HYDROGENATION OF CINNAMALDEHYDE USING CATALYSTS PREPARED FROM SUPPORTED PALLADIUM PHOSPHINE COMPLEXES

The liquid phase hydrogenation of cinnamaldehyde has been studied usin g 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 on ly) 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 i s rapidly hydrogenated selectively to hydrocinnamaldehyde and no furth er reaction occurs. When X = I, the reactions are extremely slow but a re again limited to hydrocinnamaldehyde formation. The catalysts prepa red from the chloride-containing precursors behave differently, and so me phenyl propanol is formed along with hydrocinnamaldehyde. However, the reaction mechanisms are different from that which operates with th e silica-supported palladium catalyst. The rates of the reactions are dependent on the nature of the phosphine substituent, but the selectiv ities are dependent primarily on the halide. Hydrogenation reactions o n all of the catalysts prepared from the binuclear complexes proceed o nly after an induction period during which partial reduction and/or su rface reconstruction of the supported complexes occur. This contrasts with the performance of the standard Pd/SiO2 catalysts for cinnamaldeh yde hydrogenation, which proceeds without any induction period. The re actions of all of these catalysts are thought to take place on a hydro carbonaceous overlayer which is formed rapidly from unsaturated aldehy des and alcohols. Hydrogenations of cinnamaldehyde using the unsupport ed complexes in solution proceed at much lower rates, but reveal the s ame selectivities observed with their silica-supported counterparts. ( C) 1996 Academic Press.