Advances in Chemical Conversions for Mitigating Carbon Dioxide (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 114 ADVANCES IN CHEMICAL CONVERSIONS FOR MITIGATING CARBON DIOXIDE

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Studies in Surface Science and Catalysis Advisory

Editors:

B. Delmon and J.T. Yates

Vol, 114

ADVANCES IN CHEMICAL CONVERSIONS FOR MITIGATING CARBON DIOXIDE Proceedings of the Fourth International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997

Editors T, Inui

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan

M.Anpo

Department of Applied Chemistry, Faculty of Engineering, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 593, Japan

K. Izui

Division of App/ied Biosciences, Graduate School of Agricu/ture, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan

S. Yanagida

Department of Materia/ and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565, Japan

T, Yamaguchi

Research Institute of lnnovation Technology forthe Earth (RITE), Kizu-cho, Soraku-gun, Kyoto 619-02, Japan

1998 ELSEVIER Amsterdam

m Lausanne--

New York--

Oxford m Shannon m Singapore m Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

Library of Congress Cataloging in Publication Data. A catalog record from the Library of Congress has been applied for.

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CONTENTS Preface

xv

Organization

xviii

Sponsoring

xix

Special Lecture International Energy Agency action on climate change issues M.A. Preville and H.J. Koch (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

Plenary Lecture Japan's basic strategy concerning countermeasures to mitigate climate change T. Namiki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

Research and development on new synthetic routes for basic chemicals by catalytic hydrogenation of CO 2 H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 New approaches in CO 2 reduction A. Fujishima, D.A. Tryk and T.N. Rao (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31

Development of electrocatalysts for carbon dioxide reduction using polydentate ligands to probe structure-activity relationships D.L. DuBois (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Carbon dioxide and microalgae N. Kurano, T. Sasaki and S. Miyachi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55

Perspectives of carbon dioxide utilization in the synthesis of chemicals, coupling chemistry with biotechnology. M. Aresta (Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Keynote Lecture Scope of'studies on CO 2 mitigation K. Yamada (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

77

Hydrogenation of CO 2 toward methanol Influence of the catalysts composition and preparation on the catalytic behavior R. Kieffer and L. Udron (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Photochemical carbon dioxide reduction with metal complexes: Differences between cobalt and nickel macrocycles E. Fujita, B.S. Brunschwig, D. Cabelli, M.W. Renner, L.R. Furenlid, T. Ogata, Y. Wada and S. Yanagida (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

vi

Electrochemical reduction of CO 2 at metallic electrodes J. Augustynski, P. Kedzierzawski and B. Jermann (Switzerland) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

107

Super-RuBisCO: Improvement of photosynthetic performances of plants A. Y okota (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

117

Organometallic reactions with CO 2 - Catalyst design and mechanisms E. Dinjus (Germany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

127

Oral Presentation Catalytic fixation of CO2: CO 2 purity and H 2 supply J.N. Armor (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

141

Reduction of carbon dioxide to graphite carbon via methane by catalytic fixation with membrane reactor H. Nishiguchi, A. Fukunaga, Y. Miyashita, T. Ishihara and Y. Takita (Japan) . . . . . . . . . . . . . 147 Catalytic reaction of CO 2 with C2H 4 on supported Pt-Sn bimetallic catalysts J. Llorca, P. Ramfrez de la Piscina, J. Sales and N. Homs (Spain) . . . . . . . . . . . . . . . . . . . . . . . . . . .

153

Initial transient rates and selectivities of Fischer-Tropsch synthesis with CO 2 as carbon source H. Schulz, G. Schaub, M. Claeys, T. Riedel and S. Walter (Germany) . . . . . . . . . . . . . . . . . . . . . 159 Palladium-catalyzed carboxylation of allyl stannanes and carboxylative coupling of allyl stannanes and allyl halides M. Shi, R. Franks and K.N. Nicholas (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Interaction between CO 2 and propylene on Ru-Co/A12Oa-catalysts of cluster type G.D. Zakumbaeva, L.B. Shapovalova and I.A. Shlygina (Kazakhstan) . . . . . . . . . . . . . . . . . . . . . . . 171 Photocatalytic reduction of CO 2 with H 20 on titanium oxides anchored within zeolites M. Anpo, H. Yamashita, K. Ikeue, Y. Fujii, Y. Ichihashi (Japan), S.G. Zhang, D.R. Park (Korea), S. Ehara (Japan), S.-E. Park, J.-S. Chang and J.W. Yoo (Korea) . . . . . . . . . . . . . . . . . 177 Photocatalytic reduction and fixation of CO 2 on cadmium sulfide nanocrystallites S. Yanagida, Y. Wada, K. Murakoshi, H. Fujiwara, T. Sakata and H. Moil (Japan) ......

183

Abiotic photosynthesis of amino acids, nucleic acid bases and organic acids from CO 2 dissolved in aqueous solution S. Kihara, K. Maeda, T. Hori and T. Fujinaga (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Aspects of CO 2 utilization toward the goal of emission reduction in Romania L. Dragos, N. Scarlat, M. Neacsu and C. Flueraru (Romania) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

195

CO 2 capture and utilization for enhanced oil recovery (EOR) and underground storage A case study in JiLin Oil Field, China G. Yun, D. Liu, T. Wu, J. Wu, X. Ji and Z. Li (China) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

201

Electrocatalytic reduction of CO 2 to worthier compounds on a functional dual-film electrode with a solar cell as the energy source K. Ogura, M. Yamada, M. Nakayama and N. Endo (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207

vii

Incorporation of CO 2 into organic perfluoroalkyl derivatives by electrochemical methods E. Chiozza, M. Desigaud, J. Greiner and E. Du~ach (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213

Molecular tailoring of organometallic polymers for efficient catalytic CO 2 reduction: mode of formation of the active species R. Ziessel (France) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Electroreduction of CO 2 using Cu/Zn oxides loaded gas diffusion electrodes S. Ikeda, S. Shiozaki, J. Susuki, K. Ito and H. Noda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225

Recent slow rate of CO 2 increase and vegetation activity K. Kawahira and Y. Maeda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

231

Production of PHA (poly hydroxyalkanoate) by genetically engineered marine cyanobacterium H. Miyasaka, H. Nakano, H. Akiyama, S. Kanai and M. Hirano (Japan) ................... 237 Cellulose as a biological sink of CO 2 T. Hayashi, Y. Ihara, T. Nakai, T. Takeda and R. Tominaga (Japan) . . . . . . . . . . . . . . . . . . . . . . . . .

243

Possibility of molecular protection of photosynthesis under salinity stress F. Sato, Y. Arata, K. Matsuguma, M. Shiga, Y. Kanda, K. Ifuku, K. Ishikawa and T. Yoshida (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Organometallic CO 2 complexes in supercritical CO2: a time-resolved infrared study M.W. George, D.C. Grills, X-Z. Sun and M. Poliakoff (UK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

255

Methanation of carbon dioxide on catalysts derived from amorphous Ni-Zr-rare earth element alloys H. Habazaki, T. Yoshida, M. Yamasaki, M. Komori, K. S himamura, E. Akiyama, A. Kawashima and K. Hashimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 Development of high performance Raney copper-based catalysts for methanol synthesis from CO 2 and H 2 J. Toyir, M. Saito, I. Yamauchi, S. Luo, J. Wu, I. Takahara and M. Takeuchi (Japan).. 267 Global carbon-recycling energy delivery system for CO 2 mitigation (I) Carbon one-time recycle system towards carbon multi-recycle system H. Sano, Y. Tamaura, H. Amano and M. Tsuji (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Oil extraction by highly pressurized CO 2 produced in zero emission power plants Ph. Mathieu, E. Iantovski and V. Kushnirov (Belgium) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

279

Global carbon-recycling energy delivery system for CO 2 mitigation (III) Fossil/solar energy hybridization system for utilization of carbon as solar energy carrier Y. Tamaura, M. Tsuji, H. Amano and H. Sano (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Review of measures to mitigate carbon dioxide emissions in the Slovak Republic and modes of utilization A. Moncmanov~i (Slovak Republic) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Proposal of a new high-efficient gas turbine power generation system utilizing waste heat from factories P.S. Pak, H. Ueda and Y. Suzuki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297

viii

Acetogenesis and the primary structure of the NADP-dependent formate dehydrogenase of Clostridium thermoaceticum, a tungsten-selenium-iron protein D.J. Gollin, X.-L. Li, S.-M. Liu and L.G. Ljungdahl (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 Biochemical CO 2 fixation by mimicking zinc(II) complex for active site of carbonic anhydrase K. Ichikawa, K. Nakata, M. Ibrahim and S. Kawabata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 The biological CO 2 fixation using Chlorella sp. with high capability in fixing CO 2 M. Murakami, F. Yamada, T. Nishide, T. Muranaka, N. Yamaguchi and Y. Takimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

315

Photobiological production of hydrogen gas Y. Asada (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

321

Hydrocarbon synthesis from CO 2 over composite catalysts Y. Souma, M. Fujiwara, R. Kieffer, H. Ando and Q. Xu (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

327

CO 2 for petrochemicals feedstock. Conversion to synthesis gas on metal supported catalysts. P. Gronchi, P. Centola and R. Del Rosso (Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Iron catalyzed CO 2 hydrogenation to liquid hydrocarbons R.A. Fiato, E. Iglesia, G.W. Rice and S.L. Soled (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

339

Support effects of the promoted and unpromoted iron catalysts in CO 2 hydrogenation K.-W. Jun, S.-J. Lee, H. Kim, M.-J. Choi and K.-W. Lee (Korea) . . . . . . . . . . . . . . . . . . . . . . . . .

345

Methanol synthesis from COJH 2 over Pd promoted Cu/ZnO/AI203 catalysts M. Sahibzada, I.S. Metcalfe and D. Chadwick (UK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

351

A 50 kg/day class test plant for methanol synthesis from CO 2 and H 2 K. Ushikoshi, K. Moil, T. Watanabe, M. Takeuchi and M. Saito (Japan) ..................

357

Poster Presentation

Comparison of CO 2 sources for the synthesis of renewable methanol M. Specht, A. Bandi, M. Elser and F. Staiss (Germany) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

363

Characteristics and economics assessment of power generation systems utilizing solar energy in various regions T. Kosugi, P.S. Pak and Y. Suzuki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Solar/chemical energy hybridization via Boudouard reaction H. Ono, M. Kawabe, M. Nezuka, M. Tsuji and Y. Tamaura (Japan) . . . . . . . . . . . . . . . . . . . . . . . .

371

Development of active and stable nickel-magnesia solid solution catalysts for CO 2 reforming of methane K. Tomishige, Y. Chen, X. Li, K. Yokoyama, Y. Sone, O. Yamazaki and K. Fujimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 Global carbon-recycling energy delivery system for CO 2 mitigation (II) Two possible ways for introducing solar energy M. Tsuji, H. Amano, Y. Tamaura, H. Sano and S. Maezawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . 379

ix

Efficient thermochemical cycle for CO 2 reduction with coal using a reactive redox system of ferrite T. Kodama, A. Aoki, S. Miura and Y. Kitayama (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Oxidative dehydrogenation of ethylbenzene with carbon dioxide over ZSM-5-supported iron oxide catalysts J.-S. Chang, S.-E. Park, W.Y. Kim (Korea), M. Anpo and H. Yamashita (Japan) ......... 387 Nature of CO 2 adsorbed on MgO surface at low temperatures T. Ito, J. Isawa, H. Kishimoto, H. Kobayashi and K. Toi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . .

391

CO 2 behavior on supported KNiCa catalyst in the carbon dioxide reforming of methane S.-E. Park, J.-S. Chang, H.-S. Roh (Korea), M. Anpo and H. Yamashita (Japan) ......... 395 Utilization of CO 2 in the reforming of natural gas on carbon supported ruthenium catalysts. Influence of MgO addition P. Ferreira-Apaficio, B. Bachiller-Baeza, A. Guerrero-Ruiz and I. Rodrfguez-Ramos (Spain) .......................................................................................................... 99 Catalytic conversion of carbon dioxide to polymer blends via cyclic carbonates D.W. Park, J.Y. Moon, J.G. Yang, S.M. Jung, J.K. Lee and C.S. Ha (Korea) ...........

403

The selective synthesis of lower olefins (C 2 - C 4 ) by the CO 2 hydrogenation over Iron catalysts promoted with Potassium and supported on ion exchanged(H, K)Zeolite-Y H. Kim, D.-H. Choi, S.-S. Nam, M.-J. Choi and K.-W. Lee (Korea) . . . . . . . . . . . . . . . . . . . . . . . 407 Hydrogenation of carbon dioxide over rhodium catalyst supported on silica M. Kishida, K. Onoue, S. Tashiro, H. Nagata and K. Wakabayashi (Japan) ...............

411

Dehydrogenation of ethylbenzene over iron oxide-based catalyst in the presence of carbon dioxide N. Mimura, I. Takahara, M. Saito, T. Hattori, K. Ohkuma and M. Ando (Japan) ......... 415 Promoting effects of CO 2 on dehydrogenation of propane over a SiO2-supported Cr203 catalyst I. Takahara, W.-C. Chang, N. Mimum and M. Saito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Hydrogenation of carbon dioxide over Fe-Cu-Na/zeolite composite catalysts Q. Xu, D. He, M. Fujiwara, M. Tanaka, Y. Matsumura, Y. Souma, H. Ando and H. Yamanaka (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 Fe promoted Cu-based catalysts for hydrogenation of CO 2 N. Nomura, T. Tagawa and S. Goto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

427

The effect of rhodium precursor on ethanol synthesis by catalytic hydrogenation of carbon dioxide over silica supported rhodium catalysts H. Kusama, K. Okabe, K. Sayama and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 431 Selective formation of iso-butane from carbon dioxide and hydrogen over composite catalysts Y. Tan, M. Fujiwara, H. Ando, Q. Xu and Y. Souma (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435 Vanadium-catalyzed acetic acid synthesis from methane and carbon dioxide Y. Taniguchi, T. Hayashida, T. Kitamura and Y. Fujiwara (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . .

439

Fuels and petrochemicals from CO2 via Fischer-Tropsch synthesis - steady state catalyst activity and selectivity T. Riedel, S. Walter, M. Claeys, H. Schulz and G. Schaub (Germany) ..................... 443 Effective conversion of CO2 to methanol and dimethyl ether over hybrid catalysts K.-W. Jun, M.-H. Jung, K.S. Rama Rao, M.-J. Choi and K.-W. Lee (Korea) ............

447

Characterization of CO2 methanation catalysts prepared from amorphous Ni-Zr and Ni-Zr-rare earth element alloys M. Yamasaki, H. Habazaki, T. Yoshida, M. Komori, K. Shimamura, E. Akiyama, A. Kawashima K. Asami and K. Hashimoto (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451 Hydrogenation of CO 2 over Rh ion exchanged zeolite catalysts K.K. Bando, K. Soga, K. Kunimori, N. Ichikuni, K. Asakura, K. Okabe, H. Kusama, K. Sayama and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 Interconversion of Ru-CO and Ru-rll-CO2 through reversible oxide transfer reaction K. Tsuge, K. Tanaka and H. Nakajima (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

459

Physiological properties of phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase from Rhodopseudomonas sp. No.7 T. Fujii, M. Sadaie, M. Saijou, T. Nagano, T. Suzuki, M. Ohtani and H. Shinoyama (Japan) .......................................................................................................... 63 Production of alkane and alkene from CO z by a petroleum-degrading bacterium strain HD-1 M. Morikawa, T. Iwasa, K. Nagahisa, S. Yanagida and T. Imanaka (Japan) ............... 467 Cultivation of cyanobacterium in various types of photobioreactors for biological CO 2 fixation I.S. Suh, C.B. Park, J.-K. Han and S.B. Lee (Korea) ........................................ 471 Application of photosynthetic bacteria for porphyrin production H. Yamagata, R. Matoba, T. Fujii and H. Yukawa (Japan) ....................................

475

Utilization of micro-algae for building materials after CO2 fixation T. Otsuki, M. Yamashita, T. Hirotsu, H. Kabeya and R. Kitagawa (Japan) ................

479

Photosynthetic COz fixation performance by a helical tubular photobioreactor incorporating Chlorella sp. under outdoor culture conditions Y. Watanabe, M. Morita and H. Saiki (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483 Carboxylation reaction with carbon dioxide. Mechanistec studies on the Kolbe-Schmitt reaction. Y. Kosugi and K. Takahashi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487 From carbon dioxide to C 2 organic molecules mediated by Aresta's nickel carbon dioxide complex J.K. Gong, C.A. Wright, M. Thorn, K. McCauley, J.W. McGill, A. Sutterer, S.M. Hinze and R.B. Prince (USA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 491 Methanol homologation using carbon dioxide catalyzed by ruthenium-cobalt bimetallic complex system K. Tominaga, Y. Sasaki, T. Watanabe and M. Saito (Japan) .................................. 495

Atmospheric CO 2 fixation by dinuclear Ni(II) complex, [TRANi(II)(/~-OH)2Ni(II)TPA](C104) 2 (TPA=Tris(pyridymethyl)amine) M. Ito, T. Ishihara and Y. Takita (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499 Carbon dioxide fixation with lanthanoid complex S. Inoue, H. Sugimoto, N. Ishida and T. Shima (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

503

A study on methanol synthesis through CO 2 hydrogenation over copper-based catalysts S.-K. Ihm, Y.-K. Park, J.-K. Jeon, K.-C. Park and D.-K. Lee (Korea) ....................

505

Mechanistic studies of methanol synthesis from C O 2 / H 2 o n C u / Z n O / S i O 2 catalyst D.-K. Lee, D.-S. Kim, C.M. Yoo, C.-S. Lee and I.-C. Cho (Korea) . . . . . . . . . . . . . . . . . . . . . . .

509

Highly effective synthesis of ethanol from CO z on Fe, Cu-based novel catalysts T. Yamamoto and T. Inui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

513

A study for the durability of catalysts in ethanol synthesis by hydrogenation of carbon dioxide K. Higuchi, Y. Haneda, K. Tabata, Y. Nakahara and M. Takagawa (Japan) ............... 517 Development of stable catalysts for liquid-phase methanol synthesis from CO 2 and H 2 H. Mabuse, T. Watanabe and M. Saito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

521

Ethanol synthesis from carbon dioxide and hydrogen M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa and H. Arakawa (Japan) ..............

525

New preparation method of Cu/ZnO catalysts for methanol synthesis from carbon dioxide hydrogenation by mechanical alloying H. Fukui, M. Kobayashi, T. Yamaguchi, H. Kusama, K. Sayama, K. Okabe and H. Arakawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529 Promoting effect of calcium addition to P d / S i O 2 catalysts in CO 2 hydrogenation to methanol A.L. Bonivardi, D.L. Chiavassa and M.A. Baltan~ (Argentina) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 533 Direct synthesis of gasoline from carbon dioxide via methanol as the intermediate H. Hara, T. Takeguchi and T. Inui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

537

Comparison of CO 2 hydrogenation in a catalytic reactor and in a dielecric-barrier discharge A. Bill, B. Eliasson, U. Kogelschatz and L.-M. Zhou (Switzerland) . . . . . . . . . . . . . . . . . . . . . . . . . 541 Methanol synthesis from carbon dioxide on CuO-ZnO-Al203 catalysts M. Hirano, T. Akano, T. Imai and K. Kuroda (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

545

Optimization of preparation conditions and improvement of stability of Cu/ZnO-based multicomponent catalysts for methanol synthesis from CO 2 and H 2 S. Luo, J. Wu, J. Toyir, M. Saito, M. Takeuchi and T. Watanabe (Japan) .................. 549 Effect of solvents on photocatalytic reduction of carbon dioxide using semiconductor photocatalysts T. Torimoto, B.-J. Liu and H. Yoneyama (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553

xii

Applications of electrospray mass spectrometry and high performance liquid chromatography in the elucidation of photocatalytic CO2-fixation reactions H. Hori, J. Ishiham, M. Ishizuka, K. Koike, K. Takeuchi, T. Ibusuki and O. Ishitani (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557 Photocatalytic reduction of CO 2 with HjO on Ti/Si binary oxide catalysts prepared by the solgel method H. Yamashita, S. Kawasaki, M. Takeuchi, Y. Fujii, Y. Ichihashi, Y. Suzuki (Japan), S.-E. Park, J.-S. Chang, J.W. Yoo (Korea) and M. Anpo (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561 Photoelectrochemical reduction of CO2 at a metal-particle modified p-Si electrode in nonaqueous solutions Y. Nakamura, R. Hinogami, S. Yae and Y. Nakato (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565 Infrared spectroscopic study of CO, and CO reduction at metal electrodes O. Koga, T. Matsuo, H. Yamazaki and Y. Hori (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

569

Influence of anions on the production efficiency in pulsed electroreduction of CO z on metal and alloy electrodes R. Shiratsuchi, S. Ishimaru and G. Nogami (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573 Electrochemical reduction of CO 2 by using metal supported gas diffusion electrode under high pressure K. Hara, N. Sonoyama and T. Sakata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Electrochemical reduction of carbon dioxide at a platinum electrode in acetonitrile-water mixtures Y. Tomita and Y. Hori (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581 Electrochemical reduction of CO 2 in micropores T. Yamamoto, D.A. Tryk, K. Hashimoto, A. Fujishima and M. Okawa (Japan) ...........

585

Photoelectrochemical reduction of highly concentrated CO 2 in methanol solution K. Hirota, D.A. Tryk, K. Hashimoto, M. Okawa and A. Fujishima (Japan) . . . . . . . . . . . . . . . .

589

Studies on CO 2 fixation in PNSB : Utilization of waste as the additional source of carbon for CO 2 fixation by PNSB V. Brenner, M. Inui, N. Nonoura, K. Momma and H. Yukawa (Japan) . . . . . . . . . . . . . . . . . . . . 593 Studies on CO 2 fixation in PNSB : Analysis of CO 2 metabolism in purple non-sulfur bacteria M. Inui, J.H. Roh, K. Momma and H. Yukawa (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597 Crystallization and preliminary X-ray studies of phosphoenolpyruvate carboxylase from Escherichia Coli H. Matsumura, T. Nagata, T. Inoue, Y. Nagara, T. Yoshinaga, K. Izui and Y. Kai (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601 Molecular characterization of recombinant phosphoenolpyruvate carboxylase from an extreme thermophile T. Nakamura and K. Izui (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 605

xiii

Revertant of no-active RuBisCO tobacco mutant, Sp25, obtained by chloroplast transformation method using microprojectile bombardment K. Tomizawa, T. Shikanai, A. Shimoide, C.H. Foyer and A. Yokota (Japan) .............. 609 Reductive TCA cycle in an aerobic bacterium, Hydrogenobacter thermophilus strain TK-6 M. Ishii, K. Yoon, Y. Ueda, T. Ochiai, N. Yun, S. Takishita, T. Kodama and Y. Igarashi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613 Carbon dioxide fixation and biomass production with blue-green algae Spirulina platensis S. Hirata and M. Hayashitani (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617 Chitosan-calcium carbonate composites: Biomimetic mineralization of aqueous carbonate ions into chitosan-calcium alginate hydrogels S. Hirano, K. Yamamoto, H. Inui, K.I. Draget, K.M. Varum and O. Smidsrod (Japan).. 621 Tolerance of a green alga, Scenedesmus komarekii, to environmental extremes N. Hanagata, R. Matsukawa, M. Chihara and I. Karube (Japan) ............................. Over-expressed

effect of

carbonic

anhydrase

on

CO2 fixation

in

625

cyanobacterium,

Synechococcus sp. PCC7942 M. Murakami, N. Yamaguchi, T. Nishide, T. Muranaka and Y. Takimoto (Japan) ........

629

CO2 removal by a bioreactor with photosynthetic algae using solar-collecting and light-diffusing optical devices M. Nanba and M. Kawata (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633 Isolation and characterization of a green alga Neochloris sp. for CO 2 fixation M. Kawata, M. Nanba, R. Matsukawa, M. Chihara and I. Karube (Japan) .................

637

Antioxidant activity of CO2 fixing microalgae R. Matsukawa, Y. Wada, N. Tan, N. Sakai, M. Chihara and I. Karube (Japan) ...........

641

Screening of polysaccharide-producing microalgae Y. Shishido, M. Kawata, R. Matsukawa, M. Chihara and I. Karube (Japan) ...............

645

A marine microalga utilization for a paper: Semi-batch cultivation of Tetraselrnis sp. Tt- 1 by a tubular bioreactor and the partial substitution of whole kenaf pulp for a paper Y. Samejima, A. Hirano, K. Hon-Nami, S. Kunito, K. Masuda, M. Hasuike, Y. Tsuyu and Y. Ogushi (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649 Conversion of CO2 into cellulose by gene manipulation of microalgae: Cloning of cellulose synthase genes from Acetobacter xylinum Y. Umeda, A. Hirano, K. Hon-Nami, S. Kunito, H. Akiyama, T. Onizuka, M. Ikeuchi and Y. Inoue (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 653 Ethanol production from carbon dioxide by fermentative microalgae S. Hirayama, R. Ueda, Y. Ogushi, A. Hirano, Y. Samejima, K. Hon-Nami and S. Kunito (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 657 Production of polyethylene glycol and polyoxyalkylenealkylphenyl ether microspheres using supercritical carbon dioxide K. Mishima, K. Matsuyama, Y. Taruta, M. Ezawa, Y. Ito and M. Nagatani (Japan) ...... 661

xiv Carbon dioxide separation from nitrogen using Y-type zeolite membranes S. Morooka, T. Kuroda and K. Kusakabe (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

665

Comparative study of various amines for the reversible absorption capacity of carbon dioxide Y. Nagao, A. Hayakawa, H. Suzuki, S. Mitsuoka, T. Iwaki, T. Mimura and T. Suda (Japan) .......................................................................................................... 669 Kinetic analysis of COz recovery from flue gas by an ecotechnological system M. Tabata, T. Chohji and E. Hirai (Japan) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673 Formation of peroxocarbonates from I_,3Rh(Oj)Cl and ~Ni(COj): a unique reaction mechanism with carbon dioxide insertion into the O-O bond M. Aresta, E. Quaranta, I. Tommasi, J. Mascetti, M. Tranquille and M. Borowiak (Italy). 677 Keyword Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

681

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

687

XV

Preface The global environmental problems, especially the global warming caused by the accelerative accumulation of carbon dioxide in the atmosphere, are the most crucial for human beings. The population of the world is now approaching 6 billion, and is still increasing. Developments in communication systems and transportation tools have made circulation of information, technologies and materials easy, and are bringing the rapid economic growth, particularly in the area of east and southeast Asian countries.

The natural wish of human

being to live their comfortable lives directly connects the increase in consumption of fossil fuels, and consequently, carbon dioxide emission and environmentally-burden materials increase, resulting in the acceleration of evident climate change on a global scale. According to the most recent news, the increase in carbon dioxide emission in last year was the largest among the past seven years, and the total amount of carbon dioxide emission from all over the world attained to 6.5 billion tons. Furthermore, we cannot overlook the report which appeared recently in a distinguished scientific journal, "Nature" that the floor-area of the iceberg in the South Pole has already decreased by 25% in the past five decades. The anxiety has resulted in the operation of COP (United Nation's Framework Convention of Climate Change), and the third conference on COP will be held in the beginning of December 1997 at Kyoto International Conference Hall.

Naturally, such

scenarios planned on governmental or political basis would be focused on strategic topics such as the restriction on the amount of consumption of fossil fuels through the tax for the use of carbon resources and encouraging the temperate habits in the life style of people. On the other hand, proposal and discussion on progressive technological methods to mitigate and utilize carbon dioxide, which scientists and engineers must owe, are seems to be not so clear in the topics of COP. Therefore, the scientists and engineers, who had a strong interest and wished to contribute to solve the carbon dioxide problem

from technological aspects, gathered and

discussed the countermeasures in the conference. First conference on Fixation of CO2 by Chemical Ways was held in 1991 at Nagoya Japan by late Professor Kaname Ito as the chairman, who was the president of Research Association for Chemical Fixation of CO2, which had been established in The Chemical Society of Japan. Second conference was held at Bali, Italy in 1993, which was organized by Professor Michele Aresta as the chairman.

The name of the conference was changed to the present

xvi

Intemational Conference of Carbon Dioxide Utilization (ICCDU).

On that occasion,

Professor Aresta proposed the logo of the I CCDU, and he entered upon the duties of the permanent secretary. Third conference was held at Oklahoma University of USA in 1995, which was organized by Professor Kenneth Nicholas, and the venue of the 4th Conference was decided there. The logo of the 4th Conference was designed by myself with the concept to maintain the beautiful and comfortable globe. The Organizing Committee consists of Chairpersons, Emeritus chairpersons, Advisory Board, and Executive Board.

Five members among them participated in the program

committee, who contributed to publish the proceedings from Elsevier. I have to express my grateful thanks to the invaluable contribution of sponsors listed in the following page to the conference, who made the smooth operation and progress of the conference possible.

It is noteworthy that the Commemorative Association for the Japan

World Exposition (1970) donated the significant fund to the conference, which could realize the financial assistance, even partly for the participants who come from overseas countries. A wide variety of academic society also gave the strong support and sympathy to our activities. Scientific program consists of seven sessions which cover most of possibilities of chemical conversion of carbon dioxide.

As for carbon dioxide exhausts from large

generation sources, it should be concentrated and separated, followed by the rapid conversion into valuable compounds.

Once carbon dioxide diffuses into the atmosphere, it is preferable

to be incorporated into the plant through the biochemical methods.

For six sessions, one

plenary lecture and one key note lecture were done. One special lecture was delivered by Dr. Mary Preville of IEA. The unique and important schedule was the panel discussion in this session, and summary of each session was conducted by distinguished persons; moreover, from more than ten countries, the representative speakers presented the status and perspectives of chemical approaches of CO2 reduction in each country. Technical tour to visit Research Institute of Innovation Technology for the Earth (RITE) was planned on one day moming during the period of the conference, and made a strong impression on the visitors. There are the first world demonstration plant for methanol synthesis from carbon dioxide and other related technologies such as membrane devices for separation of CO2 from the flue gas.

The visitors could see also biochemical studies

conceming CO2 fixation, and studies on total systems for CO2 mitigation.

On the aftemoon

of the same day, they visited to Todai-ji temple located in the world famous ancient capital of

xvii Japan, Nara, where the largest bronze statue of Budda among the world is situated with its accommodation of the largest wooden building.

Nobody could avoid a great and strong

impression and admiration for the high grade technologies achieved by the people in 1300 years ago and their most reverential mind. This conference had been watched with keen interest, and was open to the informal~ion media with considering the public character of this conference, and unexpected number of information media came to get good hope in the future of humankind. Over 260 active participants from 21 countries joined to this conference.

On behalf

of the Organizing Committee, I thank the authors for their high quality presentations and for contributing to this volume.

I also thank the referees for their conscientiousness which

ensured the high scientific quality of this volume. Thanks are also extended to the members of the Program Committee whose invaluable effort make the publication of the proceedings possible. Finally, I believe that the conference afforded a great success and gave a brilliant hope to the people, and the conference became an effective core for developing the strong and everlasting roles to keep our dearest globe in the pleasant place. September, 1997

Tomoyuki Inui Chairman of the ICCDU IV Editor in Chief

xviii

ORGANIZATION Organizing Committee Chairman: Tomoyuki Inui (Kyoto University,)

Vice-Chairman: Tsutomu Yamaguchi (RITE)

Emeritus Chairperson: Kaname Ito (Late) Jiro Kondo (RITE)

Advisory Board:

Executive Board:

T. Hattori (Tokyo Gas Co. Ltd.) Y. Hori (Chiba Univ.) K. Izui (Kyoto Univ.) K. Kanai (RITE) T. Kodama (The Univ. of Tokyo) R. Kurane (National Res. Inst. Biosci., Human Tech.) E. Nakanishi (Osaka Gas Co. Ltd.) H. Nakano (The Kansai Electric Power Co. Inc.) M. Misono (The Univ. of Tokyo) T. Sakata (Tokyo Inst. of Tech.) K. Tanaka (Inst. for Molecular Sci.) K. Urano (Yokohama National Univ.) H. Yoneyama (Osaka Univ.)

M. Anpo (Univ. Osaka Pref.) A. Fujishima (The Univ. of Tokyo) S. Ikeda (Nagoya Inst. of Tech.) M. Ikenouchi (RITE) M. Hattori (RITE) M. Saito (RITE) Y. Souma (Osaka National Res. Inst.) Y. Takita (Oita Univ.) Y.Tamaura (Tokyo Inst. of Tech.) S. Yanagida (Osaka Univ.) M. Y anai (RITE) H. Yukawa (RITE)

International Advisory Board (Scientific Committee) Michele Aresta (Italy), Permanent Secretary of I CCDU Donald Darensbourg (USA) Martin M. Halmann (Israel) Shohei Inoue (Japan) Tomoyuki Inui(Japan) Aaron Kaplan (Israel) Roger Kieffer (France) Lars G. Ljungdahl (USA) Kenneth M. Nicholas (USA) Giuseppe Silvestri (Italy) Ralph Thauer (Germany) Yasuyuki Yamada (Japan)

xix

SPONSORING

The Organizing Committee gratefully acknowledges financial support from:

Research Association of CO2 Chemical Fixation of the Chemical Society of Japan Research Institute of Innovation Technology for the Earth (RITE) The Commemorative Association for the Japan World Exposition (1970) New Energy and Industrial Technology Development Organization (NEDO) The Federation of Electric Power Companies Japan Automobile Manufacturers Association, Inc. The Japan Iron and Steel Federation The Japan Gas Association Petroleum Association of Japan The Japan Society of Industrial Machinery Manufacturers The Japan Chemical Industry Association Engineering Advancement Association of Japan The Cement Association of Japan

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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.

International Energy A g e n c y action on climate change issues Mary A. Preville and Hans Jorgen Koch International Energy Agency 75739 Paris Cedex 15, France 1. INTRODUCTION

The fourth International Conference on Carbon Dioxide Utilization (ICCDU IV) is very timely. Governments from around the world will meet in Kyoto in December 1997 to agree on a new Protocol or Another Legal Instrument to the United Nations Framework Convention on Climate Change (UNFCCC). The conference provides a very valuable opportunity to take stock of activities in a potentially important area of climate change responses - CO2 utilization. As in any large societal development, technology will be a key ingredient in addressing the effects of climate change. Negotiators and policy makers in the climate change arena are increasingly recognizing that technologies, including technologies for utilizing CO2, will be a key response strategy for addressing climate change. But technology does not develop in a vacuum. Technology is developed and deployed when the people, the knowledge, the financing - and most importantly- the determination to succeed,exist. The organisers of the conference should be congratulated for bringing together representatives from both developed and developing countries to work together and exchange ideas and experiences. All economies need to be involved in finding solutions to mitigate climate change. Analysis of global emission trends of greenhouse gases shows that the stabilisation of concentrations in the atmosphere cannot be achieved by the industrialised countries alone. Wider participation and efforts to limit emissions and enhance sinks will be required. Co-operation among countries to share experiences will, within the limits imposed by differing national circumstances, bring benefits ranging from replication of the actions or technologies that have proved successful in one country, to actions undertaken within a coordinated policy that might otherwise not be possible at the national level. This paper highlights some of the key activities that the International Energy Agency (lEA) undertakes in the climate change area, and highlights some of the factors which the lEA believes will set the framework for future action in the energy sector to respond to the climate change challenge. The major focus of the paper, however, is on the technology dimension to climate change. Although rising in profile, this is an area where much greater attention is needed. Not only do we need to better understand the

role of technology in our economies but we also need to develop strategies for strengthening innovation and understanding its impacts. 2. THE INTERNATIONAL ENERGY AGENCY

Society's increasing sensitivity to environmental problems is reflected in the evolving nature of the International Energy Agency's mandate. Initially, the lEA was established, in 1974, to facilitate co-operation among industrialised countries in their shared quest for energy security. Over the years, the lEA has expanded its scope to be a forum in which 24 OECD Member countries come together to cooperate on the whole range of energy policy and technology issues. The IEA's objectives have taken on three dimensions. We often refer to them as the 'Three-Es': Energy Security, Economic Growth, and Environmental Sustainability. All three objectives simultaneously guide the IEA's activities and programmes. In the area of environmental sustainability, and in particular in the area of climate change, the lEA undertakes analysis and provides advice on a wide array of issues related to the energy dimension of climate change. This includes activities ranging from modelling energy and carbon dioxide projections to policy and economic analyses which inform the decision-making process for sound energy policies and programmes, to research and development for climate friendly technologies. Energy efficiency indicators, transportation trends and competitiveness issues related to the electricity sector are a few examples of analyses undertaken by the lEA. The development of methodologies in support of the concepts of Activities Implemented Jointly (AIJ), tradeable emission permits and voluntary agreements are also undertaken. The IEA's action on climate change issues is closely linked with the implementation of the UNFCCC. The lEA is one of only a handful of international organizations, and the only non-United Nations body, to be granted the status of Partner Organisation in the United Nations Climate Convention process. The lEA has been asked to play that role in order to bring expertise on energy matters to the Convention. This is done in close co-operation with other organisations. 3. THE ENERGY DIMENSION OF CLIMATE CHANGE

The sustainable development challenge of today will need to be addressed by a multitude of actions, and instruments to mitigate climate change and promote sound development have to be flexible. Lifestyles have changed, and standards of living have increased, with the increased use of energy. However, its use entails inherent environmental effects. The challenge is to limit those effects, in a flexible way, to levels and to forms that present and future generations can live with. To meet the challenge, a deeper grasp of the complexities of the energy sector must be developed, and this understanding must be passed on to climate change policy-makers and negotiators.

In this regard, a key analytical study entitled the lEA Statement on the Energy Dimension of Climate Change (EDCC) has recently been submitted as an official document to the UNFCCC. The lEA Statement describes the principal driving forces, key underlying factors and prevailing boundary conditions relevant to the energy aspects of the global greenhouse gas problem. In particular, the lEA Statement points out that: national circumstances and energy systems vary widely among both lEA and non-lEA countries; different dynamics of the main energy services (mobility, electricity and heat) involve different infrastructures, different technologies and capital stocks, and different behaviour among end-users, and different trends in relation to GDP growth over time; the extent and age of existing energy infrastructures constrain the rate at which cost-effective reductions in energy-related emissions can be achieved in the near-term; enhanced use of the best available technologies could help reduce energy requirements and associated emissions within the possibilities of current infrastructure, but barriers to the adoption of currently available and costeffective technologies will have to be overcome; every time energy-using capital stock and infrastructure is installed anywhere in the world, there is a unique opportunity to adopt climate-friendly technologies; and early involvement of all actors concerned will help foster innovation and change in long-term trends and infrastructure and achieve emission reductions at minimum cost. The promotion of sustainable economic development of course requires providing and expanding energy services while simultaneously reducing their energy and CO2 content. Market dynamics, the rigidity of infrastructures and attitudes, and the rate of capital stock turnover define the basic parameters of viable response options, pointing both to limitations and significant opportunities. Before examining new energy technological opportunities, it is useful to briefly review the nature of the energy sector and its contributions to greenhouse gas emissions. 4. THE ENERGY AND EMISSIONS OUTLOOK

The IEA's World Energy Outlook is an annual publication offering its perspective on Global Energy Demand and Energy Supply issues. The projections in the 1996 edition illustrate how energy use will grow in the absence of effective policies to alter established patterns of energy production and use. The bulk of energy demand and

CO2 emission growth will occur in the developing world. Emissions of the countries belonging to the Organisation of Economic Co-operation and Development (OECD) are projected to grow by 26 percent between 1990 and 2010, while those in the rest of the world (excluding the FSU) are projected to grow by 126 percent in the same time period, unless effective policy measures are introduced. The most dramatic increase in energy demand is projected to occur in the developing Asian countries, which are expected to account for between 44 and 55 percent of the increase in total world energy demand. Similarly, developing Asian countries are expected to account for more than 40 percent of the incremental demand for oil, and between 36 and 52 percent of incremental electricity demand. The message of the IEA's World Energy Outlook for energy-related CO2 emissions is clearly not a happy one. In fact, whatever assumptions were made about economic growth, energy prices and energy efficiency, emissions projections in the lEA Outlook rises substantially. In the simplest terms, this message confirms that the world's economy is highly geared to the use of fossil fuels. Nevertheless, the work underlying the lEA model does suggest there is room for policies that could result in faster than expected efficiency improvements, which would reduce the rate of growth in emissions. But, even theoretical "no regrets" policies, when viewed against the projections of the IEA's World Energy Outlook, will not be adequate to stabilise, much less reduce, energy-related CO2 emissions in the OECD by the year 2010. If more is to be done to reduce greenhouse gas emissions, costs will clearly have to be incurred. At the same time, there must be realistic expectations which do not disregard the inherent rigidities in the energy system. Realistic opportunities which are properly defined will vary widely from country to country, even within the OECD. 5. TECHNOLOGY OPTIONS

In the short to medium-term this means faster deployment of existing energy technologies which emit fewer greenhouse gases, and of those which use energy more efficiently. Given current energy prices, additional policy measures will, however, be needed to enhance the market opportunities for many of these options. But, there are limits to potential energy efficiency improvements and many fuel switching options; more innovative solutions will be needed in the longer term if the current goals are to be met. The attractiveness of technology options will vary among countries. As already stated, existing infrastructures constrain the rate at which cost-effective reductions in energy-related emissions can be achieved in the near term. This infrastructure extends beyond strictly energy-related infrastructures, such as power plants.

Infrastructures such as buildings, road networks, and energy grids embody the technology choices and the patterns of energy use of an immense web of economic agents. For the most part, past technology choices did not systematically take into account their energy-use implications, still less their environmental implications. But, these past choices will shape lifestyles and patterns of energy use for decades. They will also influence the selection of longer-term technologies. Long lead times for introducing new technologies and lengthy lifetimes of plant and equipment will influence those choices. For these reasons, CO2 capture and disposal remains an attractive option for the medium to longer term if the current pattern of energy supplies continues, based on the existing fossil fuel infrastructure and reliability of associated technologies. Longer-term technological options will need considerable work to bring them into the market place as commercial options. This applies not only to CO2 capture and disposal but also to options involving, for example, hydrogen fuel systems, renewable energy technologies and biotechnologies. In the case of CO2 capture and disposal, the challenges are significant. Most of the technology options for capture and disposal involve very considerable cost increases in energy supplies. As energy producers and consumers seek to reduce greenhouse gas emissions from their activities, market focus will favour the most cost-competitive solutions. While capture and disposal has the potential to play a significant role, costs will need to come down. The environmental impacts will also need to be fully understood. This will involve assessment of Iocalised impacts as well as the regional and global impacts, particularly in the case of the innovative, ocean sequestration options. The results of these studies must also be communicated widely, so that properly informed decisions can be taken. Also, technology reliability and maintainability will need to be established - as with other emerging technologies. This means the full range of technology risks must be clearly understood as early as possible. The more significant risks must be reduced and the information communicated to potential customers for those technologies. Considerable motivation will be required to mobilise and sustain the substantial efforts needed in these areas. Experiences to date with CO2 capture, utilisation and disposal are a good illustration of the efforts required to link R&D efforts and bring them to fruition. At the national level, various groups have been studying aspects of CO2 capture for some time. Nevertheless, it took considerable effort to create the networks, stimulate interest and bring together the different players working on the various chemical, physical and biological approaches to CO2 capture. The same can be said about other groups working on disposal and utilisation options. The ICCDU conference series is fine example of these efforts to link R&D efforts. Two other examples of international collaboration in the field of CO2 capture, disposal

and utilisation include the lEA Greenhouse Gas R&D Programme and the Climate Technology Initiative (CTI). The lEA Greenhouse Gas R&D Programme has the objective to evaluate (on a full fuel cycle basis) the technical and economic feasibility and environmental impacts of technologies for the abatement, control, utilisation and disposal of carbon dioxide and other greenhouse gases derived from fossil fuel use. One of the goals of the programme is to identify targets for R&D in this field and to facilitate practical activities. The programme is also encouraging more broadly based use of a systems approach to assessment of greenhouse gas mitigation options. The lEA Greenhouse Gas R&D Programme works within a structure the lEA calls

ImplementingAgreementswhich enable countries to work together co-operatively on

projects. Implementing Agreements provides the legal contractual mechanism for establishing the objectives of the projects and the rights and commitments of its participants. There are presently forty active lEA Implementing Agreements, each with between three and twenty participating countries. The lEA Greenhouse Gas R&D Programme has fourteen lEA Member countries participating, plus the European Union, Poland and Venezuela. Industry is also actively represented in the programme. lEA Implementing Agreements are not restricted to lEA Member countries. The co-ordinated research expenditures of lEA Implementing Agreement exceeds $100 million (US dollars) per annum. Thus, the Implementing Agreement mechanism provides very substantial leveraging of domestic expenditures. Another technology collaboration vehicle which has been established by lEA Member countries is the Climate Technology Initiative (CTI). The CTI is a voluntary initiative to foster and strengthen the development and deployment of climate-friendly technologies. Its aim is to share the experience and benefits of national and international measures, practices and processes in all parts of the energy chain. It builds upon existing efforts. The CTI was launched in 1995 at the first Conference of the Parties to the UNFCCC. The CTI responds to the spirit of several UNFCCC provisions, but avoids negotiation. Its key activities are undertaken by cross-sectoral government representatives, in consultation and collaboration with developing countries, the private sector, and others, as appropriate. The CTI has undertaken to report back to the UNFCCC Conference of the Parties on the progress and development of its activities, which it will do in Kyoto in December. The CTI has both a short and a long-term focus. The shorter-term focus is on enhancing markets for currently available technologies. The longer-term focus is on stimulating the research, development and diffusion of new and improved technologies that can contribute to meeting the UNFCCC goal of stabilising concentrations of greenhouse gases in the atmosphere.

More specifically, the CTI aims to: promote awareness of technology-related activities already underway to assist with responses to climate change concerns; identify and share expertise and experiences between countries already working on particular relevant topics, sharing also with countries having limited expertise in particular areas; identify gaps in national and multilateral technology programmes which could be addressed in order to strengthen climate response strategies; strengthen and undertake practical collaboration activities between countries to make technology responses to climate change concerns more effective. One of the five CTI Task Forces set up to implement the objectives of the CTI focuses on greenhouse gas capture and disposal. This includes the role of capture and disposal options as part of a hydrogen fuel chain based on fossil fuels. This task includes an assessment of the feasibility of developing longer-term technologies in these fields and ways to strengthen relevant basic and applied research. While much of the core work on CO= capture and disposal is undertaken at the national level, the international collaboration vehicles stated above play an important role in enhancing technology progress and in improving awareness of the CO2 capture, disposal and utilisation options. Continuing international collaboration will be needed to bring these technologies successfully to the market. The benefits of international R&D collaboration are numerous, but will not be elaborated in this paper. The lEA, through its programme of collaborative research, can facilitate international cooperation as it offers a flexible mechanism for interested players to pool their scarce R&D resources for mutual benefit, and for the long-term benefit of industry and consumers alike. 6. CONCLUSION

Although the climate change issue has been actively discussed in government and industry circles now for almost a decade, discussion is only now turning actively to market and sectoral realities. Society is likely to remain highly dependent on fossil fuels for power, heat and transport; existing infrastructures will shape climate change responses for a considerable time. Innovative solutions will be needed. Technology will play an important role in achieving longer term greenhouse gas emission reductions and enhancement of sinks. Technologies to capture and dispose of greenhouse gases offer huge potential for reducing greenhouse gas emissions. While this option should not be viewed as the sole solution to greenhouse gas concerns, it does provide an option to help meet these climate change challenges.

Expanded and intensified efforts will be needed to speed up the otherwise lengthy technology development and deployment process and so realise those potential technology contributions. Enhanced international co-operation involving all of the players can speed up that process by: 9 sharing the costs of research, development and demonstration; and 9 sharing the lessons learned and so avoiding costly replication of unproductive R&D. The process also has the ability to increase awareness by decision-makers in government and industry about the potential of CO2 capture and disposal technology, and its reliability. Technology has always been a key driver for societal development, and it will be a key driver for providing options for reducing global greenhouse gas emissions. Reducing the stress on our common environment will not happen by accident. It will only happen if the necessary intellectual and financial resources are devoted to developing and deploying new and improved technologies, and this within an appropriate policy framework. Moreover, it will only happen if there is the determination and motivation to succeed. Leadership will need to come from government and industry as well as from the research community. International organisations, such as the lEA, can assist in the process.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.

Japan's Basic Strategy C o n c e r n i n g C o u n t e r m e a s u r e s to Mitigate C l i m a t e Change Tom Namiki Director-General for Environmental Protection and Industrial Location Bureau Ministry of International Trade and Industry

1. INTRODUCTION I would like to start by congratulating Professor Inui, Chairman of the Organising Committee, and all those concerned on opening the Fourth International Conference on CO2 Utilization, and to express my deep admiration for all the hard work everyone has done to prepare for the conference. I would also like to express my welcome to all the participants who have traveled to Japan from around the world to be here today. The theme of my presentation is "Japan's Basic Strategy Concerning Countermeasures to Mitigate Climate Change."

2. CURRENT STATUS OF CLIMATE CHANGE (GLOBAL WARMING) ISSUES Although a good deal of research has been carried out on global warming since the late 1980's, there is an increased sense of urgency among meteorologists and environmental researchers around the world, since there is undeniable evidence that the level of carbon dioxide in the atmosphere is continuing to rise. The Intergovernmental Panel on Climate Change (IPCC) was established to handle scientific information related to climate change as a joint proposal by the kIN Environment Program (UNEP) and the World Meteorological Organization (WMO) in 1988. The IPCC has the function of gathering and assessing scientific information and publishing reports on the results of their efforts. IPCC's first assessment report was issued in 1990 in which they announced that, although there was some tmcertainty about the problem of global warming, there was increasing international awareness that a precautional approach was necessary because it would be too late to act when real damage became apparent. As a result, the UN Framework Convention on Climate Change was adopted at the UNCED Earth Summit held at Rio de Janeiro in 1992.

l0 IPCC's second assessment report was issued in December 1995, and I will briefly summarize them. Their contents can be divided into the next five items; (1) According to the most realistic forecasts, by the end of the next century, global temperatures will increase by 2 ~ and the ocean level will rise 50 cm. (2) Climatic effects in the future depend on the volumes of greenhouse gases released from now on. Although there is a great deal of uncertainty regarding predictions, the cost of countermeasures and for damage repair may increase if countermeasures are delayed. (3) To stabilize the emission of greenhouse gases, it is necessary to limit to a fairly low level the human-induced emissions of such gases. Technologies (espeoially their promotion and transfer) have great potentiality to contribute to the decrease of such emissions. They are still practical from the financial viewpoint. (4) The more countries that carry out such measures, the greater the overall effect. (5) It is important to overcome difficulties in implementing effective policies on climate change issues, to promote the taking of measures in each country suitable to each situation, to further broaden scientific knowledge, and to develop, introduce, and transfer technologies. 3. THE BASIC POLICIES BEHIND THE JAPANESE COUNTERMEASURES FOR THE CLIMATE CHANGE

In 1990, Japan decided on an action program to arrest global warming. At the same time, by working within the UN Framework Convention on Climate Change adopted in 1992 and other treaties, we have been taking positive steps to address this issue. Considering the emergency directives for measures based on the new .scientific information given in the IPCC's second series of assessment reports and the need to conclude studies concerning specific measures for 2000 and beyond by COP3, MITI organized the Global Environmental Committee of the Industrial Structure Council from April 1996 to March 1997. The working group submitted a report in March 1997 which laid out a strategy concerning the direction in which to address the climate change issue and specific policy deployments. I would like to present my opinion on this matter, which draw on this IPCC' s report.

11 3. 1 The f i r s t

issue relates

to the future

directions

of measures

t o be t a k e n (1) Comprehensive measures encompassing environmental protection, economic growth, and energy demand/supply stabilization.

Since the major generator of CO2, which causes climate change, is the use of energy, this problem is closely related to economic growth and energy demand/supply stabilization. To continue with sustainable development worldwide, it is necessary to plan comprehensive tripartite measures for three tasks: suppression of CO2 emissions, maintenance of economic growth, and energy demand/supply stabilization, thereby realizing a socioeconomic world in harmony with the environment. In addition, while taking these measures, it is necessary to take those issues deeply related to climate change, such as population problems in developing nations, into account. (2) Consideration of overall environmental concerns, in addition to climate change.

There is a possibility that the promotion of countermeasures for climate change will affect or be influenced by other environmental problems. For example, the promotion of the use of diesel for automobiles will reduce CO2, but will increase SOx and NOx. Recycling of resources consumes energy, and may increase CO2 emissions. Accordingly, when promoting measures thoroughly, it is necessary to identify the position of the climate change problem in all environmental problems, and fully analyze the impact of such measures on the environment as a whole. (3) Importance of voluntary measures

Considering our experience in realizing significant energy conservation after the oil shock, in addition to the uncertainty of scientific information related to climate change, in my view it will be necessary to implement a range of approaches in good balance without simply immediately enforcing compulsory means to tackle climate change problems. In particular, it is important to promote voluntary actions by each economic sector. (4) Key importance of comprehensive measures by all sectors: industry, society, and transportation

The emission of C02, which is the main cause of climate change, is broadly related to human activity, and so increased emissions not only from industry but also society and the transportation sector is a serious problem. For this reason, in making a tripartite plan in the future, although it is of course essential to continue to promote policies in the industry sector, it is crucial to implement comprehensive measures that include society and the transportation sector.

12 (5) Raising the consciousness of all people. Although most people are aware of the seriousness of the climate change problem, there is as yet no consensus as to the urgency of the measures required to tackle it. Accordingly, it is important to conduct activities designed to improve people's awareness, especially by extending provision of education and information to the general public, and to promote lifestyle changes in ordinary consumers.

(6) Importance of breakthrough by development, promotion and transfer of innovative technology. To stabilize CO2 concentrations, it is necessary to see a drastic, worldwide reduction in the volume of CO2 emissions, and to realize comprehensive tripartite measures. In my view it is of key importance to achieve a breakthrough via the development, promotion and transfer of innovative technology. (7) The long--term vision In responding to the climate change problem, working toward comprehensive tripartite measures, considering the need for a breakthrough achieved by innovative technology, there must be a long-term vision.

3 . 2 Based on t h e s e f u t u r e address

specific

directions,

I would

like

to

now

policies.

(1) The direction of measures toward achieving the goals for 2000 in Japan Based on the Action Program to Arrest Global Warming, Japan has made a worldwide declaration of intent to stabilize per capita emissions of CO2 in 2000 at the 1990s level. At the same time, however, viewing the present situation, along with the continued trend in increasing emissions of CO2 centered on society and the transportation sector, there has also been a worsening trend in energy consumption ratio in the industry sector in recent years. If the present trend continues, we will face very severe difficulties in achieving the goals for the year 2000. It is of key importance to implement additional measures as soon as possible to get back on schedule as we head toward 2000. Specifically, measures concerning energy supply and demand, energy conversion, promotion of energy conservation in the industry, society and transportation sectors, and acceleration of the introduction and promotion of new types of energy that have a reduced impact on the environment, such as photovoltaic power generation, are essential. In this connection, Japan in April this year adopted "Comprehensive Energy Conservation Measures Towards the Year 2000", which strengthens the enforcement of current law concerning the rational use of energy to ensure the thorough implementation of energy conservation. Following this in June, Law on Special Measures to promote the utilization of New

13 Energy was newly stipulated to promote the introduction of new types of energy. Furthermore, from the viewpoint of promoting voluntary action by industry, the industrial environment vision was revised centering on measures against the climate change problem. (2) Domestic and international measures viewed from 2000 and after

The key aim of these measures is the promotion of technological development. To achieve the ultimate goal of countermeasures against global warming - - the stabilization of the concentration of greenhouse gases in the atmosphere m it is necessary to sharply reduce CO2 emissions over the mid- and long-term. If concentrations are to be stabilized at double the level seen before the industrial revolution, it will be necessary ultimately to reduce emissions of CO2 to less than half the current level. At the present juncture, however, the means of achieving this have not yet been found, and so in the future it will be of key importance to achieve a breakthrough in technology. Japan has made an international proposal for the New Earth 21 Program from this point of view, and the concrete deployment of this program has been to promote the development of innovative energy and environmental technologies based on the New Sunshine Program. As we promote the development of these technologies, it will become necessary to clarify the development time frame and share a long-range vision among all nations, including developing countries, to promote international cooperation. I believe it is necessary, to examine specific actions, taking into account scientific advancement, in line with the New Earth 21 Program which has recently been restructured based on statistical models. Specifically, in terms of the possibility of restrictions on the supply and demand of major energy resources in the latter half of the 21st century, it will be necessary to concentrate on R&D for technologies with less limitations in supply that can be expected to be broadly introduced and promoted via technological breakthroughs. For example, we should establish basic technologies for photovoltaic energy, hydrogen energy, superconductivity, and fuel cells by 2020-2030. By systematizing these basic technologies, we will be able to implement them on a global scale by the latter half of the 21 st century. To significantly reduce CO2 emissions over the mid-and long-term, it is also necessary to come to grips with the development of innovative environmental technologies, namely applied CO2 fixation technology, environment-friendly production technologies, and CO2 treatment technologies. It is expected that these will be firmly established by around 2020. There will be presentations here today on R&D results related to innovative environmental technologies. MITI will provide about 9.7 billion yen this year for such R&D, and we expect to further increase the budget next year. The development of innovative energy and environmental technologies is not something to be handled by Japan alone, and it is important to seek out international

14 cooperation. This makes the active utilization of the approach of the Climate Technology Initiative (CTI) essential. The objective of the CTI is to promote international cooperation in the following two areas: (i) The improvement and promotion of existing energy conservation and alternative energy technologies, and (ii) the development of innovative technologies to capture, treat and utilize greenhouse gases. CTI was jointly proposed and approved by the 24 countries in IEA/OECD at COP 1 in May 1995. Japan was appointed as task force leader for the development of innovative environmental technologies at the CTI assembly in February 1996, and in February this year we were selected as the country to chair CTI. I hope we will positively realize the initiative toward international cooperation. The second point is the importance of policies that include developing countries. Considering the dramatic increase in C O 2 emissions in developing countries, especially in Asia in the future, it is necessary to substantially promote restrictions on CO2 emissions in such countries. In particular, based on our experience in conserving energy along with economic growth, we expect to positively cooperate in working on this problem, focusing on technology transfer and promotion, taking into account the industrial structure and technological levels of developing countries. Concerning the transfer of environmental and energy technologies to developing nations, we are promoting a green aid plan whose pillars are individual projects involving discussion on policy measures and model projects. We believe it is necessary to continue to promote this plan. In addition, I consider joint implementation to be an especially effective method in further promoting specific measures. Joint implementation is a method for controlling the greenhouse gases specified in the UN Framework Convention on Climate Change. It makes it possible to collectively allot results (the volume of greenhouse gas reduce) among member countries implementing controls on greenhouse gas emissions. Concerning joint implementation, at COP 1 in 1995, agreement was reached on the start of a pilot phase for joint implementation activities, with participation understood to be on a voluntary, basis. In November 1995, Japan held two joint meetings, the Executive Committee Meeting of the Cabinet to Promote Comprehensive Energy Policies and the Council of Ministers for Global Environment Conservation. Agreements were made for a basic framework for the "Joint Implementation Activities m Japan Program." In January 1996, at the Joint Ministry Conference for Activities Implemented jointly chaired by MITI and the Environment Agency, assessment guidelines were approved. Then, Ministries related to the Joint Implementation Activities m Japan Program, centering on MITI and the

15 Environment Agency, approved the 11 plans for the first project related to Japan Program. We will proceed with negotiations with member countries and continue to work on examining conditions which are easy for developing nations to agree on, with the aim of reaching an early agreement among the nations participating in this scheme. Also to promote further positive participation at each industrial level, we are this year making a public announcement to contract out a feasibili .ty study survey on the Japan Program.

4. APPROCHES TO BUILDING FUTURE INTERNATIONAL FRAMEWORKS As you all know, COP3 will be held here in Kyoto in three months' time. Japan, as the chair of COP3, is expected to make a positive contribution toward building a new international framework for tackling the climate change problem. In COP3, I hope to create a framework which will encourage each participating country to work dedicatedly and effectively on the climate change problem, without its being merely noted as a diplomatic event. It is instead important to aim at a framework tbr concrete action. At the G7 summit meeting held in Denver this June, agreement was reached on a number of fundamental principles, which I will list, concerning COP3. (i) To agree on a meaningful, practical and equitable goal. (ii) To identify responsibilities and assure transparency in the COP3 agreement, and at the same time accept flexibility among participating nations regarding measures to achieve the goals. (iii) To recognize the need for action by developing countries to tackle climate change problems and the need for cooperation with developed countries, to include technology transfer and environmental education. At this summit meeting, Japanese Prime Minister Ryuichi Hashimoto proposed a "Global Remedy for the Environment and Energy Use (Green Initiative)," which promotes implementation of an international cooperative action program for the development and transfer of technology. This initiative was again proposed by Prime Minister Hashimoto at a special UN general meeting held in New York. I believe it is necessary for Japan to consider the following basic policies towards COP3. (1) Establishment of goals which d i f f e r e n t i a t e among developed nations Establishment of quantitative goals is eftbctive at promoting positive measures for suppressing emission of carbon dioxide by developed nations. In securing a new

16 international framework that is "equitable" considering the similarities and differences among the nations, it is essential to promote positive action by each nation. For example, the current pledge and review method obliges all countries to achieve uniform reductions based on 1990 levels. In my view, it is important to develop and propose a new index for setting different quantitative goals in line with past efforts, taking into account the fact that the levels of effort devoted by developed countries to energy conservation are far from uniform. (2) The need f o r

l o n g - t e r m a c t i o n s based on R&D

Since the problem of global warming involves continuing reductions of CO2 over the very long term, emphasis must be placed on long-term actions using technological developments. MITI proposed the New Earth 21 Project from this perspective in 1990. To promote international cooperation in future innovative technological development efforts, it is necessary for each country, including developing nations, to share a long-term vision. Based on future scientific advancement, it will be necessary to restructure the Project to realize these intentions. (3) F l e x i b i l i t y

of time frame

Looking at those countries where it is possible to make dramatic short-term reductions in CO2, the cases of the reunion of East and West Germany and the conversion from coal to North Sea natural gas by Britain show that each country has its own individual circumstances. Since these differ among countries, and technological development takes a long time, I think it will be necessary to set a more flexible time frame, allowing a longer period for some of the goals depending on their content without just imposing a uniform short time frame, such as by 2005, on each nation. (4) Importance of p o l i c i e s energy u t i I i z a t i o n

and measures t o

improve the e f f i c i e n c y

of

Reflecting on the experience of the significant conservation of energy that Japan has implemented since the 1970s, to realize a tripartite approach, policies and measures promoting the efficient utilization of energy should become more effective and efficient. In doing this, and in order for flexibility to be granted for working on policies by each country so that they are able to promote independent responses by industry and so on, and for objective evaluations of the results of work on policies encouraging the effective utilization of energy by each country, one strategy would be to establish measures aimed at increasing the efficiency of energy utilization, and a scheme established for secure reviews of the handling of such measures by each participating country. (5) Promot i on of measures i n deve Iop i ng countr i es

17 Considering the anticipated significant future increases in carbon dioxide emissions in developing countries, we should not simply debate policies for developed countries. We must study measures, including the strengthening of environmental policy dialogues, technology transtbrs from developed nations, and participation in the Annex 1 group in the semi-advanced nation UN Framework Convention on Climate Change to substantially promote as many measures as possible for developing countries within the possible negotiation range. (6) Promotion of the green i n i t i a t i v e Furthermore, I believe it will be important to make the Green Initiative that Prime Minister Hashimoto presented at the Denver Summit this past June more concrete as a comprehensive action program in relation to the promotion of introduction of energy conservation, non-fossil fuel energy sources, worldwide deployment of forest planting, and promotion of innovative technological development and technological transfer. 5. OONOLUSION It bears repeating that the climate change problem is one that is expandable in terms of both time and space, and it is one that requires independent action along a spectrum of independent bodies. In working towards a solution to the problem, viewed as a global scale issue, the key challenge worldwide is to implement environmental energy measures from a local level. Finally, to tackle global environmental problems, it is important to continue to increase scientific knowledge and vigorously promote technological developments concerning CO2. I am sure that, through the active discussions that will soon take place, this conference will contribute to the further advancement of R&D in environmen{al energy research institutes worldwide, including RITE. In closing, let me emphasize that your efforts will play a significant role in solving climate change problems. Thank you very much for your kind attention.

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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

19

R e s e a r c h a n d d e v e l o p m e n t o n n e w s y n t h e t i c r o u t e s for b a s i c c h e m i c a l s by c a t a l y t i c h y d r o g e n a t i o n of CO2 Hironori Arakawa National Institute of Materials and Chemical Research (NIMC), Higashi 1-1, Tsukuba, Ibaraki 305, Japan This paper gives a review of recent research work in catalytic hydrogenation of CO2 to various kinds of valuable chemicals and fuels. Recently, CO2 hydrogenation has been positively studied in relation to a possible countermeasure against global warming caused by CO2 emission. Advances in the effective syntheses of methanol, dimethylether, ethanol, lower paraffin, lower olefins, gasoline fraction, formic acid, acetic acid and others are introduced. Additionally the progress of solar hydrogen production for this CO2 fixation is described and its importance is pointed out. 1. INTRODUCTION Global warming caused by a remarkable increase of CO2 emission into the atmosphere is an important and urgent problem to be solved. Effective countermeasures should be carried out to reduce such a problem for sustainable prosperity of the human race. Catalytic hydrogenation of CO2 has been recently attracting considerable attention as one of chemical fixation and recycling technologies for emitted CO2.[1] Therefore, extensive studies on this technology have been conducted now. Catalytic hydrogenation of CO2 has some advantages compared with other countermeasures such as CO2 deposit and disposal. It is noted that catalytic hydrogenation can fix or convert CO2 very quickly. In addition to the reduction of CO2 emission into the atmosphere, it can produce valuable chemicals from emitted and useless CO2 and it can save carbon resources, such as petroleum and natural gas, for industrial synthesis of valuable of chemicals. Industrial process for syngas (H2/CO) conversion such as methanol synthesis gives another advantage to CO2 hydrogenation technology. Because the industrial production facilities for syngas conversion process is supposed to be easily applicable to H2/CO2 conversion process without any significant replacement of facilities. Further, CO2 hydrogenation reaction is regarded as one of artificial photosynthesis technologies. It is well known that photosynthesis by plants consists of two main reactions, that is, the light reaction and the dark reaction. In the light reaction, water is decomposed to H2 and 02 under the solar visible light irradiation. Produced 02 is emitted into the atmosphere. On the other hand, produced H2 is carried by NADP § as NADPH. In the dark reaction, adsorbed CO2 is hydrogenated by H2 from NADPH and CO2 is fixed finally as carbohydrates under the non-

20 irradiation conditions. Catalytic hydrogenation of CO2, in a sense, corresponds to the dark reaction of photosynthesis. Remarkable advances in catalytic hydrogenation of CO2 have been established by extensive research and development for last 10 years. As shown in Fig.l, various kinds of chemicals have proved to be produced effectively by CO2 hydrogenation process. This paper reviews such advances in catalytic hydrogenation of CO2.

I Lowerhydrocarbons (Etia

Ethane,.. etc.)]

] Methane ( C H 4 ) ~ r b o n [Carbon Monoxide (CO) ~

[ Higher hydrocarbons (Oil, Wax, .etc.) ]

(~x) 1~ J

(Benzene, Toluene, etc.) [ 1"Aromatics _~,,~,~,,4P'. . . . . . . . . . . .

Carbon Dioxide (CO ]1' 'i " " (CO2) Q . . . . .

[ Formic Acid ( H C ~ H ) ~ / - I " ' " " ~

-[Methanol (CH3OH) i

~

,

~ "~~ ~ m e t h y l e t h e r

'

..

( CH3OCH3) 1

! Formic Ester [ ]-Carboxyiic Acid (CH3COOH, etc.) ] [ i-ii#er A!coh.o.!s(C2HsOH, etc.)] Figure 1. Chemicals synthesized by catalytic hydrogenation of CO2 2. ALCOHOL AND ETHER SYNTHESIS 2.1. Methanol synthesis Methanol synthesis from H2/CO2 has been studied so far in relation to that from H2/CO.[2,3] One of the reasons studied before is due to an interesting phenomenon that an addition of small amount CO2 into H2/CO feed improves methanol yield significantly in the industrial process.[2] The role of added CO2 was noted. Rozovskii showed by tracer analysis study that carbon species of produced methanol originated from CO2 in the H2/CO feed , suggesting methanol was produced v/a CO2.[4] Recent researches have been aiming at the development of highly efficient catalysts for methanol synthesis for the industrial process.

Thermodynamic equilibrium for methanol synthesis Figure 2 shows thermodynamic equilibrium yields of methanol from both H2/CO2 and H2/CO at 5 Mpa. Equilibrium yield of methanol from H2/CO2 is lower than that from H2/CO at all reaction temperatures. However, experimental results show a higher yield of methanol from H2/CO2 at around 260 ~ An example over Cu-ZnO/SiO2 catalyst is shown in Fig. 3.[5]

Catalysts for methanol synthesis from H2/C02 Various kinds of metal catalysts are reported to be active for methanol synthesis from H2/CO2. Activity of metal catalyst for methanol yield increased with following order, Cu~Co=Pd=Re~Ni~Fe~Ru=Pt~Os~Ir=Ag=Rh~Au.[8] Needless to say, catalytic activity is much dependent on metal dispersion, additives and type of support. It is apparent, however, Copper is the most active metal species for methanol production. Effect of metal oxide support to 5wt%Cu catalyst was studied.[9]

21 I00

.

.

.

(a)

.

.

.

.

.

: C O + 21-I2 - CI-I301-I

2H2/CO2--*CH3OH g

3H2 = CH3OH

80

-~

~

0 /

O~~/~O

~ 9 60-

~ 9

9

0

o /

40

/

r 20 5wt%Cu-ZnO(l" 1)/Si02 150

200 250 300 Temperature (~ Figure 2. CH3OH yield at equilibrium

350

220 240 260 2so .... 9

.l

l

l

Temperature (~ Figure 3. Practical yield of CH3OH

Though methanol yield increased with following order of Cu/A1203~Cu/ZnO~ Cufl~O2~Cu/SiO2, the order of turnover frequency(TOF) was as follows, C u / Z n O ~ CufFiO2=Cu/ZrO2~Cu/Cr203=Cu/CeO2~Cu/A1203~Cu/SiO2. TOF indicates an intrinsic activity of reaction sites over Copper. Alumina and Silica improved Cu dispersion of catalyst because of high surface area of support itself and they increased methanol yield, but they did not improve intrinsic activities of reaction sites of catalysts.[9] Obtained results suggest ZnO. TiO2 and ZrO2 improve reaction sites remarkably. Doubly promoted Cu catalysts were investigated using 5wt%CuZnO(l:l) catalysts. Methanol yield in each catalyst increased with following order, 5wt%Cu-ZnO/Al203=5wt%Cu-ZnO/SiO2=5wt%Cu-ZnO/TiO2 ~ 5wt%Cu-ZnO/ZrO2 =5wt%Cu-ZnO/Cr203>5wt%Cu-ZnO/CeO2.[10] From these results, it is apparent that ZnO, TiO2, ZrO2, Cr203, CeO2, Al203 and SiO2 act as promising promoters to Cu catalyst for methanol synthesis. Concerning co-precipitated catalyst, Cu-ZnO based multi-component catalysts such as Cu-ZnO-Al203, Cu-ZnO-ZrO2 and Cu-ZnOCr203 are extensively investigated. Recently Ga203 and La2Zr207 were found to act as promising promoters for Cu catalyst by Fujitani et al. [11] and Kieffer et a1.[12], respectively. Inui et al.[13] and Lee et al.[14] have shown, respectively, that the addition of small amount of noble metals such as Pd and Rh to Cu-ZnO based catalysts improve methanol formation remarkably. As non-Copper catalysts for methanol synthesis, Ag/ZrO2115], Au/ZrO2,TiO2115,16], Pd/CeO2117], Mo2C[18] and PtW/SiO2119] have been investigated. Reaction mechanism and the structure of active site over Cu-ZnO based catalyst Many experimental results suggest methanol is produced not from CO, but from CO2 directly. As mentioned above, methanol yield from H2/CO2 is higher than that from H2/CO at a wide range of reaction conditions.[5,6,7,20] A study of influence of contact time on product distribution suggested methanol was a primary product[21]. Fujita et al.[7] and Koeppel et a1.[22] showed methanol and CO were produced

22 through parallel pathways. In situ FT-IR observations of surface species over CuZnO based catalysts showed the presence of Copper formate as well as Zinc formate.[10,21,23,24] A formate located at the Cu-Zn interface was also observed by TPD study.[25] Because of low reactivity of Zinc formate with H2 stream over catalyst, Copper formate was suggested as a main reaction intermediate.[21] Millar et al.[23]and Vanden Bussche et a1.[29] have also concluded Copper formate was the pivotal intermediate for methanol. Fujita et a1.[7,24] suggested, however, that transformation of Copper formate to Zinc methoxide by hydrogenation proceeded and Zinc methoxide, which was a observed species by in situ FT-IR, was easily converted to methanol by hydration at atmospheric reaction condition. Joo et a1.[26,27] have concluded from TPD and TPD studies that formate species migrated from Cu to ZnO and Zinc formate was converted to methanol by hydrogenation at atmospheric conditions. Various proposed reaction mechanisms were summarized in a unified reaction mechanism shown in Fig. 4. f H

oI I c /

o -

o,

CO--.CuxO

~

d /. - 9.k

o-,

;9"--9-

Ha

CH3

---->

o,

l

o

i orT

coz

.

I (Cu)m-(ZnO)n ] i Cu

Ha k

0I

/

Cu

\

0i -

Cu

IH~ z-.x

" 0 : - :0 I: :1 Cu Cu

I

>

,

0

0

Cu

Cu I

---[col

Figure 4. A unified reaction mechanism of CH3OH formation over Cu based catalyst

It is apparent that main active site of Cu-ZnO based catalyst exists on the surface of Copper particles. For example, over ZnO catalyst, methanol formation is low about two orders of magnitude compared with that over Cu catalyst.[5] Zinc oxide is just a promoter to Copper. Therefore, interfacial sites between Copper and ZnO is very important for effective methanol synthesis. Recently Fujitani et al. have found a good correlation between oxygen coverage of Copper surface of catalyst and specific activity for methanol production over various Cu based catalysts.[ll] The maximum specific activity was obtained at oxygen coverage=0.17. Oxygen coverage was determined by in situ N20 titration of catalyst. This result shows that about 20% of metallic Copper surface was oxidized and, in other words, coexistence of metallic Copper and partially oxidized Copper is essential for effective production of methanol. They have also concluded that ZnO stabilizes Cu § state of Copper particle.[28]

23

Effective methanol synthesis using Cu-ZnO based catalysts Several kinds of excellent Cu-ZnO based catalysts, such as Cu-ZnO-A1203-Cr203 [21], Cu-ZnO-TiO2 [30], Cu-ZnO-Ga203 [31] and Cu-ZnO-ZrO2-A1203-Ga203 [31], were developed so far. Table 1 shows reaction behavior of these catalysts. A fairly large amount of methanol (STY) is produced over these catalysts. Addition of small amount of CO to the H2/CO2 feed increases methanol formation significantly.[32,33] This is a favorable phenomenon for a practical methanol production from H2/CO2. Because unreacted H2, CO2 and produced CO have to be recycled in the industrial production. Methanol formation from the feed gas of H2:CO2:CO=75:22:3 is also shown in Table 1. Extremely high yields of methanol were obtained by Pd modified CuO-ZnO-Cr203-A1203-Ga203 [32] and Cu-ZnO-ZrO2A1203-Ga203 [31]. Methanol production yields from H2/CO/CO2 feed in commercial process are shown in Table 1, too.[34.35] It is apparent that methanol production from H2/CO2 feed is already competitive to industrial methanol production from syngas,H2/CO feed. Bench scale test for 50kg/day methanol production are now conducted at RITE, Japan. Table 1 Effective methanol synthesis from H2/CO2 compared with that from H2/CO Ref. Catalyst Press. Temp. GHSV CH3OHSTY (Mpa) ( ~ ) (l/h) (g/l-cat.h) H2/C02=3/1 240 20000 411 [21] CuO-ZnO-A1203-Cr203(43-20-34-3) 3 240 20000 504 [30] CuO-ZnO-TiO2(30-35-35) 5 250 4700 502 [29] La/CuO-ZnO-A1203-Cr203(25-42-32-1) 8 250 18000 738* [31] Cu-ZnO-Ga203(50-25-25) 5 H2/C02/C0=75/22/3 250 18000 785* [31] Cu-ZnO-ZrO2-A1203-Ga203 5 270 18800 1300 [32] Pd/Cu-ZnO-A1203-Ga203-Cr203 8 (lwt%/38-29-13-18-2) H2/CO/CO2/CH4=70/20/7/3 226 12000 700 ICI [34] CuO-ZnO-A1203(24-38-38) 5 CuO-ZnO-A1203(64-32-4)** 5 250 10000 300 Academic [34] CuO-ZnO-Cr203 10 253 12000 1225 Lurgi [35] *:STY:g/kg-cat.h, **:H2/CO(no CO2 in feed gas)

2.2. Dimethylether synthesis Dimethylether (DME) can be synthesized effectively from H2/CO2 feed by one-pot synthesis using hybrid catalyst. Hybrid catalyst is composed of mixture of methanol synthesis catalyst and solid acid catalyst. DME is an important and valuable chemical used as solvent, propellant and raw material to liquid fuels. As shown in Fig.l, methanol from H2/CO2 has a severe limitation of thermodynamic equilibrium compared with that from H2/CO. To overcome such a limitation, in situ transformation of methanol to DME is reasonable way to improve total methanol yield (CH3OH + DME). Table 2 shows reaction behavior to DME synthesis over hybrid

24 catalysts.[36] It is apparent that Y-zeolite and Mordenite show good performance for in situ dehydration. Total methanol yield increased 1.7 times compared with that over CuO-ZnO-A1203 catalyst alone and 55% selectivity of DME was obtained at 3 Mpa and 240~ Inui et a1.[37] obtained 70% selectivity of DME and 736g/1-cat.h of total methanol STY using the mixture of CuO-ZnO-A1203-Cr203 and SAPO-34 (1:2 in volume ratio) at the condition of 8 Mpa, 300 ~ GHSV=9400/h and H2/C02/C0=75/22/3 feed gas. Practical application of this process will be expected. Table 2 Dimethylether (DME) synthesis from H2/CO2 using hybrid catalyst Catalyst CO2 cony. Selectivity(%) Total CH3OH ~eld(%) (g/g) (%) CO DME CH3OH (DME + CH3OH) A/SIO2(1/0.56) 20.5 50.3 0.0 49.6 10.2 A/7-A1203(1/1) 21.6 46.1 16.9 37.7 11.6 A/SiO2-A1203{98-2}(1/1.6) 23.6 34.4 47.1 18.4 15.5 A/Y-Zeolite(I/0.8) 24.4 32.4 54.8 12.8 16.5 A/Mordenite(1/1.1) 25.0 31.7 55.1 13.0 17.0 1.2 ml of A catalyst was mixed with 1.7 ml of solid acid catalyst. A:CuO-ZnO-A1203(32-66-2); *:Reference 2.3. E t h a n o l synthesis Synthetic ethanol is now produced by hydration of ethylene. Ethanol is one of important chemicals. Though ethanol synthesis from H2/CO was extensively studied so far, there are a few reports about ethanol synthesis from H2/CO2. Recently, some efficient catalysts for ethanol formation from H2/CO2 were developed by Arakawa and his co-workers. Ethanol synthesis by K / Cu-Zn-Fe mixed oxide catalyst It is known that potassium promoted Fe catalyst can produce C2+-alcohols by H2/CO .[38] Therefore, the combination of reverse water gas shift reaction (1) and syngas reaction over K/Fe catalyst (2) might produce ethanol. This idea is a general concept for K/Cu-Zn-Fe catalyst development. H2 + CO2 -(shift catalyst) ..+ CO + H20(inverse shift reaction) (1) 2CO + 4H2-(K/Fe catalyst) ..+ C2H5OH + H20(syngas reaction) (2) As a result of screening test, the combination of K/CuO-ZnO and K/Fe oxide has proved to be a efficient catalyst.[39.40] The results are shown in Table 3. At reaction temperature below 250~ main product was CO. Alcohol formation, however, increased with increasing reaction temperature. Catalysts were prepared by impregnation of K2CO3 onto Cu-Zn-Fe mixed oxide. Over K/Cu-Zn-Fe(0.08/I-I-3) catalyst, ethanol was produced with 20% selectivity at 44% conversion of CO2 under the condition of 7 Mpa, 300~ GHSV=5400/h and H2/CO2=3/1. In case of GHSV=20000 using this catalyst, 270g/l-cat.h of ethanol STY was established. Similar catalyst such as Fe-Cu-AI-K/Cu-Zn-AI-K was also applied to ethanol formation. [41]

25

Table 3 Effective synthesis of ethanol over K/Cu-Zn-Fe mixed oxide catalyst Catalyst CO2 cony. SelectiviW in carbon efficiency(%) ( % ) CO MeOH EtOH C3+OH CH4 K/Fe (0. 4/2) 32.7 11.9 0.2 6.5 4.3 13.4 K/Cu-Zn-Fe(0.4/1-2-1) 39.0 8.7 1.9 13.2 6.2 12.4 K/Cu-Zn-Fe(0.08/1-1-3) 44.4 5.9 2.0 19.5 5.5 13.6 Conditions: 7 Mpa, 300~ GHSV=5400/h, H2/C02=3/1.

C2+ H.C. 63.6 57.6 53.5

Ethanol synthesis by promoted Rh/Si02 catalyst It is known that ethanol can be synthesized efficiently from H2/CO by promoted Rh/SiO2 catalyst.[42.43] Based on this result, promoted Rh/SiO2 catalysts were tested for CO2 hydrogenation to ethanol.[44,45] As a result, it has proved that Sr, Li and Fe additives are effective for ethanol formation.[46,47] Typical results are shown in Table 4.[48] It is speculated reaction proceeds v/a equation(l) and (2). Acetyl species, formed by CO insertion to methyl species on Rh surface, is supposed to be a possible reaction intermediate. Table 4 Selective synthesis of ethanol over promoted Rh/SiO2 catalyst Catalyst CO2 cony. SelectiviW in carbon efficiency(%) (%) CO MeOH EtOH CH4 5wt% Rh-Sr( 1-1)/SiO2 1.9 52.0 20.5 8.2 18.6 24.7 12.7 37.6 25.0 5wt%Rh-Li(1-1)/SIO2 4.4 5wt%Rh-Fe(l- 1)/SIO2 4.4 23.8 36.7 8.8 30.4 33.4 22.8 34.0 9.8 5wt%Rh-Fe-Li(l- 1-1)/SIO2 14.1 Conditions: 5 Mpa, 260~ GHSV=6000/h, H2/CO2=3/1

Homogeneous catalyst system Ethanol is synthesized using homogeneous catalyst in the batch reactor, too. Tominaga et al. [49] reported that the Ru3(CO)12-Co2(CO)s-KI catalyst system in NMP solvent could produce ethanol as well as methanol at the condition of 12 Mpa, 200~ H2/CO2=5/1 and 15 hrs. Reaction proceeds by homologetion of methanol to ethanol. Best result was obtained by Isaka et al.[50] Ethanol was produced with 36.2% selectivity at CO2 conversion of 42% using the Ru3(CO)12-Co2(CO)s-LiBr in Bu3PO solvent at the condition of 20 Mpa, 200~ H2/CO2=5/1 and 18hrs.

2.4. Higher alcohol synthesis Few study on higher alcohol synthesis such as propanol and butanol was conducted so far. Kieffer et al. studied higher alcohols synthesis using Co modified CuLa2Zr207 catalyst.[51] The yield of C3+-alcohols was very low compared with that from H2/CO. This is partly because of low chain propagation probability in case of H2/CO2 reaction. However, higher alcohols syntheses from H2/CO using modified Cu-ZnOA1203152], K-Mo catalyst[53] and etc. are positively studied so far. Therefore, this reaction is still of interest.

26 3.

HYDROCARBON SYNTHESIS

Catalytic hydrogenation of CO2 to hydrocarbons is classified into two categories. The one is direct hydrogenation from H2/CO2 to hydrocarbons. The other is indirect process which includes methanol synthesis from H2/CO2, followed by in situ methanol conversion to hydrocarbons using solid acid catalyst in H2/CO2 feed. Study on indirect hydrocarbon synthesis is now popular.

3.1. Lower paraffin synthesis Perfect hydrogenation of CO2 to methan is not difficult. Various kinds of supported metal catalysts are available for this reaction. Ni-La203-Ru on ceramic fiber support catalyst is known as an efficient rapid conversion catalyst.[54] Though selective and effective synthesis of C2-C5 paraffin by direct hydrogenation is difficult, indirect process is promising for selective synthesis. The concept of hybrid catalyst system for lower paraffin synthesis was firstly demonstrated by Fujimoto et al. [55] Fujiwara et al. showed the composite catalyst of Cu-ZnO-Cr203 and H-Y-zeolite could produce a high C2-C5 hydrocarbon selectivity such as 95% in total hydrocarbons produced, though CO is a major product as shown in Table 5.[56] Joon et al. reported the selective production of propane and iso-butane using hybrid catalyst composed of Cu-ZnO-ZrO2 and SAPO-44 or SAPO-5, respectively.[57] As direct hydrogenation to lower paraffin, Fe/TiO2 and Fe/ZrO2 were studied.[58,59] Table 5 Lower paraffin synthesis from H2/CO2 using hybrid catalyst Catalyst Press.Temp. GHSV CO2 cony. Conv. to(%) H.C. selectivity(%) Ref. (Mpa) (~ (l/h) (%) H.C. C O C1 C2 C3 C4 C5 C 6 + A/H-Y 5 400 3000 39.9 12.0 27.3 4.2 25.8 40.8 21.7 6.7 0.8 [56] B/SAPO-44 2.8 340 20* 25.8 8.0 17.5 5.9 23.5 43.3 23.9 2.6 0.S [57] B/SAPO-5 2.8 340 20* 25.0 9.5 15.4 3.9 5.9 18.5 54.4 12.9 4.5 [57] A:Cu-ZnO-Cr203 (3-3-1), B:Cu-ZnO-ZrO2(60-30-10), *:W/F(g-cat.h/mol) _

_

3.2. Lower olefin synthesis Selective synthesis of lower olefin by direct hydrogenation CO2 is relatively difficult in a similar manner as selective lower paraffin synthesis. However, Choi et al. reported lower olefin synthesis using Fe-K/Alumina catalyst.[60] According to their result, selectivity of lower olefin from C2 to C4 was about 44% in produced total hydrocarbons and total hydrocarbon selectivity was 95% at CO2 conversion of 68% over Fe-K(1-1)/Alumina catalyst under the condition of 2 Mpa, 400 ~ and GHSV=1900/h. In direct synthesis, Inui et al. [41] showed a selective synthesis of lower olefin using two-stage series reactor packed an effective methanol synthesis catalyst such as Pd modified Cu-ZnO-il203-Cr203 in the first stage and SAPO-34 in the following stage, as shown in Fig.5. More than 90% selectivity for C2 - C4 olefins was established. Different from H-ZSM-5, which is prefer to paraffin hydrocarbon synthesis, SAPO-34 was suitable for lower olefin synthesis because of its weak acidic and narrow pore structure.

27

3.3. Gasoline fraction synthesis U s i n g two-stage series reactor system, gasoline fraction of hydrocarbons w a s also synthesized.[41] As a m e t h a n o l conversion catalyst, MFI-type metallosilicate such as H-Fe-silicate a n d H-Ga-silicate were o p t i m u m for gasolin fraction synthesis. As shown in Fig.5, gasoline fraction w a s produced with 65% selectivity in case of HGa-silicate. H y d r o g e n inverse spillover feature of Ga p a r t in silicate suppress hydrogenation ability to paraffin formation a n d promote oligomerization of lower olefins to gasoline component. CiC2 C3

CO2-rich Syngas 22% CO,. ~ 68%H2

C~

C~

C~

A

H-Fe-silicate ff/~-r-r~_ t ' -1--:::-":.::.::--~;:.-~ I Si/Fe=400 -------~t~[~~'/,I/]",/',/< .[.::":.:..~.. ~ Pd-modified ] / 3 ~ Se146%o, STY 208 g/1.h MSCg I / MeOH cony. i00% + 33.9%CUO I ] n C2C3 C4 C5 C6 A 26.2%ZNO /1 _ IH-Ga-silicate/ ~2q-I I f:::: :::: : : ~:~ 37.8%Ah03 / ~k 300oc 15atm 0.7% Pd J ~M ' eOH CO•V. 100% . o ~ 8o a, Is A Po-34 1- - - - " SV 18,800 h Cony. to MeOH 19.4% 4500C' 1 atm MeOHSTV MeOHconv. 100% paraffin 1,028 g/l.h C2-C4 ~ olefin ,

Cl C2-4

II

Se165%, STY 294Jl.h = = +

C:2 _

_ C32 ~_C,i_C_5

Se194%, STY 425 g/1.h [ , I , l , I , 1 , I 0 20 40 60 80 100 Hydrocarbondistribution (C-wt%)

Figure 5. Lower olefins a n d gasoline synthesis u s i n g two-stage reactor s y s t e m [41] 4.

CARBOXYLIC ACID SYNTHESIS

4.1. Formic acid synthesis It is k n o w n t h a t formic acid is synthesized from H2/CO2 as ester in alcohol solvent using m e t a l complex catalysts such as HM(CO)5- (M:W, Cr,Ru) in batch reactor system.J61] However, specific activity (TOF) of these system are relatively low. Recently, Noyori et al. found a significant increase of formic acid in a supercritical mixture of H2/CO2 with N(C2H5)3 using RuH2{P(CH3)3}4 complex at the condition of 20.5 Mpa, 50~ a n d H2/CO2=1/1.4.[62] T O F increased one order of m a g n i t u d e over t h a t of conventional process because of high miscibility of H2 w i t h supercritical CO2. It is also noted t h a t m e t h y l formate produced from H2/CO2 is easily converted to acetic acid by isomerization reaction.

