Platinum Metals Rev., 1964, 8, (4), 131
Third International Congress on Catalysis
The Role of the Platinum Metals in Heterogeneous Catalytic Reactions
The Third International Congress on Catalysis, organised most efficiently by the Royal Dutch Chemical Society, took place in Amsterdam from July 20 to 25. It was attended by 828 active members from 31 countries including 141 from the USA, 29 from the USSR, and 115 from Great Britain. Of the ninety-eight papers presented, twenty-seven refer to the use of the platinum group metals as catalysts, and these are reviewed here.
In order to restrict the scope of the meeting, two themes had been selected by the organising committee: these were (i) Selectivity in Catalytic Reactions, and (ii) the Molecular Description of Catalytic Reactions. In addition to a full programme of discussion of the papers, there were six general lectures that covered some of those aspects of catalysis not included in the main themes. Two of these lectures of particular interest to those working with the platinum metals were those by Professor R. S. Nyholm on the structure and reactivity of transition metal complexes and by Professor J. Halpern on developments in homogeneous catalysis.
The proceedings of the Congress will be published by the North Holland Publishing Company, Amsterdam.
For the purpose of this short review, the papers of greatest interest have been classified under the headings: (i) Catalytic Reforming, (ii) Aromatic Ring Reduction, (iii) Reduction of Other Unsaturated Groups, and (iv) Miscellaneous Processes.
Six papers were concerned with catalysts for the reforming of hydrocarbons and the related mechanisms of reactions. J. A. Rabo, V. Schomaker and P. E. Pickert, of Union Carbide, described a technique for ionexchanging a calcium Y zeolite with [Pt (NH3)J21 to give after reduction a Pt content of 1/2per cent. This catalyst was resistant to poisoning by thiophen. The same zeolite on impregnation with H2PtCl6 to give after reduction the same Pt content was rapidly poisoned by thiophen. They concluded that the Pt in the former catalyst was atomically dispersed.
Another novel procedure was described by A. G. Goble and P. A. Lawrance of the British Petroleum Company Ltd. These authors showed that treatment of a Pt/Al203 catalyst with carbon tetrachloride vapour at 300°C resulted in the chlorination of surface hydroxyl groups, and that the resulting catalyst could isomerise n-hexane even at room temperature with remarkably high selectivity.
H. J. Maat and L. Moscou (Ketjen N.V.) showed that increasing the platinum crystallite size by sintering led to a decrease in dehydrocyclisation activity, accompanied by an increase in isomerisation activity. V. Haensel, G. R. Donaldson and F. J. Riedl of Universal Oil Products, reported that methylcyclo-pentene was a primary intermediate product in the conversion of methylcyclopentane to benzene over Pt/Al203, and that cyclohexene could be detected in the dehydrogenation of cyclohexane to benzene at high space velocities. Its yield was greatly increased on partially poisoning the catalyst with tert-butyl mercaptan. Kh. M. Minachev and G. V. Isagulyants (U.S.S.R.) were however unable to detect cyclohexene in the dehydrogenation of cyclohexane over Pd/Al203 at 480°C. S. Khoobiar, R. E. Peck and B. J. Reitzer, of the Illinois Institute of Technology, studied the kinetics of cyclohexane dehydrogenation over Pt/Al203 in an isothermal reactor, and they suggested that although reaction was initiated on the catalyst pellets it was probably propagated on the surface of the added ‘inert’ alumina.
Aromatic Ring Reduction
Five papers were devoted to the problem of the mechanisms of the hydrogenation and exchange of benzene and its methyl derivations over the platinum group metals and emphasis here also was on the role of olefinic intermediates.
