ZIF-67 templated thin-film composite forward osmosis membrane: Importance of incorporation method on morphology and performance

Abstract

In this study, a promising strategy is reported to synthesize a high water-permeable polyamide (PA) rejection layer using ZIF-67 nanoparticle as a sacrificial templating material. ZIF-67 nanoparticle was incorporated into the PA rejection layer using two different strategies (i) conventional blending interfacial polymerization (IP) and (ii) newly introduced in-situ growth methods. The primary aim was to study the effects of various incorporation strategies on template removal efficiency. In the conventional blending IP method, the pre-synthesized ZIF-67 nanoparticle was dispersed in the amine monomer solution and incorporated into the PA layer of thin-film composite membranes (TFN-ZIF-PS, modified with pre-synthesized ZIF-67). But in the in-situ method, ZIF-67 was synthesized and incorporated simultaneously with the formation of the PA layer via interfacial reaction of aqueous amine/Co+2 and organic acid chloride/Hmim solutions (TFN-ZIF-IS, modified with in-situ synthesized ZIF-67). After IP reaction, ZIF-67 nanoparticles were removed by water dissolution, which created nanovoids within the PA layer. Systematic membrane characterization confirmed that the incorporation method significantly affects template removal efficiency, where the TFN-ZIF-IS membrane shows a quite different morphology after ZIF-67 removal (membrane with nanovoids (NV), TFC-NV-IS). The effective incorporation and removal of ZIF-67 nanoparticles increased the water permeability of TFC-NV-IS to 5.6 LMH/bar, which is about 5-fold compared with the control TFC membrane. Additionally, TFC-NV-IS exhibited forward osmosis water flux that was raised about 2-fold compared with the control TFC one with a negligible decrease in selectivity. These considerably enhanced separation performances can be attributed to the improved water transport through the as-formed nanovoids. Thus, our strategies (incorporation and template removal) in this study provide a new avenue to synthesize advanced TFC-FO membranes with exceptional performance.