4.2. Acetic acid synthesis In heterogeneous system, H a t t o r i et a1.[63] observed a direct formation of acetic acid from H2/CO2 over Ag-Rh(0.2-1)/SiO2 catalyst at the condition of 2 Mpa, 200~ GHSV=12000 a n d H2/CO2=1/2. Carbon monoxide w a s a m a i n product w i t h 96% selectivity, however, acetic acid w a s produced w i t h 2.4% selectivity. They speculated t h a t direct insertion of CO2 to surface m e t h y l species on Rh led acetate formation, followed by hydrogenation to acetic acid. Acetic acid formation w a s ascribed to a

28 remarkable suppression of CO2 dissociation and desorption over highly dispersed Rh catalyst at lower reaction temperature. In homogeneous system, Fukuoka et a1.[64] found acetic acid formation from H2/CO2 and CH3I using bimetallic catalysts system such as Ru3(CO)12 + Co2(CO)8 and Ni(cod)2+Co2(eO)s in DMF solvent at the condition of 4 Mpa, 150~ H2/C02=1/1 and 24h. Acetic acid was produced with 50% selectivity and by product was mainly CO. Their proposed reaction mechanism was composed of CO2 insertion to Ru-CH3 species, followed by hydrogenation of its intermediate to acetic acid by HCo(CO)4.

5. OTHERS 5.1. Graphitic carbon synthesis Takita et al. proposed a unique conversion of CO2 to graphitic carbon v/a both direct and indirect hydrogenation processes. They found CO2 was converted to graphitic carbon with 40% selectivity at more than 60% conversion of CO2 over WO3 or Y203 catalyst under the condition of 0.1 Mpa, 700 ~ W/F=10g-cat.h/mol and H2/CO2/N2=2:l:5.[65]. Indirect process is composed of two series reactions, that is, methanation of CO2 and its decomposition to carbon and hydrogen using Ni/SiO2 catalysts. Decomposition proceeded remarkably at 500~ And this reaction was much improved using Pd membrane reactor for overcoming with the thermodynamic limitation of methane decomposition to carbon and hydrogen.[66]

5.2. Methyl amines synthesis Baiker et al. demonstrated an interesting reaction process of direct methylamines synthesis from H2/CO2/NH3.[67] Methylamines are produced now commercially from ammonia and methanol. Mono- and dimethylamine were produced effectively with by-product, CO, over 51wt%Cu/A1203 at the condition of 0.6 Mpa, 277~ GHSV=3000/h and H2/CO2/NH3=3/1/1. A new technology for amine derivatives synthesis might be developed by catalytic hydrogenation of CO2. 6.

SOLAR HYDROGEN PRODUCTION BY PHOTOCATALYST

To realize CO2 hydrogenation process as a countermeasure against global warming, solar hydrogen providing system from water should be established. Extensive studies on photocatalytic and photoelectrochemical production of H2 from water have been conducted in the world. For example, recently Sayama and Arakawa have succeeded to produce H2 from water using Na2CO3 + 5wt%NiO/TiO2 photocatalyst system under the solar light irradiation.[68] This is the first demonstration that water is decomposed to H2 and 02 stoichiometrically and steadily by a cheap powder photocatalyst under solar light. About 400ml/m 2 of H2 and 200 ml/m 2 of 02 were obtained under solar hght irradiation for 6.5 hrs in one summer day in Japan. Further, a new interesting approach using tandem system for efficient visible light water cleavage is proposed by Graetzel et a1.[69] A remarkable progress has been observed in this field, too.

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30 36. J.-L. Dubois, K. Sayama and H. Arakawa, Chem. Lett., (1992) 1115. 37. T. Takeguchi, H. Nishiyama, M. Inoue and T. Inui, Proc. of 9th Zeolite Meeting in Japan, (1993) p-65. 38. H. Arakawa and A.T.Bell, Ind. Eng. Chem., Process Dev. Div. 22(1983) 97. 39. A.Okamoto and H. Arakawa, Chem & Ind. Chem., 47 (1994)314. 40. M. Takagwa, A. Okamoto, H. Fujimura, Y. Izawa and H. Arakawa, ICCDU-IV, (Kyoto), (1997) Poster No. P-057. 41. T. Inui, Catal. Today, 29 (1996) 329. 42. H. Arakawa, T. Fukushima, M. Ichikawa, S. Matsushita, K. Takeuchi, T. Matsuzaki and Y, Sugi, Chem. Lett., (1985) 881. 43. H. Arakawa, T. Hanaoka, K. Takeuchi, T. Matsuzaki andY. Sugi, Proc. 9th Intl. Congr. On Catal., 2 (1998) 602. 44. H. Kusama, K.Okabe, K.Sayama and H.Arakawa, SekiyuGakkaishi,40 (1997) 415. 45. H. Kusama, K.Okabe and H.Arakawa, Nippon Kagaku Kaishi,(1995) 875. 46. H. Kusama, K. Okabe, K. Sayama and H. Arakawa, Catal. Today, 28 (1996)261. 47. H. Kusama, K. Okabe, K. Sayama and H. Arakawa, Energy, 22 (1997) 343. 48. H. Arakawa, and H. Kusama, Proc. of ICCDU-III (Oklahoma), (1995) p-78. 49. K. Tominaga, Y. Sasaki, M. Saito and T. Watanabe, J. Mol. Catal., 89 (1994) 51. 50. M. Isaka and H. Arakawa, Proc. of ICCDU-III (Oklahoma), (1995) p-141. 51. R. Kieffer, M. Fujiwara, L. Udron and Y. Souma, Catal. Today, 36 (1997) 15. 52. J.C. Slaa, J.G. van Ommen and J.R.H. Ross, Catal. Today, 15 (1992) 129. 53. L. Gang, Z. Chengfang, C. Yanqing, Z. Zhibin, N. Yianhui, C. Linjun and Y. Fong, Appl. Catal. A, 150 (1997) 243. 54. T. Inui, M. Funabiki and Y. Takegami, J. Chem. Soc., Farady, I, 76 (1980) 2237. 55. K. Fujimoto and T. Shikada, Appl. Catal., 31(1987)13. 56. M. Fujiwara, R. Kieffer, H. Ando and Y. Souma, Appl. Catal, A, 121 (1995) 113. 57. J.-K. Jeon, K.-E. Jeong, Y.-K. Park and S.-K. Ihm, Appl. Catal. A, 124 (1995) 91. 58. Y. Kou, Z. Suo, J. Niu, W. Zhang and H. Wang, Catal. Lett., 35 (1995) 271. 59. Y. Kou, Z. Suo, J. Niu, W. Zhang and H. Wang, Catal. Lett., 35 (1995) 278. 60. P.H. Choi, K.-W. Jun, S.-J.Lee, M.-J. Choi and K.-W. Lee, Catal. Lett.,40(1996) 115. 61. D.J. Darensbourg and C. Ovalles, Chemtech. (1985) 636. 62. P.G. Jessop, T. Ikariya and R. Noyori, Nature, 368 (1994) 231. 63. N. Ikehara, K. Hara, A. Satsuma, T. Hattori and Y. Murakami, Chem. Lett., (1994) 263. 64. A. Fukuoka, N. Gotoh, N. Kobayashi, M. Hirano and S. Komiya, Chem. Lett., (1995) 567. 65. T. Ishihara, T. Fujita, Y. Mizuhara, and Y. Takita, Chem. Lett., (1991) 2237. 66. T. Ishihara, Y. Miyasita, H.Isada, H.Nishiguchi and Y. Takita, Ionics, 21(1995) 23. 67. S.V.Gredig, R.A.Koeppel and A. Baiker, J.Chem. Soc., Chem. Commun., (1995) 73. 68. K. Sayama and H. Arakawa, J. Chem. Soc., Chem. Commun., 150(1992).;J. Phys. Chem., 97(1993)531.;J.Photochem.and Photobio.,A,77(1994) 243.;ibid,94(1996)67.; J.Chem.Soc., Farady Trans.,93(1997)1647.;The Rev. of Laser Eng., 25(1997)425. 69. M. Graetzel, Proc. of 1st Conference of future generation photovoltaic technologies, (Denver), March 24-26, (1997).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi(Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

31

N e w a p p r o a c h e s in C O 2 r e d u c t i o n A. Fujishima, D. A. Tryk, and Tata N. Rao Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan ABSTRACT

In order to make a significant impact on increasing atmospheric CO 2 levels, electrochemical and photoelectrochemical processes with both high current density and high current efficiency need to be developed. Two general approaches have been followed in our recent work. The first involves the use of gas diffusion electrodes in aqueous electrolyte in order to enhance mass transport and electrode kinetics. Recently developed high area Ni catalysts on activated carbon fibers were found to have encouragingly high current efficiencies for CO 2 reduction to CO, mimicking high pressure conditions. In the second approach, high pressure CO 2 in a methanol electrolyte has been used to enhance mass transport, which results from the high CO 2 concentration. Our work with metallic electrodes has recently been extended and now involves p-type semiconductor electrodes such as p-InP, with which we have achieved both high current densities and high current efficiencies. 1. INTRODUCTION Carbon dioxide fixation has been of global interest from both fundamental and practical viewpoints [1-3]. The conversion of CO 2 to useful chemicals has been studied intensively during the past 10 years, and the electrochemical and photoelectrochemical reduction of CO 2 have been of particular interest. The advantage of electrochemical reduction is that the reaction occurs using water as a proton donor under ambient conditions. In an aqueous solution, carbon dioxide is known to be reduced to HCOOH, CO, C H 4 o r alcohols. It is known that the conversion and the distribution of these reduction products depend strongly on the nature and catalytic activity of the electrode material [4]. For example, we have demonstrated the efficient reduction of CO 2 at hydrogen-storing materials such as unmodified and modified Pd electrodes [5, 6]. The use of the hydrogen-storing material has suppressed the evolution of hydrogen and improved the production of CH 4, HCOOH and CHgOH. In addition to control of the product distribution, the attainment of much higher reaction rates is essential for practical use. Recently, the electrochemical reduction of CO 2 with high current density has been studied by many researchers, e.g., using high pressure aqueous systems [7] and gas diffusion electrodes [8]. We

32 are also studying the high rate reduction of CO 2 in CO 2 - methanol solution [9]. The CO 2 concentrations are high, reaching a mole fraction of ~0.3 at 40 arm. In this system, the total current density of CO 2 reduction reaches values comparable to those used in typical industrial electrolyses. The CO 2 concentration is sufficiently high that the reduction rate is not limited by mass transport. The use of gas diffusion electrodes is another way to achieve high current densities. Such electrodes are used in the fuel-cell field and are typically made with porous materials. The electrocatalyst particles are highly dispersed inside the porous carbon electrode, and the reaction takes place at the gas/liquid/solid threephase boundary. CO 2 reduction proceeds on the catalyst particles and the gas produced returns to the gas compartment. We have used activated carbon fibers (ACF) as supports for metal catalysts, as they possess high porosity and additionally provide extremely narrow (several nm) slit-shaped pores, in which "nano-space" effects can occur. In the present work, encouraging results have been obtained with these types of electrodes. Based on the nanospace effects, electroreduction under high pressure-like conditions is expected. In the present work, we have used two types of gas diffusion electrodes. In one case, we have used metal oxide-supported Cu electrocatalysts, while in the other case, we have used activated carbon (ACF)-supported Fe and Ni electrocatalysts. In both cases, high current densities were obtained. Another promising way to reduce CO 2 is by photoelectrochemical means, as first reported by Halmann [10]. Although a number of groups have examined photoelectrpchemical CO 2 reduction, high photocurrent densities are difficult to achieve due to mass transport limitations. One way to overcome this limitation is the use of high concentrations of CO 2 in the electrolyte. In the present work we have used p-type InP and GaAs semiconductor electrodes in the high pressure CO2-methanol medium in order to achieve high photocurrent densities. 2. E X P E R I M E N T A L 2.1. Gas d i f f u s i o n electrodes

Metal oxide supported Cu catalysts were prepared by the alkaline precipitation method. The metal oxide powders were added to an aqueous Cu (NO3) 2 solution and were stirred at 80~ and then 0.1 M KOH was used to precipitate the Cu hydroxide. The precipitates were washed and reduced under hydrogen atmosphere at 400~ after drying. For electrode layers, a mixture of carbon black and PTFE was ultrasonically dispersed in water. The gas diffusion layer and reaction layers contained 20 wt% and 10 wt% PTFE, respectively. The prepared electrocatalysts were mixed (50 wt%) with a carbon black and PTFE mixture and then pressed, together with a stainless steel mesh to form a disk-type electrode (13 mm dia.). The exposed area of the electrode was 0.49 cm 2. The electrode was then heat-treated at 350~ under hydrogen. A 0.5 M KOH aqueous solution was used as the electrolyte. The electrodes containing ACF-supported metal catalysts were prepared in similar way, expect that the gas diffusion layer contained carbon black and PTFE in a 3:1 weight ratio, and the active layer contained carbon black, PTFE and ACF in a 9:3:1 weight ratio. Prior to the

33 preparation of the electrodes, the ACF fibers were soaked in the metal nitrate solutions overnight and washed with water. The adsorbed metal ions were reduced under hydrogen atmosphere at 350~ The electrolyte was 0.5 M KHCO 3. 2.2. High pressure COz-methanol systems The electrochemical reduction of CO 2 in the CO2-methanol solution was carried out under high CO 2 pressure. The high pressure apparatus was assembled from a SUS-316 stainless steel tube. A glass inner tube was used to avoid contact of the electrolyte with the metal apparatus. Various metal electrodes were used in this study. The details have been described previously [9]. A Pt counter electrode and a silver quasi-reference electrode were used. Reagent grade methanol was used as the solvent. Tetrabutyl or tetraethyl ammonium salts were used as supporting electrolytes. 2.3. Photoelectrochemistry For the photoelectrochemical experiments, p-type InP and GaAs wafers were cut into 0.4 cm x 0.5 cm electrodes. Ohmic contacts were made with successive vapor deposition of Zn (30 nm) and Au (100 nm), which were annealed afterward at 425~ in Ar. A stainless steel pressure vessel was equipped with a 2 cm thick quartz window for illumination. The electrolyte solution (3 c m 3, 0.3 M tetrabutylammonium perchlorate (TBAP) in C H g O H ) w a s placed in a glass cell liner in the stainless steel vessel. The gas pressure in the cell was set at the desired pressure (1 to 40 arm). A Xe lamp was used as the light source. Photoelectrolysis was carried out at 1 to 40 atm of N 2 and CO 2. A total charge of 2.2 to 10 C was passed galvanostatically at 5 to 100 mA c m -2 using a potentiostatgalvanostat. The products were analyzed using gas chromatography. 3. RESULTS A N D DISCUSSION 3.1. CO z reduction at gas diffusion electrodes Several metal oxides (ZnO, ZrO2, T i O 2, A1203, Nb205) were used as supports for the Cu metal catalyst. Table 1 shows the reduction products obtained with various electrocatalysts. The metal oxide catalysts without Cu exhibited very little activity for CO 2 reduction, and H 2 evolution was the main reaction. Similarly, low contents of Cu (5 wt%) in the reaction layer showed very little activity for CO 2 reduction. However, high contents of Cu (50 wt%) in the reaction layer produced 44% HCOOH and 4.4% CO. High current densities obtained by this method have indicated the advantage of gas diffusion electrodes. The products detected in the gas phase were carbon monoxide, methane, and ethylene, and that in the liquid phase was formic acid. Figures 1 ( a ) a n d 1 (b)show the current distribution and total current density at various electrode potentials for C u / Z r O 2 and Cu/ZnO, respectively. It is interesting to note that hydrocarbons were produced at high selectivity with the C u / Z r O 2 catalyst, i.e., ethylene was obtained with a maximum current efficiency of 20%, reaching 70 mA c m -2 partial current density at -2.2 V vs SCE. The XRD analysis of samples before and after the electrolysis has confirmed that neither degradation nor dissolution of the electrocatalyst

34 occurred. In the absence of Cu or metal oxides, the activity for C O 2 reduction was very low (mainly hydrogen evolution). These results indicate that the metal oxide supports enhance the selectivity and catalytic activity of metal catalysts. On the various supports, differences in the reaction mechanisms may give rise to the observed differences in the product distributions and catalytic activities. Table 1 Electrochemical reduction of CO 2 on various electrocatalysts a Current Efficiency (%) Catalyst Potential

Current Density c TotaF

mA cm -2

101

-199

102

-149

98

-299

80

-238

96

-299

88

-179

105

-517

2.2

95

-199

....

98

-199

V vs. SCE

H2

CO

CH 4

HCOOH

ZrO2

-1.75

101.2

0.09

0.17

....

TiO2

-1.80

99.45

0.97

0.14

1.37

Cu 5 wt%

-1.66

95.68

0.07

0.12

1.95

Cu 50 wt %

-1.60

31.41

4.41

0.07

44.20

0.2

Cu/ZnO

-1.70

50.69

45.60

0.13

Cu / ZrO2

-1.80

40.5

8.81

3.26

32.50

2.9

Cu/TiO2

-1.60

96.89

0.14

0.07

8.02

Cu / AI203

-1.64

60.38

6.68

0.7

25.40

Cu / Nb205

-1.70

72.83

0.63

0.04

24.60

C2H 4

0.07

" Reaction temperature: room temperature, electrolyte: 0.5 M KOH, charge passed: 30 C; bTotal current efficiency; CTotal current density. 100

!

v

|

|

i

|

u

!

u

u

i

i

i

|

-----~---G0

80

|

u ~

w

'''--N---H2''' '''l''''l''' ---O--- CO --. CH4 = current

.,

i - - - .-' o - - HCH4 COOH / ~" , C2H4 Itl ~ . l 60 -- ~- -current , ~ " ~ 40 r,j

.A

/

,I,,B, i~

600 500 400

~

~

m

300 ,..,~. 200

2O

100 T r,

-1.4

,

i

.~6'

'

i.'8

,

,

,

-2

,

,

,

2.2

,

,

I , , , , n ,

?.14-1.4

-1.6

T

T

, , , l , , , , u , , ,

-1.8

- 2

T I'. ,I,,,,'l

-2.2

1",3 0

- 2.4

Potential, V vs SCE

Figure 1. Potential dependency of reduction products and current density using (A) C u / Z r O 2 and (B) C u / Z n O electrocatalysts.

35 As an extension of the above work, we have used various metal catalysts supported on activated carbon fibers (ACF). The ACF fibers contain slit shape pores with widths on the order of nanometers. In the present work we have used Ni and Fe metal catalysts supported on ACF. We have also used non-activated carbon fiber supports for comparison. Table 2 shows the product distribution for various catalysts at a potential of-1.8 V vs. SCE. When ACF alone was used, hydrogen evolution was found to be very high. Even when the metal was supported on the non-activated carbon fiber (CF/Ni, CF/Fe), there was little activity showed low activity for CO 2 reduction. However, significant activity for CO 2 reduction was observed for ACF/Ni and ACF/Fe electrodes, the partial current density for CO 2 reduction at-1.8 V vs. SCE ranging from 5 to 15 mA cm-2. The effect of porosity was further demonstrated by two types of Ni catalysts supported on ACF. The ACF/Ni-2 catalyst, with a larger surface area, exhibited a high partial current density for CO 2 reduction. It also exhibited high selectivity for CO production. The highest current efficiency for CO 2 reduction to CO reached values of 70% at-1.6 V vs SCE; negligible amounts of CO are produced on conventional nickel catalysts at ambient pressures.

Table 2 Reduction products for various catalysts at-1.8 V vs. SCE Specific Current Efficiency (%) Catalyst Surface Area a H2 CO CH 4 HCOOH Total ACF only

1500

84.69

2.30

0.22

0.00

87.21

Current Density" / mA cm "2 Total C O 2 tot. b 125

2.88

CF / Fe

69.40

0.11

0.05

0.00

69.56

94

0.15

ACF / Fe

64.10

0.16

0.28

9.05

73.58

78

7.40

CF/Ni ACF / Ni-1

700

80.48 69.88

3.38 2.86

0.08 0.15

0.00 12.17

83.94 85.06

109 31

3.68 4.77

ACF/Ni-2

1300

53.14

30.31

0.14

0.00

83.66

47

14.25

a BET surface area, m 2 g - 1 ; b C O 2 reduction partial current density, mA c m -2.

We have also tested the effect of the matrix material on the activity of CO 2 reduction. Figure 2 shows the catalytic activities of A C F / N i comparing two carbon black matrix materials Vulcan XC-72 (Cabot Corp.) and Denka acetylene black (AB). When XC-72 was used, the partial current density for CO 2 reduction saturated at-2.0 V vs SCE. However, in the case of AB, the partial current density continued to increase with increasing potential. The difference may be due to the fact that AB, being more hydrophobic than XC-72, allows more CO 2 molecules to reach the catalyst pores.

36 04

350 3OO < E 25O

.,

O

09 c-

(p E)

,

!

9 Total'

~.

9 CO2 Red.

200 150

/.../".

,

9 Tota 2 Re 9 CO

,

60 d.

.

.."

c"

(D O m

o !--

/

......-'~ f

100 J 50

7

A

.

t/~

70

9"9

/"]

150

# 9, ~ ~ r 40 30

1

z/

B

10

33

~ o

~..,

~

~__. 3 > O

I

-1.6

1.8

~

,

I

-2.2 -2.4 -1.6 -1.8 Potential, V vs SCE

-2

,

,

-2

-2.2

Figure 2. Potential dependence of current density using acetylene black as the electrode matrix material.

-2.4

0

3

rb

(A) XC-72 and (B)

3.2. Electrochemical r e d u c t i o n of CO s in h i g h pressure C O s - m e t h a n o l s o l u t i o n

First, the electrochemical reduction of CO 2 on the Cu electrode was studied in a CO2-methanol medium at various pressures of CO 2. Under both atmospheric and high pressure, CO, CH 4, C 2 H 4, and H 2 w e r e detected as products in the gas phase. In the liquid phase, methyl formate, HCOOCH 3, and dimethoxymethane, C H 3 O C H 2 O C H 3 , w e r e detected. Cyclic voltammograms at various CO 2 pressures are presented in Figure 3 . The cathodic current was observed with an onset potential of-1.0 V under 1 atm of N 2. A shoulder was observed around -1.2 V during the scan in the negative direction. When N 2 w a s replaced with 1 atm of

// 4,--' C" (D

/

/

O

(a)

I

(b)(c)

-1.5

I 2 mA cm-2 I

-1.0

I

-0.5

i

0

Potential vs. Ag QRE / V Figure 3. Cyclic and linear sweep voltammograms (IR compensated) of methanol and CO2-methanol mixture. Scan rate was 50 mV s -1 Dashed line: under 1 atm of N2; solid line: under (a) 1 atm, (b) 20 atm and (c) 40 atm of CO 2.

37 CO2, a larger shoulder was observed with the same onset potential. The cathodic current under CO 2 in the more negative potential region approached the curve under N 2. This result indicates that CO 2 reduction is favorable only in the narrow potential range around -1.3 V. The magnitude of the shoulder increased with increasing CO 2 pressure, indicating an increase in reduction with increasing gas pressure. Figure 4 shows the efficiencies of product formation at various electrode potentials under 40 atm of CO 2. Under this pressure, a marked increase in the efficiency of CO 2 reduction occurred at around -1.0 V (Fig. 4A), as in the CV. CO and HCOOCH 3 were the main products. Formation of CO increased with increasingly negative potential, while that of HCOOCH 3 reached a maximum at -1.8 V. It is interesting to note that the current efficiency of CO 2 reduction did not decrease with increasing potential in the negative direction, indicating a sufficient supply of CO 2 to the electrode surface even under high current densities at high applied potentials. These results are in contrast to the observations under 1 atm or in aqueous systems, where the current efficiency decreases with increasing potential after reaching a maximum value.

100

'

'

~ ?'

'

'

'

I

'

'

'

'_

8o ~9

60

E

40

80

60

40

20 o

~ , -2.5

T -2

i

I

,

,

I

-1.5

,

,,

,-"

-2.

-

- .

-1

Potential, V vs Ag QRE

Figure 4. Effect of the electrode potential on the current efficiencies of products under 40 atm (25~ A) current efficiencies for (a) CO 2 reduction and (b) H 2 evolution; B) current efficiencies for various CO 2 reduction products; (c) CO, (d) HCOOCH 3 (e) CH 4, and (f) C2H 4. The Tafel plots obtained for the formation of different products under 40 atm are shown in Figure 5. Hydrogen evolution and HCOOCH 3 formation become diffusion-controlled at-1.4 and-1.8 V, respectively. However, the current density for CO production keeps increasing even a t - 2 . 3 V. This result demonstrates that the production of CO is not mass transport-limited in this potential range. A high current density of 500 mA c m -2 w a s recorded at -2.3 V. At this potential, the partial current densities for the production of CO and HCOOCH 3 were 134 and 173 mA cm -2, respectively. The total current density for

38 CO 2 reduction was found to be 436 mA c m -2. It is emphasized that these current density values are comparable to those used in industrial electrolytic processes. 1000

E o <E ~_ s

''

100

10

'

''

I

'

'

''

I

'

''

(b "(c ) ' ~ -

'"

-,

(a)~"--..,[3 ~,

!~"

"

..

-

. ,.,

c(!.)

0

1

,"

~,

0

m

.

e3 m

Q_

0.1 -2.5

-2

-1.5

-1

Potential, V

Figure 5. Tafel plots for (a) H 2 evolution, formation under 40 atm (25~

(b) CO formation,

and (c) H C O O C H

3

Hydrocarbon formation is more interesting in the electrochemical reduction of CO 2, since multielectron transfer is required in this process. In the electrochemical reduction of concentrated CO 2 in the CO2-methanol medium, the major products are still the two-electron transfer products, CO and methyl formate, at the Cu electrode, when tetrabutylammonium salts are used (Table 3). However, when tetraethylammonium salt was used as the supporting electrolyte, efficient formation of methane and ethylene was observed with good reproducibility. We defined the hydrocarbon selectivity (THC) as the ratio of the total current efficiency for hydrocarbon production to the total current efficiency for all CO 2reduction products. The 7HC values for were 29% at a CO 2 mole fraction (X) of 0.007 and 6.9% at X = 0.33 with the TBAT electrolyte. These results show that CO 2 reduction to hydrocarbons is less efficient with a high CO 2 concentration. Such behavior was reported using high pressure aqueous systems [11]. When TEAP was used as the supporting electrolyte, the 7HC increased to 49%. The increase in 7HC can be attributed to the effect of the cation of the supporting electrolyte. Earlier, we reported that the electroreduction of CO 2 in a CO 2methanol medium with a TBA salt yields predominantly CO, while that with a lithum salt yields mainly HCOOCH 3. We concluded that the hydrophobic atmosphere of the TBA ion and the hydrophilic atmosphere of Li ion are conducive to CO and HCOOCH 3 formation, respectively [12]. These results also indicate that the use of TEAP instead of TBAP does not affect HCOOCH 3 and H 2 formation markedly but significantly affects the hydrocarbon formation process (Table 3).

39 Table 3 Current efficiencies (q) for various products and hydrocarbon selectivity (THC) in the reduction of CO 2 in CO2-methanol at a Cu electrode a

q (%) Salt

jc

Xb

Y,c

m A . c m -2

H2

CO

HCOOCH 3 CH 4

C2H 4

%

TBATd

0.007 e

500

63.6

13.0

9.7

8.5

0.8

29.0

TBAT

0.33 f

500

4.0

46.4

34.6

3.4

2.6

6.9

TBAP

0.33 f,g

200

9.7

48.1

25.4

7.6

5.2

14.8

TEAP

0.33 f

200

6.5

24.1

27.7

40.7

9.0

48.9

TEAP

0.33 f

1000

9.3

40.9

22.0

15.8

6.9

25.3

aElectrolyses were performed at 25~ bMole fraction of C O 2 at 25~ from ref. 10; CCurrent density at a Cu cathode; dtetrabutyl a m m o n i u m tetrafluoroborate; el atm.; f40 atm; gelectrolysis was conducted at 20~ and thus the mole fraction was slightly larger than 0.33.

HCOOCH 3

t

CO

02H6

I

I

02H4

I

I

OH 4

t

I

H2

I

!

I

Sn

Pb Ag -

Zn ~

Pd

~

~

/

:

,.,, ,,

I I I I I

_

Cu

/

W Ti Pt Ni

-

0

t I I

i:~NNN ,

,

,

I

20

,

,

,

I

40

Current

,

,

,

I

60

,

,

,

I

80

Efficiency,

,

,

,

l

100

,

I

,

120

%

Figure 6. Electrochemical reduction of C O 2 at various metal electrodes. Electrolyses were performed galvanostatically at 200 mA c m -2 and 41 atm.

40 It is well known that the product distribution also depends on the electrode material used [13, 14]. We have examined the effect of various electrode materials on the product distribution in the CO2-methanol system. The current efficiencies of the reduction products are shown in Figure 6. The production of formate was fairly favorable at all electrodes, in comparison with that in aqueous systems. For example, the production of formate was more than 20% on Pt and Ni electrodes, which is much higher than that observed in aqueous systems [14]. At the metals Sn and Sb, formate production was favoured, as in the aqueous systems, but CO formation was also somewhat favored. This is due to the effect of supporting electrolyte. The electrodes Ag, Zn and Pd showed similar activities for CO production, as in aqueous systems. The efficiency of hydrocarbon formation at the Cu electrode was found to be lower, whereas that at the Ni electrode was found to be higher than that in aqueous systems. The balance of hydrogen and carbon atom concentrations on the electrode surface may explain this difference. 3.3. Photoelectrochemical reduction of CO 2 in high pressure CO2-methanol solution Photoelectrochemical experiments using p-InP and GaAs photocathodes were carried out in CO2-methanol medium under high pressure. The currentpotential curves obtained on these electrodes are shown in Figure 7. Negligibly small dark currents were observed at both electrodes. Under argon atmosphere, InP and GaAs showed photocurrent onset potentials of-1.4 V and -1.6 V vs. Ag QRE, respectively. However, under 1 atm pressure of CO 2, both electrodes showed a shift in the photocurrent onset potential in the positive direction, the shift being higher for the InP electrode. This indicates that CO 2 reduction is more favored on the InP electrode.

Ar ,"~//~CO 2 40atm I

//

c'(1)

o

Ar , / / ~ ~ O

E3

I5 mAcm2

mAcro-2

CO2 latm

COe latm I

-2.0

i

i

l

I

p-lnP I

-1.0

l

i

l

I

I

0

2 40atm

-2.o. Potential vs. Ag-QRE, V

p-GaAs -1.o

o

Figure 7. Current-potential curves for methanol under Ar and CO 2 atmospheres using p-InP and p-GaAs photocathodes.

41 The product distribution is summarized in Table 4. The potentials given in the table are IR-free potentials. Under a nitrogen atmosphere, hydrogen was found to be the only product. This is due to the decomposition of methanol. On both electrodes, the main product was CO. Methyl formate was also formed. Insignificant amounts of hydrocarbons were produced. Comparing the two electrodes, InP has higher activity for CO 2 reduction. Table 4 Photoelectrolysis product distributions Photocathode

Gas Pressure Photocurrent (mA cm -2) (atm)

Potential a

(V)

H2

Current efficiency (%) CO H C O O C H 3 Total

p-InP

N2

1

5.0

-1.5

92

0

0

92

p-Inp

CO2

40

100

-1.4

3

93

11

107

p-GaAs

CO2

1

50

-2.0

76

11

19

106

p-GaAs

CO2

40

50

-1.5

15

78

16

109

p-GaAs

CO2

40

100

-1.8

31

48

21

100

aPotential vs. Ag QRE, corrected according to the ohmic resistance measured by current interruption. 4. CONCLUSIONS The use of gas diffusion electrodes appears to be a promising approach to achieve high CO 2 reduction current densities, even at ambient pressures. Another way of achieving high current densities is to use high pressure solvent systems, e.g., CO2-methanol. In addition to high current densities, it is also necessary to consider the source of the input energy needed to drive the reduction reactions. Solar energy is the primary free energy source, if one can utilize it effectively. There are two ways to use solar energy to drive CO 2 reduction. One way is to use a highly efficient solar cell such as a state-of-the-art photovoltaic cell to generate electricity and then to use the generated electricity to drive the reduction reaction in an electrochemical cell at a metal catalyst-based electrode. Another way is to use semiconductor electrodes such as p-InP to convert solar energy directly into chemical energy in a photoelectrochemical cell. In this respect, the use of InP and GaAs in high pressure systems have provided encouraging results. Another promising electrode material for CO 2 reduction is boron-doped diamond, which can be used in both electrochemical and photoelectrochemical reduction. Highly doped diamond is a good material for electrochemical reduction, as hydrogen evolution is extremely inhibited. Lightly doped diamond is semiconducting and the photogenerated electrons in this material have strong reducing power, as the conduction band is close to the vacuum level. Use of this electrode for the photoelectrochemical reduction of CO 2 is expected to yield interesting results.

42 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 277 (1979) 637. P.G. Jessop, T. Ikariya and R. Noyori, Nature, 368 (1994) 231. K.R. Thampi, J. Kiwi and M. Gr/itzel, Nature, 327 (1987) 506. M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Soc., 137 (1990) 1772. K. Ohkawa, K. Hashimoto, A. Fujishima, Y. Noguchi and S. Nakayama, J. Electroanal. Chem., 345 (1993) 445. K. Ohkawa, Y. Noguchi, S. Nakayama, K. Hashimoto and A. Fujishima, J. Electroanal. Chem., 369 (1994) 247. A. Kudo, S. Nakagawa, A. Tsuneto and T. Sakata, J. Electrochem. Soc.,140 (1993) 1541. R.L. Cook, R. C. MacDuff and A. F. Sammels, J. Electrochem. Soc., 137 (1990) 607. T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Electroanal. Chem., 404 (1996) 299. M. Halmann, Nature, 275 (1978) 115. K. Hara, A. Tsuneto, A. Kudo and T. Sakata, J. Electrochem. Soc., 141 (1994) 2097. T. Saeki, K. Hashimoto, N. Kimura, K. Omata and A. Fujishima, J. Electroanal. Chem., 390 (1995) 77. H. Noda, S. Ikeda, Y. Oda, K. Imai, M. Maeda and K. Ito, Bull. Chem. Soc. Jpn., 63 (1990) 2459. Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochim. Acta, 39 (1994) 1833.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

D e v e l o p m e n t o f Electrocatalysts for C a r b o n Dioxide R e d u c t i o n Polydentate Ligands to P r o b e Structure-Activity Relationships

43

Using

Daniel L. DuBois National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado, 80401, USA"

This paper describes the use of polydentate ligands to optimize the performance of palladium catalysts for CO 2 reduction and to probe mechanistic aspects of catalytic reactions. Polydentate ligands can be used to precisely control coordination environments, electronic properties, and specific steric interactions that can lead to new insights into the relationship between catalyst structure and activity.

I. INTRODUCTION The major possible sources of nonfossil energy are photosynthesis, photovoltaics, wind, geothermal, hydroelectric, and nuclear fusion and fission. With the exception of photosynthesis, which produces biomass that can be converted directly to fuels, all the remaining nonfossil sources of energy produce electricity as their primary product. Today fossil fuels are burned to produce electricity, but as we rely increasingly on nonfossil energy sources, electricity will need to be converted to fuels to meet the needs for agriculture and transportation. These sectors of our economy will require fuels with high energy densities. Although biomass can produce some of the fuels that will be needed in the future, increasing food demands will likely limit the land mass available for fuels production from biomass. The efficient electrochemical reduction of CO 2 to a liquid fuel such as methanol, ethanol, or methane would provide a route to renewable fuels with high energy densities that will be important in ensuring a balanced distribution of energy sources in the future. For the electrochemical reduction of CO 2 to a fuel to be feasible, the following requirements will have to be met. 1 First, the overpotential for reduction of CO 2 should be less than 150 mV in order to achieve energy conversion efficiencies of 70% or greater. Second, the catalytic rates must be sufficiently high to permit reasonable current densities---on the order of 0.1 A/cm2. For a monolayer of a homogeneous catalyst immobilized on an electrode surface, this corresponds to a second-order rate constant for the reaction of CO 2with the catalyst of approximately "The financial support of the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division is gratefullyacknowledged. I would also like to thank my coworkers, Calvin J. Curtis, Alex Miedaner, Sheryl A. Wander, AndrewM. Herring, BryanD. Steffyand Paul R. Bernatis for their many contributions to this work.

44 104 M ' I s " 1 . Third, current efficiencies for a single product, such as methanol or methane, must be 90% or greater. Producing a variety of products will lead to lower energy efficiencies and lower energy densities. Fourth, the turnover numbers for the catalysts will have to be high-on the order of 107. This requirement is based on the estimated costs of catalysts and/or the availability of raw materials. Finally, in order to achieve high-energy-density products such as methanol or methane, multi-electron reduction of CO 2 must occur. This can be carried out in a single process or in sequential processes, all operating close to the thermodynamic potential. During the past 20 years, a number of electrocatalysts for CO 2 reduction have been 2 9 9 developed. These may be grouped into four major classes: (1) Group 9 and 10 metal complexes wath macrocychc hgands; (2) Group 7-10 metal complexes with blpyndme 9 4 9 5 9 hgands; (3) Group 10 metal complexes with phosphorus hgands; and (4) various metal electrodes. However, none of the currently known catalytic systems for CO 2 reduction meets all the rigorous requirements described above. Continued development of known and new catalysts will be required before practical systems for direct electrochemical reduction of CO 2 to high-energy-density fuels are possible. 9

9

9

3

.

6

,

.

.

.

.

2. CATALYTIC MECHANISM Our research has focused on developing transition metal phosphine complexes as catalysts for the electrochemical reduction of CO 2. A systematic study of Fe, Co, and Ni complexes containing polyphosphine ligands and weakly coordinated acetonitrile ligands 7 revealed that Ni complexes, although not catalytic, exhibited two, closely spaced, one-electron reductions that occurred at potentials of interest for possible CO 2 reduction catalysts. This led to the evaluation of palladium complexes as well, because they should have similar redox potentials. As a result of screening several palladium complexes by cyclic voltammetry, we found that [Pd(triphosphine)(PR3)](BF4) 2 complexes, 1, would catalyze the reduction of CO 2 to CO in acidic acetonitrile solutions, but not in more weakly coordinating solvents such as dimethylformamide (DMF) or acetone, s 3~p NMR studies clearly indicated that, in acidic acetonitrile, reaction 1 lies well to the right. In DMF and other poorly-coordinating solvents, however, the equilibrium lies to the left. These results indicate that complexes containing ga triphosphine ligand and a weakly coordinating solvent molecule are the true catalytic species. 2+ 2+

+

1

PR3

(1)

2

Preparation of [Pd(triphosphine)(CH3CN)](BF4) 2 complexes resulted in catalysts that were active in more weakly coordinating solvents such as DMF and acetone as well as acetonitrile. For example, CO 2 is reduced to CO with current efficiencies in excess of 95% in acidic DMF solutions using [Pd(etpC)(CH3CN)](BF4) 2 as the catalyst (where etpC is bis(2-dicyclohexylphosphinoethyl)phenylphosphine). The closely related complexes, [Pd(dppe)2](BF4) 2 and

45 [Pd(dppp)(CHaCN)2](BF4)2 (where dppe is bis(diphenyl-phosphino)ethane and dppp is bis(diphenylphosphino)propane), are not active catalysts. These results support complex 2 as the active form of the catalyst and demonstrate the utility of using polydentate ligands with different hapticities to probe the optimum number of strongly and weakly coordinating ligands. Electrochemical studies of [Pd(triphosphine)(CI-13CN)](BF4)2 complexes were carried out under both catalytic and noncatalytic conditions to probe mechanistic aspects of CO 2 reduction. The mechanism that we have proposed for this catalytic reaction is outlined in Scheme 1. In this scheme, L represents a triphosphine ligand and solv represents a solvent molecule such as DMF or acetonitrile. The data supporting the various steps shown will be summarized next. [LPd(solv)]2+ (soN)] +

1

[LPd(CO)(H20)] 2+ /

CO2 ~ 2

(

H+_~ [LPd(s~

+

#3

H

[LPd'c~OH] 2+ ~

..~.

le" N~//[LPd`s~

6

2+

4

solv + [LPd(COOH)]+ ~ Scheme 1 Comparison of the cyclic voltammograms of [Pd(triphosphine)(CHaCN)](BF4) 2 complexes under N 2 and CO 2 atmospheres indicates that the first reduction wave is decreased in height and shifted in a positive direction as shown in the left-hand graph of Figure 1. This behavior has been interpreted as evidence for the reduction of the Pd(II) complex to a Pd(I) complex, followed by rapid reaction with CO2, as shown in steps 1 and 2 of Scheme 1. Support for a one-electron reduction of [Pd(triphosphine)(CH3CN)](BF4) 2 in the presence of CO 2 comes from a comparison of the magnitude of the current for the first reduction wave of this complex to that of [Pd(triphosphine)(PRa)~](BF4)2 complexes under N2 (which have been shown to undergo two-electron reductions).- For both cyclic voltammetry and chronoamperometric experiments, the relative magnitudes of the currents for these complexes are consistent with the acetonitrile complexes undergoing one-electron reductions. Controlled potential electrolysis experiments of [Pd(tfiphosphine)(CHaCN)](BF4) 2 under CO 2 atmospheres are also consistent with one-electron reductions. Finally, recent electrochemical studies of similar palladium complexes containing triarsine ligands are consistent with two

46

0 . 5

~

I

'

I

'

I

'

2.0

i

I-

r E o I,,.. r

32 :;

0.0

(3.

............,,

-0.5

E

-2.0

0 L_ 00 .-

-4.0

-1.0 -1.5r

0.0

'

I

'

'

I

=

I

-6.0 ,

-1.4

,

,

,

-1.2

,

-1.0

,

,

-0.8

Volts vs Ferrocene

-8.0

I

,

I

,

I

,

I

,

l

-1.6 -1.4 -1.2 -1.0 -0.8 V o l t s vs F e r r o c e n e

Figure 1. Cyclic voltammograms of a 1.0 x 10"3 M solution of [Pd(IPNetpE)(CH3CN)](BF4)2 in DMF under N2 (left-hand graph, open circles) and in the presence of 0.18 CO2 (felt-hand graph, solid line). The fight-hand graph shows cyclic voltammograms of the same solution after adding acid (0.04 M HBF4) under N2 (open circles) and CO2 (solid line). sequential one-electron reductions, in which the electron transfer for the Pd(I/0) couple is significantly slower than that of the Pd(II/I) couple. ~~ All these results support a one-electron reduction of [Pd(triphosphine)(CH3CN)](BF4)2, followed by reaction with CO 2. The Pd(I) species reacting with CO 2 is probably four-coordinate. Reduction of [Pd(triphosphine)(solvent)](BF4) 2 complexes in the presence of CO 2 results in a shift of the peak potential of the Pd(II/I) couple to more positive potentials, as shown in the left-hand graph of Figure 1. This is consistent with the solvent molecule remaining bound to palladium or exchanging reversibly with bulk solvent. If an irreversible loss of solvent occurred following reduction but prior to CO2 binding, CO 2 should have no effect on the peak potential. The rate of reaction of [Pd(triphosphine)(solvent)]+ with CO 2 can be measured under noncatalytic conditions (i.e., in the absence of acid) by using the shift in peak potential. The rate can also be measured under catalytic conditions by measuring the magnitude of the catalytic current. The fight-hand graph in Figure 1 shows the current observed for [Pd(IPNetpE)(CH~CN)](BF4) 2 (where IPNetpE is bis(diethylphosphinoethyl)(diisopropylamino)phosphine) in the presence of 0.04 M HBF a under N2 and CO 2. It is clear that a catalytic wave is observed when both CO 2 and acid are present. Equation 2 shows the relationship between the peak or plateau current for a catalytic wave and the concentrations of substrate and catalyst for a simple catalytic reaction following a reversible electron-transfer. ~ ic = n F A

[cat](Dk [S] y) 1/2

(2)

In eq. 2, ic is the catalytic current, n is the number of electrons transferred, F is the Faraday constant, A is the area of the electrode, [cat] is the concentration of the catalyst, D is the diffusion coefficient of the catalyst, k is the apparent rate constant, [S] is the concentration of the substrate under investigation, and y is the order of the rate-determining reaction in substrate S. From eq. 2 it can be seen that the square root dependence of the catalytic current on CO 2 concentration (Figure 2 curve A) and its linear dependence on catalyst concentration (Figure 2 curve B) are consistent with a rate-determining step that is first order in catalyst and first order in CO 2. Within the margins of experimental error, the rate determined under

47

25.0

|

I

|

!

|

A

ca. 20.0 E o 32 15.0

E : o 32 t._

,X,

10.0 0.0

I

0.1

-

I

I

0.2

I

0.3

=

I

0.4

[002] 112 i 1/2

I

0.5

!

100.0

.

50.0

i

0.0 0.0

!

|

!

I

!

|

!

|

B

0.5 [cat]

1.0 ma

Figure 2. The let~-hand graph is a plot of the catalytic current vs. the square root of the CO2 concentration for a 2.0 x 10-4 M solution of [Pd(etpC)(CH3CN)]BF02 in acidic DMF (0.04 M HBF4). The fight-hand graph is a plot of the catalytic current vs catalyst concentration for an acidic DMF solution (0.04 M HBF0 saturated with CO2 at 620 mm Hg (0.18 M). catalytic conditions by measuring the peak current of the catal~ic wave is the same as that determined in experiments based on the shiR of peak potentials. These results indicate that the reaction of the Pd(I) intermediate with CO 2 is the rate-determining catalytic step (step 2 of Scheme 1) at high acid concentrations. In subsequent studies, we have investigated the effect of varying the substituents on the triphosphine ligand on the rate of reaction of [Pd(triphosphine)(solvent)]+ with CO2, step 2 of Scheme 1. ~2 A linear free-energy relationship exists between the redox potential of the catalyst and the log of the rate constant for the reaction of the catalyst with CO 2 (Figure 3, open circles). This is reasonable if this reaction is viewed as a nucleophilic attack of the metal on CO 2 with a transfer of charge from the metal to CO 2. A second observation is that bulky substituents on the central phosphorus atom retard the catalytic rates by approximately a factor of 2 (open squares), whereas bulky substituents on the terminal phosphorus atoms have little effect. This result can be understood from the structure of the [Pd(MesetpE)(CH3CN)] 2+ cation (where MesetpE is bis(2-diethylphosphinoethyl)mesitylphosphine) shown in the inset of Figure 3. In this structure, a methyl group of the mesityl substituent on the central phosphorus atom effectively blocks one face of the catalyst. Consequently, only one side of the catalyst is available for CO 2 binding, and the rate decreases by a factor of two. The presence of the methyl group above the palladium atom also suggests that six-coordinate intermediates can be excluded, because such complexes are not sterically feasible. This result is significant, because coordination of a sixth ligand has been suggested to play an important role in CO z binding for some Ni(I) catalysts containing macrocyclic amine ligands. ~3 The observation that bulky substituents on the central phosphorus atom of the triphosphine ligand have a greater effect on the catalytic rate than bulky substituents on the terminal phosphorus atoms indicates that CO 2 approaches the catalyst along the axis perpendicular to the plane of the catalyst. The next two steps in the catalytic mechanism, protonation (3) and a second electron transfer (4), can be inferred by comparing cyclic voltammograms of catalyst solutions saturated with CO 2 in the presence and absence of acid. In the presence of CO 2 but no acid, the peak current of the first reduction wave is consistent with a simple one-electron reduction (Figure 1, left-hand graph). When acid is added to this solution, the current increases

48 3.0 2.5 2.0

A

O

I

I

HOP, Et - ~" Me,E

'

4

A

1.5

I

J

I

I

M e s P+ , Et

Me,Cy

A

Me2N, Et

I

Bu 3P ,

N, N, Et Et Mes, Et [-7-"

,Cy h, Et

t-Bu, Et F]

Ph, Ph

Mes, Cy [3

1.0

Mes, Ph

0.5 -1.6

-1.4

-1.2

-I.0

Ell2(volts vs Ferrocene)

Figure 3. Plot of the log of the rate constant for the reaction of the Pd(I) intermediate with CO2 as a function of E~/2 for various [Pd(triphosphine)(CHaCN)](BFa)2 catalysts in DMF. Each ordered pair of substituents lists the substituent on the central phosphorous atom of the tridentate liquid first, and the terminal substituents second. BuaP + represents a tri(nbutyl)ethylphosphonium substituent, HOP is hydroxypropyl, and other substituents are defined in text or have their usual meaning. (Figure 1, right-hand graph). This implies that protonation of the CO 2 complex formed in step 2 must occur before the second electron transfer can take place at the potential of the first wave. Loss of the solvent ligand must also occur during the catalytic cycle. As discussed above, if the coordinated solvent such as acetonitrile or DMF is replaced with strongly coordinating ligands such as a monodentate phosphine ligand, catalytic activity is suppressed. In addition, dimethylsulfoxide (a strongly coordinating solvent) also suppresses catalytic activity, although reaction of the catalyst with CO 2 does not appear to be significantly affected as judged by peak shift measurements of the catalyst in the presence of CO 2 but in the absence of acid. These observations suggest that solvent loss occurs during the catalytic cycle and that it is important for cleavage of a C-O bond as shown in step 7. A biphasic dependence of the catalytic current on acid concentration is observed for these catalysts, as shown in Figure 4 for [Pd(IPNetpE)(CH3CN)](BF4) 2. For this complex, the current exhibits a first-order dependence on acid at low acid concentrations, but no acid dependence is seen at high acid concentrations. The linear dependence of the catalytic current on acid concentration is consistent with two protons being involved in the transition state (steps 3 and 6 of Scheme 1). Although the second protonation step could be rate-determining, we have interpreted this result to imply that, at low acid concentrations, the rate-determining step is the cleavage of the C-O bond to form coordinated CO and water, as shown in step 7 of Scheme 1. This bond cleavage reaction is sensitive to the number of carbon atoms in the 9

9

49 30

i0.0 tn

~' 20

1"13 O

u

-,.-.I

lO iJO0

9

9

9

9

9

.3 O ~4 tO "rq

0.i

0.15

[HBF 4 ] M

'

8.0

'

I

'

'

I

i

i

'

I

'

J

I

I

O

6.0 4.0

2.0~ V 0.05

'

l

0.000

I

i

0.020

0.040

[HBF 4 ]

Figure 4. Left-hand graph shows the dependence of the peak current on acid concentration for a 2.2 x 10.3 M solution of [Pd(IPNetpE)(CH3CN)](BF4)2 in DMF purged with N2 (solid circles) and CO2 (open circles). The fight-hand graph shows the peak current versus acid concentration for 2.3 x 10.3 M solutions of [Pd(ttpE)(CH3CN)](BF4)2 in DMF saturated with N2 (solid circles) and CO2 (open circles). backbone of the triphosphine ligand. As just discussed for [Pd(IPNetpE)(CH3CN)](BF4)2, the catalytic currents for all [Pd(triphosphine)(CH3CN)](BF4) 2 complexes with two-carbon linkages between phosphorus atoms show a linear dependence on acid concentration, consistent with the loss of water from the activated complex. For complexes with a threecarbon linkage in the triphosphine ligand, such as [Pd(ttpE)(CH3CN)](BF4)2, the catalytic current shows a square root dependence on the acid concentration consistent with a ratedetermining step that is first order in acid (Figure 4, fight-hand graph). This implies that a hydroxide ion is lost from the activated complex, and that the C-O bond is more easily cleaved than it is in the two-carbon chain analog. A structural study of [Pd(ttpE)(CH3CN)](BF4) 2 shows that a significant steric interaction exists between two of the terminal ethyl groups of the tridentate ligand and the acetonitrile ligand. ~4 We believe that a similar steric interaction between the substituents on the terminal phosphorus atoms and the coordinated hydroxycarbonyl ligand promotes the cleavage of the C-O bond in the catalytic cycle of [Pd(ttpE)(CH 3CN)]03F,)=. As discussed in the preceding paragraph, the [Pd(triphosphine)(CHaCN)](BF4) 2 catalysts exhibit a biphasic dependence on acid concentration, which is consistent with steps 2 and 7 of Scheme 1 being the rate-determining steps at high and low acid concentrations, respectively. If a steady state approximation is applied to Scheme 1 with steps 2 and 7 as the rate-determining reactions, and the resulting rate constant is incorporated into eq. 2, eq. 3 results. In this eq., id is the peak current for a reversible reduction of the catalyst under noncatalytic conditions, tr is a constant that depends on whether the second electron transfer occurs in solution (1.414) or at the electrode surface (2), ~5 n is the number of electrons involved in catalyst reduction, v is the scan rate, k~ is the second-order rate constant for the reaction of CO 2 with the Pd(I) intermediate (step 2), and k2 is a third-order rate constant, which is undoubtedly not a simple rate constant.

i d - 0.4471 n F V ~ k l [ C 0 2 ] +

k2[H+] 2

(3)

50 The solid line shown in the lett-hand graph of Figure 4 is a best-fit line to (3), and can be seen to account well for the acid dependence observed. For the complexes with a three-carbon linkage between the central phosphorus atom and the terminal phosphorus atoms of the triphosphine ligand, a similar fit can be obtained by assuming a first-order dependence on acid concentration in eq. 3 (see fight-hand graph of Figure 4). The dashed line in the fight-hand graph is the best-fit line assuming a second-order acid dependence for the second ratedetermining step. The catalytic cycle, Scheme 1, is completed by a rapid loss of CO from the Pd(II) species and solvent is coordinated (step 8). Consistent with CO loss is the observation that no CO adducts were detected by 3~p NMR spectroscopy when [Pd(triphosphine)(CH3CN)](BF4) 2 complexes were dissolved in non-coordinating solvents such as dichloromethane in the presence of CO gas (60 psi). Based on the data described above, we believe that Scheme 1 represents a good working model for the mechanism of CO 2 reduction by [Pd(triphosphine)(CH3CN)](BF4) 2 complexes. 3. SIDE REACTIONS Side reactions are of two types: one leads to products other than CO and the other to catalyst degradation. Our studies have shown that the nature of the tridentate ligand can play an important role in both types of side reactions. The major reduction product, other than CO, for all the catalysts we have studied is hydrogen. We have found that the production of hydrogen does not appreciably affect the kinetics of the complexes studied. For example, electrochemical reduction of [Pd(PCP)(CH3CN)](BF4) , 3, in the presence of CO 2 and acid produces only hydrogen. ~6 However, the catalytic current at high acid concentrations is consistent with a first-order reaction of a Pd(I) species with CO 2 and is independent of acid concentration similar to the [Pd(triphosphine)(solvent)](BF4) 2 complexes discussed above. For complex 3, CO 2 acts as a cofactor for hydrogen production, but CO 2 is not reduced. This

..'H~ H 0 P /

'-- IFPh2 P d - - NCCH3

~ ADP +-OC(O)OP(O)O22-OC(O)OP(O)O22- + NH 3 ...... > H2NCO2- + HOP(O)O22-

We have used organo-phosphorus acids [25] as promoters of the reaction of aromatic amines with dimethylcarbonate (DMC) or diphenylcarbonate (DPC) in the presence of carbon dioxide to generate N-alkyl- or aryl-carbamates. We have applied this methodology to the carbamation of aniline, naphtylamine, toluendiamine, 4,4'-diaminophenyl-methane, among others. P-acid H 2 N A r N H 2 + ROC(O)OR ........ > H2NArNHC(O)OR + ROH (R = Me, Ph) (6) H2NArNHC(O)OR + ROC(O)OR ...... > RO(O)CNHArNHC(O)OR + ROH (7) The carbonate itself can be used as solvent and the reaction is very selective as no methylation or arylation products of the amine are found. The catalyst can be easily and quantitatively recovered as arylammonium salt at the end of the reaction and recycled. Thereof, the carbamate is not contaminated with P. These features make {Ph2P(O)OH} very attractive from a practical point of view. This represents the first example of non-metal catalysis for the synthesis of carbamates from carbonates. Kinetic experiments support the "nucleophilic catalysis" depicted below. X2P(O)OH + PhNH2

X2P(O)O- +H3NPh

(R- Ph, Me; X: Ph,PhO)

X2P(O)O- +H3NPh + (RO)2C=O---> X2P(O)OC(O)OR + ROH + PhNH2 PhNH 2 + X2P(O)OC(O)OR ..... > X2P(O)OH + PhNHC(O)OR

(8) (9) (10)

This mechanism requires the intermediate formation of the carbonic diphenylphosphinic(phosphoric) mixed anhydride X2P(O)OC(O)OR. Upon reaction with the free aromatic amine, the anhydride converts to the carbamic ester, and regenerates the acid catalyst, X2P(O)OH. The mechanism is analogous to the mechanisms suggested for several enzymatic reactions that use H C O 3 - a s the carboxylating agent. The mixed anhydride X2P(O)OC(O)OR (XI) is structurally O

0

O

O

[I

II

II

II

x """"

C oR X

-0 """"

0j c OH

XI

reminiscent of carboxyphosphate (HO)OP(O)OCO22- (XII).

XII

73 In biological systems XII is the key intermediate in the carboxylation reactions. This similarity is not fortuitous as X2P(O)OC(O)OR and (HO)OP(O)OCO2 2represent the activated form of (RO)2C=O and HCO3-, respectively. The recent application of this synthetic methodology to the carbamation of diamines [25b] is of great interest as di-carbamates are the source of di-isocyanates used in the synthesis of polyamides. Interestingly, the catalyst can be used for the selective synthesis of both mono- and di-carbamates. 4. REDUCTION OF CO2 TO CO AND FIXATION OF THE REDUCED FORM. We have studied for long time the co-ordination chemistry of carbon dioxide and the reactivity of the co-ordinated molecule [26]. Also this aspect of the chemistry of carbon dioxide is relevant to biological systems. Carbon dioxide is the electron accepter in metabolic processes characteristic of several anaerobic microorganisms, methanogens and acetogens among others. Their enzymatic reaction mechanism has surprising relevance to chemical facts. An example of such similarities is given in Scheme 5. Indusld~ Processes O3) Water gas shift reactions.

Enzymatic Processes (a) CODH mediated synthesis of CO from CO2

[M] + CO2 + 2 [H+ + e'] --->

M-CO + H20

CODH mediated synthesis of the acetyl-moiety Corrin-Cl-13 + Ni--Fe(CO) ..... > Corrin + ( C ~ O 0 ) (CHo)Ni--Fe(CO) ..... > Ni--Fe-C(O)CH 3. or

(CH3)Ni--Fe(CO) ..... > Ni--Fe(CO)(GH0) ..... > N-FeC(O)Ot3 NI-Fe-C(O)CH3 ..... > CH3(O)C-Ni--Fe

CH3-G(O)-Ni--Fe

+

CoAS" .... > Ni--Fe + CoAS~O)CH3

co2+ CO+l~

H20 ~'--'~

%

CO + H2 CO2+H2.

Homologation of methanol to the acetyl-moiety. CH~ + HI -->CH31 + I..nM(CO) + 01"131 ~

co.

LnlM(CO)(CH3) ~

LnlM(CO)(CHs) LnI(CO)M-C(O)CH3

Ln represents ancillary ligands, neutral or charged. M is Co, Fe, Rh, Ru.

Scheme 5. Comparison of enzymatic and chemical reduction of CO2 to CO and fixation of the reduced form. The enzymatic mechanism presents, to date, some major points that deserve investigation: i) the existence of different sites where the reduction of carbon dioxide and the formation of the C-C bond take place, respectively, ii) The role of the Ni and Fe centers, and of-SH groups in the above mentioned processes. Under the correct conditions, model systems can contribute to shed light on enzymatic reactions. We have started an extended investigation on Fe and Ni complexes as CO2-fixation catalysts. We report here on some Ni-model systems

74 that fully mimic the enzymatic activity converting CO2 into an organic thioester, v/a reduction to CO and reaction with thiols and an olefin. For a long time, Ni has been considered to be, in the enzymatic system, the metal center binding CO and, probably, catalyzing the CO2 reduction [28]. The involved couple of oxidation states of the metal (NiLNi II, NiII-Ni III Ni0-Ni I) has been matter of discussion and investigation. If Ni or Fe is the center where CO2 is reduced to CO is still unclear, but it has been proposed that CO is bound to Fe prior the acetyl moiety is generated. Chemical models say that both Ni and Fe complexes are able to bind carbon dioxide. Concerning the CO2 reduction to CO, at the present time evidences that the Ni complex (PCy3)2Ni(CO2) (XIV) (Cy = cyclohexyl) catalyzes the CO2 reduction reaction in the presence of proton and electron donors can be found in the literature [26]. The fluxional behaviour of the carbon dioxide moiety coordinated to the metal has been unambiguously established in the case of XIV and the rotational free energy barrier (AG # = 39.6 kcal mo1-1) determined. XIV in toluene, under dinitrogen atmosphere, reacts with several Broensted acids, under electron transfer conditions, and shows a different behaviour according to the species and the reaction conditions. PhSH is a quite interesting reagent: in fact when it is added to a solution of XIV in toluene the i.r. spectrum of the solution shows the immediate disappearance of the band at 1745 cm -1 (due to coordinated CO2) while two new bands at 1980 and 1912 cm-1 indicate the formation of the bound-carbonyl species (PCy3)2Ni(CO)2 (XV). XV and (PCy3)2Ni(SPh)2 (XVI) were isolated from the reaction mixture as thermodynamic products and characterized by elemental analysis, i.r. and 13C, 31p n.m.r, spectroscopy. Water is also an end product of the reaction, while PhSSPh, that is a kinetic product with "(PCy3)2Ni(CO)" (XVII), was detected in the reaction medium but not isolated. In fact the disulphide reacts with the Ni(0) existing in solution: (11)

"(PCy3)2Ni" + PhS-SPh ...... > (PCy3)2Ni(SPh)2 The overall stoicheiometry of the reaction is:

3(PCy3)2Ni(CO2) + 4PhSH --> (PCy3)2Ni(CO)2 + 2(PCy3)2Ni(SPh)2 + CO2 + 2H20 (12) We have extended our studies on Ni complexes [17] in order to ascertain if Ni itself is able to catalyze the formation of a thioester, reproducing it alone the full enzymatic cycle that uses CO2 for the formation of the thioester group.

SPh 0

PhS~c~O

~ SPh XVIII

XIX

By reacting (PCy3)2Ni(CO2)with PhSH in the presence of 1-heptene, under dinitrogen atmosphere, the formation of thioesters was observed.

75 XVIII and XIX demonstrate that the formation of a thioester from CO2 and thiols can be p r o m o t e d by a single Ni center. The coordinatively unsaturated species "(PCy3)2Ni(CO)" (XVII), that is the kinetic product in the N i - p r o m o t e d CO2 reduction, must play a key role. In fact, when tri- or di-carbonyl species, n a m e l y (PCy3)Ni(CO)3 (XX) or (PCy3)2Ni(CO)2 (XXI) were used in the same reaction conditions, the formation of thioesters was not observed. This finding helps to explain w h y the reaction affording the thioester may give quite variable yields with the reaction conditions. XVII can either generate the dicarbonyl (Eq. 13) (that is not reactive towards olefins and the thiol), or interact with the olefin to afford 2 "(PCy3)2Ni(CO)"

..... > (PCy3)2Ni(CO)2 + "(PCy3)2Ni"

(13)

XXII (Eq. 14) that generates the thioester. "(PCy3)2Ni(CO)" + CH2=CHR . . . . >

(PCy3)2Ni(CO)(CH2=CHR) (XXII)

(14)

The reaction conditions play an important role addressing the reaction towards one or the other of the pathways. The direct utilisation of the C O D H enzymatic complex in synthetic chemistry is now one of our objectives. 5. CONCLUSIONS The utilisation of carbon dioxide in synthetic chemistry is a promising way for developing benign synthetic methodologies, avoiding toxic species and saving energy and carbon. New catalysts, that should be at the same time active and selective, must be synthesized. Metal systems are excellent cadidates for such reactions, although non-metal systems are also of interest. Nature provides very interesting examples of catalytic fixation of both the entire carbon dioxide molecule and its reduced forms. The utilisation of either biosystems or mimetic complexes is very challenging for chemists. We have found that in some cases this approach can give interesting results that might be exploited at the industrial level. REFERENCES .

2.

.

4.

.

6.

M. Aresta, I. Tommasi, Energy Convers. Mgmt, Vol. 38, (1997) 373, "and references therein". Other technological applications (refrigerators, food packaging, fumigant, fire estinguishers, soldering, moulding, dust abatement), as well as water treatment, are also quite common in industrial processes. Roughly 10 Mt per year are used for these purposes. M. Aresta, E. Quaranta, I. Tommasi, P. Giannoccaro, Gazz. Chim. Ital, 125, (1995) 509. a) T. Yatsuka, A. Ito, O. Manabe, M. Dehara, H. Hiyama, Yuki Gosei Kagaku Kyokai Shi, (1976) 30, 1030; C.A., 78, 84091b (1973);b) W. Bachmann, C. Gnabs, K. Janecka, E. Mudlos, T. Papenfuhs, G. Waese, Ger. Often. 2, 426, 850; C.A., 85, 20936t (1976); c) F. Mutterer, C.D. Weis, J. Heterocycl. Chem., 13, (1976) 1103; d) Z. Weglinski, T. Talik, Rocz. Chem., 51, (1977) 2401; C.A. 89, 43036w (1978). A. Lack, I. Tommasi, M. Aresta, G. Fuchs, Eur. J. Biochem., 197, (1991) 471. a) R. Liberio, Doctor Thesis, Department of Chemistry, University of Bali, 1997. b) M. Aresta, I. Tommasi and R. Liberio, paper in the press. M. Aresta, C. Fragale, E. Quaranta, I. Tommasi, J. Chem. Soc., Chem. Commun., (1992) 315. M. Aresta, I. Tommasi, E. Quaranta, C. Fragale, J. Mascetti, M. Tranquille, F. Galan, M. Fouassier, Inorg. Chem., 35 (1996) 4254.

76

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10. 11. 12. 13. 14. 15.

16.

17. 18. 19. 20. 21.

22. 23. 24. 5,

26. 27. 28.

B.B. Wayland, Polyhedron, 7 (1988) 1545. R. Curd, J. O. Edwuards "Activation of hydrogen peroxide by organic compounds" in Catalytic oxidations with H202 as oxidant, G. Strukul Ed.; Catalysis by metal complexes Series; Reidel-Kluwer. Dordrecht, Netherlands, (1992), Chapter 3. M. Aresta, E. Quaranta, ChemTech. 27, (1997)32. P. Adams, F. A. Baron, Chem. Rev., 65, (1965) 567. a) P. Piccardi, Chem. Ind. (Milan), 68 (1986) 108. b) T. Teh Wu, J. Huang, N.D. Arrington G.M. Dill, J. Agric. Food Chem., 35, (1987) 817; c) F. Rivetti, U. Romano, M. Sasselli, US Pat., 4 514 339, (1985) (to ECS). H. Babad, A.G. Zeiler, Chem. Rev., 73, (1973) 75. a) W. Lorenz, I. Hamman, Get. Pat., 2258805, 1972 (to Bayer AG) (Chem. Abstr., 1974, 81, 77701); b) B.A. Teicher, A.C. Sartorelli, J. Med. Chem., 23, (1980) 955; c) F. Maurer, I. Hamman, B. Homeyer, W. Behrenz, Eur. Pat., 23326, 1979 (to Bayer AG) (Chem. Abstr., 95, 43096q, 1985) a) T. Hayashi, Bull. Inst. Phys. Chem. Res. (Tokyo), 11, (1932) 133; b) H.B. Wright, M.B. Moore, J. Am. Chem. Soc., 70, (1948) 3865: c) J.K Wolle, K.L. Temple, J. Am. Chem. Soc., 70, (1948) 1414; d) K.R. Zahradnich, Chem Tech., 11, (1959) 546; e) J. B. Lallau, J. Masson, H. Guerin, M.F. Roger, Bull. Soc. Chim. Fr., 1311 (1972); f) S. Theodoropulos, Eur. Pat., 62161, 1981 (to Union Carbide Corp.) (Chem. Abstr., 98, 88842h, 1983). T.W. Martinek, US Pat. 3 061 637; 1958 (to The Pure Oil Co.) (Chem. Abstr., 58, 6700 g, 1963). F. Calderazzo, S. Ianelli, G. Pampaloni, G. Pelizzi, M. Sperile, J. Chem. Soc., Dalton Trans., (1991) 693. H. Noth, D. Schlosser, Chem. Ber., 121, (1988) 1715. R. L. Cowan, W. C. Trogler, Organometallics, 6, (1987) 2451. a) R.H. Cragg, M. F. Lappert, J. Chem. Soc., (1962) 82. b) H. Breederweld, Red. Trav. Chim. Pays-Bas, 81, (1962) 276. c) G. Oertel, H. Malz, H. Holtschmidt, Chem. Ber., 97, (1963) 891; d) T. A. Georges, K. Jones, M. F. Lappert, J. Chem. Soc., (1965) 2157; e) M. F. Lappert, B. Prokay, Adv. Organomet. Chem., (1969) 1356; f) M. R. Bandet, J. Sotge, Bull. Soc. Chim. Fr., (1969) 1356; g) R. F. Dalton, and K. Jones, J. Chem. Soc. A., (1970) 590; h) J. Koretsu, Y. Ishii, J. Chem. Soc. C., (1971) 511; i) L. K. Peterson, K. I. Th6, Can. J. Chem., 50, (1972) 562. a) M; Aresta, E. Quaranta, Tetrahedron, 48, (1992) 1515; b) M. Aresta, E. Quaranta, Ital. Patent 1 237 208, (1993). a) M. Aresta, E. Quaranta, Ital. Patent, 1 237 207, (1993); b) M. Aresta, E. Quaranta, Tetrahedron, 47, (1991)9489. M. Aresta, A. Ciccarese, P. Giannoccaro, E. Quaranta, I. Tommasi, Gazz. Chim. Ital., 125 (1995) 509. a) M. Aresta, C. Berloco, E. Quaranta, Tetrahedron, 51 (1995) 8073. b) M. Aresta, E. Quaranta, A. Bosetti, Ital Patent Appl. MI 96 A002202 (1996). For more recent works see: a) M. Aresta, E. Quaranta, I. Tommasi, R. Gobetto, Inorg. Chem., 31, (1992), 4286. b) C. Jegat, M. Fouassier, M. TranquiUe, J. Mascetti, I. Tommasi, M. Aresta, F. Ingold, A. Dedieu, Inorg. Chem., 32, (1993), 1279. a) S.W. Ragsdale, J. E. Clark, L. G. Ljungdahl, L. Lundie, H. L. Drake, J. Biol. Chem., 258 (1983),2364. b) S.W. Ragsdale, Coord. Chem. Rew., 96, (1996) 2515. M. Aresta, E. Quaranta, I. Tommasi, C. Fragale, Inorg. Chim. Acta, 1997, in press.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

77

Scope of studies on C O 2 m i t i g a t i o n

K. Yamada Dept. of Chemical System Engineering, School of Engineering, The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 1. I N T R O D U C T I O N The atmospheric concentration of CO 2, which is a main greenhouse gas, has grown significantly since pre-industrial times and caused an increase in global mean surface temperature. Therefore, CO 2 mitigation measures should intensively be implemented. There are two methods to mitigate the increase in the atmospheric CO2 concentration. One is to reduce CO 2 emissions by improving energy-efficiency (relating to energy intensity) or developing fuel substitutes (relating to carbon intensity), and the other is to collect and then immobilize atmospheric or other CO 2 in the ocean or on land. An economic revenue over the short term is most important in realizing such measures. However, results of evaluations of energy consumption and its environmental impact as well as the potentM for CO2 reduction should also be taken into account for a long term plan of such systems. In this paper, studies on CO2 mitigation options such as CO 2 capture, sequestration, biological fixation, energy conservation and renewable energy technologies are discussed. Regarding renewable energy technologies, our estimated results are shown herein. 2.

CO 2

CAPTURE TECHNOLOGY

Energy requirements to capture and sequester

CO 2 are

very high, even flue gases from fossil

fuel power plants which are large stationary sources of CO2 are treated. Moreover, capture and sequestration technologies are said to be effective for CO 2 mitigation and emergency measures. It is very important to reduce the energy penalty due to CO 2 capture for the realization of such technologies. A chemical absorption method with a monoethanolamin (MEA) solution is used in commercial CO 2 capture plants and considered to be the lowest energy requirement process. When it is applied to a conventional coal power plant, a typical energy penalty is in a range of

78 20-3O%. Mimura et al [1] found an effective amine solvent (KS-2) with a long lifetime and reported that the KS-2 solvent system could reduce the energy penalty by 20% compared with a MEA system. The power plant losses based on generator output power were calculated to be 5.4-5.8% for a natural gas-fired plant and 9.0% for a coal-fired plant at CO 2 recovery of 90%. The CO 2 capture from the flue gas of power plants always causes energy penalties, on theother hand, if CO2 is separated from the recycling gas of fuel cells, the energy merit may be obtained. Sakaki et al [2] reported that the energy efficiency of a solid oxide fuel cell (SOFC) could be increased by CO 2 separation from its recycling gas at energy requirements of less than two times the theoretical separation energy. It means the possibility of CO 2 capture in fuel cell systems without the use of additional energy. 3. CO z C H E M I C A L U T I L I Z A T I O N AND S T O R A G E T E C H N O L O G Y I E S

3.1 Chemical utilization As Aresta [3] has pointed out, the major issues for the CO 2 utilization are the determination of the actual amount of fixed CO 2 and the life of the product. When a standard of 10Mt-C/y as the fixed CO 2 and 100 years as the life is taken into consideration, it is a big challenge to find effective methods for chemical utilization. Intensive research on catalysts to convert CO 2 to methanol have been undertaken mainly in Japan. Saito et al [4] have found effective catalysts (Cu / ZnO / ZrO2 / AI203 / Ga203) with which very pure methanol (99.96%) could be produced from CO 2 and H 2. However, only a limited effectiveness for the methanol from CO 2 could be found from the viewpoint of energy efficiency.

3.2 Ocean storage technology The ocean is the largest potential sink for

C O 2.

Research on the direct injection of

CO 2

into

the ocean has been done mostly in Japan, USA and Norway. Herzog et al [5] compared and summarized five ocean storage options as shown in Table 1. Fujioka et al [11] reported cost evaluation results on ocean disposal (capture and storage) options as shown in Table 2.

79

Table 1 Comparison of ocean storage options Development

Option

Cost

Required

Environmental

Leakage to

Impact

Atmosphere

a. Dry Ice [6]

Lowest

High

Low

Low-Medium

b. Towed Pipe [7]

Medium

Low-Medium

Lowest

Medium

c. Droplet Plume [8]

Low

Low

Low-Medium

Medium

d. Dense Plume [9]

Medium

Lowest

Highest

Medium

e. CO 2 Lake [10]

Highest

High?

Low

Lowest

Table 2 Electricity and CO 2 disposal costs Option Base Relative electricity cost C O 2 disposal

cost [$/tonC-avoided]

a

b

d

e

100

350

250

230

250

0

660

370

330

370

Power plant capacity = 2 • 554MW In any case, the cost of electricity increased more than twofold by applying the

CO 2

disposal

system and this CO 2 disposal cost is calculated to be higher than $330/ton-avoidedC. Improvement in a CO 2 separation step seems an effective method to decrease the CO 2 disposal cost. In a storage step, a new method to dispose CO 2 as hydrate crystal formed at a depth of 500m in the ocean to lower the disposal cost has been proposed by Yamasaki [12].

3.3 Geological storage technology The high pointial for underground-storage and lower cost than that of ocean storage has led to geological storage as a major option for CO 2 disposal. Table 3 shows the comparison of geological storage options reported by Herzog et al [5]

80 Table 3 Comparison of geological storage options Storage Option Active oil wells (EOR)

Relative Capacity

Relative Cost

Storage

Technical

Integrity

Feasibility

Small

Very Low

Good

High

Coal beds

Unknown

Low

Unknown

Unknown

Depleted oil/gas wells

Moderate

Low

Good

High

Deep aquifers

Large

Unknown

Unknown

Unknown

Mined caverns/salt domes

Large

Very High

Good

High

4. B I O L O G I C A L F I X A T I O N Atmospheric CO 2 can be addressed from an organic viewpoint by considering biological processes using solar energy. Various kinds of biological CO 2 fixation methods such as afforestation, marine fertilization and microalgae production have been proposed. The Chem. Eng. Soc. Japan has surveyed and summarized these processes [13]. In terrestrial ecosystems, afforestation and reforestation in tropical and temperate zones are effective for CO 2 fixation from the viewpoint of energy and cost efficiency. A serious difficulty in afforestation arises in ensuring the preservation of the land. The development of the land is strongly restricted by the policies and traditions of the nation and the community. Involved in many developing countries, food shortage has become a serious problem due to the increasing population. This means that much land must be utilized for agricultural production rather than for afforestation. This is why arid land must be considered for afforestation. The most essential and serious problem in afforestation of arid land is the difficulty in maintaining a water supply. Afforestation in desert areas requires additional water supplies other than precipitation. At present, an afforestation system combined with reverse-osmosis desalination or other processes is not a CO 2 sink but a source. New sustainable systems such as precipitation enhancement [14], efficient water management or proper selection of plants should be developed for afforestation of deserts. In marine ecosystems, fertilization of the main nutrients (N. P) or a micronutrient (Fe) seems a promising process for CO 2 fixation. However, it still remains uncertain at what ratio the biomass formed by such fertilization can reach the deep ocean.

81 A study on the resulting carbon cycle in the ocean to clarify the effectiveness of fertilization is required. 5.

ENERGY-EFFICIENCY

IMPROVEMENT

TECHNOLOGY

(STATUS

OF

ELECTRIC VEHICLE) Energy-efficiency improvements and measures in energy conversion,

manufacturing,

transportation and commercial/residential sectors should be very effective for CO 2 mitigation. Energy use in transportation is projected to grow to 90-140 EJ in 2025 without new restrictions to address the 61-65 EJ in 1990. Electric vehicles (EVs) are expected to be a highly energy-efficient means of transportation compared to internal combustion engine vehicles (ICEVs) and their unit energy requirements can be half of those of ICEVs or less. However, there are significant problems in introducing EVs into commercial markets on a large scale, mainly the low energy density of the batteries inplying a shorter driving range, a long charging time and high costs. Many attempts are being made to solve these problems. Key technologies are in batteries and fuel cells. The development status of EVs is explained herein. Stula et al [15] have reported the advanced battery technologies in the USA. Nickel-metal hydride, lithium ion, lithium polymer batteries and ultracapacitors have attracted an attention and been under development. Lithium battery systems will be a strong contender as the main system for the future. Their current performance data on specific energy, specific power and cycle life and also current costs should be improved by a factor of more than two. In order to overcome the problem of the short distance range, researches on proton membrane fuel cell (PEM) technology for EV application are underway in many places. PEM has the disadvantage of requiring high purity hydrogen as fuel. Solid oxide fuel cells can use a variety of fuels and be used in the future [16], however, they are as yet in the intitial stage of development. 6. R E N E W A B L E E N E R G Y T E C H N O L O G Y

Renewable sources of energy contribute about 20% of the world's primary energy consumption at present. Most of them are fuelwood and hydropower. However, there are other renewable sources such as solar, wind, wave and geothermal energies. Their realization in commercial markets can be useful for CO 2 mitigation.

82 This commercialization requires the intensive cost reduction of energy from renewable sources. Herein, photovoltaic (PV), biomass and geothermal power generation systems are evaluated based on cost and CO2 emissions. 6.1 Photovoitaic energy systems PV systems using polycrystalline silicon (poly-Si) and amorphous silicon (a-Si) cells have been evaluated [17]. The PV systems considered here are large-scale, centralized systems directly connected to the utility grid and include the balance of system (BOS) with supporting structure, inverter, and DC control device and installed in Tokyo. Assumption for energy conversion efficiencies and PV cell production rates are shown in Table 4. Costs and energy pay-back times of present PV systems at an annual production rate of 10MW were calculated to be u (u

(u

and 5.7 years for poly-Si and u

and 6.4 years for a-Si PV systems, respectively. These cases correspond to the

present level of PV technology. Table 4 Assumption of production rate and energy conversion efficiency Annual production rate (GW)

0.01

Energy conversion efficiency (%) Poly-Si a-Si

1

100

15

17

20

8

13

16

Application of the PV system in Tokyo (dispersed system). PV systems with a-Si cells used on roof tops in Tokyo were evaluated using improved values for the weight of supporting racks and inverter efficiencies. The results are shown in Table 5. EPT, cost, and CO2 emissions could be reduced by the use of light-weight BOS components and high-efficiency inverters. The CO 2 emissions of-5g-C/kWh for PV systems were found to be very low. The electricity cost of u desirable to realize a decrease of u

is "almost competitive to current rates, however, it may be or more for commercialization.

83

Table 5 EPT, cost and CO 2 emissions fi)r PV systems using a-Si solar cells. House roof

Apartment roof

42.8

22.2

Area (km 2) Cell production rate

1GW/y

100GW/y

1GW/y

100GW/y

Maximum output (GW)

5.12

6.30

2.66

3.27

Electricity output (GWh/y)

5.12"103

6.31"103

2.66"103

3.27"103

EPT (y)

0.64

0.49

0.75

0.56

Cost (Y/W)

173

122

194

140

Cost (Y/kWh)

26

18

29

21

032 emissions (g-C/kWh)

6

5

7

5

Application of PV system in Australian desert (centralization system). The Gibson desert in Australia where the insolation energy is 2,100kwh /m2, 1.8 times higher than in Tokyo, was selected for the power plant site. The electric power generated is transmitted 1,000 km to Perth. The total power plant area of 860km 2 can provide electricity of 1.1 • 105GWh/y. Evaluation results on centralized PV systems using poly-Si cells (Production rate 100GW/y) are presented in Table 6. Table 6 EPT, cost and

CO 2

emissions for PV system using poly-Si cells in Australian desert EPT

Cost

C O 2 emissions

Without battery systems

0.9y

18Y/kWh

9g-C/kWh

With lead battery systems

1.7

101

15

With NaS battery systems

1.1

32

12

Installation of batteries which store PV electricity generated for ~,o days increases EPTs, CO 2 emissions and especially costs. A high cost with lead batteries can be reduced by use of a new type of battery such as NaS. Without battery system, the electricity cost is near a present level.

84

6.2 Power-generation systems by biomass At present, biomass power plants have low thermal efficiencies of about 20-25 %. Herein, biomass power plants with advanced gasification and liquefaction technologies [18], [19] are designed and have been evaluated [20]. The capacity of a power plant is assumed to be 100MW. Fuel for power plants is produced by the liquefaction and gasification of eucalyptus. Its heat of combustion is assumed to be 16.7MJ/kg-dry basis. The annual growth rate of biomass in a plantation area is assumed to be

1.25kg-dry wood/m 2. The plantation - harvesting cycle is 10 years. The area of the plantation site required for the 100MW plant is calculated to be about 600km 2. The evaluation results are shown in Tab. 7. As indicated in the results, thermal efficiencies are in the range of 27---31% which are 20-30% higher than for those of current biomass power plants. CO z emissions and electricity costs by biomass power plants are low compared to those by PV systems as shown in the previous section (6.1). Technology barriers to realize a new biomass power plant seem not so high, however, a high barrier is to find a plantation site. Table 7 Evaluation results (material quantities, CO 2 emissions, EPT and cost). ...................

Process

LCC

LST

GCC

Combined cycle Steam turbine Combined cycle (gas-steam

Process

(boiler)

turbines)

Thermal efficienies (%)

GST Steam turbine

(gas-steam

(boiler)

turbines)

29.0

26.7

30.5

26.7

2

3

2

3

EPT (d)

78

155

53

130

Electricity cost (u

7.1

6.5

6.9

5.2

Electricity cost (Y/kWh)**

10.1

10.0

9.9

9.9

Unit CO 2 emissions (g-C/kWh)

* Labor cost of Y1500d-lman-lat plan site

** Labor cost of u

1

85

6.3 Geothermal power generation system A geothermal power plant with a hot dr3' rock system constructed in Japan is evaluated herein. The capacity of the power plant is assumed to be 280MW and hot dry rocks (300A6) at a depth of 4km is used to generate steam. The plant area is calculated to be 1.4kin 2. The evaluation results are presented in Tab. 8. Table 8. Evaluation results on geothermal power generation system CO 2 emissions

EPT

(g-C/kWh)

(days)

3

80

Electricity cost (u 5

The evaluation results suggest the geothermal system can be a promising candidate from the viewpoint of CO 2 emissions and the electricity cost. 7. C O N C L U S I O N An overiview of verious studies

on CO 2

mitigation were presented in this paper. The research

summarized have been conducted with significant progress. However, there are no single solutions. We should investigate the near-, mid- and long-term projects considering their priority. CO 2 reduction costs and barriers for the realization of the different options discussed in this paper are shown below.

CO 2

(u CO 2 capture and sequestration

reduction cost

Barrier

avoided) 36,000

9High energy consumption 9Environmental uncertainty

Afforestation

at present

6,000

9Land-use competition

(tropical / temperate zone) PV system

at present future roof-top future centralized

800,000 0 100,000

9Conversion efficiency 9Energy storage

86 Biomass power generation system

0

9Land-use competition

Geothermal power generation system

0

9Water path structure

EV

at present

200,000

9Battery 9Short distance range

REFERENCES

1) T. Mimura et al, Energy Conrers. Mgmt Vol. 38 (1997) $57-$62 2) K. Sakaki et al, Kagakukougaku-Ronbunsyu Vol. 23, No. 2 (1997) 292-295 3) M. Aresta and I. Tommasi, Energy Confers. Mgmt Vol. 38 (1997) $373-$378 4) M. Saito et al, ibid $403-$408 5) H. Herzog, E. Drake and E. Adams, "CO 2 capture, reuse and storage technologies for mitigating climate change", DOE order No. DE-AF22-96PCO1257 (Dec. 1996) 6) N. Nakanishi et al, "Sequestering of CO2 in a Deep Ocean -- Fall Velocity and Dissolution Rate of Solid CO2 in the Ocean", CRIEPI Report (EU 91003), Japan (1991) 7) M. Ozaki et al, Energy Convers. Mgmt. 36(6-9), (1995) 476-478 8) C. Liro et al, ibid 33(5-8), (1992) 667-674 9) PM. Haugan and H. Drange, Nature 357 (1992) 318-320 10) T. Ohsumi, Marine Technology Society Journal, 29(3), (1995) 58-66 11) Y. Fujioka et al, Energy Conrers. Mgmt Vol. 38 (1997) $273-$277 12) A. Yamasaki, Reported at 1997 annual meeting of Japan Chem. Eng. Soc 13) K. Yamada, T. Kojima et al., NEDO-GET-9638 (1997) 14) D. Li and H. Komiyama, "Numerical Simulation of Limited-area Weather Modification in Australia" 1997 Australia Meteorology Soc. 15) R.A. Stula et al, Symposium Proceedings of The 13 International Electric Vehicle Vol. 1, (1996) 303-310 16) K. Sakaki, F. Nishikawa and K. Yamada, ibid, (1996) 686-692 17) K. Yamada, K. Kato and H. Komiyama, Energy Convers. Mgmt. 36, (1995) 819 18) S. Yokoyama et al., Kagakugogaku Ronbunshu, 17, (1991) 326-333 19) S. Fujinami, "Fluidized-bed gasification of cellulostic wastes", Ebara Jiho 151 (1991) 20)K. Yamada et al., "Evaluation of Electric Power Generation System by Biomass", Developments in Thermochmical Biomass Conversion (Banff- Canada, 20-24 May, 1996), 1582-1589 (1996)

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.

87

Hydrogenation of CO2 toward Methanol Influence of the catalysts composition and preparation on the catalytic behavior R. Kieffer, L. Udron. L E R C S I [UMR 7515 du CNRS] E C P M Universit6 Louis Pasteur 1, rue Blaise Pascal - 67008 Strasbourg France The influence of the preparation method of methanol catalysts composed of copper associated with rare earth oxides (eg Cu-La2Zr207 and ZnO promoted Cu-La2Zr207 systems) on the catalytic behaviour is discussed. Good activities and improved aging properties are always associated with a high copper surface area and a reasonnable crystallinity of the La2Zr207 pyrochlore. For Cu-La2Zr207, as well as for Cu-ZnO catalysts, an almost linear correlation can be observed between the methanol yield, the copper surface area and the amount of formates located on the catalyst's surface. A similar correlation cannot be evidenced on ZnO promoted Cu-La2Zr207 catalysts. The results are discussed and a mechanism for the hydrogenation of CO2 to methanol is proposed 1. I N T R O D U C T I O N CO2 hydrogenation to methanol is one of the most favorable valorization of the carbon dioxide considering that the methyl alcohol can easily be converted into hydrocarbons on zeolite type catalysts [ 1,2] even in a "one step" process using composite material. Many catalytic formulation are proposed for the hydroconversion of CO2, most of them are based on promoted copper-zinc oxides given by the long industrial experience on methanol synthesis from syngas (CO+CO2+H2) [3-6]. Specific methanol catalysts working for CO2 are proposed including promoted Cu-Zn catalysts [3,6], zirconia supported systems [7] as well as copper associated with stabilized rare earth oxides [8,9]. In the last case Cu-LaZr and CuZnLaZr catalysts were proposed and showed interesting catalytic properties in the methanol formation. The objective of the present work was to demonstrate the importance of the catalyst's preparation technique on the catalytic behavior and hence to establish the possible links between solid state chemistry and the catalytic process.

2. EXPERIMENTAL 2.1. Catalyst preparation The copper-zinc catalysts were obtained from aqueous solutions of copper and zinc nitrates using NaaCO3 (Cu-Zn [ex carbonate]) or oxalic acid (Cu-Zn [ex oxalate]) as precipitant. The filtered precipitate was washed 3 times with water, dried 14 h. at 100~ and calcined (in air) at 350~ The copper-pyrochlore catalysts were prepared from aqueous solutions of the corresponding nitrates using the precipitation with Na2CO3 (Cu-LaZr [ex carbonate]) or with oxalic acid (Cu-LaZr [ex oxalate]). The solid was washed, dried and calcined (in air) at 350~ 550~ and/or 710~ The zinc promoted catalysts were mainly obtained in the same way, i. e. by addition of zinc nitrate in the metal salts solution before precipitation. For the CuZn +LaZr

88 systems the carbonate precipitation was operate in presence of a slurry of the promoters (La2Zr207). For all the given preparations the calcination processes were monitored using TGADTA data obtained with a Setaram 92-12 TGA device.

2.2 Characterization of the catalysts BET and porosity were measured by N2 chemisorption (volumetric technique) on a Coulter SA 3100 equipment. The accessible copper surface area (SCu) is determined by the conventional N 2 0 adsorption technique on reduced catalysts (H2, 270~ 15 h). XRD measurements were performed on a Siemens D 5000 equipment using the CuKc~ radiation.

2.3. Catalytic Activities The catalytic tests were carried out in a stainless steel continuous flow reactor (6 mm inner diameter) containing 0.5 g of catalyst as described elsewhere [8]. Standard reaction conditions are : pressure = 6 MPa, CO/H2 ratio = 1/2 (CO2/H2 = 1/4), flow rate = 4 1.h-lg cat.-1. The carbon balance was always higher than 95%. 3. R E S U L T S AND D I S C U S S I O N

3.1. Copper-zinc oxide catalysts The copper-zinc [ex carbonate] catalysts, prepared by the conventional precipitation technique using Na2CO3 often described in literature [6,12], lead to efficient catalysts containing, despite washing of the precipitate, small amounts of sodium (0.05-0.1 wgt%) able to change the properties of the catalytic system [ 13]. To make a fair comparison with a Na free Cu-La-Zr catalysts Cu-Zn [ex oxalate] catalysts samples were prepared using the precipitation with oxalic acid described in the experimental part.

