Polymeric composite membranes in carbon dioxide capture process: a review

Abstract

Carbon dioxide (CO₂) emission to the atmosphere is the prime cause of certain environmental issues like global warming and climate change, in the present day scenario. Capturing CO₂ from various stationary industrial emission sources is one of the initial steps to control the aforementioned problems. For this concern, a variety of resources, such as liquid absorbents, solid adsorbents, and membranes, have been utilized for CO₂ capturing from various emission sources. Focused on membrane-based CO₂ capture, polymeric membranes with composite structure (polymeric composite membrane) offer a better performance in CO₂ capturing process than other membranes, due to the composite structure it offers higher gas flux and less material usage, thus facile to use high performed expensive material for membrane fabrication and achieved good efficacy in CO₂ capture. This compressive review delivers the utilization of different polymeric composite membranes in CO₂ capturing applications. Further, the types of polymeric materials used and the different physicochemical modifications of those membrane materials and their CO₂ capturing ability are briefly discussed in the text. In conclusion, the current status and possible perspective ways to improve the CO₂ capture process in industrial CO₂ gas separation applications are described in this review.

Introduction

Peoples are concerned about environmental and ecosystem protection to identify the possible solution to mitigate certain environmental issues like global warming and climate change, which is mainly triggered by the emission of greenhouse gases into the atmosphere (Yamasaki 2003). Carbon dioxide is a primary anthropogenic greenhouse gas for the aforementioned problems and the maximum amount persistent constituent among the greenhouse gases in the atmosphere (Yang et al. 2008). Nearly 60% and above the global warming effect is attributed to anthropogenic CO₂ emissions (Yamasaki 2003). According to the International Panel on Climate Change Sixth Assessment Report (IPCC AR6) statement, in the year 2100, the amount of atmospheric CO₂ may be reached up to 600 ppm and the effects increase the mean global temperature around 1.9 °C excess than normal and the causes increase the mean sea level about 3.5 m IPCC (2021). Hence, the IPCC stated, the global greenhouse gas (GHG) emissions must be reduced up to 50 to 80% by the year 2050; this is to be a possible solution to avoid dramatic consequences of global warming  IPCC (2021).

The exhaust gas coming from the combustion of fossil fuels and various industrial processes (thermal power plant, steel, cement, and chemical) is the major contributor source of CO₂ emission to the atmosphere (Merkel et al. 2010). In worldwide energy production, fossil fuels contribute more than 75% and above (Songolzadeh et al. 2014). The use of alternate energy sources, such as non-fossil fuels (renewable energy such as solar energy, wind energy) or gaseous fuels (e.g., biogas, syngas, and landfill gas) rather than fossil fuel, is the solution to minimize the CO₂ emission to the atmosphere (Kapoor et al. 2019). The former cases (non-fossil fuel) cannot meet the required energy fed by fossil fuels (about 1000 terawatt-hours (TWh)) World Energy Outlook (2021). In the latter case, the problem twisted by the presence of CO₂ in gaseous fuels reduces the calorific value and fuel efficiency; further, its acidic nature makes corrosion to the pipeline during transportation (Scholes et al. 2012). Therefore, considering the energy and environmental aspect, capturing CO₂ from emission sources (flue gases) and gaseous fuel is the solution to tackle the aforementioned problems (Yamasaki 2003; Zhang et al. 2013). In literature, the capture of CO₂ from various gas mixtures furnished by various technologies, such as amine absorption, solid adsorption, chemical looping, cryogenic, membranes, gas hydration, and chemical oxidation (Olajire 2010; Yeo et al. 2012; Thomas et al. 2016; Kapoor et al. 2019). Amongst membrane gas separation processes much attractive to the scientific communities in terms of succeeding advantages such as eco-friendliness, minimum energy consumption, operation compact, lesser capital cost, unit size compact, and energy efficiency (Brunetti et al. 2010; Scholes et al. 2012; Yeo et al. 2012).

In the past few decades, the membrane-based CO₂ gas separation has proven there substantial grownup, breakthroughs, and benefits over the other technology used in this field (Ramasubramanian and Ho 2011). There is a lot of variety of membranes (polymeric, inorganic and mixed matrix (MMMs)) identified and verified their effectiveness in membrane CO₂ gas separation processes in both academia and industrial point of view (Cong et al. 2007; Scholes et al. 2008; Yampolskii 2012; Baker and Low 2014; Rezakazemi et al. 2015). Among these, polymeric membranes with composite structure (polymeric composite membrane) substantially stand by in commercial purposes, due to their structural configuration making a lot of welfares, that is higher gas flux, more mechanical stability, and easy to form different membrane modules. Further, the independent collection of support and selective layer material facile to use variety of polymer material for membrane making as well as lower polymer material consumption facile to highly performed more expensive polymer material usage for the membrane making (Scholes et al. 2008; Yampolskii 2012; Dai et al. 2016). Over the long period, polymeric membranes with composite structure are extensively grownup for membrane CO₂ gas separation application, because of their process ability, material availability, flexibility to forming any modules (flat sheet (F), tubular (T), hollow fiber (HF), spiral wound (SW)), reproducibility, and inexpensive processing ability (Sridhar et al. 2007a; Yampolskii 2012).

Up to now a large number of academic reports are found from the field of polymeric composite membranes in CO₂ gas separation, while combined reviews are not yet found. From this concern, a comprehensive literature summary on various types of polymeric composite membranes, such as rubbery, glassy, co-polymers, facilitated and hydrogel polymers, and composite membranes in different CO₂ gas separation applications (e.g., CO₂/CH₄, CO₂/N₂, and CO₂/H₂), are discussed in this review. Further, the advancement of these composite membranes to overcome the negativity that persisted in the gas permeation process is also discussed. Finally, this review summarized the current perspectives and future direction in the field of the polymeric composite membrane in CO₂ gas separation.

Progress of polymeric composite membrane in carbon dioxide capture

Typically, the membrane gas separation application permeability (P) and selectivity (α) are the first two main factors that describe the performance of a membrane, and they are directly linked with the application point of view in which the productivity and purity of gas component (Amooghin et al. 2017). Polymeric membranes have some intrinsic trade-off relation between permeability and selectivity (i.e., selectivity increases, permeability decreases, and vice versa) in gas separation application (Freeman 1999). Lloyd M. Robeson has suggested that the trade-off between permeability and selectivity possesses a superior limit that provides an example of an upper bound diagram (Robeson 2008). Most of the polymeric membranes reported in the literature are occupied the place below this upper bound (Orme et al. 2001; Bum et al. 2010; Tomé et al. 2015; Mannan et al. 2016; Scofield et al. 2016; Nikolaeva et al. 2018; Zhuang et al. 2018; Sasikumar et al. 2021). If the polymeric membranes may be to overcome this upper bound, they have higher permeability with the same selectivity or same permeability with higher selectivity (Dilshad et al. 2017). This can be achieved by changing their material properties or reducing the barrier thickness or else combined to both (Rafiq et al. 2016). Then the researchers discovered that the membrane of composite form with functionalization of polymeric materials may fulfill the aforementioned criteria (Dai et al. 2016; Alqaheem et al. 2017; Maheswari and Palanivelu 2017; Bei et al. 2021). Over the decades, a lot of polymeric composite membranes have been produced with different polymeric materials (cellulose acetate (CA) and its derivatives (Shieh and Chung 2000; Mao et al. 2011; Ahmad et al. 2014; Shankar and Kandasamy 2019), polysulfones (PSf) (Scholes et al. 2010a; Rafiq et al. 2012), polycarbonates (PC) (Hellums et al. 1989; Iqbal et al. 2008), polyethylene oxides (PEO) (Suzuki et al. 1998; Wang et al. 2014; Lee and Kang 2019), perfluoropolymers (PFP)(Jansen et al. 2006; Scholes et al. 2015; Fang et al. 2016), natural or synthetic rubbers (Zhuang et al. 2018; Tseng et al. 2019), polypropylene (PP) (Teramae and Kumazawa 2007; Blinova et al. 2012), polyamines (PA) (Matsuyama et al. 1996; Kim et al. 2004; Shen et al. 2015; Chen et al. 2016), polyimides (PI) (Marek et al. 1996; Hillock and Koros 2007; Harra et al. 2019), co-polymers (Sridhar et al. 2007c; Ji et al. 2009; Chen et al. 2015), etc.) by various researchers’ extensively in CO₂ gas separation application and they are observed remarkable results in lab scale and industrial scale (Alqaheem et al. 2017; Brinkmann et al. 2017).

