Fine regulation of cellulose dissolution and regeneration by low pressure CO2 in DMSO/organic base: dissolution behavior and mechanism

Abstract

In this study, the fine regulation of the dissolution and regeneration of microcrystalline cellulose (MCC) using very low pressure (0–0.2 MPa) CO₂ in a mixed solvent of dimethyl sulfoxide (DMSO) and 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) at a very low temperature (30 °C) was achieved. The solubility of MCC in DMSO/DBU (weight ratio of DMSO W_DMSO = 0.90) could reach 9.0% at 30 °C and under CO₂ pressure of 0.2 MPa. A similar phenomenon was observed in the mixed solvent DMSO/1,1,3,3-tetramethylguanidine (TMG). Moreover, ATR-FTIR, NMR, UV-Vis, TGA, XRD and DFT computational analyses were used to investigate the dissolution mechanism. It was concluded that in the mixed solvent (DMSO and organic base), DMSO helped to dissociate ion-pairs into free ions by balancing the concentration of free ions and the number of hydrogen bonds at W_DMSO = 0.90. Interactions between CO₂ and the solvent mixture were explored, and the results indicate that the optimum CO₂ pressure not only promotes the formation of ionic bonds but also accelerates the formation of covalent bonds. In this way, these interactions prevent the MCC molecules from aggregating and facilitate the dissolving of MCC. This study gives a thorough insight into the dissolution mechanism and specificity of MCC in the CO₂–DMSO/organic base solvent system, which could be helpful for the utilization and transformation of cellulose.