摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX033

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX035

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 84(9) 291 5-31 85 (1 988) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases 2915 2933 295 1 2959 2967 2979 2987 300 1 301 5 3027 3043 3059 307 1 3079 3093 3097 3 107 96 CONTENTS Monitoring Cation-site Occupancy of Nickel-exchanged Zeolite Y Catalysts by High-temperature in situ X-Ray Powder Diffractometry J. M. Thomas, C. Williams and T. Rayment Properties of Complexes with Cobalt-Carbon Bonds formed by Reactions of Aliphatic Free Radicals with Nitrilotriacetate-Cobalt(I1) in Aqueous Solution. A Pulse Radiolysis Study Correlations of Heterogeneity Parameters for Single-solute and Multi-solute Adsorption from Dilute Solutions A. W. Marczewski, A. Derylo-Marczewska and M. Jaroniec Calculation of Nuclear Magnetic Resonance van der Waals Chemical Shifts based on a Generalized Polyatomic London Dispersion Theorem J.Homer and M. S. Mohammadi Ordered Distribution of Aluminium or Gallium Atoms in Zeolite L T. Takaishi Preparation and Electrochemical Behaviour of a Methylene Blue-modified Electrode based on a Nafion Polymer Film X-Ray Absorption (EXAFS/XANES) Study of Supported Vanadium Oxide Catalysts. Structure of Surface Vanadium Oxide Species on Silica and y- Alumina at a Low Level of Vanadium Loading T. Tanaka, H. Yamashita, R. Tsuchitani, T. Funabiki and S. Yoshida A Model for the Mass-transfer Resistance at the Surface of Zeolite Crystals M. ZoEiiik, P. Struve, K. Fiedler and M. Bulow An Aluminium-27 Nuclear Magnetic Resonance Study of Ligand Exchange. The Kinetic and Equilibrium Properties Catalytic Decomposition of Mercaptans on Metal Films of Iron, Nickel, Palladium, Aluminium and Copper Y.K. Al-Haidary and J. M. Saleh Adsorption and Catalytic Decomposition of Dimethyl Sulphide and Dimethyl Disulphide on Metal Films of Iron, Palladium, Nickel, Aluminium and Copper Y. K. Al-Haidary and J. M. Saleh The Construction and Characteristics of Drug-Selective Electrodes. Appli- cations for the Determination of Complexation Constants of Inclusion Complexes with a- and P-Cyclodextrins including a Kinetic Study N. Takisawa, D. G. Hall, E. Wyn-Jones and (in part) P. Brown Thermal Decomposition of Pyrite. Kinetic Analysis of Thermogravimetric Data by Predictor-Corrector Numerical Methods I. C. Hoare, H. J. Hurst, W. I. Stuart and T.J. White Double-layer Interaction between Spheres with Unequal Surface Potentials J. Th. G. Overbeek Double-layer Interaction between Spheres with Unequal Surface Potential. Response to the Critique Excess Enthalpies and Excess Volumes of [xCO, + (1 - x) N,O] in the Liquid and Supercritical Regions Determination of Rate Parameters in Seeded Emulsion Polymerisation Systems. A Sensitivity Analysis I. A. Maxwell, E. D. Sudol, D. H. Napper and R. G. Gilbert D. Meyerstein and H. A. Schwarz Z. Lu and S. Dong T. Jin and K. Ichikawa E. Barouch C. J. Wormald and J. M. Eyears FAR 1Contents 3 1 13 Extraframework Aluminium in Steam- and SiC1,-dealuminated Y Zeolite. A 27Al and 29Si Nuclear Magnetic Resonance Study J. Sanz, V. FornCs and A. Corma Acidic Properties of Vanadium Oxide on Titania H. Miyata, K. Fujii and T. Ono Influence of Organic Solutes on the Self-diffusion of Water as studied by Nuclear Magnetic Resonance Spectroscopy P-0. Eriksson, G. Lindblom, E. E. Burnell and G. J. T. Tiddy Excess Enthalpies and Cross-term Second Virial Coefficients for Mixtures containing Water Vapour C. J. Wormald and N. M. Lancaster Excess Molar Enthalpies of (xH,O + (1 - x)C,H,,}(g) up to 698.2 K and 14.0 MPa N. M. Lancaster and C. J. Wormald Nature of the 8-Phase of Bismuth Molybdate M. M. El Jamal, M. Forissier and A. Auroux Interactions between Metal Cations and the Ionophore Lasalocid. Part 5.-A Potentiometric, Polarographic and Electron Spin Resonance Study of Cop- per(I1)-Laslocid Equilibria in Methanol P. Laubry, G. Mousset, P. Martinet, M. Tissier, C. Tissier and J. Juillard 3121 3129 3 141 3159 3169 3 175

ISSN:0300-9599    

DOI:10.1039/F198884FP123

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

JOURNAL O F 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 II, Issue 9, is reproduced below. This issue contains the proceedings of Faraday Symposium 23 on Molecular Vibrations, held in Reading in December 1987. 1237 Introductory Lecture : Potential-energy Surfaces, Unimolecular Processes and Spectroscopy R. A. Marcus 1247 The Accurate Calculation of Molecular Properties by Ab Initio Methods R. D. Amos, J. F. Gaw, N: C. Handy and S. Carter 1263 Anharmonic Potential-energy Surfaces, Vibrational Frequencies and Infrared Intensities calculated from Highly Correlated Wavefunctions P. Botschwina 1277 Vibrational Correlation Energies for Deuterated H i Molecules G.D. Carney 1295 Beyond Ro-vibrational Separation J. Tennyson, S. Miller and B. T. Sutcliffe 1305 Near-dissociation Motions of a Model X, System A. V. Chambers, M. S. Child and R. Pfeiffer 13 15 Hamiltonians for the Internal Dynamics of Triatomic Molecules P. Jensen 1341 The Unusual Torsion-Inversion Dynamics of Carbodi-imide, HNCNH M. Winnewisser and M. Birk 1365 Calculation of Rotational-Vibrational Energies of Methane-shaped Molecules directly from an Anharmonic Potential Function 1371 Investigation of Fermi Resonances in CHX, Molecules with an Internal- coordinate Hamiltonian L. Halonen, T. Carrington Jr and M. Quack 1389 Spectroscopy and Intramolecular Dynamics of highly Excited Vibrational States of NH, K. K. Lehmann and S. L. Coy 1407 Interbond Coupling in HCN and DCN J.E. Baggott, G. L. Caldow and I. M. Mills 1423 Infrared Studies of CH and CD Stretching Anharmonicity J. L. Duncan, D. C. McKean, I. Torto, A. Brown and A. M. Ferguson 1433 High-resolution Infrared Spectroscopy of Acetylene Clusters G. W. Bryant, D. F. Egers and R. 0. Watts 1457 Vibrationally Excited Formaldehyde. The Relationship between Vibrational Structure and Collisional Properties F. Temps, S. Halle, P. H. Vaccaro, R. W. Field and J. L. Kinsey 1483 Vibrationally Mediated Photodissociation M. D. Likar, J. E. Baggott, A. Sinha, T. M. Tichich, R. L. Vander Wal and F. F. Crim 1499 How a (nearly) Free Methyl Rotor Accelerates Intramolecular Vibration Relaxation. Theory and Experiment R. J. Longfellow and Charles S. Par- menter S. Brodersen151 1 1523 1535 1555 1643 1643 7/1111 7/ 1779 7/ 1990 7/2 122 712233 7/2234 7/2235 7/2236 7/2247 Is there Chemically Significant Trapping of Vibrational Energy in Localized and Chemically Identifiable Moieties? W.P. Reinhardt and C. Duneczky A New (Cartesian) Reaction-path Model for Dynamics in Polyatomic Systems, with Application to H-Atom Transfer in Malonaldehyde B. A. Ruf and W. H. Miller Local-mode Overtones. Ultrashot Pulse Excitation, Intramolecular Relaxation and Unimolecular Reactions J. S. Hutchinson and K. T. Marshall General Discussion Index of Names List of Posters The following papers were accepted for publication in Faraday Transactions I during June 1988. The Effect of Solvent Fluctuations in the Electron-transfer Process between Two Fe+ Ions Solvent Dependence of Kinetics and Equilibria of Thallium(1) Cryptates in Relation to Solvation Free Energies of Thallium(1) Cox, B.G., Stoka, J., Schneider, I. and Schneider, H. A Kirkwood-Buff Theoretical Approach to Debye-Hiickel Theory. Interpre- tation of Electrolyte Activity Coefficients in both Dilute and Concentrated Solutions Newman, K. E. An Aluminium-27 Nuclear Magnetic Resonance Study of Exchange and Energy Relaxation Rate in Butylpyrinidium Chloride + AlCl, Melts Ichikawa, K., Jin, T., Izumi, M. and Matsumoto, T. Acid-Base Equilibria in Aqueous Micellar Solutions. Part 1 .-' Simple ' Weak Acids and Bases Acid-Base Equilibria in Aqueous Micellar Solutions. Part 2.-Sulphon- ephthalein Indicators Acid-Base Equilibria in Aqueous Micellar Solutions.Part 3 .-Azine De- rivatives Acid-Base Equilibria in Aqueous Micellar Solutions. Part 4.-Azo Indicators Drummond, C. J., Grieser, F. and Healy, T. W. The Oxidative Coupling of Methane on Lithium Nickelate(m) Hatano M. and Otsuka K. Gonzalez-Lafont, A., Lluch, J. M., Oliva, A. and Bertran, J. Drummond, C. J., Grieser, F. and Healy, T. W. Drummond, C. J., Grieser, F. and Healy, T. W. Drummond, C. J., Grieser, F. and Healy, T. w. 8/00330K/F 1 P Thermodynamics for Chemical Equilibria and Kinetics in Solution at Constant Volume AIBuquerque, Linda M. P. C. and Reis, Joao Carlos R. Ionic Transport through an Homogeneous Membrane in the Presence of Simultaneous Diffusion, Conduction and Convection Aguilella, V. M., Mafe, S. and Pellicer, Julio Structure and Reactivity of Zn-Cr Mixed Oxides.Part 3.-The Surface Interaction with Carbon Monoxide Giamello, Elio, Fubini, Bice, Bertoldi, Massimo, Busca, Guido and Vaccari, Angelo 8/0033 lI/FlP 8/00566D/FlP (ii)8/0057 1 K/ F 1 P 8/00573G/FlP 8/0061 lC/FlP 8/00923F/FlP 8/00924D/ F 1 P 8/00927I/FlP 8/00957K/F 1 P 8/00971F/FlP 8/01214H/FlP 8/01218K/FlP 8/0125 1 B/FlP 8/01258J/Fl P 8/01 390J/FlP 8/0 1454J/F1 P 8/0 1520A/F1 P 8/01 53 lG/FlP Characterization Studies of Potassium Phosphotungstate Glasses and a Model of Structural Units Selvaraj, U., Sundar, H. G. K. and Rao, K. J. Electron Spin Resonance Investigation of Ruthenium ZrO, Supported Evidence of Strong Metal-Support Interaction Cattania, M. G., Ger- vasini, A., Murazzoni, F., Scotti, R. and Strumolo, D. The Influence of the Pretreatment on the Properties of Ag/a-Al,O, Catalysts containing Large (& 1 pm) Pure and Cs-Promoted Silver Particles. Part 1 .-Extent of Oxygen and Hydrogen Sorption and T.P.D.Studies Meima, G. R., Knijff, L. M., Vis, R. J., Van Dillen, A. J., Van Buren, F. R. and Geus, J. W. Excess Molar Enthalpies of (Steam + n-Hexane) and (Steam + n-Heptane) up to 698.2 K and 12.6 MPa Al-Bizreh, Nabil, Colling, C. N., Lancaster, Neil M. and Wormald, Christopher J. A Cubic Equation of State for Mixtures containing Steam Wormald, Christopher J. and Lancaster, Neil M. Statistical Surface Thermodynamics of Quaternary Liquid Systems Pandey, Jata D., Rai, Rishi D., Shukla, Arun, K. and Sukla, Rajiv K. Electron Paramagnetic Resonance Investigation of the Copper(II)-b- Glucosidase Interaction in Aqueous Solution Laschi, Franco and Rossi, Claudio Electric Potential Developed across Langmuir-Blodgett Preparations of Proteins Conductance Studies of Alkali-metal Chlorides and Bromides in 2- Methoxyethanol at 25 "C Nandi, Debasis, Das, Susanta and Hazra, Dilip K.Mixed Metal Hydroxycarboxylic Acid Complexes. Part 2.-Formation Constants of Complexes of U"' with Al"', In"', CU" and Fe"' Manzurola, Emanuel, Apelblat, Alexander, Markovits, George and Levy, Oscar Preferential Solvation. Part 3.-Binary Solvent Mixtures Marcus, Yizhak 'H Nuclear Magnetic Resonance Studies on Cationic Reorientation and Translational Self-diffusion in Two Solid Phases, including the New High- temperature Phase of Methylammonium Sulphate Ishida, Hiro- yuki, Matsuhashi, Noritoshi, Ikeda, Ryuichi and Nakamura, Daiyu Preparation and Characterization of Ti0,-SiO, Aerosil Colloidal Mixed Dispersions Structure and Dynamic Properties of Decylammonium Chloride Micelles in Water and in Glycerol Fletcher, Paul D.I. and Gilbert, P. J. Interaction of Transitional-metal Acetylactonates with y-Al,O, Surfaces Van Veen, J. A. Rob, Jonkers, Gert and Hesselink Wim H. Tracer Diffusion of Caffeine in Aqueous Solutions at 298.15 K. The Effect of Caffeine Self-association Said, Mazen, Rosen, Dennis and Hasted, John B. Morrison, C. and Kiwi, J. Price, William E. (iii)8/01 532E/F1 P Chromia/Silica-Titania Cogel Catalysts for Ethene Polymerisation. Infrared Study of Nitric Oxide Adsorption Conway, Steven J., Falconer, John W. and Rochester, Colin H.Formation of 'CF, and 'CCl, Radicals by Unimolecular Decomposition of (CP,OR)*+ and (CCl,COR)'+ Radical Cations Symons, Martyn C. R., Rhodes, Christopher J. and Portwood, Lynn 8/01 759J/F1 P Hydrogen Exchange Reaction of Surface Deuteroxyl Groups of MgO with H, Shido, Takafumi, Asakura, Kiyotaka and Iwasawa, Yasuhiro 8/02238K/FlP Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300 "C. Part 1 .-X-Ray Photoelectron Spectroscopy Allen, Geoffrey C., Tempest, Paul A. and Tyler, Jonathan W. Characterisation of Crystalline UO, Oxidised in 1 Torr of Oxygen at 25, 225 and 300°C. Part 2.-X-Ray Diffraction and Scanning Electron Microscopy Allen, Geoffrey C., Tempest, Paul A. and Tyler, Jonathan W. 8/01 7 19K/F 1P 8/02239I/FlPCumulative Author Index 1988 Abdel-Kader, M.H., 2241 Abe, H., 511 Abraham, M. H., 175, 865, 1985 Abraham, R. J., 1911 Adachi, H., 1091 Ahluwalia, J. C., 2651 Aicart, E., 1603 Al-Haidary, Y. K., 3027, 3043 Allen, G. C., 165, 355 Amorelli, A,, 1723 Anazawa, I., 275 Anderson, S. L. T., 1897 Anpo, M., 751, 2771 Antonini, A. C. R., 1889 Aoi, H., 2421 Aoyama, T., 2209 Aracil, J., 539 Archer, G. P., 2499 Arora, K. S., 1729 Asakura, K., 1329, 2445, 2457 Auroux, A., 3169 Aveyard, R., 675 Ayyoob, M., 2377 Baba, K., 459 Back, D. M., 2585 Bagchi, S., 1501 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Barone, G., 1919 Barouch, E., 3093 Basumallick, I. N., 2697 Baulch, D. L., 1575 Bazsa, G., 215, 229 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Berry, F.J., 2783 Bertoldi, M., 1405 Beyer, H. K., 1447 Bhat, R., 2651 Binks, B. P., 675 Birch, G. G., 2635 Blandamer, A. H., 1889 Blandamer, M. J., 1243, 1889, 2703, 2906 Blesa, M. A., 9 Blinov, N. N., 1075 Bloor, D. M., 2087 Bonnefoy, J., 941 BorbCly, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Borowko, M., 1961 Bourdillon, C., 941 Brandreth, B. J., 1741 Breen, J., 293 Briggs, B., 1243, 2703 Brown, M. E., 57, 1349 Brown, P., 3059 Bruce, J. M., 2855 Brustolon, M., 2875 Brydson, R., 617, 631 Biilow, M., 2247, 3001 Burgess, J., 1243, 1889, 2703 Burget, U., 885 Burnell, E. E., 3129 Busca, G., 237, 1405, 1423 Buxton, G. V., 1101, 1113 Caballero, A., 2369 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone, A. I., 207 Caro, J., 2347 Carr, N. J., 1357 Castronuovo, G., 1919 Cavani, F., 237 Cavasino, F.P., 207 Celik, F., 2305 Centi, G., 237 Cesaro, A., 2573 Chagas, A. P., 1065 Chandra, H., 609 Chatterjee, J. P., 2697 Che, M., 751, 2771 Cheek, P. J., 1927 Cheng, V. K. W., 899 Chien, J. C. W., 1123 Chinchen, G. C., 2135 Chirico, G., 979 Christensen, P. A., 2795 Chudek, J. A., 1145, 1737 Clarke, J. K. A., 251 1 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coles, B. A., 2357 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483, 2013, 2057, 2155, 2357 Contarini, S., 2335 Cook, A., 1691 Corma, A., 3113 Costas, M., 1603 Covington, A. K., 1393 Crowther, N. J., 1211 Dadok, J., 2595 Daldrup, N., 2553 Danil de Namor, A. F., 255, 1671, 2441 Das, S., 1057 Dash, A. C., 75, 2387 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J.G., 41, 713 Day, M. J., 2013 de Bleijser, J., 293 Delben, F., 2573 Del Vecchio, P., 1919 Derylo-Marczewska, A., 295 1 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 51 1 Dong, S., 2979 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dupliitre, G., 2831 Dyster, S., 1113 Eagland, D., 1211 Eaton, G., 2181 Egawa, C., 321 Einfeldt, J., 931 Ekechukwu, A. D., 1871 Eley, D. D., 2069 Elia, V., 1919 El Jamal, M. M., 3169 Elliot, A. J., 1101 Elvidge, D., 2703 Engel, W., 617, 631 Eriksson, P-O., 3129 Eszterle, M., 575 Evans, J. C., 1723 Everett, D. H., 1455 Eyears, J. M., 1437, 3097 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Fiedler, K., 3001 Finter, C. K., 2735 Flanagan, T. B., 459 Fletcher, P.D. I., 1131 Foresti, E., 237 Foresti, M. L., 97 Forissier, M., 3169 Fornts, V., 31 13 Forni, L., 2397, 2477 Forster, H., 491 Foster, R., 1145, 1737 Fraenkel, D., 1817, 1835 Franklin, K. R., 687, 2755 Franks, F., 2595 Fubini, B., 1405 Fujihira, M., 2667AUTHOR INDEX Fujii, K., 3121 Funabiki, T., 2987 Furedi-Milhofer, H., 1301 Gal, D., 1075 Gabrail, S., 41 Gaffney, S. H., 2545 Galwey, A. K., 57, 729, 1349, Gans, P., 657 Gardner, P. J., 1879 Garrone, E., 2843 Geblewicz, G., 561 Geertsen, S., 1101 Georges, V., 1531 Giamello, E., 1405 Gilbert, R. G., 3107 Gill, D. S., 1729 Gill, J. B., 657 Gilot, B., 801 Girault, H. H., 2147 Giuliacci, M. E., 2311 Goldfarb, D., 2335 Gopalakrishnan, R., 365 Grabielle- Madelmont, C., 2609 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A.C . , 1509 Gray, P., 993 Green, P., 2109 Green, S. I. E., 41 Green, W. A., 2109 Grepstad, J. K., 1863 Griffiths, J. F., 1575 Grigera, J. R., 2603 Grigo, M., 931 Grimson, M. J., 1563 Gritzner, G., 1047 Grzybkowski, W., 155 1 Guardado, P., 1243, 2703 Guarini, G. G. T., 331 Guarino, G., 2279 Guglielminotti, E., 2195 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Guyan, P. M., 2855 Hadjiivanov, K., 37 Hakin, A. W., 1889, 2703 Hall, D. G., 773, 2087, 2215, 2227, 3059 Hall, N. D., 1889 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Handreck, G. P., 1847 Hanson, G. R., 1475 Harrer, W., 366 Harriman, A., 2109, 2795, 2821 Hasebe, T., 187 Hashimoto, K., 87 Haslam, E., 2545 Hatayama, F., 2465 Hayashi, K., 2209 Hazra, D. K., 1057 Heatley, F., 343 Hegarty, B. F., 251 1 1357 Hegde, M.S., 2377 Herley, P. J., 729 Herrmann, J-M., 261 Hertz, H. G., 2735 Hey, M. J., 2069 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Hoare, I. C . , 3071 Holzwarth, J. F., 2807 Homer, J., 2959 Hoshino, K., 2667 House, W. A., 2723 Howson, M. R., 2723 Hubbard, C. D., 1243, 2703 Hudson, B. D., 1911 Huis, D., 293 Hunter, R., 1311 Hurst, H. J., 3071 Hutchings, G. J., 131 1 Ichikawa, K., 3015 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Imamura, H., 765 Imanaka, T., 851, 2173 Inoue, A., 1195 Irinyi, G., 1075 Ishiguro, S., 2409 Ishikawa, T., 1941 Isobe, T., 1199 Ito, D., 1375 Ittah, B., 1835 Iwamoto, E., 1679 Iwasawa, Y., 321, 1329, 2445, Iyer, R. M., 2047 Jackson, S. D., 1741 Jaeger, N. I., 1751 Jaenicke, W., 366 Jaroniec, M., 2951 Jeminet, G., 951 Jens, K-J., 1863 Jin, T., 3015 Johnson, G.R. A., 501 Johnson, I., 551 Johnston, C., 309, 2001 Jonasson, R. G., 231 I Jones, A. R., 2914 Jonson, B., 1897 Jorge, R. A., 1065 Jorgensen, N., 309, 2001 Jozwiak, M., 2077 Juillard, J., 951, 959, 969, 1713, Kaizu, Y., 1517 Kakei, K., 1795 Kane, H., 851 Kaneko, K., 1795 Kanno, T., 281, 2099 Kasahara, S., 765 Kato, C . , 2677 Kato, S., 151 Katz, N. E., 9 Kawasaki, Y., 1083 2457 3175 ( 4 Kay, R. L., 2595 Keeble, D. J., 609 Keller, A., 2904 Kemp, T. J., 2027 Kermode, M. W., 1911 Kevan, L., 467, 2335 Kimura, T., 2099 Kinnaird, S., 2135 Kirby, C., 355 Kiricsi, I., 491 Kishore, N., 2651 Kiss, I., 367 Kiwi, J., 1703 Klinowski, J., 2902 Klinszporn, L., 155 1 Klissurski, D., 37 Kobayashi, A., 1795 Kobayashi, H., 1517 Kobayashi, M., 281, 2099 KoEiiik, M., 2247 KoEiFk, M., 3001 Koda, S., 1267 KodejS, Z., 2885 Koksal, F., 2305 Komatsu, H., 2537 Kondo, J., 511 Kondo, M., 2771 Kondo, S., 1941 Kondo, Y., 1 11 Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Kosugi, N., 1795 Kowalak, S., 2035 Kraehenbuehl, F., 1973 Krausz, E., 827 Krebs, P., 2241 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa, Y ., 751, 2129, 2771 Kumamaru, T., 1679 Kurimura, Y., 841, 1025 Kuroda, H., 1329, 1795 Kuroda, K., 2677 Kuroda, Y., 2421 Kusabayashi, S., 11 1 Kuwabata, S., 1587, 2317 Lahy, N., 1475 Laing, M. E., 2013 Lajtar, L., 19 Lambi, J. N., 1 Lancaster, N. M., 3141, 3159 Land, E. J., 2855 Larsson, R., 1897 Laubry, P., 969, 3175 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., 1181 Leaist, D. G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I., 229 Levine, H., 2619 Levy, A., 1817 Levy, M., 1835AUTHOR INDEX Lewis, T.J., 1531 Leyendekkers, J. V., 397, 1653 Leyte, J. C., 293 Lhermet, C., 2567 Lilley, T. H., 1927, 2545 Lincoln, S. F., 365 Lindblom, G., 3129 Lindner, Th., 631 Lips, A., 1223 Llewellyn, J. P., 153 1 Logan, S. R., 1259 Louis, C., 2771 Lu, Z., 2979 Lycourghiotis, A., 1593 Machida, K., 2537 MacKay, R. L., 1145, 1737 Mackley, M., 2910 Maezawa, A., 851 Malanga, C., 97 Malet, P., 2369 Mandel, M., 2483 Maniero, A. L., 2875 Marcandalli, B., 2807 Marcus, Y., 175, 1465 Marczewski, A. W., 2951 MarkoviC, M., 1301 Maroto, A. J. G., 9 Marsden, A., 2519 Martin, R. R., 231 1 Martinet, P., 3175 Martins, L. J. A., 2027 Maruya, K., 51 1 Mason, D., 473, 483, 2057 Mathlouthi, M., 2641 Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Matteoli, E., 1985 Maxwell, I.A., 3107 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mehta, G., 2297 Mensch, C. T. J., 65 Merkin, J. H., 993 Meunier, F., 1973 Meyerstein, D., 2933 Mills, A., 379, 1691 Mines, J. R., 1911 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miura, K., 2421 Miyagawa, S., 2129 Miyajima, K., 2537 Miyakawa, K., 1517 Miyanaga, T., 2173 Miyata, H., 2129, 2465, 2677, Mohamed, M. A-A., 57, 729, Mohammadi, M. S., 2959 Moiroux, J., 941 3121 1349 Moller, K., 1751 Morel, J-P., 2567 Morel-Desrosiers, N., 2567 Morimoto, T., 2421 Morris, J. J., 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mosseri, S., 2821 Mousset, G., 969, 3175 Muhler, M., 631 Mukai, T., 2465 Mukherjee, T., 2855 Murray, A., 2783 Murray, B.S., 871 Nagao, M., 1277 Nahor, G. S., 2821 Nakagaki, M., 2537 Nakagawa, Y., 2129 Nakamura, T., 1287 Nakamura, Y., 11 1 Nakao, N., 665 Nakayama, N., 665 Nandan, D., 2047 Napper, D. H., 3107 Narayanan, S., 521 Nazhat, N. B., 501 Neta, P., 2109 Newman, K. E., 1387, 1393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nisi, M., 2279 Nomura, H., 151, 1267 Norris, J. 0. W., 441 Northing, R. J., 2013 Noszticzius, Z., 575 Nucci, L., 97 Ohno, T., 2465 Ohshima, K., 1639 Ohtaki, H., 2409 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, K., 2317 Okamoto, Y., 851 Okubo, T., 703, 1163, 1171, Oliva, C., 2397, 2477 Oliver, S. W., 1475 Ollivon, M., 2609 Olofsson, G., 551 Ommen, J.G. van, 1491 Onishi, T., 51 1 Ono, T., 2465, 3121 Ono, Y., 1091 Oosawa, Y., 197 Overbeek, J. Th. G., 3079 Ozeki, S . , 1795 Ozutsumi, K., 2409 Page, F. M., 1145 Painter, D. M., 773, 2087 Pal, M., 1501 Pan, C.-f., 1341 Pandey, J. D., 1853 1949 (vii) Pandey, P. C., 2259 Pang, P., 1879 Paoletti, S., 2573 Pappin, A. J., 1575 Parrott, D., 1131 Passelaigue, E., 1713 Patil, K., 2297 Patterson, D., 1603 Pedatsur, N., 2821 Pelizzetti, E., 261 Pena-Nuiiez, A. S., 2181 Penar, J., 739 Penman, J. I., 2013 Perutz, R. N., 2901 Pethybridge, A. D., 2723 Pezzatini, G., 367 Pfeifer, H., 2347 Piccini, S., 331 Pichat, P., 261 Pickering, I. J., 2795 Pickl, W., 1311 Piekarski, H., 529, 591 Pilarczyk, M., 1551 Pilbrow, J. R., 1475 Pilkington, M. B. G., 2155 Plath, P.J., 1751 Pointud, Y., 959, 1713 Polavarapu, P. L., 2585 Pota, G., 215 Pradhan, J., 2387 Preston, K. F., 413 Price, W. E., 2431 Prior, D. V., 865 Pushpa, K. K., 2047 Quinquenet, S., 2609 Quist, P-O., 1033 Radulovic, S., 1243, 2703 Rai, R. D., 1853 Rajam, S., 1349 Rajaram, R. R., 391 Rao, B. G., 1773, 1779 Rao, K. J., 1773, 1779 Rao, K. M., 2195 Rayment, T., 2915 Rebenstorf, B., 1897 Rebuscini, C., 2397 Rees, L. V. C., 2911 Reller, A., 2327 Renuncio, J. A. R., 539 Rhodes, C. J., 1187 Richardson, N. V., 2909 Richardson, S. M., 2909 Richoux, M-C., 2109 Riis, T., 1863 Riva, A., 1423 Robson, B., 2519 Rochester, C. H., 309, 2001 Rojas, F., 785, 801, 1455 Rooney, J. J., 251 1 Ross, J. R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Sacchetto, G.A., 2885 Saito, M., 1025Saito, Y., 275 Saji, T., 2667 Sakaiya, H., 1941 Sakamoto, Y., 459 Sakata, Y., 51 1 Saleh, J. M., 3027, 3043 Salvagno, S., 1531 Sanz, J., 31 13 Sarkany, A., 2267 Sartorio, R., 2279 Sato, T., 275 Sauer, H., 617 Savile, G., 2907 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Scarano, D., 2327 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schonert, H., 2553 Schulz, R. A., 865 Schwarz, H. A., 2933 Schwarz, W., 1703 Scott, S. K., 993, 2904, 2908 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1 199 Senoda, Y., 1091 Sermon, P. A., 391 Serpelloni, M., 2609 Serpone, N., 261 Seuvre, A-M., 2641 Shamil, S., 2635 Sheppard, N., 2913 Shindo, H., 433 Shukla, A.K., 1853 Shukla, R. K., 1853 Sidahmed, I. M., 1153 Simmons, R. F., 1871 Sinclair, G. R., 1475 Singh, B., 1729 Singh, P. P., 1807 s’Jacob, K. J., 1509 Slade, L., 2619 Smith, E. R., 899 Smith, T. D., 1475, 1847 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Speight, J. M., 2069 Spoto, G., 2195 Stainsby, G., 871 Stange, G., 2807 Stead, K., 2905 Stearn, G. M., 2155, 2357 Steel, A. T., 2783 AUTHOR INDEX Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stocker, M., 1863 Stoeckli, F., 1973 Stone, F. S., 2843 Stone, W. E. E., 117 Stramel, R. D., 1287 Struve, P., 2247, 3001 Stuart, W. I., 3071 Subba Rao, M., 1703 Sudol, E. D., 3107 Suga, K., 2667 Sugahara, Y., 2677 Sun, L-M., 1973 Suzuki, T., 1795 Swallow, A. J., 2855 Sykes, A. F., 1575 Symons, M. C. R., 609, 1181, 1187, 2181, 2499 Szamosi, J., 917 Taga, K., 1639 Taga, T., 2537 Tagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takaishi, T., 2967 Takato, K., 841 Takisawa, N., 2087, 3059 Takriti, S., 2831 Tamaki, J., 2173 Tanaka, F., 1083 Tanaka, K., 601, 2895 Tanaka, K-i., 601 Tanaka, T., 2987 Taniewska-Osinska, S., 2077 Tardajos, G., 1603 Taylor, D.M., 1531 Taylor, P. J., 865 Tazaki, K., 231 1 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631, 2795, Thompson, J. S., 2519 Tiddy, G. J. T., 2900, 3129 Tissier, C., 951, 969, 3175 Tissier, M., 3175 Tofield, €3. C., 441 Torres-Sanchez, R-M . , 1 17 Townsend, R. P., 687, 2755 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tschirch, G., 2247 Tsuchitani, R., 2987 Tsuchiya, S., 765 Tsukamoto, K., 1639 Tummalapalli, C.M., 2585 2915 Turner, D. J., 2683 Twiselton, D. R., 1145 Uematsu, R., 111 Uma, K., 521 Unwin, P. R., 473, 483, 2057 Vaccari, A., 1405, 1423 van Rensburg, L. J., 131 1 van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., 647 Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Waller, A. M., 2013, 2357 Wang, E., 2289 Ward, J., 713 Ward, T. R., 2545 Webb, G., 2135 Wells, C. F., 815, 1153 Welsh, M. R., 1259 White, T. J., 3071 Wijmenga, S. S., 2483 Williams, B. G., 617, 631 Williams, C., 2915 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wong, J., 1773, 1779 Wood, N. D., 11 13 Wormald, C.J., 1437, 2912, 3097, 3141, 3159 Wurzburger, S., 2279 Wyn-Jones, E., 773, 2087, 3059 Yamada, M., 2457 Yamada, Y., 751 Yamamoto, Y., 2209 Yamane, T., 2173 Yamasaki, S., 1679 Yamashita, H., 2987 Yamashita, S., 1083 Yao, S., 1375 Yasugi, E., 2421 Yoffe, A. D., 2899 Yoneyama, H., 1587, 2317 Yoshida, S., 87, 2987 Yuqing, L., 2289 Zecchina, A., 751, 2195, 2327, Zeitler, E., 617, 631 Zelano, V., 29 Zibrowius, B., 2347 Zielinski, R., 151 Zundel, G., 885 2843 (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SY MPOSl U M Orientation and Polarization Effects in Reactive Collisions To be held at the Physikrentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S. Stoke Professor R.A. Levine Dr K. Burnett Professor R.N. Dixon Professor J.P. Simons Dr H. Loesch The Symposium will focus on the study of vector properties in reaction dynamics and photodissoci- ation rather than the more traditional scalar quantities such as energy disposal, integral cross-sec- tions and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interac- tions. The Symposium will provide an impetus to the development of 3-D theories of reaction dyna- mics and assess the quality and scope of the experiments that are providing this impetus. The following areas will be covered: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated ‘collisions’ in van der Waals complexes (D) Polarisation of the products of full- and half-collisional complexes. The preliminary programme 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. 87 Cata I ys is by We1 I Character ised Mate r ia I s University of Liverpool, 11-13 April 1989 Organising Committee: Professor R. W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone Dr K. C. Waugh Professor P. 6. Wells The understanding of heterogeneous catalysis is an important academic activity, which complements industry’s continuing search for novel and more efficient catalytic processes. The emergence of rele- vant, in particular in sifu techniques and new developments of well established experimental ap- proaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships.Well characterised cata- lysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and ap- propriate industrial catalysts. The elucidation of structure/function relationships and catalytic mech- anism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisa- tion techniques will also be included. The preliminary programme may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN.~ ~ ~~~~ FARADAY DIVISION INFORMAL AND GROUP MEETINGS Division jointly with Dalton Division inorganic Solids and thdr Surfaces (including the Nyholm Lecture by R. Hoffmann) To be held at the Scientific !%cleties’ Lecture Theatre, London on 22 Nowmber 1988 Further information from Mrs Y A. Fish, The Royal Society of Chemistry, Burlington House, London W1 V OBN Polymer Physics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Sodety, London on 30 November 1988 Further information from Dr M. J.Riirdson, Division of Materials, National Physical Laboratory, Queens Road, Ted&ngton, Middlesex Tw11 OLW Polar Solids Group with the Applied solid State Chemistry Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies’ Lecture Theatre, London on 2 December 1988 Further information from Dr A.E. Comyns, R & D Department, Laporte lndusties Ltd., Moorfield Road, Wdnes WA8 OQJ Theoretical Chemistry Group Beyond the Born-Oppenheimer Approximation To be held at Trent Pdyaechnic, Nottingham on 14 December 1988 Further information from Dr R. G. Woolley, Department of Physical Sciences, Trent Polytechnic, Clifton Lane, Nottingham NG118NS ~~ ~ ~ _ _ _ ~ ~ Electrochemistry Group New ideas In Electrochemistry To be held at the University of Cambndge on 15-16 December 1988 Furlher information from Dr S.P. Tyfiekl, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies’ Ledurn Theatre, London on 16 December 1988 Further information from Dr R. Buscall, ICI plc, Corporate Colloid Science Group, PO Box 11, The Heath, Runmm, Cheshire WA7 4QE High Resolution Spsctroswpy Group High Resolution Molecular Spectroscopy To be held at the University of Birmingham on 19-20 December 1988 Further information from Dr M. N. R. Ashfold, School of Chemistry, University of Bristol, Cantock‘s Close, Bnstol BS8 1TS Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S.Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX Electrochemistry Group with the Electrotechnology Group of the SCI Battery Workshop To be held at the Universtty of Oxford on 3-4 January 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB ~ Electrochemistry Group with the Organic Reaction Mechanisms Group Electron Transfer Reactions To be held in London on 5 January 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GLl3 9PBGas Kinetics Group Reactions of Ions and Free Radicals To be held at the University of Warwick on 6 January 1989 Further information from Professor R.G. Donovan, Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ Neutron Scattering Group Neutron and X-ray Scattering: Complementary Techniques To be held at the University of Kent at Canterbury on 2930 March 1989 Further information from Dr R. J. Newport, Physics Laboratory, University of Kent, Canterbury CT2 7NR Division jointly with the Colloid and Interface Science Group Annual Congress: Surfactant Interactions in Colloidal Systems To be held at the University of Hull on 4-7 April 1989 Further information from Dr J. F. Gibson, The Royal Society of Chemistry, Burlington House, London W1 V OBN Electrochemistry Group Spring Informal Meeting To be held at the University of Warwick on l a 1 2 April 1989 Further information from Dr S. P.Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB ~~~~ ~ Electrochemistry Group with the Electroanalytical Group Electroanalytical Biennial Meeting To be held at Loughborough University of Technology on 12-14 April 1989 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nudear Laboratories, Berkeley, Gloucestershire GL13 9PB lndustrial Physical Chemistry Group with the Thin Films and Surfaces Group of the IOP Materials for Non-linear and Electro-optics To be held at Girton College, Cambridge on 4-7 July 1989 Further information from The Meetings officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Polymer Physics Group Biennial Meeting To be held at the University of Reading on 13-1 5 September 1989 Further information from Dr M.J. Richardson, Division of Materials, National Physical Laboratory, Queens Road, Teddington, Middlesex lW11 OLW Division with the lnstitute of Physics Sensors and their Applications To be held at the University of Kent at Canterbury on 19-22 September 1989 Further information from The Meetings Officer, Institute of Physics, 47 Belgrave Square, London SWlX 8QX Division with the Deutsche Bunsen Gesellschaft, Division de Chimie Physique of the Socidte Franpise de Chimie and Associazione ltaliana di Chimica Fisica Transport Processes in Fluids and Mobile Phases To be held at the Physikalische Institiit, 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, Loughborough University of Technology, Loughborough L E l l 3TUJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistry or chemical physics which appear currently in J. Chem. Research, The Royal Society of Chemistry’s synopsis + microform purnal, include the following: Evaluation of the Broyden-FIetcherGoIdfarb-Shanno (BFGS) Variable Metric Method in Geometry Optimisation using Semi-empiriial SCF-MO Procedures Rzepa (1 988, Issue 3) 1. Brookes, Charles Kemball and H. Frank Leach (1988, Issue 4) Lars Carlsen and Helge Egsgaard (1 988, Issue 4) Podmore and Martyn C. R. Symons (1 988, issue 4) Dimitris K. Agrafiotis and Henry S. The Measurement of Exchangeable Hydrogen associated with Titanium Dioxide (Rutile) Beverley The Reaction between lmidogen and Elemental Carbon. An Akernative Route to Jnterstellar HCN ? Radical Cations of N,N-Dimethyluracil and N,N-Dimethylthymine Christopher J. Rhodes, Ian D. Inhibition or Acceleration of the lsomerisation of But-1 -en8 on Titanium Dioxide (Rutile) by Adsorbed Molecules Beverley 1. Brookes, Charles Kemball and H. Frank Leach (1 988, Issue 5) Methods Henry S. Rzepa (1 988, Issue 7) Cheletropic Elimination of CO and N2. A Comparison of the MNDO, AM1 and ab init# SCF-MO The Cyclopropenyl Anion: an ab inifio Molecular Orbital Study Wai-Kee Li (1 988, Issue 7) The Influence of Environmental Effects on the Geometry of Molecules in Crystals SIawomir J. Grabowski (1 988, Issue 8) (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP125