Generally, in membrane CO₂ gas separation, polymeric composite membranes are categorized by two types based on their gas transport mechanism (Wang et al. 2016). They are as follows:

• i. Solution-diffusion polymeric composite membranes

• ii. Facilitated transport polymeric composite membranes

Further, they are sub-divided by various types based on their nature of material usage in selective layer formation (Scholes et al. 2008; Du et al. 2012; Mannan et al. 2013). The schematic view of various polymeric composite membranes in CO₂ gas separation is shown in Fig. 1. The detailed progress of different polymeric composite membranes is discussed in a further section.

Fig. 1

Classification of the polymeric composite membrane in CO₂ gas separation application

Solution-diffusion polymeric composite membranes

The solution-diffusion polymeric composite membranes is well documented in gas separation application over the long period (Wang et al. 2016). In solution-diffusion polymeric composite membranes, the gas transportation is subject to solubility (S) and diffusivity (D) (i.e., P = DS) of gas molecules through the polymer selective layer, whereas solubility and diffusivity of gas molecules depend on the internal characteristic (chemical functionality) and nature (free volume, elasticity, etc.) of polymer selective layer materials (Freeman 1999; Rafiq et al. 2016). Based on the nature of polymeric material used in selective layer formation, the solution-diffusion polymeric composite membranes can be categorized into three types, i.e., rubbery polymeric composite membrane, glassy polymeric composite membranes, and co-polymeric composite membrane (Mannan et al. 2013). All these three different composite membranes experienced different CO₂ gas separation efficiency. The gas separation performances of rubbery polymers more predominate on solubility bases, while diffusion bases in glassy polymers and both together in co-polymers (Wang et al. 2016). Generally, rubbery polymers have high permeability and low selectivity, while vice versa for glassy polymers and moderate for co-polymer (Sainan et al. 2013; Chen et al. 2015). However, this inherent trade-off performance of solution-diffusion polymeric composite membrane could not meet with effective CO₂ gas separation. Further, the trade-off relation to solution-diffusion polymeric composite membrane overcome by the physicochemical modification of this polymeric material with different approaches, such as cross-linking (Liu et al. 2003; Hillock and Koros 2007; Chen et al. 2014), blending (Hosseinia et al. 2010; Zhu et al. 2015; Kim et al. 2016a, b, c), or doping (Reid et al. 2002; Xomeritakis et al. 2009; Lin et al. 2015) of other polymer and inorganic particles, surface functionalization (Xu and Coleman 1999; Kim et al. 2016a, b, c), etc. Some studies experienced enhanced results on that process (Fu et al. 2014; Halim et al. 2014; Xin et al. 2015; Nikolaeva et al. 2018). The following section of this review summarized the research progress of rubber, glassy, and co-polymeric composite membranes for CO₂ gas separation.

The rubbery polymeric composite membrane in CO₂ gas separation

Poly(dimethylsiloxane) (PDMS) is the foremost used rubbery polymeric material for gas separation application, because of its good physicochemical characteristic, i.e., mechanical and thermal stability, less aging effect, low cost, and ease of processing (Strathman et al. 1986; Wu et al. 2006; Saedi et al. 2014; Kim et al. 2016a, b, c). In 1986 Strathman and co-workers assessed the first PDMS-based polymeric composite membrane in gas separation application (Strathman et al. 1986). Later several studies (Table 1) produced the various polymer and inorganic materials supported (cellulose acetate (Wu et al. 2006), polysulfone (Li et al. 2013b; Pakizeh et al. 2013), polyvinylidene fluoride (PVDF) (Choi et al. 2010), polytetrafluoroethylene (PTFE) (Jia et al. 1991), polyimide (PI) (Qin et al. 2005), polyethersulfone (PES) (Sadrzadeh et al. 2009; Madaeni et al. 2013; Saedi et al. 2014), polyacrylonitrile (PAN) (Duval et al. 1993; Achalpurkar et al. 2007; Zhao et al. 2015a; Kim et al. 2016a, b, c), porous SS metal (PSSM) (Scholes et al. 2010b), Al₂O₃ ceramic (Sainan et al. 2013), and silica nitrate (Si₃N₄) (Firpo et al. 2015)) and PDMS composite membranes and examined their CO₂ gas separation efficiency in various gas mixture in various aspects.

Table 1 Rubbery polymeric composite membranes for CO₂ capture

For example, Merkel et al. (2000) and Peter and Peinemann (2009) reported the detailed gas transport properties of various gas mixtures in PDMS/NMS-based composite membranes. Similarly, Firpo et al. (2015) reported the thickness dependence gas permeability variability of PDMS/Si₃N₄-based membrane. Madaeni et al. (2013) studied the effect of the coating method on the gas separation performance of the PDMS/PES composite membrane.

From reported literature (Table 1), it is clear that the composite membrane made with rubbery polymers exhibited the higher gas permeability and lower gas pair selectivity in most cases (Fritsch et al. 1993; Achalpurkar et al. 2007; Sainan et al. 2013). Generally, rubbery polymers possess high chain mobility and poor size sieving ability, thus indicating the overall selectivity based on the solubility of gas molecules. Though the gas solubility should not be altered significantly, the diffusivity of gas molecules is controlled by modifying the polymer architecture with help of physio-chemical treatment. Some studies experienced the enhancement of gas pair selectivity in rubbery polymeric composite membranes after some physicochemical treatment such as cross-linking (Qin et al. 2005; Pakizeh et al. 2013), surface functionalization (Oh and Zurawsky 1996; Achalpurkar et al. 2007; Hong et al. 2017), blending of polymer (Scofield et al. 2015), and inorganic particles (Duval et al. 1993; Hussain and König 2012; Sun et al. 2016). For example, Fritsch et al. (1993) examined the non-silicon material grafting enhancing the gas permeation properties (gas pair (CO₂/N₂) selectivity) of silicon rubbery membrane. Similarly, surface modification of PDMS membrane with plasma polymers increases the performance of bare membrane which was examined by Oh and Zurawsky (1996). In other work, Sainan et al. (2013) and Pakizeh et al. (2013) reported that the cross-linking of PDMS membrane increased the gas pair (CO₂/N₂ and CO₂/H₂) selectivity with respect to cross-linking content. Similarly, Scofield et al. (2015) and Hong et al. (2017) reported that the –NH₂ group functionalization and blending or grafting of PEG polymer into PDMS membranes increase the gas pair (CO₂/N₂) selectivity from 9.8 (unmodified PDMS membranes) to 22 and 19, respectively.

On the other hand, the researchers observed overall gas separation performance of these modified membranes exceeds the Robeson trade line (Wang et al. 2016). Further, some studies explore the incorporation of the various inorganic particles (Zeolite 4A (Alavi et al. 2017), SiO₂ (Fritsch et al. 1993), ZIF-5 nanoparticles (Hussain and König 2012), GO (Alavi et al. 2017), and carbon particles (Duval et al. 1993)) into rubbery PDMS polymer matrix which enhanced the gas separation performance in terms of permeability or selectivity. Some other works (Jia et al. 1991; Saedi et al. 2014; Kim et al. 2016a, b, c) also found a similar phenomenon in the rubbery polymeric composite membrane by chemical modification. Even though the physicochemical modifications improved the gas separation performance (selectivity) of rubbery polymeric composite membranes, still this improvement is not met for the required effective CO₂ gas separation membrane criteria (> 100 selectivity) (Merkel et al. 2010). Further, the idea of the researcher to use this rubbery polymer materials as a gutter layer/productive layer formation in composite membranes production in CO₂ gas separation and many studies obtained an encouraging result (both membrane formation and gas separation performance) (Li et al. 2013a; Fu et al. 2014; Kim et al. 2016a, b, c; Scofield et al. 2016; Selyanchyn et al. 2018).

