摘要:

Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L. Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A.Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2Contents 4259 4269 4277 4287 4295 431 1 4321 4335 Protonation Constant of Caffeine in Aqueous Solution M. Spiro, D. M. Grandoso and W. E. Price Ionic Equilibria in Acetonitrile Solutions of 2-, 3- and 4-Picoline N-oxide Perchlorates, studied by Potentiometry and Conductometry L.Chmurzynski, A. Wawrzyn6w and Z. Pawlak Liquid-phase Adsorption of Binary Ethanol-Water Mixtures on NaZSM-5 Zeolites with Different Silicon/Aluminium Ratios W-D. Einicke, M. Heuchel, M. v.Szombathely, P. Brauer, R. Schollner and 0. Rademacher Influence of Oxidation/Reduction Pretreatment on Hydrogen Adsorption on Rh/TiO, Catalysts. An lH Nuclear Magnetic Resonance Study J. P. Belzunegui, J. M. Rojo and J. Sanz Volumetric Properties of Mixtures of Simple Molecular Fluids A. C. Colin, E. G. Lezcano, A. Compostizo, R. G. Rubio and M. D. Peiia Study of Ultramicroporous Carbons by High-pressure Sorption. Part 4.-Iso- thems and Kinetic Transport in Activated Carbons J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Kinetic and Equilibrium Studies associated with the Solubilisation of n- Pentanol in Micellar Surfactants G. Kelly, N. Takisawa, D. M. Bloor, D. G. Hall and E. Wyn-Jones The effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solution. Part 1 .-Maleic and Fumaric Acids R. G. Compton, K. L. Pritchard, P. R. Unwin, G. Grigg, P. Silvester, M. Lees and W. A. House 130-2

ISSN:0300-9599    

DOI:10.1039/F198985FX025

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion.The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No. 90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Orga nising Com mitte e Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Or J. W. Goodwin Professor H. Hoff mann Professor A.L. Smith Professor P. Stenius Dr Th.F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and order-disorder phenomena will form topics for discussion. The preliminary programme is now availablemay be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-18 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e ) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.

ISSN:0300-9599    

DOI:10.1039/F198985BX027

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 85(7) 151 1-1 880 (1 989) 151 1 1523 1531 1537 1545 1557 1567 1575 1585 1599 1607 1619 1631 1647 1655 1671 1685 1697 1709 JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases CONTENTS Skeletal Rearrangement of Alkanes over Ir/A1203. Transformation of n- Pentane, 2-Methylbutane and 2,2-Dimethylbutane Transformation of Methylcyclopentane over Ir/A1203 and n-Hexane over Pt- Black. Interpretation of Hydrogen Control Conductance Studies of Alkali-metal Chlorides and Bromides in 2-Methoxy- ethanol at 25 "C Study of Ultramicroporous Carbons by High-pressure Sorption. Part 1 .-N,, CO,, 0, and He Isotherms J. E. Koresh, T. H. Kim and W. J. Koros Study of Ultramicroporous Carbons by High-pressure Sorption. Part 2.- Nitrogen Diffusion Kinetics J.E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros Study of Ultramicroporous Carbons by High-pressure Sorption. Part 3.-com- plex Transport Phenomena as sensed by CO, and N, Kinetics J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros New Aspects of the Oxidation-Reduction Mechanism of the Ascorbic- Dehydroascorbic Acid System on the Dropping Mercury Electrode J. J. Ruiz and J. M. Rodriguez-Mellado Spectroscopy and Electrochemistry of Polyaniline in Non-aqueous Solution R. Jiang, S. Dong and S. Song Chromatic Reaction of Polyaniline Film and its Characterization R. Jiang and S. Dong Electron Spin Resonance Studies of Pyrroles in Aqueous Solution S. Dong, J. Ding and R. Zhan Properties of Molybdate Species Supported on Silica T-c.Liu, M. Forissier, G. Coudurier and J. C. VCdrine Effect of Temperature on the Salt-induced Sphere-Rod Transition of Micelles of Dodecyltrimethylammonium Bromide in Aqueous NaBr Solutions R. Zielinski, S. Ikeda, H. Nomura and S. Kato A B.E.T.-like Three Sorption Stage Isotherm E. 0. Timmermann Selective Lanthanide-catalysed Reactions. Catalytic Properties of Sm and Yb Metal Vapour Deposition Products H. Imamura, K. Kitajima and S. Tsuchiya Infrared Investigation of CO Adsorption on thermally Reduced Silica- supported Molybdenum Catalysts C. Louis, L. Marchese, s. Coluccia and A. Zecchina The Minimum Condition of Planck's Diffusion Potential and Goldman's Theory G. Dickel Analytical Characterization of Electrode Surface by X-Ray Photoelectron Spectroscopy. p-Pb0,-based Cathode in Voltage-compatible Lithium Cells C.Malitesta, L. Sabbatini, P. G. Zambonin, L. P. Bicelli and S. Maffi An Approach to the Problem of the Dependence of the Dissociation Constant of Weak Electrolytes on the Temperature and on the Solvent Composition in the Ethane- 1,2,-dio1-2-Methoxyethanol Solvent System G. Franchini, A. Marchetti, C. Preti, L. Tassi and G. Tosi Interactions between Metal Cations and the Ionophore Lasalocid. Part 7.-Heat Capacities and Volumes of Complexation of Lasalocid with Alkali- A. Sarkany A. Sarkany D. Nandi, S. Das and D. K. Hazra 51 F A R I1723 1743 1753 1765 1775 1787 1795 1801 1809 1821 1835 1841 1853 1861 1873 Con tents metal and Alkaline-earth-metal Cations in Methanol J. Woinicka, C. Lhermet, N. Morel-Desrosiers, J-P.Morel and J. Juillard Infrared Spectroscopic Studies of the Reactions of Alcohols over Group IVB Metal Oxide Catalysts. Part 1.-Propan-2-01 over TiO,, ZrO, and HfO, G. A. M. Hussein, N. Sheppard, M. I. Zaki and R. B. Fahim Paramagnetic Enhanced Spin-Lattice Relaxation in Micellar Solutions M. J. Hey and C. I. Raven The Nucleation and Detachment of Bubbles Photovoltaic and Photocatalytic Behaviour of a Ferroelectric Semiconductor, Lead Strontium Zirconate Titanate, with a Polarization Axis Perpendicular to the Surface Y. Inoue, K. Sat0 and K. Sat0 The Elovich Differential Equation. Hydrogen and Oxygen Adsorption on Supported Iridium M. C. Manchado, J. M. Guil and A. R. Paniego Solution of Hydrogen in Thin Films of a Palladium-Silver Alloy S. Kishimoto, N.Yoshida, T. Tanabe and T. B. Flanagan The Growth of Phase IV Ammonium Nitrate Crystals and their Transformation to the Phase I11 Structure R. J. Davey, P. D. Guy, B. Mitchell, A. J. Ruddick and S. N. Black Electronic Structures of p-Benzoquinone and Cyclohexane- 1,4-dione Radical Cations. Electron Spin Resonance Study H. Chandra, L. Portwood and M. C. R. Symons Kinetics of Reaction between Bromophenol Blue and Hydroxide Ions in Aqueous Salt Solutions at 298.15 K. Application of Pitzer’s Equations for Ionic Activity Coefficients to Kinetic Salt Effects M. J. Blandamer, J. Burgess, M. R. Cottrell, A. W. Hakin, I. M. Horn and F. Sanchez Rotating-disc Electrode Voltammetry. Digital Simulation of the Current- Voltage Behaviour of Electrode Processes involving Reversible Electron- transfer and coupled Homogeneous Kinetics R. G. Compton and P. R. Unwin Spectrochemistry of Solutions. Part 2 1 .--Inner- and Outer-sphere complexes of Lithium with Thiocyanate in Acetonitrile Solutions P. Gans, J. B. Gill and P. J. Longdon Chromia/Silica-Titania Cogel Catalysts for Ethene Polymerisation. Polymer Characteristics S. J. Conway, J. W. Falconer, C. H. Rochester and G. W. Downs Silicon-29 Shielding Tensors in some Solid Organosilicon Compounds, Studied by Slow Magic-angle Spinning Nuclear Magnetic Resonance R. K. Harris, T. N. Pritchard and E. G. Smith A Theoretical Model of the Ethane/Deuterium Exchange Reaction Catalysed by Platinum. The Nature of the ap Process Photochemical Properties of Iron Oxide Incorporated in Clay Interlayers H. Miyoshi and H. Yoneyama S. D. Lubetkin B. F. Hegarty and J. J. Rooney

