Hydrodeoxygenation of Guaiacol over Ru(0001): A DFT Study

The hydrodeoxygenation (HDO) of aromatic oxygenates over ruthenium was studied computationally on the model system guaiacol (2-methoxyphenol) on Ru(0001) using a DFT method. In addition to the adsorption geometries of the aromatic intermediates, the study focused on the energetics of elementary reaction steps that occur during the HDO of guaiacol. Bond scissions at the aliphatic side group were calculated to have barriers of at most 69 kJ mol–1. In contrast, barriers for the cleavage of the aromatic bonds Caryl–O were determined at more than 100 kJ mol–1. On the basis of calculated energetics, a reaction pathway for the HDO of guaiacol is proposed in which first the methyl group of the methoxy moiety is removed to yield catecholate. Subsequently, the oxo groups are replaced by H, yielding first phenolate and, finally, benzene. For the removal of the first oxygen center of catecholate, a substantially lower barrier (106 kJ mol–1) than for the Caryl–O cleavage of phenolate (189 kJ mol–1) was calculated. This is rationalized by the strained structure of adsorbed catecholate. The high barrier for the second Caryl–O scission step is line with recent experiments that yield phenol as the main product of guaiacol HDO over Ru/C.