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Research Note Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(Cross-Linked Poly(Ionic Liquid)−Ionic Liquid−Zeolite) Mixed-Matrix Membranes for CO2/CH4 Gas Separations Based on Curable Ionic Liquid Prepolymers Collin A. Dunn,† Zhangxing Shi,‡ Rongfei Zhou,§ Douglas L. Gin,*,†,‡ and Richard D. Noble*,† Department of Chemical and Biological Engineering and ‡Department of Chemistry, University of Colorado, Boulder, Colorado 80309, United States § College of Chemistry & Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/12/19. For personal use only.

†

S Supporting Information *

ABSTRACT: A three-component (cross-linked poly(ionic liquid) (PIL)−ionic liquid (IL)−zeolite), mixed-matrix membrane (MMM) platform based on curable IL prepolymers of controlled length has been developed for separating CO2 from CH4. Solutions of these curable prepolymers demonstrate increased resistance to support penetration compared to comparable solutions of analogous cross-linkable IL monomers. By adjusting the curable IL prepolymer chain length, it is possible to manipulate polymer susceptibility to support penetration, polymer solution gelation time, and gas separation performance in MMMs based on these materials. When a 50 wt % solution of the curable IL prepolymer with a degree of polymerization (x) of 87 was cast on an ultrafiltration support membrane, only 3.7 wt % of the polymer penetrated into the support. As the degree of polymerization of the curable IL prepolymer increases, the CO2/CH4 gas separation performance of the resulting MMM performance also improves. For example, an MMM synthesized using 64 wt % curable IL prepolymer (x = 87), 16 wt % [EMIM][Tf2N] as the IL, and 20 wt % SAPO-34 zeolite exhibited a CO2/CH4 selectivity of (42 ± 5) and a CO2 permeability of (47 ± 1) barrers. This CO2/CH4 separation performance is comparable to the previous generation of MMMs based on curable small-molecule IL monomers with the same IL and zeolite. However, this new MMM system also exhibits faster curing gelation times and the ability to be solution-cast onto a porous support for formation of thin-film composite membranes without significant selective layer soak-in.

T

MMMs maintain the superior separatory efficiency of the dispersed phase while taking advantage of the relative ease of processing associated with the polymer matrix.11−13 The dispersant phase is often a porous, highly selective material that would otherwise be too difficult to efficiently manufacture into a membrane (e.g., zeolites,7−10 metal−organic frameworks, or supramolecular organic frameworks13). A key disadvantage of these microporous materials is that improper adhesion at the dispersant/matrix interface degrades membrane performance. Interfacial voids, chain rigidification, and pore plugging can all contribute to reduced MMM performance.14 MMMs made using poly(ionic liquid) (i.e., PIL), zeolite, and free ionic liquid (i.e., IL) components have recently been reported in the literature.14−17 ILs are salts (typically organic in

he global demand for natural gas is growing, as is the demand for technologies that can improve extracted gas to pipeline grade. In 2015, the U.S. alone consumed over 24 million standard cubic feet of natural gas,1 and global natural gas production that year increased by 2.2% from 2014.2 The removal of CO2 from these extracted gas streams is paramount since the presence of CO2 depresses the heating value of natural gas and, in combination with water vapor, generates carbonic acid which corrodes pipeline equipment.3−5 Currently, membrane separation systems account for ca. 5% of the natural gas separations market, with amine scrubbing and cryogenic distillation as the dominant approaches.3,5,6 However, the energy costs associated with amine stripping and cryogenics are significant, and amine scrubbing carries an environmental risk.5 Membrane systems require less operator supervision and capital investment than the dominant technologies.4 If CO2 permeability can be increased without sacrificing membrane selectivity, fewer and smaller membrane modules can be used to process the same volume of gas. Mixed-matrix membranes (MMMs) are membranes composed of a dispersed phase combined with a polymer matrix. © XXXX American Chemical Society

Received: December 29, 2018 Revised: February 27, 2019 Accepted: March 2, 2019

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DOI: 10.1021/acs.iecr.8b06464 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

Scheme 1. Components of the Curable IL Prepolymer-Based (Cross-Linked PIL−IL−Zeolite) MMM Platforma

a

These materials form a three-component MMM, where the free IL wets the PIL interface with the zeolite component.

