Hydrogenation/Hydrodeoxygenation Selectivity Modulation by Cometal Addition to Palladium on Carbon-Coated Supports

A series of carbon-supported palladium catalysts were synthesized to study the influence of binary Pd–metal catalysts on the selectivity of condensed-phase hydrogenation reactions. Conversion of acetophenone and 1-phenylethanol using a lab-scale plug flow reactor was used to assess how the different bimetallic systems altered reaction selectivity between hydrogenation of either the aromatic ring or the carbonyl group versus hydrodeoxygenation. A variety of cometals including alkali and alkaline earth metals and transition metals were screened in this study. These results showed that the Pd–metal bimetallic catalysts led to dramatic shifts in selectivity compared to the Pd monometallic catalysts. Most notably, with the addition of iron, ethylbenzene was exclusively produced as the final product, while the addition of lithium yielded more than 80% phenyl hydrogenation products with little deoxygenation. To investigate how cometal addition alters the electronic states of Pd active sites, various characterization techniques were employed to compare differences in acidity, oxidation states, oxygen affinity, and electronic properties. The experimental results and DFT calculations suggest that the incorporation of lithium into the Pd lattice leads to the blockage of interstitial Pd hydride (Pd-H) formation through a higher formation energy compared to surface Pd-H and inhibits deoxygenation, whereas the addition of iron leads to the formation of a phase with higher affinity toward oxygen, thereby increasing the selectivity to deoxygenation. The findings of this work provide a model system to study the influence of bimetallic catalysts on reaction selectivity on carbon supports without introducing complicating factors such as pore size and heteroatoms that are difficult to control with traditional carbons used in commercial processes, and serve to provide insights that could be applied to additional processes such as selective hydrogenation of bioderived chemicals.