Mono- and di-valent ion exchange of mordenite membranes for dehydration of acetic acid by pervaporation

Abstract

The conventional Na-mordenite (Na-MOR) zeolite membranes prepared on α-Al2O3 tubes by secondary hydrothermal growth method. To improve membrane performances in dehydration of acetic acid by pervaporation, the Na+ as counter ions in MOR membranes were exchanged by a series of mono- and di-valent cations to adjusted the channel structure and water affinity. Langmuir surface areas for monovalent ion-exchanged MOR zeolites were consistent with the reverse order of ion radii as H+>Li+>Na+>K+>Cs+, while for those exchanged by divalent ions the zeolites surface areas followed the reverse order of hydrated ion radii instead of natural radii as Ba2+>Ca2+>Mg2+. The water adsorption of MOR by mono- and di-valent cations exchange were all disordered, and H+, K+ and Ca2+ exchanged MOR showed higher water capacity than that of Na-MOR. In pervaporation, monovalent ion-exchanged membranes with molecular sieving effect showed higher performances than those of divalent ion-exchanged membranes with augmented steric effect and weakened molecular sieving. Among these membranes, H-MOR showed the largest flux of 3.68 kg m−2 h−1 with a normal separation factor of 472 in pervaporation of 90 wt% acetic acid/water under 348 K due to a larger pore size and higher water adsorption, which were supplemented for acid-treated membranes. To get better separation ability, H+ exchange conditions on MOR membranes were investigated in detail. The optimized H-MOR membrane showed a flux of 1.52 kg m−2·h−1and separation factor of 1443 with good stability for 100 h. A simple water wash with a recovered performance proved a previously reported phenomenon that reduced flux with time was affected by acetic acid blockage.