HYDROGENOLYSIS OF PROPANE AND N-BUTANE OVER RHODIUM CATALYSTS

Hydrogenolysis of propane and n-butane has been studied on highly disp ersed Rh on SiO2, Al2O3 and TiO2 catalysts prepared by conventional im pregnation, using thermal cycling between ca. 410 and 520 K. Rh/SiO2 i s initially the most active for n-butane, but during thermal cycling i t deactivates the fastest, with decrease in ethane selectivity S-2. Ap parent activation energies are ca. 200 kJ mol(-1) for propane and ca. 190 kJ mol(-1) for n-butane. After the first high-temperature reductio n (HTR1), values of S-2 in the n-butane reaction are between 1.1 and 1 .6; they decrease significantly following oxidation and low-temperatur e reduction (O/LTR), but rise again after a second high-temperature re duction (HTR2). Dependence of rates of both reactions on H-2 pressure at ca. 430 K have been determined for each catalyst after HTR1, and al so for Rh/TiO2 after O/LTR. Orders in H-2 are strongly negative; the r esults are modelled by a rate expression derived from a mechanism that assumes activation of the alkane by loss of several H atoms, and valu es of constants k(1) (rate constant), K-H (H-2 chemisorption) and K-C (alkane activation), are determined. K-H generally exceeds K-C, which on Rh/TiO2 is increased by O/LTR treatment. On Rh/SiO2, S-2 for the n- butane reaction is independent of H-2 pressure, but for the other syst ems it decreases as the H-2 pressure is raised. It is proposed that th e chance of central C-C bond splitting in n-butane depends inter alia on the cleanliness of the Rh surface. Ethane selectivity in the propan e reaction shows small dependence on either temperature or H, pressure for all systems, and is typically 0.98-0.99. The lack of a marked dep endence of product selectivities on temperature and, except where note d, on H-2 pressure is attributed to the strong chemisorption of H-2 on these catalysts.