State-of-the-art polymeric membranes and polymer derived membranes for simultaneous CO₂ and H₂S removal from sour natural gas

Abstract

Natural gas is an important resource that ensures the energy supply and reduces CO₂ emissions simultaneously. However, many natural gases from well head contain a certain amount of acid gas, which must be removed to meet the pipeline requirement. Among the existing natural gas sweetening process, membrane technology is considered as a cost-effective, less energy intensive method that can remove both CO₂ and H₂S simultaneously. The membranes with high permeability, high selectivity, and good durability are developing very fast. In this review, we summarized the latest state-of-the-art membranes investigated for H₂S/CH₄ and CO₂/CH₄ separation applications, including conventional polymer membranes, polyimides, polymer of intrinsic microporosity, rubber polymers, carbon molecular sieve membranes, hollow fiber membranes, and membrane processes for H₂S and CO₂ removal from natural gas.

Graphical abstract

Keywords

natural gas purification / H₂S removal / CO₂ capture / membranes / hollow fiber

1 Introduction

The excessive greenhouse gas emissions by human activities have resulted in a rapid increase in CO₂ content in the atmosphere, thus, causing serious environmental and ecological problems that have attracted global concern. Green, renewable energy is in high demand to meet the carbon capture and storage target for different countries [1,2]. Natural gas (NG) is considered the most feasible choice to fulfill the energy consumption increment and CO₂ emission reduction simultaneously, because the CO₂ generated by NG is about of coal burning based on the same calorific value [3]. Recently, there has been a rapid growth in NG consumption (Fig.1) and the total amount reached 595 billion cubic meters in 2021 [4]. Meanwhile, over the past 10 years, the primary energy consumption ratio among coal, NG, oil, hydroelectricity, nuclear power, and renewable energies has changed, contrary to the intensive CO₂ emissions from coal and oil, the NG is the only primary energy source that are continuously increasing (Fig.1(a) and 1(b)) from 22.4% to 24.4%.

At present, there is an urgent need to develop new technologies to improve NG production efficiently [5]. Generally, raw NG from well head contains CH₄ (70%–90%) and various associate components, including acid gases (major CO₂ and H₂S), water, and a certain amount of highly condensable hydrocarbons [6]. About 40% of the global NG contains a certain amount of H₂S and CO₂, and in some cases, the H₂S percentage can reach up to 23 mol % [7]. The acid gases should be removed to increase the calorific value of NG and reduce the pipeline corrosion, e.g., the pure CH₄ has a calorific value of 9100 kcal·m^–3, and the biogas, which contains 35%–40% of CO₂, only has the calorific value of 4800–6900 kcal·m^–3 [8]. Furthermore, CO₂ needs to be separated to less than 2% to reduce the corrosion of the pipeline [9]. On the other hand, H₂S has a foul oder of rotten eggs, and the removal of sulfur impurities from NG is also called “sweetening”. H₂S removal is widely present in NG processing, petroleum refineries and geothermal energy production, posing significant public health concerns when released into the atmosphere. Apart from the unpleasant smell, H₂S is highly toxic as a gas by itself [10]. Very low concentrations of H₂S can be harmful or fatal to human beings. The LC50 is 800 ppm per 5 min [11], which must be decreased down to 4 ppm in NG to meet the pipeline requirement for safety transportation [5], whereas fuel cell applications typically need even lower H₂S content to less than 1 ppm [12].

One technique widely used in industrial for NG sweetening is scrubbing with diethanolamine (amines), sulfinol® (amines), rectisol® (methanol) or selexol™ (polyethylene glycol, PEG) that can combine/dissociate with CO₂ and H₂S by physical interactions [13,14]. This is plausible the most stable way to decrease the H₂S concentration below 4 ppm. However, the regeneration of these amines or alcohols is very energy intensive, meanwhile, the corrosion of the amine on the apparatus also makes other alternative methods very attractive. Membrane technology, a method without phase-change, offers several advantages such as a small footprint, cost-effectiveness, easy scalability, and continuous processing during separation, is developing rapidly in the industrial removal of acid gases [15]. Rezakazemi et al. [16] compared with the economics of CO₂ and H₂S removal from NG by absorption, membrane, and hybrid process, which is shown in Fig.2. The results indicated that if the flow rate is less than 100 million standard cubic feet per day (MMSCFD), and the acid gas concentration is above 5%, membrane-based acid gas removal is considered as the most economical method. He also pointed out that for H₂S separation, the rubber polymers showed more competition for highly concentrated H₂S containing NG than glassy polymers. For CO₂ removal, glassy polymers are much better than the rubber polymers.

Here is the roadmap of membranes used for NG and biogas upgrading (Fig.2(c)). In 1982, UOP pioneered in application of cellulose triacetate (CTA) membrane for CO₂/CH₄ gas pair separation [17]. Almost at the same time, Cynara tried to use CTA hollow fiber membranes for CO₂ removal in NG, and consequently, used for enhanced oil recovery [18]. Medel (now part of air liquide) and UBE also pioneered in PI membranes for CO₂ removal from NG or biogas [19]. In the 1990s, a large number of pilot scale offshore NG sweetening plants were established and the largest capacity can reach as much as 1280 MMSCFD, which was operated by Cynara in the Gulf of Thailand [20]. Recently, more than 130 biogas upgrading stations were built by Evonik, using PI membrane to upgrade NG and capture CO₂ at the same time [21].

The scope of this review is focused on the recent advances in polymers and their modified membranes for CO₂ and H₂S separation applications. Because there are plenty of reviews already for CO₂ removal from NG recently [15,22−29]. Herein, this review focuses more on the H₂S and CO₂ removal using membranes simultaneously. The membranes ranging from conventional polymers, PIs, some specially designed polymers, carbon molecular sieve membranes (CMSMs), and the hollow fiber membranes as well as membrane processes are also summarized.

2 Background

2.1 Theory and gas transport mechanism

Gas separation membranes selectively transport gas molecules based on their different speeds, which is closely related to the properties of the polymer and the gas molecules. Typically, the dynamic diameter sequence of commonly used gas molecules is CH₄ (3.8 Å) > N₂ (3.64 Å) > O₂ (3.46 Å) > CO₂ (3.3 Å) > H₂ (2.89 Å) > He (2.6 Å) [30]. In general, the mechanisms of gas transport across the membrane can be categorized into Knudsen diffusion, molecular sieving and solution diffusion [31]. In polymer membranes, the gases permeate through the dense polymer membranes following the solution-diffusion model (Fig.3(a)), in which, the gas first dissolves upstream of the membrane, and then, diffuses through the membrane toward downstream under pressure, and finally, desorption at the downstream surface of the membrane [32]. Separation of gases is achieved due to their differences in their rates of crossing the membrane.

According to Fick’s law, the amount of gas movement is proportional to the concentration gradient. The diffusion flux of gas in the membrane can be expressed by q:

(1)q=-D\frac{\mathrm{d}c}{\mathrm{d}x}=-D\frac{\mathrm{\Delta }c}{\mathrm{\Delta }x},

where q is the flux of gas through the membrane, D is the diffusion coefficient (cm²·s^–1), dc/dx is the concentration gradient, and the negative means the concentration gradient are opposite to the mass transfer. When the steady-state is reached, the gas concentration in the film becomes consistent. After integration of Eq. (1), the Eq. (2) can be obtained:

(2)q=-\frac{D(c_1-c_2)}{l},

where l is the thickness of the membrane, c₁ and c₂ are the gas concentrations upstream and downstream of the membrane, respectively.

The solubility of gas in the membrane follows Henry’s law, there is a proportional relationship between concentration c and gas partial pressure P, and the proportional constant is called the dissolution coefficient [33,34], denoted by S:

(3)c=S\times p.

When put Eq. (3) into (2), we can get:

(4)q=-\frac{D\times S\times (p_1-p_2)}{l}.

The permeability coefficient is the combination of diffusion coefficient and dissolution coefficient, which is expressed by P [32]:

(5)P=D\times S,

(6)q=-\frac{P\times (p_1-p_2)}{l}.

The permeability P_i of gas i can be determined by flux of the gas, membrane thickness l and transmembrane partial pressure ΔP (Eq. (6)), in barrer:

(7)1\mathrm{b}\mathrm{a}\mathrm{r}\mathrm{r}\mathrm{e}\mathrm{r}={10}^{-10}\frac{\mathrm{c}\mathrm{c}\left(\mathrm{S}\mathrm{T}\mathrm{P}\right)\cdot \mathrm{c}\mathrm{m}}{\mathrm{c}{\mathrm{m}}^2\cdot \mathrm{s}\cdot \mathrm{c}\mathrm{m}\mathrm{H}\mathrm{g}}.

The factors affecting the gas diffusion coefficient are as follows: (1) size and shape of gas molecules. In general, the larger the dynamic diameter of the gas, the smaller D; (2) flexibility of the polymer chain, the more flexibility of the polymer chains, the easier to move [35]; (3) fractional free volume (FFV, V_f) of the polymer. The FFV can affect its diffusion coefficient, which can be expressed by the following equation:

(8)D=A_0\times \mathrm{e}\mathrm{x}\mathrm{p}\left(-\frac{\gamma \times V^{\ast }}{V_f}\right),

where A₀ is a pre-exponential constant, which is related to the kinetic rate and molecular size of the gas, V^* is the critical free volume of the gas molecule, γ is the correction factor, V_f is the FFV of the polymer membrane, and can be calculated using the following Eq. (9):

(9)V_f=V-V_0=V-1.3V_w,

where V_w is the van der Waals volume of the polymer, which can be calculated by the Bondi’s group contribution method [36], V is the specific volume of the polymer that can be obtained by measuring the density of polymer.

The selectivity (α) of membrane between two different gases can be expressed by their permeation difference, shown as follows:

(10)\alpha =\frac{P_{\mathrm{A}}}{P_{\mathrm{B}}}=\frac{D_{\mathrm{A}}}{D_{\mathrm{B}}}\times \frac{S_{\mathrm{A}}}{S_{\mathrm{B}}},

where α is equal to the ratio of the two gas permeability coefficients, D_A/D_B is the diffusion selectivity, and S_A/S_B is the solubility selectivity.

2.2 Trade-off lines for CO₂/CH₄ and H₂S/CH₄ separation

For a special gas pair, there is a trade-off curve between gas permeability and gas pair selectivity. The correlation between single gas permeability and idea selectivity is:

(11)\mathit{P}=\mathit{k}\times {\alpha }^{\mathit{n}}.

Freeman et al. [37] theoretically analyzed this trade-off effect and pointed out the relationships between the pre-exponential parameter (k) and power parameter (n) can be expressed as the following Eqs:

(12)-\frac{1}{n}={\left(\frac{d_j}{d_i}\right)}^2-1,

(13)k^{-\frac{1}{n}}=\left(\frac{S_i}{S_j}\right)\times S_{\mathit{i}}^{-\frac{1}{n}}\times \exp \left\{\frac{1}{n}\left[b-f\times \left(\frac{1-a}{R\times T}\right)\right]\right\},

where n is related to the kinetic diameters of gas molecules, the k is related to the rigidity of the polymer chain.

To better evaluate the gas separation performance of membranes, researchers constantly generate a variety of trade-off lines. However, the H₂S/CH₄ separation trade off curves is a special case, because the H₂S/CH₄ separation results reported in literatures largely depend on testing conditions such as pressure and component ratios. The trade-off curves for CO₂/CH₄ separation were proposed by Robeson based on conventional polymers and then updated by the appearance of PIM-1 (polymer of intrinsic microporosity) in 2008 [30,38]. However, it was re-defined in 2019 by Mckeown’s group by the invention of a series of novel advanced PIMs [39]. As for the H₂S/CH₄ gas pair, George et al. [25] analyzed different membranes for acid gas removal from NG, and found that there are some trade-off effects between the H₂S permeability and H₂S/CH₄ selectivity but not specified. Later, the trade-off curve was proposed by Yi et al. [40] and Hayek et al. [41] separately from some state-of-the art polymer membranes at tri-component acid gas conditions and different pressures. Their trade-off lines and the fitted n and k parameters are shown in the following Fig.3(b) and Fig.3(c) and Tab.1. The result indicated that they are different to a large extent based on different polymers as standard.

Kraftschik et al. [42] introduced a new criterion to assess the overall acid gas sweetening performance of polymeric membranes in sour mixed-gas separation conditions for both H₂S and CO₂. The overall acid gas removal properties of membranes are the combined permeability of H₂S and CO₂, and the approximate selectivity coefficients are the sum of H₂S/CH₄ and CO₂/CH₄ selectivity. As shown in the following equations.

(14)P_{\mathrm{C}\mathrm{A}\mathrm{G}}=P_{{\mathrm{H}}_2\mathrm{S}}+P_{\mathrm{C}{\mathrm{O}}_2},

(15){\alpha }_{\mathrm{C}\mathrm{A}\mathrm{G}}=\frac{P_{{\mathrm{H}}_2\mathrm{S}}+P_{\mathrm{C}{\mathrm{O}}_2}}{P_{\mathrm{C}{\mathrm{H}}_4}}.

