Aviti Katare,Shubham Kumar,Sukanya Kundu,Swapnil Sharma,Lal Mohan Kundu,Bishnupada Mandal
Mixed Matrix Membranes for Carbon Capture and Sequestration: Challenges and Scope
Carbon dioxide (CO2) is a major greenhouse gas responsible for the increase in global temperature, making carbon capture and sequestration (CCS) crucial for controlling global warming. Traditional CCS methods such as absorption, adsorption, and cryogenic distillation are energy-intensive and expensive. In recent years, researchers have focused on CCS using membranes, specifically solution-diffusion, glassy, and polymeric membranes, due to their favorable properties for CCS applications. However, existing polymeric membranes have limitations in terms of permeability and selectivity trade-off, despite efforts to modify their structure. Mixed matrix membranes (MMMs) offer advantages in terms of energy usage, cost, and operation for CCS, as they can overcome the limitations of polymeric membranes by incorporating inorganic fillers, such as graphene oxide, zeolite, silica, carbon nanotubes, and metal–organic frameworks. MMMs have shown superior gas separation performance compared to polymeric membranes. However, challenges with MMMs include interfacial defects between the polymeric and inorganic phases, as well as agglomeration with increasing filler content, which can decrease selectivity. Additionally, there is a need for renewable and naturally occurring polymeric materials for the industrial-scale production of MMMs for CCS applications, which poses fabrication and reproducibility challenges. Therefore, this research focuses on different methodologies for carbon capture and sequestration techniques, discusses their merits and demerits, and elaborates on the most efficient method. Factors to consider in developing MMMs for gas separation, such as matrix and filler properties, and their synergistic effect are also explained in this Review.
Mixed Matrix Membranes for Carbon Capture and Sequestration: Challenges and Scope

Carbon dioxide (CO2) is a major greenhouse gas responsible for the increase in global temperature, making carbon capture and sequestration (CCS) crucial for controlling global warming. Traditional CCS methods such as absorption, adsorption, and cryogenic distillation are energy-intensive and expensive. In recent years, researchers have focused on CCS using membranes, specifically solution-diffusion, glassy, and polymeric membranes, due to their favorable properties for CCS applications. However, existing polymeric membranes have limitations in terms of permeability and selectivity trade-off, despite efforts to modify their structure. Mixed matrix membranes (MMMs) offer advantages in terms of energy usage, cost, and operation for CCS, as they can overcome the limitations of polymeric membranes by incorporating inorganic fillers, such as graphene oxide, zeolite, silica, carbon nanotubes, and metal–organic frameworks. MMMs have shown superior gas separation performance compared to polymeric membranes. However, challenges with MMMs include interfacial defects between the polymeric and inorganic phases, as well as agglomeration with increasing filler content, which can decrease selectivity. Additionally, there is a need for renewable and naturally occurring polymeric materials for the industrial-scale production of MMMs for CCS applications, which poses fabrication and reproducibility challenges. Therefore, this research focuses on different methodologies for carbon capture and sequestration techniques, discusses their merits and demerits, and elaborates on the most efficient method. Factors to consider in developing MMMs for gas separation, such as matrix and filler properties, and their synergistic effect are also explained in this Review.