ISSN:0300-9599    

DOI:10.1039/F198985FP081

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions II Molecular and Chemical Physics For the benefit of readers of Faraday Transactions I, the contents list of Faraday Transactions 11, issue 7, is reproduced below. 783 Characterization of the Ionization of Phosphoric Acid using Raman Spectroscopy A. Zwick, F. Lakhdar-Ghazal and J.-F. Tocanne 789 Solvent-induced Changes in Electronic Structure of the Exciplex between trans-9-Styrylphenanthrene and Diethylamine F. Elisei, G.G. Aloisi and F. Masetti 801 Kinetic Modelling of Methyl Radical Reactions with Formaldehyde and Isobutane. Reinterpretation of Existing Data obtained by Molecular Modulation Spectroscopy T. K. Choudhury, W. A. Sanders and M. C. Lin 809 Peroxy Radical Reactions in the Photo-oxidation of CH3CHO G.K.Moortgat, R.A. Cox, G. Schuster, J. P. Burrows and G. S. mndall 831 Dimers and Tetramers of Sic1 in a Molecular Beam from a Heterogeneous, Intermediate-pressure Reactor J. Davidsson and L. Holmlid 839 CS-INDO/CI Investigation on Rotamerism and on Excited Singlet States of 1,2’-Binaphthyl. The Appearance of Locally Excited and Twisted Intermolecular Charge-transfer States I. Baraldi 845 Acoustic Absorption in L-Cysteine Solutions at Physiological pH and Temperature using a Novel Technique A. K. Holmes and R. E. Challis 857 Comparative Study on He(1) Photoelectron Spectroscopy and Voltammetry of Ferrocene Derivatives T. Matsumura-Inoue, K. Kuroda, Y. Umezawa and Y. Achiba 867 The RHF- and UHF-AM1 Potential-energy Surfaces for the Diels-Alder Reaction of Substituted Butadienes with Substituted Ethenes.Part 58.atermination of Reactivity by MO Theory J. Y. Choi and I. Lee 879 Optimized Analytical Direct Correlation Function Approximation for Dipolar Fluids P. Sloth and L. Blum 891 Structure of Simulated Aggregates formed by Reversible Flocculation E. Dickinson, C. Elvingson and S. R. Euston 901 The Electrostatic Model of Field Gradients at Nuclei. An Application to Hydrogen-bonded Complexes of HCl J. Baker, A. D. Buckingham, P. W. Fowler, E. Steiner, P. Lazzeretti and R. Zanasi 915 Kinetics of CN(v=O) and CN(v=l) with HCl, HBr and HI between 295 and 764 K I. R. Sims and I. W. M. SmithThe following papers were accepted for publication in Faraday Transactions I during April, 1989. 8/04393K 8P4451A 8W73B 8/04834G 8/04710(3 8/049 15C 9/00126(3 9/00147F 9/002041 9/00257J 8/05051A 8/05059G 9/00369J 9/00388F 9/00587K 9/00592G 9/00858F 9/00861F 9/00862D 9/00863B 9/00879I The Capacity of Complexes of Lithium Polytetra-alkyl Borates for Oxygen Activation Bolshakov, G.F., Dmitrieva, Z. T. and Ryzhikova, I. G. Parameters of Microporous Structure of Carbonaceous Absorbents Gasified with Air or Carbon Dioxide Jaroniec, M., C homa, J., Rodriguez-Reinoso, F., Martin-Martinez, J. M. and Molina-Sabio, M. Excess Thermodynamic Properties of Some 2-Alkoxyethanol-Water Systems Douheret, G., Pal, A. and Davis, M. I. The Influence of Cr3' and Fe3+ Cations on the Structure of Alumina-Aluminium Phosphate Solids and their Catalytic Activity for N20 Decomposition Pomonis, P. J., Petrakis, D. E. and Sdoukos, A.T. Co-ordination Structures of Cu Ions in the Mixed-solvent System of Formamide and Ammonium Formate Miyake, M., Nagahara, S., Yoshikawa, Y. and Suzuki, T. Infrared Study of the Effects of Oxidation/Reduction Treatments on Pt Dispersion in F?/Al203 Catalysts Rochester, C. H., Mordente, M. G. V. and Anderson, J. A. A Physicochemical Study of Gallium@) ZSM-5 Zeolite Smith, T. D. and Handreck, G, P. Solubility in the System NiC12-ZnCl~H3BO3-H20 at 15,20 and 25 "C Balej, J. Ionic Solvation. Part 6.4tandard Potential of Ag/Ag + Cryptand (222,221 and 211) Electrode in Aprotic Media Lewandowski, A. Theoretical and Experimental Study of the Fractal Nature of the Structure of Casein Gels Bremer, L. G. B., van Wet, T. and Walsta, P. Solvent Effects on the 'h Hyperfine Coupling Constants and Spin Exchange Rates for Di-t-butyl Nitroxide and Peroxylamidisulphonate Ions Symons, M.C. R., Malik, N. A. and Smith, E. A. The Association of Caffeine in Aqueous Solution Solution: Its Effects on Caffeine Intradiffusion Harris, K. R., Price, W. E. andTrickett, K. A. Effect of Phosphate and Polyphosphonates on the Dissolution of Barium Fluoride Hamza, S. M, and El-Hamouly, S. H. An Infrared Study on the Interaction of CO with Alumina-supported Rhodium Solymosi, F. and Knozinger, H. Micropore Filling of Supercritical NO on External Surface Modified Microporous Carbons Katsumi, K. and Akihiko, M. The Inhibition of Electron-transfer Reactions at the Mercury/Acetonitrile Interface by Adsorbed Lithium Ions Compton, R. G., Northing, R. J. and Quarrell, R.E. L. Reductive Dissolution of Colloidal Ferrites by Methyl Viologen Radicals Buxton, G. V. and Cartmell, D. W. Determination of Rate Constants for the Rapid Coagulation of Polystyrene Microspheres using Photon Correlation Spectroscopy Herrington, T. M. and Midmore, B, R. 'H Nuclear Magnetic Resonance Investigations of the Cu/Zn/Al-Oxide Methanol Synthesis Catalyst Dennison, P. R. and Packer, K. J. Influence of Support on the Availability of Nickel in Supported Catalysts for Hydrogen Chemisorption and Hydrogenation of Benzene Narayanan, S. and Sreekanth, G. Direct Conversion of Methane to Methanol Burch, R., Squire, G. D. and Tsang, s. c. (ii)9/01021A 9P1068H 9/1084J 9/01579E 9/0 158 1 G 9/01584A 91015956 9B1593K 9/01598A 9/01602C 9/01605H 9/01604J 9/01607D 9X, 1632E 9/00120D Stepwise NO Chemisorption on Chrysotile Asbestos Uchiyama, H., Kaneko, K.and Ozeki, S. IR Spectroscopic Investigations of the Adsorption of Ethylene Oxide Propylene Oxide Block Copolymers from ccl4 on Pyrogenic Silica Killmann, E., Fulka, C. and Reiner, M. Synergy between Copper and Zinc Oxide during Methanol Synthesis: Transfer of Activating Species Burch, R., Chappell, R. J. and Golunski, S. E. Electron Addition to Xanthine Oxidase: An E.S.R. Study of the Effects of Ionizing Radiation Symons, M. C. R., Taiwo, F. A. and Petersen, R. L. E.S.R. Spectra of the Fe2(co)8- Radical Trapped in Single Crystals of P P C Feco(co)8- Preston, K. F., Morton, J. R. and Le Page, Y. High-temperature Superconductivity and E.S.R. Spectroscopy Morton, J.R. Mixed-ligand Complexes of Gun- 1 ,lo-o-Phenanthroline and its Analogues characterized by Computer-aided E.S.R. Spectroscopy Basosi, R., Yang, Y. and Pogni, R. Dramatic Tensor-axis Non-coincidence Effects in the E.S.R. Spectra of some Low-spin Manganese(@ Complexes Pike, R. D., Rieger, A. L, and Rieger, P. H. Quantitative Analysis of the E.S.R. Spectrum of a Uranium(m) Compound Soulie, E. J. and Lesieur, P. C. Binuclear Radical Complexes of Heavy-metal Fragments containing Ruthenium, Osmium, Rhodium and Gold Kaim, W., Kohlmann, S., Kasack, V. and Roth, E. Reactions of Ag' Ions in Alcohols after Radiolysis at 77 K Janes, R., Stevens, A. D. and Symons, M. C. R. Experimental Evidence of the Hyperfine Interaction between a Surface Superoxide Species on MgO and a neighbouring Hydroxylic Proton Giamello, E., Garrone, E., Ugliengo, P,, Che, M.and Tench, A. J. Alternatives to Field Modulation in E.S.R. Spectroscopy Hyde, J. S., Sczaniecki, P. B. and Froncisz, W. Novel E.S.R. Signals from a Six Iron Containing Fe/S Protein Hagen, W. R., Pierik, A. J. and Veeger, C. Enthalpies of Dilution of Aqueous Solutions containing 'Structure-Breaking' Solutes and Polyols Elia, V., Cascella, C., Castronuovo, G., Sartoria, R. and Wursburger, S. (iii)Cumulative Author Index 1989 Abe, M., 1493 Adachi, K., 1065, 1075, 1083 Agathonos, P., 1357 Aguilella, V. M., 223 Akitt, J. W., 121 Albery, W. J., 1181, 1189 Al-Bizreh, N., 1303 Albuquerque, L. M. P. C., 207 Allen, G. C., 55 Almeida, B. S., 1217 Amodeo, P., 621 Anderson, J. A., 11 17, 1129 Anpo, M., 609 Apelblat, A., 373 Arai, T., 929, 1451 Archer, M.D., 1027 Asakura, K., 441 Austin, J. C., 1159 Baiker, A., 999 Bald, A., 479 Barone, G., 621 Barone, V., 621 Beckett, M. A., 727 Bellotto, M., 895 Bengtsson, L., 305, 317 Berry, F. J., 467 Bertoldi, M., 237 Bertran, J., 1207 Bicelli, L. P., 1685 Black, S. N., 1795 Blandamer, M. J., 735, 1809 Bolis, V., 855, 1383 Bolton, J. R., 1027 Bond, G. C., 168 Borowko, M., 343 Bosch, H., 1425 Boss, R. D., 11 Bowker, M., 165 Brimblecome, P., 157 Biilow, M., 1501 Burgess, J., 735, 1809 Busca, G., 137, 237 Calado, J. C. G., 1217 Campbell, J. A., 843 Carbonara, M., 1257 Carlstrom, G., 1049 Caro, J., 1501 Cattania, M. G., 801 Chadwick, A. V., 166 Chandra, H., 1801 Che, M., 609 Chen, J., 829 Chen, L-f., 33 Clegg, S.L., 157 Cohen, H., 1169 Colling, C. N., 1303 Coluccia, S., 609, 1655 Comninos, H., 633 Compton, R. G., 761, 773, 977, Conway, S. J., 71, 79, 1841 Cooper, J., 1365 Copperthwaite, R. G., 633 Cottrell, M. R., 1809 Coudurier, G., 1607 Cox, B. G., 187 Cristiani, C., 895 Cristinziano, P., 621 da Costa, M. A., 907 Das, S., 1531 Datka, J., 47, 837 Davey, R. J., 1795 Dawber, J. G., 727 De Giglio, A., 23 Dell’Atti, A., 23 Dickel, G., 1463, 1671 Ding, J., 1599 Domen, K., 929, 1451 Dong, S., 1575, 1585, 1599 Donini, J. C., 91 Downs, G. W., 1841 Drummond, C. J., 521, 537, 551, Easteal, A. J., 1091 Eden, J., 991 Elisei, F., 1469 el Torki, F. M., 349 Endoh, A., 1327 Espinos, J. P., 1279 Fahim, R. B., 1723 Falconer, J. W., 71, 79, 1841 Fernandez, A., 1279 Fernandez-Pineda, C., 1019 Finch, J.A., 91 Flanagan, T. B., 1787 Fletcher, P. D. I., 147 Foerch, R., 1139 Foo, C. H., 65 Forissier, M., 1607 Forster, H., 1149 Forzatti, P., 895 Franchini, G., 1697 Frey, H. M., 167 Fubini, B., 237, 855, 1383 Gabriel, C. J., 11 Gabrys, B., 168 Gadzekpo, V. P. Y., 1027 Gans, P., 1835 Garrone, E., 585, 1373, 1383 Garst, J. F., 1245 Gasser, D., 999 Gavish, B., 1199 1821 56 1 Gervasini, A., 801 Geus, J. W., 269, 279, 293, 1267 Giamello, E., 237, 855, 1373 Gilbert, P. J., 147 Gill, J. B., 1835 Girault, H. H., 843 Gonzalez-Elipe, A. R., 1279 Gonzalez-Lafont, A., 1207 Gorner, H., 1469 Gottschalk, F., 363 Grieser, F., 521, 537, 551, 561 Guardado, P., 735 Guil, J. M., 1775 GutiCrrez, C., 907 Guy, P. D., 1795 Hagele, G., 1409 Hakin, A. W., 1809 Halle, B., 1049 Hampton, S., 773 Han, S., 829 Handreck, G.P., 645 Harland, R. G., 761 Harris, R. K., 1409, 1853 Hasselaar, M., 1267 Hasted, J. B., 99 Hatano, M., 199 Hazra, D. K., 1531 Healy, T. W., 521, 537, 551, 561 Heatley, F., 917 Hegarty, B. F., 1861 Hesselink, W. H., 389 Hester, R. E., 171, 1159 Hey, M. J., 1743 Higgins, J. S., 170 Higuchi, A., 127 Hill, W., 691 Hirai, T., 969 Holmberg, B., 305, 317 HQ~z, M., 1257 Hong, C. T., 65 Horn, I. M., 1809 Howard, J., 1233 Howarth, 0. W., 121 Hubbard, C. D., 735 Hummel, A., 991 Hunger, M., 1501 Hunter, R., 363, 633 Hussein, G. A. M., 1723 Hutchings, G. J., 363, 633 Ichikawa, K., 175 Ikeda, R., 111 Ikeda, S., 1619 Ikeda, Y., 1099 Imamura, H., 1647 Imanishi, Y., 1065, 1075, 1083 Inoue, Y., 1765AUTHOR INDEX Ishida, H., 11 1 Itaya, K., 1351 Itoh, N., 493 Iwasawa, Y., 441 Jiang, R., 1575, 1585 Jin, T., 175 Johnson, G.R. A., 677 Johnston, C . , 11 11 Jonkers, G., 389 Jorgensen, N., 11 11 Juillard, J., 1337, 1709 Jutson, J. A., 55 Kaneko, K., 869 Kanno, T., 579 Karaiskakis, G., 1357 Karger, J., 1501 Kato, S., 1619 Katoh, T., 127 Keeler, J. H., 163 Kelebek, $., 91 Kim, T. H., 1537, 1545, 1557 Kishi, R., 655 Kishimoto, S., 1787 Kitajima, K., 1647 Kiwi, J., 1043 Knijff, L. M., 269, 293 Kobayashi, M., 579 Koda, S., 957 Koresh, J. E., 1537, 1545, 1557 Koros, W. J., 1537, 1545, 1557 Kosugi, N., 869 Kotaka, T., 1065, 1075, 1083 Kozlowski, Z., 479 Kuroda, H., 869 Kuwabata, S., 969 Lancaster, N. M., 1303, 1315 Larramona, G., 907 Laschi, F,, 601 Lawrence, D. G., 1365 Lawrence, K. G., 23 Lelj, F., 621 Levy, O., 373 Lewis, T.J., 1009 Leyendekkers, J. V., 663 Lhermet, C., 1709 Li, C., 929, 1451 Liu, J.-Y., 1027 Liu, T., 1607 Lluch, J. M., 1207 Longdon, P. J., 1835 Lorenzelli, V., 137 Loudon, R., 169 Louis, C., 1655 Lowe, B. M., 945 Lubetkin, S. D., 1753 Lund, A., 421 Mafi, S., 223 Maffi, S., 1685 Maignan, A., 783 Malet, P., 1279 Malitesta, C., 1685 Manchado, M. C., 1775 Manzurola, E., 373 Marchese, L., 1655 Marchetti, A., 1697 Marcus, Y., 381 Markovits, G., 373 Maruya, K., 929, 1451 Masiakowski, J. T., 421 Matsuhashi, N., 11 1 Matsui, H., 957 Matsumoto, T., 175 Matthews, R. W., 1291 Mazzucato, U., 1469 McAleer, J. F., 783 McLure, I. A., 1217 Meima, G. R., 269, 279, 293, Merwin, L. H., 1409 Meyerstein, D., 1169 Miessner, H., 691 Mills, A., 503 Mitani, T., 1485 Mitchell, B., 1795 Miyoshi, H., 1873 Mizoe, K., 1327 Mizuno, K., 1099 Morazzoni, F., 801 Morel, J-P., 1709 Morel-Desrosiers, N., 1709 Morrison, C., 1043 Morterra, C., 1383 Moseley, P.T., 783 Mosier-Boss, P. A., 11 Mount, A. R., 1181, 1189 Mousset, G., 1337 Munuera, G., 1279 Nakagawa, T., 127 Nakamura, D., 11 1 Nakamura, T., 493 Nandi, D., 1531 Natarajan, P., 813 Nazhat, N. B., 677 Neagle, W., 429, 719 Neilson, G. W., 1365 Newman, K. E., 485 Nicholas, A,, 773 Nicol, J. M., 1233 Nomura, H., 957, 1619 Nowak, R. J., 11 Nowicka, B., 479 Nunes, M. R., 907 Ohlmann, G., 691 Ohyama, Y., 749 Okubo, T., 455, 749 Oliva, A., 1207 Oliveira Jr, 0. N., 1009 Onishi, T., 929, 1451 Orchard, S. W., 363 Osada, Y., 655 Otsuka, K., 199 Pacynko, W. F., 1397 Pandey, J. D., 331 Paniego, A.R., 1775 Pellicer, J., 223 Pereira, I., 907 Piwowarska, Z., 47, 837 Pope, C. G., 945 Portwood, L., 711, 1801 Preti, C., 1697 1267 (v) Price, W. E., 415, 1091 Pritchard, T. N., 1853 Rai, R. D., 331 Ramaraj, R., 813 Ramis, G., 137 Rao, K. J., 251 Raven, C . I., 1743 Reed, W. F., 349 Rees, L. V. C . , 33, 1501 Reis, J. C. R., 207 Reller, A., 855 Rhodes, C . J., 711 Rochester, C . H., 71, 79, 429, 719, 1111, 1117, 1129, 1841 Rodriguez-Mellado, J. M., 1567 Rooney, J. J., 1861 Rosen, D., 99 Rossi, C., 601 Rowlinson, J. S., 171, 172 Ruddick, A. J., 1795 Ruiz, J. J., 1567 Saadalla-Nazhat, R. A., 677 Sabbatini, L., 1685 Sacco, A., 23, 1257 Said, M., 99 Sakata, Y., 929, 1451 Salvagno, S., 1009 Sanchez, F., 1809 Sarkany, A., 151 1, 1523 Sato, Kazunori, 1765 Sato, Kiyoshi, 1765 Schmehl, R.H., 349 Schmidt, J. A,, 1027 Schneider, H., 187 Schneider, I., 187 Schumann, M., 1149 Scotti, R., 801 Selvaraj, U., 251 Sheppard, N., 1723 Shido, T., 441 Shindo, Y., 1099 Shukla, R. K., 331 Shot, P. J., 1425 Smith, E. G., 1853 Smith, G. W., 91 Smith, J. J., 11 Smith, M. R., 467 Smith, T. D., 645 Soares, V. A. M., 1217 Song, S., 1575 Sorek, Y., 1169 Stevens, A. D., 1439 Stewart, A. A., 843 Stirling, C . J. M., 1009 Stroka, J., 187 Strumolo, D., 801 Sugawara, S., 1351 Sundar, H. G. K., 251 Sutton, H. C., 883 Symons, M. C. R., 711, 1439, Szejgis, A., 479 Szpak, S., 11 Takagi, T., 1099 Takagi, Y., 493 1801AUTHOR INDEX Takaishi, T., 1327 Tamura, K., 1493 Tanabe, T., 1787 Taniewska-Osinska, S., 479 Tassi, L., 1697 Taylor, D. M., 1009 Terai, M., 1493 Thamm, H., I Themistocleous, T., 633 Tiddy, G.J. T., 1397 Timmermann, E. O., 1631 Tissier, M., 1337 Tosi, G., 1697 Tsuchiya, S., 1647 Tsutsumi, K., 1327 Ugliengo, P., 585, 1373 Unwin, P. R., 1821 Urch, D. S., 1139 Vaccari, A., 237 van Buren, F. R., 269, 279, 293, Van-Den-Begin, N., 150 1 van Dillen, A. J., 269, 279, 293, van Leur, M. G. J., 279 van Lith, D., 991 van Rensburg, L. J., 633 van Veen, J. A. R., 389 Vazquez-Gonzalez, M. I., 1019 Vidrine, J. C., 1607 Vink, H., 699 Vis, R. J., 269, 279 Wacker, T., 33 Walker, D. R. B., 1545, 1557 Walker, P. A. M., 1365 Waller, A. M., 773, 977 Warman, J. M., 991 Waugh, K. C., 163 1267 1267 Weale, K. E., 165 Williams, D. E., 783 Williams, G., 503 Woolf, L. A., 1091 Wormald, C. J., 1303, 1315 Woinicka, J., 1709 Yamada, Y., 609 Yarwood, J., 1397 Yeh, C-t., 65 Yoneyama, H., 969, 1873 Yoshida, N., 1787 Yoshioka, H., 1485 You, X., 829 Young, D. A., 173 Zaki, M.I., 1723 Zambonin, P. G., 1685 Zecchina, A., 609, 1655 Zhan, R., 1599 Zielinski, R., 1619Special Issues of Faraday Transactions Readers of the F a r e Transactions will be aware that we have now established Special Issues which fall broadly into two categories Collections of the r e f m p a p that have been pmented at a Scientific Conferem=e, normally run by a Subject Group of the RSC, and approved in advance by the Faraday Editorial B d . We insist, with varying degrees of success, that the papers published under this scheme shall describe origrnal wok which fully meets the n o d requirements for submitted papers.Keynote Issues which are opened by a paper written by an acknowledged authority in a particular field of interest, The Keynote author then invites colleagues known to him to contribute original papers on cognate topics, to appear in the same Keynote Issue. These p a p are refend and, as intended, have led to issues of an exceptionally high scientific standard. The initial stage of this development having been completed, Faraday Editorial Board has directed that wider invitations should be issued to the physicochemical and chemical physics community to send us original work which they would like to appear alongside papers on similar topics. Special issues in the planning stage include the tapics listed below. If you think that you have some good work coming along which could be ready for submission around the indicated date, please do write to me and let me know, when I will make the necessafy atrangements. For my part, I see no objection to adding such papers to Special Issues in both categories.David Young Scientific Editor special Issue Reactive and Inelastic Scattering Concentrated Colloidal Dispersions Structure and Activity of Adsorbed Species (with special emphasis on surface science) Closing Date 30th September 1989 15th November 1989 To be announced (vii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 88 Charge Transfer in Polymeric Systems University of Oxford, 11-13 September 1989 This Discussion aims to bring together physicists and chemists interested in the mechanism of electron and ion transport in polymeric systems.The systems indude conducting polymers, redox polymers, ion exchange membranes and modified electrodes. Discussion topics will cover experimental evidence from spectroscopy, electrochemistry and new techniques such as the quartz microbalance. Theoretical models ranging from band theory through polarons to localised chemical structures will be critically evaluated and compared with experiment. The following have agreed to participate in the Discussion: R. Murray W. J. Albery M. D. lngram M. B. Armand D. Bloor H. Cheradame P. G. Bruce R. Friend R. J. Latham A. J. Heeger A. R. Hillman P. V. Wright A. G. MacDiarmid M. Ratner M. E. G. Lyons S. Roth 0. Haas T. J. Lewis G. Tourillon C. Vincent P. G. Pickup G. Wegner A. Hamnett L.M. Peter K. Doblhofer The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHlMlE PHYSIQUE JOINT DISCUSSION MEETING 1989 Transport Processes in Fluids and in Mobile Phases Aachen, Federal Republic of Germany, 25-27 September 1989 Organised by: H. Versmold (F.R.G.) Al. Weiss (F.R.G.) M. Zeidler (F.R.G.) G. R. Luckhurst (U.K.) The purpose of the meeting is to bring together scientists working on transport and related phenomena in simple and complex fluids, colloidal and micellar systems, and surface phases.Experimental techniques considered include classical methods, optical spectroscopy, light scattering, nuclear magnetic resonance, and neutron scattering. The following persons have accepted invitations to present talks: D. Evans, Canberra; B. U. Felderhof, Aachen; D. Frenkel, Amsterdam; A. Geiger, Dortmund; W. Glaser, Grenoble; H. G. Hertz, Karlsruhe; S. Hess, Berlin; J. Jonas, Urbana; R. Klein, Konstanz; K. Lucas, Duisburg; H.-D. Ludermann, Regensburg; H. Posch, Wien; P. Pusey, Malvern; J, P. Ryckaert, Brussels; W. A. Steele, Penn State; D. J. Tildesley, Southampton; H. Weingilrtner, Karlsruhe. Further details may be obtained from: Professor H. Versmold, lnstitut fur Physikalische Chemie, RWTH Aachen, Templergraben 59, D-5100 Aachen, Federal Republic of Germany.P. Turq (France) (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 25 Large Gas Phase Clusters University of Warwick, 12-14 December 1989 Organising Committee: Professor K. R. Jennings (chairman) Professor P. J. Derrick Professor D. Phillips Dr N. Quirk Dr R. P. H. Rettschnick Dr A. J. Stace The Symposium will focus on recent developments in the rapidly expanding field of large gas phase clusters, including the preparation, structure and reaction of both neutral and ionic dusters. It is hoped that the meeting will bring together scientists working on many different types of cluster, e.g. rare gas atoms, metals, inorganic and organic species, and biomolecules, to discuss the chemistry and physics of clusters from different viewpoints. The preliminary programme is now available and may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 89 Structure of Surfaces and Interfaces as Studied using Synchrotron Radiation University of Manchester, 4-6 April 1990 Organising Committee: Professor J. N. Shewood (Chairman) Professor D. A. King Dr G. King Dr C. Norris Dr R. Oldman Dr G. Thornton The Discussion will focus on the wealth of novel information which can be obtained on the nature and structure of surfaces using the full spectral range of synchroton radiation. Emphasis will be placed on the scientific results of recent investigations rather than on technical aspects of experimentation. Papers will be welcome which shed new light on the structure of the complete range of interfaces: solidkolid, solid/gas, solidfliquid, gasniquid and "dean" surfaces including both static and dynamic in situ examinations. It is hoped that the discussion will define the utility of synchroton radiation examinations in surface science studies at a time of expansion of the availability of such sources and the inauguration of new and more powerful sources. The preliminary programme is now available and may be obtained from: Mrs Y. A.Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN.THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY ASSOCIAZIONE ITALIANA DI CHIMICA FlSlCA DEUTSCHE BUNSEN-GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE KONINKLIJKE NEDERLANDS CHEMISCHE VERElNlGlNG SOCIETE FRANGAISE DE CHIMIE, DIVISION DE CHIMIE PHYSIQUE FARADAY DIVISION GENERAL DISCUSSION No.90 Colloidal Dispersions University of Bristol, 10-12 September 1990 Organising Committee Professor R. H. Ottewill (Chairman) Professor P. Botherol Professor E. Ferroni Dr J. W. Goodwin Professor H. Hoffmann Professor A.L. Smith Professor P. Stenius Dr Th. F. Tadros Professor A. Vrij Dr D. A. Young The joint meeting of the Societies will be directed towards examining current understanding of the behaviour of colloidal dispersions. In particular, stability and instability, short range interactions, dynamic effects, non-equilibrium interaction, concentrated dispersions and orderdisorder phenomena will form topics for discussion.Contributions for consideration by the Organising Committee are invited. Titles and abstracts of about 300 words should be submitted by 30 September 1989 to: Professor R. H. Ottewill, School of Chemistry, University of Bristol, Bristol BS8 lTS, England. Full papers for publication in the Discussion Volume will be required by May 1990. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM No. 26 Molecular Transport in Confined Regions and Membranes Oxford, 17-1 8 December 1990 Experimental, theoretical and simulation studies which address fundamental aspects of molecular transport will be discussed in the following main areas: a) Transport of atoms and molecules in pores, zeolite networks and other cavities; time-dependent statistical mechanics of small systems in confined geometries b) Molecular transport through synthetic membranes, biological membranes, smectic liquid crystalline phases and Langmuir Blodgett films; the dynamics of the molecules forming the membrane c) Diffusion, reorientation, conformational dynamics, viscosity and conductivity of polymer melts, to include papers dealing with bulk systems since the segments of the polymer will move in the anisotropic environment of the complete chain d) Applications of Brownian dynamics to the study of diffusion in porous media and across membranes including the transport of flexible aggregates such as microemulsions e) The growth of crystals, colloidal aggregates and droplets on irregular surfaces and in pores Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 December 1989 to: Dr D.J. Tildesley, Department of Chemistry, The University, Southampton SO9 SNH. Full papers for publication in the Symposium Volume will be required by August 1990.~ ~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Gas Kinetics Group Developments in Gas Kinetics: New Techniques, Results and their Interpretation To be held at the Universii of York on 3 4 July 1989 Further information from Professor R. J. Donown, Deparbnent of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Industrial Physical Chemistry Group with the Thin Films and Surfaces Group of the IOP Materials for Non-linear and Electro-optics To be held at Gitton College, Cambndge on 4-7 July 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SW1X 8QX Electrochemistry Group with Electroanalyfical Group Graduate Students’ Meeting To be held at Imperial College, London on 12 July 1989 Further information from Dr G.H. Kdsall, Department of Mineral Resources Engineering, Imperial College, London SW7 2BP Polymer Physics Group Biologically Engineered Polymers 89 To be held at Churchill College, Cambndge, on 31 July to 2 August 1989 Further information from Dr M. J. Miles, AFRC Institute of Food Research, Colney Lane, Norwich NR4 7UA Carbon Group with Surface Reactivity and Catalysis Group Carbons and Catalysis To be held at toughborough University of Technology on 11-1 3 September 1989 Further information from Dr J.W. Patrick, Director, Carbon Research Group, Loughborough Consultants Ltd., University of Technology, Loughborough LE113TF Polymer Physics Group Biennial Meeting: Physical Aspects of Polymer Science 25th Anniversary To be held at the University of Reading on 131 5 September 1989 Further information from Dr G. R. Mitchell, Polymer Physics Laboratory, University of Reading, Whiteknights, Reading RG6 2AF. Colloid and Interface Science Group Inorganic Particulates To be held at Chester College on 1921 September 1989 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, Po Box 11, The Heath, Runcorn, Cheshire WA7 4QE Polar Solids Group with Low Temperature Group of the IOP and the lnstitute of Ceramics High Temperature Semiconductors To be held at the University of Birmingham on 1921 September 1989 Further information from Dr C.Greaves, Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham B15 211 ~~ ~ ~ ~~ ~ ~~ ~ ~ Division with the Institute of Physics Sensors and their Applications To be hekl at the University of Kent at Canterbury on 1922 September 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SW1X 8QXDivision with the Deutsche Bunsen Gesellschaft, Division de Chimie Physique of the Societd Franpise de Chimie and Associazione ltaliana di Chimica Fisica Transport Processes in Fluids and Mobile Phases To be held at the Physikalische Instittit, Aachen, West Germany on 2528 September 1989 Further information from Professor G.Luckhurst, Department of Chemistry, University of Southampton, Southampton SO9 5NH Division Autumn Meeting: Chemistry at Interfaces To be held at Loughborough University of Technology on 26-28 September 1989 Further information from Professor F. Wilkinson, Department of Chemistry, Loughbomugh University of Technology, Loughbomugh El1 3TU Polymer Physics Group Polymers in Motion To be held at the University of Cambndge on 14 December 1989 Further information from Dr M. J. Richardson, National Physical Laboratory, Queen’s Road, Teddington, Middlesex Tw11 OLW Colloid and Interface Science Group Wetting and Spreading To be held at the Scientific Societies Lecture Theatre, London on 15 December 1989 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runcom, Cheshire WA7 4QE Theoretical Chemistry Group Electronic Structure Calculations on Large Molecules: Novel Methods and Applications To be held in Cambridge on 1517 December 1989 Further information from Dr P. Fowler, Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD H@h Resolution Spectroscopy Group with the Molecular hams Group Spectroscopy and Molecular Beams To be held at the University of Nottingham on 18-19 December 1989 Further information from Dr P. G. Sam, Department of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD Neutron Scattering Group Applied Neutron Scattering To be held at the University of W& on 21 December 1989 Further information from Dr C. Windsor, B 521.1, AERE, Harwell, Didcot, Oxfordshire Colloid and Interface Science Group with Protein and Peptide Group, Biotechnology Group and Gesellschaft fur Biologische Chemie Stability of Proteins: Theory and Practice To be held at Gimn College, Cambndge on 2829 March 1990 Further information from Professor F. Franks, PAFRA Ltd., 150 Cambndge Science Park, Cambndge CB4 4GG Division Annual Congress: The Solid State: Reactivity and Electrical Properties To be held at Queen’s University, Belfast on 9-1 2 Apnl1990 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1V OBN Polymer Physics Group New Materials and their Applications To be held at the University of Warwick on 10-1 2 April 1990 Further information from Dr M. Ri&on, National Physical Laboratory, Queen’s Road, Teddington, Middiesex Tw11 OLW (xii)