nature) that are molten at temperatures less than 100 °C and at atmospheric pressure, with room-temperature ILs being a subset of these materials that are liquids at ambient temperature. ILs, and their polymerized derivatives, are nonvolatile, very thermally stable, and can be tuned for enhanced solubility of CO2 over other light gases.18−22 Crosslinked networks of PILs are mechanically stable and capable of forming ion gels in the presence of up to 80 wt % free IL.23,24 These ion gels display increased CO2 solubility compared to the PIL matrix alone.25−27 In a PIL-based MMM (unlike in typical MMMs based on uncharged polymers), the charged character of the PIL matrix allows superior adhesion to the charged zeolite species. The inclusion of free IL in the material as a nonvolatile, fluid, third component also enhances the interface/adhesion between the charged organic and inorganic phases.14−17 The previous generation of these (cross-linked PIL−IL− zeolite) MMMs was optimized for maximum component synergy, granting performance figures that exceeded the Robeson upper bound.14 However, all such membranes were made by cross-linking IL monomers in situ as a means of forming the desired PIL network around the IL and zeolite component. There are a number of drawbacks associated with this small-molecule IL monomer polymerization approach. For example, the viscosity of a solution of IL monomer will be lower than that of solution of a comparable PIL due to the lack of polymer chain entanglements/interactions. During typical thin-film composite (TFC) membrane casting, significant penetration of the monomer solution into the porous support occurs, resulting in selective layer soak-in with formation of defects and loss of thickness control.26 Use of a polymer- or prepolymer/oligomer-based casting solution would afford higher viscosity casting solutions. The polymerization of lowmolecular-weight (MW) monomers also requires longer set times than cross-linking of a comparable reactive polymer or

prepolymer.26 Finally, cross-linking the IL monomers typically requires the use of volatile organic cross-linking agents such as divinylbenzene (DVB).14,26 The latter two issues can be avoided by adding cross-linking functionalities directly to the polymer or prepolymer used in an initial casting solution. Herein, we show that the use of a curable (i.e., intrinsically cross-linkable) IL prepolymer of controlled length in place of a (IL monomer + cross-linker) mixture affords (cross-linked PIL−IL−zeolite) MMMs (Scheme 1) with (a) improved resistance to support penetration when casting on UF membranes, (b) faster gelation (i.e., set) times, and (c) less susceptibility to additive volatility. The resulting MMMs have a CO2/CH4 separation performance comparable to prior analogous MMMs made with an IL monomer and crosslinker. Additionally, by varying the chain length of the curable IL prepolymer used, it is possible to vary the casting solution penetration susceptibility, the gelation time, and the CO2/CH4 separation performance of the final MMM. By modifying the chain length of the curable IL prepolymer (1a−1d) used in the preparation of these MMMs, we varied the curing time and support penetration resistance of the initial casting mixture, as well as the CO2/CH4 separation capabilities of the final MMMs. This modification was done by varying the chain length of the oligomeric poly(chloromethylstyrene) precursors (2a−2d) that were prepared by reversible addition−fragmentation chain-transfer (RAFT) polymerization (see the Supporting Information for full synthesis and characterization details). To explore the possibility of synthesizing MMMs based on curable IL prepolymer starting materials, initial baseline performance trials centered on the uncontrolled polymerization of 4-chloromethylstyrene as a precursor for a curable PIL previously reported in literature.26 Once cured, this polymer displays a structure almost identical to cross-linked poly([1-styryl-3-methylimidazolium][Tf2N]), a component in some of the most high-performing MMMs B

DOI: 10.1021/acs.iecr.8b06464 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research reported in literature.14 Exploration of controlled polymerizations began when it was discovered that MMMs based on long-chain curable PILs prepared by conventional, uncontrolled free radical polymerization were too brittle to evaluate for gas separation performance. RAFT polymerization was chosen as the controlled polymerization method for producing curable IL prepolymers with controlled, low degrees of polymerization and polydispersity, since other controlled or living chain-addition polymerization methods were unsuitable for the 4-chloromethylstyrene monomer. Cyanomethyldodecyl trithiocarbonate was chosen as the RAFT agent because it was both suitable for styrene-based monomers and available as a solid. Curable IL prepolymer total degree of polymerization (x) target values of 10, 20, 40, and 80 were chosen, as they covered a range of MWs, with larger chains “diluting” the chemical and/or physical effects of their RAFT agent end groups. The actual degree of polymerization values for the synthesized curable IL prepolymers were experimentally determined to be x = 14, 17, 57, and 87, as shown in Scheme 1 (see the Supporting Information). A 25 mol % target loading of vinyl-containing side groups was chosen to ensure that the shortest prepolymers could crosslink, without approaching cross-linking densities that would result in extensive chain rigidification around the zeolite particles. 1-Ethyl-3-methylimidazolium bistriflamide ([EMIM][Tf2N]) was selected as the added IL that would act as an ionic interfacial lubricant due to its previously reported compatibility with similar IL monomers, PILs, and charged zeolite components.14,25,27 SAPO-34 was chosen as the zeolite particle additive because it has an affinity for separating CO2 from CH4 due to a combination of size sieving and thermodynamic favorability.7,9 To determine if there was a difference in support penetration between long and short curable IL prepolymer, a series of casting solution “soak-through” tests were performed using the support penetration behavior of a cross-linkable IL monomer solution of the same weight percent loading as a reference. IL monomer [VMIM][Tf2N] with 2 wt % DVB cross-linker was used to the represent older, monomer-based systems. Table 1 shows the wt % values of IL prepolymer or

