Particle size effects in the selective hydrogenation of cinnamaldehyde over supported palladium catalysts

This work investigated the catalytic performance of palladium catalysts in the selective hydrogenation of alpha, beta-unsaturated aldehydes, and especially the effect of Pd particle size on the hydrogenation of cinnamaldehyde (CAL). An unsupported nanosized Pd catalyst and a series of supported Pd catalysts using supports of activated carbon, SiO2, TiO2, gamma-Al2O3, SiC, and graphene oxide were prepared and evaluated in the selective hydrogenation of CAL. Varied sizes of Pd particles could be obtained directly by using different Pd precursors and indirectly by introducing an inactive Ag metal over a gamma-Al2O3 supported palladium catalyst. Over a reducible TiO2 support, the Pd particle size can also be controlled indirectly by the effect of strong metal-support interaction (SMSI). Combined with their TEM observations and catalytic tests, density functional theory (DFT) calculations have confirmed that smaller Pd particles favored C=C-centered adsorption of CAL leading to a higher selectivity to hydrogenation of the C=C bond, forming hydrocinnamaldehyde (HCAL), while on larger ones, the C=C-centered adsorption would be partly substituted by C=O-centered adsorption resulting in a lowered selectivity to HCAL but an increased selectivity to C=O bond hydrogenation, forming hydrocinnamyl alcohol (HCOL). It is clarified that the size dependence of the catalytic selectivity originates from the strong dependence of C=C/C=O-centered adsorption on Pd particle size. Finally, tests using solvents with different Pd-solvent interactions and alpha, beta-unsaturated aldehydes with remarkable steric effect variations were applied to further regulate the adsorption between palladium and substrate to alter the catalytic activity and selectivity.