3.1.1. Characterization of the Cu-Zn catalyst. The characteristics of the catalytic systems depend not only on the preparation technique but also on the annealing temperature. Both catalysts have a poor thermal stability and a calcination above 350~ led to very low BET and copper surface areas e.g. 3m2/g (Cu-Zn [ex carbonate]) and lm2/g (Cu-Zn [ex oxalate]) at 550~ Since the copper surface area determines for the catalytic activity only the samples calcined at T = 350~ have been used for the catalytic tests. Table 1 Characteristics of Cu-Zn catalysts Catalyst T~C Preparation Annealing Cu-Zn [ex carbonate] 350 Cu-Zn [ex oxalate] 350

SBET m2/g 64 48

SCu m2/g 37 21

3.1.2. Catalytic activity of Cu-ZnO catalysts The catalytic activity of the catalysts in presence of a CO2 + H2 mixture between 250~ and 320~ (figure 1) can be more or less related to the copper surface areas as observed in table 1. Thus the catalysts prepared using carbonate precipitation are the most active in methanol formation. This can be attributed to the higher selectivity easily related with the high copper coverage of the catalyst. It can be noted that both catalysts do not produce any heavier products and in our reaction conditions, the presence of sodium in the catalyst originated from carbonates is not able to induce any chain growth despite some literature indications [ 13].

89

MeOH Yield % MeOH Sel. % I~ Yield [carb] l~ Yield [ox] " l - Sel [carb](x 0.3) Sel [ox](x 0.3)

I... m !

-

10 i 9 i

A !

i

!

250

270

300

?~

320

Figure 1. Influence of the preparation technique on the activity of Cu-ZnO catalysts 3.1.3. Aging of the Cu-Zn catalysts. During a 70 h. run at 300~ both catalysts, originating from carbonate and oxalates precursors, show (figure 2) a decrease of the methanol yield which represents respectively 9 and 19% of the initial activity of the systems .The absence of stabilising promotors (A1203, Cr304, ZrO2 ..) can explain these deactivation in agreement with literature data [ 14]. 3.2. Copper-pyrochlore catalysts As discussed in previous papers, the conventional firing-milling technique developped by inorganic chemists for La2Zr207 preparation [ 15] cannot be used for the preparation of catalytic materials since the used calcination conditions e.g. 1000-1200~ 6-24 h. lead to samples with a BET surface area lower than 1 m2/g without any catalytic activity. To obtain well distributed copper oxide on a pyrochlore matrix two "soft chemistry" techniques were compared.

1 - precipitation of mixed oxalates from ethanolic solutions of the metal salts followed by a calcination at relatively low temperatures (e.g. 550-710~ 2 - precipitation of the precursor from aqueous solutions by Na2CO3 followed by a calcination in the conditions given previously. 3.2.1. C h a r a c t e r i z a t i o n of C u - L a 2 Z r 2 0 7 catalysts Table 2 show that the characteristics of the catalysts depend strongly on the preparation technique and on the calcination temperature. According to these results it appears that, after annealing at 550~ the Cu-La2Zr207 [ex carbonate] catalysts is poorly crystallised whereas on Cu-LaZr [ex oxalate] a crystalline pyrochlore structure was not identified, but an ordered arrangement of La and Zr in an amorphous phase cannot be excluded. The well crystallized product obtained at 900~ has an extremely low BET and copper surface areas for an use as catalyst. The increase of the copper surface area observed on CuLaZr [ex oxalate] catalysts after calcination at 710~ can be explained by a phase rejection of CuO described previously [9,16] according to the following reaction pathways : CuO + L a 2 0 3 ...... > C u L a 2 0 4 C u L a 2 0 4 + 2ZrO2 ..... > CuO + L a 2 Z r 2 0 7 .

90 These solid-solid reactions may also be possible in the carbonate catalysts but do not appear so distinctly in this experiment. The high stability of the reaction intermediate La202CO3, can hinder, in this case, the formation of CuLa204 and so the rejection process and can therefore explain these results. ATG-ADG experiments under H2 - He flow on both catalytic systems calcined at 550~ show a lower reduction temperature of Cu-LaZr [ex carbonate] (165~ than Cu-LaZr [ex oxalate] (190~ which can be related with a stronger LaCu interaction in the latter case caused by the possible presence of non crystallized CuLa204. A lower reduction temperature of the catalysts annealed at 710~ (170~ compared to that obtained at 550~ (190~ may explain the disappearence of the La-Cu interaction in accordance with the rejection of CuO at 710~ ATG experiments on the precursors of both catalytic systems show after the weight loss corresponding to the escape of CO2, an exothermic peak (very small in the case of the carbonate catalyst), in the 700-710~ temperature range which can be attributed to the formation of the crystalline La2Zr207 compound. Table 2 Characteristics of copper-pyrochlore catal~csts Catalyst Annealing SBET T~ m2/Ig Cu-LaZr 550# 60 [ex carbonate] 710 25

Cu-LaZr [ex oxalate]

SCu m2/g 12 7

900

3

100 for Ni(cyclam)Cl2. Metal(I) complexes, metal(III) hydride complexes, and metallocarboxylates such as [NiIII(cyclam)(COz2-)]+ are postulated as intermediates in the electro- and photo-chemical CO2 reduction [4]. Our research focuses on mechanistic and kinetic studies of photochemical and electrochemical CO2 reduction that involves metal complexes as catalysts. This work makes use of UV-vis, NMR, and FTIR spectroscopy, flash photolysis, pulse radiolysis, X-ray diffraction, XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine structure). Here we summarize our research on photochemical carbon dioxide reduction with metal macrocycles.

2. NATURE OF C o - C O 2 ADDUCTS We and others have characterized the interaction of low-spin d 8 CoIH]V[D + with CO2 in CH3CN [5-9] and in H20 [10,11]. Schmidt et al. [12] have characterized the binding thermodynamics as a function of organic solvents. The chiral N-H centers of the macrocycle give rise to two diastereomers, N-rac and N-meso. The CoL complexes are shown below.

98 The equilibration between the N-rac- and N-meso cobalt(II) isomers is slow in acidic aqueous and organic media, but equilibration of the two cobalt(l) isomers is relatively rapid (>2 x 10-3 s -1) in CH3CN.

N

N

~N

1~

E rac

HMD

3

meso H M D

The CO2 binding constants of the corresponding [CoHMD] + isomers are quite different: Nrac-[CoHMD] +, (1.2 + 0.5) x 104 M-I; N-meso-[CoHMD] +, 165 +_ 15 M -1 [5,6]. While hydrogen bonding interactions between the bound CO2 and amine protons of the macrocycle will tend to stabilize both adducts, the N-meso adduct is destabilized by the steric repulsion by the macrocycle methyl group. Although the N-rac-[CoHMD(CO2)] + adduct decomposes to N-rac-[CoHMD] 2+ and CO in wet CH3CN [5], it is stable enough to handle in dry CH3CN under a CO2 atmosphere. The complex is thermochromic [6,7] being purple at room temperature and yellow at low temperature (-100 ~ as shown in Figure 1. The equilibrium between five-coordinate [CoHMD(CO2)] + (purple) and six-coordinate [CoHMD(COz)(CH3CN)] + (yellow) has been studied by UV-vis, 1H NMR, FT-IR, XANES and EXAFS in CH3CN [6,7,9] [CoHMD] + + CO2 ~

[CoHMD(CO2)] +

[CoHMD(CO2)] + + CH3CN ~

[CoHMD(CO2)(CH3CN)] +

Ks = [CoHMD(CO2)(CH3CN) +] / [CoHMD(CO2) +]

(1) (2) (3)

The singular value decomposition (SVD) [13-15] spectral analysis of the temperaturedependent UV-vis data between 26 and -40 ~ is consistent with the presence of two species in CH3CN. The fit gives AH ~ -7.0 kcal mo1-1 and AS ~ -27 cal K -1 mo1-1 for eq. 2 [7]. The equilibration is rapid on the NMR time scale. The pressure dependence of the equilibrium constant shows that increasing pressure shifts the equilibrium toward the six-coordinate species with an overall reaction volume of AV ~ -17.7 + 1.0 mL mo1-1 at 15 C ~ in CH3CN [16]. The FT-IR spectra measured over the range 25 to -75 ~ in CD3CN and in a CD3CN/THF mixture indicates [7] the existence of four CO2 adducts with and without intramolecular hydrogen bonds between the bound CO2 and the amine hydrogens of the ligand: a five-coordinate, non-hydrogen-bonded form (vc=o = 1710 cm "1, VNH = 3208 cm'l), a five-coordinate hydrogen-bonded form (vC=O = 1626 cm'l), a six-coordinate non-hydrogen bonded form (vc=o = 1609 cm "1, VNH = 3224 cm-1), and a six-coordinate hydrogen-bonded form (v C=O = 1544 c m " 1, VNH - 3145 cm" 1).

99

2(){)(),() E

I

a

. L

'

:A j

"~ lOOO.o

b

51111.{1

0.11

401)

5(X)

6(X)

700

800

Wavelength, nm

Figure 1. UV-vis spectra of [CoIHMD] + (a), [CoHMD(CO2)] + at room temperature (b), and [ColIIHMD(COz2-)(CH3CN)] + at-100 C ~ (c)in CH3CN. X-ray absorption spectroscopy is an attractive tool for the characterization of metal complexes in solution. The metal coordination number, geometry, and electronic properties can be studied using XANES and the metal-ligand bond distances are obtained through analysis of EXAFS. Previous work [17-19] has also shown that the edge energy correlates with the oxidation state of the metal. The XANES spectra (Figure 2) for a series of CoHMD complexes [9] indicate that the edge positions (E()) are sensitive to the oxidation state of the metal.

1.2

-A

'

I

'

I

'

I

'

I

'

1.2 .

=""

'

I

D

'

I

'

"-

~' o.8

0.8

~

N .,.=( ,-=(

.=

'

" " = =="

"'=

t~,

.. .

A , ; .

-

6~"

--

0.4

I

/, ,;.'

-

0.4

'

Z";,:

-_


20 times faster than does the CO2. Thus the direct reduction of CO2 by TP'- plays a negligible role here and all of the photochemically generated reducing equivalents are captured by the cobalt macrocycle. The production of CO from CoL(CO2) + requires a second reducing equivalent. The source of this equivalent is of interest. Under flash photolysis conditions the TP'- has completely reacted before the CoL(CO2) + is formed. On the other hand, under continuous photolysis T P ' - can react with the ColIL 2+ or the CoL(CO2) + complexes. In the flash photolysis, where only Et2NC'HCH3 and/or CoIL + may act as the electron donor, the decomposition of CoLCO2 + is slow owing to the low concentrations of these two species. In fact, since CoLCO2 + decomposes faster with low [CO2] (i.e. higher [COL+]), CoL + is the likely electron donor under flash photolysis conditions. We suggest that reactions 10-12 are responsible for the production of CO in the photolysis. The slow step is likely to be the C-O bond breakage of the bound carboxylic acid with either Et2NC'HCH3, or CoIL + acting as electron donor. Unfortunately the UV-vis transient spectrum of [S-ColIIHMD(CO22-)] + is too weak to study the proton dependence of its disappearance.

4. P H O T O C H E M I C A L CO2 REDUCTION WITH NICKEL M A C R O C Y C L E S

4.1 Photochemical CO2 reduction In contrast to the cobalt-based system, small amounts of H2 and no CO are produced when nickel cyclam or other saturated 14-membered tetraazamacrocycles (L) in Figure 3 are used to replace the cobalt complex in the above system [22]. Flash photolysis studies indicate that the electron-transfer rate constant (k13) for the reaction of the p-terphenyl radical anion with NilI(cyclam)2+ is 4.3 • 109 M -1 s-1. However, when CO2 is added to the solution, the decay of the TP anion becomes slower! Flash photolysis studies of the acetonitrile solutions

102 suggest the existence of a minor pathway for MIL + formation that does not involve TP. When TEA (or TEOA) is used with UV excitation (~-l:5"~

0.5

-30 -40 -50 POTENTIAL (V)

Fig. 2. Cyclic voltammograms (scan rate 5 V/s) representing sequences of the activation treatment of the Cu electrode (0.28 cm 2) performed periodically at different stages of a longterm electrolysis of CO2 in 0.5 M KHCO3.

111 The electrode activation requires less than 1 percent of the total electrolysis time (i.e., 23 s every 5 or 10 min.) and consumes a negligible extra amount of electrical charge. A typical series of cyclic voltammograms corresponding to the activation sequence is shown in Fig. 2. No features directly associated with the oxidation of the poisoning species are perceptible on any of these voltammograms. An initial increase of the current on the anodic side, connected with the oxidation of the copper surface is followed, during the cathodic sweep, by a large peak due to the reverse process. The increasing area under voltammograms is indicative of a continuous increase in the true surface area of the electrode. Application of such a potential program allows high faradaic efficiencies of CH4, C2H4 and C2HsOH to be maintained over long electrolysis runs. Thus, the amount of methane formed at an activated Cu electrode increases considerably along a 24h electrolysis run (cf. Fig. 3).

200, E 0

=- lO0-

ff

I

-2.2

-2.1

I

-2

I

-1.9

-1.8

I

-1.7

-1.6

POTENTIAL (V) Fig. 3. Methane formation rates recorded at the beginning and after 24h of continuous electrolysis runs performed at increasingly cathodic potentials. Activated Cu electrode, 0.5 MKHCO3/CO2 solution, 22~ As shown in Fig. 4, no electrode deactivation was observed during a 8-days long continuous electrolysis experiment. While the total rate of hydrocarbon formation remained remarkably constant as electrolyses progressed, an increase of the amount of ethylene accompanied by a decrease of the amount of methane were in general observed. These variations in the product distribution are probably associated with the structural changes occurring at the electrode surface and, in particular, with the increasing presence of Cu+(s) species [ 19,23]. Interestingly, experiments performed using Cu electrodes modified with attached oxide (Cr203,ZrO2) particles showed a dramatic change in the distribution of the CO2 reduction products [23]. In fact, under such conditions, ethylene and ethanol are formed in larger amounts than methane (cf. Figs 5 and 6). This is in contrast with the results of electrolyses conducted using a bare Cu cathode, under otherwise identical conditions, where methane remained the major product.

112

200

60 Z u~

50

150

40

a~

30

2:

20

o~

\r"

.....

.....

................

-""*'''~176

100

.........

EtOH ca. 3.5%

5o

.,.. + . - - . . - , " .~

.

.

.

.

rj

%. . . .

rj

r .......

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r . . . . . . .

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I

1

2

. .

l

3

Z

,l

I

I

I

4

5

6

7

,

8

T I M E (days) CH4

....

C2H4

~

Total

...... Current

Fig. 4. Faradaic efficiencies of C H 4 and C2H 4 vs. time for C O 2 reduction at an activated Cu electrode (0.28 cm 2) in 0.5 M KHCO3 at 22~ ; E = -2V. Dotted line shows evolution of the apparent current density during 8 days-long electrolysis run. 40

10 j I~,Cl ~ -

r,.) 30

l

-..G. ~

-8

...13-..i=i"

7

P rj

.,.o~176 .~

~

. . . . . . . . . . . . . . . . . . . . . . . .

. ,to

20

I

I"

r~

EtOH 12 %

i

. . . x ~

'

,

0

100

-2

~,,-,''"

I

I

I

200

300

400

0 500

TIME (min.) --x--CH4

- ~ - " C2H4

. . . . . . CURRENT

Fig. 5. Faradaic yields for C2H 4 and CH 4 recorded together with cathodic current during electrolysis of CO2 at a ZrOE-modified, periodically activated, Cu electrode (0.28 cm 2) in 0.5 M K2SO 4 at 5~ ; E = -1.8 V.

113

4O

10 .D.

30

-

.D . . . . .

~i

\

.._ 1 3 - - -I:i- - - - . . . 0

13" . . . . . .

ra.-"

o.

,"/

r,.)

-8

~ ~

o o -

o

OOo

o -

oil

,,,

. . . .

ooo

2o

~ 1 7 6 1 7. 6. . . . . . ~

~ o " "

oo~

. . . . . . .

. o ~

o

~

'i

r,.)

~2 0

I

I

I

I

1O0

200

300

400

l

500

TIME (min.) ~x~

CH4

- 0--

C2H4

......

CURRENT

Fig. 6. Behaviour of a Cr203-modified Cu electrode ; all other features as in Fig. 5.

3.2 CO2 electrolysis

experiments

at a c o p p e r

cathode

conducted

under

increased

pressure

Because of a relatively small CO2 solubility in water under atmospheric pressure (0.033 mol/dm 3 at 15~ the reduction process meets limitations due to the slow transport of the reactant to the electrode. Such limitations appear clearly for an activated Cu electrode where a continuous increase in the cathodic current during prolonged electrolysis runs (cf. Fig. 4) leads finally to a decrease of the faradaic yield for hydrocarbons. To overcome this problem, several authors conducted electrolysis experiments at elevated CO 2 pressures. Ito et al. [24] examined the effect of CO 2 pressure up to 20 atm on its reduction at metals displaying high overvoltage for hydrogen evolution: Zn, In, Pb and Sn. They found that the yield of formic acid (the main reaction product at these electrodes) was increased at higher pressures. Nakagawa et al. [25] investigated the CO2 reduction at pressures up to 60 atm on group VIII metal electrodes: Fe, Co, Ni, Pd and Pt. At ambient pressure these metals yield almost exclusively hydrogen. At the CO2 pressures of J0 and 60 atm, CO was produced at all the above electrodes with faradaic efficiencies of 62 % for Pd, 37 % for Ni and 34 % for Pt. The same group of researchers investigated the effects of CO2 pressure, stirring and current density on the reduction product distribution at a Cu electrode [26,27]. They found that in order to obtain high faradaic efficiencies of hydrocarbons, a balance between CO2 supply and current density should be maintained. If the flux of CO2 was too high with respect to the current density, CO and HCOOH were the major products. If, on the contrary, CO2 was in a short supply (too low pressure for a given current or no stirring) water reduction to hydrogen prevailed. It should be stressed that all these results concern short term experiments.

114

60

I

50

-

o

40

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u_

30

Z W m 0m LL. Ill

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I

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0

2.

_

20
5

>49

2

70

_a

32

>12

100

3

74

- ~

67 a

acetonitrile

85

>95

benzene

>3

thf

>4

thf / pyridine (10/1) pyridine hexane

>2

>3

>1

20

a

sc carbon dioxide

>4

>3

90

cO

Ao

o

o

o

25

. . . . . . . A--'"

A "- "- "-,'-'-'-"-I

4-1~]0

....

450

," " "[~

500

~

...D

" " ~ " ..... J

550

,

I

600

Temperature / ~

Fig. 1 Catalytic fixation of CO into carbon via methane with conventional fixed bed reactor. temperature at CO2 methanation was fixed at 300~ (2):of CO ~ i n t o carbon @into CH4 D:into CO

3.2. CO2 fixation with conventional fixed bed reactor Figure 1 shows the dependencies of the activity and the selectivity on the temperature of CH4 decomposition, when the temperature of CO2 methanation was fixed at 300~ Carbon dioxide was completely converted into CH4, CO and C. Although the CH4 was the main product in a low temperature range below 400~ conversion into CH4 and carbon extremely decreased and mcreased, respectively, as the temperature of CH4 decomposition increased. Comparing with CO2 reduction via CO over WO3 catalyst in the previous study, 1) conversion into C was greatly increased by the methanation of CO2 and decompose it, and conversion into C attained to 60% at 700~ However, the conversion into C at 400~ was lower than 10%. This is because the decomposition of methane is restricted by the chemical equilibrium. It is expected that the activity of C H 4 conversion can be exceeded the equilibrium conversion by removing the formed H2

150

from the reaction system. We investigated on the application of m e m b r a n e reactor consisted of H2 permeable film for CH4 decomposition in order to increase the conversion into C in the low t e m p e r a t u r e range. 100

3.3. CO2 fixation with m e m b r a n e reactor

0---- :---v ~ ...... ~. --

Figure 2 shows the comparison of the conversion of CO2 into C with conventional reactor with t h a t of m e m b r a n e reactor. It is clearly shown t h a t the conversion into C drastically increased with the application of memb r a n e reactor to CH4 decomposition. Although the conversion into C was as

75

to Carbon

o~ C 0 .m (].)

-. ............. 0 .of 002 ".,, ",

50

C 0

A

into CH4

A

..'

25

low as 10% at 500~ in the conventional fixed bed reactor, it a t t a i n e d 72% on the m e m b r a n e reactor. The conversion 4"6"o 4so soo sso 600 into C was further increased with Temperature / ~ increasing the flow rate of sweep Ar, Fig. 2 Comparison of the conversion of since the permeation rate of H2 was C O 2 into carbon with conventional reactor increased. Figure 3 shows the effects of with that of membrane reactor. Temperature flow rate of sweep Ar of m e m b r a n e at CO2 methanation was fixed at 300~ reactor system on the catalytic re- closedsymbol;membranereactor duction of CO2. Although the con- open symbol; conventional fixed bed reactor version of CO2 was independent on the flow rate of sweep Ar and attained a I I I I I I

100~_b showed lactic acid production. The 2Pt 1Sn catalyst, which has a higher content of PtSn phase, showed a higher production than the 5PtlSn sample, aPtbSn catalysts with a>b have shown by XPS much higher Pt/Sn and Sn0/Sn surface ratios than aPtbSn catalysts with ab, these samples also showed the higher C2H4 hydrogenation. Table 3. Catalytic activity in the production of lactic acid from CO2, C2H4 and H20 over aPtbSn samples (a4:b).

Catalyst

F/W (ml.min- 1.g- 1 cat.)

Bmol lactic acid.g- 1 cat.min'l

5Pt 1Sn 2PtlSn 1Pt2Sn 1Pt5Sn

13

167

33 ---

157 140 133

Total Pressure: 35 bar, T=423 K, CO2/C2H4/H20=l/1/1.

Table 4. Catalytic activity in the production of lactic acid from CO 2, C2H 4 and H 2 over aPtbSn samples (a4:b).

Catalyst

lamol lactic acid.g-1 cat.min "1

C2H6/C2H 4 (%)

5PtlSn 2PtlSn 1Pt2Sn IPt5Sn

5 14 ---

11 9 2.5 0.5

F/W (ml.min-l.g "1 cat.) 18 17 17 14

Total Pressure: 35 bar, T=423 K, CO2/C2H4/H2=5/5/1.

The production of lactic acid in this case is interpreted through the reverse water gas shift reaction, which will produce the water necessary for the lactic acid synthesis. In order to compare the catalytic activity of samples in this reaction, a study was carded out at 35 bar of total pressure and 423 K. In Table 5 appears the results obtained with these catalysts in the reverse water-gas shift reaction. CO was obtained in all cases, and although in the experimental conditions used it

158 was not possible to analyze quantitatively the water produced, it was detected for 5PtlSn, 2PttSn and 1Pt2Sn catalysts. Again, aPtbSn samples with a>b showed higher activities. Table 5. Catalytic activity in the reverse water-gas shift reaction of aPtbSn samples (a#b).

Catalyst

F/W (ml.min'l.g -1 cat.)

5PtlSn 2Pt 1Sn 1Pt2Sn 1Pt5 Sn

~tmol CO g-1 cat.min-1

28 27 27 23

37 35 6 2

H20 detected detected detected non-detected

Total Pressure: 35 bar, T=423 K, CO2/H2=1/1.

4. CONCLUSIONS The catalytic activation of CO 2 and its reaction with C2H 4 and H20 occurred over silica-supported platinum-tin catalysts which contained the PtSn phase. A direct relation between lactic acid production and the content of the PtSn alloy in the catalyst is established. A negative effect of tin on the catalyst surface is observed for this process.

REFERENCES

.

5. 6. 7.

10.

A. Behr, Angew. Chem. Int. Ed. Engl., 27 (1988) 661. A. T. Ashcrott, A. K. Cheetham, M. L. H. Green and P. D. F. Vernon, Nature, 352 (1991)225. D. Walther, G. Braunlich, U. Ritter, R. Fischer and B. Sch6nocker, in Organic Synthesis via Organometallics, K. H. DOtz (ed.), Vieweg, Braunschweig, 1991, p.77. G. Burkhart and H. Hoberg, Angew. Chem. Int. Ed. Engl., 21 (1982) 76. P. Braunstein, D. Matt and D. Nobel, Chem. Rev., 88 (1988) 747. T. Tsuda, K. Maruta and Y. Kitaike, J. Amer. Chem. Soc., 114 (1992) 1498. J. Llorca, P. Ramirez de la Piscina, J. Sales and N. Homs, J. Chem. Soc. Chem. Commun., (1994) 2555. J. Llorca, P. Ramirez de la Piscina, J. L. G. Fierro, J. Sales and N. Homs, J. Catal., 156 (1995) 139. J. Llorca, P. Rarnirez de la Piscina, J. L. G. Fierro, j. Sales and N. Homs, J. Mol. Catal., A, 118 (1997) 101. J. Llorca, N. Horns, J. L. G. Fierro, J. Sales and P. Ramirez de la Piscina, J. Catal., 166 (1997) 44.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

159

Initial transient rates and selectivities of Fischer-Tropsch synthesis with C O 2 as carbon source Hans Schulz, Georg Schaub, Michael Claeys, Thomas Riedel, Stefanie Walter Engler-Bunte Institut, Universit~it Karlsruhe, Kaiserstral3e 12, 76128 Karlsruhe, Germany

Initial transient changes of conversion and selectivity during Fischer-Tropsch synthesis with a potassium-promoted iron catalyst with H2/CO 2 and H2/CO syngases were determined and several episodes of catalyst transformation distinguished. Selectivity changes are related to changes in elemental reaction steps probabilities using the model of "non trivial surface polymerisation". With the H2/CO 2 syngas the catalyst transformation episodes are extended to about one week. Transient episodes are discussed as for compositional and structural catalyst changes and related Fischer-Tropsch surface chemistry. 1. INTRODUCTION Knowledge about transient episodes in catalytic conversions may contribute to the understanding of their stationary state. In Fischer-Tropsch CO-hydrogenation, the mechanism is undoubtedly complex, and the reaction steps occur among chemisorbed species; their rates are not directly measurable, however, they can be calculated with the help of a kinetic model. Particular methods of performing the reaction and the sampling and analysis of the products are required to obtain the time resolution of reaction rates and selectivities. Early FT-literature reveals (particularly for iron) an initial catalyst transformation time [ 1]. FT-iron catalysts generally exhibit WGS-activity, which is advantageous in case of CO-rich syngas (from coal) and should be essential for FT-synthesis with a H2/CO 2 syngas. In this investigation a strongly potassium-promoted iron precipitation catalyst is used. Through momentaneous product sampling and internal calibrations, time resolved mass balances are attained [2, 3] and with the help of special GC techniques a detailed analysis of product composition is achieved, which is converted into kinetic data (reaction probabilities) for individual repeatedly at each carbon number occurring steps of surface reactions on the basis of the kinetic model "non trivial surface polymerisation" [4], the term being introduced for characterizing the FT-reaction to some extent as a heterogeneous polymerisation, however, combined with a number of further catalytic reactions in particular for synthesizing the chemisorbed monomer from CO and H 2. 2. EXPERIMENTAL

The FT-conversion has been conducted in a fixed bed reactor with the finely powdered catalyst (dp < 0.1 mm) covering larger fused silica particles (dp = 0.25-0.4 mm) as an adhering layer, the weight ratio of catalyst to fused silica particles being 1:10. By this means a very isothermal catalyst bed was provided with highly uniform flow of the gas phase, minor pressure drop and no noticeable intraparticle resistance influence. The catalyst was prepared through quick precipitation from a hot aqueous solution of the nitrates with an N H 3 solution. The precipitate was washed, dried and the potassium added (as

160 potassium carbonate solution) to yield the catalyst composition 100 Fe/13 A1203/10 Cu/25 K (weight ratios). In the reactor the catalyst (1 g of iron) was dried/calcined at 673 K for 16 hours in an Ar stream (0.1 MPa, flow = 40 ml/min (NTP)) and subjected to a reductive treatment (heated with 2 K/min to 673 K and held for 10 hours at this temperature at 0.1 MPa in a H2/Ar stream (molar ratio = 1/3) with a flow rate of 40 ml/min (NTP)). For the synthesis the reactor was cooled in Ar to the reaction temperature of 523 K, the synthesis gas flow adjusted to 30 ml/min (NTP) in the bypass line, and the reference stream of cyclopropane in nitrogen added to the reactor effluent for determination of conversions and yields from the chromatogram. A liquid product fraction was recovered in a hot trap at about 523 K before the pressure reducing needle valve. At zero time the synthesis gas flow was switched to the reactor and ampoule samples were taken from the gaseous product stream at 523 K for later GC analysis. GC analysis has been developed for high resolution of a wide product carbon number range C 1C20 and precolumn hydrogenation for olefin saturation. 3. R E S U L T S A N D D I S C U S S I O N In this section, comparatively, results obtained with the H2/CO 2- and the H2/CO-syngases are presented, regarding in particular the changes of activity and selectivity occurring with increasing duration of the experiment and discriminating distinct episodes of transformation of the catalyst, respectively, distinct episodes of transient kinetic regimes, until the stationary state of synthesis is established. The first figure shows the conversion of CO 2 (for H2/CO 2 syngas) and of CO (for H2/CO syngas) and the yields of CO respectively of CO 2, of organic c o m p o u n d s and of carbon >.-

3O

._1 uJ

I

II

m

10

i

0

100

. l01

60

c.b

40

20

i,j:

.i~i!"'~Yorg.cpds.

y

~/co :..--~.,~,.-

IV

[" !~.~..~..~:.~"

rr' uJ

Z

HI

:. H'2/C0-23

_.1 I.U >.

IV :: V

_ :~-~......... ,~

]

:~i ~'- Xco2

i :

(1I)

V

I

H2/C02=3

2O

70 d

IV

HI

....

i 103

:g

: ........

u.I ._1 u.I u~

40_

20 L ~

H2/CO=2"3 , !

I

..:#" ~! '~::~:%

L .....~esi~::'%-,~_j_. 02 ~.H C s 60 r ~ ' : i ' "

L

,

104

11/ , , r ~ ........i

I-

>_.o~

.rg.cpas.

i

. ' !~Oxygenates'Cl+l

i

i ......

101 102 103 DURATION OF E X P E R I M E N T

I

104 texp, rain.

Fig. 1; above: Conversion of CO 2 respectively CO and yield of organic compounds, carbon deposited on the catalyst and CO respectively CO 2 as function of duration of the experiment with the synthesis gases H2/CO2=3 (left) and H2/CO=2.3 (right). Transient episodes of catalyst transformation are indicated at the upper rim of the diagrams; below: Group selectivities of organic compounds in dependence of duration of experiment. (100 Fe/13 A1203/10 Cu/25 K; 523 K; 1MPa; Feed gas flow: 30 ml (N'I?) per minute and g-Fe)

161 deposited on the catalyst. This carbon yield Y zxc is determined as the difference in the carbon balance between conversion and yield of products in the reactor effluent. The yield of organic compounds comprises also the waxy products accumulated in the hot trap, which have been calculated by extrapolating the product chain length distribution. YaC reflects the carbon formation through CO-decomposition ( CO ~ C + O ). It is to be seen in Fig. ! that with both synthesis gases initially carbon deposition on the catalyst is the almost exclusive reaction. This carbiding is an intrinsic process of iron Fischer-Tropsch catalyst transformation. With the CO 2 syngas the yield of catalyst carbon is relatively high for about 2 hours (102 minutes) and then declines to about zero within one day (103 minutes). With the CO syngas carbon formation initially is higher and continues all over the experiment (as to be explained by the high carbon formation tendency of the strongly alkalized catalyst) leading to catalyst deactivation; the catalyst bed being finally plugged by the produced black carbon after 3 days. With the CO 2 syngas the CO, which is used for carbon formation, is produced through the reversible water-gas-shift- reaction ( CO 2 + 1-12 ~ CO + 1-120 ) and Fig. 1 (left) shows that in the first 100 minutes "no" CO is left, all being decomposed to carbon and oxygen. During the episode III with the CO 2 syngas, the CO yield increases and carbon deposition declines to very low values, this indicating the WGS-activity of the catalyst to increase and the catalyst carbiding process coming to an end. Then only in episode IV after about 103 minutes (about 1 day) the Fischer-Tropsch activity develops ( CO 2 + 3 l-I2 ~ (CH2) + 2 H20 ). In this episode also the CO yield increases, indicating the WGS-activity of the catalyst also to increase. It is assumed, that the now higher CO partial pressure makes the gas phase more reductive, causing increased surface reduction. It needs about 6000 minutes (4 days) until the stationary state (episode V) is attained. The decline of CO yield from about 3000 minutes onwards is not caused by a decreasing WGS-activity. Calculations show the WGS-reaction to be in equilibrium here and the decreasing CO yield being due to the decrease of its equilibrium concentration as a consequence of increasing b-T-conversion (more H20, less CO 2, less H2). It is concluded that at the stationary state the WGS-activity is very high and the reaction rate constant much higher than that of the FT reaction. It is also evident, that the Fischer-Tropsch reaction needs the CO, which is formed via the WGS-reaction, and cannot proceed through direct CO 2 hydrogenation. Then the slow establishing of the stationary FT-kinetic-regime with the CO 2 syngas results from the only slow carbiding of the iron catalyst. With the common H2/ CO=2.3 syngas (Fig. 1, right) the catalyst transformation episodes are much shorter, however, similar trends can be noticed. Additionally the episode VI of deactivation occurs after an about 1 to 2 days lasting pseudo stationary state episode. The selectivity of organic compounds, as grouped for hydrocarbons C2+, methane and oxygen compounds in dependence of duration of the experiment, is presented in Fi~. 1, below. Comparing the results with the CO 2 and the CO syngas reveals surprisingly the stationary selectivity to be almost the same for both syngases and this being true for all of the further selectivity data in Figs. 2, 3 and 4. It is concluded that the catalyst transformations as (1) Deposition of carbon from CO, (2) formation of bulk and surface carbides and elemental carbon, (3) reducing and oxidizing reactions of the iron with H 2, CO, CO 2 and H20 and (4) redistribution of the alkali (spreading on the surface) approach the same kind of FT active surface with its unique set of reaction capabilities: (1) Dissociative chemisorption of H 2, (2) dissociative chemisorption of CO, (3) transfer of activated hydrogen to chemisorbed species, (4) formation of C-C bonds and (5) inhibition of desorption reactions, particularly of associative desorption of paraff'ms (a prerequisite of the surface polymerisation mechanism).

162

100 o D~

~

80

!1~\

60

//~

uiE 40 I--

~" x 0

~"

g

20 0

!

~

2

80



H3C- CH

=O

+

H

Methane selectivity never becomes excessive as to be explained by the alkali promoter action inhibiting associative paraffin desorption. Time dependence of chain growth probability (Fi~. 3) exhibits the same trends with both synthesis gases, however more pronouncedly with the CO 2 syngas. The growth probability generally increases with time until the (almost) same curve of growth probability as a function of carbon number is attained at the stationary state in both experiments. This increase of growth probability reflects the establishing of the FTH r e g i m e . In an ideal p o l y m e r i s a t i o n scheme the growth R - C H =CH 2 + ,.j..,.. probability is carbon number independent, as represented by a Jb, horizontal line. The deviations hereof to be seen in Fig. 3, above, specifically in the episodes III and IV with the CO 2 syngas, are R- CH 2 - ~ H 2 H),H attributed to olefin readsorption on FT sites (see kinetic scheme).

163

o) CL

>i-.,J m 280 nm) at 328 K. The reaction products collected in the gas phase were analyzed by gas chromatography. The photoluminescence spectra were m e a s u r e d at 77 K using a Shimadzu RF-5000 spectrophotofluorometer. The diffuse reflectance absorption spectra were recorded with a Shimadzu UV-2200A spectrometer at 295 K. The ESR spectra were recorded at 77 K using a J E O L JES-RE2X spectrometer in the X-band mode. The XAFS spectra were m e a s u r e d at the BL-7C facility of the Photon Factory in Tsukuba. 3. R E S U L T S A N D D I S C U S S I O N

UV irradiation of powdered TiO2 and Ti-oxide/Y-zeolite catalysts in the presence of a mixture of CO2 and H20 led to the evolution of CH4 and CH3OH in the gas phase at 328 K, as well as trace amounts of CO, C2H4 and C2H6.

179 The evolution of small amounts of 0 2 w a s also observed. The rates of these photoformed products increased linearly against the UV irradiation time and the r e a c t i o n i m m e d i a t e l y ceased w h e n i r r a d i a t i o n was d i s c o n t i n u e d , indicating the photocatalytic reduction of CO2 with H 2 0 on the catalysts. The specific photocatalytic reactivities for the formation of CH4 and CH3OH are shown in Fig. 1. It is clear t h a t the photocatalytic reaction rate and selectivity for the formation of CH3OH strongly depend on the type of catalyst. It can be seen t h a t the specific photocatalytic reactivities of the Ti-oxide/Yzeolite catalysts which have been normalized by unit gram of Ti in the catalysts are much higher t h a n bulk TiO2. The ex-Ti-oxide/Y-zeolite exhibits a high reactivity and a high selectivity for the formation of CH3OH while the formation of CH4 was found to be the major reaction on bulk TiO2 as well as on the imp-Ti-oxide/Y-zeolite. Although the addition of Pt to the ex-Ti-oxide/Yzeolite is effective in i n c r e a s i n g the photocatalytic reactivity, only the formation of CH4 is promoted, accompanied by a decrease in the C H 3 O H yields. The absorption spectra of the Ti-oxide/Y-zeolite and bulk TiO2 catalysts were measured by the UV diffuse reflectance method. A significant shift to shorter w a v e l e n g t h s in the absorption band was observed with the ex-Tioxide/Y-zeolite, clearly suggesting t h a t the dispersion of the Ti-oxide species on this catalyst was higher t h a n on catalysts prepared by i m p r e g n a t i o n methods. The Pt-loaded catalyst exhibited the same spectra, indicating t h a t the local structure of the Ti-oxide species was not changed by Pt-loading. Figure 2 shows the XANES spectra of the Ti-oxide/Y-zeolite catalysts. The ex-Ti-oxide/Y-zeolite exhibits an intense single preedge peak, indicating t h a t the Ti-oxide species in this catalyst has a tetrahedral coordination [9]. 15

y:.-

I~ CH4 "i C|t3OHJ

0

& o 2,
99.0%) with a mole ratio of 3:7. The GDEs were prepared by almost the same manner as described in the case of Cu-GDEs [3]. GDEs consist of a gas layer (mixture of hydrophobic carbon black (CBphob)(Denki Kagaku Kogyo; AB-7) and polytetrafluoroethylene (PTFE) dispersion (Daikin Kogyo; D-l) and a reaction layer (mixture of catalyst powder, CBphob, hydrophilic CB (CBphil) (Denki Kagaku Kogyo; AB-11), and PTFE) laminated on a Cu gauze of 30 mm~ as a current collector. The electrodes were hot-pressed under 1.2 MPa in N; atmosphere. In the case of H2 reduced GDEs, they were reduced in an electric furnace at 300 ~ for 60 min under H2 flow at 50 cm3/min. The apparent working area of GDE was about 4.5 cm2. Electroreduction was performed at 25 ~ in 0.5 M KH2PO4 aq. soln. (pre-electrolyzed under N2 flow) potentiostatically, passing in general 200 C using a potentio-galvanostat (Hokuto Denko; HA-303), an electronic coulometer (Hokuto Denko; HF-201), and a LU-shaped Pyrex cell having one gas and two liquid chambers for anolyte and catholyte, separated by a cation exchange membrane, Nation NX90209. The cell had two lines of gas circulating systems for gas and catholyte. The counter and reference electrodes were Pt-Pt 60 i I ~ I I i 30 and Ag-AgC1 saturated with V: q (Total) ,,,/ 9 : /d KC1, respectively. The reduction 50 25 products were analyzed by gas chromatographs and a high % 40 20 performance liquid chromatograph as described in the previous paper [6]. g 30 15 3. RESULTS DISCUSSION

AND

3.1. CuO/ZnO loaded GDEs When using a GDE of (CuO / ZnO = 3 / 7 [by mole ratio]) : CB = 6 : 5 [by weight]; the standard composition, the reduction products were mainly C2HsOH (EtOH) with slight amounts of CO and HCOO-, and

o : .ooo

o 20

X

-,:co

.

9"" I

I

-1.7

I

-1.6

I

I

I

-1.5 -1.4 -1.3 V vs. Ag-AgCI

lo

I

-1.2

Figure 1. Faradaic efficiency for reduction products of CO2 on a CuO/ZnO-GDE, (CuO / ZnO = 3 / 7 ) : CB - 6 : 5, in 0.5 M KH2PO4 aq. soln. at 25 ~

227

a comparable amount of H2 a s a byI I I I I product as shown in Figure 1. 50 I A./d Faradaic efficiency (1]) maximum of 16.7% for EtOH formation with _ - 20 .IV ........ maximum selectivity of 88% was 40 V " ---V- ............................. -V observed a t - 1 . 3 2 V vs. Ag-AgC1, v . rl (Total) E where total current density and the ~ 30 o >, < faradaic efficiency for CO2 0 E 1-) H2 9 9 (CO2) reduction, 1"1 (CO2), showed maxi.0_ .mo ma. The partial current density for ~o 20 10 ~O9 r EtOH formation was 4.23 mA/cm 2, .o_ o...--~-~ 9 J O "o which is about 50 times greater than ~ r 9 "C2HsOH ~ that obtained on a sintered (CuO / u_ 10 ZnO = 3 / 7) electrode [5]. rl(H2) o m:CO increased with the potential 0 _ ~-~-~- :-~ .................... 8 0 becoming more negative. The 9 I I I I I I selectivity for EtOH formation 0 100 200 300 400 500 600 during the CO2 reduction was more Q/C than 75% in the potential range o f Figure 2. Dependence of the faradaic efficiency for CO2 1.2 to -1.7 V as shown in Figure 1. reduction products and current density on the quantity rl(COz)M,x was, however, close of electricity passed with the standard CuO/ZnO-GDE to 19% at a maximum. The in 0.5 M KH2PO4 aq. soln. at -1.30 V. distribution of reduction products, their q, and current density at -1.3 V were maintained almost constant up to 500 C as shown in Figure 2. This fact indicates that the activity of catalyst is not changed during continuing the electrolysis up to 500 C. To improve the electrode performance, the amount of catalyst of the GDE was increased to 2.5 times as much as the standard composition, i.e. (CuO / ZnO) : CB = 3 : 1. Contrary to the Table 1 Reduction products of C02 on a GDE with (CuO/ZnO):CB = 3"1 for electrolysis 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1

Current density / mA c m 2

-1.20

Faradaic efficiency / % C2H4

CO

H2

T](CO2) rl(total)

EtOH

HCOO-

4.1

4.2

0.9

0.3

1.3

20.5

6.7

27.2

-1.25

4.4

7.9

1.1

0.8

2.4

22.7

11.5

34.9

-1.30

5.2

12.8

1.4

1.5

3.3

25.6

19.0

44.6

-1.35

4.8

9.6

1.2

1.3

3.0

29.9

15.1

45.0

-1.40

3.9

7.2

1.1

0.6

2.8

34.5

11.9

46.4

228 expectations, the current density became only about 1/5 of the previous one, although CzH4 was additionally produced as listed in Table 1. This fact may indicate the decrease of gas diffusion rate in GDEs. After reduction of the GDE of (CuO / ZnO) : CB = 3 : 1 by H2 at 300~ for 60 min, the current density became 10.1 mA/cm 2 at -1.30 V and rl (CO2)Max became 34.5%, and nC3HvOH (n-PrOH) was additionally produced with rl = 9.2%. However, the selectivity for EtOH formation in the CO2 reduction became poorer than that in the case of the GDE with the standard composition as shown in Table 2. Consequently, pre-reduction of the GDE increased the 1"1(total) and rl (CO2), and did not change I"I(EtOH), but decreased the selectivity because of the produced metallic Cu, which usually leads to a variety of CO2 reduction products [6]. Table 2 Reduction products of CO2 on a H;-reduced GDE with (CuO/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq soln. at 25~ Potential / V vs. Ag-AgC1

Current density / m A c m -2 EtOH

Faradaic efficiency / % n-PrOH

HCOO-

C2H4

CO

H2

1"1(CO2) 1"1(total)

-1.20

8.2

10.1

2.9

1.0

2.1

2.3

20.0

18.4

38.4

-1.25

9.3

16.6

5.0

0.7

4.2

3.1

22.8

29.6

52.4

-1.30

10.1

11.9

9.2

1.3

7.6

4.5

25.3

34.5

59.8

-1.35

9.7

8.4

3.9

1.0

7.1

4.2

29.5

24.6

54.1

-1.40

7.8

6.8

1.3

1.1

3.8

4.0

34.8

17.0

51.8

3.2. C u / Z n O loaded GDEs To confirm the effects of Cu formed by reduction with H2 and/or in the course of the CO2 reduction in the CuO/ZnO-GDE, GDEs containing a mixture of Cu powder and ZnO have been prepared and examined. Results obtained by the as-prepared and H2-reduced GDEs are summarized in Tables 3 and 4, respectively. Table 3 Reduction products of CO; on an as-prepared GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KH2PO4 aq. soln. at 25~ Faradaic efficiency / %

Potential / V vs. Ag-AgC1

Current density / m A c m -2

EtOH

n-PrOH

HCO0-

C2H4

CO

H2

1"1(CO2)

1"1(total)

-1.20

3.9

7.2

2.4

0.8

1.5

1.8

18.2

13.7

31.9

-1.25

4.3

14.8

4.3

1.1

3.3

2.4

20.1

25.9

46.0

-1.30

5.1

11.7

7.8

0.8

5.8

3.9

24.7

30.0

54.7

-1.35

4.6

6.4

4.8

0.9

5.6

3.7

28.3

21.4

49.7

-1.40

4.2

4.3

2.9

1.1

2.7

3.1

35.1

14.1

49.2

229

n-PrOH was also produced with both GDEs. The selectivity for EtOH formation became poorer and the current density lesser than that for CuO/ZnO-GDEs of the standard composition. Faradaic efficiencies of the reduced GDE are larger than those of the as-prepared GDE. rl(EtOH)Max of 16.2% at -1.25 V, rl(COz)Max of 40.5% at -1.30 V, and rl(total)Max of 66.4% at -1.30 V were obtained with the H2-reduced GDE. These tendencies are similar to those observed for the Cu foil electrode, even though in the different electrolyte, i.e. 0.1 M KHCO3 aq. soln. [6]. Table 4 Reduction products of CO2 on a H2-reduced GDE with (Cu/ZnO):CB = 3:1 for electrolysis with 200 C passed in 0.5 M KHzPO4 aq. soln. at 25~ Potential / V vs. Ag-AgC1

Current density / mA cm 2

-1.20 -1.25

Faradaic efficiency / % H2

I"I(COz)

rl (total)

EtOH

n-PrOH

HCOO-

C2H4

7.3

9.8

3.6

1.1

3.1

3.2

21.3

20.8

42.1

8.6

16.2

6.1

1.5

6.2

4.8

23.4

34.8

58.2

-1.30

7.9

12.8

10.2

1.3

9.1

7.1

25.9

40.5

66.4

-1.35

6.8

8.3

6.5

1.1

8.6

6.9

30.2

31.4

61.6

-1.40

6.5

5.6

3.8

1.3

5.5

6.6

36.7

22.8

59.5

CO

Table 5 Summary of C02 electroreduction results for 200 C passed at Cu/Zn oxides loaded gas diffusion electrodes in 0.5 M KHzPO4 aq. soln. at 25~ CuO/ZnO-GDE Weight ratio of catalyst : CB

6:5

IdMax/mA cm -2

25.3

Pot. of rl (total)Ma,,, / V

-1.70

-1.40

1"1(total)Max / %

48.9

Pot. of rl (CO2)Max / V

3 :1 5.2

Cu/ZnO-GDE

3 : 1(red.) 10.1

3 :1

3 : 1(red.)

5.1

8.6

-1.30

-1.30

-1.30

46.4

59.8

54.7

66.4

-1.32

-1.30

-1.30

-1.30

-1.30

rl (CO2)Max / %

19.0

19.0

34.5

30.0

40.5

Pot. of rl (EtOH)Ma,,, / V

-1.32

-1.30

-1.25

-1.25

-1.25

rl (EtOH)Ma~, / %

16.7

12.8

16.6

14.8

16.2

Selectivity of EtOH / %

87.9

68.7

56.1

57.1

47.1

Pot. of rl (n-PrOH)Max / V

-

-

-1.30

-1.30

-1.30

rl (n-PrOH)Max /

-

-

9.2

7.8

%

10.2

(red.): Hz-reduced, CB: Total amount of carbon black, IdMax"Maximum of current density, Pot.: Potential vs. Ag-AgC1, rl (X)Max: Maximum faradaic efficiency for formation of X.

230 From these results, it is found that there are zinc oxide and copper oxide, lather than the metallic copper, which control the selective formation of ethanol from CO 2.

4. CONCLUSION Using ZnO/CuO-loaded gas diffusion electrodes, ethanol has been selectively produced with ca. 17% of faradaic efficiency by electroreduction of CO2 the same as with the sintered ZnO/CuO electrode in aqueous KH2PO4 solution but with about 50 times higher current density than the latter. H2-reduced GDEs or Cu/ZnO-loaded GDEs produced in addition ethylene and n-propanol, but with lower current density and selectivity. Optimum conditions for electroreduction of CO 2 on each GDE are summarized in Table 5.

REFERENCES

1. R.L. Cook, R.C. McDuff, and F. Sammells, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 39. 2. T. Ito, S. Ikeda, M. Maeda, H. Yoshida, and K. Ito, Proc. Intern. Symp. on Chem. Fixation of Carbon Dioxide (ISCF-CO2-91 Nagoya), Dec. 2-4, Nagoya, Japan, (1991) 313. 3. S. Ikeda, T. Ito, K. Azuma, K. Ito, and H. Noda, Denki Kagaku, 63 (1996) 303. 4. S. Ikeda, T. Ito, K. Azuma, N. Nishi, K. Ito, and H. Noda, Denki Kagaku, 64 (1996) 69. 5. S. Ikeda, Y. Tomita, A. Hattori, K. Ito, H. Noda, and M. Sakai, Denki Kagaku, 61 (1993) 807. 6. H. Noda, S. Ikeda, Y. Oda, and K. Ito, Chem. Lett., 289 (1989).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

231

Recent Slow Rate of CO2 Increase and Vegetation Activity K. Kawahira* and Y. Maeda** * Department of Bioscience, Fukui Prefectural University, 4-1-1 Kenjyojima, Matsuoka Town, Fukui 910-11, Japan ** Faculty of Electrical Engineering, Toyama University, 12 Gofuku, Toyama City 939, Japan Recent slower rate of the atmospheric carbon dioxide, which abruptly happened in 1989 through 1993, has been studied in relation to the vegetation activity in the Northern Hemisphere. It is found that the gradual increase of the active vegetation area was noticed after 1986. Most active vegetation was seen in July near the high latitudes around 60 ~ N. Since the growth in the vegetation was in accord with the continuous rise of the surface temperature in all seasons after 1987, the slow rate owes to growth in the active vegetation which uptakes the atmospheric carbon dioxide through photosynthesis. 1. I N T R O D U C T I O N Rapid and continuous increases in the greenhouse gases have been predicted to cause global warming at the surface by 1-4 K in the next century compared to pre-industrial time [1]. Among these gases, the atmospheric carbon dioxide increases have main role in the warming due to the anthropogenic emission by combustion of fossil fuel and cement production [1]. Contrary to the atmospheric carbon dioxide increase 'prediction', the anomaly of the increase rate, discovered by Keeling et al. [2], happened mainly in the Northern Hemisphere during the years of the 198993. The dramatic slow down in the increase rate was estimated to be large enough to uptake half as large as the total combustion of fossil fuels [3]. We hypothesize as a cause of the anomaly that the global warming induce the active period (area) of the vegetation to become longer (wider), especially over the lands in the Northern Hemisphere [4]. In the present study we focus on how the active vegetation area changed. 2. ANOMALY IN THE RECENT CO2 INCREASE In order to make the recent CO2 anomaly clear, we used the following definition? NMVj

- ((C02)j/(CO2)j. 1 -

where j mean the year.

1) x 100 ( % / year)

(1)

232 The definition is named as the normalized variation (NMV), which shows the year-to-year changes relative to the mean amount. The application of this definition to the annual changes of the observed carbon dioxide concentration at Mauna Loa [5] was made and shown in Fig. 1. 380 370

:::::::::::::::::::::::::::::::::::::::::::::::::::::::::: .. i i i i i i i i i i i i i i i i i i i i i i i i i i i i i PifiaLubd

i i

1.2

360 >

350

0.8

~, r o

EQ. 340

0.6

Q.

O 330 320

0.4

z

310 0.2

300

l

l

I

l

:

l

:

:

l

i

:

l

l

:

l

l

:

i

l

l

l

:

:

:

:

i

:

l

l

I

:

:

:

l

l

:

i ! i ! i i i i i i ! i i ! ! i ! ! ! i i i i i i ! i ! i i i ! ? ! ! i

290

l

60

l

l

62

t

t

-

64

l

J

l

66

l

l

i

68

J

l

'

70

|

L

l

72

~

l

'

74

l

l

l

76

L

I

l

~

L

'

78 80 YEAR

"

l

J

82

l

d

84

|

'

'.... |

86

'

l

88

'

l

"

90

l

0.0

'

92

94

96

Figure 1. Trend of Annual CO2 and NMV at Mauna Loa (20 N) Although the annual mean concentration increases monotonically, the NMV show variable. Sudden drop of the NMV is seen from 1988 to 89, about 0.8 to 0.4 (%/year). About 0.2 of the NMV are shown in 1992-93. In the following we determine the period of the recent CO2 anomaly as the 1989-93. Similar changes of the NMV at Mauna Loa were seen in the Point Barrow Observatory (71 N), being more remarkable compared to the Mauna Loa Observatory. 3. VARIATIONS OF THE VEGETATION AND SURFACE TEMPERATURE The surface temperature trend is an important effect on the vegetation activity. The trend is analyzed from the radiosonde observations in the world by Angell [6]. The temperature data are in a form of the anomaly, which is the deviation from the 1959-77 mean. Fig. 2 shows the long-term trend for annual mean, winter, and summer in the Northern Hemisphere. The annual mean trend shows continuous temperature rise after 1986. F u r t h e r it is interesting that the winter temperature rise is remarkable compared to the summer trend. The warming trend can bring about that the winter season in the high latitudes became a shorter period t h a n in the no warming period. The negative temperature anomaly in the 1992-93 has been explained as that the massive aerosol loading in the atmosphere due to the eruption of the Pinatubo Mountain in July 1991 causes the worldwide cooling at the surface by strong reflectance of solar radiation. This effect could induce the ocean surface temperature drop, which in turn help uptake of the CO2 [1]. However,

233 since the CO2 anomaly began in 1989, the anomaly in the 1989-91 has no relation to the eruption and~or temperature fall.

N"u

o >,

o


o

o

lOO

'

I

,

!

,

I

,

573 K Ni-3OZr-lOSm

80

60

"~

40

e.. o o

20

40 I

0

,

I

2

,

I

4

,

I

6

,

I

8

,

110

Content of rare earth e l e m e n t / a t %

0

,

I

40000

~

80000

120000

Flow rate, FAN / ml g-1 h-1 Figure 1. Change in the conversion of carbon dioxide on the Ni-Zr-rare earth Figure 2. Change in the conversion of element catalysts at 473 K with the carbon dioxide on the Ni-30Zr-10Sm content of rare earth element, catalyst with reactant gas flow rate.

264 element. Clearly, t h e n u m b e r of "-2, ~ 25 surface nickel atoms increases with i , i , i , i , i , i 0 N i-(4 O - x ) a t % Zr-x at o ~ ~ increasing rare earth element content. ~: o20 3 m Among three rare earth elements ~6 ~ 0 15 examined yttrium is the most effective element in increasing the number of ~ 10 surface nickel atoms. In particular, the 3 0 z -~ 5 _ number of surface nickel atoms in the Z Ni-30Zr-10Y catalyst is more than 3 0 0 2 4 6 8 10 times as high as that in the Ni-40Zr catalyst. Similar trends were observed Content of rare earth element / at% for BET surface areas of the catalysts. The BET surface areas of yttrium- Figure 3. Change in the number of containing catalysts were found to be surface nickel atoms in the catalysts higher in comparison with the with the content of rare earth element. corresponding other rare earth elementcontaining catalysts. . . . . . . . . i The pore size distribution of the I10 mm3 nm-1g-1 catalysts has also been examined, and o typical examples for the Ni-30Zr-10Sm and Ni-40Zr catalysts are shown in Figure 4. The Ni-30Zr-10Sm catalyst > has nanoscale pores of about 1.7 nm OZr-lOSm radius. This is quite similar to that of the Ni-40Zr catalyst, although total ~ _ _ _ Ni-40~r . . . . pore volume for the former catalyst is 1 10 larger than that for the latter catalyst. Pore radius, R / nm P Almost similar distribution of pore sizes was also observed for all the catalysts Figure 4. Pore size distribution in the containing other rare earth elements. In this manner, the addition of Ni-40Zr and Ni-30Zr-10Sm catalysts. Rp sufficient amounts of rare earth and Vp denote pore radius and pore elements is effective in increasing the volume, respectively. surface area of the Ni-Zr catalysts. A similar effect has been reported for the carbonsupported nickel catalysts containing lanthanum and cerium; the dispersion of nickel in the catalysts is increased by the addition of rare earth elements[9]. From Figs. 1 and 4, however, the increase in the conversion is not linearly correlated with the increase in the number of surface nickel atoms in the catalysts. This implies that other factors, such as rare earth oxides themselves and structural change in zirconia support, are expected to affect the catalytic activity of the catalysts. Figure 5 shows X-ray diffraction patterns of the Ni-Zr-Sm catalysts after the oxidation-reduction treatment. Reflections corresponding to metallic fcc nickel are clearly seen in the patterns of all the catalysts. Although no reflections corresponding to samarium oxide are detected, the structure of ZrO2 changes with the composition of the catalysts; in the Ni-40Zr and Ni-39Zr- 1Sin catalysts, monoclinic and tetragonal I

,

I

,

I

,

I

~

I

,

I

265

ZrO~ are present, while the catalysts containing 5 at% or more samarium contain only tetragonal ZrO~. Similar structural change was observed for the catalysts containing yttrium and cerium, although the catalyst containing 10 at% Ce contained a small amount of monoclinic ZrO2 in addition to tetragonal ZrO2. Tetragonal ZrO~ is well known to be present at and above 1373 K and is reversibly transformed to monoclinic phase at lower temperatures. However, the tetragonal phase can be stabilized at low temperatures by Zr 4§ substitution with Ca% Mg ~-§and rare earth element ions[10]. Further, metastable tetragonal phase of pure zirconia can be present even at ambient temperature by reducing the grain size to less than 30 nm[11]. In the Ni-40Zr catalyst the tetragonal ZrO2 is present, probably due to the presence of nickel as well as the formation of nano-scale grained ZrOg[7]. In the present study the tetragonal phase is further stabilized by the addition of rare earth elements and hence, this phase is predominantly formed in the catalysts containing 5 at% or more rare earth elements, although the addition of only 1 at% rare earth elements is not effective in stabilizing the tetragonal phase. Since the turnover numbers of the catalysts prepared from amorphous Ni-Zr alloys increase with an increase in the relative amount of the tetragonal phase[7], the predominant formation of the tetragonal ZrO2 also appears to be responsible for the enhancement of the catalytic activity of the catalysts containing 5 at% or more rare earth elements. 3.3. Effect o f w a t e r r e m o v a l Finally, an attempt was made to increase the conversion of carbon dioxide using two reactors connected in series. In this reaction system water produced in the first reactor was removed by bubbling the reaction gas into water at room temperature. The results are shown in Fig. 6. Clearly, the conversion on the Ni-30Zr-10Sm catalyst after passing two reactors is significantly higher than that using single reactor,

'IE

0 r-

/}

100

0 ZrO2(t)/I A v'9

~

O0

AZrO2(m .......

1

I

e
[ "1 Energy usel[II ~C02

emission

Figure 3 Renewable energy combined system with fossil fuel system (RITE type) -AE 1 the same as Fig. 2. [Syn-fuel]: synthetic fuel. (elec.): water electrolysis. The combination of two systems makes the solar energy more easily transportable for overseas, by the formation of liquid fuel with the following conventional synthetic reaction: CO2 + 3H2 -- CH3OH (liq) + H20 In this system of Fig.3, all fossil carbon will be finally emitted. Therefore, it looks like no decrease of CO2 emission, we must check the change of CO2 emission index. The ZCO2 is constant, but the ZE becomes twice for the sake of carbon-double use in the total system. ]~CO2/ZE ~ 0.42g-COz/kJ Strictly speaking, the synthesized energy on 1 mol-C/methanol is not same to the energy on 1 mol-C/coal. The former is larger (over 30%) than the latter, by these equations'

276 CH3OH(methanol)+ 1.502 -- C02 + 2H20 + 725kj CH0.800.1(average coal) § 1.1502 -- C02 q- 0.4H20 + 510kJ Then theoretical ZE in Fig.3 is about (725+510)/510-2.4. Therefore the C02 emission index of total system will decrease to smaller than half of the value of standard coal system in Fig. 1. In practice, there are considerable energy deficits in every process in this system. For example: (1) CO2 recovery process (about 20-25% of coal energy, in recent technology" a) ). (2) CO2 transportation (2-3% of synthesized methanol energy, 1000km by tanker 4a' 6) ) (3) Synthetic process of methanol (about 20% of the supplied hydrogen energy 4 a) ). These process energy deficits cause a ZE decrease, which makes the CO2 emission index larger. Considering both the merit of high calorie of the synthesized methanol and the demerit of the process energy deficits, we can expect about half value of CO2 emission index in Fig.3 comparing with that of Fig. 1. In the middle of the next century, additional severe reduction of the C02 emission index will be necessary. Such reduction wilt demand us a partial recovery of CO2 from the synthetic fuel use.

4.

RENEWABLE

ENERGY COMBINED

SYSTEM WITH DECREASING

FOSSIL

FUEL SUPPLY There are some problems to recover the C02 from synthetic fuel use, although it is technically possible. The synthetic fuel such as methanol or synthetic gasoline are not only clean fuel but high cost fuel. Mainly economical reason, these synthetic fuel will not apply to coal-type electric generation, but mostly to automobile engine fuel. The application of CO2 recovery for such small apparatus is quite unfavorable in nowadays technology.

II coALI

I coal / X L-C02_I Fuel]-q CHOH/ >

I~

PV

(combustion) ~Energy use II ' AE1 ( C 0 2 r e c o v e r y ) )

q~ Energy use II . . . . . . >~irC02 e m i s s i o n

Figure 4 Renewable energy combined system with decreasing fossil fuel system However, multi-time use of the carbon as the energy carrier is very much effective on the reduction of CO2 emission index. In Fig. 4, if half CO2 of the synthetic fuel can be

277 recycled, the CO2 emission index of the total system becomes 1/4 (about 0.21g-CO2/kJ), owing to the increase of the average carbon recycle-times in the system. Then we can control the CO2 emission index by cutting the fossil fuel consumption or by increasing the gross energy supply, either. In this point of view, a CO2 recovery from synthetic fuel use is quite desirable. How to increase a percentage of CO2 recovery from synthetic fuel use?

It is very difficult to stop the increasing of number of cars. The small size CO2 recovery apparatus for flue gas will be needed but very much difficult. Perhaps only hope will be found in fuel cell generation. In principle, fuel cell is able to avoid air-N2 dilution at the oxidation of fuel on the surface of electrode. However, at the present fuel cells, for example PAFC (Phosforic acid fuel cell) or MCFC (Molten carbonate fuel cell), the residual fuel is finally burned by the already N2-diluted exhausted gas for the heat supply in order to convert the fuel to hydrogen and CO. On the 4) other hand, SOFC (Solid oxide fuel cell) could more easily separate the CO2 recycling gas Much more research and development should be necessary for the recovery of CO2 in the fuel cell system.

5. ULTIMATE CARBON RECYCLE SYSTEM (PURE RENEWABLE ENERGY

SYSTEM) If we succeed to increase a percentage of C02 recovery from synthetic fuel use, finally, we may approach to perfect Carbon-recycle system, without an abrupt change. The ultimate scheme is shown in Fig. 5.

(CO2

[

PV

F u e l ] - - ~ CH3OH/

(comb~

recovery)
2CH3COOH + 2 C 0 2 +8H + + 8e(1) 2 C O 2 +8H + +

8e-

--> CH3COOH

Sum = C6H1206 --> 3CH3COOH

+

2H20

(2) (3)

304 The fermentation of glucose (reaction 1) proceeds by the Embden-Meyerhof glycolytic pathway. The electrons generated reduce CO2 formed in the fermentation with the formation of acetate (reaction 2), which occurs by the acetyl-CoA pathway described below. The result is the formation of three mol of acetate out of one mol of glucose (reaction 3). The ability of C. thermoaceticum to grow autotrophically was realized when it was discovered it has hydrogenase [4], and subsequently was found to grow on a mixture of H2 and CO2 or CO alone as sole source of carbon and energy [5]. Until 1967 C. thermoaceticum was the only acetogenic bacterium available. At this time Clostridiumformicoaceticum was discovered. It was followed by Acetobacterium woodii (1977), Acetogenium kivuii and Clostridium thermoautotrophicum (1981). At present over 60 different acetogenic bacteria have been described. Most of them are presented in the review by Drake [1 ], who also considered their very diverse metabolic capabilities including the use of different carbon sources as well as electron donors and acceptors. The assessment is that acetogenic bacteria are probably the most versatile anaerobes encountered [6 and chapters by B. Schink, H.L. Drake et al., J.A. Breznak, R.I. Mackie and M.P. Bryant, M.J. Wolin and T.L. Miller, Zavarzin et al. and A.C.Frazer in 1]. The impact by acetogens on the environment and in ecological settings is still being evaluated, but it appears to be enormous. Annual worldwide fixation of CO2 by photosynthesis has been estimated to be about 150 x 1 0 9 tons of dry plant mass. About 70% of this material consists of cellulose and hemicellulose, and as much as 10% of it may be converted in the anaerobic environment to methane and CO2 by consortia of anaerobic bacteria [7]. It appears as if acetogenic bacteria play a substantial role in this consortia. At the global level approximately 1013 kg of acetate is metabolized annually in the anaerobic environment and about 10% of this may be derived by CO2 fixation via the acetyl-CoA pathway. Breznak [Chapter 11 in 1] discusses the role of acetogenesis in the guts of termites from which he and his associates have isolated three acetogenic bacteria. They have estimated that bacterial formation of acetate in these guts amounts to about 1012kg annually [8]. Similarly, several different acetogenic bacteria are present in the human gut and they may form as much as 1.25 x 101~kg of acetate per year in the human population by fixation of CO2 using the acetyl-CoA pathway [Chapter 13 in 1]. Other anaerobic environments in which acetate is formed by fixation of CO2 include forest soils [9], rumen of cows [10] and intestines of other animals. Acetogenic bacteria metabolize also a number of methoxylated aromatic acids including 3,4,5-trimethoxybenzoic, syringic, ferulic, and vanillic acids. These compounds, which are lignin degradation products, undergo O-demethylation by the acetogens in the presence of CO2 or CO forming acetate with the methyl group derived from the methyl of the methoxylated aromatic acids [Chapter 17 in 1, and 11 ]. 2.THE ACETYL-CoA PATHWAY Reaction 2 above shows the reductive formation of acetate from C O 2. This occurs via the acetylCoA pathway so called because the acetyl group of acetyl-CoA is the first 2-carbon moiety in which both carbons originate from CO2. The acetyl-CoA pathway is now recognized as an autotrophic pathway of CO2 fixation. The pathway is outlined in Fig. 1. Carbon dioxide enters the pathway via two reductive reactions. One, catalyzed by an NADP-dependent formate hydrogenase (FDH), leads to the formation of formate that, in a series of reactions involving tetrahydrofolate (THF)intermediates is reduced to methyl-THF, the methyl group of which is

305 NADPH

2H++2e-

NADP ~

CO2

HCOOH ATPADP~ ~ ~

[ CODH/ACSl

THF

CiO C~I3

HCO -THF

o e-S

CH = THF

I

CoA

Celllaterial

~._

CH3COOH

THF " * - " t

NADP ~~ ~ CH 2 = THF

f~

~ CH3-THF

2H + + 2e-

Figure 1 Simplified acetyl CoA pathway of C. thermoaceticum. transferred to the cobalt atom of a corrinoid/Fe-S protein to be the precursor of the methyl group of acetyl-CoA. The properties of FDH will be discussed below. The second reaction is catalyzed by carbon monoxide dehydrogenase/ acetyl-CoA synthase (CODH/ACS). Since it is a COz fixing enzyme, some of its properties are summarized here. Ragsdale and Kumar [ 12] have recently published an extensive review of CODH/ACS. As the name of the enzyme indicates, CODH/ACS catalyzes the reversible oxidation of CO to CO2, and the final step in the synthesis of acetyl-CoA from the methyl group, CO, and CoA. CODH activity in C. thermoaceticum was first discovered by Diekert and Thauer [13]. The final conclusive evidence for the ACS activity was presented by Ragsdale and Wood [ 14]. CODH/ACS has been studied extensively by the groups ofRagsdale [ 12] and Lindahl [ 15]. The enzyme, first thought to be a hexamer, has now been shown to be a tetramer consisting of two subunits with the composition a2132. The gene acsA encodes the 13subunit consisting of 674 amino acids with a molecular mass 72,928 Da. It is followed by acsB, that encodes the c~ subunit having 729 amino acids with a molecular mass of 81,730 Da [16]. The enzyme contains two nickel, 12 iron, one zinc and 14 acid labile inorganic sulfide per al3 dimer. The metals are arranged in three clusters A, B, and C. Clusters A and C are similar with a Ni bridged to a Fe4_S4 cluster, whereas the B cluster is a regular [Fe4-S4] cluster. Clusters B and C reside in the 13subunit. Results show that the CODH

306 activity is catalyzed by the [3 subunit and involves cluster C. The ACS activity may reside in the subunit containing cluster A. The B cluster may be involved with internal electron transfer. See the review by Ragsdale and Kumar [12] for a detailed description of the clusters and other properties of CODH/ACS enzymes. 3. N A D P - D E P E N D E N T F O R M A T E D E H Y D R O G E N A S E C. thermoaceticum contains a NADP-dependent formate dehydrogenase that catalyzes the reversible reduction of CO2 with NADPH (Reaction 4). It was found in 1966 [17], and subsequently its formation was shown to be dependent on several metals present in the growth medium [18]. It was purified by Yamamoto et al. [19]. C O 2 -~- NADPH

(4)

--> HCOO- + NADP +

The enzyme has a mass of 340 kDa and consists of two hetereodimers with subunit masses of 96 kDa and 76 kDa as determined with SDS-PAGE; thus the composition is %1~2-The purified tetrameric enzyme contains per mol two tungsten, two selenium, 36 iron, and about 50 inorganic sulfide. The Se is in the form of seleno-cysteine situated in the larger Gt subunit. Tungsten exists as a pterin cofactor similar to molybdopterins or tungsto-pterins of many molybdo- and tungsto-enzymes [20, 21]. The structure of the pterin cofactor has not been established. Progress with the C. thermoaceticum 8 2 120 0.5 FDH has been slow. This is partly due to the enzyme being extremely oxygen sensitive. An apparent >, ~ 0.3 for 02 is 7.6 lam. The oxygen seems to ~6 ~ 1 60 ~ have also a secondary ~ 0.2~ slower effect involving 40 a nonreversible 5 - = ~ 0.1 inactivation of the 20 ~ enzyme. Thus all work with FDH must be 4 0 't0 0.0 0 0 20 30 40 50 60 performed in an Time (hours) anaerobic chamber. An additional problem with FDH is its Figure 2 C. thermoaceticum cultures were grown at 60~ with apparent regulation glucose (56 mM). CO2 was bubbled through the medium to maintain during the growth an anaerobic environment and as an external electron acceptor. cycle. This is shown in Optical density (O), pH (ll), glucose (A), acetate (V), and FDH Fig. 2. Cells at the specific activity ( , ) were assayed as in [19].

il~17604

307 beginning of the logarithmic growth phase have low FDH activity. During the log phase over a period of four hours a fast increase of the FDH activity occurs. It then decreases rapidly to almost nil when the cells enter the stationary phase. The regulation of FDH activity in the cells involves the metals, which are constituents of the enzyme, and also molybdenum, but additional factors which may be responsible include pH, CO2/bicarbonate ratio, and acetate concentration. The sharp regulation of the FDH activity in the cells has the practical consequence that it is difficult to harvest cells when FDH is at its highest, which is needed to obtain very active preparation of the enzyme. By using molecular biological methods involving cloning and sequencing we have recently obtained the nucleotide sequence of thefdh operon in C.thermoaceticum. The sequence has been deposited in the Genbank under the accession number U73807. The gene coding the 13 subunit,fdhB, precedes that of the a subunit,fdhA, andfdhA overlapsfdhB by eight nucleotides. Both genes are preceded by putative ribosomal binding sites andfdhB is preceded by a putative promotor sequence. Only one copy of the genes was detected in the C. thermoaceticum genome. The predicted translation product offdhA, the a subunit, has 893 amino acids with a calculated mass of 98,145 Da, and that offdhB, the [3 subunit, consists of 708 amino acids with a mass of 76,445 Da. Both values are consistent with determinations of the sizes of the subunits of the purified enzyme using SDS-PAGE. The sequence data are now being compared with those of other molybdopterin and tungstopterin enzymes and especially with those of known structures e.g. formate dehydrogenase of Escherichia coli [22], aldehyde ferredoxin oxido-reductase from Pyrococcusfuriosus [23], aldehyde oxido-reductase from Desulfovibriogigas [24], and DMSO reductase from Rhodobacter sphaeroides [25]. What has emerged is that the a subunit contains potential binding sites for four [4Fe-4S] and two [2Fe-2S] clusters whereas the 13subunit may have one [4Fe-4S] and one [2Fe2S] cluster. Thus the enzyme has the potential of binding 48 Fe, which agrees quite well with chemical analyses. It has been pointed out that the reduction of CO2 with NADPH is energetically unfavorable [19]. It is possible that the reduction is mediated through an internal electron transport chain involving the many iron-sulfur clusters. In addition to the iron centers, the 13 subunit has a binding motif for NADP(H). The tx subunit contains the selenocysteine (residue 358), which is encoded by an in frame UGA codon. It has also a molybdopterin guanine dinucleotide binding motif which presumably is the binding site for the tungstopterin cofactor. It is still not possible to predict the structure of the cofactor. ACKNOWLEDGMENTS Support for work on acetogenic bacteria from National Institute of Health grant 5 RO 1 DK 27323 and U.S Department of Energy grant DE-FG05-93ER20127 is gratefully acknowledged as is the support of a Georgia Power Distinguished Professorship from Georgia Power Company. REFERENCES 1.

H.L. Drake (ed.), Acetogenesis, Chapman & Hall, New York, (1994).

308

,

,

,

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

M.D. Collins, P.A. Lawson, A. Willems, J.J. Cordoba, J. Femandez-Garayzabal, P. Garcia, J. Cai, H. Hippe and J.A.E. Farrow, Int. J. Syst. Bacteriol., 44 (1994) 812. F.E. Fontaine, W.H. Peterson, E. McCoy, M.J. Johnson and G.J. Ritter, J. Bacteriol., 43 (1942) 701. H.L. Drake, J. Bacteriol., 150 (1982) 702. R. Kerby and J.G. Zeikus, Curr. Microbiol., 8 (1983) 27. G. Diekert and G. Wohlfarth, Antonie van Leeuweenhoek 66(1994) 209. L.G. Ljungdahl and K.-E. Eriksson, Adv. Microbial Ecology, 8 (1985) 237. J.A. Breznak and M.D. Kane, FEMS Microbiol. Rev., 87 (1990) 309. K. Ktisel and H.L. Drake, Appl. Environ. Microbiol., 60 (1994) 1370. F. Rieu-Lesme, B. Morvan, M.D. Collins, G. Fonty and A. Willems, FEMS Microbiol. Lett., 140 (1996) 281. S.L. Daniel, E.S. Keith, H. Yang, Y.-S. Lin and H.L. Drake, Biochem. Biophys. Res. Commun. 180 (1991) 416. S.W. Ragsdale and M. Kumar, Chem. Rev., 96 (1996) 2515. G. Diekert and R.K. Thauer, J. Bacteriol., 136 (1978) 597. S.W. Ragsdale and H.G. Wood, J. Biol. Chem., 260 (1985) 3970. J.Q. Xia, J.F. Sinclair, T.O. Baldwin and P.A. Lindahl, Biochemistry, 35 (1996) 1965. T.A. Morton, J.A. Runquist, S.W. Ragsdale, T. Shanmugasundaram, H.G. Wood and L.G. Ljungdahl, J. Biol. Chem., 266 (1991) 23824. L.-F. Li, L.G. Ljungdahl and H.G. Wood, J. Bacteriol., 92 (1966) 405. J.R. Andreesen and L.G. Ljungdahl, J. Bacteriol., 116 (1973) 867. I. Yamamoto, T. Saiki, S.-M. Liu and L.G. Ljungdahl, J. Biol. Chem., 258 (1983) 1826. R. Hille, Chem. Rev., 96 (1996) 2757. M.K. Johnson, D.C. Rees and M.W.W. Adams, Chem. Rev., 96 (1996) 2817. J.C. Boyington, V.N. Gladyshev, S.V. Khangulow, T.C. Stadtman and P.D. Sun, Science, 275 (1997) 1305. M.K. Chan, S. Mukund, A. Kletzin, M.W.W. Adams and D.C. Rees, Science, 267 (1995) 1463. M.J. Romeo, M. Archer, I. Moura, J.J.G. Moura, J. LeGall, R. Eng, M. Schneider, P. Hof and R.Huber, Science, 270 (1995) 1170. H. Schindelin, C. Kisker, J. Hilton, K.V. Rajagopalan and D.C. Rees, Science, 272 (1996) 1615.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

309

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Biochemical CO 2 fixation by mimicking zinc(II) complex for active site of carbonic anhydrase Kazuhiko Ichikawa *, Kou Nakata, Mohamed M. Ibrahim, and Satoshi Kawabata Division of Material Science, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060, Japan

The complex coordinated by three benzimidazolyl moieties and a single water molecule was syntesized as a model complex of the active site in carbonic anhydrase. A catalytic reaction of CO2 fixation was simulated by using the model complex.

1. INTRODUCTION The increase of CO2 in the atmosphere is the H serious problem on the global environment. In biological system CO2 is hydrated by catalytic function of carbonic anhydrase, which is an Hydrophilic ] H H "'~176 Half / i o'enzyme containing zinc(II) in its active site. / O-H~ \H f Since the native enzyme is not so suitable to use H,, H Hydrophobic as a catalyst from the view of its stability and handling, it is important to design a catalyst which fixes CO2 and the artificial model --~9 N" ~, N / complex can be used for many times as a /--\ 'N. catalyst. The crystal structure of human HN~ ~~-~~NNH"~ ~ l( His119) carbonic anhydrase II was revealed by X-ray (His 96) / (His 94) crystallography[ 1,2]. In the active site of carbonic anhydrase, zinc ion has tetrahedral Chart The active site of carbonic geometry and coordinated by three imidazoles anhydrase. of histidines and H20 (or OH-) (chart). The catalytic mechanism of carbonic anhydrase is discussed from the data of pH dependence of the catalytic activity[3-5] and X-ray crystallography[I,2]. A number of studies on the syntheses and structures of the model complexes as the active site of carbonic anhydrase have been reported[6-11], most of these complexes consist of pyrazolyl ligands[6,7] or macrocyclic amine ligands[8-10]. The kinetic studies of hydration reaction using model complexes of carbonic anhydrase active site revealed the rate constants of hydration [ 12-14]. This paper reports in vitro simulation of CO2 hydration with the aid of [LZn(OH2)] 2+ which is a model complex of the active site in carbonic anhydrase.

/.o

* the author to whom correspondence should be addressed.

310 2. E X P E R I M E N T A L SECTION 2.1. Synthesis of tris(2-benzimidazolylmethyl)amine L and [LZn(OHz)](PF6) 2 1-(PF6) z The ligand of L was p r e p a r e d from o H p h e n y l e n e d i a m i n e and nitrilotriacetic acid[15]. 1.(PF6) 2 and its D 2 0 d e r i v a t i v e I'.(PF6) 2 were synthesized according to the literature[ 16]. L

2.2. Reaction of 1' with CO2

2+ X D e p r o t o n a t i o n of 1' in D M F by base (i.e., I triethylamine, N-methylmorpholine, imidazole, or 2,6lutidine) was characterized by 2H NMR. The chemical shifts were recorded vs. acetone-d6, the shift of which was assumed to be 2.0 ppm. The dried reagent of DMF (water content is less than 0.005% ) was used as a 1 X = H20 solvent. 1' X = D20 Reaction of 1' with CO2 was monitored with the product of zinc-bound HCO 3- by 13C NMR and i.r techniques. To compare the results of the presence and absence of Et3N, the two solutions which contain I'.(PF6) 2 + Et3 N and only I'.(PF6) 2 were prepared and CO2 gas was introduced to each solution at the same time using Y-shape tube. For NMR measurements samples were made as follows, l"(PF6)2 (ca. 100 mM) with or without equimolar Et3N was dissolved into DMSO-d6. CO2 gas was bubbled for 3-5h to each solution. The new signal resulted from the hydration was clearly observed after bubbling of CO2 for 3-5h. The chemical shifts of 13C NMR in DMSO-d6 were indicated vs. TMS. For i.r. measurements samples were made as follows. After the equimolar base was dissolved into a CH3CN solution of I'.(PF6) 2 (ca. 50 mM), the solution became suspended as a result of the product of [LZn(OH)]+. The slow evaporation of the suspended solution by bubbling CO2 produced a solid compound for i.r. measurements.

3. RESULTS AND DISCUSSION We designed the model complex for the active site of carbonic anhydrase providing (i) imidazole ligand which corresponds to histidine imidazole, (ii) coordinated water molecule and (iii) hydrophobic pocket, as mentioned in Experimental Section. Since the ligand, tris(2benzimidazolylmethyl)amine L used in this work plays a role of steric hindrance, it will be able to reproduce the tetrahedral geometry which is identical with the active site of carbonic anhydrase. Furthermore, the benzene rings of benzimidazolyl groups can fix a hydrophobic pocket, in which there exists the active site of the native enzymes.

3.1. In vitro simulation of enzymatic CO2 hydration The in vitro simulation consists of the three processes of (1) deprotonation of coordinated water to give the active zinc hydroxide derivative, (2) nucleophilic attack of zinc-bound hydroxide to CO2 substrate, and (3) displacement of the bicarbonate anion by H20[4,6], as

311

shown in Scheme. 3.1.1. Deprotonation of coordinated water molecule The deprotonation of coordinated water is presented as follows 9 [LZn(OD2)] 2+ +

B ..

~ [LZn(OD)] +

+

(1)

BD +

and the equilibrium constant K of eq.(1) can be given by pK

=

pKa,D20-

(2)

pKa,BD+

Ka,D20 and Ka,BD + stand for dissociation

B

-

BH+

c o n s t a n t s o f c o o r d i n a t e d D 2 0 and (1) %,,,~j,,,4 (His)3Zn-OH (His)3Z-OH2 c o n j u g a t e acid BD+ of weak base B, respectively. The degree of deprotonation (~2) C02 HCO3of z i n c - b o u n d w a t e r m o l e c u l e is proportional to pK. The smaller pKa,D20 H~O~ ' ~ (His)3Zn-OCO2 H as well as the larger pKa,BD+ bring about Scheme The catalytic mechanism of the l a r g e r c o n c e n t r a t i o n of the zinc carbonic anhydrase. hydroxide derivative according to eq.(2). For the in vitro simulation the reagents of weak base B [ LZn(OD2) ]2+ are triethylamine, N-methyLut D+ D+ Et3N D+ ~,,,~r,fD2 O morpholine, imidazole and 2,6-1utidine. In h u m a n c a r b o n i c a n h y d r a s e II a i ! [ | i ! ! ' I ' ' couple of water m o l e c u l e s 15 lo De0 + kut, Ira, 0 connected to His 64 and Thr 199 through hydrogen bond may d e p r o t o n a t e a zincFigure 1. The evidence of deprotonation of the zincb o u n d w a t e r m o l e c u l e [ 17, bound water molecule by 2H NMR chemical shifts in 18]. F o r the c o o r d i n a t e d DMF. The concentrations of 1', DaO, and base are 50 D 2 0 of 1', the free D 2 0 , mM. Et3 N : triethylamine, MeM : N-methylmorpholine, /'+base, and D20 + base, the Lut : 2,6-1utidine, Im : imidazole. (a) 1' + Et3N, (b) 1' observed chemical shifts of + MeM, (c) 1'+ Lut, (d) 1'+ Im. 2H NMR in DMF are shown in Figure 1 : the other data of chemical shifts are also added for the salts of these bases, Et3NDC1, MeMDC1, ImDC1 or LutDBr. Figure 1 shows that (1) the larger pKa,BD+ gives rise to the smaller difference of chemical shifts between 1' +base and the salt of base, (2) the base of Et3 N provided the largest production of [LZn(OD)]+, (3) [LZn(OD2)] 2+ showed almost no d e p r o t o n a t i o n without the aid of B, and (4) free D 2 0 showed almost no deprotonation even with B.

I In~D+MeT

""orMeM (tl Et3N

3.1.2. Hydration of C O

2

The hydration of CO2 is given by

312 [LZn(OD)]+

+

CO2(g)

~

~

(3)

[LZn(OCO2D)] +

The formation of the hydrogencarbonate complex [LZn(OCO2D)] + as the reaction product of [LZn(OD)]+ with CO2 gas has been revealed by 13C NMR and i.r. studies, as shown in Figures 2 and 3. The 13C NMR spectrum (Figure 2) i n d i c a t e d the evidence of CO2 hydration. The new signal at 167 ppm observed after bubbling CO2 for 5 hours into DMSO solution of I'.(PF6)2+Et3N has been attributed to the production of z i n c - b o u n d HCO 3- and shifted to downfield from the signal of HCO3( 158 ppm ) in D M S O solution of ((Ph3P)2N)HCO 3. The yield of [LZn(OCO2D)]+ was estimated to 50% based on I'. The difference of i.r. spectra between the product from the CH3CN solutions of I'.(PF6)2+Et3N and I'-(PF6) 2 with bubbling of CO2 gas shows the two bands at 1440 cm-1 and 1675 cm-] (marked by * in Figure 3), which were assigned to the symmetric and asymmetric CO stretching bands. The separation of these two bands, Av=235 cm-1 demonstrates unidentate for HCO 3ligand[19]. The i.r. and 13C NMR results obtained from the I'-(PF6) 2 solution without Et3N did not show any evidence of CO2 hydration. In the previous studies, the ligand OHof the model complex was reacted with CO2 to produce CO32- complexes, such as [LZn(bt-CO3)ZnL], where L=HB(3,5i -Pr2pz)3[8]. Since the dimerization does not take place in vivo, these c o m p l e x e s are not preferred as an enzyme model. 1H NMR evidence showed that the [LZn(OH)], where L-HB(3-But-5-Mepz) 3 , is reacted with CO2 in benzene to produce bicarbonate complex [LZn(OCOzH)], reversibly[9].

(a)

C02

Benzimidazolyl

I

tA,,-I

,bt .t&,.~ ~ , 1 ~ ~ , J .,==~,.,=,,1., It,,w, J=J I~,l,,,l,~,~l W,,p- ~1 m,,p,,m,l~.,nm,~

(b)

'

''

'1'

I

'''

I'

''

J__L

,I

1 'i'

'''

I''

160

''

I ' ' '

140

'i

120

''

' ' 1 '

,5 (ppm)

Figure 2. 1 3 C NMR spectra after bubbling CO2 for 5h. (a) I'.(PF6)2+Et3 N solution and (b) I'.(PF6) 2 solution. The peak with * was assigned to zinc-bound HCO3-.

100 rv/'-", r e - ' - , O,i

F~AVAI~ [ //IJV ~1 F,,f'kll ~ t"~, ,',

....,.

,, ,~ A A

v

.-X-

50

2000

.3(-

1000

v / c m -1

Figure 3. I.r. spectra of the product after bubbling CO2 into I'.(PF6)2+Et3N solution ( - - ) and l"(PF6)2 solution (--). The bands with * were assigned to the symmetric and asymmetric stretching modes of HCO3- ligand.

313 3.2. Structures of 1 and characterization of coordinated water molecule The structure of 1 d e t e r m i n e d by X-ray crystallography[20] is shown in Figure 4. The O1 zinc ion in 1 was coordinated by three nitrogen atoms of benzimidazolyl groups and one oxygen atom of water molecule. The coordination N1 geometry around zinc ion is tetrahedral. The Zn-N, Zn-OH2 distances and the bond angles of N-Zn-N, N-Zn-OH 2 of 1 were consistent with the value for zinc c o m p l e x e s with four coordination[21]. The geometry around zinc Figure 4. O R T E P d r a w i n g of 1. ion in carbonic anhydrase is tetrahedral and Ellipsoids are depicted at the 50% c o o r d i n a t e d by three nitrogen atoms of probability level. Bond distances(nm) : imidazoles and H20. The observed zinc ion Znl-N1 0.2052(4), Znl-O1 0.201(3). geometry of 1 is quite similar to that of the active site in carbonic anhydrase[ 1]. The pKa value of coordinated water molecule for 1' was obtained as follows. From eq.(1) the ratio of [LZn(OD2)]2+ : [LZn(OD)]+ was indicated by [ [LZn(OD)]+ ] [ [LZn(OD2)]2+ ]

(4)

[ BD+ ] [B]

The following equation was led from eqs.(2) and (4). log(

[BD+ ]) [B]

1 = T

(5)

(pKa'BD +-pKa,D20 )

The eq.(5) m e a n s that the intercept of the linear plot of log ([ BD+] / [B] ) vs. pKa,BD+ gives the pKa of z i n c - b o u n d D 2 0 . F i g u r e 5 shows the r e l a t i o n b e t w e e n log ([ B D+] / [B] ) calculated from the chemical shifts of 13C N M R spectra in D M S O - d 6 vs. pKa,BO+. The weak base B are triethylamine, benzylamine, morpholine, 2methylimidazole, N-methymorpholine, and 2,6-1utidine. The pKa value of zinc-bound water molecule for 1' was ca. 8.6,

J9

koJ"~

o

.y, 32.4 2.1 1.1 29.2 14 3 Cu-Zn-Cr(3:3:1)/HY ~> 40.9 14.4 0.5 26.0 5 4 Cu-Zn-chromate~/HY 35.5 5.2 0.1 30.2 3 5 Fe-Zn(4:1)/HY g) 13.3 4.9 0.2 8.2 8

Distribution(C-mol%) C2__SelP C2 C3 C4 C5+ (%) 18 37 23 14 0 38 33 10 5 0 24 35 24 12 0 9 24 39 25 58 15 14 26 37 80

a) 4000C, 50 atm, SV=3000ml/g-cat-h, H2/CO2=3, b) MeOH+Me20. c) C2jC2_PC2=" d) Prepared by coprecipitation. ~) Mixing using granules, o Cu/(Cu+Zn)=0.01. g)350~ When methanol synthesis catalyst, prepared from CuO, ZnO and CrO3, was mixed with HY zeolite, C2+ hydrocarbons were obtained in a good selectivity(Run 1)[2]. The same catalyst made into granule gave better results (Run 3). The selectivities to ethane, propane and butane were higher and that to methane was lower. No olefin was observed. The hydrocarbons distribution of typical Cu-Zn-Cr/HY is shown in Figure 1. This is a typical distribution of MTG reaction, and quite different from Schultz-Anderson-Flory law.

