Tunable sour gas separations: Simultaneous H2S and CO2 removal from natural gas via crosslinked telechelic poly(ethylene glycol) membranes

Journal Pre-proof Tunable sour gas separations: Simultaneous H2S and CO2 removal from natural gas via crosslinked telechelic poly(ethylene glycol) membranes Daniel J. Harrigan, John A. Lawrence, III, Harrison W. Reid, Jason B. Rivers, Jeremy T. O'Brien, Seth A. Sharber, Benjamin J. Sundell PII:

S0376-7388(19)33857-8

DOI:

https://doi.org/10.1016/j.memsci.2020.117947

Reference:

MEMSCI 117947

To appear in:

Journal of Membrane Science

Received Date: 18 December 2019 Revised Date:

23 January 2020

Accepted Date: 7 February 2020

Please cite this article as: D.J. Harrigan, J.A. Lawrence III., H.W. Reid, J.B. Rivers, J.T. O'Brien, S.A. Sharber, B.J. Sundell, Tunable sour gas separations: Simultaneous H2S and CO2 removal from natural gas via crosslinked telechelic poly(ethylene glycol) membranes, Journal of Membrane Science (2020), doi: https://doi.org/10.1016/j.memsci.2020.117947. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Tunable sour gas separations: simultaneous H2S and CO2 removal from natural gas via crosslinked telechelic poly(ethylene glycol) membranes Daniel J. Harrigan,* John A. Lawrence III, Harrison W. Reid, Jason B. Rivers, Jeremy T. O’Brien, Seth A. Sharber, Benjamin J. Sundell* Aramco Services Company: Aramco Research Center - Boston

Abstract Gas separation studies typically investigate polymeric materials in the context of pure gas, and occasionally binary mixed gas properties, yet real world separations involve a myriad of components that can affect separation performance. The purification of sour gas demands efficient separations of both H2S and CO2 from natural gas, making realistic testing critical. In this work, we introduce a novel crosslinking scheme of the commercially ubiquitous polymer, poly(ethylene glycol) (PEG), via a facile route amenable to thin film composite formation. We crosslinked a systematic series of telechelic PEG oligomers with average molecular weights of 200-2050 g mol-1 and investigated the gas transport properties of the corresponding membranes in pure, binary CO2/CH4, and ternary H2S/CO2/CH4 sour gas mixtures. Crosslinking effectively eliminated PEG crystallinity for molecular weights up to 1000 g mol-1 and favorably increased CO2/CH4 selectivity. Though previously unexplored for H2S separations, crosslinked PEG membranes demonstrated exceptionally high H2S/CH4 selectivities ranging from 65-116 in ternary sour gas feeds. We achieved tunable transport performance of CO2/CH4 and H2S/CH4 separations by balancing PEG content with crosslink density to disrupt crystallinity. Crosslinked PEG derived from a 2050 g mol-1 oligomer showed a semi-crystalline microstructure, defining the upper limit of PEG chain length of this system for gas separation applications. Thermal and diffraction studies on the PEG membranes further revealed only amorphous morphologies up to 1000 g mol-1 and that the resulting membranes spanned both glassy and rubbery regimes, which resulted in profound differences on CO2/CH4 and H2S/CH4 transport performance. Graphical Abstract

