Surface organometallic chemistry on metals: Selective dehydrogenation of isobutane into isobutene on bimetallic catalysts prepared by reaction of tetra n-butyltin on silica-supported platinum catalyst

The selective dehydrogenation of isobutane into isobutene was studied on silica-supported bimetallic Pt-Sn. Several bimetallic catalysts were carefully prepared by selective hydrogenolysis of Sn(n-C₄H₉)₄ on Pt. Previous EXAFS studies have shown that this hydrogenolysis is a stepwise transformation of a Pt-Sn(n-C₄H₉)₃ fragment into a surface alloy. It was shown that after hydrogen treatment at 550°C, tin and platinum are in reduced form (zerovalent oxidation state) and that the tin atoms are located on the surfrace of the metallic particles. The presence of tin on platinum caused a decrease in hydrogen or carbon monoxide chemisorption, but an increase of the oxygen consumption. The decrease of H₂ and CO chemisorption is explained by the decrease of the number of accessible platinum atoms due to the increased number of surface tin atoms. The increase in the O₂ chemisorption was explained by the following reaction which represents a phase segregation: Pt_sSn_x/SiO₂+1/2(y+xy′)O₂ → (PtOy)_s(SnO_y′)_x/SiO₂. The values of y and y′ was about 1 and 2 at respectively 25°C and 300°C. Thermodesorption of adsorbed CO on several reduced PtSn catalysts showed no shift of the ν (CO) frequency, suggesting negligible electronic effect of tin atoms on the platinum atoms when both are reduced. At 550°C under atmospheric pressure of hydrogen and isobutane, the presence of tin increases drastically, both the selectivity and the activity of the isobutane conversion into isobutene (for Sn/Pt_s = 0.85, the selectivity is higher than 99% and the TOF, based on total platinum atoms, is greater than 6 s^-1). The increase in selectivity could be explained by the "site isolation effect" and the increase in activity could be due to the inhibition of the coke formation (which poisons the active surface). A mechanism of dehydrogenation and hydrogenolysis of isobutane based on elementary steps of organometallic chemistry has been proposed which accounts both for the high selectivity and activity of the bimetallic catalysts as compared to pure Pt/SiO₂.