Mixed Matrix Membranes for Natural Gas Upgrading: Current Status

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Mixed Matrix Membranes for Natural Gas Upgrading: Current Status and Opportunities Youdong Cheng, Zhihong Wang, and Dan Zhao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04796 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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Industrial & Engineering Chemistry Research

Mixed Matrix Membranes for Natural Gas Upgrading: Current Status and Opportunities Youdong Cheng, Zhihong Wang, and Dan Zhao* Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore

Abstract: In the past few decades, natural gas has attracted worldwide attention as one of the most desired energy sources owing to its more efficient and cleaner combustion process compared to that of coal and crude oil. Due to the presence of impurities, raw natural gas needs to be upgraded to meet the pipeline specifications. Membrane-based separation is a promising alternative to conventional processes such as cryogenic distillation and pressure swing adsorption. Among the existing membranes for natural gas upgrading, polymeric membranes and inorganic membranes have been extensively explored, but each type has its own pros and cons. The development of mixed matrix membranes (MMMs) by incorporating organic/inorganic fillers into the polymer matrix provides a good strategy to combine the merits of each material and fabricate novel membranes with superior gas separation performance. In this review, we first discuss the recent advances in MMMs showing potentials in natural gas upgrading. Special attention is paid to a detailed evaluation on the polymer and filler choices for acidic gas removal. After that, we analyze factors that influence the membrane separation performance and

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summarize effective strategies reported in the open literature for the fabrication of highperformance MMMs. Finally, a perspective on future research directions in this field is presented.

1. Introduction Natural gas has been recognized as an important energy source due to its clean energy release process and large reserve. It has been anticipated that natural gas will account for nearly 40% of U.S. total energy production by 2040 with an annual production incremental rate of 4% from 2016 to 2020, followed by a 1% annual production incremental rate beyond 2020.1 The total natural gas consumption has been projected to rise by 14.6% from 27.85 trillion cubic feet (TCF) in 2017 to 31.91 TCF in 2040, most of which will be used as feedstock for petrochemical industry and source for electric power generation.2 However, natural gas is a complex mixture consisting of hydrocarbons and other undesired components with exact compositions varying significantly among different sources. Table 1 summarizes the detailed composition of a typical raw natural gas. Clearly, raw natural gas needs to be further processed to remove those unwanted components and meet pipeline specifications and emission regulations. A typical natural gas purification process mainly involves acid gas removal, gas dehydration, Hg removal, natural gas liquids (NGL) recovery, and N2 rejection (Figure 1). Table 1. Summary of compositions in a typical raw natural gas and specifications for pipeline quality gas. Component

Composition range (mol%)

Specification (mol%)


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a

Methane (CH4)

29.98–90.12

>75

Ethane (C2H6)

0.55–14.22

10 wt%), GO tend to agglomerate in MMMs, leading to decreased membrane CO2/CH4 selectivity.76,81 Peinemann et al. embedded GO into a commercially available poly(ethylene oxide)-poly(butylene terephthalate) (PEO-PBT) copolymer (PolyActive™) to prepare CO2-selective MMMs. The good dispersion of GO inside the polymer matrix is achieved with the assistance of hydrogen bonds between the oxygencontaining groups both in GO and the polymer backbone. Owing to its preferential CO2 adsorption capacity, the addition of GO into the polymer matrix leads to a 22% increase in membrane CO2/CH4 selectivity. However, the long and tortuous paths generated by intergalleries of GO nanosheets sometimes hinder the fast permeation of all gases and deteriorate the membrane productivity. To solve this problem, Wu et al. functionalized GO with dopamine and cysteine (GO-DA-Cys) and added them into the SPEEK matrix (Figure 5).23 The amine groups in dopamine and carboxylic acid groups in cysteine remarkably increase the number of CO2 interaction sites on GO nanosheets, therefore leading to an increment in both CO2 reactivity and


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solubility of the composite membrane. In addition, the electrostatic interactions and hydrogen bonding between the amino groups or carboxylic acid groups on nanosheets and sulfonic acid groups from the SPEEK matrix improve their interface compatibility and promote the good distribution of GO inside the membranes. MMMs with 8 wt% GO-DA-Cys exhibit a CO2 permeability of 1247 Barrer and a CO2/CH4 selectivity of 82 at room temperature, surpassing the 2008 Robeson upper bound and showing great promise for CO2/CH4 separation. One step further has been achieved by Goh et al. in the fabrication of hollow fiber MMMs by incorporating GO into PSF polymer matrix.82 Nano-sized GO nanosheets are adopted to ensure the good dispersion of fillers within the polymer matrix. Compared with neat PSF membranes, GO/PSF MMMs show improved mechanical and thermal stability, and their CO2/CH4 selectivity is also enhanced as much as 74% even at a filler loading of only 0.25 wt%. Detailed CO2/CH4 separation performance data for carbon and its derivatives-based MMMs are listed in Table S4.


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Figure 5. (a) The grafting mechanism of dopamine and cysteine on GO nanosheets. Pure gas separation performance of the membranes for CO2/CH4 mixture (b) and CO2/N2 mixture (c). (Modified and reproduced with permission from ref 23. Copyright 2015, Royal Society of Chemistry, London.) 2.2.4 Metal oxide. Metal oxide fillers are usually impermeable to any gas due to their nonporous nature. They tend to disrupt the polymeric chain packing and create more membrane fractional free volume, which can promote the gas diffusion rate in the membranes. Table S5 lists some recent progress in metal oxide-based MMMs for CO2/CH4 separation. To maintain the CO2/CH4 selectivity at a certain level, surface modification on metal oxide is necessary to enhance its interaction with the polymer matrix and facilitate CO2 transport in the membrane. For example, Wu et al. functionalized spherical TiO2 nanoparticles with dopamine and polyethyleneimine (TiO2-DA-PEI) and mixed them with the SPEEK matrix.83 As the filler loading increases from 5 wt% to 15 wt%, MMMs show an enhancement in Young's modulus from 640 MPa to 1380 MPa, indicating a strong polymer-filler interaction originating from the attractive force between sulfonic groups in SPEEK chains and amino groups on the surface of functionalized fillers. Under humid atmosphere, these amino groups in TiO2-DA-PEI act as active sites that can react with acidic CO2, producing -HCO3 and facilitating CO2 diffusion in MMMs. MMMs with 15 wt% TiO2-DA-PEI exhibit enhancements in both the CO2/CH4 selectivity from 23 to 58 and the CO2 permeability from 565 to 1629 Barrer, compared with the pure SPEEK membranes at 1bar and room temperature. Similar strategy can be applied to the surface modification of TiO2 with EDA,84 which also provides abundant amino groups to facilitate CO2 transport in the membrane. Recent progress in the investigation of other metal


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oxide materials as fillers in MMMs, including Al2O385 and ZnO,86 reveals that this type of materials possess great potentials for effective CO2/CH4 separation. 2.2.5 MOFs. MOFs are hybrid porous materials tailored by connecting metal complexes with organic linkers via coordination bonds to obtain fascinating pore geometries and versatile frameworks.87 MOFs have gained increasing attention as potential fillers in MMMs owing to their large surface area, uniform pore channels, good thermal stability, and high affinity toward certain gases. Furthermore, their pores can be easily tuned in terms of size, shape and chemical functionality by choosing ligands with different length and functional groups during the synthesis or by post-synthetic modifications.88-90 Over the past several years, tremendous efforts have been devoted to exploring MOFs in MMMs, and good CO2/CH4 separation performance can be achieved either based on chemical or physical interactions between MOFs and CO2 or simply by molecular sieving effect induced by the narrow pores of the frameworks. Table S6 summarizes the recent progress in MOF-based MMMs for CO2/CH4 separation. Compared with zeolite, MOFs usually show better polymer-filler interactions owing to the better compatibility between their organic linkers and polymer chains.91 Up to date, the most frequently studied MOFs for CO2/CH4 separation are zeolitic imidazole framework-8 (ZIF-8),92 copper 1,3,5benzenetricarboxylate [Cu3(BTC)2],93 copper 1,4-benzenedicarboxylate (CuBDC),94 Materials Institute Lavoisier-53 (MIL-53),95 MIL-101,96 and University of Oslo-66 (UiO-66).91 ZIF-8 is made from zinc(II) cations and 2-methylimidazole anions, possessing a sodalite topology with a pore cavity of 11.6 Å and a 6-ring window aperture of 3.4 Å, which is close to the diameter of CO2 and CH4 (Figure 6a). ZIF-8 can be facilely synthesized in water with good chemical stability, high thermal stability, and excellent mechanical strength.97 The recent example of using ZIF-8 as fillers in MMMs involves the investigation of ZIF-8 with different


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particle sizes of 100 nm, 300 nm, and 500 nm in the PSF matrix.46 The penetration of membrane casting solvent, NMP, into the pores of ZIF-8 can easily cause the pore blockage and reduce channels for selective CO2 transport in the membranes, which eventually lead to the decrease in CO2/CH4 selectivity. Thermal treatment is used to activate ZIF-8 fillers and can largely improve the gas selectivity of MMMs containing ZIF-8 particles with different sizes (Figure 6b). More interestingly, it has been found that the addition of 100-nm ZIF-8 nanoparticles into the polymer matrix provides more tortuous permeation paths and consequently a significant drop in CO2 permeance. Nevertheless, larger ZIF-8 nanoparticles tend to disrupt the polymer chain packing and create more nonselective voids at the polymer-filler interface, leading to a huge loss in CO2/CH4 selectivity (Figure 6c). Other glassy polymers, such as 6FDA-durene92 and Matrimid®5218,98 also exhibit limited interactions with ZIF-8 nanoparticles. Filler agglomeration in these composite membranes can cause the decrease in gas selectivity. Hence, modifications are necessary to improve the affinity between ZIF-8 and the polymer matrix.99 Ismail et al. synthesized ZIF-8 nanoparticles in rhombic dodecahedron morphology and immersed them into ammonium hydroxide solution to prepare ZIF-8 decorated with amino groups (ZIF-8-NH2).100 The strong quadrupole-π electron attractions between CO2 and the N-H groups in modified ZIF-8 contribute to a 43% increase in CO2 permeability and a 72% increase in CO2/CH4 selectivity for the membranes after the addition of only 0.5 wt% ZIF-8-NH2. To further boost CO2 permeability in the membrane, Kim et al. creatively synthesized ZIF-8 in hollow sphere structure (H-ZIF-8) via a two-step method: solvothermal synthesis of ZIF-8 shells based on polystyrene (PS) cores, followed by a selective removal of the PS core.101 Amphiphilic graft copolymer poly(vinyl chloride)-g-poly(oxyethylene methacrylate) (PVC-g-POEM) was adopted as the matrix to simultaneously enhance polymer-filler compatibility by flexible POEM side chains and improve


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membrane mechanical strength by rigid PVC backbone. Introduction of 30 wt% H-ZIF-8 drastically increases the membrane CO2 permeability from 70 to 623 Barrer with a marginal decrease in CO2/CH4 selectivity from 13.7 to 11.2 relative to the pristine polymeric membrane.