Gases such as nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) are called greenhouse gases (GHGs) because these gases trap infrared radiation and radiate it back to the atmosphere, which leads to an increase in global temperature. Owing to rapid population growth and a sharp rise in energy demand, the GHG concentration in the atmosphere has amplified rapidly. The global mean surface temperature (GMST) was 1.0 °C higher compared to that in the preindustrial era (1850–1900) in 2017 due to the increased GHG concentration. The Intergovernmental Panel on Climate Change (IPCC) has reported that GMST will rise to 2.0 °C above that of the preindustrial era by 2035. This rise in the GMST has severely affected our ecosystems, such as the melting of ice, floods in low-level countries, and wildfires. To stabilize the rise of the GMST under 1.5 °C, CO2 capture, utilization, and sequestration (CCUS) are crucial processes that can be applied on an industrial scale. To capture the CO2 gas generated from large point sources, there are mainly four approaches available: precombustion capture, postcombustion capture, chemical-looping combustion, and oxycombustion. Oxycombustion and precombustion capture cannot be retrofitted into the existing power plants, so these two can be utilized for future power plants. Postcombustion technologies include the capture of CO2 from the exhaust gas of a fossil-fuel-fired power plant. This can be considered a preferable option, as this technology can be installed as an add-on in existing power plants, which use air for fuel combustion. There are several postcombustion CO2 capture technologies available, which include membrane separation, adsorption, absorption, and cryogenic separation. Absorption is the most effective technology widely used to remove CO2 from synthesis and natural gas. The disadvantages associated with absorption are huge costs and high energy consumption. Adsorption technologies suffer from high regeneration costs and batch operation. In cryogenic separation, the acid gas is frozen to separate the CO2 in liquid form. The major disadvantage of this technology is the requirement of enormous power to operate the refrigeration unit. Membrane separation is currently a rapidly developing technology for CO2 capture. It offers many advantages over other traditional technologies, such as ease of operation, flexibility, versatility, environmental friendliness, low energy demand, continuous operation, less maintenance costs, and lower space requirements, among others. The advantages and disadvantages associated with all the above-mentioned CO2 capture processes are listed in Table 1. Membrane separation of gases developed from a technical curiosity in the 1960s to the beginning of industrialization in the 1970s, followed by strong research efforts in the 1980s and commercialization. The ability to produce ultrathin permselective membranes (on the order of 100 nm) through advancements in the asymmetric membrane was likely the key to this achievement. Another essential component of this research was finding materials that increased the target gas permeability (P) while maintaining its selectivity. Membranes are mainly characterized based on materials used for fabrication such as polymeric, inorganic, and mixed matrix membranes, whose pros and cons are tabulated in Table 2. Polymeric membranes, being highly popular, suffer from a trade-off between selectivity and permeability, whereas inorganic membranes go through an expensive and complicated fabrication process. High permeability and selectivity are characteristic features of an ideal membrane for the maximization of its performance at low production cost. However, for membranes the trade-off effect, first introduced by Robeson, comes into the picture, i.e., high selectivity comes with an expense of low permeability and vice versa. The selectivity of a membrane is also deteriorated by an excessive increase in fractional free volume (FFV) caused by plasticization. In addition, the aging effect in the case of the glassy polymeric membranes reduces the gas permeability with the reduction in the FFV. It has been evident since the year 1990 that there is another way to improve the gas separation performance of membrane materials. The method, which is currently extremely popular, is called “mixed matrix membranes” (MMM). It involves incorporating tiny inorganic or organic–inorganic fillers into polymeric matrices, particularly with particle sizes between 10 and 100 nm. Such an approach frequently led to the enhancement of either permeability or selectivity (or frequently both). In MMMs, the addition of porous nanofillers results in an increase in permeability while maintaining the selectivity. In contrast, the selectivity of the membrane can be altered if modifications made to fillers are specific to various penetrants. These changes are often brought up by altering the diffusion coefficients and solubility coefficients. As the MMM is a selective barrier, one component from a mixture can pass through it more readily than other components. The separation performance of the MMM is judged in two terms, the selectivity and permeability of gases. The formulas for the evaluation of permeability (P) and selectivity (αij) are given below:where Pi is the permeability of the i gas, Pj is the permeability of the j gas, and αij is the selectivity of the i gas from the mixture of binary gases i and j. When examining the suitability of membranes for mixed gas separation, permeance is a crucial factor. It describes the way that gas moves through the membrane. It is defined as the ratio of gas permeability to the effective thickness of the membrane and can be expressed aswhere pi is the permeance of the i gas. The unit of permeability is Barrer, and the unit of permeance is the gas permeation unit (GPU). Since selectivity is the ratio of permeability, it is unitless. In some of the research articles, enhanced separation performance (selectivity and permeability) after including fillers in the polymer matrix has been evaluated as the ratio of the performance by the mixed matrix membrane (MMM) to the performance by the polymeric membrane itself.The current work primarily focuses on the recent scientific and technological advances for the development of high-performance MMMs for gas separation applications. First the basics of transport mechanism will be discussed to understand the various possible ways molecules pass through the membrane. Afterward, a brief description of membrane types is given to understand the need for MMMs, followed by the introduction of the overview of the state-of-the-art in MMMs utilizing various kinds of fillers. The reported MMM morphologies, chemical properties, and thermal characteristics are summarized and discussed. Taking into consideration the status, applicability, and future theoretical research in estimating the gas separation performance of MMMs, the Conclusions and Future Directions section summarizes the challenges and future perspectives for the development of high-performance MMMs.

