Acidic ionic liquid grafted PPF membrane reactor and its catalytic esterification kinetics

Electrospun polypropylene fibrous (PPF) membrane was selected as a substrate to graft ionic liquids (ILs) and avoid tedious ILs separation and recovery. A catalytic membrane, BILs-g-PPF, was prepared by long-distance and dynamic low-temperature Ar plasma flow activation and subsequent grafting with dual -SO₃H imidazole Brönsted acidic ILs (BILs), [(CH₂)₃SO₃HVIm]HSO₄. A novel multistage membrane series reactor was first developed by employing the BILs-g-PPF membrane to catalyze esterification of acetic acid with ethyl alcohol. The BILs, which supplied strong active H⁺ and benefited to overcome the chemical equilibrium due to its high hydrophilic groups, was chosen as a catalyst. The three-dimensional porous network architecture of PPF membrane provided a high specific surface area to graft considerable BILs and enhanced mass transfer in membrane. The optimization of reaction conditions was studied and the results were as followed: BILs grafting degree of 213.5 mol/m³, reaction temperature 70 °C, molar ratio of acid to alcohol 1:5, membrane flux of 21.36 L/(m²·h). In single-stage catalytic membrane system, the acid conversion achieved 66.24% in 8 h. The multistage membrane series reactor can improve the reaction conversion efficiency. For the used membrane which lost some anions, its catalytic properties could be largely restored by soaking activation using a dilute sulfuric acid aqueous solution with a mass concentration of 10%. The proposed reaction kinetic models which related the feed volume and membrane structure parameters predict the esterification process well at different conditions.