Assessing the recyclability of superbase-derived ionic liquids in cellulose processing: An insight from degradation mechanisms

The recyclability of superbase-derived ionic liquids (SILs) as a green and efficient solvent is considered to be an urgent and crucial issue to increase the economic viability of regenerated cellulose (RC) processing. Herein, two novel 1,8-diazabicyclo[5.4.0]undec-7-enium carboxylates ([DBUH][CH₃OCH₂COO] and [DBUH][CH₃CH₂OCH₂COO]) were first recovered from the coagulation bath in the cellulose regeneration, as compared to traditional 1-allyl-3-methylimidazaolium chloride (AmimCl). The degradation mechanism of ionic liquids (ILs) in the recovery process and the properties of RC prepared from recycled ILs were analyzed to investigate the solvent availability. The results indicated that the average recovery yields of the three ILs were achieved up to 95–97 % without any further process optimisation. The highest recycle times of [DBUH][CH₃OCH₂COO] (10 times) with the outstandingly thermochemical stability was obtained in the recovery process, which was twice of AmimCl (5 cycles). It can be explained that [DBUH][CH₃OCH₂COO] exhibited the highest energy gap, the lowest electrophilicity index, and the weakest tendency to nucleophilic reaction with water. Conversely, [DBUH][CH₃CH₂OCH₂COO] was only recovered for 4 cycles because of the active conjugation state between [DBUH]⁺ and [CH₃CH₂OCH₂COO]⁻, in especial the strong electron-donating capacity of –OCH₂CH₃. The C[dbnd]N in [DBUH][CH₃CH₂OCH₂COO] was prone to nucleophilic attack by water to increase the degradation degree. Furthermore, the RC obtained from recycled [DBUH][CH₃OCH₂COO] possessed higher crystallinity and thermal stability in comparison with that from recycled AmimCl, which was corresponding to the recyclability of ILs. These findings favor [DBUH][CH₃OCH₂COO] as a promising solvent in the scaling up of the cellulose processing and utilization.