Figure 6. (a) The crystal structures of ZIF-8; (b) Influence of different ZIF-8 particle sizes on the membrane relative CO2/CH4 selectivity; (c) Illustration of gas permeation in MMMs with different ZIF-8 treatment (top) and loadings (bottom). (Modified and reproduced with permission from ref 102 and 46. Copyright 2006, National Academy of Sciences, Washington, and 2014, Royal Society of Chemistry, London.) Copper based MOFs including Cu3(BTC)2 and CuBDC, constructed in the well-known paddlewheel structure via coordination bonds between copper ions and carboxylate groups in organic ligands, have been intensively studied for CO2/CH4 separation. The unsaturated copper sites in these frameworks can function as additional CO2 adsorption sites to construct CO2 permeation highways in the membrane. Besides, the suitable pore sizes of 9 Å in Cu3(BTC)2 and 5.2 Å in CuBDC are able to create more diffusion resistance toward CH4 over CO2, leading to increase in membrane selectivity. One common challenge for copper-based MOFs is their pore blockage by


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less volatile solvents. Cao et al. found that a heat treatment at 180–200 °C was necessary to effectively remove the residual casting solvents trapped inside the pores of Cu3(BTC)2.103 A dramatic increase in CO2 permeability from 47.7 Barrer to 150 Barrer is observed in PI membranes with Cu3(BTC)2 after the heat treatment, highlighting the importance of fast CO2 transport channels introduced by porous fillers in the membranes. Similar results can be found in CuBDC nanosheets, which have been reported to possess a specific surface area ranging from 53 m2 g-1 to 358 m2 g-1 depending on different solvent removal processes.94,104,105 Moreover, the filler morphology can significantly influence the polymer-filler contacting area and therefore their interfacial interactions. High aspect-ratio nanosheets are preferred in the exploration of MMMs with high CO2/CH4 separation performance.94 MIL series MOFs containing aluminum or chromium metal cations have been demonstrated as potential candidates for membrane-based CO2/CH4 separation.33,106 Among them, MIL-53 is known to exhibit a “breathing” effect, meaning it can undergo a structure change from narrow pore (np) to large pore (lp) phase upon adsorption of guest molecules (Figure 7a). This effect can be ascribed to a delicate interplay of weak dispersion force controlling the pore geometry and size of the framework. Gascon et al. utilized the flexibility of NH2-MIL-53(Al) to fabricate MMMs with improved CO2/CH4 separation performance under higher transmembrane pressures.107 Micron-sized NH2-MIL-53(Al) particles were synthesized by microwave method and then incorporated into PSF matrix (Figure 7b). CO2 adsorption tests up to 25 bar reveal a rapid CO2 uptake for MMMs with 25 wt% and 40 wt% filler loading, indicating the breathing phenomenon of fillers in the membrane. An increasing trend in CO2/CH4 selectivity with pressure in MMMs, opposite to the behavior of pure PSF membrane, is ascribed to the enhanced CO2 adsorption capacity in NH2-MIL-53(Al) at lp configuration (Figure 7c). The same group


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later explored the combination of NH2-MIL-53(Al) nanoparticles with 6FDA-DAM for CO2 separation.108 However, the rigid chains of 6FDA-DAM fail to interact strongly with fillers and lead to the formation of defects at filler loadings of higher than 15 wt%. As a result, MMMs with 20 wt% NH2-MIL-53(Al) show an 85% increase in CO2 permeability with a 10% decrease in CO2/CH4 selectivity. The poor polymer-filler interfacial interactions can be largely enhanced by choosing a polymer with flexible chains. For example, Wu et al. utilized SPEEK as the polymer matrix owing to its flexible chains that could effectively interact with MOF fillers.96 Fresh MIL101(Cr) nanoparticles were decorated with PEI in a vacuum-assisted method [PEI@MIL101(Cr)] and then dispersed into SPEEK to fabricate MMMs for CO2 separation. The electrostatic interaction and hydrogen bond between PEI chains and sulfonic acid groups in SPEEK improve the interface compatibility and enhance the membrane mechanical and thermal stability. Compared to pure SPEEK membranes, MMMs doped with 40 wt% PEI@MIL-101(Cr) show a 128.1% increase in CO2/CH4 selectivity from 50 to 71.8, with a CO2 permeability of 2490 Barrer, surpassing the 2008 Robeson upper bound.

Figure 7. (a) Illustration of the breathing effect in MIL-53; (b) Cross-sectional field emission scanning electron microscopy (FESEM) images of MMMs with 8 wt% and 40 wt% NH2-MIL-53 filler; (c) CO2/CH4 separation performance of membranes with various filler loading under


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different transmembrane pressures. (Modified and reproduced with permission from ref 109 and 107. Copyright 2014 and 2011, Royal Society of Chemistry, London.) UiO-66 is a water-stable MOF comprising octahedral and tetrahedral cages in a 1:2 ratio, which are interconnected via triangular windows with a size of 6 Å. The original UiO-66 can be easily decorated with other functional groups, including amino, methyl, carboxyl, and sulfonic groups, by choosing suitable organic linker during the MOF synthesis.91 The modulation approach is widely used in the synthesis of UiO-66 type MOFs, providing a powerful tool to tune their crystal size and particle shape for better dispersion in MMMs.110 Recently, Budd et al. prepared three isoreticular MOFs, namely, standard UiO-66, UiO-66-NH2, and UiO-66(COOH)2, and incorporated them individually into PIM-1 matrix to investigate the influence of functional groups on the gas transport.111 All membranes exhibit enhanced CO2 permeability after the incorporation of MOF fillers, and their gas transport properties can be effectively described by the Maxwell model. However, only UiO-66-NH2 based MMMs maintain their CO2/CH4 selectivity compared with pure PIM-1 membrane, possibly owing to the hydrogen bonding between the aromatic ether groups of PIM-1 and the amino group grafted to the UiO-66NH2 external surface.112 To further evaluate the interaction between UiO-66-NH2 and PIM-1 matrix, Sivaniah et al. synthesized nano-sized UiO-66-NH2 particles in a water-modulated method and incorporated them into PIM-1 to fabricate defect-free MMMs (Figure 8).91 As a comparison, large un-functionalized UiO-66 (UiO-66-ref) was synthesized in a non-watermodulated method and also added into PIM-1 matrix. Computational studies reveal stronger polymer-filler interfacial adhesion between UiO-66-NH2 and PIM-1 due to their hydrogen bonding interactions, which, however, are absent in the case of standard UiO-66. Experimental results exhibit a higher mechanical strength in MMMs with UiO-66-NH2, opposite to the


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decreasing trend in MMMs with UiO-66-ref, confirming the stronger interactions predicted in simulation results. MMMs doped with 10 wt% UiO-66-NH2 show a CO2/CH4 selectivity of 28.3, which is 106% and 97% higher than that of MMMs with 10 wt% UiO-66-ref and pure PIM-1 membranes, respectively. More importantly, the addition of UiO-66-NH2 could also increase the membrane anti-aging ability, leading to a consistent membrane selectivity performance over a period of at least one year.

Figure 8. (a) Schematic illustration of synthesis procedure of UiO-66 and its derivatives; (b) Computational study of adhesion between PIM-1 and UiO-66-NH2; (c) Gas separation performance of MMMs with large un-functionalized UiO-66 (open symbols) and small aminofunctionalized UiO-66-NH2 (filled symbols). (Modified and reproduced with permission from ref 91. Copyright 2017, Nature Publishing Group, London.)


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It is worth mentioning that MOFs are also promising candidates as fillers in asymmetric hollow fiber MMMs. Contrary to the popularity of dense symmetric membrane configurations in lab-scale fundamental studies, industrial gas separation processes prefer asymmetric membrane configurations in which thin selective layers are supported by nonselective porous supports.113-115 Until now, considerable efforts have been made to process mixed matrix membranes into asymmetric membrane configurations for gas separation. For example, Koros et al. pioneered in the preparation of asymmetric hollow fiber MMMs containing ZIF-8 fillers and Ultem® matrix. These hollow fibers exhibit an 85% increase in CO2 permeance after the addition of 13 wt% of ZIF-8 fillers, which mainly results from the high porosity of ZIF-8. Subsequently, the same group successfully fabricated ZIF-8/PI mixed matrix asymmetric hollow fiber membranes with ZIF-8 loading up to 30 wt%.116 The high polymer concentration (~25 wt %) and small ZIF-8 particle size (~100 nm) are key factors that guarantee the good dispersion of fillers inside membranes during the spinning process. However, as the ZIF-8 filler loading reaches 30 wt%, unavoidable defects start to appear in hollow fiber MMMs. Extra processes are needed to seal these defects, e.g., coating the fiber surface with a blend of PDMS and polyaramid. Other MOFs have also been processed into asymmetric hollow fiber MMMs, including Cu3(BTC)2,93 UiO66,117 MIL-53,118 etc. Ismail et al. prepared hollow fiber MMMs by coating the PSF support with a thin selective layer consisting of Cu3(BTC)2 particles and PDMS matrix via a simple dip coating method.93 The incorporation of Cu3(BTC)2 particles largely improve the membrane CO2 sorption capacity owing to the high affinity between CO2 and unsaturated copper sites in the Cu3(BTC)2 framework. Therefore, the CO2 permeance of hollow fiber membrane increases from 65 to 110 GPU after 5 consecutive coatings. Despite these progresses, mixed matrix asymmetric hollow fiber membranes are still in their infancy and technological challenges, including poor


