Orifice-ModifiedMOF-on-MOF Bilayer Membrane for HeliumSeparation

Metal–organic framework (MOF) membranes have great potential in efficient helium (He) recovery, but it is highly demanded to improve their separation performance by enhancing molecular sieving. Herein, we explored an orifice-modified MOF-on-MOF bilayer membrane prepared by growing an amino-functionalized ZIF-7 layer on a ZIF-90 layer through an aldehyde-amine condensation reaction. Due to the precise docking between the aperture windows of the crystal cells of such two MOFs, the size-sieving effect was intensified for selective He separation. The resultant membrane exhibits significantly increased selectivities for He/N₂ and He/CH₄ along with a satisfactory He permeance (∼67 × 10^–10 mol·m^–2·s^–1·Pa^–1). Specifically, the He/N₂ selectivity of the membrane can reach 16.19, which is about 5 times higher than that of the ZIF-90 membrane (3.24), ranking among the reported He-selective MOF membranes. In addition, a continuous operation for 100 h, subjected to a successive test at room temperature and 150 °C indicates that the membrane has excellent stability. The proposed concept of orifice-modified MOF-on-MOF would provide a new idea for the design and development of advanced MOF-based gas separation membranes.