Abstract
A comparison between the activities of silica-supported ruthenium, rhodium and platinum catalysts prepared from metal cluster compounds and their conventional analogues towards the activation of saturated hydrocarbons has been made. Ruthenium cluster-derived catalysts display greatly enhanced activity for the complete hydrogenolysis of straight chain aliphatic hydrocarbons to methane and provide a temperature advantage of 150°C relative to conventionally prepared ruthenium catalysts where only moderate hydrocarbon conversions are noted. The increased activity superficially correlates with the smaller metal crystallite sizes (15–20 A) reproducibly obtainable using metal cluster compounds as catalyst precursors. The highly specific activity for the hydrogenolysis of C-C bonds in saturated hydrocarbons has been applied to the selective cleavage of the alkyl group in ethylbenzene, giving toluene and methane. Conversions of up to 30% ethylbenzene have been observed at 225°C and 1 atm using a Ru3(CO)12/SiO2-based catalyst. The xylenes, particularly o-xylene, are much less susceptible to hydrogenolysis and, at 225°C, relative hydrocarbon destruction rates of 30 : 1 and 7 : 1 have been observed using mixed feeds of ethylbenzene/o-xylene and ethylbenzene/p-xylene, respectively. Such a catalyst system can, in principle, therefore provide a means of separating ethylbenzene from its mixtures with xylenes.
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