Impact of Dilute Acid Pretreatment on the Lignin Carbohydrate Complex Structure
The mode of action of dilute acid pretreatment results in hemicellulose solubilization without significant delignification, increasing the surface area of LCC and hence increasing the accessibility of sugars to the enzymes (Hu et al. 2012). It has been found that pretreatment temperature and pH play more crucial role during pretreatment as compared to the residence time to effectively solubilize hemicellu- lose (Lim and Lee 2013). Dilute acid pretreatment has been used for a wide range of feedstock including woody biomass, herbaceous crops, agricultural residues, and municipal solid waste (Zheng et al. 2009). Several studies have reported the increase in acid-insoluble lignin content of dilute acid-pretreated material (Sannigrahi et al. 2011; Hu et al. 2012). This phenomenon of increase in the lignin length is hypothesized to be due to re-polymerization of polysaccharide degradation products (such as furfural) and/or polymerization with lignin to form a lignin-like material, termed as pseudo-lignin (Li et al. 2007). Sannigrahi et al. (2011) revealed the presence of spherical droplets generated from holocellulose (hemicellulose and cellulose) during high severity dilute acid pretreatment using scanning electron microscopy (SEM). Further, NMR analysis of pretreated holocellulose indicated significant peaks originating from carbonyl, aromatic, methoxyl, and aliphatic structures, and based on these lignin functionalities, the spherical droplets were confirmed as being lignin-like material and termed as pseudo-lignin. Furthermore, the intensities of these peaks increased as pretreatment severity increased, suggesting an acid-catalyzed disproportionation mechanism accompanying pseudo-lignin formation. Donohoe et al. (2008) observed the generation of lignin droplets on the cell wall of biomass after dilute acid pretreatment as a result of lignin redistribution and condensation at lower temperatures. Hu et al. (2012) reported that during dilute acid pretreatment, polysaccharides underwent series of reactions including acid-catalyzed dehydration followed by fragmentation, rearrangement, and finally polycondensation and/or polymerization reactions to produce complex structure that consisted of carbonyl, carboxylic, aromatic, and aliphatic functionalities. It was concluded from their study that in order to avoid the formation of pseudo-lignin formation, dilute acid pretreatment should be performed at lower severity.
Pseudo-lignin being hydrophobic in nature becomes counterproductive and acts as a physical barrier to prevent enzyme access to the carbohydrate fraction as well as irreversibly bind to enzymes during enzymatic hydrolysis. Such counterproductive phenomena lower the rate of enzymatic and are found to be responsible for the higher enzyme loading during enzymatic hydrolysis (Yang and Wyman 2006).
Sannigrahi et al. (2010) reported an increase in cellulose crystallinity of loblolly pine after dilute acid pretreatment and postulated that selective degradation of the less ordered amorphous cellulose was responsible for the increase in crystallinity index; however decrease in crystallinity was observed after organosolv pretreatment (Sannigrahi et al. 2010).