Glassy polymeric composite membrane in CO₂ gas separation

In industrial CO₂ gas separation, glassy polymers are step forwarded than rubbery polymers, because they have a good balancing between permeability and selectivity along with good mechanical stability (Kim and Lee 2013; Adewole and Sultan 2019). A number of glassy polymeric composite membranes and their CO₂ gas separation performance are compiled in Table 2. In listed most of the glassy polymeric composite membranes display in well CO₂ permeability and CO₂/light gas selectivity. From the beginning, the common glassy polymeric materials of cellulose acetates (Lu et al. 2016), cellulose nitrates (CN) (Shieh and Chung 2000), polysulfones (Pakizeh et al. 2013; Mannan et al. 2016), polyamides (Petersen and Peinemann 1997), polyimides (Park et al. 2003; Weigelt et al. 2019), polycarbonates (Hellums et al. 1989), polyacetylenes (PAc)(Nakagawa et al. 1988), etc. are utilized as polymer selective layer material in glassy polymeric solution-diffusion membranes. The advancement in this field leads to different kinds of glassy polymeric materials i.e., some porous organic polymers (Kim and Lee 2013), such as thermally rearranged (TR) polymers (Berchtold et al. 2012; Kim and Lee 2014; Brunetti et al. 2017; Kim et al. 2018; Lee et al. 2020), polymers of intrinsic microporosity (PIMs) (Lasseuguette et al. 2014; Sekizkardes et al. 2016, 2018), porous organic framework (hyperbranched porous polymer (HBPPs) (Taniguchi et al. 2017) (Peter et al. 2009), metal–organic frameworks (MOFs) (Sabouni et al. 2014; Fu et al. 2016; Kim et al. 2016a; Mohamed et al. 2020), and conjugated microporous polymers (CMPs) (Lindemann et al. 2014), was utilized as polymer selective layer material in CO₂ gas separation which obtained amazing results, i.e., gas separation performance of these membrane materials goes to beyond the Robeson upper bound. Further, the continuity of this field researcher demonstrated the consequence of gas separation efficiency of the glassy polymeric membrane with the preparation method. Numerous studies (Lundy and Cabasso 1989; Chern and Wu 1997; Petersen and Peinemann 1997; Zhao et al. 2006; Teramae and Kumazawa 2007; Li et al. 2013a) explore the gas separation efficiency of glassy polymeric composite membranes improved by using different preparation methods (spin coating (SP), interfacial polymerization (IP), plasma polymerization (PP), lamination (L), co-extrusion (Co-ex), etc.). For example, Chern and Wu (1997) identified the composite membranes (polyamide membranes) with interfacial polymerization process display a better gas separation performance than some other methods (solution coating (SC) or film casting (FC), pouring (P)), because of the composite membrane produced from interfacial polymerization process has an ultra-thin selective layer thickness range from 1 µm to few nano-meter scales. Moreover, the reaction condition and reagent concentration provided the well-defined structural backbone on the polymer selective layer, which ensures the maximum gas transportation and better gas separation efficiency in membrane fabricated at IP method. Similarly, Teramae and Kumazawa (2007) and Lundy and Cabasso (1989) demonstrated that the higher gas separation efficiency contains an advanced ultra-thin (less than a 100 nm) polymer selective layer developed by plasma polymerization and lamination process.

Table 2 Glassy polymeric composite membranes for CO₂ capture

Though the glassy polymeric materials step forward to better CO₂ gas separation, the problem arises in form of CO₂ plasticization and quick physical aging during the gas transportation (Zhou et al. 2003). There are several ways to suppress the CO₂ plasticization and aging of glassy polymeric membranes like thermal treatment (Scholes et al. 2010a; Hernández-Martínez et al. 2018), cross-linking (Liu et al. 2003; Zhou et al. 2003; Hillock and Koros 2007), introducing additional functional groups, and blending of other polymers materials (Rhim et al. 2000; Hamad et al. 2001; Reid et al. 2002; Kapantaidakis et al. 2003; Hosseinia et al. 2010; Fernández-barquín et al. 2016; Kim et al. 2016a, b, c; Bei et al. 2021). In the meantime, they also considered a solution for improving the membrane CO₂ gas separation performance. For example, Scholes et al. (2010a) study the CO₂ plasticization effect of PSf/PTFE composite membrane at four different temperatures. It is observed that the plasticization potential of CO₂ decreases with increase temperature. Hillock and Koros (2007) developed the chemically cross-linked polyimide membrane for CO₂/CH₄ gas separation. The cross-linked membrane exhibited greater plasticization resistance against CO₂; moreover, they have shown significantly higher gas separation efficiency than the un-cross-linked membrane.

Likewise, Hosseinia et al. (2010) observed that the blending of poly(benzimidazole) (PBI) into polyimide composite membrane exhibited good resistance toward CO₂-induced plasticization and physical aging. Another studies with surface functionalization of sulfonated polyphenylene oxide (S-PPO) composite membranes with different metal atoms (monovalent (Li⁺, Na⁺, K⁺), divalent (Mg²⁺, Ba²⁺, Ca²⁺), and trivalent (Al³⁺)) proved that the metal atom surface modification enhanced the gas separation performance and stability (Rhim et al. 2000) and (Hamad et al. 2001). Similarly, Reid et al. (2002) reported the poly(3-(2-acetoxyethyl)thiophene) (P3AcET)-based composite membrane enriched the gas separation performance and stability on surface modification with acid and base catalytic treatment. In present days, polymeric materials are modified with sterically hindered compounds facile to reduce gas-induced plasticization and physical aging (He et al. 2015).

The physicochemical modification of glassy polymer provided some solution to the CO₂ plasticization and physical aging, but the gas permeability (gas transportation) further reduced, which makes several glassy polymeric composite membranes step backward for effective CO₂ separation (Scholes et al. 2008). Hence, new approaches are needed to improve the gas transportation of glassy polymeric membranes. The gas transport in glassy polymeric composite membranes is influenced by the number of polymer properties, such as polarity, free volume content, crystallites, average molecular weight, glass transition temperature (Tg), degree of crystallization, degree of polymerization, interchain spacing, chain orientation, composition, and defects (Marek et al. 1996; Orlov et al. 2003; Park et al. 2003; Sridhar et al. 2007b; Yampolskii 2012; Taniguchi et al. 2017; Polevaya et al. 2019). For example, the material contains a crystalline domain which adds a tortuosity factor to gas diffusion as a resulting difficulty to gas transport through this material (Yasuda and Tsai 1974). Similarly, the polymer contains highly polar groups like ether oxygen, acetates, carbonates, and nitriles to increase the solubility of polar gases (CO₂ and SO₂) (Hamad et al. 2001)(Park et al. 2003). In other words, polymer side-chain functionalization creates a high free volume polymer matrix, which contributes more selective gas diffusion to the polymer matrix (Polevaya et al. 2019). On the other hand, the gas diffusivity is related to polymer chain flexibility, and the polymer chain flexibility is directly linked with the Tg value of the polymer matrix (Yampolskii 2012). From the above statements, it was observed that the negative performance of the glassy polymeric membranes changed by changing the polymer properties chemically or physically. Polevaya et al. (2019) demonstrate the chemical modification of glassy polymer of poly(1-trimethylsilyl-1-propyne) (PTMSP) side chain with ionic liquids produce the well-defined structural CO₂-selective polymer network, which is more specific to.

CO₂ gas transport rather than N₂ and CH₄. Moreover, the membrane stability is enhanced with ionic liquid side-chain functionalization. Similarly, Park et al. (2003) reported that the different polar groups (hydroxyl and carboxyl contains diamines of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (BAPAF), 2,4-diaminophenol dihydrochloride (DAP), 3,5-aminobenzoic acid (DABA)), functionalization of 6FDA (2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride)-based polyimides, and composite membrane amend the gas separation performance with respect to the different polar groups. Marek et al. (1996) stated interfacial polymerization with different monomer precursor combinations makes ultra-thin polyimide composite membrane composed of various chain flexibilities, packing density, and fractional free volume. The chain flexibility, packing density, and fractional free volume play a major role in gas transportation, which was varied with respect to the side-chain length of the monomer precursor. It has been reported that the lower side-chain precursor produce a more ridged polyimide network on the support substrate, which makes less gas transport and more gas pair selectivity, whereas in the case of long side-chain precursor more flexible to form extensive chain packing and facilitate more gas transport and favor higher gas permeability.

