Insight into the negative effects of lignin on enzymatic hydrolysis of cellulose for biofuel production via selective oxidative delignification and inhibitive actions of phenolic model compounds

Abstract

Oxidative pretreatment of wheat straw with sodium chloride has been used for selective delignification to obtain ideal samples to understand the negative effects of lignin on cellulose accessibility of lignocellulosic biomass. Strong interactive effects has been found between hemicellulose and lignin. To achieve a high enzymatic glucan conversion (EGC) (>80% with 20 FPU/g solid) in the case of high xylan content (25–30%), a moderate degree of delignification (56.3%) with relatively low lignin content (11.9%) is necessary; while in the case of low xylan content (2.6–3.9%), slight removal of lignin by oxidative treatment can well increase cellulose digestibility with a final EGC reaching 90% even that the residual lignin content is as high as 30.9%. Delignification greatly modifies substrate surface morphology with deformation, fracture of cell wall layers and even disappearance of middle lamella. Oxidation also modifies the functional groups and surface properties of lignin, leading to reduction in the inhibitive effects. Phenolic hydroxyl group (Ph-OH) in p-hydroxyphenyl unit has been found to show the strongest inhibition to filter paper activity of cellulase cocktail. Ph-OH exerts stronger inhibition to β-glucosidase than to the endo- and exo-glucanases. Hydrogen bonding and electrostatic interaction are primarily responsible for the inhibitive action of Ph-OH.