Adsorption Science and Technology: Proceedings of the 3rd Pacific Basin Conference Kyongju, Korea May 25-29 2003

Proceedings of the Third Pacific Basin Conference on

Adsorption Science and Technology

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Proceedings of the Third Pacific Basin Conference on

Adsorption Science and Technology May 25-29,2003

Kyongju, Korea

Editor

Chang-Ha lee Yonsei University, Korea

r heWorld Scientific

.

New Jersey London Singapore Hong Kong

Published by

World Scientific Publishing Co. Re. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661 UK oflce: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

ADSORPTION SCIENCE AND TECHNOLOGY Proceedings of the Third Pacifc Basin Conference Copyright 0 2003 by World Scientific Publishing Co. Re. Ltd. All rights reserved. This book, or parts thereof; may not be reproduced in any form or by any means, electronic or mechanical, includingphotocopying, recording or any informationstorage and retrieval system now known or to be invented, without written permission from the Publisher.

For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923,USA.In this case permission to photocopy is not required from the publisher.

ISBN 981-238-349-2

Printed in Singapore

Preface The Third Pacific Basin Conference on Adsorption Science and Technology was held fiom May 25 to May 29, 2003 in Kyongju, Korea. The theme for this conference was “thinking about adsorption at a splendid, enjoyable, and sound conference.” It was the first time that an international conference on adsorption was ever held in Korea. Since the previous conferences organized by Professor K. Kaneko (1997) and Professor D. D. Do (2000) were very successful, I was very excited as well as very nervous when I was asked to organize this conference for I wanted to make this one as successful as the previous ones. The main purpose of this conference was to encourage the development of new adsorption science and technology as well as to reflect the growth of this area. The conference covered a variety of adsorption-related fields from fundamentals to applications. The conference consisted of plenary and invited sessions, oral sessions and poster sessions. And the conference areas were as follows: Fundamentals of Adsorption and lon Exchange, New Materials, Adsorption Characterization, Novel Processes, Energy and Environmental Processes. I was very happy to see many contributions from 16 countries, with more than 120 papers. The plenary lectures of Professors D. D. Do (Univ. of Queensland, Australia), K. E. Gubbins (North Carolina State Univ., USA), K. Kaneko (Chiba Univ., Japan), M. Morbidelli (ETH Ziirich, Switzerland), A. L. Myers (Univ. of Pennsylvania, USA), D. M. Ruthven (Univ. of Maine, USA), R. Ryoo (KAIST, Korea), S. Sircar (Leigh Univ. USA), M. Suzuki (United Nations Univ., Japan), and R. T. Yang (Univ. of Michigan, USA) set the tone for the theme of the conference. Also, I would like to thank Professors G.Baron, A. Neimark, and L. Zhou for their contribution as invited speakers. Plenary speakers presented an in-depth overview of key research areas: Materials, Characterization, Molecular Simulations, Equilibria, Kinetics, and Processes. Furthermore, many contributed papers were of high standard. 1 hope that this conference was a worthwhile and memorable one for all the participants. I would like to thank all the participants for all the contributions to the conference. I would like to take this opportunity to thank all the reviewers for their efforts to review papers within a very short period of time. Thanks should also go to the members of the organizing committee and secretary, the advisory and scientific committee and session chairs for their input and assistance. Also, special thanks goes to Professor J. Ritter (Univ. of South Carolina, USA) for the organization of the US side. And I would like to express my gratitude to Yeong-Joo Park, my wife, for all of her support and I thank my graduate students for all of their hard work to make this conference work. The conference would not have been possible without the generous financial support from many organizations such as the National Science Foundation, Yonsei University, Korean Institute of Chemical Engineers, KOSEF, KRF, Daesung Sanso Co., Research Institute of New Energy and Environmental Systems at Yonsei Univ., Yonsei Center for Clean Technology, Chonnam National University, Chungnam National University, NRL for Themophysical Properties, ERC for the Advanced Bioseparation Technology, NRL for Separation Process, NRL for Environmental Materials & Process. Professor Chang-Ha Lee Chairman of the Third Pacific Basin Conference on Adsorption Science and Technology Department of Chemical Engineering, Yonsei University, Korea

V

Sponsors Yonsei University Korean Institute of Chemical Engineers The Korean Federation of Science and Technology Societies Korea Science and Engineering Foundation Korea Research Foundation Daesung Sanso Co. National Science Foundation (USA)

Co-Sponsors Research Institute of New Energy and Environmental Systems at Yonsei University Yonsei Center for Clean Technology Chonnam National University Chungnam National University ERC for the Advanced Bioseparation Technology NRL for Themophysical Properties NRL for Separation Process NRL for Environmental Materials & Process

vi

Conference Chair Chang-Ha Lee

(Yonsei Univ., Korea)

Conference Advisory Committee B. H. Ha H. Lee H. K. Rhee

(Hanyang Univ.) (Yonsei Univ.) (Seoul National Univ.)

Organizing Committee S. H. Cho D. K. Choi S. H. Hyun J. W. Jang H. Kim Y. M. Koo C. S. Lee H. Moon D. S. Park S. K. Ryu Y. G. Shul J. E. Sohn J. Yi

(KIER) (KIST) (Yonsei Univ.) (SK Eng. & Construction) (Seoul National Univ.) (Inha Univ.) (Korea Univ.) (Chonnam Univ.) (Daesung Sanso Co.) (Chungnam Univ.) (Yonsei Univ.) (Dong-A Univ.) (Seoul National Univ.)

International Advisory Committee A. S. T. Chiang D. D. Do K. E. Gubbins K. Kaneko 2. Li M. Morbidelli A. L. Myers J. Ritter J. L. Riccardo D. M. Ruthven H. Tamon R. T. Yang H. Yoshida L. Zhou

Wational Central Univ., Taiwan) (Univ. of Queensland, Australia) (North Carolina State Univ., USA) (Chiba Univ., Japan) (South China Univ. of Technology, China) (ETH Zurich, Switzerland) (Univ. of Pennsylvania, USA) (Univ. of South Carolina, USA) (UNSL, Argentina) (Univ. of Maine, USA) (Kyoto Univ., Japan) (Univ. of Michigan, USA) (Osaka Prefectural Univ., Japan) (Tianjin Univ., China)

Scientific Advisory committee T. Bandosz G . Baron M. Bulow G. Carta K. Chihara C. B. Ching J. Izumi M. Jaroniec

(The City College of New York, USA) (Vrije Universiteit Brussel, Belgium) (The BOC Group, Inc., USA) (Univ. of Virginia, USA) (Meiji Univ., Japan) (The National Univ. of Singapore, Singapore) (Mitsubishi Heavy Chem. lnd., Ltd., Japan) (Kent State Univ., USA)

Vii

J. U. Keller J. M. Lee K. H. Lee N. Lemcoff M. D. LeVan M. Mazzotti G. McKay F. Meunier J. K. Moon A. Neimark Y. D. Park A. E. Rodrigues W. Rudzinski R. Ryoo A. Sakoda N. Seaton S. Sircar W. A. Steele M. Suzuki 0. Talu Y. Teraoka Y. Xie

(Univ. of Siegen, Germany) (KRICT, Korea) (Pohang Univ., Korea) (The BOC Group, Inc., USA) (Vanderbilt Univ., USA) (ETH ZUrich, Switzerland) (Hong Kong Univ. of Sci. and Tech., Hong Kong) (Laboratoire du Froid CNAM, France) (KAERI, Korea) (TRI/Princeton, USA) (FUST, Korea) (Univ. of d Porto, Portugal) (Maria Curie-Sklodowsk Univ., Poland) (KAIST, Korea) (Univ. of Tokyo, Japan) (Univ, of Edinburgh, UK) (USA) (Pennsylvania State Univ., USA) (United Nations Univ., Japan) (Cleveland State Univ., USA) (Kyushu Univ., Japan) (Peking Univ., China)

Organizing Secretary C. H. Cho S. S. Han K. T. Lee J. Y. Yang

(Youngdong Univ.) (KIER) (Yonsei Univ.) (SK Eng. & Construction)

...

Vlll

Contents

Preface

V

Plenary Papers Adsorption Equilibria of Sub-critical and Super-critical Fluids in Carbonaceous Materials D. D. Do and H. D. Do

1

FreezingMelting in Porous Carbons E R. Hung, R. Radhakrishnan, E Beguin, M. Sliwinska-Bartkowiak and K. E. Gubbins

9

Measurement of Diffusion in Microporous Solids D. M. Ruthven

17

Ordered Mesoporous Carbons with New Opportunities for Adsorption Studies R. Ryoo and S. H. Joo

27

Quantum Micropore Filling and its Application Possibility T Tanaka, I: Hattori, K. Murata, T. Kodaira, M. Yudasaka, S. I@ma and K. Kaneko

35

Adsorption in Microporous Materials: Analytical Equations for TYPE I Isotherms at High Pressure A. L. Myers

44

New Sorbents for Desulfurization of Transportation Fuels R. T. Yang, A. Hernandez-Maldonado, A. Takahashi and E H. Yang

51

Optimization of Continuous Chromatography Separations 2. Y. Zhang, M. Mauottiand M. Morbidelli

64

Adsorption Technology for Gas Separation S. Sircar

72

Carbon Composite Membranes M. Suzuki, A. Sakoda, S.-D. Bae, T Nomura and Y.-Y. Li

79

Invited Papers

On the Dominant Role of Adsorption Effects in Heterogeneous Catalysis J. E Denayer, G. V Baron, D. Devos, J. A. Martens and R A. Jacobs

iX

87

Supercritical Adsorption: Paradox, Problems, and Insights L. Zhou

91

Contributed Papers

Microwave Drying for Preparation of Mesoporous Carbon H. Tamon, T. Yamamoto, T. Suzuki and S. R. Mukai

99

Computer Simulation of Transport in Cylindrical Mesopores S. K. Bhatia and D.Nicholson

104

Multicomponent Mass Transfer Diffusion Model for the Adsorption of Acid Dyes on Activated Carbon K. K. H. Choy, J. E Porter and G. McKay

109

Sorption Thermodynamics of Nitrous OxideLSX Zeolite Systems M. Biilow, D. Shen and S. R. Jale

114

Activated Carbon Membrane with Carbon Whisker S.-D. Bae and A. Sakoda

121

Mesoporous Silica with Local MFI Structure S. P. Naik, A. S. T. Chiang, R. W Thompson, E C. Huang and H.-M.Kao

126

Infinite Dilution Selectivity Measurements by Gas Chromatography S. Gumma and 0. Talu

131

Adsorption Properties of Colloid-Imprinted Carbons M. Jaroniec and 2.-J. Li

136

On the Role of Water in the Process of Methyl Mercaptan Adsorption on Activated Carbons S. Bashkova, A. Bagreev and T. J. Bandosz

141

Studies on the Adsorption Properties of Ion-Exchanged Low Silica X Zeolite H. Jiang, W Tang, J. P. Zhang, B. I! Zhao and I! C. Xie

147

Carbonization of Organic Wastes Using Super-Heated Water Vapor and Their Adsorption Properties H. Yoshida, N. Miyagami and M. Terashima

152

Further Successful Applications of the New Theoretical Description of Adsorptioflesorption Kinetics Based on the Statistical Rate Theory #? Rudzinski and T. Panczyk

157

Characterization and Ethylene Adsorption Properties of Silver-Loaded FER Zeolite Potentially Used as Trap Material of Cold-Start Hydrocarbon Emission from Vehicles !I Teraoka, H. Onoue, H. Furukawa, I. Moriguchi, H. Ogawa and M. Nakuno

162

X

Pressure-Dependent Models for Adsorption Kinetics on a CMS Z-S.Bae, I!-K. Ryu and C.-H. Lee

167

Preparative Enantioseparation of Fluoxetine by Simulated Moving Bed H.-W Yu and C. B. Ching

172

Optimization Based Adaptive Control of Simulated Moving Beds G. Erdem, S. Abel, M. Mauotti, M. Morari and M. Morbidelli

177

Mono-Methyl Paraffin Adsorptive Separation Process S. Kulprathipanja, J. Rekoske, M. Gutter and S. Sohn

182

Chromium (VI) and (111) Species Adsorption from Aqueous Solutions by Activated Carbon Fibers 0. Astachkina, A. Lyssenko and 0. Muhina

189

Treatment of Complex Wastewaters by Biosorption and Activated Carbon: Batch Studies C. Gerente, 2. Reddad, Z: Andres, C. Faur-Brasquet and I? le Cloirec

194

Adsorption Characteristics of Protein-Based Ligand for Heavy Metals M. Terashima, N. O h ,T. Sei, K. Shibata and H. Yoshida

199

Preparative Chromatography at Supercritical Conditions A. Rajendran, M. Mauotti and M. Morbidelli

204

Adsorptive Separation of Oligosaccharides: Influence of Crosslinking of Cation Exchange Resins J. A. Vente, H. Bosch, A. B. de Haan and P. J. T. Bussmann

209

Identification and Predictive Control of a Simulated Moving Bed Process I.-H. Song, H.-K. Rhee and M. Mazzotti

214

Quick and Compact Ozonation Using Siliceous Zeolite H. Fujita, T. Fujii, A. Sakoda and J. Izumi

219

Time Resolved Multicomponent Sorption of Linear and Branched Alkane Isomers on Zeolites, Using NIR Spectroscopy A. F: P. Ferreira, M. Mittelmeijer, M. Schenk, A. Bliek and B. Smit

224

Pore Size Effects in the Liquid Phase Adsorption of Alkanes in Zeolites J. F: M. Denayer, K. de Meyer, J. A. Martens and G. K Baron

229

Detection of Freezing Point Elevation in Slit Nanospace by Atomic Force Microscopy M. Miyahara, M. Sakamoto, H. Kanda and K. Higashitani

234

xi

Modeling of High-pressure Equilibrium Adsorption of Supercritical Gases on Activated Carbons. Determination of Pore Size Distribution Using a Combined DFT and EOS E. A. Ustinov and D. D. Do

239

On the Peculiarity of the Minimum of N-Hexane Permeability in Activated Carbon J.-S. Bae and D. D. Do

244

Simplified Experimental Method to Analyse Intra-Activated Carbon Particle Diffusion Based on Parallel Diffusion Model I: Miura, I: Otake, H. T. Chang, N. Khalili, S. Iwasawa and E. G. Furuya

249

In-Situ Characterization of Ion Adsorption at Biomimetic Airwater Interfaces T. E Kim, G. S. Lee and D. J. Ahn

254

Single and Multi Component Adsorption of Volatile Organic Compounds onto High Silica Zeolites - Discussion of Adsorbed Solution Theory I? Monneyron, M. -H. Manero and J. -N. Foussard

259

Influence of VOCs Molecular Characteristics on Exothermicity of Adsorption onto Activated Carbon l? Pre, C. Faur-Brasquet and I? le Cloirec

264

The Influence of Ar and He on the Rate of Adsorption and on the Adsorption Equilibrium of Alkanes in Zeolites M. C,Mittelmeijer-Hazeleger,A. E l? Ferreira and A. Bliek

270

Modeling the Discharge Behavior of Metal Hydride Hydrogen Storage Systems S. A. Gadre, A. D. Ebner, S. A. Al-Muhtaseb and J. A. Ritter

276

The Advanced Modeling Technology for Periodic Adsorption Process: Direct Determination of Cyclic Steady State J.-H. Yun, A. C. Stawarz and E 0. Jegede

28 1

Adsorption and Desorption Characteristics of Zeolite Impregnated Ceramic Honeycomb for VOC Abatement H.-S. Kim, l!-J. Yoo, E-S.Ahn, M.-K. Park, K.-T. Chue and M.-H. Han

286

Reverse Flow Adsorption Technology for the Recycling of Homogeneous Catalysts: Selection of Suitable Adsorbents J. Dunnewijk, H. Bosch and A. B. de Haan

29 1

Molecular Simulation of Gas Separation by Adsorption Processes J. I? B. Mota

296

xii

Metal-Doped Sodium Aluminium Hydride as a Reversible Hydrogen Storage Material J. Wang, A. D. Ebner, K. R. Edison, J. A. Ritter and R. Zidan

30 1

Synthesis and Dehumidification Behaviors of Monodisperse Spherical Silica Gels with Different Pore and Chemical Structures C,H. Cho, Y. J. Yoo, J. S. Kim, H. S. Kim, Y. S. Ahn and M. H. Han

306

Production of Hard Carbons for Lithium Ion Storage by the Co-Carbonization of Phenolic Resin Precursors S. R. Mukai, T. Tanigawa, T Harada, T. Masuda and H. Tamon

313

Novel Bioactivite Carbomineral Sorbents, Including Cluster and Carbon Nanotubes for Superselective Purification of Biodiesel Fuel - Liquid Hydrocarbons and Carbonhydrate from Sulfur Containing Impurities D. I. Shvets

318

Titanosilicate ETS-10: Synthesis, Characterization and Adsorption for Heavy Metal Ions G. X. S. Zhao, J. L. Lee and f? A. Chia

324

Ordered Macroporous Materials Structurally Templated by Colloidal Microspheres Z. Zhou, ?C. -I Ong ? and G. X. S. Zhao

329

Adsorption of Nitrogen, Oxygen and Argon in Transition and Rare Earth Ion Exchanged Zeolites A and X R. K Jasra, J. Sebastian and C. D. Chudasama

334

Adsorption of Methylene Blue from Water onto Activated Carbon Prepared from Coir Pith, an Agricultural Solid Waste C. Namasivayam and D. Kavitha

339

Separation of Oxygen-Argon Mixture by Pressure Swing Adsorption X . Jin and S. Farooq

344

Dual Reflux Pressure Swing Adsorption Cycle for Gas Separation and Purification A. D. Ebner and J. A. Ritter

349

Simulation of a Coupled MembranePSA Process for Gas Separation I. A. A. C. Esteves and J. I! B. Mota

354

I3CO and l2C0 Separation on Na-LSX using Pressure-Swing Adsorption at Low Temperatures J. Izumi, N. Fukuda, N. Tomonaga, H. Tsutaya, A. Yasutake,

359

A. Kinugasa and H. Saiki

xiii

High Purity Oxygen Generation PSA Process by Using Carbon Molecular Sieve J.-G. Jee, T-H. Kwon and C.-H. Lee

365

A Study on the Preparation of Deodorizing Fibers by Coating TiOz S. W Oh, H. J. Kim and S. M. Park

370

Composite Adsorbents for the Removal of Cs and Sr Ions in Acidic Solutions J. K. Moon, C. H. Jung, S. H. Lee, E. H. Lee, H. T. Kim and I! G. Shul

375

Dehumidification Behavior of Metal(Ti, Al, Mg) Silicates Impregnated Ceramic Fiber Sheets I! S. Ahn, C. H. Cho, I! J. Yoo, J. S. Kim, H. S. Kim and M. H. Han

38 1

Synthesis of Zirconia Colloids from Aqueous Salt Solutions and Their Applications K. Lee, P. W Carr and A. V McCormick

387

Comparison of Nano-Sized Amphiphilic Polyurethane (APU) Particles with SDS, an Anionic Surfactant for the Soil Sorption and the Extraction of Phenanthrene from Soil I . 3 . Ahn, H.-S. Choi and J.-Z Kim

392

Synthesis of Mesoporous Activated Carbon with Iron Ion-Aided Activation Z Seida, K. Watanabe and Y: Nakano

398

Separation of Peptides from Human Blood by RP-HPLC S.-K. Lee, I! Polyakova and K.-H. Row

403

Separation of Acanthoside-D in Acanthopanax Senticosus by Preparative Recycle Chromatography S . 2 Hong, D.-X. Wang and K.-H. Row

408

Use of Various Forms of &aft Lignin for Toxic Metal Uptake D. R. Crist, R. H. Crist and J. R. Martin

413

Removal of Uranium Ions in Sludge Waste by Electrosorption Process C.-H. Jung, J.-K. Moon, S.-H. Lee, Z-G.Shul and W-Z. Oh

417

Ion Exchange Characteristics of Palladium from a Simulated Radioactive Liquid Waste S.-H.Lee, C.-H. Jung, J.-K. Moon, J. H. Kim and H. Chung

422

Application of Characterization Procedure for Complex Mixture Adsorption in Water and Wastewater Treatment S.-H.Kim, T.-W Kim, D.-L. Cho, D.-H.Lee and H. Moon

427

xiv

Surface Characteristics of MCM-41 on Cr(1II) and Cr(V1) Adsorption Behaviors S. J. Park, B. R. Jun and M. Han

432

Influence of Anodic Oxidation of Activated Carbon Fibers on the Removal of Heavy Metal in Aqueous Solution S. J. Park, !I M. Kim and J. R. Lee

437

Kinetics and Diffusion Processes for Reactive Dye Adsorption by Dolomite S. J. Allen, G. M. Walker, L. Hansen and J.-A. Hanna

442

Permeate Flux Behavior During Microfiltration of Protein-Adsorbed Microspheres in Stirred Cell I: Chang, S.-W Choi, T-G. Lee, S. Haam and W-S. Kim

447

Surface Fractional Dimensions of the Adsorbents from Industrial Sludge J. H. You, H. M. Wu and 2. X. Fang

452

Adsorption of Acidic Peptide on Crosslinked Chitosan Fiber: Equilibria N. Kishimoto and H. Yoshida

458

Removal of Salt and Organic Acids from Solution used to Season Salted Japanese Apricots (Ume) by Combining Electrodialysis and Adsorption W Takatsujiand H. Yoshida

463

Studies on the One-Column Analogue of a Four-Zone SMB Y. S. Kim, C. H. Lee, Y. M. Koo and I? C. Wankat

468

Advanced' Flue Gas Treatment by Novel de-SOX Technology over Active Carbon Fibers M.-A. Yoshikawa, A. Yasutake and I. Mochida

474

Adsorption of Natural Gas Components on Activated Carbon for Gas Storage Applications I. A. A. C. Esteves, M. S. S. Lopes, I? M. C. Nunes, M. E J. Eusibio, A. Paiva and J. I? B. Mota

479

Prediction of Breakthrough Curves for Toluene and Trichloroethylene onto Activated Carbon Fiber J.-M? Park, S.-S. Lee, X-W Lee and D.-K. Choi

484

Catalytic Reduction Mechanism of Nitric Oxide over ACFslCopper Catalyst S. J. Park, B. J. Kim and X S. Jang

489

xv

NO Removal of Activated Carbon Fibers Treated by Cu Electroplating S. J. Park, J. S. Shin and J. R. Lee

494

The Appliance Study of 02-PSA in the Oxygen Activated Sludge Process S. H. Lee, P. S. Yong, H. M. Moon and D. S. Park

499

Application of Solid Adsorbent for VOC Monitoring Sensor 0. J. Joung and E H. Kim

504

PSA for Solvent Recovery with USY-Type Zeolite; an Experimental and a Simulation Study K. Chihara, T. Kaneko, T. Aikou and S. O h

509

Azeotropic Adsorption of Organic Solvent Vapor Mixture on High Silica Zeolite, Experimental & Simulation K. Chihara, K. Hijikata, H. Yamaguchi, H. Suzuki and E Takeuchi

514

VOC Enrichment by a VSA Process with Carbon Beds J. Yang, M. Park, J.-W Chang and C.-H. Lee

519

Isobutane Purification by Pressure Swing Adsorption S.4. Han, J.-H. Park, J.-N. Kim and S.-H. Cho

524

Characteristics of Gas Separation by Using Organic Ternplating Silica Membrane J. Moon, S. Hyun and C.-H. Lee

529

Adsorber Dynamics of Binary and Ternary Hydrogen Mixture in Activated Carbon and Zeolite 5A Beds M.-B. Kim, J.-S. Kim, C.-H. Cho and C.-H. Lee

534

Temperature Programmed Adsorption ( P A ) of Various Hydrocarbons on Adsorbers of Honeycomb Type D. J. Kim, J. E. fie, E S. Oh and J. M. Kim

539

Non-Isothermal Dynamic Adsorption and Reaction in Hydrocarbon Adsorber System D. J. Kim, W. G. Shim, J. E. Yie and H. Moon

544

Sorption of U(V1) onto Granite: Kinetics and Reversibility M. H. Baik and P. S. Hahn

549

Adsorption of Uranium (VI) on Kaolinite: Speciation and Mechanism M. J. Kang, B. E. Han and P. S. Hahn

554

High-Temperature Adsorption of Hazardous Metal Chlorides Using Activated Kaolinite H. C. Yang, J. S. Yun, E J. Cho and J. H. Kim

559

xvi

Probing the Cut-Off for Intracrystalline Adsorption on Zeolites: Pore Mouth Adsorption R. Ocakoglu, J. E M. Denayer, J. A. Martens, G. B. Marin and G. I! Baron

564

The Low-Temperature Sorption Behaviour of Cryosorbent Materials C. Day and I! Hauer

569

Thermal and Surface Mechanism Studies on Adsorption-Temperature Programmed Desorption of Nitrogen Oxides over Chemically Activated Carbon Fiber H.-J. Kim, X-W Lee, E. Lee and D.-K. Choi

574

Surface Properties of Activated Carbons Containing Basic Hydroxide Ions and NOx Adsorption-Desorption Process X W Lee, H. J. Kim, D. K. Choi, J. W Park C. H. Lee and B. K. Na

579

Adsorption Characteristics of Nitrogen Compounds on Silica Surface H. J. Kim, C.-H. Lee, X G. Shul and W S. Min

584

Adsorption Characteristics of VOCs on Mesoporous Sorbents W G. Shim, M. S. Yang, J. W Lee, S. H. Suh and H. Moon

589

Molecular Simulation for Adsorption of Halocarbons in Zeolites K. Chihara, T. Sasaki, S. Miyamoto, M. Watanabe, C. E Mellot-Draznieks and A. K. Cheetham

595

Adsorption of BTX on MSC in Supercritical C02,a Chromatographic Study K. Chihara, N. Omi, E Znoue, T Yoshida and T Kaneko

600

Porous Alumina with Bimodal Pore Size Distribution as an Organic Adsorbent I: Kim, C.Kim, P Kim, . I . C. Park and J. yi

605

Storage and Selectivity of Methane and Ethane into Single-Walled Carbon Nanotubes X-G. Seo, B. H. Kim and N. A. Seaton

610

Hydrotalcites for Carbon Dioxide Adsorbents at High Temperature J. I. Yang, M. H. Jung, S.-H. Cho and J.-N. Kim

615

Effect of Polarity of Polymeric Adsorbents on Desorption of VOCs under Microwave Field X. Li, 2. Li, H. X. Xi and H. Wang

620

Mixed-Gas Adsorption on Heterogeneous Substrates in the Presence of Lateral AD-AD Interactions A. J. Ramirez-Pastor, E M. Bulnes and J. L. Riccardo

625

xvii

Adsorption on Correlated Disordered Substrates R. H. Lopez, E M. Bulnes, E Rojas, J. L. Riccardo and G. Zgrablich

630

Temperature Effects on the Scaling Properties of Adsorption on Bivariate Heterogeneous Surfaces E Rom*, E Bulnes, A. J. Ramirez-Pastor and G. Zgrablich

635

Adsorption of Polyatomic Species: An Approach from Quantum Fractional Statistics J. L. Riccardo, A. J. Ramirez-Pastor and E Rom’

640

Multilayer Adsorption with Multisite-Occupancy E Rom’, A. J. Ramirez-Pastor and J. L.. Riccardo

645

xviii

Plenary Papers

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ADSORPTION EQUILIBRIA OF SUB-CRITICAL AND SUPER-CRITICAL FLUIDS IN CARBONACEOUS MATERIALS

D.D.DO AND H. D.DO Department of Chemical Engineering, University of Queensland, St. Lucia, Qld 4072, Australia E-mail: [email protected] In this paper, we present an overview of a number of techniques used to characterize the adsorption equilibria of sub and super-critical fluids in nonporous carbon black and porous activated carbon. Tools such as the grand canonical Monte Carlo simulation (GCMC), Density Functional Theory (DFT), Molecular Layer Structure Theory (MLST) of Ustinov and Do, and enhanced molecular layering and pore filling proposed by Do and his co-workers will be discussed. Although the GCMC provides the brute force calculation of molecular interactions and its results have been used as a benchmark for other techniques to compare with, its extremely time consuming computation makes the other techniques more appealing to engineers and experimentalists. Despite the ever-increasing computing power of today personal computer, the advantage of simpler methods is warranted for their role in solving practical problems. Furthermore, added to this advantage is the good perfarmance in terms of the prediction power of the simpler methods. AN these tools will be discussed in this paper regarding their applications to adsorption of super and sub-critical fluids in carbonaceous materials, such as graphitid thermal carbon black and activated carbon.

1

Introduction

Adsorption equilibria and kinetics are important for the proper design of adsorption processes. The equilibria information of adsorption isotherm. is clearly the first hand information that one needs to approximately size the adsorber. Since the adsorption affinity can vary by many orders of magnitude. Ranging from very low for weakly adsorbing gases to very high for strongly-adsorbing hydrocarbon vapours, it is very important that we know the value of this adsorption affinity. Experimentally this information can be obtained from careful experimentation of adsorption isotherm measured from very low pressure (where adsorption affinity can be calculated) to very high pressure where saturation capacity can be determined. Alternatively, the adsorption affinity can be determined from some appropriate theories or computer simulation. With the advances of high speed computer and the development of modem tools to deal with inhomogeneous fluids in confined space such as micropores, the second approach is gaining ground and new theories are constantly developed allowing engineers and scientists to calculate adsorption isotherm from minimum amount of information. Nevertheless, that does not mean to say that we can make do without experimental data. We still have some ground to cover before that stage can be reached. At the present time, careful and reliable experimental data are still required for validation of theories and even confirmation of molecular simulations. In this paper we will discuss some modem tools for studying adsorption equilibria of super and sub-critical fluids on non-porous surface and in porous carbons. In particular, the tools of grand canonical Monte Carlo (GCMC) simulation [l, 21, Density Functional Theory [3], Molecular Layer Structyre Theory (MLST) [4, 51, and the enhanced molecular layering and pore filling of Do, first developed in 1998 [6]and later applied in a number of practical systems [7-101, will be discussed. Their applications to experimental systems are illustrated to highlight the advantages and disadvantages of these modem tools of equilibria characterization.

1

2

2. I

Tools of characterisation Grand canonical Monte Carlo simulation

In the molecular simulation of adsorption in confined space such as pores of adsorbent, the most widely used and successful ensemble is the grand canonical Monte Carlo simulation. In this ensemble, we specify the chemical potential of the fluid,p, that the candidate pore is immersed in, the size of the pore, and the temperature. In the GCMC, the simulation can be carried out in the same procedure suggested by Metropolis at al. [ 111, and the density distribution (hence average pore density) can be obtained as a direct result of the simulation. The GCMC simulation is usually carried out by starting with a very low value of chemical potential. Once the simulation corresponding to this chemical potential has been completed, the chemical potential is increased incrementally and the particle configuration of the last run is used as the initial configuration and the new simulation is then carried out. This process is repeated until the final chemical potential is performed. In the simulation, the simulation box is constraint by the width of the pore and the L, and L, in the x and y directions. The box lengths L, and L, are chosen large enough, and usually chosen as twice the cut-off distance. In our work we choose r, = 5ofi and periodic boundary conditions are applied in the x and y directions. The procedure of the GCMC involves three basic moves: 1. DisDlacement of Darticle: A particle is selected in random and is given a new conformation (translation and rotation). The move is accepted with a probability p = min[l, exp(-AU / kT)] where AU is the difference between the new and old configuration energies. In this move the displacement step and the rotation angle are chosen in such a manner that the acceptance rate is between 25 and 50%. 2. Insertion of Darticle: The second move is the particle insertion move. A particle is generated at a random position. Its acceptance must satisfy the probability:

-

-

1

exp& - U(N + 1)+ U(N)]/ kT) A~(N+~) where V is the volume of the simulation box and A is the thermal de Broglie wavelength. Usually in the simulation we supply the activity, z = A-’ exp(p/ kT) , instead of the chemical potential. For bulk gas phase which behaves as an ideal gas, this activity is the density of the gas phase. 3. Removal of Darticle: The third move in the GCMC simulation is the removal of a particle. A particle is chosen in random and removed. The probability of acceptance this removal is p =min 1,

The GCMC is successfully applied to many adsorption systems. For temperatures greater than the pore critical temperature, the adsorption isotherms exhibit a smooth behaviour. However, for temperatures less than the pore critical temperature, there is a possibility of transition. When there is transition, the equilibrium point may be obtained by applying the thermodynamic integration method [ 121.

2

2.2

Density Functional Theory (DFT)

The DFT method was popularized in the sixty and was increasingly modified and applied to many problems involving inhomogeneousfluids in the 80s. Among the many versions of DFT, the one proposed by Tarazona and co-workers [3] remains the popular one in solving adsorption of confined fluid in pores. Their method is the non-local DFT and is applied on a grand canonical ensemble. The starting point of the method is the grand potential of the system (la) = F(p(l)) + jP@)ucxtcr)dx - jP@>P& where p(rJ is the singlet particle density. The first term on the RHS of the above equation is the Helmholtz free energy of the system. The Helmholtz free energy is expressed as a sum of two terms. The first term is that obtained from a reference system (hard sphere fluid is chosen as one) and the other is the perturbed component (which is due to the attractive component of the intermolecular fluid-fluid interaction), that is F(PCr)) = FH(P@N+ FA-( P O The Helmholtz free energy of the reference hard sphere system, in turn,can be expressed as two terms. One is due to the ideal gas contribution (accounting for the momenta of all particles) while the other term is the repulsive interaction among the hard spheres. It is

jP@

FH(P(E)) = [In(A3P(IN - 11dr + j P ( 0 f m G(INdI where the smoothed density is given by

-

P(I) = I P W w(x-il;p(l))dzI Assuming a mean field approximation, the perturbed component of the Helmholtz free energy due to the attractive force is FA-(P(I))

1

=?j j P 0 P W %(I-d)dd

dx

(1b)

The equilibrium density profile can then be obtained by finding minimum of the grand potential as defined in eq.(la). Tarazona used the Carnahan-Starling equation to derive the excess function f”(p(I)) as (here, d is the hard sphere diameter) f”(&)) = (4y - 3 ~ ‘ )/(I 2.3

- y)’ ;

y = (nd’ 16) &)

Molecular Layer Structure Layer Theoty (MLST)

The Molecular Layer Structure Theory (MLST) was first developed [4, 51 to study the vapour liquid equilibrium and surface tension of many substances over a wide range of temperature. In this method, the fluid is considered as parallel molecular layers, whose surface densities are different for inhomogeneous fluids. Similar to the DFT method, we define the following grand potential: R= +urf - jl]

cp;k(p;)+cpj where py is the surface density, Ask)is the intrinsic Helmholtz free energy of the layer i

j,

‘pj

is the interaction energy between one molecule on the layerj and all surrounding

layers, and u;‘ is the external potential exerted on the layerj. In this method of MLST, the intrinsic Helmholtz free energy is calculated from the equation of state of homogeneous fluid, and ‘pi is calculated from the integration of 12-6 Lennard-Jones

3

potential energy. The equilibrium density is calculated by minimizing the grand potential with respect to density as well as distances between layers [4,5]. 2.4

The Do-Method

Recently, Do and co-workers [6-101 have proposed a very simple method but it does reveal the mechanistic pictures of what are occurring in pores of different size. The process of adsorption in pore is viewed as follows. Molecules in pore are constantly in motion but “statistically” there is a spatial distribution of these molecules due to the interactive forces between them and the surface atoms. We treat this spatial distribution as a step function; uniformly high density near the surface and uniformly low density in the inner core of the pore. Due to the long range interaction of the surface, the pressure of the fluid in the inner core is not the same as that in the bulk phase. Assuming a Boltzmann distribution, the pressure of the inner core is related to the bulk fluid as Pp = P.exp(- a E/ kT) (2) where E is the average potential of the inner core, which is a function of pore width and the layer thickness. The adsorption process is basically treated as a molecular layering process, and it can be described by a layering equation, for example an equation taking the same the form of the BET equation. The affinity parameter C, of this equation as the C-parameter for a flat surface. Rather they are related through the following relation c,(H) = c.~xP[(Q,(H)-Q)/RT~ (3) where Q is the heat of adsorption of the flat surface and Qp is the correspondingvalue for a pore. These values may be taken as the minimum value of the potential energy between a molecule and a flat surface and that between a molecule and a pore. The layering equation can be written as (4) t / t, = f(P, (HI, c, (HI) where ,t is the thickness of a monolayer. This layering process is followed by a pore filling process. Here the term ‘pore filling’ is used in its most general sense, that is pore is filled with molecules by either two-dimensional condensation in small pores or three dimensional condensation in large pores. We argue that this general pore filling process is governed by the equation H / 2 - t - ~ , =)* where ~0 is the position at which the solid-fluid potential energy is zero. Although this form is similar to that of the Kelvin equation, the significant difference rests on the use of the pore pressure P, in the above equation. If we substitute the pore pressure of eq.(2) into the above equation, we get P M

(H/2 - t - 2,)

For small pores, E(H) dominates the RHS of the above equation and hence the pore filling pressure in small pores is dominated by the strength of the potential field created by the overlapping of the fields of the two walls. On the other hand, for larger pores where &H) = 0 , the RHS is dominated by surface tension and this equation reduces to the well known modified Kelvin equation.

4

0

10

20 30 Pore Width (A)

40

50

Figure 1: Pore tilling p r e ~ s ~versus n pore width for argon at 87.3 K and nitrogen at 77.3 K

3

Using eqs. (2) to (5) we can readily obtain the pressure at which'a pore is completely filled with adsorbate molecules. This pressure is called the pore filling pressure. Figure 1 shows the plot of the reduced pore filling pressure (P/Po) versus pore width for nitrogen adsorption in slit pores at 77.3 K and for argon at 87.3 K. The results of DFT and GCMC are also shown as symbols, and it is seen that the agreement between the DFT., GCMC and the Do method is very good, even the minimum position of this curve.

Results and Discussion

Having presented briefly the working procedures of the various methods, we now would like to illustrate their applications to adsorption of super and sub-critical fluids on nonporous carbon surface and in porous carbonaceous solids having slit pores. But first it is worthwhile to compare the time scales of computation of these methods: Do-method < MLST A$. The experimental heats of adsorption for x-complexation are in excellent agreement with theoretical molecular orbital predictions. The sorbent capacities for thiophene at the low pressure of 2 . 3 ~ 1 0atm ~ were 0.92 moleculdCu+ and 0.42 moleculdAg', and followed the order: Cu-Y & Ag-Y >> Na-ZSM-5 > activated carbon > Na-Y > modified alumina & H-USY. For liquid phase experiments, Cu(1)-Y, Ag-Y and Na-Y zeolites were used to removal low concentration thiophene from mixtures including benzene and/or n-octane, all at m m temperature and atmospheric pressure. Sulfur-free (i.e., below the detection limit of 4 ppmw sulfur) fuels were obtained with Cu(1)-Y and Ag-Y, but not Na-Y. Breakthrough and saturation adsorption capacities obtained for an influent concentration of 760 ppmw sulfur (or 2000 ppmw thiophene) in n-octane follow the order Cu(1)-Y > Ag-Y > Na-Y and Cu(1)-Y > Na-Y > Ag-Y, respectively. Regeneration of the adsorbent was accomplished by using air at 350'C followed by re-activation in helium at 450'C. The observed adsorption behavior, in general, agrees well with the studies performed for pure component vapor phase adsorption of thiophene and benzene with the same adsorbents.

Introduction Removal of sulfur-containing compounds is an important operation in petroleum refining, and is achieved by catalytic processes at elevated temperatures and pressures [I]. The hydrodesulhkation (HDS) process is highly efficient in removing thiols and sulfides, but less effective for thiophenes and thiophene derivatives. New legislation will require substantial reductions in the sulfur content of transportation fuels. For example, the new US.EPA s u l k standards require that the sulfur contents in gasoline and diesel fuels for on-board transportation will be 30 and 15 ppm, respectively, decreased fiom the current levels of several hundred ppm. Faced with the severely high costs of compliance, a surprisingnumber of refiners are seriously considering reducing or eliminating production of on-board fuels [2]. The new challenge is to use adsorption to selectively remove these sulfur compounds from liquid fuels. Since adsorption would be accomplished at ambient temperature and pressure, success in this development would lead to a major advance in petroleum refming. However, success would depend on the development of a highly selective sorbent with a high sulfur capacity, because the commercial sorbents are not desirable for this application. First results on sorbents based on n-complexation for desulfurization have been reported recently by Yang et al. [3,4] which were shown to be superior to all previously reported sorbents. During the last decade, there have been several published accounts on using adsorption for liquid fuel desulfurization. Commercially available sorbents (i.e., zeolites, activated carbon and activated alumina) were used in all of these studies. Weitkamp et al.

51

[5]reported that thiophene adsorbed more selectively than benzene on ZSM-5 zeolite. Based on this study, King et al. [6]studied selective adsorption of thiophene, methyl- and dimethyl-thiophenes (all with one ring) over toluene and pxylene, also using ZSM-5. They showed that thiophene was more selectively adsorbed, both based on fixed bed breakthrough experiments. However, the capacities for thiophene were low (only 1-2% wt. adsorbed at 1% thiophene concentration). Both vapor phase and liquid phase breakthrough experiments were done in these studies, and the results from two phases were consistent. The pore dimensions of ZSM-5 are 5.2-5.6A. Hence organic sulfur compounds with more than one ring will be sterically hindered or excluded. Zeolites with larger pores, as well as larger pore volumes, will be more desirable than ZSM-5 as the selective sorbents. Indeed, results of Salem and Hamid [7]indicated that 13X zeolite as well as activated carbon had much higher sorption capacities for s u l k compounds. Based on the data of Salem and Hamid [7],the capacity for sulfur compounds by 13X zeolite was approximately an order of magnitude higher than that of ZSM-5, when compared with the data of King et al. [6]extrapolated to the same conditions. Modified activated alumina (Alcoa Selexsorb), which contains proprietary modifier to provide optimum adsorption of a number of polar organic compounds, has been used in an adsorption process by Irvine [8].No direct comparison has been made among these commercial sorbents. Their experiments were mostly done in fixed bed adsorbers, by measuring the breakthrough capacities. Based on the literature, the large pores zeolites (NaX or Nay) are about the same as activated carbon and alumina, in terms of adsorption of thiophene. Based on the principles of n-complexation, we have already developed a number of new sorbents for a number of applications. These include sorbents for: (a) olefdparaffm separations [9- 121,(b) diene/olefm separation or purification (i.e., removal of trace amounts of dienes fiom olefins) [ 131,and (c) aromatics/aliphatics separation and purification (i.e., removal of trace amounts of aromatics fiom aliphatics [141.Throughout this work, we have used molecular orbital calculations to obtain a basic understanding for the bonding between the sorbates and sorbent surfaces, and further, to develop a methodology for predicting and designing n-complexation sorbents for targeted molecules (e.g. Ref. 11). First results on n-complexation sorbents for desulfurization with Ag-Y and Cu(1)-Y zeolites have been reported recently [3,4]. In this work, we included the known commercial sorbents such as Na-Y, Na-ZSM5, H-USY, activated carbon and activated alumina (Alcoa Selexsorb) and made a direct comparison with Cu(1)-Y and Ag-Y which were the sorbents with n-complexation capability. Thiophene and benzene vapors were used as the model system for desulfiuization. Although most of these studies can be applied directly to liquid phase problems, Cu-Y (auto-reduced) and Ag-Y zeolites were also used to separate liquid mixtures of thiophenehenzene, thiopheneh-octane, and thiophenehenzene/n-octane at room temperature and atmospheric pressure using fixed-bed adsorptiodbreakthrough techniques. These mixtures were chosen to understand the adsorption behavior of s u l k compounds present in hydrocarbon liquid mixtures and to study the performance of the adsorbents in the desulfurization of transportation fuels. Moreover, a technique for regeneration of the adsorbents was developed in this study [4].

52

Experimental Adsorbent Preparation

Various kinds of sorbents were investigated in this work. Four as-received sorbents: Na-type Y-zeolite (Si/A1=2.43, Strem Chemical), H-type ultra-stable Y-zeolite (Si/AI=l95, TOSOH Corporation), activated carbon (Type PCB, Calgon Carbon Corporation) and modified activated alumina (Selexsorb CDX, Alcoa Industrial Chemical), were used in this study. According to the product datasheets, Selexsorb CDX is formulated for adsorption of sulfur-based molecules, nitrogen-based molecules, and oxygenated hydrocarbon molecules. Na-Y and H-USY were in powder form (binderless). Since activated carbon was in granular form and activated alumina was in pellet form, they were crushed into powder form for evaluation. Cu(1)-Y was prepared by ion exchange of Na-Y zeolites with Cu(NO3)Z followed by reduction of Cu2' to Cu'. First, as-received Na-Y was exchanged twice using excess amounts (10-fold cation-exchange-capacity(CEC) assuming that one Cu" compensates two aluminum sites) of 0.5 M Cu(NO& at room temperature for 24 hours. After the exchange, the zeolite suspension was filtered and washed with copious amount of de-ionized water. The product was dried at 100 "C overnight. Several groups have reported reduction of Cu" to Cu' in zeolite in He (i.e., auto-reduction). In this study, reduction of Cu2' to Cu' was carried out in He only at 450 "C for periods in the 1 to 18 hours range. Ag' ion-exchange Y-zeolite (Ag-Y) was prepared at room temperature for 24 h in the same manner as Cu" exchange, using 5-fold excess AgN03 (O.1M). 13X (Si/Al=1.25, Linde) was used for the preparation of Cu-X (10 fold CEC solution of Cu(NO&, ion-exchanged at 65 "C for 24 hrs, three times) and Ag-X (5-fold CEC solution of AgN03, ion-exchanged at RT for 24 hrs, twice). Na-type ZSM-5 (Na-ZSM-5) was prepared at room temperature by Na'-exchange of Nh-ZSM-5 (Si/Al=lO, ALSI-PENTA Zeolite GmbH). Vapor Phase Isotherms and Heat of Adsorption

The objective of this study is to compare the strength of adsorptive interaction between adsorbents and thiophenehenzene. Extremely low partial pressures at less than l o 5 atm would be necessary to meet this objective if isotherms were measured at ambient temperature, because the isotherms at ambient temperature are fairly flat and are difficult to compare. However, it is very difficult to obtain and control such low partial pressures experimentally. Therefore, single component isotherms for benzene and thiophene were measured at 90, 120 and 180 "C using standard gravimetric methods. A Shimadzu TGA-50 automatic recording microbalance was employed. Isosteric heats of adsorption were calculated using the Clausius-Clapeyron equation from isotherms at different temperatures. Fired Bed A&orptiodBreakthrough Experiments

All adsorption/breakthrough experiments were performed in vertical custom made quartz adsorbers equipped with a supporting glass frit. Initially, the adsorbents were loaded inside the adsorber (between 1 or 2 grams), and heated in situ (250 - 450'C) while flowing either helium or nitrogen upwards. After activation treatment, the zeolite adsorbent under study was allowed to cool down to room temperature under. Next, a sulfur-free octane or benzene solution was allowed to flow downwards through the

53

sorbent at a rate of 0.5 cm3/min.After wetting the adsorbent for about 30 minutes, the feed was changed to a mixture of CgHI8(n-octane) and/or c6H6 (benzene) containing different concentrationsof C4H4S(thiophene) also at a 0.5 cm3/minrate. Samples were collected at regular intervals until saturation was achieved, which .depended on the adsorption dynamics and amount of adsorbent. All the samples collected during the breakthrough experiments were analyzed using a Shimadzu GC (Gas Chromatography) unit equipped with a polar column, an automatic multi-sampler, and a FID detector. The minimum thiophene concentration detection was around 10 ppmw or 4 ppmw on a sulfur basis. Molecular Orbital Computational Details Molecular orbital (MO) studies on the n-complexation bonding for benzene and sorbent surfaces had been investigated recently [3]. In this work, similar MO studies were extended to thiophene and zeolites. The Gaussian 94 Program [ 151 in Cerius2 molecular modeling software [161fiom Molecular Simulation, Inc. was used for all calculations. MO calculations for thiophene and sorbent surfaces were performed at the Hartree-Fock (HF) and density functional theory (DFT) level using effective core potentials (ECPs) [17-191. The LanL2DZ basis set [20] is a double-t; basis set containing ECP representations of electrons near the nuclei for post-third-row atoms. The reliability of this basis set has been confirmed by the accuracy of calculation results as compared with experimental data. Therefore, the LanL2DZ basis set was employed for both geometry optimization and natural bond orbital (NBO) analysis. The restricted Hartree-Fock (RHF) theory at the LanL2DZ level basis set was used to determine the geometries and the bonding energies of thiophene on AgCl and CuCI. The simplest models with only a single metal chloride interacting with a thiophene molecule were chosen for n-complexation studies. The optimized structures were then used for bond energy calculations according to the following expression: Eads = Eadsorbate -k Eadsorbent - Eadsorbent-adsorbate where is the total energy of thiophene, Eadsorbent is the total energy of the bare adsorbent i.e. the metal chloride and Ea~~t-adsorb.te is the total energy of the adsorbate/adsorbentsystem. A higher value of Endscorresponds to a stronger adsorption.

Natural Bond Orbital (NBO)

The optimized structures were also used for NBO analysis at the B3LYP/LanL2DZ level. The B3LYP [21] approach is one of the most useful self-consistent hybrid (SCH) approaches [22], it is Beck’s 3-parameter nonlocal exchange functional[23] with nonlocal correlation functional of Lee, Yang and Parr [24]. The NBO analysis performs population analysis that pertains to localized wave-function properties. It gives a better description of the electron distribution in compounds of high ionic character, such as those containing metal atoms [25]. It is known to be sensitive for calculating localized weak interactions, such as charge transfer, hydrogen bonding and weak chemisorption. Therefore, the NBO program [26] was used for studying the electron density distribution of the adsorption system.

54

Modelsfor Ag-Zeolite (Agz) and Cu-Zeolite (CuZ)

The zeolite models selected for this study are similar to the ones used by Chen and Yang [33], with the molecular formula of (HO)3Si- 0 -A1(OH)3, and the cation Ag' or Cu' sits 2 - 3 A above the bridging Oxygen between Si and Al. This is a good cluster model representing the chemistry of a univalent cation bonded on site I1 (SII) of the faujasite framework (Z). Once the optimized structures of AgZ and CuZ are obtained at the B3LYPLanL2DZ level, then a molecule of thiophene (C4&S) or benzene (C6&) is added onto the cation of the zeolite model, and the resulting structure is hrther optimized at the B3LYPLanL2DZ level.

Results and Discussion Vapor Phase Adsorption

BenzenejThiophene Adsorption Isotherms Figure 1 and Figure 2 show the isotherms of benzene and thiophene on Ag-Y and Cu-Y. Curves are fitted with Dubinin-Astakhov (solid line) and Langmuir-Freundlich (dotted line) isotherms. Compared with Figure 3, these sorbents adsorbed significantly more thiophenehenzenethan Na-Y at pressures below 10" atm, and nearly the Same amounts at high partial pressures. This result was a clear indication of n-complexation with Ag'and Cu'; since Na' could not form n-complexation bonds. However, the difference of thiophenehenzene adsorption amount did not reflect the relative strengths of a-complexation between Cu' and Ag" because the Cu' exchange was not complete. Neutron activation analyses of the sorbent samples showed that the Ag' exchange was 100% but the Cu+ exchange was only 46%. According to the EPR analysis, described elsewhere [171, only a half of the Cu2+was auto-reduced to Cu' after our heat treatment at 450 "C for lhr in He. On a per-cation basis, it is seen that Cu' could adsorb higher thiophene adsorption amounts. In fact, 0.92 thiophene molecule per Cu' was obtained at 2xlO-' atm at 120°C. This amount was due to Cu' since the amount adsorbed by NaY at the same pressure was negligible. At the same pressure, only 0.42 thiopheneIAg' was obtained. This result indicated strong a-complexation bonds between both Cu' and Ag", and that the bond with Cu' was stronger. Comparison of Thiophene Adsorption on All Adsorbents Thiophene adsorption isotherms on all sorbents are compared in Figure 4. It is clearly seen that Ag-Y and Cu-Y could adsorb significantly larger amounts of thiophene even at very low pressures.

55

1 1.5

._.-

1

4.0 3.5

3.0 2.5 2.0

1.o

1.5 1.o 0.5

0.0 1.E45

--

1.E.M

1.E-

1. E m

l.E-01

(-1

Figure 1. Pure component equilibrium isotherms of benzene and thiophene on Ag-Y (SVAk2.43) at 120 OC and 180 "C. Fitted curves are not shown for benzene adsorption at 180 "C because the artificial crossovers to the curves for thiophene at 180 OC are observed.

Figure 2. Pure component equilibrium isotherms of benzene and thiophene on Cu-Y (Si/AI=2.43) at 90 "C and 120 "C.

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1.E.05

1.E.M 1.E1.E-02 PuUd Pmsun(um)

1.E-01

Figure 3. Pure component equilibrium isotherms of benzene and thiophene on Na-Y(SVAl=2.43) at 120 "C and 180 "C.

l.wE.05

1.wE.M

l.wE.03

1.oQE42

l.wE-01

P N W Raun(am)

Figure 4. Comparison of equilibrium adsorption isotherms ofthiophene at 120 "C.

Heat of Adsorption Heats of adsorption were calculated using the Clausius-Clapeyron equation fkom isotherms at two different temperatures, and are shown in Table 1. All the heats of thiophenehnzene adsorption had the tendency to decease as the loading increased. This is a common phenomenon for the sorbents such as ion-exchanged zeolites that have heterogeneous sites. The heats of adsorption on activated carbon, in particular, ranged widely fkom 23.9 kcaYmol (at OSmmoVg loading) to 8.0 kcaYmol (at 3mmoVg). Ag-Y and Cu(1)-Y exhibited the higher heats of adsorption than Na-Y for both benzene and thiophene because of .rr-complexation. More importantly, the heats of adsorption for thiophene were higher than that of benzene. These experimental results can be explained by molecular orbital calculation and NBO analysis, which will be discussed shortly. At the low loading of 0.5 mmoVg, Na-ZSM-5 showed nearly the same heats of adsorption as Na-Y for both thiophene and benzene. The different pore dimensions

A for ZSM-5 vs. 7.4 A for Na-Y) apparently had no influence on the heats of adsorption. It is not clear why the amounts adsorbed on Na-Y decreased sharply at very low pressures while that on Na-ZSM-5 maintained.

(5.2-5.6

Table 1. Heat of Adsorption (kcal/ml)calculated from isothcm at difhent temperatures. Na-Y

Ag-Y

Cu-Y

BUSY

Na-ZSM5 ActivuedC.rbon

(SilAk2.43) (SUAk2.43) (SilAk2.43) (Si/Al=195) (SilAkIO)

T y ~ PCB e

Selcxsorb CDX

8.0-23.9

16.1-17.5

Thiophc~c 19.1-19.6

21.3-21.5

20.8-22.4

7.9-11.2

(0.5-2.0)

(1.5-1.7)

(2.0-3.0)

(0.1-0.3) (0.45-0.60)

(0.5-3.0)

(0.2-1.O)

17.0-18.2

19.0-20.1

19.3-21.8

6.6-13.1

16.5-17.9

13.1-16.1

16.8-19.6

(1.5-2.0)

(1.5-1.8)

(1.8-2.5)

(0.1-0.9

(0.45-0.65)

(1.0-3.0)

(0.6-1.0)

Benzea~

18.6-19.2

AaivakdALumi~

*) Numbers in parcnthtses indicate the adsorption ~moullls (mmoVP, for calculation. **)The hcpts of adsorptiondmeased with loading in all caacs.

