Engineering gas separation property of metal–organic framework membranes via polymer insertion

We report on a novel approach to engineering the gas permeation property of metal–organic framework (MOF) membranes via polymer infiltration. This method enables MOFs of a too large intrinsic aperture size (>0.7 nm) for membrane separation of small molecules. [Zn2(bdc)2ted]n, referred to as ZnMOF, is studied as a model system for the proof of concept. This MOF has a pore limiting diameter of 7.63 Å, which is notably larger than the kinetic diameters of the gases to be separated (2.89, 3.3, and 3.8 Å for H2, CO2, and CH4, respectively). We prepare ZnMOF membranes, followed by the insertion of polyethylene glycol (PEG) via dip coating, spin-on deposition, or drop coating. Comprehensive materials characterization is performed via a number of solid-state techniques, including nitrogen physisorption, Fourier-transform infrared spectroscopy, thermogravimetric analysis, solution-state nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The results suggest that the three deposition methods all enable the infiltration of PEG in the micropores of the ZnMOF membranes. The as-made ZnMOF membrane possesses low gas selectivity (5.19 for H2/CO2 and 3.8 for H2/CH4), owing to its large pore size. With the PEG infiltration, the ZnMOF membrane presents good selectivity of H2/CO2 (26.28) and H2/CH4 (17.6). Molecular simulations also suggest that the impregnation of PEG reduces the effective pore size of the ZnMOF structure and thus renders it a higher diffusion selectivity for gas mixtures as compared to the bare PEG or the pristine ZnMOF membranes.