Hydrodeoxygenation of guaiacol and anisole over alumina supported monometallic Ni and Cu catalysts: Studies on the effect of metal sites

Abstract

Valorization of lignin-derived phenolics by hydrodeoxygenation (HDO) poses two major challenges. The first one is the reduction of oxygen content and the second one is to prevent the loss of C atoms from the feedstock while removing oxygen atoms in, -OCH₃ groups. In this study, Ni/Al₂O₃ catalysts were found to be efficient for deoxygenation, and Cu/Al₂O₃ catalysts were used to selectively carry out reactions intended toward preventing C losses. HDO over Ni/Al₂O₃ catalysts resulted in the hydrogenation of the aromatic ring of guaiacol and anisole, followed by demethoxylation, as the major reaction pathway. The demethoxylation reaction was favored at lower H₂ pressure. However, at lower pressure catalytic deactivation and the nonlinear Arrhenius nature of the aromatic ring hydrogenation limited guaiacol and anisole conversions at higher temperatures. It was found that with the increase in Ni loading the overall conversion was maximum at 10 wt% Ni. Since the demethoxylation reaction occurred only after aromatic ring hydrogenation, its rate showed a similar dependence on both Ni loading and reaction temperature as that of hydrogenation. In this work, (de)methylation reactions of guaiacol and anisole were carried out selectively over Cu/Al₂O₃ catalysts. Adsorption studies showed that alumina sites were responsible for the reactive adsorption which gave rise to the (de)methylation reactions; and the Cu sites further enhanced these reactions, especially at higher H₂ pressures. We have achieved about 65 % molar conversion of guaiacol selectively towards the formation of catechol, phenol, and methylated catechols/phenols, accompanied by small quantities of toluene and xylenes.