Enhancement of hydrogenation activity by synergistically promoting Re booster in Ni/Al<inf>2</inf>O<inf>3</inf> catalyst for selectively converting levulinic acid into γ-valerolactone

Abstract

Heterogeneous catalytic hydrogenation offers a sustainable thermochemical approach to selectively convert levulinic acid (LA) to highly valuable γ-valerolactone (GVL). In this study, an enhanced Ni/Al2O3 catalyst was designed by systematically varying the Re booster content to improve the catalytic activity for LA hydrogenation compared with those of Ni and Re benchmarks. Advanced in situ characterizations revealed the Re interacted with Ni species to form Ni–Re alloy with remaining the ReOx functioned as collaborative active sites for hydrogenation. The synergistic Re booster facilitated H2 reduction and alleviated the H2 adsorption and desorption of the Ni/Al2O3 catalyst. Compared with the monometallic Ni reference catalyst, the designed Ni–Re catalyst comprised a moderate number of Lewis acidic sites, which improved the catalytic activity. Under the optimal conditions at 140 °C and 30 bar of H2 for 2 h, the highest LA conversion and GVL yield were 100 % and 97.8 %, respectively. On the Ni–Re surface, the predominant linear CO coordination clarified the C=O bond adsorption model for LA hydrogenation. High kinetic constant and turnover frequency were obtained over Ni–Re catalyst, which are significantly higher than the values of Ni and Re catalysts owing to facilitation of rate determining step in hydroxypentanoic acid (HPA) dehydration. To elucidate the reaction mechanism, LA was hydrogenated to GVL via the key HPV intermediate. This extensive investigation provides valuable insights into the design of potential Ni–Re catalysts for application to bioresource hydrogenation at sustainable biorefineries.