MOF-containing mixed-matrix membranes for CO2/CH4 and CO2/N2 binary gas mixture separations
Highlights
► Mixed-matrix membranes (MMMs) fabricated with [Cu3(BTC)2], MIL-53(Al) and ZIF-8 and polyimide. ► Dense and asymmetric membranes showed higher permeance and selectivity than unfilled membrane. ► Dense and asymmetric membranes showed higher permeance and selectivity with increasing loadings. ► MMMs showed improved thermal and mechanical properties.
Introduction
The concern of global warming has drawn an unprecedented public attention to the issue of CO2 emission. A lot of attention has thus been on alternative resources of energy. Biogas is one such primary energy source. It mainly contains CH4 (55–60%), along with CO2 and other trace gases (H2, N2, H2S…). Biogas purification is required to reduce piping corrosion, before the gas can be used as fuel for public transport [1]. Similarly, CO2/N2 separation is important for reduction in greenhouse gas emissions generated from flue gas [2].
Gas separation processes through polymeric membranes are cost and energy effective, environmentally benign, as well as simple and versatile. It has proven to be a potential alternative to traditional processes [3], [4], in particular to amine-based wet-scrubbing [5]. Membrane technology has resulted in a 70–95% CH4 recovery rate from natural gas [6].
Unfortunately, an important constraint in the development of these polymeric membranes for gas separation applications is the trade-off between permeability and selectivity, demonstrated by Robeson in 1991 and 2008 [7], [8]. In contrary, inorganic membranes offer good thermal and chemical stability, with high selectivity and gas flux, but their wide scale applications is deterred owing to the high fabrication costs [9]. Over the last two decades, inorganic particles e.g. zeolite [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], carbon molecular sieve [26], [27], [28], [29], [30], [31] and inorganic oxides [32], [33], [34], [35] have been used to prepare mixed-matrix membranes (MMMs), also called hybrid membranes. Also, organic fillers [36], fullerenes and carbon nano tubes [37], [38] have been used to make hybrid membranes. MMMs take advantage of the superior transport properties of inorganic materials with the ease of preparing polymer-based membranes.
MMMs have been reported to have good gas separation ability. However, there are still many challenges that need to be addressed for large-scale industrial production of this type of membranes. One important challenge is to improve the interface compatibility between the fillers and the polymer. Non-selective voids at the interface are large enough for gas molecules to diffuse easily, thus reducing the overall separation efficiency of the membrane. Another important challenge is the selection of suitable fillers with respect to particle size, pore diameter, surface functional groups and affinity of the fillers towards the gases to be separated.
The recent development of metal organic frameworks (MOFs) showed promising applications as storage media, adsorbents for separations, drug delivery carriers and catalysts [39], [40], [41], [42], [43], [44], [45], [46]. They have large surface area [47], controlled porosity and affinity towards certain gases [48], [49]. At ambient temperature and pressures up to 42 bar, certain MOFs showed a CO2/CH4 sorption selectivity of 8–10 and a CO2 uptake between 5–35 m mol/g [48], [49], which is much higher than the corresponding values of zeolites and activated carbon [48]. Recently, MOFs were found to be good candidates to make MMMs, because they consist of an inorganic cluster and an organic bridge. Addition of a MOF (Cu-4,4′-bipyridine-hexafluorosilicate, Cu-BPY-HFS) to a dense Matrimid® polyimide (PI) membrane increased gas permeabilities but decreased ideal CO2/CH4 and H2/CO2 selectivities [50], indicating the presence of non-selective voids. [Cu3(BTC)2] and Mn(HCOO)2, as filler particles in PDMS and PSf were used to prepare MMMs for separation of CO2 from CH4 and N2. Little improvement in the ideal selectivity for CO2 over N2 at 10 wt% [Cu3(BTC)2] loading in PDMS and CO2 over CH4 up to 20% MOF loading were observed. [Cu3(BTC)2]/PSf MMMs showed improvement in selectivity up to 5 wt% loading only in both CO2/N2 and CO2/CH4. MMMs with a 5 wt% loading in Mn(HCOO)2 in PSf showed essentially the same CO2 selectivity as that of the pure PSf membrane. Decreased selectivity of gas pairs with increased loading is due to particle agglomeration [51]. A 30 wt% MOF-5 loaded Matrimid® showed increased single gas permeability but decreased ideal and mixed gas selectivity [52]. A ZIF-8/Matrimid® membrane showed increased mechanical stability up to 30 wt% filler loading, but the occurrence of non-selective voids was not resolved [53]. CuTPA (Cu and terephthalic acid)/PVAc (poly (vinyl acetate)) showed improvement over PVAc gas transport properties [54]. PI/[Cu3(BTC)2] MMM hollow fiber membranes showed increased H2 permeance and ideal selectivity with respect to other gases such as N2, CO2, O2 and CH4 with increased filler loadings and compared to pure PI [55]. Asymmetric [Cu3(BTC)2]/Matrimid® membrane prepared by our research group showed high CO2/CH4 selectivity and permeance compared to dense and commercial membranes. The thin selective layer together with the presence of MOFs resulted in high CO2 permeance membranes [56]. Lately, we also developed MOF-based MMMs as solvent resistant nanofiltration [57].