ISSN:0300-9599    

DOI:10.1039/F198985BP083

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chern. Soc., Furuday Trans. I , 1989, 85(7), 1511-1522 Skeletal Rearrangement of Alkanes over Ir/A1203 Transformation of n-Pentane, 2-Methylbutane and 2,2-Dimethylbutane Antal Sarkany Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 77, Hungary Transformation of n-pentane, 2-methylbutane and 2,2-dimethylbutane has been investigated as a function of the H,/hydrocarbon ratio over a 10 wt % Ir/A1203 catalyst of 14 YO dispersion. The selectivity of isomer formation has been observed to decrease with the increase of the H,/hydrocarbon ratio and follow the sequence n-pentane > 2-methylbutane 9 2,2-dimethylbu- tane. The effect of the experimental conditions on the product selectivity has been interpreted considering the actual surface state of the working catalyst.Systematic investigations with Ir including blacks,5. foil7 and supported catalysts8-’’ have revealed the remarkable hydrogenolysis activity of this metal and its ability to catalyse Ci’ lo cyclization and bond-shift skeletal rearrangement. 1-9 The selectivity of the latter reaction shows considerable scattering, which implies that a further study is desirable in this field. As pointed out by Foger and Anderson’ the selectivity of the skeletal bond-shift rearrangement over Ir is extremely sensitive for the crystallite size: large particles d 3 7 nm have shown preference for bond shift skeletal rearrangement of 2,2-dim- ethylpropane. The presence of the carbonaceous deposits also seems to be one of the controlling factors, although there is no agreement as to whether the surface carbon atoms ‘choke’ isomerization or whether they play a beneficial effect as claimed by Ponec and coworkers.12, l3 Recently Karpinski et aZ.5 did not report an increase in the isomerization selectivity upon the carburation of the Ir sites. In the present paper we report on the effect of the H,/hydrocarbon ratio in the rearrangement of n-pentane, 2-methylbutane and 2,2-dimethylbutane over an Ir/Al2O3 catalyst containing large Ir particles, d 3 7 nm. Our previous investigations with Pt- based catalystsl47 l5 have confirmed that in alkane transformation the partial pressure of hydrogen is one of the factors which control the coverage of the firmly held hydrocarbons. Considering this fact we felt that the hydrogen or the hydrocarbon sensitivity of isomer formation in a group of alkanes with increasing number of methyl groups might shed some light on the role of the trapped hydrocarbons in the isomerization process.In greater detail, the experiments on an Ir/Al,O, catalyst of 14 YO dispersion are aimed at deciding (i) whether the isomerization sites are a priori present on the surface or (ii) whether they are formed upon the partial poisoning of the fragmentation sites. The measurements at low reaction temperature and with a large excess of H, may allow clarification of point (i). The effect of the low HJhydrocarbon ratio and ‘ high ’ reaction temperature on the selectivity of isomerization may emphasize or rule out the significance of the trapped hydrocarbons.16 Experiment a1 Transformation of n-pentane, 2-methylbutane and 2,2-dimethylbutane has been investigated in an atmospheric flow system working slightly over atmospheric pressure 151 1 51-21512 Rearrangement of Alkanes over Ir/A1203 (98.5 kPa).As our intention was to study the initial activity and selectivity of an 0,- and H,-treated Ir/A1203 sample, we used the 'slug-pulse technique: ca. 100 x cm3 of hydrocarbon was injected onto the cold wall of the saturator. (Flow rates were 15-25 cm3 min-l.) The gas samples were taken at 4 min on a H,-He-hydrocarbon stream. In a few cases the initial section of the 'hydrocarbon step ' was also analysed. Prior to each catalytic run the catalyst (0.01-0.02 g) was purged in He and treated with 0, (0.65 kPa) up to 543 K. After 0, treatment the catalyst was reduced at 623 K for 30 min and then cooled back to the temperature of the next experiment. H, and H,/He mixtures taken from premixed cylinders were carefully purified over Pd/A1,0, and molecular sieve contacts.The purchased hydrocarbons contained impurities, which prevented the determination of the isomer selectivity. For this reason all the hydrocarbons used by us were passed through a preparative g.1.c. The hydrocarbons injected in the kinetic investigations showed no detectable impurities at the highest electrometer sensitivity ( The product composition was analysed on a DC 200 column (6 m long) and a 50 m long capillary column, CP Sil5CB; both columns operated at 303 K. The product selectivity (the rate of product formation divided by the rate of the consumption of the parent compound) and the reaction rate were calculated in conformity with previous definitions.12, l3 The 10 wt% Ir/A1203 sample was prepared by impregnation of y-Al,O, with an aqueous solution of H21rC16. The water was evaporated and the catalyst precursor was dried at 378 K for 18 h and then reduced in H, at 673 K for 24 h. Prior to catalytic measurements the sample was repeatedly treated with 0, (1.33 kPa) and H,. The metal dispersion of the stabilized catalyst inferred from CO chemisorption at 293 K was CO/Ir = 0.14. Hydrogen titration of Irs-0 sites formed in 0.133 kPa 0, at ambient temperature gave dispersion values between 0.1 and 0.13. The carbon coverage on the regenerated catalyst (C/Irs) measured by temperature-programmed oxidation after 30 min hydrogenation at 623 K (using 0.6 g catalyst) was less than 0.001.X-ray fluorescence detected no metal impurities. A mV-'). Results Transformation of n-pentane, 2-methylbutane and 2,2-dirnethylbutane has been investigated in the temperature range 423-495 K on 10 wt YO Ir/A1203 regenerated prior to each experiment. In the temperature range 423-495 K the main reaction is fragmentation, in agreement with the findings of previous publications. The selectivity of fragmentation (Sf,), isomerization (Si) and that of other reactions [homologation (Sn) with n-pentane and 2-methylbutane and cyclization (Sc5) with n-pentane] at different experimental conditions are summarized in tables 1-3. For 2,2-dimethylbutane the value of Si in table 3, and elsewhere in the text, refers to the sum of isomers; the isomer distribution is presented in table 4.The sum of the selectivities of the methyl shifts [chain-lengthening methyl shift (to form 3-methylpentane) and methyl displacement (to form 2,3-dimethylbutane)] is higher than that for the ethyl shifts. One has to consider, however, that there are three chances of shifting methyl and only one of shifting ethyl. In tables 1-3 the following results deserve attention. (i) At low reaction temperatures (423 K with n-pentane and 2-methylbutane, 443 K with 2,2-dimethylbutane) and at high HJhydrocarbon ratio (3 1.2 with n-pentane, 20.16 with 2-methylbutane and 56.8 1 with 2,2-dimethylbutane) no isomer formation was detected. (ii) The selectivity of isomer formation increases with increasing reaction temperature as shown by rows 1-3 and 5-7 in table 1 with n-pentane, and by rows 1-3 in table 3 with 2,2-dimethylbutane. (iii) Si increases with the decrease of the H,/hydrocarbon ratio.A .Sarkany 1513 Table 1. Product distribution from n-pentane on 10 wt YO Ir/Al,O, H2/ fragment distribution (mol YO) selectivity (%) hydro- PJkPa carbon T/K C, C, C, iC, nC, C,/nC, S,, Si Shorn Sc, 3 3 3 3 9.5 9.5 9.5 9.5 9.5 9.5 31.2 31.2 31.2 0.32 0.46 0.46 0.46 0.93 1.87 9.37 423 10.2 41.2 39.8 n.0.' 8.8 4.52 441 14.0 37.3 36.6 n.0. 12.1 3.02 473 15.1 37.6 35.2 tr 11.9 2.93 473 37.8 28.9 19.2 0.4 13.7 1.40 428 20.4 33.1 31.6 tr 14.9 2.12 473 25.6 28.5 27.9 tr 17.8 1.57 495 24.4 29.0 27.3 0.1 19.2 1.42 473 22.0 30.2 29.3 tr 18.5 1.58 473 20.1 32.7 31.6 tr 15.6 2.03 473 16.5 36.2 35.1 n.0.12.2 2.88 100 99.99 99.80 91.14 97.6 84.8 78.3 91.55 96.57 99.10 n.0. n.0. n.0. 0.01 n.0. n.0. 0.13 0.02 0.05 3.63 0.68 4.55 2.3 n.0. 0.12 9.71 0.36 5.13 15.2 0.67 5.85 6.9 0.12 1.43 3.3 0.01 0.12 0.85 n.0. 0.05 Sfr, Si, Shorn, Sc5, selectivity of fragmentation, isomerization, homologation (n-hexane + 2- methylpentane) and cyclopentane formation, respectively. a n.o., not observed. tr, trace (< 0.1 mol %). Table 2. Transformation of 2-methylbutane on 10 wt x Ir/Al,O, H21 fragment distribution (mol YO) selectivity (YO) hydro- P2,,/kPa carbon T/K C, C, C, iC, nC, S,, Si Shorn 4.65 20.16 453 47.1 6.1 5.2 41.1 0.4 100 n.0.' n.0. 4.65 20.16 473 47.1 7.7 6.8 38.1 0.3 100 n.0. n.0. 13.96 6.05 473 42.7 12.6 7.9 34.6 2.2 99.58 0.41 n.0.13.96 0.30 473 39.1 18.8 13.7 25.0 7.0 95.66 4.21 0.13 13.96 3.02 473 42.6 11.5 8.8 35.1 2.0 99.37 0.63 n.0. 13.96 2.01 473 38.3 13.7 12.1 32.3 3.6 98.79 1.21 n.0. 13.96 0.60 473 37.4 17.1 15.2 24.1 6.2 96.76 3.23 0.01 ~~~~ ' n.o., not observed; S,,, S,, Shorn, selectivity of fragmentation, isomerization and homologation (2-met hy lpen tane). Table 3. Transformation of 2,2-dimethylbutane on 10 wt O h Ir/A1203 fragment distribution (mol YO) selectivity (YO) H2 '2,2nMR /hydro- /kPa carbon T/K C, C, C, iC, nC, iC, neoC, iC,/neoC, S,, si 1.71 56.81 443 52.3 - - - - n.0.' 47.7 0 100 n.0. 1.71 56.81 463 51.1 - - - - n.0. 48.9 0 100 n.0. 1.71 56.81 493 52.8 0.8 0.1 0.6 - 0.11 45.6 2 . 4 ~ 100 n.0. 5.72 16.14 473 52.9 2.6 0.3 1.7 tr 0.21 42.3 4.9 x lo-, 99.98 0.019 5.72 1.62 473 47.9 5.6 0.7 4.3 tr 1.23 40.2 3 .0 ~ lo-, 99.97 0.031 5.72 0.81 473 48.7 7.18 1.7 7.3 tr 4.72 30.2 1.5 x 10-1 99.95 0.052 5.72 0.81 495 42.3 9.3 3.4 8.8 0.11 7.31 28.8 2.5 x lo-' 99.87 0.13 a n.o., not observed.1514 - 10 - F 0, - 5 Rearrangement of Alkanes over Ir/A1203 Table 4. Isomer selectivity formed from 2,2-dimethylbutane over 10 wt % Ir/A1203 distribution (mol YO) selectivity H,/hydrocarbon T/K 3MP 2,3DMP 2MP Si (Yo) 16.14 473 40.2 34.1 25.7 0.019 1.62 473 39.7 35.5 24.8 0.03 1 0.8 1 473 42.1 34.5 23.4 0.052 0.8 1 495 53.9 30.6 15.5 0.13 3MP : 3-methylpentane ; 2,3DMP : 2,3-dimethylbutane ; 2MP : 2-methylpentane; P2,2DMR = 5.72 kPa. 0.8 2 6 H2 /hydrocarbon 1 ' I 1 ' I 1 1 I I 0 50 100 PH2 IkPa Fig. 1. Rate of transformation of n-pentane ( x ) and the selectivity of isomerization (0) at 473 K.Pnp = 9.5 kPa. Our detailed investigations on the effect of the partial pressure of hydrogen were performed at 473 K. In fig. 1-3 the variation in the selectivity of isomer formation and of the rate of the consumption of the parent hydrocarbon (-R) are depicted as a function of the partial pressure of hydrogen. For the sake of comparison, the hydrogen orders at H,/hydrocarbon ratios of 6, 2 and 0.8 and the location of the rate maximum (-Rmax at H,/hydrocarbon) are collected in table 5. As the dependence of the isomerization selectivity upon the HJhydrocarbon ratio seems crucial from the viewpoint of the interpretation of the reaction mechanism, we have also studied the initial non-steady concentration range1'' l8 of the hydrocarbon step.In the initial section the HJhydrocarbon ratio steeply decreases with increasing hydrocarbon partialA . Sarkany 1515 0.8 2 4 1 H2 /hydrocarbon 6 0 50 100 Pi? /kPa Fig. 2. Rate of transformation of 2-methylbutane ( x ) and the selectivity of isomerization (0) at 473 K. PZMR = 13.9 kPa. Fig. 3. Rate of isomerization (0)1516 Rearrangement of Alkanes over 1r/A1203 /- e ..-. 15 30 45 60 tls Fig. 4. Transient response of n-pentane. P,, (O), partial pressure of n-pentane transformed to fragments; PZMR (x), partial pressure of isomer; 0, 100 x PZMR/4.; 0, hydrocarbon/H,; T = 473 K, 5 % H,/He, Pnp, steady = 9.5 kPa. pressure. In fig. 4 and 5 the variation of the hydrocarbon/H, ratio and that of the isomer/fragment ratio are plotted throughout the hydrocarbon step with n-pentane and 2-methylbutane, respectively. The results in fig.4 and 5, and similarly those in fig. 1-3, confirm that the selectivity of isomer formation increases with increasing hydro- carbon/H, ratio. Another interesting feature of the results shown in fig. 1-3 is that the isomerization selectivity decreases at each of the hydrocarbon ratios investigated in the order n-pentane > 2-methylbutane >> 2,2-dimethylbutane. By way of example, at H,/hydrocarbon = 2 the value of Si is 5.1, 1.2 and ca. 0.027 O h for n-pentane, 2- methylbutane and 2,2-dimethylbutane, respectively. The product selectivity of fragmentation is also affected by the reaction conditions. The effect of the H,/hydrocarbon ratio and the reaction temperatures on the fragmentation pattern are shown in tables 1-3, for n-pentane, 2-methylbutane and 2,2-dimettylbutane, respectively.In n-pentane distinction can be made between the reactivity of C,-C,, and C,,-C,, bond typesg (C, indicates primary carbon etc.). The propane/n-butane ratio in table 3 shows the variation of the ratio (rate of C,,-C,, bond rupture/rate of C,-C,, bond rupture) as a function of the partial pressure of hydrogen (rows 3, 4 and 6, 8-10). 2,2-Dimethylbutane may be fragmented via iso-unitg or C, unit mode of interaction. In table 3, the 2-methylbutane/2,2-dimethylpropane ratio (iC,/neoC,) is presented. The increase of the reaction temperature and the decrease of the H,/hydrocarbon ratio seem to favour reactions via iso-unit mode.A .Sarkany 1517 400 $ < C4t .5 2 300 - - 200 100 I5 30 45 60 tls Fig. 5. Transient response of 2-methylbutane, P,, (O), partial pressure of 2-methylbutane transformed to fragments; Pnp ( x ), partial pressure of isomer; 0, 100 x Pnp/P,,; 0, hydrocarbon/H,; T = 473 K, 5 O/O H,/He, P2MR,steady = 13.9 kPa. Discussion The absence (or presence of only very limited amounts) of isomers on our regenerated Ir catalyst, under experimental conditions which reduce the possibility of the poisoning of the surface by carbonaceous materials (low temperatures and large H,/hydrocarbon ratios, results in tables 1-3), seems to support the view12-15 that isomerization requires the partial poisoning of the surface Ir atoms. The beneficial effect of the surface-held hydrocarbons appears in a more convincing way in the investigations performed in low excess of hydrogen (fig.1-5). At 473 K and H,/hydrocarbon ratios lower than 4.6, 2 and ca. 0.8 for n-pentane, 2- methylbutane and 2,2-dimethylbutane, respectively, the deceleration of the consumption rate of the parent hydrocarbon (fig. 1-3 and table 5 ) points to the poisoning of the surface Ir atoms by dehydrogenated species (trapped hydrocarbons) owing to the low partial pressure of hydrogen. As shown in fig. 1-3, at low H,/hydrocarbon ratios with the decrease of the consumption rate Si steeply increases. This result might be interpreted in the following ways. (i) The increase of Si with decreasing partial pressure of hydrogen is the consequence of the partial poisoning of the fragmentation sites with trapped hydrocarbons, as already observed with Pt catalysts.l21 1 4 7 l9 Because the dispersion is low (14 O h ) the fragmentation sites (an ensemble of Irs atoms) can be found on low-index faces.The size of these ensembles is modified by C,H,. (ii) The decrease in the transformation rate is caused by the trapped hydrocarbons, but the poisoning is non-selective. In this case one ought to assume that isomerization1518 Rearrangement of Alkanes over Ir/A1203 Table 5. Reaction order with respect to hydrogen and gas composition at maximum ratea reaction order with respect to hydrogen at -R,,, at hydro- 6 : 1 2: 1 0.8: 1 carbon H, : hydrocarbon H,: n-pentane - 0.08 0.43 1.74 ca. 4.6 2-methyl butane - 0.45 0.0 0.64 ca. 2 2,2-dimethylbutane - 1.55 -0.1 ca.0.0 cu. 0.77 a n-Pentane (nP) at 9.57 kPa, 2-methylbutane (2MB) at 13.96 kPa and 2,2-dimethylbutane (2,2-DMB) at 5.72 kPa; T = 473 K, eot = 98.42 kPa, rn = 0.01-0.02 g 10 wt O h Ir/A1203. requires the formation of surface species which are more dehydrogenated than those participating in C-C bond rupture. This interpretation seems highly improbable. (iii) One might assume that fragmentation takes place mainly on edge and corner atoms and the isomer formation occurs on low-index (Si has been observed to increase with increasing particle size).g The increase of Si with the decrease of partial pressure of hydrogen would imply the preferential poisoning of edge and corner atoms, a conclusion which cannot be supported with the existing eviden~e.'~? 2o The formation of trapped hydrocarbons should be suppressed in large excess of hydrogen because of the inhibiting effect of hydrogen on the rupture of C-H bonds as indicated by the negative hydrogen orders in fig.1-3 (numerical values in table 5). At large HJhydrocarbon ratios the selectivity of isomer formation is small; the absence of the carbonaceous materials apparently increases only the selectivity of fragmentation, which tends towards 100% upon the increases of the hydrogen pressure. In the transient section of the hydrocarbon step the selectivity of isomerization and that of fragmentation change in the opposite direction: the selectivity of isomer formation increases as the partial pressure of hydrocarbon increases in the 5 O h H,-He stream. With both n-pentane and 2-methylpropane the formation of fragments commences earlier than the formation of isomers and the rate of formation of fragments decreases sooner than that of isomers.The results seem to indicate that (a) on the hydrogen-covered surface fragmentation prevails and (b) the poisoning of iridium sites caused by the increase of the hydrocarbon/H, ratio affects the fragmentation process to a greater extent than it does isomerization. All these findings support interpretation (i) rather than (ii) or (iii). Accepting that under the reported conditions the presence of the firmly held hydrocarbons promotes the formation of isomers, the question which arises is how this conclusion can be reconciled with the mechanisms of isomerization. A variety of isomerization mechanisms have been proposed and systematically reviewed.Two reaction mechanisms seem to be acceptable (fig. 6). (i) Gault and co-workers21~22 have suggested the formation of reaction intermediates consisting of a 7c-adsorbed alkene and a carbenoid species [fig. 6(a)] similar to those suggested by Chauvin and H e r r i s s ~ n ~ ~ for metathesis. Foger and Anderson l6 have also explained the features of the rearrangement of 2,2-dimethylpropane over iridium-gold catalysts by means of an alkene metathesis mechanism. (ii) Rooney and c o - w ~ r k e r s ~ ~ have interpreted the bond-shift isomerization via the formation of a a-alkyl species and a transient state with three centre orbitals stabilized by dz-p,* charge transfer [fig. 6(b)]. A crucial step of the bond-shift isomerization mechanism proposed by Gault andA .Sarkany 1519 , CH3 Fig. 6. Proposed mechanisms for 1,2-bond-shift (a) metathesis of carbene,20 (b) Rooney-Samman mechanism.22 Muller2'T22 is the activated rotation of the alkene formed by dismutation of the metallocyclobutane ring. On hydrocarbon modified surfaces the rotation of alkene might be strictly limited owing to the steric blockage caused by the firmly held hydrocarbons (C,H,). One would also expect25 that the mechanism is only important when there is substantial homologation and cracking to methane. The results with n-pentane and 2-methylbutane (tables 1 and 2, respectively) show that homologation is only detected in small amounts. It might be assumed therefore that the alkene metathesis mechanism is unlikely to contribute to the isomer formation to a considerable extent under the experimental conditions reported.The rearrangement of caged hydrocarbon^^^? 25 has provided firm evidence for a reaction mechanism proposed by Rooney et al. The experimental results in this paper do not support the opinion that a a-alkyl species would be sufficient for the formation of isomers. As shown in fig. 1-5 the presence of a large excess of hydrogen, which would be a necessary requirement to ensure the formation of a-alkyl from our alkanes, is not a favourable condition for skeletal bond shift. It is very likely that the rearrangement requires the formation of a localized multicarbon interaction (e.g. a y species diadsorbed on one metal atom), as already assumed by Clarke and Rooney,26 otherwise the transient state shown in fig.6(b) will not be formed. Considering the fact that the selectivity of isomerization has been observed to decrease with the increase of the H,/hydrocarbon ratio one should conclude that although the increase of the hydrogen coverage decreases the ensemble size of Ir atoms, as suggested by Frennet et ~ l . , ~ ' the chemisorbed hydrogen cannot force our alkanes to participate in localized multicarbon interactions. The partial blocking of the surface atoms by trapped hydrocarbons increases the probability of finding isolated sites,12-15 and thus instead of multisite interactions leading to C-C bond rupture (e.g. Gcap species adsorbed on three metal atoms) localized interactions come into prominence and might result in the formation of isomers.The electronic alteration oP1y26 the residual sites caused by firmly held hydrocarbons should act in a direction opposed to isomerization. Under the experimental conditions employed, Si is a monotonous function which seems to indicate that the electronic effect of C,H, owing to its low coverage does not counteract the geometric/steric effect. A remarkable result of our experiments is that the selectivity of isomerization (or the1520 Rearrangement of Alkanes over Ir/Al2O3 (4 (6) Fig. 7. (a) Interaction of 2,2-dimethylbutane on an Ir surface partially covered with hydrogen. Interaction of 2,2-dimethylbutane on a partly C, H, (trapped hydrocarbon) covered surface ; catalyst is in the H-Ir-C,H, state. (6) the rate of isomer formation, R x Si/lOO) at a given HJhydrocarbon ratio (fig.1-3) does not follow the sequence which would be expected for a reaction mechanism closely akin to that of carbonium-ion rearrangement. The observed order n-pentane > 2- methylbutane g 2,2-dimethylbutane may be interpreted if one assumes that there are different reaction routes to form the transient state in fig. 6(b), as proposed by Clarke and Rooney.26 As the variation of Si as a function of the H,/hydrocarbon ratio can be attributed to the self-poisoning of the surface iridium atoms with firmly held hydrocarbons, one feels a temptation to suggest that the very small Si value with 2,2- dimethylbutane is in part the consequence of the very low poisoning activity of this hydrocarbon. Reforming studies over Pt based catalysts2* have confirmed the high poisoning effect of hydrocarbons which contain a C, ring or are able to form a C, ring.The hydrogen orders in table 5 may provide a further argument in favour of our proposal. In the H,/hydrocarbon range investigated the consumption rate of 2,2- dimethylbutane continuously increases up to ca. 0.8, whereas those of n-pentane and 2- methylbutane begin to decrease owing to self-poisoning at H,/hydrocarbon ratios lower than 4.6 and 2, respectively. Finally we focus our attention on the variation of the fragmentation pattern caused by the change in the surface composition of the catalyst. In large excess of hydrogen with 2,2-dimethylbutane the main reaction route is fragmentation via C, unit mode in conformity with earlier observation^.^ Upon the decrease of the partial pressure of H, there is a definite tendency to interact via iso-unit modeS (ay interaction).The ratio of 2-methylbutane/2,2-dimethylpropane (and 2-methylpropane/2,2-dimethylpropane) increases together with the isomers. Our observations are in accordance with the findings of Ponec and co-workers12 on Ir samples poisoned deliberately with carbonaceous materials. On the basis of the isomerization results we interpret our findings with the steric/geometric effect of the firmly held hydrocarbons rather than with an increase in the degree of dissociation of C-H bonds in the reactive species. A model of the working Ir surface indicating the effect of the partial pressure of hydrogen on the surface composition of the catalyst is presented in fig.7. The firmly held deposits are proposed to govern the orientation of the metal-hydrocarbon interaction as it is presumed in fig. 7. In the transformation of n-pentane the selectivity of the internal rupture (C,,-C,,) is much greater than that of the terminal one (Cl-Cll) at large H,/hydrocarbon ratios and low reaction temperatures (first row, table 1). The propane/n-butane ratio is 2.93 at 473 K, with 3 kPa n-pentane and 93.6 kPa H,. This value is close to those reported by Foger and Anderson’ with Ir particles of 7 nm. With the decrease of the partial pressure of hydrogen the terminal rupture becomes conspicuous : the propane/n-butane ratio is decreased to 1.57 in 5 % H,/He and 9.5 kPa n-pentane (sixth row, table 1). The steric crowding on Ir sites seems to force n-pentane to interact with the remaining sites via theA .Sarkany 1521 chain end. (An ap or cxy interaction with the participation of a methyl group should be sterically less hindered than the interactions via internal carbon atoms owing to the decrease of the substrate-substrate repulsion energy.) The chain end preference, which has also been frequently observed in alkane dehydrogenation,26- 29 is likely to originate from geometric/steric effects although electronic effects cannot be entirely disregarded. Conclusions (i) The variation of the isomerization selectivity (and that of the fragmentation pattern) as a function of the H,/hydrocarbon ratio can be interpreted if one accepts that the hydrogen coverage regulates the coverage of the trapped hydrocarbons1** l5 which in turn for steric/electronic reasons modify the surface Ir atoms.The reactants themselves determine the actual surface state of the working catalyst. (ii) The investigations in large excess of hydrogen confirm that the hydrogen most likely because of its high mobility and low steric volume, cannot ensure the formation of the localized multicarbon interactions which are thought to be required for skeletal bond shift. (iii) Considering the fact that the isomers are practically absent in large excess of hydrogen but they appear at low H,/hydrocarbon ratios, one might tentatively suggest that fragmentation sites have been transformed to isomerization sites upon the self- poisoning of the surface Ir atoms. On the Ir catalyst investigated the isomerization sites have been created by the reactant hydrocarbons.The modest level of isomerization selectivity in comparison with that of Pt is likely to stem in part from the resistance of Ir to the formation of carbonaceous depo~its.'~' 30 The scattering of Si values observed in the literature may tentatively be explained by the different degrees of poisoning of Ir sites caused by carbonaceous materials. References 1 J. R. Anderson and N. R. Avery, J. Catal., 1966, 5, 446. 2 T. J. Plunkett and J. K. A. Clarke, J . Cataf., 1975, 35, 330. 3 Z. Karpinski and J. K. A. Clarke, J . Chem. SOC., Faradaj) Trans. 1, 1975, 71, 2310. 4 0. F. Finlayson, J. K. A. Clarke and J. J. Rooney, J . Chem. SOC., Faraday Trans. I , 1984, 80, 191. 5 Z. Karpinski, W.Juszczyk and J. Pielaszek, J . Chem. SOC., Faraday Trans. I , 1987, 83, 1293. 6 A. Sarkany, K. Matusek and P. Tetknyi, J . Chem. Soc., Faraday Trans. I , 1977, 73, 1699. 7 D. I. Hagen and G. A. Somorjai, J. Catal., 1976, 41, 466. 8 M. Boudart and L. D. Ptak, J . Catal., 1970, 16, 90. 9 K. Foger and J. R. Anderson, J . Catal., 1979, 59, 325. 10 F. Weisang and F. G. Gault, J . Chem. SOC., Chem. Commun., 1979, 519. 11 C. O'Donohoe, J. K. A. Clarke and J. J. Rooney, J . Chem. SOC., Faraday Trans. 1, 1980, 76, 345. 12 M. W. Vogelzang, M. J. P. Botman and V. Ponec, Faraday Discuss. Chem. SOC., 1981, 72, 33. 13 J. G. van Senden, E. H. van Broekhoven, G. T. J. Wreesman and V. Ponec, J . Cataf., 1984, 87, 468. 14 A. Sarkany, J . Chem. SOC., Faraday Trans. I , 1988, 84, 2267. I5 A. Sarkany, J . Mol. Cataf., 1989, 51, 239. 16 K. Foger and J. R. Anderson, J . Catal., 1980, 64, 448. 17 J. Margitfalvi, P. Szedlacsek, M. Hegediis and F. Nagy, Appl. Catal., 1985, 15, 69. 18 P. Szedlacsek, T. Talas, M. Hegediis and J. Margitfalvi, in Heterogeneous Catalysis Proc. 6th Znt. 19 L. Guczi, A. Sarkany and P. Titinyi, J . Chem. SOC., Faraday Trans. I , 1974, 70, 1971. 20 A. Sarkany, in Catalyst Deactivation, ed. B. Delmon and G. Froment (Elsevier, Amsterdam, 1987), p. 125. 21 J. M. Muller and F. G. Gault, J . Cataf., 1972, 24, 361. 22 F. G. Gault, Adv. Catal., 1981, 30, 1. 23 Y. Chauvin and J. L. Herrisson, Makromol. Chem., 1971, 141, 161. 24 M. A. McKervey, J. J. Rooney and N. G. Samman, J . Catal., 1973, 30, 330. 25 V. Amir-Ebrahimi and J. J. Rooney, J . Chem. SOC., Chem. Commun., 1988, 260. Symp. Sojia, 1987, ed. D. Shopov (Publishing House of BAS, 1987), p. 82.1522 Rearrangement of Alkanes over Ir/A1203 26 J. K. A. Clarke and J. J. Rooney, Adv. Catal., 1976, 25, 125. 27 A. Frennet, G. Lienard, A. Crucq and L. Degols, J. Catal., 1978, 53, 150. 28 C. G. Myers, W. H. Lang and P. B. Weiss, Ind. Eng. Chem., 1961, 53, 299. 29 H. Zimmer, Z. Paal and P. TCtCnyi, Acta Chim. Acad. Sci. Hung., 1982, 111, 513. 30 J. H. Sin felt, U S . Patent, 3953 368. Paper 8/00583D; Received 17th February, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501511

