Nanotechnology in Mesostructured Materials

studies in Surface Science and Catalysis 146 NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS

Studies in Surface Science and Catalysis 146

NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS

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studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Series Editor: G. Centi Vol. 146

NANOTECHNOLOGY IN MESOSTRUCTURED MATERIALS Proceedings of the 3*^^ International Mesostructured Materials Symposium, Jeju, Korea, July 8-11, 2002

Edited by Sang-Eon Park \ Ryong Ryoo ^, W h a - S e u n g Ahn ^ and Chul W e e Lee ^ and Jong-San Chang^ ^ Catalysis Center for Molecular Engineering, KRICT, Yusung, Taejon, 305-600, Korea ^ National Creative Research Initiative Center for Functional and Department of Chemistry, KAIST, Yusung, Taejon, 305-701, Korea

Nanomaterials

^ School of Chemical Science & Engineering, Inha University, Inchon, 402-751, Korea "^ Advanced Chemical Technology Division, KRICT, Yusung, Taejon 305-600, Korea

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CONTENTS

Preface Organizing committee

xxxi xxxiii

International advisory board

xxxiv

Local advisory board

xxxiv

Supporting organizations

xxxv

Financial support

xxxv

I. Synthesis and materials A new family of organic-bridged mesoporous materials

\

S. Inagaki Strategies to fabricate large-pore three-dimensional mesoporous materials with versatile

9

applications C Yu. B. Tian, J. Fan, X. Liu, H. Yang, L Wang. S. Shen, B. Tu and D. Zhao Mesostructured solid acids

15

S. Hamoudi, D. Trong on and S. Kaliaguine Template synthesis and catalysis of metal nanowires in mesoporous silicas

23

A. Fukuoka. Y Sakamoto, H. Araki, N. Sugimoto, S. Inagaki, Y. Fukushima and M. Ichikawa Mesostructured silica films with crystalline domains and structural features on multiple length

29

scales Y.-S. Lee, JR. Archer, andJ.F. Rathman Synthesis of mesoporous carbons with various pore diameters via control of pore wall thickness of

33

mesoporous silicas J.-S. Lee, S.H. Joo and R. Ryoo Ordered mesoporous carbon molecular sieves with functionalized surfaces

37

S. Jun, M.K. Choi. S. Ryu, H.-Y Lee and R. Ryoo Characterisation of ordered mesoporous carbons and their MCM-48 silica templates obtained by the replication technique using different carbon infiltration processes

41

J. Parmentier, C. Vix-Guterl, P. Gibot, M. Iliescu, J. Werckmann and J. Patarin Morphological control of highly ordered mesoporous carbon

45

C. Yu, J. Fan, B. Tian, F. Zhang, G.D. Stucky and D. Zhao Thermally induced structural changes in SBA-15 and MSU-H silicas and their implications for

49

synthesis of ordered mesoporous carbons S.H. Joo, R. Ryoo, M. Kruk and M. Jaroniec Regeneration of mesoporous inorganic materials using ordered mesoporous carbon as the template

53

J.M. Kim, M. Kang, S.H. Yi, J.E. Yie, S.H. Joo andR. Ryoo A novel preparation route for palladium-carbon composite materials - pore filling of SBA-15

57

mesoporous molecular sieve H.H.P Yiu, l.J. Bruce, F McGuinness and PA. Wright Structure of ultra-thin RbBr "Solution" in carbon nanospace

61

T. Ohkubo, H. Kanoh, Y. Hattori, T. Konishi and K. Kaneko Synthesis and characterization of mesoporous silica films by spin-coating on silicon:

65

photoionization of methylphenothiazine and photoluminescence of erbium 8-Hydroxyquinolinate in mesoporous silica films J. Y Bae, J.-I. Jung, O.-H. Park, B.-S. Bae, K.T. Ranjit andL

Kevan

Synthesis of 2D hexagonal mesoporous silica thin films via phase transition from lamellar structure

69

C.-W. Wu, K. Miyazawa and M. Kuwahara Nanostructured silicate film templated by discotic CT-complex column

73

A. Okahe, T. Fukushima, K. Ariga and T. Aida Mesoporous titania thin film with cubic mesostructure using photocalcination

77

U.-H. Lee. Y.K. Hwang and Y-U. Kwon Preparation of tin modified silica mesoporous

film

81

B. Yuliarto, H.-S. Zhou, T. Yamada. I. Honma and K. Asai Novel non-lithographic large area fabrication method to generate various polymeric nanostructures

85

W. Lee, M.-K. Jin, W.-C. Yoo and J.-K. Lee Mesoporous anodic alumina mbembrane with highly ordered arrays of uniform nanohole

89

CIV. Lee, C.I. Lee, Y Lee, H.S Kang YM. Hahm and YH. Chang Preparation and characterization of poly(ester)-silver and nylon-silver nanocomposites S.-H. Choi, K.-P LeeandS.-B.

93

Park

Synthesis of ordered three-dimensional large-pore mesoporous silica and its replication to ordered

97

nanoporous carbon J. Fan, C. Yu, L. Wang, Y Sakamoto, O. Terasaki, B. Tu and D. Zhao Morphology control of mesoporous SBA-16 using microwave irradiation Y.K. Hwang J.-S Chang Y-U. Kwon andS.-E. Park

101

One-Step synthesis of mesoporous silica SBA-15 with ultra-high microporosity

105

S.-C. Hung, H.-R Lin and C.-Y. Mou Controlling the pore sizes of SBA-15 mesoporous silica by the addition of poly(propylene oxide)

109

J.C. Park, J.H. Lee, P. Kim and J. Yi Synthesis of mesoporous silicas with different pore size by using EOmMAn diblock copolymers of

113

tunable block length as the templates Y.-T. Chan, H.-P Lin, C.-Y Mou and S.-T. Liu Polypropylene glycol as a swelling agent for the synthesis of mesoporous silica (SBA-15) by

117

amphiphilic block copolymer templating X. Cui, J.-H. Ahn, W.-C Zin, W.-J. Cho and C.-S. Ha Thermal decomposition-precipitation inside the nanoreactors high loading of W-oxide

121

nanoparticles into the nanotubes of SBA-15 L. Vradman, Y Peer, A. Mann-Kiperman and M. V. Landau Phase transition of SBA-1 induced by embedded heteropoly acids

125

S.H. Lim, H. Yoshitake and T. Tatsumi A further investigation on effect of basic media on the synthesis of MCM-41

129

C Yang, S. Ge and N. He Cationic templating with organic counterion for superstable mesoporous silica

133

P. Reinert, B. Garcia, C Morin, A. badiei, P. Perriat, O. Tdlement and L. Bonneviot. The synthesis of mesoporous materials with semicrystalline microporous walls

137

S.L Cho, YK. Kwon, S.-E. Park and G-J. Kim Synthesis of a mesoporous molecular sieve with hydrothermal stability

141

YK Kwon, G-J. Kim, J.H. Lim, D.H. Kim and B.D. Choi Diffusive characterization of large pore mesoporous materials with semi-crystalline zeolitic

145

framework H. V. Thang, A. Malekian, M. Eic, D. Trong On and S. Kaliaguine Synthesis of cubic mesoporous aluminosilicates with enhanced acidity

149

G Li, Q. Kan, T. Wu, C. Hou, F.-S. Xiao and J. Huang Synthesis and characterization of supersurface MCM-41 zeolite using additives

153

C.-M. Song Z.-F. Yan and H.-P Wang Preparation of large pore high quality MCM-48 silica by a simple post-synthesis hydrothermal

157

treatment J. Sun and M.-O. Coppens Synthesis and properties of aluminosilicate mesoporous material with adjustable pore structure

161

Y Zhang D. Wu, YH. Sun, S. Y Peng D. Y Zhao, Q. Luo and F Deng Variation of the pore properties of mesoporous silica after washing by water and ethanol-water

165

solutions L. Pasqua, F. Testa, R.Aiello, F. Di Renzo and F. Fajula Sythesis of ordered lamella mesophase from helix layered silicate (HLS)

169

M.-G. Song, J.-D. Kim and Y. Kiyozumi Sythesis of monolithic nanostructured silicate family materials through the lyotropic liquid

173

crystalline mesophases of non-ionic surfactant S.A. El-Safty and T. Hanaoka Synthesis and characterization of a new mesoporous molecular sieve

177

Q. Liu, C. Han, W. Sun, J. Yang and Y Zhou Direct- and post-hydrothermal treatments in ammoniated solution for the morphogenesis of

181

mesoporous silica nanotubes Z.-Y Yuan, B.-L. Su and W. Zhou Generalized homogeneous precipitation method for precisely controlled synthesis of mesoporous

185

silicas J. Rathousky and A. Zukal Sythesis of mesoporous silica particles preapared by using multiple emulsion

189

C Oh, J.-H. Park. S-i. Shin and S-G Oh Preparation and characterization of mesoporous silica spheres by polymerization induced colloid

193

aggregation method C.I. Lee, S. W. Lee, Y Lee, YH. Chang and YM. Hahm Preparation of mesoporous solids by agglomeration of silica nanospheres Y.K. Ferreira, M. Wallau and E.A.

197

Urquieta-Gonzdlez

Ordered mesostructured materials with composite walls of decavanadate and silica

201

Y-Y Chang, YK. Hwang, H. Choi and Y-U. Kwon Nanoporous alumina formation using mulit-step anodization and cathodic electrodeposition of

205

metal oxides on its structure / Oh, Y Jung. J. Lee and Y Tak Synthesis of mesoporous y-aluminas of controlled pore properties using alkyl carboxylate assisted

209

method Y Kim. C Kim. J. W. Choi. P Kim and J. Yi Synthesis and characterization of mesoporous alumina molecular sieves using cationic surfactants

213

HJ. Kim, H.C Lee, D.H Choo, H.C Lee. S.H. Chung. KH. Lee andJ.S. Lee Synthesis and characterization of mesoporous alumina molecular sieves with cationic surfactants in

217

the presence of formamide H.C. Lee. H.J. Kim. D.H Choo, H.C. Lee. S.H. Chung, KH. Lee andJ.S

Lee

Structure and properties of porous mesostructured zirconium oxo-phosphate with cubic(Ia-Id)

221

symmetry F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki and F. Schuth Synthesis and characterization of mesoporous titanium oxide

227

J.-L. Tsai, H.-W. Wang andS. Cheng Improvement of thermal stability of Ti-Zr mesoporous oxides using CTAB surfactant templates

231

mixed with auxiliary organic additives W. Li, X. Yang, Y. Zhang and W. Chu Synthesis and characterization of mesoporous zirconia

235

Y-W. Suh, J.-W. Lee and H.-K. Rhee A novel method to prepare mesoporous nano-zirconia

239

X.-M. Liu, M. G.Q. Lu, Z.-F Yan Control of ordered mesoporous molecular sieves synthesis using non-ionic surfactants by

243

incorporation of transition metal ions in the micellar solution A. Leonard, J.L. Blin, G. Merrier and B.-L. Su Texture of chromia aerogels and structure of their nanocrystals

247

M. Abecassis-Wolfovich, H. Rotter, M.V. Landau, E. Korin, A./. Erenhurg, D. Mogilyansky and E. Garshtein Preparation of ordered mesoporous NbTa mixed oxide with crystallized wall

251

T. Katou, B. Lee, D. Lu, J.N. Kondo, M. Hara and K. Domen Compositional effects of bimodal mesopore silica synthesized by a base-catalyzed ambient pressure

255

sol-gel processing X.-Z. Wang. W.-H. Li, T. Dou and B. Zhong A direct template synthesis of highly ordered mesostructured carbons using as-synthesized MCM-

259

48 as template S.B. Yoon, J. Y. Kim, Y.-S. Ahn, H.-S. Kim and J.-S. Yu

II. Characterization Gas adsorption: a valuable tool for the pore size analysis and pore structure elucidation of ordered

263

mesoporous materials M. Jaroniec and M. Kruk Three-dimensional transmission electron microscopy of disordered and ordered mesoporous

271

materials K.P. deJong, A.M. Janssen, P. van der Voort and A.J. Koster Structures of silica-mesoporous crystals and novel mesoporous carbon-networks synthesized within the pores

275

O. Terasaki, Z. Liu, T. Ohsuna, T. Kamiyama, D. Shindo, K. Hiraga, S.H.Joo, T.-W. Kim and R. Ryoo Phase transformations involved during silica, modified silica, and non-silica mesoporous

281

organized thin films deposition. The role of evaporation D. Grosso, E.L Crepaldi, GJ.de A. A. Soler Illia, F. Cagnol, N.Baccile, F. Babonneau, P.A. Albouy, H. Amenitsch and C. Sanchez Comparison of the mechanical stability of cubic and hexagonal mesoporous molecular sieves with

285

different pore sizes M. Hartmann and A. Vinu Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as

289

microchemical reactor S. Y. Ryu and MJ. Yoon Detailed investigation of the microporous character of mesoporous silicas as revealed by small-

295

angle scattering techniques B. Smarsly, K. Yu and CJ. Brinker X-ray diffraction analysis of mesostructured materials by continuous density function technique

299

LA. Solovyov, O. V. Belousov, A.N. Shmakov, V.I. Zaikovskii, S.H. Joo, R. Ryoo, E. Haddad, A. Gedeon and S.D. Kirik Influence of aluminium, lanthanum and cerium on the thermal and hydrothermal stability of MCM-

303

41-type silicates M. Wallau, R.A.A. Melo and E.A.

Urquieta-Gonzalez

Enhanced acidity and hydrothermal stability of mesoporous aluminosilicate with secondary

307

building units characteristic of zeolite beta W. Guo, L. Kong, C.-S. Ha and Q. Li Chemical coating of the aluminum oxides onto mesoporous silicas by a one-pot grafting method

311

Y.-H. Liu, H.-P. Lin. C.-Y Mou. B.-W. Cheng and C.-F Cheng Acidity and temperature effect on the synthesis of SBA-1 M.-C

315

Liu, H.-S. Sheu and S. Cheng

HMS materials with high Al loading: a joint FT-IR and microcalorimetric study of their

319

acidic/basic properties B. Bonelli, B. Onida, B. Fubini, J.D. Chen, A. Galarneau, FD. Renzo and E. Garrone Effect of cations addition for the highly ordered mesoporous niobium oxide

323

B. Lee, D. Lu, J.N. Kondo and K. Domen Synthesis of zirconium-containing mesoporous silica Zr-MCM-48 membranes with high alkaline

327

resistance for nanofiltration D.-H. Park, H. Saputra, N. Nishiyama. Y. Egashira and K. Veyama Synthesis of siliceous MCM-41 grafted with transition metal carbonyls

331

R.-S. Raul, J.M. Dominguez, R. Feridand T.C. Alvarez Surface and pore structures of CMK-5 ordered mesoporous carbons studied by nitrogen adsorption

335

and surface spectroscopic methods H. Darmstadt, C. Roy, S. Kaliaguine, T.-W. Kim andR. Ryoo A comparison of the sorption properties of mesoporous molecular sieves MCM-41 and MCM-48

339

J.C. Vartuli, W.J. Roth, J.D. Lutner, S.A. Stevenson and S.B. McCullen Argon and nitrogen adsorption on ordered silicas with channel-like and cage-like mesopores:

343

implications for characterization of porous solids M Jaroniec and M. Kruk Mesopores developed in KL zeolite dealuminated with (NH4)2SiF6 solution

347

N. He, C. Yang, J. Tang and H. Chen Time-resolved in situ grazing incidence small angle x-ray scattering experiment of evaporation

351

induced self-assembly A. Gibaud, D. Doshi, B. Ocko, V. Goletto and C.J. Brinker Small angle neutron scattering study on the formation mechanism of mesostructures during sol-gel

355

processing Y.K. Kwon, D.H. Kim, G.-J. Kim, Y.-S. Han and B.-S. Seong Preparation of mesoporous silica anchored Mo catalysts and in-situ XAFS characterization under

359

propene photometathesis reaction N. Ichikuni, T. Eguchi, H. Murayama, K.K. Bando, S. Shimazu and T. Uematsu In-situ XAFS observation of formation of Pd-Pt bimetallic particles in a mesoporous USY zeolite

363

K.K. Bando, T. Matsui, L. Le Bihan, K. Sato, T. Tanaka, M. Imamura, N. Matsubayashi, and Y. Yoshimura Investigation of the internal pore structures of Beta/MCM-41 and ZSM-5/MCM-41 composites by

367

'^^Xe NMR W. Guo, L. Huang, C.-S. Ha and Q. Li Study of chromium species in the Cr-MCM-48 mesoporous materials by Raman spectroscopy

371

C Pak, H.S. Han and G L. Haller Covalent bonding of Disperse Red 1 in HMS silica: synthesis and characterization

375

B. Onida, L. Borello, S. Fiorilli, C Barolo, G Viscardi, D.J. Macquarrie and E. Garrone Accessibility of dye-molecules embedded in the micellar phase of hybrid mesostructured MCM41-

379

type materials B. Onida, B. Bonelli, L. Borello, S. Fiorilli, S. Bodoardo, N. Penazzi, C Otero Aredn, G Turnes Palomino and E. Garrone Influence of surface properties of MCM-48 on the formation of a nanocomposite structure based on MCM-48 and PVA J. He, J. Yang, S. Zhang, D. G Evans, X. Duan

383

A study on the structure of Si-O-C thin films with nano size pore by ICPCVD

387

T. Oh, K.-M. Lee and C.K. Choi Template effects on low k materials made from spin-on mesoporous silica

391

C.-Y. Ting, D.-F. Ouyan, W.-F. Wu and B.-Z. Wan Porosity tuning of single-wall carbon nanohoms with gaseous activation

395

E. Bekyarova, K. Murata, K. Kaneko, D. Kasuya, M. Yudasaka and S. lijima

III. Modification and composite Expanding horizons of mesoporous materials to non-siliceous systems

399

F Schiith, T. Czuryskiewicz, F. Kleitz, M. Linden, A. Lu, J. Rosenholm, W. Schmidt, A. Taguchi Structure and shape control in functional mesostructured materials from block copolymer

407

mesophases U. Wiesner Strategies for spatially separating photoactive molecules in mesostructured sol-gel silicate

films

413

R. Hernandez, P. Minoofar, M. Huang, A.-C. Franville, S. Chia, B. Dunn andJ.L Zink Design of supported catalysts by surface functionalization of micelle-templated silicas

419

D. Brunei, AC. Blanc, P.-H. Mutin, O. Lorret, V. Lafond, A. Galarneau, A. Vioux and F. Fajula Proteosilica-mcsopoTous silicates densely filling amino acid and peptide assemblies in their

427

nanoscale poresK. Ariga, Q. Zhang, M. Niki, A. Okahe and T. Aida Counteranion effect on the formation of mesoporous materials under acidic synthesis process

431

S. Che, M. Kaneda. O. Terasaki and T. Tatsumi Influence of alumination pathway on the steam stability of Al-grafted MCM-41

435

R. Mokaya Macroporous titanium oxides: from highly aggregated to isolated hollow spheres

439

P. Reinert. C Graillat, R. Spitz and L. Bonneviot Nanostructured mesoporous Ti02, Zr02 and Si02 synthesis by using the non-ionic Cm(EO)n -

443

inorganic alkoxyde system : toward a better understanding on the formation mechanism J. L. Blin, A. Leonard, L. Gigot, O. Provoost and B. L. Su Morphology control of hierarchically ordered ceramic materials prepared by surfactant-directed

447

sol-gel mineralization of wood cellular structures Y. Shin, L.-Q. Wang, J.H. Chang, WD. Samuels. GJ. Exarhos A NH3-responding material based on Reichardt's dye-impregnated mesoporous silica

453

B. Onida, S. Fiorilli, R. Gobetto, A. Russo, D.J. Macquarrie and E. Garrone Preparation and redox behavior of ordered porous zirconium oxide loaded with cerium

457

H.-R. Chen, J.-L Shi, J.-N. Yan, H.-G. Chen and D.-S. Yan Direct synthesis of bi-fimctionalized organo-MSU-X silicas

461

Y Gong, Z. Li, D. Wu, Y. Sun, B.H. Dong, F. Deng High-density modification of mesoporous silica inner walls with amino acid function by residue

465

transfer from template Q. Zhang, K. Ariga, A. Okabe and T. Aida The synthesis of optically active amino acid over Pd catalysts impregnated on mesoporous support

469

K.H. Chang, YK. Kwon and G.-J. Kim Sulfonic acid-functionalized periodic mesoprous organosilicas

473

S. Hamoudi and S. Kaliaguine Functionalized periodic mesoporous organosilicas with sulfonic acid group

477

X. Yuan, H.I. Lee, J. W. Kim, J.E. Yie andJ.M. Kim Exclusive incorporation of aluminum into tetrahedral site of the framework of periodic mesoporous

481

organosilica S.S. Park, J.H. Cheon and D.H. Park Functionalization of hexagonal mesoporous silica and their base-catalytic performance

485

C Yang X. Jia, Y Cao and N. He Microstructure of the organo-modified SBA-15 (Vinyl-SBA 15) prepared under different pH

489

B.-G Park, J. Park, W. Guo, W.-J. Cho and C-S Ha Surface coating of MCM-48 via a gas phase reaction with hexamethyldisilazane (HMDS)

493

A. Daehler, ML. Gee, F. Separovic, G W. Stevens and A.J. O'Connor Reacitvity of silica walls of mesoporous materials towards benzoyl chloride

497

L. Pasqua, F. Testa and R. Aiello Catalytic activitiy of chiral phosphinooxazolidine ligands immobilized on SBA-15 for the

501

asymmetric allylic substitution PH. Chong, YK. Kwon. C Y Lee and G.-J. Kim Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic

505

activity in Knoevenagel condensation K.-S. Kim, J.H. Song, J.-H. Kim and G Seo MCM-41-supported norephedrine ligand for ruthenium-catalyzed asymmetric transfer

509

hydrogenation of ketones M.-J. Jin, S.-H Kim, S.-J Lee and W.-S Ahn Synthesis of silica support for biocatalyst immobilization

513

J.-K Kim, J.-K. Park and H.-K Kim Mesostructured materials for controlled macromolecular and supramolecular architectures M. Ikegame, K Tajima and T. Aida

517

Nanocomposites of MCM-41 and SBA-15 with polyaniline for electrorheological fluid

523

M.S. Cho, H.J. Choi, K. Y. Kim and W.S. Ahn Functionalized mesoporous adsorbents for Pt(II) and Pd(II) adsorption from dilute aqueous solution

527

T. Kang, Y. Park, J.C. Park, YS. Cho and J. Yi Environmentally benign removal of pollutant oxyanions by Fe adsorption center in functionalized

531

mesoporous silica T. Yokoi, T. Tatsumi and H. Yoshitake How can nanoparticles change the mechanical resistance of ordered mesoporous thin films?

535

E. Craven, S. Besson, M. Klotz, T. Gacoin, J.-P. Boilot and E. Barthel Nanoporous Si02 films prepared by surfactant templating method - a novel antireflective coating

539

technology H.-T. Hsu. C.-Y Ting, C.-Y Mou andB.-Z. Wan Textural and structural properties of Al-SBA-15 directly synthesized at 2.9 < pH < 3.3 region

543

M.S. Mel'gunov, E.A. Mel'gunova, A.N. Shmakov, V.I. Zaikovskii Fabrication of nanostructured SiC and BN from templated preceramic polymers

547

I.-K. Sung, T.-S. Kim. S.-B. Yoon, J.-S. Yu and D.-P Kim

IV. Application and catalysis Mesoporous solids for green chemistry

^^.

/ H. Clark Ultrastable acidic MCM-48-S assembled from zeolite seeds

^^-^

P.-C Shih. H.-P Lin and C.-Y Mou Acidic zeolite coated mesoporous aluminosilicates

^^^

D. Trong On and S. Kaliaguine Stable ordered mesoporous titanosilicates with active catalytic sites

c^r

F.-S. Xiao. Y Han. X. Meng. Y Yu. M. Yang, and S Wu W/Zr mixed oxide supported on mesoporous silica as catalyst for n-pentane isomerization

^^g

T. Li. S.-T. Wong. M.-C. Chao. H.-P Lin. C.-Y Mou andS. Cheng Au and Au-Pt bimetallic nanoparticles in MCM-41 materials: applications in CO preferential

^^^

oxidation S. Chilukuri. T. Joseph. S Malwadkar. C Damle. SB. Halligudi. B.S. Rao, M. Sastry and P. Ratnasamy Effective inclusion of chlorophyllous pigments into mesoporous silica for the energy transfer

^^^

between the chromophores H. Furukawa and K. Kuroda Biological applications of organically functionalised mesoporous molecular sieves and related

t:o^

materials H.H.R Yiu and IJ. Bruce One pot synthesis of mesoporous ternary V205-Ti02-Si02 catalysts

585

V. Pdrvulescu, V.I. Parvulescu, M. Alifanti, S. M. Jung and P. Grange Photocatalytic hydroxylation of benzene on Ti-modified MCM-41 with both framework and non

589

framework Ti- centers Z. Guo, J. He, S. Zhang, D. G. Evans, X. Duan The relationship between the local structures and photocatalytic reactivity of Ti-MCM-41 catalysts

593

Y. Hu, G Martra, S. Higashimoto, J. Zhang, M. Matsuoka, S. Coluccia and M. Anpo Photocatalytic epoxidation of propene with molecular oxygen under visible light irradiation on V

597

ion-implanted Ti-HMS and Cr-HMS mesoporous molecular sieves H. Yamashita, K. Kida, K. Ikeue, Y. Kanazawa, K. Yoshizawa, and M. Anpo Mesostructured Ti02 films as effective photocatalysts for the degradation of organic pollutants

601

J. Rathousky, M. Slabovd and A. Zukal Comparisons of the structural and catalytic properties of Ti-HMS synthesized using the

605

hydrothermal and molecular designed dispersion methods T. Williams and GQ.(Max) Lu Oxidation of methyl-propyl-thioether with hydrogen peroxide using Ti-SBA-15 as catalyst

609

DC. Radu, A. Ion, V.I. Pdrvulescu, V. Cdmpeanu, E. Bartha, D. Trong On and S. Kaliaguine Direct synthesis of hydrothermally stable mesoporous Ti-MSU-G and its catalytic properties in

613

liquid-phase epoxidation P. Wu, H. Sugiyama and T. Tatsumi Mesoporous V-containing MCM-41 molecular sieves: synthesis, characterization and catalytic

617

oxidation C.-W. Chen

andA.-N.Ko

Catalytic oxidation of H2S to elemental sulfur over mesoporous Nb/Fe mixed oxides

621

S.J. Jung, M.H. Kim, J.K. Chung, M.J. Moon, J.S. Chung, D. W. Park and H.C Woo Fe-MCM-41 catalyzed epoxidation of alkenes with hydrogen peroxide

625

Q. Zhang, Y. Wang, S. Itsuki, T. Shishido and K. Takehira Highly selective oxidation of styrene with hydrogen peroxide catalyzed by mono- and

629

bimetallic (Ni, Ni-Cr and Ni-Ru) incorporated MCM-41 silicas V. Parvulescu, C. Dascalescu and B.L. Su Mixed (Al-Cu) pillared clays as wet peroxide oxidation catalysts

633

S.-C. Kim, S.-S. Oh, G.-S. Lee, J.-KKang, D.-S. Kim and D.-K. Lee Finely-dispersed Ni/Cu catalysts supported on mesoporous silica for the hydrodechlorination of chlorinated hydrocarbons

637

Y.G Park, T.W. Kang, Y.-S. Cho, P. Kim. J.-C. Park and J. Yi New SO2 resistant mesoporous mixed oxide catalysts for methane oxidation

641

D. Trong On, S. V. Nguyen and S. Kaliaguine Decomposition of VOCs using mesoporous Ti02 in a silent plasma

645

W.-H. Hong. K.-S. Choi. G-J. Kim andD.-W. Park Preparation of mesoporous 12-tungstophosphoric acid HPW/Si02 and its catalytic performance

649

Z. Zhu. W. Lu and C. Rhodes Catalytic properties of heteropolyacids supported on MCM-41 mesoporous silica for hydrocarbon

653

cracking reactions J. N. Beltramini Preparation, characterization and catalytic activity of heteropolyacids supported on mesoporous

657

silica and carbon Z. Zhao. W. Ahn and R. Ryoo Novel SBA-15 supported heteropoly acid catalysts for benzene alkylation with 1-dodecene H.-O. Zhu. J. Wang. C.-Y Zeng andD.-Y

661

Zhao

Aluminum containing periodic mesoporous organosilicas: synthesis and etherification

665

J.-W. Kim, H.I. Lee. J.M. Kim, X.D. Yuan andJ.E. Yie Friedel-crafts alkylation over Al-incorporated mesoporous honeycomb

669

Y-S. Ahn. H.S. Kim, M.H. Han. S. Jun, S.H. Joo, R. Ryoo and S.J. Cho Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity K.-K Kang and H-K

673

Rhee

Asymmetric dihydroxylation catalyzed by MCM-41 silica-supported bis-cinchona alkaloid

677

S.-H. Kim and M.-J. Jin Roles of pore size and Al content on the catalytic performance of Al-MCM-41 during

681

hydrocracking reaction W.-H. Chen. Q. Zhao. S.-J. Huang, C-Y Mou andS.-B Liu HDS of FCC gasoline: Mesoporous modified support catalyst and its effects on the hydrogenolysis

685

reaction selectivity G.H. Tapia, T. Cortez. R. Zarate. J. Herbert and J. L. Cano Synthesis of mesoporous carbon nanotubes and their application in gas phase benzene

689

hydrogenation DC Han. Z.Q. Zhu. A.M. Zhang. J.Z. Zhu andJ.L Dong Characteristics and reactivities of cobalt based mesoporous silica catalysts for Fischer-Tropsch

693

synthesis W.S. Yang. H. W. Xiang, YY Xu and Y.-W Li Physicochemical characteristics of Ti-PILC as a catalyst support for the reduction of NO by NH3

697

H.J. Chae. I.-S. Nam and S.B. Hong Performance of double wash-coated monolith catalyst in selective catalytic reduction of NOx with

701

propene H.-G. Ahn andJ.-D.

Lee

Copper loaded MCM-41. An alternative catalyst for NO reduction in exhaust gases ? - Study of its

705

acidic and redox properties M.S. Batista, M. Wallau, R.A.A. Melo and E.A.

Urquieta-Gonzdlez

Studies on synthesis and activity for selective catalytic reduction of NO over Pt supported MCM-48

709

J.-S.Yang, S.-C. Lee and S.-J. Choung Synthesis of titania-pillared clays and their application as catalyst supports for selective catalytic

713

reduction of NO with ammonia S.~C. Kim, J.-K. Kang, D.-S Kim and D.-K. Lee Hydrogenation of aromatics on Pt/Pd bimetallic catalyst supported by Al-containing mesoporous

717

silica S.-Y.Jeong High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and

721

performance in hydrodesulfurization and hydrogenation L. Vradman, M. V. Landau, M. Herskowitz, V. Ezersky, M. Talianker, S. Nikitenko, Y. Koltypin, A. Gedanken Cr-MCM-41 for selective dehydrogenation of lower alkanes with carbon dioxide

725

Y. Wang, Y Ohishi, T. Shishido, Q. Zhang and K Takehira Methane reforming on molybdenum carbide on Al-FSM-16

729

M Nagai, T. Nishivayashi and S. Omi Preparation of carbided WO3/FSM-16 and Al-FSM-16 and its catalytic activity

733

M Nagai, K. Kunieda, S. Izuhal and S. Omi AMI study on the catalytical isomerization of 1-hexene to 2-hexene on the surface of

737

aluminosilicate molecular sieves M. Pu, Z.~H. Li, S.-R. Zhai, D. Wu, Y.-H. Sun Isomerization and hydrocracking of n-decane over Pt/MCM-41//MgAPO-n composite catalysts

741

S.P. Elangovan and M. Hartmann Catalytic properties of mesoporous aluminosilicates and lanthanum containing mesoporous

745

aluminosilicates studied by m-xylene isomerisation M. Wallau, R.A.A. Melo and E.A.

Urquieta-Gonzdlez

Diels-Alder reaction catalyzed by ordered micro- and mesoporous silicates

749

Y. Kubota, H. Ishida, R. Nakamura and Y. Sugi Isospecific polymerization of propylene with metal-MCM-41

753

W.Z. Shen, J.T. Zheng, Y.L. Zhang, J.G Wang andZ.F. Qin A possibility of block-copolymer templated mesoporous silica films applied to surface photo

783

voltage (SPV) type NOx gas sensor T. Yamada, H.S. Zhou, H. Uchida, M. Tomita, Y. Ueno, Y. Katsube and I. Honma Proton conducting silica mesoporous/heteropolyacid-PVA/SSA nano-composite membrance for

787

polymer electrolyte membrane fuel cell Y.-H. Chu, J.-E. Lim, H.-J. Kim, C.-H. Lee, H.-S. Han and Y.-G Shut Hydrothermal synthesis of titania nanotube and its application for dye- sensitized solar cell

791

S. Uchida, R. Chiba, M. Tomiha, N. Masaki and M. Shirai Preparation of hydrophobic Ti-containing mesoporous silica by the F-modification and their

795

photocatalytic degradation of organic pollutant diluted in water H. Yamashita, H. Nakao, M. Okazaki and M. Anpo Synthesis of functionalised silicas for immobilisation of homogeneous catalysts

799

S.A. Riddel, W.R Hems, A. Chesney andS.R. Watson Author index Subject index Other volumes in the series

811 815

PREFACE The 3rd International Mesostructured Materials Symposium (IMMS 2002) was held successfully in Jeju Island, Korea from July 8 to July 11, 2002 under the auspices of the International Mesostructured Materials Association (IMMA). We would like to express sincere thank all the members of the IMMA council and the International Advisory Board for their active supports for the conference. The attendance in this conference was very encouraging with respect to the futuristic perspective of the scientific field in mesostructured materials and their applications. Four years ago, the first International Symposium on Mesoporous Molecular Sieves (ISMMS) was held in Baltimore. This symposium was followed by the second meeting two years later in year 2000 at Quebec, and the International Mesostructured Materials Association was organized following the success of the Quebec symposium. The title of the symposium was also changed to the International Mesostructured Materials Symposium (IMMS) in order to accommodate the rapidly expanding field of various types of mesostructured materials such as organic polymers, metals, organic-inorganic nanocomposite, and ordered mesoporous carbons. During the 4 day meeting of the IMMS 2002, 5 plenary lectures, 18 keynote lectures, and 25 papers were presented orally in 4 sessions and 174 papers as well as 19 recent research reports were presented as posters. Their topics of the IMMS 2002 covered the followings: synthesis and characterization of periodic mesoporous silicas and other metal oxides organic-inorganic hybrids with mesoscopic periodicity sol-gel approach for mesostructured materials synthesis and applications of mesoporous carbons synthesis of new nanostructured materials using mesoporous templates mesostructured and mesoporous organic polymers, pore size analysis and structure modeling host-guest interaction and molecular imprinting on mesoporous materials

-

catalytic applications of mesoporous materials adsorption and separation using mesoporous materials application of mesostructured materials for optical, electronic, electric, and magnetic

devices. We believe that the IMMS 2002 provided you with the most recent research results and stimulating scientific discussions with opening of novel and diverse mesostructured materials. This book was conceived as the proceedings of the IMMS 2002, which reflect various aspects of synthesis, characterization, and applications of mesostructured materials exhibiting

a mesoscopic periodicity. Recently, mesostructured materials including periodic mesoporous materials have been receiving much attention due to their potential uses in nanotechnology. Actually these materials are considered to be the promising candidates for designing nanoscopically-engineered materials due to their well-defined pore structures, tailor-made synthetic ability, and hosting ability for guest species exhibiting catalytic, optical, electronic and magnetic properties. In these proceedings the reader will indeed find regular papers from many groups worldwide, covering the most recent advances in mesostructured materials to give future perspective of nanotechnology. The organizers wish to express sincere appreciation to attendees of the IMMS 2002 and the authors for submitting their manuscripts to the proceedings. We are grateful to the outstanding scientists who accepted our invitation to overview vital research areas in plenary lectures and the keynote lectures that introduce the important topics of each session covered by the conference. We are also grateful to Prof Chang-Sik Ha, Prof Jong-Sung Yu, Dr. Jong-San Chang, Mr. Sang Hoon Joo, Prof Ji Man Kim, Dr. Sung June Cho and Prof Dong Ho Park who have spent so much time and efforts for the success of the symposium IMMS 2002. Furthermore, we wish to thank members of catalysis center for molecular cngineering(CCME), KRICT, especially Dr. Soo Min Oh, and members of center for functional nanomatcrials, KAIST, who very efficiently helped in the preparation of the proceedings. Finally we wish to acknowledge the help and generous financial support by co-operating organizations and sponsors from industry.

Jcju, July 2003 Sang-Eon Park Ryong Ryoo Wha-Seung Ahn Chul Wee Lee

Organization Organizing Committee Chairman Ryong Ryoo

KAIST, Korea

Co-Chairman Sang-Eon Park

KRICT, Korea

Secretary Wha-Seung Ahn

Inha Univ., Korea

Treasurer Chang-Sik Ha

Pusan National Univ., Korea

Scientific committee Jong-Sung Yu, Chair Dong Ho Park Ji Man Kim Jinwoo Cheon Jong-Ho Kim Kookheon Char Kyung Byung Yoon Seong-Geun Oh Seung-Kyu Park Sung June Cho Taeghwan Hyeon Yong Gun Shul Young-Uk Kwon

Hannam Univ., Korea Inje Univ., Korea Ajou Univ., Korea Yonsei Univ., Korea Chonnam National Univ., Korea Seoul National Univ., Korea Sogang Univ., Korea Hanyang Univ., Korea LG Co., Korea KIER, Korea Seoul National Univ., Korea Yonsei Univ., Korea Sungkyunkwan Univ., Korea

Program committee Chul Wee Lee, Chair Byung-Sung Kwak Chanho Pak Duk-Young Jung Geon-Joong Kim Wha Jung Kim Jung Hwan Park Jong-San Chang

KRICT, Korea SK Co., Korea SAIT, Korea Sungkyunkwan Univ., Korea Inha Univ., Korea Konkuk Univ., Korea Zeobuilder Co., Korea KRICT, Korea

Kwang Ho Park Myongsoo Lee Sang Sung Nam Soon-Yong Jeong Sun Keun Hwang

LG Co., Korea Yonsei Univ., Korea KRIC, Korea KRICT, Korea Aekyung PQ Adv. Mater Co., Korea

International committee Chung-Yuan Mou Dongyuan Zhao Osamu Terasaki Takashi Tatsumi

National Taiwan Univ., Taiwan Fudan Univ., China Tohoku Univ., Japan Yokohama National Univ., Japan

International Advisory Board Abdelhamid Sayari Avehno Corma C. N. R. Rao Charles T. Kresge Francois Fajula G. Q. Max Lu Galen D. Stucky George S. Attard Ilyun-Ku Rhee Jackie Y Ying James C. Vartuli Kazuyuki Kuroda Kenneth J. Balkus Klaus K. Linger Larry Kevan Laurent Bonneviot Masakazu Anpo Michael W. Anderson Mietek Jaroniec Mingyuan He Pierre A. Jacobs Serge Kaliaguine Shilun Qiu Shinji Inagaki Thomas J. Pinnavaia

Univ. Ottawa, Canada Univ. Politencnica de Valencia, Spain Jawaharlal Nehru Centre, India Dow Chemical Co., USA ENSCM, France Univ. Queensland, Australia Univ. California, Snata Barbara, USA Univ. Southampton, UK Seoul National Univ., Korea MIT USA Mobil, USA Waseda Univ., Japan Univ. Texas, Dallas, USA Johannes Gutenberg Univ., Germany Univ. Houston, USA Ecole Normale Superieure de Lyon, France Osaka Prefecture Univ., Japan UMIST UK Kent State Univ, USA Beijing Petro. Chem. Eng. Inst., China Katholieke Univ.. Leuven, Belgium Laval Univ., Canada Jilin Univ., China Toyota Central R&D Labs., Japan Michigan State Univ., USA

Local Advisory Board Baik-Hyon Ha Gon Seo

Hanyang Univ., Korea Chonnam National Univ., Korea

Hakze Chon Seong Ihl Woo Son-ki Ihm Yang Kim Young Sun Uh

KAIST, Korea KAIST, Korea KAIST, Korea Pusan National Univ., Korea Young Lin Instrument Co., Korea

Supporting Organizations Korean Ministry of Science and Technology Korea Advanced Institute of Science and Technology Financial Support The Organizing Committee gratefully acknowledges the receipt of financial support from R&D Center of SK Corp., Korea ATI Korea Co., Ltd. Korea I. T. S. Co., Ltd. Protech, Korea Young Lin Instrument Co., Ltd., Korea

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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

A new family of organic-bridged mesoporous materials Shinji Inagaki Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan A new family of hybrid mesoporous materials containing a variety of bridging organic groups inside the pore walls is reported. A short review about the previous studies on the synthesis of organic-bridged mesoporous materials along with our recent research works on the formation of crystal-like ordered structure in the pore walls of organic-bridged mesoporous materials as well as the functionalization of ordered pore-walls of mesoporous benzene-silica with sulfuric acid groups is summarized. 1. INTRODUCTION The periodic mesoporous materials'"^^ have definite advantages of possessing uniform pores whose sizes are larger than those of zeolites, the high stability and the diversity in controlling framework composition and morphologies. The functionality of pore-wall surface was generally poor in previous mesoporous materials because the pore walls are composed of amorphous materials. The amorphous nature of the pore walls also narrows a range of application of the mesoporous materials. Various efforts have been made for the functionalization of pore walls of mesoporous materials by different approaches such as introduction of organic groups in the framework^"" ^ and crystallization of pore walls.'^''^^ The mentioned -Si(0R)3 approaches would be indeed {R0)3Sieffective to improve the functionality of pore walls. The

organic-bridged

mesoporous material has uniformly distributed orga

Fig.l Synthesis of organic-bridged mesoporous material and the pore-wall structure.

nic and inorganic moieties in the framework that are covalently bonded to each other and form stable framework (Fig.l). The materials have been synthesized from pure organosilane precursors (100%) having two or more silyl groups attached to the organic groups in the presence of surfactants. The organic-bridged mesoporous materials are quite distinct from conventional organic-grafted mesoporous materials having terminal organic groups in the pore space. In other reports, the attempts have been made to crystallize the pore walls of mesoporous materials include the synthesis of mesoporous transition metal oxides such as titania, zirconia with partially crystallized pore walls^^'^"^^ and the synthesis of mesoporous aluminosilicates composed of crystal seeds of zeolite in the framework.^'^'^''^ However, so far we have had no report on periodic mesoporous material possessing ordered structure in the whole region of pore walls. In this article, I, present a short review on the previous studies on the synthesis of organic-bridged mesoporous materials along with our recent works on the formation of crystal-like ordered structure in the pore walls of organic-bridged mesoporous materials as well as the functionalization of ordered pore-walls of mesoporous benzene-silica with sulfuric acid groups. 2. A SHORT REVIEW ON ORGANIC-BRIDGED MESOPOROUS MATERIALS Tabic 1 Hybrid mesoporous materials prepared from 100% of organic-bridged silane. Bridging organic8(-R-) -CHjCHj-

-CHjCHj(Block copolymer) -CH=CH-

-O^

-o^

Mesophases

Authors

2D & 3D-hex. Inagaki'^) Disrodered Stein 2") Cubic(film) Brinker^'^) Cubic Pm-3n Inagak'^' 2D-hex,Cubic Sayari'*** 2D-hex. Park' ) Brinker^^), Char^''), Roziere^^', Froba2«), Burleigh^'^), Jaroniec^"' Stein^') Disordered 2D-hex. Ozin^i) Ozin22) Disordered Inagaki'*2) 2D-hex. 2D-hex.

Inagaki^')

Disordered

Ozin22)

2D-hex.

Ozin2^)

2D-hex.

Ozin2^)

(PPO-PEO-PPO)

^Q
^

The

synthesis

of

organic-bridged

mesoporous materials from the pure organic-bridging precursors (100%) is summarized in Table 1. We were the first to reported the synthesis of organic-bridged mesoporous materials from ethane-bridged precursor using alkyltrimethylammonium as a surfactant (Fig. 2).^^'*^^ Control of the synthesis temperature and alkyl-chain length of surfactant resulted in the formation of three mesophases of two- and three-dimensional hexagonal and cubic Pm-3n with highly ordered structures. The ethane-bridged mesoporous materials

showed

single

crystal-like

well-defined particle morphologies of hexagonal rod, spherical, and decaoctahedral shapes, whose shapes reflect the underlying the pore-arrangement symmetries.^^'^"^^ After our

(MeO)3Si-CH2CH2-Si(OMe)3

% NaOH/HgO Ci8H37N(CH3)3CI

Fig. 2 The first synthesis of organic-bridged mesoporous material.^^^ publication, Stein's and Ozin's groups reported the similar organic-bridged mesoporous materials including ethylene^°'^^\ benzene^^\ thiophene^^\ methane^"'^ toluene^"^^ etc. Drinker's group reported thin films of ethane-bridged mesoporous materials and spherical particles of benzene-bridged mesoporous materials by evaporation-induced self-assembly method.^^^ Recently, several groups described the synthesis of ethane- and benzene-bridged mesoporous materials with large pore size and wall thickness using nonionic triblock coplymers.^^-^^^ Table 2 lists the synthesis of bifunctionalized mesoporous materials by co-condensation of bridging and terminal (or TEOS) precursors. The co-condensation approach resulted in the synthesis of various mesoporous materials containing both of bridging organic moieties inside the walls and terminal groups protruding into the channel space.^^'"^''^ The bifunctional mesoporous materials have unique structure in which bridging organics play a structural and Table 2 Hybrid mesoporous materials mechanical role while the terminal groups are prepared from the mixtures of organicbridged and terminal(or TEOS) silanes. readily accessible for chemical transformation. Bridging organics(-R-)

Terminal organic(-R')

Alvaro et al. reported a mesoporous material containing viologen units in the framework by Ozin^2) -CH=CH- + -CH=CH2 co-condensation with TEOS."'^^ The pore walls -CHjCHj- + Burleigh^^ ^^) of the mesoporous material should show 'N/^NH^NH2 unique optoelectrical properties because viologenes are the most widely used electron acceptor units in a variety of charge transfer complexes and electron transfer processes. The bridged mesoporous materials containing Garcia"^^) + TEOS amine complexes in the framework have been Corriu^^) + TEOS also synthesized by co-condensation with » Mercier^**) + T E O S TEOS.^^'^^^ The reports also exist on the 'S/^N'^NA/^ organic-bridged mesoporous materials Inagaki'^'*) (-SO3H) incorporating Al and Ti in the framework.^'^ '^"^ By combining these previous synthesis approaches it is possible to design unique mesoporous catalyst containing hydrophobic and hydrophilic sites, acid sites and organic functional sites.

-O-O"

-o-

--0

Authors

2. FORMATION OF CRYSTAL-LIKE PORE WALL STRUCTURE The crystal-like periodic structure in the pore walls was first observed for benzene-bridged mesoporous material prepared under the controlled synthesis conditions."^^^ The benzene-bridged mesoporous material was synthesized by condensation of 100% of l,4-bis(triethoxysilyl)benzene [(C2H50)3Si-C6H4-Si(OC2H5)3, BTEB] in the presence of octadecyltrimethylammonium chloride surfactant under basic condition. The ^^Si and '^C NMR study revealed that the condensation 9000 reaction of the silylbenzene precursor proceeded ideally to form benzene-silica hybrid structure in the framework without any Si-C bond cleavage during the synthesis process. X-ray diffraction of the benzene-bridged 30 40 26 (degree) mesoporous material showed several Fig. 3 XRD pattern of benzene-bridged remarkable sharp reflections of d=7.6, 3.8, mesoporous material (surfactant free).^'* 2.5 and 1.9 A at medium-scattering angles in addition of low angle reflections of d= 45.5, 26.0 and 22.9 A due to the hexagonal mesostructure (Fig. 3). The medium angle reflections that never been observed for previously reported conventional mesoporous materials, were assigned as a lamellar structure with a basal spacing of 7.6 A. Transmission electron microscopy image showed that many lattice

^;..V

;-

*4!^!!^>{- -

(c)

>f%^^S*i^f^^-^^-.

':^" "A

''•«fcij^^^/-5r

Fig. 1. XRD patterns of as-synthesized Fig. 2. TEM (a), SEM (b) and AFM (c) of (a,c) and calcined (b,d) mesoporous silica calcined mesoporous silica films films

67

(a) Thin Film

1500

1550

1600

Wavelength (nm)

Fig. 3. ESR spectra of methylphenothiazine Fig. 4. PL spectrum of mesoporous silica in mesoporous silica film (a) and powder (b). film with incorporated ErQ at 300 K. Mesoporous silica films with impregnated methylphenothiazine show a weak ESR signal before ultraviolet irradiation [3,7]. After being irradiated by 320 nm light at 100 K for 20 min, the samples showed a large ESR signal as shown in Figure 3. Figure 3 shows the ESR spectra of impregnated methylphenothiazine in mesoporous silica film (a) and powder (b) after being irradiated by 320 nm light at 100 K for 20 min. These ESR spectra are an asymmetric partially resolved sextet at g = 2.0055. The photoyield of methylphenothiazine cation radical is about 35 % higher in the film compared to the powder. The relative high efficiency of the formation and stabilization of methylphenothiazine cation radical in mesoporous silica films suggest that such films are promising materials for various applications. Figure 4 shows the photoluminescence (PL) spectrum of mesoporous silica film with incorporated ErQ. It is expected that the peak at 1475nm is due to the gratings in monochromator. The main luminescence peak is at 1545nm. The bandwidth at half-maximum is 72nm. This is much wider than for any other Er-doped materials [8]. The wide bandwidth is obtained by emission from Er atoms in different local environments. Such a broad spectrum enables a wide gain bandwidth for optical amplification. Therefore it is considered that the mesoporous silica film is a good matrix to be doped by a rare-earth complex homogeneously. 4. CONCLUSIONS Transparent mesoporous silica films with hexagonal and cubic phases are formed by control of the surfactant concentration. Transparent mesoporous silica films are fairly homogeneous and relatively easy to produce. The mesoporous silica films with hexagonal and cubic phases show possibilities of application as advanced materials. The incorporation of methylphenothiazine into mesoporous silica films shows successful photoionization by ESR and the incorporation of erbium 8-hydroxyquinolinate (ErQ) into mesoporous silica films is characterized by PL and isothermal nitrogen physisorption studies. Such mesoporous films with impregnated photo-functional materials may find application as sensor, optical devices, nanoreactors, and hosts for large organic molecules. REFERENCES 1. D. Zhao, Z. Luan and L. Kevan, Chem. Commun., (1997) 1009. 2. T. Kimura, Y. Sugahara, K. Kuroda, Chem. Commun., (1998) 559.

68

3. J.Y. Bae, K.T. Ranjit, Z. Luan, R.M. Krishna, L. Kevan, J. Phys. Chem. B, 104 (2000) 9661. 4. J.Y. Bae, L. Kevan, Microporous Mesoporous Mater., 50 (2001) 1. 5. D. Zhao, R Yang and G.D. Stucky, Adv. Mater., 10 (1998) 1380. 6. O.-H. Park, J.Y. Bae, J.-I. Jung, B.-S. Bae, Submitted for publication. 7. Z. Luan, J.Y. Bae and L. Kevan, Chem. Mater., 12 (2000) 3202. 8. W. J. Miniscalco, Lightwave J. Techn., 9 (1991) 234.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

69

Synthesis of 2D hexagonal mesoporous silica thin films via phase transition from lamellar structure Chia-Wen Wu, Kunichi Miyazawa and Makoto Kuwabara* Department of Materials Engineering, University of Tokyo, 7-3-1 Hongo, Tokyo, Japan. A phase transition from lamellar to 2-dimensional (2D) hexagonal structure has been observed to occur in silica thin films during heat treatment up to 80 °C in a humid atmosphere. By way of this phase transition, transparent highly ordered 2D hexagonal mesoporous silica thin films have been successfully synthesized from a silicon alkoxide (tetraethoxysilane: TEOS) precursor solution using self-assembly triblock copolymer PI23 (E02oP07()E02()) as a template. The heat treatment conditions that allow the phase transition were investigated in detail with respect to heating rate and the hold temperature and time. Characterization of the obtained mesoporous films was made using SA-XRD, FE-SEM and TEM. 1. INTRODUCTION There have been many reports [1-3] dealing with the synthesis of silica powders and thin films with various mesostructures, such as lamellar, hexagonal and cubic ones, by controlling synthetic or aging conditions. It has been shown that the mesostructures formed in powders or thin films can be estimated by some phase determining factors such as the surfactantpacking factor and the charge density matching between the surfactant and silica species [4-5]. Moreover, the mesostructurc of the as-made silica materials can be transformed (for example from lamellar to hexagonal) by controlling these phase determining factors. The phase transformation in mesoporous materials has been widely studied mainly in silica/surfactant composites in a powder form, by putting as-made samples into hot water [4-7], but there have been little reports on thin films. In this paper, we report the phase transformation from lamellar to highly ordered 2-D hexagonal in silica thin films by heating lamellar structured films under water vapor hydrothermal conditions. 2. EXPERIMENTAL Silica/surfactant mesostructured thin films were prepared as follows: tetraethyl orthosilicate (TEOS, 98%, Wako) was partially hydrolyzed under a strong acidic condition (pll~2) [8] at 60 °C for 2 h, and then mixed with a triblock copolymer (E02()P07()E02(), Mav = 5800, Aldrich) cthanol (EtOH) solution. In this study, the final precursor solution with the

70

compositions of ITEOS: 0.01 EO20PO70EO20: 30EtOH: 0.12HC1: II.5H2O (in molar ratio) was prepared and spin cast coated on glass substrates to form thin films. Lamellar structured silica thin films were obtained by aging the as-made films from room temperature (R.T.) to 120 °C for 24 h in a dry atmosphere. Hexagonal structured silica thin films were obtained by either aging the as-made films or the lamellar structured films under water vapor hydrothermal conditions. The water vapor hydrothermal conditions were carried out by placing the as-made or lamellar structured thin film samples in a glass container with an ample of water, and heating from R.T. to 150 °C with varied ramp rates (1-3 °C/min) in an electric furnace (Miwa, MT-1100). Inside the glass container, the film sample was exposed to water vapor atmosphere, which was produced as the temperature increased. Mesostructures of the synthesized silica thin films were characterized with small angle X-ray diffractometry (SA-XRD, Rigaku-Rint2000, Cu Ka ), field-emission scanning electron microscopy (FESEM, Hitachi S5000), and transmission electron microscopy (TEM, HitachiSOO, 200kV). 3. RESULTS AND DISCUSSION Transparent mcsoporous silica thin films with a lamellar structure can be synthesized when the deposited films were aged at a temperature from R.T. to 120 °C in a dry atmosphere for 24 hours. The presence of strong (001) and (003) diffraction peaks and a weak (002) peak of the as-synthesized film indicate the formation of highly ordered alternating silica/P123 layers (Fig. la) [9]. The (001) d spacing and intensity of lamellar structured silica films is about 8.5 nm while aging at R.T. and decreases to 4.7 nm while aging at higher temperatures (Fig.2). The 2D hexagonal structured silica thin films can be synthesized when the deposited films were aged at a temperature from R.T. to 80 °C under water vapor hydrothermal conditions for 6 hours. The formation of a highly ordered 2D hexagonal mesoporous structure in the films was clearly demonstrated by five well resolved (100), (200), (300), (400) and (500) diffraction peaks in Fig. lb: the silica film was calcined at 450°C to remove the template.

5

6

Fig. 1. The mcsoporous silica films with (a) a lamellar and (b) a 2D hexagonal structure.

100

120

H*at tr«a(in«nt temp (*C)

Fig. 2. The shift of the d spacing and intensity of lamellar structured silica f ilms as function of aging temperature for 24 hours

71 The (100) d spacing and intensity of the hexagonal structured silica films is about 9 nm. While heating lamellar structured silica films at the ramp rate of l~3°C/min from R.T. under water hydrothermal conditions, the lamellar-to-hexagonal phase transformation was observed (Fig.3). The phase transformed, 2D hexagonal mesoporous silica films show the highly ordered structures that pore channels in the hexagonal mesoporous structure are highly oriented to be parallel to the surface of the substrate [10,11]. This can indeed be seen from a TEM image recorded on the silica thin film, as show in Fig. 4a,b. The FE-SEM images (Fig. 4c, d) also show the smooth surfaces and continuous mesostructures. In summary, we have synthesized lamellar and hexagonal structured silica films by controlling the aging conditions, and observed the phase transformation from lamellar to 2D highly ordered hexagonal in mesostructured silica thin films by heating as-made film samples under water vapor hydrothermal conditions. XRD patterns show the gradual transition process, and the SEM and TEM images prove the transformed films are transparent with smooth

(D) (C)

^^^^/rv.^.|^,^^^;^

20(deg.)

Fig. 3. Small-angle XRD patterns corresponding to a lamellar-to-hexagonal phase transformation in mesostructured silica thin films synthesized using EO20PO70EO20 surlaetant. The sample was heated under a water vapor hydrothermal condition, where the temperature was ramped at a rate of 1 °C/min from 25 °C (trace A), to 80 °C (trace B), 110 °C (trace C), and 150 °C (trace D), respectively.

Fig. 4. a,b) TEM images of hexagonal structured films after phase transformation alon g a) [110] zone axes and b) along [001] zone axes. c,d) SEM micrographs of c) surface and d) cross section of phase transformed hexagonal silica films after calcination.

72

surface and highly ordered mesostructures. The exact mechanism of this phase transition has not been clarified yet, but one may recognize that water vapor played the most important role in the phase transition. Water vapor strongly affects the hydrolysis and condensation reactions of alkyl silica groups, resulting in the reduction of the interfacial charge density leading to the phase transformation. This phase transformation process not only benefits us to understand the mesophase formation mechanism during the synthesis of mesoporous silica thin films with block copolymer as template, but also is expected to supply a novel pathway to prepare highly ordered mesoporous thin films via precise control of the phase transformation process.

REFERENCES 1. M. Ogawa and N. Masukawa, Micro. Meso. Mater., 38 (2000) 35. 2. D. Zhao, Q. Iluo, J. Feng, B. F. Chmelka, and G D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 3. S. Besson, C. Ricollcau, T Gacoin, C. Jacquiod, and J. P. Boilot, J. Phys. Chem. B., 104 (2000) 12095. 4. Q. Huo, D. I. Margolcsc, and G D. Stucky, Chem. Mater., 8 (1996) 1147. 5. S. H. Tolbert, C. C. Landry, G D. SUicky, B. F. Chmelka, P. Norby, J. C. Hanson, A. Monnier, Chem. Mater, 13 (2001) 2247. 6. C. C. Landry, S. H. lolbcrt, K. W. Gallis, A. Monnier, G D. Stucky, P. Norby, and J. C. Hanson, Chem. Malcr., 13 (2001) 1600. 7. K. W. Gallis and C. C. Landry, Chem. Mater., 9 (1997) 2035. 8. S. Yun, K. Miyazawa, 11. Zhou, I. Honma, and M. Kuwabara, Adv. Mater., 18 (2001) 1377. 9. Y. Lu, Y. Yang. A. Sellingcr M. Lu, J. Huang, H. Fan, R. Haddad, G Lopez, A. R. Bums, D. Y Sasaki, J. Shclnutt, and C. J. Brinker, Nautre, 410 (2001) 913. 10. I. A. Aksay, M. Trau, I. Honma, Y L. Zhou, P. Fenter, P. M. Eisenberger, and S. M. Gruner, Science, 273 (1996) 892. 11. D. Zhao, R Yang, N. Mclosh, J. Feng, B. R Chmelka, and G D. Stucky, Adv. Mater., 16 (1998) 1380.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

73

Nanostructured silicate film templated by discotic CT-complex column A. Okabe, T. Fukushima, K. Ariga and T. Aida ERATO Nanospace Project, JST, National Museum of Emerging Science and Innovation Bldg., 2-41 Aomi, Koto-Ku, Tokyo 135-0045, Japan Nanostructured

silicate

films

were prepared

composed of a triphenylene-based

using charge-transfer

disk-like molecule as templates.

(CT)

complexes

A triphenylene

derivative having hydrophilic triethyleneglycol groups was newly synthesized as an electron donor that can form CT complexes with various kinds of electron acceptors, and transparent nanostructured silicate films were obtained with these CT complexes by various coating techniques, i.e., spin-coating, dip-coating and casting.

The CT columns immobilized in the

mesopores are stable against external perturbation, and possibly have long-range structural ordering. 1. INTRODUCTION Various macroscopic morphologies of mesoporous silica have been so far reported [1-4]. Among them, mesoporous silica films containing various functional molecules have been especially paid much attention as materials for a wide-range of application including electro-optical devices [2].

Although mesoporous silicate films can be easily obtained

through solvent evaporation by dip-coating [3] or spin-coating [4], lack in methodologies to immobilize functional

molecules limits preparation of functionally-attractive

materials.

Here we propose use of CT complexes of a disk-like molecule for mesoporous silica syntheses, because columnar structures of the CT complexes [5] are appropriate for the mesoporous silica templates.

In the obtained nanocomposites, the CT columns are isolated

in the mesopores and would express various novel and attractive properties and functions [6]. In this report, fabrication of novel nanostructured silicate films using triphenylene-based CT complexes is demonstrated (Figure I) [7]. 2. EXPERIMENTAL An amphiphilic triphenylene derivative TP was newly synthesized (Figure 1). complexes

were

prepared

by

mixing

with

electron

acceptor

molecules

such

CT as

74 R = 2,4,7-trinitro-9-fluorenone (TNF), Donor -{CH2)io—(OCH2CH2)30CH3 2, 3, 6, 7, 10, 11- hexacyanohexaazatriphenylene (HAT), 7, 7, 8, TP 8tetracyanoquinodimethane (TCNQ) chloranil (CA), and 1, 2, 4, Acceptors 5- tetracyanobenzene (TCNB) in TNF, HAT, TCNQ, benzene solution. Tetrabutoxy CA, TCNB silane (TBOS) was partially hydrolysed and polymerized in the presence of the CT complexes with small amount of H2O (H20/Si = 5) in HCl/ethanol solution at room temperature for 2-48 h. The solution was then coated on substrates by spin-coating, dip-coating, or casting, followed by drying at room temperature for CT Column 12 h. Glass, mica and graphite sheets were used as the substrates. Fig. 1. Schematic representation of mesoporous silicate The obtained films were calcined films templated by CT column. at 723 K for 3 h after dried at 373 K for 12 h. The structure of the films were characterized by XRD and TEM. The CT columns immobilized in nanostructured silicate films were characterized by electronic spectroscopy.

3. RESULTS AND DISCUSSION The silicate films prepared on glass substrates by spin-coating, dip-coating, or casting from an equimolar mixture of triphenylene derivative TP and TNF (TP/TNF/TBOS/ethanol = 1/1/20/4600 in molar ratio), showed XRD peaks that were apparently different from those of non-silicate triphenylene assemblies (Figure 2a). However, these peaks disappeared after calcination, indicating that the formed phase was lamellar. This structural characteristic was also comfirmed by TEM observations (Figure 2b). Modification of the preparative conditions by increasing TBOS/TP ratio and decreasing ethanol content (TP/TNF/TBOS/ethanol = 1/1/60/1540 in molar ratio) induced the structure featured by hexagonal XRD patterns. Unlike the former case, the (100) peak remained even after the template removal by calcination (Figure 3a). TEM observations also showed hexagonally aligned pore arrays (Figure 3b). Hexagonal structures were similarly obtained

75 a)

1 25000 cps (100)

I 2500 cps

(110)

(200) uncalcined

(200)

JI

.

ft

11

uncalcined calcined

1

,

,

,

I

,

calcined

. ', „..J

4 6 2 theta / degree

4 6 2 theta / degree

50 nm Fig. 2. a ) XRD pattern and b) TEM image of lamellar cast film on glass.

Fig. 3. a ) XRD pattern and b) TEM image of hexagonal cast film on glass.

from the CT complexes of the other acceptors. The obtained siHcate films were all highly transparent, and stained in blue to red colors depending on used acceptors. Table 1 summarizes absorption maxima that are characteristics of CT interaction, and the maxima showed red-shifted features compared with those of the corresponding CT complexes in nonstructured media. The latter fact may indicate long-range structural ordering of the CT columns in mesopores. In addition, the colors of the CT complexes in the hexagonal mesoporous silicate films stably remained even when the films were immersed in acetnitrile solution containing the other acceptor molecules. In contrast, the lamellar composite film was immediately decolored upon exposure to acetnitrile. These Table 1 results indicate that the hexagonally Absorption maxima (nm) of mesoporous silicate films -arranged silica framework is an containing CT colomns. appropriate medium for stable TNF TCNB HAT CA TCNQ accommodation of the one -dimensional CT columns in 490 548 615 700 890,410 well-ordered pore structures.

76

4. CONCLUSION We successfully demonstrated the first example of immobilization of one-dimensional columnar CT complexes into the pores of transparent mesoporous silicate film. The CT columns segregated by the hexagonally-arranged silica framework are highly stable and possibly have long-range structural ordering. The films obtained in this research would be highly useful for nano-fabricated devices based on various electro-optical properties. REFERENCES 1. Q. Huo, D. Zhao, J. Feng, K. Weston, S. K. Buratto, G. D. Stucky, S. Schacht and F. Schuth, Adv. Mater., 9(1997)974. 2. Y. Lu, Y. Yang, A. Sellinger, M. Lu, J. Huang, H. Fan, R. Haddad, G. Lopez, A. R. Bums, D. Y. Sasaki, J. Shelnutt and C. J. Brinker, Nature, 410 (2001) 9131. 3. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. Zink, Nature, 389 (1997) 364. 4. M. Ogawa, Chem. Comm., (1996) 1149. 5. H. Bengs, M. Ebert, O. Karthaus. B. Kohne, K. Praefcke, H. Ringsdorf, J. H. Wendorff and R. Wustefeld, Adv. Mater., 2 (1990) 141. 6. N. Boden, R. J. Bushby, J. Clements, B. Movaghar, J. Mater. Chem., 9 (1999) 2081. 7. Okabe, T. Fukushima, K. Ariga and T. Aida, Angew. Chem. Int. Ed., in press.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

Mesoporous Titania Photocalcination

Thin

Film with

77

Cubic

Mesostructure

using

U-Hwang Lee, Young Kyu Hwang and Young-Uk Kwon Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkvy^an University, Suwon 440-740, Korea. FAX: +82-31-290-7070. Well ordered mesoporous titania thin films with cubic structures were prepared by evaporation-induced self-assembly of a mixture composed of titania nanoparticles and diblock copolymers followed by UV-irradiation treatment to remove the surfactant molecules. This new strategy of using nanoparticles shows enhanced reproducibility over the others reported for mesoporous materials of non-silica compounds. The photocalcination provides further improvement by having less structural distortion upon the organic template removal process. 1. Introduction Ti02 thin films are used in dye-sensitized photoelectrochemical cells, as antireflection material in solar cells, as gas sensors, in photoelectrocatalysis, in photocatalysis, and in luminescence.''^ However, attempted syntheses of mesoporous titania have achieved only limited success in terms of the control of the mesostructure and reproducibility. Especially, the low reproducibility of reported synthesis methods for non-silica mesoporous materials is a serious problem for the future development of this class of materials. It is partly because the condensation of the inorganic precursors does not occur in harmony with the self-assembly process of the template surfactant molecules. Previously, we have reported a new approach of using titania nanoparticles to solve these problems.'^ Because the condensation step and self-assembly step were separately controlled, this approach gives reproducible results and easy control of the mcsostructures. However, the thermal calcination to remove the organic molecules accompanies structural distortions, which is an important drawback of this process. In order to solve this problem, we have employed the photocalcination technique in the place of thermal calcination and have synthesized mesoporous titania thin films with little structural distortions from the ideal cubic or hexagonal mcsostructures.'^^ 2. Experimental Section 2.1 Synthesis The synthesis of mesoporous titania is achieved as described in our previous paper except that

78

the removal of the surfactant molecules is performed by photocalcination instead of thermal calcination at 350-450°C. The synthesis of mesoporous titania is achieved in four steps of 1) synthesis of nanoparticles, 2) blending nanoparticles with template molecules into thin films, 3) aging the blended mixtures into mesostructures under appropriate conditions, and 4) calcination to remove the organic templates by UV-irradiation treatment. Stock solutions of Ti02 nanoparticles were prepared according to the literature procedure with slight modifications.'^ TiCUwas dissolved in absolute ethanol to make the final concentration 20 wt. %. A mixed solution of cone. HCl and 35% H2O2 was added and the solution was refluxed at 80°C for 2h. A Brij-type block copolymer, CnH2n+i(OCH2CH2)yOH, with n/y= 16/20, was dissolved into the Ti02 stock solution. The molar composition of the final solution was TiCl4/CnEOy/HCl/H202/EtOH/H20 = 1/0.083/3.8/0.97/6.1/15. The solution was dip-coated on silicon substrates, and the resultant thin films were dried and aged at 18°C under a controlled huminity of 80%. Finally, the as-made thin films whose mesostructures were confirmed with powder X-ray diffraction were photocalcined with a UV-ozone lamp (253.Inm + 184.9nm, 5W) at room temperature. 2.2 Characterization Characterization of the thin film by X-ray diffraction (XRD) was carried out with a Rigaku D/max-RC. FT-IR spectra were measured by using a Nicolet 1700 FT-IR spectrometer. TEM images were obtain by a HRTEM (JEOL-3011, 300kV). 3. Result and disscusion The selective removal of organic surfactant using the UV-irradiation treatment was verified by FT-IR measurements. The as-made (before calcine) thin films show pattern for the C-H stretching frequencies at about 2800cm ', methylene (-CH2) and methyl (-CH3) groups

( d )

""^

-''

( c )

^,/ /

\p H 3 0 0 0

w

( b )

\/' '' ( a )

- H 2 5 0 0

a v c n u m

b c r

( c m

')

Fig 1. FT-IR spectra of mesoporous Ti02 thin films after UVirradiation a) Omin, b) 15min, c) 30min, d) 45min

79

bending frequencies at about 1300 - 1600 cm ~' and bending frequencies of many CH2 groups in a open chain (long chain) at about 650 - 750 cm ' from the surfactant molecules. (Figure la) Figure lb ~ d show the gradual decrease of absorption bands as a function of UVirradiation time. In the sample treated with UV-irradiation over 45 minute, these peaks are absent, indicating complete removal of the organic molecules.

( b ) 5 .5 ni

( a ) 7 .1 n 1

20/degree

Fig 2. XRD patterns of mesoporous Ti02 thin films prepared using C16EO20 templates aged at 18 °C (a) as-made (b) after UV-irradiation for 45min. The as synthesized film prepared with C16EO20 block copolymer formed a mesostructure as evidenced by the XRD peak with d = 7.1 nm. (Figure 2a) The XRD peak was shifted to d = 5.5 nm upon UV-irradiation indicating a lattice contraction. (Figure 2b) Although the XRD patterns do not reveal the details of the structure with only one peak, the observed TEM images in Figure 3 show a cubic mesostructure view in the [110] direction with duo = 3.9 nm in direct correspondence with the XRD result which give dioo == 5.5 nm in an excellent agreement. The TEM images displayed a regular pore structure with the mean pore size of about 3nm. The pore walls are continuous with a thickness of about 2.5 ~ 3nm. Unfortunately,

Fig 3. TEM images of the calcined thin film that show a cubic structure Ti02thin film after UV-irradiation

80

our TEM and XRD data of UV-irradiated Ti02 thin films did not provide any evidence of crystalline Ti02 in the walls, probably because the particles were not well crystallized or were too small in size. The UV-irradiated Ti02 thin film shows less contraction of the dioo-space as well as decrease in the intensity of the (100) peak than thermally calcined Ti02 film. These results show that photocalcination of mesostructured Ti02 thin film has less structural distortion than thermal calcinations. 4. Conclusion Compared with our previous results with thermal calcination of the same material that produced a distorted cubic mesostructure, the advantage of the photocalcination for producing well-ordered mesoporous titania is evident. The effectiveness of the photocalcination can be assessed with the infrared spectra of the film materials before and after the UV-irradiation. The characteristic absorption peaks for the organic molecules disappeared completely. Probably, the photocatalytic effect of titania for decomposing organic molecules also has contributed to the almost complete removal of the surfactant molecules. This work was supported by KOSEF(SRC, CNNC) and School of Molecular Science through BK21 project. References (1) Y. Matsumoto, Y. Ishikawa, M. Nishida and S. li, J. Phys. Chem. B 20(X), 104,4204. (2) A. Hagfcldt and M. Griitzcl, Ace. Chem. Res., 2000, 33, 269. (3) W. Lin, W. Pang, J. Suh and J. Shen, J. Mater. Chem. 1999, 9, 641. (4) Thompson, D. W. and Meyer, G. Langmuir 1999, 15, 650 (5) Doeswijk, L. M. and Rogalla, H. M. Appl. Phys. A 1999, 69, S409 (6) Lin, H. M. and Tung, C. Y. Nanostruct. Mater. 1997, 9, 747. (7) Ichikawa, S. and Doi, R. Thin Solid Film 1997, 292, 130. (8) Xagas, A. P. and Falaras, P. Thin Solid Film 357, 173 (9) Y. K. Hwang, K. C. Lee and Y. U. Kwon, Chem. Commun., 2001, 1738. (10) Keene, M. T. J. and Llewellyn, P.L. Chcm.Commun.1998, 20, 2203 (11) H. K. Park, D. K. Kim and C.H. Kim, J. Am. Ceram., Soc, 1997, 80, 743

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights resei*ved

Preparation of Tin Modified Silica Mesoporous Film Brian Yuliarto'', Hao-Shen Zhou***, Takeo Yamada'', Itaru Honma'', and Keisuke Asai'* ^ Department of Quantum Engineering and System Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,Bunkyo, Tokyo 113-8656, Japan. ^ Energy Material Group, Energy Electronic Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba-shi, Ibaraki 305-8568 Japan.

Tin modified silica mesoporous films have been prepared by direct synthesis for the first time. The films were synthesized using cetyltrimethylammonium chloride as structure-directing agents and deposited on glass substrates by spin coating and finally calcined at 400^^ C. Characterization including small axis x-ray diffraction patterns, nitrogen absorption desorption and atomic force microscope were observed. The resulting films with the Sn/Si ratios of 0.005-0.03 have shown to maintain mesoporous structure after calcinations. The pore size uniformity was confirmed by the nitrogen absorption desorption isotherm although the hexagonal pore arrangements were disordered as the increasing amount of tin loaded into the solution. This novel tin modified silica mesoporous film has promising application in catalyst as well as sensor devices. 1. INTRODUCTION The unique and prospective of mesoporous silica properties, which contain large surface area and porosity, uniform pore size distribution, order pore arrangement, and possible surface engineering, has stimulated the development of application in various fields [1-3]. Recently, the research related to mesoporous silica has been conducted to improve the performance by modifying the structure of it. The modification by incorporating transition metal ions into the mesoporous material is of great interest for catalytic application as well as sensing devices. In oxide condition tin would be very interesting phenomena, because tin oxide has semi conductive properties and has been widely used as a catalyst for oxidation of organic compounds, optical electronics and especially as a gas sensors [4]. To date, the previous result of tin containing silica mesoporous material concerned only in powder forms [5,6]. Further more, it is well known that film is an ideal morphology for the applications of mesoporous materials to optical, electronic and sensing devices. As a consequence, it is necessary to synthesize tin modified mesoporous silica materials in thin film form. In this paper, we report the first synthesis (to our knowledge) of tin modified mesoporous silica film using sol gel methods. The molar ratio of tin and silica is varied between 0.005 and 0.05 molar. Moreover, the effect of amount of tin added into silica mesoporous thin film was investigated. Corresponding author. Tel.: •81-298-61-5795; fax: < 81-298-61-5829. E-mail: hs.zhour«)-aist.go.jp

82

2. EXPERIMENTAL SECTION Tetradecyltrymethylammonium chloride (abbreviated as C16TAC) was obtained from Tokyo Kasei Industries Co. Tetraetyl orthosilicate (abbreviated as TEOS), tin (IV) chloride, 1-propanol, 2-butanol, and hydrochloric acid were purchased form Wako Pure Chemicals Industries, Ltd, All materials are used without further purification. The tin modified mesoporous silica film was synthesized according to our group published procedure with quite alteration to introduce tin material [7,8]. The samples were prepared in the following way. The certain amount of TEOS was mixed with 1 -propanol and stirred for several minutes. The TEOS was hydrolyzed via the addition of a solution of previously mixed HCl and water, after that 2-butanol was added and stirred. In addition, the SnCU with variation of concentration was added while the solution was stirred. The C16TMA-CI surfactant solution was slowly added under stirring into the previously prepared sol. Finally the solution was the spin coated on a glass substrate. The final gel composition is 1 TEOS, 6.4981 1-Propanol, 2.6505 2-Butanol, 6.8472 H2O, 0.2632 IN HCl, and xSnCU, with x = 0.005, 0.01, 0.02, 0.03, and 0.05. Calcinations for the resulted films were performed at 400" C for 60 minutes. Atomic force microscopy (AFM) analysis was carried out to investigate surface morphology of the films on AFM SPA300HC from Seiko Instrument Inc. The obtained sample films were also characterized by X-ray diffraction (XRD), and Nitrogen absorption desorption isotherm. The small angle X-ray diffraction (SAXRD) pattern were observed for both after synthesis and after calcinations sample on a Mac Science M03XHF22 using CuK a radiation operated at 40 kV and 50 mA. Nitrogen absorption desorption of the calcined films were measured using Bell Sorp 18+ (Bell Japan Inc.) at 77 K. The sample for absorption desorption measurement was prepared on cover glass with several micrometer thickness. The Brunauer-Emmet-Teller (BET) calculation and Dollimore-Heal (DH) method were applied to calculate the specific surface area and the pore size distribution, respectively.

CI)

(b)

Ui

Fig. 1. AFM surface morphology of tin-modified mesoporous film with Sn/Si - 0 (a), 0.01 (b), 0.03 (c).

3. RESULTS AND DISCUSSION The film produced from the sol gel method was transparent film even after addition for all percentage of loading SnCU- The thickness of film can be adjusted by controlling the spinning rate of coating process. According to the AFM record, the film surface is rougher as tin content increases, as shown in Figure 1. The x-ray diffraction record for as synthesis samples with different Sn content are shown in Figure 2-1. From the picture, it can be observed that all samples with ratio between 0.005

83

and 0.05 reveal peak as synthesized, indicates that mesostructure was formed. However the peak intensity became weak as the increasing of Sn/Si ratio, except it was quite strong when the amount of Sn/Si was 0.03. [2-1]

[2-2]

(0 (e)

M JSl (b) (a) 3

4

26 L"J

3

4

20 ["]

Fig. 2. X-ray difiraction pattern of the as-synthesizcd [2-1] and as calcined [2-2] tin-modified silica mesoporous film with Sn/Si - 0 (a), 0.005 (b), 0.01 (c), 0.02 (d), 0.03 (c), and 0.05 (0-

The x-ray diffraction pattern for Sn-containing silica thin film of the as calcinations in the different Sn contents are shown in Figure 2-2. Nevertheless, when the percentages of SnCU reached to 0.5% of TEOS, the solution was no longer homogenous, and the resulting film tend to slight turbid. This phenomenon indicates that the films have a homogeneous structure until 3% of Sn/Si ratio, however when the ratio reach 5 %, the tin oxide was being isolated particle so that the heterogeneous solution was formed. Therefore, although the diffraction peak was observed in samples up to 0.003 of Sn/Si ratio, the peak became broad with the increase in the loaded Sn amount. Finally the peak did not disappeared at all for 0.05 of Sn/Si ratio, showing that mesoporous structure was broken. TTie values of d spacing increased as the increasing of Sn/Si ratio. Furthermore, at the same ratio condition, the d values decreased upon calcinations. This fact indicates that wall structure of film sample shriveled after removal of surfactants. Nitrogen absorption desorption isotherms of as-calcined films in all ratio of Sn/Si are shown in Figure 3-1. From the absorption desorption records, it is clear that the isotherm are type IV as identified by TUPAC for all loading Sn amount. This indicates that mesoporous structure was formed within the film samples. TTie adsorption isotherm reveals a large inflection in the partial pressure (P/Po) range around 0.2, which is the typical graph of capillary condensation within uniform mesoporous. The surface areas according to the BET calculation method are 1150 m'^g, 661 m"'^g, 620 m~^g, 514 m^ g, and 312 m~^g for pure silica, 0.5%, 1%, 2%, and 3% respectively. The calculations according to DH plots of the derivative of the pore volume per unit weight with respect to the pore radius are performed as shown in Figure 3-2. A narrow pore size distribution is observed in all samples ratio. Nevertheless, the pores size distribution after tin incorporating is shifted towards lower values as the increasing

84 [3-1]

1.107

[3-2]r - • - 0% Sn / Si -•-0.5%Sn/Si - A - 1 % Sn/Si - • - 2% Sn/Si - • - 3% Sn/Si

mrli-M I i 1.0

0.2

0.4

0.6

0.8

10

R e l a t i v e pressure [P/P^I

0

1

2

3

Pore Radius [nm]

Fig. 3. Nitrogen adsorplion/desorption isotherm [3-1J and DII pore distribution [3-2J of the calcined tin-modified silica mcsoporous film in all condition

of Sn/Si ratio, indicates that the pore structure is being disturbed as the increasing of tin, which is consistent with x-ray diffraction pattern. Additionally, these four samples posses a similarity of peak location at 2.2 A of diameter. This fact indicates that the addition of tin into silica mesoporous does not change the pore size. In addition, the phenomenon also explain that tin disperse within the framework of the film sample in spite of in the pore. 4. CONCLUSION It is concluded that the synthesis modification of silica mesoporous film allows to introduce tin into the mesoporous silica film. The film properties of tin-modified silica mesoporous samples depend on tin loading. According to the evidences, the mesoporous silica structure suffer degradation their structure after modification of 3% ratio of Sn/Si upon direct synthesis. Moreover, the direct synthesis to prepare the samples gives convenience method for metal transition incorporating into mesoporous silica film. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmidtt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B.Higgins, J.L. Schlenker, J.Am. Chem. Soc. 114(1992) 10834. 3. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc, Chem. Commun. (1993) 680. 4. F. Chen, M. Liu, Chem. Commun. (1999) 1H29. 5. G. Li, S. Kawi, Sensors and Actuator B 59 (1999) 1. 6. Y. Teraoka, S. Ishida, A. Yamasaki, N. Tomonaga, A. Yasutake, J. Izumi, I. Moriguchi, S. Kagawa, Microporous and Mesoporous 48 (2001) 151. 7. H.S. Zhou, D. Kundu, I. Honma, J. of European Cer. Soc. 19(1999) 1361. 8. Honma, H.S. Zhou, D. Kundu, A. Endo, Adv. Mater. 12 (2000) 1529.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

85

Novel non-lithographic large area fabrication method to generate various polymeric nanostructures Woo Lee, Mi-Kyoung Jin, Won-Cheol Yoo, and Jin-Kyu Lee School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151 -747, Korea. FAX: +82-2-882-1080. E-mail: [email protected] A simple and completely non-lithographic route has been developed to fabricate freestanding nanostructured polymeric films or polymer/nanoparticle composite films with a close-packed hexagonal array of nanolenses or nanoposts by using electrochemically prepared textured aluminum sheets or mesoporous anodic aluminum oxides (AAOs) as a replication master. TEM, FE-SEM, and AFM analyses revealed that our nanofabrication procedure could provide a convenient route to produce multiple copies of polymeric nanostructures over several square centimeters, even in an ordinary laboratory where one could not make routine access to state-of-the-art lithography facilities. 1. INTRODUCTION Fabrication of the materials with nanometer-scale periodic array is of utmost importance due to their potential technological applications in high-density magnetic memories, singleelectron devices, and optical media.''^ The most common techniques used for generating periodic array in nanometer scales so far are lithographies (i.e., ion- and electron-beam lithography, x-ray lithography, probe-tip based lithography, and etc.). However, they have some fundamental drawbacks, such as low throughput and high cost requiring state-of-the-art facilities. Herein we report facile and completely non-lithographic routes for fabricating large area nanostructured polymeric films with a two-dimensional vast array of nanolenses or spatially well-separated nanoposts using electrochemically prepared textured aluminum sheets or mesoporous AAO. We also demonstrated that the textured aluminum sheets could be used as a replication master for the fabrication of the polymer/nanoparticle composite films, whose structural features are characterized as a close-packed hexagonal array of polymeric nanoembosses containing nanoparticles. The financial support from the Interdisciplinary Research Program (Grant No. 1999-2-121-004-5) of the KOSEF is greatly acknowledged.

86

2. EXPERIMENTAL Textured aluminum masters with 2-D hexagonal array of approximately hemispherical concaves on their surfaces were prepared by the two-step electrochemical oxidation of Al using 0.3 M H2SO4 (10 ^'C), 0.3 M H2C2O4 (17 °C), and 10 wt. % H3PO4 (-3 °C) to give different sizes of concaves, followed by the complete removal of porous AI2O3 films using an aqueous acid mixture of 1.8 wt. % chromic acid and 6 % H3PO4. On the other hand, mesoporous AAO replication master was prepared by briefly anodizing textured aluminum for 100 s. Fabrication of free-standing thin films of polystyrene (PS) replicas of respective replication master has been realized by spin-on assisted replica molding or nanoimprint pattern transfer, followed by stripping of nanostructured polymeric films. The structures of the replication masters and the replicated polymeric films have been investigated by using AFM, FE-SEM, and TEM. 3. RESULTS AND DISCUSSION FE-SEM investigation revealed that the surface of the textured aluminum consists of closepacked hexagonal arrays of approximately hemispherical concaves (i.e., a honey-comb structure). The radius (r) of each concave varies as a function of the anodization voltage with r = 2.6 nm/V; the average radii of concaves in the textured aluminums prepared from H2SO4 (25 V), H2C2O4 (40 V), and H3PO4 (160 V) are 61 nm, 111 nm, and 420 nm, respectively (Fig. 1).

Fig. 1. FE-SEM images of (a ~ c) anodic aluminum oxide (AAO) and (d ~ e) textured aluminum master produced from (a and d) 25 V H2SO4, (b and e) 40 V H2C2O4, and (c and f) I6OVH3PO4.

87

On the other hand, further anodization of the textured aluminum master produces highly ordered mesoporous AI2O3 film with cylindrical channels at the precise center of hemispherical concaves, generating another replication master. In this case, the interpore distance of the mesoporous AI2O3 film is predefined by the center-to-center distance of concaves on a textured aluminum and the length of channels depends on the anodization time. Fabrication of a free-standing thin films of polymer replica of the present masters has been realized by spin-on assisted replica molding or nanoimprint pattern transfer technique. In spin-on assisted replica molding method, the replication master was supported on the chuck of a conventional spin-coater and then polymer solution was placed on the surface of the master. Typically we used commercial polystyrene (PS) (M.W. = 1 x 10^) dissolved in methylene chloride (10 wt. %). By subjecting the master to a specified spinning rate (typically, 3000 rpm for 30 s) the solvent of the polymer solution was allowed to evaporate. The free-standing polymer thin film with vast arrays of nanostructure was easily separated from the replication master by simply immersing the sample into distilled water. In nanoimprint pattern transfer (see Schemel 1.), the replication master was placed directly on the polymer substrate. A pressure was applied to hold the master against the polymer substrate. The whole assembly was heated uniformly to a temperature slightly above the glass transition temperature (Tg) of polymer, and then cooled down to room temperature. The master was easily removed from the polymer substrate to give the large area nanostructured polymer surface. (a)

textured . aluminum'

,„......„.^^^^

substrate (\)\

^

p^^^^^^^

heating (T >'l\ of polymer)

^

(rrr^

cooling and master removal

Schemel 1. Schematic illustration of nanoimprint pattern transfer using (a) textured aluminum sheet and (b) mesoporous AAO as replication master According to our AFM and FE-SEM studies, the surface structures of polymeric films replicated from the textured aluminums or mesoporous AAO masters are mainly characterized as a 2-D hexagonal arrangement of nanolenses and nanoposts, respectively (Fig. 2); the surface of the replicated polymeric films have exactly complementary structures of the replication masters, manifesting the high fidelity of the spin-on assisted replica molding or nanoimprint pattern transfer technique for producing a periodic nanostructures over large area.

Fig. 2. FE-SEM micrographs of freestanding polystyrene (PS) films with 2-D hexagonal array of (a and b) nanolenses and (c and d) nanoposts.

Fig. 3. FE-SEM image of polystyrene/ferrite nanoparticle composite film; TEM image of the sample is also presented as an inset.

Replication masters could also be used to produce various nanocomposite films, whose structural features are characterized as a close-packed hexagonal array of nanoembosses containing inorganic nanoparticles. This has been simply achieved by stepwise spin-on assisted loading of nanoparticles and polymer solution, followed by stripping of nanocomposite films. In Figure 3, we presented a typical FE-SEM image of polystyrene/ferrite nanoparticle composite film produced by this procedure, together with TEM micrograph as an inset. The present electron micrographs reveal clearly that each nanoemboss contains several ferrite nanomagncts (ca. 10 nm), forming close-packed hexagonal arrangement. It is worthy to note here that this novel process could provide ample variations of the constituents of the nanocomposite. The nanoparticles could be nanometersized polymer beads, quantum-sized metals, catalytically important semiconductor nanoparticles, or oxide nanomagncts. The polymeric matrices could also varied from elastomers, thermoplastics, to conducting polymers. Considering a broader perspective, the utilization of textured aluminum sheets or mesoporous AAOs as replication master presents a novel and exciting methodology for preparing various surface nanostructures. In addition, it is expected that our process will open up the new possibility of generating various functional materials for many interesting applications (for instances, anti-reflection, anti-fogging, electromagnetic wave shielding, and etc.) and that it will attract much attention from the academic and industrial communities. REFERENCES 1. Black, C. T. et al., Science, 290 (2000) 1131. 2. Postma, H. W. Ch. et al.. Science, 293 (2001) 76.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Mesoporous anodic alumina membrane with highly ordered arrays of uniform nanohole C.W. Lee', C.I. Lee^ Y. Lee^ H.S. Kang^ Y.M. Hahm' and Y.H. Chang^* 'Department of Chemical Engineering, Dankook Univ., Seoul, 140-714, Korea. ^Department of Chemical Engineering, Inha Univ., Inchon, 402-751, Korea. The mesoporous anodic alumina(AA) membrane with highly ordered arrays of uniform nanohole was prepared by anodic oxidation process in an aqueous solution of sulfuric acid at 20 °C. Morphology, pore size, pore size distribution and thickness for mesoporous AA membrane was examined with several anodizing conditions; reaction temperature, current density, electrolyte concentration, amount of additive, etc. The pore density in the array was approximately 3.5-5.8 X lO'"* m"^ with pore diameter and membrane thickness of approximately 25-35 nm and 50 j^m. 1. INTRODUCTION The properties of materials or devices can be tailored by controlling their microstructure on atomic level, which has become an emerging interdisciplinary field based on solid state physics, chemical, biology and material science[l]. Recently, the fabrication of quantum dot or nanodot arrays has attracted considerable attention, because these nanostructures show not only the novel physical properties, but also the potential application in the electronic devices, catalysts, gas absorption/separation membranes and efficient sensors[2-4]. In general, porous AA membranes are used in a number of diverse applications, such as filtration, bioreactors, analytical device including sensor and as supports for active materials[5-7]. However, most fabrication methods are not satisfactory due to some drawbacks, such as low uniformity of the shapes and sizes of pores, etc. Moreover, preparation of mesoporous A A membrane with highly ordered arrays of uniform nanohole was very difficult. We reported herein the mesoporous AA membrane with highly ordered arrays of uniform nanohole was prepared by anodic oxidation using DC power supply in an aqueous solution of sulfuric acid at 20°C. 2. EXPERIMENTAL The aluminum plate used in this study has 99.8% purity (size: 30X70X0.6mm). Prior to anodic oxidation, sample was washed several times using distilled water and acetone to eliminate the impurities on the surface. After washing, thermal oxidation was executed for 15 min at 580 °C to make better formation of pores. Subsequently, a chemical polish was made with a solution of H3P04(3.5 vol%)-Cr03(45 g/L) for 10 min at 80°C. Electrochemical polish

90

was made at a constant current of 2.87A with a solution of H3P04(85wt%)-H2S04(98wt%)H20(7:2:lby volume), which contained 35g/L CrOs, for 10 min at 40°C and washed once more in distilled water. In order to prepare PAA membrane by anodic oxidation, the sample was sealed with silicon rubber except reaction side. In order to mesoporous AA membrane with highly ordered arrays of uniform nanohole was examined with several anodizing conditions; reaction temperature, current density, electrolyte concentration, amount of additive, etc. The anodic oxidation was carried out at constant current density. To prevent the chemical dissolution of mesoporous AA membrane during anodic oxidation, aluminum sulfate and aluminum nitrate as additive were added to electrolytic solution of sulfuric acid at a reaction temperature of 20°C. The remaining Al substrate of the PAA membranes was removed by 0.1 M CuCb solution containing 32wt% HCl at a room temperature. Scanning electron microscope (SEM) photographs were obtained with a Jeol JSM-5800, N2 adsorption measurements were performed at 77K using a Micromeritics ASAP 2010 analyzer utilizing Barrett-Joyner-Halenda (BJH) calculations of pore volume and pore size distributions. 3. RESULTS AND DISCUSSION Figure 1 shows the scanning electron microscopy (SEM) photographs of the surface of mesoporous AA membrane prepared by anodic oxidation at various reaction temperature in an aqueous solution of sulfuric acid. As can be seen from this figure, we get the mesoporous A A membrane with smooth surface and uniform pore diameter at low temperature.

Fig. 1. SEM photographs of mesoporous AA membrane prepared at various temperature in sulfuric acid. [cone. : 15wt%, current density : 20 mA/cm^](a) 20°C, (b) 10°C, (c) 0°C In case of high reaction temperature, degree of chemical dissolution increased because of high ion activity. Forward reaction rate for second ionization reaction of sulfuric acid is reduced at lower temperature (see Van't Hoff Eq.[8]). Hydrogen sulfate (HSO4) as weak acid with lower dissolution activity was formed since second ionization constant of sulfuric acid was decreased. It plays an important role in electrode reaction of anode and it reduces degree of chemical dissolution because of space charge formation by proton. Figure 2 shows the SEM photographs of the surface of mesoporous AA membrane prepared by anodic oxidation in an aqueous solution of sulfuric acid containing Al2(S04)3 as an additive at 20 °C reaction temperature. The mesoporous AA membranes, with highly ordered arrays of uniform nanohole, was obtained.

91

Fig. 2. SEM photographs of mesoporous AA membrane prepared according to current density. [Cone. : 5wt%, Al2(S04)3: 10 g/£] (a) 10 mA/cm^', (b) 30 mA/cm^ (c) 50 mA/cm^ Figure 3 and 4 shows effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various current density and electrolyte concentration. The amount of aluminum sulfate was increased with concentration of electrolyte increasing. Namely, 15g additive per 1 L electrolyte was proper. Pore diameter scarcely varied by current density and electrolyte concentration with the use of additives during anodic oxidation.

l -

1

•

• A -'-^ ^

1

# • •

1

1

-

•

•

.A •

•

30 mA/cm 40 mA/cm' 50 mA/cm

Fig. 3. Effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various current density.[Conc. : 15wt%]

1

-

-L-^

•

1

-•5wt% - • - 10wt% A 15wl% 1

1

1

1

10

15

20

AI,(SO,)3(g/l|

Fig. 4. Effect of additive quantity on pore diameter of mesoporous AA membrane prepared at various electrolyte concentration, [current density : 30 mA/cm^]

92

Mesoporous AA membranes were prepared with pore sizes in the range of approximately 25 to 35nm, and corresponding pore density in the range of approximately from 3.5X10'"* to 5.8X io'"* m"^ and membrane thickness of 50 ^m. 4. CONCLUSION It was made an attempt to get mesoporous anodic alumina membrane by adding an additive in sulfiiric acid during anodic oxidation at 20 °C and aluminum sulfate as an additive suppressed the degree of chemical dissolution. Moreover, the amount of aluminum sulfate was increased with concentration of electrolyte increasing. Namely, 15g additive per 1 L electrolyte was proper. Pore diameter scarcely varied by current density and electrolyte concentration with the use of additives during anodic oxidation. It was possible preparing of anodic alumina membrane with highly ordered arrays of uniform nanohole. The pore density in the array was approximately from 3.5 xio'"* to 5.8Xl0''*m'^ with pore diameter and membrane thickness of approximately 25 ~35 nm and 50 Mm. REFERENCES 1. H. Gleiter, Acta Mater., 48, (2000), 1. 2. I. Amlani, et al.. Science, 282, (1988), 1473. 3. S. Fafard, et al., Phys. Rev., B 59, (1999), 15368. 4. S. Drecker, et al., J. Am. Chem. Soc, 118, (1996), 12465. 5. G. Graff, Science, 253, (1991), 1097. 6. W. R. Bowen, and D. T. Hughes, J. Membrane Sci., 51, (1990), 189. 7. S. K. Dalvie, and R. E. Baltus, J. Membrane Sci., 71, (1992), 247. 8. P. W. Atkins, Physical Chemistry, 4th Ed., 219, 1989.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Preparation and characterization of poly(ester)-silver and nylon-silver nanocomposites Seong-Ho Choi^, Kwang-Pill Lee^* and Sang-Bong Park'' ^Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea. Polymerization R&D Team, R&D Center, Kolon Industries, Inc., Kumi, 730-030, South Korea. The carboxylic acid-modified Ag nanoparticles were prepared by reaction of Ag colloidal nanoparticles and mercaptosuccinic acid.

The carboxylic acid-modified Ag nanoparticle was

precipitated

bonding

by

means

mercaptosuccinic acid.

of

hydrogen

of

an

carboxylic

acid

group

onto

The Ag nanocomposites were also prepared by polymerization of

poly(ester) and nylon in the presence of the carboxylic acid-modified Ag nanoparticle.

In a

poly(ester)-Ag nanocomposite, the Ag nannoparticle was aggregated in the poly(ester) matrix, whereas the Ag nanoparticle was dramatically dispersed in a nylon matrix. 1. INTRODUCTION The nano metal particle-organic polymer composites have attracted considerable interest in recent years.

These composites not only combine the advantageous properties of metals and

polymers but also exhibit many new characteristics that single-phase materials do not have. They have a wide range of applications including electromagnetic inference shielding, heat conduction, discharging static electricity, conversion of mechanical to electrical signal, and the like.'-^ The nano metal particle-polymer composites can be simply prepared by homogenizing polymer and nano powder.

In order to homogenize nano metal and polymer, combination

between the hydrophilic properties of nano metal and the hydrophobic properties of the polymer matrix can be considered. In this study, the Ag colloidal nano particles were synthesized by y-irradiation using silver salt in H2O in the presence of radical scavenger and colloidal stabilizers.

The surface of the

Ag nanoparticle was simply immobilized by the self-assembling of the thiol group in •Corresponding author. Tel.: +82-53-950-5901; fax: +82-53-952-8104. E-mail address: [email protected]

94

mercaptosuccinic acid. The mercaptosuccinic acid-modified Ag nanoparticle powder was analyzed by XRD and TEM. The poly(ester)-Ag nanocomposite and nylon-Ag nanocomposite were prepared by a homogenizing method. The characteristics of the poly(ester)-Ag nano composite and nylon-Ag nanocomposite are discussed. 2. EXPERIMENTAL 2.1. Preparation of silver colloidal nano particle."^'^ The preparation procedure of the Ag nanoparticle and the carboxylic acid-modified Ag nanoparticle. The solution (500 mL) containg the AgNOs (25.3g), 2-propanol (3.3 mL) as radical scavenger, and PVP (0.5g) as stabilizer was prepared, oxygen was removed by bubbling with pure nitrogen for 30 min. and then irradiated by Co-60 y-ray source. 2.2. Immobilization of carboxylic acid onto surface of silver colloidal nanoparticle. In order to obtain Ag nanoparticles powder in Ag colloidal solution, the carboxylic acid group was introduced onto the surface of an Ag colloidal nanoparticle. A typical preparation procedure was described below. The mercaptosuccinic acid solution (I.OXIO"^ M) was prepared and then added to the Ag colloidal solution by sonicating. The Ag nanoparticle was precipitated and separated by centrifuge. 2.3. Synthesis of poly(ester)-Ag nanocomposite and nylon-Ag nanocomposite. Poly(ester)-Ag nanocomposite. After the condensation reaction of the dimethylterephthalate and ethylene glycohol at 140 ~ 230 °C using magnesium acetate as catalyst, the ethylene glycohol dispersed Ag nano powder was added to the reaction solution. The reaction solution was reacted in the presence of arsenic(lll) oxide (AS2O3) as catalyst at 280 °C for 3 hrs in a vacuum state. Nylon-Ag nanocomposite. The e-caprolactam dispersed with an Ag nanoparticle was maintained on 15 kgf/mm^ at 260 °C for 1 hr and repeatedly reacted in previous condition after decompressed to normal pressure for 1 hr. 3. RESULTS AND DISCUSSION Figure 1 shows the UV spectra of the silver colloidal solution prepared by y-irradiation. The band at about 400 nm, which is due to colloidal nano silver, which is due to silver cluster plasmons. Fujita et al.^ began the synthesis of metal aggregates by the radiolytic reduction of metal cations in solution. In order to obtain Ag nanoparticles powder, the author selected the compound containing the carboxylic acid group (-COOH) and thiol group (-SH) because

95

carboxylic acid groups have hydrogen bonding sites in solution and the thiol group bonded the surface of the metal particle. Figure 2 shows XRD spectra of the Ag nanoparticle (a) and carboxylic acid-modified Ag nanoparticle (b). XRD patterns show the products are metallic silver. The average size crystallite sizes calculated from peak broadening of XRD patterns by the Scherrer equation. The average size of the Ag nanoparticle and the carboxylic acid-modified silver was to be 18.8 and 5.4 nm, respectively. The size of the silver nanoparticle precipitated by centrifuge higher than that of carboxylic acid-modified Ag silver. It may be considered that the large size Ag particle was precipitated by centrifuge of 45000rpm/min. Figure 3 shows TEM image of the Ag nanoparticle (a) and the carboxylic acid-modified Ag nanoparticle. The shape of Ag nanoparticle and the carboxylic acid-modified Ag nanoparticle was spherical-type powder. Figure 4 shows the SEM image of the poly(ester)-Ag and nylon-Ag nanocomposite: surface of poly(ester)-Ag nanocomposite (a), surface of poly(ester)-Ag nanocomposite etched by plasma (b), surface of nylon-Ag nanocomposite (c), and surface of nylon-Ag nanocomposite (d). The Ag nanoparticle were aggregated onto a poly(ester) matrix, whereas the Ag nanoparticle was dramatically dispersed onto a nylon matrix. For these reasons, the carboxylic acid-modified Ag nanoparticle were dispersed onto nylon with a hydrophilic backbone chain, whereas the carboxylic acid-modified Ag nanoparticle was aggregated on poly(ester) with a hydrophobic backbone chain.

10

20

30

40

50

60

20 (cleg.)

Fig. 1. UV spectra of Ag nanoparticle prepared by y-irradiation.

Fig. 2. XRD spectra of Ag nanoparticle (a) and carboxylic acid-modified Ag nanoparticle (b).

96 ;.5i-::>;..'^vrv,i:.-

J\ •

(a)

(b)

Fig. 3. TEM image of Ag nanoparticlc (a) and COOH-modificd Ag nanoparticlc (b).

Fig. 4. SEM image of surface of the poly(estcr)-Ag (a), surface of the poly(estcr)-Ag etched by plasma (b) surface of the nylon-Ag (c), and surface of nylon-Ag nanocomposite etched by plasma (d).

ACKNOWLEDGEMENT This work was supported (in part) by the Ministry of Science & Technology (MOST) and the Korea Science and Engineering Foundation(KOSEF) through the Center for Automotive Parts Technology(CAPT) at Keimyung University.

REFERENCES 1. S.T. Selvan, T. Hayakawa, and M. Nagami, J. Phys. Chem. B, 103 (1999) 7441. 2. X. Xu, Y. Yin, X. Ge, H. Wu, and Z. Zhang, Materials Letter, 37 (1998) 354. 3. J.-C. Huang, X.-F. Qian, J. Yin, Z.-Kang Zhu, and Il.-J. Xu, Mater. Chem. Phys., 69 (2001) 172. 4. H. S. Nalwa, Handbook of Nanostructured Materials and Nanotechnology, ACADEMIC PRESS, INC., New York, 2000, chapter 9. 5. H. Fujita, M. Izawa, and H. Yamazaki, Nature, 196 (1962) 666.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Synthesis of Ordered Three-Dimensional Large-pore Mesoporous Silica and Its Replication to Ordered Nanoporous Carbon Jie Fan^, Chengzhong Yu^, Limin Wang^, Yasuhiro SakaInoto^ Osamu Terasaki*"^, Bo Tu^, Dongyuan Zhao^* ^ Department of Chemistry, Fudan University, Shanghai 200433, P. R. China, ^ Department of Physics, Tohoku University Sendai 980-8578, Japan "^CREST, Japan Science and Technology Corporation, Tohoku University Sendai 980-8578, Japan Ordered three-dimensional (3D) large-pore mesoporous channels have been fabricated based on mesoporous silica SBA-15 and SBA-16 by a new synthesis strategy, which involves the introduction of organic co-solvents followed by a high temperature hydrothermal process. The previous small entrances (2.3 nm) for caged cubic mesoporous SBA-16 can be enlarged (up to 10.8 nm) by using this approach. These highly ordered mesoporous silica materials with large entrances have been verified to be suitable templates for the synthesis of ordered cubic carbon replicas with a novel ball-type pore structure. 1. Introduction Highly ordered large pore mesoporous silica shows great importance for many applications, such as catalysis, separation, adsorption and fabrication of nanostructured matcrials.|l-5| Compared with ID channel MCM-41, 3D mesoporous materials have the advantage in the mass diffusion and transport because of their interconnecting networks. Except for bicontinuous cubic MCM-48 {Ia3d), 3D mesoporous materials, including cubic SBA-1 (/^w3^), 3D hexagonal SBA-2 and SBA-12 {P63/mmc\ and cubic SBA-16 and FDU-1 {Im3m) structures, have caged mesostructures, in which small entrances block the large pore channels. Erom a standpoint of applications, it is oi^ great importance to break the channel dimensional limitation for original ordered mesoporous silica SBA-15, as well as tailor the cavity and entrance dimension for SBA-16 (specially enlarge their original small entrances). |6| Recently, many efforts have been taken to the synthesis of ordered mesoporous carbons templated from mesoporous silica templates for their potential applications in advanced electronic devices, shape-selective catalysts, and hydrogen-storage systems. [7-10] Various ordered mesoporous silicas have been chosen as the templates for the variation of the

98

mesostructure of mesoporous carbon. Up to date, mesoporous carbon with 3D cubic {ImSm) structure has not been reported. Scheme 1

•:?,!;"\"™:r''

Scheme 2 window size < 4 nm

window size 10 8 nm

MX

lanocrystal in caged mesoporous silica

Schematic illustration for the pore structure of 1) 3D mesoporous SBA-15 and 2) entrances expanding of 3D cubic caged mesoporous silica SBA-16. 2. Experimental Section The modified 3D mesoporous SBA-15 was synthesized from Pluronic P123 {MOK^O-IOMOIO) under an acidic condition. Organic co-solvent (1,3,5-trimethylbenzene, fMB) was introduced into embryo mesostructured material (if desired), and the as-synthesized products were under a high temperature hydrothermal treatment (up to 150°C). The caged cubic mesoporous SBA-16 with large entrances (denoted as SBA-16L) was synthesized by using triblock copolymer with long V\0 segments (such as HOio^POvoP^Oio^, 1'127) as a template. Different from that for 3D mesoporous SBA-15, TMB acts as co-surfactant in the presence ol' inorganic salts. Dispersed Au nanocrystals were prepared by using the large window mesoporous silica as the hard templates according to an easily loading approach reported previously. 111 j The synthesis of carbon using SBA-16L as a hard template was similar to that reported by Ryoo and co-workers | 7 | except for difference of silica-to-sucrose ratios. 3. Results and Discussion 3.1 3D SBA-15 3D large-pore mesoporous SIiA-15 can be prepared by a high temperature hydrothermal process, which involves the introduction of '1MB as an organic co-solvent into embryo mesostructured SBA-15, illustrated in Scheme 1. XRD patterns show that 3D modified mesoporous silica SBA-15 has an average mesostructure of hexagonal space group symmetry p6m. [12] HREM imgaes show that 3D SBA-15 has many nanosized ( 2 - 8 nm) connections /tunnels that are randomly distributed between the ID-channels (Figure 1). The presence of the interconnected tunnels results in the formation of 3D large pore (average pore size up to 22.3 nm) networks.

99

Figure 1. TEM images of calcined 3D mesoporous SBA-15 viewed along a) [100] and b) [110] direction. 3.2 SBA-16 with Large Entrances (SBA-16L) Highly ordered large pore (15.4 nm) mesoporous silica SBA-16 with large entrances (up to 10.8 nm) has successfully been synthesized at high temperature (130°C) according to Scheme 2. XRD patterns and TEM images show that the SBA-16L with large entrances has excellent structural ordering for a cubic space group {Jm3m) with a cell parameter (a) of 21.8 nm (Figure 2a, b). The nitrogen sorption isotherms show the entrance size of SBA-16L can be large up to 10.8 nm, suggesting that the window of SBA-16 can be enlarged after high temperature hydrothemal process in the presence of inorganic salt such as NaCl or KCl. The

_caicined

\ 0

V a 1

2

as-synthesized 3

4

2 Theta value

100 nm

5

60nhi 2 Theta value

Figure 2. XRD patterns (a, c) and TEM images (b, d) for silica SBA-16L (a, b) and its carbon replica (c, d).

100

negative diluted Au nanocrystals prepared from SBA-16L products as a hard templates can be used to image the opened connectivity of the neighbouring spherical cavities, as well as entrance dimension. 3.3 Cubic Mesoporous Carbon Using SBA-16L as Hard Template Such caged cubic mesoporous silica SBA-16L with large entrances has unblocked 3D pore networks and facilitates to prepare stable mesoporous carbon. Here we have successfully synthesized a highly ordered cubic mesoporous carbon with a novel ball-type pore structure by using above large entrance sized SBA-16L as a template. XRD pattern of the mesoporous carbon (Figure 3a) shows characteristic of body center cubic mesostructure, similar to the silica template. TEM images for the carbon sample reveal that it has the same structural symmetry with the silica SBA-16L template (Figure 3b). Such fully ordered carbon products can not be obtained by using conventional SBA-16 with small entrances as a template. We think that the interconnected channels (less than 2.3 nm) of mesoporous carbon prepared from conventional SBA-16 is quite small, resulting in that the mesostructure of the carbon is unstable during the preparation. 4. Conclusions Ordered 3D large-pore (22.3 nm) mesoporous SBA-15 and cubic caged SBA-16 with large entrances (up to 10.8 nm) have been successfully synthesized. Such large entrances SBA-16L facilitates to prepare stable mesoporous carbon with a novel ball-type pore structure. This work was supported by NSF of China (Grant No. 29925309 and 20173012), Shanghai Promote Center (0/52nm029). State Key Basic Research Program ofPRC

Nanotech.

(G2000048001).

