Recent advances in the conversion of lignocellulosic biomass and its degraded products to levulinic acid: A synergy of Brønsted-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 Brønsted-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 and thermodynamic study are outlined to provide a tentative directive framework of bifunctional catalysts. [Display omitted] •Bifunctional catalysts improve the levulinic acid production in tandem reactions.•These catalysts show the interplay of Brønsted-Lowry acid and Lewis acid.•Synergistic effects of bifunctional catalysts vary as the properties change.•Analyzing synergistic effects is vital for a directive framework of catalysts.