Bond Energies, Geometries and NBO Results The energies of adsorption are summarized in Table 2. The theoretical calculations indicate that the n-complexation strengths follow the order CuZ > AgZ and more importantly,thiophene > benzene. This trend is in agreement with the experimental data, in Table 4. In fact, the molecular orbital results on CuZ and AgZ are in excellent agreement with the experimental data. Both chloride and zeolite models were used as the anion in the theoretical calculations, while only zeolite framework was the anion in the experiment. It is known fiom our previous work that the anion has a large effect on the n-complexation bonds [11,27]. The bond energies on the zeolites (Z)are significantly higher than that on the chlorides (Table 2). This result indicates that the zeolite anion is more electronegative than the chloride anion, which has been already revealed by Chen and Yang in their ab initio molecular orbital calculations [27]. In the optimized structures of thiophene-MC1complexes, the distance between the thiophene molecule and Cu ion is about 0.3 A shorter than that of thiophene and Ag ion for chloride. The NBO analysis for thiophene adsorption is summarized in Table 3. There is some donation of electron charges from the n-orbital of thiophene to the vacant s orbital of metals known as ci donation and, simultaneously, back donation of electron charges fiom the d orbitals of metals to x* orbital (i.e., anti-bonding x orbital) of thiophene or x back-donation. It appears that the o donation is more predominant for thiophene and the x back-donation is more important for benzene (published elsewhere). Comparing the two anions, zeolite anion and chloride anion, the NBO results show that both ci donation and d z* backdonation are significantly stronger with the zeolite anion bonded to Agf or Cu'. The charge transfer results again confirmed the experimental data that the relative strengths of the n-complexation bonds follow the order: thiophene > benzene and Cu' > Ag'.

-

57

Table 2. Summary of energies of adsorption for thiophene and benzene in kcaUmol calculated from molecular orbital theory (Z denotesZeolite anion) E,,(Thiophene) E,,(Benzene) CUCl 13.5 12.4 9.0 8.6 AgCl 21.4 20.5 CUZ A@ 20.0 19.1 Table 3. Summary of NBO analysis* of n-complexationbetween thiophene and MCVMZ C+M interaction (adonation)

M+C interaction (d x'backdonation)

Mx

91

e

CUCl AgCl CUZ

0.037 0.022 0.112

-0.022 -0.014 -0.063

0.101

-0.086

A@ ~

-

~~

Net Change 91+e 0.015 0.008 0.049 0.015

~~

~

~~

* q I is the amount of electron population change in valence s orbitals of the metal, and e is the total amount of electron populationdecrease in valence d orbitals of the metal.

Liquid Phase Aakorption

Figures 5 and 6 show breakthrough curves obtained for 2000 ppmw thiophene (760 ppmw s u l k basis), for n-octane as solvent (the breakthrough curve for Cu(1)-Y is shown in a separate figure for clarity since the abscissas are quite different). All adsorbents showed remarkable selectivity towards C41&S, indicating that CgHIg adsorption is not competitive. Saturation adsorption capacities calculated from the breakthrough curves were 1.05 and 0.90 mmoVg for Na-Y and Ag-Y, respectively. However, for Na-Y, the breakthrough of thiophene molecules occurred earlier, at about 2.84 cm3/g compared to 22.50 cm3/g in Ag-Y. This was evidence of weak adsorbate-adsorbent interactions on Na-Y, which did not have the ability for n-complexation as in the case of Ag-Y. This agrees very well with the pure vapor phase adsorption data reported above. The saturation adsorption amount in Na-Y was higher than that Ag-Y due to pore volume differences and difference in the densities of zeolites. For the same feed conditions described above, Cu(1)-Y showed again the highest selectivity and capacities among the adsorbents studied. The saturation capacity was 2.55 mmoVg, which was more than twice the amount found for the other adsorbents, indicating superior interaction with the thiophene molecules. For about 2 grams of Cu(I>Y, it took more than 300 minutes for the thiophene molecules to break through the adsorbent at a feed rate of 0.5 cm3/min (refer to figure 6). Saturation was reached after 600 minutes, which was remarkable for such a small amount of adsorbent. A large amount of Cu2+ions must have been reduced to Cu'. As mentioned earlier, Takahashi et al. and others [14] have reported 50% auto-reduction of copper under helium/vacuum atmospheres after just 1-2 hours. The adsorbent used in this part of the work was exposed to helium at 450'C for no less than 18 hours. Possibly longer activation time increased the amount of reduced copper ions, while the adsorption behavior already indicates that the auto-reduction process yields promising results.

58

Figure 7 shows breakthrough curves for Cu(1)-Y for an influent containing 500 ppmw thiophene (190 ppmw sulfur> in n-octane. The saturation capacity was reduced to 1.28 mmoVg, which was about 50% of the amount obtained previously with the 2000 ppmw thiophene feed. This indicates that the equilibrium adsorption isotherm was not "rectangular" in shape at low concentrations and rather showed a noticeable decrease in adsorbed amount as one decreased the concentration. Despite this, the observed saturated amount was not low, when taking into account that the thiophene concentration was 75% less than the case discussed previously (i.e., 2000 ppmw). Figure 7 also shows breakthrough curves after Cu(1)-Y adsorbent regeneration (second cycles). Under an atmosphere of nitrogen at 350'C, the regenerated adsorbent did not recover the original capacity. The new capacity for the adsorbent at saturation was 0.80 mmoVg, which was more than a 30% reduction fkom the original capacity. In fact, the color of the adsorbent remained black, which indicated the presence of copper thiophene complexes. Meanwhile, regeneration under air at 350'C followed by reactivation under helium at 450'C recovered almost all of the original capacity. For this case, the observed saturation capacity was about 1.20 mmoVg, which was only a 5% reduction from the original capacity. 1

0.8

0"

0

"

0

0.2 0

0 10 20 30 40 50 60 70 cmVg (cumdative efauent volumdsorbentweight)

Figure 5. Breakthrough of thiophene in a fixed-bed adsorber with Ag-Y 0 or Na-Y (0) adsorbents, with a liquid feed containing 2000 ppmw (Ci) of thiophene in octane, at room temperature.

0 20 40 60 80 100 120 140 160 180 200 cd/g (cumdative effluent volumdsorbent weid

Figure 6. Breakthrough of thiophene in a fixed-bed adsorber with Cu(1)-Y adsorbent, with a liquid feed containing 2000 ppmw (Ci) of thiophene in n-octane, at room temperature.

0.4 o 0.2 o

0 50 100 150 200 250 300 350 400 cmVg (cumdative d u e n t vohmdsorben~weight

Figure 7. Breakthrough of thiophene in a fixed-bed adsorber with fresh (0) and regenerated (0,[7, Cum-Y adsorbent, with a liquid feed containing 500 ppmw (Ci) of thiophene in natane, at room temperahue. Adsorbent regenerated in nitrogen at 350’C followed by re-activation in helium at 450’C. 0 Adsorbent regenerated in air at 350’C followed by re-activation in helium at 450’C.

(a)

o 0

.

6

5 0 1 0 0 1 5 0 # ) 0 2 5 0

cmV.3 (clumllative dfiucst v o l d s & t

weight)

Figure 8. Breakthrough of thiophene in a fixed-bed adsorber with Cu(I)-Y adsorbent, with a liquid feed containing 500 ppmw (C,) of thiophene, 20 wt?hbenzene and 80 wt% n-octane, at room temperature.

For the final part of this study, it was desired to use mixtures with compositions similar to that of transportation fuels. Gasoline contains about 20-30 wtoh aromatics, many thiophenic compounds and 70-80% alkanes such as n-hexane and n-octane. The aromatic contents in diesel and jet fuels are < 20%. Thus, a mixture containing 20 wt% benzene, 80 wt% n-octane, and 200 ppmw sulfiu (ca. 500 ppmw thiophene) was used to simulate gasoline. Figure 8 shows a breakthrough curve for thiophene in such mixture after adsorption at room temperature on Cu(1)-Y. The sulfur adsorption capacity was 0.44 mmoVg or about 1.4 wt% sulfur. The results are promising when compared to other adsorbents used in previous studies. Ma et al. studied fixed-bed adsorption of thiophene compounds fiom diesel and jet fuels using an undisclosed transition metal compound ( 5 wt% loading) supported in silica gel [28,29]. They obtained a saturation adsorption capacity of 0.015 g of sulfur per cm3 of adsorbent which can be compared directly to our results. Assuming that the density of the Cu(1)-Y is close to that of Na-Y (- 1.3 g/cm3), which is lower than the actual value because sodium is lighter than copper, then the observed saturation capacity in our case is approximately 0.018 g of sulfur per cm3 of adsorbent. Ma et al. also showed that breakthrough occurs at about 20 cm3 effluent volume for about 3.2 cm3 of the metal loaded silica gel compared to 30 cm3 effluent volume for about 0.75 cm3 volume of Cu(1)-Y. Therefore, our adsorbent is capable of processing more fuel with very low sulfur streams with less adsorbent material. It should be mentioned that jet fuel and other fuels contain heavier thiophene compounds such as benzothiophenes, dimethylthiophenes, and dimethylbenzothiophenes and these should adsorb strongly in Cu(1)-Y. Breakthrough results on gasoline and other transportation fuels will be published elsewhere shortly.

Conclusions In this work, vapor-phase benzene/thiophene adsorption isotherms were investigated to develop new sorbents for desulfurization. Among the sorbents studied, Cu(I>Y and Ag-Y exhibited excellent adsorption performance (capacities and separation factors) for desulfurization. This enhanced performance compared to Na-Y was due to the x-complexation of thiophene with Cu' and Ag'. Molecular orbital calculations confirmed the relative strengths of n-complexation: thiophene > benzene and Cu' > Ag'. This work has also demonstrated that copper (auto-reduced) and silver exchanged Y-type zeolites are excellent adsorbents for removal of thiophene tiom aromatic and/or hydrocarbon mixtures, based on fixed-bed adsorption experiments. Both adsorbents were capable of reducing sulfur content to values < 4 ppmw sulfur for long periods of time. Cu(I>Y provided the best adsorption capacity both at breakthrough point and at saturation, surpassing all other adsorbents by more than 50%. Regeneration of the copper based adsorbents can be accomplished in air at 35072 which recovered almost all of the original adsorption capacity. More studies will be needed in order to fully understand the effect of copper loading and to include heavier thiophenes, which are abundant in liquid fuels. Breakthrough results on gasoline and other transportation fuels will be published elsewhere shortly.

Acknowledgements

-

Supports from NSF and DOE are acknowledged. A U.S. Patent is pending filed with U.S. and foreign Patent Offices.

References 1. Farrauto, R. J.; Bartholomew, C . H. 2. 3.

4.

5.

Fundamentals of Industrial Catalytic Processes, Chapman and Hall, New York, 1997. Parkinson, G., Diesel Desulfurization Puts Refiners in a Quandary. Chemical Engineering, 2001, February issue, 37. Yang, R. T.;Takahashi, A.; Yang, F. H., New Sorbents for Desulfurization of Liquid Fuels by n-Complexation. Ind Eng. Chem. Res., 2001,40,6236. Yang, R.T.; Takahashi, A.; Yang, F.H.; Hernandez-Maldonado, A. Seldctive Sorbents for Desulfurization of Liquid Fuels. U.S. and foreign Patent applicationsfiled, 2002. Weitkamp, J.; Schwark, M.;Emest, S. Removal of Thiophene Impurities from Benzene by Selective Adsorption in Zeolite ZSM-5. J, Chem. SOC.Chem. Commun.,

1991,1133. 6. King, D. L; Faz,C.; Flynn, T. Desulfurization of Gasoline Feedstocks for Application

in Fuel Reforming. SAE Paper 2000-01-0002, SOC.Automotive Engineers, Detroit , 2000.

7. Salem, A. S. H.; Hamid, H. S. Removal of Sulfur Compounds fiom Naphtha Solutions Using Solid Adsorbents. Chem. Eng. Tech., 1997,20, 342. 8. Irvine, R L.,Process for Desulfirizing Gasoline and Hydrocarbon Feebtocks, U. S .

Patent 5,730,860(1998).

61

9. Yang, R.T.; Kikkinides, E. S. New Sorbents for Olefin-Param Separations by Adsorption via p-Complexation AZChE J., 1995, 4I , 509. 10. Rege, S. U.; Padin, J; R; Yang,, R T. Olefm-Paraffin Separations by Adsorption: Equilibrium Separation by n-complexation vs. Kinetic SeparationAZChE J., 1998,44, 799. 1 1 . Huang, H.Y.; Padin, J.; Yang, R. T. Ab Initio Effective Core Potential Study of Olefin/Paraffin Separation by Adsorption via n-Complexation: Anion and Cation Effects on Selective Olefin Adsorption J. P h s . Chem. B., 1999,103,3206. 12. Padin, J.; Yang, R T. New Sorbents for Olefin-Parafin Separations by Adsorption via n-Complexation: Synthesis and Effects of Substrates Chem. Eng. Sci., 2000, 55, 2607. 13. Jayaraman, A; Yang, R. T.; Munson, C. L.; Chinn, D. Deactivation of n-Complexation Adsorbents by Hydrogen and Rejuvenation by Oxidation, Znd Eng. Chem. Res., 2001,40,4370. 14. Takahashi, A.; Yang, R T. New Adsorbents for Purification: Selective Removal of Aromatics, AIChE J., 48,1457 (2002). 15. Frisch, M. J. et al., Gaussian 94, Revision B.3, Gaussian, Inc., Pittsburgh, PA., 1995 16. Bowie, J. E., Data Visualization in Molecular Science: Tools for Insight and Innovation; Addison-Wesley Pub. Co.: Reading, Mass., 1995; chapter 9. 17. Hay, P. J.; Wadt, W. R Ab Inifio Effective Core Potential for Molecular Calculations. Potentials for the Transition Metals Atoms Sc to Hg. J. Chem. Phys. 1985,82,270 18. Wadt, W. R.; Hay, P. J. Ab Inifio Effective Core Potential for Molecular Calculations: Potentials for Main Group Elements Na to Bi. J. Chem. Phys. 1985, 92, 284 19. Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potential for Molecular Calculations: Potentials for K to Au Including the outermost core orbitals. J. Chem. Phys. 1985,82,299 20. Russo, T. V.; Martin, R. L.; Hay, P. J. Effective Core Potentials for DFT Calculations. J. Phys. Chem. 1995,99, 17085 21. Becke, A. D. Density Functional Thermochemistry. 111. The Role of Exact Exchanges. J. Chem. Phys. 1993,98,5648 22. Becke, A. D.; A New Mixing of Hartree-Fock and Local Density-Functional Theories. J. Chem. Phys. 1993,98, 1372 23. Becke, A. D.; Density-Functional Thermochemistry. 11. The Effect of the Perdew-Wang generalized-gradient correlation correction. J. Chem. Phys. 1992, 97, 9173 24. Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev., 1988, B37,785 25. Reed, A. E.; Weinstock, R.B.; Weinhold, F. Natural Population Analysis. J. Chem. Phys. 1985, 83 (2). 735 26. Glendening, E. D.; Reed, A. E.; Carpenter, J. E., Weinhold, F. NBO Version 3. I , 1995. 27. Chen, N.; Yang, R. T. Ab Inifio Molecular Orbital Study of Adsorption of Oxygen, Nitrogen, and Ethylene on Silver-Zeolite and Silver Halides. Ind. Eng. Chem. Res. 1996,35,4020.

62

28. Ma, X.; Sun, L.; Yin, Z.; Song, C. New Approaches to Deep DesulfUrization of Diesel Fuel, Jet Fuel, and Gasoline by Adsorption for Ultra-Clean Fuels and for Fuel Cell Applications. Am. Chem. SOC.Div. Fuel. Chem. Prepr. 2001,46,648. 29. Ma, X.; Sprague, M.; Sun, L.; Song, C. Deep Desulfurization of Liquid Hydrocarbons by Selective Adsorption for Fuel Cell Applications. Am. Chem. Sac. Div. Pet. Chem. Prepr. 2002,47,48.

63

OPTIMIZATION OF CONTINUOUS CHROMATOGRAPHY SEPARATIONS ZIYANG ZHANG AND M. MORBIDELLI Swiss Federal Institute of TechnologyZurich, Laboratoriumfir Technische Chemie/LTC, ETH-HiinggerbergIHCI, CH-8093 Ziirich, Switzerland E-mail: [email protected]

M.MAZzOTrI ETH Zurich ,Institut f i r Verfarenstechnik, Sonneggstrasse3, CH-8092 Zurich, Switzerland E-mail: manotti@ivuk mavt.ethz.ch Two recent developments of the simulated moving bed chromatographic separation units, i.e. the Varicol and the PowerFeed processes, are addressed. The performances of these three processes are compared with reference to two chiral separation systems taken from literature, using a multiple objective optimization technique based on a genetic algorithm. The performance of each process has been optimized in a wide range of operating conditions, and from their comparison a good assessment of their relative potential has been made. The optimization results have been discussed in the frame of equilibrium theory and the N I ~ S of optimal design of the Varicol and PowerFeed processes have beem discussed.

1

Introduction

Simulated Moving Bed (SMB) is an established technology for performing continuous chromatographic separations covering all scales of possible interest in applications, particularly in optical enantiomer separations. The SMB unit has been originally devised as a practical realization of a true moving bed (TMB) unit where the two phases move countercurrently. A schematic diagram of a typical 4-section SMB is shown in Figure 1(a). The countercurrent movement of the solid and the fluid is simulated by moving synchronously the inlet (feed and eluent) and outlet (raffinate and extract) ports by one column in the direction of the fluid flow, with a predetermined period or switching time, 5. The design and optimization of this unit can be done in the frame of equilibrium theory, using the so called triangle theory [ 1,2] or using more detailed simulation models in connection with various optimization strategies [3-71. In order to make SMB units more economically efficient and competitive, several new operation modes have been introduced. These include supercritical fluid SMB [&lo], temperature gradient SMB [ 111 and solvent gradient SMB [12-14]. The basic idea is to change the adsorption strength of the solute on the stationary phase in the different sections by creating along the unit a gradient of either pressure or temperature or solvent composition, respectively. Another direction which has been taken to improve SMB performance is related to the idea of somehow forcing its dynamics. In this context, the SMB unit is not regarded as an approximationthrough appropriate discretization of the TMB unit, but is considered as a unit with many degrees of freedom that can be optimized to improve its performance. The first step in this direction is the Varicol unit [ 151, where the inlet and outlet ports are shifted asynchronously. An alternative operation has been proposed by Kloppenburg and Gilles [ 161 and more recently by Zang and Wankat [171, who considered fluid flow rates changing during the switching period as shown in Figure l(b). This will be referred to in the following as “PowerFeed” operation. It is worth noting that, these two ideas, Varicol and PowerFeed, can be regarded in some sense as having a common root, i.e.

64

changing within the switching period the flow rates, of the solid and the fluid, respectively. In this sense, as mentioned above, they do not try to better approximate the TMB unit from which the SMB is derived.

t ----

SMB Powdd

s

sectiw 2, Q

Figure 1. (a) Operating diagram of a four section SMB unit; (b) Fluid flow rates schemes in the SMB and PowerFeed operations during one switching period, t,

In this paper, we present and compare the optimal performances of the SMB,Varicol and PowerFeed operations, using a multiobjective optimization technique, on two chiral separation systems from the literature. The aim is to provide a clear picture, although inevitably confined to the cases examined, of the relative potentials of these three operation modes. 2

Optimization of SMB,Varicol and PowerFeed processes

Optimization of the SMB,Varicol and PowerFeed processes is very complex due to the large amount of continuous and discrete parameters involved. These parameters include fluid flow rates, switching time, unit configuration (total number of columns, column distributionand column dimension), feed conditions, size of the packing particles and so on. These parameters might have different values in the subintervals of one switching period in the Varicol and PowerFeed operations. Depending upon the specific application, the best process performance may be achieved by maximizing the product quality (mostly in terms of the purity of either extract or raftinate stream or both) under fixed cost and productivity, or by minimizing the cost and at the same time maximiig the productivity under some given specification on product quality, or by other combinations of practical interest. Note that in most cases, two of the objective hctions to be optimized are conflicting, as for example: productivity increases if the unit operates at higher system fluid flow rates which, however, imply a decrease in column efficiency, and therefore in the product purities. Furthermore, various practical constraints on the column or entire unit pressure drop, switching time, pump flow rates and product quality etc. complicate the optimization problem. In our earlier works [6,7],we have developed a new optimizationprocedure based on a genetic algorithm, i.e. the non-dominated sorting genetic algorithm (NSGA), that allows to handle these complex optimization problems. A more detailed description of this global search and Optimizationtechnique is available elsewhere [18, 191.

65

Results and discussion

3 3.1

Comparison of SMB and Varicolfor diflerent total number of columns

An important aspect in comparing SMB and Varicol processes is the effect of the total number of columns, Ned.It is in hct expected that as N,, increases, the discretizationof the movement of the solid in the SMB improves and thereforethe main advantage of Varicol, i.e. the possibility of better tuning the distribution of the columns in the sections by using non integer values, becomes less effective. We consider in this work a single objective optimization problem using as a model system the chiral separation reported by Biressi et al. [3], aiming at investigating the effect of NW1on the performance of these two processes, according to the following statement of the optimization problem:

XI

Maximize Subject to

J = PE[QI, ml, m2, m4, P R 2 90% and APdt 5 70 bar

Vsotid=120ml, R=l cm2,F=0.7 ml/min, C T ~ gA, = ~dp=30 pn, N,,=4

-8

where the extract purity, PE is the objective to be maximized under the following constraints: minimum 90% raffiate purity, PR,maximum 70 bar pressure drop along the entire unit, Munit, given total solid volume, Vlolid,given production rate (i.e. faed feed flow rate, F and total feed concentration, C,”) and given packing particle diameter, d,. Having fixed also the column cross section a,changes in NCdimply changes in column length. The optimization variables are the flow rate in section 1, Q1,the flow rate ratios, ml,m2 and m4 [2], and the unit configuration represented by the parameter, x. For the SMB this can be represented in the form of n1/n2/n3/n4where njrepresents the number of columns in section j. For the Varicol operation the decision variables are the same, but now x can attain a much larger number of values, which is in fact determined by the number of subintervals considered in each switching interval. Thus for example, the complete configuration of a 4-subinterval Varicol unit with NC.l=5 can be described by the configuration sequence 2/1/1/1-1/2/1/1- 1/1/2/1-1/1/1/2 fiom the first subinterval to the last. The equilibrium stage model reported by Zhang et al. [7], which accounts for the influence of fluid flow rate on the column efficiency, has been used for the optimization simulations. The optimization results for a SMB and a 3-subinterval Varicol unit are compared in Table 1 for various values of NWI. Table 1. Optimization results of SMB and Varicol processes for various values ofNml

Process

I I I

SMB

varicol

N ,

,L

Qi

rnl

rnz

mc

(crn)

(mumin)

4 5 6 7 8 4

30 24 20 17.14 15 30

30.328 30.747 35.126 34.735 36.595 29.502

1.550 1.567 1.512 1.584 1.643 1.747

0.867 0.843 0.900 0.888 0.900 0.837

0.754 0.738 0.594 0.525 0.444 0.680

5

24

35.761

1.538

0.895

0.663

011~11-012/111-1111111;** 1/1/2/1-112/111-1/2/1/1

66

x

m u n t i

PR

PE

1/1/1/1 1/2/1/1 1/2/2/1 1/3/2/1 1/3/3/1

(barj 52.79 52.14 60.47 57.86 59.86 48.40

% 90.02 90.00 90.07 90.02 90.06 90.01

% 86.49 91.68 96.04 97.13 97.29 91.55

61.81

90.00

94.22

* **

It is seen that PEincreases with increasing number of columns, particularly at the lower values of N d . PEincreases in fact by almost 10%from 86.5% for Nc0,=4to 96.0% for NcOl=6. A further increase in total number of columns has a smaller influence on PE.In addition, from a practical point of view, it is worth noting that the short switching time values caused by the short column lengths may lead to difficulties in the operation of the recycle pump. With respect to column configuration x, the results in Table 1 indicate that all the additional columns tend to distribute equally between sections 2 and 3. This is because, once the lower bound on ml and the upper bound on m4 are satisfied, sections 2 and 3 are the most important in determining PEand PR.The mivalues in Table 1 indicate that indeed in the cases under examinationboth such constraints are satisfied. In Table 1 the performances of a 3-subinterval Varicol with 4 and 5 columns are also shown. It is seen that Varicol For Nco1>5, the outperforms SMB both for NCoI=4,and to a lower extent, for NeoI=5. optimization technique has not been able to find a Varicol configuration which improves PE over the corresponding SMB value. This means that, as expected, for increasing total number of columns, the Varicol configuration is not worthwhile anymore. It is worth noting that in the case of N,,=4, the following five possible configurations have been considered for Varicol: 0/1/2/1, 0/2/1/1, 1/2/1/0, 1/1/2/0 and l/l/l/l. The first four of such configurationshave only three sections, which actually were not considered in the cases where NmI>4.The configuration 0/1/2/1~0~/1/1-1/1/1/1 was found to be optimal for the 3-subinterval Varicol, which improves PEby about 5% over the corresponding SMB with a 1/1/1/1 configuration. It is interesting to note that in the first two subintervals there is no column in section 1; the fourth column moves in fact from section 3 in the first subinterval to section 2 in the second and eventually to section 1 in the last subinterval. This corresponds to the time averaged configuration 0.33/1.33/1.33/1, which compared to the configuration 1/1/1/1 ofthe SMB, indicates that in this particular case Varicol improves the performance of the separation by reducing the length of section I and increasing that of sections 2 and 3. 3.2

Comparison of SMB and Varicolfor direrent product purity requirements

Another situation of interest in applications is one where the product purities are fixed, and the objectives for optimal process operation are to reduce operating costs and increase production. Hence, in this case, for a fixed target product purity of both extract and raffiate streams, we seek the optimal process parameters for a SMB and a 4-subinterval Varicol unit, which maximize production using minimum amount of eluent, for another chiral separation system taken fiom literature [6,15]. The optimization problem is represented mathematically as follows: Max Min Subject to

J I = F CQz, F, D,t XI Jz = D [Q2, F, D,4, XI PE= x f 0.002, x = 0.90,0.95,0.99 P R = Xf0.002,~=0.90,0.95,0.99 Q1= 27.5 mYmin, NWI = 5 , Lcol= 0.1 m, NNTP=80

where Q1,the column flow rate in section 1, is set at 27.5 ml/min to fix the maximum column pressure drop, and the total solid used is also set by fixing total number of columns, NmIand column length, L,I. The decision variables are the column flow rate in section 2, Q2, feed flow rate, F,eluent flow rate, D, switching time, t, and column configuration parameter, x. Note that the two variables F and D appear also in the objective functions. The Pareto

67

optimal solutionsare shown in Figure 2 for purity requirementsfor both extract and rafiinate streams of 90%, 95% and 99%. P. and PI

0

= M%

PrandPm=SB%

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

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

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

8

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

0

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

................... 2.3

2.4

2.5 2.8 C. r U m b

2.7

1.5

2.0

1.8

.T

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

.o.............. 0

2.2

5.8

1.7

1.8 F, mVhlln

vuisol

5.4 0.4

2

1.9

0.8 F, mumln

0.8

1

1.2

Figure 2. Optimal solutions for the SMB and Varicol processes with 9004 95% and 99% purity requirements.

In all cases, using 5 columns, the optimal column configurationsare found to be 1/2/1/1 and 1/1/1/2-1/1/2/1-1/1/2/1-1/2/1/1 for the SMB and 4-subinterval Varicol unit, respectively. It can be observed that for fixed purity specifications, both the SMB and the Varicol processes require to increase the eluent consumption in order to increase the feed flow rate. Secondly, the Varicol process consumes less eluent, D than the SMB process for the same feed flow rate, F;or equivalentlyfor the same eluent consumption, D,the Varicol process can treat more feed, F.However, the extent of improvementdepends on the purity specifications. The more stringent the purity requirement, the larger the improvement achieved by Varicol over SMB. For example, at D = 5.6 mymin, the improvement in production rate, F of Varicol over SMB is lo%, 25% and 127% for a purity requirement both in the extract and in the d m a t e streams of 90%, 95% and 99%, respectively. Finally, it is seen fkom Figure 3 (for the case of purity requirement of 95%) 42

-

4.2 VIriCol 4 ..............................................................

SYB 4 .............. ............ 0............ ?.......................... ~

3.8 .............................................................

. e

em’

3.8 .......................................................

3.6 ............................................................

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

E 3.4 ............................................................. A A A 3 2 ......................................................................

f

0

1.5

1.8

3 ............x............ 2.8

g............0...........................

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

1.8

F, mllmln

1.0

...... . . ~..... ~L d ..... ....

0

0

0

0

3 ................... ‘I’...... x.............x .....x ..... x... r ......

1 1.7

*---.

....

32

X

4

....

3.4

2.8

A 1.5

1.8

1.7

1.8

F. mllmln

1.9

2

Figure 3. Flowate ratio parameters ml to nu for the SMB and Varicol processes with 95%purity requirement.

that the optimal values of the flow rate ratios in sections 2 and 3, m2 and m3, in the SMB and in the Varicol process are very similar (although the corresponding optimal performances are different) and change very little as the feed flow rate increases. This is consistent with the triangle theory, which indicates that the optimal operating point in the (mz, m3) plane is independent of the feed and eluent flow rates [1,2].

68

3.3

Comparison of SUB, Varicol and PowerFeed

The separation problem examined in this case requires the simultaneous maximization of the rafiate (PR)and the extract purity (PE)for a given feed flow rate, F,eluent flow rate, D and fixed configuration of the unit, for the same chiral separation considered in section 3.2. This optimization problem in the case of PowerFeed operation can be represented mathematically as follows: Max MaX Subject to

= PR[Qz,~,.*., Qz,s, Fi, Jz = PE[Qz,i, .**, Qz,sy FI, *-) PR2 90% Ji

Fs-I,DI,*-,b - 1 , t XI Fs-I,Di,--, Dsi, 4, XI

P E 90% ~ Q1=27.5ml/min, N,I=~ or 6, D,,=6.24 ml/min, Fa,, NNTp where Q2s, Fi and DI represent the flow rate of section 2, the feed flow rate and the eluent flow rate in the P subinterval,respectively. The average feed and eluent flowrates (Fave and Dave respectively) are fixed; therefore the flow rates in the last subinterval are not independent since they are determined by the corresponding values in the previous subintervals. Note that in this case it has been assumed that the flowrates Q2, F, D and consequently Q3, Q4,R and E, change in S subintervals. The problem can be simplified by changing in time less flow streams, e.g., only F,which implies the change of Q3 and R.It is to be noted that in the problem above both the SMB and the Varicol operation modes have three decision variables, i.e. Qz,t,and x. The same stage in series model described by Zhang et al. [6]has been adopted to simulate SMB, Varicol and PowerFeedprocesses, with a slight revision, which enables the column flowrates to change in time. A comparison of the SMB, Varicol and PowerFeed operations has been conducted for two sets of operatingconditions, one with FBve=1.62ml/min and NNTP=80and the other with F,,,=2.2 ml/min and Nm=60. Two different PowerFeed configurations, one varying F in 4 subintervals ( S 4 ) and the other varying Q2,F and D in 3 subintervals(S=3), are considered for the two cases, respectively. In the first configurationthe decision variables reduce to Q2, , ~ JF1, , F2, DI, D2, t, and x. The FlyF2, F3, t,and x, while in the second they are Qz,,, Q ~ J Q optimization results of a 5-column PowerFeed unit are compared to those of the corresponding SMB and Varicol units in Figure 4(a) and 4(b) for the two sets of operating conditions, respectively. It is interesting to observe that firstly, in both cases for the SMB, Varicol and PowerFeed units,we obtained Pareto solutions. Secondly, each Pareto has at least one discontinuity due to change in column configuration x. This is due to the fact that different purity requirement requires different column configuration. Thirdly, the 5-column Varicol and PowerFeed processes perform better than the corresponding 5-column S M B process. Finally, as the difficulty of the separation increases, as shown in Figure 4(b), the PowerFeed performs better than the Varicol, and is even comparable to the 6-column S M B at the two ends of the Pareto curve. A better understanding of these results can be achieved by reasoning in terms of the flowrate ratio parameter, m as shown in Figure 5, taking as an example the separation case shown in Figure 4(a). It can be observed from Figure 5(a) that m2 and m3for the 5-column SMB, Varicol and PowerFeed units are almost constant, although they tend to decreases slightly as PRincreases. This is in agreementwith triangle theory as discussedby Bang et al. [6].For the 5-column PowerFeed unit, the m3 values changing in time are shown in Figure 5(b). It is interestingto note that the m3of the SMB process, goes very closely to the average m3 of the PowerFeed process, m3ave and to a slightly less degree to the m3of the Varicol

69

process due to the column configurationchange of the Varicol operation, as shown in Figure 5(a). Therefore, the optimal design of the Varicol and PowerFeed processes can be replaced by the optimal design of the corresponding SMB process given by the equilibrium theory, followed by the search of the optimal ports switching and flow rates variation schemes within the switching period, respectively. 0.99

--"

0.98

-.

0.87

..

OD=-

O O O

0.98

0.96 --

0.96

2" 0.95 -. 0.94 -. 0.83

0.94

-.

0.92 -.

0.92

0.91 -. 0.94 0.9

:

:

:

0.91

0.92

0.93

~

0.94

; 0.85

.

,i

:

i

i

0.97

0.88

0.99

:

0%

0.9

I 1

0.9

0.82

0.94

0.96

0.98

1

PR

PRI'

Figure 4. Optimal solutionsfor the SMB, Varicol and PowerFeedprocesses (x changes with increasing PR:5-col SMB and PowerFeed: (a) and (b) 1/2/111-+1/1/2/1; 6-column SMB: (a) 1/212/1+211/2/1, (b) 1/2/2/1; 5-col Varicol: (a) 111/~1-111/2/1-112/1/1-1/2/111+1/1/211-1/1/2/1-1/1/211-1~/1/1~1/112/1-1/11211-1/11211-2111111, (b) 1~11211-112/1~1-1/2/111+1/1/2/1-1/1/211-1/2/1/1). 3.8

3.9

(a) comparison of the m2 and m3

3.6

3.5

"

(b) m,in lhe 4 subintervals oflhe P o w i f f e e d process

3.7

3.7

w

W

'

A

S

~

$

=

M

3.5

E 3.3

E" 3.4

U

(.3.1

E

3.3

2.8

3.2

A SMB, m3

2.7

0

AVarkol n0 Varkol nQ

SMB,nQ

rFvwerFeed.ave tr3 FuwerFeed, nQ

3.1

2.5

3

0.8

0.91

0.92

0.93

0.94

0.95

0.88

0.97

0.88

0.99

1

0.9

0.91 0.92 0.93 0.94

0.95 0.88 0.97 0.88 0.99

1

PR

PR

Figure 5. Comparison of (a) the average m 2 and m3 among the 5-column SMB, the Varicol and the PowerFeed processes and (b) Values of m3 in the 4 subintervals of the PowerFeed process corresponding to the points in Figure 4(a).

4

Conclusions

Our results show that the Varicol and the PowerFeed operations improve the performance of the SMB process, particularlywhen the total number of columns is small and the separations are difficult. The optimal design of the Varicol and PowerFeed processes can be replaced by the optimal design of the corresponding SMB process given by the equilibrium theory, followed by the search of the optimal ports switching and flow rates variation schemes within the switching period, respectively.

70

References

1. Storti G., Mazzotti M., Morbidelli M. and Carra S., Robust design of binary countercurrent adsorption separation processes, AICHE J. 39 (1993) pp. 471-492. 2. Mazzotti M., Storti G. and Morbidelli M., Optimal operation of simulated moving bed units for nonlinear chromatographic separations, J. Chromafogr. A 769 (1997) pp. 3-24. 3. Biressi G.,Ludemann-HombourgerO., Mazzotti M., Nicoud R.M. and Morbidelli M., Design and optimization of a simulated moving bed unit: role of deviations from equilibrium theory, J. Chromatogr. A 876 (2000)pp. 3-15. 4. Klatt K.U., Hanisch F. and Dunnebier G., Model-based control of a simulated moving bed chromatographic process for the separation of hctose and glucose, J. Process Contr. 12 (2002) pp. 203-219. 5. Ruthven D.M. and Ching C.B., Counter-current and simulated counter-current adsorption separation processes, Chem. Eng. Sci. 44 (1989) pp. 1011 1038. 6. Zhang Z., Hidajat K., Ray A.K. and Morbidelli M., Multiobjective optimization of simulated moving bed system and Varicol process for chiral separation, AZChE J. (2002) in press. 7. Zhang Z., Mazzotti M. and Morbidelli M., Multiobjective optimization of SMB and Varicol processes using genetic algorithm,J. Chromafogr.A (2002) in press. 8. Mazzotti M., Storti G.and Morbidelli M., Supercritical fluid simulated moving bed chromatography,J. Chromafogr.A 786 (1997) pp. 309-320. 9. Di Giovanni O., Mazzotti M., Morbidelli M., Denet F., Hauck W. and Nicoud, R.M., Supercritical fluid simulated moving bed Chromatography 11. Langmuir isotherm, J. Chromafogr.A 919 (2001) pp. 1-12. 10. Denet F., Hauck W., Nicoud R.M., Di Giovanni O., Mazzotti M., Jaubert J.N. and Morbidelli M., Enantioseparation through supercritical fluid simulated moving bed (SF-SMB) chromatography,Ind. Eng. Chem. Res. 40 (2001) pp. 4603-4609. 11. Migliorini C., Wendlinger M., Mazzotti M. and Morbidelli M., Temperature gradient operation of a simulated moving bed unit, Ind. Eng. Chem. Res. 40 (2001) pp. 2606-2617. 12. Jensen T.B., Reijns T.G.P.,Billiet H.A.H. and van der Wielen L.A.M., Novel simulated moving-bed method for reduced solvent consumption, J. Chromafogr.A 873 (2000) pp. 149-162. 13. Antos D.and Seidel-MorgensternA., Application of gradients in the simulated moving bed process, Chem. Eng. Sci. 56 (2001) pp. 6667-6682. 14. Abel S., Mazzotti M. and Morbidelli M., Solvent gradient operation of simulated moving beds I. Linear isotherm, J. Chromafogr.A 944 (2002) pp. 23-29. 15. Ludemann-Hombourger O., Nicoud RM. and Bailly M., The Varicol process: a new multicolumn continuous chromatographic process, Sep. Sci. Technol. 35 (2000) pp. 1829-1862. 16. Kloppenburg E. and Gilles E.D., A new concept for operating simulated moving-bed processes, Chem. Eng. Technol. 22 (1999) pp. 8 13-817. 17. Zang Y. and Wankat P.C., SMB operation strategy-Partial feed, Znd. Eng. Chem. Res. 41 (2002) pp. 2504-25 1 1. 18. Bhaskar V., Gupta S.K. and Ray A.K., Applications of multi-objective optimization in chemical engineering,Rev. Chem. Eng. 16 (2000) pp. 1-54. 19. Srinivas N. and Deb K., Multiobjective function optimization using nondominated sorting genetic algorithms, Evol. Compuf.2 (1 995) pp. 22 1-248.

-

71

ADSORPTION TECHNOLOGY FOR GAS SEPARATION SHIVAJI SIRCAR Department of Chemical Engimering, Lehigh University 11 1 Research Drive, Iacocca Hall, Bethlehem, Pa 18015-4791, USA. E-mail: [email protected]

Separation and purification of gas mixtures by selective adsorption on micro-meso porous solid adsorbents such as zeolites, activated carbons, silica and alumina gels, polymeric sorbents, etc., has found numerous commercial applications in the chemical, petrochemical, environmental, medical, and electronic gas industries. Table 1 lists some of the key uses of this technology [I]. Two generic cyclic process concepts called Temperature Swing Adsorption (TSA) and Pressure Swing Adsorption (PSA) are generally employed. Each of these concepts have numerous variations depending on (a) the product specifications, (b) the energy of separation, (c) the sequences and the modes of operation of the steps of the process, (d) the types of adsorbent used, etc. The gas purification applications typically use the TSA processes except for gas drying and solvent vapor recovery applications where both the TSA and the PSA processes are used. The bulk gas separation applicationsexclusively use the PSA processes. Table 1. Key Commercial Applications of Gas Separation and Purification by Adsorption Technology T

'lut R val Trace Organicand inorganic Impurity Removal hDrYing Air PollutionControl Nuclear Waste Management Solvent Vapor Recovery Electronic Gas Purification

Air Separation (4 and NZfrom Air) Hydrogen and Carbon Dioxide Roduaion from Steam-Methane Reformer W G a s Roduction of Ammonia SynthesisGas Hydrogen Recovery fromRefinery off Gases Methane-CarbonDioxide w o n from Landfill Gas Carbon Monoxide-HydrogenSeparation N o d Isoparaffn Scparahon Alcohol Dehydration

-

rn

The interest and growth in the research and development of adsorptive separation processes have been phenomenal. Table 2 lists the number of U.S. patents cited by the Derwent Chemical Patent index between the years of 1980 and 2000 under the keywords given in the table [I]: Table 2. Results of U.S. Patent Search Between 1980 and 2000 Kevwords Gas Separation by Adsorption Adsorption for Air Pollution Ressun swing Adsorption Temperature Swing A m t i o n

NumberofP@nts 3050 1164 608 60

m r d s Air Separation by PSA Hydrogen Purification by PSA Gas Drying of PSA

m

a

l

t

s

391 185

32

Gas adsorption has become the state-of-the-art technology for (a) trace impurity removal, (b) small to large scale (1 40,000 SCFH) gas drying, (c) small to medium scale (0.0 1-1 00 TPD) production of O2 (90+%) and N2 (99+%) from air, and (d) small to large scale

72

(1-100 MMSCFD) production of high purity H2 (99.999+%) from steam-methane reformer and refinery off-gases. Research and development on adsorptive processes has primarily been directed towards (a) producing purer products (single or multiple) from a feed gas at higher recoveries (b) lowering the capital and operating costs of separation, (c) designing novel hardware and process control systems, and (d) increasing the scale of applications. Some of the key achievements include (a) lowering the specific power ( 4 2 KW/T/D) for production of 90% O2 from air below that of conventional cryogenic distillation air separation route, (b) direct production of high purity N2 ( 4 0 0 ppm 02) from air, and (c) production of high purity H2with high recovery (90+%). The primary reasons for this spectacularp w t b in this area are given below [2]: (a) There is an extra degree of thermodynamic freedom for describing adsorption systems compared to those for conventional gas separation methods like distillation and absorption. This introduces an immense flexibility in the design and operation of adsorptive separation processes. (b) Numerous families of porous adsorbents are available which offer multiple choices of core adsorptive properties (equilibria, kinetics, and heats) for a given separation application. (c) A successfuladsorptive process is generally a good marriage between the optimum adsorbent and the efficient process design. (d) There can be many different paths (combinationsof materials and processes) for achieving the same separation goals. The above reasons are also the key driving forces for promoting innovations in this area.

The points discussed above can be demonstrated by the case of simultaneous production of O2 and N2 enriched gases from ambient air. Air can be fractionated by selectively (thermodynamic) adsorbing N2 over O2 and Ar on a zeolite [3], or selectively (kinetic) adsorbing O2over N2 and Ar on a molecular sieve carbon [4,5]. Furthermore, many different process schemes for air separation can be developed using different zeolites and molecular sieve carbons having different adsorptive properties. For example, a zeolite like Na-mordenite having a moderate N2 selectivity of -4 over O2 at ambient conditions can be used in a four-step PSA process [3]. The cyclic steps would include (a) adsorption of N2at near ambient conditions by flowing air through a packed bed of the zeolite, while producing the O2enriched product gas at feed air pressure (PA),(b) rinsing the adsorber cocurrently with a stream of essentially pure N2 and venting the air-like effluent, (c) evacuating the adsorber counter-currently to pressure PD and withdrawing a N2 rich product gas, a part of which is used in step (b), and finally (d) counter-currently pressurizing the adsorber from PD to PA with a part of product gas generated by step (a). The N2 rinse step is needed to displace the co-adsorbed and the void 0 2 remaining in the column at the end of step (a) so that the desorbed gas in step (c) is essentially pure N2. This step can be eliminated if the N2 selectivity over O2by the zeolite is high (say >8) as in the case of CaX zeolite. The desorbed gas, in this case, can be hctionated in order to

73

reject the N2lean earlier part, and collect the N2rich latter part as the N2product gas [3]. Higher Nz selectivity of an adsorbent allows the desorbed gas fractionation concept to be practical, which leads to a simpler three-step process scheme, while meeting the product specificationsof the former four-step cycle. The cyclic steps of one of the PSA processes using the molecular sieve carbon as the adsorbent consist of (a) flowing compressed air through a packed bed of the carbon so that 0 2 can diffuse and adsorb into the carbon pores faster than N2and Ar and produce a N2 rich product gas at feed air pressure (PA), (b) pressure equalizing the adsorber with a companion adsorber, (c) counter-currently depressuring the adsorber to near ambient pressure to produce the O2 enriched gas, (d) pressure equalizing with another adsorber, and finally (e) repressurizingthe adsorber to PAwith feed air [4]. Table 3 shows an example of the comparative performances of these three processes. All of them can produce a 99+%N2enriched product gas. The two zeolite processes also produce a 85-90% O2 enriched product gas. The O2 product purity of the carbon sieve process is, however, low. This shows that different adsorbents can be married with different process schemes to obtain similar product purities but different process performances (recovery, productivity, product pressure, etc.). Table 3. ComparativePerformancesof Various Air Fractionation Processes

Process (a)

(b) (c) Process (a)

(b) (c)

Adsorbent Na-Mordenite Ca-X Carbon Molecular Sieve Adsorbent Na-Mordenite Ca-X Carbon Molecular Sieve

Purity (%) 84. I 90.0 33.8

Purity 99.0 99.0 99.0

Productivity (SCfh/P) 31

20 144

Productivity 122 83 92

Recovery

(W

Pressure (Psi@

63.0 24.0 98.1

2 2 0

Recovery

Pressure

65.0 30.0 49.4

0 0 104

TRENDS IN FUTURE ADSORPTIVE PROCESS DEVELOPMENTS

Two areas of development have attracted considerable attention in recent years. They are: (a) Rapid PSA cycles for bulk gas separation (b) Novel adsorbet designs RaDid PSA The concept is to use faster cycle times (seconds) than those used by the conventional PSA cycles (minutes) in order to obtain a step change in the productivity of the process (volume of productholume of adsorbenthour). Some of these designs use a conventional PSA cycle but they are operated faster by appropriate changes in the mechanical designs [6]. Others, propose novel process schemes in order to accommodate fast cycle times [7-91. An example of the second case is a RPSA process for air separation where a single adsorber vessel is packed with two or more (even numbers) shallow layers of small zeolite particles (-0.5 mm). The layers are separated

74

by screens which act as pressure drop devices. The cycle steps consist of (a) simultaneouslypressurizing and adsorbing N2from air on one layer of the adsorbent while

producing an O2 enriched product gas, and then (b) simultaneously depressurizing and back purging the layer with a part of the O2 enriched product gas produced by a companion layer [9]. Using a 5A zeolite as adsorbent and a total cycle time of 10 seconds (compared to conventional PSA cycles of 60-240 seconds), the RPSA process could produce a 27.5% O2enriched gas stream with an O2recovery of -64.1% from feed air at a pressure of 2.22 atm [3]. The 0 2 productivity rate was -2300 sfi?/ft3/hrwhich was an order of magnitude higher than that of a conventional PSA. The process was found to be suitable for producing 2340% 02 from air for enhanced combustion applications in cupolas, metallurgical furnaces, etc. [lo]. Novel Designs Conventional adsorber designs include vertical (length/diameter >1) or horizontal (lengtlddiarneter 4)packed beds. The maximum permissible gas flow rates through these vessels is governed by pressure drop and the possibility of local fluidization and channeling [ 1 13. Some of these problems are solved by novel designs such as (a) radial bed and (b) rotary bed adsorbers. Radial Bed Adsorbers (PSA and TSA) The adsorbent is placed in an annular section between two co-axial cylinders. The walls of the cylinders are perforated for gas flow in a radial direction. The entire assembly is enclosed inside the adsorber vessel with gas inlet and outlet conduits. Many different designs are patented [ll-131. The adsorbers allow faster cycle times, lower pressure drops, and higher gas throughputs without fluidization. However, the equipment design is more complex and costly.

Rotarv Bed Adsorbers (TSA) The rotary bed adsorber (also called adsorption wheel) provides a truly continuous TSA system. It uses a shallow wheel-shaped adsorption bed that continuously turns about an axis inside a fixed supporting frame. A section of the wheel is continuously used for adsorbing impurities 60m a gas while the other section is continuously regenerated by heating it with an impurity free gas. The adsorbent is made from a honeycomb-shaped alumina substrate that can be coated with layers of silica gels, activated carbons, or zeolites [14]. It has been used for gas dehumidification, solvent vapor recovery, VOC removal, and deodorization of a gas stream. NOVEL ADSORPTIVE GAS SEPARATION CONCEPTS Two novel hybrid concepts for gas separation using adsorption technology have emerged in recent years. They include (a) adsorbent membranes, and (b) simultaneous adsorption and reaction. Adsorbent Membranes

75

They consist of a thin layer ( 4 0 pm) of a nanoporous (3-lOA) carbon filmsupported on a meso-macroporous inorganic solid (alumina) or on a carbonized polymeric structure [151. They are produced by pyrolysis of polymeric films. The following two types of membranes are produced: Molecular Sieve Carbon (MSC) Membranes The MSC membranes are produced by carbonization of PAN, polymide, and phenolic resins. They contain nanopores, which allow some of the molecules of a feed gas mixture to enter the pore structure at the high pressure side, adsorb, and then diffise to the low pressure side of the membrane, while excluding the other molecules of the feed gas. Thus, separation is based on the difference in the molecular sizes of the feed gas components. The smaller molecules preferentially d i f i e through the MSC membrane as shown by Table 4 [16,17]. Table 4. Separation Performance of Various MSC Membranes

Selective Surface Flow (SSF) Membranes The SSF membranes, which are produced by carbonization of PVDC, contain nanopores that allow all of the molecules of a feed gas mixture to enter the pore structure. However, the larger and more polar molecules are selectively adsorbed on the carbon pore walls at the high pressure side, and then they difiiise selectively to the low pressure side. The smaller molecules are enriched at the high pressure side. These membranes can be used to enrich H2 from mixtures with CI-C4hydrocarbons or from mixtures with C02 and CH4. They can also be used to separate C&-H2S and H2S-H2 mixtures. Table 5 compares performances of SSF carbon and polymeric PTMSP membranes for H2 enrichment from FCC off gas [15]. Clearly, the SSF membrane is much superior for this application. Table 5. Separation Performancesof SSF and PTMSP Membranes

Gas Components C3H8 c3H6

CZH6 c 2 H 4

CH4 Hz

Commnent Reiections SSF PTMSP 98.2 86.0 98.8 86.0 94.1 76.0 93.3 72.0 52.0 46.0 40.0 40.0

Hi& Pressure Product Comwsitions (%) SSF PTMSP 1.16 6.33 I .49 12.20 2.30 6.33 2.03 5.19 41.46 32.73 51.56 36.19

SimultaneousAdsomtion and Reaction

76

The conversion of reactants to products, as well as the rate of product formation, of an equilibrium controlled reaction can be increased by removing a product from the reaction zone, according to the Le Chatelier's principle. Adsorption has been used to achieve this goal by using admixtures of catalysts and adsorbents in packed bed reactors. Process concepts called "pressure swing reactors" have been proposed when the adsorbent is periodically regenerated by using the principles of PSA [181. Recently, a novel scheme called "Sorption Enhanced Reaction Process (SERP)" was developed for direct production of essentially carbon oxide free ( 4 0 ppm) H2 enriched gas stream (90+%) containing CH., as the primary impurity by steam-methane reforming (SMR). The process could be operated at a much lower temperature (400-5OO0C) than that needed by the conventional SMR reactor (800-900°C), and yet produce high conversion of CH4 to H2 [19-201. The cyclic steps of the SERP consisted of (a) sorption-reaction, where a gaseous mixture of CH., and H20 is passed through the reactor and a stream of COXfree H2 enriched gas is directly produced at feed gas pressure (PA), counter-current depressurization of the reactor to near ambient pressure and venting the effluent gas, (b) counter-current evacuation of the reactor to pressure PD and purging the reactor with steam at PD while discarding the C02 rich effluent gas, and finally (c) counter-currently pressurizing the reactor fiom PD to PA with steam. The reactor is maintained at 400-5OO0Cthroughout the entire cycle. Table 6 shows an example of the cyclic steady state performance of the SERP concept using an admixture of a SMR catalyst (noble metal on alumina) and a C02 chemisorbent (KzC03promoted hydmtalcite) in the reactor [20]. The reactor temperature was 4 9 O O C . The feed H20:CH4ratio was 6: 1. The concept can directly produce -95% H2 product (dry basis) with a CH., to H2 conversion of 73%. The trace impurities in the product gas contained less than 40 ppm COP The corresponding product gas composition (thermodynamic limit) of a SMR reactor operated without the C02 chemisorbent will be -67.2% H2, 15.7% CH.,, 15.9% C02, and 1-2% CO (dry basis), and the CHI to H2 conversion will be only 52%. Thus, the SEW concept may be attractive for direct production of a CO free H2 stream for fuel cell applications. Table 6. Performance of the SEW Concept for H2 Production

Feed

1-

Press. (psia)

Feed

Purge Steam

Hydrogen Product

26.2

0.60

1.88

0.25

Hvdroeen Product Puritv IDrvl

H2 (9'0

cH4 (YO

94.4

5.6

40

Methane Conv. CO m Not detected

ToH dro en 9'0 73

Removal of Bulk COi from a Wet HiPh TemDerature Gas

Another interesting example of using a chemisorbent (Na20 supported on alumina) in a PSA process is direct removal and recovery of bulk C02 from a wet high temperature feed gas without pre-drying or cooling the feed [21]. A gas stream at 200 C containing 10 mole % C02 and an inert component (dry basis), and which is saturated with water, can be treated to simultaneously produce a COz depleted stream (MFI(39.5), Ag( 1.8)-BEA(37.0) and Ag(2.O>FER(60.2) showed the C2I& adsorption property characteristic to Ag species idon zeolites, namely, forming strongly adsorbed species with the VJAg ratios between 0.7 and 0.9 and desorbing it above 200 "C. As described above, the impact of the hydrothermal treatment on Ag(2.0)FER(60.2) was not so severe. For the other two samples of MFI and BEA zeolites, on the other hand, the adsorption capacity of both strongly and weakly adsorbed species considerably decreased. Powder X-ray diffraction measurements showed that the hydrothermal treatment caused the partial destruction of the zeolite structure for MFI and BEA systems, but not for the FER system. Such structural stability was also recognized from the change of the specific surface area, S, (Table I). The S, values of fresh and aged Ag-FER was almost the same, while those of Ag-MFI and Ag-BEA decreased considerably after the hydrothermal treatment. 3.4

Characteristics of Ag-FER as cold-HC trap material

The results reported in this paper clearly show that Ag-FER(60.2) is the promising material for cold-HC trap with the following characteristics. (1) superior selectivity to olefins (C2H4, C3H6)over a parafin (C2H6) (2) high adsorption capacity with the VJAg ratio in the range of roughly 0.7-0.9 (at p/po=O.15) (3) desirable storage ability desorbing the strongly adsorbed species above 200 "C (4) high hydrothermal stability Selectivity (l), capacity (2) and storage ability (3) originate from the characteristics of Ag itself, while the stability (4) of Ag-FER(60.2) is contributed by the structure stability of the FER(60.2) and insensitivity of the adsorption property of Ag to its existing states. The higher SiO2/AI2O3ratio of FER(60.2) is the most probable reason for the zeolite-structure stability. The insensitivity of Ag is a favorable property as -a stable HC trap material, and its origin will be elucidated by a future study. References 1.

2. 3. 4. 5.

6.

Burk P. L., Hockmuth J. K., Anderson, D. R., Sung, S, Punke, A, Dahle, U, Tauster S. J., et al., Stud. S u Sci. ~ Catal., 96 (1995) pp.9 13-939. Nishizawa K., J. of Soc. of Automotive Engineers of Jpn. (Japanese), 50 (1996) pp. 61-65. Ballinger, T. H., Manning W. A., Lafyatis, D. S., SAE Paper (1997) pp.27-31 (970741). Lyfyatis, D. S., Ansell, G. P., Bennett, S. C.,Frost, J.C., Millington P. J., Rajaram, R. R., Walker A. P., Ballinger, T.H., Appl. Catal. B, 18 (1998) pp.123-135. Czaplewski, K. F., Reits T. L., Kim, Y. J., Snurr, R. Q., Microporous and Mesoporuos Materials, 56 (2002) pp.55-64. Bogdanchikova, N. E., Dulin, M. N., Toktarev, A. V., Shevnia, G. B., Kolomiichuk, V. N., Zailovskii V. I., Petranovskii, V. P.,Stud Surf: Sci. Catal., 84 (1994) pp. 10671074.

