Size sieving and electrostatic exclusion synergetic strategy for high efficiency recovery of superbase-derived ionic liquids via nanofiltration from the spinning process

Abstract

Superbase-derived ionic liquids (SILs) as the promising green solvents for cellulose spinning process should be urgently and deeply evaluated with respect to their recyclability. Herein, size sieving and electrostatic exclusion synergetic strategy was adopted for recycling four SILs ([DBUH][CH3CH2OCH2COO], [DBUH][CH3OCH2COO], [DBNH][CH3CH2OCH2COO], and [DBNH][CH3OCH2COO]) from aqueous solutions by the nanofiltration membranes (NF270 and NF90) at the first time. NF270 with larger pore and more negative charge was conducive to recycle SIL on account of a higher volume flux and the remarkably high IL rejection (＞95 %) compared to NF90. Rising the pressure and temperature played the positive role in the permeate flux of the membranes, whereas the elevated SIL concentration markedly decreased the electrostatic exclusion, leading to a decline in the recovery efficiency. Benefiting from the size sieving effect, the [DBNH][CH3OCH2COO] with small cation and anion structure could be recovered efficiently due to the high energy gap and big IL aggregates. Conversely, the strong electrostatic attraction was appeared between [DBUH][CH3CH2OCH2COO] (low energy gap and high positive charge density) and the membranes, leading to a decline in recovery efficiency. Furthermore, the combination of nanofiltration and evaporation to recover SIL from spinning wastewater effectively reduced the total recovery cost (1.51 $/Kg) in comparison with the evaporation only. Insight gained from this work suggested a high-efficiency and economical approach could be used in the recovery of SILs with small cations and anions size via large pore nanofiltration membranes during the industrialized cellulose spinning process.