1-ethyl-3-methyl imidazolium acetate, hemicellulolytic enzymes and laccase-mediator system: Toward an integrated co-valorization of polysaccharides and lignin from Miscanthus

Abstract

The pretreatment of Miscanthus x giganteus with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]), followed by enzymatic hydrolysis and oxidative depolymerization, resulted in the efficient production of monomeric sugars and selective production of phenolic intermediates. The liquid fractions containing monomeric sugars were obtained with high yields (>80%), while a solid fraction enriched with lignin (>50% w.w−1) was isolated. The phenolic monomers released during [Emim][OAc]-pretreatment were identified and quantified by HPLC. A decrease in the content of vanillin, vanillic acid, and p-hydroxybenzaldehyde was observed during pretreatment, suggesting a selective solubilization of guaicyl and p-hydroxyphenyl units from the lignin polymer in [Emim][OAc]. The decrease in p-coumaric and ferulic acids content suggested a cleavage of lignin-carbohydrate complexes during pretreatment. Both pretreatment and enzymatic hydrolysis affected the content of ferulic acid ether and ester bonds. The NMR structural study (2D HSQC) of the isolated lignin showed the preservation of β-O-4′ interunit bonds, while 31P NMR analysis revealed a high content of aliphatic hydroxyl groups. The oxidative depolymerization of the isolated lignin, catalyzed by the laccase mediator system in the presence of 5% v.v−1 recycled [Emim][OAc] in buffer, mainly led to the production of free p-hydroxybenzaldehyde as a promising platform molecule. The mass balance of glucan, xylan and lignin from Miscanthus was validated throughout this strategy.