Recent advances in the conversion of lignocellulosic biomass and its degraded products to levulinic acid: A synergy of Bronsted-Lowry acid and Lewis acid

Levulinic acid (LA) imparts carboxylic acid and ketone bifunctionality, serving as a highly versatile platform chemical for the production of various value-added products. The sustainable production of LA through the biorefinery route especially from the lignocellulosic biomass feedstock is particularly attractive in view of its lower cost and environmentally benign process. However, a large scale of biomass conversion faces several bottlenecks typically due to the recalcitrance of raw material, low selectivity of catalyst and limitation of mass transferring. Bifunctional catalysts emerging from the synergistic coupling of Bronsted-Lowry acid and Lewis acid have significantly improved the cellulose conversion to LA in tandem reactions, which may even be accomplished in a one-pot conversion which is otherwise difficult to achieve. The variability arises from the complementary functional sites allowing ample opportunity for tuning in favor of the needs, shedding light in breaking through the bottlenecks. This paper reviews the current-state-of-art and development of various bifunctional catalysts including mineral acid-based, heteropolyacid-based, ionic liquid-based, carbon-based and silica-based catalysts, with respect to their synergistic effects on the conversion of lignocellulosic biomass and its degraded products to LA. The pertinent factors such as the catalyst acidity, surface area and porosity as well as reaction conditions are discussed alongside. Lastly, the methods of analyzing synergistic effects via synergy factor, frontier orbital theory , thermodynamic study are outlined to provide a tentative directive framework of bifunctional catalysts.

Automated summary

Levulinic acid (LA) is a highly versatile platform chemical that can be used for the production of various value-added products. The sustainable production of LA from lignocellulosic biomass through the biorefinery route is attractive due to its lower cost and environmentally friendly process. However, there are several bottlenecks in large-scale biomass conversion, including the recalcitrance of raw material, low selectivity of catalyst, and limitation of mass transferring. Bifunctional catalysts, which combine Bronsted-Lowry acid and Lewis acid, have been shown to significantly improve the conversion of cellulose to LA, even in a one-pot conversion. This paper reviews the current state-of-art and development of various bifunctional catalysts and discusses their synergistic effects on the conversion of lignocellulosic biomass to LA. The paper also outlines methods for analyzing the synergistic effects and provides a tentative framework for bifunctional catalysts.