The present study provides the incorporation of [Cu3(BTC)2], ZIF-8 and MIL-53(Al) MOFs in Matrimid® PI as both dense and asymmetric membranes for two binary gas mixture (CO2/CH4 and CO2/N2) separations. These MOFs were selected because of their different properties and mechanism of separation. The separation of gases in MOFs is based on (i) size-exclusion involving differences in MOF pore aperture and kinetic diameter of gas molecules and (ii) different interaction strengths between guests and the MOF framework [58]. In [Cu3(BTC)2], the unsaturated Cu coordination sites act as the centre of guest binding [58]. In addition, there are two kinds of domains in the structure: (i) small tetrahedron side pockets with strong electrostatic interaction and (ii) large square shaped channels with weak electrostatic interactions [49]. Gas molecules occupy the small tetrahedron side pockets followed by the unsaturated Cu atoms [59]. In ZIF-8, the energetically favored sites for CO2 adsorption are regions close to (i) imidazole rings and (ii) windows connecting pores [60]. CO2 adsorption in MOFs can be complex by changes in the structure. In MIL-53(Al) for instance, a framework contraction and reduction of the pore dimensions take place initially [61]. Further CO2 adsorption at higher pressure (⩾6 bar) reopens the framework and allows adsorption of additional CO2 molecules in the newly formed spaces [62].
The crystal structure of the selected MOFs has been mentioned elsewhere [57]. [Cu3(BTC)2] has a three-dimensional network with intersectional pores with a tetrahedral side pockets of 0.6 nm and a cage size of 1.0–1.2 nm. The tetrahedral pockets and main channels are connected by triangular windows of 0.35 nm diameter [44], [63], [64], [65], hence exactly the dimension to separate gas molecules. ZIF-8 exhibits a topology formed by four-ring and six-ring ZnN4 clusters. ZIF-8 has large pores of 1.16 nm, which are accessible through small apertures of 0.34 nm [66], and MIL-53(Al) exhibits a three-dimensional microporous framework with one-dimensional diamond shaped channels of 0.4 × 0.4 nm and 0.77 × 0.77 nm, having an internal diameter of 0.85 nm [67]. Table 1 describes the performance and properties of the three MOFs.
Section snippets
Reagents
PI Matrimid®9725 [56] was kindly provided by Huntsman (Switzerland). PDMS monomer and crosslinker (RTV 615 A and B) were acquired from General Electric (USA) and the non-woven polypropylene/polyethylene fabric Novatexx 2471 from Freudenberg (Germany). N-methylpyrrolidinone (NMP) was purchased from Acros Organics (Belgium), isopropanol (IPA) and cyclohexane from VWR (France), dichloromethane from Fisher Scientific (UK) and 1,3-dioxolane from Sigma–Aldrich. All solvents were analytical grade and
MOF characterization
All three adsorption isotherms of [Cu3(BTC)2], ZIF-8 and MIL-53(Al) are of type I with a steep initial N2 uptake and without hysteresis [56], [57], indicating high microporosity in the structure. Table 2 shows some relevant MOF properties. It indicates a high (surface area)/(pore volume) ratio of MOFs (2160–2600 m2/cm3) [57]. The crystalline MOF structures were confirmed by XRD patterns. Fig. 1 shows the SEM images of MIL-53(Al) and ZIF-8, a distinct crystalline structure and particle size of
Conclusions
The influence of three different MOFs in MMMs for binary gas mixture separations was studied by preparing dense and asymmetric Matrimid® membrane filled with/[Cu3(BTC)2], ZIF-8 and MIL-53(Al). The priming protocol to prepare the MMMs resulted in a good distribution of fillers in the polymer matrix. The possible non-selective voids in the asymmetric membranes were sealed using a PDMS coating. Incorporation of fillers in PI-membranes resulted in increased thermal and mechanical properties of the
Acknowledgements
S.B. acknowledges the MIP-project (Milieu- and Energietechnologie – Innovatieplatform, Berchem, Belgium) for a Grant as a doctoral research fellow. A.C. acknowledges the Departament d’Universitats, Recerca i Societat de la Informació (DURSI) from the Generalitat de Catalunya for the postdoctoral scholarship awarded. We thank K.U. Leuven for support in the frame of a GOA grant and the CECAT excellence, as well as the Flemish Government for the Methusalem funding and the Federal Government for an
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