References I . e . T. Krcsge, M. \\. Lconowicz, W. J. Roth, J. C. VartuH, J. S. Beck, Nature 1992, 359. 710. 2. J. Wu, A. F Gross, S. H. lolbert,./ Phys. Chem. B 1999, 103. 2374. 3. S. S. Kim, W. / . Zhang, X. J. Pinnavaia, Science 1998, 2H2. 1302 4. Z. Zhang, S. Dai,./. Am. Chem. Soc. 2001, 123. 9204. 5. D. Zhao, Q. Muo, J. Feng, B. \\ Chmelka, G. D. Stucky,./. Am. Chem.Soc. 1998, 120, 6024. 6. a) Y. Sakamoto, M. Kancda, O. Terasaki, D. Zhao, J. M. Kim, G. D. Stucky, H. J. Shin, R. Ryoo, Nature 2000, 40S, 449. b) M. Kruk, V. Antochshuk, J. R. Matos, L. R Mercuri, M. JaroniccJ. Am. Chem. Soc. 2002, 124. 768. 7. R. Ryoo, S. H. Joo, S. Jun,./. Phys. Chem. B 1999, J03. 7743. 8. S. H. Joo, S. J. Choi, 1. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 2001, 412. 169 9. J. Lee, S. Yoon, T. Hyeon, S. M. Oh, K. B. Kim, Chem Commun 1999, 2177. 10. S. Kim, T. J. Pinnavaia, Chem. Commun. 2001, 2418. 11. Y. J. Han, J. M. Kim, G. D. Stucky, Chem. Mater 2000, 12. 2068. 12. J. Fan, C. Yu, L. Wang, i^. Tu, D. Zhao, Y. Sakamoto, O. lerasaki, ./ Am. Chem. Soc. 2000,/2J, 12113.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Morphology control of mesoporous SBA-16 using microwave irradiation Young Kyu Hw^ang^'^, Jong-San Chang^, Young-Uk Kwon^, and Sang-Eon Park^* ^Catalysis Center for Molecular Engineering, KRICT, PO Box 107, Yusong, Taejon, 305-600, Korea ''Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkwan University, Suwon 440-740, Korea. FAX: +82-42-860-7676. Well ordered mesoporous SBA-16 was successfully synthesized within an one hour by employing microwave-irradiation (MI). Morphologies of these materials prepared by controlling the aging time of silica sol, can be addressed with decaoctahedron and spherical shape. Particle size of mesoporous mateials is also controlled by an effective heat transfer agent as a microwave active material, such as ethylene glycol. 1. INTRODUCTION Well ordered large pore mesoporous materials which have the pore-size distribution from 2 to 30nm have been researched for their applications such as catalysis, separation and nanoscience.''^ Moreover, three dimensional cubic mesoporous silicas have advantage compared to hexagonal mesoporous material with the one-dimensional channels. In addition to the pore size tuning, morphology control of mesoporous silica have been reported such as fibers, spheres, hollow tubulars, and monoliths. Compared with conventional hydrothermal method, microwave-synthesis of nanoporous material have the advantages of the rapid crystallization time and homogeneous nucleation. In this regards, microwave irradiation technique has been widely introduced to the synthesis of nanoporous materials such as zeolite A, Y, ZSM-5 and MCM-41'^"'^ Recently, ordered hexagonal mesostructured SBA-15 under acidic condition by employing microwave has been reported by Kormaneni and co-workers'^^ In our previous work, we reported synthesis of MCM-41 using microwave heating, and mechanism study for mesoporous MCM-41.'' Herein, we report the microwave-synthesis of mesoporous SBA-16 with the morphology of dodecahedron and spherical shapes obtained by controlling the aging time and amount of the ethylene glycol.

2. EXPERIMENTAL 2.1. Synthesis Mesostructures of silica-polymer were obtained following the synthesis procedure reported elsewhere ^ except for the use of the microwave synthesis. In a typical synthesis, 1.6g of EOi()6P07oEOi()6 polymer mixture was dissolved in 41.2g of distilled water and then 4.7 Ig of the gel was stirred for x Min.(x=0, 30, 120). The gel obtained was loaded into microwave

102

oven to increase degree of silanol group condensation under microwave irradiation condition at 373K for 1 hour. The molar composition of the final gel mixture was 1.0 Si02 : 3.17x10'"^ F127; 6.68 HCl : 137.9 H2O : 1.4-8.4 EG. Microwave synthesis was performed using MAR5(CEM Corp., Matthews, NC) microwave digestion system. We denote samples depending on stirring tiems and microwave irradiation times, such as stirring for 30min and microwave irradiation for 60min(S30/M60). 2.2. characerization Mesostructures were monitored by X-ray diffraction (XRD, Rigaku D/max-RC). Transmission electron microscopic (TEM) images were taken using a JEM-3011 instrument (JEOL) equipped with slow-scan CCD camera operating at 300 keV. Scanning electron microscopic (SEM) images were collected with a JEOL 630-F microscope operating at 5 kV. N2 adsorption-desorption isotherms were obtained using a Micromeritics ASAP 2040 apparatus at liquid N2 temperature. 3. RESULTS AND DISCUSSION 3.1. Structure and morphology of SBA-16 XRD pattern of the as-made and calcined samples obtained by stirring for 30min and MI 120min (S30/M60), can be indexed as a cubic mesophase with (110), (200), and (310) diffractions (Im3m space group, a=155A and 133 A for assynthesized and calcined samples, respectively) which is consistent with the reported for SBA-16.^ Although 3 the XRD patterns do not reveal the < details of the structure with these diffraction patterns, the TEM image of the calcined material, in Figure Ic, 50nm C can be explained with Im3m cubic o structure with the [11 l]-direction, a = 130A. On the contrary, sample which is not seen grown without stirring (S0/M60) has a single intense peak at 1 2 3 4 5 6 7 a d spacing of 112 A, typical of the 2 6 I degree peak corresponding to disordered Fig. 1. XRD patterns of (a) as-made and (b) calcined and mesostructured silica prepared by nonionic surfactant templating.'^ The TEM image of (c) calcined SBA-16 N2 adsorption and dcsorption isotherms of calcined sample give a BET(Brunauer-Emmett-Teller) surface area of 822.9 m^g" ' and a pore volume of 0.73 cm^g'. The morphology of well ordered mesoporous SBA16(S30/M60) has a decaoctahcdron shape, with 6 squares and 12 hexagon, which has a relatively uniform size of ~2 //m.(Figure 2). The morphology of the observed crystals of this material is also consisted with a cubic structure.'^ The formation of a well-defined external morphology of the SBA-16 suggests that the mesostructured material has a highly ordered structure and a low level of imperfections or defects in the lattice. When samples prepared without stirring (S0/M60) have spherical shape.

103

• ^

*

3.2. Controlling particle of disordered materials M e s o s t r u c t u r e d material having disordered spherical shape prepared without stirring before MI has an irregular size of 0.5-5 (im in Figure 3a was obtained. When the EG as a heat transfer agent under MI is added into the synthetic solution, particle size distribution of spherical shape of disordered mesostructured materials having smaller one can be controlled depending on an amount of ethylene glycol Fig. 2. SEM image of as-made SBA-16(S30/M60) as can be s e e n from the SEM images(Figure 3). In Figure 3a, the size of the mesoporous materials have wide ranges of distribution from 1 fim to 10 fim. In a EG/H2O ratio of 0.04, particle size has been almost homogeneously distributed with a ~ 2 //ni. As M *^Jf

^jL^-y-.J: ^\ -f^ w^ i >*^-«

I: LK'

X^^d^

».'-•• v,"^:/?". /j^?»' o

CL

0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/PJ

Pore Diameter (nm)

Fig. 1. Nitrogen adsorption/desorption isotherms (left) and pore size distributions (right) of (a) SBA-15; (b) PPOl-SBA; (c) PP02-SBA; and (d) PP04-SBA. The nitrogen adsorption/desorption isotherms and pore size distributions calculated by BJH method using desorption branch of the isotherm are shown in Fig. 1. The hysteresis loop of the PPO added samples showed almost the same behavior with the typical SBA-15 mesoporous silica with a little shift in the position to a higher relative pressure. The pore size distributions showed that all the materials have micropores less than 2 nm and the peak position in the mesopore region was shifted to a larger diameter with the amount of added PPO in the synthetic mixtures. The SAXS patterns of the samples are shown in Fig. 2. In this figure, SBA-15 and PPO added samples showed an intense primary peak around 26 = 0.9 and the ratios between the 1000A three peaks are close to 1:1.73:2, which / Id 800supports that these samples have the highly ordered hexagonal pore 600structures. In addition, TEM images of i: the SBA-15 and PP04-SBA sample in 400 H ' b Fig. 3 shows the particle morphologies 200^ and the highly ordered hexagonal pore structures of these samples. In this 0^ figure, it was shown that the polymer added PP04-SBA samples have the 20 same morphologies with the SBA-15 mesoporous silica. However, PP04Fig. 2. SAXS patterns of (a) SBA-15; (b) PPOl- SBA sample showed the larger particle sizes in the axial direction. SBA: (c) PP02-SBA: and (d) PP04-SBA.

1\ lie

L

—

'

—

1

—

'

?..

—

1

112

(b)

(a) 4009C'3fe

(c) (d) Fig. 2. T¥M images of 2()(),()0() limes (left) and 5,000 times (right) magnification, (a) and (b) for the SBA-15; and (C) and (d) for the PP04-SBA 4. CONCLUSIONS Poly(propylene oxide) was added to the synthetic mixtures of the SBA-15 type mesoporous silica. The added polymer alTected the pore properties of the synthesized mesoporous silicas and the results were an increase in the pore diameter, surface area and pore volume, especially, the micropores were remarkably increased. Also, the polymer addition has afTected the morphologies of the synthesized particles and thus, the particle size was increased in the axial direction. However, the highly ordered hexagonal pore structures of the SBA-15 type mesoporous silica were conserved with the amount of added polymer.

REFERENCES 1. A. Galameau, D. Desplantier-Ciiscard, V. D. Renzo and F. I-ajuIa, Catalysis Today, 68 (2001) 191. 2. Y. S. Cho, J. C. Park, W. Y. Lee and J. Yi, Catalysis Letters, in press (2001) 3. Y. S. Cho, J. C. Park, B. S. Choi, J. Moon and J. Yi, Stud. Surf. Sci. Catal., 133 (2001) 559 4. H. G. Karge and J. Wcitkamp, Molecular Sieves: Synthesis, Springer-Verlag, New York, 1998 5. Y. Wang, M. Noguchi, Y. lakahashi and Y Ohtsuka, Catalysis Today, 68 (2001) 3

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

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Synthesis of mesoporous silicas with different pore-size by using EOmMAn diblock copolymers of tunable block length as the templates Yi-Tsu Chan^, Hong-Ping Lin^, Chung-Yuan Mou^'^ and Shiuh-Tzung Liu^* ^ Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. ^ Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan. '^ Center of Condensed Matter, National Taiwan University, Taipei 106, Taiwan. The EOmMAn diblock copolymers with various unit number of MA segment can be synthesized via a typical ATRP method and used as the templates for preparation of mesoporous silicas with well-ordered hexagonal mesostructures. With the feasible adjustment of hydrophobic MA parts, the pore size of mesoporous silicas can be tuned in a wide range from 4.0 to 20.0 nm. 1. INTRODUCTION Since the discovery of M41S mesoporous silicas by Mobil Oil Corp. in 1992, this area of research has received much attention due to the high surface area(~ 1000 m^/g), tunable pore size (1.0-10.0 nm) and uniform as well as stable pore structures of these materials.[l] Besides of quaternary ammonium surfactants, the low-cost, biodegradable, and natural friendly neutral polyethylene oxides surfactants recently have been extensively performed to synthesize mesoporous silicas with various mesostructures and morphologies. It is well known that mesostructure, wall thickness, morphology and porosity of the mesoporous silicas rely on surfactant micelles and liquid-crystal arrays of micelles as structure-directing agents.[2] However, the amphiphilic surfactant used for this function is not well understood because few polyethylene oxide surfactant sources are available. [3] Recently, the synthetic methods leading to well-controlled copolymers have been developed, which raises the interest in design of diblock amphiphilic polymers for surfactant to build the mesoporous materials. Herein, we reported a synthetic approach via atom transfer radical polymerization (ATRP) method to obtain various poly(ethylene oxide)-/7-poly(methyl acrylate) diblock copolymers (denoted as EOmMAn) with various polymerization degree of MA segment, [4-6] which were used for construction of mesoporous silica with different mesostructures and pore sizes in a wide dimension of 4.0-20.0 nm. 2. EXPERIMENTAL 2.1. Synthesis of EOmMAn diblock copolymers The CuBr/MceTREN (tris[2-(dimethylamino)ethyl]amine) and the poly(ethylene oxide)-2-bromoisobutyrate were used as the catalyst and the macroinitiator for the polymerization of MA, respectively. The detail synthetic procedures, chemical composites and reaction condition have been reported in elsewhere.[5-7]

114

2.2. Preparation of mesoporous silicas The EOmMAn-silica mesostructural composites were synthesized in acidic media as that for SBA-15 siHcas."^ The as-synthesized mesoporous siHca products were obtained after 1 day agitation at 25-50 °C. The final gel composition (in gram) is: (0.3~0.5)g EOmMAn: (20.0~25.0)g H2O : (4.0~6.0)g 37%HC1 : (l~2.5)g TEOS. In order to increasing the mesostructural ordemess and stability, 1.0 g dried acid-made mesoporous silicas was combined with 50.0 g water (pH «7.0) and then hydrothermally treated at 100 °C for 24 hr. [8] 2.3. Measurement The powder x-ray diffraction patterns (XRD) were taken on Wiggler-A beamline (k = 0.1326 nm) of Taiwan Synchrotron Radiation Research Center. The mesostructures of mesoporous silicas were recorded on Hitachi S 7100 transmission electron microscope (TEM) with an operating voltage of 100 keV. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus, and the pore size distribution was calculated from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. 3. RESULTS AND DISCUSSION Fig. lA shows the XRD patterns of the calcined mesoporous silicas synthesized from the EO17MA12, EO45MA72, and EO45MA94-TEOS-HCI-H2O compositions. One can clearly see that all these calcined mesoporous silicas exhibit 3 XRD peaks at low angle range of 0.3-2.0°. All these calcined mesoporous samples have the representative (100), (110) and (200) peaks indicating of hexagonal mesostructure. Moreover, every mesoporous silica sample possesses a sharp capillary condensation in each N2 adsorption-desorption isotherm (Figure IB). 1500 -

T

1400 -

A

B

r^J^

1200 1 1 00 -

900 -

(110) (100)

700 600 -

II

500 -

II

.^ Cl'j^O) 00)

300-

•i-J

I

KOO -

1

\lOO)

III

1 '• t

/

200 -

(••0)

(2,„

100-

ill

0 2 0.5

1.0

1.5

2.0

2.5

0 4

3.0

2e/degree

Fig. 1. (A) The XRD patterns and N2 adsorption-desorption isotherms of the mesoporous silica synthesized from E0mMAn-TE0S-HCl-H20 composition. I. EO45MA94; 11. EO45MA72; III. EO17MA12.

115

•S'S

Fig. 2. The TEM micrographs of the calcined mesoporous silica using different EOmMAp diblock copolymers. A. EO17MA12; B. EO45MA72; C. EO45MA94. However, the capillary condensation occurs at different relative pressure (P/Po). Using the Barrett-Joyner-Halenda calculation method, the pore size of the mesoporous silica from EO17MA12, EO45MA72, and E045MA94are 3.8, 12.0 and 19.1 nm, respectively. In Figure 2, the TEM micrographs demonstrate the well-ordered hexagonal array of the nanochannels of the mesoporous silicas in Figure 1. This TEM result is parallel to that of XRD. The existence of the mesostructure pattern of the calcined sample suggests that this mesostructure is thermal stable as those of SBA-15 mesoporous silicas prepared by using Pluronic 123 triblock copolymer."* With an approximate comparison and measurement on the pore dimension, we found the measured pore size is close to that of N2 adsorption-desorption isotherms and increase in the order of EO17MA12 < EO45MA72 < EO45MA94. According to the prediction of the core-shell model, [9] it was supposed that the H pore size and d-spacing increase with the increase of the hydrophobic units in block Number oflMA units copolymers. In EOmMAn diblock copolymers, Fig. 3. The plot of the pore size vs. the the hydrophobic part is MA segment. To confirm this ideal, we performed various number of the MA units in E045MAn E045MAn diblock copolymers with varying diblock copolymers. MA units as the templates to synthesize the mesoporous silicas. After the analysis the pore size from the N2-adsorption branches, a plot of pore size vs. MA units was illustrated in Figure 3. The pore size linearly increases with the increase of the number of MA units and the slope is about 0.25/MA unit. While doing an extra-plot, the intercept is about 1.92 nm, ascribed to the contribution of 45 EO units. Consequently, the hydrophobic MA plays the major role on controlling the pore size rather that hydrophilic EO segments. These results

t *

+

116

almost match the core-shell model. Thus, changing the hydrophobic MA units upon the synthesis of EOmMAn diblock copolymers, the pore size can be fme-tuned. In addition, the pore size of the mesoporous silicas can be feasibly swollen to 20.0 nm at MA unit =110 without the addition of hydrocarbon expanders, which have been used to expand the pore size of SBA-15 orMCM-41 mesoporous silicas. From further analysis of the N2 adsorption-desorption isotherms, it shows that all calcined mesoporous silica samples aforementioned have the advantages of high surface area (400-800 m^/g), tunable pore size (4.0-20.0 nm) and thick wall thickness (2.5-4.0 nm). In summary, the EOmMAn diblock copolymers are a new family of organic templates to generate the high surface -area mesoporous materials with the desired pore size and porosity. 4. CONCLUSION In brief, the ATRP synthetic method provides a convenient way to control the composition and combination of the neutral diblock or triblock copolymers. With a well tuning in the surfactant micellar properties and liquid-crystal phases, one can design the porosity, pore size, morphologies and mesostructures for extending the applications of mesoporous materials. [10] ACKNOWLEDGEMENT We thank Mr. Chung-Yuan Tang and Ching-Yuan Lin for helping the TEM micrographs preparation. This research was financially supported by National Science Council of Taiwan (NSC-90-2113-M-002-038). REFERENCES 1. C. T. Krcsge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. D. Zhao, R Yang, Q. Hou, B. R Chmelka and G. D. Stucky, Current Opinion in Solid State and Materials Science, 3 (1998) 111. 3. (a). D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F Chmelka, and G. D. Stucky, Science, 279 (1998) 548.; (b). S. A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269(1995) 1242. 4. Y T. Chan, H. R Lin, C. Y Mou and S. T. Liu, unpublished result. 5. R. N. Keller and H. D. Wycoff, Inorg. Synth., 2 (1946) 1. 6. M. Ciampolini and N. Nardi, Inorg. Chem., 5 (1966) 41. 7. K. Jankova, X. Y Chen, J. Kops and W. Batsberg, Macromolecules, 31 (1998) 538. 8. D. Zhao, Q. Huo, J. Feng, B. F Chmelka and G. D.Stucky, J. Am. Chem. Soc, 120 (1998) 6024. 9. J. H. Chang, L. Q. Wang, Y Shin, B. Jeong, J. C. Bimbaum and G. J. Exarhos, Adv. Mater., 14(2002)378. 10. F S. Bates and G. H. Fredrickson, Phys. Today, 52 (1999) 32.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

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Polypropylene glycol as a swelling agent for the synthesis of mesoporous silica (SBA-15) by amphiphilic block copolymer templating Xiuguo Cui^, Joong-Hyun Ahn^, Wang-Cheol Zin^, Won-Jei Cho ^, and Chang-Sik Ha^* ^Department of Chemical Engineering, Yanbian University, Yanji 133002, P. R. China ^Department of Materials Engineering, Pohang University of Science and Technology, Pohang 790-390, Korea. ''"Department of Polymer Science & Engineering, Pusan National University, Pusan 609-735, Korea. The role of poly(propylene glycol) (PPG) as a polymeric swelling agent in the synthesis of mesoporous silica, SBA-15, by amphiphilic triblock copolymer templating was investigated. Two different molecular weights of PPG were compared. It was found that even a low concentration of PPG expands effectively the pore size of the SBA-15 in the presence of the triblock copolymer, poly(ethylene oxide)-poly(propylene oxide)-poly(propylene oxide) (PEOPPO-PEO) without sacrifying the wall thickness of the framework and the original morphology of pores. 1. INTRODUCTION The traditional methods to control the pore size of mesoporous materials are the postsynthesis hydrothermal treatments[l,2], the addition of an organic swelling agent (i.e. 1, 3, 5trimethylbenzene, TMB)[3] as well as the use of templates with different lengths of hydrophobic chains. In order to obtain mesoporous silica with large-sized pores, amphiphilic triblock copolymers have been also utilized as a template[4]. However, unlike low molecular weight surfactants whose hydrophobic chain length could be finely adjusted, the length of the block segment in the amphiphilic triblock copolymer, PEG-PPG-PEG, is not easily controlled in a small-scale region without the use of delicate and thorough synthetic skills. Furthermore, it is not easy to obtain commercially available block copolymers that have various defined block lengths. Here we report the role of a polymeric swelling agent, poly(propylene glycol) (PPG) with two different molecular weights in the synthesis of SBA-15 by the triblock copolymer templating. 2. EXPERIMENTAL The template solutions were prepared by dissolving PEG2o-PPG7o-PE02o(average molecular weight of 5800, abbreviated as EPE5800), and a swelling agent, PPG(average molecular weight of 2000 and 2700, abbreviated as PPG2000 and PPG2700, respectively), in de-ionized water under moderate stirring at 308K for 6h, then a 2M HCl solution was added into the template solution. Gnce the PPG suspension was adsorbed and a transparent solution was obtained, tetraethyl orthosilicate (TEGS) was dropped slowly into the acidic template

solution while stirring. In a typical synthesis, a mixture of 2.03g of EPE5800 and 0.4g of PPG2000 dissolved in 30g of de-ionized water was added to 60g of a 2M HCl solution. Then, 11.1ml of TEOS were dropped into the template solution that has a pH of 0.7, while being stirred at 308K. Other procedure is similar as that for the SBA-15. The mesoporous silicas were characterized using small angle X-ray scattering (SAXS) and N2 sorption experiments. 3. RESULTS AND DISCUSSION Figure 1 illustrates the N2 adsorption/desorption isotherms for mesoporous silica prepared with or without hydrophobic PPG. The isotherms exhibit a typical type IV curve, which characterize properties of mesoporous materials exhibiting a capillary condensation step and a hysteretic loop. The BJH pore size distributions (inset in Figure 1) exhibit a single narrow peak. The mesoporous silica synthesized in the absence of PPG has a pore size of 39 A , which is smaller than that of SBA-15 prepared at 408K for 20h[4], because this sample was prepared at 408K for 8h without any post-treatment procedure. Table 1 Physicochemical Properties of Mesoporous Silica Prepared Using Hydrophobic PPG as a Swelling Agent and Amphiphilic Copolymer as a Structure-directing agent Sample Content of PPG: Pore Wall ABJH ABI;T ^Langmuir dioo ao Size Thickness /g(io"Wir') (mV) (mV") (mV) (A) (A)

0

1044

382

522

(A) 39

(A)

80.8

93.3

54.3

PPG2000/0.4g 501 681 1434 42 83.2 96.1 54.1 (2.0) PPG2000/1.0g 58.9 1266 108.9 50 443 601 94.3 (5.0) PPG2700/0.54g 517 711 60 1367 94.4 109.0 49.0 (2.0) PPG27001.08g 62 540 744 1438 96.1 111.0 49.0 ^ _ . . (4.0) ABJH, ABI:T, ALangmuir arc surface areas obtained from adsorption branch results calculated by software; the pore sizes were determined from the BJH pore size distribution; a()=2di()()/V3; and the wall thickness^ao-porc size A condensation step was displayed at P/Po=0.45 (Figure la). In contrast, adding 2.0x10'^ mol.r' of PPG2000 resulted in pore sizes of 42 A (see inset in Figures lb), and shifts of mesoporous adsorption steps from P/Po=0.45 to near P/P()=0.5 in the isotherm. Hysteretic loops in the sorption isotherm for sample 1 is type H2, which is different from type H I , whose adsorption and dcsorption branches should be almost vertical[4]. Similarly, a clear H2 - type and a modified H2 - type hysteretic loop, respectively corresponding to mesoporous silica prepared with PPG2000 and PPG2700, were observed in the isotherms. The BET surface area and the other characteristic parameters obtained from the N2 adsorption/desorption experiment are summarized in Table 1. These results indicate that the mean pore size of mesoporous silica was enlarged by increasing the amount of PPG, and for expanding the pore size of mesoporous silica, PPG2700 with a higher molecular weight is more effective than PPG2000 with a lower molecular weight.

119 400

3.9nm

5,2.5

350

^ 1.5

Q.

feaoo •tlaso

2 0.5 o ^0.0

T3 200

0

250

T

80 160 240 320 400

Pore Diameter, (Angstroms)

50 0.2

0.4

in 0 100 200 300 400 50r

Pore Diameter, (angstrom*?)

150

0.6

0.8

' Adsorption Desorption

100 50

(a)

0

s

200

Adsorption Desorption

0.0

EPE580(>+PPG200Q (0.4g)

300

(A

3

4.2nm

350

I 1.0

•s O 150

E

400

EPE5800+PPG (Og)

E 2.0

(b)

0.0

1.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure (P/PQ) Relative Pressure (P/PQ) Fig. 1. N2 adsorption and desorption isotherms of mesoporous silicas prepared with(a) or without(b) polymer swelling agent. Insets are the BJH pore size distributions. (100)

E P E 5 8 0 0

( 2 0 0 ) E P E 5 8 0 0

+ 0.54g

P P G 2 7 0 0

+

P P G 2 0 0 0

0.4g

>|^ ( 2 1 0 )

h k I 10 0 2 0 0 2 10 E P E 5 8 0 0

d (n m ) 8.0 8 4.0 5 3.0 5 + Og P P G

S (n m -1 )

Fig. 2. Small angle X-ray scattering patterns of mesoporous silicas prepared using a polymer swelling agent with different molecular weights. Small angle X-ray scattering patterns of mesoporous silicas are shown in Figure 2. The results demonstrate that mesoporous silicas synthesized with EPE5800 as a structure directing agent and PPG as a swelling agent exhibit a well-ordered hexagonal pore shape. In the absence of PPG, the SAXS pattern of mesoporous silica shows three well resolved peaks with d spacings of 80.8 A, 40.5 A, and 30.5 A (Figure 2A). These three peaks display a d value ratio of Vl: V4: V?, which are indexable as (100), (200) and (210) reflections, respectively, in the hexagonal space group. Thus, the unit cell distance ao between pore centers can be

120

calculated by the formula ao=2^ioo/V3, and the thickness of the framework wall determined by subtracting the mean pore size from ao is 54.3 A. Similarly, three peaks were shown in the SAXS pattern when adding 2.0x10'^ mol.l'' of PPG2000 in the synthesis of the mesoporous silica. These peaks were also assigned to (100), (200), and (210) reflections, respectively (Figure 2B). The mesoporous silica prepared by using EPE 5800 as a template and 2.0x10'^ mol.r' of PPG2700 as a swelling agent showed a well-resolved hexagonal SAXS pattern (Figure 2C). Three peaks were observed with d spacings of 94.4A, 54.2 A, and 47.3 A {dvalue ratio of Vl: V3: V4), indexing as (100), (110), and (200) reflections, respectively. It is clear that the d spacing increases with the increase in the molecular weight of the hydrophobic polymer used for a swelling agent. These results are identical with those of the N2 adsorption/desorption experiments. In summary, a new polymer swelling agent, hydrophobic poly(propylene glycol) (PPG), was used to prepare the mesoporous silicas possessing fme-controllable pore size with a hexagonal porous structure in the presence of a triblock copolymer, PEO-PPO-PEO as a structure-directing agent. The synthetic procedure did not involve a post-synthesis treatment at high temperature. With the addition of PPG of two different molecular weights in different amounts, mesoporous silicas maintained the original porous structure and a thick framework wall. The pore size of mesoporous silicas increased when the amount of the swelling agent was increased. The PPG with the molecular weight of 2700g.mor^ is more effective for expanding the pore size of mesoporous silica, than one with 2000g.mor'. The engineering of pore size control can be completed at a low concentration of the swelling agent (below 2wt%), and without a treatment procedure for the swelling. The advantages of a polymer-swelling agent reported here have allowed us to approach other mesoporous materials to fmely control their pore size. ACKNOWLEDGEMENTS The supports of the Center for Integrated Molecular Systems, POSTECH, Korea and the Brain Korea 21 Project are gratefully acknowledged. Prof. Cui thanks to the Program of Young Scientist Exchange of Korea-China in 1999.

REFERENCES 1. Khushalani, D. M.; Kupperman, A.; Ozin, G; Tanaka, A. K.; Garces, J.; Olken, M. M.;Coombs, N. Adv. Mater. 1995, 7, 842. 2. Huo, Q.; Margolese, D. I.; Stuck, G D. Chem. Mater. 1996, 8, 11477. 3. Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487-491. 4. Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G H.; Chmelka, B. F.; Stucky, G D. to'cwe 1998, 279, 548.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

121

Thermal decomposition-precipitation inside the nanoreactors. High loading of W-oxide nanoparticles into the nanotubes of SBA-15. L. Vradman, Y. Peer, A. Mann-Kiperman and M. V. Landau. Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. The solution thermal decomposition-precipitation (ThDP) of oxide precursor inside the nanotubes (nanoreactors) of mesoporous silica support under atmosphere saturated with solvent at the oxide precursor decomposition temperature was explored for loading the Woxide nanoparticles into SBA-15. ThDP of W-Ethoxide solution in decalin yielded WO3/SBA-I5 composites with W-phase located exclusively inside the pores in form of nanocrystals strongly blocking the pores. ThDP of W(C0)6 yielded W-phase up to 32 wt% spread as an amorphous monolayer on the pore walls with minimal pore blocking. 1. INTRODUCTION Since the discovery of MCM-41 and related materials [1], many attempts were done to employ them as supports for catalytic phases dispersions [2]. However, the conventional impregnation methods yielded poor dispersion of the active phase and significant pore blocking even at relatively low loading of active components [3]. Moreover, at least part of the active phase was formed outside the mesopores. In the present study we tried to overcome this limitation by thermal treatment of the support impregnated with W-oxide precursor solution in a closed reactor with a gas phase saturated with a solvent. In this way the solution was forced to remain inside the nanotubes during decomposition of the precursor. Hence the precipitation of an active phase occurred exclusively inside the mesopores. 2. EXPERIMENTAL The wide-pore pure silica SBA-15 material with surface area of 800 m^/g, uniform mesopore diameter of 6.5 nm and pore volume of 1 cc/g was prepared according to published procedure [4] modified by increasing the duration of the hydrothermal treatment at 100 ^C to 3-7 days to decrease the micropore contribution to less than 10%. I g of SBA-15 was impregnated with decalin solution of W-Ethoxide or W(C0)6 at 80°C to increase the solubility. After solid separation by filtration, the sample together with 5 ml of pure decalin were introduced into the different areas of the 50 ml stainless steel vessel and pressurized to 20 atm with air. ThDP was performed in two steps. First, the temperature was increased up to 350 °C and kept for 3 h. Next, the temperature was increased up to 450 °C and the vessel pressure was released. Due to the limited solubility of W(C0)6 in decalin, the concentration of WO3 in the composite was about 7 wt% after one ThDP step and subsequent ThDP steps were necessary to increase the loading. Reference sample was prepared by conventional wet

122

impregnation of SBA-15 with water solution of ammonium tungstate, followed by drying at 120 *^C and calcination at 550 °C. Unsupported WOx was prepared by similar procedures in absence of SBA-15. The chemical composition of the solid catalysts was measured by EDS analysis with a JEOL JEM 5600 microscope (SEM-EDS). Surface areas, pore volumes and pore size distributions were obtained from N2-adsorption-desorption isotherms using conventional BET and BJH methods. XRD patterns were recorded on a Phillips diffractometer PW 1050/70 (CuKa radiation) equipped with a graphite monochromator. HRTEM micrographs were obtained on a JEOL FasTEM-2010 electron microscope operating at 200 kV and equipped with an analytical EDS-system for composition analysis. 3. RESULTS AND DISCUSSION The XRD patterns showed that conventional impregnation led to formation of crystalline WO3 phase (Figure 1, a). The average crystals size of 15 nm was much higher than SBA-15 pore diameter suggesting that at least part of the WO3 phase was located outside the pores of SBA-15. It was also confirmed by TEM and TEM-EDS measurements. Furthermore, the pore blocking extent, calculated from normalized surface area [3], was 57% as a result of partial pore blocking. ThDP of W-Ethoxide yielded WO3 phase with average crystals size of 5.5 nm (XRD) (Figure 1, b). This suggests that entire WO3 phase was located only inside the nanotubes of SBA-15 since the WO3 phase obtained by ThDP of W-Ethoxide at the same conditions in absence of SBA-15 has much higher average crystal size (>20 nm). This was also confirmed by TEM-EDS. The blocking extent, however, was also high (73%) as a result of blocking the SBA-15 mesopores with WO3 nanocrystals. This correlates well with HRTEM investigations (Figure 2, a) where large nanoparticles could be recognized at the openings of the hexagonally arranged nanotubes of SBA-15 support. Some of the pores seems to be completely blocked in agreement with strong reduction of the measured surface area of the 32 wt% WOx/SBA-15 sample prepared by ThDP of W-Ethoxide (145 m^/g) compared with parent SBA-15 (800 m'/g). ThDP of W(C0)6 yielded highly dispersed almost XRD amorphous WOx phase (Figure 1, c). WOx phase obtained by the same method in absence of SBA-15 had an average crystal size of 7.1 nm. Therefore, ThDP after impregnation of SBA-15 yielded WOx phase located exclusively inside the SBA-15 nanotubes that was also confirmed by TEM-EDS. Furthermore, the pore blocking extent was minimal (14%) which correlates well with XRD data. HRTEM micrograph of 32 wt% WOx/SBA-15 sample obtained by ThDP of W(C0)6 (Figure 2, b) clearly demonstrates the openings of SBA-15 nanotubes that are not blocked with any particles as opposed to the sample obtained by ThDP of W-Ethoxide (Figure 2, a). At the same time, several EDS analysis, taken from this area with 15-25 nm probe size, yielded an average WOx concentration similar to that measured be SEM-EDS. This means that WOx phase is spread on the SB A-15 pore walls in form of amorphous layer in agreement with XRD and N2-sorption measurements. This demonstrates high efficiency of thermal decomposition-precipitation method for loading the transition metal oxide into the mesopores without blocking them by using the optimal precursor.

123

(a)

rt 32 wt% W0,/SBA-15, CS=15 nm, SA=235 m^/g

3

Unsupported WO^, CS>20 nm, SA=10 m^/g

c

0)

\L^ j (b)

(1 32wt%WO,/SBA-15,CS=5.5nm,SA=145m2/g

Unsupported WO^, CS>20 nm, SA=7 m^/g c c

32 wt% W0,/SBA-15, CSC for 1 hour. The molar ratios of CTAB to EOLWFOTOEOLW in the mixtures were 16, 40, 56, 72 and 100. Then they were added into the ZSM-5 precursor solution with stirring at the same temperature and aged at 100 "(^ for 1 day. The resultant inorganic-organic solutions were cooled to room temperature and the pH was adjusted to approximately 11 by dropwise addition of aqueous HCl with vigorous stirring. Then the solutions were heated again at 100 "C for additional 2 days. The pH adjustment was repeated several tim(\s during additional aging, due to NaOH, produced during the reaction that shifted the solution pH toward a strong base. After aging at 100 "(\ th(^ half of the precipitates was filtered, washed and dried for analyti(!al measurements and the remains were transferred to a Teflon-coated autoclave for further heat treatment for the formation of zeolite seeds at 175 "C for 12 hours. The as-made samples were calcined at 500 "C for 4 h in air. For convenience, the calcined samples synthesized with the CTAB/ K02()P07oE02() molar ratios of 16, 40, 56, 72, 100 were denoted as C16, C4(), (^56, C72 and ClOO. 3. RESULTS AND DISCUSSION X-ray diffractometer (XRD) scan data (Fig. 1 a) of C56 shows an intense Bragg peak near 29 = 2" and a series of weak high order peaks in the range of 2.6 < 20(degrees) < 7.5, indicating the existence of a mesoscopic ordering of the pore structure. To measure the hydrothermal stability of the C56, it was placed in boiling water for 24 hrs and 1 N NaOH solution for 3 hrs. Fig. 1 b shows a typical XRD data of C16 after heat-treating in boiling watei- for

143

24 hrs. The data show t h a t the intense small angle peak was still observed near the same 20 as t h a t seen in Fig. 1 a. The peak intensity was decreased by approximately 35 % which may be ruptured in boiling water. After treating in I N NaOH, the main peak decreased and slightly shifted to the wide angle region. These data indicated that the part of the mesostructure was dissolved under the basic condition and the distance between neighboring pores became smaller.

c

03

> 15

1.5

^H

25

35

45

55

65

20 (degrees)

8.5

15.5

29

22.5

29.5

(degrees)

Fig. 1. XRD data of C56: (a) after calcination; (b) after dipping in boiling water; (c) after dipping in IN NaOH solution. The main intense peak of the XRD data for the specimens prepared without heat treatment at 175"C was almost disappeared due to the collapse of the amorphous framework. By addition of EOL^OPOTOEO^O in CTAB micellar solutions, the electrostatic molecular interactions between EOLWFOTOEOLU) and CTAB enhance the molecular packing density especially in the hydrophobic core, leading to a hard-gel type micellar formation. With the tightly-coiled molecular conformation, the organic-inorganic templates may not be ruptured even during the solid phase conversion of the MFI-type zeohte precursors into the ZSM-5 seeds at high temperatures and pH ^ 12. The successful incorporation of non-ionic EO^OFOTOEOL'O in the cationic CTAB solution depends on the molar ratio of two components with given molecular weights which changes the molecular packing density or interfacial curvature of the mixed micelles which determines the hydrothermal stability of the organic-inorganic templates during treatment of ZSM-5 precursors. To analyze the pore geometry and structure, N2 adsorption-desorption isotherms were measured. In Fig. 2, the adsorption and desorption branches of C72 gradually increases and decreases with pressure, indicative of the

144

absence of mesoporosity of these compounds. The desorption branch of C5() was steeper than the adsorption branch, typical of mesoporous structures with

1(f) Ui

E o •a

/ OU

600

•

500

.

400

o

^ ^ o (A

^ M"^^.-"'

''

/^

.-l-^^j^feafeM

_^^^^I^C^MHHH

i^^n^pTHnf^BggiiMHI 't^^nSsSmKK^^K^^A

;'^&^KB^^^^HI

200

o

1 no

245

"E O 210

" a

300

•a


4 nm), and impregnated with TPAOH as a template for zeolite crystallization. The second stage involved solid state crystallization at 130^C and different times of aging (08 days) ''^. Some alumina was also added in the precursor solution for the UL-ZSM-5 synthesis (Si/Al=100). Structural data of the samples are shown in Table 1. Table 1 Structural properties of Al-SBA-15 (precursor) and UL-Zeolite samples Micropore Mesopore Sample* SBI:T SBJH ^micropore volume volume (m'/g) (m'/g) (mVg) (cmVg) (cmVg) 0.65 0.058 144 Al-SBA-15(100/0) 914 770 0.089 1.25 188 UL-ZSM5(lOO/2) 974 786 0.145 371 UL-ZSM5( 100/6) 904 533 2.19 0.151 0.48 373 UL-ZSM5( 100/8) 479 106 309 0.133 UL-Silicalite(oo/6) 421 112 0.18 *The numbers in parenthesis indicate Si / Al ratio, e.g., 100 and crystallization e.g., 0,2,6 etc.

Mesopore diameter(A) 38 110 195

-

40 time in days ,

4. RESULTS AND DISCUSSION Diffusion measurements, according to the standard ZLC method, were conducted at low concentrations (partial pressures), e.g., 0.1-0.2 Torr, which are considered to be within the linear range of adsorption isotherms. In one of the earlier studies'* it was reported that the diffusion measurements under these conditions revealed parallel microporous structure of MMS. The ZLC desorption curves shown in Figure 1 for toluene in different UL-ZSM-5 samples, as well as the reference (precursor) Al-SBA-15 sample indicate interesting and

147

distinctive features regarding mass transfer processes occurring in these samples at low concentration levels. Two UL-ZSM-5 samples with different times of crystallization, e.g., 2 and 6 days, as well as the Al-SBA-15 sample show typical ZLC curves of the Fickian diffusion. The original micropore structure from Al-SBA-15 precursor is retained in the ULzeolite structure, and is exposed to diffusing sorbate molecules in a fashion similar to micropore diffusion in zeolites. Taking an average diameter lOfim for a single particle, as revealed by SEM images, one can obtain diffusivities in the range of 10''^ to 10"'^ m^/s range from the data presented in Tables 2 and 3, which are typical of micropore diffusion in zeolites^. However, the ZLC curve for Ul-ZSM-5 ((100/8), top curve), which has the highest crystallinity or the longest aging time (eight days) showed a linear behavior on the semi-log scale for the entire desorption time range indicating a response typical of the surface barrier controlled process, as described by Eqn. (4). A plausible explanation for this type of the transport mechanism could be related to the collapse of meso-structure, as is evident from the structural data presented in Table 1, i.e., a drastic reduction of the mesopore area (SBJH) and mesopore volume when inter-grown zeolite crystallinity becomes close to 100 % (8 days of aging). This collapse of the mesopore structure is likely to cause obstruction of micropores within the original SBA-15 precursor's structure. The exactly same pattern of behavior was also observed for n-heptane diffusion in the UL-ZSM-5 samples. Figure 2 illustrates toluene diffusion in composite UL-silicalite particles in comparison with single silicalite crystals. Effective time constants (Dco/R^) determined from these curves confirm faster diffusion process in the composite UL-silicalite particles (3-4 times). Similar results were obtained for other comparative systems involving n-heptane and toluene in ULZSM-5 particles and ZSM-5 crystals. Generally all these results confirmed facilitated mass transport involving UL-zeolite composite particles. Summary of the results presented in Tables 2 and 3 shows that toluene effective time constants are 2-3 times higher in the UL-ZSM- 5 (2 and 6 days) compared to the UL-silicalite sample. This is in contrast to diffusivites involving zeolite single crystals, where diffusion of the same or similar sorbates is always larger in silicalite than ZSM-5 due to the absence of active acid centers in the former. Property data shown in Table 1 indicate very small mesopore volume of UL-silicalite (0.18 cmVg) in comparison with 2.19 cmVg associated with the UL-ZSM-5 (100/6) which could be indicative of a partially collapsed mesoporous structure in the UL-silicalite, and possibly some obstruction of the micropores. In addition these results show significantly higher effective time constants for n-heptane in UL-ZSM-5 than toluene as expected, due to interactions between the acid sites of the adsorbent with aromatic ring of toluene. 5. CONCLUSIONS AND RECOMMENDATIONS At the low concentration levels diffusion processes in UL-zeolite samples are entirely governed by micropores that are associated with the precursor, SBA-15 structure. Effective diffusion time constants for the samples with developed meso-stucture are generally much higher in comparison to the corresponding zeolite single crystals. If a mesoporous structure is destroyed due to a shrinkage of mesopore volume, then the transport process is entirely controlled by a surface barrier mechanism

148

Fig. 1. ZLC curves of toluene in different samples of UL-ZSM-5 and SBA-15 at 80°C l^*'.^

^ • ^ ^ ^ ^ ^^>toLr*^Cfc»

i

'"'-^Mx>^. "^''**'^.'-v'v.'„

,..-'^^**^J'^^*'-S8A15(100) UL-Z8M5(100/8)

y

^ ^ « ^ , „ ^ ^ Silicalite "" ^ ^ l ^ ^ ' ^ H i t ^ ^

C/Co ' •"^,,

^ ^ ^ ^ ^ f e j B - j .

J

I

^^ ^

UL-ZSM5(100/t)

xsT"^'^--^ ^ v '"^^'->-1

Fig. 2. ZLC curves of toluene in silicalite and UL-silicalite samples at 80"C

':

"-K

UL-silicaiite

^ " " ^

^^^^^

^^^^S^^^Sikaft

UL-ZSM5(100/2)

0

too

30

200

50

0

Table 2 Summary of the ZLC results for toluene in different adsorbents Sample AI-SBA-15 (100) UL-ZSM-5 (6) UL-Silicalitc (6) Silicalite

ZSM-5 (100)

60 80 100 80 100 120 60 80 100 60 80 100 120 60 80 100

(xlO^) 5.9 10.7 22.3 13.0 22.6 49.1 2.5 5.4 10 5 0.7 1.5 3.1 6.2 1.8 2.9 4.5

150

200

250

300

35

Table 3 Summary of ZLC results for n-heptane in different adsorbents Sample

Temp.

CO

100

T i m e (sec)

Time(sec)

Temp.

(kJ/mol) 32.2

Deo/R'

H. (kJ/mol)

("C) Ai-SBA-15

60

4.6

(100)

80

11.4

44.53

100 38.16

UL-ZSM-5 (6)

37.60

UL-Silicalite (6)

29.36 Silicalite 23.05

ZSM-5 (100)

60

15.1

80

24.9

100

51.4

60

2.2

80

4.3

100

8.1

120

19.6

60

1.23

100

6.9

80

1.9

100

2.9

120

4.1

21.20

39.23

21.20 21.91

The ZLC measurements involving higher (non-linear) concentration levels of sorbates arc recommended in a future work to assess a role of mesoporcs and inter-grown nano-crystals of zeolites in the overall diffusion process. REFERENCES 1. D. Trong On, D. Lutic and S. Kaliaguine, Microp. Mesop. Mat., 44-45 (2001) 435. 2. D. Trong On and S. Kaliaguine, Angew. Chem.lnd. Ed., 40 (2001) 3248. 3. F. Stallmach, A. Graser, J. Karger, C. Krause, M. Jeschke, U. Oberhagenmann and S. Spange, Microp. Mesop. Mat., 44-45 (2001) 745. 4. D.S. Campos, M. Eic and M.L. Occelli, Stu.Surf.Sci.Cat., 129, A. Sayari et al. eds., Elsevier Science (2000) 639. 5. M. Jiang, M. Eic and D.M. Ruthven in Fundamentals of Adsorption 7, K. Kaneko et al. eds.. International Adsorption Society (2002) 732 6. J. Karger and D. M. Ruthven, Diffiision in Zeolites and Other Microporous Materials, J. Wiley and Sons, New York (1992).

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

149

Synthesis of cubic mesoporous aluminosilicates with enhanced acidity Gong Li, Qiubin Kan*, Tonghao Wu, Changmin Hou, Feng-Shou Xiao, Jiahui Huang College of Chemistry, Jilin university, Changchun 130023,RR.China. Cubic mesoporous aluminosilicate(AlMB48) with enhanced acidity was synthesized by two-step crystallization and characterized by XRD, N2 physical adsorption-desorption, ^^Al MAS NMR , IR and NH3-TPD methods. A1MB48 possessing stronger acid centers showed higher activity for the cumene cracking and the catalytic alkylation of 2,4-ditert-butylphenol with tert-butanol than conventional mesoporous materials. 1. INTRODUCTION M41S molecular sieves have only weaker acid strength owing to the amorphous character of the pore wall, which limited their applications for the catalytic conversion of large hydrocarbons or other organic molecules. Much effort has been undertaken to synthesize new types of materials which combined the advantages of mesoporous and microporous molecular sieves^''^'. Kloetstra et al'"^^ have tried to synthesize the mesosoporous materials by recrystallization of MCM-41 and IIMS in the presence of tetrapropylammonium cations and found that the acidity and catalytic activity of both materials were improved after recrystallization of the pore wall. Karlsson '^' and Huang et al '^'' have ever tried other ways in which microporous and mesoporous composite matetials with hexagonal ordered structure could be prepared in the presence of a mixed template and using a dual templating method through a process of two-step crystallization. Zhang et a r ' a n d Liu '^'synthesized exceptionally acidic and steam-stable hexagonal aluminosilicate mesostructures from protozeolitic nanoclusters. However, the mesoporous materials mentioned above are all hexagonal mesostructures and the cubic mesoporous materials with stronger acidic strength have never been published up to date. Here we make use of the precursor containing the structure units of zeolite Beta at low concentration of cetyltrimethylammonium bromide to synthesize a cubic mesoporous aluminosilicate designated AIMB48 with enhanced acid centers in the mesoporous wall and improved catalytic activity for the reaction of larger molecules. 2. EXPERIMENTAL General procedure for preparing AIMB48 is as follows: 0.1906g sodium aluminate was dissolved in 2.9 mL water and then 14.3 mL25% aqueous solution of tetraethylammonium hydroxide(TEAOH) and 2.25 mL of 3.70 moll'* HCl were respectively added, followed by addition of 2.5g fumed silica under vigorous agitation. The whole mixture was stirred for 1 h at room temperature to form a homogeneous gel with the composition of Si02: O.O2AI2O3: 0.028Na2O: 0.6TEAOH: 0.2HCI: 2OH2O. The gel mixture was loaded into a teflon-lined •Corresponding author, E-mail address: qkanr^f-mail.ilu.edu.cn. Fax: 86-431-8949334

150

Stainless steel autoclave and heated at 140 °C for 22 h. The products which were verified by XRD and IR spectra not to contain the zeolite phases but to embody the secondary structure units of zeolite were cooled to room temperature, stirred for 25 min and kept on for second step of crystallization. The above precursor was combined with 9 ml of 19.28 % aqueous solution of cetyltrimethylammonium bromide (CTMAB) and 1.12 ml of 3.70 M HCl and stirred for 1 h to form homogeneous composition of SiOz: O.O2AI2O3: 0.028Na2O: 0.6TEAOH: 0.14CTMAB: 0.3HC1: 33H2O. This mixture was heated in autoclave under static condition at 140 °C for 24 h. After cooling to room temperature,the solid product was recovered by filtration, washed with deioned water, dried in air at ambient temperature and calcined at 540 °C (1 °C /min) for Ih in flowing nitrogen followed by calcination in flowing air for 8h. The obtained mesoporous sample was designated as A1MB48(25), where 25 in parentheses represented the Si/Al ratio of the reactant mixture. As-synthesized sample was exchanged with 2 molL"' NH4NO3 solution (pH=3) for three times at 80 ""C for 5 h and calcined at 540 ""C to generate H-form, HMB48(25). For comparison purposes, AlMCM-48(25) was prepared according to proceures reported in literature^ \ Samples were measured by X-ray diffraction, N2 adsorption and desorption, ^^Al MAS NMR, IR and NH3-TPD. Cumene cracking reactions were performed in a pulsed microreactor with 50mg catalytst at 350 °C. Hydrogen was used as carrier gas at a flow rate of 50 ml /min, the amount of cumene injected for each test was l|iL. The catalytic alkylation of 2,4-ditert-butylphenol with tert-butanol was investigated by a continuous flow fixed bed reactor. The reaction was carried out with 500 mg of catalyst and 2,4-ditert-butylphenol / tert-butanol molar ratio of 1:2 at 120 "C. The WHSV was 2.20 h-^ and average conversion was reported in 10 h. 3. RESULTS AND DISCUSSION The design of A1MB48 is based on two-step crystallization procedure. The zeolite precursor was firstly prepared. This step does not allow to form complete structure of zeolite phase but should generate secondary structure units of zeolite which possess stronger acidic strength than amorphous aluminosilicatc. The second crystallization in the presence of organic surfactant (CTMAB) makes the precursor prepared in the first step to construct the framework of mesoporous materials. We have investigated the crystallization kinetics of zeolite Beta and found that the Beta precursor with Si/Al=25 containing structure units could be formed within 20-24hat 140"C. The XRD patterns of as-synthesized and calcined A1MB48(25) in Fig. 1 show the characteristic cubic (la3d) structure of mesoporous materials''"' without observing any diffraction peaks of the zeolite in 20 region 10-50^\ Calcination of the sample leads to the contraction of the unit cell from 101.0 to 94.0 A. The TMAB/ Si02 ratio of reactant mixture in the second crystallization of A1MB48 is much lower than that in the synthesis of conventional mesoporous MCM-48'^'""'^'. The two-step crystallization procedure can be used to synthesize cubic mesoporou A1MB48 with various Si/Al ratios from 15-100. ^^Al MAS NMR spectroscopy of as-synthesized and calcined A1MB48 in Fig.2(a,b) show a single resonanse at ca. 54ppm which not only indicates the tetrahedral aluminum environment but implies the absence of zeolite Beta crystal which will present a peak at ca. 60-63 ppm. Although NH4^ exchange treatment causes migration of aluminum from the framework to outside of A1MB48, most aluminum is present in the framework of HA1MB48 as tetrahedral coordination(see Fig. 2c). IR spectra in Fig.3 show a vibrational bond at 550-600 cm'' region for A1MB48(25), which is characteristic of five-membered rings'*^^, indicating the presence of

151

the secondary structure units of zeolite. On the other hand, AlMCM-48(25) which has amorphous pore wall only shows a very weak absorption at this range.

At 4 6 20/degrees

8

Fig. 1. XRD patterns of (a) assynthesized and (b) calcinedd A1MB48(25).

50 6/ppm

0

Fig. 2. ^^Al MAS NMR spectra of (a) as-synthesized and (b) calcined A1MB48(25), (c) H A1MB48(25).

The nitrogen adsorption-desorption isotherms of calcined A1MB48(25) are typical Type IV curves of mesoporous materials (Figure not shown here). The sharp step between p/po=0.3 and 0.4 indicate a narrow distribution of mesopore. The pore diameter (DBJH), cumulative pore volume (VBJH) and BET surface area (ABHT) of A1MB48(25) are 28.48A, 0.75cm^g'' and 881m^g'', respectively. There are two peaks at ca.360 "C and 185 ''C in the NH3-TPD profiles of HA1MB48(25) as shown in Fig.4, which imply the presence of stronger acid centers besides weaker acid sites on HA1MB48(25). This is not the case for HAlMCM-48(25) on which only weaker acid sites exist. The acidic amounts of HA1MB48(25) and HAlMCM-48(25) is 0.80 and 0.72 mmolg"', respectively.

(b)AIMCM-48(25)

561cm

400

^ (a)AIMB48(25)

600 800 1000 J 200 Wavenumber/cnn'

Fig. 3. FTIR absorption spectra of (a) A1MB48(25) and (b) A1MCM-41(25).

100 200 300 400 500 600 Temperature/"C Fig.4. NH3-TPD profiles of (a) HAlMCM-48(25) and (b) HA1MB48(25).

The catalytic tests also announce the difference between HA1MB48(25) and HAlMCM-48(25). For the standard reaction of cumene cracking, the conversion of cumene

152

over HA1MB48(25) (89.7%) is much higher than that over HAlMCM-48(25) (52.9%) under the same conditions. For alkylation of 2,4-ditert-butylphenol with tert-butanol producing 2,4,6-tritert-butylphenol with larger molecules size, the results are shown in table 1 and the orders of activity and selectivity are as HAlMB48(25)>HAlMCM-48(25)>HBeta(25), indicating the advantage of HA1MB48(25) for this reaction. Table 1 Alkylation of 2,4-ditert-butylphenol with tert-butanol ^ . Conversion of "^P 2,4-ditert-butylphenol / % HA1MB48(25) 22.7 HAlMCM-48(25) 15.7 HBeta(25) 14^2

selectivity of 2,4,6-tritert-butylphenol / % 45.6 35.9 14J

ACKNOWLEDGMENT We thank financial support by the Natural Science Foundation of China (29973001). REFERENCES 1. C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt, J. Am. Chem. Soc, 122 (2000) 7116. 2. M. J. Verhoef, P. J. Kooyman, J. C. van der Waal, M. S. Rigutto, J. A. Peters, H. van Bekkum, Chem. Mater, 13 (2001) 683. 3. Y. Liu, W.Zhang, T. J. Pinnavaia, J. Am. Chem. Soc, 122 (2000) 8791. 4. K. R. Kloetstra, W. van Bekkum, J. C. Jansen, J. Chem. Soc, Chem. Commun., (1997) 2281. 5. A. Karlsson, M. Stocker, R. Schmidt, Microporous Mesoporous Mater., 27 (1999) 181. 6. L. M. Huang, W. R Guo, R Deng, Z. Y. Xue, and Q. Z. Li, J. Rhys. Chem. B, 104 (2000) 2817. 7. Z.-Z. Zhang, Y. Han, L. Zhu, R.-W. Wang, Y. Yu, S.-L. Qiu, D.-Y. Zhao, and R-S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 8. Y. Liu. W. Zhang, T. J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 9. A. A. Romero; M. D. Alba; J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 10. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 11. J. Xu, J. Luan, H. He, W. Zhou, L. Kevan, Chem. Matter., 10 (1998) 3690. 12. A. Sayari, J. Am. Chem. Soc, 122 (2000) 6504. 13. J. C. Jansen, R J. van der Gaag, H. van Bekkum, Zeolite, 4, (1984), 369. 14. C. E. A. Kirschhock, R. Ravishankar, F. Verspeurt, P. J. Grobet, P. A. Jacobs, J. A. Martens, J. Phys. Chem. B, 103 (1999) 4965.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

153

Synthesis and characterization of supersurface MCM-41 zeolite using additives Chun-Min Song

Zi-Feng YAN*

Huai-Ping Wang

State Key Laboratory for Heavy Oil Processing, the Key Laboratory of Catalysis, CNPC, University of Petroleum, Dongying, Shandong , China, 257061 1. INTRODUCTION MCM-41 is one member of a new family of mesoporous materials, designated as M41S and discovered by Mobil^''^^ The two most investigated materials, MCM-41 with a 2-D hexagonal structure and MCM-48 with a 3-D cubic structure, are synthesised using rt-alkylammonium salts as templates. It has attracted considerable attention for potential application as catalyst supports or adsorbents because of its high surface area and large pore volume^'"^l At present, the synthesized MCM-41 materials usually have the specific surface area of about lOOOm^.g''. For the MCM-41-supported catalyst, it is rather important that the active sites in such mesoporous zeolite could be well dispersed, which requires that the supported MCM-41 zeolite possess large surface area. Furthermore, super-high surface MCM-41 with larger meso-porosity might be potentially used as the multi-way catalyst carriers as well as shape-selective adsorbents. However, studies for the MCM-41 zeolites with super high surface area have not yet been reported in the literature. The aim of the current study was to develop the novel method to obtain MCM-41 zeolites with super-high surface area and uniform mesoporosity by means of XRD and nitrogen adsorption techniques. 2. MATERIAL AND EXPERIMENTS 2.1. Materials and synthesis process The chemicals used in the experiment were hexadecyltrimethyl ammonium bromide (CieTMABr, A.R.), sodium silicate (Na2Si03.9H20, A.R.), sodium aluminate (C.R), sulfuric acid (A.R.), acetic acid (A.R.), ammonium citrate (A.R.), ammonium nitrate (A.R.), etc. The experimental procedure has been described in our previous work^'^l For the sake of completeness, it is briefly restated here. The mesoporous MCM-41 molecular sieves were prepared with starting gel compositions of 1.0SiO2: O.O5AI2O3: (0.25 ~ 0.125) CieTMABr: 6OH2O: (0 ~ 0.3) additives at 373K for different times, where additives are some compounds such as ammonium citrate, ammonium nitrate etc. The solid products were recovered by filtration, washed with deionized water, and dried at 373K for 24 h. The samples were calcined at 823K for 6 hours to remove the template. These samples were denoted as MCM(t), where t stands for the crystallization time in hours. The other two samples obtained from the same composition, with temperatures of the treatment of 373 and 413K, respectively, are denoted as MCM-41(373) and MCM-41(413). * Corresponding author.

Email: zfyancat(a^hdpu.cdu.cn

154

2.2. Characterization Powdered X-ray diffraction (XRD) were performed on a Rigaku D/MAX-IIIA X-ray powder diffractometer using Cu Ka radiation, and operated at 40kV and 40mA. The X-ray diffraction pattern was recorded over the range from 1 to 10° 26. Nitrogen adsorption measurements were performed at 77K on an ASAP-2010 volumetric adsorption analyzer manufactured by Micromeritics. Before the adsorption analysis, the calcined samples were outgassed for 4 h at 673K in the degas port of the adsorption analyzer. The BET specific surface area was calculated using nitrogen adsorption data in the relative adsorption range from 0.04 to 0.2. The mesopore diameter was evaluated using the BJH method^^^.

u 1

ij

^ 11

1

^

^1

c

ft as-synlhesized

1

3

4

5

fi

7

2

3

4

5

6

7

2

:)

4

5

6

7

2

3

4

5

fi

7

2

3

4

5

6

Fig. 1. XRD patterns of MCM-41 samples with different crystallization time (a) 28 h, (b) 32 h, (c) 48 h, (d) 96 h, (e) 120 h Table 1 Sample MCM(28) MCM(32) MCM(48) MCM(96) MCM(120)

Unit cell parameter (nm) 4.40 4.64 4.31 4.60 4.60

BET specific surface area 903 1005 1282 1245 1104

Pore volume (cm\g-') 0.92 1.02 1.31 1.23 1.23

Pore diameter (nm) 3.46 3.22 3.28 3.12 3.46

3. RESULTS AND DISCUSSION 3.1. X-ray diffraction analysis XRD spectra of the synthesized samples are shown in Figure 1. It can be seen that the XRD spectra of the samples bear four well-resolved reflection peaks indexed as (100), (110), (200), and (210) at 26 range of 1-7°, based on a hexagonal lattice for high quality MCM-41 mesoporous materials. MCM-41 synthesized gives one smart and strong (100) peak, which is indicative of a highly ordered material while the disordered amorphous silica gel showed a broad peak in the X-ray diffraction pattern. The subsequent calcination resulted in the peak intensity to became stronger while the peak location shifted toward higher 2lvalues and the d spacings became smaller. It indicated that further partial cross-linking and reconstruction of aluminosilicate species has been occurred to give better-organization of the mesostructures while calcination. Simultaneously, as the crystallization times increases, the XRD peak intensity and resolution initially increases and subsequently decreases, but the unit cell parameter varies only slightly. Figure 1 displayed that the higher crystallinity is obtained

155

when crystallization times are in the 32 ~ 100 h range. It revealed that the speed of crystallization is relatively rapid and considerably constricted by thermal dynamics of this process. The crystallizing products are thermodynamically metastable and will further turn into a more stable amorphous phase if crystallization time is excessively extended.

700-



X)


\ treatment in water at 80 °C for 5 days, and no evident changes in morphology was ... ,, crNii^oTT) -Si-O ( thicker wall observed. The particle surface is as smooth thinner wall Fig. 4. A schematic model for postas that of initial mesoporous materials. This hydrothcrmal-synthesis of mesoporous silicas indicates that ammonia may take an in ammonia solution. important role in the changes of morphology and structural property of mesoporous silicas during post-synthesis hydrothermal process. Khushalani et al. [ 6] believed that pore expansion is mainly due to the penetration of water into the pores during the posthydrotreatment. Our present study is carried out at low-temperature (80 °C). Due to the volatility of ammonia, ammonia molecules should be easier to penetrate inside the nanochannels than water, and then the channels were swelled up, resulting in the pore expansion. The possible diffusion of volatile ammonia species in the nanochannels upon mild hydrothermal treatment would drive the movement of surfactant-silica species, inducing the extrusion of some silica nanotubes on the particle surface, accompanied with the decrease of wall thickness (Fig. 4). Therefore, the formation mechanisms of mesoporous silica nanotubes during direct- and post-hydrotreatments are quite different. The growing surfactant/silicate aggregates are invoked to explain the larger pore diameter, thicker and highly condensed pore walls and the paintbrush-like morphology observed in direct-hydrothermal synthesis. Whereas the penetration and diffusion of ammonia may be the cause of the pore expansion, pore wall reduction and the extrusion of silica nanotubes in post-synthesis treatment. The findings may be significant for the design and synthesis of mesostructured materials with controllable pore system and morphology. REFERENCES 1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. Z. Yuan and W. Zhou, Chem. Phys. Lett., 333 (2001) 427. 3. H.P. Lin, C.Y. Mou and S.B. Liu, Adv. Mater., 12 (2000) 103. 4. W. Zhou and J. Klinowski, Chem. Phys. Lett., 292 (1998) 207. 5. W. Zhou, R. Mokaya, Z. Shan and T. Maschmeyer, Prog. Nat. Sci., 11 (2001) 33. 6. D. Khushalani, A. Kuperman, G.A. Ozin, K. Tanaka, J. Garces, M.M. Olken and N. Coombs, Adv. Mater., 7 (1995) 842.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

185

Generalized homogeneous precipitation method for precisely controlled synthesis of mesoporous silicas Jifi Rathousky and Amost Zukal* J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic A simple synthesis pathway has been developed which affords organized mesoporous silicas using inexpensive sodium metasilicate as the silica source and nonionic surfactants as the structure-directing agents. Due to their precipitation from an alkaline medium, these materials exhibit markedly different structure features from those prepared in an acid medium, being non-microporous. The precipitation should be carried out at strictly quiescent conditions, as stirring leads to an inevitable decrease in the pore ordering. 1. INTRODUCTION Recently, we have developed a new procedure for the synthesis of organized mesoporous silica (OMS), which is based on the homogeneous precipitation in an alkaline medium of a water solution of sodium metasilicate and quaternary ammonium surfactant [1,2]. The decrease in pH, which causes the formation of a solid product, is achieved by the hydrolysis of a suitable ester of acetic acid. The acidification of the reaction mixture occurs under quiescent conditions without local variations. Due to the possibility to control both the rate of the decrease in pH and its final value, this procedure enables to prepare siliceous mesoporous materials with different structure features as well as the long length scale control of the particle formation. Nonionic alkyl poly(ethylene oxide) surfactants and poly(alkylene oxide) triblock copolymers are important families of surfactants, being low-cost, nontoxic and biodegradable. The synthetic procedures based on the use of nonionic surfactant and tetraalkyl orthosilicate are not as commercially viable as they might be due to the high cost of this silica source. Therefore, syntheses starting from sodium silicate solutions as an inexpensive inorganic silica source were recently reported [3-7]. In all these procedures an amount of acid is added to the reaction mixture to lower its pH; in some cases pH is then adjusted to the desired value. This study aims at the development of a new procedure for the synthesis of OMS, which combines the advantages of both the precipitation under quiescent conditions and the application of nonionic surfactants. Unlike above cited procedures, the formation of the silica mesophase occurs at gradually decreasing pH from highly alkaline to neutral region. Typical results obtained with three different types of nonionic surfactants are reported in this contribution.

•Corresponding author; Telephone: +4202-66053865; Fax: +4202-86582307; E-mail: [email protected].

186

2. EXPERIMENTAL 1 g of nonionic surfactant and 2.5 g of Na2Si03 were dissolved in 900 mL of H2O. The decrease in pH, which caused the precipitation of OMS, was due to the addition and ensuing hydrolysis of 5 mL of ethyl acetate at 35 - 80 °C. The resulting solid was recovered by filtration, washed with water, dried and calcined at 600 °C in air. As structure directing agents, alkyl poly(ethylene oxide) surfactant Brij 56, poly(alkylene oxide) triblock copolymer Pluronic P 123 and poly(ethylene oxide) sorbitan monostearate Tween 60 were used. (The designation of samples and synthesis temperatures are given in Table 1.) Adsorption isotherms of nitrogen at 77 K were taken with a Micromeritics ASAP 2010 instrument. Powder X-ray diffraction (XRD) patterns were collected on a Siemens D 5005 diffractometer. Scanning electron microscopy (SEM) was performed using a JEOL JSM5500LV microscope. 3. RESULTS AND DISCUSSION It was found that stirring should be only applied to achieve homogenization of the reaction mixture after adding ethyl acetate, while the synthesis proper should be carried out at strictly quiescent conditions. If this requirement is not observed, either less well-organized or even disordered material is obtained depending on the concentration of reaction mixture. The diffractograms of all the samples (not shown) exhibited only single reflection in the low 20 range. These diffractograms are typical for the mesopore structure with wormhole motifs; the reflection can be interpreted as an indication of the distance between nearest neighbours, rather than as distance between lattice planes. Adsorption isotherms on samples prepared at the temperature 80 "C are shown in Fig. 1 A. It is obvious that the structure of OMSs, which were prepared with Brij 56 or Pluronic P 123, is not substantially influenced by the type of surfactant. On the other hand, the structure of OMS is influenced by the specific shape of the Tween 60 surfactant molecule with a short hydrophobic chain compared with a large hydrophilic head made of three free poly(ethylene Table 1 Synthesis conditions and material parameters Sample

856/35 856/50 856/65 856/80 P123/80 Tw60/80

Surfactant

Brij 56 Brij 56 Brij 56 Brij 56 Pluronic P 123 Tween 60

Synthesis temperature (^C)

Sm-.i

^Ml-SO

^Mi':sc)

(m'g')

(cm^g-')

(nm)

35 50 65 80 80 80

546.4 485.4 477.4 439.6 559.7 415.4

0.584 0.777 0.957 1.062 1.099 1.351

4.1 6.5 9.5 12.2 6.4 7.1; 24

The mesopore volume ^^MESO and mean mesopore diameter DMESO were obtained from the desorption branch of the nitrogen isotherm using the BJH method.

187

-|

0.0

0,2

1

1

T"

0,4

0,6

0,8

1,0

PlPo

Fig. 1. (A) Adsorption isotherms of nitrogen at 77 K on samples PI 23/80 (a), B56/80 (b) and Tw60/80 (c). (B) BJH pore size distribution of samples B56/35 (a), B56/50 (b), B56/65 (c) and B56/80 (d). oxide) chains and one linking the ring to the hydrophobic tail leads to a distinctly differing product. The size of mesopores can be tailored by the choice of the synthesis temperature. Increasing step-by-step the synthesis temperature over the range of 35 - 80 "C leads to a substantial increase in the mesopore size and volume as illustrated by the samples prepared withBrij 56 (Figure IB). The texture parameters (Table 1) were evaluated from adsorption data using the BET and BJH methods (Table 1). They confirm the influence of the surfactant nature and synthesis temperature on the mesopore size and volume. The maximum in the pore size distribution of the sample Tw60/80 at 7.1 nm and the shoulder at 24 nm illustrate the specific role of the Tween 60 surfactant as a structure directing agent. The analysis of adsorption isotherms performed by means of the comparison plot method has shown that neither of the materials prepared contains micropores. The shape of adsorption isotherms and the absence of micropores represent a marked difference from analogous materials prepared in strong acid media [8,9], where the ethylene oxide (EO)n moieties of the surfactant associate with hydronium ions forming units, which can be designated as S^H^. Below the aqueous isoelectric point of silica, the assembly of OMS proceeds through an intermediate in the form (S"H^)(X 1^) where X" is an anion such as CI' in the HCI medium and r is a protonated Si-OH moiety [8]. However, the assembly of OMS in the alkaline media is based on another type of interaction between the nonionic surfactant and silicate species. OMSs are formed from silicate anions I" in a reaction pathway, which can be denoted as (S'^M^)r, wherein electrostatic forces are introduced into the assembly process through (EO)n complexation of small metal cations M"^ (such as Na^ cations for Na2Si03 used as the silica source) [10].

188

•

g

' (^^'; " ^ V '» ,•*-

Sjiiii

^

•

Fig. 2. Scanning electron micrographs of samples B56/50 (A), B56/80(B) and Tw60/80(C). Scanning electron micrographs of samples 356/50, 856/80 and Tw60/80 reveal that the nature of the surfactant strongly influences the assembly pathway. All the samples prepared with Brij 56 are characterized by spherical particles (Fig. 2A and 2B), which proves that the particles are liquid-like after their assembly and solidify only due to their subsequent aging. At lower synthesis temperatures the coalescing of particles can be observed (Fig. 2A). On the contrary, the sample Tw60/80 is characterized by large irregular particles, from which it follows that they grow as a solid phase from the very beginning. 4. SUMMARY The process described in this contribution provides a new insight into the synthesis of OMS in an alkaline medium using nonionic surfactants as structure-directing agents. The materials prepared exhibit structure features, which differ from silicas tcmplated by nonionic surfactants in an acid medium. ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (WA 1116/7-1). REFERENCES 1. G. Schulz-Ekloff, J. Rathousky and A. Zukal, Microporous Mcsoporous Mater., 27 (1999) 273. 2. G. Schulz-Ekloff, J. Rathousky and A. Zukal, J. Inorg. Mater., 1 (1999) 97. 3. L. Sierra, B. Lopez, H. Gil and J.-L. Guth, Adv. Mater., 11 (1999) 307. 4. L. Sierra and J.-L. Guth, Microporous Mcsoporous Mater., 27 (1999) 243. 5. J.M. Kim and G.D. Stucky, Chem. Commun., 2000, 1159. 6. S.-S. Kim, T.R. Pauly and T.J. Pinnavaia, Chem. Commun., 2000, 1661. 7. C. Boissiere, A. Larbot and E. Prouzet, Chem. Mater., 12 (2000) 1937. 8. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 9. M. Kruk, M. Jaroniec, C.H. Ko and R. Ryoo, Chem. Mater., 12 (2000) 1961. 10. S.A. Bagshaw, T. Kemmitt and N.B. Milestone, Microporous Mcsoporous Mater., 22 (1998)419.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

189

Synthesis of mesoporous silica particles prepared by using multiple emulsion Chul Oh, Jae-Hyung Park, Seung-il Shin and Seung-Geun Oh* Division of Chemical Engineering and Center for Ultramicrochemical Process System(CUPS), Hanyang University, 17 Haengdang-Dong, Seongdong-gu, Seoul, 133-791, Korea The spherical silica particles with meso- and macropores at the surface and inside of particles were prepared

in the hexane/water/n-decyl

alcohol

multiple-emulsion.

Also

micrometer-sized hollow silica particles could be prepared by controlling the viscosity of the aqueous phase in W / 0 emulsion with polyethylene glycol (PEG). The morphology of silica particles was influenced by the concentration of PEG, HPC polymer and the external oil phase (O2). 1. INTRODUCTION The preparations of mono-dispersed silica particles have attracted more and more attention in the recent years because of their wide technological applications [1]. Specially, hollow silica particles are widely applied to drug delivery system (DDS), catalysis, composite materials, and protecting sensitive agents, etc., owing to their low density [2]. Generally, for the preparation of mono-dispersed silica particles from aqueous solution of silicon alkoxides, the silica particles are formed by two-step of hydrolysis and condensation [3]. Emulsions are dispersions of two immiscible fluids such as water and oil. Multipleemulsion may have either of the water-in-oil-in water type (W/O/W) or of the oil-in-water-inoil type (0/W/O) [4]. 0 / W / O multiple-emulsion is a good reaction medium for the preparation of particles which have meso- and macropores. In this study, we prepared the silica particles with meso- and macropores at the surface or inside of the particles and the hollow particles like pin-pong ball shape. As changing the reaction environments, the variation of size and morphology of particles were investigated by field emission scanning electron microscopy (FE-SEM).

* Corresponding author : e-mail ([email protected])

190

2. EXPERIMENTAL 2.1. Materials Tetraethyl orthosilicate (TEOS, 98%) as a silica source, hydroxylpropyl cellulose (HPC) as a stabilizer, n-hexane (99%+) as an oil phase (Oi) and Tween 20 as high HLB surfactant were purchased from Aldrich Chemical Company. N-decyl alcohol (minimum 98%) as an external oil phase (O2), Span 80 as low HLB surfactant and polyvinyl-pyrrolidone (PVP) were purchased from Sigma Chemical Company. 1-Octanol as an oil phase (O2) and polyethylene glycol (PEG) as a water stabilizer were obtained from Junsei Chemical Company. Also, ammonium hydroxide as a catalyst and ethanol as a washing reagent were purchased from Acros and Teamin Chemical Company. All chemicals were used as received without further purification. The water was deionized by Milli-Q Plus system (Millipore, France). 2.2. Preparation methods and characterization A multiple-emulsion preparation procedure can be described as followings: In the first step, Oi/W emulsion was prepared by dispersing n-hexane in an aqueous solution containing Tween 20. After stirring with magnetic stirrer, NH4OH and PEG were added to the water phase of Oi/W emulsion. To make an external oil phase (O2), HPC and Span 80 was solubilized in n-decyl alcohol at 50°C. In the second step, Oi/W emulsion solution of 10 wt% was added to an external oil phase. And then we mixed the multiple emulsion using the magnetic stirrer for Ih at 40°C. In order to prepare spherical silica particles in O1/W/O2 multiple emulsion, TEOS was added into the O2 phase. After reaction for an appropriate time, the samples were centrifuged at 2,500 rpm for 10 minutes to obtain the silica particles. The obtained particles were washed with ethanol 2 times. In the case of preparation of hollow silica particles, n-hexane in the water phase was excepted. So, the multiple-emulsion was changed to W/0 emulsion. FE-SEM was used to investigate the morphology of silica particles. 3. RESULTS AND DISCUSSION 3.L Preparation of silica particles with dimple structure by using multiple emulsion Though general emulsion techniques such as microemulsion method were utilized to obtain spherical silica particles, it was difficult to prepare the particles which had various-sized pores at the surface and inside of particles. For this reason, the multiple-emulsion technique was applied to this system. In the previous work, spherical silica particles with meso- and macropores inside were prepared by using the o/w/o multiple-emulsion as reaction media [5]. In order to increase the stability of multiple-emulsion and control the distribution of macropores in silica particles, HPC and PEG were employed. HPC played the key role in

191

growing the primary particles into the spherical silica particles in the range of l-3|im through the aggregation, while PEG affects the morphology of surface of spherical silica particles. Without HPC, the primary particles ranging 30-40nm didn't have the spherical shape and resulted in the flat form with irregular pores. When HPC concentration increase from 0.5wt% to 0.7wt%, the spherical particles were obtained and the size were tailored from 5|im to Ijim. When both HPC and PEG were added into the multiple-emulsion, the spherical silica particles with meso- and macropores at the surface and inside of particles were formed. As shown in figure 1, under the condition of Rw=4, 2wt% PEG, and 0.7wt% HPC, the surface structure like the dimpled surface of golf ball was observed very well and the particle size was more or less larger than the other samples. As PEG concentration increased to 6wt%, dimple structure was exchanged to the structure with pores inside of particles. (Figure 2)

^Sr •1,1

.1 I

r

.',^^'•

U\.UC

-.rn

..VV,

. rr

Fig. 1. SEM micrograph of silica particles with dimpled surface

If

^

i

Fig. 2. SEM micrograph of silica particles with many pores inside

3.2. Synthesis of hollow silica particles in W/O emulsion with PEG and HPC To prepare hollow silica particles, water/1-octanol (w/o) emulsion with PEG and HPC was used as reaction matrix. The preparation of hollow silica particles is based on the hydrogen bonding of water-PEG interaction and viscosity effect in the aquous phase. According to the reaction conditions, hollow and micrometer-sized spherical silica particles were obtained. As shown in figure 3, the sample prepared under conditions of 2wt% PEG didn't have the hollow structure and the size distribution was more or less broad. Though hollow silica particles didn't exist in the samples, the density of the aggregation among primary particles in a particle is lower than other samples prepared by emulsion method without PEG polymer. When PEG concentration was changed from 2wt% to up to 6wt%, the hollow silica particles as shown in figure 4 were obtained. The size of hollow silica particles was nearly same as that of the particles in figure 3. The shell thickness ranging from 200 to 500nm could be observed through the magnifying SEM image of the hollow silica particles. This variation of shell

192

width resulted from the diversity of concentration of PEG polymer which exist in each water droplets. Because of influence of HPC and PEG polymers, many mesopores exist at the surface of hollow silica particles.

H

1,

Fig. 3. SEM micrograph of silica particles with dense structures

..^.^.^

^ir,

iir

Fig. 4. SEM micrgraph of silica particles with hollow structures

ACKNOWLEDGEMENT This work was supported in part by Center for Ultramicrochemical Process System (CUPS). REFERENCES 1. F. Garbassi, L. Balducci, and R. Undrarelli, J. Non-Cryst. Solids, 223 (1998) 190. 2. F. Caruso, R.A. Caruso, and H. Mohwald, Science, 282 (1998) 1111. 3. W. Stober, A. Fink, and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. 4. J. Sjoblom (eds.). Encyclopedic Handbook of Emulsion Technology, Marcel Dekker, New York (2001) 5. M.H Lee, S.G. Oh, S.K. Moon, and S.Y. Bae, J. Colloid Interface Sci., 240 (2001) 83.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

193

Preparation and Characterization of Mesoporous Silica Spheres by Polymerization Induced Colloid Aggregation Method C. I. Lee'\ S. W. Lee', Y. Lee', Y. H. Chang'and Y M. Hahm'* 'Department of Chemical Engineering, Dankook Univ., Seoul, 140-714, Korea. ^Department of Chemical Engineering, Inha Univ., Inchon, 402-751, Korea. Mesoporous silica spheres having a diameter in the 3 to 15 micron range were produced by polymerization induced colloid aggregation method. Uniform sized silica spheres with a narrow pore size distribution were controlled by reaction conditions such as composition of polymeric materials, concentration of colloidal silica, temperature of heat treatment and amount of acidic catalysts. 1. INTRODUCTION Mesoporous silica spheres have been used as versatile catalysts, catalyst supports, packing materials for normal phase chromatography, etc 11-2]. It is now apparent that the morpiiological control as well as handling and texture oi^ mesoporous silica is extremely important for these applications. Mesoporous silica spheres with a narrow pore size distribution are expected to use as a packing material in chromatography or an easy-to-handle from orMCM-41 for catalytic purpose. Various techniques for synthesis of spherical silica developed such as oil emulsion, spray drying, sol-gel, etc. Ihese methods have produced a polydisperse collection of spheres ranging from 0.5 to 500j.mi in diameter. Another method for preparation of spherical porous particles was referred to polymerization induced colloid aggregation (PICA) because polymer growth occurs along with colloid aggregation. In this method, acid-catalyzed polymerization take place and the oligomer so formed adsorbs onto the surface of the colloid particles causing them to aggregates. It may proceed by the formation of polymer linkage between colloids | 3 | . Both inorganic and organic acid as acidic catalysts used for control of surface characteristics of silica spheres |4]. Here, we report the variations of particle size and shape, structural characteristics of pores according to reaction conditions such as composition of polymeric materials, concentration of colloidal silica, temperature of heat treatment and amount of acidic catalysts.

194

2. EXPERIMENTAL SECTION The preparation of the mesoporous siHca spheres was performed under acidic condition using polymerization induced colloid aggregation. Colloidal silica containing 40 wt.% of Si02 was diluted with ultra pure water and the pH adjusted to 1-3 by adding concentrated hydrochloric acid (HCl) or various organic acid while stirring rapidly. Urea and 35 wt.% formaldehyde were added and stirred until dissolved. pH of mixture was again adjusted to 1-3. Within a few minute, the mixture had turned white and opaque, due to the formation of spherical particles of a complex of silica and polymer. After the reaction, the clear aqueous supernatant liquid was discarded and remained white cake washed with water. The washed product, in the form of wet settled cake, was dried in vacuum at 60 "C for 24 hours. The vacuum dried material was then heated in tube furnace at 400-1000 °C in air atmosphere, rising the temperature slowly, to burn off organic material. Scanning electron microscope (SHM) photographs were obtained with a Jeol JSM5800, FT-IR spectra were obtained with a shimadzu DR-8011, N2 adsorption measurements were performed at 77K using a Micromeritics ASAP 2000 analyzer utilizing Brunauer-Hmmett-Teller (BHT) calculations of surface area and Barrett-Joynerllalenda (BJII) calculations of pore volume and pore size distributions. Ihermogravimetric analysis (TCiA) was performed on a lA instruments 2100 analyzer with temperature rate of lOTVmin in air. 3. RESULTS AND DISCUSSION Micromcler-sized mesoporous silica spheres can be synthesized in present of a mixture oi^ polymer and colloidal silica by PICA method, figure 1 shows the PT-IR spectra of urea-formaldehyde resin (UP resin) and synthesized spherical silica. UI* Resin has three strong absorption peak at 1670, 3300 and 1200 cm" , which are assigned to C=(), N-Il and C-N bond. After heat treatment of synthesized silica sphere, shows three inherent ab.sorption peak of amorphous silica at 480. 800, 1 100 cm"'.

\ CD 0 C 03

E

CO

c

2 h=

\

\y

/•~"'

-—--. ""^-v

_

••'

i

\y

I

,

1

figure 1. I'l-IR spectra of UP resin and synthesized silica sphere. (a) UP resin, (b) UP resin & silica composite before heat treatment, (c) pure silica after heat treatment

195

Thermogravimetric analysis (TGA) of UF^ resin, silica composited with polymer and heat treated silica provides information about the weight loss steps corresponding to physically adsorbed water, amounts of polymeric material, weight percentage of silica (see Figure 2). As can be seen from this figure the UF resin removed at 600 °C completely. Hence, heat treatment temperature fixed 600 °C, or higher.

1

UI' Resin

j

Ul- R c s i n - S i l i c a j Silica

Figure 2. TGA curves of UF silica, and their composite. 0

200

400

600

800

resm.

1000

rcnipciaturc | V \

SEM images (Figure 3, 4) of the silica spheres show their impacts to sphere shape with relatively uniformed micrometer-sized. The effect of molar ratio (silica/polymeric materials) was insignificant (see Figure 3). The size of silica spheres is depended on both primary particle size of colloidal silica and pi I of solution (see I^igure 4). With increase of the pi I of solution from 0.7 to 2.8, the average particle size of mesoporous silica spheres increases from 4 to 10 |.un.

Figure 3. SEM photographs of mesoporous spherical silica with molar ratio, polymeric material/silica, (a) 0.5 (b) 1.0 (c) 2.0 (d) 3.0

Figure 4. SEM photographs of mesoporous spherical silica with pH of solution, (a) 0.7(b) 1.5(c) 2.0(d) 2.8

196

Figure 5 shows nitrogen adsorption/desorption isotherm and pore size distribution in surface of spherical silica prepared with inorganic acid (HCl) or organic acid (L-tartaric acid) as acidic catalyst. In case of using organic acid, specific surface area, pore size and pore volume increased. Pore diameter and pore volume could be adjusted by addition of organic acid and heat treatment conditions.

—•— —I'— —T— —V—

Inorganic Acid (ads ) Inorganic Acid (dcs.) Organic Acid (ads ) Organic Acid (dcs.)

,

(a) E

Ji^p^

200

ij\ iji^^

jj;3[^.^ ^ ^>->^-^-^ ^'-^ 0.2

0.3

0.4

0.5

0.6

0.7

Relative Pressure |l*/Po|

0.8

0.9

1.0

KM) Pore D i a m e t e r j A |

Figure 5. BET analysis for silica sphere prepared with inorganic/organic acid (a) nitrogen adsorption/desorption isotherm, (b) pore size distribution. 4. CONCLUSION Polymerization induced colloid aggregation method has been introduced to control the morphology and the porosity of micrometer size silica spheres. Urea and formaldehyde were used as polymeric materials, colloidal silica as a silica source, organic and inorganic acid as a catalyst. The particle size could be adjusted in range of 3-15 \xm by pH of solution, amount of colloidal silica and stirring conditions. Pore diameter and pore volume could be adjusted by addition of organic acid and heat treatment conditions. These spherical particles are used as packing materials for various separation techniques such as High Performance Liquid Chromatography (IIPLC) or as supports for catalysts.

REFERENCES 1. R. K. Her, The Chemistry of Silica, John Wiley & Sons, New York, 1979. 2. Qian Luo, et al.. Studies in Surface Sci. Catalysis, 129 (2000) 37. 3. U. Trudinger, et al., Chromatogr., 535 (1990) 111. 4. H. Izutsu, et al., J. Mater. Chcm., 7 (1997) 1519.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

197

Preparation of mesoporous solids by agglomeration of silica nanospheres Yuri K. Ferreira, Martin Wallau and Ernesto A. Urquieta-Gonzalez*' ^ Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905 Sao Carlos - SP, Brasil Silica nanospheres were synthesized with diameters in the range from 40 to 170 nm. The size and shape of the spheres were determined by Photon Correlation Spectroscopy (PCS) and Scanning Electron Microscopy (SEM). The nanospheres were agglomerated by centrifugation and solvent evaporation and characterised by SEM and nitrogen adsorption. Partly regular mesostructures could be observed by SEM. However, these structures easily crumbles when they are handle and therefore no surface area related to the formation of mesopores was detected by nitrogen sorption. 1. INTRODUCTION Ordered mesoporous materials have great potential for application as catalysts, adsorbents and as host material for the preparation of electronic and optical devices. Besides mesoporous materials derived from the M41S family, recently porous materials using monodispersed nanospheres as cast have attracted much interest [1]. Silica nanoshperes firstly described by Stober et al. [2] are obtained by ammonia catalysed hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol and can be modified by lining with functionalised organic molecules [3]. Silica nanospheres form after agglomeration mesoporous solids with pore diameters varying approximately between 0.15 to 0.5 times of the particle diameter, depending of the structural arrangement of the nanospheres. Transformation of the sphere surface into zeolitic structures permits the preparation of hierarchical porous materials [4]. Here we describe the synthesis of silica nanospheres and their agglomeration into mesoporous structures. 2. EXPERIMENTAL A 3^ factorial assay of the experiments were used to study the factors influencing the particle diameter and its standard deviation. While the TEOS concentration (0.14 mol/L) and the reaction time (24 h) were kept constant, the reaction temperature and the concentration of ammonia and water were varied as it is indicated in Fig. 1. The samples were denominated in the form E([NH3]/[H20])[T], with the ammonia concentration [NH3] and the water concentration [H2O] given in (mol/L) and the reaction temperature [T] in (°C). The sphere diameter and its standard deviation were determined by Photon Correlation Spectroscopy (PCS). Selected samples were further characterised by Scanning Electron Microscopy (SEM). Agglomerates were prepared by evaporation of the supernatant alcoholic solution at room temperature or by centrifugation (1000 - 3000 rpm). The obtained solids were subsequently dried in an exsiccator and characterised by SEM and nitrogen sorption (BET) corresponding author: FAX: +55-16-260-8266. E-mail: [email protected] ^ Acknowledgements are given to CNPq Brazil for the financial support (proc. 461444/00-3 and 300373/01-5) and to Professor Fernando Galembeck, IQ/Unicamp-Brazil for the opportunity to realise the PCS measurement.

198

3. RESULTS AND DISCUSSION The dependency of the nanosphere diameter (0sph.) from the reaction conditions is demonstrated in Fig. 1. It can be seen that, in general, the diameter increases with increasing water [H2O] or ammonia concentration [NH3] but decreases with increasing temperature [T]. The influence of the parameters can be described by the equation 1.

• 20**c D 40 *C H 60**C

[NH3] (mol/L)

IH201 (mol/L)

Fig. 1. Influence of the reaction parameters on the sphere diameter determined by PCS. 0,ph.(nm) = 17.31 + 235.17 [NH3] + 16.63 [H2O] -0.78 [T] (1) The influence of the reaction parameters on the sphere diameter might be explained considering that the nanospheres formation comprises hydrolysis of TEOS, nucleation and particle growth. It was observed that the TEOS hydrolysis is the rate limiting step in the formation of the nanospheres and that particle growth proceeds mainly via addition of monomers and small oligomers on particle nuclei [5]. However, as suggested by van Blaaderen et al., in the early stage of the particle formation, aggregation of nanometer sized sub-particles also occurs [5]. At low NH3 concentration, small particles are stabilised and the aggregation of sub-particles decreases. Therefore, a larger number of particle nuclei leading to smaller sized spheres is present at low NH3 concentration. It was found by Weres et al. [6], that the critical nuclei size is inverse proportional to the reaction temperature Therefore increasing temperature further stabilises small nuclei, resulting also in small particles. ->' In general it could be observed that larger spheres are more uniform. The uniformity of the nanospheres is r\ demonstrated in the SEM micrograph of a typical sample shown in Fig. 2. Using the standard deviation (00) of the mean diameter, as a measure of the i uniformity, the influence of the reaction parameters may be described by equation 2.

if

It can be seen from Fig. 3, that evaporation of the solvent leads to compact but irregular agglomerates of the spheres.

^1

Fig. 2. SEM micrograph of E(0.2/6.())2() (scale bar = 300 nm; 0 = 160.9 nm).

00 (%) = 17.13 - 38.18 [NH3] - 6.95 [H2O] +0.23 [T] + 2.24 [NH3]^ + 0.77 [HzO]^ + + 0.0008 [T]^ + 7.13 [NH3][H20] - 0 . 3 3 [NH3][H20] - 0.33 [NH3KT] - 0 . 0 3 [H20][T]

(2)

199

On the other hand, as it can be seen from the micrographs shown in Fig. 4, the agglomerates obtained by centrifugation are more ordered. These micrographs reveal further that the • uniformity of the agglomerates increases with increasing the rotation frequency. The agglomeration of the nanospheres is influenced by directed gravitational sedimentation and undirected dislocation due to Brownian motion. It can be calculated for silica spheres with diameter in the range of pig. 3. E(0.3/4.0)40 ( 0 = 131.8 nm) 170 to 130 nm in ethanol at 20 °C, that agglomerated by evaporation (bar = 1 pm). the displacement of the spheres due to the Brownian motion is around 110 to 220 times higher than the displacement caused by the gravitational sedimentation. For nanospheres of sample E(0.2/6.0)20, with diameters around 160 nm, the ratio Brownian dislocation to sedimentation dislocation (B/S) is decreased to 21 and 3 when they are centrifuged at 1000 and 3000 rpm, respectively. This explains why the agglomerate shown in Fig. 4b is more ordered. The predominance of the Brownian motion for small particles ( 0 « 40 nm) explains why these particles could not agglomerated, even at 4500 rpm where B/S is still around 50. (a)

^^^

(b)

Fig. 4. E(0.2/6.0)20 ( 0 = 160.9 nm) agglomerated (a) 1000 rpm, (b) 3000 rpm (bar = 300 nm). Although the agglomerates prepared by centrifugation show a higher uniformity than that prepared by evaporation of the solvent, they are brittle and therefore difficult to handle. This fragility might be the reason for the unexpected adsorption behaviour. Although that one would expect from the dense arrangement of the spheres shown in Fig. 4b, that the agglomerate could possess mesopores in the range of 24 to 66 nm (0.15 - 0.41 x 0sphcrc)^ it shows an adsorption isotherm, given in Fig. 5, classified as type II, which is typical for nonor macroporous materials. This could have been caused by disintegration of the agglomerate during the sample preparation. As for different packing structures of spheres with a density of 2.17 g/cm"^, pore volumes in the range of 0.2 to 0.9 cmVg are expected, the disintegration of the agglomerate structure (shown in Fig. 4) is strengthened by the low observed pore volume of 0.039 and 0.088 cm^g for the samples agglomerated at 1000 and 3000 rpm, respectively.

200

The observed BET surface area of 20.4 and 21.2 m^/g for these agglomerates is higher than the expected specific surface area of 17.2 m^/g, calculated for spheres with diameters of 160.9 nm and density of 2.17 g/cm"'. The observed specific surface area would correspond to a density of 1.87 g/cm"'. Giesche [7] observed by helium pycnometry also densities lower than 2.0 g/cm^ for silica nanospheres prepared by the Stober method [2] and found that the density of the particles increases to values typical for amorphous silica (~ 2.2 g/cm"') after calcination at temperatures above 800 °C. Therefore he concluded that these nanospheres, prior to calcinations, still contain difficulty accessible micropores with pore diameters around 0.3 nm [7] which are not accessible for nitrogen molecules (kinetic diameter = 0.36 nm).

k

loose 96

g94t

8 92+ n

S

90+

esj eel 0,4

0,6

1,0

p/R

Fig. 5. Isotherm of N2 sorption on sample E(0.2/6.0)20 agglomerated at 3000 rpm.

200

400

600

800

Temperature [°C]

Fig. 6. Thermogravimetry of sample E(0.2/6.0)20 agglomerated at 30(X) rpm.

A typical example of thermoanalysis of the agglomerates is shown in Fig. 6. It can be observed that the weight loss of the nanospheres occurs in two steps. Desorption of physically adsorbed water (9.2 %) until approximately 190 **C and desorption of ammonia accompanied by dehydroxylation of the surface hydroxyl groups (4.0 %) between 250 and 700 °C, as it is schematised in equation 3. -0-Si

—^-* 2NH, + 2HO - Si -

(3) -^INH.+H^O + Si- O-Si The number of surface (Si-O')-groups (77 jxmol/m^), estimated from equation 3 and the observed weight loss, is much higher than that reported [8] for commercial silica (7.0 - 9.5 Hmol/m^). This do not only indicate the presence of hydroxyl groups in micropores, which cannot be detected by nitrogen adsorption, but also that no or only a small number of covalent Si-O-Si bonds exist between the nanospheres agglomerated by centrifugation. Then they must be connected only by H-bridging bonds, thus explaining their low mechanical stability. 2NH;

REFERENCES 1. A. Stein, Microporous Mesoporous Mater., 44-45 (2001) 227. 2. W.Stober, A. Fink and E. Bohn, J. Colloid Interface Sci., 26 (1968) 62. 3. C. Beck, W. Hartl and R. Hempelmann, Angew. Chem., I l l (1999) 1380. 4. M.W. Anderson, S.M. Holmes, N. Hanif and C.S. Cundy, Angew. Chem., 112 (2000) 2819. 5. A. van Blaaderen, J. Van Geest and A. Vrij, J. Colloid Interface Sci., 154 (1992) 481. 6. O. Werres, A. Yee and L. Tsao, J. Colloid Interface Sci., 84 (1981) 379. 7. H. Giesche, J. Eur. Ceram. Soc, 14 (1994) 189. 8. K. Unger, Angew. Chem. Int. Ed. Engl., 11 (1972) 267.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

201

Ordered mesostnictured materials with composite walls of decavanadate and silica Yoon-Young Chang, Young Kyu Hwang, Hyuk Choi, and Young-Uk Kwon* Department of Chemistry and BK-21 School of Molecular Science, Sungkyunkwan University, Suwon 440-740, Korea. FAX: +82-31-290-7075 E-mail: [email protected] Ordered mesostructured materials composed of decavanadate-silica composite wall materials were synthesized from the reactions of decavanadate-silica composite sol solution with a structure-directing agent MTAB (CH3(CH2)i3N(CH3)3Br). The sol solution was prepared by inducing hydrolysis of silica in the presence of decavanadate ions. Controlled reactions of the process parameters such as composition, aging time of the sol solution, and pH produced mesostructured materials with the ID hexagonal symmetry of a = 4.32nm. While thermal calcination even at low temperatures destroyed the mesostructure, photocalcination of these materials provides a viable means to generate mesoporous materials with vanadia-silica composite wall. 1. INTRODUCTION There have been many attempts to modify the walls of mesoporous silica materials with metal ions for catalytic purposes. In most of the cases, the metal ions are impregnated onto the walls of preformed mesoporous silica. We have recently synthesized a composite material in which polyoxometalate ions are encapsulated by silica layers to form nanoscopic sol particles,

-h

Scheml. Reaction scheme of the formation of a mesostructure with POM-silica composite wall materials.

202

which may be a useful precursor to synthesize mesostructured materials. In this study, we have explored this possibility with decavanadate ions stabilized by silica layers. 2. EXPERIMENTAL The vanadia-silica mesostructured materials were synthesized by using MTAB, sodium metavanadate and sodium silicate solution. NaVOs was dissolved in distilled-water and 2M HCl was added until pH 4.5 to prepare a decavanadate solution. A sodium silicate solution was added to the decavanadate solution. After the vanadia-silicate solution was stirred for Ih at room temperature, a 6wt% MTAB solution was added. A yellow precipitates formed immediately. 2M HCl solution was added to adjust the pH to some designated values shown in Table 1. The resultant mixture was rapidly stirred at room temperature for 12hr. The yellow precipitates were aged for 2-3 days at 80°C in an oven. The precipitate was filtered, washed, and vacuum dried. The surfactant molecules of assynthesized vanadia-silica materials were removed either by UV irradiation or thermal calcination. The mesophases are characterized by powder X-ray diffraction (RIGAKU D/max-RC) and transmission electron microscopy (JEOL-3011, 300kV). 3. RESULTS AND DLSCUSSION For the successful synthesis of pure mesostructured materials with vanadia-silica composite wall, the important parameters were aging time, stirring time, p\\ and surfactant/inorganic precursor ratio. Fig. 1 shows the progression of the mesostructure formation from a pH = 4.5 reaction as a function of aging time. Just before the aging, there arc unreactcd crystalline MTAB and a small amount of what appears to have the desired hexagonal mesostructure. After 1

2

3

4

2G(degree)

5

Fig. 1. XRD patterns of as-synthcsizcd materials with various aging time, (a) MTAB only (b) before aging (c) aging 8hr (d) aging 48hr

aging for 8hr, there grew a sharp peak at 20 ^ 3° that may be assigned to a mesostructure composed of decavanadate and MTA ions in addition to the hexagonal mesostructure and the MTAB. It takes over 48 hr of aging to form a pure hexagonal mesostructure in this system.

203

Tablel Peak indices, peak positions for as-synthesized materials with various pH conditions pH

4.5

5.5

6

8

(100)

38.37

36.78

36.48

37.40 23.35

(110)

22.18

21.25

20.96

(200)

19.11

18.39

18.32

21.43

2e(degree) Fig. 2. XRD patterns of as-synthesized materials with various pH conditions

As shown in Figure 2, characteristic peaks for a hexagonal mesostructure were observed in materials synthesized below pH 7, but a mixed phase was observed at the higher pH. We have tried to remove the surfactant to obtain mesoporous materials. However, unfortunately the structure collapsed under thermal calcination at 200°C, probably because of the low thermal stability. On the other hand, the hexagonal mesostructure appears to be intact under photocalcination condition as evidenced by the TEM image (Fig.3) and XRD pattern (Fig.4). However, the IR spectrum shows that there is some residual organic materials C-H stretching peak (2850~3000cm"^) remaining even after photocalcination.

(100)

d CO

(110) (200) 'yv___vN...^___,_^

• • - *

CO

cCD

(a)

c

(b) 1

.

1 ^

^$§:

^

2 e ( d e g r e e ) Fig. 3. XRD patterns of a) as-synthesized hexagonal materials and b) after photocalcination

Fig. 4. TEM image photocalcination

after

204

4. CONCLUSIONS We have synthesized mesostnictured materials with vanadate-silica composite walls by using decavanadate ions. The decavanadate ions are encapsulated by silica layers and are kinetically stabilized at all pH range we have studied. Although we have failed in obtaining pure mesoporous materials from this approach primarily because of the instability of the composite wall material, this approach may be utilized in synthesizing mesostructured materials with composite walls of various polyoxometallate ions. REFERENCES 1. Hyuk Choi, Young-Uk Kwon, and Oc Hee Han, Chem. Mater., 1999, 11, 1641. 2. Matthew T. J. Keene, Renaud Denoyel, Philip L. Llewellyn, Chem. Commun., 1998,11, 2203. 3. Theotis Clark, Jr., Julia D. Ruiz, Hongyou Fan, C. Jeffrey Brinker, Basil I. Swanson, and Atul N. Parikh, Chem. Mater., 2000,12, 3879. 4. Abdelhamid Sayari, Ping Liu, Microporous Materials 1997, 12, 147. 5. Victor Luca and James M. Hook, Chem. Mater., 1997, 9, 2731.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Nanoporous alumina formation using multi-step anodization and cathodic electrodeposition of metal oxides on its structure Jaeho Oh, Youngwoo Jung, Jaeyoung Lee^, and Yongsug Tak Department of Chemical Engineering, Inha University, Incheon 402-751, Korea. ^Water Protection Research Team, Research Institute of Industrial Science & Technology, Pohang 790-330, Korea Highly ordered nanoporous alumina structure was fabricated with a solution pretreatment method and a time-efficient anodization process and, this structure was tested as a template for the electrochemical synthesis of metal oxides, CU2O and ZnO. Cathodic electrodeposition of oxides inside pores was retarded because of the variation of pore structure caused by the pH increase, but nano-thin oxide films was formed at pore mouth. 1. INTRODUCTION Template synthesis method has been widely used for the fabrication of nano-size materials and a highly ordered nano-porous alumina has been actively applied as a template structure*. Partly hydrated alumiunm oxide, which consists of thin barrier layer and porous overlayer, is prepared by anodization in an acid electrolyte. Thickness of two-layer depends on the cell voltage applied and the regularity of pore distribution depends on the anodization time. Masuda et. al. developed a repetitive formation and dissolution of oxide layers to obtain the honeycomb structures of anodic alumina. Longer first anodization time at 40 V is requisite for the ideally arranged structure^. This nanopore structure has been used as a template for the fabrication of ceramic nanowires, LiCo02^ and LiCoo.5Mno.5O2'*, using sol-gel process. Electrochemical deposition of metal oxides has advantages over conventional methods due to higher deposition rate, precise morphology control, and low synthesis temperature^. ZnO and CU2O are potential materials for application in solar cells due to semiconducting behaviors and can be electrodeposited by the control of surface pH. In this work, different pretreatment and anodization steps were utilized to control the nanopore array structure of aluminum oxide. This structure was utilized for the electrochemical preparation of nanostructural metal oxides. This work was supported by the Korea Science & Engineering Foundation under a grant from the Engineering Research Center for Energy Conversion and Storage.

206

2. EXPERIMENTAL A high purity aluminum plate (Aldrich 99.999%) was employed in this experiment. Prior to anodization, aluminum metal was electropolished and pretreated with IM NaOH solution. Specimen was embedded in a Teflon holder with exposed surface area of 1.18 cm^ and electrolyte was rigorously stirred and maintained at S'C during anodization. Basically, two-step electrochemical anodization process is used for obtaining a porous alumina structure. Copper and zinc nitrate solutions were used for the formation of metal oxides by applying a direct cathodic current. Surface morphologies of the anodic alumina and metal oxides were investigated with a scanning electron microscope (SEM, Hitachi s-4300). 3. RESULTS AND DISCUSSION Prior to anodization, aluminum metal was electropolished to remove an air-formed oxide and smooth the surface. Its surface roughness was estimated to be 0.7335 nm over 3 |im^. Figure 1(a) shows the irregularly ordered pore arrangement after anodization, however, the ordered region exists at the bottom of pores.^ When the irregular porous alumina was removed, the concave pore bottom remails and this textured structure works as pore initiation sites in the following anodization steps. Similar concave structure is found during uniform aluminum dissolution in an alkaline solution because of the hydrogen gas evolution. In this work, electropolished surface was chemically pretreated with NaOH solution and it results in the regularity of pore distribution after the first anodization. Figure 1(b) show the regularly spaced hexagonal pore structure after 12 hrs of second anodization. Cross-section views of specimen indicate the straight and parallel pores of which has a high aspect ratio over 1,000, as shown in Fig. 1(c). It has been known that pore structures, interpore distance and barrier layer thickness, are dependent on the applied cell potential and electrolyte composition. In oxalic acid solution, long-range ordering takes place at 40 V of anodization,^ which requires a time-consuming process. At less anodization voltage, the bottom of pores is less ordered and it results in less ordered pore distribution. Three-step anodization process including the second removal of oxides gives a better pore distribution and a reduced anodization time, and it provides the control method of pore length by simply adjusting anodization time without sacrificing ordered pore distribution. Figure 2 shows the cross-section views of 200 nm-long pores, formed after 30 sec of the third anodization. Ordered pore structure was used as a template for the formation of semiconducting oxides, CU2O and ZnO. Figure 3 shows a linear sweep voltammogram in zinc nitrate solution after the removal of barrier film at the pore bottom.

207

Fig. 1. Porous structure of alumina prepared by anodization; (a) first anodization, (b) second anodization, and (c) cross section view of straight proes. 0

E

-5 •10 -15

O 0 55158

i0.0kv

X60 . ek'•'ae'erirn

Fig. 2. Porous alumina prepared 30 sec of anodization time in three-step process.

-20

:

- J

:

r^

^

-J 1

-2.5

.

1

-2.0

.

1

-1.5

.

1

-1.0

.

1

-0.5

.

1

1

0.0

Potential ( V vs. SCE ) Fig. 3. Linear sweep voltammogram in obtained in zinc nitrate solution with porous alumina electrode.

Obtained potential-current curve shape is similar to a reported polarization curve measured on a flat electrode but the potential is shifted to cathodic direction.^ Its behavior can be ascribed to the existence of thick porous alumina. Current plateau around -1.6 V indicates the formation of passivating layers or mass transfer limited current and electrodeposition of ZnO was performed at this plateau potential. Figure 4(a) shows a tortuous pore structure, compared to Figure 2. Cathodic reaction takes place at the pore bottom and it is considered to be in a neutral solution. 2H20 + 2e' -> H2 + 2OH" As the result, pH of the electrode surface increases and it makes straight pore into tortuous by dissolving alumina. On the other hand, Figure 4(b) shows that pore mouth was covered with a transparent nano-thin ZnO film. It suggests that the dissolution of alumina occurs ahead of the precipitation of ZnO. Similar phenomena were observed during cathodic deposition of CU2O and Figure 5 shows the twisted pore structure. When a buffering agent, diammonium hydrogen citrate, is added during electrodeposition, destruction of pore wall is diminished. Crystalline CU2O is only nucleated around pore mouth.

208

nucleated around pore mouth. (a) ,vy'>^>r- .

.
-qCPt*^

[>^ 00

26 (Degrees)

rTTT

0.2

0.4

0.6

0.8

Relative Pressure (P/Po)

1.0

r^

0

50

100

150

200

Pore diameter (A)

Fig. 2. Dependence of N2 adsorption/desorption isotherms (A) and pore size distribution (B) on surfactant chain length, (a) P-AMS-1, (b) P-AMS2, (c) P-AMS-3.

216

tOOnm 10

20

30

40

50

60

70

80

90

29

Fig. 3. HRTEM image of P-AMS-3 after calcination 773 K for 4 h

Fig. 4. XRD patterns for P-AMS-3. (a) before calcinations, (b) after calcination at 773 K, (c) 873 K, (d) 973 K for 4 h.

Figure 3 shows HRTEM image for P-AMS-3. In the HREM image, there seems to be no discemable long range order of the pore structure, although it shows pores with a quite regular diameter. Thus, the packing of pore in P-AMS-3 seems to be rather random with "wormholelike" or "sponge-like" morphology, which is typically observed for disordered mesoporous silicas and aluminas.^'^ The XRD patterns in Figure 1 and 4 imply that the wall of P-AMS materials after calcination consists of bulk y-alumina with a low crystallinity. We could conclude that the thermally stable mesoporous alumina molecular sieves could be synthesized in a much simple manner with only cationic surfactant under hydrothermal condition without any additives.