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filler dispersion, interfacial defects, reduced thermal and mechanical stability, should be directly addressed in future studies. Further efforts are still needed to explore the potential of this type of membrane in the application of gas separation with excellent separation performance. 2.2.6 COFs. COFs are porous crystalline materials in which pure organic building blocks are covalently extended into periodic structures with well-defined skeletons and ordered pores. Since their first discovery in 2005,119 COFs have gained increasing interests in the applications of gas storage and separation, catalysis, electronics and photoconductivity due to their highly ordered structures with an opportunity to be easily functionalized at the molecular level.120 In contrast with other porous materials containing inorganic components such as zeolite and MOFs, COFs made from pure organic building blocks may exhibit better compatibility with the polymer matrix, avoiding the formation of interfacial voids in the membrane. Table S7 lists the recent progress in COF-based MMMs for CO2/CH4 separation. Our group reported the first example of applying two COF nanosheets (NUS-2 and NUS-3, NUS stands for National University of Singapore) for the fabrication of MMMs for efficient CO2/CH4 separation.36 Given that NUS-2 possesses pores with a diameter of 8 Å, much smaller than the pore size of 18 Å in NUS-3, better CO2/CH4 discrimination is achieved in MMMs with 20 wt% NUS-2, exhibiting 14% and 60% higher CO2/CH4 selectivity compared to MMMs with the same loading of NUS-3 and pure polymeric membranes, respectively. Subsequently, Kharul et al. incorporated two chemically stable isoreticular COFs, namely, TpPa-1 and TpBD, into PBI-BuI matrix to prepare free standing MMMs.121 The existence of hydrogen bonding between the COF filler and PBI-Bul matrix ensures their good interface compatibility even when the filler loading reaches as high as 50 wt%. The relative large pores in these two COFs (15 Å for TpPa-1 and 18 Å for TpBD) boost the membrane CO2 permeability from 2.3 Barrer to 13.1 Barrer for TpPa-1 and 14.8 Barrer for


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TpBD at the cost of 15–30% loss in CO2/CH4 selectivity. To further explore the potential of COFs in MMMs, Gascon et al. prepared a microporous azine-linked COF (ACOF-1) in homogeneous spherical structure with a particle diameter of 350 ± 30 nm.122 After doping with 16 wt% ACOF-1, the composite membrane shows a 130% increase in CO2 permeability contributed by additional gas transport pathways in porous COF fillers, and a slightly higher CO2/CH4 selectivity owing to the strong dipole–quadrupole interactions between CO2 and the Nrich COF fillers. Considering that previous COF based MMMs only involve polymers with intrinsically low CO2 permeability, Jiang et al. combined a microporous COF (SNW-1) with highly permeable PIM-1 to fabricate MMMs for fast CO2/CH4 separation.123 The addition of SNW-1 into PIM-1 matrix creates more gas selective fractional free volume and provides fast CO2 transport channels owing to the uniform 0.5-nm pores of SNW-1, allowing a 116% increase in CO2 permeability and an increment of 27.4% in CO2/CH4 selectivity for MMMs at 30°C and a transmembrane pressure of 2 bar.

Figure 9. Chemical compositions of various COFs including aldehyde units (a) and amine units (b). (c) The chemical structure of NUS-2. 2.2.7 Hybrids. Hybrids involve the combination of at least two types of materials as fillers inside the membranes. Considering that only one type of fillers may not be sufficient to boost the


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membrane separation performance, hybrids can provide a good strategy to tackle this problem. Different hybrids have been achieved in the literature, including carbon-silica,124 zeolite-MOF,125 mesoporous silica-MOF,126 GO-MOF,127-129 CNT-MOF,130 and CNT-GO.131 Table S8 lists the recent progress in hybrid filler-based MMMs for CO2/CH4 separation. For example, Vankelecom et al. synthesized porous carbon–silica nanocomposite (CSM) fillers and incorporated them into Matrimid® matrix to prepare MMMs for CO2 capture.124 The high porosity of mesoporous silica (MCM-41) improves the membrane fractional free volume, leading to a high CO2 permeability. Meanwhile, carbon trapped inside silica pores can enhance the filler affinity to CO2, resulting in a better CO2/CH4 selectivity for MMMs. After the incorporation of 30 wt% CSM with 18.4% carbon loading, the membranes experience a CO2 permeability increase from 5.94 to 38.9 Barrer and a CO2/CH4 selectivity increase from 30.9 to 41.9, confirming the synergistic effects between these two materials. Coronas et al. pioneered the combination of zeolite (silicalite-1) and MOFs (HKUST-1 and ZIF-8) inside PSF matrix to fabricate MMMs for gas separation.125 They synthesized zeolite and MOF crystals separately and physically blended them inside the membrane casting solution. These porous fillers tend to disrupt polymer chain packing and increase the membrane fractional free volume. As a result, membranes containing hybrid fillers show enhanced CO2 permeability compared to pure polymeric membranes. However, the lack of strong interactions between zeolite and MOF crystals weakens their synergistic effects, which eventually leads to the decrease in CO2/CH4 selectivity for MMMs. One possible strategy of enhancing interactions in different fillers is to grow hybrid core-shell structures. For example, Téllez et al. selectively grew ZIF-8 on the outer surface of mesoporous silica spheres (MSS-S8) and combined the hybrid filler with PSF matrix into MMMs for CO2/CH4 separation.126 The micropores in ZIF-8 function as selective barriers and the mesopores in silica can provide high


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gas permeation pathways. MSS-S8 shows a 770% higher CO2 adsorption capacity compared to that of pure PSF, suggesting a higher CO2 solubility in the membranes after the filler addition. Besides, the organic linkers in ZIF-8 can improve the polymer-filler compatibility, avoiding the formation of non-selective voids in the membranes. With the incorporation of 32 wt% MSS-Z8, membranes experience a 300% increment in CO2 permeability from 6.1 to 24.4 Barrer with a nearly constant CO2/CH4 selectivity. Recently, Wu et al. dispersed CNTs and GO nanosheets in a Matrimid® matrix to fabricate MMMs with improved CO2 separation performance.131 The smooth walls of CNTs function as gas permeation highways, and GO nanosheets act as sizeselective barriers, which may be able to achieve a simultaneous increase in CO2 permeability and CO2/CH4 selectivity in MMMs. Compared to pure Matrimid® membranes, MMMs doped with 5 wt% CNTs and 5 wt% GO nanosheets display a 331% and 149% increase in CO2 permeability and CO2/CH4 selectivity, respectively. Considering the unlimited combinations of different fillers, the exploration of more hybrids with better synergistic effects may provide a powerful tool to fabricate MMMs with better separation performance. A summary of major features and future challenges for the above-mentioned fillers is presented in Table 2. Currently, microporous fillers, especially MOFs and COFs, have gained most attention and they will become even more popular in the foreseeable future owing to their rich chemical functionality, which can offer more opportunities to achieve better size-selective separation. Figure 10 presents the CO2/CH4 separation performance of recent MMMs containing different fillers plotted on the 2008 Robeson upper bound, indicating that only around 20% MMMs can surpass the upper bound limit. More than 60% of MMMs exhibit a CO2 permeability lower than 100 Barrer, which is mainly due to the usage of low-permeable polymers, such as PSF and Matrimid®5218. Therefore, combinations between highly selective fillers and highly


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permeable polymer matrix are highly encouraged for the exploration of MMMs with excellent separation performance. Table 2. Main features and future challenges for various types of fillers used in MMMs for natural gas upgrading. Filler type

Main features

Future challenges

Zeolites

High porosity, good stability

Poor polymer-filler interactions, moderate chemical functionality

Mesoporous silica

Fast gas permeation in the pores, good stability

Low chemical functionality, hard to achieve size-selective separation

Nonporous silica

Disruption on polymer chain packing Large length/diameter or Carbon and aspect ratios, high its derivatives mechanical strength, good stability Disruption on polymer chain Metal oxide packing

MOFs COFs Hybrids

Nonporous nature, easy to generate nonselective voids in the membranes Low chemical functionality, low affinity to polymer matrix Poor polymer-filler interactions, low chemical functionality, nonporous nature

High porosity, rich in chemical functionality

Low chemical stability, moderate polymerfiller interactions

Strong polymer-filler interactions, high porosity Combining merits of multiple materials

Relatively large pore size, irregular morphology Weak or no mutual interactions


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Figure 10. CO2/CH4 separation performance of MMMs containing various types of fillers. 2.3 Plasticization. In membrane-based CO2/CH4 separation process, plasticization is a pressure dependent phenomenon in which the dissolution of condensable CO2 in the polymer matrix swells the interstitial place among polymer chains and further lowers the chain packing density. The loosely packed polymer chains allow the fast diffusion of both CO2 and CH4, resulting in a significant loss in CO2/CH4 selectivity. CO2-induced plasticization still remains as a big challenge in natural gas industry. Promisingly, investigations on MMMs reveal that the existence of fillers possessing effective interactions with the polymer matrix could impart better plasticization resistance to MMMs (Table 3). Chung et al. reported a straightforward strategy to suppress the membrane plasticization by the addition of polyhedral oligomeric silsesquioxane (POSS) nanoparticles.132 POSS outperforms other porous or nonporous fillers (e.g., zeolite, silica, MOFs, and COFs) in terms of its good solubility in common solvents and easiness to be functionalized with various organic groups at its surface. The positron annihilation lifetime spectroscopy (PALS) results confirm the increase in membrane factional free volume and the restriction of polymer chain stiffness after the addition of POSS. As a result, no plasticization is observed for MMMs with a 2 wt% loading of POSS during the mixed CO2/CH4 gas tests at a


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total pressure up to 30 bar. Nijmeijer et al. synthesized a series of MOFs, including MIL-53(Al), ZIF-8, and Cu3(BTC)2, and incorporated them into Matrimid®5218 to evaluate their influence on membrane CO2/CH4 separation performance.95 All MOFs are tightly wrapped by the Matrimid®5218 chains owing to the strong polymer-filler interactions, which further decrease the chain mobility and enhance membrane anti-plasticization ability. Besides, additional active sites provided by porous MOF fillers can favor CO2 adsorption and transport in the membranes and hence increase the CO2/CH4 selectivity. The pristine Matrimid®5218 membrane exhibits a fast increase in CO2 permeability with a significant loss in CO2/CH4 selectivity when the transmembrane pressure reaches 10–12 bar in the mixed gas tests. Regardless of the MOF type, all MMMs with 30 wt% fillers show suppressed plasticization phenomena even at a pressure up to 20 bar. Recently, Long et al. demonstrated that the incorporation of MOF Ni2(dobdc) nanocrystals into various PIs could potentially preserve membrane CO2/CH4 separation performance at a pressure up to 55 bar (Figure 10), which is close to pressures required in actual CO2 removal conditions in natural gas industry (60 bar).133 A 6–10°C increase in Tg was observed in all PI membranes after the addition of Ni2(dobdc), suggesting the existence of polymer rigidification caused by effective polymer-filler interactions. The rigidified polymer chains are less likely to be swelled by CO2 even at high pressures. Additionally, Ni2(dobdc)/6FDA-DAT composite membranes exhibit a 7% increase in CO2/CH4 selectivity when switching from single gas tests to mixed gas tests, which is opposite to the 20% decrease for pure 6FDA-DAT membrane, highlighting the importance of competitive adsorption induced by Ni2(dobdc) in the membranes. Table 3. Comparison of plasticization pressure between pure membranes and MMMs. Polymer

Filler

Filler

Plasticization pressurea (bar)

Ref.


