Mixed matrix membranes consisting of ZIF-8 in rubbery amphiphilic copolymer: Simultaneous improvement in permeability and selectivity
Graphical abstract
Introduction
Recently, rapid global warming and climate change have become the greatest threats to humanity and are linked to the survival of our next generation. Carbon dioxide is the main contributor, accounting for the largest percentage of greenhouse gases; its atmospheric concentration exceeded 400 ppm in 2017 (Faralli et al., 2017). Various approaches have been taken to address this problem, based on efficient, economical technologies for carbon capture, storage, and utilization. Among them, separation technologies through polymeric membranes have received much attention due to their lower operation cost, high energy efficiency, and environmental friendliness (Tomé et al., 2014; Sreenivasulu et al., 2015) in comparison to other technologies, such as absorption and adsorption (Kumar and Kumar, 2018; MacInnes et al., 2017; Zhang et al., 2017; Wang et al., 2018; Álvarez-Gutiérrez et al., 2017). For these reasons, the market for polymeric gas separation membranes has been constantly growing, and novel materials and strategies have been intensively researched and developed not only to improve performance of polymeric membranes, but also to fulfill industrial, environmental demands (Jiang and Ladewig, 2020; Deng et al., 2019; Sanaeepur et al., 2019; Zhang et al., 2018a; Xie et al., 2018; Liang et al., 2019).
Metal-organic frameworks (MOFs), which comprise metal centers surrounded by organic linkers, are attractive hybrid materials in the field of membrane science because they have remarkable chemical properties, physical stability, and porous structure with a large surface area (Banerjee et al., 2008; Wang et al., 2008). Among various MOF materials, zeolite imidazole frameworks (ZIFs), a subclass of MOFs, show extraordinary thermal and chemical stability (Pan et al., 2011). ZIF-8 is composed of zinc metal ions and 2-methylimidazole linkers, having a pore size of 3.4 Å, which is between the kinetic diameters of CO2 (3.3 Å) and N2 (3.6 Å) (Tsai et al., 2018). Therefore, if it is suitably distributed in a polymer matrix, the resultant mixed matrix membranes (MMMs) could be effective in separating CO2/N2 gas mixtures via a molecular sieving mechanism. Furthermore, a recent research showed that the incorporation of ZIF-8 into polymer matrix can effectively control polymer structures through restraining interchain interactions of polymeric chains followed by enhanced separation performance (Japip et al., 2019). To achieve development of MMMs with improved performance, various kinds of polymer matrices for ZIF-8 nanoparticles have been widely researched, such as polyimide (e.g. Matrimid) and poly(ether-block-amide) under the trade name of Pebax (Nafisi and Hägg, 2014; Hamid and Jeong, 2018).
However, it is still challenging to achieve the simultaneous improvement of CO2 permeability and CO2/N2 selectivity due to the difficulty of controlling the interface and interaction between the filler and matrix. Recent studies have illuminated compatibility between fillers and polymers through experimental and simulation methods (Amooghin et al., 2018; Semino et al., 2018). It has been revealed that typical glassy polymers do not interact well with inorganic fillers due to their rigid and stiff structure, causing poor compatibility between two phases and deterioration of membrane performance. In contrast, Pebax, which is well-known as a high-performance thermoplastic elastomer, has shown good mechanical strength as well as excellent separation and adhesion properties, which stem from microphase-separated structure into polyamide (PA) and polyethylene oxide (PEO). However, the cost of Pebax is high, and very strong interaction between the filler and matrix often results in significant reduction of CO2 permeability (Wang et al., 2008). Hence, it is necessary to develop a low-cost polymer matrix that has an appropriate interface and the desired interactions with inorganic fillers (Pan et al., 2011).
In this study, we prepared a series of MMMs based on ZIF-8 nanoparticles dispersed in a rubbery amphiphilic copolymer matrix, namely, poly(styrene-b-butadiene-b-styrene)-g-poly(oxyethylene methacrylate) (SBS-g-POEM). The SBS-g-POEM was synthesized through a mass-producible, one-pot free-radical polymerization process, while the ZIF-8 was obtained via a facile solution-synthesis method without hydrothermal reaction. The structure, morphology, and interactions of the MMMs were characterized by Fourier-transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), small-angle X-ray scattering (SAXS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and universal testing machine (UTM) measurements. The gas permeation properties were measured using both the time-lag and continuous-flow techniques at 35 °C.
Section snippets
Materials
Polystyrene-block-polybutadiene-block-polystyrene (SBS, styrene 30 wt%, Mw = 140,000 g mol−1), poly(ethylene glycol) methyl ether methacrylate (POEM, Mn = 950 g mol−1), zinc nitrate hexahydrate (99%), 2-methylimidazole (MeIm, 99%), and dicumyl peroxide (DCP, 98%) were purchased from Sigma–Aldrich. Methanol (MeOH, 99.5%), toluene (99.5%), and tetrahydrofuran (THF, 99.5%) were purchased from J.T. Baker.
Synthesis of SBS-g-POEM rubbery amphiphilic copolymer
First, 8 g of SBS was dissolved in 100 mL of toluene in a vial with vigorous stirring for more than 2 h at
Synthesis of SBS-g-POEM and ZIF-8 nanoparticles
It is challenging to achieve the simultaneous improvement of permeability and selectivity through MMMs due to the difficulty of controlling the interface and interaction between the filler and matrix. In this study, an amphiphilic SBS-g-POEM amphiphilic copolymer to have good interactions with the inorganic filler was synthesized using free-radical polymerization (Scheme 1), which represents a high potential for scaling up. The ZIF-8 nanoparticles were obtained using zinc nitrate and MeIm in
Conclusion
We presented an approach to prepare robust MMMs with simultaneous improvement of permeability and selectivity for CO2/N2 separation. The polymer matrix used for the MMMs was SBS-g-POEM rubbery amphiphilic copolymer, which was synthesized through a mass-producible free radical polymerization process. The inorganic filler, namely, ZIF-8 nanoparticles, was synthesized by a facile method. Our FT-IR and XRD analysis results indicate secondary bonding interactions and successful fabrication of the
Conflicts of interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers’ bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in
Acknowledgements
This work was supported by a National Research Foundation grant, funded by the Ministry of Science, ICT and Future Planning (grant numbers NRF-2017R1A4A1014569, NRF-2017R1D1A1B06028030, NRF-2019M1A2A2065614).
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