Keyword: sour gas separation, poly(ethylene glycol), PEG, isocyanate crosslinking, crosslinked membrane 1. Introduction Global energy production continues to shift away from coal and oil towards cleaner-burning natural gas (CH4) [1]. Current projections estimate natural gas will remain a primary means of electricity generation through 2050, especially as a bridge towards renewable technologies. One challenge facing the growing natural gas sector is the depletion of “sweet” natural gas wells that feature low quantities of contaminants and require relatively little refining, which consequently drives the need to produce from “sour” gas wells that contain significant levels of corrosive and/or toxic gases, including quantities of hydrogen sulfide (H2S) from 4 ppm up to 30% by volume and high levels of carbon dioxide (CO2) [2]. Pipeline specifications require that H2S concentrations remain below 1 ppm and CO2 concentrations under 3% by volume, demanding significant “sweetening” of sour gas prior to its transportation and distribution[2]. Over 40% of natural gas reserves in the United States are sour, and this figure is substantially higher in the Middle East and many other parts of the world[3]. Membranes generate high industrial interest as low energy separation solutions to remove H2S and CO2 from natural gas. Separations of these contaminants from natural gas, however, are frequently governed by divergent transport mechanisms, making the simultaneous separation of H2S and CO2 a particularly challenging problem. Gas transport through dense polymer membranes follows the solution diffusion model, in which permeability depends on both the solubility of a gas into the material, related to gas condensability and diluent-polymer interactions, and diffusion of the gas through the polymer, determined by differences in gas molecule size and polymer free volume[4, 5]. Polymeric material development for a particular separation typically focuses on exaggerating differences in either solubility or diffusion selectivities. For example, polybenzimidazoles are rigid, low free volume polymers that exaggerate differences in gas diffusion resulting in preferential permeation of H2 over CO2[6-8]. Conversely, flexible polymers, such as MTR’s PEG-based Polaris™ membrane, reduce differences in gas diffusion while taking advantage of the superior condensability of CO2 to facilitate preferential permeation of CO2 over H2[9, 10]. These separations can be classified by their dominant mechanism. The H2/CO2 separation via polybenzimidazoles is diffusion controlled, whereas a CO2/H2 separation utilizing PEG is solubility controlled. With few exceptions, glassy polymers are typically deployed in diffusion controlled separations and rubbery polymers for solubility controlled separations[11-13]. This makes membrane development for the simultaneous separation of CO2 and H2S from CH4 difficult because CO2/CH4 separations are often diffusion controlled, while H2S/CH4 separations are usually solubility controlled. Most polymers prefer one sour gas component over the other and their separation performance can vary widely. Despite this challenge, novel membranes with improved simultaneous separation capabilities must be developed to address treatment of sour gas wells. One material with proven commercial success in sour gas separations is cellulose acetate (CA), currently available through UOP’s Separex™ membrane modules. CA separation efficiency, often characterized by selectivity for one gas component over another, is rather modest with typical H2S/CH4 and CO2/CH4 selectivities of 30[14]. Although CA exhibits equivalent H2S and CO2 separation from CH4, most polymers prefer one sour gas component over the other. Pebax® (poly(ether-b-amide)) is another commercial polymer that has been thoroughly evaluated for sour gas separations. Despite exhibiting high selectivities of 50-80 for H2S/CH4, the flexibility of Pebax® limits diffusion selectivities and results in overall CO2/CH4 selectivities of 10-15[15-17]. Rubbery poly(urethane urea) membranes tuned for H2S/CH4 separations succumb to similar limitations[18]. Glassy polyimide-based membranes help define upper bounds for CO2/CH4 due to their

