Lignin Hydrothermal Liquefaction into Bifunctional Chemicals: A Concise Review

Lignin, the second largest biomass after cellulose is underutilized. Yet, it remains the only natural source of aromatic, and phenolic compounds. It is imperative to, amidst the expanding interest on biomass conversion, to accord the necessary attention towards lignin degradation into value added chemicals. Specifically, its phenyl, guaiacyl, and syringyl derivatives. Understanding lignin degradation chemistry, goes a long way in its selective valorization into fuels and chemicals via thermochemical routes such as hydrothermal liquefaction (HTL). Therefore, development of technologies targeting value addition of products and by-products from lignin, would undoubtedly give way to emerging markets in the industry. Previous review papers focused on the general HTL of biomass, food waste, algae, and their model compounds. However, review on HTL of lignin is scarcely available. This paper presents the detailed literature analyses of the current trend in lignin degradation via HTL. Effect of HTL conditions including temperature, heating rate and catalyst has been reviewed. In-depth discussion on use of ionic liquids as catalyst for HTL of lignin has also been compiled. Other lignin degradation techniques such as pyrolysis and hydrolysis were also discussed. This is aimed at bringing together an up-to-date information on lignin degradation into selected chemical intermediates.