Hydrogenation of cinnamaldehyde to cinnamyl alcohol with metal phosphides: Catalytic consequences of product and pyridine doping

Selective hydrogenation of alpha,beta-unsaturated aldehydes is challenging due to the competition between unsaturated functional groups (C=C and C=O) with the catalyst. This study probes the use of metal phosphides as selective catalysts for cinnamaldehyde hydrogenation. Monometallic phosphides (MP; M = Ni, Co, Ru) showed high affinity to C=C hydrogenation, with 98 % selectivity to hydrocinnamaldehyde with both Ni2P and Co2P. Bimetallic RuMoP improved the cinnamyl alcohol (COL) selectivity up to 91 %, while bimetallic NiMoP preferred C=C hydrogenation to hydrocinnamaldehyde. Density functional theory (DFT) suggested greater charge transfer between the carbonyl oxygen and Mo sites on the surface leading to a lower activation energy barrier for cinnamaldehyde hydrogenation to COL. Product and pyridine doping studies resulted in an increased selectivity to COL possibly through surface coverage effects. Lastly, diffuse reflectance infrared Fourier transform spectra and DFT provided insights into the CAL adsorption mode and the reaction mechanism that supported the experimental observation.