The solution-diffusion mechanism (SDM) assumes no voids in the membrane and no particular reactions between CO2 and functional groups of membrane materials. The domination of SDM has been seen frequently in nonporous polymeric membranes. According to SDM, there are three steps involved during the transport of gas molecules across the membranes: at first, the gases are adsorbed in the membrane on the feed side, then diffuse across the membrane due to the driving force and finally escape from the membrane on the sweep side (Figure 1). Separation occurs due to the difference in the solubilities and diffusivities of the gases. Permeability is expressed in terms of the diffusion coefficient (D) and the solubility coefficient (S) as given below:As mentioned in eq 2, selectivity is the ratio of individual gas permeability; therefore, permeability and selectivity are related to the diffusion and solubility coefficients. The affinity and condensability of gas molecules in the membrane govern the solubility coefficient. For the gas to have a higher solubility coefficient, it should have a higher critical temperature. The diffusion coefficient of the membrane is dependent on the kinetic diameter of the gases, the polymer chain spacing, and the random motion of the chain segment in the membrane. The diffusivity coefficient increases with the decreasing kinetic diameter and increasing chain spacing and random motion of the chain segment in the membrane. Considering CO2, CH4, N2, and H2, CO2 possesses the highest critical temperature (304.2 K) among all four gases, which helps CO2 to dissolve in the membrane. Additionally, CH4 and N2 have larger kinetic diameters than CO2, which help in the faster transport of CO2 across membranes. H2 has faster transport than CO2 in the membrane due to its smaller kinetic diameter compared to CO2. Between CO2 and H2, CO2 has higher solubility and H2 has higher diffusivity, thus making it difficult to get high CO2/H2 selectivity considering only solution-diffusion.

The porous structure of inorganic fillers considerably aids the separation of the gases according to the molecular sieving mechanism. When the diameter of the pores is between the kinetic diameters of the gases in the gaseous mixture to be separated, the gases with a kinetic diameter smaller than the pore size are transported across the MMMs while the gases with a greater kinetic diameter than pore size get blocked (Figure 2). As the kinetic diameters of CO2 and other gases vary very little, the pore diameter should be precisely controlled as per the requirement. Microporous materials like porous graphene oxide, metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), and carbon molecular sieves (CMS) with pore sizes on a molecular scale and fine pore size distributions can be used to get higher selectivity for the separation of gases as filler in the MMMs.

This mechanism is inspired by biological membranes, where active and passive transport take place. The facilitated transport membrane has specific interactions between CO2 and the amine groups. The reversible chemical reaction between CO2 and a few functional groups helps to enhance the passage of CO2 from upstream to downstream across the membrane (Figure 3). The most common functional groups that acts as carriers for CO2 separation are the amine groups. CO2 reactions with primary, secondary, and tertiary amines are depicted as follows:It can be seen from the equations mentioned above that CO2 reacts with primary and secondary amines in either the presence or absence of H2O. In contrast, a tertiary amine only reacts with CO2 in the presence of water. In primary and secondary amines, 0.5 mol CO2 reacts with 1 mol amine to form the stable carbamate ion without water. The amount of CO2 reacting with primary and secondary amines increases from 0.5 to 1 mol for each mole of amine with water, leading to the increase in the reaction rate. The tertiary amine is a weak basic catalyst for the hydration reaction of CO2 in the presence of water, resulting in bicarbonate formation. Basic groups other than amine that act as CO2 carriers are PO43–, F–, CO32–, and −COO–. The reaction between CO2 and the carboxyl group is given as follows:where R denotes the alkyl group. As explained above, the presence of amine functional groups is very crucial to facilitate the transport of CO2 across the membrane. To achieve facilitated transport effects, there are various methods available to modify the fillers of MMMs, such as blending, grafting, conjugation, post-synthetic modifications, and the use of ligands that contain amine functional groups in their molecules during MOF, COF, and POF synthesis.

