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.