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Loading (%)

Pure membrane

MMM

Cellulose acetate

NaY zeolite

32

12.76

16.86

134

Matrimid®

Cu3BTC2

30

10–12

~20

95

Polyethersulfone

NaA zeolite

50

27

>35

21

6FDA-DAMb

Ni2(dobdc)

23

~10

>25

133

6FDA-dureneb

Ni2(dobdc)

21

~7

~10

133

a. Plasticization pressure refers to the CO2 partial pressure that induces plasticization phenomena. b. Abberviation: 6FDA: 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride; DAM: 2,4,6-trimethyl-1,3phenylenediamine; durene: 2,3,5,6-tetramethyl-1,4-phenylenediamine.

Figure 10. Variation of CO2/CH4 permselectivity as a function of the total feed pressure of a binary gas mixture measured at 35°C for each membrane material studied. Open circles represent the neat polymeric membranes and closed circles represent the Ni2(dobdc)-loaded membranes. (Modified and reproduced with permission from ref 133. Copyright 2016, Royal Society of Chemistry, London.)


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2.4 Aging. Aging frequently occurs in glassy polymers not in thermodynamic equilibrium below their Tg. Polymer chains tend to reach their equilibrium states, and the dynamic voids formed due to the fast solvent removal in membrane preparation will diminish during aging. It is anticipated that the gas diffusivity coefficient will decline quickly in the membrane during aging, resulting in decreased gas permeability. Research work on aging reveals that highly permeable polymers would experience faster aging process in thin film structures.135 PIM-1 represents a typical glassy polymer with exceptionally high CO2 permeability owing to its twisted or bent spirobisindane and dibenzodioxane linkages that prevent efficient chain packing. For example, Paul et al. prepared thick PIM-1 membranes (~30 µm) by a solution casting method and thin PIM-1 membranes (~220 nm) by a spin coating method.136 They observed that thin membranes experience a 67% decrease in CO2 permeability after aging for 1000 h, which is higher than the 53% decrease in thick membranes over the same period. The incorporation of fillers provides an effective strategy to reduce the aging rate in highly permeable polymeric membranes. Cooper et al. investigated the influence of cage fillers on the gas separation performance and the aging process of PIM-1 membranes.35 All MMMs were treated with ethanol before gas permeation measurements to remove residual solvents and open up the micropores in PIM-1 structures. The incorporation of 30 wt% crystalline CC3 cages into PIM-1 matrix via in situ generation strategy leads to a triple increase in the CO2 permeability from 11,400 to 37,400 Barrer. Even after an aging period of one year, PIM-1/CC3 composite membrane still possesses a CO2 permeability of 13,000 Barrer with a CO2/CH4 selectivity of around 10, surpassing the 2008 Robeson upper bound limit. Besides cage fillers, the porous aromatic frameworks (PAFs) also demonstrate the capability to mitigate aging in glassy polymers for CO2 transport.137,138 PAF-1 is a highly porous organic polymer with a Langmuir surface area of up to 7100 m2 g-1. It demonstrates a high CO2


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uptake capacity (1300 mg g-1 at 298 K, 40 bar) and outperforms MOF materials in terms of chemical stability owing to its strong covalent bonds.139 Hill et al. synthesized highly porous PAF-1 fillers and added them into PIM-1 matrix to provide additional gas sorption sites and maintain gas transport highways in the membranes (Figure 11). Small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) reveal the maintenance of polymer interchain distance in MMMs after aging for 400 days. On the contrary, pure PIM-1 membranes experience a decrease in the interchain distance from 11 Å to 9.5 Å after the same aging period. This antiaging phenomenon was investigated by the change in mixture solution viscosity and composite membrane

13

C solid-state NMR. It was found that the penetration of PIM-1 bulky dimethyl

chains into the large pores (48 Å) of PAF-1 could reduce chain mobility, indicating the rigidification effect at the polymer-filler interface. However, the cyano groups of PIM-1 still remain flexible and mobile even after aging. The combination of rigidified and flexible PIM-1 chains maintains fast permeation channels for small molecules and creates more resistance to large molecules, which therefore leads to higher gas pair selectivity. To overcome the challenges of expensive reagents and harsh reaction conditions for PAF-1 synthesis, Wood et al. shifted to hypercrosslinked polymers (HCPs) with much lower costs and milder synthesis conditions as potential fillers to fabricate ageless membranes.137 They synthesized a highly dispersible and scalable HCP based on a,a′-dichloro-p-xylene (p-DCX) and combined them with super glassy polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP), to fabricate anti-aging MMMs. After the addition of 10 wt% p-DCX, membranes exhibit a 48% increase in CO2 permeability and a 46% decrease in CH4 permeability owing to the strong CO2 affinity and the selective pore size of pDCX. The resultant MMMs maintain the CO2 permeability of up to 40,000 Barrer after aging for 60 days, which is two-fold higher than that of pure PTMSP membranes with the same treatment.


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This type of scalable and inexpensive fillers may pave the way for developing high performance membranes with long-term stability for CO2/CH4 separation in natural gas industry.

Figure 11. (a) The synthetic scheme of PAF-1 (COD = 1,5-cyclooctadiene); (b) Schematic illustration of the anti-aging performance of MMMs containing PAF-1; (c) Structural formula of PIM-1; (d) SAXS and WAXS of as-cast PIM-1, aged PIM-1, and aged PIM-1/PAF-1 membranes. (Modified and reproduced with permission from ref 138. Copyright 2015, WileyVCH, Weinheim.) 3. Other Natural Gas Upgrading Applications 3.1 Dehydration. Natural gas from production reservoirs is often saturated with H2O, which will form gas hydrates or ice during natural gas purification process. Gas hydrates or ice may block pipelines and equipment, interrupt operations and sometimes even arouse safety concerns


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if severe solid agglomeration occurs in the system. Besides, H2O may condense in the pipelines and cause corrosion problems. Therefore, H2O should be removed from natural gas before it is transported to downstream pipelines. The specification of < 0.1 ppm (mole) H2O in natural gas is normally adopted for liquefied natural gas production units. Currently, the most widely used technology for natural gas dehydration is triethylene glycol (TEG)-based physical absorption owing to its high efficiency.39 The TEG-based dehydration process is favored at a high pressure and low temperature condition so as to reduce the required inner diameter of the dehydrator and minimize the concentration of TEG needed to fulfill the task. However, this process requires high energy input to regenerate TEG after it is saturated with water during which the desiccant may decompose and bring extra impurities into the downstream natural gas. In addition, foaming is frequently encountered in the operation process as a result of mechanical or chemical disturbance, causing the huge desiccant loss and decreasing the dehydration unit efficiency.140 Membrane technology provides an alternative for natural gas dehydration, as demonstrated by the high H2O permeability (more than 50,000 Barrer) and high H2O/CH4 selectivity (more than 5000) in PEO-containing copolymers.6 However, this technology is currently only used for niche dehydration applications. The major challenge is that a significant amount of CH4 (4–5%) will be lost in the permeate side. Baker proposed the usage of a secondary CH4 drying loop to generate dry CH4 as a sweep gas to reduce the CH4 loss to 1%, which, however, largely increases the complexity of the membrane unit as well as the operational cost.6 Moreover, membranes are sensitive to contaminants in natural gas, which may cause fast membrane aging or plasticization during the water removal process. So far, study on MMMs for water separation from natural gas is still scarce. However, considering that approximately 42,000 units of glycol dehydrators are now in service in the United States,6 there is still a great opportunity for membrane technology.


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A proper choice of porous fillers with high H2O uptake capacity and suitable pores that reject the permeation of CH4 may largely enhance the H2O permeability and H2O/CH4 selectivity of resultant MMMs. For example, Wang et al. reported the development of porous MOF-801 with a water uptake capacity of up to 2.8 L kg-1 for water harvesting from air, which may be a potential filler in MMMs for H2O removal in natural gas industry.141 Process and module design also plays an important role in achieving the optimum dehydration performance of membranes with minimum footprint and operational cost. 3.2 N2 Rejection. N2 can naturally occur in high concentration in natural gas wells. Approximately 14% of natural gas wells in the United States are high in N2 content and would not meet the pipeline specification (< 4%).8 The existence of N2 in natural gas not only increases the overall compression and transportation cost, but also reduces the heating value of the delivered fuel due to its inert property. The most widely used technology for N2 rejection in natural gas industry is cryogenic distillation, which, however, requires complex feed gas pretreatment, large capital investment, and high energy penalty.5 Membrane-based separation technology is an attractive alternative owing to its easy operation, low energy consumption, low capital cost, and small carbon footprint. It is well-established that glassy polymeric membranes preferentially permeate N2 over CH4 because of the smaller diameter of N2 that allows it to diffuse faster in the membrane. Therefore, glassy membranes are denoted as N2/CH4 selective membranes. On the other hand, rubbery polymeric membranes exhibit CH4/N2 selectivity owing to the higher solubility of CH4 in their polymer chains. Given that CH4 will be pressurized at the downstream process for easy transport purpose, N2/CH4 selective membranes are preferred at this stage to keep CH4 as the retentate to maintain its high pressure.142 As estimated by Baker et al., for a 10% N2 in CH4 mixture, single-stage membrane with a N2/CH4 selectivity of 17 can