2.3 Plasticization effect of CO₂ and H₂S in membrane-based NG sweetening

The gas separation properties of polymer membranes suffer from the plasticization effect, which originated from the average chain distance of the polymer chains become larger than its prototype under high pressure of condensable gases or vapors, and thereafter, enhances its permeability whereas decreases their gas pair selectivity [44−46]. In real NG separation conditions, the upstream pressure of the mixture can be as much as 800–1000 psi, and the partial pressure for both CO₂ and H₂S can be over 10 atmosphere pressure. The plasticization is a very common phenomenon in NG sweetening, especially for CO₂/CH₄ and H₂S/CH₄ separations. The point at which this happens is called the “plasticizing pressure”, as illustrated in Fig.3(d). Numerous investigations have explored how to alleviate the plasticization effect, such as thermal annealing, physical and chemical crosslinking [47−49]. All these approaches can significantly reduce the plasticization effect of CO₂ to some extent. However, the H₂S has a higher critical temperature (373.5 K) than CO₂ (304.2 K), which has a higher solubility coefficient than CO₂, under the same pressure, the H₂S will induce a more serious plasticization effect. Meanwhile, the kinetic diameter of H₂S (3.62 Å) is closer to CH₄ (3.84 Å) than that of CO₂ (3.3 Å, Tab.2), this will lead to a much smaller diffusion selectivity of H₂S/CH₄ than that of CO₂/CH₄, as a result, the H₂S/CH₄ separation is more dependent on solubility selectivity whereas the CO₂/CH₄ separation relay on both solubility and diffusion selectivity. The plasticization for CO₂/CH₄ resulted in a huge decreased selectivity, however, the plasticization can improve both the H₂S permeability but also H₂S/CH₄ selectivity due to the sorption effect (Fig.3(e)) [43].

3 Dense membrane for H₂S removal from NG

Polymeric membranes are the most successful membranes in industrial applications for H₂S separation due to their high chemical stability, robustness, and flexibility in bulk separation, applicable to both high and low levels of H₂S. The effectiveness of H₂S removal by these membranes depends on their chemical and physical structures, and thus, a large number of polymers have been reported for H₂S and CO₂ removal. Here, membranes such as glassy polymer membranes, rubber polymer membranes, and carbon molecular sieving membranes are summarized.

3.1 Glassy polymers for H₂S removal from NG

Glassy polymers are a kind of polymers that have their glass transition temperatures (T_g) above room temperature [51]. Due to the rigidity, good film formability, modest permeability, high selectivity for CO₂/CH₄ and can simultaneously remove H₂S, which is widely investigated as membranes for separation applications. Thousands of glassy polymers are synthesized for sweetening of NG, and these polymers can be classified into polysulfone (PSF), CA, polycarbonate, polyphenylene oxide (PPO), PI, PIMs, and their modified polymer membranes.

3.1.1 CA for H₂S and CO₂ separation

Cellulose is a polysaccharide consisting of a linear chain with hundreds to thousands of β-(1,4)-linked D-glucose units, which is the most abundant organic material in nature. Cellulose can be made into celluloid, paper, cloth, and consumables, pharmaceuticals, and so on [52]. CA membranes have been extensively used for acid gas removal from natural or biogas since the mid-1980s, representing ~80% share in the NG processing membrane market [53]. There are different types of CA membranes, depending on the acetyl group substitution content (Fig.4(a)). The prototype is CA. The strong hydrogen bonding and high crystallinity prevent gas molecules from permeating. The commonly used one is CTA with acetyl groups concentration of ~2.84 [54]. Considering its low cost, high stability and good vapor resistance, CTA membrane was chosen for NG sweetening for a long period. In 1989, Schell et al. [55] reported the CO₂ and H₂S separation performance of CTA in the presence of concentrated CO₂ and H₂S with the component ratio of CH₄/CO₂/H₂S (38/45/17) at 200 psi, a modest selectivity of 10–14 for CO₂/CH₄ and 15–20 for H₂S/CH₄ were obtained during 1500 h of testing (Tab.3). Chatterjee et al. [56] reported the selectivity and permeability of CTA for CO₂/CH₄ and H₂S/CH₄ under a mixed-gas (CH₄/CO₂/H₂S = 65/29/6) feed pressure of 10 bar. The dense CTA membrane has a pretty low CO₂ and H₂S permeability of ~2.4 barrer of each, combined with very close CO₂/CH₄ and H₂S/CH₄ selectivity of 22.1 and 19.4, respectively. Crosslinking of the CTA is a good method to improve its CO₂ and H₂S separation properties, because crosslinking can disrupt its crystallinity, enhance permeability, and improve resistance to plasticization simultaneously. Achoundong et al. [57] reported a series of vinyltrimethoxysilane (VTMS) modified CTA to adjust its H₂S and CO₂ separation performances as well as the membrane plasticization resistance (Fig.4(a)). The resulting crosslinked CTA membranes showed a slightly higher H₂S/CH₄ (34.3 vs. 29.7) and similar CO₂/CH₄ selectivity of 21 than the pristine CTA membrane under high gas feed pressure of 500 psi with CH₄/CO₂/H₂S component ratio of 60/20/20, however, the CO₂ and H₂S permeabilities are more than 10 times higher than the neat CTA membrane, that is, 204 vs. 8.71 barrer for H₂S and 129 vs. 8.66 barrer for CO₂ (Tab.3). Meanwhile, under the upstream pressure of 700 psi in a ternary CH₄/CO₂/H₂S (60/20/20) mixture, the VTMS-modified CA membranes showed a high H₂S/CH₄ (~28) and CO₂/CH₄ (~20) selectivity, and pretty high H₂S and CO₂ permeability of above 190 and 136 barrer, respectively. The VTMS-modified CTA demonstrated a very promising NG sweetening potential for removing CO₂ and H₂S at the same time.

Recently, removing H₂S and CO₂ from humidified and hydrocarbon contained NG was reported by Peters et al. [58], who studied the stability of CTA dense membranes in humid NG feed streams with a ternary CH₄/CO₂/H₂S feed component ratio of (80/5/15). Meanwhile, there are trace amounts of butane and toluene up to 4500 ppm. The CTA membranes were evaluated at 30 and 50 °C in the pressures of 20, 35, and 50 bar. The results showed that the introduction of butane and toluene in the high H₂S feed mixture resulted in an inhibition of both the CO₂ and H₂S permeability, which is ascribed to the highly condensable gases not only competing with H₂S and CO₂ by sorption but also hinder their diffusion. However, after a period of equilibrium region, the CTA membranes demonstrated excellent stability even when the testing time was extended to 800 h.

3.1.2 PAI for H₂S and CO₂ separation

PI is a kind of aromatic heterocyclic polymer containing an imide group in the main chain, known for its excellent heat resistance, electrical insulation properties, chemical resistance and mechanical strength. So, they are widely used in gas separation fields [59−66]. PI is synthesized by the polycondensation of diamine and dianhydride, their properties can be fine-tuned by changing the structure and component ratios of the monomers. By sophisticated design, PIs are often used in NG sweetening both in academic and real industrial [67−72]. Among them, PIs based on 4,4'-(hexafluoroisopropylidene) diphthalic anhydride (6FDA) have received widespread attention due to their ease of solution processing, relatively good thermal and chemical stability, and excellent mechanical properties. In addition, the presence of a bulky CF₃ group in the PI main chain also results in high free volume and enhances chain stiffness, as a result, 6FDA derived PIs exhibit high selectivity and permeability as compared to conventional PIs. However, 6FDA-based PIs are susceptible to plasticization, especially in the presence of H₂S. To mitigate the H₂S and CO₂ plasticization behavior of 6FDA-based PIs, an amide bond (–CO–NH–) with strong hydrogen bonding was introduced to form PAI to enhance the inter- and intra-molecular interaction. For example, Vaughn et al. synthesized a series of 6FDA-based PAI membranes (6F-PAI-1, 6F-PAI-2, 6F-PAI-3) by reacting 6FDA with three diamines, and their structures are shown in Fig.4(b) [73]. All membranes demonstrated much higher H₂S and CO₂ permeability than Torlon (Fig.4(b)). For example, the 6F-PAI-1 showed an H₂S permeability of 6.2 barrer, which is 31 times higher than Torlon. After annealing at 200 °C for 24 h, although there was a slight decrease in CO₂ permeability, there is almost no plasticization till CO₂ pressure up to ~1000 psi. This is attributed to the annealing treatment promoting the intra- and intermolecular charge transfer (Fig.4(b)) of the polymer main chain, and improved the plasticization resistance of the material to CO₂ and H₂S.

The 6F-PAI-1 showed higher stability in swelling than 6F-PAI-2 and 6F-PAI-3 when using pure H₂S gas as feed (5.5 vs. 7 bar). This is due to the reduced solubility of H₂S in a highly fluorinated polymer matrix. The concentration of H₂S in the feed gas also has a huge influence on the acid gas removal performances.

3.1.3 PI for H₂S and CO₂ separation

Vaughn and Koros [74] analyzed the H₂S, CH₄, and CO₂ transport properties of 6F-PI-1 membrane under two acid gas concentrations with H₂S rich CH₄/CO₂/H₂S (60/20/20) and H₂S lean CH₄/CO₂/H₂S (50/45/5) mixtures. The CO₂ permeability continuously decreased as the H₂S concentration increased but their CH₄ permeability changed very slightly as the H₂S concentration changed. At the same time, although the permeability of H₂S in the mixed-gas is lower than the pure-gas, the H₂S permeability in the H₂S rich mixed-gas is much higher than the H₂S lean condition and continuously increases as the partial pressure of H₂S increases (Fig.5(a) and Fig.5(b)). Surprisingly, the selectivity of H₂S/CH₄ gradually increased as the feed pressure increased in the H₂S rich feed gas whereas the H₂S/CH₄ continuously decreased in the H₂S lean mixed-gas till the feed pressure up to 800 psi (Fig.5(c)). This is due to the competitive sorption that restricted the H₂S Langmuir sorption capacity in the large amount of CO₂ in the H₂S lean component, and thus, gives lower solubility selectivity of H₂S/CH₄.

Another way to improve the H₂S and CO₂ separation performance of PI is copolymerization. Copolyimides exhibit the advantages of different monomers, which can fine tune the H₂S and CO₂ removal performances from NG at the molecular level. For random copolymers, the properties can be adjusted by changing the proportion of different monomers. For block copolymers, their properties can be adjusted by varying the lengths and proportions of different blocks. Yahaya et al. [75] conducted an in-depth study on block copolyimide membranes based on (6FDA-mPDA (4,4'-m-phenylenediamine))-(6FDA-durene) for sour gas separation, which showed H₂S/CH₄ and CO₂/CH₄ selectivity of 23 and 27, respectively. The membranes demonstrate excellent stability and separation performance over a wide range of temperatures and pressures, maintaining high H₂S/CH₄ and CO₂/CH₄ separation factors even at H₂S concentrations up to 20 wt %. Notably, as the feed H₂S concentration and partial pressure increase, there is a reciprocal selectivity trend for CO₂/CH₄ and H₂S/CH₄, in which, the CO₂/CH₄ gradually decreases whereas the H₂S/CH₄ continuously increase in the opposite way. This is due to the highly condensable gases, such as CO₂ and H₂S, competing with each other for the sorption sites. Since H₂S has a higher affinity than CO₂ it greatly reduced the CO₂ solubility, leading to a lower CO₂ permeability. On the other hand, CH₄ is not greatly affected because of its much lower affinity for the sorption sites than both CO₂ and H₂S.

Later, Yahaya et al. [77] introduced the CARDO building block into random copolyimides to inhibit interchain stacking and intra-chain packing, the resulting membrane showed an increased permeability without reducing selectivity. In the multi-component feed gas mixtures with H₂S concentration up to 20 vol %, The 6FDA-durene/CARDO (1:3) ideal selectivity of CO₂/CH₄ and H₂S/CH₄ are 19–24 and 21–24, and the permeability of CO₂ and H₂S are 32–55 barrer and 46–53 barrer (Tab.4), respectively. Alghannam et al. [78] conducted an in-depth study on the gas separation performance of block copolyimides using CARDO as a building block, who found that the block copolyimide 6FDA-CARDO/6FDA-durene membrane was highly stable in aggressive environments. Among them, 6FDA-CARDO/6FDA-durene (2500/2500) membrane performs well in harsh acidic gases (mixtures of CO₂, CH₄, N₂, C₂H₆, and H₂S), with H₂S permeability of 275.8 barrer and H₂S/CH₄ selectivity of 20.1 at 36 vol % H₂S and feed pressure of 27.6 bar. Hayek et al. [79] studied the gas separation performance of random and block copolyimides of 6FDA-6FpDA-durene, the results showed that block copolyimides performed better than random copolyimides in gas separation. When the 6FpDA/durene molar ratio changed from 25% to 80%, the ideal selectivity coefficient of CO₂/CH₄ increased to approximately 47, while the CO₂ permeability coefficient gradually decreased to 65 barrer. Further increasing the content of 6FpDA to 80%, the mixed gas permeability of CO₂ decreased to 45 barrer coupled with CO₂/CH₄ selectivity of 39 for 6FDA-6FpDA/6FDA-durene (4:1) block copolyimide membrane. However, when the polymer was exposed to a gas mixture of H₂S/CO₂/CH₄ with 20 vol % H₂S content, the CO₂/CH₄ selectivity value dropped to approximately 20.8 and the H₂S/CH₄ selectivity coefficient was 13.1 at upstream pressures up to 500 psi. The permeability coefficients of CO₂ and H₂S were 41.7 and 26.4 barrer (Tab.4), respectively.

Yahaya et al. [76] synthesized three random copolyimides using 6FDA with DAM (2,4,6-trimethyl-m-phylenediamine) and another three diamines of 6FpDA, CARDO, and ABL-21 at the ratio of 1:3 (Fig.5(d)). Without H₂S in the feed gas, the 6FDA-DAM/6FpDA (1:3) random copolyimide membrane exhibited the highest CO₂ permeability of ~89 barrer and CO₂/CH₄ selectivity of ~35 at 55 bar. When the 20% H₂S was introduced, the CARDO-based diamine showed both higher H₂S permeability and H₂S/CH₄ selectivity than both the 6FpDA- and ABL-21-based diamines, however, the 6FpDA-based copolyimide showed the highest CO₂ permeability and also pretty high CO₂/CH₄ selectivity. As a result, the 6FDA-DAM/6FpDA (1:3) membrane showed the best CAG separation property, even at upstream pressures up to 34.5 bar at 25 °C (Fig.5(e)). The block copolyimide of 6FDA-DAM/6FDA-CARDO (1:1) in sour mixed-gas condition showed much higher permeability than the random 6FDA-DAM/CARDO (1:3) copolyimide and also maintained high CO₂/CH₄ and H₂S/CH₄ selectivity. For example, the CO₂, and H₂S permeability coefficients increase about 57% and 93%, and the CO₂/CH₄ and H₂S/CH₄ selectivity only dropped 23% and 6%, respectively. This performance exceeds that of glass polymers currently used in the industry, making this membrane a potential candidate for NG sweetening.

The bulky substitution of the diamine can also improve the H₂S/CH₄ separation performance of the PI membranes. MDEA has gained attention due to the steric hindrance provided by the tetraethyl-substituted benzene ring, which restricts the free rotation of the central methylene group [72,80,81]. Hayek et al. [82] reported the 6FDA-durene/MDEA random copolyimides with different molar ratios of durene and MDEA (3:1, 1:1, and 1:3), and studied their pure gas permeation properties. Compared to the pure PIs (6FDA-MDEA and 6FDA-durene, Fig.5(f)), these copolyimides showed significant improvements in gas separation performance. For example, at a pure gas feed pressure of 100 psi, 6FDA-durene/MDEA (3:1) obtained the best CO₂ permeability of 234 barrer, and CO₂/CH₄ selectivity of 17.8. For a mixture consisting of 10% CO₂, 59% CH₄, 30% N₂, and 1% C₂H₆, 6FDA-durene/MDEA (1:3) showed a CO₂ permeability of 87.7 barrer and a CO₂/CH₄ selectivity of about 24 at elevated pressures of 400 psi (Tab.4). On the other hand, the substitution at the pendent group also has a prominent effect on the CO₂ and H₂S separation properties of the resulting PI membrane, for example, Hayek et al. [83] introduced two large side groups, dimethylamino group and tert-butyl group, into 4,4'-diaminotriphenylamine-based PIs (Fig.5(g)), and found that both side groups can increase the FFV in the polymer membrane matrix, thereby improving the gas diffusion, but the solubility coefficient is almost the same. Later, Hayek et al. [84] successfully introduced a tert-butyl group into the 6FDA-CARDO molecular structure through Friedel-Crafts alkylation reaction, which significantly increased the FFV of the resulting membrane material. The gas separation performance of 6FDA-CARDO (t-Bu) membrane was superior to that of 6FDA-CARDO. The 6FDA-CARDO (t-Bu) membrane showed an H₂S permeability of 148 barrer and an H₂S/CH₄ selectivity of 16.1 at 500 psi. Tert-butyl substituted CARDO-based copolyimides were also reported by using durene as the second diamine (Fig.5(h)) [85]. The resulting 6FDA-durene/6FDA-CARDO (t-Bu) demonstrates an H₂S and CO₂ permeability coefficients of 456 and 238 barrer, respectively, with selectivity of 20.4 for H₂S/CH₄ and 10.6 for CO₂/CH₄, which exceeds most glass PIs and has great hope in NG deacidification applications.

An industrial applicable membrane material must be highly efficient and stable in the desired separation. For high condensable gases such as CO₂ and H₂S, which often resulted in plasticization. In general, due to the higher condensability of H₂S over CO₂, it showed much lower plasticization pressure. However, the plasticization had a positive effect on both H₂S permeability and H₂S/CH₄ selectivity. For example, Kraftschik et al. [42] investigated the dense PI membranes by copolymerization of 6FDA-DAM/DABA (3,5-diaminobenzoic acid) with different ratios (3:2; 1:1; 1:2) for aggressive sour gas H₂S/CO₂/CH₄ (9.95/19.9/70.15) separations till high pressure. In which, the permeability of H₂S continuously increased as the pressure increased up to 300 psi (Fig.6(a)), at the same time, the H₂S/CH₄ selectivity further enhanced from ~18 to 25 (Fig.6(b)). On the other hand, the CO₂/CH₄ separation is totally different from H₂S separation, e.g., after plasticization, there is a huge improvement in CO₂ permeability (Fig.6(c)), however, the CO₂/CH₄ selectivity gradually decreased to more than half till the upstream pressure of over 700 psi (Fig.6(d)). Plasticization of PI is beneficial for H₂S/CH₄ separation by promoting sorption coefficient and selectivity. However, due to the loss of molecular sieve separation effect, it is not conducive to CO₂/CH₄ separation.

Crosslinking either physical or chemical is an efficient way to enhance the H₂S/CH₄ separation and anti-plasticization properties. The 6FDA-DAM/DABA (3/2) is a good candidate material due to its inherent chemical, mechanical, and thermal stability. In addition, the crosslinking capabilities of the COOH group in the PI provide a good way to improve resistance to plasticity and may even enhance the overall performance. Kraftschik et al. [42] introduced a COOH group in the PI backbone by reacting the DABA with 6FDA to form the 6FDA-DAM:DABA (3:2) membrane. The results show that the introduction of H₂S into the feed stream results in a significant difference in plasticization resistance. In pure H₂S, when the feed pressure is lower than 1 bar, the plasticizing effect occurs. Thermal annealing at sub T_g can effectively reduce the plasticizing effect. E.g., the ideal selectivity value for H₂S/CH₄ is 90% higher than that of binary and ternary gas penetration tests with H₂S partial pressure greater than 2 bar. Liu et al. [86] discovered that the increase in DAM ratio led to a higher plasticization tendency of 6FDA-DAM:DABA PI membrane. The increased DAM:DABA ratio can provide a higher H₂S permeability, while the H₂S/CH₄ selectivity is almost unchanged. They also reported the H₂S/CH₄ and CO₂/CH₄ separation performance of 6FDA-DAM and 6FDA-DAM/DABA (3:2) under various conditions [43], and found that when H₂S is present in the feed, PI membrane plasticization can actually provide a huge performance advantage for H₂S/CH₄ separation, while unfavorable to CO₂/CH₄ due to the loss of molecular sieve separation effect. For example, when a gas mixture (H₂S/CO₂/CH₄ = 20/20/60) was used as feed gas at 46 bar, the 6FDA-DAM shows an H₂S permeability of 495 barrer and H₂S/CH₄ selectivity of 31, and a CO₂ permeability of 301 barrer combined with CO₂/CH₄ selectivity ~19 (Tab.4).

To improve the stability of the membrane, Kraftschik and Koros [87] prepared a series of crosslinked membrane materials based on the 6FDA-DAM:DABA (3:2) PI backbone. Short-chain PEG molecules are used as crosslinkers in esterification reactions from ~230 to 280 °C such as dietheylene glycol (DEG), trietheylene glycol (TEG) and tetraethylene glycol (TetraEG).

Compared to the unmodified 6FDA-DAM:DABA (3:2), higher acid gas selectivity and membrane stability are achieved under harsh feed conditions. For pure H₂S, the un-crosslinked 6FDA-DAM:DABA (3:2) showed a plasticization pressure of approximately 2 bar, and as the crosslinker size increased from DEG, TEG, to TetraEG, the plasticization pressure gradually increased from 6 to 8 bar. The plasticization caused by pure CO₂ only occurs when the feed pressure is greater than 25 bar. The crosslinked membranes showed good mixed-gas separation properties, under a mixture of H₂S/CO₂/CH₄ (20/20/60) at 62 bar, the selectivity of TetraEG crosslinked membrane exhibited H₂S/CH₄ and CO₂/CH₄ selectivities of 21 and 29 (Fig.7, Tab.4), respectively. This combination of results showed very promising in real highly sour gas removal in NG sweetening, indicating that crosslinking is an efficient way to enhance the membranes for NG sweetening under harsh conditions.

The CO₂, H₂S permeability and H₂S/CH₄, CO₂/CH₄ selectivity of recently reported PIs are summarized in Tab.4. It can be observed that most of the PI membranes show H₂S permeability of 3 to 1076 barrer, and the H₂S/CH₄ selectivity of PI ranging from 7 to 49. Notably, the permeability and selectivity of the H₂S/CH₄ can be improved at the same time by careful molecular design. Additionally, as the feed pressure and H₂S concentration increase, there is also an enhanced H₂S permeability combined with H₂S/CH₄ selectivity increase. This anti-trade-off effect gives PI membranes a great opportunity for H₂S removal from NG at high pressure feed gas mixtures.

3.1.4 PIMs and modified PIMs for H₂S and CO₂ separation

PIMs are a class of polymers characterized by high rigidity and microporosity [88−91]. PIMs have been extensively studied in the field of O₂/N₂ and CO₂/CH₄ separation membranes due to their high free volume and good processability, very high permeability with 2 orders of magnitude than conventional polymers and modest gas pair selectivity [92−95], however, in the field of H₂S/CH₄ separation, due to the lack of interaction between polymer chains, most PIM membranes exhibit low H₂S/CH₄ selectivity in high-pressure gas mixtures, and the structures of some reported PIMs investigated for H₂S separation are shown in Fig.8.

The poly(1-trimethylsilyl-1-propyne) (PTMSP) is the first generation of highly microporous polymer, which was synthesized by Masudu et al. [96] in the 1980s. Merkel tested the temperature dependent H₂S permeability of PTMSP, which demonstrated exceptional H₂S permeability up to 21400 barrer at the pressure of 20 bar [97]. The H₂S permeability of PVTMS was 350 barrer evaluated by Malykh et al. [98] However, its H₂S/CH₄ selectivity is merely 1.6. Although these polymers showed high H₂S permeability, their H₂S/CH₄ selectivity still needs to be improved. In 2015, Yi et al. [99] reported a hydroxy-functionalized PIM PI (PIM-6FDA-OH) with inherent micropores, which showed slight plasticization up to 8 bar of pure H₂S feed. The PIM-6FDA-OH showed significant resistance to pure gas CO₂ plasticization at feed pressures up to 28 bar. In the ternary mixed-gas (H₂S/CO₂/CH₄ = 15/15/70) system, with the increase of feed pressure from 6.9 to 48.3 bar, the permeability of H₂S rapidly increased to 63.5 barrer (Tab.5), but the permeability of CO₂ decreased to 52.6 barrer due to the higher competitive sorption of H₂S than CO₂, whereas the permeability of CH₄ remained at ~1.9–2.1 barrer, and the H₂S/CH₄ separation factor quickly increased to 29.9. Later, Yi et al. [40] reported an amidoxime functionalized PIM-1 (AO-PIM-1, Fig.8(a)) as membranes for H₂S separation, which has a larger Brunauer-Emmett-Teller surface area than PIM-6FDA-OH. A ternary feed mixture of H₂S/CO₂/CH₄ (20/20/60) was used as feed gas from low pressure up to 77 bar. The AO-PIM-1 membrane showed an unprecedented H₂S permeability of over 4000 barrer and H₂S/CH₄ selectivity of up to 75, this figure-of-merit is among the highest H₂S/CH₄ separation membranes, which is more than one order of magnitude higher in H₂S permeability, and its H₂S/CH₄ selectivity is much higher than those of PI and conventional polymer membranes. The overall H₂S/CH₄ separation, CAG removal performances of the AO-PIM-1 and PIM-1 membranes are shown in the following Fig.9(a)–Fig.9(d). The pristine PIM-1 also showed an H₂S permeability increase with increasing feed pressure, when the upstream pressure went up to 77 bar, the H₂S permeability of PIM-1 reached over 13800 barrer and an H₂S/CH₄ selectivity of 26 (Tab.5).

Aside from amidoxime group, the amine based microporous polymers have great potential in H₂S separation applications. Mizrahi Rodriguez et al. [100] and Dean et al. [101] studied the effects of amine-functionalized PIM-1 (PIM-NH₂) and microporous polyaryl ether (PAE-NH₂) on polymer packaging structure and specific transport behavior of CO₂ and H₂S through mixed gas adsorption tests (Fig.8(a)). It was found that adding primary amine to the microporous polymer could improve the mixed gas separation performance through competition. For example, compared with the nitrile functionalized PAE-CN, the pure gas adsorption rate for PAE-NH₂ was 69% higher for CO₂ and 26% for H₂S at 1.013 × 10⁵ Pa and 35 °C, indicating an increased affinity for both CO₂ and H₂S after amine functionalization. The mixed-gas sorption analysis results indicated that the PIM-NH₂ and PAE-NH₂ showed a much higher solubility selectivity of CO₂/CH₄ and H₂S/CH₄ than their pristine PIM-1 and PAE-CN, indicating the amine functionalized microporous membranes have great potential in the future acid gas removal from NG. Dean et al. [102] circumvent these manufacturing constraints while maintaining competitive-sorption enhancements by synthesizing eight representative microporous PAEs with tertiary amines.

The overall H₂S/CH₄ separation properties and CAG separation performances of PIs and some reported glassy polymers are listed in the following Fig.9(e) and Fig.9(f). Till now, the amidoxime-functionalized PIM membranes show the most promising H₂S/CH₄ separation, combined (H₂S + CO₂)/CH₄ separation performance, with an H₂S permeability of 4000 barrer and an H₂S/CH₄ selectivity of over 70 (Tab.5), which is even higher than the 2021 H₂S/CH₄ upper bound.

TR polymer membranes are also another type of highly porous polymer membranes, which were reported by Lee et al. [103] in 2007, who thermally treated a PI with a –OH or –SH group in the ortho position of imide bond at 450 °C, the PI changed to polybenzoxazole (PBO). These PBO membranes significantly improved the permeability and selectivity [28,104−110]. Scholes et al. [111] reported the HD and HDDB series of PI precursors and thermally treated them at 400 °C for 1 h. The membranes were tested at a pressure of up to 1 bar. For cross-linked TR (XTR) membranes (HDDB), their H₂S permeability was observed to decrease with increasing partial pressure, while for the non-cross-linked TR membrane (HD), a constant H₂S permeability with pressure increased was observed (Fig.8(b)). This is attributed to the permeability within the XTR membrane being H₂S solubility dominated, while in the non-crosslinked TR membrane, permeability was H₂S diffusivity dominated.

Quantum mechanics and simulation techniques to analyze the properties of polymer membranes at the atomic scale are of great significance for understanding the behavior of side groups in gas separation. Ghasemnejad-Afshar et al. [112] used ab-initio calculation, molecular dynamics, and Monte Carlo simulation techniques to study the structural properties of triptycene-based microporous PI with different side groups (C₄H₉, C₃H₇, CH₃, and CF₃) shown in Fig.8(c). It is found that PIM membranes with larger side groups are more rigid, and the free volume in the polymer chain structure increases, which is conducive to the diffusion and penetration of gases. In particular, membranes with C₄H₉ side groups showed the highest selectivity of all the binary gas mixtures studied. For the binary mixture of CO₂/CH₄ and H₂S/CH₄, the membrane with the CF₃ group (PIM-CF₃) is more selective than PIM-C₃H₇. Although there are some deviations between the permeability and selectivity of the simulation and the experimental results, these simulation results provide useful guidance for synthesis and modification.

3.1.5 Other glassy polymers for H₂S and CO₂ separation

Novel glassy polymer membranes are always desirable in searching for advanced membranes for acid gas removal from NG. Recently, H₂S and CO₂ separation properties of some polyoxadiazole-based polymer derivatives have also been studied by Hayek et al. [113], who synthesized a series of polyazole-based polymer membranes (POz-CF₃ and FPT-Ph(OH)) for H₂S separation with their structures shown in Fig.10(a).

Both membranes showed good plasticization resistance up to 700 psi. The FPT-Ph(OH) exhibited a very high H₂S permeability of 119 barrer and H₂S/CH₄ selectivity of 21.4. This figure-of-merit is significantly higher than that of conventional PIs. Meanwhile, the two polymer membranes showed a big difference in both permeability and selectivity. Specifically, the FPT-Ph(OH) membrane not only showed 2 and 3 times higher CO₂ and H₂S permeability than POz-CF₃, but also 1.6- and 4.1-fold higher CO₂/CH₄ and H₂S/CH₄ selectivity (Tab.6). This can be ascribed to the H₂S-induced plasticization that hugely improved the permeability of H₂S in FPT-Ph(OH) membranes, but the enhancement in selectivity still needs to be analyzed in detail.

Lawrence et al. [114] investigated the performance of alkoxysilane-substituted VAPNBs for CO₂ and H₂S separation from NG (Fig.10(b)). They studied the acid gas separation performances of 10 different VAPNBs were studied by varying the alkoxysilane-substituted groups. Most of these membranes showed extremely high CO₂ and H₂S permeability ranging from 1000 to 4200 barrer combined with relatively high CO₂/CH₄ and H₂S/CH₄ selectivity. Among them, the P7 with 2-methoxy-ethoxy-silicyl group exhibited an H₂S permeability of up to 3130 barrer, and H₂S/CH₄ selectivity of 47.8 (Tab.6). VAPNBs have demonstrated excellent performance in acid gas separation, making them one of the most promising materials in the field of H₂S removal from NG.

Perfluorinated polymers are very promising candidates due to their exceptional thermal and chemical stability. They can be dissolved only in certain perfluorinated solvents to fabricate into thin-film composite membranes. The perfluorinated polymers demonstrate lower solubility coefficients for both CO₂ and H₂S than those conventional hydrocarbon polymers, and thus, they show much higher CO₂/H₂S solubility selectivity. The Teflon AF 2400 and Teflon AF 1600 showed very high H₂S permeability of 410 and 100 barrer, respectively. Merkel and Toy [115] found out that both Cytop and Teflon on membranes were capable of absorbing more CO₂ than H₂S, resulting in much higher CO₂/H₂S solubility selectivity ~1.2 to 1.5. They also point out that fluorinated polymers are more plasticization-resistant as compared to non-fluorinated polymers as they are highly resistant to chemical degradation which allows them to withstand acidic conditions without a decrease in performance.

Humidified H₂S and CO₂ separation performances were also studied by a PFSA (Aquivion® E87-12S) membrane with the mixed-gas of H₂S/CO₂/CH₄ (9/10/81) at 50 °C and upstream pressures up to 10 bar, and the RH changes from 20% to 90% (Fig.10(c)) [116]. The results showed that there is a strong dependence of PFSA separation performance on water activity and temperature (Fig.10(d)). There is a sharp increment in H₂S permeability as the temperature and RH increase, for example, the H₂S permeability reaches 500 barrer at 50 °C and 80% RH. Notably, the CO₂, CH₄ and H₂S permeability enhancement by the humidity for Aquivion® E87-12S is much larger than the temperatures and upstream pressures parameters. Moreover, under the humidified conditions, both CO₂ and H₂S simultaneously enhanced sharply but the CO₂/CH₄ and H₂S/CH₄ selectivity are almost kept constant, which hugely improves its acid gas removal ability.

3.2 Rubber polymers for H₂S and CO₂ separation

Rubber polymers are a type of polymers with their glassy transition temperatures below room temperature. Rubber polymers are widely applied for H₂S separation, because the rubber polymers have very high solubility selectivity for H₂S/CH₄, which always results in pretty high H₂S/CH₄ selectivity. Recently, most of the rubber polymers used in sour gas removal are generally composed of a soft part and a hard segment to keep the shape of the membrane whereas maintain its flexibility.

Polyurethane is a typical rubber polymer that contains –NHCOO– groups that are synthesized by the reaction of isocyanate with aliphatic or aromatic di-alcohols, and widely used in furniture, bedding, and seating, thermal insulation, elastomers and coatings due to its flexible, durable and adjustable properties. Chatterjee et al. [56] synthesized two polyether urethane (PU1 and PU3) and two PU urea (PU2 and PU4) membranes by adjusting the polyether (PE) and 1,5-heptanyl-diol or 1,5-heptanyl-diamine linkages for H₂S separation from “low-quality” NG (Fig.11(a)). The results demonstrated that the PU4 membrane exhibited an H₂S/CH₄ selectivity of 76 at 35 °C and over 100 at 20 °C and 10 bar under mixed gas H₂S/CO₂/CH₄ (70.8/27.9/1.3) (Tab.7), which is one of the highest values reported at the time, and the higher H₂S/CH₄ selectivity of PU4 than PU3 is probably due to the more polar of urethane urea group. When the H₂S concentration increased from 1.3% to 12.5%, the H₂S/CH₄ selectivity decreased from 102 to 95 (Tab.6), which is most probably due to the competitive sorption of H₂S.

Sadeghi et al. [117] introduced a novel PU (polypropylene glycol-hexamethylene diisocyanate-1, 4-butane diol, PPG-HDI-BDO) membrane that synthesized from PPG, HDI, and BDO, which showed a very high H₂S permeability of 790 barrer, and H₂S/CH₄ selectivity of 27.2 for ternary CH₄/CO₂/H₂S (H₂S% of 0.66%) gas mixtures, respectively. This result is the highest among the reported PU membranes. Meanwhile, it also showed a CO₂ permeability is 473 barrer combined with CO₂/CH₄ selectivity of 16.3. This study also revealed that as operating pressure and temperature increased, both the permeability and selectivity for H₂S and CO₂ improved. These figure-of-merits confirmed the excellent CAG removal ability for this PPG-HDI-BDO. Mohammadi et al. [118] investigated the permeation behavior of H₂S, CO₂, and CH₄ through poly (ester urethane urea) (PEUU) membranes under varying operational conditions. As pressure increased, the permeability of H₂S and CO₂ decreased, which is likely due to the compressive effects on the membrane. However, higher temperatures significantly enhanced gas permeability, indicating that the permeation behavior was primarily diffusion-controlled. Although the membrane performed excellent H₂S separation properties, its selectivity was slightly inferior to that of Pebax® and PEO-based membranes.

Harrigan et al. [119] reported a novel type of crosslinked PU (PTI-PEG) by reacting the methylidyne tri-p-phenylene triisocyanate and PEG. The H₂S/CH₄ and CO₂/CH₄ separation performance of the PEI-PEG membrane can be adjusted by the crosslinking density by adjusting the molecular weight of PEG (200 vs. 1000) (Fig.11(b)). The findings revealed that by crosslinking PEG of varying molecular weights, membranes made from low-molecular-weight PEG were more suited for CO₂/CH₄ separation, whereas those made from high-molecular-weight PEG performed better in H₂S/CH₄ separation. The small molecular weight PEG induced a high crosslinking density, which makes the membrane more like a glassy polymer. However, with high PEG molecular weight, the crosslinking density is reduced, and more rubber effects on the H₂S/CH₄ separation properties are observed.

Pebax® is a thermoplastic elastomer developed by Arkema Group, and frequently utilized in the preparation of gas separation membranes. It is composed of block copolymers of PE and PA, where the PE segment provides flexibility, elasticity, and high gas permeability, while the PA segments provide mechanical strength and chemical resistance (Fig.11(c)). An examination of the ether-associated oxygen content in the various Pebax membranes were reported by Kraftschik et al. [42] and shown in Fig.11(d), the almost linear regression curve indicates that a strong correlation between the ether content in the Pebax with H₂S/CH₄ selectivity exists. Harrigan et al. [120] investigated the transport properties of various Pebax® membranes in high-pressure NG separation, focusing on acidic gases such as H₂S and CO₂. The results demonstrated that Pebax® 1074 and 1657 membranes, which contain 50% and 60% PEO, respectively, exhibited H₂S/CH₄ selectivity of 79.9 and 80.6, and CO₂/CH₄ selectivity of 12.0 and 12.8, under a high-pressure mixed gas environment containing 20% H₂S (Tab.7). As pressure increased, the CO₂/CH₄ selectivity of all membranes decreased by an average of 24%–42% (Fig.11(e) and Fig.11(f)), with a more pronounced effect in high H₂S environments. The study indicated that membrane stability was linked to hydrogen bonding, with PEO-containing membranes demonstrating enhanced anti-plasticization properties due to stronger hydrogen bonding interactions. This research underscores the importance of testing membrane materials under realistic gas conditions, highlighting the potential of Pebax® membranes for NG treatment in environments with elevated H₂S concentrations.

PEG materials exhibit flexibility and tunable physical properties in the separation of acid gases. Wong et al. designed a series of melamine-crosslinked PEG membranes (Fig.11(g) and Fig.11(h)), synthesized via azide-alkyne cycloaddition (click chemistry), for the efficient separation of acid gases (H₂S and CO₂) [121]. The resulting membrane exhibited exceptional permeability and selectivity for H₂S and CO₂ under high pressure conditions (55 bar), particularly in acid gas removal and enrichment processes. The study also found that the molecular weight of the PEG chains had a significant impact on membrane performance, with high-molecular-weight PEG (such as PEG 1100) exhibiting superior selectivity and permeability for H₂S/CO₂ separation, which is consistent with the findings of Harrigan et al. [119].

Vaughn and Koros [74] conducted a comparative analysis of the acid gas separation performance of the glassy polymer 6F-PAI-1 and the rubbery polymer Pebax® in NG purification. The results showed that 6F-PAI-1 achieved a CO₂/CH₄ selectivity of 40 and an H₂S/CH₄ selectivity of 20. Notably, the selectivity of Pebax® remained largely unaffected by variations in acid gas concentrations, which is consistent with the behavior of rubbery materials. The superior anti-plasticization performance of 6F-PAI-1, particularly under high CO₂ concentrations, makes it highly suitable for maintaining stable separation performance in complex environments.

Compared with the high H₂S selective rubber material, which shows a higher H₂S separation ability (Fig.11(i)). However, the glassy PI membranes have much higher CO₂/CH₄ selectivity, which will benefit the reduction of CH₄ loss in the downstream. Therefore, under the low H₂S feed condition, the glassy polymer with higher CO₂ selectivity is more suitable, while at the high H₂S content, the rubber polymer is preferred to deacidification for NG separation. On the other hand, glassy PI membranes also have a plasticization benefit in high-pressure of H₂S/CH₄ separation, which will also facilitate their H₂S removal performance.

3.3 CMSMs for H₂S and CO₂ separation

The CMSMs possess extreme rigidity, separation performances for conventional gas pairs, and utmost thermal and chemical stabilities [123−129]. However, these membranes for H₂S separations are quite limited. Tronci et al. [130] theoretically analyzed gas transport through nanoporous graphite membranes for NG purification. And the results indicated that the pore size, shape, and functionalization of graphene severely affect the selectivity of the membrane toward CO₂/CH₄ and H₂S/CH₄ gas pairs. Four different pore structures and functionality are used as model compounds for simulating the H₂S, CO₂, and CH₄ transport, and their pore structures as well as the three-dimensional structures of the molecules are shown in the following Fig.12. The results indicated that the P1 showed the best H₂S/CH₄ selectivity of 20 combined with a pretty high CO₂/CH₄ selectivity. This result suggests that the circular pore for the separation of H₂S and CO₂ from CH₄ with graphene membranes is 5.90 Å.

Haider et al. [131] analyzed the CTA derived carbon molecular sieve hollow fiber membranes (CHF) for long-term acid gas removal from NG. Different storage conditions were carried out to evaluate the long-term permeability stability. When the CHF was exposed to high (150 to 2400 ppm) H₂S conditions, the resulting CHF showed an improved CO₂ permeability but decreased CH₄ permeability after storage for the first 30 d. However, after 4 months, the CO₂ permeability decreased by almost 70%, which is due to the pore blockage of H₂S.

4 Hollow fiber and thin film composite (TFC) membranes for H₂S and CO₂ separation

4.1 CA hollow fiber membrane for H₂S separation

Hollow fiber membrane is a fiber-like membrane with self-supporting structure, ultra-thin circulated skin layer, and porous supported layer, excellent surface area/volume ratio due to its high packing density is widely applied for real gas separation applications [132]. The application of polymeric membranes for NG deacidification has been widely investigated for a long time, in which, CA membranes due to their partially crystallized and robust that can withstand extremely high pressure and some associated components in the NG. Up to now, CA hollow fiber membrane is still the major membrane for acid gas removal in industries. Some representative products are listed in Cynara (Fig.13), in which, the single Cynara CA membrane module can reach as much as 30 inches in diameter with a weight of 3.5 t.

Morisato and Mahley et al. [133] reported the effect of feed pressure and temperature on the separation performance of asymmetric CTA hollow fiber membranes. The permeance of CTA hollow fiber membrane increased with the increase of feed pressure, and the selectivity of H₂S/CH₄ was basically stable at low pressure, while it declined sharply when the pressure was above 30 bar. The mixed gas permeability of H₂S and CO₂ increased with the decrease in feed temperature. The CTA hollow fiber membrane showed the best performance at 15 °C and 39 bar, the permeability of H₂S and CO₂ were 180 and 133 GPU, and the selectivity of H₂S/CH₄ and CO₂/CH₄ was 23 and 16, respectively. CTA hollow fiber membranes also showed good performance for H₂S separation under practical application conditions, with excellent H₂S high pressure and temperature tolerance.

Peters et al. [58] used a dry-wet spinning method to make the CA hollow fiber membrane using the component ratio and spin conditions (Tab.8). The resulting CA hollow fiber membrane showed a dense spongelike cross-section image (Fig.13(b)), which has a H₂S permeance of 84 GPU and H₂S/CH₄ selectivity of 19.8 under 15% H₂S in the feed condition at 20 bar and the stage-cut of 5% (Tab.9). When the stage cut decreased to 1%, both H₂S permeance and H₂S/CH₄ selectivity increased to 91.3 and 21.8. Notably, the results also confirmed that there is almost no change of H₂S permeance as well as H₂S/CH₄ selectivity under humidified feed gas, with H₂S permeance of ~80 GPU and H₂S/CH₄ selectivity of 23 at 1.5% stage-cut under humidified gas condition.

Lu et al. find that the plasticization for CTA hollow fiber membrane can be benefit for H₂S/CH₄ separation in raw NG sweetening [134]. The separation performance of CTA hollow fiber membrane was tested in NG mixtures containing high concentrations of H₂S, CO₂, C₂H₆, C₃H₈, and trace amounts of toluene. When the upstream pressure increased from 7 to 31.3 bar, the CTA hollow fiber membrane exhibited an H₂S permeance increased from 90 to 140 GPU and H₂S/CH₄ selectivity enhanced from 25 to 28 at 35 °C. However, the CO₂ permeance increased from 90 to 115 GPU, whereas the CO₂/CH₄ selectivity decreased from 25 to 22 (Tab.10).

4.2 PPO hollow fiber for H₂S and CO₂ separation

The PPO is a linear amorphous thermoplastic with T_g ~490 K. Because of the phenyl rings, PPO is hydrophobic in nature and has excellent resistance to water, acids, bases, and alcohols. Chenar et al. [136] investigated the H₂S removal from NG in a series of bench-scale experiments using a commercial PPO membrane module (Parker Filtration and Separation B.V., The Netherlands), at the H₂S concentrations ranging from 101 to 401 ppm, with a stage cut of 5%. The H₂S permeance of PPO increases with the feeding pressure of H₂S, from 50 to 100 psi, the H₂S permeance increases from 46 to 64 GPU when the H₂S concentration is 198 ppm. Meanwhile, the higher the H₂S concentration, the larger the H₂S permeance, at the same time, their H₂S/CH₄ selectivity was not affected to be ~4. Furthermore, hollow fiber CTA membranes was also studied at various H₂S concentrations (100–5000 ppm) in CH₄/H₂S binary synthetic gas mixtures and a real NG sample obtained from a gas refinery by Niknejad et al. [135]. When the H₂S concentration increased from 968, 3048 to 5008 ppm in NG. There is a huge increased H₂S permeance of ~2–3 times and good enhancement in CH₄ enrichment was observed. When the H₂S feeding is 3048 ppm, the retentate H₂S decreases to 2033 ppm, whereas the permeate H₂S is 6751 ppm. Notably, when the temperature increased from 293.15 to 313.15 K, there was an increased separation performance of the PPO membrane for H₂S removal. The H₂S concentration of 3048 ppm, the retentate H₂S decreased from 2033 to 1815 ppm.

Chenar et al. [136] compared the performance of commercial PPO (supplied by Aquilo Gas Separation B.V., the Netherlands) and Cardo-type PI (supplied by RITE, Japan) hollow fiber membranes in separating H₂S from CH₄ at different concentrations (structures shown in Fig.13(c)). As the feed pressure increased from 50 to 100 psi, there is an improvement for H₂S permeance for both PPO (from 60 to 74 GPU) and Cardo-PI (from 15 to 25 GPU) at the H₂S concentration of 198 ppm. In the presence of H₂S the permeance of CH₄ declined for the Cardo type PI membranes, whereas for the PPO membranes it remained relatively unchanged. The H₂S/CH₄ separation factors are ~4 for PPO and ~6 for Cardo-PI hollow fiber membranes and these value increases as the concentration of H₂S increase. Meanwhile, at high operating temperatures, these membrane materials can still maintain their separation properties.

Saedi et al. [137] prepared an asymmetric PES membrane via a phase inversion method and coated it with polydimethylsiloxane (PDMS) for NG decarboxylation and desulfurization. The DMF with the highest solubility parameter give the lowest CO₂ permeance of 4.7 GPU and highest CO₂/CH₄ selectivity of 19 than DMAc and NMP. The H₂S permeance of PDMS coated PSF asymmetric membrane is 32.8 GPU and H₂S/CH₄ selectivity of 11.4, however, both the H₂S permeance and H₂S/CH₄ selectivity decreased sharply as the feed pressure increased.

4.3 PI hollow fiber membrane for H₂S and CO₂ separation

PI hollow fiber membranes are also investigated for H₂S and CO₂ separation applications. Liu et al. [138] made a 6FDA based PI hollow fiber membrane that is semi-rigid, cross-linked, and defect free, aiming to improve the separation efficiency of H₂S and CO₂ in acidic NG. The membrane was evaluated in pure H₂S and three simulated acidic NG environments. The experimental results show that, due to its rigid molecular structure, the subtle asymmetric nanostructure of the membrane is maintained and the H₂S permeability is not decreased during the crosslinking process. In acidic NG environments with low concentrations of H₂S (0.5% H₂S, 20% CO₂, 79.5% CH₄), the crosslinked hollow fiber membrane achieved an H₂S permeance of 32 GPU and H₂S/CH₄ selectivity of 30 at 450 psi and 35 °C. At the same time, the permeance of CO₂ and the selectivity of CO₂/CH₄ are also effectively maintained, with 35 GPU and 32 respectively (Tab.10). Under more challenging acidic NG environments (e.g., 25% H₂S, 5% CO₂, 70% CH₄ and 20% H₂S, 20% CO₂, 60% CH₄), un-crosslinked CF₃ hollow fibers showed plasticization. Interestingly, this plasticization is beneficial for H₂S separation, with un-crosslinked samples showing slightly higher H₂S permeance and H₂S/CH₄ selectivity than cross-linked samples. In addition, the effect of acid gas composition on the properties of the membrane was also discussed. The results revealed that the increase of H₂S concentration in acidic NG has a significant effect on the permeability and selectivity of H₂S and CO₂, compared with the increase of CO₂ concentration in feed. Liu et al. [139] also studied the H₂S and CO₂ removal efficiency of triethylene glycol monoesterification crosslinked (TEGMC) hollow fiber membrane under actual NG sweetening conditions (H₂S (25%), CO₂ (5%), CH₄ (64%), C₂H₆ (3%), C₃H₈ (3%) and toluene (100 ppm)). The resulting TEGMC-crosslinkable hollow fiber membranes still show good gas separation properties after 7 years of storage. The permeance for CO₂ and H₂S is ~32, and 10 GPU combined with their CO₂/CH₄ and H₂S/CH₄ selectivity of 55 and 19, respectively. Unexpectedly, the presence of various hydrocarbons in the feedstock gas, namely C₂, C₃, and toluene, increased the selectivity of H₂S/CH₄ and CO₂/CH₄. The author proposed that the infiltration competition may be a useful tool rather than a matter of adjusting membrane properties. Shalabi et al. [140] studied the potential separation performance of 6FDA-durene/6FDA-CARDO (5000:5000) hollow fiber membranes. Preliminary screening studies show good pure gas performance, with a CO₂/CH₄ selectivity coefficient of about 28 and a CO₂ permeance between 310 and 350 GPU. To evaluate the performance of the membrane under simulated industrial environmental conditions, a quaternary gas mixture was tested with feed pressures up to 900 psi and stage cuts from 5 to 20 percent. As the feed pressure increased to 900 psi, the gas mixture permeability and CO₂/CH₄ selectivity of all gases remained relatively stable. The CO₂ permeance and CO₂/CH₄ selectivity of the mixed gas decreased with the increase of stage cut. As expected, due to the trade-off between recovery and purity, as CO₂ recovery is maximized, CO₂ purity is minimized, and CH₄ loss increases with stage cut.

By comparison of the CTA hollow fiber and PI hollow fiber membranes for CO₂ and H₂S removal (Fig.13), we can clearly conclude that the permeance of CTA is still the best membrane with pretty high CO₂ and H₂S permeance as well as CO₂/CH₄ and H₂S/CH₄ selectivity.

PI is also used for TFC membranes for H₂S separation applications. Yahaya et al. [141] also successfully coated a thin layer of 6FDA-durene/6FDA-CARDO block copolyimide onto a mesoporous polyacrylonitrile acrylate substrate, providing an effective solution for efficient separation of acid gases from NG. The membrane demonstrated excellent separation performance under laboratory conditions, achieving high levels of CO₂/CH₄ and H₂S/CH₄ selectivity of 8 to 10 and 15 to 19, respectively, while maintaining CO₂ and H₂S permeability of 122 and 223 GPU (Tab.10). For rubber polymers, the TFC membrane for acid gas removal is also applied to rubber polymers. Mohammadi et al. [118] reported a TFC membrane made of a PEUU supported on a porous Teflon substrate. The structure of the TFC is shown in the below SEM (Fig.13(d)). The average thickness of the active layer is 25 μm. The membranes can withstand the upstream pressure of 30 bar without plasticization and also a variety of temperature ranges. The membrane showed an average permeance of 95 and 45 GPU for H₂S and CO₂, with the H₂S/CH₄ and CO₂/CH₄ selectivity of 43 and 16.

5 Membrane process for H₂S/CH₄ and CO₂/CH₄ separation

The membrane process is very important in the real deacidification of NG. Bhide and Stern [143] investigated the economic feasibility of asymmetric CA membranes for NG desulfurization, who designed seven different membrane separation processes and examined the effects of various process and economic parameters, including feed composition, flow rate, and pressure, membrane stage cut, pressure ratio, recovery rate, and the influence of membrane material selectivity on economic performance. Bhide and Stern [143] further investigated the economic feasibility of a hybrid process for acid gas separation that combines membrane separation with diethanolamine absorption. They examined the impact of various process and economic parameters on the total processing cost, and thereafter, selected two streams of feed gas at about 35 MMSCFD, the stream A is H₂S free and stream B contains H₂S of 5000 ppm, and the feed pressure was set at 800 psi to mimic the real NG condition. CTA was adopted as a membrane with H₂S/CH₄ and CO₂/CH₄ selectivity of 19 and 21, respectively. The membrane system used is a three-stage membrane system shown as follows (Fig.14).

The result indicated that the total separation cost for the H₂S free feed is the lowest (0.244 $·MMSCFD^–1). If there is H₂S in the feeding component, the hybrid system is the most energy efficient way to deacidification of the NG (0.296 $·MMSCFD^–1). The author also investigated the feed flow rate and pressure on the cost of the acid gas removal from NG [144]. The result indicated that the hybrid system is very sensitive to the flow rate whereas the membrane system is highly dependent on the feed pressure of the system.

Hao et al. [145,146] studied the upgrading low-quality NG with two polymer membranes 6FDA-HAB and PEUU, the PEUU is H₂S selective (H₂S/CH₄ = 75) and the 6FDA-HAB is CO₂-selective (CO₂/CH₄ = 60). The NG component ratios studied is CO₂/CH₄/H₂S (40/10/50) at the feed flow rate of 35 MMSCFD. Seven processes were proposed for this acid gas removal (Fig.15). Under conditions without considering recycle flow, different gas compositions correspond to different process configurations. For instance, under conditions with a low amount of H₂S (≤ 10 mol %) and CO₂, the cost of using a single H₂S membrane is the lowest. In contrast, when CO₂ is present in medium amounts (≤ 40 mol %) whereas H₂S is low (≤ 8 ppm), the cost of a single CO₂ membrane is the lowest. However, when both H₂S (≤ 10 mol %) and CO₂ (≤ 40 mol %) are present in relatively low concentrations, a series of two-stage membranes is most suitable.

The impacts of recycle flow, feed flow rate, pressure variations, and membrane module costs on the processing costs of low-quality NG were also discussed. In the absence of recycle flow, the cost mainly depends on CH₄ loss. With recycle flow, for low acid gas content, the separation cost is influenced by CH₄ loss, whereas for higher acid gas content, the separation cost depends on the cost of gas recycling. The actual acid gas content also depends on the feed composition and operating conditions. Hao et al. [146] summarized the membrane separation costs for different CH₄/CO₂/H₂S ratios, as shown in Fig.15(f). In which, A represents a single-stage CO₂/CH₄ selective membrane. B represents two-stage membrane in series without recycle, first stage H₂S/CH₄ selective membrane and second stage CO₂/CH₄ selective membrane. C represents a single-stage H₂S/CH₄ selective membrane. D shows a two-stage membrane in series with recycle. For treating NG with 0–40 mol % CO₂ and 0–10 mol % H₂S to meet pipeline specifications for both acidic gases, different processes are required.

In 2011, Peters et al. [147] investigated amine absorption and membrane acidic gas removal using Aspen Hysys software and estimated the total capital investment for each system. For amine absorption, with an acid gas recovery rate of 90%, the purity of the acid gas was 98%. In membrane separation, the scheme of the membrane process is shown in Fig.16, the performance of PVAm/PVA membranes was used to simulate the separation of acidic gases. Compared to amine absorption, the CO₂ produced by membrane separation meets the sales gas standards (< 2% CO₂ in the sales gas), but the CO₂ purity is lower than that produced by amine absorption. However, the cost of membrane-based acidic gas separation is lower.

Harasimowicz et al. [8] simulated the enrichment of CH₄ from biogas using a mixture of CH₄, CO₂, and H₂S. In this study, the test was conducted on an A-2 hollow fiber membrane module from UBE Europa GmbH. The feed gas flow rate was 0.2 Nm³·h^–1, feed pressure was 0.60 MPa, retentate side pressure was 0.58 MPa, permeate was at atmospheric pressure, and the QP/QR ratio was 1:1, with temperatures ranging from 10 to 60 °C (Fig.17(a)). The results indicated that using a capillary module with PI membranes could enrich the CH₄ concentration from 55%–85% to 91%–94.4%. The membrane material is resistant to low concentrations of acidic gases and ensures a reduction in H₂S and water vapor concentrations.

Seong et al. [148] reported on the design, construction, and optimization of a pilot-scale 3-stage membrane system operated by a single compressor, which was used for producing high-purity CH₄ and CO₂ from raw biogas. The effects of feed pressure and temperature on the gas separation performance of the developed membrane modules were explored. Additionally, the impact of various operating conditions on the purity and recovery rate of CH₄ was reported. Using simulation programs and gas permeation data, a 3-stage membrane system capable of processing 100 Nm³·h^–1 of raw biogas (65–75 vol % CH₄) was designed and constructed (Fig.17(b)). It was confirmed that the final purity and recovery rates of CH₄ and CO₂ obtained in the constructed 3-stage pilot plant were influenced by the operating conditions. Under optimized operating conditions, the 3-stage membrane system demonstrated excellent biogas separation performance, with high CH₄ purity and good recovery rates (98.4% and 98.7%, respectively), as well as high CO₂ purity and recovery rates (97.2% and 97.0%, respectively). This study demonstrated that a 3-stage pilot-scale system using PSF hollow fiber membranes can produce high-purity CH₄ and CO₂ from low-purity biogas simultaneously.

6 Conclusions and outlook

Membrane-based NG sweetening process has shown a great perspective in both energy saving and footprint reduction. Different membranes can be considered as potential candidates for CO₂ and H₂S removal simultaneously. Conventional polymers such as CTA, PPO, PSF, and PI membranes are excellent candidates due to their high robustness and reliability in real industrial gas separations. However, advanced membranes with higher CO₂ and H₂S permeance and CO₂/CH₄ as well as H₂S/CH₄ selective membrane should be developed.

The rubber polymers exhibited much higher H₂S permeability and H₂S/CH₄ selectivity than the conventional glassy polymers, however, the thin film formation ability of the rubber polymers still needs to be strengthened to get a better permeance for acid gas removal. Meanwhile, the combination of glassy polymers for CO₂ separation while using the rubber polymers for H₂S removal seems one of the best choices for efficiently removing both the CO₂ and H₂S in NS sweetening technique.

The PIM based polymers, especially the AO-PIM-1 has the best CO₂/CH₄ and H₂S/CH₄ separation performances, which is attributed to its highly rigid glassy polymer properties and large microporosity that ensure the high solubility selectivity of H₂S/CH₄, making this kind of polymers ideal for acid gas removal in NG. The CMSMs are also considered as potential for H₂S and CO₂ removal simultaneously. Both high permeability and selectivity as well as high pressure resistance make it a potential good candidate for NS gas purification.

The hollow fiber membrane is the only membrane format that practically used for NS deacidification, however, hollow fiber membrane procedure, excellent membrane materials are still need to be further developed. Additionally, thin-film composite membranes are also in great perspective in future H₂S removal applications. Furthermore, for a good membrane separation, the membrane process also needs to be optimized based on the properties of advanced membrane module.

References

[1] Liu Q , Zhu D , Jin Z , Tian H , Zhou B , Jiang P , Meng Q , Wu X , Xu H , Hu T . . Carbon capture and storage for long-term and safe sealing with constrained natural CO₂ analogs. Renewable & Sustainable Energy Reviews, 2023, 171: 113000

[2] Hanssen S V , Daioglou V , Steinmann Z J N , Doelman J C , Van Vuuren D P , Huijbregts M A J . The climate change mitigation potential of bioenergy with carbon capture and storage. Nature Climate Change, 2020, 10(11): 1023–1029

[3] Li H , Zhao J , Zhang R , Hou B . The natural gas consumption and mortality nexus: a mediation analysis. Energy, 2022, 248: 123577

[4] Hafezi R , Akhavan A , Pakseresht S A , Wood D . Global natural gas demand to 2025: a learning scenario development model. Energy, 2021, 224: 120167

[5] Baker R W , Lokhandwala K . Natural gas processing with membranes: an overview. Industrial & Engineering Chemistry Research, 2008, 47(7): 2109–2121

[6] Ma Y , Guo H , Selyanchyn R , Wang B , Deng L , Dai Z , Jiang X . Hydrogen sulfide removal from natural gas using membrane technology: a review. Journal of Materials Chemistry A: Materials for Energy and Sustainability, 2021, 9(36): 20211–20240

[7] Boschee P . Taking on the technical challenges of sour gas processing. Oil and Gas Facilities, 2014, 3(6): 21–25

[8] Harasimowicz M , Orluk P , Zakrzewska-Trznadel G , Chmielewski A G . Application of polyimide membranes for biogas purification and enrichment. Journal of Hazardous Materials, 2007, 144(3): 698–702

[9] Burgers W F J , Northrop P S , Kheshgi H S , Valencia J A . Worldwide development potential for sour gas. Energy Procedia, 2011, 4: 2178–2184

[10] Aminuddin M S , Bustam M A , Johari K . Latest technological advances and insights into capture and removal of hydrogen sulfide: a critical review. RSC Sustainability, 2024, 2(4): 757–803

[11] Reiffenstein R J , Hulbert W C , Roth S H . Toxicology of hydrogen sulfide. Annual Review of Pharmacology and Toxicology, 1992, 32(1): 109–134

[12] Du Z , Liu C , Zhai J , Guo X , Xiong Y , Su W , He G . A review of hydrogen purification technologies for fuel cell vehicles. Catalysts, 2021, 11(3): 393

[13] Qi M , Liu Y , He T , Yin L , Shu C M , Moon I . System perspective on cleaner technologies for renewable methane production and utilisation towards carbon neutrality: principles, techno-economics, and carbon footprints. Fuel, 2022, 327: 125130

[14] Khan U , Ogbaga C C , Abiodun O A O , Adeleke A A , Ikubanni P P , Okoye P U , Okolie J A . Assessing absorption-based CO₂ capture: research progress and techno-economic assessment overview. Carbon Capture Science & Technology, 2023, 8: 100125

[15] Scholes C A , Stevens G W , Kentish S E . Membrane gas separation applications in natural gas processing. Fuel, 2012, 96: 15–28

[16] Rezakazemi M , Heydari I , Zhang Z . Hybrid systems: combining membrane and absorption technologies leads to more efficient acid gases (CO₂ and H₂S) removal from natural gas. Journal of CO₂ Utilization, 2017, 18: 362–369

[17] Liu C , Greer D W , O’Leary B W . Advanced Materials and Membranes for Gas Separations: UOP Approach. Nanotechnology: Delivering on the Promise, 2016, 2: 119–135

[18] Bernardo P , Tasselli F , Clarizia G . Gas separation hollow fiber membranes: processing conditions for manipulating morphology and performance. Chemical Engineering Transactions, 2013, 32: 1999–2004

[19] AndersonC L. Case Study: Membrane CO₂ Removal From Natural Gas. Regional Symposium on Membrane Science & Technology, 2004.

[20] Alcheikhhamdon Y , Hoorfar M . Natural gas purification from acid gases using membranes: a review of the history, features, techno-commercial challenges, and process intensification of commercial membranes. Chemical Engineering and Processing, 2017, 120: 105–113

[21] Bettenhausen C . Evonik boosts gas separation membrane capacity. Chemical and Engineering News, 2021, 99(33): 14

[22] Hasan R , Scholes C A , Stevens G W , Kentish S E . Effect of hydrocarbons on the separation of carbon dioxide from methane through a polyimide gas separation membrane. Industrial & Engineering Chemistry Research, 2009, 48(11): 5415–5419

[23] Galizia M , Chi W S , Smith Z P , Merkel T C , Baker R W , Freeman B D . 50th anniversary perspective: polymers and mixed matrix membranes for gas and vapor separation: a review and prospective opportunities. Macromolecules, 2017, 50(20): 7809–7843

[24] Brunetti A , Scura F , Barbieri G , Drioli E . Membrane technologies for CO₂ separation. Journal of Membrane Science, 2010, 359(1–2): 115–125

[25] George G , Bhoria N , AlHallaq S , Abdala A , Mittal V . Polymer membranes for acid gas removal from natural gas. Separation and Purification Technology, 2016, 158: 333–356

[26] Chen X , Liu G , Jin W . Natural gas purification by asymmetric membranes: an overview. Green Energy & Environment, 2021, 6(2): 176–192

[27] Sanders D F , Smith Z P , Guo R , Robeson L M , McGrath J E , Paul D R , Freeman B D . Energy-efficient polymeric gas separation membranes for a sustainable future: a review. Polymer, 2013, 54(18): 4729–4761

[28] Sun L , Xu Z , Huang L , Wang H , Zhang H , Li J , Wang Y , Ma X . Significantly enhanced gas separation properties of membranes by debromination and thermal rearrangement simultaneously. Journal of Membrane Science, 2024, 698: 122619

[29] Weng Y , Li N , Xu Z , Huang J , Huang L , Wang H , Li J , Wang Y , Ma X . Super high gas separation performance membranes derived from a brominated alternative PIM by thermal induced crosslinking and carbonization at low temperature. Separation and Purification Technology, 2023, 314: 123548

[30] Robeson L M . Correlation of separation factor versus permeability for polymeric membranes. Journal of Membrane Science, 1991, 62(2): 165–185

[31] BakerR W. Membrane Technology and Applications. 2nd ed. Hoboken: John Wiley & Sons, 2004

[32] Wijmans J G , Baker R W . The solution-diffusion model: a review. Journal of Membrane Science, 1995, 107(1–2): 1–21

[33] Barrer R M . Diffusivities in glassy polymers for the dual mode sorption model. Journal of Membrane Science, 1984, 18: 25–35

[34] CrankJ. The Mathematic of Diffusion. Oxford: Oxford at the Clarendon Press, 1975

[35] YampolskiiYPinnauIFreemanB. Materials Science of Membranes for Gas and Vapor Separation. Hoboken: John Wiley & Sons, 2006

[36] Bondi A . van der Waals volumes and radii. Journal of Physical Chemistry, 1964, 68(3): 441–451

[37] Freeman B D . Basis of permeability/selectivity tradeoff relations in polymeric gas separation membranes. Macromolecules, 1999, 32(2): 375–380

[38] Robeson L M . The upper bound revisited. Journal of Membrane Science, 2008, 320(1–2): 390–400

[39] Comesaña-Gándara B , Chen J , Bezzu C G , Carta M , Rose I , Ferrari M C , Esposito E , Fuoco A , Jansen J C , McKeown N B . Redefining the Robeson upper bounds for CO₂/CH₄ and CO₂/N₂ separations using a series of ultrapermeable benzotriptycene-based polymers of intrinsic microporosity. Energy & Environmental Science, 2019, 12(9): 2733–2740

[40] Yi S , Ghanem B , Liu Y , Pinnau I , Koros W J . Ultraselective glassy polymer membranes with unprecedented performance for energy-efficient sour gas separation. Science Advances, 2019, 5(5): eaaw5459

[41] Hayek A , Shalabi Y A , Alsamah A . Sour mixed-gas upper bounds of glassy polymeric membranes. Separation and Purification Technology, 2021, 277: 119535

[42] Kraftschik B , Koros W J , Johnson J R , Karvan O . Dense film polyimide membranes for aggressive sour gas feed separations. Journal of Membrane Science, 2013, 428: 608–619

[43] Liu Y , Liu Z , Liu G , Qiu W , Bhuwania N , Chinn D , Koros W J . Surprising plasticization benefits in natural gas upgrading using polyimide membranes. Journal of Membrane Science, 2020, 593: 117430

[44] Dong G , Li H , Chen V . Plasticization mechanisms and effects of thermal annealing of Matrimid hollow fiber membranes for CO₂ removal. Journal of Membrane Science, 2011, 369(1–2): 206–220

[45] Bos A , Pünt I G M , Wessling M , Strathmann H . CO₂-induced plasticization phenomena in glassy polymers. Journal of Membrane Science, 1999, 155(1): 67–78

[46] Minelli M , Oradei S , Fiorini M , Sarti G C . CO₂ plasticization effect on glassy polymeric membranes. Polymer, 2019, 163: 29–35

[47] Low B T , Chung T S , Chen H , Jean Y C , Pramoda K P . Tuning the free volume cavities of polyimide membranes via the construction of pseudo-interpenetrating networks for enhanced gas separation performance. Macromolecules, 2009, 42(18): 7042–7054

[48] Kratochvil A M , Koros W J . Decarboxylation-induced cross-linking of a polyimide for enhanced CO₂ plasticization resistance. Macromolecules, 2008, 41(21): 7920–7927

[49] Qiu W L , Chen C C , Xu L R , Cui L L , Paul D R , Koros W J . Sub-T_g cross-linking of a polyimide membrane for enhanced CO₂ plasticization resistance for natural gas separation. Macromolecules, 2011, 44(15): 6046–6056

[50] Rowe B W , Robeson L M , Freeman B D , Paul D R . Influence of temperature on the upper bound: theoretical considerations and comparison with experimental results. Journal of Membrane Science, 2010, 360(1–2): 58–69

[51] OdianG. Principles of Polymerization. 4th ed. Hoboken: John Wiley & Sons, 2004

[52] Klemm D , Heublein B , Fink H P , Bohn A . Cellulose: fascinating biopolymer and sustainable raw material. Angewandte Chemie International Edition, 2005, 44(22): 3358–3393

[53] Bashir Z , Lock S S M , Hira N E , Ilyas S U , Lim L G , Lock I S M , Yiin C L , Darban M A . A review on recent advances of cellulose acetate membranes for gas separation. RSC Advances, 2024, 14(27): 19560–19580

[54] Puleo A C , Paul D R , Kelley S S . The effect of degree of acetylation on gas sorption and transport behavior in cellulose acetate. Journal of Membrane Science, 1989, 47(3): 301–332

[55] Schell W J , Wensley C G , Chen M S K , Venugopal K G , Miller B D , Stuart J A . Recent advances in cellulosic membranes for gas separation and pervaporation. Gas Separation & Purification, 1989, 3(4): 162–169

[56] Chatterjee G , Houde A A , Stern S A . Poly(ether urethane) and poly(ether urethane urea) membranes with high H₂S/CH₄ selectivity. Journal of Membrane Science, 1997, 135(1): 99–106

[57] Achoundong C S K , Bhuwania N , Burgess S K , Karvan O , Johnson J R , Koros W J . Silane modification of cellulose acetate dense films as materials for acid gas removal. Macromolecules, 2013, 46(14): 5584–5594

[58] Peters T A , Ansaloni L , Tena A , Karvan O , Visser T , Chinn D , Bhuwania N . Performance and stability of cellulose triacetate membranes in humid high H₂S natural gas feed streams. Journal of Membrane Science, 2024, 693: 122324

[59] AldhawiZ A. BinSharfan I I, Abdulhamid M A. Chapter 1 Polyimide-based membranes for gas separation applications. In: Polymer Membranes: Increasing Energy Efficiency. Boston: De Gruyter, 2024: 1–44

[60] MaX HYangS Y. Polyimide Gas Separation Membranes. In: Advanced Polyimide Materials. Amsterdam: Elsevier, 2018: 257–322

[61] Wang Y , Alaslai N , Ghanem B , Ma X , Hu X , Balcik M , Liu Q , Abdulhamid M A , Han Y , Eddaoudi M . . Hydroxyl-functionalized polymers of intrinsic microporosity and dual-functionalized blends for high-performance membrane-based gas separations. Advanced Materials, 2024, 36(51): 2406076

[62] Guo W , Sun L , Chu J , Liu L , Li J , Ma X . Huge improved gas separation performance of carbon molecular sieve membrane by forming a double crosslinked polyimide precursor. Journal of Membrane Science, 2024, 711: 123218

[63] Chen J , Cai M , Han Z , Chen Z , Sun L , Liu H , Zhang S , Cui T , Min Y . Ultra-high selectivity carbon molecular sieve membrane derived from PI/SPANI blends for efficient gas separation. Separation and Purification Technology, 2025, 354: 128797

[64] Ye C , Luo C , Ji W , Weng Y , Li J , Yi S , Ma X . Significantly enhanced gas separation properties of microporous membranes by precisely tailoring their ultra-microporosity through bromination/debromination. Chemical Engineering Journal, 2023, 451: 138513

[65] Zhao W , Li K , Ma Y , Gao Y , Zhang J , Zhou L , Wang W , Wang J , Ma Y , Guo M . . Simultaneously enhanced gas separation and anti-aging performance of intrinsic microporous polyimide by dibromo substitution. Journal of Membrane Science, 2023, 687: 122081

[66] Zhao W , Zhang J , Liu C , Ma Y , Li K , Guo M , Jiao L , Ma X , Yang L , Yang S . . Fine-tuning gas separation performance of intrinsic microporous polyimide by the regulation of atomic-level halogen substitution. Journal of Membrane Science, 2024, 692: 122317

[67] Ma X H , Abdulhamid M , Miao X H , Pinnau I . Facile synthesis of a hydroxyl-functionalized Tröger’s base diamine: a new building block for high-performance polyimide gas separation membranes. Macromolecules, 2017, 50(24): 9569–9576

[68] Ma X H , Abdulhamid M A , Pinnau I . Design and synthesis of polyimides based on carbocyclic Pseudo-Tröger’s base-derived dianhydrides for membrane gas separation applications. Macromolecules, 2017, 50(15): 5850–5857

[69] Zhang S , Xu Z , Weng Y , Cai M , Wang Y , Zhu W , Min Y , Ma X . Remarkable gas separation performance of a thermally rearranged membrane derived from an alkynyl self-crosslinkable precursor. Journal of Membrane Science, 2023, 672: 121464

[70] White L S , Blinka T A , Kloczewski H A , Wang I F . Properties of a polyimide gas separation membrane in natural gas streams. Journal of Membrane Science, 1995, 103(1–2): 73–82

[71] Tanaka K , Kita H , Okamoto K , Nakamura A , Kusuki Y . Gas permeability and permselectivity in polyimides based on 3,3′,4,4′-biphenyltetracarboxylic dianhydride. Journal of Membrane Science, 1989, 47(1–2): 203–215

[72] Tanaka K , Okano M , Toshino H , Kita H , Okamoto K I . Effect of methyl substituents on permeability and permselectivity of gases in polyimides prepared from methyl-substituted phenylenediamines. Journal of Polymer Science. Part B, Polymer Physics, 1992, 30(8): 907–914

[73] Vaughn J T , Koros W J , Johnson J R , Karvan O . Effect of thermal annealing on a novel polyamide-imide polymer membrane for aggressive acid gas separations. Journal of Membrane Science, 2012, 401–402: 163–174

[74] Vaughn J T , Koros W J . Analysis of feed stream acid gas concentration effects on the transport properties and separation performance of polymeric membranes for natural gas sweetening: a comparison between a glassy and rubbery polymer. Journal of Membrane Science, 2014, 465: 107–116

[75] Yahaya G O , Qahtani M S , Ammar A Y , Bahamdan A A , Ameen A W , Alhajry R H , Sultan M M B , Hamad F . Aromatic block co-polyimide membranes for sour gas feed separations. Chemical Engineering Journal, 2016, 304: 1020–1030

[76] Yahaya G O , Hayek A , Alsamah A , Shalabi Y A , Ben Sultan M M , Alhajry R H . Copolyimide membranes with improved H₂S/CH₄ selectivity for high-pressure sour mixed-gas separation. Separation and Purification Technology, 2021, 272: 118897

[77] Yahaya G O , Mokhtari I , Alghannam A A , Choi S H , Maab H , Bahamdan A A . Cardo-type random co-polyimide membranes for high pressure pure and mixed sour gas feed separations. Journal of Membrane Science, 2018, 550: 526–535

[78] Alghannam A A , Yahaya G O , Hayek A , Mokhtari I , Saleem Q , Sewdan D A , Bahamdan A A . High pressure pure- and mixed sour gas transport properties of Cardo-type block co-polyimide membranes. Journal of Membrane Science, 2018, 553: 32–42

[79] Hayek A , Yahaya G O , Alsamah A , Panda S K . Fluorinated copolyimide membranes for sour mixed-gas upgrading. Journal of Applied Polymer Science, 2019, 137(5): 48336

[80] Ghanem B S , McKeown N B , Budd P M , Selbie J D , Fritsch D . High-performance membranes from polyimides with intrinsic microporosity. Advanced Materials, 2008, 20(14): 2766–2771

[81] Lee M , Bezzu C G , Carta M , Bernardo P , Clarizia G , Jansen J C , McKeown N B . Enhancing the gas permeability of Tröger’s base derived polyimides of intrinsic microporosity. Macromolecules, 2016, 49(11): 4147–4154

[82] Hayek A , Yahaya G O , Alsamah A , Alghannam A A , Jutaily S A , Mokhtari I . Pure- and sour mixed-gas transport properties of 4,4′-methylenebis(2,6-diethylaniline)-based copolyimide membranes. Polymer, 2019, 166: 184–195

[83] Hayek A , Alsamah A , Qasem E A , Alaslai N , Alhajry R H , Yahaya G O . Effect of pendent bulky groups on pure- and sour mixed-gas permeation properties of triphenylamine-based polyimides. Separation and Purification Technology, 2019, 227: 115713

[84] Hayek A , Alsamah A , Yahaya G O , Qasem E A , Alhajry R H . Post-synthetic modification of CARDO-based materials: application in sour natural gas separation. Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2020, 8(44): 23354–23367

[85] Hayek A , Alsamah A , Saleem Q , Alhajry R H , Alsuwailem A A , Jassim F I . Modified CARDO-based copolyimides with improved sour mixed-gas permeation properties. ACS Applied Polymer Materials, 2022, 4(12): 9257–9271

[86] Liu Z , Liu Y , Qiu W , Koros W J . Molecularly engineered 6FDA-based polyimide membranes for sour natural gas separation. Angewandte Chemie International Edition, 2020, 59(35): 14877–14883

[87] Kraftschik B , Koros W J . Cross-linkable polyimide membranes for improved plasticization resistance and permselectivity in sour gas separations. Macromolecules, 2013, 46(17): 6908–6921

[88] Budd P M , Ghanem B S , Makhseed S , McKeown N B , Msayib K J , Tattershall C E . Polymers of intrinsic microporosity (PIMs): robust, solution-processable, organic nanoporous materials. Chemical Communications, 2004, (2): 230–231

[89] Rose I , Bezzu C G , Carta M , Comesana-Gandara B , Lasseuguette E , Ferrari M C , Bernardo P , Clarizia G , Fuoco A , Jansen J C . . Polymer ultrapermeability from the inefficient packing of 2D chains. Nature Materials, 2017, 16(9): 932–937

[90] Carta M , Malpass-Evans R , Croad M , Rogan Y , Jansen J C , Bernardo P , Bazzarelli F , McKeown N B . An efficient polymer molecular sieve for membrane gas separations. Science, 2013, 339(6117): 303–307

[91] Lai H W H , Benedetti F M , Ahn J M , Robinson A M , Wang Y , Pinnau I , Smith Z P , Xia Y . Hydrocarbon ladder polymers with ultrahigh permselectivity for membrane gas separations. Science, 2022, 375(6587): 1390–1392

[92] Chen L , Han X , Chen G , Sun R , Li W , Lin Z , Pang J , Jiang Z . A structure-property study for the effect of methyl substituents on the gas separation of spirodifluoranthene-based polymers of intrinsic microporosity. Journal of Membrane Science, 2023, 683: 121817

[93] Chen L , Li W , Chen G , Lin Z , Pang J , Jiang Z . In situ hydrolyzed microporous polymer membranes for additional free volume to facilitate CO₂ permeation. Journal of Membrane Science, 2025, 713: 123379

[94] Sun L , Xu W , Zhang H , Chu J , Wang M , Song K , Wu W , Li J , Wang Y , Wang Y . . In-situ formation of three-dimensional network intrinsic microporous ladder polymer membranes with ultra-high gas separation performance and anti-trade-off effect. Angewandte Chemie International Edition, 2025, 64: e202420742

[95] Yu C , Wang Y , Xia Y , Luo S , Ma X , Yin B H , Wang X . Polymers of intrinsic microporosity with internal dihedral lock for efficient gas separation. Advanced Membranes, 2024, 4: 100097

[96] MasudaTIsobeEHamanoT. Synthesis of poly[1-(trimethylsilyl)-1-propyne] with extremely high molecular weight by using tantalum pentachloride-triphenylbismuth (1:1) catalyst. Macromolecules. 1986, 19(9): 2448–2450.

[97] Merkel T . Mixed-gas permeation of syngas components in poly(dimethylsiloxane) and poly(1-trimethylsilyl-1-propyne) at elevated temperatures. Journal of Membrane Science, 2001, 191(1–2): 85–94

[98] Malykh O V , Golub A Y , Teplyakov V V . Polymeric membrane materials: new aspects of empirical approaches to prediction of gas permeability parameters in relation to permanent gases, linear lower hydrocarbons and some toxic gases. Advances in Colloid and Interface Science, 2011, 164(1–2): 89–99

[99] Yi S L , Ma X H , Pinnau I , Koros W J . A high-performance hydroxyl-functionalized polymer of intrinsic microporosity for an environmentally attractive membrane-based approach to decontamination of sour natural gas. Journal of Materials Chemistry A: Materials for Energy and Sustainability, 2015, 3(45): 22794–22806

[100] Mizrahi Rodriguez K , Dean P A , Guo S , Roy N , Swager T M , Smith Z P . Elucidating the role of micropore-generating backbone motifs and amine functionality on membrane separation performance in complex mixtures. Journal of Membrane Science, 2024, 696: 122464

[101] Dean P A , Mizrahi Rodriguez K , Guo S , Roy N , Swager T M , Smith Z P . Elucidating the role of micropore-generating backbone motifs and amine functionality on H₂S, CO₂, CH₄, and N₂ sorption. Journal of Membrane Science, 2024, 696: 122465

[102] Dean P A , Wu Y , Guo S , Swager T M , Smith Z P . Tertiary-amine-functional poly(arylene ether)s for acid-gas separations. JACS Au, 2024, 4(10): 3848–3856

[103] Park H B , Jung C H , Lee Y M , Hill A J , Pas S J , Mudie S T , Van Wagner E , Freeman B D , Cookson D J . Polymers with cavities tuned for fast selective transport of small molecules and ions. Science, 2007, 318(5848): 254–258

[104] Yerzhankyzy A , Ghanem B S , Wang Y , Alaslai N , Pinnau I . Gas separation performance and mechanical properties of thermally-rearranged polybenzoxazoles derived from an intrinsically microporous dihydroxyl-functionalized triptycene diamine-based polyimide. Journal of Membrane Science, 2020, 595: 117512

[105] Zhuang Y B , Seong J G , Lee W H , Do Y S , Lee M J , Wang G , Guiver M D , Lee Y M . Mechanically tough, thermally rearranged (TR) random/block poly(benzoxazole-co-imide) gas separation membranes. Macromolecules, 2015, 48(15): 5286–5299

[106] Calle M , Jo H J , Doherty C M , Hill A J , Lee Y M . Cross-linked thermally rearranged poly(benzoxazole-co-imide) membranes prepared fromortho-hydroxycopolyimides containing pendant carboxyl groups and gas separation properties. Macromolecules, 2015, 48(8): 2603–2613

[107] Cai M , Chen J , Liu H , Sun L , Wu J , Cui T , Zhang S , Ma X , Min Y . Unveiling the mystery: how TR precursors lead to exceptional gas separation performance in CMSMs. Journal of Membrane Science, 2025, 713: 123287

[108] Cai M , Chen J , Liu H , Sun L , Wu J , Han Z , Chen Z , Cui T , Zhang S , Ma X . . Remarkably enhanced molecular sieving effect of carbon molecular sieve membrane by enhancing the concentration of thermally rearranged precursors. Separation and Purification Technology, 2024, 341: 126945

[109] Cai M , Liu H , Chen J , Sun L , Wu J , Chen Z , Han Z , Cui T , Zhang S , Min Y . . Breaking the permeability-selectivity trade-off: advanced carbon molecular sieve membranes derived from thermally rearranged mixed-matrix membrane precursors. Separation and Purification Technology, 2024, 335: 126163

[110] Sun L , Chu J , Zuo H , Wang M , Wu C , Riaz A , Liu L , Guo W , Li J , Ma X . The influence of debromination and TR on the microstructure and properties of CMSMs. Separation and Purification Technology, 2025, 352: 128167

[111] Scholes C A , Dong G , Kim J S , Jo H J , Lee J , Lee Y M . Permeation and separation of SO₂, H₂S and CO₂ through thermally rearranged (TR) polymeric membranes. Separation and Purification Technology, 2017, 179: 449–454

[112] Ghasemnejad-Afshar E , Amjad-Iranagh S , Zarif M , Modarress H . Effect of side branch on gas separation performance of triptycene based PIM membrane: a molecular simulation study. Polymer Testing, 2020, 83: 106339

[113] Hayek A , Alsamah A , Alaslai N , Maab H , Qasem E A , Alhajry R H , Alyami N M . Unprecedented sour mixed-gas permeation properties of fluorinated polyazole-based membranes. ACS Applied Polymer Materials, 2020, 2(6): 2199–2210

[114] Lawrence J A III , Harrigan D J , Maroon C R , Sharber S A , Long B K , Sundell B J . Promoting acid gas separations via strategic alkoxysilyl substitution of vinyl-added poly(norbornene)s. Journal of Membrane Science, 2020, 616: 118569

[115] Merkel T C , Toy L G . Comparison of hydrogen sulfide transport properties in fluorinated and nonfluorinated polymers. Macromolecules, 2006, 39(22): 7591–7600

[116] Signorini V , Giacinti Baschetti M , Pizzi D , Merlo L . Permeation of ternary mixture containing H₂S, CO₂ and CH₄ in Aquivion® perfluorosulfonic acid (PFSA) ionomer membranes. Membranes, 2022, 12(11): 1034

[117] Sadeghi M , Talakesh M M , Arabi Shamsabadi A , Soroush M . Novel application of a polyurethane membrane for efficient separation of hydrogen sulfide from binary and ternary gas mixtures. ChemistrySelect, 2018, 3(11): 3302–3308

[118] Mohammadi T , Moghadam M T , Saeidi M , Mahdyarfar M . Acid gas permeation behavior through poly(ester urethane urea) membrane. Industrial & Engineering Chemistry Research, 2008, 47(19): 7361–7367

[119] Harrigan D J , Lawrence J A III , Reid H W , Rivers J B , O’Brien J T , Sharber S A , Sundell B J . Tunable sour gas separations: simultaneous H₂S and CO₂ removal from natural gas via crosslinked telechelic poly(ethylene glycol) membranes. Journal of Membrane Science, 2020, 602: 117947

[120] Harrigan D J , Yang J , Sundell B J , Lawrence J A III , O’Brien J T , Ostraat M L . Sour gas transport in poly(ether-b-amide) membranes for natural gas separations. Journal of Membrane Science, 2020, 595: 117497

[121] Wong D A , Haddad E E , Lin S , Sharber S A , Yang J , Lawrence J A III , Harrigan D J , Wright P T , Liu Y , Sundell B J . Rational design of melamine-crosslinked poly(ethylene glycol) membranes for sour gas purification. Journal of Membrane Science, 2024, 709: 123082

[122] Zhang P , Ma X , Tu Z , Zhang X , Hu X , Wu Y . Constructing ether-rich and carboxylate hydrogen bonding sites in protic ionic liquids for efficient and simultaneous membrane separation of H₂S and CO₂ from CH₄. Green Energy & Environment, 2024, 10(3): 560–572

[123] Zhang C , Koros W J . Ultraselective carbon molecular sieve membranes with tailored synergistic sorption selective properties. Advanced Materials, 2017, 29(33): 1701631

[124] Sanyal O , Hays S S , Leon N E , Guta Y A , Itta A K , Lively R P , Koros W J . A Self-consistent model for sorption and transport in polyimide-derived carbon molecular sieve gas separation membranes. Angewandte Chemie International Edition, 2020, 59(46): 20343–20347

[125] Hazazi K , Wang Y , Ghanem B , Hu X , Puspasari T , Chen C , Han Y , Pinnau I . Precise molecular sieving of ethylene from ethane using triptycene-derived submicroporous carbon membranes. Nature Materials, 2023, 22(10): 1218–1226

[126] Shiflett M B , Foley H C . Ultrasonic deposition of high-selectivity nanoporous carbon membranes. Science, 1999, 285(5435): 1902–1905

[127] Hu L , Bui V T , Krishnamurthy A , Fan S , Guo W , Pal S , Chen X , Zhang G , Ding Y , Singh R P . . Tailoring sub-3.3 Å ultramicropores in advanced carbon molecular sieve membranes for blue hydrogen production. Science Advances, 2022, 8(10): eabl8160

[128] Enrico D, Giuseppe B, Enrico D, Giuseppe B. Membrane Engineering for the Treatment of Gases: Gas-separation Problems with Membranes. United Kingdom: The Royal Society of Chemistry, 2011.

[129] WuZ. Membrane Separation Principles and Applications. Amsterdam: Elsevier, 2019

[130] Tronci G , Raffone F , Cicero G . Theoretical study of nanoporous graphene membranes for natural gas purification. Applied Sciences, 2018, 8(9): 1547

[131] Haider S , Lindbråthen A , Lie J A , Hägg M B . Regenerated cellulose based carbon membranes for CO₂ separation: durability and aging under miscellaneous environments. Journal of Industrial and Engineering Chemistry, 2019, 70: 363–371

[132] Li G , Kujawski W , Válek R , Koter S . A review —the development of hollow fibre membranes for gas separation processes. International Journal of Greenhouse Gas Control, 2021, 104: 103195

[133] Morisato A , Mahley E . Hydrogen sulfide permeation and hydrocarbon separation properties in cellulose triacetate hollow fiber membrane for high hydrogen sulfide contained natural gas sweetening applications. Journal of Membrane Science, 2023, 681: 121734

[134] Lu H T , Liu L , Kanehashi S , Scholes C A , Kentish S E . The impact of toluene and xylene on the performance of cellulose triacetate membranes for natural gas sweetening. Journal of Membrane Science, 2018, 555: 362–368

[135] Niknejad S M S , Savoji H , Pourafshari Chenar M , Soltanieh M . Separation of H₂S from CH₄ by polymeric membranes at different H₂S concentrations. International Journal of Environmental Science and Technology, 2017, 14(2): 375–384

[136] Chenar M P , Savoji H , Soltanieh M , Matsuura T , Tabe S . Removal of hydrogen sulfide from methane using commercial polyphenylene oxide and Cardo-type polyimide hollow fiber membranes. Korean Journal of Chemical Engineering, 2011, 28(3): 902–913

[137] Saedi S , Madaeni S S , Shamsabadi A A . PDMS coated asymmetric PES membrane for natural gas sweetening: effect of preparation and operating parameters on performance. Canadian Journal of Chemical Engineering, 2014, 92(5): 892–904

[138] Liu Z , Liu Y , Liu G , Qiu W , Koros W J . Cross-linkable semi-rigid 6FDA-based polyimide hollow fiber membranes for sour natural gas purification. Industrial & Engineering Chemistry Research, 2020, 59(12): 5333–5339

[139] Liu Y , Liu Z , Kraftschik B E , Babu V P , Bhuwania N , Chinn D , Koros W J . Natural gas sweetening using TEGMC polyimide hollow fiber membranes. Journal of Membrane Science, 2021, 632: 119361

[140] Shalabi Y A , Yahaya G O , Choi S H , Alsamah A , Hayek A . Copolyimide asymmetric hollow fiber membranes for high‐pressure natural gas purification. Journal of Applied Polymer Science, 2023, 140(21): e53866

[141] Yahaya G O , Choi S H , Sultan M M B , Hayek A . Development of thin-film composite membranes from aromatic Cardo-type co-polyimide for mixed and sour gas separations from natural gas. Global Challenges, 2020, 4(7): 1900107

[142] Liu Y , Liu Z , Morisato A , Bhuwania N , Chinn D , Koros W J . Natural gas sweetening using a cellulose triacetate hollow fiber membrane illustrating controlled plasticization benefits. Journal of Membrane Science, 2020, 601: 117910

[143] Bhide B , Stern S A . Membrane processes for the removal of acid gases from natural gas. I. Process configurations and optimization of operating conditions. Journal of Membrane Science, 1993, 81(3): 209–237

[144] Bhide B , Voskericyan A , Stern S . Hybrid processes for the removal of acid gases from natural gas. Journal of Membrane Science, 1998, 140(1): 27–49

[145] Hao J , Rice P , Stern S . Upgrading low-quality natural gas with H₂S-and CO₂-selective polymer membranes: Part I. Process design and economics of membrane stages without recycle streams. Journal of Membrane Science, 2002, 209(1): 177–206

[146] Hao J , Rice P , Stern S . Upgrading low-quality natural gas with H₂S-and CO₂-selective polymer membranes: Part II. Process design, economics, and sensitivity study of membrane stages with recycle streams. Journal of Membrane Science, 2008, 320(1–2): 108–122

[147] Peters L , Hussain A , Follmann M , Melin T , Hägg M B . CO₂ removal from natural gas by employing amine absorption and membrane technology: a technical and economical analysis. Chemical Engineering Journal, 2011, 172(2–3): 952–960

[148] Seong M S , Kong C I , Park B R , Lee Y , Na B K , Kim J H . Optimization of pilot-scale 3-stage membrane process using asymmetric polysulfone hollow fiber membranes for production of high-purity CH₄ and CO₂ from crude biogas. Chemical Engineering Journal, 2020, 384: 123342

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