REFERENCES 1. S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science 1995, 2(59, 1242-1244. 2. S. A. Bagshaw, T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 1996, J5, 1102-1105. 3. F. Vaudry, S. Khodabandeh, M. E. Davis, Chem. Mater. 1996, 8, 1451-1464. 4. M. Yada, M. Machida, T. Kijima, Chem. Commun. 1996, 769-770. 5. S. Cabrera, J. E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. D. Marcos, P. Amor6s,^Jv. Mater. 1999, //, 379-381. 6. S. Valange, J. -L. Guth, F. Kolenda, S. Lacombe, Z. gabelica, Micropor Mesopor Mater 2000, 35-36, 597-607. 7. V. Gonzalez-Pefia, I. Dias, C. Marques-Alvarez, E. Sastre, J. Perez-Pariente, Micropor Mesopor Mater 2001, 44-45, 303-310. 8. E. R Barett, L. G. Joyner, R R Halender, J. Am. Chem. Soc. 1951, 73, 373-380.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

217

Synthesis and characterization of mesoporous alumina molecular sieves with cationic surfactants in the presence of formamide Hyun Chul Lee,^ Hae Jin Kim,^ Dae Hyun Choo,^ Hee Cheon Lee,^ Soo Hyun Chung,^ Kyung Hee Lee^ and Jae Sung Lee^ ^Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea* ''Department of Chemistry and Division of Molecular and Life Science, Pohang University of Science and Technology, Pohang, 790-784, Korea '^Korea Institute of Energy Research, Daejeon, 305-343, Republic of Korea We have synthesized alumina mesoporous molecular sieves with a cationic surfactant (CTAB) exhibiting thermal stability and high surface area and were also able to synthesize the mesoporous alumina from aluminum alkoxide precursor with cationic surfactant under hydrothermal condition in the presence of formamide as an external additive. The pore sizes and surface area could be controlled by changing of the amount of formamide. The HRTEM image of P-AMS-F showed the disordered interwoven network of the pore architecture. The ^^Al NMR results clearly demonstrate the existence of the different electronic environment around resonant aluminum nuclei. 1. INTRODUCTION Considerable interests have been devoted to developing new synthetic methods, pathways, and also to characterizing nano-structured molecular sieves since the discovery by Mobil researchers of the ordered mesoporous M41S family by means of the templating of surfactant micelle structures.' The synthetic strategy used for the silica-based materials has been extended to the preparation of non-siliceous mesoporous oxides. In spite of extensive efforts recently devoted to the synthesis of mesoporous alumina with an ordered pore structure^"^, the well-established procedures used for the synthesis of siliceous mesoporous * This work has been supported by BK-21 program of Korea Ministry of Education and ERC

218

and NRL program of Korea Ministry of Science and Technology. materials have often failed. Nevertheless, there are a few successful examples of mesoporous alumina molecular sieves having thermal stability. It was first achieved by Cabrera et al^ for the mesoporous alumina molecular sieves with a cationic surfactant by adding triethanolamine as a 'hydrolysis retarding agent' of the aggregates in the mother solution. All of these mesoporous alumina molecular sieves showed 'wormhole-like' or 'sponge-like' morphologies. Here we report synthesis of mesoporous alumina molecular sieves with cationic surfactants in the presence of formamide. 2. EXPERIMENTAL The method of preparation for the mesoporous alumina molecular sieves in the presence of formamide (denoted as P-AMS-F) is as following: formamide and distilled water was added very slowly to a homogeneous mixture of cationic surfactants ([CH3(CH2) i5N(CH3)3]Br, CTAB) and aluminum tri-sec-butoxide dissolved in 1-butanol solvent while stirring. The molar composition, (surfactant: Al: formamide: water) of the resulting gel, was 0.5: 1: x: 2 (x=0-l). After stirring until the homogeneity was obtained, the resulting well-mixed gel was put into Teflon-lined autoclave vessel. Then, hydrothermal reaction was followed at a desired temperature for 24 h under autogenous pressure and static condition. The product was washed with absolute ethanol, dried and then calcined at 773 K for 4 h in the flow of air. Characterizations of the mesoporous materials prepared through this method were carried out with XRD (MAC Science Co, M18XHF diffractometer), HRTEM (JEOL JEM 201 OF, Field Emission Electron Microscope), nitrogen adsorption (ASAP 2010, Micromeritics) and solid state NMR (Varian Unity Inova 300 MHz spectrometer equipped with a 7mm Chemagnetics MAS probe head using a sample rotation rate of 6 KHz). 3. RESULTS AND DISCUSSION The XRD experiments showed that P-AMS-F consisted of aluminum oxide such as gamma alumina and oxyhydroxide species with a low crystallinity. Table 1 shows the characteristics of mesoporous alumina molecular sieves synthesized with cationic surfactant (CTAB) in the presence of formamide (P-AMS-F) after removal of surfactant by calcination at 773 K. As the amount of formamide increased from 0 to 0.45 (mol ratio to Al), the average pore size in mesoporous alumina decreased from 6.7nm to 3.0 nm and its distribution became sharper while calculated BET specific surface area increased. These results suggest that the pore size might be controlled to some extent by adjusting the amount of the formamide.

219

Table 1 Effect of the amount of formamide on the synthesis of mesoporous alumina molecular sieves with cationic surfactant under hydrothermal conditions at 373 K for 24 h. Amount of formamide (mol/ Al mol)

Materials

BET Surface Area

BJH Pore Size

Sg(m'/g)

(nm)

P-AMS-F (0)

0

337

6.7

P-AMS-F(O.l)

0.1

370

3.8

P-AMS-F (0.2)

0.2

388

3.7

P-AMS-F (0.45)

0.45

404

3.0

Figure 1 shows the dependence of N2 adsorption/desorption isotherms and pore size distribution on the amount of formamide added to synthesize P-AMS-F materials. The narrower pore size distribution was observed as the adding amount of formamide raised. The packing of channel systems in P-AMS-F (Fig. 2) appears to be an interwoven network of pore architecture, rather than so called 'wormhole-like' or 'sponge-like'^'"^ morphology often observed for disordered mesoporous silicas and aluminas in general, and P-AMS without formamide addition. The HRTEM images for the P-AMS-F showed no discemable long range order in the pore structure. The representative ^^Al solid-state NMR spectra of calcined P-AMS-F are depicted in Figure 3. ^^Al MAS NMR spectra (Figure 3a) show two well-resolved ^^Al NMR peaks in all samples, which can be assigned to Al centers coordinated to a donor atom with tetrahedral and octahedral geometries, respectively. In addition to these two peaks, P-AMS-F show an additional very weak NMR signal at 33 ppm which is assigned to a penta-coordinated aluminum site.^ This result implies the existence of amorphous domains with a poor crystallinity as defects arising from distorted octahedrally coordinated Al in P-AMS-F.

O.025 0.020 "

1. ^

Q

0.O15

0.010 0.005 0.000

0.2

OA

0.6

0.8

R e l a t i v e Rnessure (P/Po)

f\\ /y-^ "^

(d)

\ ^

_

(c)

(b)

^^-~—(£>. 50 100 150 F=tane clamE*er (/>^

Fig. 1. N2 adsorption/desorption isotherms and pore size distribution of the samples. Amount of formamide (mol/Al mol)= FA, (a) FA=0, (b) FA=0.1, (c) FA=0.2, (d) FA=0.45.

220

(a) 3

^, z''' (b) •^':v=-'"--''

^fe^,.

r\''.; ^ i,-••-*-'

i^lS-

•J.?..

-"^..jigte ^ ..g^

1 50nin

if^

200

100

0

-100

-200200

100

0

-100

-200

Chemical Shift from AliU^O)^^"^

Fig. 2. HRTEM images after calcination 773 K

Fig. 3. ^^Al NMR spectra. (A)

for 4 h.

AMS-F(0.45), (B) P-AMS-F(O).

P-

(a) P-AMS-F (0), (b) P-AMS-F (0.45)

(a) ^^Al MAS NMR, (b) ^^Al CPMAS NMR.

Also, ^^Al CPMAS (Cross Polarization Magic Angle Spinning) NMR spectrum (Figure 3b) of the calcined P-AMS-F exhibits three well-resolved NMR peaks at 72, 33, and -1 ppm. The cross polarization effect will increase the relative intensity of penta-coordinated aluminum center, largely due to the magnetization transfer from proton to the aluminum center. Nevertheless, the peak from penta-coordinated aluminum site at 33 ppm nearly did not change for P-AMS-F(0.45) compared with that of P-AMS-F(O). Thus, from the resuh of ^^Al NMR experiments, the amorphous domains in P-AMS-F are relatively small and not directly involved with proton sites. In conclusion, we could successfully control the pore size in mesoporous alumina molecular sieves with a variation of the amount of formamide. They showed a morphology of an interwoven network of pore architecture. In addition, we confirmed the different electronic environments in aluminum sites via ^^Al NMR experimental techniques.

REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992, 359, 710. 2. S. A. Bagshaw, T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 1996, 35, 1102. 3. F Vaudry, S. Khodabandeh, M. E. Davis, Chem. Mater. 1996, 5, 1451. 4. S. Cabrera, J. E. Haskouri, J. Alamo, A. Beltran, D. Beltran, S. Mendioroz, M. D. Marcos, P Amoros, Adv. Mater. 1999, 7/, 379. 5. S. Valange, J. -L. Guth, F Kolenda, S. Lacombe, Z. gabelica, Micropor Mesopor Mater 2000, 35-36, 597.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

221

Structure and properties of porous mesostructured zirconium oxophosphate witli cubic (Ia3d) symmetry Freddy Kleitz^^ Stuart J. Thomson^^, Zheng Liu*', Osamu Terasaki^ and Ferdi Schiith^* ^Max-Planck-Institut fiir Kohlenforschung 45470 Miilheim an der Ruhr, Germany. ^Japan Science and Technology Corporation (CREST) and Department of Physics, Tohoku University, Sendai 980-8578, Japan The synthesis and characterization of the first porous zirconium oxo-phosphate material structured on the nanoscale with a cubic Ia3d symmetry is described. The new ordered porous material was obtained in aqueous solution by the self-assembly of a simple cationic surfactant combined with the inorganic zirconium sulfate precursor. The cubic zirconium oxo-phosphate was characterized by X-ray diffraction (XRD), high resolution electron microscopy (HREM), N2 sorption and FTIR spectroscopy. 1. INTRODUCTION The developments in the field of non-siliceous mesostructured and mesoporous materials have recently been reviewed.''^ In particular, transition metal-based ordered mesoporous materials have been synthesized on the basis of titanium, zirconium, niobium or tantalum, most of them being either hexagonally ordered or rather disordered.''^ However, considerably less attention has been given to non-hexagonal structures,^ mainly due to the higher difficulty in achieving stable well-ordered porous solids."*'^ We previously reported the synthesis of mesoporous zirconium 0x0phosphates with 2-D hexagonal phase.^'^ These well-ordered and thermally stable zirconium oxo-phosphate materials, show relatively large adsorption capacity, high surface area, and Lewis and Bronsted acidity. The desire to create porous materials combining acid-base properties and the advantages of a well-defined 3-D structure led us to develop the synthesis of a cubic Ia3d mesoporous zirconium-based analogue.^ However, in the initial study we were not able to remove the template without structural collapse. By carefully examining the synthesis conditions and the method used for the template removal, we have now succeeded in removing the template without destroying the structure.^ The present report focuses on the characterization of this newly synthesized material. 'Author for correspondence. E-mail: [email protected] ^Present address: Center for Functional Nanomaterials, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea. ^Present address: Materials Division, Australian Nuclear Science and Technology Organisation (ANSTO) PMB 1, Menai, NSW, Australia, 2234. The European Community (project HPRN-CT-99-00025) and the Japan Science and Technology Corporation are gratefully acknowledged forfinancialsupports.

222

2. EXPERIMENTAL SECTION The addition of an aqueous solution of Zr(S04)2«4H20 to N-benzyl-N,Ndimethyloctadecylammonium chloride in water leads to rapid formation of a zirconium sulfate-surfactant composite mesophase. The reactants molar ratio of the reagents used was Zr(S04)2: C18BDAC : H2O =\ I r I All. Two surfactant to zirconium sulfate molar ratios (r) were studied, r = 0.40 and r = 0.54. The mesostructured material obtained under acidic conditions was then hydrothermally aged (3 days) and subsequently posttreated with an aqueous solution of phosphoric acid (0.5 M), following a method described previously.^'' As a comparison, hexagonal analogous materials were synthesized, with r - 0.40 and r = 0.54, according to a method described previously.'' Template-free products were obtained after air calcination in a box furnace with a plateau at 300°C for 3 hours followed by 3 hours at 500°C. Slow heating rates (0.5°C/min) were used as this has been shown to have a critical effect on the mesostructure.*^'^ Full details of the syntheses and characterization procedures have been recently reported.^ 3. RESULTS AND DISCUSSION The XRD patterns of the assynthesized samples show reflections suggesting a cubic Ia3d symmetry (Fig. la and lb). Only reflections within 2-8 ° (20), which are due to the ordering of the pores, are observed. This indicates that no condensed crystalline phases arc present. The unit cell parameter of the cubic lattice, calculated from d(211), is generally about ao = 9.9 nm for assynthcsizcd materials. It has been shown previously that the surfactant to zirconium ratio range, where the cubic Ia3d mesophase with a well-resolved diffraction pattern is obtained, is rather 2 theta (•] 2 theta ['] narrow, between r = 0.40 and r = 0.67.^ No significant variations in Fig. 1: XRD patterns, a) and b) As-synthesized cubic zirconium oxo-phosphates. c) and d) As-synthesized the (211) d-spacing (d = ca. 4 nm) hexagonal zirconium oxo-phosphates. e) and f) Calcined is observed. However, even within cubic materials, g) and h) Calcined hexagonal materials. this r range only the two first The dashed lines materialized low angle scattering reflections, assigned to the cubic intensity cut by beam block. space group Ia3d, arc well defined. The higher order reflections appear with low signal-to-noise ratios. In contrast, materials obtained under the same synthetic conditions using CI STAB as a template, exhibit well-resolved XRD patterns of 2-D hexagonal p6mm phase (Fig. Ic and Id).^'^ This highlights the unique role of the surfactant molecular geometry to direct the final

223

mesophase structure. The unit cell parameter of the hexagonal phase is usually around 5.3 nm.^The HREM images of an as-synthesized sample synthesized in presence of Nbenzyl-N,N dimethyloctadecylammonium ions with r = 0.54 reveal domains of highlyordered mesostructure (Fig. 2). Images of the [111] and [100] zone axis are presented in Fig. 2a and 2d, respectively. In the electron diffraction (ED) pattern (Fig. 2b), only diffuse rings are observed indicating that the wall structure of the as-prepared samples is amorphous. Fig. 2c, which is the Fourier diffractogram obtained from the HREM image in Fig. 2a, suggests that the material is commensurate with Ia3d symmetry. As-prepared samples synthesized with r = 0.40 show similar features. In agreement with the XRD, the HREM investigations confirm that the architecture of the zirconium oxo-phosphate surfactant mesophase is characteristic of the cubic Ia3d phase.

®D

202

' o6o 121 ]

•fl

' 2nm'^

0.2nm"^

Fig. 2: Typical HREM image and electron diffraction (ED) pattern of an as-prepared sample with r = 0.54. Fig. 2a) HREM image taken along the [111] zone axis. Fig. 2b) Electron diffraction pattern. Fig. 2c) Fourier diffractograms obtained the area labeled by 1. Fig. 2d) HREM image taken along the [100] zone axis.

The samples were carefully calcined as described. Fig. le and If show the XRD patterns recorded for calcined samples synthesized with r = 0.40 and r = 0.54, respectively. Generally, the structure shrinks drastically and the (220) reflection appears only as a shoulder. No higher order reflections can be detected. The sample synthesized with r = 0.54 undergoes a larger shrinkage (about 30%, acaicined-o.54 = 7 nm) than that with r = 0.40 (about 25%, acaicined-o.4o = 7.5 nm). On the other hand, the shrinkage is slightly less pronounced for hexagonal phase materials (21% for r = 0.40, 25% for r = 0.54) and the reflections at higher 2 theta angles are retained. But a lower ordering is evidenced in all cases. The samples synthesized with less surfactant (r = 0.40) are more stable and undergo less contraction upon calcination. Although the X-ray diffraction patterns recorded for the calcined cubic materials are poorly resolved (Fig. le and If), the HREM image reveals large domains of highlyordered mesostructure (Fig. 3a and 3d). The HREM images presented in Fig. 3a and Fig. 3d are consistent with the Ia3d symmetry and show the uninterrupted channels along the observation direction. In the electron diffraction pattern (Fig.3b), one can observe diffuse electron diffraction rings, indicating that the walls remain amorphous after calcination. This is also supported by the absence of wide-angle reflections in the XRD pattern. The Fourier diffractogram (Fig. 3c) indicates that the zirconium oxophosphate material is also commensurate with the Ia3d symmetry after calcination. Therefore, Fig. 3 gives the clear evidence that the cubic Ia3d mesostructure is retained after the removal of the template by thermal treatment. The sample with r = 0.40, investigated by EM shows similar well-resolved cubic domains.

224

[ ^

202 .

2nm"'

0^0

I 0.2nrn"^

-121

^^

Fig. 3: Typical HREM image and electron diffraction (ED) pattern of a sample with r = 0.54 after calcination at 500°C. Fig. 3a) HREM image taken along the [111] zone axis. Fig. 3b) Electron diffraction pattern. Fig. 3c) Fourier diffractograms obtained from the HREM image in Fig. 3a. Fig. 3d) HREM image taken along the [100] zone axis (inset is the ED pattern).

The N2 sorption isotherms are similar to Type I isotherms characteristic for microporous materials, and likely correspond to pore sizes in the upper micropore range or lower mesopore range.^ In general, the total nitrogen adsorption capacity decreases rapidly with increasing surfactant-to-zirconium sulfate ratio. The highest adsorption capacity is measured for r = 0.40. This cubic zirconium oxo-phosphate sample exhibits total nitrogen adsorption capacity of up to 130 cm^/g and has a pore volume of up to 0.20 cmVg. In addition, the physisorption data indicate a smaller pore size for the cubic zirconium oxo-phosphate compared to Wavenumber [cm^] the corresponding hexagonal phase Fig. 4: Typical FTIR spectra recorded on a material.^ zirconium-based cubic mesophase. a) Zirconium The FTIR spectra recorded on a cubic sulfate mesophase. b) As-synthesized zirconium zirconium-based mesophase (r = 0.40), oxo-phosphate. c) Calcined zirconium oxoprior to and after the phosphation step, phosphate. Samples in KBr. Offset is for clarity. and after removal of the template by calcination, are detailed in Fig. 4a, 4b and 4c, respectively. The broad unresolved peak observed for all synthesis stages at about 3200-3600cm'' is characteristic of hydrogenbonding from 0-H groups. The peak observed at 1630-1640 cm'^ is due to the bending mode of water adsorbed on the sample surface, which also contributes to the broad 0-H stretching band above 3200 cm"'.'''^ The absorption bands observed around 1470 cm"' and 2800-3000 cm', in Fig. 4a and 4b, originate from the surfactant species and are due the C-H hydrocarbon deformation and stretching modes, respectively. In addition, the weak absorption bands observed around 3065 cm' originate from the aromatic ring of the surfactant head group. All these bands disappear after calcination (Fig. 4c).'^Several absorption bands attributed to the sulfate groups in the zirconium sulfate-surfactant mesophase are observed between 900-1300 cm' (Fig. 4a). After phosphation of the sample, an intense broad

225

band centered at 1040-1060 cm"^ assigned to the stretching region of phosphates^ ^''^ is observed at the same frequency range (Fig. 4b). Furthermore, the spectrum in Fig. 4a exhibits medium intensity peaks around 610-650 cm"'. After phosphation, these signals are reduced (611 cm'^ Fig. 4b), and a new absorption band appears at ca. 515 cm'. After thermal treatment at 500°C, the bands at 610-650 cm' seem to vanish, while the band at ca. 515 cm'' is retained. The appearance of all peaks in the phosphated sample in Fig. 4b likely suggests therefore the presence of both sulfate and phosphate species, which may act to increase the disorder in the zirconium-based fi-amework.^ In addition, a weak absorption peak is observed around 742 cm"' for the calcined zirconium oxophosphate (Fig. 4c), and might be due to the presence of pyrophosphate groups"''^ (P0-P bending) suggesting phosphate condensation during calcination. The intensity around 2440 cm"' is probably due to overtone and combination bands. Pyridine sorption followed by IR spectroscopy shows that the samples contain both Bronsted and Lewis acid sites, the concentrations of which depend on the synthesis parameters.^ In terms of relative peak intensities, the largest Bronsted : Lewis (B : L) peak ratio, determined using the ratio of the 1540 cm"' (B) and the 1446 cm' (L) peak, was observed in the 0.54 sample. The sample with r = 0.40 has more Bronsted acidic bridging OH groups. This sample has the highest pore volume (0.20 cmVg), and the highest thermal stability. 4. CONCLUSIONS The cubic structure inferred from XRD is confirmed for the template free materials by HREM, which enables precise structure assignment. The porous zirconium oxophosphate described is therefore one of the first transition metal-based analogues of MCM-48-type materials. The zirconium oxo-phosphate exhibits total nitrogen adsorption capacity of up to 130 cm^/g and has a pore volume of up to 0.20 cmVg, with pore sizes reaching the upper micropore range. Pyridine sorption followed by IR spectroscopy shows that the samples contain both Bronsted and Lewis acid sites. As prospects, it could be expected that such high surface area ordered porous zirconium oxo-phosphates could find interest as metal or metal sulfide catalyst supports for hydrotreatment processes'^ or low-temperature methanol decomposition reactions.'^ 5. REFERENCES 1. A. Sayari, Chem Mater., 8 (1996) 1840. 2. F. Schuth, Chem. Mater., 13 (2001) 3184. 3. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Chem. Mater., 11 (1999) 2813. 4. D.M. Antonelli, A. Nakahira and J.Y. Ying, Inorg. Chem., 35 (1996) 3126. 5. H. Hatamaya, M. Misono, A. Tagushi and N. Mizuno, Chem. Lett., (2000) 884. 6. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger and F. Schuth, Angew. Chem. Int. Ed. Engl., 35 (1996)541. 7. U. Ciesla, M. Fr6ba, G.D. Stucky and F. Schuth, Chem. Mater., 11 (1999) 227. 8. F. Schuth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo and G. Stucky, Mater. Res. Bull., 34 (1999) 483. 9. F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki and F. Schuth, Chem. Mater., in press. 10. F. Kleitz, W. Schmidt and F. Schuth, Microporous Mesoporous Mater., 44-45 (2001) 95. 11. D.E.C. Corbridge and E.J. Lowe, J. Chem. Soc, (1954) 493. 12. K. Segawa, Y. Kurusu, Y. Nakajima and M. Kinoshita, J. Catal., 94 (1985) 491. 13. M.S. Wong and J.Y. Ying, Chem. Mater., 10 (1998) 2067. 14. Y. Sun, P. Afanasiev, M. Vrinat and G. Coudurier, J. Mater. Chem., 10 (2000) 2320. 15. M. Ziyad, M. Rouimi and J.L. Portefaix, Appl. Catal. A, 183 (1999) 93 16. M.P. Kapoor, Y. Ichihashi, W.-J. Shen and Y. Matsumura, Cata. Lett., 76 (2001) 139.

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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Synthesis and characterization of mesoporous titanium oxide Jia-Long Tsai, Hsiao-Wan Wang and Soofin Cheng* Department of Chemistry, National Taiwan University, Taipei 106, Taiwan *Fax No: +886-2-2363-6359; Email: [email protected] Mesoporous Ti02 powders were synthesized in the presence of surfactant-type pore-directing agents. Monolaureth phosphate was used as the pore-directing agent because of its low cost and industrial convenience. Mesoporous Ti02 was successfully obtained by stabilizing the titanium source with acetylacetone and by hydrolyzing it in strong acidic condition. The crystallinity of hexagonal arranged meso-structure was improved by adding butanol as a co-surfactant and post-treatment with ammonia solution. Several methods were used to remove the organic templates in the structure, and their effect on the porous structure and surface area was compared. The photo-catalytic activity of the resultant porous TiOz in degradation of phenol was also studied. 1. INTRODUCTION Titanium oxide has been extensively studied because of its attractive properties and numerous applications. It has high refractive index and commonly used in pigments. It is a semiconductor and used as catalyst supports or photo-catalysts. Mesoporous Ti02 is attractive as a result of its additional potential applications in chemical sensor, photonic crystal and solar cell. Mesoporous materials are usually synthesized by self-assembling of surfactants or block copolymers as templates. Mesoporous Ti02 was first prepared by Antonelli et al. [ 1] by using titanium isopropoxide as starting material and potassium tetradecylphosphate as pore-directing agent, but phosphate was still present in the structure after calcination at 350°C. Block copolymer templating syntheses were also used to prepare mesoporous TiO: [2,3]. Aging time of seven days was required while employing titanium chloride as titanium source. Besides, a variety of surfactants were also applied to prepare mesoporous TiO: [4,5]. However, the materials were suffered from low thermal stability. Here, we report an easy synthesis method using a convenient and low-cost surfactant. We also compare the influence of different types of surfactants on the structure of resultant Ti02 products. 2. EXPERIMENTAL Anionic monolaureth phosphate (MLP) was used as pore directing-agent. An aqueous solution of titanium /so-propoxide and acetyl acetone was added into a solution containing MLP, «-butanol and HCl. After stirring at room temperature for 16 h, the precipitate was separated by filtration. The resultant powders were heated with 0.5M NH3 solution at 80 C for 2 days to obtain mesoporous Ti02. Several methods, including ion-exchange with 0.5 M NaCl(aq), calcination, and irradiation with UV light, were applied to remove the phosphate templates. This work was supported by the Ministry of Education and the National Science Council of Taiwan.

228

XRD patterns were recorded on a Scintag XI instruments. BET surface area was obtained using a Micrometric ASAP 2000 physisorption system. IR data were taken using a Bomem MBIOO spectrometer. The elemental analysis was obtained using an Allied Analytical System (Jarrell-Ash), Model IC AP9000 ICP-AES. The morphology and pore structure were examined with a Hitachi S-2400 SEM and a Hitachi H-7100 TEM, respectively. 3. RESULTS AND DISCUSSION Fig. 1. compares the structures of Ti02 obtained by using surfactants of different charged natures: cationic CTMABr (cetyltrimethylammonium bromide), neutral hexadecylamine, and anionic MLP. The XRD patterns show that highly crystalline meso-structure was obtained when using MLP as pore-directing agent. On the other hand, a meso-structure with a very broad XRD peak was obtained when applying neutral surfactant, hexadecylamine as template, and no precipitation could be seen when using cationic surfactant, CTMABr. Moreover, anatase phase Ti02 was formed if the solution of CTMABr was neutralized with base. These results imply that the titanium precursor formed in the synthesis solution containing titanium /50-propoxide and acetyl acetone is likely a cationic complex, and an anionic surfactant would be a proper pore-directing agent to synthesize mesoporous Ti02 materials.

I \^y*4^^^f^^ -^->-w^___

K^.

(b)

(c) 10

2 0

Fig. I. XRD patterns of Ti02 samples prepared with different surfactants as templates: (a) monolaureth phosphate, (b) sample (a) after NH^-treatment, (c) C16H33NH2 amine, (d) sample (c) after NH.vtreatment, and (e) CTMABr

Fig. 2. XRD patterns of Ti02 samples synthesized (a) with w-butanol, (b) with /i-butanol but without HCl, and (c) without «-butanol.

Addition of/i-butanol and HCl into the synthesis gel was found to improve the crystallinity of the mesoporous Ti02 (Fig. 2). «-Butanol was considered to play the role of co-surfactant, which probably interacts with the hydrophilic end of MLP and helps the formation of rod-shaped micelles. On the other hand, HCl can slow down the hydrolysis of titanium complexes and prevent the formation of dense Ti02 structure. It can be seen that a material of poor crystallinity was obtained when n-butanol and HCl were not added to the synthesis gel.

229

(a) (b)

r^ Fig. 3. XRD patterns of Ti02 products synthesized with (a) MLP, and (b) treated with 0.5M NH3 solution at 80°C for 2 days.

5

(c) (d)

*

-JJJL^ —//

20

- — 1

30

* 40

(e) X, 50

2 9

Fig. 4. XRD patterns of Ti02 products (a) synthesized with MLP, (b) ion-exchange with 0.5M NaCl(aq) for 16 h, (c) irradiated with 300 nm UV for 65 h, (d) calcined at 500°C for 6 h, and (e) calcined at 800°C for 6 h. * anatase TiOz, ^ TiP207.

The XRD patterns in Fig. 3 show that before ammonia-treatment, the structure of the as-synthesized TiOz product is more like lamellar compound. After ammonia-treatment, the lamellar structure seems to reorganize and transform to hexagonal arranged mesoporous structure. Several methods were used to remove the organic template, including ion-exchange, calcination and irradiation with UV light. The extent of template removal was examined by the C-H stretching intensity in the IR spectra. The organic phosphate template cannot be completely removed by ion-exchange with NaCl, probably due to the strong interaction between Ti and phosphate. A nearly complete removal of the template was achieved by calcination at 500°C. UV light irradiation could also decompose the organic template, depending on the irradiation period. However, the hexagonal arranged structure collapsed when the template was removed, as shown in Fig. 4(c) and 4(d). When the as-synthesized sample was calcined at 800°C for 6 h, a cubic phase TiPzOv [6] and anatase TiOz formed, as shown in Fig. 4(e). These results imply that the hexagonal arranged meso-structure TiO: has strong interaction with the phosphate template. In other words, the meso-structure material is Table 1 Photodegradation activity of TiOz compounds. Conversion (%) Catalyst Eg(eV) CO2 yield (%) 68 anatase 3.09 83 26 89 rutile 2.85 18 27 3.38 K2Ti409 48 67 Meso-Ti02 after UV radiation* 3.22 28 31 Meso-Ti02 after UV radiation 3.36 50 mLof 0.5 mM phenol solution over 0.01 g catalyst, radiated with 300nm UV for 6 h. * without NH3 treatment.

230

likely a composite of titanium oxide and titanium phosphate. The elemental analysis of meso-Ti02 samples with ICP-AES showed the presence of P. The P/Ti atomic ratio in the as-synthesized sample is ca. 0.58. That value decreased to ca. 0.29 after 300 nm UV irradiation for 65 h, indicating that a large portion of phosphorus was also removed by UV irradiation. Although the structure loses its crystallinity, irradiation with UV light is a promising method to remove phosphate template from meso-Ti02. Fig. 5 shows the N2 adsorption-desorption isotherms of the meso-TiOz samples after different post-treatments. A sample with surface area of 68 m^/g was obtained with UV irradiation and 125 m^/g for the sample calcined at 500°C. In contrast, relatively high surface areas (150-360 m^/g) were obtained for the samples ion-exchanged with NaCl. The hexagonal arranged pore structure was detected on the ion-exchanged samples but hardly seen on the calcined sample due to the collapse of ordered-structure during heat treatment. Fig. 6 shows the TEM images of TiOa products after ion-exchange. Table 1 shows that the meso-Ti02 materials demonstrated photo-catalytic activities in degradation of phenol. The meso-Ti02 samples were irradiated with 300 nm UV light to decompose the organic templates before they were used as photo-catalysts. As shown in Table 1, porous Ti02 demonstrated higher photo-catalytic activities than commercial rutile or K2Ti409 but lower than that of anatase. Besides, the meso-Ti02 has larger energy gap than that of anatase and rutile. The blue-shift in energy gap comparing with pure Ti02 is because the meso-Ti02 is a composite of titanium oxide and titanium phosphate. 100 nm

*

—

•

.

.

_

_

B ^

Fig. 5. N2 adsorption-desorption isotherms of meso-Ti02 products (a) calcined at 500^^0 for 6 h, and (b) irradiated with 300 nm UV light.

Fig. 6. TEM image of meso-TiOz products after ion-exchange with NaCl.

REFERENCE 1. 2. 3. 4. 5. 6.

Antonelli, D. M. et al., Angew. Chem. Int. Ed. Engl. 34 (1995) 2014. Yang, P D. et al.. Nature 396 (1998) 152. Yang, P D. et al., Chem. Mater. 11 (1999) 2813. Antonelli, D. M. et al.. Micro. Meso. Mater. 30 (1999) 315. Khushalani, D. J. Mater. Chem. 9 (1999) 2491. Joint Committee for Powder Diffraction Standard, Powder Diffraction File No. 38-1468. (JCPDS International Center for Diffraction Data, 1987)

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

231

Improvement of thermal stability of Ti-Zr mesoporous oxides using CTAB surfactant templates mixed with auxiliary organic additives Weibin Li , Xufei Yang, Yu Zhang, Wenbo Chu The Environmental Catalysis Group, Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China. Several auxiliary organic additives such as dodecylamine, Triton-X 100, triethanolamine and hexamethylenetetramine (HMTA) were all found to be able to improve thermal stability of the 1:1 molar ratio Zr/Ti mesoporous oxides prepared from inorganic salt precursors using cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. Particularly, a high surface area, i. e., 386 m^/g was available on the Ti-Zr mesoporous oxides prepared from CTAB and HMTA after calcinations at 450 °C, meanwhile the mesoporous structures still retained on the calcined sample. 1. INTRODUCTION Mesoporous transition metal oxides arc very attractive in the fields of catalysis and gas adsorption because of their unique pore structures and redox properties [1]. However, as compared to siliceous MS41 mesoporous materials, it is much difficult to keep their mesoporcs after removing organic templates due to their easily collapsed structures. Several attempts has been made to stabilize their mesoporous structures including using amphiphilic poly (alkylcne oxide) block copolymers [2], addition of sulfate and phosphoric acid during the gel reaction by Ciesla et al [3] and Ying et al [1], respectively. Post-synthesis treatment with phosphoric acid was also employed on mesoporous Ti-Zr oxides by Chen et al [4]. But it is difficult to remove phosphor or sulfur species on the final samples, and hence limiting their applications on some catalytic reactions or adsorption process because of the poisoning [5-6]. In this presentation, several auxiliary organic components were chosen to be mixed with cetyltrimethylammonium bromide (CTAB) template solution in an attempt to improve thermal stability of the mesoporous Zr-Ti oxides during the synthesis process. 2. EXPERIMENTAL The Ti-Zr mesoporous mixed oxides were synthesized from titanium sulfate and zirconium nitrate through templating by CTAB and other auxiliary organic components, i.e. dodecylamine (DDA), triethanolamine, TritonX-100, and hexamethylenetetramine (HMTA) in aqueous To whom correspondence should be addressed. E-mail: wbli(a'mail.tsi^^hua.cdii.cn

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solution at 100-110°C for 2 days; the Ti/Zr molar ratio in the starting gel mixture was varied from 20-80 mol% of the titanium; the molar ratio of DDA/CTAB, triethanolamine /CTAB, TritonX-lOO/CTAB, and HMTA/CTAB, and CTAB/Ti-Zr in the gel mixture with 1:1 Ti/Zr molar ratio was 0.2, 0.55, 0.12, 5.2, and 0.5, respectively. After stirring for 2 hours, the Ti-Zr containing gel mixtures were transferred into Teflon autoclave, and subsequently heated at 100°C for 2 days. After filtration and washing, the powders were dried at 100°C in air, a part of dried sample was chosen for small angle XRD test, the remaining part of the sample were further calcined at 350 or 450°C in nitrogen followed by in oxygen. Small angle X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max RB X-ray diffractometer using Cu K a radiation. TEM images were obtained on a JEM-200C transmission microscope. Nitrogen adsorption/desorption isotherm was determined at 77K by means of Micromeritics ASAP 2010 surface area analyzer. Elemental analysis was done with X-ray fluorescence (XRF) analyzer on Shimadzu XRF-1700 spectroscopy. 3. RESULTS AND DISCUSSION 3.1. Effect of the Ti/Zr ratio

Figure 1. XRD patterns of various Ti-Zr samples

Figure 1 shows XRD patterns of the various Ti-Zr samples prepared from CTAB dried at 100°C, indicating that the crystallinity depends strongly on the Ti/Zr molar ratio in the gel mixtures. The results show that the intensity of XRD peak first increased and then decreased with an increase in the Ti/Zr molar ratios. The most intense and sharp peak at 2.3° 2 0 , with dioo=3.84 nm was observed on the sample with the 1:1 Ti/Zr molar ratio. It is likely that the better ordered mesostructure could be obtained on the Ti-Zr mixed oxides as compared to that on the pure Ti or Zr oxide., Similiarly, Chen et al recently reported that doping with 10 mol % titania could significantly increase the thermal stability of Ti02-Zr02 samples after post-synthesis treatment with phosphoric acid solution [4].

3.2. Effect of auxiliary organic components Figure 2 shows that addition of DDA could improve the mesopore structure of the 1:1 Ti/Zr mixed oxides at 350 "C remarkably, and the effect was also observed slightly for the addition of Triton-X 100 and triethanolamine. Additionally, the peak position of XRD pattern was shifted to a lower 2 9 value after addition of triethanolamine during the synthesis process, which indicated a wider pore diameter was obtained on the sample. Further heating these samples up to 450 "C unfortunately led to the disappearance of XRD peak at a small 2 0 angle, which indicated the collapse of the mesoporous structure. As for the sample prepared with the CTAB template mixed with HMTA, the XRD peak at a

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small angle still retained after the sample was calcined at 450 °C for 2h as shown in Figure 3. It is clear that HMTA could improve the thermal stability of Ti-Zr mesoporous oxides more pronouncedly than other auxiliary organic components such as dodecylamine (DDA) did.

3 4 5 6 7 8 9

2 Theta (° )

10

Fig. 2. XRD patterns of the Ti-Zr samples prepared with and without (A) auxiliary organic components after calcination at 350 °C : DDA (B), triethanolamine (C), TritonX-100 (D).

Fig. 4. TEM images of the Ti-Zr sample prepared from CTAB and DDA after calcination at 350°C.

1 2

3

4

5

6

2 Theta (" )

7

Fig. 3. XRD patterns of Ti-Zr samples calcined at 450 "C (A), 350 "C (B) and 100 ''C (C) with the molar ratio of HMTA/CTAB/Ti+Zr as 5.2:1:0.5.

Fig. 5. TEM images of the Ti-Zr sample prepared from CTAB and HMTA after calcination at 450°C.

Figure 4 shows a TEM image of the Ti-Zr sample prepared from the mixture of CTAB and DDA after calcination at 350°C, indicating a lamellar ordered channels with continuous walls were clearly found on the sample; While Figure 5 shows a TEM image of the Ti-Zr sample prepared from the mixture of CTAB and HMTA after calcination at 450°C, illustrating a "worm-like" mesoporous structure was obtained. The two different mesopore structures imply that HMTA was possibly playing different role in the synthesis of Ti-Zr mesoporous oxides with DDA did. Apparently, the partial pyrolysis of HMTA and the resulting change in pH value of the gel acidity during the hydrothermal synthesis process may be one of the reasons for the difference. Figure 6 shows the Nitrogen adsorption/desorption isotherm and the BJH pore size distribution of the Ti-Zr sample prepared from the mixture of CTAB and HMTA after calcination at 450 °C. It revealed that the adsorption/desorption loop at relative pressure (p/po)

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of 0.4-0.7 and 0.75-1.00, indicating the sample structures are of bimodal mesoporous, its BET specific surface area is 386 m^/g. The pore size distribution calculated from BJH equation is centered at 2.5 nm and 55 nm was also shown in Figure 6. The first peak was associated with the primary mesopores and the second peak was due to the secondary mesopores formed by secondary ^O) 3 0 0 particle aggregates [7], which was consistent with the TEM image in Figure 5. The similar worm-like Pore Diameter (nmV..'^.-' pore was observed on mesoporous alumina as reported by Bagshav et al [8] Elemental Analysis by XRF shows that only desorption minor amount of sulfur species, i. e., 0.026% of Relative Pressure (p/Po) sulfur by weight was detected on the calcined sample as compared to the higher amount of sulfur Fig. 6. Nitrogen adsorption/ desorption isotherm and pore size distribution from BJH species, i. e., 5-8 wt% sulfate for the sample (inset) for the Ti-Zr oxide sample. reported by Ciesla et al [9]. 4. CONCLUSIONS A well-ordered mesoporous Zr-Ti mixed oxide sample with the 1:1 Zr/Ti molar ratio was synthesized from inorganic salt precursors using either cetyltrimethylammonium bromide (CTAB) or a mixture of CTAB and an auxiliary organic component. It was also found that thermal stability of the mesoporous Zr-Ti oxides could be improved by the presence of dodecylamine, Triton-X 100, tricthanolamine, or hcxamethylenctctraminc (HMTA) under hydrothcrmal conditions. It is noteworthy that a high surface area, i. c., 386 mVg after calcinations at 450 °C was available on the Ti-Zr mesoporous oxides prepared from CTAB and IIMTA, moreover the mesoporous structures could remain on the calcined sample with only minor amount of sulfur species, i. e., 0.026% of sulfur by weight. ACKNOWLEDGEMENT National Natural Science Foundation of China (#29907003) is gratefully acknowledged. REFERENCES 1. D. M. Antonelli and J. Y. Ying, Angew. Chem. Int. Ed. Egnl., 34 (1995) 2014. 2. P. D. Yang, D. Y. Zhao, D. I. Margolese et al. Nature, 396 (1998) 152. 3. U. Ciesla, S. Schacht, G. D. Stucky, K. linger and F. Schuth , Angew. Chem., Int. Ed. Engl., 35(1996)541. 4. H. R. Chen, J. L. Shi, Z. L. Hua et al. Mater. Lett., 51 (2001) 187. 5. D. Trong On, Langmuir, 15(1999) 8561. 6. H. Fujii, M. Ohtaki and K. Eguchi, J. Am. Chem. Soc, 120 (1998) 6832. 7. M. L. Occelli, S. Biz, A. Auroux, G. J. Ray, Mesopor. Mesopor. Mater., 26 (1998) 193. 8. S. A. Bagshav, T. J. Pinnavaia, Angew. Chem. Int. Ed., 35 (1996) 1102. 9. U. Ciesla, M. Froba, G. D. Stucky and F Schuth , Chem. Mater 11 (1999) 227.

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Synthesis and characterization of mesoporous zirconia Young-Woong Suh, Jung-Woo Lee and Hyun-Ku Rhee School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Seoul 151-742, Korea* Mesoporous zirconia has been synthesized using zirconium chloride and PEO nonionic surfactant, Triton X-100, as a zirconium source and a structure directing agent, respectively, in aqueous medium. From XRD, BET, SEM and TEM analyses, it can be known that the material treated with UV and ozone has a wormhole structure and a spherical morphology with uniform size. 1. INTRODUCTION Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates [1], a number of studies have been reported concerning the preparation conditions, synthesis mechanism, characterization and use of these materials as catalysts and catalyst supports for various reactions [2]. This surfactant templating procedure was extended to the formation of non-silica mesoporous oxides [3], e.g., titania, niobia, tantala, alumina, manganese oxide, ceria, hafnia and zirconia. Among these non-silica oxides, zirconium oxide is of particular interest for acid catalysis [4]. Hence, much effort has been directed to the preparation of mesoporous zirconia using cationic quaternary ammonium [5-7], anionic surfactants [3,8] and primary amines [9] as the structure directing agents. More recently, Stucky and co-workers [10] prepared mesoporous Zr02 using PEO-PPO-PEO block copolymers and zirconium chloride in a nonaqueous medium. This material was reported to have a two-dimensional hexagonal structure with a semicrystallinc wall. They utilized inorganic salts as metal precursors and carried out the synthesis of mcsostructure in a nonaqueous medium such as ethanol solution, because the presence of excess water makes the hydrolysis and condensation of the reactive metal alkoxides as well as the subsequent mcsostructure assembly process difficult to control. In this study PEO nonionic surfactant with alkyl and aryl groups is used as the structure directing agent. Mesoporous zirconia is prepared in an aqueous medium in contrast to the work of Stucky and co-workers [10]. Finally, the material obtained in this work is compared to the one synthesized in a nonaqueous medium. 2. EXPERIMENTAL 2.1. Synthesis of mesoporous zirconia Triton X surfactants have structures given as (CH3)3CCH2CH(CH3)C6H40(CH2CH20);,H, where x = 8 (TX-114) or 10 (TX-lOO). The latter surfactant was utilized as the structure directing agent in the synthesis of mesoporous zirconia. In a typical preparation, 0.002 mol of 'Address for correspondence: E-mail, hkrhccq/'snu.ac.kr

Fax. +82-2-888-7295

Tel. +82-2-880-7405

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Triton X-100 was dissolved in 100 g of water. To this solution, zirconium chloride precursor in anhydrous ethanol (10 mL) was added very slowly with vigorous stirring. The molar ratio of Zr/surfactant was made equal to 8. The mixture was stirred in a thermostatic oil bath maintained at 100 °C for 48 h. Then, it was aged at 120 °C for 48 h to favor the mesostructure stabilization. After aging, the powder was obtained by the centrifiigation at 12,000 rpm for 30 min, washed with ethanol and dried at room temperature overnight. Instead of calcination, the material was finally treated with ultraviolet (UV) light and concomitantly generating ozone at room temperature to remove the occluded surfactants. It has been suggested in recent studies that UV/ozone treatment is an effective method for the removal of the template surfactants from bulk three-dimensional (3D) MCM-41 materials [11] or two-dimensional mesoporous silica thin films [12]. TX-lOO templated materials will be designated as TX-lOO—Zr02. 2.2. Characterization Powder X-ray diffraction patterns in the 26 range of 1—10° were collected at ambient temperature using Cu-Ka radiation, X = 1.54056 A, on a Philips X'Pert MPD diffractometer operating at 40 kV and 30 mA. Nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 system after the samples were vacuum-dried at 100 °C overnight. Surface areas were determined by the BET method in the 0.05-0.2 relative pressure range. The pore size distribution was determined by the Barrett-Joyner-Halenda (BJH) method. Transmission electron microscopy (TEM) studies were carried out on a JEOL JSM2000EXII electron microscope operating at 200 keV. The samples for TEM were mounted on a microgrid carbon polymer supported on a copper grid by placing a few droplets of a suspension of ground sample in ethanol on the grid, followed by drying at ambient conditions. Field emission scanning electron microscopy (FE-SEM) was performed on a JEOL JSM6700F microscope. 3. RESULTS AND DISCUSSION SEM image showing the particle texture of TX-lOO—Zr02 is shown in Figure 1. This surfactant provides the zirconia with well-defined elementary spherical morphology with a mean size of 200 nm. This particle texture was also observed in mesoporous Zr02 synthesized with Tween-20 surfactant [13]. These particles are much smaller than those usually obtained with MCM-41-type materials (mean size ~2 jim) [1]. Therefore, it is expected to obtain a textural porosity within the partial pressure range of 0.8 to 1. Figure 2 presents the XRD pattern for TX-100-ZrO2 treated with UV light and in-situ generating ozone at W[) 3.0mm ?.OkV X30,000 room temperature. The pattern resembles those obtained with MSUX materials with a single correlation Fig. 1. SEM image of UV/ozonc-treated TXpeak due to the 3D wormhole porous 100-ZrO2.

237

o-o o^_o^:0;*^-at«**^

:

y^ J

1..

. 1

0.0 2 theta /degrees

Fig. 2. XRD pattern of UV/ozone-treated TX-100-ZrO2.

0.2

0.4

0.6

'^w-^.-A-

1

Pora dlarTMl^r (nm)

0.8

1.0

Relative pressure (P/PQ)

Fig. 3. N2 physisorption isotherm of UV/ozone-treated TX-100-ZrO2. Inset: BJH pore size distribution.

framework structure [14]. This single peak pattern is typical of materials possessing uniform diameter pores in the mesoporous range, indicating that either the pore architectures of the materials are non-symmetrical or the particle sizes are small [14]. Since TX-lOO—Zr02 particles are relatively large [14], the single peak XRD pattern indicates that the particles have non-symmetrical worm-like pores. The peak is observed in the d spacing of 29.1 A, similar to the one (29.8 A) obtained from mesoporous zirconia synthesized with Tween-20 surfactant [13]. Mesoporosity of TX-100—Zr02 is illustrated by the N2 adsorption/desorption isotherms and pore size distribution as shown in Figure 3. The material exhibits a broad, but well-defined step in the adsorption isotherm and a clear hysteresis in the desorption isotherm over the relative pressure range of 0.4 to 0.8, which is indicative of the filling of the frameworkconfined mesopores. The existence of textural mesoporosity is evidenced by the presence of a hysteresis loop above PfP{) = 0.8. Some necking of the pore structure is suggested by the sharp curvature in the desorption leg of the hysteresis loop. Surface area determined by the BET method is 290m^/g, very high when compared to that of the conventional zirconia. The BJH model applied to the %. desorption branch of the isotherms verifies the expected bimodal framework (3.88 nm) and textural (21.7 nm) pore size distribution (see the inset of Figure 3). TEM image showing the ordered character of UV/ozone-treated TX100—Zr02 is presented in Figure 4. The spherical particles are observed in accordance with SEM analysis. It is Fig. 4. TEM image of UV/ozone-treated TXnoticed that no apparent order in the 100-ZrO2. pore arrangement exists, which is in

IIHL'''

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good agreement with the absence of extra peaks in the X-ray diffraction patterns. In fact, the pore packing can be well described as wormhole-like or possibly sponge-like. Similar pore distributions have been observed for disordered mesoporous silicas and also aluminas when nonionic surfactants were used [14]. 4. CONCLUSIONS A mesoporous zirconia, UV/ozone-treated TX-lOO—Zr02, is synthesized using zirconium chloride and Triton X-100 in an aqueous medium. Apparently, the material is composed of elementary spherical particles with a mean size of 200 nm. It has both the framework and textural mesoporosities and a wormhole structure. In contrast to the work of Stucky and coworkers [10], the mesoporous zirconia of this study is synthesized in an aqueous medium using different kind of PEG nonionic surfactant. ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support of LG-Caltex Oil Corporation and the partial aid from the Brain Korea 21 Program sponsored by the Ministry of Education.

REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. P.L. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. Unger, Stud. Surf. Sci. Catal., 84 (1994) 2013; A. Corma, M. Iglesia and F. Sanchez, Catal. Lett., 39 (1996) 153; P.T. Tancv, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 3. M.S. Wong and J.Y. Ying, Chem. Mater., 10 (1998) 2067 and references therein. 4. T. Yamaguchi, Catal. Today, 20 (1994) 199. 5. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger and F. Schuth, Angew. Chem., Int. Ed. Engl., 35 (1996) 541; U. Ciesla, F. Froba, G.D. Stucky and F. Schuth, Chem. Mater., 11 (1999) 227. 6. P. Liu, J.S. Reddy, A. Adnot and A. Sayari, Mat. Res. Soc. Symp. Proc, 431 (1996) 101. 7. J.A. Knowles and H.J. Hudson, J. Chem. Soc, Chem. Commun., 2083 (1995). 8. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, Chem. Commun., 491 (1997). 9.N. Ulagappan, Neeraj, B.V.N. Raju and C.N.R. Rao, Chem. Commun., 2243 (1996); Y.-Y. Huang, T.J. McCarthy, W.M.H. Sachtler, Appl. Catal. A, 148 (1996) 135. 10. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152; P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Chem. Mater., 11(1999) 2813. 11. M.T.J. Keene, R. Denoyel and P.L. Llewellyn, Chem. Commun., 2203 (1998). 12. T. Clark Jr., J.D. Ruiz, H. Fan, C.J. Brinker, B.I. Swanson and A.N. Parikh, Chem. Mater., 12 (2000) 3879. 13. Y.-W. Suh and H.-K. Rhee, Stud. Surf. Sci. Catal., 141 (2002) 289. 14. S.A. Bagshaw and T.J. Pinnavaia, Science, 269 (1995) 1242; P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865.

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A novel method to prepare mesoporous nano-zirconia Xin-Mei Liu''\ Max G. Q. Lu", Zi-Feng Yan"'^ ""Department of Giemical Engineering, University of Queensland, Brisbane 4072, AUSTRALIA ^State Key Laboratory for Heavy Oil Processing, Key Laboratory of Catalysis, CNPQ University of Petroleum, Dongying 257062, CHINA A novel method to prepare mesoporous zirconia was developed. The synthesis was carried out in the presence of PEO surfactants via solid-state reaction. The materials exhibit strong diffraction peak at low 2-theta angle and their nitrogen adsorption/desorption isotherms are typical of IV type with H3 hysteresis loops. The pore structure examined by TEM can be described as wormhole domains. The tetragonal zirconia nanocrystals are uniform in size (around 1.5nm) and their pores center at around 4.6nm. The zirconia nanocrystal growth is mainly via an aggregation mechanism. This study also reveals that the PEO surfactants can interact with the Zr-O-Zr framework to reinforce the thermal stabiHty of zirconia. The ratio of NaOH to ZrOCb, crystalhzation and calcination temperature play an important role in the synthesis of mesoporous zirconia. 1. INTRODUCTION Mesoporous nano-Zirconia is of particular interest recently because of its potential applications in chemical sensors of oxygen, solid oxide electrolyte of fuel cell, oxide electrode materials, and catalysis. The uniform mesoporosity of nano-zirconia is necessary to control the transport of the reactant molecules to active sites and determine the length of the triplephase boundary where charge transfer occurs for an electronically conducting electrode and is expedient to the percolation of electrons throughout the electrode microstructure '''"*'. The fme particle zirconia bears the better wear resistance '^' and the weaker diffusion resistance, which could be feasible to use the inner active sites in the catalyst and obtain the higher reaction conversion ratio. Simultaneously, the nanosize zirconia has higher adsorptive capacity, which exhibits the potential application in adsorption or separation. Thus very recently, nanosize zirconia with mesoporous texture has attracted considerable interest because of its large surface areas, unusual adsorptive properties, surface defects and fast diffusivities. Most zirconium oxides were generally synthesized via the sol-gel or precipitation processing using surfactant as template or scaffold agent in previous research. In this paper, a novel method combining solid-state reaction and in-situ crystallizing with polyethylene oxide surfactant to prepare the nanosize zirconia with mesoporous structure is tentatively presented. 2. EXPERIMENTAL 2.L Preparation of nano-zirconia The nanosize zirconia was prepared via solid-state reaction using zirconyl chloride (ZrOCl2-8H20) as precursors. Several procedures were investigated to elucidate the influence

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of Zr/NaOH ratios, the calcinations and crystallization temperature, and the role of surfactants. Firstly, the ZrOCl2-8H20 and NaOH were milled to fine and mixed them at ambient temperature. Then the mixture were transferred to the autoclave and kept it at desired temperature for certain time. After that the mixture were washed with deionized water until free of CI' ions, and then washed with ethanol for two times to remove the water involved in the solid. Finally, the samples were dried at 383K for overnight and calcined at temperatures of 523K~ 773K in the fiimace for 20h using heating rate of 2^C/min. 2.2. Characterization of the samples The synthesized samples were characterized by nitrogen adsorption analyzer. X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (TG-DTA). 3. RESULTS AND DISCUSSION 3.1. XRD investigation

Fig. 1. XRD patterns of zirconia with the different of calcination temperature. Insert: High-angle peaks

Fig. 2. The XRD pattems of zirconia with the different of crystallizing temperature

Of interest is a rather broad low-angle peak occurred in XRD pattems of as-synthesized zirconia shown in Figure 1 and the other four peaks appeared at high 20 degrees, which shows that the synthesized samples are single-phase zirconia with tetragonal structure. It also means that such as-synthesized zirconia actually bears mesoporous skeleton. The broad shape of the XRD peaks means that the as-synthesized samples may possess the less ordered mesoporous structure and the particle may be ultrafine. The particle size estimated by the Sherrer equation is about 1.5nm as perceived from extensive comparison of TEM images. Figure 1 also illustrated that the XRD peaks tended to be sharp and strong with the increase of the calcinatiion temperature. It indicated that the agglomeration and surface reconstruction of as-synthesized nano-zirconia samples occurred in the process of calcinations. Such agglomeration and surface reconstruction might result in the growth of mesoporous nanozirconia particle sizes. The growth of the particle size can be attributed to condensation of the abounding surface hydroxyls groups, which causes the nucleation of new oxide crystals and the growth of the existing one at higher temperature ^^l It is noteworthy that the crystallizing temperature plays an important role on the crystal phase of the zirconia. Figure 2 illustrates that the sample crystallized at ambient temperature exhibits broader diffraction peaks with rather weak intensity, which shows that the sample is the amorphous pattern and particle size is ultrafine. As indicated by the position of the main diffraction peak and the ticks corresponding to tetragnol zirconia, the amorphous structure had

241

the tetragonal zirconia local order. The weak XRD peak means that mesoporous nano-zirconia samples bear tetraganol skeleton but enrich many defects and/or lattice vacancies. Such large number of lattice vacancies and local lattice disorder result in weak in diffraction intensity and even to disappear of crystal planes ^'^\ Upon heating at elevated temperatures, the zirconyl clusters can agglomerate each other and generate many small nuclei ^^\ This results in the larger particle size and more ordered nanocrystalline at higher temperature. However, the monoclinic phase can be formed when the temperature up to 200°C although the signal is not apparent. This means the target crystal phase can be obtained by controlling the crystallizing temperature.

100 200 300 400 500 600 700 800 900 100 Temperature / °C

Fig. 3. Nitrogen adsorption/desorption isotherm of zirconia with different calcinating temperature

Fig. 4. Profile of the TGA spectrum of as- prepared Zr02

3.2. Nitrogen adsorption isotherm The isotherms of the samples all are typical IV isotherms with type H3 hysteresis loops just as shown in Figure 3. It means that the zirconia prepared with this novel method is comprised of the aggregate of plate-like particles forming slit-like pores. It also exhibits that the calcinating temperature plays an important role in the pore structure formation. At the elevated temperature the thermal lattice contraction might occur and the particle size can grow, which results in the larger mesopore generation. NaOH/Zr ratio is another key factor to synthesize mesoporous nano-zirconia. An increase of ratio from 2 ~ 4.0 resulted in an enormous increase of the adsorption capacity of synthesized zirconia. However, the adsorption capacity will be slightly decreased when the NaOH/Zr ratio is above 4.0. Consequently, the specific surface area changed from 182.3 m^/g to 363.9 m^/g, and then decreased to 314.8 m^/g when the ratio is up to 5.0. Of interest is that the inception point of the hysteresis loop shifts to the lower pressure region with the increase of the NaOH/Zr ratio. It indicates that the mesopore diameter of synthesized zirconia obviously shrink with the increase of the NaOH/Zr ratio. It shows that the pore sizes of zirconia prepared with solid-state reaction can be tuned by choosing various ratio of NaOH to ZrOCb. 3.3. Thermal analyses Two weight loss stages were observed in TGA profile of nano-zirconia sample illustrated in Figure 4. The first one that located at low 373K corresponded to the evaporation of the water adsorbed in the sample. The weight loss presented between 523K and 773K is the removal of the terminal hydroxyl groups bonded on the surface of zirconia. Such great weight loss between 523 and 723K means that many hydroxyl groups enriched on the surface of synthesized nano-zirconia. When the samples were annealed above 773K, no further weight

242

loss was observed. This means that the thermal stability of the zirconia prepared with solidstate reaction is well. 3.4. TEM images

^dTZ"

Fig. 5. TEM images of zircoinia.

''l^...i

TEM imagines of nano-zirconia sample depicted in figure 5 positively supported the acquired XRD and nitrogen adsorption/desorpotion results. It confirmed synthesized zirconia samples actually have uniform mesopore and nano-crystalline particles. The mesopore architecture of these zeolite-like is best described as the worm hole. These pore structure have been noted in catalysis and adsorption owing to its greater accessibility to surface sites for gaseous species ^^\ The lattice images exhibit the necking between crystallites while a void region representing the pores winds extensively throughout the structure. 4. CONCLUSION The mesoporous nano-zirconia can be initiatively synthesized by solid-state reaction. 1) The pore size can be tuned by changing the NaOH/ZrOCb ratio. 2) The different crystal phase can be formed at different crystallizing temperature. 3) The particle growth is mainly via an aggregation mechanism 4) The nanostructure is strongly influenced by the NaOH/ZrOCb ratio, calcinating and crystallizing temperature. REFERENCES 1. M. Mamak, N. Coombs, G. Ozin, J. Am. Chem. Soc. 122 (2000) 8932. 2. H. Verveij, Adv. Mater 10 (1998) 1483. 3. A. Ziehfreund, U. Simon, W. F. Maier, Adv. Mater. 8 (1996) 424. 4. F.P.F. van Berkel, F.H. van Heuveln, J.P.P. Huijsmans, Solid state Ionics, 72 (1994) 240. 5. Y.J. He, A.J.A. Winnubst, A. J. Burggraaf, H. Verweij, PG. van der Varst, and B.G. De With, J. Am. Ceram. Soc. 9 (1996) 3090. 6. J.A.Wang, M.A. Valenzuela, J. Salmones, etc., Catal. Today, 68 (2001) 21. 7. G.G. Siu, M.J. Stokes, Y.L. Liu, Phy Rev. B, 59 (1999) 3173. 8. Michael Z.-C. Hu, Michael T. Harris, Charles H. Byers, J. Colloid and Interface Science, 198(1998)87. 9. M. Yoshimura, Am. Ceram. Soc. Bull. 67 (1998) 1950.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Control of ordered mesoporous molecular sieves synthesis using non-ionic surfactants by incorporation of transition metal ions in the micellar solution A. Leonard ^ J.L. Blin, G. Herrier ^ and B.-L. Su* Laboratoire de Chimie des Materiaux Inorganiques, ISIS, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium Phone : +32-81-72-45-31, Fax : +32-81-72-54-14, e-mail: [email protected] Ordered mesoporous silica molecular sieves were obtained using a series of non-ionic Cm(EO)n surfactants. The control of the hexagonal structure was achieved by adding transition metallic cations to the micellar solutions. It has been shown that highly organised CMI-2 and 4 materials could be obtained with Ci6(EO)io and Ci8(EO)io whereas disordered wormhole-like mesostructures were reached with Ci3(EO)6 and Ci3(EO)i2. 1. INTRODUCTION The control of the internal structure and texture as well as external morphology is essential in the design of new materials such as nanobiosensors and opto-electronic devices and their application in industrial processes. Large-pore mesoporous materials were recently prepared with use of polyoxyethylene alkyl ether surfactants [1-5]. This is a more environmentalfriendly way for synthesis as these surfactants are less toxic and more biodegradable than their ionic analogues generally used in the preparation of MCM-41. Besides, it appears that the recovery of the template is easier and so a further re-utilisation could be envisaged. Previous studies using this kind of surfactants have shown that the textural, structural and morphological features of the final mesoporous compounds were strongly affected by physico-chemical variables such as the surfactant / silica molar ratio, pH of the synthesis gel, stirring duration and hydrothermal treatment conditions [6, 7]. Especially the control of the structure of the materials is important, since the 3-dimensional structure of MSU could be more appropriate for catalysis. Whereas the fabrication of semi-conducting wires [8] would require a regular array of long straight channels. One way allowing the combination of the advantages of PEO-type surfactants with the yield of highly ordered materials was proposed by Pinnavaia et al. These authors induced an electrostatic control of the surfactant - silica assembly process by complexing small transition metallic cations by the hydrophilic oxyethylene heads of the template [9]. In this work, we have investigated if this method is effective in the obtention of highly ordered mesoporous molecular sieves by using a series of non-ionic Cm(EO)n surfactants.

^ : FRIA fellow : Corresponding author

244

2. EXPERIMENTAL 2.1. Synthesis Micellar solutions with defined weight percentages of Cm(EO)n were prepared by dissolving the surfactant at a temperature below its cloud point value in an aqueous solution containing cobalt chloride. The cation / surfactant molar ratio was varied from 0.25 to 4.00. Sulfuric acid was then added to decrease pH to a value of 2. After homogenization, TMOS was added dropwise in order to reach a surfactant / silica molar ratio of 1.50. After stirring during 1 hour, the synthesis gel was poured in teflon-lined cartridges sealed in stainless steel autoclaves and submitted to hydrothermal treatment. The recovered gel was then extracted using a Soxhlet apparatus, dried and calcined at 550°C under nitrogen and oxygen. 2.2. Characterization XRD measurements and Transmission Electron Microscopy using a Siemens D-5000 diffractometer and a Philips Technai lOOkV microscope respectively assessed structural features. For TEM observations, the powders were embedded in an epoxy resin and sectioned with an ultramicrotome. The final morphologies were observed using a Philips XL-20 Scanning Electron Microscope. The textural characteristics of our compounds were evaluated by nitrogen adsorption-desorption measurements with a volumetric adsorption analyzer ASAP 2010 or Tristar 3000, both manufactured by Micromeritics. The pore size distributions were calculated by the BJH method applied to the adsorption branch. 3. RESULTS AND DISCUSSION 3.L C,6(EO),oand Ci8(EO^,o surfactants. The introduction of Co^ cations leads to the formation of organized materials obtained with 50 wt.% Ci6(EO)io micellar solutions as can be seen from TEM micrographs. Working in the same concentration domain without cations usually leads to wormhole-like structures [6]. The inserted FFT's (Fig. 1) show that ordering gets better if the concentration in cations ,„^ ..,,....._ ...-, ... is raised. The X-ray diffraction ^i':;. _ • ^::j- ; ~*L*J^"~ resolved secondary reflections, - V ' • / \ indicating that organisation is not perfect. The morphology (not - i'.: ' shown here) is also affected. Indeed, in this case, the particles .•. ^(),ljn ^^^^ ^h^ appearance of splitted .I . . "™ '"' *^'^' " " ' -'•—~ leaves compared with the smooth Fig. 1. TEM micrographs of compounds prepared with blocks that were obtained in the Co^^/Ci6(EO)io molar ratios of a: 0.25 and b : 1.25. absence of transition metallic cations. Using Ci8(EO)io, well-ordered materials can be prepared if Co^^ cations are added to the micellar solution. In the absence of these cations, hexagonal materials were obtained at concentrations below 30wt.% [10]. In this case, the compounds become organised below 40wt.% micellar solutions (Fig. 2b-f). Indeed, the diffractograms show secondary reflections, which can be indexed in a hexagonal system. The TEM pictures showing the honeycomb-like channel array also confirm this arrangement. This suggests that the domain of existence of isolated cylindrical micelles allowing the obtention of ordered materials through a cooperative mechanism is enlarged in the presence of the cations. The presence of the cation

245

probably induces some changes in the packing parameter of the surfactant. Its value decreases if the relative surface of the hydrophilic head becomes larger by complexation of the cobalt ion, leading to the transition from a hexagonal phase to isolated cylindrical or spherical micelles. Regular hexagonal materials are then formed by a cooperative mechanism involving the assembly of silicate covered cylindrical micelles, as observed for CMI-1 and 3. The formers are also stabilised by the rigidification induced by complexation of the cation by the oxyethylene part of the template.

4

^

'

•

29 n

6

^ n pit^i Fig. 2. XRD patterns of compounds prepared with solutions containing Fig. 3. TEM micrograph of a compound 0.705M Co^' as a function of C,8(EO),o prepared with a Ci8(EO)io concentration of weight percentage : a : 50, b : 40, c : 30, 5 wt.% in the presence of Co^^ ions. d : 20, ein: the 10 and : 5 (d-spacings in Concerning texture, for both surfactants, there is a decrease poref size as the content in cobalt is raised (Table 1). The specific surface area of the materials however remains very high. When oxyethylene heads surround the cations, their conformation is frozen. This rigidification could be accompanied with a retraction of the hydrophilic head when increasing the cobalt chloride concentration. This contracted conformation could then account for the smaller pore sizes of the obtained materials, especially for the higher amounts of added transition metal.

3.2. Ci3(EO)i2and Ci3(EO)6 surfactants. Using Ci3(EO)i2, variable amounts of cobalt chloride were added to 15wt.% micellar solutions. Indeed, the materials obtained with 50wt.% were supermicroporous with pore sizes below 2.0 nm. From XRD measurements, it can be seen that secondary reflections appear and become more intense as the amount of added Co^"^ cations increases (Fig. 4). However, these Table 1 Evolution of pore sizes (in nm) as a function of the Co^^ / surfactant molar ratio for different Cm(EO)n surfactants C0^VCn.(E0)n 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 3.00 4.00 molar ratio 4.0 Ci6(EO),o5^w/.% 3.8 2.5 5.5 2.7 3.7 Cis{EO)io 50 wt.% 3.2 4.3 Ci8(EO),o 15 wt.% 2.9 500 m^^- After endothermic dehydration in N2 or vacuum the nanoparticles contained two-dimensional fragments (clusters) of a -CrOOH crystals built on [Cr(OH)303] octahedra without bonding along the Z-axis (Fig.5). The dimensions of the octahedtra corresponded to the d-spacings calculated from the X-ray patterns (Fig3-2,4) of dehydrated aerogel are shown in Fig 5. After endothermic dehydration in air followed by exothermic Cr(III) ^Cr(IV) oxidation at 450-593 K the material consisted on almost amorphous Cr02 nanoparticles (fig 3-3) with texture similar to that of aerogels dehydrated in oxygen-free atmosphere. At higher temperatures depending on the treatment atmosphere, exothermic dehydration-recrystallization (CrOOH, N2, vacuum, > 773 K) or decompositioncrystallization (Cr02, air, >650 K) resulted in an exothermic glow transition into large (- 50 nm) crystals of well-defined a-Cr203 (Fig.3-5) with a low surface area of

CD

^ €

10.0 nm

:^;-?-

0 •

0

02

04

oe P/Pu

>2. 3r '(/) c

_

0)

^

_J

2

•

1

^- — 1

r

1

3 4 5 26 (degree)

'Mi^

1

Fig. 1. Low-angle XRD pattern (left), N2 sorption isotherm and representative TEM image and diffraction pattern (right) for NbTa oxide sample prepared under the optimized conditions of water and metal sources. It is noted that the wall thickness estimated by assuming a hexagonal structure was ca. 2.6 nm. Crystallization of the sample after calcination of 923 K was confirmed by wide-angle XRD pattern (not shown).

Fig.2(a) displays a typical TEM image of NbTa-TIT-2, crystallized

NbTa oxide with periodical mesoporous structure.

The mixed spot ED pattern (inset of

Fig.2(a)) obtained from a whole particle reveals the presence of multi crystal phases.

In

order to estimate the size of a single crystal domain, ED patterns were collected from places of different sizes.

The ED pattern (Fig.2(b)) taken from a 100-nm range (shown image)

shows a spot ED pattern, which indicates a single crystal phase in the range.

From high

resolution TEM (HRTEM) image of walls displayed in Fig.2(c), lattice fringes in a limited place, where ordered pores are directly observed, run in the same direction.

From these

results, it is considered that the ED pattern of mixed spots (inset of Fig.2(a)) taken from a whole particle is resulted from the fact that a crystallized particle with the ordered mesoporous structure consists of phases by ca. 100-nm ranged single crystal domains.

This

means that the ordered crystallized mesoporous NbTa oxide, NbTa-TIT-2, is different from single crystal particles of worm-hole mesoporous NbTa oxide, NbTa-TIT-1, in the size of a single crystal domain.

In the case of worm-hole structure, single crystal domain spreads to

several hundreds nanometer size, whereas ordered mesoporous structure

254

m

:5^" ii--#

-•K-

SO.Onm

(^\

Fig. 2. Typical TEM images and ED patterns of NbTa-TIT-2.

(a) A particle with periodical

mesoporous structure (inset : ED pattern collected from whole particle), (b) periodical mesoporous structure in 100-nm ranges single crystal domain (inset : ED pattern collected from the image) and (c) HRTEM image of walls. suppresses the size to ca. 100 nm.

It is also mentioned that in the case of the

2D-hexagonally ordered mesoporous NbTa oxide calcined under the same condition without re-filling template, the crystallized sample as NbTa-TIT-1 was obtained.

Therefore, the

presence of re-filling template appeared to be effective for preserving mesoporous structure during the crystallization. In conclusion, it was found that NbTa-TIT-2 prepared by the use of furfuryl alcohol as re-templating source possessed ca. 100-nm ranged single crystal domains in a particle with the original 2D-hexagonally ordered mesoporous structure.

We expect that this strategy

would be improved and become one of the general methods applicable to various materials. REFERENCES 1. U. Ciesla and F. Schiith, Microporous and Mesoporous Materials, 1999, 27, 131. 2. Y. Takahara, J. N. Kondo, T Takata, D. Lu and K. Domen, Chem. Mater., 13, 1200. 3. M. Uchida, J. N. Kondo, D. Lu and K. Domen, Chem. Lett., 498 (2002). 4. B. Lee, D. Lu, J. N. Kondo and K. Domen, Chem. Commun., 2001, 2118. 5. B. Lee, T. Yamashita, D. Lu, J. N. Kondo and K. Domen., Chem. Mater, 2002, 14, 867. 6. T. Katou, D. Lu, J. N. Kondo and K. Domen, J. Mater. Chem., 2002, 12 1480.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved

255

Compositional effects of bimodal mesopore silica synthesized by a basecatalyzed ambient pressure sol-gel processing X.- Z. Wang," ^ * W.- H. Li,^ T. Dou' and B. Zhong^ ^Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China E-mail: \[email protected] ''State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China The effects of tetraethylorthosilicate (TEOS) concentration and TEOS/surfactant molar ratio on the synthesis of bimodal mesopore silica (BMS) were studied. It was found that the BMS silica can be synthesized in a wide range of component concentration and its secondary mesopore size is more sensitive to the change of precursor concentration than its primary mesopore size and the secondary mesopore volumes can be up to 2.0 or more times as large as the primary mesopore volumes. The controllability of the bimodal mesopore size distributions, in particular the secondary mesopore size of BMS silica is of great interest to catalysis because they greatly facilitate mass transport to the primary mesopore. 1. INTRODUCTION The synthesis of inorganic frameworks with hierarchically structured pores, and an accurately controlled pore texture at different length scales is of potential importance in catalysis[l], separation technology[2] and biomaterials enginecring[3]. Since the first synthesis of mesoporous MCM-41 materials[4.5], there has been an unparalleled activity in the design and synthesis of a variety of mesoporous solids with different structural characteristic. In earlier investigations[6.7], we showed that careful controlling alkalinity affords a novel porous materials with well-defined bimodal mesopore size distribution (designed as BMS) in a cationic surfactant-contained synthesis system at ambient conditions, which used usually to prepare MCM-41 mesoporous materials. Further investigation showed that the first key factor for the formation of BMS silica was to control the relative rate of the hydrolysis and polycondensation of TEOS and then gelation[8]. Thus, any variation of the reaction components and its concentration may influence the reaction kinetics of sol-gel alkoxides and then influences the mesostructure of the resultant silica gel. Unquestionably, the unique bimodal mesopore structure and fairly thermal stability, more specifically the fine controllability of bimodal mesopore structure can be of great value in designing BMS materials as catalyst supporter, catalysts, adsorbents and sensor materials. Accordingly, in the present study we examine systematically the influence of TEOS concentration and TEOS/ surfactant molar ratio on the bimodal mesopore structure of BMS silica. At the same time we give a full account of the trend of pore size adjustment and an anew insight into the formation mechanism of BMS silica.

256

2. EXPERIMENTAL The synthesis procedure for BMS sihca was described elsewhere[6.7] and the standard molar ratio of the reaction gel mixtures was 1.0 Si02: 0.185 Ci6H33N(CH3)3Br : 0.6 NH3 : 115H2O. For the purposes of probing the effect of TEOS concentration and TEOS/surfactant molar ratio on the bimodal mesostructure porosity, BMS silicas were prepared over a wide range of TEOS concentration from 9.9wt% to 24.8wt% while holding the TEOS/CTAB molar ratio constant at 5.4, or holding other component constant to make the TEOS/CTAB molar ratio increasing from 5.4 to 16.3, and the pH values of the reaction mixture were adjusted with aqueous ammonia. All of the BMS reaction products were washed repeatedly with distilled water in a centrifliger, dried in air at 353K and finally calcined in air at 2K min'' to 823K for 6h to remove the template. The powder X-ray diffraction patterns (XRD) were recorded using a D/max-2500 powder diffractometer with Cu-Ka radiation (40kV, 100mA), 0.02"step size and 1 s step time over the range 1°< 2 9 < 8°. N2 adsorption isotherms were measures at -196°C using a ASAP2000 analyser. The volume of adsorbed N2 was normalized to standard temperature and pressure. Prior to the experiments, samples were dehydrated at 350°C for 12h.The pore-size distribution was calculated using the desorption branches of the N2 adsorption isotherm and the Barrett-Joyner-Halenda (BJH) formula. 3. RESULTS AND DISSCUSSION XRD patterns of all BMS silica samples prepared under different component 1000 concen- tration conditions exhibit qualitatively equivalent diffraction features. Figure 1 provides the representative X-ray B 500 powder diffraction patterns for the calcined BMS silicas prepared under different TEOS/ CTAB molar ratios. The patterns all contain a single strong, relatively broad 0 2 4 6 8 reflection at low 2 0 angle. However, the ZTheta positions of the intense reflection are dependent by the TEOS/CTAB molar ratios of the reaction medium. As the Fig. 1. Powder X-ray diffraction patterns of TEOS/CTAB molar ratios increased, the calcined BMS silicas prepared from dioo values gradually increased from different TEOS/CTAB molar ratio: (a) 5.4; 4.49nm to 5.74nm. An analogous increase {b)7.6;(c) 13.3; (d) 16.3. in the dioo value with increasing the TEOS concentration at constant TEOS/CTAB molar ratios was also observed. The basal spacings represented by the strong diffraction line are correlated with the BJH pore sizes, even though the framework lacks regular long-range order. Figure 2 shows the corresponding N2 adsorption isotherms and the BJH pore size distributions for calcined BMS samples mentioned above. As can be seen from the adsorption plots, the samples all exhibit type IV isotherms as expected for mesopore silica but with a characteristic hysteresis loop lifted up

257

0

0.2

0.4

0.6

0.8

10

1

100

1000

10000

Pore Size (A)

Relative Pressure (p/po)

Fig. 2. N2 adsorption-desorption isotherms and BJH pore size distributions of calcined BMS silicas prepared with different TEOS/CTAB molar ratio: (a) 5.4;(b) 7.6;(c) 13.3;(4) 16.3. sharply in the p/po region of 0.8-1.0, corresponding a bimodal mesopore size distribution was observed in the BJH plots. Following the increase of TEOS/CTAB molar ratio, the adsorption step at the position of p/po=0.8-1.0 is shifted gradually to higher relative pressure, but the adsorption step at the position of p/po=0.25-0.45 is not shifted obviously. Corresponding, the secondary mesopore size of BMS silicas increased systematically with increasing TEOS/CTAB molar ratio from I8.9nm at 5.4 to 45.5nm at 16.3 and the primary mesopore size was not changed obviously. An analogous shift in the bimodal mesopore size distributions with increasing the TEOS concentration at constant TEOS/CTAB molar ratio can be also observed in Figure 3, however, the increasing extent of the secondary mesopore size (such as from 18.9nm to 37.7nm) was far lower than that of the former and Table 1 provided the relevant structure parameters. It is clear that the secondary mosopore size of BMS silica is more sensitive to the change of precursor concentration than that of its primary mesopore size. Since the primary framework mesopore Table 1 Physical parameters for calcined BMS silicas prepared under different TEOS concentration and TEOS/CTAB molar ratio. CTEOS

(wt %) 9.9 13.3 19.4 24.8 13.3 19.4 24.8

TEOS/CTAB (molar ratio) 5.4 7.6 13.3 16.3 5.4 5.4 5.4

dioo (nm) 4.49 4.85 5.29 5.74 4.62 4.81 5.15

Primary mesopore

Secondary mesopore

(m'/g)

(cmVg)

Dp (nm)

(m'/g)

ABET

Vs (cm'/g)

Ds (nm)

1064.6 806.5 592.1 608.8 908.5 843.5 886.2

0.66 0.50 0.37 0.40 0.60 0.58 0.63

2.80 2.70 2.70 2.65 2.71 2.70 2.94

243.1 230.7 184.1 137.5 255.2 273.2 220.8

1.18 1.50 1.74 1.14 1.12 1.41 1.94

18.9 26.0 43.0 45.5 20.0 22.7 37.7

ABET

258

007

^5

_

a

n

lie

h^ij4

1 1 ™B»P*^i ttAAMoUr1 1 1 1 mill

0

0.2 0.4 0.6 0.8 Relative Pressure (p/po)

1

10

100

1000

10000

PbreSEE(/^

Fig. 3. N2 adsorption-desorption isotherms and BJH pore size distributions of BMS silicas prepared with different TEOS concentration at constant TEOS/CTAB molar ratio: (a) 9.9wt%; (b) 13.3wt%; (c) 19.4wt%; (d) 24.8wt%. of BMS silica results from the removal of surfactant template and the secondary textural mesopore results from the interparticle porosity[8], the above results indicate that the change of compositional concentration used here main affects the relative rate of the hydrolysis and condensation of TEOS and then affects the particle sizes of resultant silica gel, but has a little effect on the micelle size, which decides the primary mesopore size. On the contrary, the element that can alter the micelle size, such as by altering the surfactant alkyl chain length or adding an auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) into the reaction systems, can alter also simultaneously the relative rate of the hydrolysis and condensation of TEOS, and the fmal result is that the bimodal size distributions of DMS silicas can be well-matched adjusted in certain range at the same time[9]. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.20073029) and the Province Youth Science Foundation of Shanxi (Grant No.981007).

REFERENCES 1. P.T. Tanev, M.Chibeve and T.J.Pinnavaia, Nature., 368(1994)321. 2. R.Burch, N.Cruise, D.Gleeson and S.C.Tsang, J.Chem.Soc.,Chem.Commun., 1996, 951. 3. R.M. Barren Hydrothermal Chemistry of Zeolites. Academic, London, 1982. 4. J.S.Beck, J.C.Vartuli, W.J.Roth, M.E.Leonowicz, C.T.Kresge, K.D.Schmitt, C-T.W.Chu, D. H.Olson, E.W.Sheppard, S.B.McCullen, J.B.Higgins and J.L.Schlenkere, J.Am.Chem.Soc, 114(1992)10834. 5. C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature.,359( 1992)710. 6. X.Z. Wang, T.Dou and Y.Z.Xiao, Chem.Commun., 1998,1035. 7. X.Z. Wang, T.Dou, Y.Z.Xiao and B.Zhong, Stud.Surf Sci.Catal., 135 (2001) 199. 8. X.Z. Wang, T.Dou, D.Y.Zhao and B.Zhong, submitted to Chem.Mater., 9. X.Z. Wang, T.Dou, D.Wu and B.Zhong, Stud.Surf Sci.Catal., 2002, Nanoporous Mater-III.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

259

A direct template synthesis of highly ordered mesostructured carbons using as-synthesized MCM-48 as template S. B. Yoon,' J. Y. Kim,' Y.-S. Ahn,'' H.-S. Kim^ and J.-S. Yu'* ''Department of Chemistry and Institute of Infor-Bio-Nano Materials, Hannam University, Taejon, 306-791, Korea Functional Materials Research Team, KIER, Taejon, 305-343, Korea A direct template carbonization using as-synthesized MCM-48 as template provides a simple and efficient synthetic method for highly ordered mesostructured carbons with great mechanical. 1. INTRODUCTION Porous carbons have been greatly studied as adsorbents and electrode materials [1]. Various porous carbon materials have been fabricated using inorganic templates including zeolites [2J, opals [3J and silica gels [4]. Recently, a new class of mesoporous carbons was reported using mesoporous materials as templates [5]. In these previous works, before the carbonization, the surfactant molecules in as-synthesized templates were completely removed by calcination process. Such process may often cause some partial lattice collapse or shrinkage of mesoframework as observed by line broadening or signal shift in their powder x-ray diffraction. The process also wasted the expensive surfactants, usually organic hydrocarbons or block copolymers, which can be a good carbon source. To help this end, we report here a simple synthetic method called "a direct template synthesis" of porous carbons using as-synthesized mesostructures as templates. The surfactant in the as-synthesized host was also used as a carbon source. This work can save extra labor, time and energy required for the calcinations process, and yet is found to be an efficient way of synthesizing high quality nanoporous carbons with great mechanical stability. 2. EXPERIMENTAL Mesoporous silica MCM-48 was prepared using hexadccyltrimcthylammonium bromide (Ci6H33N(CH3)3Br) and Brij 30 (polyoxyethylene (4) lauryl ether, Ci2(EO)4) as surfactants and colloidal silica Ludox HS40 as a silica source [5]. As-synthesized silica MCM-48 template is

260

denoted as AM48T in this work. For comparison purposes, some of the as-synthesized MCM-48 was calcined in air at 823 K to remove surfactant molecules. The calcined silica MCM-48 template is named as CM48T. Each of AM48T and CM48T was transferred to a reaction flask in a dry box and dried under vacuum at 373 K for 3 h prior to introduction of carbon precursor. Divinylbenzene (DVB) with a free radical initiator, azobisisobutyronitrile (AIBN) (DVB/AIBN mole ratio D 24) was used as a carbon precursor. The carbon precursor was incorporated into the mesopore of the dried MCM-48 templates. Although the composites in the as-synthesized form are relatively dense, DVB molecules still can enter the pores. The large inner inorganic/organic surface can provide an area for the filling of the carbon precursor solution. The resulting template/polymer composites were then carbonized under argon gas flow by heating at ca.I273Kfor7h. 3. RESULTS AND DISCUSSION Fig. 1 shows powder X-ray diffraction (XRD) patterns of the silica hosts and the resulting carbons, respectively. The AM48T shows the first intense (211) XRD signal at 26^ = 2.1. Calcination process used in this work caused framework shrinkage as indicated in a slight shift of the first signal io 20 = 2.2 as shown for CM48T. Two intense signals ai 269 = 1.4 and 2.4, and 2(-) = 1.5 and 2.5 were observed for AM48T-C (carbon) and for CM48T-C, respectively. The overall XRD intensity of the AM48T-C (formed from both surfactant and DVB as carbon precursors) was usually better than that of the CM48T-C.

20

20

Fig. 1. Powder X-ray diffraction patterns using Cu Ka radiation of (a) as-synthesized MCM-48 (AM48T) and (b) calcined MCM-48 (CM48T) and the resulting nanoporous carbons prepared from (a) AM48T and (b) CM48T

261

The first new (110) intense signal not seen in the MCM-48 host was the result of the phase transition of a cubic MCM-48 to a new cubic phase upon removal of the silica framework [5]. The same XRD signal was also observed for mesostructured polymers templated in MCM-48 [6]. Interestingly, the two intense signals of the CM48T-C as compared with those of the AM48T-C were found to shift to higher 2 theta values by about the same 10 = 0.1 as the shift of the first (211) signal of the CM48T in comparison with that of the AM48T. Transmission electron microscope (TEM) images show highly regular arrays of holes separated by walls, indicating equally great structural integrity and order for the both carbons. The values of unit cell parameter, BET surface area, total pore volume and pore diameter are listed in Table 1. Interesting pore size changes were observed from morphological alterations during the replication process, in which the pores and walls of the silica host were transformed to the walls and pores in the resulting carbon network, respectively. The AM48T-C has a greater unit cell dimension and slightly smaller pore size distribution as compared with those of the CM48T-C. The greater unit cell (or d interplanar spacing) of the former stems from direct template use of the intact AM48T. The framework shrinkage observed in the CM48T is considered to occur mainly in the pore, which will be filled by carbon precursor, rather than in silica wall, thus resulting in thin wall in the corresponding CM48T-C. At least 7 % thicker cross-sectional wall diameter was observed for the AM48T-C as compared with that of the CM48T-C, thus allowing one way of a fine-tuning for carbon wall thickness control. Mechanical strength was measured by monitoring XRD intensity changes after pressurizing Table 1 Structural properties of the AM48T and CM48T silica hosts and the corresponding nanoporous AM48T-C and CM48T-C unit cell , total pore ^ sample d spacing' BET surface pore size parameter ^ volume name (nm) ^ area (nr/g) (nm) ao (nm) (ml/g) AM48T 4.2 10.3 63 0.15 CM48T 3.9 9.6 1130 1.15 3.3 AM48T-C 6.3 8.9 1116 0.94 2.3 CM48T-C 5.9 8.3 1147 0.88 2.4 ''The d spacings were determined from (211) and (110) reflections for the MCM-48 templates and corresponding carbon replicas, respectively. ^XRD unit cell parameter equal to 6'''xd(211) for AM48T and CM48T and equal to 2'''xd(l 10) for AM48T-C and CM48T-C, respectively. ^Maximum value of the BJH pore size distribution peak calculated from the adsorption branch of the N2 isotherm.

262

pelletized carbons at each of different pressures. The relative intensity decreases mainly at low pressure range less than 120 MPa and slowly decreases at higher pressure range. The intensity of the CM48T-C decreased more rapidly than that of the AM48T-C against pressure with ca. 72 % and ca. 85 % of their corresponding initial intensity after 470 MPa, respectively, indicating the latter showed much better mechanical stability. This may be mainly due to the difference in wall thickness. With an assumption of cylindrical shape for the wall, simple calculations indicates at least 14 % larger cross-sectional wall area for the AM48T-C as compared with that of the CM48T-C. In contrast to the carbon replicas, the CM48T silica with high structural order maintained only 38 % of the initial intensity after 470 MPa. 4. CONCLUSIONS It has been demonstrated that the direct synthesis method using as-synthesized MCM-48 as templates and divinylbcnzene as a carbon precursor is simple and energy-saving, and yet also an efficient way of synthesizing ordered nanoporous carbons. The composite carbon formed from both surfactant and DVB showed no structural instability and defects from the heterogeneity, and together with direct use of the intact as-synthesized hosts, rather greatly increased its structural integrity and mechanical stability as compared with the carbon templated in the calcined hosts. ACKNOWLEDGEMENT Authors thank KOSBF for support (Project No. RO1-2001-00424) and Korea Basic Science Institute (in Taejon) for TliM pictures. REFERENCES 1. F. Rodriguez-Reinoso, in Introduction to Carbon Technology, cd. 11. Marsh, \i. A. Ileintz and P. Rodrigucz-Reinoso, Univcrsidad dc Alicante, Secretariade de Pub. Alicante, (1997) p35. 2. Z. Ma, T. Kyotani and A. Tomita, Chcm. Commun., (2000) 2365. 3. A. A. Zakhidov, R. H. Boughman, Z. Iqbal, C. X. Cui, I. Khayrullin, S. O. Danta, L. Marti and V. G. Ralchcnko, Science, 282 (1998) 897. 4. (a) J.-S. Yu, S. B. Yoon and G. S. Chae, Carbon, 39 (2001) 1442. (b) S. B. Yoon, K. Sohn, J. Y. Kim, C. H, Shin, J.-S. Yu and T Hycon, Adv. Mater., 14 (2002) 19. (c) S. Kang, J.-S. Yu, M. Kruk and M. Jaronicc, Chcm. Commun. (2002) 1670. 5. (a) S. B. Yoon, J. Y Kim and J.-S. Yu, Chem. Commun., (2001) 559. (b) R. Ryoo, S. H. Joo and S. Jun, J. Phys. Chem. B, 103 (1999) 7743. 6. J. Y Kim, S. B. Yoon, F. Kooli and J. -S. Yu, J. Mater. Chem., 11 (2001) 2912.

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Gas adsorption: a valuable tool for the pore size analysis and pore structure elucidation of ordered mesoporous materials Mietek Jaroniec and Michal Kruk Department of Chemistry, Kent State University, Kent, Ohio 44242, USA A overview and outlook is presented for the application of ordered mesoporous materials (OMMs) in the development of accurate and reliable methods for the determination of pore size distributions (PSDs) and the elucidation of the pore connectivity. Current status of the use of MCM-41 as a model adsorbent, including the evidence of suitability of adsorption branches of isotherms for the PSD calculations, and the methods for the consistent evaluation of PSDs from nitrogen adsorption at 77 K and argon adsorption at 77 and 87 K are discussed. Opportunities in the use of OMMs with various porous structures to develop PSD calculation methods are outlined. Recently proposed methods for the pore entrance size determination are overviewed and emerging opportunities in the pore connectivity elucidation based on gas adsorption isotherms are discussed. 1. INTRODUCTION The determination of the pore size distribution (PSD) and pore connectivity is one of crucial aspects of characterization of adsorbents, and heterogeneous catalysts [1-3]. Gas adsorption has been an important tool for the elucidation of these important structural properties [1-4]. Many methods to calculate PSD from gas adsorption data have been developed and some of them have been extensively applied [1-4]. Unfortunately, different methods proposed to determine PSDs from gas adsorption data often produce inconsistent results [1,4-6], so much so that in some relatively common cases, completely different estimates of the number, shape and position of peaks on PSDs are indicated by different PSD calculation methods. There were also numerous attempts to elucidate the pore connectivity from gas adsorption data [7], although so far, none of the elaborated methods has gained much practical importance. Experimental verification of the methods to determine PSDs and pore connectivity from gas adsorption data was hampered by the lack of mesoporous solids with well-defined pore shape, size and connectivity. This situation changed dramatically during the last decade thanks to the discovery of ordered mesoporous materials (OMMs) [8-10] that are now available in a wide range of structure types and pore sizes. These structural features can be determined using methods based primarily on X-ray diffraction (XRD), and transmission electron microscopy (TEM) [11-13], which are independent from adsorption methods of the PSD calculation and pore connectivity characterization. Therefore, it is now possible to use OMMs to experimentally test the methods for PSD calculations based on adsorption data and to elaborate new ones [14-18] that would provide the accuracy and reliability required in the emerging nanotechnology

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research. The challenge remains in realizing all the opportunities in the elaboration of methods for PSD evaluation that arose from the discovery of OMMs. Moreover, adsorption methods to elucidate the pore connectivity can now be put to the test [19], and a better understanding of the opportunities and limitations in the use of gas adsorption data for the characterization of pore connectivity is expected to emerge from studies of OMMs with well-defined cage-like pores [11,20]. 2. DISCUSSION In 1997, we first demonstrated how MCM-41 silicas with a wide range of pore sizes can be used as model adsorbents to develop a practical method to calculate PSDs for silicas with cylindrical pores [14]. The MCM-41 pore sizes were determined on the basis of a geometrical relation that involves the XRD interplanar spacing and volume of ordered pores [12,13]. Subsequently, experimental relations between the capillary condensation pressure and the pore size as well as between the capillary evaporation pressure and the pore size were determined for nitrogen adsorption at 77 K. It was found that the capillary condensation pressure tended to gradually and systematically increase as the pore diameter increases. On the other hand, the relation between the capillary evaporation pressure and pore size was more complicated in the adsorption-desorption hysteresis region. In particular, a relatively narrow range of capillary evaporation pressures close to the lower limit of hysteresis corresponded to an appreciable range of pore diameters. It was also apparent that the capillary evaporation tended to be somewhat delayed in MCM-41 samples of lower degree of structural ordering, which we attributed to "single-pore" pore blocking effects similar to those observed in porous networks with constrictions [7], but related to the variations in diameter along single channel-like pores [14,21]. Subsequent studies of argon adsorption at 87 and 77 K [16,18] as well as nitrogen adsorption at 77 K on an extensive set of MCM-41 silicas [4] confirmed that our initial observations were representative for adsorption behavior of different gases at different temperatures on MCM-41 silicas. It has become clear that adsorption branches of isotherms are suitable for PSD calculations because of a well-defined relationship between the capillary condensation pressure and pore size, whereas PSD calculations from desorption branches of isotherms are inherently difficult and may be highly unreliable. Based on this work, we have developed a practical method to calculate PSDs [14] using the well-known Barrett-Joyner-Halenda (BJH) algorithm [22], which we implemented in a rigorous way without approximations originally proposed. We have found that consistent PSD assessment can be made from adsorption branches of nitrogen adsorption isotherms at 77 K, and argon adsorption isotherms at 87 and 77 K [16,18], although argon at 77 K does not allow one to evaluate PSDs for pores of diameter above about 15 nm [18]. Our work mostly involved the use of MCM-41 as a model adsorbent, therefore the developed PSD calculation method was primarily suited for silica-based materials with cylindrical pores. However, this method can be readily extended on materials with different surface properties, for instance on silicas with chemically bonded organic groups [15,17], The pore diameter of MCM-41 samples used in the above studies was restricted to 6.5 nm and we intended to extend the pore size range for the model adsorbents used. SBA-15 silica appeared to be very promising from this point of view, as it can be synthesized with

265

pores much larger than those of MCM-41 [23], but SBA-15 was found to exhibit connections between 2-D hexagonally ordered pores [24], thus making it less suitable as a model adsorbent. Recently, much progress has been made in the synthesis of other OMM structures with tailored pore size, including cubic Ia3d structure of MCM-48 with channel-like branched pores [8], cubic Pm3n structure of SB A-1 and SBA-6 with cage-like pores [11], and cubic Im3m structure of SB A-16 with cage-like pores [11]. These structures have recently been elucidated in detail using electron crystallography [11,25] and their pore size can be estimated from XRD and pore volume data using simple geometrical equations [19,26,27]. The use of these OMMs as model adsorbents for the development of methods for the PSD calculation is anticipated. During the last two years, methods for the elucidation of the pore entrance size in OMMs with cage-like pores were developed [11,20]. One of them was based on the electron crystallography, which solves the 3-D structure of OMM, providing a wealth of information about pore diameter, pore entrance size and pore connectivity [11]. The other method was based on the modification of the OMM surface with ligands of different size to determine the smallest size of ligand that renders the pores inaccessible to gas molecules [20]. Specifically, the OMM with cage-like pore structure is modified with several ligands of gradually increasing size and adsorption isotherms for the resultant modified materials are measured. These ligands can be selected among organosilanes that are commercially available in a wide range of structures and sizes of organic groups. However, one needs to ensure that the modifier forms a monolayer of a predictable thickness on the surface rather than an ill-defined multilayer with thickness that is difficult to predict or is spatially inhomogeneous. This restricts the choice of organosilanes to monofunctional ones, such as organomonochlorosilanes. In addition, the modification conditions need to be chosen in such a way that sufficiently high coverage of surface groups is introduced. From our experience, a high surface coverage results when template-free siliceous OMM is modified with organochlorosilane in the presence of pyridine under reflux conditions [15,20,24], Typically, cage-like pores of OMMs are accessible after the surface modification with smaller organosilanes (such as trimethylchlorosilane), but become inaccessible after the surface modification with larger silanes. For instance, an FDU-1 sample synthesized at room temperature, the pores were accessible after the introduction of trimethylsilyl ligands, but the introduction of triethylsilyl groups made most of the cage-like pores inaccessible [20]. Another sample that was synthesized in the same manner and additionally subjected to hydrothermal treatment for 6 hours at 100°C exhibited accessible porosity after the modification with much larger butyldimethylsilyl ligands, but the pores were inaccessible after the modification with octyldimethylsilyl ligands. In this case, the modification with hexyldimethylsilyl groups, whose size was between the sizes of the two aforementioned ligands, resulted in a partial pore blockage [20]. In judging the degree of pore accessibility after the modification, it is important to keep in mind that in the case of any rigid pore system, the successftil surface modification reduces adsorption capacity and pore diameter of the material. The degree of such a reduction for a material with cage-like pores that are accessible after modification can be estimated from the results for modification of channel-like pores of MCM-41 [28] or SBA-

266

15 [24]. When such a comparison is made, it is suggested to select the channel-like material with pore diameter similar to the cage diameter of the material with cage-like pores. It is important to be able to relate the size of the surface ligand that causes the pore blockage to the pore entrance size. The pores are expected to become inaccessible when the thickness of the layer of bonded groups on the surface is larger than the pore radius minus the radius of adsorbed gas atom or molecule. In the case of the modification with organochlorosilane groups, the maximum possible thickness of the bonded layer can be readily related to the structure of the silane on the basis of the bond angles and bond lengths. However, many organosilanes useful for the considered modifications exhibit much structural flexibility related to the fact that they feature long hydrocarbon chains without branching. Therefore, the maximum possible thickness of the bonded layer is likely to be larger than the actual thickness. To investigate this effect, MCM-41 silicas with cylindrical pores were used and it was found that the pore blocking is observed in cases where the maximum possible ligand size is about the same as the pore radius minus of the radius of the adsorbate atom or molecule [20]. So, in the case of pore entrances that have a geometry close to circular, one can readily relate the size of the surface modifier that causes the pore blocking to the pore entrance size. It should be noted that in general, flexible surfacebonded ligands are not expected to adopt fully extended geometry, because they can coil. The pore size reduction observed in the case of modifications with ligands much smaller than the pore radius provides the confirmation of this contention [28]. However, in the case of high surface coverage of ligands, whose maximum extension is close to the pore radius, bonded on the surface of cylindrical or spherical pore, the geometrical constraints (related to the fact that there is more space close to the surface than in the center of the pore) may force at least some of the ligands to adopt fully extended configurations, which would explain the experimental findings for pore-blocked MCM-41. Therefore, the assumption that the pore blocking takes place for ligands whose maximum extension is equal to the pore radius minus the adsorbate molecule radius appears to have both experimental and theoretical basis lor cylindrical pores. Using the methodology discussed above, it was concluded that the pore entrance diameter of FDU-1 synthesized at room temperature is larger than 1.2 nm, but most of the entrances have diameters smaller than 1.4 nm. On the other hand, FDU-1 that was additionally subjected to heating at 100°C for 6 hours had pore entrances larger than 1.9 nm and smaller than 2.9 nm in diameter [20]. This methodology for the assessment of size of entrances to cage-like pores is expected to be particularly useful for silicas and organosilicas with hybrid organic-inorganic frameworks, which are two important types of materials with cage-like pores. The maximum pore entrance diameter that can be assessed using this method is likely to be about 5 nm on the basis of the size of commercially available organosilanes [20]. It is also expected that the accuracy of the pore entrance size evaluation will be lower for larger entrance sizes because of higher uncertainty in estimation of the size of the surface groups. The fact that the above method of the pore entrance size evaluation is not likely to be suitable for entrances larger than about 5 nm in diameter does not appear to be a major limitation, as the sizes above this limit can be characterized simply on the basis of the shape of desorption branches of isotherms in the adsorption-desorption hysteresis region. This opportunity arises from the fact that the capillary evaporation in pores with constrictions

267

takes place either (i) at pressure where the constrictions themselves exhibit capillary evaporation or (ii) at lower pressure limit of adsorption-desorption hysteresis. The first scenario is a widely adopted hypothetical mechanism of delayed capillary evaporation in ink-bottle pores [3]. However, it needs to be kept in mind that the second scenario is often prevalent. The unambiguous evidence of the validity of the second scenario was obtained for FDU-l silicas, whose sizes were assessed via surface modification to be below 3 nm, or even in the micropore range (below 2 nm). These FDU-l samples exhibited capillary evaporation at a relative pressure of about 0.48, whereas the capillary evaporation from the constrictions of size below 2-3 nm is expected to take place at much lower relative pressures. So, the phenomenon of delayed capillary evaporation does not provide any specific information about the constrictions in which capillary evaporation takes place below the lower limit of adsorption-desorption hysteresis. However, this phenomenon is expected to provide information about constrictions that exhibit capillary evaporation above the lower limit of hysteresis. For instance, it was reported [19] that FDU-l silicas subjected to extended hydrothermal treatments at 100°C exhibit nitrogen capillary evaporation at 77 K that starts to take place above the lower limit of adsorption-desorption hysteresis (relative pressure of 0.48). We attributed this behavior to the formation of defects in the pore entrance structure as a result of overly extended hydrothermal treatments [19]. Therefore, the examination of the shape of the adsorption-desorption hysteresis loop for large-pore OMMs with cage-like pores promises to be useful in the investigation of defects in the pore opening structure and in the pore entrance size elucidation in general. However, when nitrogen adsorption at 77 K is used, this method appears to allow one to study openings of sizes down to only about 5 nm (the lower limit of adsorption-desorption hysteresis at a relative pressure of 0.48 corresponds to the capillary evaporation from uniform cylindrical pores about 5 nm in size), which is beyond the typical range of sizes of entrances to mesoporous cages. There is a strong incentive to find gases and experimental conditions that would allow one to obtain information about practically important pore entrance sizes from the shape of hysteresis loops of adsorption-desorption isotherms. To this end, argon adsorption at 77 K was identified as promising, because in this case, the adsorption-desorption hysteresis extends to somewhat smaller pore sizes than in the case of nitrogen at 77 K. We expect that the use of argon at 77 K allows one to probe pore entrance sizes down to about 4 nm on the basis of the shape of the desorption branch of the hysteresis loop. We currently investigate this possibility. 3. CONCLUSIONS Gas adsorption is an important tool in the characterization of ordered mesoporous materials. The determination of pore size distributions can be accomplished by analyzing adsorption branches of isotherms. Well-ordered OMMs with simple pore geometry and a wide range of pore sizes assessed using independent methods can conveniently be used as model adsorbents suitable for the testing and development of methods to calculate PSDs. We have used MCM-41 silicas for this purpose and achieved consistent PSD estimates from nitrogen adsorption data at 77 K and argon adsorption data at both 77 and 87 K. However, we found that the use of argon at 77 K in PSD calculations is restricted to pores of diameter below about 15 nm. In favorable cases, the examination of desorption branches

268

of hysteresis loops allows one to gain insight about the size of entrances to cage-like pores. In the case of nitrogen adsorption at 77 K, we expect that the desorption branch may provide information about entrances above 5 nm in diameter, whereas the use of argon adsorption at 77 K is suggested to allow one to obtain information about entrances of diameter above 4 nm, thus providing additional information about entrances in the 4-5 nm interval. The pore entrance size below 5 nm in silicas and organosilicas with cage-like pores can be assessed in another way. Namely, the surface modification of the sample with cage-like mesopores with monolayer of ligands of gradually increasing sizes allows one to find the smallest ligand size that causes a complete pore blocking, which results from the reduction of the pore entrance size to that below the size of the adsorbate molecule. The pore accessibility is monitored by gas adsorption and the size of smallest ligand that caused the pore blocking is used to assess the pore entrance size. Consequently, gas adsorption can conveniently be use to determine pore size distributions of OMMs and may provide information about the pore entrance size, which can be accomplished on the basis of either desorption branches of hysteresis loops or changes in adsorption properties of the material after modification with surface groups of gradually increasing size. 4. ACKNOWLEDGMENTS The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research. This work was also supported in part by NSF Grant CHE-0093707. REFERENCES I. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982 2. M. Jaroniec and R. Madey, Physical Adsorption on Heterogeneous Solids, Elsevier, Amsterdam, 1988. 3. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press, San Diego, 1999. 4. M. Kruk and M. Jaroniec, Chem. Mater. 13 (2001) 3169. 5. P. Selvam, S. K. Bhatia and C. G. Sonwane, Ind. Eng. Chem. Res., 40 (2001) 3237. 6. P. I. Ravikovitch, D. Wei, W. T. Chueh, G. L. Haller and A. V. Neimark, J. Phys. Chem. B, 101 (1997)3671. 7. H. Liu, L. Zhang and N. A. Seaton, J. Colloid Interface Sci., 156 (1993) 285. 8. J. S. Beck, J. C. Varluli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 9. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 10. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc, Chem. Commun., (1993) 680. II. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408 (2000) 449. 12. T. Dabadie, A. Ayral, C. Guizard, L. Cot and P. Lacan, J. Mater. Chem., 6 (1996) 1789. 13. M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 101 (1997) 583.

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14. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 15. M. Kruk, V. Antochshuk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 103 (1999) 10670. 16. M. Kruk and M. Jaroniec, Chem. Mater., 12 (2000) 222. 17. M. Kruk and M. Jaroniec, Microporous Mesoporous Mater., 44-45 (2001) 725. 18. M. Kruk and M. Jaroniec, J. Phys. Chem. B, 106 (2002) 4732. 19. J. R. Matos, L. P. Mercuri, M. Kruk and M. Jaroniec, Langmuir, 18 (2002) 884. 20. M. Kruk, V. Antochshuk, J. R. Matos, L. P. Mercuri and M. Jaroniec, J. Am. Chem. Soc. 124(2002)768. 21. M. Kruk, M. Jaroniec and A. Sayari, Adsorption, 6 (2000) 47. 22. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc, 73 (1951) 373. 23. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 24. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 104 (2000)11465. 25.M. Kaneda, T. Tsubakiyama, A. Carlsson, Y. Sakamoto, S. H. Joo and R. Ryoo, J. Phys. Chem. B, 106(2002) 1256. 26. P. I. Ravikovitch and A. V. Neimark, Langmuir, 16 (2000) 2419. 27. P. I. Ravikovitch and A. V. Neimark, Langmuir, 18 (2002) 1550. 28. C. P. Jaroniec, M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B., 102 (1998) 5503.

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Three-Dimensional Transmission Electron Microscopy of Disordered and Ordered Mesoporous Materials

K. P. de Jong^, A.H. Janssen^, P. van der Voort^ and A.J. Koster*^ ^Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands ^Laboratory of Adsorption and Catalysis, University of Antwerp, Universiteitsplein 1, B-2610 Wilrijk, Belgium '^Molecular Cell Biology, Utrecht University, Padualaan 8, 2584 CH Netherlands

Utrecht, The

The use of 3D-TEM, in particular electron tomography, for the characterisation of mesoporous materials is introduced. In 3D-TEM a tilt series of the specimen collected in bright field mode comprises typically 150 images over tilt angles ranging from - 7 0 ° to +70°. The tilt series are used to calculate a full 3D- image reconstruction of the specimen in question. The first example delt with comprises the study of zeolite Y crystals that contain mesopores. The pore shape, size and connectivity of the zeolite Y crystal is obtained with great clarity and detail. The second example involves SBA-15 materials. The curved nature of the pores in the particles of SBA-15 is clearly demonstrated from tilt series. 1.

INTRODUCTION

Transmission Electron Microscopy (TEM) is one of the most powerful techniques to characterize mesoporous materials. Typical images from MCM-41 obtained by TEM are displayed in Figure 1. The side-on view of the mesopores (left) combined with a view into the pores (middle) has led to the general belief that MCM-41 can be considered as to consist of hexagonally-packcd straight channels (right). It should be realized though that in a transition electron microscope the image obtained essentially is a 2D projection of the 3D object. Recently, in materials science great strides have been made to obtain a 3D reconstruction from 2D images obtained by TEM. For an overview of the several modes of 3D-TEM we refer to a recent paper [1]. Here we focus on a particular mode of 3D-TEM, viz. electron tomography. First wc will describe briefly the essential features of electron tomography. Second, we present a study on mesopores in zeolite Y and in SBA-15 using 3D-TEM.

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-^t in|> From 14s, together with the micelle curvature diffusion ring and the lamellar phase peak, characteristic diffraction peaks corresponding to the H(002) (d()02 = 53A), H( 101) and H(IOO) reflections of the 3D-hexagonal P63/mmc are recorded, confirming that spherical micelles are present and organise in the latter Fig. 3. Model of Pm3n thin film formation from compact structure. After 16s, the TEOS/CTAB initial sol (see Table 1 sample C). characteristic diffraction pattern of the Pm3n cubic structure begins to overlay the 3D-hexagonal and the lamellar ones, while the diffusion ring is not visible any longer. The characteristic C(211) diffraction is located at d2ii = 50 A and on the in-plane profile line, suggesting that the domains have their (211) planes parallel to the film surface. At this stage one may assume that the whole film is

283

organised in three different mono-oriented mesostructures. At 20 s and at 21 s the lamellar and the 3D-hexagonal phases respectively disappear, while the cubic structure remains the only phase present in the dry film. It is lost if the film is allowed to stay in an ethanol saturated atmosphere (within the sealed dip-coater) for more than 25 min. During this process, the location of the phases with respect to both interfaces concords with the general behaviour of surfactant in composition phase diagrams : isotropic -^ arrangement of spherical micelles -> arrangement of cylindrical micelles -^ lamellar, with increasing concentrations. A model of such film formation is draw in Figure 3. Similar experiments performed on the other systems showed that (i) the organization occurs via a disordered to ordered transition and is also governed by the presence of both substrate/film and film/air interfaces for non ionic surfactants[8], (ii) the same mesostructure as pure silica is obtained for organically modified silica despite a different phase transition sequence, (iii) a similar mechanism applies for silica and non-silica systems leading to 2D-hexagonal structure despite their difference in chemical properties. Results concerning all the studied systems are given in Table 1. Table 1 Final structures for each system with related chemical and processing parameters. LP. : Inorganic precursor; S/M, EtOH/M, H2O/M, and HVM: molar ratios of surfactant, ethanol, water and proton to metal respectively; HR: relative humidity during dip-coating; F.S.: final film structure (D: disordered, H3: P63/mmc, CI: Pm3n, H2: P6m, C2: Im3m; + for phase coexistence).* similar results for Brij or Pluronic surfactants with different S/M.** 85% TEOS + 15% (C6H5)Si(OEt)3. Sample LP. Surf S/M EtOH/M H2O/M H V M Sol age HR F.S. A TEOS CTAB 3 nm. The mean pore diameters for MCM-41/16 and MCM-41/14 are determined to 5.0 nm and 4.2 nm, respectively, which is about 2 nm larger than the results from BJH analysis of the adsorption branch of the respective nitrogen isotherm. It is, however, widely accepted that BJH analysis underestimates the pore diameter by about 1 nm in this mesopore range [7,8]. Furthermore, the inaccuracy of mercury porosimetry in the range of such narrow mesopores is probably also ca. 1 nm, so that the pore diameters calculated from both methods are comparable within experimental error. The pore diameter decreases slightly with increasing pelletizing pressure and the pore size distribution broadens. The second maximum around 0.2 ^m and 1 fim for MCM-41/14 and MCM-41/16, respectively, (Figure 4.36) is ascribed to the filling of the void spaces (macropores) between the primary particles. The study of the intergranular porosity shows that the particles are getting closer under pressure. Therefore, the size and the number of these pores decrease with increasing pelletizing pressure. For the pellets, a new maximum at ca. 100 |im is observed which represents the toroidal void space of a collection of solid particles [9]. The size of these macropores mainly depends on the pellet size and, hence, is independent of the pelletizing pressure and the catalyst under investigation.