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barely reduce N2 concentration to 4% in the product side with a CH4 recovery of around 93%.8 However, the state-of-the-art N2-selective glassy membranes (PIs) only show a maximum N2/CH4 selectivity of 2.3 with a N2 permeability as low as 3.1 Barrer, which is still far from the economical requirement.39 Pure inorganic membranes, such as zeolite membranes, have demonstrated excellent N2/CH4 separation performance (N2/CH4 selectivity of 11.3 and N2 permeability of around 3380 Barrer for SAPO-34) owing to their intrinsic porous nature and suitable pore size.143 However, their further application in natural gas industry is still limited by their brittleness, high cost and low reproducibility. The exploration of MMMs for N2 rejection is still in its infancy, and preliminary results validate it as a promising strategy for developing novel membranes with decent N2 rejection performance. For example, Golemme et al. incorporated amino-functionalized SBA-15 (SBA-15-NH2) into poly(styrene-b-butadiene-b-styrene) (SBS) matrix to fabricate MMMs with good CH4/N2 separation performance, which would be useful for the purification of natural gas with high N2 content.144 MMMs with 10 wt% SBA-15-NH2 filler exhibit a 83% increase in CH4/N2 selectivity from 4.0 to 7.3 compared with the pure polymeric membranes. More efforts are still needed to explore fillers with appropriate pore sizes and effective gas-filler interactions to further enhance the membrane performance, making membrane technology more competitive against cryogenic distillation. 3.3 C3+ Separation. Most natural gas sources contain certain amounts of C3H8, n-C4H10, and other higher hydrocarbons, which need to be separated. The separation of heavy hydrocarbons is necessary to prevent the formation of hydrocarbon liquids in the pipeline. In addition, heavier hydrocarbons purified from raw natural gas can be mixed with liquid oil, increasing the amount of transportable hydrocarbon liquids that can be sold separately. The current technologies to separate hydrocarbons from natural gas are refrigeration and lean oil absorption, which are


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complicated to operate with high capital costs. Membrane technologies are competitive in this separation process. Currently, silicone rubber, with mixed-gas C3H8/CH4 selectivity of 3–5 and C3H8 permeability of 7,500 Barrer, has been processed into selective layers on a polyacrylonitrile support for hydrocarbon separations.145 These membranes are manufactured as disk shaped envelops and stacked into a module of around 400 m2/m3, comparable to a spiral wound module.39 Another typical example of polymer for hydrocarbon separation is PTMSP, which shows an n-C4H10/CH4 selectivity of 30 with a CH4 permeability of 1,800 Barrer. However, PTMSP has limited resistance toward heavy hydrocarbons, and its poor long-term stability still prevents it from being fully commercialized. As for MMMs, the addition of porous fillers that can selectively separate CH4 from other hydrocarbons will enhance their separation performance and increase their long-term stability. In summary, membrane technology is promising for hydrocarbon separation in the foreseeable future. 4. Factors Influencing the Performance of MMMs Even though the development of MMMs provides a good strategy to improve the separation performance of current membranes, challenges still exist in the fabrication of defect-free MMMs with desired gas separation performance, excellent long-term stability, and good chemical, thermal and mechanical properties. Several factors can significantly influence the MMM performance, such as filler sedimentation and agglomeration, filler size, filler morphology, polymer-filler interfacial interactions, filler pore blockage, and polymer chain rigidification. These factors can play important roles in determining the dispersion state of fillers inside the polymer matrix and influencing the microenvironment at the polymer-filler interface. In the following section, we will describe the detailed effects of these factors on membrane separation performance, and summarize the recent progress on the fabrication of defect-free MMMs.


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4.1 Filler Sedimentation and Agglomeration. During the MMM fabrication process, the sedimentation or agglomeration of fillers are frequently encountered, especially for the membrane with a large polarity or density difference between the filler and the polymer matrix. A large number of fillers, including fumed silica,146 zeolite,147 GO,23 MOFs,148 etc., have been reported experiencing this problem, which becomes even worse as the filler loading increases in the membrane. This may lead to reduced polymer-filler interactions and cause pinholes or defects in the resulting membranes. Up till now, several effective strategies have been proposed to solve this problem. 4.1.1 Filler Drying-Free. Fillers tend to agglomerate and fuse together during the drying and activation under high temperature, and it is difficult to re-disperse them in the casting solvent. One effective solution to this challenge is the direct utilization of fillers suspended in solvents without a drying process. As shown in Figure 12a, Wu et al. synthesized ZIF-8 nanoparticles in water and directly added them into the poly(vinylalcohol) (PVA) solution without an intermediate drying process.149 The excellent colloidal stability of the ZIF-8 suspension without drying is confirmed by the high transmittance recorded by a UV spectrophotometer, which is opposite to the serious agglomeration in the ZIF-8 suspension with a drying process (Figure 12b). Due to the existence of water functioning as lubricating medium during the membrane fabrication process, PVA could adopt its flexible chains around ZIF-8 nanoparticles and strongly interact with them through the hydrogen bonding between hydroxyl groups in PVA chains and nitrogen elements in ZIF-8. The ZIF-8 loading in this case can be as high as 39 wt% in the MMMs without interfacial voids, which is much larger than the 20 wt% ZIF-8 in the MMMs prepared in a drying and mixing process.150


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Figure 12. (a) Preparation of PVA/nano-ZIF-8 MMMs from ZIF-8 suspensions with and without drying; (b) The transmittance at ߣ=650 nm of ZIF-8 suspensions as a function of standing time. (Modified and reproduced with permission from ref 149. Copyright 2016, Wiley-VCH, Weinheim.) 4.1.2 “One-Pot” Synthesis. Contrary to the conventional MMM fabrication process that divides the filler synthesis and dispersion into two stages, the one-pot MMM synthesis combines them in the polymer solution as one simple step. This process is economically attractive, while a big challenge exists in finding a suitable solvent that can dissolve the polymer and promote the filler crystallization simultaneously. Coronas et al. pioneered to directly synthesize nano-sized MOF MIL-68(Al) in tetrahydrofuran (THF) in the presence of PSF.37 The good crystallinity of MIL-68(Al) is evidenced by the strong X-ray diffraction peaks originating from crystals embedded in the membranes and further confirmed by the high Brunauer−Emmett−Teller (BET) surface area (1180 m2 g-1) of crystals separated from the membrane casting solution. Compared with conventional drying and sonication method, “one-pot” synthesis in this study could produce MOF fillers in smaller size (diameter < 100 nm) and MMMs without filler sedimentation or agglomeration even at a filler loading of 16 wt%. MMMs with 8 wt% MIL-68(Al) show 17.4% and 31.7% increase in CO2/CH4 selectivity compared with pure PSF membranes and


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conventional casting MMMs with the same filler loading, respectively. Other fillers, such as ZIF8151-153 and silica,51 can also be mixed with the polymer matrix to fabricate defect-free MMMs with excellent gas separation performance via this method. 4.1.3 Solution Viscosity Adjustment. A dilute solution is of great importance in generating high shear rate during the vigorous stirring or sonication, which disrupts the filler agglomeration and thereby enhances the polymer-filler interactions. However, dilute solution may cause serious filler sedimentation owing to its low viscosity that fails to maintain fillers at the fixed position during the membrane drying process. One effective strategy to prevent the filler sedimentation is to increase the solution viscosity before the membrane casting, thus restricting the filler mobility and retarding its sedimentation. Urban et al. concentrated the mixed solution before casting it onto the flat plate and obtained MMMs with a homogeneous distribution of fillers inside the whole membranes even at the MOF loading of as high as 50 wt% (Figure 13).154 They found that a percolative MOF network is achieved and the CO2 permeability dramatically increases from 18 to 46 Barrer when the MOF loading increases from 30 wt% to 40 wt% with the remaining of selectivity over CH4 and N2. It must be noted that the choice of a suitable polymer matrix with intrinsically high mechanical strength is central to the successful fabrication of MMMs with high filler loadings. Alternatively, choosing the solvent with a low boiling point for the fast formation of MMMs is another effective strategy. Solvents, such as THF, chloroform, and dichloromethane (DCM), are good choices for membrane casting owing to their good dissolvability to various polymers and fast evaporation speed under mild conditions. A good example is that homogeneous dispersion of silane modified NaY zeolite micro-sized particles in Matrimid®5218 matrix can be obtained in chloroform.147 The fast removal of chloroform leads to increase in solution viscosity and consequently slows the filler sedimentation process.


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Figure 13. SEM cross-section images of (a) PSF homopolymer and (b–f) MMMs containing (b) 10 wt%, (c) 20 wt%, (d) 30 wt%, (e) 40 wt%, and (f) 50 wt% UiO-66-NH2, respectively; Diffusion coefficients (g) and solubility coefficients (h) of CO2 (triangles), N2 (squares), and CH4 (circles) at 3 bar and 35°C as a function of UiO-66-NH2 loading in MMMs. (Modified and reproduced with permission from ref 154. Copyright 2016, Royal Society of Chemistry, London.) 4.1.4 Sequential Deposition. Contrary to the conventional physical blending of fillers and polymer, sequential deposition method involves the deposition of fillers onto porous substrates first, followed by an interfacial polymerization of the polymer phase. This method prevents the pre-mixing of two phases and avoids the fill agglomeration in the final membranes. Filler deposition process can be achieved by in situ growth,155 dip-coating,156 or Langmuir−Schaefer


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transfer technique,38 which can also provide an accurate control on the thickness of the filler layer. Subsequently, an ultrathin polymer layer can be prepared through the interfacial polymerization, covering the filler layer and sealing gaps between filler particles. Membranes prepared using this method exhibit high permeance and selectivity in organic nanofiltration.38 Therefore, this also offers a promising methodology of fabricating gas separation membranes with enhanced separation performance. 4.2 Filler Size. At the initial stage of developing MMMs, most studies report the usage of large fillers with sizes in micrometer range, typically resulting in enhanced permeability and decreased selectivity. Interestingly, the same combination of fillers and polymer matrix could result in different membrane performance owing to the variation in filler size. One of the pioneering work in systematic investigation of filler size effect dates back to 2000. TantekinErsolmaz et al. incorporated the same amount of zeolite silicate with various filler sizes (0.1, 0.4, 0.7, 0.8, 1.5, and 8 µm) into PDMS matrix to prepare MMMs.157 They found that fillers with smaller sizes could lead to a reduction in membrane permeability owing to their increased contacting area with the polymer matrix. This larger interface area causes an enhancement in gas mass-transfer resistance and therefore leads to an increase in CO2/N2 and CO2/O2 selectivity owing to the smaller kinetic diameter of CO2 compared with that of N2 and O2. A more recent study conducted by Chung et al. reveals that there exists an optimum filler size in MMMs.158 They incorporated ZIF-71 particles with different diameters (30, 200, and 600 nm) into 6FDADurene and explored gas transport properties of resulting MMMs. MMMs with 200-nm ZIF-71 fillers exhibit the best H2/CH4 separation performance owing to the good polymer-filler interface compatibility and well-developed filler crystallinity. Modified Maxwell models further confirm that inefficiently packed polymer chains lead to the interfacial voids and deteriorate membrane


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performance in MMMs with 600-nm ZIF-71 fillers. In a similar manner, nano-sized (~55 nm) hypercrosslinked polystyrene (HCP) was incorporated into highly permeable PIM-1, leading to a 250% increase in CO2 permeability compared with that of pure PIM-1.48 As shown in Figure 14, MMMs with 21.3 wt% HCP fillers only show a 39.9% decrease in CO2 permeability after aging for 150 days, which is much lower than the 66.3 wt% loss of CO2 permeability in pure PIM-1 membranes over the same period. The addition of HCP fillers largely improves the membrane anti-aging ability owing to their rigid pore structure, therefore preserving the membrane fractional free volume during the polymer chain relaxation process.

Figure 14. (a) Molecular structure of PIM-1 and HCP filler and schematic illustration of effect of filler on overall performance of MMMs; (b) The single gas CO2 permeability and ideal CO2/N2 selectivity of PIM-1 based MMMs plotted against Robeson’s 2008 upper bound; (c) Permeability and aging characteristic of MMMs after ethanol treatment (black: N2 permeability


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on first day; red: N2 permeability on 150th day; green: CO2 permeability on first day; blue: CO2 permeability on 150th day). (Modified and reproduced with permission from ref 48. Copyright 2016, Royal Society of Chemistry, London.) The dispersion of fillers in the polymer matrix at the molecular level is highly desired for defect-free MMMs. This will largely improve polymer-filler interfacial interactions and ensure the homogeneous distribution of fillers inside the membranes. Musselman et al. synthesized highly soluble metal-organic polyhedral-18 (MOP-18) and added it into Matrimid®5218 matrix to prepare MMMs.159 The long alkyl chains in MOP-18 help to improve its solubility in common organic solvents. The good solubility of MOP-18 in the casting solvent largely improves its interactions with the polymer matrix and promotes the formation of defect-free MMMs, as confirmed by membrane cross-sectional SEM images. Moreover, MMMs with filler loadings as high as 80 wt% can still be obtained due to the strong affinity between polymer chains and alkyl chains of MOP-18. Gas solubility experiments demonstrate that the inaccessible pores of MOP18 crystals could be accessible to gas molecules at pressures higher than 30 bar in the membranes, further indicating the molecular level dispersion of MOP-18. As a result, the pore, core and alkyl chains of embedded MOP-18 synergistically introduce new sorption sites for gases and lead to the fast increase in membrane gas permeability. Following this, other soluble MOPs, such as Na6H18-[Cu24(5-SO3-1,3-BDC)24(S)24]·xS (where S = methanol/N,N’dimethylacetamide)160 and [Cu24(5-tBu-1,3-BDC)24(S)24],161 have been explored as potential fillers to prepare MMMs. Besides MOPs, another type of newly developed porous materials with good processability is porous organic cages (POCs).35,162,163 Bushell et al. reported the first example of utilizing POCs in MMM fabrication, whereby the CC3 cages were introduced into PIM-1 matrix.35 CC3 was initially dissolved in PIM-1 solution and tended to crystallize as the


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solvent slowly evaporated. The relatively large CC3 crystals in the final membranes lead to increase in gas permeability at the cost of slightly decreased gas selectivity. Notably, the field of POCs as fillers in MMMs is still in its infancy, and how to prevent the cage crystallization in MMM fabrication process is worth further exploring. 4.3 Filler Morphology. Filler morphology is strongly related to the filler dispersion and the polymer-filler interface compatibility in the membranes. Up to now, fillers with various morphologies have been investigated in MMMs, including nanoparticles,121,164 needles,108 cubes,165 tubes,76 spheres,60 and sheets.36 The effect of filler morphology on the membrane microstructure and performance has been studied. One good example of the systematic study on this topic was demonstrated by Gascon et al. on NH2-MIL-53(Al) in 6FDA-DAM matrix for CO2/CH4 separation. As shown in Figure 15, they synthesized NH2-MIL-53(Al) crystals in three different morphologies, including nanoparticles, nanorods, and microneedles. After fabricating MMMs with these fillers, they discovered that nanoparticles lead to an increase in CO2 permeability, opposite to the decreasing trend in the latter two fillers. This may originate from the better disruption of the polymer chains by the nanoparticles, providing more fractional free volume in the resulting MMMs. We recently observed similar results in the case of a copper MOF [Cu2(ndc)2(dabco)]n in polybenzimidazole (PBI) matrix.165 Three different morphologies of MOF fillers including bulk crystals, nanocubes, and nanosheets were prepared and added into the PBI matrix to fabricate MMMs for pre-combustion CO2 capture. The optimum gas separation performance was achieved in the MMMs containing nanosheets due to their good dispersion and better polymer-filler interactions in the membranes. It is worth mentioning that MMMs with 20 wt% nanosheets exhibit a H2 permeability of 6.13 Barrer and a H2/CO2 selectivity of 26.7, exceeding the 2008 Robeson upper bound.


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Figure 15. TEM micrographs of (a) NH2-MIL-53(Al) nanoparticles; (b) NH2-MIL-53(Al) nanorods, and (c) NH2-MIL-53(Al) microneedles. FESEM images of the MOF with various morphologies: (d) bulk crystal; (e) nanocube; (f) nanosheet. (Modified and reproduced with permission from ref 108 and 165. Copyright 2016, Wiley-VCH, Weinheim, and 2015, Royal Society of Chemistry, London.) The dispersion of MOF bulk crystals and nanosheets in MMMs was investigated with focused ion beam scanning electron microscopy (FIB–SEM) by Gascon et al. in 2015.94 They creatively adopted a bottom-up method that relies on the diffusion-mediated growth mechanism to synthesize CuBDC in nanosheet morphology. MMMs incorporating either CuBDC bulk crystals or nanosheets were prepared in the same solution-casting method and analyzed by FIB-SEM, as illustrated in Figure 16. Despite identical filler loading amount (8 wt%), striking differences in the nanostructure are immediately evident. Compared with the regular CuBDC crystals that agglomerate together and leave a significant fraction of the polymer matrix unoccupied in the


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membrane, CuBDC nanosheets are uniformly distributed over the inspected volume, strongly demonstrating the good dispersion of nanosheets in the membranes. As a result, MMMs with CuBDC nanosheets show 30–80% and even 75% to 8-fold increase in CO2/CH4 selectivity compared with the pure polymeric membranes and MMMs with CuBDC bulk crystals, respectively. This sharp selectivity increase is attributed to the good polymer-nanosheet interfacial interactions and the occupation of the gas permeation pathways by the molecular sieve. A subsequent study further validates this conclusion104 and we found that the centrifugal force generated during the spin coating process could better align the nanosheets to expose their size-selective pores to the gas diffusion direction.105

Figure 16. (a) CO2/CH4 separation performance of pure polymeric membranes and MMMs containing CuBDC with different morphologies; (b) Surface-rendered views of the segmented FIB–SEM tomograms for MMMs containing bulk-type (left) and nanosheet (right) CuBDC MOF embedded in PI; (c) Histogram of the efficiency with which the individual MOF nanosheets cover the membrane cross-section, defined as the ratio between the area of the MOF


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lamellae (Alam) and that projected on the plane perpendicular to the gas flux (Aproj), as schematically depicted in the inset. (Modified and reproduced with permission from ref 94. Copyright 2015, Nature Publishing Group, London.) Besides nanoparticles and nanosheets, spherical fillers can also be promising for the improved polymer-filler surface contact and filler dispersion. Numerous materials with spherical structures, including mesoporous silica,60 TiO2,83 zeolite,166 MOFs,167 and COFs122 have been evaluated as potential fillers in MMMs for gas separations. One of the most prominent advantages of spherical fillers is their homogenous orientation of selective pores to the gas diffusion path. On the contrary, the random dispersion of nanoparticles or nanosheets, especially those with one dimensional channels, will deteriorate their separation performance in MMMs.168 Dai et al. proposed the idea of synthesizing hollow carbon spheres (HCS) with microporous shells functioning as molecular sieving pathways and incorporated them into the block copolymer polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (PS-PEB-PS) matrix.169 PSPEB-PS copolymer consists of rigid polystyrene segments, which provide good mechanical strength, and soft ethylene-ran-butylene segments, which offer high chain flexibility. The additional fractional free volume provided by the hollow structure facilitates gas permeation in the resulting MMMs, and the size selective pores in the spherical fillers maintain or even slightly increase the gas selectivity, as shown is Figure 17. The CO2 permeability is enhanced from 55.3 to 468.6 Barrer when the HCS loading is increased from 0 wt% up to 30 wt%, accompanied by a increase of CO2/N2 selectivity from 16.9 to 24 at the same time. Moreover, the polymer-filler interface compatibility is largely improved by the spherical morphology of HCS and the softrigid polymer chains in the PS-PEB-PS matrix, leading to the defect-free morphologies as evidenced by the FESEM images. Nonporous spherical fillers containing functional groups that


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strongly interact with nonpolar gases, such as CO2, can also be added into the membranes to facilitate the transport of these gases across the membranes. For example, Wu et al. synthesized carboxylic acid nanogels (CANs) in spherical shape with an average diameter of 400 nm and added them into the Pebax matrix to fabricate MMMs for CO2/CH4 and CO2/N2 separations. The strong hydrogen bonds between the carboxylic groups on the surface of spherical CANs and Pebax polymer chains enhance the interfacial interactions, resulting in the polymer chain rigidification around the CAN spheres. Besides, CANs can increase membrane water retention ability and create additional CO2 transport sites in the membranes owing to their carboxylic acid groups. As a result, MMMs with 30 wt% CANs show a simultaneous increase in CO2 permeability (4.1-fold) and CO2/CH4 and CO2/N2 selectivity (1.7-fold and 1.6-fold, respectively) compared with the pure Pebax membranes, surpassing the 2008 Robeson upper bound limit.

Figure 17. (a) Schematic illustration of the HCS MMMs for CO2/N2 separation; (b) CO2/N2 permeability as a function of HCS loading; (c) Ideal CO2/N2 selectivity as a function of HCS loading. (Modified and reproduced with permission from ref 169. Copyright 2017, Wiley-VCH, Weinheim.) 4.4 Polymer-Filler Interfacial Interactions. Strong polymer-filler interfacial interactions are critical to the formation of defect-free MMMs. Otherwise, interfacial voids are unavoidable and


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they will severely deteriorate the separation performance of the membranes.170,171 Common strategies that can enhance the interfacial interactions include the suitable combination of polymer/filler, priming protocol, surface modification of polymer/filler, thermal annealing, melt processing and incorporation of additives. These strategies largely promote the wetting process of polymer chains at the external surface of fillers and enhance their interactions either by hydrogen bonds or covalent bonds. 4.4.1 Suitable Combination of Polymer/Filler. The purpose of fabricating MMMs is to achieve better gas separation performance by highly selective pores in porous fillers. The polymer matrix mainly provides good mechanical support and fast gas diffusion pathways, suggesting that a highly permeable polymer material is desired for high-performance MMMs. In early studies of MMMs, silicon rubber with flexible backbone chains that can effectively interact with the inorganic fillers is used as the polymer matrix.58,172,173 With the development of novel fillers consisting of pure organic species, such as POCs and COFs, glassy polymers with rigid backbones can also interact strongly with these porous fillers and form defect-free MMMs. We have reported the first example of utilizing two dimensional COFs in glassy polymers, including poly(ether imide) (Ultem) and PBI, to prepare MMMs with defect-free structures.36 Two waterstable COFs named NUS-2 and NUS-3 were exfoliated into nanosheets and incorporated into commercial Ultem or PBI to fabricate MMMs for H2/CO2 and CO2/CH4 separations. The good interface compatibility in the membranes, which was confirmed by the cross-sectional FESEM images, leads to robust membranes at a COF loading as high as 30 wt%. MMMs with 20 wt% fillers show around 1-fold (for NUS-2) and 6-fold (for NUS-3) increase in CO2 permeability compared with pure Ultem membranes at a transmembrane pressure of 5 bar, demonstrating the successful generation of CO2 transport highways by one dimensional channels in these porous


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fillers. The same strategy can be extended to inorganic fillers such as zeolite or MOFs to enhance their compatibility with the membrane matrix. Chen et al. designed and prepared novel MOFbased MMMs comprising of organosilica as the membrane matrix, in which MOF particles and organosilica networks match very well due to their similar organic–inorganic hybrid nature.174 As shown in Figure 18, no interfacial defect between organosilica and inorganic fillers is visible owing to the inorganic nature of the organosilica matrix. The as-prepared ZIF-8 and MIL-53NH2 incorporated MMMs demonstrate good performance for highly H2-(H2/CH4 selectivity = 26.5, H2 permeance = 3160 GPU ) and CO2-selective (CO2/CH4 selectively = 18.2, CO2 permeance = 430 GPU) separations, for H2/CH4 and CO2/CH4 mixtures at room temperature, respectively, surpassing most MOF-based MMMs. This work offers a simple and general concept of broadening the matrix choice to enhance the membrane interfacial interactions, paving the way for developing useful MMMs with high separation performance for industrial gas separations.

Figure 18. (a) SEM images of the surface and cross-section view of the as-prepared (top) ZIF8/organosilica, (middle) MIL-53-NH2/organosilica, and (bottom) CAU-1-NH2/organosilica


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membranes; (b) Single gas permeance of H2, N2, CO2 and CH4 through the as-prepared (A) ZIF8/organosilica, (B) MIL-53-NH2/organosilica, and (C) CAU-1-NH2/organosilicamembranes. (Modified and reproduced with permission from ref 174. Copyright 2017, Royal Society of Chemistry, London.) 4.4.2 Priming Protocol. To promote the polymer-filler interface compatibility, a surface priming protocol was suggested by Koros et al., in which the fillers were coated with an ultrathin layer of the matrix polymer.175 In a typical process, fillers are firstly dispersed in solvent under sonication or stirring, followed by a slow addition of a small portion of the polymer solution, leading to the coating of a thin polymer layer onto the filler surface. Finally, the remaining polymer solution is poured into the “primed” solution under continuous stirring. This strategy is applicable to various fillers, such as zeolite,176 TiO2,177 mesoporous silica (MSS),178 and MOFs.46,110,179 Koros et al. pioneered to investigate the influence of membrane preparation protocol on final membrane microstructures by utilizing zeolite 4A-poly(vinyl acetate) (PVAc) system.175 They found that MMMs prepared from direct polymer-filler mixing showe a “sievein-a-cage” morphology owing to the poor contact between the filler and the polymer on the interface. On the contrary, after being primed with a small amount of PVAc, zeolite 4A fillers exhibit better dispersion in the membrane casting solution. As a result, dense and defect-free MMMs can be prepared with a filler loading up to 40% in this way. It gradually becomes a standard process to fabricate MMMs with enhanced polymer-filler interactions. For example, Coronas et al. adopted the priming protocol to improve the dispersion of porous fillers in commercial glassy polymers at a filler loading up to 16 wt% (Figure 19).178 The priming process largely prevents the filler agglomeration and promotes the polymer-filler interactions.


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Figure 19. Scheme of the MMM preparation using the priming protocol. (Modified and reproduced with permission from ref 178. Copyright 2014, Elsevier, Amsterdam.) 4.4.3 Surface Modification of Polymer/Filler. To overcome the problems of weak polymerfiller interfacial interactions, techniques such as polymer chain modification and filler surface modification are always needed before mixing these two phases. This strategy not only greatly broadens the scope of polymer and filler choices, but also further enhances the membrane performance due to extra functional groups introduced during the modification process. Polymer chain modification involves interchain crosslinking180 and chemical grafting.181,182 PIM-1 has been shown to possess an interchain crosslinking behavior after thermal treatment or UV-light treatment, which leads to an enhancement in gas pair selectivity.183,184 Following this idea, Chung et al. reported the example of UV-light treatment-induced photo-oxidative PIM-1 based MMMs comprising ZIF-71 fillers for gas separation.180 An ultrathin selective layer is formed at the membrane surface after the UV treatment without any visible defects, as confirmed by the FESEM and positron annihilation spectroscopy (PALS). Under mixed gas tests, the UV-light treated MMMs containing 20 wt% ZIF-71 exhibit a CO2 permeability of around 2000 Barrer and a CO2/CH4 selectivity of 32, surpassing the performance of original PIM-1 and UV-light treated PIM-1 membranes.


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Compared with the modification of polymer chains, the surface modification of fillers is more widely studied owing to the rich chemistry in fillers. Silane coupling agents,147 amino groups,110 polyethylene glycol monomethyl ether (PEG) and polyethylenimine (PEI) groups,52 and sulfonic acid groups185 have been adopted in the modification of fillers, among which amino groups have received the most attention owing to their attractive forces between CO2 for the acidic gas removal process. Li et al. successfully fabricated robust MMMs based on an aminofunctionalized ZIF-7 (ZIF-7-NH2) and crosslinked poly(ethylene oxide) rubbery polymer with a filler loading of up to 36 wt% (Figure 20).186 The amino-functionalization not only improves the polymer-filler interfacial interactions, but also changes the breathing effect of pure ZIF-7 fillers. Original ZIF-7 exhibits a gate opening effect in the CO2 adsorption isotherm in the temperature range of 273 – 308 K (Figure 20a), originating from the change in its benzimidazole (BzIM) linker conformation. After substituting 70 wt% BzIM linkers with amino-functionalized BzIM linkers, the as-prepared ZIF-7-NH2 (70) fillers show a typical Langmuir type adsorption isotherm owing to the enlargement of the framework aperture size induced by amino groups (Figure 20b). Besides, these amino groups enhance filler affinity to CO2, as confirmed by the higher CO2 isosteric heat of adsorption in ZIF-7-NH2 (70). These effects synergistically result in a significant improvement of CO2 solubility in MMMs, leading to a simultaneous increase in membrane permeability and selectivity for CO2/CH4 separation. Fillers in nanosheet-structure can be more easily functionalized as a result of their high aspect ratios. Graphene oxide (GO) shows great potentials in gas separations because of its high aspect ratios, easy surface functionalization, high production yields, and good thermal and chemical stability.79,187,188 Wu et al. prepared a novel multi-permselective MMM by incorporating GO fillers with PEG and PEI groups (PEG-PEIGO) for CO2 capture.52 As illustrated in Figure 21, the functionalization process can greatly


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enhance the amino group contents in the original GO, improving the membrane solubility selectivity toward CO2/CH4 and CO2/N2 separation. Compared with the pristine GO, functionalized GO can be well-dispersed in the membranes and form smooth microstructures without any visible voids. MMMs with 10 wt% PEG-PEI-GO show a CO2 permeability of 145 Barrer and a CO2/CH4 selectivity of 24, which are 2.7-fold and 2.4-fold higher than those of pristine membranes, respectively.

Figure 20. CO2 adsorption isotherms of ZIF-7 (a) and ZIF-NH2 (70) (b); (c) Equimolar CO2/CH4 mixed-gas permeation results; (d) Comparison between experimental data and the Maxwell model predicted permeation results. (Modified and reproduced with permission from ref 186. Copyright 2017, Wiley-VCH, Weinheim.)


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Figure 21. (a) Illustration of the preparation of PEG−PEI−GO; (b) Pure gas permeability and ideal CO2/CH4 selectivity of the humidified membranes with different fillers. (Modified and reproduced with permission from ref 52. Copyright 2015, American Chemical Society, Washington.) 4.4.4 Thermal Annealing. Thermal annealing provides another powerful strategy to relax the polymer chains around the surface of fillers. Under high temperatures, polymer chains can fluctuate and flow around fillers more freely, leading to mitigation of defects at the polymerfiller interface. However, prior to the membrane thermal annealing, the thermal stability of polymers and fillers should be evaluated by thermal gravimetric analysis (TGA) or dynamic mechanical thermal analysis (DMA). Previous studies reveal that a proper temperature, which can induce the mobility of polymer chains without deterioration to the polymer matrix or fillers, is critical to the successful preparation of MMMs with enhanced interface compatibility.189 Besides, thermal annealing could further remove the trapped solvents or guest molecules in pores of fillers and make the filler pores accessible to gases during membrane separation process. Sivaniah et al. incorporated ZIF-8 nanoparticles into Matrimid®5218 matrix to fabricate MMMs for gas separation by solution casting.190 Thermal annealing was applied to eliminate the residual


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solvents trapped among the polymer chains and interfacial defects between two phases. MMMs with 30 wt% ZIF-8 annealed at 180 °C show a 2-fold increase in CO2 permeability and a 25-fold increase in CO2/CH4 selectivity compared with membranes annealed at 150 °C owing to the activation of selective pores in ZIF-8 at a higher temperature. However, further increasing temperature to 300 °C can cause the degradation of ZIF-8 fillers and make the membranes too brittle to be tested. The importance of thermal annealing is highlighted for the cross-linking between the polymer matrix and fillers by a recent work from Vankelecom et al.99 As shown in Figure 22, MMMs composed of ZIF-8 fillers and Matrimid®5218 show a color change from light yellow to dark brown under thermal annealing at temperatures ranging from 100 °C to 350 °C. The prolonged thermal annealing at a temperature higher than 160°C could induce the polymer chain oxidation and cross-linking, as convinced by ATR-FTIR spectra, membrane solubility, glass transition temperature (Tg), and mechanical strength change. During the thermal annealing process, ZIF-8 partially decomposed and became amorphous, which largely facilitates the polymer-filler cross-linking and the formation of the transition layer. Besides, it can create a highly porous and interconnected molecular sieve network in the membranes, enhancing the membranes’ thermal stability and plasticization resistance at high pressures up to 40 bar. It is worth mentioning that MMMs with 30 wt% ZIF-8 fillers annealed at 350°C show a CO2/CH4 selectivity of 162, which is one the highest CO2/CH4 selectivity data reported in the literature.


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Figure 22. (a) Visual tests of the thermally treated membranes. The annealing influences the visual properties of the Matrimid and MMMs with 20–30–40 wt% ZIF-8 loading (from top to bottom); (b) The suggested cross-linking mechanism for the PI and the MMMs; (c) Comparison of the gas separation performance between MMMs prepared in this work and other membranes reported in the literature. (Modified and reproduced with permission from ref 99. Copyright 2017, Royal Society of Chemistry, London.) 4.4.5 Incorporation of Additives. In addition to polymer matrix and fillers, a third component that can effectively interact with them can also be incorporated into the membranes to further enhance the polymer-filler compatibility and eliminate their interfacial voids. This idea was firstly proposed by Park et al. in 2001.59 The addition of a small molecule with three amino


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groups, 2,4,6-triaminopyrimidine (TAP), into MMMs comprising Matrimid®5218 and zeolites (4A or 13X), helps to form hydrogen bonds between the polymer matrix and the fillers. It has been observed that the threshold amount of TAP to depress the void formation is related with the number of external hydroxyl groups of zeolite particles. MMMs show a remarkable increase in CO2/CH4 selectivity from 2.23 to 617 for zeolite 4A and 6.86 to 133 for zeolite 13X after the incorporation of TAP, respectively. The larger selectivity increase for zeolite 4A might originate from its smaller pore size compared with that of zeolite 13X (3.8 Å versus 7.4 Å). Ionic liquids (ILs) can also be used as additives in MMMs owing to their strong affinity to CO2, high boiling points, and good chemical stability.191-193 Yang et al. creatively synthesized ZIF-8 in a room temperature IL (denoted as IL@ZIF-8) and tailored its cage size from 1.12 nm to 0.59 nm by confining the IL inside its SOD cages (Figure 23a, b).192 Besides, these narrow cages are interconnected through small apertures (0.34 nm), which are able to exhibit a molecular sieving effect for CO2 (0.33 nm) over other bulky molecules, such as N2 (0.364 nm) and CH4 (0.38 nm). A crowding-out phenomenon was observed for CH4 and N2 in the MMMs containing 6 wt% IL@ZIF-8 due to the narrowed pores of ZIF-8 fillers with IL, cutting down the permeability of CH4 and N2. On the contrary, the CO2 permeability is improved by 50% in these MMMs compared with the pure polymeric membranes, leading to a 76% and 334% increase in CO2/CH4 and CO2/N2 selectivity, respectively. More interestingly, the selectivity of MMMs with 6 wt% IL@ZIF-8 is increased from 42 to 66 for CO2/CH4 and from 130 to 152 for CO2/N2 at transmembrane pressures ranging from 6 bar to 10 bar, which can be ascribed to the molecular sieving capacity of IL@ZIF-8.


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Figure 23. (a) Schematic illustration of the cavity-occupying concept for tailoring the molecular sieving properties of ZIF-8 by incorporating RTILs; (b) Mixed gas separation performance toward equimolar CO2/CH4 mixture in membranes with different fillers; (c) Schematic illustration for interface design of ZIF-8@PD-PI MMM; (d) Variations of H2 permeability and H2/CH4 ideal selectivity with aging time of ZIF-8-PI (20%) and ZIF-8@PD-PI (20%) MMMs. (Modified and reproduced with permission from ref 192 and 194. Copyright 2015 and 2016, Wiley-VCH, Weinheim.) 4.5 Pore Blockage and Polymer Chain Rigidification. When blending porous fillers with polymer matrix in the solution, polymer chains may penetrate into the pores of fillers and decrease the membrane gas permeability. In some cases, this pore blockage may improve the gas pair selectivity by reducing the relatively large pore sizes into a small range where molecular sieving mechanism can occur.65,195 This further leads to the polymer chain rigidification owing to the reduced thermal fluctuation of polymer chains penetrated in the pores, causing a reduction in


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their fractional free volume. Mohammadi et al. predicted that the average absolute relative error on CO2 permeability in the PVAc-zeolite 4A system could reach 29.8% based on the ideal Maxwell model.196 This large error represents a huge permeability loss in non-ideal MMMs, which mainly comes from the penetration of polymer chains into zeolite pores and polymer chain rigidification. Therefore, the prevention of such two phenomena is of great importance in MMMs. Chung et al. modified zeolite surface with a silane coupling agent, (3-aminopropyl)diethoxymethyl silane (APDEMS), introducing a gap of 5–9 Å between polymer chains and zeolite surface and reducing the extent of the partial pore blockage and polymer chain rigidification. This thin APDEMS layer improves the polymer-filler interactions and provides extra functional groups (e.g., CH2 and CH3) that could effectively interact with CO2. As a result, a simultaneous increase in CO2 permeability and CO2/CH4 selectivity is observed in MMMs with modified zeolite as fillers compared with those containing unmodified zeolite. Following this strategy, Jin et al. coated ZIF-8 nanoparticles with an ultrathin polydopamine (PD) layer (ZIF8@PD) hoping to protect the micropores of ZIF-8 from blocking.194 Moreover, PD can largely enhance the polymer-filler compatibility by forming hydrogen bonds with the polymer chains through abundant amino groups in these two structures (Figure 23c, d). Under the same filler loading, MMMs with ZIF-8@PD show a higher H2/CH4 selectivity compared with MMMs with pure ZIF-8, highlighting the importance of the PD layer on the prevention of pore blockage and polymer chain rigidification. 5. Summary and Perspectives Great efforts have been devoted to the development of novel MMMs with enhanced gas separation performance for natural gas purification over the past several years. The synergistic effects originating from the good processability of polymers and decent separation characteristics


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of porous/nonporous fillers offer MMMs great advantages compared to pure polymeric membranes. Specifically, porous fillers, such as zeolites, mesoporous silica, MOFs, and COFs, can provide additional gas transport pathways through their pores, therefore improving the membrane permeability. Strong affinity between the target gas with fillers and/or the small pore size of fillers that fall in the molecular sieving range will enhance the membrane gas pair selectivity. Nonporous fillers, such as nonporous silica and metal oxide, can create more fractional free volume in MMMs by disrupting the polymer chain packing, leading to higher gas permeability. Grafting these nonporous fillers with functional groups that strongly interact with target gases provides an effective strategy to further improve the membrane selectivity. Besides, membrane preparation processes also play important roles in revealing the full potential of MMMs. Different strategies, including filler drying-free, “one-pot” synthesis, high casting solution viscosity, and sequential deposition, have been demonstrated to be effective in preventing the filler sedimentation and agglomeration. Nano-sized fillers with functional groups are preferred in fabricating defect-free MMMs by enlarging polymer-filler contacting area and enhancing their interfacial interactions, especially in the cases of asymmetric hollow fiber MMMs where ultrathin selective layer is required for the high gas flux. Moreover, various techniques, such as priming, thermal annealing, and incorporation of additives, can further help to improve the polymer-filler compatibility and broaden the possible combinations of polymers and fillers. However, contrary to polymeric membrane systems in natural gas industry for acidic gas removal and heavy hydrocarbon recovery, MMMs still mainly remain in lab-scale studies, indicating the co-existence of opportunities and challenges. Here, we provide several possible


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future directions for MMMs to be further developed as reliable technologies in industrial applications. Firstly, future studies are encouraged to explore the membrane separation performance under more realistic conditions. Currently, lab-scale membrane testing conditions largely deviate from real operational conditions in natural gas industry. The existence of impurities may influence membrane separation performance. As pointed out by Baker et al., the feed gas for CO2/CH4 separation membranes in natural gas industry can contain about 1000 ppm of water and up to 500 ppm of BTEX aromatics (benzene, toluene, ethylbenzene, and xylene), and these substances tend to condense and dissolve in membranes at high operational pressures (up to 60 bar).197 These substances can result in sever membrane plasticization and softening, largely undermining the membrane separation performance. However, in current lab-scale studies, high-purity gases are widely used and only few membrane systems have been examined in the presence of water vapor.23,52,198 Besides, high pressures are needed in natural gas industry, which, however, are seldom realized in lab-scale studies. Therefore, the anti-plasticization and anti-aging ability of membranes can hardly be evaluated with good accuracy based on permeation data collected at low pressures. Secondly, asymmetric hollow fiber MMMs are economically desirable for industrial natural gas purification. Despite the promising permeation results obtained from dense MMMs with membrane thickness of higher than 50 µm, the gas separation performance of corresponding thin MMMs (0.1–1 µm) mainly remains unknown. It has been demonstrated that thin membranes will experience an accelerated aging process compared with their thick counterparts.136 Besides, as membrane layer becomes thinner, fillers with smaller particle size (

Mixed Matrix Membranes for Natural Gas Upgrading: Current Status

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