Pathways for Reactions of Esters with H2 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 (C 5 H 10 O 2 ) in the presence of hydrogen (H 2 ) over Pd nanoparticles supported on high-surface-area Nb 2 O 5 . Over Pd-Nb 2 O 5, C 5 H 10 O 2 reacts by three competing primary pathways: hydrogenation to form ethyl propyl ether (C 5 H 12 O) 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 C 5 H 10 O 2 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 Brønsted acid sites by exchange with K + ions suppresses mass-averaged rates of C 5 H 12 O formation by sevenfold, and physical mixtures of Pd-SiO 2 and Nb 2 O 5 give rates more than 10 times lower than Pd-Nb 2 O 5 . These results demonstrate that C 5 H 12 O formation requires Brønsted 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 Brønsted 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.