rigidity and high free volume, but their relatively inert backbones compared to polyethers typically limits H2S/CH4 selectivity below 30[12, 19-22], and swelling at elevated pressures with condensable gases leads to decreased selectivities [23, 24]. Recent developments in polymers of intrinsic microporosity (PIMs) improved H2S and CO2 transport properties by enhancing the solubility of H2S through intermolecular interactions with polar polymeric backbone moieties resulting in selectivity values of 15-25 for CO2/CH4 and 20-30 for H2S/CH4 [3, 25]. Importantly, materials that achieve simultaneously high CO2 and H2S separation feature strong intermolecular interactions between polar polymer backbones/moieties and CO2 and H2S gas molecules while still taking measures to increase CO2/CH4 diffusivity selectivity. Given these considerations, membranes featuring PEG represent promising candidates for simultaneous separations given its favorable interactions with H2S and CO2. PEG is widely employed in medical applications, manufacturing, and even membrane applications due to its highly polar polymer backbone[26-28]. PEG membranes may also be tuned with various membrane structures to meet solubility and diffusion criteria in simultaneous separations. While the incorporation of polyether segments through crosslinking[29], copolymerizing[18], and/or blending[30] with other polymer components have shown promise in sour gas applications, neat PEG membranes have not been specifically investigated for H2S/CH4 separations to our knowledge. Crystallinity in PEG inhibits gas transport, though this has been circumvented by Freeman and Lin et al. by photocrosslinking PEG-acrylate comb polymer membranes to form wholly amorphous morphologies[26, 31]. Owing to the flexible nature of PEG, these materials exhibited impressive solubility selectivity, approaching and surpassing the CO2/H2 and CO2/N2 upper bounds, respectively. Unsurprisingly, diffusivity selectivities were much more modest, due to the high chain flexibility that prevents effective size discrimination. In addition to limitations in diffusion controlled separation and mitigating crystallinity, PEG membranes often suffer from reduced mechanical stability and intractable film forming properties. [32, 33]. Therefore, strategies to rigidify and improve the film-forming properties of PEG while minimizing crystallinity should provide a viable pathway to improve the separation capabilities of PEG-based membranes. Herein, we describe a novel crosslinking strategy to enhance diffusion-based separations for CO2/CH4 while maintaining PEG’s high polar affinity for the simultaneous separation of H2S and CO2 from CH4. Employing a trifunctional isocyanate enabled robust crosslinking through formation of a urethane-bridged network. The resulting rigidification of the PEG network improved CO2/CH4 selectivity by circumventing plasticization challenges in diffusion-controlled separations. Given the variation in PEG phase behavior across a relatively narrow range of molecular weights (amorphous solids at Mn = 1000 and 600, liquid at Mn < 400), modulation of PEG molecular weight offered a means to vary gas transport as a function of material properties. Membrane rigidification, achieved by reducing PEG chain length, greatly enhanced CO2/CH4 selectivity and produced high combinations of H2S/CH4 and CO2/CH4 selectivities at all molecular weights. By taking advantage of a relatively simple design principle, membrane rigidification, these easily fabricated materials showed profound improvements in CO2 selectivity for simultaneous separations of H2S and CO2 from CH4. This work not only demonstrates the utility of PEG membranes toward simultaneous sour gas separations, but also introduces a successful design principle for improving diffusion control within a predominantly solubility controlled membrane.

Scheme 1. Initial reaction between telechelic PEG and trifunctional crosslinker results in urethane bond formation, continued reaction produces the highly crosslinked network

2. Experimental 2.1. Materials Varying molecular weights of PEG were procured with average molecular weights of 200 g mol-1, 300 g mol-1, 400 g mol-1, 600 g mol-1, 1000 g mol-1, 2050 g mol-1, and ultra-high molecular weight (UHMW, 1,000,000 g mol-1) from Sigma Aldrich. Methylidynetri-p-phenylene triisocyanate (PTI) was supplied from Biosynth and stored at 0 °C under an inert gas. Ethyl acetate was obtained from VWR International. Poly(acrylonitrile) (PAN) ultrafiltration membrane supports (150k nominal MWCO) with a woven polyester backing were obtained from Nanostone. 2.2. Synthesis and film formation Scintillation vials were charged with 2.0 g of PEG oligomer. For molecular weights above 400 g mol-1 (waxy solids), the PEG oligomer was heated above its melt temperature (50 °C) before being introduced to the reaction. Separately, 1.5 molar equivalents of 27% PTI in ethyl acetate (as received) were carefully added to a separate scintillation vial equipped with a stir bar. N2 flow was used to evaporate and largely remove solvent from the crosslinker solution in order to produce sufficiently concentrated solutions for film casting. Once solvent was removed, PEG oligomer was added via syringe to initiate the reaction. This was the case for all molecular weights except 2050 g mol-1, where the stock PTI crosslinker solution was added directly to the melted polymer without any pre-concentration. Varying reaction times were used and viscosity was closely monitored so that the viscous solutions could be cast on microporous supports before complete gelation occurred. Individual reaction conditions and times are reported in Table 1 below. Varying molecular weights were examined, as the starting molecular weight of the telechelic PEG oligomer also dictated Mc, crosslink density, PEG crystallinity, and the amounts of PTI crosslinker used. Table 1. Crosslinking reaction conditions and resulting membrane gel fractions. Theoretical mass fractions are included for fully crosslinked PEG structures