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/

' 'V^ /' ^

•o

III! 1 1 1 I

11

g,2

/

i f\

IIIIM 1 1 T

MCM-41-52 MCM-41-130 MCM-41-260-

-^0.4 :

/

H

llllll 1 ( 1

\

\

]

100

10 1 0.1 0.01 0.00 100 10 1 0.1 0.01 0.00 Pore diameter / |jm Pore diameter / fjm Fig. 2. Mercury porosimetry analysis of MCM-41/14 (left) and MCM-41/16 (right). In Figure 3, the relative pore volumes of the compressed cubic phases MCM-48 and SBA-1 are compared. The mechanical stability of these materials differs only slightly with MCM-48 being more stable. The difference might be attributed to the different structure of the two cubic phases. For the hexagonal phases MCM-41 and SBA-1, the mechanical stability decreases with decreasing wall thickness / unit cell size (w / (w + dp) (Figure 4). Consequently, SBA-15 is mechanical less stable compared to MCM-41/16 despite its significantly thicker walls (2 nm vs. 1.1 nm). 1 .z

I

1

I

1

I

I

I

I

I

I

^o ^ 10^ ^ ' ^ ^ ^ ^ E D #^^^^^^ o 0.8 > * \ ^ ^ 0

o

-

0.6

§ ^ ^ \,^^

0)

> '% 0.4

-

-

ff

0 1

0

1

100

1

1

200

1

1

300

400

,

500

, 600

Pressure / MPa

Fig. 3. Comparison of the relative pore volumes V/Vo of pelletized (in the calcined form) SBA-1 (closed symbols) and MCM-48 (open symbols) molecular sieves calculated from the adsorption of nitrogen (o), benzene (V), cyclohexane (n) and A7-heptane (A). In comparison to MCM-41/14, which exhibits a pore volume of 0.58 cm'^ g"' (68 % of the parent material) and a BET surface of 960 m^ g"' after application of 260 MPa , the MCM-48 materials tested in our study are more stable. A significant lower stability is reported for other MCM-41 materials [1], which were probably of inferior quality compared to the materials prepared in our study. The higher stability of MCM-48 is most likely due to the threedimensional arrangement of the pore system, which also profits from a higher wall thickness as compared to MCM-41. Nevertheless, the mesoporous materials investigated in this work exhibit inferior mechanical stability compared to other adsorbents and catalysts such as

288

zeolites, silica and alumina [10,11], which may result from their large porosity and the absence of a stabilizing crystal structure.

50

100

150

200

300

Pressure / MPa Fig. 4. Comparison of the relative pore volumes V/Vo of pelletized (in the calcined form) SBA-15 and MCM-41 molecular sieves with different pore sizes calculated from the adsorption of nitrogen. 4. CONCLUSIONS Data on the mechanical strength of mesoporous materials have been obtained from nitrogen and organic vapor adsorption as well as from mercury porosimetry. For these materials, it has been shown that the loss of pore volume takes place without a significant decrease of the pore diameter. It is, therefore, surmised that a fraction of the pores is completely destroyed, while the rest of the material is unaffected by the compression. . The mechanical stability increases with decreasing pore diameter and with increasing wall thickness. Mesoporous molecular sieves were found to possess rather low mechanical stability as compared to other materials, viz. zeolites and aluminophosphates which are crystalline

REFERENCES 1. V. Y. Gusev, X. Feng, Z. Bu, G.L. Mailer and J.A. O'Brien, J. Phys. Chem., 100 (1996) 1985. 2. T. Tatsumi, K.A. Koyano, Y. Tanaka and S. Nakata, J. Porous Mater. 13 (1999) 6. 3. Desplanticr-Giscard, O. Collart, A. Galarneau, P. Van dcr Voort, F. Di Renzo and F. Fajula, Studies in Surface Science and Catalysis 129 (2000) 665. 4. M. Hartmann, and C. Bischof, J. Phys. Chem. B 103 (1999) 6230. 5. M. Hartmann, S. Racouchot, and C. Bischof, Microp. Mesop. Mater., 27 (1999) 309. 6. A. Galarneau, D. Desplantier-Giscard, F. Di Renzo and F. Fajula, Catal. Today, 68 (2001) 191. 7. A. Sayari, M. Kruk and M. Jaroniec, Catal. Lett., 49 (1997) 147. 8. P.I. Ravikovitch and A.V. Neimark, Langmuir, 16 (2000) 2419. 9. R. Mayer, and R.A. Stowe, J. Phys. Chem., 70 (1966) 3867. 10. V. Bosacek, M.M. Dubinin, O. Kadlets, K.O. Murdmaa and V. Navratil, Dokl. Chem., 174(1967)305. 11. S.J. Gregg, and J.F. Langford, J. Chem. Soc, Faraday Trans., I 73 (1977) 747.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

289

Fs-time-resolved diffuse reflectance and resonance Raman spectroscopic studies on MCM-41 as microchemical reactor Su Young Ryu and Minjoong Yoon* Department of Chemistry, Chungnam National University, Daejon, 305-764, Korea We have encapsulated porphyrin derivatives into MCM-41, TiMCM-41, and Cu°-A1MCM41, and its photoinduced electron transfer were investigated by steady state and ultrafast timeresolved spectroscopy. All spectroscopic results measured for tetraphenyl Porphine Manganese (III) Chloride (Mn(III)TPPCl) encapsulated in MCM-41 and TiMCM-41 reveal that framework modification by incorporating the Ti02 into the MCM-41 enhances the electron accepting ability of the MCM-41 framework. And also, Raman and UV-Vis spectroscopic investigations for Zn" tetraphenylporphyrin (Zn^TPP) encapsulated into MCM41 and CU"A1MCM-41 allow us to conclude that Zn"TPP in MCM-41 is oxidized to a considerable extent. Furthermore, central metal ion exchange of Zn"TPP encapsulated into CU"A1MCM-4 1 gives Cu" tetraphenylporphyrin (CU"TPP) with almost unit transformation efficiency. In conclusion, metal ion exchanged MCM-41, Cu°AlMCM-41, might be used for microchemical reactor metal-ion exchange reactions of the porphyrin derivatives. 1. INTRODUCTION Among the heterogeneous hosts, a mesoporous silica, MCM-41, whose pore size is between 2 and 10 nm, has been known to have promising usage for catalysis due to its regular hexagonal array of uniform silica tube with a narrowly distributed diameters.' These unique properties of MCM-41 might allow one to selectively ionize porphyrin macrocycles and reduce the back electron transfer rate by separating the donor and acceptor, which will eventually increase the ionization efficiency."^ If the tetrahedral Si^"* of the mesoporous silica is replaced with transition metal, it is known to be more acidic as well as more reactive with adsorbates than that of MCM-41.^ Due to the catalytic potential of transition metal ions adsorbed, the synthesis and characterization of MCM-41 modified by exchanging the metal ions is one of active research fields."* In this work, we have attempted to investigate the fast electron or charge-transfer processes between metallo-porphyrin and mesoporous silica by using the ultrafast time-resolved diffuse reflectance spectroscopy. And Zn^TPP encapsulated into CU^'A1MCM-41, was also investigated with respect to the utility of mesoporous silica as a michrochemical reactor to control the product selectivity of the central metal exchange reaction in porphyrin macrocycles. 2. EXPERIMENTAL 1.

The synthesis procedure and characterization of the mesoporous silica used in this work are already reported elsewhere.^ X-ray diffractograms were recorded by M03X-HF diffractometer. The diffuse reflectance UV-VlS spectra were recorded by using a Shimadzu

To whom correspondence should be addressed. This work was financially supported by the Korea Research Foundation and the Korea science and Engineering Foundation.

290

UV-3101PC spectrometer. The details of femtosecond diffuse reflectance spectroscopic system have been reported elsewhere.^' ^ Briefly, a light source of self-mode-rocked Ti:sapphire laser pumped by an Ar^ laser and a Tiisapphire regenerative amplifier system with Q-switched Nd:YAG laser. Resonance Raman signal upon photoexcitation at 442 and 458 nm from HeCd and Ar ion laser, respectively, was dispersed by a Raman UIOOO double monochromator and detected with cooled photomultiplier in a photon counting configuration. To determine which metals were altered by the encapsulation of Zn°TPP into Cu AIMCM41, the metal concentration in the toluene solution extracted from the used mesoporous silica was measured by inductively coupled plasma mass spectroscopy(ICP-MS). 3. RESULTS AND DISCUSSION 3.1. The photoinduced electron transfer of Mn (III) chloride tetraphenylporphine incorporated TiMCM-41 and MCM-41 The efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which completes with other reactive pathway of the generated radical ion pairs. The large pore molecular sieves like MCM-41 is used to provide an appropriate microenvironment for retarding dramatically the back electron transfer and increasing enormously the lifetime of the photogenerated ion pairs. The ground-state absorption spectra of Mn°^TPP(Cl) encapsulated into MCM-41 and TiMCM-41 show a dramatically change compare to that in benzene, i.e. i) the Q-band was blue-shifted, ii) the ratio of Soret bands to Q-bands was reduced, and iii) the Soret bands became broader. These absorption spectral changes indicate that Mn"' TPP(Cl) molecules are adsorbed well onto MCM-41 and TiMCM-41, and that the porphyrin Ji-electrons interact with the surface hydroxyl group of MCM-41 and TiMCM-41.^'' ^^^ To understand the microenvironmental effects on the photophysical dynamics of Mn TPP(Cl), we have performed the femtosecond diffuse reflectance photolysis. Figure la shows the transient absorption spectra of Mn"' TPP(Cl) in benzene at delay times 1 ps with an excitation of the soret band at 390 nm. Since Mn'" TPP(Cl) has a d"^ ground-state electron configuration, the (ji, ji*) transition states of complex are consist of a quintet singlet state ( S i ) and a "tripmultiplet" manifold (^Ti, '^Ti, ^Ti). The transient species at 450-500 nm with a decay time of approximately 6 ps in benzene is assigned that the tripquintet, Ti (;i, JT ), which is apparently formed rapidly via intersystem crossing from the lowest singquintet, Si(jr, JT ) decaying very rapidly (2-3 ps).'^''^ The transient spectra in MCM-41 and TiMCM-41 were observed greatly different with that in benzene as shown fig. 1(b) and 1(c) : In MCM-41 and TiMCM-41, its spectral feature shows the broad transient absorption not only around 450-500 nm but also in the low energy region around 550-800 nm. According to suggestion by Holtern et al.,^^' ^^ we ascribe the transient absorption around 550-800 nm to a

400

500

600

700

wavetengji/nm

450

500

550

600

650

Wavetength/nm

700

750

450

500

550

600

650

Wav^engih/nm

Fig 1. Transient absorption spectra Mn"'TPP(Cl) in benzene (a), in MCM-41(b), and in TiMCM-41(c).

^CO 750

291

quintet CT state. The temporal decay profile of the transient absorption indicates that two different transient species (c.a. 10 ps and c.a. 80 ps) were observed in MCM-41 and TiMCM41. The short-lived component should be originated from relaxation of a "tripmultiplet" state and the longer-lived component should be attributed to the spin-forbidden relaxation via a quintet CT state. And also, we found that the longer-lived component in TiMCM-41 is even more enhanced than in MCM-41, indicating that the framework of TiMCM-41 could be more easily interacted with Mn^TPP(Cl) compared with one of MCM-41, and the spin-forbidden relaxation via a quintet CT state were favorable in the order TiMCM-41> MCM-41. After irradiation, MnTPPCl^* radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays good electron acceptor. Furthermore, it has been found that the formation MnTPPCr* is easier in TiMCM-41 than in MCM-41, indicating that framework modification by incorporating the Ti"^^ into the MCM-41 enhances the electronaccepting ability of the MCM-41 framework. Therefore, those photogenerated electrons and Mn TPP(Cl) cation radical in this system could be applied to the photocatalytic reaction. 3.2. Resonance raman studies on Zn" Tetraphenylporphyrin encapsulated into MCM-41 and Cu^AlMCM-41: catalytic ionization of Zn^TPPand its central metal ion exchange We have encapsulated Zn°TPP into MCM-41 and Cu°AlMCM-41. Fig. 2(a) shows the resonance Raman spectrum of Zn"TPP encapsulated into MCM-41.The resonance Raman spectra of Zn"TPP in crystal and toluene were also displayed in Fig. 2(b) and 2(c) for comparison. The frequency and half-width of the Raman band in crystal are identical to those in toluene solution. The most evident change due to encapsulation of Zn"TPP into MCM-41 is the enhancement of u 4 mode intensity as well as the broadening of u 2 mode to the lower frequency region. It is interesting to compare Raman spectrum of Zn"TPP-MCM-41 with that of Zn"TPP radical cation electrochemically generated in solution because mesoporous silica is well known to have an oxidative catalytic properties, and the radical cation is rather stabilized to be long-lived.^' ^ The metallo-porphyrin radical cation like Zn^TPP^ and Cu"TPP^exhibit a rather strong enhancement in the intensity of the Raman modes related to phenyl substituent such as u 4 and u 1 modes as well as appreciable down Shift of u 2 modes.''' These observations were consistent well with the results of the observed Zn"TPP-MCM-41. All the

1300 1400 1500 R a m a n S h ift ( c m ' ' !

350 400 450 500 550 600 650 700 750 800 W a v e l e n g t h (nm )

Fig. 2. Resonance Raman Spectra of Zn'^P-MCM-

Fig. 3. UV-Vis absorption spectra of Zn"TPP-MCM-41

41(a), Zn"TPP crystal (b), Zn°TPP-Cu"AlMCM-

(a), ZnVPP-Cu°AlMCM^l (b), sample in toluene

41(c), Zn"TPP (d), and Cu°TPP in solution (e).

extracted &om Zn°TPP-MCM41 (c), and Zn'^PCU"A1MCM-41 (d), neat Cu°TPP in toluene (e).

292

results led us safely to propose that MCM-41 can efficiently oxidize Zn"TPP encapsulated. Fig. 3a shows absorption spectra of Zn"TPP-MCM-41, the Soret absorption band maximum of Zn"TPP was observed blue shifts accompanied with an appreciable broadening compare to that in toluene solution. In addition to this, the red shifts of Q-band with a new band at 650 nm could be also observed. The characteristic features in the electronic spectra of porphyrin cation radical compared to those of neutral porphyrins are a diminished intensity, the broadening of the Soret band and the appearances of new visible bands at 600-700nm.^^ Therefore, The broadening and blue shifts observed in the Soret band and the additional band at 650 run in Q-band can be explained in terms of the catalytic oxidation of Zn°TPP into MCM-41. Raman and UV-Vis spectroscopic investigations allows us to conclude that Zn TPP in MCM-41 is oxidized to a considerable extent. Tabel 1 Resonance Raman Frequencies (cm"^) for Zn"TPP(crystal), Zn"TPP in solution, Zn"TPPMCM-41, Zn"TPP-Cu'^AlMCM-41, and CU"TPP in solution. Zn"TPPMClVI-41

Zn"TPP-

Zn"TPP in solution

Cu"TPP in solution

CU"AIMCM-41

(a)

Zn"TPP (crystal) (b)

(c)

(d)

(e)

1234

1234

1236

1234

1238

1292

-

-

-

-

1351

1352

1368

1348

1366

1414 1467

-

-

1492

1490

-

1490

-

Vll

1545

1545

1561

1548

1563

V2

1593

1592

1601

1597

1601

4

assignment

Vi

" V4

V2«

The encapsulation of Zn"TPP into CU"A1MCM-41 exhibits a broad Soret band at 410 nm (Figure 3 (b)), which is lower wavelength compared to that of Zn"TPP-MCM-41. The Q-band of Zn"TPP-Cu"AlMCM-41 also exhibits the spectral features quite different from that of Zn"TPP-MCM-41. The resonance Raman spectrum in CU"A1MCM-41 is shown in Figure 2 (c). Of very interest it is to note that o 2 and u 4 modes is up-shifted more than 10 cm ' compared to those for Zn"TPP in crystal and toluene. It should be also noticed that the spectrum is quite different from that of Zn"TPP-MCM-41 (See Table 1). Surprisingly, both UV-Vis absorption and Raman spectral feature of Zn"TPP-Cu"AlMCM-41 were almost identical to the previous reported spectra of Cu" TPP in solution.^^ To obtain further spectroscopic data on this peculiar system, UV-Vis spectrum of the extracts with toluene from the solid Zn"TPP-Cu"AlMCM-41 is shown in Figure 3 (d). For comparison. Figure 3 (c) shows the electronic absorption spectrum observed from the toluene extracts from Zn"TPPMCM-41. Of quite interest is to note that the two spectra are different from each other even though the same porphyrin, Zn"TPP is initially encapsulated. To gain better understandings of the interesting central metal ion exchange occurred in mesoporous silica, we have measured UV-Vis as well as Raman spectra on the extracted solutions from both Zn"TPP-Cu"AlMCM41 and Zn"TPP-MCM-41. Zn"TPP is recovered without any noticeable changes in both UV-

293

Vis and Raman spectrum after encapsulation into MCM-41. However, the extracted solutions from Zn"TPP-Cu ' A I M C M - 4 1 shows that the spectroscopic observations are identical to those from C U " T P P dissolved in toluene. From the ICP-MS studies the relative ratio between Cu" to Zn" quantity in mole fraction is found to be (32.5±1.0) (not shown). Central metal ion exchange of Zn^TPP encapsulated into Cu°AlMCM-41 gives Cu ° tetraphenylporphyrin with almost unit transformation efficiency. All the experimental results in addition to the above considerations led us to safely suppose that the mobile Cu" ions in CU"A1MCM-41 might replace Zn" ions from the porphyrin macrocycles, if the resultant C U " T P P is stable in the mesoporous silica (see Scheme 1).

ClPTPP Scheme 1. Microreactor controlled metal-ion exchanged reactions of porphyrin derivatives 4. CONCLUSION Mesoporous MCM-41 and TiMCM-41 molecular sieves are found to be promising hosts for photoinduced charge separation of adsorbed Mn"'TPP(Cl). In MCM-41 or TiMCM-41, Mn'" TPPCl *' radicals are generated by irradiation, indicating that the mesoporous silicate framework plays good electron acceptor. The Mn'" TPPCl *' generation increases in the order MCM-41 < TiMCM-41, indicating that framework modification by incorporating the Ti^^ into the MCM-41 enhances the electron-accepting ability of the MCM-41 frame work. Also the metal ion exchanged MCM-41 and Cu°AlMCM-41 might be used for microchemical reactor controlled metal-ion exchange reactions of the porphyrin derivatives.

REFERENCES P Selvam, S.K. Bhatia, C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Chem. B 101 (1997) 10455 J. Xu, J.-S. Yu, S.J. Lee, B.Y. Kim, L. Kevan, J. Phys. Chem. B 104 (2000) 1307. S. Sinlapadech, R.M. Krishna, Z. Luan, L. Kevna, J. Phys. Chem. B 105 (2001) 4350. Y. Kim, J.R. Choi, M. Yoon, A. Furube, T. Asahi, H. Masuhara, J. Phys. Chem. B 105 (2001)8513. 6. A. Furube, T. Asahi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem B 103 (1999) 3120-3127. T. Asahi, A. Furube, H. Fukimura, M. Ichikawa, H. Masuhara, Rev. Sci. Instrum. 69 (1998)361. H. M. Sung-Suh, Z. Luan, L. Kevan, J. Phys. Cheln. B 101 (1997) 10455-10463. R A. Leennker, H. T. Thomas, L. D. Weis, R C. James, J. Am. Chem Soc. 88 (1966) 5075.

1. 2. 3. 4. 5.

294

10. A. Ron, M. Folman, 0. Schnepp, J. Phys. Chem 36 (1962) 2449. 11. L. Mochida, K. Tsuji, H. Fujitsu, K. Takeshida, J. Am Chem.Soc. 84 (1980) 3159. 12. M. Goutennan, In The Polphyrins, Dolphin. D., Ed., Academic: New York. Vol.3 (1978) 1-165. 13. M. P. Irvine, R. J. Harrison, M. A. Stahand, G. S. Beddard, Bunsen-Ges. Ber, Phys. Chem 89 (1985) 226-232. 14. X. Yan, C. Kinnaier, D. Holten, Inorg. Chem. 25 (1986) 4774-4777. 15. D. Holten, M. Gouterman, In Optical Properties and Strucfure of Tetrapyrroles, Blauer, G. Sund, H., Eds., de Gruyter: Berlin. (1985) 64. 16. L. Pekkarinen, H. Linschitz, J. Am. Chem Soc. 82 (1960) 2407-241150 17. R.S. Czernuszewicz, K.A. Macor, X.-Y. Li, J.R. Kincaid, T.G. Spiro, J. Am. Chem. Soc. I l l (1989) 3860. 18. R.H. Felton, D. Dolphin, The Porphyrins, Academic Press, New York, 5 (1978) Chapter 3.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

295

Detailed investigation of the microporous character of mesoporous silicas as revealed by small-angle scattering techniques B. Smarsly"', K. Yu^ and C. J. Brinker''^ ^University of New Mexico, Center for Microengineered Materials, Advanced Materials Laboratory, 1001 University Blvd., SE, Albuquerque, NM 87106, USA ^Sandia National Laboratories, MS 1349, Albuquerque, NM, 87185, USA 1. INTRODUCTION Mesoporous

silica bulk materials, prepared

by templating with

amphiphilic

block

copolymers, have recently been shown to contain a substantial degree of additional micropores, located in the silica walls. Significant experimental evidence from sorption and SAXS/SANS (Small-Angle X-ray/Neutron Scattering) suggest that these micropores originate from the hydrophilic poly(ethylene oxide) (PEO) chains being tightly embedded in the matrix and creating voids of similar dimension after template removal [1-2]. This microporosity was found in bulk silicas obtained from several polymer templates such as PEO-PEO-PEO (SBA15) and poly(styrene)-/?-poly(ethylene oxides) (PS-PEO). While the degree of microporosity seems to be tunable by a variation of the preparation conditions, the determination of micropores in the presence of mesopores still is an experimental challenge. In particular, standard evaluations of microporosity based on nitrogen sorption measurements have to be regarded as inappropriate for these materials. T-plot and as-plot analyses do not have the required accuracy because of the lack of suitable reference materials. It was recently demonstrated that suitable SAXS/SANS techniques provide a practical and

accurate

determination of microporosity, and even the micropore size could be determined. Thus, this analytical tool allows studying the influence of different preparation methods on the microporosity. In this study, mesoporous silica films were prepared by using PS-PEO polymers as structure directing agents and applying the pathway of evaporation-induced self'correspondence author: [email protected] This work was supported by Sandia National Laboratories, a Lockheed Martin Company, under Department of Energy Contract DE-AC04-94AL85000, the Air Force Office of Scientific Research Award Number F49620-01-1-0168, the DOE Office of Basic Energy Sciences, the DOD MURI Program Contract 318651, and Sandia's Laboratory Directed Research and Development Program.

296

assembly. Starting from a diluted solution of the polymers in THF/H2O, containing TEOS as precursor, this preparation method is characterized by a very low polymer concentration at the beginning of the evaporation, which is different from the procedures for powder materials. The objective of this study is two-fold. Firstly, it was attempted to produce well-defined mesostructured porous silica films, which were characterized by TEM and SAXS in grazing incidence geometry (GISAXS). Secondly, detailed GISAXS and sorption studies were carried out in order to check for microporosity due to PEO chains. The GISAXS data will be compared with corresponding analyses on powder materials obtained also from PS-PEO templates. 2. EXPERIMENTAL In a typical synthesis, PS(22)-Z7-PEO(70) diblock copolymer, which has 22 styrene units and 70 ethylene oxide units, was dissolved in tetrahydrofiiran (THF) at 1 wt.%. Afterwards, a certain amount of tetraethoxysilane (TEOS), hydrogen chloride (HCl), as well as water (Milli Q) were added to the dilute copolymer solution in THF [3-4]. The quantity of the silica precursor, namely MTES added was such as to achieve a weight ratio of ca. 1 copolymer : 7 precursor. The total amount of HCl and water added was such as to achieve molar ratios of 1 TEOS : 0.004 HCl : 5 H2O. After 30 minutes of sonication, one drop of the solution was cast to obtain a silica/diblock thin film on a silicon wafer. Calcination in Argon (with a heating rate of l°C/min to 400 °C for 3 hours) removed the diblock, as confirmed by thermogravimetric analysis, and produced mesostructured porous silica films. The GISAXS measurements were performed directly on cast thin films, using the 5-meter pinhole instrument in the SAXS laboratory at the Center of Micro-Engineered Materials at the University of New Mexico, with an available range of s values between 0.08 - 1.4 nm', where .v = 2sin(0)/A. and 20 is the scattering angle and X the CuKa wavelength (0.154 nm). 3. RESULTS AND DISCUSSION Fig. 1 shows a TEM (Transmission Electron Mikrograph) picture of a mesoporous silica film, obtained by using a PS(22)-Z7-PEO(70) block polymer. Tilting experiments indicate a cubic lattice. The structure corresponds to a bcc or fee structure, but a differentation between them was not possible based on TEM. It is seen that the pore size is about 5nm assuming spherical mesopores, but the determination of mesopore sizes from TEM involves certain inaccuracies. It could be shown by thermogravimetric analysis that by the calcinations step almost all of the polymer is removed at 400 degrees Celsius. Interestingly, these materials showed almost no absorption in nitrogen sorption measurements, indicating that the mesopores are isolated voids.

297

Fig. 1. TEM of a mesoporous silica film obtained from PS(22)-Z7-PEO(70) polymer. The scale bar corresponds to 50nm. A and B correspond to different tilting angles, the difference is 15 degrees.

/^. 1000 J

y

[200]

100 J CO

10

[110]

[110]

14 0.07 0.10

0.33

0.67 1.00 -ii

scattering vector s [nm" ] Fig. 2. A. GISAXS data of a mesoporous silica film, templated with PS(22)-6-PEO(70) (curve 1, crosses), mesoporous silica film without any template (curve 2, open circles), SAXS data of mesoporous SEIOIO silica powder according to ref [1] (curve 3, crosses), and Porod's law (s"^). B. 2D GISAXS pattern corresponding to A, curve 1. The peak indexing is based on a bcc cubic structure. The two non-indexed peaks are due to the primary reflected beam.

298

Fig. 2 shows SAXS and GISAXS data from silica films and powder materials. Curve 1 corresponds to a mesoporous silica film prepared by using a PS(22)-Z?-PEO(70) block copolymer template described by the procedure above. The curve was obtained by averaging the 2D GISAXS data in Fig. 2B in the region of the [110] reflections, if the GISAXS pattern is interpreted in terms of a cubic bcc structure. Curve 2 represents the GISAXS data of a silica film prepared under identical conditions as the sample belonging to curve 1, without using a template. The non-templated film was studied to study the microstructure of the silica matrix: if micropores were present, this should be apparent at larger scattering vectors. Since the two GISAXS data sets were obtained under comparable conditions (angle of incidence, etc), the relative scattering intensities can be directly compared. It is seen that curves 1 and 2 are almost identical in shape and intensity at scattering vectors beyond ca. s = 0.3 nm'. This identity at larger s already suggests the absence of additional micropores due to PEO. This assumption is further support by comparing the GISAXS data of the thin films with SAXS data obtained on mesoporous silica powders, which were also obtained from PS-PEO templates (Fig. 2A, curve 3). This material was obtained from a PS(9)-b-PS-23) polymer and shows several reflections in SAXS patterns, resulting from ordered mesopores. These materials were shown to contain a significant degree of microporosity. It was concluded that SAXS curves at larger scattering vectors s significantly differ from a theoretical mesostructure without any microporosity. The microporosity gets apparent by a significant deviation from Porod's law (I(s) a s'^) at larger s. A comparison with the GISAXS patterns in Fig. 2A obtained from the silica films therefore furthermore indicates the absence of additional microporores, which were shown to have sizes of about 0.5-1.5nm. Based on the microporosity in mesoporous silica porous, determined in our previous publications [1-2], the upper limit for the micropore volume can be estimated to be about 0.01 ml/g. The reason for the absence for microporosity in the mesoporous silica films may be related to the significantly different preparation conditions, compared to mesoporous bulk silicas: the self-assembly is carried out over a much longer period of time (about 1 day) and in THF, which is a good solvent for PS. Our results suggest that during the solvent evaporation the polymer continuously gets more insoluble, while the silica framework is still fragile enough to rearrange. Hence, the PEO chains may retract from the matrix as a result of the solvent removal. REFERENCES 1. C. G. Goltner, B. Smarsly, B. Berton, M. Antonietti, Chem. Mater., 13 (2001) 1617. 2. B. Smarsly, C. Goltner, M. Antonietti, W. Ruland, E. Hoinkis, J. Phys. Chem. B, 10 (2001)831. 3. K. Yu, A. J. Hurd, C. J. Brinker, A. Eisenberg, Langmuir, 17 (2001) 7961. 4. B. Smarsly, K. Yu, C. J. Brinker, Mat. Res. Soc. Symp. Proc, SI.9 (2002) 728.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

299

X-ray diffraction analysis of mesostructured materials by continuous density function technique L.A. Solovyov^ O.V. Belousov^ A.N. Shmakov\ V.I. Zaikovskii^ S.H. Joo", R. Ryoo', E. Haddad^ A. Gedeon'^ and S.D. Kirik' ^Institute of Chemistry and Chemical Engineering, K.Marx av., 42, Krasnoyarsk 660049, Russia. E-mail: [email protected] ^Boreskov Institute of Catalysis, Novosibirsk 630090, Russia "^Department of Chemistry (School of Molecular Science-BK21) Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea "^Pierre and Marie Curie University, Paris 75252, France A continuous density function technique has been developed for X-ray diffraction (XRD) structural investigations of mesostructured materials. The technique is designed for the analysis of the density distribution in materials exhibiting nanoscale (2-50 nm) ordering of structural elements without atomic long-range order. The results of structure investigations of a series of silicate, metallosilicate and carbon mesostructured materials with hexagonal and cubic symmetry are presented. 1. INTRODUCTION The X-ray powder diffraction is one of the main methods applied to the characterization of mesostructured materials. However, apart form a number studies published, this technique is mostly used for the qualitative ascription of materials to known structural types and for determining the lattice dimensions. The detailed analysis of mesostructures basing on the diffraction peak intensities still presents a challenge due to the severe disordering of these substances at atomic level. Recently, a novel XRD structure analysis method has been proposed and applied in the structural investigations of a series of two-dimensionally ordered mesoporous materials [1, 2]. Here we report further development of this technique for threedimensionally ordered mesostructures and present the results of its application to the structure analysis of different materials, including MCM-41, SBA-15 (2-D hexagonal p6mm), MCM48 (3-D cubic Ia3d) and carbon molecular sieves. 2. EXPERIMENTAL X-ray powder diffraction data were collected on a laboratory diffractometer DRON-4 (CuKa radiation) and on a high-resolution diffractometer located in Siberian Center of Synchrotron Radiation. The scheme of the continuous density function technique used for structural investigations is described as follows. The averaged density distribution in the material is modeled by

300

flexible analytical function p with adjustable parameters. The intensities of the diffraction reflections are calculated by the Fourier-transform of the model function. The adjustable parameters of yoare refined by minimizing the difference between calculated and experimental powder diffraction profiles using the Rietveld technique [3]. The structure characteristics are determined from the refined model parameters. Additional unknown structure details can be revealed from the density distribution maps calculated by the inverse Fourier-transform based on the reflection intensities extracted from the powder diffraction profile and the initial phases derived from the refined model function. The two-dimensional hexagonal mesostructures were modeled by the density distribution function proposed in [1]. To simulate the density distribution in cubic MCM-48 material and its carbon derivative CMK-1 [4] we designed a density function [5] based on the nodal approximation of the "Gyroid" periodic minimal surface [6]. The structural disorder was allowed for by the Debye-Waller factor. 3. RESULTS AND DISCUSSION The application of the developed approach to the structure investigations of a series of mesoporous silicate, alumosilicate, titanosilicate and carbon mesostructured materials allowed their comprehensive structural characterization. In Fig. 1 some results of structure modeling of three samples of MCM-41 silicate mesoporous material are presented. Sample 1 was synthesized at room temperature from an aqueous mixture of cetyltrimethylammonium bromide (CTAB), Ethanol, NH3 and Tetraethoxysilane (TEOS). Sample 2 was obtained by heating sample 1 in the mother liquor for 2 hours at 383 K, and sample 3 was a calcined form of sample 2. The structure characteristics obtained for samples 1, 2, 3 are: unit cell - 4.37(1), 4.36(1), 4.26(1) nm; pore diameter - 3.67(4), 3.32(4), 3.32(4) nm; wall thickness - 0.70(4), 1.04(4), 0.94(4) nm; pore hexagonality - 67, 54, 37 %. As seen, the material formed at room temperature has the least wall thickness and the most hexagonal pores. After the thermal

3 5 20 (degree)

Fig. 1. Experimental (circles) and calculated (solid line) XRD powder patterns of three MCM-41 materials studied. Respective density distribution maps are shown.

1 2 20 (degree)

Fig. 2. Experimental and calculated XRD powder patterns of SBA-15 aluminosilicate at two stages of hydrothermal treatment and respective density distribution maps.

301

treatment in the mother Uquor the wall thickened without unit cell expansion. The room temperature sample 1 was found to be unstable after calcination and boiling in water, but sample 3 was stable in boiling water and strong hydrochloric acid. We believe that this improvement of the material stability was due to the silica polycondensation and wall thickening without unit cell expansion. The surfactant distribution in the mesopores was found to be not uniform with distinct minimum in the pore centers. This feature was also observed in our previous studies [1, 2] for as-made silicate and metal-silicate MCM-41 materials obtained from different media. In Fig. 2 the results of XRD structural studies of SBA-15 aluminosilicate mesoporous materials obtained at two stages of hydrothermal treatment are presented. The materials were synthesized in the presence of Pluronic 123 surfactant via hydrothermal treatment at 373 K for 16 (sample 1) and 48 (sample 2) hours. Both materials exhibited cylindrical mesopores of nearly the same diameter 9.85(5) nm and their unit cells were determined to be 13.39(3) and 13.66(2) nm respectively. The density distribution maps demonstrate the elution of surfactant from mesopores during the hydrothermal treatment. Our structural studies of different silicate and metal-silicate SBA-15 materials show that their pores are, basically, of less hexagonality than those of MCM-41 and the surfactant within the pores of SBA-15 is distributed more uniformly. The structural studies of MCM-48 mesoporous material were carried out on samples synthesized by the well known procedure [7] from an aqueous mixture of CTAB, NaOH and TEOS at 383 K. The samples were highly ordered, exhibiting XRD reflections with d-spacing up to 1 nm. The results for the as-made form of the material are illustrated by Fig. 3. The refined structure model provided nearly perfect XRD profile fitting. The as-made and calcined materials were found to have the same wall thickness of 0.83(5) nm and unit cells of 9.57(1) and 9.00(1) nm respectively. The analysis of the Fourier density maps revealed that the distribution of surfactant in the pores of MCM-48 was not uniform (Fig. 3) with a central

1 3

5 7 20 (degree)

Fig. 3. Experimental (circles) and calculated (solid line) XRD powder patterns of as-made MCM-48. Sections (100) and (211) of the 3-D density distribution map are shown.

2 3 20 (degree)

1 2 20 (degree)

Fig. 4. XRD powder patterns and density distributions for CMK-1 (left) and CMK-3 (right) carbon molecular sieves.

302

minimum similar to that observed for MCM-41. This feature may be ascribed to the strong interaction between the cationic surfactant and inorganic wall in MCM-type materials. The technique was also applied in structural investigations of mesostructured carbon molecular sieves CMK-1 and CMK-3 [4] obtained by replication of the mesoporous silicas MCM-48 and SBA-15 with carbon. The results are illustrated by Fig. 4. The structure of CMK-1 was characterized as an ordered interwoven assembly of two enantiomeric carbon sub-frameworks reproducing the shape of the MCM-48 mesopores. It was shown that the subframeworks are displaced with respect to one another to form contacts without significant distortions. The structure model was explicitly confirmed by the transmission electron microscopy. The effective thickness of the frameworks (2.9 nm) and their mutual displacement distance (1.4 nm) were determined by the XRD structure model refinement [5]. The structure of CMK-3 material was described as fairly long-range ordered hexagonal arrangement of cylindrical carbon nanorods with average diameter of 7.1 nm [8]. The results were consistent with both TEM and adsorption data. The density distribution Fourier-map for this material displayed bridges of non-zero density between the nanorods (Fig. 4), which could be attributed to the carbon matter providing the mesostructure connectivity derived from former complementary porosity of the SBA-15 template. 4. CONCLUSIONS The developed continuous density function technique is demonstrated to be a powerful tool for comprehensive and precise XRD structure analysis of mesostructured materials, providing important information on the mesostructure characteristics. The technique is flexible and can be applied to substances with different anatomy and composition. ACKNOWLEDGEMENTS This work is supported by the INTAS Fellowship grant for Young Scientists YSF 2001/2-3, INTAS grant 01-2283 and joint grant KRSF-RFBR 02-03-97704. REFERENCES 1. L.A. Solovyov, S.D. Kirik, A.N. Shmakov, and V.N. Romannikov, Microporous and Mesoporous Mater., 44-45 (2001) 17. 2. L.A. Solovyov, V.B. Fenelonov, A.Y. Derevyankin, A.N. Shmakov, E. Haddad, A. Gedcon, S.D Kirik and V.N. Romannikov, Stud. Surf Sci. Catal., 135 (2001) 287. 3. H.M. Rietveld, J. Appl. Cryst., 2 (1969) 65. 4. R. Ryoo, S.H. Joo, M. Kruk and M. Jaroniec, Adv. Mater., 13 (2001) 677. 5. L.A. Solovyov, V.I. Zaikovskii, A.N. Shmakov, O.V. Belousov and R. Ryoo, J. Phys. Chem. B, submitted. 6. P.J.F. Gandy, S. Bardham, A.M. Mackay, J. Klinowski, Chem. Phys. Lett., 333 (2001) 427. 7. A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, B.F. Chmelka, Science, 261 (1993) 1299. 8. L.A. Solovyov, A.N. Shmakov, V.I. Zaikovskii, S.H. Joo and R. Ryoo, Carbon, in press.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

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Influence of aluminium, lanthanum and cerium on the thermal and hydrothermal stability of MCM-41-Type silicates Martin Wallau, Rogerio A. A. Melo* and Ernesto A. Urquieta-Gonzalez^' * Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos - SP, Brasil MCM-41 were prepared in the presence of aluminium (Al-MCM-41), lanthanum (LaMCM-41) or cerium (Ce-MCM-41) and lanthanum and aluminium (La/Al-MCM-41). Aluminium in Al- and La/Al-MCM-41 and lanthanum in La-MCM-41 were incorporated. Otherwise cerium in Ce- and lanthanum in Al/La-MCM-41 was found as extra-framework species. With respect to Si-MCM-41, cerium decreases the thermal and increases the hydrothermal stability, while Al-MCM-41 show similar stability. Extraframework lanthanum below 5 % (w/w) in Al/La-MCM-41 enhances its stability but decreases it above this content. 1. INTRODUCTION MCM-41 is widely studied as adsorbent, catalyst and catalyst support [ 1 ]. In these applications thermal and hydrothermal stability are crucial. However, the reported data arc relatively lacking for MCM-41 synthesised with hetero-elements other than aluminium. Although some works report that lanthanum enhances the MCM-41 stability [2], little work was published concerning the effect of cerium on MCM-41 stability. Here we describe the preparation of MCM-41 with aluminium and/or lanthanum or cerium and discuss the influence of these metals on the thermal and hydrothermal stability of MCM-41. 2. EXPERIMENTAL MCM-41 samples were prepared at 373 K using the gel compositions given in Table 1. The solids, calcined at 823 K, were characterised by elemental analysis before and after ion exchange, which was developed with NH4CI (1 mol/L) for Al-MCM-41 or with HCl (0.1 mol/L) for Ce-, La-, and La/Al-MCM-41. To verify their stability, the calcined samples before ion exchange, were treated thermally and hydrothermally. In the first case in dry air at 1153 K for 2 hours and in the second in a water saturated nitrogen flow at 933 K for 1 hour. All samples were characterised before and after specified treatments by XRD and nitrogen sorption (BET). One Al-MCM-41 (sample III) was additionally characterised by "'^Si and " Al MAS NMR, while Ce-MCM-41 was supplementary examined by ESR spectroscopy.

* Present adress: Centro Universitario do Sul de Minas, Faculdade de Engenharia Quimica, Av. Gel. Jose Alvcs. 37010-540 Varginha - MG, Brasil ^ Corresponding author: FAX: +55-16-260-8266. E-mail: [email protected] * This work was financially supported by CNPq (461444/00-3; 300373/01-5), FAPEMIG (TEEC - 1241/01).

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Table 1 Elemental composition of the synthesis gels and of the calcined MCM-41 samples before and after ion exchange. Gel* Calcined Solid XAI2O3 yMzOs before ion exchange after ion exchange 0 (I) 0 0 (II) 0.017 Nao.027 [ S io.960AI0.040O2] Nao.023[Sio.958Alo.04202] 0 (III) 0.05 Nao.035[Sio.923Alo.07702] Nao.o79[Sio.9ooAlo.ioo02] 0.017^ (IV) 0 Naz[Sio.986Lao.oi402] Naz[Sio.985Lao.oi502] 0.017* NazCeo.oo9[Si02] (V) 0 NazCeo.oo7[Si02] 0.005^ (VI) 0.05 Nao.o6Lao.ooi5[Sio.9Alo.i02] Nao.o3Lao.ooo7[Sio.9Alo. 102] 0.018^ (VII) 0.05 Nao.o5Lao.oi3[Sio.92Alo.o802] Nao.02Lao.001 [Sio.93 AI0.07O2] 0.035^ (VIII) 0.05 Nao.o4Lao.o2o[Sio.94Alo.o602l Nao.03Lao.004iSio.95Alo.05O2] *Si02 : 0.07 NazO : 0.03 CTMA^ 0.14 TMA^ : X AI2O3: y M2O3: 100 H2O •••M = L a ; * M = Ce.; CTMA^ = cetyltrimethylammounium ; TMA^ = tetramethylammonium. 3. RESULTS AND DISCUSSION The idealised sum formulae of the calcined solids before and after ion exchange are given in Table 1 in the form NawM'x[SiyM"z02] where M ' means extraframework cations (/. e. Ld^^ and Ce"^^) and M " framework incorporated cations (/. e. A\^^ and La^^). It can be seen that the aluminium content of the samples II, III and VI do not differ significantly from that of the synthesis gel, while the lanthanum and cerium content in the samples IV-VIIl are much lower. This indicates that aluminium is easier incorporated into the wall structure than lanthanum and cerium. However, the presence of lanthanum seems to hamper the aluminium incorporation in sample VII and VIII. However, this cannot be taken as an indication for competitive framework incorporation of lanthanum, because ion exchange decreases the lanthanum content dramatically (see Table 1), and therefore lanthanum is probably present in extra-framework sites in the La/Al-silicates (VI-VIII). On the other hand, in these samples (VI-VIII), as well as in the Al-MCM-41 (II, III), the aluminium content is not affected by ionexchange, this indicating its framework incorporation. In sample III aluminium in framework position is confirmed by the ^^Si - and ^^Al MAS NMR spectra shown in Fig. 1. -106 -102 -

\A

-110

b

,

S3

1

-1

-91

1

1

^ 1

1

-100

-120

ppm (TMS)

1^**^""^^ 11

100 90

80

70

\-*L

i

1

60

50

I—k—k

1

40

30

20

10

"T'^^T"^

-10 -20 -30 -40

ppm (AI(H20)J

Fig. 1. MAS NMR spectra of calcined sample (III): (a) ""Si; (b) '^Al. While the peaks at -110, -102 and the shoulder at -91 ppm are usually attributed to (-0-)4Si (Q4), (-0-)3SiOH (Q3) and (-0-)2Si(OH)2 (Q2) units [3], respectively, the peak at -106 ppm can be attributed to (-0-)3Si-0-Al units [3]. The presence of Al in tetrahedral coordinated framework positions is confirmed by the peak at 53 ppm in the ^^Al MAS NMR. As it was shown by Janicke et al. [ 4 ] , aluminium atoms incorporated into the MCM-41

305

framework can be co-ordinated to two other water molecules resulting in a peak at around 0 ppm. Therefore, the peak at - 1 ppm due to octahedral co-ordinated aluminium, can not be taken as a conclusive proof for the presence of extra-framework aluminium. The lanthanum content in La-MCM-41 (sample IV) remains unchanged after ion exchange, which might indicate the incorporation of lanthanum into the framework. On the other hand in La/Al-MCM-41, where the lanthanum content decreases after ion exchange, the lanthanum is probably be present as extra-framework species. It can be calculated from Table 1, that for the calcined La/Al-MCM-41 before ion exchange, the ratio (Na^ + 3 La^^)/Al is 0.65, 1.11, and 1.67 for sample VI, VII and VIII, respectively. This show, that in sample VI and VII, the positive charges of the sodium and lanthanum cations did not exceed significantly the negative framework charge, expected for the complete incorporation of all aluminium atoms. Otherwise, for sample VIII, the number of cations is much higher than the number of the expected negative framework charges. This suggest the presence of lanthanum cations located on ion exchange sites in sample VI and VII, while sample VIII will also contain neutral lanthanum species, probably as La203. From the absence of any signal in the ESR spectrum (not shown) of Ce-MCM-41, we conclude that Ce"^^ was oxidised during the synthesis to Ce"^^, which might form insoluble Ce02. Therefore, the nearly unchanged cerium content after acid treatment, do not allow to conclude that cerium was incorporated into the MCM-41 framework. Only around 20 % of the cerium used in the synthesis gel is present in the obtained solid, indicating that its incorporation is hampered or impossible. 1200 The XRD patterns reveal the presence of before thermal a hexagonal mesostructure by a broad peak ^ 1000 treatment around 2 °(20) (dioo) and additional, sometimes overlapped peaks (duo, d2oo), ^ 800i between 4 and 5 °(20). As a typical after thermal 600 example, the XRD patterns of Si-MCM-41 treatment (Sample I) before and after thermal ^ 400 treatment are shown in Figure 2. After the thermal treatment at 1153 K the intensity 200H of the dioo peak is decreased to 73 %. Decreased intensity of the X-ray 0 reflections is often taken as an indication 0 8 10 2e' of structural degradation [2]. However, it Fig. 2. XRD patterns of Sample 1. was pointed out, that for mesostructured materials an intensity loss of the XRD peaks cannot necessarily be interpreted as a loss of "crystallinity" [5]. Therefore we used the relative change of the specific surface area as a measure of the thermal and hydrothermal stability of studied MCM-41 type materials. The results shown in Figure 3 indicate that the thermal and hydrothermal stability of Al-MCM-41 (Sample II, III) is in the same order or slightly better than that of Si-MCM-41 (Sample 1). A higher stabilisation of the mesoporous structure is observed for La-MCM-41 (Sample IV) and for La/Al-MCM-41 with lanthanum contents below 5 % (w/w) /. e. Sample VI and VII. On the other hand, while the mesostructure of Ce-MCM-41 (sample V) is completely destroyed after thermal treatment at 1153 K, it shows a slightly better hydrothermal stability than SiMCM-41. The presence of lanthanum amounts higher than 5 % (w/w) strongly decreases the thermal and hydrothermal stability of La/Al-MCM-41 (Sample VIII). While our observation confirms the higher stability of La-MCM-41 claimed by He et al. [2], the observed thermal

306

and hydrothermal behaviour of Al-MCM-41 (sample II and III) is in contrast with the results of these authors, who observed lower stability for Al-MCM-41. As it can be seen from Table 2, the Al-MCM-41 studied here possess thicker walls (13 to 30%) than the sample studied by He et al. [2], which might have caused the enhance in the stability. The wall thickness of LaMCM-41 is only slightly enhanced, and its higher stability might be due to lanthanum incorporation. As it was discussed above, Sample V and Sample Fig. 3. Relative specific surface area (BET) of the calcined VIII might contain the Me-MCM-41 ( • ) and after thermal ( • ) and hydrothermal strong Ce02 and La203 (CZD treatment (sample identification as in Table 1). bases, respectively, and as known, even weak bases Table 2 in the mesopores of MCM Wall thickness (s) of MCM-41 samples -41 leads to structure MCM Si Al* Al^ Al^ La^ La** degradation [6] by hydroThis work 1.50 1.75 2.04 1.62 lysis of the T-0- T bonds. 1.41 Ref[2] 1.45 1.49 T bonds. The elemental Si/M = *29; '^24; ^9; §66; **42. analysis of a La/Al-MCM41 studied by He et al. [2] with ca. 4 % (w/w) of lanthanum also suggest the presence of La203, which would explain the low observed stability. Then, the enhanced stability of La/AlMCM-41 containing less than 5 weight % (Sample VI and VII), might be attributed to the presence of lanthanum cations located on ion exchange sites, which can react with the framework structure: (a) La^^ + Si-(0- )-Al + H2O -^ La(OH)^^ + Si-(OH)-Al; (b) La^^ + 2 SiOH -> La(H20)^^ + Si-O-Si, thus preventing hydrolysis of the T-O-T bonds. The stabilising effect of cerium in the hydrothermal treatment of Ce-MCM-41 (Sample V) is still unclear. A possible explanation might be an enhancement of the hydrophobicity as it was observed for Ce-X [7]. REFERENCES 1. F. Schuth, Stud. Surf Sci. Catal., 135 (2001) 1. 2. N.-Y. He, S.-L. Bao, Q.H. Xu, Stud. Surf Sci. Catal., 105 (1997) 85. 3. Z. Luan, C.-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 4. M.T. Janicke, C.C. Landry, S.C. Christiansen, D. Kumar, G.D. Stucky, B.F. Chmelka, J. Am. Chem. Soc, 120 (1998) 694. 5. B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Microporous Mater., 6 (1996) 375. 6. C.N. Perez, E. Moreno, CA. Henriques, S. Valange, Z. Gabelica, J.L.F. Monteiro, Microporous Mesoporous Mater., 41 (2000) 137. 7. Q.J. Chen, T. Ito, J. Fraissard, Zeolites 11 (1991) 239.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Enhanced acidity and hydrothermal stability of mesoporous aluminosilicate with secondary building units characteristic of zeolite Beta Wanping Guo^'^, Lingdong Kong^, Chang-Sik Ha^ and Quanzhi Li^* ^Department of Chemistry, Fudan University, Shanghai 200433, P. R. China ''Department of Polymer Science and Engineering, Pusan National University, Pusan 609-735, Korea Ordered hexagonal mesoporous aluminosilicate (SBU-MCM-41) with secondary building units characteristic of zeolite Beta has been characterized and compared with the corresponding MCM-41 by means of XRD, SEM, N2 adsorption, IR spectra of pyridine, catalytic cracking activity and hydrothermal treatment. The results indicate that the introduction of secondary building units characteristic of zeolite Beta into the mesopore wall is an effective route to improve the acidity and hydrothermal stability of mesoporous material. 1. INTRODUCTION The development of mesoporous material MCM-41, which possesses highly regular arrays of uniform-sized pore channels ranging from 1.5 to 10 nm in size, large surface area and good thermal stability, has inspired a great deal of interest in processing large molecules [1,2]. The acid strength of mesoporous aluminosilicate is, however, weaker than that of traditional zeolites owing to the amorphous character of mesopore wall, which limits potential applications in petroleum industry [1]. In order to improve the framework property of mesoporous material, it is practicable to introduce zeolite-like structural order into the mesopore wall. Up to now, there have been only a few articles involving this issue [3-7]. Our recent study [8] reported that a series of ordered hexagonal mesoporous molecular sieves (SBU-MCM-41) with various Si02/Al203 ratios, of which the mesopore wall was constructed by a lot of secondary building units characteristic of zeolite Beta, were prepared through pretreating the aluminosilicate gel in the absence of alkali metal cations. Here we present the characterization of the SBU-MCM-41 mesoporous aluminosilicate compared with the corresponding mesoporous material in terms of structure, acidity, catalytic performance and hydrothermal stability.

308

2. EXPERIMENTAL The preparation of the SBU-MCM-41 mesoporous aluminosilicate was carried out under hydrothermal condition avoiding the presence of alkali metal cations according to the procedure described elsewhere [8]. As a reference, a corresponding MCM-41 mesoporous material was prepared using the same procedure as the SBU-MCM-41 except pretreatment process. 3. RESULTS AND DISCUSSION The XRD patterns of calcined SBU-MCM-41 and the corresponding MCM-41 are shown in Fig. 1. The typical hexagonal lattice for the SBU-MCM-41 and the corresponding MCM-41 can be verified by the observation of a strong peak (100) at very low angle although the (110) and (200) peaks for both samples are ill-defined and overlap to give a single broad peak. After calcination, there is shrinkage of 7.0 % (from 3.98 nm to 3.70 nm) in dioo spacing of the SBU-MCM-41 while the dioo spacing of the corresponding MCM-41 shrinks by 11 % (from 3.77 nm to 3.35 nm). Fig. 2 displays scanning electron micrographs of the SBU-MCM-41 and the corresponding MCM-41. It is found that the SBU-MCM-41 appears in spindle "crystal-like" morphology and the corresponding MCM-41 shows typical loose aggregates. From the N2 adsorption-desorption isotherms and the BJH pore size distribution plots for the SBU-MCM-41 and the corresponding MCM-41, it can be seen that both samples exhibit a well-expressed hysteresis loop at relative pressure P/PQ of 0.2-0.4, characteristic of framework-confined mcsoporcs [7]. The pore structure data of the SBU-MCM-41 and the corresponding MCM-41 are presented in Table 1. It can be seen that the BET surface area and Table 1 Pore structure data of the SBU-MCM-41 and the corresponding MCM-41 Sample BET surface Pore volume Pore diameter ao (nm) area (m^ g"') (cm^ g'') D (nm) SBU-MCM-41 942 1.54 2.75 4.27 Corresponding MCM-41 1178 1.48 2.72 3.87

t(nm) 1.52 1.15

pore volume of the SBU-MCM-41 are as high as 942 m^ g'^ and 1.54 cm^ g\ respectively. It should be noted that the pore wall thickness (1.52 nm) of the SBU-MCM-41 is much higher than that (1.15 nm) of the corresponding MCM-41. The IR spectra of pyridine adsorbed on protonated samples in the region 1600-1400 cm' show that both Bronsted and Lewis acid sites of the SBU-MCM-41 are much higher than those of the corresponding MCM-41 at different desorption temperatures. A good correlation can be observed between the cumene cracking activity and the number of acid sites (see Table 2). Therefore, the SBU-MCM-41 with much more Bronsted and Lewis acid sites contributed by many secondary building units characteristic of zeolite Beta in the mesopore wall exhibits much higher catalytic activity for cumene cracking than the corresponding MCM-41. The test of hydrothermal stability of calcined samples refluxed in water at 373 K shows That

309

1.5 2

4

_

,

6

10

2 tneta

Fig. 1. Powder XRD patterns of calcined samples (a) the SBU-MCM-41 and (b) the corresponding MCM-41.

a

•' "' •

'

-

^

' •"•'' " ^

l i — ' — '

•'""

^

' • ' ' ^ ' • -

'

Fig. 2. SEM images of (a) the SBU-MCM-41 and (b) the corresponding MCM-41.

Table 2 Acidity and catalytic activity of the SBU-MCM-41 and the corresponding MCM-41 Sample SBU-MCM-41 Corresponding MCM-41

Acidity (x lO'^g') Bronsted 393K 453K 513K 3.11 2.59 1.76 2.13 1.65 0.89

Cumene conversion (%) Lewis 393K 453K 7.29 5.05 5.88 3.56

513K 2.94 2.13

523K 15.4 7.3

573K 36.1 20.4

623K 60.4 30.8

673K 89.9 53.4

310

the hexagonal pattern of the corresponding MCM-41 almost disappears after hydrothermal treatment for 16 h. By contrast, hydrothermal treatment for the same time has little effect on the structural integrity of the SBU-MCM-41. Moreover, the SBU-MCM-41 refluxed for 48 h is still well ordered. 4. CONCLUSIONS The ordered SBU-MCM-41 mesoporous aluminosilicate has been investigated and compared with the corresponding MCM-41 for their structure, acidity, catalytic performance and hydrothermal stability. The much enhanced acidity and hydrothermal stability of the SBU-MCM-41 can be attributed to the presence of zeolite-like structural order (secondary building units characteristic of zeolite Beta) in the framework wall. ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (Grant No. 29733070) and financial support from the Center of Integrated Molecular Systems, POSTECH, Korea and the Brain Korea 21 Project is gratefully acknowledged. REFERENCES 1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. J.Y. Ying, C.P Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 3. Y. Liu, W. Zhang and T.J. Pinnavaia, J. Am. Chem. Soc, 122 (2000) 8791. 4. Y Liu, W. Zhang and T.J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 5. Z. Zhang, Y Han, F.S. Xiao, S. Qiu, L. Zhu, R. Wang, Y Yu, Z. Zhang, B. Zou, Y Wang, H. Sun, D. Zhao and Y Wei, J. Am. Chem. Soc, 123 (2001) 5014. 6. Z. Zhang, Y Han, L. Zhu, R. Wang, Y Yu, S. Qiu, D. Zhao and F.S. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 7. W. Guo, L. Huang, P. Deng, Z. Xue and Q. Li, Microporous Mesoporous Mater., 44-45 (2001)427. 8. W. Guo, L. Kong and Q. Li, Stud. Surf Sci. Catal., in press.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Chemical coating of the aluminum oxides onto mesoporous silicas by a one-pot grafting method Yi-Hsin Liu^ Hong-Ping Lin^* Chung-Yuan Mou^'', Bo-Wen Cheng"^ and Chi-Feng Cheng"^ ^Department of Chemistry National Taiwan University, Taipei 106, Taiwan. ^Department of Chemistry, National Cheng Kung University, Tainan 701, Taiwan. '^Center of Condensed Matter Science, National Taiwan University, Taipei 106, Taiwan. ^Department of Chemistry, Chung-Yuan Christian University, Chung- Li, Taiwan. Using a suitable solvent (such as, 1-propanol) that prevents the self-condensation of aluminum /^o-propoxide and extract the quaternary ammonium surfactant out of the nanochannels, a high-coverage aluminum oxide (Al/Si = 0.3-0.5) can be chemically coated onto acid-made mesoporous silica. The aluminum oxide coated mesoporous silica has the both advantages of aluminum oxide (highly hydrothermal stability) and mesoporous materials (large pore size of 2.7 nm, high surface area of around 800 mVg, and well-ordered mesostructure). With the well-dispersity, high accessibility and la»*ge pore size, the aluminum oxide coated mesoporous silica was supposed to be an acid-catalyst for large molecules. 1. INTRODUCTION Mesoporous silica with the advantages of high surface area (1000 m^/g), tuneable pore size (1.0-30.0 nm) have reasonably been considered as a superior support that can promote well-dispersed and high accessibility of active sites for catalytic applications [1]. Typically, the metal oxides-grafting on calcined mesoporous silica must be done in an extremely dry condition to avoid the fast hydrolysis and self-condensation of metal alkoxides [2]. Basically, one should think the metal alkoxides grafting as a simple chemical surface modification of mesoporous silica via covalent bonding of metal alkoxide and surface silanol groups [3]. By using this idea, it is possible that a convenient method for high grafting of metal oxides onto the mesoporous silica could be achieved by choosing a proper solvent, and precursor of metal oxide via judicious synthesis processes. Here, we provided a convenient one-pot strategy for aluminum /^o-propoxide grafting onto the acid-made mesoporous silica in a low-toxicity 1-propanol solution at low reaction temperature of 80 °C [4]. The high-coverage of aluminum oxide makes the aluminum oxide-coated mesoporous silica highly hydrothermal stable, and demonstrates a high catalytic activity of cumene cracking reaction. 2. SYNTHESIS AND METHOD 2.1. One-Pot grafting of aluminum oxide on acid-made mesoporous silica The acid-made quaternary ammonium surfactant micelle-templated silicas were prepared by typical method reported in the previous literatures,[5] and the detailed procedures and

312

chemical compositions were demonstrated elsewhere. The one-pot process of aluminum /i-o-propoxide grafting is described as followed: 1.0 g dried acid-made mesoporous silica was added into 40-100 g 1-propanol (99.5%, Acros) solution containing the suitable amount of aluminum /5o-propoxide (Al(/-OC3H7)3, Acros, 98%). Then, that solution was re fluxed at 80 ^C for 24-60 hr. Filtration, washing, and drying recovered the aluminum wo-propoxide grafted mesoporous materials. Finally, the aluminum oxide coated mesoporous silicas products were obtained after calcined at 560 °C to remove the unreacted /5o-propoxide. 2.2. Measurement The powder x-ray diffraction patterns (XRD) were collected on Scintag XI diffractometer using Cu Kct radiation (X = 0.154 nm). The mesostructural observations of mesoporous silica were recorded on Hitachi S 7100 or Philips CM 200 transmission electron microscope (TEM) with an operating voltage of 100 and 200 keV, respectively. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus, and the pore size distribution was calculated from the adsorption isotherms using the Barrett-Joyner-Halenda (BJH) method. 2.3. Cumene cracking reaction The cumene cracking reaction was performed in a continuous-flow fixed bed system at different temperature from 250 to 400 °C. The reactant, cumene (saturated vapor pressure at 0 ^C), was continuously mixed into the nitrogen carrier gas stream of flow rate = 20 ml/min, and the catalyst weight is 0.05 g. The products were analyzed on-line by a Shimadzu GC-7A gas-chromatograph. 3. RESULTS AND DISCUSSION Fig. lA shows the XRD patterns of the Al(/-OC3H7)3 grafted mesoporous silica. The Al(/-OC3H7)3 grafted mesoporous sample has the well-ordered hexagonal mesostructure as the original acid-made mesoporous silica. Combining the analysis of N2 adsorption-desorption isotherm (Figure 2), the uncalcined aluminum oxide grafted mesoporous silica possesses a sharp capillary condensation at P/Po « 0.38 and high porosity (~ 0.9 ml/g) as that of the surfactant-free mesoporous materials. The result indicates that the quaternary surfactants (S^X) in nanochannels of acid-made mesoporous silicas are almost completely taken out during the chemical grafting of Al(/-OC3H7)3. To confirm the successful grafting of Al(/-OC3H7)3, we performed a Induced Couple Plasma-Atomic Emission Spectrometer to analyze the Al content in the Al(/-OC3H7)3 grafted mesoporous silica, and the Si/Al ratio is around 2.3. Consequently, this grafting reaction could be regarded as a energy-favored interaction transformation from weak hydrogen-bonding of silica and surfactant to chemical bond of silanol and Al(/-OC3H7)3. Moreover, the avoidance of high-temperature calcination, the high silanol group density can be remained to promote the high loading of metal-oxides. Therefore, this one-pot grafting method provides a new and convenient way to chemically grafting the well-dispersed aluminum oxide onto the acid-made mesoporous silica. For converting the Al(/-OC3H7)3 grafted mesoporous silica to the aluminum oxide coated mesoporous silica, a high-temperature was directly used. One can see that the XRD pattern, pore size and porosity are preserved as the uncalcined one (Figure 1).

313

P/P„

Fig. 1. (A) XRD patterns and (B) N2-adsoption-desorption isotherms of aluminum wo-propoxide grafted and aluminum oxide coated mesoporous silica samples. (I). Aluminum /50-propoxide grafted mesoporous silica. (II). Aluminum oxide coated mesoporous silica. (Ill) Aluminum oxide coated mesoporous silica after a hydrothermal reaction at 100 °C for 20 hr. With the examination of the high-angle XRD pattern and TEM micrographs, no diffraction peaks and images characteristic of nano-sized AI2O3 clusters as byproduct were found in the aluminum oxide coated mesoporous silica. Owing to high content of aluminum oxides (Si/Al ratio = 1.8-2.5) and uniform pore size, it was reasonably supposed that the aluminum oxide was homogeneously coated on the silica nanochannels. It should be mentioned that the aluminum oxide coated mesoporous silica demonstrate the thicker wall thickness (~ 2.5 nm) than the un-grafted one(~ 2.0 nm), and the difference between these values is about 0.5 nm. Because the aluminum oxide layer behaves like a protecting film, the aluminum oxide coated mesoporous silica shows highly hydrothermal stability,[6] which can stand for at least 20 hr in boiling water without the apparent mesostructural damage (Fig. lA). In contrast, the mesostructures of the calcined acid-made mesoporous completely collapsed after 6 hr. To investigate the effects of Al(iOPr)3 concentration and reflux time on the physical properties and Al content, we adjusted both factors to get a proper reaction condition with relatively low Al(iOPr)3 concentration and short reflux time. In Table 1, one can find that the high aluminum content can be almost achieved by using a low Al(iOPr)3 concentration of around 50 mM and refluxing for 24 h. High Al(iOPr)3 concentration can not increase the Al Table 1 Physical properties of aluminum oxide coated mesoporous silicas samples prepared from different Al(iOPr)3 concentration and reflux time. Reflux time Si/Al" [Al(iOPr)3] dioo Dwall' ^porc /m'/g Ik /mM /hr Ik Ik 48.4 43.3 157.0 417.0

19 60 19 19

898 844 877 854

42.4 43.3 42.8 42.9

24.8 24.2 23.8 23.8

24.2 25.8 25.6 25.7

2.38 2.10 2.07 1.97

a. Dwall = 2d 100/ 3 - Rporc. b. The Si/Al ratio was obtained from Induced Couple Plasma-Atomic Emission Spectrometer.

314

content, but may induce the formation of AI2O3 nano-clusters out of the nanochannels. The aluminum oxide coated mesoporous siHca was recommended as one kind of acid-catalyst. [7] Thus, the catalytic activity of aluminum oxide coated mesoporous silica toward cumene cracking has been investigated at different temperature. The high conversion (78 %) at 400 °C showed the high accessibility and well-dispersity of the active sites. In contrast, the cumene conversion of Al Time (/lO min) incorporated MCM-41 (Si/Al = 20) is only 20 Fig. 3. A plot of cumene conversion vs. %. As the decreasing reaction temperature, the time at 250-400 °C. catalytic activity decreases. However, the activity completely recovered upon another temperature-raising process. The high activity can maintain for longer than 24 h. 4. CONCLUSION The one-pot grafting method is convenient and efficient to coat aluminum oxide to mesoporous silica. To extend this concept, other metal oxides also can be coated to mesoporous silicas by choosing suitable precursors and solvent. Combining with the acid-treatment process,[8] coating of metal oxides on the alkaline-made MCM-41 and MCM-48 mesoporous silicas could be achieved. In brief, the desired surface property and activity of the mesoporous silica based catalysts possibly will be obtained to fit the applications by using well-designed post-treated processes. ACKNOWLEDGEMENT This research was financially supported by National Science Council of Taiwan (NSC-90-2113-M-002-038). REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. A. Sayari and S. Hamoudi, Chem. Mater., 13 (2001) 3151. 3. R. Ryoo, S. Jun, J. M. Kim and M. J. Kim, Chem. Comm., (1997) 2225. 4. H. P. Lin, L. Y. Yang, C. Y. Mou, S. B. Liu and H. K. Lee, New, J. Chem., 24 (2000) 253. 5. H. R Lin, C. R Kao, C. Y. Mou and S. B. Liu, J. Phys. Chem. B, 104 (2000) 7885. 6. R. Mokaya, Chem. Comm., (2001) 633. 7. Y Han, R S. Xiao, S. Wu, Y Sun, X. Meng, D. Li and S. Lin, J. Phys. Chem. B, 105 (2001) 7963. 8. H. P. Lin, P. C. Shih, Y. H. Liu and C. Y. Mou, Chem. Lett., (2002) 566.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

315

Acidity and temperature effect on the synthesis of SBA-1 Ming-Chang Liu," Hwo-Shuenn Sheu'' and Soofin Cheng*" ""Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. FAX: +886-2-2363-6359. '^Research Division, Synchrotron Radiation Research Center, Hsinchu 300, Taiwan The formation of cubic phase SBA-1 is strongly influenced by the synthesis condition. In-situ XRD experiments showed that SBA-1 crystallized more quickly in higher acidic conditions, and no phase transformation was observed in the synthesis gel when varying either the reaction period or the acidity (HCl: TEOS molar ratio = 2~5). Moreover, the cubic phase was less stable when the solid was separated from the mother liquid. A phase transformation from cubic to hexagonal was observed on the precipitates formed in short crystallization period when they were dried at room temperature. To the best of our knowledge, it was the first time in the literature to report that the phase transformation was correlated to the removing of the solvent from the precipitate. The stability of SBA-1 crystal structure was greatly improved by post-treatment of the wet products with ammonia solution. 1. INTRODUCTION Since the discovery of the M41S family of mesoporous molecular sieves, a great effort has been focused on their possible applications as catalysts and adsorbents. Cubic phase SBA-1 (Pm3n) possesses uniformly sized pore structure and open windows [1-2]. It is considered to be suitable for catalytic reactions because of its high surface area and three-dimensional inter-connected pores. However, the material is suffered from its relatively low stability. Recently, Che et al. [3] reported that a phase transformation from hexagonal to cubic occurred during the synthesis ofSlBA-l and a co-solvent 1,3,5-trimethylbenzene could slow down the transformation. Phase transformation was also reported on other meso-structurc materials [4-7]. These transformations were all occurred in hydrothermal condition at reaction temperatures greater than 100"C. This subject is of great interest and importance for the purpose of synthesizing the desired meso-structure material. However, the mechanism of surfactant-templated phase transformation reaction was not well understood yet. In the present study, the synthesis condition of SBA-1 and the factors affecting the phase transformation were examined by ex- and in-situ x-ray diffraction techniques. The stability of SBA-1 was also improved by a post-treatment. 2. EXPERIMENTAL SBA-1 was synthesized following the procedures in ref 2. The molar composition of the synthesis gel was TEGS/ CTEABr/ HCl/ H20= 1/ 0.13/ x/ 125, where x= 2-5. The reaction temperature was maintained at ca. 0-2°C, if not specified. In ex-situ study, powder XRD

316

patterns were recorded using a Scintag XI instrument with a Cu Ka radiation. In-situ XRD studies of the synthesis gels were conducted at the Synchrotron Radiation Research Center, Hsinchu, Taiwan. The patterns were recorded in the transmission mode with X= 1.32633nm radiation (1.85 GeV and 200 mA). 3. RESULTS AND DISCUSSION SB A-1 was synthesized under strong acidic condition. The effect of acidity was examined by varying the amount of hydrochloric acid used. Ex-situ powder XRD was utilized to determine the crystalline structure of the precipitates dried at 100°C overnight. For precipitates synthesized in higher acidic conditions (x^ 5), only cubic phase was observed. However, for those formed in lower acidic conditions (x= 2- 4), the hexagonal phase was

LA t

-(k)

(I) (k) (J)

ML

(i)

-(h)

(h)

-(g) -(0 -(e) -(d)

(g)

(0 (c) (d)

-(c)

(c)

-(b)

(b)

-(a)

2TH

Fig. 1. In-situ XRD patterns of SBA-1 gel (x-3 at 0"C) (a) O.lh, (b) 0.3h, (c) 0.6h, (d) 0.9h, (e) 1.4h, (f) 1.6h, (g) 1.9h, (h) 2.2h, (i) 2.4h, Q) 2.6h, (k) 4.8h

(a)

2TH

Fig. 2. In-situ XRD patterns of SBA-1 (x=3, crystallized for 2.8h) precipitate exposed to air at room temperature for (a) 5m, (b) 30m, (c) l.lh, (d) 1.5h, (e) 1.6h, (f) 1.68h, (g) 1.77h, (h) 1.85h, (i) 1.92h, (j)2.0h, (k)3.5h,(I) I2h.

observed for the samples crystallized for short period time. As the reaction prolonged, samples of cubic phase were obtained. These results deduced that increasing the acidity of the synthesis gel would accelerate the appearance of cubic phase and the resultant crystal structure would be more stable. In other words, strong acidity was favorable for the formation of cubic crystalline phase. These results are consistent with those reported in the literature [3,8]. The in-situ XRD experiments of the synthesis gel showed that only cubic phase was formed in the gel when the HCl concentration was varied from x= 2-5. The results were contradictory

317

to those observed on the dried samples. The crystallinity of the gels increased with the aging period, as shown in Fig. 1. No hexagonal phase was observed even the synthesis gel was in low acidic conditions. On the other hand, the HCl concentration would affect the rate of crystallization. For synthesis gel with HCl content x^ 2, 3, 4, and 5, it took 7 h, 1.4h, 0.7h, and 0.6h, respectively, for the resolved cubic phase to appear. The reaction temperature was also examined with in-situ experiment. For the gel with HCl content x= 3 condition, cubic phase was the only phase observed when the synthesis gel was heated from 0 to 80"C, and the crystallinity increased with the gel temperature. In another experiment, the wet precipitate crystallized for short period of time was sealed in between two tapes. The cubic phase of SBA-1 was retained when heating the precipitate from room temperature to lOO^C. All these results demonstrate that acidity and temperature are not the factors for phase transformation. Fig. 2 shows that a phase transformation from cubic to hexagonal occurred when the wet precipitate crystallized for short period of time was left in open-air environment. The phase transformation apparently has to do with the solvent evaporation. In other words, the cubic SBA-1 phase, which is stable in the presence of solvent, transforms to hexagonal phase when it is dried up. However, for the precipitate crystallized for long period of time, the cubic phase was retained even the precipitate is dry. The phenomenon of phase transformation at room temperature has never been reported on mcsoporous silica. The solvent evaporation would lead to the surfactant and the acid being concentrated. The spherical micelles probably transform to rod-shape micelles when the concentration of surfactant increases. That accounts for the phase transformation observed in this system. ^\Si MAS NMR studies confirmed that

(b)

20

AH

60

80

100

IM

1«

Iffl

-180

-140

-160

-180

Q3/"i

(a)

I ', Q4

2TH

(ppm)

Fig. 3. Ex-situ XRD patterns of SBA-1 (x=5) samples: (a) as-synthesis, (b) post-treatment with 0.1 M NH3 for 2 days at lOO^'C, (c) hydrothermal treatment of sample (b) with water at lOOTfor 1 day.

Fig. 4. ''^Si MAS solid state NMR spectra of SBA-1 (x=5) samples: (a) as-synthesis, (b) post-treatment with IM NH3 at lOOT for 2days.

318 Q4 peak (-llOppm) increased significantly after solvent evaporated. This implied that silica condensation continued when the precipitate left the mother solution. The thermal stability of the SBA-1 products could be improved by hydrothermal treatment of the wet samples with O.IM NH3 at lOO^'C for 2 days (Fig. 3). After NH3 treatment, the XRD intensity of the sample was retained while the diffraction peaks shifted slightly toward lower angles. This result implies that the crystal lattice is expanded slightly during the ammonia treatment. In the hydrothermal stability test, O.lg of the samples in powder form were suspended in lOg of water and boiled at 100"C for 1 day. The crystalline structure of SBA-1 collapsed no matter the experiment was started with wet as-synthesized samples or the calcined ones. In contrast, the crystalline structure retained for the NH3 post-treated samples. The effect of post-treatment was examined by taking the ^'^Si MAS NMR spectra on the SBA-1 samples before and after ammonia treatment. Fig. 4 shows that the Q2peak at -91 ppm disappeared and the intensity of the Q4peak at -110 ppm increased after NH3 post-treatment. This implies that NH3 post-treatment should make the silica condensation more complete. The resultant silica framework of relatively low defects was more tolerable to hydrothermal treatment. REFERENCES 1. Q. Huo,D.l. Margolcsc and G.D. Stucky, Chcm. Mater. 8 (1996) 1147. 2. S. Che, Y. Sakamoto, H. Yoshitakc,0. Tcrasaki and T. Tatsumi, J. Phys. Chcm. B 105 (2001) 10565. 3. S. Che, S. Kamiya, O. Tcrasaki and T. Tatsumi, J. Am. Chcm. Soc. 123 (2001) 12089. 4. J. Xu, Z. Luan, H. He, W. Zhou and L. Kcvan, Chem. Mater. 10 (1998) 3690. 5. C.F. Cheng, D.H. Park and J. J. Klinowski, Chem. Soc, Faraday Trans. 93 (1997) 193. 6. M.T. Anderson, J.E. Martin, J. G Odinck, and P.P. Newcomer, Chcm.Matcr. 10 (1998) 311. 7. A.F. Adam, E.J. Ruiz and S.H. Tolbcrt, J. Phys. Chem. B 104 (2000) 5448. 8. M.J. Kim and R. Ryoo, Chcm. Mater. 11 (1999) 487.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

319

HMS materials with high Al loading: a joint FT-IR and microcalorimetric study of their acidic/basic properties B. Bonelli,^ B. Onida,^ B. Fubini,^ J.D. Chen,' A. Galameau,' F. Di Renzo' and E. Garrone^ ^Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, Torino, 1-10129, Italy ^Dipartimento di Chimica Inorganica, Fisica e dei Materiali Universita degli Studi di Torino, V. Pietro Giuria 7,1-10125, Torino, Italy '^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, ENSCM, 104, rue de la Galera - 34097 Montpellier, France HMS materials with Si/Al ratio = 2.5 have been synthesised and characterized. "^^Al NMR spectra showed minimal occurrence of octahedral phase after template removal. FT-IR and microcalorimetric results showed that, though having the same chemical composition of Y zeolites, such materials exhibit drastically different acidic/basic properties. 1. INTRODUCTION Since their discovery in 1992, ordered mesoporous materials have attracted much interest because of their high surface area and uniform distribution of mesopores diameters.' HMS (hexagonal mesoporous silicate) materials are usually synthesised by means of a "neutral route", in which the template is a primary amine. They may be prepared with different Al contents: as the Si/Al ratio decreases, however, the fraction of Al not incorporated in the lattice increases and only few papers deal with micelle-templated silicates with a low Si/Al ratio, with Al sitting stably in tetrahedral coordination.^ This work reports on the synthesis and the characterization of three HMS samples with Si/Al ratio = 2.5, either in the H form or after partial exchange with Li^ or Na^ cations. 2. EXPERIMENTAL The parent H sample was synthesised in ethanol (95%, Carlo Erba) at neutral pH with dodecylamine, TEGS and aluminium isopropoxide in molar ratios 1.0/2.5/1.0, respectively. Li- and Na-exchanged samples were obtained by contacting H sample with LiCl and NaCl alcoholic solutions, and finally washing with alcohol/water 95% mixture, in order to remove most of the template. Remaining template was removed by successive thermal treatments. Exchange was not complete, as the final ratios were Li/Al = 0.43 and Na/Al = 0.82. The parent H sample, containing a large template amount, was calcined for 8 hours in dry-air fiow at 723 K (temperature ramp = 5°/min). After calcination, ^^Al NMR only featured the peak of tetrahedral Al, with minimal contribution of the octahedral phase and ^Si NMR indicated a

320

thorough dispersion of Al. To allow FT-IR measurements, powders were pressed into thin, self-supported wafers and out-gassed at 573 and 873 K in an IR cell equipped with KBr windows. The acidic/basic properties of the specimens have been characterized by means adsorption of i) CO at 77 K; ii) propene and CO2 at RT. On samples Li and Na outgassed at 873 K, adsorption of CO2 was followed also by means of a Tian-Calvet microcalorimeter (Setaram) operated at 303 K. 3. RESULTS AND DISCUSSION 3.1. Hydroxyls population of samples outgassed at 573 and 873 K Figure 1 compares normalised spectra in the region of 0-H stretch of the three samples outgassed at 573 K (section a) and 873 K (section b). The intensity of the absorptions shows that, after thermal treatment and exposure to air, re-hydration takes place to a large extent, especially on H sample (section a). The prominent peak at 3742 cm' is due to free silanols and the broad absorption at lower frequencies to hydroxyls interacting via H-bonding. On Li and Na samples, such absorption is markedly less intense: exchange with alkali-cations brought about a change in surface properties, in that Li and Na samples show a different pattern of re-hydration. After treatment at 873 K (Figure lb), a sharp peak, due to OH stretch of isolated silanols, is seen at 3748 - 46 H 0.6 J cm'. On H sample, such peak is tailed on the low frequencies side (arrow), showing that several types of hydroxyls 8 with different acidic strength are C Li expected, whereas after ionic exchange the more acidic species have probably exchanged a proton for Li^ or Na' cations. The disappearance of the tailing when passing from H sample to ^"

2

4 6 2 e (deg.)

Fig. 1. XRD pattern of mesoporous niobium oxide with (a) and without Ca^"^ addition (b).

Fig. 2. N2 gas adsorption-desorption isotherm of mesoporous niobium oxide with (a) and without Ca^^ addition (b).

326

a lOnm?,'/;

.i10nml^'

V ^ rf y

*

I

'>:

t #

10nm

Fig. 3. TEM image of 3D hexagonal mesoporous niobium oxide collected from different zone axes. The electron diffractions are shown in inset of each image: [0001] zone axis (a), [1213] zone axis (b), and [0111] zone axis (c).

REFERENCES 1. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater., 11 (1999)2813. 2. T. Sun and J. Y. Ying, Nature, 389 (1997) 704. 3. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc, 120 (1998)6024. 4. P. T. Tanev and T. J. Pinnavaia, Science, 267 (1995) 865. 5. S. A. Bagshaw, Chem. Commun., (1999) 1785. 6. W. Zhang, B. Glomski, T. R. Pauly and T. J. Pinnavaia, Chem. Commun., (1999) 1803. 7. Q. Huo, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268 (1995) 1324.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

Synthesis of zirconium-containing mesoporous silica membranes with high alkaline resistance for nanofiltration

327

Zr-MCM-48

Dong-Huy Park, Hens Saputra, Norikazu Nishiyama, Yasuyuki Egashira and Korekazu Ueyama Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531, Japan Mesoporous silica containing zirconium (Zr-MCM-48) membranes were synthesized on a porous alumina support. The Zr-MCM-48 membranes showed high stability in the alkaline solution of pH 12, indicating that only 3 % of Zr effectively enhanced the alkaline resistance. The permeation of gases through the calcined Zr-MCM-48 membrane was governed by the Knudsen diffusion mechanism. There was no contribution of viscous flow, which occurs in large pinholes. The result of permporometry measurements suggested the narrow pore size distribution of the membrane. 1. INTRODUCTION Mesoporous silica MCM-48 is an attractive material for many possible applications such as catalysis, sensors and membrane separations because MCM-48 has a three-dimensionally accessible pore structure. We have synthesized mesoporous silica MCM-48 membranes for pervaporation and nanofiltration [1-3]. However, silicate materials dissolve in water and alkaline solutions, which decreases the possibility of practical use. Some researchers [4,5] have prepared porous glass films [4,5] and membranes [6,7] containing zirconium with high resistance against water and alkaline solutions. We have reported that the introduction of zirconium into MCM-41 and MCM-48 powders effectively enhances their stability in alkaline solutions [8]. In the present study, we synthesized zirconium-containing MCM-48 (Zr-MCM-48) membranes on a porous alumina support. Structural stability in alkaline solutions and gas permeation characteristics of Zr-MCM-48 membranes were studied. Pore size distributions of Zr-MCM-48 powders and membranes were measured using N2 adsorption and permporometry, respectively. 2. EXPERIMENTAL A Zr-MCM-48 membrane was prepared as follows. A porous a-alumina support (NGK Insulators, Ltd.) with an average pore diameter of 0.1 im was placed in a tetraethyl orthosilicate (TEOS) and zirconium propoxide (ZrPr) mixture. A solution which consists of the quaternary ammonium surfactant, Ci6H33(CH3)3NBr (C16TAB), NaOH, and deionized

328

water was added to TEOS and ZrPr containing the a-alumina support. The molar ratio of the mixtures was 0.97 TEOS: 0.03 ZrPr: 0.4 CieTAB: 0.5 NaOH: 61 H2O. After the mixture was stirred for 2 h, the mixture and support were transferred to an autoclave. The reaction was carried out at 423 K for 24 h. The product was calcined at 773 K for 7 h. The product was identified by X-ray diffraction (XRD). The alkaline resistance of the Zr-MCM-48 membranes was evaluated by the XRD measurements before and after treatments in alkaline solutions with pH 10 ~ 12 at 303 K for 3 h. Gas permeation measurements using an as-synthesized and a calcined Zr-MCM-48 membranes were carried out with N2, He and H2 gases. The pore size distributions of MCM-48 and Zr-MCM-48 powders were calculated from N2 adsorption isotherms using the BJH method. The permporometry of Zr-MCM-48 membranes was carried out by monitoring N2 flux in the presence of capillary condensation of water vapor. 3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of MCM-48 and Zr-MCM-48 membranes before and after the treatment in the alkaline solution of pH 10-12. The peaks of the MCM-48 membrane disappeared after the alkaline treatment (Fig. 1(a)). This means that the structure of MCM-48 membrane cannot be maintained in the alkaline solution of pH 10. Fig. 1(b) shows that the structure of the Zr-MCM-48 membrane was maintained even after the alkaline treatment. The improvement of alkaline resistance seems to be caused by the strong Si-O-Zr network near the surface of the pore wall.

1 2

3 4 5 6 2 Theta [degree]

2

3 4 5 6 7 2 Theta [degree]

8

Fig. 1. X R D patterns of (a) M C M - 4 8 and (b) Z r - M C M - 4 8 m e m b r a n e s before and after treatment in the alkaline solutions

In the N2 gas permeation measurement using the as-synthesized Zr-MCM-48 membrane, no gas permeation was observed. This suggests that Zr-MCM-48 membrane has no pinholes or cracks before calcination. Fig. 2 shows the permeance of gases as a function of pressure drop through the a-alumina support and the Zr-MCM-48 membrane at 295 K. The permeance of N2 through the a-alumina support was proportional to the pressure drop, showing the characteristic of viscous flow. On the other hand, the permeance of gases through the calcined

329

Zr-MCM-48 membrane was constant with pressure drop, indicating that the gas permeation is governed by the Knudsen flow. There was no contribution of viscous flow to the permeation of gases, which occurs in large pinholes.

M_

c S 0.5

ZJp [kPa]

zip [kPa]

Fig. 2. The permeance of gasses as a function of the pressure drop through (a) a-alumina support and (b) Zr-MCM-48 membrane (295 K). 0:N2, DiHj, A: He.

0.16




2 1

J

1

1

1

20

30

40

50

60

70

Benzene Vapor Pressure (torr)

20

10

20

30

40

50

60

Benzene Vapor Pressure (torr)

Fig. 1. MCM-41/48 benzene sorption isotherms 3. RESULTS AND DISCUSSION The benzene isotherms of MCM-41 and MCM-48, exhibit three unique characteristics. The first is the exceptionally high hydrocarbon sorption capacity (>50 wt.% benzene at 50 torr). The second characteristic is the sharp inflection of the isotherm indicative of capillary

70

341

condensation within uniform pores. The third feature is the position of the inflection point at relatively high partial pressure (p/po) suggesting large diameter pores. The total benzene sorption capacity and the partial pressure associated with the inflection point increases with increasing pore size. For the MCM-41 samples the total benzene capacity was 51% for the MCM-41 (CI2) sample, 62% for the MCM-41 (CI4) sample and 78% for the MCM-41 (CI6) sample. The total sorption capacity of the MCM-48 (CI6) sample (45%)) is about 30% less than that of the MCM-41 sample with a similar pore diameter. The benzene sorption rates are also 50% greater for the MCM-41 materials compared to those of the MCM-48 (see Figure 1 for data). The wall thicknesses were calculated to be approximately 8A for all three MCM-41 samples, suggesting that the thickness of the pore walls is independent of alkyl chain length. The pore wall thickness of the MCM-48 sample is estimated to be approximately lOA, or 25% larger than that observed for the MCM-41 samples. These values are in agreement with those previously reported by Chen et al. [8]. The triisopropylbenzene (TIPB) adsorption profiles are consistent with the benzene sorption characteristics. For the MCM-41 samples, the TIPB uptakes increase with increasing pore size (52% for the MCM-41 (CI2) sample, 58% for the MCM-41 (CI4) sample and 78% for the MCM-41 (CI6) sample). The MCM-48 (CI6) sample sorbed 45%, again approximately 30% less than the corresponding pore sized MCM-41 sample. The TIPB sorption rates are also about 50% greater for the MCM-41 materials compared to those of the MCM-48. The fact that the uptakes of benzene and TIPB are essentially the same suggests that the pore channels of both MCM-41 and MCM-48 are readily accessible for the molecules up to ~9 A. The kinetic diameter of benzene is 6 A and that of TIPB is 8.5 A. MCM-41 and MCM-48 demonstrate extraordinary sorption capacity for both benzene and triisopropylbenzene. For similar pore diameters, the MCM-41 samples have approximately 30% greater capacity for both hydrocarbons than MCM-48. The sorption rates are also 50% greater for the MCM-41 materials compared to those of the MCM-48. However, these rates and capacities differences are due to differences in macropore diffusion rates rather than differences in diffusion rates in the mesoporous channels.

REFERENCES 1. 2. 3. 4. 5. 6. 7.

Beck, J.S., et. al., J ACS, 114(27) (1992), 10834. Beck, J.S., et. al., Chem. Mater., 6(10) (1994), 1816. Branton, P. J., et. al., J. Chem. Soc. Faraday Trans., 90(19) (1994), 2965. Rathousky, J., et. al., J. Chem. Soc. Faraday Trans., 90(18) (1994), 2821. Thommes, M., et. Al., Studies in Surface Science and Catalysis, 135 (2001), 2893. Vartuli, J.C, et. Al., Microporous and Mesoporous Materials, 44-45 (2001), 691. Vartuli, J.C, et. al.. Zeolites and Related Microporous Materials: State of the Art 1994 (Proceedings of the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, 7/17-22/94), J. Weitkamp, H.G. Karge, H. Pfeifer, and W. Holderich, eds., Elsevier Science, 53(1994). 8. Chen, C.Y., et. al., Microporous Mater., 2 (1993), 17.

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Argon and nitrogen adsorption on ordered silicas with channel-like and cage-like mesopores: implications for characterization of porous solids Mietek Jaroniec and Michal Kruk Department of Chemistry, Kent State University, Kent, Ohio 44242, USA Argon adsorption isotherms at 77 and 87 K and nitrogen adsorption isotherms at 77 K were measured for ordered silicas, including MCM-41 with channel-like mesopores. Nitrogen adsorption isotherms at 77 K were also measured for FDU-1 silica with cage-like mesopores. The studies of MCM-41 silicas with a wide range of pore sizes strongly suggest that adsorption branches of isotherms are suitable for the calculation of pore size distributions (PSDs). Moreover, for a given porous solid, similar PSD estimates can be obtained from adsorption branches of isotherms of different gases (nitrogen and argon at 77 K), or the same gas at different temperatures (argon at 77 and 87 K). On the other hand, in cases where adsorption-desorption hysteresis is observed, desorption branches of the isotherms are usually less suitable or even not suitable at all for the PSD determination. However, the desorption branches may provide important information about size of constrictions in the porous structure, as inferred from nitrogen adsorption data for some FDU-1 silicas. 1. INTRODUCTION The discovery of ordered mesoporous materials opened a new era in the field of gas adsorption in porous media. This discovery has made it possible to experimentally verify theories and reexamine empirical knowledge about gas adsorption in porous solids. In particular, it is now possible to experimentally study gas adsorption behavior in uniform channel-like and cage-like pores of different sizes and to provide a definite answer as to how adsorption data can be used to reliably assess pore size distributions (PSDs) [1,2]. Our earlier studies [1,3] demonstrated that in the cases of nitrogen adsorption at 77 K and argon adsorption at 87 K in channel-like pores of MCM-41, the capillary condensation pressure gradually increases as the pore diameter increases, whereas the relation between the capillary evaporation pressure is much more complicated in the pressure range of adsorption-desorption hysteresis. We have proposed to use the experimental relation between the capillary condensation pressure and the pore diameter for MCM-41 in PSD calculations [1,3], and demonstrated that when this approach is adopted, very similar PSDs are assessed from nitrogen adsorption data at 77 K and argon adsorption data at 87 K. We also observed that in the case of FDU-1 silicas with large cage-like pores, the capillary evaporation relative pressure is about 0.48 and varies very little for samples synthesized under different conditions, which suggests that this pressure does not reflect the pore diameter [4]. This relative pressure value would correspond to a pore diameter of about 4 nm when one assumes the validity of the Kelvin equation for hemispherical meniscus. We have demonstrated [5] that it is possible to synthesize FDU-1 samples that exhibit pore entrance size on the borderline between micropore (width below 2 nm) and mesopore (width between 2 and 50 nm) ranges. Therefore, the relative pressure of 0.48 is related to neither the pore diameter nor the pore entrance

344

diameter. Herein, we further examine the suitability of adsorption and desorption branches of gas isotherms in the pore size analysis, with particular emphasis on argon adsorption at 77 K [6]. 2. EXPERIMENTAL MCM-41 silicas with pore diameters below 5 nm were synthesized using alkylammonium surfactants of different structures and alkyl chain length from 8 to 22 carbon atoms, as reported in [7-9], where their properties were described. MCM-41 silicas with pores above 5 nm were synthesized using the postsynthesis hydrothermal restructuring method, and their properties are described elsewhere [10]. The synthesis of FDU-1 silica was carried out as originally proposed by Zhao et al. [11] using a poly(ethylene oxide)-poly(butylene oxide)poly(ethylene oxide) triblock copolymer and the details are described in [4]. Argon adsorption isotherms at 77 and 87 K, and nitrogen adsorption isotherms at 77 K were measured on a Micromeritics ASAP 2010 volumetric adsorption analyzer [1,3,4,6]. Before the measurements, the samples were outgassed for 2 hours at 473 K in the port of the adsorption analyzer. 3. RESULTS AND DISCUSSION Argon adsorption isotherms were measured at 77 K for MCM-41 silicas with pore diameters from 2 to 6.5 nm in order to establish the experimental relation between the pore diameter and the capillary condensation/evaporation pressure for cylindrical pores [6|. fhe capillary condensation pressure tended to increase gradually as the pore diameter was increased, which allowed us to establish a relation useful in PSD calculations. To extend this relation on the pore diameters larger than 6.5 nm, argon adsorption isotherms for SBA-15 and disordered large-pore silicas were also acquired and examined [6J. It was shown that PSDs calculated for MCM-41 from adsorption branches of argon isotherms at 77 and 87 K. and nitrogen isotherms at 77 K were very similar [6]. However, argon adsorption at 77 K was found suitable to study pores of diameter below 15 nm, because there is no capillary condensation in larger pores. The desorption branches of argon isotherms at 77 K were found to provide much less information about the pore size, because the relation between the pore diameter and capillary evaporation pressure exhibited much scatter. Moreover, in many cases, the steepness of the desorption branches of isotherms did not reflect the degree of structural ordering of the materials. These observations based on argon adsorption data at 77 K are the same as those made earlier in cases of nitrogen adsorption at 77 K [1] and argon adsorption at 87 K [3], but in the case of argon adsorption at 77 K, the unsuitability of desorption branches of isotherms was particularly striking. Therefore, we recommend that PSDs be calculated from adsorption branches of gas isotherms and we discourage the use of desorption data in the pore size analysis even for materials with channel-like pores [12]. It will be discussed below that in cases where adsorption-desorption hysteresis is observed, desorption data are certainly not suitable for the determination of pore size distributions for materials with cage-like pores. Nitrogen adsorption at 77 K on FDU-1 silicas was also studied. It was found earlier that in the case of nitrogen adsorption, the analysis of the desorption branch of the isotherm may provide important insight into the pore connectivity in FDU-1 and related materials [4]. Namely, good-quality FDU-1 (pore diameter of at least 8.5 nm) exhibits capillary evaporation delayed down to the lower limit of adsorption-desorption hysteresis, which in this case

345

corresponds to a relative pressure of about 0.48. It is clear that this relative pressure does not correspond to the actual pore entrance size, because FDU-1 samples with pore entrances in the micropore range, or in the lower end of the mesopore range (below 3 nm) all exhibit very similar capillary condensation pressure, as inferred from the results reported elsewhere [5]. This result suggests that a classical picture of delayed capillary evaporation related to constrictions in the porous structure [13,14] is largely valid, but needs to be modified to reflect the behavior at the lower limit of adsorption-desorption hysteresis [13]. One can conclude that capillary evaporation from a large cage-like pore with narrow entrance is delayed either to the pressure at which the capillary evaporation in the entrance takes place or to the lower pressure limit of adsorption-desorption hysteresis, whichever pressure is higher [4]. Therefore, the examination of the delayed capillary evaporation can provide information only about pore entrances that exhibit capillary evaporation above the lower limit of adsorption-desorption hysteresis in the wider pore parts. In the case of nitrogen at 77 K. this corresponds to pore entrances of diameter above about 5 nm. FDU-1 samples subjected to extensive hydrothermal treatment at 373 K or higher temperatures can serve as examples. In their cases, the onset of capillary evaporation of nitrogen at 77 K takes place relative pressures higher than 0.48, which suggests the presence of pore connections of size larger than 5 nm between some of the large, uniform mesopores. The occurrence of such large pore entrances, which is not observed for FDU-1 samples synthesized at lower temperatures or at short heating times, can serve as an indication of structural degradation of uniform cage-like structure during extended hydrothermal treatments [4]. The information about the loss of uniformity of the pore entrance size is not easy to extract from adsorption branches of nitrogen isotherms, so the analysis of desorption branches is much more informative in this case. However, in the case of nitrogen adsorption at 77 K, this methodology is expected to be suitable for studies of pore openings of diameter above about 5 nm. It is expected that argon adsorption at 77 K may offer important advantages over nitrogen in studies of pore connectivity in cage-like structures of large pore diameter. This is because it is known for silicas with channel-like pores that the adsorption-desorption hysteresis extends to appreciably lower pore sizes in the case of argon at 77 K, when compared nitrogen adsorption at 77 K [6]. But it needs also to be kept in mind that argon adsorption at 77 K is not expected to be suitable for studies of entrances to pores of diameters much larger than 15 nm, because argon at 77 K does not exhibit capillary condensation in such large pores [6]. 4. CONCLUSIONS The studies of ordered mesoporous materials with channel-like and cage-like porous structures allowed us to conclude that adsorption branches of isotherms are suitable for the calculation of pore size distributions, whereas desorption branches are often not particularly good, or even are completely unsuitable for PSD calculations. This is to large extent because of the lack of a clear relation between the capillary evaporation pressure and the pore diameter in the adsorption-desorption hysteresis region, which is particularly striking for cage-like pores, but observed also to some extent in channel-like pores. On the other hand, the position of the capillary evaporation step of the hysteresis loop can be related to the pore entrance size, unless the step happens to be at the lower limit of adsorption-desorption hysteresis. It is suggested that pore entrance sizes above 5 nm can be elucidated from desorption data in the hysteresis region in the case of nitrogen at 77 K. The use of argon adsorption at 77 K is

346

promising for the extension of the characterization capabilities to somewhat lower pore entrance diameters. ACKNOWLEDGMENTS Professors Ryong Ryoo and Abdelhamid Sayari are gratefully acknowledged for providing MCM-41 silicas. The authors also thank Professor Jivaldo R. Matos for the synthesis of FDU1 silicas. The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research. This work was also supported by NSF Grant CHE-0093707. The authors also thank Dr. Rene Geiger from Dow Chemicals for providing the triblock copolymer suitable for the synthesis of FDU-1 silica. REFERENCES 1. 2. 3. 4. 5.

M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. P. I. Ravikovitch and A. V. Neimark, Langmuir 18 (2002) 1550. M. Kruk and M. Jaroniec, Chem. Mater., 12 (2000) 222. J. R. Matos, L. P. Mercuri, M. Kruk and M. Jaroniec, Langmuir, 18 (2002) 884. M. Kruk, V. Antochshuk, J. R. Matos, L. P. Mercuri and M. Jaroniec, J. Am. Chem. Soc. 124(2002)768. 6. M. Kruk and M. Jaroniec, J. Phys. Chem. B 106 (2002) 4732. 7. R. Ryoo, I.-S. Park, S. Jun, C. W. Lee, M. Kruk and M. Jaroniec, J. Am. Chem. Soc. 123 (2001)1650. 8. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo and C. H. Ko, J. Phys. Chem. B, 104(2000)292. 9. M. Kruk, M. Jaroniec, H. J. Shin, R. Ryoo, Y. Sakamoto and O. Terasaki, Microporous Mesoporous Mater., 48 (2001) 127. 10. A. Sayari, P. Liu, M. Kruk and M. Jaroniec, Chem. Mater., 9 (1997) 2499. 11. C. Yu, Y. Yu and D. Zhao, Chem. Commun., (2000) 575. 12. M. Kruk and M. Jaroniec, Chem. Mater. 13 (2001) 3169. 13. M. Kruk, M. Jaroniec and A. Sayari, Adsorption, 6 (2000) 47. 14. S. J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press. London, 1982.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

347

Mesopores developed in KL zeolite dealuminated with (NH4)2SiF6 solution Nongyue He^ ^, Chun Yang^, Jianxin Tang^ Hong Chen^ ^Key Laboratory of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China ^Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China. '^Department of Chemistry, Nanjing Normal University, Nanjing 210024, P. R.. China KL zeolite was dealuminated with a solution of (NH4)2SiF6 for secondary synthesis of mesoporous KL zeolite materials. In the absence of NH4AC as a buffer agent, some mesopores were generated but the distribution in pore size was broad. In the presence of NH4AC, or elongation of corrosion time, not only a higher crystallinity of samples was obtained, but also the mesopore size distribution was narrowed. Moreover, an empiric linear relationship between the 1)770 frequency and XAI was found. 1. INTRODUCTION Since a new family of mesoporous materials designated as M41S was first introduced to the scientific community by the scientists from Mobil Corporation [1,2], much attention has been paid to the synthesis, modification, characterization and application of the materials, and a great deal of work has been reviewed [3-6]. We have found that this kind of material showing very high alkylation activity and very strong photoluminescent effect [7-8]. In deed, this kind of novel material shows many very interesting new phenomena and promising application in many related fields. However, if the pore size distribution is not strictly required, the approach to develop nano-metric mesopores from conventionally used microporous zeolite molecular sieve materials by "secondary synthesis" is still a very useful tool owing to the low cost and the stability of the obtained products. We have treated KL of a one-dimensional channel structure with (NH4)2SiF6 solution and investigated the effect of corrosion conditions on the pore distribution in detail. 2. EXPERIMENTAL The treatment of KL with (NH4)2SiF6 solution was referred to that described previously [9]. X-ray powder diffraction (XRD) patterns were taken with a Rigaku D/max-yA instrument.

348

Pore size distribution was analyzed on a Micromeritics ASAP 2000 instrument at 77 K with N2 adsorption, following the Barrett-Joyner-Halenda algorithm [10]. The framework vibration infrared (IR) spectra of samples were reported for the wafers of mixture of 1% sample in KBr. The mixtures were ground by hand in pestle and mortar for 5 min and were then pressed at 4 tons to give a pellet (15 mm in diameter). Spectra were ao/c ) recorded on a Nicolet 51 OP FT-IR spectrometer Fig. 1. XRD patterns of parent KL(a) with a resolution of 2 cm'. Compositions were and modified KL sample (b). Si/Al: determined by conventional chemical analysis. a-2.90, b-3.94. 3. RESULTS AND DISCUSSIONS Figure 1 shows the XRD patterns of the KL samples before and after treatment. We can find that the obtained KL sample remained a good crystallinity. The good retention of the crystallinity after the modification is also demonstrated by the relative crystallinities of the samples listed in Table 1. A shift of the XRD peaks toward higher 20 degree direction was ascribed to the substitution of the shorter Si-0 bond for the longer Al-0 bond [11-12], but no linearity between the shift and XAI was gained after a systematic study. Interestingly, from the framework vibration FT-IR spectra (not shown here) and Table 1 wc find that the frequency for the internal tetrahedron symmetric band at -770 cm' increased significantly with the raise of Si/Al ratio and the frequency maximum position can always be determined. We plotted the frequency maximum of this band against the atom fraction of Al (XAI) in the framework tetrahedral sites, an empiric linear relationship between the frequency and XAI is obviously shown in Figure 2:

Table 1 Molar compositions and frequencies of framework IR spectra for samples. Sample

Si/Al (mole)

(mole)

1 2 3 4 5 6 7 8

2.75 2.90 2.95 3.41 3.94 4.98 6.14 7.37

0.267 0.257 0.254 0.227 0.203 0.167 0.140 0.119

XAI

Relative Frequency (cm' ) Crystallinity Asymmetric Symmetric T-0 bend (%) 1024.33 767.77 725.33 476.48 101 100 1028.19 768.71 727.26 476.48 769.70 726.29 478.41 105 1027.23 1030.21 97 773.01 727.45 478.41 1034.46 776.45 729.67 477.44 105 99 1035.40 782.23 726.11 477.44 indiscernible 785.12 730.19 478.83 88 99 indiscernible 788.02 729.19 476.46

349

XAI = -7.309x10-^ (1)770-760) + 0.3242 This linear relationship was verified by a series KL samples whose compositions were carefully determined by conventional chemical analysis in our investigation. Moreover, upon investigating the relationship between the XAI and D770, we found the dealumination was limited and approximately 50% of Al in framework can be removed (not shown here), in accordance with the estimate by Vansant et al [13]. If one hopes to dealuminate more than 50% of Al atoms in the framework of KL, a partial breakdown of the framework of KL is inevitable. That partial breakdown together with the above dealumination makes some micropores evolve into nano-metric mesopores. However, the pore size distribution of the developed mesopores is usually very broad as shown in Figure 3 (a) without the introduction of NH4AC.

790 \ \

780 770 760

0

0.1

0.2

0.3

XAI /(Al/(Si+Al)) Fig. 2. Frequency of the internal tetrahedron symmetric band at -770 cm' vs. the atom fraction of Al (XAI) in the framework for KL zeolite molecular sieves samples.

Upon the introduction of buffer agent NH4AC, the pore size distribution was narrowed obviously as shown in Figure 3 (b). That is due to that the release of F' ions was controlled and, therefore, the severe collapse of framework resulting from the serious depletion of Al atoms was suppressed and the insertion of Si was enhanced [9, 14]. When the aging time was elongated (Figure 3 (c), NH4AC/KL =0.6, aging for 9 h), the mesoporc size distribution was further narrowed owing to the more complete replenishment of Si species into framework. After the NH4AC/KL ratio further increased to 1.3 (Figure 3 (d)), the mesopore size

0.20


'*;;v:"^>"^ .

morphology. Figure 1 illustrates the preserved SWNH structure after Irealmcnt in CX)2 at 1273 K for 1 hour. In this study physical adsorption analysis was applied to understand the elTect oftiic

•

•, • ' ^ • .

; • > . * * ' V

•

'••\'\ . — ^'S' ' • " - ^ ''"^S''' "'" h-SWNH-l 273/1.

gaseous treatment on the nanohorn opening. The nitrogen adsorption isotherms of SWNHs treated in O2 and CO2 are given in Figure 2 {a and b). The isotherms clearly indicate an enhanced adsorption capacity after the gaseous treatment, suggesting that both gases, O2 and CO2, open the nanohorns at the applied treatment conditions. Additionally, we have found that treatment in CO2 for 10 h changes the slope of the isotherm at moderate and high pressures. The steeper uptake at relative pressures between 0.4 and 0.9 suggests the presence of larger amount of mesoporcs. The mesopore size distributions (PSD) given in Figure 2 (c and d) confirm that treatment in different oxidizing atmospheres has a different effect on the mesopore structure of SWNHs. Thus, there are no changes in the PSD curves of the nanohorns treated in O2 independently on the temperature of treatment (Figure 2c). On the contrary, the PSD curve becomes broader after treatment in CO2 and the higher the temperature of treatment the broader the distribution (Figure 2d). Whereas the development of

397

micropores and pores of about 3 nm is associated with the nanohom opening, mesopores larger than 3 nm could be formed only between adjacent spherical assemblies. Respectively, development of such pores suggests rearrangement of the spherical assemblies during the treatment due to detachment of the assemblies and increasing the space between them. Table 1 Pore structure parameters estimated by as-method. Sample

at

Vme

Vmi

d

(m'/g)

(cm'/g)

(cmVg)

(nm)

b-NH

320

021

on

-

b'NH-623

600

0.29

0.23

1.8

b-NH-693

830

0.32

0.34

1.9

b-NH-1173/1

566

0.23

0.22

1.5

b-NH-1173/5

558

0.22

0.22

1.7

b-NH-1173/10

560

0.38

0.22

2.1

b-NH-1273/1

668

0.34

0.26

2.0

b-NH-1273/5

670

0.48

0.27

2.4

b-NH-1273/10

820

0.62

0.35

2.7

The pore structure parameters estimated by the subtracting pore effect method for as-plot [5] are given in Table 1. The specific surface area (a,) and the micropore volume (Vmi) increase after the oxidation due to opening of the nanohoms. Consequently, both parameters, a, and I 'mi, increase with the temperature as a result of the higher reaction rate and the opening of larger number of nanohoms. The enhancement of the microporosity is more pronounced for SWNHs treated in O2. The data reveal that a significant development of the mesoporosity (Vme) is achieved after a long treatment in CO2. Additionally, the diameter (d) of the internal pores of the open nanohoms could be controlled by the temperature and time of treatment in CO2. The treatment in O2 has little effect on the average diameter of the nanopores. This is associated with the different reactivity of O2 and CO2 towards carbon. The oxidizing agent etches away carbon atoms from the nanohom walls creating nanowindows. The number and the size of the nanowindows depend on the reactivity of the graphitic walls towards the oxidizing agent. The slow reaction between CO2 and carbon allows formation of nanowindows of different size depending on the time and temperature of treatment. In summary, treatment of SWNHs in oxidizing gaseous atmosphere allows tuning of the

398

pore structure by control of the temperature and time of treatment.

800 •

600

b-SWNH-623 jfl

• b-SWNH-693 400 200

1600

(a)

is

:^4

^

^1200

b-SWNH-1273/10

I

b-SWNH-1173/10 pristine J

800

™ 400

pristine

0 0.0

0.2

0.4 0.6 P/Po

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

40

50

PlPo

•o

0

10 20 pore size, x (nm)

30

0

10

20

30

pore size, x (nm)

Fig. 2. Nitrogen adsorption isotherms at 77 K of SWNMs treated in O2 (a) and COiih). The PSD curves estimated by Barrett-Joyner-Halenda method are plotted in (; L: observed in SBA-15 which had been the starting point of the synthesis. The XRD " pattern reveals the typical low angle reflections with the (11) and (20) ^" reflection discernible, the sorption isotherm shows the typical step in the . . . ' " mesopore range (BET surface areas ._...„_ 200 nm around 500 m^/g). The quality of the resulting material was found to depend Fig. 3: TEM of a typical NCS-l material critically on the quality of the starting obtained as a SiOo nanocast from CMK-3.

