Ultem®/ZIF-8 mixed matrix hollow fiber membranes for CO2/N2 separations
Highlights
► Formation of mixed matrix hollow fiber membranes containing metal organic framework (MOF) fillers. ► Incorporation of a MOF sub-class, zeolitic imidazolate framework (ZIF) fillers, into polymer matrices. ► Improved separation performance compared to pure polymer membranes for CO2/N2 separations. ► Platform development for future polymer/MOF mixed matrix membranes.
Introduction
The impact on global warming caused by emissions of CO2 from various sources is a major concern. According to the Environmental Protection Agency, the U.S. emitted 6.1 billion metric tons of CO2 to the atmosphere in 2007 [1]. One large source of carbon dioxide is fossil fuel combustion. Alkaline-based sorbents and scrubbing solutions can be employed to remove CO2 from various gas mixtures. These methods have critical drawbacks, however, due to the large volume and low pressure of flue gas. Carbon dioxide capture methods have been reviewed by a number of authors [2], [3]. Membranes for the selective removal of CO2 in the presence of CO, H2, H2O, and H2S (fuel gas) or N2, O2, H2O, SO2, NO, and HCl (flue gas) would be potentially advantageous relative to other approaches [4], [5].
The well-known challenges of low selectivity and permeability in polymer membranes are being addressed with some degree of success with advanced engineering polymers [6], [7]. There is no guarantee, however, that these materials can be processed into cost-effective asymmetric hollow fiber membranes. The “spinnability” of a polymer is often undermined by the lack of adequate stability of the nascent fiber, which prevents extrusion and take up at economical rates [8].
In addition to challenges associated with the hollow fiber spinning, selectivity enhancement via the introduction of new polymers offers slow progress as is evidenced by rather marginal increases in selectivity and permeability over the previous decade [9]. In contrast to polymers, inorganic and carbon based molecular sieves offer very high selectivities and permeabilities. These materials are, unfortunately, prohibitively expensive to fashion into working membranes with current technology. A promising route to enhanced transport properties involves the formation of hybrid or mixed matrix membranes (MMM) combining the processibility of spinnable polymers with the excellent transport properties of molecular sieves. In principle, adhesion of a continuous polymer matrix to a dispersed zeolite can create a hybrid material with appropriate properties [10], [11]. One class of inorganic materials that has been considered extensively for this purpose is zeolites. Two major drawbacks of zeolite-based MMM must be overcome. First, the number of zeolite types identified to produce successful MMM is limited. Second, the inorganic surface chemistry of zeolites leads to additional membrane formation challenges when attempting to create a defect-free morphology [12].
Metal organic frameworks (MOFs) are a relatively new class of microporous materials comprised of transition metals and transition metal oxides connected by organic linkages to create one-, two- and three-dimensional microporous structures. MOF particles offer an attractive alternative to zeolite particles in MMM applications. Like zeolites, MOFs have characteristically high surface areas, sorption capacities and selective affinities for gases. The “tailorability” or flexibility of the MOF structure introduces controlled pore sizes, surface functionalities and chemical properties which are very desirable properties in a hybrid membrane system [13], [14], [15]. In short, MOFs appear to be attractive fillers in MMMs [16], [17]. Thousands of crystalline MOF structures have been identified [18], [19]. Zeolitic imidazolate frameworks (ZIFs) are a class of MOFs with tetrahedral networks that resemble structure types of conventional zeolites. As one of the many different such structures, ZIF-8 crystallizes in the sodalite topology (SOD) and consists of Zn(II) tetrahedrally coordinated by four 2-methylimidazolate linkers [20]. The rate-limiting step for the diffusion of guest molecules is the passage through the six-membered windows connecting each cavity. The window size is estimated to be 3.4 Å by X-ray diffraction structure analysis [20], [21]. ZIF-8 is currently the only ZIF that is commercially available.