PEG MW (g mol-1) 200 300 400 600 1000 2050

Volume PTI mass of Ethyl Reaction Reaction Gel PEG mass PTI mass solution PEG Acetate time Temp Fraction ± (%) fraction fraction (mL) (g) (mL) (min) (°C) (%) (%) (%) 10.2 6.8 5.1 3.4 2.0 1.0

2.0 2.0 2.0 2.0 2.0 2.0

0 0 0 0 0 0.5

5 10 5 10 10 100

25 25 70 70 70 70

106.4* 101.3* 94.5 95.3 92.2 93.5

0.12 0.34 0.36 0.12 0.10 0.14

45.0 55.1 62.0 71.0 80.3 89.3

55.0 44.9 38.0 29.0 19.7 10.7

* Gel fractions exceeding 100% may result from the hydrophilicity of the PEG networks, which promotes water sorption under ambient conditions

Highly viscous and partially crosslinked solutions were knife cast on PAN supports using a 1 mil gap height, which resulted in membranes with selective layer thicknesses of 5-10 µm. Dense films were produced by pouring partially crosslinked solutions into PFA casting dishes for mechanical, thermal, and structural characterization. All films were dried under ambient conditions for 16 h to further vitrify and crosslink, then dried in vacuo at 70 °C for 16 h to remove residual solvent. Films were gradually cooled to room temperature and stored in the desiccator. 2.3. Membrane characterization Fourier transform infrared spectroscopy (FTIR) spectra were collected and analyzed with a Thermo Scientific Nicolet IS50R system with attenuated total reflectance (ATR) capabilities and OMNIC software. Background subtracted spectra were obtained with 64 scans at a resolution of 4 cm-1. Interchain spacing within each polymeric sample was determined through wide-angled X-ray diffraction (WAXD) using a Bruker D8 Discover diffractometer. Monochromatic radiation was emitted through a copper tube at 1.5418 Å, passed through a 0.3 mm collimator and micromask, and recorded by a VANTEC-500 2-D detector. Scanning was performed with 2θ ranging from 10-55° at 15° intervals. Postprocessing and analysis was performed with Diffrac.Eva 3.0 software. The interchain spacing, also referred to as d-spacing, was correlated to the peak scattering angles through Bragg’s Law: dBragg’s=

(1)

Thermogravimetric analysis (TGA) was performed with a TA Discovery Series™ TGA with a heat rate of 20 °C min-1 up to 700 °C under nitrogen gas. TGA results can be found in the SI. Thermal characterization via Differential Scanning Calorimetry (DSC) was performed using a TA Instruments Discovery Series DSC with liquid nitrogen serving as both the coolant and gas source. Samples were first heated to 200 °C at 10 °C/min, cooled to -90 °C at 10 °C/min to erase thermal history, and then heated once more to 200 °C at 10 °C/min for analysis. Membrane cross sections were imaged using a JEOL 7100 F Scanning Electron Microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Samples were prepared according to the detailed procedure reported elsewhere[34]. Representative SEM images can be found in the SI. 2.4. Permeation testing Gas transport properties of PEG membranes were measured using a custom designed permeation system as described in greater detail previously[17, 35]. Permeation measurements were performed using the constant volume, variable pressure technique[36]. The permeability, P, of gas, i, was calculated by:

= 10

∙

∙

∆

where dpi/dt is the slope of the steady state pressure rise for the permeate, V is the permeate volume, R is the ideal gas constant, T is the permeate temperature, l is the membrane thickness measured by crosssectional SEM, A is the membrane surface area calculated from photographs of the mounted membranes using ImageJ image processing software, and ∆fi is the fugacity difference across the membrane, which is approximately equal to the pressure difference for low pressure pure gas testing. Fugacity coefficients for high pressure multicomponent feeds were calculated with the Peng-Robinson equation of state using

thermodynamic parameters listed in Table S1[37, 38]. Permeate concentrations were analyzed using a Shimadzu 2014 GC for mixed gas tests. Selectivity was defined as the ratio of permeabilities: ⁄