penetrated the support the least when photocured immediately after application. Interestingly, an uncured solution of curable IL prepolymer 1a (x = 14) penetrated less than an uncured solution of the 87-mer 1d. This inconsistency likely lies in the error of the measurement method. These data suggest that even relatively short curable IL prepolymers are significantly more capable of resisting support penetration than their monomer counterparts. Full experimental details for these “soak-through” studies are available in the Supporting Information. Qualitative studies to determine the relative gelation rates of IL solutions featuring [VMIM][Tf2N] + 2 wt % DVB and those containing curable IL prepolymers 1a and 1d dissolved in [EMIM][Tf2N] were conducted by comparing the ability of the solutions to “flow” after exposure to UV light, which initiates radical cross-linking. This employed a method used previously for such tests from the literature.26 This method involves adding drops of the different solutions to cuvettes, turning the cuvettes on their sides, and observing how the curable solutions moved as a function of time before flow stopped (i.e., onset of gelation). Using this method, it was observed that only the solution of curable IL prepolymer 1d (x = 87) formed an immobile gel after 300 s of UV light exposure. With all UV light exposure times, it was clear that IL prepolymer 1d is more resistant to flow than prepolymer 1a (x = 14), which in turn is more resistant to flow than the [VMIM][Tf2N] + 2 wt % DVB monomer solution. After leaving the solution covered for 1 week, the IL monomer solution still had not gelled, while curable IL prepolymer 1a had. Details of these experiments and photographs of the flow behavior of the irradiated samples are available in the Supporting Information (see Figure S3). These results suggest that the longer-chain curable IL prepolymers more rapidly approach and reach gelation than their shorter counterparts and that even the smaller curable prepolymers gel more quickly than a comparable monomer solution. Ideal (i.e., single-gas) permeabilities for CO2 and CH4 were determined for a series of MMM samples with different component loadings (given as weight percent of curable IL prepolymer−IL−zeolite) and different IL prepolymer numberaverage degree of polymerization (x) values. These data are summarized in Table 2. It was found that MMMs made with curable IL prepolymer 1a (x = 14) performed poorly in terms of both CO2/CH4 gas selectivity and CO2 permeability. Membranes made using the slightly longer curable IL prepolymer 1b (x = 17) demonstrated an improvement in performance compared to the parent zeolite-free PIL−IL ion gels (i.e., 80/20/0 (w/w/w) loading), but they exhibit little improvement when 20 wt % SAPO-34 is added to them. Furthermore, the zeolite-free membranes based on curable IL prepolymers 1c (x = 57) and 1d (nx = 87) show virtually identical CO2/CH4 gas transport performances compared to the zeolite-free MMM based on prepolymer 1b (x = 17). This suggests that by the time the curable IL prepolymers are approximately 20 repeat units long, they already display most of the gas transport properties of longer polymer chains. Adding 20 wt % SAPO-34 particles into the composition based on prepolymer 1c (x = 57) almost doubles the permeability of CO2, leaving the selectivity unchanged. The MMM based on IL prepolymer 1d (x = 87) exhibits further improvements in permeability and selectivity and is comparable in gas separation performance to a previously reported MMM based on an IL monomer that had the same zeolite, IL, and component

Table 1. Mass Penetration of 50 wt % Solutions of Curable IL Prepolymer or ([VMIM][Tf2N] Il Monomer + 2 wt % DVB) in Acetone into an Ultrafiltration Membrane with a 30 kDa MW Cutoff curable IL prepolymer or IL monomer system

mass % penetration into porous support

[VMIM][Tf2N] + 2 wt % DVB, uncured [VMIM][Tf2N] + 2 wt % DVB, cured 1a, uncured 1a, cured 1d, uncured 1d, cured

70 ± 20 67 7 6 11 3.7

± ± ± ± ±

9 1 2 8 0.9

monomer added to a support that could not be recovered due to penetration. From the data presented in Table 1, it is clear that 50 wt % solutions of ([VMIM][Tf2N] + DVB) in acetone, with or without UV curing, readily penetrate into the underlying porous support structure rather than forming a distinct, continuous film on top of it. Upon comparison of the curable IL prepolymer solution performances, a 50 wt % solution of the curable IL prepolymer 1d (x = 87) in acetone C

DOI: 10.1021/acs.iecr.8b06464 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

Table 2. Comparison of Gas Transport Properties for MMMs with Different Curable IL Prepolymer Chain Lengths and Component Loadingsa curable IL prepolymer used

