Improving Enzymatic Saccharification and Ethanol Production from Hardwood by Deacetylation and Steam Pretreatment: Insight into Mitigating Lignin Inhibition

Lignin component in lignocellulosic biomass always adversely affects the efficiency of its enzymatic saccharification and subsequent bioethanol production. To mitigate the lignin inhibition, this work thoroughly evaluated the effectiveness of deacetylation and steam pretreatment for improved fermentable sugars and bioethanol production from hardwood as well as quantified the relative contribution of lignin toward physical blockage effect and unproductive adsorption effect on cellulose hydrolysis. It was found that deacetylation reduced the formation of acid catalyst, thus significantly mitigating lignin inhibition on cellulose hydrolysis. The addition of Na2S in deacetylation was more liable in improving biomass porosity, and the addition of Na2SO3 was more favorable for removing surface lignin; both of these factors contributed to the decreased physical blockage effect of lignin. Besides, X-ray photoelectron spectroscopy (XPS) results indicated that deacetylation distinctly suppressed the undesirable lignin repolymerization and the formation of phenolic hydroxyl groups, which could alleviate the unproductive adsorption effect of lignin. When lignin was isolated from the deacetylationsteam-pretreated substrates and added to the enzymatic hydrolysis of Avicel, a distinctly lower lignin inhibition was observed compared to the samples without deacetylation. Besides, the lignin inhibition showed a strong correlation with the extent of lignin repolymerization, proving the lignin inhibition mitigating effect of deacetylation through modification of the lignin structure.