Deep eutectic solvents (DES) are an emerging class of solvents composed of a hydrogen bond donor (HBD) and a halide or quaternary ammonium salt such as choline chloride (CC) salt. DES are considering green solvents due to their null or negligible vapor pressure, easy to synthetize, cheap, and non-toxic properties, which allows them to be used as solvents in a broad range of applications such the use, pre-treatment, and transformation of the lignocellulosic (cellulose, lignocellulose, and lignin) biomass in order to obtain valuable derivative compounds. Cellulose pre-treatment is a key step in the biomass valorization. The pre-treatment process depends on the molecular interactions from solvent that are capable to disrupt the recalcitrant structure of polymer, allowing then both preparing the cellulose for third incoming molecules (preconditioning) or dissolving the cellulose crystallite in their glucan chains. Thus, in this work, the role of four DES solvents: Choline-Chloride (CC) Etileneglycol (CCEtg), CC Oxalic acid (CCOx), CC Urea (CCU), and CC Levulinic acid (CCLev) in their ability to precondition a cellulose crystallite at room temperature was evaluated using classical molecular dynamics simulations. Radial distribution functions (RDFs) for CC and HBD solvent molecules indicate that there is an increase of the g(r) values in the mixture, without changing their corresponding interaction distances from pure behavior. Furthermore, RDFs between DES and cellulose crystallite indicate that the hydrogen atoms from HBD molecules are slightly better positioning towards the oxygen atoms located on the hydroxyl moieties in glucose units than hydrogen atoms from choline cation. On the other hand, the hydrogen atoms of the hydroxyl moieties are better related towards chlorine atom, regardless the HBD in the mixture. Finally, the number of hydrogen bonds (HBs) between DES and cellulose crystallite explains the ability of DES to break the cellulose interchain HBs by reducing the common HBs occupancies of the cellulose crystallite and replacing them with other more weak, leaving key interactions sites such as hydroxyl oxygen atoms free. This ability of DES solvents is consistent with their corresponding hydrogen bond basicity (β Kamlet-Kaft) parameter as reported in the literature. We hope that this work provides insightful information on the emerging process of the both cellulose treatment and cellulose preconditioning using DES solvents.

Evaluation of four deep eutectic solvents for the preconditioning of a cellulose I-β crystallite at room temperature, using molecular dynamic simulations

ACS Spring Meeting 2021

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