Eco-friendly additives in acidic pretreatment to boost enzymatic saccharification of hardwood for sustainable biorefinery applications

Abstract

Pretreatment of renewable lignocellulosic biomass is essential to produce fermentable sugars and biofuels in a sustainable biorefinery. However, lignin repolymerization during pretreatment was reported to intensify lignin inhibition in the enzymatic hydrolysis of cellulose. Seeking green additives to mitigate lignin inhibition is of great significance for sustainable lignocellulosic biorefinery. In this work, green additives, mannitol and syringic acid, were thoroughly evaluated for improving the dilute acid pretreatment efficacy of hardwood. The results showed that these additives used in dilute acid pretreatment altered the structure and physicochemical properties of lignin, thus mitigating lignin inhibition on cellulose hydrolysis. In detail, ³¹P NMR results showed evidently higher aliphatic OH groups of lignin after mannitol addition. The 2D HSQC NMR analysis exhibited more β-O-4 and fewer β–β structures, while GPC analysis showed a lower molecular weight of lignin. Results indicated that mannitol was incorporated into the lignin structure to form etherified lignin with plenty of hydroxyl tails, which suppressed lignin repolymerization. As a result, the surface barrier effect of lignin was largely diminished by mannitol addition, leading to improved ease of cellulose hydrolysis by up to 53%. As far as we know, mannitol as an additive in biomass pretreatment has not been reported. This work presented novel green additives to suppress lignin repolymerization for boosting enzymatic saccharification of cellulose in a sustainable lignocellulosic biorefinery.