_

.

• ,

^ j „ ^^i^^

^

^n

^ ^^ •u .

materials used as molds as well as on the processing conditions, especially on the thermal treatment. After having been successful with silica, the process was extended to alumina. So far, a material with the same high degree of order as the silica was not obtained, but the results already show, that the process is transferable also to other oxides. As aluminum source we used different aluminumalcoholates. Fig. 4a shows the isotherm for one example. Fig. 4b a TEM, which demonstrates that the pore system is still rather disordered, in agreement with the single, relatively broad low angle reflection in the XRD. The pore size as seen in the TEM, however, corresponds to the pore size calculated from the isotherm.

^nm 0.2 0.4 0.6 0.8 Relative pressure (P/Po)

Fig. 4: (a) Sorption isotherm of a nanocasted alumina NCA-1 obtained from CMK-3 and aluminun tributylate (b) TEM of the same sample.

406

4. CONCLUSIONS Great advances in the synthesis of non-siliceous mesoporous materials have been made over the last years, and the field is still expanding. It is probably no great risk to state that in the years to come repeated nanocasting will be one of the main pathways by which even more unusual framework compositions will become accessible. 5. REFERENCES 1. F. Schuth, Chem. Mater. 13 (2001) 3184. 2. F. Schuth, Stud.Surf.Sci.Catal. 135 (2001) 101. 3. A.H. Lu, W. Schmidt, A. Taguchi, B. Spliethoff, B. Tesche, F. Schuth, submitted. 4 . J.M. Kim, M. Kang, S.H. Yi, J.E. Yie, S.H. Yoo, R. Ryoo, 3rd IMMS, July 8-11, 2002, Jeju, Korea, PA-7. 5 . C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 6. T. Yanagisawa, T. Shimizu, K. Kuroda, C Kato, Bull.Chem.Soc.Jpn. 63 (1990) 988. 7. D. Honicke, E. Ditsch, Anodic Alumina, in: F. Schuth, K.S.W. Sing, J. Weitkamp (eds.). Handbook ol Porous Solids, Wiley-VCH, Weinheim, 2002. 8. M. Eddouadi, J. Kim, N. Rosi, D. Wodak, J. Wachter, M. O'Keeffe, O.M. Yaghi, Science 295 (2002) 469, with further references. 9. D.M. Antonelli, J.Y. Ying, Angew.Chem.Int.Ed.Engl. 35 (1996) 426. 10. J. Livage, M. Henry, C. Sanchez, Progr.Sol.State Chem. 18 (1989) 259. 11. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schuth, Angew.Chem.Int.Ed.Engl. 35 (1996) 541. 12. U. Ciesla, M. Froba, G.D. Stucky, F. Schuth, Chem.Mater. 11 (1999) 227. 13. F. Schuth, U. Ciesla, S. Schacht, M. Thieme, Q. Huo, G.D. Stucky, Mater.Res. Bull. 34 (1999) 483. 14. F. Kleitz, S.J. Thomson, Z. Liu, O. Terasaki, F. Schuth, Chem.Mater., in print. 15. M. Linden, J. Blanchard, S. Schacht, S.A. Schunk, F. Schuth, Chem.Mater. 11 (1999) 3002. 16. J. Blanchard, F. Schuth, P. Trens, M. Hudson, Microporous and Mesoporous Mater. 39 (2000) 163. 17. T. Czuryskicwicz, J. Rosenholm, F. Kleitz, F. Schiith, M. Linden, in preparation 18. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J.Am.Chem.Soc. 120 (1998) 6024. 19. U. Ciesla, F. Schuth, Microporous and Mesoporous Mater. 27 (1999) 131. 20. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.A. Albouy, A. Brunet-Bruncau. A.R. Belkcnende, Adv.Mater. 13 (2001) 1085. 21. H.S. Yun, K. Miyazawa, H. Zhou, I. Honma, M. Kuwabara, Adv.Mater. 13 (2001) 1377. 22. T. Hyodo, T. Nishida, Y. Shimizu, M. Egahira, Sensors and Actuators B 83 (2002) 209. 23. T.T. Emons, J. Li, L.F. Nazar, J.Am.Chem.Soc, in print. 24. C. Serre, M. Hervieu, C. Magnier, F. Taulelle, G. Ferey, Chem.Mater. 14 (2002) 180. 25. J. El Haskouri, S. Carbrera, F. Sapina, J. Latorre, C. Guillem, A. Beltran-Porter, D. Bellran-Porlcr, M.D. Marcos, P. Amoros, Adv.Mater. 13 (2001) 192. 26. D. Farrusseng, K. Schlichte, B. Spliethoff, A. Wingen, S. Kaskel, J. Bradley, F. Schuth, Angew.Chem. 113 (2001) 4336. 27. G.S. Attard, C.G. Goltner, J.M. Corker, S. Henke, R.H. Templer, Angew.Chem.Int.Ed.Engl. 36 (1997)1315. 28. I. Nandhakumar, J.M. Elliott, G.S. Attard, Chem.Mater. 13 (2001) 3840. 29. R. Ryoo, S.H. Joo, S. Jun,.J.Phys.Chem.B 103 (1999) 7743. 30. M. Kaneda, T. Tsubakiyama, A. Karlsson, Y. Sakamoto, T. Oshuna, O. Terasaki, S.H. Joo, R. Ryoo, J.Phys.Chem.B 106 (2002) 1256. 31. R. Ryoo, S.H. Joo, S. Jun, T. Tsubakiyama, O. Terasaki, Stud.Surf.Sci.Catal. 135, Elsevier, Amsterdam 2001. 32. S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Oshuna, O. Terasaki, J.Am.Chem.Soc. 122(2000)10712. 33. S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu., O. Terasaki, R. Ryoo, Nature 412 (2001) 169. 34. J. Lee, S. Yoo, T. Hyeon, S.M. Oh, K.B. Kim, Chem.Commun. (1999) 2177. 35. J.Y. Kim, S.B. Yoon, F. Kooli, J.S. Yu, J.Mater.Chem. 11 (2001) 2912.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Structure and shape control in functional mesostructured materials from block copolymer mesophases Ulrich Wiesner Department for Materials Science & Engineering, Cornell University, 329 Bard Hall, Ithaca, NY 14850, USA 1. INTRODUCTION The synthesis of functional mesostructured organic-inorganic hybrid materials using organic molecules as structure-directing agents or templates is an area of rapid growth w^ith diverse applications, such as separation technology and catalysis. Block copolymers can be regarded as macromolecular analogues of low molecular weight surfactants.^'^ Use of block copolymers has recently been shown to extend the structural feature size of mesostructured hybrid materials as well as the pore sizes of the resulting ordered porous silica to hundreds of Angstroms.^^'^^ Combined principles of polymer, colloidal and inorganic chemistries have been used to synthesize materials with uniform and adjustable pore sizes, with thick, hydrothermally stable walls and with both, 2-dimensional (2D) hexagonal structures as well as 3-dimensional (3D) cubic morphologies with more accessible pores.^^"''^ Unprecedented morphology control is obtained for mesostructured materials by changing from conventional silicon precursors to organically modified ceramic (ormocer) precursors in the block copolymer directed synthesis.^"^^ Most of the mesophase morphologies observed in block copolymers or copolymer-homopolymer mixtures have been obtained for such organicinorganic hybrid materials.^'''^^ The basis for this morphological control is a unique polymerceramic interface which can be characterized by solid state NMR techniques.^'^^ The hydrophilic blocks of the amphiphilic copolymers are completely integrated into the ceramic phase analogous to what is found in biological hybrid materials. This leads to a "quasi two phase system" allowing for a more rational hybrid morphology design based on the current understanding of the phase behavior of block copolymers and copolymer-homopolymer mixtures.^'"^^ Through the unique interface the ceramic phase is plasticized by the polymers thus generating an approach to a novel class of materials referred to as 'flexible ceramics' in the following. In the present paper, after elucidating the structural control obtained through the block copolymer-ormocer approach, several examples illustrate areas for potential applications of these hybrid materials. They include applications as solid hybrid polymer (SHyP) electrolytes, polymer-hybrid nanocomposites, and mesoporous materials.

408

2. PHASE SPACE OF ORMOCER DERIVED MESOSTRUCTURED HYBRIDS (H3CO)3SP

In ormocer derived meso-structured '4J^>======^NL^^ AI(0*Bu)j hybrids the morphology of the final material is mainly a function of the weight of the added inorganic fraction components and essentially independent of the microstructure of the block copolymer.^''^ This allows access to a wide variety of morphologies starting from a single block copolymer by simply mixing in the inorganic components. This is Fig.l. Schematic drawing of approach for demonstrated schematically in Figure 1 for synthesizing mesostructured hybrid materials: the ormocer precursors, (3-glycidyloxyp- Left: the morphology of the precursor polymers; Right: the resulting morphologies ropyl)-trimethoxy si lane (GLYMO) and after addition of various amounts of metal aluminum ^ec-butoxide (Al(0^Bu)3) in a alkoxides. molar ratio of 80:20, with the diblock copolymer poly(isoprene-block-ethyleneo-xide), (PI-b-PEO), as structure directing agent (for details of the procedure see reference 11 and references therein). Representative transmission electron microscopy (TEM) micrographs of selected hybrid morphologies are depicted in Figure 2. Starting from a PI-^-PEO block copolymer with bcc structure (1.410'* g/mol, /PEG ~ 0.13), increasing the content of GLYMO and Al(0^Bu)3 leads to spheres (WINORG ^ 0.23, Fig. 2A), hexagonally packed cylinders (WINORG = 0.32, Fig.2B), lamellae (WJNORG = 0.45, Fig. 2C),

the inverse cylinder morphology (WINORG = 0.65,

Fig.2D)

and

randomly packed wormlike micelles of PI in an inorganic-rich matrix (vviNORG = 0.82). Employing a block copolymer exhibiting a hexagonal array of cylinders (M= 16400, /PEG = 0.38) as structure directing agent, lamellae (WINGRG = 0.53), a bicontinuous cubic Plumber's Nightmare structure^'^^ Fig.2. TEM micrographs of some of the mesostructured (wiNGRG = 0.56, Fig.2E), an hybrid materials. If not otherwise shown, magnifications inverse cylinder morphology are as depicted by the bar in (A). Except for (E) all and inverse images were taken under bright-field conditions (bright (wiNGRG = 0.73) (WINORG = 0.79, Fig.2F) spheres inorganic and dark polymer phase). In (E) the contrast is are found. inverted (dark-field conditions). When comparing the sequence of hybrid morphologies in Figure 1 with that observed for the pure PI-^-PEO and other block copolymers^'"^'^^^ it is evident that the overall structural control is similar and the sequence of morphologies indeed closely follows what is expected ft*om block copolymer phase diagrams. Nevertheless, subtle differences are observed. First,

409

only for the block copolymer with larger PEO content a bicontinuous cubic structure could be obtained. As described in an earlier publication,^^^^ SAXS and TEM data on this bicontinuous structure does not agree with the double gyroid morphology expected from block copolymer phase diagrams^'"^^ but rather is consistent with a so called "Plumber's nightmare" morphology. Second, also the inverse spherical morphology could only be reached through addition of inorganic material to the copolymer with larger PEO content. In contrast, addition of large amounts of inorganic material (WINORG > 0.8) to the block copolymer with bcc morphology leads to wormlike rather than spherical micelles. This morphology has been reported for A2Bmictoarm star polymers^'^^ and diblock copolymer/homopolymer (AB/A) mixtures.^'^'^^^ The occurrence of worm-like micelles instead of spheres arranged on a cubic lattice has been ascribed to interface-curvature constraints.^'^^ 3. SOLID HYBRID POLYMER ELECTROLYTES Solid polymer electrolytes (SPEs) are potential materials for application as electrolytes and separators in secondary lithium and lithium-ion batteries. The prototypical SPE is polyethylene oxide (PEO) blended with the lithium salt of a large soft anion. Cross-linking can improve the mechanical strength of such SPEs. The addition of nanoscale ceramic materials inhibits the recrystallization of PEO, increases the cationic conductivity and stabilizes the Li electrolyte interface. The present PEO/Al-GLYMO composites when combined with a lithium salt provide a novel type of lithium ion conductor with an unprecedented combination of properties: a Solid Hybrid Polymer (SHyP) electrolyte.^''^^ Because of the molecular-scale mixing of the components, crystallization of PEO is completely suppressed, while strength, conductivity, and lithium transference numbers all are high compared to prototype Fig. 3. Energy Filtering (EF) TEM SPEs. Cyclic voltammetry shows that lithium micrograph (right) of isolated nano can be plated and stripped from these cylinders of a PI-b-PEO/Al-GLYMO electrolytes and suggests a reasonable cycling composite and its molecular interpretation efficiency. Finally, as demonstrated in Figure (^'g^^; ^.PP^^ P^^' ^ark-field image - , . -^ .-' , ,^ • J • reveahng silicon; lower part: carbon map. 3, this composite can be self-organized using o r r diblock copolymer technology into nanometer scale plates and rods, paving the way to making lithium-conducting cables, for example, and hence solid-state electrochemical devices of sizes down to 10 nm. 4. MODEL BLOCK COPOLYMER NANOCOMPOSITES Using the block copolymer assisted sol-gel synthesis a series of silica based fillers with well-defined shapes, i.e. spheres, rods and plates, and surface potentials ("hairy objects") can be synthesized, see scheme in Figure 4 (see also Figure 3).^'^^ These filler particles can subsequently be incorporated into, e.g., a lamellar poly(styrene-b-isoprene) block copolymer matrix to generate model nanocomposites. The influence of filler dimensionality on orderdisorder phase transition of the block copolymer matrix can then be studied.^^^^ The addition

410

of as little as 0.5wt. % fillers drastically alters the thermodynamic properties of the nanocomposites. The order-disorder transition temperature is lowered by 1523°C and is accompanied by a significant broadening of the transition temperature window. The dimensionality of the fillers plays a significant and non-trivial role in the process of the order-disorder phase transition. Rod-like fillers induce the largest depression and broadening of the phase transition. The findings can be rationalized based on varying defect energy density arguments as also supported by recent computer simulations. Experimental work to further elucidate the origins of the observed behavior is now in progress in our laboratory.

^Mfs^

- c r ^ AI(OBu\

0.5%

99.5%

5. MESOPOROUS MATERIALS Fig. 4. Schematics of how to generate model nanocomposites. Mesostructured hybrid materials with an inverse cylindrical (cf Figs. 1 & 2D) and a Plumber's Nightmare morphology (cf Figs. 1 & 2E) can be converted successfully to the corresponding mesoporous materials after heat treatment in several stages up to 600 ^c.^'^'"^ A schematic representation of the resulting structures as well as the corresponding TEM micrographs of the calcined materials are shown in Figure 5. In case of the bicontinuous cubic Plumber's Nightmare, after calcinations the bulk material consists of a particularly fascinating morphology with an aluminosilicate matrix interwoven with two discrete, continuous nano-channel systems that do not touch each other (see Scheme in Fig.5C). Because of its interwoven and regular, branched cubic bulk structure, the resulting mesoporous material is expected to provide excellent mass transfer kinetics Fig. 5. Schematic representations (left) in catalytic and separation technologies. From NMR and TEM micrographs (right) of studies about half of the aluminum is incorporated in mesoporous materials with hexagonal and bicontinuous cubic the silicon network as fourfold coordinated (top) aluminum. Calcinations of the as-made materials Plumber's Nightmare morphologies leads to an even larger amount of such in-frame (bottom). aluminum with respect to the precursor material thus providing a pathway to materials with acid catalytic activity (data not shown). It is striking that the structures are well preserved after calcinations even though the unit cell volumes sometimes decrease by as much as 75%! The large mass loss and shrinkage is due to the large fraction of organic moieties even in the inorganic phase of the hybrids. Preservation of the structures indicates that the bonding network formed by the inorganic precursors is extremely robust. This may be a general feature of the present block copolymer-ormocer derived hybrid materials. Both mesoporous materials exhibit a nitrogen sorption isotherm of

411

type IV according to BDDT classification with specific surface areas typically around 300 m^/g (according to the Brunnauer-Emmett-Teller (BET) method) for materials with pore diameters of about 9 nm. It is interesting to note that, e.g., in the hexagonal mesoporous materials with about 12 nm the wall thickness is about doubled with respect to that of materials described in the literature.^^'^'^ This should lead to significantly improved stability. Furthermore, the pore sizes (as well as the wall thicknesses) of the present materials can be varied through a simple variation of the molecular weight of the precursor PI-Z?-PEO obtained through anionic polymerization. As an example, from a PI-^-PEO sample with A/n = 8.4-10'^,/pEo ^ 0.08 and a polydispersity, Mw /Mn =1.05, a hybrid material with inverse hexagonal morphology was prepared at (WINORG =0.28) and then calcined. Analysis of the corresponding nitrogen sorption isotherm revealed a pores size of about 50 nm.

REFERENCES 1. B. M. Discher, Y. Y. Won, D. S. Ege, J. C. M. Lee, F. S. Bates, D. E. Discher, D. A. HamrnQT, Science 1999,25^,1143. 2. S. A. Bagshaw, E. Prouzet, T. J. Pinnavaia, Science 1995, 269, 1242. 3. C. G. Goltner, M. Antonietti, Adv. Mater. 1997, P, 431; C. G. Goltner, S. Henke, M. C. Weissenberger, M. Antonietti, Angew. Chem. 1998, 110, 633-636; Angew. Chem. Int. Ed. ^•wg/. 1998, J7, 613. 4. M. Templin, A. Franck, A. Du Chesne, H. Leist, Y. Zhang, R. Ulrich, V. Schadler, U. Wiesner, Science 1997, 278, 1795. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science 1998, 279, 548. 6. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 1998,120, 6024. 7. D. Zhao, P. Yang, N. Melosh, J. Feng. B. F. Chmelka, G. D. Stucky, Adv. Mater. 1998,10, 1380. 8. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature 1998, 396, 152. 9. P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Nature 1998, 282, 2244. 10. A. C. Finnefrock, R. Ulrich, A. Du Chesne, C. C. Honeker, K. Schumacher, K. K. Unger, S. M. Gruner, U. Wiesner, Angew. Chem. Int. Ed. 2001, 40, 1208 11. P. F. W. Simon, R. Ulrich, H. W. Spiess, U. Wiesner, Chem. Mater. 2001, 13, 3464. 12. R. Ulrich, A. Du Chesne, M. Templin, U. Wiesner, Adv. Mater. 1999, //, 141. 13. S. M. De Paul, J. W. Zwanziger, R. Ulrich, U. Wiesner, H. W. Spiess, J. Am. Chem. Soc. 1999, 121, 5727. 14. The Physics of Block Copolymers, I. W. Hamley, Oxford University Press, Oxford 1998. 15. G. Floudas, B. Vazaiou, F. Schipper, R. Ulrich, U. Wiesner, H. latrou, N. Hadjichristidis, Macromolecules 2001, 34, 2947. 16. D. J. Pochan, S. P. Gido, S. Pispas, J. W. Mays, A. J. Ryan, J. P. A. Fairclough, I. W. Hamley, N. J. Terrill, Macromolecules 1996, 29, 5091. 17. T. Hashimoto, H. Tanaka, H. Hasegawa, Macromolecules 1990, 23, 4378. 18. D. J. Kinning, K. I. Winey, E. L. Thomas, Macromolecules 1988, 21, 3502. 19. R. Ulrich, J. W. Zwanziger, S. M. De Paul, A. Reiche, H. Leuninger, H. W. Spiess, U. Wiesner, Adv.Mater. 2002, in press. 20. A. Jain, J. S. Gutmann, C. Garcia, Y. Zhang, M. Tate, S. Gruner, U. Wiesner, Macromolecules 2002, 35, 4862. 21. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Chem. Mater. 1999, 7/, 2813.

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Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

Strategies for spatially separating mesostructured sol-gel silicate films

413

photoactive

molecules

in

Raquel Hernandez, Payam Minoofar, Michael Huang, Anne-Christine Franville, Shinye Chia, Bruce Dunn and Jeffrey I. Zink Department of Chemistry and Biochemistry, University of California, Los Angeles, 405 Hilgard Ave., Los Angeles, California 90095 USA Three strategies for placing molecules in designated regions of mesostructured thin films made by the sol-gel dip-coating technique are demonstrated. These strategies all involve one-step syntheses where all of the components are present in the sol from which the substrate is pulled. Silicate films templated by ionic surfactants contain three spatiallyseparated regions: a silicate framework, an organic region formed by the hydrocarbon tails of the surfactants, and an intervening ionic interface formed by the charged surfactant head groups. Luminescent molecules are placed in each of these regions, and the formation is monitored spectroscopically. 1. MESOSTRUCTURED FILMS The sol-gel process is a technique used for preparing transparent inorganic glasses at room temperature. The process used in this paper begins by reacting a silicon alkoxide such as tetraethoxysilane with water. The alkoxide undergoes hydrolysis reactions Si(0R)4 + H2O ^ H0-Si(0R)3 + ROH. The hydrolyzed molecules then undergo condensation polymerization reactions to produce a three dimensional network: (0R)3Si-0H + H0-Si(0R)3 -^ (0R)3Si -0-Si(0R)3 + H2O -> -> Si02. We discovered methods of making mesostructured sol-gel thin films formed by a rapid dip-coating method with structures that possess a high degree of long-range order [1]. Macroscopic materials having nanostructured long range order are fabricated on a support or substrate. The desired structure is built into films by using molecular templates such as surfactant molecules. Nanostructured sol-gel films are formed by the co-assembly of the inorganic silica and the organic surfactant molecules. The self assembly of the surfactants forms the desired structure which is locked in place by the silica [1,2]. The specific structure that is obtained depends on the type and concentration of surfactant used. For example, a lamellar layered structure can be produced by SDS (sodium dodecyl sulfate) surfactant and a hexagonal array of rods is formed when CTAB (cetyltrimethylammonium bromide) is used. These structures are shown in Figure 1. 2. STRUCTURAL REGIONS On the molecular level, the structures that are formed have three distinct regions as sketched in Figure 1. The solid transparent sol-gel silicate structure that holds the material together is called the framework. In most of the materials in this paper the framework is on the order of 20 A thick. The region containing the hydrophobic long-chain hydrocarbon tails of the surfactant is called the organic region. It is a planar sheet in the lamellar materials and a rod in the hexagonal materials. The dimensions of the organic region are controlled by the length of the hydrocarbon tail of the surfactant used in the preparation.

414

When dodecyl sulfate surfactant (twelve carbon chain length) is used, the organic region in the lamellar structure is about 20 A thick and the measured lattice spacing is 39 A. The third region that contains the ionic head group of the surfactant and the counterions is called the ionic region. It is a few Angstroms thick and forms the interface between the framework and the organic region. In the as-pulled films, it also Fig. 1. Schematic diagram of the processes that occur contains residual water during film pulling. The left side shows a film with as well as ions from the lamellar mesostructure templated by SDS, the right side a acid catalyst and buffer 2-d hexagonal structure templated by CTAB. The three (if they were used in the spatially separated regions of the final mesostructured preparation.) The films—the silicate framework, the organic interior of the thickness of the ionic micelles, and the ionic interfaces between the framework layer cannot be readily and the surfactant head group—are shown in the center. controlled, but its composition can be modified by the choice of surfactant and the salts added to the initial sol used to pull the film. The total thickness of the film is measured by profilometry and is usually 100-200 nm thick. 3. STRATEGIES OF DELIBERATE PLACEMENT IN SPATIALLY SEPARATED REGIONS Three strategies for placing molecules preferentially in any one of the three distinct regions [3] depicted in Figure 1 are demonstrated here. The first is iermcd philicity, and is summarized by "like dissolves like." Organic compounds are dissolved by the hydrophobic interior of micelles and ionic compounds will accumulate at the ionic interface. The second strategy is termed bonding. Bonding entails the use of molecules derivatized with trialkoxysilyl groups that can co-condense with the silica precursors in the sol to become fully integrated into the silicate framework of the final film. In other cases, the bonding strategy may apply to covalent attachment of functional molecules to surfactants, thereby placing the dopants in the hydrophobic core of the films. The third strategy is termed bifunctionality, and it applies to molecules that simultaneously incorporate both of the above strategies. These molecules possess both a physical affinity for a particular region and the capability of bonding to another. 4. EXAMPLES OF FILMS SYNTHESIZED BY USING THE STRATEGIES The focus of this paper is on the placement of luminescent molecules in structured films. In all cases, the structure is verified by x-ray diffraction. The formation of long-range

415

ordered structures is a delicate balance involving formation of micelles and the ordered phase of the template and also hydrolysis and condensation of the silicate. 4.1. Philicity The philicity strategy is a useful approach to the deliberate placement of molecules into certain regions of the mesostructured sol-gel materials [4-8]. The most common application involves non-polar molecules that reside preferentially in the hydrophobic interiors of micelles. The first example of the use of this strategy involved pyrene, a non-polar luminescent molecule, as a probe of micelle formation during film pulling. In this study, a TEOS sol is prepared and an anionic surfactant sodium dodecylsulfate (SDS) is used as the templating agent. The final mesostructure is lamellar with alternating surfactant micelle layers and silicate layers (Figure 1). As the film is pulled, the non-polar hydrophobic pyrene molecule becomes incorporated into the micellar interior when the micelle first forms, and the change in luminescence is used to monitor the formation process of the mesostructured sol-gel films [9,10]. The intent of these studies is to monitor the dynamics of film formation in real time, and the final film contains pyrene deliberately placed in the organic region. To further characterize the dynamic properties, interferometry is used to monitor the film thickness at the same time that in-situ photoluminescence spectroscopy of the pyrene probe monitors micelle formation. Monochromatic light is used to illuminate the film. Because of the continuous decrease in the film thickness during the dip-coating process, constructive pyrene band III to band I ratios

percentage of H2O in the sol solvent

1-2-hour-oldfilm R = 1.45 ±0.11 0th fringe R = 1.21 ±0.03 10 R = 1.03 ±0.07

R= 1.39 ±0.29 5

-\R = 1.54 ±0.22

R = 1.12 ±0.12

Fig. 2. Dynamic changes occurring during film pulling. The light and dark interference fringes on the film are used to measure the thickness. The time of the process and the distance above the sol are shown on the left. The pyrene molecule is incorporated in the surfactant by the philicity strategy. The ratios R of the vibronic bands in the luminescence spectra monitor micelle formation (R=1.54), reorganization (R = 1.03) and final lamellar mesostructure formation (R > 1.21).

416

and destructive interference leads to the appearance of light and dark fringes on the developing film that can be used a convenient scale to monitor the thickness. The positions of the fringes on the moving film do not change, A schematic diagram of the results of pyrene luminescence spectroscopy and interferometry of SDS sol-gel films is shown in Figure 2. Changes in the relative vibronic band intensities (band III to band I ratios) in the luminescence spectra are related to changes in the polarity of the probe environment. Low band III to band I ratios correspond to a polar environment, whereas a high III/I ratio is indicative of a non-polar environment. The results show that micelles are formed early in the film formation process (III/I ratio increases from 1.12 to 1.54 within 5 sec.) and that pyrene is incorporated into the micellar interior and experiences a non-polar environment. Then the ratios gradually decrease (III/I ratio decreases to 1.03) and finally increase again at the end of the process (III/I ratios increase to 1.21 and eventually to 1.45). During this period, the initially-formed micelles undergo a phase transformation as the surfactant concentration continues to increase. Micelles break up and pyrene becomes re-exposed to the polar solvents. Finally when the micellar reorganization is complete and the final lamellar phase is formed, pyrene becomes incorporated into the surfactant layer and again reports a non-polar environment. This example shows that pyrene's philicity can be used to incorporate it selectively into the micellar region of the mesostructured sol-gel films, and in the process it can be used to probe the micelle formation and its transformation into the mesostructured sol-gel films. 4.2. Bonding The bonding strategy for incorporating luminescent molecules in the framework requires molecules that contain at least two alkoxysilane substituents on opposites sides [11-13]. Trialkoxysilyl groups undergo condensation with TEOS, the silica precursor in the starting sol, to become integrated into the silicate framework. In all of the examples discussed in this paper, six trialkoxysilyl substituents that surround the molecule in three dimensions are used. An example of a ligand that binds luminescent lanthanide ions is shown in Figure 3 [13]. The incorporated complexes exhibit both the characteristic lanthanide emission spectra and excitation spectra consistent with the absorption-transfer-emission (ATE) mechanism of luminescence typical of these complexes. The Eu complex depicted in Figure 3 is incorporated into the framework of hexagonally structured thin films [13]. This localization is evidenced by the relative intensities of the 616 nm and 592 nm Eu"^^ emission peaks that are the same in both silicate (no surfactant) and nanostructured films. In addition, the Eu"'^ emission lifetimes are the same in both types of films. The silylated ligand enables other 500 550 600 650 700 lanthanides to be deliberately placed in Emission Wavelength (ran) the framework. Terbium and cerium Fig. 3. Placement of a silylated europium have also been studied; Tb^^ emission complex in the framework by using the lifetimes were used to demonstrate the bonding strategy. The molecular structure and placement of the terbium [14]. If this the placement are sketched on the left; the ligand is synthesized without the luminescence spectra of both the silicate groups, its lanthanide mesostructured and silicate films are shown. complexes are hydrophobic and the Films were excited at 330 nm.

417

SDS Templated Film

CTAB Templated Film

li-:iiiU'U(irk

Fig. 4. Placement of a silylated ruthenium complex by using the bifunctional strategy. The molecular structure is sketched at the top. When the surfactant is anionic, the positively charged ruthenium extends into the ionic interface, but when a cationic surfactant is used the ruthenium is repelled and located in the framework.

philicity strategy can be used to place them in the organic region.In another example of the bonding strategy, a silylated Ru(bpy)3(PF6)2 complex that has six condensable trialkoxysilane groups is incorporated into the silicate framework [14]. The luminescence spectra is the same in both CTAB templated films and silicate films indicating that the ruthenium complex is located in the same region, the silicate matrix, in both films. This bidentate ligand strongly binds many transition metals and can be used to place other metals in the framework.

4.3. Bifunctionality of The strategy bifunctionality requires a molecule that has characteristics of two of the regions within the mesostructured film. An example of a bifunctional molecule is the singly silylated Ru(bpy)2ATT (Figure 4) that has the ability to bond one end to the silicate matrix and also has an ionic end with an affinity for the ionic region of the film [3]. The position of the luminescence band maximum is sensitive to the metal's environment and is used to characterize its location. In the first studies of the bifunctional strategy with silylated Ru(bpy)2ATT, the molecule was incorporated in films templated with the anionic surfactant SDS (Figure 4). The emission band maximum is at 650 nm. In a control experiment, the molecule is incorporated in a silica film (no surfactant). In this film, where the molecule is located in the silicate region, the band maximum is at 665 nm. The shift of the emission band maximum to shorter wavelengths shows that the metal-containing end of the 320 360 400 440 molecule is not in the framework but Wavelength (nm) instead extends into the ionic interface Fig. 5. Simultaneous placement of region. In subsequent studies, silylated paraterphenyl in the organic region by using Ru(bpy)2ATT was incorporated in the philicity strategy and the silylated CTAB templated films. The emission europium complex in the framework by the bonding strategy. The molecular structures, band maximum, 665 nm, is the same in and luminescence spectra, collected with both the pure silicate and the 266 nm excitation, are shown. mesostructured film, and shows that the

418

ruthenium resides in the same type of environment, the silica region, in both films. The above studies suggest that the charge on the surfactant's head group plays a major role in the final location of the cationic metal end of the bifunctional molecule. When anionic SDS is used, the metal is attracted into the interface region, but when cationic CTAB is used, the metal is repelled from the interface and becomes incorporated into the framework. 4.4. Dual placement: philicity and bonding Simultaneous incorporation and separation of two different luminescent molecules in structured thin films is possible when the molecules take advantage of two different strategies. An example is the simultaneous incorporation of the silylated Eu complex in the framework by the bonding strategy, and of paraterphenyl (PTP), a hydrophobic laser dye, in the organic region by the philicity strategy [13]. Figure 5 shows emission spectra obtained from a mesostructured thin film containing both the Eu complex and PTP prepared in a one-step, one-pot synthesis from a sol containing both luminescent molecules. This experiment demonstrates that two dopants can be incorporated into nanostructured thin films and simultaneously be directed to specific regions of the same films. The dual placement can produce functional films that undergo intermolecular energy or electron transfer. 5. SUMMARY Luminescent molecules are deliberately placed in one of the three spatially separated regions of mesostructured films by using the strategies of philicity, bonding or biftinctionality. All of the components are present in the sol from which the film is pulled. The structure is confirmed by x-ray diffraction and the location of the molecule is determined by luminescence spectroscopy. Simultaneous placement of two different molecules in two different regions is also demonstrated. Functional films that undergo intermolecular electron transfer and energy transfer can be synthesized in a one-pot, onestep procedure.

REFERENCES 1. Lu, Y.; Gangull, R.; Drewlen, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Quo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364-368. 2. Sellinger, A.; Weiss, P. M.; Nguyen, A.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256. 3. Hernandez, R.; Franville, A.-C; Minoofar, P.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2001,723, 1248-1249. 4. Franville, A.; Dunn, B.; Zink, J. I. J. Phys. Chem. B 2001, 105, 10335-10339. 5. Ogawa, M. Langmuir 1995, 11, 4639. 6. Ogawa, M. Chem. Mater. 1998, 10, 1382. 7. Wimsberger, G.; Scott, B. J.; Chmelka, B. F.; Stucky, G. D. Adv. Mater. 2000,12, 1450. 8. Wu, J.; Abu-Omar, M. M.; Tolbert, S. Nano Letters 2001, /, 27-31. 9. Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I. Langmuir 1998, 14, 7331. 10. Huang, M. H.; Dunn, B. S.; Zink, J. I. J. Am. Chem. Soc. 2000, 122, 3739-3745. 11. Lebeau, B.; Fowler, C. E.; Hall, S. R.; Mann, S. Journal of Materials Chemistry 1999, 9, 2279-2281. 12. Li, H.; Fu, L.; Wang, S.; Zhang, H. New J. Chem. 2002, 26, 674. 13. Minoofar, P.; Hernandez, R.; Franville, A.; Dunn, B.; Zink, J. Journal of Sol-Gel Science and Technology 2002, in press. 14. Minoofar, P.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I.; Franville, A. submitted.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

419

Design of supported catalysts by surface functionalization of micelletemplated silicas D. Brunei'*, A.C. Blanc", P-H. Mutin^ O. Lorret", V. Lafond^ A. Galameau', A.Vioux'' and F. Fajula"

'Laboratoire des Materiaux Catalytiques et Catalyse en Chimie Organique - UMR-5618CNRS-ENSCM, 8, rue de I'Ecole Normale, F-34296 -MONTPELLIER Cedex 5, France. FAX: +33-4-67-14-4349. e-mail: [email protected]. ^Laboratoire de Chimie Moleculaire et Organisation du Solide - UMR-5637-CNRS-UM II, Place Eugene Bataillon, F-34095 -MONTPELLIER Cedex 5, France. The functionalization of the surface of micelle-templated silicas with catalytic sites opened up a wide range of opportunities for advanced applications in supported catalysis. In this field, the control of the different modification steps is critical in the catalyst design. This presentation deals with the best control of the surface coverage of the mineral surface with organic silane bearing chiral ligand or guanidine, by surface sol-gel polymerization process. This strategy provides more efficient enantioselective catalyst and/or catalyst stability. On the other hand, the enhancement of the chemical stability of base supported catalysts by other coupling strategies will allow their application in fine organic chemistry. 1. INTRODUCTION Increasing interest is renewed on the use of immobilized homogeneous catalysts on mineral supports owing to the possibility of the easy recovery and reuse of the catalyst. Moreover, the discovery of the highly structured silica such as M41-S family and others has opened up new possibilities for their use as nanostructured supports '. In particular, these materials offer larger accessible surface than zeolites, and therefore present considerable advantages for their applications as catalyst supporting solids in fine organic chemistry. Hence, the surface of these materials have been covalently grafted with organic moieties bearing catalytic sites. Depending on the nature of the tethered catalyst, these hybrid materials are potential heterogeneous catalysts for either acid or base or hydrogenation or oxidation reaction. Besides the advantages provided by the particular texture of the micelle-templated silicas (MTS), which often avoid difflisional limitation, the homogeneous chemical nature of their surface could conveniently allow dispersion of the catalytic sites, hence their isolation towards mutual site-site interaction. Nevertheless, the activity and selectivity of such supported catalysts could be strongly changed compared to that of their homogeneous couterparts due to sitesurface interactions which depends on the type of the used anchoring methodogy. This aspect is developed here. Morover, one of the drawbacks of such solids which could hamper their

420

widespread use as catalysts, is their chemical instability towards hydrolysis or solvolysis particularly during their use under basic conditions.^ However, up to now, although some materials obtained recently by post-synthesis treatments exhibited a considerable improvement of their chemical stability,^ this has remained as a challenge since there was no effective methods for the preservation of the texture of MCM-41 containing strong bases. In this paper, we present two possible routes to increase the stability of the hybrid organic-silicic mesoporous materials and some applications in catalysis. 2. EXPERIMENTAL 2.1. Materials MCM-41 materials were synthesized from alkaline silicate solution in the presence of hexadecyltrimethylammonium bromide"*. Different pore sizes were achieved by adding various amounts of swelling agent: 1,3,5-trimethylbenzene. 2.2. Surface modifications of MCM-41 Three different methods were investigated: Type 1 Silylation: The organosilane chains were grafted on dehydrated surface of MCM-41 samples by reaction of organotrialkoxysilane, mainly 3-chloropropyltrimethoxysilane, in anhydrous refluxing toluene. Type 2. Coating: MCM-41 samples were functionalized by surface sol-gel polymerization of chloropropyltrimethoxysilane molecules which were firstly adsorbed on MCM-41 surface (5 molecules/nm^), according to a procedure leading to an optimal surface coverage on silica^. Type 3. Two-step modification'. The MCM-41 surface was first covered by TiOx overlayers according to a method reported in ref 6, then functionalized by grafting with 3-bromopropanphosphonic acid. 2.3. Anchorage of catalytic sites Ephedrine or guanidine was anchored by nucleophilic substitution of chlorine atom born by the differently grafted organosilane chains, or (in type 3 modification) by substitution of bromine atom born by the grafted alkylchain phosphonate groups. 2.4. Characterization Hybrid mesoporous materials were characterized by a battery of techniques including XRD, sorption measurements, ^'^Si and *^C NMR, IR, UV-Vis, TGA and elemental analyses. 2.5. Catalytic tests 2.5.1. Enantioselective additions of diethylzinc to benzaldehyde were conducted in presence of stirred dispersions of various ephedrine-grafted MCM-41 samples in anhydrous toluene at 273 K. 2.5.1. Transesterification reactions were performed by addition of ethylpropionate to stirred dispersions of various guanidine-grafted MCM-41 samples in butanol at 373 K. 3. RESULTS AND DISCUSSION MCM-41 possessing different mesopore sizes have been functionalized with 3aminopropyl- and 3-halogenopropylsilane by surface grafting according to silylation- or coating-type methods. The amine-containing MCM-41 revealed base-catalysis efficiency in Knoevenagel condensation and fatty acid addition on glycidol and the chloropropane -

421

containing MCM-41 was an useful precursor for the preparation of various supported homogeneous catalysts. For example, other organic bases such as piperidine or guanidine and different ligands or metal transition complexes have been covalently anchored on MCM-41 by halogen substitution reaction.

^


CI > Br > NO3 [12]. Thus, it is reasonable that H2SO4 leads to facile formation of 3d-hcxagonal^ Pdj/mmc mesophase with a smaller g parameter, and that HNO3 favors the formation of /a3 dry grafted > wet grafted. We propose that the difference in steam stability between supercritical, dry and wet grafted Al-MCM-41 materials is due to the way in which the Al interacts with the host silica framework and in particular the extent to which the Al is sorbed onto rather than into the framework. 1. INTRODUCTION Mcsoporous aluminosilicates are currently attracting considerable research effort due to their potential use as heterogeneous solid acid catalyst, especially for large molecule transformations.'"^ Although, in general, mesoporous aluminosilicates are only moderately acidic compared to zeolites, they are however potentially useful catalysts for large molecule transformations that do not require very strong acid sites. A key requirement for their successful use as solid acids (or ion exchangers), is good hydrothermal stability in hot aqueous solutions and under steaming (high temperature hydrothermal) conditions."^ Recent work has shown that mesoporous aluminosilicates with improved hydrothermal stability may be prepared via post-synthesis grafting routes or from zeolite seeds as inorganic framework precursors."''^ Mesoporous aluminosilicates prepared via post-synthesis alumination routes offer certain advantages over similar but directly synthesised materials with respect to accessibility to active (Al) sites and structural ordering. Post-synthesis alumination is therefore fast becoming an attractive alternative route for the preparation of mesoporous aluminosilicates derived from various forms of mesoporous silicas. Previous studies on the hydrothermal stability of Al-grafted MCM-41 have shown that the post-synthesis alumination pathway (i.e., grafting in either aqueous or non-aqueous media) does not have any significant effect on the level of stability in boiling water.^ We have now investigated the high temperature hydrothermal (steam) stability of Al-grafted mesoporous aluminosilicates and show here that the post-synthesis alumination pathway is a critical factor in determining the steam stability of Al-grafted MCM-41. In particular we show that the solvent used during post-synthesis alumination of pure silica MCM-41 has a significant effect on the steam stability of the resulting Al-containing MCM-41 materials.

436

2. EXPERIMENTAL The Al-grafted materials were prepared, at a target Si/Al ratio of 10, via an aqueous, nonaqueous or supercritical fluid (SCF) mediated alumination method; in the aqueous (or wet) method 1.0 g of calcined purely siliceous MCM-41 was added to a 50 ml solution of aluminium chlorhydrate (ACH) and stirred for 2 hours. The solid was obtained by filtration and thoroughly washed with distilled water (until free of CI ions), dried at room temperature and calcined in air at 550^0 for 4 hours to obtain the (wet) Al-grafted material which was designated HIO. In the non-aqueous (or dry) route 2.0 g of Si-MCM-41 was dispersed in 50 ml dry hexane and added to 150 ml dry hexane containing the appropriate amount of aluminium isopropoxide. The resulting mixture was stirred for 10 minutes and allowed to react at room temperature for 24 hours. The obtained powder was filtered, washed with dry hexane, dried at room temperature and calcined at 550"C for 4 hours to yield a sample designated PIO. For supercritical fluid mediated alumination, the required amounts of calcined mesoporous silica and aluminium isopropoxide were placed in a 60 ml magnetically stirred, high pressure autoclave and while under vigorous stirring, the temperature was raised to llO^C before pressurization with supercritical propane (150 bar). Vigorous stirring was continued for 19 hours after which the autoclave was depressurized slowly over 15 min. The autoclave was allowed to cool to room temperature and the (dry) sample recovered. The dry sample was then calcined at 600"C for 4 hours to obtain the Al-grafted material designated SIO. The high temperature hydrothermal (steam) stability of the Al-graftcd materials was evaluated by subjecting them to heat treatment at 900"C for 4 hours in a flow of nitrogen saturated with water vapour at room temperature. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of the wet, dry and supercritically grafted materials (all with a Si/Al ratio of ca. 10) before and after steaming at 900"C for 4 hours. The patterns of the Al-graftcd materials are typical of well ordered MCM-41; all three samples exhibit an intense (100) diffraction peak and some well resolved higher order (110), (200) and (210) peaks. The influence of the alumination pathway on steam stability is clearly illustrated in Figure 1. The XRD patterns of the steamed samples clearly show that despite a similar Al content, the supercritically grafted sample is much more steam stable compared to the other two samples. It is also clear that the dry grafted sample is more stable compared to the wet grafted sample. The wet grafted sample (HIO) is severely degraded after the hydrothermal treatment. The supercritically grafted sample (SIO) on the other hand, still retains excellent structural ordering after steaming. The dry grafted sample (PIO) also exhibits considerable stability, which is intermediate between that of the wet-grafted and supercritically grafted samples. Table 1 shows the textural properties of the three samples before and after steaming (the steamed samples are designated as SHIO, SPIO and SSIO). After steaming, the wetgrafted sample retained only 10% of its original surface area and 20% of pore volume. The dry grafted sample, on the other hand, retained 78% of its original surface area and 53% of pore volume. The supercritically grafted sample retained much of its surface area (88%) and

437

pore volume (79%) after steaming. Another indicator of stability is the extent to which the pore size reduces after steaming. As shown in Table 1, steaming induced reduction in pore size is lowest for the supercritically grafted sample.

J 0

2

(a)

4

2^/°

6

8

0

2

4

6

2 91°

8

0

2

4

6

2er

Fig. 1. Powder XRD patterns of (a) wet-grafted, (b) dry-grafted and (c) supercritically grafted Al-MCM-41 before (top) and after (bottom) steaming at 900°C for 4 hours. Table 1 Textural properties and acidity of Al-grafted materials before and after steaming Pore APD[a] Sample Surface Volume Area

(A) (cm'/g) (m'/g) HIO 0.73 35.0 835 0.15 SHIO 86 36.8 0.83 PIO 902 0.44 25.0 SPIO 705 0.92 36.0 SIO 833 729 0.73 33.1 SSIO [^IAPD = Average Pore Diameter estimated using the relation APD = 4V/S, where V is the mesopore volume. The pore size estimates are given here only as an indication of the trend and extent of reduction after steaming. Figure 2 shows the nitrogen sorption isotherms of the three samples before and after steaming at 900'^C. All three samples exhibit sorption isotherms characteristic of well ordered MCM-41. This is consistent with the XRD traces in Figure 1 and confirms that the alumination pathway does not have any significant influence on the structural ordering. This is a key observation in a comparative study - and we can rule out structural ordering as a factor in determining steam stability. The sorption isotherms indicate that the wet grafted sample completely losses its mesoporous structure - the sorption isotherm of the steamed wet grafted sample (SHIO) does not exhibit any mesopore filling step. The isotherm of the steamed dry grafted sample (SPIO) has a much-reduced mesopore-filling, which indicates considerable degradation but still retains some mesoporous character. The supercritically grafted sample, on the other hand, presents very high steam stability; the isotherms of both the parent and steamed sample exhibit a sharp mesopore filling step characteristic of wellordered MCM-41 materials.

438

n

1iji

E (0 03

600

(a)

600

(b)

600 (c)

400

400

400

200

200

200

"o >

0 00

0.5

1 0

0 0.0

0.5

1 0

0.0

0.5

1.

Partial pressure (P/Po) Fig. 2. Nitrogen sorption isotherms of (a) wet-grafted, (b) dry-grafted and (c) supercritically grafted Al-MCM-41 before (top) and after (bottom) steaming at 900°C for 4 hours. The results show that Al-grafted MCM-41 materials prepared via dry grafting exhibit considerably higher steam stability compared to materials grafted in aqueous media. The difference in stability between dry grafted and wet grafted Al-MCM-41 materials is most likely due to the way in which the grafted Al interacts with the host silica framework and in particular the extent to which the Al is sorbed onto rather than into the framework. Under dry grafting conditions it is likely that the Al is sorbed mainly on the outermost surface of the host Si-MCM-41 while under wet (aqueous) grafting conditions the Al may penetrate the framework (due to greater hydrolysis of the host silica framework) and occupy both surface and near surface sites. The extent to which Al is sorbed into (penetrates) the host silica framework is expected to be lower when the grafting is performed under dry conditions. This is because the host silica framework does not undergo any significant hydrolysis during the 'dry' grafting procedure. For supercritically grafted samples, it is likely that the low solvating power of SCFs ensures even more efficient deposition of Al onto rather than into the silica framework. No hydrolysis of the host silica framework occurs during the SCF mediated alumination. Furthermore, better dispersion of Al achieved under SCF conditions can be expected to coat efficiently the surface of the host Si-MCM-41 with a protective aluminosilicate layer. Removal of Al (i.e., dealumination) which occurs during steaming is therefore more detrimental to the structural integrity of wet grafted samples due to extraction of Al sited deeper within the framework. Steam stable Al-grafted MCM-41 materials are therefore best prepared via alumination pathways that efficiently coat the outermost parts (i.e., pore wall surfaces) of the host pure silica material with Al without introducing Al deep into the silica (pore wall) framework. REFERENCES 1. J.Y. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 2. D.T. On, D. D. Giscard, C. Danumah and S. Kaliaguine, Appl. Catal. A, (2001) 299. 3. R. Mokaya, Angew. Chem. Int. Ed., 38, (1999) 2930. 4. R. Mokaya, Chem. Commun. (2001) 633; S.C. Shen and S. Kawi, Chem. Lett. (1999) 1293 5. Y. Liu and T.J Pinnavaia, J. Am. Chem. Soc, 122, (2000) 8791. 6. Z. Zhang, et al., Angew. Chem. Int. Ed., 40, (2001) 1258. 7. D.T. On and S. Kaliaguine, Angew. Chem. Int. Ed., 41, (2002) 1036. 8. R. Mokaya, ChemPhysChem, 3, (2002) 360.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

439

Macroporous titanium oxides: from highly aggregated to isolated hollow spheres P. Reinert^ C. G^aiIlat^ R. Spitz^ and L.Bonneviot^ ^Laboratoire de Chimie Theorique et Materiaux Hybrides, Ecole Normale Superieure de Lyon, 46 allee d'ltalie, 69364 Lyon et Institut de Recherches sur la Catalyse, 2 av A. Einstein, 69626 Villeurbanne. ^Laboratoire de Chimie et Procedes de Polymerisation, Ecole Superieure de Chimie Physique Electronique de Lyon, 43 boulevard du 11 novembre, 69616 Villeurbanne The control of both poly condensation of the titanium alkoxide, [Ti(OR)4], and alkoxide coverage of the templating monodispersed polystyrene (PS) beads in suspension lead to various morphologies of macroporous solids after calcination. By increasing the ratio H20/[ri(OR)4], the solids progressively evolve from a dense interconnected macroporous network to almost isolated hollow spheres characterized using SEM. 1. INTRODUCTION In the process to further improve the diffusion of liquid or solid within a porous solid (absorbent or catalyst), the search for hierarchical porosity is a very challenging topic [1]. In this newborn field, the control at the macroscopic scale among that of the meso- and microscopic scales is the newest [2-5]. A better knowledge on the conditions required to generate connected macropores is necessary. The sedimentation-aggregation method may lead to v£irious stages of aggregation of the templating PS beads and, therefore, to solids of v£irious density. The preparation of hollow spheres aggregates of low connectivity generate a superporosity at a higher scale while isolated spheres may find interesting applications. There are a variety of methods used to fabricate hollow spheres and one of the newest is based on polystyrene bead templating. One of these method developped by Caruso et al [6] consists of coating nanoparticles of oxides on the beads by using layers of charged polymers, that allows the adherence of the successive inorganic layers and control their thickness. Encapsulation has also been envisaged by Zhong et al. who obtained hollow spheres using a gelification process in the presence of polystyrene beads entrapped between two glass plates [7]. Other authors [8] performed the poly condensation of the alkoxide precursor using a carbon template and are able to move from macroporous to hollow spheres of oxides by changing the number of infiltration. The present method consists of an impregnation of the PS beads by titanium alkoxide in solution followed by a sedimentation-aggregation process [9] that allows also a control of the morphology of the solid. The packing of the impregnated beads depends on the hydrolysis rate of the alkoxide controlled here varying both water/alkoxide ratio and stirring rate during the addition of the alkoxide in the suspension of the PS beads.

440

2. EXPERIMENTAL Non-crosslinked PS beads were synthesized by emulsifier-free emulsion polymerization. The polymerization was carried out in a 1 -L glass reactor equipped with an anchor stirrer and filled with water heated to 70 °C and degassed under N2. Ammonium persulfate was used as initiator. The beads obtained have an average size of 630 nm and their monodispersity was determined by light-scattering measurement and also estimated using SEM. The beads were kept in water and were transferred in absolute ethanol just before the synthesis and their size increases up to 710 nm. Titanium ethoxide (diluted in absolute ethanol) was slowly added under stirring to the suspension of polystyrene beads in absolute ethanol. Water may be added at this stage in order to control the condensation of the oxide precursor. After 20 min of stirring, the mixture was kept at room temperature to allow the sedimentation of the beads to proceed. After 5 days, the supernatant solution was eliminated and the solid dried at 80°C overnight. Polystyrene was removed from the solid by calcination in air at 500°C for 8 hours. 3. RESULTS AND DISCUSSION Various synthesis parameters such as water content in the mixture, alkoxide content or stirring time were studied (Table 1). For all the samples, the beads diameter measured after the drying step is larger (730 nm) than the initial one (710 nm). This suggests that the titanium alkoxide has reacted with water giving a layer of titanium oxide precursor on the beads (amorphous according to XRD measurement). Moreover this diameter increases from 730 to 770 nm when H20/Ti molar ratio goes from 0.2 to 1.9 for the same amount of alkoxide (compare samples A to E). This shows that thicker walls are obtained at higher degree of polycondensation of titanium oxide (higher H20Ari ratio). For ratios H20Ari ^ 1, the titanium oxide obtained after calcination at 500°C (anatase phase) is a macroporous solid (samples A and B). Table Synthesis conditions and morphological characteristics of the titanium oxides Coated beads Calcined solid (500°C) H20/Ti x^ Sample diameter ^ (nm) molar ratio Pore diaWall thickMorphology -meter ^ (nm) -ness ^ (nm) 02 730 500 macroporous solid A 2x50 1 750 macroporous solid 500 2x50 B 1.4 765 500 55 C hollow spheres 765 hollow spheres 1.6 560 D 65 1.9 770 hollow spheres 545 E 55 0.2 1.25 765 macroporous solid 520 F 2x50 780 550 hollow spheres 0.15 1.5 C} 55 0.1 2 H dense gel / / / 1 I^ 0.2 770 hollow spheres 490 60 ^ weight composi tion of he s tarting mixture: 1 PS : x TET : 18 Ethanol ^ estimated from SEM observations ^ stirring time : 16 hours

441

This material is characterized by cavities (average size of 500 nm) which are connected through windows of about 110 nm diameter (Figure 1 b). These windows resuh from the numerous contact points existing between beads at low hydrolysis rate. This can be observed on the beads reported in Figure la, which surfaces present imprints resulting from the contact with other spheres. These cavities in the solid are delimited by double-wall of about 2x50 nm large. It should be noted that the complete merging of these walls can be achieved during the synthesis procedure with a centrifiigation step just after decantation. The framework contraction calculated for this morphology is about 20%. For higher ratios, 1< H20/Ti Ph (L-lsomer) H

H

0

0

1

BrL-isomer

0

1 2 3 4

H

6

5

1

Aj^A,0(CHj),5CH3 .W H 0

^

(^

.^ N

f Br

0

N ..^.-.J: N C A 0

^f' L-isomer

1 ^> . H 6

.0(CHz),5CH3 6

Fig. 1 (A) Schematic illustration of mesoporous silica modification by the residue transfer method from template. (B) Surfactants used in this research. 2. EXPERIMENTAL SECTION Mesoporous

silica

was

prepared

using

tetramethoxysilicate

(TMOS)

or

tetraethoxysilane (TEOS) as silica sources under conditions described later. The obtained silica/surfactant composites (50 mg) were hydrolyzed by conc-HCl (0.5 g) in rcfluxcd THF (30 mL) for 8 hours.

Parts of the obtained materials were calcined at 500 °C for 6 hours.

3. RESULTS AND DISCUSSION The XRD patterns of the obtained materials from 1 are summarized in Table 1.

The

patterns assigned to thermally-stable mesoporous silica in hexagonal and cubic phases can be prepared under TEGS-containing acidic condition and TMOS-containing basic conditions, respectively.

As summarized in Table 2, mesoporous silica can be obtained

from surfactants containing various amino acid residues under the optimized conditions. Especially, in the cases of 1, 2, and 4, both hexagonal- and cubic-structured materials were obtained upon the selection of the catalyst and the silica source. structures obtained from 1 were confirmed by TEM observation.

The cubic and hexagonal

467

Table 1 XRD patterns of mesoporous silica templated by 1 under various conditions Silica

Catalyst

Source

XRD Peaks

XRD Peaks

d

(Uncalcined)

(Calcined)

1 nm

Assigned Structure

TMOS

HCl"

(100)

(100)

A.T

Hexagonal

TEOS

Hcr

(100), (110), (200)

No Peaks

A.y

Hexagonal (Unstable)

TEOS'^

HCl"

(100), (110), (200)

(100), (110)

A.y

Hexagonal

TEOS'

HCl"

(100)

(100)

4.5''

Hexagonal

TMOS

NaOH''

(211), (220), (332)

(211), (220), (332)

3.y

Cubic

TEOS

NaOH''

(211)

No Peak

3.6"

Cubic (Unstable )

4.5'' TEOS'' NaOH'' (100) Hexagonal (100) "Reaction Condition: 1/H20/Silica Source/HCl/EtOH = 0.13/131/1 /8.8/3 in molar ratio, 4h reaction at room temperature. ''TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 2h. ' TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 7h. ''Reaction Condition: 1/H20/Silica Source/NaOH = 0.12/141/1/0.27 in molar ratio, 4h reaction at room temperature. '' Value of (100) peak for uncalcined sample. ^Value of (211) peak for uncalcined sample.

Table 2 XRD patterns of mesoporous silica templated by various surfactants Surfactant

Catalyst

XRD Peaks

XRD Peaks

d

(Uncalcined)

(Calcined)

1 nm

Structure

(100), (110)

4.3'

Hexagonal Hexagonal

(100), (110), (200)

Assigned

2

HCl"

3

HCl"

(100), (110)

(100)

4.4'

4

HCl"

(100), (110)

(100)

4.5'

Hexagonal

5

HCl"

(100), (110)

(100)

4.7'

Hexagonal Disordered

6

HCl"

(100)

No Peaks

4.8'

2

NaOH''

(211), (220), (332)

(211), (220), (332)

3.5''

Cubic

3

NaOH''

(100)

No Peaks

4.0'

Disordered

4

NaOH''

(211), (220), (332)

(211), (220), (332)

3.6''

Cubic

5

NaOH''

(100), (200)

(100)

4.0'

Hexagonal

(100) NaOH'' (100), (200) 6 4.4' Hexagonal "Reaction Condition: Surfactant/H.O/TEOS/HCl/EtOH = 0.13/131/1/8.8/3 in molar ratio, 4h reaction at room temperature. ''Reaction Condition: Surfactant/HjO/TMOS/NaOH = 0.12/141/1/0.27 in molar ratio, 4h reaction at room temperature. TEOS was prehydrolyzed in aqueous ethanol with HCl at 70 °C for 2h. 'Value of (100) peak for uncalcined sample. ''Value of (221) peak for uncalcined sample.

468

Maintenance of regular structures of hexagonal and cubic silica from 1 during the hydrolysis process was also revealed by XRD measurement.

Transfer of the amino acid

residue from the template to silica backbone was next investigated by FT-IR spectroscopy. In IR spectrum of the cubic mesoporous silica composite from 1, characteristic peaks for V3XCH2), v(CO, ester), and v(CO, amide) were detected at 2925, 1742, and 1685 cm', respectively.

Selective removal of the alkyl tails by hydrolysis with HCl under refluxed

condition was confirmed by disappearance of v^,(CH2) peak and preservation of v(CO, amide) peak.

The peak originally observed at 1742 cm"' was shifted to 1734 cm"' that can

be assigned to CO stretching vibration for COOH group.

The similar spectral features

were observed for hexagonal silica prepared from 1. The composition of the silica/1 composites were analyzed by TGA, and 44% and 46% of organic components were detected for the cubic and hexagonal samples, respectively. The organic components in hydrolyzed silica decreased to 26% and 27% for the cubic and hexagonal ones, respectively.

The latter values are in good agreement with theoretical

values (22% for cubic and 23% for hexagonal) calculated with molecular weight change upon hydrolysis of the ester linkage.

These thermal analytical data confirm that the alkyl

chains were selectively removed only with ignorablc decomposition of the amino acid moiety. 4. CONCLUSION In conclusion, mesoporous hybrid preparation using the novel kinds of the surfactants and subsequent selective removal of alkyl chains provide mesoporous silica with inner surface densely covered by amino acid residues.

This residue transfer method

will be also applied to effective immobilization of the other biological functions to inorganic silica mesopore structure and be highly useful for preparation of bio-inorganic nanocomposites. REFERENCES 1. A. Stein, B. J. Melde and R. C. Schroden, Adv. Mater., 12 (2000) 1403. 2. A. Sayari and S. Hamoudi, Chem. Mater., 13 (2001) 3151. 3. K. Mollcr and T. Bcin, Chcm. Mater., 10 (1998) 2950.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

469

The synthesis of optically active amino acid over Pd catalysts impregnated on mesoporous support Kyung Hye Chang^, Yong Ku Kwon'' and Geon-Joong Kim^ ^ Department of Chemical Engineering, Inha university, Incheon 402-751, Korea ^ Department of Polymer Sciene and Engineering, Inha university, Incheon 402-751, Korea Pd metals immobilized on SBA-15 and NaY were applied as catalysts in the synthesis of amino acid. These catalysts afford a high level of enantioselectivity in the asymmetric hydrogenation of a-keto acids to corresponding amino acids. Indeed, this reaction has been investigated and reported only using Pd metals immobilized on active carbons. 1. INTRODUCTION Almost enantioselective catalysts are soluble metal complexes containing some type of chiral ligands. It appears that such catalysts are effective because of the chiral environment created around the active metal center by the chiral ligands. The heterogeneous cataysts are usually produced by attaching ligands to an insoluble matrix and then using these insoluble ligands to complex with the active metal species. The other type of chiral heterogeneous catalyst can be either a supported metal which has been treated with a chiral modifier or an active metal on a chiral support. Asymmetric syntheses of a-amino acids from their corresponding a-keto acids have been reported[l]. Hiskey and Northrop[2] have demonstrated the synthesis of optically pure a -amino acids by catalytic hydrogenation and subsequent hydrogenolysis of the Schiff bases of a-keto acids with chiral a-methylbenzylamine. Harada[3] reported the syntheses of optically active amino acids in a way principally similar to those done by Hiskey but by the use of a-phenylglycine in alkaline aqueous solution (optial purity 40-65%). These reactions are interesting because they are essentially a kind of asymmetric transamination reaction performed by catalytic hydrogenation and hydrogenolysis. To date, however, very few publications have dealt with the use of zeolites as catalysts for asymmetric reactions. The aim of this study is to demonstrate the aptitude of Pd-containing mesoporous materials for enantioselective catalytic hydrogenation. In this study, Pd metals immobilized on SBA-15 and NaY were applied as efficient catalysts in the synthesis of amino acid. These catalysts afford a high level of enantioselectivity in the asymmetric hydrogenation of a-keto acid to corresponding amino acids. Indeed, this reaction has been investigated and reported only using Pd metals immobilized on active carbons.

2. EXPERIMENTAL The (S)-(-)-a-Methylbenzylamine(2.42g, 0.02 mole) in ethanol(30 iii^) was added to pyruvic acid(0.88g, 0.01 mole) in cold ethanol (40 ni^). The mixture was allowed to stand for 30 min at room temperature. To the solution was added palladium on supports, and then it was

470

CHj

Pd / Support Hydrogenation COOH

'NH

Pd(0H)2 Hydrogenoloysis coo-

U H3C'

Scheme 1. Reation pathway to synthesize chiral amino acid hydrogenated for 8 hr at room temperature. The catalyst was removed by fikration and washed with hot water. The combined solution was evaporated to 20 mL To the concentrated solution was added 30 % aqueous ethanol (50 iii^ and palladium hydroxide on charcoal. The hydrogenolysis was carried out at room temperature for 12 h using Pd(0H)2 as shown in Scheme 1. The filtrate was concentrated to 5 111^ in vacuo. (S)(+)-Alanine was obtained (0.07g), and ee% was determined by instrumental analysis. 3. RESULTS AND DISCUSSION In this work, Hiskey-type reaction was carried out in order to screen the effect of support. Initially comparative investigations were carried out under the given reaction conditions to establish the suitability of the prepared Pd-containing catalyst for hydrogenation. The optical purities of the resulting amino acids were dependant on the kinds of supports and the enantiomeric excess values vary according to the composition of zeolitic materials. Figure 1 shows the relation between the initial Pd wt% and enantioselectivity. In this case, the reaction was conducted with a hydrogen pressure of 3.5 atm. The ee(%) increased with the increase in the loading amount of palladium on the support. An optical purity of about 81% was obtained on the 10%Pd/SBA-15, and the highest optical yield of 88% was obtained

100

100

80

80

60

60

40

—•—Pd/C —0—Pd/N«Y —A— Pd / AljOj —A—Pd/SBA-15

201-

4

6 Wt

8

10

:^^^^^\

3 ^

40 20 0

•

—•—Pd/( —0—Pd/NaN —A—Pd/AljO, -A—Pd/SBA-15

2

3

4

%

Fig. 1. Relation beteen the initial Pd Wt % and ee %

Fig. 2. The effect of hydrogen pressure on the product ee%

471 COOH HjN——H

1?K

NHT^O

icture I.Major structure

PhR"

R (S)

II. Minor structure

Scheme 2. Conformation of substrates when NaY was used as the support. 10%Pd/Active Carbon and Na-Mordenite gave a relatively low enantioselectivity of around 65 ee% for the synthesis of alanine from Pyruvic Acid. The unsupported Pd black itself also gave a low enantioselectivity, showing 52 ee(%). When using acidic support such as HY, as compared to zeolite Y in sodium form, a decrease in the optical yield was investigated. No improvement in enantioselectivity was achieved by using acidic supports in the hydrogen form. In Figure 2, the plots show the effect of hydrogen pressure on the product ee%. Optical yields mainly depended on hydrogen pressure. As shown in Figure 2, the maximum ee% of the product was found at the hydrogen pressure of 3.5atm. The effect of Pd metal size was also investigated in this reaction. As mentioned above, the enantioselectivity was influenced by the loading amount of Pd on supports. This result indicates that larger crystallite size of Pd would provide the suitable surfaces for the effective enantio-differentiation in the hydrogenation. Figure 3 shows the TEM images of Pd metal supported on the mesoporous materials. The Pd metals were observed to be apparently aggregated, and the mean size of metal particle became larger with the increased amount of Pd on the supports. Nitta el al.[4] have reported that the catalyst with the larger crystallite size gave the higher optical yield in the enantio-differentiating hydrogenation of methyl acetoacetate. They predicted

1%. (a) 10 % Pd loading on MCM-41

(b) Enlargement of photograph(a)

Fie. 3. TEM imaee of Pd-loaded MCM-41

472

that the catalyst with a larger crystallite size had regularlyH R arranged metal atoms on the _ / catalyst surface providing sites for ••S^'//, / \ a strong and regular adsorption of 0 ^N Ph the modifier, propitious to obtain \ / a high optical yield. (Pd)n When this fact was taken into account, the results in Fig. 1 indicate that larger metal would Scheme 3. Major coformation adsorpted on Pd provide the appropriate surface metals for the enantio-differentiating hydrogenation of Pyruvic Acid to (S)-alanine. The enantioselective mechanism proposed in the literature stated that the structure I might be the most predominant structure and structure II might be a minor structure. Structure I resulted in (S)-amino acid when (S)-amine was used. On the other hand, structure II resulted in (R)-amino acid when (S)-amine was used. When the alkyl group of keto acid is methyl(pyruvic acid), conformation of reactant might be composed mainly of structure 1, therefore resulting in highly optically active alanine as indicated in Scheme 2. However, according to the experimental results, structure 1 seems to be a major conformation in this reaction. The structure I might form a five-membered cyclic structure on Pd metal and then the structure would be adsorbed at the less bulky side of the molecule. On the other hand, structure II might not form such a cyclic structure because of the steric hindrance. Ihe difference in the ease of formation of the cyclic complex between structure 1 and II might be an important factor why structure I is a major conformation in the reaction. It is assumed that the adsorpted state of reactants as structure I or II may be influenced by the reaction conditions such as the Pd metal size, resulting in the different enantioselectivity. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Reseach Program of the Korea Science & Engineering Foundation. REFERENCES 1. K. Harada and K. Matsumoto, J. Org. Chem. 32 (1967) 1794. 2. R.G.Hiskey and R.C.Northrop, J. Am. Chem. Soc. 83 (1961) 4798. 3. K. Harada, Nature, 212 (1966) 1571. 4. Y.Nitta, F.Sekine, T.Imanaka, and S. Teranishi, Bull.Chem.Soc.Jpn. 54 (1981) 980.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

473

Sulfonic acid-functionalized periodic mesoporous organosilicas S. Hamoudi and S. Kaliaguine Department of Chemical Engineering, Laval University, Quebec, GIK 7P4, Canada Mesoporous ethane-silica materials containing sulfonic acid bearing groups were synthesized using bis(trimethoxysilyl)ethane (BTME) and mercaptopropyltrimethoxysilane (MPTMS) as framework precursors under acidic or basic conditions. Pluronic PI23 or polyoxyethylene (Brij-56) were used as surfactants for the acidic synthesis, while cetyltrimethylammonium chloride (CTAC) was used for the basic synthesis. Conversion of the mercaptopropyl groups into sulfonic acid moieties was achieved via oxidation using hydrogen peroxide. Ordered hexagonal mesostructures with high surface areas (up to 1180 mVg) and narrow pore size distributions (up to 5.4 nm) were obtained. 1. INTRODUCTION At the end of the last decade, a new class of mesostructured silica cumulating organic and inorganic moieties within the framework was discovered [1-3]. These novel periodic mesoporous organosilica (PMO) materials exhibited a homogeneous distribution of organic fragments and inorganic oxide within the framework and demonstrated highly ordered structures and uniform pores. In order to confer to PMO materials additional functionalities, a combination of both bridging organic functional moieties in the framework and terminal organic functional groups protruding into the pores stands for a judicious strategy. The main objective of the present work is to take advantage of both surface functionalization and framework modification to design sulfonic acid bearing hybrid mesoporous ethane-silica materials. In fact, there has been considerable interest in the development of mesostructured solid acid materials for their potential use as advanced materials or as acid catalysts. This opens a new route for the engineering of acidic mesoporous materials with hydrophobic properties, wherein the amount of hydrophobic groups is not limited as is the case for conventional mesoporous materials. Indeed, the combination of both functionalities (hydrophobic and acidic) may result in interesting surface properties facilitating for instance diffusion and/or adsorption of reactants and products in acid catalyzed reactions. 2. EXPERIMENTAL Sulfonic acid functionalizcd mesoporous ethane-silica materials were prepared under acidic conditions using either PI23 copolymer or polyoxyethylene Brij-56 as surfactants. The resulting materials are denoted here as SAF-MES-Al and SAF-MES-A2, respectively. When PI23 copolymer was used as surfactant, the synthesis method used herein was recently described by Burleigh et al. [4], except that in our work a supplementary step for the incorporation of MPTMS was added. Therefore, the synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; PI23, 0.05; HCl, 36; H2O, 1000. In the presence of Brij-56, the synthesis procedure was adapted from the method reported by Coleman and Attard for all-

474

silica materials [5]. The synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; Brij-56, 0.24; HCl, 83; H2O, 9260. For the material prepared under basic conditions (SAFMES-B), a synthesis procedure modified from method II described by Inagaki et al. was adopted [1]. Hence, the synthesis gel molar composition was: BTME, 0.75; MPTMS, 0.25; CTAC, 0.57; NaOH, 2.36; H2O, 353. For all the materials reported herein, conversion of the thiol groups into sulfonic acid moieties was carried out on the solvent-extracted samples by oxidation with hydrogen peroxide [6]. Adsorption measurements were performed on a Coulter Omnisorp 100 gas analysis apparatus. Pore size distributions were calculated using the Barrett-Joyner-Halenda (BJH) method. XRD spectra were obtained on a Philips X-ray diffractometer. Thermogravimetric analysis was carried out on a Perkin Elmer 7 scries thermal analyzer from ambient temperature to 800 °C at a heating rate of 5 °C/min under nitrogen atmosphere. The acid capacity of the SAF-materials was determined by NaOH titration. The proton conductivity was assessed using impedance spectroscopy. 3. RESULTS AND DISCUSSION Nitrogen adsorption analysis for extracted materials displayed type IV isotherms with marked hysteresis loop for SAF-MES-Al material and sharp adsorption step at relative pressures ranging between 0.5 and 0.8 (Figure 1). Furthermore, the SAF-MES-A2 and SAFMES-B exhibited quite similar adsorption isotherms without hysteresis loop. The corresponding BJH pore distributions were reasonably narrow, and centered around 3.5-5.4 nm (Figure 1, inset). As reported in Table 1, the BET surface areas ranged from [520 to 1180 mVg], whereas the pore volumes reached ca. 0.64 to 0.69 cm7g. Moreover, the SAF-MES-A2 and SAF-MES-B materials exhibited prominent sharp peaks in the diffraction patterns at approximately 20 = 1.6° and 2°, respectively (Figure 2), characteristic of hcxagonally ordered mcsoporous materials. Table 1 Pore Pore S03H^'^ (mcq/g) size volume (nm) (cmVg) 0.69 5.4 522 SAF-MES-Al 0.83 1184 0.64 SAF-MES-A2 3.3 0.93 0.64 882 3.5 SAF-MES-B 0.62 ^'^Propyl-sulfonic acid loading determined from TGA peak

Sample

SBET

(m'/g)

SOiH^^^ (mcq/g) 0.62 0.77 0.53 at 450 °C.

Proton conductivity^"^ (S/cm) 1.31 X 10-^ 1.38 X 10-' 4.78 X 10-^

^^^ Determined by titration and defined as mmol HVg Si02. ^"'^ Proton conductivity at ambient temperature and relative water content of 60 %. As depicted in Figure 3, thermogravimetric analysis showed that all the sulfonic acid modified materials displayed a peak centered at 100 °C attributed to the desorption of water. Above this event, slightly different thermal behaviors depending on the surfactant used were observed. Indeed, both the samples synthesized under acidic conditions were thermally stable until ca. 300 °C, whereas the SAF-MES-B exhibited moderate weight loss (3 %) with a maximum at 240 °C assigned to the partial thermal decomposition of the alkyl-sulfonic acid groups leading to SO2 release [6]. Subsequently, all the three materials exhibited comparable

475

thermal profiles. A more or less marked weight loss (ca. 7 %) taking place between 280 and 380 °C was assigned to the pyrolysis of the pendant unreacted mercaptopropyl groups in the pore systems. A subsequent event occurring between 380 and 520 °C was attributed to the thermal decomposition of the whole alkyl sulfonic acid groups [7]. The last event occurring above 520 °C was ascribed to the partial decomposition of the ethane bridging groups in the framework [3, 7]. Impedance spectroscopy analysis was performed at ambient temperature on the SAF-MES materials at different water contents. As depicted in Figure 4, an increase in the samples water content yielded a continuous rise in the proton conductivity exceeding 10'^ S/cm at water to solid ratio above 100 %. Furthermore, the registered profiles clearly indicated that the materials synthesized under acidic conditions exhibited the highest proton conductivities if compared to their homologue synthesized under basic conditions. Moreover, the acid capacity of the different samples in agreement with their propyl-sulfonic acid amount determined by TGA also followed the same tendency (see Table 1). Such behavior was attributed to a better conversion of the thiol groups into sulfonic acid moieties reached for the materials synthesized under acidic conditions, as previously reported for all-silica materials [8].

SAF-MHS-A2

3 5 2-Thcta (Degree)

Fig. 1. Nitrogen adsorption/desorption isotherms and BJH pore size distributions (inset).

7

Fig. 2. X-ray diffraction patterns.

4. CONCLUSION Sulfonic acid bearing ethane-silica mesostructurcd materials were synthesized for the first time. The procedure involved the synthesis of mercaptopropyl-attached materials, followed by conversion of the mercaptopropyl groups into sulfonic acid moieties using hydrogen peroxide. Different synthesis procedures under acidic and basic conditions led to ordered mesostructurcs. The sulfonic acid modified materials synthesized under acidic conditions were shown to be thermally stable up to 300 °C. These materials exhibited appreciable sulfonic group concentrations neighboring 1 meq HVg as well as high proton conductivities beyond 10'^ S/cm at ambient temperature.

476 A 100

:^.^-^

'? ie-44.

B

SAF-MES-Al SAF-MES-A2 SAF-MES-B

^

Water content (w/w %)

Fig. 4. Room temperature proton conductivity as function of water content. 200

400

600

Temperature (°C)

Fig. 3. (A) Thermogravimetric weight loss curves and (B) derivative plots for (a) SAF-MES-Al; (b) SAF-MES-A2 and (c) SAF-MES-B.

REFERENCES 1. S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki and S. Inagaki, J. Am. Chem. Soc, 121 (1999)9611. 2. B. J. Melde, B. T. Holland, C. F. Blanford and A. Stein, Chem. Mater., 11 (1999) 3302. 3. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 4. M. C. Burleigh, M. A. Markowitz, E. M. Wong, J. Lin and B. P. Gaber, Chem. Mater., 13 (2001)4411. 5. N. R. B. Coleman and G. S. Attard, Micropor. Mesopor. Mater., 44-45 (2001) 73. 6. S. Mikhailenko, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Micropor. Mesopor. Mater., 52 (2002) 29. 7. D. Margolese, J. A. Melero, S. C. Christiansen, B. F. Chmelka and G. D. Stucky, Chem. Mater., 12 (2000) 2448. 8. D. Trong On, D. Desplantier-Giscard, C. Danumah and S. Kaliaguine, Appl. Catal. A. Gen., 222 (2001) 299.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

477

Functionalized periodic mesoporous organosilicas with sulfonic acid group Xingdong Yuan^'', Hyung Ik Lee^, Jin Won Kim*^, Jae Eui Yie'^ and Ji Man Kim^* ^ Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea ^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China '^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea Periodic mesoporous organosilicas (PMO) functionalized with sulfonic acid group have been successfully synthesized by co-condensation of bis(triethoxysily)ethane and 3mercaptopropyltrimethoxysilane in the presence of octadecyltrimethylammonium chloride as the structure-directing agent under basic conditions. The PMO materials have been characterized by nitrogen adsorption, powder X-ray diffraction, IR, and thermogravimetric analysis. The results indicate that the materials exhibit well-ordered mesostructures, high surface areas and solid acid properties. 1. INTRODUCTION Mesoporous silicas functionalized with organic groups, which can be designed for uniform and inorganic framework, have attracted much attention because of new catalytic and adsorption functions [1,2]. Both post-synthetic grafting and co-condensation methods have been used for preparation of various kinds of organically functionalized mesoporous materials. The organic functional groups not only give a new function to mesoporous materials but also enhance their hydrophobicity. However, the materials may exhibit weak hydrothermal stability for catalytic applications in the presence of water because the organic groups exist only on the surface of pore wall so that the framework structures (-Si-O-Si-) may be disintegrated during the application under hydrothermal conditions [3]. Recently, the development of PMO has led to great interest in the field of mesoporous materials [4,5]. The synthesis of PMO materials combines the structural properties of ordered mesoporous materials with the chemical properties of both silica and the organic bridging groups. The presence of organic groups within the frameworks is expected to give these materials a lot of favorable properties: structural rigidity and a degree of hydrophilic character that are useful for applications in aqueous systems. However, there are few reports on the modification and applications of PMO materials [6]. In the present work, we describe the direct synthesis of thiol-modified PMO materials (PMO-SH) and preparation of PMO containing sulfonic acid moieties (PMO-SO3H) through subsequent oxidation. The synthetic conditions for the functionalized PMO materials will also be discussed.

478

2. EXPERIMENTAL Bis(triethoxysily)ethane (BTSE), 3-mercaptopropyltrimethoxysilane (MPTMS) and octadecyltrimethylammonium chloride (ODTMACl) were used as received from Aldrich and Kogyo, Co. LTD. PMO-SH was synthesized using BTSE as the main framework source and MPTMS as functional group and ODTMACl as the structure-directing agent[7] , The synthesis procedure was as follows: 1.0 g of ODTMACl was added to 31.7 g of doubly distilled water under stirring in a polypropylene bottle to give a clear solution, and subsequently 0.47 g of NaOH was added to the surfactant solution at room temperature. 1.55 g of BTSE and 0.15 g of MPTMS were mixed in a separate vial. The framework source mixture was then added to the surfactant solution, and the resulting mixture was stirred at room temperature for 20 h. The gel compositions were (\-x) BTSE : x MPTMS : 0.57 ODTMACl : 2.4 NaOH : 350 H2O (;c = 0 - 0.25). The reaction mixture was heated at 95 °C in an oven for 21 h under static condition. The white precipitate solid was filtered off, washed with doubly distilled water and dried at 60°C overnight.. In order to remove the surfactant, 1.0 g of solid product was treated with 150 ml of a mixture of ethanol and HCl at 70 °C for 12 h. The product was filtered, washed with ethanol and dried at 60 °C for 12 h. This extraction procedure was repeated one more time to remove the surfactant completely. The PMO-SH was oxidized with H2O2 at room temperature for 24h. Finally the solid product was acidified by H2SO4 to produce sulfonic moieties (PMO-SO3H). Ion-exchange capacities of the PMOSO3H materials were determined using aqueous solution of NaCl (2.0 M) as exchange agent. In a typical experiment, 0.05g of PMO material was added to 10 g of NaCl solution. After reaching to equilibrium, the suspension was titrated using an aqueous solution of NaOH (0.01 M). A pure silica PMO material (Si-PMO) was synthesized under the similar conditions without MPTMS to compare with the PMO-SO3H 3. RESULTS AND DISCUSSION

2

3 4 5 2-Theta(degree)

Fig. 1. Powder X-ray diffraction patterns of (a) Si- PMO, (b) PMO-SH (0.1), (c) PMOSH(a 15), (d) PMO-SH (0.2), (e) PMO-SH (0.25) and (f) PMO-SO3H (0.25).

Figure 1 shows XRD patterns for the PMO materials after the extraction of surfactant. All the materials exhibit a very intense Bragg peak at low-angle and two or more weak peaks, which are characteristic of 2-d hexagonal {P6mm) mesostructures. There are no significant changes upon the oxidation of -SH group to -SO3H group, as shown in Figure 1(f). The t/100 intensities in XRD patterns decrease as the amount of thiol precursor increase. This may be related to the fact that the MPTMS contains fewer hydrolysable groups, so that when its amount increases, the degree of cross-linking within the framework decreases [8]. When x is above 0.2, the PMO-SH materials exhibit somewhat broad XRD patterns and a shoulder at low angle, which indicates that the materials are mixture with disordered material or different from perfect 2-D hexagonal structure.

479

1600 I

1

These results may be due to alkylthiol group of MPTMS that results in the disturbance 1400 for interaction between the surface of surfactant micelles and framework sources. 1200 IR spectra indicate that all the functionalized PMO materials exhibit strong 1000 bands at 2920 and 2890 cm'^ assigned to CH stretching and deformation vibrations, 800 1410 and 1270 c m ' corresponding to C-H deformation vibrations of the framework i 600 organic group. The peaks at 780 and 690 cm"' assigned to Si-CH2 stretching 400 vibrations. A weak peak at 2580 cm' for the PMO-SH materials, corresponding to S200 h H stretching vibration is disappeared after oxidation to the sulfonic acid group. N2 adsorption-desorption isotherms for P/P' the PMO-SH materials after surfactant Fig. 2. N2 sorption isotherms for extracted PMO extraction are shown in Figure 2. The materials: (a) Si-PMO, (b) PMO-SH (0.1), (c) isotherms for the PM0-S03H materials PMO-SH (0.15), (e) PMO-SH (0.2) and (f) PMO- coincide with data in Figure 2. When x = 0 SH (0.25). 0.2, the materials exhibit type IV isotherms without hysteresis loops, which are the wellknown characteristics of 2-d hexagonal mesoporous materials. A well defined step of the adsorption and desorption appears between partial pressures pIpQ of 0.3 ~ 0.4. The materials exhibit a very narrow pore size distribution, which means well-defined uniform pore dimensions. However, the PMO-SH {x = 0.25) gives somewhat flat and broad step in the mesoporous range, indicating that a disordered materials is formed as expected from XRD results. Table 1 summarizes BET surface areas, total pore volumes and pore sizes for the materials. The decrease in the surface areas and pore sizes after x = 0.2 also means disordered nature of the materials. Table 1 Structural properties of PMO materials Sample Si-PMO PMO-SH (0.1) PMO-SH (0.15) PMO-SH (0.2) PMO-SH (0.25)

SBET (mVg)

1050 1043 1116 873 783

Vp(cmVg) 0.838 1.133 0.852 0.759 0.646

Pore size (nm) 2.87 2.98 2.95 2.81 2.61

Figure 3 shows thermogravimetric analysis (TGA) results under nitrogen atmosphere for as-synthesized PMO materials. A weight loss of 2 ~ 5 wt% below 120°C is attributed to the loss of small amounts of residual water adsorbed to the materials. This is followed by weight loss of 30 - 35 wt% from 120 to 250°C due to surfactant decomposition. The PMO-SH material exhibits a weight loss around 350°C, which is the decomposifion of thiol group and is not observed from Si-PMO material. The PMO-SH materials with higher x values result in more weight loss in this range. An additional weight loss of 5 - 7 wt% above 500°C indicates

480

the decomposition of organic bridging group within the framework. The TGA results mean that the PMO materials containing functional group can be used below than 350°C. The PMOSO3H material can be used over 24 hr without any loss of catalytic activity for alkylation of phenol at 150°C, which means the material has also excellent hydrothermal stability. Table 2 shows the results obtained 1 • I 100 200 300 400 500 600 700 from ion exchange with NaCl and titration with NaOH to investigate the Temperature (°C) amount of acid sites for the PMOSO3H materials. All the acid capacities Fig 3. Thermogravimetric weight loss curves defined as mmol of H^ per g catalysts. Table 2 indicates that the acid capacities of the PMO-SO3H materials are good agreement with x value in the synthesis gel mixtures. The MPTMS in the initial mixture can be grafted on the surface when the x is lower than 0.15, whereas some thiol precursor may be remained as soluble species or incorporated within the framework. The authors are grateful for support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. 100

PMO-SH(O.l) Si-PMO

Table 2 Acid capacities of PMO-SO3H materials Sample

Calculated (mmol H^/g)

Titrated (mmol HVg)

Incorporation degree (%)

PMO-SOjHCO.l) PMO-SO3H(0.15) PMO-S O3H (0.2) PMO-S O3H (0.25)

0.57 0.85 1.15 1.45

0.57 0.84 0.93 1.07

100 98.8 80.9 73.8

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

I. Diaz, F. Mohino, P P Joaquin and E. Sastre, Appl. Catal. A, 205 (2001) 19. X. H. Lin, G. K. Chuah and S. Jaenicke, J. Mol. Catal. A, 150 (1999) 287. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, et al, J. Am. Chem. Soc, 121 (1999) 9611. S. Hamoudl, Y. Yang, I. L. Moudrakovskl, S. Lang, et al., J. Phys. Chem., B, 105 (2001) 9118. S. Y Guan, S. Inagaki, T. Ohsuna, O. Terasaki, Micro. Meso. Mater, 44 (2001) 165. T. Asefa, M. J. Maclachlan, H. Grondey, N. Coombs, et al., Angew. Chem. Int. Ed., 39 (2000) 10. M. C. Burieigh, M. A. Markowitz, M. S. Spector, and B. P Gaber, J. Phys. Chem. B., 105 (2001)9935. M. C. Burleigh, M. A. Markowitz, M. S. Spector, and B. P Gaber, Chem. Mater, 13 (2001) 4760.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

481

Exclusive incorporation of aluminum into tetrahedral site of the framework of periodic mesoporous organosilica Sung Soo Park^, Jong Hyeon Cheon'' and Dong Ho Park^* ^National Science Research Institute, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea ^Department of Chemistry, Inje University, Kimhae, Kyongnam, 621-749, Korea.

Al-containing periodic mesoporous organosilicas (Al-PMO) with hexagonal symmetry have been synthesized with varying the source and concentration of aluminum. Unlike MCM41, all Al species used in this study, except of aluminum hydroxide, were exclusively incorporated into the tetrahedral site of framework of PMO synthesized under the optimized reaction condition. The incorporation of aluminum into framework was identified by ^^Al and ^^Si - MAS NMR. The strength of Bronsted acid site of Al-PMO was weaker than that of Al-MCM-41, which is calculated by using HF/6-31G(d) basis set in Gaussian 98 program. 1. INTRODUCTION Surfactant-mediated synthesis method for mesoporous materials, which was initiated by Mobil group[l], made it possible to produce periodic mesoporous organosilicas[2]. These organic-inorganic hybrid materials containing of covalently-linked organic groups to Si inside the channel open up the possibility for many applications such as catalysis, sensing, enantioselective separation, asymmetric syntheses, chromatographic supports and so on[3,4|. With potential catalytic applications in mind, much attention has been given to isomorphous incorporation of heteroatoms into the silicate framework of MCM-41. Unlike MCM-41, the literatures relevant to heteroatom-incorporated PMO are limited yet. Since the Bronsted acidity of hydroxyl groups associated with 4-coordinate Al incorporated into framework is essential for acid catalytic application of PMO, the effort for synthesis of framework Al containing PMO is necessitated. We have found that all Al species except of aluminum hydroxide are exclusively incorporated into the tetrahedral site of framework of PMO under our synthetic condition, while the ratio of framework vs. nonframework Al were changed depending on Al source and concentration, in case of MCM-41. Here we report the exclusive incorporation of Al into tetrahedral site of the framework of PMO. The acidic property of Al-PMO was theoretically compared with that of Al-MCM-41 by Gaussian 98 program.

Corresponding author. Fax. +082-55-321-9718; e-mail: chempdhfglijnc.lnie.ac.kr; research grant: No. R052002-00922-0 from the Basic Program of the Korea Science & Engineering Foundation.

482

2. EXPERIMENTAL The synthesis of the Al-PMO was performed under basic condition with sodium hydroxide(Aldrich) using l,2-bis(trimethoxysilyl)ethane (BTME, Aldrich), cetyltrimethylammonium bromide (CTABr, Aldrich), and various aluminum source such as aluminum isopropoxide(Fluka), aluminum sulfate(Aldrich), aluminum nitrate(Aldrich), aluminum phosphate(Aldrich), aluminum acetylacetonate(Aldrich), aluminum hydroxide(Fluka), and sodium aluminate(Kokusan) at 95 °C for 21 h with varying the concentration of Al sources. X-ray powder diffraction (XRD) patterns were obtained by a Rigaku Miniflex 2200 diffractometer using Cu Ku radiation. Nitrogen adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 instrument. Surface areas were determined by the Brunauer-Emmett-Teller (BET) method and the pore size distribution curve was obtained using Barrett-Joyner-Halenda (BJH) method from the adsorption branch isotherms. The chemical environment of aluminum and silicon was characterized by ^^Si and ^^Al MAS NMR spectra using a Bruker DSX400 spectrometer in air at 6 kHz and 12 kHz, respectively. In order to compare the acidic properties of Al-PMO with that of Al-MCM-41, the electron density in modeling compound [Ai(OSi(OH)3)4]"' for Al-MCM-41 and [Al(OSi(OH)2CH3)4]'' for Al-PMO, respectively, is calculated by HF/6-31G(d) in Gaussian 98 program. 3. RESULTS AND DISCUSSION Al-PMO's were synthesized under the optimized reaction condition[5] from reaction mixture with the molar composition of 1.0 BTME - (0.0-0.1) Al source - 0.57 CTABr - 2.73 NaOH - 380 H2O. XRD patterns of as-prepared Al-PMO from reaction mixtures with Si/Al 53 2 (tEtrahedrai) molar ratio on the range of 10-300 reflect p6mm I hexagonal symmetry lattice (not shown). As compared with the XRD of aluminum-free PMO, n si/Ai even low level of aluminum incorporation \\ molar ratio affects the quality of XRD pattern, which is SBA-3>DDA-HMS and ED>AM>P/, respectively, depending on the pore volume, molecular size and reactivity of organosilane. These functionalised samples are effective catalysts with high activity for the Knoevenagcl condensation reaction of benzaldehyde with ethyl cyanoacetate. However, the over-loading is unfavorable to the initial activity. 1. INTRODUCTION Grafting organic functional groups onto the internal surface of mesoporous materials has become an effective method to modify these materials for catalysis and other applications in recent years [1-5]. Based on this approach, one can obtain novel solid base catalysts by functionalizing the surface with basic groups. Here we modify the hexagonal and hexagonal-like mesoporous silica materials SBA-3 and HMS with three N-containing organosilanes, 3-aminopropyltriethoxysiIane (AM), 3-ethyldiaminopropyltrimethoxysilane (ED) and 3-piperazinylpropyltriethoxysilane (PZ), to prepare solid base catalysts. An investigation on the effects of the sizes of organosilanes and the pore volumes of mesoporous materials on the loadings of functional groups is made. Then the base-catalytic properties of the functionalised samples were studied using the model Knoevenagel condensation reaction.

486

2. EXPERIMENTAL The SBA-3 was synthesized as previously described [6]. HMS were prepared using dodecyl amine (DDA) and octadecyl amine (ODA) as templates, respectively, at a composition ratio: lTEOS:0.27DDA(ODA):6.5EtOH:36H2O. The resulted samples are designated as DDA-HMS and ODA-HMS, respectively. These mesoporous materials were used as mother samples for further functionalization after calcinations to remove the templates. The samples grafted organosilane were prepared following the procedures: the mother samples were mixed with the given organosilane in toluene, followed by stirring for 3 h at given temperature. The resulted samples were filtered and the extra organosilane were extracted with CH2CI2 in a Soxhlet apparatus twice. The obtained samples are designated as AM (ED, PZ)-SBA-3 (DDA-HMS, ODA-HMS). The XRD patterns were recorded on Rigaku D/max-yC X-ray diffractometer, N2 sorption measurements were performed on Micromeritics ASAP 2000 instrument after evacuation at 573 K and 5x10"^ mmHg. The element analyses for C and N were conducted on Perkin-Elmer 2000 instrument to determine loading levels. The catalytic activities of samples for Knoevenagel condensation were investigated at 353 K and in toluene solvent. Dosage of catalyst was 4.5% of the total weight of reactants, and benzaldehyde and ethyl cyanoacetate were adopted 8 mmol each. The reaction mixtures were analyzed by gas chromatography (Varian 3400). 3. RESULTS AND DISCUSSION Listed in Table 1 are some structural parameters of the mother samples. ODA-HMS possesses the largest pore size and pore volume. As expected, the amount of used organosilane influence the loading level (see Figure 1). When the dosage of AM is increased from 0 to 1 mmol/g mother sample, the loading (mmol/g mother sample) increases rapidly and all of AM is grafted onto the surface. No significant difference in AM loading is observed for the three different mother samples at this stage because the influence of pore size on diffusion o( organosilane is inconsiderable at low loading level. When the AM dosage further increases, the raise in loading slower and the sequence of loading level on three mother samples is ODA-HMS>SBA-3>DDA-HMS, consistent with that of their pore volume. Table 1 Some parameters of mothei' samples Sample

dioo/nm

ao/nm ^

D/nm^

L/nm'

SBHi/m-g-''^

V/mLg''

SBA-3 DDA-HMS ODA-HMS

3.24 3.56 4.80

3.74 4.11 5.54

2.12 2.35 3.04

1.62 1.76 2.50

1276 943 718

0.66 0.53 0.86

^ ao=2 dioo/V3 , ^BJH desorption pore diameter, '^Thickness of pore wall, L = ao - D, ^ Bl: surface area and ^ Total pore volume.

487

The loading level of organosilanes is less 00 influenced by reaction temperature and time. Figure 2 shows the functionalization of SBA-3 with different organosilanes. It is found that the sequence of loading level is ED>AM>PZ, especially at high loadings. Two reasons are responsible for the sequence. o (1) The bulky PZ molecules diffuse into the channel more difficultly than the smaller ED < and AM molecules do. On the other hand, larger size makes the number of PZ 0 12 molecules anchored in the same area of Dosage of organosilane / mmol g surface less than that of ED and AM Fig. 1. Effect of dosage of organosilane on molecules. (2) The reactivity of RO- group of loading of AM. (Reaction temp.: 120°C) organosilane influences the loading level. The CH3O- group of ED molecule is more active than the C2H5O- group of AM molecule [7] and, therefore, the loading 0 4 h amount of ED is greater. The same sequence 0 /^ for loading level is observed on DDA-HMS d m "^ < (not shown here). OQ c C/3 0 On ODA-HMS, however, the loading of 00 0 ^0 2 h .c 3 AM is similar to that of ED (not shown here). 6 This is attributed to the larger pore size of 0 b 1 h C/) G/) ODA-HMS, which allows the entrance of C more AM molecules even though the Ti cd 0 reactivity of C2H5O- group is lower. It should J I II III IV also be mentioned that the loading levels of Reaction conditions all organosilanes arc higher on ODA-HMS than on SBA-3 and DDA-HMS, indicating Fig. 2. Loadings of different organosilanes that the larger pore size favors the grafting of on SBA-3. functional groups on the surface. Reaction temp, and dosage of organosilanes: Knocvenagel condensation reaction of (1) r.t., 1 mmol/g SBA-3, (Il)80°C,5mmol/gSBA-3, benzaldchyde with ethyl cyanoacetate was (1I1)120°C, 5 mmol/g SBA-3 used to investigate the base-catalytic (1V)120"C, 12 mmol/g SBA-3 properties of the functionalizcd samples. The results on HMS samples are shown here as an example. Since the selectivity for condensation product, a a, P-unsaturated ester, is 100%, the yield of this product can be considered as the activity of reaction. Figure 3 exhibits the change of activity as a function of reaction time on DDA-HMS grafted with AM. No notable activity is detected on the mother sample (Figure 3(1)); but very high yields are observed after grafting AM molecules onto the surface. For most of the samples, the yields approximate 100% after 2.5-3 h of reaction time and the difference in activity appears only at initial stage ^-1

488

of reaction. In order to compare the catalytic performance of various samples, we investigated the activities at 0.5 h of reaction time and show those of ED-grafted samples in Figure 4 as a representative. It can be seen from Figure 4 that the change of yield with loadings is not monotone and a maximum exists for both DDA-HMS and ODA-HMS, suggesting that over-loading is unfavorable to the initial activity, especially for DDA-HMS with a smaller pore size. This is because the pore volume decreases (not shown here), i.e., the channels are partly blocked by organic molecules at the higher loading levels so that the diffusion of reactants and products is hindered and accessible active sites decrease. This effect of diffusion occurs only under higher loadings and similar activities are observed for DDA-HMS and ODA-HMS at lower loading levels as shown by the yields before reaching the maximum in Figure 4.

lUU

80

S2

a

;(3)

"

~^V^

60

-o 40

(2)

/(5)

-53 ^

r.

20 n { U '

(1)

1 2 Reaction time / h

3

Fig. 3. Activities change with reaction time on AM-DDA-HMS at loading of (1) 0 mmol/g, (2) 0.49 mmol/g, (3) 0.95 mmol/g, (4) 1.80 mmol/g, (5) 2.60 mmol/g.

Loadings/mmol g" Fig. 4. Effect of loading on initial activity for (1) ED-DDA-HMS, (2) ED-ODA-HMS

REFERENCES 1. X. Feng, G.. E. Fryxell, L.-Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 276 (1997)923. 2. I. Diaz, C. Marquez-Alvarez and F. Mohino et al, J. Catal., 193 (2000) 283. 3. D. Brunei, Micro. Meso. Mater., 27 (1999) 329. 4. W. A. Carvalho, M. Wallau and U. Schuchardt, J. Mole. Catal. A, 144 (1999) 91. 5. S. Jaenicke, G. K. Chuah, X. H. Lin and X.C. Hu, Micro. Meso. Mater., 35-36 (2000) 143. 6. X.-P. Jia, C.Yang, N.-Y He and Z.-H. Lu, Chinese J. Inorg. Chem., 17 (2001) 256. 7. Z.-D. Du, J.-H. Chen, X.-L. Bei and C.-G. Zhou, Chemistry of Organosilicon Compounds, Beijing, Higher Education Press, 1990:195

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

489

Microstructure of the organo-modified SBA-15 (Vinyl-SBA 15) prepared under different pH Byimg-Gyu Park, Jiyong Park, Wanping Guo, Won-Jei Cho, and Chang-Sik Ha* Department of Polymer Science & Engineering, Pusan National University, Pusan 609-735, Korea. Microstructure of the organo-modified SBA-15(vinyl SBA-15) prepared under different pH was investigated using a scanning electron microscopy(SEM). It was found that the morphology of the organo-modified mesoporous materials depended on pH conditions during sol-gel reaction. The periodic mesoporous structure was formed when the materials are obtained in rod-like shapes. 1. INTRODUCTION Much progress has been made in the last years in the development of organo-modified periodic nanoporous materials[l-3]. Chemical functionalization of the inorganic framework of porous materials through the covalent coupling of an organic moiety is a promising approach to specific pore surface properties such as hydrophobicity, polarity, and catalytic, optical, and electronic activity. Silsesquioxanes or bridged silsesquioxanes are used as coprecursors with tetraalkoxy silane for the surface modification of organo-modified periodic nanoporous silica materials. A few attempts have been made to use porous inorganic materials with their superior thermal and mechanical properties as the carrier for the preparation of organo-modified silica gels. Hybrid catalysts with organic groups attached to the support by standard silica functionalization techniques have been proposed. It is apparent that materials with wide pores are required in order to accommodate the functional groups and to allow easy access of reactants to the active sites. We prepared organo-modified SBA-15 by introducing silsesquioxane, triethoxy vinyl silane(TEVS), to the silica framework by the direct synthesis method. In this paper, we report on the microstructure of the organo-modified mesoporous SBA-15 like materials prepared by using silsesquioxanes under different pH conditions. 2. EXPERIMENTAL For a typical synthesis, a triblock copolymer, poly(ethylene oxide)-poly(propylene oxide)poly(propylene oxide) (PE02o-PP07o-PE02o;EPE) was dissolved in water and stirred at 40 °C

490

for 3 hours. A catalyst, such as HCl or NaOH, was added to this solution, then the mixture of TEOS and TEVS of a given mole ratio was put into the solution under different pH conditions and stirred for 40 hours. After reaction, the precipitated powder products were filtered and dried in air at ambient temperature for 1 day, then put into an oven at 60 °C for 4 days. The products prepared under acidic conditions were washed with distilled water before drying. As-synthesized samples were extracted by acidic solution containing hydrochloric acid and methanol at 80 °C for 48 hours. Table 1 summarizes samples prepared in this work. Small angle X-ray scattering(SAXS) patterns were obtained on 4C2 beam lines with a Co Ka radiation operated at 2.5 GeV and 140m(wavelength, Ah2=2--diphenylposphinobenzaldehyd e, THF solvent

•etermi nded by HPLC with chiralcel OD column(25cm x 0.46cm) : 1% 2-propanol in hexane, flow rate=0.5mL/min, tR(min)=25.6(R), 27.5(S)

504

As summarized in Table 1, the enantioselective catalytic activities of the phosphinooxazolidines immobilized on solid supports are slightly lower than those of the corresponding homogeneous phosphino-oxazolidines. SBA-15-supported catalysts gave much higher reacon rates and higher asymmetric induction than silica gel-supported ones. Highly ordered mesoporous silica supports were found to be better inorganic support than amorphous silica gel. The ligands prepared from 2-pyridinealdehyde and 2-thiophenecarboxaldehyde afford 320% ee. On the basis of asymmetric allylic substitution reaction, the chiral complexes immobilized on mesoporous material by the present procedure can be applied as an effective heterogenized homogeneous catalyst for the asymetric reactions. 4. CONCLSIONS New heterogeneous catalysts employing various amino alcohols immobilized on SBA-15 have been synthesized and they were applied to the asymmetric allylic substitution. The enantioselectivity was strongly dependent on the structure of amino alcohol and the enantiomeric excess varied substantially from one amino alcohol to another. SBA-15 has served as a potential support for the heterogenized chiral catalysts in the asymmetric reduction of aromatic ketones to alcohols. ACKNOWLEDGMENT This work was supported by grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and partially by Inha Technical College. REFERENCES 1. A. K. Ghosh, P. Mathivanan and J. Cappiello, Tetrahedron Asymmetry, 9 (1998) 1. 2. H. Steinhagen, M. Reggelin and G. Helmchen, Angew. Chem. Int. Ed. Engl. 36 (1997) 2108. 3. B. J. Nagy, P. Sutra, F. Fajula, D. Brunei, P. Lentz, G. Daelen, Colloids and Surfaces. 158 (1999)21. 4. G. Giffels, J. Beliczey, M. Felder and U. Kragel, Tetrahedron; Asymmetry, 9 (1998) 691. 5. G.-J. Kim and J.-H. Shin, Tetrahedron Lett., 40 (1999) 6827. 6. S. W. Kim, S. J. Bae, T. Hyeon and B. M. Kim, Microporous and Mesoporous Materials, 44(2001)523. 7. N. Bellocq, D. Brunei, M. Lasperas, P. Moreau, Stud. Surf Sci. Catal., 108 (1997) 485. 8. B. M. Trost and D. L. van Vranken, Chem. Rev. 96 (1996) 395.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

505

Preparation of guanidine bases immobilized on SBA-15 mesoporous material and their catalytic activity in knoevenagel condensation Keun-Sik Kim, Jong Hun Song, Jong-Ho Kim and Gon Seo. Department of Chemical Technology & The Research Institute for Catalysis, Chonnam National University, Gwangju, 500-757, Korea. Guanidine was immobilized on SBA-15 mesoporous material by a consecutive addition reaction of precursors and a condensation reaction between presynthesized guanidinecontaining silane and hydroxyl groups of supports. Immobilized guanidine was thermally stable and showed the high activity in the Knoevenagel condensation between cyclohexanone and benzylcyanide. 1. INTRODUCTION Guanidine, non-ionic organic base, is widely employed as active base catalysts in various organic synthesis because of its strong basicity and high miscibility with organic reactants [1 ]. The difficulty in the separation of guanidine from products, however, reduces its economic feasibility by increasing separation expense. In addition, heating for distillation accelerates the formation of by-products, lowering the purity of desired products. Organic bases can be immobilized by the reactions between bases and chlorinated polystyrene supports [2]. Although immobilized bases show reasonable activity in basecatalyzed reactions, their low thermal stability and easy breaking of benzylic groups of polymer inhibit to achieve high activity and multiple use. Reaction of alkoxysilane with hydroxyl groups of solid silica supports provide an effective way to immobilize organic bases on them. Exceptional thermal stability of silica and strong Si-C chemical bond promise better performance of silica as catalyst support. In this study, three different kinds of immobilized guanidine base catalysts were prepared following the procedures shown in Scheme 1: through the stepwise reaction of 3-amino propyltriethoxysiliane (APTS) and N, N'-dicyclocarbodiimide (DCC) consecutively, and the reaction of presynthesized guanidine-containing silanes with hydroxyl groups of SBA-15 mesoporous material. The physico-chemical property and catalytic activity of guanidineimmobilized catalysts in Knoevenagel condensation were discussed relating to the basic character of immobilized guanidine. 2. EXPERIMENTAL SBA-15 mesoporous material was synthesized using an acidic reactant composing of tetraethoxysilane, polyalkylene oxide copolymer (Pluronic-123), trimethylbenzene (TMB), and hydrochloric acid [3]. Calcinated SBA-15 mesoporous material was used as a catalyst support in this study and guanidine was immobilized through the procedure described in Scheme 1.

506

O-NCNHQ -OH

-(gSi

r-BuOH, reflux, 24 h

toluene, reflux, 12 h

Nil

•6

[guan(step)/SBA]

OEt r, ^ . . ^ ^.. /-BuOH, reflux, 24 h

HN-(3

Art 1000 m^/g) and the close tuning of pore sizes, are considered to be useful for various fields such as catalytic applications and adsorbents. In addition, they can be utilized in preparing organic-inorganic composites having well-defined nanosized structure. Composites between a mesoporous host and other guest materials such as carbon, polymer and metal, have been investigated after establishing the nano-scale alignment of the encapsulated molecules in the host channel [1|. l:leclrical properties of nanostructured conducting polymers and carbon, in particular, have been actively investigated by means of implementing ''molecular wires" in electronic devices |2|. As one of the most spectacular smart materials, ER fluids, which can be transformed into a solid-like state by an applied electric field, are composed of dispersions of polarizable or semiconducting particles in insulating oils and represent a unique class of electroaclive intelligent materials that exhibit drastic change in rheological and electrical properties |3. 4|. Various semiconducting polymers and inorganic materials have been used as particulates in the ER fluids [5-7] and recently, MCM-41 suspension in silicone oil was reported to show ER properties [8]. In this study, we prepared an organic-inorganic nanocomposite in which the conducting polyaniline (PANI) is located insides the mesoporous silicas, MCM-41 and SBA-15. and its potential use as an ER fluid system was investigated.

524

2. EXPERIMENTAL MCM-41 was prepared following the procedure of Ryoo et al. [9]. Ludox AS-40 (Si02 40wt% colloidal silica in water, Dupont) was added under vigorous stirring to a 40wt% TEAOH (tetraethylammonium hydroxide) solution. This solution mixture was then combined with a 25wt% CTMACl (cetyltrimethylammonium chloride) solution, and the gel obtained was stirred at room temperature for an hour. Subsequently, the mixture was placed in an autoclave and kept at 373 K for 24 h in a conventional oven for drying. The reaction mixture was cooled to room temperature, and acetic acid was added dropwise under vigorous stirring until the pH reaches 10.2. This mixture was then heated again to 373K at 24h. The pH adjustment and subsequent heating were repeated twice more. The solid product obtained was filtered and surfactant was extracted from MCM-41 in EtOH-HCl solution at room temperature (Ig solid / EtOH 20ml + HCl 2ml). After drying, it was calcined in air at 823K. Concurrently, SBA-15 was synthesized using nonionic triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic PI23, BASF) as a template according to the method reported[10]. In a typical synthesis, lOg of Pluronic PI23 was added to 380ml of 1.6M HCl. After stirring for Ih, a clear solution was obtained. 21.3g of tetraethylorthosilicate (TEOS, 98%, Aldrich) was then added to the solution with vigorous stirring for lOmin. Resulting mixture was left for 24h at 308K, and subsequently heated for 24h at 373K. The solid product obtained was filtered without washing, and dried overnight al 373K. After drying, product was calcined at 823K for 4h to remove the surfactant. SBA-15 was further dried before use at 473K under vacuum for 2 h. To synthesize PANI/MCM-41 and PANI/SBA-15 nanocomposite, the hosts were contacted with aniline gas at 313K for 24 h [11]. Either MCM-41 or SBA-15 containing aniline was then immersed in 0.2M HCl aqueous solution and the same mole of oxidant initiator ammonium peroxysulfate as absorbed aniline was added to the reaction system with stirring at room temperature [12]. The polymerization was conducted for 24 h. The PANI/MCM-41 was washed several times with aqueous HCl solution and methanol, and it was dried at room temperature under reduced pressure. To prepare as a dispersing phase in ER fluid system, the nanocomposites were further dried at 383K, and then mixed with electrically insulating silicone oil to the concentration of 10%(w/w).

ER property of PANI/MCM-41 suspension in silicone oil was obtained b> a

rotational rheometer (Physica, MCI20) equipped with a DC high voltage generator.

The

measuring geometry was a concentric cylinder and all measurements were conducted at 25''C. Dielectric spectrum of ER fluids were also measured by an impedence analyzer using a measuring fixture for liquids, in order to investigate their interfacial polarization.

