Theoretical Mechanism on the Cellulose Regeneration from a Cellulose/EmimOAc Mixture in Anti-Solvents

Abstract

The experiments on cellulose dissolution/regeneration have made some achievements to some extent, but the mechanism of cellulose regeneration in ionic liquids (ILs) and anti-solvent mixtures remains elusive. In this work, the cellulose regeneration mechanism in different anti-solvents, and at different temperatures and concentrations, has been studied with molecular dynamics (MD) simulations. The IL considered is 1-ethyl-3-methylimidazolium acetate (EmimOAc). In addition, to investigate the microcosmic effects of ILs and anti-solvents, EmimOAc-nH₂O (n = 0-6) clusters have been optimized by Density Functional Theory (DFT) calculations. It can be found that water is beneficial to the regeneration of cellulose due to its strong polarity. The interactions between ILs and cellulose will become strong with the increase in temperature. The H-bonds of cellulose chains would increase with the rising concentrations of anti-solvents. The interaction energies between cellulose and the anions of ILs are stronger than that of cations. Furthermore, the anti-solvents possess a strong affinity for ILs, cation-anion pairs are dissociated to form H-bonds with anti-solvents, and the H-bonds between cellulose and ILs are destroyed to promote cellulose regeneration.

Keywords: cellulose; ionic liquids; regeneration; theoretical calculations.

Figure 1

The initial configuration of cellulose (16 × 8) with 16 glucan chain system and a degree of polymerization (DP) = 8.

Figure 2

The electrostatic potential on the van der Waals surfaces of (A) Emim, (B) OAc, (C) H₂O, (D) CH₃OH, and (E) CH₃CH₂OH.

Figure 3

The number of H-bonds between cellulose chains in different 80 wt% anti-solvents in 293 K.

Figure 4

The (50–100 ns) average number of H-bonds between cellulose chains and ILs in different temperatures (80 wt% water).

Figure 5

Radial distribution functions for cellulose around O atoms of OAc in different temperatures (80 wt% water).

Figure 6

(A) The number of H-bonds and (B) average (50–100 ns) number of H-bonds between cellulose chains in the cellulose/EmimOAc/water mixtures at 20, 40, 60, 80, and 100 wt% water.

Figure 7

Density functional calculated structures of the solvent-separated ion pairs (SIPs) and contact ion pairs (CIPs) for EmimOAc-nH₂O (n = 0–6) clusters.

Figure 8

DFT calculated (A) intermolecular bond r_CH…O(anion) and r_CH…O(water); (B) intramolecular bond r_C…H of cation; (C) total binding energies for EmimOAc-nH₂O (n = 0–6) clusters.