Site requirements of supported W2C nanocatalysts for efficient hydrodeoxygenation of m-cresol to aromatics

Abstract

Selective hydrodeoxygenation of lignin derivatives into aromatic compounds is a promising route for the upgrading of lignin feedstocks. Metal carbide catalysts have exhibited excellent selectivity in hydrodeoxygenation reactions, while their structure-activity relationship is still in ambiguity. Herein, a liquid-phase atomic layer deposition method was employed to synthesize W₂C/SiO₂ catalysts with uniform and size-controllable W₂C nanoparticles. For gas-phase hydrodeoxygenation of lignin-derived m-cresol at 350 °C, these W₂C/SiO₂ catalysts showed superior toluene selectivities (>95%) regardless of the W₂C particle size. An optimal W₂C particle size of ~7 nm was obtained for achieving the highest W₂C-based hydrodeoxygenation rate. In contrast, the turnover rate per surface W site increased almost monotonously as the W₂C particle size increased within 0.7‒15 nm, attributable to high-index planes appeared on the larger W₂C nanoparticles. Kinetic effects of m-cresol and H₂, taken together with temperature-programmed desorption of probe molecules and theoretical treatments, further indicate that the W₂C surface is nearly saturated by adsorbed m-cresol or its derivates under the reaction condition and the H-addition of the C₇H₇* intermediate to form toluene, instead of the initial C-O cleavage in m-cresol, acts as the rate-determining step. A side-by-side comparison between W₂C(102) and W₂C(001) catalyst surfaces in theoretical simulations of m-cresol hydrodeoxygenation verifies that high-index planes can stabilize kinetically-relevant transition states more effectively than the low-index ones, as a result of more available less-coordinated active sites on the former. The above findings bring new mechanistic insights into the site requirements of supported W₂C nanocatalysts, distinct from those metal-catalyzed hydrodeoxygenation of oxygenates.