Hydrogenolysis of alkanes. Part 5.—Effect of metal dispersion in ruthenium/alumina catalysts on the hydrogenolysis of propane and of n-butane
Turnover frequencies for the hydrogenolysis of propane and of n-butane decrease with increasing dispersion for a series of Ru/Al2O3 catalysts prepared from Ru(acac)3, having values of H/Ru between 0.07 and 1.05. Activation energies for reactant removal do not vary greatly or systematically with dispersion, but product selectivities found with n-butane do. Thus at 480 K, selectivity to methane (S1) decreases markedly, and those to ethane (S2) and to propane (S3) increase correspondingly, with increasing dispersion; and most notably the very highly dispersed catalyst maintains its high value of S2 to > 520 K, at which temperature the catalyst of lowest dispersion produces almost pure methane.
These variations are interpreted in terms of a combination of geometric and electronic factors; an ensemble-size requirement determines the rate, but an increasing electron deficiency with increasing dispersion favours a 2,3-adsorbed C4 intermediate, which preferentially forms ethane. The results are compared with those in the literature for other supported Ru catalysts, and it is concluded that the method of preparation, as well as the kind of support and the particle size, affect turnover frequency.