Evaluation of organosolv processes for the fractionation and modification of corn stover for bioconversion

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Abstract
Corn stover was pretreated for compositional fractionation and structural modification for maximum conversion of carbohydrate to soluble sugars. The process scheme consisted of three steps: (1) mild prehydrolysis in dilute sulfuric acid, (2) delignification with various organosolv solvents, and (3) enzymatic hydrolysis in an agitated bead reactor. Prehydrolysis of corn stover can be achieved at temperatures ranging from 95 to 120°C, which is a much milder condition than must be applied to wood. Various organosolv solvents, including several alcohols with acid as catalyst, ethylene glycol, and its derivatives, and amines were used for delignification of the prehydrolyzed corn stover. Aromatic alcohols were found to be more effective in solubilizing the prehydrolyzed corn stover than were the aliphatic alcohols. Butanol was the most effective among the aliphatic alcohols; on the other hand, phenol was the best among the aromatic alcohols. Ethylene glycol, methylcellosolv, and ethylcellosolv were effective in solubilizing the prehydrolyzed corn stover but not for enhancing the enzymatic hydrolysis. Various amines achieved delignification at the mild temperature of 95°C, but they tended to solubilize substantial amounts of carbohydrate in addition to lignin. n-Butylamine was effective in enhancing the conversion during enzymatic hydrolysis; it was a good delignifying agent as well as one that achieved a concomitant swelling of the cellulose structure. The low enzymic conversion (20–37%) of prehydrolyzed and solvent-extracted corn stover that was achieved implies that lignin is not the only major barrier for enzymatic hydrolysis. Modification of cellulose structure also should be accomplished to achieve a high degree of conversion. Enzymatic hydrolysis in the agitated bead system increased the rate and extent of conversion of corn stover substantially compared to systems without beads.