Development of novel nanoporous hexagonal tungsten oxide membrane for separation of water/acetic acid mixtures via pervaporation

A novel nanoporous hexagonal tungsten oxide (NP-h-WO3) membrane was developed for separating water/acetic acid (H2O/AcOH) mixtures. The nanopores on the c-plane of h-WO3 are larger than H2O molecules and smaller than AcOH molecules and thus should allow for their effective separation. Moreover, h-WO3 exhibits high stability in acids (pH < 4). Therefore, c-axis-aligned NP-h-WO3 membranes are promising for the separation of H2O/AcOH mixtures. Given this background, we report, for the first time, the synthesis of c-plane-aligned dense NP-h-WO3 membranes on porous ceramic supports via the hydrothermal method using a precursor solution containing AcOH. The presence of AcOH and the concentration of Na2WO4 in the precursor solution are the key factors affecting crystal growth and alignment along the c-plane. A high Na2WO4 concentration is necessary for obtaining membranes with a high degree of c-plane alignment. Thus, dense, c-plane-aligned NP-h-WO3 membranes could be fabricated by precoating the support with amorphous WO3 seeds and subsequent hydrothermal synthesis in a precursor solution of Na2O/WO3/AcOH/H2O (1/1/0.88/201) at 180 °C for 24 h. The fabricated membranes exhibited a maximum separation factor of 42.6 during the pervaporation of a H2O/AcOH mixture (10/90 (w/w)) at 80 °C. Thus, these NP-h-WO3 membranes are highly suited for H2O/AcOH separation.