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. 1, 1988, 84(9), 2915-2931 Monitoring Cation-site Occupancy of Nickel-exchanged Zeolite Y Catalysts by High-temperature in situ X-Ray Powder Diffractometry John M. Thomas* and Carol Williams? Davy Faraday Research Laboratory, The Royal Institution, 21 Albemarle Street, London W1 X 4BS and Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 I EP Trevor Rayment Department of Physical Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EP Procedures based on the Rietveld method of refinement of X-ray powder profiles have been used to determine the sites and occupancies of extra- framework cationic nickel, potassium and sodium in typical zeolitic catalysts at temperatures in the range 400-500 "C.The siting of Ni2+ and other ions at high temperature differs from that at room temperature previously reported. There are also significant differences in the location of cations and in the reducibility of Ni2+ which depend upon the nature of the treatment (e.g. steam treatment, treatment with an alkaline solution) to which the as- prepared zeolite is subjected prior to dehydration and/or reduction. Transition-metal exchanged zeolites, both in their unreduced and reduced form, are powerful catalysts for a wide range of organic reactions, including selective oxidation, hydrogenation and isomerization. The precise nature of the catalytic activity, selectivity and stability is, however, critically dependent upon the pretreatment, especially the thermal history to which the metal-exchanged zeolite has been subjected.This, in itself, is not surprising, since there are many distinct crystallographic sites available for occupancy in any given zeolitic framework. Thus in the faujasitic zeolites (X and Y) the various sites are shown in fig. 1. These sites have been identified because room- temperature single-crystal X-ray studies of zeolites A and Y can be carried out. Such studies are not, however, feasible for real catalysts, which are polycrystalline and are used at temperatures % 400°C. As a consequence, there is an acute paucity of information pertaining to the siting of the catalytically active cations, or their reduced analogues, inside the zeolite under actual reaction conditions. The catalytic properties of nickel-containing zeolites have been widely studied in recent years, as they show activity for methanation,lP4 hydrogenation5 and de- In addition, nickel in the cationic form is a potential catalyst for hydrocarbon transformation ;8 on dehydrogenation, cationic nickel interacts with the framework oxygens but can also possess free coordination sites for interaction with reactants.The catalytic properties of these cations will obviously be governed by their location in the intrazeolitic cavities and upon their coordination to the framework. Many studies have focused on the reducibility of nickel zeolites as well as on a number of inter-related factors such as the precise framework structure, nature and location of co-cations, presence of acidic groups and reduction conditions, all of which in greater or lesser measure influence the ease and degree of reduction.'* '-12 Notwithstanding these t Present address : Institute of Catalysis, Academy of Sciences Siberian Branch, Novosibirsk, U.S.S.R.2915 96-22916 X-Ray Powder Profiles of Ni-Exchanged Zeolites Fig. 1. Illustration of the structure of zeolite Y, showing the four types'( 1,2,3,4) of oxygen atoms (a), and the principal cations sites (I, 1', II', 11). Possible location of type 111 sites, associated with the four-membered rings in the supercage, are also shown. Ring (1) : four-membered rings formed by 0(1) and O(2); ring (2): four-membered rings formed by O(3) and O(4). earlier studies, the mechanism of reduction is still not clear, and the influence of different pretreatments on the reducibility and catalytic activity is imperfectly understood.The accessibility of intrazeolitic cationic nickel to reactant molecules and the ease of its reduction clearly depend on its precise location. Exchangeable charge-balancing cations can, in general, be located bonding either (a) to framework oxygen atoms only, or (b) to ligand molecules (e.g. water) only, with no direct contact to the walls of the cages, or (c) coordinated to both framework oxygen atoms and ligand molecules. The number of cations per unit cell is usually less than the number of sites available; so the cations will be distributed among the different sites, depending upon factors such as temperature, degree of hydration and zeolitic structure. The location of extra- framework cations is most directly determined by diffra~ti0n.l~ Nickel cation locations have previously been determined in as-prepared, dehydrated and reduced faujasitic zeolites, using single-crystal X-ray studies14 or integrated intensity studies of powdered ~amp1es.l~ In all cases, data were collected at room temperature after dehydration and/ or reduction at higher temperatures.However, several applications of zeolites, including their use as catalysts, require elevated temperatures, and so it is important to gather information about their structure and cation locations at elevated temperatures. Thus Uytterhoeven et a1.16 studied the influence of temperature on the location of Ca2+ ions in zeolite Y; analysis of X-ray diffraction data collected over a temperature range from room temperature to 800 "C showed increasing occupancy of site I with increasing temperature.In this work we obtain the distributions of cationic nickel in NiY zeolites at temperatures of 400 "C or higher using Rietveld profile refinement of X-ray powder data. The samples were dehydrated in flowing N, and then reduced in flowing H,. In situ studies have enabled us to record diffraction patterns after dehydration and both during and after reductions, and hence to evaluate the cation distribution at temperatures of 400 "C or higher after dehydration and/or reduction. The influence of various pretreatments, such as prior exposure to steam or to alkaline solution, on the ease of reduction and the particle sizes of resulting metallic nickel has been investigated by temperature-programmed reduction (t .p.r.) and transmission electron microscope (TEM) studies.The distribution of cationic nickel in these pretreated samples has been determined by X-ray diffraction and found to differ significantly from the distribution in as-prepared samples. Zeolite Y is built up of so-called sodalite or ,&cages that are linked to adjacent onesJ . M. Thomas, C. Williams and T. Rayment Table 1. Principal cation sites in zeolite Y site no. per nomenclature unit cell environment of cation I 16 I' 32 11' I1 nside the double-six ring (D6R) with octahedral coordination to six framework oxygens nside the sodalite cage on a six-membered ring of oxygen ions, adjacent to site I. The cations bond with three of the oxygens in the six-membered ring also inside the sodalite cage, on a six-membered ring between the sodalite cage and the supercage in the supercage, bonding to three of the oxygens in a six- membered ring between a sodalite cage and a supercage 32 32 2917 via hexagonal prisms (or doublc-six rings, D6R) (see fig. 1).The supercages possess an intern!l diameter of ca. 12.5 A, which is accessible through four 12-membered rings (,ca. 7.4 A in diameter). The principal sites for the extra-framework cations are designated17 sites I, 1', 11' and I1 (fig. 1 and table 1). A further site, 111 or 111', is situated in the vicinity of the four-membered rings that form the walls of the supercages.16 For a specific cation-exchanged faujasitic zeolite, the precise positions of cations may differ from the idealised positions shown in fig.1. Experimental Sample Preparations The starting material was a Na+-exchanged zeolite Y which had a %/A1 ratio of 2.5. The K+- and NHi-exchanged forms were prepared by ion exchange with 2 moldm-3 solutions of the respective chlorides at 80 "C, with the exchange procedure repeated three times. Ni2+-containing samples (NiNaY and NiKY) were prepared by ion exchange with 0.005 mol dm-3 NiCl, solution and 0.5 g zeolite per dm3 solution so as to minimize the degree of hydrolysis during the exchange procedure." Portions of the normal NiY samples were then modified either by heating in a steam environment at 400 "C for 5 h (NiNaY, steam treated) or by stirring in a 0.1 mol dmP3 potassium hydroxide solution for 3 h at 60 "C (NiKY, KOH-treated).Temperature-programmed Reductions (T.P.R.) A t.p.r. system, kindly made available by Drs C. J. Adams and C. Marsden, Unilever Research Laboratory, Port Sunlight, was set up by modifying a Quantachrome sorption system. The sample (ca. 150 mg) was subjected to a linear heating rate (10 K min-l) in a stream of N, containing 5% H,. The rate of reduction was monitored by measuring the change in concentration of H, in the gas stream with a thermal conductivity detector. The area under the t.p.r. profile is proportional to the amount of H, consumed during the reduction process. Samples could be subjected to t.p.r. either with no pretreatment,2918 X-Ray Powder Profiles of Ni-Exchanged Zeolites (ii) (iii) 1 1 1 1 1 1 1 , 300 600 900 TIo C Fig. 2. Temperature-programmed reduction (t.p.r.) profiles for various nickel-containing zeolite Y samples, which were heated in a mixture of H, and N, at 10 K min-' either with no pretreatment, i.e.hydrated (h), or after dehydration in a flow of N, gas at 450 "C (d): (i) as prepared, normal NiNaY (h); (ii) steam-treated NiNaY (h); (iii) as prepared, normal NiKY (d); (iv) KOH-treated NiKY (h); (v) NiHY (from NiNH,Y) (d). Table 2. Temperature-programmed reduction results for NiNaY samples ~ ~ ~~ no. of Ni2+ cations reduced per unit cell low- high- cations reduced (from chemical total Ni2+ content per unit cell total number of sample temperature temperature per unit cell analysis) NiNaY 3.9 15 NiNaY 10.5 7.5 (as prepared) (steam- treated) 17.5 i.e. hydrated, or after overnight dehydration in a flow of N, at 400-500 "C, or after heating in a closed environment of their own water of hydration.The respective labels h, d and s are used to denote the treatment prior to t.p.r. Transmission Electron Microscopy Measurements Particle size distributions of metallic nickel in the reduced zeolite samples were investigated using a JEOL 200 CX transmission electron microscope. Samples wereJ. M. Thomas, C. Williams and T. Rayment 2919 Table 3. Average particle sizes for metallic nickel particles (as determined by transmisson electron microscope), after temperature-programmed reduction up to 1000 "C sample particle size range/A NiKY 400-600 NiKY, KOH- treated 100-350 NiKY, steam-treated 700-800 NiKX 100-200 and 300-400 NiKL 400-700 2et0 Fig. 3. X-Ray diffraction patterns for dehydrated NiNaY samples : (a) normal NiNaY (Y450) ; (b) steam-treated NiNaY after dehydration at 400 "C (YS400); (c) steam-treated NiNaY after dehydration at 400 and 600 "C (YS600).Data were collected at 450,400 and 400 "C, respectively. prepared by grinding the zeolite powder in acetone followed by ultrasonification to disperse the particles. Drops of the resulting suspension were placed on a grid which was then suspended on a microscope probe. X-Ray Diffraction X.r.d. data were collected on a Philips vertical X-ray diffractometer (model PW 1050) fitted with a high-temperature attachment (Anton Parr HTKl 0). The powdered sample rested on a platinum bar (10 cm long) which was heated resistively. A Pt/Pt-10% Rh thermocouple welded under the central region of the bar yielded the temperature, which could be controlled to better than f 1 "C at the thermocouple.Since the temperature gradient across the length of the bar is approximately parabolic, only the central portion of the bar was used. A thin covering of powdered sample was sieved onto the bar2920 X-Ray Powder ProJiles of Ni-Exchanged Zeolites 10 L 2 e p peaks from platinum bar / Fig. 4. Diffraction patterns recorded during isothermal reduction of dehydrated NiNaY at 450 "C showing initially a two-phase mixture of reduced and unreduced zeolite regions, which is gradually converted to a single-phase reduced sample: (a) 3, (b) 6, (c) 12, ( d ) 19 and (e) 23 h. E ." 531 28 I" Fig. 5. Fast scans (2 s per point) for NiNaY in the range 16-36 O (269, showing more clearly the presence of two phases during reduction.J .M . Thomas, C. Williams and T. Rayment 292 1 Fig. 6. Diffraction patterns recorded during dehydration and reduction of steam-treated NiNaY : (a) after overnight dehydration in N, at 400 "C; (b) after reduction in H, at 400 "C for 12 h; (c) after prolonged reduction at 400 and 500 "C. All data recorded at 400 "C. through a fine wire mesh to reduce preferred orientation of the crystallites. A steady flow of dry gas (N, or H,, 2 dm3 h-l) passed over the sample. Cu Ka radiation was used, with 1 " collimating and receiving slits and a graphite diffracted beam monochromator. Diffraction patterns were recorded at temperatures of 400 "C (or higher) after dehydration, or during and after reduction, with a step of 0.02 (28) and a counting time of 10s per point for long scans and with 0.025 (28) and 2 s per point for faster scans.Results Temperature-programmed Reduction and Transmission Electron Microscopy Typical t.p.r. profiles are presented in fig. 2 for nickel-containing zeolite Y. They show the influence of pretreatment, co-cation and zeolite structure on the ease of reduction. From the peak areas which yield the amount of H consumed during the reduction process, the number of Ni2+ cations converted to Nio during the reduction process represented by that peak can be determined, on the assumption that the stoichiometric reaction is Nizlolite + H, --+ Nio + 2H:e0,ite. Results for normal (as prepared) and steam-treated NiNaY are compared in fig.2(a) and (b) and in table 2. Samples subjected to t.p.r. up to 1000 "C were cooled to room temperature in a flow of reducing gas, exposed to the atmosphere and transferred to the electron microscope2922 20000 $ 15000 s 10000 5000 X-Ray Powder ProJiles of Ni- Exchanged Zeolites 1 1 1 i - I . . I I + - * ' - . - - ' - . * . ' to study particle size distributions of the metallic nickel crystallites. Results are summarized in table 3 for some zeolite samples (all with K+ as the co-cation). They show the influence of pretreatment and zeolite structure on the sintering behaviour of the metallic particles. X-Ray Powder Diffraction High-temperature X-ray diffractograms were collected both for the as-prepared NiY samples (with Na+, K+ or H+ as the co-cation) and for modified samples (steam-treated NiNaY and KOH-treated NiKY) after dehydration in a flow of N,.Significant differences were observed in the relative intensities of the diffraction peaks of as- prepared and modified samples with the same co-cation, suggesting that the cation distribution of the nickel in the zeolite channels undergoes changes (see fig. 3 for the as- prepared and steam-treated NiNaY samples). In situ reduction studies (both isothermal and temperature-programmed) were carried out on these various samples. Examples of diffraction patterns collected during the reduction process are shown in fig. 4-6. Structural refinement of the X-ray diffraction data was carried out using the Rietveld powder profile refinement method as described el~ewhere.l~-~~ Briefly, the DBW29 program of Wiles and Young23 was used, with the modified Lorentzian-2 peak-shape function.Backgrounds were estimated by linear interpolation between points in regions between the peaks, and peaks due to the platinum support and metallic nickel (in the case of reduced samples) were excluded from the refinement. All data were refined in the space group F d m , with starting framework coordinates and cation positions taken from ref. (14) and (1 5 ) . A typical profile fit obtained from the refinement procedure is shown in fig. 7, while cation occupancies and some bond distances are summarized in tables 4 and 5 . A more detailed discussion of the refinement process for each sample and the full results of the refinement procedure will be presented elsewhere.24J. M . Thomas, C. Williams and T. Rayment 2923 Table 4. Site occupancies and selected bond distances for dehydrated and reduced Ni,,,,H,.,Na,,Y as-prepared as-prepared and reduced steam-treated steam-treated sample Y450 Y450H YS600 YS400H Y (neutrons) I I' 11' I1 a I1 O(5) I 1'-0(3) 1'-0(5) I1 a-0(2) I1 a-O( 5) 11-O(2) T-O( 3) (T = Si or Al) average (T-0) unit cell ./A 1 3Ni2+ - 5.6Ni2+ 8.9Na+ - 2.21 2.34 2.14 2.47 1.73 - - 1.65 24.39 sites 3.9Ni2+ or 6. 2Ni2+ 2.9Ni2+ or 5.6Ni2+ 8.7Na+ 6.4Na+ - - - 7. 5Ni2+ 8.7Na' 9.3Na' 6.9 - bond distances/A 2.57 2.45 2.19 2.82 - 1.75 2.09 2.63 2.34 2.40 1.66 1.66 - - 1.66 1.65 24.74 24.52 2.4Ni2+ or 5.4Na+ 3.8Ni2+ or 8.4Na+ - 10.7Na' 2.55 2.13 - - 2.50 1.68 1.66 24.69 1 4.7Ni2+ 3.1Ni2+ 1 .3Ni2+ 19.3Na+ 6.6 - 2.26 2.67 2.01 2.03 2.27 1.68 - 1.65 24.39 data collection time dehydration reduction step/ per abbreviation sample conditions conditions gas T/"C "(28) point/s Y450 as-prepared 450 "C, N,, 24 h Y450H NiNaY 450 "C, N,, 24 h Y S600 steam-treated 400 "C, N,, 11 h NiNaY +600 "C (6 h) +400 "C (5 h) Y S400H - 400 "C, N,, 10 h Y (neutrons) as-prepared evacuate 300 "C NiNaY (2 h)+250 "C overnight - N, 450 0.025 15 450"C, H,, H, 450 0.02 10 32 h - N, 400 0.02 10 400"C, H,, H, 400 0.02 10 13 h + 500 "C (4 h) - neutron - diffraction evacuated 200 K 0.05 Discussion From the results of refinement we can identify the presence of scattering matter at particular sites in the zeolite channels, but for actual assignment of the scattering matter to Ni2+, Na+ or K+ cations or non-framework oxygen atoms, the following factors must2924 X-Ray Powder ProJles of Ni-Exchanged Zeolites Table 5.Site occupancies and selected bond distances for NiKY KOH- treated sample NiKY NiKY site I 14.4 Ni2+ I' I1 7.4 K+ 111 a 7.9 K,' bond distances/A I-0(3) 2.24 11-O(2) 2.65 I11 a-O( 1) 2.72 I11 a-0(4) 2.68 T-0(3) 1.71 (T-0) 1.65 - 1'-0(3) - uni t-cell a/ A 24.36 p!rameter, 9.0 K+ 9.2 K+ 26.2 K+ - 2.76 2.85 2.76 - - 1.68 1.65 24.79 data collection sample time step/ per dehydration conditions gas T/"C "(28) point/s Nill3Kl8Hll.J 400 "C, N,, 12 h N, 400 0.02 10 KOH-treated NiKY 400 "C, N,, 2 h + 500 "C (NJ, 13 h N, 500 0.02 10 be taken into consideration. (a) The cation-framework oxygen bond distances should be reasonablF values for the particular cation (ionic radii of Ni2+, Na+ and K+ are 0.69,0.97 and 1.33 A, respectively).(b) The number of cations determined from refinement should correspond to that known from chemical analysis. (c) Observing a change in scattering density at a particular site after reduction may help to distinguish nickel from other cations. (d) One should make a comparison with the literature concerning site preferences of various cations. In this way it is possible to identify different cations at various sites and arrive at the cation distributions shown in tables 4 and 5. However, as noted in the tables, we cannot exclude the possibility of some mixed occupancies, e.g. a mixture of Na+ and Ni2+ cations present at the same site. We now proceed to deal separately with the dehydrated and reduced samples. Dehydrated Samples (NiNaY, NiKY and NiNaY, Steam-treated ; NiKY, KOH-treated) NiNaY The results for the as-prepared, dehydrated NiNaY sample (table 4) may be compared with reported single-crystal results14 and integrated intensity powder results,15 where in both cases, data were collected at room temperature. The preference of Ni2+ for site IJ .M . Thomas, C. Williams and T. Rayment 2925 sodalite cage supercage ------ ‘ \ \ .’ b$ Fig. 8. Illustration of the relative positions of sites 11, IIa, 11’ and the centre of the six-membered ring (denoted by 0); sites IIa and I1 are both in the supercage, while site 11’ is in the sodalite cage. and the distortion of the double-six ring to coordinate more effectively tp the Ni2+ cations (as shown by the increase in the T-p(3) bond distance to 1.73 A, and the contraction of the unit-cell parameter to 24.39 A) are in agreement with results found at room temperature.However, the cation distribution over the other sites in the zeolite cavities differs significantly from the room-temperature results. Thus at 450 “C in a flow of N, we have located ca. 50% of the Na+ cations known to be present from chemical analysis at site 11. This compares with location of almost all the Na+ cations at site I1 both at room temperature, by Gallezot et al.,15 and at 200 K, as determined by our neutron diffraction study of the present sample.22 In addition we have located Ni2+ at a site IIa, which is slightly into the supercage, but close to the plane of the six-membered ring between the sodalite and supercage. There is no scattering matter at site 11’.The relative positions of sites I1 and IIa and the centre of the six-membered ring are illustrated in fig. 8. The remaining 50% of Na+ cations must be situated in the supercages, but because of the weak scattering of X-rays by sodium, and because of the large number of available sites in the supercage, these Na+ cations have very little effect on the relative intensities of the zeolite peaks. Thus a good fit is obtained between observed and calculated profiles without inclusion of these cations in the structural model. In order to locate the missing cations, we have studied a NiKY sample (see below). The much stronger X-ray scattering power of K+ compared with Na+ means that K+ ions will have a greater in- fluence than Na+ ions on the relative intensities of zeolite peaks, and on the profile fit. NiKY The results for dehydrated NiKY (table 5 ) also show a high occupancy of site I by Ni2+, distortion of the double-six ring, and again ca.50% of the K+ ions in the sample are situated at site 11. There is no scattering matter at site IIa. The remaining K+ ions (known to be present from chemical analysis) must be situated in the supercages. Single- crystal studies25 of Cu-faujasite have located Cu ions on type I11 sites at the edge of a four-membered ring in the supercage (fig. 1). Inclusion of scattering matter at site IIIa (figure 1 and 9) resulted in a significant drop in the refinement R,, value (refinement2926 X-Ray Powder Profiles of Ni-Exchanged Zeolites Fig.9. Illustrations of the position of the IIIa site in zeolite Y, and a possible path of migration for cations from site I1 + I11 + I1 (i.e. 6-ring + 4-ring -+ 4-ring + 4-ring + 6-ring). Site I = Ni2+; site I1 = K'; site IIIa = K+. weighted profile), and the refined occupancy of sites I1 and IIIa was in good agreement with the potassium content, determined by chemical analysis (table 5). The bond distances from sites I1 and IIIa to framework oxygens are typical values for K+ cations. Similar K occupancies and decreases in R,, values were obtained when the cations were placed initially at site IIIb or IIIc (fig. 1) and constrained to stay on the mirror planes during refinement. If the x, y and z coordinates of a site 111 b cation were allowed to vary independently during the refinement procedure, the cation moves to the IIIa site.Thus for NiY samples at high temperature, the Kf or Na+ ions are equally distributed over site I1 and site 111, unlike the case at room temperature, where Na+ ions are primarily located at site 11. It is likely that the Na+ or K' cations are migrating along the walls of the supercages at these higher temperatures with the path from site I1 + I11 + I1 (i.e. 6-ring + 4-ring + 4-ring + 4-ring + 6-ring) as shown in fig. 9. KOH-treated NiKY From the results of refinement of data for the KOH-treated NiKY sample, the distribution of scattering matter is found to, be different from that determined for NiKY. On the basis of bond distances of 2.7-2.8 A to framework oxygen atoms, the scattering matter at all three sites (I, I' and 11) has been assigne4 to K+ cations.The undistorted double-six ring and large unit-cell parameter of 24.79 A also support the assignment of site I scattering matter to K+ cations. In contrast to the as-prepared NiKY sample, site I1 is now almost fully occupied, with no cations at site 111. This cation distribution is similar to that determined recently for KY at 723 K,26 and to that predicted for KY fror energy calc~lations.~~ Thus in this KOH-treated sample, the nickel is no longer present as normal charge-balancing cations but as an oxy- or hydroxy-precipitate either in the supercages or on the outer surface of the zeolite crystals. No separate nickel phases areJ . M. Thomas, C. Williams and T. Rayment 2927 observed in the X.r.d.pattern, suggesting that the nickel particles are small. A recently published EXAFS study of NiNaY has suggested the presence of Ni-OH-Ni linkages after treatment with NaOH solution, and the formation of oligomeric nickel oxide in the supercage after calcination. 28 Steam- treated NiNaY Comparison of the cation distribution for steam-treated NiNaY with that found for the as-prepared sample (table 4) shows a much lower occupancy of site I by Ni2+; site I occupancy is limited to ca. 6 per unit cell even after prolonged heating at 700 “C in nitrogen. Most of the Ni2+ ions are located in the supercage (IIa) and sodalite cages (1’). Nickel cations at site I’ interact weakly with three framework oxygen [ 0 ( 5 ) , x = y = z = 0.161. This oxygen is unlikely to be adsorbed water, since it is not observed for the as- prepared NiY samples.It must arise from hydrolysis of hydrated nickel ions during the steam treatment with the formation of (NiOH)+ species or (Ni-0-Ni)”+ type clusters in the sodalite cages. Such hydrolysis during normal dehydration has been observed for LaY.29y30 Reduced Samples T.P.R. The t.p.r. studies (fig. 2) show the influence of both pretreatment and zeolite structure on the reducibility of the Ni2+ cations. The observed differences can be explained on the basis of the cation distributions determined for the dehydrated samples. For the as- prepared NiNaY sample, ca. 20% of the Ni2+ cations were reduced in the low- temperature region (table 2). This is in reasonable agreement with the relative number of Ni2+ cations located at site IIa after dehydration (table 4), suggesting that the low- temperature t.p.r.peak corresponds to reduction of site IIa Ni2+ ions, while the high- temperature peak corresponds to reduction of site I and I’ Ni2+ ions. Steam treatment results in a different distribution of Ni2+ ions, with most of the cations at site IIa (supercage) or I’ (sodalite cage). This again suggests an assignment of the low- temperature t.p.r. peak to reduction of supercage (IIa) nickel ions, and the high- temperature peak to the less accessible sodalite and double-six ring sites. The t.p.r. profile of the NiKY-KOH-treated sample is quite different from that observed for normal and steam-treated NiY samples, displaying a rather broad low-temperature reduction profile.Refinement results for this sample (table 5) show that nickel is not present as charge-balancing cations, but probably as a precipitated hydroxide or oxidic species. Reduced NiNaY Samples Cation distributions determined after reduction of NiNaY (both the normal and steam- treated samples, table 4) show a complete loss of scattering at the IIa site, while scattering density at site I1 is hardly changed from that determined for the dehydrated samples. This confirms the assignment of scattering matter at site IIa to Ni2+ ions and that at site I1 to Na+ in the dehydrated samples. Similarly, for the steam-treated NiNaY sample the decrease in scattering density at the I’ site after reduction supports the assignment of I’ scattering matter to Ni2+ ions in the dehydrated sample.During refinement of data for reduced NiNaY samples, all the non-framework scattering matter was initially treated as Na+ cations. However, refinement of site occupancies located a total of ca. 24 Na+ cations per unit cell, while chemical analysis shows only 18 Na+ cations per unit cell. Thus a fraction of the non-framework sites must be occupied by2928 X- Ray Powder ProJiles of Ni- Exchanged Zeolites unreduced nickel cations. Given the preference shown by Ni2+ for site I, we have assigned the scattering matter at site I in the reduced NiNaY samples to unreduced .Ni2+ cations (although there may be some Na+ also present at this site). The scattering matter at site I' may be a mixture of Na+ and Ni2', although the 1'-0(3) distance is more typical of a Ni-0 bond distance than Na-0.On the basis of the factors listed at the beginning of the discussion section, in the reduced NiNaY samples we assign site I1 scattering matter to Na+, site I to unreduced nickel ions and site I' to Na+ ions, with a small fraction of unreduced nickel ions. Reduced NiKY, KOH-treated During reduction of this sample at 500 "C, a gradual growth of a very broad metallic nickel (1 11) peak was observed; however, there was no change in the relative intensities of the diffraction peaks emanating from the zeolite. Even after ramped heating (10 K min-l) to 850 "C in H,, the nickel (1 11) and (200) peaks remain broad, and there is no visible loss of crystallinity of the zeolite. This contrasts with the observation for normal untreated NiKY or NiNaY under similar conditions.In these cases reduction results in significant changes in the zeolite peak intensities (as Ni2+ ions are removed from different sites, owing to reduction to NiO); ramped heating to 850 "C results in intense narrow metallic nickel peaks and partial destruction of the zeolite lattice. Thus treatment with KOH solution has resulted in the formation of non-charge-balancing nickel species (probably small nickel oxide particles) on the outer surface of the crystallites, and hence reduction causes no change in the cation distribution in the extra framework sites, since all the sites are occupied by K+ ions. On reduction, the metal particles formed are much smaller than those produced from untreated NiKY, as seen by X-ray line broadening.Metallic nickel from the KOH-treated sample is much more resistant to sintering at high temperatures, as shown by the TEM studies (table 3). This may arise because for the KOH-treated sample the charge-balancing cations after reduction are still K+ and the zeolite framework retains its crystallinity at high temperatures (e.g. ramped heating to 850 "C). For untreated samples the charge-balancing cations after reduction are Na+ or K+ and H+. The presence of charge balancing protons means that there is partial breakdown of zeolite framework at such high temperatures, and this may enable the metallic nickel particles to grow in size. Mechanism of Reduction Diffraction patterns collected during reduction of as-prepared NiNaY (fig. 4 and 5 ) show the presence of two phases (the reduced and unreduced zeolite) at intermediate stages of the process.Initial fast growth of peaks were observed for the reduced zeolite; these appear as shoulders at the low-angle side of the main peaks. This is followed by a slower, gradual conversion over a period of ca. 30 h to the final single-phase sample. For the steam-treated NiNaY sample there is no evidence in the X.r.d. patterns of such a two-phase reduction process (fig. 6). The cation distribution for these NiNaY samples after dehydration and reduction are summarised in table 4. It is seen that a large proportion of the Ni2+ cations reduced in the normal NiNaY sample is located in site I prior to reduction. For the steam-treated sample most of the Ni2+ cations that are reduced were located in sites I1 a and I' prior to reduction ; very little (if any) Ni2+ cations originally present at site I are reduced in this case.This strongly suggests that the 'two- phase' reduction process (which is observed for normal NiY but nor for steam-treated NiY) arises from the reduction of Ni2+ cations located at site I prior to reduction. If reduction involved a gradual, uniform removal of Ni2+ cations from site I throughout the whole sample, then we would expect to see a gradual shift in the zeolite peak positions to lower 28, as the unit-cell size increased (owing to a lesser degree ofJ. M. Thomas, C. Williams and T. Rayment 2929 distortion in the double-six rings as Ni2+ cations are removed from these sites). In fact, we observe two overlapping diffraction patterns, one from the unreduced sample with peak intensities characteristic of high Ni2+ occupancy of site I, the other for the reduced sample, with peak intensities characteristic of low Ni2+ occupancy of site I.This implies that in some regions of the samples, most or all of the Ni2+ (site I) cations have been reduced, while in other regions most or all of the nickel cations are still located in site I as charge-balancing cations. (This two-phase behaviour was observed for untreated NiNaY samples of different sample length and bed depth, thus excluding any effect from a temperature gradient across the sample.) The reason for the two-phase behaviour is not clear, but it suggests that it is more favourable for the zeolite to have either very high or very low occupancy of site I by Ni2+, but not an intermediate occupancy.The presence of Ni2+ in Sites I causes the double-six ring to distort and become strained, in order that it may coordinate more effectively to the small cations. In the refinement of the finally reduced NiNaY, there are very few Ni2+ cations still in site I, and so the double-six rings have relaxed to their original conformation. A possible explanation for the ' two-phase ' reduction behaviour is that reduction of Ni2+ in site I is difficult, but that once a Ni2+ cation in site I has been reduced, and the double-six ring has relaxed to its original conformation, it may be more favourable for neighbouring double-six rings to relax also, instead of staying in their rather strained conformation owing to interaction with Ni2+ cations.This would be achieved by reduction of these Ni, cations as well. If this is so, then reduction of the first Ni, makes it more favourable for Ni, at neighbouring sites to be reduced, and so on. In this way a region of reduced material could grow rapidly outward from the point of reduction of the first nickel cation. A number of such Ni, cations in different regions of the sample may be reduced and act as nuclei for this outward growth of the reduced region. This would give rise to a mixture of reduced and unreduced regions in the intermediate stages of reduction and a gradual complete conversion to the one-phase reduced material. Initially, the rate of reduction will be fast and increase with time as the reaction front increases.However, once reduction has started, the kinetics of reduction will be complicated by the formation of products, metallic nickel which agglomerates on the surface and OH groups in the zeolite matrix. For the steam-treated NiNaY sample, only three Ni2+ cations are located in site I after dehydration, so only a few double-six rings will be distorted to coordinate to the small cations, and there will be much less chance of neighbouring sites I being occupied. Thus the influence of the relaxation of one double-six ring on other strained ones will be less because occupied I sites are further away from one another on average than they are in normal NiNaY. In fact, profile refinement of the reduced steam-treated NiNiY shows that the occupancy of site I is unchanged after reduction.Since Ni, cations are not involved in the reduction in this sample, we would not expect to see a 'two-phase' reduction process. Conclusions Our results have shown the importance of high-temperature in situ studies for locating nickel cations under reaction conditions and for monitoring the reduction process both by changes in the zeolite diffraction peaks and by appearance and growth of peaks from metallic nickel. In particular, we have shown : (I) The importance of temperature on cation locations in NiY. Thus for untreated, dehydrated NiNaY and NiKY we find the Na+ or K+ cations distributed equally over sites I1 and I11 at high temperatures (400 "C), but almost exclusively in site I1 at low temperature. In addition, some Ni2+ cations have been located at a IIa site, just inside the supercage, where they are easily accessible to reactant molecules.Such site I1 a nickel ions are potentially catalytically active sites.2930 X-Ray Powder Projiles of Ni-Exchanged Zeolites (2) The influence of steam or alkaline solution pretreatments on the state, location and reducibility of nickel. The effect of steam treatment is particularly important in relation to the use of such a zeolite for large-scale catalysis. For example, if a large quantity of sample (with a large bed depth) were dehydrated in a gas flow, with the water staying in contact with the catalyst for some time, the conditions of dehydration might be rather similar to those encountered during steam treatment, and hence the nickel cation distribution would be significantly different to that expected from the ideal case of a shallow-bed sample dehydrated in vacuo.Reduction of samples after treatment with alkaline solution results in smaller metallic nickel particles and a higher resistance to sintering at high temperatures. (3) The presence of a ‘two-phase’ reduction process, from X-ray data collected during reduction. Also, location of cations in the finally reduced samples enable us to estimate the degree of reduction and help confirm our assignment of scattering matter in the dehydrated samples of Ni2+, Na+ or K+. (4) That X-ray Rietveld refinement is a viable method for following changes in cation locations at high temperature for the cubic zeolite Y. The method can now be applied with confidence to study non-cubic systems where considerable peak overlap makes integrated intensity methods impracticable.In addition we have observed that the relative intensities of some of the zeolite peaks e.g. 1(531)/1(440) are particularly sensitive to the number of cations in site I (determined from refinement). As a result, it is possible to estimate the site I occupancy at intermediate stages in a reaction by monitoring the intensities of say the (531) and (440) peaks, rather than having to undertake a full-scale refinement on each data set. In this paper we have dealt exclusively with characterizing the state and location of cations in dehydrated Ni-exchanged zeolite Y as a function of pretreatment, and studying the reduction of these cations. The correlation of such structural attributes with the catalytic performance of these materials and with energy calculations on preferred cation locations will be the subject of subsequent papers. We are grateful to the S.E.R.C.and Unilever (Port Sunlight Laboratory) for a CASE award (to C. W.), and to Drs C. hlarsden and C. J. Adams (Unilever) for valuable help. References 1 S. Bhatia, J. F. Matthews and N. N. Bakhshi, Acta Phys. Chem., 1978, 24, 83. 2 H. Schrubbers and G. Schulz-Ekloff and G. Wildeboer, in Metal Microstructure in Zeolites, ed. P. A. 3 D. J. Elliot and J. H. Lunsford, J. Catal., 1979, 57, 1 1 . 4 V. Patzelova, A. Zukal, Z. Tvaruzkova and 0. Malicek, in Structure and Reactivity of Modified 5 K. G. Ione, V. N. Romanikov, A. A. Davydov and L. B. Orlova, J . Catal., 1979, 57. 126. 6 H. Bremer, W. P. Reschetilowski, F. Vogt and K-P. Wendlandt, in Structure and Reactivity of Modified 7 M. Suzuki, K. Tsutsumi and H. Takahashi, Zeolites, 1982, 2, 193. 8 I. E. Maxwell, Adv. Catal., 1982, 31, 1 . 9 C. Mirodatos, J. A. Dalmon, E. D. Garbowski and D. Barthomeuf, Zeolites, 1982, 2, 125. Jacobs (Elsevier, Amsterdam, 1982), p. 261. Zeolites, ed. P. A. Jacobs (Elsevier, Amsterdam, 1984), p. 367. Zeolites, ed. P. A. Jacobs (Elsevier, Amsterdam, 1984). 10 M. Suzuki, K. Tsutsumi and H. Takahashi, Zeolites, 1982, 2, 51. 1 1 M. Suzuki, K. Tsutsumi and H. Takahashi, Zeolites, 1982, 2, 87. 12 M. Suzuki, K. Tsutsumi and H. Takahashi, Zeolites, 1982, 2, 185. 13 W. J. Mortier, Compilation of Extra-framework Sites in Zeolites (Butterworth, London, 1982). 14 D. H. Olson, J . Phys. Chem., 1968, 72, 1400. 15 P. Gallezot and B. Imelik, J . Phys. Chem., 1973, 77, 652. 16 E. Dendooven, W. J. Mortier and J. B. Uytterhoeven, J . Phys. Chem., 1984, 88, 1916. 17 J. V. Smith, Ado. Chem. Ser., 1971, 101, 171. 18 R. A. Schoonheydt, L. J. Vandamme, P. A. Jacobs and J. B. Uytterhoeven, J . Catal., 1976, 43, 292. 19 H. M. Rietveld, Acta Crystallogr., 1967, 22, 151.J. M. Thomas, C. Williams and T. Rayment 293 1 20 H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 65. 21 A. K. Cheetham and J. C. Taylor, J. Solid State Chem., 1977, 21, 253. 22 C. Williams, Ph.D. Thesis (University of Cambridge, 1986). 23 D. B. Wiles and R. A. Young, J . Appl. Crystallogr., 1981, 14, 149. 24 J. M. Thomas and C. Williams, unpublished work. 25 I. E. Maxwell, J. J. de Boer and R. S. Downing, J. Catal., 1980, 61, 493. 26 J. J. I. Van Dun, W. J. Mortier and J. B. Uytterhoeven, Zeolites, 1985, 2, 257. 27 M. J. Sanders and C. R. A. Catlow, Proc. Sixth Znt. Zeolite Conf., ed. D. Olson and A. Bisio 28 M. Sano, T. Maruo, H. Yamatera, M. Suzuki and Y. Saito, J. Am. Chem. SOC., 1987, 109, 52. 29 A. K. Cheetham, M. M. Eddy and J. M. Thomas, J. Chem. SOC., Chem. Commun., 1984, 1337. 30 M. L. Costenoble, W. J. Mortier and J. B. Uytterhoeven, J. Chem. SOC., Faraday Trans. 1, 1978, 74, (Butterworths, London, 1984). 466. Paper 71897; Received 19th May, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402915