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1989, 85(7), 1523-1529 Transformation of Methylcyclopentane over Ir/A1203 and n-Hexane over Pt-Black Interpretation of Hydrogen Control Antal Sarkany Institute of Isotopes of the Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 77, Hungary Hydrogen sensitivity of MCP ring opening on Ir/A1203 and that of n- hexane transformation on Pt black have been investigated. Comparison of the results on 0,-H,-treated and 'carbon '-precovered catalysts confirms that in the interpretation of the hydrogen effect on product distribution one has to regard the steric/electronic effect of trapped hydrocarbons on the surface metal atoms. The role of hydrogen in the transformation of alkanes over metals has been the subject of systematic investigation^.'-^ It has been found that the partial pressure of hydrogen in the reaction mixture significantly affects both the reaction rate of the transformation and the selectivity of individual reaction r~utes.l-~ The hydrogen sensitivity of product formation has been demonstrated by a large group of metals and alkanes.lP7 The variation of the product selectivity has been inter~retedl-~ by the influence of the hydrogen partial pressure on the degree of dissociation of the surface intermediate for a given reaction.The authors con~idered'**~~ the role of hydrogen as an astoichiometric component and its inhibiting effect on the rupture of C-H bonds. This approach cannot explain why the formation of alkenes requires a lower partial pressure of hydrogen than aromatization and moreover, if hydrogenolysis involves multidissociated species why fragmentation is favoured at higher hydrogen pressures than, e.g.alkene formation. Theoretical considerations', on kinetics of alkane transformation based on the ensemble size model make also questionable that the reaction order would reflect the degree of dissociation in the reacting intermediate. The problem of surface composition is also evident in 1,5-cyclisation and ring opening.1°-15 The selective cyclic mechanism (SCM) seems to operate on hydrogen covered catalysts, whereas in a low excess of hydrogen or in the presence of carbonaceous materials the non-selective cyclic mechanism (NSCM) might prcyail. The aim of the present study is to present evidence that in the interpretation of the hydrogen effect on the product selectivity one has to consider the steric/electronic effect of the surface-held hydrocarbons the coverage of which is governed by the partial pressure of hydrogen.In this paper the selectivity of low temperature transformation of methylcyclopentane (MCP) over 10 wt O/O Ir/A1203 and that of n-hexane (nH) over Pt- black is discussed. The experiments with methylcyclopentane (partial pressure 4.62 kPa) over 10 wt YO Ir/Al,03 (dispersion 12%) were carried out at 493 K. The product selectivity was measured at 2 min time on stream unless otherwise stated. In fig. 1 the nH/2MP and the nH/3MP ratios are presented together with the rate of MCP consumption. In H, stream (93.8 kPa H,) the ring opening is 'selective' in agreement with the investigations of Gault n-hexane appears only in amounts of 1.4 0.4 %.Upon decreasing the partial pressure of hydrogen the nH/2MP (or nH/3MP) ratio increases indicating that the 15231524 Transformation of Methylcyclopentane and n-Hexane 0.6 0.5 0.4 2 s ", z = 0.2 5 B 0.3 0.1 0.0 , . . . . . . . . . 1 2 log (PH? /kPd -6 e w 9 -7 -8 -9 Fig. 1. Ring opening of methylcyclopentane on 10 wt % Ir-A1203: a, nH/3MP; D, nH/2MP; @, rate of MCP consumption in mol g$ s-'; T = 493 K ; pHC = 4.62 kPa; 0.0185 g catalyst + 0.2 g A1,0,. (a) First experiment, (6) 7 min on stream, (c) 20 min on stream (catalyst 0, and H, treated/523 K, 0.65 kPa 0,, 5 min and 573 K, 98.4 kPa H, for 30 min) prior to each measurement. 0.6 0.5 3 % Q 0.4 s 3 0.3 cz 0.2 3 0.1 0.0 -7 e - e M - -9 Fig.2. Ring opening of methylcyclopentane on carbon poisoned 10 wt% Ir-A1203 (0, nH/3MP; D, nH/2MP; (>, rate of MCP consumption; T = 493 K, pHC = 4.62 kPa, 0.253 g catalyst poisoned with 2,2-dimethylbutane in 1 YO H,-He at 633 K).A . Sarkany 1525 rupture of secondary-tertiary C-C bonds comes into prominence. With the decrease of the H,/HC ratio the reaction rate first increases, passes through a maximum and then decreases, suggesting that in the absence of sufficient hydrogen the coverage of the chemisorbed hydrocarbon, which under the given H,/MCP ratio remains trapped on Ir sites, increases: the lower the partial pressure of H, the larger is the hydrocarbon coverage held firmly on Ir sites. In considering the reasons for the observed hydrogen sensitivity of MCP ring opening in fig.1 the following choice of explanations might be suggested : (i) One might assume that the surface-held hydrocarbon which causes the decrease of the reaction rate upon the decrease of the H,/MCP ratio behaves as a non-selective poison, i.e. the nature of the remaining centres is not affected and the variation of the product selectivity is caused by the increase in the degree of dissociation of C-H bonds in MCP as proposed in ref. (lH6). According to this explanation the size of the working ensembles should increase with decreasing hydrogen coverage. (ii) The variation of the product selectivity is the consequence of the increase in the coverage of trapped hydrocarbons which for geometric/electronic reasons modify the Ir sites. One might propose that the ensemble size decreases with decreasing hydrogen coverage.The selectivity measurements at a given H,/MCP ratio over poisoned Ir catalysts12 and the time on stream experiments in fig. 1 [(a), (b) and (c) were measured at 2,7 and 20 min] confirm that the selectivity of MCP ring opening can be influenced by trapped hydrocarbons. The variation of the reaction pathways and the possible reaction intermediates involved are discussed in detail in ref. (1 2)-( 15). However, these papers do not give information about the role of hydrogen on a hydrocarbon modified surface. In fig. 2 the rate and the hydrogen sensitivity of MCP ring opening is presented on our poisoned Ir/A1203 catalyst (2,2-dimethylbutane at 633 K). Two features of the results deserve attention.First, the rate of MCP consumption is almost independent of the partial pressure of hydrogen which might point to the presence of ‘ small ’ ensembles. Secondly, once the surface-held hydrocarbons ensure the ‘ small ’ size of the working ensembles on an originally ‘clean ’ surface (at low H,/MCP ratio this condition is fulfilled as shown by the value of R in fig. 1) the variation of the H, pressure exerts only a slight influence on product selectivity, which in turn seems to confirm that (i) might not be the main reason for the change in the product selectivity in fig. 1. Apparently, the role of hydrogen at low H,/MCP ratios is that it permits the very rapid initial poisoning of some Ir sites so that further on the hydrogen availability and the degree of dissociation of C-H bonds are governed essentially by the hydrocarbon coverage.Transformation of n-hexane, our second example, has been investigated on a Pt black at 563 K. Prior to the catalytic runs, the Pt black was repeatedly 0, and H, treated up to 633 K in order to remove carbonaceous materials. On the ‘clean’ sample the amount of carbon measurxl by temperature-programmed oxidation (t.p.0.) after 30 min hydrogenation at 633 K was < 0.005 cm3 (s.t.p.) CO, g;,’. The number of surface sites has been inferred from five repeated sequences of 0, (OTl), H, (HTl) and 0, (OT2) titrations on the regenerated black: OT1 = 0.23k0.02, HT1 = 0.63+0.008, OT2 = 0.305+0.01 cm3 (s.t.p.) g;,‘. Using the value of OT2 the number of surface sites, Pt’, is taken as 1.08 x 1019 g;:.The hydrocarbon coverage on the working Pt black and the corresponding product selectivity pattern measured in each experiment at 5 min on H,-He-n-hexane stream as a function of the H, content of the carrier gas are presented in fig. 3 and 4. The ‘total’ carbon coverage, C/Pts (curve 3 in fig. 3) was measured by t.p.0. after removing the reactant mixture from the reactor by Ar (3 min at 563 K). The fraction of free sites, lOOf (this term corresponds to 1 -BCZHy) represents the surface sites which are able to chemisorb 0, at 293 K on the hydrocarbon covered surface (curve 2 in fig. 3). In separate experiments after 5 min on H,-He-n-hexane stream the n-hexane stream was1526 100 J 80 - ’ 60- h 2 3 *=I % 40- w” 20- 0- Transformation of Methylcyclopentane and n-Hexane 30 !S 3 E 15 0 2o %H2inHe loo Fig.3. Transformation of n-hexane on Pt-black at 563 K. 1 ( x ), rate of n-hexane consumption; 2 (a), fraction of ‘free’ sites on the working surface; 3 (O), Ctot/Pts, nH-H,-He is purged from the reactor by Ar for 3 min; 4 (O), after nH-H,-He stream catalyst is hydrogenated by H,-He for 3 min and 5 (a), for 15 min (0.253 g catalyst, PnH = 4.92 kPa, ptOtal = 98.42 kPa). 0 20 %.H2in He’ 80 100 Fig. 4. Selectivity of the transformation of n-hexane on Pt-black at 563 K. (Conditions in fig. 3.) 0, Sf,.; 0, S,; 0, S Z M p + 3 M p ; 0, S,,p*A . Sarkany 100 - 80 - n E - 8 0 60- .I .- > 2 40- 0- 20- 0- 1527 0 20 %HHZinHe 80 Id0 Fig. 5. Selectivity of the transformation of n-hexane on a carbon poisoned Pt-black at 563 K (at 93.5 kPa, H,, 100 x f = 23.9 and C/Pts = 1.49) 0.253 g catalyst,p,,, = 4.92 kPa, eota, = 98.42 kPa.Symbols as for fig. 4. plugged and the surface was hydrogenated with the corresponding H,-He mixture for 3 and 15 min, curves 4 and 5 in fig. 3, respectively. The hydrocarbons that remained on the surface can be regarded as firmly held hydrocarbons. The difference between curves 3 and 4 in fig. 3 (or between 3 and 5 ) represents chemisorbed species bound reversibly to Pt sites in the presence of H,. A sharp dividing line between reversibly bound and firmly held forms cannot be drawn as even the firmly held forms can be slowly hydrogenated (compare curves 4 and 5). It is clear, however, that the coverage of the trapped hydrocarbons increases steeply and that of the reactive forms decreases with decreasing partial pressure of hydrogen.Similarly to the surface composition of the catalyst, the product selectivity in fig. 4 is remarkably influenced by the variation of the partial pressure of hydrogen. In large excess of hydrogen fragmentation is observed to dominate. With the decrease of the H, content in He, non-destructive reactions such as MCP and benzene formation come into prominence. Formation of hexenes has only been observed in 0.13 % H,-He: the product selectivity, not shown in fig. 4, is 45.1 YO benzene, 24.3 % MCP, 13.2% fragments, 10.8 % 2MP + 3MP and 6.6 YO hexanes. The variation of the product selectivity in fig. 4 might be interpreted if one assumes that upon decreasing the partial pressure of hydrogen some of the surface sites become occupied by firmly held hydrocarbons and as a consequence of Pt site blocking non- destructive reaction of n-hexane appears.The role of trapped hydrocarbons is obvious if one compares the product selectivities in fig. 4 with those observed on our 'carbon' precovered Pt-black in fig. 5. The Pt-black poisoned deliberately by carbonaceous residues shows definitely less fragmentation and higher isomerization (2MP + 3MP), MCP or benzene-forming selectivities than the originally 0,-H,-treated Pt-black, as a function of the H, pressure. The decrease of the fragmentation selectivity was also observed in other systems.16-1H Inspection of fig. 4 and 5 confirms that the variation of the product selectivity cannot be explained by the direct effect of hydrogen on the size of the working ensembles as that would assert the assumption that the hydrocarbon poisoning is non-selective.19-21 If that were the point the individual selectivity curves in fig. 4 and 5 would be identical as non- selective poisoning affects only the number of working ensembles. As this is not the case one has to accept that surface-held hydrocarbons, or at least a certain type of deposits,1528 Transformation of Methylcyclopentane and n-Hexane are able to affect the product selectivity pattern and the hydrogen sensitivity appears because the partial pressure of hydrogen affects the coverage of the firmly held hydrocarbons which in turn governs the geometric/steric and electronic state of the working sites. The crowding on Pt sites might exert steric effects upon the configuration of reactive species : the extensive dissociation of C-H bonds is likely to be restricted and the metal-carbon bond might become localised rather than delocalised.22 The binding energy of the C-Pt bond like that of H-Ptl’ should be decreased as-a consequence of electronic and geometric effects. An interesting feature of the product selectivity curves is that only the selectivity of fragmentation increases monotonically with increasing partial pressure of hydrogen. The experimental results seem to indicate that at extremely large H,/nH ratios the only reaction which could be observed is fragmentation. The same conclusion appears to be valid with a group of Pt-Al,O, and Pt-SiO, catalysts of differing di~persion.~~ As in large excess of H, the formation of trapped hydrocarbons is prevented (fig.3, curves 4 or 5 ) one might conclude that the surface of a Pt-H system consisting of Pt’ and hydrogen covered Pt’ is reactive only in C-C bond rupture. On such an ideal surface the direct effect of chemisorbed hydrogen on the ensemble size (or on the probability of finding free sites for chemisorbed n-hexanes) manifests itself in the decrease of the rate of n-hexane consumption but not in the variation of the product selectivity. Summing up, on the basis of the presented evidence we conclude that the surface-held hydrocarbons whose coverage is a function of the partial pressure of hydrogen contribute to the observed hydrogen sensitivity of the product selectivity. The trapped hydrocarbons owing to geometric/electronic reasons modify the surface state of the working catalysts.The experiments with methylcyclopentane over Ir-A1203 have confirmed that in the presence of trapped hydrocarbons distributed probably randomly on the surface a non-selective cyclic mechanism prevails and the selectivity is not very much affected by the partial pressure of hydrogen. The experiments with n-hexane show that the presence of the firmly held hydrocarbons ensures the occurrence of non- destructive reactions like C,-cyclisation, aromatization and alkene formation. As these reactions can only be observed in the presence of trapped hydrocarbons one has to propose that on the surface of clean Pt the reaction centres catalysing non-destructive reactions are ‘created’ by the hydrocarbon present in the catalytic system.References 1 Z. Paal and P. G. Menon, Catal. Rev. Sci. Eng., 1983, 25, 229. 2 Z. Pail, Adv. Catal., 1980, 29, 273. 3 H. Zimmer, M. Dobrovolszky, P. TCtenyi and Z. Paal, J . Phys. Chem., 1986, 90,4758. 4 Z. Paal, H. Zimmer and P. TCtCnyi, J . Mol. Catal., 1984, 25, 99. 5 H. Zimmer, Z. Padl and P. Titenyi, Acta Chim. Acad. Sci. Hung., 1982, 111, 513. 6 Z. Paal and P. TCtenyi, Appl. Catal., 1981, 1, 9. 7 A. Sarkany, J . Catal., 1984, 89, 14. 8 P. Parayre, V. Amir-Ebrahimi, F. G. Gault and A. Frennet, J . Chem. SOC., Faraday Trans. I , 1980,76, 9 A. Frennet, G. Lienard, L. Degals and A. Crucq, Bull. SOC. Chem. Belg., 1979, 88, 621. 1704. 10 F. Weisang and F. G. Gault. J . Chem. SOC., Chem. Commun., 1979, 519. 1 1 F. G. Gault, V. Amir-Ebrahimi, F. Garin, P. Parayre and F. Weisang, Bull. Soc. Chem. Belg., 1979,88, 12 J. G. Van Senden, E. H. Van Broekhoven, C. T. J. Wreesman and V. Ponec, J . Catal., 1984, 87, 468. 13 0. E. Finlayson, J. K. A. Clarke and J. J. Rooney, J . Chem. SOC., Faraday Trans. 1, 1984, 80, 191. 14 F. G. Gault and J. J. Rooney, J. Chem. SOC., Faraday Trans. I , 1979, 75, 1320. 15 J. J. Rooney, J. Mol. Catal., 1984, 26, 132. 16 A. Sarkany, L. Guczi and P. TltCnyi, React. Kinet. Catal. Lett.. 1974, 1, 169. 17 L. Guczi, A. Sarkany and P. Tetinyi, J. Chem. Soc., Faraday Trans. I , 1974, 70, 1971. 18 P. P. Lankhorst, H. D. De Jongste and V. Ponec, in Catalyst Deactivation, ed. B. Delmon and G. F. 475. Froment (Elsevier, Amsterdam, 1980), p. 43.A. Sarkany 1529 19 S. M. Davis, F. Zaera and G. A. Somorjai, J. Catal., 1982, 77, 439. 20 S. M. Davis, F. Zaera and G. A. Somorjai, J. Phys. Chem., 1982, 104, 7453. 21 S. M. Davis, F. Zaera and G. A. Somorjai, J. Catal., 1984, 85, 206. 22 M. W. Vogelzang, M. J. P. Bcltman and V. Ponec, Furaday Discuss. Chem. Soc., 1981, 72, 33. 23 A. Sarkany, Catal. Today, submitted. Paper 8/02044B ; Receiued 23rd Muy, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501523