166

PRESSURE-DEPENDENT MODELS FOR ADSORPTION KINETICS ON A CMS Youn-Sang Bae, YoungKi Ryu, and Chang-Ha Lee' Dept. of Chem. Eng., Yonsei University, Seoul, Korea Tel.: +82-2-2 123-2762, Fax: +82-2-3 12-6401, E-mail: leechk2vonsei.ac.h

An adsorption kinetic model was developed to evaluate the adsorption rates of five pure gases (Nz, 02, Ar, CO, and Ch) on a Takeda-3A CMS over a wide range of pressures up to 15atm. The kinetic characteristics of adsorptionon the CMS were studied by using the adsorption equilibrium of five pure gases measured at three different temperatures and their physical properties. Since the diffisional time constants of all the components showed much stronger dependence of pressure than those expected by the traditional Darken relation, a structural diffusion model was applied to predict the strong pressure dependence. The proposed model successfully predicted the diffisional time constant up to high pressure on the CMS.

1

Introduction

Carbon molecular sieve (CMS) is useful in air separation processes because of its ability to selectively discriminate on the basis of molecular size and hence adsorb the smaller oxygen molecule over nitrogen. The difference in the adsorption kinetics of various gases aIlows the separation of gas mixtures into pure components using pressure swing adsorption (PSA). Generally, the kinetic rate constants on adsorbents increase with increasing surface coverage. The reason is probably related to surface diffusion (Reid and Thomas, 1999). Ruthven (1992) pointed out that the diffisivity of oxygen increased with adsorbate loading on CMS more or less in terms of Darken's equation. The pressure-dependencesof D/? are generally predicted by Darken-relation, but in some cases the pressuredependeces are so strong that it cannot be predicted by traditional Darken-relation. Hence, in these cases, the model that can predict these strong pressure-dependencesis needed. In this paper, the isotherms and diffisivities of five pure gases @I2, 02,Ar, CO, and CH4)in CMS were studied in the range of 293-313K, 0-15atm. 2

Experimentals

The volumetric method was used to obtain the data of the adsorption equilibrium and the adsorption kinetics. The adsorbent used in this study was Carbon Molecular Sieve (Takeda Co.) and has an average pore size of 3A. The adsorbates were 99.99%-purity gases. Prior to the measurements, the adsorbent was regenerated by evacuation at 423K during 12hr. The CMS is loaded with an adsorbate in a stepwise procedure; equilibrium and kinetic data are obtained in each step. In determining the size of each step, we considered the pressure range in which linear isotherm can be applied.

167

3

Mathematical Models

3. I

Equilibrium Models

Langmuir isotherm:

L F isotherm: Toth isotherm:

D-R isotherm:

bP l+bP

c, = c,

b PI'" 1 + b P"" bP c, = c, (1 + (bP)" ) I / "

c, = c,

P

C , = C, -exp[- a' x(In(-))2 P O

]

(4)

3.2 Pressure-dependent Modelsfor Adsorption Kinetics The exponential increases of the effective diffusional time constant might be explained by the following Darken-relation and this relation was derived under the assumption that the chemical potential gradient is the driving force of the diffusion (Ruthven, 1992). Darken-relation :

If above Darken-relation (Eq.(5)) is combined with the Langmuir, L-F, Toth, and D-R isotherms, the resulted models are as follows: t

Darken-Langmuir:

D, = D,, ( 1 + bP )

(6)

I

Darken-LF:

D,= D,, [ 1 + bP"" ]

Darken-Toth: Da rken-DR.

D, = Dfl

168

I

ln(4 / P)

(7)

4

4.1

ResultsandDiscussion

Adsorption Equilibrium

Adsorption isotherms of five gases at 293K for C M S determined from the volumetric experiments are shown in Fig. 1. Adsorption capacities of each gas are as follows: Nz,4, Ar < co ACF-3 >ACF-2 >ACF-1 and can be attributed to the self-catalicale reduction-oxidation reactions between carbon and Cr(V1) species and/or oxygen-containing groups on carbon (-OH, -COH) and Cr(V1) ions. The similar effect was described in our previous paper for the platinum and gold species adsorption on ACF [7]. The data presented in table 3 and fig2 show the abatement of Cr(V1) concentration, AC of ACF and the growth of Cr(II1) concentration at pH 2.5-2.9. A large increase in the amount of Cr(V1) transformed to Cr(II1) and in the amount of chromium removed is produced as the pH decreases. Furthermore the amount of Cr(1ll) (fig.2) rises up to

190

maximum in the initial phase of ACF/solution contact (time up to 2 h) and than falls to the minimum (time up to 48 h). It indicates that the Cr(II1) species could be adsorbed onto ACF when the ACF are oxidized during the adsorptiodreduction of Cr(V1) ions. The presence of oxidized form of ACF is expected to facilitate the Cr(1II) elimination but they are not effective for Cr(V1) removal. The ACF-2ox/Cr(VI) solution contact confirm low capacity of oxidized fibers for chromium (VI) ions adsorption. Table 2. chromium (vi) species adsorption at initial pH - 4.7 as a function of time.

A. B.

Cr(V1) concentration, mg/l Total mount of chromium (VI) and (Ill) removed - AC, mg/g

0

0

0,s

1

1.5

a

2

X

r

a

24

48

h

-

Figure 1. Chromium (111) concentrationvariation as a function of time. Initial pH 4.7; temperature 20 "C.

Table 3. chromium (vi) species adsorption at initial PH - 2.5 as a function time.

A. B.

Cr (VI).concent&on, mg/l . Total mount of chromium (VI) and (Ill) removed - AC, mg/g

0

A

m

0

0.5

I

1.5

2

81me. h

x

ax

X

X

24

48

-

Figure 2. Chromium (III) concentrationvariation as a function of time. Initial pH 2.5; temperature 20 'C.

191

3.2

Kinetics of Cr(II4 removal

As from the previous experiments it was impossible to determine the adsorption capacity in regard only to Cr(1II) ions theirs adsorption onto different ACF was examined from Cr@IO3), solution with initial concentration of Cr(II1) - 100 mg/l. Fig.3 shows the amount of Cr(II1) species removed at pH 3.2 - 3.9 temperature 20-25 "C and at different ACF/solution contact time. tACF-4 +ACF-3

t ACF-2 JC-ACF-1 Jlt ACF-ZOX

0

1

2

3

4

time, h

5

Figure 3. Chromium (111) removal by ACF at 20 "C as a hnction of time.

According to the experiments carried out the maximum adsorption capacity to Cr(lI1) ions can be affirmed for oxidized sample ACF-2ox. The Cr(lI1) removal by non-oxidized ACF is more than twice less. The adsorption equilibrium for all ACF was reached in a period of time close to 3 h.

3.3

The influence of temperature on Cr(VI) species adsorption

Figure 4 compares the amount of total chromium removed by ACF-1 at different temperatures for saturation time 48 h, pH=2 and initial concentration of Cr(V1) in solution 200 mg/l.

0 # ) 4 0 6 0 8 0

T

Figure 4. Chromium (VI) removal by ACF-I as a hnction of temperature (T, "C).

The inverse relationship between the amount of chromium ions removed and the temperature was found. This fact can be explained in the consideration that the amount of Cr(V1) reduced to Cr(II1) increases with the increase of the temperature and that the adsorption of Cr(II1) ions on ACF-1 is relatively low. So, more the Cr(lI1) ions appears in the solution during ACF and Cr(V1) species contact less the total amount of chromium removed. Additionally it can be mentioned that the three states of chromium (Cr20;2, Cr203, C i 3 ) on surface of ACF were detected by X-ray photoelectron spectroscopy. According

192

to the data obtained the direct adsorption of Cr(V1) and Cr(Il1) species (if they are both in the solution) as well as Cr(V1) reduction to Cr203 and to Cr (111) ion with the subsequent adsorption can be supposed. 3.4

The influence of concentration

The relationship between the initial concentration of Cr (VI) species in the solutions and adsorption capacity of ACF-1, ACF-2 and ACF-3 at pH 2.5, temperature 20 'C, after 48 h is presented in figure 5.

2

0

600 800 lo00 initial concentration,mgh 200

400

Figure 5. Chromium (VI) adsorptionon ACF-las a function of initial concentration

The amount of chromium increases with the increase of initial concentration of Cr(V1). The adsorption capacity of 800 mg/g can be reached when the initial Cr(V1) concentration is 1000mg/l. The adsorption capacity of oxidized sample ACF-2ox at the same conditions was not more than 310 mg/g. The amount of Cr(II1) removed by ACF20x from the solution with Cr(II1) initial concentration 1000 mg/l was only 140 mg/g. Thus, the factors which affect to Cr(V1) and Cr(II1) removal by ACF are not only temperature and initial concentration but also the oxidationheduction ability of fibers and theirs oxidation state before and during adsorption. The ACF used in the study can be useful to remove Cr(V1) and Cr(II1) from aqueous solution. To ameliorate the rate of chromium removal it seems better to use at the same time the oxidized and non-oxidized ACF. The most usefid porous texture of ACF is when the total pore volume is more than 0.4 and less 0.5 cm3/gand contains 70-80 % of micropores.

References 1 . Lalvani S.B., Wiltowsk T., Hubner A., Weston A., Mandich N., Carbon 36 (1998) p. 1219. 2. Morozova A. A., Zh. Prikl. Khimii 68 (1995) p. 770. 3. Brown P.N., Jayson G.G., Thomson G., Wilkinson M.C., Carbon 27 (1989) p. 821. 4. Lyssenko A., Carbon-based media for water purification, The international magazine for technical textile users 38 N24 (2000) p. 33. 5. Chand S., Agarwal V.K., Kumar P., Indian J., Enviror HLTH 36 Ne3 (1994) p. 15 1. 6. Aggarwal D., Goyal M.,Bansal R.C., Carbon 37 (1 999) p. 1989. 7. Lyssenko A., Simanova S., Abstracts, At 7th International conference of Fundamentals of Adsorption, Nagasaki, Japan, May (200 1) p. 38.

193

TREATMENT OF COMPLEX WASTEWATERS BY BIOSORPTION AND ACTIVATED CARBON : BATCH STUDIES C. GERENTE, Z. REDDAD, Y. ANDRES, C. FAUR-BRASQUET, AND P. LE CLOIREC Ecole des Mines de Nantes, GEPEA, UMR CNRS 6144, BP 20722, 44300 Nantes Cedex 03, France E-mail: [email protected] In order to treat effluents characterized by metallic and organic pollutants, an association of two different adsorbents, sugar beet pulp and granular activated carbon, is investigated. In a first step, equilibrium data are determined for each adsorbent and mono-component solutions. Then. multimetallic and organic-metal solutions are tested to determine some inhibitions or special selectivities. Finally, it is shown that the association of sugar beet pulp for metal removal, and activated carbon for organic elimination, is efficient to treat complex wastewaters.

1

Introduction

Industrial wastewaters are often complex aqueous mixtures containing various pollutants: heavy metal ions, organic molecules, dyes, etc. and some of them are toxic or undesirable to many living species. Previous investigations were mainly focused on the use of low-cost sorbents [ I ] as a replacement for costly methods of removing heavy metals from solution. Commonly, it concerns inorganic materials like fly-ash [2] as well as chitosan [3,4], biomass [5,6], sewage sludges [7], peat [8] ... For these latter of biological origin, the term of “biosorption” is used to encompass contaminant uptake via physico-chemical mechanisms such as adsorption or ion exchange. The low-cost biosorbent used in this paper is a byproduct of the sugar industry: the sugar beet pulp. This material is very cheap (1 00 € per metric tonne) and its production reaches 14.106 tomes of dry matter each year in the European Community [9]. Sugar beet pulp is a natural polysaccharide and composed of 20 % and more than 40 % of cellulosic and pectic substances respectively. These latter contain polygalacturonic acids which cany carboxyl functions and consequently exhibit good capacities to retain metal ions [10,1 I]. Activated carbon, within its different forms, is commonly used in water treatment for organic pollutant removal [12,13,14]. Nevertheless, some applicationshave showed its ability in metal removal [ 151. In this work, the treatment of a synthetic wastewater composed by metals ions (Cu2+, Ni2+ and Pb2+) and organic molecules (benzoic acid, benzaldehyde and phenol) is investigated with a mixture of two sorbents, sugar beet pulp and granular activated carbon. In a first step, equilibrium data are determined in a batch reactor for each adsorbent and mono-component solutions. Then, the pollutants are mixed to treat binary and ternary systems of metal ions, or a combination of Cu2+with organics. A second part focuses on an association of the activated carbon with the polysaccharide for the treatment of a solution containing Cu2’ and phenol.

194

2

Materiels and Methods

The adsorbents Raw sugar beet pulp was provided by Lyven (France). Its preparation has been previously 2.1

described by GBrente et al. [ 101 and the fixation of several metals have been studied and published in [11,16]. The granular activated carbon, Pica NC60 from Pica Co. (France), presents a high specific surface area (1200 mz.g-') coupled with a large microporosity (94.5% VOI.).

2.2

Sorption experiments

Experiments were performed in batch reactors at 21 f 1°C with a continuous stirring at 500 rpm and some ratio solidsolution fixed at 2.26 g.L-' for dry pulp and 0.5 g.L-' for activated carbon. A pre-hydration of 90 min of the pulp was necessary and the pH of the solution was stabilized at 5.5. Equilibrium times were deduced from the kinetics. The mixed metallic solutions had equimolar initial concentrations (8.1 O4 mol.L-'). The influence of benzaldehyde, benzoic acid and phenol on the fixation of CU" onto the pulp was conducted using 100 mg.L-' (expressed in TOC) of organic compounds. The adsorption on the mixture of sorbents of phenol and Cu2' ions was carried out with 50 mg.L-' of each components. 3 3.1

Results and Discussions

Fixation of Cu", Ni2' and Pb2' in mono and multi-metallicsolutions

--

............. &!.

alone

......

CuZ'

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

"

-12% - 5 0 % -42%

-10%-40% -43%

-56% -47% -61%

% of decrease

Figure 1. Ion competition experiments, conducted at initial equimolar ionic concentrations (8.1O4 mo1.L-I)

In the case of single metal ions adsorption, it has been seen (Fig.1) that the adsorption capacities order following Pb" > Cu2' > Ni2' [ 161. Smith and Martell [171 and Makridou et al. [181 have shown that Pb2'presents a higher stability constant with galacturonic acid than Cu". Moreover, no value are featured for Ni2'and Makridou et al. [ 181 assert that a complex between this metal and galacturonic acid does not exist. These results confirm

195

the order obtained above. Some experiments are then performed to study the competition of adsorption of these metallic species. Experiments conducted with two different cations show that Ni2' ion exhibits the lowest competitive effect: whereas its presence induces decreases of adsorption capacities of 12 % for Pb" and 10% for Cu", these last two cations prevent the Ni2'fixation at 47 and 56 % height respectively. Pb" seems to have a higher effect than Cu". In a mixed solution of copper and lead, the influence of the two metals is slightly different: Pb2' induces 40 % decrease in Cu2+fixation, and vice-versa the percentage reaches 50 %. As Pb2' is supposed to have the main preference to the pulp, this value should be abnormally great, especially as the addition of the third metal induces a lower decrease (- 42 %). The clear sorption preference for Cu2' and Pb2+is always marked in the three metal solution: the addition of Ni2' seems to have no effect on the other metals fixation.

3.2

Influence of organic compounds on Cu" fwation onto raw sugar pulp

The selected organic compounds were simple aromatic molecules (phenol, benzaldehyde, benzoic acid) which could simulate moieties present in natural organic matters. Preliminary, removal kinetics of these organic compounds have been performed using the same concentrations; they showed that no fixation occurred (data not presented). Secondly, isotherms of Cu2' ions sorption have been performed with a high constant organic charge and they are plotted on Figure 2. In comparison with the fixation of copper alone in solution, the presence of benzoic acid, at a concentration of 180 mg.L-', induces a reduction of about 30% on the fixation capacity. The two other compounds decrease it lower but the experiment does not enable to separate their influence. An explanation about this special decrease in the presence of benzoic acid could be a complexation in solution between Cu2+ions and benzoic acid, which would prevent the cation to be fixed on the pulp. The stability constant given by Smith and Martell [ 171 between these two species has a value of 1.6, which is close to that obtained with the complex Cu2'-galacturonic acid (1.8). The effect of competition is obvious and these results confirm a strong affinity of the carboxylic functions towards Cu2+ ions. Taking into account that the organic concentrations are relatively important, it can be supposed that less concentrations would have little effect on copper elimination. 3.3

Association ofpulp and activated carbonfor the removal of Cu2"and phenol

Preliminary experiments were carried out on each sorbent. As a little part of the organic content of sugar beet pulp was soluble in water (close to 35 mg.L-' expressed in TOC for a ratio pulp/water of 2.26 g.L-'), experiments with activated carbon were carried out in a liquid medium of pulp-water. It was verified that this specific soluble organic matter, probably composed of great molecules, was not adsorbed on NC60. The results are presented in Table 1. In a first approach, it was confirmed that phenol was not removed by sugar beet pulp at this concentration. The removal of copper was efficient with and without the presence of phenol since the removal percentages reached a value close to 60 % in both cases. This interesting result confirms those obtained above and shows that copper ions could be treated with pulp, even with a moderate organic charge. As far as activated carbon is concerned, on one hand the efficiency of this kind of material is verified towards the organic molecule since NC 60 exhibits a high removal percentage for phenol (73 %) and low for Cu" ions (14 %). On the other hand, the presence of metal

1%

decreases around 20 % the fixation of phenol and the presence of phenol slightly affects the copper fixation. The mechanisms of adsorption must be different, favoring a chemisorption based on an attraction with the surface functions for metal fixation and a physisorption, highly influenced by steric hindrance when another pollutants, for example hydrated cations, are present in solution. 0.3

0.25

ECD 3

3

0.2

2

20 -ca

0.15

E

0.1

-ild

0.05

% Cu alone Cu + 130 mg.L-1 of benzaldehyde Cu + 180 mg.L-1 of benzoic acid Cu + 140 mg.L-I of phenol

0 4 X

0

Figure 2 Organic influence of high concentrationon Cu2+fixation on pulp

Table 1. Removal percentage of copper and phenol, on pulp, NC 60 and a mixture of the two adsorbents (Initial concentration of copper and/or phenol 50 mg.L-'; pulp ratio 2.26 g.L'; NC 60 ratio 0.5 g.L").

cu2+ Cu2'(with presence of phenol) Phenol Phenol (with presence of Cu2+)

Pulp 63 60 0 0

NC60 14 11 73 54

Pulp+NC60 67 66 74 53

When these two different adsorbents are used together, the results show that their respective properties are conserved. In other terms, the sugar beet pulp exhibits a high affinity with copper ions (67 YO), even if phenol is present (66 %), and NC 60 keeps its efficiency with phenol (74 %) and is greatly influenced by the presence of copper.

To conclude, these preliminary results have shown the important ability of a low-cost sorbent, the sugar beet pulp, to remove metal ions from aqueous solution. When several metals are present in solution, a selectivity can be highlighted. The polysaccharide exhibits high affinities towards Cu" and Pb2+whereas organic molecules are not retained on it. The influence of organic matter on metal fixation occurs at high concentration or if a complexation between species is possible. Finally, if the effluent contains organic and metallic pollutants, the association of two kinds of adsorbent, namely sugar beet pulp and activated carbon, seems to be efficient since they would keep their respective properties in

197

terms of adsorption. Further investigations would confirm these results in a dynamic pilot unit and with real industrial wastewaters.

References , 1.

Bailey S. E., O h T. J., Bricka R. M. and Adrian D. D., A review of potentially lowcost sorbents for heavy metals, Wat. Res. 33 (1999) pp. 2469-2479. 2. Ricou P., Lecuyer I. and Le Clouec P., Removal of heavy metallic cations by fly ash in aqueous solution, Environ. Technol. 19 (1998) pp. 1005-1016. 3. Guibal E., Milot C. and Tobin J. M., Metal-anion sorption by chitosan beads: equilibrium and kinetic studies, Ind Eng. Chem. Res. 37 (1 998) pp. 1454-1463. 4. Gerente C., Andres Y. and Le Cloirec P., Uranium removal onto chitosan: competition with organic substances. Environ. Technol.,20 (1 999) pp. 5 15-521. 5. Volesky B., Advances in biosorption of metals: selection of biomass types. FEMS Microbiol. Rev., 14 (1994) pp. 291-302. 6. Texier A. C., An&& Y. and Le Cloirec P., Selective biosorption of Lanthanide (La, Eu, Yb) ions by Pseudomonas aeruginosa. Environ. Sci. Technol. 33 (1999) pp. 489495. 7. Solari P., Zouboulis A. I., Matis K. A. and Stalidis G. A. Removal of toxic metals by biosorption onto nonliving sewage sludge, Sep. Sci. Technol., 31 (1996) pp. 10751092. 8. Ho Y. S. and McKay G., The sorption of lead(l1) ions on peat, Wat. Rex, 33 (1999) pp. 578-584. 9. Dronnet V. M., Renard C. M. G. C., Axelos M. A. V. and Thibault J.-F., Binding of divalent metal cations by sugar-beet pulp, Carbolydr. Polym., 37 (1997) pp. 73-82. 10. Gerente C., Couspel du Mesnil P., Andres Y., Thibault J.-F. and Le Cloirec P., Removal of metal ions fiom aqueous solution on low cost natural polyssacharides: sorption mechanism approach, React. Funct. Polym., 46 (2000) pp. 135-144 . 11. Reddad Z., Gerente C., Andres Y. et Le Cloirec P., Ni(I1) and Cu(I1) binding properties of native and modified sugar beet pulp, Carbohydrate Polymers, 49 (2002) pp. 23-3 1. 12. Crittenden B., Thomas WJ., Adsorption technology and design (1998), ButterworthHeinemann, Boston 13. Economy J., Lin R.Y., Adsorption characteristics of activated carbon fibers, Applied Polym. Symposium, 29 ( 1976) pp. 199-21 1. 14. Faur-Brasquet C., Metivier-Pignon H., Le Cloirec P., Activated carbon cloths in water and wastewater treatments, Res. A h . in Water Res., 2 (2002) pp. I - 19. 15. Faur-Brasquet C, Reddad Z, Kadirvelu K, Le Cloirec P, Modelling the adsorption of metal ions (Cu2',Ni2',Pb2+) onto activated carbon cloths using surface complexation models, Applied Surface Science, 196 (2002) pp. 356-365. 16. Reddad Z., GBrente C., Andres Y. and Le Cloirec P., Adsorption of several metal ions onto a low-cost biosorbent : kinetic and equilibrium studies, Environ. Sci. h Technol., 36 (2002) pp. 2067-2073. 17. Smith R. M. and Martell A. E. Critical Stability Constants (Plenum Press, New York, 1989). 18. Makridou C., Cromer-Morin M. and Schatff J.-P., Complexation de quelques ions metalliques par les acides galacturonique et glucuronique, Bull. Soc. Chim. Fr., (1977) pp. 59-63.

198

ADSORPTION CHARACTERISTICS OF PROTEIN-BASED LIGAND FOR HEAVY METALS MASAAKI TERASHIMA, NORIYUKI OKA, TAKAMASA SEI, KAZUYA SHIBATA, AND HIROYUKI YOSHIDA Department of Chemical Engineering, Graduate School of Engineering, Osaka Preficture Universi& 1-1, Gakuen-cho.S a k i CiQ, JAPAN

E-mail: terasimaachemeng. osakfu-u.ac.jp A fusion protein was engineered from maltose binding protein (pmal) and human metallothionein (MT). The recombinant protein (pmal-MT) expressed in E. coli was purified, and immobilized on ChitopearlTMresin. As expected from a tertiary structure of metallothionein, the prnal-MT ligand adsorbed 12.1 cadmium molecules per one molecule of the ligand at pH 5.2. We have found that the prnal-MT ligand also bound 26.6 gallium molecules per one molecule of the ligand at pH 6.5. Adsorption isotherms for the both ions were correlated by Langmuir-type equation. Two types of binding sites have been elucidated based on HSAB (hard and soft acid and base) theory: gallium ion specifically binds to amino acid residues containing oxygen and nitrogen atoms, while cadmium ion binds to specific binding sites formed by multiple cysteine residues. The pmal-MT protein bound these metals in the concentration range of 0.2 - 1.O mM, and the bound metal ions could be eluted under relatively mild condition (pH 2.0). The pmal-MT ChitopearlTMresin was stable and could be used repeatedly without loss of binding activity. Thus, this new protein-based ligand would be useful for recovery of toxic heavy metals and/or valuable metal ions from various aqueous solutions.

Introduction

Recently, the recovery and reuse of valuable metal ions such as rare earth metals, from process waste water of electronic industries and waste electronic devices, is strongly desired for saving precious resources and for achieving sustainable development. While synthetic ligands or chelators are widely studied, biosorbents prepared from biomass of bacteria, fungi, and algae have several advantages over synthetic chemical ligands. The biosorbents, for example, show high selectivity to various ions depending on their tertiary structures, and require only relatively mild conditions for adsorption and desorption [ 11. Peptides and proteins could be efficient metal binding ligands, because they have the functional groups for metal binding in their amino acid residues, and they can be produced at low cost by recombinant technologies. While many peptides and proteins are known to work as metal transport proteins in biological systems, metallothioneins (cysteine rich proteins with molecular weight of ca. 7 kDa) have attracted researchers’ attention for decades because they bind heavy metals in vivo [2]. The metallothioneins are considered to be involved in detoxication and metabolism of heavy metals. In this work, a fusion protein has been engineered from maltose binding protein (pmal) and human metallothionein (MT). The fusion protein (pmal-MT) has been expressed in E. coli, and purified with an amylose column. The purified fusion protein was immobilized on a solid matrix, and its characteristics as metal binding ligand have been studied. We have found that the pmal-MT ligand efficiently binds gallium ion, one of the valuable rare metals desired to be recovered from aqueous solution [3]. Different binding mechanisms for two metal ions have been elucidated based on HSAB (hard and soft acids and bases) theory [4].

199

Methods Preparation of p-ma1 MT protein, and immobilization of the gmal MT protein on ChitopearlTMresin were described in detail in the previous work [3]. Chitopearlm resin inmobilizing p-ma1 MT protein (pmal-MT ChitopearlTM),ChitopearlTMimmobilizing p-ma1 protein (pmal ChitopearlTM),and ChitopearlTMresins have been prepared. The latter two resins are prepared as negative controls. The ChitopearlTMresin was packed in a glass column (inner diameter 1. 4 cm, bed height 4.3 cm). The column was first equilibrated with a 20 mM MES buffer containing 20 mM NaCl and 10 mM 2-mercaptoethanol. Then 60 ml of the MES buffer containing metal ion was applied at the flow rate of 0.5 ml/min. Adsorption capabilities of the ligands were examined for cadmium, gallium, cupric, zinc, or nickel ion. Afier the column was washed with MES buffer, the adsorbed metal ion was eluted with the MES buffer (pH of which was adjusted to pH 2.0). In order to examine effects of pH on the adsorption, pH of the MES buffer was varied from pH 5 to pH 9. The eluted solution was collected as several fractions of 10 ml each, and the metal concentration of each fraction was determined with atomic adsorption analysis (SAS 7500A, Seiko Instruments, Japan). Total amount of the eluted metal ion was defmed as the adsorbed metal ion on the resin. The total amount of the adsorbed metal ion was divided by the total amount of immobilized protein to calculate the number of metal molecules bound to one mole of the protein. The adsorption experiments were carried out multiple times, and the maximum experimental error was 25%.

Results Amounts of the protein immobilized on the ChitopearlTMresin were 3.55 (mg/g-wet resin) for the pmal and 1.51 (mg/g-wet resin) for the pmal-MT. The optimal pH for cadmium binding was pH 5.2 (data not shown). Figure 1 shows an adsorption isotherm at 298 K for cadmium adsorption on the pmal-MT ChitopearlTMresin at pH 5.2. Neither the pmal ChitopearlTMresin nor the ChitopearlTMresin adsorbed cadmium ion under the employed experimental condition. These results clearly show that cadmium ion binds to the metallothionein moiety of the pmal-MT ligand. The adsorption equilibrium was correlated by a Langumuir-type equation. The equilibrium constant K, and adsorption capacity for cadmium binding Q were 15.74 [mM-'] and 3.76 x 1 0 ' [mol/g-wet resin], respectively. The maximum amount of adsorbed cadmium ion per metallothionein molecule is 12.1 (mol cadmium/mol metallothionein), which is relatively close to a theoretical value 7 confirmed by NMR [ 5 ] . These results strongly suggest that the methallothionein moiety of the h i o n protein bind cadmium as it works in vivo. Figure 2 shows effect of NaCl concentration in the metal solution applied to the column on the cadmium adsorption. The amount of adsorbed cadmium ion drastically decreased at NaCl concentration about 45 mM, suggesting that the tertiary structure of the metal binding site probably change at this salt concentration, and thus the ligand lose its binding ability for cadmium ion. The binding ability, however, was easily recovered by washing the column with the MES buffer (pH5.2).

200

0.2

a4

0.6

QI)

1

0

Cadmium eweem. fmMl

50

100

150

m

NaCI ancen. [mMl

Figure 1 Adsorption isotherm for cadmium ion

Figure 2 Effect of NaCl on cadmium adsorption

We have found in this work that the pmal-MT ligand also binds a valuable rare metal gallium ion. Adsorption characteristics of the pmal-MT for gallium ion, however, were different from those for cadmium ion. An optimal pH for gallium ion adsorption was pH 6.5, which was different fiom that for cadmium ion (pH 5.2). This result suggests that .the conformation of metallothionein suitable for binding of cadmium ion is not preferable to gallium ion, and vise versa. An adsorption isotherm for gallium ion at 298 K is shown in Figure 3. Unlike the case of cadmium ion, the pmal ChitopearlTMresin and the ChitopearlTMresin adsorbed gallium ion about 4.0 x lom7(mol/g-wet resin) at pH 6.5. Since these results suggest that gallium ion bind to the proteins non-specifically, non-specific binding of gallium ion to the proteins was examined by comparing the adsorption of gallium ion on BSA. The numbers of gallium ions adsorbed on BSA (mol gallium iodmol BSA) were 0.0335 at pH 5.2 and 0.114 at pH 6.5, while those of cadmium ions (mol cadmium iodmod BSA) were 0.392 at pH 5.2 and 0.293 at pH 6.5. These results showed that BSA did not adsorb cadmium ion and gallium ion under the employed experimental condition, and suggest that both ions do not bind to proteins by simple ion-exchange effects. In order to evaluate the adsorption on metallothionein moiety, the amount of gallium ion adsorbed on the base matrix was subtracted from the resin. The corrected result is shown in experimental data for the pmal-MT ChitopearlTM Figure 4, and adsorption equilibrium is correlated by Langmuir-type equation. The equilibrium constant K, and adsorption capacity for cadmium binding Q were 5.2 1 [mM'] and 9.09 x [moVg-wet resin], respectively. It should be noted that the maximum amount of adsorbed gallium ion per metallothionein molecule reached 26.6 (mol gallium iodmol metallothionein), which is much higher than that of cadmium ion. Effect of NaCl concentration in the metal solution on the gallium adsorption is shown in Figure 5. The drastic decrease in metal binding at NaCl45 mM, which was seen for cadmium ion, was not observed in the case of gallium ion. These results also strongly suggest that the binding mechanism of gallium ion is different from that of cadmium ion. The amounts of adsorbed metal ion on the pmal-MT ChitopearlTMfor various ions are summarized in Table 1. These results show that the metal binding on metallothionein is highly selective.

201

I,

0

0.2

0

0.4

0.6

OS

.

I

0

0.4

0.2

0.6

M

1

Gallium C O I I C ~ . [mMl

Gallium cancea. [mMl

Figure 3 Adsorption of gallium ion

Figure 4 Adsorption isotherm for gallium ion Table 1 Amount of adsorbed metal ion for various ions Metals Cia Ni Zn

Cd 0

50

100

150

Adsorbed amount” 26 0.3 0.2 12.1

Ionic radius [x 1 P m] 62 69 74 91

Hard Little hard Little soil

Soil

200

NaCI canem. [mMl

Figure 5 Effect of NaCl on gallium adsorption

Discussion We have found that metallothionein, which selectively binds cadmium ion in vivo, binds gallium ion. The number of the gallium ion bound to one molecule of metallothionein, 26.6, was about twice as large as that of cadmium ion. The results of adsorption experiments to BSA, and the adsorption of various metal ions show that metallothionein selectively binds gallium ion. The specificity is very high because other metals which as similar ionic radius did not bound to the metallothinein as shown in Table 1.

A binding mechanism of metallothionein for cadmium ion and gallium ion is elucidated as follows. According to HSAB (hard and soft acid and base) theory, metal ions are classified into hard acid (ion) and soft acid (ion). Cadmium ion is classified as soft acid that has strong binding affinity to S and P atoms. On the other hand, gallium ion classified as hard acid that has strong binding affinity to N and 0 atoms. Therefore, cadmium ions bind to S atom of cysteine residues of the metallothionein as indicated by NMR. The conformation of binding sites formed by multiple cysteine residues should be

202

strongly affected by the change in tertiary structure of the metallothinein. Drastic change of the amount of adsorbed cadmium ion by the increase of NaCl concentration (Figure 2) suggests that the large conformation change of the cadmium binding sites of metallothionein. On the other hand, gallium ion should bind to N and 0 atoms of amino acid residues of the metallothionein molecule. Human methallothionein posses 3 Asp, 1 Glu, 7 Lys, 8 Ser, 2 Thr,1 Gln,1 Asn residues per molecule. Some of these negatively charged residues are probably located the outer surface of metallothinein might form binding sites. As elucidated fiom the adsorption experiment on BSA, adsorption of gallium ion by simple ion-exchange effect is negligible. The characteristicsof metal ions such as ionic radius and hardness/sobess, and the conformation of the metallothionein probably affect the selectivity of metal adsorption. The understanding of the mutual interactions among those factors would be a key factor in designing the protein-based ligand suitable for a specific metal ion.

References 1.

Gutnick, D. L., and Bach, H., Engineering bacterial biopolymers for the biosorption of heavy metals; new products and novel formulations,Appl Microbiol Biotechnol. 54 (2000) pp. 45 1 460 Romero-Isart, N. and Vasak, M., Advances in the structure and chemistry of metallothioneins,J. Inorg. Biochem. 88 (2002) pp.388-396 Terashima, M., Oka,N., Sei, T. and Yoshida, H., Adsorption of cadmium ion and gallium ion to immobilized metallothionein fusion protein, Biotechnol. Progress, in press (2002) Pearson, R. G., Hard and soft acids and bases, J. Am. Chem. SOC.85 (1963) pp. 3533-3539 Messerle, B. A., Schaffer, A., Vasak, M., Kagi, J. H., and Wuthrich, K. Three-dimensional structure of human [ 1 13Cd7]metallothionein-2 in solution determined by nuclear magnetic resonance spectroscopy, J Mol Biol. 214 (1990) pp. 765-779

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2. 3.

4. 5.

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PREPARATIVE CHROMATOGRAPHY AT SUPERCRITICAL CONDITIONS ARVIND RAJENDRAN, MARC0 MAZZOTTI AND MASSIMO MORBIDELLI ETH Swiss Federal Institute of Technology, CH 8092 Zurich, Switzerland E-mail: [email protected] The basic issues in packed bed supercritical fluid chromatography (SFC) are: i) characterizing adsorption equilibrium; ii) charracterizing mass transfer dynamics; iii) predicting column dynamics and iv) the effect of polar modifiers. The first three issues have been addressed through experiments. Experiments have been performed on a preparative column with significantpressure drop using pure COz as an eluent and phenanthrene dissolved in toluene as solute. Parameters relating to retention behaviour and mass transfer characteristics were measured. The observations are reported and the deviations with respect to HPLC and GC behaviour are highlighted and discussed.

1

Introduction

Preparative chromatography is a proven technology for the separation of specialty chemicals mainly in food and pharmaceutical industries, particularly the enantioseparation of chiral compounds on chiral stationary phases. The potential of preparative chromatographic systems were further increased by the development of continuous chromatographic processes like the simulated moving bed (SMB) process. Compared to the batch column chromatography, the SMB process offers better performance in terms of productivity and solvent consumption [2]. Supercritical fluids poses properties that lie in between those of liquids and gases. These properties, which are functions of their density, can be tuned according to the process requirements. Supercritical fluid chromatography (SFC) is a proven analytical tool which allows one to exploit the advantages of GC (faster and better separation) for non-volatile substances which are usually analysed using HPLC. The solvent power of the supercritical fluid is a function of its density. Hence, when a solute dissolved in a supercritical fluid is in contact with a stationary phase, i.e. a solid adsorbent, the affinity of the solute to the stationary phase, which under linear conditions is characterized by the Henry’s constant Hi, depends on the density of the supercritical fluid. At higher densities, the supercritical fluid has a higher solvent power and hence the solute has a lower Hi value and vice versa. This property allows for the establishment of a solvent gradient in a SMB unit, thereby enhancing separation performance. In fact it was shown that the supercritical fluid simulated moving bed (SF-SMB) process operated under the pressure gradient mode (where the pressure in each section is regulated by a back pressure regulator) offers a productivity improvement by a factor of 3 compared to the isocratic mode (without any pressure regulation) [l]. In this context, it is therefore important to study and understand the fundamentals of SFC under conditions where the pressure drop along the column is significant, because these are the conditions in preparative applications, particularly in SF-SMB. The pressure gradient in the column causes substantial density gradient, which in turn leads to a gradient in velocity. Since the mass transfer properties are a function of density, they change at every point along the column. Moreover, unlike HPLC and GC systems, where the Henry’s coefficient, Hi, does not depend on the pressure, in SFC systems the retention characteristics are a function of pressure (density) [l]. Hence, the dynamics of an injected pulse is governed by several factors. This establishes the need to develop a

204

model, which could be used to simulate the pulse response of a packed column under supercritical conditions with substantial pressure drop. A dynamic model is especially useful, since it can be easily extended to simulate the SF-SMB process. In the present study, experiments have been performed on a preparative column using pure COz as the mobile phase. Experiments have been performed at four different back pressure levels. For each back pressure setting, runs were performed both at low flow rates, i.e. where the pressure drop was negligible and at high flow rates. The main process aspects, i.e. pressure drop, mass transfer and retention have been experimentally evaluated and analysed.

2

Experimental setup and procedure

Carbon dioxide (99.995% pure, obtained from PanGas, Switzerland) was used as the mobile phase. Phenathrene @urity>97%) dissolved in Toluene (purity>99.7%, both

100

80

- +- BP =I80 bar

+-BP =210 bar

n L

lu

a LI

60-

E!

0

a

2

40-

v) v)

2

n 20

-

0 I 0 I

I

I

I

I

I

3 4x1Om2 Mass flow [g/s] 2

Figure 1. Pressure drop characteristicsof the SFC column at different back pressure levels.

obtained fiom Fluka, Switzerland) was used as a solute. A Lichrospher 100 RP-18 column (Merck, Darmstadt) 125x4 mm, with an average particle size of 5pm, was used for the experiments. The experimental set-up consists of a syringe pump (Isco 260D) capable of producing a continuous flow of COz which flows through an injection valve (Valco

205

C14W) that has an internal loop volume of 60 nL. The mobile phase then enters the column at the end of which is a UV detector (Jasco W-1570).The pressure in the system is determined by a back pressure regulator (Jasco BP1580-81) which is located downstream of the U V detector. The column and the injection valve are housed in a temperature controlled water bath. Upstream and downstream pressures are measured using pressure transducers (Trafag 8891). The experiments are performed by setting the back pressure regulator at the desired level and programming the syringe pump to operate at a given flow rate. The system is then allowed to reach a steady state. Once the pressure profile in the system is established, a mixture of phenanthrene in toluene (2% w/w) is injected into the column through the injection valve and the data acquisition is started simultaneously. For each setting, the experiment is repeated more than three times to ensure reproducibility. 3

Experimental Results

Both high flow rate and low flow rate experiments were performed at 4 different back pressure levels, namely 130, 150, 180 and 210 bar. Some low flow rate experiments were also perfomed at intermediate back pressure settings. The operating temperature for

0 BP=150 bar A B P 4 8 0 bar V BPe210bar

0

400

200

600

800

(mass flow ratej’[(g/sj’] Figure 2. Retention time against the inverse of mass flow rate at different back pressure levels.

206

all the runs was 64°C.The pressures upstream and downstream of the column and the UV signal were measured. From the UV detector readings, the retention time and HETP values were calculated. The HETP was calculated using the formula

N = 5.45(t,/~)~ HETP = L/N where t R is the retention time, w the width of the peak at half peak height, N the number of plates and L the length of the column. These equations have been used to describe the HETP behaviour though they have the limitation that they assume the velocity to be constant along the column. The pressure drop characteristicsfor the four different sets of experiments are plotted against the mass flow-rate in Fig. 1. The mass flow rate is used as the independent variable since it is the only variable which remains constant throughout the column. At higher flow rates there is a deviation from linearity. For a given mass flow rate, the runs at a low back pressure setting show a larger pressure drop. The measured retention times are plotted against the inverse of mass flow rate in Fig. 2. It can be seen that the points corresponding to a particular back pressure setting fall on a straight line and the slope of the line depends on the back pressure setting. The line corresponding to a lower back pressure setting has a larger slope than the one at a higher back pressure setting. This is in contrast to HPLC where for a given temperature, under linear conditions, all points, irrespective of the back pressure setting, will fall on one straight line whose slope is proportional to Hi. This shows that in the case of SFC, Hi,is a

6o 50 -

-

40 -

E3.

Y

n F w I

3020 -

r

10 -

0

0

10

20

30

Mass Flow [glsJ Figure 3.HETP values at different back pressure levels.

207

40

0 50x10"

function of density. Further at high flow rates, there is a velocity gradient and a density gradient in the system and these affect the retention time. Hence, the observed, or the apparent, Hi is a combined effect of these two gradients. The mass transfer characteristics, which are described by the HETP, are shown in Fig. 3, where the HETP is plotted against the mass flow rates for different back pressure levels. In general, the HETP curve has the typical shape of the well-known van Deemter plot. Though plotting the HETP in this fashion (i.e. grouping runs with the same back pressure setting) does not offer the provision to extract the mass transfer parameters 6om the van Deemter equation, it nevertheless offers a qualitative picture of the mass transfer kinetics. In general, at low flow rates, the H E P values fall with increasing flow rates, reach a minimum, and gradually rise. Let us focus on the part of the curve after the respective minima. The curve is flat for the runs with a back pressure of 130 bar compared to those corresponding to 150 and 180 bar. The slopes of the later part of the curves increase with increasing back pressure. It can also be seen that the curves show cross over at larger flow rates. For the curve corresponding to 210 bar, it was however, not possible to perform high flow rate experiments as the upstream pressures rose beyond the maximum allowable pressure of the syringe pump. 4

Conclusion

Experiments have been performed on a preparative SFC system using pure CO2 as the mobile phase under significant pressure drop. The retention times, pressure drop characteristics and the mass transfer behaviour were studied. The trends observed differ 6om the behaviour of HPLC systems. These trends also emphasize the complexity involved in analyzing the data for SFC measurements, which imply in turn greater complexity of the SFC model as compared to standard liquid chromatography model. Reference 1. Denet, F., Hauck, W., Nicoud, R. M., Di Giovanni, O., Mazzotti, M., Jaubert, J. N.

and Morbidelli, M., Enantioseparation through supercritical fluid simulated moving bed (SF-SMB) chromatography, Ind. Eng. Chem. Res. 40 (2001) pp. 4603-4609. 2. Juza, M., Mauotti, M. and Morbidelli, M., Simulated moving-bed chromatography and its application to chirotechnology, Tren& Biorechnof.18 (2000) pp. 108- 1 18.

208

ADSORPTIVE SEPARATIONOF OLIGOSACCHARIDES INFLUENCE OF CROSSLINKING OF CATION EXCHANGE RESlNS JOHANA. VENTE'.~,HANS BOSCH', ANDRE B. DE HAAN', PAUL J.T. BUSSMANN~ I

Separation Technology Group, Faculty of Chemical Technology, Universiw of Twente, P. 0.Box 21 7, 7500 AE Enschede, The Netherlands TNO-MEP, P.O. Box 342, 7300A H Apeldoorn, The Netherlands The influence of crosslinking on the sorption propetties of poly(styrene-co-divinylbenzene) (PSDVB) strong acid ion exchanger in K ' or Ca2+form was studied. Isotherms of sugars were determined for resin containing 2, 4 and 8% DVB. Sorption was strongly influenced by the degree of crosslinking. At increasing crosslinking the amount of sorption by (non-selective) distribution decreases and complexation is the dominant sorption mechanism. Resin with 2% DVB was hardly selective, but resin with 8% DVB was very selective. A selectivity of 7.0 for fructose/sucrose and Ca" loaded resin was obtained due to a combination of size exclusion and complexation. However, sorption capacity decreared with increasing crosslinking. Chromatograms of fructo- and galactooligo-saccharides (0s)showed that the separation of the monosaccharidesfrom the 0s was best for 8% DVB but the fractionationof the 0s was best for 2%DVB.

1

Iatroductioa

Oligosaccharides (0s)are applied as functional food and feed ingredients. Fructo-OS (FOS) are made by partial hydrolysis of inulin and galacto-0s (GOS) by transgalactosylation of lactose. They are produced as mixtures of different types of carbohydrates and further separation is required for most applications. Examples of desired separations are the removal of monosaccharides in order to decrease the amount of calories and the sweet taste. A technique to perform the desired separation is chromatography. The adsorbent used in a chromatographic separation has a large influence on the performance of the separation process. Fructose/glucose is separated on process scale with Ca" loaded crosslinked poly(styrene-co-divinylbenzene) (PS-DVB) strong acid ion exchange resins. To investigate the influence of resin properties on the separation, we selected PS-DVB resins. There are two chemical properties of the PS-DVB resin material to optimise: (1) the type of cation and (2) the degree of crosslinking. Earlier work focussed on the choice of the cation. K ' was selected as the cation for GOS separation and Ca" for FOS separation [1,21. The work presented here focussed on the degree of crosslinking. The degree of crosslinking of the PS-DVB resin can be varied by the amount of DVB used during the synthesis of the resin. Crosslinking decreases the elasticity of the resin and thereby the swelling and equilibrium water content [3]. Saccharides can be sorbed into the resin, either by distribution of the saccharides between the liquid inside the resin and the liquid outside the resin or complexation [4]. To be able to distribute, the saccharide molecules have to be smaller than the size of the interstices of the resin. The size of hydrated monosaccharides is in the same order of magnitude as the size of the pore diameter of a resin [5]. The separation of monosaccharides from 0 s may be improved by choosing a DVB content such that disaccharides and 0s are excluded, but the monosaccharides are still able to be sorbed by the resin. Higher water content increases the sorption of the saccharides by distribution. The amount of cations per volume unit water increases with increasing DVB content. Therefore, complexation driven sorption

209

might improve relative to the sorption by distribution with increasing DVB content. The DVB content not only influences the equilibrium sorption properties of the resin, but also the sorption kinetics and mechanical properties such as elasticity and attrition resistance. Published data illustrate the influence of the degree of crosslinking of PS-DVB resins on the separation of sugar alcohols [6], monosaccharides [7],and malto-OS [8]. Resins with DVB contents between 3% and 8% are suitable for the separation of two hexoses [7]. For sugar alcohols the optimal DVB content is 7% DVB for Ca2' form resins [6]. The separation of glucose from maltose improves with increasing DVB content. For malto-OS and DVB contents between 2% and 6%, the separation improves with increasing DVB, but is completely lost for a DVB content of 8% due to size exclusion of the larger 0s by the resin. Resin with 6% DVB was optimal for the separation of malto0s with a degree of polymerisation of 1 to 7.It can be concluded from these literature data that crosslinking has a large effect on the sorption and separation properties of PSDVB resin and that the effects are dependent on the type of sugar. However, no information could be found on the separation of FOS or GOS. The goal of the work presented in this paper was to determine the influence of the crosslinking on sorption properties of PS-DVB cation exchange resins for FOS and GOS separation. Improved understanding of the interactions of saccharides with sorbents may lead to the development of highly selective sorbents for low cost separations. 2

Materials and methods

Gel type strong acid PS-DVB cation exchange resin, Dowex 50W (particle diameter 3874 pm), was used with different DVB contents. The resin was ion exchanged into the desired cation form. Table 1 summarizes the properties of the columns (internal diameter 0.160 m) packed with resin. Porosity was calculated from the retention time of Dextran T2000 (Pharmacia, Sweden). Dry substance content was measured by air-drying of filtrated resin at 105°C until constant weight. Isotherms and chromatograms were measured at 60°C in a column set-up, as was described earlier [2]. Glucose (0-300 gll), galactose (0-10 fructose (0-300 sucrose (0-300 lactose (0-200 and mixtures of FOS (Raftilose@60, Orafti, Belgium), containing fructose, sucrose and 0s and GOS (Elixor@259, Borculo Domo Food Ingredients, The Netherlands), containing glucose, galactose, lactose and 0s were used. The isotherms were correlated with q=uz+bc, with q the saccharide concentration in the resin, a and b fitparamters and c the concentration in the liquid phase. The chromatograms were plotted as a fimction of dimensionlesstime, defined as: (r-r,mcer)/r,mce,., with rrmer the retention time of dextran and r the time. The selectivity of component i relative to component j , was calculated as:

a),

a),

a),

a)

(qh)4@$ Table 1: Properties of columns packed with strong acid PS-DVl3 cation exchange resin (i.e.=ionexchange).

210

3 3. I

Results and discussion Isotherms of saccharides on cation exchange resins

Fig. 1 shows the single sugar isotherms for K+ and Ca2' loaded resins with different degrees of crosslinking. High sugar concentrations were applied, because concentrated sugar solutions are used in commercial processes. The isotherm data were correlated with the equations in Table 2. A stronger crosslinked resin, resulted in less sorption of sugars. This result can be explained by the decreased elasticity and swelling of the resin with an increased crosslinking. This results in lower water content of the resin (see also Table 1) and decreasing sorption of saccharides by distribution. Moreover, crosslinking had a larger effect on sorption than the type of cation [I, 21. For K ' loaded resin, the observed order of adsorption was galactose2fructose>glucose>lactose>sucrose. The sorption order of the sugars was for Ca" loaded resin the same as for K" loaded resin, except that fructose2galactose. Fructose forms a complex with a Cat+ ion [9], which explains that fructose sorbed better than glucose or galactose in highly crosslinked Ca" loaded resins. For low crosslinking however, the monosaccharides sorbed almost to the same extent. The explanation for the loss of selectivity may be that at low crosslinking the resin contains more water and the amount of cations per volume unit resin is lower. Consequently, more distribution may occur and the amount of fructose sorbed due to complexation may decrease relative to the amount sorbed by distribution. In that case, the sorption of non-complexing monosaccharides such as glucose or galactose is favoured relative to fructose, as was observed experimentally. At equal degree of crosslinking, the disacharides lactose and sucrose sorbed less than the monosaccharides due to their larger size and hence the restricted accessibility to the resin interstices. The sorption of the disaccharides decreased strongly with increasing crosslinking. For 8% DVB the sorption was almost vanished. At increasing crosslinking less water is available in the resin for distribution. In addition, part of the water is hydration water of the cations [lo]. Apparently, for 8% DVB almost no water was available for distribution of sugar. The effect of crosslinking was stronger for sucrose than for lactose. Lactose (0-PD-galactopyranosyl-(1,4)-D-gIucopyranose)and sucrose (0-a-Dglucopyranosyl-(1,2)-PD-fructofuranoside)differ in the constituent monosaccharides and the bond between the monosaccharides. Although sucrose contains a fructose unit, it is not able to complex with Ca" in the same way as the monosaccharide fructose, because the glycosidic bond of sucrose occupies the complexation site of the fructose unit. However, the differences in sorption may be the result of differences in the structure of the molecules. Lactose is able to convert via the open chain form to another anomeric form, whereas sucrose does not. Sucrose exhibits two interresidue intermolecular hydrogen bonds in aqueous solution [I 13. These structural differences may result in a larger effective size of sucrose compared to lactose, hence increased size exclusion and less sorption. Table 2: Isotherm correlations for 2,4 and 8% crosslinking of PS-DVBresin loaded with K' or Ca*' at 60°C.

211

0

1w

200

300

100

0

m

300

400

Fig. 1: Isotherms of glucosc for 2%,4%and 8% DVB on PSDVB rcsin with K ' (left) or Caz+(right) at 60°C

3.2 Selectivity of cation exchange resins Table 3 lists selectivities for sugar concentrations at 200 g/l, which were calculated from the single sugar isotherms in Table 2. Besides the fiuctose/glucoseselectivity, Table 3 includes the selectivities for glucose/lactose and fructose/sucrose, which are indicative for the separation of glucose from GOS and fructose from FOS, respectively. All selectivities increased with increasing crosslinking. The increase was already explained for fructose/ glucose in the previous section. The selectivity of glucose/lactose increased with increasing crosslinking due to the larger size of lactose compared to glucose. The selectivity of fructose/sucrosereflects a combination of complexation of fructose with Ca2' cations and size exclusion of sucrose and resulted therefore in the highest selectivity. Selectivities as high as up to 2.5 for glucosellactoseand up to 7.0 for fhctoselsucrose were obtained. Table 3: Selectivityof resin with P?, 4%and 8% crosslinking at an individual sugar concentration of 200 gll.

1.1 1.2

3.3

1.3 1.7

1.5 3.8

1.1 1.4

1.3 2.4

2.5 7.0

Chromatograms

Fig. 2 presents chromatograms of GOS on K ' resin and FOS on Ca" resin. Due to the use of a short column and a relatively large resin particle diameter, baseline separation was not achieved. However, qualitative effects of different DVB content of resins could be obtained from the chromatograms. It appeared that with decreasing DVB content of the resin, saccharides eluted over a longer period and the peaks became wider. The components in the mixtures that were not retained, eluted first and with increasing time, more retained components eluted. Glucose was the last eluting peak in the chromatogram of GOS and fructose the last peak for FOS. From injection of pure components it appeared that the retention times of the saccharides, which were determined only by equilibrium effects, were in agreement with the measured isotherms [I]. Fig. 2 clearly shows that, due to increased sorption capacity, the elution times increased with decreasing crossliiking. Separation of fructose was in particular good for 8% DVB, as was expected from the high selectivity of fructose/sucrose. Furthermore, Fig. 2 shows that 4% and 2% DVB improved the hctionation of OS, which resulted in more peaks on the chromatogram. Although 0s eluted over a longer period, the peaks on the chromatogram

212

for 2% DVB were wider than the peaks on the chromatogram for 4% DVB. Especially in the case of GOS this resulted in poor separation. The long elution times lead to excessive product dilution, which is for large-scale applications an economical disadvantage.

GOS m 4% DVB nrinmK+fan

0.0

0.5 1.0 1.5 20 25 dk*nsM...nktbnk.(J

0.0

0.5

1.0

-

1.5

20

-13

dlnm-

25

1 0.0

GOS m 2% DVB rcsin m K' fam

0.5

1.0 1.5 20 dkmnimhu-mC)

25

:

Fig.2: Chromatograms of G O S (K' resins) and FOS (Ca resins) with different DVB content (8?4 4%, and 2% DVB), injection volume 1 ml, concentration 100 g synrp/l, flow rate 0.545 ml/min, temperature 60°C. *+

Conclusions

4

The isotherms of sugars on PS-DVB resin showed that increased crosslinking resulted in more selective sorption of sugars at the cost of sorption capacity. The effect of crosslinking on capacity and selectivity is larger than the effect of cation type [2]. Nonselective sorption of sugars decreases with increasing crosslinking, due to a decrease in available space in the resin for distribution. Also, with increasing crosslinking, the relative contribution of complexation to the sorbed mount increases and as a result the selectivity increases. The chromatogramsof FOS and GOS showed that a DVB content of 8% is better than 4% DVB for monosaccharide removal. A PS-DVB resin with 8% DVB is best for the removal of fructose from FOS.However, if the goal is to separate the FOS in several fractions with different degree of crosslinking, then it is better to use 4% DVB. For separations including molecules with different size, it is recommended to select first the optimal degree of crosslinkingbefore optimising the cation type.

