The importance of Brønsted acid sites on C[sbnd]O bond rupture selectivities during hydrogenation and hydrogenolysis of esters

Abstract

Esters represent an important class of reagents and intermediates for the production of fine chemicals and polymers. In prior studies of homogeneous catalysis, molecular complexes have been reported to selectively cleave either carbonyl CO or acyl CO bonds of ester to ether or alcohols, respectively. In contrast, reactions of esters and H2 upon heterogeneous catalysts typically cleave acyl CO bonds and produce alcohols. Here, we demonstrate that the proximity of Brønsted acid sites to Pd nanoparticles and the thermodynamic strength of these Brønsted acid sites influence the rates and selectivities toward CO bond cleavage pathways either to ethers or alcohols during reactions of esters over bifunctional solid catalysts. The combined results from rate measurements as functions of H2 pressure, ester conversion, and methods to combine Pd nanoparticles and acidic supports; kinetic isotope effects; in situ Brønsted acid site titrations; and calorimetric assessments of Brønsted acid strength provide evidence for bifunctional pathways that require proximity between Brønsted acid sites and Pd atoms to form ethers. These findings suggest that Brønsted acid sites near Pd nanoparticles and with lower deprotonation energies promote the direct reduction of esters to ethers by cleaving the carbonyl CO bond. Taken together, these data indicate that direct reduction of esters to ethers involves the hydrogenation of the ester reactant to form hemiacetal at Pd nanoparticles, followed by dehydration of the hemiacetal at proximal acid sites, and subsequent hydrogenation of the enol ether. In comparison, hydrogenolysis of acyl CO bonds of the ester reactant involves the reaction of a hydrogenated intermediate (plausibly hemiacetal) upon Pd nanoparticles to form the corresponding alcohol and aldehyde products. Among the materials examined, Pd nanoparticles supported on WO3 catalyze the direct reduction of esters and lactones to corresponding ethers and remain stable over extended periods of time on stream (∼42 h). These findings offer a foundation for further design and improvement of heterogeneous catalysts for the selective reduction of esters and other challenging hydrogenations.