525

3. RESULTS AND DISCUSSION In the PANI/MCM-41 nanocomposite, the PANI content was ca. 10% (w/w) as confirmed by a TGA thermogram. Its conductivity was 10'^ S cm'' measured by 2-probe method using a pressed disk of PANI/MCM-41. The conductivity of doped PANI is generally known to be about 1 S cm"'. Based on the observation of much lower conductivity of PANI/MCM-41 compared with PANI, we can indirectly come to the conclusion that all the synthesized PANI is located inside the MCM-41 channel. In the case of the PANI/SBA-15 nanocomposite, the PANI content was ca. 35%(w/w) with its conductivity of 10"^ S cm"'. This indicates that under the same condition, SEA-15 contains more polyaniline than MCM-41, because pore size and pore volume of the SBA-15 is larger than that of the MCM-41. Meanwhile, since its conductivity is too high to use it as dispersed phase for ER fluids, causing electrical short under the applied electric field, the conductivity of PANI/SBA-15 particles was lowered about 10'^ S cm"' through a dedoping process. The polymerization confined within channel was also confirmed by a nitrogen sorption experiment. The residual pore volume of PANI/MCM-41 is reduced by 0.63 ml/g from 0.96 ml/g for the empty MCM-41 (Fig. 1(a)) and that reduced to 0.53 ml/g from 0.98 ml/g for PANI/SBA-15 (Fig. 1(b)), respectively.

600

•

„u..*—*

(b) SBA-15

Q

fe 500-

V)

™ 200

0.2

0.4

0.6

Relative pressure ( P / P J

Fig.

0.8

'

/ /

E^ 400 -2 300 o

/

m*^ _,^jmr

/

/ PANI/SBA-15

J ^ 0.2

0.4

0.6

08

10

Relative pressure ( P / P Q )

N2-adsorption isotherm curves before (square) and after (circle) aniline polymerization. Solid symbols represent adsorption process and open symbols desorption process.

Figure 2 shows flow curves of PANI/MCM-41 and MCM-41 ER fluids under an applied electric field of 3kV/mm. Flow curves of empty MCM-41 host materials are also shown for comparison. Polarizing species inducing ER characteristics under electric fields are conducting polymer PANI for PANI/MCM-41 ER fluid and absorbed moisture for MCM-41 ER fluid [8]. The developed stress for PANI/MCM-41 by an applied electric field was found to be larger in whole shear rate regime than that of MCM-41 alone.

526

nD°

PANI/MCM-41

10^

Uooooo o

Ooooo ooooo

•4

oo^

MCM-41

Shear rate (1/sec)

Fig. 2. Shear stress as a function of shear rate under 3kV/mm, with a particle concentration of 10%(w/w).

ACKNOWLEDGEMENT This study was supported by research grants from the KOSEF through the Applied Rheology Center at Korea University, Korea. REFERENCES 1. K. Moller and T. Bein, Chem. Mater. 10 (1998) 2950. 2. M. J. MacLachlan, P. Aroca, N. Coombs, I. Manners and G. A. Ozin, Adv. Mater. 10 (1998) 144. 3. I. S. Sim, J. W. Kim, H. J. Choi, C. A. Kim and M. S. Jhon, Chem. Mater. 13 (2001) 1243. 4. H. J. Choi, M. S. Cho, J. W. Kim, C. A. Kim and M. S. Jhon, Appl. Phys. Lett. 78 (2001) 380. 5. W. H. Jang, J. W. Kim, H. J. Choi and M. S. Jhon, Colloid Polym. Sci. 279 (2001) 823. 6. J. W. Kim, H. J. Choi, S. H. Yoon and M. S. Jhon, Int. J. Mod. Phys. B 15 (2001) 634. 7. M. S. Cho, H. J. Choi, I. J. Chin and W. S. Ahn, Micropor. Mesopor. Mater. 32 (1999) 233. 8. H. J. Choi, M. S. Cho, K. K. Kang, W. S. Ahn, Micropor. Mesopor. Mater. 39 (2000) 19. 9. R. Ryoo, C.H. Ko, and R.F. Howe, Chem. Mater. 9 (1997) 1607. 10. D. Y. Zhao, J. T. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka. (}. D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. 11. C. G. Wu and T. Bein, Science 264 (1994) 1757. 12. J. H. Lee, M. S. Cho, H. J. Choi, M. S. Jhon, Colloid Polym. Sci. 277 (1999) 73.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Functionalized mesoporous adsorbents for Pt(II) and Pd(II) adsorption from dilute aqueous solution Taewook Kang, Younggeun Park, Jong Chul Park, Young Sang Cho* and Jongheop Yi** **School of Chemical Engineering, Seoul National University, Seoul, 151-742, Korea *Korea Institute of Science and Technology, Seoul, 136-791, Korea The surface of the SBA-15 was functionalized with imidazole or thiol functional group via grafting method. Binding behaviors of the adsorbents toward Pt(II) and Pd(II) were examined. The properties of the adsorbents such as pore structure and pore uniformity were also investigated. The pore structure of as-synthesized adsorbents was conserved throughout the preparing steps. The results showed that imidazole- or thiol-functionalized adsorbents showed a high affinity for Pt(II) and Pd(II) metals in aqueous solution. 1. INTRODUCTION For the metal extractions from dilute aqueous solution, solid-phase adsorbents have greater applicability than traditional solvent extraction. The recent discovery of mesoporous molecular sieves have stimulated a renewed interest in developing a novel adsorbcnt.^'^ However, little researches have been reported for the adsorption of noble metal ions using mesoporous silica."* It was reported that polymeric extractants with heterocyclic amine units exhibited efficient adsorption of Pt(II) and Pd(II) from aqueous solutions, and also reported that polymers containing functional groups with donor N and S atoms were the promising reagents toward noble metal ions.^'^' In this study, mesoporous adsorbents functionalized with chelating ligands (imidazole group or thiol group) via grafting method were synthesized and we investigated their binding capability for noble metal ions such as Pt(II) and Pd(II). 2. EXPERIMENTAL The synthesis of hexagonally ordered SBA-15 was performed as described in the literature.^ In order to graft metal adsorptivc functional group containing silanc on the mesopore wall, SBA-15 was silanized with N-(3-triethoxysilylpropyl)-4,5-dihydroimidazole (Imidazole, Gclest Inc.) or 3-mcrcaptopropyltricthoxysilane (MPTES, Gelcst Inc.). In the specific synthesis, 2.0 g of SBA-15 was refiuxed for 20 hr with 60 mL of dry toluene and 3 mL of silanes containing imidazole or thiol functional group. Solid products were filtered off and washed with solvents in the order of toluene, acetone and ethanol. The materials were labeled as Imi-SBA-15 and Thio-SBA-15, where Imi represents imidazole functional group and Thiol Corresponding author: ivid^/^snu.ac.kr Financial support by the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged.

528

4000

2thcta

3500

3000

2500 2000

1500

1000

500

Wavenumber, [1/cm]

Fig. 1. SAXS patterns (right figure) of (a) SBA-15, (b) Thio-SBA-15 and (c) ImiSBA-15

Fig. 2. FT-IR spectra of (a) SBA-15, (b) Thio-SBA-15 and (c) Imi-SBA-15

groups of SBA-15 was decreased and a very weak S-H stretching peak was seen at 2572-2589 cm"' for Thio-SBA-15. For Imi-SBA-15, the intensity of-OH stretching band of silanol groups was also reduced, while 1544-1734 cm"' absorption peak (C=N stretching), which resulted from the grafting of imidazole functional group, appeared.'^ The amounts of functional groups, based on the elemental analysis, were determined to be 2.54 mmol/g SBA15 (Imi-SBA-15) and 0.58 mmol/g SBA-15 (Thio-SBA-15), respectively. Metal adsorption experiments were carried out using SBA-15, Imi-SBA-15 and Thio-SBA15 in buffer solution at pH 4. The amounts of adsorbed metal ions are shown in Table 2. The extent of metal adsorption capability can be represented by distribution coefficient, K^, which is defined as the ratio of the amount of metal ions in solid matrix to those in liquid matrix as listed in Table 2. The Kd value of Imi-SBA-15 was 16000 for Pt(II), 4300 for Pd(II) and the Kd value of Thio-SBA-15 was 38000 for Pt(II), 990000 for Pd(II) in single solution. A Kj value of 990000 for Thio-SBA-15, to our knowledge, the highest value reported for metal ions adsorption in similar conditions although target metal ions were different.'°'" Table 2 Physicochemical properties of surface functionalized mesoporous silicas Pd(II)

Pt(II) Adsorbent

Uptake

/ %

mlg"

Capacity/ mmolg"'

SBA-15

20.9

26

0.019

Imi-SBA-15

99.4

16000

Thio-SBA-15

99.7

38000

Kd/

Uptake

mlg-'

Capacity/ mmolg'

13.5

16

0.012

0.091

97.7

4300

0.093

0.095

99.9

990000

0.098

/ %

Kd/

The results (Table 2) showed that SBA-15 had binding affinity for Pt(II)and Pd(II) metals. Probably oxygen atom in the silanol group of the surface interacted with Pd(II) and Pt(II) by

529

denotes thiol functional group. A variety of properties of as-synthesized adsorbents were characterized with SAXS (BRUKER), FT-IR (Jasco), elemental analysis (MT-2, Yanaco) and N2 sorptometry (ASAP 2010, Micromeritics). A batch technique was applied to determine the metal binding ability of as-synthesized adsorbents. Typically, 0.1 g of adsorbent was equilibrated with 10 mL of ca. 1 mM K2PdCl4 or K2PtCl4 (pH 4.01 buffer) in vials, and these mixture was shaken for 12 hr and the metal ion uptake was determined by analyzing supernatant solution using Inductively Coupled PlasmaAtomic Emission Spectrometer (ICP-AES). 3. RESULTS AND DISCUSSION N2 adsorption/desorption isotherms of SB A-15, imidazole functional group grafted SBA-15 (Imi-SBA-15) and thiol functional group grafted SBA-15 (Thio-SBA-15) showed irreversible type IV adsorption isotherms with a HI hysteresis loop as defined by lUPAC. The physical properties of SBA-15 and the functionalized silicas were listed in Table 1. Surface area, pore diameter of the SBA-15 decreased due to the grafting of organic functional group. Moreover, the decrease in surface area and pore diameter was observed in the order of size of functional group. Surface area and pore diameter of imidazole-functionalized SBA-15 was more sharply decreased than thiolated analogue. Pore size distributions ofSBA-15, Imi-SBA15 and Thio-SBA-15 were similar except for decreasing pore diameter approximately 1-2 nm throughout the preparing steps. No change occurred in hexagonal mesoporous structure of the Tabic 1 Physical properties of the samples Functional group/ BET surface area/ *Pore diameter/ Sample mmolg rn g nm SBA-15 721 8.3 Imi-SBA-15 2.54 161 5.8 Thio-SBA-15 0.58 437 7.3 *Pore size calculated from the desorption branch using BJH formula SBA-15 through the preparing steps. This conservation of the mesoporous structure is confirmed precisely by the SAXS data (Fig. 1). The SAXS pattern of SBA-15, Imi-SBA-15 and Thio-SBA-15 showed three reflections, respectively. The X-ray diffraction pattern of ImiSBA-15 and Thio-SBA-15 showed a very intense peak (100) and two additional high order peaks (110, 200) with lower intensities. This result was characteristic of a hexagonal pore structure. The functional groups contained by three samples were identified using FT-IR (Fig. 2). Silanol groups on the silica surface exists as several types, such as isolated, hydrogenbonded, and geminal types of silanol.^ The IR absorption bands of these silanol groups are corresponding to the peaks at 3738 cm"', 3200-3600 cm' and 3738 cm'. The results showed that the surface silanol group was mainly of the hydrogen-bonded type, IR absorption bands observed at 3200-3600 cm"'. The siloxane, -(SiO)n-, peak appeared at 1000-1100 cm"'. Si-0 bond stretching was detected at 960 cm"'. The intensity of-OH stretching band of silanol

530

ion-pairing mechanism with K^ ion as balancing counter-ion.^ The maximum loading capacities of Imi-SBA-15 were 1.1 mmol/g for Pt(II), 1.0 mmol/g for Pd(II) and the maximum loading capacities of Thio-SBA-15 were 0.60 mmol/g for Pt(II) and 0.88 mmol/g for Pd(II). Metal/functional group ratios were approximately 0.5 (both Pt(II) and Pd(II)) for Imi-SBA-15 and 1 (Pt(II)), 0.7 (Pd(II)) for Thio-SBA-15. In summary, we have shown that the introduction of chemical functional groups such as thiol and imidazole, to the mesoporous silica support leads to remarkable increase of the binding capacity for Pt(II) and Pd(II). REFERENCES 1. J. S. Kim and J. Yi, Separ. Sci. Technol., 34 (1999) 2957. 2. H. Lee and J. Yi, Separ. Sci. Technol., 36 (2001) 2433. 3. B. Lee, Y. Kim, H. Lee and J. Yi, Micropor. Mesopor. Mat., 50 (2001) 77. 4. T. Kang, Y. Park, J. C. Park, Y. S. Cho and J. Yi, The Korean J. of Chem. Eng., 19 (2002) 5. R. Liu, Y. Li, H. Tang, J. Appl. Polym. Sci., 83 (2002) 1608. 6. G. G. Talanova, L. Zhong, O. V. Kravchenko, K. B. Yatsimirskii, R. A. Bartsch, J. Appl. Polym. Sci., 80(2001)207. 7. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky. Science, 279(1998)548. 8. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B, 102 (1998) 1556. 9. G. Socrates (2"^^), Infrared Characteristic Group Frequencies: Tables and Charts. John Wiley & Sons Ltd., Chichester, 1994. 10. S. Dai, M. C. Burleigh, Y. H. Ju, H. J. Gao, J. S. Lin, S. J. Pennycook, C. E. Barnes, and / . L. Xue, J. Am. Chem. Soc, 122 (2000) 992. 11. M. C. Burleigh, S. Dai, E. W. Hagaman, and L. S. Lin, Chem. Mater., 13 (2001) 2537

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Environmentally benign removal of pollutant oxyanions by Fe adsorption center in functionalized mesoporous silica Toshiyuki Yokoi^, Takashi Tatsumi and Hideaki Yoshitake^* ^Graduate School of Environment and Information Sciences, Yokohama National University. 79-7 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. ''Division of Materials Science and Chemical Engineering, Graduate school of Engineering, Yokohama National University. 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan corresponding author, [email protected]

The specific molecular adsorption sites were built on the pore wall of MCM-41 by means of fixation of ethylenediamine group followed by cationization with Fe ^. The iron-anchored surface sites lead to selective adsorptions of arsenate, chromate, selenate and molybdate. The used adsorbent can be regenerated by an environmentally benign process.

1. INTRODUCTION Recently, the pollution of groundwater by arsenic has attracted a considerable attention and a trace amount of pollutants in groundwater need to be removed for human health. Precipitation methods have been utilized as a removal technique of the pollutant in water environment. However, those methods are not satisfactory and have several disadvantages, such as large amount of secondary waste products. Although the adsorption on solid surfaces can be an efficient method for removing pollutants from water, the specific adsorption of the oxyanions is generally difficult because As is shelled by four oxygen atoms (arsenate HAs04^' or H2ASO4') and several kinds of anions, such as sulfate and chloride often compete. Mesoporous silica with a well-ordered structure and a high specific surface area has been expected to be applicable to catalyses and adsorptions [1-3]. In addition the high density of silanol groups on the surface is beneficial to the introduction of functional groups with high coverage by silylations [4]. We report here the synthesis and the utilization of Fe(in)-chelated ligands immobilized on the surface for a high performance and environmentally benign adsorbent for pollutant oxyanions.

532

2. EXPERIMENTAL 2.1. Synthesis of adsorbent The surface of MCM-41, which was synthesized by a conventional method [5], was modified with an organosilane containing amino groups, l-(2-aminoethyl)-3-amino propyltrimethoxysilane (NN-MCM-41) [6]. Fe(III) ion was coordinated with amino Hgands to form a stable complex on the surface in the pores (Fe/NN-MCM-41). The composition and structure of MCM-41, NN-MCM-41 and Fe/NN-MCM-41 were characterized by CHN and ICP elemental analyses, XRD, nitrogen adsorption, ^'^Si-NMR and FT-IR spectroscopy. 2.2. Adsorption experiments Fe/NN-MCM-41 was utilized as an adsorbent for arsenate, chromate, selenate and molybdate. Typical adsorption experiments were carried out by using 50 mg of Fe/NN-MCM-41 stirred in 10 ml of aqueous solutions containing the oxyanion, KH2ASO4, K2Cr04, K2Se04 and K2M0O4 for 10 h at 298 K. The concentration of initial and residual oxyanion in the solution was analyzed by ICR

3. RESULTS AND DISCUSSION 3.L Structural properties of adsorbent XRD patterns demonstrated that NN-MCM-41 and Fc/NN-MCM-41 retained the original ordered mcso structure of MCM-41, though the surface area, pore volume and pore size decreased (Table 1). Thus the high accessibility of ions from the outside of the pore to Fe(lll) cations center in the pore is likely to be maintained. *^'^Si-NMR and FT-IR spectra of NN-MCM-41 showed the presence of Si-C bonds and amino groups of the organosilane. According to the elemental analyses, the molar ratio of N / Fe ^ was 4, suggesting that one Fe * was tethered to four N atoms and also coordinated with Cf or H2O ligands. Table 1 Characteristics of Functinalized MCM-41. A HI./

MCM-41 NN-MCM-41 Fc/NN-MCM-41

(m'g') 1283 586 310

v,.^ (cnr^g"') 1.05 0.50 0.25

2R,.^ (nm) 2.9 2.6 2.2

Fc^' content (mmol g ' )

-

0.55

C/N N content'' (molar ratio) ( m m o l g ' )

-

2.65 2.73

2.76 2.09

"AHI;T: B E T specific surface area. V|.: primary mesopore volume. '^Rp: pore radius (by the BJH method). ''Assuming that -NH groups content is equal to nitrogen atoms content.

533

3.2. Adsorption behavior for Oxyanions The adsorption isotherms of various oxyanions are shown in Figure 1. The maximum adsorption amount reached as large as 1.56, 0.99, 0.81 and 1.29 mmol g'' for arsenate, chromate, selenate and molybdate, respectively. The maximum leaching of Fe(in) cations during the adsorption of oxyanions was less than 7 wt%. The molar ratio of the cations

-As(V) -Se(VI)

-Cr(VI) -Mo(VI)

to

-r-l S

0 2 4 6 8 10 (Fe-^^) to the anion (HAs04^', Cr04 Equilibrium concentration of oxyanions Se04^" or Mo04^') at the adsorption /mmol r' saturations was almost in agreement with Fig. 1. Adsorption isotherms of oxyanions, # the electric charge balance. This nearly A s ( V ) , B C r C V l X ^ S c C V I ) , A M o ( V I ) , by Fe/NN-MCM-41. Reaction conditions; 50 mg stoichiometric adsorption demonstrated adsorbent, 10 ml H2O as a solvent, reaction time the molecular nature of adsorption site. 10 h and reaction temperature 298 K. When the density of the organic groups was increased, the surface area and the structural order of the silica framework decreased,

resulting in lower adsorption capacity. Purely inorganic silica (MCM-41) showed negligible adsorption capacity compared to Fc/NN-lVICM-41, indicating that the complexation was indispensable to adsorptions of anions. 3.3. Inhibition by competing anions Several kinds of anions in the hydrosphere can be adsorbed competitively during removal of the pollutant oxyanions. We carried out coadsorption experiments in the presence of sulfate and chloride anions at low initial concentration of oxyanions (0.5 mmol l') in order to clarify the inhibition effect on the adsorption capacities. The results showed that in the presence of sulfate or chloride with the concentration of 10 molar times as high as all four kinds of oxyanions, over 80 and 90 % of the adsorption capacities were maintained, respectively, which implies that Fc/NN-MCM-41 will work as an effective adsorbent of oxyanions effectively in a real environment. The strong resistance against competing anions is attributed to the strong specific affinity between Fe(lII) cation center and the target oxyanions. 3.4. Regeneration of used adsorbent We also explored the reusability of our adsorbent. Table 2 shows the result of the

534

regeneration of arsenate-saturated Fe/NN-MCM-41, where the complete desorption of arsenates from the adsorbent was achieved. However, this process did also simultaneously lead to the leaching of almost all the Fe(in) cations from lSrN-MCM-41 probably due to the strong acidity of the treatment solution. Because most of the organic groups were retained after the HCI treatment, we re-introduced Fe(III) cation into the fiinctiolized silica to restore the adsorption sites (Table 2). The adsorption of arsenates on the regenerated Fe/NN-MCM-41 showed 84 % (1.31 mmol g'') of the initial adsorption capacity. The decrease in the adsorption amount was nearly in accordance with the decrease in the adsorption site. Table 2 Regeneration of Fe/NN-MCM-41 by Treatment with 1 M HCI aq.. Residual oxyanion" (mmol g')

Fe^^ content N content N / Fe^^ (mmol g"') (mmol g') (molar ratio) Before adsorption 0.55 2.09 3.8 1.50 1.98 3.9 After adsorption of arsenate 0.51 0.001 1.53 0.03 After HCI treatment 0.001 0.42 1.51 After re-incorporation of Fe^^ 3.6 "Calculated from solid analysis. Treatment conditions: 1 M HCI aq. 200 ml g' , treatment time 10 h, treatment temperature 298 K.

4. CONCLUSIONS Fe(in)-cation coordinated with the amino ligands on the surface of mesoporous silica work as excellent adsorbing sites for pollutant oxyanions, arsenate, chromate, selenate and molybdate. The selectivity to the target oxyanion was not interfered by abundant sulfate and chloride anions, suggesting the strong specific affinity between Fe(III) cations and the target oxyanions. Adsorbed oxyanions were successfully desorbed by a simple acid treatment and the degree of regeneration was found satisfactory to a recycled use of the adsorbent. REFERENCES 1. L. Mercier, T. J. Pinnavaia, Environ. Sci. Technol., 32 (1998) 2749. 2. H. Yoshitake, T. Yokoi, T. Tatstumi, Chem. Lett., 6 (2002) 586. 3. G. E. Fryxell, J. Liu, T. A. Hauser, Z. Nie, K. F. Ferris, S. Mattigod, M. Gong, R. T. Hallen, Chem. Mater., 11 (1999) 2148. 4. Zhao, X. S.; Lu, G. Q.; Whittaker, A. J.; Millar, G. J.; Zhu, H. Y. J. Phys. Chem. B, 101(1997)6525. 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J.S.Beck, Nature, 359 (1992) 710. 6. K. Moller; T. Bein, Chem. Mater. 10 (1998) 2950.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

535

How can nanoparticles change the mechanical resistance of ordered mesoporous thin films ? Edward Craven', Sophie Besson'' ^, Michaela Klotz', Thierry Gacoin^, Jean-Pierre Boilot^ and Etienne Barthel' ^Laboratoire CNRS / Saint-Gobain "Surface du Verre et Interface" UMR 125 ;39 Quai Lucien Lefranc ; F- 93303 Aubervilliers, France ^Laboratoire de Physique de la Matiere Condensee, UMR CNRS 7643 ;Ecole Polytechnique ; F-91128 Palaiseau, France Nanoindentation was performed on mesoporous thin films in order to investigate their mechanical behaviour. It has been found that the empty mesoporous films behave plastically while films filled with nanoparticles exhibit more elastic deformations. This contrasfing behaviour is consistent with the different indent morphologies observed by optical and electronic microscopy. KEYWORDS : mesoporous thin films, nanoindentation, elastic modulus and hardness 1. INTRODUCTION Mesoslructured thin films arc studied for their potential applications in a variety of fields including separation technology, sensors and catalysis. These films are formed by the association of sol-gel chemistry and a templating mesophase. Combining the advantages of each component, one can form a crack free thin film that exhibits a beautifully ordered mesostructure. Most studies aim at the understanding of the formation mechanism and fiinctionalisation of these films. However, the mechanical behavior - most prominently the resistance to wear - has to be considered for industrial applications. In this contribution, we present the results of indentation tests performed on two different types of mesostructured films: a mesoporous film presenting an isotropic hexagonal micellar structure and the same structure uniformly filled with CdS particles. The results will be correlated to the structural properties of the thin film. 2. EXPERIMENTAL Mesoporous thin films are prepared under acidic conditions using cetyltrimethylammonium bromide as a template. The synthesis procedure has previously been reported'. CdS particles are subsequently grown in situ by impregnation of the mesoporous film with a cadmium containing solution of pH 9.5. The film is then treated with H2S leading to the precipitation of CdS particles. These steps were repeated until film saturation. The complete preparation procedure and characterization is reported in ^. An additional sample is prepared following the same procedure except that the pH 9.5 solution used for impregnation is prepared using ammonium hydroxide. This chemical treatment of the film is thus considered equivalent but will not lead to the precipitation of nanoparticles. In the following, we will refer to this sample as "base treated".

536

Indentation tests are performed with an XP Nano Indenter (MTS). The experiments are carried out using Berkovich type tip (three sided pyramid). In order to study the mechanical properties of the film and suppress the influence of the substrate, only indents of 1/10^^ of the film thickness are considered. 3. RESULTS AND DISCUSSION The mesoporous thin films are highly textured with single domains occupying the entire film thickness. They exhibit a 3D hexagonal structure (P63/mmc) described in more detail in '. The total pore volume was determined by ellipsometry measurements^. The approximative part of mesoporosity and microporosity of the walls was deduced by simple calculation knowing the pore diameter and the structural parameters of the structure. The results are summarized in table 1. The detailed characterization of the nanocomposite formed by in situ growth of CdS particles^ show that the 3D hexagonal structure is maintained during the filling process. The nano-crystals were found to always be located within the pores and 100% of the pores were filled. It was also found that the particle diameters, determined from UVvisible absorbance spectra, were equal to the initial pore size of 3.5 nm. Table 1 Structural and porous properties of the empty mesoporous thin film Structural parameters a

c

5.6 nm 6.1 nm

Porosity

Total Pore Mesopore Microporosity Film Pore thickness diameter volume volume within the walls 300 nm

3.6 nm

55%

25%

40%

The Young's modulus and hardness (table 2) for the films can be derived from the nanoindcntation force curves (Figure 2): The empty 3D hexagonal film was found to behave as an almost perfectly plastic material, with a low yield stress, and negligible elastic recovery. The SEM image (Figure la) shows no other surface effect than the trace of the plastic deformation of the material. Comparatively, the CdS filled structure exhibits a sizably larger modulus (+80%) and hardness (+100%), resulting in a more brittle layer than the empty one. Identical penetrations in the CdS filled layer lead to delamination along with the formafion of flakes (Figure Ic). Finally, the results on the base treated sample shows that the influence of the chemical treatment during the growth process is limited in terms of mechanical behavior.

^

a) Empty mesoporous film b) base treatment Fig. 1. SEM images of the residual trace of l)im indents

-

_

-

^

c) CdS filled

537

.4x10 1.2 1.0

• • A •

' I ' ' ' ' I ' ' ' ' I '

silica Pyrex CdS filled empty base treatment

£0.20

0.8 0.6

0.25 f-'

t0.15 E ra ^0.10 o

.,jr

-^•^

0.4

1




.

1

1

1

' ' ' 1 • ' ' ' 1 ' ' ' ' 1 ' ' ' Lrt

silica Pyrex CdS filled empty base treatment

T3

0.2

o 0.05

0.0 Vyy'^^ffr: ^^v^»,'^-,-r-T 0 5 10 15 20 25 Displacement into surface (nm)

tJ»iyyfy'(i.f>.yJT •

30

5

0.00

a) Harmonic contact stiffness

10 15 20 25 Displacement into surface (nm)

b) Load on sample

Fig. 2. Nanoindentation curves for 300 nm thick layers on a glass substrate. Table 2 Elastic modulus and hardness of the different films Elastic Modulus Hardness Sample (GPa) (GPa) Silica

71.6

8.35

Empty

11.0

0.75

Base treatment

10.9

0.49

CdS filled

19.7

1.45

The results can be correlated to the structure of the thin film by the use of simple models. First, wc used Voigt's model'^ to determine the elastic modulus of the walls from the elastic modulus of the empty film. This model supposes equal strain in the 2 phases composing the material. Then

£

tilm

={\-(l) V

)E ,,+(/)

' ni.vo/xir.' /

wall

T mifd/uirf

E

mesupore

Knowing that the film is composed of 25% of mesopores and that the elastic modulus of the void mesopores equals to zero, we determined that the elastic modulus of the walls is 14.5 GPa. This value is much lower than for dense silica. This is attributed to the important part - 40% - of microporosity in the walls. The previous model is not applicable in the case of the CdS filled thin film. We suppose in this case that the walls and the CdS particles get equal stress^ during the nanoindentation experiment. Then

1

E ..

E

The elastic modulus of CdS is 45GPa. Using the elastic modulus of the walls previously determined, one can calculate the theoretical value of the nano-particle filled sample: 17.3 GPa. This result is in good correlation with the experimental value.

538

4.CONCLUSION The mechanical behavior of mesostructured thin film have been studied. Empty mesoporous thin films were found to behave like a very plastic material. The elastic modulus of the walls have been deduced. The calculated values are much lower than those of dense silica, which can be correlated to the microporosity within the walls. The filled layer behaves like a more brittle material. The film's capacity to store stresses causes delamination instead of a distributed plastic deformation as observed for the empty film. The chemical treatment which leads to the formation of the CdS nano-cristals was shown to have a weak impact on the mechanical behavior of the empty film, especially for the Young's modulus. The film's elastic modulus correlates well with a simple additive model, thus, the modification of the behavior is mainly due to the presence of the nano-particles. The mechanical properties of the mesoporous thin film could be improved by the in-situ growth of nano-cristals. However, the filled film is more sensitive to delamination which causes visible optical defects.

4. REFERENCES l.S. Besson et al., J. Mater. Chem., vol. 10, p. 1331,2000 2. S. Besson et al., Nano Lett., vol. 2, p. 409, 2002. 3. S. Besson, PhD thesis, Ecole Polytechnique, Palaiseau (France), 2002 4. W. Voigt, Lehrbuch der Kristallphysik, Teubner, Leipzig, 1910 5. A. Rcuss, Z. Angcw. Math. Mech., p9-49, 1929

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Nanoporous SiOi films prepared by surfactant templating method - a novel antireflective coating technology Heui-Ting Hsu^, Chih-Yuan Ting^, Chung-Yuan Mou^ and Ben-Zu Wan^* ^Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, R.O.C. ^Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C. A sol-gel derived antireflection (AR) coating by surfactant templating method is presented. The spin coating process was used to deposit Si02 thin films on glass. The pore size and the pore volume of the film were controlled by the size and the volume of the template (i.e. Tween 80). It was found from this research that the transmittance was increased from 91.7% to above 99.0% either by a single-layer or by a double-layer porous Si02 antireflection coating, on both sides of the glass pane at a specific wavelength. 1. INTRODUCTION Antireflection coatings can reduce the intensity of light-reflection (or increase the intensity of transmittance) and increase the quality of optical lens system. Porous Si02 antireflection films arc commonly prepared by sol-gel deposition.' Reflectance can be minimized at a particular wavelength /l,, at normal incidence, when refractive index and thickness satisfy the following two conditions': (1).Light amplitudes reflected at air/film and film/substrate interfaces must be equal. That is

where nc, no, and ns arc refractive indices of film, air and substrate, respectively. (2). Film thickness (tc) must be 1/4 of a reference wavelength in the film, for the reflected light to interfere destructively. That is t,=Z,/(4n^.)

(2)

Therefore, when /l,)~510nm is chosen and a glass (ns=1.52) is used as a substrate, the optimum refractive index and thickness of coated antireflection film can be calculated as nc~1.23 and tc~100nm, respectively. Because the refractive index of a dense Si02 film is also 1.52, the desired film refractive index must be reduced by adjusting the porosity of the Si02 film. Therefore in this paper, a sol-gel derived antireflection (AR) coating by surfactant templating method is introduced. The results of antireflection (or enhanced-transmittance) is demonstrated.

540

2. EXPERIMENTAL SECTION Silica sols were made from Tween 80, ethanol, H2O, TEOS, and HCl. Tween 80 is a non-ionic surfactant and act as a template in the sols. The spin coating was used for the deposition of sols on silicon wafers and glass substrates^ The coated substrates were baked on a hot plate at 106°C, then were calcined at 400°C for 3 h. Later, the Si02 surface was grafted with silane by immersing the samples in a HMDS/toluene solution at 80°C, in order to increase the surface hydrophobicity. On the other hand, extraction of template molecules and simultaneous grafting of silane were also studied by treating uncalcined samples in HMDS/ethanol solution at 50 °C (direct surface modification). For the transmittance measurement, a spectrophotometer (Hitachi, UV-visible 3410) was employed to record transmittance of the AR coated glass in a 200-800 nm wavelength range. Refractive index and thickness of the thin film was measured with a n&k analyzer 1200. Surface morphologies were characterized by a scanning electron microscope (SEM, Hitachi S-2400). 3. RESULTS AND DISCUSSION 3.1. Controlling refractive index and thickness of the film For an antireflection coating, it is necessary to control the refractive index and the thickness of the films. In the case of nanoporous Si02 films, both can easily be achieved. The refractive index of the film can be tuned by varying the film porosity, which can be achieved by varying the Table 1 Refractive indices and thicknesses of weight ratio of Tween 80 to TEOS in the coating solution. Table 1 shows the refractive indices and the films prepared by different ratios of the thicknesses of the films made from different Tween 80/TEOS Tween 80 Refractive Thickness weight ratios of Tween 80/TEOS, when the molar Index /TEGS ratios of TEOS/E1OH/M2O/HCN 1/60/4.2/0.24 arc 0.13 1.43 74 fixed. It was found that the refractive indices 0.41 93 1.25 decreased from 1.43 to 1.19, when the weight 1.21 0.62 103 ratio was increased from 0.13 to 0.83. Fig. 1 118 0.83 1.19 shows the empirical dependence of the refractive index on Tween 80 mixing ratio, which allows precise adjustment of the film refractive index in the later experiments. The thickness of the film can be controlled by varying the concentrations of TEOS, Tween 80 and ethanol in the coating solution. Changing the spin-coating speed is another way for varying thickness. It is concluded from this research that when the molar ratio of TEOS /EtOH/H20/HCl is 1/48-72/8/4.2/0.24 and the weight ratio of Tween 80/TEOS is 0.13-0.83 in the coating solution, the Tween 80/TEOS desired film thickness for this research can be obtained. Fig.I. Refractive indices as a function of the ratios of Tween 80/TEOS

541

3.2. Single-layer AR coating Substrate Pretreatment

Prepare coating solutiai

^ i n coating Baking(106-'120°C)

Calcinations(400°C)

400

500 600 Wavelength(nm)

800

Surface i m3dification(80°C)

Treating uncaldned sanple in HMDS/BOH solution at 50°C

Fig.3. Experimental procedure

Fig. 2. Transmittance of porous Si02 AR coatings prepared by (a) calcinations or by (b) direct surface modification in HMDS/EtOH solution and (c) uncoated ^lass.

The transmittance spectrum in Fig.2.(a) shows a broad-band AR from a single-layer coated glass, calcined at 400°C. It can be found that the transmittance of more than Table 2 Refractive indices and thicknesses of films 96% is over the whole visible spectrum. The prepared by different ways for removing transmittance at 475 nm even reaches 99.3%. The refractive index and the thickness of the template film are 1.22 and 90 nm, which are listed in Refractive Thickness Table 2. The high transmittance Index (nm) demonstrates that the surfactant-templated (a)Calcinations 1.22 90 nanoporous film is a potential candidate for (b)HMDS/EtOH 1.29 120 AR coating. However, it should be noted that the high temperature (400"C) may damage the glass substrate and cause the coated film to shrink. In order to prevent these, the direct surface silyl modification'^ to remove the templates in the solution at low temperature was developed. Experimental procedure are shown in Fig. 3. And the transmittance spectrum from the coated glass is shown in Fig.2.(b). It can be found that the transmittance spectrum shows a broad-band AR in the range of 600 to 800 nm. The refractive index and thickness listed in Table 2 are 1.29 and 120nm, respectively. The highest transmittance (98.5%)) appears at 710 nm. It is apparent that the different wavelength at the highest transmittance in Fig.2. (a) and (b) results from the different film thicknesses. And the difference of their maximum transmittance in Fig.2 (a) and (b) are caused by the different refraction indices in the films. 3.3 Double-layer AR coating Single-layer AR coatings normally cover narrow bandwidth. In order to achieve broadband optical performance, a multi-layer stack is usually necessary. Multi-layers coating can further

542

reduce the reflected light but require layers of refractive index below 1.2. In this research, the optimal optical parameters for the coatings were determined from a theoretical calculation^. Fig. 4(a) shows the measured transmittance of a glass substrate coated with double-layer AR coating. The glass was calcined at 400°C. It consists of two porous Si02 layers with refractive indices 1.19 and 1.44 and with thickness 141 and 86 nm, respectively. It can be found that the transmittance spectum in 570~800nm wavelength range increased from 91.7% to >99%, and the transmittance is 99.4% at ~713nm, which are better than those from a single-layer coating. Moreover, Fig. 4. (b) is the result form computer simulation according to the experimental parameters (refractive index and the thickness). The similar trend between the experimental and the simulation results suggests that the surfactant tcmplating method is one of the effective ways for the preparation of antireflectivc films.

400

500

600

700

800

Wavelength(nm) Fig. 4. Transmittance of (a) glass substrate coated with two porous Si02 layers, (b) transmittance from theoretical calculation and (c) uncoated glass.

4. CONCLUSION Surfactanl-templatcd nanoporous silica films have been applied as AR coatings for the first time. The refractive index and the thickness of the films are easily adjusted via coating solution composition and spin coating speed. The transmittance of a single-layer AR coated glass can be enhanced from 91.7% to 99.3% measured at a single wavelength. The transmittance of the double-layer AR coated glass is increased from 91.7% to an excellent value of >99% in the 570~800nm wavelength range and approaches to 99.4% at ~713nm. The film preparation process reported in this research is simple and cheap. Both the high performance of the film and the simplicity of the process make the reported technology a potential candidate for AR coatings application in the future. REFERENCES 1. C. J. Brinker, G.W. Scherer, Sol-Gel Science, Academic Press, Boston, 1990. 2. H. A. Macleod, Thin-Film Optical Filters, Mcgraw-Hill, New York, 1986. 3. S. Walheim, E. Schaffer and J. Mlynek, U. Steiner, Science, 283 (1999) 520. 4. H. P. Lin, L.Y. Yang, C.Y. Mou, S. B. Liu and H.K. Lee, New J. Chem., 24 (2000 ) 253.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Textural and structural properties of Al-SBA-15 directly synthesized at 2.9 < pH < 3.3 region Dedicated to memory of Dr. V.N. Romannikov Maxim S. Mel'gunov, Elena A. Mergunova, Alexander N. Shmakov, Vladimir I. Zaikovskii Boreskov Institute of Catalysis, SB RAS, Novosibirsk, 630090, Russia. FAX: +7-3832-343056. E-mail: 2max(a)bk.ru Implantation of Al in a carcass of SBA-15 silicate directly during its synthesis at pH in a range of 2.9-3.3 using sodium silicate and aluminum sulfate as precursors results in formation of a well organized hexagonally structured mesophase. The effect of mesophase perfection improvement due to Al implantation is reported. The obtained materials demonstrate high thermohydrostability and mechanical strength. 1. INTRODUCTION Various synthetic procedures for the preparation of Al-SBA-XX silicates have already been reported. There are two methods of Al incorporation into a SBA mesophase, including directand post-synthesis. The latter is generally based on impregnation of a pre-prepared SBA mesophase with an Al-containing precursor followed by conversion of the precursor to the surface grafted AlOx. However, this method results in formation of relatively big AlOx clusters weakly bounded to the mesophase surface, thus the final material has low catalytic activity. The direct synthesis allows implantation of Al in a mesophase carcass resulting in higher dispersion and stability of AlOx clusters, increasing catalytic activity. Catalytic and hydrothermal properties of Al-SBA mesophase can also be improved when synthesis proceeds in weak acid conditions. For example, recently Yue et al [\] have reported the direct synthesis of Al-SBA-15 using tetraethyl orthosilicate and aluminum tri-/er/-buthoxide as precursors at ambient temperature and pH of 1.5 followed by hydrothermal treatment at 373K and calcination at 823K. The main disadvantage of this method is usage of metal-organic precursors that results in low commercial viability. To bypass this, Kim and Stucky [2] have offered sodium silicate as Si02 precursor for the synthesis of pure siliceous mesophases with various structure at pH of-0.6. Following these trends, in this paper we report the synthesis and textural and structural study of Al-SBA-15 materials prepared at 2.9t=0.154 nm) radiation. N2 adsorption-desorption isotherms were obtained on a Micromeritics ASAP 2010 sorptometer at 77 K. The infrared spectra were measured on MAGNA-IR 500 spectrometer in the range of 400-1000 cm' with a resolution of 2cm'. The NH3-TPD-MASS curves were determined in the range 110-800"C at a temperature-increasing rate of 10"C/min on a AutoChem 2910 and Thermo ONIX ProLab system. HRTEM micrographs were taken with Philips CM200 Microscope. 3. RESULT AND DISCUSSION 3.1. X-ray diffraction patterns By controlling the right pH value range (-11.2) and reaction temperature, we can successfully synthesize the MCM-48-S with Si/Al ratio between 40 and 60(Fig. 1.). They cannot get good structures when the aluminate's content is too much. The XRD patterns (Fig. 2.) show the MCM-48-S has well-ordered cubic {laSd) mesostructures, even treated in boiling water for 10 days or after calcination at 900"Cfor 6 h they show only limited decay of structure. 3.2. N2 adsorption-desorption isotherm BET surface area and pore volume (Table 1) shows a 30-40% reduction after 5-days hydrothermal reaction and keeps nearly the same to 10 days. According to Schumacher's method [7], for calculating the wall thickness of MCM-48-S we found the wall thickness of MCM-48-S is about 10 A close to typical MCM-48. It indicates that the gained stability is due to a stronger rather than thicker wall. Table 1 lists the physical properties of the calcined products (Si/Al ratio is 60) that treated in boiling water for different time.

559

800

29/degree

29/degree

Fig. 1. The XRD patterns of samples with different Si/Al ratio.

700

600

500

400

Wavenumber (cm')

Fig. 2. The XRD patterms of calcined MCM-48-S with different times in boiling water (Si/Al=60).

Fig. 3. Infra-Red spectrum of calcined MCM-48-S (Si/Al=50).

Table 1 Comparison of physical characteristics of calcined products (Si/Al ratio is 60) with different rehydrothermal time (A) non rehydrothermal, (B) 24 h, (C) 48 h, (D) 5 days, (E) 10 days, (F) MCM-48 Sample

Surface area (mVg)

A

1312

B

Pore volume (cm^/g)

d-spacing

(A)

Ao(A)

h(A)

22

0.97

34.76

85.14

8.25

1213

19

0.81

33.70

82.54

8.60

C

1206

18.5

0.80

34.22

83.82

8.95

D

959

15

0.57

34.22

83.82

9.8

E

931

15

0.58

33.96

83.18

9.7

F

1322

23.5

0.87

34.22

83.82

8.70

Pore size

(A)

3.3. IR spectroscopy We further took an IR spectrum (Fig. 3) of MCM-48-S. An absorption shoulder at 550 cm"' was observed indicating the presence of five-member ring structure for the siloxanc connections. 3.4. Transmission electron microscopy The HRTEM image (Fig. 4) of the calcined samples shows excellent periodic structure. The distance between the pores is in good agreement with that determined from XRD pattern.

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3.5. Acidity Measuring from the NH3-TPD-MASS curves of the H-form MCM-48-S (Fig. 5.), NHs-desorption lasts until a fairly high temperature of 500 °C which is much higher than that of typical MCM-48 at 320 °C. Similar to the desorption temperature of the acidic HZSM-5, we thus have a fairly acidic mesoporous aluminosilicates in MCM-48-S. The combination of 3-D interconnecting channel system, strong acidity and highly hydrothermal stability will be useful in many catalytic applications.

200

Fig. 4. High Resolution Transmission Electron Microscopy (HRTEM) image of calcined MCM-48-S. (Si/Al=60).

300 400 500 600 700

Temperature/ C

Fig. 5. The NH3-TPD-MASS curves of HMCM-48-S (Si/AH37).

4. CONCLUSIONS The MCM-48-S has been synthesized by the reaction of CTAB solution and zeolite seed. By the characterization of XRD and NH3-TPD-MASS, we prove the material possesses highly hydrothermal stability and strong acidity. In future, it would be very useful for catalytic reaction. REFERENCES Kresge, C.T., Leonowicz, M.E., Roth, W. J., Vartuli, J.C, Beck, J. S., Nature 359 (1992) 710. Y. Liu, W. Zhang, and T J. Pinnavaia, T. J., J. Am. Chem. Soc. 122 (2000) 8791. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao, and F. S. Xiao, Angew. Chem. Int. Ed. 40(2001) 1258. Z. Zhang, Y. Han, F. S. Xiao, S. Qiu, L. Zhu, R. Wang, Y Yu, Z. Zhang, B. Zou, Y Wang, H. Sun, D. Zhao, and Y. Wei, J. Am. Chem. Soc. 123 (2001) 5014. Y Liu, and T. J. Pinnavaia, Chem. Mater. 14 (2002) 3. Y Han, F. S. Xiao, S. Wu, Y Sun, X. Meng, D. Li, and S. Lin, Phys. Chem. B 105 (2001) 7963 K. Schumacher, P. I. Ravikovitch, A. Du Chesne, A. V. Neimark and K. K. linger, Langmuir, 16(2000)4648.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Acidic zeolite coated mesoporous aluminosilicates D. Trong On and S. Kaliaguine Department of Chemical Engineering, Laval University, Ste-Foy, Quebec G I K 7P4, Canada, A new approach to the synthesis of unusual zeolite coated mesoporous aluminosilicate (ZCMAS) using a diluted clear gel solution containing primary zeolite units is reported. Hydrothermally ultra-stable and highly acidic ZCMAS are achieved due to the nanocrystalline zeolitic nature of their pore wall surface, which opens up new opportunities for the use of these materials as high temperature acid catalysts. 1. INTRODUCTION Because of their amorphous wall character, the acidity and hydrothermal stability of mesoporous aluminosilicates (MAS) are relatively low compared to those of zeolites, which limits their potential applications as catalysts.' One might expect to improve both the stability and acidity of these materials if zeolite-like order could be introduced into the mesopore walls. The use of zeolite seeds as precursors for the assembly of mesoporous aluminosilicates was reported.^ Furthermore, recent results from our group showed the preparation of a new type of materials (UL-zeolites) with semi-crystalline zeolitic mesopore walls. The results indicated that nanocrystals were embedded in the continuous amorphous inorganic matrix to form semicrystalline wall structures while preserving the mesoporous structure. Herein, we describe another approach namely the production of zeolite coated mesoporous aluminosilicates (ZCMAS) using diluted clear solutions containing primary zeolite units."* 2. EXPERIMENTAL The synthesis of zeolite coated mesoporous materials involves two steps: The first step consists of the preparation of the MAS precursors such as SBA-15 ^ or a mesocellular foam (MCF) ^ and the desired clear zeolite gel containing primary nanocrystal units. The second step is the coating of zeolite nanocrystals on the MAS surface using the diluted clear zeolite gel. The calcined mesoporous precursors after contacting the diluted ZSM-5 gel under vigorous stirring at room temperature for 1 h were filtered, washed with distilled water and dried at 80°C. The resulting materials were subsequently suspended in glycerol and then transferred into a Teflon lined autoclave and heated at 130°C for 24 hours. Finally the solid product was filtered, washed with distilled water, dried at 80°C and calcined at 550°C. The materials were characterized using BET, FTIR, and TEM. Solid-state ^^Al and ^^Si MAS NMR spectra were recorded at room temperature using a Bruker ASX 300 spectrometer.

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3. RESULTS AND DISCUSSION It is of special concern that due to the size of primary ZSM-5 units templated by tetrapropylammonium ions (28 A in diameter), the pore diameter of the mesoporous precursor molecular sieves should be higher than 30 A. Mesoporous aluminosilicate precursors, such as SBA15 ^ and meso-cellular foam (MCF) ^ were used in this context. The N2 adsorption/desorption isotherms of the SBA-15 aluminosilicate presursor and ZSM-5 coated SBA15 exhibit the typical behavior of a mesoporous molecular sieve with a mesopore volume of-1.56 and 0.78 cmVg (BJH surface area: 800 and 465 m^/g), respectively (Fig. 1). Further, a significant decrease in pore diameter (from 70 to 54 A) and a narrower pore diameter distribution of the coated sample compared to that of the parent sample could conceivably be ascribed to the ZSM-5 nanocrystals coated inside the mesopore channels of the host. Similar results were also obtained for a MCF aluminosilicate precursor before and after coating (Fig. 2). The mesopore volume and the pore diameter decrease from 2.4 to 0.7 cm^/g and 315 to 175 A, (surface area from 875 to 435 m^/g), respectively.

k

.—7^

I

XV

s

a

Fig. 1. N2 sorption isotherms and BJH pore diameter distributions from the desorption branch a) SBA-15 and b) ZSM-5 coated SBA15.

^

Fig. 2. N2 sorption isotherms and BdBFHH pore diameter distributions from the adsorption branch a) MCF and b) ZSM-5 coated MCF.

The ZSM-5 coated samples of both SBA-15 and MCF show a FTIR absorption band at 550 cm'\ which is essentially not present in the parent samples. The band around 550 cm"' is characteristic of five-membered ring units in pentasils indicating that ZSM-5 nanocrystals are present within the mesopore walls. The acidity of the materials after coating is enhanced. The FTIR spectra of adsorbed pyridine on the parents, ZSM-5 coated samples in H-form and on H-ZSM-5 with almost the same Al content indicate that strong Bronsted and Lewis acid sites are created in the ZSM-5 coated samples. These sites are only slightly weaker than in ZSM-5 itself showing however more Lewis sites (not shown). The order of the acid strength is as follows: the parent samples « ZSM-5 coated MCF < ZSM-5 coated SBA-15 < ZSM-5.

563

0

25

50

75

100

125

150

175 200

Pore diameter (A)

Fig. 3. Si^^ MAS NMR spectra a) parent and b) ZSM-5 coated SBA-15 samples.

Fig. 4. BJH pore size distributions of ZSM-5 coated SBA-15a) before and b) after steaming.

Table 1 Physico-chemical properties of the parent mesoporous aluminosilicates (SBA15) and ZSM-5 coated SBA15 (ZC-SBA15) samples before and after hydrothermal treatments. N" Materials Treatment SBIT SBJH Mesop. Vol. BJH pore time (days) (m^/g) (m^/g) (cm^/g) diameter (A) Boiling water at 100"C SBA15-0-W* 1080 1 0 70 800 1.56 SBA15-2-W 415 375 1.72 2 120 2 465 0.78 ZC-SBA15-0-W 495 52 0 3 485 0.85 ZC-SBA15-2-W 55 475 2 4 58 495 1.35 ZC-SBA15-5-W 485 5 5 Steaming of 20% vapor water in N2 at 800"C ZC-SBA15-1-S 1 445 400

0.70

53

* SBA15-x-y where: x treatment time in days, y: boiling water (W) or steaming (S) treatment

The ^^Si MAS NMR spectrum of the parent SBA-15 sample (atomic Si/Al= 65/1) shows a ^'^Si MAS NMR spectrum typical of mesoporous alumosilicates, which contains four main features (Fig. 3). Two main resonances at -112 and -100 ppm and a weak peak at -92 ppm correspond to Si(0Si)4 (Q'*), (H0)Si(0Si)3 (Q^), (HO)2Si(OSi)2 (Q^) silicate species, respectively; a shoulder at -105 ppm has been assigned to (A10)iSi(0Si)3 species due to the tetrahedral aluminum structure. However, the ^^Si MAS NMR spectrum of the ZSM-5 coated SBA-15 sample (atomic Si/Al= 50/1) shows a main resonance centered at -112 ppm, which is attributed to Q'* silicon of the silicalite framework and a shoulder (A10)iSi(0Si)3 band at -105 ppm (Fig. 3). Only a weak resonance attributable to Q^ silicon from surface hydroxyl groups is observed at ~100 ppm. The increase in intensity of the Q"* resonance and

564

concomitant decrease in intensity of the Q"^ and Q^ resonances reflect the transformation of the hydrophiHc surface of the precursor into a more hydrophobic one upon coating. The same trend was also observed for the MCF aluminosilicate sample before and coating (not shown). The hydrothermal stability in boiling water at 100°C and steam stability with water vapor of the parent SBA-15 and ZSM-5 coated SBA-15 samples were also studied. In boiling water, the mesopore structure of the parent sample was collapsed after 2 days. However, no significant collapse of the mesopore structure was observed for the coated sample after 2 days in the same treatment conditions. Even after 5 days, the mesopore structure was still uniform. The coated sample was also steamed with 20% water vapor in N2 at 800°C (Fig. 4) and showed no essential change in the pore size distribution after one day of steaming indicating that the coated sample is hydrothermal I y ultra-stable. The remarkable hydrothermal stability of the coated sample observed here involves the zeolite seeds coated on the mesopore surface, which act to heal defect sites in the mesopore surface, and should be associated with the lowered silanol surface concentration. 4. CONCLUSION We demonstrate a general approach to coating zeolite nano-units within the mesopore structure. This synthesis methodology can be extended beyond the coating by ZSM-5 seeds, since primary units with diameters of 28 A for MFI/MEL structures, 26 A for zeolite beta, 15 A for ZSM-12 and 16 A for zeolite sodalite have been reported. They can potentially be used to prepare a large variety of zeolite coated mesoporous molecular sieves. This new type of materials has potential application as high temperature acid catalysts. REFERENCES 1. D. Trong On, D. Desplanticr-Giscard, C. Danumah, S. Kaliaguinc, Applied Catalysis A: General, im\, 222,299. 2. Y. Liu, W. Zhang, T. J. Pinnavaia, Angew. Chem. Int. Ed. 2001, 40, 1255. 3. D. Trong On and S. Kaliaguine, Angew. Chem. Int. Ed. 2001, 40, 3248. 4. D. Trong On and S. Kaliaguinc, Angew. Chem. Int. Ed. 2002, 41, 1036. 5. D. Zhao, Q. Huo, J. Feng, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 1998, 120, 6024. 6. P. S. Winker, W. W. Lukens, P. Yang, D. I. Margolese, J. S. Lettow, J. Y. Ying, G. D. Stucky, Chem. Mater. 2000, 12, 686.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Stable ordered mesoporous titanosilicates with active catalytic sites Feng-Shou Xiao*, Yu Han, Xiangju Meng, Yi Yu, Miao Yang, and Shuo Wu Department of Chemistry & State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, China Stable ordered mesoporous titanosilicates with highly active catalytic sites (MTS-9) have been successfully synthesized from assembly of pre-formed titanosilicate precursors with polymer surfactant (PI23) in strongly acidic media. Mesoporous MTS-9 shows good hydrothermal stability in boiling water (over 120 hours). Catalytic data show that MTS-9 is very active catalyst in catalytic conversion of both small molecule of phenol and bulky molecule of 2,3,6-trimethylphenol. The MTS-9 samples were characterized with infrared, UV-Raman, and TEM techniques. The results suggest that the good hydrothermal stability of MTS-9 is attributed to thicker mesoporous wall and zeolite-like connectivity of TO4 (T=Si, Ti) in the mesostructure. The high catalytic activity of MTS-9 are due to the TS-1-like enviromcnt of the Ti species in MTS-9. 1. INTRODUCTION Since the discovery of microporous TS-1 [1], a series of microporous titanosilicates have been reported, which exhibit remarkable catalytic properties in oxidation of alkanes, epoxidation of alkenes, and hydroxylation of phenol [2,3]. However, one disadvantage of these microporous titanosilicate catalysts is that their pores are too small for access by bulky reactants. Recent progress in solving this has been the substitution of titanium ions into the silicon sites of mesoporous materials (MCM-41) [4-6]. These mesoporous titanosilicates have pore diameters of 30-200 A and exhibit catalytic properties for the oxidation of bulky reactants under mild conditions, but unfortunately, when compared with TS-1, the oxidation ability and hydrothermal stability are relatively low [7]. The low oxidiation ability and hydrothermal stability can be attributed to the amorphous nature of the mesoporous wall [7]. On the other hand, microporous crystals of zeolites are very stable, and are widely used commercial catalysts [8]. Recently, it has been reported the successful synthesis of titanosilicate nanoclusters with zeolite primary and secondary structural building units [9]. More recently, there had been great progress in the preparation of mesostructured materials assembled from nanoclusters such as mesoporous aluminosilicate nanoclusters [10,11]. In our preliminary work [12,13], we have briefly reported the synthesis of an ordered mesoporous titanosilicate (MTS-9) via self-assembly of pre-formed titanosilicate precursors with triblock copolymers in a strong acidic media. We demonstrate here that MTS-9 shows excellent

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hydrothermal stability and very high activity for the oxidation of the smaller molecules of phenol and styrene and also of the bulky molecule of 2,3,6-trimethylphenol (TMP). 2. EXPERIMENTAL The preparation of MTS-9 have been described in elsewhere [12,13]. X-ray diffraction (XRD) patterns were obtained with a Siemens D5005 diffractometer. Transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were recorded on a JEOL2010FEG. The nitrogen isotherms at -196 °C were measured using a Micromeritics ASAP 2010 system. Infrared (IR) spectra of the samples were recorded on a Perkin-Elmer FT-IR spectrometer (PE 430). UV-Raman spectra were recorded on an UV-Raman spectrometer built by the State Key Laboratory for Catalysis, Dalian, China. Catalytic experiments were run in a 50 ml glass reactor and stirred with a magnetic stirrer. Phenol hydroxylation, styrene epoxidation, and hydroxylation of 2,3,6-trimethylphenol were performed at 80 °C for 4h, 45 °C for 3h, and 80 °C for 2h with molar ratio of reactant/H202 at 3/1, respectively. The products were analyzed by gas chromatography (GC-17A, Shimadzu). 3. RESULTS AND DISCUSSION 3.L X-Ray diffraction The small-angle X-ray diffraction pattern for a typical as-synthesized MTS-9 sample shows well-resolved peaks that can be indexed as (100), (110), and (200) reflections associated with the hexagonal symmetry [4,6]. The (100) peak reflects a d spacing of 112 A (a()=130A). No diffraction peak was observed in the region of higher angles 10-40°, which indicates the absence of large microporous crystals in the sample, suggesting that MTS-9 sample is a pure phase. Interestingly, after treatment of the sample in boiling water for more than 120 h, the XRD patterns still show those peaks assigned to the hexagonal symmetry, suggesting that MTS-9 is extremely hydrothermally stable, as compared to Ti-MCM-41 and SBA-15. 3.2. Transmission electron microscopy TEM image of MTS-5 exhibits ordered hexagonal arrays of mesopores with uniform pore size [4,6]. From high-dark contrast in the TEM image of the sample, the distance between mesopores is estimated to be 120 A. Furthermore, we observed that the wall thickness of MTS-9 is greater than that of SBA-15 reported in the literature. 3.3. Adsorption isotherms The N2 adsorption-desorption parameters of various samples are presented in Table 1. Notably, MTS-9 shows that the BET surface is 980 m^/g and 8.0 nm. Ti-MCM-41 and SBA-15 give the values at 1080 mVg, 2.7 nm, 870 m^/g and 7.6 nm, respectively. After treatment in boiling water, only MTS-9 keeps its structure

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Table 1 MTS-9, Ti-MCM-41, & SBA-15 samples before and after treatment in boiling water for 120 h Pores:size (nm) Wall thickness (nm) Surface area (m^/g) Samples After Before After Before After Before 8.9 4.8 4.3 980 720 8.0 MTS-9 ___ _-_ 1080 55 Ti-MCM-41 1.5 2.7 3.6 870 187 7.2 SBA-15 — — 3.4. IR spectroscopy. IR spectrum of SBA-15 shows a borad band at 460 cm', which is similar to those of amorphous materials. However, MTS-9 exhibits 550 cm'' band, which are similar to those of 5-membered rings of T-O-T (T=Si or Al) in microporous zeolites [8]. These results suggest that MTS-9 has zeolite primary and secondary building units. 3.5. UV-vis spectroscopy UV-vis spectrum of Ti-MCM-41 exhibits the relatively broad band centered at 230 nm, which is assigned to the distorted 4-coordinated Ti specie. However, MTS-9 gives at 215 nm, suggesting that both Ti species in TS-1 and MTS-9 are the same (4-coordinated) [1]. 3.6. UV-Raman spectroscopy UV-Raman spectroscopy is very sensitive to the coordination environment of titanium species. The framework titanium ions of TS-1 have been identified with UV-Raman spectroscopy by the asymmetric stretching of Ti-O-Si species, and the asymmetric stretching vibration mode of the tetrahedral framework titanium of TS-I is found to be at 1125 cm"' [14]. UV-Raman spectrum of MTS-9 shows the peak at 1122 cm', indicating that the coordinated environment of Ti in MTS-9 is very similar to that of TS-1 [14]. On the contrary, Ti-MCM-41 exhibits the peak at 1110 cm"' [15], suggesting that the coordinated environment of Ti in MTS-9 and Ti-MCM-41 is distinguishable. 3.7. Catalytic tests Catalytic activities for the oxidation of aromatics by H2O2 over MTS-9, Ti-MCM-41, and TS-1 catalysts are summarized in Table 2. In phenol hydroxylation, Ti-MCM-41 shows very low catalytic activity, but MTS-9 exhibits very high catalytic activity, with a phenol conversion of 26% which is comparable with TS-1 [1]. In styrene epoxidation, MTS-9 shows activity and selectivity similar to those of TS-1. In 2,3,6-trimethylphenol hydroxylation, Ti-MCM-41 is inactive due to the relatively low oxidation ability of Ti species in the amorphous wall of Ti-MCM-41, and TS-1 is also inactive due to the inaccessibility of the small micropores of TS-l to the large diameter of a bulky molecule like 2,3,6-trimethylphenol. However, MTS-9 is very active for this reaction with a conversion of 18.8%, indicating that MTS-9 is an effective catalyst for the oxidation of bulky molecules.

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Table 2 Catalytic activities in oxidation reactions by H2O2 over MTS-9, Ti-MCM-41, and TS-1 samples Sample Reaction TOF Conv. H2O2 Product Selectivity"^, % (%) Eff.^ PI P2 __P3__ MTS-9 Phenol Hydroxylation*^ 6.8 26.3 79.5 0.7 59.5 39.8 Ti-MCM-41 Phenol Hydroxylation® 60.1 0.5 2.5 7.6 1.9 38.0 84.8 48.6 5.5 28.0 50.4 Phenol Hydroxylation® 1.0 TS-1 Styrene Epoxidation 9.4 56.4 56.4 29.3 MTS-9 42.7 28.0 Styrene Epoxidation^ 5.2 54.6 54.6 58.3 29.0 TS-1 13.3 Styrene Epoxidation^ 48.3 Ti-MCM-41 48.3 6.1 100 — — 18.8 68.3 MTS-9 TMP Hydroxylation*^ 7.4 66.7 21.1 12.2 4.1 20.9 TMP Hydroxylation*^ 1.4 Ti-MCM-41 4.6 25.5 69.8 TMP Hydroxylation*^ 1.2 4.2 71.1 0.3 11.3 TS-1 17.6 #: The efficiency conversion of H2O2 was calculated as follows: H2O2 eff. conv. = IOOXH2O2 (mols) consumed in formation of products/total H2O2 (mols) added. +: The product selectivity: PI (or P2 or P3)/(P1 + P2 + P3). @: The products are catechol (PI), hydroquinone (P2), and benzoquinone (P3). $: The product are styrene epoxide (PI), phenylacetaldehyde (P2), and benzaldehyde (P3). &: The product are trimethylhydroquinone (PI), trimethylbenzoquinone (P2), others (P3).

REFERENCES 1. M. Taramasso, G. Perego and B. Notari, U.S. Patent 4,410,501 (1983). 2.T. Blasco, A. Corma, and J. Perez-Pariente, J. Am. Chem. Soc. 115 (1993) 11806. 3. B. Notari, Catal. Today 18 (1993) 163. 4. C. T. Krcsge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 352 (1992) 710. 5. P T. Tanev, M. Chibwe & T. J. Pinnavaia, Nature, 368 (1994) 321. 6. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F Chmelka, G. D. Stucky, Science 279 (1998) 548; Z. Luan, J. Y. Bae, and C. Kcvan, Chem. Mater., 12 (2000) 3202. 7. A. Corma, Chem. Rev. 97 (1997) 2373. 8. H. Van Bekkum, E. M. Flanigen, P. A. Jacobs, J. C. Jansen (Eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 2002. 9. X. Xu, M.S. Thesis, Jilin University, China, 1999. 10. Y. Liu, W. Z. Zhang, T. J. Pinnavaia, J. Am. Chem. Soc. 122 (2000) 8791; Angew. Chem. Int. Ed. Engl. 40(2001) 1255. 11 .Z. Zhang, Y Han, L. Zhu, R. Wang, Y Yu, S. Qiu, D. Zhao, and F.-S. Xiao, Angew. Chem. Int. Ed. Engl. 40 (2001)1258; J. Am. Chem. Soc, 123 (2001) 5014. 12. Y Han, F-S. Xiao; S. Wu; Y Sun; X. Meng; D. Li; S. Lin, F Deng, X. Ai, J. Phys.Chem. B 105(2001)7963. 13.F-S. Xiao, Y. Han, Y. Yu, X. Meng, M. Yang, S. Wu, J. Am. Chem. Soc, 124 (2002) 888. 14.C. Li, G. Xiong, Q. Xin, J. Liu, P Ying, Z. Feng, J. Li, W. Yang, Y Wang, G. Wang, X. Liu, M. Lin, X. Wang, E. Min, Angew. Chem. Int. Ed. 38 (1999) 2220. 15.G. Xiong, C. Li, H. Li, Q. Xin, Z. Feng, Chem. Commun. (2000) 677.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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W/Zr mixed oxide supported on mesoporous silica as catalyst for «-pentane isomerization Tao Li, She-Tin Wong, Man-Chien Chao, Hong-Ping Lin, Chung-Yuan Mou and Soofin Cheng Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. FAX: +886-22363-6359. E-mail: [email protected] W03/Zr02 mixed oxides supported on various porous silica were prepared. For the mesoporous silica, SBA-15 was found to retain the crystalline structure better than MCM-41 after loading W/Zr mixed oxides. Tungstated zirconia is mainly dispersed into the mesoporous channels of SBA-15, which causes the dramatical decrease in the surface area and pore volume. All the mesoporous siliceous materials supported solid acid samples showed strong acidity. The SBA-15 supported W03/Zr02 materials promoted with Pt were highly efficient in catalyzing the isomerization of n-pentane with a high selectivity of isopentane. SBA-15 with 1 %Pt/20%WO3/40%ZrO2 gave the highest catalytic activity. 1. INTRODUCTION Due to the hazardous properties of liquid acids such as HF and H2SO4 commonly employed in petrochemical industry, a great of effort has been focused on the development of more environmentally friendly strong solid acids [1]. Mesoporous silica, such as MCM-41 and SBA-15 have uniformed hexagonal arrays of mesopores and very high surface area [2,3]. However, the material itself has no acidity and low catalytic activities. The objective of this work is to introduce acid function on the mesoporous materials by supporting W/Zr mixed oxide on them and to examine their catalytic activities in isomerization of n-pentane. 2. EXPERIMENTAL Pure siliceous MCM-41 and SBA-15 were synthesized according to the literatures [4,5], The W/Zr mixed oxides were supported on the mesoporous silicas by co-impregnation of zirconium(IV) acetylacetonate and ammonium metatungstate hydrate. The resultant calcined materials were impregnated by the incipient wetness method with aqueous platinum tetrachloride and calcined at 773K in air. The resultant catalyst was characterized with various techniques such as XRD, N2 physisorption, HRTEM and DRIFTS of pyridine. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out on a Micromcritics AutoChcm 2910 instrument. The desorption process was monitored by a Quadruple Mass Spectrometer and the mass number 16 was followed to obtain the TPD profiles of NH3. The catalytic activities in the isomerization of ^-pentane were carried out in a fixed-bed micro-reactor at atmospheric pressure. On-line product analysis was done using a GC equipped with a FID detector. This work was supported by China Petroleum Corporation and National Science Council of Taiwan.

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3. RESULTS AND DISCUSSION 3.1. Characterization It was found that SBA-15 could keep its mesoporous structure better than MCM-41 after supporting W/Zr mixed oxide. The XRD patterns showed that the structures corresponding to hexagonal SBA-15 were still retained for the samples calcined at 800°C with the WO3 and Zr02 contents as high as 20wt% and 40wt%, respectively, while the MCM-41 lost its hexagonal structure with the same amount of WO3 and Zr02 loadings. For the SBA-15 with WO3 and Zr02 loadings up to 20wt% and 40wt%, respectively, only pure tetragonal Zr02 phase was observed and nearly no peaks due to WO3 crystallites could be seen. When the WO3 and Zr02 loadings were further increased, the crystalline WO3 phase and a little monoclinic Zr02 phase would appear. The physical properties measured from nitrogen adsorption-desorption isotherms of some solid acid catalysts are depicted in Table 1. Since all the SBA-15 supported mixed oxide samples were calcined at 800°C, the parent SBA-15 sample calcined at such a high temperature was compared. It has surface area, pore volume and pore diameter of 452 m^/g, 0.62 cmVg and 7.5nm, respectively. After supporting tungstated zirconia on SBA-15, the BET surface area and pore volume decreased gradually with the metal oxide loading. These results imply that the supported oxides should be dispersed onto the internal surfaces of the mesopores of SBA-15. Moreover, the supported catalysts have much larger surface areas (>92mVg) than the unsupported W03/Zr02 (24m^/g). The HRTEM photographs confirmed that the hexagonal arranged mesopores of SBA-15 were still retained and tungstated zirconia was mainly dispersed inside the pores. In comparison, the decreases in surface area and pore volume are more drastic for MCM-41 loaded with W03/Zr02. The temperature-programmed desorption of ammonia (NH3-TPD) was performed to determine the amount and strength of acid sites on the catalysts. Figure 1 compares the Nn3Table 1 Physical properties of some solid acid catalysts Sample

W03/Zr02 (wt/wt)

Zr02/support (wt/wt)

-

-

0.50 0.50 0.50 0.50 0.50 0.20 0.35 0.50 0.65

0.10 0.25 0.54 1.0 1.86 1.0 1.0

Si02 gel W03/Zr02/Si02 W03/Zr02 MCM-41 W03/Zr02/MCM-41

-

-

0.50 0.17

1.0

SO/-Zr02/MCM-41

-

SBA-15 WO^/ZrOj/SBA-lS WO^/ZrO./SBA-lS W03/Zr02/SBA-15 W03/Zr02/SBA-15 WOj/ZrOz/SBA-lS W03/Zr02/SBA-15 WOj/ZrO./SBA-lS W03/Zr02/SBA-15 W03/Zr02/SBA-15

0.50

1.0 1.0

1.0 1.0

Calc. Temp. ("C) 800 800 800 800 800 800 800 800 800 800 560 800 800 560 800 680

BHT area (m'/g) 452 411 301 201 137 92 216 166 137 125 660 141 24 1085 162 476

BJH pore volume Pore diameter (nm) (cm-Vg) 0.62 0.60 0.42 0.30 0.21 0.14 0.25 0.22 0.21 0.20 0.67 0.17 0.03 1.0 0.11 0.41

7.5 8.0 7.0 6.8 6.2 6.2 6.3 5.8 6.2 6.9

26.8 21.3 25.0

571

200

3(K)

400

500

6(X)

7(K)

800 Calcination temperature (°C)

Temperature ("C)

Fig. I.NH3-TPD profiles of (a) l%Pt/50%SZr02/MCM-41, and 1 %Pt/20%WO3/40%ZrO2 onvarious supports (b) SBA-15, (c) MCM-41, (d)silica gel, and (e) l%Pt/17%W03/Zr02.

Fig. 2.Catalytic activities of l%Pt/20% WO3/40%ZrO2/SBA-15 catalyst as a function of calcination temperature of the supported mixed oxides.

TPD profiles of W03/Zr02 supported on various silica supports, SBA-15, MCM-41, and silica gel, the unsupported V^IO^IZrOi and sulfated Zr02 supported on MCM-41. The WO3 and Zr02 contents in the three supported samples were similar, 20%WO3 and 40%ZrO2. The results demonstrate that W03/Zr02 supported on either SBA-15, MCM-41 or silica gel contains much more acid sites than the unsupported sample, while the acid strength does not have much change. In comparison to the MCM-41 supported sulfated zirconia with the same Zr02 loading, the acid strength of the supported mixed oxides is slightly weaker. These results accompanying with the BET results in Table 1 suggest that supporting W03/Zr02 on various silica can fonn mixed oxide of high dispersion and generate large amount of acid sites with medium acid strength. 3.2. Catalytic studies The catalytic properties of SBA-15 supported W03/Zr02 were investigated in the isomerization of /7-pentane to /.vo-pentane. The Pt-promoted W/Zr mixed oxide supported on SBA-15 was found to be efficient catalysts in the A7-pentane isomerization. For the Pt-free catalysts, nearly no activity was observed. Introducing a small amount of Pt (0.5wt%) onto the catalyst can cause a great increase in both the conversion of ^-pentane and selectivity to /.vo-pentane. The catalytic activity increases with the increase in Pt loading, while the selectivity of/.sY;-pentane keeps constant around 97%. For the catalyst with Pt loading higher than lwt%, no significant increase in promotion effect of Pt was observed. Under our reaction conditions, the major product was /.vr;-pentane. The main by-products were cracking products such as methane, ethane, propane, A2-butane and /.vc^-butane. Figure 2 shows that the «-pentane conversion over l%Pt/20%WO3/40%ZrO2/SBA-15 was dependent on the calcination temperature of the W03/Zr02/SBA-15 sample before impregnation of Pt. The optimal activity was observed on the catalyst calcined at SOO'^C. Figure 3 compares the catalytic activities in «-pentane isomerization of Pt-promoted W/Zr mixed oxide without support and that supported on various silica materials. It shows that SBA-15 support gives the highest A2-pentane conversion. In contrast, MCM-41 and silica gel supports give relatively low conversions. The BET surface areas of the three supported catalysts were in a close range of 140-160 m^/g, but the BJH pore diameter of SBA-15

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TOS(h)

Fig. 3. Conversion of A?-pcntane versus time on stream over different catalysts: (a) l%Pt/ 20%WO3/40%ZrO2/SB A-15, (b) unsupported l%Pt/17%W03/Zr02, (c) l%Pt/50%SZrO2/ MCM41, (d) l%Pt/20%WO3/40%ZrO2/MCM-41 and (e) l%Pt/20%WO3/ 40%ZrO2/SiO2.

TOS (h)

Fig. 4. Conversion of n-pentane versus time on stream over l%PtAV03/Zr02/SBA-15 with W03/Zr02 (wt/wt) ratio of 0.50 and different Zr02/SBA-15 (wt/wt) ratio (a) 1.0, (b) 1.86, (c)0.54, (d) 0.25 and (e) 0.10.

catalyst was ca. 6 nm and much larger than 2 nm of MCM-41. Therefore, the easier diffusion of gaseous reactants and products in SBA-15 probably accounts for its higher activity. The unsupported W/Zr mixed oxide gives similar conversion as that over SBA-15 supported catalyst. However, the unsupported catalyst decays with time-on-strcam more easily than the supported ones. These results imply that the supports play a role in stabilizing the catalytic active centers. The MCM-41 supported sulfated zirconia shows very high initial activity, but it decreases abruptly with time-on stream. These results are elucidated by that the acidity of supported sulfated zirconia is too strong and leads to severe coking and decay of the catalyst. Figure 4 compares the catalytic performance of l%Pt/W03/Zr02/SBA-15 samples with W03/Zr02 (wt/wt) ratio of 0.50 and different Zr02/SBA-15 (wt/wt) ratio. For catalysts with low Zr02 loadings (profiles d and e), the conversions of A7-pentanc are lower than 2%, and isopentane selectivities are lower than 50%. The catalytic activity increases drastically with Zr02 content. The optimal catalytic activity was observed on the catalyst with Zr02/SBA-15 wt/wt ratio of 1.0 (20wt% WO3 and 40wt% ZrO:). Further increase in Zr02 content would cause the decrease in /7-pentane conversion. This maybe due to the block of the mesoporous pores by too large amount of mixed oxide. These results also imply that the crystallites of tungstated zirconia must meet the required size to become the active sites for /7-pentane isomerization.

REFERENCES 1. A. Corma and A. Martinez, Catal. Rev.-Sci. Eng. 35 (1993) 483. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. 3. D.-Y. Zhao, J.-L. Feng, Q.-S. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. . Stucky, Science 279 (1998) 548. 4. D. Das, C.-M. Tsai and S. Cheng, Chem. Commun. (1999) 473. 5. C.-P. Kao, H.P.Lin and C.Y. Mou, J. Phys. Chem. Solid, 62 (2001) 1555.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Au and Au-Pt Bimetallic Nanoparticles in MCM-41 materials: Applications in CO Preferential Oxidation Satyanarayana Chilukuri', Trissa Joseph', Sachin Malwadkar', Chinmay Damle^, S.B. Halligudi\ B.S. Rao', Murali Sastry^and Paul Ratnasamy Catalysis' and Materials^ Chemistry Divisions, National Chemical Laboratory, Pune-411 008, India, Abstract Nanosized Au and Au-Pt bimetallic particles of different atomic ratios were synthesized from HAuCU and HPtCU inside the channels of amine functionalized MCM-41. These were characterized through chemical analysis, XRD and TEM. The size of the bi-metallic particles was found to be in the range of 2-4 nm. Their catalytic activities were evaluated in simulated gas mixtures that typically contain 0.5 and 0.96% CO in presence of large proportions (-74%) of H2. The catalysts that contain higher concentrations of Pt were found to be active and offer good CO preferential oxidation activity. Catalysts that contain an optimum amount of Au along with Pt have shown highest activities at lower temperatures. 1. INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFC's) require hydrogen, free from CO impurities (s^ prompted us to improve an efficient adsorption method of Chls on mesoporous silica for the highly efficient ET Energy transfer systems. In this study, to suppress pigment M Entry Symbol Re RA RB denaturation, alkanediol modified FSM Mg COOMe Me 1 Chi a O-C20H39 materials were synthesized and the ET 2 Chi 6 Mg COOMe CHO O-C20H39 3 Zn-APTES-ChI Zn H Me NH(CH2)3Si(OEt)3 behavior between Chls in mesopores 4 Cu-APTES-ChI Cu H Me NH(CH2)3Si(OEt)3 was investigated by fluorescence Fig. 1. Molecular structures of chlorophyllous ' Corresponding author. pigments. (Inset) Schematic representation of ET E-mail address: [email protected] in the mesopores.

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measurements. Moreover, to improve the ET efficiency in the mesopores, chlorophyll derivatives possessing triethoxysilyl groups were also synthesized and grafted on the silica surface. 2. EXPERIMENTAL Synthetic procedures of FSM were described in the literature [3, 5]. Dried FSM was refluxed in 1,6-hexanediol (HD) for 24 h under an N2 atmosphere. The esterified product (HD-FSM) was washed to remove unreacted diol and dried [3, 5]. Extraction procedure of Chls was reported previously [6]. The prescribed amount of Chi (1, 2, or the mixture of 1 and 2) was dissolved in toluene, and then HD-FSM was dispersed in the Chi solution. The mixture was stirred at room temperature for 3 h in the dark, and centrifuged to remove supernatant and dried in vacuo [5]. The amidation of Chi is as follows [7]: zinc C13^-demethoxycarbonyl-chlorophyllide a was added to a mixture of 3-aminopropyltriethoxysilane (APTES), dimethylaminopyridinium tosylate, and l-(3-dimethylaminopropyl)-3-ethylcarbodiimide in dry CH2CI2. The mixture was stirred for 12 h under N2 at room temperature. A crude compound was purified by a preparative high-performance liquid chromatography (HPLC). FSM powders grafted with Chls were prepared by the following method. An amidated Chi (3, 4, or the mixture of 3 and 4) and APTES, a modifier to suppress denaturation of Chls, were dissolved in dry CH2CI2. Then FSM powders were dispersed in this solution. The mixtures were stirred at room temperature for 24 h in the dark, centrifuged, washed with acetone, and dried. 3. RESULTS AND DISCUSSION The XRD pattern of HD-FSM had four peaks which are characteristic of a typical hexagonal array of mesopores, indicating that mesostructure did not change after the modification. Based on the data by solid-state NMR, CHN elemental and N2 adsorption analyses, the pore structure parameters of FSM materials were obtained (Table 1). This clearly shows that a large part of silanol groups were esterified by diols. The adsorption of Chi a (1) in the FSM pore system was performed by liquid-phase adsorption. The absorption spectral shape of the FSM/1 compounds was similar to that of Chi a in acetone. The suppression of the pigment degradation in the mesopores was confirmed by the analysis of the extracts from FSM/1 by HPLC. These findings indicate that Chls were adsorbed into FSM without denaturation. Next, to mimic some functions of photosynthesis, such as energy transfer from Chi h to Chi Table 1 Pore characterization of FSM and HD-FSM. BET Pore Pore size ^^ Number of grafted Number of silanol surface area volume silanol groups groups "^ ( m^ • g"') ( mL • g"') ( "tn ) ( groups • nm"^) ( groups • nm ") FSM 1020 0.82 3.2 2.8 HD-FSM 610 042 TT lA 1) Calculated by the BJH method 2) Based on the ^IQ^ ratio of ^"^Si MAS NMR measurements

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a (2 to 1), both Chi a and Chi b (1 and 2) were co-adsorbed into the HD-FSM pore system. When the ratio of 2/1 was 1/10, the peak area of the emission from Chi b (2) was about 0.3 times smaller than that from the single 2 system (Fig. 2a). With the decreasing of emission at around 655 nm, new emission in the wavelength region longer than the 655-nm band was 650 700 750 also observed in the difference spectrum 800 Wavelength / nm (Fig. 2b), though the fluorescence Fig. 2. (a) Fluorescence emission spectra of quantum yield of Chi a (1, ca. 0.2 [8]) is FSM/Chls. (b) Difference spectrum of (a). relatively small. This indicates that the Forster type D/A ET should occur. In the case of 2/1 ratios were 1/4 and 1/1, however, the peak areas of the emission from 2 were about 0.65 and 0.9 times smaller than that from the single 2 system. The results suggest that the energy transfer becomes more difficult as the 2/1 (D/A) ratio increases. This leads that it is necessary to optimize the adsorption states of the Chls in the mesopores. The arrangement between neighboring chromophores in mesopores was hardly controlled by simple liquid-phase adsorption, which is not suited for the efficient ET. To overcome this problem, the compounds 3 and 4 (Fig. 1) were synthesized and grafted on the silica surface. The advantages of grafting are as follows: (i) the interaction between adsorption site on silica surface and Chls is decreased, and (ii) the grafted chromophores may be centered in the mesopores. All grafted FSM powders were scarcely bleached by repeated washing with acetone. This is in sharp contrast with the fact that Chls incorporated into HD-FSM by simple liquid-phase adsorption were easily desorbed from mesopores by washing with acetone. This finding strongly supports that the amidated Chls (3 and 4) were grafted onto the surface of mesopores as well as APTES (modifier) molecules. Fig. 3 depicts the visible absorption spectra of the FSM powders grafted with the Chi derivatives. The top and middle curves illustrate 3 and 4 onto FSM, respectively. In the co-adsorbed state, the spectral shape was roughly interpreted as a summation of those of 3 and 4. For investigation of the ET behavior, the fluorescence emission spectra were recorded. The emission peaks of Zn-APTES-Chl (3, donor) and Cu-APTES-Chl (4, acceptor) on FSM were located at 666 and 670 nm (data not shown). The emission intensity of 4 was much weaker than that of 3 in FSM due to the small quantum yield of Cu-Chl. In the co-adsorption systems the emission intensity of 3 was decreased with an increase in the amount of 4. The ET efficiency from 3 to 4 is approximately estimated by the decrease in the emission area in comparison with that of FSM/3 complex (90% for 3/4 = 1/2; 50% for 1/1). Based on the pore volume and pore size of grafted FSM, the distance between neighboring chromophores (RQ) is calculated to be ca. 12 nm. It is of interest that this efficiency is much higher than that presented above (ET efficiency was ca. 70% for 2/1 = 1/10), though the pigment concentration and RQ in mesopores are basically same. Taking

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into account the trend that the highly efficient ET is obtained in this grafted system in comparison with the conventional adsorption method, the planes of chromophores are probably perpendicular to the silica walls in the mesopores (Fig. 1 inset). 4. CONCLUSION

400

500

600

700

Wavelength / nm

Fig. 3. Visible absorption spectra of Chls grafted We have demonstrated that the use of on FSM powders. FSM has enabled us to fabricate new inorganic/organic nanocomposite containing no solutions in the system. The modification on silica surface is also effective techniques for inclusion of Chls without denaturation. The ET efficiency of the system that Chls were incorporated by liquid-phase adsorption was not so high in comparison with that of the grafted system. We believe that the grafting of Chi on FSM surface has allowed us to optimize the pigment orientation for efficient ET. ACKNOWLEDGEMENTS The authors are grateful to Prof. T. Watanabe (Univ. of Tokyo) for HPLC analyses and Messrs. T. Shigeno and S. Murata (Waseda Univ.) for their experimental assistance. This work was partially supported by a Grant-in-Aid for COE Research, Japan.

REFERENCES 1. A.G. Tweet, W.D. Bellamy, and G.L. Gaines, J. Chem. Phys., 41 (1964) 2068. 2. K. Colbow, Biochim. Biophys. Acta, 314 (1973) 320. 3. S. Murata, H. Hata, T. Kimura, Y. Sugahara, and K. Kuroda, Langmuir, 16 (2000) 7106. 4. H. Furukawa, K. Kuroda, and T. Watanabe, Chem. Lett., (2000) 1256. 5. S. Murata, H. Furukawa, and K. Kuroda, Chem. Mater., 13 (2001) 2722. 6. H. Furukawa, T Oba, H. Tamiaki, and T. Watanabe, Bull. Chem. Soc. Jpn., 73 (2000) 1341 7. H. Furukawa, T. Watanabe, and K. Kuroda, Chem. Commun., (2001) 2002. 8. H. Scheer (ed.). Chlorophylls, CRC press, Boca Raton, 1991.

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Biological applications of organically functionalised mesoporous molecular sieves and related materials Humphrey H.P. Yiu and Ian J. Bruce School of Chemical and Life Sciences, University of Greenwich, Wellington Street, Woolwich, London, SE18 6PF, U.K. The adsorption behaviour of mesoporous molecular sieves SBA-15 and MCF was studied using a family of polysaccharide molecules, dextrans. The results provide some insight into the accessibility of the mesopores to biological molecules. The effect of an amine functionalised surface of the solid was also examined. 1. INTRODUCTION The discovery of the mesoporous molecular sieves M41S in the early 1990s had opened up a new research direction for materials sciences. The breakthrough regarding the limitation of microporous dimensions of zeolites permits the potential use of molecular sieves with larger molecules, such as biomolecules. Early candidates of mesoporous molecular sieves such as MCM-41 and MCM-48 (pore diameter 25 to 35 A) were considered to be unsuitable for biological applications but with the discovery of SBA-15 which possess relatively large pore dimensions (pore diameter ca. 60 A), biological applications of mesoporous molecular sieves became possible and the biomolecular adsorption behaviour of SBA-15 had been studied previously [1]. Research into the biological application of mesoporous molecular sieves is still limited. Enzyme immobilisation [2] and, more recently, protein separations [3] are the two major areas which have been explored. We are reporting a new route for analysis the accessibility of mesoporous molecular sieves SBA-15 and MCF by biomolecules. Dextrans, a family of polysacharride molecules, have been used to develop model systems to study adsorption which are relatively simple molecules compared with other biomolecules such as proteins. 2. EXPERIMENTAL 2.L Siliceous and amine functionalised SBA-15 The method for the preparation of pure siliceous SBA-15 has been reported previously [4]. The PEO-PPO-PEO template was removed by calcination. Propylamine functionalised SBA15 (PrNH2-SBA-15 with 1 mol % propylamine) was prepared following the in situ route already reported in the literature [2]. The PEO-PPO-PEO template was removed by extraction with ethanol at 78°C for 5 hours three times and the solid product was filtered, washed with ether and air-dried. * Corresponding author: Tel. +44 20 83318215. E-mail address: [email protected]

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2.2. Siliceous and functionalised MCF materials The method for the preparation of siliceous and amine functionaUsed MCF materials has been reported previously [3]. Two oil to surfactant ratios (0.3 and 0.6) were used and the products were denoted as MCF-3 and MCF-6 respectively. 2.3. Dextran adsorption to solid supports Dextran standard solutions ranging from 0 to 2 mg cm"^ were prepared by dissolving dextrans (Sigma and Fluka) in potassium phosphate buffer solutions (pH = 5 to 9). Dextrans with 4 different molecular weights were used (see table 1). The solid supports (20 mg) were suspended in the dextran solution in a microcentrifuge tube and incubated in a rotating disc for 16 hours at 25°C to ensure an equilibrium in adsorption of dextran molecules. The dextran content of the supernatant was determined colorimetrically (see below). Table 1 The notations for dextrans used in the adsorption experiment. Dextran Average molecular weight Dextran A 10,400 15,000--20,000 (or 17,500) Dextran B 40,000 Dextran C 68,800 Dextran D

Supplier Sigma Fluka Fluka Sigma

2.4. Colorimetric assay for dextrans Dextran content of the supernatants was determined using the anthronc assay for sugars [6]. A standard anthronc solution was prepared by dissolving 20 mg of anthronc (Sigma) in 100 cm"^ 80% w/w sulphuric acid. 0.3 cm"^ of supernatant was added to a test tube followed by 0.3 cm"^ cone. HCl and 30 |.il 90% formic acid. After the solution was thoroughly mixed, 2.4 cm^ anthronc standard solution was added slowly. The solution was then heated at 90°C for 5 minutes and cooled in a water bath immediately. The absorbance of the solution at 630 nm was measured. 3. RESULTS AND DISCUSSIONS 3.1. Molecular weight and adsorption to solid support The dextran adsorption isotherms of SBA-15 are shown in Figure 1. The amount of dextran molecule adsorbed to the solid support appeared to decrease with increasing dextran molecular weight in the range of 10 kD to 40 kD. Increase in molecular weight above this showed no significant difference in the adsorption isotherm. This could be because dextran molecules with molecular weights higher than 40 kD are too large to fit inside the mesopores of SBA-15. As a result, larger dextran molecules (C and D) can only be adsorbed to the outer surface of SBA-15. The result is in agreement with the protein adsorption isotherms from literature [I].

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0.01

0.02

0.03

0.04

0.05

equilibrium concentration (mM)

Fig. 1. Dextran adsorption isotherms of SB A-15. (Key: • dextran A (av. mw = 10.4 kD), • dextran B (av. mw = 17.5 kD), x dextran C (av. M.w. = 40 kD) and A dextran D (av. M.w. = 68.8 kD) 3.2. Effect of pore size on dextran adsorption Since the majority of biomolecules have dimensions larger than the pore diameter of SBA15, supports with pore diameters larger than those are needed for more general biological applications. Recently MCF materials formed by microemulsion have been used in the immobilisation of chloroperoxidase [7] and, as a result, were selected to study the effect of pore size on dextran adsorption. The dextran B and C adsorption isotherms are depicted in figure 2. From the dextran B adsorption isotherms, MCF-3 adsorbed a larger amount than molecules than SBA-15 (figure 2a). This could be because the pore dimension of MCF-3 is larger and so more molecules could be packed inside its cages. However, on increasing the pore dimension, MCF-6 appeared to adsorb a lower amount than MCF-3. A possible reason for this was that the pore size of MCF-6 was too large dextran B molecule packing to occur and only a monolayer of molecules was adsorbed. On increasing the molecular weight of dextran to 40 kD, the amount adsorbed by SBA-15 decreased significantly possibly because the molecules were too large to enter its pores. MCF-3 still adsorbed a considerable amount but less than the amount adsorbed MCF-6. It seemed that when the size increased to a certain extend, molecules could be packed inside the pores of MCF-6. 3.3. Effect of the surface chemistry on dextran adsorption Propylamine functionalised mesoporous materials have been used in protein separation [3] because the amine group on the surface can possess anion exchange property. However, in our experiment, no effect on the adsorption and desorption of dextran was observed.

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0

0.02

0.04

0.06

equilibrium concentration (mM)

0.01 0.02 0.03 0.04 equilibrium concentration (mM)

Fig. 2. Dextran adsorption isotherms of SBA-15, MCF-3 and MCF-6. Figure 2a is the comparison for the adsorption behaviour of the three supports using dextran B (av. M.w. = 17.5 kD) and dextran C (av. Mw. = 40 kD) was used in figure 2b. (Key: • SBA-15, D MCF3 and x MCF-6) 4. CONCLUSIONS Dextran adsorption isotherms have been used as a model to study the accessibility of the porous structure of mesoporous molecular sieves by biomolecules. The pore size of the supports and the molecular weight of dextrans have significant effect on the adsorption isotherms. Because of their larger pore size, MCF materials seem to be more useful for biological applications than SBA-15. ACKNOWLEDGEMENT We would like to thank BASF for kindly supplying the Pluronic P-123 surfactant, and EU for funding HIIPY (project no. G5RD-CT-2001-00534). REFERENCES 1. H.H.P. Yiu, C.H. Botting, N.P. Bottign, P.A.Wright, Phys. Chem. Chem. Phys. 3 (2001). 2. H.H.P. Yiu, P.A.Wright, N.P. Botting, J. Mol. Catal. B: Enzym. 15 (2001) 81. 3. Y.J. Han, G.D. Stucky, A. Butler, J. Am. Chem. Soc, 121 (1999) 9897. 4. H.H.P. Yiu, P.A.Wright, N.P. Botting, Microporous Mesoporous Mater. 44 (2001) 763. 5. P. Schmidt-Winkel, C.J. Glinka, G.D. Stucky, Langmuir, 16 (2000) 356. 6. Lisa M. Higgins, M.Ph. Thesis, University of Greenwich, 1996. 7. Y.J. Han, J.T. Watson, G.D. Stucky, A. Butler, J. Mol. Catal. B: Enzym. 17 (2002) 1.

Studies in Surface Science and Catalysis 146 Park et al (Editors) ©2003 Elsevier Science B.V. All rights reserved

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One pot synthesis of mesoporous ternary V205-Ti02-Si02 catalysts V. Parvulescu^ V. I. Parvulescu^*, M. Alifanti^'", S. M. Jung^ and P. Grange'^ ^Institute of Physical Chemistry of the Romanian Academy of Sciences, Splaiul Independentei 202, Bucharest, Romania. ^University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: [email protected]. c

Universite Catholique de Louvain, CATA, Croix du Sud 2/17, 1348 Louvain-la-Neuve, Belgium. Mixed vanadia-titania-silica catalysts (3 or 6w^t% V2O5, and 16-34wt% Ti02) were one pot prepared by sol-gel and hydrothermal methods in the presence of different surfactants. Tetraethylortosilicate (TEOS) was used as precursor for silica, tetraisopropylorthotitanate (TIPOT) for titania, and vanadyl sulfate for vanadia. As surfactants were used cetyltrimethylammonium bromide (CTMABr) or octadecyl-trimethylammonium bromide (ODCABr). The catalysts were characterized by adsorption and desorption curves of N2 at 77 K, NH3-DRIFTS, H2-TPR, XRD, in-situ Raman spectroscopy, XPS, and TEM. 1. INTRODUCTION Almost all the preparations concerning ternary vanadia-titania-silica considered the deposition of vanadium on mixed Ti02-Si02 oxides prepared either by co-precipitation or by sol-gel method. Shikada et al. [1] and Odenbrand et al. [2] prepared mixed Ti02-Si02 oxides by co-precipitation and vanadium was subsequently introduced by impregnation with NH4VO3. Shikada et al. [1] and Vogt et al. [3] proposed a procedure in which both titanium and vanadium were introduced by impregnation from several precursors. Rajadhyaksha et al. [4] prepared silica-supported titania samples and then V2O5 was impregnated on these supports. Handy et al. [5] also prepared V205-Ti02-Si02 catalysts by reacting vanadyl triisopropoxide with sol-gel prepared Ti02-Si02 mixed oxide. Reiche et al. reinvestigated the structural properties of these catalysts [6] by preparing vanadia grafted on Ti02-Si02. Very recently, Sorrentino et al. [7] attributed the performances of vanadia grafted on Ti02-Si02 catalysts to the size of V2O5 clusters. Here we report for the first time on the one pot synthesis of mesoporous ternary vanadiatitania-silica catalysts for reduction of nitrogen oxides with ammonia. Several questions like the location and the state of vanadia, thermal and hydrothermal stability of the catalysts were adressed.

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2. EXPERIMENTAL VTS1-VTS4 catalysts were obtained by the polymeric sol-gel method when the molar ratio of Si02/Ti02A^205 was 1.00/0.14/0.022. The molar ratio of surfactant/silica was 0.2/1. For the VTSl sample, the sol-gels of silica (A) and titania (B) were obtained by two different modalities. As for (A), a mixture of TEOS, ethanol and water (Si02/EtOH/H20 for a molar ratio of 1/5.2/5.2) was refluxed (pH=l) at 353 K for 2h. Separately for (B), a mixture of TIPOT, 2-propanol and acetic acid (TIPOT/C3H7OH/CH3COOH molar ratio: 1/9.5/1.5.2) was stirred for 3h. CTMABr was added as mixture (C). This resulted by mixing CTMABr with water for Ih. Vanadyl sulfate was added, under stirring, to the mixture (B). Then the (A+B+C) mixture was mixed and the gelation was carried out at room temperature. Acetylacetone was added to mixture (B) after the addition of vanadyl sulfate (the amount of acetylacetone was calculated as function of Ti02; the molar ratio Ti02/acetylacetone was 1/0.15). VTS2-VTS4 samples were prepared by mixing firstly the silica sol-gel (A) with the titania sol (B). For these catalysts, the mixture (B) was obtained by mixing a solution of TIPOT in 2-propanol with acetylacetone and vanadyl sulfate. The solution (C) was added to the mixture (A+B) and the gelation was carried out at room temperature. The composition of the gel was 1.00 Si02: 0.14 Ti02: 0.022 V2O5 for VTS3 and VTS4, and 1.00 Si02: 0.14 Ti02: 0.06 V2O5, for VTS2. For VTS4 the surfactant was ODCABr. The catalysts were characterized by adsorption and desorption curves of N2 at 77 K, NH3DRIFTS, H2-TPR, XRD, in-situ Raman spectroscopy, XPS, and TEM. Activity measurements were performed in a continuous flow fixed bed reactor operating at atmospheric pressure on 0.08g of the sample. The total flow rate was lOOml/min and feed composition was: nitric oxide 0.1vol%; ammonia 0.1vol%; 3vol% oxygen, in helium. The inlet and outlet gas compositions were measured using a quadrupole mass spectrometer QMC 311 Balzers coupled to the reactor. 3. RESULTS AND DISCUSSIONS The use of the sol-gel method in the presence of surfactants led to very high surface area materials (Table 1). The structures resulted in these preparations were better organized, resulting in MCM-41 textures as for VTS2, VTS3 and in a small extent for VTSl and VTS4. TEM analysis confirmed such a behavior (Fig. 1). Monomodal pore size distribution was determined for these samples with a BJH diameter between 2 and 6 nm (Fig. 2). However, differences exist between these procedures, and that used for the VTS2-VTS4 catalysts led to more organized textures. Actually, this procedure seems to provide a more intimate interaction of titanium and vanadium in the silica matrix. The agglomeration of vanadium is supposed to determine ruptures in the MCM channels, and as a direct consequence, smaller surface areas. The differences between VTS2-VTS3 and VTS4 are related to a direct effect of the surfactant. These above textures seemed to be fairly stable because no change was observed after 6 h reaction, neither from the adsorption-desorption curves of N2 at 77K nor from the TEM analysis. Vanadium is better dispersed in these structures as result both from the values of the XPS binding energies and from the Raman evidence of mono-oxo tetrahedral monomeric vanadyl species. The same XPS investigation showed that titanium is tetrahedrally coordinated in these samples. H2-TPR profiles indicated a very small hydrogen consumption with peaks centered at high temperatures, namely over 773 K. These results come in the same

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Table 1 Chemical composition and textural characteristics of the investigated catalysts Catalyst Chemical composition, wt.% Surface area Pore size V2O5 Ti02 Si02 m^ g'^ , nm 16 81 709 VTSl 5.5 16 81 1206 VTS2 3.7 32 62 994 VTS3 2.1 34 63 873 VTS4 3.3 Table 2 XPS binding energies, relative XPS and chemical ratios Catalyst Binding energy, eV Atomic XPS ratios V/(Ti+Si+V) Si2D Ti2p3/2 V2p3/2 0.17 459.5 517.5 103.3 VTSl 0.20 459.4 517.4 VTS2 103.8 0.26 459.5 517.8 103.8 VTS3 517.4 0.17 459.3 103.6 VTS4

Atomic chemical ratios V/(Ti+Si+V) 0.023 0.023 0.047 0.023

line with XPS and Raman spectroscopy data indicating that for these catalysts both titanium and vanadium are more rigidified in a defined surrounding. NH3-DRIFT spectra indicated for all the investigated catalysts the existence of both Bronsted (1430 cm"') and Lewis (1610 cm"') acid sites. The source of the acidity might be either the linkage between Ti and Si or V and Si or V and Ti. The increase of the temperature leads to a decrease of the intensity of the band located at 1430 cm"', which almost disappeared at 523 K indicating that in such conditions the Bronsted acid contribution is very low. A

Pore diameter, Fig. l.TEM picture of VTS3

Fig. 2. Pore size distribution for VTS3

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100 T

Fig. 3. NO conversion over the investigated catalysts comparatively to the commercial catalyst (total flow rate lOOml/min, 80 mg catalyst, 0.1vol% nitric oxide, 0.1 vol% ammonia, 3vol% oxygen, in helium, 350 °C).

Fig. 4. Selectivity of the investigated catalysts (total flow rate lOOml/min, 80 mg catalyst,0.1vol% nitric oxide, 0.1vol% ammonia, 3vol% oxygen, in helium, 350

decrease of the intensity also occured for the bands due to ammonia adsorbed on Lewis acid sites, but these are still present after the temperature was raised to 523 K. Figs. 3-4 indicate the catalytic performances of the above catalysts. Except for the commercial V205/Ti02 catalyst, a direct relation between the conversion and the surface area of these materials has been determined. This means that, indeed, in terms of turnover the commercial catalyst is still more active, but in terms of productivity the new catalysts are more effective. These data may also confirm the dispersion of vanadia inside the mesoporous structure. The selectivity of these catalysts was found to be superior to that exhibited by the V205/Ti02 catalyst (Fig. 4). In conclusion, one pot synthesis of mesoporous ternary V205-Ti02-Si02 systems led to catalysts which exhibit a good productivity in selective catalytic reduction of NO with ammonia. In addition, these systems exhibit a good stability under the investigated catalytic conditions and a good selectivity. REFERENCES 1. T. Shikada, K. Fujimoto, T. Kunugi, H. Tominaga, J. Chem. Technol. Biotechnol. A33 (1983)446. 2. C. U. I. Odenbrand, S. T. Lundin, L. A. H. Anderson, Appl. Catal., 18 (1985) 335. 3. E. T. C. Vogt, A. Boot, A. J. Van Dillen, J. W. Geus, F. J. J. Janssen, F. M. G. van den Kerkhof, J. Catal., 114(1988)313. 4. R. A. Rajadhyaksha, G. Hausinger, H. Zeilinger, A. Ramstetter, H. Schmelz, H. Knozinger, Appl. Catal., 51 (1989) 67. 5. B. E. Handy, A. Baiker, M. S. Marth, A. Wokaun J. Catal., 133 (1992) 1. 6. M. A. Reiche, E. Orteli, A. Baiker, Appl. Catal B., 23 (1999) 187. 7. A. Sorrentino, S. Rega, D. Sannino, A. Magliano, P. Ciambelli, E. Santacesaria, Appl. Catal. A: General, 209 (2001) 45.

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Photocatalytic hydroxylation of benzene on Ti-modified MCM-41 with both framework and Non-framework Ti centers Zongying GUO' Jing HE"* Shichao ZHANG*' D.G EVANS" Xue DUAN"* "The Key Laboratory of Science and Technoloy of Controllable Chemical Reactions, Beijing University of Chemical Technology, Ministry of Education, Beijing 100029. ^Beijing University of Aeronautics and Astronautics, Xuanyuan Lu, Beijing. Ti-modified MCM-41 possessing well ordered long-range and pore structures has been prepared and characterized by XRD, low temperature N2 adsorption-desorption measurements, AAS, UV-visible diffuse reflectance spectroscopy and XPS technique. The hydroxylation of benzene was carried out using Ti-MCM-41 containing both framework and non-framework Ti centers as photocatalyst with molecular oxygen as oxidant in the absence of any co-oxidant species. 1. INTRODUCTION There is currently great interest in catalytic oxidation technologies for the production of bulk and fine chemicals that are cleaner, safer and more environmentally friendly than traditional methods''"^'. Using redox molecular sieves as catalysts and H2O2 in aqueous solution as the oxidant, organic compounds can be transformed without the production of environmentally unfriendly side products. The high cost and low efficiency of H2O2 however, limits its applications on an industrial scale. In previous research^"'' we found that Ti-substituted MCM-41 exhibited high initial conversion of benzene and selectivity to phenol in the hydroxylation process. The benzene conversion decreased with the number times the catalyst was reused however, resulting most probably from the changes in the long-range and pore structures of Ti-substituted MCM-41 and, especially, the local structure of the Ti sites. According to the possible mechanism reported for TS-l'^', tetrahedral framework Ti centers are the active sites for selective oxidation reactions. The transformation of some framework Ti to non-framework Ti centers will therefore result in the decrease in the selective oxidation activity of Ti-substituted MCM-41. In this paper we investigate whether the hydroxylation in the liquid phase will result in the occurrence of non-framework Ti centers and whether these Ti centers can be used as photocatalytic sites on which molecular oxygen is transformed to the active oxidant species either for the hydroxylation of benzene on framework Ti centers or to interact directly with benzene to produce phenol. A previous report ^^^ on the photocatalytic oxidation of hydrocarbons in the liquid phase to give oxygenates indicates that the photocatalysis may be a potential low-temperature alternative for selective oxidation processes with molecular oxygen as oxidant. 'This work is supported by the State Major Basic Research Project of China (2000048009) and NSFC (20173003) *Co-corresponding authors E-mail: iinghe(g',263.net.cn or duanx(g),mail.buct.edu.cn

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2. EXPERIMENTAL Ti-substituted MCM-41 samples (also denoted as fresh Ti-MCM-41) were synthesized as described in our previous paper^^l Fresh Ti-MCM-41 was used as the catalyst for the hydroxylation of benzene at 65 °C with 30% H2O2 in aqueous solution as oxidant and acetone as solvent, to produce the spent catalyst (also denoted as used Ti-MCM-41). MCM-41 supported Ti02 was prepared by physically mixing of Ti02 with MCM-41. The total Ti contents determined by AAS are close to 4wt.% in all three samples. The atomic absorption spectra (AAS) measurements were carried out on a HITACHI Z-8000 atom absorption spectrometer. Powder XRD patterns were obtained using an XRD-6000 diffractometer with Cu Ka radiation, step size of 0.02° and scan rate of l°/min. The low-temperature N2 adsorption-desorption experiments were carried out using a Quantachrome Autosorb-1 system. The XPS spectra were obtained using a VG ESCA LAB 5 spectrograph with Al K^ radiation (9 kV, 18.5 mA). Diffuse reflectance UV-visible spectra were recorded by means of a TU-1221 spectrometer equipped with an integrating sphere attachment using BaS04 as background. Photocatalytic reaction was performed in a quartz reactor using a high-pressure mercury lamp (100 W, >t=200-700 nm) as UV-light source. Air was bubbled into the reactor during the reaction time with a flow rate of 80 ml/min. H2O2 yield was determined by titration with KMn04 standard solution after deionized water(100 g) and catalysts(0.1 g) were irradiated by UV light at 40°C for 3h. OH yield was determined as described in literature'^'. Photocatalytic hydroxylation of benzene was performed on the same photocatalytic reactor using acetic acid as solvent. Benzene (6 g, 77 mmol), acetic acid (15 g, 250 mmol) and deionized water (5 g, 278 mmol) were added to a specified quantity (0.1 g if not indicated) of catalyst. The mixture was vigorously stirred with irradiation at 40°C for 3h with an air flow of 80ml/min. The products were analyzed using a TU-1221 UV-visible spectrometer. 3. RESULTS AND DISCUSSION 3.1. Structural characteristics of catalysts The structural characteristics of catalysts were characterized by XRD, N2 adsorption experiment, UV-Visible diffuse reflectance spectroscopy and XPS technique. The XRD patterns (shown in Fig. 1) indicate that the used Ti-MCM-41 retains the well-ordered long-range structure of the fresh material. The results obtained from N2 adsorption-desorption isotherms given in Table 1 show that, the used Ti-MCM-41 sample possesses pore structure characteristic typical of mesoporous materials including a narrow distribution of pore diameters, high mesopore volume and specific surface area. The properties of the used catalyst are similar to those of the fresh catalyst and the Ti02/MCM-41 mixture. The XPS results (see Table 1) show that the Ti 2p3/2 B.E of used Ti-MCM-41 lies between the B.E values of fresh Ti-MCM-41 in which the Ti centers are located largely in the framework and Ti02/MCM-41 in which the Ti centers can be completely attributed to non-framework Ti sites. The XPS results indicate that the Ti centers in the used Ti-MCM-41 probably comprise both framework and non-framework Ti as predicted. The UV-Visible diffuse reflectance spectrum of fresh Ti-MCM-41 (Fig.2 (a)) shows a maximum absorption at about 240 nm which can be assigned to tetrahedral framework Ti (IV) centers'^''^'. For the used Ti-MCM-41 (Fig.2 (b)), the maximum shifts to about 270 nm with a

591

Table 1 Structural characteristics of Ti-modified MCM-41 Catalyst UsedTi-MCM-41 Fresh Ti-MCM-41 Ti02/MCM-41

Pore diameter at the maximum

Pore volume

Specific surface area

Surface Ti/Si molar ratio

2.7 2.7 2.7

0774 0.78 0.93

1042 1055 1258

0.12 0.07 0.07

An^l

Ti 2p3/2

B.E (eV) 458.6 459.9 458.1

shoulder at 240 nm. The origin of UV-visible absorption maxima at about 260-280 nm has been the subject of some dispute in the literature ^^'^l Calcination of used catalyst did not give a material with an absorption maximum at the same position as that of the fresh catalyst however (Fig.2(c)), indicating that the absorption maximum at 260-280 nm most probably results from non-framework Ti (IV) centers. Comparison of the spectrum of used Ti-MCM-41 (Fig. 2(b)) with that of Ti02/MCM-41 (Fig. 2(d)) indicates that bulk Ti02 is not present in the used catalyst. The diffuse reflectance UV-Visible spectra indicate that the used Ti-MCM-41 contains both framework and non-framework Ti centers, consistent with the XPS results discussed above. c di

u

c en

X)

b a

O

C/3

X3


t > 250 nm observed for Ti-MCM-41(2.00) can be attributed to a minor moiety of the Ti(IV) centers in pentaand/or octahedral coordination [6]. In order to monitor the relative amount of the Ti-oxide species exposed at the surface of the wall of the various Ti-MCM-41 materials, the IR spectra of ammonia irreversibly adsorbed at room temperature were recorded. No bands due to the adsorbed species were observed for pure MCM-41. Conversely, the spectra of the Ti-MCM-41 samples in contact with ammonia exhibit, in

215 1

iq08

c

l\ 1 \

D CD

Q)

n r 205\ D

0.02 A.u

230

d

o

\c

A'

1 1

200

MCM-41

^

250 300 Wavelength / nm

350

Fig. 1. Diffuse reflectance UV-vis spectra of the Ti-MCM-41 with different Ti contents (a) 0.15, (b) 0.60, (c) 0.85, (d) 2.00 wt%.