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(9), 2933-2949 Properties of Complexes with Cobalt-Carbon Bonds formed by Reactions of Aliphatic Free Radicals with Nitrilotriacetate-Cobalt@) in Aqueous Solution A Pulse Radiolysis Study Dan Meyerstein" Chemistry Department, Ben-Gurion University of the Negev, Beer-Sheva, Israel Harold A. Schwarz Brookhaven National Laboratory, Upton, New York 11973, U.S.A. The reactions of the aliphatic free radicals 'R ['R = 'CH,, 'CH,OH, 'CH(CH,)OH, 'C(CH,),OH, 'CH(CH,)OC,H,, 'CH,CO; and COJ with Co"(nta)(H,O); in aqueous solutions yield the unstable transients (nta)Co"I-R(H,O)- with cobalt-carbon u bonds. The complexes with 'R = 'CH,, 'CH,OH, 'CH(CH,)OH, 'C(CH,),OH and 'CH(CH,)OC,H, decompose via homolytic cleavage of the cobalt-carbon bond. The kinetics of reaction of these free radicals with (nta)CoII'-R(H,O)- are reported.The only organic products of the latter reactions are the dimers R-R for 'R = 'CH, and 'CH,OH, whereas a mixture of disproportionation products and dimers is formed for 'R = 'CH(CH,)OH and 'C(CH,),OH. For 'R = 'CH,OH, 'CH(CH,)OH and 'C(CH,),OH the pK, values for the acidic dissociation of the alcoholic group in the complexes are reported. For 'R = 'CH,OH the activation parameters for the homolytic dissociation of the cobalt-carbon bond and the acidic dissociation are reported. The ultraviolet-visible spectra of all the transients are- reported. Properties of complexes containing transition-metal-alkyl bonds in aqueous solution are of interest in the context of various homogeneous catalytic proce~ses,~-~ including several biochemical systems.' The suggestion was made that processes catalysed by coenzyme B,, are triggered by the homolytic dissociation of a cobalt-carbon 0 bond.,9 3 9 This idea has generated interest in the kinetics of reaction of aliphatic free radicals with divalent cobalt complexe~~-~~ and the determination of the factors affecting the rate of the homolytic cleavage of the cobalt-carbon bond thus formed.2r61 7 e 7 16-,0 The complexes studied so far have all contained ligands which are considered coenzyme B,, analogues, i.e.unsaturated ligands bound to the cobalt ion in a planar configuration, such as corrins, 2~ ' phthalocyanines, l4 bis-dimethylglyoximato, 2. NJV"' disalicylene- 0-phenilenediamine, 2 * unsaturated tetra-azamacrocyclic ligands12* l6 etc.These ligands stabilize the cobalt in three oxidation states, + 1, + 2 and + 3. The stabilization of monovalent cobalt limits the studies to 'R groups which do not reduce the cobalt. Thus only a few studies with 'CR,R,OH radicals were reported and in most of these the L-CoIr1-CR1R,OH decomposed into L-Co' and RlR,C0.10~12* l4 We have studied complexes of cobalt and reducing radicals formed in the reaction Co'I(nta)(H,O); + ' R e (nta)Co'"R(H,O)- + H,O (1) [where nta is nitrilotriacetate, N(CH,CO;),]. The nta is a strong o donor ligand and is expected to stabilize cobalt in the + 2 and + 3 oxidation states. The reactions of 'CH,C(CH,),OH and C0;- with Co"(nta)(H,O), were reported to lead to the forma- 29332934 Properties of Complexes with Cobalt-Carbon Bands Table 1.Rate constants of radical reactions reaction k / l 0-9 dm3 mol-1 s-' ref. eiq + N,O 2 O *OH + N, +OH- 'OH + CH,OH -+ 'CH,OH + H,O 'OH + C,H,OH -+ 'CH(CH,)OH + H,O (87 Yo) + 'CH,CH,OH + H,O (1 3 Yo) 'OH + CH(CH,),OH -+ 'C(CH,),OH + H,O (86 O/o) -+ 'CH,CH(CH,)OH + H,O (1 3 Yo) 'OH + CH,CO; -+ 'CH,CO; + H,O 'OH + (C,H,),O + 'CH(CH,)OC,H, 'OH + (CH,),SO -, 'CH, + CH,SO,H 2'CH, -+ C,H, 2'CH,OH -+ (CH,OH), 'OH + HCO; -+ C0;- + H,O 2'CH(CH,)OH -+ CH,CHO + C,H,OH 2'C(CH,),OH + (CH,),CO + CH(CH,),OH 2c0,- -+ c,o,2- 8.7 0.7 1.7 2.3 0.085 2.9 2.6 7.0 1.6 1.5 0.7 0.70 0.85 22 23 23 23 23 23 23 24 25 25 25 25 25 tion of short-lived intermediates with cobaltxarbon bond^.'^ Complexes with 'R = 'CH,, 'CH,OH, 'CH(CH,)OH, 'C(CH,),OH, 'CH(CH,)OC,H5, 'CH,CO, and C0;- are reported here.The stability of the cobalt*arbon bond towards homolytic cleavage was found to be in the order CH, > CH,OH > CH(CH,)OH > C(CH,),OH > CH(CH,)OC,H,. Complexes with alcohol radicals were found to exhibit a pK,: (nta)Co"*R(H,O)- g (nta)Co111R(H20)'2- + H+ (2) where the prime indicates loss of a proton from the alcohol group. Experimental All materials were analytical grade : CoSO, - 7H,O, Matheson, Coleman and Bell; nitrilotriacetic acid and HClO,, G. F. Smith ; N-hydroxyethylenediamine triacetic acid (HEDTA), Sigma; sodium formate, sodium acetate and propan-2-01, J. T. Baker; Dimethyl sulphoxide (DMSO), Fischer Scientific ; methanol and ethanol, Burdick and Jackson ; diethyl ether and sodium hydroxide, Mallinkrodt.The pulse radiolysis was performed with 2 MeV electrons from a Van de Graaff accelerator. The dose per pulse was such that the initial free radical concentration was 2 x lo-' to 2 x lo-, mol drn-,. The optical pathlength was 2.0 or 6.1 cm, depending on the nature of the experiment. All solutions were thermostatted at 25 "C unless otherwise stated. Steady-state radiolysis was carried out with a 'OCo pray source in which radicals were produced at 1.2 x lo-, mol dm-, min-l (1 800 rad min-l). Methane and ethane were determined by gas chromatography using a molecular-sieve 5A column. Heavier organic products were determined by gas-chromatography-mass- spectroscopy on a 30 m OB-wax column. The radiolysis of water may be summed up by the equation H,O -+ eiq, 'H, 'OH, H,, H,O,, H,O+ in which the hydrogen, hydrogen peroxide and H atoms are minor products.2' The dominant reactions and their rate constants for eiq and OH are given in table 1.In allD. Meyerstein and H. A . Schwarz 2935 I I 1 I I 1 200 300 400 500 600 700 Fig. 1. Spectra of Co"I(nta) (solid lines) and Co"I(nta)Br- (dashed lines) at pH 4.9. cases, 95% or more of e& reacts with N,O to produce OH radicals. The solute concentrations were such that OH reacts with substrates to produce the radicals of interest well within s. The H atom reactions are not given but they react with solutes (with the excention of DMSO) to nroduce the same radicals as OH. but at rate constants 1 W O O times slower.26 The H atoms are a small fraction (8%) of the total radicals and their reactions were also complete before the intermediate of interest was observed.The total H atom and OH radical yield available for reaction with solutes was taken to be constant at 6.8 radicals per 100eV for all solutions. The assumption of constancy is probably accurate to & 5 O/O. Results The Spectrum of Co'"(nta)(H,O), The reaction of Br; with CoII(nta)(H,O); has been reported to yield Co"'(nta)Br(H,O)-, which then reacts by slow loss of Br- to give C0"I(nta)(H,0),.~~ Spectra of these products are of interest for comparison with radical adduct spectra. The spectra we obtained for Co"'(nta)Br(H,O)- and Co"'(nta)(H,O), are given in fig. 1 and table 2. At pH 10.4 A,,, was below 240nm for both Co"'(nta)aq [a mixture of Co"'(nta)(OH)(H,O)- and C~"'(nta)(OH)i-]~~ and Co11'(nta)Br(OH)2-.The rate constants of the aquation reaction were 5.6 and 610 s-l at pH 4.9 and 10.4, respectively. Equations for the Equilibria and Kinetics An observed extinction coefficient is defined in terms of the absorption change of a solution following pulse radiolysis, the optical path length and the total concentration of oxidized species, calculated from the known production rates of OH, e,, and H: A = IE,~~~[ox].2936 Properties of Complexes with Cobalt-Carbon Bands Table 2. Absorption maxima (and extinction coefficients)a of Co(nta)R(H20)- H2O CH3 C(CH3),OH Br- CH20H CH(CH ,)OH CH20- CH(CH,)O- CH(CH3)OC,H, CH,CO; C(CH3)20- co, < 230 (1.2) 290 (1.2) 245 (1.4) 256 (0.93) 262 (0.90) 263 (1.1) < 230 < 245 d 240 (0.6) 272 (0.68) 275 (0.87) 266 (0.7) 395 (150) 410 s (200) 390 (170) 396 (310) 410 (580) 412 (530) 445 (230) 450 (230) 430 s (220) 400 (320) 420 (790) 380 s (200) 560 (190) 580 (1 50) 600 (90) 600 (30)" 570 (40)" 550 (80) 600 (30)" 620 (50)" 600 (40)" 600 (30)" 580 (40)" 610 (90)" a 2 in mm, E in dm3 mol-' cm-l.are small and diffuse. s denotes a shoulder. " Maxima are approximate as absorptions Spectrum of first intermediate observed; see text and fig. 7. If the absorbance is measured at the completion of equilibration of reactions (1) and (2) and before subsequent decay of the species, then E-, + ~ ~ [ ~ O " ( ~ ~ ~ ) ( ~ 2 ~ ) ~ ] ( E c o ~ ~ ~ R + E C o ~ ~ ~ R ~ K,/[H+]) 1 + K,[Co"(nta)(H,O),]( 1 + K,[H+]) 'obsd = (3) Two equivalent forms of eqn (3) were used.At constant [H+] E., + E'K'[CO"( n ta)( H ,0)2] 1 + K'[CoT1(nta)(H,O),] 'obsd = where K' is Kl[( 1 + K,/[H+] and E' is ( E ~ ~ I I I ~ + E ~ ~ I I I ~ , K,[H+])/( 1 + K,/[H+]). At constant [ Co I I (nta)( H ,O)J E" + &CoR, K"/[H+] 'obsd = 1 + K"/[H+] where K"is {K1[Co"(nta)(H,0),]/( 1 + K,[Co"(nta)(H,O)J)} K, and E" is (E., + E ~ ~ I I I ~ Kl x [CoI'(nta)(H,O),]/( 1 + K,[Co"(nta)(H,O)J). Thus K' is determined at constant, preferably high, [H+], K" is determined at constant, preferably high, [Co"], and Kl and K, are evaluated from K' and K". E., is determined directly from eqn (3a) at constant [H+], and is always small. E ~ ~ I I I ~ , is determined directly from eqn (36) and E ~ ~ I I I ~ can be calculated from either E' or E". Reaction (1) is rate-limiting below pH 4.5 at all cobalt(r1) concentrations used.The approach to equilibrium is expected to be pseudo-first-order, with First-order growth of the absorption was seen in most solutions to at least 99% completion with no observable subsequent decay (i.e. < 3 %). The final absorbance used to calculate eobsd in these solutions, A,, was found by fitting the absorbance to A = exp(-kobsd t ) + A F where A , is any initial absorbance. At low CoII concentrations such that equilibrium (1) lies to the left, slower growth and more rapid disappearance is observed. In the worst case (the equilibration of the ethanol radical with 10-4mol dm-3 CoII) the decayD. Meyerstein and H. A . Schwarz 2937 wave length/ nm Fig. 2. Spectrum of (nta)CoCH,(H,O)- (solid lines and circles). The subsequent growth around 320 nm (dashed lines and squares) is attributed to formation of a Co(nta)- adduct with (CH,),SOH [see ref.(29)]. The second-order decay of (nta)CoCH,(H,O) is shown in the inset. amounted to 15 % at the time kobsd t = 5, or > 99 % equilibration. This small amount of decay was corrected for by adding a linear time term, -at, to the growth equation. In more basic solutions the equilibration time for reaction (2) is comparable to that of reaction (l), and pseudo-first-order growth is not expected. The absorbance should vary with time with two exponential terms, and one example of this will be given. The increase in complexity of the kinetics with increasing pH is compensated for by a large increase in lifetime of the equilibrated solution before decay, and so A,, the absorbance of the equilibrated solutions can still be accurately determined.One or two exponential fits were used as the data seemed to require. The kinetic parameters were not used for any other purpose. The Reaction of Methyl Radicals with Co"(nta)(H,O); When N,O-saturated solutions of 5 x 10-5-1 x mol dm-, nta, 0.2-1 mol dm-, (CH,),SO, pH 4.0-9.7, were irradiated the formation of a long-lived intermediate was observed (fig. 2).? The rate of formation, rate of disappearance and spectrum were independent of pH, and nta and (CH,),SO concentrations, The rate of formation was first order in Co''(nta)(H,O), concentration and the rate of disappearance was second order, as seen in the insert of fig. 2 and confirmed at various initial radical concentrations. The second-order rate constant for disappearance varied inversely with Co"(nta)(H,O); concentration between 5 x 1 0-5 and 1 x lo-, mol dm-, CoI'. Under the experimental conditions, e& reacts with N20 to produce OH, and the OH reacts with (CH,),SO to produce methyl radicals (as may be seen from the reactions in mol dm-3 CoSO,, 1 x 10-3-5 x j' The small band around 320 nm was more apparent at pH 4.0 than at pH 9.7.Its formation is pseudo- first-order in [Co(nta)(H,O),]. During this reaction, which occurs after all the methyl radicals reacted, no change in the spectrum at the other three bands is observed. This band is tentatively attributed to the product of reaction between Co(nta)(H,O); and (CH,),SOH. The latter radical is formed in the reaction 'H+ (CH,),S0.2s2938 Properties of Complexes with Cobalt-Carbon Bands Table 3. Rate and equilibrium constants for (nta)Co"'R(H,O)- species kl k-1 Kl K2 R /lo-* dm3 mol-' s-' s-l /dm3 mo1-1 /mol dmP3 - 7.0 x 107 - 1.6 2.0 3.9 5100 2.2 x 10-5 CH3 CH(CH,)OH 0.97 10.1 1100 1.2 x 10-4 0.23 3.7 500 2.9 x 10-5 CH(CH,)OC,H5 0.49 40 110 CH,CO; 0.15 < 0.1 > 1.5 x 104 CH,OH - > 1 x 107 - C(CH3)20H co, 0.73 - - table I).The formation of the intermediate observed here must be attributed to reactions between methyl radicals and Co"(nta)(H,O);. The spectrum of the intermediate and the kinetics of its decomposition differ considerably from those of CO~~[-O,CCHN(CH,CO;),]~~ which would be formed by hydrogen abstraction from Co"(nta)(H,O);. The spectrum is not that of Co"'(nta),, (fig.1). We conclude that the reaction is (1') for which k, = 1.6 x lo8 dm3 mol-1 s-l. The reverse rate constant, k-,, could not be measured (< lo3 s-'). The inverse dependence of the second-order rate constant for disappearance of Co"'(nta)(CH,)(H,O)- on Co"(nta)(H,O); concentration indicates that reaction (1') is reversible and is followed by Co"(nta)(H,O); + 'CH, + (~~~)CO'~'(CH,)(H,O)- + H,O 'CH, + (nta)Co"'(CH,)(H,O), -+ C2H6 + Co"(nta)(H,O),. ( 5 ) The pre-equilibrium reaction (1') and reaction ( 5 ) give, at sufficiently large Co''(nta)(H,O), that K1[Co*'] % I , - 2k5 [(~~~)CO"'CH,(H,O)-]~ - d[(nta)Co"'CH,(H,O)-] dt Kl [ Co 'I( n t a)( H,O),] as observed, with 2k5/K1 = 28 s-'. If the disappearance was due to then the CoI' dependence would be inverse square, which was not observed.The value of 2k5 should be equal to or larger than the value for similar reactions of the CH20H complex (1 x lo9 dm3 mol-l s-l) or CH(CH,)OH complex (0.6 x I O9 dm3 mol-1 s-l) to be discussed later. If 2k, is taken as 2 x lo9 dm3 mol-1 s-l, then Kl is 7 x lo7 dm3 mol-l. This value is entered in table 3. N,O-saturated solutions containing 0.005 mol dm-, CoSO, and 0.005 mol dm-, nta, 0.5 mol dm-, (CH,),SO at pH 4.5 were irradiated in the 6oCo source. The yields of C,H6 and CH, were about equal, i.e. ca. of the CH, radicals end up as ethane. The low dose rate in the 6oCo source (ca. mol dm-, radical per second) results in very long (nta)"'CoCH,(H,O)- lifetimes for second-order decay, ca. 30 s, thus increasing the probability of side reactions such as hydrogen abstraction to form methane.A similar solution, but lo-, mol dm-, in CoSO, and 2 x low3 mol dm-, in nta, was irradiated with a series of electron pulses from the Van de Graaff. Under these high dose rate conditions the lifetime is much shorter, and the ethane yield was 5 times the methane yield, i.e. 90 % of the methyl radicals yielded ethane.D. Meyerstein and H. A. Schwarz 2939 8 3 P q 4 N E * 2 0 200 300 400 500 600 700 w avelengt h/nm Fig. 3. Spectra of (nta)CoCH,OH(H,O)- (circles) and (nta)CoCH,0(H,0)2- (squares). The effect of pH on the apparent extinction coefficient at 260 nm produced in mol dm-3 Co" solution is shown in the inset. Reactions of a-Hydroxyl Radicals with Co"(nta)(H,O), Upon irradiation of N,O-saturated solutions containing 0.2-1 mol dm-, alcohol [CH30H, C,H,OH, or (CH,),CHOH] all primary radicals, including H atoms, are converted rapidly into 'R radicals [mainly 'CH,OH, 'CH(CH,)OH and 'C(CH,),OH, although some 'CH,CH,OH and 'CH,(CH,)CHOH are formed in solutions of ethanol and propan-2-01, table 13.These radicals react with added Co"(nta)(H,O), (1 x 10-*-5 x lo-, rnol dm-3) to produce transients which are attributed to (nta)Co"'R(H,O)-, formed by reaction (1). The kinetics of formation and decay of (nta)Co"'R(H,O)- depended on pH and Co"(nta)(H,O), concentration, and the spectra changed with pH, as shown for the 'CH,OH species in fig. 3. The second transient, seen at high pH, is attributed to (nta)Co'1'R(H,0)'2- produced in reaction (2).kobsd for equilibration and E~~~~ at 260 nm for the three alcohol radical complexes are shown as functions of [Co''(nta)(H,O);] in fig. 4. The expected dependences are observed and the curves drawn in the inset to fig. 3, and in fig. 4 are calculated from eqn (3) using parameters given in tables 2 and 3. The rate constant plots are interpreted according to eqn (4), and k, and k-, are recorded in table 3. In each case the ratio k,/k-, is in adequate agreement with the values of K, determined from E ~ ~ ~ ~ . For the methanol system the experiments were repeated at 7 and 55 "C. The AH" and ASo for reactions (1) and (2) and AH+ and A S t for k , and k-, were calculated from the results and are given in table 4. At high pH and in the absence of buffer, reactions (1) and (2) are of comparable rate and the growth of (nta)Co1''CH2OH(H,O)- and subsequent decay to (nta)C0"'CH,0(H,0)~- can be observed, as is seen in fig.5. The two rate constants were 4.4 x lo5 s-'and 1.5 x lo5 s-l. The rate constant expected for approach to equilibrium of reaction (1) is 1.7 x lo5 s-l and the value expected for k, is 6 x lo5 s-', based on an assumed k-, of 4 x 10'' dm3 mol-' s-l and the pK. The oberved rate constants do not give directly the two relaxation rates, but should be comparable to them, as is observed. The disappearance of all three transients obeyed second-order rate laws. The2940 5 4 - I w3 w 0 - 1 3 a 4" Properties of ComplexeJ I I I I I 0, 0 0.001 0002 OD03 0.004 0.005 uith Cobalt-Carbon Bands I 1 1 I I I 10-4 I 0-3 10-2 Fig.4. The variation of /cobs,, with cobalt(I1) concentration according to eqn (4) and the variation of final observed extinction with cobalt(1r) concentration according to eqn (3); 0, R = CH,OH; 0, R = CH(CH,)OH; 0, R = C(CH,),OH. rate constants decreased with increasing pH and with increasing concentration of CoII(nta)(H,O),. These results suggest a mechanism similar to that for (nta)CoII'CH,(H,O)-, i.e. reactions (I) and (2) followed by 'R + (nta)Co'IIR(H,O)- + Co"(nta)(H,O); + products 'R + (nta)Co111R(H20)'2- -, CoII(nta)(H,O); + products (6) (7) 2'R -+ products. (8) The fractions of total species present as 'R, (nta)CoI'IR(H,O)- and (nta)Co11*R(H,0)'2- are and the second-order rate constant can be expressed as 2kobsd = 2k8ft, + 2k6f RfCoR + 2 k 7 f RfCoR'-' (10) The results indicate that k6 and k7 are about equal, at least for methanol and ethanol, so eqn (10) can be written as 2kobsd 2 k 8 f 2 , + 2k6$R:,(1 -fR)' (1 1) The logarithm of 2kOb,, is plotted against the logarithm ofJR in fig. 6.If 2k6 = 2k,, then the data in fig. 6 should fall on straight lines with unit slope. This is nearly the case for ethanol solutions, and almost as good for methanol solutions. The curves in fig. 6 forD. Meyerstein and H . A . Schwarz 294 1 Table 4. Effect of temperature on R = CH,OH reactions" Kl K2 k , T/OC /dm3 mol-' / 1 0-5 mol dm-3 /dm3 mol-' s-' k , / s - ' 1.36 x 104 1.40 9.7 x 107 6.9 x 103 25 5.1 x 103 2.2 2.0 x lo8 3.9 x 104 55 8.4 x lo2 5.6 4.1 x lo8 5.5 x 105 7 AHO(AH') - 10.7 5.3 4.8 16.0 A S "( A s ) - 19.1 - 3.4 -4.6 16.3 a Units are kcal mol-1 for AH and cal K-l for AS.Expected errors are 2 0.5 kcal mol-' for AH and & 2 e.u. for A S (1 e.u. = 4.184 J K-' mol-l). 0.0 I C 0 .4 z 2 -2 t I I I I 1 I I 0 10 20 tlPS Fig. 5. Growth of (nta)CoCH,OH(H,O) from 0.002 mol dm-3 Co(nta)- and 'CH,OH and subsequent decay to (nta)CoCH,0(H20)2- at pH 7.2 and 7 "C. 30 0 ' these two systems are calculated using 2k, = 2.3 x lo9 dm3 mol-' s-l and 2k, = 1.0 x lo9 dm3 mol-' s-l for methanol and 2k, = 1 .O x lo9 dm3 mol-1 s-' and 2k6 = 0.6 x lo9 dm3 mol-'s-l for ethanol. These values of 2k, are close to the literature values in table 1. The average deviation of the CH,OH species rate constants from the curve is - + 10 YO. That for the CH(CH,)OH species is f 20 %. If 2k, is negligible compared to 2k,, then the slope of the curve in fig.6 should be 2. This is nearly the case for the propan-2-01 system forfR > 0.01. The values actually used in calculating the curve were 2k, = 1 .O x lo9 dm3 mol-' s-l and 2k, = 3 x lo7 dm3 mol-l s-l. The data can be fitted almost as well with 2k, = 1.3 x lo9 dm3 mol-' s-l and 2k6 = 0. Note that the data atfR -c 0.003 do not fall on the curve at all. The data in this region were collected at pH values of 6-8, well above the pK values of the intermediates, but the obvious assumption that k, 9 k, for the propan-2-01 species will not fit all the data either. The actual intermediate lifetimes for propan-2-01 solutions in this region are ca. 1 s, so it is possible that the rates are determined by impurity effects. The disappearance kinetics observed for (nta)Co"'(CH(CH,)OH)(H,O)- and (nta)Co"'(C(CH,),OH)(H,O)- were not as clean as those that were observed for2942 Properties of Complexes with Cobalt-Carbon Bands to4 lo3 to2 Id' 1 f*R Fig.6. Variation of second-order rate constants for disappearance of (nta)CoROH(H,O) as a function of the fraction of total ROH present as the free radical, 'ROH: e, ROH = CH,OH; 0, ROH = CH(CH,)OH; 0, ROH = C(CH,),OH. (nta)Co"'CH,OH(H,O)-. In particular, for the ethanol system another disappearance step ca. 20 times faster was seen at higher pH values. This step accounted for ca. 25% of the total absorbance, and the rate constant was proportional to the total intermediate concentration. This result is probably due to the 13 YO of primary radicals, 'CH,CH,OH and 'CH,CH(CH,)OH, in the two ~ystems.'~ The effect was too small to be studied in detail here.We plan to study the properties of these species by producing the free radicals via OH addition to olefins. Final products were examined by g.c.