Research Progress on Metal–Organic Framework Composites for Greenhouse Gas Adsorption and Separation

Abstract

The excessive emission of greenhouse gases (CO₂, CH₄, SF₆, and CF₄.) is a primary driver of global climate change, making the development of efficient adsorption and separation technologies critically important for achieving carbon reduction goals. Metal–organic frameworks (MOFs) have attracted considerable attention in this field due to their crystalline porous structures, ultrahigh surface areas, and tunable pore architectures. However, pristine MOFs face significant bottlenecks including poor water stability, high bed pressure drops caused by their powdered form, and limited mass transfer, which severely hinder their industrial application. The integration of MOFs with functional materials such as carbon materials, polymers, metal oxides, and porous SiO₂ offers a synergistic strategy to overcome these limitations. Carbon materials provide hydrophobic barriers and mesoporous transport channels, polymers enhance processability and mechanical strength, metal oxides introduce basic sites for enhanced chemisorption, and MOF-on-MOF heterostructures enable atomic-level interfacial integration and pore synergy. This review systematically summarizes recent advances in MOF composites for the separation of CO₂, CH₄, and fluorinated greenhouse gases (SF₆, CF₄.), with an emphasis on design strategies, structure–performance relationships, and synergistic mechanisms across different composite types. Finally, the current challenges including scalable synthesis, long-term stability, and separation performance under realistic conditions are discussed, and future directions toward rational design and functional synergy for industrial carbon capture and fluorinated gas emission reduction are envisioned.

1. Introduction

According to the IPCC Sixth Assessment Report, the excessive emission of greenhouse gases has become a critical factor driving global warming and disrupting ecological balance. A commonly used metric for evaluating the greenhouse effect is the global warming potential (GWP), which comprehensively reflects both the heat-trapping ability of a gas and its atmospheric lifetime [1]. The Kyoto Protocol and subsequent international conventions have identified carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases (SF₆, CF₄) as the primary greenhouse gases requiring control. Among these, CO₂, CH₄, and N₂O are the primary drivers of climate change [2]. CO₂, as the reference gas with a GWP of 1, mainly originates from fossil fuel combustion and industrial processes [3,4]; CH₄ has a GWP approximately 28 times that of CO₂ and is primarily emitted from natural gas extraction and agricultural livestock [1,5]; N₂O has a GWP approximately 265 times that of CO₂ and also poses the dual hazard of depleting the stratospheric ozone layer [6]. SF₆, CF₄, and C₃F₈ are typical electronic specialty gases widely used in semiconductor manufacturing, electrical equipment insulation, and refrigeration systems [7]. Although these fluorinated gases play an irreplaceable role in industrial technologies, their environmental impact is severe. For example, SF₆ has a GWP approximately 23,500 times that of CO₂ and an atmospheric lifetime of up to 3200 years, while CF₄ has a GWP of 6630 times that of CO₂ and an atmospheric lifetime of up to 50,000 years [8,9]. Once released, these gases cause long-term and irreversible damage to the Earth’s radiative balance [1].

Currently, the separation and recovery technologies for greenhouse gases such as CO₂, CH₄, N₂O, and fluorinated gases (e.g., SF₆, CF₄, C₃F₈) mainly include cryogenic distillation, membrane separation, and adsorption separation [10,11]. Among them, cryogenic distillation can achieve high-purity products but suffers from extremely high energy consumption and poor economic efficiency when treating low-concentration greenhouse gases [11]. Membrane separation is operationally simple but limited by the separation selectivity and stability of membrane materials [12]. In contrast, adsorption separation is considered one of the most promising technologies for greenhouse gas recovery due to its mild operating conditions, low energy consumption, and environmental friendliness [13]. The core of adsorption separation technology lies in the design and development of high-performance adsorbents. Although traditional porous materials such as activated carbon and zeolite molecular sieves have been widely used in the field of gas separation, their single pore structures and poorly tunable surface chemical properties limit their adsorption capacity and selectivity for greenhouse gases [14,15,16]. Therefore, the development of novel high-efficiency adsorbents has become the key to advancing greenhouse gas recovery technologies.

Metal–organic frameworks (MOFs) are a class of crystalline porous materials formed by the self-assembly of inorganic metal nodes (metal ions or metal clusters) and organic linkers through coordination bonds [17,18,19]. This unique “molecular building block” structure endows MOFs with exceptional designability in terms of pore size, pore shape, and chemical environment on pore surfaces, all of which can be precisely tuned by selecting different metal nodes and organic linkers. Their pore sizes typically range from 0.3 to 10 nm [20,21]. Benefiting from their highly ordered pore architectures, MOFs exhibit extremely high specific surface areas, 7000 m²/g. In addition, MOFs also possess features such as high porosity, structural diversity, and the ability to introduce functional sites through post-synthetic modification [22,23].