In recent times, a slow and steady increase in the usage of membranes in separation procedures has taken place. It is anticipated in the current situation where, notwithstanding the unpredictability of energy costs, membrane GS is contemplated for minimizing the ecological footprint and energy requirement costs of industrial operations. In this section, a detailed description is provided for membranes that can be categorized as inorganic, polymeric, and mixed matrix membranes; their morphological features are shown in Figure 4.

In recent years, inorganic membrane development has advanced pretty quickly at laboratory scale. The permselectivity of the porous inorganic membranes is influenced by many factors, which include the microstructures of the membrane and the support composite, the porosity, the tortuosity, and the affinity of the permeating species. Membranes constructed of inorganic materials like zeolites, metals, and ceramics function better and are more thermally and chemically stable. Sharma et al. fabricated an amino-silicate membrane integrated on a cost-effective macroporous industrial alumina-based ceramic support. The synthesized ceramic membrane displayed a CO2 permeance of 46.44 GPU with a CO2/N2 selectivity of 12.5 at 80 °C. However, owing to the challenges of the mass-production of defect free inorganic membranes, polymeric membranes and mixed matrix membranes predominate in commercial uses.

The nonporous polymeric membrane gas separation ideas were developed before any other membrane separation techniques. According to research by Mitchell et al., even in the absence of pores, mass transfer in polymeric and liquid films is possible. Graham studied the permeability of several gases through various membranes, such as films made of natural rubber, and observed that carbon dioxide showed the greatest penetration rate among all the measured gases and had the highest solubility in membrane materials. It was widely believed until the 1970s that glassy polymeric membranes had poor permeability but potentially strong permselectivity. This theory was invalidated after the discovery of poly(vinyl-trimethyl silane) (PVTMS) and, more notably, poly(trimethylsilylpropyne) (PTMSP). The design of these polymeric matrix membranes, which incorporate significant Si(CH3)3 and have great permeability coefficients, such as that obtained for PTMSP compared to all previously known polymers (primarily polyacetylenes), greatly influences the performance of gas separation and motivates the search for more advanced materials with high permselectivity. So far, a variety of polymers like chitosan, PVAm, carboxymethyl chitosan, matrimid, PEBAX, PVAm, PVA, polysulfone, and polyurethane, among others (as shown in Figure 5 and Table 3) have been employed commercially or at laboratory scale for CO2 capture. However, the counter connection between permeability and selectivity places limitations on polymeric membranes despite the fact that they are less expensive and easier to construct than inorganic membranes. In a permeability against selectivity plot, polymeric membranes frequently straddle the Robeson upper bound limit. Another significant issue with membrane-based CO2/N2 separation is CO2-induced plasticization. Different penetrants dissolve into the polymer matrix to cause plasticization, which destroys the structure of the polymer. Swelling of the membrane is yet another critical problem for the separation of CO2/N2. It is the most crucial parameter that must be considered while deciding on the kind of membrane material. When the polymer dissolves in the specified solvent or as a result of feed moisture, swelling happens. Owing to these and many other drawbacks associated with conventional membranes, researchers’ attention has been diverted toward the mixed-matrix membranes (MMMs), the design of which is one of the promising approaches to improve the performance of conventional polymeric and inorganic membranes.

Since the ground-breaking assessment by Okumus et al. on the possibilities of MMMs, a number of important review papers have been released. These materials classify MMMs in three categories as liquid–polymer, solid–polymer, and solid–liquid polymer MMMs. The fabrication of solid-polymer MMMs has drawn interest in recent years. Solid fillers incorporating polymer matrix MMMs can be traditional (such as carbon molecular sieves (CMSs), silicas, and zeolites) or novel (such as carbon nanotubes (CNTs), metal–organic frameworks (MOFs), graphene oxides (GO), spherical materials, layered materials, or delaminated materials). The structures of some of these materials are shown in Figure 6. The membranes composed of pure MOFs for gas separation include IRMOF-1, MIL-100 (Fe), ZIF-22, UIO-66 (Zr), Cu-(hfipbb)(H2hfipbb)0.5, MOF-5, ZIF-8, IRMOF-3, HKUST-1, MMOF, ZIF-7, ZIF-8, ZIF-90, and Co3(HCOO)6, among others. Due to the sheer improvement in gas separation performance after the incorporation of fillers in the polymer matrix, there is considerable interest in discussing various fillers and their behavior in the membranes toward CO2 capture as alternatives.

