Synthesis of C-Plane Oriented Hexagonal Tungsten Oxide Membranes on Tubular Substrates and Their Acetic Acid/Water Separation Performances.

Hexagonal tungsten oxide (h-WO) membrane is a novel candidate for dehydration of acetic acid (CHCOOH)/water mixtures owing to its molecular sieving property and acidic resistance. Meanwhile, c-plane orientation is an important factor for h-WO membranes because the pores of h-WO run along its c-axis. However, so far, high c-plane orientation has not been successful on tubular substrates. Here, the effect of synthesis conditions of h-WO membranes on tubular substrates against c-plane orientation and CHCOOH/water separation performance are investigated. The h-WO membranes were prepared by hydrothermal synthesis from a precursor sol containing various amounts of sodium tungstate (NaWO) in the presence of tubular substrates with seeds embedded on their outside surface. The seeding method and the amount of NaWO in the precursor sol significantly affected both crystal orientation and densification of the membrane. A precursor sol with appropriate amounts of NaWO was essential to simultaneously satisfy high c-plane orientation and densification of the membrane while excess NaWO drastically decreased the degree of c-plane orientation. A highly c-plane oriented h-WO membrane was successfully obtained under the optimized condition, which exhibited a maximum separation factor of 40.0 and a water permeance of 1.53 × 10 mol·m·s·Pa in a 90:10 wt % CHCOOH/water mixture. The water permeance approximately doubled compared to the previous report, possibly owing to the significantly higher degree of c-plane orientation. Furthermore, it was found that its separation ability can be maintained while stored in 90:10 wt % CHCOOH/water mixture with pH < 0 for more than 500 h.