UNIQUE SELECTIVITY FOR THE DIMERIZATION OF PROPENE ON RHSN SIO2 CATALYSTS - IMPLICATIONS ON THE MECHANISM OF C-C BOND FORMATION AND CLEAVAGE ON BIMETALLIC CATALYSTS/

A series of bimetallic catalysts RhSnx/SiO2 (x=0.4, 0.7, 0.9, and 1.4) were synthesized by the reaction of the monometallic catalyst Rh/SiO2 with Sn(n-butyl)(4) under hydrogen. Various chemical and spectroscopi c methods indicated that the metals present were fully reduced, and th at tin atoms rest on the surface, very slightly increasing particle si ze and producing isolated rhodium sites. The catalytic reactions of pr opylene/hydrogen mixtures in the presence of these bimetallic catalyst s are compared with those of the monometallic Rh/SiO2 catalysts. The m echanistically interesting reactions observed are those of carbon-carb on bond formation and cleavage. For the monometallic catalyst, olefin homologation and hydrogenolysis were observed, reactions which invoked the transfer of C-1 fragments from one olefin to another. For the bim etallic catalysts, a marked increase in the selectivity for C-6 produc ts was observed. The presence of hydrogen is necessary to this reactio n but selectivity for C-6 is enhanced when hydrogen is in deficit with respect to propylene. Selectivity for C-6 increases with the surface rhodium to tin, Rh-s/Sn, ratio to a maximum at 0.9. Low temperature fa vors the formation of C-6 and C-2 products.