Many researchers considered various aspects to improve the gas separation performance of the glassy polymeric composite membrane by blending of polymers and inorganic particle into polymer matrix which display some positive results. Further developments focused on hybrid membranes (glassy polymers with inorganic particles or other polymers) for effective CO₂ gas separation process (Basu et al. 2010b; Hosseinia et al. 2010; Sadeghi et al. 2011; Koschine et al. 2015; Friebe et al. 2016; Kim et al. 2016a, b, c, 2018; Mei et al. 2020). Friebe et al. (2016) prepared the NH₂-MIL-125 MOF and Matrimid polymer composed of mixed matrix thin film composite membrane (MM-TFCM) for H₂/CO₂ separation expressed the gas separation efficiency at the higher temperature of 150 °C. Moreover, the gas separation performance of the MM-TFC membrane exceeded the H₂/CO₂ Robeson trade limit. Besides, free NH₂ groups presented on pore system major contributor to the better gas separation performance. Another work by Awad and Aljundi (2018) prepared a carbide-derived-carbon filler (CDC) incorporating polyamide/PSf composite membrane by interfacial polymerization process with piperazine (PIP) and isophthaloyl chloride (IPC) precursors. The prepared membrane improved the gas separation performance with respect to the increment of CDC conc. In addition, the layer-by-layer (LB) deposition of the IP process declines the gas permeance and upturns the gas pair selectivity. Likewise, the pressure and temperature studies also revealed the better gas separation efficiency of the prepared MM-TFC membrane.

Further, the gas separation performance of glassy polymeric MM-TFC membranes is well footprinted in literature (Awad and Aljundi 2018; Hosseinia et al. 2010; Friebe et al. 2016; Mei et al. 2020), but the problems arise in the form of interfacial vide between glassy polymers and inorganic particles declined the gas separation performance (gas pair selectivity), which might be the reason to the poor adhesive nature of most inorganic particles into glassy polymers. Some studies have proven that the physicochemical modification of the filler particles (filler particles surface modifications with appropriated functional groups) improves the adhesive between polymer and filler particle, consequence of non-appearance of interfacial vide, and enhancement in gas separation efficiency (Khan et al. 2012; Kinoshita et al. 2017; Aliyev et al. 2018; Fauzan et al. 2020; Kanehashi et al. 2015). Khan et al. (2012) demonstrated that the MWCNTs + PIM-1-based MM-TFC membrane enhances the CO₂ solubility selectivity and good interfacial adhesive between polymer and filler particles by on PEG functionalization of MWCNTs surface. Kinoshita et al. (2017) reported that the polyhedral oligomeric silsesquioxane (POSS) composed of PIM-1-based molecular sieving MM-TFC membrane possess superior gas separation for H₂/CO₂ mixture and well-defined interfacial interaction between polymer and POSS nanoparticles by amine and nitro functionality modification of POSS particle. Moreover, the additional functionalization of POSS particle retards the physical aging phenomenon of the MM-TFC membrane. Likewise, some other studies (Aliyev et al. 2018; Chung et al. 1999; Fauzan et al. 2020; Kanehashi et al. 2015; Kim et al. 2003, 2016a, b, c; Kusakabe et al. 1996; Sanchez-Lainez et al. 2018a, b; Suzuki et al. 1998) also demonstrated and obtained the moderate results on that physicochemical modification of polymer and filler particle in the glassy polymeric composite membrane, but still upgrading needs composite membranes with glassy polymer for real industrial CO₂ gas separation application (Lillepärg et al. 2014).

Co-polymeric composite membrane in CO₂ gas separation

Further to the growing concern of solution-diffusion polymeric composite membrane in CO₂ gas separation, the researchers establish new aspects by using co-polymers as a polymer selective layer (Blume and Pinnau 1990; Reid et al. 2002; Liu et al. 2004). Blume and Pinnau (1990) patented the first co-polymer-based composite membrane in gas separation in 1990. While using co-polymer as selective layer material, the inherent property of copolymers such as both rubbery soft segment and glassy hard segment make an ultra-thin film and well-defined structural backbone for better gas transport pathway on hard support. This contributes to the superior gas separation performance (permeability and selectivity) via both solubility and diffusivity bases. Another advantage of co-polymers is that it exhibits desirable mechanical and thermal stability over a long period (Liu et al. 2004). These aspects were motivating the researchers to make several co-polymeric composite membranes in CO₂ gas separation and thus strengthened the field of the co-polymer composite membrane in the gas separation process (Liu et al. 2004, 2005; Sridhar et al. 2007c; Yave et al. 2009, 2010b; Ji et al. 2010; Ren et al. 2012; Khalilinejad et al. 2015; Karunakaran et al. 2017). The currently existing various copolymers composed of composite membranes and their gas separation performance are provided in Table 3. It is clear that most of the copolymer used in selective layer formation was the soft or rubbery segment that encompasses ether-oxygen groups and hard segments designed by amine groups (Liu et al. 2004). These ether-oxygen and amine groups are more CO₂-philic in nature and trigged the CO₂ gas molecular transport more and specifically. This is an important key point for using copolymer for membrane CO₂ gas separation (Sridhar et al. 2007c). In literature, these functionality bases are generally named as PEBAX (Marcq et al. 2005; Zhao et al. 2008; Sridhar et al. 2007c). Liu et al. (2004, 2005) prepared a composite membrane composed of PEBAX 2533/PSf and PEBAX 2533/PEI thin-film hollow fiber composite membranes that show remarkable performance in CO₂/N₂ gas separation. Similarly, Sridhar and co-workers (Sridhar et al. 2007a) prepared another kind of PEBAX (PEBAX 1657)/PVDF composite membrane for CO₂/CH₄ separation. The performance of PEBAX 1657/PEI and PEBAX 3533/PEI thin-film hollow fiber composite membranes for sour gas separation (SO₂/CO₂/N₂) was also examined by other research groups (Kim et al., 2013a, b and Ren et al. 2012). The membrane shows stable and vestal gas separation performance at even the presence of other impurity gases. Similarly, some other studies (Car et al. 2008; Ahmadpour et al. 2014; Khalilinejad et al. 2015; Scofield et al. 2015) also found that the PEBAX composed of co-polymeric composite membrane offers better performance in CO₂ gas separation.

Table 3 Co-polymeric composite membranes for CO₂ capture

Apart from PEBAX co-polymer, Yave and co-workers (Yave et al. 2010a) prepared a polyethylene oxide-polybutylene terephthalate (PEO-PBT)-based co-polymeric composite membrane for CO₂ gas separation in various gas mixtures (N₂, CH₄, and H₂). The prepared membrane exhibits outstanding gas separation performance and stability. Gu and Lodge (2011), synthesized the triblock co-polymer ion gel composite membrane composed of poly (styrene-b-oxy-ionic liquid-amide-b-styrene)/PVDF for CO₂ gas separation from N₂ and CH₄ gases. The membrane parades higher gas separation efficiency and more stability in a given environment. Ji and co-worker (Ji et al. 2009, 2010) prepared a new kind of PDMAEMA-PEGMEMA/PSf co-polymeric composite membranes for CO₂/N₂ separation via interfacial polymerization process; the membrane exhibits better gas separation efficiency along with good CO₂ plasticization resistance and physical aging. Another work by Karunakaran et al. (2017) introduced a PAN-r-PEGMA/PAN co-polymeric composite membrane for CO₂, N₂, CH₄, and H₂ separation; the membrane shows remarkable stability.

Based on the various studies, it can be concluded that the numerous co-polymers composed of composite membrane show higher gas separation efficiency and thus occupied the top position in Robeson upper bound 2008 compared to glassy and rubbery polymers (Ji et al. 2010; Yave et al. 2010a). But still they have some stability issues that need to be addressed, namely physical aging and the CO₂ plasticization phenomenon. Cross-linking the polymer network (Sridhar et al. 2007c) or changing the functionality of co-polymer segments with different functional groups (metals, amino, imido, fluro, etc.) addresses the aforementioned issues. Wang et al. (2006) prepared a CO₂-philic co-polymeric composite membrane composed of poly(N-viny-γ-sodium aminobutyrate-co-sodium acrylate) (PVSA-SA)/PSf. The membrane displays outstanding performance with good stability against CO₂ plasticization in CO₂/CH₄ mixture. Shen et al. (2006) developed the metal functionalized poly(2-N,N-dimethyl aminoethyl methacrylate-co-acrylic acid sodium) (PDMAEMA-Na) co-polymer shown there effective CO₂/CH₄ separation performance and metal substitution increased the stability and gas transportation of CO₂ gas molecules with a specific interaction. Similarly, fluorinated poly(ethylene glycol)-block-poly(pentafluoropropyl acrylate) diblock copolymer network for CO₂/N₂ gas separation was synthesized by another group (Scofield et al. 2016). After fluorination of co-polymer network enhanced the membrane stability against temperature, pressure, and CO₂ plasticization. Nguyen et al. (2013) synthesized the alkyl-imidazolium-based RTIL block copolymer (BCP) composite membrane for CO₂/N₂ gas separation application. They claimed alkyl and imidazolium combination created the well-ordered nano-structural polymer network and the formed nanostructure greater affinity with CO₂ gas molecules, which enhanced the CO₂ gas transportation through this polymer network.

