Abstract
In order to confirm the contribution of delignification to the increase in lignocellulosic cellulose digestibility, several laboratory oxidative pretreatments under mild conditions, including alkaline-hydrogen peroxide (AP), two-step alkaline/peracetic acid (APAA) and sodium chlorite (SC) pretreatments were employed to achieve selective delignification of sugarcane bagasse and retained most of the hemicelluloses (xylan) in the pretreated solids. Four commercial cellulase cocktails were used to test the enzymatic hydrolyzability of pretreated substrates. Results revealed that delignification indeed could greatly improve the final (120 h) cellulose hydrolysis with relatively high final (120 h) glucan conversion (> 90%) by different cellulase cocktails even if the substrates still had a high hemicelluloses content. However, the xylan conversion seemed to be more greatly dependent on the pretreatments and cellulase cocktails used. AP and APAA pretreatments resulted in the disappearance of middle lamella and liberation of cellulose fibers with significant etching, deformation and fracture of cell wall structure. SC pretreatment greatly modified the sugar bagasse surface morphology to make the surface much coarser. The cell wall also underwent serious fracture and deformation with some middle lamella disappearing. However, no significant alteration on the structure of pure cellulose was observed by SC oxidative pretreatment of filter paper. Oxidative pretreatment might also modify lignin structure and surface properties thus greatly reducing the non-specific adsorption of enzymes. The obtained results strongly support the conclusion that delignification under mild pretreatment condition can be very helpful to improve the enzymatic hydrolysis of lignocellulosic cellulose by commercial cellulase cocktails even if the substrates has a high hemicelluloses content.
Highlights
Lignocelluloses is one of the most promising renewable resources for biorefining to produce various biofuels, biochemicals and biomaterials
alkaline-hydrogen peroxide (AP) and alkaline/peracetic acid (APAA) pretreatments removed about 40% of hemicelluloses, while glucan solubilization was lower than 6%
The results revealed that delignification could greatly improve cellulose hydrolysis and relatively high glucan conversion could be obtained though there was some difference for the hydrolysis rate by different cellulase cocktails
Summary
Lignocelluloses is one of the most promising renewable resources for biorefining to produce various biofuels, biochemicals and biomaterials. To analyze the effect of lignin on biomass recalcitrance, it is necessary to obtain samples under mild conditions to achieve selective removal of lignin, but minimizing removal of hemicelluloses and modification of cellulose. In this aspect, sodium chlorite (SC) pretreatment and combination of alkaline and oxidative delignification, such as alkaline-oxygen (Kallioinen et al 2013), alkaline-hydrogen peroxide (AP) (Dutra et al 2018) and alkaline/peracetic acid (APAA) (Zhao et al 2009) seems to be good choice, because they usually can be operated under mild conditions to retain most of hemicelluloses in the pretreated solids and achieve a high degree of delignification (DD). The obtained findings would provide insightful information to understand the negative effects of lignin on cellulose accessibility for fermentable sugar production
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