Zeolites are crystalline aluminosilicates having molecular-sized pores and gaps. On a molecular level, zeolite crystals are porous materials. They have a consistent network of pores and channels (between 3 and 15 Å), which form a nanoscale labyrinth that might potentially store water or other guest molecules. These molecular sieving capabilities caused zeolites to be employed as fillers in mixed matrix membranes for CO2 capture applications. Many researchers have shown the well-documented study of zeolite as a conventional inorganic filler in MMMs. The ability of zeolite-filled MMM to transport gas is significantly influenced by the intrinsic properties of the zeolite particles and the dense polymer phase. The anticipated enhancement happens when dispersed zeolite particles are incorporated into the dense polymer matrix. Yu et al. prepared a membrane using a polyether sulfone-grafted polyethylene glycol (PES-g-PEG) comb-like copolymer as matrix and malleable MFI zeolite as inorganic filler. At 35 °C, the MMM has a CO2 permeability and selectivity of 66.9 Barrer and 9.6, respectively. The enhancement in performance is achieved due to the interaction between the filler and the polymer matrix, which is supposed to have disrupted the packing of the polymer chains, enlarged the void, and facilitated gas diffusion. It is also believed that the filler’s mesoporous structure reduced the resistance to gas flow. Likewise, a large number of other studies have been conducted and published; Table 3 summarizes their results.

A graphene-based material, graphene oxide, was created in 1840, but it was not until 2004 that it gained popularity. Similar to graphite, GO has layers and numerous functional groups, such as lactol, carbonyl, carboxylic acid, and epoxy groups, that are covalently attached to its basal plane and edges. A complicated, inhomogeneous system made up of a hybrid sp2/sp3 system is generated as a result of the interlayer distance expansion. This also explains why graphene oxide has tunable optical, chemical, and electrical properties. Graphene oxide and graphene oxide-based hybrid materials are frequently used for CO2 capture. It has been shown that graphene and GO significantly increase CO2 transport through their frictionless nanochannels while blocking N2 or CH4 to produce high permeance and selectivity. Katare et al. synthesized graphene oxide nanosheets and utilized them as fillers in a chitosan matrix. The group conducted the postsynthetic modification (PSM) of GO sheets by conjugating them with lysine amino acid and then incorporating them into CS. The CO2/N2 gas permeation test revealed a significant enhancement in the separation performance of synthesized MMM when compared to that of neat CS. The presence of GO nanosheets increased the surface roughness of the membrane and consequently aided the active survey area available for molecules to travel across the membranes. Prasad et al. incorporated graphene nanoparticles (GNPs) as fillers and silk fibroin (SF) mobile carriers into the CS matrix and observed a significant improvement in gas separation performance compared to that of neat CS, as the CO2 permeance increased from 12 to 159 GPU and the CO2/N2 selectivity increased from 54 to 93. The cooperative effect of the carrier amines from SF supporting the facilitated transportation and the assistance of GNP in the solution-diffusion mechanism boosted the performance factors of the CS/SF/GNP MMM. Lee et al. engineered the surface of GO sheets via grafting with polyethylene glycol (PEG) and embedding with a poly(oxyethylene methacrylate) (POEM) monomer, and the solution was then casted onto a porous polysulfone (PSf) support along with a poly(trimethylsilyl)propyene (PTMSP) gutter layer. The highest CO2 permeance of 3169 GPU and CO2/N2 selectivity of 37.4 were achieved in the synthesized MMMs. A selective CO2 pathway was provided by the interlayer spacing between GO layers as obtained from the optimization of the Lewis acid–base interaction. Upon the construction of a very thin selective layer on the PSf support, an exclusive CO2 separation performance was achieved.