MMM compositionb

1a 1a 1a 1a 1b 1b 1c 1c 1d 1d

80/20/0 64/16/20 50/20/30 40/30/30 80/20/0 64/16/20 80/20/0 64/16/20 80/20/0 64/16/20

CO2 permeability (barrers) 9.3 4 14.55 26.6 24 24 23.1 43.3 22.89 47

± ± ± ± ± ± ± ± ± ±

CH4 permeability (barrers)c

0.2 1 0.05 0.2 1 1 0.1 0.1 0.02 1

0.38 0.12 2.57 3.23 0.99 0.85 0.71 1.5 0.89 1.1

± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.05 0.02 0.07 0.21 0.06 0.4 0.05 0.1

CO2/CH4 selectivity 25.5 30 5.67 8.20 24 30 32 30 26 42

± ± ± ± ± ± ± ± ± ±

0.1 10 0.03 0.07 2 8 3 7 2 5

All measurements were taken with 1 atm with a feed gas pressure at 21 °C. Reported values are the average of triplicate tests. bComposition reported as wt % values given as (curable IL prepolymer−[EMIM][Tf2N]−SAPO-34). cFor membranes with very low CH4 permeability, it is difficult to distinguish the CH4 permeation from the small intrinsic leak rate of the gas permeation test system. Greater uncertainty in selectivity is tied to the uncertainty of detecting methane. a

loadings.14 While increasing the chain length of the curable IL prepolymers does not improve the performance of membranes without zeolite, it does substantially improve the performance of zeolite-containing MMMs. Procedures for synthesizing and testing the MMMs are available in the Supporting Information. Analyzing the gas separation performances of the MMMs based on curable IL prepolymers of different lengths revealed a link between prepolymer MW and improved MMM performance. A MMM made with an 87-mer of these materials performs competitively with similar MMMs based on crosslinked poly([1-styryl-3-methylimidazolium][Tf2N]), the PIL used for some of the best-performing (cross-linked PIL−IL− zeolite) MMMs yet reported.14 Additionally, longer curable IL prepolymers were observed to gel/solidify more quickly than shorter ones (all else held constant), and even short oligomers of these curable IL prepolymers better resist support penetration when compared to analogous solutions of IL monomers. These new (cross-linked PIL−IL−zeolite) MMMs formed from curable IL prepolymers have yet to be optimized in terms of their component loadings, cross-linking group loadings, and zeolite choice. In future work, we aim to perform these optimization studies as well as evaluate these MMMs in highpressure, mixed-feed-gas environments.

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Funding for this work from Total, S.A. (FR0008001) is gratefully acknowledged. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Z. V. Singh for his insights into MMM casting, Dr. M. G. Cowan for his advice on curable IL monomer and prepolymer synthesis, and Prof. Wei Zhang for use of his group’s GPC instrument.

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ABBREVIATIONS USED IL = ionic liquid PIL = poly(ionic liquid) MMM = mixed-matrix membrane TFC = thin-film composite RAFT = reversible addition−fragmentation chain transfer VMIM = 1-vinyl-3-methylimidazolium

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b06464. Synthesis and characterization procedures and data for the curable IL prepolymers and the MMMs described in this work and procedures for conducting gas permeability testing, soak-through testing, and gelation studies (PDF)

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REFERENCES

(1) U.S. Energy Information Administration. https://www.eia.gov/ dnav/ng/ng_cons_sum_dcu_nus_a.htm (accessed April 5, 2018). (2) British Petroleum. http://oilproduction.net/files/especial-BP/ bp-statistical-review-of-world-energy-2016-full-report.pdf (accessed March 7, 2019). (3) Scholes, C. A.; Stevens, G. W.; Kentish, S. E. Membrane gas separation applications in natural gas processing. Fuel 2012, 96, 15− 28. (4) Baker, R. W.; Lokhandwala, K. Natural Gas Processing with Membranes: An Overview. Ind. Eng. Chem. Res. 2008, 47, 2109−2121. (5) Mokhatab, S.; Poe, W. A. Handbook of Natural Gas Transmission and Processing, 2nd ed.; Elsevier, Gulf Professional Publishing: Waltham, MA, 2012. (6) Bara, J. E.; Carlisle, T. K.; Gabriel, C. J.; Camper, D.; Finotello, A.; Gin, D. L.; Noble, R. D. Guide to CO2 Separations in Imidazolium-Based Room-Temperature Ionic Liquids. Ind. Eng. Chem. Res. 2009, 48, 2739−2751. (7) Li, S.; Falconer, J. L.; Noble, R. D. Improved SAPO-34 Membranes for CO2/CH4 Separations. Adv. Mater. 2006, 18, 2601− 2603.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Douglas L. Gin: 0000-0002-6215-668X D

DOI: 10.1021/acs.iecr.8b06464 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Research Note

Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.8b06464 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX