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 wer e carefully prepared by selective hydrogenolysis of Sn(n-C4H9)(4) on P t. Previous EXAFS studies have shown that this hydrogenolysis is a ste pwise transformation of a Pt-Sn(n-C4H9)(3) fragment into a surface all oy. It was shown that after hydrogen treatment at 550 degrees C, tin a nd platinum are in reduced form (zero-valent 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 carbo n monoxide chemisorption, but an increase of the oxygen consumption. T he decrease of Wt and CO chemisorption is explained by the decrease of the number of accessible platinum atoms due to the increased number o f surface tin atoms. The increase in the Oz chemisorption was explaine d by the following reaction which represents a phase segregation: PtsS nx/SiO2 + 1/2(y + xy')O-2 --> (PtOy)(s)(SnOy')(x)/SiO2. The values of y and y' was about 1 and 2 at respectively 25 degrees C and 300 degree s C. Thermodesorption of adsorbed CO on several reduced PtSn catalysts showed no shift of the v (CO) frequency, suggesting negligible electr onic effect of tin atoms on the platinum atoms when both are reduced. At 550 degrees C under atmospheric pressure of hydrogen and isobutane, the presence of tin increases drastically, both the selectivity and t he 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 p latinum atoms, is greater than 6 s(-1). The increase in selectivity co uld be explained by the ''site isolation effect'' and the increase in activity could be due to the inhibition of the coke formation (which p oisons the active surface). A mechanism of dehydrogenation and hydroge nolysis of isobutane based on elementary steps of organometallic chemi stry has been proposed which accounts both for the high selectivity an d activity of the bimetallic catalysts as compared to pure Pt/SiO2. (C ) 1998 Academic Press.