Moreover, the form ordered nanostructure stable against CO₂ plasticization and physical aging problems. Some other studies also reported the improvement in the membrane stability and performance by changing the functionality of the co-polymer segment (Yave et al. 2010a; Zhu et al. 2015; Karunakaran et al. 2017).

Though the physical aging and CO₂ plasticization problems of co-polymer were minimized by changing the functionality of the co-polymer segment, still the issues were not toggled fully. Further, the researcher identified another solution to improve the gas separation performance and stability of co-polymer composite membrane by blending of polymers or inoculation of inorganic particles into the copolymer matrix and multilayer formation (Sridhar et al. 2007c; Murali et al. 2010; Li et al. 2013a; Halim et al. 2014; Scofield et al. 2015, 2016; Peng et al. 2017; Sarmadi and Salimi 2020; Taheri et al. 2021). In the above former cases (blending or inoculation), extra functionality arises to the co-polymer matrix which promotes the solubility and diffusivity of CO₂ gas molecules and also enhanced the membrane stability; on the other side of latter cases (multilayer formation), it helps to make a defect-free ultra-thin polymer selective layer. The identification of suitable polymer blend, inorganic particles, and gutter or productive layer material are more essential to the effective membrane preparation because the right polymer blends or inorganic particles facilitate the greater gas separation efficiency as well as improve the thermal, mechanical, anti-plasticization, and physical aging characteristic of the copolymer matrix (Ji et al. 2009, 2010; Yave et al. 2010a; Jomekian et al. 2011, 2017a; Kim et al. 2013a, b; Scofield et al. 2015; Karunakaran et al. 2017). The effective blending of polymer (e.g., PDMS, PEG, PEI, PI) into co-polymer matrix considers as fine miscible of polymers on a molecular level and possesses both good mechanical strength and gas separation efficiency (Car et al. 2008; Ji et al. 2009; Yave et al. 2009, 2010a; Kim et al. 2013a, b; Ahmadpour et al. 2014). Similarly, incorporation of filler particles (e.g., metal oxide, zeolites, CMS, CNTs, MOFs, and ZIF) to the copolymer matrix needs good adhesive nature, a defect-free filler–polymer interface which after greater CO₂ gas separation efficiency (Murali et al. 2010; Jomekian et al. 2011, 2017b; Li et al. 2013a, b, c; Fu et al. 2014; Chen et al. 2015; Friebe et al. 2016; Dai et al. 2019a). Many studies proved that blending of polymer or addition of filler to the co-polymer matrix enhanced the CO₂ gas separation efficiency with good stability. The separation performances of blend and multilayer co-polymeric membranes are also listed in Table 3.

Sridhar et al. (2007a) produced the silver incorporated PEBAX2533/PVDF co-polymeric composite membrane for CO₂/CH₄ separation. The Ag- incorporation enhanced the gas pair selectivity compare to pure PEBAX2533 and also the gas separation efficiency of membrane feasible for commercial purposes. Murali et al. (2010) prepared composite membranes composed of MWCNTs-PEBAX 1657/PSf, which shows remarkable stability against thermal and CO₂ plasticization as well as good gas separation efficiency after incorporation of MWCNTs. Moreover, the authors claimed that the gas separation efficiency of modified co-polymer membrane proved their suitability in various gas separation applications, such as CO₂/N₂ separation in the power plant industry, H₂/NH₃ for purge gas recycling, O₂ from the air in medical application, and CO₂ from water gas shifting for H₂ purification. Similarly, Li et al. (2013b) prepared the ZIF-7 nanoparticles incorporated into PEBAX1657/PAN composite membrane which exhibit good separation efficiency, as well as better mechanical stability in CO₂/CH₄ gas mixture combination. Another work by Fu et al. (2014) introduced the PEG-based cross-linked soft nanoparticles incorporated into the PEBAX2533/PAN composite membranes for CO₂/N₂ separation. The membrane exhibited the highest CO₂ permeability with reasonable selectivity and the performance of the membrane denoted membrane suitable for up-scaling in post-combustion carbon capture, such as power plant CO₂ capture. Further, research continued with the blending of polymer. Halim et al. (2014) prepared a co-polymeric composite membrane composed of PEGand PEG-b-PDMS polymer blend PEBAX2533/PAN showing remarkable CO₂ flux after blending of polymer, and the membrane shows long time stability in CO₂/N₂ separation. Similarly, Car et al. (2008) prepared a PEG200 blended PEBAX1657/PAN co-polymeric composite membrane for CO₂/N₂/H₂ separation; the membrane exhibited good gas separation efficiency in both CO₂/N₂ and CO₂/H₂ gas mixture up to 20 bar. Yave et al. (2010a) introduced the different types of PEGpolymer blended PEO-PBT/PAN co-polymeric composite membrane which show outstanding CO₂ separation performance over N₂ and H₂. The gas separation performances of this new kind of polymer combination boost up the co-polymeric composite membranes for commercial purposes. Similarly, some other studies (Anson et al. 2004; Kim et al. 2006; Yave et al. 2009; Chen et al. 2015; Shen et al. 2016; Park et al. 2019; Sanchez-Laınez et al. 2019; Jomekian et al. 2020; Scofield et al. 2015, 2016) proved that the blending or incorporation of inorganic particles, such as activated carbon (CA), MOF, ILs, PEG, PEG-b-PPFPA, and ZIF, into co-polymer network, improves their gas separation performance and stability. The overall literature studies denoted that the blending or incorporation polymer or inorganic particle into co-polymer network offers essential membrane stability without compromising the gas separation ability, thus literally membranes are gone for behind the Robeson upper boundary 2008.

The aforementioned different strategies improve the gas separation performance and long-term stability of membrane (anti-CO₂ plasticization and physical aging), but the improvements in CO₂ gas separation performance are not adequate to replace the currently existing amine absorption process (Yang et al. 2008; Basu et al. 2010b; Widjojo et al. 2012). Further, researchers realize that the polymeric composite membrane with solution-diffusion mechanism is still not much approachable for effective industrial CO₂ gas separation process in later research. Hence further research focused on an alternative solution to overcome the persisting problems than the researchers’ move to facilitate transport phenomenon in CO₂ gas separation.

Facilitated transport polymeric composite membranes (FTPCMs)

The idea of facilitated transport is originated from the biological process since the molecular transport from the bloodstream into cells is accomplished by the facilitated transport phenomenon. Thereby researchers applied the concept of facilitated transport phenomenon in polymeric composite membranes for various gas separation processes (CO₂/N₂, CO₂/CH₄, H₂/CO₂, O₂/N₂, etc.) (Yamaguchij et al. 1995; Kim et al. 2004; Dong et al. 2006; Sandru et al. 2010; Yu et al. 2010; Yao et al. 2012; Chen et al. 2016; Prasad and Mandal 2017; Saeed et al. 2021). In the gas separation process, FTPCMs exhibit fairly higher CO₂ permeability and selectivity than the solution-diffusion polymeric composite membrane. Because the polymer selective layer contains the CO₂ reactive carrier molecules in their backbone trigger the CO₂ gas transport by both facilitation transport and solution-diffusion manner. While the non-interacting gases (N₂, H₂, and CH₄) are transported only by the solution-diffusion, which is facile to achieve high CO₂/light gas selectivity without sacrificing permeability (Scholes et al. 2008; Rafiq et al. 2016; Maheswari and Palanivelu 2017). In the following functional groups, -NH₂, PO₄³⁻, F⁻, CO₃²⁻, and COO⁻, some metals (Ag, Zn, Fe, etc.) are employed as CO₂ reactive carriers in many pieces of literature (Matsuyama et al. 1999a; Kim et al. 2004; Wang et al. 2007; Bara et al. 2010; Mao et al. 2011; Yao et al. 2012; Jie et al. 2015; Tomé et al. 2015; Shankar and Kandasamy 2019). Amongst the functional groups, amine (-NH₂) is considered as most effective CO₂ reactive carrier in FTPCMs for gas separation process (Yamaguchij et al. 1995; Kim et al. 2004; Dong et al. 2006; Yu et al. 2010; Yuan et al. 2011; Shen et al. 2015; Prasad and Mandal 2017). Due to its fast CO₂ reaction rates, high absorption capacity, and ease of regeneration, the CO₂ gas transportation of amine CO₂ carrier presented polymers is accomplished by a reversible reaction manner. From the literature, it is observed that possible reversible reactions between CO₂ and amine carrier molecules present in the polymer selective layer are given below (Eqs. 1, 2, 3, 4, and 5).