Carbon nanotubes (CNT), often known as Bucky tubes, are a family of nanomaterials that were first discovered in 1991. They are constructed of carbon atoms arranged in a two-dimensional (2D) hexagonal lattice. CNTs are 1D nanomaterials with excellent mechanical and thermal capabilities. They can be divided into two broad categories: single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs). SWCNTs are cylindrical sheets of rolled graphite with a maximum diameter of 1.5 nm. MWCNTs have a high surface area with interstitial channels and are made up of two or more concentrically manufactured cylinders separated by 0.35 nm, which is equivalent to the basal plane spacing. The ability of CNT fillers to capture and transport CO2 has led to an increase in their use in the membranes for gas separation. Thus, the incorporation of CNT in polymeric membranes has attracted interest as a result of the development of novel gas-separation membrane technologies: MMMs. Borgohain et al. synthesized a mixed matrix membrane with a different loading of MWCNTs into carboxymethyl chitosan (CMC) and utilized it for a CO2/N2 gas separation application. The fabricated MMM displayed a CO2 permeance of 43 GPU and a CO2/N2 selectivity of 45. It has been discovered that adding wrapped CNT to CMC produces thermally stable MMM, which considerably boosted the CO2/N2 selectivity. Similarly, many other researchers have also utilized CNT for CO2 capture, and their findings are summarized in Table 3.

Metal–organic frameworks (MOFs), a unique class of crystalline porous materials, are gaining increasing attention in the field of gas storage and separation. According to reports, MOFs can be modified before and after synthesis to achieve a strong affinity for CO2. They also offer the advantages of a tiny aperture size and tunable physical and chemical properties. Moreover, the high porosity of MOFs hastens the movement of gases through the membrane layer. The resulting MMM makes use of the superior qualities of both the polymer and the MOFs, and the chemical bonds created between the polymer chains and the MOFs boost the separation efficiency. Over time, MOFs have drawn increased attention as a result of these characteristics. Basu et al. produced PDMS membranes with a variety of fillers, including MIL-53(Al), MIL-47(V), HKUST-1, and ZIF-8. To separate CO2 from diverse gas mixtures, Basu et al. also fabricated analogous MMMs by encapsulating the MIL-53(Al), ZIF-8, and HKUST-1 in matrimid. They demonstrated that, when compared to ZIF-8, the HKUST-1 and MIL-53(Al) had improved selectivity with a 0–30 wt % loading. Moreover, they asserted that this outcome was caused by the potent CO2 interactions between HKUST-1’s unsaturated metal sites and its breathing behavior while interacting with MIL-53(Alhydroxyl) groups. Coterillo et al. proposed a ternary system by incorporating ZIF-8 in a membrane made of chitosan and [Emim][Ac] and suggested a fascinating method for improving the interface morphology. They claimed that the existence of the ionic liquid at the interface boosted the chitosan–ZIF-8 phase adhesion, which led to the greatest CO2/N2 separation performance at low ZIF-8 loading (5 wt %). Hybrid membranes with amine-functionalized UiO-66–NH2 and 5 to 20 wt % pure UiO-66 have been developed. These membranes have a size range from 60 to 80 nm on average. The amount of inorganic phase enhanced the CO2 permeability for both types of nanoparticles in a proportional manner, achieving an improvement of about 2.5× over the value of the pristine membrane (Table 3). These findings revealed that the exceptional affinity of UIO-66 for CO2 was caused by an −OH coordinated bond related to the Zr cluster. The CO2/N2 selectivity, which showed a peak between 7.5 and 10 wt % loadings, showed a different trend. Compared to the pristine MOF, the UiO-66–NH2 has a higher affinity, as the polymer matrix allowed for a greater selectivity enhancement (88% vs 42%). Interestingly, mixed matrix membranes made with UiO-66-NH2 continued to perform consistently even when humidity was present. In a recent study, Lei et al. reported high-performance CO2 adsorption by a Ni-based metal–organic framework (MOF-74(Ni)). The high capture is associated with the abundantly available adsorption site and narrow micropore channels. These MOFs provide high CO2 selectivity (CO2/N2= 49), high capture efficiency, and well-tuned isosteric heat of CO2 adsorption. Guo et al. reported the synergetic effect of adsorption and diffusion for separating CO2/CH4 using a ZIF-8-based mixed matrix membrane. The selective adsorption and molecular sieving properties of ZIF-8 nanoparticle fillers improve membrane separation performance. Many research studies have been conducted on the flat sheet mixed matrix membrane rather than thin-film hollow fiber mixed matrix membranes because the incorporation of fillers on the flat sheet membrane significantly improved the gas separation performance. However, hollow fiber membranes have a potential future in various gas separation processes because of their benefits, such as their self-supporting structure, high packing density, better separation factor, mechanical strength, and thermal and chemical resistivity. Li et al. investigated the formation of the selective layer containing a MOF-based hollow fiber membrane support and its gas separation performance. Amine-functionalized UiO-66 nanoparticles were incorporated into the Pebax 2533 thin selective layer on the polypropylene (PP) hollow fiber supports by a dip coating process. The selectivity (CO2/N2) and permeance of the CO2 of the hollow fiber mixed matrix membrane containing 10 wt % UiO-66 were reported to be 37 and 26 GPU, respectively. The hollow fiber configuration gives exceptional mass transfer properties due to its high surface area and selectivity. Therefore, these configurations are more significant for industrial gas separation applications. Hollow fiber mixed matrix membranes (filler incorporated) with thin selective layers are recommended, as the thickness of the membrane is important for the separation of gas.