=

Pure gas permeability coefficients were measured for CH4 and CO2 at 100 psi at ambient temperature (25°C) in triplicate, and average performance values were reported. Room temperature mixed gas permeability coefficients were measured under two different mixed gas feeds as shown in Table 2 at 800 psi: a 20/80 binary mixture of CO2/CH4 frequently used to evaluate acid gas separation performance, and a 5/3/92 ternary sour gas mixture of H2S/CO2/CH4 representative of moderately H2S-contaminated natural gas. Retentate flow was measured using a Restek ProFlow 600 for non-H2S-containing feeds and an Omega FMA 1800 series mass flow meter for H2S-containing feeds. The retentate stream was metered to maintain a <1% stage cut, i.e. the retentate flow was greater than 100 times the permeate flow such that the upstream composition is nearly equivalent to the feed composition[36]. Duplicate testing was performed on each molecular weight of PEG to determine average sample performance. Table 2. Gas compositions for permeability testing

Gas Species CO2 H2S CH4

Pure Gas 100 -100

Binary Mixture 20 -80

Ternary Mixture 3 5 92

3. Results 3.1. Membrane Characterization Composite films of six different PEG molecular weights were successfully produced through crosslinking PEG of varying molecular weight with PTI (scheme 1). FTIR confirmed the polymer structure, as shown in Figure S1. Predictably, the peak heights associated with urethane bonds increased as PEG molecular weight decreased due to greater incorporation of the isocyanate crosslinker. With the exception of minor peaks in the 200 g mol-1 PEG sample, peaks associated with isocyanate functional group were nonexistent, indicating the complete reaction of the trifunctional crosslinker with PEG. Crosssectional SEM imaging also demonstrated homogenous dense film morphologies with uniform skin thicknesses, examples of which are shown in Figure S2. These techniques were useful in verifying membrane structure, but offered little information on separation performance. Therefore, thermal and crystallographic characterization via DSC and WAXD were employed to draw structure-propertyperformance relationships in crosslinked PEG membranes.

Figure 1. DSC thermograms of varying molecular weight PEG. Lower molecular weights and increased crosslink density led to increased Tg, indicated by yellow circles Table 3. Glass (Tg) and melting (Tm) transition temperature measured by DSC

PEG MW 200 300 400 600 1000 2050 UHMW

Tg (°C) 70.0 73.2 36.6 -14.0 -32.6 -25.6 -58.3

Tm (°C) -----55.2 62.5

The polymer glass transition temperatures (Tg) in Figure 1 showed a distinct trend of increasing Tg with lower molecular weights of PEG. This increase is expected and likely arises from two phenomena, increased crosslink density in lower molecular weight PEG restricts segmental mobility[3941], and increased crosslinker loading at low molecular weights reduces the amount of flexible PEG content. These effects result in Tg values for crosslinked PEG networks that span the spectrum of glassy and rubbery polymers. Crosslinked networks of PEG at 200 to 400 g mol-1 yielded glassy membranes at room temperature, while PEG at 600+ g mol-1 yielded membranes that retained the rubbery characteristics of neat high molecular weight PEG. This dramatic variability in polymer rigidity should strongly influence the gas transport properties within the series of membranes, as rigidification reduces gas diffusion within the polymer and ultimately lowers permeability. However, as discussed earlier, glassy membranes exhibit greater diffusion-controlled permeation compared to rubbery membranes. Ultimately, DSC confirmed the successful rigidification of PEG via trifunctional crosslinking. With the exception of 2050 g mol-1 PEG, the DSC thermograms lacked features above their respective Tgs, indicating that for molecular weights of 1000 g mol-1 or less, high extents of crosslinking are sufficient to inhibit PEG crystallization. Inhibiting crystallinity is critical in gas separation applications since polymeric crystallinity introduces significant regimes of closely packed chains that are

generally impermeable to small molecules, restricting effective transport pathways to reduce overall permeability [42, 43]. In such semi-crystalline systems, gases permeate exclusively through the amorphous phase. This detrimental semi-crystalline behavior is apparent in 2050 g mol-1 PEG, as evidenced by a large melting endotherm (Tm) at 55.2 °C. Ultrahigh molecular weight (UHMW) PEG was included in characterization studies as a semi-crystalline control, which exhibited a characteristic Tm at 62.5 °C. Considering the two orders of magnitude difference in amorphous and semi-crystalline PEG permeabilities shown by Lin et al.[44], we expect a significant change in gas transport properties as semicrystallinity is introduced.

