Hydrodeoxygenation of guaiacol as a model compound of bio-oil in methanol over mesoporous noble metal catalysts

The liquid phase hydrodeoxygenation (HDO) of guaiacol (GUA), a model compound of bio-oil, was studied on bimetallic (PtPd) and monometallic (Ru) catalysts supported on mesoporous aluminosilicate of Al-HMS(X) type with different Si/Al (X) ratios and on mesoporous zirconia modified with silica (m-ZrO2-SiO2) in the presence of methanol as a solvent. The catalysts were characterized by NH3-TPD, TEM, XPS, 27Al and 29Si solid-state NMR and N2 adsorption–desorption methods. The influence of сatalyst loading, temperature, solvent/guaiacol ratio and contact time on the catalytic performance was investigated. It was established that, decreasing the Si/Al ratio and, correspondingly, increasing the acidity of the catalysts based on Al-HMS led to increasing conversion of guaiacol. Phenol, catechol, and their methylated derivatives were the main products of guaiacol HDO reaction in methanol at low catalyst loading (guaiacol/metal ratio, 800). It was found that the fraction of completely hydrodeoxygenated products (cyclohexane and methylcyclohexane) greatly increased as the catalyst loading grew (guaiacol/metal ratio, 160). Conversion of guaiacol on PtPd/m-ZrO2-SiO2 catalyst was higher than that on PtPd/Al-HMS(10), in accordance with the larger number of acid sites on the catalyst surface; however, the undesirable heavy fraction of methylated by-products was also higher. Ru-based catalysts exhibited the highest catalytic activity and showed unusually high selectivity toward fully hydrodeoxygenated products (cyclohexane, methylcyclohexane) in the HDO of guaiacol in the presence of methanol. Guaiacol can be efficiently converted into alkanes, with quantitative conversion and selectivity to cyclohexanes of 78% over Ru/Al-HMS(10) catalyst under relatively mild conditions (200 °C, 5 MPa H2). Methylation under the influence of methanol, deoxygenation on acid sites and aromatic ring hydrogenation on metal sites proceeded in a parallel way according to the suggested reaction pathways.