MMMs require the homogeneous dispersion of inorganic fillers at the nanoscale level in a polymer matrix to incorporate both the advantageous properties of the polymer and filler materials. Their success relies heavily on the polymer matrix, the inorganic filler, and the interaction between the two phases. The choice of fillers, which affects the gas separation wellness of a membrane, poses the most challenges during the development of MMMs. In case of a defect-free MMM, the minimum separation performance is determined by the polymer phase, while the upgradation of membrane permeability and selectivity is determined by the filler. The following considerations should be taken into account while choosing proper fillers for MMM: 1. Porous materials, such as MOFs or zeolites, can typically be utilized as nanofillers because they act as molecular sieves and boost selectivity due to their carefully designed architectures. 2. To improve the target gas solubility and selectivity in the MMM, the desired gases, such as CO2, are made to interact with the filler’s surface functionality, such as −NH2. 3. The development of high-performance MMMs depends on the compatibility of the filler phase and the polymeric matrix. 4. The use of MOFs with organic linkers equivalent to the functional groups of polymer chains is appropriate to satisfy the screening requirements for MOFs as MMM fillers. 5. MOFs with amino groups or surfaces containing ammonia are widely used to improve the contact between the filler and the polymer. 6. Hydrogen bonds can be created between the functional groups and polymer chains in MOFs with surface functionalization to improve the compatibility and interactions between the filler and the polymer. 7. By combining two distinct fillers in one polymer, the capacity to separate gases can be enhanced. The following considerations should be taken into account while choosing a polymer for efficient MMM: 1. The foremost criteria is the film-forming ability; the selected polymeric material should have properties of forming a free-standing stable and rigid film/membrane. 2. Flexibility of the particular polymer during the solvent removal process is the main concern. The glass transition temperature (Tg) is a factor that can be indirectly related to the flexibility of polymeric materials, i.e., the lower the Tg, the more flexible the material. Hence, the matrix’s Tg should be considered while selecting of polymer material for the matrix. 3. In order to prevent the blockage of the accessible travel path for gas molecules, the polymer must not pierce the pores of the fillers, which will impair the separation performance. 4. The polymer and filler particles need to get along for the transportation of gas across the polymer. A noticeable improvement in the gas separation performance is achieved when the permeabilities of the faster-permeating component through the polymer and the filler are substantially comparable. 5. Another factor that influences the selection of materials is the polymers’ processing capabilities after the addition of filler particles. 6. Another important factor in the development of flawless membranes is the particle dispersion inside the matrix. Polymers should have the ability to embed the filler particles into their matrixes. So far various MMMs have been created using zeolites, MOFs, CNTs, GO, activated carbon, CMS, metal oxides, and other filler particles. 7. The polymer should be mechanically, thermally, and chemically stable to withstand harsh environmental conditions in applications such as flue gas separation. 8. For the mixed matrix approach to be fully effective, the parameters of the polymer matrix should be close to the “upper bound” on the Robeson plot.

