Towards understanding of delignification of grassy and woody biomass in cholinium-based ionic liquids

Abstract

The molecular level details of dissolution of lignin in certain ionic liquids (ILs), such as cholinium-based ILs, are a relatively underexplored area and several key details to comprehend the dissolution mechanism are yet to be discovered. To understand, answer, and connect the missing links in the delignification mechanism during biomass pretreatment using cholinium-based ILs we employ COnductor like Screening MOdel for Real Solvents (COSMO-RS) and molecular dynamics (MD) simulations to evaluate the interactions between lignin-like model compounds and the anion and cation of several cholinium-based ILs. Initially, lignin dissolution was studied for cholinium-based ILs containing five different carboxylate anions ([For], [Ace], [But], [Hex], and [Oct]) and were compared with lysinate as the anion. The microscopic properties such as interaction energies, activity coefficient, radial and spatial distribution functions (RDF/SDF), and hydrogen bonds and their dynamics were assessed to characterize lignin dissolution in these ILs and were validated with experimental data. Among the anions studied, both octanoate and lysinate containing ILs demonstrated better lignin dissolution; lysinate being the best. The simulation data suggested that [Ch][Lys] has higher affinity for ether linkages of lignin (e.g., β-O-4) than for C-C linkages, which explains the higher delignification of hardwood and grassy biomasses (60-80% C-O-C linkages) in [Ch][Lys].