Effects of Zeolite Framework on Direct Reduction of Esters to Ethers Using H2 on Pt-Zeolite Catalysts

Abstract

The direct reduction of esters to ethers using H2 represents a fundamental selectivity challenge for heterogeneous catalysis and offers a method to increase the hydrothermal stability of chemicals and fuels derived from renewable resources. Prior studies revealed that ethers derive from a series of reactions that involve in situ formation of hemiacetals by hydrogen transfer on metal nanoparticles, followed by diffusive transport to Brønsted acid sites that cleave the carbonyl C–O bond. The competing hydrogenolysis pathway proceeds by a similar bifunctional mechanism; however, hydrolysis and transesterification occur by direct reactions of esters solely at Brønsted acid sites. Here, we find that rates and selectivities reflect the intrinsic reactivity at individual Brønsted acid sites, which depends upon the topology of the aluminosilicate support. Pt-Faujasite (FAU) provides ether formation rates at least an order of magnitude greater than smaller pore zeolites that possess similar Pt loadings and Si/Al ratios (e.g., mordenite framework inverted (MFI), beta (BEA), and ferrierite (FER)). Correlations between formation rates and activation energies for ether formation with heats of adsorption of propyl acetate across Pt-zeolites and Pt-AlMCM41 suggest that dispersive interactions among the zeolite pore wall, surrounding molecules (e.g., propyl acetate), and transition states contribute to differences in performance among these catalysts. These effects appear responsible for Pt-FER providing the highest ether selectivities observed (∼50%), although the specific structural features responsible for this behavior remain uncertain and may reflect framework-specific confinement and transport effects. However, ether selectivities also appear sensitive to other zeolite properties, including channel dimensionality, which may influence diffusive transport between Pt and Brønsted acid sites. These findings illustrate that the topology of the aluminosilicate support impacts rates and selectivities for ester reduction by orders of magnitude: these differences reflect contributions of inter-function transport reminiscent of other bifunctional catalysts for hydrotreating reactions but also evidence from comparisons to titration calorimetry that the local solvation of reactive intermediates at Brønsted acid sites influences catalysis.