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1989, 85(7), 1531-1536 Conductance Studies of Alkali-metal Chlorides and Bromides in 2-Methoxyethanol at 25 "C Debasis Nandi, Susanta Das and Dilip K. Hazra* Department of Chemistry, North Bengal University, 734 430 Darjeeling, India The conductances of a number of alkali-metal chlorides and bromides, MX(M+ = Li, Na, K, Rb, Cs and X- = C1, Br) have been measured in 2-methoxyethanol (ME) at 25 "C and the data have been analysed by the 1978 Fuoss Conductance equation in terms of the limiting molar conduct- ance AO, the association constant, KA and the association distance, R. Single ion conductivities have been determined using Bu,NBPh, and Bu,NBBu, as reference electrolytes. Strong association was found for all these salts in ME and the limiting conductances (Ao) were in the order Li > Na < K < Rb < Cs and C1 > Br.Evaluation of Stokes radii indicates that NaCl and NaBr form ion pairs with fully solvated ions while in other cases some solvent molecules are perhaps included in the ion pair. 2-Methoxyethanol (ME) has attracted much attention in recent years as a solvent medium for various electrochemical investigations. ' Although a number of precise conductance measurements of alkali-metal halides in other solvents are known in the literat~re,~-' relatively few studies of such a nature have been made in 2-methoxyethanol. In this paper an attempt has been made to reveal the nature of ion-solvent interactions of alkali-metal halides with 2-methoxyethanol through the measurement of their conductances at 25 "C.Since transference number data for solutions of these salts are lacking in ME, single ion conductivities have been evaluated using two reference electrolytes, uiz. Bu,NBBu, and Bu,NBPh,, in an effort to provide reliable values of ionic mobilities. Experimental 2-Methoxyethanol (G.R.E. Merck) was distilled twice in an all glass distillation set before use. The purified sample had a density of 0.96002 g ~ m - ~ , viscosity 1.5414 cPt and a specific conductance of ca. 1.01 x lop6 C1 cm-' at 25 "C. The alkali-metal chlorides and bromides (Fluka) were of purum or puriss grade; they were dried in uacuo for a long time immediately prior to use and were used without further purification. Tetrabutylammonium tetraphenylborate (Bu,NBPh,) was prepared by mixing equi- molar quantities of NaBPh, and Bu,NBr as described in the literature.' Tetra- butylammonium bromide (Bu,NBr) and tetrabutylammonium tetrabutylborate (Bu,NBBu,; Alfa) were purified as suggested by Lawrence et a1.' Conductance measurements were made by a Pye-Unicam PW 9509 conductivity meter at a frequency of 2 kHz using a dip-type cell of cell constant 0.73 cm-'.Measurements were made in an oil bath maintained at 25 kO.005 "C. The details of experimental procedures have been described earlier." Solutions were prepared by weight for the conductance runs, the molalities being converted to molarities by the use of densities. Several independent solutions were prepared and runs were performed to ensure the reproducibility of the results. All data were corrected at 25 "C with the specific t 1 CP = 10P Pa s.15311532 Conductance Studies of Alkali-metal Halides Table 1. Equivalent conductances and corresponding molarities of the alkali-metal halides in 2-methoxyethanol at 25 "C A/W1 cm2 C/10-4 mol dm-3 mol-' 19.229 14.422 12.889 11.572 9.614 7.692 5.769 19.226 14.420 1 1.263 9.614 8.653 7.691 6.721 7.694 6.732 5.771 4.808 3.846 2.885 1.923 9.741 8.654 7.692 6.731 5.769 4.807 4.423 49.999 40.001 30.001 20.001 14.998 10.001 7.999 10.002 8.997 8.002 6.999 6.001 4.998 4.00 1 LiCl 24.54 26.38 27.04 27.53 28.47 29.48 30.58 KC1 26.90 28.32 29.62 30.18 30.49 30.92 31.39 CSCl 32.90 33.30 34.75 35.65 36.70 38.60 40.6 1 LiBr 29.20 29.75 30.28 30.82 31.29 31.87 32.24 KBr 25.60 26.37 27.53 29.10 30.1 1 31.10 32.01 CsBr 33.32 34.40 35.22 36.01 36.61 37.59 38.50 C/ lop4 mol dm-3 A/Q-' cm2 mol-' NaCl 38.459 28.844 25.169 19.229 14.422 9.614 8.653 RbC1 7.694 6.732 5.771 4.808 3.846 2.885 1.923 NaBr 79.998 60.001 49.998 45.001 40.002 34.998 30.01 1 25.179 14.422 12.546 10.572 8.654 7.694 5.771 21.81 23.3 1 23.93 25.14 26.08 27.28 27.57 30.60 31.15 32.06 33.20 33.90 35.10 37.12 22.07 22.97 23.49 23.80 24.1 1 24.42 24.74 RbBr 28.85 31.52 32.08 32.71 33.42 33.70 34.53D.Nandi, S. Das and D. K. Hazra 1533 conductance of the solvent. The corrected values were analysed by means of the Fuoss conductance equation. The dielectric constant of ME was taken from the literature." Results The measured equivalent conductances and the corresponding molarities for alkali- metal salts in 2-methoxyethanol are given in table 1 .The data were analysed with the Fuoss conductance e q ~ a t i o n ~ ' . ~ ~ using the following set of equations : e2 EkB T p = - - K , = K,/( 1 -a) = K,( 1 + Ks) (6) where R, and EL are relaxation and hydrodynamic terms, respectively, as derived by Fuoss, and the other terms have their usual significance. The parameters A', K , and R were obtained by solving the above equations. The calculations were performed on a Wipro 2-650 computer using the program devised by Fuoss. Initial Ao values for the iteration procedure were obtained from Shedlovsky extrapolations of the data. Calculations were carried out by finding the values of A' and a which minimize o2 = C [Aj(calc) - A,(~bs)]~/(n - 2) , (7) for a sequence of R-values, and then plotting YO = 100a/Ao against R; the best fit R corresponds to a minimum of the a% us.R curve. First approximate runs over a fairly wide range of R values were made to locate the minimum and then a fine scan around the minimum was made. Finally the minimizing value of R was read into the computer and the corresponding values of A' and a were calculated. However, since the rough scan using unit increment of R values from 3 to 15 gave no significant minima in the a%-R curve except for RbCl and CsCI, the computations in other cases were done by fixing R at P/2. The values of A', K , and R obtained by this procedure are reported in table 2. The limiting conductances of alkali-metal ions based on the values of Bu,NBPh, and Bu,NBBu, have been shown in table 3. The A' values of Bu,NBPh, and Bu,NBBu, were taken from our previous w0rk.l' The Walden product and the Stokes radii of the ions are also given in table 3.Discussion Table 2 shows that with the exception of the sodium salts, the limiting equivalent conductances (Ao) of the alkali-metal chlorides and bromides increase as the size of the alkali ion increases, the values being greater in the case of chlorides than bromides. The results show that the conductance of the Na+ ion is much too low on the basis of its dimension. Similar behaviour has been observed in acetone.6 The explanation offered for this is that, although the Na+ ion has a lower surface charge density than that of the Li+ ion, it is experiencing a greater interaction between the charge on the ion and the dipoles of the adjacent solvent molecules, which leads to a reduction in mobility.In other cases, the structure-forming effect increases with decreasing dimension and consequently1534 Conductance Studies of Alkali-metal Halides Table 2. Conductance parameters of alkali-metal halides in 2-methoxyethanol at 25 "C salts LiCl NaCl KCI RbCl CSCl LiBr NaBr KBr RbBr CsBr Bu,NBr Bu,NBBu, Bu,NBPh, ~__ A,-,/Q-' cm2 mo1-1 k,/dm3 mol-' 39.94 (k 0.47) 35.42 ( f 0.23) 40.12 (k0.17) 42.94 (f 0.38) 48.13 (k 0.48) 38.22 ( IfI 0.26) 33.71 (k0.31) 38.40 (k0.14) 41.22 (k 0.23) 46.42 (k 0.37) 36.74 (k 0.27) 28.84 (k 0.13) 27.64 ( f 0.28) 587 (k42) 290 (+31) 410 ( 5 13) 762 (k 52) 973 (k 63) 328 (k 25) 127 (& 10) 175 ( f 8 ) 240(+16) 524 ( f 34) 372 ( & 24) 142 (k 11) 369 (k 21) Walden product 0.616 0.543 0.6 18 0.662 0.741 0.589 0.521 0.591 0.634 0.720 0.566 0.445 0.426 R / A ~~ 7.50 7.85 8.22 8.80 8.30 7.50 7.99 8.34 8.51 8.69 11.97 8.00 8.60 0 0.16 0.24 0.06 0.19 0.2 1 0.05 0.09 0.15 0.15 0.10 0.20 0.12 0.14 Table 3.Limiting ionic conductance, Walden pro- ducts and Stokes radii of alkali-metal (chlorides and bromides) ions in 2-methoxyethanol at 25 "C Cl- Li' Na+ K' Rb' CS' Br- Li' Na' K' Rb' CS' 24.04 15.90 11.38 16.08 18.90 24.09 22.32 15.90 11.39 16.08 18.90 24.10 0.370 0.245 0.175 0.247 0.291 0.371 0.344 0.245 0.175 0.247 0.29 1 0.371 2.21 3.34 4.67 3.30 2.81 2.20 2.38 3.34 4.67 3.30 2.8 1 2.20 mobility in the reverse order. This explains the order observed for the limiting equivalent conductances of alkali-metal chlorides and bromides in methyl cellosolve.We find that all these salts are moderately associated in this solvent media. This is quite expected owing to the low dielectric constant ( E = 16.93)'' of the solvent. The most outstanding feature of the association constant given in table 2 is the fact that the salt containing larger ions shows a considerable amount of association. Furthermore, the process of ionic association in ME does not exhibit the simple dependence upon ionic size predicted by electrostatic theory. This can be seen more clearly in fig. 1, where log KA for salts in ME is plotted against the reciprocal of the sum of the estimated crystallographic radii.14 However, the association constant decreases with the increasing size of the anion; the decrease appears to be greater in the case of large cations than in that of small cations. This can be accounted for by the assumption that the destabilization of the anion in ME increases in the order C1 > Br and the stabilizations of the cations are in the order Li > Na < K < Rb < Cs.Thus the behaviour of theD. Nandi, S. Das and D. K. Hazra 3.2 I 3.0 -7 2.8 - 2 s (? 2 2.6 1 4 M 4i 2.4 2.2 - - - - - h a 8.0 ' 7.0 $ 6.0 P - I d 2 k - 1 4.5 1535 - - I 1 1 I I 1 1 I 2.0' ' I I I I 1 5.5 6.0 7.0 8.0 9.0 10.0 11.0 12.0 ?-;'/ti-' Fig. 1. Plot of association constant (K,) for the alkali-metal chlorides (A) and bromides (0) as a function of crystallographic radii (rx) at 25 "C. Fig. 2. A plot of Walden product against 1 /rx for tetra-alkylammonium and alkali-metal bromides in 2-methoxyethanol at 25 "C.alkali-metal salts in ME are not consistent with the measurements in alcohols4 where KA increases with the increasing size of the anion. The single-ion conductances have been evaluated from the division of A' values of Bu,NBBu, and Bu4NBPh, using the following relationships : 3,,(Bu,N+) = R,(BU,B--)~*~~ (8) and (9) The A* values of the reference electrolytes in ME were taken from our earlier w0rk.l' The A: values of alkali-metal ions from eqn (8) are presented in table 3. Values derived from eqn (9) differed from eqn (8) only by + or -0.13 for anions and cations, respectively. This suggests that either of the two methods can be used to calculate the limiting ionic conductances in organic solvents.1536 Conductance Studies of Alkali-metal Halides The Walden product, A;qo and Stokes radii (rs) of alkali-metal ions are reported in table 3.The dependence of Aoqo upon ionic size in ME at 25 "C can be seen in fig. 2, the corresponding values for tetra-alkylammonium ions were taken from our previous work.l0 The Walden products for alkali-metal halides in ME are substantially lower than those in aqueous solutions4 and also show considerably less variation with crystal- lographic size than the corresponding values for aqueous solutions and are also grouped closely together with the exception of Li+ ion. The apparent excess of mobility in aqueous solution has been attributed to far greater solvation in the non-aqueous solvents. It is generally accepted that larger alkali and halide ions possess an excess mobility in aqueous solution owing to their ability to break hydrogen bonds in their immediate vicinity and thereby reduce the local v i s ~ o s i t y .~ ' ~ Thus in fig. 2, the change from Li' to Cs+ may be considered to have been associated with the change of ions from strongly solvated to structure breaking. Also the trend from Cs+ to Et4N+ was associated with 'iceberg' formation1' induced in ME by tetra-alkylammonium ions. Table 3 shows that the Stokes radii decrease with the increasing size of the cations (with the exception of Na+) and is most likely due to greater ionic mobilities of the cations. The order of halide-ion conductances in ME is Cl > Br, which is the same as that found in acetone.6 The Stokes radii for these ions in ME are greater than the crystallographic radii, indicating that they may be solvated in this solvent. However, with the exception of the Na+ ion, the Stokes radii ar: much less than the sum of the radii of the ion and the solvent molecule (rME = 3.14 A)," indicating that they are only slightly solvated in this solvent medium, though nothing can be said definitely in the absence of precise transference number data.We thank Prof. R. M. Fuoss for furnishing the computer programming of this equation. D.N. thanks the U.G.C., New Delhi, for the award of a research fellowship. References 1 G. Roux, G. Perron and J. E. Desnoyers, J. Solution Chem., 1978, 7, 639. 2 J. P. Butler, H. I. Schift and A. R. Gordon, J. Chem. Phys., 1951, 19, 752. 3 R. L. Kay, J. Am. Chem. SOC., 1960, 82, 2099. 4 R. L. Kay and D. F. Evans, J . Phys. Chem., 1966, 70, 2325. 5 J. Thomas and D. F. Evans, J. Phys. Chem., 1970, 74, 3812. 6 L. G. Savedoff, J. Am. Chem. SOC., 1966, 88, 664. 7 P. Bruno and M. D. Monica, J. Phys. Chem., 1972, 76, 1049. 8 M. Castagnolo, A. Sacco and A. D. Giglio, J. Chem. SOC., Faraday Trans. I , 1984, 80, 2669. 9 K. G. Lawrence and A. Sacco, J. Chem. Soc., Faraday Trans. I , 1983, 79, 615. 10 D. Dasgupta, S. Das and D. K. Hazra, J. Chem. SOC., Faraday Trans. 1, 1988, 84, 1057. 1 1 H. Sadek, Th. F. Tadros and A. A. El-Harakany, Electrochim. Acta, 1971, 16, 339. 12 R. M. Fuoss, Proc. Natl. Acad. Sci. U.S.A., 1978, 75, 16. 13 R. M. Fuoss, J. Phys. Chem., 1978, 82, 2427. 14 M. F. C. Ladd, Theor. Chim. Acta, 1968, 12, 533. 15 D. S. Gill and M. B. Sekhri, J. Chem. SOC., Faraday Trans. I , 1982, 78, 119. 16 B. S. Krumgalz, J. Chem. SOC., Faraday Trans. I , 1980, 76, 315. 17 Wen-Yang Wen, Water and Aqueous Solutions : Structure, Thermodynamics and Transport Processes, 18 V. Viti, P. L. Indovina, Podo, L. Radics and G. Memethy, Mol. Phys., 1974, 27, 521. ed. R. A. Home (Wiley-Interscience, 1972), chap. 15. Paper 8/01214H; Received 20th June, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501531

