Ionic Liquids: Enzymatic Hydrolysis of Lignocellulose

Abstract

Abstract Lignocellulose is an abundant and renewable resource with potential to replace fossil raw materials. The general recalcitrance of lignocellulose makes it difficult to depolymerize plant cell wall polysaccharides to fermentable monosaccharides. Thus, efficient pretreatments are needed prior to polysaccharide hydrolysis, which, in current second generation bioethanol plants, is done by enzymatic hydrolysis. Ionic liquids (ILs) are salts with low melting points (<100 °C) and some ILs have been found to dissolve cellulose or even lignocellulose. It has been shown that different pretreatments with ILs, in which the whole lignocellulose sample, or components of it, are dissolved, lead to significantly increased enzymatic hydrolysis yields and kinetics. After IL pretreatment, the substrate is typically washed, but the washing is a very water‐intensive process. To skip process steps and improve water economy, a one‐pot procedure has been proposed, in which the IL pretreatment and enzymatic hydrolysis of lignocellulose are done subsequently in the same vessel without the intermediary step of IL removal. Cellulose‐dissolving ILs inactivate cellulases and hemicellulases, the enzymes used in plant cell wall polysaccharide hydrolysis. This fact severely limits the applicability of the one‐pot hydrolysis procedure. The inactivation mechanisms of cellulases in ILs have not been fully elucidated so far, but proposed reasons for inactivation include structural unfolding, stripping of essential water molecules, reduced substrate‐binding affinity, competitive inhibition, and pH effects due to the basicity of cellulose‐dissolving ILs. In order to overcome cellulase inactivation in the presence of IL, several strategies have been tested. Cellulose‐dissolving ILs have been designed with special focus on features to ensure better enzyme compatibility. Enzymes with better IL tolerance have been screened from metagenomic libraries and organisms living in extreme environments, or enzyme IL tolerance has been improved by different directed evolution endeavors, mutagenesis or point mutation approaches. In addition, chemical modification of protein surfaces and immobilization have been reported as effective methods to increase enzyme IL tolerance. Impressive progress has been made in the area with several different approaches for providing enzyme‐compatible hydrolysis mixtures in the presence of even high IL concentrations.