Figure 2. WAXD diffractograms of varying molecular weights of crosslinked PEG and UHMW PEG control

Diffraction studies were implemented to further probe membrane microstructure. Again, the UHMW PEG control demonstrated significant crystallinity, exhibiting characteristic diffraction peaks from 2θ = 13°-40° [44]. The crystalline peaks of 2050 g mol-1 PEG strongly overlap with UHMW PEG, confirming the presence of crystalline PEG domains within the crosslinked sample. Peak broadening was also observed, resulting from crosslinking that limited the size and regularity of crystallites within the membrane. On the other hand, WAXD studies confirmed the completely amorphous morphology of crosslinked PEG of 200-1000 g mol-1 by showing broad regions of chain packing between 2θ = 10-30° and no visible evidence of crystallinity. While there is no significant shift in the amorphous peak between different molecular weights, the lack of crystallinity in these crosslinked membranes is expected to promote gas transport relative to semi-crystalline membranes. 3.2. Ternary sour gas permeation While typical transport comparisons focus on pure, and occasionally binary gas separations[21, 23, 45], these membranes were designed for the simultaneous removal of H2S and CO2 from CH4. Therefore, the sour gas transport properties of the PEG films were measured using a ternary natural gas mixture containing 3% CO2, 5% H2S, and 92% CH4. Figure 3 compares the CO2/CH4 selectivity versus the H2S/CH4 selectivity to demonstrate the simultaneous separation potential of these PEG membranes. Here, we temporarily ignore the importance of permeability, which is often highly tunable through film thickness/permeance relationships[46-48]. Similar to asymmetric CA membrane modules, the synthesis

and casting of the studied PEG membranes are amenable to thin film composite formation with submicron thick selective layers to achieve industrially attractive permeances.

Figure 3. Sour gas selectivity combination plot comparing CO2/CH4 and H2S/CH4 selectivities of crosslinked PEG (colored shapes) with other polymer materials developed for sour gas separations under the same testing conditions (black squares) and other results from literature (white boxes[32]). A selection of popularly studied research materials are also included and quantified in the SI. A dashed line was added to visualize a performance trend related to PEG molecular weight.

The PEG membranes exhibit high combinations of H2S/CH4 and CO2/CH4 selectivities when compared with other materials studied for sour gas separations (Figure 3, black boxes and white boxes[32]). In general, non-microporous glassy polymers, such as polyimides, succeed in diffusion controlled separations[13, 49] (CO2/CH4) whereas rubbery polymers, such as Pebax and PDMS, outperform their glassy counterparts in separations highly influenced by solubility selectivity (H2S/CH4)[13, 17, 43]. The H2S/CH4 = CO2/CH4 boundary represents a useful generalization typically associated with rubbery and glassy regimes. The crosslinked PEG membranes reside in the rubbery regime, exhibiting ternary mixed gas H2S/CH4 selectivities as high as 116. However, as PEG molecular weight decreases, separation performance tends towards the glassy regime, following the trend observed in glass transition temperature. Rigidification in these crosslinked systems results in a trade-off relationship between H2S/CH4 and CO2/CH4 selectivities. In the case of 200 g mol-1 PEG, the crosslinking scheme successfully increased CO2/CH4 selectivity (58.0) while maintaining high H2S selectivity (64.7).

Figure 4. H2S/CH4 selectivity (filled shapes) and CO2/CH4 selectivity (open shapes) plotted versus respective sour gas permeabilities (H2S and CO2). Dashed lines are added to guide visualize performance trends related to molecular weight