[-7 Paraffin ~" 3 O

E

0

1

2

I

3

4

5

I

6

C number Figure 1. Hydrocarbons distribution over Cu-Zn-Cr/HY

The yield of hydrocarbons (14.4%) were higher than that of equilibrium conversion of C O 2 to methanol (ca 7% at 400~ 50 atm). It means that methanol formation was accelerated by MTG reaction. When methanol synthesis catalyst was prepared by coprecipitation, the yield of hydrocarbons decreased (Run 2). This seems to be due to the deactivation of zeolite by the sodium remaining after 5 times wash. Similar tendency was observed on the hydrocarbon synthesis between two Cu-Zn/HY composite catalyst, in which one Cu-Zn catalyst was precipitated by Na2CO3, and another Cu-Zn catalyst was precipitated by oxalic acid[3]. When methanol synthesis catalyst was prepared by sodium compound, remaining sodium deactivate an active site of zeolite on MTG reaction.

330

Secondly, Zn-chromate catalyst, which is useful for the reduction of ester and does not hydrogenate coexisting double bond, was applied for composite catalyst[5]. A Cu-Znchromate catalyst containing 1% of Cu was most effective among various composite catalysts (Run 4). The formation of ethene was observed, and the selectivity of higher hydrocarbon increased comparing to Run 1. It was shown that higher hydrocarbons were obtained via olefin. Iron catalyst is well-known as Fisher-Tropsh catalyst to give hydrocarbons with Schulz Flory law. However, hydrocarbons distribution changed drastically with the addition of zeolite(Run 5)[6]. The selectivity to ethene was high, and the selectivity to higher hydrocarbons increased comparing to those of other composite catalysts. The main hydrocarbons produced were branched. Detail studies were carried out on Fe-ZnO and Fe-ZnO/zeolite catalyst further more. Hydrocarbon synthesis from CO2 over various Fe-ZnO/zeolite catalyst is shown in Table 2. By the addition of HY zeolite, the formation of olefins and the selectivity of C2§ hydrocarbon increased and that of methane decreased. The hydrocarbon distribution of Fe-ZnO and FeZ n O / H is shown in Figure 2. Hydrocarbon distribution followed Schulz-Flory rule over FeZnO catalyst, indicating F-T reaction took place. However, it changed to the formation of higher hydrocarbons over Fe-ZnO/HY, indicating MTG reaction took place. Table 2. The effect of zeolite on the formation of hydrocarbons Catalyst CO2 Conv. Yield(%) Sel. of olefin (Fe:Zn=4:1) (%) CI-I4 C2. Oxya CO Cz= C3= Fe-ZnO 17.2 7.6 3.1 0.5 6.0 1 4 Fe-ZnO/HY 13.3 0.4 4.5 0.2 8.2 80 30 Fe-ZnO/HM 11.5 0.2 3.0 2.6 5.7 90 73 FeZnO/H-ZSM-5 14.4 0.0 2.8 0.0 11.6 40 11 Condition: 350~ 50 atm, SV=3000ml/g-cat. h, H2/CO2=3, 6h. a MeOH + Me20.

2

(A) Fe-ZnO(4:1)/HY

(B) Fe-ZnO(4:1)

r-1 Paraffin I I Olefin

o~" 1.5 !

0

!

1'

.,.,.,

.~_

>" 0.5 ,

1 Figure 2.

I

I.

2 3 4 5 6 Carbon Number

I

7

-

1

2 3 4 5 6 Carbon Number

7

Hydrocarbons distribution over Fe-ZnO(4:I)/HY (A) and Fe-ZnO(4:I)(B).

331

The effect of the zinc content on the catalytic behaviors of Fe-ZnO is shown in Figure 3(A). When the zinc content is higher than 33%, methanol was obtained in up to 3% yield and the yields of hydrocarbons including methane was very low. These Fe-ZnO catalysts acted as methanol synthesis catalyst rather than F-T catalyst. On the other hand, Fe-ZnO catalysts with a zinc content lower than 33% produced hydrocarbons in good yields. The distribution of hydrocarbons approximately followed the Schulz-Anderson-Flory model, indicating that those catalysts work as F-T catalysts. The dependence of the catalytic behaviors of Fe-ZnO/HY on the zinc content is also shown in Figure 3(B). Fe/HY (without zinc) and Zn/HY (without iron) gave hydrocarbons in very poor yields. However, Fe-ZnO/HY with various zinc contents produced hydrocarbons in up to 5% yields, and the selectivities of C2+ hydrocarbons in all hydrocarbons were high. Although the zinc content was not a crucial factor in the hydrocarbon synthesis, the best yield of hydrocarbons was observed with Fe-ZnO (4:I)/HY. XRD Patterns for Fe-ZnO catalysts containing less than 33% ZnO before the reaction showed two phases : a-Fe~O3 and ZnFe204. After the reaction, the diffraction patterns of a-Fe203 disappeared completely while those of ZnFe204remained. This suggests that a Fe~O3 was transformed to other iron species effective for F-T reaction, and ZnFe204 was not active for F-T reaction. On the other hand, the diffraction patterns of ZnO and ZnFe204 were detected in Fe-ZnO with a zinc content higher than 33%. After the reaction, all peaks still remained but became sharper. The catalytic activity for methanol synthesis of Fe-ZnO(l:2) was higher than that of ZnO as shown in Figure 3(A), indicating that not only ZnO but also ZnFe204 is responsible for methanol synthesis. 30 (A)

c,

(B)

20

10

5

4~

15 ......

20

.....

002

I- / ~

~-




cony.

I

2~

Figure. 3.

20

40

60

Zn Fe+Zn (%)

80

~

o , ~

"']~176176 ~. . . . . . . . .

2 ~] ,.,....

0

.=.........e...

,

MeOH

0 100

o_.

O 5 0 0

20

40

60

80

=~0 100

Zn Fe+Zn (%)

Effect of zinc content on the activity of Fe-ZnO(A) and Fe-ZnO/HY(B). Reaction conditions: 350~ 5MPa, SV=3000ml/g-cat. h, H2/CO2=3 [6]

The Fe-ZnO catalyst shows two kinds of catalytic sites, that is, iron species effective for F-T reaction formed from a-Fe203(Fe304) and Fe promoted ZnFe204 effective for methanol synthesis. In the absence of zeolite, the F-T reaction sites are very active to produce hydrocarbons with the Schulz-Anderson-Flory distribution. On the other hand, the sites for F-T reaction are deactivated and the sites for methanol synthesis ZnFe204 exhibit the catalytic activity in the case of the composite catalyst. Therefore, hydrocarbons were obtained by MTG reaction with a non-Schulz-Anderson-Florv distribution over F e - Z n O / H (Figure 4).

332

,.,,,.,

CO ,...~...t,

tjk.)2~ t ~ "1-

H2

i

..........

~ . . . . . . .: . . . . . . . .. . . . . . "

F-T Reaction - ~ . . . .' -

~ c a t a l y s t

~

[._~_~_~-~..

MeOH

z' z - .".'.....~. ., . . . . . . . . . .' .: .

,

..,: :.1

:.....: :~ ~ " + ~

,,Ee7(fr0m Fe3o4) '

I Hydrocarbons] Schulz-AndersonFiery Distribution

(High Methane Selectivity) Non-

Schulz-AndersonFiery

Distribution

(Low MethaneSelectivity) Figure 4.

Reaction scheme of hydrocarbon synthesis over Fe-ZnO/HY [6]

4. CONCLUSION Hydrocarbons were obtained from CO2 by MTG reaction using various composite catalysts. Cu-Zn-Cr oxide prepared from Cue, ZnO and CrO3 was an effective component of the composite catalyst for the synthesis of light paraffins. Methanol synthesis was accelerated on composite catalyst, and much hydrocarbons were obtained above equilibrium conversion to methanol. Cu-Zn-chromate/HY or Fe-ZnO/HY, which gave ethene or propene, were good catalysts to give higher hydrocarbons by oligomerization of olefin. The main hydrocarbons produced were branched. The XRD analysis showed that Fe-ZnO has two active sites: iron species formed from Fe304 for F-T reaction and ZnFe204 for methanol synthesis. When Fe-ZnO was used alone, F-T reaction over iron species was predominant. The addition of HY zeolite to Fe-ZnO deactivated the iron species for F-T reaction, and thus MTG reaction over ZnFe204 became predominant.

REFERENCE

1. K. Fujimoto, T. Shikada, Apple. Catal., 31, (1987)13. ; T. Inui, T. Takeguchi, Catal Today, 10, (1991) 95. 2. M. Fujiwara, Y. Souma, J. Chem. Soc., Chem. Commun., (1992)767. 3. M. Fujiwara, R. Kieffer, L. Udron, H. Ando, Y. Souma, Catal. Today, 29, (1996)343. 4. M. Fujiwara, R. Kieffer, H. Ando, Y. Souma, Appl. Catal. A, 121, (1995) 113. 5. M. Fujiwara, H. Ando, M. Tanaka, Y. Souma, Appl. Catal. A, 130, (1995)105. 6. M. Fujiwara, R. Kieffer, H. Ando, Q. Xu and Y. Souma, Appl. Catal. A, 154, (1997)87.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

333

Studies in Surface Science and Catalysis, Vol. 114 1998 Elsevier Science B.V.

C02 for petrochemicals feedstock. Conversion to synthesis gas on metal supported catalysts. P. Gronchi, P. Centola, and R. Del Rosso Industrial Chemistry and Chemical Engineering Department, CIIC, Politecnico di Milano, P.za Leonardo da Vinci 32, 20133 Milano, Italy.Tel/Fax+39-2-23993274. E-mail Paolo. [email protected]

1. INTRODUCTION The direct conversion of methane to synfuels or petrochemicals is a very arduous reaction and few research papers appear in the literature [ 1] the passage through the synthesis gas remains the obligatory industrial route towards Cn (n> 1) organic compounds (Fig. 1). -- -

CH3OH

~[ .... ALCOHOLS C

4

/

,,

--

Cn

•

L

-..

f-F i - ,

/'~,, New ,~"'\ "H20

]

+ nil2

ETHYLENE/PROPYLEN~,~

- -

GAsoL.,,~E 4

-

~J

Fig.1. Synfuels and Cn (n> 1) organic compoundsproduced trough synthesis gas from CI-L.

/, C 0 2 ' i WAX

.J- 02 /.- .,

Synthesis gas

-

-

--II~[

INTERMEDIATE

,

Due to the large natural gas reserve (same order of magnitude than crude [2]) methane appears the most cheap and available carbon source. The catalytic steam reforming is actually the most frequently industrial route to synthesis gas. However the carbon and thermal efficiency deeply depend on the CO2. The dioxide is formed during the reaction from the hydrocarbon partial oxidation or from CO, or it may be present in the natural gas (some reserves have till 40% of CO2). As a consequence the reactivity of pure CO2 in the reforming conditions must be studied at first to optimise the reforming catalyst and at second to design industrial reforming processes with CO2 and H20 mixture as reactants from which synthesis gas with customised CO/H2 ratio can be obtained for specific organic hydrocarbon synthesis [3]. Carbon dioxide weakly adsorbs at high temperature and in presence of CH4 and H20 on the traditionally used metal catalyst (Ni supported on A%03). A relevant role can be however

334 assumed by the support when the basic character prevails [4]. In fact by using a basic support the adsorbed CO2 could become more reactive and more available to the near metal active site [5]. In this paper we investigate the behaviour of Ni metal catalyst on pure SiO2 and on La203 (very recently investigated also by other authors [6]) pure or as additive of SiO2, with the aim of more deeply understanding the surface reaction.

2. EXPERIMENTAL

Catalysts. SiO2 (70-230 mesh, Merck 7734, 60~ 430 m2/g BET method), and La203 (Fluka; La 99,98%; 9-12 m2/g BET method) were used as support following two different procedures of the Ni deposition. Using the first one, Ni was deposited by wet impregnation starting from Ni(NO3)2.6H20 alkaline solution of NH3. Alternatively a solution of Ni(NO3)2.6H20(1 ml/g of support) was slowly feeded to a SiO2 fluidized bed maintaining silica at incipient wetness (dry method). Both the obtained catalysts were dried under vacuum at 333K, and successively in an oven at 383K (24h) and finally calcined at 873K (2h). FTIR analysis does not reveals nitrate adsorption. The La203-SiO2 supports also were obtained by wet impregnation of La(NO3)3 (Prolabo) alkaline solutions on SiO2 or dry impregnation as described for Ni impregnation. All the catalysts were used fresh prepared and reduced with H 2 before the catalytic tests. A Philips P-W1130 diffractometer was used for the X-ray powder analysis. Reaction Apparatus. A quartz tube flow reactor (10 mm in diameter, 400 mm overall length) filled with 200-500 mg of catalyst between two layers of carborundum, placed in a ventilated oven, with a thermocouple located inside the catalytic bed, was used. The flow rates of the gases (>_100 ml/min, CH4 + CO2, diluted with N2) were controlled by mass flow meters (Brooks 5850).The concentration of reagents and products was determined using a GC (Dani 3800) equipped with a TCD and a Carbosieve S II packed column. Blank tests showed negligible conversions of the reagents. The discrepancy from 100% mass balance was ascribed to coke and tar formation. A direct measurement of the amount of deposited coke was made by an extensive hydrogenation of the catalyst to CH4 after each experiment in the 823-923K temperature range. Thermal analysis. A Mettler TA2000 system was used for TGA. The analyses, on supports and catalysts, were performed under CO2+ CH4 (1:1) at atmospheric pressure in isothermal conditions at 823, 873,923 K with the H2 initial reduction and H2 final treatment (60 min He purge after each gas change). XPS analysis. A M-Probe XPS instrument [7] was used for the analysis of catalyst surface that was treated under the reaction conditions at 873K.

3. RESULTS 3.1. Preparation methods. Two different routes were used for the preparation of Ni/SiO2 catalysts. The starting material was always Ni(NO3)2 but following the wet impregnation the metal hydroxide is the precursor.

335 The nitrate salt itself is the precursor following the dry impregnation procedure. The CO production rate vs. the reaction time is reported in Fig. 2; the data indicate that the catalyst obtained using the hydroxide precursor has a very stable activity for more than 2 hours and that, on the contrary, the nitrate precursor causes a quick loss of activity. We tested also the effect of the time to reach the calcination temperature from ambient temperature, by the comparison between the calcination performed using a tubular reactor with a 5~ of increasing programmed temperature and that using a large oven, which has a time to final temperature about 8 times longer. No effect was ascertained on the wet prepared catalysts. The time of calcination (873K) is a determining parameter only on the activity of the catalysts prepared by the dry impregnation method as the CO production rate of the other materials (not reported in the figure) is not modified by 2 or 3 times longer calcination time..

20 18 ;

[CO production rate ]

16 l

/

.mmo]_E~

h.gNi

1412I 10,,.7-..

......

-

•

=

-

-

,~ W E T

IMPREGNATION

t

6 ;] ~,%~.'\,'kDRY IMPREGNATION 4 ~- -,,~.!~,. ,,~ 2 ~ : : : ~ . : : . . ~ : ......."~:

DRY IMPREGNATION + LONG CALCINATION

0 20 40 60 80 100120140 Time (rain) Fig. 2. CO production rate dependence on the impregnation and calcination method of Ni/SiO2 catalysts.

The physical characteristics of the prepared catalysts are reported in Table. 1. The pore distribution is monomodal and the medium pore size does not change from that of the support when nitrate is used as precursor and while it increases using Ni(OH)2. Tab. 1. Comparison of the physical characteristics of catalystsa. Catalysts

Precursor

Ni (%w/w) // 3.2 3.2 4.0 4.0

Surfaceareab (m2/g) 550 491 438 317 252

Porosity (mediumvalue; A) 31 27 26 48 47

SiO2 // Ni/SiO2c Nitrate Ni/SiO2 Nitrate Ni/SiO2 Hydroxide Ni/La203Hydroxide Si02d a)Calcination at 873k (3h); b) BET area; c) Calcinated for 8h at 873K d) La203 (2% w/w) was obtained by dry impregnation (from La(NO3)3) Useful indications can be also acquired from the XRD analysis. Using the wet deposition the powder does not show NiO reflections at low Ni per cent (5% max.); a 30% minimum amount of Ni is necessary in order for the reflections appear. On the contrary following the dry method just 4% of Ni give NiO reflection.

336 3.2. XPS analysis Cls XPS spectra recorded on catalysts at incipient carbon deposition [8] after 7 and 16 hours of reaction are presented in Fig. 3. Three carbon species can be observed: the first at low binding energy range, (284-285 eV) attributable to -CHx- species or adventitious carbon, the second at slightly high binding energy ( at about 286-287 eV) probably due to carbon linked to -O- or -OH groups, and at last that in the 288-289 eV range characteristic of-CO3= groups[9]. 5000

4o00-

Incipient C deposition

Fig. 3. Cls XPS binding energies at increasing reaction time (T=823K; P= 101KPa; CH4/CO2/N2=40/40/20 ml/min): Aincipient carbon deposition, B- after 7 hours, C-after 16 hours.

Ni/La203 Ni/SiO 2

c 0

0

3000

-

2000

-

1000

--

// / / / / / //

/ / U

I

I

I

I

eV

284 285 286 287 288 289 290

10000

50000

420 min

8000u) ~9 6000 c

o r

4000

30000

-

20000

20000

I

284

960 min

40000

285

|R.

10000

I

I

I

286

287

288

289

eV

284

I

I

I

I

285

286

287

288

289

3.3. Effect o f the catalyst r e d u c t i o n

The analyses have been performed with the same apparatus of the catalytic tests using the H2reduced and the unreduced catalysts (Fig.4); at first the reactants were feeded together ( A serie) and then separately (B serie), The data refer to the GC analysis at 5 min. of reaction time. The catalysts used were Ni(4%)/SiO:, Ni(3.2%)/SiO2-La:O3(2%) and Ni(4.4%)/La203 all prepared by the wet method. The degree of reduction of the catalyst surface influences the conversion of CO2 and CH4. The conversion of CO2 appears strongly different between the two serie of analyses (A and B). If CO2 is feeded together with CH4 the conversion is always greater than in the separate reaction, while with the same type of feeding, comparing the reduced and the unreduced materials, the conversion is greater on the unreduced La203-SiO2 support that on the same reduced one. The figure shows the same conversion pattern as CO2 for the CH4 but all the values are less than those of CO2. On the contrary when CO2 and CH4 are feeded separately CH4 conversion is greater than CO2 and it is significantly higher on the unreduced La203-SiO2 and La203 supports when CO2 is absent that on the reduced one. With the pure lanthana

337 support the CH4 conversion ratio is much higher than with the reduced one when CO2 is absent. Conversion 513

Reduced [77"~ Unreduced 25

CH 4

00 2

A

N iS iLa T-~ NiLa ~i'.~i

:;

Conversion (%)

F--"-!

(%)

-

20

C02

-

l ~ !OH 4

B

l

I

Reduoed Unreduced

NiSiLa

ii

I

rt

zl 15

-

10

-

NiSiLa NiSi

NiSi

]

r 6

~;

""

-

s.,

Fig. 4. C02 and CH4 conversions at 5 min. and 873 K, on reduced (white bar) and unreduced catalysts when the reactants are co-feeded (A) and separately feeded (B). 4. DISCUSSION By using as precursor the Ni(OH)2 obtained following the wet impregnation method, we observed a more stable activity than with the Ni nitrate salt (dry impregnation). The behaviour could be related to the type of the surface metal: the hydroxide seems to close the small pores while the nitrates penetrate them. The lower decomposition temperature of hydroxides than nitrates determines a better dispersion together with a more abundance in the macropores which have a greater accessibility. The easy CO desorption preserves the metal site from an excess of carbon formation from Boudouard reaction and the dispersion promotes the reaction of the adventitious (or "status nascendi") carbon with the surface oxygen before its ageing or transformation into less reactive form [8]. In fact it is well known that the CHx (where x=0-3) fragments, from which originate the carbon polymers, take place from both the CI-I4 dissociation and the CO disproportionation deactivating the catalyst [10]. We very recently reported a mechanistic hypothesis where the Ni-surface oxygen takes a relevant role [11]; others reported the importance of the metal-oxide perimeter on the activity [12]. Moreover the data presented here confirms the ox-red mechanism of the CH4 reforming with CO2 as the conversion of both the reactants is greater when they are feeded at the same time than alone. CO2 acts as an oxidant and becomes converted on the reduced catalyst but more extensively on that treated with CH4 (A serie) than H2 (B serie). The conversion occurs both on the CH4 reduced metal (see the Ni/SiO2 catalyst) and on the CH4 reduced support (Ni/La203) as it appears that low amount of easy reducible La203 increases the CH4 conversion when it is feeded alone, probably due to the reduction of the oxide surface. Similarly on CeO2 Japanese researchers observed an CO2/CH4 ox-red reaction [13] suggesting that low ox-red potential of the support may play an important role.

338 CH4 reacts both on the oxidised surface of the support giving CO and on the reduced metal producing CHx specie. The conversion is lower than CO2 when co-feeded and, on the contrary, higher than CO2 when it is feeded alone. Really other reactions have to be considered; La203 easily forms carbonate specie or, at reduction conditions, cover the metal surface thus, due to the stability of carbonate groups or the excessive metal decoration, inhibiting the CH4 reaction. At this respect as appears from XPS analysis, the oxygenated C ls population on La203 seems more numerous than on the SiO2 support, at the initial reaction time being carbonate and successively C-O-X or C-O-H. The carbon is also formed more easily on the lanthana containing support, probably due to the oxide promotion of the decomposition of carbonate species into CO which disproportionate to C specie on the metal.

5. C O N C L U S I O N We investigated two methods of preparation observing that the wet impregnation produces a more stable catalyst with an increased dispersion of the metal due to the Ni(OH)2 precursor. About the support, La203 used as pure support or as a promoter on SiO2 puts in evidence the support effect as it modifies the conversions and the carbon deposition. Indeed investigating more deeply about the differences of CO2 reactivity between silica and lanthana supported Ni catalysts, particularly from the tests on reduced and unreduced catalyst, it appears that the oxidation degree of the catalytic surface plays a relevant role on the reactivity. A concerted ox-red scheme of reaction between CH4 and CO2 is confirmed involving the surface oxygen of the support. At low temperature, lanthana promotes carbon formation more than the silica support.

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

E.E. Wolf, Natural Gas Conversion IV, Kruger National Park (South Africa), Dec. 1995, Closing remarks D.Sanfilippo, ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.IV, pag.2219-2220 J.Rostrup-Nielsen,ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.I, pag.327-330 E. Ruckenstein and Y.Hang Hu, J.ofCatal., 162, 230-232 (1996) P.Gronchi, R.Del Rosso, and P.C6ntola, Applied Catalysis A, 152 (1997) 83-92 Z. Zhang and X. Verykios, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M.de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M.S.Scurrell (eds.), Vol. 107, pag.511-516, Elsevier (1997). 7. C.L.Bianchi, M.G.Cattania, V.Ragaini, Mat.Chem. & Phys, 29 (1991) 297-302 8. P.Gronchi, D.Fumagalli, R.Del Rosso, and P.C6ntola, J. of Thermal Anal., 47, 227-235 (1996). 9. J.Chastain, (ed.), Handbook of XPS. Eden Praire: Perkin-Elmer (1992) 10. A.Erdtshely. K.Fodor and F.Solymosi, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M.de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M.S.Scurrell (eds.), Vol. 107, pag.525-530, Elsevier (1997) 11. P.Gronchi, P.C6ntola, and R.Del Rosso, ECCE-1, Florence (Italy), May 4-7, 1997, Proceedings Vol.I, pag.375-378 12. J.H. Bitter, PhD Thesis, University ofTwente (NL), (1997) 13. K.Otsuka, E.Sunada, T.Ushiyma and I.Yamanacha, Natural Gas Conversion IV, Studies in Surface Science and Catalysis, M. de Pontes, R.L.Espinoza, C.P.Nicolaides, J.H.Scholtz and M. S. Scurrell (eds.), Vol. 107, pag.531-536, Elsevier (1997)

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

339

Iron Catalyzed CO2 Hydrogenation to Liquid Hydrocarbons Rocco A. Fiato* E. Iglesia, G. W. Rice and S. L. Soled Exxon Research and Engineering Company, Florham Park, New Jersey 07932 USA

Many of the catalysts which are useful in Fischer-Tropsch synthesis are also capable of catalyzing the hydrogenation of CO2 to hydrocarbons. Our structure-function studies have shown that it is possible to control the selectivity of CO2 hydrogenation by specific iron-based catalysts to generate yields of C5+ hydrocarbons that are comparable to those produced with conventional CO based feedstocks.

1. INTRODUCTION Catalyst structure-function studies have shown that alpha-olefins are the primary products of CO2 hydrogenation, with normal paraffms being formed by secondary hydrogenation. Catalysts that contain mainly iron oxide, or iron carbide with substantial concentrations of matrix carbon, favor secondary reactions and limited chain growth. The relative concentrations of these phases depend on the ease of reduction of the starting catalyst precursor, which can be changed to some degree by substitution of various cations into the initial formulation. In cases where excess matrix carbon is introduced into the reduced/carbided catalyst system, we are able to demonstrate the importance of diffusion constraints on overall process selectivity. We have developed proprietary iron catalysts that are essentially free of metal oxide and/or matrix carbon phases, with unprecedented selectivity toward C5+ olefinic products.

2. BACKGROUND ON CO AND CO2 HYDROGENATION Studies on CO2 hydrogenation can be traced back to the early 1900's [1] with emphasis on cobalt, nickel and iron-based catalysts. While this work clearly demonstrated the overall reactivity of CO2 under a wide range of reaction conditions, the primary product(s) consisted of methane, CO and other light hydrocarbons. Emmett and others have evaluated the large body of experimental work that was performed through the early 1960's and concluded that CO2 hydrogenation to higher molecular weight hydrocarbons remained a significant challenge. Selectivity control continues to be a critical issue in Fischer-Tropsch chemistry, a catalytic process that dates back more than seventy years [ 1]. Operating conditions can be adjusted to control selectivities but overall effects are limited [2-4]. During Fischer-Tropsch synthesis with conventional bulk iron catalysts, various phases, including metal, metal carbides and metal oxides are present at steady-state catalytic conditions [5-7]. * Corresponding author

340 Previous studies indicate that alkali increases the heat of CO chemisorption while decreasing the heat of H2 chemisorption [8,9]. As a result of alkali promotion on iron catalysts, the average chain length and the olefin content of the products increase, whereas the activity and methane selectivity decrease [ 10,11 ]. Recent studies indicate that ot-olefins, the major primary products formed during FischerTropsch synthesis, participate in secondary reactions [12]. Chains can terminate either by 13-hydrogen abstraction to form an ct-olefin or by H-addition to form a paraffin [13,14]. Olefins can undergo secondary reactions by subsequent readsorption leading to isomerization or hydrogenation. We observe selectivity relationships that are consistent with Egiebor's proposal that significant secondary hydrogenation reactions can occur on iron catalysts [ 12].

3. EXPERIMENTAL ON CO AND CO2HYDROGENATION The details of the preparation of the iron oxide catalyst precursors are described elsewhere [1517]. For those carbides made by exsitu carburization, the oxide was loaded into a 1" diameter quartz tube and heated in a 1/1 H2/CO mixture at a space velocity of 10,000 v/v/hr at 350~ for 24 hours. Iron carbide catalysts were also prepared by laser pyrolysis of iron carbonyl and ethylene using a 150 watt continuous wave CO2 laser to provide both a rapid high temperature reaction (-.1 sec with T-1000~ and quench [18]. The procedures used to produce the wet chemical and laser generated iron carbides of this paper have been disclosed in detail [22]. Catalyst tests were performed in a 300 cc Parr stirred tank reactor with octacosane as the slurry medium. Wide-angle powder x-ray diffraction identified catalyst phases present. Thermogravimetric reductions were recorded on a Mettler TA-2000~ using 100-200 mg of sample and a heating rate of 8 deg/min. Gravimetric titrations were performed using a specially designed heated entry port enabled injection of the pyridine titrant [19]. Mossbauer spectra were collected with an Austin Sciences instrument with a radiation source consisting of Co 57 diffused into Rh (New England Nuclear).

4. RESULTS AND DISCUSSION Unpromoted iron oxide catalysts (surface area 30-50m2/gm) and iron oxide converted ex-situ to iron carbide were compared under standard reaction conditions. Figure 1 illustrates the sequential conversion of an iron surface in 1:1 H2:CO at 1 atm under programmed heating conditions. In region 1 CO adsorbs onto the metal surface. At about 300~ Fe begins to convert to (Haag) carbide and at slightly higher temperature amorphous carbon begins to grow. Gravimetric acidity titrations with 3,5 dimethylpyridene at 250~ showed an acid site concentration of 99 ~t-moles/grn catalyst for the oxide and only 27 for the carbided catalyst. These numbers compare with --250 ~t-moles/grn for a solid acid like ~/-A1203. Figure 2 summarizes the different product selectivities measured at CO conversions of-50%. In the carbided system we find ethylene present as well as a higher olefmic content in Ca, apparently as a result of inhibited secondary hydrogenation. Consequently, a carbided iron surface produces a more olefmic, heavier product, consistent with the observations of Egiebor and Cooper who suggest that acid sites on a precipitated FeOx/SiO2 catalyst lead to secondary reactions of tz-olefins [ 12].

341

Alkali titrates acid sites on the iron surface and also increases the strength of the CO-surface interaction. Figure 3 compares the effect of potassium promotion on the iron carbide catalyst where we had substantial amounts of matrix carbon. A cementite Fe3C phase was synthesized by laser pyrolysis with a carbon content corresponding closely to the Fe3C stoichiometry, i.e., without any excess matrix carbon. Some of it was heated in H2/CO mixtures (1/1) at 350~ in order to introduce 40-50% by weight matrix carbon. Figure 4 shows that the excess carbon reduced olefin selectivity while favoring CH4 and lighter products. The pure carbide, as prepared, does not respond to alkali treatment, Figure 5, suggesting that potassium is only needed when removal of the olefins is not fast enough to prevent secondary hydrogenation. The oxide catalyst reduces at lower temperature with substitution of Co into Fe304. Figure 6 shows the onset of reduction is initiated 20~ lower with the cobalt substitution. This may explain the enhanced olefin selectivity attributed to iron-cobalt catalysts in the literature [20,21]. Mossbauer spectroscopy on spent catalysts suggests that oxide/carbide phase formation in iron catalysts is also sensitive to reactor configuration (extent of backmixing). In integral fixed bed reactors, we find that iron has partitioned into carbide in the front of the bed but shows increasing amounts of oxide near the exit, whereas the same catalyst in the stirred tank reactor remains all iron carbide, Figure 7. Comparative tests were conducted on CO2 hydrogenation over a conventional coprecipitated Fe/Cu/K/Si catalyst versus a wet chemical derived sample of FesC2, see Figure 8. At 7/1 H2/CO2 feed ratio, the carbide exhibits significantly higher selectivity to C2+ products and higher olefin yields than the conventional catalyst. The higher molecular weight fractions produced from the 7/1 H2/CO2 feed consisted of a mixture of alpha- and beta-olefins, n-paraffins and n-alcohols together with large fraction of methylbranched and internal olefin isomers, see Figure 9, while a 2/1 mixture of H2/CO operated at >80% CO conversion (i.e., effective H2/CO ratios >10/1) over this catalyst would produce nearly 65% alphaolefin, 15% n-paraffm, 1-3% n-alcohol and 15% methyl-branched and internal olefin isomers. Laser generated carbides that contain virtually no matrix carbon overlayer have been tested and show much higher selectivity to desired products than wet chemical analogs. The Zn and Mn containing systems required potassium promoters for optimum selectivity, see Figure 10. A series of 13-CO2 labeling studies were conducted to determine the extent of CO2 conversion to hydrocarbons in the presence of varying quantities of CO, see Figure 11. These studies showed that no CO2 was converted in mixtures where >5% carbon atom CO was present, and that CO2 conversion to hydrocarbons did not occur until virtually all of the CO was consumed. Preliminary kinetic studies showed that product formation rates in the presence of low levels of unlabeled CO were consistent with a mechanism involving dissociative CO2 chemisorption followed by "CO" and surface "C" formation with hydrogenation to hydrocarbons, Figure 12.

342 REFERENCES

M. E. Dry, "The Fischer-Tropsch Synthesis" in Catalysis, Science and Technology. Vol. 1, p. 159, ed. J. R. Anderson and M. Boudart, NY (1981); "The Fischer-Tropsch and Related Synthesis," H. Storch, N. Golumbic and R. Anderson, Wiley NY 1951. C. D. Frohning and B. Cornils, Hydrocarbon Processing, p. 143, Nov. 1974. B. Comils, B. Buessmeier and C. D. Frohning, Inf. Ser..-Alberta Res. Council, 85, 126 (1978). B. Buessemeier, C. D. Frohning and B. Comils, Hydrocarbon Processing, p. 105, Nov. 1976. F. Fischer and H. Tropsch, Bennstoff-Chem. 7, 97 (1926). V. V. Niemanstuerdiert, A. M. vanderKraan, W. L. Van Dyk and H. S. vanderBaan, J. Phys Chem. 84, 3363 (1980). F. Blanchard, J. P. Reymond, B. Pommier and S. J. Teichner, 3rd International Symp on Scient Basis for Prep. ofHet. Catal. (Belg), Sept. 1983 and J. Mol. Catal. 176, 171 (1982). .

9.

M. E. Dry, T. Shingles, L. J. Boshoff and G. J. Ostuizien, J. Cat. 15, 190 (1969). J. Benziger and R. J. Madix, Surf. Sci. 94, 119 (1980).

10.

D. L. King and J. B. Peri, J. Cat. 79, 164 (1983).

11.

H. Storch, N. Golumbic, and R. B. Anderson, "Fischer-Tropsch and Related Synthesis", Chap. 6, Wiley N. Y., 1951.

12.

N. O. Egiebor and W. C. Cooper, Appl. Catal. 17, 47 (1985).

13.

P. Biloen, J. N. Helle and W. Sachtler, J. Cat. 58, 95 (1979).

14.

G. Henrici-Olive and S. Olive, Angew. Chem. Int. Ed. 15, 136 (1976).

15.

R. A. Fiato and S. L. Soled, U.S. Pat. 4,618,597 (1986).

16.

S.L. Soled and R. A. Fiato, U.S. Pat. 4,544,671 (1985).

17.

S. L. Soled and R. A. Fiato, U.S. Pat. 4, 584, 323 (1986).

18

G. Rice, R. A. Fiato and S. L. Soled, U.S. Pat. 4,788,222 (1988).

19.

S. L. Soled, G. McVicker and B. DeRites, Proc. 1lth North American Thermal Analysis Conference, 1981.

20.

M. Nakamura, B. B. Wood, P. Y. Hou and H. Wise, Proc. 7th Int Conf of Catal., part 7a, June '80.

21.

R. M. Stanfield and W. N. Delgass, J. Cat. 72, 37 (1981).

22.

R. A. Fiato, S. L. Soled, G. W. Rice and S. Miseo, U.S. Pat. 4,687,753 (1987) and U.S. Pat. 5,140,049 (1992).

ACKNOWLEDGMENTS The authors would like to thank Professor Carl Lund for the Mossbauer measurements and Professor Charles Mims for the 13-CO2 labeling experiments.

343

Figure 1 T H E R M A L C O N V E R S I O N O F I R O N TO IRON C A R B I D E ,

,

l

.

F i g u r e 2. OXIDE SURFACE SHOWS M O R E SECONDARY PRODUCTS THAN CARBIOED SURFACE % seioctlvlty

70

,

Fe in HdO0 at 1 atm

i "~I

oo" 50-

~

~

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40301~0

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0

l

C2 olefln/C2 total

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C4 olefln/C4 total

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C5 plus

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F i g u r e 4. E X C E S S MATRIX C A R B O N D E C R E A S E S S E L E C T I V I T Y ON L A S E R G E N E R A T E D Fe3C

% selectivity

/i--I.

i

Condition*: 270~ 75 l~i, 2/1 H2/CO 1500 vlg~r, conv 9SO%,CSTR

F i g u r e 3. A L K A U P R O M O T I O N I N H I B I T S SECONDARY HYDROGENATION IO0

i t

/

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J

I

100

% selectivity

li- +-.c+!

'~ ! "~''"~

J

I

-J

I

60

40

C2 olefln/ C2 total

C4 olefln/ C4 total

C10 olefln/ C10 total

C:2 olefln/ C2 totaJ

CS plus

F i g u r e 5. POTASSIUM P R O M O T I O N H A S M I N O R EFFECT ON L A S E R - G E N E R A T E D IRON C A R B I D E S % sek~-tJv~

~"

:

C4 oleftn/ C4 total

//'-++"L m

C10 olefln/ C10 total

Conditions: 270~ 75 p~, 2/1 H2/CO

8000 vlg/hr, CSTR

C10 C10

CS plus

total

CX4

Figure 6 COBALT SUBSTITUTION FACIUTATES R E D U C T I O N OF F e 3 0 4 I

:1//

total

Conditions: 270"(:, 7513411,2/1 H2/CO 1000-0000 v/Whr, CSTR L a i r G4menm~ Cata~W

Conditions: 270~ 75 psi, 2/1 H2/CO lSOOvlg/hr, cow > 50%, CSTR

100]

C10 olefin/ C10

CS plus

I m ~ 3 c

Thm111~ Grlvlmetry: Fe304 R e d ' ~

i

CH4

9 ....

i ....

i ....

i ....

i ....

:

. . . .

~ Loss Spinel

/

I

....

! .... 260

+X

! .... 270

1 .... 280

1 .... 290

I .... 300

310

T (~

Onset 9 of Reduction Lowertd by 20~ with Co Substitution

344

Figure 7

Figure 8

MOSSBAUER SPECTROSCOPY REVEALS DIFFERENT PHASES PRESENT IN SLURRY VS. FIXED BED REACTORS Re~eve zmn~y

CO2 HYDROGENATION PERFORMANCE OF CARBURIZED F5C2/2% K

~red~ S ~ ~mW ~

r, _

~!.~',

_

--Top

IS0A

,

I

,

i

9

7.0

7.0

3.0

1.7

21

37

23

13

CH4

64

16.5

6.2

4.2

C.~

36

83.5

93.8

95.8

28

80

95

99

Figure 9

Figure 10

CO2 HYOROGENATION PROOUCES WlOE RANGE OF C10+ PROOUCTS

LASER GENERATED IRON CARBIDES FOR SELECTIVE CO2 HYDROGENATION

FeC

FeZn-C

FeMn-C

Colllplrstlve Fe/Cu/K/Si

Conversion

22

25

31

21

Seie~lviW (based on Ct+1 CH4 C2+ % Oleftn in C2-C4

5.5 94.5 94.0

5.8 942 93.0

S.1 94.9 96.0

64 36 28

Catalyst 9

FesC2/2%K with > 50% wt. Matrix Carbon

% CO2 Conversion

37

S e k ~ i v ~ (based on c1+) CH4 C2+ % Oiefin in C2-C4

16.5 83.5 80

C|Q Product Dlstrlbutlon (% wt) r,-olefln I~-olefin n-paraffin n-alcohol CH3-Branched4ntsmal Isomers

36.7 0~ 13.3 14.2 28.4

Figure 11 REACTION MECHANISM STUDIES VIA 13CO2 L A B E U N G

Reaction

Fes C212% K With >50% wL Matrix Carbon

Conditions: 260-270~ 3800 cm31gFe/hr,75 psig, 10% vol catalyst in octacosane, Parr CSTR, 30-50% vol N2 In Feed gas

,

~0 0.0 s.0 10.0 vek~(nv~)

Catalyst:

H2/CO2 Feed Ratio % CO2 Conversion

% Oiefln in C2-C4

~ t'~,~~ '~ ~,~.~ i

Comparative Fe/Cu/IOSI

Selectivity (based on C1+)

~!!? l

Catalyst

CO/'CO2 + H2 --~ --CH2-- + --CH2*-- + H20

9 Feeds with >5% (Carbon Atom) CO Feed Show No Label In Product 9 Mbmd Feeds Show Labeled Product After All CO is Consumed 9 CO2/tt 2 Alone Yields Mixture of Hydrocarbon Plus CO as s By-Product 9 Preliminary Kinetics Suggest Two Routes for Hydrocarbon Formation - Reverse Shift Followed by "CO" Hydrogenation - Oissocistive CO2 Chemisorptlon Followed by "C" Hydrogenation

~, r

Conditions: 270~ 7/1 H2/CO2 3800 cm31gFe/hl, 75 psig, 26% N2 vol In CSTR Feed 9Laser generated FeZn and FeMn carbides contain .

", x ! /

!i i

~ 40

0

Solar photovoltaic system ', ' ~ ' ~ ',,

'~.\

Hybrid system

"-~.~:,, -

~ 30

"

!

g

.

'

o 20

o 20

D

Hybr,d system . i i

10

i

..

2

Figure 1. Estimated unit cost where low and high construction costs are assumed.

/

~,

g o

,

60

50

"\.~

/,

>.,

50

cQ)

/, / ,

._ E

i / '

CF = I.5 yen/MJ

60 Ni(3 moi%)/MgO found to have very high and stable activity o for a long period (100 days). 3mo1% ~ 40 Ni/MgO did not so high activity, but rather -=_ stable. 3 mol% Ni/ml20 3 catalyst ~deactivated very rapidly and finally the 20 Ni(3 mol%)/AI203 reactor was plugged with the deposited carbon. We measured the amount of total 0 0 20 40 60 80 100 carbon species on the catalyst surface after Time on stream / day the reaction for 120 h. The results were 0.1 wt% on Ni 0.03Mg0.97O, 1.6 wt% Ni/MgO Figure 1. Reaction time dependence of and 10 wt% on Ni/A1203 catalysts. From methane reforming with carbon dioxide. Reaction condition: 1123 K, W/F=1.2 these results, it was found that Ni0.03Mgo.970 gh/mol, CHJCO2=I/1, 0.1 MPa, 0.1 g. catalyst has high resistance to carbon deposition in CO 2reforming of methane. Figure 2 shows the TPH results on Nio.03Mgo.970 and 3.0 mol% Ni/MgO after the reaction at 773 K, and the activity was listed in Table 1. Under this reaction condition, methane conversion is far from the thermodynamic equilibrium level. Two peaks were observed in the TPH profiles. One appeared at 550 K-700 K (a-carbon), and the other above 873 K (fl-carbon). It is found that the peak intensity of a-carbon was almost constant, while that of fl-carbon increased linearly with the time on stream. From the behavior and reactivity, fl-carbon is ascribed to deposited carbon, fl-carbon formation rate and selectivity were also in Table 1. Selectivity to carbon is much related to the dispersion of Ni metal particles. This suggested that carbon formation tended to proceed on the larger Ni particles. And carbon was formed on solid solution catalysts with higher Ni content. O

i

i

i

i

377 Table 1. Catalyst properties of nickel-magnesia solid solution and supported catalysts. Catalyst BET O2 a) HEb) Ni'~ c) D , Rco~ fl-carbon rate ~ selP /mE/g /btmol/g-cat /% /% /btmol g~s -~ /bt C-mol g~s ~ /% 0.00 Nio.03Mg0.970 22 10.5 3.1 2.9 29.5 31 0.00 0.019 Nioa0Mgo.900 33 130.4 14.9 11.4 11.4 155 0.03 Ni/MgO h) 25 21.8 2.4 58.6 11.0 87 0.02 0.023 Ni/MgO i) 25 226.5 3.9 62.4 1.7 138 0.15 0.109 a: adsorption temperature 873 K, b: adsorption temperature 298 K, c: reduction degree of Ni was estimated by 2x(amount of 0 2 adsorption )/(total amount of Ni), d: dispersion of Ni was estimated by (amount of H 2 adsorption)/(amount of 0 2 adsorption), e: CO formation rate in the reforming of c n 4 with CO 2 under 773 K, 0.1 MPa, CHJCO2=I/1, W/F=0.1 gh/mol, catalyst weight: 0.05 g, f: r-carbon formation rate under the same reaction condition, g: r-carbon selectivity is estimated by ( r-carbon rate)/( r-carbon rate + CO formation rate), h, i: Ni loading of supported catalyst was 0.3 and 3.0 mol%, respecitively.

3 mol% N i / M g ~

Nio.03Mgo.970

9

_./x.__

3

9

s I

270

1

I

470 670 870 Temperature / K

I

1070

I

I

l

1

270 470 670 870 1070 Temperature / K

Figure 2. TPH profiles on the samples after being exposed to cn4-1-CO 2 for (1) 2, (2) 30, and (3) 60 min. Reaction conditions: 773 K, W/F=0.1 gh/mol, CH4/CO2=1/1, 0.1 MPa, 0.05 g. Figure 3 shows FUR spectra of CO adsorption on nickel magnesia catalysts. On Ni0a0Mg0.900 and Ni/MgO catalysts, linear (2100-2000 cma), bridge (2000-1850 cm -~) and physisorbed Ni(CO)4 (2057 cm -~) were mainly observed. In contrast, on Nio.03Mgo.970 nickel monomer and dimer carbonyl species which are interacted with MgO were mainly observed as previously reported[10]. These species were increased with the CO pressure, therefore they are found to be formed via CO induced structural change. On Nio.o3Mgo.970 solid solution, Ni metal particles seem to be highly dispersive. Figure 4 shows TEM image of reduced Ni 0.03Mg0.970. Large cube and small sphere on it were observed. Large cube is nickel-magnesia solid solution, small sphere is a nickel metal particle. The average size of small particle is about 4 nm. This is larger than that estimated by the dispersion as listed in Table 1. In addition, the number of metal particles are much smaller than that of solid solution cube, and that expected by reduction degree as listed in Table 1. This means that a lot of solid solution cubes with no metal particles were observed, and most nickel atoms can not be observed in TEM image. This is because the metal particle size is beyond the detection limit of TEM, or very small metal particle is oxidized during TEM sample preparation. It is strongly suggested that Ni o.o3Mgo.97O has more highly dispersive nickel metal particles than other three catalysts.

378

3.0 mol% Ni/MgO Figure 3. FTIR spectra of CO adsorption on the samples. Pco=13.3 kPa, 298 K.

:!;~

0.3 m o l % N i / M g O

,~'.~

,, ~ _

~: ..,;.

Ni 0.10Mgo.900

Nio.03 Mgo.97 O

100 nm Figure 4. TEM image of reduced Ni 0.03Mg0.97 O . c G ~,4

r

~ ~ ---.

c G --.

Wavenumber/cm-t

4. CONCLUSION

Ni0.03Mgo.970 solid solution catalyst has high resistance to carbon deposition in C O 2 reforming of methane. From the characterization results, this catalyst was found to have highly dispersive nickel metal particles with the interaction with support surface. The inhibition mechanism is suggested to be the activation of adsorbed CO 2 at the interface between metal and support surface and rapid supply of oxygen species to nickel surface. REFERENCES

1. J.R. Rostrup-Nielsen, Catalysis Science and Technology, J. R. Anderson, M. Boudart (eds), Germany, Berlin, Springer, 5 (1984) 3. 2. A.T. Ascchcroft, A. K. Vermon, M. L. H. Green and P. D. E Vermon, Science 352 (1991) 225. 3. J.R. Rostrup-Nielsen and J. H. B. Hansen, J. Catal., 144 (1993) 38. 4. J.T. Richardson and S. A. Paripatyadar, Appl. Catal., 61 (1990) 293. 5. O. Yamazaki, K. Omata, T. Nozaki and K. Fujimoto, Chem. Lett. (1992) 1953. 6. O. Yamazaki, K. Tomishige and K. Fujimoto, Appl. Catal. A:General 136 (1996) 49. 7. K. Tomishige, Y. Chen, K. Yokoyama, Y. Sone and K. Fujimoto, Shokubai, 39 (1997)70. 8. Y. Chen, K. Tomishige and K. Fujimoto, Appl. Catal. A:General, in press. 9. Y. Chen, O. Yamazaki, I~ Tomishige and K. Fujimoto, Catal. Lett., 39 (1996) 91. 10. A. Zecchina, G. Spoto, S. Coluccia and E. Guglielminotti, J. Chem. Soc., Faraday Trans. 1, 80 (1984) 1891.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

379

Global c a r b o n - r e c y c l i n g e n e r g y delivery s y s t e m for C 0 2 m i t i g a t i o n (II) Two p o s s i b l e w a y s for i n t r o d u c i n g solar e n e r g y M. Tsuji a, H. Amano ~, Y. Tamaura a, H. Sano b and S. Maezawa c aTokyo Institute of Technology, Research Center for Carbon Recycling and Utilization, Ookayama, Meguro-ku, Tokyo 152, Japan bLaboratory Office of Global Energy System, Makioti, Minoo 562, Japan cResearch Institute of Innovative Technology for the Earth, Toyokaiji-Bldg., Nishi-shinbashi, Minato-ku, Tokyo 105, Japan

Two possible ways of H2 production technologies were studied for starting up a global carbon-recycling energy delivery system (GCRED-system). They are based on conventional electrolysis using electric power produced by solar energy. Solar/electricity conversion methods studied include solar thermal power generation (STP) and PV technology. The installation cost, levelized electricity cost, solar/electricity conversion efficiency and specific CO2 emissions by these solar power stations were compared for feasibility of their practice. These considerations showed that the STP technology is more cost-effective and has approximately the same specific CO2 emissions level as the PV technology.

1. INTRODUCTION In the global carbon-recycling energy delivery system (GCRED-system), solar energy is carried in form of CH3OH [1-4]. Cost-effective H2 production from renewable energies has an essential role for the feasibility of the GCRED-system. In the GCRED-system, the H2 produced by electrolysis of solar electricity will be used for chemical conversion of CO2 into solar methanol transportable for end use from world's sunbelt. It is needed to close the economic gap between the low prices of fossil fuel and the higher prices of the solar methanol. The solar energy is the only major renewable energy that can meet the primary energy requirement on a global scale. There are two possible ways to produce solar electricity by PV and STP. The commercial STP (Solar Electricity Generation System, SEGS plant in California, Table 1, STP a) uses trough-shaped

380 mirror collectors and is currently operated using concentrated solar energy as its primary heat source and fossil back-up fuel in the boiler (solar share 75%). LEC for THESEUS project of SEGS plant in Crete (Greece) (1998-2000 construction term) will be 0.085ECU/kWh [7] (Table 1, STpc). The present paper compares key parameters such as solar/electricity conversion efficiency, solar share, levelized electricity cost and installation cost of these two possible ways.

2. COMPARISON EMISSIONS

OF

ELECTRICITY

COST

AND

SPECIFIC

CO2

Solar/electricity conversion efficiency, capital cost and levelized electricity cost (LEC) are briefly compared in Table 1. The solar/electricity conversion efficiency is an important factor determining land required for both systems, though nonconcentrated sunlight is used in PV power plant, while concentrated sunlight is employed in solar thermal power plant. The conversion efficiency ranges between 14 and 21% for the STP technology, and between 8 and 12 for the PV technology, depending on the plant capacity in MW. The capital cost of STPs is one quarter to a half of that of PV power plant (Table 1). The LEC is defined by the total cost, [(capital cost)x(fixed charge rate) + O&M + fuel cost], per annum net electricity production in kWh. It varies depending on the plant scale and solar share. The LEC ranges between $0.05kWh and $0.12/kWh for the STP, and attains $0.4/kWh-$1/kWh for PV electricity. Thus, the solar H2 production cost by PV and STP electricity is not comparable in terms of capital cost and LEC. However, the cost of STP electricity is still not competitive with that of a conventional coal-fired power plant. To close the price gap, the solar thermal parabolic trough power plant industry has introduced integrated solar combined cycle systems (ISCCS) which are able to offer a competitive base-load electricity cost of $0.05-0.07/kWh at solar shares of 15-25% [7] (Fig.l). Direct solar steam generation (DISS) is being developed as more cost-effective STP system[8]. It does not use oil, but steam as heat transfer medium. The expected benefit is a 30% reduction in the electricity generation cost. It may be able to close the economic gap coming from fossil fuel price and be competitive with a conventional power plant (Table 1 STP e, Fig. 1). Other technology is Solar Concentration Off-Tower optical configuration with Combined Cycle (SCOT/CC) [9] (Table 1 STP d, Fig.l). The SCOT design offers several advantages having better collection optics, stable flux distribution and ground-level plant. As to PV technology, though silicon-cell systems are currently best developed, several other technologies are in various stages of development and it is unclear which of these will prove least expensive under what conditions in the future. Among them, polycrystalline thin films are expected to reduce PV module costs to $0.50-1.00 per peak Watt. In this case, flat-plate thin-film systems may be capable of producing

381 Table 1 Costs of construction and electricity by STP and PV electric power generation System

Capacity (MW)

Solar/elec. convers, eft.

(%)

Solar share

Capital cost* ($/kW)

LEC (S/kWh)

(%)

STP a STC b

80 100

14.0

55

3,011 4,683

0.07-0.12 0.068

STP c STP d STP e

55 34 200

15.9 21.3

55.2 80-90 70-85

2,462 2,460

0.085 0.06-0.11 0.05-0.08

PV f Coal-fired b

0.01 100

8- 12

100 0

10,000 2,440

0.4- 1 0.032

Coal-fired g

700

0

2,467

*Cost is given in US$, unless described. 1US$=u 1ECU=IUS$. Sources: a: Solar Electric Generating System (SEGS VIII, 1989). b: Solar Thermochemical Power Plant with Solar/Gas Turbine Combined Cycle[6]. c: THESEUS plant in Frangokastello, Sfakia, Crete, Greece [7]. d: Solar Concentration OffTower/Combined Cycle (SCOT/CC) [8]. e: Direct Solar Steam Generation in the Solar field (DISS) [9]. f: [5]. g: a conventional coal-fired power plant (Japan, 1988).

electricity for $0.04-0.08/kWh [5] (where detailed evaluation is not given, therefore this is not included in Fig. 1). Thus, in comparison of the present costs (* in Fig.l), the STP system is superior to the PV system. CO2 emissions level is comparable between PV and STP (Fig.2). These findings recommend the solar H2 production using STP electricity for the GCRED-system. Further study on the solar/electricity conversion using STP and STC-systems should be also conducted while being compared with the solar H2 production by PV electricity. The other possible way for producing the solar H2 will be a direct- or two-stepwater splitting by the solar thermochemical process (STC-process). In the STCprocess, the maximum solar/chemical (H2) conversion efficiency of the solar radiation is 70% at 1000sun. This high conversion efficiency in STC-system suggests that the solar H2 production by the STC-process will be one of the promising options for the GCRED-system. Thus, the solar energy conversion s y s t e m s u s i n g c o n c e n t r a t e d solar beam give a h i g h e r efficiency in solar H2 production, which will enable to lower the economic gap from fossil fuel price.

382

* at present # (at the year

PV*---- ""

5.2

given in Figure) o

0.4

~ !,,,,,i r~

3.8

4

o

~ t

SCOT/CC#(2010) Trough _ \ (thermal m e d i a = o i l ) ~ ........ ~

o

rr O1

9

.

9 1,,,,,i

r N

Coal-fired i

,

0

roug DISS (Steam) I

t

[

~

i

t

I

50 Solar Share (%)

100

1

1.1

PV

STP

Oil

Coal

Figure 2. Comparison of specific CO2 emissions by various power sources.

F i g u r e 1. Change in levelized electricity cost (LEC) with increase in solar share

REFERENCES 1. Y. Tamaura and M. Tsuji, Proc. Int. Conf. Energy, Economy and Environment, Osaka, (1996) 59. 2. M. Tsuji and Y. Tamaura, Proc. Int. Conf. Energy, Economy and Environment, Osaka (1996) 65. 3. Y. Tamaura, Kinou-zairyou, 16 (1996) 31. 4. Y. Tamaura and M. Tsuji, Proc. Symp. CO2 Fixation, Chemical Society of Japan, (1996) 6. 5. N. Nakicenovic(ed.), Energy-The International Journal, 18 (1993) 437. 6. J. H. Edwards, K. T. Do and A. M. Maitra, Energy Convers. Mgmt., 37 (1996) 1339. 7. Pilkinton Solar Int'l: Status Report on Solar Thermal Power Plant, Pilkington Solar Int'l, January 1996. 8. A. Kribus, A. Segal, R. Zaibel, D. Carey and S. Kusek, Proc. ECE/WEC/UNESCO/MOST Workshop on the Use of Solar Energy, Bet Berl, Israel, Aug. 1995. 9. E. Zarza, J. I. Ajona, K. Hennecke, Proc. 8th Int. Symp. Solar Thermal Concentrating Technologies, K(iln, Germany, October 6-11 (1996) 397.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

383

Efficient thermochemical cycle for C O 2 reduction with coal using a reactive redox system of ferrite Tatsuya Kodama, Akira Aoki, Satoshi Miura and Yoshie Kitayama Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-21, Japan A two-step thermochemical process using a redox system of metal oxides for converting CO 2 and coal to CO was studied on a number of iron-based oxides (ferrite) for the purpose of utilizing solar high-temperature heat. Reactions are done in a two-step redox cycle in which ferrite is reacted with coal powder at high temperatures to produce COx, H2 (the lst-step reaction) and metals which are reoxidized with CO2 to generate CO at lower temperatures in a separate step (the second-step reaction). In the lst-step reaction, pure magnetite (Fe304) was readily reduced to FeO but the reduction from FeO to o~-Fe scarcely proceeded by a short time reaction (30 min) with coal below 900~ It was found that Ni(II)- or In(III)-substitution for Fe(II) or Fe(III) in magnetite strongly enhances the phase transition from FeO to metallic phase to improve the efficiency for coal conversion. The efficiency of the reaction by the two-step cycle using a reactive ferrite is superior to that by a conventional single-step CO2 gasification of coal. Our findings suggest that highly endothermic CO2 reduction with coal will be conducted using concentrated solar radiation as the energy source for high-temperature process below 900~ 1. I N T R O D U C T I O N Efficient conversion of solar high-temperature heat below 1000~ to fuels or chemicals is a current subject for research[ 1]. The goal is an industrially important endothermic reaction which can be driven by high-temperature heat. On the other hand, the mitigation of CO2 emission has been a major environmental concern. Recently, a two-step thermochemical process using a redox system of metal oxide is proposed for steam gasification of coal utilizing concentrated solar radiation[2]. Reactions are done in a two-step redox cycle in which metal oxide is reacted with coal powder at high temperatures to produce CO, H 2 and reduced metal oxide (or metal) which is reoxidized with H20 to generate H2 at lower temperatures in a separate step. The net reaction is represent by: CHn (coal) + H 20 ~ CO + (n/2+ 1)H2. This two-step process will be applied for a thermochemical reduction of CO2 to CO with coal as follows: metal oxide + CHn ~ metal + xCO + (1-x)CO2 + n/2H2 metal + (2-x)CO2 ~

metal oxide + (2-x)CO

(1) (2)

384 The net reaction is so-called CO2 gasification of coal, CHn + CO2 ~ 2CO + n/2H2. The coal gasification is conventionally performed directly in a single step using steam or CO2 as an oxidant for coal[3]. It is highly endotherlnic and, in the conventional single-step gasification, the process heat is supplied by direct combustion of fossil fuels which release large amounts of

CO2. In the proposed two-step process, it is important to attain high efficiencies for conversion of coal to COx (CO + CO2) in the first-step reaction and then for conversion of CO2 to CO in the second-step reaction by an external heat input. From the thermodynamic conditions and the low cost, the redox pair of Fe304/c~-Fe was one of the promising redox systems for the two-step process, but it still required the operating temperature above 1200~ It is well known that many kinds of metal ions can be incorporated into the spinel lattice structure of magnetite by replacing ferrous or ferric ions. There is the possibility that metal-substitution for Fe z+ or Fe 3+ in magnetite causes a phase transition to the metallic phase, which proceeds readily even at low temperatures and improves the conversion efficiencies of coal and CO2 to CO in the two-step process. In the present work, a number of redox systems of ferrites were studied to find the most reactive working materials. Efficient two-step process using the most reactive In(III)-doped ferrite was demonstrated below 900~ 2. E X P E R I M E N T A L STUDIES

Ultrafine particles of magnetite and metal-bearing ferrites (>50nm) were synthesized by coprecipitation method. The molar ratio M]Fetotal in the starting solution was 0.50 (except for In 3+) to synthesize stoichiometric MFe204. The synthesis of In(III)-bearing ferrite was carried out at an In/Fetotal molar ratio of 0.15 because high level In 3+-substitution for Fe 3+ in magnetite is difficult in this method. Ferrite thus prepared was mixed with Australian bituminous coal pulverized smaller than 3001am[5]. The reactivities for the coal-gasification step [equation (1)] and the CO2-reduction step [equation (2)] were examined for the ferrite. Detailed procedure have been reported on the previous paper[5]. 3. R E S U L T S AND D I S C U S S I O N

To study the variations of reactivities and selectivities of metal oxides with temperature in the coal-gasification step, the mixture of coal and metal oxide was slowly heated from 300 to 900~ at 3.3~ min -1 under N2 gas flow. Figure 1 shows a typical temperature variations of the partial pressures of evolved CO and CO2 for magnetite. Evolution of CH4 was also observed but the partial pressure was negligibly small. The values of Pco

o~" 30

o:g 09

=

)

CO

09

15

oe~

:5 m~

0

~27

300

k..J

500 700 Temperature(~

900

Figure 1. Evolution profiles of CO and CO 2 during the coal-gasfication step using the magnetite

385

and Pco2 increased with increasing temperature and reached a maximum at 900~ For the other ferrites, the maximum peaks for CO and CO2 evolutions were also observed at 900~ except for Ni(II)-ferrite. With Ni(II)-ferrite, the maximum peaks for both CO and CO2 evolution appeared at 800~ The formation rates of COx (CO + CO2) at the maximum level of Pcox are shown in Table 1. For Mg(II)-, Mn(II)- and Zn(II)-ferrites, the formation rate of COx at 900~ were rather lower than that for magnetite. However, this rate was greatly increased with In(III)-ferrite and was 1.6 times as fast as that with magnetite. For Ni(II)-ferrite, nearly the same value of COx formation rate as that for magnetite at 900~ was obtained at a lower temperature of 800~ These results indicate that the Ni(II)-, and In(III)-ferrites are promising materials for the two-step process. The mixture of coal and magnetite, Ni(II)-, or In(III)-ferrite was rapidly heated to 900~ (heating rate = 30~ min -1) and the coal-gasification step was carried out for 30 min at a constant temperature of 900~ Ferrite was mixed with coal (0.2g) at a molar ratio of oxygen in the ferrite to carbon in coal - 1.2. Table 2 shows the conversions and selectivities for products. Only coal reacted directly with CO2 under similar reaction Table 1 Reactivities of metal oxides in the coal-gasification step. Material Magnetite Ni(II)-ferrite M g(II)- ferrite

(~

Products b (mol %) CO CO2

Max. formation rate of COx c (ktmol min -1)

900 800 900

23.7 16.1 13.9

11.5 15.1 4.42

270 225 136

Th a

Mn(II)-ferrite

900

13.3

4.43

125

Zn(II)-ferrite

900

14.1

5.99

148

In(III)-ferrite

900

39.0

8.12

437

CO2

900

19.9

-

-

aTh is the temperature at which the highest level of the formation of CO and CO2 w e r e observed. bThe partial pressures of CO and CO2 when the reactor reached to the temperature of Th. CCOx-production rate when the reactor reached to the temperature of Th. Table 2 Coal conversions to CO,CO2 and CH4 and the selectivities in the coal-gasification step for 30 min at 900~ Material a In(III)-ferrite Ni(II)-ferrite Magnetite CO2

selectivity ( % )

coal conversion to (%)

CO 48.7 16.8

CO 2 18.6 26.7

CH4

9.5

9.3 -

13.1

CO 70.8 36.0

CO2 27.0 56.7

CH4

3.4

total 68.8 46.9

2.6

21.4

44.4

43.3

12.3

0.4

13.5

-

-

1.5

2.2 7.5 -

aMetal oxide was mixed with coal (0.2g) at a molar ratio of oxygen in metal oxide to carbon in coal = 1.2.

386 conditions to carry out the direct single-step C O 2 gasification of coal (coal-CO2 reaction) and the total coal conversion was only 14% (Table 2). A higher conversion of 21% was obtained with the coalmagnetite reaction. With Ni(II)-ferrite, it was improved to 47%. However, the In(III)-ferrite reaction showed a much higher conversion of 69% which was 5 times larger than that by the single-step coal-CO2 reaction. Figure 2 shows the XRD patterns of the solid phases of magnetite, Ni(II)- and In(III)ferrites following the coal-gasification step at 900~ The magnetite showed strong peaks due to wustite (FeO) along with very small peaks of ~-Fe (Fig. 2a). The spinel peaks disappeared completely. This indicates that magnetite was readily reduced to FeO

a) Magnetite tO

~

0 FeO

/."~-Fe o

b)Ni(II)-ferrit~ 9

I

9 Ni-Fe alloy 0 FeO

ooll

i'

o

~.~_'.a::~......._L~I~,,,,~ _~. . . . . .

~.......... _,1

cI2'II'-e'te 1 1__

30

I

40

I

I

50 60 20CuKo t (deg.)

I

70

Figure 2. XRD patterns of magnetite but the reduction from FeO to ~-Fe scarcely and Ni(II),In(III)-ferrite after use for proceeded by a short time reaction (30 rain) with coal the coal-gasification step for 30min. below 900~ On the other hand, in the XRD pattern The O/C molar ratio in the mixture = of the Ni(II)- or In(III)-ferrite, the spinel peaks also 1.2. disappeared but strong peaks of metallic phases appeared. The peaks of FeO were small or little here. Strong peaks of Ni-Fe alloy appeared with small peaks of FeO in the XRD pattern of Ni(II)-ferrite (Figure 2b). For In(III)-ferrite, the peaks of FeO were scarcely observed and the strong peaks of a-Fe appeared with peaks due to metallic In (Figure 2c). The presence of Ni(II) or In(III) in the ferrite phase enhances the phase transition from FeO to the metallic phase. From these results, we concluded that the most reactive iron-based oxide for the proposed two-step process was the In(III)-doped ferrite. After performing the coal-gasification step, the reduced In(III)-ferrite reacted with CO2 at 800~ to carry out the CO2-reduction step. Significant amount of CO was evolved and the evolution was completed within 30 rain. In the XRD pattern of the ferrite after the CO2reduction step, the peaks of the metallic o~-Fe and In disappeared completely, and strong peaks of a spinel appeared with small peaks of In203. The metallic phases formed in the coalgasification step were almost reoxidized to the oxidized phases such as the ferrite. This twostep process was twice repeated in the temperature range of 800-900~ and more than 80% of coal used could be converted to CO in total. REFERENCES 1 .E.A. Fletcher, J. Minnesota Academy of Science, 49 (1983) 30. 2. Y. Tamaura, Y. Wada, T. Yoshida and M. Tsuji, EnergymThe International Journal, 22 (1997) 337. 3.J. Kellar, Fuel Process. Technol., 24 (1990) 247. 4.T. Kodama, Y. Wada, T. Yamamoto, M. Tsuji and Y. Tamaura, J. Mater. Chem., 5 (1995) 1413. 5. T. Kodama, S. Miura, T. Shimizu and Y. Kitayama, Energy--The International Journal, in press.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

387

O x i d a t i v e d e h y d r o g e n a t i o n o f e t h y l b e n z e n e with carbon dioxide over Z S M - 5 supported iron oxide catalysts Jong-San Chang a, Sang-Eon Park a, Woo Young Kim a, Masakazu Anpo b and Hiromi Yamashita b alndustrial Catalysis Research Laboratory, Korea Research Institute of Chemical Technology (KRICT), Taejon 305-606, Korea bDepartment of Applied Chemistry, Osaka Prefecture University, Osaka 593, Japan Carbon dioxide was proposed as an oxidant in dehydrogenation of ethylbenzene over zeolite-supported iron oxide catalyst, which was highly dispersed in zeolite matrix. The dehydrogenation was mainly proceeded under the oxidative pathway in the presence of carbon dioxide. The presence of carbon dioxide contributed to remarkable enhancement not only in dehydrogenation activity of catalyst but also of its coke resistance. 1. INTRODUCTION Great efforts and considerable attention have recently being given to the transformation of carbon dioxide into valuable products through catalytic reactions [ 1,2]. These transformations are mainly based on the reduction of carbon dioxide as a carbon source. However, in other way, carbon dioxide has been also utilized as an oxygen source or oxidant since this could be considered a nontraditional or mild oxidant and oxygen transfer agent [3-5]. As an example of this view, it was reported that oxidizability in the gasification of coke is as follows: O2(105) > H20(3) > CO2(1) > H2(0.003) where the numbers in parenthesis indicated the relative ratios of the gasification with oxidants [6]. This oxidizability suggests that carbon dioxide can play as an oxidant for the oxidative transformation of hydrocarbons. Large amounts of styrene are commercially produced by dehydrogenation of ethylbenzene (EB) in the presence of steam using iron oxide-based catalysts. Carbon dioxide, small amounts of which are formed as a by-product in the ethylbenzene dehydrogenation, was known to depress the catalytic activity of commercial catalyst [7,8]. However, it has been recently reported that several examples show the positive effect of carbon dioxide in this catalytic reaction [5,9,10]. In this study, we investigated the effect of carbon dioxide in dehydrogenation of ethylbenzene over ZSM-5 zeolite-supported iron oxide catalyst.

2. E X P E R I M E N T A L

Zeolite-supported iron oxide catalysts were prepared by precipitation of aqueous suspension of Fe(II) hydroxide in slightly alkaline solution at 333 K under N2 atmosphere.

388 The support used was high siliceous NaZSM-5 zeolite (Uetikon PZ-2/980, Si/A1 = 1900). These newly prepared catalysts were dried in vacuo at 353 K followed by calcination under N2 flow at 673 K for 3 h. Loading of iron oxide, Fe304 on the catalysts was in the range of 1.5 - 20 wt.%, which were designated as FeNaZ catalysts hereinafter. For example, 5FeNaZ denotes 5 wt.% loading of iron oxide supported on NaZSM-5 zeolite. Bulk Fe304 oxide prepared by the above method without zeolite support was compared the catalytic activity with FeNaZ catalyst. Dehydrogenation of ethylbenzene into styrene was carried out in a conventional flow-type reactor made of a quartz tube (10 mm i.d., 300 mm length) at 873 K and atmospheric pressure. Ethylbenzene during the reaction was fed to the reactor as passing carbon dioxide or nitrogen through ethylbenzene saturator kept at 298 K. Several methods such as powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and BET isotherms from N2 adsorption-desorption were selected to characterize iron oxide in zeolite-supported catalysts. 3. RESULTS AND DISCUSSION The XRD patterns of zeolite-supported catalysts containing iron oxide below 10 wt.% loading did not show crystalline phases of iron oxides but the crystalline pentasil-type structure. It was found that XRD peaks related to Fe304 oxide in zeolite-supported catalysts was appeared when loading ratio is more than 20 wt.%. No change in surface area of catalyst exhibited until up to 10 wt.% loading of iron oxide. Characterizing zeolite support by BET full isotherms from adsorption-desorption cycle of N2, it showed hysteresis loop at p/po = 0.2 - 0.4, indicating mesopores in the range of 30 - 40 A. As presented in Figure 1, the Fe 2p spectra of zeolite-supported iron oxide 3 catalysts showed the intrinsic pattem of Fe304 oxide without the significant satellite structure at 719.1 eV which is present in the a ,~. , ~ (a) oxidized surface of bulk Fe304. In addition, the O ls binding energy was observed at 2 530.3 eV to support the presence of iron oxide, while that of zeolite lattice was shown at 532.4 eV. These results explain that iron ~ oxide phase in zeolite-supported catalysts r~ 1 would be Fe3Oa-like and highly dispersed on mesopore and external surface of zeolite support below the level of critical dispersion 719.1 capacity which denoted the border line to start the appearance of crystalline oxide 0 phase. 730 720 710 700 740 Dehydrogenation activity on ethylbenzene Binding Energy (eV) was measured by using catalysts treated with nitrogen at 873 K for 1 h before the reaction. Figure 1. XP spectra of Fe 2p core levels: (a) Among zeolite-supported iron oxide catalysts the highest catalytic activity was obtained in 1.5FeNaZ, (b) 3FeNaZ, (c) 5FeNaZ, and (d) the case of 5.0 wt.% loading of iron oxide Fe304 (oxidized). -

I

,

l

a

I

,

I

389 onto zeolite. As shown in Figure 2, the presence of carbon dioxide over ZSM-5-supported iron oxide (5FeNaZ) catalyst induced the enhancement of the activity as compared with the condition of nitrogen stream. Dehydrogenation of ethylbenzene with carbon dioxide over FeNaZ catalysts produced styrene, water and carbon monoxide along with small amount of hydrogen and stoichiometrically only less than 25 % of carbon monoxide produced. Moreover, even small amount of oxygenated by-product such as benzaldehyde was formed together with benzene and toluene. Formation of water as well as carbon monoxide in this reaction implied that CO2 molecule could be dissociated into CO and surface oxygen which could abstract hydrogen from ethylbenzene forming water. On the other hand, water would be also produced partially via reverse water-gas shift reaction. It was known that the alleviation of the equilibrium limitation in the dehydrogenation of ethylbenzene was achieved by effective removal of produced hydrogen through coupling reaction by the reverse water-gas shift reaction [10]. When the reaction was carried out under an inert nitrogen stream, unsupported Fe304 oxide showed similar dehydrogenation activity to 5FeNaZ catalyst. However, this Fe304 oxide exhibited considerable decrease of catalytic activity in the presence of carbon dioxide [ 11 ]. In the presence of CO2, styrene yield on FeNaZ catalyst was more than 2.5 times comparing with that on bulk Fe304. This result showed that dehydrogenation over zeolite-supported iron oxide catalyst was predominantly enhanced by oxidative pathway and carbon dioxide played a role on the dehydrogenation of ethylbenzene. The activity of 5FeNaZ catalyst under a N2 stream decreased monotonically with reaction time and the reaction was abruptly stopped by plugging the reactor after 5 h due to coke deposition but the catalytic activity under a CO2 stream maintained without significant decay of the activity [ 11 ]. To examine the effect of iron oxidation state on the dehydrogenation the activity of the iron oxide catalyst was compared as in various pretreatment. As presented in Figure 3, the catalyst pretreated with nitrogen showed rather high activity compared to those pretreated with carbon ~.

60

[ 9 EB Conv. 50

|

CO2

.~_

EB Conv. ~ Styrene Yield C02

40

40

~

30

o.,

t...

I-,

~1

~.

~

20

2O

r,.) 1.o

0 i

3

s

Reaction Time (h) Figure 2. Catalytic activity in dehydrogenation of ethylbenzene depending on carrier gas at 873 K. W/F=298 g.h/mol; CO2 (or N2)/EB=80 (mol. ratio).

N2

CO 2

H2

Air

Pretreatment G a s e s Figure 3. Activity of 5FeNaZ catalyst depending on pretreatment condition at 873 K. Reaction conditions: see Figure 2. Pretreatment: heating at 873 K for 10 min (H2, air) or 60 min (N2, CO2), respectively.

390 dioxide, air or 5% H2 in N2 stream. This result indicates that partial reduction during heat treatment under nitrogen flow reaches a certain optimum oxidation state of iron oxide which has oxidation state of +2 and +3 although the optimum population between two oxidation states is not obvious now. Since the reaction occurs in the presence of hydrogen, ethylbenzene and carbon dioxide it is possible for the iron oxide to undergo both partial oxidation by carbon dioxide and partial reduction by hydrogen and ethylbenzene. Thus this population would be controlled properly in gaseous mixture of ethylbenzene, hydrogen and excess carbon dioxide during the reaction. Oxygen deficiency of iron oxide in FeNaZ catalysts also had influenced in the catalytic activity [11]. The highly dispersed iron oxides in zeolite matrix had much more isolated oxygen-deficient sites comparing with that on unsupported Fe304. It is speculated that oxygen deficient sites of zeolite-supported iron oxide stimulate the CO2 dissociation into CO. The dissociated oxygen on the catalyst surface would abstract hydrogen of ethylbenzene or convert hydrogen molecule produced into water. Dispersion of the oxygen deficient sites or oxygen defects seemed to be more efficient for the CO2 dissociation and thereby the oxidative dehydrogenation of ethylbenzene. Therefore, it can be concluded that carbon dioxide was utilized as an oxidant on dehydrogenation of ethylbenzene to improve its activity and that an active phase of iron oxide in the catalyst is considered to be rather reduced iron oxide, like Fe304, dispersed in zeolite matrix. ACKNOWLEDGMENTS We acknowledge the financial support by Korea Electric Power Research Institute (KEPRI), Ministry of Environment (MOE), and Ministry of Science and Technology (MOST) in Korea. REFERENCES

1. M.M. Halmann (ed.), Chemical Fixation of Carbon Dioxide, CRC, FL, Boca Raton, 1993. 2. M. Aresta, C. Fragale, E. Quaranta and I. Tommasi, J. Chem. Soc., Chem. Commun., 315 (1992). 3. O.V. Krylov and A.Kh. Mamedov, Ind. Eng. Chem. Res., 34 (1995) 474. 4. J.S. Yoo, P.S. Lin and S.D. Elfline, Appl. Catal., 106 (1993) 259 5. S.-E. Park, J.-S. Chang and M.S. Park, Prepr. Am. Chem. Soc., Div. Fuel Chem., 41(4) (1996) 1387. 6. C.H. Bartholomew, Chem. Eng., Nov. 12, (1984) 96. 7. T. Hirano, Appl. Catal., 26 (1986) 65. 8. J. Matsui, T. Sodesawa and F. Nozaki, Appl. Catal., 67 (1991) 179. 9. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and M. Suzuki, Appl. Catal., 121 (1995) 125. 10. S. Sato, M. Ohara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 11. J.-S. Chang, S.-E. Park and M.S. Park, Chem. Lett., accepted (1997).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

391

Nature of COz adsorbed on MgO surface at low temperatures T. Ito, a J. Isawa, a H. Kishimoto, a H. Kobayashi, b and K. Toi c aDepartment of Social Information Studies, Otsuma Women's University, Karakida, Tama, Tokyo 206, Japan bDepartment of Chemical Technology, Kurashiki University of Science and the Arts, Tsurajima, Kurashiki, Okayama 712, Japan CDepartment of Chemistry, Tokyo Metropolitan University, Minami-Osawa, Hachioji, Tokyo 192-03, Japan

Among five types of species formed by CO2 adsorption on MgO, dynamic behavior of unidentate-type carbonate (species-l) upon desorption was investigated. Surface diffusion, O-exchange, and molecular substitution appreciably take place prior its desorption even below the room temperature. Since these dynamic processes necessitate bond rupture in species, MgO may be useful in CO2 conversion at low temperatures.

1. INTRODUCTION Magnesium oxide may be expected to function as an active catalyst for CO2 utilization since CO2 is easily chemisorbed on MgO [1-3]. Most works so far reported on this adsorption system were carried out above the room temperature, and recent works have demonstrated the occurrence of oxygen isotope exchanges between adsorbed species and surface 02. [4-6]. However, dynamic behaviors of adsorbed species, especially below the room temperature, are still open to question.

2. EXPERIMENTAL

An MgO sample used was a smoke-type with a specific surface area of 10-15 m 2 g-1 and was pretreated in vacuo at 1123 K before each run. CO2 gas below 133 Pa was introduced onto MgO surface at and below the room temperature. The isotopic concentrations of labeled gases are 88.3-180 and 99-13C atom % for C1802 and 13C02, respectively. Other details have been described elsewhere [6].

392 3. RESULTS AND DISCUSSION

3.1. Types of adsorbed species TPD curves measured after the adsorption of a small amount of CO2 at a low temperature indicate the presence of five types of chemisorbed species, species-0 to -4. Their desorption temperature and structure which has been determined from observed IR bands, theoretical models [6], and isotopic studies are summarized in Table 1. In species-0, CO2 is adsorbed as a base molecule but all the other species are basically carbonate-type, CO32. Species-1 has a unidentate structure, while species-3 and -4 have a bidentate structure which are known as major species in CO2 adsorption and stable above the room temperature. The difference between species-3 (type b) and -4 (type a) seems to come from the coordination number of the adsorption sites. Their adsorption models are illustrated in Figure 1. Dynamic behaviors of species-1 which is stable only below room temperature are mainly described hereafter. 3.2. TPD observations for species-1 Figure 2(A) shows TPD curves observed after the C1802adsorption at 110 K, where species-1 is desorbed with the nearly same isotopic composition as the source gas and the difference in desorption temperature among isotopic species is within 10 K as shown in Table 2. However, species-1 formed by the C1802 adsorption on the surface carrying species-3 and -4, which was previously formed by the C~602 adsorption at the room temperature, is desorbed in a complex manner [Two-step adsorption, Figure 2(B)]. For example, the isotopic composition of species-1 is considerably diluted by 160, and C1802 species which Table 1 Adsorbed species of CO z on MgO Species Structure Species-0 Linear CO 2 on Mgcc 2+ Species-1 Unidentate CO32Species-2 (Unknown) Species-3 Bidentate CO32-(type b) Species-4 Bidentate CO~2- (type a)

0

\ /

0

oN

C

---OLc

Desorption temperature / K 190 230 300 390 ca. 800

2+

MgLc

Unidentate C032 (Species-I) Figure 1. Adsorbed models.

/

C ~0

---0~

I 2+

MgLc

Bidentate C032 (Species-3,4)

IR band / cm -1 2355 1274, 1711 951, 1274, 1626 1022, 1326, 1660

393

!

0.8

!

|

(B)

(A)

3

_

1

~-6 t--

_C1802,,,,~~

0.6

3 L._

~4 ~t - 9 0.4

Or) fO

O c N

--=2

0.2

ii .

o 100

200

300 Temperature / K

400

500

lOO

200

300

400

500

Temperature / K

Figure 2. TPD curves observed after adsorption of 1.3-Pa C1802 at 110 K on MgO carrying (A) no preadsorbed species and (B) preadsorbed species-3 and -4 (not shown in this figure) consisting of C1602 (Two-step adsorption). Table 2 Desorption temperatures of species-1 observed for TPD in two-step adsorption a Adsorption pressure in the Desorption temperature / K first step b /Pa C16Oz C160180 C18Oz 0 (No first step) 220 225 230 1.3 223 237 250 6.6 227 246 257 27 228 246 266 aIn the second step 1.3-Pa C1802gas was admitted at 110 K. bc1602 gas was admitted under indicated pressure at the room temperature. was directly formed in the second step is desorbed at a temperature close to that observed in the single step adsorption of the C1802 gas while the desorption of C160180 and C1602 species is shifted to much higher temperatures in this order. The degrees of this dilution by 160 and the shift in desorption temperature are enhanced with increase in the amount of previously adsorbed species-3 and -4 (cf. Table 2). These facts indicate that a part of species-1 can diffuse on the surface, through which O isotope is exchanged with surface O 2, and then tends to be desorbed at a higher temperature in TPD process. The longer the diffusion distance is, the larger both the content of 160 included in species-1 and the shift in the desorption temperature become. A similar two-step adsorption using the 13CO2 gas in the second step also gives complex TPD curves. Especially the appearance of species-1 consisting of 12CO2, which is also desorbed at a higher temperature than ]3CO2, strongly suggests that molecular substitution between adsorbed species occurs in the TPD process since no species-1 can be present in the first step.

394

3.3. IR observations for species-1 IR spectra observed after two-step adsorption using C1802in the second step at 180 K show that isotopic composition of species-1 formed is nearly same as that of the original gas. This fact corroborates that, during the adsorption at low temperatures, first, no CO2 in the gas phase undergoes O-isotope exchange with surface 02. or previously adsorbed speices-3 and -4, and second, no molecular substitution occurs between gaseous CO2 and preadsorbed species. The IR bands due to species-1 formed in the single step adsorption at 180 K completely diminishes with increasing temperatures up to 270 K while the bands due to species-3 and -4 are much increased in intensity with this temperature increase. This indicates that a part of species-1 is readsorbed as these more stable species through surface diffusion upon heating. 3.4. Dynamic behaviors of species-1 The TPD and IR observations mentioned above indicate that many dynamic processes for species-1 can occur even below the room temperature. The fate of this species upon heating can be classified into the following four cases. First, species-1 is directly desorbed from the adsorbed site, which ideally leads to no isotope exchange and no temperature shift in desorption. The other three cases are all accompanied by surface diffusion of species-1 during which an O-exchange takes place between the diffusing molecule and surface 02. . Second, the diffusing molecule is desorbed as species-1 without changing the adsorption state after diffusion, which causes a temperature shift in desorption. Third, the diffusing molecule undergoes molecular substitution with a more stable species (e.g., species-3), and consequently the diffusing molecule (species-l) remains as a more stable species and, instead, the substituted molecule is desorbed as species-1 with a temperature shift. Fourth, the diffusing molecule is readsorbed as a more stable species on a vacant active site, resulting in no desorption of species- 1. Each molecule adsorbed as species-1 is diminished through any of the four cases mentioned above during heating. These dynamic processes occur through the bond rupture of species, and hence MgO catalysts seem to be able to convert CO2 into other useful substances at low temperatures when a second component is admitted. REFERENCES 1. J. V. Evans and T. L. Whateley, Trans. Faraday Soc., 63 (1967) 2769. 2. Y. Fukuda and K. Tanabe, Bull. Chem. Soc. Jpn., 46 (1973) 1616. 3. R. Philipp and K. Fujimoto, J. Phys. Chem., 96 (1992) 903 5. 4. Y. Yanagisawa, Y. Shimotama, and A. Ito, J. Chem. Soc., Chem. Commun., 1993 (1993)610. 5. H. Tsuji, T. Shishido, A. Okamura, Y. Gao, H. Hattori, and H. Kita, J. Chem. Soc., Faraday Trans., 90 (1994) 803. 6. Y. Yanagisawa, K. Takaoka, S. Yamabe, and T. Ito, J. Phys. Chem., 99 (1995) 3704.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

395

C02 b e h a v i o r on s u p p o r t e d K N i C a catalyst in the carbon dioxide r e f o r m i n g of methane Sang-Eon Park a, Jong-San Chang a, Hyun-Seog Roh a, Masakazu Anpo b and Hiromi Yamashita b aIndustrial Catalysis Research Laboratory, Korea Research Institute of Chemical Technology (KRICT), Yaejon 305-606, Korea bDeparment of Applied Chemistry, Osaka Prefecture University, Osaka 593, Japan

CO2 behavior in the carbon dioxide reforming of methane over supported KNiCa catalyst was investigated by using 'in-situ' FT-IR, CO2-TPD and pulse reaction method. The role of alkali metals on the catalyst was to improve catalyst stability through a high surface coverage of adsorbed CO2 and formation of surface carbonates. It was demonstrated that the oxidation step of surface carbon with adsorbed CO2 or surface oxygen on the catalyst contributes to high catalyst stability by the effective removal of surface carbon species. 1. INTRODUCTION There has been an increasing interest in the catalytic transformation of carbon dioxide, a greenhouse gas, into more valuable compounds [1]. The carbon dioxide reforming of methane (so-called "dry reforming") to produce CO-rich synthesis gas (H2 and CO) is considered to be one of the promising ways of CO2 utilization. Previously, we suggested that zeolite-supported KNiCa catalyst (KNiCa/ZSI) exhibited high activity and coke resistance for the CO2 reforming of methane into synthesis gas [2]. To understand detailed chemistry in the dry reforming, it is essential to elucidate the behavior of CO2 in the reaction and on the catalyst surface. The aim of the present study is to examine the adsorption behavior of CO2 on our designed catalyst and the role of CO2 in the reaction. 2. E X P E R I M E N T A L

Zeolite-supported Ni and KNiCa catalysts were prepared by molten-salt method [2], which were designated as Ni/ZSI and KNiCa/ZSI, respectively, hereinafter. The support was a high siliceous ZSM-5 zeolite (UOP S-115) mixed with an alumina. Experiments for pulse reaction on reduced KNiCa/ZSI catalyst were conducted with a gas sampling valve in a pulse microreactor, which was incorporated between the sample inlet and the column of gas chromatograph. FT-IR spectroscopic studies were performed in a quartz vacuum cell using self-supported wafers which were treated under vacuum or underwent the

396 same treatment as catalyst testing. The chemisorbed amounts of C02 on reduced catalysts were measured by a static vacuum volumetric system (Micromeritics' model ASAP 2000C) with stainless steel tubing greaseless valves. Temperature-programmed desorption (TPD) of CO2 on reduced catalysts was performed by IGA (Intelligent gravimetric analyzer, Hiden IGA-002). 3. RESULTS AND DISCUSSION In the previous study, KNiCa/ZSI catalyst exhibited high activity and high resistance to coke in the CO2 reforming of methane at 700 ~ [2]. Its high activity showed near equilibrium conversions of CO2 and CH4 as well as near equilibrium yields on CO and H2, which were unchanged during over 140 h. On the other hand, Ni/ZSI catalyst was also highly active, but this showed severe coke deposition within several hours. Upon the in-situ FT-IR analysis of KNiCa/ZSI catalyst, no C-H bands were detected in the region of 2700 - 3100 cm l corresponding to CHx adsorbed species after the reaction and CH4 introduction at 700~ into in-situ cell. The adsorbed species formed upon CH4 adsorption was postulated mainly as a type of Ni-C. Figure 1 shows FT-IR spectra of reduced Ni/ZSI and KNiCa/ZSI catalysts before and after CO2 adsorption, and under the CO2-reforming conditions. The behavior of adsorbed CO2 on KNiCa/ZSI was different from that on Ni/ZSI. FT-IR spectrum of KNiCa/ZSI even after reduction at 700 ~ showed two evident bands at 1480 and 1410 cm 1 which did not appear that on Ni/ZSI catalyst. These bands could be assigned to mainly asymmetric (Vas(OCO)) and symmetric stretching vibration (vs(OCO)) modes of monodentate carbonate species coordinated to Ca oxide or K oxide of the catalyst, respectively [3]. The intensities of these bands on KNiCa/ZSI increased by the C02 .12 adsorption at 25 ~ or upon the CO2-reforming (d) reaction, but on Ni/ZSI no changes were detected by above treatment. This formation of carbonate species formed on the basic catalyst -,2~ surface by the CO2 adsorption in air or under vacuum would lead to the high catalyst stability of KNiCa/ZSI due to reducing the formation of coke on surface. In other words, the enrichment -.52of surface oxygen upon CO2 adsorption as carbonate species could play a role on the elimination of coke by converting it into CO, which gave high CO/H2 ratio as near 1. -.84 ........... i .....1 The metallic Ni and CaO sites adjacent to 1800 1600 1400 1200 reduced nickel metal sites on KNiCa/ZSI W a v e n u m b e r (cm-1) catalyst were considered as major sites of CO2 Figure 1. FT-IR spectra of (a) Ni/ZSI after chemisorption. The chemisorbed amounts of reaction at 600~ KNiCa/ZSI after (b) CO2 on KNiCa/ZSI catalyst were increased to a reduction at 700~ (c) 20 Torr CO2 adsorption value 40% higher than that of Ni/ZSI catalyst. at RT followed by evacuation at 400~ and (d) As shown in the CO2 desorption profiles in the reformingreactionat600~ Figure 2, the integrated amounts of CO2

i

397 desorption on KNiCaJZSI catalyst were higher than those on Ni/ZSI catalyst which corresponded to the result of volumetric 5 CO2 adsorption. In addition, significant difference in the CO2-TPD profiles 4 o between Ni/ZSI and KNiCa/ZSI catalysts ~ ~ 6 ~~ was observed in the high temperature region. CO2-TPD profile of KNiCa/ZSI 2 exhibited a strong desorption peak above 600~ which was not observed on Ni/ZSI. The desorption peak at higher temperature indicated the formation of surface 0 , i , i , i , i , i , i , i carbonate, mainly on Ca promoter due to 0 100 200 300 400 500 600 700 its great amounts of composition, and the Desorption Temp. (~ presence of this peak seemed to be directly related with excellent stability of Figure 2. CO2-TPD profiles of reduced (a) Ni/ZSI KNiCa/ZSI catalyst compared to Ni/ZSI. and (b) KNiCa/ZSI catalysts (13 = 10 ~ Results of pulse reactions showed that the dissociation of reactant molecules and the formation of CO on KNiCa/ZSI catalyst might greatly affect the state of catalyst surface. Figure 3 displays the amounts of CO produced as a function of ordinal number of CO2 and CH4 pulses dosed at 600~ over reduced KNiCa/ZSI catalyst. At the first several CO2 pulses the formation of CO was great but decreased fastly and then reached to steady state with an increase of CO2 pulse number, showing steady production of CO after 7th pulse. This catalyst exposed to several CO2 pulses was subjected to CH4 pulse. Then, CO and H20 were generated together with H2. The amounts of CO formed were 4.3 pmol at the 1st CH4 pulse although they were getting decreased later. This means that surface oxygen species on the catalyst were formed and remained on catalyst surface during CO2 pulses. These species played a role as oxidant for methane oxidation. The formation of H2 from CH4 pulse after the reaction with several CO2 pulses was lower than that on the freshly reduced catalyst. This is ascribed to the loss of hydrogen by its oxidation into water with the formed 16 oxygen species from CO). When o the catalyst was exposed again E 4 o CO2 pulses 12 E 2 pulses:H 4 puL, es ~ O ::I. to the CO2 pulse after sequential 0 3 9 CO2 pulses and CH4 pulses, the 8 amounts of CO increased with 4 ~= 2 = times higher than those of the freshly reduced catalyst at initial 4 100A) hematite species was formed by the aggregation of iron oxides on the supports.

CO2 and H2 chemisorption _

CO2 and H2 chemisorption studies were performed on supporting materials and iron supported on HY & KY zeolite catalysts to determine the relative basicity. The results were listed in Table 2. No chemisorption of CO2 was observed on the HY and KY zeolite. However, the chemisorbed amount of CO2 increased with increasing the iron content on the supports. By the way, iron supported on potassium ions in zeolite-Y catalyst showed a much higher chemisorption capacity of CO/. From these results, it is concluded that CO2 appears to chemisorb on the free iron surface and on the iron surface on the potassium present in the zeolite matrix. The addition of potassium into Fe/HY and Fe/KY catalysts slightly increased the chemisorption amount of CO2 due to the electron donating ability of potassium to neighboring surface iron atoms. On the other hand, the chemisorbed amount of H2 did not show considerable difference in all samples.

409 Table 2. The chemisored amounts of CO2 & H2 o n prepared catalysts samples

CO2 uptake(/.t mol/g)

HY KY - - ~'e/H-Y-Fe/KY Fe/HY Fe/KY Fe-K/HY Fe-K/KY -

H2 uptake(/.t mol/g)

6 .-5-. . . . . . . . . 25.0 13.8 54.2 17.4 58.5

(F~ 5-wt.%3. . . . . . . . (Fe" 5wt.%) (Fe: 17wt.% fie: 17wt.%) (Fe: 17wt.% fie: 17wt.%)

2.1 1.7 1.8 1.3 1.7 1.1 1.5 1.2

not observed

Temperature Programmed Reduction(TPR) Temperature programmed reduction experiments of prepared catalysts were done to understand the strength of metal-support interaction. The results were shown in Fig. 1. There are two peaks on each profile. As shown in Fig. l-b, both peaks in Fe/KY catalysts are shifted toward higher temperature region compared with Fe/HY. This implied that iron component interacted more strongly with potassium ion exchanged zeolite-Y than hydrogen exchanged zeolite-Y. TPR profile of potassium added in Fe/KY catalyst (Fig. l-c) was very similar to that of Fe/KY catalyst. This indicated that potassium addition to the catalysts did not strongly affect the interaction between iron and KY zeolite. 440

460

I

~

I

I

I

250

3 0

450

550

600

isothermal

Temperature / *C Fig. 1. TPR profiles of Fe & Fe-K catalysts a supported on HY and KY.

410

Activities and selectivities of various catalysts in CO2 hydrogenation The results of catalytic activity and selectivity were presented in Table 3. Iron supported on HY could not yield olefin and was mainly responsible in producing methane due to the strong acidity of hydrogen exchanged zeolite-Y. This was well coincident with the low level of CO2 chemisorption uptake and reduction degree from the TPR profile. When the potassium was added to the Fe/HY catalyst, it also could not change selectivity to lower olefins. For the Fe/KY catalyst, potassium ions in zeolite-Y bifunctionally acted as a support to increase catalyst surface basicity as well as a chemical promoter to increase C2+ hydrocarbon distribution. It can be explained by the fact that it can enrich the local electron density towards active iron metals 3. The addition of potassium into Fe/KY catalysts slightly increased the catalytic activity (21% conversion) and C2-C4 olefin selectivity (80 %). Therefore, it is suggested that not only the electron donating ability of the chemical promoter but also the basicity of the support are very important factors to synthesis C2+ hydrocarbons and olefins from CO2 hydrogenation. Table 3. C02 hydrogenation over Fe and Fe-K (2:1 atomic ratio) catalysts a supported on ion exchanged Y zeolites. (*SFe 5wt. % loading, *17Fe 17wt. % loading) CO2 Selectivity Catalysts conv. (c mol %)

Hydrocarbon Distribution (C mol %)

(%) CO HC C1 C2= C2 C3= C3 C4: C4 C5> Fe/HY .5 3.15 79.06 20.94 75.70 0.00 17.29 0.32 4.90 0.00 1.80 0.00 Fe/KY .5 14.33 62.16 37.84 36.77 0.96 14.79 3.79 10.45 4.21 5.45 23.40 -Fs qT- 10.14 Fe/KY *17 17.95 Fe-K/HY *17 11.90 Fe-K/Ky*1721.28

39.14 31.35 33.87 26.53

60.86 66.49 66.13 69.35

72.56 12.54 52.70 11.17

0.02 8.94 0.14 9.12

15.23 3.18 16.91 2.08

0.07 13.02 0.79 13.62

7.64 2.98 1.49 3.24 10.40 3.83 44.50 12.72 0.69 7.81 8.24 2.33 10.76 2.75 47.64

O1.(%)b /(Ol.+Pa,) C2-C4 1.30 22.48 0.36 75.93

-47i .

.

.

.

.

.

82.38

aco: hydrogenation at 1900 ml/g/h, 573 K, and 10 atm, bSelectivity to olefins (C mol %)

CONCLUSIONS Prepared catalysts showed iron was highly dispersed on the supporting materials and they are free from impurities. Chemisorption studies suggest that CO2 appears to chemisorb on the free iron surface and on the iron surface on the potassium present in the zeolite matrix. TPR profiles implied that the iron component interacted more strongly with potassium ion exchanged zeolite-Y than with hydrogen exchanged zeoliteY. Potassium ions in zeolite-Y bifunctionally acted as not only a support to increase the catalyst surface basicity but also as a chemical promoter to shift C2+ hydrocarbon distribution and olefin selectivity. From these results, we proposed that the basicity of catalyst surface is a very important factor in CO2 hydrogenation. 4.

REFERENCES 1. Pyung-Ho Choi, Ki-Won Jun, Soo-Jae Lee, Myung-Jae Choi, Kyu-Wan Lee, Catalysis Letter, 40, 115-118 (1996) 2. U.S. Patent, 3-130-009 (1970) 3. H. P. Bonzel, H. J. Krebs, Surf. Sci., 117, 639 (I982)

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998.Elsevier Science B.V. All rights reserved.

411

H y d r o g e n a t i o n of c a r b o n dioxide over r h o d i u m catalyst s u p p o r t e d on silica Masahiro Kishida, Kyotaro Onoue, Shizuka Tashiro, Hideo Nagata, and Katsuhiko Wakabayashi. Chemical Engineering Group, Department of Materials Process Engineering, Graduate School of Engineering, Kyushu University, Higashiku, Fukuoka, 812-81, Japan. In this work, it was found that the surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600m2/g by using the preparation method which we have developed using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. By using these catalysts, it was also found that turnover frequencies for CO2 hydrogenation increased linearly with the catalyst surface area. 1. INTRODUCTION Although it is important, it is difficult to study the effect of catalyst surface area on the catalytic behavior because it has been impossible to control only catalyst surface area by using a conventional method for catalyst preparation. For example, the metal particle size of the catalyst probably becomes small if the metal salts were impregnated on the support with a large surface area. On the other hand, we have developed a novel method for catalyst preparation using water-in-oil (w/o) microemulsion. By using our method, the metal particle size of the catalyst has been controlled regardless of metal content [ 1,2]. The objectives of this work are to control the catalyst surface area of silica-supported rhodium catalysts and to investigate the effects of the catalyst surface area on the catalytic behavior in the hydrogenation of carbon dioxide. 2. EXPERIMENTAL

Rh/SiO2 catalysts were prepared using cetyltrimethylammonium chloride (CTAC) / 1hexanol / RhC13 aq. w/o microemulsion in the same manner as reported previously [ 1-3]. The concentration of CTAC in 1-hexanol and the aqueous concentration of RhC13 were 0.5 and 0.19 mol/dm 3, respectively. The water-to-surfactant molar ratio in the starting microemulsion was 12. The Rh-N2H4-complex nanoparticles were formed in the microemulsion by adding hydrazine directly. After tetraethylorthosilicate (TEOS) as the silica source and a diluted ammonium solution were added to the microemulsion containing the nanoparticles, silica gel containing the nanoparticles were precipitated by the hydrolysis of TEOS. The precipitates were filtered, thoroughly washed by ethanol, dried at 80 ~ overnight, and calcined under air flow at 500 ~ for 2 h. The catalyst thus obtained was pelleted, crushed, sized to ca. 16-24 mesh, and reduced at 450 ~ for 2 h. This preparation method will be denoted by ME method. The catalyst surface areas were determined using the BET equation from the nitrogen isotherms at 77 K. The rhodium particle was characterized by X-ray diffraction (XRD, Rigaku,

412 RINT2500), transmission electron micrography (TEM, Nihon Denshi, JEM-2000FX). Rhodium particle size was determined by the broadening technique. The CO2 hydrogenation was carried out at 220~ under a pressure of 5.0 MPa using a reactant gas mixture composed of H2/CO2/Ar=60/30/10. The flow rate of the gas mixture was adjusted to obtain the CO2 conversion in the range between 4 and 6%. The reactants and products were analyzed with on-line gas chromatographs (Shimadzu GC-4BPT, GC-7BPT, GC-8A). 3. RESULTS AND DISCUSSION 3.1 Effects of hydrolysis conditions on catalyst surface area In the ME method, silica as a support was formed by the hydrolysis of TEOS. Thus, in order to control a catalyst surface area, it is necessary to prepare catalysts changing the hydrolysis conditions of TEOS and to investigate the change in surface area of the resulting catalysts. Table 1 shows the relationship between hydrolysis conditions and the catalytic properties. The amount of TEOS charged considerably affected the BET surface area of the catalysts prepared by the ME method. It was found that the BET surface area increased with decreasing the amount of TEOS. The silica yield, which is defined as {weight of silica obtained }/{ weight of silica when all the TEOS charged were converted into silica }, was independent of the amount

Table 1 Catalyst preparation condition and the catalytic properties. Composition before hydrolysis

Catalyst

Solution a) [cm 3]

TEOS [g]

NH3 aq. [Vol % ]

[NH3]aq b) [mol/dm 3]

Silica yield c) [%]

Rh cont. [wt%]

ABETd) [m2/g]

191 191 191 191

10 20 30 50

30 30 30 30

13.5 13.5 13.5 13.5

94 94 98 93

6.3 3.1 2.1 1.2

420 240 140 31

191 191 191 191 191 191 191

50 50 50 50 50 50 50

30 30 30 30 30 30 30

2.9 4.7 6.8 8.8 10.3 11.7 13.5

55 69 78 80 92 86 93

2.0 1.6 1.4 1.4 1.2 1.3 1.2

700 605 413 327 141 68 31

a) Volume of the solution before adding TEOS. b) The aqueous concentration of NH 3 in water-pool. c) The ratio of the weight of silica obtained to the weight of silica when all the TEOS charged was converted into silica. d) The N2-BET surface area of the catalyst.

413

of TEOS. This result indicated that the amount of silica obtained was approximately proportional to the amount of TEOS charged. Here, the amount of rhodium contained in the starting solution was kept constant at 4.56x10 -3 mol. Consequently, the rhodium content of the resulting catalyst increased with decreasing the amount of TEOS. The aqueous concentration of NH3 before adding TEOS also greatly affected the BET surface area of the resulting catalysts. It was noteworthy that the BET surface area extremely increased with decreasing the concentration of NH3 and was 700 m2/g at the NH3 concentration of 2.9 mol/dm 3. Here, the amount of TEOS charged was always 50 g, but the rhodium content was changed because silica yield was changed with the NH3 concentration. As it was generally known that both increasing the amount of TEOS and the concentration of NH3 promote the hydrolysis rate, these results show that the silica prepared at a faster hydrolysis rate has a smaller BET surface area. The surface area of the catalyst was changed in this way, however the rhodium content was also changed at the same time. Thus, we made an attempt to control only the surface area at a constant rhodium content. From Table 1, the catalyst surface area and the rhodium content were found to be a function of both the amount of TEOS and the NH3 concentration as follows; (1) (2)

(Surface area) = - 2 . 2 x (amount of TEOS) + 62 x (NH3 conc.) + 1071 (Rh cont.) =-0.027 x (amount of T E O S ) - 0.024 x (NH3 conc.) + 2.91

The amount of TEOS and the NH3 concentration were determined by the eqs. (1) and (2) for the purpose of controlling the catalyst surface area with a constant Rh content. Consequently, as shown in Table 2, the catalyst surface area could be controlled in the range between 68 and 605 m2/g. The rhodium particle sizes and the rhodium contents of all the catalysts were approximately 5 nm and 1.6 wt%, respectively. 3.2 Effect of catalytic surface area on catalytic behavior Next, the effects of the catalyst surface area on the catalytic behavior in CO2 hydrogenation were investigated using the catalysts with a controlled surface area. The time dependence on turnover frequencies (TOF's) for CO2 hydrogenation were shown in Fig. 1. The product was methane only. The deactivation was observed in all the catalysts, Table 2 Control of catalyst surface area. Target

Condition

Result

Rh cont. [wt%]

BET S.A. [m2/g]

TEOS [Vol%]

[NH3]aq [mol/dm 3]

Rh cont. [wt%]

BET S.A. [m2/g]

Rh size [nm]

1.6 1.6 1.6 1.6 1.6

50 150 300 400 600

32.4 33.9 36.1 37.6 40.6

15.4 13.7 11.1 9.6 6.0

1.5 1.6 1.6 1.7 1.6

68 118 299 371 605

4.9 5.0 4.4 4.8 5.0

414

0.15

'

I

'

O

'7

O

o t'-

(D

0.1

O

ABET [m2/g] 605 371 299 118 68 O

_

O

O

O

A

A

0.08

'

0.06 o to

0.04 >

a~ > 0.05 o t-

A

x_

D

9

9

9

9

F-I

0

200

,

I

'

I

o 0.02

o00

9

\7 i

'

././

"7,

x.,.

x.,.

I

540 min on stream

9

I

9

S

0 ,

400

600

Time on stream [min]

Fig. 1 Time dependence on turnover frequency for C O 2 hydrogenation.

,

0

I

200

,

I

400

,

I

,

600

800

BET surface area [m2/g]

Fig.2 Relatiionship between the surface area and the turnover frequency for the catalyst.

however the change in activity became smaller at 540 min on stream. Since carbon was detected in all the catalysts subjected to the reaction, the deactivation was suggested to be due to carbon deposition on the catalyst surface. It was noteworthy that the TOF increased with an increase in the catalyst surface area. The TOF at 540 min on stream was found to increase linearly with the surface area, as shown in Fig. 2. This result suggests that the species adsorbed on silica surface play an important role in the CO2 hydrogenation over the silica-supported Rh catalyst. 4. C O N C L U S I O N 1. The catalytic surface area of silica-supported rhodium catalysts could be controlled in the range between 60 and 600 m2/g by the novel preparation method using water-in-oil microemulsion. The catalysts with a controlled surface area had the same average size of rhodium particles. 2. The effects of the surface area on the catalytic behavior of silica-supported rhodium catalysts in CO2 hydrogenation were examined. It was found that the turnover frequencies for CO2 hydrogenation increased lineally with the catalytic surface area.

REFERENCES 1. M. Kishida, K. Umakoshi, W.Y. Kim, T. Hanaoka, H. Nagata, K. Wakabayashi, Kagakukogaku Ronbunshu, 21, 990 (1995). 2. M. Kishida, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Catal Today, 29, 355 (1996). 3. M. Kishida, T. Fujita, K. Umakoshi, J. Ishiyama, H. Nagata, K. Wakabayashi, Chem. Soc., Chem. Commun., 1995, 763.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

415

D e h y d r o g e n a t i o n of e t h y l b e n z e n e o v e r i r o n o x i d e - b a s e d c a t a l y s t in t h e p r e s e n c e of c a r b o n dioxide N. Mimura a, I. Takahara ~, M. Saito ~, T. Hattori b, K. Ohkuma c, M. Ando d aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi,Ibaraki 305, JAPAN bDepartment of Applied Chemistry, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-01, JAPAN cSystems Research & Development Institute of Japan, 16-5 Tomihisacho, Shinjuku-ku, Tokyo 162, JAPAN dJapan Fine Ceramics Association, Halifax Onarimon Bldg. 6F, 3-24-10, Nishishinbashi, Minato-ku, Tokyo 105, JAPAN

The energy required of a new process using CO2 for the dehydrogenation of ethylbenzene to produce styrene was estimated to be much lower than that of the present process using steam. A Fe/Ca/A1 oxides catalyst was found to exhibit high performance in the dehydrogenation of ethylbenzene in the presence of CO2.

1. INTRODUCTION Styrene is one of the most important substances as a raw material of polymers. In Japan, 1.5 million tons of styrene is produced every year. It is commercially produced by the dehydrogenation of ethylbenzene (equation 1), which is made from benzene and ethylene (equation 2). C~H6 + C2H4 ---> C6Hs-C2H5 C6Hs-CzH5 --> C6H~-C2H3 + H2

(i) (2)

A large quantity of high temperature steam (steam/ethylbenzene=7-12 mol/mol) is used in the commercial plant. The important roles of the steam in the dehydrogenation of ethylbenzene are considered as ibllows. (1) The medium for supplying heat to the endothermic dehydrogenation

416 (2) Dilution of ethylbenzene to increase equilibrium conversion (3) Avoiding coke deposition on the catalyst. It has been pointed out that latent heat of condensation of steam is lost at a separator in a commercial process. Recently, dehydrogenation of ethylbenzene in the presence of carbon dioxide instead of steam has been studied 1)z). It is considered that carbon dioxide can play above-mentioned three roles. In this paper, we will also report a result of calculation of energy required to produce styrene in comparison between a present process using excess steam and a new process using carbon dioxide, and some experimental results of the dehydrogenation of ethylbenzene in the presence of CO2.

H20

CO2

One step pathway

Step Step(H)

Catalyst ~iv

H 2"[-CO 2 ~ H20 + CO Two step pathway

2. E X P E R I M E N T A L

The iron oxide-based catalysts were prepared by Figure 1. Pathways of dehya coprecipitation method. In a typical experiment, drogenation of ethylben1.4 g of catalyst (0.18-0. 30mm) was set in a quartz zene in the presence of C02 tube reactor. Ethylbenzene was fed through a vaporizer, and was mixed with CO2. The flow rate was 130 ml/min. The dehydrogenation was conducted at 550~ under atmospheric pressure. The product was analyzed by GC. 3. R E S U L T and D I S C U S S I O N

1.0

One step pathkvay COz/EB=9 \ - / ' ~ - f o " 3.1. T h e r m o d y n a m i c consideration of 0.8 the d e h y d r o g e n a t i o n of e t h y l b e n z e n e Two step ,/~//,"(" 0.6 - p a t h w a y ~ / ,'" . \ There might be two possible pathways COz/EB=9 7 , / ,~ Simple. for the dehydrogenation in the presence 0.4 ~ / / dehydrogenation of CO2, as shown in Figure 1. Figure 2, "/~'/s" "~ H20/EB=9 which shows the effect of temperature on 0.2 the equilibrium yield of styrene, clearly indicates that the yield of styrene in the 0.0 presence of CO2 is much higher than that 300 400 500 600 700 Temperature fC of the process using steam. On the other hand, at given temperature, the two step Figure 2. Equilibrium yield of styrene pathway appears to be more favorable for in the dehydrogenation of ethylbenthe yield of styrene, zene o~,,~

417 3.2. E s t i m a t i o n of e n e r g y r e q u i r e d for d e h y d r o g e n a t i o n processes Figure 3 shows model flow sheets for a typical present commercial process and new process using CO2. Table 1 gives some basic parameters corresponding to the production of styrene ~ a steam process and CO2 process. In the presence of CO2 the temperature of the dehydrogenation was assumed to be 50K lower than that for commercial process on the basis of equilibrium data shown in figure 2. Table2 summarize the estimated energies required to produce styrene by dehydrogenation of ethylbenzene in the presence of CO2 as well as in the presence of steam. The quantity of energy required for the new process using CO2 is much lower than that for the present process, mainly because a large quantity of latent heat of water condensation cannot be recovered in the commercial process. Consequently the dehydrogenation in the presence of CO2 should be a energy saving process. Table1 Basic parameters of the model process Present process New process 630 580 Atmospheric pressure b) H20/EB=9, 25~ CO2fEB=9, 25~ Simple One step dehydrogenation pathway Yield of styrene (Selectivity= 100%) R1:35%, R2:35%, Total:70% a) Temperature at the top of R1, which is indicated by * in Fig.4 and 5 b) The pressure in commercial plant is about 0.5-0.8 atm Reaction temperature(~ Pressure Feed gas and temperature Reaction mechanism

'aporator EB 25~

Heat excnanger EB 25~

Water Separator

,!

/~, Off Gas Product 40~

"

Separator

R1

R2

,~Off Gas Product 40~

~oiler

J~BoilLer

Water~___._~ :~~~_ C02 r25~ Present process

_

25~

New process

Figure 3. Model flow sheets of a present commercial process and a new process

418 Table 2 Energy required for producing styrene Commercial process New process 10 8 cal / t-styrene 10 8 cal / t-styrene Input Boiler 17.8 12.2 (1) Evaporator 2.2 Output Combustion of off gas 5.0 5.9 (2) Surplus energy 4.4 Energy required (1)-(2) 15.0 1.9(6.3 a)) a) Without the surplus energy recovered by heat exchanger (el. Fig.3) 3.3. D e v e l o p m e n t of c a t a l y s t s 6o I 9 :Fe/Ca/Ai(14/10/76wt%) Figure 4 shows that activities of several I: Fe/AI(14/86wt%)a) kinds of iron oxide based catalysts. A 5o I'[ -~. &: Ca/Al(10/86wt%) Fe/Ca/A1 oxides catalyst exhibited the best ~ 4o performance among the catalysts tested. Fe/Ca/A1 and Fe/A1 oxides catalysts were "~ 30 highly active, whereas Fe/Ca and Ca/A1 ox20 ides catalyst were extremely low in activity, lO The selectivities of Fe/A1 oxides and Fe/Ca/A1 I o oxides catalysts were almost the same (97% at o 1 2 3 4 5 5.25 h), and the main by-products were benTime / h zene and toluene. Therefore the addition of an optimum amount of CaO to Fe/A1 based cata- Figure 4. Activity of Fe oxidelyst could suppress the deactivation of the based catalysts catalyst during long term reaction. Further Feed gas" CO2/EB=ll experiment are under achievement to eluci- a)catalyst weight=l.0g date precisely the role of CaO.

3. C O N C L U S I O N The energies required for the process using steam and for the new process using CO2 were estimated to be 1.5xl0%al/t-styrene and 6.3xl0Scal/t-styrene, respectively. Therefore, the new process using CO2 should be a "energy-saving process." A Fe/Ca/A1 oxides catalyst was found highly active and promising catalyst for the dehydrogenation of ethylbenzene in the presence of CO2. REFERENCES 1. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. ikenaga, and T. Suzuki, Appl.Catal.A, 121(1995)125 2. S. Sato, M. Ohhara, T. Sodesawa, and F. Nozaki, Appl.Catal.,37 (1988) 207

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

419

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Promoting effects of C O 2 supported Cr203 catalyst

on

dehydrogenation of propane over a SiO2-

I. Takahara a, W.-C. Chang b, N. Mimura a and M. Saito a aNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan bjKF (Japan-Korea Industrial Technology Onogawa, Tsukuba-shi, Ibaraki 305, Japan

Co-operation Foundation)

researcher,

16-3

The effects of carbon dioxide on the dehydrogenation of C3H8 to produce C3H6 were investigated over several Cr203 catalysts supported on A1203, active carbon and SIO2. Carbon dioxide exerted promoting effects only on SiO2-supported Cr203 catalyst. The promoting effects of carbon dioxide over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation.

I. I N T R O D U C T I O N The utilization of carbon dioxide has recently received much attention since the global warming mainly due to carbon dioxide was recognized as one of the most serious problems in the world. The catalytic hydrogenation of CO2 to produce methanol, hydrocarbons, etc., and the CO2 reforming of methane to syngas have been extensively studied. Furthermore, it has been reported that CO2 has several promoting effects on the conversion of hydrocarbons, for example, oxidative coupling of methane [1], aromatization of propane [2] and dehydrogenation of ethylbenzene [3, 4]. The authors have investigated the effect of CO2 on the dehydrogenation of propane over several supported Cr203 catalysts, and found that CO2 has promoting effects on a silica-supported Cr203 catalysts. 14 2. EXPERIMENTAL

12

B

C

~lO Several supported Cr203 catalysts were prepared by an impregnation method using an aqueous solution of chromium nitrate. The supports used were y- A1203, active carbon (AC) and SiO2. The catalysts prepared were calcined at 823 K in air for 2 h. The catalysts were characterized by X-ray diffraction (XRD). The XRD pattems of CrzO3/A1203 and CrzO3/AC showed the diffraction lines ascribed only to the phases of respective

=~ 8 o 6 -~ ~ 4 2 06

~ x

2

4 6 0 2 4 0 ' t/h t/h 2/h Figure 1. Catalytic activities of several supported Cr203 catalysts as a function of time on stream. supports. In the case of Cr203/SiO2, a Cr203 phase A=Cr203(5wt%)/Ai203, B=Cr203(5wt%)/AC' and an amorphous SiO2 phase were observed. The C-Cr203(Swt%)/SiO2' 823 K, W/F=2 g-cat.h/mol, dehydrogenation of C3H8 was conducted under Feed gas:C3H8/CO2=l/l(O), C3H8/Ar=-I/I(O)

420 100

atomospheric pressure of C3H8+CO2 (At) at 823 K by using a fixed bed flow reactor. TPR/D studies were carried out for elucidating the behavior of Cr203 in the catalyst during treatment with H2 and CO,. Pulse reaction technique was also employed for examining the initial activity of the catalyst.

A

88~

B

~

-''-

.

_.

~:~ 60 Q ~ .,,,~ ..,.4

3. R E S U L T S A N D D I S C U S S I O N

"~ 20 r~

3 . 1 . Effects of CO2 on d e h y d r o g e n a t i o n o f C3Ha

%

The main products of the conversion of C3H8 in the presence of Ar were C3H6 and H2, while those in the presence of CO, were C3H6, H2, and CO. Since the selectivities for C3H6 were more than 90%, the dehydrogenation of C3H8 to C3H6 should be the main reaction both in the presence of CO2 and in the absence of CO2. The yield of H2+CO was found higher than C3H6 yield on all catalysts used in the present study. There might be three possible routes for CO formation; the first one via the successive reactions (1) and (2), the second o n e via the reaction (3) and the third one via CO2 reforming of C3H8 (reaction 4) as shown below.

' 2 ' 4 6 L 8 ' 10 Yield of C3H6 / % Yield of C3H 6 / %

Figure 2. Selectivities of H2 and CO in the

reactionof C3H8+CO2as a function of C3H6 yield. A=Cr203(5wt%)/Al203, B=Cr203(5wt%)/Si02, Selectivity" H2(ll), CO(O),823 K, W/F=2g-cat-h/tool, Feed gas:C3H8/CO2=l/1

7 6 ~

/

~

~5 ~:~4

~

rS 3

~

f a'~,,,,o

/

o .,-,2

C3H8- C3H6 + H2 CO2 + H2 = CO + H20 C3H8 + CO2 = C3H6 + CO + H20 C3H8 + 3CO2 = 6CO + 4H2 Figure

1 shows

the activities

(1) (2) (3) (4)

of

>' 1 06

:~

3, t/h ~i

~

10

Figure 3. Change in the C3H6 yield with alternate several feeds of C3Hs/Ar and C3Hs/CO2/Ar over a Cr203/SiO 2.

supported Cr203 catalyst for the dehydrogenation C3H8/CO2/Ar=I/2/7(O) ' C3Hs/Ar=I/9(O) ' of C3H8 in the presence of CO2 as well as in the 823 K, W/F----0.62g-catoh/mol ~. . 1.3.~ absence of CO2. The activity of the Cr203]AI203 catalyst was much lower in the presence of CO2 l ~ ,~ than that in the absence of CO2. The activity of the 8 Cr203/AC was independent of the presence of CO2. ~ - t.2~ ~:1~6~.~' ~,~ L) On the other hand, the activity of Cr203/SiO2 catalyst in the presence of CO2 was surprisingly ~2/~\ "~ found to be 40% higher than that in the absence of .~ 4 t ~ "d CO2. ~ ~.~r~ Figure "~ C shows the O selectivities _ for HE and ~2t x~~__ ~ ~ as a function of C3H6 yield. In the case of 0/ i t ~ ~ t 1 o Cr203/AI203 catalyst, the, selectivity for H2 0 20 Cr20340/wt% 60 "= decreased with an increase in C3H6 yield, whereas Content of the selectivity for CO increased with an increase in Figure 4. Yield of C3H6 in the dehydrogenation of C3H8in the presence of CO2(l) and in the C3H6 yield, as shown in Figure 2A. On the other absenceof CO2(O) and their ratio(O) as hand, the selectivities of Cr203/SiO2 catalyst for a function of Cr203 contenton a Cr203/SiO2. CO and H2 did not change irrespective of C3H 6 C3Hs/CO2(Ar)=I/1,823K, W/F=2g-catoh/mol

421

yield, as shown in Figure 2B. These findings suggest that CO might be formed via successive reactions (1) and (2) over Cr203/A1203 catslyst, whereas both the reaction (1) and the reaction (3) could take place simultaneously over Cr203/SiO2 catalyst. In order to study the contribution of CO2 in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst, catalytic tests with alternate feeds of CaH8/Ar and C3H8/CO2/Ar were carried out. The results shown in Figure 3 clearly indicate that the presence of CO,,_ markedly improved the yield of C3H6. This catalytic performance is a proof that CO,, plays a promoting role in the conversion of C3H8 to C3H6 over a Cr203/SiO2 catalyst. Furthermore, the ratio of C3H6 yield in the reaction in the presence of CO2 to that in the absence of CO 2 decreased with increasing Cr203 content in the Cr203/SiO2 catalyst, as shown Figure 4. This suggests that the boundaries between Cr203 and SiO2 particles might have an important role in the promoting effect of CO2. The effect of CO2 addition on the deactivation of a Cr203/SiO2 catalyst was also examined (Figure 5). The reaction conditions for the both cases with and without CO2 were the same except the catalyst weight, which was 50% larger in the case of the reaction without CO2, in order to obtain the same initial yield of C3H6. The decrease in the yield of C3H6 was found much less in the presence of CO~ than in the absence of CO2. This finding suggests that the addition of CO2 could suppress the deactivation of the catalyst.

lo

8 :~ 6 e~

~ g7. O

2 I

I

I

2 4 6 0 t/h Figure 5. Effect of C O 2 o n the deactivation of Cr203(5wt%)/SiO2. Reaction conditions: 823K 9 : C3H8/CO2/Ar=1/2/7, W/F=0.62g-catoh/mol O: C3Hs/Ar=-1/9, W/F=0.93g-cat.h/mol

(A) in He -I

~B) in H2(10)/Ar

~ .~

3.2. TPR/D result Figure 6 shows TPR/D profiles during various treatments of Cr203/SiO2 catalyst. The Cr203 was (C) in He after CO2 treatmentat 823 K found to be reduced in a stream of H2. And smaller peaks were observed when the catalyst, which was reduced with H2 and then treated with CO2, was re(D) in H2(10)/Arafter (C) reduced with H2. This suggests that CO2 could 323 423 523 623 723 823 oxidize some part of the surface of Cr203 in the catalyst. Accordingly, these findings suggest that Temperature / K --823K= the surface of Cr203 on a Cr203/SiO2 catalyst could Figure 6. TPR/D profile for treatment be reduced during the dehydrogenation of C3H8 in A---*B----C----Dover a Cr203/SiO 2 catalyst. the absence of CO,_, whereas the surface of Cr203 Operation conditions 9H2(10)/Ar(90), 10 K/min might be maintained partially oxidized during the reaction in the presence of CO2. ,.._.___

-

- -

_.

_

422

3.3.

Pulse

reaction

A pulse reaction technique was employed tbr | 50 examining the initial activity of the catalyst. Figure 25 H2 treatment CO2 treatment / 7 shows the conversion of C3H8 and the yield of I hydrocarbon products as a function of the number 20 ~-._~:~ :i 40~. of C3H8 pulses on Cr203/SiO2 catalyst. The yield of C3H6 reached a maximum of about 21% at the - 30~ second pulse. After the second pulse, the yield of ~15 0 C3H6 slightly decreased with increasing pulse ~ "~ number. The yield of C3H6 decreased to 17% by ~,10 20-~ the treatment of the catalyst with H2 after filth pulse, o and remained constant from sixth pulse to ninth 5 10~ pulse. The treatment of the catalyst with CO2 after ninth pulse raised the yield of C3H6 to 19%, but the 0 yield of C3H6 gradually decreased with increasing 0 0 2 4 Pul6enumber810 12 14 pulses number from 10th pulses. These findings 7. Conversionof C3H8 and yield of confirm that partially oxidized Cr203/SiO2 catalyst Figure hydrocarbon products as function of could be more active for the dehydrogenation of the number of C3H8 pulse on Cr203/SiO2. C3H8 than a reduced catalyst. The results obtained Reaction conditions: 823K, He carrier from pulse reaction studies and from TPR/D studies Yield: CH4(i"I), C2H4(A), C2H6(O), C3H6(. ) might explain slower deactivation of the catalyst Conversion: C3H8(1) during the dehydrogenation in the presence of CO2. Furthemore, pulse reaction studies were performed for comparing between the yield of C3H6 during the reaction in stream of CO2 and in a stream of He. The C3H 6 yield at the first pulse and the second pulse in the presence of CO2 was found to be higher than those in the absence of CO2. This strongly suggests that CO2 could enhance the rate of the dehydrogenation of C3H8 over a Cr203/SiO2 catalyst. This promoting effects of CO2 might occur at the boundaries between Cr203 and SiO2 particles, as described in 3.1. 4. C O N C L U S I O N The effects of CO2 on the dehydrogenation of C3H8 to produce C3H6 exhibits only on SiO2-supported Cr203 catalyst. The promoting effects of CO, over a Cr203/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation. The partially oxidized Cr203 supported on SiO2 catalyst is active for the dehydrogenation of C3H8. In the presence of CO2, the surface of Cr203/SiO2 might be maintained partially oxidized during the reaction.

REFERENCES

1. T. Nishiyama and K. Aika, J. Catal., 122 (1990) 346. 2. S. Yamauchi, A. Satsuma, T. Hattori and Y. Murakami, Sekiyu Gakkaishi ( J. of Jpn. Petrol. Inst.), 37 (1994) 278. 3. S. Sato, M. Ohhara, T. Sodesawa and F. Nozaki, Appl. Catal., 37 (1988) 207. 4. M. Sugino, H. Shimada, T. Turuda, H. Miura, N. Ikenaga and T. Suzuki, Appl. Catal. A:General, 121 (1995) 125.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

423

H y d r o g e n a t i o n of c a r b o n dioxide over F e - C u - N a / z e o l i t e c o m p o s i t e catalysts Qiang Xu a, Dehua He b, Masahiro Fujiwara a, Mutsuo Tanaka a, Yasuyuki Matsumura a, Yoshie Souma a, Hisanori Ando a and Hiroshi Yamanaka a aOsaka National Research Institute, AIST, MITI, Midorigaoka, Ikeda, Osaka 563, Japan bDepartment of Chemistry, Tsinghua University, Beijing, P. R. China.

Physical mixing Na-rich Fe-based catalysts with zeolites gave rise to a great improvement of the activity for the hydrogenation of carbon dioxide at 250~ Branched and higher hydrocarbons were obtained over such modified composite catalysts. Sodium migration from the surface of the iron-based catalyst to the zeolite via the solid-solid reaction accounts for the change of catalytic activity. Evidence for Na migration was obtained by XRD measurements of the catalysts before and after the reaction. 1. I N T R O D U C T I O N Hydrogenation of carbon dioxide is an important reaction for the utilization of CO2 as a carbon resource [1]. The hydrogenation of carbon dioxide over Fe catalysts, the typical Fischer-Tropsch (F-T) catalysts, produces mainly linear hydrocarbons in the Anderson-SchulzFlory distribution [2]. Composite catalysts with zeolites have been employed to improve the distribution [3-5]. We have reported in a preliminary communication that physical mixing of zeolites with Na-rich Fe-Cu catalysts improved the activity of the F-T catalysts and enhanced the generation of branched and higher hydrocarbons [6]. The idea of Na-migration from the Fe-based catalyst to the zeolite has been proposed to explain the improvement of catalytic activity. Evidence for Na migration has been obtain by examining the reductive behavior of the catalysts by TG measurements [7]. In the present paper, we report evidence for Na migration obtained by XRD studies and describe the influence of the solid-solid interaction between the iron-based catalysts and the zeolites on the catalytic activity and on the distribution of products in the hydrogenation of carbon dioxide.

2. E X P E R I M E N T A L Fe-Cu-Na catalysts were prepared by coprecipitation from the iron and copper nitrates with sodium hydroxide. Sodium contents were controlled by washing the precipitates. The composite catalysts were obtained by physical mixing of equal amounts of Fe-Cu-Na oxides and zeolites. After reducing the samples in a flow of 10% H2/N2 for 6 h at 350~ the catalysts were kept at 250~ in a flow of reaction gas under 20 atm. The reactants and products were analyzed with an online gas chromatograph system. The XRD powder patterns of the catalysts before and after the reaction were obtained with CuKa radiation at 40 kV and 30 mA on a RIGAKU X-ray diffractometer.

424

3. RESULTS AND DISCUSSION Table 1 summarizes the results of the hydrogenation of carbon dioxide at 250~ catalyzed by Fe-based catalysts and their composite catalysts. Over the Fe-based catalysts without zeolites physically mixed (C1-C4), the distribution of hydrocarbons in the products follows the Anderson-Schulz-Flory law [2]. Na addition resulted in (1) an increase in the average molecular weight of hydrocarbon products, (2) an increase in olefin selectivity, (3) an increase of CO selectivity, and (4) an increase in hydrocarbon yield at low alkali concentrations (C2), followed by a decrease at higher levels of Na addition (C3 and C4). These observations are consistent with the previous reports on the effects of alkali promotion on iron catalysts for the hydrogenation of monoxide (F-T synthesis) [8,9]. The results can be interpreted in terms of electronic effects from alkali, the base (electron donor), which weakens the bonds between the metals and hydrogen (electron-donor), lowers the degree of reduction and therefore decreases the CO2 conversion. The electronic effects from alkali also lead to low activity for hydrogenation of olefins and therefore high selectivities of olefins. Table 1 Hydrogenation of carbon dioxide over Fe-Cu-Na/zeolite composite catalysts Cat

Fe-Cu-Na a zeoliteb, c

No.

Conv. of

Convert to C-mol%

C=

Cbrch

CO2/%

CO

C1

C2+

C1

99:1:0.06

-

13.8

20.8

26.5

47.5

Oxy d Ratio e Ratiof 5.2

2.9

7.2

C2

99:1:0.17

-

14.4

19.5

27.6

48.4

4.5

3.2

6.2

C3

99:1:1.45

-

6.8

69.0

4.7

23.4

2.9

70.5

2.9

C4

99:1:27.0

-

6.7

71.9

4.7

21.2

2.1

64.5

3.3

C5

99:1:0.06

HY(4.8)

C6

99:1:0.06

US-Y(10.7)

C7

99:1:1.45

HY(4.8)

C8

99:1:1.45

C9

99:1:1.45

CI0 Cll

8.4

25.5

33.5

41.1

0

0.6

61.5

12.5

14.8

35.1

50.1

0

0.5

76.4

8.2

33.4

21.3

45.3

0

1.1

69.6

HMOR(9.8)

11.8

23.7

17.6

58.4

0.3

15.7

44.0

US-Y(10.7)

11.5

22.7

24.0

53.3

0

0.3

77.3

99:1:1.45

HZSM-5 (25)

12.3

19.6

22.9

57.5

0

3.1

64.6

99:1:1.45

HZSM-5 (1000)

7.9

54.6

8.4

31.0

6.0

41.5

3.1

C 1 2 99:1:1.45 NAY(4.8) 12.3 18.5 20.4 54.4 6.7 6.0 4.2 250~ 20 atm, SV=3000 ml/g-cat./h, H2/CO2=3. Results after 5 h. aMolar ratio, bRatio, SIO2/A1203, in the parentheses. CReference catalysts of the Catalysis Society of Japan. dMeOH+EtOH+PrOH, eOlefin/(olefin+paraffin)/% of C2, C3 and C4. fBranched/(branched+linear)/% of C4, C5, and C6. Mixing HY zeolite with the Na-poor catalyst (C1) resulted in deactivation of the F-T activity of Fe-Cu catalysts (C5). The H-type zeolite with a higher Si/A1 ratio, such as US-Y zeolite (C6), has less acidic sites, which deactivated the F-T activity to a slight degree, and the Natype one, which has no acidic sites, did not deactivate it. These facts indicate the deactivating process is closely related to the acidic sites. Physical mixing of zeolites with Na-rich catalysts of Fe-Cu-Na(99:l:l.45) (C3) drastically improved the activity (C8 -C10). It has been proposed that sodium migration from the surface of the metal catalyst to the zeolite via the solid-state reaction accounts for the change of catalytic activity [6,7]. Na migration resulted in a decrease of the inhibitory effect of Na on reduction of the iron oxide, giving rise to an increase in F-T activity. Light olefins were formed on the surface with a moderate Na level,

425 and branched and higher hydrocarbons were then formed via the acidic carbon-homologation of the light olefins on the partially Na + ion-exchanged H-type zeolites. The slight change in the activity observed in the reaction with HY (C7) arose from the deactivation of the F-T sites by the large number of acidic sites on the zeolite which competed with the enhancement of reduction arising from Na migration. A large increase of CO2 conversion was observed in the reaction with US-Y zeolite, which has a relatively small amount of acidic sites to deactivate the F-T sites. The zeolite having higher Si/A1 ratio allowed only a small amount of Na to migrate in and therefore no apparent change in activity appeared with HZSM-5(1000) ( C l l ) . However, a remarkable enhancement of activity was observed in the reaction of the Na-type zeolites (C12).

4-

+

A 4-I, ,4-+/ 4-l+

A

A

A

A

+

b 4-

4-4-

+§ 0

I+

20

a

Oo 40

60

80

20 / degree Figure 1. XRD patterns of (a) the catalyst of Fe-CuNa(99:1:2.9)/HY before the reaction; (b) the catalyst of Fe-Cu-Na(99: l:2.9)/HY after the reaction; and (c) the catalyst of Fe-Cu-Na(99:l:27)/HY after the reaction. o, Fe203; Lx,Fe304; +, HY zeolite. In the present study, we obtained evidence for Na migration by measuring the X-ray diffraction of the catalysts before and after the reaction. Figure 1 shows the powder patterns of X-ray diffraction for (a) the catalyst of Fe-Cu-Na(99: l:2.9)/HY before the reaction; (b) the catalyst of Fe-Cu-Na(99:l:2.9)/HY after the reaction; and (c) the catalyst of Fe-CuNa(99:1:27)/HY after the reaction. The reflections of Fe203 and Fe304 were observed before and after the reaction, respectively. The absence of Fe after the reaction may be due to the reoxidation of the samples, which were exposed to the air after the reaction and during the XRD measurements. Although no distinct differences appeared in the XRD patterns of the HY zeolite for the composite catalyst containing a small amount of Na before and after reaction (Figs. 1a and lb), the crystallinity of the HY zeolite decreased during the reaction over Fe-Cu-

426 Na(99:1:27)/HY, the composite catalyst containing a large amount of Na (Fig. l c). This observation indicates that a large amount of Na migrated from the metal oxides with a high Na content to the zeolite, resulting in a lower hydrothermal stability and destruction of the crystal structure of zeolite underthe reaction conditions [ 10]. For the composite catalysts with a low Na content, it is reasonable to consider that Na migration occurred between the metal oxide catalysts and the zeolites, although the small amount of Na did not cause the destruction of the crystal structure of the zeolites. Similar solid-solid reactions have been reported for alkaline, and alkaline-earth metal chlorides/zeolite systems [11-14]. We have recently revealed evidence for Na migration from the surface of the metal-oxide to the zeolite by examining the reductive behavior of the catalysts [7]. It was found that there was almost no effect on the reduction rate as a result of physically mixing the HY zeolite with the Na-poor catalyst. On the other hand, physical mixing increased the rate of reduction of the Na-rich catalyst as a result of the increased efficiency of Na migration. The effect was more significant for zeolites of either of H- and Na-type having lower SIO2/A1203 ratio, indicating that the Na affinity is affected by the polarization effect of [A104]-; it does not depend on the structure of the pore in the zeolite or whether the zeolite is of the H-type or not. These observations supported the results of the hydrogenation of carbon dioxide over the composite catalysts as described above.

4. C O N C L U S I O N We presented a facile route for the modification of zeolites and for the preparation of bifunctional catalysts possessing both acidic and hydrogenation functions via solid-solid reaction. Branched and higher hydrocarbons were obtained over such modified composite catalysts. Sodium migration from the surface of the iron-based catalyst to the zeolite during the solid-solid reaction accounts for the change of catalytic activity. XRD measurements exhibited evidence for Na migration.

REFERENCES 1. W. M. Ayers, Catalytic Activation of Carbon Dioxide, American Chemical Society, New York, 1988. 2. R. B. Anderson, The Fischer-Tropsch Syntheses, Academic Press, New York, 1984. 3. M. Fujiwara, H. Ando, M. Tanaka and Y. Souma, Appl. Catal. A, 130 (1995) 105. 4. K. Fujimoto and K. Yokota, Chem. Lett., (1991) 559. 5. P. D. Caesar, J. A. Brennan, W. E. Garwood and J. Ciric, J. Catal., 56 (1979) 274. 6. Q. Xu, D. He, M. Fujiwara and Y. Souma, J. Mol. Catal., 120 (1997) L23. 7. Q. Xu, D. He, M. Fujiwara, M. Tanaka, Y. Souma and H. Yamanaka, J. Mol. Catal., submitted. 8. R. A. Dictor and A. T. Bell, J. Catal., 97 (1986) 121. 9. S. L. Soled, E. Iglesia, S. Miseo, B. A. DeRites and R. A. Fiato, Topics in Catalysis, 2 (1995) 193. 10. T. Masuda, M. Ogata, T. Ida, K. Takakura and Y. Nishimura, J. Japan Petrol. Inst., 26 (1983) 344. 11. H. G. Karge, Stud. Surf. Sci. Catal., 83 (1994) 135. 12. H. G. Karge, H. K. Beyer and G. Borbely, Catalysis Today, 3 (1988) 41. 13. H. K. Beyer, H. G. Karge and G. Borbely, Zeolites, 8 (1988) 79. 14. M. S. Tsou, H. J. Jsiang and W. M. H. Sachtler, Appl. Catalysis, 20 (1986) 231.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

427

Fe p r o m o t e d C u - b a s e d c a t a l y s t s for h y d r o g e n a t i o n of CO2 Naofumi Nomura, Tomohiko Tagawa and Shigeo Goto Department of Chemical Engineering, Nagoya University, Chikusa, Nagoya, 464-01, J a p a n

The effects of compositions and reaction conditions on product distribution were investigated over various metal promoted Cu-based catalysts to improve the performance for synthesis of hydrocarbons. The formation of carbon monoxide was suppressed and the formation of hydrocarbons increased with the increase in the amount of Fe. The synergetic effect between copper and iron was required for hydrocarbon synthesis.

1. INTRODUCTION Hydrogenation of CO2 to produce valuable chemicals has received much attention in recent years. Not only synthesis of methanol as the energy carrier but also syntheses of hydrocarbons and higher alcohols as the carbon resources will be possible. CO is usually formed as a sideproduct at the same time. Therefore, the control of the product distribution is important. We have carried out hydrogenation of CO2 over copper-based catalysts [1-4]. These results showed t h a t the formate species formed on copper was a common reaction intermediate to both methanol and CO and that CuO-ZnO/TiO2 was the most effective for methanol from the viewpoint of activity and selectivity [1,2,4]. Furthermore, we have found that Fe added catalyst reduced CO with increasing hydrocarbon products [3]. In this study, the effects of compositions and reaction conditions on product distribution were investigated over various Fe promoted Cu-based catalysts.

2. EXPERIMENTAL Table 1 s u m m a r i z e s the tested catalysts. CuO-ZnO/TiO2 catalyst (cat. A) was prepared by precipitating CuO-ZnO onto TiO2 powder. Metal promoted catalysts (cat. B) were prepared by impregnating solution of a metal compound at d i f f e r e n t m o l a r r a t i o s to CuO-ZnO/TiO2. Then, t h e y were d r i e d a n d c a l c i n e d in air. Fe/TiO2 c a t a l y s t (Cu free) w a s p r e p a r e d in t h e s a m e procedure as cat. B (cat. C). In addition, the c a t a l y s t (cat. D) which was

428 Table 1 Tested catalysts Cat.

Composition (weight fraction)

A B

CuO-ZnO/TiO2 (30:30:40) M-CuO-ZnO/TiO2

C D

Fe/TiO2 (0.3:99.7) CuO-ZnO/TiO2 + Fe/TiO2 (mixing of A and C) (50:50)

Molar ratio (Cu:M)

1:0.005 (M=Fe) 1:0.01 (M= Fe, Co, Ni, Ru, Rh, Pd) 1:0.05 (M=Fe)

p r e p a r e d by physical mixing of cat. A and cat. C was also tested. After p r e t r e a t m e n t by hydrogen at 623K for 1 hour, the reactions were started. Reactions were carried out with a conventional continuous flow reactor at a pressure of 1.0MPa. The reaction conditions were as follows: mole ratio of HJC02=4/1, W/Fco2,o=570kg-cat's/mol and T=553K. The reaction mixture was analyzed by a TCD gas c h r o m a t o g r a p h connected to the reactor t h r o u g h a gas sampling valve.

3. RESULTS AND D I S C U S S I O N 3.1. Effect of addition of metals Several metals (cat. B) with the hydrogenation activity were added to CuOZnO/TiO2. The r e s u l t s are s u m m a r i z e d in Tables 2 and 3. H y d r o c a r b o n s

Table 2 Effect of addition of noble metals Additive a)

Conversion

None Ru Ru b) Rh Rh b) Pd Pd b) a) Cu:M=I:0.01

Selectivity (%)

(%)

CO

CH30H

CH 4

C2H6

23.0 12.4 7.7 13.3 8.4 0.9 0.8

97.6 79.8 63.9 92.0 84.1 20.0 18.5

2.4 10.0 20.4 3.8 7.8 trace trace

0.0 10.2 15.7 4.2 8.1 80.0 81.5

0.0 trace trace 0.0 0.0 0.0 0.0

b) p r e t r e a t m e n t by hydrogen at 673K.

429 Table 3 Effect of addition of t r a n s i t i o n metals Additive a)

Conversion

None Fe Fe only b) Co Ni a) Cu:M=I:0.01

Selectivity (%)

(%)

CO

23.0 23.4 2.6 24.9 23.1

97.6 60.5 61.3 87.0 97.7

CH3OH CH4 2.4 5.2 9.3 7.0 2.3

0.0 17.3 29.4 4.6 trace

C2H~ C,~Hs C4H,o 0.0 6.6 0.0 1.1 0.0

0.0 5.8 0.0 0.3 0.0

0.0 4.6 0.0 0.0 0.0

b) cat. C (Fe/TiO2, Cu free).

were not formed without additives. Addition of noble metals (Table 2) decreased conversion markedly, but formed m e t h a n e and suppressed CO formation. C2+ hydrocarbons were not produced. Since these catalysts were prepared by using the chlorides as raw materials, they were also calcined at higher t e m p e r a t u r e (673K) to remove residual chlorine. However, they were deactivated due to the sintering. On the other hand, addition of t r a n s i t i o n metals (Table 3) did not decrease conversion. Ni did not show the promotion effect. Co mainly promoted the formation of methanol and methane. Fe produced C2+ hydrocarbons. Furthermore, Co and Fe suppressed CO formation. However, Fe/TiO2 catalyst (cat. C) showed poor activity. This indicates t h a t Cu is also necessary for hydrocarbon synthesis. It was shown from these results t h a t addition of Fe to CuO-ZnO/TiO2 was the most effective for the formation of C2+ hydrocarbons. Thus, the effects of Fe addition were further investigated in the following study. 3.2. Effect of a m o u n t of Fe

Figure 1 shows the effect of a m o u n t of Fe. The a m o u n t of Fe was varied from 0 to 5%. Hydrocarbon selectivity increased and CO selectivity decreased with the increase in the a m o u n t of Fe. On the other hand, conversion and methanol selectivity were almost independent on the a m o u n t of Fe. Therefore, the addition of Fe allowed the shift from CO formation to synthesis of hydrocarbons. 3.3. Effect of method of Fe addition

Figure 2 shows the effect of method of Fe addition on product distributions. CuO-ZnO/TiO2 (cat. A) w a s active for m e t h a n o l s y n t h e s i s , b u t it w a s not effective for the s y n t h e s i s of h y d r o c a r b o n s . This i n d i c a t e s t h a t Cu species alone is not e n o u g h to produce h y d r o c a r b o n s . On t h e c o n t r a r y , F e - b a s e d c a t a l y s t s are k n o w n as h y d r o c a r b o n s y n t h e s i s c a t a l y s t s from CO, t h a t is, Fischer-Tropsch reaction. However, Fe/TiO2 c a t a l y s t (cat. C) showed poor

430 activity in this study of CO2 hydrogenation, since the pressure in this study (1.0MPa) I ~ 9 8O was quite low compared to the t ~f Fischer-Tropsch reaction o~20 & conditions (5 10MPa). This c60 O indicates that Cu is also > o3 necessary for hydrocarbon (D o > 40 csynthesis. In the case of /x o10 CO physically mixed catalyst (cat. o D), the product distribution was 20 almost the same as that of CuOI ,[~E? ZnO/TiO2 (cat. A). Therefore, ,~ ] synthesis of hydrocarbons was Amount of Fe [%] not a simple consecutive route that CO formed on Cu-Zn Figure 1. Effect of amount of Fe: O - c o n v . ; catalyst was converted into A - s e l . of CO; C ] - s e l . of CHaOH; hydrocarbons on Fe catalyst. ~ - sel. of CnHm. Only when copper and iron were supported simultaneously (cat. B), hydrocarbons 2522.4 22.7 were produced and CO 20formation was suppressed. This strongly 14.2 o'~ 15 _ suggests that the synergetic "o effect between copper and ~J lO8.0 iron is required for synthesis of hydrocarbons. In F-T synthesis, the synergetic 0 8 1"6 effect between copper and iron is explained as Cu being Cu-Zn Cu-Zn-Fe Fe Cu-Zn + Fe a reduction promoter for Fe. (A) (B) (C) (D) 3q~

.

.

.

.

I

100

'

~

,

_

Me0H ~CnHm ~ C 0 Figure 2. Effect of method of Fe addition.

REFERENCES 1. 2. 3. 4.

Y. T. T. T. 21

Amenomiya Tagawa, M. Tagawa, N. Tagawa, N. (1995) 193.

and T. Tagawa, Proc. 8th Intern. Congr. Catal., 2 (1984) 557. Shimakage and S. Goto, ISCF-CO2 (1991) 409. Nomura and S. Goto, Proc. ICCDU (1993) 369. Nomura, M. Shimakage and S. Goto, Res. Chem. Intermed.,

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

431

The effect of rhodium precursor on ethanol synthesis by catalytic hydrogenation of carbon dioxide over silica supported rhodium catalysts Hitoshi Kusama, Kiyomi Okabe, Kazuhiro Sayama and Hironori Arakawa National Institute of Materials and Chemical Research (NIMC), Tsukuba, Ibaraki 305, Japan

Ethanol synthesis by CO2 hydrogenation was carried out over Rh/SiO2 catalyst. Several catalysts were prepared using acetate hydrate, nitrate and chloride precursors of rhodium, and the effects of the precursor on reaction behavior were investigated. The remarkable effect of Rh precursor on the ethanol selectivity as well as activity of reaction was observed. The highest ethanol selectivity was obtained over the 5 wt% Rh/SiO~ catalyst prepared from rhodium acetate hydrate precursor. It has been suggested that the difference in Rh precursor changed the mean particle size of Rh/SiO2 catalysts, resulting in the change in the ethanol selectivity.

1. I N T R O D U C T I O N We have been studying the feasibility of CO2 hydrogenation to oxygenates over silica supported rhodium catalysts based on the results obtained through CO hydrogenation. In previous papers, we reported that the effect of additives to Rh/SiO2 catalysts. As a result, we found that only four additives ( Li, Fe, Sr, and Ag ) showed ethanol formation [1-3]. It has been reported that the precursors of catalyst components over promoted Cu catalysts influence reaction behavior of CO2 hydrogenation. Nitta et al. has reported that the precursor components of the precipitated Cu-ZrO2 catalysts have a great influence on the methanol selectivity as well as CO2 conversion [4]. Based on these findings through CO2 hydrogenation, it is considered that appropriate choice of the precursors in the preparation of supported metal catalyst might improve ethanol formation in CO2 hydrogenation. In this study, the catalysts using different Rh precursors were prepared and the effect of the Rh precursors of Rh/SiO~ on reaction behavior was investigated. Prepared catalysts were characterized by various kinds of methods, in order to

432 elucidate the relation b e t w e e n the p r o p e r t i e s of the c a t a l y s t s a n d reaction behavior.

2. E X P E R I M E N T A L Silica gel ( Fuji-Davison, g r a d e / / 5 7 ), sieved into 16-32 mesh size range and e v a c u a t e d at 473 K for 2 h, was i m p r e g n a t e d with an a q u e o u s solution of Rh(CH~COO)3.5/2H20 ( acetate h y d r a t e , Soekawa Chemicals ) or Rh(NO~)3 ( nitrate, Soekawa Chemicals ) or RhCh~ ( chloride, Wako P u r e Chemical I n d u s t r i e s ) by the incipient wetness method to form Rh/SiO2 catalysts. After drying at 473 K in vacuo, the catalyst was reduced at 623 K for 1 h in an H2 flow of 100 cmS/min. X-ray photoelectron spectra of catalysts were m e a s u r e d using a Shimadzu ESCA-850 after p r e t r e a t m e n t at 623 K for 0.5 h in an H2 flow of 200 cm3/min within the p r e c h a m b e r of the a p p a r a t u s . The binding energies of XPS were referred to the evaporated Au on the surface as the internal s t a n d a r d with the Au 45/2 level at 83.8 eV. H2 adsorption was determined at 308 K using a Micromeritics ASAP 2000 in order to measure dispersion and mean particle size of Rh. Hydrogenation of COs was conducted using a pressurized fixed-bed, flow-type micro-reactor. One gram of pre-reduced catalyst was packed in the reactor tube and was pretreated in-situ at 623 K for 0.5 h in an H2 flow of 200 cm3/min. After cooling to room temperature, the gas was switched to H2 - CO2 premixed gas ( H2 / CO2 = 3 ) containing 1% of Ar as an internal standard for GC analysis, and the reaction was carried out u n d e r a p p r o p r i a t e conditions. The effluent gas was analyzed by on-line gas chromatography with a PEG-1500 column ( 3 m, FID ) and a 2 wt% squalene/active carbon column ( 3 m, TCD ). The tubings from the catalyst bed to the GC were kept hot to avoid condensation of all products.

3. R E S U L T S A N D D I S C U S S I O N 3.1. R e a c t i o n b e h a v i o r Three 5 wt% Rh/Si02 catalysts with different Rh precursors were prepared. Table 1 shows the effects of Rh precursors on reaction behavior. The turnover frequency of C02 conversion increased in the order: nitrate precursor < acetate hydrate precursor < chloride precursor. The remarkable influence of Rh precursor of catalyst on product selectivity was observed. While the main product was CO over the catalysts prepared from acetate hydrate precursor and nitrate precursor, t h a t was m e t h a n e over the catalyst p r e p a r e d from chloride. E t h a n o l was not detected over the catalyst prepared from chloride precursor. The highest ethanol

433 and methanol selectivity was obtained over the catalyst p r e p a r e d from acetate hydrate precursor.

Table 1 Effect of Rh precursor on C02 hydrogenation over 5 wt% Rh/Si02 catalysts a Rh precursor

Rh mean Selectivity in carbon efficiency Turnover % particle size c frequency b h -1

MeOH

EtOH

CO

CH4

nm

Rh(CH3COO)3.5/2H20

44.9

6.0

1.2

89.3

3.3

2.82

Rh(N03)~

12.4

5.4

0.5

68.6

25.5

3.27

144.5

0.1

0

0.1

99.8

5.50

RhCh~

a Reaction temperature = 533 K, pressure = 5 MPa, flow rate = 100 cm3/min, H2/ C02 ratio = 3. b, c Determined by H2 adsorption.

3.2. Characterization of catalysts In order to clarify the reason why Rh precursor influenced the product selectivity, the catalysts were characterized. The residue such as chlorine coming from Rh precursor were not detected on the surface of catalysts by XPS analysis. Table 1 also shows the Rh mean particle size determined by H2 adsorption. The Rh mean particle size of catalysts increased in the order: acetate h y d r a t e precursor < nitrate precursor < chloride precursor. CO selectivity decreased and m e t h a n e selectivity increased d r a m a t i c a l l y in the same order. F r o m these results, it has been suggested t h a t the difference in Rh precursor changed the particle size of Rh, resulting in the change in the product selectivity consequently. 3.3. Correlation b e t w e e n Rh particle size and ethanol selectivity It was shown t h a t Rh p r e c u r s o r influenced ethanol selectivity of C02 hydrogenation over Rh/SiO2 catalysts. Therefore, the catalysts with different Rh dispersion were prepared by changing both Rh precursor and the a m o u n t of Rh loaded on silica supported catalysts. The a m o u n t of Rh loading r a n g e d 0.1 - 10 wt% to Si02. The turnover frequency of C02 conversion decreased with increasing Rh particle size ( correspond to acetate hydrate precursor in Table 1 ), attained a minimum ( correspond to nitrate ), and then increased ( correspond to chloride ). Figure 1 shows the effect of Rh particle size determined by H2 adsorption on ethanol selectivity. The highest ethanol selectivity was obtained when the mean size of Rh particle was about 2.5 nm. The reason why ethanol selectivity reaches

434

to maximum over Rh/Si02 having about 2.5 nm of Rh mean particle size is now under investigation in detail.

2.5

2.0

I

--

I

I

I

I

9

Acetate hydrate precursor

9

Nitrate precursor

l

9 .

Chloride precursor .

.

.

,m

ID

1.5

u

O

1.0

-

0.5

0.0

9

_

I

v

1

9

2

3

v

4

I

5

m

mm

I

6

l

mm

7

Rh m e a n particle size d e t e r m i n e d by H2 a d s o r p t i o n / n m Figure 1. Effect of Rhodium mean particle size on ethanol selectivity. Reaction temperature = 533 K, pressure = 5 MPa, flow rate = 100 cm3/min, H2/CO2 ratio = 3.

REFERENCES 1. H. Kusama, K. Sayama, K. Okabe, and H. Arakawa, Nippon Kagaku Kaishi, 1995 (1995) 875. 2. H. Kusama, K. Okabe, K. Sayama, and H. Arakawa, Catal. Today, 28 (1996) 261. 3. H. Kusama, K. Okabe, K. Sayama, and H. Arakawa, Energy, 22 (1997) 343. 4. Y. Nitta, T. Fujimatsu, Y. Okamoto, and T. Imanaka, Catal. Lett., 17 (1993.) 157.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

435

Selective formation of iso-butane from carbon dioxide and hydrogen over composite catalysts Yisheng Tan, Masahiro Fujiwara, Hisanori Ando, Qiang Xu and Yoshie Souma

Osaka National Research Institute, AIST, MIT1, 1-8-31 Midorigaoka, Ikeda, Osaka563, Japan

The hydrogenation of carbon dioxide was studied using composite catalysts comprised of Fe-Zn-M (M = Cr, AI, Ga, Zr) catalysts and the HY zeolite, where the methanol synthesis and the methanol-to-gasoline (MTG) reaction are combined. The results show that light olefins are important intermediates for iso-butane formation. In all of the cases, the selectivity of isobutane, which can be used as a reactant in subsequent methyl-tert-butyl ether (MTBE) synthesis, was the highest in hydrocarbons. 1. INTRODUCTION The increase of carbon dioxide in atmosphere is believed to be the cause of the serious global warming problem. Therefore, the development of new technologies which suppress the emission of carbon dioxide or convert it into useful chemicals has been intensively studied in recent years [ 1,2]. Especially, the hydrogenation of carbon dioxide is an important method for the utilization of carbon dioxide. We have already reported that the liquefied petroleum gas [3], ethylene and propylene [4,5] could be synthesized from carbon dioxide and hydrogen over various composite catalysts. Recently, methyl-tert-butyl ether (MTBE) has received much attention as an octane-enhancing and exhaust gas-cleaning additive, of which the increased demand has caused a lack of iso-butylene as a key reactant for MTBE synthesis [6]. Conversion of carbon dioxide to iso-butane and subsequent dehydrogenation must be a potential effective process for obtaining iso-butylene. 2. EXPERIMENTAL

The Fe-Zn-M catalysts were prepared by coprecipitation from the corresponding solutions of nitrates with NaOH solution. The two solutions were concurrently added to a 1000 mL beaker and continuously stirred at 65 ~ the pH value of which was kept at 7. The precipitates were aged for 2 h, then filtered and washed with distilled water 10 times. The gels were dried at 120 ~ overnight and calcined at 400 ~ for 4 h. The sodium contents in all catalysts were determined to be less than 0.01% (mass) by atomic absorption spectroscopy (Hitachi Z-8200). The composite catalysts were obtained by physical mixing with the HY zeolite (JRC-Z-HY4.8, SIO2/A1203=4.8) of which the weight ratio of Fe-Zn-M and the HY zeolite was 2:1.

436 The catalytic test was performed in a fixed-bed flow reactor[5]. The reactor was made of a stainless-steel tube with an inner diameter of 9 mm. 1 gram of the catalyst was packed in the reactor, and reduced in a stream of 5% H2 (95%N2) at 340 ~ for 14 h. A reaction gas (H2/CO2=3) was then introduced into the reactor under 5 MPa. All effluent gases were analyzed by an on-line gas chromatograph system [5]. All results in this paper were obtained after reaction at 360 ~ for 6 h. Powder X-ray diffraction was carried out using CuKa radiation at 40 kV and 30 mA on a Rigaku X-ray diffraction meter. 3. RESULTS AND D I S C U S S I O N

Table 1 Hydrogenation of carbon dioxide over Fe-Zn-M/HY composite catalysts a Catalysts Conv. of Selectivity Distribution Ratio of Yield of C02 (%) (%) of hydrocarbons(%) olefins (%) i-C4_ HC Oxy b CO C1 i-C4 Other c C2=d C3=e (C-mol%) Fe-Zn-Zrf 13.9 3.0 27.0 70.0 100 0 0 0 0 0 Fe-Zn-Zr/HY 15.5 40.8 0.7 58.5 2 34 64 72 7 2.5 Fe-Zn-Zrf/HY 17.2 46.8 0 53.2 3 38 59 79 6 3.0 Fe-Zn-Zr/HYg 15.2 42.5 2.8 54.7 6 32 62 74 24 2.1 Fe-Zn-Cr/HY 18.9 31.1 0 68.9 3 39 58 74 6 2.3 Fe-Zn-A1/HY 17.5 35.3 0 64.7 3 39 58 67 3 2.4 Fe-Zn-Ga~Y 15.0 43.3 2.0 54.7 7 33 60 75 24 2.2 Cu-Zn-A1/HY 30.5 6.3 9.3 84.4 10 trace 90 0 0 0 Fe-Zn/HY h 13.3 36.8 1.5 61.7 8 23 69 80 30 1.1 a 360 ~ 5 MPa, S V = 3000 ml/g-cat./h, H2/CO2=3, Fe-Zn-M =1:2:1 (in atomic ratio), Fe-Zn-M/HY=2: l(in weight ratio), b MeOH+MeOMe. c C2H4/(C2H4+C2H6). d C3H6/(C3H6+C3H8). e Other (C2-C7, except for iso-butane), fFe-Zn-Zr =1"11 (in atomic ratio), gFe-Zn-Zr(I'I'I)/HY=I'I, hRef. [5], Fe-Zn(4:I)/HY=II, 350~ Table 1 shows the results of the hydrogenation of carbon dioxide over various composite catalysts. All the composite catalysts except for the Cu-Zn-A1/HY gave considerable amounts of olefins and exhibited high selectivity of iso-butane (32-39%) with low content of methane (2-7%). For the Fe-based composite catalysts, the selectivities of hydrocarbons (31.1-46.8%) depended on the third metal added to the Fe-Zn catalyst, while the conversions of CO2 were relatively constant at 15-18%. In the series of Fe-Zn-M(1:2:1)/HY composite catalysts, the highest yield of iso-butane was observed in the Zr-containing composite catalyst (2.5 C-mol%). The selectivity of hydrocarbons (46.8%) and the yield of iso-butane (3.0 C-mol%)for Fe-Zn-Zr(I:I:I)/HY, as far as we know, are the best for the selective production of iso-butane from carbon dioxide and hydrogen. We compared our novel catalysts with a commercial methanol synthesis catalyst Cu-ZnA1 (Cu-Zn-Al=42:45:13, in atomic ratio). As shown in Fig.la, although the conversion of CO2 for the Cu-Zn-A1/HY composite catalyst was the highest (30.5%), the selectivity of hydrocarbons was the lowest (6.3%) in our study. The decomposition of methanol to CO at high temperatures accounts for the remarkable decrease in the selectivity of hydrocarbons. Moreover, because olefins are easily hydrogenated into paraffins over Cu-based catalysts [7], no olefins and only a trace of iso-butane appeared in the products. The Fe-Zn-Zr (1:1:1)

437 catalyst produced methanol and methane exclusively, although Fe-Zn gave C2+ hydrocarbons as well [5]. For the Fe-Zn/HY composite catalyst, the maximum selectivity of hydrocarbons was 33.8% and the content ofiso-butane only 23% in hydrocarbons [5]. On the other hand, in the case of Fe-Zn-Zr/HY, both of them increased to 46.8% and 38%, respectively. The distribution of hydrocarbons was completely different from the Schulz-Anderson law, indicating that the hydrocarbons are not formed by F-T reaction. We have reported that the Fe-Zn catalyst acts as a methanol synthesis catalyst in the case of the composite catalyst, although the Fe-Zn catalyst is a typical F-T catalyst [5]. However, the Fe-Zn-Zr catalyst without the zeolite behaved as a methanol synthesis catalyst and hydrocarbons were formed via the M T G reaction over the HY zeolite. The XRD powder patterns indicate that the aFe203 and ZnFe204 spinel exist in the Fe-Zn/HY composite catalyst. ZnFe204, ZrO2 and a-Fe203 exist in the fresh Fe-Zn-Zr/HY composite catalyst. However, after the reaction, ZnO appeared and a-Fe203 disappeared. It is known that ZnFe204, ZnO and ZrO2 [8] are favorable for methanol synthesis. It seems that the improvement of iso-butane selectivity by adding the third metal is due to the different behavior of Fe-Zn and Fe-Zn-Zr catalysts, the former as a F-T catalyst and the latter as a methanol synthesis catalyst. 1.0

4.0

I-i

Paraffin

3.5

2

HC Sel. 6.3% Oxy Sel. 9.3% ield (1.92 C-tool%)

0.8

~0.6

CO Conv. 30.5%

~3.0

HC Yield 8.05 (C-mol%) CO Conv. 17.2% m

2

HC Sel. 46.8% _

I

I

I

i

r]

Paraffin

1 ~

Olefin Iso-butane

,E 2.5

B

c..)

-~2.o

(a)

(D

(b)

~0.4 1.0

0.2

0.5

0.0

1

2

3 4 5 6 Carbon Number 360 ~

7

0.0 1

2

3

4

5

6

7

Carbon N u m b e r

5 MPa, SV = 3000 ml/g-cat./h, H2/CO2=3

Figure 1. Hydrocarbon distribution over the composite catalysts of (a) Cu-Zn-A1/HY and (b) Fe-Zn-Zr(l 1 1)/HY The importance of olefins to the distribution of hydrocarbons and the synthesis of isobutane is obvious. In the case of the Cu-Zn-A1/HY composite catalyst, the immediate hydrogenation of the olefins such as ethylene and propylene prevents the oligomerization to higher hydrocarbons. Therefore, C4+ hydrocarbons were scarcely obtained. On the contrary, in the case of the Fe-Zn-Zr/HY composite catalyst (Fig.l, b), the selectivity of methane was

438 low and that ofiso-butane was the highest in all hydrocarbons. Ethylene and propylene were also observed. These olefins seem to be important intermediates to form iso-butane via carbon homologation. Therefore, as predicted in our previous paper [4], to use a metal oxide catalyst which has high activity for methanol synthesis but low activity for the hydrogenation of olefins, is essential for obtaining a high selectivity of iso-butane on the composite catalysts. Our composite catalyst system is also advantageous to produce branched hydrocarbons, especially iso-butane, because of the acidic M T G mechanism. This was confirmed by the predominant formation of iso-butane to n-butane. On the other hand, linear hydrocarbons are formed exclusively by the carbene polymerization mechanism in the F-T reaction. We now wish to propose a plausible reaction path from carbon dioxide to isobutane over the composite catalysts (eq-1). At first, carbon dioxide is hydrogenated to methanol over the Fe-Zn-Zr catalyst, followed by the conversion of methanol to hydrocarbons in the acidic sites of the HY zeolite according to the mechanisms of chain growth [9]. A main route to iso-butane formation is the reaction between methanol and propylene [ 10]. -H20 CO 2 + H 2

~ CHsOH

Fe-Zn-Zr

~CHsOCH s

HY CHsOH C2H 4

~.. C3H6

~ iSO-C4Hlo

(eq-1)

4. CONCLUSION The hydrogenation of carbon dioxide produced iso-butane over Fe-Zn-M/HY (M = A1, Cr, Ga, Zr) composite catalysts with high selectivities. The mechanism of iso-butane formation combines the methanol synthesis reaction and the MTG reaction. The olefins were formed to be important intermediates for iso-butane formation. In order to obtain high selectivity of iso-butane, we found it essential to prepare a composite catalyst which has high activity for methanol synthesis but low activity for the hydrogenation of olefins.

REFERENCES

[1] T. Inui, K. Kitagawa, T. Hagiwara and Y. Makino, Appl. Catal. A., 94 (1993) 3. [2] T. Fujitani, M.Saito, Y. Kanai, T. Kakumoto, T. Watanabe, T. Nakamura and T.Uchijima, Catal. Lett., 25 (1994) 271. [3] M. Fujiwara, R. Kieffer, H. Ando andY. Souma, Appl. Catal. A, 121 (1995) 113. [4] M. Fujiwara, H. Ando, M. Tanaka and Y. Souma, Appl. Catal. A, 130 (1995) 105. [5] M. Fujiwara, R. Kieffer, H. Ando, Q. Xu and Y. Souma, Appl. Catal. A, 154 (1997) 87. [6] A. Sofianos, Catal. Today, 15 (1992) 149. [7] B. Denise and R. P. A. Sneeden, J. Mol. Catal., 37 (1986) 369. [8] B. Denise, R. P. A. Sneeden, B. Beguin and O. Cherifi, Appl. Catal., 30 (1987) 353. [9] W. W. Kaeding and S. A. Butter, J. Catal., 61 (1980) 155. [10] C.D. Chang, Catal. Rev.-Sci. Eng., 25 (1983) 1.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

439

V a n a d i u m - c a t a l y z e d acetic acid synthesis from m e t h a n e and carbon dioxide Yuki Taniguchi, Taizo Hayashida, Tsugio Kitamura, and Yuzo Fujiwara Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Fukuoka 812-81, Japan I. INTRODUCTION Carbon dioxide is one of the natural Cl-resources which is being watched with keenest interest as a substitute of toxic CO in the C~-chemistry. The chemical fixation of CO2, a process which is appealing for both scientific and environmental reasons since methane and CO2 are well known as the greenhouse gases, is an important task for the human being though it is difficult because of low reactivity of CO2. Alkane activation/functionalization by transition metals under mild conditions is also one of the most challenging problems in modern chemistry since small alkanes including methane, ethane, and propane are the most abundant natural resources of hydrocarbons. In continuing studies on C-H bond activations [1], we have found that methane, ethane, and propane give rise to the corresponding acetic, propionic, and butyric acids, respectively in good yields when allow to react with CO using the Pd(OAc)2/Cu(OAc)2/K2S2Os/CF3COOHcatalyst system [2]. We have also reported that oxygen can be used as an oxidant in lieu of K2S2Os in the Pd/Cu system, and that more interestingly CO2 can also react with methane to give acetic acid [3]. On the acetic acid synthesis from methane and CO2, there is only one example [4] in addition to ours [3]. We have found that carbon dioxide can also react with methane to give acetic acid in the presence of vanadium catalysts.

CH 4

+

CO 2

V cat., K2S20 8

CF3COOH

~

CH3COOH

2. EXPERIMENTAL In a 25-mL stainless steel autoclave fitted with a magnetic stirring bar, VO(acac)2 as catalyst, K2S2Os and CF3COOH were added. The autoclave was closed and then pressurized to 5 atm with CH4 and 5 atm with CO2. The mixture was heated with stirring at 80 ~ for 20 h. After cooling the autoclave was opened and the mixture was analyzed by GLC.

440

3. RESULTS AND DISCUSSION At first, we examined the reaction of methane (5 atm) with CO 2 (5 atm) in the presence of K2820 8 (5.0 mmol) in trifluoroacetic acid (TFA) (10 mL) using various transition metal compounds at 80 ~ for 20 h, and the results are summarized in Table 1. As is apparent from the table, VO(acac)2 (acac: acetylacetonate) gives the highest turnover number (TON) and the highest yield of acetic acid (entry 2). Vanadium(III) oxide is also effective in this reaction (entry 5). In the absence ofK2S208, the TONs of the catalyst are extremely low (entries 2-4), which suggests that K28208 acts as an oxidizing agent. Some vanadiumcontaining heteropolyacids also act as catalyst in this reaction (entries 6-9). This reaction requires strong acid as a solvent. The solvent effect for the synthesis of acetic acid is in the order: TFA (TON = 8.91) >> 2N TFA (1.60)- 2N HCI (1.36)-~ 2N HzSO 4 (1.07) < 2N NaOH (0.36) z 0 o

50

f

9 Xco - O Xco2

0

I

I

I

25

50

75

100

nco2 / ( nco + nco2 ), tool-%

nco2 / ( n c o + nco2 ), mol-%

Fig. 2: Conversion (X) of CO and CO 2 as a function of the molar CO 2 content of the feed gas; 100Fe/13A1203/10Cu/1 OK (left), 100Co/60MnO/147 SIO2/0.15Pt (right); (For further information see Fig. 1)

~,

1

100 Fe/13AI203/10Cu/1 OK

-'i 0.75 m 131 < m 0.5

H2/CO/CO2 9 7/3/0 O 7/O/3

-r" 0.25 I--

0

1

100Co/60M nO/147Si02/0.15Pt

0.75

0 iI

0

o,

I

I

I

I

I

2

4

6

8

10

CARBON N U M B E R ,

0.5 0.25

~/~%.,.cro - o ]/

9 6/3/0

v---

V' 6 / 2 / 1 -


4

O

3

(.9

~9

> = o

Ga modification

0 10000

30000 50000 70000 Space velocity (h -~)

Fig. 4 Effect of space velocity on the conversion to each product Fe:Cu:Zn:AI:K:Pd:Ga = 1:0.53:0.5:2.75:0.7:0.02:0.16 C02/H2- 1/3, 80 atm, 330~

2 ' 20000 3;000 40000 5;000 6;000 70000 Space velocity(h1) Fig. 5 Effect of Pd- and Ga-modification on conversion CO2 to ethanol Fe:Cu:Zn:AI:K:Pd:Ga -1:0.53:0.5:2.75:0.7:0.02:0.16 C02/i--I2 = 1/3, 80 atm, 330~

REFERENCES [1] T. Inui, [2] T. Inui, [3] T. Inui, [4] T. Inui, (1982).

M. Inoue, T. Takeguchi, J. Lee, Catal. Lett., to be submitted. M. Funabiki, M. Suehiro, T. Sezume, JCS Faraday I, 75, 787(1979). H. Hara, T. Takeguchi, J. Kim, Catal. Today, 36 (1997) 25-32. M. Suehiro, Y. Saita, T. Miyake and Y. Takegami, Appl. Catal., 2,389

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

517

A study f o r the durability of catalysts in ethanol synthesis by h y d r o g e n a t i o n of c a r b o n dioxide* Katsumi Higuchi a, Yoko Haneda b, Kenji Tabatab, Yoshiko Nakahara b, and Makoto Takagawa a aCorporate Research Laboratory of Mitsubishi Gas Chemical Co. Inc. (MGC) 22, Wadai, Tsukuba, Ibaraki, 300-42 Japan bResearch Institute of Innovative Technology for the Earth (RITE) 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto, 619-02 Japan The durability of catalysts in ethanol synthesis by the hydrogenation of C O 2 w a s investigated by means of XRD, TEM, and EDS. The K/Cu-Zn-Fe oxides catalyst was deactivated by the segregation of catalyst components to FeCO 3, ZnO, and Cu during the reaction. The segregation was prevented by the addition of Cr component to the catalyst. Consequently, the K/Cu-Zn-Fe-Cr oxides catalyst indicates long catalytic life. 1. I N T R O D U C T I O N It was reported that the K/Cu-Zn-Fe oxides catalyst efficiently converted a mixture of C O 2 and H 2 into ethanol by Mitsubishi Gas Chemical and National Institute of Material and Chemical Research [1]. However, the catalyst was deactivated quickly during the reaction. To improve the catalytic life, an addition of various kinds of components was tried. It was found that the addition of Cr component to the catalyst prevented the deactivation of catalyst [2]. In this paper, we describe the effect of the addition of Cr component to the catalyst from the results of XRD analysis, transmission electron microscope observation (TEM), and energydispersive X-ray microanalysis (EDS) of the catalysts before and after the reaction. 2. E X P E R I M E N T A L

The catalysts were prepared by co-precipitation method from aqueous solution of metal nitrates of Cu, Zn, Fe, and Cr and NaOH aqueous solution. Potassium was impregnated to the precipitate with K2CO 3 aqueous solution. The composition of catalysts were as follows; CAT A: K/Cu-Zn-Fe=0.077/1-1-3, CAT B: K/Cu-Zn-Fe-Cr=0.077/1-1-3-0.1. The hydrogenation of CO 2 was performed with a conventional flow reactor for about 150 hours at 300~ and 7.0MPa. The structures of catalysts were identified by means of Rigaku RINT 2000 X-ray diffractometer. The observation of catalyst particles and the micro analysis of their compositions were carded out by means of Hitachi HF-2000 field emission transmission electron microscope and Kevex DELTA plus 1 energy-dispersive X-ray spectrometer. ,

We wish to thank MITI and the Japan Alcohol Association for their support and approval to the presentation of this paper.

518 3. R E S U L T S A N D D I S C U S S I O N

The results of hydrogenation of C O 2 w e r e shown in Figure 1. In the reaction using K/CuZn-Fe oxides catalyst (CAT A), the CO 2 conversion at the start was 45%, and the ethanol selectivity was 19%. However, the CO 2 conversion fell down quickly. After 114 hours, the CO 2 conversion was 30 %, and the ethanol selectivity was 16%. The study concerning catalytic life revealed that the addition of Cr component to K/Cu-Zn-Fe oxides catalyst prevented the deactivation of catalyst. In the reaction using K/Cu-Zn-Fe-Cr oxides catalyst (CAT B), the CO 2 conversion at the start was 37%, and the ethanol selectivity was 19%. The catalytic activity of CAT B at the start was lower than that of CAT A. However, after 20 hours, the catalytic activity of CAT B reached a steady state, 35% CO 2 conversion and 16% ethanol selectivity, and kept these conversion and selectivity even after 170 hours run. 50

50 --0--

CO2 Conv. of C A T A

40

~

.,..~

~o

30

I*

c7 ~

ztoIIS~ ~ATB I

20

lo

rj

-0

50

1 O0

150

2t

Time / h Figure 1. Hydrogenation of CO 2 on CAT A and CAT B.

I

'l

"

....

I ' ' ' ' I ' ' ' ' I

\

I , , , , I , ' , , I ' ' ' ' I ' '

9

I,a)

I

9

9

'I'''

'.

9 Fe304

"

0 FeCO3

-"

] ~zu~

-

. I

. '''I'' .

.

. I ' ' ' ' I.

.

.' ' ' I

.

. ''I'

I

'

'.

O)Fe304 FeCO 3

(a)

Ocu

A ZnO

::

O

9

~

.

....

9

..o

9

(b)

(b 9

9

9

9

I0

20

30

~O~I)

40

50

60

70

20 / deg. Figure 2. XRD profiles of CAT A. (a) Before reaction. (b) After reaction for 114 h.

80

90

I0

20

30

40

50

60

70

80

20 / deg. Figure 3. XRD profiles of CAT B. (a) Before reaction. (b) After reaction for 190 h.

90

519

%~.~,

~,

,"

.~..-

.

,, ~....

.i' ~ ~" ~.

m m

:.~

9

.

,

. , . j.:!

Figure 4. Transmission electron micrographs of CAT A. (a) Before reaction. (b) After reaction for 114 h.

m

10 n m

(a)

.~.,,

9

i:,'

,.

. ~:,.':~: i..~!

,-'"L?~ .........

.._,

Figure 5. Transmission electron micrographs of CAT B. (a) Before reaction. (b) After reaction for 190 h.

m

10 n m

520 To clarify the reason of slow deactivation rate in the reaction using CAT B, we characterized the catalysts before and after the reaction by means of XRD analysis, transmission electron microscope observation, and energy-dispersive X-ray microanalysis. The XRD profile and the TEM of CAT A were shown in Figure 2 and Figure 4, respectively. The structure of CAT A before reaction was the same as that of Fe304 which has a spinel type structure. The small peaks assigned to CuO and ZnO were observed. The existences of metallic Cu and Zn could not be ascertained by XRD analysis. On the TEM observation, the CAT A before reaction consisted of the particles of uniform size, 10-20 nm diameters. The compositions of particles were uniform also. These results suggest that Cu and Zn components are dissolved into the spinel type structure. The CAT A after the reaction for 114 hours was identified as the mixture of Fe304, FeCO3, ZnO, and Cu by XRD analysis. On the TEM observation, the various sizes and shapes of particles were observed. On the microanalysis of composition, the segregation of Cu, Zn, and Fe components was observed. Based on these results, it is cleared that the K/Cu-Zn-Fe oxides catalyst is segregated to FeCO3, ZnO, and Cu during the reaction. This fact suggests that the deactivation of catalyst is caused by the segregation of catalyst components. The XRD profile and the TEM of CAT B were shown in Figure 3 and Figure 5, respectively. The structure of CAT B containing Cr component before reaction was as same as that of CAT A before reaction. Regardless of the long reaction time for 190 hours, the CAT B after reaction retained the spinel type structure. The existences of FeCO 3 and Cu were observed a little. On the TEM observation, the CAT B before reaction consisted of the particles of uniform size, 10 nm diameters. The compositions of particles were uniform. After the reaction, the particles of similar size, 10-20nm diameters, were observed. Most of the panicles had the same composition as that of panicles before reaction. The segregation of catalyst components was observed on only a few panicles. Based on these results, it is concluded that the addition of Cr component to the K/Cu-Zn-Fe oxides catalyst prevents the segregation of catalyst components during the reaction. This fact can explain slow deactivation rate in the reaction using CAT B. It is considered that the Cr component stabilizes thermally the structure of catalyst. 4. C O N C L U S I O N S We make clear the following facts in this study. The K/Cu-Zn-Fe oxides catalyst has a spinel type structure. The K/Cu-Zn-Fe oxides catalyst is deactivated by the segregation of catalyst components to FeCO3, ZnO, and Cu during the reaction. The segregation is prevented by the addition of Cr component to the catalyst. The long life of K/Cu-Zn-Fe-Cr oxides catalyst can be explained by its slow segregation rate. It is considered that the Cr component stabilizes thermally the spinel type structure of the catalyst.

