Influence of the hydrocarbon structure on the hydrogenolysis of alkanes over rhodium/alumina catalysts

Abstract

The influence of particle size has been investigated on the hydrogenolysis of n-hexane, methylpentanes, 2,2-dimethylbutane, 2,2,3,3-tetramethylbutane and methylcyclopentane using a series of Rh/Al 2O 3 catalysts of widely varying dispersion. Specific activities and selectivities vary in a limited range for n-hexane, whereas large variations are observed with 2,2,3,3-TMB and MCP. In all cases specific activities for hydrogenolysis increase with particle size. Some general rules appear for the influence of size on the splitting of the different bonds found in these alkanes: the cleavage of bonds involving only primary, secondary or tertiary carbon atoms is less sensitive to a change in particle size of the metal, since activities vary by less than a factor of ten. The activation energy for these reactions is lower than that observed for the hydrogenolysis of bonds involving a quaternary carbon atom. The splitting of these bonds is highly sensitive to the metallic dispersion, since specific activities can vary by four orders of magnitude between catalysts of high and low dispersion, for the splitting of biquaternary C-C bonds. These different behaviours can be rationalized assuming different types of adsorption of the hydrocarbon depending on the size of the metallic particles and the substitution of the carbon atoms.