Alkaline Pretreatment Methods
Alkaline pretreatment involves the use of bases in the form of slurry or an aqueous solution for pretreatment of biomass. The most
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Table 5.8 Representative examples for sulfur dioxide pretreatment of different types of cellulosic biomass [180].
a G: glucose, X: xylose, % yields are sugar recovered based on sugar available; g/100 g is yield of sugar recovered from 100 g of dry biomass |
widely used bases are lime and sodium or potassium hydroxide. The conditions for alkaline pretreatments are usually less severe than other pretreatments. It can be performed at ambient conditions, but longer pretreatment times are generally required rather than at higher temperatures. The alkaline process involves soaking the biomass in alkaline solutions and mixing it at a target temperature for a certain amount of time. A neutralizing step to remove lignin and inhibitors (salts, phenolic acids, furfural, and aldehydes) is required before enzymatic hydrolysis. The use of a strong alkali in the pretreatment causes saponification of intermolecular ester bonds crosslinking lignin and part of the hemicellulose, resulting in structural alteration of lignin. In addition to this, partial decrystallization of cellulose [187, 188, 41] and partial solvation of hemicellu — lose [189, 187] can occur during the alkaline pretreatment process.