REFERENCES 1. H. Arakawa and A.Okamoto, Chem. Chemical Ind., No.47, (1994) 1314. 2. M. Takagawa, A. Okamoto, H. Fujimura, Y. Izawa, and H. Arakawa, Proceedings of ICCDU IV, P-057, Kyoto, Japan, 1997, Elsevier.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

521

D e v e l o p m e n t o f stable catalysts for liquid-phase methanol synthesis from CO2 and H2 H. Mabu sea,T. Wat anab ea,M. Sait o b "Research Institute of Innovative Technology for the Earth(RITE) 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto,619-02, Japan bNational Institute for Resources and Environment(NIRE) 16-3, Onogawa, Tsukuba-shi, Ibaraki, 305, Japan

This work focuses on the investigation of the stability of catalytic activity in the liquid phase methanol synthesis process. Novel catalysts with a long-term stability have been developed by the addition of hydrophobic materials. The addition of hydrophobic materials were effective for slowing down the crystallite size growth and inhibition of deactivation of catalyst as compared with the original catalyst without modification.

1. INTRODUCTION Methanol synthesis from CO2 and H2 has received much attention as one of the most promising processes to convert CO2 into chemicals. Gas-phase methanol synthesis process should recycle a large quantity of unconverted gas and furthermore the single pass conversion is limited by the large heat release in the reaction. Liquid-phase methanol synthesis in solvent has received considerable attention, since temperature control is much easier in the liquid phase than in the gas phase. Several types of reactors have been proposed such as the liquid entrained reactor(I) and the Trickle bed reactor(2). The authors have been studying a liquid-phase methanol synthesis process in order to develop a new technology as an alternative for a gas-phase process, and reported that a new process employing liquid-liquid separation of the products from the solvent has several advantages in practical methanol synthesis(3). Lee et a1.(4,5,6) have studied the phenomenon of crystallite size growth and the post-treatment using carbon dioxide in Cu/ZnO-based methanol catalysts. They have concluded that the produced water is one of the most strongly suspected species promoting the crystallite size growth and the existence of ZnCO3 slows down the rate of crystallite size growth in the liquid phase methanol synthesis. In the present study, novel catalysts with a long-term stability for the liquid-phase methanol synthesis process have been developed by the addition of hydrophobic materials.

522 2. E X P E R I M E N T A L 2.1. Catalyst A Cu/ZnO-based multicomponent catalyst(Cu/ZnO/ZrO2/Al203) prepared by a conventional coprecipitation method was used in the present study. A mixture of aqueous solution of metal nitrates and an aqueous solution of Na2CO3 were added dropwise to distilled water. Subsequently, the precipitate was filtered out, washed with distilled water, dried in air at 393K overnight, calcined in air at 623K for 2 hr. Two kinds of hydrophobic silica and the special silicone oil replaced a part of methyl group in dimethyl silicone oil by hydrogen were employed for the hydrophobic treatment of the catalyst. Two kinds of hydrophilic silica were used for comparison. The hydrophobic silica and hydrophilic silica were mixed physically with the powder of calcined catalyst. The special silicone oil diluted with isopropyl alcohol to the prescribed concentration was impregnated with the powder of calcined catalyst, removed isopropyl alcohol in air at room temperature and polymerized in air at 523K. The catalysts were pelletized to ca. 5 x 20mm cylindrical pellets under a pressure of 20MPa and crushed to the size of 1-2ram.

2.2. Apparatus and procedures The activity of the catalyst was examined using a liquid-phase continuous reactor described elsewhere (3). The reaction conditions were as follows:temperature=523K,total pressure=15 Mpa, H2/CO2=3/1, recycle rate of solvent=100 l-solvent/l-cat./hr. XRD measurements were performed to analyze the structure of the catalyst. The contact angle between the catalyst and water was measured in order to evaluate the hydrophobicity of the catalyst. The thermal treatment in presence of mixture of water, methanol and solvent(n-dodecane) was performed in order to evaluate the modified catalyst in a brief period.

3. RESULTS AND DISCUSSION 3.1. Crystallite size of Cu The results in Table 1 show the contact angles before the thermal treatment and the crystallite sizes of Cu after the thermal treatment. The contact angles of the catalyst modified with the hydrophobic materials were larger than that of the catalyst without modification. The Cu crystallite sizes of the catalysts modified with the hydrophobic materials were smaller than

Table 1 Contact angles and Crystallite sizes of various modified catalysts Additive Hydrophobic silica A Hydrophobic silica B Special silicon oil Special silicon oil Hydrophilic silica A Hydrophilic silica B Original catalyst

Component (wt%) 10 10 5 10 10 2 m

Contact angle (degree)

Crystallite size of Cu (nm)

30 60 85 150 m ---

22.4 20.0 12.6 9.2 28.1 22.0 26.1

523 that of the catalyst without modification. The special silicone oil was most effective among another hydrophobic materials. It seems that the hydrophobicity of materials are connected with the sintering of Cu with the exception of the hydrophilic silica B. Another experiment showed that water caused a great growth of Cu crystallite size of the catalyst,while methanol had a small effect. These findings suggest that the addition of the hydrophobic materials to the catalyst could suppress the sintering of Cu particles in the catalyst. 3.2. A c t i v i t y

A long-term methanol synthesis test was performed using the catalyst modified with the special silicon oil(5wt%), the catalyst mixed physically with the original catalyst without modification and the catalyst modified with the special silicon oi1(10wt%)(50/50), and the catalyst modified with the hydrophilic silica B(l%). It was performed using the original catalyst without modification for comparison. Figure 1 shows the change in the activities of these catalysts. The activity of the original catalyst without modification decreased gradually with time. The deactivation of the catalyst modified with the hydrophilic silica B was lower than that of the original catalyst. On the other hand, the activities of the catalysts modified with the special silicon oil were relatively stable at higher level.

8O0 [ 7OO

/muI-i._....

ur

~

o+,-4b

/ 600


98%), the activity of those materials where calcium was added on pre-reduced Pd did not differ from that of the unpromoted Pd/SiO2 The present piece of work, in an effort to contribute in establishing a correlation between catalytic performance, catalyst structure and surface composition, presents combined results of reactivity in activation of syn-gas (CO/H2) and CO:M: mixtures. Hydrogen chemisorption (HChS), TEM, and XPS and Raman spectroscopic techniques have also been used.

Thanks are given to Universidad Nacional del Litoral and to CONICET. The continuous support of the Japanese International Cooperation Agency (JICA) is gratefully acknowledged.

534 2. EXPERIMENTAL

Palladium acetate in aqueous ammonia (pH = 11) was used to ion exchange Pd (2 % w/w) onto the pre-purified, calcined support. After air drying in stove, the Pd tetraammine complex was decomposed to the diaammine one at 423K. Part of the stock was used to prepare a first set (S Series) of Ca promoted catalysts where maximum Ca-Pd interaction was expected; a second part was H2 reduced instead (723K @ 2 K/min) to minimize the said interaction (R Series). Different amounts of Ca(AcO)2 were added by incipient wetness, in vacuo, to aliquots of both stocks (Ca/Pd = 0.1, 0.2, 0.5, 1.0, and 2.0 at/at) and then water was sublimated. Both series were calcined at 673K (@ 2 K/min) in dry, CO2-fi'ee air and then reduced in H2 at 723K. Catalytic activity was evaluated at 3.0MPa, 493-523K, using the following binary mixtures: CO/HE: 1/2.5 [B1], CO2/H2" 25/75 [B2] in a copper-plated differential microreactor using always the same W/F ratio. The reaction products were analyzed by GC (FID/TCD). Surface composition and oxidation states were determined with an ESCA PHI 5300 unit (Mg anode; pass energy of 8 eV; BE values referred to the Si 2p peak). TEM studies were performed in a JEOL-100 CX microscope. Raman spectra were obtained using a Jasco Laser Raman System (Model TRS-600SZ-P) equipped with a CCD 9000 detector. The 514.5-nm line of a 200 mW powered argon laser was used to excite the sample.

3. RESULTS AND DISCUSSION

The values of total surface area by BET and mean pore diameters indicate that the structure of the different materials is independent of the CaJPd ratio and/or the preparation method (Table 1). On the other hand, the size of the palladium crystallites is not modified by the presence of calcium only on the R Series catalysts. On the S Series materials, though, the larger the Ca contents is the larger the Pd crystallite diameter (TEM) becomes. The agreement between TEM and hydrogen chemisorption particle size data is very good. Table 1 Catalysts structure vs. Ca content and catalysts preparation method. Code Ca/Pd Stot(BET) Dpore dp(TEM) (*) (at/at) m2/$ (nm) (nm) SiO2 G-59 271 16.6 R0 0.0 266 16.8 1.4 R2 0.2 266 16.5 1.4 R3 0.5 259 1.4 R Series R4 1.0 259 1.5 R5 2.0 255 16.5 1.4 SO 0.0 274 16.7 1.5 $2 0.2 264 16.9 1.4 S Series $3 0.5 259 1.4 $4 1.0 256 1.5-2.0 (:l:) $5 2.0 232 17.2 1.4-3.5($)

FE (t) 0.70 0.69 0.71 0.68 0.65 0.75 0.75 0.71 0.61 0.41

dp(FE)

(nm) 1.6 1.6 1.6 1.6 1.7 1.5 1.5 1.6 1.8 2.7

(*) 2.0 % Pd w/w. (~') Fraction of exposed Pd (hydrogen chemisorption). (~) Bimodal distribution. The Pd 3d5/2 BE of the catalysts in the R Series was always 335.5 eV, and constant, which corresponds to small Pd metal particles [3]. Instead, the BE values decreased steadily in the S

535 Series, from 335.5 eV for Ca/Pd = 0 to 335.1 eV when Ca/Pd = 2. The latter result is consistent with TEM findings, as well as HChS data, which indicate that the Pd crystallites are larger the higher the Ca to Pd ratio is (Table 1). The Ca 2p3a signal was always typical of Ca 2+ (347.5 eV), with consistently shifted values (+ 0.7 eV) from those of bulk CaCO3 or CaSiO3, which can be attributed to final or initial state effects quite similar to those observable whenever an oxide is dispersed onto an inert support [4]. The presence of a C ls signal at 289.3 eV only in Ca-containing samples, and in both series, indicates the possible association of the carbonate anion with C a 2+ cations. The Raman spectra of high Ca loading Pd/SiO2 catalysts are shown in Fig. 3 together with the ones of the silica and reference materials.The band at 964 cm~ for the catalysts of both Series can be assigned to the support. The band at 1088 cm1 for sample R5 strongly resembles the features observed in CaCO3 spectra due to the A~g symmetric stretching for calcite [5]. However, the spectrum of $5 catalyst showed a broad band appearing at 1058 cm~ and a weak shoulder at 634 crn~ which are difficult to assign. While it is expected that the calcination of Ca(AcO)2 will proceed to calcium carbonate, the formation of a calcium carbonate silicate can not be ruled out. Therefore, these results together with XPS results confirm the presence of calcium carbonate on R Series and suggest the association of the carbonate and silicate anions with calcium cations on S Series. 9

_

I

9

I

9

i

~

A

,

.

S5

6

..aldlr

~ I

9

CalPd=2

CalPd=2

CalPd=d

v

,m

w c 0

G -59

m

. C a C

1400

0

s---;

'

. . . . . . . . . . . . .

1

2100

'

-~:-

.........

I

_. . . . . . . . . . . . . . . . . . . . .

,

1000 V

(cm

I

800

,

_. . . .

I

600

,

400

"1 )

Figure 1. Raman spectra of R5 and $5 catalysts (Ca/Pd=2.0) and the support (G-59) referred to calcite (CaCO3) and wollastonite (CaSiO3). TEM results on the series S showed small (1.5-2.0 nm) Pd particles on the unpromoted Pd/SiO2 and larger ones (3-5 nm) for Ca/Pd = 2; the former were structurally unresolved. Nevertheless, the XPS spectra gave identical C/Si surface signals in both series. Moreover, the HChS measurements of exposed metal fraction indicate that the average size of the Pd crystallites coincides with that evaluated from TEM data, and allows us to conclude that the noble metal is not significantly covered by either C nor Ca. As said above, we have reported recently that the catalytic activity toward methanol synthesis from a [B 1] mixture is strongly affected by both the method of addition of the Ca promoter to the Pd-loaded silica and the Ca/Pd ratio. Catalysts of the S Series can be 40-fold

536 more active (Ca/Pd=2) than unpromoted Pd/SiO2 or any from the R Series. Using the [B2] mixture, however, the increase in the TORcH3OHowing to the Ca addition was independent of the Ca/Pd ratio or the preparation method (Figure 2.a). a

40-

b

60-

rlCO~ i COZfl~

II

30-

~1~ 45 20

F

]

Dcom2

.1, 4

0 R0 R2 R4

S0

S2

S4

R0 R2 R4

S0

S2

S4

Figure 2. Activity to methanol as a function of the catalysts preparation method and the reaction mixture (T=523 K, P= 3 MPa, SV = 104 hl). a) Turnover rates; b) Specific rates. Yet, as the fraction of exposed Pd at pseudo steady-state conditions was severely reduced upon exposure to the [B1] mixture (FE _:

Recycl i ng rat io " 4 maN/maN, GHSV " 5000 h-1 Make-up gas composition " H2/C02=75/25 mol%

6o

A D'

Pressure [MPa]

9

lo ,,

~ ' s2o

--A--Z~

6_ 51o E Q~

I"1

I--. 500

i

4O0

I

I 800

I

I 1200

I

I 1600

Time

Fig.4.

I

I 2000

I

I 2400

I

I 2800

I

3200

[h]

Result of durability test on M-2 catalyst underrecycledunreacted gases

4. CONCLUSIONS Two kinds of CuO-ZnO-A1 2 0 3 catalyst with different A1 z O 3 content named M-1 and M-2 for methanol synthesis from carbon dioxide were prepared. Both the catalysts exhibited higher activity than the commercial methanol synthesis catalyst and the commercial CO shift catalyst. The initial activity of M-1 and M-2 catalysts were neary equal to the equiliblium methanol yield (25%) at 523K. M-2 catalyst containing 5 wt% A1 2 0 a showed better durability than M-1 catalyst containing 8 wt%A1 2 0 3 . Those results suggest that minor difference of A1 2 0 3 contents in catalyst composition cause considerable difference in durability despite a little difference in initial activity. Additionally, M-2 catalyst showed about 95% methanol yield for 3000 hours by gradually raising the temperature from 513K to 521K at 9MPa with recycling of the unreacted gases. REFERENCES

1. H.Kumazawa, Kemikaru Enjiniaringu, 2 (1993) 22-26 2. Y.Arakawa, Shokubai, 35 (1993) 96-97 3. T.Fujitani, M.Saito, Y.Kanai, M.Takeuchi, K.Moriya, T.Watanabe, M.Kawai and T.Kakumoto, i b i d , 35 (1993) 92-95 4. M.Saito, i b i d , 35 (1993) 485-491 5. T.Inui, Kemikaru Enjiniaringu, 2 (1993) 61-65

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

549

Optimization of preparation conditions and improvement of stability of Cu/ZnObased multicomponent catalysts for methanol Synthesis from CO2 and H2 S. Luo a, J. Wu a, J. Toyir a, M. Saitob, M. Takeuchi c and T. Watanabe e aResearch Institute for Innovative Technology for the Earth (RITE), 16-30nogawa, Tsukubashi, Ibaraki 305, Japan* bNational Institute for Resources and Environment (NIRE), 16-30nogawa, Tsukuba-shi, Ibaraki 305, Japan eRITE, 9-2 Kizukawadai, Kizu-cho, Soraku-gun, Kyoto 619-02, Japan

The preparation conditions of Cu/ZnO-based multicomponent catalysts were optimized. The temperature during copreciptation should be less than 313 K, and the removal of Na from the catalyst by washing the precipitates is most important. The addition of a small amount of silica to the catalyst greatly improved the stability of the catalyst.

1. INTRODUCTION Catalytic hydrogenation of CO2 to produce various kinds of chemicals and fuels has received much attention as one of the most promising options for mitigating CO2 emission. In particular, methanol synthesis by CO2 hydrogenation has been considered to play an important role in the transportation of hydrogen energy produced from natural energy such as solar energy, hydropower and so on. At ICCDU-III held at Oklahoma in 1995, the authors reported that highly active Cu/ZnO-based multicomponent catalysts had been developed after elucidatng the role of metal oxides contained in a Cu/ZnO-based catalysts [1 ]. In the present conference, the effects of the conditions for preparing Cu/ZnO-based multicomponent catalysts on their methanol synthesis activities and the effect of silica on the stability of the catalyst in a long-term methanol synthesis operation will be reported.

2. EXPERIMENTAL Cu/ZnO-based multicomponent catalysts (Cu/ZnO/ZrO2/A1203/Ga203 and Cu/ZnO/ZrO2/A1203) were prepared by a coprecipitation method using Na2CO3 as a precipitant, as described in detail elsewhere [2]. The operation conditions for preparing the catalyst such as the temperature during coprecipitation, the time of aging precipitate, the extent of washing the precipitate and so on were varied. A small amount of silica was added New Energy and Industrial TechnologyOrganization (NEDO)research fellow

550 to the catalysts by using a colloidal silica supplied by Nissan Chemical Co. A fixed bed flow reactor was used both for short-term methanol synthesis tests and for long-term tests. The surface areas of the catalysts were measured by N2 adsorption with a flow method. The Cu surface areas of the catalysts were measured by the same way as described elsewhere [2]. Xray diffraction measurements were performed for analyzing the structure of the catalysts.

3. RESULTS AND DISCUSSION

3.1. Optimization of preparation conditions of the catalyst Among the various operation conditions for preparing the precipitate, only the temperature during the coprecipitation had a significant effect on the activity of the multicomponent catalyst. This finding is favorable for preparing a practical catalyst. No significant difference has been observed among the activities of the catalysts prepared at temperatures between 273 K and 313 K, whereas the activity of the catalyst prepared at 333 K was slightly (only 7%) lower, as shown in Figure 1. XRD measurements showed that the crystallite size of the precipitate prepared at 333 K was slightly larger than those of the precipitates prepared at temperatures ranging from 273 K to 313 K. Sodium (Na) remaining in the catalyst after washing the precipitate with distilled water greatly decreased the activity, as shown in Figure 2. The precipitates after being washed different times and then dried overnight at 383 K gave almost the same XRD patterns, whereas the XRD patterns for the catalysts calcined at 623 K became sharper with the increase in the amount of Na in the catalyst. These findings clearly indicate that the Na remaining in the catalyst causes the crystallization of the components in the catalyst, and thus decreases the surface area, the Cu surface area and the activity of the catalyst.

30

800

~" 800 -

O

o 600

2o~

30

600 q |

"~ 400

*6 400

t~

8 o

~200

200

0

0

273 293 313 333 Temperature during coprecipitation (K)

Figure 1. Effect of temperature in coprecipitation on the activity (D) and Cu surface area (e) of a Cu/ZnO/ZrO2/A1203/Ga203 catalyst, Reaction conditions : 523 K, 5 MPa, SV=I 8,000, feed gas composition=CO2(25)/H2(75 )

0

5 1 2 3 4 Amount of Na in the catalyst (wt%)

Figure 2. The activity (o) and the Cu surface area (e) of the multicomponent catalyst as a function of the amount of Na in the catalyst. Reaction conditions were the same as shown in Figure 1.

551

3.2. Improvement of long-term stability of the catalyst The addition of a small amount of silica to the catalyst was found to improve a long-term stability of the catalyst. Calcining the catalyst at high temperatures of around 873 K is also important for stabilizing the catalyst activity. The activity of a Cu/ZnO/ZrO2/A1203 catalyst containing silica and calcined at 873 K decreased by 10% during initial 40 h in methanol synthesis, but after that no significant decrease in the activity was observed until 500 h, as shown in Figure 3. On the other hand, the activity of the catalyst without silica decreased monotonously and was not stabilized until 500 h. Table 1 shows both the surface area and Cu surface area of the catalysts without and with silica as a function of time on stream in methanol synthesis. Both the surface area and the Cu surface area changed in the same manner during the methanol synthesis as the catalytic activity.

80o 700~A~

o

%00

.~,~~o,~o

600 rk,,

A~ Z ~ A

1w1-~

\--x-I~1-x-- ~ CO+OH"

-~ 0

1 O0

"10 0 i,..

80

',0 tO

60

9-

20

a

Solvation

H

--~ HCO0

b) high - polar solvents

r

1 O0

0 L

80

'*-0

60

b

c

d

e

(B)

g 40

.~

~U_

20

a

Scheme 1 Illustration of hypothetical reduction pathway of CO 2 on photocatalyst surface. M: metal ion site on the surface.

(A)

40

a) l o w - polar solvents

I

HCOOH

fl co

b

c

d

e

Photocatalyst

Figure 3 The fraction of C O 2 reduction products obtained by irradiation of various surface-modified CdS photocatalysts in acetonitrile (A), and dichloromethane (B). Photocatalysts of (a) to (e) were the same as those given in Table 1.

556 Table 1 The molar ratio of adsorbed thiol to CdS determined by elemental analyses.

Catalyst

Surface-modification condition

a

none

b

Ratio of adsorbed thiol to CdS (mol%)

surface coverage (%)

0

0

0.1M NH2(CH2)2SH

0.60

29

c

0.1M n-C 12H25SH

0.65

30

d

0.1M NaSO3(CH2)2SH

0.90

45

e

0.5M n-C lzH25SH

1.30

65

In order to confirm the above hypothesis, photoreduction experiments of C O 2 w e r e performed with use of the surface-modified CdS particles. Figure 3 shows the fraction of formate and carbon monoxide in CO 2 reduction products obtained for use of various kinds of CdS photocatalysts in acetonitnle and dichloromethane. It is clearly seen that the fraction of formate was varied greatly depending on the kind of CdS photocatalyst used, and in both solutions, it increased in the order of photocatalyst (a) to (e). Though the same modifier of 1-dodecanethiol was used in the photocatalysts (c) and (e), the fraction of CO 2 reduction products was greatly different. Table 1 shows the molar ratio of the adsorbed thiol compounds to CdS determined by elemental analyses, and also listed is the surface coverage with thiol compounds which were obtained by using the molar ratio of thiols to CdS and the average diameter of 50 nm of CdS particles and by assuming that the surface density of Cd 2+ sites was the same as that of (0001) facet of hexagonal CdS. Comparing the results in Figure 3 with the surface coverage by thiol compounds on the CdS surface shown in Table 1, the fraction of formate production seems to be related to the degree of the surface coverage, and the presence of the functional groups o f - N H 2 a n d - S O 3- in thiol compounds did not greatly alter this tendency. It seems likely that CO2o-anion radicals in low polar solvents tend to predominantly adsorb on the positively charged Cd 2+ sites, resulting in carbon monoxide formation. Since the thiol compounds should be bound to the surface Cd 2+ sites, the amount of the adsorbed CO,~oon Cd 2+ sites must decrease with increase of the degree of surface modification, and then formate production becomes predominant with increasing the surface coverage. REFERENCES

1. 2. 3. 4. 5. 6

T. Inoue, A. Fujishima, S. Konishi, and K. Honda, Nature 277 (1979) 637. A. Henglein, M. Guttieretz, and C. H. Fischer, Ber. Bunsenges. Phys. Chem. 88 (1984) 170. H. Inoue, R. Nakamura, and H. Yoneyama, Chem. Lett. (1994) 1227. M. Kanemoto, M. Nomura, Y. Wada, T. Akano, and S. Yanagida, Chem. Lett. (1993)1687. Z. Goren, I. Willner, A. J. Nelson, and A. J. Frank, J. Phys. Chem. 94 (1990) 3784. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B. 101 (1997) 2632. 7. H. Inoue, T. Matsuyama, B. -J. Liu, T. Sakata, H. Mori, and H. Yoneyama, Chem. Lett. 653 (1994).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

557

Applications of electrospray mass spectrometry and high performance liquid chromatography in the elucidation of photocatalytic CO2-fixation reactions H. Hori a, J. Ishihara a, M. Ishizuka a, K. Koike a K. Takeuchi a T. Ibusuki a, and O. Ishitani b aNational Institute for Resources and Environment, 16-3, Onogawa, Tsukuba 305, Japan bGmduate School of Science and Engineering, Saitama University, Urawa 338, Japan Photocatalytic CO2-fixation using [Re(bpy)(CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), PEt3 (2), PPh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), P(OEt)3 (6)] was examined by electrospray mass spectrometry and high performance liquid chromatography. The complexes 1, 2, 4, $ and 6 remained at close to initial levels during the catalytic CO formation and a formate complex Re(bpy)(CO)3OC(O)H (7) was detected after prolonged irradiation. On the other hand, 3 changed completely to solvent (DMF, triethanolamine) complexes with a quantum yield of >>1 before catalytic CO formation, followed by the generation of 7. The ligand exchanges with solvent molecules were explained in terms of a chain reaction mechanism.

1. INTRODUCTION Rhenium bipyridine complexes have received a great deal of attention because they act as photoreduction catalysts from CO2 to CO in the presence of amine with high selectivity and high efficiency [ 1-3]. The first step of the reaction is an electron transfer from the amine to the excited complex. However, the subsequent processes are not clear. Difficulties in the elucidation of the reaction mechanisms are ascribed to the instability of the intermediates. Furthermore, the presence of a highly non-volatile amine, e.g., triethanolamine (TEOA), makes it more difficult to isolate the intermediates. FJectrospray mass spectrometry (ESMS) is a powerful tool for identifying unstable complexes and high performance liquid chromatography ( ~ ) is also useful for the quantification of relatively stable species such as the initial and f'mal complexes during the reaction. A great advantage of these techniques is that there is no requirement for pretreatment, in other words, the reaction solutions can be directly subjected to the measurements. In this work, we examined CO2-photoreduction by six rhenium complexes [Re(bpy)(CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), PEt3 (2), VPh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), P(OEt)3 (6)] using ESMS, HPLC, and in situ UV/vis spectral measurements. 2. EXPERIMENTAL The complexes [Re(bpyXCO)3L] + were prepared according to a previous paper [4]. A high-pressure mercury lamp with suitable glass filters was used to obtain fight at 365 or >330 nm. As a typical photochemical run, a TEOA-DMF (1 : 5, v/v) solution of the complex was placed in a glass cell. The solution was purged with CO2 and then irradiated. After irradiation, the gas was analyzed by gas chromatography while the liquid was analyzed by HPLC and

558 ESMS [3]. In situ UV/vis spectra were measured by a spectrometer connected to the photochemical cell through optical fibers. e0 3. R E S U L T S AND D I S C U S S I O N 60 3.1. Photocatalytic reduction of CO2 All of the complexes caused catalytic reduction of ~ 40 CO2 to CO. Fig. 1 shows the time conversion plots of CO formation by 365 nm irradiation. After J so irradiation started, CO was generated linearly with irradiation for about 4 hours, and then the rate o m reduced. The quantum yields of CO formation (Oco) 10 of 1-6 in the steady CO formation period were 0.02, 0.01, 0.05, 0.20, 0.20, and 0.17, respectively (fight o - 0 4 8 12 16 intensity = 1.27 • 10 .8 einstein s- 1). The O c o values Irradiation time I h of the phosphine complexes (1-3) were much lower Fig. 1. than those of the phosphite complexes (4-6). It is Rots of CO fommtion against irradiation time. A TEOA-DMF solution of each known that the electron-acceptor strength of phosphite complex (2.6 mM) was irradiated (365 nm) is higher than that of corresponding phosphine. The under CO2. tendency observed here indicates that the elecmm2.0 acceptor strength of L significantly affects the catalysis. 1.5

I

3.2. In situ UV/vis spectra Fig. 2 shows the in situ UV/vis absorption spectra of $. When irradiation started, new bands around 400 and 500 nm appeared. These bands are attributable to the one-electron reduced species $ ' , comparable with the spectrum of $" produced by an electrochemical technique [2]. The spectral changes of 1, 2, 4, and 6 were similar to that of $. The reduced species were generated with yields of 20 100 % during irradiation, indicating that they act as precursors for CO formation. On the other hand, the in situ spectra of 3 were much different (Fig. 3). After accumulation of 3", another absorption maximum at -380 nm was found and no further changes occurred. This final species was extracted with dichloromethane- water and then purified by silica gel chromatography using methanol - ethyl acetate as an eluenL It was identified as the formate complex Re(bpy)(CO)3OC(O)H (7) on the basis of its NMR, IR, and UV/vis spectra.

1.0 O.5 0.0 8O0

.

4OO

SO0

eO0

Wavelength / nm

Fig. 2.

In silu UWvis spectra of TEOA-DMF

solution containing $ under CO2 during irradiation (365 nm). Timeinterval is 20 s 2.O

1.5

,~1.0

:

-.-~,

0.5

o,o

300

Fig. 3.

400

~::-~---500

600

Wavelength I nm

In sire UV/vis spectra of TEOA-DMF

solution containing 3 under CO2 during irradiation (365 nm) for (a) 0, (b) 70 and (c) 230 s.

559 3.3. H P L C Fig. 4 shows a HPLC chromatogram of $ after prolonged 365 nm irradiation. The starting complex $ remained at close to initial levels during the steady CO production. After prolonged irradiation, a peak corresponding to 7 appeared. The yields of 7 after 13 h were 3-11% for the complexes except 3. It is known that 7 has low CO producing ability [ 1,3]. Hence, a reason for the decrease in CO production rate after prolonged irradiation is the formate complex formation. Comparatively, the complex 3 showed drastic changes in H ~ C with irradiation (Fig. 5). When irradiation started, 3 showed an 81.6 % loss even after 5 s of >330 nm irradiation accompanied by the a ~ of new peaks, X and Y. Further irradiation caused decreases in X and Y, which resulted in the formation of 7. When the yield of 7 attained a maximum (52.2 % based on 3 used) and all of the other species disappeared, the turnover number (TN) of CO formation was 1.03. On the other hand, the overall CO formation process occurred with a TN reaching 12. These findings indicate that 7 acts as a real photocatalyst of CO2 reduction using 3. As for the species corresponding to peaks X and Y, they could not be isolated due to their instability. However, when excess CI- was added and then warmed after the solution had been irradiated until X and Y attained maxima, Re(bpy)(CO)3Cl formed with a 78.3 % yield. Therefore, these two species have an easily removable ligand, such as a solvent molecule. Consistently, [Re(bpy)(CO)3(DM~] + (8) showed the same retention time as that of X. To obtain more information on these intermediates, ES-mass spectra were measured. Solvent peaks Solvent peaks 5

[

,__

Fig. 4. HPLC chromatogram of $ after 13 h irradiation (365 nm). Retention time of $ is 32.5 min.

i, -l i

,~'l[t

'%"

b

I m

..

7

3.4.

ESMS Fig. 6 shows changes in the ES-mass spectra of 3 upon irradiation. Before irradiation, a sole peak of 3 (m/z = 689) was detected. After irradiation, another peak (m/z = 576) for a single positively-charged species was detected. This is clearly attributable to [Re(bpyXCO)3(TEOA)] + (9). Although the presence of 8 was expected from HPLC, no corresponding peak was observed. Presumably, 8 should be labile enough to undergo substitution of the DMF Ugand with an anion such as OMe- or OH- present in the mobile phase. This ligand substitution produces neutral complexes, which are hard to detect by this method. In fact, the neutral species 7 gave no peaks

C I

_ I

I

_I'-,~ jl ,

~. 0

5

/,_

~0

,'-7--'-~., /

~s

Retention

'~,s

,

,"

50

_

I.

ss

time / min

Fig. 5. Changes in HPLC chromatograms due to irradiation for (a) 0, (b) 5, (c) 600 and (d) 1200 s. A TEOA-DMF solution containing 3 was irradiated (>330 nm) under CO 2.

560 in the ES-mass spectra. Therefore, Y was identified as the TEOA complex 9 whereas X is supposed to be derived from the DMF complex 8. :i The photochemical fommtion of 8 and 9 from 3 should be a chain reaction because the quantum yield of the decrease in 3 was 16.9 upon irradiation at 365 i nm with an intensity of 8.30 • 10-10 einstein s-1. The mechanism can be e x p ~ as follows. 300 3 + hv -* 3* ( 3 ~ ) 3* + TEOA ~ 3" + TF~A .+ 3" + DMF or TEOA ~ 8" or 9" + PPh3 3 + 8 " org" --- 3" + 8 o r 9

(1) (2) (3) (4)

689

,.

,

400

500

,

,

nVz

600

b

700

800

689

d 576

The excited species 3" generated by irradiation (eq. 1) undergoes an electron transfer with TEOA to give 300 the reduced species 3" (eq. 2), which then undergoes the substitution of the PPh3 ligand with TEOA or c DMF to give S" and 9" (eq. 3). Subsequent electron :i exchange of 8" and 9" with another 3 leads to the d formation of g and 9 accompanied by the regeneration of 3" (eq. 4). I

.......

t.,al&,_l.=.

400 ,

9 ._

500

s.

m/z

L~..I

600

,

I._.

. . . . .

700

,

800

,

576

J

4. C O N C L U S I O N 700 800 The photocatalytic CO2-fixation using [Re(bpy)- 300 400 500 m/z 600 (CO)3L] + [bpy = 2,2'-bipyridine, L = P(n-Bu)3 (1), Fig. 6. PEt3 (2), PVh3 (3), P(OMe)Ph2 (4), P(Oi-Pr)3 ($), Changes in ES-mass spectra of 3 due to P(OEt)3 (6)] was examined by ES-mass specUv- irradiation for (a) O, (b) 5 and (c) 600 s. metry and HPLC. In the case of 1, 2, 4, $, and 6, the initial complexes remained largely undimimshed during the catalytic CO formation, and a formate complex 7 produced after prolonged irradiation which caused a decrease in the catalytic activity. In contrast, 3 was rapidly substituted with solvent to form [Re(bpy)(CO)3(DMF)] + (8) and [Re(bpy)(CO)3(TF.DA)] + (9) prior to catalytic CO formation by a chain reaction mechanism, followed by the formation of 7. These differences indicate that the nature of the phosphorus ligands (steric bulk, electron-accepter strength, etc.) is very important for the design of CO2-reduction photocatalysts. REFERENCES 1. J. Hawecker, J.- M. Lehn and R. Ziessel, Helv. Chim. Acta, 69 (1986) 1990. 2. H. Hod, F. P. A. Johnson, K. Koike, O. Ishitani and T. lbusuki, J. Photochem. Photobiol. A: Chem., 96 (1996) 171. 3. H. Hod, F. P. A. Johnson, K. Koike, K. Takeuchi, T. Ibusuki and O. Ishitani, J. Chem. Soc., Dalton Trans., (1997) 1019. 4. H. Hod, K. Koike, M. Ishizuka, K. Takeuchi, T. lbusuki and O. Ishitani, J. Organomet. Chem., 530 (1997) 169.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

561

Photocatalytic reduction of CO2 with H20 on Ti/Si binary oxide catalysts prepared by the sol-gel method Hiromi Yamashita a, Shinchi Kawasaki a, Masato Takeuchi a, Yo Fujii a, Yuichi Ichihashi a, Yasuo Suzukib, Sang-Eon Park c, Jong-San Chang c, Jung Whan Yoo c, Masakazu Anpo a* a'Department of Applied Chemistry, Osaka Prefecture University, Gakuen-cho, Sakai, Osaka 599, Japan bIon Engineering Research Institute Corporation, Hirakata, Osaka 577, Japan CKorea Research Institute of Chemical Technology, Daeduck Science Town, Daejeon, 305-606, Korea

Titanium-silicon (Ti/Si) binary oxides prepared by the sol-gel method with different Ti contents exhibit high photocatalytic reactivity for the reduction of CO2 with H20 to form CH4 and CH3OH. A dramatic enhancement in the photocatalytic reactivity was found in regions of lower Ti content. In situ spectroscopic investigations of these Ti/Si binary oxides indicate that the titanium oxide species are highly dispersed in the SiO2 matrices and exist in tetrahedral coordination. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the oxides as a function of the Ti content clearly indicates that the high photocatalytic reactivity of the oxides having a low Ti content is associated with the high reactivity of the charge transfer excited state of the titanium oxide species in tetrahedral coordination. 1. I N T R O D U C T I O N

Thc utilization of solar energy through chemical storage can be achieved by the photocatalytic activation of light-sensitive catalytic surfaces. The photocatalytic reduction of CO2 with H20 is of special interest and one of the most desirable goals in this field [1]. Although, recently, the photocatalytic reduction of CO2 with gaseous H20 to form CH4 and CH3OH has been found to proceed on the titanium oxide catalysts highly dispersed on silica and zeolites at room temperature [2-4], the efficiency of the photocatalytic reaction is still low. It has been shown that the "sol-gel method" is an effective and fascinating way to design highly active photocatalysts [4-6]. The present study deals with the preparation of Ti/Si binary oxide photocatalysts by the sol-gel method and the relationship between the local structure of the titanium oxide species and its photocatalytic properties in the photoreduction of CO2 with H20 at 328 K.

562 2. E X P E R I M E N T A L

Ti/Si binary oxides having different Ti contents were prepared by the sol-gel method from mixtures of tetraethylorthosilicate and titaniumisopropoxide. Ti/Si gels were obtained by keeping the mixture at room t e m p e r a t u r e for several days, washed with sufficient amounts of boiled w at er and then calcined in dry air at 725 K for 5 h. The Ti/Si binary oxides were crushed and sieved to 0.25-mm-size particles. Prior to spectroscopic measurements and photocatalytic reactions, the catalysts were treated with 02 at 725 K for 2 h and then evacuated for 2 h at 475 K. The photocatalytic reduction of CO2 with H 2 0 was carried out with the catalysts (150 mg) in a quartz cell connected to a conventional vacuum system. UV irradiation of the catalysts in the presence of CO2 (24 pmol) and gaseous H20 (120 pmol) was carried out using a high-pressure Hg lamp (~ > 280 nm) at 328 K. The reaction products collected in the gas phase were analyzed by gas chromatography. 3. R E S U L T S AND D I S C U S S I O N XRD patterns of the binary oxides exhibited only diffraction lines which were attributed to the crystalline anatase phase of TiO2. When the Ti content was decreased, these X-ray diffraction lines decreased in intensity and finally disappeared, indicating t h a t the t i t a n i u m oxide species were present in amorphous structure within the SiO2 matrices. FT-IR spectra of the oxides showed a peak due to the stretching of the Si-O- of the Si-O-Ti bond at around 950 cm -1. The binding energy of the Ti(2p3/2, 5/2) XPS bands shifted to higher values when the Ti content was decreased, especially the less than 20 wt% as TiO2. This shift could be attributed to the smaller relaxation energy for the highly dispersed titanium oxide species as compared to powdered bulk TiO2. UV-Vis absorption spectra of the Ti/Si binary oxides with low Ti content exhibited a large shift towards shorter wavelength regions. This large shift was attributed to the size quantization effect arising from the presence of extremely small t i t a n i u m oxide particles and/or the presence of highly dispersed titanium oxide species having a low coordination number. These results obtained by XRD, IR, XPS and UV-Vis absorption m e a s u r e m e n t s clearly show that a decrease in the Ti content causes the crystalline structure of the titanium oxides in SiO2 matrices to change from aggregates in an a n a t a s e phase to ultrafine t i t a n i u m oxide species with an amorphous structure and eventually to isolated titanium oxide species having a local coordinate geometry different from those of the crystalline anatase TiO2. As shown in Fig. 1, the Ti K-edge XANES and FT-EXAFS spectra of the binary oxides with 80 wt% TiO2 content are similar to those of pure anatase TiO2. On the other hand, the oxides with a low TiO2 content (< 20 wt% TiO2) exhibit a single and intense preedge peak in the XANES spectra, indicating t h at the titanium oxide species are present in tetrahedral coordination. The corresponding FT-EXAFS spectra indicate the presence of isolated titanium oxide species. ESR signals due to the Ti 3+ ions g e n e r a t e d by the photoreduction of the oxides with H2 at 77 K also indicated the presence of

563 f, tetrahedrally coordinated 81 Ti 0 FT-EXAFS t i t a n i u m oxide species in the XAN ES ~ 1 wt,,y~ oxides with low Ti content, c O 5 As shown in Fig. 2, the Ti/Si ~ 4 L i wt% b i n a r y oxides h a v i n g low Ti o content of less t h a n 20 wt% as < TiO2 exhibit a c h a r a c t e r i s t i c N p h o t o l u m i n e s c e n c e s p e c t r a at E around 490 nm upon excitation TiO2 at around 280 nm at 77 K. The z observed photoluminescence --if| J_ spectra are in good a g r e e m e n t 0 2 4 6 4960 5000 5040 with those of highly dispersed Distance / Energy/eV tetrahedrally coordinated t i t a n i u m oxides anchored onto Fig. 1. The Ti K-edge XANES and FTVycor glass where the EXAFS spectra of the Ti/Si binary oxides. a b s o r p t i o n of UV l i g h t at around 280 nm brought about 250 an electron t r a n s f e r from the lattice (A) (a) H20 oxygen (O/2-) to the t i t a n i u m ion (Ti/4+) 5 to form a charge t r a n s f e r excited state, (Ti3+mO-) * [1,2]. These findings clearly .~ 125 show the presence of highly dispersed .m t i t a n i u m oxide species in the oxides c" h a v i n g a t e t r a h e d r a l coordination. On cthe o t h e r h a n d , Ti/Si b i n a r y oxides o c 0 having a large Ti concentration did not 0 exhibit any photoluminescence. (B) C02 (a) . .c_" As shown in Fig. 2, the addition of H20 E or CO2 molecules onto the Ti/Si binary 0 oxides leads to an efficient quenching of ~r " 100 the photoluminescence and shortening of I% its lifetime, their extent depending on the a m o u n t of a d d e d gasses. Such an efficient quenching of the 0 I p h o t o l u m i n e s c e n c e w i t h CO2 or H 2 0 300 475 650 i n d i c a t e s t h a t a d d e d CO2 or H 2 0 Wavelength / nm interacts and/or reacts with the titanium Fig. 2. Photoluminescence oxide species in the excited state. For the spectrum (a) of the Ti/Si binary quenching of the photoluminescence in oxide (5 wt% as TiO2) upon its i n t e n s i t y and lifetime, CO2 is less excitation at 280 nm at 77 K effective t h a n H20, indicating t h a t the and the effects of addition of i n t e r a c t i o n of CO2 w i t h the c h a r g e gasses. The a m o u n t of added t r a n s f e r excited s t a t e of the t i t a n i u m gasses: (A) H20; b, 0.1, c, 0.5, d, oxide species is weaker t h a n H20. 1.0, e, 5.0, f, 10, (B) CO2; b, 0.5, c, UV i r r a d i a t i o n of the Ti/Si b i n a r y 1.0, d, 5.0, e, 10 Torr. oxides in the p r e s e n c e of a gaseous

oF A ' I P

564 mixture of CO2 and H 2 0 was 150 found to lead to the reduction of : 6 Yields of C1 products I 5 CO2 to produce CH4 and CH3OH APotoluminesc,ence Intensity ] ~ as the m a i n products. The .~ .zphotocatalytic reaction rate and 100 ~f::: E 4 selectivity for the formation of =L f:: u C H 3 O H strongly depended on I {9 the Ti content of the oxides. The t-(D {9 oxides having a low Ti content ~" 2 50 ~ exhibits high r e a c t i v i t y and r E selectivity for the formation of CH3OH (22 mol% on the binary ~ oxide at 1 wt% as TiO2) while the ~_ 0 formation of CH4 was found to be o, 0 20 40 60 80 100 the major reaction on bulk TiO2 Ti Content / wt% asTiO2 (selectivity of CH3OH formation: F i g . 3. The effects of the Ti/Si 1 mol%) as well as on the oxides composition of Ti/Si binary oxides on having a high Ti content. As their photoluminescence yields and shown in Fig. 3, the parallel specific photocatalytic reactivities for relationship between the specific the photocatalytic reduction of CO2 photocatalytic reactivity of the with H20 to produce CH4 and CH3OH titanium oxide species and the at 328 K. photoluminescence yields of the oxides clearly indicates that high photocatalytic reactivity and selectivity for the formation of CH3OH is closely associated with the high reactivity of the charge transfer excited complex of the highly dispersed tetrahedrally coordinated titanium oxide species. XAFS and photoluminescence investigations of the oxides indicated that these catalysts can sustain tetrahedral coordination of the titanium oxide species until the Ti content reaches up to approximately 20 wt% as TiO2. Thus, it can be seen that the Ti/Si binary oxides having a Ti content of less t h a n 20 wt% can be successfully utilized as active photocatalysts for the efficient reduction of CO2 with H20 at 328 K. (9

O

e-

9

0 r-

REFERENCES

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

M. Anpo and H. Y a m a s h i t a , in Heterogeneous Photocatalysis, M. Schiavello. (ed.), John Wiley & Sons, London, 1997 in press. M. Anpo and K. Chiba, J. Mol. Catal., 74 (1992) 207. M. Anpo, H. Yamashita, Y. Ichihashi, Y. Fujii, and M. Honda, J. Phys. Chem. B, 101 (1997) 2632. M. Anpo, H. Yamashita Y. Ichihashi, and S. Ehara, J. Electroanal. Chem., 396 (1995) 21. S.C. Moon, M. Fujino, H. Yamashita, and M. Anpo, J. Phys. Chem. B., 101, (1997) 369. H. Yamashita, S. Kawasaki, Y. Ichihashi, M. Harada, M. Anpo, M. A. Fox, J. M. White, C. Louis, and M. Che, Chem. Mater, 1997 (in press.).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

565

Photoelectrochemical reduction of CO2 at a metal-particle modified p-Si electrode in non-aqueous solutions Yasushi Nakamura, Reiko Hinogami, Shinji Yae, and Yoshihiro Nakato* Department of Chemistry, Graduate School of Engineering Science, and Research Center for Photoenergetics of Organic Materials, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560, Japan A p-type silicon (p-Si) electrode modified with copper particles (particulate-Cu/p-Si) was applied to photoelectrochemical (PEC) reduction of carbon dioxide (CO:) in acetonitrile electrolyte solutions with and without 3.0 M H20. The particulate-Cu/p-Si electrode generated high photovoltages of 0.50 to 0.75 V, and produced methane, ethylene, etc., under addition of 3.0 M H:O, similar to a Cu metal electrode, indicating that the particulate-Cu/pSi electrode acted as an efficient electrode for the PEC reduction of CO2 in non-aqueous solutions. 1. INTRODUCTION Photoelectrochemical (PEC) reduction of CO: with a p-type semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from a view-point of the global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made [ 1], but the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. We reported [2-4] that a p-Si electrode modified with small metal (Cu, Au and Ag) particles worked as an ideal-type electrode for the PEC reduction of CO2 in aqueous solutions. In the present paper we will report that the electrode of this type is also effective for the PEC reduction of CO: in non-aqueous solutions which have high CO2 solubility. ///sio2 A schematic cross-section of D - ...... h v the present-type electrode is shown ...... in Figure 1. Metal is deposited on p....." Si sparsely in the form of small D -metal particle "#~,," ~ " ,,:~ % , particles. The ideal size of the Cu e t c Si/metal contact areas and their separation are estimated I theoretically to be about 5 and 20 CH4 etc. nm, respectively [5]. Photogenerated holes in p-Si enter into its interior, D C O 2 + H§ whereas photogenerated electrons non-a q u e o u s come out to the surface due to the electrolyte presence of band bending near the pSi surface. The electrons then Figure 1 A schematic cross section of a transfer into the metal particles and metal-particle modified p-Si electrode.

~

566 reduce CO2 at the surface which has high catalytic activity. This-type electrode has prominent features in that it not only generates a high photovoltage but also has high catalytic activity for the electrode reactions. 2. E X P E R I M E N T A L

Single crystal p-Si(100) wafers [CZ, 1.37 - 1.7 s Shin-Etsu Handotai Co., Ltd.] were used. Ohmic contact was made with In-Ga-Zn alloy. Metal (Cu) particles were deposited photoelectrochemically in a 0.01 M CuSO4 acidic solution. Details of the electrode preparation were described in our previous papers [2-4]. Copper metal electrodes were prepared using a Cu sheet (99.994 %). They were electrochemically polished at 2.0 V vs. Cu plate counter-electrode in 85.0 % phosphoric acid for ca. 1 min. The PEC reduction of CO2 was performed using an H-shaped Pyrex cell, with an Ag/AgC1 electrode as the reference electrode and a Pt plate as the counter electrode. The cathode and anode compartments were separated with a cation exchange membrane (Nation 117) in order to avoid mixing of products at the cathode and the anode. The electrolyte was acetonitrile (infinity pure, Wako Chemical) solutions containing 0.1 M tetrabutylammonium perchlorate (TBAP; 99 %, Acros Organics) with and without 3.0 M H20. Current-potential curves were measured with a potentiostat (Nikko-Keisoku NPOT-2501) and a function generator (Hokutodenko HB-III). Because the electrolyte has a relatively high resistivity, the iR drop was corrected by measuring the solution resistance by an iR compensation instrument (Hokutodenko HI-203). Photoelectrolyses for product analyses were performed under a sealed condition; namely highly pure CO2 (99.99 %) was bubbled into the stirred electrolyte until the air in the cell was completely substituted for CO2, then the cell (cathode compartment) was sealed and the photoelectrolysis was carried out potentiostatically under illumination with a tungsten-halogen lamp (light intensity: 100 mW/cm2). The temperature of the catholyte was kept at 20.0 + 0.1 ~ Reduction products in the gas phase of the cathode compartment were analyzed by gas chromatography, and those in the catholyte were analyzed by high performance liquid chromatography and gas chromatography. Special-grade chemicals and water purified with a Milli-Q water purification system (Nihon Millipore Kogyo) were used for all experiments. 3. RESULTS

Figure 2 shows the photocurrent (j) - potential (U) curves in a CO2 bubbled acetonitrile (AN) solution containing 0.1 M TBAP and 3.0 M H20. The j-U curves for Cuparticle modified p-Si electrodes (denoted as Cu/p-Si in Fig.2) varied from electrode to electrode, probably due to a variety of the form of deposited copper particles or the thickness of silicon oxide on p-Si, but the onset potentials of photocurrents were almost unchanged (0.5 V vs. Ag/AgC1). In Fig. 2, two typical curves (ordinary and best) are shown. The potentials at j = -5 mAcm -2 for the particulate-Cu/p-Si lay 0.50 - 0.75 V more positive than that for a Cu electrode, showing that high photovoltages were generated in the particulateCu/p-Si electrodes. The j-U curve for a naked p-Si electrode was almost the same as that for the particulate-Cu/p-Si(ordinary) in Fig. 2, but the saturated photocurrent of the particulateCu/p-Si(ordinary) was decreased due to a decrease in incident light intensity by the deposited Cu particles. Table 1 shows current efficiencies of products for CO2 reduction on various electrodes. Note that the results in Table 1 (and also Fig. 2) are obtained at high current densities compared with those in aqueous solutions previously reported by us, by taking account of high CO2 concentration in non-aqueous solutions. In an AN electrolyte solution with no addition of H20 (first row), a Cu metal electrode produced mainly CO as reported [6]. Small amounts of H2 and HCOOH were also produced due to a trace amount of H20

567

(0.016 %) contained in AN used in the present Potential vs. Ag/AgC1 / V work. With the addition -2.0 -1.5 -1.0 -0.5 0.0 -2.5 of 3.0 M H:O, a Cu electrode produced CH4 and C2H4together with CO, HCOOH and H2. C1 - 5 This result indicates (d~ ~) that H20 acted as a -10 proton donor in an AN Cu/p-Si (best) ~. s o l u t i o n for h y d r o -15~ carbon production. A naked p-Si -20 naked p-Si electrode / gave mainly H2 together with small amounts of -25 ~. CO, HCOOH and CH4, Figure 2 Photocurrent-potential curves for CO 2 reduction indicating that the Si surface has low in a CO 2 bubbled 0.1 M TBAP + 3.0 M H20/AN solution. catalytic activity for CO2 r e d u c t i o n . The parti c ul ate- Cu/p- S i (ordinary) generated CH4 and C2H4, similar to the Cu metal electrode, showing that the copper particles on p-Si acted as a good catalyst. The production of a large amount of H2 for this electrode, however, suggests that the reaction proceeds also on the naked Si surface. The total current efficiency for all the reduction products, especially that for the electrodes producing CH4 and C2H4(second and fourth rows), does not reach 100 %. This is partly because the electrolysis experiments were performed in a sealed cell for long periods of time of 30 min and thus parts of the reduction products escaped from the cell, e.g., through the Oring seals and the cation exchange membrane, and partly because certain amounts of CH4

Table 1. Current efficiencies of products of CO2 reduction in CO2 bubbled 0.1 M TBAP + 3.0 M H20/AN solutions a~. Potential

Electrode

Current Efficiency [%]

j b~

HCOOH CH4 C2H4 [mA/cm 2]

[V vs. Ag/AgC1]

H2

CO

Cu c~

-2.08

6.4

75.2

5.1

0.0

0.0

16.55

Cu

-2.21

3.9

24.8

7.4

11.8

9.9

28.71

naked p-Si

-2.68

87.8

1.5

2.1

1.5

0.0

28.37

Cu/p-Si(ordinary) d~

-1.68

40.3

20.8

6.6

2.1

4.7

12.98

a) The electricity which passed during the electrolysis is 10 C. The area of the electrodes is 0.5 cm 2. b) The average current density during the electrolysis. c) A result without any addition of H20. d) This is a particulate-Cu/p-Si electrode. The meaning of "ordinary" is shown in Fig. 2. Most of the tested electrodes showed the "ordinary" j-U curve.

568 and C2H4 were left dissolved in the AN electrolyte solution and not detected. However, the above-mentioned essential features for the reduction products were well reproduced by several-time experiments. 4. DISCUSSION The results described in the preceding section indicate that the particulate-Cu/p-Si electrodes generate high photovoltages and reduce CO2 at 0.50 - 0.75 V less negative potentials, producing methane and ethylene as reduction products due to catalytic effect of the deposited copper particles, though a naked p-Si electrode produces mainly H2. Thus, the particulate-Cu/p-Si electrode works as an effective electrode for the PEC reduction of CO2 in non aqueous solutions. The mechanism of the efficient CO2 photoreduction on particulateCu/p-Si was explained in detail in our previous papers [2-4]. The important point is that a high energy barrier height is formed at the p-Si/Cu/solution contact, irrespective of the barrier height of the p-Si/metal contact. A problem remains in the present result: As already mentioned, the current efficiency of H2 for the particulate-Cu/p-Si(ordinary) electrode is much higher than that on a Cu electrode (Table 1), indicating that the reaction proceeds not only on the Cu particles but also on the naked Si part of the particulate-Cu/p-Si(ordinary) electrode. This effect should be much smaller on the particulate-Cu/p-Si(best) as is understood from Fig. 2, but it is not easy to obtain such an electrode reproducibly. To reduce the H2 evolution may be possible by suppressing the reaction on the naked Si part, e.g., by making a silicon oxide layer on it or by increasing the amount of Cu particles. The photovoltages of 0.50 to 0.75 V obtained in the present work are fairly larger than those obtained in aqueous solutions (-- 0.5 V) [2-4]. This may be due to a decrease in surface carrier recombination centers (such as surface penetrated H atoms) in p-Si by a decrease in the H § concentration in solution, or due to a beneficial effect of TBA+ cations on the CO2 reduction reported in the literature [7-9]. Further studies are now in progress. REFERENCES

1. I. Taniguchi, in: Modern Aspects of Electrochemistry No. 20, Eds. J. O'M. Bockris, Ralph E. White and B. E. Conway, (Plenum press, New York, 1989) Chap. 5, and papers cited there. 2. R. Hinogami, T. Mori, S. Yae, and Y. Nakato, Chem. Lett., (1994) 1727. 3. R. Hinogami, Y. Nakamura, S. Yae, and Y. Nakato, Appl. Surf. Sci. (Proc. of 6th Iketani Intern. Conf. on "Surface Nano-control of Environmental Catalysts and Related Materials", Nov. 25 - 27, 1996, Waseda University, Tokyo), 121/122 (1997) 301. 4. R. Hinogami, Y. Nakamura, S. Yae, and Y. Nakato, J. Phys. Chem., submitted. 5. Y. Nakato, K. Ueda, H. Yano, and H. Tsubomura, J. Phys. Chem., 92 (1988) 2316. 6. S. Ikeda, T. Takagi, and K. Ito, Bull. Chem. Soc. Jpn., 60 (1987) 2517. 7. I. Taniguchi, B. Aurian-Blajeni, and J. O. Bockris, Electroanal. Chem., 161 (1984) 385. 8. J. O'M. Bockris and J. C. Wass, J. Electrochem. Soc., 136 (1989) 2521. 9. T. Saeki, K. Hashimoto, N. Kimura, K. Omata, and A. Fujishima, J. Electroanal. Chem., 77 (1995) 390.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

569

Infrared spectroscopic study of CO2 and CO reduction at metal electrodes O. Koga, T. Matsuo, H. Yamazaki, and Y. Hori Department of Applied Chemistry, Faculty of Engineering, Chiba University, Yayoi-cyo, 1-33, Inage-ku, Chiba, 263, Japan In order to elucidate intermediate species of electrochemical reduction of CO2 and CO to hydrocarbons, surface species at Cu, Ni, and Fe electrodes were investigated by infrared spectroscopy. The results show that adsorbed CO is intermediate species to hydrocarbons. The reduction activities of adsorbed CO on metal electrodes were discussed. 1. I N T R O D U C T I O N

Electrochemical reduction of CO 2 is one of the most important topics in connection with environment, energy and natural resources. A few papers have reported spectroscopic detection of intermediate species. CO2 or adsorbed CO is detected in the electroreduction of CO2 on a Pb or a Pt electrode.[1,2] However, these electrodes scarcely produce hydrocarbons from CO2 or CO. Cu electrodes electrochemically produced CH4, C2H4 and alcohols from CO2 and CO in aqueous media at high current densities, and Ni electrodes at lower current densities.[3-7] Fe, not active in CO2 reduction, can reduce CO to hydrocarbons.[8] It is thus interesting to reveal intermediate species on these electrodes in the electrochemical reduction of CO2 and CO by in-situ spectroscopic method. 2. E X P E R I M E N T A L

Two kinds of phosphate buffer solutions were used as the electrolyte after preelectrolysis purification with a platinum black cathode overnight; the one is 0.10 M KH2PO4 + 0.10 M K2HPO4 (1 M = 1 moi dm 3) saturated with CO and Ar or N2 (pH 6.8), and the other one, initially 0.05 M KH2PO4 + 0.15 M K2HPO4, gives pH 6.8 after equilibration withsaturated CO 2. The electrochemical equipments are a potentiogalvanostat (Tohogiken Co. Ltd., 2001) and a function generator (Tohogiken Co. Ltd., FG-02T). The electrode potentials were measured with respect to an Ag/AgCI reference electrode, and the potential values are given against SHE in this article. The counter electrode is a platinum wire. A copper sheet (99.999 % purity, 8 x 10 mm, thickness 0.2 mm), a nickel sheet (99.99 % purity, 10 x 12 mm, thickness 0.2 mm) and an iron sheet (99.99 % purity, 10 x 12 mm, thickness 0.1 mm) attached with a lead strip of the same metal were used as electrodes in infrared measurements. The electrodes were polished to a

570

mirror finish with alumina compounds down to 0.05 pm, degreased with acetone and electropolished in 85 % phosphoric acid. The infrared-electrochemical cell, originally designed by Bewick and his coworkers, was partly modified to introduce an electrode from the upper part of the cell. The front side of the cell is attached with a CaF 2 optical window, and the backside with a glass syringe which pushes the electrode against the window. The Fourier transform infrared measurements were conducted at 0 ~ for Cu electrodes and at ambient temperature for Ni and Fe electrodes by JIR-6000 (Nihon Densi, Co. Ltd.) externally equipped with an MCT (mercury-cadmium-telluride) detector. Infrared spectra were acquired by the subtraction of two spectra reflected from the electrode at different potentials (SNIFTIRS). The other details were described previously.[9] 3. R E S U L T S 3.1 I n f r a r e d s p e c t r a of a d s o r b e d s p e c i e s on metal electrodes.

CO 2 is not reduced at potentials more positive than -0.4 V at Ni electrodes. After the potentials were scanned negative direction to-1.2 V in the solution saturated with CO 2, the cathodic hydrogen formations were significantly suppressed. The previous studies showed that a reduced species is formed and adsorbed on the surface.F] The adsorbed species prevents cathodic currents. The reduced species in CO2 reduction was investigated by the following procedure. The Ni electrode, initially separated from the CaF 2 window, was polarized at-1.2 V in CO2 saturated solution. Hydrogen evolution is rapidly suppressed by this treatment. The electrode was subsequently pushed against the window and the IR spectroscopic measurements were carried out. In the same way, cathodic currents on Cu electrodes were also significantly prevented when the electrode potentials were kept at-1.0 V in CO 2 atmosphere. The reduced species on the Cu electrode was measured by the similar procedure as indicated above. When CO was introduced into the solution, the cathodic currents were suppressed on Fe electrodes. The adsorbed species on Fe in CO atmosphere was measured by infrared spectroscopy. The resulting infrared spectra of adsorbed species on each metal are given in Fig. 1. There

a)

2081

b)

. 1884

1986'-"

V1851 !

c)

v 1955 I

2200

I

I

I

2000

I

I,

1800

w a v e n u m l ~ r / c m "1

Fig. 1. SNIFTIRS spectra of adsorbed species: a) the reduced CO2 on Cu (-0.4 V vs.-1.0 V); b) the reduced CO 2 on Ni (-0.3V vs. -0.5 V); c) the adsorbed CO on Fe (-0.5 V vs. -0.7 V).

571

are one monopolar peak on Cu, two bipolar peaks on Ni and one bipolar peak on Fe in the wavenumber region 2300~1700 cm ~. The absorption peaks above 2000 cm 1 are assigned to Vco of linearly adsorbed CO and those around 1900 cm 1 are assigned to Vco of bridgedly adsorbed CO. CO adsorbs linearly on Cu and adsorbs both linearly and bridgedly on Ni. The peak at a little lower than 2000 cm 1 on Fe is assigned to Vco but is not clear due to linear or bridged adsorption. Thus adsorbed CO is evidently formed on Cu and Ni electrodes during the CO2 reduction and on Fe electrodes during the CO reduction. The downwards peaks correspond to the absorption of adsorbed species at more negative potentials and upwards peaks vice versa. The bipolar peaks on a Ni and a Fe electrode mean that CO exists on the electrode surface at both potentials and the absorption bands shift lower wavenumbers as more negative potentials are applied. Two types of CO on Ni and one type on Fe are stably adsorbed. The monopolar downwards peak on Cu indicates the adsorbed CO exists on the surface at more negative potentials but does not at less negative potentials. CO may be easily desorbed from the surface of Cu electrodes at less negative potentials.

3.2 Reduction of adsorbed CO at a Ni electrode by negative polarization. The electrochemical and infrared spectroscopic measurements have confirmed that the adsorbed CO is formed during CO 2 reduction. However, it still remains ambiguous whether the adsorbed CO is a real intermediate species to produce hydrocarbons or stable poison species. Thus negative polarization of CO adsorbed on Ni electrode was studied by infrared spectroscopy at ambient temperature. CO adsorbed at-0.4 V with the Ni electrode kept apart from the IR window in CO atmosphere and the dissolved CO was subsequently purged with N2. Then the Ni electrode was polarized at -1.0 V in N2 atmosphere. After the polarization, the electrode was maintained a t - 0 . 4 V and pushed against the window. The S NIFTIRS operation was conducted between -0.3 and -0.5 V. The results are shown in Fig. 2. The heights of two bipolar peaks decrease and disappear after the polarization 15 min. Figure 2b)indicates the linear-CO species decreases rather faster than the bridge-CO species. The adsorbed CO is probably reduced to hydrocarbons at-1.0 V on the electrode. Free CO will not be desorbed at the negative potential, since the products from CO2

a)

2031

2025

b)

A 1905

1898 A Jl

lx10"aa

1867

c)

I

I

I

I

I

2200 2000 1800 wavenumber I cm 1 Fig. 2. SNIFTIRS spectra of adsorbed CO on Ni affected by the cathodic polarization at-l.0 V. Polarization time: a) 0 s; b) 40 s; c) 15 min.

572

reduction do not contain any trace of CO.[7] Thus the CO adsorbed on Ni electrodes is stable at-0.4 V but easily reduced at-1.0 V. 3.3 Nature of the adsorbed CO as an intermediate species in CO reduction. We can compare the results on three metals, Cu, Ni and Fe. The order of wavenumbers of adsorbed CO at-0.9 V is Cu:2080 cm -1 > Ni: 2010 cm 1 (linear) > Fe:1960 cm ~ > Ni:1880 cm ~ (bridged). Blyholder related the C-O stretching band frequency of adsorbed CO with the adsorption strength to the metal.[10] Weak adsorption will lead to high C-O stretching frequency, approaching the value of free CO molecule, i.e. 2140 cm ~. Thus the adsorption strength of CO on the electrodes in the aqueous solutions may be given in the following order; Cu < Ni (linear) < Fe < Ni (bridged) . The CO molecule on Cu electrodes is easily desorbed from the surface, as discussed in 3.1. The infrared bands of adsorbed CO may be related to the activity in the electrochemical reduction. Ni and Fe electrodes reduce CO to methane, ethylene and ethane electrochemically with the current efficiency 3 % at a constant current density 2.5 mA / cm 2 (the electrode potentials are around-1.5 V ) i n 0.1 M KHCO3.[8,11] The Cu electrode effectively reduces CO to hydrocarbons with the current efficiency 50 % at 2.5 mA /cm 2 (the potential is-1.4 V).[4,5,11] Thus the activity order in CO reduction is Cu > Ni ,~ Fe. The linear CO is more easily reduced than bridged one on the Ni electrode. The order of the electrochemical activity of metals in CO reduction roughly agrees with the reverse order of the adsorption strength of CO. We demonstrated that the adsorption strength between CO and electrode metals relates closely with the product selectivity in CO and CO2 electroreduction, and that moderate strength of CO adsorption on Cu electrode is suitable for production of hydrocarbons.[11] The present results of IR spectroscopy evidently confirm the validity of our hypothesis. REFERENCES 1. A. W. B. Aylmer-Kelly, A. Bewick, R. Cantrill, A. M. Tuxford, Faraday Discuss., Chem. Soc., 5 6, 96 (1973). 2. B. Beden, A. Bewick, M. Razaq, J. Weber, J. Electroanal. Chem., 139, 203 (1982). 3. Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett., 1985, 1695. 4. Y. Hori, A. Murata, R. Takahashi, S. Suzuki, J. Am. Chem. Soc., 109, 5022 (1987). 5. Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 1, 85, 2309 (1989). 6. Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta, 3 9, 1833 (1994). 7. Y. Hori, A. Murata, Electrochim. Acta, 3 5, 1777 (1990). 8. A. Murata, Y. Hori, Denki Kagaku, 5 9,499 (1991). 9. Y. Hod, O. Koga, Y. Yamazaki, T. Matsuo, Electrochim. Acta, 40, 2617 (1995). 10. G. Blyholder, A. C. Allen, J. Am. Chem. Soc., 91,3158 (1969). 11. Y. Hori, A. Murata, R. Takahashi, S. Suzuki, Chem. Lett., 1 987, 1665.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yarnaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Influence of anions on the production efficiency in pulsed electroreduction of on metal and alloy electrodes

573

CO 2

R. Shiratsuchi a, S. Ishimaru b and G. Nogami a aDept, of Elect. Engn., Kyushu Inst. ofTech., Tobata-ku, Kitakyushu 804, Japan bDept.of Elect.Engn., Ariake Nat. Coll.of Tech., Higashihagiomachi, Omuta 836, Japan

A pulsed electrolysis of C O 2 o n Au, Ag and Cu and their alloyed electrodes was performed, laying special emphasis on an improvement of the selectivity of reduction products. The effect of anions (CI-, Br-, I-) intentionally added to a KHCO3 blank solution on the selectivity was investigated. The pulsed electrolysis was found to have a remarkable effect in enhancing hydrocarbonization reactions on Ag and Cu, while little on Au. It is concluded that oxide surface formed during an anodic period may play a key role in the selectivity of reduction products of CO2.

1. INTRODUCTION As reported elesewhere [1-3], a galvano/potentiostatic electroreduction of C O 2 o n Cu electrode shows poisoning effect of electrode due to graphitic carbons produced as one of the final products of CO2. We proposed a pulsed electroreduction which has an advantage over the conventional methods in the following two respects [4-7]: i) the high selectivity of the reduction products could be obtained by selecting optimum bias conditions, ii) long term electrolysis could be run probably because the poisoning effect can be avoided. Improvement of the selectivity of electrode reaction is undoubtfully of vital importance in electrochemistry. Uniqueness of the pulsed method may be a temporal interruption of chemical reaction sequences from intermediates to final products by a sudden bias change from cathodic to anodic, with which some of competitive reaction pathways may be interrupted. Therefore, an anodic bias can be a key parameter in the pulsed electrolysis. When bias is changed from cathodic to anodic, charges should be forced to redistribute across the interface. Electrolyte anions may also take part in charge redistribution which can afl'ect the electrode reaction taking place during the next cathodic period. The influence of the electrolyte anion and cation on the reduction products of CO2 under the static condition has been reported by Y. Hori et.al. [8] and G. Kyriacou et.al. [9]. In the present paper, the influence of anions (F, CI-, Br-, I, SO42-, NO3-, and C10;) intentionally added to a CO2-saturated 0.1M KHCO3 solution on the pulsed reduction products of CO2 was investigated for Au, Ag, Cu, and their alloyed electrodes. 2. EXPERIMENTAL Metal wires used as an electrode material were Au (99.95%), Ag (99.99%) and Cu

574 (99.999%) purchased from Nilaco Co. Ltd.. CuAu, AuAg, and AgCu electrodes were prepared by melting these metal wires in vacuum. The weight ratio for the alloy electrodes was Au:Cu=59:41, Au:Ag=57:43, and Cu:Ag=68:32. The experimental procedure, analytical method, and electrolysis system were the same as before [7]. Cathodic period and anodic period in the pulsed electroreduction were both set at 5s. All potentials were referred to Ag/AgC1 electrode. The electrolyte used (blank solution) was a CO2-saturated 0.1M KHCO3 buffer solution (pH 6.8) at 10~ The electrolyte containing a certain anion was prepared in each experiment by adding KF, KC1, KBr, KI, KNO3, K2SO4, or K C 1 0 4 to the blank solution. A conventional H-type gastight glass cell was divided by an ion exchange membrane (Nation 417). The Ag/AgC1 reference electrode was also separated from the working electrode compartment by the membrane so as to avoid a contamination of KC1. A gas chromatograph (Yanaco G-3810) was equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). Molecular Sieve 5A and Porapak Q were used for CO and H2 analysis in the TCD and CH4 and C2H4 analysis in the FID, respectively. Soluble products such as CH3OH , CH3CHO , and C2H5OH were analyzed by the FID after electrolysis for 5 h. Formate ions and other anions in the solution were analyzed by means of an ion chromatograph(Dionex DX-100) equipped with an anion exchange column (IonPac ICE-AS1), an anion exchange micromembrane suppressor, and a conductivity detector module. 3. RESULTS The electrolyses for the solutions containing lmM of CI-, F, Br-, or I- and the blank solution were performed at Vc=-I.5V and V,=-0.9~ 1.2V. Au: Figure 1 shows the faradaic efficiencies for CO (77 (CO)) and H2( 77(H2)) as a function of the anodic bias with the additives. 80

"~

60

'

':'

o

Bla,~

[]

KBr

Q

[] KF

9

60"

o

,' o~ 0

Blank KC1 KBr

m

40

o

4O

'

CO

CH 4

.o

.o

20

20 -

o

-65 Va vs.

'

0

H2

-| .....

'

Ag/AgC1

9

0:5

/V

Figure 1. Faradaic efficiency 77 (CO) for CO and 77 (H2) for H 2 o n an Au electrode in pulsed reduction at a cathodic bias of Vc= -1.5V vs. Ag/AgC1, as a function of anodic bias with and without the addition of lmM halogen ion.

-0.5 V,

vs.

0 Ag/AgC1

0.5 /V

Figure 2. Faradaic efficiency 77 (CH4) for CH4 and 77 (CzH5OH) for C z H s O H o n a Cu electrode in pulsed reduction at a cathodic bias of Vc =-2.25V vs. Ag/AgC1, as a function of anodic bias with and without the addition of 1 mM KC1 or KBr.

575

Table 1 Typical faradaic efficiencies(%) for pulsed electroreduction products of CO z Electrode Vc Va Anion H2 CO CH4 C2H4 CzHsOH CH3CHO -2.25 -0.6 blank 36.2 tr 20.1 5.8 8.2 11.0 -2.25 -0.6 cr 47.4 tr 12.7 7.8 7.2 5.0 Cu -2.25 -0.25 cr 26.7 tr 30.4 4.0 4.8 3.5 -2.25 0 C113.0 tr 26.0 1 0 . 1 10.3 6.1 -2.25 -0.6 blank 9.1 15.1 17.4 1.8 7.5 3.4 -2.25 -0.6 cr 7.6 49.7 0.2 0.2 2.7 4.2 Ag -2.25 -0.5 cr 3.8 47.8 3.5 2.5 8.6 14.9 -2.25 -0.4 C110.8 1 5 . 1 13.0 1.4 9.3 5.5 CuAu AgAu AgCu

-2.25 -2.25 -2.25 -2.25 -2.25 -2.25

-0.25 -0.4 -0.6 -0.5 -0.25 -0.25

blank C1blank cr blank C1-

45.1 45.7 44.8 60.6 30.4 44.2

7.6 5.7 17.9 17.4 tr tr

6.7 3.8 tr 1.3 23.1 18.9

3.1 2.4 tr 1.1 3.8 2.8

6.2 7.2 1.4 3.9 4.0 2.4

13.8 11.2 tr 10.2 tr 5.3

HCOOH 6.1 3.7 1.4 1.3

Total 87.4 82.8 70.8 66.8

14.5 7.7 7.4 9.1

68.8 72.3 88.5 64.2

3.8 2.4 5.8 2.2 15.8 7.6

86.3 78.4 69.9 96.7 77.1 81.2

77 (CO) was dependent on these anions over the anodic bias range between -0.6V and 0.9V. 77 (CO) at Va=0V was the maximum value for I-(57.6%) and the minimum value for F (34.5%). However, 77 (CO) for the solution with SO42-, NO3-or C104- was almost the same value as that of the blank solution at Va=0V. 77 (CO) for the blank solution was monotonously increased with increasing the anodic bias. 77 (H2) was suppressed by adding these anions to the blank solution. Cu: Figure 2 shows the dependence of the faradaic efficiency, 77 (CH4) and 77(CzHsOH) on anodic bias for the blank solution and the solution containing lmM KC1 or lmM KBr. The addition of KC1 or KBr gave a maximum value of 77 (CH4)=30.4% at Va=-0.25g. 77(CzHsOH) reached about 10% for the pulsed reduction at Va=0V and was almost independent of the addition of anion. The dependence of the yield of CzH 4 and CH3CHO on V, was similar to that observed for77 (CzHsOH). The total faradaic efficiency of hydrocarbonization reactions to CH4, C2H4, CzHsOH and CH3CHO was 53.8%. 77(CH4) in the blank solution was independent of Va between Va=-0.25 and 0.25V probably because of the surface oxide layer produced by the reaction between Cu metal and OH-. Although main reduction product of CO2 on Ag under galvano/potentiostatic electrolysis is CO, CH4 was found to be effectively formed under the pulsed electrolysis at Vc=-2.25V and Va =-0.6~ -0.4V. The present result shows that the peak potential at which 77 (CH4) and 77 (CO) become a maximum shifts positively. Amaximum faradaic efficiency of about 30% for hydrocarbonization reaction on Ag electrode was achieved as shown in Table 1. The yields of hydrocarbons increased while 77 (CO) decreased. In the meanwhile, the addition of C1- has little effect on alloy electrodes. Only exception is that hydrocarbonization reaction may be enhanced on the AgAu electrode when C1- is added. The production of CO for the pulsed reduction on the CuAu electrode occurred, while that on the Cu electrode and the CuAg electrode is trace. The increase in the production of HCOOH on the AgCu electrode reflected the behavior of the Ag electrode.

576 4. DISCUSSION It has been reported that the concentration of proton and adsorbed hydrogen can be controlled by adjusting the anodic and cathodic bias in the pulsed method [7]. The hydrogen adsorbed on the electrode surface seems to interrupt the reaction for the electrochemical reduction of CO2. The CO2 coverage on the electrode surface may be increased by the elimination of adsorbed hydrogen during anodic period. In the subsequent cathodic period, the electron transfer to CO2 was promoted, yielding CO2 radical anions. The selectivity of products for the electrochemical reduction of CO2 was determined in association with electrode material and CO2 radical anion [10,11]. CO is intermediate species in the reaction process of hydrocarbonization [8]. Main product of CO2onAu and Ag electrode under galvano/potentiostatic electrolysis is known to be CO. Even under a pulsed electrolysis, this is also the case for Au electrode. On the contrary, the pulsed method has a remarkable effect for Ag electrode. The largest difference between pulsed and conventional methods lies in that, in the former, anodic reactions and/or charge redistribution can take place during the anodic period. As well known, Cu and Ag are more easily oxidized to form an oxide layer than Au. Therefore, the oxide layer formed during the anodic period may play a key role in the selectivity of reduction products. In addition, Au has another disadvantage over the hydrocarbonization reactions because hydrogen adsorption is reported to be quite limited on Au surface[12]. The effect of anions was pronounced both on Au and on Ag. The fact that r/(CO) on Au and Ag increases when halogen ions are added implies that the amount of adsorbed hydrogen may be decreased by a specific adsorption of anions to the metal electrodes. This may be the reason why r/(CO) on Au and Ag depends on anions.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

R.L. Cook, R. C. MacDuff, and A. E Sammells, J. Electrochem. Soc., 135 (1988) 1320. D.W. DeWulf, T. Jin, and A. J. Bard, ibid., 136 (1989) 1686. G. Kyriacou and A. Anagnostopoulos, J. Electroanal. Chem., 322 (1992) 233. R. Shiratsuchi, Y. Aikoh, and G. Nogami, J. Electrochem. Soc., 140 (1993) 3479. G. Nogami, Y. Itagaki, and R. Shiratsuchi, ibid., 141 (1994) 1138. S. Ichikawa, and R. Doi, Catal. Today, 27 (1996) 271. R. Shiratsuchi, and G. Nogami, J. Electrochem. Soc., 143 (1996) 582. Y. Hori, A. Murata and R. Takahashi, J. Chem. Soc., Faraday Trans. 1, 85 (1989) 2309. G. Kyriacou, and A. Anagnostopoulos, J. Appl. Electrochem., 23 (1993) 483. M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, J. Electrochem. Soc., 137 (1990) 1772. Y. Hori, H. Wakebe, T. Tsukamoto, and O. Koga, Electrochim. Acta, 39 (1994) 1833. G.M. Schmid and M. E. Curley-Fiorino, in: Encyclopedia of Electrochemistry of The Elements Vol.IV, Ed. A.J. Bard (Marcel Dekker, New York 1975) p. 131.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

577

E l e c t r o c h e m i c a l r e d u c t i o n of CO2 by using m e t a l s u p p o r t e d gas diffusion electrode under high pressure Kohjiro Hara, Noriyuki Sonoyama and Tadayoshi Sakata

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan The electrochemical reduction of CO2 under high pressure was carried out using various metal supported gas diffusion electrodes (GDEs). By using Pt supported GDE in reverse arrangement, in which the Pt catalyst layer of GDE is directed toward CO 2 gas phase, methane was produced at faradaic efficiencies of 38.8 %, while in the normal arrangement in which the Pt catalyst layer of GDE is directed toward the electrolyte hydrogen was the main product. By using Ag and Pd supported GDEs, CO was produced at faradaic efficiencies of 57.5-86.0 %. The partial current density of CO formation at Ag supported GDE reached 3.0 A cm-2. 1, INTRODUCTION We have investigated the electrochemical reduction of CO2 under high pressure on various metal electrodes in aqueous electrolytes.[1-2] The effect of CO2 pressure on the electrochemical CO2 reduction on metal electrodes was studied. The CO2 reduction at large current density could be achieved on metal electrodes by the high pressure electrolyses due to high concentration of CO2 in the aqueous electrolyte. By using the gas diffusion electrode (GDE) which have been developed and employed for fuel cells, a large current density electrolysis can also be achieved. The electrochemical CO2 reduction on GDEs supported by various metals and metal compounds under 1 atm were conducted by several workers.[3-5] They reported the achievement of larger current density for CO2 reduction, compared to the electrolyses in aqueous electrolyte using metal electrodes. In the present study, we have carried out electrochemical reduction of CO2 under high pressure using various metal supported GDEs to demonstrate the efficient electrochemical conversion of CO2 in high current density. 2. EXPERIMENTAL The electrolyses under high pressure were carried out in a stainless steel autoclave. The apparatus for the electrolyses is shown in Figure 1. A GDE and Pt supported GDE are purchased from Tanaka Noble Metal, Ctd. Other metals were supported on the GDE by the impregnation method. These metal supported GDEs were used as working electrodes. The aqueous electrolyte was 0.5 mol dm-3 KHCO3 and 2.0 mol dm-3 KOH aqueous solution, which was prepared from reagent-grade chemicals and distilled water (Wako Pure Chemical Industries, Ltd.) The electrolyte was purified by the pre-electrolysis with a Pt black cathode at

578 a current density of 0.25 mA cm-2 to eliminate heavy metal impurities. A Pt wire was used as an anode, and a reference electrode was Ag/AgCl/saturated KC1. High purity CO2 (99.99%) was introduced into the autoclave. The electrolyses were carried out galvanostatically at 25~ using a potentiostat/galvanostat (Hokuto, HA-501) connected in series with a coulometer (Hokuto, HF-201). The electrode potential was corrected with an IR compensation instrument (Hokuto, HI-203). Electrolysis products such as hydrocarbons, ethanol, CO, and hydrogen were analyzed quantitatively by a gas chromatograph, and formic acid was determined by an HPLC. Other details were described in previous papers.[6-8]

3. RESULTS AND DISCUSSION 3.1. Electrolyses of CO2 by using Pt supported GDE Table 1 shows the faradaic efficiencies of the reduction of CO2 under 20 atm on GDEs with and without Pt catalyst at a constant current density of 600 mA cm-2 for passed charge of 150 C. Where, "Normal" and "Reverse" are arrangements of GDEs. "Normal" means the arrangement that the Pt catalyst layer of GDE is directed toward the electrolyte while the gas diffusion layer faces CO2 gas phase and "Reverse" means the arrangement that the Pt catalyst layer of GDE is directed toward CO2 gas phase. In the case of the normal arrangement, CO and formic acid were formed at faradaic efficiencies of 0.6 and 4.0 %, respectively, and hydrogen was predominantly formed. In contrast to this, CO2 was efficiently reduced and methane was the predominant product formed at the faradaic efficiency of 38.8 % in the case of the reverse arrangement. Moreover, CO and formic acid were produced at faradaic efficiencies of 3.8 and 6.3 %, respectively. From these results, it is clear that the Pt catalyst and the

Highpressure / ~ \ Pressure 002111' L" )gauge ~' yr~ Sampling

[I

II

I II Ill

80

Autoclave n

6O

o

o

E =L

r 40 0

Ag/A ~ /

~-

E

ii1~-~'~ ~ ~ e l e c t r o d e I ~ - - " - ~ ~ L ~unter)

Gasd/iiffusi0n electrode

).8 E E

\ ~

Electrolyte Magneticstirrertip

Figure 1. Schematic diagram of the electrolysis equipment.


2 mol dm-3) are necessary. The concentration of protons, which can be involved in H2 generation, decreases as the CO2 pressure is increased. The range of proton concentrations, however, was found to be relatively small and thus the influence on the product distribution was negligible. p-InP and p-GaAs exhibited differing types of product distributions. On p-InP photocathodes, CO production was predominant at every CO2 pressure (Fig. 2a), while on pGaAs hydrogen evolution was the principal reaction at lower CO2 pressures (1 to 10 atm, Fig. 2b). This difference may be due to differing catalytic activities for CO2 reduction of the two different semiconductor surfaces. Further studies are now in progress in our laboratory.

REFERENCES

1. M. Halmann, Nature, 275, 115 (1978). 2. M. Halmann, Chemical Fixation of Carbon Dioxide - Methods for Recycling CO2 into Useful Products, CRC Press, Boca Raton, Florida, 1993. 3. See, e.g., I. Taniguchi et al., Elecrochim. Acta, 29, 923 (1984). 4. H. Noda et al., Chem. Lett., 1757 (1990). 5. T. Saeki et al., J. Electrochem. Soc., 141, L130 (1994). 6. T. Saeki et al., J. Phys. Chem., 99, 8440 (1995). 7. T. Saeki et al., Chem. Lett., 361 (1995). 8. E. Brunner et al., J. Chem. Thermodynnamics, 19, 273 (1987).

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 O 1998 Elsevier Science B.V. All rights reserved.

593

Studies on CO2 fixation in PNSB : Utilization of w a s t e as the a d d i t i o n a l source of c a r b o n for CO2 fixation b y PNSB V.Brenner, M.Inui, N.Nunoura, K.Momma, and H.Yukawa Molecular Microbiology and Genetics Lab., Research Institute of Innovative Technology for the Earth (RITE), 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto, 619-02, JAPAN

1. INTRODUCTION Global warming is thought to be caused to a large part by CO2 accumulation in the atmosphere and poses serious environmental threat. A variety of different technologies are being developed to decrease the CO2 emissions. We are convinced that biotechnology could significantly contribute to the solution of this problem. This time we present a new concept for complex utilization of different wastes in biotechnology employing purple non-sulfur photosynthetic bacteria (PNSB) under anaerobic light conditions for chemical production and for SCP production with subsequent fixation of CO2 released during industrial processes. PNSB have great bioindustrial potential due to their metabolic flexibility and relatively rapid growth [1]. They also utilize various organic compounds as electron donors for the fixation of carbon dioxide and molecular nitrogen under anaerobic light conditions [2]. Large volumes of agroindustrial wastes are discharged all over the world and their accumulation causes environmental problems in the agricultural districts. Our experiments presented in this paper were performed to obtain preliminary data for setting parameters to suggest convenient technology for PNSB mediated CO2 fixation during waste treatment. Therefore, waste related carbon source utilization by model PNSB, their optimal light range for the growth, and their efficiency regarding CO2 utilization, were studied. 2.EXPERIMENTAL

Growth of PNSB on a broad range of selected carbon sources was monitored by measuring absorbance at 650nm . The optimal light range for the growth of PNSB was measured by LI- 192 SA pyranometer sensor with the light intensity 100W/m 2 during the growth on ethanol at 35oc. Changes in UV absorption spectra of the PNSB diluted culture filtrates during the growth on aromatic acids (AA) were monitored on spectrophotometer Beckmann DU 640. CO2 fixation rates were calculated by TOC 5000A analyzer.

594

3.RESULTS AND DISCUSSION 3.1.Metabolism of aromatic acids (AA) in PNSB O u r p o i n t in s t u d y i n g of A A p h o t o m e t a b o l i s m in PNSB w a s to find o u t the s p e c t r u m of A A u t i l i z a t i o n (Table 1) a n d to i n v e s t i g a t e m a j o r m e t a b o l i c r o u t e s in the b e s t p e r f o r m i n g strain. Lignin d e r i v a t i v e s k n o w n as c i n n a m i c acids are u b i q u i t o u s in a n y w a s t e c o n t a i n i n g p l a n t m a t e r i a l [3] . C h a n g e s in UV a b s o r p t i o n spectra of PNSB d i l u t e d culture filtrates d u r i n g the g r o w t h on AA w e r e m o n i t o r e d . F r o m t h e s e d a t a a n d f r o m the m e t a b o l i c s t u d i e s t w o m a j o r m e t a b o l i c r o u t e s of A A in the best p e r f o r m i n g AA d e g r a d e r , strain No.7, w e r e suggested: Ph en yl v al era te--p h e n yl p r o p i o n a te-- ci n n a m a te--benzoate .......... m i n e r ali z a ti on p- cou m ara te 4-hyd roxybenzoa t e.......... m i n e r ali z a ti on Table 1 G r o w t h of PNSB on a r o m a t i c acids Su-bstrate- . . . . . . . . . . Benzoate Cinnamate o-Coumarate p-Coumarate Ferulate 4- H y d r o x y b e n z o a t e Mandelate Vannilate Salicylate Phenylpropionate Phenylvalerate

R.pa!u. stris # 7 + ++ + +. + +. -+ + ++

R.capsulatus-R~spheroides # 11166 # 17023 + + + -+ + . . . -+ + . . . -+ + + + +

R.rubrum #11170 + + +-+ -+

3.2.Growth of PNSB on other carbon sources Bacteria w e r e g r o w n a n a e r o b i c a l l y on M M m e d i a in i l l u m i n a t e d c o n d i t i o n s . The e v a l u a t i o n of g r o w t h is c h a r a c t e r i z e d by scale w h e r e u s e d s y m b o l s h a v e this meaning: - no g r o w t h ( O . D . 6 5 0 < 0 . 1 ) ; - + negligible g r o w t h ( 0.2 >O.D.650>0.1) +- m o d e r a t e g r o w t h (0.4 >O.D.650>0.2); + fair g r o w t h (1.5>O.D.650>0.4); ++ excellent g r o w t h (O.D.650> 1.5 ). D a t a are s u m m a r i z e d in Table 2 a n d d i s c u s s e d in the section "Conclusion". 3.3. Effect of light range on the growth of Rhodopseudomonas palustris No.7 In the i l l u m i n a t i o n i n t e n s i t y of 1 0 0 W / m 2 w e h a v e s t u d i e d the effect of light r a n g e o n the g r o w t h of s t r a i n Rhodopseudomonas palustris No.7. F r o m the data obtained, the o p t i m a l light r a n g e 850-950nm, w a s e s t i m a t e d (Fig.2). 3.4 Analysis of CO2 fixation by Rhodopseudomonas palustris No.7 C O 2 fixation rate in the strainR.palustris No.7 w a s m e a s u r e d in p h o t o a n a e r o b i c c o n d i t i o n s . The h i g h e s t fixation rate p e r r e a c t o r (0.43g/L/hr) w a s o b t a i n e d d u r i n g the g r o w t h on e t h a n o l u n d e r i l l u m i n a t i o n 4 1 6 W / m 2. CO2 fixation rates nn

cliffprpnt

earhnn

~nllreoq

arp

(~cwnnaroct

in FiCrllro 1

595

Table 2 Selected waste

related

carbon

Substrate

R.palustris

substrate

utilization

R.capsulatus

#7

# 11166

by

PNSB

R.spheroides

R.rubrum

# 17023

11170

VFAs acetate

+

+

+

+

propionate

+

++

+

+-

butyrate

+

+

+

+

malate

++

+

+

+

succinate

+

+

+

+

glutarate

++

+-

+

+

pyruvate

+

++

++

+

caproate

++

+

+

+

SUGARs sucrose

-

-+

-+

fructose

-

+

++

glucose

-

+

++

xylose

-

-

+ +

mannose

-

-

mannitol

-

-

+

sorbitol

-

-+

+

+-

ALCOHOLs +

ethanol

++

-+

-+

n-propanol

++

-+

-+

+-

n-butanol

+

-

-

-+

glycerol

-

+-

benzylalcohol

+

-

+

1.5

ethanol f,'valerate

5.0"

I::

//

4.0"

acetate

:..:z'propionate

acetate te

r

///btlyratea

3.0"

2.0'

L~

~.~..~

/;:

rJ

5

1.0.

