Efficient cellulose dissolution in a tertiary [EHEMIM]-[EMIM]OAc-water system

Abstract The ionic liquid 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc) is a wide spread cellulose solvent, however solubilization times of 12–36 h, to achieve concentrated cellulose solutions are still a major drawback. Repeated recycling of [EMIM]OAc during our previous published vinyl ester-based cellulose laurate synthesis, led to a much faster cellulose dissolution. The effect is an increasing conversion of 1-ethyl-3-methylimidazolium (EMIM) to 1-ethyl-2-(hydroxyethyl)-3-methylimidazolium (EHEMIM) by the addition of an EMIM-carbene species to the by-product acetaldehyde. The formation of EHEMIM-cations was analyzed by 1H- and 1H,1H-COSY NMR-studies and monitored through all synthesis-recycling cycles, correlating with significant faster cellulose dissolution, from 16 h to 5 min. The presence of a small amount of water in the [EMIM]-[EHEMIM]OAc mixtures is crucial for superior cellulose dissolution. Major increased cellulose-solubilization capability was obtained at an optimized ratio of 50 cation-% EHEMIM, 50 cation-% EMIM and 8.5% w/w water, solubilizing 14% w/w cellulose within 3 h at 80 °C.