References 11 J. A. Vente, et al., to be submitted, 2002. 2!I J. A. Vente. et al. in: AIChEAnnual Meeting - Symposium on industria/App[icationsof I4,dkorption and Ion Exchange. 2001. Reno (NV), USA: AIChE, p. 786. 3 J. Tiihonen, et al., Journal of Applied Polymer Science, 2001.82: p. 1256. 4 S. Adachi, et al., Bioscience Biotechnology and Biochemistry, 1997.61( 10): p. 1626. 5 M.Saska et al., Journal of Chromatography, 1992.590: p. 147. 6 H. Caruel, et al., Journal of Chromatography, 1992.594: p. 125. 7 S. Adachi, et al., Journal of Chemical Engineering of Japan, 1999.32(5): p. 678. a S. Adachi, et al., Agricultural and Biological Chemistry, 1989.53(12): p. 3193. J R. W. Goulding, Journal of Chromatography, 1975.103: p- 229. 1 9 R. S. D. Toteja et d.,Langmuir, 1997. 13(11): p. 2980. 1 I] S. Immel and F. W. Lichtenthaler, Liebigs Annalen, 1995(1 I): p. 1925. 1

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IDENTIFICATION AND PREDICTIVE CONTROL OF A SIMULATED MOVING BED PROCESS IN-HYOUP SONG AND HYUN-KU W E E School of Chemical Engineering h Institute of Chemical Processes Seoul National Universiw, Kwanak-ku, Seoul, 151 742, Korea E-mail: [email protected]

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MARC0 MAZZOTTI Institute of Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland E-mail: Mazzotti@ivuk mavt.ethz.ch Identification technique is applied to a simulated moving bed (SMB)process for chiral separation of enantiomers of TrOger’s base and an advanced predictive controller is designed on the basis of the identified model. To obtain the identified model, an artificial continuous system is constructed by keeping the discrete events such as the switching time and the number of columns to be switched constant. The SMB process is identified as an inputloutput data-based prediction model, which is then used to design a linear predictive controller. In this study the internal flow rate ratios are chosen as the input variables whereas the pair of product purities are taken as the controlled outputs. It is demonstrated by simulation studies that the designed predictive controller performs satisfactorily for the disturbance rejection as well as for the setpoint tracking in the SMB process.

1

Introduction

In the chemical industry, chromatographic separation process is an emerging technology for the separation of pharmaceutical products, food and fine chemicals. To improve the economic viability, a continuous countercurrent operation is often desirable but the actual movement of the solid leads to a serious operating problem. Therefore, the simulated moving bed (SMB) process is an interesting alternative option. In recent years, several researchers have applied some advanced control strategies to simulated moving bed units to treat the dynamic operation of SMB processes, ranging from the nonlinear control strategies such as the input-output linearizing control [4] to the repetitive model predictive control [S]. One of the shortcomings of these control strategies is that they use the first principles model. Although these controllers may be effective to treat various control problems, it may be difficult to implement them to actual SMB processes because of their heavy computational load and complex design procedure. To overcome these drawbacks of the controller based on first principles model, various identification techniques were applied. Neural network based model predictive control was used for the dynamic control of SMB unit [ 11, however, its implementation to actual process can be very difficult because of the complexity of identified neural net model. In this study we identify an SMB process using the subspace identification method. The well-known inputloutput data-based prediction model is also used to obtain a prediction equation which is indispensable for the design of a predictive controller. The discrete variables such as the switching time are kept constant to construct the artificial continuous input-output mapping. With the proposed predictive controller we perform simulation studies for the control of the SMB process and demonstrate that the controller performs quite satisfactorilyfor both the disturbance rejection and the setpoint tracking.

214

2

Brief description for SMB process

In this study, the SMB process is divided into four sections, each of which consists of 2 columns of chromatographyplaying a specific role in the separation. Ethanol solution of the racemic Troger's base is taken as the feed stream and unsupported microcrystalline cellulose triacetate(CTA) bead is used as the stationary phase. The separation is carried out in the two central sections. For the reference conditions of simulation study, one may refer to the previous work[7]. The principle of operation can be best described with reference to the equivalent true countercurrently moving bed (TCC) configuration. Since the two configurations are equivalent, i.e., they achieve the same separation performance provided the geometric and kinematic conversion rules are fulfilled, the simpler model of the equivalent TCC unit can be used to predict the steady state separation performances of SMB units, in particular, for design purposes. The first principles model of the SMB unit is constructed with reference to the previous works[4,6] and considered to be the actual plant.

3

Subspace identification of SMB process

Figurel. Validation of identified model. In order to solve the first principles model, finite difference method or finite element method can be used but the number of states increases exponentially when these methods are used to solve the problem. Lee et d [ 8 ] used the model reduction technique to reslove the size problem. However, the information on the concentration distribution is scarce and the physical meaning of the reduced state is hard to be interpreted. Therefore, we intend to construct the input/output data mapping. Because the conventional linear identification method cannot be applied to a hybrid SMB process, we construct the artificial continuous inpudoutput mapping by keeping the discrete inputs such as the switching time constant. The averaged concentrations of rich component in raffinate and extract are selected as the output variables while the flow rate ratios in sections 2 and 3 are selected as the input variables. Since these output variables are directly correlated with the product purities, the control of product purities is also accomplished.

215

We adopt the inputloutput data-based prediction model using the subspace identification technique. To find the correlation between the inputs and outputs, we need to obtain the input and output data. On the basis of the triangle theory[6], the optimal feed flow rate ratios at steady state are calculated. Then, the pseudo random binary input signal is generated on the basis of this optimal value. Figure 1 compares the output from the identified model (dot) with that from the first principles model (solid curve). Clearly, we observe that the identified model based on the subspace identification method shows an excellent prediction performance. The variance accounted for (VAF) indices for both outputs are higher than 99%. The detailed identification procedure can be founded in the literature [3,5,9,10].

Predictive control for SMB process

4

The inputloutput data-based predictive controller based on the identified model is designed and applied to a MlMO control problem for the SMB process. We use the input/output data-based prediction model in the MPC algorithm. The QP method is used to obtain the control input u,by minimizing the objective function defined as minJ(u f ) = ( L p

+ L,wp - r f ) T@Lu,u, + L , W ~- r,)+u;Ru,

"f

where w p is defined as OPT u ~with ~ the) past ~ values of inputs up and output yp , and rj denotes the set-point trajectory. Here, Q and R are the weighting matrices for the output and input, respectively and the controller parameters L,, and L," are determined during the identification procedure. It is to be noted that there is no need to explicitly calculate the state estimate or the state space model. The complete separation region in the triangle theory is considered as the input constraint, and the output constraint comes from the requirement for the purity. One may refer to our previous work[9] for the details of the controller design procedure. Here we shall treat two typical control problems of practical interest; one is the disturbance rejection and the other is the setpoint tracking. First, we assume that the feed pump stops during 40 minutes after 40th switching. These may be considered as unmeasured disturbances introduced to the process. Figure 2 shows that the controller successfully rejects the unmeasured distu rbance.

216

B

j

g .r t

B Figure 2. Disturbance rejection performance.

I

Figure 3. Tracking control performance.

In the second case the setpoints for the average concentrations of A at extract and that of B at raffiate are changed simultaneously after 20 switching times as shown in Figure 3. For the control purpose, the prediction and control horizons are set equal to 5 and 2 switching periods, respectively. The weighting matrices are tuned by the trial and error method. Here it is noticed that the control inputs act predictively to bring the control output to their new respective setpoints. It is clearly seen that the control performance is quite satisfactorily.

217

5

Conclusions

An SMB process is identified by using the subspace identification method. The

inputloutput data-based prediction model is used to obtain the prediction model. The identified model exhibits an excellent prediction performance. The inputloutput data-based predictive controller based on the identified model is designed and applied to MIMO control problems for the SMB process under the presence of the input and output constraints. The simulation results demonstrate that the controller proposed in this study shows an excellent control performance not only for the di'sturbance rejection but also for the setpoint tracking.

References 1. Wang, C.; Engell, S.; Hanisch. F.; Triennial World Congress, Barcelona, Spain 2002,1145-1 150. 2. Dunnebier, G.; Fricke, J.; Klatt, Karsten-Ulrich.; Ind. Eng. Chem. Res. 2000,39, 2290-2304. 3. Favoreel, W.;De Moor, B.; Van Overschee, P.; Gevers, M., Proceedings of the American Control Conference 1999,3372-3381. 4. Kloppenbwg, E.; Gills, E.D. Journal ofProcess Control 1999,9,41-50. 5 . K.-Y. Yoo; H.-K. Rhee, AZChE J. 2002,48(9), 1981-1990. 6. Migliorini, G.; Mazzotti, M.; Morbidelli, M., Journal of Chromatography 1998, A, 827, 161-173. 7. Pedefem, M.; Zenoni, G.; Mazzotti, M.; Morbidelli, M., Chem. Eng. Sci., 1999, 54, 3735-3747. 8. Natarajan, S.; J. H. Lee, Computers and Chemical Engineering 2000,24, I 127-1 133. 9. Song, I.-H.; K.-Y. Yoo; H.-K. Rhee; Ind. Eng. Chem. Res. 2001,40,4292-4301. 10. Verhaegen, M.; Dewilde, P. Inr. J. Control 1992,5, 1187-1210.

218

QUICK AND COMPACT OZONATION USING SILICEOUS ZEOLITE Hirotaka Fujita*, Akiyoshi S&da*, Taka0 Fujii* and Jun Inmi** *Institute of Industrial Science, University of Tokyo 4-6-1 Komaba,Meguro-ku, Tokyo, Japan **Nagasaki R&D Center, Mitubishi Heavy Industries, Ltd. 5-717-1 Fukahori-cho. Nagasaki, Japan We developed a novel omnation using high silica mIites as an adsorptive concentrator of ozone and processing organics, resulting in a significant increase in reaction rate. Throgh TCE degradation test, it was found that the ozone reaction toward TCE was significantly increased. Key words: ozone. adsorption, high silica zeolite

Introduction Drinking water resoufrces are inmasingly contmimted with chlorinated pollutants such as hichloroethene (ICE), etc. ozonation is an oxidation process extensively applied to water treatments for eliminating such pollutants. However, single omnation isn't always effective in terms of time required for technical pcess. To enhance the omnation effectiveness, many studies have focused on AOP ( a d v d oxidation pmcesm) such as omnation combined with H&[ Beltran et al., 1998 I, etc, in which OH radicals, a much more reactive specie than omne itself, play a main role. 'Ihe present study performs a new trial aiming at the enhancement of the omne mction,designing a novel oxidationprocess in which the omne reaction rateis sigruficantly increased through the use of an omne &orbent. We found in our pvious work [ Fujita et al, ux)2] that high silica zeolitespcwxseda m n g abiity to adsorb omne. In addition to that,it has been reposed that dissolved organics were also highly adsorbed onto these zeolites [ Giaya et al., 20001. The above findings will raise the possibility that a considerable increase of reaction rate can be achieved due to the adsorptive enrichment of omne and target Organics inside the zeolites. The objectives of this work are to investigate the effectivenessof the novel omnation process proposed and to elucidate the fundamental phenonena throughTCE degradation tests using a tubular flow mtor..

Materials and methods

lsMelIligb~zeolitesUsed

Adsorbents employed are listed in Table

TCE was selected as a

model substance to be decomposed and its adsorption property was examined by batch and brealahrough tests. In order to

investigate TCE degradation, the experimental appmlus shown in figure 1 was employed. Ozone solution was produced by bubbling gaseous omne generated by an ozone generator (POX-10, Fuji Electric Co., Ltd). A constant concenhation of TCE solution was prepad, miXing hued water and high comntrationof TCE solution fed by a syringe type microfeeder (KDS-100, Kd Scientific Inc.,). From different paths, Ozone solution and TCE solution were pumped into the mixing vessel before the inlet to the fixed bed, using a dual plunger pump without pulsating c m n t

219

(NP-KX-120, Nihon Seinritsu Kagaku Co., Ltd), and this mixed solution was fed into the glass column. TCE concentrations at the inlet and oudet of die column at a steady state were detected, and die conversion of TCE, x, at a steady state was given. For die determination of TCE, samples were withdrawn into an aqueous solution phase of

1

saturated SQs2 tor die destruction of residual ozone, simultaneously extracting TCE in die aqueous phase into hexane phase, and analyzed on a gas chromatograph equipped with an electron capture detector. For die analysis of chloride ion, samples were mixed with a little amounts of phenol solution for die destruction of residual ozone, and analyzed using an ion chromatograph (PIA-1000, Shimadzu GLC Ltd) equipped with a Shim-pack IC-A3 column (Shimadzu GLC Ltd) and a conductivity detector. The transfer phase was an aqueous solution of 8mM p-Hydroxy benzoic acid (Wako) and 3.2mM Bis (2-hydroxcyediyl) iminotris (hydroxymethyl) methane (Wako). The concentration of aqueous ozone was monitored with an UV spectrometer (UV-1600, Shimadzu Co., Ltd) at 258 run.

Fig. 1 Schematic diagram of experimental apparatus employed for the test of TCE deradation (J) Oxygen tank @Ozonizer(DOzone stock solution distilled water ©Mixing vessel ©Dual plunger pump without pulsating current © syringe pump with high concentration of TCE solution ©Packed column with an adsorbent ©Column packed with wet activated carbon for the degradation of exhausted ozone

Result and Discussion /. Adsorption Property of TCE Adsorption isotherms of TCE are presented in Figure 2. The adsorption performances were strongly influenced by SiQ/AljQ ratio and the pore structure of die zeolites. Amount adsorbed increases in loop the following order • ZSM-5(SiOj/Al2O5ratio:3000) Mordenite 100 • ZSM-5(SiOj/Al2O3ratio:30) ratio: 10) < A Mordenile(SiOj/Al2O3ratio:90) Mordenite (SiQ/AlA ratio: 20) < ZSM-5 ratio: 30) Mordenite ratio: 90) •3SM-5 (SiCVAlA ratio: 3000). This order corresponded

closely

10

O Mordenitc(SiOj/Al2O3 ratio: 20) D MordenitefSiOj/AljOjratio:10)

0.1

0.01 0.1

OrjP 1

10

Equilibrium concentration [mg/L]

Fig..2 Adsorption isotherms of TCE

220

with that of ozone [ Fujita et al., 2002 1. 2. Dep&&n of TCE F d y , the ratio of Chloride ion to TCE disappeared was examined. Almost the Stoichiornehic release of chloride ion was found to occuc when using ZSM-5(SiO#d2@ ratio 3ooo) , which pvided the evidence that TCE was decomposed in the column and the steady state was achieved. D Reaction time [s] TCE decomposition behaviors in the column with and without the Fig. 3 TCE degradation with and without siliceouszeoItte &orbent were examined. As Wow rate:lO mL/mtn. Particle d i a m e k 5 0 0 - 5 9 0 ~ . AqueousTCE concentratlon:1.27-3.0 mg/L) shown in figure 3, much fastex TCE decomposition was observed in the column packed with ZSM-5 (SiWAl2Q ratio: 3000) in comparison to bulk reaction. The d e w o n behavim when using different adsorbents were examined. TCE conversion increased with higher adsorption performance of ozone and 'ICE, which indicates that the incmse of reaction rate is likely due to the high enrichment of ozone and TCE inside the adsorbent With the further increase of adsorption perfomance, TCE conversion converged to a certain value, independent of high silica zeolite species. In other words, a maximum limit of the increase of reaction rate existed. Apparent TCE demmposition behavior in the packed column would be expressed in following Equation(l), which was based on simple mass balanceequation : r,, = u

d[TCE] =4 , [O, 1" [TCElp dz

(1)

For the determination of these unknown parameters, a , B , Kob,, the dependence of decompsition upon ozone concentration and n=E concentration was examined using ZSM-5 (SiOJAlfi ratio : 3000) which gives the above-mentioned maximum limit. A semi-logarithmic plot of fICEll4TCE]~vs z was found to be approximately b, which meatls a is approximately equal to 1.0. As shown in f i p 4, ozone concentration has little effect on TCE conversion, h m which we can determine pis approximately equal to 0. As a result, Equation(1) is consequently changed into Equation(2). x = 1-exp(- k,a,.r)

(2)

Intereshgly, this results suggests that n=E conversion, x, was apparently i n f l u e d only by reaction time r , not by ozone concentration, which differs h m the bulk reaction directly influenced by ozone concenttation moigne et d, 1983 I. This suggests that there will be conclusive factors that have a strongly effect on the apparent reaction behavior, independent of the Charactenstl ' 'csof the adsorbent natu~. Thus,the effect of other factorssuch as particle size (biier

221

@cle size), flow rate should be investigated in order to elucidate ihese phenomena with smallea @cle size, hi* conveasions~fwndto be attained. 'Ihe effect of the particle size on the apparent reaction rate is likely due to the

0.2

'

mass-transfer resistance.

-

0

2

4

6

8

1

0

Reaction time, r [sl

Fig. 4 El€& of ozone concentrationon TCE degradation

Fkhemme,theincrease of flow rate gave higher

(Flow rate:loml/min.particle diameter: 5 0 0 - 5 9 0 ~ .

TCE concentratlon: 1.27-3.Omg/L)

TCE conversion, which raised the possibility that film transfer resistance between the adsorbent surface and current w i l l strongly influence the apparent d o n . The limitation of reaction enhancement was likely due to the mass transfer limitation of tila Taking these into consideration, a kinetic TCE destruction model was propod as follows. Apparent reaction rate r-hcan be described in Equation(3) 1 is equal , to 0, r-hcan give the maximum reaction rate, r h , , as described in When ~ Equation(4) r, = - h a , ([TCa - [TCEJ,) r,-

=-k,a,[TCEI

(3) (4)

?his equation can be transformed kto the following Equation (5). k can be guessed by the equation proposed by carbery et al[Caheny et aL, lW].and that proposed by W h n W h n et al., 1%6]., while a,can be given by Fiquation(6). x = 1-exp(- k,a,T)

(5)

a, =- 3P,

%PP

We assumed that this equation would give the maximumvalue of TCE conversionobserved in the experimental results when the adsorption performanas for TCE and ozone were high enough. Close concordances were exjmxsed between the experimental results and the kinetic model. .As a result, it can be deduced the adsohen& wexe kept almost in a virgin state. Aqueous ozone mncentration has no effect on TCE conversion under our experimental condition as observed in the experimental results due to the relatively fast reaction in the pores in comparison to TCE film diffusion. However,such acase will be very specific. In the case that the substance to be degraded is more Unreactive or is adwrbed less, or in the case that ozone CoIlCentration is much lower, other states would occur.

222

CONCLUSION

Remarkable increase in ozone reaction rate toward TCE was observed in the presence of high silica zeolite in comparison to bulk reaction. TCE conversion increased with the increase in adsorption of ozone and TCE, which provided the evidence that this significant increase in reaction rate is likely due to the high enrichment of ozone and TCE inside the adsorbent With further increase of adsorption, TCE conversion converged to a certain value, which indicated a maximum limit of reaction improvement exists. The kinetic model provides an accurate description of the experimental results, which suggested the limit was due to the limitation of mass transfer in film. This suggested that ZSM-5 (SiO/AlAratio: 3000) were kept almost in virgin state due to the fest ozone reaction with TCE inside the adsorbent NOMENCLATURE

w:Linear flow rate[ ms"1 ], L:Column lengthf m], r :Reaction time defined as L/u,[ s ], (TCE]:TCE Concentration [ molm"3, or mgL"1 ], (TCE]i,:lhfluent TCE Concentration[ molm"3, or mg/L ], [TCEL^Effluent TCE Concentration[ molm"3, or mg/L ], gLength[ m ], k^: apparent Reaction rate constant[m3mor1s"1],[O3]:Ozone Concentration [ molm"3, or mg/L ], a,: Surface area of adsorbent per unit volume[ nfari3 ], kL :Mass transfer coefficient between liquid and solidf ms"1 ], rah,: Apparent reaction rate[ molL's"1 ], Dm :Difrusion coefficient of TCE in water[ mV ] Reference

Beltran FJ., Encinar. J. M., Alonso M. A. (1998) A Kinetic Model for Advanced Oxidation Processes of Aromatic Hydrocarbons in Water Application to Phenanthrene and Nitrobenzene, bid. Eng. Chem. Res., 37,32-40 Carberry J. J,.(1960) A boundary-layer model of fluid-particle mass transfer in fixed bed. AIChE J., 6,460-463 Fujita H., Sakoda A., Fujii T., Izumi J., (2002) Adsorption and decomposition of water-dissolved ozone on high silica zeolite. Wat Res., submitted Hoigne J., Bader H. Wat Res. (1983) Rate constants of reactions of ozone with organic and inorganic compounds in water-1 non-dissociating organic compounds. Wat Res. 17,173-183 Wilke C. R., P. Chang. (1955) Correlation of diffusion coefficients in dilute solutioa AIChE J., 1, 264-270 Wilson E. J., Geankoplis CJ. (1966) Liquid mass transfer at very low Reynolds numbers in packed beds. I&EC Fundamentals, 5(1), 9-14

223

TIME RESOLVED MULTICOMPONENTSORPTION OF LINEAR AND BRANCHED ALKANE ISOMERS ON ZEOLITES, USING NIR SPECTROSCOPY ALEXANDRE F. P. FERREIRA, M. MITTELMEIJER, M. SCHENK, A. BLIEK AND B. SMIT. University of Amsterdam, Department of Chemical Engineering, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlad E-mail: a.ferreira@cience. uva.nl Snap shot oE

- butane on MFI

- iso-butane on MFI

Pressure Swing Adsorption (PSA) unit is a dynamic separation process. In order to create a precise model of the process and thus an accurate design, it is necessary to have a good knowledge of the mixture’s adsorption behaviour. Consequently, the diffusion rates in the adsorbent particles and the mixture isotherms are extremely vital data. This article intends to present a new approach to study the adsorption behaviour of isomer mixtures on zeolites. In a combined simulation and experimental project we set out to assess the sorption properties of a series of zeolites. The simulations are based on the configurational-bias Monte Carlo technique. The sorption data are measured in a volumetric set-up coupled with an online Near Infta-Red (NIR) spectroscopy, to monitor the bulk composition. Single component isotherms of butane and iso-butane were measured to validate the equipment, and transient volumetric up-take experiments were also performed to access the adsorption kinetics.

1

Introduction

Branched hydrocarbons are preferred to linear hydrocarbons as ingredients in petrol because they enhance the fuel octane number. By catalytic isomerisation linear hydrocarbons are converted into mono and di branched hydrocarbons, and it becomes necessary to separate the mixture. A variety of zeolites may be used for this purpose, either on the basis of sorption thermodynamics or on the basis of sorption kinetics. Such data are relevant to the development of sorption based separation methods, but also they provide key information regarding the catalytic isomerisation over zeolites themselves. Zeolites are crystalline materials with a well-defined system of micro-pores. Zeolitic materials are used in a variety of applications, one of the majors is in the area of separation processes because of their unique porous properties. The size and structure of the pores as well as the molecular structure of the adsorbate determines the adsorption capacity and selectivity [S]. Simulation data on mixture adsorption can be used to screen zeolites as adsorbents, but experimental data are necessary to validate the simulations and to accurately design the separation process. The first step of the process design is to obtain such data. However, the experimental assessment of multi-component adsorption equilibria and kinetics is not straightforward and is highly time-consuming. As a result, some theories have been developed that predict adsorption behaviour for a mixture based on the pure component equilibria [1,3]. The isotherm data have to be correlated before their use in a design model

224

for easier handling. Therefore, the experimental systems have to be measured accurately over a wide range of pressures and temperatures. Gravimetric or manometric techniques have been used to establish adsorption data of gases on zeolites. Both techniques present problems, manometric equipment has an accumulation of the error; and data obtained by the gravimetric method are influenced by effects associated with flow patterns, bypassing, and buoyancy. In the mixture’s adsorption behaviour, isomers mixtures have the highest degree of difficulty to study. Isomers can not be differentiated in standard commercial adsorption equipment. This problem has been solved in this study by coupling a manometric apparatus with an NIR spectrometer, which allows us to measure the gas phase composition (in time, if necessary). In this paper we report this new approach to study the adsorption of mixtures of butane and iso-butane. 2

ExperimentalSection

In a combined simulation and experimental project we set out to assess the adsorption properties of a series of zeolites. In the present work the adsorption properties of n-butane and iso-butane on MFI are being studied. The experimental part consists in the validation of the molecular simulation model, by confirming its results. The experiments were performed in a constructed in-house manometric apparatus coupled with a NIR spectrometer (Perkin Elmer, FT-IR system, GX Spectrum). Figure 1 is a scheme of the experimental set-up. Liquidpump

Figure 1. Scheme of the manometric apparatus coupled with the NIR spectrometer.

Figure 2. SEM picture of MFI.

A sample of commercial MFI from ZeoIist was used, with a silicon aluminium ratio (SYAl) of 100, the crystals do not present a regular shape. On figure 2 Scanning Electron Micrograph (SEM) of MFI crystals is presented. The sample was calcined for 6h at 873K. The adsorptive gases used were n-butane with a 99.5% purity, and i-butane with 99.95% purity from Praxair. No further treatment was performed on them before their admittance into the experimental set-up. For single component and mixtures calibration curves, the NIR spectra of pure gases are recorded at several pressures and the data gathered are treated as explained in the result section. To measure a point of the single component isotherm, pure gas is admitted to the setup with valve 2 closed (figure 1). By closing valve 1 and opening valve 2 the pressure

225

starts to drop, and the final value is measured after equilibrium is reached, and the loading can be then calculated. Transient volumetric up-take experiments are performed following similar procedure of measuring one point of the isotherm, but before opening valve 2, NIR spectra are recorded in regular intervals. After a short period valve 2 is opened to start the adsorption. Spectra are recorded until the equilibrium is reached.

3

Results and discussion

Experimental data on single component adsorption isotherms of normal-butane and isobutane, on MFI zeolite, at 373K, for a pressure range of OSkPa to 200kPa,were obtained. 1.o 1.8

=-

1.6 1.4

-

0.9

0.8 s0.7 E 0.6 1.0 E 0.5 v ; 0.8 0.4 3 0.6 0.3 > 0.4 0.2 0.2 0.1 on . 0.0 1 .OE-05 1 .OE-03 1 .OE-01 1.OEM1 1 .OEM3 1 .OE-05 1 .OE-03 1.OE-0 1 1 .OEM 1 1.OE+03 P(kPa) P(kPa)

2?

5

1.2

-

3

Figure 3. nButane and i-butane isotherms on MFl at 373K. (- Mol. Simulations, 0 Exp. Results, Results ref [ 6 ] )

- - - Exp.

In the figure 3 we present data on single component adsorption isotherms and simulation results. Data obtained from the literature [6] are included for comparison. The increase of the loading observed on the high pressures region can be explained by capillary condensation in the exterior secondary pore system, in particular between the crystals. This has been observed also by other authors [5]. Using the presents used manometric set-up coupled with a NIR spectrometer produced results that are in agreement with literature data, and in agreement with simulations ( simulations details are provided on ref [4]). Near Infra-Red spectroscopy is a non-intrusive technique that allows to monitor the composition of the gas phase (differentiate isomers) and its changes in a time resolved manner. In the NIR spectra (figure 4) some small differences between iso-butane and nbutane can be observed. We can also observe that mixtures exhibit a spectral behaviour that is a linear composition of the pure component spectra. It is necessary to quantify the spectral differences of the two isomers, so that the composition of the mixtures can be determined by NIR spectroscopy. Spectra of pure n-butane and pure i-butane are recorded at several pressures. The single component calibration curves can be calculated by integrating the spectra in a certain wave length interval obtaining the total spectral intensity (T.S.I.) for that same interval (figure 5). After some preliminary studies we conclude that the interval of [6700,7350]+[8200,8700] cm-' is the best to use for the calibration, since it presents the lowest degree of non-linearity. We did correct all spectra for baseline drift and water bands. In the figure 5 we present the single component curves that were obtained.

226

0.8 h

0.6

v

0

e

0.4

w 9

0.2

0 5000

7000 Wave lengh (cm-1)

9000

Figure 4. n-Butane, i-butane and 50/50% mixture NIR spectra.

2

7

1

i-butane

n-bu tnae -1.5

:

- 1

-!

m

& 0.5

0 Experimental -Cahbratio n

-Calibration

0 0

60

120

180

240

0

60

p Wa)

120 P (kPa)

180

240

Figure 5. Single component calibration curve. 0.6

0.6 4 0.5 f0.4 s 0.3 > m 0.2

n-butane

h

j 0.5

h

2 0.1

0 0

60

180

240

0

60

120

P (kPa)

180

240

Figure 6. Multi-componentcalibration curves - NAS value.

To calculate the amount of each component in one mixture we use the Net Analyte Signal (NAS) theory [2]. In figure 6 we can see the NAS calibration curves for n and ibutane. In figure 7 we present the volumetric up-take experiment results. The time-base spectra for n-butane are recorded. We can see a overlapping of spectra on the higher region, that correspond to the initial part before opening valve 1, so the pressure is kept constant for 150s. On the lower part we see high number of spectra overlapping, that correspond to the equilibrium. These two regions can also be seen on the total up-take curve, the line at OmmoVg for the initial part between 0 and 15Os, and the flat part at 0.96moI/g for the equilibrium, reached after 1000s. Figure 8 shows adsorption data for n-butane and i-butane mixture. The total loading is represented by the grey symbols, the black symbols represent the partial n-butane loading and the open symbols the partial i-butane loading. We can observe that the separation between the two isomers increases with the pressure, but we need to measure more points in the high pressures region.

227

0.21

1.2

~

Total Uptake -final pressure 3.90kPa

,

t = loo to 700s

c?. I

3 0.14

0

v

3> 0.4

n P 0.07




0.0

l.E-03

0.2

0 l.E-01

l.E+01 P(kPa)

l.E+03

0

400 time (s) 800

1200

Figure 8. Mmnm equilibrium data and partial uptake for mixture components versus time.

4

Conclusions

We conclude that with a manometric set-up coupled to a NIR spectrometer it turns out to be possible to measure equilibrium and kinetic data for single components and mixtures.

5

Acknowledgements

This research was carried out within the project CW/STW 349-5203. The authors thank the Stichting Technische Wetenschappen for their financial support. References 1. Krishna R., Diffusion of Binary Mixtures in Zeolites: Molecular Dynamics Simulations versus Maxwell-StefanTheory. Chem. Phys. Lr. 326 (2000) pp 477-484 2. Lorber A., Faber K., Kowalski R., Net Analyte Signal Calculation in Multivariate Calibration. Anal. Chem. 69 (1997) pp 1620-1626 3. Ruthven D. M., Past Progress and Future Challenges in Adsorption Research. I n d Eng. Chem. Res. 39, (2000) pp2 127-2131 4. Schenk M. et al., Sep. of alkane isomers by exploiting entropy effects during adsorption on silicalite-1: a CBMC simulation study. Lungmuir 17 (2001) pp 1558-1570 5. Stach H., Lohse U., Thamm H. and Schirmer W., Adsorption Equilibria of Hydrocarbons on Highly Dealuminated Zeolites. Zeolites 6 (1986) pp 74-90 6. Zhu W. et d.,Adsorption of Light Alkanes in Silicalite-1: Reconciliation of Exp. Data and Mol. Simulations. Phys. Chem. Chen Phys., 2 (2000) pp 1989-1995

228

PORE SIZE EFFECTS IN THE LIQUID PHASE ADSORPTION OF ALKANES IN ZEOLITES JOERI F.M. DENAYER', KURT DE MEYER', JOHAN A. MARTENS' AND GIN0 V. BARON' 'Departmentof Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussel, Belgium. Joeri.deMverO.vub.ac.be Centerfor Surface Science and Catalysis, Katholieke UniversiteitLeuven, KasteelparkArenberg 23, B-3001 Leuven, Belgium. The liquid phase adsorption of C5-C22 linear alkanes on ZSM-5 was studied using a batch adsorption technique. Saturation capacities of the alkanes depend strongly on the chain length. The CHI packing density in the pores of ZSM-5 increases from pentane to heptane, then decreases steeply to octane, and subsequently increases gradually to reach a plateau where the theoretically highest packing density is observed. Binary adsorption experiments show pronounced selectivity effects between alkanes with different chain length. For most binary mixtures, the longest alkane is adsorbed preferentially, but for certain binary alkane mixtures, the adsorptionselectivity is inverted.

1

Introduction

The adsorption of the homologous alkane series on zeolites and other adsorbents has been extensively studied by many research groups. These studies demonstrate the existence of linear relationships between adsorption enthalpy and entropy, and the carbon number [ 141. Contrarily, there are few studies dealing with the adsorption of alkanes in liquid phase. This can be explained by the lack of selectivity effects that occur in the adsorption of alkanes on adsorbents in liquid phase. Indeed, classical stationary phases for HPLC show no separation of alkane mixtures, as a result of the rather weak interactions between the molecules and the force field exerted by the surface of the amorphous material. When the alkane molecules are trapped in the pores of a zeolite, much stronger energetic interactions occur compared to those on amorphous surfaces. Pulse chromatographic experiments in liquid phase showed a slight increase of the alkane retention with carbon number on a column packed with a large pore Y zeolite [5]. When ZSM-5 was used, a zeolite with much smaller pores than Y, very large differences in adsorption between short and long alkanes were observed in liquid phase. Among all zeolites, ZSM-5 is undoubtedly the most studied one, both from the adsorption and catalysis point of view. Several experimental studies reported the occurrence of a kink in the pure component adsorption isotherms of certain alkanes in gas phase on ZSM-5 [6-81, and an inflection point in the adsorption enthalpy and entropy versus zeolite loading curves [9,10]. Generally, the two-step behavior of these linear and branched alkanes is interpreted in terms of the ZSM-5 channel geometry. For example, it has been observed that at low partial pressures, isobutane is adsorbed preferentially in the channel intersections of ZSM-5, while at higher pressures, these branched molecules are "pushed" into the channel segments, resulting in a significant loss of entropy and a kink in the adsorption isotherm [ 1 11. The packing and sitting of alkanes in the pore system of ZSM-5 has been investigated by computational techniques, such as Configurational-Bias Monte Carlo simulation [12-14]. These simulations all confirm the importance of entropy effects in the adsorption of alkanes on ZSM-5. In the present work, we have investigated the liquid phase adsorption behavior of nalkanes on a ZSM-5 zeolite. The saturation capacity of linear C5-C22 alkanes was determined to study alkane packing effects. Adsorption isotherms of binary mixtures were

229

measured to determine the effect of alkane packing on the competition between short and long alkanes. 2

Experimental

The H-ZSM-5 zeolite sample used in this study was obtained by deamoniating a NH4ZSM-5 from Zeolyst (CBV 8014, SiOZ/A1203 mole ratio of 80) at 673 K in a muffle oven in presence of air for 48h. Experimental adsorption isotherms were obtained by means of batch experiments. Zeolite samples (-I g) were put in 10 ml glass vials and weighed with a balance. After regeneration overnight at 673 K, the vials were immediately sealed with a cap with septum in order to avoid water uptake from the air, and were weighed to determine the regenerated zeolite mass. Mixtures of two linear alkanes or only a pure alkane in solvent were added to the sample (each 20 ml). Iso-octane (99% purity, Acros) was used as a solvent in the adsorption measurements. Immediately after sealing the cap and weighing the sample, about 10 ml of the mixture was injected through the septum into the zeolite containing vials, and another 10 ml was added to a vial without zeolite, to be used as blank sample. Samples were kept at 277 K, to be sure no compounds could evaporate, and stirred frequently. Liquid samples were taken with 2 ml syringes after 24 h and 48 h, to verify if equilibrium between adsorbed and bulk phase was achieved, and analyzed in a gas chromatograph with flame ionization detector. For every binary mixture, a calibration line was obtained by analysis of the blank samples, for which the concentration of the components is exactly known. For each sample the amount adsorbed (qwhte) at equilibrium was obtained by calculation of the mass balance.

-

3

Results and Discussion

Fig. 1 shows the uptake of pure hexane and decane from their mixture with iso-octane on ZSM-5. The amounts adsorbed in the zeolite remain constant after 20 hours, indicating that the experiments were performed under equilibrium conditions. In order to verify that iso-octane can be used as “inert” solvent, a comparative experiment was performed in which the binary adsorption isotherm of hexane and decane was determined using isooctane and 1,3,5-trimethylbenzene as respective solvents. The same adsorption isotherms are obtained with both solvent (Fig. 1b), demonstrating that iso-octane does not interfere with the adsorption of the linear alkanes, and that the relative adsorption of the short and long n-alkanes is not influenced by the nature of the solvent, given that the solvent is not able to enter the pore system. Fig. 2a gives the saturation capacity of the C5-C22 n-alkanes, expressed in number of molecules adsorbed per unit cell. Fig. 2b gives the total number of carbon atoms per unit cell. For pentane and hexane, about 7.7 molecules are adsorbed per unit cell of ZSM-5. This corresponds to 39 C atoms for pentane and 45 C atoms for hexane. Although the number of adsorbed carbon atoms is the same for heptane as compared to hexane, only about 6.4 heptane molecules are adsorbed per unit cell. A sudden drop is observed between heptane and octane: only 4.5 octane molecules are adsorbed per unit cell. From octane on, the number of C atoms adsorbed per unit cell increases steadily, to reach a plateau of about 55 C atoms adsorbed unit cell.

230

0

w

m

40

20

l W l 2 0 1 4 0

LO

02

LB

04

U r n lh)

08

10

xw

Fig 1a: uptake of hexane and decane in ZSM-5 fiom their mixture with iso-octane

Fig 1b: Binary adsorption isotherm of C6 and C10. Open symbols: mesitylene solvent; closed symbols: iso-octane solvent . ....................... ..........

t i 1.............................................................

1

a 4

e 2

5

6

7

8

9

10

11

12

13

14

15

18 20

P

5

C.rbmnumbm

8

7

0

9

10

11

12

13

14

15

18 20

P

Urbnn m b r

Fig 2a: Saturation capacities of n-alkanes on ZSM-5 (molecules/UC)

Fig 2b: Saturation capacities of n-alkanes on ZSM-5 (C-atoms/UC)

Obviously, the saturation capacity depends both on the dimensions of the adsorbing molecules, and the geometry of the pore system. The pore system of ZSM-5 is constituted of linear channels, with a fiee pore diameter of 5.6 x 5.3 A, intersecting with sinusoidal channels, with a free diameter of 5.5 x 5.1 A (see Fig. 5a). The length of the linear channel segments between two intersections is equal to 4.5 A, while the length of the sinusoidal channel segments between two intersections equals 6.65 A. Each unit cell contains 4 intersections, 4 sinusoidal segments and 4 linear segments. An octane molecule has a length of 11.1 A, exceeding the length of a linear channel segment and an intersection, but is also longer than a sinusoidal channel segment. As a result, not all channel segments can be occupied by C8 molecules, explaining the lower saturation capacity compared to C5-C7. Even with these shorter molecules, no use is made of all the available space for adsorption. This is shown in Fig. 2b, where the total number of Catoms adsorbed per unit cell is plotted as a function of the carbon number. Only from C 13 on, a plateau is reached. When the number of adsorbed molecules per unit cell is multiplied by their molecular length, a plateau value of about 70 A is obtained. This is longer than the total length of the intersections, linear and sinusoidal channels (66.2 A), which seems contradictory at fmt sight. However, it should be considered that the kinetic diameter of the n-alkanes is 4.3 A, leaving an additional 1.1 A at every intersection for adsorption. This gives a maximal length available for adsorption of 66.2 A + 4* 1.1 A = 70.6 A, which corresponds very nicely to the plateau value. This 100 % occupancy of the ZSM-5 pores is only possible if the long molecules are able to bend, and occupy both linear and sinusoidal channels.

231

1

0.8

0.6

E 0.4

02

0

0.0

0.2

0.6

0.4

0.0

0

1.O

02

0.8

0.4

0.8

1

XCO

XCO

Fig 3a: Binary adsorption isotherm of CB and C 12 on Z S M J 1.4

Fig 3b: Selectivity diagram for the competitive adsorption of C8 and C 12

1

..

ao

02

0.4

0.8

08

10

0.0

0.2

0.4

0.0

0.8

1.0

XQ) XQ)

Fig 4a: Binary adsorption isotherm of C6 and C8 on ZSM-5

Fig 4b: Selectivity diagram for the competitive adsorption of C6 and C8

In Fig 3%the binary adsorption isotherms of a CB/C12 mixture is shown. C12 is adsorbed in a very selective way from the mixture, as can be seen in the selectivity diagram (Fig 3b), in which x and y represent the molar firactions in the liquid and adsorbed phases respectively. This selectivity for the longer chain can be explained by its higher interaction with the zeolite.

Fig 5a: Pore structure of ZSM-5

Fig 5b: Packing of C6 and CS molecules from their mixture in the pores of z5m-5

232

A peculiar behavior is observed with a C6K8 mixture (Fig 4), where, at low C6 concentrations (thus a high C8 concentration), the lightest component is adsorbed

preferentially over the heavier alkane. This adsorption selectivity reversal is again explained by the packing of the molecules in the pores. Since octane can only adsorb in a !+action of the channel segments, vacancies remain available in which C6 can adsorb, as is shown in Fig. Sb, where an example is given how the octane molecules can pack in the linear channels, leaving open space in the sinusoidal channels for hexane. Selectivity inversion has been observed with a range of binary mixtures, but these data will be treated elsewhere. 4

Conclusions

The adsorption of linear alkanes on ZSM-5 is governed by geometric and packing effects, which result in pronounced selectivity effects, and even in an inversion of the normal adsorption selectivity. These effects are certainly important with respect to catalytic and separative applications, and will be studied in further work.

Acknowledgements This research was financially supported by FWO Vlaanderen (G.0127.99). J. Denayer is grateful to the F.W.0.-Vlaanderen, for a fellowship as postdoctoral researcher. 5

References

1. Bond, C.G., Keane, M.A., Kral, H., Lercher, J.A., Catal. Rev. - Sci. Eng., 42(3), 323383,2000. 2. Ruthven, D.M., Kaul, B.K., Adsorption, 4,269-273, 1998. 3. Eder, F.; Lercher, J.A. J Phys. Chem. B 1997,101, 1273-1278. 4. Hampson, J.A.; Jam, R.V.; Rees, L.V.C., Characterization of porous solids I1 Rodriguez, F. et al, Amsterdam 1991, 509,517 5. Denayer, J.F.M., Bouyermaouen, A., Baron, G.V., Ind. Eng. Chem. Res., 37 (9),

1998,3691. 6. Richards, R.E., Rees, L.V.C., Langmuir, 1987,3,335-340. 7. Zhu, W.,van der Graaf, J.M., van den Broeke, L.J.P., Kapteijn, F., Moulijn, J.A., Ind. Eng. Chem. Res., 1998,37,1934-1942. 8. Sun,M. S., Talu, 0. Shah, D. B., J. Phys. Chem., 1996, 100(43), 17276-17280. 9. Yang, Y., Rees, L.V.C., Microporous materials, 12, 1997, 117-122. 10. Millot, B., Methivier, A., and Jobic, H., J. Phys. Chem. B, 1998, 102(17), 32103215. 11. Zhu, W.,Kapteijn, F., Mouiijn, J.A., PCCP, 2000,2, 1989-1995. 12. Maginn, E.J., Bell, A.T., Theodorou, D.N., J. Phys. Chem., 1995,99,2057-2079. 13. Smit, B., Siepmann, J.I., J. Phys. Chem, 1994,98, 8442-8452. 14. Krishna, R, Paschek, D., PCCP, 2001,3,453-462.

233

DETECTION OF FREEZING POINT ELEVATION IN SLIT NANOSPACE BY ATOMIC FORCE MICROSCOPY M. MIYAHARA, M. SAKAMOTO,H. KANDA AND K . HIGASHITANI Department of Chemical Engineering, Kyoto University, Kyoto 606-8501, Japan E-mail: [email protected] An experimental trial for finding the freezing point elevation phenomena was conducted, employing the so-called colloidal-probe Atomic Force Microscopy. The elevated freezing point had been predicted in the earlier molecular simulation work by the authors, which is thought to be caused by the attractive potential energy from pore wall. To make up a strongly attractive nanospace, a carbon microparticle was attached to the top of the cantilever tip, and its interaction force with cleaved graphite was measured within a liquid cell filled with organic liquid, controlled at a desired temperature above the bulk freezing point of the liquid. The two surfaces will form a slit-shaped nanospace because the radius of the particle is far larger than the separation distance concerned. For two kinds of liquids, freezing behavior has been detected above the bulk freezing point. Though the extent of the elevation itself was rather small, the finding of the definite existence of the elevation, not only in the micropores but also in a nanospace with the size of a few nanometers, would be of much importance in the research field of the phase behavior in nanopores.

1

Introduction

Solid-liquid phase transition (fieezing) in confined space, which is of importance not only in adsorption but in the nanomaterial fabrication, nanotribology and in the pore size determination, has recently been explored by several research groups including ours. In contrast to the long-believed phenomena of “freezing point depression in pores”[ 11, some molecular simulation studies have clarified that the freezing point in slit nanospace in equilibrium with saturated vapor or liquid in bulk can be higher than the bulk, depending on the potential energy of the confining walls [2,3]. Note that the pore shape other than slit geometry, and the equilibrium vapor-phase pressure less than the saturated one would bring depressing effect in freezing point [4,5]. The present understanding of the freezing in nanospace is roughly reviewed below. Our study on the first point [2] clarified the following. Depending on the strength of the attractive potential energy from pore walls, fluid in a slit pore in equilibrium with saturated vapor showed freezing point elevation as well as depression, and the critical strength to divide these two cases was the potential energy exerted by the fluid’s solid state. The “excess” attraction relative to the critical one was considered to bring the confined liquid to a higher-density state that resembles a compressed state, which would result in the elevated freezing point. The above result is in accord with other recent studies. Dominguez et al. [6] examined freezing of LJ fluid in slit pores of purely repulsive and weakly attractive walls, employing thermodynamic integral technique to determine true equilibrium points. The freezing points showed significant downward shift, relative to the bulk, in purely repulsive walls, while the downward shift was much smaller in magnitude for weakly attractive walls. Further, Radhakrishnan and Gubbins [3] used a different approach of determination of freezing point in slit pores employing the Landau free energy calculation, and simple fluid in strongly attractive slit pore was shown to exhibit elevated freezing points.

234

Now the research effort goes toward experimental verification of the elevation phenomena in the simplest geometry, a slit. Our main interest is in the range of a few to several nanomenters. Some experimental studies have already reported fieezing point elevation in slit pores [7-91,but the materials used were activated carbon fibers (ACFs), which have only micropores less than 2nm. In such small pores the first layer adjacent to the attractive pore wall, which is known to form an ordered phase at a temperature well above the bulk freezing point, will occupy most of the pore spaces, and the freezing behavior in the interior of the pore space is difficult to be detected. Further, there may still remain some controversy if a liquid confmed in a larger nanopore would exhibit elevation unless an experimental verification is made for such a pore. The employed technique for this purpose was the so-called colloidal-probe AFM (Atomic Force Microscopy). The results clearly demonstrate that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes at a temperature above the bulk freezing point. Though the extent of the elevation itself might look rather small, we believe that the finding of the definite existence of the elevation would be of much importance in the research field of the phase behavior in nanopores. 2

Experimental

An AFM apparatus, PicoSPM manufactured by Molecular Imaging (MI), which is schematically shown in Fig.1, was used. The temperature of the liquid cell can easily be controlled from the bottom side of the cell, onto which Peltier cooling device is equipped. MI guarantees temperature control with 0.1 “C accuracy. A slit-like nanospace can be made up by applying so-called the ‘‘colloidal probe A F M technique, which was in its origin developed to measure the force between a solid surface and a particle of micrometer size. The particle was glued on the cantilever tip, and this colloidal probe is to be used instead of the usual cantilever. The vertical scanning of the cantilever controls the distance between the particle surface and a solid surface, and the usual manner of detecting the bending of cantilever gives the AFy

Piezo synner/Laser unit

f

Laser

f

figure 1. Schm@ic drawing of AFM apparatus and image of the nanospace formed between surfaces.

235

forcedistance relation. If we look at the system with the separation distance of single nanometers, the space between the two surfaces would be almost slit geometry because the radius of the particle (order of 10' pn) is far larger than the separation distance. A graphite plate (HOPG) and carbon particles, which were made from phenolic resin by pyrolyzing above 2000"C, were used to form a nanospace with carbonaceous surfaces. The graphite plate was freshly cleaved before measurement by using Scotch tape. Cyclohexane, and octamethylcyclotetrasiloxane(OMCTS), whose bulk freezing point were 6.4"C and 18°C respectively, were used for the examination of freezing behavior. The reasons of the choice were: i)aBity to carbonaceous surface, ii)freezing point near ambient temperature, and iii)almost spherical molecular shape. Methanol, with bulk 6eezing point of -98"C, was also used as a reference.

3

Results and discussion

3. I

Ultrahighsensitivity of vdWforce on temperature and its trick

Figure 2 illustrates the force curves between the carbon particle and graphite plate immersed in cyclohexane at various temperatures above the bulk freezing point. (Note that the distances shown in the figures are those detrmined from usual mannar of AFM measurement, and is not the distances between the center of nuclei of surface atoms, which are often used for simulation results.) As is expected the force at higher temperature, e.g., at 18.6"C, exhibits typical van der Waals (vdW) force that acts mainly in the single nanometer range. On the other hand what was NOT expected is its ultrahigh sensitivity on the temperature. Only about 10 degree C of temperature change brought multi-fold variation in the force. From theoretical point of view the effect of temperature on the vdW force would be only from the variation in density of solids and liquid, which at most would affect only a few percent in this small range of the temperature examined.

I

0.3

'

0

17.0"C

c.l

+ ll.0"C

-s

A

10.0"C 9.0T 8.0"C

E z 0.2 E 0.1

0

.

3

0

0-0.1

f.4

-0.2

-0.3I 0

I

10

I 20

I 0

I

10

I

20

I 30

Distance [nm]

Distance [nm]

Figure 3. Force curves between carbon-graphite surfaces immersed in methanol

Figure 2. Force curves between carbon-graphite surfaces immersed in cyclohexane for various

temperatures above freezing point (6.4"C)

As a matter of fact, the same measurement in methanol gives the results shown in Fig. 3: Quite naturally the force curves do not show any detectable difference against the temperature variation. We should, then, not take the abnormal temperature dependence

236

as is apparently shown, but should interpret it on the basis of the insensitivity of vdW force on temperature. The observed abnormality can be reasonably interpreted if we look at the trick in the determination of the separation distance in the AFM measurement, which follows below. Unlike the surface force apparatus, the AFM measurement does not have a direct method to detect the distance between the surfaces. Instead, one will take the linear signal of cantilever deflection against sample displacement as the origin of the surface distance: The linear signal results because the two surfaces are in contact and move together. This manner of “wall detection” usually works well. However, what would result if the “freezing point elevation” is the case? Suppose that the liquid between the two surfaces may freeze when the two surfaces come close to a certain distance, at which the superposition of the potential energy from each wall exceeds a critical strength that would be needed for the liquid to freeze. The two surfaces, then, can never go any closer because of the steric repulsion of the frozen phase (Fig. 4). The system replies with a linear signal as if it reached the situation of real contact. The observed force curve would then represent a part of the real vdW force cut out at this distance, taking this point as the origin of the surface distance, A “weak” force thus appears. 0.3 CI

ul

o.2

0.1

a 3

0

0

-0.1

. 8

0

-0.2

-0.3

0

10

20

Distance [nm] Figure 4. Schematic illustration of “contact” with frozen phase between surfaces.

3.2

Figure 5. Shifted force curves and emerged “walls” for cyclohexane.

Force curves against real separation distance: fieezing point vs. distance relation

Now we try constructing the real force curves plotted against real separation distance. The key assumption for this trial would be the insensitivity of the vdW force on the temperature. The attractive force before the freezing or the apparent contact should stay almost unchanged. Thus each force curve was slid laterally to exhibit a best fit in the region of attractive force with long separation distance. The results are shown in Fig. 5. Each force curve converges into a single curve in longer distance region, which is in line with the insensitivity of the vdW force on temperature and stands for the validity of the above assumed interpretation of the apparent phenomena. Then the “walls” standing at certain distances for lower temperatures would be a direct reflection of the freezing at a temperature above the bulk freezing point. Namely, the graphiticlcarbonaceous surfaces

237

in their nature exert strongly attractive potential energy to cyclohexane confined within, and the superposition of the two potential energies grows with decreasing distance between them. At the position where the “wall” stands in Fig. 5 , the potential energy in the nanospace exceeds the critical value for the confined liquid to freeze. Thus the distance of the “wall” gives the “pore” size needed for the freezing at each elevated temperature. Quantitative discussion will further be presented in the conference. We observed similar “walls” also for the case with OMCTS, though the page limitation does not allow us to show the results here. The superposition of the potential energy is typically the case for the micropores. Note that, however, superposition of potential energy in some larger pores of a few nanometers would still be sufficient to cause detectable elevation in the freezing point because the potential energy of carbon surfaces itself shows quite a large value in the unit of temperature when converted with Boltzmann’s constant: It will be more than hundreds of Kelvins in the vicinity of the surface and can easily be tens of Kelvins even with a distance of a few nanometers.

4

Conclusion

An experimental trial for finding the freezing point elevation phenomena was conducted, employing the so-called colloidal-probe Atomic Force Microscopy. A carbonaceous nanospace with slit geometry was successfully made up by this technique. The results demonstrated that cyclohexane in the nanoscale slit space between the carbonaceous solids freezes when the distance comes down to 4 nm, even at 8.4”C,which is above the bulk freezing point of 6.4”C. Measurements with octamethylcyclotetrasiloxane, bulk freezing point being 18”C, also detected the freezing behavior at an elevated temperature of 22°C. The detected elevation of a few degree C might be felt as a matter of less significance, but we believe that the finding of the definite existence of the elevation would be of much importance in the research field of the phase behavior in nanospaces.

References 1.

2. 3. 4.

5. 6. 7. 8. 9.

e.g., Patric W. A. and W. A. Kemper, J. Chem. Phys., 42 (1938) 369; Durn J. A., N. J. Wilkinson, H. M. Fretwell, M. A. Alam and R. Evans, J. Phys. Cond. Matter, 7 (1995) L713. Miyahara M. and K. E. Gubbins, J. Chem. Phys., 106 (1997) 2865. Radhakrishnan R. and K. E. Gubbins, Mol. Phys., 96 (1999) 1249. Kanda H., M. Miyahara and K. Higashitani, Langmuir, 16, (2000) 8529. Miyahara M., H. Kanda, M. Shibao and K. Higashitani, J. Chem. Phys., 112 (2000) 9909. Dominguez H., M. P. Allen, and R. Evans, Mol. Phys., 96 (1999) 209. Kaneko K., A. Watanabe, T. Iiyama, R. Radhakrishnan and K. E. Gubbins, J. Phys. Chem. B, 103 (1999) 7061. Watanabe A. and K. Kaneko, Chem. Phys. Lett., 305 (1999) 71. Sliwinska-Bartkowiak M., R. Radhakrishnan and K. E. Gubbins, Mol. Simul., 27 (2001) 323.

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MODELJNG OF HIGH-PRESSURE EQUILIBRIUM ADSORPTION OF SUPERCRITICAL GASES ON ACTIVATED CARBONS. DETERMINATION OF

PORE SIZE DISTRIBUTION USING A COMBINED DlV AND EOS E. A. USTINOV St. Petersburg State Technological Institute (Technical University). 26 Moskovsky prospect, St. Petersburg, 198013 Russia E-mail: [email protected]

D.D.DO Department of Chemical Engineering, University of QueensIand,St Lucia, QLD 4072, Australia E-mail: [email protected] The density functional theory modified by including the Bender equation of state is applied to the analysis of high-pressure (up to 50 MPa) adsorption of argon, nitrogen, methane and helium in activated carbon Norit R1 in temperature range from 298 to 343 K. The approach allows us to determine the pore size distribution (PSD) and the adsorbent density without recourse to helium experiments. The PSD obtained with nitrogen at 77.35 K and those obtained with the high-pressure argon, nitrogen and methane are in fair agreement with each other. However in the latter case the PSD shows small pores of about 0.3 nm (physical pore width), which do not appear in the 77.35 KPSD due to diffusional limitations. It was shown that the adsorption of helium is not negligible at room temperature and it cannot be used as an inert gas to determine the adsorbent density. The approach can be easily extended to near critical gases such as carbon dioxide and ethane.

1

Introduction

Adsorption of supercritical gases at high-pressure has some features, one of which is that the adsorption excess vs pressure exhibits a maximum. There are some simplified theories involving Langmuir, Langmuir - Freundlich, Toth, Dubinin - Astakhov equations for the absolute adsorption isotherm and 2-Dequation of state (EOS) to the adsorbed phase. Some approaches were based on Ono - Kondo equations for lattice gas and the simplified local density theory that utilizes the van der Waals EOS and the Peng - Robinson EOS modified for the case of confined slit pores. Brief review of these models was presented earlier [ 13. There are some investigations of supercritical and near to critical temperature adsorption of gases on the basis of the density functional theory (DFT) [2,3] and the grand canonical Monte Carlo (GCMC)simulations [4-71. In both cases the PSD obtained using high temperature and high-pressure gas adsorption and that determined with nitrogen at 77 K for the same sample of activated carbon was shown to be markedly different. A group of small pores of the PSD obtained with near critical carbon dioxide and supercritical methane is not revealed with the low temperature nitrogen adsorption, which is attributed to diffusion limitations or even blocking these pores by frozen nitrogen 161. By this reason the PSDs obtained with C02are considered to be more robust and reliable than those obtained with N2at 77 K. Thus it might suggest that supercritical adsorption is a suitable means to derive PSD. But there exists one obstacle to this approach. Since the capillary condensation does not occur in supercritical adsorption the dependence of amount adsorbed expressed in terms of excess per unit surface area on pressure is nearly the same for sufficientlywide pores (wider than 6 collision diameters in the case of methane adsorption [4]). It does not allow us to reliably determine the PSD in the region of large pore widths and even the total pore volume. This problem may be

239

overcome by increasing the pressure range for experimental isotherms and specifying the total pore volume, which could be evaluated from N2 adsorption at 77 K. In this paper we use a modified DFT approach to the data on Ar, N2, CH4 and He adsorption on activated carbon Norit R1 at pressures up to 50 MPa and four temperatures: 298, 313, 328 and 343 K. Experiments were carried out at Institute of Non-classical Chemistry of University of Leipzig, Germany, using a magnetic suspended gravimetric system (Rubotherm PrtlzisionsmesstechnikGmbH, Bochum, Germany) [11. Our aim was to develope a method of the PSD analysis from high-pressure adsorption data without recourse to helium adsorption to determine the skeletal density of the adsorbent. 2

Model

The widely used non-local density functional theory [8-101 is known to involve a number of assumptions. The Helmholtz free energy of a fluid confined in a pore is divided by two parts, which account for separately attractive and repulsive forces. The former is usually calculated by integrating the Weeks - Chandler - Andersen (WCA) fluid - fluid potential over the confined pore on the basis the mean field approximation (MFA). The latter is modeled by the equivalent hard sphere fluid using the Camahan - Starling equation of state (CS EOS). The Helmholtz free energy of the hard sphere fluid is supposed to be a function of a smoothed density, which is defined as a weighted average according to the T a w n a prescription [el. The solution for a given pore at a specified temperature and a bulk phase pressure is obtained by minimizing the grand thermodynamic potential with respect to the density profile across the pore. Mathematically the problem is reduced to a system of highly nonlinear equations with respect to the set of densities at different distances from the pore wall, which can be solved by an iteration scheme. In the sub critical region one may obtain two or more density profiles depending on initial conditions. In such a case the true density distribution corresponds to the minimal value of the grand potential. In the case of supercritical gas adsorption only one density profile is obtained at each bulk phase pressure. The parameters for the fluid - fluid interaction (the potential well depth m, the collision diameter aff and the equivalent hard sphere diameter dHS)are chosen in such a manner that the agreement between the bulk phase properties (dependence of the saturation pressure and densities of coexisting phases on temperature) and the surface tension and their respective experimental values is reasonably well. The solid - fluid parameters are calculated by the empirical Berthelot mixing rule, which sometimes can be corrected using the data in the Henry law region. For the solid - fluid potential the 10 - 4 - 3 Steele potential is usually used. Application of the NLDFT to the case of high-pressure adsorption, being principally the same as in the case of sub critical adsorption, is however accompanied by an additional difficulty. The problem is that the bulk phase density is comparable with that of the adsorbate phase and must be described as a function of pressure very precisely. The Carnahan - Starling EOS does describe this dependence for Ar, Nz and CH4 reasonably well but its accuracy is not enough to lead to reliable results, especially in the PSD analysis, which is known to be very sensitive to small experimental errors and hence to small changes in the governing equations. Moreover, the bulk phase properties near to critical gases such as COz and C2& the CS EOS cannot fairly correlate at all. The question is whether the bulk gas could be described by any more accurate equation than the CS EOS. In the case of high-pressure adsorption the contribution of the central part of a sufficiently wide slit pore to the excess must tend to zero due to the solid - fluid interaction becomes very weak, Obviously, it is possible only in the case when the same

240

EOS is applied to the adsorbed phase and the bulk phase. It suggests the following scheme of a modification of the NLDFT. The Carnahan - Starling EOS is replaced by the

much more accurate Bender EOS [ 111 and then the corresponding molar Helmholtz free energy is split into three parts: (i) the ideal component, (ii) the excess fiee energy associating with only repulsive forces and (iii) the component accounting for only attractive forces. The latter is assumed to be described by the WCA potential in just the same way done in the original NLDFT. Then the excess free energy is obtained by subtracting the ideal part and the attractive part (defined for homogeneous fluid) from the molar Helmholtz fiee energy corresponding to the Bender EOS. At a specified temperature the excess free energy is considered to be the single-valued function of the smoothed density, which is defined according to the Tarazona prescription in the case of inhomogeneous fluid. The Bender EOS is given by p = pRT + Bp2 + Cp' + Dp4 + Ep' + Fp6 + (G + Hp2)p3exp(-a,,p2) (1) where p and p are the pressure and density, respectively. The constants B, ..., H are temperature-dependent. Employing this equation, after subtracting the ideal component and that accounting for fluid - fluid attractive interactions for the molar excess Helmholtz free energy one can obtain: F,(p) = Bp + Cp212 + Dp3 /3 + Ep4 / 4 + FpS15 (2) +(Z/2)[G +ZH-(G +ZH+HpZ)exp(-a,,p2)]+C,p where Z = l/azo,CO= (2'"16/9)lre&, and NA is the Avogadro number. The term --Cop is the contribution of attractive interactions in the case of homogeneous fluid. In order to determine the potential well depth Q and the collision diameter uffthe CS EOS was used as a first approximation:

p = RTdl +(4 P. Transitionmetal fbnctionalized adsorbents proved the adsorption of the PPh3according to: Ag' > Co".

1

Introduction

Homogeneous transition-metal catalysts offer a number of advantages [I] when compared to heterogeneous catalysts. Higher selectivities are achieved due to the well-defined and adaptable ligand structures of the homogeneous catalyst. Mass transfer resistances are negligible because of the high degrees of dispersion of the reactants, products and the homogeneous catalyst in one single reaction phase. And consequently, homogeneous catalyzed processes are performed at relative mild reaction conditions in comparison to heterogeneous catalyzed processes. In spite of these advantages, homogeneous catalysis is still not as common in use as heterogeneous catalysis due to the various draw-backs during the usual methods for the recovery and recycling of homogeneous catalysts. Various processes have been proposed for the recovery of homogeneous catalysts [I]: decomposition, distillation, extraction, membrane filtration and - more recently - phase transition by using fluorous media [2]. However, these separations include additional solvents and/or are operated at process conditions that negatively influence the stability of the homogeneous catalyst. Recovery of homogeneous catalysts by adsorption excludes the need for additional solvents. The combination of a reversible adsorption with the reverse flow technology [3] - Reverse Flow Adsorption - is a potential method for the integrated recovery and recycling of homogeneous catalysts. With the right choice of adsorbent, the stability of the homogeneous catalyst is preserved as the adsorption can be carried out within the stability region of the homogeneous catalyst, for instance at reaction conditions. The catalyst is separated from the product flow by adsorption downstream the reactor. In the subsequent step, the catalyst is recycled by desorption from the saturated adsorbent by reversal of the process flow (figure I).

Figure 1. Homogeneous catalyst recycling by Reverse Flow Adsorption (feed alternatesi between A, and I

In this paper, the Hard and Soft Acids and Bases (HSAB) theory [4] is applied to select potential adsorbents for the reversible adsorption of transition-metal complexes. 2

Approach

In actual homogeneous catalyzed processes, a homogeneous transition-metalcatalyst is in equilibrium with its free transition-metal center and ligands. An excess of ligands is normally added [l] to decrease the amount of free transition-metal which negatively influences the selectivity of the reaction. Therefore, to apply Reverse Flow Adsorption, a combination of two adsorbents has to be used to reversibly adsorb: the transition-metal center and the excess of ligands. The above mentioned equilibrium - o-bondn-backbond - also exist between the free transition-metal- or ligand - and its immobilized counterpart if one of the components is bound to a solid carrier (figure 2). The HSAB theory gives a first-order prediction for the strength of the interaction between a transition-metal and its ligand. For Co(II), a transition-metal with borderline acid strength, it is expected that the interaction with a group V element containing ligand decreases according to: N > P > As > Sb. For a given soft ligand, the trend is predicted to decrease according to: Ag' > Co' > Fez+> Co2+. Ligand +Metal - Ligand

Ligand

- Metal + Ligand

Metal - Ligand

Metal + Ligand

(b) Figure 2. (a) Transition-metal adsorption by an immobilized ligand and (b) ligand adsorption by an immobilized transition-metal.

Cobalt and tipbenylphosphine (PPh3) ligands are commonly encountered in homogeneous catalyzed processes. Therefore, dichlorobis(triphenylphosphine)cobalt(II) has been selected as a homogeneous model catalyst. In 1-butanol, this complex is in

equilibrium with the flee Co(II)CI2 and PPh3 ligands.

292

We studied two groups of adsorbents, based on their interactions with the Co(I1) transition-metal center or PPh3 ligands: 0

Nitrogen and phosphor functionalized adsorbents for the adsorption of Co(I1). Hereby, Amberlyst A21 was selected for its nitrogen functionality. It is a macroreticular polystyrene - crosslinked by divinylbenzene - anion exchange resin fimctionalized with an alkylamine group. As a phosphorous functionalizedadsorbent, polymerbounded PPh3has been selected. It is a gel-type polystyrene - crosslinked by 2 [%I divinylbenzene - resin functionalized with a PPh2group. Ag' and Co2+functionalized adsorbents for the PPh3 adsorption. These transitionmetal functionalizedadsorbents were prepared by immobilizing Ag' and Co2' onto a solid carrier, for which Amberlyst 15 has been selected. Amberlyst 15, a macroreticular polystyrene - crosslinked by divinylbenzene - sulfonated cation exchange resin, has been selected as carrier because of its large pore diameter of approximately 100 [nm]. These macropores ensure the accessibility for the relatively large PPh3 ligands.

3

Experimental

The three selected carriers were: Amberlyst A21 (4.8 [mmol N/g dry], Sigma-AIdrich), polymerbounded PPh3 (3.0 [mmol P/g dry], Sigma-Aldrich) and Amberlyst 15 (4.7 [mmol H/g dry], Sigma-Aldrich). The Amberlyst 15 was firstly washed with de-ionized water (Millipore) in a column. The functionalization of the Amberlyst 15 was done by contacting the resin with either 0.1 [mM] CoCl2 (98 [%I, Sigma-Aldrich) or 0.1 [mM] AgN03 (extra pure, Sigma-Aldrich) aqueous solutions. During the ion-exchange, the hydrogen of the Amberlyst 15 was exchanged for the transition-metal. The exchange was done until maximum loading was reached. All four adsorbents were pre-rinsed with deionized water. Then, the remaining water was rinsed out of the resin with methanol (p.a., Merck). The remaining methanol was rinsed with 1-butanol (p.a., Merck). The adsorbents thus prepared were taken fiom the column and used in the adsorption experiments. The adsorption characterizations of the various adsorbents were done via batch adsorption experiments. The series of nitrogen and phosphorous functionalized = 0.3 [gr]) were contacted in erlenmeyer flasks with 10 [ml] of adsorbents dichlorobis(triphenyIphosphine)cobalt(II) (98 [%I, Sigma-Aldrich) at various concentrations of 1, 2, 4 and 8 [mM]. The Ag' and Co2+functionalized adsorbents were conctacted with 10 [ml] PPh3 solutions of 2, 4, 8 and 16 [mM]. The erlenmeyer flasks were then equilibrated at 90 ["C] in a thennostated shaking water bath for 15-16 [hr] (approximately 5 times the real equilibration time). The liquid phases were decanted and analyzed. UVNis spectroscopy (Shimadzu 2501) was used for the determination of the PPh3 concentrations at 265 [nm]. The Co(I1) concentrations have been analyzed by AAS (Varian Specrtaa 1 10). After adsorption, all equilibrated samples were contacted with 10 [ml] of fresh I-butanol for 15-16 [hr] at 90 ["C] to investigate the reversibility of the adsorption. The equilibrium concentrations after desorption of the relevant components were measured as described above. The amounts adsorbed were calculated from the differences in initial and equilibrium amounts. The loading of the adsorbents after adsorption and desorption are expressed with respect to the number of functional sites - N, P, Ag' or Co2' - in the adsorbents.

293

4

Experimental results

The results of the Co(II) adsorption and desorption experiments over the nitrogen and phosphor functionaiized adsorbents are presented in figure 3. The Co(II) loading onto these two selected adsorbents is shown as a function of the equilibrium concentration of Co(I1) in the liquid phase.

0.0

2.0

4.0

6.0

Go@) [&I Figure 3. Co(1I) adsorption (closed squares) and desorption (open squares) onto nitrogen functionalized Amberlyst A21 and Co(II) adsorption (closed triangles) and desorption (open triangles) onto phosphorous functionalized polymerboundedPPh,.

It can be concluded from figure 3, that the Co(II) adsorption onto Amberlyst A21 (closed squares) is strong. This adsorption is reversible, as the desorption results (open squares) are located on the adsorption isotherms. The poIymerbounded PPh3 adsorbent shows a less strong, but reversible Co(II) adsorption (closed triangles). The stronger Co(II) adsorption by the nitrogen functionalized adsorbent was expected from the HSAB theory. For both adsorbents, no PPh3 adsorption has been observed. The experimental results of the PPh3 adsorption and desorption on both transitionmetal - Ag+ and Co" - functionalized adsorbents are shown in figure 4 as a function of the equilibrium PPh3 concentration. Because the immobilized Ag+ can be exchanged for Co2+from the homogeneous model catalyst, the transition-metal functionalized adsorbents have only been contacted with PPh3 solutions. Thus, to avoid the exchange of an immobilized transition-metal, Reverse Flow Adsorption requires two separate adsorption beds. The transition-metal center has to be recovered before the adsorption of the ligands. Figure 4 demonstrates that the PPh3 adsorption (closed triangles) onto Co2+functionalized Amberlyst 15 is weak. The low degree of adsorption is caused by two effects: 1) the interactions between Co2+ and PPh3 - as predicted by the HSAB theory - are small and 2) the steric effects of the SOigroups with the relatively large PPh3. One Co2+is immobilized onto two SOi groups. As predicted by the HSAB theory, the Ag+ functionalized Amberlyst 15 shows a larger PPh3 adsorption (closed squares) compared to the immobilized Co2+. The experimental results - the open symbols - indicate that the PPh3 desorbes from the transition-metal functionalized adsorbents. However, no complete desorption was observed, indicating that the desorption time has been taken to short.

294

0.0

2.0

4.0

6.0

8.0

CPPh3

10.0

12.0

14.0

16.0

[d]

Figure 4. PPh3 adsorption onto (closed squares) Ag' and (closedtriangles) Co2' and desorption from (open squares) Ag' and (open triangles) Co2' functionalized Ambedyst 15.

5

Conclusion

To apply Reverse Flow Adsorption, a combination of two adsorbents has to be used for the reversible adsorption of a homogeneoustransition-metal catalyst. The transition-metal center can be adsorbed by a suitable ligand immobilized onto a solid carrier, while the ligand is adsorbed by an immobilized transition-metal. Two groups of adsorbents have been studied, based on the HSAB predictions on the interactions with the Co(I1) transition-metal center or PPh3 ligands: 0

0

Nitrogen and phosphor - group V elements - functionalized adsorbents showed to reversibly adsorb Co(I1) according to the HSAB theory: N > P. The PPh3 ligands were adsorbed - as predicted by the HSAB theory - by Ag+ and Co2+functionalized adsorbents according to: Ag+ > Co".

To avoid the exchange of the immobilized transition-metal, for the transition-metal of the homogenous catalyst, Reverse Flow Adsorption requires two separate adsorption beds. The first bed for the recovery of the transition-metal center and the second bed for the ligand adsorption. References 1. S. Bhaduri, et al, Homogeneous Catalysis; Mechanisms and Industrial Applications, Wiley-Interscience,200 1 2. A. Behr, et al, Temperature Depended Solvent Systems; An Alternative Method for Recycling Homogeneous Catalysts, proc. ECCE3,200 1 3. J. Dunnewijk, H. Bosch, A.B. de Haan, Reverse Flow Adsorption Technologyfor Homogeneow Catalyst Recovery, proc. ISMR-2,200 1 4. R. G. Pearson, Absolute Electronegativity and Hardness: Application to Inorganic Chemistry, Inorg. Chem., 27,734-740, 1988

295

MOLECULAR SIMULATION OF GAS SEPARATION BY ADSORPTION PROCESSES J. P.B. MOTA Departamento de Quimica. Cenm de Quimica Fina e Biotecnologia, F a l a h i e de Ciincias e Tecnologia. Universidade Nova de Lisboa, 2829-51 6 Caparica,Portugal A new molecular simulation te-chnique is developed to solve the pernubation equations for a multicomponent, isothermal stirred-tank adsorber under equilibrium controlled conditions. The method is a hybrid between the Gibbs ensemble and Grand Canonical Monte Carlo methods, coupled to macroscopic material balances. The bulk and adsorbed phases are simulated as two separate boxes, but the former is not actually modelled at the atomistic level. To the best of our knowledge, this is the first attempt to predict the macroscopic behavior of an adsorption process from knowledge of the intermolecular forces by combining atomistic and continuum modelling into a single computational tool.

1 Introduction

Process modelling is a key enabling technology for the development, design and optimization of every adsorption process. However, its success is critically dependent upon the accurate description of adsorption equilibriumand kinetics. Molecular simulation has now developed to the point where it can be useful for quantitative prediction of those properties. Although there are several molecular simulation methodologies currently available, bridging techniques, i.e. computational methods used to bridge the range of spatial and temporal scales, are still largely underdeveloped. Here, we present a new molecular simulation method that bridges the range of spatial scales, from atomistic to macroscale, and apply it to solve the perturbation equations for a multicomponent, isothermal stirred-tank adsorber under equilibrium controlledconditions.

2 Problem formulation Consider an isothermal stirred-tank adsorber under equilibrium-controlledconditions. q is the bulk porosity (volumetric fraction of the adsorber filled with fluid phase), qp is the porosity of the adsorbent, Fi 2 0 is the amount of component i added to the adsorber in the inlet stream, and Wi 2 0 is the correspondingamount removed in the outlet stream; both fi and Wi represent amounts scaled with respect to the adsorber volume. The differential material balance to the ith component of an m-component mixture in the adsorber yields

where ci and qi are the concentrationsin the fluid and adsorbed phases, respectively. Since the fluid phase is assumed to be perfectly mixed, dWi =yidW = c i d C ,

(2)

where yi is the mole fraction of component i in the fluid phase and dG is the differential volume of fluid (at the conditions prevailing in the adsorber)removed in the outlet stream, scaled by the adsorber volume. Substitution of Eq. (2) into Eq. (1) gives q dci

+ (1 - q)qp dqi = d E - Ci dG.

296

(3)

When Eq. (3) is integrated from state n obtained:

- 1 to state n, the following material balance is

In Eq.(4) the superscript denotes the state at which the variable is evaluated and

is the average concentration of component i in the volume AG(") of fluid removed in the outlet stream. If AG(")is small enough, then a first-order implicit approximation for Eq. ( 5 ) holds,

and Eq. (4) can be approximated as

Given that the inlet value A FYI is an input parameter, the terms on the r.-h.-s. of Eq. (7) are known quantities. To simplify the notation, the r.-h.-s. of Eq. (7)is condensed into a single parameter denoted by wi and the superscripts are dropped. Eq. (7)can be written in this shorthand notation as (q

+ AG)ci + (1 - q)qpqi =

Wi.

(8)

This equation requires a closure condition which consists of fixing the value of either AG or the pressure P at the new state. Here we show that Eq. (8), together with the conditions of thermodynamic equilibrium for an isothermal adsorption system (equality of chemical potentials between the two phases), can be solved using the Gibbs ensemble Monte Car10 (GEMC) method in the modified form presented in the next section.

3 Simulation method In the GEMC method' the two phases are simulated as two separate boxes, thereby avoiding the problems with the direct simulation of the interface between the two phases. The system temperature is specified in advance and the number of molecules of each species i in the adsorbed phase, Nip, and in the bulk, N ~ Bmay , vary according to the constraint NjB Nip = Ni, where Nj is fixed. and Nip, the following expression is obtained: If Eq. (8) is rewritten in terms of N ~ B

+

where NA" is avogadro's number and Vp is the volume of the box simulating the adsorbed phase. The value of Cj has been expressed as a function of Vp instead of the volume VB of the box simulating the bulk fluid. The reason for this is that Vp is always fixed, whereas, as we shall show below, VB must be allowed to fluctuate during the simulation when the

297

pressure is an input parameter. Obviously, for Eq. (9) to be valid the values of VB and Vp must be in accordance with the relative dimensions of the physical problem, i.e.

Since the GEMC method inherently conserves the total number of molecules of each species, Eq. (9) is automatically satisfied by every sampled configuration provided that each Ci turns out to be an integer number. This feature makes the Gibbs ensemble the natural ensemble to use when solving Eq.(9). Unfortunately,in general it is not possible to size VB and Vp according to Eq. (10) and Eq. (9) so that each Ci is an integer number. To overcome this problem, Eq. (9) is satisfied statistically by allowing Ni to fluctuate around the target value Ci so that the ensemble average gives (Ni) = Ci.

(11)

This approach is different from that employed in a conventional GEMC simulation in which Ni is fixed. When AG is an input parameter, the sizes of the two simulation boxes are fixed and their volumes are related by Eq. (10). On the other hand, when the pressure of the bulk fluid is imposed, the volume VB must be allowed to fluctuate during the simulation so that on average the fluid contained within it is at the desired pressure. Once the ensemble average ( VB) is determined, the value of AG follows from Eq. (10):

It is shown in detail elsewhere2 that if an equation of state for the fluid phase is known, the bulk box does not have to be explicitly modelled computations on the bulk box amount to just updating the value the NiB as the configuration changes. Thermodynamic equilibrium between the two boxes is achieved by allowing them to exchange particles and by changing the internal configurational of volume Vp. The probability of acceptance of the latter moves (molecule displacement, rotation, or conformational change) is the same as for a conventional canonical simulation: min{1, exp(-pAU)],

(13)

where B = l / k ~ Twith , kg the Boltzmann's constant, and AU is the internal energy change resulting from the configurational move. The transfer of particles between the two boxes forces equality of chemical potentials. The probability of accepting a trial move in which a molecule of type i is transferred to or from volume Vp is, respectively,

298

where U ( S ~ ~ Pis+ the ~ ) internal energy of configuration s ~ ~ P in+ volume ~ Vp, NB = [ N I B ,. ., N ~ B ]and , ~ ( N Bk ), is the fugacity of species i in a gas mixture at temperature ?' and mole-fraction composition NiB k NIB ... , yi = ... , ym = NNmB Yl = N B + k ' B+k' NB+k'

.

+

-

-

How the equation of state is actually employed to compute depends on the type of problem being solved. If AG is an input parameter, Vg is fixed throughout the simulation and the gas mixture is further specified by its number density p ~ ~ +=k (NB k)/ VB. If, on the other hand, the pressure is fixed, its value defines the state of the mixture. The statistical mechanical basis for Eqs. (14) and (1 5) is discussed elsewhere.2 All that remains to completethe simulation procedure is to generatetrial configurations whose statistical average obeys Eq. (1 I). To get the best statistics Ni = NiB Nip must fluctuate with the smallest amplitude around the target value Ci, which is the case when Ni takes only the two integer values int(Ci) or int(Ci) 1. It is straightforward to derive that for Eq. (1 1) to hold, the probability densities of finding the system in one of the two configurationsmust be

+

+

+

nl(N + int(Ci)) O( 1 - S i ,

N ( N i + int(Ci)

+ I ) oc S i ,

(17) where 6i = Ci - int(Ci). In order to sample this probability distribution, a new type of trial move must be performed which consists of an attempt to change the system to a configuration with int(Ci) or int(Ci) 1 particles. It is highly recommended that the box for insertionhemoval of the molecule always be the bulk box (except for the rare cases that NiB becomes zero). This choice is most suited for adsorption From the gas phase where, in general,the bulk phase is much less dense than the adsorbed phase and, therefore, more permeable to particle insertions. Furthermore, given that the bulk box is not actually modelled the molecule insertionhemovalmove amounts to just updating the value of N ~ B .

+

4 Application example Due to lack of space, the few results presented here are primarily intended to demonstrate the validity of the proposed method. The pore space of the adsorbent is assumed to consist of slit-shaped pores of width 15 A, with parameters chosen to model activated carbon. The porosity values are fixed at q = 0.45 and q p = 0.6. The feed stream is a ternary gas mixture of H ~ / C H ~ / C ZThe H ~ .vapor-phase fugacities were computed from the virial equation to second order, using coefficients taken from Reid et aL3 Methane and ethane were modelled using the TraPPESunited-atom potential in which CH4 and the CH3 group are considered as single Lennard-Jones interaction centers. The LJ parameters for hydrogen, ~ / k g= 38.0 K and 0 = 2.915 A, were taken from Turner et aL6 The potential cutoff was set at 14 A, with no long-range corrections applied. The interactions with the carbon walls were accounted for using the 10-4-3 Steele p~tential.~ The cross-term LJ contributionsbetween all molecules were calculated using the LorentzBerthelot mixing rules. The simulations were equilibrated for 1O4 Monte Carlo cycles, where each cycle consists of N attempts to change the internal configurations of volume VP (equally partitioned between translations, rotations and conformational changes) and N / 3 attempts to transfer molecules between boxes. Each particle molecule attempt was followed by a trial move to adjust the total number of molecules of that type.

299

Table 1. Results of simulations.The subscripts indicate the estimated error in the last digit of the value.

The simulations reported here consisted of pressurizing an initially evacuated adsorber with four mixtures of different compositions. These simulations are very much like the traditional flash calculations of chemical engineering thermodynamics applied to an adsorption system. The first set of runs, which we refer to as set G-NVT,is equivalent to solving Eq. (4) with c!') = 0, 41') = 0, AF!') = yj"AF(') 2 0, closure condition A G ( ' ) = 0, and the equilibriumpressure as output of the simulation. Then, a second set of runs (G-NPT) was performed with the same feed mixtures and the pressure fixed at the values obtained from the G-NVT runs. In this case ( VB)is an output of the simulation. Finally, a third set of runs (GCMC) was camed out to check the validity of our simulation technique. These runs consisted of standard multicomponent GCMC simulationswith mixture fugacities calculated from the virial equation of state using the pressures and gas-phase compositionsobtained in the G-NVT runs. The results obtained are listed in Table 1. The three sets of runs give the same results to within the statistical uncertainty of the simulations, which attests to the viability of the proposed method. The total number of molecules employed in the G-NVT and G-NPT runs is given in the 8th column of the table. They were purposively set to noninteger values to test the efficiencyof the method in generatingtrial configurationswhose statistical average obeys Eq. (1 1). As can be easily verified, there is very good agreement between (NB) (Np) and the imposed N value for every run. The theoretical approach presented here represents a successful attempt to develop an ab-initio or first-principles computational methodology to predict the macroscopicbehavior of an adsorption process from knowledge of the intermolecularforces and structural characteristicsof the adsorbent.

+

References 1. A.Z. Panagiotopoulos,Molec. Phys. 61,8 13 (I 987). 2. J.P.B. Mota, J. Chem. Phys, submitted (2002). 3. R.C. Reid, J.M. Prausnitz, and B.E. Poling, The Properties of Gases and Liquids, 4th ed. (McGraw-Hill, Singapore, 1987). 4. W.A. Steele, f i e Interaction of Gases wih Solid Surfaces (Pergamon,Oxford, 1974). 5. M.G. Martin and J.1. Siepmann,J. Phys. Chem. B 102,2569 (1998). 6. C.H. Turner, J.K. Johnson,andK.E. Gubbins,J. Chem. Phys. 114,1851 (2001).

METAL-DOPED SODIUM ALUMINIUM HYDRIDE AS A REVERSIBLE HYDROGEN STORAGE MATERIAL JUN WANG'. ARMIN D. EBNER~,KEITH R. EDISON', JAMES A. RIT"ER' AND RAGAN ZIDAN' 'Department of Chemical Engineering, Swearingen Engineering Center University of South Carolina, Columbia, SC 29208, USA,E-mail: [email protected] zWestinghouse Savannah River Company Savannah River Technology Center, Aiken, SC 29804. USA In an ongoing effort to reduce the kinetic limitation of the dehydrogenation of NaAIH4, while maintaining sufficient HZcapacity, the effect of different transition metal catalysts (Ti, Zr, Fe) in various combinations have been investigated using thennopvimhic analyses. The Ti doped systems, in all cases, exhibited the lowest Hz desorption temperam, with the HZdesorption kinetics improving with an increase in the Ti loading, but at the expense of decreasing the Hz capacity. In all samples doped with 4 mole% combinations of Ti, Zr and Fe, the Ti played the most important role; however, an interesting synergistic behavior was revealed when doping NaAIb with 1 mole% Fe and 3 mole% Ti. Overall, these results continue to prove that doping NaAW with transition metals, especially Ti, improves the Hz dehydrogenationkinetics, but much more research needs to be done.

1

Introduction

Metal-doped N a A l b is becoming a very promising material for H2 storage because it contains a high concentration of useful hydrogen (5.6 wt%). At standard conditions, the dehydrogenation of N a A l b is thermodynamically favorable, but it is kinetically slow and takes place at temperatures well above 200°C in a two-step process involving the following reactions:I4 3NaAlh

+ Na3A1H6+ 2A1+ 3H2

(1)

The first work on the doping of NaAlK with Ti used solution chemistry techniques, whereby nonaqueous solutions of N a A l b and either TiC13 or Ti(0Bu"k catalyst precursors were decomposed to solid Ti-doped NaAlb.' Zidan et aL2 and other investigators3-' discovered later that a further lowering of the dehydrogenation temperature was highly dependent on the doping and homogenization procedures. They also found that Zr when mixed with Ti improved the dehydrogenation reversibility of N a A l b over Ti alone. These favorable effects of using mixed metals as the dopant generated interest in trying other combinations of mixed metal catalysts. The objective of this study is to show the effects of Ti, Fe, Zr and their combinations on the H2 desorption kinetics of NaAlh. 2

Experimental

TiC13 (Aldrich), FeC13 (Aldrich, 99.99%. anhydrous) and ZrCb (Aldrich, 99.9%) were used as received as the catalyst precursors. Crystalline NaAl& (Fluka) was purified fiom a THF (Aldrich, 99.9%,anhydrous) solution and vacuum dried. The dried N a A l b was mixed with

301

a predetermined amount of catalyst in THF to produce a doped sample in the desired concentration up to 4 mole96 total metal. Samples containing a single catalyst or a combination of them were all prepared in this manner. The THF was evaporated while the NaAlh and the catalyst were mixed manually for about 30minutes using a mortar and pestle, or until the samples were completely dry. These mixtures were then ball-milled for 2 h, using a high-energy SPEX 8O00 mill. The above procedures were carried out in a N r laden glove box free of oxygen and moisture. A Perkin-Elmer thermogravimetric analyzer (TGA) was used to determine the hydrogen desorption kinetics at atmospheric pressure. This instrument was located in another glove box under nitrogen atmosphere to prevent any exposure of the samples to air and moisture. Samples were heated to 250°C at a ramping rate of 5"Umin under 1 atm of He, using an initial 1 minute delay to ensure an environment of pure He. Approximately 10 mg of sample were used in the TGA.

3

Resultsanddiscussion

Figure la shows the TGA results for catalyzed NaAl& with 1 to 4 mole% TiC13. The 4 mole% Ti sample exhibits the best behavior with respect to the H2 desorption kinetics, while the 1 mole% Ti sample has the highest H2 capacity. In the recent study by Sandrock et al,6 they found that the TiC13 was completely reduced by Na in the NaAl& to form NaCl and most likely zero-valent Ti. This solid state reaction can be written as: (1-x)NaAl&+xTiC13+( 1-4x)N~+3xNaCl+xTi+3xAl+6xH~

(3)

where x is the mole fraction of TiC13 in the NaAl&. This reaction shows that the H2 capacity depends on the amount of TiC13 in the sample. Theoretically, after doping with 4 mole% Ti, the H2 capacity decreases to 4.6 wt%; the experimental value obtained here is very close to this value at 4.5 wt%. Clearly, the Tic13 loading has a negative effect on the H2 capacity. In contrast, the TiC13 loading has a positive effect on the H2 desorption kinetics, which increases with increasing TiC13loading. Figure 1b shows the TGA analyses for NaAl& doped with 4 mole% each of the three different metal chlorides. The 4 mole% Ti sample exhibits the best behavior with respect to the H2 desorption kinetics, followed by 4 mole% Zr and then 4 mole% Fe. This result confirms that Ti by itself is the best catalyst with respect to the kinetic behavior. Figure 2a shows the TGA analyses for NaAlH., catalyzed with 4 mole% metal, but in different combinations and with each containing with 1 mole% Fe. The 1 mole% Fe-3 mole% Ti sample exhibits the best behavior with regard to the Hz desorption kinetics and again the kinetics increase with increasing Ti loading. Figures 2b, 3a and 3b compare the 1 to 3

302

5.0 A

f 4.0

[

s

3.0

1

2.0 1.o

0.0

50

150

100

200

cc) Figure 1. TGA analyses of NaAIH4doped with a) 1 to 4 mole% Ti; and b) varying amounts of the three pure metal chloride catalysts.

mole% Ti samples with different amounts of Fe and Zr and Ti itself. All the mixed metal samples with the same Ti loading have similar profiles, i.e., the samples with 1 or 2 mole% Ti exhibit similar kinetics and Hzcapacity. However, by comparing with Ti alone at the same loading, the 1 or 2 mole% Ti mixed with different metals show an improved kinetic profile, while losing some Hzcapacity. Surprisingly, the 1 mole% Fe-3 mole% Ti sample is better than the 4 mole% Ti sample with respect to HZdesorption kinetics, but it does nothing for improving the kinetics of the second reaction depict in eq 2. This synergistic behavior with the Fe-Ti mixed catalyst system is very interesting and needs to be explored in more detail. In general, however, all the samples containing Ti exhibited the best behavior.

303

f

4.0

1.o

0.0

5.0

4.0

1

1

3.0

i

20 1.o

0.0

Figure 2. TGA analyses of NaAI& doped with 4 mole% metal in different combinations with each containing a) 1 mole% Fe; and b) 1 mole% Ti.

4

Conclusions

These results continue to prove that doping NaAlh with transition metals, especially Ti, improves the Hz dehydrogenation kinetics. However, more research needs to be done to lower the dehydrogenation temperature even further, especially for the second reaction depict in eq 2. In this study, the effect of the different transition metals played an insignificant role in reducing the temperature or increasing the kinetics of the second reaction. Other metals and metal combinations are currently being explored for this reason, and to further reduce the temperature (increase the kinetics) of the first reaction.

304

5.0

f

4.0

I 1::

2KTklxzr+l%F.

ao

0.0

1

4.0

3.0

g

2.0

1

1.0

0.0

Figure 3. TGA analyses of NaAlH4doped with a) 4 mole% metal with at least 2 mole% Ti and varying amounts of Fe and Zr; and b) 1 mole% Fe-3 mole% Ti, and 4 mole% Ti. 5

Acknowledgements

Financial support was provided by SCURJZF/WSRC/DOE under contract WEST052, KGO9725-0. and the NRO under contract NRO-00-C-0134. References

1 2 3 4

5

6

B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253 (1997) 1. B. Bogdanovic, R. A. Brand, A. Marjanovic, M. Schwickardi, I. TolIe, J. Alloys Comp. 302 (2000) 36. R. A. Zidan, S. Takara, A. G. Hee C. M. Jensen, J Alloys Comp. 285 (1999) 119. C. M. Jensen, R. Zidan, N. Mariels, A. Hee, C. Hagen, Inter. J. Hydrogen Energy 24 (1999) 461. C. M. Jensen, K. J. Gross, Appl. Phys. A Mat. Sci. Proc. 72 (2001) 213. G. Sandrock, K. J. Gross, G. Thomas, J Alloys Comp. 339 (2002) 299.

305

SYNTHESIS AND DEHUMIDIFICATION BEHAVIORS OF MONODISPERSE SPHERICAL SILICA GELS WITH DIFFERENT PORE AND CHEMICAL STRUCTURES

C.H.CHO, Y.J. YOO,J. S.KIM, H.S.KIM, Y.S.AHN AND M.H.HAN Centerfor Functional Materials Research, Korea Institute of Energy Research, 71-2 Jang-dong, Yusong-gu, Taejon 305-343, Korea E-mail: [email protected] Monodisperse spherical silica gels were prepared by aging a gel precursor in different basic conditions. The precursor was synthesized by the SFB process and was monodisperse, spherical and 200nm sized in diameter. The specific surface area of the precursor was so small ( I 6m2/g). In the precursor, there was no pore except the micropores of which average diameter was around 16A. On the aging process, there was no spherical morphology change. As the basic condition strengthened, the specific surface area minutely increased to 25m2/g. In addition to the pore structure, the chemical structure of the precursor didn't change in the aging process. All the silica gels prepared in the present study showed the same Si-NMR spectrum in which main Q2 and Q3 peaks and minor Q' peak appeared. Clearly the reaction to form anhydrous silica has not gone to completion as evidenced by the complex distribution of Q' through Q' species. In the present study, effect of synthesis and aging conditions on the pore and chemical structures of silica gels will be investigated and then the relationship between pore and chemical structures and dehumidification behavior will be discussed.

1

Introduction

Colloidal silica gels have been applied to numerous industrial fields such as thermal insulation, catalyst supports, filters and sorbents[11. Their industrial performance is dependent not only on their chemical structures but also on their physical pore structure, which comprises pore sue, pore size distribution, pore volume and surface area. Therefore, it is important to elucidate the relationship between the pore and chemical structures and the performance efficiency. From the viewpoint of materials processing, the pore and chemical structures are affected by three kinds of processing conditions: one is concerned to the parameters of sol-gel based synthesis (hydrolysis and condensation) route[2-5], another is related to the aging and drying conditions of the wet gels[5-10], and the third is in close relationship to the calcination conditions of the dried gels[ 1 11. C. J. Brinker et al. showed that the pore size of silica gel increased by the simple immersion in a basic solution[4]. R. K. ller has summarized the dependence of solubility of amorphous silica in the aqueous solvent as a hnction of pH[1]. The solubility gently decreases at the low pH values, reaches a minimum at pH 7-8, and shows a steep increasing behavior above pH 8, as pH increases. It is expected that the aging of silica gel in basic solutions increases the pore size due to the activated particle growth by the Ostwald ripening process. Therefore, it is important to investigate the structural and chemical change of silica gels during the aging in basic conditions. In the present study, monodisperse spherical silica gels were prepared by aging a monodisperse spherical silica gel precursor in different basic conditions, and then the effect of the basic strength in aging process on the pore and chemical structures were investigated.

306

2

Methods

2. I

Synthesis of Silica Gels Silica gels with different particle size were synthesized by the SFB process[ 121, a base-catalyzed synthesis route. In the present study, the particle size was controlled by changing the volume ratio of H20 to EtOH and the addition amount of NH3, a base-catalyst. In the synthesis process, tetraethoxysilane(TE0S) was used as a silica source material. Detailed preparation procedures of silica gels are as follows. At first, a mixed solvent of ethanol and water was prepared by mixing the anhydrous ethanol(EtOH, 99.9%, Hayman Chemical Co., England) with the lab.-made DI water. The volume ratio of H20to EtOH was controlled to be from 0 to 1.5, and the total volume of mixed solvents was constant to be 100ml. In the mixed solvent, H20 simultaneously plays both roles of a solvent and a reactant. As a synthesis catalyst, NH3 aqueous solution (30%, Junsei Chemical Co., Japan) was added in the mixed solvent and then homogenized by a vigorous stirring process. The addition amount of NH3was changed from 2 to 12ml. After the homogenization process, a 2ml of TEOS (99.9%, Aldrich Chemical Co., USA) was added and mixed into the catalyst-included mixed solvents. As reaction time goes on after the TEOS addition, the hydrolysis and condensation reactions initiated, so that the transparent solution became white and white. The reaction rate highly depended on the reaction conditions such as the volume ratio of H20 to EtOH and the addition amount of NH3. The synthesis temperature and time were room temperature and 4hrs, respectively. After the synthesis reaction had proceeded for 4 hours, the synthesized silica gel was washed with water three times by repeated centrifuging and dispersion in water, and then dried at 110°C for 72hrs. All the chemicals used in the present study were used without any fkthermore purification. 2.2 Aging of Silica Get%h Basic Conditions A silica gel with 400nm in diameter was used as a precursor for the as-following aging process in basic conditions. The 0.4g of dried silica gels were dispersed in lOOml NH3 aqueous solutions with different contents of NH3 aqueous solutions (0, 0.01, 0.1, 0.5, 1, 2, 4, 6 and lOml), and then aged in slow stirring mode for 24hrs. The aged gels were water washed at three times by repeated centrifuging and stirring process, and then dried again at 110°C for 72hrs. 2.3 Characterizationof Silica Gels Particle morpholoa, size and distribution, phase, pore structure and chemical structure of the synthesized silica gels were characterized by SEM (XL30, Phillips, Holland), Laser Scattering(ELS-8000, Otsuka, Japan), XRD(DIMAX2000, Rigaku, Japan), BET(ASAP2400, Micrometrics, U.S.A.1 Autosorb I, Quanta chrone Instrument, USA.) and Si-NMR (DSX-300,Bruker, Germany) analysis, respectively. Before the BET analysis, the silica gels were degassed at 210°C for 4 hrs.

3

Results and Discussion

Generally, it is known that in the SFB process, particle size of silica gels is affected by the processing parameters such as temperature, time, [NH3], [TEOS] and volume ratio of H20 to EtOH. In the present study, the particle size was controlled to be 50 to 50Onm

307

by changing the volume ratio of H2O to EtOH and mH3], the addition amount of the base catalyst. Figure 1 represents the average particle size of the synthesized silica gels as a function of the volume ratio of H20 to EtOH and mH3]. As the volume ratio of H20 to EtOH increased, the particle size increased to maxima at about 0.1 of volume ratio of H 2 0 to EtOH, and then again decreased. In addition to the volume ratio, the content of NH3 affected the particle size. The particle size increased as the content of NH3 increased.

VokM. ntio of H,O Lo EIOH

Fig. 1 . Average particle diameter of silica gels as a function of the volume ratio of H20 to EtOH and the addition amount of NH3, a base-catalyst.

Figures 2(a) to (c) represent SEM images of the silica gels synthesized in the mixed solvents with (a) 0, (b) 0.25 and (c) 0.67 volume ratio of H20to EtOH, respectively. In all cases, the addition amount of NH3 in the synthesis process was 6ml. Without relation to the volume ratio of H20 to EtOH, all the synthesized silica gels were monodisperse and spherical. Also it was clearly shown that the particle size evaluated by the SEM analysis was well in accord with the particle size results characterized by the laser scattering method as shown in Fig. 1.

Fig. 2. SEM images of the silica gels synthesized in the mixed solvents with (a) 0, (b) 0.25 and (c) 0.67 volume ratio of H 2 0 to EtOH.

G. H. Bogush ef al. have suggested that the growth of silica gel in the SFB process should be governed by an aggregative growth mode1[13-14]. This model states that the particle growth occurs due to an aggregation of primary particles that are nucleated in a

308

supersaturation of silica, and nowadays their model is generally accepted for the growth of silica gel in a sol-gel process. If the growth of the synthesized spherical gel is governed by the agglomeration model, it is expected that the silica gels have high specific surface area, because the spherical gels are composed of primary nanogels. R. K. Iler has systematically summarized the dependence of solubility of amorphous silica on pH[ 11. As pH increases, the solubility of amorphous silica gently decreases at the lower pH, reaches a minimum at pH 7-8, and shows a steep increasing behavior above pH 8. The growth of primary particles in Ostwald ripening process will be activated by the increment of NH3 content in the aging process. It is expected that the pore size can be systematically controlled by aging the gels in different basic conditions, in other words, by controlling the growth rate of primary nanogels. From the SEM analysis, it was known that there was no spherical morphology change before and after the aging process in basic conditions. To confirm that the spherical silica gel comprises of primary nanogels, the pore structure was analyzed by the BET analysis. Contrary to the expectations, the precursor and aged silica gels have relatively very low specific surface area. In Table I, the specific surface area of the precursor and aged silica gels was presented. As the addition amount of NH3 increased, the specific surface area increased. Table I Specific surface area of the precursor and aged gels Addition amount of NH3 0 0.01 fml\ Specific surface area (m2/g)

16.6

16.6

0.1

1

10

17.6

19.9

24.1

The fact that the silica gels had low specific surface area means that the synthesized silica gels were so dense. Therefore, the aggregative growth model suggested by G. H. Bogush et al. might be impertinent for the silica gel prepared in the SFB process.

Fig. 3. Cumulative pore volume curves of precursor and aged silica gels as a function of average pore diameter.

The cumulative pore volume curves as a hction of average pore diameter were calculated to elucidate that the spherical silica gels are dense, and represented in Fig. 3. It

309

was clearly shown that all the silica gels have micropores with the pore diameter less than 20A and diffuse mesopores with the 10 to 20Onm of pore diameter. The diffuse mesopores originated in the randomly loose packing of the spherical silica gels. To investigate the detailed size distribution of the micropores, the BET analysis was minutely conducted at the very low pressure of N2.Fig. 4 represents the detailed incremental pore volume curves as a function of average pore diameter. It was clearly shown that the pore diameter of the micropores was about I5.k irrespective of the aging conditions.

-

Fig. 4. Incremental pore volume pore diameter curves for silica gels aged with (a) 0.5. (b) 2 and (c) 4 ml of N H 3 .

It is interesting to answer the question where did the micropores originate in?. Fig. 5 represents the XRD patterns of the silica gels before and after the aging process. It is obvious that there was no trace for the progress of crystallization during the aging process. Therefore, it was concluded that the micropores didn't originate in the lattice pore in crystalline phase.

,r I.0

I.

I,

(0

I.

I0

I

2

Fig. 5. XRD patterns of silica gels aged in basic conditions.

310

As mentioned in the introduction part, gas adsorption and desorption behaviors of the porous solids depends not only on the pore structure but also on the chemical structure. Therefore, it is interesting to investigate if the aging process can affect the chemical structure of silica gels or not. Fig. 6 represents the Si NMR spectra of the precursor and aged silica gelss. All the synthesized silica gels were mainly composed of ' and Q ' component, and also contained small amounts of Q'. The aging process the Q didn't affect the chemical structure of silica gels. Clearly, in the synthesis and aging processes, the reaction to form anhydrous silica has not gone to completion as evidenced by the complex distribution of Q' through Q3species.

Fig. 6. Si-NMR spectra of the silica gels (a) before and (b) after the aging process with 6 ml of N H 3 aqueous solution.

4

Conclusion

Under the consideration that silica gels prepared by SFB process were so dense, the growth of silica gels in SFB process was governed by Ostwald ripening process rather than the agglomeration model. Silica gels prepared by base-catalyzed routes such as the SFB process are not suitable for the preparation of adsorbents due to the small specific surface area. Also, the aging process in basic conditions is not suitable to control the pore and chemical structures of silica gel. In the presentation, the effect of synthesis and aging conditions on the pore and chemical structures of silica gels will be introduced and then the relationship between pore and chemical structures and dehumidificationbehavior will be discussed.

5

Acknowledgements

This work was financially supported by National Research Laboratory Program (Korea Ministry of Science and Technology).

31 1

References 1. R K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry (John Wiley & Sons, New York, 1979) 2. C. J. Brinker and G. W. Scherrer, Ultrastructure Processing of Ceramics, Glasses, and Composites (Wiley, New York, 1984). 3. C. J. Brinker and G. W. Scherrer, Sol-gel Science (Academic Press, New York, 1990). 4. A. Yasumori, M. Anma and M. Yamane, Phys. Chem. Glasses 30 (1989) 193. 5. D. C. L. Vasconcelos, W. R. Campos, V. Vasconcelos, W. L. Vasconcelos, Muter. Sci. & Eng. A334(2002) 53. 6. M. Yamane and S . Okano, Yogyo-Kyokai-Shi87(8) (1979) 56. 7. R. Takahashi, K. Nakanishi and N. Soga, J. Non-Cryst. Solids 189( 1995) 66. 8. P. J. Davis, C.J. Brinker and D. M.Smith, J. Non-Ctyst. Solids 142(1992) 189. 9. P.J. Davis, C.J.Brinker, D. M.Smith and R. A. Assink, J. Non-Cryst. Solids 142(1992) 197. 10. J. H. Harreld, T. Ebina, N. Tsubo and G. Stucky, J. Non-Cryst. Solids 298(2002) 24 1. 11. Z. J. Li, C. R. Liu and Q. S. Zhao, J. Non-Ctyst. Solids 265(2000) 189. 12. W. Stiiber, A. Fink, E. Bohn, J. Colloid & Inter. Sci. 26(1968) 62. 13. G. H. Bogush and C. F. Zukoski IV,J. Colloid& Inter. Sci. 142(1991) 1. 14. G. H. Bogush and C. F. Zukoski IV, J. Colloid & Inter. Sci. 142(1991) 19.

312

PRODUCTION OF HARD CARBONS FOR LITHIUM ION STORAGE BY THE COXARBONIZATION OF

PHENOLIC RESIN PRECURSORS S. R. M U M , T. TANIGAWA, T. HARADA AND H. TAMON Dept. of Chem. Eng., Grad. School of Eng., Kyoto University Yoshida-Honmachi,Sahyo-ku, Kyoto 606-8501,Japan E-mail: [email protected]~p T. MASUDA Div. of Materials Sci. and Eng. Grad. School of Eng..,Hokkuido UniversityNI3W8 Kita-ku, Sapporo 060-8628,Japan E-mail: takaeeng.hokudai.ac.jp

.

Hard carbons were synthesized by carbonizingvarious combinations of phenolic resin precursors in order to obtain a material with a structure suitable to be used as the anode material in lithium ion battery systems, i.e. a carbon with a large pore volume and small pore openings. The lithium ion capacities of thus obtained carbons were also measured. From the obtained results, strategies to obtain hard carbons with large reversible capacities and small irreversible capacities are proposed.

1

Introduction

Lithium ion batteries have dominated the market of portable secondary batteries, due to their higher energy densities and longer shelf lives. In this battery system, carbon materials that can store a significant amount of lithium ions within their structure are used as the anode material. There are numerous types of carbon materials, but in commercial cells, graphitic carbons are mostly used due to their high stability. However, as there is a limit in the lithium ion capacity in this type of carbon (372 mAhg-’), and materials with capacities close to this limit have already been developed, worldwide scale research is in progress to find alternative anode materials that possess higher capacities. Among various types of carbon materials, hard carbons, which are predominantly formed by graphene sheets stacked like a “house of cards” [4], is a potential alternative. However, although most hard carbons possess large lithium ion reversible capacities, their irreversible capacities are also rather large, making them difficult to be used in commercial batteries. It is widely recognized that a large part of this irreversible capacity arises from the formation of a passivation layer on the outer surface of the carbon [2]. In hard carbons, such layers are also likely to be formed within its pores, which leads to an increase in irreversible capacity. Previously, we showed that the irreversible capacities of hard carbons highly depends on their pore structures, and hard carbons which pore openings are small enough so that C02cannot penetrate into them tend to have smaller irreversible capacities [ 5 ] . However, the pore volume of such hard carbons tends to be small, and it is hard to expect large reversible capacities from such materials. As there are a wide variety of hard carbon precursors, there is a high possibility to obtain a hard carbon with a large pore volume and small pore openings by combining precursors of different natures. In this work, hard carbons were synthesized by carbonizing combinations of various phenolic resin precursors. The lithium ion capacities of thus obtained carbons were also measured. From the obtained results,

313

strategies to obtain hard carbons with large reversible capacities and small irreversible capacities are proposed. 2

Experimental

One feature of phenolic resins is that they are usually synthesized via several stages. First phenols react with formaldehyde by the catalysis of an acid catalyst and novolac resin is formed. These novolacs are usually cured with agents such as hexatnethylenetetramine and a thermosetting resin is obtained. Hard carbons can be obtained by carbonizing this thermosettingresin. A wide variety of hard carbons can be obtained by carbonizing mixtures of phenolic resins derived fiom different phenols that are at different synthesis stages. In this work, first various phenolic resins were synthesized from combinations of different phenols and formaldehyde. The phenols employed were pure phenol, o-cresol and 3,5-xylenol. Note that the relative reactivities of these phenols with formaldehyde are 1:0.26:7.55, respectively. Next mixtures of phenolic resins derived from different phenols that are at different synthesis stages were combined and carbonized at 1273 K for 1 h, yielding various types of hard carbons. The pore volumes of the obtained hard carbons were measured using the molecular probe method [3]. Adsorption isotherms of the probe molecules were measured at 298 K using an adsorption apparatus (Be1 Japan, Belsorp 28). The employed probe molecules were C02, C2H6, n-C4Hlo and i'C4H10 (minimum molecular dimensions: 0.33, 0.40, 0.43 and 0.50 nm, respectively). By applying the Dubinin-Astakhov equation (n-2) [11 to the measured isotherms, the limiting micropore volumes corresponding to the minimum size of the adsorbed molecules were determined. Measurements of the lithium ion reversible and irreversible capacities of the samples were conducted using a two-electrode cell at a constant current of 25 mAg-'. Cut off voltages were set to 0 and 2.5 V. Lithium metal was used as the counter (reference) electrode. The carbon electrodes were constructed by supporting ball-milled carbon to copper foil using PVDF. The electrolyte used was a 1 M LiC104-EC/DEC(1: 1) solution (Mitsubishi Chemicals). Celgard 2400 (Hoechst Celanese) was used as the separator. 3

Results and Discussion

Through preliminary experiments, it was found that the pore structures of carbonized phenolic resins differ significantly when different phenols are used for synthesis. When the reactivity of the phenol with formaldehyde is high, the resulting carbon tends to have small pore openings, and if low, the pore volume tends to become large. Therefore the carbonization of a combination of phenolic resins, one derived from phenols with high reactivity and the other fiom phenols with low reactivity, is expected to give hard carbons with large pore volume and small pore openings.

314

By testing various resin combinations, it was found that the carbonization of a mixture of 0-cresol derived phenolic resin at the thermosetting stage (OCN-R)with the 3,5-xylenol derived phenolic resin at the novolac stage (3,5XN) gave a hard carbon which has a large pore volume and small pore openings.