Despite the clear benefits that MMMs offer and the vast area of additional research, numerous issues still need to be resolved. There is no precise technique for the fabrication of nonporous membranes or composite membranes based on MMMs. Thus, there is uncertainty about making membranes with uniform nanoparticle distribution with as low of agglomeration as possible. According to conventional thinking, it is challenging to get a better affinity between fillers and the polymer matrix without adequate functionalization. Furthermore, because the additive-free volume is formed at the interface between nanoparticles and the matrix, imperfect compatibility may harm the achievement of greater permeability and diffusivity enhancements. Poor adhesion causes interface problems such voids, a rigidified polymer, and pore blockage. Moreover, excess inorganic filler dispersion results in its aggregation in the polymer matrix and degrades the properties of the membrane when dealing with high loading compositions of inorganic filler. Moreover, if the polymer matrix is loaded with more inorganic fillers and/or incorrect particle sizes, it leads to pinholes and voids that degrade separation properties of the membrane. There have been no proposed methods for determining the surface energy of the injected nanoparticles beforehand to determine the variety of nanoparticle and polymer matrix combinations. Polymers used as matrices in fabricated MMMs must logically have appropriate permeability and permselectivity even in their raw state. As a result, the introduction of nanoparticles boosts the separation performance even further. Nevertheless, overcoming the Robeson upper bound curve remains a significant challenge. The separation factors and permeability coefficients must be at specific optimized values in practice. Therefore, picking random combinations of polymers and nanoparticles is not advisable. It is necessary to consider the requirements of future industrial applications.

The uses of several membrane types for CCS applications are enumerated in this Review. The significance of mixed matrix membranes over other conventional membranes, namely inorganic and polymeric, for gas separation applications is discussed in this paper. MMMs are a wonderful invention that outperforms the limitation of inorganic and polymeric membranes toward permeability and selectivity. Even though there are many accessible reviews of MMMs, research will continue until new efficient inorganic filler materials are created. Inorganic fillers in MMMs help to achieve higher selectivity and permeability in gas separation applications. Therefore, MMMs comprising various inorganic fillers are found to be a potential research direction for CO2 separation. The thermal, chemical, and mechanical strength, permeability, and selectivity of the membrane can be tailored by changing both the inorganic and organic phases. More and more studies have been published every year demonstrating exceptional interactions of polymer and filler materials with good gas separation performances. However, industrial engineering research in this field is still in its early stage. This provides many future research and development opportunities to address these engineering challenges. Recent research has mainly been focused on two-dimensional porous materials like MOF, COF, and graphene oxide nanosheets as filler materials in the polymer matrices. The data of gas separation by various filler-based MMMs revealed that agglomeration in the polymer matrix occurred at higher concentrations of filler in the matrix, resulting in significant deterioration in the gas separation performance. To manage pore size and prevent filler agglomeration, an appropriate amount of fillers in the polymer matrix is crucial. According to research, MMMs can transport gases with exceptional efficiency by increasing the matrix–filler dispersion and employing lower filler concentrations. Therefore, it is essential to develop thin membranes to attain the maximum flux to reduce the cost of separation. Considering sonication and ball milling as the most widely used exfoliation methods, a high-performance homogeneous membrane can be produced with the proper shear forces obtained from sonication or ball milling. For the creation of a thin membrane with excellent performance, the choice of a proper filler–matrix pair is crucial. However, the higher price of filler materials restricts their use on a large scale. In addition, the industrialization of MMMs for CCS applications has necessitated the use of renewable, cost-effective, and naturally occurring polymeric materials. However, the fabrication and reproducibility of thin, defect-free, and biopolymeric MMM instead of a synthetic membrane on a large scale is still challenging.