Figure 4 illustrates a more typical upper bound plot to compare structure-property-transport relationships in CO2 and H2S permeabilities and selectivities. As molecular weight increases up to 1000 g mol-1, both H2S and CO2 permeabilities increase due to increases in flexible PEG content and corresponding decreases in crosslink density. This result is consistent with other studies on crosslinking in rubbery polymers[50]. A distinct exception occurred for the 2050 g mol-1 sample that exhibited permeabilities an order of magnitude lower than 1000 g mol-1 crosslinked PEG, indicative of the semicrystallinity in 2050 g mol-1 crosslinked PEG discussed in the characterization section. The ternary mixed gas results in Figure 4 also show that CO2/CH4 selectivity negatively correlates with CO2 permeability, commonly referred to as an upper bound tradeoff relationship[21, 22, 51]. Conversely, H2S selectivity positively correlates with H2S permeability, leading to a widening gap between CO2/CH4 and H2S/CH4 selectivities as molecular weight increases. The different selectivity trends stem from the dominant terms in the solution diffusion model for CO2/CH4 separations (diffusion controlled) versus H2S/CH4 separations (solubility controlled). H2S permeation is driven by the condensability of H2S, which is facilitated in the more permeable, less crosslinked PEG membranes with higher starting molecular weights. In addition, hydrogen bonding interactions between H2S and oxygen atoms in the PEG backbone contribute to a monotonically increasing selectivity with PEG molecular weight (and therefore increased ratios of PEG : crosslinker)[17]. This is also observed in the 2050 g mol-1 membrane that exhibits enhanced H2S/CH4 selectivity despite the reduced permeability compared to the 600 and 1000 g mol-1 membranes with lower PEG content. At low molecular weights of crosslinked PEG, polar interactions and hydrogen bonding influence the separation to a lesser extent. Instead, increased crosslinking density rigidified the polymer network and promoted diffusion-controlled behavior. These

elevated CO2/CH4 selectivities with the preservation of high H2S/CH4 selectivities highlight the utility of crosslinked PEG for simultaneous sour gas separations. 3.3 Binary CO2/CH4 permeation Ternary mixed gas permeation experiments provided benchmark membrane performance for sour gas separations, but the complexity of simultaneous transport phenomena can convolute the analysis of material properties through feed effects, such as plasticization and competitive sorption[24, 52]. To examine these effects, pure gas permeation tests were performed, however, the selectivity comparisons were complicated due to low CH4 permeation rates below the detection limit for the two lowest molecular weight samples. Instead, binary mixed gas permeation experiments were utilized to reduce the feed complexity and further elucidate structure-property relationships.

Figure 5. Comparison of mixed gas CO2 permeability plotted as a function of PEG molecular weight in ternary and binary mixed gas feeds. Dashed lines connecting the data are included to guide the eye. See table S3 and S4 for permeation data

Figure 5 also depicts the trend of increasing permeability with PEG molecular weight in both binary and ternary feeds as well as a sharp decline in permeability above 1000 g mol-1 due to the presence of PEG semi-crystallinity. The bell-shaped curve suggests that a maximum permeability was achieved through optimizing polyether content while inhibiting PEG crystallinity. Specifically, a minimum amount of crosslinker is required to inhibit crystallinity. As crosslinker loading increases, the relative amount of PEG in the crosslinked network is reduced, and the polymer chains in the network become less flexible, reducing diffusion of gas penetrants and CO2 permeability. With the exception of the 300 g mol-1 sample, CO2 permeability measured using the binary feed closely matches the CO2 permeability using ternary gas feeds, especially at higher molecular weights of PEG. This result suggests that crosslinked PEG membranes have similar permeation properties in the presence and absence of H2S, a condensable and highly plasticizing gas, at high pressures. Such behavior indicates that crosslinking stabilizes the membranes to prevent swelling and plasticization, processes that

typically increases permeability for all gas penetrants in polymeric membranes. Plasticization resistance should also result in a consistent CO2/CH4 selectivity in both the binary and ternary feed mixtures.