0 0.30

0.35

0.40

0.45

0.50

0.55

Minimum Dimensionof Probe Molecule [nm] Figure 1 Accumulated micropore volume distributionsof the obtained samples

Figure 1 shows typical micropore volume distributions of the carbonized mixtures along with those of carbons obtained from the carbonization of 0-cresol derived phenolic resin (Sample A) and 3,5-xylenol derived phenolic resin (Sample D). Sample A has a large pore volume but the sizes of the pore openings are rather large. On the other hand, the pore openings of Sample D are extremely small, and it is natural to assume that its pore volume is also small. These structures reflect the reactivity of the phenols used for resin synthesis. By carbonizing a mixture of 10 wt?? 3,5XN and 90 wt?? OCN-R, the pore openings of the resulting carbon (Sample B) becomes smaller, but it maintains a pore volume close to that of Sample A. When the amount of 3,5XN is increased to 20wt?h, the pore openings of the resulting carbon (Sample C) also become smaller, with a slight sacrifice of its pore volume.

315

500

IReversibleCapacity1

[IrreversibleCapacity

1

Theoretical 400 Capacity ---- -372 mAh g-l

300 200 100

0 Sample A

B

C

D

A

B

C

D

Figure 2 Lithium ion capacities of the obtained samples

Figure 2 summarizes the results of the electrochemical measurements of the obtained carbons. Sample A has a large lithium ion capacity, but the proportion of its irreversible capacity is also large due to its large pore openings. The lithium ion capacity of Sample D is not so large, but the proportion of the irreversible capacity is small which also reflects its pore structure. When compared with Sample A, a large reversible capacity increase, and a slight irreversible capacity decrease was observed in Sample B. When the amount of 3,5XN was increased to 20 wt'?! (Sample C), the reversible capacity decreased but there was a significant decrease in irreversible capacity. These results are consistent with the pore structures of the tested carbons. It is obvious that the narrowing of the pore openings is an effective way to minimize the irreversible capacities of hard carbons. We believe that it is much more feasible to directly synthesize a hard carbon with small pore openings rather than narrowing the pore openings of a synthesized hard carbon. However, the pore volumes of hard carbons which pore openings are small tend to be small when the carbons are synthesized using typical methods. The carbonizing of mixtures of phenolic resins derived from different phenols that are at different synthesis stages gives a wide variety of hard carbons with various pore structures. This method is thought to be a promising method to obtain hard carbons which pore structures lead to large reversible and small irreversible capacities for lithium ion insertion. 4

Acknowledgements

This research was partially supported by Industrial Technology Research Grant Program in '01 from New Energy and Industrial Technology Development Organization of Japan.

316

References 1.

2.

3. 4. 5.

Dubinin M. M. and Astakhov V. A., Description of adsorption equilibria of vapors on zeolites over wide ranges of temperature and pressure. A&. Chem. Series 102 (1 97 1) pp. 69-85. Fong R., Sacken U. and Dahn J. R., Studies of lithium intercalation into carbons using nonaqueous electrochemical cells. J. Electrochem. Soc. 137 (1990) pp. 2009-13 Lamond T.G.,Metcalfe J. E. 111 and Walker Jr. P. L., 6A molecular sieve properties of saran type carbons. Carbon 3 (1965) pp. 59-63. Liu Y., Xue J. S., Zheng T. and Dahn J. R., Mechanism of lithium insertion in hard carbons prepared by pyrolysis of epoxy resins. Carbon 34 (1996) pp. 193-200 Mukai S. R., Masuda T., Tanigawa T., Harada T., and Hashimoto K., Structure of Hard Carbons which Leads to Small Irreversible Capacities for Li Insertion. International Symposium on Carbon Science and Technology for New Carbons (Tokyo) (1998) pp. 305-306.

317

NOVEL BIOACTIVITE CARBOMINERALSORBENTS, INCLUDING CLUSTER AND CARBON NANOTUBES FOR SUPERSELECTIVE PURIFICATION OF BIODIESEL FUEL LIQUID HYDROCARBONS AND CARBONHYDRATE FROM SULFUR CONTAINING IMPURITIES

-

DMITRY I. SHVETS Institute for sorption and problems of endoecology NAS of Ukraine.13. General Nawnov Str... Kiev-I64,03680, Ukraine E-mail: [email protected]: [email protected] The properties of bioactive carboncontaining sorbents are consided during clearing liquid hydrocarbonaceouspropellant from sulfurcontaining substances. It was revealed, that the superselectivity is stipulated by availability in bioactive carboncontaining sorbents of clusters and carbon nanotubes. It was shown, that a type of clusters, their concentration, natm and the sizes of nanotubes are defining at sorption of sulfurcontaining substances. It was found, that the purified solar oil contains minimum amount of admixtures and conforms the requirements of the world standads. The mechanism of superselective sorption with allowance of nanoclaster structuresis discussed.

1.

Introduction

The diesel fuel recently involves the increasing notice both technologists, and consumers. For making solar oil use alcohols, carbohydrates etc. Despite of the successes, achieved in this direction, rather acute there is a problem of quality of propellant, namely contents of toxic admixtures, first of all elementcontaining. It is stipulated by that element (Cl, 0, N, S) - containing liquid carbohydrates and the hydrocarbons (i.e. propellant with admixtures) at combustion will derivate toxic products of the special danger, which one harm as the person, and ozone layer of the Earth. Therefore, problem of purification of liquid carbohydrates and hydrocarbons from toxic admixtures is one of most significant for today for a propellant industry. The technologies applied for clearing of propellant from toxic admixtures are rather diverse; however degree of clearing remains insufficiently high and does not correspond to increasing requirements. Not subjecting criticism any of existing methods, in operation is considered an opportunity of clearing of components of solar oil - liquid carbohydrates and hydrocarbons from sulfurcontaining substances with usage new of bioactive carboncontainingsorbents with nanocluster structures in porous space.

2. Methods As objects of study there were used new modifications of carbon materials vegetative (carbon material with heterostructures of a radical type), organocarbon (nanotube), carbon (puffed up graphite) nature and also combined carboncontaining composite. Modification of sorbents conducted during their synthesis (heat treatment at fixed temperature) or by processing carbon materials by special (ecologically clean) reagents at ambient temperature. As toxic matters used petroleum, oil products, and also various fuels containing toxic substances of a various type - aromatic substances, asphaltenes, resin, sulfurcontaining substances, dispersible sulfur etc. Analysis of properties of carbon materials and sorbed products carried out with usage of physicochemical methods, atomic-adsorptive analyzer, and also with applying of methods ESR-,IR-, UV-, X-ray spectroscopy, Zpotentiometry etc. As the object of investigation diesel fuel @F) was used. DF- is the product of cokechemical production, containing asphaltenes, soot’s, paraffin’s, sulfur-, nitrogen- and

oxygen-containing compounds. Purification of DF was done by method of their passing through sorption column, which containing material with sorption-catalyticproperties on 318

the base of natural type's aluminosilicatesof common formula (SiO~)m(Al~O~),, (Meox),, . y(0H). z(H20). Estimation of purification degree was done by visual method by changing of DF color in comparison with DF, corresponded to technical specifications. Efficiency of the process of purification was estimated by quantity of volumes of DF, purified by one volume of compositional material.

3. Results and Discussion

To obtain the composite meeting our requirements we varied its composition, taking different amounts of the components and modifying their properties. Experimental results showed that carbonmineral composites are much better than others adsorbents, for example, the mineral one. A good selectivity of carbon materials made us to assume that it is a carbon substance is responsible both for selectivity and synergistic effect of adsorption too. From our point of view one of the reason of such a behavior could be specially organized carbon structures such as intermediate complexes (clusters), which possess peculiar electron properties only to them. Probably similarly toxic substances are adsorbed, such as phenols, cresols, quaiacol, aldehydes, polyatomic alcohols, ethers etc. (Table 1). Table 1. Efficiency of decontaminationof some aqueous solutions from organic pollutants by &on andcarbonrmn ' eral* materials

I

modified

concentration,m@l POUUtPDt

Phenol Cresol Quaiacol Ethers* Aldehydes* Polyatomic alcohols*

Initial

I

Final

2.1 4.8

Decontamination level, %

100 100

lo4

4.0. lo4 279.2

100

13.2 0.036

0.4

95 92 100

3,7

High efficiency of developed composite carbonmineral materials was demonstrated in the processes of purification of water-alcohol mixtures. The results of experiment demonstrated more effective extraction of toxic impurities from solution by our material in comparison with known technology (Table 2). TaMe 2. Efficiency of purification in relation to kind of sorption material

The results of comparative study of sorption properties of carbomineral sorbents (initial and modified form) on the purification of technological solution are demonstrated on Fig. 1-2. It is seen that carboncontainingsorbents provide efficient purification of iquid hydrocarbons and diesel oil from toxic impurities chlorophyll and carotene. The absence of toxic impurities in liquid carbohydrate - vegetable oil (from rape) provides, as it was stated by us experimentally, its high heating capacity as a component of biodiesel fuel. As

-

319

can be seen from fig.3, the most cleaning degree is reached at use sorbent of mixed type only, where synergic effect is maximal. Efficiency of refining different mixtures with the usage of the combined sorbents was established to be significantly higher than in the case of initial unmodified sorbents. The observed synergism proposes new approaches to the selection of components of the carbomineral sorbents. A. X

'9

4% 100 80 1 -NSP

60

2.Nsz

3-Nso

40 20

&NST

S-WW

0

1

2

3

4

5

6

S-NYNM

Fig.1 Influence of natun of dispersed material on purificationdegree of oil (rape) from impurities

1

2

3

4

5

6

7

8

9

Rg. 2. Influence of sorbents type and contents of compositional mixtures on type of DF purification from toxic admixtures: 1.2.5.6.8 - natural sorbents; 3 . 4 - two-component mixtures; 7 fourcomponent mkm; 9 two-stage process of purification

-

-

Modification of carbomineral sorbents causes not only change of sorption properties resulted in increasing the quantity of sorbate, but also significant improving of selectivity. It should be noted that increasing of the adsorption ability of carbomineral sorbents towards toxic pollutants of different nature, such as ions of heavy metals, organic compounds, was attained by thorough selection of complexing agents. High efficiency of developed composite carbomineral materials was demonstrated in the processes of purification of water-alcohol mixtures. The results of experiment demonstrated more effective extraction of toxic impurities from solution by our material in comparison with known technology. It was established that the use of diffusion vortex affecting on heterogenous system, composed from a liquid medium and composite sorbing materials allows to reach nonadditive effect: it lies in abnormal raise of an extent of hydrocarbon fuel purification from admixtures, as in case of a stream passage through fixed sorbent layers, as in case as sorbent of composition mixture each component separately. Nonadditive effects are characteristic for the processes of water purification from oil (Fig 3) and benzene [5], also hydrocarbon fuel from toxic admixtures [6](Fig.4 ). The perspective alternative by technology of purification of liquid mediums from toxic impurities represents by usage of systems in a superextreme state [7,8].The used term %systemin an extreme state D is similar to the term "stress" offered prof. E. Thukin [9] or model of disastrous considered by us in [lo]. Specificity of such state is that the system passes in a superextreme state at usual stresses and temperatures, and the supercriticality of a state is caused by applying (in our case) combination of dispersible materials with different significances of a power charge (first of all of opposite sign). At such situation there is such system condition, when the separation of components of a liquid phase (or extraction of one component from an admixture) proceeds practically instantly through a stage "of stress", i.e. through a stage of instantaneousrough "reacting" by addition in a liquid phase of dispersible materials. Confirmation of a developed hypothesis are indicated below experimental data received by us recently. However, as

our investigationshave shown, not only mixed sorbents capable to show superselectivity and provide high purifying degree of fuel from impurities. As it is seen from fig.5, similar 320

1

1

2

3

4

5

6

7

8

9

Fig. 3. Influencecondition of heat treatmenton the type of DF purification by sorbents of different type (2.5.8 heat treatment of s o h t by stages;3.6,9 -heat treatment of sorbents mixture; 1.4.7 - sorbent without heat treatment)

+petroleum -)Ioil 1

2

3

4

5

6

Material type Wg.5. The influence of CadJoncontaining nanotubes n m (1 - 3 cadJon nature. 4 - 6 organic nature)on the efficiency of the oilproducts adsorption from water surface

Fig.4. The purification variation of liquid hydrocarbons from toxic admixture depending on sorbent nature: 1 - carbon. 2 - mineral. 3 carbonmineral

-

effect of superselectivity is observed for carbon nanotubes. The reasons of the anomalous effect of such type are yet to be explained, however even today we can say about it value for practical purposes for obtaining of high pure ecologically harmless kinds of biofuel.

Surface’s nature of aluminosilicate’s sorbents On the base of data analyses Fig.3 and Fig.4 it can be suggested, that surface’s nature of alumosilicate’s sorbents plays certain role in purification process. It’s significant, effect of purification can be observed in two situations, on the one hand-sorbents, containing acidic centers, on the other hand-sorbents with basic’s properties. Accept acidic-basic properties of alumosilicate’s surfaces, another factor influence on effectivity of purification. Confirmation of rightness of such suggestion is the fact, at thermo-handling of natural alumo-silicates loosing of water and also dehydroxilation of surface take place. But, as we see from Fig.3, while increasing of water loosing purification degree of DF is achieving maximum, and after that is falling down, that connected with releasing of such called structural or “zeolite” water from pore’s space. Simultaneously, the character of dependence of purification degree on thermo-handled samples is opposite to data on Fig.2. More detail analyses show, that among natural surface (acidic and basic centers) as a sorption, such a structural water play important role. Exactly “zeolite” water is maximally releasing from alumosilicate’ssorbents at thermo-handling in the range of 350 C , in other wards, while going away of water releasing from pore’s space take place. In these pore‘s

321

space additional sorption of toxic substances can be happened. It’s possible that impact of such factor is significant, but it’s role in process of purification (as the role of OH-groups and “acidic” centers) is not dominant. Nature of surface’sions also play significant role in purification process Confmnation for benefit of further mentioned argument is the fact, that exactly after the thermo-handling of natural materials degree of purification of DF is increasing, but type of natural sorbents is greatly influencing ). And we can suggest, that with structural-sorption parameters nature of surface’s ions also play significant role in purification process. In sorption process one materials saturated by toxic admixtures of DF and changes their color; another one doesn’t change their color. This fact confirms possible role of surface’s ions nature. This fact takes a great attention, because it allows to suggest, that surface’s centers are not only the sorption centers, but in the same time they are catalytically manyfunctional centers of cluster’s type, Independent confirmation of these suggestion is a point, that quantity of purified fuel in counting of it per lgramm (>20 g/g) exceeds any variants (practical and theoretical), from volume of sorption pores Vs = 0,6-0,7 cm3/g and density of admixtures d=0,8 g/cm3for sorption of 20 g of admixtures 25 cm3 of volume is needed, and really system has 1,8-2,0 cm3, i. e. only sorption scheme of purification is unreal. Relatively to the possibility of proceeding of the sorption-catalyticprocess, it is shown by the fact , that functional-depended thermo-handling strictly increasing purification degree, as in absolute values, such in effectivity. 4. Conclusions

So. ?resented experimental results shows, that at first time we’ve found the effect of overselectivity, based on fact, that in oxide’s systems of mixed type, passed through the functional-depended thermo-handling, it is formed, clusters center and carboncontaining nanotubes nature , providing the possibility of proceeding of over selective sorptioncatalytical process of purification of liquid hydrocarbons from toxic admixtures

References 1. Shvets D. I., Adsorption Science and Technology 17 (1999) pp. 709-714 2. Shvets D. I., Carboncontaining sorbents of mixed type: properties and applying in extreme situations, Curbon-02 In Prossiding International Conference on Carbon, September 15-20,2002 Beijing, China. ISBN 7-900352-03-7/16-03. 3. Shvets D. I., Kravchenko O.V., Urvant O.S. ,The applying of mixed carboncontaining sorbents for removal of oil products from the water surface, Curbon-02 In Prossiding International Conference on Carbon, September 15-20, 2002 Beijing, China. ISBN 7900362-03-7/IG-03. 4. Shvets D.I., Lapko V.V., Urvant O.S., Sorption-catalytic purification of diesel fuel from toxic admixtures by oxides systems of mixed type, Adsorption Science and Technology 21 (2003) (in press). 5. Shvets D. Biocatalysys on the basis of carbon - and carbon-mineral sorbents and their property.Ros. 5’ European Congressjn Catalysis. Limeriick, Iceland. 2001. p.129. 6. Baiker A. Chem. Rev., 99 (1999), pp. 453-454. 7. Sagave P.E.Chem. Rev., 99 (1999), pp. 603-605. 8. Shchukin E, Amelina E, Izmailova V. In Roc. NATO Advanced Research Workshop “Role of interface in Environmental Protection”, Miskolc - Hungary, 2002, p. 6.

322

Shvets D In Pmc. NATO Advanced Research Workshop “Role of interface in Environmental Protection”, Miskolc - Hungaru, 2002, p. 161. 10. Shvets D.I., Chochlova L.I., Kravchenko O.V. et al.The physico-chemicalaspects of oil sorptiol by the carbon sorbents from water surface. Chemical and Technology Water, 24 (2002), pp.22-31 (Rus).

9.

323

TITANOSILICATEETS-10: SYNTHESIS,CHARACTERIZATIONAND ADSORPTION FOR HEAVY METAL IONS [GEORGE)X.S . ZHAO*, J. L. LEE AND P.A. CHIA Department of Chemical and Environmental Engineering, National University of Singapore, I0 Kent Ridge Crescent, Singapore I 1926; E-mail: cliens0,nus.edu.sg

Microporous titanosilicate ETS-I0 was synthesized in the absence of organic template and characterized using XRD, FTIR, Raman, and nitrogen adsorption. The adsorption properties of heavy metal ion PbZ+on ETS-I0 were studied by measuring the adsorption kinetics and equilibria using a batch-type method. Highly pure ETS-I0 was obtained without the presence of ETS-4. The adsorption rate of heavy metal ions on ETS-I0 is extremely rapid, less than 5 seconds is required to attain maximum adsorption capacity in a 10 moYL solution with a batch factor of 200 m u g . The kinetic data can be fitted by pseudo-second-order model whereas the equilibrium data is better fitted to Langmuir isotherm than to Freundlich isotherm. The maximum adsorption capacity of Pb2+and Cu” as predicted by the Langmuir equation was 1.12 and 0.578 mmol/g, respectively. The remarkable adsorption rate coupled with the high adsorption capacity promise potential applications of ETS-I0 for the removal of heavy metals present in drinking water and wastewater.

1

Introduction

Heavy metals such as lead (Pb) are common groundwater contaminants that must be controlled to an acceptable level according to the increasingly stringent environmental regulations. The heavy metals, especially Pb present in drinking water are extremely detrimental to human beings. Depending on the existing form of the metals, they can be removed by different technologies such as chemical precipitation, membrane filtration, ion exchange, and adsorption [l]. Unfortunately, none of them affords reducing the heavy metals to an acceptable low level at a minimal contact time, which is of significance in the treatment of waters, especially in purification of drinking water. ETS-10, a microporous titanosilicate ETS-10 discovered by Engelhard in 1989 [2] is zeolite material with a pore-opening size of 0.8 nm [2-4]. The basic anhydrous formula of as-synthesizedETS- 10 is Na1.5&.5TiSi5013. Unlike conventional zeolites, the framework of ETS-I0 is constituted fiom SiO, tetrahedra and TiOs octahedra by corner-sharing oxygen atoms [3]. The presence of each tetravalent Ti atom in an octahedrum generates two negative charges, which are balanced by exchangeable cations Na+ and K+.Such a unique framework property manifests itself a promising and potential ion exchanger for many cationic metal ions that are present in waters such as Pb2+,Cd”, Cu2+,Zn2+,etc. However, adsorption data of heavy metals on ETS-10 have been hardly available [5,6]. Al-Attar and Blackbum compared the uptake properties of uranium on ETS-10 materials synthesized with different Ti sources and noted that the method of ETS-10 preparation has a considerable effect on the uptakes of uranium [5]. Kunicki and Thrush [6] observed that ETS- 10 and ETAS- 10 (Al-containing ETS- 10) displayed an extraordinarily rapid adsorption rate towards Pb2+.The concentration of Pb2’ was reduced to a negligible amount fiom 2000 ppm in a very short contact time at a liquid to solid ratio of 100:2.4 (g:g). Unfortunately, adsorption equilibrium data were not available.

324

Motivated by the work of Kunicki and Thrush [6], we have carried out a systematic study on the adsorption equilibria and kinetics of several heavy metal ions including PbZ+,

Cd2+,Cu”, Zn2+and Ni2+on ETS-10 using a batch-type technique. Our observations have not only confirmed that ETS- 10 does exhibit a remarkable adsorption rate towards heavy metal ions but also demonstratedthat the maximum adsorption capacity of Pb” on ETS- 10 is as high as 1.12mmoVg according to the prediction of Langmuir model. This is the highest uptake that has been observed on zeolite materials [ 11. In this paper, we present the unusual adsorption properties of ETS- 10 towards heavy metal ions Pb2+and Cu2+.Adsorption equilibrium and kinetic data are reported. Fitting of the experimental equilibrium results to both Langmuir and Freundlich isotherms and the kinetic data to both pseudo-fist- and pseudo-second-orderkinetic models is described. 2

Methods

The method of synthesis of ETS-10 was similar to that reported by Yang and co-workers [7]. The synthesis recipe was 8NaOH:2KOH:TiF.,:5.7Si02:350H~0. Sodium silicate solution (Merck) and TiF4(Aldrich) were used as the Si and Ti source, respectively. All chemicals were used as received. Samples were characterized by using X-ray diffraction (XRD) on a Shimadzu XRD-6000 diffractometer (CuKa radiation), physical adsorption of nitrogen on a Quantachrome NOVA 1000, Fourier transform infrared (FTIR) spectroscopy on a Biorad spectrometer using the KBr method, Raman spectroscopy on a Bruker FRA 106/S FT-Raman spectrometer, and scanning electron microscopy (SEM) on a Joel JSM-5600LV. Adsorption of heavy metal ions on the ETS-10 sample was conducted using a batch-type method at room temperature (23 OC). For kinetic measurement, 1 g of air-dried ETS-10 was added to 100 ml of solution pre-acidified by nitric acid under shaking so as to generate a solution of pH = 5.8. Then, 100 ml of 20 mmoVL Pb(NO& (or Cu(N03)z) solution was added to obtain a mixture with an initial Pbz+(or Cu2> concentration of approximately 10 mmoVL, a final pH value of about 5.0 and a batch factor (ratio of liquid volume to solid mass) of about 0.2 L/g. 5 ml of the mixture was withdrawn at an appropriate time interval by using a 5 ml syringe and rapidly filtered through a 0.2 pm nylon membrane filter. The filtrate was collected in a sample valve and analyzed for Pb (or Cu), Na and K concentrations using a spectrometer (Perkin-Elmer Analyst 300). The amount of metal adsorbed at time t (s), qc(mmovg), was deduced from the mass balance between the initial concentration (Co)and concentration at time t (CJ. The experimental data were fitted to pseudo-second-order equation (t/q, =l/vo +t/q,) [8], where k (g/mmoVs) is the adsorption rate constant, qc(mmoVg) is the amount of metal adsorbed at equilibrium, and vo (mmol/g/s) is the initial adsorption rate which is kq;. Adsorption equilibrium data were collected in a similar way as the kinetic measurement. The equilibrium time was 10 min, which, according to the kinetics data, was found to be sufficiently long to attain adsorption equilibrium. The experimental data were fitted to both the Langmuir isotherm ( q , = q,,,bC,/(1+ bC,) ), where qm (mmol/g) is the maximum adsorption capacity, C, (mmol/L) is the equilibrium concentration of the heavy metal ion in solution, and b (L/mmol) is the Langmuir constant, and the Freundlich isotherm (4,= KC:‘”),where K and n are constants.

325

Results and discussion

3 3. I

Characterization of Em-I0 sample

Fig. 1 depicts the XRD pattern, Raman and ETIR spectra of the ETS-I0 sample used in this study. The XRD pattern is identical to that of ETS-10 materials reported previously [2,7], showing that the sample is a pure ETS-I0 phase without the presence of ETS-4 impurity (it has been shown that ETS-4 is a thermodynamicallymore stable phase than ETS-10 and it is normally present in an ETS-I0 product [9]). The Raman spectrum (left-hand insert) further confirms the purity of the sample. A most intense peak at 728 cm", assigned to Ti-0-Ti stretching in comer-shared Ti06 chains [lo], and a small band at 305 cm-', attributed to Ti-0-Si bending [1I] can be seen. The absence of any peak above 800 cm-' on the Raman spectrum further confirms the inexistence of ETS-4 in this sample [10,111. The observation of a main band at about 1024 cm-' due to Si-0 stretchingand a few small bands at 668,570 and 434 cm-' because of Ti-0 stretching, Si-0 rocking and 0-Ti-0 bending, and 0-Si-0 and 0 - T i 4 bending and Ti-0 rocking, respectively, on the FTIR spectrum (right-hand insert) is consistent with the literature data of ETS-10 [121. SEM image (not shown) displays cuboid-shape crystals of about 5 pm, but incomplete crystal growth was also observed. The specific surface area of this sample calculated by using BET model was about 258 m2/g. I

c

I

728

a E

g

Ba

r .-

200

l

600 800 1000 Raman shift (cm-l)

1200

400

400

aoo 1200 Frequency (cm-l )

l

I

I

I

I

I

I

5

10

15

20

25

30

35

40

Two theta Figure 1. XRD pattern, Raman spectrum (insert,left) and FTIR spectrum (insert, right) of

as-synthesized ETS-10 3.2

Adsorption kinetics of h e w metal ions on E n - I 0

Fig. 2 shows the adsorption kinetics of Pb2+ on ETS-10 together with the pseudo-second-order kinetic curve. It is seen that the adsorption rate is extremely fast. Under the experimental conditions, less than 10 s was required to attain saturation adsorption. When the concentration of Pb was about 2.5 mmoVL, Pb2+was not detected

326

after 5 s. This rapid-adsorption behavior of ETS-10 towards Pb is of interest and significance in terms of purification of drinking water as Kuznicki and Thrush suggested [6]. This unusual adsorption behavior of ETS-10 towards heavy metal ions is currently being investigated. It is believed that the rapid adsorption rate must be related to the unique structure of ETS-10. The experimental data fit well to the pseudo-second-order equation. The experimental data were fitted to pseudo-second-order equation (t/q,=\/v0+t/qe),7 where k (g/mmol/s) is the adsorption rate constant, qe (mmol/g) is the amount of metal adsorbed at equilibrium, and v0 (mmol/g/s) is the initial adsorption rate which is kq*.

O

Experimetnal data Langmuir isotherm

Experimetnal data Pseudo-second-ordvr model

40

60

BO

0.00

100

0.05

0.10

0.15

ce (m mol/L)

Figure 3. Adsorption isotherms of Pb2+ on ETS-10 (23 °C)

Figure 2. Adsorption kinetics of Pb2+ on ETS-10 (C0 = 10 mmol/L, V/m = 0.2 L/g)

3.3

Adsorption isotherms of heavy metal ions on ETS-10

Fig. 3 shows the adsorption isotherm of Pb2+ on ETS-10. The experimental data were fitted to both Langmuir and Freundlich isotherms and the results are included in Figure 2 as well. The parameters derived from the two models are presented in Table 1. As can be seen, the Langmuir isotherm predicts the experimental data much better than the Freundlich isotherm. The maximum adsorption capacity of Pb2+ on ETS-10 as predicted by the Langmuir isotherm is 1.12 mmol/g or about 232 mg/g. Such a high adsorption capacity of Pb2+ on zeolite materials had never been observed [1]. The adsorption of Pb2+ on a commercial zeolite NaY sample (Si/Al = 2.45) was measured as well and the results showed that its maximum adsorption capacity towards Pb2+ was about 56.3 mg/g, much less than ETS-10. The adsorption of other heavy metals including Cd2+, Cu2+ and Zn2+ on ETS-10 and zeolite NaY was also studied and compared. A similar adsorption behavior was observed, namely, the adsorption rate of these metal ions on ETS-10 was extremely fast and the adsorption capacity was much higher on ETS-10 than on zeolite NaY. The maximum adsorption capacity of Cd, Cu and Zn on ETS-10 was found to be all around 0.5 mmol/g while it became about 0.2 mmol/g on zeolite NaY. Table 1. Langmuir and Freundlich Parameters for Pb2* Adsorption on ETS-10

Freundlich model n A:(mmol L 1/n g 1 ) 1.50 0.102 M/n

/v 0.932

Langmuir model qm (mmol g"1) b (L mmol"1) 1.12 480

327

I? 0.992

3.4

Adsorption mechanism of heavy metal ions on Em-10

During the measurementsof adsorptionkinetics, the concentrations of both Na+and K+were also monitored. It was observed that the decrease in Pb2+was at the expense of the increase in Na+and K+. In addition, the concentration of K' in the solution was always one third of that of Na+, indicating the equal opportunity of ion exchange of Pb2' with the two alkali metal ions. Furthermore, the concentration sum of Na' and K' in the solution exactly doubled the concentration of Pb2+at any time, suggesting that each Pb2+ion replace the pair of (1.5Na++0.5K?. These results suggest an ion exchange mechanism. 4

Conclusion

In conclusion, we have demonstrated the unusual adsorption behaviors of microporous titanosilicate ETS-I0 towards heavy metal ions with an extremely fast rate and in a large adsorption capacity, showingthe application potentials of ETS- 10 for water and wastewater treatment. The adsorption is most likely via ion exchange. It is believed that the unique compositional framework together with the large pore size of ETS-10 play a vital role in determining its remarkable adsorption properties towards heavy metal ions. References 1. Bailey S. E., Olin T. J., Bricka R. M. and Adrian D. D., A review of potentially low-cost

sorbents for heavy metals, Wat. Res. 33 (1992) pp. 2469-2479. 2. Kunicki S. M., Large-pored crystalline titanium molecular sieve zeolites, US Patent 4853202 (1989). 3. Anderson M. W., Terasaki O., Ohsuna T., Philippou A. MacKay S. P., Ferreira A., Rocha J. and Lidin, S., Structureof the microporous titanosilicate ETS-10, Nature 367 ( I 994) pp. 347-351. 4. Rocha J. and Anderson M. W., Microporous titanosilicates and other novel mixed octahedral-tetrahedral framework oxides, Eur. J. Inor. Chem. (2000) pp. 80 1-818. 5. Al-Attar, L., Dyer, A. and Blackburn, R., Uptake of uranium on ETS-I0 microporous titanosilicate, J. Radioanal. Nucl. Chem. 246 (2000), pp. 45 1-455. 6. Kunicki S. M. and Thrush K. A., Removal of heavy metals, especially lead, ffom aqueous systems containing competing ions utilizing wide-pored molecules of the ETS-I0 type, US Patent 4994191 (1991). 7. Ho Y. S.and McKay G., The kinetics of sorption of divalent metal ions onto sphagnum moss peat, Water Res. 34 (2000), pp. 735-742 8. Yang X.,Paillaud J.-L., van Breukelen H. F. W. J., Kessler H. and Duprey E., Synthesis of microporous titanosilicate ETS- 10 with TiF4and Ti02, Micropor. Mesopor. Mater. 46(2001)pp. 1-11. 9. Xu H., B a n g Y., Navrotslq A., Enthalpies of formation of titanosilicates ETS-4 and ETS-10, Micropor. Mesopor. Mater. 47 (2001), pp. 285-291. 10. Kim W. H.,Lee M. C.,Yo0 J. C. and Hayhurst D. T., Study on rapid crystallization of ETS-4 and ETS-10, Micropor. Mesopor. Mater. 41 (2000), pp. 79-88. 11. Su Y., Balmer M. L. and Bunker B. C., Raman spectroscopic studies of silicotitanates, J. Phys. Chem. B 104 (2000), pp. 8160-8169. 12. Mihailova B., Valtchev V., Mintova S. and Konstantinov L., Vibrational spectra of ETS-4 and ETS-10, Zeolites 16 (1996), pp. 22-24.

ORDERED MACROPOROUS MATERIALS STRUCTURALLYTEMPLATED BY COLLOIDAL MICROSPHERES Z. Zhoy W.C. Ong, (George) X. S. Zhao* Department of Chemicaland Environmental Engineering, National University of Singapore, Singapore 119260: E-mail: [email protected]

Ordered macroporous materials (OMMs)are a new family of porous materials that cm be synthesized by using colloidal microspheres as the template.‘” The most unique characteristicsof OMMs are their uniformly sized macropores arranged at micrometer length scale in three dimensions. Colloidal microspheres (latex polymer or silica) can self assemble into ordered arrays (synthetic opals) with a threedimensional crystalline structure. The interstices in the colloidal crystals are infiltrated with a precunor material such as metal alkoxide. Upon removal of the template, a skeleton of the infiltrated material with a three-dimensionally ordered macroporous structure (inverse opals) is obtained. Because of the 3D periodicity of the materials, these structures have been extensively studied for photonic applications.’ In this paper, the synthesis and characterization of highly ordered macroporous materials with variouS compositions and functionalities (silica, organosilica, titana, titanosilicate, alumina) are presented. The application potential of OMMS in adsorptionlseparation is analyzed and discussed.

1

Introduction

Porous materials can be classified, according to the pore size, into microporous materials with ore size smaller than 20 A, mesoporous materials with pore size between 20 8, and 500 and macroporous materials with pore size larger than 500 A. Microporous and mesoporous materials have been studied extensively and have found wide applications in many areas, such as adsorption, separation and catalysis. One of the frequently used methods of synthesizing porous materials is template strategy. A well-known example is the Mobil’s liquid-crystal templating mechanism by which many mesoporous materials can be made. This discovery provides an opportunity of treating and processing relatively large molecules. However, when macromolecules such as enzymes are dealt with, macroporous materials with an ordered pore structure and uniform pore size are desired. OMMs were first synthesized for the purpose of photonic a lications because of their 3D spatial structure with periodically varied reflective index!‘Since then, OMMS with various chemical compositions have been prepared and they have been demonstrated to find wide applications in the fields other than photonics? With the availability of porous inorganic-organic materials: and the successful synthesis of surfactant-mediated highly ordered mesoporous organosilica materials,’ organic-inorganic macroporous composite materials would afford high application potentials in adsorptiodmembrane separation. In addition, OMMs are a suitable material for processing macromolecules such as enzymes. In this paper, the synthesis and characterization of highly ordered macroporous materials with various chemical compositions and functionalities (silica, organosilica, titana, titanosilicate, aliumina) are presented. The application potential of the OMMs in adsorptiodseparationare analyzed and discussed.

1,

329

2

2. I

Methods Synthesis of OMMS by using self-assemblystrategy

The synthesis strategy of OMMs is similar to the conventional template method (see Fig. 1). The template used is self-assembled microspheres instead of single molecule or surfactant micelles. Colloidal microspheres with uniform size and morphology are induced to spontaneously organize into a crystalline lattice (artificial opals), which could have a face-centered cubic (fcc) or a hexagonal-closed-pack (hcp) structure or the combination of them depending upon experimental conditions. The artificial opals are slightly annealed to improve the stability of the crystal structure and to form connecting necks between the adjacent spheres. Then the voids among the opals are tilled with another material such as oxides, polymers, and hybrid materials, etc. Finally, the artificial opals which act as the template are removed, leaving behind an OMM.

li

Self Assembly

a

Witration

removai

Figure 1. Illustration of preparation procedures of OMMs using self-assembled template 2.2

Synthesis of uniform-size microspheres

Polystyrene (PS) spheres were prepared with emulsifier free emulsion polymerization. Silica spheres were prepared following the modified Sttiber method! All chemicals were used without further purification.

330

2.3

Ctystallization of the microspheres

The formation of artificial opals was achieved by using a number of methods including sedimentation, filtration, evaporation, and drip method. 2.4

Opal annealing

After the opal forming, annealing was used. The PS opal was heated in oven at 110 ' C (slightly higher than the glass temperature of PS) for 5 to 10 min.

2.5

Opal injiltration

Infiltration of the artificial opals was carried out by either filtration, or chemical vapor deposition, or soaking method. 2.6

Template removal

The template was removed either by calcination at 550 OC for the PS opal or by chemical etching in HF solution for silica spheres. 2.7

Characterization

Samples were characterized by using scanning electron microscope (SEM) (JEOL JSM5600LV), transmission electron microscope (TEM), physical adsorption, FTIR, Raman.

3 3.1

Results and discussion Opalformation

Highly crystalline opals can be obtained by sedimentation, filtration, evaporation, and drip methods as demonstrated by the SEM images shown in Fig. 2. Thermodynamically, atoms or molecules tend to adopt the structure with the lowest Gibbs free energy. When colloidal microspheres are allowed to self assemble at a closeto-equilibrium state, they tend to form closely packed crystalline structures, such as face cubic center (fcc) and hexagonal closed packed (hcp) lattices. As a result, maintenance of equilibrium plays a vital role in obtaining a highly crystalline lattice. In addition, because the fcc structure has the lowest Gibbs free energy it is always the observed structure during self assembly as can be seen from Fig. 2. Another important factor having influence on the self assembly process is the size distribution of the spheres. It was observed that a size derivation of larger than 5% destroyed the long-range order. 3.2

Opal annealing

The opals obtained by self-assembly are mechanically unstable because there is only Van der Waals force between spheres. The subsequent infiltration process could easily destroy the ordered colloid arrays. So we annealed the opals of polymer sphere to increase their stability. As a result, there would form interconnections between spheres, which come from the slight melting of the sphere surfaces. These necks can provide the opal with necessary mechanical stability. In addition, they are important for producing inverse opal structure. After infiltration, when the samples are treated with calcinations, these necks can act as channels for the transport of the products formed during calcination like COz.

331

Figure 2. Self-assembled artificial opals fabricated with different methods: (A) sedimentation, (B) filtration, ( C ) evaporation, and (D) flow-controlled evaporation.

3.3

Opal infdtration and template removal

Figure 3. SEM images of (A) PS opal infiltrated with SiOz, (B) after removal of template of (A), (C) macroporous Ti02 prepared via core-shell method, and (D) macroporous organosilica materials

332

Macroporous materials with an ordered structure of various framework compositions were prepared by using both PS and silica spheres as the template. Shown in Fig 3 (A) is the SEM image of PS opal infiltrated with Si02. It is seen that the self-assembled opal was essentially completely inverted with SiOz. After removal of the PS spheres by calcinations, polystyrene decomposed into C02 and a reverse opal structure was left behind (Fig. 3 B). Fig. 3 (C) shows the SEM image of macroporous Ti02prepared by self assembly of core-shell spheres. PS spheres were first hctionalized, coated with a layer of TiOz on the surface to obtain PS-Ti02 composite spheres, followed by self assembly to obtain an order structure. Upon removal of the PS spheres, a macroporous shell was obtained. Using similar approach, macroporous organosilica materials was also obtained (Fig. 3 D) by using silica spheres as the template.

4

Conclusion

In conclusion, we have successfully fabricated OMMs with different pore size by using opal templated method. The pores are in micron scale and have narrow size distribution. We also studied the synthesis conditions during each step and found a relative feasible route to prepare OMMs.

References 1. Imhof A. and Pine D. J., Ordered macroporous materials by emulsion templating, Nature 389 (1997), pp. 948-951. 2. Vlsov Y.A., Xiang Z. B., Sturm J. C. and Norris D. J., On-chip natural assembly of silicon photonic bandgap crystals, Nature 414 (2001), pp. 289-293. 3. Xia Y.,Gates B., Yin Y.and Lu Y., Monodispersed colloidal spheres: old materials with new applications,A h . Muter. 12 (2000), pp. 693-713. 4. Subramania G.,Constant K., Biswas R., Sigalas M. M. and Ho K.-M., Inverse facecentered cubic thin film photonic crystals, A h . Muter. 13 (2001), pp. 443-446. 5. Stein A., Sphere templating methods for periodic porous solids, Micropor. Mesopor. Muter. 44-45 (2001), pp. 227-239. 6.Loy D. A. and Shea K. J., Bridged polysilsesquioxanes. Highly porous hybrid organic-inorganic materials, Chem. Rev. 96 (1995), pp. 1431-1442. 7. Inagaki S., Guan S., Ohsuna T. and Terasaki O., An ordered mesoporous organosilica hybrid material with a crystal-like wall structure, Nature 416 (2002), pp. 304-307. 8. T. Okubo, T. Miyamoto, K. Umemura and K. Kobayashi, Colloid Polym. Sci., 2001, 279,1236. 9.Okubo T., Miyamoto T., Umemura K. and Kobayashi K., Seed polymerization of tetraethyl orthosilicate in the presence of colloidal silica spheres, Colloid Polym. Sci. 279 (2001), pp. 1236-1240.

333

ADSORPTION OF NITROGEN, OXYGEN AND ARGON IN TRANSITION AND RARE EARTH ION EXCHANGED ZEOLITES A AND X RAKSH VIR JASRA, JINCE SEBASTIAN AND CHINTANSINH D. CHUDASAMA Discipline of Silicates & Catalysis, Central Salt & Marine Chemicals Research Institute, G.BMarg, Bhavnagar 364 002 INDIA E-mail: [email protected]

Adsorption ofN2,O~and Ar in zeolite A and X exchanged with silver, cerium and europium at 15 and 30°C temperatures has been studied. Silver exchanged zeolite A show higher adsorption capacity and selectivity for nitrogen compared to Zeolite CaA.Enhanced Nitrogen interactions of nitrogen molecules with AgA are also observed in very high heat of adsorption (38kJmor'). AgA also exhibits argon selectivity over oxygen atypical of zeolites. Both cerium and europium exchanged Zeolite X show oxygen selectivity over nitrogen in Henry region which has been attributedto interaction of oxygen with non-stoichiometric oxides of these cations apparentlyformed inside zeolites cavities.

I

Introduction

Zeolites are of immense interest in gas and chemical industries for purification and separation due to their unique adsorption properties. The extra framework cations invariably present in zeolites play significant role in determining their adsorptive properties [l]. In particular, if coordinately unsaturated metal ions can be incorporated inside the zeolite cavities, novel adsorption behavior may be fashioned on the basis of coordination of guest molecules. Exchangeable transition metal ions in activated zeolites are generally coordinately unsaturated and readily form complexes with a variety of guest molecules [2]. This is due to the van der Waals and Coulombic interactions between the extra framework cations and the guest molecules. Synthetic zeolites of type A, X and mordenite having alkali and alkaline earth metals as the extra framework cations have been extensively studied and are mainly used as the nitrogen selective adsorbents for the adsorptive separation of oxygen from air [3,4,5,6,7,8]. However, there are few studies on the adsorption behavior of transition or rare earth cation exchanged zeolites. In the present paper, we report the adsorption of nitrogen, oxygen and argon in some transition and rare earth metal ion exchanged zeolite A and X. 2

Methods

2. I Cation Exchange Commercially available zeolite A and X from Zeolites and Allied Products Mumbai India was used as the starting material without any further purification. For exchangingwith transition and rare earth metal ions, the zeolite was mixed with 0.1 M aqueous solution of the specific cation and refluxed at 80°C for 4 hours. The zeolite was filtered and washed with distilled water until the washings were free fiom ions and used for the adsorption measurements after drying. The percentage of ion exchange was determined by Atomic Adsorption Spectroscopy after acid digesting the sample.

334

2.2 X-ray Powder Difiaction

Structuralchanges due to the cation exchange, if any, was determinedby measuring the X-ray powder diffraction with Philips X'Pert M P D system using Cu Kctl(l= 1.54056) in the 20 range 5 to 65. 2.3 Adsorption Isotherms

Oxygen, nitrogen and argon adsorption at 15°C and 3OoCwas measured using a static volumetric system (Micromeritics ASAP 20 10, USA), after activating the sample at 350°C under vacuum for 8 hours. Dosing of the adsorbate gas was done at volumes required to achieve a targeted set of pressures ranging from 0.5 to 850mmHg. The adsorption and desorption were completely reversible, hence it was possible to remove the adsorbed gases by simple evacuation. 2.4 Heat of Adsorption

Isosteric heat of adsorption was calculated from the adsorption data collected at different temperatures using Clausius - Clapeyron equation. &dH" = R [alnpl/[a( 1IT)] 1 0 where R is the universal gas constant, 8 is the fraction of the adsorbed sites at a pressure p and temperature T. A plot of Inp against 11T gives a straight line with slope of A,,+H"/R. 3.

Results

3. I The Silver exchanged Zeolites A & X Silver exchanged zeolite A shows anomalousadsorption behavior towards nitrogen and argon. The adsorption isotherms measured at 303K on AgA are compared with those measured in NaA and CaA at the same conditions in figure 1. The nitrogen adsorption capacity for AgA is higher than (1.5 times at about 1 atmosphere pressure) that of CaA, which has been reported to have highest adsorption capacity for nitrogen among Zeolite A based adsorbents. The completely silver exchanged form shows an adsorption capacity of 22.3cclg for nitrogen, 4.36 cclg for oxygen and 6.25 cclg for argon at 30°C and 765mmHg. AgA shows argon selectivity (around 2) over oxygen, which is not generally observed in other zeolite adsorbents. The high heat of adsorption value (-38 klmol-') for AgA compared to 25 klmol-' for CaA shows that the nitrogen molecules undergo chemisorption-assisted physorption with the silver species inside the zeolite cavities. However the silver exchange on zeolite X increases the nitrogen adsorption capacity only by 20% compared to sodium form. The adsorption isotherms on A@, NaX and CaX is given in figure -2. But the N2 selectivity over O2 is as high as 20 in the Henry's region. The heat of adsorption value for nitrogen is high in the low-pressure region.

335

Nitrogen Adsorption Isotherms

0

200

400

600

800 Pressure in mmHg

Figure -1 Adsorption Isotherms measured at 30°C ~

A%r M

2

12

Nitrogen Adsorption Isotherm 1

30 25 20

15 10

5 0

Pressureinnun€& Figure

- 2 Adsorption isotherms measured at 30°C

3.2 Cerium & Europium Zeolite X Cerium and Europium exchanged zeolite X having different cerium and europium loading was prepared and N2,O2and Ar adsorption measurementswere carried out at 15°C and 3OOC. The heat of adsorptionvalue for oxygen is found as high as -69 klmol-' in cerium exchanged zeolite X. Samples having specific cerium loading displays oxygen adsorption selectivity over argon and nitrogen. However, form our measurements extending up to 850mmHg equilibrium pressure, oxygen selectivity over nitrogen is limited to lower pressure only ( 6 0 0 mmHg), i.e., Henry's region as which is shown in Fig.3.

336

Europium exchanged zeolite X also shows higher heat of adsorption (-47 klmof') value for oxygen. Samples having specific Europium loading displays oxygen adsorption selectivity over argon and nitrogen in Henry' s region only which is shown in Fig.3.

II

CeX

Eux 0.7

0.6 0.5 0.4

0.3 0.2 0.1 0

0

II It

200400600800

0

20

40

60

80

Pressure in mmEIg Figure -3 Adsorption Isotherm measured at 3OoCon CeX & EuX

The isosteric heat of adsorption on various zeolite adsorbents are given in table 1 Table-I Heat of Ahorption in kJ mot'K' at O.04mmoUg Coverage

Sample

Nitrogen

Oxygen

I

Argon

25.01 20.51

4

AgA

38.77F

NaX

22.45

15.62

13.98

CaX

29.15

15.23

14.12

Agx

37.86

15.84

14.76

CeX

17.57

69.48

EUX

16.10

46.90

I

15.60

Discussion

The various interactions contributing towards total energy of physical adsorption include dispersion, polarization, field-quadmpole and close range repulsion interactions. Framework oxygens and extra M e w o r k cations are the principal sites for interactionswith

337

the adsorbates molecules for zeolitic adsorbents. However, in case of zeolite adsorbents used for the air separation, the main factors influencingthe nitrogen adsorptioncapacity and selectivity are the difference in the quadruple moment of the adsorbate molecules and accessibilityand the charge density of the cations [4,5]. Silver ions are reported to form neutral and charged silver clusters in the zeolite cavities on vacuum dehydration at higher temperature [9].These coordinately unsaturated species interact very strongly with nitrogen molecule [5].The other major factor contributing towards the interaction of silver zeolite with nitrogen is enhanced polarization interactions between AgA and nitrogen molecules. Ag+ ion with higher polarizing power compared to Na'/Ca'2 ions and nitrogen molecules with relatively high polarizability interacts strongly with each other resulting into enhanced adsorption for nitrogen. The argon selectivity showed by the silver zeolite A can be explained in terms of the difference in polarizability values. Argon having polarizability higher than that of oxygen shows relatively high adsorption capacity and selectivity. Higher interaction of oxygen molecules with CeX may be explined in terms of interaction of oxygen molecules with non-stochiometricoxides of cerium probably formed inside zeolite cavities due to interaction of cerium with zeolitic framework oxygens. Cerium is known for its non-stochiometric oxides, which are oxygen selective.

5

Acknowledgements

We grateful to Dr.P. K. Ghosh, Director, CSMCRI and Department of Science and Technology for the financial assist and support.

References 1. R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic

Press: London, (1978). 2. E.Y.Choi, Y. Kim and K.Seff, Microporous and Mesoporous Materials, 41 (2000), 61-68 3. S. Sircar, Ind Eng. Chem. Res., 41 (2002) 1389-.I392 4. R.V.Jasra, N. V. Choudary and S.G.T.Bhatt, Separation Science and Technology,26 (1991)885-.930 5. R.T.Yang, Y,D.Chen, J.D.Peck and N.Chen , Ind Eng. Chem. Res., 35 (1996) 3093-3099. 6. S.Sucar, R. R. Conrad and W. J. Ambs, US Patent 4,557,736,1985. 7. C. C. Chao US Patent 5,454,857,1995 8. J. Sebastian and R. V.Jasra PCTApplication (2002) 9. T.Sun and K Seff Chemical Reviews 94 (1994)857 - 870. 10. J.G.Nery, Y.P.Mascarenhas, Bonagamba and N.C.Mello, Zeolite, 18 (1997)44-49. 11. E.F.T.Lee and L.V.C.Rees ,Zeolite,7 (1987)446-450.

338

ADSORPTION OF METHYLENE BLUE FROM WATER ONTO ACTIVATED CARBON PREPARED FROM COIR PITH, AN AGRICULTURAL SOLID WASTE C. Namasivayam * and

D. Kavitha

Environmental Chemistv Lab, Department of Environmental Sciences,

Bharathiar University, Coimbatore -64 I 046, iNDIA *Comespon&ng author: Tek 91422422222; F m +92422425706 E-mail: [email protected] (C. Namasivayam) The adsorption of methylene blue by coir pith carbon was carried out by varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature. Equilibrium adsorption data obeyed Langmuir isotherm. Adsorption kinetics followed a second order rate kinetic model. The adsorption capacity was found to be 5.87 mg dye per g of the adsorbent. There was no significant change in the per cent removal with pH. The pH effect and desorption studies suggest that chemisorption might be the major mode of the adsorption process. Key wordr: Methylem blue, adrorption, coir pith carbon. isotherm, pH efeci, desorption studies

1. Introduction

The conventional methods for removal of dyes using alum, femc chloride, coconut shell based activated carbon etc., are not economical in the Indian context. Pollard el QL, [l] and Namasivayam [2] have reviewed non-conventional adsorbents used for the removal of dyes and heavy metals. Adsorption of acidic and basic dyes by activated carbon and bone char [3] acid dye on bagasse pith [4] and orange peel [5], acidic and basic dye by biogas residual sluny [6] and banana pith [7] have been reported. Industrial solid wastes like Fe(III)/Cr(III) hydroxide for the removal of congo red [8] and red mud for the removal of procion orange [9] have been investigated. Dyeing wastewater treatment using organic coagulant and Fenton’s reagent [lo] and ozone [ l l ] have also been examined. The photocatalytic oxidation and photoelectrocatalytic oxidation of rhodamine B using the Ti/TiOz have been investigated and compared [12]. Biosorbents like chitosan beads for reactive dye [13] and mixed culture of bacillus species and pseudomonas stutzeri for methyl red [14] have been studied. Coir pith is a waste byproduct of coconut coir industries in southern India. It is a soft biomass separated from the coconut husk during the preparation of coir fiber. The purpose of this work was to investigate the removal of methylene blue by coir pith carbon as a model study. 2. Experimental

2.1. Materials and Methods Carbonized coir pith was prepared fkom dried coir pith powder (250-500pm) using a muffle furnace at 7OO0C for 1 h. Adsorption experiments were carried out by agitating 300 mg of carbon with 50 nil of dye solution of desired concentration and pH at 200 rpm, 35OC in a thennostated rotary shaker (ORBITEK, Chennai, India). Dye concentration was estimated spectrophotometrically by monitoring the absorbance at 660 nm using UV-Vis spectrophotometer (Hitachi, model U-32 10, Tokyo). pH was measured using pH

339

meter(Elic0, model LI-107, Hyderabad, India). The dye solution was separated fiom the adsorbent by centrihgation at 20,000 rpm for 20 min and its absorbance was measured. Effect of pH was studied by adjusting the pH of dye solutions using dilute HCl and NaOH solutions. Effect of adsorbent dosage was studied with different adsorbent doses (25-600 mg) and 50 ml of 10,20,30,40 mg& dye solutions. For desorption studies, the adsorbent that was used for the adsorption of 10,20 mg/L of dye solution was separated from the dye solution by centrifugation. Then the spent adsorbent was agitated with 50 ml of distilled water, adjusted to different pH values for 60, 80 min. The desorbed dye was estimated as before. For temperature studies, adsorption of 10 mg/L of methylene blue by 200 mg of adsorbent was carried out at 35,40, 50 and 6OoC in the thermostated rotary shaker.

3. Results and discussion 3.1 Effects of agitation time and concentration of dye on adsorption The amount of dye adsorbed (rng/g) increased with increase in agitation time and reached equilibrium. The equilibrium time was 40 and 60 rnin for 10 and 20 mg/L dye concentration, respectively and 120 min for both 30 and 40 mg/L dye concentration.. The amount of dye removal at equilibrium increased from 1.4 to 5.4 mg/g with the increase in dye concentration fiom 10 to 40 mg/L. It is clear that the removal of dyes depends on the concentration of the dye. 3.2. Adsorption dynamics 3.2.1 Adsorbent rate constant The rate constant of adsorption is determined from the first order rate expression given by Lagergren [6]: log (qe -9) = logq, - kl t / 2.303 (1) where qe and q are the amounts ofdye adsorbed (mg/g) at equilibrium and at time t (min), respectively and kl is the rate constant of adsorption (Ymin).The disagreement between the calculated and experimental qe shows that the adsorption of dye onto coir pith carbon is not a first order reaction. The second-order kinetic model [151 is expressed as t/q=l/kzq:+t/qe (2) where kz (g/mg/min) is the rate constant of second order adsorption. The calculated qe values agree very well with the experimental data. This indicates that the adsorption system belongs to the second order kinetic model. The second order rate constants were in the range 0.61-0.07g/mg/min. Similar phenomena have been observed in the adsorption of Congo red on coir pith carbon [ 161. 3.3. Effect of adsorbent dosage Increase in adsorbent dosage increased the per cent removal of dye, which is due to the increase in absorbent surface area. The per cent removal was quantitatively at 300,400,500 and 600 mg/50ml adsorbent dose for 10,20,30 and 40 mg/L dye concentration, respectively. 3.4. Adsorption isotherms Langmuir isotherm is represented by the following equation [171: CJqe = 1/Qob + C e IQo (3) where C, is the concentration of dye solution (mg/L) at equilibrium. The constant Qo signifies the adsorption capacity (mg/g) and b is related to the energy of adsorption (L/mg). Qoand b were found to be 5.87 mg/g and 0.93 L/mg, respectively. Adsorption equilibrium data do not follow the Freundlich isotherm [ 181.

340

3.5. pH effect.

The per cent removal was >90% in the pH range 2-1 1. At pH 2, though positively

charged surface sites on the adsorbent do not favor the adsorption of dye cations due to the electrostatic repulsion, dye removal was still high (>90%). As the pH increased, the removal increased slightly. This suggests that the chemisorption might play a major role in the adsorption process. A similar trend was observed for the adsorption of methylene blue by clay [19] and rhodamine B by biogas residual slurry [20]. The per cent desorption was 5 0

0 0.0

0.2

0.4

0.8

1.0

0.8

Relative Pressure

Fig. 3.

