Biomass and Cellulose Dissolution—The Important Issue in Renewable Materials Treatment

Sustainable development strategies, as well as the shift toward a circular bioeconomy, has led to high interest in the development and implementation of technologies that efficiently utilize biomass as a raw material. Switching from fossil-based to bio-based resources requires the consideration of many new challenges and problems. One of the crucial issues is the solubility of lignocellulose or at least its ingredients. According to the trends and legislation, the selected chemicals and methods of dissolution/treatment should also be environmentally friendly. The pretreatment processes prepare biomass for further transformations (e.g., chemical, thermal including pyrolysis, or biological) to valuable products such as biofuels, bio-oils, Fine Chemicals, solvents, plastics, and many others. This review discusses the latest findings on the dissolution of biomass and its ingredients. The application of novel, green solvents such as ionic liquids or deep eutectic solvents is discussed in detail. The impact of the composition and structure of these solvents on the biomass/cellulose dissolution process, as well as the mechanism of cellulose–ionic liquid interaction, is presented. Some novel achievements in the usage of inorganic salts and specific metal complexes are also overviewed.