-m.s. analysis of N,O-saturated solutions containing 5 x lo-, mol dm-, CoSO,, 6 x lop3 mol dmp3 nta and 0.5 mol dm-3 alcohol, at pH 5.5, which were irradiated in a 6oCo source to 106rad. In methanol solution (CH,OH), was the only organic product observed. In ethanol solutions the major product was CH,CHO, but a small amount of butane-2,3-diol was detected. In propan- 2-01 solution only acetone was observed. The same solutions were irradiated with a series of electron pulses from the Van de Graaff, and in methanol solutions (CH,OH), was again the only product.In ethanol solutions, acetaldehyde was still the major product, but butane-2,3-diol was comparable. In propan-2-01 solution acetone was again the major product, but some 2,3-dimethylbutane-2,3-diol was detected.D. Meyerstein and H. A . Schwarz 2943 wavelengt h/nm Fig. 7. Spectra of intermediates produced from acetate radical : 0, (nta)Co111CH2COO(H,0)2- at end of reaction of acetate radical with (nta)Co"(H,O),; 0, spectrum after first-order change for which k = 880 s-'. Reaction of 'CH,OH with Co(hedta)(H,O)- Studies similar to those with nta ligand were made with N-hydroxyethylenediamine triacetate ligand. The spectrum of (hedta)Co"I(CH,OH)- consists of two bands at 270 nm (E = 14000 dm3 mol-' cm-l) and 380 nm (E = 230 dm3 mo1-l cm-l), and a possible third band at 530450 nm with E NN 50 dm3 mol-1 cm-l.Other results were k, = 1.25 x lo7 dm3 mol-1 s-l, k-, < 60 s-l, K, > 2 x lo5 dm3 mol-1 and K, = 1 .O x lo-' dm3 mol-' (pK, = 9.0). The disappearance of the intermediate was first-order with a rate constant of 0.018 and 0.010 s-l at pH 4.1 and 10.3. The mechanism for disappearance is different from that for the nta complexes and was not studied further. Reaction of 'CH(CH,)OC,H, with Co(nta)(H,O), N,O-saturated solutions containing (1-5) x lop3 dm3 mol-' CoSO,, 6 x dm3 mol-' nta and 0.5 dm3 mo1-1 (C,H,),O at pH 4.0 were irradiated and the results are given in tables 2 and 3. The disappearance kinetics were second order, with 2k,,,, = 1.5 x lo9 dm3 mol-1 s-' for a solution containing 5 x mol dm-3 CoSO,.This complex is more weakly bound than the others, and the concentration of radicals is greater than that of the complex by 2 to 1, so the decay is most probably mainly due to reaction (8), the reaction between two radicals. Reaction of 'CH,COO- with Co(nta) The acetate radical reacts with Co''(nta)(H,O), at 1.5 x lo7 dm3 mol-1 s-', the smallest value of k, observed in this work. The spectrum of the resulting complex is shown in fig. 7. The small value of k, limited the useful Co"(nta)(H,O); concentration to above mol dm-3, but the rate constant for the reaction of OH with CH3C0, (table 1) is so small that even in 1 mol dm-3 acetate 15% of OH reacts with Co''(nta)(H,O), at 0.004 mol dmP3. In this range the rate constant for production of the complex was pro- portional to [Co"(nta)(H,O),], with no observable intercept, so k-, is < lo3 s-', and2944 Properties of Complexes with Cobalt-Carbon Bands thus Kl 2 1.5 x lo4 dm3 mol-'.The apparent extinction coefficients could not be used to estimate K,, as the OH reaction was not quantitative. A first-order reaction of the acetate radical complex, k = 880 s-', was observed to give a new species with a very similar spectrum to the first above 350 nm, also shown in fig. 7. The principal maximum of the second species is below 250 nm. The lifetime of the second species was ca. 100 s. The nature of the change is not known, but the results suggest that it is an isomerization of the transient. Such a process could be a conversion between complexes with carbon bound acetate group being cis and trans to the nta nitrogen.However, such a process was not observed for the other transients studied. It is tempting to suggest that valence tautomerization - 5H2 . - 2- 1 0- I . a .c .* \ * * OH . * Co(nta) or hyperc~njugation~' after loss of the water ligand (nta)Co"'CH,CO; (H,O),- 2- 7 1 2- dH, Po- Co(nta) L- describes the process observed. Such processes would also explain the blue shift of the U.V. band (see below). Reaction of C0;- with Co(nta)- This reaction was studied earlier by Bhattacharyya and Srisankar. l5 Our spectrum of (nta)Co"'(H,O)CO~- and value of k, are in excellent agreement with their data, but we do not agree on the mode of decay of the intermediate. They reported second-order decay with 2k = 1.9 x lo8 dm3 mol-1 s-l.We find the disappearance of the spectrum is much more complex and is autocatalytic. The study of the decay will continue, but an upper limit on a second-order component is lo6 dm3 mol-1 s-l in lop4 mol dm-, CoII(nta)(H,O);, 0.2 mol dm-3 formate solution. If the rate constant for CO, + (nta)CoI1I(H,O)CO;- + product is assumed to be lo9 mol dmP3 s-l, then [CO;]/[(nta)Co"'(H,O)CO~-] < or K, > 7 x lo7 dm3 mol-l. Discussion The results described above point out that nitrilotriacetate indeed stabilizes transient complexes of the type (nta)Co''I-R(H,O)- as expected. With a proper choice of ligands analogous transient complexes might be observed for most transition-metal cations. Indeed (nta)CuI"-R- (R = CH,, CH,OH, CH(CH,)OH and C(CH3),0H),3i (nta)Mn'IICH,(H,O)-~ (nta)Fe1''CH,(H,0)-32 and (nta)Ni"'CH3(H,0)iq,32 were recently observed.Of course, reaction (1) might yield two isomers : one with the R groupD. Meyerstein and H. A . Schwarz 2945 trans to the nitrogen of the ligand or one with the R group trans to one of the acetate groups. There was no evidence of two species being present from either spectra or kinetics, with the possible exception of the acetate radical complex. Either equilibration between the species is rapid or one isomer is thermodynamically preferred. In the analogous chromium complexes the observed isomer is most probably that with the methyl bound trans to the nitrogen ligand.33 Spectra of the Transients The general features of the spectra of the (nta)Co"'-R(H,O)- complexes resemble those of Co"'(nta)(H,O), and (nta)Co"'Br(H,O)- (fig.l), in accord with their identification as cobalt (111) complexes. The spectra of the (nta)Co"'R(H,O)- complexes also resemble those of the analogous (H,0)5CrR2+34 complexes. However, the results suggest that the dependence of 1, on the nature of R is smaller for the cobalt complexes. The strong absorption band in the U.V. is attributed to a ligand to metal charge-transfer transition by analogy with the corresponding chromium complexes.34 The values of 1, for the two types of complexes are similar. This is plausibly due to a compensation between two factors: Co"*(nta)(H,O), is a better oxidizing agent than Cr(H,O)if and therefore 1, should be red shifted for the cobalt complexes. On the other hand, the lowest charge- transfer transition for the cobalt complexes is 0 --+ eg and for the chromium complexes 0 + t,, therefore 1, should be blue-shifted for the cobalt complexes.Note that A, is usually red-shifted when K, decreases, as it does in the series :CR,R,O-, :CH,, :CH,OH, :CH(CH,),OH and :CH(CH,)OC,H,. The complexes with :CO, and :CH,CO; are exceptions to this rule. The correlation between 1, and K , is in accord with the assignment of 1, to an o+eg transition, as the (T orbital is clearly stabilized when K , increases. The observation that 1, behaves differently for the complex with :CO, is perhaps connected to the fact that CO; is a considerably more powerful reducing agent than the other radicals. The U.V. absorption band of (hedta)C0'~'-CH,0H(H,0)- is red shifted relative to that of (nta)Co'"-CH,OH(H,O)-, although K , is larger for the former complex.However, one should remember that the change from nta to hedta has other effects on the properties of the cobalt complex. Thus Co'II(nta)(H,O), is a stronger oxidizing agent than Co"'(hedta)(H,O), which would cause a blue shift of the charge-transfer band. On the other hand, the tzg6eg1 electronic configuration of the cobalt", formed in the charge- transfer transition, is an excited configuration. The excitation energy A. - P depends on the ligand and is larger for the hedta complex. This excitation energy probably causes the red shift observed. Note that the intensity of the second absorption band, 1, z 400 nm, increases when K, decreases (table 2).For the analogous chromium complexes34 this absorption band is attributed to a d-d transition with some interaction with the charge-transfer state. Such an interaction is expected to increase with the increase in the covalent nature of the cobalt-carbon bond in accord with observations. Equilibrium Constant for Cobalt-Carbon Bond Formation The data in table 3 show that Kl decreases along the series R = 'CH,, 'CH,OH, 'CH(CH,)OH, 'C(CH,),OH and *CH(CH,)OC,H,.?. This order is similar to that The values for R = .CO, and 'CH,CO, cannot be derived from the data and only lower limits of Kl are reported. C0;- clearly differs considerably from substituted alkyls and therefore a comparison of Kl for *CO; and other results is unjustified. From redox potentials 'CH,CO; is expected to have a Kl value between those of R = 'CH, and R = 'CH,OH.However, electronic factors might increase Kl considerably, as :CO; is a strong electron-withdrawing Hyperconjugation and/or formation of a n complex as discussed above might also affect K l . 91 FAR 12946 Properties of Complexes with Cobalt-Carbon Bands reported for the analogous equilibrium constant for the (H,0),Cr"'-R2+ complexes,36* 37 with the difference that Kl for R = CH(CH3)OC2H, falls between those for R = 'CH(CH,)OH and R = 'C(CH,),OH in the latter case. For the chromium complexes k-, was nearly proportional to I/K, over seven orders of magnitude, i.e. k, was very weakly dependent on K,. A much smaller range is covered in the present study, but the variation of k, here is similar to the chromium case.For the chromium complexes a correlation between l/K, and the rate of the homolytic dissociation of the carbon-carbon bond in the similarly constituted ethanes, R-R, was pointed out. Clearly K , for the formation of the cobalt-carbon bond will show a similar correlation, with the complex with R = 'CH(CH,)OC,H, being an exception. A correlation between K, and the redox potentials of the free radicals 'R is also expected. This expectation stems from the fact that the metalkarbon bond is expected to be more ionic, and less covalent, as 'R becomes a stronger oxidizing agent. The oxidation potentials of the free radicals used here, E"(R', H+/RH), will parallel the free energy of dissociation of the RH bond and so should be in the order CH; > 'CH20H > 'CH(CH,)OH x 'CH(CH,)OC,H, > 'C(CH,),OH. This order is the same as that found for the equilibrium constants K,, except that the species with R = 'CH(CH,)OC,H, is the weakest bound.Reactions (1) and (2) and the ionization of the radicals 'ROH e 'RO- + H+ (12) (13) can be combined to give Co(nta)(H,O), + 'RO- g Co(nta)RO(H20)2- for which K13 = Kl K,/K,,. For R = 'CH,OH, for example, K,, = 2 x so K13 = 6 x lo9 dm3 mol-', a factor of lo6 larger than K l . The oxidation potential for the radical ion, EO('CH,O-, H+/CH,O-), is considerably smaller than for the radical, as 'CH,OH is a stronger acid than CH30H. Thus the stability of the basic species relative to the acidic one is in the opposite direction to that expected from oxidation potentials. The ionized forms might be stabilized via partial bonding of the oxygen to the cobalt centre owing to the electrostatic interaction : H C- 0- H2 H Note that K, is considerably larger for (hedta)Co"'-CH,OH- than for (nta)(H,O)Co"'-CH,OH-.This finding is in accord with the redox potentials of Co"(nta)(H,O), and Co"(hedta)(H,O)-. Heat of Reaction The AH" for the reverse of reaction (1) is sometimes referred to as a bond dissociation energy, although it is really the difference between the dissociation energy of the ROH complex and that of the water complex. For the hydroxymethyl complex AH"( - 1) was found to be 10.7 kcal mol-l, 5.3 kcal mol-1 less than the enthalpy of activation forD. Meyerstein and H . A . Schwarz 2947 reaction (- l).? It has been assumed that AH? for k, is sma11,2*17.36 2 kcal mol-1 or less, so that AH? for k-, would be a good measure of AH".In fact this is a serious overestimate of AH", as the activation energy for the forward reaction (l), is not negligible. Acid Dissociation Constants for the Cobalt Complexes with a-Hydroxyalkyl Residues The a-hydroxyalkyl complexes studied here are the first for which a pKa of the alcoholic group has been observed. The values of K , for the (~~~)CO~~'-CR,R,OH(H,O)- complexes (table 3) are at least 12 orders of magnitude larger than those for the corresponding alcohols. The results thus point out that the (nta)Co"'(H,O)- radical exerts a large inductive effect on the alcoholic residue. This result might seem to be surprising as the pK, of the acetic acid residue in cobaloxime complexes of the type (D,H,)LCo'I'-CH,CO,H is higher than that of free acetic acid.30 The latter observation was attributed to hyperconjugation : OH : +a Co(D,H,)L .No such effect is expected for the a-hydroxyalkyl complexes. The large difference in K2 for (~~~)CO'~~-CH,OH(H,O)- and (hedta)Co"'-CH,OH-, K2 = 2.2 x lop5 and 1.0 x mol drn-,, respectively, suggests that K, depends strongly on the degree of covalency of the metal-carbon bond. The more the ionic nature of the bond the higher the pK, for the alcoholic group, as observed. The inductive effect of the methyl substituents on the alcoholic residue is expected to affect the acidity of the alcoholic group so that K , will decrease along the series :CH,OH, :CH(CH,)OH and :C(CH,),OH. On the other hand, K2 is expected to depend on the strength of the covalent bond, i.e.on K,, and this effect is expected to cause a reverse order of the acidities. The observed order of K , which decreases along the series :CH(CH,)OH, :C(CH,),OH and :CH,OH seems to be a compromise between these two tendencies. No analogous pK, values for other a-hydroxyalkyl complexes were observed so far. The only system studied in detail is that of the (H,0)5Cr"'ROH2+34 and (nta)Cr"'ROH(H,0)-33 complexes. The chromium-carbon in these complexes has clearly a more ionic nature than that of the intermediates observed in the present study, as Cr(H,O):+ and Cr(nta)(H,O); are considerably stronger reducing agents than Co(nta)(H,O);. Thus one expects that the pKa for the chromium-a-hydroxyalkyl complexes will be observed only at higher pH values than those in which these complexes were studied.It has been suggested that the transients observed in the reactions of 'CR,R,OH with Fe"(protoporphyrin), Cu'l(nta);q and CU;~ are (pr~toporphyrin)Fe~~'-CR,R,OH,~~ (~~~)CU"I-CR,R,OH-~~ and CUII-CR,R,OH+,~~ respectively. The possibility that the intermediates observed were (proto- porphyrin)Fe"'-CR,R,O-, (nta)Cu"'-CR,R2O2- and Curr-CR,R,O cannot be ruled out, and these systems will have to be reinvestigated. Note that it has been suggested that the mechanism of reduction of Co(NH,):+ by the 'CR,R,OH radicals in slightly acidic solutions occurs via a transient with a pK, between t 1 cal = 4.184 J. 97-22948 Properties of Complexes with Cobalt-Carbon Bands pH 4 and pH 6.39 The exact nature of the transient is not clear, but the results suggest that the inductive effect of the central tervalent cobalt ion increases considerably the acidity of the alcoholic group.Reactions of the 'R Radicals with the (nta)Co"'-R(H,O)- complexes All complexes except those with 'CH,CO, and C0;- decomwse by second-order routes consistent with reactions (St-(S). No first-order routes, which would yield either (nta)Co12- or (nta)Co'II, were observed. For R = :CH3 and :CH,OH the only products observed are the dimers, C,H6 and HOCH,CH,OH. For :R = :CH(CH,)OH and :C(CH,),OH a mixture of the dimers and disproportionation products is obtained. The small rate constant for reaction of *C(CH,),OH with its complex, 2k6 z 3 x lo7 dm3 mol-1 s-' or less, is probably a steric effect.The results thus point out that carbon-carbon bond formation occurs via reactions of aliphatic free radicals with complexes with metal-carbon 0 bonds. Such reactions have been studied kinetically only in a few though implicated in many cobalt catalysed reaction^.^^ The formation of the carbon*arbon bond might occur via two mechanisms: (a) attack of the free radical on the carbon bound to the central metal cation; (b) attack of the free radical on the central cation followed by reductive elimination. 'R + (nta)Co"'-R(H,O)- -+ (nta)CoIV-(R), -+ (nta)Co"(H,O); + R-R. (1 5) We had hoped that the measurement of the kinetics of decomposition of (hedta)Co'II- CH,OH- would give evidence as to the detailed mechanism. However, as the mechanism of decomposition of this transient differs from that of (nta)ColI1- CH,OH(H,O)-, no relevant information regarding the detailed mechanism was obtained.This research was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. We thank E. Norton for performing the GC-MS analyses. References 1 J. Halpern, Acc. Chem. Res., 1982, 15, 238. 2 J. Halpern, Pure Appl. Chem., 1979, 51, 2171. 3 (a) J. K. Kochi, in Free Radicals, ed. J. K. Kochi (Wiley, New York, 1973), vol. 1, chap. 11; (b) J. K. Kochi, Organometallic Mechanisms and Catalysis (Academic Press, New York, 1978); (c) R. A. Sheldon and J. K. Kochi, Metal Catalysed Oxidations of Organic Compounds (Academic Press, New York, 1981).4 M. D. Johnson, Acc. Chem. Res., 1983, 16, 343. 5 R. Scheffold, Chimia, 1985, 39, 203. 6 J. Halpern, Pure Appl. Chem., 1983, 55, 1059. 7 (a) B. M. Babior, Acc. Chem. Res., 1975, 8, 376; (b) R. H. Abeles and D. Dolphin, Acc. Chem. Res., 1976, 9, 114; (c) G. N. Schrauzer, Angew. Chem., Znt. Ed. Engl., 1977, 16, 233; ( d ) A. W. Johnson, Q. Rev. Chem. SOC., 1980, 125; (e) R. G . Finke, D. A. Schiraldi and B. J. Mayer, Coord. Chem. Rev., 1984, 54, 1 ; (f) N. Bresciani-Pahor, M. Foriolin, L. G. Marzilli, L. Randaccio, M. F. Summers and P. J. Toscano, Coord. Chem. Rev., 1985, 63, 1. 8 P. B. Chock and J. Halpern, J. Am. Chem. Soc., 1969, 91, 582 and references therein. 9 (a) T. S. Roche and J. F. Endicott, Inorg. Chem., 1974, 13, 1575; (b) T.S. Roche and J. F. Endicott, J. Am. Chem. SOC., 1972, 94, 8622; (c) J. F. Endicott and G. J. Ferraudi, J. Am. Chem. SOC., 1977, 99, 243. 10 (a) A. M. Tait, M. Z. Hoffman and E. Hayon, J . Am. Chem. SOC., 1976,98, 86; (6) A. M. Tait, M. Z. Hoffman and E. Hayon, Znt. f. Radiat. Phys. Chem., 1976, 8, 691. 11 R. Blackburn, M. Kyaw, G. 0. Philips, f. Chem. SOC., Faraday Trans. I , 1975, 71, 2277. 12 H. Elroi and D. Meyerstein, f. Am. Chem. Soc., 1978, 100, 5540.D. Meyerstein and H. A. Schwarz 2949 13 (a) J. H. Espenson and A. H. Martin, J. Am. Chem. SOC., 1977,99, 5953; (b) K. Yoshino, Y . Ohkatsu and T. Tsuruta, Bull. Chem. SOC. Jpn, 1979, 52, 2028; (c) G. M. Schrauzer, M. Hashimoto and A. Maihub, 2. Naturforsch., Teil b, 1980, 35, 588; ( d ) J.Halpern and H. U. Blaser, J. Am. Chem. SOC., 1980, 102, 1684; (e) V. E. Stel’mashok and A. L. Poznyak, Russ. J. Znorg. Chem., 1981, 26, 1324. 14 (a) W. A. Mulac and D. Meyerstein, J. Am. Chem. SOC., 1982, 104, 4124; (b) Y. Sorek, H. Cohen, W. A. Mulac, K. H. Schmidt and D. Meyerstein, Inorg. Chem., 1983, 22, 3040; (c) G. Ferraudi and L. K. Patterson, J. Chem. SOC., Dalton Trans., 1980, 476. 15 S. N. Bhattacharyya and E. V. Srisankar, J. Chem. SOC., Faraday Trans. 1, 1978, 74, 2089. 16 (a) C. Y. Mok and J. F. Endicott, J. Am. Chem. SOC., 1978, 100, 123; (b) J. F. Endicott, K. P. Balakrishnan and C. L. Wong, J. Am. Chem. SOC., 1980, 102, 551. 17 (a) J. Halpern, F. T. T. Ng and G. L. Rempel, J. Am. Chem. Soc., 1979, 101, 7124; (b) F. T. T. Ng, G. L. Rempel and J.Halpern, J. Chem. SOC., 1982, 104, 621 ; (c) F. T. T. Ng, G. L. Rempel and J. Halpern, Znorg. Chim. Acta, 1983,77, L165; ( d ) J. Halpern, S. H. Kim and T. W. Leung, J. Am. Chem. SOC., 1984, 106, 8317. 18 (a) S. M. Chemaly and J. M. Pratt, J. Chem. SOC., Dalton Trans., 1980, 2274; (b) L. Randaccio, N. Bresciani-Pahor, R. J. Toscano and L. G. Marzilli, J. Am. Chem. SOC., 1980, 102, 7373; (c) B. T. Golding, C. S. Sell and P. J. Sellars, J. Chem. SOC., Perkin Trans. 2, 1980, 962; (e) H. B. Gjerde and J. H. Espenson, Organometallics, 1982, 1, 435. 19 (a) R. G. Finke, B. L. Smith, B. J. Mayer and A. A. Molinero, Znorg. Chem., 1983,22, 3679; (b) R. G. Finke and B. P. Hay, Znorg. Chem., 1984, 23, 3043. 20 (a) I. Ya. Levitin, A. L. Sigan, R. M. Bodnar, R. G.Gasanov and M. E. Vol’pin, Znorg. Chim. Acta, 1983,76, L169; (b) A. D. Ryabov, I. Ya. Levitin, A. T. Nikitaev, A. N. Kitaigordskii, V. I. Bakhmatov, I. Yu. Gromov, A. K. Yatsimirsky and M. E. Vol’pin, J. Organometal. Chem., 1985, 292, C4. 21 M. S. Matheson and L. M. Dorfman, Pulse Radiolysis (M.I.T. Press, Cambridge, MA, 1969), chap. 6. 22 M. Anbar, M. Bambeneck and A. B. Ross, Nut1 Bur. Stand. Ref Data Ser., 1973, NSRDS-NBS 43. 23 Farhataziz and A. B. Ross, Nut1 Bur. Stand. Ref Data Ser., 1975, NSRDS-NBS 59. 24 D. Veitwisch, E. Janata and K-D. Asmus, J. Chem. SOC., Perkin Trans. 2, 1980, 146. 25 A. B. Ross and P. Neta, Natl Bur. Stand. Ref Data Ser., 1982, NSRDS-NBS 70. 26 M. Anbar, Farhataziz and A. B. Ross, Nut1 Bur. Stand. Ref: Data Ser., 1975, NSRDS-NBS 51. 27 J. Lati and D. Meyerstein, J. Chem. Soc., Dalton Trans., 1978, 1105. 28 A. A. Martell and R. M. Smith, Critical Stability Constants (Plenum Press, New York, 1974), vol. 1, 29 S. A. Chaudhri, M. Gobl, T. Freyholdt and K. D. Asmus, J. Am. Chem. Soc., 1984, 106, 5988. 30 (a) K. L. Brown and E. Zabonyi-Buda, J. Am. Chem. SOC., 1982, 104, 4117; (b) K. L. Brown, S. Ramamurthy and D. S. Marynick, J. Organometal. Chem., 1985, 287, 377. 31 M. Masarwa, H. Cohen and D. Meyerstein, Znorg. Chem., 1986, 25, 4897. 32 H. Cohen and D. Meyerstein, Znorg. Chem., in press. 33 A. Rotman, H. Cohen and D. Meyerstein, Znorg. Chem., 1985, 24, 4158. 34 H. Cohen and D. Meyerstein, Znorg. Chem., 1974, 13, 2434. 35 Y. Sorek, H. Cohen and D. Meyerstein, J. Chem. SOC., Faraday Trans. 1, 1985, 81, 233. 36 J. H. Espenson, Advances in Inorganic and Bioinorganic Mechanisms, 1982, 1, 1 . 37 G. W. Kirker, A. Bakac and J. H. Espenson, J. Am. Chem. SOC., 1982, 104, 1249, 38 M. Freiberg, W. A. Mulac, K. H. Schmidt and D. Meyerstein, J. Chem. Soc., Faraday Trans. 1, 1980, 76, 1838. 39 (a) K. R. Olson and M. Z. Hoffman, J. Chem. SOC., Chem. Commun., 1974,938; (6) H. Cohen and D. Meyerstein, J. Chem. SOC., Dalton Trans., 1977, 1056; (c) J. H. Espenson, M. Shimura and A. Bakac, Znorg. Chem., 1982, 21, 2537. 40 (a) R. C. McHatton, J. H. Espenson and A. Bakac, J. Am. Chem. Soc., 1982, 104; 353; (b) W. A. Mulac, H. Cohen and D. Meyerstein, Znorg. Chem., 1982, 21, 4016. 41 (a) R. Scheffold, Chimia, 1985, 39, 203; (b) J. K. Kochi and F. F. Rust, J. Am. Chem. Soc., 1964, 83, 2017; (c) G. N. Schrauzer, J. W. Sibert and R. J. Windgassen, J. Am. Chem. SOC., 1968, 90, 6681 ; (d) T. Funabiki, B. D. Gupta and M. D. Johnson, J. Chem. SOC., Chem. Commun., 1977, 653; (e) P. Bougeard, A. Bury, C. J. Cooksey and M. D. Johnson, J . Am. Chem. SOC., 1982, 104, 5230; (f) R. Scheffold, G. Rytz, L. Walder, R. Orlinski and Z. Chilmonezyk, Pure Appl. Chem., 1983,55, 1791 ; ( g ) Y. Murakami and Y. Hisaeda, Bull. Chem. SOC. Jpn, 1985,58,2652; (h) S . Fukuzumi, K. Ishikawa and T. Tanaka, Chem. Lett., 1985, 1355; (i) S. Fukuzumi, K. Ishikawa and T. Tanaka, J. Chem. SOC., Dalton Trans., 1985, 899. p. 142. Paper 71943 ; Received 29th May, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402933

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(9), 2951-2957 Correlations of Heterogeneity Parameters for Single-solute and Multi-solute Adsorption from Dilute Solutions Adam W. Marczewski, Anna Derylo-Marczewska and Mieczyslaw Jaroniec* Institute of Chemistry, M. Curie-Sklodowska University, 20031 Lublin, Poland Solute adsorption from dilute solutions is considered in terms of a new description of physical adsorption from multicomponent mixtures on heterogeneous solids. This description requires that the type of adsorption energy variability is similar for all components of the mixture on different surface sites. The theoretical equations are used to predict the heterogeneity parameter for a multi-solute adsorption system by means of the het- erogeneity parameters which characterize the suitable single-solute ad- sorption systems.This prediction procedure is examined by using the experimental isotherms of organic solute adsorption from dilute aqueous solutions on activated carbons. Several methods for estimating the parameters which characterize multicomponent adsorption equilibria have been proposed in the literature [see ref. (1H5) and references therein]. One of these methods was formulated in terms of the adsorption theory on heterogeneous solid surface^.^^ The basic equation of this theory, the integral equation, which defines the relative partial adsorption of the ith solute from a dilute aqueous solution containing n solutes, is as follows: 9 i ( n ) , t = I, / 9 i ( n ) , l ( c , ~ ) x ( ~ ) d ~ ; i = 172, * * * , n (1) where, 9i(n),t is the relative adsorption of the ith solute for a heterogeneous solid, 9i(n),l is the local relative adsorption, c = (cl, c,, ..., c,) is the one-dimensional matrix of solute concentrations, E = (El, E,, .. ., En) is the one-dimensional matrix of reduced adsorption energies E, = EJRT for i = 1,2, ..., n, Ei is the reduced adsorption energy of the ith solute with respect to solvent, x(E) is the n-dimensional distribution function of adsorption energies and AE is the integration range. Eqn (1) may be solved analytically for the multi-Langmuir type local isotherm and different energy distribution^.^ These solutions contain the parameters, which may be evaluated from the experimental data of single-solute adsorption, and they are useful for predicting the multi-solute adsorption equilibria.However, it should be emphasized that these equations were obtained by using some mathematical approximations4 and simplifying assumptions, e.g. identical shape of the adsorption energy distribution functions for all solutes;4 this assumption limits the usefulness of the above solutions. In this paper a new method for predicting the values of heterogeneity parameters for multi-solute adsorption by means of suitable parameters obtained from single-solute adsorption systems is presented. This method is based on the recent description of gas and liquid adsorption on heterogeneous ’ Although this description assumes the existence of a certain correlation between adsorption energies for all solutes and surface sites, in comparison to the previous methods4v5 it is more general.295 12952 Heterogeneity Parameters Theory Let us assume that the adsorption energies for all solutes on different types of surface sites change in such a way that the mutually synonymous dependences for the adsorption energies of two solutes i and j exist: Ej = Ej(Ei) and Ei = Ei(Ej). (2) These dependences are equivalent to the following dependence between the adsorption energies and distribution functions for the components i and j adsorbed on the site *: xj(Ej) dEj = F(E,*) = F(E7) = F* (3) where F(EJE ( 0 , l ) is the integral distribution function. The assumptions (2) and (3) allow considerable simplification of the integral eqn (1) : s" xi(Ei)dEi = p: E<, min Ej, min where Koi is the entropy factor and Ei(F) is obtained from eqn (3).Introducing into eqn (4) the function zi(F) = Ei(F) - Ei ( 5 ) we obtain O 1 + 2 Ki ci exp [zj(F)] J j-1 where Ki is the constant which describes solute-solvent phase-exchange reaction and corresponds to the mean adsorption energy E,: Ki = Koi exp (Ei). (7) Comparison of eqn (1) and (6) shows that eqn (6) is considerably simpler than eqn (1). Moreover, this equation contains only the parameters which may be obtained from the experimental data of single-solute adsorption. Thus, eqn (6) may be very useful for predicting the parameters of multi-solute adsorption. Eqn (6) describes the competitive solute-solvent adsorption. For high solute surface coverages the role of solvent in adsorption process may be neglected and then the competitive adsorption between solute molecules dominates.In this case, the bi-solute adsorption is described by the following integral equations : where Eqn (8) corresponds to the distribution function x12(E12 = El -E2) with an energy dispersion'l ' If the energy distributions xi(Ei) for both solutes correspond to the Langmuir- Freundlich-type isotherm, then their competitive adsorption is well represented by the equation*. Ki2 ci2 = Ki ci/(K2 c2), zi2(F) = zi(F) -z2(F). (9) 0 1 2 = 1% - 0 2 1 . (10) %2,, t A 2 L t = ( K l 2 C12)m" (1 1)A . W. Marczewski et al. where The energy dispersion cri is equal to 2953 (12) where o12 and m,, describe the heterogeneity effects for competitive adsorption of two solutes. For the special case of dilute solution, when all solutes are characterized by similar energy distribution functions, sum of the surface coverages i9i(n,,t for all solutes may be or for Azi(F) NN 1 d F i-1 e, = 1 + K, exp [$F)] 2 Ril ci exp [Azi(F)] i-1 where q F ) is the mean distribution function, Azi(F) = zi(F) - 8 F ) and lAzi(F)I < q F ) .Eqn (14b) is formally identical to the known isotherm for adsorption of many solutes characterized by the distribution functions of the same shape, but is shifted on the energy axis only. For quasi-Gaussian distributions of the Langmuir-Freundlich type and adsorption of two solutes, the isotherm eqn (14b) has the following mathematical form : e, = [Kl(Cl+ K21 C2)Y 1 + [K1(c1 + K21 c2)]% - In fig. 1 the model isotherms are drawn in the coordinates of eqn (15) Ot us. [In(cl + g2, c,)].The solid lines show the course of these isotherms for single-solute adsorption from dilute solutions, which are characterized by the heterogeneity parameters (energy dispersions) ml(ol), m,(o,) and the equilibrium constants K1, K,. The thick dashed line denotes course of the isotherm (1 5) for bi-solute adsorption. The asymptotes for all the isotherms and for the adsorption isotherm of the second solute against its concentration are also drawn in fig. 1 (thin dashed lines). Analysis of the curves in fig. 1 leads to the conclusion that the general eqn (6) and (14a) describe the whole range of intermediate adsorption (c, # 0 and c, # 0) which is placed between the isotherms of single solutes plotted in the coordinates Ot us. 1n(c1+K2, c,). The isotherm (15) obtained from eqn (14b) describes only the centre part of this range (i.e.the comparable adsorptions of both components). In this case the isotherm (15) (m, = m2) is only an analytical approximation of eqn (14a), thus making possible a simultaneous usage of eqn (1 1) for description of competitive adsorption of both solutes and eqn (1 5 ) for approximating the global adsorption of all components. Results and Discussion In order to confirm assumptions (2) and (3) of our model and to investigate the usefulness of the equations, ten experimental systems of bi-solute adsorption from dilute2954 C L E - Heterogeneity Parameters 0 ln(c1+ 221 C2) Fig. 1. Model Langmuir-Freundlich isotherms and their asymptotes for adsorption of single solutes (-) and their mixtures (dashed area), and the isotherms (15) (---).The curves are presented in the coordinates of eqn (15). (a) rn2 In K,, (b) ~FI In K,, (c) rn, In K2. Table 1. Experimental systems adsorbed from dilute aqueous solutions on activated carbons code experimental system adsorbent T / K ref. A 'B C D E F G H I J phenol( 1 )-p-ni trophenol(2) p-nitrophenol( 1 )-p-chlorophenol(2) p-nitrophenol( 1 )-benzoic acid(2) p-ni trophenol( 1 )-o-phenylphenol(2) p-chlorophenol( 1 Fphenylacetic acid(2) 2,4-dichlorophenol( 1 )-dodecylbenzo yl acetone( 1 kpropionitrile(2) p-cresol( 1 )-p-chlorophenol( 2) phenol( 1)-benzoic acid(2) p-chlorophenol( 1 )-phenol(2) sulphonic acid(2) B10 BlO BlO B10 B10 BlO F300 F300 F400 F400 293 293 293 293 293 293 298 298 298 298 10 10 10 10 10 10 11,12 11,12 13 13, 14 aqueous solution^^^-^^ and the suitable single-solute adsorption systems have been analysed.The experimental systems studied are summarized in table 1. The single-solute adsorption isotherms have been analysed by the following linear form of the Langmuir-Freundlich equation : In ni = mi InKi+mi h e i ni, max - ni where ni is the adsorption of the ith component and ni,max is its maximum adsorption. The heterogeneity parameters m, and adsorption energy dispersions oi calculated by optimizing eqn (16) for all single-solute adsorption systems (table 1) are compared in table 2. Table 2 also contains the values of the heterogeneity parameters and energy dispersions evaluated by means of eqn (1 1) (m12, opt, o12,0pt) and eqn (1 5 ) (mopt, gOPt) fromA.W. Marczewski et al. 2955 Table 2. Comparison of heterogeneity parameters mopt and dispersions oopt obtained by optimizing the experimental data according to eqn (11) and (15) with the suitable values calculated theoretically from the dependences meal =A@ = (a, +0,)/2] and m12,cal =f(b,, = lo1 --n2l) ~ A B C D E F G H I J ~ 0.38 (4.42) 0.28 (6.22) 0.28 (6.22) 0.28 (6.22) 0.34 (5.02) 0.22 0.67 0.31 (5.48) 0.25 (7.02) 0.215 (8.24) (8.04) (2.01) 0.28 (6.22) 0.34 (5.02) 0.41 (4.03) 0.19 (9.37) 0.30 (5.77) 0.35 (4.85) 0.71 (1.80) 0.27 (6.55) 0.29 (6.08) 0.25 (7.02) 0.71 (1 $0) 0.83 (1.20) 0.64 (2.19) 0.50 (3.15) 0.92 (0.75) 0.49 (3.19) 0.99 (0.21) 0.86 (1.07) 0.89 (0.95) 0.83 (1.21) 0.77 (1.50) 0.91 (0.83) 0.81 (1.31) 0.66 (2.06) 0.74 (1.65) 0.40 (4.16) 1 .oo (0.00) 0.83 (1.17) 1.18 0.75 (1.60) (0) 0.33 (5.32) 0.31 (5.62) 0.345 (5.12) 0.226 (7.80) 0.32 (5.40) 0.271 (6.45) 0.69 (1.91) 0.29 (6.02) 0.27 (6.55) 0.23 (7.63) 0.33 (5.19) 0.34 (5.02) 0.35 (4.85) 0.180 (9.91) 0.33 (5.19) 0.35 (4.85) 0.66 (2.06) 0.28 (6.22) 0.25 (7.12) 0.24 (7.40) the bi-solute adsorption data in comparison to the suitable parameters calculated from the single-solute adsorption data and by using eqn (1 0), (1 2) and (1 3) (m,,, cal, a,,, cal) and by averaging the single-solute dispersions [g,,, = (a, + a,)/2].In fig. 2 the dispersion values evaluated directly from the bi-solute adsorption data and predicted by means of the single-solute parameters are compared. In fig. 2(a) a comparison of the values of the average dispersion uopt evaluated from the bi-solute adsorption data (see table 1) and the average dispersion [(a, + a,)/2] calculated from the dispersions for single solutes is presented.The dispersion values for adsorption of single solutes are also shown in fig. 2 (the ends of the vertical segments). The dashed line corresponds to a straight line of the orthogonal regression. The high correlation coefficient (0.9893) and the closeness of the slope to unity (1.020) confirm the correctness of the simplifying assumptions [eqn (14b) and (15)] and the basic assumptions [eqn (2) and (3)] concerning correlation between adsorption energies of all solutes. For comparison, in fig. 2(a) the points referring to the values o12, opt (m,,, opt) calculated by means of eqn (1 1) are also presented. The slopes of the straight lines of orthogonal regression : z O.26[(al + a,)/2] (correlation coefficient = 0.83) and Q ~ ~ , ~ ~ ~ x 0.24@0pt (correlation coefficient = 0.78) (lines are not plotted) and the suitable values of the correlation coefficient prove that dependences between the averaged dispersions for single-solute adsorption and the mixtures [eqn (1 5)], and the dispersion characterizing competitive solute adsorption [eqn (1 l)] are not simple.In fig. 2 (b) the theoretical values of dispersions calculated from eqn (I 1) for a competitive adsorption of two solutes 012,cal and the optimized values according to eqn (11) for the experimental systems from table 1 (0) are compared. A statistical deviation is a result of many factors, such as: error in the calculation of heterogeneity parameters m,, m,, miopt, m12, opt ; approximation of real2956 8 6 U 4 2 0 Heterogeneity Parameters ac A ./ / 0 0 / / / / / Fig.2. (a) Comparison of the optimized average dispersions for bi-solute adsorption [eqn (1 5)] with the average dispersions for single solutes sea, = (a, + a2)/2 (0) (tables 1 and 2) ; the straight line of orthogonal regression: = 1.020~7~~~ (correlation coefficient r = 0.9893) (a) the optimized values of dispersion a,,,,,,(rn,,,,,,) [eqn (1 l)] for competitive adsorption of mixture components. (b) Comparison of theoretical values of dispersions calculated from eqn (12) for competitive adsorption, with the optimized values a12,0pt(rn12,0pt) [eqn (1 l)] for experimental data (tables 1 and 2); the straight line of orthogonal regression: = 1.014a,2,,,t (correlation coefficient r = 0.9242).energy distribution functions by Langmuir-Freundlich- type distributions ; neglect of the effects of differences in molecular sizes, dissociation, multilayer formation and molecular interactions ; only partial fulfillment of assumptions (2) and (3) concerning the existence of commonly synonymous relations between the adsorption energies ; the approximate character of eqn (1 1) for competitive adsorption in comparison to the exact eqn ( 6 ) ; eqn (1 1) neglects the solvent effect (it is partly justified by very high adsorption of organic solutes from aqueous solutions). Despite this deviation slope of the straight line of the orthogonal regression is close to unity (1.014) (correlation coefficient = 0.9242) and confirms the usefulness of the adsorption model proposed and the correctness of assumptions (2) and (3).References 1 C. E. Brown and D. H. Everett, Colloid Science (Specialist Periodical Report, The Chemical Society, 2 D. H. Everett and R. T. Podoll, Colloid Science (Specialist Periodical Report, The Chemical Society, 3 J. Davis and D. H. Everett, Colloid Science (Specialist Periodical Report, The Chemical Society, 4 M. Jaroniec, A h . Colloid Interface Sci., 1983, 18, 147. 5 A. Derylo-Marczewska and M. Jaroniec, Surf. Colloid Sci., ed. E. Matijevic (Wiley-Interscience, New 6 A. W. Marczewski, A. Derylo-Marczewska and M. Jaroniec, Monatsh. Chem., 1988, 119, in press. 7 A. W. Marczewski, A. Derylo-Marczewska and M. Jaroniec, Chem. Scr., in press. 8 A. Derylo and M. Jaroniec, Chem. Scr., 1982, 19, 108. 9 M. Jaroniec, A. Derylo and A. W. Marczewski, Monatsh. Chem., 1983, 114, 393. London 1975), vol. 2. London 1979), vol. 3. London 1983), vol. 4. York, 1987), vol. 14.A . W. Marczewski et al. 2957 10 F. A. DiGiano, G. Baldauf, B. Frick and H. Sontheimer, Chem. Eng. Sci., 1978, 33, 1667. 11 C. J. Radke and J. K. Prausnitz, AZChE J., 1972, 18, 761. 12 C. J. Radke and J. K. Prausnitz, Znd. Eng. Chem. Fundam., 1972, 11,445. 13 K. Fukuchi, H. Hamaoka and Y. Arai, Mem. Fac. Eng. Kyushu Univ., 1980, 21, 339. 14 K. Fukuchi, F. Yamashita, J. Hirayama and Y. Arai, Enu. Cons. Eng., 1981, 10, 297. Paper 71964; Received 1st June, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402951

