Synthesis and gas transport properties of novel sandwich membranes based on the MOF/PIM-1/MOF structure for gas separation

Mixed matrix membranes (MMMs) form a novel class of membrane materials that combine the mechanical properties of polymers with the high permeability of porous materials such as zeolites, mesoporous silica or metal organic frameworks (MOFs). A negative aspect of these materials is that they can suffer from bad dispersion of the filler, poor adhesion between filler and matrix or other problems that lead to poor performance or low mechanical stability. Therefore, the development of MMMs based on polymers of intrinsic microporosity remains a challenge.[1,2] The present work presents an alternative approach, in which the MOFs are not dispersed within the polymer matrix, but they are grown as a continuous layer onto the surface of a polymer film. This leads to sandwich-like structures of MOFs on polymer membrane (Figure 1). The substrates used for the fabrication of composite polymer membranes are based on unmodified and modified polymer of intrinsic microporosity PIM-1. Layers of either one of two different MOFs, ZIF-8 and HKUST-1, were grown on the both sides of neat PIM-1, amide surface-modified PIM-1 and HMDA surface-modified PIM-1 membranes. In this way a novel membrane design was obtained with a sandwich-like structure. The surface-grown crystals were characterized by a combination of several techniques, including X-ray diffraction, infrared spectroscopy and scanning electron microscopy to investigate the effect of the growth conditions. The pure gas permeabilities of He, H2, O2, N2, CH4, CO2 were studied to understand the effect of the surface modification on the basic transport properties and evaluate the potential use of these membranes for industrially relevant gas separations. The pure gas transport was discussed in terms of permeability and selectivity, highlighting the effect of the MOF growth on the diffusion coefficients of the gas in the new composite polymer membranes. The results confirm that the growth of MOFs on polymer membranes can enhance the selectivity of the appropriately functionalized PIM-1, without a dramatic decrease of the permeability.