Pyridinium protic ionic liquids: Effective solvents for delignification of wheat straw

Abstract

Lignocellulosic biomass covers a vast area of the globe and contains many valuable components that can be envisaged for numerous renewable products. The valorization of these components is hindered by its cumbersome isolation due to the recalcitrant nature of lignocellulosic biomass. Acidic ionic liquids (AILs) have emerged in this field as effective pretreatment solvents. The unique property of disrupting lignin-cellulose-hemicellulose bonds and dissolving lignin makes these ILs significant for the separation of lignin and cellulosic constituents. In this study three protic ionic liquids based on pyridinium cation and hydrogen sulfate anion (HSO4−) have been synthesized; [PyH][HSO4] (IL1), [PyH][HSO4. (H2SO4)] (IL2), and [PyH][HSO4 . (H2SO4)3] (IL3) and utilized for the isolation of lignin from wheat straw. The synthesized ILs were characterized by NMR and FTIR analyses. The lignin yield was optimized with respect to reaction temperature, time, biomass loading, and type of IL. The lignin removal efficacy of the ionic liquid with highest ratio of acid [PyH][HSO4.(H2SO4)3] (IL3) is indicated by high delignification (79%) and lignin recovery (77%) under mild conditions (60 °C, 2 h). The cellulose rich material (CRM) and isolated lignin were characterized by FTIR, SEM, TGA, GPC, and HSQC NMR to assess their structure, molecular weight and stability. The cellulosic part was further hydrolyzed enzymatically to evaluate the capability of the recovered pulp to generate glucose. An appreciable yield of reducing sugars (85%) substantiates high lignin removal.