Hydrogen bonding in ternary mixtures of N-Methyl morpholine Oxide, water and Dimethyl sulfoxide for enhanced cellulose dissolution capabilities

Abstract

N-Methylmorpholine-N-oxide (NMMO) is the solvent of choice in the lyocell process because of the interesting biodegradable, non-toxic and non-ecotoxic features that the full recovery of NMMO provides to the process. NMMO is used in its monohydrate form (1NMMO:1H2O) and cellulose dissolution occurs via hydrogen bond (HBs) formation between the oxygen at the oxide group (as the HB acceptor, HBA) and the hydroxyl groups of the cellulosic units (as the HB donor, HBD). Interestingly, HO is a serious competitor for NMMO to form HBs with cellulose so the number of HO-cellulose-HBs should be kept under control. Addition of co-solvents has become a common practice and the challenge is studying the capability of this additional co-solvent to balance the NMMO-cellulose-HBs versus HO-cellulose-HBs and thus modify the solvent capabilities of the resulting ternary mixture. Herein, we prepared ternary mixtures by addition of the respective equivalents of HO and DMSO (e.g., 1:0.5, 1:1 and 0.5:1) to anhydrous NMMO to obtain 1NMMO:1HO:0.5DMSO, 1NMMO:1HO:1DMSO and 1NMMO:0.5HO:1DMSO. We correlated the solvent capability with the HB formed among the components in the different three ternary mixtures. 1NMMO:0.5HO:1DMSO controlled the number of HO-cellulose-HBs better than the other two mixtures and this helped to maintain the resulting cellulose solution in the liquid form at room temperature.