Primary amine:

$${{\varvec{D}}{\varvec{r}}{\varvec{y}}\boldsymbol{ }{\varvec{s}}{\varvec{t}}{\varvec{a}}{\varvec{t}}{\varvec{e}}: CO}_{2}+{2RNH}_{2}\rightleftharpoons {RNHCO}_{2}^{-}\left(carbamate\right){+RNH}_{3}^{+}$$

(1)

$${{\varvec{W}}{\varvec{e}}{\varvec{t}}\boldsymbol{ }{\varvec{s}}{\varvec{t}}{\varvec{a}}{\varvec{t}}{\varvec{e}}: 2CO}_{2}+{2RNH}_{2}+{H}_{2}O\rightleftharpoons {RNHCO}_{2}H+{ RNH}_{3}^{+}+{HCO}_{3}^{-}$$

(2)

Secondary amine:

$${{\varvec{D}}{\varvec{r}}{\varvec{y}}\boldsymbol{ }{\varvec{s}}{\varvec{t}}{\varvec{a}}{\varvec{t}}{\varvec{e}}: CO}_{2}+{2R}_{2}NH\rightleftharpoons {R}_{2}{NCO}_{2}^{-}\left(carbamate\right)+{R}_{2}{NH}_{2}^{+}$$

(3)

$${{\varvec{W}}{\varvec{e}}{\varvec{t}}\boldsymbol{ }{\varvec{s}}{\varvec{t}}{\varvec{a}}{\varvec{t}}{\varvec{e}}: 2CO}_{2}+{2R}_{2}NH+{H}_{2}O{\rightleftharpoons {R}_{2}NCO}_{2}H+{R}_{2}{NH}_{2}^{+}+{HCO}_{3}^{-}$$

(4)

Tertiary amines:

$${{\varvec{W}}{\varvec{e}}{\varvec{t}}\boldsymbol{ }{\varvec{s}}{\varvec{t}}{\varvec{a}}{\varvec{t}}{\varvec{e}}: CO}_{2}+{R}_{3}N+{H}_{2}O\rightleftharpoons {R}_{3}{NH}^{+}+{HCO}_{3}^{-}$$

(5)

The reversible reaction between CO₂ vs primary (Eqs. 1 and 2) and secondary amines (Eqs. 3 and 4) functionality polymer selective layers is happening by carbamate intermediate ion formation in dry state and bicarbonate intermediate formation in a wet state. Whereas in the case of reversible reaction between CO₂ vs tertiary amines (Eqs. 5), functionality polymer selective layers are deferent from primary and secondary amines, the formation of carbamate intermediate ion is not possible in a dry state, an alternative bicarbonate formation is happening at wet state, and so tertiary amines in the CO₂ reversible reaction are only possible at the presence of water content. Therefore, the performance of ternary amine composed polymeric composite membranes is much more dependent on the environment humidity. Further information about the facilitated transport membrane is found in the reviews of Rafiq et al. (2016) and Maheswari and Palanivelu (2017).

Normally, polymers used in FTPCMs are categorized into two types: fixed-site polymers and hydrogel polymers. Based on the polymer used, the FTPCMs are classified as fixed-site polymeric composite membrane and hydrogel polymer composite membrane. The recent research developments in the area of the fixed site and hydrogel polymeric composite membranes for CO₂ gas separation are discussed in a further session.

Fixed site polymeric composite membranes for CO₂ gas separation (FSPCMs)

Over the decades, more than thousands of papers have been published in the field of a fixed-site polymeric composite membrane in gas separation applications. Various FTPCMs and their gas separation performances are furnished in Table 4. Through the literature polyvinyl amine (PVAm), polyallylamine (PAAm), polyethyleneimine (PEI), chitosan, PAMAM, and amine-functionalized co-polymers are the most used fixed-site polymers in CO₂ gas separation application (Matsuyama et al. 1999b; Kim et al. 2004; Yu et al. 2010; Duan et al. 2012; Zhao and Ho 2012; Li et al. 2015b; Shen et al. 2015). Certain literature described these fixed-site polymers cannot sustain without support material and they are stable in composite form because the unsupported polymers are sticky and less permeable; moreover, they are not able to stand in self-standing form during the operation conditions (Matsuyama et al. 1999b; Kim et al. 2004; Duan et al. 2012; Zhao and Ho 2012). For example, Kim et al. (2004) reported that the FT polymer of PVAm is a topmost CO₂ carrier polymer; however, its nature is like a high viscosity fluid to sticky the self-standing form; hence, they are not capable to handle without support materials. On the other hand, a lot of FTPCMs drop their better gas separation ability in a short time period, due to some interior characteristics (crystallinity, free volume, CO₂ and carrier reaction rate, etc.) of the polymer matrix. Some studies proved that this may be overcome by using different support materials (Kim et al. 2004; Sandru et al. 2010), cross-linking, and copolymerization with various CO₂ selective molecules (Zhang et al. 2002; Kim et al. 2004; Dong et al. 2006; Yi et al. 2006; Li et al. 2015b; He et al. 2016), different treatment process (pH control (Kim and Lee 2013), preparation method (Yu et al. 2010)), and operation mode (sweep or vacuum mode) (Sandru et al. 2010). For example, Kim et al. (2004) examined the PVAm composite membrane for CO₂/CH₄ separation with the different cross-linking agents and various support materials. Among the various cross-linker and support materials used, ammonium fluoride (NH₄F) and PSf support show remarkable higher performance, due to the higher electronegativity of the fluoride ion anticipated more CO₂ facilitated transport through the PVAm membrane; moreover, the film formation of NH₄F/PVAm polymer composition is better on PSf support surface. Similarly, Dong et al. (2006) and Yi et al. (2006) studied the GA cross-linked PVAm/PSf hollow fiber membrane for CO₂/CH₄ separation. The cross-linked membrane shows well separation efficiency along with good stability (6 days stable performance) at lower pressure range (96 cmHg). In another work, Kim et al. (2013a, b) demonstrated the CO₂/N₂ separation efficiency of PVAm/PSf composite membranes with three different pH of PVAm casting solution. From the experimental results, they observed that pH of casting solution directly influence the membrane characteristic (degree of free amine groups and viscosity of casting solution) and CO₂ gas separation efficiency. The higher pH of casting solution increase the degree of free amine groups, which facilitated more CO₂ transport through the membrane; likewise, the lower pH of the casting solution increase the viscosity of casting solution, thus reducing the gas transportation. Yu et al. (2010) prepared a novel amine contains fixed-site membranes via an interfacial polymerization on porous polysulfone support (DNMDAm-TMC/PS) for CO₂/N₂ and CO₂/CH₄ mixture separation. The membrane formed ultrathin (l = 300 nm) defect selective layer shows remarkable higher CO₂ permeability and selectivity over CH₄ and N₂, and the study revealed the performance of membrane depending on concentration of TMC in IP process.

Table 4 Facilitated polymeric composite membranes for CO₂ capture

Li et al. (2015b) synthesized a new kind of co-polymer as a selective layer for the separation of CO₂ from N₂, CH₄, and H₂ mixture. The membrane material contains three types of CO₂-reactive molecules (primary amino groups, quaternary ammonium groups, and carbonate groups) that make more CO₂ facilitation to the membranes. Further, the study exposed membrane has long-term stability and resistance to impurity gases. Similarly, other studies (multilayer composite membranes formation (Li et al. 2015a), sterically hindrance (Zhao and Ho 2012), and different support usages (Kim et al. 2004; Sandru et al. 2010), implied the stability of facilitated transport composite membranes. Later researchers found the aforementioned process addresses the stability issues of facilitated transport polymeric composite membrane, but the gas separation efficiency is not adequate to beat the existing trade-off relationship between permeability and selectivity. This was over-come by immobilization of mobile carriers (small organic and inorganic molecules that possess CO₂ facilitation transport nature, e.g., amines, amino alcohols, salts of amino acids, dendrimers, enzymes, metal carbonates, and hydroxide) into polymers of fixed-site carrier composite membranes.

A moment ago, many authors (Yamaguchij et al. 1995; Yi et al. 2006; Duan et al. 2008; El-azzami and Grulke 2009; Yuan et al. 2011; Zhao et al. 2012; Qiao et al. 2013; Jie et al. 2015; Chen et al. 2016; Yu et al. 2016; Prasad and Mandal 2017) demonstrated that the addition of mobile carrier into the polymer matrix enhanced the membrane permeability and selectivity, because the addition of mobile carrier increased the number of CO₂ reactive site and also easily move CO₂-carrier complex through the membrane, which resulting higher gas transport and higher gas separation efficiency. Massive mobile carrier incorporated facilitated transport polymeric composite membranes drop their gas separation performance earlier during continuous operation, due to the carrier loss and degradation of the mobile carrier at given operation condition (El-azzami and Grulke 2009)(Yuan et al. 2011). From an applications’ point of view, selection of suitable membranes are based on separation ability along with stability and lifetime of membrane. Therefore, the researcher step forwarded to the grownup the improvement of stable and longer lifetime of mobile carrier containing FTPCMs for CO₂ gas separation application. Then they are introduced hyperbranched or sterically hindered molecules to attain the maximum carrier stability in a polymer matrix (Yu et al. 2011; Zhao and Ho 2012; Duan et al. 2014; He et al. 2015). Alternatively, the researchers (Yi et al. 2006; Cai et al. 2008; Deng et al. 2009; Zhao et al. 2012, 2013, 2015b; Shen et al. 2013, 2015; Casado-Coterillo et al. 2015; Li et al. 2015a; Wang et al. 2015; Prasad and Mandal 2017; Bharali et al. 2020) identified the blending of another polymer or inorganic particles into the facilitated polymer matrix enhanced both separation efficiency and membrane stability.

Yi et al. (2006) prepared a PEG blended PVAm/PES facilitated transport composite membranes for CO₂/CH₄ separation. The author observed the addition of PEG decreases the crystallinity of the PVAm matrix, thus increasing mechanical stability and gas separation ability. Similarly, Shen et al. (2013) investigated the CO₂/N₂ separation ability of PEI blended carboxymethyl chitosan/PSf membrane. The author indicated that this membrane shows remarkable higher separation performance along with good stability than the other facilitated transport membranes. Another work by Zhao et al. (2012, 2013) demonstrated the incorporation of polyaniline nanoparticles (PANI nono particles) into the PVAm matrix, and the membrane displays higher CO₂ permeance (up to 1200 GPU) and higher CO₂/N₂ selectivity (up to 120). Moreover, the performance deterioration was not observed up to 300-h operation; hence, the study denoted membrane has on long time stability and suitable for effective CO₂ separation. Similarly, some other research works also performed with different polymer and inorganic materials incorporated fixed-site facilitated polymeric composite membrane for CO₂ gas separation applications in various gas mixtures, and they are shown well throughput in given operation conditions (Gao et al. 2018; Han et al. 2018; Wang et al. 2019, 2020; Xu et al. 2019; Janakiram et al. 2020a; Zhang et al. 2021). Most recently, some studies demonstrated the facilitated polymeric composite membranes in a field trial in the form of high membrane surface area comprised hollow fiber and spiral wound modules (Salim et al. 2018; Dai et al. 2019c; Han et al. 2019a; Janakiram et al. 2020b). The authors observed the outstanding gas separation performance with long-term stability in-field operation; thereby, they suggested FTPCMs are a good candidate for industrial CO₂ gas separation operation.

Hydrogel polymeric composite membrane (HPCMs) in CO₂ capture

On the other side, researchers demonstrated that hydrogel polymeric composite membranes are another class of facilitated polymeric composite membranes used in CO₂ gas separation (Bai et al. 1988; Ito et al. 1997; Bae et al. 1998; Matsuyama et al. 1999b; Zhang et al. 2002; Adams et al. 2011; Mao et al. 2011; Liu et al. 2014; Saedi et al. 2016; Jahan et al. 2018; Borgohain and Mandal 2019; Lee and Kang 2020). In HPCMs, the polymer selective layer is made of hydrogel polymeric materials. Normally, hydrogel polymeric materials are defined as cross-linked hydrophilic polymer network (cross-linked polyvinyl alcohol (Moulay 2015), polyvinyl acetate (PVAc) (Semsarzadeh and Ghalei 2012), polyvinyl pyrrolidone (PVP) (Wenthold et al. 1987), chitosan (Ito et al. 1997), and cellulose (Mao et al. 2011)), which is swelling significantly in presence of solvent (water as a common solvent) but not dissolving to it (Scholes et al. 2008). The various works on hydrogel polymeric composite membranes are provided in Table 5. Generally, in hydrogel polymeric composite membranes, gas separation ability is poor in dry condition, but after being swollen by water, the performance of the membranes is remarkably high which indicated water molecules one of the key crucial in hydrogel membranes for gas separation process (Ito et al. 1997; Bae et al. 1998; Deng and Hägg 2010; Mao et al. 2011; Mondal and Mandal 2014a; Saedi et al. 2016). For example, Ito et al. (1997) reported that the CO₂ permeability of the chitosan membrane is very lower (0.4 barrer) in a dry state, but after being swollen by water, the CO₂ permeability increased subsequently (up to 200 barrer at 20 °C). Here they observed the swollen membranes contains bounded or sorbed water molecules, which disturbed the chain packing of polymer matrix as a resultant broaden the free-volume (gas transport channels) of CS polymer network and facilitate more gas transport through the CS matrix. From that, they are concluded that water molecules play an important role on CO₂ gas transport through hydrogel polymeric composite membranes. Further studies explore the water molecules take part in this process are classified in two types: free water molecules and bound water molecules. The CO₂ gas transportation in free water molecule is assisted by solution-diffusion whereas in the bound water molecule it is mainly by facilitated transport. Liu et al. (2008) discover the gas transport of hydrogel polymeric composite membranes anticipated by dual ways; one is channel transport, and the other is water passage way. Increase in the CO₂ gas separation performance could be achieved by constructing more number of channels to the polymer matrix as well as increase in the amount of water content during the process in HPCMs.

Table 5 Hydrogel polymeric composite membranes in CO₂ capture

Though the higher CO₂ permeability is obtained in hydrogel polymeric composite membrane with high water content, on the other side problem facing by falling down the gas pair selectivity and membrane stability by excess water content, this can be overcome by controlling the degree of cross-linking to the polymer matrix with effective techniques (Philipp and Hsu 1979). Nevertheless, the higher degree of cross-linking makes membranes brittle and hard to handle, which may also aid poor performance (CO₂ permeability) of hydrogel polymeric composite membrane in CO₂ gas separation (Huang et al. 2008; Deng et al. 2009; Uma Maheswari and Palanivelu 2017). Further, an improvement in the gas separation performance of the hydrogel membrane is observed by the addition of a CO₂ facilitating carrier (amine, amino acids, liquid enzymes, etc.) to the hydrogel polymeric network.

The gas permeation results of some CO₂ facilitating carrier incorporated HPCMs are listed in Table 5. In recent times, various authors (Francisco et al. 2007; El-azzami and Grulke 2008; Huang et al. 2008; Yao et al. 2012; Saeed and Deng 2015; Yu et al. 2016; Uma Maheswari and Palanivelu 2017; Dai et al. 2019b; Klemm et al. 2020; Lee and Kang 2020) demonstrated that the addition of CO₂ facilitated carriers (liquid enzymes, both fixed and mobile carriers) into hydrogel polymer network enhanced the CO₂ permeability and selectivity. For this, enhanced gas separation efficiency denoted HPCMs as a good candidate for CO₂ gas separation application, but still, there are some limitations alleviated in terms of stability issues (instability of CO₂ facilitating carriers into hydrogel polymer matrix) not able to go for field application. Further researches address the existing carrier instability problems via the addition of some inherent properties (low volatile, high polarity and ionic conductivity, thermal stability, steric hindrance, etc.) which contain ILs (Casado-coterillo et al. 2014) (Kamio et al. 2020b)(Klemm et al. 2020) and hyperbranched dendrimer networks (Kai et al. 2008)(Duan et al. 2012) (Kunalan et al. 2021) as CO₂ facilitating carrier into the hydrogel polymer matrix. The aforementioned solution has not fully resolved the instability problems of CO₂ facilitated carriers. Further, researchers (Wang et al. 2007; Kouketsu et al. 2007; Duan et al. 2012, 2014; Deng and Hägg 2014; Ansaloni et al. 2015; Shen et al. 2015; Casado-Coterillo et al. 2015; Saedi et al. 2016; Jahan et al. 2018; Borgohain and Mandal 2019; Torstensen et al. 2019; Lilleby Helberg et al. 2020) established that another aspect by blending of polymers or inorganic particle into HPCMs enhanced the gas separation performance without compromising the membrane stability.

Shen et al. (2015) prepared a PEI hyperbranched GO nanofiller incorporated PVAm-CS/PSf composite membranes for CO₂/N₂ separation. The HBPEI-GO-PVAm-CS/PSf membrane shows double the performance than PVAm-CS/PSf membrane, which may happen by GO addition providing more CO₂ channel to the PVAm-CS/PSf composite membrane. Moreover, the stability study revolved the membrane possesses in long time stability against given operating conditions. Similarly, Ansaloni et al. (2015) demonstrated that the amine-functionalized MWCNT incorporated into cross-linked polyvinylalcohol-polysiloxane/amine blend/PSf composite membrane for CO₂ separation from H₂/N₂/CH₄ mixture. The authors observed that the gas separation performance of the MMM-composite membrane was incredibly higher after the addition of MWCNTs. Moreover, the addition of amine-functionalized MWCNTs enhanced the polymer filler comfortability and mechanical strength remarkably. The highest CO₂ permeability and selectivity along with greater mechanical stability offers this MMMs-composite membrane a good candidate for industrial CO₂ gas separation application. One more work by Deng and Hägg (2014) developed a CNT-PVAm-PVA/PSf composite membrane for CO₂/CH₄ separation. The membrane displays long durability with good CO₂/CH₄ separation ability. Some other studies (Bai and Ho 2011; Duan et al. 2013; He et al. 2014; Zhao et al. 2014; Casado-Coterillo et al. 2015; Saedi et al. 2016; Jahan et al. 2018; Barooah and Mandal 2019; Borgohain and Mandal 2020) also demonstrated the incorporation of filler particle to the facilitated hydrogel polymeric composite membrane for CO₂ gas separation from the different gas mixture, and the membranes show remarkably high gas separation performance and stability.

Since the great potential for the development of facilitating transport polymeric composite membranes of fixed-site polymeric composite membranes and hydrogel polymeric composite membranes in CO₂ gas separation application, they have some limitations to the application point of view. Due to the higher CO₂ partial pressure (above 5 bar), these composite membranes drop their gas separation performance (both permeability and selectivity). On the other side, low CO₂ concentration, low CO₂ partial pressure, and desirable temperature (up to 100 °C), the FPCMs, and HPCMs show extraordinary gas separation efficiency and membrane stability. In this point of view, the flue gas (10–15%) CO₂ capture applications facilitated transport polymeric composite membranes more favorable than conventional solution-diffusion polymeric composite membrane because the pressure and temperature range of flue gas is less than 2 bar and 100 °C. At the same time, the functionalized inorganic filler incorporated facilitated transport composite membranes sustained at higher pressure (up to 30 bar) and higher temperature (150 °C), so the hybrid facilitated transport polymeric composite membranes (HFTPCMs) are suitable for fuel gas separation (biogas, natural gas, syngas).

The merits and demerits of the polymeric composite membranes in CO₂ gas separation

The overall investigation on the polymeric composite membrane in CO₂ gas separation from various gas mixtures revealed that each type of polymeric composite membrane exhibited different CO₂ gas separation efficiency. Table 6 provides information about various polymeric composite membranes’ merits, demerits, and possible ways to overcome the problems.

Table 6 Types of polymeric composite membranes and their approaches to overcoming the existing problem

Whereas in solution-diffusion polymeric composite membranes, the gas separation performance of rubbery polymer materials suggested that the rubbery polymer material is the most suitable for gutter and productive layer formation. On the other side, composite membranes composed with glassy or co-polymers material offer a better gas separation performance by blending with other polymers or inorganic materials (MMMs), but the gas separation performance has not yet reached up to the level of real field application. Further, the composite membrane composed with facilitated transport polymers exhibited the highest CO₂ permeability and CO₂/light gas selectivity than polymeric composite membranes relating to solution-diffusion. Even though the higher gas separation performance of FPCMs are not yet commercialized due to the gap between lab results and results obtained from the industrial condition though their gas separation performance is favourable. Therefore, the developing facilitated polymeric composite membranes exhibiting a larger CO₂ absorption capacity along with a long lifetime at given industrial operation conditions are more preferred for effective implementation in-field application and compete for the current existing CO₂ capturing system (amine absorption process).

Critical outlook and the new approaches to improve the existing membrane system

In CO₂ gas separation amine-based absorption process well-established technology in over the long period. Nowadays, membrane technology also competes with the current mature technology of the amine absorption process intensively in terms of energy and cost. A review of the overall literature results points out that each type of polymeric composite membrane has some interstice trade-offs like selectivity-permeability, synthesis route, processability, and membrane stability. However, the efficiency of polymeric composite in CO₂ captures somewhat greater effort in the current scenario. The well efforts on the polymeric composite membranes in CO₂ gas separation yield a good positive result on lab scale, but still, there is a large gap between labs to industrial scale. If the polymeric composite membranes have to compete with the current existing CO₂ amine absorption capturing technology, the membrane systems need to improve in the following aspects.

• ✓ Defect-free membrane formation through guttering or productive layer formation and using advanced membrane fabrication system (lamination, plasma polymerization, layer-by-layer deposition, etc.)

• ✓ Hybrid membrane formation is done by the controlled atmosphere, with an appropriate pre-treatment process. Further, the membrane system was adopted with some pre-treatment systems (i.e., gas compression, particulate matter removal, and desulfonation).

• ✓ In most of the studies, the efficiency of polymeric composite membranes was determined only with simple gases streams (single and binary gas mixture), and further research needs to evaluate the effect of traces impurities (NO_x and SO_x) on gas separation performance.

• ✓ The viability of polymeric composite membranes in real field harsh environmental conditions (e.g., feed gas composition, higher pressure, and temperature) has not been performed systematically, and also the long-term stability of membranes in those conditions has not yet been reported and so the study toward on membrane stability in harsh environmental condition to be addressed.

• ✓ Since in industrial-scale enormous volume of the gaseous stream needs to be processed, at this stage a large membrane area is required for this process, using hollow fiber and spiral wound module satisfied the purpose. But, effective studies have yet not been performed in hollow fiber and spiral wound membrane modules.

If the aforementioned points are addressed systematically, the membrane technology will outperform the current conventional process of the amine absorption process.

Conclusion

Through the literature studies with polymeric composite membrane for CO₂ gas separation, the polymeric composite membrane with facilitated transport shown their good throughput in CO₂ capture process than solution-diffusion membranes. In the case of SDPCMs, the combination of polymer matrix and inorganic filler (mixed matrix) partially fulfill the required CO₂ gas separation ability along with membrane stability, but the gas separation performance is not at the level of facilitated transport polymeric composite membranes. On the other side, FTPCMs well-adapted power plant industrial flue gas CO₂ capture, due to suitability of operation environmental (CO₂ concentration, partial pressure, and temperature). Further, it is hoped that the polymeric composite membrane better choose to real field CO₂ capture in the future. The review discussed all those various kinds of approaches and some of the requirements (defect-free layer formation, impurity present, harsh condition, and module type) that are more important for implementation than only success with the membranes for gas separation process in real field application.