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. Soc., Faraday Trans. 1, 1988, 84(9), 2959-2966 Calculation of Nuclear Magnetic Resonance van der Waals Chemical Shifts based on a Generalized Polyatomic London Dispersion Theorem John Homer* and Mansur S. Mohammadi Department of Chemical Engineering and Applied Chemistry, Aston University, Aston Triangle, Birmingham B4 7ET A recently published theory for net attractive polyatomic London dispersion forces has been used to calculate 'H n.m.r. gas-to-solution chemical shifts. Good agreement with experimental data was found. The problem of explaining solvent effects on n.m.r. chemical shifts is one of long- standing difficulty. 1v Even the gas-to-solution shifts of isotropic molecules have proved difficult to characterize quantitatively until recently. Homer and Perciva13 have shown that these van der Waals shifts can be explained with reasonable quantitative precision by using a reaction-field approach together with the treatment of a new 'buffeting' intermolecular force.Their approach has been revised recently to produce a generalized theory for polyatomic London dispersion forces that places no direct reliance on empirical scaling factor^.^ This recent theory, which embraces both attractive and repulsive forces between molecules at equilibrium separations, has been used to predict with precision the heats of vaporization for an extensive range of compo~nds.~ The present paper presents an adaptation of the generalized London theory for the calculation of n.m.r. van der Waals chemical shifts. Theoretical It is well known that when corrected for the effects of volume magnetic susceptibility, observed gas-to-solution chemical shifts for isotropic solutes and solvents yield the van der Waals chemical shift,'.ow. The van der Waals screening constant is given by5. ow = - B ( E 2 ) (1) where B is the classical screening coefficient and ( E 2 ) is the mean-square electric field producing 0,. Although the acceptance of eqn (1) is widespread, the methods of calculating ( E 2 ) are numerous [see e.g. ref (2) and references therein]. The recently proposed method of characterizing dispersion forces4 affords a new route to ( E 2 ) . A simple adaptation of this method reveals that ow can be given by ow = - 2BZ, R-%xj (m2)j F(i,j). (2) where xi is the number of atoms of type j in the solvent molecule and 2, is the number of nearest solvent neighbours that surround the solute at infinite dilution in the solvent.lt has been suggested4 that the relationship between the nearest numbers of neighbours for a single (solvent) molecule in its pure liquid and solid (S) phases may be given by It will be assumed that eqn (3) applies to the number of nearest neighbours surrounding a solute molecule at infinite dilution in various solvents. The factor F(i, j ) accounts for the averaging of the inverse sixth power of the distance 2, % zs- 1. (3) 29592960 N.M.R. van der Waals Shifts I I Fig. 1. Schematic definition of parameters relevant to the London dispersion interaction between two C(CH,), molecules. r i , j between atoms i and j in the solute and solvent during random molecular rotation^,^ in terms of the equilibrium intermolecular separation R through (ri,;) = F(i,j) R-6.(4) Whilst an explicit expression for F(i, j ) has been ~ b t a i n e d , ~ the simpler form, obtained numerically, is given4 by F(i,j) = (1 +0.727q- 17.5O9q2+25.550q3)-' ( 5 ) where q = (di+dj)/2R (6) with dt being the distance of the centre of the atom i from the centre of mass of the solute, and dj being the corresponding parameter for atom j in the solvent molecule. The atomic mean-square electric moment required for eqn (2) may be deduced from London's treatment' as where a and I are the polarizability and first ionization potential, respectively. In fact, eqn (7) is applicable strictly to isolated atoms of the inert-gas type. When considering molecules, the polarizability and ionization potential for bonded atoms are required to define (m2). Although without rigorous theoretical justification, it has been dem- onstrated recently* that the value of (m2) for bonded atoms may be adequately approximated to the value of (m2) for the inert gas that is closest to the bonded atom in the periodic table.Eqn (2) is strictly applicable to those molecules that rotate more rapidly than they translate in the liquid phase.4 Consequently, eqn (2) will be used to predict ow for molecules expected to fall in this class. To illustrate the implementation of eqn (2) the case of ci, for the lH resonance ow of pure C(CH,), is considered. Relevant features of this system are included in fig. 1. In this case eqn (2) becomes ow = - 2 8 2 , R-6[(m2),1 F(H, C,,) +4(m2),, F(H,C2) + 12(rn2), F(H, H)] (8) wheres (m2)>, = (m'),, and (m'), = (m'),, and 2, from eqn (3) equals 11 because'' Zs = 12.The values of F(H,j) are determined by using di and dj, as defined in fig. 1, in either the explicit expression4 for F ( i , j ) , where applicable, or in the slightly less precise (m2) = 3ccI/2 (7)J . Homer and M . S. Mohammadi 296 1 Table 1. Calculated and experimental values of -cw (ppm) for pure compounds at 30 "C ( Z , , = 11. B = 0.54 x 10l2 cm3 ppm erg-l) compound R / A &',/Aa RIA-J/A H,/kJ mol-' expt12 calc. H2 3.89 4.3 4.9 1 C2H4 4.82 C6H12 6.34 C(CH3), 6.3 Si(CH,), 6.72 Ge(CH,), 6.72 Sn(CH,), 6.94 Pb(CH,), 7.01 CH4 'ZH6 3.89 4.23 4.9 1 4.82 6.27 6.67 6.92 6.78 6.8 6.74 3.27b 4.28" 4.98" 4.67d 6.89d 6.78d 6.70d 6.84d 6.96d - 1.05" 8.1P 1 5.63e 13.53s 30.05e 22.36" 26.91 " 29.76" 33.01" 36.96" - 0.230s 0.279h 0.29 1' 0.203,O. 192 0.229,0.217 0.228,0.205 0.260 0.297,0.3 10 0.358 0.092 0.276 0.324 0.290 0.20i 0.219 0.226 0.260 0.294 0.360 a The volumes used in eqn (9) are at 30 "C except for the first four compounds which are at their melting points. S. Bretsznjder, Prediction of Transport and Other Physical Properties of Fluids (Pergamon Press, Oxford, 1971). R. Reid and T. K. Sherwood, The Properties of Gases and Liquids (McGraw-Hill, New York, 1958). F. H. A. Rummens, W. T. Raynes and H. J. Bernstein, J . Phys. Chem., 1968, 72, 21 11. " Handbook of Physical Chemistry and Physics (CRC Press, Cleveland, Ohio, 53rd edn, 1972-73). Lange's Handbook of Chemistry (McGraw-Hill, New York, 12th edn, 1979).Calculated by extrapolating the gaseous data (-0.482~) to the liquid at its melting point ( V = 33.6 cm3 mol-'). ' The dependence of uw on density for C2H6 and C,H, are given (S. Gordon and B. P. Dailey, J . Chew. Phys., 1961,34, 1084) as -0.488~ and -0.515~ ppm, respectively, from which the entries in the table are deduced. eqn (5); To obtain ow in ppm, B, R and (m2)inert should have units of 10l2 cm3 ppm ergp1, A and In order to proceed further with the general use of eqn (2) it is necessary to have a satisfactory method of deducing R for the liquid state. One way is to make use of the knowledge that whilst the number of nearest neighbours changes on melting, R changes to a smaller extent." Accordingly, where values of R are known from studies of the solid state these may be used in eqn (2).Where solid-state values of R are not known an alternative approach may be used. cm3 erg, respectively. For pure compounds it has been found4.' that R may be obtained from R = 2(0.17 Y,); (9) where Vm is the molecular volume, and the factor of 0.17 accommodates the random packing of particles in a similar manner to that first used by Hertz.12 It has been suggestedg that if the solute is either very much smaller or larger than the solvent, the value of R is obtained from (R,+Rj)/2. Alternatively, if the solute and solvent are of similar size, R is deduced from solvent data, i.e. the solvent cavity size is used for R. Whilst for many molecules eqn (9) proves sati~factory,~?~ it yields spurious values for R for some Group IV tetramethyls (sec table 1).For example, from the liquid-state (20 "C) density of C(CH,), aovalue of 6.67 A is obtained for R using eqn (9), whereas the solid- state value islo 6.21 A. These two values are inconsistent with the suggestion that while 2 changes on melting, R changes to a smaller extent." In view of the success in predicting latent heats of vaporization el~ewhere,~ an alternative way of calculating R is to use the potential-energy expression on which eqn (2) is based to deduce R frop experimental latent heats of vaporization. For C(CH3), this approach yields R = 6.3 A, which agrees well with the solid-state value. This approach to R (given in the data column of table 1) will be used hereafter, and is discussed later.It should be emphasised that whilst this approach is adopted to permit overall consistency, it could be interpreted2962 N.M.R. van der Waals Shifts Table 2. Calculated and experimental values of - a,(ppm) for CH, in several solvents at 30 "C (ZI, = 11, B = 0.54 x cm3 PPm erg-') CH212 5.68 CHBr, 5.83 CBr, 6.32 Br2 4.82 CH,I 5.22 CBrCl, 6.08 CCl, 5.90 CH,Br 5.04 CHCl, 5.67 CH2CI, 5.27 SiCl, 6.40 0.769, 0.767" 0.646, 0.652" 0.594 0.556 0.505, 0.547" 0.533, 0.542" 0.443, 0.4722 0.355, 0.445" 0.407, 0.420" 0.398, 0.407" 0.301, 0.347, 0.765 0.641 0.592 0.502 0.562 0.541 0.540 0.486 0.458 0.527 0.48 1 a Deduced from volumes at 30 "C using eqn (9), and for the pure solvents give the precise latent heats of vaporization. The values of R for the shift calculations are obtained from R = (Rsolvent +R,,,,,e)/2.F. H. A. Rummens, Can. J. Chem., 1976, 54, 254. " W. T. Raynes, J. Chern. Phys., 1969, 51, 3138. to bias the justification of eqn (2). If, however, the values obtained for R from latent heats of vaporization not only permit the calculation of ow but are also in agreement with values obtained for R by other independent means, this will provide some justification for the present theory. The use of eqn (2) depends on an unambiguous and universal value for B. Kromhout and Linder13 have shown quantum mechanically that B = 0.59 x cm3 ppm erg-' for CH;..CH, interactions and Yonemoto14 has similarly shown that B = 0.54 x cm3 ppm erg-' for H;.-H,. In view of the fact that Rummens'~~~ empirical work confirms the latter value, this will be adopted here.The values of di and dj representing actual molecular parameters for the various systems considered here are either given in ref. (4) or have been calculated by the method described therein. Discussion Gas-to-liquid Chemical Shifts Inspection of table 1 reveals satisfactory agreement between the calculated and experimental values of ow for several pure compounds. The fact that for these compounds the values of ow lie within a relatively small screening range is to be expected from the work of Homer and Per~ival.~ They demonstrated that ow should be influenced significantly by the nature of the peripheral atoms of the solvent. For the compounds in table 1 the peripheral atoms are always hydrogen. For solvents with heavier peripheral atoms the present approach, and that of Homer and Per~ival,~ requires that these solvents should give rise to enhanced ow.This is borne out by the data in table 2 which relate to CH, in a range of halogenated solvents. Again the agreement between the calculated and experimental values of ow is generally satisfactory. Data for further mixed solute-solvent systems are presented in table 3. The agreements between calculated and experimental data are again satisfactory. In view of the success of the present approach in predicting pure van der Waals gas- to-solution shifts, it is interesting to consider the case of anisotropic solvents. For these, the (susceptibility-corrected) experimental gas-to-solution shifts can be corrected for van der Waals contributions using eqn (2) to yield estimates of the solvent neighbourJ.Homer and M. S. Mohammadi 2963 Table 3. Calculated and experimental2 values of - a,/ppm at 30 "C for several solute-solvent systems (2, = 1 1, B = 0.54 x cm3 ppm erg-') C6H12 o C(CH3L0 Si(CH31, (R = 6.72 A) CCl, 0 (R = 5.9 A) (R = 6.34 A) solute exptl calc. exptl calc. exptl calc. exptl calc. ( R = 6.3 A) H2 0.485 C2H6 0.305 0.310 0.345 0.370 'GH6 0.394 (R = 6.18 A). 0.420' 0.443 C6H12 0.267 0.265 C(CH314 0.290 0.307 0.320 Si(CH3)4 0.267 0.299 0.322 0.360 C2H4 0.474 0.470 0.500 0.394 - - - - - 0.278 0.273 - - 0.285 0.419 0.300 - 0.195 0.413 0.210 - 0.225 0.394 0.240 - 0.257 0.310 0.203 - 0.203' 0.201 0.192' - 0.230 0.233 0.220 - 0.187 - 0.233 0.239 0.270 - 0.282 - - 0.383 - 0.378 - 0.400 - - - - - 0.397 - 0.385 - - - 0.407 0.268 0.291 - - 0.243 0.220 - - 0.187 0.223 0.229 0.219 - - 0.217 - - - 0.240 0.216 0.255 - 0.182 0.175 0.185 0.168 0.212 - - - 0.228 0.226 0.205 - a Calculated from its latent heat of vaporization: RL.J = 6.32 A.' F. H. A. Rummens, Can. J. Chem., 1976, 54, 254. anistropy screening, oa. Table 4 represents estimates of oa for four common solvents. It can be seen that the signs and magnitudes of oa are as expected.l*16 Moreover, it can be seen that, in general, the magnitude of oa increases as the size of the solute decreases. l6 Equilibrium Intermolecular Separations Eqn (2) derives from a theorem for a net attractive polyatomic London dispersion potential which has been shown elsewhere4 to arise from the resultant of the attractive and repulsive forces. By using a self-consistent approach it provides a satisfactory explanation of latent heats of vaporization and van der Waals nuclear screening constants.Nevertheless, it has to be acknowledged that its rigorous validification depends principally on the consideration of three factors. The first relates to the mean- square electric moment of bonded atoms and the hypothesis that this may be approximated by the value of this parameter for the corresponding inert-gas atom. Although this hypothesis is without theoretical justification, it has been tested extensively.'. It has been shown that the approach permits the satisfactory prediction of molecular first-ionization potentials and the discrimination between molecular structures. The second and third factors concern the methods of calculating 2, and R.Moelwyn-HughesL7 gives the approximate relationship 2, = w- HFIHS) (10) (1 1) which is equivalent to 2, = Z,Hv/(Hv + HF) where HF, Hs and H , are the heats of fusion, sublimation and vaporization, respectively.Table 4. Estimation of the neighbour-anistropy screening, ca, from experimental gas-to-solution shifts and calculated ow for several anistropic solvents (Z,, = 11 except for CS, for which Z,, = 7,) % CH,CN(R = 4.8 A) k B solute exptl" calc. CJa exptl" calc. =a exptl* calc. c a exptlb calc. 6, 3 CH4 C6H,(R = 6.18 A) C(NO,),(R = 6.57 A) CS,(R = 5.14 A) 0.124 -0.432 0.556 -0.053 -0.580 0.530 -0.583 -0.414 -0.169 -0.416 -0.317 -0.099 -0.452 -0.394 -0.058 0.235 -0.379 0.614 -0.120 -0.447 0.327 0.272 -0.225 0.497 0.043 -0.298 0.341 -0.398 -0.206 -0.192 -0.285 -0.241 -0.044 - -0.300 - C6H6 C(CH,), 0.213 -0.218 0.431 0.028 -0.292 0.264 -0.426 -0.201 -0.225 -0.317 -0.234 -0.083 2 Si(CH,), 0.15 1 - 0.229 0.380 0.035 -0.290 0.255 -0.520 -0.198 -0.322 -0.343 -0.237 -0.106 9 h a F.H. A. Rummens, Can. J . Chem., 1976, 54, 254. W. T. Raynes and M. A. Raza, Mol. Phys., 1969, 17, 157.J . Homer and M. S. Mohammadi 2965 Comparison of the values obtained for 2, using eqn (10) and (1 1) with accepted values for Zs demonstrate the validity of approximation (3). If the methods of calculating 2, and (m2) are accepted it follows that values for R may be deduced from, for example, latent heats of vaporization, and these should compare favourably with corresponding values deduced independently.The ex- perimental values for latent heats of vaporization given in table 1 can be deduced using the values given as R in column 1 (these have been used to calculate the values of ow). Note that the values given as R,,,, from eqn (9) are generally in good agreement with those calculated from latent heats of -Japorization. More importantly, the values, given as RL.J, that have been deduced from viscosity data, using the Lennard-Jones potential, are in good agreement with those deduced here. It appears, therefore, that the theorem for a net attractive polyatomic London dispersion p~tential,~ from which eqn (2) derives, is suitable for explaining the properties of molecular systems at equilibrium molecular separations. This in itself raises the important question concerning the reason why the Lennard-Jones potential requires an R-12-dependent term to achieve the same ends as that on which the present work is based, and which does not require such a term.Although necessarily based on speculation, it is interesting to address this question. The potential function at the heart of this work is based on the assumption that at equilibrium intermolecular separations, the contribution from overlap repulsion is negligible and that the longer-range dipolar repulsions are accommodated in the net attractive dispersion term. The evidence so far suggests that if bonded-atom properties and molecular rotation [through F(i,j)] are properly accounted for, it becomes unnecessary to account separately for an overlap repulsion term based on, say, R-12.The choice of alternative powers of n in the general R-"-dependent repulsion term by some workers could be taken to indicate that this is an empirical scaling term that has become necessary, simply because the attractive R-6 term has not been characterized properly. Of course, at very small intermolecular separations where overlap is important it will be necessary to introduce an additional R-" repulsion term. Conclusions The polyatomic London dispersion potential presented el~ewhere,~ and used to characterize latent heats of vaporization, is shown here to characterize n.m.r. van der Waals chemical shifts. It appears, therefore, that this may be an authentic intermolecular potential function that is capable of more extensive use.The satisfactory use of the explicit polyatomic London theory in predicting ow when using values of R deduced from experimental heats of vaporization and a value of B = 0.54 x lo-'' cm3 ppm erg-' that has been proposed independentlyl43 l5 again indicates the consistency of the present approach. We are grateful to Prof. W. R. McWhinnie for the provision of facilities and M. S. M. thanks Evode Ltd (Stafford) and the International Family Service for financial support. References 1 J. Homer, Appl. Spectrosc. Rev., 1975, 9, 1 . 2 F. H. A. Rummens, van der Waals Forces in NMR Intermolecular Shielding, Vol. 10, NMR Basic 3 J. Homer and C. C. Percival, J. Chem. Soc., Faraday Trans. 2, 1984, 80, 1. 4 J. Homer and M. S. Mohammadi, J . Chem. SOC., Furuday Trans. 2, 1987, 83, 1957. 5 T. W. Marshall and J. A. Pople, Mol. Phys., 1958, 1, 199. 6 M. J. Stephen, Mol. Phys., 1958, 1, 223. 7 F. London, Trans. Faraday SOC., 1937, 33, 8. Principles and Progress, ed. P. Diehl, E. Fluck and R. Kosfield (Springer-Verlag, Hiedleberg, 1976).2966 N.M.R. van der Waals Shifts 8 J. Homer and M. S. Mohammadi, J. Chem. SOC., Faraday Trans. 2, 1987, 83, 1975. 9 M. S. Mohammadi, Ph.D. Thesis (Aston University, 1986). 10 A. H. Mones and B. Post, J . Chem. Phys., 1952, 20, 755. 11 E. A. Moelwyn-Hughes, Physical Chemistry (Pergamon Press, Oxford, 2nd revised edn, 1961). 12 P. Hertz, Math. Ann., 1909, 67, 387. 13 R. A. Kromhout and B. Linder, J. Magn. Reson., 1969, 1, 450. 14 T. Yonemoto, Can. J. Chem., 1966, 44, 223. 15 F. H. A. Rummens, Mol. Phys., 1971, 21, 535. 16 J. Homer and D. L. Redhead, J. Chem. Soc., Faraday Trans. 2, 1972, 68, 1049. 17 E. A. Moelwyn-Hughes, Chemical Statics and Kinetics of Solutions (Academic Press, New York, 1971). Paper 71966; Received 1st June, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402959

