Process intensification strategies for lignin valorization

Nowadays, the increasing concern about the declining fossil fuel reserves and the environmental impact derived from their use has put considerable interest in lignocellulose exploitation as a renewable source of biofuels and biomaterials, according to the biorefinery concept. Several processes and technologies have been extensively studied in order to optimize biomass treatments aiming to enhance the recovery of its main products: cellulose, hemicelluloses and lignin. Lignin is, in fact, considered a valid substitute to petroleum as a source of aromatics, thanks to its abundance in nature. However, its complex and highly resistant structure limits its further applications, therefore, lignin upgrading is considered extremely challenging: various processes have been developed in recent years, but their feasibility at industrial scale still represents a bottleneck. Recently, process intensification has gained considerable attention in the design of sustainable procedures for lignin valorization. In particular, non-conventional technologies such as Ball milling, Ultrasounds (US) and Microwaves (MW) have recently shown promising results in biomass exploitation, thanks to their ability in generating specific high-energy microenvironments which could enhance process efficiency: mechanochemical and US activation have been mostly applied to biomass pre-treatment, in order to separate its components and enhance lignin extraction yield, while MW have been exploited as a means for lignin depolymerization, achieving higher yields of aromatics in milder reaction conditions. However further efforts should be done to improve profitability through new processes, aiming to reduce the cost associated to bio-derived products. In the present review, recent approaches to lignin valorization are discussed, focusing on new alternative methodologies for process intensification, besides their challenges and feasibility at industrial scale.