Another category of solvent pretreatment involves the use of cellulose-dissolving solvents, such as cadoxen, concentrated mineral acids, DMSO, and zinc chloride (10, 12). While these agents can be effective at directly releasing sugars from the carbohydrate fractions of biomass and/or producing a solid residue containing cellulose that is highly digestible by enzymes, the use of such solvents in pretreatment processes for the production of fuels and commodity chemicals from biomass will be challenging due to the expense of such catalysts, catalyst recycle requirements, and the requirement for clean process streams for subsequent biological conversions.

14.5.4 Supercritical fluid pretreatments

Biomass pretreatment processes using supercritical fluids to extract lignin from biomass feedstock have been investigated. A number of different supercritical fluids (alone or in mixtures) have been investigated, although the most common approaches utilize water, carbon dioxide, or ammonia (14, 68). While supercritical pretreatment conditions can effectively remove lignin and produce pretreated biomass that exhibits good enzymatic digestibility, the economic viability and practical operation of processes at supercritical operating conditions have not been effectively demonstrated. Of greatest concern are the extremely high-pressure requirements (generally above 10 MPa) of these processes.