ZIF-8 was predicted by Haldoupis et al. to have extraordinarily high membrane selectivity for CO2/CH4 mixtures in theoretical calculations based on a rigid crystal structure [22]. Bux et al. however have observed much lower actual selectivity in a pure ZIF-8 film [23]. This is consistent with the idea that internal vibrations in ZIF-8 are sufficient to enable the window between cages to allow molecules to pass through even though the molecules are “larger” than the pore size that would be assigned from a rigid crystal structure. Although flexibility in the windows of ZIF-8 has not been observed directly, Zhou et al. have shown the existence of a quasi-free rotational potential for CH3 groups in the ZIF-8 structure [24]. Although a full description of the effect of framework vibrations on different molecules in ZIF-8 is not available, this material is nonetheless a reasonable candidate for membrane-based separation because of its relatively small pores. MOFs that have only large pores (i.e. when diffusion is controlled by constrictions 1 nm across or larger) are unlikely to show appreciable diffusion-based selectivity for CO2/N2 and are therefore not as interesting as smaller pore materials in membranes for this separation [22], [25].
There has been a growing interest in MMMs containing MOF fillers as seen in the literature reports [13], [16], [17], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Additionally, there have been a few reports in recent literature of MMMs containing a ZIF-8 [26], [27], [29], [31], [35]; moreover, a few reports exist in the open literature on asymmetric mixed matrix zeolite hollow fiber spinning for gas separations [28], [36], [37], [38], [39], [40], [41], [42]. Only a single report was found on MOF/polymer asymmetric mixed matrix hollow fibers for H2 separations [28].
The objective of this study was to create mixed matrix asymmetric hollow fiber membranes comprising ZIF type fillers in a commercially available glassy polymer matrix. In this work Ultem® 1000 (a polyetherimide) was chosen for the matrix because it is has acceptable intrinsic separation properties and has been used successfully for creating mixed matrix hollow fibers [37], [38]. This publication, to the best of our knowledge, is the first report of successful ZIF/polymer mixed matrix hollow fibers for gas separation.
Section snippets
Materials
Ultem® 1000 polymer was purchased from GE Plastics (Pittsfield, MA). Anhydrous N-methyl-2-pyrrolidione (NMP), tetrahydrofuran (THF), iso-propanol (IPA), methanol (MeOH), hexane, lithium nitrate (LiNO3), heptane and ZIF-8 were all purchased from Sigma-Aldrich (Milwaukee, WI). Polydimethylsiloxane (PDMS, Sylgard 184) is produced by Dow Chemicals. All chemicals and polymers were used as received without any further purification.
Mixed matrix dense membrane preparation
ZIF-8/Ultem® dense membranes were made to study the concept prior to
SEM
SEM was used to study the membrane cross section morphology as well as probe the particle-polymer interface. Sample images from both sonicated pure ZIF-8 and 13 wt% (17 vol%) ZIF-8 loaded films are shown in Fig. 1. The crystal size of the BASF ZIF-8 sample was around 200 nm and crystals were generally not aggregated. SEM images of the dense film membrane cross sections show a homogeneous dispersion of the ZIF-8 material in the polymer matrix and good adhesion between the ZIF-8 and polymer. This
Conclusions
Organic–inorganic hybrid (mixed matrix) asymmetric dual layer hollow fiber membranes were spun via a dry jet-wet quench process containing the metal organic framework sieve ZIF-8 in an Ultem® polymer matrix. The fibers reported in this work contained 17 vol% ZIF-8 in the selective skin layer of the asymmetric membrane. Good adhesion was observed between the bulk polymer and ZIF-8 particles in the membrane. This was confirmed by the enhancement in the permeation properties of the post-treated
Acknowledgments
The authors would like to thank Dr. Yougui Huang for his assistance with XRD measurements, Liren Xu for his assistance with SEM imaging and Wei Long for her assistance with XPS characterization. Additionally, the authors would like to acknowledge the DOE ARPA-E IMPACCT Program for financial support under contract DE-AR0000074. This publication is based in part on work supported by Award No. KUS-I1-011-21 made by King Abdullah University of Science and Technology (KAUST).
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