287043 Nanocomposite Membrane of a Polymer of Intrinsic Microporosity and Zeolitic Imidazolate Frameworks for Gas Separation
Developing energy-efficient and environmentally-friendly separation processes has become an important research topic in dealing with global issues, such as carbon capture, natural gas production, and water purification. Membrane separation technology has great potential for solving these challenges as an alternative to conventional industrial processes. Nanocomposite membranes, also known as mixed matrix membranes, are promising materials for gas separation with potentially enhanced permeability and selectivity. In this study, we report the preparation and gas separation performance of nanocomposite membranes combined of zeolitic imidazolate frameworks (ZIF-8) nanocrystals in a polymer of intrinsic microporosity (PIMs).
The PIMs polymers present microporosity due to their unique contorted and rigid molecular structure, and have shown great potential for a wide range of applications, such as membrane for gas separation, gas storage, and catalysis. As membrane for gas separation, PIMs have shown high gas permeability but have selectivity that need to be enhanced to a satisfactory level for industrial applications. Zeolitic imidazolate frameworks (ZIFs), a sub-family of MOFs, have attracted significant attention with tuneable pore sizes, exceptional chemical stability, and versatile structures analogous to that of inorganic zeolites. The ZIFs nanoparticles with sub-nanometer channels could present synergetic effects as fillers for mixed matrix membranes: (1) disruption of the packing of polymer chains; and (2) molecular sieving effect. The PIMs/ZIFs composite membrane is a representative example of combination of amorphous microporous polymer and crystalline microporous MOFs. Therefore, understanding the physical and chemical properties of this composite, such as porosity, free volume, thermal stability, and gas sorption properties, is not only important for developing high performance composite membrane for gas separation, but also useful for many other applications, such as adsorbent and catalysis.
In this study, the as synthesised ZIF-8 nanoparticles, dispersed as colloids in solvent solution, were incorporated into the PIM-1 polymer matrix forming a nanocomposite membrane, which gives uniform dispersion of ZIF-8 in the polymer matrix. A series of composite membrane were fabricated from solution casting of the PIM-1/ZIF-8 solution with loading of ZIF-8 up to the significantly high values of ~40 wt%. The nanocomposite membranes were further treated with various techniques, such as methanol treatment, thermal treatment, and photochemical modification, aiming to improve the permeability or selectivity. The pure gas permeation properties were evaluated in a constant-volume variable-pressure apparatus (time lag method). Mixed gas permeation was tested in a similar membrane cell equipped with a gas chromatography.
For the nanocomposite membrane without methanol treatment, the gas permeability decreased with the loading of ZIF-8 nanoparticles, while the selectivity was maintained without significant loss. After methanol treatment, the permeability of nanocomposite membrane increased with the loading of ZIF-8 nanocrystals and surpassed the Robeson's upper bound. The effects of annealing temperature (100-300°C) on the gas permeation performance of pure polymer and nanocomposite membrane were monitored. For the pure polymer membranes annealed at high temperature (up to 300°C), the gas permeability slightly decreased while the selectivity was found to be stable. For the PIM-1/ZIF-8 nanocomposite membranes, the gas permeability decreased with the annealing temperature, interestingly, the selectivity could be significantly enhanced. Particularly, for the composite membrane with 20 wt% loading of ZIF-8 annealed at 300°C, the CO2 permeability could reach approximately 1000 Barrer with selectivity of both CO2/N2 and CO2/CH4 as high as 30. The ideal selectivity of other gas pairs, such as O2/N2, H2/N2 and H2/CH4, were all enhanced surpassing the Robeson's upper bound.
Gas sorption properties of these nanocomposites were also examined with various types of gases, so their potential as adsorbents or gas storage materials were also evaluated.
We acknowledge the NPRP grant from the QNRF, the Engineering and Physical Sciences Research Council (EPSRC, UK), and the China Scholarship Council.
Notes: Corresponding author: firstname.lastname@example.org (Dr. Easan Sivaniah)
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