Hydrogenolysis and related reactions of hydrocarbons (C3 to C5) on silica-supported Rh-Pt bimetallic catalysts

J. A. Oliver, C. Kemball
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引用次数: 5

Abstract

The reactions with hydrogen of propane, butane, 2-methylpropane, pentane, 2-methylbutane, 2, 2-dimethylpropane and cyclopentane have been studied in a static reactor using a series of highly dispersed rhodium-platinum catalysts supported on high-area silica. The main reaction over all catalysts was hydrogenolysis involving the breaking of a single carbon-carbon bond. Most of the compounds reacted at similar rates over platinum with activation energies in the range 132-144 kJ mol-1. With rhodium, rates varied with hydrocarbon structure by factors of more than 102. The selectivities for the formation of the various products showed that the relative rates of breaking different carbon-carbon bonds over rhodium were S-S > P-S > S-T > P-T > P-Q (P = primary, Q = quaternary, S = secondary and T = tertiary). At temperatures above 455 K, there was evidence of a change of rate-determining step over rhodium with the overall reaction becoming controlled by the rate of desorption of methane. Probable mechanisms over platinum and rhodium are discussed. Rhodium was more active than platinum by factors of about 200 for branched hydrocarbons and of 103 or more for straight-chain compounds. Patterns of activity with metal composition are interpreted in terms of active sites consisting of ensembles of about six metal atoms. But an alternative description of the active site as a single metal atom with properties influenced by a number of nearest neighbours cannot be excluded.
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硅负载铑铂双金属催化剂上碳氢化合物(C3 ~ C5)的氢解及相关反应
采用高面积二氧化硅负载的高分散铑-铂催化剂,在静态反应器中研究了丙烷、丁烷、2-甲基丙烷、戊烷、2-甲基丁烷、2,2 -二甲基丙烷和环戊烷与氢的反应。所有催化剂的主要反应是氢解反应,包括破坏单个碳-碳键。大多数化合物在铂上的反应速率相似,活化能在132-144 kJ mol-1之间。对于铑,反应速率随碳氢化合物结构的不同而变化,影响因子大于102。不同产物形成的选择性表明,铑上不同碳碳键断裂的相对速率为S-S > P-S > S-T > P-T > P-Q (P =一级,Q =四级,S =二级,T =三级)。在455 K以上的温度下,有证据表明铑的速率决定步骤发生了变化,整个反应由甲烷的解吸速率控制。讨论了铂和铑的可能机理。对于支链烃,铑的活性比铂高约200倍,对于直链化合物,铑的活性比铂高103倍或更多。具有金属成分的活性模式是根据由大约6个金属原子组成的系综组成的活性位点来解释的。但是,不能排除将活性位点描述为单个金属原子,其性质受若干近邻的影响。
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