Investigation of the activity and selectivity of supported rhenium catalysts for the hydrodeoxygenation of 2-methoxyphenol

Abstract

Despite the recognized efficacy of rhenium (oxide) for promoting hydrogenolysis of the CO bond of biomass-derived oxygenates when combined with a hydrogenation metal such as Pd or Ru, the hydrogenation function of rhenium itself is relatively unexplored. Herein, we investigated the activity and selectivity of rhenium nanoparticles supported on various metal oxides for the hydrodeoxygenation of guaiacol, i.e., 2-methoxyphenol. Catalytic experiments performed at 280 °C and 20 bar H2 in a batch reactor revealed that rhenium itself adequately catalyzes the demethoxylation and dehydroxylation of guaiacol, producing completely deoxygenated products of cyclohexane/benzene. The reduced and unreduced rhenium catalysts (Re vs. ReOx) exhibited similar catalytic activity, and both bulk rhenium oxide (ReO3 and Re2O7) and supported rhenium oxide, i.e., ReOx/SiO2, were completely reduced to metallic rhenium after the reaction, suggesting that metallic rhenium is mainly responsible for the hydrogenolysis of guaiacol. Among the catalysts tested, Re/SiO2 exhibited the highest guaiacol conversion (98%) and the highest selectivity for fully deoxygenated cyclohexane (58%), followed by TiO2, Al2O3, ZrO2, and CeO2. The catalytic activity was generally correlated with the rhenium dispersion as measured by transmission electron microscopy and CO-chemisorption. Despite its high activity, sintering and leaching of rhenium occurred for SiO2 support, whereas the second most active TiO2-supported rhenium catalyst was leaching-resistant. Using Re/TiO2 as a benchmark catalyst, the effects of the rhenium loading, the mesoporosity of TiO2, and reaction conditions on the conversion of guaiacol were further explored.