Lignocellulosic biomass conversion to levulinic acid via acid catalysis: Current methods, opportunities and challenges for self-sustaining biorefineries

• Cheaper levulinic acid from lignocelluloses requires full by-products valorisation. • Nature of solid by-products depend on feedstock composition and catalytic variables. • Heterogeneous/homogeneous catalyst synergies are positively correlated. • Solid residues from acid hydrolysis of lignocellulose are the largest by-product. • Hydrochar use as promoter of anaerobic digestion is a novel valorisation route. In recent years, integrated biorefineries have received growing interest as to maximise the extractable value from biomass in the form of green platform chemicals in order to support the establishment of a low carbon economy. Research and development in this area has focused on the conversion of lignocellulosic biomass, regarded as second-generation feedstocks, into a multitude of biochemicals from numerous different plant-based feedstocks with a wide range of catalytic systems, particularly the production of levulinic acid. This work critically reviews the most significant and pertinent literature of the methods and challenges of second-generation feedstock for levulinic acid production, identifying heterogeneous/homogeneous catalyst synergies can improve yields between 28 and 58 % depending on mineral catalyst promoter used. In addition, areas of particular focus of this review regards solid residues potential applications in a circular biorefinery context, revealing the novel role of hydrochar as booster additive in anaerobic digestion, which results in increased methane yields and, potentially, better soil amending digestate. The review also highlights the gaps in knowledge and synthesise new concepts to bridge these gaps in research, most pertinently applied to the solid residue formation and properties, with key links between multiple fields defined for the first time.