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

Hexagonal tungsten oxide (h-WO₃) membrane is a novel candidate for dehydration of acetic acid (CH₃COOH)/water mixtures owing to its molecular sieving property and acidic resistance. Meanwhile, c-plane orientation is an important factor for h-WO₃ membranes because the pores of h-WO₃ 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-WO₃ membranes on tubular substrates against c-plane orientation and CH₃COOH/water separation performance are investigated. The h-WO₃ membranes were prepared by hydrothermal synthesis from a precursor sol containing various amounts of sodium tungstate (Na₂WO₄) in the presence of tubular substrates with seeds embedded on their outside surface. The seeding method and the amount of Na₂WO₄ in the precursor sol significantly affected both crystal orientation and densification of the membrane. A precursor sol with appropriate amounts of Na₂WO₄ was essential to simultaneously satisfy high c-plane orientation and densification of the membrane while excess Na₂WO₄ drastically decreased the degree of c-plane orientation. A highly c-plane oriented h-WO₃ 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^-7 mol·m^-2·s^-1·Pa^-1 in a 90:10 wt % CH₃COOH/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 % CH₃COOH/water mixture with pH < 0 for more than 500 h.