Plant polyphenol intermediated metal-organic framework (MOF) membranes for efficient desalination

Synthesis of a continuous and intergrown metal-organic framework (MOF) membrane remains a challenging issue due to the grain boundary defects and intercrystalline cracks in MOF thin film. In this study, a ZIF-8 membrane was fabricated by using a plant polyphenol tannic acid (TA) layer as an intermediate layer to overcome this issue. Specifically, a TA layer was constructed on a polyethersulfone substrate via a facile and environmentally friendly aqueous coating process. Thereafter, the above modified membrane was immersed into a Zn(Ac)2 and Hmim mixture solution for in-situ growing a ZIF-8 thin film on membrane surface. It was found that the abundant pyrogallol groups in TA layer could coordinate with Zn2+ ions, promote heterogeneous nucleation and contribute to a continuous and intergrown ZIF-8 thin film. The as-prepared ZIF-8 membrane showed a pure water permeance of 3.6 L m−2 h−1 bar−1 with NaCl and Na2SO4 rejections of 64.7% and 92.2% respectively, overwhelming the controlled ZIF-8 membrane without TA as intermediate layer in both permeability and selectivity. This membrane is one of few MOF membranes that have a divalent salt rejection higher than 90%, demonstrating its great potential application in efficient desalination.