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(9), 2967-2977 Ordered Distribution of Aluminium or Gallium Atoms in Zeolite L Tetsuo Takaishi Toyohashi University of Technology, Toyohashi 440, Japan The ordered distribution of aluminium or gallium atoms in the framework of zeolite L has been determined by analysing 29Si magic-angle-spinning (m.a.s.) n.m.r. spectra. As a first step, an ideal crystal with no defects was investigated ; subsequently a small number of defect atoms were introduced and a best fit was obtained between calculated and observed 29Si n.m.r. spectra. Zeolite L has a doubled c constant and belongs to the space group symmetry P6,. The proposed model explains the geometries and properties of extra-framework cation sites. Zeolite L was first synthesized by Breck and Acara in 1958,l and its framework structure was proposed by Breck and Flanigen,2 and reinvestigated and corrected by Barrer and coworker^.^? Recently Newsam carried out structure refinements of zeolite L and galliated zeolite L, using neutron diffraction technique^,^^ and proposed an uneven distribution of aluminium atoms in zeolite L.He also measured 29Si m.a.s.n.m.r. spectra of these zeolites, but did not fully analyse the spectra. In this paper, his 29Si n.m.r. data have been analysed to obtain a detailed knowledge of the ordered distribution of aluminium atoms in the framework. Structural and N.M.R. Data The typical unit cell content of zeolite L is (K,Na),A1,Si27072 * nH20 and its Si/Al ratio varies in the range 2.6-3.5. The structural analysis was carried out by assuming the space group symmetry P6/mmm (of, or no.191), in which the difference between Si and A1 is neglected or a random distribution of Si and A1 was assumed. The true symmetry of zeolite L is lower than this if Si and A1 are distributed regularly. The highest possible symmetry assigned to zeolite L is P6, (G or no. 173) with a doubled c constant. The doubled c constant produces super-structure lines, i.e. (OOOr) lines with half-integer values of 1. On the other hand, the extinction conditions for P6, are hkil, no conditions and 0001, I = 2n. These two effects compensate each other, and no half-integer lines are observed, in accordance with experimental observation. The framework of zeolite L is shown in fig. 1. Cancrinite cages (&-cages) and double- six rings (D6R) are alternatively linked along the c-axis, constituting a chain of -D~R-E-D~R-E- [fig.1 (a)]. Six such rings are cross-linked to form flat 12-membered oxygen rings (12-rings) which constitute a large pore channel parallel to the c-axis [fig. 1 (b)]. The edges of the 12-ring are called the T4 sites, and the edges of the D6R are called the T6 sites. F(T4) and F(T6) are the fractional aluminium or gallium occupancies of T4 and T6 sites, respectively. F(T4)/F(T6) was found to be 1.4 and 1.0 for aluminium zeolite (Al-L) and gallium zeolite L (Ga-L), respectively, through detailed analysis of neutron diffraction Newsam did not show in his paper5v6 the 29Si m.a.s.n.m.r. spectra of these zeolites, which are shown in ref. (7) along with detailed discussions.(A spectrum obtained by us 2967Distribution of A1 and Ga in Zeolite L Fig. 1. Framework of zeolite L. (a) Chain of cancrinite cages and double 6-rings along the c-axis. A, A1 atoms; A, A1 atoms located on a hidden site. (b) Projection of the framework on the c-plane. Table 1. Populations of A1 (or Ga) atoms and Si(v-Al) units [or Si(v-Ga) units] in the unit cell of zeolite L Si(4-Al) Si(3-Al) Si(2-A]) Si( 1 -Al) Si(0-A1) or or or or or F(T4) species A1 or Ga Si(4-Ga) Si(3-Ga) Si(2-Ga) Si( 1 -Ga) Si(0-Ga) F(T6) observeda A1-L 9.26 0.00 1.60 10.27 11.64 3.22 1.4 Ga-L 9.80 0.26 1.76 10.75 11.37 2.06 1 .o ideal 9.00 0.00 0.00 12.00 12.00 3 .OO 1 .o model I 9.00 0.00 0.00 12.00 12.00 3 .OO 1 .o model I1 9.00 0.00 0.00 15.00 6.00 6.00 1 .o (i) ALL 9.26 0.00 1.61 10.29 11.63 3.21 1.19 model I with defectsb (ii) Ga-L 9.78 0.28 1.78 10.60 11.46 2.10 1.01 a Data from Newsam.’ Concentrations of defects, see table 3.is shown in the Appendix.) Lippma et al. showed that the central 29Si atoms in the 29Si(OA1),(OSi),., unit, with v = W, have different chemical shift values in their m.a.s.n.m.r. spectra.8 (Applications of 29Si m.a.s.n.m.r. to zeolite science has been reviewed by Klino~ski.~) On this basis, the spectra of Al-L zeolite is deconvoluted into four components referred to as Si(3-A1), Si(2-A1), Si( 1-Al) and Si(0-AI), where Si(v-Al) designates the central Si in the Si(OAl)v(OSi)4-, unit. The populations of each component deduced are given in table 1. The 29Si atoms on T4 and T6 sites have different chemical shift values, but each component of Si(v-Al) observed could not be further deconvoluted into their respective parts, referred to as T4 and T6 sites.The spectrum of Ga-L zeolite is deconvoluted into five components referred to as Si(v-Ga), with v = 0-4. Their populations are also given in table 1.T. Takaishi 2969 Table 2. Possible partitions of 18 A1 atoms into eight 6-rings and two lZrings, which are contained in a doubled unit cell 6-rings pattern neighbouring numbering given chain chain 12-ring Fig. 2. Distribution of A1 triads along the E-D~R chain. A, A1 atom; A, A1 atom located on a hidden site; e, Si(3-Al). (a) Unallowable configuration, (b) chain I, (c) chain 11, (d) chain 111.2970 Distribution of A1 and Ga in Zeolite L Fig. 3. Three kinds of models for the ordered distribution of A1 atoms in the framework of zeolite L.A, A1 atom; A, A1 atom located on a hidden site; 0, Si(3-Al). (a) Model I, (b) model 11, (c) model 111. Distributions of A1 and Ga Atoms in an Ideal Crystal of Zeolite L without Defects The zeolite sample used by Newsam contained fractional numbers of A1 or Ga atoms per unit cell; namely, Al-L zeolite contained 9.26 A1 atoms and Ga-L zeolite contained 9.80 Ga atoms, in contrast to the stoichiometric value of 9.00. Let us consider, as a first approximation, an ideal crystal with a composition of (K,Na),AI,Si,,O,, having noI L622972 Distribution of A1 and Ga in Zeolite L Table 3. Changes in the populations of A1 (or Ga) atoms and Si(v-Al) units [or Si(v-Ga) units] produced by various defects A1 or Ga Si(4-A1) Si(3-Al) Si(2-A1) Si( 1 -Al) Si(0-Al) defects on T4 on T6 Si(4-Ga) Si(3-Ga) Si(2-Ga) Si(1-Ga) Si(0-Ga) or or or or or (9 (ii) 0.44 0.28 0 1 -1 0 1 0 -0.18 0.50 0 3 0 3 0 2 0 1 1 4 0 0 mixture 0.00 1.61 0.28 1.78 -2 -2 -4 -1 -3 -3 - 1.71 - 1.40 - 1 -1 2 -1 -2 6 -0.37 -0.54 -1 - 1 0 1 -2 -3 -0.21 - 0.90 ~~ The compositions of the mixtures : (i) 0.26 D, + 0.18 D, + 0.47 D3, (ii) 0.22 Di + 0.28 D, + 0.04 D,.2. The geometry of the distribution of A1 triads is discussed by using a D~R-E-D~R-E chain in fig. 1 and 2. Two A1 triads cannot be located on the same D6R, because six Si(3- Al) units are produced in such a configuration, as can be seen in fig. l(a). This configuration is unavoidably produced in partitions V and VI, as three 6-rings out of four contain A1 triads, hence these partitions must be excluded.An uneven distribution of A1 atoms between two 12-rings destroys the P63 symmetry, hence partitions I1 and I11 are excluded. In partition I, half the numbers of T4 sites in a D~R-E-D~R chain are occupied by A1 atoms. If an A1 triad is further located on a 6-ring of T6 sites, three Si(3- Al) units are produced. Such a situation is shown in fig. 2 (a). Partition I is thus excluded. Consequently the only allowable partition is partition IV. In partition IV, there are several patterns of stacking three A1 triads along the chain-axis. In the configurations of two A1 triads shown in fig. l(a), six or three Si(3-Al) units are produced, and these configurations must be avoided. Allowable alignments of three A1 triads in the chain are shown in fig.2(b )-(d). When these chains are cross-rinked to form 12-rings, three kinds of frameworks, models I, I1 and 111, are obtained from chains I, I1 and 111, respectively, as shown in fig. 3. Model I11 contains many Si(3-A1) units at the linking sites, and must be discarded. Models I and I1 contain no Si(3-A1), and populations of Si(v-Al) in them are easily counted by inspection and given in table 1. By comparing their population spectra with that in the row 'ideal', we conclude that only model I gives the correct distribution of A1 atoms. The same is the case with Ga atoms in Ga-L zeolite. Distribution of A1 Atoms in the Real Crystal Let us consider three kinds of defects, D,, D, and D,, shown in fig. 4. Changes in the populations of Si(v-Al), produced by these defects, are given in table 3.If a crystal contains 0.26 D,, 0.18 D, and 0.47 D, per unit cell, the population spectrum in the crystal becomes that given in the second row from the bottom in table 1. This spectrum agrees with the observed spectrum for A1-L in the first row, within experimental error.T. Takaishi 2973 Fig. 5. Three kinds of defects in model I of the framework of galliated zeolite L. A, Ga atom; A, Ga atom located on a hidden site; 0, Si(3-Ga); 0, Si(4-Ga); 0, site to which Ga atom is introduced. (a) Defects Di and D,. (b) Defect D,. A1 triads, located in the lower part than the plane 'cut', are rotated by 60".2974 Distribution of A1 and Ga in Zeolite L Fig. 6. Cation sites in zeolite L.A, A1 atom; A, A1 atom located on a hidden site; 0, site A; 0, site B’; a, site B”; 0, site C; @, site D’; (>, site D”; ::I, site E. Distribution of Ga Atoms in the Real Crystal The Ga-L zeolite studied by Newsam contained a small amount of Si(4-Ga), so that we must further take into consideration other kinds of defects, D;, D, and D,, which are shown in fig. 5. Changes in the populations of Si(v-Ga) units, produced by these defects, are given in table 2. If a crystal contains 0.22 Di, 0.28 D, and 0.04 D, per unit cell, the populations of Si(v-Ga) units in the crystal are given by those in the bottom row in table 1. This spectrum agrees with that of Ga-L zeolite in the second row of table 1, within experimental error. Discussion Barrer and Villiger3 considered five kinds of extra-framework cation sites, which are shown in fig.6 along with A1 (or Ga) atoms in the framework of model I. The coordinates of these sites are given in table 4. In the present model, the unit cell is doubled owing to the superstructure of the ordered distribution of A1 (or Ga) atoms. Let us investigate the effect of the ordering on properties of the cation sites. Site A is located at the centre of D6R, one 6-ring of which contains no A1 atoms. A cation on site A experiences a weaker asymmetric crystal field, in contrast to a cation at the centre of D6R of zeolite X containing six A1 atoms. Sites B, located at the centre of &-cages, must be divided into two sub-types, B’ and B”, as site B’ is surrounded by six A1 atoms and has a strong affinity to a cation, while site B” is surrounded by three A1 atoms and has a moderate affinity to a cation.Site C is surrounded asymmetrically by three A1 atoms, and has a moderate affinity to a cation. Sites D, on z = 0, are divided into two sub-types, D’ and D”. Site D’ is surrounded by three A1 atoms, while site D” has one neighbouring A1 atom. Site E, on z = 0, has two neighbouring A1 atoms. A cation may have a weak affinity to sites D’ and E, and the weakest affinity to site D”. The unit cell (original small cell) contains nine A1 atoms, one B’ site, one B” site, three D’ sites, three D” sites and three E sites. It is considered that cations on sites B’ and B” prevent other cations from occupying the A site located between them, and that eight alkali-metal cations preferentially occupy one B’, one B”, three C and three D’ sites.OneT. Takaishi 2975 Table 4. Extra-framework cation sites and numbers of cations on them numbers of cations symbol and coordinate Wyckoff notationa Barrer Wright for sites X Y Z et aL3 et al.l0 Newsam‘ this work A 2c 1/3 2/3 0.0 1.4 0.0 0.0 0.0 B 2d 1/3 2/3 1/2 2.0 2.0 2.0 2.0 c 3g E 3 f 0.0 1/2 0.0 0.0 0.0 0.0 0.0 0.0 1/2 1/2 2.7 2.9 (1) 3.0 3.0 4.0 + xc D 6j 0.0 yb 0.0 3.6 5.3 (1) 5.09(13) a Notations in P6/rnmrn symmetry. b y has a value of ca. 0.32f0.01. composition, K,+,(Al,Ga),+,Si2,-,0,2 of zeolite L. x is defined by the residual alkali-metal cation may reluctantly occupy site D” with the weakest affinity, because three cations on neighbouring C and D’ sites prevent another cation from occupying an E site.The distance between sites D’ and E is smaller than that between sites A and B, so that the electrostatic repulsion between cations on sites D’ and E is very strong. The proposed cation distribution is compared with experimental results in table 4. The sample used by Barrer et al. was hydrated, while others used dehydrated samples. The agreement between the observed and proposed distributions is satisfactory, as far as the dehydrated zeolite L is concerned. If the above model is accepted, the following important conclusion is derived. The D” site has the weakest proton affinity. In other words, site D” may be a strong acid site. There are three D” sites per unit cell, but not all of them act as strong acid sites. Let the composition of zeolite L be (H,K)9+z(Al,Ga)9+zSi2,-x0,2, then the number of proton on D” sites, [H+/D”], is given by [H+/D”] = 1 +x, per unit cell.At x < 0, the number of strong acid is just 1 +x, if there is no defect. For x > 0, some amount of defects D,, D; and others must be introduced. Defects D, and D; introduce one A1 (or Ga) atom in the neighbourhood of a D” site as seen in fig. 4 and 5, respectively, i.e. they introduce only a weak acid. Defect D, introduces one A1 atom in the neighbourhood of two D” sites to produce a pair of weak acid sites. Summarizing the above, the number of strong acid sites does not exceed 1 per unit cell, irrespective of the composition of zeolite L. According to Newsam,” conventional zeolite L with an Si/Al ratio close to 3 did not have any long-range coherence, indicating an ordered distribution of A1 (or Ga) atoms.Synthetic zeolites are usually crystallized at a higher growth rate and contain a large amount of defects, some of which are observed by the imaging electron microscope.12 In such a crystal the size of the domain of the ordering of A1 (or Ga) atoms may be too small to produce a detectable long-range coherence, but may be large enough for 29Si n.m.r. to detect the ordering of A1 (or Ga) atoms. With zeolite L, the random distribution of A1 atoms contradicts the observed n.m.r. data, and we consider that the above situation is realized. It seems that diffraction techniques are not totally conclusive in the problem of the ordering of A1 (or Ga) atoms in zeolitic frameworks. Finally, we do not claim that the above models are the only solutions, as the defects are introduced in an arbitrary manner to obtain the best fit to the experimental data.However, further refinements of the models are not worthwhile, because one cannot at present observe all the defects experimentally. It is concluded, in spite of this shortcoming, that the present models do not contain any inconsistency and are highly probable.2976 Distribution of Al and Ga in Zeolite L ( c ) k --- I-,-- -71.1 - 83.4 -95.7 - 107.9 -1 20.2 6 (PPm) Fig. Al. m.a.s.n.m.r. spectrum of zeolite L. (a) Observed spectrum (JEOL), (b) spectrum calculated by Dr M. Kato, (c) difference between (a) and (b). Table Al. Results of analysis of 29Si m.a.s.n.m.r. spectrumn of zeolite L synthesized by I tabashi A1 Si(3-AI) Si(2-AI) Si( 1 -Al) Si(0-AI) peak position (ppm) relative integral intensity population per unit cell ideal crystal 0.50 D, or D; 0.14 D, 1.01 D, mixture observed - 0.1058 9.50 2.80 calculated population 9.00 0.00 0.50 1 S O 0.00 0.28 0.00 1.01 9.50 2.79 - -91.8 -96.6 - 0.3553 9.42 12.00 - 1 .oo -0.56 - 1.01 9.43 101.7 - 0.4065 10.77 12.00 -0.50 0.28 - 1-01 10.77 106.9 0.1324 3.51 3 .OO - 0.50 0.00 1.01 3.51 Chemical shift values are given against a reference of tetramethy lsilane.Appendix Recently, we obtained zeolite L synthesized by Itabashi of Toyo Soda. The 29Si m.a.s.n.m.r. spectrum of the zeolite was measured by JEOL and is shown in fig. Al. The spectrum can be fitted as a sum of four Gaussians, whose intensities, positions and breadth are treated as independent variables (a total of 12 parameters).The results of the analysis are summarized in table Al. The agreement between the observed and calculated population spectra of Si(v-Al) units is satisfactory. It is concluded that the experimental data from different sources are well described by the present theory. This is an indication of the correctness of the theory.T. Takaishi 2977 The author thanks Dr J. M. Newsam, Exxon Research and Engineering, who supplied n.m.r. data of zeolite L in advance of publication, and Prof. N. Yotsukura, of Toyohashi University of Technology, for language correction. The present work was financially supported by Toso (previously, Toyo Soda). References 1 2 3 4 5 6 7 8 9 10 11 12 D. W. Breck and N. A. Acara, U S . Patent, 71 1, 565. D. W. Breck and E. M. Flanigen, Molecular Sieves (SOC. Chem. Industry, London, 1968), p. 47. R. M. Barrer and H. Villiger, Z . Kristallogr., 1969, 128, 352. Ch. Berlocher and R. M. Barrer, Z . Kristallogr., 1972, 136, 245. J. M. Newsam, J . Chem. SOC., Chem. Commun., 1987, 123. J. M. Newsam, Mater. Res. Bull., 1986, 21, 661. J. M. Newsam, Solid State Zonics, ed. K. R. Poeppelmeier and S. T. Wilson (ACS, Washington D.C., 1988), in press. E. Lippma, M. Nagi, A. Samoson, G. Engelhardt and A. R. Grimmer, J . Am. Chem. SOC., 1980, 102, 4889. J. Klinowski, Prog. Nucl. Magn. Reson. Spectrosc., 1984, 16, 237. P. A. Wright, J. M. Thomas, A. K. Cheetham and A. K. Nowak, Nature (London), 1985, 318, 611. J. M. Newsam, personal communication. J. M. Thomas, G. R. Millward and S. Ramadas, Intrazeolite Chemistry, ACS Symp. Ser., 1983, 218, 181. Paper 7/1009; Received 9th June, 1987 98 FAR 1