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1989, 85(7), 1537-1544 Study of Ultramicroporous Carbons by High-pressure Sorption Part 1.-N,, CO,, 0, and He Isotherms Jacob E. Koresh,* T. H. Kim and W. J. Koros Department of Chemical Engineering, University of Texas at Austin, Austin, T X 78712, U.S.A. Adsorption-desorption isotherms for nitrogen, carbon dioxide, oxygen and helium on as-received TCM- 128 ultramicroporous carbon are reported for pressures up to 60 atmt at 35 "C. Evidence is presented that suggests that there are regions in the carbon that are composed of tiny hydrophobic constrictions in series, which are hardly penetrated, and more open pores inside. At room temperature, water molecules cannot penetrate these constrictions in reasonable time due to a clustering effect, while the much bigger, but unclustered nitrogen and carbon dioxide molecules do penetrate these constrictions at a measurable rate.These regions are responsible for unexpected hysteresis observed for nitrogen and carbon dioxide at 35 "C and the unusually large amount of helium adsorbed. In earlier papers, we have reported studies under a variety of conditions of a carbon sorbent referred to as TCM- 1 28.'-11 Room-temperature water sorption equilibria were measured in as-received and activated samples of this material as a means of probing the total available pore volume, including tiny pores not easily accessible to larger penetrants such as nitrogen and o ~ y g e n . ~ - ~ * ~ ' Low-temperature sorption kinetics and equilibria for various gases were also measured for the as-received and activated material^.^ Pore regions that were detectable with water sorption in the unactivated as-received TCM-128 carbon at room temperature were not penetrated by nitrogen at a detectable rate at - 80 "C.Two TCM-128 samples with slightly different degrees of activation showed no difference in nitrogen sorption uptake at -80 "C. On the other hand, sorption levels were two orders of magnitude smaller at - 196 "C for the less-activated sample compared with the slightly more activated material.2 This molecular sieving results from the inability of the low-temperature nitrogen to penetrate the rigid-pore system of the less-activated carbon at an observable rate. Based on previous studies, basic features of the morphology of the TCM-128 material have been post~lated.l*~ Specifically, the material is believed to consist of rather open pores with periodic constrictions that occur in series.The intervening passageways between the constrictions are believed to be sufficiently large to offer little resistance to movement, so the essential resistance to transport consists of the series of constrictions. This morphology explains the observed coexistence of a molecular sieving ability and a high effective degree of molecular mobility in the TCM. Because of the paucity of data on the as-received substrate material, we chose to focus our attention on it to establish a basepoint for later comparisons. The present study extends our characterization of TCM- 128 to include high-pressure t 1 atm = 101 325 Pa.15371538 Study of Ultramicroporous Carbons by High-pressure Sorption pressure transducer A pressure transducer B w Fig. 1. Schematic of the volumetric high-pressure adsorption systei m. sorption/desorption equilibria data for nitrogen, carbon dioxide, oxygen and helium in the as-received material near ambient temperature. The data lead us to an improved understanding of the complex morphological nature of this ultramicroporous material. In this respect, the different size penetrants serve as ultrafine probes of the structure of the material. Experimental Materials A fibrous carbon cloth TCM-128 was supplied by Carbone Lorraine, France, and was used as received. The material was obtained from the same sample lot as the material that was considered earlier in our sorption measurements with water vapour.The nitrogen, carbon dioxide, oxygen and helium gases used were obtained from Linde at a purity of greater than 99.9% and were used as received. Adsorption Cell Gas sorption measurements were made up to 60 atm at 35 "C using a volumetric system described before12 but with some modification. The system is schematically shown in fig. 1. The carbon cloth was placed in a chamber of volume VB. After an evacuation for 24 h, valve B was closed and gas was introduced into chamber A through valve A at the desired pressure. The amount of gas introduced was calculated by: n = - ' A 'A zRT where z , the compressibility factor, was calculated for each pressure from a PVT data source.13 During the experiment, valve B was opened to allow gas into the sample for sorption, or to remove a controlled amount for incremental desorption.The pressure in the sample chamber was recorded continuously as a function of time until the rate of change of pressure decay became undetectable in all cases except for the higher pressure (> 35 atm) nitrogen adsorption runs. In these cases, a very protracted long-term uptake was apparent, as will be described later. In these high-pressure nitrogen runs, the experiments were arbitrarily terminated when the change in pressure over a 24 h period was less than 0.5% of that observed over the first 24 h period of the incremental sorption run. Typically, this meant that run times for each high-pressure incremental nitrogen sorption point were carried out for 6 days.The amount of gas adsorbed in each incremental sorption or desorption step was calculated by subtracting the remaining amount of gas in each volume (A and B) fromJ . E. Koresh, T. H. Kim and W. J. Koros 1539 60 1 .' 04 I I I 0 2 0 4 0 60 Platm Fig. 2. High-pressure nitrogen adsorption-desorption isotherm on TCM- 128 as-received carbon at 35 "C. 4, desorption; ., adsorption. the cumulative amount injected or removed. Since the system has a separate pressure transducer for each chamber, a material balance is maintained on all gas in the cell; therefore, equilibrium calculations are not affected by the length of time, or the extent to which valve B is opened. Results and Discussion Nitrogen Isotherms Fig. 2 shows the high-pressure adsorption and desorption isotherms for nitrogen at 35 "C on the as-received carbon TCM-128 after evacuation at 35 "C for 24 h.The iso- therm has a type I shape, without reaching a plateau, thereby indicating that pore volume filling is not complete over the pressure range studied. The as-received TCM- 128 is an ultramicroporous adsorbent and does not adsorb measurable amounts of nitrogen at liquid nitrogen temperature over experimentally accessible time scales.' Moreover, it hardly adsorbs the tiny CO, molecule at -78 "C,' but does adsorb water7.' as well as nitrogen near room temperature as shown in fig. 2. Under these conditions, it is reasonable to assume a mass density for adsorbed water equivalent to the density of water in the liquid state (e.g. 1.0 g ~ m - ~ ) . For comparison, it is useful to assume that the mass density for adsorbed nitrogen is equivalent to the density of nitrogen in the liquid state at - 196 "C (0.808 g cme3).The nitrogen-accessible pore volume calculated in this fashion from the sorption level at 60 atm is 0.066 cm3 g-' (TCM- 128) as compared with 0.120 cm3 g-' (TCM- 128) determined from the apparent plateau value of the water isotherm on the same batch of TCM-128.7i10 This is an interesting result, since the nitrogen isotherm is clearly still not saturated at 60 atm, so more capacity exists for nitrogen if higher pressures could be reached. Moreover, note that correction of the liquid density for the ambient temperatures used here will lead to even higher estimates of accessible pore volumes for the nitrogen, so the use of the 0.808 g cm-3 value provides a conservatively low estimate of the pore volume accessible to nitrogen at 35 "C.Thus, the fractional saturation of the pore volume (estimated from water sorption plateau) by nitrogen at 60 atm and 35 "C is, therefore, about 0.55.1540 Study of Ultramicroporous Carbons by High-pressure Sorption Further perspective on the large extent of penetration of the as-received TCM- 128 by the high-pressure nitrogen at this ambient temperature can be offered by considering earlier nitrogen adsorption studies at 20 and -78 "C on an active carbon (AC) with less discriminating constrictions in the pore network than the TCM- 128? The nitrogen uptake at 60atm and 20 "C in the AC corresponds to filling an apparently smaller fraction of the pore volume, as in the TCM; note that the AC pore volume was sensed by the -78 "C nitrogen sorption uptake plateau instead of by the room temperature water uptake plateau as in the TCM.Under equivalent pressure conditions on any adsorbent, however, nitrogen at -78 "C, clearly adsorbs much less than at liquid- nitrogen conditions where a high affinity constant allows saturation of all pore volume that is not 'closed' by molecular sieving exclusion. For example, on slightly activated TCM-128 at - 196 "C, the asymptotic nitrogen uptake was more than three times higher than at -78 "C (see fig. 2 in ref. 2). It was shown earlier that essentially the same pore volume was available to nitrogen at liquid nitrogen temperature in a slightly activated TCM-128 sample as was available to water at room temperature on the as-received TCM (comparing fig.2 in ref. 2 with fig. 1 in ref. 10). The kinetic restrictions to penetration of the standard pore regions by nitrogen were eliminated by the slight activation. Without the slight activation, essentially no nitrogen penetration of the TCM-128 was possible and the pore volume for the as-received TCM could be assessed only from the plateau level of the water isotherm. Therefore, we can conclude that on an equivalent basis, the 60 atm nitrogen uptake in the present study corresponds to very significantly higher fractional filling of the total volume as compared with that occurring in the AC under similar pressure and temperature conditions. At least two hypotheses are possible to explain these results.In fact, phenomena related to both hypotheses may actually be responsible for the observed results; however, they are presented independently here for ease of discussion. The first hypothesis relies upon the conventional wisdom that a more energetically favourable thermodynamic environment may exist in the TCM- 128, since the attractions from both pore walls experienced by adsorbates in the fine-pored TCM produces a higher effective affinity constant compared with the more open AC in which dual wall interactions of individual penetrants are not anticipated. l5 A second viable hypothesis is more speculative, and suggests that nitrogen at 35 "C and at high concentrations or pressures can penetrate pore volume in the TCM- 128 that is not accessible to water at the same temperature.In this case, since the water is excluded from certain regions in the TCM- 128, one tends to underestimate the available pore volume and overestimate the fractional saturation of the total volume. This situation should not be present in the more thoroughly oxidized and open AC where water has essentially full access to all regions. The above hypothesis is based on the fact that a potential clustering mechanism exists for the hydrogen-bonding water molecule which is not a factor for nitrogen. Clearly, an individual water molecule has a smaller sieving dimension than a corresponding individual nitrogen molecule ;Is however, when clustering occurs, this simple picture no longer applies. Even in hydrophobic polymers, clustering of water is well known to cause reduced effective water diffusion ~0efficients.l~ In the case of carbons, polar surface oxide groups provide a more compatible environment for the individual water molecules, and thereby should suppress water cluster formation. Kinetic factors related to the tremendously slower penetration of clustered water, compared with the unclustered species, may make water penetration through tiny hydrophobic constrictions unobservable under realistic time scales. Mechanistically, the reduced mobility can be described in terms of the additional energy of activation needed for dissociation to individual molecules to be able to fit through the smallest constrictions.Overcoming a hydrogen bond strength of the order of 5 kcal mol-1 whichJ .E. Koresh, T. H. Kim and W. J. Koros 1541 is typical of water in such a process would account for a 3500-fold lower diffusion coefficient of the clustered material. Clearly in this case, one would not observe such extremely slow penetration of tiny hydrophobic pores. Hydrophobic tiny Rores could easily arise from defects in unoxidized graphite interlayers whose 3.4 A effective spacing is smaller than the standard constrictions which presumably have oxide surfaces. Evidence for the presence of oxidized surfaces on the standard constrictions has been reported.' Specifically, low-temperature (300 "C) activation of TCM led to CO, emission with a negligible resultant weight loss and caused an increase of four orders of magnitude in the rate of adsorption of CO, at -78 "C.These results suggest that, while the internal surfaces of the standard pores responsible for most of the sorption capacity have oxide groups, the standard molecular sieving constrictions must also have oxide groups, since the gentle activation which removes small amounts of these oxides led to such a large increase in the uptake rates of CO,. The hysteresis shown for the nitrogen isotherms reflects a gradual merging of the desorption curve with the adsorption curve. This form of hysteresis differs from that observed when desorption curves drop rapidly to meet the adsorption curve at a certain characteristic pressure, depending upon bottle necks or constriction dimensions. Hysteresis similar to that shown in fig. 2 for the as-received TCM-128 is not typical of nitrogen or CO, high-pressure isotherms at ambient temperature with microporous carbons of zeolites.1 4 7 18. l9 For an earlier room- temperature high-pressure methane isotherm on an activated TCM-128, with larger pore dimensions that were still in the microporous range, no hysteresis was observed at all. Such a hysteresis response, referred to as low-pressure hysteresis, has been reported by Bailey et al. for sorption of strongly interacting organic vapour on high- and low-activated carbons.20 In the present study, long equilibration times were allowed (typically 5 days, even though the samples appeared to be essentially at equilibrium after only 2 days). During desorption, a secondary process was observed at high pressures and long times, which suggests a protracted time-scale for diffusive uptake into regions whose access is controlled by tiny constrictions.The secondary process was observed as a slow but easily detectable readsorption of nitrogen after the simple desorption process had apparently been completed for high-pressure desorption steps. The rate of readsorption moderated by uptake through the tiny constrictions decreased as the pressure decreased, and it completely stopped at 40 atm. At this point, therefore, it appears that true equilibrium saturation had been achieved in both the regions, accessible through only tiny pores as well as through the larger pores. During subsequent desorption, we waited for extremely long periods of time beyond that when the majority of the desorption process occurred. Usually we waited for 2-3 days for each point to reach equilibrium and have waited an extra 3 days for some points and found that the additional amount desorbed is negligible (about 4% of the total amount desorbed at that pressure) and far from closing the gap between the adsorp- tion-desorption curves.In summary, we believe that the restricted regions have essentially reached desorption equilibrium, and in this case, the true equilibrium adsorption levels of both the restricted and the more open pore regions are well represented by the desorption curve below 40 atm. Conversely, the sorption uptake in regions whose access is moderated by dif- fusion through standard pores is approximately represented by the adsorption curve up to 40 atm. Based on the above discussion, above 40 atm, presumably both sorption and desorption isotherms represent a complex mixture of uptake into the standard and 'closed porosity' regions determined by the pressure and exposure time of the sample to high-pressure gas.52 FAR I1542 Study of Ultramicroporous Carbons by High-pressure Sorption 0 4 8 0 2 0 4 0 60 Platm Fig. 3. High-pressure carbon-dioxide adsorption-desorption isotherm on TCM- 128 as-received carbon; inset lower pressures isotherm. H, adsorption ; +, desorption. Carbon Dioxide Isotherms The adsorption4esorption isotherms for CO, in fig. 3 show a hysteresis that is similar to that seen for nitrogen; however, the desorption curve does not meet the adsorption curve. As in the preceding discussion of nitrogen, the CO, hysteresis is believed to reflect uptake into regions at high pressures whose access is limited by movement through tiny constrictions.Unlike N,, for the smaller CO, molecule these regions are able to saturate during the extended high-pressure adsorption runs prior to beginning the desorption process. This fact is reflected by the observation that for carbon dioxide there was no sign of the readsorption phenomenon at the higher desorption pressures mentioned for desorption runs with N, for pressures between 60 and 40 atm. This observation is reasonable, since the ratio of the diffusion coefficients (and hence equilibration times) in the standard pores relative to those in the tiny constrictions would be nearer unity for the smaller CO, compared to N,. At about 55-60 atm, both the sorption and desorption isotherms indicate that saturation of the entire pore volume has occurred.By following the same approach as in the N, case, and assuming the liquid-like density of 1.031 g cm-3 for CO, measured at -20 "C21 along with the pore saturation value taken from fig. 3, an accessible pore volume of 0.146 cm3 g-l results. This estimate is more than 20 % higher than the pore volume calculated from the plateau amount sorbed in the water isotherm at 25 "C. Again, this is a conservative estimate, since the adsorbed density at 35 "C may be somewhat less than that of liquid CO, at - 20 "C. As in the case of N,, the foregoing discussion suggests that carbon dioxide molecules can penetrate into regions which are not accessible to water over the time scale of a typical sorption experiment and thus giving the more accurate higher pore volume.In the case of the water sorption measurements, sorption equilibration appeared to be reached in 3-4 h, so a much slower (> 3000 times) penetration of clustered water wouldJ. E. Koresh, T. H. Kim and W. J. Koros 1543 60 n O 40 v 3 0 0 20 n c D v 5 0 0 10 2 0 3 0 0 20 4 0 6 0 Platm P l a n Fig. 4. Fig. 5. Fig. 4. High-pressure oxygen adsorption-desorption isotherm on TCM- 128 as-received carbon. m, adsorption ; +, desorption. Fig. 5. High-pressure helium adsorption4esorption isotherm on TCM- 128 as-received carbon. A, adsorption; + , desorption. not be apparent. For non-clustering penetrants such as nitrogen and carbon dioxide, experiments that are only four to five times longer than necessary for saturation of the standard pore volume allow observation of at least measurable extents of saturation of the regions whose access is moderated by diffusion through tiny constrictions.Oxygen and Helium Isotherms High-pressure adsorption-desorption isotherms on TCM- 128 carbon are shown in fig. 4 and 5 at 35 "C for oxygen and helium, respectively. Unlike the two previous penetrants, no hysteresis is apparent. It is likely that measurements at higher pressures with oxygen would have shown results between those of nitrogen and carbon dioxide, since oxygen's minimum dimension falls between those of nitrogen and carbon dioxide. We performed the oxygen measurements only up to 25 atm, due to concerns about safety in our external gas handling equipment. Based on the preceding discussions, it is likely that the oxygen results represent adsorption equilibria only into the standard pores with essentially no contribution from the tiny pores probed by both nitrogen and carbon dioxide at high pressures and long exposure times.The helium data are interesting in terms of the extremely large uptake that is apparent for this low condepibility penetrant. We presume that the extremely small minimum dimension (ca. 2.6 A) of helium allows it to probe essentially the entire accessible volume of the solid; however, due to helium's low sorption affinity constant, saturation would require studies at higher pressures than our equipment can presently reach. Comparable high-pressure adsorption levels of helium on active carbon were observed previously only at liquid nitrogen temperature.14 52-21544 Study of Ult ram icroporous Carbons by High-pressure Sorption Conclusions Unexpected hysteresis, observed in the adsorption-desorption isotherms for nitrogen and carbon dioxide, can be explained in terms of the existence of tiny constrictions that moderate uptake into these regions in the as-received TCM-128.These new features were able to be observed due to the different experimental techniques used in this study which combined high pressures and ambient temperature capabilities with protracted run times. Comparison of the nitrogen, carbon dioxide and helium data in this study with earlier water adsorption measurements in the same material suggests that these gases have access to regions not available to water.This phenomenon can be explained in terms of clustering of water near tiny hydrophobic constrictions, moderating access to these regions. Mechanistically, the clustering adds significant activation energy to the diffusion coefficient for water, thereby greatly suppressing the ability of this otherwise tiny molecule to penetrate hindered environments. References 1 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. I , 1980, 76, 2457. 2 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 2472. 3 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 3005. 4 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 2507. 5 J. Koresh, J. Colloid Interface Sci., 1982, 88, 398. 6 J. Koresh and A. Soffer, J. Colloid Interface Sci., 1983, 92, 517. 7 S. S. Barton and J. E. Koresh, J. Chem. SOC., Faraday Trans. 1, 1983, 79, 1147. 8 S. S. Barton and J. E. Koresh, J. Chem. Soc., Faraday Trans. I , 1983, 79, 1165. 9 S. S. Barton and J. E. Koresh, J. Chem. SOC., Faraday Trans. I , 1983, 79, 1173. 10 J. E. Koresh, A. Soffer and H. Tobias, Carbon, 1985, 23, 571. 1 1 S. S. Barton, M. J. B. Evans, J. E. Koresh and H. Tobias, Carbon, 1987, 25, 663. 12 W. J. Koros and D. R. Paul, J. Polym. Sci. : Polym. Phys. Edn, 1976, 14, 1903. 13 F. Din, Thermodynamic Function of Gases (Butterworths, London, 1961), vol. 3. 14 A. von Antropoff, Kolloid Z., 1952, 129, 1 1 ; Kolloid Z., 1954, 137, 105, 108. 15 D. A. Everett and J. C. Powl, J. Chem. SOC., Faraday Trans. I , 1976, 72, 619. 16 D. W. Breck, Zeolite Molecular Sieves (Wiley, New York, 1974), p. 636. 17 V. T. Stannett, G. R. Ranade and W. J. Koros, J. Membrane Sci. 1981, 10, 219. 18 S. S. Barton, J. R. Dacey and D. F. Quinn, in Fundamentals of Adsorption, ed. A. L. Myers and 19 P. G. Menon, Chem. Rev., 1968, 68, 277. 20 A. Bailey, D. A. Cadenhead, D. A. Everett and A. J. Miles, Trans. Faraday SOC., 1971, 67, 231. 21 Handbook of Chemistry and Physics (CRC Press, Boca Raton, Florida, 64th edn, 1983-84), p. C-219. G. Belfort, Proceedings of Engineering Foundation Conference, West Germany, May (1983), p. 65. Paper 81012755; Received 30th March, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501537