Figure 6. Comparison of mixed gas CO2/CH4 selectivity plotted as a function of PEG molecular weight under ternary and binary mixed gas feeds. Dashed lines connecting the data were included to guide the eye. See table S3 and S4 for permeation data

As expected for plasticization resistant materials, Figure 6 shows overlapping CO2/CH4 selectivity values for crosslinked PEG exposed to the two different feeds. H2S induced plasticization is expected to enhance segmental mobility of polymer chains and result in depressed, diffusion controlled selectivities[23, 53]. However, crosslinking imparts a resistance to chain swelling that counteracts this effect. Since plasticizing feed effects were mitigated, the trends in CO2/CH4 selectivity holds for binary mixed gas permeation as they did for ternary mixed gas permeation. The mixed gas CO2/CH4 selectivity decreases as PEG molecular weight increases from 200 to 1000 g mol-1 since the CO2/CH4 separation is primarily diffusion-controlled and diffusion selectivity is reduced with increasing chain mobility between crosslinks. Interestingly, we observed an uptick in selectivity for the 2050 g mol-1 sample under both binary and ternary feed mixtures, likely due to the re-introduction of crystallinity in this high molecular weight sample. The CO2/CH4 selectivity increase is likely related to tortuosity and chain immobilization factors where crystalline phases act as additional physical crosslinkers in the PEG network[44]. This immobilization could result in enhanced CO2/CH4 diffusivity selectivities. The gas transport results demonstrated the ability to tune crosslinked PEG membrane by adjusting the molecular weight between crosslinks to achieve a range of sour gas separation properties. The crosslinking motif successfully rigidified a traditionally rubbery, solubility-controlled polymer to enhance the performance in diffusion-controlled separations. This approach to membrane design and synthesis resulted in H2S/CH4 selectivities ranging from 64.7 to 116.0 and CO2/CH4 selectivities ranging from 22.1 to 58.0 under a high pressure ternary mixed gas feed. Given the trade-off in H2S/CH4 and CO2/CH4 selectivity, membrane selection, i.e. the choice of PEG molecular weight, depends on the

intended application and level of sour gas contamination. Permeability varied over two orders of magnitude between the membranes studied, although the synthesis technique is suitable for thin film composite formation to improve membrane permeance. 4. Conclusions In summary, we introduce a series of crosslinked PEG membranes suitable for the simultaneous separation of H2S and CO2 from natural gas. Membrane performance was highly tunable, with low molecular weight PEG derivatives achieving mixed sour gas H2S/CH4 and CO2/CH4 selectivities > 60, and higher molecular weight derivatives achieving H2S/CH4 selectivities > 110 at the expense of reduced CO2/CH4 performance. Gas permeability through these PEG membranes varied over two orders of magnitude as a function of PEG molecular weight and crosslink density, suggesting that a careful balance of crosslinking to disrupt crystallinity while maintaining high loadings of PEG content is critical for achieving optimal sour gas membrane performance. Isocyanate crosslinking proved an excellent route towards increasing and stabilizing CO2/CH4 transport properties in sour gas feeds, effectively preventing plasticization in binary and ternary mixed gas feeds. The crosslinked membranes were also fabricated on support materials, providing a path towards the more industrially relevant thin film composite formation. These are some of the first H2S properties reported for PEG membranes and their high H2S/CH4 selectivities and H2S permeabilities encourage further development. This general crosslinking motif to enhance diffusion-controlled separation in rubbery membranes through rigidification can be extended to a myriad of other polymer/crosslinker systems, which may be useful for sour gas or other separation applications that require simultaneous contaminant removal. AUTHOR INFORMATION Corresponding Authors *Email: [email protected], [email protected] Acknowledgements No external funding was provided for this research.

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Highlights • • • • •

PEG membranes were developed for the simultaneous separation of H2S and CO2 from natural gas In one example, ternary mixed gas selectivities of CO2/CH4 = 58 and H2S/CH4 = 65 were achieved Various molecular weights of PEG were crosslinked through a facile reaction at room temperature Increased weight of initial PEG oligomers leads to higher gas permeability, until crystallization occurs The resulting membranes span both rubbery and glassy regimes, rubbery favoring H2S/CH4 and glassy favoring CO2/CH4

CRediT author statement Daniel J. Harrigan: Conceptualization, Methodology, Investigation, Writing – Original Draft, Writing – Review and Editing, Visualization, Supervision. John A. Lawrence III: Investigation. Harrison W. Reid: Investigation. Jason B. Rivers: Investigation. Jeremy T. O’Brien: Investigation. Seth A. Sharber: Investigation. Benjamin J. Sundell: Supervision, Project Administration, Writing – Original Draft, Writing – Review and Editing

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: