Alkaline Pretreatment

The alkaline pretreatment method is one of the most effective delignification methods, owing to its effectiveness on breaking the anisole bond between lignin and xylan, and ability to degrade glycosides (Li et al., 2010), destroy the structure of lignin, and reduce the degree of polymerization and crystallinity of cellulose (Zhang et al., 2020b). Alkaline reagents, such as NaOH, Na₂CO₃, Ca(OH)₂, KOH, and ammonia, were reported in the alkaline pretreatment process (Sun et al., 2014; de Carvalho et al., 2016; Mirmohamadsadeghi et al., 2016; Yuan et al., 2018; Júnior et al., 2020). The mechanism of alkaline hydrolysis is that the uronic ester bond that functions as cross-link is broken by saponification, and the crosslinks between the xylan chain and other polymerization units are broken. Finally, lignin is removed and cellulose and hemicellulose are retained (Tarkow and Feist, 1969). Figure 2 illustrates the main reaction mechanism under alkaline conditions (Rinaldi et al., 2016). de Carvalho et al. (2016) pretreated bagasse at 175°C for 1.5 h with a 15% NaOH solution, and more than 90% lignin removal rate was obtained. Meanwhile, the hemicellulose and cellulose removal rates were achieved at 45.3 and 11.1%, respectively. Furthermore, the yield of glucose could reach up to 51.1% by the saccharification of pretreatment liquid for 24 h. In addition, interesting studies have shown that under room temperature and normal pressure conditions, soaking corn stover in aqueous ammonia for 10–60 days could remove 55–74% of lignin without stirring, retaining 85% of the xylan and with the glucan remaining almost unchanged (Kim and Lee, 2005).

The cleavage of βaryl ether bonds under alkaline conditions.

Compared with the acid pretreatment method, alkaline pretreatment requires milder conditions, is a relatively easier process, and has better treatment effect, which can remove most of the lignin and part of the hemicellulose, swell the cellulose, and improve enzymolysis rate (Sindhu et al., 2014). However, it has disadvantages such as longer treatment time and having a more complicated post process (Bali et al., 2015). Moreover, the low purity of lignin separated by alkali pretreatment significantly limits its application in actual production (Zhao et al., 2017).