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. 1, 1989, 85(7), 1545-1556 Study of Ultramicroporous Carbons by High-pressure Sorption Part 2.-Nitrogen Diffusion Kinetics Jacob E. Koresh", Tae Han Kim, David R. B. Walker and William J. Koros Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, U.S.A. Sorption-desorption kinetics for nitrogen in the as-received TCM 128 ultramicroporous carbon fibre are reported at 35 "C over a wide range of pressures. The nitrogen sorption kinetics at low pressures follow the Fickian model. As the pressure increases, the deviations from the model become more pronounced, and at sufficiently high pressure a sigmoid kinetic response shape is observed which is indicative of a non-Fickian diffusion process. An additional timescale must be active to account for the non- Fickian transport behaviour.This second timescale process may correspond to adsorbate surface rearrangements leading to locally time-dependent clearing of constrictions at a rate with a characteristic kinetic constant. The nitrogen desorption kinetics are found to be less affected by the non-Fickian transport, thus leading to higher apparent diffusion coefficients for the same average pressures. At high pressures and very long sorption times, slow protracted uptake becomes apparent into restricted regions. The kinetics for this process can be described by a barrier model and are sufficiently slow to allow treatment independent of the processes occurring in the more open pore system. In a previous study' with permanent gases under cryogenic temperatures for several activated samples of TCM 128 carbon fibres, direct measurements of the molecular diffusion coefficients were not made. In this case, in fact, severe non-Fickian sorption behaviour was observed. Moreover, the as-received carbon was not studied, because these gases are effectively non-sorbing at cryogenic temperatures for the as-received materia1.2~3 It was an objective in this study to determine if Fickian sorption kinetics could be observed at higher temperatures such as those of interest in actual separation applications.It was desired to measure actual molecular diffusion coefficients for nitrogen movement within the as-received TCM 128 under different degrees of saturation of the micropores. Also, because of the paucity of data on the as-received substrate material, we chose to focus our attention on it to establish a basepoint for later comparisons with more activated samples. We studied the ambient temperature range, since work with only slightly activated carbons had demonstrated22 the increased ability to penetrate restricted pore environments as temperature increases.The present study, therefore, extends our characterization of TCM 128 to include high-pressure nitrogen sorption-desorption kinetic data in the as-received material near ambient temperature. The data lead us to an improved understanding of the complex morphological nature and energetic heterogeneity of this ultramicroporous material. In this respect, the nitrogen penetrant serves as an ultrafine probe of the structure of the material.15451546 Nitrogen Difusion in Ultramicroporous Carbon Experimental The fibrous carbon cloth, TCM 128, and the high-pressure sorption apparatus and procedures used in this work were the same as described in Part 1 of this series dealing with equilibrium aspects of gas sorption4esorption. The nitrogen gas used was obtained from Linde at a purity of 99.999% and was used as-received. Theory Assuming a homogeneous continuum for the carbon fibres, Fick’s second diffusion equation in cylindrical coordinates becomes : - ac = --[iz(c)rg] 1 a at r a r where 9(C) is an effective local diffusion coefficient that may be concentration dependent owing to at least two factors. The most obvious of these factors is simple energetic site heterogeneity which causes the local diffusion coefficient to increase with increasing degree of site saturation. This effect occurs because of the higher activation energy for diffusion of the molecules occupying the more energetic sites.Obstruction of critical constrictions, on the other hand, causes the local diffusion coefficient to decrease, rather than increase with degree of site saturation. In this case, a literal ‘traffic jam ’ can occur at the constrictions where the molecular sieving process tends to occur. The concentration dependence of this obstruction function modifies the concentration dependence of the inherent diffusion coefficient which arises from simple energetic heterogeneity. Since a constant-volume sorption cell was used in the present study, the pressure drops slightly as the adsorption proceeds; however, the pressure in the gas phase is always uniform throughout the cell.For the case where 9 ( C ) NN D (a constant) the solution of eqn (1) with a changing boundary condition has been rep~rted.~ After integration with respect to r one obtains the expression for M,/M,, the normalized amount of material sorbed (or desorbed) in time t relative to the amount of material sorbed (or desorbed) at equilibrium ( t = 00) during a particular incremental run: where the q, are the positive, non-zero roots of and a = Kell/( Kdsorbent K ) the ratio of the volumes of adsorption cell and the adsorbent modified by the partition function, K = C(carbon)/C(gas). J , and J1 are the Bessel functions of the first kind of order zero and one, respectively.The qn values were obtained by solving eqn (3) with the experimental value of a for our particular system. The a value used in the above equation for each incremental sorption or desorption run was determined using the equilibrium IC values for each run as calculated from the sorption or desorption equilibrium isotherms reported in Part 1. The simple analytical expression given by eqn (2) in terms of a constant diffusion coefficient, D, has been shown to be quite effective for describing sorption kinetics over the entire sorption kinetic run if an average effective diffusion coefficient, D, is used for the concentration range of interest. The diffusion coefficients were obtained by curve- fitting of the experimental data up to M,/M, < 0.5 with eqn (2) using 32 qn values.J. E.Koresh, T. H. Kim, D. R. B. Walker and W. J . Koros 1547 1 .o 0.8 0.6 0.4 0.2 0.0 1 .o 0.8 0.6 0.4 0.2 0.0 0 1 0 2 0 3 0 4 0 0 1 0 2 0 3 0 4 0 5 0 6 0 J;/min: Fig. 1. Nitrogen adsorption kinetics at 35 "C (a model fit): (a) 1.3 atm, (b) 18 atm. Solid line without superimposed experimental data is the Fickian model. The average diffusion coefficient for use in eqn (2) is appropriately defined as: D = /I:9(C)dC/(C2-Cl) (4) where C, and C, refer to the concentrations in the carbon at equilibrium at the end and beginning of a sorption (or desorption) run, re~pectively.~ Clearly, the average diffusion coefficient in eqn (4) reflects the complex effects of site heterogeneity and obstruction factors discussed in terms of the local diffusion coefficient, 9(C).For cases in which even more complex effects cause additional time-dependent phenomena not linked simply to local concentration, the preceding Fickian analysis is insufficient. It is anticipated that relaxation of the rigid carbon matrix is highly unlikely. Nevertheless, non-Fickian effects which will be presented in the experimental results suggest that an additional process with a timescale longer than that of the local diffusive process is occurring. Results Nitrogen Kinetics at Low and Medium Pressures Sorption Fig. 1 (a) and 1 (b) show typical N, sorption kinetics at low (ca. 1 atmt) and medium pressure (ca. 18 atm). The lines through the points, corresponding to the Fickian model using the D values, fit the experimental data very well up to 0.65 of the normalized amount adsorbed at low pressure and only up to 0.45-0.5 at high pressure before starting to deviate.Fig. 2(a) shows the trend of the nitrogen kinetics as a function of pressure increase during the time interval where the Fickian model fits. The apparent diffusion coefficients as calculated using the curve-matching technique t 1 atm = 101 325 Pa.1548 Nitrogen Difusion in Ultramicroporous Carbon 0.6 1 0.4 0.2 0.0 1 .o 0.8 0.6 0.4 0.2 0.0 0 2 4 0 2 0 4 0 6 0 Fig. 2. Nitrogen adsorption kinetics at 35 "C for low and medium pressures: (a) short times, (6) long times; (0) 1.3 atm, (A) 8.5 atm, (0) 13.4 atm, ( x ) 17.9 atm. described above from eqn (2) for the data in fig. 2(a) are plotted as a function of the average pressures? in fig.3. The initial slope of the kinetic curves tends to increase with increasing pressure, as reflected by the D values in fig. 3. On the other hand, at intermediate and large normalized uptake ( M J M , > 0.4) a complex crisscrossing of the kinetic responses occurs with a general trend for the approach to final saturation to be more protracted as the average pressure increases [fig. 2(b)]. In order to explain the above behaviour, it is useful to consider an additional factor besides the simple energetic site heterogeneity that is generally believed to cause the increasing trend in D as a function of the extent of pore volume filling. Specifically, in addition to such penetrant-pore interactions, the mobility of a given molecule through the partially filled pores may decrease owing to steric blockage of free motion within the pore by the presence of other sorbed and diffusing penetrants as pressure increases.If the kinetic constant for establishment of such impediments is sufficiently fast compared to the local diffusion coefficient divided by the square of a characteristic length such as the fibre radius, the sorption kinetics will be Fickian. This factor will tend to cause the diffusion coefficient to decrease with increasing adsorbate concentration. On the other hand, when time-dependent adsorbate rearrangements occur over longer timescales than that of the local diffusion process, an even more complex non-Fickian case occurs owing to time dependence in the diffusion coefficient. If the observed negative deviations at long times from eqn (2) were due only to concentration dependence, the diffusion coefficient for sorption at high pressure should be a strongly decreasing function of pressure.In this case, however, at the end of run i, the same 'morphology ' should exist as at the very beginning of run i + 1. Clearly, since the long-time slopes decrease with increasing pressure, more resistance to motion exists at the end of the run than at the beginning. In this case the initial slope for the next run 7 The average pressure refers here to the average of the pressures at the beginning and end of the sorption or desorption run being considered.J . E. Koresh, T. H . Kim, D. R. B. Walker and W. J. Koros 0 10 2 0 30 4 0 4, /am Fig. 3. Adsorption diffusion coefficient as a function of average pressure.0.6 0.4 0.2 0.0 1549 0 2 4 6 Jtlrnin 5 Fig. 4. Nitrogen desorption kinetics for low and medium pressures: (0) 4.2 atm, (0) 6.7 atm, (a) 11.3 atm, (H) 19.6 atm, (0) 31.9 atm, (A) 42.7 atm. should be smaller than the preceding one, which is opposite to the observed results in fig. 2(a). These facts, therefore, suggest that the deviations are actually due to time- dependent rearrangements that relax out by the time the next sorption run starts. Presumably, energetic heterogeneity produces a concentration-dependent diffusion coefficient up to the point at which sufficient penetrant loading of the constrictions1550 Nitrogen Diffusion in Ultramicroporous Carbon 1 .o 0.8 0.6 0.4 0.2 1 .o 0.8 0.6 0.4 0.2 0.0 0 2 0 4 0 6 0 8 0 0 2 0 4 0 6 0 8 0 Jt/m;lf Fig.5. Nitrogen desorption kinetics at 35 "C (a model fit): (a) 4.2 atm, (b) 19.6 atm. Solid line without superimposed experimental data is the Fickian model. makes the time-dependent rearrangements strongly influence the further movement into the pores. Desorption Fig. 4 shows the normalized desorption kinetics for different pressures. Qualitatively these kinetics resemble the behaviour of the adsorption kinetics at short times, namely a rate increase with pressure reflecting the heterogeneous energetics of the sites. In the desorption data the deviations from the Fickian model all begin to occur at approximately the same value of M J M , (fig. 5 ) and therefore, the desorption kinetic response curves (fig. 4) do not cross as in the adsorption curves (fig.2). This indicates that the desorption kinetics are affected less by the time-dependent reduced mobility in the pores believed to be responsible for the deviations at long times in the sorption runs. The desorption diffusion coefficients calculated from eqn (2) (fig. 6) reveal much larger values at higher pressures for the desorption than the corresponding adsorption runs at the same average pressure. This is inconsistent with a Fickian response in which the local diffusion coefficient increases monotonically with concentration or pressure. This is a general conclusion based on the fact that in cases where diffusion coefficients increase monotonically with increasing local penetrant concentration the desorption rate is lower than the sorption rate.6 Clearly, at low sorption levels, steric obstruction effects like those indicated in the sorption results at long times and high pressures will be of minimum importance. Thus at low pressures the kinetics should approach the Fickian limit in which the sorption response occurs more rapidly than that of desorption.The data in fig. 6 are consistent with this expectation, indicating a higher effective diffusion coefficient for the sorption versus the desorption experiment in the limit of sufficiently low pressures. If the form of the local concentration-dependent diffusion coefficient displays a maximum at low concentration^,^ behaviour such as ours with a larger desorption diffusion coefficient as compared to the sorption coefficient for the same incremental concentration interval occurs.While the use of such average coefficients could obscure aJ . E. Koresh, T. H . Kim, D. R. B. Walker and W. J . Koros 1551 0 10 2 0 30 P., /am Fig. 6. Adsorption-desorption diffusion coefficient as a function of average pressure : (0) adsorption, (+) desorption. maximum occurring in local diffusion coefficients for a single run spanning the local concentration range over which the maximum occurs’ this cannot be the cause of the observed results, since the average diffusion coefficient data in fig. 6 increase monotonically with increasing concentration. Clearly, to maintain the trend of higher desorption us. sorption rates, even the average diffusion coefficient must decrease with increasing pressure above the concentration corresponding to the maximum in the local coefficient.No such maximum occurs in fig. 6, indicating that no form of concentration dependence alone can explain the observed data. Therefore, both the sorption and desorption observations indicate strongly that the deviations from eqn (2) are due to time-dependent rearrangements rather than concentration dependence of the diffusion coefficient. We believe that no relaxation of the rigid carbon matrix occurs at medium pressures; however, some additional timescale process must be active to account for the non- Fickian transport behaviour. Specifically, adsorbate present in the vicinity of critical constrictions may inhibit the random walk of other penetrants through the passages. If these obstructing adsorbates move from these easily accessible high-energy sites into less accessible (and less obstructing) low-energy sites, the random walking of other penetrants again becomes less obstructed.Therefore, we suggest that the two time scales controlling movement within the carbon correspond to : (i) concentration-dependent diffusion as discussed earlier in the context of time-independent obstruction and site heterogeneity ; (ii) obstruction-clearing rearrangements that have a characteristic kinetic constant not linked to the diffusion process, without which diffusive passage through local constrictions is obstructed to various degrees.1552 0.6 - 0.4 - \ s x 0.2 - 0.0 Nitrogen Difusion in Ultramicroporous Carbon . 0.0 - 0 1 2 3 4 5 0 1 2 3 4 5 Jrlminf Fig. 7. Nitrogen kinetics at 35 "C at high pressures.(a) adsorption: (A) 31.5 atm, (0) 45.3 atm, (0) 57.2 atm; (b) desorption: (A) 42.7 atm, (0) 52.5 atm, (0) 60.3 atm. If the kinetic constant for the adsorptive rearrangements is at least an order of magnitude smaller than the diffusion coefficient divided by the square root of the fibre radius, the sorption kinetics will tend to be non-Fickian and strongly affected by the adsorption rearrangement kinetics. In the limit where the rearrangements required to allow diffusion are very slow, they could totally control the uptake. As discussed above, time-dependent rearrangements affect the sorption process more than the desorption process as indicated by the earlier onset with increasing pressure of deviations from eqn (2) for sorption compared to desorption.This is consistent with the higher apparent diffusion coefficient for desorption as compared to sorption. Although the kinetic data can be fitted using these apparent diffusion coefficients in eqn (2) for a certain range of MJM,, we believe the response is non-Fickian over the entire range for pressures above 10 atm. This conclusion is based on the earlier argument that the sorption response must lie above that for desorption for Fickian kinetics with a monotonically increasing diffusion coefficient. Since this is clearly not observed in our data above 10 atm (fig. 6), the apparent diffusion coefficients are actually complicated measures of the overall diffusion and relaxation processes. Rigorous decoupling of these effects would require either steady-state permeation or transient sorption measurements using samples with different characteristic dimensions.Neither of these options was available to us with the current material. High-pressure Sorption and Desorption Kinetics The sorption and desorption responses for high pressures (> 30 atm) are shown in fig. 7 (a) and 7 (b). Clearly the qualitative shapes of these responses show a marked increase in sigmoid nature of the kinetic response and differ significantly from those in fig. 1 for the low and medium pressure ranges. This sigmoid shape starts to appear at about 30 atm for the sorption kinetics and at ca. 50 atm for the desorption. It is another indication that the desorption kinetic process is less affected than the sorption process by the factors responsible for the non-Fickian response at high pressure.J .E. Koresh, T. H. Kim, D. R. B. Walker and W . J. Koros 1553 High-pressure Sorption Kinetics into Restricted Regions At pressures above 35 atm a very slow adsorption into restricted regions occurs through tiny constrictions which are responsible for the unusual sorption-desorption isotherm hysteresis discussed in our earlier paper.* Three important points relevant to the current study were found in this earlier paper: (i) the kinetic sorption rates into these restricted regions are orders of magnitude lower, (ii) the small amount adsorbed in these regions is much less than that for the more open pores for the timescales studied and (iii) no desorption from the restricted regions was observed even at the lowest desorption pressure studied (4 atm).Quantitative estimation of the diffusion coefficient for uptake into these regions will be given below. Since the kinetics of adsorption into restricted regions is very slow, we can describe the high-pressure sorption as composed of two parallel uptake processes with two different average diffusion coefficients. The restricted-regions kinetic rate does not interfere with the uptake process for the more open pores which saturate much more quickly; however, these regions do affect the total amount adsorbed. To avoid incorrectly scaling the M J M , response for the faster process, the total apparent M , must be corrected to eliminate the contribution of the restricted regions. Correction of the M , could be done in two independent ways: (a) M , for adsorption could be evaluated from the desorption isotherm.Since there is no long-time desorption apparent from the restricted regions, taking the amount desorbed, from the isotherm, at the same pressure interval for the adsorption point, should give the corrected M,; or (b) extrapolation of the last points of M, us. square root of time to zero time for the sorption run. The second method could give an erroneous M , value for the following reasons. Towards the end of the high-pressure runs the order of magnitude of the adsorption rate into both the open and restricted regions become very similar. This phenomenon occurs because the driving gradient is largely dissipated for the uptake into the more open pores. The tremendous adsorption transport resistance of the restricted regions, on the other hand, forces the uptake rate in this region always to be small in spite of the significant driving force.The first method clearly distinguishes the amount adsorbed in the restricted regions. Corrections for the two highest pressure points which were found to be the only cases affected by the restricted-region penetration, were made. The corrections of other points were found to be negligible, consistent with our earlier conclusion that adsorption in restricted regions can occur to a significant extent only above 35 atm. The corrected kinetic responses corresponding to the results in fig. 7(a) are given in fig. 8. It is clearly seen that the curves in fig. 7 ( a ) are shifted upwards without changing the nature of the sigmoid shape of the high-pressure kinetic responses.A plot of the kinetics of uptake into restricted regions is shown both as a function of time and as a function of square root of time over the entire period of the high-pressure incremental sorption run at 52 atm [fig. 9 ( a ) and (b)]. The linearity of the plot versus time is striking and suggests that the additional uptake process is not simply a diffusion- controlled sorption into a network of pores with uniformly smaller diffusion coefficient. If this latter situation occurred, the response would be expected to be linear in square root of time with a zero intercept unlike the observed behaviour in fig. 9(b). The linear behaviour in fig. 9(a) is, in fact, consistent with a situation that has physical significance relative to the morphology of the as-received carbon’s restricted regions. Specifically, we believe that the bulk of the volume of these regions is similar to the more open regions in terms of typical pore diameters.These regions are, however, restricted due to a very small number of limiting constrictions that forbid ready access to the considerable volume bordered by them. In other words, a barrier exists to penetration into the restricted regions, and an effective permeability could be assigned to this barrier based on the number and size of the tiny limiting constrictions. An indication of the magnitude1554 Nitrogen Difusion in Ultramicroporous Carbon 0.8 0.6 0.4 \ s 0.2 0.0 0 1 2 3 4 5 6 J;lmin: Fig. 8. Nitrogen adsorption kinetics at 35 "C at high pressures after M , correction.0 10 2 0 30 4 0 5 0 6 0 7 0 filmin; 0 .o 0 1000 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 tlmin Fig. 9. Nitrogen adsorption kinetics at 35 "C at high pressures into restricted regions. ( a ) 2's. \ t. (b) vs. t .J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros 1555 of these regions can be seen by results of earlier studies where incremental activation from 1 to 2% weight loss compared to the as-received TCM increased the volumetric uptake by 60% even at liquid nitrogen temperatures [compare fig. 2 ref. (3) with fig. 3 ref. (9)]. Since these tiny constrictions can be destroyed to a sufficient degree to allow access to the restricted regions that was previously precluded, the small amount of activation has a tremendous effect on the sorption level.Based on the above barrier interpretation, the slope of the plot in fig. 9(a) will be a function of the driving pressure across the limiting tiny constrictions as well as the number of such constrictions. As time progresses, the dissipation of the driving force between the restricted regions and the more open porosity will cause an eventual breakdown in the Mtl us. t relationship; however, this has clearly not occurred to a significant degree in fig. 9(a) even after 5000 min. For a barrier model under pseudo- steady-state conditions, an exponential time response would be observed as the pressure difference between the open and restricted regions seeks to equalize itself.' The exponential behaviour appears as the observed linear time response because only a small fraction of the percentage equilibration has been achieved.The slope of fig. 9(a) normalized by the capacity of the restricted regions? gives the effective time constant for the exponential function governing the equilibration of these regions. For example consideration of the 52 atm point indicates a time constant of 7 x lo4 min. The time to reach 90 % equilibration of the restricted regions would be 1.6 x lo5 min. This is much more than two orders of magnitude longer than the time to reach an equivalent degree of equilibration in the more open pores, therefore it is clear why this extremely long- timescale process does not interfere with the kinetics of the normal sorption process in the open pores.Conclusions Sorption4esorption kinetics for nitrogen in the open-pore system of the as-received TCM are complicated by concentration-dependent diffusion and additional slow kinetic rearrangements. No form of concentration dependence of the local diffusion coefficient can explain the observed data without also invoking time dependence of this coefficient. This time dependence may occur when a molecule adsorbs at an easily available but relatively high-energy site that can obstruct easy diffusion of other molecules. Rearrangement from such an unstable mode into a lower energy and unobstructing site presumably produces this additional time-dependent effect. The apparent diffusion coefficients at pressures above ca. 10 atm are complicated measures of the combined diffusion and relaxation processes which cannot be further decoupled with the presently available data. In addition to the above effects in the open porosity, protracted sorption into restricted regions occurs in a process moderated by tiny barrier constrictions. This supports the idea that the majority of these regions are composed of pores whose dimensions are similar to those in the accessible open porosity, but whose access is precluded by a limited number of tiny constrictions. t The restricted regions capacity is conservatively estimated to be 40 YO of the more open pore capacity at the same pressure.1556 Nitrogen Diflusion in Ultramicroporous Carbon References 1 J. Koresh and A. Soffer, J. Chem. SOC., Faraday Trans. 1, 1980, 76, 3005. 2 J. Koresh and A. Soffer, J . Chem. SOC., Faraday Trans. 1, 1980, 76, 2457. 3 J. Koresh and A. Soffer, J . Chem. SOC., Faraday Trans. 1, 1980, 76, 2472. 4 J. Crank, The Mathematics of Diffusion (Clarendon Press, Oxford, 2nd edn, 1975), p. 77. 5 V. T. Stannett, G. R. Ranade and W. J. Koros, J . Membrane Sci., 1981, 10, 219. 6 Ref. (4), p. 183. 7 Ref. (4), p. 186190. 8 J. Koresh, T. H. Kim and W. J. Koros, J . Chem. SOC., Faraday Trans. I , 1989, 85, 1537. 9 J. Koresh and A. Soffer, J . Colloid Interface Sci., 1983, 92, 517. Paper 8/02361A; Received 16th August, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501545

出版商:RSC    

年代:1989

数据来源: RSC

摘要:

