Surface organometallic chemistry on metals: kinetics of the hydrogenolysis of tetra n-butyl tin on silica and alumina supported rhodium catalysts

The stoichiometry and the kinetics of the hydrogenolysis of tetra n-butyl tin on silica and alumina supported rhodium catalysts has been determined at 298 K in n-heptane. Various Rh/SiO₂ and Rh/Al₂O₃ catalysts have been prepared by exchange from [RhCl(NH₃)₅] (OH)₂ and by impregnation from RhCl₃ respectively. After reduction, metallic dispersions were found to vary from 0.45 to 0.9 as determined by hydrogen chemisorption measurements. Concerning the hydrogenolysis of the organotin complex, the amount of grafted tin was found to depend on the amount of tetra n-butyl tin introduced: since this amount is lower than the value corresponding to a ratio Sn/Rh_s of 0.7, all the tin complex is grafted and the amount of grafted tin is directly proportional to the number of surface rhodium atoms; on average, each surface rhodium atom is able to graft 0.8 Sn(n-C₄H₉)_x fragment. Above this ratio, the amount of grafted tin is almost independent of the amount of complex introduced. These results strongly support the hypothesis that the reaction occurs selectively on the metallic surface. The stoichiometry of the hydrogenolysis, determined by the amount of butane evolved per mole of grafted tin, depends on the coverage of the rhodium surface: for a low coverage, all the butyl groups are hydrogenolyzed, showing the formation of 'naked' tin atoms. For a full coverage, some butyl groups remain fixed on the surface showing the presence of grafted organotin species on the rhodium surface. The stoichiometry of the hydrogenolysis also depends on the reaction time for a given concentration of tetra n-butyl tin: at short reaction times which correspond to low coverages of the metallic particles, all the butyl groups are removed whereas this phenomenon is not observed at longer reaction times for which alkyl groups remain on the metallic surface. Kinetics of the hydrogenolysis indicates that the reaction is first order in 'free surface adsorption site' and zero order in tetra n-butyl tin.