Pd and PdZn supported on ZnO as catalysts for the hydrogenation of cinnamaldehyde to hydrocinnamyl alcohol.

Adsorbed HCAL molecules control the adsorption of CAL and enhance the selective hydrogenation to COL over PdZn catalyst. [Display omitted] • The catalytic function of PdZn alloy for cinnamaldehyde (CAL) hydrogenation is largely different from metallic Pd. • PdZn ally more selectively produces cinnamyl alcohol than Pd. • The product of hydrocinnamaldehyde is suggested to promote the hydrogenation of aldehyde group of CAL. Liquid phase selective hydrogenation of cinnamaldehyde (CAL) was investigated over ZnO-supported Pd and PdZn catalysts different in the Pd loading. The former monometallic catalyst was less selective to the formation of cinnamyl alcohol (COL) irrespective of the Pd loading (5 and 30 wt.-% Pd). When the Pd loading was small (5 wt.-%), PdZn catalyst (PdZn-5) indicated similar catalytic actions. However, PdZn catalyst containing Pd in a larger content of 30 wt.-% (PdZn-30) showed different results: the COL selectivity was about 20% at low conversion but it increased with CAL conversion, reaching to >50% at a conversion of 60%. The COL selectivity was likely to change depending on the concentration of a product of hydrocinnamaldehyde (HCAL). The coadsorption of HCAL should control the orientation of CAL molecules adsorbed on the PdZn-30 catalyst. This may assist the adsorption of CAL via its aldehyde group on the surface of catalyst, resulting in an increase in the COL selectivity. Unique catalysis of PdZn-30 may result from structural features of the surface of its large PdZn particles, which are different from those of PdZn-5 having smaller PdZn particles. [ABSTRACT FROM AUTHOR]