ISSN:0300-9599    

DOI:10.1039/F19888402967

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I, 1988, 84(9), 2979-2986 Preparation and Electrochemical Behaviour of a Methylene Blue-modified Electrode based on a Nafion Polymer Film Ziling Lu and Shaojun Dong* Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin 130021, People's Republic of China A methylene blue (MB) chemically modified electrode has been prepared by incorporating MB molecules into a Nafion polymer film on a glassy carbon surface. The electrochemical behaviour of the MB-modified polymer film electrode is discussed in detail. The electrode reaction of MB bound to the polymer film shows a reversible, two-electron transfer process with good stability and reproducibility. The equation of half-wave potential E; us. pH was deduced theoretically and was proved to be reasonable experimentally by the effect of solution pH on the MB-modified polymer-film electrode.The influence of supporting electrolytes on the electrode is discussed. The chemically modified electrode (CME) has been rapidly developed in recent years.l-'O Potential new materials for modification and preparation of stable polymer-film electrodes have been widely studied. Research on dye-modified electrodes has centred on the development of electrochromic materials,' spectroelectrochemistry * and applications such as electrocatalysis and electroanalysis. Here we report the preparation and electrochemical behaviour of an MB-modified electrode based on a Nafion polymer. The redox process and some effects of MB-modified polymer-film electrodes are discussed. Experiment a1 Materials A Nafion 117 (DuPont) membrane was used and dissolved according to the procedure of Martin et al.l0 to obtain the polymer solution (ca.4 mg cmP3). Methylene blue (MB) was an indicator reagent, purified via chromatography on silica using methanol as eluent, the purity was checked using t.1.c. (silica, MeOH :acetic acid 9 : 1). There was no difference in electrochemical behaviour of the MB-modified electrodes prepared using purified and unpurified MB. MB was dissolved in water to obtain MB solution. All other reagents were of analytical-reagent grade. Doubly distilled water was used. Apparatus A conventional single-compartment cell equipped with a platinum-wire counter- electrode and saturated potassium calomel reference electrode (SCE) was employed. The working electrode was a home-made Teflon-shrouded glassy carbon disc electrode (geometric area 0.13 cm2) polished to a mirror with MgO powder.Electrochemical measurements were made with model CV 47 voltammograph (BAS) and recorded on a home-made model LZ3-204 X- Y recorder. Chronoamperometric response was recorded by a home-made model MS- 1650B digital memoryscope. Unless otherwise mentioned, 0.09 mol dmP3 H,SO, was used as a supporting electrolyte solution. All potential values are given us. SCE. 2979 98-22980 MB-modified Polymer Electrode ( b ) , -0.2 0.0 0.2 0.4 0.6 -0.2 0.0 0.2 0.4 0.6 EIV EIV Fig. 1. Cyclic voltammograms of the MB-modified electrodes made by (a) a one-step method; (b) a two-step method: lo-’ cm3 Nafion solution coated glassy carbon electrode in the blank solution after immersing in rnol dmV3 MB solution.Supporting electrolyte 0.09 mol dm+ H’SO,, scan rate = 20 mV s-l. Results and Discussion Preparation of MB-modified Electrodes based on Nafion Polymer MB-modified electrodes can be prepared by a one-step method, i.e. lo-’ cm3 Nafion and (5-10) x lop3 cm3 MB solutions are mixed and added to the surface of glassy carbon electrode, then allowed to evaporate. The uniformly and tightly attached polymer coatings are obtained by the association between MB cations and the SO, sites of Nafion. The film electrode is rinsed several times with doubly distilled water to remove unbound MB molecules. This electrode can be used for electrochemical studies, and the cyclic voltammogram is shown in fig.1 (a). We also tried a two-step preparation: glassy carbon was initially coated with Nafion solution, evaporated and then immersed in MB solution to obtain an MB-bound Nafion polymer-film electrode. Unfortunately, MB-modified electrodes prepared by the latter method present an irreversible or asymmetric cyclic voltammogram with poorly defined shape, shown in fig. 1 (b). This is attributed to a non-homogeneous distribution of bound dye within the polymer. Thus we used the one-step method to prepare MB-modified polymer film electrodes as they exhibit well defined redox peaks over the MB concentration range from 5 x to 1 x lop2 mol dm-3. The peak current (i,) of the polymer-film electrode increases with increasing concentration of MB solution and gradually reaches a constant value, showing the saturation of the association of MB cations with SO, sites of Nafion polymer film.El remains constant and does not relate to the concentration of MB; the slope of log i, us”. log ZI is ca. 0.8. Electrochemical Behaviour of MEmodified Polymer-film Electrodes From the cyclic voltammogram of MB-modified electrodes in 0.09 mol dm-3 H2S0, supporting electrolyte solution [fig. 1 (a)] it can be seen that E; = 0.18 V, ip,/ipc. = 1 and i, increases with scan rate v, and AE, = 35 mV at v = 20 mV s, which slightly increases with increasing v, resulting from the IR drop of the polymer-film electrode. This suggests that the electrode reaction of bound MB is a reversible redox process. Let us consider the structure of the MB bound Nafion polymer-film electrode: there exist both hydrophobic and hydrophilic domains in Nafion.The -S03H groups of Nafion in the hydrophilic domain can dissociate to form -SO, anions in aqueous solution and these can associate with large MB cations by ion-exchange and bind MB from solution onto the Nafion polymer film (scheme 1).Z . Lu and S. Dong 298 1 GC hydrophobic domain + MB+ 9ByT Scheme 1. GC Table 1. Comparison of cyclic voltammetric data of MB- modified electrode with those for MB solution bound MB 0.200 0.165 0.18 35 1 0.8 free MB 0.215 0.185 0.20 30 1 0.5 a a = 3 log ip/a log u. Supporting electrolyte 0.09 mol dmP3 H,SO, solution. Scan rate u = 20 mV s-I. We consider that the bound MB should be situated in the hydrophilic domain of the Nafion polymer film and that the redox processes of bound MB are controlled by the charge transfer in the polymer film.When the MB-modified electrode comes into contact with aqueous solution the properties of the Nafion polymer film and the redox processes of the MB-modified electrode should be greatly affected by conditions such as pH and supporting electrolyte etc. From the experimental data in table 1, it can be seen that the redox reaction of the MB-modified electrode is in agreement with that of MB in solution. Based on the redox reaction of MB in aqueous solution" we can infer the overall reaction occurring at MB in the modified polymer-film electrode : (film) + 2H+ + 2e Me2N N Me, ox H Red Supposing that the redox reaction of MB-modified Nafion polymer-film electrode occurs in the hydrophilic domain similarly to that in bulk aqueous solution, we can deduce the half-wave potential equation as : Ef = E;' + (RT/2F) In [H+]&[Ox],/[Red], c",x = [OXHI, + [OX], = [OX],(' + [H+la/&) Cfed = CRedH21, + [RedHIf + [Red], = [Red],(] + [H+Ia/Kr, + [H+I3Kr1 Kr2) (2) (3) (4) where aq represents aqueous solution and f the polymer film2982 0.4 MB-modiJied Polymer Electrode - (a1 1 3 5 7 9 11 PH 0.2 - 3 f 0.0 - ;u" rA > \ - 0 .2 * 0 2 4 6 8 10 Fig. 2. pH effect for MB-modified Nafion polymer-film electrode (a) E; us. pH, (6) iPa us. pH. Britton-Robinson buffer was used. PH where KO, K,, and Kr2 represent the dissociation constants'l of the oxidised and reduced states, respectively. Therefore El = constant + (RT/2F) In ([H+]z + K,,[H+]: + K,, Kr2[H+]:)/(K0 + [€-I+],).(6) Because KO is very small, EI = constant + (RT/2F) In ([H+]: + K,,[H+]: + K,, K,,[H+],). (7)2. Lu and S. Dong 2983 Table 2. Effect of the composition of the electrolyte ~~~ LiCl -0.11 -0.23 120 -0.17 NaCl -0.11 -0.20 100 -0.15 KCI -0.12 -0.23 110 -0.18 CaCl, -0.08 -0.24 160 -0.16 KBr -0.10 -0.20 100 -0.15 KI -0.11 -0.23 120 -0.17 Na,SO, -0.15 -0.24 90 -0.195 KReO," 0.035 -0.025 60 0.005 a 0.02 mol dm-3, others 0.1 mol dmT3. Scan rate = 50 mV s-l. These electrolytes are electro- inactive in the region of interest. If the redox reac ion of bound MB conducts in the hydrophilic d main of the polymer film, we must prove the validity of eqn (7) by experiment. It can be seen from fig. 2 that the experimental results conform very well with the half-wave potential equation (where E is in mV).When pH < 5.6, [H+I3 $ Kr,[H+]2+Kr,K,2[H+] 4 = constant + (RT/2F) In [H+I3 = constant - 90 pH. thus When pH > 5.6, K,, Kr2[H+] 9 [H+I3 + Kr1[H+l2 thus The slopes of EL us. pH in fig. 2(a) are -90 and -30 mV per pH unit, respectively, at pH < 5.6 and p h > 5.6. The relationship between i, and pH is quite similar to that between E; and pH. The above results reveal that the nature of the hydrophilic domain is close to that of bulk aqueous solution and the supposition is reasonable that the redox reaction of MB- modified Nafion polymer-film electrode takes place in the hydrophilic domain. In fact, there are two more interactions for .the bound MB sites: one is an electrostatic interaction between cation-bound MB and the anionic SO, site of the Nafion polymer, in which the higher the charge is, the stronger is the interaction.The other is a hydrophobic interaction between the large organic MB molecule and the hydrophobic domain of the Nafion polymer. These interactions make bound MB different from free MB in aqueous solution. It can be seen from table 1 that E5 1 o f the bound MB is more negative than that of free MB in aqueous solution in the same cases. Furthermore, both interactions make the MB-modified electrode more stable. = constant - 30 pH. Effects of Supporting Electrolytes The movement of counter-ions from or to the polymer film must be accompanied by the redox process of the polymer-film electrode to maintain charge equilibrium in the film.Thus the electrochemical behaviour of the modified electrode will be affected by the nature of counter-ions and their concentrations in solution. Table 2 gives the cyclic voltammetric data of MB-modified Nafion polymer-film electrode in different electrolyte solutions. It can be seen that the voltammetric data (&, i, and AE,,) are quite similar for Li+, Na+, K+ and Ca2+ with the same anion in the electrolyte. ALE, is a little large for2984 MB-modified Polymer Electrode 1 J -0.4 - 0 . 2 0.0 0.2 - 0.2 0.0 0.2 0. L Et v Fig. 3. Cyclic voltammograms of the MB-modified electrode. Supporting electrolyte (a) 0.1 mol dmP3 NaCI, (b) 0.02 mol dm-2 KReO,. Scan rate = 50 mV s-l. Table 3. Effect of the concentration of the electrolyte 0.0 1 0.05 0.5 1 .o 5.0 0.005 0.05 0.1 0.5 1 .o 0.002 0.005 0.02 0.05 - 0.080 -0.105 -0.100 -0.105 - 0.035 -0.135 -0.145 -0.150 -0.160 -0.185 - 0.005 0.020 0.035 0.03 NaCl -0.175 90 -0.180 75 -0.175 75 -0.185 80 -0.125 90 Na,SO, -0.220 85 -0.235 90 -0.240 90 -0.240 80 -0.275 90 KReO, - 0.09 85 -0.065 85 -0.025 60 -0.03 60 -0.13 -0.14 -0.14 - 0.145 - 0.08 -0.178 -0.19 -0.195 -0.20 - 0.23 - 0.048 - 0.023 0.005 0.00 2.1 2.3 2.8 2.8 2.3 2.0 2.2 2.2 2.6 2.65 3.0 3.2 4.05 6.1 Scan rate = 50 mV s-l .Ca2+, possibly because of strong ion association between Ca2+ and the SO; groups of Nafion. For different anions, voltammetric data are also similar for halogen ions (Cl-, Br- and I-), but different for some anions such as SO:-, ClO, and ReO;. The redox peaks of the MB-modified electrode are not well defined in LiC10, solution.E; shifts negatively in Na2S0, solution. It is interesting to see that in KReO, solution, ip apparently increases, E; shifts positively and AEp decreases with respect to other electrolytes. Fig. 3 shows the cyclic voltammograms of MB-modified electrode in NaClZ. Lu and S. Dong 2985 80 60 9 40 2 20 0 1 2 3 4 5 r-3 1s-3 Fig. 4. Current us. t-i for the oxidation of the MB-modified electrode in 0.09 mol dm-3 H,SO, solution. The potential was stepped from -0.2 to 0.5 V. and KReO, solutions. The results suggest that when the redox process of the MB- modified electrode takes place, the counter-ions involved are anions not cations. Furthermore, we conclude that increasing the radius of the counter-ion (anion) makes E; more positive (e.g.ReOJ and that counter-ions with higher charge numbers like SO:- make Ei more negative. This may result from bound MB Nafion polymer-film electrode having a good selectivity for ReO,. Table 3 shows the effects of concentrations of supporting electrolytes on the voltammetry of MB-modified electrode. It can be seen that i, for the MB-modified electrode increases with increasing concentration of electrolyte. However, i, can also decrease when the concentration is too high (e.g. 5 mol dm-3 NaCl), similar to the case of polyvinyl ferrocene film electrodes.12 For NaCl Ei changes slightly with the concentration; for Na2S0, E; shifts slightly to more negative values with increasing concentration, but for KReO, as electrolyte, E; shifts apparently to more positive values and ip is much higher than it is in other electrolyte solutions (although the concentration of KReO, is much lower owing to its solubility).The voltammogram of the MB-modified electrode can be restored to its original form when the electrode is taken out of the ReO; solution and rinsed with doubly distilled water. Stability of the MB-modified Electrode 'The MB-modified electrode is stable in air or solution. No change appears after it is dried in air for several days. i, of the electrode remains constant after immersion in ReO, electrolyte for one day, and only a small decrease appears after two days. In 0.09 mol dm-3 H,SO, the cyclic voltammograms are the same during consecutive scans, but ip decreases slightly when the electrode is immersed for a longer time, indicating the possible dissociation of MB molecules from the polymer film in aqueous solution.The good stability makes the MB-modified electrode useful in applications such as electrochromism and analysis etc. Related work is in progress.2986 MB-modijied Polymer Electrode Chronoamperometric behaviour of MB-modified Nafion Polymer-film Electrode Chronoamperometry can be used to investigate the diffusion processes of electroactive materials bound in polymer-film electrodes. When a potential step from -0.2 to 0.5 V was applied to the Nafion-MB/GC electrode in 0.09 mol dmP3 H,SO, for complete reductionl to complete oxidation, the current us. time transient was recorded. The plot of i us. 1-5 (fig. 4) shows a linear r5gion with zero iptercept (Cottrell region).Thelslope of the linear region yielded CD;pp (= slope x nu/nFA) = 9 x mol cm-2 s-5 and Dapp = 8 x lo-’ cm2 s-l (the quantity of MB attached is estimated from the charge, Q, consumed in complete oxidation or reduction of the film; the film thickness was calculated using 1.58 g cm-3 as the wet Nafion 117 film densities of the 1100 eq.wt polymer13 based on the Cottrell equation and assuming an average concen- tration of the electroactive MB molecules distributed uniformly throughout the film of the layer (these may yield some error in calculating Dapp). The deviations from Cottrell equation which appear at longer times are due to thin-layer effects. References 1 S. Dong, Fenxi Huanxue (Analytical Chemistry), 1985, 13, 870. 2 H. S. White, J. Leddy and A. J. Bard, J. Am. Chem. Soc., 1982, 104, 481 1. 3 C. R. Martin, 1. Rubinstein and A. J. Bard, J. Am. Chem. Soc., 1982, 104, 4817. 4 D. A. Buttry and F. C. Anson, J, Am. Chem. Soc., 1982, 104, 4824. 5 A. E. Keifer and A. J. Bard, J. Phys. Chem., 1986, 96, 868. 6 F. F. Fan and A. J. Bard, J. Electrochem. Soc., 1986, 133, 301. 7 S. Dong and F. Li, J. Electroanal. Chem., 1986, 210, 31; 1987, 217, 49. 8 R. Memming, Prog. Surf. Sci., 1984, 17, 7. 9 E. Yeager, Electrochim. Acta, 1984, 29, 1527. 10 C. R. Martin, T. A. Rhoades and J. A. Ferguson, Anal. Chem., 1982, 54, 1639. 11 Edmund Bishop, Indicators (Pergamon Press, Oxford, 1972), pp. 503-504. 12 G. Inzelt and L. Szabo, Electrochim. Acta, 1986, 31, 1381. 13 C. R. Martin and K. A. Dollard, J. Electroanal. Chem., 1983, 159, 127. Paper 7/1156; Received 29th June, 1987

ISSN:0300-9599    

DOI:10.1039/F19888402979

出版商:RSC    

年代:1988

数据来源: RSC