0.0 :/

; ~

0

10

. 20

30

.

. 40

. 50

0 60

Culture time (hr) Figure

eth and

....;'

1. G r o w t h

i

0

i

10

.

i

20

,

i

30

i

i

40

!

l

50

i

i

60

Culture time (hr)

and CO2 consumption

Rhodopseudomonas palustris

70

No.7

on selected substrates

by the strain

,

70

596 1.6 m

..

> 700 nm

A

-

> 800 nm

-

> 900 nm

1.4 1.2 l

500-600 nm

o

0.8

----O---

9

600-700 nm

0.6 700- 800 nm

0.4

800-900 nm

0.2

900-1000 nm 0

24

48

72

96

120 144 168

---~--

Control

Growth time (hr) Figure 2. Influence of selected light range on the growth of the strain

Rhodopseudomonas palustris No.7 4. CONCLUSION The strain Rhodopseudomonas palustris No.7 is good utilizer of lower alcohols , volatile fatty acids (VFA), and aromatic acids and therefore may be useful for the treatment of wastes containing either material from plants (such as bean industry), or containing ethanol (breweries), and VFAs (complex wastes after anaerobic treatment). Sugars are the major components of wastes released by food industry and agroindustry. They were preferentially utilized by the strain Rhodopseudomonas spheroides 17023 which in opposite did not grow solidly on alcohols. The optimal light range for the strain No.7 was found in the interval between 850nm and 950nm. The CO2 fixation rate for the same strain in bioreactor was 0.43g C O 2 / L / h r , measured at 35~ pH 7.5, at light intensity 4 1 6 W / m 2. Two alternative models , the first one considering anaerobic pretreatment of waste with subsequent utilization of prevalently created VFAs by PNSB in a laser operated photobioreactor [4], and the second one with direct application of waste into the photobioreactor, were suggested. REFERENCES

[11 K.Sasaki,

N.Noparatnaraporn, S.Nagai, Bioconversion of Waste Materials to Industrial Products, Martin A.M. eds., Chapter 7, 223 (1991) [2] M. Kobayashi, M.Kurata, Process Biochem., _1_3,27, (1978) [3] P.G.England, D.A.Pelletier, M.Dispensa, J.Gibson, C.S.Harwood, Proc.Natl.Acad. Sci.USA, 94, 6484, (1997) [4] H.Sawada, P.L.Rogers, J.Ferment.Technol. _55, 311, (1977) This work was supported by a grant from the New Energy and Industrial Technology Development Organization.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

597

S t u d i e s on CO2 fixation in PNSB : A n a l y s i s of CO2 m e t a b o l i s m in p u r p l e n o n - s u l f u r bacteria M. Inui, J. H. Roh, K. Momma a, and H. Yukawa Molecular Microbiology and Genetics Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-Soraku, Kyoto 619-02, JAPAN aResearch Institute of Food Science, Kyoto University, Gokasho, Uji, Kyoto 611, JAPAN 1. INTRODUCTION Purple non-sulfur bacteria (PNSB), anoxygenic phototrophic prokaryotes, are a phenotypically and genotypically diverse set of species. They are able to grow aerobically as chemoheterotrophs, anaerobically using respiration or fermentation, and photosynthetically in an autotrophic or heterotrophic mode. Therefore, PNSB exhibit metabolic diversity reflecting the complex modes of growth. CO2 fixation in most PNSB is catalyzed by ribulose 1,5-bisphosphate carboxylase /oxygenase (RubisCO) in the Calvin pentose phosphate cycle. Besides the plant like formI RubisCO, which is composed of both large(L) and small subunits(S) and has an L8S8 structure, formII RubisCO containing only the large subunit exists in PNSB. Both forms of RubisCO are differentially expressed according to the culture conditions. In addition to CO2 fixation by RubisCO, anaplerotic enzymes such as p h o s p h o e n o l p y r u v a t e carboxylase(PEPC), phosphoenolpyruvate carboxykinase (PEPCK), and pyruvate carboxylase(PC) also play important roles in carbon metabolism, supplying intermediates for energy production and cell materials. Understanding the expression and regulation of these enzymes will furnish important information for utilizing PNSB in biotechnological applications. In this study, we have constructed genomic disruptants of formI and formII RubisCO, PEPC, and PEPCK, and analyzed the function of each enzyme. 2. EXPERIMENTAL

The genes encoding the two RubisCOs, PEPC, and PEPCK were cloned and sequenced from Rhodopseudomonas palustris No. 7 [1]. Each cloned gene was inactivated by insertion of a kanamycin(Km) cassette. Inactivated genes were transferred from Escherichia coli to R. palustris No. 7 by conjugation. Homologous replacement took place in vivo between the chromosomal gene and the Km inactivated copy carried on the suicide vector, pGP704 [2]. Exoconiugants

598 were initially selected by Km resistance because the suicide vector can not replicate in R. palustris No. 7, and then tested for double crossover with the loss of vector-mediated gentamycin resistance. The integration of the Km cassette via a double crossover was confirmed by Southern hybridization experiments. Growth and the enzyme activities of the disruptants were investigated under various growth conditions as previously reported [3]. 3. RESULTS A N D DISCUSSION

3. 1. Characterization of R u b i s C O deficient mutant. R. palustris No. 7 possesses two forms of RubisCO, formI and formII, as reported for Rhodobacter(Rba) species. In Rba. sphaeroides, formI RubisCO is mainly e x p r e s s e d u n d e r photoautotrophic and formII u n d e r photoheterotrophic conditions [4]. When ethanol is used as a carbon source of R. palustris No. 7 under anaerobic light condition, addition of NaHCO3 is needed for an electron acceptor to maintain the redox balance. Although mutants deficient for either RubisCO did not show any change in growth rate compared to wild type under this condition, the RubisCO activity in the formII deficient mutant decreased to 45% of the wild type, while a formI deficient mutant did not(Fig. 1). This result indicates that both forms of RubisCO are functional for CO2 fixation in this strain and active enough to support the photoheterotrophic growth of either disruptant. To investigate the regulation of the expression of two RubisCO, studies are in progress utilizing both p h o t o a u t o t r o p h i c conditions and p h o t o h e t e r o t r o p h i c conditions. 10[~ Table 1. RubisCO activity of R. pahistris No. 7, FormI and FormI! deficient mutant E Strains

-

Activity(nmol/mg/rain)

R. pahlstris No. 7

6.9 (:t:0.2)

FormI deficient mutant

7.3 (•

FormlI deficient mutant

3.2 (•

O

n R. palustris No. 7 9 FormI deficient mutant [] FormII deficient mutant 50

100

Time(hr) Figure 1. Growth of R. palustris No. 7 and the RubisCO deficient mutants on 20mM ethanol plus 20mM NaHCO3 medium under anaerobic light conditions.

599

A

o

B

1

R.p

.1

o

.1

ient mutant I

I

I

30 Time(hr)

I

I

I

60

i

I

I

30

I

I

I

60

Time(hr)

Figure 2. Growth of PEPC deficient mutant on pyruvate(A) and pyruvate plus 0.05 % malate(B) as a carbon source under anaerobic light conditions.

3. 2. Characterization of PEPC d e f i c i e n t mutant. PEPC is an anaplerotic CO2 fixing enzyme catalyzing the irreversible conversion of phosphoenolpyruvate to oxaloacetate which is an intermediate of the TCA cycle and a precursor of many cellular compounds. Most PNSB contain at least one anaplerotic enzyme, PC or PEPC, to supply oxaloacetate. A PC deficient mutant of Rba. capstllatus where PC is the sole anaplerotic enzyme in the wild type did not grow on glucose unless the m e d i u m was s u p p l e m e n t e d with TCA cycle intermediates [5], but a PEPC deficient mutant of Corynebacterium glutamicum where the wild type contains both PC and PEPC shows the same growth when compared to wild type on glucose m e d i u m [6]. The PEPC deficient mutant of R. palustris No. 7 showed reduced growth rate under anaerobic light condition on pyruvate or substrates such as lactate and gluconate metabolized via pyruvate(Fig. 2). However, in the presence of a TCA cycle intermediate, growth is almost the same as that of wild type. Therefore, PEPC has an important function in R. palustris No. 7. 3. 3. Characterization of PEPCK deficient mutant. PEPCK of most bacteria appears to function as a gluconeogenetic enzyme rather than as an anaplerotic one. The PEPCK deficient mutant of R. palustris No. 7 showed a similar growth rate to that of wild type on p y r u v a t e and on TCA cycle intermediates necessary for gluconeogenesis(Fig. 3). PEPCK mutants of most bacteria show reduced growth on pyruvate and TCA cycle intermediates, but malic enzyme and PEP synthase, or pyruvate phosphate dikinase and PEP synthase are able partly to support the growth by forming an alternative pathway to synthesize PEP [7]. The existence of NAD+-dependent malic enzyme activity suggests that R. palustris No. 7 utilizes this alternative pathway for gluconeogenesis.

600

a

1.0

:

R. palustris No. 7

1.0 i

B R. palustris No. 7

~D

9 0.5 9

:~

....~

0.5"

nt

.

PEPCK dificient mutant |

9

40

,

|

,

|

80 Time(hr)

.

i

,

i

'

'

120 Time(hr)

Figure 3. Growth of PEPCK deficient mutant on pyruvate(A) and succinate(B) as a carbon source under anaerobic light condition. 3. 4. PC of R. palustris N o . 7. PC is a biotin containing anaplerotic enzyme catalyzing the conversion of pyruvate to oxaloacetate. PC activity of R. palustris No. 7 was detected by coupling the production of oxaloacetate with the oxidation of N A D H using malate dehydrogenase. NADH oxidation was dependent on acetyl-CoA, ATP, pyruvate, NaHCO3, and MgC12. In addition, specific band with a molecular mass of 130kDa was detected by western blot analysis for biotinylated protein(data not shown). Purification experiment are in progress to clarify the existence of PC in this microorganism. REFERENCES

1. T. Fujii, A. Nakazawa, N. sumi, H. Tani and A. Ando, Agric. Biol. Chem., 47 (1983) 2747. 2. V. L. Miller and J. J. Mekalanos, J. Bacteriol., 170 (1988) 2575. 3. M. Inui, V. Dumay, K. Zahn, H. Yamagata and H. Yukawa, J. Bacteriol., 179 (1997) 4942. 4. D. L. Falcone, R. G. Quivey Jr. and F. R. Tabita, J. Bacteriol., 170 (1988) 5. 5. J. C. Willson, J. Gen. Microbiol., 134 (1988) 2429. 6. P. G. Peter-Wendisch, B. J. Eikmanns, G. Thierbach, B. Bachmann and H. Sahm. FEMS Microbiol. Lett., 112 (1993) 269. 7. M. Osteras, B. T. Driscoll and T. M. Finan, Microbiology, 143 (1997) 1639.

This work was supported by a grant from the New Energy and Industrial Technology Development Organization.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

601

C r y s t a l l i z a t i o n a n d p r e l i m i n a r y X - r a y s t u d i e s of p h o s p h o e n o l p y r u v a t e c a r b o x y l a s e f r o m E s c h e r i c h i a Coli H. Matsumura a, T. Nagata a, T. Inoue a, Y. Nagara a, T. Yoshinaga b, K. Izui c, Y. Kai ~ aDepartment of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan bDepartment of Public Health, Faculty of Medicine, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan cDepartment of Agricultural Biology, Faculty of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan We have crystallized the enzyme and determined the X-ray structure of PEPC by a multiple isomorphous replacement method. Our current structural model suggests that PEPC forms a 'dimer of dimers' and provides the mechanism for allosteric regulation. 1. I N T R O D U C T I O N

Phosphoenolpyruvate carboxylase (PEPC) catalyzes the fixation of bicarbonate ion to form oxaloacetate and inorganic phosphate in the presence of Mg 2§ The native enzyme is present in all plants and in a variety of bacteria but is absent in mammals and fungi. PEPC is a tetramer comprising of four identical subunits with molecular weight of about 100 kDa. The enzyme plays a key role in the initial CO 2 fixation of photosynthesis in C4 and CAM plants. The PEPCs from most sources are allosteric, and their activities are regulated by a variety of effectors. The Escherichia coli enzyme is regulated by four different activators: acetyl-CoA, fructose 1,6-bisphosphate, GTP, and long chain fatty acids, as well as by the inhibitors Laspartate and L-malate. In order to study its biological function and allosteric regulatory properties based on the three dimensional structure, we have determined the X-ray structure of PEPC. 2. S T R U C T U R E D E T E R M I N A T I O N

Crystals of PEPC were grown by the hanging drop vapor diffusion method as

602 reported previously [1]. Cell constants and a space group are determined by precession photographs. Crystals are of the space group/222 with the unit cell dimensions of a = 117.6, b = 248.4, and c = 82.7 A. The asymmetric unit contains one PEPC monomer. The statistics of diffraction data and initial phases determined by the multiple isomorphous replacement method are summarized in Table 1. Although the electron density map at 2.8 h. resolution was sufficient enough to recognize secondary structure, it was further improved by a series of density modification methods. The final model includes 6,875 non-hydrogen protein atoms and 120 solvent molecules. The final R-factor and free R-factor [2] for the 25,652 independent reflections between 10.0 and 2.8 A resolution with F>2(~(F) is 0.224 and 0.290, respectively. Table 1 Crystallographic data statistics Diffraction data Resolution

Completeness

Native

2.8 A

91.3%

7.0%

CH3HgC1

2.8 .~

67.2%

11.3%

mersalyl acid

3.5

60.2%

7.7%

EMTS

3.5 A

90.1%

9.8%

No. of heavy-atoms

Rcullis* 0.68 0.77 0.80

Phasing power + 1.38 1.05 0.87

Re*

Heavy-atom phasing statistics

CH3HgC1 mersalyl acid EMTS

6 3 4

Refinement statistics Resolution limits

10- 2.8 s 22.4% / 29.0%

Rfactor / Rfree R.m.s deviaton in bond length

0.009 A

R.m.s deviaton in bond angle

2.0 ~

*Rmerge=21 I-/average I/ Z I

*RCun~=21 IFpH- FpI- Fr~(calc) I/ ~E IFpH- FpI +Phasing power=2 IFn(calc) I/2 I IG.- G I- IF~(calc) I 3. D E S C R I P T I O N OF T H E S T R U C T U R E .

As shown in Figure la, PEPC monomer has overall dimensions of approximately

603 42 x 42 x 60 ,~. The monomer is abundant in a-helices and consists of the 13barrel structure with eight ~-strands. Whereas the 13barrel region including the active site was fitted well on the map, nine residues (K702- G708) near the region are disordered and could not be modeled. The high mobility of these residues along with three disordered residues (M1- E3) at the N-terminus might contribute to the high overall temperature factor for X-ray data that is estimated as 60 ~2 by Wilson scaling [3]. PEPC t e t r a m e r has an overall size of approximately 60 x 110 x 140 A. The four subunits of PEPC are related by the crystallographic 222 symmetry. The entire PEPC molecule forms a 'dimer of dimers' (Figure lb), which corresponds to the results of biological experiments.

a)

~' ,

b)

t

!' )

Figure 1. Three-dimensional structure of phosphoenolpyruvate carboxylase from E. coli. a) Subunit structure of PEPC. b) The entire PEPC molecule. 4. A L L O S T E R I C R E G U I ~ T I O N

PEPC has a highly conserved unique sequence, 578-FHGRGGSIGRGGAP-591, in which a GRGG motif is repeated twice with two intervening residues. Aspartic acid in one of the effector molecules for the allosteric regulation of PEPC. As shown in Fig. 2, aspartic acid binds to PEPC through the hydrogen bonds to R587, K773, R832, and N881. As reported from previous studies [4], some Lys and Arg residues are essential for catalytic activity and allosteric regulation. One of them, Arg587 in a highly conserved sequence is the residue to cause the inactivation of PEPC by site-directed mutagenesis. Based on these results, the highly conserved loop region including R587 may be regulated by the binding of Asp even though the region is far from the active site.

604

R832

K773 N881

K773 N881

Figure 2. Binding structure of asparate in the regulatory site. 5. M E T H O D S 5.1 C r y s t a l l i z a t i o n a n d d i f f r a c t i o n d a t a

PEPC from E. coli was crystallized as described previously [1] with minor modifications. X-ray diffraction data were collected at station BL-6B of the Photon Factory, Japan. Intensity data were obtained using a Weisssenberg camera for macromolecule crystallography and imaging plates as a detector [5]. The data were processed using DENZO and scaled by the program SCALEPACK [6]. The crystal structure was determined by multiple isomorphous replacement method. 5.2 M o d e l b u i l d i n g a n d r e f i n e m e n t Interpretation of the electron density map allowed the chain tracing for most parts of the polypeptide chain. Refinement has been carried out using X-PLOR and REFMAC, and manual model rebuilding resulted in an R-factor of 22.4% for all 2c F data between 10.0 and 2.8 A resolution. Using a 5% reflection test set (1,380 reflections) the free R-factor [2] value is 29.0%. REFERENCES

1. Inoue, M., Hayashi, M., Sugimoto, M., Harada, S., Kai, Y. and Kasai, N. J. Mol. Biol. 208 (1989) 509. 2. Briinger, A. T. Nature 355 (1992) 472. 3. Wilson, A. J. C. Acta Crystallogr. 2 (1949) 318. 4. Terada, K., Murata, T. and Izui, K. J. Biochem. 109 (1992) 49 5. Sakabe, N. Nucl. Instrum. Methods Phys. Res. A303 (1991) 448. 6.Otwinowski, Z. & Minor, W. in Proc. CCP4 Study Weekend, 29. Jan. (1993)

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

605

M o l e c u l a r characterization of r e c o m b i n a n t p h o s p h o e n o l p y r u v a t e c a r b o x y l a s e f r o m an e x t r e m e t h e r m o p h i l e Tsutomu Nakamuraa, * and Katsura Izuib aGraduate School of Science and bGraduate School of Agriculture, Kyoto University, Sakyoku, Kyoto 606-01, Japan Phosphoenolpyruvate carboxylase (PEPC) of Thermus sp., was characterized and its mechanism of stabilization was studied by means of site-directed mutagenesis. A divergent sequence at a Gly-rich region of Thermus PEPC was revealed to contribute to the activity at high temperature but not to the tolerance to the irreversible heat inactivation. 1. INTRODUCTION Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) catalyzes the reaction of phosphoenolpyruvate (PEP) with HCO3- to form oxaloacetate (OAA) and orthophosphate in the presence of Mg2+, and plays an anaplerotic role by replenishing C4-dicarboxylic acids in the citric acid cycle [1]. In C4 plants such as maize and sugarcane, PEPC plays a key role in the photosynthetic CO2-fixation [2]. PEPC consists of four identical subunits with a molecular mass of around 100 kDa, and its activity is, in most cases, regulated by wide variety of allosteric effectors. Since PEPC utilizes HCO3-, which is chemically less reactive than CO2, as a substrate, its reaction mechanism has been attracting much attention. According to the stepwise reaction model of PEPC-reaction, formation of carboxyphosphate (CP) and enolate anion of pyruvate (Pyr), which must be very unstable in water, is supposed to precede the carboxylation [3, 4]. In order to demonstrate CP and to convert CP to useful compounds, use of organic solvent-resistant PEPC seems more advantageous. Enzymes of thermophiles are potentially suited to application because of their high stability. PEPC from an extreme thermophile, Thermus sp., was highly stable to heat and also to high concentrations (up to 4 M) of dioxane. In order to study thermostable PEPC using genetic engineering technique, we have cloned and sequenced the gene for PEPC from Thermus sp. [5]. The polypeptide of Thermus PEPC consisted of 857 amino acid residues and its molecular mass was 95,632 Da. In the present study, Thermus PEPC was expressed in E. coli and enzymological characterization was performed. Furthermore, site-directed mutagenesis was carried out to identify the residues which contributes to the expression of catalytic activity at high temperature.

2.1. Expression and purification of recombinant Thermus PEPC The gene for Thermus PEPC was inserted to an expression vector pTVll9N and expressed in E. coli JM109. Although the expression level in the E. coli cells was low, Thermus PEPC *Present address: National Institute of Bioscience and Human-Technology, 1-1 Higashi, Tsukuba 305, Japan.

606 was purified to homogeneity with ammonium sulfate fractionation, heat treatment at 65~ for 10 min, column chromatography with Butyl-Toyopearl, Mono-Q, and Superose 12 [6]. Thermus PEPC had a strong interaction with hydrophobic-interaction column as reported for PEPCs from the other thermophilic organisms [7, 8].

2.2. Catalytic and regulatory properties of Thermus PEPC The activity of Thermus PEPC was enhanced by acetyl-CoA (CoASAc), as is the case with the E. coli enzyme. Acetyl-CoA affected not only maximum velocity (Vmax) but also halfsaturation concentrations (S0.5) of PEP and Mg2+. Thermus PEPC was inhibited by aspartate and malate. Unexpectedly, it was also inhibited by various phosphorylated compounds such as fructose 1,6-bisphosphate and GTP which are known to be activators of E. coli PEPC. The inhibition by these compounds showed tendencies to be released by increasing CoASAc. These properties were essentially the same with those of native Thermus PEPC.

2.3. Behavior of Thermus PEPC at high temperature The activity of Thermus PEPC was highest at 80~ when monitored with Vmax in the presence of I mM CoASAc (cf. Fig. 3). Above the optimum temperature, Thermus PEPC showed decreased activity. This decrease of activity was reversible and was not due to irreversible heat inactivation. Inactivation of Thermus PEPC at high temperature proceeded slowly even at 95~ Fig. 2) and the inactivation was negligible during measurement of enzyme activity which took about 5 min. Thus, the mechanism for loss of catalytic activity prior to irreversible heat denaturation remains to be elucidated.

2.4. Identification of the region contributing to the expression of activity at high temperature Alignment of Thermus PEPC with the enzymes of mesophilic organisms showed a significant divergence of Thermus PEPC in a Gly-rich conserved region. PEPCs of mesophilic sources have highly conserved and unique sequences with repeated Gly-rich motifs (GRGG) and two intervening residues (Fig. 1, ref [9]). This region is presumed to be exposed to the solvent and to form a flexible loop since it is rich in Gly and polar residues. Previous studies by means of site-directed mutagenesis of His579 and Arg587 in E. coli PEPC showed the involvement of this region in the catalytic activity [10, 11]. In Thermus PEPC, the amino acid sequence in the corresponding region is 562-FHGRGTSTARGGGP575. At three sites (underlined above), two Gly residues are substituted by bulkier amino acids, Thr and Ala, and an aliphatic residue by hydroxylated one, Thr. These substitutions seem to decrease the flexibility of the Gly-rich region of Thermus PEPC. In order to examine the role of the divergent sequence of Thermus PEPC in the behavior at high temperature, we prepared a mutant Thermus PEPC in which Thr567, Thr569, and Ala570 were replaced with the residues of the E. coli enzyme, Gly, lie, and Gly, respectively, and characterized it [12].

(A)

Thermus sp. 562-FHGRGTSTARGGGP-575 E s c h e r i c h i a coli FHGRGGSIGRGGAP Coryn eba ct eri um FHGRGGTVGRGGGP gl u tami cum A n a c y s t i s nidulans FHGRGGSVGRGGGP S o r g h u m vulgare (C4) FHGRGGTVGRGVGP Zea mays (C4) FHGRGGTVGRGGGP

(s)

Mutant Thermus PEPC

562-FHGRGGSIGRGGGP-575

Fig. 1 Gly-rich highly conserved region of PEPCs.(A) Amino acid sequences of the Gly-rich region of Thermus PEPC and other 17 ones aligned by Toh et al. (1994). (B) Amino acid sequence of the Gly-rich region of mutant Thermus PEPC. Mutagenic residues are indicated by boldfaces.

607 As shown in Fig. 2, rate of heat inactivation at high temperature (90 and 950C) was almost the same with the wild-type and the mutant. Thus, the tolerance to heat inactivation of Thermus PEPC was not affected by the mutation. On the other hand, catalytic activity of Thermus PEPC and its dependence on the temperature was affected by the mutation (Fig. 3). When assayed below 65~ Vmax and S0.5 of PEP of the mutant Thermus PEPC were around 70% and 15-fold, respectively, of those of the wild-type. The optimum temperature was lowered, from 80~ to 65~ by the mutation. Therefore, the Gly-rich region of Thermus PEPC does not contribute to heat stability of the enzyme, but does to its activity at high temperature. A |

1~176 t

i

,

I

,

&

,

o

200

100

Incubation time (min)

Fig. 2 Heat inactivation of Thermus PEPCs. Heat treatments were carried out with 0.1 mg/ml of wild-type ( e , A) and mutant (O, A) Thermus PEPCs at 90 (0, O)and 95~ (A, A). The residual activities were assayed at 60~ in the reaction mixture containing 10 mM potassium PEP, 10 mM KHCO 3, 10 mM MgSO4, 0.3 mM NADPH, 1.0 mM CoASAc, 0.1 M Ches-KOH, pH 8.6, 2.0 U malate dehydrogenase from Thermus sp., and the enzyme. Relative activities were plotted against incubation time.

300 0")

E c

20o

X m 100

040

50

60

70

80

Temperature (~

90

oo

Fig. 3 The effect of temperature on the activity of wild-type (4,) and mutant () Thermus PEPCs. Vmax was calculated from the saturation curves for PEP. Enzyme assay was performed in the reaction mixture described in the legend to Fig. 2 except for the concentration of PEP.

2.5. Subsidiary activity of Thermus PEPC As described in "INTRODUCTION," postulated reaction intermediates of PEPC, CP and enolate anion of Pyr, are extremely unstable in water. They are hydrolyzed to produce HCO3and Pi, or protonated to Pyr, respectively. Thus, impairment of the partial reaction of the latter step of PEPC-reaction leads to the formation of Pyr due to bicarbonate-dependent hydrolysis of PEP. For example, the mutant enzymes of E. coli PEPC in which His137 or Arg587 are replaced with Asn or Ser, respectively, catalyze the subsidiary bicarbonatedependent hydrolysis of PEP[3, 11]. Figure 4 shows the authentic OAA-forming and subsidiary Pyr-forming activities of wild-type and mutant Thermus PEPCs. In mutant Thermus PEPC whose Gly-rich region had been converted to that of the E. coli enzyme, higher ratio of Pyr/OAA-formation was observed than the wild-type especially at high temperature. Judged from the sequences, the Gly-rich region seems to be more flexible in the mutant than in the wild-type enzyme. Thus, exceeding flexibility in the Gly-rich region may cause the subsidiary activity of Thermus PEPC by exposing the intermediates to the solvent.

608 3. CONCLUSION In Thermus PEPC, stability of the enzyme can be expressed in two ways, activity at high temperature and tolerance to heat inactivation. Site-directed mutagenesis of Gly-rich region of Thermus PEPC revealed that the divergent sequence at the region contributed to the former but not to the latter. Moreover, the divergence at the region was suggested to be important for avoiding the subsidiary activity of Thermus PEPC at high temperature.

.2oo

. . . . . . . . .

"~ 8o

. . . . . . . . .

il

ut

,oo

4o._ 20

"N

o

0

20

40

60

80

100

Temperature (~ lOO

(B)

60

40

a. 20 0 40

50

60

70

80

Temperature (*C)

0

20

40

60

80

100

Temperature (~

80

0

o

90

100

Fig. 4 0 A A - and Pyr- forming activities of wild-type and mutant Thermus PEPCs. (A) OAA-forming activity (0, A)or Pyrforming activity (O, A)were assayed with wild type (left) or mutant (right) Thermus PEPC in the presence (e, O)or absence (A, A)of 1.0 mM CoASAc. (]3) Relative Pyr-forming activity to OAAforming activity in the presence of 1.0 mM CoASAc is represented in % for wild type (l I) and mutant (r-l) Thermus PEPC.

REFERENCES 1. M.F. Utter and H.M. Kolenbrander, in The Enzymes (Boyer, P.D., ed,) 3rd ed., Vol. 6, 117, Academic Press, New York 1972. 2. R. Chollet, J. Vidal and M.H. O'Leary, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996) 273. 3. K. Terada and K. Izui, Eur. J. Biochem., 202 (1991) 797. 4. J.W. Janc, J.L. Urbauer, M.H. O'Leary and W.W. Cleland, Biochemistry, 31 (1992) 6432. 5. T. Nakamura, I. Yoshioka, M. Takahashi, H. Toh and K. Izui, J. Biochem., 118(1995)319. 6. T. Nakamura, S. Minoguchi and K. Izui, J. Biochem., 120 (1996) 518. 7. Y. Sako, K. Takai, A. Uchida and Y. Ishida, FEBS Lett., 392 (1996) 148. 8. K. Takai, Y. Sako, A. Uchida and Y. Ishida, J. Biochem., 122 (1997) 32. 9. H. Toh, T. Kawamura and K. Izui, Plant Cell Environ., 17 (1994) 31. 10. K. Terada, T. Murata and K. Izui, J. Biochem., 109 (1991) 49. 11. M. Yano, K. Terada, K. Umiji and K. Izui, J. Biochem., 117 (1995) 1196. 12. T. Nakamura and K. Izui, Biotechnol. Lett., 19 (1997) 335.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversionsfor Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

609

Revertant of no-active RuBisCO tobacco mutant, Sp25, obtained by chloroplast t r a n s f o r m a t i o n method using m i c r o p r o j e c t i l e b o m b a r d m e n t Ken-Ichi Tomizawa a, Toshiharu Shikanai b, Ayako Shimoide a, Christine H. Foyerc and Akiho Yokota a,b aplant Molecular Physiology Laboratory, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizu-cho, Kyoto, 619-02, Japan bGraduate School of Biological Sciences, Nara Institute of Science and Technology (NAIST), Takayama-cho, Ikoma, Nara, 630-01, Japan CLaboratoire de Metabolisme, INRA, F78000 Versailles, France 1. I N T R O D U C T I O N Ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBisCO, EC 4.1.1.39) is the key regulatory enzyme of CO2 fixation in photosynthesis [1]. RuBisCO from higher plants is composed of eight large (L) and eight small (S) subunits; the L subunits contain the active site of the enzyme. RuBisCO activities have been enhanced in some bacteria and lower plants by introducing single amino acid substitutions into the L subunit by techniques of in vitro mutagenesis [2]. This approach has not been used, to date, to engineer higher plant RuBisCO because reliable techniques have not been available to introduce foreign DNA sequences (such as an altered rbcL, the chloroplast gene for the L subunit) into the plastid genome. In addition, appropriate hosts that lack rbcL and/or RuBisCO activity have not been available. In this report we take advantage of recent advances in chloroplast transformation technology to introduce a wild type rbcL gene from Nicotiana tabacum into Sp25, a tobacco mutant that lacks RuBisCO activity [3]. Sp25 was generated by EMS mutagenesis and is maternally-inherited. Compared to the wild type gene, rbcL in the mutant has a single nucleotide change, resulting in an amino acid substitution of Gly-322 to Ser-322 [4]. We show that transformation of the mutant with the wild type rbcL gene can restore RuBisCO activity. This suggests that the amino acid substitution in rbcL is the sole reason for the loss of RuBisCO activity in Sp25. These experiments further represent the first successful attempt to genetically engineer higher plant RuBisCO by altering the sequence of the L subunit. 2. MATERIALS AND METHODS 2 . 1 . Chloroplast transformation A vector was constructed that contained coding and flanking regions (5' and 3' ) of the wild type N. tabacum rbcL gene. The aadA gene, conferring spectinomycin resistance was introduced into the 3'-flanking region of rbcL for purposes of selection. Gold particles were coated with vector sequences and then applied to mature Sp25 leaves (2-4 cm 2) by particle bombardment, according to the methods of Svab and Maliga [5]. After bombardment, transformants were screened on a solidified MS medium containing spectinomycin. Because it

This work was partly supportedby PEC/MITI

610 is photosynthetically incompetent, Sp25 (and putative transformants) were maintained on sucrose-containing medium.

2.2. Analyses of rbcL gene sequences Total cellular DNA was prepared by the method of Taylor and Powell [6]. PCR (polymerase-chain reaction) was performed using two oligonucleotides, 5'AACATATACCACTGTCAAGG-3' and 5'-TCACTCAGAAAAGAATGATC-3', corresponding to ca. 300 bp upstream and ca. 1150 bp downstream, respectively, of the coding region of the tobacco rbcL gene [7]. The PCR products were fractionated by electrophoresis, purified and then subjected to restriction enzyme analysis. A small aliquot of the PCR reaction mixture was used for DNA sequencing using the dye terminator cycle sequencing FS ready reaction kit (Perkin-Elmer Japan Corp., Tokyo, Japan) with a synthetic oligonucleotide, 5'-AACAAAATCATCACGCAGTA-3'. The products were analyzed using an ABI PRISMTM377 DNA Sequencer (Perkin-Elmer Japan Corp., Tokyo, Japan). 2.3. Analyses of RuBisCO content and activity To determine RuBisCO contents, leaf tissues were quick-frozen in liquid nitrogen then homogenized with a mortar and pestle in a buffer containing 50 mM HEPES-KOH (pH 7.6), 1 mM dithiothreitol, 2% polyvinylpolypyrrolidone, 1raM phenylmethylsulfonylfluoride and 10~tM leupeptin. The homogenate was passed through two layers of cheesecloth, centrifuged at 20,000 x g for 10 min, then fractionated on a PD-10 gel filtration prepacked column (Pharmacia Biotech, Uppsela, Sweden). The fractionated sample was analyzed by SDS-PAGE To analyze RuBisCO activities, 200 ~1 of the soluble protein sample (lmg/ml) was added to 0.5ml of a buffer containing 100mM HEPES-KOH (pH 8.0), 20mM MgC12, 0.1mM DTF, lmM EDTA, and 20mM NaHCO3. The mixture was incubated at 25 ~ for 10 min (for RuBisCO activation to occur), then 25 ~tl of 20mM RuBP was added for 10 min (for the carboxylation reaction to occur). The sample was then treated with Dowex and the filtrate (5~tl) was analysed using a Dionex DX-AQ 1110 anion-exchange chromatographic system [8]. PGA was detected at about 6 min.

2.4. Analysis of chlorophyll content and photosynthetic activity Chlorophyll contents were determincd by Arnon's method [9]. In brief, fresh leaf tissue was ground to a fine powder in liquid nitrogen. After adding 5 volumes of 80% acetone, thc suspension was incubated overnight at 4 ~ thcn centrifuged. Pigment contents werc determined spectrophotometrically on the supernatants. Photosynthetic activities were measured using a portablc photosynthcsis system (LI6400, LI-COR, Lincoln, NB, USA) according to thc manufacturcr's instructions. 3. R E S U L T S AND D I S C U S S I O N Wild type rbcL sequences were introduced into tobacco Sp25 leaves by the biolistic method, and the transformed plants were regenerated on sucrose-containing medium supplemented with 500~tg/ml spectinomycin. Out of 120 leaves that were bombarded, 8 independent putative transformants were identified that survived three rounds of screening on the spectinomycin. Non-bombarded Sp25 leaves served as controls. Total cellular DNA was prepared from the control and putatively-transformed plants, and PCR amplification was carried out using the two primers. It would be predicted that a

611 single 2.9 kbp rbcL-containing band would be amplified from Sp25 plants; this band should not contain a HindIII site. On the other hand, it would be predicted that the primers should amplify a 4.2 kbp band from the transformed plants (rbcL + aadA); this fragment should contain two HindIII sites, giving rise to subfragments of 2.0kbp, 1.4kbp and 0.8kbp. One of these HindIII sites arises from the Gly-322 (versus the Ser-322) in the wild type rbcL sequence. Of the 8 transformants, four behaved like Sp25; these are likely spontaneous mutants, as described by Svab and Maliga [5]. Three of the transformants were heteroplasmic and contained the three HindIII restriction fragments, as well as faint amounts of the 2.9 kbp band. The final transformant contained a single HindIII site and likely suffered a deletion of the introduced rbcL gene, but not of the aadA gene. Only the three heteroplasmic transformants will be further discussed in this report. Direct sequence determination of the 4.2kbp product in the three transformants confirmed that it contained a Gly-322 instead of Ser-322, as well as the HindIII site (Fig. 1).

Hindlll

9

!~Kpnl

!i W

Fig. 1. DNA sequence analysis of a portion of the 4.2 kbp rbcL-containing fragment in the transformed plants. Both strands of the amplified sample were sequenced. The Ser-322 to Gly-322 change, as well as the new HindIII site, are underlined.

Soluble proteins from the transformed and Sp25 plants, as well as from wild type N. tabacum plants, were separated by SDS-PAGE. As illustrated in Figure 2, RuBisCO L and S amounts are nearly the same in the wild type and transformed plants. This contrasts with their virtual absence in Sp25. RuBisCO activity assays revealed that PGA is detectable after about 6 min in the products of the transformed plants, but that it is not present in Sp25 (data not shown). Taken together, these data indicate that RuBisCO content and activity are restored to wild type levels in the transformed plants. Consistent with these findings, the in vivo photosynthetic activities of the transformants, as assayed by gas exchange, were normal. Compared to the pale green color of Sp25 leaves, the transformants also had dark green leaves and their chlorophyll contents were 1.4 times higher than those of Sp25. It is clear that the loss of RuBisCO in Sp25 has a pleiotropic effect on the physiology of the plant. The results in this report strongly support the hypothesis that the conversion of Gly-322 to Ser-322 leads to the loss of RuBisCO activity in Sp25. These results also pave the way for the introduction of mutations into the tobacco rbcL gene. This will allow us to investigate the

612 structure-function relationships of higher plant RuBisCO, as is being done for the prokaryotic RuBisCO [10].

:3-,' .,,.

, ~ . ~ ~,~ ~ , ~.

A

B

C

Fig. 2. SDS-PAGE of soluble proteins. Lane A: N. tabacum (wild type). LaneB: Sp25. Lane C: Transformant REFERENCES 1. T.J. Andrews and G.H. Lorimer. M.D. Hatch and N.K. Boardman (eds), The Biochemistry ofPlants, vol. 10 (1987) 131 2. G.Schneider, Y. Lindqvist and C.I. Brand6n, Ann. Rev. Biophys. BiomoL Struct., 21 (1992) 119 3. C.H. Foyer, A. Nurmi, H. Dulieu, M.A.J. Parry, J. Experi. Bot..,44 (1993) 1445 4. T. Shikanai, C.H. Foyer, A. Nurmi, H. Dulieu, M.A.J. Parry, A. Yokota, Plant Mol. BioL, 31 (1996) 399 5. Z. Svab, P. Maliga, Proc. Natl. Acad. Sci. USA, 90 (1993) 913 6. B, Taylor and A. Powell, Focus, 4 (1982) 2 7. K. Shinozaki, M. Ohme, M. Tanaka, T. Wakasugi, N. Hayashida, T. Matsubayashi, N. Zaita, J. Chunwongse, J. Obokata, K. Yamaguchi-Shinozaki, C. Ohto, K. Torazawa, B.Y. Meng, M. Sugita, H. Deno, T. Kamogashira, K. Yamada, J. Kusuda, F. Takaiwa, A.Kato, N. Tohdoh, H. Shimada and M. Sugiura, EMBO J 5 (1986) 2043 8. K. Uemura, Y. Suzuki, T. Shikanai, A. Wadano, R.G. Jensen, C. Wendy and A. Yokota, Plant Cell Physiol., 37 (1996) 325 9. D.I. Amon, Plant Physiol, 24 (1949) 1 10. S. Gutteridge and A.A. Gatenby, The Plant Cell, 7 (1995) 809

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi(Editors) Advances in Chemical Conversionsfor Mitigating CarbonDioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Reductive TCA cycle in an aerobic bacterium, thermophilus strain TK-6

613

Hydrogenobacter

Masaharu Ishii, Ki-Seok Yoon, Y a s ~ Ueda, Toshihiro Ochiai, Nare Yun, Seiichi Takishita, Tohru Kodama*, and Yasuo Igarashi Department of Biotechnology, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan

Introduction Hydrogenobacter thermophilus strain TK-6 is an aerobic thermophilic hydrogen-oxidizing bacterium isolated from hot spring in Izu, Japan [1]. As for C02 fixation pathway of the strain, 14C02 labeling experiment [2], in vitro enzyme assays [2], and purification of ATP:citrate lyase [3] in our group supported the idea that the reductive TCA cycle is operative in strain TK-6. However, identification of a strong reductant which is needed for the pyruvate synthase and 2-oxoglutarate synthase and clarification of an electron transport system were definitely needed. H. thermophilus was shown to belong to a very early branching order, the Aquificales [4]. Also, strain TK-6 was isolated from hot spring, which implies that the strain may have its original ecological niche underground where little or no oxygen exists. Taking these things into consideration, it is of great interest to examine if the strain has its ability to grow anaerobically. Results and Discussion 1. Purification and characterization of ferredoxin [5] Purification was performed aerobically by adding octyl- B-glucoside to buffers. Columns of Q-Sepharose Fast Flow, DEAE-5PW, and Superdex 75 were used in these order. The ferredoxin had a molecular mass of 13000. The sequence of Nte rminal amino acids was MKDWKIYEKKLGELKDYLEKNYATNPDVEFRLLXPYDXGF. The sequence had a long stretch at the N-terminal region like those from Thermoplasma acidophilum [6], Sulfolobus [7], and Halobacterium [8]. However, identity was less than 20%. 2. Purification and characterization of pyruvate synthase (Pyruvate:ferredoxin oxidoreductase (POR)) [9] POR activity was determined spectrophotometrically by following the *Faculty of Textile Science and Technology, Shinshu University, Ueda-city, Nagano 386, Japan

614 pyruvate-dependent reduction of methyl viologen under nitrogen gas atmosphere at 70 ~ Buffers used throughout the purification procedure contained 0.1% Triton X-100. Purification was performed by using columns of DEAE-Sepharose CL-6B, PA-QA, Hydroxyapatite, and Superdex-200 in these order. The enzyme had a molecular mass of 135 kDa and was composed of four subunits (46, 31.5, 29, and 24.5 kDa). The enzyme did not react with 2-oxoglutarate. The enzyme reacted with ferredoxin isolated from strain TK-6, FMN, and FAD as an electron acceptor. However, NAD, NADP, and ferredoxin from Chlorella spp. or Clostridium pasteurianum were not effective.

3. Purification and characterization of 2-oxoglutarate synthase (2-Oxoglutarate:ferredoxin oxidoreductase (OGOR)) [10] OGOR activity was determined spectrophotometrically by following 2oxoglutarate-dependent reduction of methyl viologen under nitrogen gas atmosphere at 70~ The buffers used throughout the purification procedure contained 0.1% Triton X-100. For the purification of OGOR, the order of columns usage was the same as that for POR. OGOR and POR could be separated by the first column; DEAE-Sepharose CL-6B. OGOR had a molecular mass of 105 kDa, and was composed of two subunits (70 and 35 kDa). The enzyme only slightly reacted with pyruvate (less than 0.4% relative to that with 2-oxoglutarate). The enzyme reacted with ferredoxin isolated from strain TK-6, FMN, and FAD as an electron acceptor. NAD, NADP, and ferredoxin from Chlorella spp. or Clostridium pasteurianum were not effective. 4. Carboxylation reactions of POR and OGOR [11] Demonstration of synthetic reaction of pyruvate or 2-oxoglutarate by using only purified components has not been successful probably because of the instability of acetyl-CoA and succinyl-CoA. In the next step, attempt was made to measure carboxylation reactions (14CO2 incorporation) of POR and OGOR by using ATP:citrate lyase and succinyl-CoA synthetase, respectively, within the cell extract. The results are given in Tables I and II. Also, the CO2 fixation products were analyzed by using HPTLC and the Bio-imaging analyzer system. The major products of 14CO2 fixation by POR were pyruvate and oxalacetate and the major one by OGOR was 2-oxoglutarate. The reason why oxalacetate was detected in the carboxylation reaction of POR is because pyruvate carboxylase which exists within the cell extract carboxylates pyruvate to produce oxalacetate. These results strongly demonstrate that the reductive TCA cycle is operative as the CO2 fixation pathway in strain TK-6. 5. Electron transport system Formerly, we isolated a new quinone from strain TK-6 (Methionaquinone; 2- m ethyl thio-3-VI,VII-t etrahydro multipr enyl 7-1,4naphthoquinone) [12] and demonstrated that the quinone is involved in the

615 m e m b r a n e electron t r a n s p o r t system of strain TK-6 [13]. Purification of membrane-bound hydrogenase was performed as follows; the hydrogenase was solubilized by 0.5% Triton X-100 at pH 10, and the solubilized enzyme was successively put on columns of Butyl-Sepharose, Mono-Q, and Hydroxyapatite. The enzyme was composed of two subunits (63 kDa and 32 kDa). Recently, we d e m o n s t r a t e d t h a t the activities of soluble NAD-reducing hydrogenase and NADH:Fd reductase exist within cell extract. Based on these results, we proposed a mechanism for energy generation in strain TK-6 [14]. Table 1. The carboxylation reaction of POR Treatment

[14C]incorporated (cpm)

Complete

22600

Citrate omitted

103

CoA omitted

101

ATP omitted

1060

Ferredoxin omitted

3000

NaH14C03 omitted

35

Table 2. The carboxylation reaction of OGOR Treatment Complete

[14C]incorporated (cpm) 10500

Succinate omitted

114

CoA omitted

145

ATP omitted

1150

Ferredoxin omitted

1500

NaH14CO3 omitted . . . . . . . . . .

36

,

6. Terminal electron acceptor Elemental sulfur, thiosulfate, or sulfate was tested as a terminal electron acceptor for growth of strain TK-6. Among these, the strain was able to grow on elemental sulfur; the strain produced H2S during its growth and the obtained cells were able to grow aerobically again (H2:02:CO2=75:15:10). These facts show that the strain can grow anaerobically. It is widely believed that there was little or no oxygen on the earth when the

616 first living cell emerged. Because the strain TK-6 is thought to belong to a very early branching order and because the strain can grow anaerobically, the strain might evolve so that it can use and tolerate a little amount of oxygen. In fact, when the cultivation of the strain is performed, much care should be taken so that dissolved oxygen concentration is around 1 ppm.

Conclusions (1) Purification and characterization of components involved in the reductive TCA cycle in H. thermophilus strain TK-6 confirmed the operation of the cycle. (2) Analysis of energy transport system demonstrated the overall energy flow in the strain. (3) The strain was shown to grow anaerobically.

REFERENCES 1. T. Kawasumi, Y. Igarashi, T. Kodama, and Y. Minoda. Int. J. Syst. Bacteriol., 34 (1984) 5. 2. H. Shiba, T. Kawasmni, Y. Igarashi, T. Kodama, and Y. Minoda. Arch. Microbiol., 141 (1985)198-203. 3. M. Ishii, Y. Igarashi, and T. Kodama. J. Bacteriol., 171 (1989) 1788. 4. C. Pitulle, Y. Yang, M. Marchiani, ERB moore, JL Siefert, M. Aragno, P. Jurtshuk Jr, GE Fox. Int. J. Syst. Bacteriol., 44 (1994) 620. 5. M. Ishii, Y.Ueda, K.-S. Yoon, Y. Igarashi, and T. Kodama. Biosci. Biotech. Biochem., 60 (1996) 1513. 6. S. Wakabayashi, N. Fujimoto, K. Wada, H. Matsubara, L. Kerscher, and D. Oesterhelt. FEBS Lett., 162 (1983) 21. 7. T. Iwasaki, T.Fujii. T. Wakagi, and T. Oshima, Biochem. Biophys. Res. Commun., 206 (1995) 563. 8. T. Hase, S. Wakabayashi, H. Matsubara, M. Mevarech, and M.M. Werber, Biochim. Biophys. Acta, 623 (1980) 139. 9. K.-S. Yoon, M. Ishii, T. Kodama, and Y. Igarashi, Arch. Microbiol., 167 (1997) 275. 10. K.-S. Yoon, M. Ishii, Y. Igarashi, and T. Kodama, J. Bacteriol., 178 (1996) 3365. 11. K.-S. Yoon, M. Ishii, T. Kodama, and Y. Igarashi, Biosci. Biotech. Biochem.,61 (1997) 510. 12. M. Ishii, T. Kawasumi, Y. Igarashi, T. Kodama, and Y. Minoda, J. Bacteriol., 169 (1987) 2380. 13. M. Ishii, T. Omori, Y. Igarashi, O.Adachi, M. Ameyama, and T. Kodama, 55 (1991) 3011. 14. K.-S. Yoon, Y. Ueda, M. Ishii. Y. Igarashi, T. Kodama, FEMS Microbiol Lett., 139 (1996) 139.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

617

Carbon dioxide fixation and biomass production with blue-green algae Spirulina platensis S a t o s h i H i r a t a a b and M a s a o H a y a s h i t a n i a b a R e s e a r c h I n s t i t u t e of I n n o v a t i v e Technology for the E a r t h (RITE) 9-2, Kizugawa-dai, Kizu-cho, Kyoto 619-02, J a p a n bAkashi Technical I n s t i t u t e , K a w a s a k i H e a v y I n d u s t r i e s , Ltd., 1-1 Ka was ak i- cho, A k a s h i 673, J a p a n * 1. I N T R O D U C T I O N

In p h o t o a u t o t r o p h i c culture of microalgae, CO2 is a sole c a r b o n source for the growth. CO2 supplied to the s u s p e n s i o n culture of microalgae is first dissolved to the medium and utilized by the cells. In u s u a l culture o p e r a t i o n of microalgae, gas mixture containing CO2 is c o n t i n u o u s l y supplied to the medium. As CO2 supply r a t e is more t h a n CO2 c o n s u m p t i o n r a t e by the cells, m o s t of supplied CO2 e s c a p e s to the atmosphere. In the p r e s e n t study, with the regulation of gas supply r a t e , the efficiency of CO2 utilization by the cells is improved. In microalgal cells, CO2 is fixed as organic c o m p o u n d s such as protein, sugars and lipids. For the p u r p o s e of fixation and utilization of CO2 in the e x h a u s t gas from t h e r m a l power generation, a basic r e s e a r c h of biological CO2 fixation process using microalgae was carried out. 2. E X P E R I M E N T A L

A f ila men to u s blue-green alga, Spirulina platensis N I E S 3 9 was u s e d i n this study. This algal s t r a i n was provided from Global E n v i r o n m e n t a l Forum, J a p a n . For the algal cul t i vat i on, SOT medium [ 1] was used. The pH value of the medium was a d j u s t e d at 9.0 with 1 mol dm -3 NaOH. The medium was a u t o c l a v e d at 121 ~ for 15 min prior to use. P r e c u l t u r e of the cells was p e r f o r m e d at 30 ~ in a 0.3 dm 3 E r l e n m e y e r f l a s k for 7 - 10 days. The flask was s h a k e n at 50 rpm on a r o t a r y s h a k e r u nd er irradiation with white fluorescent lamps. The cells h a r v e s t e d by c e n t r i f u g a t i o n were us ed as an inoculum in s u b s e q u e n t culture experiments. For culture e x p e r i m e n t s of S. platensis, a cylindrical culture a p p a r a t u s The present work was supported by a grant from the New Energy and Industrial Technology Development Organization (NEDO), Japan. *Corresponding address.

618 (12 c m in d i a m e t e r , 16 cm in h e i g h t , 1.8 dm 3 in c u l t u r e v o l u m e ) was used. The s c h e m a t i c d r a w i n g o f t h e c u l t u r e a p p a r a t u s is s h o w n i n F i g u r e 1. A f t e r t h e i n o c u l u m was added to t h e m e d i u m , a b a t c h c u l t u r e s t a r t e d a t 30 ~ The light of w h i t e f l u o r e s c e n t l a m p s was c o n t i n u o u s l y i l l u m i n a t e d to the c u l t u r e a p p a r a t u s from t h e side. The m e d i u m was a g i t a t e d a t 60 r p m u s i n g a s t i r r i n g propeller. Air c o n t a i n i n g 10 or 20 % CO2 was s u p p l i e d to t h e m e d i u m t h r o u g h a g l a s s t u b i n g . In t h e c a s e of i n t e r m i t t e d s u p p l y of t h e g a s , w h e n the p H v a l u e of the m e d i u m was m o r e t h a n 9.1, t h e g a s e o u s m i x t u r e was i n t r o d u c e d to t h e m e d i u m . I n c o n t i n u o u s s u p p l y of t h e gas, t h e g a s e o u s m i x t u r e w a s also i n t r o d u c e d a t a c o n s t a n t flow r a t e of 0.54 dm 3 h-1. Cell c o n c e n t r a t i o n in the m e d i u m was e s t i m a t e d as dry cell weight. The h a r v e s t e d cells were c o l l e c t e d b y c e n t r i f u g a t i o n ( 1 2 , 0 0 0 • g, 4 ~ for 15 min) a n d w a s h e d w i t h distilled w a t e r . The cells dried u n d e r r e d u c e d p r e s s u r e were s u b j e c t e d to c h e m i c a l a n a l y s i s . The calorific v a l u e of t h e cells was m e a s u r e d u s i n g a n a u t o m a t i c b o m b c a l o r i m e t e r (CA-3, S h i m a d z u , J a p a n ) . E l e m e n t a l c o m p o s i t i o n of the cells w a s m e a s u r e d u s i n g an automatic elemental analyzer (EAll08, Fisons Instruments Co., Italy). 3. R E S U L T S

AND DISCUSSION

3.1. E f f e c t of i n t e r m i t t e n t s u p p l y of CO~ for algal g r o w t h I n o r d e r to e x a m i n e the effect of i n t e r m i t t e n t s u p p l y of C 0 2 for algal g r o w t h , b a t c h c u l t u r e s of S. platensis were c a r r i e d out for 39 d a y s w i t h t h e i r r a d i a t i o n of light i n t e n s i t y of 18 W m "2. T i m e c o u r s e s of cell c o n c e n t r a t i o n X were s h o w n in Figure 2. T h e r e was little difference b e t w e e n t h e profiles of cell c o n c e n t r a t i o n c h a n g e in i n t e r m i t t e n t s u p p l y and

Intermittent Continuous supply supply X [kg m"3] o Vt [dm3] 1.5

. . . . . . .

"I~ = 18 w m 2 ~ t ~ 1.o

,1 5OO

400

..."

300

0.5 (~) cylindrical culture apparatus, (~) pH probe, (~) white fluorescent lamp, (~)pH controller, (~) CO2 cylinder, (~) compressor, (~) gas mixer, (~) mass flow controller, (~) flow meter, Figure 1. Schematic drawing of a culture apparatus for S. platensis.

o o

lO 20 3o Culture time [days]

39

Fugure 2. Batch cultures ofS. platensis with different gas supply methods.

619 c o n t i n u o u s s u p p l y of CO2. I n F i g u r e 2, t o t a l v o l u m e s of C O 2 - e n r i c h e d air s u p p l i e d to t h e m e d i u m , Vt, w e r e also i n d i c a t e & I n t h e c a s e of c o n t i n u o u s s u p p l y , C O 2 - e n r i c h e d air of 8.7 t i m e s a s m u c h a s in i n t e r m i t t e n t s u p p l y w a s s p a r g e d to t h e m e d i u m . The e f f i c i e n c y of CO2 u t i l i z a t i o n b y t h e cells w a s e s t i m a t e d as 34.5 % in i n t e r m i t t e d s u p p l y a n d 4.2 % in c o n t i n u o u s s u p p l y , r e s p e c t i v e l y . F r o m t h e s e r e s u l t s , it w a s f o u n d t h a t i n t e r m i t t e n t s u p p l y of CO2 c o r r e s p o n d i n g to t h e p H v a l u e of t h e m e d i u m w a s a v a i l a b l e in a p h o t o a u t o t r o p h i c c u l t u r e of a l g a l cells. I n b a t c h c u l t u r e s s h o w n in F i g u r e 2, light i n t e n s i t y w a s s u p p o s e d to r e s t r i c t t h e cell g r o w t h . T h e r e f o r e , b a t c h c u l t u r e of S. platensis w a s p e r f o r m e d u n d e r i r r a d i a t i o n of light i n t e n s i t y of 87 W m -2 w i t h i n t e r m i t t e n t s u p p l y of CO2 g a s . C o n c e n t r a t i o n of CO2 w a s r e g u l a t e d a t 20 %, w h i c h is a p p r o x i m a t e l y t h e s a m e c o n c e n t r a t i o n a s in t h e e x h a u s t gas f r o m t h e r m a l p o w e r g e n e r a t i o n . T i m e c o u r s e s of cell c o n c e n t r a t i o n a n d t o t a l a m o u n t of CO2 s u p p l i e d to t h e m e d i u m are s h o w n in F i g u r e 3 a n d t h e c a l c u l a t i o n r e s u l t s from t h e c u l t u r e a r e also s h o w n in T a b l e 1. The c o n c e n t r a t i o n of t h e cells i n c r e a s e d w i t h e l a p s e d t i m e a n d r e a c h e d 10.9 k g m "3 a f t e r 70 d a y s . The m e a n g r o w t h r a t e of t h e cells d u r i n g 70 d a y s w a s c a l c u l a t e d as 0.16 k g m -3 d -1. S i n c e t o t a l v o l u m e of CO2 s u p p l i e d to t h e m e d i u m for 70 d a y s w a s 55.8 dm 3, it w a s also c a l c u l a t e d t h a t

lO

~i0=87 w m 2 .

I

o' o o

400 3oo

o

200 i00

,~. ~-~

o

0

20

40

60

80

Culture time [h] Figure 3. Batch culture of S. platensis with intermitted supply of 20% CO2. Table 1. Calculated results from batch culture of S. platensis with intermitted supply of 20% CO2. 0.16 kg-dry cells m "3 d -1 Mean growth rate of the cells 0.28 kg-CO2 m 3 d -1 Mean CO2 fLxation rate by the cells 55.8 dm 3 Total volume of C02 supplied to the medium for 70 days 19.7 g-dry cells Produced cells in the culture for 70 days 49% Carbon contents of the cells 32.3 % Efficiency of CO2 utilization by the cells for 70 days 3.25 x 107 J Furnished light energy for 70 days 1.94 x 107 J kg-l-dry cells Calorific value of the cells 1.2 % Efficiency of conversion from light to chemical energies

620 32.3 % of supplied CO2 was utilized by the cells and fixed as organic compounds. The c a r b o n c o n t e n t of the cells was 49 % and the calorific v a lu e of the cells was 1.94• J kg -1 on dry weight basis. The m e a n CO2 fixation r a t e c a l c u l a t e d from the c a r b o n c o n t e n t was 0.28 kg-CO2 m -3 d "1. The efficiency of conversion from light energy to c h e m i c a l energy was e s t i m a t e d as 1.2 %. From t h e s e r e s u l t s , it was possible t h a t CO2 in the e x h a u s t gas from t h e r m a l power g e n e r a t i o n was efficiently r e m o v e d with p h o t o s y n t h e s i s of microalgae with r e g u r a t i o n of gas supply r a t e to the medium.

3.2. C o m p o s i t i o n of p r o d u c e d c e l l s Table 2 r e p r e s e n t s gross c h e m i c a l compositions of produced cells in b a t c h c u l t u r e s for 39 days. The cells were rich in p r o t e i n s and the p r o t e i n c o n t e n t was e s t i m a t e d as a p p r o x i m a t e l y 60 % of the dry weight. C a r b o h y d r a t e s including sugars and fibers was a b o u t 13 % of the dry weight and lipids was also a b o u t 8 %. These r e s u l t s r e f l e c t e d t h e p o t e n t i a l of the cells as h u m a n food, a n i m a l feed and fertilizers. In this e x p e r i m e n t s , a lot of a s h e s were d e t e c t e d in the cells. The sodium and p o t a s s i u m c o n t e n t s were a b o u t 5.7 % and 2.4 % of the dry cell weight, r e s p e c t i v e l y . It was s u p p o s e d t h a t the n u t r i e n t s from the medium r e m a i n e d i n the sam pl e u sed for the chemical a n a l y s i s . Table 2. Gross chemical composition of produced cells Composition g per 100g of dry cells

Composition g per 100g of dry cells

Protein Lipids Sugars Fibers Ashes

P Fe Ca Na K

60.1 7.6 12.2 1.2 18.9

1.29 0.054 0.077 5.69 2.35

4. C O N C L U S I O N (1) In p h o t o a u t o t r o p h i c c u l t u r e s of microalgae, the ratio of C02 fixed by the cells was i m p r o v e d by m e a n s of regul at i ng gas supply r a t e corresponding to the pH value of the medium. (2) In a b a t c h culture o f S . platensis for 70 days with the r e g u l a t i o n ofgas supply r a t e , cell c o n c e n t r a t i o n b e c a m e 10.9 kg m -3 and the a v e r a g e C02 fixation r a t e c a l c u l a t e d on the b a s i s of the carbon c o n t e n t of the cells was 0.28 kg-CO2 m "3 d -1. (3) The produced cells were rich in p r o t e i n s , c a r b o h y d r a t e and lipids. It was s u p p o s e d t h a t foods, feed and fertilizer were p r o m i s i n g for utilization of microalgal b i o m a s s produced using CO2.

REFERENCES 1. Marquez et al., J. F e r m e n t . Bioeng., 76 (1993) 408.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors)

Advances in Chemical Conversions for Mitigating Carbon Dioxide

621

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Chitosan-calcium Biomimetic into

carbonate

mineralization

chitosan-calcium

Shigehiro Hirano,Koichi Kjell M. V a r u m , b a n d O l a v ~Department University, bDepartment

Norway

composites: of

alginate

Biotechnology,

University

carbonate

ions

hydrogels

Yamamoto, ~ tliroshi Smidsrod b

of A g r i c u l t u r a l Biochemistry Tottori 680, Japan, of

aqueous

and

of

Inui, -

Kurt

I.

Biotechnology,

Trondheim,

Draget,

b

Tottori

N-7034,

Trondheim,

Two hydrogels of c a l c i u m a l g i n a t e and chitosan-calcium alginate composite were prepared f r o m Na a l g i n a t e and its composite with chitosan by soaking in O.1M a q . C a C l z . A biomimic fixation of a q u e o u s C 0 3 2 - i o n s a s CaC03 was performed. E a c h of t h e h y d r o g e l s was p u t i n t o a d i a l y s i s membrane tube, and was s o a k e d in 0.1M a q . Na2C03 (pH 11) a t room t e m p e r a t u r e for 1 day to afford a white precipitate o f CaCOa or c h i t o s a n - C a C 0 3 and a watersoluble sodium alginate. The d r y c h i t o s a n - C a C 0 3 composite (60% c h i t o s a n and 23% C 0 3 2 - ) w as s t a b l e in aqueous alkaline solutions and unstable in aqueous acidic solutions. 1

.

I N T R O D U C T

I ON

Chitin, a (l~4)-linked N-acetyl-i~-D-glucosaminan, and chitosan, the Ndeacetylated derivative of chitin, are widely found in shells of many marine animal such as crabs and shrimps. Alginic acid, a heteropolysaccharide consisting of ( l ~ 4 ) - l i n k e d ~-D-mannuronic acid and ~-L-guluronic acid residues, is found in various seaweeds. Calcium carbonate is the main structural c o m p o n e n t of c r a b s h e l l s , seashells and coral reefs [1]. The c r a b s h e l l i s o n e of n a t u r a l composite materials of CaCOa, chitin and p r o t e i n as the main components [2]. Several artificial composite materials of c h i t i n or c h i t o s a n with inorganic salts have been prepared [3]. In t h e e c d y s i s of c r a b s h e l l s [4-6], CaC03 i n t h e shells is solubilized into the body fluids and into the specific organs, and the solubilized Ca i o n s a r e u t i l i z e d for the mineralization of a q . HC03ions as CaC03 in t h e h y d r o s p h e r e to f o r m new s h e l l s . Crabs assemble chitin fibrils into a cylindrical architecture at the endocuticle layer, using Ca 2§ i o n s in t h e body fluids and aq. C032- ions in the hydrosphere. The calcification of HC03- i o n s i n t h e h y d r o s p h e r e has been reported in coral reefs [7]. Sea water is slightly alkaline ( pH 7 . 5 - 8 . 5 ) due to p r e s e n c e of several metal cations including Na § Mg 2 § a n d K§ [ 8 ] . In our previous works [9-11], f r e e C a C l 2 was u s e d a s a c a l c i u m source for the fixation of HCOaand COa 2 - i o n s i n w a t e r . However, the greater part of CaCl2 was released out of t h e g e l a t i n o u s matrixes of c h i t i n and chitosan, and the maximum 0 . 4 4 m o l e C032§ i o n s p e r GlcN ( o n l y a b o u t 10% o f t o t a l weight) was mineralized as CaC03. Zhang and Gonsalves [12] reported a crystal growth of CaC03 on c h i t o s a n surface in a s u p e r s a t u r a t e d CaC03 solution. Taking these previous reports into consideration, it is essential to develop an efficient and mild method for the mineralization o f a q . COa 2 and HC03-

622

ions in water. We w i s h t o r e p o r t a novel biomimic mineralization o f a q . C0~ 2 - i o n s i n t o a Ca a l g i n a t e hydrogel and into a chitosan-Ca alginate hydrogel in water. 2 . MATER I ALS AND METHODS Crab shell chitosan (Flonac N, K y o w a T e c h n o s , Co., Ltd. Chiba) was treated one t i m e w i t h a q . 40% Na0H a t 100 ~ for 4 h to give a purified sample of chitosan, MW 3 x 1 0 5 ; d . s . 0 . 1 f o r NAc; [ ]D 2~ - 6 ~ (C 0 . 8 % , 2% a q . Ac0H); p ~ x KBr 1 6 2 0 cm -~ (NH2) a n d no a b s o r p t i o n s were detected a t 1 6 5 0 arid 1550 cm-1 (NAc). Sodium alginate (MW 3 6 x 1 0 s , 69% L-guluronic acid) was a product of Protan Co., Ltd., Norway (Protanal L F I 0 1 6 0 SLP 4 6 6 ) , and its source was from a brown algae, Laminaria. hyperborea. FTIR spectra were recorded on a J a s c o F T I R 5 3 0 0 s p e c t r o m e t e r (Jasco Company, Tokyo). 3

.

3.1

EXPER

.

I

MENTAL

C a l c i u m

a l g i n a t e

h y d r o g e l

A solution o f Na a l g i n a t e ( 0 . 5 0 g) i n w a t e r (20 m l ) w a s p u t i n t o a d i a l y s i s membrane tube (diameter 1.5 cm), and soaked in aq. 0.1M CaC12 (500 ml) at room temperature overnight to afford a rigid hydrogel. The hydrogel was dialyzed against running water and against distilled water to give a transparent Ca a l g i n a t e hydrogel in the tube. 3 . 2 .

T r e a t m e n t

N a 2 C O 3

of

the

Ca

a l g i n a t e

h y d r o g e l

in

O.1

M

aq.

s o l u t i o n

The hydrogel obtained above was put into a dialysis membrane tube, and was soaked i n 5 0 0 ml o f a q . 0 . 1 M Na2C03 solution (pHll) with gentle stirring at room temperature for 1 day. Calcium carbonate appeared as a white precipitate ( 0 . 1 0 g ) , a n d Na a l g i n a t e was solubilized. The precipitate was collected by filtration and dried to afford CaC03 in 0 . 1 0 g y i e l d . ~ma• in yacuo, 1450, 8 7 0 a n d 710 c m - 1 ( C 0 3 2 - ) . The filtrate was concentrated and 3 volumes of ethanol was added to afford Na a l g i n a t e as a precipitate. 3 . 3 .

C h i t o s a n - - c a l c

ium

a l g i n a t e

gel

A chitosan ( 0 . 3 0 g) s o l u t i o n in aq. 0.2% acetic acid (6 m l ) was diluted with water (17 m l , pH a b o u t 6.5). To t h e s o l u t i o n , w a s a d d e d a Na a l g i n a t e ( 0 . 4 0 g) s o l u t i o n i n 20 ml d i s t i l l e d water to afford a viscous transparent solution. The solution was put into a dialysis membrane tube (diameter 1.5 cm), and was soaked in 0.1M aq. CaCl, solution (500 ml) overnight to give a rigid hydrogel. The hydrogel was soaked in distilled water overnight to afford a transparent hydrogel of chitosan-Ca alginate composite. The hydrogel was stable in boiling water, but unstable in aq. strong acid and alkaline solutions. A part of the solution was lyophilized. ~ ms• KBr 3 5 0 0 (OH), 1640 (C00-), 1600 ( N H a ) , a n d 1100 ( C - 0 ) c m - 1 ; A n a l . N 3.73% (43% chitosan). 3 . 4 . in

T r e a t m e n t O.1

M

aq.

of N a z C O 3

a

c h i t o s a n - - C a

a l g i n a t e

h y d r o g e l

s o l u t i o n

The hydrogel obtained above was put into a dialysis membrane tube, and was soaked in 500 ml o f 0 . 1 M a q . N a z C 0 3 s o l u t i o n (pH c a . ll) with gentle stirring at room t e m p e r a t u r e for 1 day. A chitosan-CaC03 composite was

623

obtained as a white precipitate, a n d Na a l g i n a t e was solubilized. The composite was collected and dried to afford a white chitosan-CaC03 composite in 0.39 g yield. ~ ms• KBr (KBr) 1 6 0 0 ( N H z ) , 1 4 2 6 , 8 7 6 a n d 7 1 2 c m -1 (C032-). Anal. N 5 . 2 2 % (60% c h i t o s a n ) . The composite contained 23% CO~ 2 as estimated f r o m C/N r a t i o of the elemental analyses. The filtrate was c o n c e n t r a t e d in yacuo, and 3 volumes of ethanol was added to afford Na alginate

as

a precipitate.

Fig. 1. ~ i~hot~graphic scerlc of a chitosan-CaCO~ composite. Left, an a i r - d r i e d slice of the chitosan-CaC03 composite hydrogel; an a i r - d r i e d chitosan of the same size as the composite hydrogel.

4.

R e s u l t s

and

Right,

D i s c u s s i o n

In t h e Ca a l g i n a t e hydrogel, Ca 2§ i o n s a r e b o u n d a s s a l t to the region of contiguous L-guluronic acid residues (G-block) more stronger than those of contiguous D-mannuronic acid (M-block) and alternative L-guluronic and Dmannuronic acid (GM-block) residues [1]. The a l g i n a t e sample used in the present study contains 69% o f t h e G-block. The chitosan-Ca alginate hydrogel was s o a k e d in O.1M a q . Na2CO~ s o l u t i o n to afford a chitosan-CaC03 composite as a precipitate, a n d Na a l g i n a t e is solubilized into water. The c a l c i f i c a t i o n reaction o f a q . COs 2 - i o n s i n t h e Ca a l g i n a t e hydrogel is as followings: Na a l g i n a t e + C a C I 2 ---> Ca a l g i n a t e Ca a l g i n a t e + N a 2 C 0 3 ----> CaCOo The hydrogel

calcification reaction is as followings:

of

+ NaC1 + Na a l g i n a t e

CO~ 2 -

ions

in

the

Chitosan-Na alginate + C a C 1 2 ---> C h i t o s a n - C a alginate Chitosan-Ca alginate + Na2C03 ---> C h i t o s a n - C a C 0 3

chitosan-Ca

alginate

+ 2NaCI + Na a l g i n a t e

One m o l e o f Ca 2§ i o n r e a c t s with one mole of C032- ion, and one mole of CaC03 is produced in the reaction with NazC03 as demonstrated here. However, o n e m o l e o f Ca 2§ i o n r e a c t s w i t h two m o l e s o f H C 0 3 - i o n s , and one m o l e o f C a C 0 3 a n d o n e m o l e o f COe a r e p r o d u c e d in the reaction with NaHC03 as demonstrated previously [13]. Fig. 1 shows a photographic scene of a dry piece of the chitosan-CaC03 composite, which did not shrink after drying, although a chitosan piece shrank significantly. The d r y chitosanCaC03 c o m p o s i t e is white and elastic, and stable in alkaline solutions, and

624

unstable in acidic solutions. The c o m p o s i t e is probably usable as a new material f o r mimic c r a b s h e l l s and a s a f u n c t i o n a l feed additive of a n i m a l s and f i s h e s an d a s a g r i c u l t u r a l materials [14]. The r e c o v e r e d alginate is repeatedly usable for the preparation of c h i t o s a n - C a alginate hydrogel. ACKNOWLEDGMENTS

The work was supported by r e s e a r c h g r a n t from the the Industrial Technology Development Organization (NEDO), Research Institute of I n n o v a t i v e Technology for the Earth

New E n e r g y and Tokyo, and the (RITE), Kyoto.

R E F E R E N C E S

1. 2.

0. S m i d s r o d and A. H a u g , A c t a Chem. S c a n d . 22 ( 1 9 6 8 ) 1 9 8 9 - 1 9 9 7 . T. K. P a r s o n s , M. T a k a h a s h i and B. H a r g r a v e , Biological Oceanographic Processes, 3rd Ed., vol. 1, 5 5 - 5 8 ( 1 9 8 4 ) 3. S. H i r a n o , H. I n u i , K. Yamamoto, E n e r g y . C o n v e r s . Mgmt. 36 ( 1 9 9 5 ) 7 8 3 786. 4. I . Yan o, N i p p o n Suisan Gakkaishi 38 ( 1 9 7 2 ) , 7 3 3 - 7 3 9 . 5. I . Y a n o , N i p p o n S u i s a n G a k k a i s h i 40 ( 1 9 7 4 ) 7 8 3 - 7 8 7 . 6. I . Y a n o , K a g a k u to S e i b u t s u 15 ( 1 9 7 7 ) 3 2 8 - 3 3 6 . 7. J. K o n d o , T. I n u i and K. Wasa ( E d s ) , P r o c e e d i n g s of t h e 2nd International C o n f e r e n c e on C a r b o n Dioxide Removal, Pergamon Press, 1995. 8. J . P. R i l e y and G. S k i r o w , C h e m i c a l O c e a n o g r a p h y , Academic Press, New York, 1975. 9. S. H i r a n o , K. Yamamoto, H. I n u i , E n e r g y . C o n v e r s . Mgmt. 38 ( 1 9 9 7 ) $ 5 1 7 $521. 10. S. H i r a n o , K. Yamamoto, H. I n u i , M. J i , M. Z h a n g , M a c r o m o l . Symp. 105 (1996) 149-154. 11. S. H i r a n o , K. Yamamoto, M. Yamada, H. I n u i , M. J i , A d v a n . C h i t i n Sci. 1 (1996) 137-142. 12. S. Z h a n g , K. E. G o n s a l v e s , J . A p p l . P o l y m . S c i . 56 ( 1 9 9 5 ) 6 8 7 - 6 9 5 . 13. S. H i r a n o , K. Yamamoto, H. I n u i , K. I . D r a g e t , K. M. Varum a nd O. Smidsrod, 7th Int. Conf. Chitin/Chitosan, Lyon, Sept 3-5, 1997. 14. S. H i r a n o , A p p l i c a t i o n s of C h i t i n a n d C h i t o s a n . Ed. M . F . A . G o o s e n , Technomic, Lancaster, 1997, pp. 237-258.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

625

T o l e r a n c e o f a g r e e n alga, S c e n e d e s m u s k o m a r e k i i , to e n v i r o n m e n t a l e x t r e m e s N. Hanagata a'b, R. Matsukawa c'd, M. Chihara c'e and I. Karube c a Research Institute of Innovative Technology for the Earth (RITE), Minato-ku, Tokyo 105, Japan b Chiba Laboratory, Mitsui Engineering & Shipbuilding Co., Ltd., Ichihara, Chiba 290, Japan c RCAST, University of Tokyo, Meguro-ku, Tokyo 153, Tokyo d New Energy and Industrial Technology Development Organization (NEDO), Toshima-ku, Tokyo 170, Japan e

Japanese Red Cross College of Nursing, Shibuya-ku, Tokyo 150, Japan

The capacity of a bark-inhabiting Scenedesmus komarekii to tolerant environmental extremes was examined. The optimum temperature for growth was 40~

The alga had high viability even

after an incubation period at 45~ for 3 days and remained after freezing at-20~ for 2 days. This alga was able to grow after the cells were dried at 25~ and 45~

alga could tolerate

an atmosphere up to 30% CO 2. The capacities of the strain to tolerate a high CO 2 concentration was enhanced by preculturing under slightly low CO 2 concentrations. I. I N T R O D U C T I O N Some freshwater algae can adapt to aerial or subaerial habitats although environmental conditions are generally harder on land than in water. Bark-inhabiting green algae are exposed to high and low temperatures, sometimes freezing, dry conditions, air pollutant gases, and strong sunlight. Furthermore, the environmental changes are more rapid on land. To survive in the terrestrial environment, the algae have to be able to tolerate these environmental conditions. In this study, we examined the capacity of bark-inhabiting Scenedesmus komarekii to tolerate environmental extremes and determined which environmental factor was the most important as regards this species. Footnote: This work was in part supported by a grant from the New Energy and Industrial Technology Development Organization to the Research Institute of Innovative Technology for the Earth, Japan.

626 2. M A T E R I A L S AND M E T H O D S

2.1. Algal strain Scenedesmus komarekii Hegewald was isolated from tree bark (1). The culture conditions have been previously described (2).

2.2. Tolerance of the algae to environmental extremes During high temperature tolerance experiment, ten ml of the sub-sample from the preculture was transferred to a test tube and incubated at 45~ for 3 days. The algal suspension was inoculated into a 100-ml Erlenmeyer flask containing 40 ml medium and cultured at 25~ on a rotary shaker (130 rpm). During freezing tolerance experiment, ten ml of the sub-sample from the preculture was transferred to a test tube and frozen at-20~ for 2 days. The frozen algal culture was defrosted and inoculated into a 100-ml Erlenmeyer flask containing 40 ml medium. The culturing was carried out at 25~ on a rotary shaker (130 rpm). In the dryness tolerance experiment, the precultured alga was harvested by centrifugation at 3,000 rpm for 10 min and washed twice with distilled water. The pellet was resuspended in sterilized water so that the optical density was 1.0 at 660 nm. One ml of the suspension was filtered using a filter paper of 2.5 cm in diameter. Two filter papers were prepared. One was dried at 25~ for 7 days, the other was dried at 45~ for 2 days. Each of the dry filter papers was transferred into 40 ml medium in a 100-ml Erlenmeyer flask and cultured at 25~ on a rotary shaker (130 rpm). Cell growth was determined by the optical density of the culture medium at 660 nm. 3. R E S U L T S

AND D I S C U S S I O N

3.1. Effect of temperature on the cell growth Figure 1 shows the effect of temperature on the growth rates. Increasing the incubation temperature from 25~ to 40~ increased the growth rate. The growth rate drastically decreased at 42~

and no growth occurred at 43~

40~ and 42~

The optimum and growth limiting temperatures were

respectively.

3.2. Tolerance to temperature and to dry conditions The alga showed a high recovery of its viability after incubation at 45~ (Figure 2). This result indicates that this alga can maintain cell viability at 45~ Figure 3 shows the growth after incubation at -20~

although no growth occurs.

The alga grew after a 5-day lag. This alga

had 3-day and 5-day lags before growth after drying 25~ and at 45~

respectively (Figure 4).

627

, -|

0.3

2.0

"o "7, ..J

E to r r (g a

0.2 "o O) 0.1

r

0

L

0 ~

0.0 2O

25

30

35

40

1.0

0.5

t"

r

1.5

0.0 q

v

I

Temperature (~

I

I

3

0

45

. 6

.

.

.

.

9

. . 12

15

18

Time (d)

Figure 1. Effect of incubation temperature

Figure 2. Tolerance of high temperature.

on the growth rate

The alga incubated at 45~ for 3 days was cultured at 25~

The lag time shows the cell damage injured by these treatments. The longer lag time and the lower growth rate indicate that freezing is more harmful than high temperatures for this alga. 0.3

E c:

o r r

CI O

1.5

E

0.2

1.0

o ~D ~D 0.1

m a o

v

0.0

.

0

.

5

.

10 Time

.

.

15

0.5

0.0 c

20

0

3

(d)

6

9

12

Time

(d)

15

18

21

Figure 3. Tolerance to freezing. The alga

Figure 4. Tolerance to dry conditions at

frozen to -20~ for 2 days was cultured

25~ and 45~

at 25~

7 days and at 45~ for 2 days was

The alga dried at 25~ for

cultured at 25~ 3 . 3 . E f f e c t o f CO 2 c o n c e n t r a t i o n o n the c e l l g r o w t h

The effects of the

CO 2

concentration of the aeration gas on the cell growth at the optimum

temperature is shown in Figure 5. The alga was precultured in air. The growth rates of the air-adapted alga were not affected by CO 2 concentrations of 10% and 20% in the bubbling air. A four-day lag before growth was observed in the culture bubbled with 30% CO z in the air. The

628 alga was precultured with air containing 20% grow in

CO 2

CO 2 .

Consequently, the CO2-adapted alga could

concentrations of 40% and 50% (Figure 5). This result shows that the alga is able

to adapt to higher concentrations of

CO 2

after preculture in a high concentration of

CO 2 .

Hanagata et al. (3) and Kodama et al. (4) also reported that the capacities of Chlorella sp., S. acuminatus and Chlorococcum lithorale to tolerate a high CO 2 concentrations were enhanced

after preculturing high concentrations of CO 2 . Although S. komarekii used in this study could also adapt to the high concentration of CO 2, the mechanism is still not clear. Further studies will be needed on the mechanism of adaptation to higher concentrations of CO 2. 0.5 "7, ..J

A I-"

air-adapted alga

0.4

0.5

"o ol

0.3

~" o1

0.3

m (n

0.2

m

0.2

t~

m

E

=

CO2-adapted alga

0.4

E

0.1

0.1

0

0.0 I 0

0.0 2

4

6

8

10

12

, 0

, 1

Time (d)

Figure 5. Effect of

CO 2

,

, 2

9 3

9 4

Time (d)

concentration on the algal growth. The alga preincubated in air (left) and

CO2-riched air (fight) were cultured in air (open circle), 10%

CO 2

(open square), 20% CO 2

(open triangle), 30% CO 2 (closed circle) and 40% CO 2 (closed square) in air. The cultivation temperatures were maintained at 40~

REFERENCES

1. N. Hanagata, I. Karube and M. Chihara, J. Jpn. Bot., 71 (1996) 87. 2. N. Hanagata, T. Takeuchi, Y. Fukuju, D. J. Barnes and I. Karube, Phytochemistry, 31 (1992) 3345. 3. M. Kodama, H. Ikemoto and S. Miyachi, J. Mar. Biotechnol., 1 (1993) 21.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

629

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

Over-expressed effect of carbonic anhydrase cyanobacterium, Synechococcussp. PCC7942

on CO2 fixation

in

M. Murakami .'b, N. Yamaguchi ~b, T. Nishide ~, T. Muranaka ~band Y.Takimoto "b a Research Institute of Innovative Technology for the Earth (RITE), Minato-ku, Tokyo 105, JAPAN b Biotechnology Laboratory, Sumitomo Chemical Co., Ltd., Takaram~a, Hyogo 665, JAPAN Over-expressed effect of carbonic anhydrase (CA) on CO2 fixation in cyanobacterium, Synechococcus sp. PCC7942, was evaluated. CA overexpression in carboxysomes induced elevated levels of growth rate and CO2 fixation rate, suggesting the possibility to improve photosynthetic performance. 1. INTRODUCTION Cy anob acterium is a pro c ary oti c i.~'''"'r"''''''~ organism with the ability of oxygenic Cettwatt photosynthesis. In cyanobacterium, Cell membrano~~ the external dissolved inorganic carbon Carboxysom (DIC), CO2 and/or bicarbonate, enters the cells and accumulates in the ~ c / cytoplasm via energy-dependent Hi Oa" I system(s). Regardless of whether HCO; r~ T [~ ,.-.,.,, 9 ~ HCOa" r~ bicarbonate or CO2 is supplied, bicarbonate is the predominant species Periplasmic CA in the cytoplasm. Bicarbonate emers the carboxysomes, polyhedral subcellular C02 bodies, where localized carbonic anhydrase (CA)catalyzes the formation of CO2 from bicarbonate, increasing the Figure 1. Schematic view of concentration of substrate at the site of inorganic carbon transportation carboxylating enzyme, ribulose 1,5and concentration mechanisms biphosphate carboxylase/oxygenase in cyanobacteria. (RuBisCO) (Fig. 1) [1 ]. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO).

630

When the cDNA encoding human CA was introduced into Synechococcus sp. PCC7942, CA was expressed in the cytoplasm and the" transformant showed a high CO2 requiring phenotype, possibly due to a rapid conversion of cytoplasmic bicarbonate into CO2, followed by a CO2 leakage from the cells to the medium[2]. This showed the importance of CA localization in the cells. In order to evaluate the effect of increased CO2 supply to RuBisCO on CO2 fixation, we constructed transformed Synechococcus sp. PCC7942 which overexpressed CA at the site of RuBisCO. In this report, we examined the effect of CA over-expression on CO2 fixation rate. 2. MEIItODS 2.1. Construction of CA over-expression vector and transformation Plasmids containing the cyanobacterial CA gene (icfA)[3], promoter and terminator derived from RuBisCO and the ampicillin resistant gene were constructed (Fig.2). The resulting plasmids were introduced into Synechococcus sp. PCC7942 [4]. 2.2. Western blot analysis of CA protein Total soluble protein of Synechococcus sp. PCC7942 was used in SDS-PAGE and Western blotting analysis using a Multigel 10/20 (Daiichikagaku) and an ECL Western

blotting detection kit (Amersham). Rabbit

pUG118

Sac-~,~.~

Xho l

icfA ~ T ~ P v u

II

Sac I--~ g%UH24-pUC-CA-R] ori ~ X h o~l . ~ , , ~ , M pUH24

Bg/ll

P : Synechococcus sp.PCC7942

RuBisCO promoter

T : Synechococcus sp.PCC7942

RuBisCO terminator

icfA : Synechococcus sp.PCC7942

CA gene

Figure 2. Construction of CA expression

antiserum against cyanobacterial CA protein vector in Synechococcus sp.PCC7942. was used for Western blot analysis. Total protein was measured by Lowry method. 2.3. Tolerance to acetazolamide Cells were grown aerated (2vvm) with ordinary air in BG11 medium with several concentrations of acetazolamide (AZA, CA inhibitor) [5]. Ratio of packed cell volume (PCV) with AZA to PCV without AZA was calculated after culturing the cells for one day. PCV was determined by using a COULTER MULTISIZER (COULTER). 2.4. Growth rate of transformed Synechococcussp. PCC7942 Cells were grown in 15 ml test tubes with 8 ml of culture medium under a photon flux density of approximately 150/zE/m2/s and were aerated with ordinary air at 4 ml/min or 16 ml/min. Cells were also cultured under the same conditions except for CO2 concentration in air (20 ppm). The PCV change during the growth of cells was measured by COULTER MULTISIZER

631

(COULTER). The linear growth rate (mg dry w e i g h t / ~ ) was calculated by using PCV to dry cell weight converting factor. 2.5. CO2 fixation rate of CA over-expressed transformants

Cells were grown in 15 ml test tubes under a photon flux density of approximately 150tzE/m~/s and were aerated (2vvm) with ordinary air containing 20 ppm CO2. Linear growth rate was converted to CO2 fixation rate (gCO2/1/day) multiplying the conversion factor (1.65). The conversion factor is calculated as follows: 44(CO2)/12(carbon)X0.45(carbon content of cells). 3. RESULTS AND DISCUSSION CA over-expression vector was introduced into Synechococcus sp. PCC7942 resulting ampicillin resistant transformants. Southern blot analysis revealed the existence of introduced gene (icfA) in transformants. The antiserum against icfA was raised and was used for Western blot analysis. Western blot analysis revealed increased levels of the 30kDa icfA protein in eleven transformants, but not in five vector-control clones (Fig.3). vector-control

CA over-expression

IB 1 2 3 4 5 1 2 3 4 5 ,",'

,,' ". ,

,~,,,'c'

,

'

'

,

,

, ,

6 7 8 910111B

,' ",, '

, '

,

"'

, , , ", ',

',,"i

,i

,~"'~"~''~'~~~~'*''~ ~ , , , , ~ ~ , ~ " " " ,"91--- 30kDa .

.

.

.

.

Figure 3. Western blot analysis of CA over-expressed in Synechococcus sp.PCC7942. IB, 30kDa icfA protein in inclusion bodies produced by transformed E. coli

Transformants showed increased tolerance to AZA comparing to control cells, suggesting that over-expressed CA exerts its function. By immunological electron microscopy, CA protein was found in carboxysomes of transformants, where most of RuBisCO protein was also found, but CA protein was not found in cytoplasm, showing that the site specific expression of CA in the cells [6]. In order to clarify the effect of over-produced CA in carboxysomes on photosynthetic performances, growth rate and CO2 fixation rate were evaluated. CA over-expressed transformants when cultured under the CO2 concentration in the air, showed elevated levels of growth rate comparing with wild type or vector-control strains. This effect was enhanced as the aeration rate was decreased or under the concentration of 20ppm CO2 in the culture (Fig.4), suggesting that over-expressed CA is especially important under the limited supply of DIC.

632

360ppm C 0 2 (16ml/min) 360ppm C02 (4ml/min) ,, 20ppm C02 w~//~//7//~///////~ ~ ,,'HHHHHHH~ (16ml/min) 0 50 15 10 5 0 Growth rate % increase of growth rate (mg dry weight/I/hr) in CA over-expressed transformants(%) BB CA over-expressed transformants r-! Wild type and vector-control

Figure 4. Growth rate of transformed

Synechococcussp.PCC7942.