N2 dsorption-desorption isotherms of silica, magnesium silicate, aluminum silicate and titanium silicate-impregnated ceramic sheets.

.....

10

100

lo00

Average Pore Diameter (Anstroms)

Fig. 4. Pore structures of silica, magnesium silicate, aluminum silicate and titanium silicate-impregnatedceramic sheets.

4

Conclusion

The ceramic sheets with aluminum silicate or titanium silicate adsorbents showed better dehumidification behavior than the silica and magnesium silicate-impregnated ceramic sheets. The superior dehumidification efficiency originates om the well developed micropores.

385

5

Acknowledgements

This work was fmancially supported by National Research Laboratory program (Korean Ministry of Science and Tschnology).

References 1. Kuma, Shirahama, Izumi, US. Patent 5,753,345 (1998). 2. Kuma, Toshima, Shirahama, Noriaki, Izumi, Horaki, US. Patent RE37,779 (2002) 3. Kuma, Tosimi, Okano, Hiroshi, US.Patent 4,911,775 (1990) 4. Kuma and Hirose, J. Chem. Eng. Japan, Vol. 29, No. 2,376 (1996). 5. Dinnage, Paul A., Tremblay, Gerard, U.S.Patent 5,505,769 (1996)

386

SYNTHESIS OF ZIRCONIA COLLOIDS FROM AQUEOUS SALT SOLUTIONS AND THEIR APPLICATIONS KANGTAEK LEE Department of Chemical Engineering, Yonsei University,Seoul, Korea E-mail: [email protected] ALON v. MCCORMICK' AND PETER w.c m 2 Department of Chemical Engineering and Materials Science' and Chemistd, University of Minnesota, Minneapofis,M N 5.5455, USA E-mail:;[email protected] We monitor the synthesis of submicron zirconia colloids from dissolved ZrQCI2.8HzO using quasielastic light scattering. We investigate the effects of both the precursor salt concentration and of the pH on the final colloid size distribution. We find that the pH has the strongest effect on the final colloid size. These colloids are used in the polymerization-induced colloid aggregation process to produce micron-range particles, and the performance is compared with commercial colloids.

1

Introduction

Monodispersely-sized submircon zirconia colloids are useful starting material for ceramics, catalysts, and chromatographic stationary phases. One such process (polymerization-induced colloid aggregation or PICA process) requires entirely reproducible aqueous zirconia sols with no surfactants [1,2]. In the PICA process developed by Iler and McQueston [3], the concentrated (-20 wt. %) 100 nm zirconia colloids are aggregated by urea-formaldehyde polymerization reaction to produce the porous zirconia particles in the size range of 4 6 pm [ 1,2]. Two classes of precursor have been demonstrated to prepare such zirconia colloids: salts [4-71 and alkoxides [&lo]. Given the constraints above, the hydrothermal synthesis from zirconium salts [4,7] is especially attractive. The salt is easier to protect and handle than alkoxide precursors, and the product colloid does not require the removal of organics or dispersing agents before firther aqueous processing. Bleier and Cannon [7] showed that one can easily make 80 nm monodisperse aqueous sols with no dispersing agents. Unfortunately, the procedures reported to date do not show how one might easily control independently the average size and the final concentration. For instance, the process to make 80 nm colloids gives only ca. 2.5 wt.% sol [7], while the PICA process requires higher concentrations [1,2]. Since dialysis is time consuming and evaporation would require some care to avoid flocculation, it would be advantageous to produce more concentrated sols with the hydrothermal salt method while maintaining the average size ca. 100 nm and maintaining monodispersity. In this paper, we show that the average size is primarily controlled by pH, and we show that important limitations are imposed by increasing solubility at high initial salt concentration due to the lower resultant pH. We investigate the effect of salt concentration and of added acid on the final average size, polydispersity, and yield. We use these colloids to make PICA particles and make a comparison with the commercially available colloids.

-

387

2

2. I

Methods Zirconia Cottoit&@nthesis

Crystalline zirconium oxy-chloride octahydrate (ZrOCI2-8H20)was used as received from three sources - Aldrich, Acros, and Alfa. ZrOC12-8H20 was dissolved in distilled and deionized water so that the final concentration was 0.1,0.2, 0.4, and 0.6 M.Dissolution at room temperature required ca. three hours. Then, the solution was boiled (up to -105 "C at these concentrations)under reflux to allow reaction. Samples were taken at different reaction times and immersed in a water bath to quench the reaction. The following tests were performed size measurement using QELS (quasi-elastic light scattering, Coulter Model N4 SD)and yield measurement. For QELS, the samples were diluted with distilled and deionized water and the run time was 400 s. For the yield measurement, the sample was centrifuged at 12,000 rpm for at least 15 min, then the mass of the dried solid was measured. For some samples, a nitrogen sorptometer (Micromeritics) was used to measure the surface area after drying. 2.2

PICA Reaction of the Synthesized Colloids

When 0.4 M ZrOCI2.8H20 was used, the weight fraction and the pH of the final colloidal solution were -5 wt.% and -0.5, respectively. To compare the PICA performance of the synthesized colloids with that of the Nyacol colloids, it was concentrated by reverse-osmosis. For reverse osmosis, stirred cell from Amicon with an ultrafiltration membrane (50,000 MW cut-off) was used and pressure up to 50 psi was applied. After reverse osmosis, water was added to increase the pH. By repeating this as many times as necessary, 20 wt.% colloids at pH 1.75 were prepared. For a PICA experiment, urea (U) from Fisher Scientific company was added to a new 30 ml polystyrene reactor. Formaldehyde solution (F) from Mallinckrodt company in a separate beaker was added to a reactor to start the reaction. ([U]+[F])/[Zr02] and [U]/[F] ratios were kept at 1.5 and 0.75, respectively. Reaction was quenched with water when the secondary particles started appearing (-15 min). Particles were washed with water three times, then with isopropanol twice. The particles were, then, filtered and dried in a vacuum oven. At this point, the weight of particles was measured. The particles were burned at 350 OC for two hours to remove polymers on the surface, then sintered at 750 OC for six hours and 900 "C for three hours. After burning and sintering, the weight of particles was again measured to get yield and polymer content. The surface area and the pore size distribution of these particles were measured using Micromeritics nitrogen sorptometer. S-800 SEM (scanning electron microscopy) from Hitachi was used to observe the final particles. 3 3. I

Results Zirconia Colloids Synthesis

We find that the synthesis as reported by Bleier and Cannon [7], but changing the salt concentration, is capable of reproducibly making 100 - 250 nm (average diameter) zirconia colloids with a narrow distribution (standard deviation always smaller than -50

nm). However, we note that there is a maximum in the final particle size at 0.2 M ZrOC12.8H20 (for which the pH is 0.9). Figure 1 shows the effect of the initial salt concentration on the final particle size. Note that there is a correlation between the salt concentration and the pH of the solution; the pH falls with higher salt concentration because of zuconia ion hydrolysis. If too much salt is supplied (> 0.4 M),the yield is limited to L 80 %. The highest concentration we obtain (pH < 0.9) is ca. 5 wt. %. 300 250

-

0

200

-

-

150

-

100

50

-

i

0' 0.0

I

I

0.2

0.4

I

0.6

0.8

[ZrOCI;8H20] (M) 1.1

0.9

0.5

0.4

PH Figure 1. Final particle size vs. initial [ZrOC12.8H20] (the axis at the bottom indicates the initial pH of the solution).

3.2

PICA Reaction of the Synthesized Colloi&

In order to compare the PICA performance of the synthesized colloids with that of commercial colloids from Nyacol, 5 wt.% colloids were concentrated to 20 wt.% and pH was adjusted to 1.75 as described in the experimentalsection. After urea and formaldehyde were added to our colloids, small particles appeared in -5 minutes, and they grew until the secondary particles appeared in -15 minutes. This is shilar to the PICA performance of the Nyacol colloids [1,2]. After sintering, the yield was -7 % and the polymer content was -70 %. The surface area of these particles was -20 m2/g. Figure 2 shows the SEM pictures of final PICA particles made from our colloids; clearly, they are very porous and show the irregular shapes. 4

4. I

Discussion Zirconia Colloid Synthesis

One spec%= goal was to see whether simply adjusting the salt concentration could provide starting marerid for PICA process. Of our trials, a solution of 0.4 M ZrOC12.8H20with HCI is optimal for the procftrction of monodisperse -100 nm colloids because:

t 0p.m Figure 2. SEM micrograph of PICA particles made from the synthesized colloids.

1) It produces a narrow distribution. 2) The fmal concentration of colloids is high, at -5 wt. %. 3) The yield reaches 100% in less than a week.

Since we would ideally like to produce even more concentrated sols (ca. 20 wt. %). we wish to better understand the effect of the initial salt concentration. It is helpful to consider the solubility of zirconia in water. The solubility of zirconia is reported by Baes and Mesmer [ 111. At low pH range the solubility increases sharply with acidity. For the systems with no added acid, the limited yield - when the initial concentra$on of salt is higher than 0.4 M (pH 50.4) - may be attributed to the very high solubility of zirconia at low pH. 4.2

PICA Reaction of the Synthesized Colloids

The differences of the final PICA particles using the synthesized colloids and the Nyacol colloids are summarized in Table 1. It should be noted that the reactant concentrations and the burning procedure were optimized for the Nyacol colloids, but not for our colloids. The different polymerization kinetics in the PICA reactions using our colloids instead of Nyacol colloids causes the very high polymer content, which also leads to lower yield, smaller particles, bigger pores, irregular shape, and a broader distribution. We also ascribe big pores to the unoptimized burning procedure. Thus, in order to reproduce PICA particles made from Nyacol colloids, it is essential to reoptimize the reactant concentrations of PICA reactions using our colloids.

Table 1 Comparison of PICA particles using the synthesized and the Nyacol (V-66 batch) colloids

5

Synthesized colloids

Nyacol colloids (V-66)

Yield (%)

7.0

15.0

Polymer content (%)

70.0

40.0

Pore size (angstrom)

1000.0

400.0

Final distribution

broad

narrow

Shape

irregular

spherical

Final particle size (prn)

1.O 4.0

-

3.0 5.0

-

Acknowledgements

This study is supported by Korea Research Foundation Grant (KRF-2001-005-E00030).

References 1. Amen M.J., Kizhappali R., Cam, P.W. and McCormick A.V., J. Muter. Sci.29 (1994) pp.6123-6130. 2. Sun L., Annen M.J., Lorenzano-Porras F., Can P.W. and McCormick A.V., J. Colloid Intevace Sci. 163 (1994) pp. 464-473. 3. Iler R.K. and McQueston H.J., US Patent No. 4,010,242 (1977). 4. Blumenthal W.B., The Chemical Behavior of Zirconium (D.Van Nostrand Company, Inc., New York, 1958) pp. 125-132. 5 . Matsui K. Suzuki H. and Ohgai M., J Am. Ceram. SOC.78 (1995) pp. 146-152. 6. Aiken B. Hsu W.P. and Matijevic E., J. Muter. Sci. 25 (1990) pp. 1886-1894. 7. Bleier A. and Cannon R.M., In Better Ceramics Through Chemisny 11, ed. by C.J. Brinker, D.E. Clark and D.R. Ulrich (Materials Research Society, Pittsburgh, 1986) pp. 71-78. 8. Fegley B., White P. and Bowen H.K., Am. Ceram. Bull. 64 (1985) pp. 1115-1120. 9. Bartlett, J.R.,Woolfiey J.L., Percy M., Spiccia L. and West B.O., J. Sol-Gel Sci. Tech. 2 (1 994) pp. 2 15-220. 10. Lerot L., Legrand F. and De Bruycker P., J Muter. Sci. 26 (1991) pp. 2353-2358. 11. Baes C.F. and Mesmer R.E., The Hydrolysis of Cations (John Wiley & Sons, New York, 1976) pp. 152-159.

391

COMPARISON OF NANO-SIZED AMPHIPHILIC POLYURETHANE (APU) PARTICLES WITH SDS, AN ANIONIC SURFACTANT FOR THE SOIL SORPTION AND THE EXTRACTION OF PHENANTHRENE FROM SOIL IK-SUNG, AH" AND HEON-SIK, CHOI Dept. of Chem. Eng., Yonsei University, Seoul, Korea E-mail: [email protected] JU-YOUNG, KIM Dept. of materials engineering, Samchok National University,Samchok, Korea E-mail: JUYOUNGK@amchok. ac.kr Understanding of surfactant sorption onto soil is needed to assess surfactant mobility in soil and surfactant-facilitated transport of hydrophobic pollutants in soiVaqueous systems. Micelle-like amphiphilic nano-sized polyurethane (MU)particles synthesized from amphiphilic urethane acrylate anionomers could solubilize a model hydrophobic pollutant, phenanthrene within their hydrophobic interiors. Batch experiments were conducted with soil slurries to compare APU Sodium Dodecyl Sulfate (SDS), anionic surfactant for the sorption onto soil. APU particles (K,@.2 mug) were weakly adsorbed onto the sandy soil compared to SDS ( K d 1 . 3 mug), due to their chemically crosslinked structure. Compared with SDS, APU particles exhibited the higher extraction efficiency to remove phenanthrene from the contaminated sandy soil.

1 Introduction

Contamination of soil and groundwater by hydrophobic organic carbons (HOCs) is caused by leakage fiom storage tanks, spillage, or improper disposal of wastes. Once in the soil matrix, HOCs can act as a source of dissolved contaminants[1-31. Among HOCs, PAHs are of special interest because they are strongly sorbed to soil or sediment, as a consequence, sorbed PAHs may act as a long-term source of groundwater contamination. So many researchers have been using various surfactants to enhance desorption of sorbed PAHs from soil through solubilization of sorbed PAHs in surfactant micelles[4-8]. Surfactant-enhanced remediation techniques have shown significant potential in their application to the removal of PAHs in the soil remediation process. Some of the disadvantages of these techniques are micelle breakage and loss of surfactant through sorption to soil. In addition, Surfactant-enhanced desorption and washng process is only effective when surfactant dose is greater than its critical micelle concentration (CMC), because most of surfactants molecules below CMC are sorbed onto soi1[6-8, 9-11]. So, recent research has been directed toward the design of surfactant that minimizes their losses and the development of surfactant recovery and recycling technique. The purpose of this research is to compare micelle-like polymeric particles with a surfactant with respect to the sorption to soil and the extraction of an organic soil contaminant. Nano-sized polyurethane (APU) particles synthesized from amphiphilic urethane acrylate anionomers were used as model micelle-like polymeric particles. Employing APU particles with various degrees of hydrophobicity, the effects of hydrophobicity on the soil sorption and the phenandvene extraction from soil of polymeric particles were studied. Sodium Dodecyl Sulfate (SDS)was used as a model conventional surfactant. Phenanthrene was used as a model soil contaminant.

392

2 Materials and Methods

2. I Materials The soil used in all experiments was obtained from coal-mine region in Samchok, Korea. Soil sample was air-dried and screened through a US standard No. 10 mesh (2mm) sieve to remove coarse firagments. The fraction of organic carbon in the soil sample was determined to be 0.14% from TOC analysis using MULTI N/C-300 Total Carbon Analyzer (Analytic Jeni. AG., Germany). Phenanthrene. SDS and radio-labeled phenanthrene (9-14C, 13. IrICV-hnol) were purchased from Sigma Chemical Co.(St. Louis, MO USA). APU particles were synthesized by polymerization of urethane acrylate anionomers (UAA) as described by Kim et al. [12]. Table 1 shows the molar ratios of reagents used in the synthesis of the UAA precursors. The hydrophilicity and the hydrophobicity of the synthesized polymer can be modified by changing these molar ratios. APU made from UAA 2:8 has the highest hydrophilicity, while that from UAA 6:4 has the highest hydrophobicity.

Table 1. The molar ratio of reagents in the synthesis of various APU particles

mn-m-p

Types of APU UAA 218 UAA5:5 UAA 6:4

--

Reagents PTMG/DMPA/TDI/2-HEMA PTMGIDMPAITDI/2-HEMA PTMGIDMPNTDIR-HEMA ----*

Molar ratio 0.2/0.8/1.5/1.5 0.5/0.5/1.5/1.5

0.6/0.4/1.5/1.5

-*,?,**vc-BHAC>after desorption in order. It is deemed that the lower distribution of NO; after adsorption than BHAC may be attributed to the fact that N hnctional group, having existed on BHAC, was bonded with surface oxygen and came to be partially desorbed from the surface when temperature increased. Figure 2 C shows sputter depth profile of OK. Distribution of OH- is found to be the highest in BHAC. On BHAC, OH- ions provide selective adsorption sites and react with NO2, as seen in Eq. (l), and evaporate into H20. Accordingly, after adsorption of NO2, a substantial reduction of O K was observed regardless of surface depths. However, after desorption, O K ions increased on the surface which may be explained by two possibilities: A

KOH(s)

+ K 2 0 ( a )+ H,O(g) He

(7)

1) As KOH in BHAC, non-reacted selective adsorption sites, was decomposed, as seen Eq. (7), to produce H20 upon desorption at a high temperature, and H20 evaporated to enable a hydrogen bond between H with surface oxygen; and 2) H bond was broken away from some deficient carbon due to a high temperature and H was bonded with surface oxygen. 4

References

1. Lee Y. W., Choi D. K., and Park J. W., Surface chemical characterization using AES/SAM and ToF-SIMS on KOH-impregnated activated carbon by selective adsorption of NO, Ind Eng. Chem. Rex 40 (2001) pp. 3337-3345. 2. Lee Y. W., Choi D. K., and Park J. W., Characteristics of NOx adsorption and

582

surface chemistry on impregnated activated carbon, Sep. Sci. Technol. 37 (2002) pp. 937-956.

3. Lee Y. W., Choi D. K., and Park J. W., Performance of fixed-bed KOH impregnated activated carbon adsorber for NO and NOz removal with oxygen, Carbon 40 (2002) pp. 1409-1417. 4. Lee Y. W., Choung J. H., Choi D. K., and Park J. W., NOx adsorption on impregnated activated carbon, Fundamentals of A&orption, IK International Ltd. (2002) pp. 154-161. 5. Lee Y. W., Park J. W., Choung J. H., and Choi D. K., Adsorption characteristics of SO2 on activated carbon prepared fkom coconut Shell with potassium hydroxide activation, Environ. Sci. Technol.36 (2002) pp. 1086-1092. 6. Lee Y.W., Park J. W., Yun J. H., Lee J. H., and Choi D. K., Studies on the surface chemistry Based on competitive adsorption of NO,-SOz onto a KOH impregnated activated carbon in excess 02,Environ. Sci. Technol.,36 (2002)pp. 4928-4935. 7. Lee Y.W., Park J. W., Kim H. J., Park J. W., Choi B. U., and Choi D. K.,Adsorption and reaction behavior for simultaneous adsorption of NOx and SOz over carbon-supported potassium catalysts, Carbon, in revision. 8. Lee Y. W., Kim H. J., Choi D. K., Yie J. E., and Park J. W., Temperature programmed reaction and regeneration studies of NOx over potassium hydroxide-containingactivated carbon, Emiron. Sci. Technol, in revision.

583

ADSORPTION CHARACTERISTICSOF NITROGEN COMPOUNDS ON SILICA SURFACE HYUN JONG KIM,CHANG HA LEE AND YONG GUN SHUL Department of Chemical Engineering, Yonsei Universiw, Seoul, Korea E-mail: [email protected] WHA SIK M M SK R&D Center, Daejeon, Korea E-mail: [email protected] The interaction between silica surface and nitrogen compounds was studied by using mainly solid state NMR. The quinoline as basic nitrogen cornpound and carbazole as non-basic nitrogen compound were adsorbed on the dry or wet silica Both of them made a hydrogen bonded hydroxyl proton on the surface of silica. Surface water on the silica might effect on the interaction between silica surface and nitrogen compounds.

1

Introduction

A strive toward a cleaner environment has led to the global tightening of the sulfur content

in automotive diesel fuel. For example, the sulfur limit of 500ppm has been adopted by EEC since 1996, and in Japan since 1997[1]. More severe specifications with a sulfur content of 35Oppm S is now practiced, and a sulfur level as low as 5Oppm or even at least lOppm is being proposed in Europe for the year 2005[2]. For this reason, there are considerable efforts being expended to develop new technologies for the production of clean fuels, like adsorption, extraction, oxidation, alkylation, and bioprocessing[2]. Currently, however, hydrodesulfiuization(HDS) appears to be the technologically preferred solution. The most practical method to produce 10 ppm ultra low sulfbr diesel is considered to be the two-stage HDS process. In this process, the sulfur levels are reduced down to around 250 ppm in the first stage and are further reduced down, after the hot gas that contains the inhibiting components are removed, to below 10 ppm in the second stage. This second stage requires much higher pressure, hydrogen to feed ratio, hydrogen purity and lower space velocity compared to the conditions used in most of the conventional HDS processes. It could increase the capital investment[3]. The performance of HDS is lowered by organic hetercompounds and polyaromatic hydrocarbons, for which the following order of inhibition has been reported Nitrogen compounds > organic sulfur conpounds > oxygen compounds > monoaromatic hydrocarbons[4]. SK Corporation in Korea has been developed a unique technology, called SK HDS Pretreatment Process, to produce the lOppm ultra low sulfur diesel[3]. The technology is based on the adsorption technique, and is designed to remove nitrogen compounds fiom the feedstock to the HDS process. It improved the performance of conventional HDS units, by removing effectively the components that inhibit the desulfiuizationreaction. The nitrogen compounds have been characterized among the strongest HDS inhibitors. Many researchers reported a strong inhibiting effect on the thiophene and DBT HDS reactions[5-8]. It is well known that the basic nitrogen compounds could poison the

584

acidic sites of catalysts and, consequently, retard desulfurization efficiency[5,61. Recently, it is reported in some papers that the non-basic nitrogen compounds can also strongly inhibit hydroprocessing reactions through competitive adsorption[7,8]. In this study, the nitrogen compounds in light gas oil was removed by adsorption technology, similarly with SK HDS Pretreatment Process. And the local interaction between nitrogen compounds and adsorbent was explored by solid-state NMR.For the analysis, we used model light gas oil containing the basic or non-basic nitrogen compounds, 2

Experimentals

The nitrogen compound in the light gas oil was classified with basic nitrogen compound and non-basic compound. For the practical condition,quinoline was used as basic nitrogen compound and carbazole was as non-basic nitrogen compound. And normal hexane was selected as model light gas oil. And, silica was used for the adsorbent of nitrogen compounds. The model light gas oil was prepared with 300 ppm nitrogen compounds in hexane. For the adsorption of nitrogen compounds, the adsorbent was immersed in the model light gas oil for 6 hours. The filtered silica particle was dried in the room temperature for 4 hours. The 'H M A S NMR was performed on a JNM-ECP300 JEOL spectrometer with a spinning rates of ca. 5.5 kHz, for the understanding of the local interaction between nitrogen compound and adsorbent. 1H NMR spectra was carried out at 300.53 MHz using single-pulse excitation. The d2 pulse width and pulse delay were 4 p and 10s.

3

Results

Before the adsorption of nitrogen compounds, the silica was exposed in the moistured air to have different water contents. Figure 1 shows the 'H MAS NMR spectra of silica used in this study. Figure l(a) is the N M R spectrum of dry silica powder. The narrow peak at 6 = 1.8 ppm is assigned the isolated silanol. It showed general spectrum of dry silica as reported in many studiesf9-1I]. After the exposure in wet condition, the resonance at 3.5 ppm which means the physically adsorbed water was appeared. The water content of silica was increased by exposure in wet air up to 0.11 1glg.silica . These dry and wet silica were used for adsorption of nitrogen compounds.

20

15

10

5

0

-6

40

-15

-20

shin (6. m)

Figure 1. 'H MAS N M R spectra of silica (a) before the exposure in moisture and @I) after the'exposure in

mosture.

585

20

15

10

5

-5

0

-10

Chmicpl shift (S, ppm)

Figure 2. 'HMAS NMR spectra of dry silica (a) before the adsorption of quinolineand (b) after the adsorption of quinoline.

Quinoline as basic nitrogen compound was adsorbed on dry silica. As shown in figure 2, the 'H N M R spectrum of the silica with adsorption of quinoline was considerably changed. The peaks at k 8 . 5 , 7.9 and 7.5 ppm were newly appeared. These peaks could be assigned to the hydrogen of quinoline. It means that quinoline was adsorbed on the silica surface. The intensity of peaks was not changed above 130 ppm of quinoline in hexane. The adsorbed amounts of quinoline was 0.06 g/g. And, the isolated silanol signal at k1.8 ppm was disappeared and the resonance at k 5 . 0 ppm was highly increased. The broad peak at W . 0 ppm is the hydrogen bonded hydroxyl proton. With adsorption of quinoline on the silica surface, the hydrogen bonded silanol was increased. It might be because quinoline could make strong interaction with silica surface.

20

15

10

5

0

.5

-10

Chemical shirt (6. ppm)

Figure 3. 'H MAS NMR spectra of wet silica (a) before the adsorption of quinoline and (b) after the adsorptionof quinoline.

586

Quinoline was also adsorbed on the wet silica. In the 'H MAS NMR spectra of figure 3, the originations of peaks are same to dry silica with quinoline in figure 2. The resonance at k8.5.7.9 and 7.5 ppm could be due to the adsorbed quinoline. And, isolated silanol group was disappeared. However, the signal at k5.0 ppm which means hydrogen bonded hydroxyl proton was more increased than that of figure 2(b). The reason why hydrogen bonding was increased by surface adsorbed water is not obvious. Water might effect on the interaction between silica surface and quinoline. The detailed analysis is now in progress. In the case of carbazole(non-basic nitrogen compounds), there are no characteristic signal of carbazole. However, the peak of isolated silanol was disappeared and that of hydrogen bonded hydroxyl proton was highly increased. It could mean that small amount of carbazole might be adsorbed on the silica. And, carbazole might also strongly interact with silica surface. 4

Acknowledgements

This wore was supported by SK corporation and Korea Research Foundation. References 1. Koltai T., Macaud M., Guevara A., Schulz E., Lemaire M., Bacaud R. and Vrinat M., Comparative inhibiting effect of polycondensed aromatics and nitrogen compounds on the hydrodesulfurization of alkyldibenzothiophenes, Appl. Caral. A: Gen. 231 (2002) pp. 253-261. 2. Wang X.,Clark P. and Oyama S. T., Synthesis characterization and hydrotreating activity of several iron group transition metal phosphides, J. Caral. 208 (2002) pp. 321-331. to make ultra low sulfur diesel, KIChE annual meeting (2002). 4. Kwak C., Lee J. J., Bae J. S. and Moon S. H., Poisoning effect of nitrogen compounds on the performance of CoMoS/Al203 catalyst in the hydrodesulfurization of dibenzothiophene 4-methyldibenzothio phene and 4-6-dimethyldibenzothiophene, Appl. Catal. B: Environ. 35 (2001) pp. 59-68. 5. Laredo G. C., Reyes 3. A., Can0 J. L.and Castillo J. J., Inhibition effect of nitrogen compounds on the hydrodesulfurization of dibenzothiophene, Appl. Catal. A: Gen. 207 (2001) pp. 103-112. 6. Nagai M.and Kabe T., Selectivity of molybdenum catalyst in hydrodesulfurization hydrodenitrogenation and hydrodeoxygenation: effect of additives on dibenzothiophenehydrodesulfurization,J. Catal. 81 (1983) pp. 4 4 0 4 9 . 7. LaVopa V. and Satterfield C. N., Poisoning of thiophene hydrodesulfurization by nitrogen compounds, J. Catal. 110 (1988) pp. 375-387. 8. Nagai M., Sata T. and Aiba A., Poisoning effect of nitrogen compounds on dibenzothiophene hydrodesulfurization on sulfide NiMo/Alz03catalysts and relation to gasTphasebasicity, J. Caral. 87 (1986) pp. 52-58. 9. Liu C. C. and Maciel G. E., The fumed silica surface: A study by NMR,J. Am. Chem. SOC.118 (1996) pp. 5103-5119.

3. Min W. S., A unique way

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10. Siahkali A. G., Philippou A, Dwyer J. and Anderson M. W., The acidity and catalytic activity of MCM-41 investigted by MAS NMR FIX2 and catalytic tests, Appl. CuraL A: Gen. 192 (2000) 57-69. 11. Mariscal R., Lopez-Granados M.,Fierro J. L. G., Sotelo J. L., Martos C. and Van Grieken R., Morphology and surface properties of titania-silica hydrophobic xerogels, Langmuir 16 (2000)9460.9467.

588

ADSORPTION CHARACTERISTICSOF VOCS ON MESOPOROUSSORBENTS W.G. SHIM', M.S. YANG', J.W. LEE', S.H. SUH3AND H. MOON' 'Faculty of Applied Chemisq, Chonnam National University,Gwangiu 500- 757, Korea E-mail: [email protected] 'Department of Chemical Engineering, Seonam University,Namwon 590-711, Korea E-mail:[email protected] 'Department of Chemical Engineering, Keimyung University.Daegu 704-701,Korea E-mail: [email protected]

Adsorption equilibria of VOCs on mesoporous sorbents (MCM-48) synthesized in our laboratory are measured at several temperaturesusing a quartz spring balance equipped in a high vaccum system. A new hybrid isotherm model for mesoporous sorbents is proposed by combining both the Langmuir isotherm at low pressure and the Sips isotherm in multilayer region. Since mesoporous sorbents have narrow pore size distribution between 20 to 46 A, capillary condensation of VOCs can be observed at p / po4 . 2 4 3 . Also, the adsorbed amount decreased considerably with increasing pelletizing pressure. The isosteric heat of adsorption was calculated using experimental data,

1

Introduction

Due to the significanteconomic and environmentalimplications of disposing VOCs, much attention has been recently directed towards cost-effective pollution - prevention techniques aimed at reducing VOCs emissions from industrial facilities [l]. There are many techniques available to control VOCs emissions with different advantages and limitations [2]. One of the most effective methods for controlling VOCs is an adsorption process [3]. Among adsorptiodseparation technologies, the activated carbon has been widely used in adsorption processes due to higher micropore volume, easy operation, low operating cost and efficient recovery of most VOCs. However, it often encounters some problems such as combustion, pore blocking, and hygroscopicity. As a result, alternative adsorbents have been receiving much attention. For example, hydrophobic zeolites are found to be one of advancement in controlling VOCs because of their advantage [4,5]. On the other hand, recent discovery of a new family of mesoporous molecular sieves named M41S has been receiving a great deal of attention after introducing by Mobil researchers [6]. The M4 1S family is classified into several members: MCM-4 1, MCM-48 and other species. Their synthesis and utilization have been investigated by many researchers because of their peculiar characteristics such as large internal surface area, uniformity of pore size, easily controlled size of pore, and high thermal stability. These mesoporous materials may be useful as adsorbents, supports, and catalysts. Moreover, interwoven and branched pore structures of MCM-48 provide more favorable mass transfer kinetics than MCM-41. Therefore, MCM-48 seems to be a better candidate as an adsorbent in separation techniques or as a catalyst support than MCM-41 [7,8]. In the past few years, some works have been done on the synthesis and characterization, mechanical stability and adsorption characteristicsof MCM-48. Recently, Hartman and Bischof reported the experimental adsorption isotherm and breakthrough

589

curves of VOCs on MCM-48 without quantitative prediction. A few isotherms have been developed for the adsorption of condensable vapors and reviewed elsewhere. However, none of the models yields a complete description of adsorption isotherms over a wide pressure range with a uniform set of parameter [9,10]. Therefore, in this paper, we used a simple hybrid isotherm for interpreting the adsorption isotherms of VOCs on MCM-48 (unpressedpressed) as a function of temperature. The simple isotherm model is constructed by combining both Langmuir equation at low pressure and Sips equation at regions of multilayer and capillary condensation. The reason for the selection of Sips equation to explain capillary condensation is based on an engineering point of view, namely, both flexible fit of isotherm data and saving calculation time in dynamic simulation although its physical meaning is somewhat lack. MCM-48 are prepared by conventional hydrothermal syntheses and characterized by X-ray dihction (XRD) and nitrogen adsorption and desorption. Also, the isosteric heat of adsorption is calculated using experimental data. 2

2. I

Experimental

Synthesis of mesoporous materials

MCM-48 sample was synthesized as follows. 12.4 g of cethyltrimethylammonium bomide (CTMABr, CI9Hd2BrN,Aldrich), 2.16 g LE-4 (polyoxyethylene lauryl ether, C12H25 (OCH2CH2)40H,Aldrich) were dissolved in Teflon bottle containing 130 g of deionized water at 333 K. This aqueous solution was added dropwise to a another aqueous solution in Teflon bottle containing 40 g of Ludox AS-40 (Du Pont, 40 wt% colloidal silica in water), 5 g of NaOH, and 130 g of deionized water under vigorous stirring. The solution mixture was preheated in a water bath kept at 3 I3 K and was stirred at 500 rpm for 20 min. The resultant gel was loaded to autoclaves, and the mixture was hydrothermally treated at 373 K for 78 h. The mixture was then filtered and washed with 500 mL deionized water. The washing procedure was repeated 4-5 times to assure the complete removal of the bromide and other free ions. After drying at 333K for overnight, the dried solid was then calcined in air at 873 K. 2.2

Measurement of mechanical stability

To test the adsorption property of MCM-48, the samples were compressed using a hand-operated press. The pelletized MCM-48 diameter is 10 mm and the external pressure applied is 0, 50, 100, 200, 300, 400 and 500 kg/cm2. Subsequently, the obtained pellet was crushed and sieved to obtain pellets with a diameter of 0.1 to 0.2 mm that were used for adsorption equilibrium and fixed bed studies. 2.3

Characterization

In designing an adsorption column, the characterization of adsorbents should be done prior to experiments. In particular, one should know not only the specific area but also the pore size distribution of the adsorbent in order to confirm that it would be proper for a given purpose. Nitrogen adsorption and desorption isotherms, BET surface areas, and BJH (Barren, Joyner and Halenda) pore size distributions of the synthesized sorbents

590

were measured at 77 K using a Micromeritics ASAP 2000 automatic analyzer. Prior to measurment, the samples were outgassed at 623 K for 10 h. X-ray powder diffhction data of mesoporous sorbents were collected on Phillips PW3 123 difictometer equipped with a graphite monochromater and Cu K, radiation of wavelength 0.154 nm. XRD patterns were obtained between 2’ and 50’ with a scan speed of l’/min. 2.4

Aakorption stu*

The adsorption amounts of VOCs vapor were measured by a quartz spring balance, which was placed in a closed glass system. A given amount of MCM-48 particles were placed on the dish, which was attached to the end of quartz spring. This system was vacuumed for 15 hours at 10” Pa and 250’C to remove volatile impurities from the mesopoprous sorbents. The variation of weight was measured by a digital voltmeter that was connected to the spring sensor. The adsorption equilibrium was usually attained within 30-60min. Equilibrium experiments were carried out at different temperatures.

3

Results and Discussion

Figure 1 presents the nitrogen adsorption and desorption isotherms with BJH pore size distribution curves for MCM-48. The isotherms are type IV according to the IUPAC

800

E UJ “9 Is00

B

0400

-f s

200

0.0

02

0.4

0.6

0.8

1.o

ppo

Figure 1. Nitrogen adsorption and desorption isotherms on MCM-48 at 77 K

classification. Also, the isotherms exhibit sharp steps in the relative pressure P/Po = which are associated with capillary condensation in channels of MCM-48 structure. The sharpness of the capillary condensation steps indicates uniformity of pore channels and their narrow size distribution. The isotherm is reversible and does not exhibit hysteresis between adsorption and desorption. The surface area of MCM-48 was about 1100 m2/g. And the maxima in the pore size distribution curves indicate the uniform mesopores of approximately 32 A. The BJH data showed approximately20 A distribution 0.2-0.4,

591

from the mean pore diameter for MCM-48, and this matches well with other distribution curves reported earlier. Adsorption isotherms play a key role in either the design of the adsorption-based process for the disposal of wastes containing VOCs or modeling the catalytic oxidation process. The equilibrium data for mesoporous sorbents are fitted to combined model of h g m u i r and Sips equations. This hybrid isotherm model with four isotherm parameters (4,,b, ,b, ,n) is as below:

q=qm

[l+b,P+ blP

1

1+b2P" b2P"

The isotherm parameters were determined using Ne1der:Mead simplex method by minimuig the sum of residual, namely, the differences between experimental and estimated adsorption amount. Figure 2 showed the adsorption isotherms of TCE on MCM-48 at 303,308,3 13,323 K. As one can be expected, the adsorption capacity was decreased with increasing temperature. The hybrid isotherm model for a pure adsorbate was found to fit the individual isotherm data very well. The parameters of the hybrid equations are listed in Table 1. Table 1. Hybrid equation parameters for different temperatures

4m bl b2 n 15

Temperature 303 K 6.260EM0 1.825E-01 9.303E-40 9.848EM 1

I

0

Temperature 311 K 6.36084-00 8.592E-02 8.441E-32 4.448E4-01

Temperature 308 K 5.968EMO 1.764E-01 3.297E-25 4.695E4-01

Temperature 323 K 5.978E4-00 7.522E-02 1.993E-49 5.544E4-01

I

3

6

9

12

0

2

4

6

8

10

Amount adsorbed, mmoUg

Pressure, kPa

Figure 2. Adsorption isotherms of Figure 3. lsosteric heat of adsorption with TCE for different temperatures respect to amount adsorbed

592

It has been known that condensation pressure depends on the adsorbate, temperature, pore size, and geometry of sorbent. As increasing temperature, the capillary condensation pressures is increased and adsorption capacity is decreased. This fact implies that adsorption and desorption can be easily achieved by only little adjustment of pressure and temperature. Therefore the effective removal of VOC can be done by pressure swing adsorption (PSA) or thermal swing adsorption (TSA) processes. As a useful thermodynamic property, the isosteric heat of adsorption has been generally applied to characterize the adsorbent surface. The isosteric heat of adsorption is evaluated simply by applying the Clausius-Clapeyron equation if one has a good set of adsorption equilibrium data obtained at several temperatures.

where 4,, is the isosteric heat of adsorption, R is the gas constant, and N is the amount adsorbed. In Figure 3,, the isosteric heats of adsorption for vapors studied are plotted as a function of the amount adsorbed. The isosteric heat is approximately 40 kJ/mol between the adsorption amounts of 0.5 to 2.0 mmol/g, but that is 45 kJ/mol between the adsorption amount of below 2.0 m o V g and above 8.0 mmoVg. This difference comes from the capillary condensation. Because of the joint effects of the energetic non-uniformity of the adsorbent surface and the interaction of adsorbate molecules in the adsorbed film itself, the heat of adsorption in general varies significantly with the amount adsorbed. The isosteric heat of adsorption can be divided into two sections, namely, low and capillary 12 0 Pmsun-100

A Pmssure-200

Pmsun-300

0 Pmssunr-400

m

...-....

10

8

2 g

6

P

U

4

2 0 0.0

1.5

3.0

4.6

6.0

7.5

Pressure, kPa

Figure 4. Adsorption isotherms of benzene on MCM-48 at 300K

593

0

I00

200 300 Pressure, kg/cm2

400

Figure 5. Comparison of the amount adsorbed of MCM-48 with pelletizing pressure

500

condensation regions. Especially, the isosteric heat of adsorption changes rapidly in capillary condensation range. To test the mechanical stability, MCM-48 samples were pressed into pellets using six different pressures. Figure 4 showed the effect of pelletizing pressure on adsorption capacity. The pelletizing pressure did not affect the capillary condensations although benzene adsorbed amount changed dramatically. The adsorption amount of sample pressed at 500 kg/cm*corresponded to 60 % of the origin sample as shown in Figure 5.

4

Acknowledgements

This work was supported by grant No. (R-01-2001-000-00414-0)from the Basic Research Program of the Korea Science & Engineering Foundation. References 1. Khan, F.I. and A.K. Ghoshal., Removal of Volatile Organic Compounds from polluted air. J. Loss. Prevent. Proc. 13 (2000) pp.527-545. 2. Clausse, B.; Garrot, B.; Cornier, C.; Paulin, C.; Simonot-Grange M.-H.; Boutros, F. Adsorption of Chlorinated Volatile Organic Compounds on Hydrophobic Faujasite: Correlation between the Thermodynamicand Kinetic Properties and the Prediction of Air Cleaning. Micro. and Meso Mat 25 (1998) pp.169-177. 3. Kim, D. J., Shim, W.G. and Moon, H., Adsorption Equilibrium of Solvent Vapors on Activated Carbon. KJChE 18 (2001) pp.518-524. 4. Takeuchi, Y.; Hino M.; Yoshimura, Y.; Otowa, T.; Izuhara H.;Nojima T. Removal of Single Component Chlorinated Hydrocarbon Vapor by Activated Carbon of Very High Surface Area. Sep. and Puri! Tech. 15 (1999) pp. 79-90. 5. Chintawar, P. S.; Greene, H. L. Adsorption and Catalytic Destruction of Trichloroethylene in Hydrophobic Zeolites. AppZ Cat B. 14 (1997) pp. 37-47. 6. Beck, J. S.; Vartuli, J. C.; Roth, W.J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. A n Chem SOC.114 (1992) pp 10834-10843. 7. Zhao, X. S.; Lu, G. Q. Organophilicity of MCM-41 Adsorbents Studied by Adsorption and Temperature-Programmed Desorption. Colls and SurfA 179 (2001) pp 26 1-269. 8. Ryoo, R.; Joo, S.H.; Kim, J.M. Energetically Favored Formation of MCM-48 from Cationic-Neutral SurfactantMixtures. J. Phys. Chem. B 103 (1999) pp 7435-7440. 9. Hartmann, M.and C. Bishof, Mechanical Stability of Mesoporous Molecular Sieve MCM-48 Studied by Adsorption of Benzene, n-Heptane, and Cyclohexane. J. Phys. Chem. B. 103 (1999) pp. 6230-6235. 10. Sonwane, C.G. and Bhatia, S.K.Adsorption in mesopores: A molecular-continuum model with application to MCM-41 Chem Eng Sci 53 (1998) pp. 3143-3156

594

MOLECULAR SIMULATION FOR ADSORPTION OF HALOCARBONS IN ZEOLITES Kazuyuki Chihara*’’, Tsuyosbi Sasaki”, Shingo Miyamoto”. Michi Watanabe”,

Caroline F.Mellot-Draznieks2’,Anthony K.Cheetham3’ 1 ) Department of Industrid Chemistry,Meiji Universio 1-1-1 Higashi-mita,Tama-ku, Kawasaki, Kanagawa 214-8571 JAPAN *Correspondingauthor: Far: 044-934-7197. E-mail:chihara0isc.meijiac.jp 2) Institut Lavoisier, Universite de VersailIes,Saint-Quentin-en-Yvelines45, Avenue &s Etats-Unis, 78035, Versailles Cedex, France 3 ) Material Research Laboratory, University of California, Santa Babara, CA93106-5130, USA The Grand Canomcal Monte Carlo (GCMC) method is simulation method for solving a phenomenon from a microscopic level, and it is turning into the powerful analysis technique in the field of the enginering. However, information on forcefield parameters and charges are often inadequate, even in systems where the structure is well known. From the environmental point of view, the adsorption of chlorinated hydrocarbons by the use of zeolites may have some potential utility in ground water or soil remediation and other areas. It is becoming possible to interpret the adsorption characteristic in a molecule level rationally, and to predict the macroscopic characteristicsuch as adsorption isotherms in recent years using Computer modeling In this study, equilibria and isosteric heat of adsorption for the system of chlorinated hydrocarbons and Ytype zeolite were obtained with gravimetric method and chromatographic method. By comparing an experiment result with a molecular simulation result, the validity of forcefield parameters and zeolite model was examined

1. Introduction

Molecular simulation has now become powerful tool for the study of adsorbed molecules in zeolites, and the Grand Canonical Monte Carlo (GCMC) method is especially useful for predicting adsorption equilibria. However, information on forcefield parameters and charges are often inadequate, even in systems where the structure is well known. From the environmental point of view, the adsorption of chlorinated hydrocarbons by the use of zeolites may have some potential utility in ground water or soil remediation and other areas. Mellot et al. [3] recently reported new forcefield parameters and charges for chlorinated hydrocarbons in the faujasite zeolite: NaX, NaY and siliceous Y. These yield heats of adsorption that are in good agreement with calorimetric data [2]. In this study, their forcefield was used to simulate adsorption isotherms and isosteric heats of adsorption for chloroform, trichloroethylene and tetrachloroethylene in USY-type zeolite, separately. The results were compared with gravimetric and chromatographic experiments. 2. Experimental

2.1. Gravirnetric Method Fig.1 shows experimental apparatus for gravimetric analysis. The zeolite sample (about Ig) was placed in a quartz basket (G). Then the adsorbate in flask (B) was fed to

595

adsorption tube (N). The whole apparatus was in a constant temperature air bath. The temperature range was 303-323 K. The amount adsorbed was measured correspondingto the pressure of the vapor in the tube. The pressure was measured by pressure sensor (P) at higher pressure range (2 0.013 atm) and baratron (0)at lower pressure range (O.l am), all the simulations were coincident and almost Table2 Meat ot Adsorption [kJ/moll correspond to gravimetric data. At lower (PQ-USY-chloroform) pressure, simulation for the acid site model was good agreement with chromatographic Exp. Chromato. 44.25 data and baratron data. Experimental heat of adsorption obtained Pure Siliceous Y 34.05 by chromatography at zero coverage is Silanol nest 33.66 compared with simulated heat of adsorption for 3 models in Table 2. Here simulation Acid site 39.98 for the acid site model was closer to the experimental value than the pure siliceous model and the silanol nest model. In Fig.6, experimental adsorption isotherms for tetrachloroethylene in PQ-USY at 303 K are shown. ~~

