Grafting cellulose acetate with ionic liquids for biofuel purification membranes : Influence of the anion

Membranes made from cellulose acetate grafted with imidazolium or ammonium ionic liquids (ILs) containing different anions were considered for ethyl tert-butyl ether biofuel purification by pervaporation. The new cellulosic materials were obtained after bromide (Br^-) exchange by different anions (Tf₂N^-, BF₄^-, AcO^-). IL structure-membrane property relationships revealed that the membrane properties were strongly improved by varying the anion structure, molecular size and hydrogen bonding acceptor ability β in the Kamlet-Taft polarity scale. The grafted ammonium IL with AcO^- anion combined the highest parameter β with big cation/anion sizes and finally led to the best membrane properties with a normalized pervaporation flux of 0.41 kg/h m² (almost 20 times that of virgin cellulose acetate) for a reference thickness of 5 μm and a permeate ethanol content of 100%. Such properties thus corresponded to an outstanding separation factor at 50 °C.