Green solvent for lignocellulosic biomass pretreatment: An overview of the performance of low transition temperature mixtures for enhanced bio-conversion

In parallel with the departure from fossil fuel to more renewable resources, a more environmental-friendly and sustainable alternative, lignocellulosic biomass (LCB) has gained much research attention recently since it can be converted to numerous by-products, such as biofuels, biomaterials, and biochemicals. LCB, however, has recalcitrant nature, which often hinders its effective fractionation and subsequent conversion. Low transition temperature mixture (LTTM) is a relatively new class of green solvent that exhibits biodegradability, biocompatibility, and low toxicity, which can be prepared from abundantly available hydrogen bond acceptor and donor in nature. Given its high lignin solubility, LTTM has been applied to the LCB pretreatment to delignify the biomass and extract the lignin. This review seeks to overview the progress made over the last decade in LCB pretreatment using LTTM. Discussions are also presented on the mechanisms of the lignin extraction and the effect of the operating conditions with a summary of the performances from various LTTMs to provide a concise guide as to which LTTM at what operating condition can perform well for specific LCB. Despite its potential, the relatively young status of this technology warrants future works to enhance its commercialisation through (i) lowering the viscosity of LTTM by the addition of cheap solvent; (ii) enhancing the recyclability of LTTM; (iii) developing indicator to assess LTTM applicability for a specific LCB; and (iv) techno-economic and life cycle analyses.