Towards understanding the role of acidity on levulinic acid conversion using alumina-niobia mixed oxide-supported catalysts

Abstract

The catalytic transformation of levulinic acid (LA) represents a pivotal advancement towards establishing a sustainable biorefinery industry. Hence, this study investigated the intricate role of the catalyst acidity, which remains relatively unexplored, in the catalytic transformation of LA. A range of alumina-niobia supported nickel-copper catalysts, with varying alumina (Al) to niobia (Nb) ratios, was synthesized and characterized. The results of characterization techniques revealed that the strength, quantity, and nature of the acidic sites of the catalysts were modified by the variations in the Al to Nb ratios. Further, a correlation between the acidic properties of the catalysts and the result of the reaction runs and poisoning experiments was established. Specifically, Lewis acid sites were responsible for the conversion of LA into angelica lactone (AL), which can be subsequently hydrogenated to gamma-valerolactone (GVL). Conversely, Brønsted acid sites are required for the ring-opening reactions of GVL which led to the formation of 2-methyltetrahydrofuran (MTHF) and valeric acid (VA). Notably, selectivities as high as 85.0% for MTHF and 67.0 % for VA were achieved with Ni-Cu/Al-Nb (6) and Ni-Cu/Nb2O5, respectively. To the best of our knowledge, this represents the highest MTHF selectivity in solvent-free catalytic conversion of LA. Additionally, based on our findings, we propose a comprehensive reaction pathway employing alumina-niobia-supported nickel-copper catalysts, shedding light on the intricate interplay of acidity in the LA conversion process. This study not only deepens our understanding of LA transformation but also holds great promise for optimizing catalytic processes in pursuit of sustainable and eco-friendly biorefinery technologies.