Zeolite NaA Membrane: Preparation, Single-Gas Permeation, and Pervaporation and Vapor Permeation of Water/Organic Liquid Mixtures

Abstract

Zeolite NaA membranes were prepared reproducibly by a one-time-only hydrothermal synthesis with a short reaction time of 3 h at 373 K using a gel with the composition Al2O3:SiO2:Na2O:H2O = 1:2:2:120 (in moles) and porous α-alumina support tubes seeded with zeolite NaA crystals. A dense intergrown zeolite crystal layer of about 30 μm in thickness was formed on the outer surface. The zeolite NaA membranes were highly permeable to water vapor but impermeable to every gas unless dried completely. The completely dried membranes displayed gas permeation behavior attributed to Knudsen diffusion, indicating the presence of interstitial spaces between the zeolite crystal particles, or nonzeolitic pores. The membranes displayed excellent water-permselective performance in pervaporation (PV) and vapor permeation (VP) toward water/organic liquid mixtures. With an increase in temperature, both the permeation flux Q and the separation factor α increased, and the membrane performance was much better for VP than for PV. For VP at 378 K and 10 wt % of feedwater, Q values were 4.5, 3.5, and 7.8 kg/(m2 h) and α values were >30000, 5700, and >9000 for the water/ethanol, /methanol and /dioxane systems, respectively. A mechanism of PV and VP based on the capillary condensation of water in the zeolitic and nonzeolitic pores and the blocking of other molecules from entering the pores was proposed.