1800

1700 1600 1500 1400 Wavenumber (cm"') Fig. 2. FT-IR spectra of NH3 molecules adsorbed on MCM-41 and Ti-MCM-41 (a, 0.15; b, 0.60; c, 0.85; d, 2.00 Ti wt%) observed after admission of 10 Torr NH3 and subsequent outgassing at room temperature for 1 minute.

595

the 1800-1350 cm" range, a characteristic band at 1608 cm' (5asym NHsads) due to the NH3 Lifetime (ms) molecules irreversibly adsorbed on the Ti(IV) 0.110 r "^ V b centers of the tetrahedrally coordinated 0.098 Ti-oxide species [7], as shown in Fig. 2. The Sy 0.027 increase in the Ti content leads to an increase in \ 0.024 yd the intensity of this band, indicating that the amount of tetracoordinated Ti(IV) sites exposed at the surface walls of the channels of the Ti-MCM-41 materials increased with the Ti / ^ loading. The bands at 1708, 1660, 1453 cm' can be assigned to the vibrational bending / / ^ modes of NR^"^, formed by a protonation of the ammonia molecules by some acidic surface dir''^^ 1 1 1 r "^ hydroxyl groups. The band at 1553 cm' is 350 400 450 500 550 600 650 700 attributed to the Ti-NH2 or Si-NH2 bending Wavelength /nm mode formed by the irreversible reaction of Fig. 3. The photolumincsccncc spectra of Ti-MCM-41 with different Ti contents (a) NH3 with the Si-O-Ti bridges or distorted 0.15, (b) 0.60, (c) 0.85, (d) 2.00 wt% measured surface Si-O-Si bridges formed due to the at 295 K. incorporation of Ti into the Si-O-Si networks [7]. As shown in Fig. 3, the Ti-MCM-41 catalysts exhibit a photoluminescence spectrum at around 470 nm upon excitation at around 240 nm at 295 K. The observed photoluminescence spectra were attributed to the radiative decay process from the charge transfer excited state to the ground state of the isolated Ti-oxides in tetrahedral coordination [8-10]. As also shown in Fig. 3, the overall intensity of the photoluminescence increases with an increase in the Ti content up to 0.60 wt% and then decreases sharply for the higher Ti content. Furthermore, it was found that an increase in the Ti content from 0.60 wt% to 0.85 wt% leads to a decrease in the phosphorescence lifetime from 0.1 ms to 0.025 ms. On the basis of these data, it can be proposed that, when photoexcited, only the isolated tetrahedrally coordinated Ti-oxide species, which are more abundant for Ti contents up to 0.60 wt%, can stay in the excited state long enough to allow the occurrence of some radiative decay to the ground state. The dimeric and/or oligomeric tetrahedrally coordinated Ti-oxide species, which are likely to be the overwhelming species present at higher Ti loadings, decay quickly to the ground state through not-radiative, vibrational relaxation processes, which apparently are favoured by their clustered structure. We have found that UV irradiation of 0.60 0.85 Tiwt% Ti-MCM-41 in the presence of NO leads to Fig. 4. Relationship between the yields of N2, the the formation of N2 and O2, their yields intensities of the IR band of NH3 molecules increasing linearly against irradiation time. adsorbed on tetrahedral Ti(IV)-oxides (a) and These results clearly showed that the photoluminescence spectra (b) of Ti-MCM-41 photocatalytic decomposition reaction of NO with various Ti contents.

596

proceeds on Ti-MCM-41 at 295 K. As shown in Fig. 4, the efficiency of the photocatalytic decomposition of NO on these catalysts under UV irradiation at 295 K, which did not occur under dark conditions or by UV irradiation on the pure MCM-41, appears to be correlated to the intensities of the photoluminescence spectra. Thus, the amount of the isolated tetrahedrally coordinated Ti-oxide species, not the total amount of the Ti-oxide species exposed at the surface of the walls of the Ti-MCM-41 channels as monitored by the intensity of the 6asymNH3 band, appeared to play an important role. These results suggest that only the highly dispersed isolated tetrahedrally coordinated Ti-oxide species act as active sites in the photocatalytic decomposition ofNO into N2 and O2. 4. CONCLUSIONS The characterization of mesoporous Ti-MCM-41 prepared at ambient temperature with Ti contents in the 0.15-2.00 wt% range, using various spectroscopic methods such as XAFS, UV-vis, FT-IR, photoluminescence, showed that for Ti contents up to 0.60 wt%, isolated tetrahedrally coordinated Ti-oxide species are formed, while at higher Ti contents, dimeric or oligomeric Ti-oxide species, still with Ti(IV) in tetrahedral coordination, became overwhelming. The comparison between the intensity of the photoluminescence spectra, which are due only to the isolated tetrahedrally coordinated Ti-oxide species, and the yield in the photocatalytic decomposition of NO into N2 and O2 allowed us to conclude that only these species are responsible for such photocatalytic reactivity. REFERENCES 1. M. Anpo, Stud. Surf Sci. Catal., 130, 12th Int. Congr. Catal., Part A, A. Corma, F.V. Melo, S. Mendioroz, J.L.G. Fierro (Eds.), Elsevier, Amsterdam, (2000) 157. 2. M. Anpo, S. Higashimoto, Y. Shioya, K. Ikeue, M. Harada and M. Watanabe, Stud. Surf. Sci. Catal., A. Gamba, C. Colella, S. Coluccia (Eds.), Elsevier, Amsterdam, 140 (2001) 27. 3. J.-L. Zhang, M. Minagawa, T. Ayusawa, S. Natarajan, H. Yamashita, M. Matsuoka and M. Anpo, J. Phys. Chem. B., 104 (2000) 11501. 4. W. Zhang, M. Froba, J. Wang, P. T. Tanev, J. Wong and T. J. Pinnavaia, J. Am. Chem. Soc, 118(1996)9164. 5. L. Le Noc, D. Trong On, S. Solomykina, B. Echchahed, F. Beland, C. Cartier dit Moulin and L. Bonneviot, Stud. Surf. Sci. Catal., 101, 11th Int. Congr. Catal., Part A, J.W. Hightower, W.N. Delgass, E. Iglesia, A.T. Bell (Eds.), Elsevier, Amsterdam, (1996) 611. 6. L. Marchese, E. Gianotti, V. Dellarocca, T. Maschmeyer, F. Rey, S. Coluccia and J.M. Thomas, Phys. Chem. Chem. Phys., 1 (1999) 585. 7. L. Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia and J.M. Thomas, II Nuovo Cimento, 19D(1997) 1707. 8. M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis and E. Ciamello, J. Phys. Chem., 89 (1985)5017. 9. J.-L. Zhang, Y. Hu, M. Matsuoka, H. Yamashita, M. Minagawa, H. Hidaka and M. Anpo, J. Phys. Chem. B., 105 (2001) 8395. 10. M. Anpo and M. Che, Adv. Catal., 44 (1999) 119.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Photocatalytic epoxidation of propene with molecular oxygen under visible light irradiation on V ion-implanted Ti-HMS and Cr-HMS mesoporous molecular sieves Hiromi Yamashita*, Keiko Kida, Keita Ikeue, Yukiya Kanazawa, Katsuhiro Yoshizawa, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan FAX: +81-72-254-9287. E-mail: [email protected] The metal ion-implantation with V ions was effective to shift the absorption band of Ticontaining mesoporous molecular sieves (V/Ti-HMS, V/Ti-MCM-41) to the longer wavelengths. V/Ti-HMS and V/Ti-MCM-41 performed the photoepoxidation of propene with molecular oxygen under UV irradiation with the longer wavelength (K> 340 nm), while no reaction proceeded on the original Ti-HMS. Cr-containing mesoporous molecular sieves (CrHMS) could absorb visible light and showed the photoepoxidation under visible light irradiation (\> 450 nm). The charge transfer excited state of the tetrahedral chromium oxide moieties dispersed on mesoporous silica played a significant role in the photocatalytic reaction. 1. INTRODUCTION Ti-containing mesoporous molecular sieves exhibit high photocatalytic reactivity for the epoxidation of propene with molecular oxygen under UV irradiation (220-260 nm). It is vital to develop a photocatalyst that can operate efficiently under visible light irradiation [1,2]. In the present study, we have investigated on the photocatalytic reactivities of Ti-containing mesoporous molecular sieves implanted with V ion by an ion-implantation method (V/TiHMS, V/Ti-MCM-41) and chemically produced Cr-containing mesoporous molecular sieves (Cr-HMS) for the photoepoxidation under UV irradiation with the longer wavelengths (K > 340 nm) and visible light irradiation {\> 450 nm). 2. EXPERIMENTAL Ti-HMS, Ti-MCM-41 (Si/Ti=100) and Cr-HMS (Si/Cr=50) were synthesized using tetraethylorthosilicate, titaniumisopropoxide and Cr(N03)-9H20 as the starting materials, respectively, and templates: dodecylamine (HMS), cetyltrimethylammonium bromide (MCM41). The metal ion-implantation with V ions (0.66 p mol/g-cat) to the parent Ti-HMS and TiMCM-41 was carried out using an ion-implanter consisting of a metal ion source, mass analyzer, and high voltage ion accelerator (150 keV). Prior to the photocatalytic reactions, the

598

catalysts were degassed and calcined in 02 at 723 K for 2 h, then degassed at 473 K for 2 h. The photoluminescence were measured at 77 K and XAFS spectra was measured in the fluorescence mode at BLOIBI of SPring-8 (project No.: 2002A0613). The photocatalytic oxidation of propene (16 u mol) with 02 (32 y mol) was carried out with the catalysts (50 mg) using a high-pressure Hg lamp through a UV cut filter (A > 250 nm, A, > 340 nm, X > 450 nm) at 295 K. The products collected in the gas phase and after heating of catalysts at 573 K were analyzed by g.c. 3. RESULTS AND DISCUSSION 3.1. V ion-implanted Ti-HMS molecular sieves The results on the XRD patterns and the BET surface area of the Ti-HMS and Ti-MCM-41 indicated that these catalysts have a hexagonal lattice having mesopores larger than 20 A with a high BET surface area. The results obtained using XAFS analysis indicated that the local structure of the titanium oxide species of the Ti-HMS and Ti-MCM-41 is highly dispersed and exists in tetrahedral coordination (tetra-Ti-oxide), while Ti02 powder have the octahedral coordination. This tetra-Ti-oxide can exhibit the unique photocatalytic reactivity. Although the tetra-Ti-oxide included within mesoporous silica can exhibit the high selectivity for epoxide formation in alkene oxidation, they can only absorb and utilize UV light at around 220-260 nm to form the charge transfer exited state as active species. Fig. 1 shows the effect of metal ion implantation on the diffuse reflectance UV-Vis absorption spectra of Ti-HMS having the tetra-Tioxide. As shown in Fig. 1, the V ion-implanted TiHMS can absorb the light at the longer wavelengths (-450 nm) while the original un-implanted Ti-HMS absorbs the UV light at around 220-260 nm. These results indicate that the metal ion-implantation is effective to modify the Ti-HMS to absorb the light 200 250 300 350 400 450 500 with the longer wavelengths and exhibit the Wavelength/nm photocatalytic reaction under irradiation at the longer Fig. 1. UV-Vis absorption spectra of wavelengths. the V ion implanted Ti-HMS. Table 1 shows the results of the photocatalytic oxidation of propene with 02 under Table 1 The products in the photocatalytic oxidtion of propene irradiation of light with the various with O2 on the various catalysts under the light irrdiation with wavelengths ( \ > 250 nm, \> 340 the various wavelengths. Selectivity / % Light Conv. PO-yield nm, A,> 450 nm). Under UV Catalysts /nm /% /% PO HC COx HO irradiation with the longer wavelengths (K> 340 nm), the photo-epoxidation of propene with 02 to form propylene oxide (PO) proceeded on the ion-implanted V/Ti-HMS and V/Ti-MCM-41, while no reaction occurred on the original un-implanted Ti-HMS.

V/Ti-HMS >340 V/Ti-MCM-41 >340 >340 TI-HMS Ti-HMS >250

0.8 0.9 0 7

0.2 0.2 0 1.5

26 24 0 22

66 17 0 57

8 60 0 11

0 0 0 10

Cr-HMS

>450

10

1.2

12

63

4

20

CrS-1

>450

3.3

0.1

4

67

21

25

HO: propanal+acetone+acrolein+ethanal+alcohols, HC: hydrocarbons, CO,: CO2+CO

599

3.2. Chemically produced Cr-HMS mesoporous molecular sieves The results of XRD analysis indicated that ACF-O Cr-O-Cr the Cr-HMS have the structure of HMS R/A N 1.98 5.5 mesoporous molecular sieves and the Croxide moieties are highly dispersed in the framework of HMS and the CrS-1 have the structure of MFI zeolite. As shown in XAFS spectra (Fig. 2), Cr-HMS and CrS-1 exhibit CO n a sharp and intense preedge peak in the Ow XANES regions which is characteristic of < Cr-oxide moieties in tetrahedral coordination (tetra-Cr-oxide). In the FTEXAFS spectrum, only a single peak due to 5990 6010 6030 6050 Energy / eV Distance / A the neighboring oxygen atoms (Cr-0) can be observed and the curve fitting analysis Fig. 2. XANES (A-C ) and FT-EXAFS ( a-c ) spectra. indicated that tetra-Cr-oxide existed as in an (a) imo-Cr/HMS. (b) Cr-HMS. (c) CrS-1. isolated state with two terminal bonds {Cr=0) in the shorter distance of 1.57 A and two single bonds (Cr-0) of 1.82 A, while CrS-1 has tetra-Cr-oxide with the high Td symmetry (four oxygen atoms at 1.78 A). As shown in Fig. 3, the UV-Vis spectra of Cr-HMS exhibits three distinct absorption bands at around 280, 370 and 490 nm which can be assigned to charge transfer from O^" to C/'^ of the tetra-Cr-oxide. The tetra-Cr-oxide in Cr-HMS with the low Td symmetry exhibits absorption band at 490 nm (forbidden Ai-Ti transition), while it was not observed with CrS-1 having tetra-Cr-oxide in the high Td symmetry. Cr-HMS exhibited a photoluminescence spectrum at around 550-750 nm upon excitation of the absorption (excitation) bands at around 250-550 nm. Fig. 4 shows the 0.8 \l photoluminescence spectra of Cr-HMS observed at Ky V a 77 K upon the excitation at 280, 370, 500 nm, 1 ^"^ respectively. These three spectra were observed 0 1 h ^ ^^^*'«T ^ 1 1.2 at the same position. In the excitation spectrum 1 A of Cr-HMS monitored at 640 nm (Fig. 4), three excitation bands are observed in the same wavelengths to those observed in the UV-Vis 0 I 1 1 ^^^~"'-'— absorption spectra (Fig. 3). These results suggest 200 300 400 500 600 that the photoluminescence occurs as the radiation Wavelength / nm decay process from the same excited state Fig. 3. UV-Vis absorption spectra of CrHMS(a), Cr-HMS with 25 Torr H2O (b), independently to the excitation wavelength. These CrS-1 (c),and Cr04^" in K2Cr04 solution (d).

Am

600

0^-

V

a

Scheme. 1 Charge transfer process of the tetrahedrally coordinated Cr-oxide moieties.

absorption and photoluminescence spectra can be attributed to the charge transfer processes on the tetra-Cr-oxide involving an electron transfer from O^' to Cr^^ and a reverse radiative decay [3,4], respectively, as shown in scheme. 1. After the addition of reactants onto the Cr-HMS the efficient quenching of the photoluminescence was found, their intensity depending on the amount of added gases accompanied by the shortening of the emission lifetime of the excited triplet state. These results indicated not only that the charge transfer excited state of the tetra-Cr-oxide easily interact with the added gases and plays a significant role in the photocatalytic reaction but also that the large mesoporous cavities are significant in inducing for efficient photoreactions. 550

800 650 700 750 Wavelength / nm Fig. 4. The photoluminescence spectra of Cr-HMS at 77K. Excitation wavelength : 370 nm (a), 500 nm (b), 280 nm (c).

600

Under visible light irradiation (A> 450 nm), the photoepoxidation of propene with molecular oxygen to form propylene oxide proceeded on the both Cr-HMS and CrS-1 and CrHMS exhibits the highest PO yields among the present catalysts. 4. CONCLUSIONS The metal ion-implantation with V ions was effective to shift the absorption band of Ticontaining mesoporous molecular sieves to the longer wavelengths. The ion-implanted V/Ti-HMS and V/Ti-MCM-41 were found to exhibit the photoepoxidation of propene with molecular oxygen even under UV irradiation with the longer wavelength ( \ > 340 nm ). Chemically produced Cr-HMS can absorb visible light and act as an efficient and selective photocatalysts under visible light irradiation. The charge transfer excited state of the tetrahedral chromium oxide moieties dispersed on mesoporous silica are responsible for the efficient photocatalytic reactivities. REFERENCES 1. 2. 3. 4.

Yamashita, H., Yoshizawa, K., and Anpo, M., Chem. Commun., 2001, 435. Murata, C , Yoshida, and H., Hattori, T., Chem. Commun., 2001, 2412. M. Anpo, I. Takahashi, and Y Kubokawa, J. Phys. Chem., 86 (1982) 1. M. F. Hazenkamp and G Blasse, J. Phys. Chem., 96 (1992) 3442.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Mesostructured TiOi films as effective photocatalysts for the degradation of organic pollutants Jifi Rathousky*, Marketa Slabova and Amost Zukal J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. A well-organized film, whose photocatalytic activity is comparable with that of the most active commercial anatase powders, is formed only from a completely hydrolyzed precursor. 1.

INTRODUCTION

In the field of organized mesoporous materials most of the published experimental research has focused on silica as the inorganic framework constituent. The application of the reaction schemes originally developed for siliceous materials has been found much less successful for the synthesis of mesoporous transition metal oxides, especially titanium dioxide. Recently, a novel approach has been developed, enabling to obtain mesoporous titanium dioxide with highly interesting structural properties based on the usage of amphiphilic poly(alkylene oxide) block copolymers as structure directing agents in non-aqueous solution for organizing the network-forming titanium dioxide species [1]. Only very recently first successful preparations of mesoporous titania films have been reported [2-5]. In this communication the preparation of mesoporous titania films based on the mentioned approach will be analyzed with respect to the effects of decisive processing parameters and the obtained films will be tested in the photocatalytic destruction of an important organic water pollutant, viz. 4-chlorophenol. 2.

EXPERIMENTAL

2.1. Preparation of mesoporous titanium dioxide using block copolymers First, 0.9 g of Pluronic P-123 (BASF) were dissolved in 11 mL of ethanol. To this solution, 1 mL of titanium tetrachloride was added under vigorous stirring. The mixture was maintained in an open beaker at 40"C for 5 days, the evaporated ethanol being filled up every 12 h. Thus prepared clear yellowish solution could be stored at room temperature for several weeks without apparent changes. Films of different thickness were prepared by spreading various amounts of the stock solution on the glass support. The liquid layer was subsequently •Corresponding author; Telephone: +4202-66053865; Fax: +4202-86582307. E-mail: [email protected].

602

gelled in air at 40°C for 7 days and calcined at 400°C for 5 h in air. Finally, the titania films were peeled off the support (samples I-A, I-B, I-C, I-D). 2.2. Measurements Scanning electron micrographs were obtained by a Hitachi S-900 apparatus. Powder X-ray diffraction data were collected with a Siemens D 5005 diffractometer in the Bragg-Brentano geometry using CuKa radiation. Adsorption isotherms of nitrogen and krypton were measured at -196°C with an ASAP 2010 instrument (Micromeritics). UV/Vis were measured with an Perkin Elmer Lambda 19 spectrometer. ESCA analysis was performed with a Scienta 310 instrument (Gammadata AB). Photocatalytic activity of the Ti02 samples was studied using 4-chlorophenol as model pollutant. Photodegradation of this compound was examined employing a tube photoreactor where Ti02 was dispersed in water. After illumination by a medium pressure arc mercury lamp with the dominating 366 nm line, 4-chlorophenol follows three separate reaction pathways: hydroxylation, substitution and direct charge-transfer oxidation forming 4-chlorocatechol, hydroquinone and non-aromatic compounds as primary intermediates, respectively. The reaction rate was calculated according to the first-order kinetics. 3. RESULTS AND DISCUSSION The stock solution for the film preparation contains an ethoxide-modified titanium chloride, formed by the reaction: TiCU + X EtOH -^ TiCl4-x(0Et)x + x HCl, where x « 2. The formed TiClx(OC2H5)4-x species, which are rather stable against hydrolysis, associate preferentially with poly(ethylene oxide) moieties to produce a self-assembling complex. The necessary prerequisite for the formation of ordered material is the hydrolysis of titanium-containing species. Due to their stability, this process is strongly dependent on such parameters as the sufficient supply of water vapor and the length of the hydrolysis. Finally, calcination in air removes quantitatively the organic template. Chemical analysis by XPS has confirmed that the product does not contain any detectable amounts of elements other than titanium and oxygen, i.e. the removal of the organic component was complete. Because of the intended application in the continuous effluent decontamination and the aimed study into the effect of the completeness of the hydrolysis on the structure properties of mesoporous titania, the samples were prepared in the form of films of variable thickness. With thin films (samples I-A and I-B, density of 2 mg/cm^ and 4 mg/cm^, respectively), the full hydrolysis occurs due to a good accessibility for the air humidity during the aging. This ensures the creation of a highly uniform and regularly arranged porous structure with a narrow pore size distribution as has been proved by SEM and N2 adsorption (Fig. 1). With medium (sample I-C, density of 6 mg/cm^) and thick films (sample I-D, density of 8 mg/cm^) the hydrolysis is far from being complete. Consequently, larger pores are formed in addition to smaller ones during the calcination of the non-hydrolyzed fraction. This leads to the formation of a bimodal porous structure (Fig. 1, right). The structure parameters of all the films studied are given in Table 1. X-ray diffractograms and the Raman spectra (not shown here) evidence that all the samples contain a pure anatase phase. The presence of an amorphous titania component is probable because X-ray diffractograms exhibit decreased intensity of reflections due to anatase in comparison with

603

Table 1 Structure parameters of films prepared using block copolymers d^ Sample SBET (mg/cm^) (mVg) (nm) I-A 105 4.4 I-B 94 4.6 I-C 127 4.0, 5.2 I-D 104 6.2,16.0 ^density of the film, BET surface area, ^ mean pore size (two values correspond to a bimodal porous structure).

Fig. 1. SEM image of the thinnest film I-A (left) and adsorption isotherms of N2 at -196°C (right). The start point of each isotherm is shifted by p/po = 0.4. The solid symbols denote desorption. a pure reference material (Bayer). This component does not seem, however, to exhibit any characteristic Raman signal, which would distinguished it from anatase. As the absorption spectra in the UV range (not given here) show that there is no shift in the position of the absorption edge due the size-quantization effect, the obtained films are suitable for photocatalyzing the mineralization of 4-chlorophenol due to illumination with given light source. It was recently demonstrated that mesoporous titania prepared using ligand assisted templating methods has low photocatalytic activity compared to the crystalline phase despite its high surface area [6]. This low activity is due to the incomplete extraction of the surfactant and the amorphous titania channel walls. The authors conclude that partially crystallized titania is essential for obtaining high photocatalytic activity. It this study we have found that by optimizing the synthesis condition a highly active photocatalysist can be synthesized using block copolymers, whose activity compares well even with the best commercial materials (such as PKP 09040, Bayer). There are, however, severe requirements, which should be met. The preparation of a highly active photocatalyst requires the complete hydrolysis of the precursor, as that is the case with samples I-A and I-B. Consequently such a photocatalyst is characterized by a regularly arranged porous structure

604

with a narrow pore size distribution. Rate constants of the decomposition of 4-chlorophenol calculated according to the first-order kinetics are given in Table 2. Table 2 Decomposition of 4-chlorophenol Rate constant of the decomposition of 4-chlorophenol Sample I-A I-B Non-optimum films Bayer

(loS-') 3.49 3.42 1.4-2.4 3.37

4. CONCLUSIONS The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powder. ACKNOWLEDGMENT This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (contract No. A4040804) and the Deutsche Forschungsgcmeinschaft (WA 1116/7-1). REFERENCES 1. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 2. L. Kavan, J. Rathousky, M. Gratzel, V. Shklover and A. Zukal, J. Phys. Chem. B 104 (2000)12012. 3. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.-A. Albouy, A.Brunet-Bruneau and A.R. Balkenende, Adv. Mater. 13 (2001) 1085. 4. H.-S. Yun, K. Miyazawa, H. Zhou, I. Honma and M. Kuwabara, Adv. Mat. 13 (2001) 1377. 5. J. Yu, J.C. Yu, W. Ho and Z. Jiang, New. J. Chem. (2002) 607. 6. V.F. Stone and R.J. Davis, Chem. Mater. 10 (1998) 1468.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Comparisons of the structural and catalytic properties of Ti-HMS synthesized using the hydrothermal and molecular designed dispersion methods Tristan Williams and G. Q. (Max) Lu ^The NanoMaterials Centre, School of Engineering, The University of Queensland., St Lucia, Queensland, 4072, Australia. Fax: +64-7-3365-6074. E-mail: [email protected] Titanium containing wormhole-like mesoporous silicas, denoted Ti-HMS, synthesized both via the hydrothermal synthesis route and the post synthesis grafting technique, known as molecular designed dispersion, have been successfully applied in the gas phase oxidation of Toluene to CO and CO2. Selectivity towards CO2 for all catalysts, at temperatures between 400-600"C, was above 80%. Benzene and benzaldehyde were observed at temperatures above 450°C, but in very low concentrations. The conversion of toluene was shown to increase significantly when the VIHX/VMHSO ratios were increased from 0.07 to 0.84. No significant difference in catalytic activity was observed for catalysts prepared via the different synthesis techniques. The catalytic activity also depends on the concentration of tetrahedrally coordinated titanium atoms and not on the total concentration of titanium in the catalyst. 1. INTRODUCTION Ti-HMS materials exhibit greater catalytic activity than their corresponding Ti-MCM-41 counterparts. Some researchers [1, 2] have attributed this to the presence of textural mcsoporosity which facilitates access of reactant molecules to the active Ti sites. Others [3] hoverer, have observed very little difference in textural mcsoporosity between Ti-HMS and Ti-MCM-41 and have sought other factors to explain this difference. Since spectroscopic techniques cannot discriminate between the two materials, one such factor could be the location of Ti active sites on the silica wall. It has been observed that samples prepared by grafting Ti onto the surface of MCM-41 mesopore walls are just as active as Ti-HMS. Thus, they suggested that in contrast to Ti-MCM-41 the Ti species in Ti-HMS are concentrated on the surface of the mesopores and not randomly distributed in the silica framework. In this work we have compared the catalytic activity of the Ti-HMS synthesized via these two different routes. The effect of textural mcsoporosity on catalytic activity was also investigated. 2. EXPERIMENTAL The hydrothermal synthesis of Ti-HMS was performed as described by Zhang et al. [2] and the molecular designed dispersion method was performed as described by Schrijnemakers et

606

al. [4]. The molecular designed dispersion technique involved first making HMS samples with the desired structural properties and then grafting them with the desired amount of Titanium. N2 sorption measurements were performed on a Quantachrome Autosorb IC; UV-vis diffiise reflectance measurements and fixed wavelength absorbance measurements were performed on a Jasco UV-vis spectrometer. The catalj^ic tests were carried out at atmospheric pressure in a continuous flow, fixed-bed quartz microreactor. The reactor was loaded with 0.05 g of calcined catalyst and had a fixed bed volume of 0.25 cm' . Toluene was introduced into the reactor by flowing air through a saturator maintained at 95°C. The volumetric flow through the catalyst was held constant at 100 ml min"' and the concentration of toluene was 1000 ppm. The reactions were carried out at 400, 450, 500, 550 and 600°C. Before each reaction the catalyst was activated in air flowing at 400 °C for 30 min. The reactants and the reaction products were analyzed using two Shimadzu GC-17A gas chromatographs. Toluene and its incomplete oxidation products, benzene and benzaldehyde were measured online, using a 30 m DB-5 capillary column connected to a flame ionization detector. The O2, N2, CO and CO2 were analyzed on the second gas chromatograph fitted with a thermal conductivity detector and were separated using a 30 ft Porapak Q packed column. 3. RESULTS AND DISCUSSION The degree of textural mesoporosity of the Ti-HMS catalysts can be quantified using the VTI.:X/VMHSO ratio. This ratio is simply the textural mesopore volume, VTHX, divided by the mesopore volume VMI:SO. The textural pore volume is the difference between the total pore volume measured at a partial pressure of 0.98 and the mesopore volume. In Table 1 the properties of the catalysts investigated in this study are reported. Figure 1 compares two isotherms of Ti-HMS, one with low textural mesoporosity and the other with high textural mesoporosity. The high textural mesoporosity is indicated by the large increase in volume of nitrogen adsorbed above partial pressures of 0.9. Table 1 Physical and Chemical Properties of Catalysts Tested. Ti BJH Mesopore Total Textural VTEXA/MESO Synthesis BET Volume Volume Method ;Surface Pore Pore Loading Volume Area Radius cc/g cc/g Wt% A m'/g cc/g 1.07 0.42 17.0 MDD 0.65 6.99 768 0.65 1.30 0.69 0.77 0.53 5.93 909 15.5 MDD 1.01 HydroThermal 1152 0.12 0.14 13.7 3.73 0.89 0.47 15.4 1.30 HydroThermal 973 0.56 0.83 3.69 0.07 0.83 0.05 HydroThermal 939 13.8 0.78 6.69 1.71 0.84 17.2 0.78 HydroThermal 873 6.22 0.93 All catalysts are mesoporous and have high surface areas varying between 768 to 1159 m^/g. The titanium content varies between 3.69 Ti Wt 5 and 6.99 Ti Wt %.

607

1800

80 70

1500

-^^-3.7TlHMS-0.13 --3.7TiHMS-0.56 -X - 6.7TiHMS-0.06

- High Textural Porosity - Low textural Porosity

60 ^ -^>-6.2TIHMS-0.84

1200

50

o

c .2

I 900

12

- A - MDD5.93TiHMS-0.5:^ -•.-.MDD6.99TiHMS-0.69

40

0)

g 30 o o

> 600

20

300

0.0

10 i

0.2

0.4

0.6

P/Po

0.8

1.0

Fig. 1. N2 sorption Isotherms of Ti-HMS.

200

300

400

500

Temperature (C)

Fig. 2. Conversion of Toluene.

Figure 2 shows the behavior of the catalysts in the oxidation of toluene to CO and CO2. Samples prepared via the molecular designed dispersion are indicated with the prefix MDD. The numerical prefix indicates the titanium content as a weight percent while the numerical suffix indicates the VTHx/VMiiso ratio. For all catalysts this oxidation gives rise to greater than 80% conversion to CO2 at all temperatures between 400 and 600"C. Benzene and benzaldehyde were observed at temperatures above 450°C, but below the calculated minimum detectable limit of the FID. Comparison of the catalysts 3.7Ti-HMS-0.13, 3.7Ti-HMS-0.56, 6.7Ti-HMS-0.06 and 6.2Ti-HMS-0.84, all prepared hydrothermally, show that under these flow conditions an increase in textural porosity from around 0.1 to above 0.5 results in a near doubling of toluene conversion at SOO^C. To investigate the effect the synthesis method has on toluene conversion, we compare catalyst 6.2Ti-HMS-0.84 with catalysts MDD5.93Ti-HMS-0.53 and MDD6.99Ti-HMS-0.69. Catalyst 6.2-Ti-HMS-0.84, synthesized hydrothermally, is less than 5% more active at 500°C than MDD5.93Ti-HMS-0.53, but this minor increase may be possibly attributed to its increased textural mesoporosity. Catalyst MDD6.99Ti-HMS-69, synthesized via molecular designed dispersion, however, is almost 10% more active than 6.2-Ti-HMS-0.84 at 500°C. This slight increase may possibly be attributed to its greater titanium content. In either case, the differences in toluene conversion are very small so it appears that the type of synthesis method has negligible impact on the activity of the catalysts. Another trend observed for all catalysts was that the titanium content has very little effect on the over all activity of the catalyst. To fiirther investigate the effect of titanium loading, it was necessary to perform UV-vis diffuse reflectance measurements on the catalysts. Figure 3 contains the diffuse reflectance UV-vis spectra of all 6 catalysts. Figures 3a, b and c clearly show 2 distinct peaks. The first is a sharp peak at 210 nm, which corresponds to Ti

608

species in the tetrahedral site-isolated form. The second much broader peak at 260-270 nm corresponds to site isolated Ti atoms in a penta- or octahedral coordination [2, 5]. r\ A

3.7TiHMS-0.14 3.7TiHMS-0.56

0.3 -

K

0.6 -

250 300 350 Wavelength (nm)

0.7 -1

0.6

0.5 -

0.5

80.4 -

80.4

\

5°^ 1

0.1 -

200

6.7TiHMS-0.07 1 6.2TiHMS-0.84 |

400

0.2

0.2 -

0.1 -

0.1 -

0 2C)0

250

300

350

W a v e l e n g t h (nm)

400

0 200

\

MDD5.971HMS-0.69 MDD6.9TiHMS-0.65

250

300

350

400

W a v e l e n g t h (nm)

Fig. 3a. UV-vis spectra of Fig. 3b. UV-vis spectra of Fig. 3c. UV-vis spectra of TiTi-HMS synthesized by the Ti-HMS synthesized by the HMS synthesized by the hydrothermal method. hydrothermal method. MDD method. It can be seen in Figures 3a, b and c that as the titanium content increases, the maximum absorbance measured at 210 nm varies only slightly, while the absorbance at 260-270 nm of the much broader peak increases significantly. This suggests that the catalytic activity of the catalysts is dependent on these tetrahedrally coordinated titanium atoms, and not on the total concentration of titanium in the catalyst. 4. CONCLUSIONS The catalytic oxidation of toluene has been investigated for a series of Ti-HMS catalysts. Toluene is oxidized to CO and CO2. Selectivity towards CO2 is approximately 80% and is not affected by changes in temperature. Benzene and benzaldehyde were the only other incomplete oxidation products observed, but in very low concentrations. Greater textural mesoporosity results in higher catalytic activity, while the type of synthesis method employed has negligible impact. UV-vis diffuse reflectance measurements show that catalytic activity depends on the concentration of tetrahedrally coordinated titanium atoms, and not on the total titanium concentration.

REFERENCES 1. Tanev, P.T., Chibwe, M., Pinnavaia, T.J., Nature, 1994. 368: p. 321-323. 2. Zhang, W., Froba, M., Wang. J., Tanev, P.T., Wong, J., Pinnavaia, T.J., J. Am. Cham. Soc, 1996. 118: p. 9164-9171. 3. Tuel, A., Microporous and Mesoporous Materials, 1999. 27: p. 151-169. 4. Schrijnemakers, k. and E.F. Vansant, J. Porous Mater, 2001. 8: p. 83-90. 5. Blasco, T., Corma, A., Navarro, M.T., Pariente, J.P., J. Catal., 1995. 156: p. 65.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

609

Oxidation of methyl-propyl-thioether with hydrogen peroxide using Ti-SBA-15 as catalyst D. C. Radu^ A. Ion\ V. I. Parvulescu^*, V. Campeanu^ E. Bartha^ D. Trong On' and S. Kaliaguine" ^University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: V [email protected]. ^Institute of Organic Chemistry of the Romanian Academy of Sciences, Splaiul Independentei 202, Bucharest, Romania. Department of Chemical Engineering, Laval University, Ste-Foy, Quebec, GIK 7P4 Canada. A series of Ti-SBA-15 catalysts (atomic ratio Ti/Si = 1.5%) with various average pore diameters was prepared. These catalysts were tested in chemoselective oxidation of methylpropyl-thioether with hydrogen peroxide. The modification of these catalysts with tartaric acid was found to improve the chemoselectivity to sulfoxide. No leaching of Ti was detected under the investigated conditions. 1. INTRODUCTION Heterogeneous oxidation of organic thioethers and sulfoxides received a special attention in recent years. Chemo- but mainly stereoselective oxidation of these substrates may provide very useful intermediates and products. Both kinds of reactions were carried out in the presence of titanium catalysts. Chemoselective oxidation of thioethers and sulfoxides over several zeolite-type catalysts was reported.[1-2] Choudary et al.[3] indicated titanium-pillared montmorillonite modified with tartrates as a very efficient heterogeneous Sharpless catalyst. Very recently, Iwamoto et al.[4] reported the asymmetric oxidation of sulfides to sulfoxides using tartaric acid modified Ti-containing mesoporous silica catalysts. Our group reported the oxidation of various pyrimidine-derivatives on titania-silica mixed oxides prepared by sol-gel [5] Although the chemoselectivity was excellent, the modification with tartaric acid led to very poor stereoselectivity. The present study reports on the chemoselective oxidation of methyl-propyl thioether on Ti-SBA-15 catalysts. The oxidized substrate is a very effective intermediate in synthesis of flmgicides, acaricides, etc. Several questions like the titanium leaching and the stability of the catalysts under the investigated conditions were addressed.

610

2. EXPERIMENTAL A series of Ti-SBA-15 catalysts (atomic ratio Ti/Si of 1.5%) with various pore diameters were prepared in a strong acidic medium (2M HCl solution) using HO(CH2CH20)2o(CH2CH(CH3)0)7o(CH2CH20)2oH (Pluronic P-123, BASF) as the surfactant and tetraethyl orthosilicate and tetrapropyl orthotitanate as silicon and titanium sources. The modification of the above catalysts was made in situ by treating 10 mg catalyst with 3 mg tartaric acid in hydrogen peroxide solution 12wt% in dioxane at 298 K for 2.5h. The catalysts were characterized using adsorption-desorption isotherms of N2 at 77K, XRD, and XPS. The sulfides were prepared following a reported procedure [5]. The catalytic tests have been carried out in a glass-flask. Standard experiments used 10 mg catalyst and 100 mg sulfide under inert atmosphere. The reactions were carried out for a sulfide: H2O2 ratio between 1:1.2 and 1:12, temperatures between 293 and 323 K, and reaction times between lOmin and 5h. Dioxane was used as solvent. The analysis of the products was done by ' H and '^C NMR operating at 300 MHz for ^H and 75 MHz for '^C. The optical purity of produced sulfoxide was assessed by ' H NMR using the Pirkle's reagent. This was expressed as enantiomeric excess. 3. RESULTS AND DISCUSSIONS 3.1. Catalysts characterization Table 1 compiles the synthesized Ti-SBA-15 catalysts and the characteristics of these materials. All the catalysts contain the same amount of titanium. The catalysts pore diameter and surface area were adjusted from 3.6 to 5.6 nm and 610 to 920 m^/g by varying the heating temperature and time of the gel mixture during the synthesis process The SB A structure was well evidenced from XRD patterns. The binding energies corresponding to Ti2p.? 2 species were very close to those of Ti(!V) [6]. In addition, these data may give some information about the coordination of titanium ions. According to several authors [7-9] the binding energy of 459.5 eV to 460 eV is typical for tetrahedrally coordinated titanium. Table 1. Chemical composition, textural properties and Ti2p.v2 XPS binding energies of Ti-SBA-15 catalysts Ti loading. Surface area. Pore size. Ti2p3 2 XPS binding Catalyst DOl D02 D03 D04

mol % 1.5 1.5 1.5 1.5

m^g' 610 715 830 920

nm 3'6 4.2 5.1 5.6

energy, eV 459.4 459.5 459.4 459.5

3.2. Catalytic activity in oxidation of methyl-propyl-thioether The oxidation of methyl-propyl-thioether may occur selectively to sulfoxide or nonselectively to sulfone (Scheme 1). The stereocontrol of the first step to one of the two stereoisomers makes this reaction even more interesting leading to valuable products.

611

610

715

830

920

BET surface area (m g' )

BET surface area (m^g'^) Fig. 1: Selectivity to sulfoxide as a function of the surface area of the Ti-SBA-15 catalysts (298 K, 100% conversion)

Fig. 2: Selectivity to sulfoxide as a function of the surface area of the tartaric acid modified Ti-SBA-15 catalysts (298 K, 100% conversion)

On the investigated catalysts oxidation was complete after 2.5 h. The selectivity to the corresponding sulfoxide is presented in Fig. 1. Except for the sample with 830 m^ g\ the selectivity was higher than 84% for a conversion of 100%. These data indicated almost no dependence of selectivity on the texture characteristics. The hydrogen peroxide efficiency was over 60% in all the cases. O

/ 101

,c-^N,c,-c„,^

"3C

,

CH^CH^-CH-,

H3C

- i ^ U H3C

CH^- CH^- CH3

CH^- CH^- CH3

Scheme 1. Oxidation of methyl-propyl-thioether Similar tests in the absence of the catalysts showed that the conversion reached 100% after 5h reaction with a 100% selectivity to sulfone. These data clearly evidenced the contribution of the catalysts, indicating that the chemoselective oxidation of the thioether is a heterogeneous catalytic mediated reaction. Robinson et al. [2] also investigated the oxidation of small thioethers using a TS-1 catalyst. These authors reported that the selective sulfoxidation was achieved merely as a homogeneous catalytic step. Actually, under the conditions of our investigation no leaching

612

was observed. Further oxidation of the liquor separated after the hot filtration of the catalysts indicated only an enhancement of conversion identical with that observed for oxidation of the same substrate without any catalyst. The reuse of the catalysts for five times indicated the same performances. Such a behavior might be well correlated with the tetrahedrally coordinated titanium state found by XPS. Previous data [5] using sol-gel prepared mixed titania-silica catalysts in which part of titanium existed as octahedrally coordinated species showed that under these conditions part of titanium may leach to the solution. No e.e. was observed under these conditions. The modification of the catalysts with tartaric acid caused no changes in the catalysts activity, the conversion being also completed after 2.5 h. But, as it can be observed from Fig. 2 the modification with tartaric acid determined important changes in the chemoselectivity. Except for the sample with surface area of 920 m^ g\ the selectivity was increased. However, the e.e. remained very small being less than 3% in all cases. These results might again be correlated with the coordination state of titanium. For tetrahedrally coordinated species, titanium belonging to the solid network has not enough free coordinating valences to bond both the ligand and the substrate. This is completely different from the case of the sol-gel prepared mixed titania-silica catalysts where e.e. of maximum 30% were obtained because these catalysts contained a part of titanium as octahedrally coordinated species. The stability of titanium in the network of Ti-SBA-15 under the investigated conditions might also be appreciated from the fact that the presence of tartaric acid, which is a corrosive reactant, caused no leaching of titanium, as was determined both from the chemical analysis of the used catalysts and the reproducibility of catalytic results. 4. CONCLUSIONS In conclusion, heterogeneous oxidation of thioethers on Ti-SBA-15 catalysts indicated these catalysts as selective systems, leading with a good chemoselectivity to the synthesis of sulfoxides. No Ti leaching has been detected under the investigated conditions. The modification of the catalysts with optically active tartaric acid did not yield a stereoselective oxidation Both the characterization and catalytic data indicated that Ti is well rigidified in these catalysts as tetrahedrally coordinated species. REFERENCES 1. V Hulea, P. Moreau, F. DiRenzo, J. Mol. Catal. A \\\ (1996) 325 2 D J Robinson, L Davies, N.McMorn, D.J Willock, GW Watson, P C B Page, D.Bethell, G.J.Hutchings, Phys. Chem. Chem. Phys., 2 (2000) 1523. 3 B. M. Choudary, V.L.K. Valli, A. Durga Prasad, Chem. Commun, (1990) I 186. 4 M. Iwamoto, Y.Tanaka, J.Hirosumi, N.Kita, Chem Lett., (2001) 226. 5 D C Radu, V.Parvulescu, V. Campeanu, E. Bartha, A. Jonas, P. Grange, VI Parvulescu, Appl. Catal. A., in press. 6. M. A. Stranick, M. Houalla, D. M. Hercules, J. Catal., 106 (1987) 362. 7 M. A. Reiche, E. Orteli, A. Baiker, Appl. Catal B., 23 (1999) 187. 8. A. Y. Stakheev, E. S. Shpiro, J. Apijok, J. Phys. Chem , 97 (1993) 5668. 9 S. Kaliaguine, Stud. Surf Sci. Catal., 102 (1996) 191.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Direct synthesis of hydrothermally stable mesoporous Ti-MSU-G and its catalytic properties in liquid-phase epoxidation Peng Wu, Hiroyuki Sugiyama and Takashi Tatsumi Division of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Ti-MSU-G has been directly synthesized to investigate its hydrothermal stability and catalytic properties by comparing to pure silica MSU-G and Ti-MCM-41. Ti-MSU-G with ordered mesostructure and containing mainly tetrahedral Ti species in the silica framework is successfully synthesized in the Si/Ti ratio range of 30-70. Showing superior hydrothermal stability to MCM-41, Ti-MSU-G withstands the treatment in boiling water up to 75 h. No Ti leaching occurs when Ti-MSU-G catalyst is used repeatedly in the epoxidation of cyclohexene with either hydrogen peroxide or tert-huty\ hydroperoxide (TBHP). Nevertheless, the states of tetrahedral Ti species of Ti-MSU-G prove to be more stable in TBHP than in aqueous H2O2. 1. INTRODUCTION Titanium-containing mesoporous molecular sieves such as Ti-MCM-41 [1], having ordered mesopores and extremely high specific surface area, has received researchers' much attention because of their potential apphcations to the oxidation of bulkier substrates in comparison to microporous titanosilicates of TS-1, Ti-MWW and Ti-Beta. Nevertheless, Ti-MCM-41 suffers a distinct disadvantage of low hydrothermal stability due to its thin wall thickness and high hydrophilicity derived from abundant surface silanol groups. Although trimethylsilylation modification significantly improves both the hydrophobicity and catalytic activity of Ti-MCM-41 by removing the surface silanols [2], it is desirable to prepare mesoporous titanium molecular sieves hydrothermally stable by themselves. Based on this consideration, we have prepared Ti-SBA-15 of thick silica walls by a postsynthesis method, and verified that Ti-SBA-15 exhibits not only superior hydrothermal stability but also Ti stability against leaching in actual liquid-phase epoxidation reactions [3]. Recently, MSU-G mesoporous silica, synthesized with electrically neutral gemini surfactant, is reported to have superior hydrothermal stability even much higher than SBA-15 because of its high degree of silica framework cross-linking [4]. Moreover, its vesicular morphology promises that MSU-G may serve as an excellent support of catalytic species. With the purpose of applying MSU-G to the catalysis, the incorporation of Al has been carried out by a post-synthesis method to prepare acid catalyst [5], but still no researches have been reported on the incorporation of transition metals. We report here for the first time the hydrothermal synthesis of Ti-MSU-G and its catalytic properties in comparison to Ti-MCM-41.

614

2. EXPERIMENTAL Ti-MSU-G was hydrothemally synthesized using tetraethyl orthosilicate (TEOS), tetrabuthyl orthotitanate (TBOT) and neutral gemini surfactant of CioH2iNH(CH2)2NH2by modifying the procedures reported on pure sihca MSU-G [4]. In a typical run, deionized water and surfactant were mixed and stirred at room temperature for 1 h. The solution of TEOS and TBOT was then added under vigorous stirring to obtain a gel with a molar composition of I.O Si02 : x Ti02 • 0.3 surfactant : 78 H2O where x was 0-0.033. This gel was then heated statically under autogenous pressure at 373 K for 48 h. The solid product was gathered by filtration and calcined in air at 873 K for 10 h to remove the surfactant. The physico-chemical properties of Ti-MSU-G were characterized by XRD, ICP, UV-visible spectroscopy, and N2 or H2O adsorption measurements. Its catalytic properties were tested for the epoxidation of cyclohexene with H2O2 or TBHP in liquid-phase at 333 K. 3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of Ti-MSU-G samples with various Ti content after the calcination. The incorporation of Ti into the structure showed no obvious influence on the sharpness of the 001 diffraction peak, verifying the presence of highly ordered mesostructure in the samples. The N2 adsorption-desorption isotherms of pure silica MSU-G and Ti-MSU-G were characteristic of mesoporous materials (Fig. 2 a). The hysteresis loops at higher relative pressure is probably attributable to the vesicular particle morphology of this type of mesoporous material [4]. The D-H pore size distribution curves showed that the samples contained highly ordered mesopores of ca. 2.5 nm in diameter (Fig. 2b). All the Ti-MSU-G samples exhibited in 800 600

C/5 (DO

1 ^

M S U ^ c

400

JO O

200 o

en

T3

4 6 10 2theta Fig. 1. XRD patterns of calcined pure silica MSU-G (a), and Ti-MSU-G synthesized from the gel with Si/Ti ratio of 70 (b), 50 (c) and 30 (d) 0


Ti02 catalyst > only plasma discharge. In addition, presence of catalysts improved the CO2 selectivity and suppressed the formation of N2O. 1. INTRODUCTION Many researchers have reported that nonthermal plasma chemical reactions are effective for decomposing most volatile organic compounds (VOCs)[l]. Chemical reactions in silent plasma reactors, such as surface-discharge reactors, dielectric-barrier discharge reactors, and packed-bed type discharge reactors, lead to removal of NOx, decomposition of VOCs, ozone generation, flue gas cleaning, and in door cleaning[l-3]. Compared to other technologies, a silent plasma chemical processing has many practical advantages; relatively low-temperature processing, achieving high decomposition efficiencies of dilute hazardous air pollutants (HAPs)[3]. However, silent plasma reactor has many disadvantages, including low energy efficiencies, poor selectivity to CO2, and byproduct formation[ 1,4-5]. To overcome this problem, we tried the combination of silent plasma and catalyst (hybrid reactor) in decomposing benzene in air stream and controlling discharge byproducts. 2. EXPERIMENTAL

•

a

^

Oscilloscope

MFClj] High Voltage Power Supply

Fig. 1. Schematic diagram of experimental setup

Mesoporous Ti02 catalyst was obtained by following synthesis. H2O, ethanol (EtOH), TiCU, and HCl were mixed and refluxed at 373K for Ih when mole ratio was TiCl4:H20:EtOH = 1:2:4. To this solution, 1.51g of Poly(alkylene oxide) block copolymer HO(CH2CH20)2o(CH2CH(CH3)0)7o(CH2 CH20)2oH (designated as E02()P07oE02o; Pluronic P-123, BASF) was dissolved

646 with vigorous stirring and refluxed at room temperature for 2h. The resulting sol solution was soaked in glass wool and dried at 338K in air for 1-7 days, during which the inorganic precursor hydrolyses and polymerizes into a metal oxide network. And BASF in the prepared glass wool was removed by 0.2M NaOH solutions. V205/Ti02 catalysts were prepared by impregnation technique. The schematic diagram of experimental system is shown in Figure 1. The reactant gas of 200 ppm benzene balanced with air was introduced into the non-thermal plasma reactor with a gas flow rate. The gas flow rate was adjusted with mass flow controllers. The rectangular type DBD plasma reactor consisted of parallel-plate electrodes. These electrodes were made of copper plate (150 mm x 100 mm). The parallel-plate electrodes were covered with a dielectric barrier, such as glass plate. The glass wool such as each attached Ti02 and V205/Ti02 was inserted between two glass plates in the plasma reactor to investigate the effects of the catalyst. The arrangement of silent plasma reactor had an effective discharge area of 140 mm x 90 mm. The AC high voltage with a frequency of 60 Hz was applied to one electrode, and the other electrode was grounded. Discharge voltage was varied from 10 to 14 kV. The discharge voltage and the current were measured by digital oscilloscope (Tektronix, TDS 3012). The concentrations of benzene at inlet and outlet in the DBD plasma reactor were measured by gas chromatograph (Agilent 6890). The on-line byproduct analysis in the outlet was performed by Fourier transform infrared spectrometer (FT-IR, BOMEM inc. MB-104). The experimental conditions are shown in table 1. Table 1 Experimental conditions Discharge voltage Electrode gab Initial C6H6 concentration Residence time Catalyst

lOkV- 14kV 5 mm 200 ppm 5s~ 15s Ti02 lwt%V20s/Ti02

3. RESULTS AND DISCUSSION Figure 2 shows TEM image and Figure 3 shows N2 adsorption and desorption isotherms

* W * > 50nm

0.2

0.4

0.6

0.8

1.0

Relative Pressure, (PIPJ

Fig. 2. TEM mesoporous Ti02

image

of

Fig. 3. Nitrogen adsorption-desorption isotherms and pore-size distribution plot (insets) for Ti02

647

and pore-size distribution for TiOi. A mesostructured Ti02 powder was formed, based on ^^^•-^— TEM results (Fig. 2). This mesostructured Ti02 § 80 o powder exhibited an average pore diameter of 85A, •k o 60 • a BET surface area of 203 m^/g. (Fig. 3). Many c o microdischarge emitting blue light were observed •55 40 —•— No catalyst from 10 to 14kV. The current increased with o Q. increasing discharge voltage. And breakdown in i 20 • ^4—iwt%VjO/rioJ o dielectric layer is started at 10 kV. The discharge Q 10 11 12 13 14 was slightly changed when the catalyst was attached. Discharge voltage (kV) Figure 4 shows that the decomposition efficiency of Fig. 4. Decomposition efficiency of CeHe as a function of discharge voltage for different C6H6 as a function of discharge catalysts. After adsorption equilibrium, plasma voltage (CeHe concentration: 200ppm, discharge was kept during 20 min. We have known flow rate: 400 ml/min) that catalysts such as Ti02, V205/Ti02, improved the decomposition efficiency for CeHe in the silent plasma and C6H6 decomposition efficiency greatly increased with increasing discharge voltage. These results mean that increasing of discharge voltage leads to increase the intensity of high-energy electrons and the activation of the catalyst. The identification and suppression of secondary hazardous byproducts which accompany C6H6 decomposition are as important as the achievement of high C6H6 decomposition efficiency. The elimination of benzene from gas stream leads to the formation of inorganic byproducts such as CO, CO2, and N2O. The formation of these byproducts depends on reaction conditions and various catalysts. Figure 5 shows FT-IR spectra in the DBD plasma reactor without catalyst. Byproducts such as CO, CO2, and N2O, were produced by a discharge. It means that 200 ppm benzene balanced with air is oxidized into CO and CO2 by plasma discharge. And the amount of CO and CO2 increased with increasing discharge g 100 >» o

(a)

' Yf^

0)

(a)

X-

(b)

-A. r^-

u c

u c cs

(c)

CO,

E c

CO

^

4000

3000

1 CO 1000

Wavenumber (cm'^)

Fig. 5. Typical FT-IR spectra in DBD plasma reactor without catalyst for benzene decomposition experiments (a) before discharge, (b) after discharge (12 kV), (c) after discharge (14 kV) (benzene concentration: 200 ppm, gas flow rate: 400 ml/min).

4000

'A. f ^

-ji—A r^Y-

•^

-^Kj:—\ COj

N,0

2000

yy-v

--.

3000

N,0

'

CO,

03^1

2000

1000

Wavenumber(cm'^)

Fig. 6. Typical FT-IR spectra in DBD plasma reactor for benzene decomposition experiments (a) without catalyst, (b) Ti02, (c) lwt% V205/Ti02 (benzene concentration: 200 ppm, discharge voltage: 12 kV, gas flow rate: 400 ml/min).

voltage. A significant amount of ozone also is generated by dielectric barrier discharge. Figure 6 shows FT-IR spectra in the DBD plasma reactor with catalyst placed in the discharge zone. After adsorption equilibrium, plasma discharge was kept during 20 min. The use of the catalysts such as Ti02 and V205/Ti02 resulted higher CO2 selectivity than when no catalyst was used. This result means that catalysts in the silent plasma reactor play an important role for the improvement of CO2 selectivity. The amount of N2O increased with increasing discharge voltage in DBD reactor without catalysts, as shown in Figure 5. These results indicate that N2 molecules nearly are not dissociated by electron impact. However, electron impact converts N2 molecules into metastable N2 molecules. And decomposition efficiency of C6H6 with catalysts was higher than without catalysts. But amount of N2O was not increased. From these results, it may be suggested that the suppression of N2O formation is brought by the properties of catalysts. 4. CONCLUSIONS Silent plasma chemical decomposition of CeHa was investigated with a DBD-catalyst hybrid system in air stream. The catalyst of Ti02, V205/Ti02, obtained the highest decomposition efficiency in low C6H6 concentration. And its decomposition efficiency decreases in the following order: 1 wt% V205/Ti02 catalyst > Ti02 catalyst > only plasma discharge. The discharge byproducts are CO, CO2 and N2O. The selectivity of CO2 with catalyst was higher than when no catalyst was used. The use of catalysts also suppressed N2O formation. The combination of plasma and catalyst are useful for VOCs treatment. ACKNOWLEDGEMENTS This work was supported by the Inha University post-doctorial fund in 1999. REFERENCES 1. Atsushi Ogata, Toshiaki Yamamoto, IEEE Trans. Ind. Appl., 35 (1999) 1289 2. Azuchi Harano, Masayoshi Sadakata, The Society Chem. Eng., 31 (1998) 700 3. Aihua Zhang, Shigeru Futamura, J.air&Waste Manage. Assoc, 49 (1999) 1442 4. Toshiaki Yamamoto, J. Electrostatics, 42 (1997) 227 5. Atsushi Ogata, Toshiaki Yamamoto, IEEE Trans. Ind. Appl., 37 (2001) 959

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

649

Preparation of mesoporous 12-tungstophosphoric acid HPW/SiOi and its catalytic performance 'Zhirong Zhu*, 'Wenkui Lu, ^Colin Rhodes 'shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China. FAX: +86-21-68482283. E-mail: zhuzhirong(5),vahoo.com ^Cardiff University, Cardiff CF10 3TB, Wales UK. The included HPW/Si02 with mesoporous structure is prepared with nonionic surfactant AEO as a template, and Keggin structure of HPW included in Si02 matrix is retained, hi addition, it shows much stronger acidity than Al-containing MCM-41. The included HPW/Si02 is of high catalytic activity and selectivity, especially a good stability for reaction of esterification. 1. INTRODUCTION Supported 12-tungstophosphoric acid (HPW), as an important heteropolyacid catalyst, is of greater value for practical applications than pure HPW in reactions relating to surface area and pore structure [1]. Si02 is the most common and efficient support thanks to its relative inert, large surface area and available resources. MCM-41 was considered to be an ideal support for HPW in some acid-catalyzed reactions owing to its large surface and special pore structure [2]. However, HPW on the surface of supports is easy to lose during reactions, especially in polar reactants or products. It was reported that the HPW included in Si02 matrix from hydrolysis of tetraethyl-orthosilicate (TESO) shows high catalytic activity and selectivity, especially a good stability for many reactions such as hydrolysis, esterification and alkylation [3]. The mesoporous materials with pore diameters of 2-8 nm greatly enlarge the accessibility of zeolite materials for molecules of reactants to perform catalytic organic syntheses. However purely siliceous MCM-41 materials show a very limited application in catalysis due to the lack of its acidity and capacity of ion-exchange. Although mesoporous molecular sieve, as an acid catalyst, has been prepared by incorporation of Al into its framework [4, 5], its acidity is weaker than ordinary zeolites [6]. On the other hand, the incorporation of Al in framework may result in lowing uniform mesoporous structure [7]. Therefore it is important to discover a new way to obtain acidic mesoporous materials to perform reactions catalyzed by strong acids. Recently other acidic mesoporous materials were prepared by post-synthesis, generally with modification of mesoporous surface [8, 9]. In this paper, the included HPW/Si02 with mesoporous structure was prepared with alkyl alcohol polyoxyethylene ether (AEO) as a template. The catalytic performance of included HPW/Si02 was measured through reactions of esterification.

650

2. EXPERIMENTAL 2.1. Preparation Tetraethyl-orthosilicate was added into HPW aqueous solution containing nonionic surfactant AEO C16H33 (C2H50)60H at the ratio of 5, 10 or 20 wt.% HPW/Si02, and stirred at 323K for 24 h. After included HPW/Si02 obtained above was dried at 393K, and was extracted by acetone and ethanol. Then it was pretreated at 523K and 673K respectively. 2.2. Characterization The XRD characterization was carried out by using D/MAX-2400 diffractometer with Cu target Ka-ray. FT-IR spectrum of samples was obtained with Pekin-Elmer 2000 FT-IR equipment and a self-supported wafer of sample. The surface area and pore distribution were determined by N2 adsorption at 77K, with automatic Microporous 2500 apparatus. The pore size distribution was calculated from the desorption branch of N2 desorption isotherm using the conventional Barrett-Joyner-Halenda(BJH) method [10]. Temperature peogrammed desorption (TPD) of NH3 was conducted with automatic Altamira-100 Characterization System (USA). The sample was pretreated in helium at 673 K for 2 h, and TPD was carried out in the helium flow of 25 ml/min from 373 K to 973 K with a heating rate of 12"C/min. 2.3. Catalytic test The synthesis of dioctyl phthalate was performed under atmosphere pressure at 383K in a multi-necked flask with magnetic stirrer. The mixture of either 1.2 g HPW/Si02 catalyst powder or 0.24g hexadydrate HPW, 15.6 g n-octylanol and 7.5 g phthalic anhydride reacted for 4 h. The products of esterification reaction were analyzed by GC with FID. 3. RESULTS AND DISSCUSSION 3.L Preparation and structural characterization Though most of mesostructured materials are synthesized in the basic medium, some mesoporous silica has been successfully synthesized with nonionic surfactant Triton X-100 or PO-EO, under acidic condition [11, 12]. 12-tungstophosphoric acid dissociates into 12-tungstophosphate anions with negative charges and protons to make the synthetic medium acidic (PH

15 10 5

0 -11 •=

/ ''"^ ' // // / / 1

\~f^

V

M.41

\

'^* 1

*

*

>

^

1

Carbon Number of Components

Fig. 3. Yield (wt%) of product of cracking of high density polyethylene over different catalysts at 450 °C. As can be seen the lightest product was obtained over the HPW impregnated MCM-41 followed by those obtained over pure MCM-41. The product distribution of polyethylene cracking strongly indicates a carbenium ion mechanism. The levels of isobutane and isobutene are very high relative to the levels of methane and ethane as compared with pure MCM-41 test runs. It is proposed that the formation and stabilization of carbenium ion in the pores of the MCM-41 be due to the adsorption between the polyethylinic fragments with the surface of the channels where the HPW is adsorbed . 4. CONCLUSION It has been shown that addition of HPW to all silica MCM-41 catalyzes the cracking reaction of hydrocarbons. The product distribution of cracking reactions strongly indicates a carbenium ion mechanism, associated with acid sites created by the HPW lining the channels of MCM-41.

REFERENCES 1. A.Corma, M. Iglesia and F. Sanchez, J. Chem. Soc. Chem. Comm., 1635, (1995). 2. J.A. Diaz, J.P Osegovic and R.S. Drago, J. Catal., 183, 83, (1999). 3. Y. Izumi, N. Natsume, H. Takamine, I. Tamaoki and K. Urabe, Bull. Chem. Soc. Japan, 62, p. 2159,(1989). 4. Blasco, T, Corma, A, Martinez, A. and Escolano, P, J. Catal. 177 (1998), 306.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

657

Preparation, characterization and catalytic activity of heteropolyacids supported on mesoporous silica and carbon Zhenbo Zhao^'^ , Whaseung Ahn^ and Ryong Ryoo'^ ^Catalysis Laboratory, School of Chemical Science and Engineering, Inha University, Incheon, 402-751, Korea ^Department of Light Industry and Textile Engineering, Jilin Institute of Technology, Changchun, 130012, R R .China '^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea HPA was impregnated on mesoporous silica (SBA-15, MCM-41), commercial silica (ML369) and activated carbon, as well as on mesoporous carbons (CMK-1, CMK-3). The resulting materials were investigated by means of XRD, BET and catalytic probe reactions of liquid-phase esterification of hexanoic acid with propanol-1 and acylation of 2-methoxynaphthalene with acetic anhydride. The catalytic performance was optimal with loading of around 40wt% HPA. H3PW12O40 (PW) was more active than H4SiWi204o (SiW) for esterification, irrespective of the supports used. Solvent used in the impregnation and hydrophobic/hydrophilic nature of the support could influence the performance of the catalyst in these acid catalyzed reactions. INTRODUCTION Heteropolyacids(HPAs) have stimulated considerable research in both heterogeneous and homogeneous catalysis [1-3]. Among them, 12-tungstophosphoric acid, H3PWi204() (PW), the strongest and the most stable acid in the Keggin series of HPAs, has attracted the most attention. The main drawback of such materials for catalytic application is their low specific surface area. Thus, direct dispersion of the bulk HPAs on a mesoporous silicate support such as MCM-41 [4], commercial silica, and activated carbon [5] has been attempted. SBA-15 [6] possessing larger pore size and CMK-1,3 [7] are mesoporous materials newly emerged recently, and have not been tested as a carrier for supporting HPAs. Here we report the comparison of mesoporous silica (SBA-15, MCM-41), commercial silica (ML369), activated carbon and mesoporous carbon (CMK-1, CMK-3) supported HPAs as catalysts for the liquid phase esterification and acylation reactions. 2. EXPERIMENTAL MCM-41 was synthesized according to the literature recipe [4]. SBA-15 was synthesized by using the triblock copolymer, EO20-PO70-EO20 (Pluronic 123, BASF) as the surfactant and the tetraethylorthosilicate (TEGS, 98% Aldrich) as silicon source [6]. The supported HPAs catalysts were prepared by impregnation of HPAs on various carriers following the procedure

658

of Kozhevnikov et al [2]. Typically, the required amount of HPAs was dissolved in various solvent and a proportional amount of the support material were added. The mixture was stirred for 18h at room temperature. Subsequently, solvent was removed in a rotary evaporator, yielding the HPA-impregnated catalysts. The material was dried and mildly calcined at 403K and stored in a desiccator until use. XRD patterns were obtained with a CuKa X-ray source (Rigaku Miniflex instrument, 45OW). N2-adsorption isotherms were obtained at 77K using a Micromeritics ASAP2000. The samples were outgassed at 403K and 0.1 Pa for 12h before measurements were performed. Specific surface areas were obtained with the BET equation; the mean pore sizes by the BJH method. Esterification of hexanoic acid with propanol-1 was carried out at reflux temperature in a glass vessel equipped with a magnetic stirrer and a Dean Stark trap for water removal. Toluene was used as a solvent. Acylation of 2-methoxynaphthalene with acetic anhydride was carried out at 323 K using chlorobenzene as a solvent. Analysis was performed with GC (ShimadzuGC-14A) equipped with Shimadzu column Hicap CBP1-M25-025 with a flame ionization detector. 3. RESULTS AND DISCUSSION As shown in Fig.l, the introduction of 12-tungstophosphoric acid, H3PW12O40 (PW) to mesoporous silica SBA-15 or mesoporous carbon CMK-3 resulted in little decreases in intensities of the XRD reflections of the mesostructures, which suggests that the structural order of the host materials is maintained, ki addition, no peaks corresponding to HPA were detected indicating highly dispersed state of HPA impregnated. The preparation conditions such as the heteropoly anionic species, physicochemical properties of the supports, the impregnation solvent, and the loadings of HPAs were shown to have pronounced influences on catalytic performances (shown in Figs. 2-5). H3PW12O40 (PW) was more active HPA than H4SiWi204o(SiW)for esterification, irrespective of the supports used. Catalytic performance was optimal with loading of around 40wt% HPA. Apparently, the higher the HPA loadings, the higher would be the catalytic activity due to generation of increased active sites, but loadings of 50% or more must have caused blockage in the support channels and also resulted

20%

PW / S B A - 1 5 ( a q )

_3_0^o_ P W / S B A - 1 5 ( a q ) 40%

20%

PW/CM

K-3(aq1

3 0%

PW/CM

K-3(aq)j

40%

P W / C M K - 3 (a q

P W / S B A - 1 5(aq)

4

2 theata

6

4

6

2 theata

Fig. 1. XRD Patterns after different loadings of H3PW,204o on SBA-15 and CMK-3

659

Table 1 Surface areas and porosities of SBA- 5 impregnated with PW BET surface area BJH average diameter Samples (A) (m'/g) 64.1 30%PW/SBA-15(aq) 425 61.0 40%PW/SBA-15(aq) 292 59.8 50%PW/SBA-15(aq) 207_

Pore volume (cc/g) 0.60 0.44 0.31

in poor dispersion. As shown in Table l,when the amount of PW introduced to SBA-15 was 30wt%, 40wt%, 50wt%, the surface area and pore volume varied correspondingly as 425, 292, 207mVg and 0.60,0.44,0.3 Iml/g, respectively. These changes can be explained by the large molecular size of the Keggin anion(1.2nm in diameter) and its interaction with the amorphous wall of the host material. In the hexanoic acid esterification, following catalytic activity order of PW supported on different silica impregnated in aqueous phase was obtained: PW/SBA-15 > PW/MCM-41 > PW/silica. This difference can be explained in terms of the larger pore size of SBA-15 than MCM-41 and bigger surface areas of the mesoporous materials than

y^

•--^

-a

80-

80 H

a 604020-

/ •

/

•''

•''

1//

o X

JC

o c o

/

52

—•—40% SiW/SBA-l5(aq) —•—40% PW/SBA-15(aq) 1

^ o

r

u

Reaction time / min

Fig. 2. Catalytic performance of different HPAs on SBA-15.

1

y^^

y^ / /•^ A

HO-

^ (^)4 0 -

^ ^ n - ^ ' ^ ^ ^ . ^ • ^ ^ ^

/.-'•

0-

(J

^.-^-'^

20-

0-

/

/

1 /

F

^

u

^

180

240

Reaction time / min

Fig. 4. Effect of impregnating solvent for HPA on catalytic performance (SBA-15 support)

. Reaction time / min

. _ ^ . ^ - -

- . - 4 0 % PW/SBA-15(CH,0H) —•—40% PW/SBA-15(aq) 120

''

- • - — 4 0 % PW/silica369(aq) - « — 4 0 % P W / M C M - 4 1 (aq) - - * - - 4 0 % PW/SBA-I5(aq)

^-'^*^^^'

r_

•

Fig. 3. Effect of different silica supports on catalytic performance.

40

JL_

^m

/x .

/ C

^---^ /^ ,_--——' ^-"^^^^ •-'"^^ .--""^

">

•

fL 0

60

^

^

—•—30%PW/SBA-15(aq) — • — 4 0 % PW/SBA-15(aq) —-*^-50%PW/SBA-lS(aq) 120

180

240

300

Reaction time / min

Fig. 5. Effect of HPA loadings on catalytic performance (SBA-15 support)

1 |

660

Table 2 Conversion of hexanoic acid over various PW supported catalysts (loading: 40% PW reaction time: 3h) CMK-1 CMK-3 Activated Carbon O.lMHCl 42.9 54.4 61.4 CH3OH 33.7 36.8 58.4 commercial silica. As shown in Table 2, commercial activated carbon was a better carrier than mesoporous carbons when impregnated in methanol or HCl but aqueous impregnation resulted in poor conversion; the former having the larger surface area (ca. 1500 mVg) than mesoporous carbons (ca. 1000 mVg) and due to more hydrophobic nature of activated carbon. Water or methanol was equally acceptable solvent for HPA for mesoporous carbon CMK-3, and this shows different surface nature of mesoporous carbons from commercial one. For acylation, again mesoporous materials produced better catalytic performance than commercial silica, and HPA on CMK-3 with larger pores performed slightly better than commercial carbon or CMK-1 (Fig.6 and Fig.7).

40% PW/CMK-3(aq) -40%PW/MCM^l(aq) 4()%PW/SBA-15(aq) 40% PW/silica(ML369)(aq)

Reaction time/min Fig. 6. Comparison of different supports on acylation activity

• - 40% PW/CMK-1 (0.1M I ICl) • 40% PW/CMK-3(0.1M HCl) ^ 40% PW/Carbon(0.1M I ICl)

Reaction time / min

Fig. 7. Comparison of different carbon supports on acylation activity

ACKNOWLEDGEMENTS This work was supported by grant 2000-1-30700-3 from the basic research program of the Korea Science & Engineering Foundation. REFERENCES 1. M. Misono, N. Norjiri, Appl. Catal. 64 (1990) 1 2. I.V.Kozhevnikov, A.Sinnema, R.J.J.Janse, K.Pamin, H.Van Bekkum, Catal. Lett. 30 (1995) 241 3. T. Blasco, A. Corma, A. Martinez, P J. Martinez-Escolana, J. Catal., 177 (1998) 396 4. C.T. Kresge, J. E. Leonowicz, W.J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710 5. Y. Izumi, et al., J. Catal., 84 (1983) 402 6. D. Zhao, J. Feng, Q. Huo, W. Melosh, G.H. Fredrichson, B. F. Chmelka, G. D. Stucky, Science, 279 (1998) 548 7. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999) 7743

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

661

Novel SBA-15 supported heteropoly acid catalysts for benzene alkylation with 1-dodecene Hai-Ou Zhu^, Jun Wang^'*, Chong-Yu Zeng^ and Dong-Yuan Zhao^ ^Jiangsu Key Laboratory of Chemical Engineering and Technology, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China ^Chemistry Department, Fudan University, Shanghai 200433, China Phosphotungstic acid (PW) catalysts supported on the mesoporous molecular sieve SBA-15 have been prepared, characterized and evaluated in the alkylation reaction of benzene with 1-dodecene. SBA-15 supported PW catalysts exhibit much higher catalytic activity, stability and selectivity compared with HY zeolite. It is proposed that the high dispersion of PW on SBA-15, high surface area, mesoporosity and specific acidity of the catalyst could responsible for its catalytic performances. 1. INTRODUCTION Linear monoalkylbenzene (LAB) is the primary raw material for detergent. The manufacture of LAB conventionally involves HP as a catalyst, which is a source of pollution and equipment corrosion. Thus, many studies on solid acid catalysts have been carried out to solve this problem, among which zeolite type catalysts are mostly measured, and only a few and isolated literatures have dealt with heteropoly acid (HPA) catalysts [1]. Pure HPA is known to possess the strong bronsted aicidity, and has been widely investigated in numerous acid catalyzed reactions [2]. However, owing to its very low surface area and high solubility in polar solvent, supported HPA catalysts attract much research attention recently [3,4]. Since the newly synthesized silica mesoporous molecular sieve, SBA-15 [5], has a very high surface area, large pore volume and satisfied stability, with the uniform pore size being large enough to implant HPA molecules, we consider it as a potential qualified support for HPA. In this work, the much higher catalytic activity, stability and selectivity in the benzene alkylation with 1-dodecene are achieved over the SBA-15 supported PW catalyst, compared with HY zeolite. Catalyst performances are discussed based on their physicochemical characteristics. 2. EXPERIMENTAL SBA-15 supported PW catalysts, m%PW/SBA-15, were prepared by an impregnation method, where m stands for the percentage of PW in the catalyst by weight. Before introducing into the reactor, the catalysts were dried at 333 K and then calcined at required temperatures, and HY zeolite was calcined at 773 K for 5 h. * Corresponding author. E-mail address: [email protected].

662 1

1

1

i PW 90%PVWSBA-1^5

_ ^L

I

•j

70%PW/SBA-15

JJIf-—

t=

60%PW/SBA-15

U

1 ,Jv^- , 4

8 12 Pore diameter /nm

16

Fig, 1. Pore size distribution 50%PW/SBA-15.

50%PW/SBA-15 40%PW/SBA-15 30%PW/SBA-15

K. V 20

for

0

20%PW/SBA-15 10%PW/SBA-15 SBA-15

5

10

15 20/"

20

25

30

Fi£;. 2 . XRD patterns for PW/SBA-15

catalysts.

Catalytic tests for the alkylation of benzene with 1-dodecene were carried out in a glass flask reactor equipped with a magnetic stirrer and a condenser. The reaction temperature was 353 K, with typically a 5 (ml/ml) ratio of benzene to 1-dodecene and a 20 (ml/g) ratio of 1-dodecene to catalyst. Reaction products were analyzed by the gas chromatograph with FID as the detector furnished with a 30 m SE30 capillary column. Physicochemical properties of catalysts were measured by X-ray powder diffraction (XRD, Bruker D8 Advance), temperature programmed desorption of ammonia (NH3-TPD, home-made) and N2 adsorption (Coulter Omnisorp lOOCX) techniques. 3. RESULTS AND DISCUSSION Table 1 shows the BET surface area (SB!:T), mean pore diameter (d) and pore volume (Vp) of the three selected samples. It reveals that the catalyst surface area and pore volume decrease gradually with the increase of PW loading. However, even at a high PW loading of 50%, the material still retains a rather large surface area (> 300 m^.g"'), with the pore size only decreasing slightly. Moreover, Fig. 1 indicates a very narrow pore size distribution for 50%PW/SBA-15, demonstrating a uniform mesoporosity of this catalyst. Table I Surface area and porosity of PW/SBA-15 catalysts Catalyst SBHT/m .g' SBA-15 540 30%PW/SBA-15 433 50%PW/SBA-15 315

d/nm 6.6 6.4 6.4

Vp/ml.g-'

0.94 0.69 0.47

Fig. 2 displays XRD patterns for various PW/SBA-15 catalysts. It can be clearly seen in Fig. 2 that the SBA-15 support employed here possesses a typical hexagonal mesoporous structure. When PW is loaded on SBA-15, no clear diffraction peak from PW crystal phase appears until the PW loading is as high as 70%. This indicates that PW can highly disperse on the surface of SBA-15 support with the mesoporosity unaltered by the loaded PW. This can be reconfirmed by the result in Table 1 and Fig. 1.

663 100

80

O

60 1

8

-o

•o ^ c g (0 (D

100

200

300

400

500

Temperature /°C

Fig. 3. NH3-TPD profiles for (1) 20%PW/SBA-15; (2) 40%PW/SBA-15; (3) 60%PW/SBA-15 and (4) HY.

—••

i

>

•

40

•

\

\

•

20

' - ' 1

—»—40% PW/SBA-15 1 —•—HY

2

3 4 5 Reaction cycle

6

Fig. 4. Comparison of catalytic stability between PW/SBA-15 and HY catalyst (1-dodecene/catalyst = 5 (ml/g)).

The catalyst acidity is shown in Fig. 3. Only one broad ammonia desorption peak in a temperature range from about 100 °C to 300 °C is observed for PW/SBA-15 catalysts, while two peaks for HY zeolite are found with the maximum at about 220 °C and 350 °C, respectively. This phenomenon directs the existence of weak and medium acid sites on PW/SBA-15, and medium and strong acid sites on HY zeolite. It is further revealed that both acid strength and acid number increases with the increase of PW loading. The stabilized conversion of 1-dodecene and product selectivity of PW/SBA-15 and HY catalysts are shown in Table 2. 2~6-P (2~6-phenyldodecane) and 2~6-dodecene are the only products detected here, with the later being considered as the unconverted reactant when calculating the reaction conversion. 2-P is the mostly desired isomer due to its better emulsibility and biodegradability. It can been seen in Table 2, by introducing the heteropoly acid into inactive pure SBA-15, all supported catalysts exhibit considerable catalytic activities, and the activity increases with the increase of PW loading up to 60%. Further increase of the PW loading results in a decrease of activity. The highest activity of 89.7% is found on 60%PW/SBA-15 catalyst, which is higher than that of HY by 28%, meanwhile, its selectivity for 2-P (37.3%) is also higher than that of HY by 7%. Fig. 4 compares the catalytic stability between PW/SBA-15 and HY. It reveals a much slower deactivation rate for PW/SBA-15 than that for HY catalyst. At the fourth reaction cycle, the activity of HY decreases sharply to 20%, while 40%PW/SBA-15 still exhibits a high activity of 90%. Benzene alkylation with 1-dodecene proceeds via the carbenium ion mechanism. While the secondary dodecylcarbenium ion is electrophilically attacked by benzene to produce 2-P, it also tends to react with other olefins to generate polymer, which is the source of coke, or transform into other dodecene isomers by intramolecular double bond isomerization and finally give 3~6-P products via attacking to benzene. Based on the above consideration, the high activity of the SBA-15 supported PW catalyst could be ascribed to its larger number of acid site over that of HY zeolite (Fig. 3), as well as its mesoporous channel, in which the acid site is much more easier of approach by the reactant and thus more favorable to the generation and diffusion of products in comparison with the 12-membered ring channel of HY zeolite. However, at very high PW loadings (> 60%), the activity of PW/SBA-15 begins to drop off. This is supposed to arise from the poor dispersion of PW on SBA-15 support due to the occurrence of PW crystal phase on its surface (Fig. 2).

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Table 2 Conversion of 1-dodecene and product selectivity over PW/SBA-15 and HY catalysts Catalyst Conversion Product selectivity /% /% 2-P 4-P 3-P 5-P 61.7 30.7 13.7 16.6 HY 17.1 6.4 3.0 68.3 16.9 10%PW/SBA-15 5.7 45.8 44.1 21.7 12.9 20%PW/SBA-15 12.0 38.8 21.6 13.9 78.5 30%PW/SBA-15 14.6 14.1 84.5 40%PW/SBA-15 38.3 21.5 13.8 85.2 37.7 21.7 14.3 14.4 50%PW/SBA-15 89.7 37.3 22.0 14.3 60%PW/SBA-15 13.8 37.8 22.1 14.3 77.6 70%PW/SBA-15 13.6 32.3 49.3 21.3 11.6 10.4 90%PW/SBA-15

6-P 21.9 2.7 9.3 11.2 12.4 11.9 12.6 12.2 7.4

On the other hand, Y zeolite has a three-dimensional channel with the pore diameter of 0.74 nm and the interval supercage of 1.3 nm. This microporosity implies that the produced coke on medium and strong acid sites of HY tends to block the pore channel entrance and/or cover the acid sites inside the channel, resulting in a rapid deactivation, as shown in Fig. 4. By contrast, the mesoporosity of PW/SBA-15 is proposed to be responsible for its much stable catalytic performance. 2-P is the slimmest one among the produced isomers, and one tends to use zeolite as the target catalyst to improve the selectivity for 2-P due to its shape selectivity. However, we observed here an higher selectivity for 2-P on the mesoporous PW/SBA-15 catalyst than that on HY. It is known that 3~6-P are secondary reaction products in the whole reaction network, with the double bond isomerization of 1-dodecene taking place first. This isomerization reaction occurs simultaneously and competitively with the desired direct alkylation of benzene with 1-dodecene at same acid sites. Consequently, the catalytic selectivity for 2-P relates not only to the size of catalyst pore, but also to the catalyst acidity itself. It is thus deduced here that the weak and medium acidity on PW/SBA-15 can be more favorable to the direact alkylation of benzene with 1-dodecene with the produce of 2-P, compared with the strong acid sites on HY zeolite which is considered to be more beneficial to the double bond isomerization of 1-dodecene, leading to more 3~6-P products. ACKNOWLEDGEMENTS This work was supported by Jiangsu Natural Science Foundation (BK99122) and NSF (29925309) of China, and by Jiangsu High Technology Project (BG2001044) of China.

REFERENCES 1. 2. 3. 4. 5.

J.A. Kocal, V.V. Bipin and I. Tamotsu, Appl. Catal. A: General, 221 (2001) 295. L. Marosi, G. Cox, A. Tenten and H. Hibst, J. Catal., 194 (2000) 140. T. Blasco, A. Corma, A. Martinez and P Martinez-Escolano, J. Catal., 177 (1988) 306. L. Pizzio, P Vazquez, C. Caceres and M. Blanco, Catal. Lett., 77 (2001) 233. D. Zhao, J. Feng, Q. Huo, N. Melosh, GH. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 279 (1998) 548.

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Aluminum containing periodic mesoporous organosilicas: synthesis and etherification Jin-Won Kim^, Hyung Ik Lee*', Ji Man Kim^, Xingdong Yuan^'^ and Jae Eui Yie^* ^Catalyst and Surface Laboratory, School of Chemical Engineering and Biotechnology, Ajou University, Suwon, 442-749, Korea ^Functional Materials Laboratory, Department of Molecular Science & Technology, Ajou University, Suwon, 442-749, Korea '^Department of Petrochemical Technology, Fushun Petroleum University, Fushun, 113001, China Aluminum has been successfully incorporated within the frameworks of periodic mesoporous organosilicas (Al-PMO) by co-condensation of bis(triethoxysilyl)ethane and dibutoxyaluminotriethoxysilane. The Al-PMO materials exhibit highly ordered 2-d hexagonal structures, high surface areas, and narrow pore size distribution in the mesoporous range. The Al-PMO catalysts result in excellent catalytic activity and selectivity for etherification reaction between 2-naphthol and ethanol, which is comparable with those of beta zeolite. 1. INTRODUCTION Recently, periodic mesoporous organosilicas (PMO) have been synthesized by condensation of bridged silsequioxane in the presence of structure-directing agents, and attracted much attention due to their well-ordered mesostructures and noble framework structures [1,2]. The presence of organic groups within the frameworks is expected to give hydrophobic character and hydrothermal stability to the mesoporous materials. These properties are very important for the applications under hydrothermal conditions and organophilic reactions systems, compared with those of normal mesoporous silicas such as MCM-41 and MCM-48. However, the PMO materials constructed with organosilica frameworks (Si-PMO) are of limited use in catalysis, due to the lack of acidity and ion exchange sites. Incorporating other elements such as Al, Ti, Mn, Fe, V, etc. into the organosilica frameworks can improve the properties, which are important for the applications as catalysts and adsorbents. So far, there are a few reports on the modification with organic functional groups and on their applications [3]. It seems to be difficult to incorporate heteroatoms within the PMO frameworks with conventional methods that have been generally used in preparation of metal containing MCM-41. Etherification reaction of 2-naphthol is very important because the products have been extensively used in the fine chemical industry [4]. For example, 2-naphthyl methyl ether has been used in perfumery, which is traditionally manufactured from 2-naphthol and methanol in the presence of sulfuric acid. However, the drawbacks of such a process include corrosion, safety hazards, separation procedures, and environmental problems due to the use of sulfuric

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acid. A PMO material with solid acid properties is expected to be an excellent heterogeneous catalyst for this reaction due to its mesoporosity and hydrothermal stability. In the present work, incorporation of aluminum into the PMO frameworks has been successfully carried out by co-condensation between bis(triethoxysilyl)ethane (BTSE) and dibutoxyaluminotriethoxysilane (DBATES) in the presence of structure-directing agents, and the possibility of the materials for catalytic applications to etherification are investigated. 2. EXPERIMENTAL Al-incorporated PMO materials (Al-PMO) were synthesized by modified procedures described elsewhere [1] using BTSE as the framework source, DBATES as the aluminum source and octadecyltrimethylammonium chloride (ODTMACl) as the structure-directing agent. A typical gel compositions was 1 BTSE : 0.067 DBATES : 0.57 ODTMACl : 2.4 NaOH : 350 H2O : 10 EtOH : 0.012 HCl. BTSE and DBATES were prehydrolyzed and oligomerized under acidic conditions before mixing with surfactant solution. To investigate the effect of aluminum source on the materials, the Al-PMO materials (Si/Al — 30) were synthesized by using various kinds of aluminum sources such as A1(N03)3, Al(i-OC3H7)3 and NaA102. The resulting mixture was magnetically stirred at room temperature for 20 hr, and subsequently heated in an oven at 368 K for 20 hr. The precipitate was recovered by filtration, washed with doubly distilled water and dried at 373 K for 6 h. The as-made products were refluxed in an excess acidified ethanol with HCl to remove the surfactant. The products were obtained by filtration, washed with ethanol and dried at 373 K for 10 h. The solvent-extraction procedures were repeated three times. The Al-PMO materials were characterized by powder X-ray diffraction (XRD), N2 adsorption, FT-IR, solid-state MAS ^^Al NMR spectroscopy, thermogravimetric analysis (TGA). Etherification reactions between 2-naphthol and ethanol were carried out in a down flow fixed bed reactor at 453 K. The reaction conditions were 0.1 g of catalysts, ethanol/2-naphthol = 10/1, and reactants flow rate = 1.0 cc/h. 3. RESULTS AND DISCUSSION Figure 1 shows XRD patterns for the Si-PMO and the Al-PMO obtained by using DBATES as the aluminum source, before and after surfactant extraction. The XRD patterns for PMO materials before extraction (Figure la and Ic) give a very intense diffraction peak and two or more weak 2 4 6 8 peaks, which are characteristic of 2-d hexagonal 20/ degree (P6mm) mesostructure [5]. There are no significant Fig. 1. X-ray diffraction patterns for (a) changes upon removal of surfactant except for the as-made, (b) washed Si-PMO, (c) as- expected increase in XRD peak intensity. The AlPMO after surfactant removal (Figure Id) gives made and (d) washed Al-PMO.

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(210) and (300) peaks, which indicates excellent textural uniformity of the material. TEM image also indicates that the material has a highly ordered 2-d hexagonal structure. Lattice parameters (a), calculated from dwo spacings, for the Si-PMO and Al-PMO materials are 4.67 nm and 4.96 nm, respectively. Line broadening and large lattice parameter of the Al-PMO material, compared with those of Si-PMO, may be due to the Al incorporation within the frameworks. Nitrogen adsorption isotherms indicate that the BET surface areas of the Si-PMO and Al-PMO materials are 1050 mVg and 1692 m^/g, respectively. The pore sizes for the materials obtained by BJH model are 2.6 nm and 2.9 nm. From the lattice parameters and pore sizes, framework thickness for the materials is very similar (2.1 nm). According to IR spectra, all the PMO materials after surfactant — I — — I — removal exhibit strong bands at 2920 and 2890 cm' 100 -100 -200 200 assigned to C-H stretching and deformation Chemical shifts(ppm) vibrations, 1410 and 1270 cm' corresponding to C-H Fig. 2. 'Al MAS NMR spectra for Al- deformation vibrations, which means the presence of PMO materials obtained with (a) organic bridging group within the frameworks. Figure 2 shows ^''AI MAS NMR spectra of the AlA1(N03)3, (b) NaA102 (c) Al(i-OC3H7)3 (d) DBATES (Si/Al = 30) and (c) PMO materials obtained with different aluminum DBATES (Si/Al = 8) sources. NMR peak around 50 ppm and 0 ppm can be assigned to a tetrahedrally coordinated aluminum species within the framework and an octahedrally coordinated extraframework aluminum species, respectively. The NMR results in Figure 2 clearly show that the extraframework aluminum species are present in the Al-PMO materials obtained with A1(N03)3 and NaA102. In case of DBATES and Al(i-OC3H7)3, there is no NMR peak around 0 ppm, indicating that all the aluminum species are incorporated within frameworks. However, a significant amount of octahedrally coordinated aluminum species appears as the Si/Al ratio decreases when Al(i-OC3H7)3 is used as the aluminum source, whereas DBATES results in only framework aluminum species till Si/Al = 8 (Figure 2e). The results show that aluminum incorporation into the PMO frameworks is highly dependent on the nature of aluminum source. Figure 3 shows TGA results under nitrogen atmosphere for the Al-PMO 100 200 300 400 500 600 material before and after surfactant TerriDerature / °C Fig. 3. TGA diagrams for the Al-PMO material (a) removal. Before solvent extraction, weight loss of 5 wt% below 120 °C is attributed before and (b) after surfactant extraction

668 to the loss of small amounts of residual water adsorbed to the materials. This is followed by a weight loss of 30 wt% from 120 to 250 °C due to surfactant decomposition. An additional weight loss of 5 - 7 wt% above 500 °C indicates decomposition of organic bridging group within the framework. Figure 3b shows that there is little weight loss in the temperature range for surfactant decomposition (120 - 250 °C), indicating that the surfactant within the mesopores can be removed completely through the solvent extraction. The weight loss above 500 °C also appears in Figure 3b. According to the TGA results, the Al-PMO materials synthesized in the present work may be used below 500 °C without loss of organic bridging group within the frameworks. Figure 4 shows catalytic activities of 80 the etherification reaction between 2naphthol and ethanol. All the 70 materials give 100 % selectivity for 2naphthylethylether. As shown in ^ 60 Figure 4, beta (Si/Al = 13.5), c 50 mordenite (Si/Al = 15), HY (Si/Al = g 3) and ZSM-5 (Si/Al = 30) zeolites 40 0) result in 66 %, 43 %, 4 % and 1 %, > c 30 respectively. The Al-PMO material o O (Si/Al = 30, DBATES) exhibits 58 % 20 conversion and 100% selectivity for 10 the etherification reaction. The catalytic activity and selectivity are comparable with those of beta zeolite h . ^KO ^ K)®^•N^' .^^^^ ^ that is the best one among various 6®^ ^^' ^\'.?^ ^'P^' kinds of solid acids catalysts in the Fig. 4. Catalytic activities of the materials for present work. etherification reaction between 2-naphthol and In summary, the highly ordered Alethanol. PMO material with framework aluminum can be successfully prepared using DBATES as the aluminum source. The material thus obtained is an excellent solid acid catalyst. The present synthetic strategy may be very useful for the rational design and preparation of PMO materials containing other elements within the frameworks. The authors are grateful for financial support by the Research Initiation Program at Ajou University (20012010) and Department of Molecular Science & Technology through Brain Korea 21 Project. REFERENCES 1. S. Inagaki, S. Guan, Y. Fukushima, T Ohsuna and O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. 2. T. Asefa, M. J. MacLachlan, N. Coombs and G. A. Ozin, Nature, 402 (1999) 867. 3. C. B. Mark, A. M. Michael, S. S. Mark and P G. Bruce, Chem. Mater., 13 (2001) 4760. 4. G. D. Yadav and M. S. Krishnan, Ind. Eng. Chem. Res., 37 (1998) 3358 5. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vatuli and J. S. Beck, Nature, 359 (1992) 710.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Friedel-crafts alkylation over Al-incorporated mesoporous honeycomb Young Soo Ahn^, Hong Soo Kim^, Moon Hee Han^, Shinae Jun^, Sang Hoon Joo^, Ryong Ryoo and Sung June Cho^ ^Functional Materials Research Center, Korea Institute of Energy Research, Taeduk Science Town, Taejon 305-343, Korea. ^National Creative Research Initiative Center for Functional Nanomaterials and Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea, ^Catalytic Combustion Research Center, Korea Institute of Energy Research ,Taeduk Science Town, Taejon 305-343, Korea. Catalytic activity of Friedel-Crafts alkylation of toluene with benzyl alcohol has been measured over the Al-incorporated mesoporous honeycomb. The honeycomb was fabricated using MCM-48 and pseudobohemite as inorganic binder and the incorporation of aluminum was performed either by direct implementation of AICI3 or by slurry mixing before the extrusion. Hydrothermal stability and compressive strength can be improved with the increase of the aluminum content. High catalytic activity of Friedel-Crafts alkylation was observed for the honeycomb containing Al initially in the slurry mixture. 1. INTRODUCTION The catalyst powder should be fabricated into a certain type of structure that can allow a facile diffusion of reactants to catalytically active sites. Honeycomb is the most common commercially available structure that accommodates catalysts at the surface of each small rectangular structure. Recently, the silica-based mesoporous molecular sieves has been investigated extensively as a substrate for catalytic conversion of large molecules inside its uniform pore of which the surface area is ranging from 2 to 30 nm [1]. Their high hydrothermal stability is comparable to those of conventional aluminosilicate zeolites. Ahn et al. showed that the honeycomb can be fabricated from the MCM-48 powder [2]. The integrity of such a mesoporous structure in the honeycomb can be retained during the hydrothermal treatment. Here, we report the results of the catalytic activity of the FriedelCrafts alkylation over the honeycomb containing aluminum that is incorporated different ways. 2. EXPERIMENTAL The MCM-48 silica powder was synthesized following the method described in the previous reports using a surfactant mixture of cetyltrimethylammoniumbromide and tetraoxyethylene dodecyl ether. The MCM-48 samples containing the surfactants as synthesized were further treated with an aqueous solution of NaCl, in order to improve the

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hydrothermal stability. The samples were then dried in an oven at 100°C, washed with an ethanol-HCl mixture to remove as much surfactant as possible, and finally calcined in air under static conditions at 550°C. The bath composition of the slurry containing the MCM48 powder was controlled to the 80 wt % of MCM-48, 20 wt % of inorganic binder, 1 5 - 2 5 wt % of organic binder and the above 100 wt % of water on the basis of the total weight of MCM-48 and inorganic binder. The fabrication of the honeycomb follows a typical procedure consisting of powder mixing, wet mixing, aging, kneading, extruding and final sintering. The test method for the hydrothermal stability, the characterization and the catalytic activity of Friedel-Crafts alkylation of toluene and benzyl alcohol can be found in elsewhere [3]. 3. RESULTS AND DISCUSSION Employing pseudobohemite as inorganic binder increased the mechanical stability of the mesoporous honeycomb. The pseudobohemite contains Al itself, which can also be act as an acid catalyst. Fig. 1 shows the compressive strength and surface area depending on the calcination temperature. The increase of the aluminum incorporation in the honeycomb during the slurry formation decreased the surface area but improved the compressive strength. The incorporated Al may also act as inorganic binder like the pseudobohemite. In addition to the mechanical stability, the hydrothermal stability is a important factor for the design of mesoporous honeycomb. In this work, the hydrothermal stability was measured from the XRD patterns before and after the treatment of the honeycomb in the boiling water for 12 h. 660 680 700 720 740 The XRD diffraction patterns Calcination temperature (C) indicated that the mesoporous structure Fig. 1. Change of Surface area and connpressive strength was retained after the treatment and the as a function of the calcination temperature: (I ), 0 wt% AICI3; (m), 5.6 wt% AICI3; (t ) 11.2 wt% AICI3. Al incorporation led to more hydrothermal ly stable mesoporous honeycomb. It was shown that the hydrothermal stability was increased by the incorporation of alkali or alkaline earth ion to the mesoporous material. It seems that the incorporated Al increased the hydrothermal stability in addition to the mechanical stability. The local environment of Al in the mesoporous honeycomb was probed with ^^Al NMR depending on the incorporation methods of Al. Fig. 2 illustrates the NMR spectra of the mesoporous honeycombs. The direct implementation of an acid fiinction to the surface of the mesoporous channel was reported to be another viable method for the catalyst preparation. The spectral intensity of the peak corresponding to the tetrahedral Al site increased for the sample containing the direct implementation of AICI3. The calcination of the sample at 650 °C resulted in the similar ^^Al NMR spectrum to that of the mesoporous honeycomb containing AICI3 in slurry mixture initially. This suggested that the incorporation method of

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150

100

50

-50

-100

Chemical Shift/ppm Fig. 2. (a) the Al impregnated mesoporous honeycomb, (b) the sample (a) calcined at 650 °C and (c) the mesoporous honeycomb containing Al in the slurry, after calcination at 700 °C.

Al did not affect the local environment of Al sites, which can be attributed to the large amount of inorganic binder, pseudobohemite. Fig. 3 shows the microstructure of the mesoporous honeycomb. During the fabrication of the honeycomb, all the components were mixed thoroughly to get homogeneous slurry for the extrusion, which can result in the breaking or destruction of crystalline shape. Indeed, the MCM-48 had a crystalline shape in powder form but the honeycomb had an irregularly tough surface microstructure due to the mixing step as shown in Fig. 3. The increase of Al incorporation led to the increase of mechanical stability and hydrothermal stability. However, in the scanning electron micrograph of the honeycomb sample, there is no significant difference in the surface structure. The catalytic activity of the Friedel-Crafts alkylation was measured over the honeycomb samples in a similar way reported in the literature. Fig. 4 shows the effect of the Al-incorporation method on the catalytic activity. The honeycomb without Al direct implementation or impregnation gave a comparable catalytic activity for the alkylation of toluene with benzyl alcohol. The conversion of toluene increased up to 40 % for 2.5h.

^

Fig. 3. Scanning electron micrographs of the honeycomb sample calcined at 700 °C: (a), 0 wt% AlCb; (b), 5.6 wt% AlCb; (C) 11.2 wt% AICI3.

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Honeycomb Al imprcg Honeycomb Honeycomb pre Al

60

80

100 120 140 160

Time / min

Honeycomb Al impreg Pellet Al imprcg Honeycomb Pellet

60

80

100 120 140 160

Time / min

Fig. 4. Catalytic activity of the Friedel -Crafts alkylation between toluene and benzyl alcohol. The activity measurement was performed in a similar way reported in the literature [3]. The reason is that the pseudobohemite was added to the slurry as an inorganic binder for the extrusion of the honeycomb. The Al incorporation method affected the catalytic activity of the alkylation. The honeycomb containing the Al in the slurry mixture showed the better catalytic performance compared to that of the Al-implemented or -impregnated honeycomb. This might be due to the masking of the active surface by Al or the agglomeration of the impregnated Al. The mixing of AICI3 in the slurry mixture was more effective for catalyzing the honeycomb for the alkylation, which can be attributed the homogeneous distribution of the catalytically active sites. From comparison of the catalytic performance with those of pellet, it has been suggested that the large open area of the honeycomb provided the better catalytic activity for alkylation due to the thin wall thickness of the honeycomb. In summary, this work suggested that the integrity of mesoporous structure can be retained during the fabrication of honeycomb and the incorporation of aluminum without the pore blockage or masking. This means that the catalyzing the honeycomb can be done successfully on the honeycomb containing mesopores for the alkylation of toluene. This work was supported by National Research Laboratory Program and Creative Research Initiative Program in the Korean Ministry of Science and Technology, KOREA. REFERENCES 1. J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 2. Y. S. Ahn et al.. Stud. Surf. Sci. Catal., 135 (2001) 318. 3. S. Jun and R. Ryoo, J. Catalysis, 195 (2000) 237.

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Heterogenization of AICI3 on mesoporous molecular sieves and its catalytic activity Kyoung-Ku Kang and Hyun-Ku Rhee* School of Chemical Engineering and Institute of Chemical Processes, Seoul National University, Kwanak-ku, Seoul, 151-742, Korea. Aluminum containing mesoporous materials were prepared by direct hydrothermal synthesis and AICI3 immobilization. All the samples were characterized by well-established methods. According to the results of XRD and N2 physisorption, all the mesoporous molecular sieves, pure silica and aluminum substituted samples, have a long-range order structure. The catalytic performance of AICI3 immobilized mesoporous materials in the liquid phase alkylation of benzene is compared with those of other aluminum containing mesoporous materials. The AICI3 immobilized mesoporous materials are more active than other materials and the selectivity to the mono-alkylation product increases as the chain length of olefin molecules becomes large or as the pore size decreases. 1. INTRODUCTION Linear alkyl benzenes (LABs), which are used in the production of biodegradable surfactants, are synthesized commercially by benzene alkylation with linear alkenes. This reaction is usually carried out in the liquid phase in the presence of Lewis acid (AICI3 and ZnCh) or using Bronsted acid (HF and H2SO4). However, this reaction system suffers from several disadvantages such as the corrosive nature, potential environmental hazards and difficulties in separation, recycling and disposal of the spent catalysts. To overcome such problems, heterogeneous processing using solid acid catalysts is highly desirable and thus an extensive effort has been directed to the heterogenization of homogeneous catalysts using clay minerals and zeolites as supports. For example, heterogeneous Friedel-Craft catalysts based on AICI3 and ZnCb immobilized on montmorillonite and silica gel have been reported to show a high catalytic activity for the alkylation reaction [1, 2]. The H^ form zeolite beta has also been known to have a good catalytic activity for the liquid phase alkylation of benzene with light olefins [3]. In this study, alkylation of benzene has been carried out with three olefins, which have different chain lengths, using heterogeneous Lewis acid catalysts prepared by modification of Si-MCM-41 and Si-SBA-15 with AICI3. We have also prepared Al-MCM-41 and Al-SBA-15 by the direct synthesis method and compared their catalytic activities with those of the former. •Adress for correspondence: E-mail. hkrhcc(a)snu.ac.kr Fax. +82-2-888-7295 Tel. +82-2-880-7415 ** This work was supported by Grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and also partially by the Brain Korea 21 Program sponsored by the Ministry of Education.

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2. EXPERIMENTAL 2.1. Preparation of mesoporous materials The Si-MCM-41 was prepared using a cationic surfactant (cetyltrimethyl ammonium bromide), as a template and sodium silicate solution as a silica source and following the synthesis procedure reported elsewhere [4]. The Si-SBA-15 was obtained by hydrothermal synthesis in the presence of PI23 (BASF: triblock copolymer) as template [5]. All the samples were washed, dried at 373 K and calcined in air at 823 K. The direct synthesis of Al-MCM-41 and Al-SBA-15 in aluminosilicate form was realized by applying almost the same procedure as for the pure silica, except for the addition for aluminum source. The remainder of synthesis procedure is the same as the one for pure silica materials. To obtain the HAl-MCM-41 and HAl-SBA-15 catalysts, the calcined Al-MCM-41 and Al-SBA-15 were converted to the H^ form through NH4^ ion exchange and subsequent calcination. 2.2. Immobilization of AICI3 Anhydrous AICI3 was dissolved in dry benzene. The pure silica samples were heated in a flask at 473 K for 24 h under vacuum condition. The dried Si-MCM-41 was cooled to room temperature under dry N2(g). The AICI3 solution and dried benzene were added to the silica samples. The resulting mixture was refluxed under nitrogen for 48 h, the solvent was eliminated by syringe, and the solid was repeatedly washed with dry solvent more than five times. All the immobilization processes was carried out in a glove box under dry N2(g). Finally, AICI3 immobilized MCM-41 and SBA-15 catalysts were dried at 373 K for 24 h. 2.3. Catalyst characterization All the samples were characterized by various analysis techniques. The small-angle X-ray scattering (SAXS) patterns were measured at room temperature using a Bruker GADDS diffractometer. The N2 adsorption isotherm was measured at liquid N2 temperature with a Micromeritics instrument (ASAP 2010). The specific surface area and pore size were calculated by using BET method and BJH algorithm. 2.3. Alkylation The alkylation of benzene was carried out in the liquid phase with magnetic stirring under refluxing condition for 1-3 h. Under the atmosphere of nitrogen, 100 mmmol of each of the alkenes (1-hexene, 1-octene and 1-dodecene) was added over a period of 30 min to a reactor containing 200 mmol of dried benzene and Ig of catalyst. The AICI3 immobilized catalysts were recycled. The conversion of alkene was analyzed by gas chromatography. 3. RESULTS AND DISCUSSION The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defined reflections of hexagonal structure as reported [4, 5]. The SAXS patterns for aluminosilicate MCM-41 and SBA-15 prepared by the direct synthesis procedure showed almost the same SAXS pattern and intensity as those of pure silica sample. The AICI3

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immobilized mesoporous samples exhibit nearly the same SXAS patterns and the intensities remain almost the same as those for their parent pure silica samples as shown in Figures 1 and 2. These results indicate that the incorporation of aluminum has no influence on the hexagonal structure formed during the direct synthesis procedure. i AICI3-MCM-4I (Si/AI=25)

\

AI-MCM-41 (Si/AI=25) AI-MCM-41 (Si/AI=50) Si-MCM-41

- AICI3-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=25) •AI-SBA-15(Si/AI=50) -Si-SBA-15

(A C 4)

"c

.>0) "(3

0^

J

.1 2 theta

Fig. 1. SAXS patterns of MCM-41

Fig. 2. SAXS patterns of SBA-15

The results of N2 physisorption for all the mesoporous samples registered surface areas over 800 m^/g and narrow pore size distributions, being typical of mesoporous molecular sieves (c/ Table 1). The results of XRD and N2 physisorption analyses confirmed that the structural integrity of the mesoporous materials remained intact after heterogenization with AICI3. All the aluminum containing samples except for HAl-SBA-15 were found to be effective for the liquid phase alkylation of benzene with olefins as may be noticed from Table 1. The conversion of olefins over HAl-SBA-15 synthesized by the direct synthesis method is very low. Especially, the alkylation of benzene did not progress at all over HAl-SBA-15 with Si/Al=50. Since the SBA-15 was synthesized under acidic condition with 1.6 M aqueous HCl solution, the acidic condition caused the elution of aluminum to the reaction mixture. Therefore, the Al-SBA-15 prepared by direct synthesis contains less aluminum than the initial reactant gel and shows a lower activity. The selectivity to the mono-substituted alkyl benzene increased as the chain length of the olefin molecules becomes large or as the pore size decreases. It should also be noted that AICI3 immobilized mesoporous samples exhibited an enhanced catalytic activity in comparison to HAl-MCM-41 and HAl-SBA-15 with the same Si/Al ratio, respectively, and these catalysts could be re-used three times without loss of catalytic activity.

676

Table 1 The structural data and the catalytic reaction results Si/Al

BET surface area (m^/g)

BJH adsorption average pore size (A)

00

1107.1

37.7

-

-

-

Si-MCM-41'' Al-MCM-4r Si-SBA-15' Al-SBA-15'

1 -hexene

1-dodecene conversion / selectivity

(%)

50

963.8

39.5

49.7/73.1

39.8/77.1

41.3/95.7

25

1013.1

42.04

29.9/79.9

32.3/79.7

31.1 /93.2

00

891.4

63.2

-

-

-

50

851.1

60.9

25

824.6

58.7

13.1 /62.1

10.3/61.7

11.7/77.3

70.1/76.5

53.8/92.8

67.4/78.3

48.3/94.1

85.0/60.0

73.8/75.9

81.9/63.1

71.9/74.8

25 853.4 69.7/74.9 35.6 (fresh) AICI3MCM-41 25 62.9/77.0 (recycled) 25 748.3 61.5 83.9/61.3 (fresh) AICI3SBA-15 25 79.8/63.9 (recycled) The alkylation of benzene was carried out under rcfluxing condition for 3 h. " direct synthesis, ^ selectivity to linear alkyl benzene

4. CONCLUSIONS The SAXS patterns of pure silica and aluminum incorporated mesoporous samples exhibited well defmed reflections of hexagonal structure with their surface areas and pore sizes being typical of mesoporous molecular sieves. The results of SAXS and N2 physisorption analyses confirmed that all the samples have well developed hexagonal mesoporous structure. All the aluminum containing MCM-41 and AICI3 immobilized samples were found effective for the liquid phase alkylation of benzene with olefins. Among various samples the AICI3 immobilized catalyst is the most active and the selectivity to mono alkyl benzene increases as the chain length of olefin molecules becomes large or as the pore size decrease.

