Pathways for Reactions of Esters with H-2 over Supported Pd Catalysts: Elementary Steps, Site Requirements, and Particle Size Effects

The direct reduction of esters to ethers offers an efficient pathway to ethers from renewable intermediates. This chemistry previously required homogeneous catalysts and utilized costly and unstable hydride reagents. Here, we elucidate pathways for reactions of propyl acetate (C5H10O2) in the presence of hydrogen (H-2) over Pd nanoparticles supported on high-surfacearea Nb2O5. Over Pd-Nb2O5, C5H10O2 reacts by three competing primary pathways: hydrogenation to form ethyl propyl ether (C5H12O) by apparent C=O bond rupture, hydrogenolysis to form acetaldehyde and propanol (C-acyl-O bond rupture), and hydrolysis to form acetic acid and propanol. Secondary reactions yield other alcohols, esters, ethers, and hydrocarbons. Hydrogenation and hydrogenolysis rates do not change with the pressure of C5H10O2 and increase with a sublinear dependence on H-2 pressure. Furthermore, these dependencies and apparent activation enthalpies remain similar for Pd nanoparticles with different mean diameters (4-22 nm), which shows that the extent of undercoordination of Pd does not significantly affect the mechanism or kinetics for C-O bond rupture steps. Ether formation rates remain constant when D-2 replaces H-2 as the reductant, which together with the rate dependence on H-2 suggests that ethers form by kinetically relevant C-O bond cleavage in a partially hydrogenated intermediate (e.g., hemiacetal). Ex situ titration of Bronsted acid sites by exchange with K+ ions suppresses mass-averaged rates of C5H12O formation by sevenfold, and physical mixtures of Pd-SiO2 and Nb2O5 give rates more than 10 times lower than Pd-Nb2O5. These results demonstrate that C5H12O formation requires Brensted acid sites that reside in close proximity to Pd nanoparticles. Collectively, these observations suggest a reaction mechanism for the reduction of esters that hydrogenates the carbonyl by stepwise addition of H* atoms to form a hemiacetal that dehydrates at proximal Bronsted acid sites or cleaves the C-acyl-O bond to form lower carbon number products. These findings reveal a pathway to convert renewable oxygenates, such as carboxylic acid derivatives, into value-added chemicals useful as surfactants and solvents.