Unique selectivity for the dimerization of propene on RhSn SiO₂ catalysts: implications on the mechanism of CC bond formation and cleavage on bimetallic catalysts

A series of bimetallic catalysts RhSn_x 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 spectroscopic methods indicated that the metals present were fully reduced, and that tin atoms rest on the surface, very slightly increasing particle size and producing isolated rhodium sites. The catalytic reactions of propylene/hydrogen mixtures in the presence of these bimetallic catalysts are compared with those of the monometallic Rh SiO₂ catalysts. The mechanistically interesting reactions observed are those of carbon-carbon bond formation and cleavage. For the monometallic catalyst, olefin homologation and hydrogenolysis were observed, reactions which invoked the transfer of C₁ fragments from one olefin to another. For the bimetallic catalysts, a marked increase in the selectivity for C₆ products was observed. The presence of hydrogen is necessary to this reaction but selectivity for C₆ is enhanced when hydrogen is in deficit with respect to propylene. Selectivity for C₆ increases with the surface rhodium to tin, Rh^s Sn, ratio to a maximum at 0.9. Low temperature favors the formation of C₆ and C₂ products.