REFERENCES 1. V. V. Veselosky, A. S. Gybin, A. S. Lozanova, A. M. Moiseenkov, W. A. Smit, and R. Caple, Tetrahedron Lett., 29 (1989) 175. 2. J. H. Clark, A. P. Kybett, D. J. Macquarrie, S. J. Barlow, and P. Landon, J. Chem. Soc, Chem. Commun., 1353 (1989). 3. E. Armengol, A. Corma, H. Garcia, and J. Primo, J. Appl. Catal. A., 149 (1997) 177. 4. K-K. Kang and H-K Rhee, Stud. Surf. Sci. Catal., 141 (2002) 101. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, and G.D. Stucky, Science, 279(1998)548.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

677

Asymmetric dihydroxylation catalyzed by MCM-41 silica-supported biscinchona alkaloid Sang-Han Kim and Myung-Jong Jin School of Chemical Science & Engineering, Inha University, Inchon, 402-751, Korea MCM-supported bis-cinchona alkaloid complexed to osimium was examined as a chiral catalyst in asymmetric dihydroxylation of olefins. The desired diols were obtained in high yield with high enantiomeric excesses of up to 99%. High ordered MCM-41 silica was found to be better inorganic support for the catalytic system than amorphous silica gel. 1. INTRODUCTION Osmium-catalyzed asymmetric dihydroxylation (AD) of olefins has emerged as an attractive method for the synthesis of optically active diols. ^ Cinchona alkaloid-based osmium complexes are known to be the most effective catalysts for AD reaction in terms of both reactivity and enantioselectivity."^ However, for large scale synthesis the high cost and toxicity of the osmium catalyst must be taken into consideration. For this reason the development of chiral heterogeneous catalysts is a field of great interest. One approach which has been shown to be highly fruitful is the attachment of the catalyst to an insoluble polymer support, which then allows easy separation and reuse of the catalyst. Recently, silica gel and mesoporous silica have been successfully used as supports for the immobilization of the osmium catalysts."^"^ Our interest in the field led to prepare a MCM-41 silica-supported biscinchona alkaloid 3. Herein, we report our preliminary results on the AD reaction of olefins using the MCM-supported chiral ligand 3. 2. RESULTS AND DISCUSSION

E

0;Si' O

"'^>i> MeO

Reaction of MCM-41 with an excess of (3-mercaptopropyl)trimethoxysilane in refluxing toluene gave mercaptopropylsilanized MCM 2. The MCM-supported bis-cinchona alkaloid 3 was prepared by the reaction of l,4-bis(9-0-quininyl)phthalazine monomer 1 with

678

mercaptopropylsilanized MCM 2 under radical condition (Scheme 1). With the heterogeneous chiral Ugand 3 in hand, we then performed investigated the AD reactions of various olefins. The results are summarized in the Table 1. hi most cases, the products were obtained in high yields with high enantiomeric excesses. The AD reaction of stilbene catalyzed by 1 or 2 mol% of 3 and 1 mol% of OSO4 at room temperature for 24 h proceeded to afford the corresponding diol in 95% conversion with 99% ee (entries 3 and 4). Satisfactory e.e. was also obtained in the reaction of styrene. It has been known that styrene substrate gives low enantioselectivity. The MCM-supported ligand 3 offered somewhat better asymmetric induction than silica gelsupported bis-cinchona alkaloid.^ The improved stereochemical outcome of the reaction seems to be attributed to crystalline structure of MCM support. The MCM framework allows Scheme 1

^ ^ ^ K2CO3 KOH Toluene, reflux

MeO

N-N 1,4-bis(9-0-quininyl)phthalazine ]

AIBN,CHC1

an ordered array of chiral catalytic sites on the pore surface. The ordered array leads to elegant site-isolation,^ which may result in enhanced enantioselectivity. These results are comparable to those of its homogeneous counterpart. The MCM-supported was easily filteredft-omthe reaction mixture. It is noteworthy that the heterogeneous system the MCMsupported alkaloid-Os04 complex can be reused by the easy filtration from the reaction mixture with only moderate loss of reactivity and enantioselectivity. In conclusion, we have achieved excellent results in the heterogeneous catalytic AD using MCM-supported bis-cinchona alkaloid 3. Moreover, The MCM-41 could be served as a potential support for the heterogeneous chiral ligand. Efforts for the synthesis of further MCM-based chiral ligands are currently underway in our laboratory

679

Table 1 Heterogeneous AD of olefins using MCM-41-supported-bis-cinchona alkaloid 3^ MCM-41-supported Ugand 3 cat. OSO4 K3Fe(CN)6-K2C03 in/-BuOH-H20(l:l)

Entry

R

Time (h)

1

H

22

2

U

3

Yield (%)

[a]D(c, solvent)

% ee^

Config^

94

+32.4 (2.5, EtOH)

83

S

22

80

+28.4 (2.5, EtOH)

73

S

Ph

24

93

-92.0 (1.0, EtOH)

99

S,S

4'

Ph

24

93

-92.1 (1.0, EtOH)

99

S,S

5"^

Ph

24

82

-85.6(1.0, EtOH)

92

S,S

6

COzMe

24

94

+10.4 (1.1, CHCI3)

97.6

2R,3S

7

CH3

24

91

+32.0 (0.8, EtOH)

96

2S,3S

''The reaction was carried out at RT; Molar ratio of olefin/ OSO4/ MCM-41-supported ligand = 1/0.01/ 0.02. ^% Ee and absolute configuration were determined by comparison of [a]D with literature value."' "Molar ratio of olefin/ OsOV MCM-41-supported ligand = 1/0.01/0.01. '^Reaction was carried out with 3 which was used in entry 3 without further addition of OSO4. 3. EXPERIMENTAL 3.1. Preparation of l,4-Bis(9-0-quininyl)phthalazine 1 A-100 mL three-neck round-bottom flask equipped with a Dean-Stark-condenser was charged with 1.56 g (4.82 mmol) of quinine, 0.5 g (2.51 mmol) of 1,4-dichlorophthalazine, 1.02 g (7.38 mmol) of K2CO3, and 50 mL of anhydrous toluene. After 2 hrs reflux under nitrogen atmosphere, 0.42 g (7.38 mmol) of KOH pellet were added and then the reaction was continued for 20 h. The light orange solution was mixed with water and then extracted with EtOAc. Recrystallization from Et20 gave 1.75 g of white powder L

680

3.2. Preparation of MCM-41 silica 2 MCM-41 silica^ (1.0 g) was treated with 0.87 g of (3-mercaptopropyl)trimethoxysilane in 12 ml of anhydrous toluene. The mixture was heated at 110°C for 24 hours. The powder was collected by filtration and washed with methylene chloride. After drying in vacuo at 50 °C, mercaptopropylsilanized MCM 2 was obtained. Elemental analysis and weight gain showed that 2.9 mmol of (3-mercaptopropyl)trimethoxysilane was anchored on 1.0 g of MCM-41. 3.3. Preparation of MCM-41-supported bis-cinchona alkaloid 3 This derivatized MCM 2 (0.75 g) was suspended in chloroform and refluxed with 1,4bis(9-0-quininyl)phthalazine 1 (0.56 g) and a,a'-azoisobutyronitrile (AIBN, 26 mg), as radical initiator, for 48 hours. The powder was collected by filtration and washed with methanol and methylene chloride until the l,4-bis(9-0-quininyl)phthalazine in excess was completely removed. After drying in vacuo at 50 °C, MCM-41-supported alkaloid 3 was obtained. Elemental analysis and weight gain showed that 0.52 mmol of l,4-bis(9-0quininyl)phthalazine 1 was anchored on 1.0 g of the MCM 2. 3.4. Typical procedure for the asymmetric dihydroxylation using MCM-41-supported bis-cinchona alkaloid 3 To a mixture of MCM-41-supported bis-cinchona alkaloid 3 (45 mg, 0.02 equiv.), potassium ferricyanide (0.58 g, 3.0 equiv.), potassium carbonate (0.24 g, 3.0 equiv.), and OSO4 (1 mole %, 0.5 M in water) in 5mL of ^er/-butyl alcohol-water mixture (1:1, v/v) at room temperature, the olefin (5 mmol) was added at once. The reaction mixture gradually changed from a heterogeneous to a homogeneous solution in 22-24 h. Solid sodium sulfite (0.47 g) was added, and the mixture was stirred for an additional hour. The MCM 3 was removed either by filtration or centrifugation and washed with ether. The combined organic extracts were then evaporated; the residue was dissolved in CH2CI2 (20 mL), washed with brine (10 mL) and dried (Na2S04). The residue was purified either by chromatography or distillation. Enantiomeric excess of the diol was determined by comparison of [ajo with literature value.^ This work was supported by the Center for Advanced Bioseparation Technology, Inha University. REFERENCES 1. Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H. In Comprehensive Asymmetric Catalysis II, Springer-Verlag, Berlin, 1999. 2. Sharpless, K. B. Tetrahedron 1994, 50, 4235. 3. Song, C. E.; Yang, J. W.; Ha, H, J. Tetrahedron: Asymmetry 1997, 8, 341. 4. Lee, H. M.; Kim, S. W.; Hyeon, T. H.; Kim B. M. Tetrahedron: Asymmetry 2001, 12, 1537. 5. Vanppen, D. L. A.; De Vos, D. E.; Genet, M. J.; Rouxhlet, P. G.; Jacobs, P. A. Angew. Chem. Int. Ed. Engl. 1995, 34, 560. 6. Ryoo, R.: Jun, S. J. Phys. Chem. B, 1997, 101,317.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

681

Roles of pore size and Al content on the catalytic performance of Al-MCM41 during hydrocracking reaction Wen-Hua Chen ^ Qi Zhao ^ Shing-Jong Huang ^ Chung-Yuan Mou ^, and Shang-Bin Liu ^^* ^Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106, R. O. C. ^ Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan 106, R. O. C. The catalytic properties of Al-MCM-41 materials having varied Al contents and pore sizes, were evaluated by means of 1,3,5-triisopropylbenzene (1,3,5-TiPB) cracking reaction. It is found that, while the overall activity increases linearly with increasing Al content of the AlMCM-41, the catalytic ability per active site is mainly controlled by the dispersion of acid sites. 1. INTRODUCTION It is well known that the activity of a catalyst depends mainly on its acidity and masstransport limitations. The former is normally manipulated by the concentration and distribution of Al species, whereas the latter is controlled by steric constrains imposed on the structural porosity of the catalyst. In particular, Mesoporous aluminosilicate Al-MCM-41 materials, being less acidic compared to most microporous zeolites, possess highly ordered mesoporous channels and hence are most suitable for cracking large molecules during which only weak acidity is required.'"^ Al-MCM-41 materials, first discovered by Mobil researchers in 1992,"* typically possess prominent properties, such as high surface area (~ 1000 m^/g), hydrocarbon sorption capacities (> 0.7 ml/g), and thermal and hydrothermal stability. Moreover, the pore size of these materials can be tailored (in the range of 1.5-10 nm) and they can be prepared in a wide range of framework Si/Al ratios thus render the manipulation of their acidic and catalytic properties during material synthesis. The objective of this study is to investigate the roles of Al content and pore size on the catalytic performances of Al-MCM-41 during 1,3,5-TiPB cracking reaction. In particular, the variation of 1,3,5-TiPB initial activities, a parameter used to reflect the concentration of acid sites, for Al-MCM-41 materials with two different pore sizes and various Si/Al ratios were examined. Corresponding author (SBL: [email protected]); the support of this work by the Nation Science Council, R O C. (NSC 90-2113-M-OO1-065 to SBL) is gratefully acknowledged

682

2. EXPERIMENTAL Powdered, particulate MCM-41 molecular sieves with varied Si/Al ratios (15-oo) and pore diameters (2.6 and 3.0 nm) were synthesized by the "delayed neutralization" procedure.^ Their structural features and physical properties were confirmed by powder XRD, SEM/TEM, N2 adsorption/desorption (77 K) and ^^Si NMR spectroscopy. The Characteristics of the samples were shown in Table 1. Reagent 1,3,5-TiPB (A.R. grade, ACROS) was purified by molecular sieve 4A before use. Catalytic reactions were conducted in a continuous flow, fixed-bed flow reactor under the standard conditions, namely Tr = 573 K; WHSV = 15.25 h ' ; pressure = 1 atm; carrier gas: N2; N2/EB = 2.0 mol/mol, and time-on-stream (TOS) = 0-3 h. The catalyst was prepared by mixing the palletized MCM-41 sample (10-20 mesh; ca. 1 g) with quartz (ca. 20-30 mesh). Prior to the reaction, sample was first activated in air at 723 K for 8 h; the reactor was then cooled under N2 stream down to the desired reaction temperature. The composition of the reactor effluents was analyzed by gas chromatography (Shimadzu GC-9A) using a packed column (5% SP-1200 + 1.75% Bentone 34 on 100/120 Supelcoport, 6 ft). All products were identified using the internal standard method.

Table 1 Characteristics and catalytic properties of the MCM-41 samples.

Si/Al

Pore size (nm)^

Pore volume {m\/gf

c, r Surface area

MCM-C16/15

15

2.62

0.98

1015

29.2

38.3

0.81

67.5

MCM-C16/46

46

2.64

1.06

1032

24.3

21.4

0.76

45.7

MCM-C16/60

60

2.58

1.02

1135

22.6

20.6

1.04

43.2

MCM-C 16/120

120

2.61

0.98

1093

17.4

20.4

0.79

37.8

MCM-C 16/370

370

2.58

0.98

1027

7.7

27.8

0.87

35.5

MCM-C 18/15

15

2.98

0.97

1061

16.5

28.1

0.75

44.6

MCM-C 18/37

37

3.04

1.21

1028

11.4

17.1

0.58

28.6

MCM-C 18/60

60

2.80

1.06

1019

2.0

20.8

0.46

22.8

MCM-C18/120

120

2.82

1.12

1118

5.1

13.4

0.93

18.5

MCM-C 18/370

370

2.85

0.98

1044

2.1

14.0

0.13

16.1

Samples

Deactivation parameters'^

"^ Data obtained by the BJH method based on the desorption curve of N2 adsorption/desorption isotherms (77 K). ^ Determined by N2 isotherms at p/po = 0.96. ^' Results obtained from data fitting of Eq. 1. "^ Represent initial conversion (TOS = 0 h); in unit of wt%.

683

3. RESULTS AND DISCUSSION The catalytic activities of various Al-MCM-41 samples were assessed by the conversion of 1,3,5-TiPB, which was catalyzed to produce mainly mono- and di-substituted isopropylbenzenes. Except for the pure siliceous MCM-41, which revealed the expected null activity, the 1,3,5-TiPB conversion obtained from the two series of Al-MCM-41 samples (pore sizes 2.6 and 3.0 nm; varied Si/Al ratio) during cracking reaction were found to obey the first-order exponential decay function: Xt=Xo + k e"

(1)

where Xt represents the conversion at a given time-on-stream (TOS) t, Xo and k are constants, and the exponent a is a parameter accounts for the deactivation rate (by coking).^ The related deactivation parameters derived are depicted in Table 1. The initial activities of 1,3,5-TiPB observed in various Al-MCM-41 samples were used to evaluate the acid properties and catalytic performances of the catalysts. The variations of 1,3,5-TiPB initial conversion (i.e., Xo + k; at TOS = 0 h) with Al content of Al-MCM-41 samples are depicted in Fig. 1 and Table 1. It is obvious that the initial conversions of 1,3,5TiPB decrease exponentially with the Si/Al ratio of the Al-MCM-41. For the two sample series respectively with the pore size of 2.6 or 3.0 nm, the initial conversion curves tend to reach a plateau at ca. 37 and 18 wt%, as their respective Si/Al ratio exceeding ca. 120. It has been shown^ that, while the concentration of the acid sites decreases with increasing Si/Al I • 1 • I ' l l ratio of the Al-MCM-41, the acidic strength OU" remains practically unchanged. Upon initial Al-MCM-41 1] reaction, the 1,3,5-TiPB reactants are • MCM-C16 (2.6 nm) 1 ^ n • MCM-C18(3.0nm) MCM-C18(3 0nmi |I immediately catalyzed to form products or carbonaceous residues, which tend to deposit C 60on the acid sites. Thus, the reaction is readily o \ diffusion controlled. For samples with Si/Al > 120, it is plausible that the feed reactants have % 40covered all of the active sites in Al-MCM-41, c resulting a plateau in the observed initial o o conversion of 1,3,5-TiPB. Note that this effect should also depend on the contact time or •(5 WHSV applied. 20Figure 2 displays the correlation between the 1,3,5-TiPB initial conversions and sample 1 « 1 • r^— Al concentration (expressed in terms of Al 100 200 300 400 molar fraction). A linear correlation is evident regardless of the sample pore diameter, Si/Al ratios indicating the overall catalytic activity increases with increasing Al content for each Fig. 1. Correlations of 1,3,5-TiPB sample. The results indicate that all acid sites initial conversion with Si/Al ratio of are well isolated and apparently having similar Al-MCM-41 for two different sample catalytic activity. This is thus in line with the series during cracking reaction. results obtained in our previous investigation

T

f-V

-- 1

684

on the acid properties of Al-MCM-41 using solid-state ^'P MAS NMR of the adsorbed trimethylphosphine oxide (TMPO) as the probe molecule^ It was found that the ^'P chemical shifts remain practically unchanged upon varying sample Si/Al ratios of Al-MCM-41 samples indicating that the strength of the acid sites is invariant with the sample Al content. In addition, by comparing the results obtained from samples with varied pore sizes but having the same Al content, it is clear that sample with smaller pore size has a higher initial conversion. This is ascribed due to the fact that more Al per unit surface area is available in larger-pore sample. Thus, the hydrocracking ability per acid site is mainly controlled by the dispersion of acid sites on the internal surface of the Al-MCM-41. 4. CONCLUSIONS

80 i

0.00

• •

Al-MCM-41 MCM-C16 (2.6 nm) MCM-C18 (3.0 nm)

0.02

0.04

0.06

0.08

Al cone. Fig. 2. Correlations of 1,3,5-TiPB initial conversion with the Al concentration of AlMCM-41 for two different sample series during cracking reaction.

We have demonstrated that the initial catalytic conversion of 1,3,5 TiPB during cracking reaction over Al-MCM-41 can be used to reflect the distribution of acid sites in the mesoporous molecular sieve. While the overall activity increases linearly with increasing total Al content of the Al-MCM-41, the catalytic ability per acid site is mainly dictated by the dispersion of acid sites. Moreover, it is conclusive that 1,3,5-TiPB cracking reaction is more favorable for Al-MCM-41 having the smaller pore size and a greater acid site concentration. REFERENCES 1. (a) Reddy, K. M.; Song, C , Catal. Lett. 1996, 36, 103. (b) Reddy, K. M.; Song, C , Catal. Today \996,3\, 137. 2. Chen, X. Y., et al., Catal. Lett. 1997, 44, 123. 3. Siahkali, A.G. et al., Appl Catal. A 2000, 192, 57. 4. C. T. Kresge, M. G. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature 1992, 359,710. 5. Lin, H. P. et al., Microporous Mater. 1996. 10, 111. 6. Chen, W. H. et al, Microporous and Mesoporous Mater., submitted (2002). 7. Zhao, Q. et al, Stud. Surf. ScL Catal 2002, 141, 453.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

685

HDS of FCC gasoline: Mesoporous modified support catalyst and its effects on the hydrogenolysis reaction selectivity Gonzalo H. Tapia ^, Teresa Cortez ^, Rene Zarate ^, Javier Herbert ^, Jose L. Cano ^ ^Mayan Crude Program, Mexican of Petroleum Institute, Eje Central Lazaro Cardenas 152 c.p. 07730, Col. San. Bartolo Atepehuacan, Mexico, D.F., e-mail: [email protected]. A synthesis methodology strategy for the systematic control of surface properties has been developed for HDS catalyst using alkaline metals and rare earth metals. The strategy is useful for change the acid-base balance on catalysts surface. The mesoporous modified catalysts have been used to HDS of a selected heavy cut of FCC gasoline to study the activity and selectivity reactions, in order to adapt the conditions that allows to reduce the sulfur content with minimum octane loss. 1. INTRODUCTION In a near future scene, the sulfur reduction in fuels will be the most important action to take into account to get the ambient legislation, considering to reduce the SOx and NOx emissions[l]. In this way, the tightening regulations and the incrisely product demands, becomes the FCC naphtha hydrotreatment attractive to achieve the sulfur level target, considering it contributes with 80 - 90 Wt % of the total sulfur in the gasoline pool [2]. The primary method to remove sulfur is hydroprocessing This methodology will likewise play an essential role in reducing FCC gasoline sulfur. A modified catalysts systems with systematic control of surface properties has been applied for HDS catalyst using alkaline metals and rare earth metals. Recent results obtained [3,4] support the supposition that after addition of small amounts of alkali oxides the acidity and basicity balance of the material surface were modified. The strategy is useful for change the acid/basic sites balance on catalyst surface without change on textural properties. In the present study the authors tried to examine the effect of basic metal oxides added to mesoporous support on acid-base surface properties in order to improve the activity and selectivity to hydrogenolysis reactions, that allows to reduce the sulfur content in heavy cut of FCC gasoline with minimum octane losses. 2. EXPERIMENTAL The synthesis of the mesoporous modified support were performed under basic conditions using lanthanum and potassium nitrates, in order to modified their surface properties. The ions (La^"^ and K^), were incorporated to the support before extrusion. Active metals were added in simultaneous way using ammonium hydroxide solution. The resulting catalysts were characterized by X-ray powder diffraction (XRD) with a DRX Siemens instrument model D5005. Pore size distributions was analyzed with N2 adsorption in a Micromeritics ASAP 2405

equipment. Acid-base surface properties were determined by temperature programmed desorption of C 0 2 (TPD-C02) in a Zeton-altamira-AMI-3 equipment and FTIR of pyridine in a Nicolet 170-SX equipment. The activities of hydrogenolysis and hydrogenation were estimated in a fixed-bed stainless steel tubular reactor at pilot plant level, using a selected heavy cut of FCC gasoline. Catalysts were pelleted and particles were diluted with a-alumine before being charged into the reactor. The catalysts were presulfided before HDS reaction. The reaction conditions for the HDS reaction were temperature, 270-340 OC; total pressure, 19 Kg/cm2; Hzlfeed ratio, 850 scfib and LHSV, 4 H-'. The reaction products were separated in a gas-liquid separator to collect the liquid products. The gas and liquid products were analyzed by a chromatograph with sulfur detector using a commercial capillary column. To select pilot plant feed, FCC gasoline and their cuts were characterized. Physical and chemical properties were determined and sulfur compounds identified. The olefins and sulfur distribution of full range FCC gasoline were employed in order to select the optimal cut temperature. 3. RESULTS AND DISCUSSION

The XRD pattems showed that all the supports and their corresponding synthesized catalysts have structures corresponding to the y-alumina. This result suggest that the ions added are highly dispersed on the surface of the support and the major effect is on the surface properties. Figure 1 shows that the modified supports are constructed with mesoporous with very narrow size distribution and the pore diameters were systematically controlled from 60 to 120 A. A poresize maximum for the A1201 and A1203-La201supports at 68 Angstrom has been found. The pore size variation of the K' modified support indicates that exist different interaction between the support and the added ion. It can be seen that the pore volume decreases lightly with potassium addition, : : ""'r:o,.r suggesting that the support sinterization is favored by calcination temperature. Support modification IOR . with lanthanum ions ( ~ a " ) do no change the pore .. 01 size distribution, moreover, it was found that ~ a ~ ' ions bring about an increase in thermal stability of the 00 alumina support. Physical properties of the prepared 10 100 IWO catalysts were reduced after depositing P m ZI-F. A distribution of the metal species. The decreases in specific surface Fig. 1. Port mesoporous modified supports from area and pore volume result partly from the density alumina with N2 at 77.3 K. increase by depositing the metal species and partly from the pore blocking by the species. Support modifications with lanthanum and potassium ions affects the acid-base balance on catalysts surface. Acid-base properties were determinate by FTlR of pyridine and temperature programmed desorption of CO2. Figure 2 shows desorption of C02 patterns for unmodified and modified supports. The addition of basic metals oxides resulted in a marked increase in the CO2

,)k'" A-

j:l /

-,.st).bO

P

..

687 desorption. K2O exhibited the most pronounced effect for increasing the signal intensity. Quantitative evaluation is needed for the effect of the added basic metal oxides on CO2 desorption; thus the micromoles of CO2 desorbed were quantified . Acid site distribution in solid supports is usually determinated by adsorption-desorption studies of basic probes molecules . Pyridine is the most common used for oxides. Pyridine adsorption on the supports gives rise to an IR band at 1450-1453 cm"' due to a superposition of two absorption: piridine H-bonded and coordinatively bonded to a lewis acid site. Weak, intermediate strong

100

200

300

I " I

400 500 600 Temperature, °C

WAVKNIJMBK.R

Fig. 2. CO2 desorption of modified support with alkaline earth oxide and alkali metal.

Fig. 3. FT-IR spectra of modified support with Alkaline earth oxide and alkali metal.

In addition, AI2O3-K2O and Al2O3-La203 modified supports exhibit IR bands of less intensity at 1450-1453 cm' compared to unmodified support ("d" line in figure 3), due to a reduction in acid sites density. Similar effect occurs on the IR espectra for catalysts. Modification of support properties by adding Lanthanum and Potassium ions has a significant effect on the activity of cobalt-molybdenum catalysts in the applied reactions of HDS of the heavy cut of FCC gasoline. Pilot plant results of catalytic activity of the discussed modifications in HDS reactions are presented at figure 4 and 5. The effect of modification of the support with lanthanum and potassium ions on the activity of cobalt-molybdenum are similar. Catalyst modified with potassium and lanthanum revealed high HDS activity and low hydrogenation selectivity (measured like octane losses) when compared to a commercial catalyst. Modification of the support with lanthanum and potassium ions decreases acidity of the catalysts, which is the reason why the hydrogenations reactions are low, and the octane losses too. On the other hand, the high activity to hydrogenolysis reactions indicates an increase of the basicity of the catalysts surface.

688

98

.--

94 90 86 82 78 74

72

t*'

:—^ -. CoMo/Ab034.a203 ^ I CoMo/AI}0>«:0 HI, Commercial

-

1

270

280

1

I

290 300

I

I

310

1 320

330 340

Temperature, ' C

Fig. 4. HDS activity of modified and unmodified Catalysts with alkaline earth oxide and alkali metal.

270 280

290 300

310

Temperature, "C

320 330 340

Fig. 5. Octane losses of heavy cut of FCC gasoline after HDS. 19 kg/cm^ 4 H ' , 850 scf^ H2/HC ratio.

In the process of support modification, lanthanum and potassium ions react with the surface OH groups, causing a decrease of the support acidity[6-7], they also react with Lewis centers[89]. In conclusion, a change in acid-base properties of a support causes similar changes in the acidic and basic properties of the surface of catalysts obtained on the basis of thus modified support. 4. CONCLUSIONS Modification of support with alkaline metals (potassium) and rare earths metals (lanthanum) decreases the acidity of the Cobalt-molybdenum catalysts, thus increasing their basicity without changes on textural properties. The effect of this modification on the activity of cobaltmolybdenum are similar. Catalyst modified with potassium and lanthanum revealed high HDS activity and low hydrogenation selectivity (measured like octane losses) when compared to a commercial catalyst. To reduce octane loss by HDS on the heavy cut of FCC gasoline, is convenient to use modified catalysts, which have high activity and low hydrogenating function.

REFERENCES 1. M. Seris, Outlook for European Demand of ULS gasoline and diesel and consequences for US imports. NPRA Annual Meeting, march 17-19, San Antonio Texas, 2002. 2. W.K. Shiflett and L.D. Krenzke. Consider improved catalyst technologies to remove sulfur. Hydrocarbon processing, February 2002, 41-43. 3. T. Horiuchi, H. Hidaka, T. Fukui, Y. Kubo, M. Horio, K. Suzuki, T. Mori. Applied Catalysis A: General 167 (1988)195-202. 4. M. Lewandowski and Z. Sarbak. Applied Catalysis A: General 173 (1988)87-93 5. S.W. Golden, D.W. Hanson and S.A. Fulton. Use better fractionation to manage gasoline sulfur concentration. Hydrocarbon processing, February 2002, 67-72. 6. L.Vordonis, P.G. Koutsoukos, A. lycourghiotis. J. Catal. 98 (1986) 296 7. L.Vordonis, P.G. Koutsoukos, A. lycourghiotis. Colloids and Surface 50 (1990) 353 8. R. Fiedorow, I. G. Dalla Lana. J. Phys. Chem. 84(1980)2779. 9. M. Lewandowsky and S. Zarbak. Applied Catal. 168 (1998) 179-185

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

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Synthesis of mesoporous carbon nanotubes and their application in gas phase benzene hydrogenation Dong Cheng Han, Zhi Qing Zhu, Ai Min Zhang , Jian Zhong Zhu, Jia lu Dong, Department of Chemistry, Nanjing University, Nanjing, P.R. China The multi-wall carbon nanotubes with pore diameter of 30-50 nm, synthesized by chemical vapor deposition over Co-La catalyst via decomposition of acetylene at 700 °C, were employed as carrier of Ni-loading catalysts and exhibited excellent conversion of benzene and selectivity for cyclohexane in the gas phase benzene hydrogenation under atmospheric pressure. 1. INTRODUCTION Since their discovery in 1991 [1], carbon nanotubes have been presented as a very promising material in a wide range of potential applications [2]. Many researchers have reported their mechanical properties, superior thermal and electric properties. These exceptional properties of carbon nanotubes have been corroborated for devices such as field-emission displays, scanning probe microscopy tips and micro-electronic devices. With the large-scale synthesis of carbon nanotubes, attention is now being directed to their potential application in various fields of materials. Catalysis is a nanoscale phenomenon that has been the subject of research and development for many decades, but only recently become a nanoscale science of materials and chemistry involving more investigations on the molecular level. In the field of heterogeneous catalysis, various carbon materials have been used to disperse and stabilize metallic particles [3]. However, the carbon nanotubes, different from general carbon materials, exhibit exceptional properties such as uniform pore diameter, high length-diameter ratio, ability of very high H2 uptake [4], and large specific surface area, and the hydrophobic or hydrophilic character of the surface can be controlled by chemical treatment or modification [5]. These properties especially the unusual ability of H2 uptake suggest enormous potential applications of carbon nanotubes as novel materials for the catalyst carrier in hydrogenation reactions. The hydrogenation of benzene to cyclohexane is used probe the activity for reactions taking place on metal sites. Moreover, this reaction has arose practical interest as special attention has recently been focused on the hydrogenation of aromatic compounds and there in an increasing demand for suppression of the benzene content in petroleum fuels and especially in gasoline and diesel in near future. In this study, we characterized as-synthesized and treated CNTs with TEM, XRD, BET, and TG/DTA. Then prepared Ni-supported catalyst by impregnation in ethanol solution, and investigated the catalytic performance of nickel-supported carbon nanotubes in the reaction of gas phase benzene hydrogenation under atmospheric pressure. * Corresponding author.. E-mail address: [email protected]

690

2. EXPERIMENT 2.1. Preparation of carbon nanotubes and Ni-supported carbon catalyst The multi-walled carbon nanotubes were obtained by chemical vapor deposition of acetylene over Co-La catalyst at 973K following the procedure reported previously [8]. The main impurities coexistent with multi-walled carbon nanotubes were metal particles and amorphous carbon. In order to remove these impurities, the as-synthesized carbon nanotubes were first suspended in concentrated HNO3 solution with stirring and refluxing at 333K for some hours. After filtering, washing and drying, the treatment with nitric acid was repeated twice for surface oxidation of carbon nanotubes. Subsequently, the dried oxidized carbon nanotubes were impregnated with NiN03-6H20 dissolved in ethanol with stirring for 5 h, dried at 323K for 15 h and heated in vacuum at 409K for 2 h. Finally, the samples were oxidized in air and reduced in H2 atmosphere. 2.2. Characterization The morphometries of carbon nanotubes were observed with the JEM-200 CX transmission electronic microscope (TEM). The specimens for TEM were first mulled in agate bowl, then dispersed in aqueous solution containing 50% alcohol by ultrasonic treatment and dropped onto holey grids. The specific surface area was measured by the method of nitrogen physisorption at liquid nitrogen temperature using a Micrometritics ASAP 2000 apparatus. XRD patterns were taken with a D/MAX X-ray diffraction instrument by using CuKa in the voltage of 40 kV and current of 50 mA. 2.3. Activity for hydrogenation of benzene Gas phase hydrogenation of benzene was carried out using a tubular 4 mm ID flow micro-reactor at 473 or 453K. 50mg of the sample with the particle size of 20/40 meshes was put into the U-shape quartz tube. Before reaction, the catalyst was pretreated by heating in nitrogen flow at a constant rate of 10 Kmin'^ to 773K and held at this temperature for 2 h. After reduction in hydrogen flow for 2 h, the reactor was cooled down to reaction temperature. Then, the mixture of H2 saturated with benzene was passed through the reactor at a constant flow rate of 15 ml/min. Reactants and hydrogenated products were analyzed by on-line gas chromatograph GC-1102 with FID detector and Proparak QS column. 3. RESULTS AND DISCUSSIONS 3.L Purification and characterization of carbon nanotube For large-scale synthesis of carbon nanotubes, the as-prepared carbon nanotubes usually contain a large amount of impurities such as metal particles, amorphous carbon and multi-shell carbon nanocapsules. These impurities bring about a serious impediment to the detailed characterization of carbon nanotubes and catalytic properties, so it is very important to purify the carbon nanotubes in order to obtain ideal catalysts. The TEM image of the as-synthesized carbon nanotubes is shown in Fig. 1. They are multi-walled carbon nanotubes and have outer diameter of about 30 nm, inner diameter of about 5 nm and length of several tens of |xm. From Fig. 1, we notice that there are many dark metal agglomerates around carbon nanotubes. The purified carbon nanotubes after treatment with acid are shown in Fig. 2. Those dark metal agglomerates have been removed and the

691

walls of carbon nanotube become thinner than before. Which is due to the occurrence of oxidation on the surface of carbon nanotubes at the same time of removing the impurities with concentrated nitric acid. The oxidized carbon nanotubes were favored to support active metals. The specific surface areas of as-prepared nanotubes were about 100 mVg but the value was increased to about 120 VOL'I g after treatment with acid. It has shown that the part of the nanotubes originally closed has opened. The XRD pattern shows only one sharp peak at 26= 26.06, indicating that the carbon nanotubes have a uniform pore size and graphited-well structure and there are no impurities.

Fig. 1. TEM image of the as-synthesized CNTs

Fig. 2. TEM image of the purified CNTs

3.2. Activity of Ni-supported carbon nanotubes for hydrogenation of benzene Three Ni-supported carbon nanotubes samples were obtained, with the Ni loads being 5.0%, 10.0% and 12.0 wt %, respectively. The activities of benzene hydrogenation measured at 200 °C under atmospheric pressure were shown in Figure3, in which the conversion of benzene as a function of reaction time over these catalysts. It is obviously that the conversion of benzene increases with the increase of Ni load. The products analysis showed didn't detected any other hydrogenated products except cyclohexane, the selectivity of cyclohexane achieved 100 % over these samples. Hydrogen spillover is a phenomenon that occurs in many heterogenous catalytic reactions and has received significant attention recently[6]. The term spillover is applied to the transport of active species from one surface to another in which the second surface does not form the active species under the same condition. It has been claimed that hydrogen spillover plays an important in aromatic hydrogenation on supported metal catalysts. When the carbon nanotubes with high ability of H2 uptake was employed as carrier of metal, the hydrogen in the inside of nanotubes was activated by nearby Ni metal, will react with benzene molecules adsorbed on the carbon nanotube sites in the form of carbonium ions are hydrogenated by the spillover hydrogen. Here the interparticle region of nickel and carbon nanotube may be an excellent acceptor of H species spilled over the metal nickel particle. In addition the carbon nanotubes have the trend for adsorbing benzene due to their organophilic property. So the high activities of benzene hydrogenation and high selectivity for cyclohexane could be attributed to the following contributions: the first is the contribution of metal nickel in Ni/CNTs; the second is that acid sites created on carbon nanotubes during the pretreatment with nitric acid; the third is that the carbon nanotubes adsorb preferential for the benzene due to their organophilic property; and forth is that the high ability for hydrogen uptake so that to supply much more H2 as precursor of spillover hydrogen. In the case they are also easy form the saturated cyclohexane instead of unsaturated cyclohexene.

692

The influence of the reaction temperature on the conversion of benzene over Ni/carbon nanotubes is shown in Fig.4. It can be seen that the conversion of benzene increase with the decrease of reaction temperature. Some researchers reported the optimal temperature of most high conversion of benzene is 200 "C due to the thermodynamic limitation. In our experiment we found the conversion of benzene is higher at the temperature of 180°C than that of 200 °C and they did not show the decline tendency for long time (from Fig.4). CD

100

CD N CD JD

80 _60 •5.0 wt% •10.0 wt% 12.0 wt %

p ^40 *CO CD

20

> c o

O

0 0

50

100 150 200 250 300 350 Reaction time (min)

Fig. 3. Conversion of benzene in benzene hydrogenation reaction as a function of reaction time over three different Ni load catalysts at 473K. 5 e

CD

120 100

f 80 ^^60 i 40 § 20 c

R

0

•• • • 200 "C -»-180°C 100

200

• •

•

300

400

Reaction time (min) Fig. 4. Conversion of benzene in benzene hydrogenation reaction over 12 % Ni/CNTs catalyst as a function of the reaction time at different temperature. REFERENCES 1. lijima S. Nature;354 (1991) 56. 2. R.F.Service, Science 281 (1998) 940. 3. Tans,S.J., Devoert, M.H., Dai, H., Thess, A., Smalley, R.E., Geerligs, L.J., and Dekker, C, A^a/wre, 386 (1997) 474. 4. Dillon, A.C., Jones, K.M., Bekkedahl, T.A., Kiang, C.H., Bethune, D.S., and Heben, M.J., A^amre, 386 (1997) 377 5. Fan, S., Chapline, M.G., Franklin, N.R., Tombler, T.W., Cassell, A.M., and Dai, H., Science, 283(1999)512. 6. D. Duprez, Stud. Surf Sci. Catal. 112 (1997) 13.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Characteristics and Reactivities of Cobalt Based Mesoporous Silica Catalysts for Fischer-Tropsch Synthesis W. S. Yang, H.W. Xiang*, Y.Y. Xu, Y.-W.

Li

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001 P. R. China, E-mail: hwxiang(a),sxicc.ac.cn Performance in Fischer-Tropsch synthesis (FTS) and characteristics were investigated using Co-based HMS, MSU-1, and SBA-12 mesoporous silica catalysts. Due to the surface compound resistant to reduction for Co/MSU-1 and Co/SBA-12 and the decreasing number of the active site for Co/HMS, Co/SBA-12, Co/MSU-1 respectively, FTS activities of the catalysts decreased for Co/HMS, Co/SBA-12, and Co/MSU-1 respectively, and Co/HMS showed the lowest methane selectivity about 8.0 (wt)%. L Introduction Catalyst productivity and selectivity to C5+ hydrocarbons arc critical design criteria in the choice of FTS catalyst [1], and an active cobalt catalyst is usually prepared by depositing a cobalt salt on a support (Si02, AI2O3, TiOj, etc.) with second metal (Re, Rh, Ru, Pt, etc.) and another oxide (ZrOj, Th02, etc.) [2,3]. Recently, mesoporous silica with pore size ranging from 2 to 50 nm has been extensively applied as supports for catalysts [4-9], and Co/HMS catalyst showed the good FTS stability [9]. Now the present article tries to investigate the catalyst structure and FTS reactivities of Co/HMS, Co/MSU-1, and Co/SBA-12. 2. Experiment 2.1 Catalyst preparation HMS was prepared in the presence of ethanol and isopropyl alcohol as the co-solvent by our previous report[9] and the literature [10] (5'H,.T^890 mVg; Kp=1.07 cmVg); MSU-1 was synthesized by the hydrolysis of TEOS in the presence of AEO9 and water according to literature [11] (5i3i.j=761 mVg; Kp=0.49 cmVg); mesoporous SBA-12 was obtained by using TEOS, Brij 76 (Aldrich), water and HNO, from the literature [12] (5BET=728mVg; Vp=OAS cmVg). All catalysts (15% cobalt loading) in this study were prepared by aqueous incipient impregnation and cobalt nitrate hexahydrate as the cobalt source; the incipient wetness impregnation was performed at a single step, and followed by air-drying (room temperature for 12 h), then drying (313 K for 24h), and calcination (723 K for 4) [9]. 2.2 Catalysts characterization and FTS tests The X-ray diffraction (D/max diffractometer with Cu-Ka radiation; A = 0.154 nm) and N2 adsorption (micromeritics ASAP 2000 system) were used for the characterization of catalysts. XPS spectra (Reference energy value C 285.00 eV) were collected with a PHI-5300 ESCA spectrometer, and a Al x-ray source was used. The temperature programmed reduction (TPR) was carried out by passing a mixture of 5% H2 in N2 (temperature increase rate: 5K/min), after

694 5 A molecular sieve removed the water of the effluent gas passing through the micro-reactor, a TCD monitored the effluent gas with Nj as a reference, and the weight of the catalyst was 80 mg. The dihydrogen temperature programmed desorption (H2-TPD) was performed in a tubular quartz reactor, after loading the 150 mg sample. The catalyst was reduced with Hj at 673 K for 6 h and then cooled down to the room temperature in flowing H2; the Ar gas was adjusted for the sample, and the H2-TPD results were recorded at the temperature increase rate: lOK/min. For the reaction test, firstly the catalyst was crushed and sieved into 20-40 meshes, and then 5ml catalyst was loaded and reduced in situ for 16 h (P=0.2MPa, T=612> K, H2/CO=2.0, GHSV=500 h') with pure hydrogen in a integral fixed-bed reactor made of stainless steel. After reduction, syngas was introduced and the pressure was adjusted to 2.0MPa. The analysis of the outlet gases CO2, CO, H2, N2, and C, to Q hydrocarbons was done by off-line GC; the solid and liquid hydrocarbon products were analyzed after the end of the test. CO conversion was defined as the percent of the converted CO at total CO, and CO2 mole selectivity was defined as the percent of the CO converted into CO2. Hydrocarbon distribution was the percent of the component / weight at the total hydrocarbon product. 3. Result and discussion 3.1 Activity and hydrocarbon distribution of the catalysts for FTS FTS reactivities of three catalysts were shown in table 1. The gradually increasing CO conversion and the increasing methane selectivity for the cobalt catalyst were found with the increasing reaction temperature, which agreed well with FTS thermodynamics [13]. Co/MSU-1 catalyst was inactive in FTS, while Co/IIMS catalyst showed the highest activity with the lowest methane selectivity, besides it was found that the C5.,x hydrocarbon selectivity of Co/I IMS, about 70 wt% at total hydrocarbon product, was higher than that of Co/MSU-1 and Co/SBA-12. Generally Co/HMS catalyst presented the best reactivities, and indicated a potential application for liquid fuel production. Table 1 Effect of temperature on reactivities of F-T synthesis CO conv. CO2 sel. Hydr. Distr. (wt%) (mol%) (mol%) c, C.s-11 C12-1K C 19.25 C?.4 C7; 35.34 30.09 13.10 2.91 76.98 8.78 9.77 0.63 473 88.18 483 38.11 31.04 12.52 2.52 7.48 8.33 0.87 Co/HMS 90.89 34.83 28.28 15.37 4.90 7.70 2.59 493 8.93 43.04 25.55 9.41 2.84 2.39 96.26 503 11.81 7.81 27.84 33.21 18.84 6.39 4.82 493 63.78 -0.00 8.90 26.54 31.29 18.77 4.74 Co/MSU-1 77.50 0.59 503 12.33 6.33 31.66 28.91 13.65 4.05 86.71 1.43 15.20 6.52 513 73.40 -0.00 11.50 7.81 26.96 31.11 15.38 7.24 493 84.09 22.77 29.09 15.87 11.87 Co/SBA-12 503 0.17 12.06 8.35 513 88.06 16.56 8.67 33.23 26.15 10.79 4.60 1.71 3.2 N2 adsorption-desorption for the samples HMS, MSU-1, SBA-12, and three oxidized catalysts presented the characteristic type IV shape isotherm like MCM-41 [4]. Among three used catalysts, the only Co/MSU-1 and Co/SBA-12 performed the typical IV shape isotherm. So it was concluded that the mesoporous framework of Co/MSU-1 and Co/SBA-12 was partially retained and was more Catals

695

Stable than that of Co/HMS, corresponding to the literatures [11] and [12]. 3.3 XRD for the samples Three supports exhibited the low angle reflection, and transmission electron micrograph (HRTEM: JEM-200CX, not shown in this article) further confirmed the regular array of channels for SBA-12 like the literature [12] and the worm-like channels for HMS and MSU-1 [11]. The used Co/MSU-1 and Co/SBA-12 presented the small angle pattern with lower intensity strength due to the partial collapse of their mesoporous framework in FTS, while the small angle diffraction of the used Co/HMS disappeared due to the complete collapse of its mesoporous framework, and it was previously reported that the mesoporous structure collapsed completely after 24.00 hours from the beginning of FTS for Co/HMS [9], so the better structure stability for Co/SBA-12 and Co/MSU-1 was further confirmed. All oxidized catalysts contained C03O4, and the used catalysts showed the reflection of wax product and the metal cobalt. More importantly the oblivious CoO phase from XRD was found for Co/MSU-1 and Co/SBA-12, which was maybe connected with the lower FTS activity due to the lower reduction than Co/HMS. 3.4 XPS data for the oxidized catalysts Table 2 XPS data for the oxidized catalysts Catalysts Cobalt binding energy (eV) % Co (2py2) Surface Co/Si atomic ratio 780.81 0.04 Co/HMS 0.98 Co/SBA-12 5.82 780.32 0.26 Co/MSU-1 10.32 779.98 0.48 XPS data for the oxidized catalysts were presented in table 2. The presence of C03O4 phase for the catalysts was further confirmed from the binding energy; both the gradually increasing surface cobalt and increasing Co/Si ratio were found with the order of Co/HMS, Co/SBA-12, and Co/MSU-1, and it was suggested there was the oblivious surface structure difference such as the dispersion of cobalt among three catalysts needed to be further confirmed by the other techniques. 3.5 TPR and H^-TPD for the samples

300 400 500 600 700 800 900 1000

T/K

300 350 400 450 500 550 600 T/K

Fig. 1 TPR and M2-TPD spectroscopy for the catalysts (A. TPR; B. Hj-TPD)

a. Co/HMS; b. Co/MSU-1; c. Co/SBA-12 Two reduction peaks at low temperature for three catalysts corresponded to two step

696

reduction: C03O4 -^ CoO -^ Co [14], but cobalt reducibility shown in Fig. 1 were obliviously different. The reduction peak at 742K for Co/HMS should be related to the further reduction of CoO and Co-Si compound from the literature [15]. TPR peak at higher temperature for Co/SBA-12 and Co/MSU-1 indicated that the more difficult reduction of CoSi complex was formed in the preparation and calcination. Thus the reduction degree of Co/MSU-1 and Co/SBA-12 was lower than that of Co/HMS, which was further confirmed by the weak diffraction of CoO for the XRD patterns of the used catalysts. Generally it was concluded that the lower reducibility of Co/MSU-1 and Co/SBA-12 was maybe responsible for the lower FTS activity. H2-TPD spectroscopy presented a single peak at about 353 K and the similar peak shape. The peak area decreased with the order of Co/HMS, Co/SBA-12, and Co/MSU-1.It was suggested that the available cobalt active sites for the catalysts proportional to the peak area decreased with the same order, and this result agreed well with the activity difference shown in table 1. 4. Conclusion The cobalt based mesoporous silica catalysts were prepared, and FTS performance and characteristics of the catalysts were investigated. (1) It was found by Nj adsorption-desorption and XRD pattern that the mesoporous framework of Co/SBA-12 and Co/MSU-1, except Co/HMS, was partially kept after FTS. (2) The difference of the cobalt reducibility and the cobalt active sites uncovered by TPR and Hj-TPD spectroscopy confirmed the different FTS activity among three catalysts. (3) Co/HMS showed the lowest methane selectivity about 8.0 (wt)%, and indicated a potential application for liquid fuel production. Financial supports from the Key R&D Project (China) G1999022402 and Shanxi Science Foundation (China) 20021024 are highly acknowledged REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

E Iglesia, Appl. Catal. A 161(1997) 59. R. Oukaci, A. H. Singleton, J. G. Goodwin Jr., Appl. Catal. A 186(1999) 129. B. Ernst, L. Hilaire, A. Kiennemann, Catalysis Today, 50(1999) 413. C. T. Kresgc, M. E. Lconowicz, W. J. Roth, ct al.. Nature 359(1992) 710. P. T. Tanev, M. Chibwc, T. J. Pinnavaia, Nature 368(1994) 321. R. T. Yang, T. J. Pinnavaia, W. B. Li, et al., J. Catal. 172(1997)488. S. Kim, S. U. Son, S. I. Lee, et al., J. Am. Chcm. Soc. 122(2000) 1550. D. H. Yin, W. H. Li, W. S. Yang, et al., Micr. Meso. Mater. 47(2001)15 W. S. Yang, H. Y. Gao, H. W. Xiang, et al.. Acta Chimica Sinica 59(2001) 1870. P T. Tanev, T. J. Pinnavaia. Science 267(1995) 865. S. A. Bagshaw, E. Prouzct, T. J. Pinnavaia. Science 269(1995) 1242. D. Y. Zhao, Q. S. Huo, J. L Feng, et al., J. Am. Chem. Soc. 120(1998) 6024. R. B. Anderson, The Fischer-Tropsch Synthesis, New York, Academic Press, 1984. B. A. Sexton, A. E. Hughes, T. W. Tumey, J. Catal. 97(1986) 390. A. Kogelbaucr, J. G. Goodwin, Jr., R. Oukaci, J. Catal. 160(1996) 125.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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Physicochemical characteristics of Ti-PILC as a catalyst support for the reduction of NO by NH3 Ho Jeong Chae,^ In-Sik Nam^* and Suk Bong Hong^ ^Department of Chemical Engineering/School of Environmental Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea ''Division of Chemical Engineering, Hanbat National University, Taejon 305-719, Korea A pillared interlayered clay (PILC) intercalated by titania has been prepared as a catalyst support in an attempt to overcome the drawbacks of titania. The morphological, thermal, and surface properties of Ti-PILC have been particularly examined to use as a support for a NO SCR catalyst. The Ti-PILC prepared here was found to exhibit higher surface area and stronger acidity and thermal stability than the titania used as a common catalyst support. 1. INTRODUCTION The anatase type of titania has been widely employed as a catalyst support in the field of heterogeneous catalysis. However, titania as a catalyst support suffers from several disadvantages such as limited surface area and pore structure, weak mechanical strength, and poor thermal stability. Especially, the anatase type of titania reveals the poor thermal stability at high temperatures, which is of vital importance to determine the catalyst life. To overcome these drawbacks of titania, a composite material, titania with silica or alumina has been developed [1,2]. In the present study, the physicochemical characteristics of Ti-PILC have been examined as an alternative catalyst support to titania. The selective catalytic reduction (SCR) of NO by NH3 over V2O5 supported on Ti-PILC has been examined as a probe reaction to evaluate the performance of Ti-PILC as a catalyst support. 2. EXPERIMENTAL Ti-PILCs were prepared with Ti/clay ratios (mmol Ti/g of clay) in the range from 2 to 20 and are referred to as Ti-PILC(w), where m is mmol Ti per gram of clay used for the preparation of Ti-PILCs. Final products were calcined in the temperature range 300800 ""C for 5 h. JRCl (100% anatase) and P25 (70% anatase + 30% rutile, Degussa) as reference titania were employed for the comparative study. A series of vanadia/Ti-PILC catalysts were prepared by the conventional impregnation method. N2 adsorption experiments were carried out by a Micromeritics ASAP 2010 analyzer. Powder XRD patterns were measured on a MAC Science M18XHF diffractometer with CuKii radiation. TPD of NH3 was recorded on a fixed-bed, flow-type apparatus attached to a VG QMS quadrupole mass spectrometer. The IR spectra of adsorbed pyridine were measured on a Perkin-Elmer 1800 FT-IR spectrometer, using self-supported wafers of Ti-PILCs prepared here. XANES spectra were taken at the Ti K-edge using the 3C1 E-mail: isnam(a)Dostech.ac.kr. Fax: 82-562-279-8299.

698

beam line at the Pohang Acceleratory Laboratory (PAL) in Pohang, Korea. The catalytic activity and sulfur tolerance of V2O5 catalyst supported on Ti-PILC for NO SCR reaction were examined in a fixed-bed, continuous flow reactor. The concentration of NO was analyzed on-line by a Thermo Electron 42C chemiluminescence NO-NO2 analyzer. Details of the reactor system and the operating conditions employed are given elsewhere [3]. 3. RESULTS AND DISCUSSION 3.1. Morphological and textural properties of Ti-PILC One of major characteristics required for a promising catalyst support is its textural property such as surface area and pore structure. The advantage of PILCs over the conventional catalyst support is the diversity of their physical and structural characteristics with respect to the method of preparation. As shown in Table 1, the BET surface area and pore volume of Ti-PILC catalysts were found to considerably increase with increasing the content of titania, as well as with varying the method of the catalyst preparation. Especially when the freeze-drying method is applied, Ti-TILC with a considerably high surface area (> 200 mV') was obtained after intercalation of titania into the interlayer of the clay. Even after the calcination at 800 °C, in addition, Ti-PILC still maintains the surface area higher than 120 m^g'. It should be noted here that our Ti-PILC contains not only micropores but also meso- or macropores. Our recent TEM and PSD studies have shown that the macropores formed by the freeze-drying method are induced by the delamination of the layers [4]. Table 1 Physicochemical properties of Ti-PlLCs prepared in the present study Ti02 SA." Pore Vol." Cal. Temp. S.A. of Ti-PILC(IO)'''-' (wt%) (m^g') (cm^g-') CO (m^g') 25 100 320 KNB' 0.08 146 300 261 15 Ti-PILC(2)'^ 0.17 223 30.7 500 Ti-PILC(5)' 47.5 199 182 0.23 600 Ti-PILC(IO)' 47.5 0.27 156 230 700 Ti-PILCF(10r^ 49.3 Ti-PILC(20y 0.29 128 169 800 JRC-1 65 100 50 100 P25 ^The natural bentonite from Kyongju, Korea. ^ Determined after calcination at 500 ^C. "^ The values in parentheses are the Ti concentration (mmol Ti/g clay) used for their preparation. '^Prepared by the freeze-drying method. '^Material obtained from a different batch. 3.2. Acidic and thermal properties of Ti-PILC The surface acidity is one of the most important properties required for a NO SCR catalyst. From the NH3 TPD profiles in Figure lA, it can be seen that the pillaring of titania into the clay significantly enhances the surface acidity of the catalyst, compared to the original clay and commercial titania. To further identify the nature of acid sites, the IR spectra of adsorbed pyridine on the catalyst surface have been measured and are shown in Figure IB. The parent montmorillonite exhibit no IR bands associated with the pyridine adsorbed at Bronsted or Lewis acid sites. For P25 titania, in addition, only one band around 1455 cm"' typical of Lewis acid sites are detected. However, the IR

699

L

^^ c

4 c O)

L (£l

E 3

/ /

(OZ/>

L+

^

P\

3

B

Spectrum of the TiPILC(IO) sample after pyridine adsorption clearly shows the presence of both Bronsted and Lewis acid sites on its surface, which is mainly due to the formation of Ti-0Si (or Al) bonds by pillaring of TiOz- This strongly suggests that the Ti-PILC prepared here may have high potential as a support for the NO SCR catalyst.

\

\

// ^^^^^^^\ ^ ^"^ /-^-——-^ ^ ^ ^^ '

' ^-^-A \ //">o^^/''^-VX \T/ \

(b)

(A

\ ^ \A ^

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(a)

if

.V-,

r>

_c

(a)

100

200

300

400

500

1700

Temperature {°C)

1600

1500

1400

Wavemunber (cm'^)

Fig. 1. (A) NH3 TPD profiles and (B) IR spectra of pyridine adsorbed: (a) KNB, (b) Ti02(P25), and(c)Ti-PILC(10). Another fatal drawback of the anatase-type titania in applications to a variety of catalytic processes may be its weak thermal stability, due to the transformation into the rutile phase at elevated temperatures. Figure 2 shows the powder XRD patterns of JRC-1 and P25 tinania, and Ti-PILC(IO) treated at different temperatures. It is clear that in cases of JRC-1 and P25, the phase transformation of anatase into rutile begins at 600 °C and the formation of rutile phase becomes evident as the thermal temperature increases. Thus, there is no indication of the presence of anatase phase in these titanias after calcination at 800 "C. As seen in Figure 2, however, the majority of Ti-PILC(IO) still remains as an anatase phase even after calcinations at the same temperature. This again shows the high applicability of Ti-PILC to a variety of the catalytic process, particularly to the high temperature reactions. .anatase

5

1

rutile

rutile

(A)

icy

rutile

jpi / ^ (c)

JLLUJLLI.

A ^ (•) 20

40 60 2-meta

80

Fig. 2. Powder XRD patterns of (A) Ti-PILC, (B) P25 TiOz, and (C) JRC-1 TiOz after calcination at different temperatures: (a) 300, (b) 500, (c) 600, (d) 700, and (e) 800 °C.

700

A similar result can be also observed from the XANES spectra of Ti-PILC(IO) materials treated as a function of temperature (Figure 3). Based upon the shapes of the bands appearing in the pre-edge and edge regions, it is clear that the anatase phase with octahedral Ti is predominant for Ti-PILC(IO) even after calcination at 800 ""C. 3.3. Reduction of NO by NH3 NO SCR reaction has been employed as a probe reaction to evaluate the performance of TiPILC as a catalyst support. The catalytic data listed in Table 2 Energy (eV) Energy (eV) Fig. 3. XANES spectra of references and Ti-PILC reveal that the VzOs/Ti-PILCClO) as a function of temperature: (a) Ti foil, (b) catalyst exhibits stronger sulfur tolerance as well as higher initial anatase TiOz, (c) rutile Ti02and Ti-PILC(IO) after activity than the commercial heating at (d) 100, (e) 300, (f) 500, (g) 600, (h) V205-W03/Ti02 catalyst [5]. This 700, and (i) 800 T . can be attributed to the higher acidic properties of Ti-PILC and high redox ability of V205/Ti-PILC [3]. Table 2 also shows that the sulfur tolerance of freeze-dried V205/Ti-PILC is distinctive, which is mainly due to its unique pore structure of the catalyst support. Table 2 NO conversion with respect to the reactor on-stream time^ SO2 Deactivation Time (hr) Catalyst 5 15 25 40 0 0.68 0.57 0.50 0.46 V2O5/Ti-PILCF(10) 0.82 0.45 0.83 0.65 0.51 0.40 VjOs/Ti-PILCClO) 0.46 0.52 0.62 0.54 0.38 VjOs-WOj/TiO. 0.32 0.77 0.55 0.38 V205/Ti02

55 0.40 0.32 0.31

^Reaction conditions: space velocity, 100,000 hr^ reaction temperature, 250 "C; NO = NH3 = 500 ppm; O2 = 5%; SO2 = 5000 ppm. 4. CONCLUSION A series of Ti-PILCs have been prepared, characterized and evaluated as NO SCR catalyst support. These materials exhibit high surface area and acidity, strong thermal stability and multi-modal pore structure as compared to the well-known P25 titania. It is found that the NO removal activity and sulfur tolerance of V205/Ti-PILC is superior to those of the commercial V205(W03)/Ti02.

REFERENCES 1. H. K. MatraHs, M. Ciardelli, M. Ruwet and P. Grange, J. Catal., 157 (1995) 368. 2. B. M. Reddy, I. Ganesh and E. P. Reddy, J. Phys. Chem. B, 101 (1997) 1769. 3. S. W. Ham, H. Choi, I.-S. Nam and Y. G. Kim, Ind. Eng. Chem. Res., 34 (1995) 1616. 4. H. J. Chae, I.-S. Nam, H. S. Yang, S. L. Song and I. D. Hur, J. Chem. Eng. Japan, 34 (2001) 148. 5. I.-S. Nam and H. J. Chae, H. S. Yang, S. L. Song and I. D. Hur, Korean Patent No. 2000-0020980 (2000).

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

701

Performance of double wash-coated monolith catalyst in selective catalytic reduction of NOx with propene H.-G. Ahn and J.-D. Lee Department of Chemical Engineering and Nanotechnology

Center, Sunchon

National

University, School of Applied Materials Engineering, #315 Maegok-dong, Suncheon-city, Jeonnam, 540-742, Korea. Highly dispersed AU/AI2O3 and Pt/A^Oa catalysts were applied to the lower layer of double wash-coated monolith catalyst for selective catalytic reduction (SCR) of NOx with C3H6. Hmordenite, Cu-mordenite or MCM-41 was coated as the upper layer. The catalytic performance was investigated in the presence of oxygen. The double wash-coated catalysts were more active than the catalyst with only zeolite or without upper layer. Temperature window of the double wash-coated catalyst was broadened, and catalytic performance was remarkably improved. The role of each layer and a reaction mechanism were discussed. The combined noble metal monolith catalyst with zeolite was effective in removing NOx by SCR with hydrocarbons. 1. INTRODUCTION Nitrogen oxides (NOx) in the exhaust of both automobile and stationary sources arc of critical concern because these byproducts are toxic environmental pollutants that lead to acid rain and ozone formation. Due to these effects, scientific and technological challenges have been poured to remove them. To alleviate NOx emission, variety of approaches has been applied such as direct catalytic decomposition of NOx, selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), and so on [1]. Especially, SCR was much attracted to us because it has much advantage. Addition of reducing agent is required for the selective conversion of NO to N2 in the presence of O2. However, most catalysts have the narrow temperature window and low conversion of NOx at lower temperature. Obuchi et al. [2] have * This work was supported by Jeonnam Regional Environmental Technology Development Center of Yosu National University.

702

applied double-layered catalysts to SCR of NOx with hydrocarbon. The results showed that the combination of Pt/SiOz with H-ZSM-5 showed high performance at lower temperature. The oxidation of NO to NO2 occurs in the lower layer, and C3H6 adsorbs in the upper layer, so the lower layer catalyst may require a properly active constituent. On the other hand, gold was scarcely employed in heterogeneous catalysis because of its less affinities to any chemical species. It was however reported that gold nanoparticle on metal oxides was highly active in oxidation of carbon monoxide, ethylene, and benzene even at low temperature [3,4]. In this study gold supported on AI2O3 was applied to the lower layer of double wash-coated monolith catalyst, and the upper layer was H-mordenite, Cu-mordenite, and MCM-41. The catalytic performance was examined in SCR of NO with C3H6 in the presence of oxygen, and the role of each layer was discussed. 2. EXPERIMENTAL Mini-size honeycomb type monolith (M) as a support was prepared by cutting out of a honeycomb with 400 cell/in^. Diameter of the monolith was ca. 20 mm (12 g). The monolith samples was coated by first immersing it into 50 (w/v)% solution of aluminum or cobalt nitrate, followed by drying and calcining at 600 '^C for 3 h. Loading of AI2O3 or C03O4 was ca. 10 wt% with respect to monolith. Gold and platinum in this layer were coated by deposition using NH4HCO3 and impregnation method, respectively. The upper layer was coated by immersing the lower layered monolith in well-mixed water slurry composed of a zeolite and colloidal silica (Aldrich Chem.), followed by drying and calcining at 500 °C for 3 h. Hmordenite (HM), Cu-mordenite (CuM), and MCM-41 were respectively used as the upper layer. The weight of coated zeolite was ca. 0.25 g. We denote the double layer catalysts as HM//AU/AI2O3/M (upper//lower layer). Also, HM or CuM was directly coated on bare monolith. Gold particle was observed using TEM (Phillips). Catalytic activity of the mini-size monolith catalysts was measured by using a flow type reactor under atmospheric pressure. Reactant was composed of 5000ppm NO, 2.5 mol% O2, and 5000ppm C3H6 balanced with helium at a flow rate of 60 ml/min. The concentration of NO and NO2 was analyzed with chemiluminescence NOx analyzer (Eco Physics), and C3H6, N2, N2O, and CO was analyzed by gas chromatography (Shimadzu). 3. RESULTS AND DISCUSSION Gold particle on AU/AI2O3/M with only lower layer was examined with TEM. Coated gold

703

particles were uniformly dispersed on AI2O3/M, and its average size was about 5nm. Gold and platinum particles could be highly dispersed on monolith similarly to on powder AI2O3 [3,4]. From SEM image for the double wash-coated catalysts, the zeolite in upper layer of all catalysts was well coated by colloidal silica as a binder. Catalytic activity in NO+C3H6+O2 reaction was investigated over HM/M, CuM/M, and MCM-41/M coated on bare monolith. Their activities were very poor. NO conversion on AU/AI2O3/M and Pt/AbOs/M catalysts with only lower layer was very low because oxidation of C3H6 proceeded preferentially. Fig. 1 shows variation of NO and C3H6 conversion with reaction temperature over AU/AI2O3/M, MCM-41/M, and MCM-4I//AU/AI2O3/M. Combination of AU/AI2O3 with MCM-41 led to increase the activity considerably. Fig. 2 shows variation of NO and C3H6 conversion with reaction temperature over HM//AU/AI2O3/M and CUM//AU/AI2O3/M. Mordenite (especially CuM) in upper layer of AU/AI2O3/M was effective in increasing the activity. The maximum activity on CUM//AU/AI2O3/M was obtained at ca. 350 °C that was lower than 450 °C on AU/AI2O3/M. Fig. 3 shows effect of reaction temperature on conversion respectively over Ft/AhOsM, HM//Pt/Al203/M, and CuM//Pt/Al203/M that in which platinum was used instead of gold as noble metal of lower layer. Conversion of NO or C3H6 was greatly enhanced by coating mordenite (especially CuM) as upper layer. In all experiments, NO conversion began to decrease when C3H6 was nearly consumed in the course of reaction. o o

100

80

52 >

-#^4- • -O-A- O

Au/AI/M(N()) MCM41/M(NO) MCM41//Au/Al/M(N0) Au/Al/M(C3H(,) MCM41/M(C'3H(,) MCM41//Au/A1(C,HJ

80

60

>

#• eB-

HM//AuyAI/M(NO) CuM//Au/Al/M(NO) HM//Au/AI/M(C:3H6) CuM//Au/Al/M(C3H6)

40

X

u

60

o U

c o U vo

-

o =^ 20

20

o

o

0

200

300

400

500

Reaction temperature [^C]

100

200

300

400

500

600

Reaction temperature [^C]

Fig. 1. Effect of reaction temperature on

Fig. 2. Effect of reaction temperature on

conversion over AU/AI2O3/M, MCM-41/M,

conversion over HM//AU/AI2O3/M and

and MCM-4I//AU/AI2O3/M.

CUM//AU/AI2O3/M.

704

The activity of AU/C03O4/M coated with HM or CuM in upper layer was poor because of high activity of AU/C03O4 in oxidation. It was therefore considered that the role of zeolite in upper layer is selective permeability among the reactant components and/or capacity for C3H6 adsorption.

MCM-41

coated

on

AU/AI2O3/M do not adsorb selectively C3H6 to pass through the upper layer, and C3H6 react rapidly with NO2 formed by NO+O2

reaction

in

lower

layer.

Improvement of catalytic performance of Au (or Pt)/Al203/M coated with Cumordenite may be explained by proper

200

300

400

500

600

Reaction temperature PC] Fig. 3. Effect of reaction temperature on conversion over HM//AU/AI2O3/M and CUM//AU/AI2O3/M.

permeability and adsorption capacity. In other words, the upper layer is considered to be a membrane that has substantially different permeability and adsorption capacity to the various reactants and products. 4. CONCLUSIONS Catalytic performance of double wash-coated monolith catalysts was examined for SCR of NOx with C3H6. The double wash-coated catalysts were more active than the catalyst with only zeolite or without the upper layer. Temperature window of CUM//AU/AI2O3/M and CuM//Pt/Al203/M was broadened and shifted towards lower temperature. It was known that two-functional monolith catalyst was effective in controlling NOx in exhaust gas by SCR with hydrocarbons. REFERENCES 1. M. Iwamoto, T. Zengyo, A.M. Hernandez, H. Araki, Appl. Catal. B, 17 (1998) 259. 2. A. Obuchi, I. Kaneko, J. Uchisawa, A. Ohi, A. Ogata, G.R. Bamwenda, S. Kushiyama, Appl. Catal. B., 19(1998) 127. 3. M. Haruta, N. Yamada, T. Kobayashi, and S. Ijima, J. Catal. 115, 301 (1989). 4. H.-G. Ahn and D.-J. Lee, Research Chemical Intermediates, in press.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

705

Copper loaded MCM-41. An alternative catalyst for NO reduction in exhaust gases? - Study of its acidic and redox properties Marcelo S. Batista, Martin Wallau, Rogerio A. A. Melo* and Ernesto A. Urquieta-Gonzalez'" * Departamento de Engenharia Quimica, Universidade Federal de Sao Carlos, Caixa Postal 676, 13565-905, Sao Carlos - SP, Brasil MCM-41 and ZSM-5 were exchanged with copper cations and tested as catalyst for the NO reduction with propane. ZSM-5 contains highly active isolated copper cations on ion exchange sites while inactive CuO was formed in the pores of MCM-41. 1. INTRODUCTION From its discovery, MCM-41 is being widely studied as catalysts and catalyst support [1], as a possible alternative to microporous zeolites in the processing of bulkier molecules or in processes which do not require shape-selectivity. Since the last decade [2], a wide range of transition metal exchanged zeolites are studied as catalysts for NO reduction by hydrocarbons. In this process molecular sieves might be important catalysts for the reduction of harmful nitrogen oxides (NOx) emitted by internal combustion engines. Here we will describe the properties of copper exchanged MCM-41 as catalyst for the reduction of NO with propane and compare the obtained results with that observed for Cu/ZSM-5, in order to judge the potential of MCM-41 type catalysts in environmental applications. 2. EXPERIMENTAL The precursor Na/ZSM-5 and Na/MCM-41 were prepared by conventional hydrothermal synthesis [3,4] and the copper containing catalysts by ion exchange of the parent sodium form at 25 °C using a solution of copper acetate (20 mmol/L; pH = 5.5) and a Cu/Al ratio of 1.3, subsequently drying at 110 °C for 12 h and calcining for 2 h at 520 °C. Also a physical mixture of CuO and Na/ZSM-5 zeolite was prepared, which was calcined in air for 2 h at 520 °C. The samples were denoted as Cu/ZSM(x/y) or Cu/MCM(x/y), x meaning the Si/Al ratio and y the copper content in weight %. The sample prepared by physical mixture is indicated by adding the letter M. The samples were characterised by XRD, nitrogen sorption (BET), UVA^IS, and H2-TPR. The catalytic reduction of NO with propane was developed in a fixed be reactor using 50 mg catalyst mixed with quartz powder (150 mg) activated for 1 h at 520 °C in air flow. A mixture of 0.3 % NO, 0.32 % C^Hg and 1.7 % O2 in helium, a GHSV of 42,000 h"^ and temperatures varying from 100 to 500 °C were used.

Present address: Centre Universitario do Sul de Minas, Faculdade de Engenharia Quimica, Av. Cel. Jose Alves. 37010-540 Varginha - MG, Brasil ^ corresponding author: FAX: +55-16-260-8266. E-mail: [email protected] ' Financial support: CNPq (461444/00-3; 300373/01-5), FAPESP (98/02495-5), FAPEMIG (TEEC- 1241/01).

706

3. RESULTS AND DISCUSSION The elemental composition of the molecular sieves before and after ion-exchange is given in Table 1. It can be seen, that the exchange degree for the former is less than the theoretically expected Cu/Al ratio of 0.5, but more than ten times higher for the latter. This behaviour is probably due to the large hydration sphere of the Cu^^ cations which hampers the adsorption onto the microporous Na/ZSM-5 but not onto the mesoporous Na/MCM-41. The Cu/Al ratio of 5.7 observed for the Cu/MCM-41 suggests the CuO formation on its surface, while for copper exchanged ZSM-5, Cu^^ cations on ion exchange sites seem to be more likely. Table 1. Elemental composition of the studied molecular sieves. Sample Si/Al Exchange x time [h] Cu/Al 0.46 Cu/ZSM(11/4.8) 11 3 X 24 Cu/ZSM(23/0.7) 3x6 0.28 23 0.15 Cu/ZSM(11/1.6)M 11 5.7 2 X 24 23 Cu/MCM(23/9.0)

Cu content [%] 4.8 0.7 1.6 9.0

Table 2. Physical-chemical properties of Na/MCM-41 Table 2 reveals that before and after ion exchange. after copper exchange the pore diameter (dp) and the sample ao [A] dp [A] Sobs, fm^/gl Scai.'^ fm-/g] measured specific surface Na/MCM-41 45 30 910 819 area (Sobs.) of the Cu/MCM(23/9.0) 45 26 682 561 mesoporous MCM-41 are *Scal. = Sgeom./(Vgeom.p) = 8dp/[2«r/ - 2flr;dp)p] significantly decreased. As the unit cell parameter ao remains unchanged, the decrease in the pore diameter is rather due to the deposition of CuO species on the pore walls than to the degradation of the mesopore structure. It can be seen from the calculated specific surface area (Scai), obtained supposing an ideal hexagonally mesoporous structure and using the observed unit cell parameter and pore diameter demonstrated in Table 2, that the decrease in the specific surface area results from the decrease in the pore diameter. Furthermore, one should consider the higher density of CuO (6,5 g/cm"^ for crystalline CuO) in comparison to amorphous (Al,Si)0: (« 2.17 g/cm ) which also influences the specific surface area. The XRD patterns of the Cu/ZSM(11/4.8), Cu/ZSM(11/1.6)M and crystalline CuO are shown in Fig. 1 and for Na/MCM-41 and Cu/MCM(23/9.0) in Fig. 2. Although solid state ion exchange might be occurred in Cu/ZSM(11/1.6)M, the presence of reflections of CuO (Fig. Ic) in the pattern of Cu/ZSM(11/1.6)M (Fig.lb) suggests that in this sample Cu"^ cations on ion exchange sites are unlikely. By XRD no crystalline CuO can be observed in Cu/ZSM( 11/1.6), suggesting that in this catalyst, the copper cations are located on ion exchange sites. Although the high Cu/Al ratio in Cu/MCM(23/9.0) strongly indicates the presence of non ionic copper oxide species, no reflections, which could be attributed to CuO, are observed for this sample. It was outlined by Carniti et al. [5], that small CuO crystals (< 3 nm) may not be detected by XRD. Therefore, elemental analysis and XRD results suggest for Cu/MCM(23/9.0) the presence of finely dispersed CuO. It was discussed above that nitrogen sorption did not indicate degradation of the MCM-41 structure after ion exchange. Therefore the decreased intensity of the XRD reflections of the MCM-41 after ion exchange (Fig. 2b) is rather due to the adsorption of the radiation by the deposited copper species than to structural

707

degradation. This is also suggested by the decreased intensity of the broad peak around 23°(20), typical for amorphous material, which should be increased by structure degradation.

Fig. 1. XRD pattern: (a) Cu/ZSM(11/4.8); Fig. 2. XRD pattern: (a) (b) Cu/ZSM(11/1.6)M; (c) CuO. (b) Cu/MCM(23/9.0).

Na/MCM-41;

215 . 260

A

(A 750 —

(c) ^^-~:rr^^

11A \

\

a>)

_,

(a) 600 >t ( n m )

200

400

600

Temperature [°C]

Fig. 3. UV/VIS spectra: (a) Cu/ZSM(23/0.7); Fig. 4. H2-TPR: (a) Cu/ZSM(11/1.6)M; (b) (b) Cu/ZSM(11/4.8); (c) Cu/MCM(23/9). (c) Cu/ZSM( 11/4.8); Cu/MCM(23/9.0); (d) Cu/ZSM(23/().7). A UV/VIS band around 890 nm confirms the presence of CuO in Cu/MCM(23/9.()) (see Fig. 3c). The UV/VIS bands observed for all copper exchanged samples (Fig. 3) at 215 and 260 nm are due to charge transfer transitions and the broad band around 750 - 800 nm, can be attributed to d-d transition of Cu^^ in octahedral symmetry [6]. The results of the H2-TPR (Fig. 4) reveal that in Cu/MCM(23/9.0), as well as in Cu/ZSM(11/1.6)M, the copper(II) cations are reduced in one step, as it is typical for CuO,

708

thus confirming that this compound is the only copper species present in those samples and that no solid state ion exchange occurred after thermal treatment of the physical mixture of CuO and Na/ZSM-5. Small CuO crystals in Cu/MCM(23/9.0) are indicated by the decreased reduction temperature, which is in accord with the absence of X-ray reflections attributable to CuO mentioned above. The H2-TPR (fig. 4) shows four and three peaks for the Cu/ZSM(11/4.8) and Cu/ZSM(23/0.7), respectively. Following the results reported by T^ ui ^ r» j .• . . ^ r.• • ^civ/i c Wichterlova et al. [71, we , ., , , . ,^ Table 3. Reduction temperatures for Cu cations in ZSM-5. attributed these peaks, as -— T: r-^^—i n TT 1 7 n" it is demonstrated in Cu/ZSM Cup^VCup- CupVCup" CuJVCu.^ Cu.VCu,/' Table 3, to the step wise 01/4.8) 210 °C 400 °C 600 °C 800 »C reduction of two different (23/0-7) 220:C 490:C 60(rC : kinds of copper cations (Cua and Cup). The Cua"^ specie in Cu/ZSM(23/0.7) is probably reduced at temperatures above 900 °C and it was not observed under the used conditions. Wichterlova et al. [7] identified both species as isolated copper cations on ion exchange sites. Cua, co-ordinated to two aluminium atom, possesses a high positive charge density, is difficult to reduce and preferentially observed on ZSM-5 with low copper content. Cu|i, coordinated to one aluminium atom possesses a low positive charge density, is easier to reduce and preferentially observed on ZSM-5 zeolites with high copper loading [7]. The are observed frequencies (TOF) depicted turnover against the reaction -Cu/ZSM(23/0.7) -Q-Cu/ZSM(11/4.8) 16 r-Cu/ZSM(11/1.6)M -•-Cu/MCM(23/9.0) temperature in Fig. 5. It can be seen, that t'^ Cu/ZSM(11/1.6)M and Cu/MCM(23/9.0) l^o are nearly inactive for the reduction of N O . l a On M C M - 4 1 large amounts of copper J e acetate are impregnated during ion ^ 4 exchange which after thermal treatment results in the formation of catalytic so iso 250 350 inactive C u O , as it is also present in T«mp«nrtur« ['C] Cu/ZSM(11/1.6)M. The catalytic activity of the Other two ZSM-5 samples, increases Fig- 5. Turnover frequencies for the continuously from 300 to 500 °C. This reduction of NO (GHSV: 42,(XK) h" ). beahivour indicates that isolated Cu"^ cations are the catalytic active species for the reduction of NO. The presented results might suggest lower activity of Cu"^ in Cu/ZSM(11/4.8). However, over this catalyst, 100 % of the reducing agent propane are already consumed at 450 °C, preventing further increase of NO conversion. Doubling the GHSV to 84,000 h ', similar TOF are observed for Cu/ZSM(23/0.7) and Cu/ZSM( 11/4.8), concluding that the different Cu*^^ species found in these catalysts do not differ in their catalytic activity. REFERENCES 1. F. Schuth, Stud. Surf. Sci. Catal. 135 (2001) 1. 2. E. Kikuchi, K. Yogo, Catal. Today 22 (1994) 73. 3. M.S. Batista, Master Thesis, DEQ/UFSCar, Sao Carlos, Brazil, 1997. 4. R.A.A. Melo, M.V. Giotto, J. Rocha, E.A. Urquieta-Gonzalez, Mater. Res. 2 (1999) 173. 5. P. Carniti, A. Gervasini, V.H. Modica, N. Ravasio, Appl. Catal. B 28 (2000) 175. 6. C. Dossi, A. Fusi, S. Recchia, R. Psaro, G Moretti, Microporous Mesoporous Mater. 30 (1999) 165. 7. B. Wichterlova, J. Dedecek, Z. Sobalik, A. Vondrova, K. Klier, J. Catal. 169 (1997) 194.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. Allrightsreserved

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Studies on synthesis and activity for selective catalytic reduction of NO over Pt supported MCM-48 Jae-seung Yang, Sung-Chul Lee and Suk-Jin Choung School of Environmental and Applied Chemistry, KyungHee University, Suwon, Kyungkido, 449-701 Korea In this study, in order to overcome drawbacks in the zeolite catalytic system for selective catalytic reduction of NOx, platinum metal was dipped in MCM-48. To confirm MCM-48 structure, various characterization methods such as XRD, B.E.T. surface areas, and XPS were carried out. Additionally simulated flue gases mixing and activity test were performed. Similar to specific charactertics of alumina supported platinum metal catalyst, NO reduction activity was improved in low temperature range under 300 °C on Al-MCM-48, and more improved activity was obtained when platinum was dipped over aluminum substituted MCM48 than silica type MCM-48. 10wt% water vapor injected on Pt/Al-MCM-48catalyst, and it was found that the NO reduction activity was not interfered by water vapor contents. When compared with the activity performance of Pt-ZSM-5, still Pt/Al-MCM-48 has superior resistance to water vapor and sulfur contents in flue-gases than Pt-ZSM-5. 1. INTRODUCTION There are several draw-backs such as sudden catalyst deactivation at high temperature range, difficulty in storage and transportation of NH3, and formation of ammonium sulfate in commercial SCR process with NH3 as reducing agent. To overcome those problems, the use of hydrocarbons as an alternative reducatnt has been studied. The S.C.R process-using hydrocarbon as a reducing agent under excess oxygen condition was already proposed at 1988 by Toyota, Japan. Many kinds of catalysts based on zeolite were tested until now (e.g Cu/ZSM-5[1], Co-Pt/ZSM-5[2]). However it also have several problems like diffusion resistance and hydro-thermal stability. To overcome these weaknesses, we tried new SCR catalyst using MCM-48 that is mesoporous molecular sieve. MCM-48 has been known as mesopore molecular sieve, which has a little diffusion resistance and relatively good thermal stability. From our experiment, we could expect that the disadvantages of common zeolite catalyst for S.C.R. would be overcome by using MCM-48.

710

2. EXPERIMENTAL MCM-48 and Al/MCM-48 were synthesized by the conventional procedure given in Ref [3]. Pt/MCM-48 has been prepared by incipient wetness method. The powder X-ray diffraction(XRD) measurement was collected using M18XHF(MAC Science) with Cu Ka radiation at 1.0 deg/min scan speed over a 1.5°PtPdDAY-30>PtPdKIT-60> PtPdMMS-75> PtPdMMS-25. Pt/Pd supported on the DAY-30 and Al-MMS-50 gives a higher selectivity towards the hydrogenation of the second ring of naphthalene, as can be seen from the ratio between the first, ki, and second hydrogenations, k2+k3. These above results well illustrate that application of Pt-Pd/Al-MMS as catalyst for selective aromatic hydrogenation. Table 1 Compositions of products after hydrogenation of naphthalene using various catalysts(wt%) PtPdMMS-75 PtPdMMS-50 PtPdMMS-25 PtPdKIT-60 PtPdDAY-30 Naphthalene 0.02 0.02 0.01 0.01 0.05 Tetralin 3.46 2.68 4.87 3.18 0.42 0.04 0.23 0.19 1.01 Cis-decaline 0.53 0.52 1.65 0.05 1.52 Trans-decaline 1.07 2.46 6.24 Unknown 0.86 0.66 1.57 94.91 93.98 92.33 93.64 n-Hexadecane 91.58 Operating conditions : 300°C, 5.0MPa, 200ppm sulfur, Ig catalyst, lOOg feedstock(5wt% naphthalene, 95wt% n-Hexadecane), Ihour. Table 2 Kinetic rate constants for the hydrogenation of naphthalene (ki), and tetraline to cis-and transdecaline (k2+k3) obtained on the different Pt/Pd-supported catalysts ki(hr-') k,/(k2+k3) Sample Name (k2:+k3 ) X 1 0 V ) PtPdMMS-75 3.56 4.89 7.28 PtPdMMS-50 4.33 12.39 3.52 PtPdMMS-25 0.34 4.75 139.71 4.51 PtPdKIT-60 9.01 5.01 1.20 9.62 PtPdDAY-30 1.25 REFERENCES 1. Van den Berg, J. P., Lucien, J. P., Germaine, G., Thielemans, G. L. B. Fuel Process. Technol. 35, 119(1993). 2. Corma, A., Martinez, A., Martinez-Soria, V. J. Catal. 169, 480(1997). 3. Armor, J. N. Appl. Catal. 112, N21(1994). 4. Heinerman, J., Vogt, E., PCT Int. Pat. Appl. W094126846(1994). 5. Jeong, S. Y, Suh, J. K., Lee, J. M., Kwon, O. Y, J. Colloid and Interface Sci 192, 156(1997) 6. Voegtlin, A. C , Ruth F., Guth J. I., Patarin, J., Huve, L., Microporous Materials, 9, 95(1997). 7. Ryoo, R., Jun, S., Kim, J. M., Kim, M. J. M. Chem. Commun. 2225(1997). 8. Girgis, M. J., Gates, B. C. Ind. Eng. Chem. Res. 30, 2021(1991). 9. Frye, C. G., Weitkamp, A. W. J. Chem. Eng. Data, 14, 372(1969). 10. Lin, S. D., Song, C. Catal. Today 31, 93(1996).

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. All rights reserved

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High loading of short W(Mo)S2 slabs inside the nanotubes of SBA-15. Promotion with Ni(Co) and performance in hydrodesulfurization and hydrogenation. L. Vradman*, M. V. Landau*, M. Herskov^itz*, V. Ezersky^, M. Talianker^, S. Nikitenko^ Y. Koltypin^ A. Gedanken^. * Blechner Center for Industrial Catalysis and Process Development, Chemical Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. ^ Materials Engineering Department, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. ^ Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel. Layered nanoslabs of a M0S2 and WS2 phases with a well-defined hexagonal crystalline structure were inserted into the nanotubular channels of SBA-15 at loadings up to 60 wt%. Sonication of a slurry containing SBA-15 in a W(Mo)(CO)6-sulfur-diphenylmethane solution yielded an amorphous W(Mo)S2 phase inside the mesopores that was transformed into hexagonal crystalline W(Mo)S2 nanoslabs by further sulfidation. The nanoslabs were distributed exclusively inside the mesopores in a uniform manner (HRTEM, local quantitative microanalysis), without blocking the pores (N2-sorption). The Ni(Co) promoters were introduced into the W(Mo)S2/SBA-15 composites by impregnation from an aqueous solution of nickel (cobalt) acetate. The activity (based on the volume of the catalyst loaded into reactor) of the optimized Ni-W-S/SBA-15 catalyst in hydrodesulfurization (HDS) of dibenzothiophene (DBT) and hydrogenation (HYD) of toluene was 1.4 and 7.3 times higher, respectively, than that of a sulfided commercial C0-M0/AI2O3. The HDS activity of Co-MoS/SBA-15 catalyst was 1.2 times higher than that of commercial catalyst. After promotion with Co, the directly introduced M0S2 slabs and M0S2 slabs prepared by sulfidation of Mooxide monolayer spread over SBA-15 displayed similar HDS performance. 1. INTRODUCTION Since the discovery of MCM-41 and related materials [1], many attempts were done to employ them as supports for catalytic phases dispersions [2-5]. However, it was shown that the main problem is to combine the formation of a well-defined nanocrystalline catalytic phase at high loading (>30 wt%) inside the mesopores with high accessibility of the nanocrystals to the reacting molecules (low blocking extent). It was shown previously [6, 7] that ultrasonication of the Mo(CO)6 solution in decalin in presence of MCM-41 support yielded closed packed monolayer of Mo-oxide spread at silica surface without blocking the

722

mesopores. In the present study the ultrasonication method was employed for direct introduction of M0S2 and WS2 crystalline phase into mesopores of SBA-15 material. 2. EXPERIMENTAL The catalysts were prepared by sonication of a slurry containing W(C0)6 or Mo(CO)6, elemental sulfur and SBA-15 in diphenylmethane at 90 °C for 3 h under argon with a highintensity ultrasonic probe [6]. The dried solid was transferred to the tubular reactor and sulfided in situ with a 1.5% dimethyldisulfide (DMDS)-toluene mixture at 320 °C and 5.4 MPa under hydrogen flow for 24 h. The Ni(Co) component was introduced into the W(Mo)S2/SBA-15 composite after sulfidation by impregnation with an aqueous solution of nickel (cobalt) acetate and drying under vacuum at room temperature. Reference Mo-oxide monolayer spread over SBA-15 (M0O3/SBA-I5) and Co-Mo-0/SBA-15 samples were prepared by ultrasonication as described in [6]. After sulfidation, the activity of the catalysts in HDS of DBT and HYD of toluene were measured as described in [6] and [8], respectively. 3. RESULTS AND DISCUSSION The pore volumes and BET surface areas of the different samples are listed in Table 1. The surface areas and pore volumes, normalized to SBA-15 contribution [6], were high for all loaded samples, which is evident for the small pore blocking effect. XRD data showed the amorphous nature of the ultrasonically deposited W(Mo)S2 phases. Treatment with the DMDS-toluene mixture under hydrogen led to the formation of small crystals of hexagonal WS2 and M0S2 phases. Direct evidence for the location the W(Mo)S2 phase nanocrystals within the SBA-15 nanotubes was obtained by HRTEM. The micrographs (Figure 1) clearly show the nanoparticles occluded within the nanotubes at the side view (a, c, e) and at the front view (b, d, f) of the hexagonally ordered nanotubes. Parallel fringes running across the nanoparticlc images have a periodicity of 6.2 A, which corresponds to the well-known distance between the atomic layers packed along the c-axis in the hexagonal WS2 or M0S2. An examination of 15 different 85x85 |am areas of the sample indicated no W(Mo)S2 phase outside the SBA-15 particles. Thus the nanocrystals were located only inside the mesopores of SBA-15 support. Table 1 Texture of the samples derived from N2-sorption. Sample Pore volume cm^/g SBA-15 20wt%WS2/SBA-15 60wt%WS2/SBA-15 32wt%MoS2/SBA-15 50wt%MoS2/SBA-15 42 wt% M0O3/SBA-I5 before sulfidation after sulfidation (47 wt% M0S2)

Normalized

BET surface area m'/g

Normalized

1.0

1.0

800

1.0

0.68

0.85

509

0.80

0.28 0.52

0.70

230 424

0.72

0.76

0.34

0.68

296

0.78 0.74

0.51 0.42

0.88 0.79

394 332

0.78

0.85

723

Fig. 1. HRTEM micrographs of the 60 wt% WS2/SBA-I5 (a, b) and 50 wt% M0S2/SBA-I5 (c, d) both prepared by direct insertion of sulfide, 47 wt% M0S2/SBA-I5 prepared by sulfidation of oxide monolayer (e, f).

724

Table 2 Comparison of catalysts performance in dibenzothiophene HDS and toluene HYD. Catalyst

W(Mo) (wt%)

Ni(Co) (wt%)

Slab length (nm)*

Stacking number*

C0-M0/AI2O3

17.6

4.5

-

Ni-W-S/SBA-15 Co-Mo-S/SBA-15

44.5 30.0

5.7 9.2

Co-Mo-O/SBA-15

28.0

8.9

^HYD

kHDS

TONHDS

-

(h-^) 0.6

(h-^) 38

(h-^) 1.26

3.6

3.2

4.4

54

0.90

3.5 3.4

2.6 2.4

(commercial) 50 1.23 1.29 47 0.74 7.7 5.8 26.9 16.6 5.1 28 Ni-W/Si02 16.4 0.9 26.4 0.78 7.3 2.4 28 Ni-W/AbOs * Average value obtained from HRTEM statistics performed as described elsewhere [9]. Increasing the Ni content in the Ni-W-S/SBA-15 catalyst increased both HDS and HYD activity up to Ni/W ratio of about 0.4 followed by a slight decrease at Ni/W ratio of 0.8. The optimal Co/Mo ratio was found to be close to 0.5 for both M0S2/SBA-I5 samples prepared by direct insertion of M0S2 or sulfidation of Mo-oxide monolayer. Furthermore, after promotion with Co, the HDS activity of both catalysts was similar (Table 2). This is a result of the same texture of the sulfided samples (Table 1) as well as the same structure and dispersion of the M0S2 nanocrystals as follows from HRTEM (Figure 1) and XRD data. Table 2 compares the HDS and HYD performances of the ultrasonically prepared catalysts with a commercial C0-M0/AI2O3 catalyst (KF-752, Akzo Nobel Chemicals) and with optimized Ni-W catalysts deposited on conventional y-AbOs and Si02 supports by impregnation [9]. The HDS activity of the SBA-15 supported Ni-W and Co-Mo catalysts was higher than that of commercial Co-Mo catalyst and Ni-W catalysts supported on conventional supports as a result of higher loading of W(Mo)S2 in the SBA-15. Ni-W-S/SBA-15 catalyst displayed HYD activity close to silica-supported Ni-W due to the lower stacking number [9] and much higher compared with the C0-M0/AI2O3 and Ni-W/Al203. Thus, the high loading Ni-W-S/SBA-15 has excellent potential for application in deep HDS of petroleum feedstocks.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.

U. Ciesla, F. Schuth, Micropor. Mesopor. Mater., 27 (1999) 131. F. Schuth, A. Wingen, J. Sauer, Micropor. Mesopor. Mater., 44-45 (2001) 465. J. Sauer, F. Marlow, B. Spliethoff, F. Schuth, Chem. Mater., 14 (2002) 217. M. Froba, R. Kohn, G. Bouffaud, Chem. Mater., 11 (1999) 2858. A. Ghanbari-Siahkali, A. Philippou, J. Dwyer, M. W. Anderson, Appl. Catal. A, 192 (2000) 57. M. V. Landau, L. Vradman, M. Herskowitz, Y. Koltypin, A. Gedanken, J. Catal., 201 (2001)22. A. Gedanken, X. Tang, Y. Wang, N. Perkas, Yu. Koltypin, M. V. Landau, L. Vradman, M. Herskowitz, Chem. Eur. J., 7 (2001) 4546. L. Vradman, M. V. Landau, M. Herskowitz, Catal. Today, 48 (1999) 41. L. Vradman, M. V. Landau, Catal. Letters, 77 (2001) 47.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B.V. Allrightsreserved

725

Cr-MCM-41 for selective dehydrogenation of lower alkanes with carbon dioxide Ye Wang^, Yoshihiko Ohishi*', Tetsuya Shishido^, Qinghong Zhang^ and Katsuomi Takehira'' ^State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China ''Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-hiroshima 739-8527, Japan Cr-MCM-41 synthesized by both direct hydrothermal (DHT) and template-ion exchange (TIE) methods is studied for dehydrogenation of lower alkanes including C2H6, C3H8 and i-C4Hio with CO2. Both methods lead to Cr species highly dispersed on the wall surface of MCM-41 and exhibited similar catalytic behaviors. Selectivity higher than 90% to each alkene has been achieved, and the presence of CO2 enhances the conversion of alkane. 1. INTRODUCTION Supported chromium oxide is used as catalyst for the dehydrogenation of C3H8 or i-C4Hio to alkene in industry [1]. There exist many reports on the development of catalysts for the oxidative dehydrogenation of lower alkanes with O2 since the highly endothermic dehydrogenation process consumes a large amount of energy [2]. However, the selectivity to alkenes with O2 is generally low due to the formation of COx. Recently, a few studies have reported the coupling of CH4 [3] and the dehydrogenation of C2H6 [4] or CsHg [5] with CO2. On the other hand, MCM-41, a typical mesoporous silica, which possesses a hexagonal array of uniform mesopores and high surface area may result in high concentration of uniformly distributed active sites if an appropriate method is used to introduce the catalytically active sites to MCM-41. Cr-MCM-41 has been synthesized by either the DHT method [6, 7] or an impregnation method [8], and has been applied to the liquid phase oxidation with H2O2 [9] and the oxidative dehydrogenation of CaHg with O2 [8]. However, the selectivity and yield to C3H6 in the latter reaction were very low. In this study, we apply the Cr-MCM-41 synthesized by both the DHT and the TIE methods to the dehydrogenation of lower alkanes with CO2. In the

726

TIE method, the Cr source is introduced to MCM-41 by an ion-exchange between the template cations embraced in the as synthesized MCM-41 and the Cr^^ in the aqueous solution. This method has been used for the synthesis of Mn- [10, 11], V- [12] and Fe-MCM-41 [13], and the metal ions introduced by this method show different coordination environments with those by the DHT method [11-13].

2. EXPERIMENTAL Cr-MCM-41-DHT was prepared by hydrothermal synthesis at I50°C for 48 h using the synthesis gel containing sodium silicate, chromium nitrate and hexadecyltrimethyl- ammonium bromide. Cr-MCM-41-TIE was synthesized by exchanging the template cations embraced in the as synthesized MCM-41 with the Cr^^ ions in aqueous solution at 80°C for 20 h. After hydrothermal synthesis or template ion exchange, the solid was recovered by filtration and then washed with deionized water, followed by drying at 40"C in vacuum and calcination in a flow of air at SSO^'C for 6 h. XRD and N2 adsorption at 77 K were measured to obtain information about the mesoporous structure. The diffuse reflectance UV-Vis and UV-Raman (exciting source 325 nm) spectroscopic studies were performed to characterize the coordination environment of Cr species. The catalytic reactions were carried out with a conventional fixed-bed flow reactor using lower alkanc and CO2 as rcactants. No reaction occurred without catalyst.

3. RESULTS AND DISCUSSION 3.L Properties of catalysts XRD measurements showed that the diffraction lines of (100), (110), (200) and (210) at 20 degrees of ca. 2.2, 3.6, 4.3 and 5.7" indexed to the hexagonal regularity of MCM-41 were observed for the Cr-MCM-41 samples by both the DHT and TIE methods, suggesting that the hexagonal array of mesopores of MCM-41 was sustained after the introduction of Cr with both methods. The peak intensity was not significantly changed with increasing Cr content to 1.7 wt% (Si/Cr= 50), but a further increase in Cr content to 3.4 wt% (Si/Cr= 25) decreased the peak intensity. The porous properties obtained from N2 adsorption measurements at 77 K are shown in Table 1. Narrow pore size distribution around 2.5-3.0 nm was observed for all the samples. The surface area and pore volume gradually decreased with an increase in Cr content up to 1.7 wt%, and became remarkably low for the sample with Si/Cr ratio of 25, indicating the decrease in structural regularity at high Cr content. The change of color of all the Cr-MCM-41 samples from pale green to pale yellow after

727

Table 1 Properties of Cr-MCM-41 synthesized with both the DHT and the TIE methods Sample^ MCM-41 Cr-MCM-41-DHT( 100) Cr-MCM-41-DHT(50) Cr-MCM-41-DHT(25) Cr-MCM-41-TIE( 100) Cr-MCM-41-TIE(50) Cr-MCM-41-TIE(25)

(m'g')

Pore vol. (mlg')

Pore dia. (nm)

1025 878 780 629 961 885 624

0.89 0.79 0.70 0.36 0.92 0.83 0.85

2.7 2.7 2.7 2.7 2.7 2.7 2.5

SBET

Color of sampl e As synthesized White Pale green Pale green Pale green Pale green Pale green Pale green

Calcined White Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow Pale yellow

^The number in parenthesis are the Si/Cr atomic ratio. calcination suggests that Cr^^ in the as-synthesized sample was transformed to Cr^^ during calcination. This indicates that most of the Cr species after calcination exist as chromate species on the wall surface of MCM-41 but not as Cr^^ in the framework of MCM-41. Fig.l shows the diffuse reflectance UV-Vis spectra of Cr-MCM-41 synthesized by the two methods. UV bands at 280 and 370 nm were mainly observed for both kinds of samples. These bands could be assigned to O-Cr(VI) charge transfers of a chromate species. A weak shoulder around 440 nm was also observed particularly for the TIE samples, probably suggesting the existence of polychromate species. An intense band at 980 c m ' ascribed to the dehydrated monochromate was observed in the UV-Raman spectra for the DHT samples recorded at 200"C in N2 atmosphere, whereas bands around 1000-1200 cm' were observed in addition to that at 980 cm"' for the TIE samples. Thus, only monochromate species exist in the DHT samples, while the TIE samples contain monochromate and polychromate species.

300 400 500 600 700 800 Wavelength /nm Fig. 1. UV-Vis spectra of Cr-MCM-41. (a) DHT, Si/Cr=50;(b) DHT, Si/Cr=25; (c)TIE, Si/Cr=lOO; (d) TIE, Si/Cr=50;

1 2 3 Cr content /wt% Fig. 2. Dehydrogenation of C3H8 over Cr-MCM-41 by DHT(a,c)and TIE(b,d) methods. W=0.4g, P(C3H8)=12.2 kPa, P(C02)=68.9 kPa, T=823 K, F=50ml/mi n.

728 3.2. Catalytic properties of Cr-MCM-41 Fig. 2 shows the effect of Cr content on the catalytic properties of CsHg dehydrogenation with CO2 over both DHT and TIE samples. Both series of catalysts showed similar performances; C3H8 conversion increased almost linearly with an increase in Cr content and C3H6 selectivity was kept at 92-95%. This result indicates that the monochromate and the polychromate species exhibit similar catalytic effect on the dehydrogenation of C3H8 with CO2. Table 2 Table 2 shows that although CjHg could be converted to C3H6 in the absence of CO2, but C3H8 conversion increases remarkably with the partial pressure of CO2, suggesting that CO2 plays a cmcial role in the dehydrogenation of C3H8 to C3H6. The dehydrogenation of C2H6 and i-C4Hio occurred also effectively on the same catalyst with CO2 as shown in ja5ie 2.

Dehydrogenation of lower alkanes over Cr-MCM-41 P(C02)

Alkane

Alkene

/kPa

conv.%

select./%

9.8

93.2

Catalyst

Akane

TIE (50)

C3H8

0

TIE (50)

C3H8

68.9

17.4

95.5

DHT(50)

C3H8

0

9.4

90.0

DHT (50)

C3H8

68.9

17.0

93.1

DHT (50)

C2H6

68.9

11.5

99.7

DHT (50)

i-C4Hio

68.9

18.3

90.4

^= 823 K, W= 0.4 g, P(alkane)= 12.2 kPa, F= 50 ml/min.

REFERENCES 1. B.M. Weckhuysen, I. E. Wachs and R. A. Schoonheydt, Chem. Rev., 96 (1996) 3327. 2. G. Centi and F. Trifiro, Appl. Catal. A: General, 143 (1996) 3. 3. Y. Wang, Y. Takahashi and Y Ohtsuka, J. Catal., 186 (1999) 160. 4. K. Nakagawa, C. Kajita, K. Okumura, N. Ikenaga, M. Nishitani-Gamo, T. Ando, T. Kobayashi and T. Suzuki, J. Catal., 203 (2001) 87. 5.1. Takehara and M. Saito, Chem. Lett., (1996) 973. 6. N. Ulagappan and C.N.R. Rao, Chem. Commun., (1996) 1047. 7. Z. Zhu, Z. Chang and Larry Kevan, J. Phys. Chem. B, 103 (1999) 2680. 8. J. Santamaria-Gonzalez, J. Merida-Robles, M. Alcantara-Rodriguez, P. Maireles-Torres, E. Rodriguez-Castellon and A. Jimenez-Lopez, Catal. Lett., 64 (2000) 209. 9. A. Sakthivel, S.E. Dapurkar and R Selvam, Catal. Lett., 77 (2001) 155. 10. M. Yonemitsu, Y. Tanaka and M. Iwamoto, Chem. Mater., 9 (1997) 2679. 11. Q. Zhang, Y Wang, S. Itsuki, T. Shishido and K. Takehira, J. Mol. Catal. A: Chem., (2002) in press. 12. Q. Zhang, Y Wang, Y Ohishi, T. Shishido and K. Takehira, J. Catal., 202 (2001) 308. 13. Y Wang, Q. Zhang, T. Shishido and K. Takehira, J. Catal., 209 (2002) 186.

Studies in Surface Science and Catalysis 146 Park et al (Editors) © 2003 Elsevier Science B. V. All rights reserved

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Methane reforming on molybdenum carbide on Al-FSM-16 Masatoshi Nagai, Toshihiro Nishibayashi, and Shinzo Omi Graduate School of Bio-applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan Methane reforming on the carbided 12% Mo/Al-FSM-16 catalysts with Si/Al ratios of 30, 50, and 80 was performed at 973 K under atmospheric pressure. The characterization was carried out by N2 adsorption, XRD, and ^^Al MAS NMR. AI-FSM-16 with a Si/Al ratio of 30 exhibited an implantation of aluminum into the Si02 structure of FSM-16. The 873 K-carbided 12% Mo/Al-FSM-16 catalyst was more active than the oxidized catalyst and the catalysts carbided at a higher carbiding temperature. The largest amounts of hydrogen and benzene were formed using the catalyst with the Si/Al ratio of 80. P-M02C on the catalyst was formed during the carbiding and methane reforming. 1. INTRODUCTION Recently, methane reforming has been extensively studied for effectively utilizing natural gas resources. Mo/ZSM-5 catalysts are very active in methane reforming. The implantation of aluminum into mesopore FSM-16 is expected to be used as a catalyst support by generating acid sites [1]. Mesoporous materials having a high surface area and heat tolerance promoted the reaction with a fast molecular diffusion in the mesopores. In this study, 12% M0O3/AI-FSM-I6 is carbided by a temperature-programmed reaction in a stream of 20% CH4/H2 [2], and analyzed by N2 adsorption, NMR, and XRD. The effects of the Si/Al ratio and preparation procedure on the structure were studied. The catalytic activity is determined during methane reforming using the 12% Mo/Al-FSM-16 catalysts with three different Si/Al ratios. 2. EXPERIMENTAL Sodium silicate and sodium aluminate (Si/Al=30, 50, and 80) were added to a small amount of water and the mixture was stirred at 333 K for 3 h. The solution was dried at 353 K (and 413 K) to yield a sodium aluminosilicate glass which was then calcined at 973 K for 3 h. The layered sodium silicates containing aluminum at three Si/Al ratios were dispersed in an aqueous solution of [Ci6H33N(CH3)3]Cl and stirred at 343 K. The solid products were separated from the solutions by suction filtration (or decantation), and dried at 353 K (Method II). The sample (Method I) was dried at 353 K and subsequently calcined at 813 K in air . The 12 wt% M0O3/AI-FSM-16 catalyst was prepared by an incipient wetness method after Al-FSM-16 (Method I or II) was added to an aqueous solution of (NH4)6Mo7024-4H20. The resulting product was dried at 353 K, calcined at 573 K, and carbided by the temperature-programmed reaction in 20% CH4/H2 at a flow rate of 66.7 ml min' from 573 to 873 (-1073) K at a rate of 1 K min''. The catalyst was maintained at this temperature for 3 h. The BET surface area of the samples was measured at 77 K using a Beckman-Coulter adsorption apparatus. The structure of the samples before and after pretreatment and carburidation was measured by XRD analysis. Diffraction patterns were determined using a RAD-II (Rigaku Co.) equipped with Cu-Ka radiation with slits of (ds) 1/2°, (rs) 0.3 mm, and (ss) 1/2°. The solid state MAS NMR spectra were measured on a JEOL JNM-EX400 spectrometer. ^^Al MAS NMR spectra were recorded at 6 kHz spinning. Methane reforming was carried out using a continuous-flow quartz reactor (0.03 g) in streams of methane and helium with a 15 mlmin"' rate at 973 K. The reaction mixtures were analyzed using a Balzer quadrupole mass spectrometer.

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3. RESULTS AND DISCUSSION

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3.1. Preparation of Mo/Al-FSM-16 The N2 isotherms of the Al-FSM-16, 12% Mo/Al-FSM-16 (I), and 12% Mo/Al-FSM-16 (II) are shown in Figure 1. The predominant increase in the adsorptions of Al-FSM-16 and 12% Mo/Al-FSM-16 (II) were observed at P/Po = 0.25 ~ 0.4, which was Y^^ characteristic of the N2 adsorption in mesopores, while these characteristic 0.2 1.0 peaks were not observed for the 12% Mo/Al-FSM-16 (I) after molybdenum P/Po loading. This result indicated that the Fig. 1. N2 Adsorption/desorption is oth erms loading of molybdenum destroyed the of(#)Al-FSM-16, (x)Mo/Al-FSM-16(l), and structure of 12% Mo/Al-FSM-16 (I), while the structure of the 12% ( • ) Mo/Al-FSM-16(11). Volume(gas)l.56Xl0 3 Mo/Al-FSM-16 (II) was uniformly maintained even after calcination at 813 K. The BET surface areas of the Al-FSM-16 are 715, 799, and 1275 m^ Table 1 g"' for the Si/Al ratios of 30, 50, and 80, BET surface area, spacing dioo, and unit cell respectively, showing that the surface dimension ao (100) of each sample area increased with the increasing Si/Al ratio. After drying the sample (II), the XRD Surface area Sample surface area after loading the dioo/nm ao/nm /m^g' molybdenum species decreased from 715 to 514 m^ g ' in Table 1, but 1444 FSM-16 3.73 4.31 maintained the structure of the support. 3.84 4.43 Al-FSM-16 715 The surface area of the sample (I) l2%Mo/Al-FSM--16(1) 269 decreased much more than that of the 12%Mo/Al-FSM-16(1) 247 sample (II). The decrease in surface carbide at 973 K area of the sample (I) was due to l2%Mo/Al-FSM-16(II) 514 3.73 4.31 plugging of the molybdenum oxides in the micropores of Al-FSM-16 and destroying the FSM-16 structure. The XRD patterns of FSM-16, Al-FSM-16, and 12% Mo/Al-FSM-16 (I, II) are shown in Figure 2. The (100), (110), and (200) phases were observed for FSM-16, but only the (100) phase was seen for Al-FSM-16. The Al-FSM-16 exhibited the implantation of the aluminum atom into the SiOz structure of FSM-16 by having an irregular structure. The surface area of FSM-16 was two times greater than that of Al-FSM-16, supporting the result of the structure regularity by XRD. Thus, the structural regularity was likely to affect the surface area. The 12% Mo/Al-FSM-16 (II) was prepared by calcination after loading the molybdenum compound which resulted in retaining the structure of the (100) phase. Since carbization of the sample (II) slightly decreased the surface area, the structure of the support was not changed before and after the carbization. In Table 1, FSM-16 and Mo/Al-FSM-16 (II) had the same unit cell dimensions as the value (4.31 nm) in the literature [3]. This result showed that Al-FSM-16 and 12% Mo/Al-FSM-16 maintained the mesoporous structure of the 16 carbon chains. The pore sizes of Al-FSM-16 were 2.8 and 4.2 nm. The former pore size was due to FSM-16 and the latter due to the formation of bridging of the aluminum with silica in the preparation of Al-FSM-16. The 12% Mo/Al-FSM-16 (II) contained micropores of 2.8 nm more than Al-FSM-16. The XRD pattern of the impregnated 12% Mo/Al-FSM-16 is shown in Figure 2e. Al-FSM-16 had a peak of Si02. 12% Mo/Al-FSM-16 carbided at 973 K had the peak of P-M02C but no peaks for the oxide form. P-M02C had agglomerated outside the pores

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of the support in flowing 20% CH4/H2 at high carbiding temperatures. This result showed that molybdenum oxides on the surface of the Al-FSM-16 were loaded more than that inside of the micropores. 3.2. Properties and structure The ^^Al MAS NMR spectra for the Al-FSM-16 with the three Si/Al ratios are shown in Figure 3. The Al-FSM-16 sample (filtration) had a peak at 50 ppm, which is ascribed to four coordinated sites, while the Al-FSM-16 sample (decantation) had the peak at 8 ppm for the six coordinated sites. The formation of six-coordinated alumina is due to the more basic solution of sodium silicate and sodium aluminate at a pH of about 12.4. These compounds were precipitated and changed to the six-coordinated compounds containing aluminum sources. The ratio of the six-coordinated octahedral to four-coordinated tetrahedral aluminum increased with the decreasing aluminum content (high Si/Al ratio). This result suggested that the implantation of aluminum in the Si02 body required a certain amount of aluminum in the feed solution. The uniform implanting of aluminum into the SiOi structure needs an excess amount of aluminum. For Al-FSM-16 with Si/Al = 80, the large ratio of the hexahedral aluminum to the tetrahedral aluminum was observed more than those with Si/Al = 30 and 50 from the decantation preparation. The decantation cannot completely remove the dissolved feed (sodium aluminate). The XRD analysis confirmed maintaining of the hexagonal structure after the molybdenum oxides were loaded and subsequently carbided in a stream of 20%CH4/H2. Thel2%Mo/Al-FSM-16 (II) with good hexagonal structure exhibited a higher surface area than 12% Mo/Al-FSM-16(I).

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