Kinetics of Hydrogenation of Aqueous Levulinic Acid over Bimetallic Ru–Ni/MMT Catalyst

Abstract

γ-Valerolactone is one of the value-added products obtained from levulinic acid hydrogenation, having tremendous applications in fuels and chemicals. In this work, several monometallic and bimetallic compositions of Ru and Ni were prepared and evaluated for levulinic acid hydrogenation in an aqueous medium. The optimum composition of 0.5 wt % Ru and 5 wt % Ni on MMT was found to achieve 91% levulinic acid conversion with complete selectivity to γ-valerolactone within 5 h at 220 °C and 250 psig H2 pressure. XPS and H2-TPR studies clearly showed that the electronic interactions between Ru–Ni leading to the synergism enhanced the catalytic efficiency of the Ru–Ni bimetallic catalyst. The average metal particle size by HRTEM was found to be in the range of 1.5–4.5 nm, having mixed cubic, oval, and spherical morphology, dispersed on the surface as well as within the layers of MMT. NH3-TPD showed a higher acidity due to both Lewis and Brønsted sites of the 0.5 wt % Ru–5 wt % Ni/MMT catalyst than individual MMT and monometallic catalysts. The presence of higher Lewis acidic sites preferred selective formation of GVL. Reaction kinetics was studied to propose the rate equation for this reaction based on which a plausible reaction pathway is proposed involving a first step dehydration of levulinic acid to an α-angelica lactone intermediate which then undergoes hydrogenation by dissociatively adsorbed H2 to form γ-valerolactone. The stability of our catalyst was proved by its recycle studies as well as by a hot filtration test.