Solvents and ions for pretreatment in lignocellulosic biorefineries

Satisfactory conversion of lignocellulosic biomass into bioproducts requires a pretreatment. Often, the pretreated biomass is hydrolyzed enzymatically into sugars. However, the sugar yields highly depend on the pretreatment effectiveness. The pretreatment liquid is composed of acids, bases, salts and solvents, which are more costly than the biomass in biorefinery processes. Thus, to develop cheap and efficient pretreatment concepts, it is important to know which strategies result in high sugar yields. This review focuses on liquid-based pretreatment in combination with enzymatic hydrolysis. We classify pretreatment phenomena (e.g., disintegration, changes of composition) according to biomass modifications on different length scales. By analyzing these phenomena, we identify relevant mechanisms that influence the overall sugar yield. To this end, the correlation of pretreatment phenomena with parameters for the description of pretreatment processes and pretreatment liquids (e.g., severity factor, Kamlet-Taft) is evaluated. Furthermore, the role of components in pretreatment liquids acting as solvent or reactant during pretreatment is discussed, including effects of ion species and concentration. We conclude that the development of more effective pretreatment strategies requires further research to link multiple, independent aspects of the discussed parameters for the description of pretreatment liquids with pretreatment phenomena and mechanisms.