Hydrogenation of cinnamaldehyde using catalysts prepared from supported palladium phosphine complexes

The liquid phase hydrogenation of cinnamaldehyde has been studied using catalysts prepared from the binuclear palladium complexes [Pd(2)X(4)(PR(3))(2)] (X = Cl, Pr or I; R = Me, Et, and Pr-i or Bu for X = Br only) on silica, and the results have been compared with the performance of a standard Pd/SiO2 catalyst prepared from Pd(NO3)(2) and with that of the unsupported molecular complexes. When X = Pr, cinnamaldehyde is rapidly hydrogenated selectively to hydrocinnamaldehyde and no further reaction occurs. When X = I, the reactions are extremely slow but are again limited to hydrocinnamaldehyde formation. The catalysts prepared from the chloride-containing precursors behave differently, and some phenyl propanol is formed along with hydrocinnamaldehyde. However, the reaction mechanisms are different from that which operates with the silica-supported palladium catalyst. The rates of the reactions are dependent on the nature of the phosphine substituent, but the selectivities are dependent primarily on the halide. Hydrogenation reactions on all of the catalysts prepared from the binuclear complexes proceed only after an induction period during which partial reduction and/or surface reconstruction of the supported complexes occur. This contrasts with the performance of the standard Pd/SiO2 catalysts for cinnamaldehyde hydrogenation, which proceeds without any induction period. The reactions of all of these catalysts are thought to take place on a hydrocarbonaceous overlayer which is formed rapidly from unsaturated aldehydes and alcohols. Hydrogenations of cinnamaldehyde using the unsupported complexes in solution proceed at much lower rates, but reveal the same selectivities observed with their silica-supported counterparts. (C) 1996 Academic Press.