Simple purification of ionic liquid solvents by nanofiltration in biorefining of lignocellulosic substrates

Ionic liquids are excellent but expensive solvents to dissolve lignocellulose for biofuel production. Since it is presumed that ionic liquids cannot be cost-effectively purified in large-scale processes, they are simply avoided as solvents. Thus, this study aims to investigate the feasibility of ionic liquid purification via nanofiltration membranes with regards to permeate flux and rejection performance. Two commercially available polyamide and one polyimide membranes were used to separate saccharide products from various feed concentrations of the ionic liquid 1,3-dimethylimidazolium dimethylphosphate. At high concentrations of ionic liquid, there was a marked decrease in permeate flux for all tested membranes due to low permeability of the ionic liquid and due to osmotic pressure differences. At low feed concentrations of this ionic liquid, the concentration of products in the permeate was significantly lower using the polyamide membranes than with the polyimide membranes. Conversely, at high feed ionic liquid concentrations, the contaminant concentrations significantly decreased using the polyimide membrane. In conclusion it is feasible to recover ionic liquid solvent up to a purity of 80% by using both polyamide and polyimide membranes. Since they are non-volatile and environmentally friendly in general, ionic liquids are very suitable candidates to replace conventional organic solvents in lignocellulose refining.