Preparation hierarchical porous MOF membranes with island-like structure for efficient gas separation

Abstract

The development of polymer supported metal-organic frameworks (MOF) membranes with supernal gas permeance and good gas selectivity can satisfy the economic and technical requirements of membrane industry application that is still a huge challenge. This work, we prepared an hierarchical porous ZIF-8 (IZIF-8) membranes with Ag+ ion doped, in which isolated-mesopores connected by continuous-micropores, like island structure. On one hand, the membranes could shorten the diffusion path and reduce the diffusion barrier, which possessed higher gas permeance compared to the ZIF-8 membranes. During the crystallization of MOF membranes, the Ag+ ion was squeezed out because it could not coordinate with the ligand molecule, which etched the polycrystalline structure of MOF membranes due to the strong oxidation and corrosion in aqueous solution at the same time. Thus bi-continuous porous island-like structures in nanometers could be formed. On the other hand, the IZIF-8 membrane had abundant open metal sites contributing to the high gas selectivity due to the doping of Ag+ ions and Ag metal clusters. More importantly, the island-like structure could endow additional gas adsorption sites due to the introduction of dispersed defective-mesopores. Herein, the IZIF-8 membranes achieved the synergistic improvement of permeance and selectivity for gas separation by the special island-like structure. The obtained membranes displayed excellent N2 permeance of 5158 GPU with favorable N2/CH4 selectivity of 7.8 and outstanding CO2 permeance of 25820 GPU with acceptable CO2/CH4 selectivity of 35, which had good potential for application in the field of CH4 purification.