Mechanism of Guaiacol Hydrodeoxygenation on Cu (111): Insights from Density Functional Theory Studies

Abstract

Understanding the mechanism of the catalytic upgrade of bio-oils via the process of hydrodeoxygenation (HDO) is desirable to produce targeted oxygen-deficient bio-fuels. We have used calculations based on the density functional theory to investigate the reaction mechanism of HDO of guaiacol over Cu (111) surface in the presence of H2, leading to the formation of catechol and anisole. Our analysis of the thermodynamics and kinetics involved in the reaction process shows that catechol is produced via direct demethylation, followed by dehydrogenation of –OH and re-hydrogenation of catecholate in a concerted fashion. The de-methylation step is found to be the rate-limiting step for catechol production with a barrier of 1.97 eV. Formation of anisole will also proceed via the direct dehydroxylation of guaiacol followed by hydrogenation. Here, the rate-limiting step is the dehydroxylation step with an energy barrier of 2.07 eV. Thermodynamically, catechol formation is favored while anisole formation is not favored due to the weaker interaction seen between anisole and the Cu (111) surface, where the binding energies of guaiacol, catechol, and anisole are -1.90 eV, −2.18 eV, and −0.72 eV, respectively. The stepwise barriers also show that the Cu (111) surface favors catechol formation over anisole as the rate-limiting barrier is higher for anisole production. For catechol, the overall reaction is downhill, implying that this reaction path is thermodynamically and kinetically preferred and that anisole, if formed, will more easily transform.