Advancements and challenges in catalytic hydrothermal liquefaction of lignocellulosic biomass: A comprehensive review

•Renewable biomass: key alternative for eco-friendly energy.•Innovative use of microwave-assisted heating in HTL processes.•Examination of Co-HTL as a strategic approach to enhance economic viability.•Comprehensive analysis of homogeneous and heterogeneous catalysts for HTL.•Identification of research needs and opportunities for further exploration. Hydrothermal liquefaction (HTL) is one of the leading technologies for converting lignocellulosic biomass waste into liquid bio-oil to meet the growing energy demands and alleviate environmental crises. In this comprehensive review, an examination and comparison were conducted on three distinct technologies: HTL, Microwave-Assisted Hydrothermal Liquefaction (MA-HTL), and Co-hydrothermal Liquefaction (Co-HTL). The assessment encompassed various parameters, including production efficiency, bio-oil quality, production cost, and operational conditions. Furthermore, the pivotal role of catalysts in enhancing the bio-oil quality was explored. The corresponding catalytic attributes, spanning homogenous, heterogeneous, and combined catalysts, along with the associated mechanisms such as depolymerization, decarboxylation, decarbonylation, hydrogenation, dehydration, deoxygenation, and hydrodeoxygenation, were thoroughly examined. Additionally, the review delved into upgrading technologies and potential applications of bio-oil to broaden its utility. The current challenges and future prospects concerning bio-oil production using HTL technologies were also addressed.