Lignocellulose depolymerization

Various processes have been developed for hydrolyzing lignocellulose to its major constituents, i.e., to sugars and (partly) depolymerized lignin. The lignin is usually precipitated from the aqueous solution and either used as chemical feedstock or burned as process fuel. The aqueous sugar solution is then applied for fermentation to ethanol after neutralization and purification. 

Various solvents are being investigated to dissolve lignocellulosic materials. Some approaches focus on the selective depolymerization and extraction of lignin and hemicellulose as pre-treatment to produce clean cellulose fibers for subsequent fermentation or for pulping. Other approaches attempt to dissolve the whole lignocellulose with or without depolymerization. The liquefaction processes that are carried out at high temperature (>300 °C), and produce a complex oil mixture, are discussed above with the pyrolysis processes. 

Extraction of lignocellulosic materials with dioxane has been used for quite some time as a method for lignin isolation at atmospheric pressure (1) or, as recently reported, at high pressure and with supercritical C02 mixtures (2-5). In this case, it was possible to extract from wood lignin oligomers with a low degree of chemical modification, hemicelluloses were also depolymerized and extracted, but cellulose remained without significant mass losses. 

We are investigating various pretreatment processes to increase the pyrolytic yield of levoglucosan from herbaceous biomass. This work is motivated by our effort to convert lignocellulosic material into simple sugars suitable for fermentation. In particular, we are investigating thermal depolymerization of com stover and switchgrass into anhydrosugars and their subsequent hydrolysis and fermentation to lactic acid. 

Carbon dioxide explosion is a pre-treatment process that uses supercritical carbon dioxide to break down the biomass structure. In aqueous solution, carbon dioxide forms carbonic acid which depolymerizes lignocellulosic materials. As a small molecule, carbon dioxide can penetrate into the pores of the biomass better than ammonia. When carbon dioxide explodes due to the change of pressure, it breaks the cellulosic structure. This process is usually operated under high pressure but low temperature to prevent monosaccharide degradation. But in comparison to steam explosion and ammonia explosion processes, the sugar recovery yield from this process is... 

Early efforts for the efficient conversion of biomass into monomers were concentrated on the acidic transformation and on the fermentation of sugars obtained from sucrose, or from glucose after depolymerization of starch materials (potato, com). During the last two decades, however, more attention has been focused on other constituents of plant materials, such as cellulose and lignocellulosic residues. The current trend is indeed to avoid competition with food, offering new routes to C6 and C5 carbohydrates, for further transformation into value-added platform molecules. 

Zhao (2005) described the possibility of SCO production from lignocellulose hydrolysates, the biomass-to-biodiesel three-step plan lignocellulose biomass depolymerization into fermentable sugars, their conversion into microbial lipids by oleaginous microorganisms, and the chemical transformation of their lipids into biodiesel. 

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