A highly water-selective carboxymethylated cellulose nanofiber (CNF-CMC) membrane for the separation of binary (water/N2) and ternary (water/alcohols/N2) systems in vapor-permeation

Abstract

A separation top layer was successfully deposited using cellulose nanofiber (CNF) and carboxymethylated cellulose nanofiber (CNF-CMC) on a cellulose acetate (CA) microfiltration membrane via the casting method since the filtration method resulted in the penetration of the coating solution into the membrane support. The CNF-CMC-based separation top layer displayed a uniform non-porous structure with a thickness of approximately 1.3 μm thus indicating that most of the membrane pore substrate was covered by CNF-CMC. In contrast, the CNF-based membrane exhibited a non-uniform porous structure within the range of 10–50 nm formed by aggregated CNFs, which is unsuitable for water vapor separation. The CNF-CMC membrane fabricated by the casting method exhibited a high water vapor permeation and H2O/N2 separation factor of 1.6 × 10−6 mol/(m2 s Pa) and 490, respectively, owing to the high dispersibility of the CNF-CMC solutions along with a reduction of macrovoids and microvoids. In ternary separation systems consisting of H2O/alcohols/N2 mixtures, the rich phase of carboxylate groups on the CNF-CMC enhanced the preferential sorption behavior of water vapor while preventing the permeation of isopropyl alcohol (IPA) and N2. The highest separation performance was observed in an H2O/IPA/N2 mixture with a high water vapor permeance level of 1.5 × 10−6 mol/(m2 s Pa) and an H2O/IPA permeance ratio exceeding 79,000.