S. Siegel and V. Ku (University of Arkansas) have detected them during the liquid-phase hydrogenation of the xylene isomers: concentrations varied between 0.02 and 0.7 mole per cent. They conclude that virtually every saturated molecule formed passes through the desorbed olefin stage. F. Hartog, J. H. Tebben and C. A. M. Weterings, of Dutch State Mines, studied the hydrogenation of benzene over platinum, palladium and ruthenium catalysts, using both hydrogen and deuterium, with full product analysis in the latter case. They concluded that C6X6 and C6X7 species (X=H or D) were common intermediates for both hydrogenation and exchange, and that cyclohexene was an intermediate, but even in the most favourable case, which is ruthenium, only about 1 per cent desorbs from the surface. The paper presented by R. J. Harper and C. Kemball (The Queen’s University, Belfast) dealt with the exchange and hydrogenation of p-xylene over films of platinum, palladium and tungsten. Exchange of the methyl groups was the most rapid process, followed by the slower ring exchange and hydrogenation processes. No olefinic intermediates were detected in this work. H. A. Smith and W. E. Campbell, of the University of Tennessee, measured the rates of reduction of benzene and a series of methylbenzenes alone and in competition over Rh/Al203, and from their results were able to calculate the relative strengths of adsorption of the molecules.
K. Hirota and T. Ueda (Osaka University) observed that in the exchange of p-xylene with D2O, platinum and palladium catalyse the exchange of both ring and methyl hydrogens ; iridium, ruthenium and rhodium do so less efficiently, while nickel and cobalt can only exchange the methyl hydrogens.
Reduction of Other Unsaturated Groups
Comparatively few papers were devoted to the hoary old problem of the mechanism of the hydrogenation of olefins catalysed by the platinum group metals. J. Turkevich, F. Nozaki and D. Stamires (Princeton University), in what was widely agreed to be one of the most impressive papers of the Congress, examined the electron spin resonance of Pd/Al203, and on this basis proposed a donor-acceptor model for catalytic hydrogenation. J. L. Carter, P. J. Lucchesi, J. H. Sinfelt and D. J. C. Yates (Esso Research and Engineering Co) studied the hydrogenation of ethylene over Pt/Al202 and concluded that migration of active intermediates occurred between the Pt and A1203 centres. G. V. Smith and J. A. Roth (Illinois Institute of Technology) investigated the addition of deuterium to dimethylitaconate and its isomers, and obtained evidence for an intramolecular hydrogen transfer. G. C. Bond and J. S. Rank (Johnson Matthey and Co Ltd) showed that the rate of double-bond migration in I-pentene relative to its rate of hydrogenation was similar for palladium on a number of supports, and for Pd/C in a number of solvents. They also studied the liquid-phase hydrogenation of the pentynes and penta-dienes using platinum, palladium, ruthenium, rhodium and iridium catalysts, and obtained product distributions. J. J. Phillipson, P. B. Wells and D. W. Gray, of the University of Hull, studied the gas-phase hydrogenation of 2-butyne using platinum, rhodium and iridium supported on alumina, and discussed reaction mechanisms in detail.
Two interesting papers concerned the mechanism of the hydrogen-oxygen reaction. V. Ponec, Z. Knor and S. Cerny (Czechoslovak Academy of Science) followed the chemi-sorption of hydrogen and of oxygen, and their interaction, on films of rhodium, palladium, nickel and molybdenum by electrical conductivity changes. S. Z. Roginsky (Institute for Chemical Physics, Moscow) summarised a great deal of published work, chiefly concerning the isotope effect in this reaction.
J. R. Anderson and N. J. Clark, of the University of Melbourne, studied the reactions of hydrogen cyanide on evaporated films of platinum, palladium and a number of other metals. Kobozev, Krilova and Shashkov, of the Moscow State University, investigated the exo-electron emission of platinum on several supports: this novel and simplf technique merits further application.
The limitations imposed by the organisers of the Congress on its scope resulted in at overall impression of a strong ‘academic flavour: only the general lecture by Professor Boreskov on the theoretical bases of selection preparation and use of industrial catalysts served partially to restore the balance. In the papers reviewed here, there were two re- current themes, first, the detection and description of reaction intermediates, using ; variety of techniques and approaches, and secondly the role of the support, with par ticular reference to the possible activation of the support by the metal, for which concep there is now much indirect evidence. It re- mains to be seen to what extent these issues are clarified before the next Congress, which it is hoped will be held in the Soviet Union in 1968.