597

~

~~~

~~

In this system, all simulation results Table3 Heat of Adsorption [kJ/moll with three models were coincident, (Pa-USY-t etrachloroethylene) and almost correspond to chromatographic data and gravimetric Exp. Chromato. 43.64 data. This is thought to be because tetrachloroethylene is different from Pure Siliceous Y 41.40 chloroform and it i s a non-polar molecule. Silanol nest 41 30 In Table 3, all simulations show the heat of adsorption corresponding to Acid site 43.68 chromatographic data for the same reason, and especially acid site model show the best agreement in the three models.

exp. ad. 303K

1

3

0

Acid site model

0

0.001 0.m1

o.woo1 0.m1

A

-

0.01

=

exp. ad. 303K Baramon exp. 303K chromato. PureSiliceousY model Silanol nest model

8

0.1

0.m1 0.001

0.01

0.1

1

Pressure [am] Fig3 Comparison between exp. and simulations for adsorption isotherms of PQUSY-Chloroform system 4 exp. ad. 303K

exp. ad. 303K

Baratoron -exp. 0

303K

chromto. Pure Siliceous Y &I

A S i b 1 nest model 0 Acid site model 0 . m 1 0.m1 0.001

0.01

0.1

1

pressure [atm]

Fig.6 Comparison between exp. and simulations for adsorption isotherms of PQUSY- Tetrachloroethylene system

598

4. Conclusions

Equilibrium and isosteric heat of adsorption for the system of chloroform-Y-type zeolite were studied. The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method was used to get the initial slope of the isotherms. In the simulation, GCMC method was used to calculate amounts adsorbed for various conditions. When the experiment result and simulation result of chloroform are compared, the simulation for the acid site model was most agreement with chromatographic data and baratron data. The simulation result of tetrachloroethylene with three models corresponded mostly for the non-polar molecule, and above all the acid site model was the closest to the experiment result. Therefore, to get better coincidence between experimental data and simulation, it was found to be necessary to account for aluminum rather than silanol nest. As a conclusion, FF parameters were confidently applied. And modified structure model are effective for simulation. Nomenclature K* :adsorption equilibrium constant [cc/g] M : molecular weight [glmol] q : amount adsorbed [g/g] R :gas constant [JK moll R-Min : Bond Length Equilibrium [A] T :temperature [K] : potential energy minimum [eV] Epsilon K :Boltzmann constant [JK] References 1. K. Chihara, M. Suzuki, K. Kawazoe, Adsorption rate on Molecular Sieving Carbon by Chromatography, AIChE J., 24,237 (1978) 2. C. F. Mellot, A. K. Cheetham, S. Harms, S. Savitz, R. J. Gorte, A. L. Myers, Calorimetric and Computational Studies of Chlorocarbon Adsorption in zeolites, J. Am. Chem. Soc., 120,5788. (1998a) 3. C. F. Mellot, A. M. Davidson, J. Eckert, Adsorption of Chloroform in NaY Zeolite: A Computational and Vibrational Spectroscopy Study, J.Phys. Chem. B, 102,2530 (1998b)

599

ADSORPTION OF BTX ON MSC IN SUPERCRITICAL COz, A CHROMATOGRAPHIC STUDY

Kazuyuki Chihara, Naoki Omi, Yusuke Inoue, Takuji Yoshida, Takashi Kaneko

Department of Industrial Chemistry,Me& University 1-1-1. Higashi-Mita, Tama-Ku, Kawasaki, 214-8571, Japan tel&fa: 044-934-7197,e-mail:[email protected] Chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on molecular sieving carbon (MSC) in supercritical fluid C02 mixed with organics. Supercritical chromatographpacked with MSC was used to detect pulse responses of organics. Adsorption equilibria and adsorption dynamics for organics were obtained by moment analysis of the response peaks. Dependences of adsorption equilibrium constants, K*,and micropore diffusivity,D, on amount adsorbed were examined.

Introduction Adsorption equilibria and adsorption dynamics in supercritical fluids have been reported recently and it will be possible to apply the supercritical fluid to some new adsorptive separation processes. Fundamental informations on adsorption under supercritical conditions are necessary to design such processes. Supercritical chromatography has been used for study on the adsorption equilibria and adsorption dynamics.Adsorption of organics, i.e., benzene, toluene and m-xylene, respectively, on MSC under supercritical conditions has already been reported in reference (Chihara, 1995). In the previous study, chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on MSC in supercritical COz mixed with benzene, toluene and m-xylene respectively. Moment analysis of the chromatogram was carried out. In the study, the organics used in the form of pulse were the same as organics mixed with supercritical COz. The dependencies of adsorption equilibrium and micropore diffisivities on the amount adsorbed were obtained. In the present study, supercritical COz chromatograph packed with MSC was again used to detect the pulse responses of organics, and the moment analysis technique was used to analyze. Equilibrium and dynamics were studied for benzene, toluene and mxylene, respectively, -MSC systems in the supercritical COz mixed with organics which were different fiom that used in the form of pulse. Furthermore, the dependence of adsorption of the organics on the amount adsorbed of other organics was discussed. Experimental procedure and conditions The experimental apparatus (Super 200-type 3; Japan Spectroscopic Co., LTD) was shown in Fig.1. The carrier fluid of the chromatograph was supercritical COz (critical temperature 304K, critical pressure 7.3 MPa) and it’s mixture with the above-stated

600

organics (benzene, toluene or m-xylene) respectively. The adsorbates used in the form of pulse were different fiom organics mixed with supercriticalC02. For example, in the case of C02 mixed with benzene, the organic used in the form of pulse was toluene or mxylene. In the previous study, pulse organics were the same as organics in the carrier. The volumes of the pulse were fixed to be 8 x 10m9m3 as liquid. MSC 5A (Takeda chemicals Co., HGK882.)was crushed and screened to obtain particle size between 1.49 x lo4 1.77 x. 104m (an average particle radius of 8.12 x. lo-’ m). 4.82 x 104kg of these particles were packed into the chromatographic column of 5 x 10-*mlong and 4.6 x lO”m in diameter. The void hction, E, of the bed was determined to be 0.355. The properties of MSC5A are shown in Table 1 in reference (Chihara, 1978). Flow rate of supercritical C 0 2 was 1.33 x 10-’m3/s at 268K and at 15.0,20.0 and 25.OMPa respectively and flow rate of adsorbate (benzene, toluene or m-xylene) was 1.67 x 10-’0m3/s,5.00 x lo-’’ m3/s and 1.00 x m3/s as liquid at room temperature (298K). The column pressure was kept at 15.0, 20.0 and 25.0 MPa respectively. The pressure drop across the adsorbent bed was estimated to be about 0.1Mpa and was assumed to be negligible. The experimental column temperature was kept at 313, 333 and 353 K respectively. Before experimental runs started, the adsorbent particles were regenerated and stabilized by feeding pure C02 for 2 hours at the experimental pressure and temperature. Pulse responses were detected using a multi-wave length UV detector (Multi-340; Japan Spectroscopic Co., LTD.) (195350 nm). Response data were processed by a personal computer. A : Liquid C02 Cylinder B : Valve C : Cooler D1.2: Pump E : MixingColumn F : Adsorbate (Liquid) G : InjectionColumn H : Six-way Valve 1 : Valve J : PackedColumn K : Valve L : UV Detecter (MULTI-340) M : Back Pressure Regulator N : PersonalComputer 0 : Fraction Collector P : Vent

Fig. 1 Experimentalapparatus Moment analysis of supercritical fluid chromatogram was tried, and the apparent adsorption equilibrium constant, K* and time constant of micropore diffusivity, D/a2 obtained from first and second moment of response peak, as in references (Chihara, 1993; Chihara, 1995).

601

Result and discussion Figure.2 shows adsorption isotherm of toluene at 313K. According to Fig.2, The amount of adsorption increased with increases of molarity of toluene, and reached to saturation. The amounts adsorbed became larger with decreases of column pressure. It was considered that the situation is competitive adsorption and amount adsorbed of toluene decreases as COz adsorption increase with increases of column pressure.

+

15MPa 20MPa 25MPa

H

p 1.5

A

4 1.0

5

adsorbate : toluene solute : toluene

a5

E

~~

0

Concentration [ml/ma] 20 40

80

Fig. 2 Adsorption isotherm : toluene at 3 13K 1

toluene benzene A m-xylene

y 0.1 1 E

Y

1

2 0.01 awt

a5

0

1.0

1.5

temperature :3 13[K] pressure :2O[MPa]

20

Amount adsorbated of toluene [ml/kgl

Fig. 3 Dependencies of K* on the amount adsorbed of toluene Figure.3 shows dependency of adsorption equilibrium c nstar s, K*, for benzene, toluene, and m-xylene on amount adsorbed of toluene at 15Mpa. This is reasonably decreasing, which corresponds to Fig.2. 0 H

A

toluene benzene in-xylene

temperature : 353[K1 pressure : 20CMPal

0

0.5

1.a

1.5

. Amount adsorbated of toluene

2.0

[mol/kd

Fig. 4 Dependencies of D&exp(d) on the amount adsorbed of toluene

602

Figure.4 shows dependency of micropore diffusivity, D/a2exp(o2), for benzene, toluene, and m-xylene on amount adsorbed of toluene at ISMPa. The increase of D/a^expCo2) for toluene could be reasonably explained by chemical potential driving force. However, as for dependency of D/a^expCo2) of benzene and m-xylene on amount adsorbed of toluene, further discussion would be necessary. Molecular simulation A simulation is assuming a molecule on a computer and performing various kinds of physical chemistry calculation. It was with the molecular design support system Cerius2 (Version4.2) made from MSI. The purpose of performing simulation is to elucidate an adsorption mechanism on the molecule level. The simulation was performed on the same conditions as an experiment in order to compare with an experiment. MSC68-RC1 and MSC84-RC1 model were used as adsorbent. There is 6.8A and 8.4A of distance between adsorption spaces, respectively.

KS

p

Adsorption state *•" In the beginning, we will examine how molecules of adsorbate is benzene was used for the adsorbate here. MSC68RC1 model The results are shown in Figure 5. We see from Figure 5 that benzene is adsorbing to the adsorption space reproduced micro pore. Here, benzene is adsorbed to layer in parallel in MSC68 model, MSC84RC1 model on the other hand, Fig.5 Adsorption state of benzene It is adsorbed aslant in 0.0025 MSC84 model

S

01

1 -° i a

Adsorption isotherm The simulation was carried out the single component Benzene is CDO used for adsorbate. Conditions are 313K and ISOatm. The results are OMSO68-RC1 shown in Figure 6. DMSC84-RC1 As for Fig.6, adsorbed amount for MSC84 model is larger than 06 MSC68 model. The cause of this 0 0.00002 0.00004 0.00006 0.00008 result is that the amount adsorbed is molar concentration [mol/ni] dependent on the size of the Fig.6 Adsorption isotherm of benzene adsorption space. According to Fig.6, The amount of adsorption increased with increases of molarity of benzene, and reached to saturation. Figure 7 shows comparison of adsorption isotherm for a molecular simulation different force field and an experiment. Conditions are 313.15K. and ISOatm. The simulation was carried out the two components system. UNIVERSAL 1.02, London, and DREIDING2.11 were used for the force field, respectively. As for the amount adsorbed, in every figure,

603

the simulation was small rather than the experiment. The difference of an experiment and a simulation is large, so that molarity becomes large. In the simulation using the force field parameter of London, although morlarity was increasing by MSC84 model, the result that the amount adsorbed decreases sharply was shown. In the two components system, not only the interaction of an adsorbent model and adsorbate model but the interaction of adsorbate model is added, calculation becomes more complicated, it is thought that such a result arose.

Conclusion Adsorption equilibrium and adsorption dynamics on MSC were evaluated for each organics in supercritical C02 fluid mixed with adsorbate by chromatographic measurement. The dependencies of adsorption equilibrium constants, K*, and micropore diffiivity, D, of toluene, benzene and m-xylene, on molarity of toluene with each parameters of temperature or pressure were obtained. It was found that the values of K* and D for an organic substance depended on the amount adsorbed of other organics strongly. The state of the molecule could be observed by the molecular simulation. As for the amount adsorbed, the simulation is small, in comparison with the experiment.

-

2 e, 3

0.0014 R C l-80.0012

I 0 MSC84-RC1

0.001

Y A

A Exp:

0.0008

4

0.0006

c

5

0.0004

P

0.0002 0

,M

ij

0.00002 0.00004 0.00006 molar concentration [molhl] UNIVERSALI.

0.00008

0.0014 0.0012 0 MSC84-RC1

0.001 0.0008

3I 0.0006 1 0.00041

~

j

,

B 0.0002

0'

-

-" i:

0.0014 0.0012

4

Y

0.0008

4- 0.0006

0 MSC84-RC1

A

0.001

I

-0

.

A ~

3

0.0004

Q

6

0.0002

G

-

0 0

_

0 v

A

A Exp. _

-

I-I

0

0

0.00002 0.00004 0.00006 0.00008 molar concentration ImoVml]

Londo Fig.7Comparison with experiment and molecular simu1ation:Adsorption isotherm

Reference Chihara, K., Kawazoe, K.,Sumki, M., Seisan Kenkyu, AIChE J, 24,237-246. (1978) Chihara, K., Aoki, K., AIChE, Annual Meeting, (1993) Chihara, K., Oomori, K., Kaneko, R., Takeuchi, Y., AIChE, Annual Meeting, (1995)

604

POROUS ALUMINA WITH BIMODAL PORE SIZE DISTRIBUTION As AN ORGANIC ADSORBENT YOUNGHUN KIM, CHANGMOOK KIM, PIL KIM,JONG CHUL PARK AND JONGHEOP YI' School of Chemical Engineering, Seoul National University,Seoul 151-742, Korea E-mail: [email protected] Hierarchical channel or wellconnected smaller and larger pore networks show multiple advantages for application in catalysis or adsorbent in aqueous condition. Micro- and mesopores may provide size or shape selectivity for guest molecule, while additional macropores can reduce transport limitations and enhance the accessibility to the active sites. In this study, we proposed a novel method to prepare bimodal p o r n aluminas with meso- and macropores with narrow pore size distribution and well defmed pore channels. The framework of the porous alumina is prepared via a chemical templating method using alkyl carboxylates. Polystyrene beads are employed as physical templates for macropore. We examined PDDA treated aluminas as organic adsorbent in aqueous solution. Most anionic dye is removed within 10 min, and the adsorption rate of PDDNP4 is faster than that of PDDA/P2 because macropore of P4 may have reduced transport limitation and enhanced the accessibility to the active site, cationic charge.

1

Introduction

Porosity is one of the important factors that influence the chemical reactivity and the physical interactions of solids with gases and liquids [l]. Physical properties such as density, surface area and strength are dependent upon the porosity and the pore structure of a solid. Especially for industrial applications, the control of porosity with well-ordered structural pore networks is of great importance, for example, in the design of catalysts, adsorbents, membranes, structural materials and ceramics. Researches on uniform pore structure of nanometer to micrometer dimensions have been progressed with great interest due to a variety of possible application in catalysis, molecular separation, membranes, structural materials and adsorbent [2]. Recently, ordered inorganic structures with macropore have been prepared using physical templating method [3], latex sphere and emulsion droplets, or chemical templating method [4] by post-hydrothermal treatment and primary particle seeding. Micro- and mesopores provide size or shape selectivity for guest molecule, while additional macropores can reduce transport limitations and enhance the accessibility to the active sites [5]. As theoretically proven by Levenspiel, the bimodal catalyst can guarantee high diffusion efficiency [6]. For example, cobalt catalysts supported on bimodal silicas show remarkably high activity in liquid-phase Fischer-Tropsch synthesis [7]. Relatively few studies on the synthesis of mesoporous alumina have been reported to date [8]. One of the limitations of the reported synthetic strategies is that the rate of hydrolysis (and condensation) reaction of aluminum alkoxide are much faster than that of silicon alkoxide. In this study, we proposed a novel method to prepare bimodal porous aluminas with meso- and macropores with narrow pore size distribution and well-defined pore channels. The b e w o r k of the porous alumina is prepared via a chemical ternplating method using a w l carboxylates. Here, self-assemblied micelles of wboxylic acid were used as a chemical template. Mesoporous aluminas were prepared through careful control of the reactants pH, while the procedures are reported elsewhere [9].

605

2

Experimental

Polystyrene beads (PS) are employed as physical templates for macropore. The emulsifier-fiee emulsion polymerization method used here allows for the synthesise of nearly monodisperse latex beads of PS in the size of ca. 100 nm [101. PS beads were prepared using 700 ml degassed water, 54 ml styrene monomer, 0.65 g potassium persulfate as initiator, and 20 ml divinylbenzene as cross-linking agent. PS beads were obtained at 7OoC and 350 rpm, and dried under ambient condition.Aluminum sec-butoxide and stearic acid were separately dissolved in parent alcohol, sec-butyl alcohol, at room tempature, and then the two solutions were mixed. Appropriate amount of HN03 solution was dropped into the mixture at a rate of I mVmin to acidify and hydrolyze the aluminum precursor. PS beads were added into aluminum hydroxide solution after stirring for 10 h. The fmal pH of the reactant was approximately 7. Organic templates, both stearic acid and PS bead, were easily removed fiom dried aluminum hydroxide by calcination. The overall synthetic procedure is as shown in Fig. 1. An adsorption test for dye was performed using model solution, which was prepared with acid red 44 (crystal scarlet, Aldrich). Prepared aluminas (P2and P4)were added into 2%polydiallyldimethylammonium chloride (PDDA, Aldrich) solution, and stirred for 3 hr. 0.1 g of PDDA treated aluminas were stirred with 10 ml of dye solution (50 ppm). Depletion of dye was determined by UV (510 nm) spectrometry (HP8453, Hewlett Packard).

*!

Stearic acid

-b

___, calcination

macropore

Figure 1. Schematics for the synthesis of porous alumina with bimodal pore size distribution. Templates removal steps are followed by dotted-arrow for polystyrene beads and solid-arrow for silica gels as physical templates.

3 3. I

Results and discussion Synthesis of bimodal alumina with meso- and macropore

Mesoporous aluminas prepared using only chemical template (P2)show a narrow pore size distribution, adjustable pore diameter (2-7 nm) and 300-500 m2g-' of surface area [9].Based on the XRD, DSC, and TGA analysis, the phase of P2 calcined at 420°C and for 3 h is y-fom, and thermally stable up to %Or. The mesopore size of P2 is dependent upon the carbon tail length of alkyl carboxylate, the ratio of water to aluminum precursor, calcination condition, and the pH of reactants. Oxolation reaction of aluminum alkoxide was minimized at IEP (ca. 8-9), and P2 prepared at pHIEPshowed poorly organized mesostructure [11,12], while P2 prepared (Table 1 and Fig. 2) at pH 7 had a relatively well organized mesostructure. Therefore, we adjusted the final pH of reactants using HN03. The XRD patterns of P2 show only one peak at low angle, which indicates that the pore system is lacking in a long-range order. It is in good aggreement with the

606

TEM result that less ordered worm-like pore distribution was formed in mesoscale (Fig. 3b). Table 1. Characteristics of bimodal porous aluminas Pore size [nm] V [cm3g-'] SA[m2g-l] Damla1 DBni M DWB-FHH Icl PI 4.2 0.15 149 0.53 485 P2 6.8 3.4 51 0.24 216 P3 6.4 P4 6.2 3.5 50 0.35 239 [a] Gurvitsch (4VIA) model. [b]BJH model on the desorption. [c] BdB-FHH spherical model on the desorption. Sample

Important trends in N2isotherm when the PS beads are used as a physical template are shown in Table 1 and Fig. 2. In Table 1, PI is the alumina prepared without any templates, P2 is prepared without physical template (PS bead), P3 is prepared without chemical template (stearic acid), and P4 is prepared with all templates. For above 10 nm of pore size and spherical pore system, the Barrett-Joyner-Halenda (BJH) method underestimates the characteristics for spherical pores, while the Broekhoff-de Boer-Fre~el-Halsey-Hill(Bdl3-FHH) model is more accurate than the BJH model at the range 10-100 nm [13]. Therefore, the pore size distribution between 1 and 10 nm and between 10 and 100 nm obtained fiom the BJH model and BdB-FHH model on the desorption branch of nitrogen isotherm, respectively. N2isotherm of P2 has typical type IV and hysteresis loop, while that of P3 shows reduced hysteresis loop at P/Po cu. 0.5 and sharp lifting-up hysteresis loop at P/Po> 0.8.This sharp inflection implies a change in the texture, namely, textural macro-porosity [4,14]. It should be noted that P3 shows only macropore due to the PS bead-fiee fiom alumina h e w o r k .

--Pz

0.0

0.2

0.4

0.6

0.8

P3 P4

1.0

PPO

Figure 2. Nitrogen adsorption-desorption isotherms of the porous alumina prepared using stearic acid and/or PS bead as a template (curve are shifted for clarity). The inset shows the corresponding pore size distribution from the desorption branch.

When neither chemical nor physical template is used in preparation of alumina, PI, the resulting material shows less ordered pore size distribution, while P4 shows bimodal pore systems with both mesopore (3.5 MI)and macropore (50 nm) after thermal treatment.

607

As shown in Fig. 3% the size of PS bead is approximately 100 nm, and dried PS beads show an aggregate form. However, the pore sizes of calcined P4 and P3 are approximately 50 nm. Calcination resulted in polymer-he metal oxide strucures, with spherical voids smaller than the PS bead diameter, by 50%. This contraction phenomenon is obviously related to the reorganizaiton of the surrounding walls during the calcination process as evidenced in Fig. 3c and Fig. 3d. The part of the wall retracts by the burning PS bead [I I]. This thermal treatment causes the link of metal oxide in the structure (Fig. 3d), and it is also affected by the calcination temperature.

Figure 3. E M micrographs of (a) colloidal PS bead and (b) calcined Pt,and SEM images of (c) as-made P4 and (d) calcined P4.

In SEM micrographs, unimodal mesoporous alumina (P2) shows smooth surface, while the surfaces of calcined P3 and P4 were coarse. As-made P4 materials show aggregated form of PS beads surrounded by aluminum hydroxide, and this is similar to that of pure PS beads aggregates. After calcination, the morphology of the calcined P4 changes into inverse shape of as-made P4. As shown in Fig. 3d, several dark spots of about 40-70 nm diameter can be observed inside the alumina structure. These dark spots, macropre, surrounded by mesopore framework and connected with neighboring macropore. Therefore, multi-pore structure shows interlinked cylindrical mesopore with spherical macropore, like a spongy shape. 3.2

Organic adsorption test

As a model case, we examined the adsorption capability of organics in aqueous solutions using the alumina developed here. Wang et al. [151 showed that siliceous materials, which were treated with cationic polymer and oppositely charged mixed surfactant micelles, adsorbed organophilic compound. Such organic removal reaction is known to be very fast and reversible ion-exchange reaction. Cationic charge site (N3 in PDDA reacts with anionic charge site (SO’? of dye through charge matching. The alumina sample of P4 was prepared from attaching PDDA onto the surface and contacted with the Acid Red 44 dissolved in aqueous solution. Most anionic dye is removed within 10 min, and the adsorption rate of PDDA/P4 is faster than that of PDDNP2 because macropores of P4 reduced the transport limitation and enhanced the accessibility to the active site, as shown in Fig. 4. The maximum uptake of dye was dependent upon the amount of attached PDDA on the surface, while the elusion of adsorbed dye was easy and fast when 0.5 M HN03 solution was applied.

Financial support by the National Research Laboratov WRL) of the Korean Science and Engineering Foundation (KOSEF) is gratefully acknowledged

608

1.2,

1.0 0.8 .

3

0.6 -

0.4

~

0.0 J

I

0

20

40

BO

80

i

m

m

time (nin)

Figure 4. The concentrationchange of acid red 44 as a function of time.

References 1. U. Schubert, N. Husing, Synthesis of Inorganic Materials, Wiley-VCH, 2000. 2. A. Stein, B. J. Melde ,R. C . Schroden, A h . Muter. 2000, 12, 1403; B. Lee, Y. Kim, H. Lee, J. Yi, Micropor. Mesopor. Mat. 2001,50,77. 3. B. Lebeau, C. E. Fowler, S. Mann, C. Farcet, B. Charleux, C. Sanchez, J. Mater. Chem. 2000, 10, 2105; C. F. Blanford, H.Yan, R. C. Schroden, M. Al-Daous, A. Stein, Adv. Muter. 2001,13,401. 4. X. Wang, T. Dou, Y. Xiao, Chem. Commun. 1998, 1035; J. Sun, Z. Shan, T. Maschmeyer, J. A. Moulijn, M.-0. Coppens, Chem. Commun.2001,2670. 5 . - M.-0. Coppens, J. Sun,T. Maschmeyer, Catal. Today, 2001,69,331; Y. S. Cho, J. C. Park,W. Y. Lee, J. Yi, Catai. Lett. In press, 2002. 6. 0. Levenspiel, Chemical Reaction Engineering, 3rd Ed. John Wiley & Sons, 1999. 7. N. Ysubaki, Y. Zhang, S. Sun, H. Mori, Y.Yoneyama, X. Li, K. Fujimoto, Catal. Commun.2001,2,3 1 1. 8. F. Vaudry, S. Khodabandeh, M. E. Davis, Chem. Muter. 1996,8, 1451; S . Cabrera, J. El Haskouri, J. Alamo, A. Beltran, S. Mendioroz, M. D. Marcos, P. Amoros, Adv. Muter. 1999, 11,379. 9. Y. Kim, B. Lee, J. Yi, The Korean J. of Chem. Eng. 2002, 19(5). 10. S. Vaudreuil, M. Bousmina, S. Kaliaguine, L. Bonneviot, A h . Muter. 2001, 13, 1310. 11. S. Valange, J.-L. Cuth, F. Kolenda, S. Lacombe, Z. Gabelica, Micropor. Mesopor. Mat. 2000,35-36,597. 12. C. J. Brinker, G. W. Scherer, Sol-Gel Sience, Academic press, 1990. 13. M. Kruk, M. Jaroniec, Langmuir 1997, 13, 6267; W. W. Lukens, P. S. Winkel, D. Zhao, J. Feng, G. D.Stucky, Langmuir 1999,15,5403. 14. W. Lin, J. Chen, Y. Sun, W. Pang, J. Chem. SOC.Chem. Commun. 1995,2367; K. R. Kloetstra, H. W. Zandbergen, J. C. Jansen, H. B. Bekkum, Microporous Muter. 1996, 6,287. 15. Y. Wang, J. Banziger, P. L. Dubin, G. Filippelli, N. Nuraje, Environ, Sci. Technol. 2001,35,2608.

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STORAGE AND SELECTIVITY OF METHANE AND ETHANE INTO SINGLE-WALLED CARBON NANTOUBES YANG GON SEO Division of Applied Chemical EngineeringIERI, GyeongsangNational Universiy 900 Gajwa-dong,Jinju 660-701, Korea E-mail:[email protected] BYEONG HO KIM Department of Environmental Protection, Gyeongsang National University 900 Gajwa-dong,Jinju 660-701, Korea E-mail:[email protected] NIGEL A. SEATON School of Chemical Engineering, University of Edinburgh King's Buildings, Edinburgh EH9 3JL, UK E-mail: [email protected] The adsorption equilibria of methane, ethane and their mixture into single-walled carbon nanotubes (SWNTs)were studied by using a Grand Canonical Monte Carlo (GCMC)method. The equilibrium isotherms of methane and ethane and the selectivity from their equimolar mixture were reported.

1

Introduction

Although carbon nanotubes (CNTs) have been recently discovered [9], they have been attracting a great deal of scientific interest due to their potential application in areas such as adsorbents and composite materials. CNTs have the number of graphite sheets in tube walls can vary fkom 1 for single-walled nanotubes (SWNTs) to over 50 for multi-walled nanotubes (MWNTs), and inner diameter raging from Inm to 5nm with a definite diameter [ 1,3]. The possibility of controllable pore size and distribution suggests that CNTs might be used applications such as gas storage and selective separations from gas mixtures. Up to now, numerous studies have been conducted on their synthesis [9,10], treatment [5,13] and physical properties [4]. However only limited number of studies has been carried out on the adsorption of gas in CNTs, including experimental works [8,1 I ] and molecular simulations [3,7,14-151. Adsorption behavior depends strongly on the microporous structure of the particular adsorbent. In this work the effect of pore size on the adsorption behavior is of interest. The adsorption equilibria of methane, ethane and their mixture into SWNTs were studied by using a Grand Canonical Monte Carlo (GCMC) method. We reported equilibrium isotherms of methane and ethane, and the selectivity from their equimolar mixture. 2

Model and Simulation

CNTs are cylindrical structures and retain their cylindrical shape when their internal diameters are less than lnm. CNTs flatten to form a honeycomb structure when the

610

internal diameters exceed 2 . 5 ~ 1[12]. Although the nanotubes typically have their ends capped, selective oxidation can remove their end caps and reveal their hollow central cavities [5,13]. A high-resolution transmission electron microscope (TEM) image of SWNTs (manufactured by Iljin Nanotec, Korea) is shown in Figure I (a).

0

(4

Figure 1. A high-resolution TEM image(a) and a segment of an armchair mode of single-walled carbon nanotubes(b).

There are two modes of rolling graphite sheet, which give rise to the armchair and saw-tooth configurations. We construct SWNTs according to the armchair mode of rolling. SWNTs constructed in this manner have only certain allowed diameters. SWNTs at different allowed diameters can be produced by the saw-tooth mode of rolling. Figure 1 (b) illustrates a segment of an armchair SWNT of diameter D=l.496nm. D is the centerto-center distance of two diametrically opposite carbon atoms on the nanotubes walls. The boundary condition of axial direction was applied. We used a length of 7.87nm for all simulationsreported in this work. The C-C bond length of 0.142nm corresponding to that of graphite was used. The intermolecular interactions between two molecules and fluid-wall interactions in SWNTs were given by a 12-6 Lennard-Jones (LJ) potential. Methane was modeled as a spherical LJ molecule and ethane as two LJ sites with the unified methyl group. The interactions were cut at 2.286nm which corresponding to 5 times the methane o parameter. In each step, one of these was chosen with equal probability at random. For each point on the isotherm, the system was allowed to equilibrate for 5 ~ 1 steps 0 ~ before collecting data. After equilibration, the simulation continued for 2 ~ 1 steps 0 ~ in order to calculate the average values of the extent of adsorption. Further details of the simulations are given elsewhere[2,6]. 3

Results and Discussion

In Figure 2 the pure-component adsorption isotherms of methane and ethane in SWNTs are presented as an amount adsorbed per unit volume of the pore. At low pressures the greatest adsorption occurs in the small pores. This is due to smaller pores having larger adsorbate-adsorbent interaction potentials. Small pores fill rapidly, even low pressure, due to the presence of a strong wall potential function. The complex variation between the isotherms for different pore sizes is caused by a trade-off between the strength of methane-SWNTs interaction and the ability of SWNTs to accommodate methane

611

molecules. For D=O.678nm storage of methane was very low due to sieving - capacity effect of small pore.

. .-

8

10 x(2.035~11

t 0.678~11

0.950~11 0.814nm

5 0

0

0

10

20

30

40

L

0

Prrasurc [bar]

10

20

30

40

Pressure [bar]

Figure 2. Simulated isotherms for methane and ethane adsorption in single-walled carbon nanotubes of different diameters.

The most important parameter from the point of view of mixture separation is the selectivity. Figure 3 shows the binary selectivity at 298.2K with a 50% methane, 50% ethane bulk-gas mixture over a range of pressures and pore diameters. The curves show behavior with an initial increase in selectivity due to cooperative interactions between the ethane molecules as the pressure is increased. As pressure increases further the adsorbate densifies, which imposes an ordering of the adsorbate. The selectivity is a strong function of pressure and pore width. The most commonly used approach to predict multicomponent adsorption is Ideal Adsorbed Solution Theory (IAST) based on a classical thermodynamic analysis. This approach requires pure component adsorption isotherms, at the proposed temperature of operations of the adsorption unit, for all of the components in the mixture. The pure-component isotherms form the inputs to IAST, and the binary predictions are plotted in Figure 3. The binary simulation results by GCMC are in good agreement with IAST.

612

GCMC

0

80

0

lAST

A - 9 .

D[m] 1.628 2.035 2.713

20 0

Figure 3. Comparison of GCMC simulation and IAST predictions of selectivity derived from simulated single component isotherms.

4

Conclusions

The grand canonical Monte Car10 (GCMC) method was applied to calculate adsorption equilibria of methane, ethane and their mixture. At low pressure small pores filled rapidly due to strong wall potentials. The selectivity strongly depended on pressure and pore width.

5

Acknowledgments

This work was supported by Korea Research Foundation Grant (KRF-2000-005-D00005).

References 1. Ajayan P. M., Stephan O., Colliex C. and Trauth D., Aligned carbon nanotubes arrays formed by cutting a polymer resin-nanotube composite. Science 265 (1994) pp. 1212-1214. 2. Allen M. P. and Tildesley D. J., Computer simulation of liquids (Clarendon, Oxford, England, 1987). 3. Ayappa K. G., Simulation of binary mixture adsorption in carbon nanotubes: Transitions in adsorbed fluid composition. Lungmuir 14 (1998) pp. 880-890. 4. Dujardin E. Ebbesen T. W., Hiura H. and Tanigaki K., Capillarity and wetting of carbon nanotubes. Science 265 (1 994) pp. 1850-1 852. 5. Ebbesen T. W., Ajayan P. M., Hiura H. and Tanigaki K., Purification of nanotubes. Nature 367 (1994) p. 5 19. 6. Frenkel D. and Smit B., Understanding molecular simulation from algorithms to applications(Academic Press, San diego, 1996).

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7. Gu C., Gao G. H., Yu Y. X.and Nitta T., Simulation for separation of hydrogen and carbon monoxide by adsorption on single-walled carbon nanotubes. Fluid Phase Equilibria 194-197 (2002) pp. 297-303. 8. Hilding J., Grulke E. A., Sinnott S. B., Qian D., Andrews R. and Jagtoyen M., Sorption of butane on carbon multiwall nanotubes at room temperature. Langmuir 17 (2000) pp. 7540-7544. 9. Iijima S.,Helical microtubulesof graphitic carbon. Nature 354 (1991) pp. 56-58. 10. Iijima S. and Ichihashi T., Single-shell carbon nanotubes of 1-nm diameter. Nature 363 (1993) pp. 603-605. 11. Mackie E. B., Wolfson R. A., Arnold L. M., Lafdi K. and Migone, Adsorption studies of methane films on catalytic carbon nanotubes and on carbon filaments, Langmuir 13 (1997) pp. 7 197-7201. 12. Tersoff J. and Ruoff R. S.,Structure properties of a carbon-nanotube crystal. Phys. Rev.Lett. 73 (1994) pp. 676-679. 13. Tsang S. C., Chen Y. K., Harris P. J. F. and Green M. L. H., A simple chemical method of opening and filling carbon nanotubes, Nature 372 (1994) pp. 159-162. 14. Yin Y.F., Mays T. and McEnaney B., Molecular simulations of hydrogen storage in carbon nanotubes arrays. Langmuir 16 (2000) pp. 10521-10527. 15. Zhang X. and Wang W., Methane adsorption in single-walled carbon nanotubes arrays by molecular simulation and density functional theory. Fluid Phase Equilibria 194-197 (2002) pp 289-295.

614

HYDROTALCITES FOR CARBON DIOXIDE ADSORBENTS AT HIGH TEMPERATURE

J.I.YANG,M.H.JUNG,S.H.CHOANDJ.N.KIM Separation Process Research Center, Korea Institute of Energy Research, 71-2, Jang-don, Yusung-gu,Taejeon, 305343, Korea E-mail:[email protected] Carbon dioxide adsorbing capacity was investigated using hydrotalcites as high temperahue adsorbents. A gravimetric method was used to determine the Co2 adsorbing capacities of the hydrotalcites, and the temperature was 450 "C. Hydrotalcite took higher adsorbing capacity compared to other basic materials such as MgO, Alza. To increase the Co2 adsorbing capacity of the hydrotalcite, MglAl ratios, preparation methods, and K2C03 impregnation were. checked. As a result, the hydrotalcite prepared by high supersaturationwith MglAl=2 showed desirable adsorptiondesorption pattern and a higher C@ adsorbing capacity. Furthermore, KzC03 impregnation on the hydrotalcite increased the C a adsorbing amount and the optimum value of &Ca impregnation was 20 wt%. The hydrotalcite prepared by high supersaturation with Mg/Al=2 and 20 wt% K z C a impregnation took the highest C@ adsorbing capacity of 0.77 mrnol/g at 450 "C and 800 mmHg.

1

Introduction

Hydrogen is commonly produced by the endothermic steam methane reforming (SMR) reaction that is generally carried out in a catalytic (Nily-AlzO3) reaction at a temperature of 750-900 "Cand a pressure of 50-600 psig. According to Hufton et al. [l], the hydrogen production was improved by sorption-enhanced reaction process (SEW) that used an admixture of a S M R catalyst and an adsorbent. The key requirement for practical use of the SEW for HZproduction is development of the adsorbent that has a high COz working capacity at moderately high temperatures (300-500 "C). Although zeolite A, zeolite X and activated carbon are frequently used as carbon dioxide adsorbents at room temperature, it is very difficult to use them at high temperature. Recently hydrotalcite-like compounds were reported to have good features for carbon dioxide adsorbent at high temperatures

PI. Hydrotalcite is a double-layered material that is composed of a positively charged brucite-like layer and a negatively charged interlayer. Hytrotalcites are represented by the wherein, M(II)=Mg, Cu, general molecular formula, [M(II)I.xM(III)x(OH)~"'A~n~yH~O Ni, Co, Zn; M(III)=Al, Fe, Cr,V; A" is any interlayer anion such as CG", Cl-, NO', S O : - and x=0.1-0.33 [3]. The basicity of hydrotalcite depends on chemical composition (type of cation, ratio of Mz'/Mh, type of anion existing in the interlayer) and activation conditions. For example, calcination of the hydrotalcite above 450 "C results in mixed metal oxides with strong basicity and high surface area. Thus, they can be used as base catalysts or catalyst supports. In this research we prepared several hydrotalcites with different preparation methods, varying Mg/Al ratios and adding different amounts of K2C03for the purpose of using them as high temperature COzadsorbents.

615

2

2.1

Methods Preparation of hydrotalcites.

Two different processes were used for preparing hydrotalcites; low supersaturation and high supersaturationaccording to the methods described in the literature [4]. 2.2

Evaluation of C02 adsorbing capacity.

A gravimetric method was used to determine the C02adsorbing capacities of the prepared hydrotalcites. The amount of sample loaded into the TGA unit (C-1100,CAHN INSTRUMENTS.INC.)was 65 mg. The sample was pretreated in flowing He at 500 "C for 3 h. After the temperature was lowered to 450 "C,C02 was introduced to the balance and the weight change was measured. The range of adsorption pressure was between 0 to lo00 mmHg.

3 3.I

Results Comparison of hydrotalcite with other C02 adsorbentsat high temperature.

C02 adsorbing capacities of a pure MgO, a pure A1203 and a hydrotalcite (Mg/Al=2, prepared by high supersaturationmethod) were measured at 450 "C,and their adsorptive isotherms are shown in Figure 1. Both of individual components of hydrotalcite, MgO and Al2O3, showed appreciable C02 adsorbing abilities even at high temperatures due to their basicity. However, the hydrotalcite adsorbed the significantly more amount of C02than MgO and A 1 2 0 3 3.2

Effect of aluminum contents.

Figure 2 shows COz adsorbing capacities of hydrotalcites prepared by the low supersaturation method with different Mg/Al ratios. The hydrotalcite with Mg/AI ratio of 2 adsorbed the largest amount of C02 among hydrotalcites with Mg/Al ratios higher than 2. As the ratio of MglAl increased to 5 , C02adsorbing capacity decreased. However, as the Mg/Al ratio increased further, hydrotalcites adsorbed more C02and the amount of C02adsorption by the hydrotalcite with Mg/N ratio of 10 became almost equal to that of hydrotalcite with Mg/Al ratio of 2. Yong et al. [2] also observed the similar valley shaped results with the effect of aluminum content on C02 adsorbing capacities of Mg/Al hydrotalcites. When the ratio of Mg/Al equaled two, the high layer charge density of the hydrotalcite due to larger Al contents enabled higher C02 adsorbing capacity. However, as the content of Al decreased with the increasing Mg content, the C02adsorbing capacity decreased due to the loss of the layer charge density. On the other hand, the number of basic sites increased with the increase in Mg content, resulting the increased C02 adsorption on the hydrotalcite with Mg/Al=lO.

3.3

Effect of preparation methods.

Hydrotalcites were prepared by two different methods of the differing precipitation rate during the preparation of hydrotalcite precursor. Although both hydrotalcites showed almost same adsorbed amount of C02, the hydrotalcite prepared by the high supersaturation nethod had the higher desorption capacity. The hydrotalcite of high supersaturation is known to have a low crystallinity, many crystalline nuclei and high surface area due to its small particle size. And the higher surface area of hydrotalcites

616

prepared by the high supersaturation method may yield the desirable adsorptiondesorption behavior. 0.50,

.. . 0

100

2w

300

4M)

800

6w

7w

800

ow

1000 1100

Peq. mmHg

Figure 1. COzadsorption isotherms of various adsorbents ( T 4 5 0 "C, + Hydrotalcite(Mg/AI=2.high), + MgO , -A- AIzO3).

0.30

. 2 0

0.25

E

g

0.20

a

5 0.15 3 0.10 ' 0.05 d U

U

0

0.09

2

5

3

7

10

MglA ratio

Figure 2. Adsorbed amount of COz on hydrotalcites with varying ratios of Mg/Al (T=450 "C,prepared by low supersaturation method). 3.4

Effecr of &COj content.

Hoffman et al. [5] reported that the COz adsorbing capacity of the hydrotalcite could be increased by KzCO3 impregnation in the presence of steam according to the reaction (K2C03+COz+HzO=2KHC03).Furthermore, the increased basicity due to the impregnated alkali metal carbonate may provide positive effect of adsorbing more COz to the hydrotalcite even without steam. Based on that assumption, different amounts of KzCO3 were impregnated on the hydrotalcite and their COZ adsorbing capacities were

617

investigated. It should be pointed out that COPadsorptionsin this experiment were carried out without s t e m l'O

t

0.1 02t 0.0 I

0 5 1 0 1 5 2 0 2 5 r ) 3 5 4 0 4 5 ! h C 0 , loading amount, wt %

Figure 3. Effect of

the KzCO3 loading on COz adsorption ( T 4 5 0 "C, P=800 mmHg, MglAl=2, prepared by high supersaturation).

As shown in Figure 3, the amounts of KzCO3 up to 48 wt% were impregnated on the hydrotalcite with Mg/Al=2 prepared by the high supersaturation method, which was proved to be more effective in the COz desorption. As a result, the COZadsorbing capacity was dramatically increased with the loading of KzCO3 compared to the hydrotalcite without KzC03 impregnation. And there was an optimum amount of K&03 loading. Therefore, the COz adsorbing capacity was 0.77 mmoVg in 20 wt96 KzCO3 impregnation. If the loadiig amount increased further, the COz adsorbing capacity decreased. Therefore, it was c o n f i i that high loading of KzCO3 could be favorable to COZadsorption due to its increased basic sites, but the pores for COz adsorption were blocked with the loading of KzCO3. 4

Conclusion

The hydrotalcite with the composition of Mg/Al=2 was a suitable COZadsorbent at high temperature, viz. 450 "C, and its C02 adsorbing capacity was 0.28 mmoVg. A high supersaturation method for making a hydrotalcite was desirable because the structure of the hydrotalcite prepared by the method was proper for regeneration by pressure swing operation. The KzC03 impregnation on the hydrotalcite was favorable to COz adsorbing capacity because it made chemical properties of the hydrotalcite more basic than that of the hydrotalcite without KzC03 impregnation. The 20 wt% K2CO3 impregnation in the hydrotalcite (Mg/Al=2, high supersaturation) took the highest COZadsorbing capacity of

618

0.77 mmoVg at 450 "C and 800 d g . Above the amount of K2Ca impregnation, the

Cot adsorbing capacity decreased. References

1. Hufton J. R., Mayorga S. and Sircar S., Sorption-enhanced reaction process for hydrogen production. AIChE J. 45 (1999)pp. 248-256. 2. Yong Z.,Meta V. and Rodrigues A. E., Adsorption of carbon dioxide onto hydrotalcite-like compounds (HTls) at high temperatures. I n d Eng. Chem Res. 40 (2001)pp. 204-209. 3. Mckenzie A. L., Fishel C. T. and Davis R. J., Investigation of the surface structure and basic properties of calcined hydrotalcites. J. Cutul. 138 (1992)pp. 547-561. 4. Narayann S.and Krishna K..Hydrotalcite-supportedpalladium catalysts. Appl. Cutul. A 174 (1998)pp. 221-229. 5. Hoffman J. S. and Pennline H. W., Investigation of C02 capture using regenerable sorbents. Proc. of the I ThAnnual Intemutional Pittsburgh Coal Conference. 2000.

619

EFFECT O F POLARITY OF POLYMERIC ADSORBENTS O N DESORPTION OF VOCs UNDER MICROWAVE FIELD XIANG LI ZHONG LI*

HONG XIA XI HUAN WANG

College of Chem. Eng., South China University of Technology Guangzhou, 510640 I! R.China. Email:[email protected] In this work, desorption of volatile organic compounds (VOCs)from polar, weak polar and non-polar polymeric adsorbents using microwave was investigated experimentally. Benzene and toluene were separately used as adsorbates. Results showed that the application of microwave to regenerate the polymeric adsorbents not only got higher regeneration efficiency in comparison with the use of heat regeneration, but also made the temperatures of the fixed beds much lower than that when using the heat regeneration. The weaker the polarity of a polymeric adsorbent,the easier its regenerationwas.

1. Introduction

Recent years, the pollution of volatile organic compounds (VOCs) has attracted much attention. The VOCs have become important pollutants of air"]. The leakage of the VOCs from chemical and pharmaceutical manufacturing, printing processes, paint and adhesive manufacturing and applications, composites and fiberglass molding etc. is the source of the pollutants. A number of adsorbents are capable of capturing a wide range of VOCs. However, the conventional process of regenerating the adsorbents by using organic solvents or thermal fluid still poses a major challenge in this field, notably because of the high expense or secondary pollution. Thermal regeneration involves higher temperature, which results in excessive burnout of the carbon (Grant 1990) It]. Chemical regeneration usually requires the use of organic solvent and involves inevitably a secondary separation or pollution. Recently, some efforts, including supercritical regeneration 13], ultrasound regeneration [4751 and bioregeneration of an adsorbent, have been underway. During the 1980s a number of researchers investigated the otential for using microwave heating to regenerate adsorbents. Burkholder (1986) [6 found that applying microwave energy enhanced the desorption of ethanol without heating silicalite because it selectively heated the ethanol without heating adsorbent. Schmidt (1993) ['I used microwave to desorb water from activated alumina and from Type 4A and 13X zeolites, molecular sieves, as well as methanol from 13X zeolite. Initial results indicated that for particular adsorbentladsorbate systems, microwave heating could dramatically reduce re eneration time. Further, heating was uniform within a fixed bed. Opperman and Brown (1999) firstly compared the microwave regeneration and the thermal regeneration of activated carbons. Their experimental results showed that the temperature required to regenerate completely the activated carbon when the microwave method was used was much lower than that when the thermal regeneration method. Turner (2000) proved that the use of microwave can make the adsorbate with the greater microwave absorptivity be desorbed selectively, and the surface and adsorbed species can be heated selectively. The objective of this work is to use microwave energy to regenerate polymeric adsorbents saturated with benzene and toluene respectively.

P

2. Principle of microwave desorption

Different substances have different abilities of adsorbing microwave energy and

620

converting into heat. The property that describes how well a material or an adsorbate adsorbs microwave energy and convert into heat is the effective dielectric loss factor (c> ). Generally, the heat -up rate of an actuating medium in an applied electric field is in proportion to its dielectric loss fator, and the frequency and the intensity of the electric field, and on the other hand, its heating rate under microwave field is in inverse proportion to its density and its specific heat capacity, as indicated in equation (1) [91. Where M is mass of the substance or actuating medium, c i is an effective dielectric loss factor of the actuating medium under microwave field, P is density of the actuating medium, C, is specific heat capacity of the actuating medium, f is fiequency of microwave, and E is intensity of the electric field. It means that the smaller the dielectric loss factor €of the actuating medium, the less the amount of microwave energy adsorbed by the medium is, and thus the slower the its heating rate is.

t If the dielectric loss factor c> of a material is equal to zero or very small, then the material is transparent or semi-transparent to microwave, which means that microwave can easily penetrate through the material without being adsorbed. For the fixed bed in which the adsorbent has saturated with the adsorbate, when the use of microwave heats or regenerates it, if the effective dielectric loss factor of the adsorbate is much larger than that of the adsorbent, the adsorbate would be rapidly and selectively heated and then desorbed, while the main body of the fixed bed, the adsorbent, would not be obviously heated. It implies that in this case the most of microwave energy is used to heat the adsorbate instead of the adsorbent It not only gets high efficiency of regeneration but also decreases consume of the energy required to regenerate the adsorbent. As a result, it is an important advantage of the application of microwave to regenerate the fixed bed. Usually, the dielectric loss factors of polymeric adsorbents are relative small, i.e. the polymeric adsorbents are transparent or semi-transparent to microwave. So they are suitable to be used to adsorb VOCs and then be regenerated by microwave. 3. Experimental section 3.1. Reagent and Materials Benzene (AR), Toluene (AR), N2 (purity: 99.5%). Polymeric resins: NKA 11 resin (a polar resin), AB-8 (a weak polar resin) and D4006 (a non-polar resin), which were purchased fiom Nankai Chemicals plant, Tianjin. Diameter of the resins ranged from 0 . 3 m to 1.0mm. 3.2. Device Instrument WD8OO microwave oven whose power intensity was 0.734W/cm2, which was made in Tianjin; FNJA electronic scale with 0.0001g of accuracy, which was made in Shanghai; Adsorption column made of Teflon, which was 7cm long, and 1.3cm in diameter. 3.3. Adsorption of VOC on Polymeric Resins Firstly, the resin was filled in the adsorption column, and then was dried by N2 under microwave field for 10-15 minutes. Secondly, gassy mixture of dried air and VOC was introduced into the column at constant flow rate 0.16 m3/h until the resin packed in the

621

column was saturatedby the VOC.The adsorption column was put on the electronic scale with 0.0001g of accuracy. Its weight increased gradually as the VOC adsorbed on the resin, and then got constant when the resin was saturated. The weights of the column before and after the adsorption of the VOC can be measured using the electronic scale. Finally, knowing the weights as well as the weight of the resin packed in the column, ones can calculate the amount adsorbed on the resin, qo(g/g).

3.4. Microwave and Thermal Desorption For the microwave regeneration, firstly, put the adsorption column packed with the polymeric adsorbent on which the saturated amount adsorbed of VOCs was 40 in microwave oven, and subsequently, use synchronously microwave to heat the column and N2to sweep the VOCs desorbed from the adsorbent out. Regeneration efficiency of the adsorbent was measured every thirty seconds by weighing method until regeneration operation ended. For the thermal regeneration, firstly, put the fixed bed packed with the polymeric adsorbent on which the saturated amount adsorbed of the VOCs was qo in temperature-maintaining container. Then, heat the column and maintain its temperature at 9O'C. After that, use N2 to sweep the VOCs desorbed from the adsorbent out. The regeneration efficiency of the adsorbent can be measured using previously stated method. Regeneration efficiency of the polymeric resin or the desorption efficiency of the adsorbate, %%, can be found out according to equation (2). Where qo (g/g) was initial amount adsorbed of the adsorbate on the resin before regeneration, and qt (glg) was transient amount adsorbed of the adsorbate on the resin.

4. Results and discussion 4.1 Comparison between Microwave and Thermal Regeneration

Figures 1-3 showed the comparison between the microwave and thermal regeneration. It can be seen that the application of microwave to regenerate the polymeric adsorbents can get higher regeneration efficiency than the application of the thermal regeneration method. For three kinds of the polymeric resins that had adsorbed benzene, the 0 L . n . . . . . . . . . * J 0 50 100 150 200 250 300 350 400 regeneration efficiency obtained by t (6) using microwave was up to go%, while the regeneration efficiency obtained by Fig. I Comparison between Microwave and Thermal using microwave was merely about Regeneration of NKA-11 Resin. Adsorbate: Benzene, I

60-70%.

.

.

NZFlow Rate: 0.16 m3/h

Table 1 showed the time dependence of temperatures, Tb, of three kinds of the fixed beds respectively packing with the NKA I1 resin, the AB-8 resin and the D4006 resin. It can be seen that the temperatures of the fixed beds when using the microwave regeneration were relative low in comparison with the temperature (90%) of the fixed bed when using the thermal regeneration. A interesting result was that not only the

622

temperatures of the fixed beds when using the microwave regeneration were much lower than that when using the heat regeneration, but also the regeneration rate of the polymeric adsorbents obtained by using the microwave regeneration were much higher than that by using the heat regeneration method. It implied that these polymeric adsorbents were semi-transparent to microwaves, allowing the microwave energy to be applied efficiently throughout the adsorbent bed, and meanwhile, under the microwave field the activated energy of desorption of these organic compounds from the resins became decreased, which made the regeneration of the adsorbents easy in comparison with the heat regeneration.

-

0

0

50

100 150 200 250 300

Fig. 2 Comparison between Microwave and Thermal Fig. 3 Comparison between Microwave and Thermal Regeneration0fAB-8 Resin. Adsorbate: Benzene, N2 Regenerationof MOO6 Resin. Adsorbate: Benzene, N2 Flow Rate: 0.16 m'h Flow Rate: 0.16 m'h

On the other hand, it should be noted that, the temperature of the fixed bed packing with the D4006 resin was the lowest one, while the temperature of the fixed bed packing with the NKA I1 resin was in the highest one. That is to say, the weaker the polarity of the polymeric resin was, the lower the temperature of the fixed bed corresponding to the resin was. It meant that the dielectric loss factor of the polymeric resin was in proportion to its polarity. It should be mentioned that since a thermocouple or thermometer can not be used to measure directly the temperature of the fixed bed under microwave field. The temperatures of the fixed beds was measured by means of a special thermometer and Microwave workstation Mar5 purchased from CEM company, US. Table 1 Temperature Variance of the Fixed Beds Packed with Different Resins under Microwave Field Microwave radiation time (s)

d

60

240

300

400

D4006 Fixed bed (C)

28

31.5

37

38

40.5

AB-8 Fixed bed (TI

28

41

55

58

61

NKA-I1 Fixed bed ( C )

28

40

69

75

80.3

4.2 Effect of different polymeric adsorbents on microwave desorption efficiency In this work, three kinds of the polymeric adsorbents were used, of which the NKA I1 resin was polar, the AB-8 resin was weak polar and the D4006 resin was non-polar. Figures 4-5 showed the regeneration efficiency of the polymeric adsorbents obtained when using the microwave regeneration. It can be seen that whatever the adsorbate used was, benzene or toluene, the regeneration efficiency of the D4006 resin was the highest, while the regeneration efficiency of the NKA I1 resin was the lowest. The regeneration

623

efficiency of the AB-8 resin was intervenient. It implied that adsorption affinity between these VOCs and the polymeric adsorbents decreased with the decrease of adsorbent polarity. The weaker the polarity of the adsorbent, the lower the corresponding desorption activated energy of the VOCs on the adsorbent was, which would make the VOCs more easier to desorb from the adsorbent. For example, compared with the regeneration efficiency of the NKA I1 resin, not only the regeneration efficiency of the D4005 and the AD-8 resin was more than 90%, but also the regeneration rates were higher. 100-

80

-

A

d

2obdsp/

-0-Adsorbent: D4000 -A- Adsorbent: AB-0 -0Adsorbent: NKA-II

--

0

--o-Adsorben(: D4000 -0-Adsorbent: AB-0 4 Adsorbent: NKA-II

100

200

300

400

500

t (S)

Fig. 4 Effect of Different Resins on the Microwave RegenerationEficiency. Adsorbate: Benzene, NZFlow Rate: 0.12 m’h.

Fig. 5 Effect of Different Resins on the Microwave Regeneration Eficiency. Adsorbate: Toluene, NZ Flow Rate: 0.12 m3/h

5. Conclusion The application of microwave to regenerate the polymeric adsorbents can not only get higher regeneration efficiency in comparison with the use of conventional heat regeneration, but also make the temperatures of the fixed beds be much lower than that when using the heat regeneration. The weaker the polarity of a polymeric adsorbent, the easier its regeneration was. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 29936100) and the Natural Science Foundation of Guangdong Province. References 1. Opperman S.H. and Brown R. C., Pollution Engineering, 3 l(1) (1999) p58 2. Grant T. M. and King C. J., Ind. Eng. Chem. Res., 29 (1 990) p264 3. Xie Lanying, Xi Hongxia, LI Xiangbin, Li Zhong, Ion Exchange and Adsorption, 16(5) (2000) p413 4. Rege S.U. Yang R.T. and Cai C.A., Desorption by Ultrasound: Phenol on Activated Carbon and Polymeric Resin”, AIChE Journal, 44(7) (1998), pp1519-1528 5. LI Zhong, LI Xiangbin, XI Hongxia, Effects of Ultrasound on Adsorption equilibrium of Phenol on Polymeric Resin, Chemical Engineering Journal, Vo1.86 (2002) pp375-379. 6. Burkholder H. R., Fanslow G. E., and Bluhm D. D., Ind. Eng. Chem. Fund , 25: (1986) p414 7. Schmidt Philip S. and James R. Fair, Waste Management, 13(5-7) (1993) p25 8. Turner Michael D., Laurence R. L. and Yngvesson K. S., AIChE Journal, 46(4) (2000) p758 9. JIN Qinhan, Microwave Chemistry, Beijing: Science Press, 1999

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MIXED-GAS ADSORPTION ON HETEROGENEOUS SUBSTRATES IN THE PRESENCE OF LATERAL AD-AD INTERACTIONS A. J. RAMIREZ-PASTOR, F. M.BULNES AND J. L. RICCARDO Laboratorio de Ciencias de Superficiesy Medios Porosos. Dpto. ak Fisica. UniversidadNacional de San Luis - CONICET. Chacabucoy Pedernera. 5700- San Luis. Argentina. E-mail: [email protected]

Adsorption of binary gas mixtures in the presence of ad-ad interactions is studied through grand canonical Monte Carlo simulation in the framework of the lattice-gas model. The disordered surface has been characterized by patches of shallow and deep sites, arranged in a chessboard-like topography. The adsorption process is monitored through partial and total isotherms, and differential heats of adsorption. Interesting behaviors have been observed depending on ad-ad interactions and energetic disorder.

1

Introduction

Adsorption of gas mixtures on solid heterogeneous substrates has received an increasing interest in the last decades, due to its importance in relation with new technological developments, like gas-separation and purification [ 1,2]. The description of real adsorption requires to take into account two main effects on the calculation of the thermodynamic quantities: lateral ad-ad interactions, and characteristics of the energy surface. In addition, in the case of multicomponent adsorption, different species could "see" different disordered topographies. Previous works dealing with disordered surfaceshave been dedicated mainly to random, or correlated topographies. In the latter case, the combination of heterogeneity and ad-ad interactions effects produce complex behaviors on the equilibrium properties. An exact statistical mechanical treatment is unfortunately not yet available and, therefore, the theoretical description of adsorption has relied on simplified models. One way of circumventingthis complication is the Monte Carlo (MC) method, which has demonstrated to be a valuable tool to study surface processes [3,4]. In this work, the adsorption of binary mixtures is studied through MC simulation in the context of the lattice-gas model. The topography has been characterized by shallow and deep sites, arranged in a chessboard-like structure. The disorder has been associated to one of the species, while the other component interact with an homogeneous substrate. The process is monitored through partial and total isotherms, and differential heats of adsorption, which appear as sensitive to both lateral interactionsand energetic disorder. 2

Model and Monte Carlo Simulation

The substrate has been represented by a square lattice of M=LxL adsorbing sites with periodical boundary conditions. The heterogeneityhas been introduced by considering the two referred kinds of sites. This is a so-called bivariate surface, in equal concentration, forming hd patches distributed in a chessboard-likeordered topography. Let us introduce the site occupation variable ci: ci=O if the site i is empty, and ~ = - l [%=I] if the site i is occupied by a B[A]-atom. The variable a labels the different kind of

625

sites; a 4 [a=1] represents a shallow [deep] site. Each component occupies only one adsorption site. The energies involved in the process are: [EBD]: adsorption energy for an A p]particle on a deep site. [EM]:adsorption energy for an A [B] particle on a shallow site. W M [WBB] nearest neighbor (NN) energy interaction between an ad-pair AA [BB]. WAB, NN energy interaction between an ad-pair AB (or BA). &AD EAS

Under these considerations, the Harniltonian H of the system, is given by

where 6 is the Kronecker delta function, pA [ p ~ is] the chemical potential of specie A [B], and I",i means that for a given site i, the sum runs over its four NN sites. The binary mixture adsorption is simulated by assuming an ideal AB-gas at fixed T, pA and p ~In. equilibriumthere are two ways to perform a change of the system state: adsorbing (desorbing) one molecule onto (fiom) the surfkce. For a given topography, an elementary MC simulation step (MCS) is as follows: 1) Set PA, p ~T, , and an initial state by placing randomly N molecules on the lattice. 2) Choose randomly one of the components of the mixture -*X ( X=A or B). 3) Choose randomly one of the M sites 4 i; generate a random number 5 E [0,1]: i) if the site i is empty, and 5 5 W,, ,then an X particle is adsorbed on i. Otherwise, the transition is rejected. Wadsis the transition probability fiom a state with N particles to a new state with N+1 particles ii) if the site i is occupied by an X particle, and 5 S wd, , then the X particle is desorbed fiom i. Otherwise, the transition is rejected. wda is the transition probability fiom a state with N particles to a new state with N-1 particles. 4) Repeat fiom step 2)M times.

W* and Wda were obtained in the Metropolis scheme [5]. Then, the total and partial isotherms are obtained as simple averages over m successive configurations: @LA,~B)= =(N)/M, eA(pA,pB)=(NA)/M, and ~PA,CIB)=(NB)/M,where (...) means the time average throughout the simulation, and Nx (X=A or B) is the number of adsorbed X-particles (N=NA+NB).The differentialheat of adsorption qi for the i-specie is [6]:

where U is the energy of the adsorbate and pil/kBT. The simulations were developed for square LxL lattices, with L=200,and periodic

626

boundary conditions. Finite size effects are negligible. The first mo=2x105MCS were discarded to allow equilibrium. The next m=2x105 MCS were used to compute averages. The chemical potential of one of the components is fixed through the process, p~=0,while the other one is variable. 3

Results and discussion

Figure 1 shows the effect of WBB on adsorption isotherms and differential heats of adsorption, for a strongly heterogeneous substrate (A.EA=EAD-EAS=-32hT) and repulsive and T WAB=O. For pA+ -00 (being p~=0),0 ~ and 4 0~ is a function of interactions W M ~ C B WBB. 8B(pA+ -00 ,p~=o) diminishes with WBB, since the B species behaves as a repulsive single gas on an homogeneous system. In fact, the range of variation of 8B(pA+ -00 ,pB=O) lies between 0.5 and 0.226, for WBB=O and WBB + 00, respectively. 40

30

4,

20

10 0

-10 -4.00

*.

-20 -40

-20

0 P A

Figure I: W

M ~ WA& ,

-40

-20

0 P A

20 0.0

I ,O

0.5 QA

and differentWBB’S. a) total and b) partial isotherms; c) differentialheats of adsorption.

As pAincreases, some A particles are adsorbed at expenses of B particles which results in a decreasing of the B coverage. Both A and B particles tend to form 42x2) interpenetrating ordered structures as WBB increases. Owing to WAB=O,8A(pA) and qA(pA), do not depend on WBB. As eA=O.75,the B isotherms (for different WBB’S) collapse to a limit curve. In this conditions, the A particles have completed the deep patches and are forming a 42x2)phase in the shallow patches. On the other hand, the B particles occupy the holes in the shallow patches, surrounded by four NN A particles. It is interesting to note that total and B partial curves are contained between two limit ones, corresponding to wBB=O and WBB+~O.

In order to analyze the effect of the A-B interaction, Fig. 2 shows the behavior of ~ different WAB’S. As isothermsand differential heats of adsorption for wM=4kBT,W B B and pA+ -00, the surface is half covered by B particles, distributed at random. With increasing PA, NA increases, which produces a decreasing h 6 B owing to the WAB repulsions. In fact, the probability of fmding a pair AB (or BA) adsorbed on NN sites diminishes with wAB.In the limit wAB+ w every A particle on the lattice excludes five sites for the adsorption of B particles (the occupied site plus its four NN sites). For 0'

n — ^ 7*> 1^ ft ftl

10

-5

Figure 1. Adsorption isotherms for homonuclear dimers adsorbed on a square lattice with attractive nearest neighbor interactions. The symbols represent the results from MC simulations and the lines represent the results from QF Statistics. Figure 1 and 2 correspond to attractive and repulsive dimers respectively. For the attractive case, which is the most interesting for experimental systems, it is observed that the value of g obtained comes very close to 4, and w agrees very well with the one set in the computer experiment, within a small statistical uncertainty, g = 4 is also consistent with the number of states that an isolated dimer on a square lattice excludes to others.

1.00.8-

.•'A '

fi

0.6-

A

Repulsive Dimers

MC Simulation • w/k B T = 0

• A

0.4-

w/k B T = 2 w/k B T = 4

QF Statistics

-0.06 ±0.02; g = 3.83 ±0.011.94±0.08; g = 4.01±0.01 w/k B T= 4.27±0.21; g = 3.96±0.07

0.20.0

10

20

30

40

Figure 2. As figure 1 for repulsive nearest neighbor interactions. This results means that the adsorption isotherm within the framework of the present theory may provide us of a reliable tool to obtain and interpret the adsorption configuration and lateral interactions of polyatomic adsorbates.

643

With regards to the repulsive case, the model adsorption isotherm reproduces the MC data fairly well for w / k ~ T d The . discrepancy in the case w/kT=4 links to the orderdisorder phase transitions that dimers develop on a square lattice at w k T , ' d , 8,'=1/2 and w/k~T:nr5, €l:=20 (see ref. [5]). These transitions can not be reproduced by the approximation (mean field) used in eq. (8). As it is clear from Figure 2, the simulated isotherm for w/k~T=4display a plateau (characteristicof an ordered phase) at €l=1/2 since w/k~T,'