J . Chern. SOC., Furaduy Trans. I, 1989, 85(7), 1557-1566 Study of Ultramicroporous Carbons by High-pressure Sorption Part 3.-Complex Transport Phenomena as sensed by CO, and N, Kinetics Jacob E. Koresh,*? Tae H. Kim, David R. B. Walker and William J. Koros Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712, U.S.A. High-pressure sorption kinetics for N, and CO, in as-received TCM carbon, introduced in Part 2 of this series (J. E. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros, J. Chern. Soc., Furuday Trans. I , 1989, 85, 1545), is elaborated here. Fickian processes are apparent for both gases, and provide the background against which the following complicated transport phenomena are overlaid. The most dramatic of these additional phenomena is C0,-induced constriction-dilation, which causes a decrease over four orders of magnitude in the equilibration time as pressure increases from 0 to 60 atm.$ Desorption equilibration times at lower pressures following the CO, exposure also reflect residual dilation.Higher desorption rates compared to adsorption rates were also observed for N,; however, the differences are much less extreme and are believed to be due to immobilization of adsorbate near constrictions instead of constriction- dilation. Evidence is presented to indicate the existence of a weak, transient barrier at the pore entrances at the beginning of sorption runs if appropriate conditions are chosen for each of the two gases. In a previous paper,' we have analysed data for nitrogen sorption kinetics in as-received samples of TCM 128 carbon-fibre materials.The sorption kinetics were influenced by both concentration-dependent Fickian diffusion and a slower process in which penetrant molecules are believed to rearrange to clear transient impediments to transport through critical molecular-sieving constrictions. This latter kinetic process was consistent with local rearrangements of penetrants from easily accessible high-energy sorption sites near critical constrictions to essentially co-located lower-energy sites. The rearrangements permitted clearing of apparent impediments that formed toward the end of kinetic runs at intermediate pressures, but which cleared prior to the initiation of the next run at the next higher pressure. The scale of the rearrangement was suggested to be localized and to preclude simple readsorption at the same high-energy sites responsible for the previous episode of clogging and clearing. The time t o reach effective saturation of the main pore system increased monotonically in the intermediate pressure range, where the kinetic rearrangement process had a significant influence on the sorption process.At higher average pressures the sorption- kinetic responses changed in appearance from the above pattern. Specifically, a sigmoid shape was found for the kinetics, whereas the low- and intermediate-pressure ranges were characterized by concave shapes when plotted versus the square root of time. Moreover, the kinetic response time for equilibration actually began to decrease with increasing pressure. These two observations suggested that a new non-Fickian process had come into play at higher pressure.This paper will present additional data to elaborate on this latter phenomenon. t Permanent address : Nuclear Research Centre, Negev, Chemistry Division, P.O. Box 900 1, Beer Sheva, Israel. $ 1 atm = 101 325 Pa. 15571558 Sorption Study of Ultramicroporous Carbons l o 3 .2 \ 01 9 10 * 10' 0 2 0 4 0 6 0 Pes 1- Fig. 1. Time needed to reach M J M , = 0.9 for nitrogen sorption and desorption us. the average pressure : , adsorption ; 0, desorption. Experimental The fibrous cloth TCM 128 and the high-pressure sorption apparatus and procedures used in this work were the same as described in Parts 1 and 22 of this series dealing with equilibrium and some of the kinetic aspects of sorption.The sorption measurements were all made at 35 "C. The nitrogen gas used was obtained from Linde at a purity of 99.999%, while the carbon dioxide gas was Instrument grade (99.99%) and was also used as received. Results and Discussion The data in fig. 1 present the time required for achieving M,/M, = 0.9 in both sorption and desorption kinetic runs for nitrogen in as-received TCM 128. The to.9 parameter reflects the interplay between the two processes of concentration-dependent diffusion and clogging/clearing rearrangements believed to compete for control of the transport kinetics in the sorption and desorption processes. The monotonically decreasing values of to.9 with increasing pressure for the desorption runs suggest that the concentration- dependent diffusion process is dominant over the entire period of all of the desorption runs.The monotonically decreasing value of to.9 reflects the heterogeneity of the carbon and the resultant increasing diffusion coefficients with increasing pressures. As noted earlier, however, the non-Fickian deviations at long times are evident even in the desorption responses. The to.9 data for the adsorption process show a maximum versus average pressure, reflecting a more complex interplay between the processes. The monotonically increasing tendency of to.9 at low and intermediate pressures suggests that the sorption kinetics are controlled by the timescale of the constriction-clearing rearrangements under these conditions. Based on the monotonically decreasing values of to.9 for high-pressure sorption kinetics (30-60 atm), concentration-dependent diffusion would appear toJ .E. Koresh, T. H. Kim, D. R. B. Walker and W. J . Koros 1559 dominate the process, in the same way as seen for the desorption data over the entire pressure range. This is, however, an oversimplified view, as will be discussed later. The behaviour of to,9 data for the sorption kinetics complements the picture proposed earlier to explain the additional time-dependent process responsible for deviations from the Fickian model at long times. Specifically, sorption at easily accessible high-energy sites near critical constrictions blocks the passage through these points until slower time- dependent clearing occurs in that region.The fact that the constriction-clearing rearrangements cease to be dominant at high pressures sheds further light on transport- limiting processes occurring at the constrictions. If the kinetic clearing process involves a localized rearrangement of the adsorbed molecule, without vacating the site, then subsequent adsorption at this site will be precluded. In this case, subsequent reblockage of these same constrictions will be more difficult even at relatively high pressures, since the easily accessible high-energy sites will be less available. Therefore the timescale of the rearrangement affects the overall diffusion processes much less at high pressures, and the concentration-dependent diffusion process appears to be dominant. has a sigmoid shape rather than the simple concave form seen at low and intermediate pressures.l In the context of the previous discussion of factors affecting the to.s parameter, the appearance of the extreme non-Fickian sigmoid response, concommitant with the decreasing values of to.s, seems anomalous.In order to explain this anomaly, an additional non-Fickian process should be active besides the clogging/clearing rearrangements discussed above. Specifically, as noted above, the constriction-clearing rearrangements cease to be dominant mass-transfer limitation factors at high pressures, since the low-energy sites near constrictions are less available for adsorption. As a consequence, we believe that the sigmoid kinetic response is indicative of a new non- Fickian transport process which becomes active at short times following the imposition of a pressure step.Two possible explanations for the sigmoid response can be postulated: (i) a ‘short-time barrier’ which is consistent with a large body of related information, including cryogenic sorption kinetic^,^ and (ii) a time-dependent relaxation process that allows a subtle dilation of critical constrictions to allow more rapid sorption in the intermediate and long time phases of sorption. The physical picture of a barrier involves the formation of a transient resistance inside the pores near the outer surface of the carbon. Such a transient barrier presumably could form immediately after applying a pressure step at high pressure when the pores are highly occupied. The barrier is assumed to be composed of low-energy quasi-condensed assemblages of sorbates, providing structures which significantly increase the resistance to downstream movement of molecules. These structures, however, are fragile in the case of nitrogen, because only weak intermolecular interactions are possible with this sorbate.Even if such a barrier forms, the pressure decay early in the sorption kinetic run is expected to destabilize these structures in the case of N,. At low and intermediate pressures with N2,. quasi-condensed structures are not favoured, owing to the less concentrated adsorption under these conditions. Even at a high pressure, where the amount of adsorption is potentially sufficient to form such structures, the barrier would be observable only if the transport through the downstream pores is fast relative to the time for a penetrant to traverse the quasi-condensed structures.As shown above, the rearrangement process ceases to play a significant role, and transport becomes faster at high pressure, thereby potentially allowing the observation of such a barrier. The kinetic response derived for a barrier moderating adsorption in a constant- volume system in the Henry’s law region is a simple exponential f ~ n c t i o n . ~ For p = 0 at t = 0 eqn (9) in ref. (3) could be rewritten as The nitrogen sorption-kinetic response for pressures beyond the maximum in 1 - [(b + c)/bMt] Mt = exp { - [(b + c)/bc] KA t }1560 0.8 0.6 $ 0.4 s 0.2 0.0 Sorption Study of U1 t ram icropo r ous Carbons 0 1 2 3 4 5 J;/min: -0.0 -0.2 4.4 5! P S 4.6 5 -0.8 -1 .o 0 1 2 3 4 5 rlmin Fig.2. Nitrogen kinetic response on TCM as received at 35 "C: ., pa, = 57 atm; +, pa, = 45 atm. (a) M J M , us. t1I2 ; (b) In ( 1 - MJM,) us. t. where b is the Henry's law constant, c = V/RT, Vis the unoccupied cell volume, is the initial amount of adsorbate in the gas phase and KA is a parameter equal to the product of an effective permeability and the projected area of the pore openings. At t = 00, M , from eqn (1) will be bMt/(b+c). Substituting M , in eqn (1) yields 1 - MJM, = exp { - [(b + c)/bc] KAt). (2) For the kinetic response for the second and subsequent points, where at t = 0 a certain amount M, is already adsorbed, eqn (2) would be 1 - M , / M , = (1 - M,/M,) exp { - [(b + c)/bc] KAt). (3) The sigmoid kinetic adsorption response of N, at high pressures shown in fig.2(a) excludes the possibility of Fickian diffusion. Fig. 2(b), on the other hand, indicates that the barrier model [eqn (3)] fits the data up to MJM, z 0.33 for the two high-pressure runs. The results are therefore consistent with the presence of transient barriers at the pore entrances which become unstable as the pressure drops during the adsorption process, thereby allowing more facile penetration as time progresses. Clearly, solely on the basis of these two isolated runs, such a conclusion is tenuous, since the complexity of the high- pressure kinetic response does not allow a complete decoupling of the various effects. Prior to more in-depth consideration of such situations, the issue of possible time- dependent dilation of constrictions as the source of the sigmoid response will be considered.Intuitively, the sigmoid shape of a kinetic run, taken by itself, might suggest swelling of the matrix as the cause of the non-Fickian response, as commonly seen in polymeric system with organic penetrants. Clearly the faster transport for N,, as indicated by the smaller values at high pressures (fig. 1) would also be consistent with a dilation of the critical constrictions in the carbon. This issue is best discussed after reviewing results presented below for CO,. If the preceding hypotheses concerning a transient barrier formation in the case of N, are realistic, one could anticipate situations in which barriers could form even at low pressures, where the kinetic response is much simpler.J.E. Koresh, T. H. Kim, D. R. B. Walker and W . J. Koros 1561 0 . 0 2 . 0 4 . 0 6 . 0 I I 1 0 2 0 4 0 6 0 platm Fig. 3. Adsorption isotherms of nitrogen and CO, on TCM as received at 35 "C. Inset: low- pressure isotherms; +, CO,; ., N,. The following conditions should favour such low-pressure barrier formation : (i) using adsorbates with high critical temperatures, thereby promoting formation of condensed- like structures at the pore entrances in the presence of a pressure step, (ii) using adsorbates with high diffusion coefficients in the pores, thereby minimizing resistance to transport in the internal pore system to accentuate the barrier at the pore entrance, and (iii) choosing an appropriate amount adsorbed during a given pressure step, to result in a low equilibrium pressure at the end of the given run, preventing obstruction of the diffusion process. CO, as an adsorbate on the as-received TCM at 35 "C seems to satisfy the first two criteria, and conditions can be set to satisfy the third one also.CO, has much higher critical temperature than N,, and is also a much smaller molecule. CO, can penetrate tiny pores from which N, is completely excluded, and the kinetic rate of CO, on slightly activated carbon could be increased by an order of magnitude, while nitrogen does not penetrate into the pore s y ~ t e m . ~ Therefore, higher CO, diffusivity as compared to that of N, could be expected. For the same sample and reservoir sizes, since the slope of the CO, isotherm is much steeper than that of nitrogen, one can impose a larger relative pressure step and end up with a lower equilibrium penetrant pressure for CO, as compared to nitrogen.This1562 Sorption Study of Ultramicroporous Carbons 0.8 0.6 < ' 0.4 0.2 0.0 0.0 h ?! -0.5 Y - W +i -1 .o -1.5 0 1 2 3 4 0 1 2 3 4 Jtlmini rlmin Fig. 4. CO, kinetic response on TCM as received at 35 "C for a W.3 atm pressure step. (a) M J M , us. P I 2 ; solid line: the Fickian model; (b) In (1 - MJM,) us. t , barrier model. produces conditions conducive to the formation of a quasi-condensed structure at the outer pores, while maintaining adequate sink capacity at the downstream face of the hypothetical barrier. Fig. 3 shows the adsorption isotherms of CO, and N, on as-received TCM at 35 "C. For example, the amount of CO, adsorbed at 4 atm is roughly equivalent to that of N, adsorbed at 50 atm.By applying a relatively small pressure step, 0-4 atm of CO,, the pore entrances at the beginning of the kinetic response would be filled with a similar amount of penetrant to that present in the case of nitrogen at 50 atm, where sigmoid kinetics were seen. Since CO, has both a greater tendency to form condensed-like structures and also will be shown below to have a higher diffusion coefficient inside the pores, the barrier model should fit CO, kinetic data over a larger range of reduced mass uptakes, MJM,, than was the case with N, [fig. 2(b)]. Fig. 4(a) shows the CO, kinetic response after applying a pressure step from 0 to 4.3 atm, which decayed to a low equilibrium pressure of 0.56 atm.The sigmoid experi- mental data in this figure are inconsistent with the ' best-fit ' Fickian model represented as the solid line in this figure. On the other hand, fig. 4(b) shows the corresponding barrier model, which fits the experimental points up to MJM, z 0.5. The second pressure step was from 0.56 to 5 . 3 atm, and the equilibrium pressure stabilized at 2.9 atm, thereby providing a very substantial adsorbed concentration [20cm3 (s.t.p.) per cm3 carbon] prior to the beginning of the run and ending at an highly concentrated level of adorbate [ca. 50 cm3 (s.t.p.) per cm3 carbon]. Under these conditions the Fickian model fits the experimental points in fig. 5(a) up to a value of M J M , = 0.6, while the barrier model [fig. 5(b)] is completely ineffective even at small values of MJM,.These two sequential runs underline the fragility of the barrier and the difficulty in observing its influence. In the second run, presumably the sorbed concentration not the quasi-condensed structures was sufficient to impede transport into the pores and thereby violate the third criterion for obtaining a barrier at low pressure. The diffusion coefficient, as calculated from the Fickian model by least-squares fit of1 .o 0.8 0.6 \ s * 0.4 0.2 J . E. Koresh, T. H . Kim, D. R. B. Walker and W. J. Koros 0.0 * OS I 0.0 n s 3 CI -Os5 v S -1 .o Q 1563 0 1 2 3 4 0 2 4 6 8 filmin: tlmin Fig. 5. CO, kinetic response on TCM as received at 35 “C for a 0.56-5.3 atm pressure step. (a) M J M , us. t”,; (b) ln(1 -M,/M,) us. t. the kinetics for the second CO, run up to M J M , = 0.5 with eqn (2) of ref.(l), is 1.2 x cm2 s-l. This value is more than an order of magnitude greater than the corresponding value for nitrogen at the same pressure. As the pressure increases, the onset of deviation from the Fickian model starts at lower values of MJM, (fig. 6) in a similar way to that seen with the nitrogen kinetics. An argument against swelling phenomena at low pressures can be offered based on the trends seen in CO, sorption kinetics. Specifically, as the CO, pressure increases the sigmoid shape of the kinetics disappears rather than becomes more exaggerated, as would be expected if swelling were the source of the sigmoid non-Fickian phenomenon. On the other hand, data discussed later suggest that some deformation process might occur at high pressures, especially with CO,, leading to less obstructed penetration of critical con striction s.- - - . - - __ - - - __ - - - - - - - - __ - - We speculate that the physical basis of the previously noted slow obstruction-clearing process and a constriction-deformation process are in fact related and are occurring during the long time period of the kinetic response. Specifically, the slow adsorption at low-energy sites slightly deforms the constrictions, producing a generally lower-energy adsorbate-adsorbent pair, with drastically lower restriction to passage by other adsorbates. While such local deformations may affect the critical dimensions of transport-regulating constrictions, it is anticipated that the bulk of the more open rigid matrix is essentially unchanged.This situation is different from ‘swelling’, in which a general dilation of the entire matrix is envisioned. Since constriction dilation is presumably a slow process it does not obscure the observation of any short-time transient barrier. Fig. 7 shows the adsorption and desorption times for CO, to reach MJM, = 0.9. Although both the CO, adsorption and desorption curves resemble the behaviour of N,, the difference between the two adsorbates is clearly seen. For adsorption, at low pressures in the range 1-2 atm the value of to.g for CO, is over an order of magnitude faster than for N,. The decrease in as the pressure increases1564 Sorption Study of Ultramicroporous Carbons 1 .o 0.8 0.6 $ z 0.4 0.2 0.0 0 1 2 3 4 Ji/min: Fig. 6.CO, kinetic response on TCM as received at 35 "C for a 2.9-18.3 atm pressure step; solid line, the Fickian model. 10 l o 2 10' .8 6 \ m ,o 10 10- ~ 0 Fig. 7. Time needed to reach M J M , = 0.9 for CO, sorption and desorption us. the average pressure : , adsorption ; 0, desorption.J . E. Koresh, T. H . Kim, D. R. B. Walker and W. J . Koros 1565 0 4 8 1 2 Pav Fig. 8. Fickian diffusion coefficients for CO, as a function of the average pressures. beyond the maximum for CO, at ca. 10 atm (fig. 7) is two orders of magnitude larger than the corresponding decrease in lo.g for N, beyond its maximum, which occurs at ca. 40 atm (fig. 1). During desorption the value of ro.g for CO, at 30 atm was too fast to be monitored accurately in our system (< 10 s cf 500 min for nitrogen). If one extrapolates the desorption data in fig.7 to 30 atm, the ratio of the values for N, and CO, is ca. 5 x lo5. The further increase of the ro.g ratio by more than two orders of magnitude during desorption is presumably a consequence of the exposure of the as-received carbon to CO, at 60 atm. ratio between nitrogen and CO, at high pressures, especially during desorption, it is reasonable to assume that the constrictions are dilated by exposure to CO, at high pressures. The onset of such deformations is suggested above 10 atm by the adsorption data. Fig. 8 shows the Fickian adsorption and desorption diffusion coefficients for CO, at low pressures and short times up to ca. M,/M, z 0.5. The first adsorption data point in fig. 8 is obtained from the best-fit line for the Fickian model shown in fig.4(a). The point appears consistent with the trend of the data in fig. 8; however, the model fit for this first run is inappropriate. For all the other points in fig. 8 the fit is satisfactory. The CO, desorption diffusion coefficients are always higher than the adsorption ones, contrary to the expected behaviour at low pressures if the diffusion coefficients are monotonically increasing with pressure in a Fickian model. This is another indication that CO, is dilating the constrictions, thus giving rise to higher desorption diffusion coefficients even at low pressures where the carbon matrix is not relaxing. For nitrogen it was shown' that the expected behaviour of lower desorption diffusion coefficients at low pressures as compared with the adsorption ones was observed, which certainly argues against the possibility of dilated constrictions.The time-dependent clearing rearrangements from easily accessible high-energy sites to low-energy sites near the constrictions at intermediate pressures were suggested to be responsible for the To account for the tremendous increase in the1566 Sorption Study of Ultramicroporous Carbons higher desorption as compared to adsorption diffusion coefficients for nitrogen. In this mechanism the molecules at the low-energy sites near the constrictions are not easily desorbing, and therefore there is no impedance to the desorption process due to the rearrangement clearing effect. While the possibility of extremely small extents of N,- induced constriction deformation above 40 atm cannot be excluded, it is unnecessary to invoke this phenomenon to explain all of the observations for N,.On the other hand, the tremendous rate of change of with pressure for CO, makes the dilation hypotheses more tenable for this gas. Conclusions Evidence has been provided to indicate that high-pressure CO, is a constriction-dilating agent similar to an organic vapour known to swell polymers4 or moderately rigid carbons. In the case of nitrogen, either strongly adsorbed molecules in the low-energy sites near critical constrictions or a minute constriction-dilation process alone could explain the faster desorption kinetics as compared with the adsorption ones. We tend to believe that constriction dilation does not occur in the case of nitrogen, even at pressures above 40 atm. Evidence was seen for the transient barrier phenomenon in the case of both nitrogen at high pressure and CO, at low pressure. The difference in the conditions of observation of such a barrier phenomenon was rationalized in terms of the relative condensibilities and transport rates in the carbon. Finally, the process of CO, interaction with the TCM carbon, leading to dilation of constrictions, was differentiated from a more extensive process that would normally be termed ' swelling '. Constriction dilation is used here to indicate deformation of critical transport-regulating constrictions without significant disturbance to the bulk of the carbon matrix per se. We anticipate the possibility of distinguishing the differences between the subtle constriction deformation we believe to be active in the as-received TCM and a true swelling process by characterizing progressively more activating TCM carbons. References 1 J. Koresh, T. H. Kim, D. R. B. Walker and W. J. Koros, J . Chem. SOC., Faraday Trans. I , 1989, 85, 2 J. Koresh, T. H. Kim and W. J. Koros, J. Chem. SOC., Faraday Trans. I , 1989, 85, 1537. 3 J. Koresh and A. Soffer, J. Chem. SOC., Faraday Trans. I , 1981, 77, 3005. 4 D. J. Enscore, H. B. Hopfenberg and V. T. Stannett, Polymer, 1977, 18, 793. 5 J. Koresh and A. Soffer, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 2457. 6 A. Bailey, D. A. Cadenhead, D. A. Everett and A. J. Miles, Trans. Faraday Soc., 1971, 67, 231. 1545. Paper 8/03202E; Received 4th August, 1988

ISSN:0300-9599    

DOI:10.1039/F19898501557

出版商:RSC    

年代:1989

数据来源: RSC