Ester reduction on bifunctional metal-acid catalysts: Effect of metal to acid ratio

Abstract

Ethers are highly desirable commodities in food, cosmetics, rubber, and coatings due to their low chemical reactivity, stability, and miscibility with other organic solvents. The direct reduction of esters to ethers by reactions with H2 provides a pathway to create nonsymmetrical ethers from biomass-derived esters. Propyl acetate (PA) and H2 react over platinum nanoparticles supported on Brønsted acidic faujasite zeolite (Pt-FAU) by two primary pathways: hydrogenation to form ethyl propyl ether and water and hydrogenolysis to form acetaldehyde and propanol. Here, we evaluate how rates and selectivities for these pathways depend on the ratios of the metal to Brønsted acid sites and the densities of Brønsted acid sites within Pt-FAU. X-ray absorption spectroscopy and infrared spectroscopy of adsorbed CO show that Pt species exist in a metallic state in the presence of H2 at elevated temperatures. The vibrational frequencies of chemisorbed CO on surface Pt atoms (Pts) do not vary significantly with differences in Pt contents or Brønsted acid site (H+) density. Temperature programmed reactions of amines protonated upon Brønsted acid sites demonstrate that the fractions of Brønsted acid sites accessible to organic reactants decrease with increasing Pt content, which implies intimate contact between Pt nanoparticles and acid sites. Comparisons between electron microscopy and rate measurements indicate that the proximity between Pt and H+ matters even when Pt nanoparticles appear to be within pores. Even though rates for reactions between PA and H2 over Pt-FAU decrease significantly during the initial use of the catalysts, analysis of initial rates provides mechanistic insight into elementary steps that determine rates. Initial hydrogenation rates increase by 13-fold, and hydrogenolysis rates increase by 33-fold when the atomic ratio of Pts to H+ increases from 0.08 to 0.6. Hydrogenation and hydrogenolysis rates increase with Pts density but do not correlate with H+ densities, which indicates kinetically relevant steps occur on Pt sites. These elementary steps likely involve the formation of hydrogenated intermediates (i.e., hemiacetal) that diffuse and react rapidly upon Brønsted acid sites, which generates intraparticle hemiacetal gradients. These correlations confirm that ester reduction rates and selectivities depend sensitively on the metal to acid ratio and proximity between these functions in a manner reminiscent of bifunctional catalysts used for hydroisomerization and hydrocracking catalysis. Consequently, increasing the number of stable Pt sites close to H+ sites should increase rates per mass catalyst by increasing the concentration of hemiacetals at Brønsted acid sites.