The enhanced CO2 fixation rate in the transformants was further confirmed under the concentration of 20ppm CO2 in the culture, using independent eleven clones of CA over-expressed transformants and five vector-control clones. As a result, CA over-expression induced elevated levels of CO2 fixation rate, suggesting the possibility to improve photosynthetic performances (Fig. 5). 200 A

o or E

G)

Q

150

lo0

c o

.x "-O4 o o

I

CA over-expressed transformants , J L . 131 -I-23.6 Vector-control transformants

.----82.4___8.14 O

50

* Mean -I- SD of eleven clones # Mean :1: SD of five clones

Figure 5. CO2 fixation rate of CA over-expressed transformants.

REFERENCES 1. D. A. Bryant (ed.), The Molecular Biology of Cyanobacteria, Kluwer Academic Publishers, The Netherlands, 1994 2. G.D.Price and M.R.Badger, Plant Physiology, 91, (1989) 514. 3. H.Fukuzawa et al, Proc. Natl. Sci. USA, 89, (1992) 4437. 4. Methods in Enzymology, 153, (1987) 199. 5. S.Bedu et al, Plant Physiol., 90, (1989) 470. 6. M. Murakami et al, in preparation.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversionsfor Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

633

CO2 removal by a bioreactor with photosynthetic algae using solarcollecting and light-diffusing optical devices* M a s a r u N a n b a a'b and Miyuki K a w a t a ~'b a Hitachi, Ltd. Hitachi Research L a b o r a t o r y 1-1, Omika-cho 7-chome, Hitachi-shi, Ibaraki-ken, 319-12 J a p a n b Research Institute of Innovative Technology for the E a r t h (RITE) 2-8-1, Nichi-shinbashi, Minato-ku, Tokyo 150, J a p a n

A photo-bioreactor was designed and constructed, in which solar light was supplied by light-diffusing optical rods connected to solar-light collector through optical fiber cables. The performance of biological CO2-fixation with photosynthetic microalgae, Chlorella sp. UK001, under the natural rays of sunlight was estimated. A r a t e of CO2-fixation was obtained of 24 g m2d -1 on a sunny day, in which energy density of direct-sunlight was 780 W m 2. Photosynthetic efficiency of converting light energy to biomass was e s t i m a t e d as 1.6% for total incident energy on the surface of the solar-light collector, and 12% for photosynthetically available visible-region. 1. I N T R O D U C T I O N The atomospheric concentration of so-called greenhouse effect gases such as carbon dioxide (CO 2) is increasing as a result of the rapid increase in the use of energy and other resources [1]. Microalgal photosynthesis has increasingly been paid attention as a m e a n s of producing industrially valuable compound and reducing the emmission of CO 2 into the atomosphere. Mass production of photosynthetic microalgae such as Chlorella or S p i r u l i n a have been realized for commercial purposes. Various types of photobioreactors have been used or proposed, such as open-pond types, raceway types and tublar types [2, 3]. Light supply as an energy source would be the limiting factor of microalgal production, ff the reactor was properly operate& For efficient solar energy utilization, a novel illumination s y s t e m b a s e d on light-diffusing optical fibers or rods have been proposed in order to increase the ilumination area to culture volume ratio [4, 5]. These systems have supposed the solar light collector. We have designed a photo-bioreactor, in which solar light was supplied by light-

* This study was supported by New Enegy and Industrial TechnologyDevelopment organaization.

634 diffusing optical rods connected to solar-light collector through optical fiber cables. In this work, the performance of biological CO~-f~xation with photosynthetic microalgae under the natural rays of sunlight is presented. 2. MATERIALS AND M E T H O D S 2.1 P h o t o - b i o r e a c t o r Figure 1 shows the schematic diagram of the photo-bioreactor

O2-CO2 ITemp 9 I Gas-meter x ~ ~, Icontrollerl I ~ ~.L__/., ~ ~ ~[.~ o ~ ~][ ;::--I.Rec()rder I ; Solar col ~ " | ~- ,, (Reflecting Mirror , , r , , , , . . ,, ,.r~--, .~ ~,::~--m @300mm) I~1 I IIll]' Ir-~J'l (~~~11 Light-difusin'(rn.~~~l ~L 02 il opticalrods ' ~ ~ I!!!!I~~N2 (013x300m o

used in this study. T h e r e a c t o r was constructed in the form of a cylindrical glass vessel 500 mm tall with a n internal diameter o f o Sensors io o o (.HDOTemn) 300 m m (20 L in culture Reactor vesse .. . ' ' 9 volume). Solar light or artificial (O300x500mm, k, . . : - ~ - / compressor culture volume2L) Gas-dispersion / plate light from a metal-halide lamp

'8r~ | e!.oI li

could be transmitted through Figure 1. Schematic diagram of the photooptical fibers into light-diffusing bioreactor and control devices. optical rods (Bridgeston Co. Ltd.), 13 mm in diameter and 300 mm in length, which were arranged in the reactor vessel. The radiation surface area to volume ratio was 18 m -1. A mixed air with CO~ was used for aeration at the bottom of the reactor. Gas flow rate was 0.5L min -1 , not otherwise state& CO 2 concentrations in the supply and exhaust gases were measured by infrared gas analyzer (VIA-510, Horiba, Ltd.) and used to estimate CO~ removal. Solar light collector, developed by Asahi Glass Company, was constructed with reflection mirror (300 mm in diameter) and designed to concentrate solar light into the input end of 18 optical glass fiber cables. Photosyntbetically active radiation with wavelengths between 400 and 700 nm was transmitted into the light-diffusing optical rods. Light transmission efficiency through the solar collector to the optical rods was determined to be 34% for visible region of the sun light. 2.2 Culture strain A green alga, Chlorella sp. UK001, was obtained from M. Murakami et al. (Sumitomo Chemical Co., Lt&) and used in this study. This strain possesses high capability in fixing CO2 at high concentration of CO2 (10-40%) in aeration gas and high temperature (30-40 ~ The Closterium medium was prepared at a pH of 7.5, and maintained at a temperature of 35~ throughout the experiment. CO 2concentration of 10% (v/v) in air was aerated into the medium at a flow rate of 0.51.0 L/min.

635 3. R E S U L T S AND D I S C U S S I O N 12 3.1 CO2 r e m o v a l a n d b i o m a s s y i e l d

1

Figure 2 shows the cell growth curve .i : of Chlorella sp. UK001 strain using a n :', ', ', '11 artificial light source of m e t a l halogen i .... ', ' , , , i lamps. Light illuminations were carried "~ ' "' ' out under periodic llight-dark cycles, 10 light period of 10 h and dark period of ~14 h. Cells were grown a t light periods " o~ " c, 0.5 Cell density decreased s o m e w h a t in =, the dark, especially a t higher cell ~ 9 o density t h a n 0.2 g L ~. The growth r a t e ~ a t the light periods showed a c o n s t a n t ~ value of 0.26 g h ~ by dry weight. Thus, 8 the r a t e of CO9 fmation was t a k e n as 0.48 g h ~ from the carbon content of the cells, which was analyzed as 50% 0 7 0 50 100 150 in weight ratio. Culture Time (h) At the experiment shown in Figure 2, a mixed air with CO2-concentration of 10.5% was a e r a t e d into the culture Figure 2. Changes of cell density and vessel. CO 2 concentration in the outlet CO2-concentration in outlet gas of the Chlorella sp. UK001 culture vessel. gas from the vessel decreased during strain was cultured in the photothe light illumination. CO~ removal was bioreactor, under condition of light (10h)calculated from CO 2 concentration in dark (14h) cycles with artificial halogen CO 2 concentration of supply and exhaust gases and the feed lamps a t 35~ rate. The rate of CO~ removal was aeration gas into the vessel was 10.5%, and gas flow rate was 1 L min -1. Light t a k e n as 0.47 g h ~ , which agreed well intensity at the surface of light-diffusing with the rate of CO9 fixation obtained optical rods was 160 ~E m 2 s 1 above. It was concluded t h a t almost all the CO 2 removed from the feed gas was fixed as algal biomass in Chlorella sp. UK001 strain. 3.2 CO~ r e m o v a l w i t h s u n l i g h t

Figure 3 shows some examples of changes of CO 2 concentration in the outlet gas of the culture vessel, when solar light collector was used as a source of light-supply. The CO 2 concentration in the outlet gas was decreased t h a n t h a t in the inlet gas, while direct-incident rays of sunlight was obtained. Daffy a m o u n t of CO 2 removed in the feed gas was calculated by integration of difference between inlet- and outlet-gases in a whole day (see Fig. 3, shadowed area

636 in upper part). Table 1 summarized the performance of the photobioreactor used in this study. 1.7 g of CO 2 was fixed as algal biomass in a free day. The rate of CO2-fmation per unit area of the sun-light collector was 24 g m2 d-i. Photosynthetic efficiency of converting light energy to chemical energy of algal cells was 1.6% based on the total incident energy. Photosynthetically useful radiation (400-700 nm) is 39% of the solar energy, and 34% of that is transmitted to the photobioreactor. So, the photosynthetic efficiency was estimated as 12% based on the photosynthetically available visible-region of sunlight. The efficiency would be higher than that of conventional type reactors, considering that the energy loss of solar light collector used here was 66%. It may be safe to say that lightcollecting type photo-bioreactor is helpful to produce industrially valuable compound and reducing the emmission of CO 2 into the atomosphere.

v

o m

o

~

~ ~.~-

I

c/.oudy .~-fin~

"

" " ~ " -nine

0.8

................

0.6

=,,.,,,.,,.,,.~,.....,,.,,

& .................

" ~'":"

,~ . . . . . . . . . . . . . . . .

- ..........

,,,.,,,~,,

cloudy

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-:

,,,,,,~,,,.

= = , 0.4 .~_.~v o.2 ~ 0 . ~ om

"7.'-

oo

20

40 60 Culture time (h)

80

100

Figure 3. Changes of CO2-concentration in outlet gas of the culture vessel and the light intensity of direct irradiation of sun-light. Chlorella sp. UK001 strain was cultured in the photo-bioreactor under the natural rays of sun-light. Cultural temperature was controlled at 35 ~ CO 2 concentration of aeration gas into the vessel was 9.8%, and gas flow rate was 0.5 L min -~. Table 1. Summary of culture of Chlorella sp. UK001 strain using sunlight. Fixed CO 2 through a day

[g-CO2/d ]

Light collection area Fixed CO 2 per unit area Photosynthetic efficiency of light energy to biomass Light irradiation area to culture volume ratio

[m 2] 0.071 [g-CO2/m2/d] 24.1 [%] [m-l]

Direct sun-light irradiation [MJ/m2/d] Averaged light intensity [W/m 2] Efficiency of solar collector [%]

1.7

1.6 18 21 778 34

REFERENCES

1. S. H. Schneider, Science, 243 (1989) 771. 2. E. A. Laws, US DOE Rep. SERI-SP-231-3071 (1987) 209. 3. A. Vonshak and A. Richmond, Biomass, 15 (1988) 233. 4. K. Mori, H. Ohya, I~ Matsumoto, H. Furuune, K. Isozaki and P. Siekmeier, Adv. Space Res., 9 (1989) 161. 5. H. Takano, H. Takeyama, N. Nakamura, K. Sode, J. G. Burgess, E. Manabe, M. Hirano and T. Matsunaga, Appl. Biochem. Biotech., 34/35 (1992) 449.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

637

Isolation and characterization of a green alga Neochloris sp. for CO 2 fixation* M. Kawata a,b, M. Nanba a,b, R. Matsukawa c,d, M. Chihara c,e and I. Karube c a Hitachi Research Laboratory, Hitachi Ltd, 7-1-10mika-cho Hitachi-shi, Ibaraki 319-12, Japan b Research Institute of Innovative Technology for the Earth, 2-8-1, Nishi-shinbashi, Minato-ku, Tokyo 105, Japan c Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, 153 Tokyo, Japan d New Energy and Industrial Technology Development Organization*, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, Japan e Japanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, Japan A green alga Neochloris sp. TS4F isolated from a pond at Ueno Park, Tokyo, is tolerant of the concentration of 10% CO 2 bubbling at 35~ This microalga can fix CO 2 at the rate of 0.027 g L - l h -1 by photosynthesis at 30 ~ and produces lipid as 24% by dry weight when grown under nitrogen starvation. 1. I N T R O D U C T I O N The increase in the carbon dioxide (CO 2) concentration in the atmosphere is suspected to be causing global warming via the greenhouse effect [1]. Atmospheric CO 2 has been increased by burning of fuels, especially fossil fuels, in association with the increase in world population. The use of photosynthetic microalgae to convert CO 2 into usable materials should be expected to contribute to decrease the CO 2 concentration in the atmosphere. We have studied CO 2 fixation of the exhaust gas from thermal power stations by microalgal photosynthesis. In order to utilize the resulting large quantities of biomass for fuel, compost, feed and other useful chemical substances, we have screened microalgae which produce lipids or hydrocarbons. In this study, we report the isolation and characterization of a green alga tolerant of the concentration of 10% CO 2 at 30~

*This study was supported by New Energy and Industrial Technology Development Organization (NEDO).

638 2. MATERIALS AND METHODS 2.1. Selection of oil-producing microalgae We collected water or soil samples containing microalgae from ponds and swamps in Japan, on May, 1995. They were cultured in modified Fitzgerald medium[2]. Firstly, microalgae were screened under atmosphere of air at 25 or 30 ~ CO 2 concentration in the feed gas was increased in sequence to the concentration of 10% CO 2 at each temperature. Secondly, a cytochemical staining procedure for semiquantitative estimation of hydrophobic materials of algae was employed [3]. Cells were stained with a solution of Nile Red [3], a fluorescent reagent for lipids. The oil droplets or lipid storage centers of the cells appeared as yellowish-orange structures, while the remaining cell structures were stained deep red. Nile Red-dyed cells were isolated on agar plates. Optical density of algal suspension was used as a measure of cell density. Specific growth rate was determined at logarithmic phase of the cell growth by dry weight. 2.2. Mass culture and chemical profiles of selected microalga After the unialgal culture was established, the strain was cultured under various conditions in order to investigate the effects of temperature, pH, light/dark cycle and nutrient deficiency in the medium. Cell suspensions were inoculated into 0.8 L (~ 90 mm ) or 4 L (~ 160 ram) of cylindrical glass vessel, which contained MDM medium [4]. The medium was enriched with 3200-4200 mM potassium nitrate (the original amount of nitrogen) and 200-600 mM magnesium sulfate heptahydrate and 360-460 mM dipotassium hydrogen phosphate. The culture medium was continuously stirred mechanically and by bubbling with 10% carbon dioxide in air. Continuous light (160 or 200 /1 E m-2s-1)was provided by cool-white fluorescent light. To determine the optimum temperature, cultures were grown at 25, 30 and 35~ To promote good lipid accumulation in the cells, the culture was transferred to a nitrogen-absent medium. The rate of depletion of soluble potassium nitrate in culture medium was determined with Hitachi HPLC L6000. And the fluorescent intensity of the Nile Red-dyed microalgal cells was measured by Hitachi fluorescent spectrophotometer UV-2000. Stainability with Nile Red was determined as fluorescent intensity at 575 nm per cell density. After the cultivation, cells were harvested by centrifugation, washed twice with distilled water and lyophilized. Cellular lipid was extracted by the method of Bligh and Dyer [5] and determined gravimetrically. Elemental cell compositions were analyzed with Yanagimoto CHN analyzer MT-3 and heat values of the biomass were analyzed with Shimazu bomb calorie meter CA4PJ or Ogawa OS meter 150.

639

0.04 i

c~

0

~ ,,.,.i

0.03 0.02 0.01 I

I

I

25 30 35 Culture temperature (~ Figure 1. Neochloris sp.TS4F.

Figure 2. Effect of temperature on the specific growth rate of Neochloris sp. TS4F.

3. RESULTS AND DISCUSSION 3.1. Selection of oil-productive microalgae Among 273 samples collected from ponds and swamps, 236 samples grow at the concentration of 10% CO 2 at 30~ Only 28 samples show yellowish-orange structures in the Nile Red-dyed cells. Finally, we obtained eight algal samples, because 20 samples were diatom and died out. Among the eight green algal strains, one microalgal strain with the highest growth rate was selected and isolated. It was temporarily identified as Neochloris sp. TS4F (Chlorococcales Neochloridaceae) with light and electron microscopic observations (Figure 1). Neochloris sp. TS4F isolated from Shinobazu-ike in Ueno Park, Tokyo, has the following characters ; (1) cells are spherical, with 5-18 ju m in diameter; (2) zoospores are without walls and with two flagella; (3) vegetative cells have one or more pyrenoids in the chloroplast; (4) vegetative cells are sometimes multinucleate. Additionally, Neochloris sp. TS4F strain tends to aggregate with hundreds of cells, so its sedimentation rate was higher than that of other green algae such as Chlorella. This characteristic should be advantageous at harvest of cells after cultivation. 3.2. Mass culture and chemical profiles of selected microalga The optimum growth condition of Neochloris sp. TS4F was determined with 10% CO 2 at 30~ and pH 7.5. Specific growth rate at 30 ~ was 0.03 h -1, which meant doubling time of 23 h (Figure 2). Under continuous lighting condition, biomass yield after eight days culture was 1.45 times that of 16:8 h light/dark cycle. This results showed that the biomass yield was proportional to the lighting time. The growth rate of Neochloris sp. TS4F strain was 0.018 g L-lh -1 based on dry cell mass, under light intensity of 160 ,u E m-2s -1 and 30 ~ The rate of CO2-fixation was estimated to be 0.027 g-CO 2 L-lh-1, according to the carbon content of cells.

640 Cultivated cells in a nitrogen-rich medium were harvested by centrifugation, and inoculated into a nitrate-deficient medium. Figure 3. shows the effect of nitrogen starvation on the growth rate, stainability with Nile Red and cellular content of total lipids and nitrogen. The growth of alga slowed gradually and stopped finally. In contrast, the stainability with Nile Red increased gradually. It shows that nutrient deficiency in the medium caused the increase of the fluorescent intensity of algal cells stained with Nile Red. The total lipid content was found to increase with the depletion of nitrogen in the medium. Cellular lipid content increased from 13% to 24% by dry weight. It means that lipid accumulation caused by nitrogen starvation requires continuous cultivation for at least 140 hours under nitrogen starvation. Cellular lipid content by nitrogen starvation became considerably high, a heat of combustion increased from 5.3 kcal g-1 to 6.1 kcal g-l, as cellular lipid content increased. 20

0.02

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~

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2

( o

~

Z 0

~

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1~

0 0

50 100 150 200 Nitrogen starved time (h)

0

0 50 100 150 200 Nitrogen starved time (h)

Figure 3. Effect of nitrogen starvation on the growth rate and stainability with Nile Red (A), and on the cellular total lipid and the cellular nitrogen content (B) of Neochloris sp. TS4F. In conclusion, the feasibility of increasing the lipid content of Neochloris sp. by nitrate depletion in the medium suggests the possibility of using this strain in a biological CO2-fixation system and as a source of liquid fuels or feed in the future. REFERENCES

1. S. H. Schneider, Science 243 (1989) 771. 2. E. O. Hughes, P. R. Gorham and A. Zehnder, Can. J. Microbiol. 4 (1958) 225. 3. D. Berglund, B. Cooksey, K. E. Cooksey and I. R. Priscu, 1986 Aquatic Species Program Annual Report, No. SERI / sp-231-3071 41-52. Solar Energy Research Institute, 1987. 4. A. Watanabe, J. Gen. Appl. Microbiol. 6 (1960) 283. 5. E.G. Bligh and W. J. Dyer, Can. J. Biochem. Physiol. 37 (1959) 911.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

641

A n t i o x i d a n t activity of CO2 fixing microalgae* R. Matsukawaa, b, Y. WadaC,d, N. TanC,d, N. SakaiC,d, M. Chiharaa, e and I. Karube a aResearch Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, 153 Tokyo, Japan bNew Energy and Industrial Technology Development Organization, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, Japan CResearch Institute of Innovative Technology for the Earth, 2-8-11, Nishishinbashi, Minato-ku, Tokyo 105, Japan dToyo Engineering Corporation, Research Engineering, Biotechnology, 1818 Fujimi, Togo, Mobara-shi, Chiba 297, Japan ejapanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, Japan Microalgae from hot spring and ponds in Japan were screened for antioxidant activity using a lipoxygenase inhibition test. The ethanolic extracts of some microalgae showed high lipoxygenase inhibition activity. The ethanolic extracts of isolated Chlorella species with tolerance to high temperatures and high concentrations of CO2 possessed high radical scavenging activity. These results suggest that CO2 fixing microalgae may be potential sources of natural antioxidants. 1. INTRODUCTION Carbon dioxide (CO2) is the principle greenhouse effect gas and the increase in the atmospheric CO2 concentration is suspected to be causing global warming via the greenhouse effect [1]. In order to decrease the CO2 concentration in the atmosphere, the development of CO2 fixation and removal systems, in conjunction with a system which converts the fixed CO2 to useable material, is desirable. "This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) in Japan.

642 It is well known that photosynthetic plants convert CO2 to organic matter using solar energy. Photosynthesis is more efficient in microalgae than in terrestrial higher plants and CO2 from industrial exhaust could be a useful carbon source for growing algae. In order to fix excessive atmospheric CO2, microalgae could be cultured and then be exploited as a useful resource. Photosynthesizing plant cells, including microalgae, are exposed to a combination of intense light and high oxygen levels [2, 3], leading to the formation of free radicals and other strong oxidizing agents. The ability of microalgae to thrive under such extremely oxidizing conditions led us to focus our investigation on identifying potential antioxidant activity in microalgae. In this study, we report the screening of microalgae for antioxidant activity and the radical scavenging activity of microalgae with tolerance to high temperatures and high concentrations of CO2. 2. MATERIALS A N D METHODS 2.1 Preparation of microalgal extract Over 100 species of microalgae were collected from hot springs and ponds in Japan. Some of these were isolated and cultured in suitable media. The microalgae were separated from the medium by centrifugation at 3,000 rpm for 10 min. After washing the solid algal pellets with distilled water, a volume of solvent (water, ethanol or methanol) weighing ten times the sample weight was added to the pellets. The sample was then homogenized in a sonicator for 30 rain in 30 sec intervals. The homogenate was then centrifuged at 3,000 rpm for 10 rain, and the supernatant was used as the algal extract. 2.2 Determination of antioxidant activity As an assay for antioxidant activity, we examined the inhibition of lipoxygenase activity and radical scavenging activity using the o~, cz-diphenyl-~picrylhydrazyl (DPPH)decolorization test. Aqueous, ethanolic and methanolic extracts of microalgae were used in the assays. The assay of lipoxygenase activity was carried out using the method of BenAziz et al. [4]. The enzyme reaction was carried out in the cuvette of a spectrophotometer monitored at 234 nm until the reaction rate reached a steady state. That wavelength is the peak of absorption for the hydroperoxides generated by the action of lipoxygenase on linoleic acid, with the uptake of oxygen. The extent of inhibition was defined as the ratio of the rate of increase in OD234 in the absence of algal extract, to that measured in the presence of the sample. The assay of radical scavenging activity was determined using of a stable free radical, DPPH, according to the method of Blois [5]. The decrease in absorbance due to DPPH was measured at 540 nm using a microplate reader. The activity was defined as the ratio of OD540 in the absence of algal extract, to that measured in the presence of the sample.

643 3. RESULTS AND DISCUSSION

3.1 Screening of microalgae for antioxidants Several microalgae samples from hot springs and ponds showed lipoxygenase inhibition activity in both aqueous and ethanolic extracts. The ethanolic extracts of Chlorella sp. were observed to be more effective than aqueous extracts. The methanolic extract of another Chloretla sp. showed lipoxygenase inhibition. The inhibition of lipoxygenase by the extracts of microalgae suggested the existence of a cellular mechanism protecting them from oxidation in vivo. 3.2 Effects of temperature on cell growth Our previous paper reported the isolation and growth characteristics of Chtoretla strains H-84 and A-2 with tolerance to high temperature and high concentrations of CO2 [6]. Figure 1 shows the effects of temperature on the growth of Chlorella sp. H-84. The strains showed a high growth rate at 40~ but the growth rate was significantly decreased at 30~ and above 45~ The algae was unable to grow at temperatures higher than 50~ Thus, the optimum temperature for this alga seemed to be around 40~ The isolated Chlorella spp. were identified as Chlorella sorokiniana. Previously, the thermophilic Chloretla species known was only Chlorella sorokiniana [7]. 3.3 Effects of CO2 concentration on cell growth Growth of Clorella sp. H-84 was examined under different CO2 concentrations as shown in Figure 2. The alga grew well at concentrations of CO2 from 5 to 40%. It was able to grow in the culture medium with CO2 concentrations up to 60% (data not shown). The optimum concentration of CO2 is about 20%. Similar results were obtained for Clorella sp. A-2. These results suggest that these microalgae may be suitable for the biological fixation of CO2 from

' 30"C 9 40"C A

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Figure 2. Effects of CO2 concentration on cell growth.

644 industrial flue gases.

3.4 Radical scavenging activity The radical scavenging activities of aqueous, ethanolic and methanolic extracts of microalgae with tolerance to high temperatures and high concentrations of CO2 are shown in Table 1. The stable free radical DPPH was oxidized and decolorized in the presence of extracts of the microalgae. The ethanolic extracts of Chlorella sp. A-2 and Chlorella sorokiniana UTEX-1230 showed high radical scavenging activity. Ethanolic extracs of Chlorella sp. H-84 showed a radical scavenging activity of about 53%. These species of microalgae have antioxidant activity and may be used for purposes such as food, animal feed, pharmaceuticals and cosmetics.

Table 1 Antioxidant activity of extracts of Chlorella species Chlorella sp.

Activity (%) aqueous

ethanolic

methanolic

H-84

39.2

52.8

43.9

A-2

32.4

98.5

77.1

52.7

98.3

97.5

sorokiniana

REFERENCES 1. S.H. Scheider, Science, 243 (1989) 771. 2. A. Abeliovich and M. Shilo, J. Bacteriol., 111 (1972) 682. 3. R.R. Bidigrare, M.E. Ondrusek, M.C.II., Kennicutt, R. Iturriaga, H.R. Harvey, R.W. Hoham and S.A. Macko, J. Phycol., 29 (1993) 427. 4. A. Ben-Aziz, S. Grossman, I. Ascarelli, P. Budowski, Anal. Biochem., 34 (1970) 88. 5. M.S. Blois, Nature, 181 (1958) 1199. 6. N. Sakai, Y. Sakamoto, N. Kishimoto, M. Chihara and I. Karube, Energy Convers. Mgmt., 36 (1995) 693. 7. C. Sorokin and J. Myers, Science, 117 (1953) 330.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

645

Screening of polysaccharide-producing microalgae Yoshiko Shishido a b, Miyuki Kawata a c , Ritsuko Matsukawa Mitsuo Chihara d f and Isao Karube d

de

Research Institute of Innovative Technology for the Earth (RITE), 2-8-11, Nishi-Shinbashi, Minato-ku, Tokyo 105, J a p a n b Sumitomo Heavy Industries, Ltd., 1-15, Kuryo-zutsumi, Hiratsuka-shi, Kanagawa 254, J a p a n c Hitachi Research Laboratory, Hitachi, Ltd., 7-1-1, Omika, Hitachi-shi, Ibaraki 319-12, J a p a n a Research Center for Advanced Science and Technology, University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo 153, J a p a n New Energy and Industrial Technology Development Organization, Sunshine 60, 21F, 3-1-1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170, J a p a n f Japanese Red Cross College of Nursing, 4-1-3, Hiro-o, Shibuya-ku, Tokyo 150, J a p a n

e

1. INTRODUCTION The study has been carried out for the purpose of investigating the fLxation of CO 2 in exhaust gas from thermal power stations by microalgae and the effective utilization of microalgal products. It is important to develop technologies to convert photosynthesized organic products into useful substances such as fuels, feed, manure, materials for plastics and concrete, and fine chemicals which have a high added value. We have screened microalgae which produce polysaccharides. In this study, we report a method for the screening and detection of polysaccharide-producing microalgae, which tolerate atmospheres with high CO 2 concentrations and high temperatures.

2. METHODS AND MATERIALS Samples were collected from ponds, marshes, hot springs and softs at various places in Japan. Collected samples were cultivated on 6 kinds of

* This study was supported by the New Energy and Industrial Technology Development Organization (NEDO).

646 Table 1. Cultivation media medium Fitzgerald 1) 9

pH

I microalgae

usage

7.0

general

BG-11 2~

9.0

blue-green algae

CSi 3)

7.0

diatoms

CC ~)

3.0

red & green algae

MC 4)

6.0

green algae

Pro 4)

6.0

green algae

]

cultivation under 10% CO 2,3O ~

dying with Chinese ink .

,

,

precipitation by ethanol

*Vitamins were added in the medium. Isolation Figure 1. Method of screening and detection media (Table 1).

2.1. Screening of polysaccharide-producing microalgae The method of screening and detecting polysaccharide-producing microalgae is shown in Figure 1. Firstly, microalgae were screened under an atmosphere of 10% CO 2 at 30 ~ Secondly, microalgae growing under these conditions were selected by dying in Chinese ink and microscope observation. Thirdly, we picked out microalgae which secreted a non-toxic polymer from the cell. Lastly, the presence of polysaccharides was determined by precipitation with ethanol. Microalgae which deposited a precipitate were selected as polysaccharide-producing microalgae and isolated.

2.2. Growth of polysaccharide-producing microalgae Microalgae were cultured in a test tube containing 5ml of medium with s h a k i n g at 120 rpm under an atmosphere of 10% CO 2 at 30 ~ The growth of microalgae in the test tube was m e a s u r e d directly by means of the optical density at 690 nm. The growth velocity was defined as a relative rate by calculation of the slope of the growth curve during the linear phase.

3. RESULTS AND DISCUSSION

3.1. Screening of polysaccharide-producing microalgae We screened 273 microalgae in total, as shown in Table 2. Twenty six samples of polysaccharide-producing microalgae were selected and were

647 classified into 17 green algae and 9 blue-green algae by microscopic observation. These results show that the combination of dying using Chinese ink and precipitation of polysaccharides with ethanol is useful for the easy detection of polysaccharide-producing microalgae. 3.2. Growth characteristics of polysaccharide producing microalgae Three typical growth curves of polysaccharide-producing microalgae are shown in Figure 2.

Table 2. Results of screening for polysaccharide-producing microalgae. polysaccharide-producing microalgae

total number medium

total

of sample

Fitzgerald BG-11 Csi CC MC Pro

80 69 67 19 30 8

13 0 6 1 6 0

Total

273

26

green algae

blue-green algae

17

_

m

OT51F

9 NY7F 1.5-

O

9 NY5C

1

v

9

J

0.5

A

0

A

A

A

A

120 240 CULTURE TIME(hr)

360

F i g u r e 2. G r o w t h c u r v e s of p o l y s a c c h a r i d e producing microalgae

648 0.05 m green algae

0.04 . . . .

0

D blue-green algae

!!

0.03 0.02

~ O.Ol o

ii

L

lllli, illll lli ,,l" n l

',' ~ ~ ~.~,

i CD

0

0 o

0

0

0

O0

0

MICROALGA STRAINS F i g u r e 3. R e l a t i v e g r o w t h r a t e of p o l y s a c c h a r i d e producing microalgae

The relative growth rates of the 26 strains is shown Figure 3. The results show that blue-green algae grew faster than green algae. The strains OK72F1, ON1F, OT51F and ON2F1, which were classified as blue-green algae by microscopic observation, were rapidly growing microalgae. More examination is needed to establish the absolute growth rate and CO 2 fixation rate. The simple examination of growth characteristics is useful for screening for rapidly growing microalgae. We are continuing the characterization of polysaccharide-producing green algae, and the function of the polysaccharides in order to find a way to utilize these metabolites.

REFERENCES

1. H. Tamiya and A. Watanabe (eds.), Sourui Zikkenhou(in Japanese), Nankoudou Ltd., Tokyo, 68-104, 1965 2. R. Rippka, J. Deruelles, J. B. Waterbury, M. Herdman and R. Y. Stanier, J. Gen. Microbiol., 111 (1979) 1-61. 3. National Institute for Environmental Studies, LIST OF STRAINS, 1994 4. K. Nishizawa & M. Chihara (eds.), Methods in phycological studies, Kyouritsu shuppan Ltd, Tokyo, 281-305, 1979 5. H. Miyashita, H. Ikemoto, N. Kurano, S. Miyachi and M. Chihara, J. Gen. Appl. Microbiol., 39 (1993) 571. 6. M. S. Blois, Nature, 181 (1958) 1199.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

649

A marine microalga utilization for a paper: Semi-batch cultivation of Tetraselmis sp. Tt-1 by a tubular bioreactor and the partial substitution of whole kenaf pulp for a paper Y. Samejimaa, A. Hirano a, K. Hon-Nami a, S. Kunito a, K. Masuda b, M. Hasuike b, Y. Tsuyuki~, and Y. Ogushib aEnergy and Environment R&D Center, Tokyo Electric Power Co., 4-1 Egasaki-cho, Turumi-ku, Yokohama 230, Japan bHiroshima R&D Center, Mitsubishi Heavy Industries, Ltd., 4-6-22 Kan-on-shinmachi, Nishi-ku, Hiroshima 733, Japan ~Chemical Plant Engineering & Construction Center, Mitsubishi Heavy Industries, Ltd., 3-3-1 Minatomirai, Nishi-ku, Yokohama 220, Japan A semi-batch cultivation of a Prasinophyceae, Tetraselmis sp.Tt-1, was examined using a 50L scale tubular bioreactor under lamps with a view to estimating its performance. The mean productivity of more than 20g/m2/day could be obtained by fixing the flow rate of the culture medium at 0.5m/s, which was the maximum in this apparatus, by drawing a partial medium once in two days, and by adjusting an initial cell concentration to 0.5-1.0g/L. A long-term and stable cultivation for more than two months could be carried out by this system. Our previous study, on the other hand, has shown that cell bodies of this microalga are of use as the partial substitute of a wood pulp paper [ 1]. In the present study, whole kenaf pulp paper including algae obtained by the above cultivation was made and its properties were examined. It was indicated that they also could be used as an agent for surface improvement of kenaf pulp paper in addition to a partial substitute for the pulp as in the case of a paper made from wood pulp. 1. INTRODUCTION The increase of CO2 in the atmosphere has been considered to be one of the causes of global warming. The amount of CO2 exhausted from the thermal power stations of electric power companies has reached approximately one fourth of the total amount of CO2 exhausted in Japan. Therefore, as one of the measures to cope with this problem, technologies to fix CO2 using the photosynthetic ability of the microalga have been studied [2]. The establishment of the utilization technology of CO2 which is fixed by microalga is also

650 indispensable [3]. In the present study, it is reported the performance of a tubular bioreactor system having advantages such as a higher alga-productivity and a lower liability for a contamination with various germs compared to an open pond system. In our previous study [ 1], on the other hand, it was shown that Tetraselmis sp. Tt-1 was useful as a partial substitute to wood pulp for a paper. Kenaf, an annual herbaceous plant of the genus Hibiscus, grows relatively quick and absorbs more CO2 compared to other herbaceous plants. Since its fiber is long and strong, the use of this fiber as an alternative of wood pulp for a paper has already been tried in order to contribute to the preservation of forest resources [4]. We also describe some properties of microalga-added paper using kenaf pulp of its whole stem. 2. EXPERIMENTAL PROCEDURES A microalga used in this study was Tetraselmis sp.strain Tt-1 [ 1] which was

isolated from seawater ladled at the Sagami Bay in Japan. The f/2 medium [5, 6] was prepared as described previously [7], and

gas inlet

was used with a slight modification; concentrations of nitrate and phosphate were enriched 4 times and 8 times, respectively. The semi-batch cultivation of this strain was carried out using a tubular Figure 1. The drawing of a tubular bioreactor similar to that described in [7] bioreactor. (Fig. 1). The inner diameter of the tubes made of transparent acryl resin was 70mm. The size of the apparatus was 4,350mm(L), 235mm(W), and 2,270mm(H). A working volume and an irradiation area were 50L and 0.84m 2, respectively. The tubular reactor was put in a room controlled at 25~ with metal haloid lamps (M700-L/BU-SC, Matushita Denki k.k., Osaka,). Illuminance was maintained at about 30klux and the irradiation time was 12 hours a day. The CO2 concentration injected was set at 1.8%. Alga cells harvested were washed with deionized water before use. Microalga-added paper was prepared according to the method stipulated by the Japanese Industrial Standard (JIS) P8209 [8]. Indexes of paper quality, including tensile strength, smoothness, air permeability and receptivity to printing ink were measured by the methods of JIS series P 8113, P 8119, P 8117 and J.TAPPI No.46, respectively [8]. 3. RESULTS AND DISCUSSION 3.1. Indoor cultivation using a tubular bioreactor

With a view to optimizing the cultivating conditions in this apparatus, three parameters were considered; the flow rate of the medium, the drawing frequency of grown cells, and the initial cell concentration, which is designated in this paper as the cell concentration after

651

the partial drawing of grown cells, followed

25

by the addition of a flesh medium. The flow rate tested were 0.3m/s and 0.5m/s. The drawing frequency tested were once a day, once in two days, and once in three days. The initial cell concentration were changed

E

20

~0

~15 ,. 10

among 0.3g/L and 2.0g/L. As shown in Fig.2, the productivity depended on the all these parameters; each optimum value for the flow rate, the drawing frequency, and the initial concentration were 0.5m/s, which

~0

5 0

i

ii

iii

iii

0.5 1.0 1.5 2.0 2.5

Concentration [g/L]

was the maximum in this apparatus, once in

Figure 2. Relationship between the cell

two days, and 0.5-1.0g/L, respectively. The

productivity and the initial cell concentration

mean productivity up to more than 20g/m2/day was obtained when each optimal

at two kinds of the flow rate and at various

conditions were employed simultaneously.

drawing frequencies. All closed symbols show the flow rate of 0.5m/s. Drawing frequencies of symbols O, I1, and A are once a day, once in two days, and once in

It was suggested that a long-term and stable cultivation in this system would be possible from the fact that a stable semi-

three days, respectively. Open circle(O)

batch cultivation for more than two months

shows 0.3m/s and once a day for the flow

was carried out (data not shown).

rate and the drawing frequency, respectively.

3.2. Use of cultured microalgae for a paper

Figure 3 shows the mixture of the microalgae and kenaf pulps, which indicates that the size of this microalga is extremely smaller than that of kenaf pulp fiber. The surface of microalgae-added paper looks to be smoother than that of pure kenaf-pulp paper, as shown in Fig.4.

Figure 3. Light micrograph of Tetraselmis

Figure 4. Electron micrographs of the

sp.Tt- 1 cells and kenaf pulp fibers. • 100

surface of pure kenaf-pulp paper (A) and microalgae-added paper (B). The content of alga, Tetraselmis sp. Tt- 1, was 15%.

652 As shown in Fig.5, some physical properties of microalgae added-kenaf pulp paper clarified that the paper quality could be improved by the addition of these algae to the pulps. With the increase in the content of microalgae, the tensile strength was slightly declined without any problem in its practical use. Both the air permeability and the smoothness were increased, while the receptibility to printing ink was decreased 9It was also demonstrated that one of the advantages of kenaf pulp paper, that is, being stronger than that of hardwood pulp paper, was still maintained after the substitution of pulps even 15% algae (Fig.5). In summary, Tetraselmis sp.Tt-1 could be used as a partial substitute of kenaf pulps and an agent for surface improvement of kenaf pulp paper as in the case of hardwood pulp paper.

o o

o =

o

[-2

5 10 Microalga content (%)

15

100 ~" 80 60 o 40 o 20 0

~ r~

0

~'~

60 40

0

10 5 Microalga content (%)

15

o o

~.,~lOO ;~ 80 ,.Q o

~ 20 o 0

.m

0

o

~ 10 04

04 Z 04

10

z

04

0 0

9 0

I

10-7

.

. ......l

CO 2 permeance

10-6

,

, ......

10-5

[m01-m'2.s-l-pa -1]

I

10-9

I

10-8

........

I

10-7

10-6

N2 permeance [m01.m-2-s-l.pa -1]

Figure 3. Relationship between CO2/N 2 selectivity and permeances to CO 2 and N 2 at 30~

668 The free aperture of the main 100 channels in Y-type zeolite is 0.74 nm [7] and is much larger than the diameter of CO2 and Nz molecules. If the concentrations of CO z and N 2 in the micropores of > the Y-type zeolite membrane are equal to o those in the outside gas phase, these 10 t/) molecules permeate through the membrane r at a low C O z / N 2 selectivity. However, this Z Membrane length o,! was not the case. Carbon dioxide moleO o 3cm cules adsorbed on the outside of the o membrane migrate into micropores by sur9 20cm 1 ........ I ....... face diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micro10-7 10-6 10-5 pores by translation-collision mechanism CO 2 permeance [mol.m-2-s-l-pa -1] from the outside gas phase. The mouth of the micropores may Figure 4. Effect of membrane length be narrowed by adsorbed CO 2 molecules, on separation performance at 30~ which block N 2 molecules from entering the pores. Furthermore, C O 2 diffuses in pores of the zeolite membrane at a faster rate than N z. This selection mechanism is plausible for micropores with a width of up to six molecules [7]. When CO 2 molecules are strongly adsorbed on the pore wall, the CO 2 permeation rate will be low even if CO 2 is concentrated in the pore. If the pore size is close to the size of molecules, CO z molecules cannot pass N 2 molecules. Thus, balances between pore size and molecule size and between adsorptivity and mobility as well as difference in polarity of competitive species are important to attain both high permeance and selectivity. '

'

'

' ' " ' 1

'

'

'

' ' "

gs oo

.

m

4. CONCLUSIONS A Y-type zeolite membrane was formed on a porous a - a l u m i n a support tube. The membranes produced separated CO 2 from N2 at a permeance of the order of 10 -7 mol.m-2-s-l.pa-1 and a selectivity of 20-100 at 30~ This rapid and selective permeation was due to the pore-size controlled adsorption. REFERENCES 1. 2. 3. 4. 5. 6. 7.

J.-i. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe and S. Morooka, Ind. Eng. Chem. Res., 35 (1996) 4176. B.-K. Sca, M. Watanabe, K. Kusakabe, S. Morooka and S.-S. Kim, Gas Sep. Purif., 10 (1996) 187. K. Kusakabe, S. Yoneshige, A. Murata and S. Morooka, J. Memb. Sci., 116 (1996) 39. K. Kusakabe, T. Kuroda, A. Murata and S. Morooka, Ind. Eng. Chem. Res., 36 (1997) 649. C. Bai, M.-D. Jia, J.L. Falconer and R.D. Noble, J. Memb. Sci., 105 (1995) 79. D. Li and S.-T. Hwang, J. Memb. Sci., 66 (1992) 119. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conrersions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

669

Comparative study of various amines for the reversible absorption capacity of carbon dioxide Y. Nagao a, A. Hayakawa a, H. Suzuki a, S. Mitsuoka b, T. IwakP, T. Mimura c and T. Suda c aDepartment of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo-ku, Kyoto 606-01, Japan bHiroshima Research Institute, Mitsubishi Heavy Industries, Ltd. Kan-on-shin-machi, Nishi-ku, Hiroshima 733, Japan CTechnical Research Centre, The Kansai Electric Power Co. Inc. Nakoji, Amagasaki 661, Japan 1. A B S T R A C T

Aqueous solution of a-amino amides has been found to exhibit good reversible absorption capacity of carbon dioxide compared to amino alcohols when carbon dioxide is absorbed under 1 atm. However, the absorption capacity of (x-amino amides is highly dependent on the partial pressure of carbon dioxide, the absorption capacity being considerably decreasing when carbon dioxide is absorbed under 0.1 atm. 2. I N T R O D U C T I O N

Global warming is one of the most serious present-day issues for the human to overcome. The enhanced greenhouse effect was mainly caused by the rapid increase of carbon dioxide in the atmosphere since the Industrial Revolution. To resolve the problems about carbon dioxide emissions, several new methods to recover and utilize carbon dioxide have recently been developed [1]. Alkanolamine solutions are generally used as chemical absorbent for the recovery of acid compounds from flue gases. Although the reaction between carbon dioxide and a variety of amino compounds, represented by monoethanolamines, has been widely studied [2],[3],there is still a great need to develop more effective and chemically more stable absorbents. In the present study, we have prepared several different types of amino compounds and compared their capacity for the reversible absorption of carbon dioxide at different temperatures and under different pressures of carbon dioxide.

670

3. RESULTS AND D I S C U S S I O N 3.1. Comparison of amino alcohols and a-amino amides With the aim to develop more effective and chemically more stable absorbents than conventionally used amino alcohols, we have prepared a-amino amides 1 ~8 and compared their capacity for the reversible absorption of CO 2 with those of commonly used amino alcohols. Absorption capacity was examined by bubbling CO2 under 1 atm. As seen in Table 1, CO 2 loading of a-amino amides at both 40 and 100 ~ was generally smaller than those of amino alcohols. The solution of 8 absorbed CO 2 more slowly than 9 and 10 at 40 ~ as shown in Figure 1, but the same solution desorbed CO 2 considerably faster than that of 9 at 100 ~ as shown in Figure 2. These results mean that compound 8 was better in the desorption ability of CO 2 than compound 9, though the absorption ability of a-amino amides for CO 2 was found to be generally lower than that of amino alcohols. This difference in the reversible absorption capacity between amino alcohols and a-amino amides is noteworthy and may be attributed to the difference of nucleophilicity and coordination ability between two functional groups involved, i.e. hydroxyl and amide groups. Table 1 Comparison of CO2 loading (%) of amino alcohols and amino amides Substrates

Loading (%)

Capacity (%)c

40 ~

100 ~

EtNHCH2CONH 2 (1) ~PrNHCH2CONH 2 (2)

62

13

66

12

54

nBuNHCH2CONH2 (3)

57

14

43

tBuNHCH2CONH2 (4)

74

11

63

49

Me2NCH2CONH 2 (5)

50

7

43

Et2NCH2CONH 2 (6)

67

8

59

Me2NCH2CONMe 2 (7) Me2NCH2CONEt2 (8) N H2CH2CH2OH (9)

80 74

12 13

68 61

74

33

41

EtNHCH2CH2OH (1 0)

93

44

49

Et2NCH2CH2OH (1 1) 97 51 46 aAmount of CO2 absorbed within 1 h under 1 atm of CO2. bAmount of CO2 remaining after 1 h. ~ in loading values at 40 and 100 ~ As shown above, the reversible absorption capacity of a-amino amides was comparable with those of amino alcohols when CO 2 was absorbed under 1 atm. However, aqueous solutions of compounds 1 ~7 slowly underwent hydrolysis when they were kept at 100 ~ for a long time. Chemical stability is one of the

671

indispensable prerequisites for the chemical absorbents. So, at first sight, o~-amino amides may be thought to be unsuitable for the present purpose. But taking advantage of the property that o:-amino amides desorbs CO 2 with less thermal energy, we synthesized a compound 8, which has a hindered amide function alkylated by two ethyl groups and is quite difficult to be hydrolyzed even heated for prolonged time at high temperatures. 100

O O

80 ~ 70 v 60 .c_ 50

o

A

401 30

[]

A

D

O

O

O

A

A

I

II

D

c]3

/

20-1 10 0 0

1.

9

A9

98 o 10

70 ~. 60~50= 40:5 30o ,.,,.,

1'0 20 3'0 4'0 50 6'0

absorption time (min) Figure 1. Absorption behavior of compounds.3, 8~ 10

e8 •

,~

A 9 I I A

A

9

101 0

5 10 15 20 25 30 35 40 desorption time (min) Figure 2. Desorption behavior of compounds 8 and 9.

3.2. C o m p a r i s o n under different pressures of C O 2 As discussed above, the reversible absorption capacity of the solution of amide 8 was better than those of amino alcohols under 1 atm of CO2, though the absorption rate of the former compound was slower. However, when CO2 absorption was carried out by bubbling a gaseous mixture of CO2 and N 2 in a ratio of 1 : 9, the CO2 loading value of the solution of 8 significantly dropped as compared to that of monoethanolamine, as shown in Table 2. The absorption ability of the solution of 8 became poor and its reversible absorption capacity was no better than that of monoethanolamine, either, under these conditions employed. This may be interpreted that the absorption ability of the tertiary amide 8 was highly dependent

Table 2 Comparison of reversible CO 2 loading (%) Substrates a

Me2NCH2CONEt 2 (8)

Loading (%)

Capacity (%)~

40 ~

100 ~

29 (74) e

13

16 (61)'

NH2CH2CH2OH (9) 59 (74) e 33 26 (41)' aUsed as 1M aq. solution, bAmount of CO2absorbed within 2 h under 0.1 atm of CO2.CAmount of CO2 remaining after 1 h; see Table 1. dDifference in loading values at 40 (under0.1 atm of CO2)and 100 ~ "Amount of CO2 absorbed within I h under 1 atm of CO2. 'Difference in loading values at 40 (under 1 atm of CO2)and 100 ~;.

672

on the partial pressure of CO2. Thus, a-amino amides are suitable as the absorbent for CO2 under a normal partial pressure of CO 2, but not so satisfactory under a low partial pressure of CO2.

4. EXPERIMENTAL

SECTION

A 1 M aqueous amine solution was placed in a two-necked flask, through which CO 2 was bubbled with stirring for 1 hour at a flow rate of 50 ml/min at 40 ~ under 1 atm. After the end of CO 2 bubbling, the solution was heated to 100 ~ and kept at this temperature for an additional hour. The amount of CO2 remaining in the amine solutions at 40 ~ and 100 ~ was determined by an apparatus SHIMADZU TOC10B and was shown here as the "loading capacity" in mol COJmol amine (%). Similar determinations of CO 2 Ioadings were made under a CO 2 partial pressure of 0.1 atm, using a 1 : 9 gaseous mixture of CO 2 and N2. Compounds 1--6 were prepared from 2-chloroacetamide and the respective amines, while compound 7 was obtained from chloroacetyl chloride and dimethylamine and compound 8 from chloroacetyl chloride, diethylamine and dimethylamine, as previously reported [4],[s]. 5. C O N C L U S I O N a-Amino tertiary amides were found to be attractive as a potential candidate for the new CO2 absorbent with respect to their high stability toward oxidative degradation as well as thermal economy. The CO 2 loading of c(-amino amide 8 under 1 atm of CO 2 was comparable with that of monoethanolamine, but the former compound exhibited better desorption ability than the latter. REFERENCES 1. D. H. Gibson, Chem. Rev., 96 (1996) 2063. 2. P. V. Danckwerts and M. M. Sharma, Chem. Engng., (1966) CE244. 3. G. F. Versteeg, L. A. J. Van Dijck and W. P. M. Van Swaaij, Chem. Eng. Commun., 144 (1996) 113. 4. M. M. Werber and Y. Shalitin, Bioorganic Chemistry, 2 (1973) 202. 5. V. P. Y. Gadzekpo, J. M. Hungerford, A. M. Kadry, Y. A. Ibrahim, R. Y. Xie and G. D. Christian, Anal. Chem., 58 (1986) 1948.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

673

K i n e t i c a n a l y s i s o f C 0 2 r e c o v e r y f r o m flue g a s b y a n e c o t e c h n o l o g i c a l system M. Tabata, a T. Chohji a and E. Hirai b aDepartment of Ecomaterials Engineering, Toyama National College of Technology, 939 Toyama-si, Hongou-mati 13-banti, Japan bLibrary Center, Hokuriku University, 920-11 Kanazawa-si, Taiyougaoka 1-1, J a p a n Kinetic analysis of Ca 2§ desorption due to the displacement of Ca 2§ from cation exchange resin by H § which is generated by bubbling CO~. to the resin containing water at room temperature has been studied. Ca 2§ was rapidly displaced by H § at room temperature. This indicates that cation exchange resin or soil which acts as cation exchange resin may be good material for CO2 recovery from flue gases. 1. I N T R O D U C T I O N Development of CO2 removal system is the one of the urgent problems to be solved[I]. The CO2 removal system should satisfy the following requirements. (1) Consume low energy. (2) Using no or less harmful materials to environment. (3) Products of CO2 removal system is non toxic and/or commercially valuable materials. Recently, we have proposed a new system to recover CO2 from flue gas in the form of carbonate salts which satisfy the above requirements[2, 3]. The system uses water as absorbent of CO2 and soil as cation exchanger. The principal of the new system consists of the following four steps. (1) Dissolution of CO2 separated from the flue gas into soft(or cation exchange resin)-containing water which is accompanied by a formation of carbonate ions and a decrease in the pH value of the soil-containing water. (2) The decrease in the pH value of the soil-containing water results in the enhancement of the displacement of alkaline earth metal ions (such as Ca 2§ adsorbed on the soil (or resin) with H § by ion exchange reaction. (3) Separation of the soil (or resin) from the soil-containing water and aeration of the supernatant. (4) Carbonate ions react with alkaline earth metal ions to form carbonate salts which are insoluble in water. Although a portion of dissolved CO2 is released after the aeration step, soluble and/or insoluble carbonates are eventually formed in the soil-separated solution. All the materials used in the system are not harmful to the environment and all the reaction undergo at room temperature. We have tested this new system for CO2 recovery using soil samples taken in Hokuriku district, Japan[4, 5], and have

674 found that CO2 was fixed in the form of calcium carbonate. However, kinetic analysis of this reaction system has not been analyzed. In the present paper, reaction rate for Ca ~-+ desorption step during bubbling pure CO2 gas to the resincontaining water is analyzed. 2. E X P E R I M E N T A L Schematic experimental procedure is shown in Figure 1. All the chemicals used were of analytical grade, and ion-exchanged distilled water was used for all the procedure. Amberlite IRC-76 (Organo K.K.) was used for cation exchange reactions. Its cation exchange capacity for 1 dm 3 of wet resin is 200 g of CaCO3. The resin was treated in the diluted HC1 solution to displace Na + by H +, and then treated in saturated CaCO3 solution to displace H + by Ca 2+. After washing with the distilled water, 1 cm 3 of wet Ca2+-resin was dispersed in the 300 cm 3 of the distilled water. Pure CO2 gas was introduced into the resin-dispersed solution at the constant flow rate (10 cm3.minl). Time variation of the pH value and Ca 2+ concentration of the resin-dispersed solution was analyzed by using pH / ion meter (Horiba K.K. model F-23 with pH and calcium ion electrodes).

mixing with diluted HC1 solution -i 9

l Na+.resin

i

-~

H+-resin

I

i

I addition of CaZ+-eontaining solution .....................

C02 bubbling

CO2 f i x a t i o n a n d r e l e a s e of Ca 2+

lI

t

Ca2+.resin

:

'.

Figure 1. Schematic diagram of the present reaction system

3. R E S U L T S A N D D I S C U S S I O N

The pH value of the resin-dispersed solution decreased from 7 to 3.5 when C02 was introduced. Immediately after few seconds of C02 introduction, the pH value increased to 4.5, and the pH value kept constant. The decrease in the pH value just after C02 introduction is due to that ionization of C 0 2 by the following equation.

675 C02

+

C02-H~O

H20

~-

~-

HCOa- ~--

C02"H20

(1)

COa 2-

(3)

+

H+ H§ +

HCO3-

(2)

The increase in the pH value indicates that some of H § formed by dissolving CO2 into the resin-dispersed solution were displaced by cation exchange reaction with Ca 2§ High pH value of C02-bubbled resin-dispersed solution compared to 3.5, which is attained by bubbling CO2 into pure water, is attributed to t h a t most of the H § formed by CO2 bubbling continuously displaced with Ca 2§ released from the resin. Time variation of the Ca 2§ concentration in the resin-dispersed solution in Figure 2 shows that the Ca 2§ concentration rapidly increased after introducing CO2 gas; the Ca 2§ concentration increased to 1 ppm in few seconds and to 1000 ppm in 10 min. The evolution of Ca 2§ is due to that pH value is increased because of CO2 dissolution. Variation of the Ca 2+ increase is not linear to the CO2 bubbling time. This suggests that rate determining step for Ca 2§ evolution is not CO2 dissolution step, since the CO2 bubbling rate was kept constant. From the fact that pH value of the resin-dispersed solution kept constant value, displacement of cation on the surface of cation exchange resin seems to be proceeded at a constant rate. The rate determining step may then be among following reaction steps. Ca2+-resin + 2 H + ~ -- H+-resin-H + Ca 2+ + CO32" ~-- CaCO3

40000

30000 o 20000 o9 r

9 ~9 + r

i0000

~

0

A . ~

A.

A . A .

5

-

_

10

15

20

Time (min) Figure 2. Time variation of Ca 2+ concentration in the resin-dispersed solution

+

Ca 2+

(4) (5)

Because the amount of Ca 2+ evolved by 10 minutes C O 2 bubbling was larger t h a n 1000 ppm, the present system may sufficiently fix CO2 from flue gas. Ca 2+ displacement by H + will proceed fast under acidic condition. This indicates that CO2 fixation rate will be enhanced when the pH value of resin-dispersed solution is low. Flue gas contains not only CO2 but also nitrogen oxide compounds, so that when flue gas is bubbled into the resin-dispersed solution instead of pure CO2 gas, evolution rate of Ca 2§ from the resin is expected to be enhanced. At the same time, if the pH value of resin-dispersed solution is too low, CO2 will not be dissolved in the solution. Further study on the reaction kinetics is necessary.

676 4. COUCLUSIONS Present C02 recovery system using materials harmless to global environment and mild reaction condition (ecotechnological system) was examined for the practical application from the viewpoint of reaction kinetics. Cation exchange reaction of Ca 2§ by H § generated by bubbling pure CO2 proceeded very fast, indicating that cation exchange resin may be applied for the present ecotechnological C02 recovery system. Rate determining step of the cation exchange reaction is either cation exchange step or the formation of insoluble CaC03 step. REFERENCES

1. C. Hendriks, Carbon Dioxide Removal from Coal-Fired Power Plants, Kluwer Academic Publishers, The Netherlands (1994). 2. T. Chohji and T. Korenaga, Proceedings of the International Symposium on C02 Fixation and Efficient Utilization of Energy, Tokyo, (1995) 227. 3. T. Chohji, M. Tabata and E. Hirai, Energy, 22 (1997) 151. 4. C. Nakagawa, T. Chohji and E. Hirai, Mizu Kankyou Gakkai-Si (J. Japan Soc. Water Environ.), 16 (1993) 114. 5. C. Nakagawa, T. Chohji and E. Hirai, Mizu Kankyou Gakkai-Si (J. Japan Soc. Water Environ.), 16 (1993) 175.

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide

677

Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

F o r m a t i o n of p e r o x o c a r b o n a t e s from L3Rh(O2)CI a n d L2Ni(CO2) : a u n i q u e r e a c t i o n m e c h a n i s m w i t h c a r b o n d i o x i d e i n s e r t i o n into t h e OO bond. Michele Aresta, a Eugenio Quaranta, a Immacolata Tommasi, a Jo/~lle Mascetti b, Michel Tranquille, b Marek Borowiak. c a Dipartimento di Chimica and Centro CNR-MISO, Universitb di Bari, Bari, Italy, b Laboratoire de Spectroscopie Mol(~culaire et Cristalline, Talence, France c Industrial Chemistry Research Institute, Warsaw, Poland. 1. INTRODUCTION Transition-metal peroxocarbonates of formula LnM(CO4)X m (L = ancillary ligand; n = 2, 3; X = halogen; m = 0, 1) are known since a long time but, despite the cospicuous synthetic work available in the literature, [1] very little was reported about their mechanism of formation. If the peroxocarbonate complexes are prepared by reaction of dioxygen complexes of transition metals LnM(O2)X m (M = P d , Pt, Rh, Ir) with carbon dioxide, [la] in principle, two ways are possible, that imply the formal insertion of carbon dioxide into the O-O (route 1, Scheme 1) or M-O (route 2, Scheme 1) bond. Scheme 1

route 1 / *0 M

/ *0

or

\

(+) route

2

M

COz

M

\.0 -

.ol*

d"

/ *0(+)

~.0( -)

-) d

~

/0~0.

M

/

~.< ) ~ C ~ O .

.

/~

COz

~

M

([

\ o / ~o

The mechanism of formation of RhCI(CO4)(PEt2Ph) 3 [2] from CO 2 and RhCl(~!2O2)(PEt2Ph) 3 and of Ni(CO4)(PCy3)2 from Ni(CO2)(PCy3)2 and 02 has been ascertained through the comparison of a normal coordinate analysis for the fragment [RhCI(CO4)(PC3)3] and [Ni(CO4)(PC3)2] respectively and the IR spectra of the labeled analogues of peroxocarbonate complexes obtained through the following reactions : RhCI0802)(PEt2Ph)3 + CO2 (reaction 1A) RhCI(O2)(PEt2Ph)3 + 0 8 0 2 (reaction 1B) Ni(CO2)(PC~3)2 + 1802 (reaction 2A) Ni(C1802)fPCy3)2 + 02 (reaction 2B)

678 2. EXPERIMENTAL SECTION General P r o c e d u r e . Unless otherwise stated, all reactions and manipulations were conducted under a dinitrogen or CO 2 atmosphere by using vacuum-line techniques. All solvents were dried as described in the literature [2] and stored under dinitrogen. CO 2 (>99.95%) and 02 (>99.998%) were from SIO SpA, C1802 080 97.7%) was from CEA-ORIS, 1802 080 99%) from SIC. FTIR spectra were recorded using a Bruker 113V Fourier transform interferometer. Frequencies are accurate to • cm-1. Solid samples were studied as Nujol mulls (Nujol was previously dried on sodium wire and bubbled with argon). Structural parameters and initial force field were taken from literature [3] and the program used to calculate the generalized valence force field was similar to Schachtschneider's one. [4]

Synthesis of ClRh(-160-lsO-C(1sO)160)(PEt2Ph)3 and ClRh(-laO-160C060)aaO)(PEt2Ph)3 A solid sample of RhCl(~I2-1802)(PEt2Ph)3 (0.300 g, 0.45 mmol) was dissolved in toluene (30 mL) saturated with C1602 at 243 K. The solution was then added with pentane (50 mL) and stored overnight at 243 K. The yellow solid that separated was filtered under Ar, washed with pentane and dried in vacuo. It was characterized by elemental, IR and NMR analyses. O-Transfer to Phosphine Ligand 0.150 g (0.21 mmol) of C1Rh(-180-160-C(160)lso)(PEt2Ph)3 were dissolved in 5 mL of CH2CI2. The solution was stirred at 313 K for I h and analyzed by GC-MS. The GC-MS analysis showed the formation of 180=P(Et2Ph) 3 and 160=P(Et2Ph) 3 in molar ratio 85:15. The experiment was repeated in the same conditions using CIRh(-1.60-180-C080)160)(PEt2Ph)3 . In this case, 180=P(Et2Ph) 3 and 160=P(Et2Ph) 3 were formed in 15:85 molar ratio.

Synthesis of Ni(-a60-lsO-C(180)160)(PCy3)2 and Ni(-180-160C060)180)(PCy3)3 0.150 g (0.36 mmol) of Ni(CO2)(PCy3)2 (solid state) were put in a Schlenk tube under 1802 atmosphere at 278 K. The system was allowed to react, u n d e r continuous stirring, until the initial yellow color of the powder was changed into pink-salmon. The same procedure was used by reacting Ni(C1802)(PCy3)2 with 160 2. Samples of equilibrium gas were withdrawn at regular time intervals and examined using the FTIR technique. The solid resulting sample was characterized by Infrared spectroscopy and elemental analysis. 3. RESULTS AND DISCUSSION 3. 1. The vibrations due to the peroxocarbonate moiety The analysis of the FTIR spectra of solid RhCI(CO4)(PEt2Ph)3 and its 180 labeled derivatives obtained through reactions 1A and 1B allowed a preliminary assignement of the spectroscopic vibrations due to the RhOOC(0)O moiety. The strong absorption present in the IR spectrum of RhCI(CO4)(PEt2Ph) 3 at 1665 cm -1 is assigned to the carbonyl stretching v(C--O).

679 Others absorption shifted by C1802 labeling are the medium band at 1249 cm q associated to one of the v(C-O) modes and the two sharp bands at 579 and 545 cm q which have isotopic effects of 6 and 10 cm q, respectively (assigned to the two stretching modes v(Rh-O) ). The medium strong sharp absorption at 963 cm q, which is shifted to 943 cm -1 upon 1802 labeling is assigned to the stretching of the O-O bond and the assigment of the bands at 579 and 545 cm q to v(Rh-O) is confirmed by their isotopic effects of 23 and 17 cm-1 by 1802. Bands at 808, 779, 380 and 322 cm -1 also show weak isotopic effects by C1802 a n d / o r 180 2 labeling and are therefore associated to vibrations of the I~hOOC(O)C) moiety. [5] In the case of the Ni(CO4)(PCy3)2 complex the bands sensitive to isotopic labeling were the strong absorption at 1685 cm-1 (assigned to v(C=O)), the medium bands at 1258 and 1250 cm -1 (assigned to v(CO)) and the medium band at 995 cm -1 (assigned to v(O-O)). 3.2. Mechanism of formation of peroxocarbonates from "Rh~2-O2 '' and CO2: a normal mode analysis Chart 1 shows all derivatives obtainable from RhCI(CO4)(PEt2Ph) 3 by isotopic labeling with 180 2 (molecules 2 and 3, Chart 1) and C180 2 (molecules 4 and 5, Chart 1) Chart 1

Rh(~lz-leOz) + C160z P

P. I ,,o

Rh(2 1802) + C160z (Reaction 1A)

P

-

p

P-I,,,"80--. 0 P

2

A

Rh(~lZ-160z) + C180 (Reaction 1B)

1

p

P-.I ,,'-0 "0 P

o

"E, c/

3

p

P'I

P

4

'~--~0

A

2

P

P . I .... 0--9 P

5

A normal mode coordinate analysis on the RhCl(CO4)(PEt2Ph)3complex has shown that from all derivatives obtainable the observed frequencies are in agreement only with the formation of species 2 and 4. As reported in the Experimental Section the Ni(CO4)(PCy3)2 complex is prepared by reacting Ni(CO2)(PCy3)2 with 02 according to reaction 2A or 2B. Also in the case of the Ni-complex 6 there were, in principle, 4 possible labeled analogues of Ni(CO4)(PCy3)2 species that could be observed. (Chart 2) Interestingly, a normal mode coordination analysis of the Ni(CO4)(PC3)2 fragment has shown that also in the case of the Ni-complex only species 7 and 9 are formed which can form only by insertion of the CO2 molecule in one O-O bond. This result and the finding that the FTIR spectrum of the gas evolved during the reaction have shown the presence of the CO2 free molecule, bring to

680

Chart 2

Ni( Z_ClsOz ) + lSOz R,,N i""Oz'--/04 P" 6

Ni('qZ-C160z) + 180z (Reaction 2A) P.

"

.,

0----0

CI'''N~'~'~)'~'/ 9 7

A

%0

P'"N -." O ~ " o

Cl'" ~0---/, 8

%0

Ni('qZ-C180z) + 1602 (Reaction 2B)

A

P.,

CIf N 9

o

,,, 0 ~ 0 CI~Ni'~'" / ~'O---C~ 0

10

the conclusion that the Ni-CO2 complex reacts with 02 by releasing CO2. Dioxygen is co-ordinated with formation of a Ni-O2 complex that undergoes CO2 insertion into the O-O bond affording the peroxocarbonate species. The described Nichel reactions are among tke very few solid-state organometallk reactions reported in the literature. 3. 3. Oxygen Transfer from Peroxocarbonate to Oxophiles When ClRh(-180-lSO-C(O)O)(PEt2Ph) 3 was dissolved in CH2CI 2 at roorr temperature the O-transfer reaction to phosphine took place according tc equation (1). ClRh(-180.160.C(O)180)(P)3 - - - - - >

RhCI(CO3)(P)2 + p=180

(1)

The GC-MS analysis of the solution has allowed to determine that phosphinc oxide formed contained more than 85-90% of 180. This suggests that the oxyger atom of the peroxo group transferred is that linked to Rh, more than that linkec to carbon. This result is confirmed also by the reaction of ClRh(A60-180 C(180)160)(PEt2Ph)3 that affords RhCI(CO3)(P) 2 and P=160 when dissolved ir CH2CI 2 at room temperature with a similar selectivity (85-90 % 160 on phosphin( oxide). [2] REFERENCES 1. a) P. J. Hayward, D.M. Blake, G. Wilkinson, C.J. Nyman, J. Am. Chem. Soc., 92 (1970) 5873; b) L. Malatesta, R. Ugo, J. Chem. Soc. (1963) 2080; c) M. Aresta, C.F. Nobile, J. Chem. Soc., Dalton Trans. (1977) 708; d) M. Aresta, E. Quaranta, A. Ciccarese, C 1 Mol Chem., 1, (1985) 267. 2. D.D. Perrin, W.L. Amarego, D.R. Perrin, Purification of Laboratory Chemicals; Pergamon press: Oxford England, 1986. 3. a) M. Fouassier, M.T. Forel, J. Mol. Struct., 26 (1975) 315; b) A. Nakamura, Y. Tatsuno, M. Yamamoto, S. Otsuka, J. Am. Chem. Soc., 93, (1971) 6052; c) C. Jegat, M. Fouassier, J. Mascetti, Inorg. Chem., 30 (1991) 1521; d) C. Jegat, M. Fouassier, M. Tranquille, J. Mascetti, I Tommasi, M. Aresta, F. Ingold, A. Dedieu, Inorg. Chem., 32 (1993) 1279. 4. J.H. Schachtschneider, R.G. Snyder, Spectrochim. Acta, 19 (1963) 117. 5. M. Aresta, I. Tommasi, E. Quaranta, C. Fragale; J. Mascetti, M. Tranquille, F. Galan, M. Fouassier, Inorg. Chem., 35 (1996) 4254.

681

Keyword Index A abiotic photosynthesis of amino acids acetic acid synthesis Acetobacter xylinum acetogenesis acetonitrile-water mixtures active site of carbonic anhydrase aerobic bacterium alloy 9 electrodes allyl halides allyl stannanes amorphous Ni-Zr alloys amorphous Ni-Zr-rare earth element alloys analysis of CO2 metabolism antioxidant activity Aresta's nickel CO2 complex aspects of CO2 utilization atmospheric CO2 fixation

B biochemical CO2 fixation biological CO 2 fixation biological sink of CO2 biomass production biomimetic mineralization of aqueous carbonate ~ons bioreactor biotechnology blue-green algae Spirulina platensis Boudouard reaction building materials C C 2 organic molecules cadmium sulfide nanocrystallites carbon dioxide behavior carbon dioxide capture carbon dioxide fixation carbon dioxide fixation by PNSB carbon dioxide fixation in PNSB carbon dioxide fixing microalgae carbon dioxide for petrochemicals feedstock. carbon dioxide hydrogenation carbon dioxide insertion into the O-O bond carbon dioxide methanation carbon dioxide mitigation carbon dioxide recovery from flue gas carbon dioxide reduction carbon dioxide reforming of methane carbon dioxide separation from nitrogen carbon dioxide utilization carbon multi-recycle system carbon-recycling energy delivery system carboxylation of allyl stannanes

189 439 653 303 581 309 613 573 165 165 451 261, 451 597 641 491 195 499 309 315,471 243 617 621 633 65 617 371 479 491 183 395 201 503,637 593 593,597 641 333 345, 407, 505, 533, 541 677 451 77, 273 673 31,43 375,395 665 65 273 379 165

682 carboxylation reaction with CO2 catalyst design catalytic behavior catalytic CO2 reduction catalytic conversion of CO2 catalytic fixation of CO2 catalytic hydrogenation of CO2 catalytic reaction of CO2 cellulose cellulose synthase genes chitosan-calcium alginate hydrogels chitosan-calcium carbonate composites Chlorella sp. chloroplast transformation method climate change cluster cobalt macrocycle composite catalysts conversion of CO 2 into cellulose copper-based catalysts countermeasures Cr203 catalyst crystallization Cu/Zn oxides Cu/ZnO catalysts Cu/ZnO/SiO 2 catalyst Cu/ZnO-based multicomponent catalysts cultivation of cyanobacterium CuO-ZnO-A1203 catalysts cyanobacterium cyclic carbonates

487 127 87 219 403 141 19,431 153 243 653 621 621 315, 483 609 1,9 171 97 327,435 653 505 9 419 601 225 529 509 549 471 545 629 403

D

dehydrogenation of ethylbenzene dehydrogenation of propane dielecric-barrier discharge dimethyl ether dinuclear Ni(II) complex durability of catalysts

415 419 541 447 499 517

E

economics assessment of power generation systems ecotechnological system effect of solvents electrocatalysts electrocatalytic reduction of CO2 electrochemical methods electrochemical reduction of CO2 electroreduction of CO2 electrospray mass spectrometry energy delivery system Escherichia Coli ethanol production from CO 2 ethanol synthesis ethanol synthesis from CO2 and H 2

367 673 553 43 207 213 107, 577, 581,585 225 557 285 601 657 431,517 525

683

ethene extreme thermophile

153 605

F

Fe promoted Cu-based catalysts Fe, Cu-based novel catalysts Fe-Cu-Na/zeolite composite catalysts fermentative microalgae Fischer-Tropsch synthesis formate dehydrogenase of Clostridium thermoaceticum formation of peroxocarbonates from L3Rh(O2)C1 and l_,zNi(CO2) fossil/solar energy hybridization system fuels and petrochemicals from CO2 functional dual-film electrode

427 513 423 657 159, 443 303 677 285 443 207

G gas diffusion electrodes gas turbine power generation system gene manipulation of microalgae global carbon-recycling energy delivery system graphite carbon green alga Neochloris sp.

225 297 653 273 147 637

H

helical tubular photobioreactor high performance liquid chromatography highly pressurized CO2 hybrid catalysts hydrocarbon synthesis from CO2 hydrogenation of CO2 Hydrogenobacter thermophilus strain TK-6

483 557 279 447 327 87,411,423,427,455, 517 613

I

IEA action incorporation of CO2 influence of anions influence of MgO addition infrared spectroscopic study initial transient rates lnterconversion of Ru-CO and Ru-h 1-COz ion exchanged(H, K)Zeolite-Y iron catalysts iron catalyzed CO2 hydrogenation iron oxide catalysts iron oxide-based catalyst

1

213 573 399 569 159 459 407 345, 407 339 387 415

K

kinetic analysis KNiCa catalyst Kolbe-Schmitt reaction

673 395 487

L lanthanoid complex light-diffusing optical device

503 633

684

liquid hydrocarbons liquid-phase methanol synthesis

339 521

M

marine cyanobacterium marine microalga utilization for a paper mechanical alloying mechanisms membrane reactor metal complexes metal electrodes metal supported gas diffusion electrode metal-particle modified p-Si electrode metallic electrodes methanation of CO2 methane methanol methanol as the intermediate methanol homologation using CO2 methanol solution methanol synthesis from CO2 and H 2 MgO surface microalgae micropores microprojectile bombardment mitigate CO 2 emissions molecular characterization molecular tailoring

237 649 529 127 147 97 569 577 565 107 261 147, 439 87, 447 537 495 589 267, 351, 3 57, 505, 509, 529, 533,545, 549 391 55 585 609 291 605 219

N

nature of CO 2 new synthetic routes for basic chemicals nickel macrocycle nucleic acid bases

391 19 97 189

O oil extraction oil recovery orgam c acids organic perfluoroalkyl derivatives organometallic CO2 complexes organometallic polymers organometallic reactions over-expressed effect of carbonic anhydrase oxidative dehydrogenation of ethylbenzene

279 201 189 213 255 219 127 629 387

P

Pd promoted Cu/ZnO/A1203 catalysts Pd/SiO 2 catalysts Pd-catalyst petroleum-degrading bacterium strain HD-1 phosphoenolpyruvate carboxykinase phosphoenolpyruvate carboxylase photobiological production of H 2

351 533 165 467 463 463, 601 321

685 photobioreactors photocatalytic CO2-fixation photocatalytic reduction of CO 2 photochemical reduction of CO 2 photoelectrochemical reduction of CO2 photosynthesis under salinity stress photosynthetic algae photosynthetic bacteria photosynthetic COg fixation photosynthetic performances plants platinum electrode polydentate ligands polymer blends polysaccharide-producing microalgae porphyrin production production of alkane production of alkene Production of poly hydroxyalkanoate production of polyethylene glycol microspher production of polyoxyalkylenealkylphenyl ether microsphere promoting effect of calcium addition promoting effects of CO 2 propylene Pt-Sn bimetallic catalysts pulsed electroreduction of CO 2 purity of CO2 purple non-sulfur bacteria

471 557 177, 183,553,561 97 565, 589 249 633 475 483 117 117 581 43 403 645 475 467 467 237 661 661 533 419 171 153 573 141 597

R

Raney copper-based catalysts recombinant phosphoenolpyruvate carboxylase redox system of ferrite reduction of COz reductive TCA cycle reforming of natural gas reversible absorption capacity of CO2 reversible oxide transfer reaction Rh ion exchanged zeolite catalysts rhodium catalyst rhodium catalysts Rhodopseudomonas sp. No.7 Ru-Co/AI2 O3-catalysts RuBisCO ruthenium catalysts ruthenium-cobalt bimetallic complex system

267 605 383 147 613 399 669 459 455 411 431 463 171 609 399 495

S

Scenedesmus komarekii selective formation of iso-butane semi-batch cultivation semiconductor photocatalysts sol-gel method solar cell solar energy

625 435 649 553 561 207 367, 379

686 solar energy carrier solar/chemical energy hybridization solar-collecting device Sp25 stable catalysts stable nickel-magnesia solid solution catalysts strategy structure-activity relationships super-RuBisCO supercritical CO2 supply of H 2 support effects Synechococcus sp. PCC7942 synthesis gas synthesis of chemicals synthesis of ethanol from CO2 and H 2 synthesis of gasoline from CO2 synthesis of lower olefins synthesis of renewable methanol

285 371 633 609 521 375 9 43 117 255, 661 141 345 629 333 65 513 537 407 363

T test plant tetraselmis sp. Tt- 1 thermochemical cycle Ti/Si binary oxide catalysts time-resolved infrared study titanium oxides tobacco mutant tolerance of a green alga tungsten-selenium-iron protein

357 649 383 561 255 177 609 625 303

U underground storage utilization of micro-algae

201 479

V

vanadium catalyst vegetation activity

439 231

W

waste heat from factories

297

X

X-ray studies

601

Y Y-type zeolite membranes

665

Z

zeolites zero emission power plants zinc(II) complex ZSM-5

177 279 309 387

687

Author

Index

A Akano, T. Akiyama, E. Akiyama, H. Amano, H. Ando, H. Ando, M. Anpo, M.

Arata, Y. Aresta, M. Armor, J.N. Asada, Y. Asakura, K. Asami, K. Augustynski, J.

545 261,451 237,653 273,285, 379 327, 423,435 415 177,387,395, 561 383 19,431,455, 525, 529 249 65, 677 141 321 455 451 107

B Bachiller-Baeza, B. Baltan~, M.A. Bandi, A. Bando, K.K. Bill, A. Bonivardi, A.L. Borowiak, M. Brenner, V. Brunschwig, B.S.

399 533 363 455 541 533 677 593 97

Aoki, A. Arakawa, H.

C Cabelli, D. Centola, P. Chadwick, D. Chang, J.-S.

Chiozza, E. Cho, I.-C. Chohji, T. Choi, D.-H. Choi, M.-J. Claeys, M.

97 333 351 177,387,395, 561 419 375 533 625, 637, 641, 645 213 509 673 407 345, 407, 447 159, 443

D Del Rosso, R. Desigaud, M.

333 213

Chang, W.-C. Chen, Y. Chiavassa, D.L. Chihara, M.

Dinjus, E. Draget, K.I. Dragos, L. DuBois, D.L. Dufiach, E.

127 621 195 43 213

E Ehara, S. Eliasson, B. Elser, M. Endo, N. Ezawa, M.

177 541 363 207 661

F Ferreira-Aparicio, P. Fiato, R.A. Flueraru, C. Foyer, C.H. Franks, R. Fujii, T. Fujii, Y. Fujimoto, K. Fujimura, H. Fujinaga, T. Fujishima, A. Fujita, Z. Fujiwara, H. Fujiwara, M. Fujiwara, Y. Fukui, H. Fukunaga, A. Furenlid, L.R.

399 339 195 609 165 463,475 177, 561 375 525 189 31,585, 589 97 183 327, 423,435 439 529 147 97

G George, M.W. Gollin, D.J. Gong, J.K. Goto, S. Greiner, J. Grills, D.C. Gronchi, P. Guerrero-Ruiz, A.

255 303 491 427 213 255 333 399

H Ha, C.S. Habazaki, H. Han, J.-K. Hanagata, N. Haneda, Y. Hara, H. Hara, K. Hashimoto, Kazuhito

403 261,451 471 625 517 537 577 585, 589

688 Hashimoto, Koji Hasuike, M. Hattori, T. Hayakawa, A. Hayashi, T. Hayashida, T. Hayashitani, M. He, D. Higuchi, K. Hinogami, R. Hinze, S.M. Hirai, E. Hirano, A. Hirano, M. Hirano, S. Hirata, S. Hirayama, S. Hirota, K. Hirotsu, T. Horns, N. Hon-Nami, K. Hori, H. Hori, T. Hori, Y.

261,451 649 415 669 243 439 617 423 517 565 491 673 649, 653,657 237, 545 621 617 657 589 479 153 649, 653,657 557 189 569, 581

Ishitani, O. Ishizuka, M. Ito, K. Ito, M. Ito, T. Ito, Y. Iwaki, T. Iwasa, T. Izawa, Y. Izui, K.

557 557 225 499 391 661 669 467 525 601,605

J Jeon, J.-K. Jermann, B. Ji, X. Jun, K.-W. Jung, M.-H. Jung, S.M.

505 107 201 345, 447 447 403

I Iantovski, E. Ibrahim, M. Ibusuki, T. Ichihashi, Y. Ichikawa, K. Ichikuni, N. Ifuku, K. Igarashi, Y. Iglesia, E. Ihara, Y. Ihm, S.-K. Ikeda, S. Ikeuchi, M. Ikeue, K. Imai, T. Imanaka, T. Inoue, S. Inoue, T. Inoue, Y. Inui, H. Inui, M. Inui, T. Isawa, J. I shida, N. Ishihara, J. Ishihara, T. Ishii, M. Ishikawa, K. Ishimaru, S.

279 309 557 177, 561 309 455 249 613 339 243 505 225 653 177 545 467 503 601 653 621 593,597 513,537 391 503 557 147, 499 613 249 573

Kawabata, S. Kawabe, M. Kawahira, K. Kawasaki, S. Kawashima, A. Kawata, M. Kedzierzawski, P. Kieffer, R. Kihara, S. Kim, D.-S. Kim, H. Kim, W.Y. Kishida, M. Kishimoto, H. Kitagawa, R. Kitamura, T. Kitayama, Y. Kobayashi, H. Kobayashi, M. Koch, H.J. Kodama, Tatsuya Kodama, Tohru Koga, O. Kogelschatz, U. Koike, K. Komori, M. Kosugi, T. Kosugi, Y. Kunimori, K.

K Kabeya, H. Kai, Y. Kanai, S. Kanda, Y. Karube, I.

479 601 237 249 625, 637, 641, 645 309 371 231 561 261,451 633, 63 7, 645 107 87, 327 189 509 345, 407 387 411 391 479 439 383 391 529 1 383 613 569 541 557 261, 451 367 487 455

689 Kunito, S. Kurano, N. Kuroda, K. Kuroda, T. Kusakabe, K. Kusama, H. Kushnirov, V.

649, 653,657 55 545 665 665 431,455, 529 279

Moil, H. Moil, K. Morikawa, M. Morita, M. Morooka, S. Murakami, M. Murakoshi, K. Muranaka, T.

183 357 467 483 665 315, 629 183 315, 629

L Lee, C.-S. Lee, D.-K. Lee, J.K. Lee, K.-W. Lee, S.B. Lee, S.-J. Li, X. Li, X.-L. Li, Z. Liu, B.-J. Liu, D. Liu, S.-M. Ljungdahl, L.G. Llorca, J. Luo, S.

5O9 505, 509 403 345, 407,447 471 345 375 303 201 553 201 303 303 153 267, 549

N Nagahisa, K. Nagano, T. Nagao, Y. Nagara, Y. Nagata, H. Nagata, T. Nagatani, M. Nakahara, Y. Nakai, T. Nakajima, H. Nakamura, T. Nakamura, Y. Nakano, H. Nakata, K. Nakato, Y. Nakayama, M. Nam, S.-S. Namiki, T. Nanba, M. Neacsu, M. Nezuka, M. Nicholas, K.M. Nishide, T. Nishiguchi, H. Noda, H. Nogami, G. Nomura, N. Nonoura, N.

467 463 669 601 411 601 661 517 243 459 605 565 237 309 565 207 407 9 633,637 195 371 165 315, 629 147 225 573 427 593

O Ochiai, T. Ogata, T. Ogura, K. Ogushi, Y. Ohkuma, K. Ohtani, M. Okabe, K. Okamoto, A. Okawa, M. Onizuka, T. Ono, H. Onoue, K. Otsuki, T.

613 97 207 649, 657 415 463 431,455, 529 525 585, 589 653 371 411 479

M

Mabuse, H. Maeda, K. Maeda, Y. Maezawa, S. Mascetti, J. Masuda, K. Mathieu, Ph. Matoba, R. Matsuguma, K. Matsukawa, R. Matsumura, H. Matsumura, Y. Matsuo, T. Matsuyama, K. McCauley, K. McGill, J.W. Metcalfe, I .S. Mimura, N. Mimura, T. Mishima, K. Mitsuoka, S. Miura, S. Miyachi, S. Miyasaka, H. Miyashita, Y. Momma, K. Moncmanov& A. Moon, J.Y.

521 189 231 379 677 649 279 475 249 625, 637, 641, 645 601 423 569 661 491 491 351 415, 419 669 661 669 383 55 237 147 593,597 291 403

690 P

Pak, P.S. Park, C.B. Park, D.R. Park, D.W. Park, K.-C. Park, S.-E.

297,367 471 177 403 505 177,387,395, 561 505 255

Park, Y.-K. Poliakoff, M. Preville, M.A. Prince, R.B.

491

Q Quaranta, E.

677

1

R

Rama Rao, K.S. Ramfrez de la Pisca, P. Rao, T.N. Renner, M.W. Rice, G.W. Riedel, T. Rodrfuez-Ramos, I. Roh, H.-S. Roh, J.H.

447 153 31 97 339 159, 443 399 395 597

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Sadaie, M. Sahibzada, M. Saijou, M. Saiki, H. Saito, M. Sakai, N. Sakata, Tadayoshi Sakata, Takao Sales, J. Samejima, Y. Sano, H. Sasaki, T. Sasaki, Y. Sato, F. Sayama, K. Scarlat, N. Schaub, G. Schulz, H. Shapovalova, L.B. Shi, M. Shiga, M. Shikanai, T. Shima, T. Shimamura, K. Shimoide, A.

463 351 463 483 267, 357, 419, 495, 549 641 577 183 153 649, 657 273,285, 55 495 249 431,455, 195 159, 443 159, 443 171 165 249 609 503 261,451 609

415, 521,

Shinoyama, H. Shiozaki, S. Shiratsuchi, R. Shishido, Y. Shlygina, I.A. Smidsrod, O. Soga, K. Soled, S.L. Sone, Y. Sonoyama, N. Souma, Y. Specht, M. Staiss, F. Suda, T. Sugimoto, H. Suh, I.S. Sun, X-Z. Susuki, J. Sutterer, A. Suzuki, H. Suzuki, T. Suzuki, Yasuo Suzuki, Yutaka T Tabata, K. Tabata, M. Tagawa, T. Takagawa, M. Takahara, I. Takahashi, K. Takeda, T. Takeguchi, T. Takeuchi, K. Takeuchi, Masami Takeuchi, Masato Takimoto, Y. Takishita, S. Takita, Y. Tamaura, Y.

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463 225 573 645 171 621 455 339 375 577 327, 423,435 363 363 669 503 471 255 225 491 669 463 561 297, 367 517 673 427 517, 525 267, 415, 419 487 243 537 557 267, 357, 549 561 315, 629 613 147, 499 273,285, 371, 379 641 435 459 423 439 661 411 491 391 495 243 375 581 609

691

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677 553 267, 549 677 31,585, 589 459 273,285, 371, 379 649

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87 297 657 613 653 357

Tommasi, I. Torimoto, T. Toyir, J. Tranquille, M. Tryk, D.A. Tsuge, K. Tsuji, M.

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693 S T U D I E S IN SURFACE SCIENCE A N D CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume

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Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings ofthe 32nd International Meeting ofthe Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, u Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.l. Yermakov, B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Ldzni~ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P. Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties- Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P. Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilomar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G,I, Golodets

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Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings ofthe Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P.JinX, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited byJ. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~evar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11, 1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine

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Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F. Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary ofthe Catalysis Society of Japan edited byT. Inui Transition Metal Oxides. Surface Chemistry and Catalysis byH.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, Wfirzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T, Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings ofthe Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura

696 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F. Trifiro Olefin Polymerization Catalysts. Proceedings of the International Symposium Volume 56 on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Heterogeneous Catalysis and Fine Chemicals I1. Proceedings of the 2nd Volume 59 International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Chemistry of Microporous Crystals. Proceedings of the International Symposium Volume 60 on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Characterization of Porous Solids I1. Proceedings of the IUPAC Symposium Volume 62 (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P. Grange and B. Delmon New Trends in CO Activation Volume 64 edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. (Shlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf0red, September 10-14, 1990 edited by L.I. Simandi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13, 1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Experiments by M. Kiskinova Volume 55

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Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings ofthe 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and P.Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings ofthe Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5-8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.E Howe New Developments in Selective Oxidation II. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids II1. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger

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Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis I1. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by u Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F. Vansant, P. Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, I-lungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution Control II1. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Quebec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J. Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dajbrowski and V.A. Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995 edited by M. Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 1lth International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 1lth ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J. W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon, S.I. Woo and S. -E. Park Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzifiski, W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 1lth International Zeolite Conference, Seoul, Korea, August 12-17, 1996 edited by H. Chon, S.-K. Ihm and Y.S. Uh

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Hydrotreatment and Hydrocracking of Oil Fractions Proceedings ofthe 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment, B. Delmon and P. Grange Natural Gas Conversion IV Proceedings of the 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23, 1995 edited by M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings of the 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12, 1996 edited by H.U. Blaser, A. Balker and R. Prins Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium, Antwerp, Belgium, September 15-17, 1997 edited by G.F. Froment and K.C. Waugh Third World Congress on Oxidation Catalysis. Proceedings of the Third World Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings ofthe 7th International Symposium, Cancun, Mexico, October 5-8, 1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings of the 4th International Conference on Spillover, Dalian, China, September 15-18, 1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4, 1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide Proceedings of the Fourth International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11, 1997 edited by 1".Inui, M. Anpo, K. Izui, S. Yanagida and T. Yamaguchi

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