However, pristine MOF materials still face several bottlenecks in practical applications. First, insufficient water stability is one of the key issues limiting their application. The coordination bond nature of MOFs makes them sensitive to moisture. Water molecules not only compete with target gases (e.g., CO₂) for open metal sites (OMS) but can also directly attack metal–linker bonds, leading to framework hydrolysis and structural collapse [24,25,26]. Second, engineering problems arising from the powdered form cannot be ignored. MOFs synthesized by conventional solvothermal methods are typically nano- or micrometer-sized powders. When used directly in fixed-bed adsorption columns, they can cause excessively high bed pressure drops, severe dust pollution, and pipeline clogging, greatly increasing the difficulty of industrial application [27,28]. Third, the mass transfer performance of MOFs is also limited. Pristine MOF materials are predominantly microporous, which is unfavorable for the rapid diffusion of large molecules or mixed gases [29]. Moreover, the pores of some MOFs tend to collapse upon removal of guest molecules, further compromising their adsorption performance [30,31,32]. Meanwhile, the long-term stability of MOF materials under harsh conditions such as acidic, alkaline, and high-temperature environments still requires improvement [33].

Faced with the bottlenecks of pristine MOF materials in terms of stability, processability, and mass transfer performance, researchers have gradually recognized that the inherent properties of a single MOF can hardly simultaneously meet the multiple requirements of practical applications for adsorbents [34,35]. Against this background, the strategy of compositing MOFs with functional materials has emerged, aiming to overcome the performance limitations of single-component materials through synergistic effects among multiple components [36]. MOF composites refer to novel materials formed by combining MOFs, either as the host or guest, with one or more functional materials (such as carbon materials, polymers, metal oxides, porous silica, or other MOFs) through physical mixing, in situ growth, or chemical bonding [37]. Their synthesis strategies are diverse, mainly including one-pot synthesis, surfactant-assisted synthesis, precipitation, in situ synthesis, template synthesis, and stepwise synthesis [38]. Through such compositing, significant synergistic enhancement effects can be achieved, resulting in overall performance that is fundamentally different from that of individual components [36]. Therefore, the synergistic combination of MOFs and functional components endows the composites with unique structural advantages and functional diversity, rendering them promising for a wide range of applications including gas adsorption and separation, catalysis, energy storage, and sensing [39,40].

In recent years, researchers worldwide have published a series of important reviews on MOF composites in the field of gas adsorption and separation. Regarding the research themes, existing publications exhibit the following characteristic: most reviews focus on CO₂ as a single gas system, while systematic summaries on CH₄ purification and the separation of fluorinated greenhouse gases (SF₆, CF₄.) remain relatively scarce. Against this background, this review focuses on the research progress and synergistic mechanisms of MOF composites in greenhouse gas adsorption and separation, and discusses the prospects and challenges for their industrial applications (

Table 1. Comparison of this review with recent review articles on MOF composites for gas adsorption and separation.

Feature	This Review	Other Reviews
MOF composites for CO2 capture	√	Many
MOF composites for CH4 purification	√	Few
MOF composites for fluorinated gases (SF6, CF4)	√	Very few
Systematic summary across multiple greenhouse gases	√	Rare

).

2. Types and Design Strategies of MOF Composites

Based on the nature of the functional component, MOF composites can be broadly classified into four representative categories: carbon-based materials (graphene, carbon nanotubes, biochar, porous carbon), polymers (cellulose, chitosan, PVDF, Pebax), metal oxides (MgO, CaO, ZnO), and MOF-on-MOF heterostructures (core–shell, yolk–shell) (Figure 1). Other types, such as MOF/porous SiO₂ and MOF/ionic liquid (IL) composites, are briefly discussed in Section 2.5, but the four categories above represent the most widely studied and developed classes. Their design strategies focus on achieving synergistic effects to overcome the limitations of pristine MOFs, including poor water stability, low processability, and mass transfer bottlenecks. Key strategies include interface engineering to enhance component compatibility, hierarchical pore construction to improve diffusion, and surface functionalization to introduce additional adsorption sites. These approaches collectively enable the rational design of MOF composites with enhanced performance for greenhouse gas adsorption and separation.

This review primarily focuses on MOF composites as adsorbents for greenhouse gas capture. Membrane separation and shaping strategies (e.g., pelletization, granulation, packed columns) are briefly discussed only where they directly support or relate to the adsorption performance of MOF composites. A detailed discussion of membrane-based separation or industrial shaping processes is beyond the scope of this review. For readers unfamiliar with the nomenclature of metal–organic frameworks,

Table 2. Nomenclature of metal–organic frameworks (MOFs) discussed in this review.

MOF Acronym	Full Name	Metal Cluster	Organic Linker
ZIF-7	Zeolitic Imidazolate Framework-7	Zn2+	Benzimidazole
ZIF-8	Zeolitic Imidazolate Framework-8	Zn2+	2-Methylimidazole
ZIF-67	Zeolitic Imidazolate Framework-67	Co2+	2-Methylimidazole
HKUST-1	Hong Kong University of Science and Technology-1	Cu2+	1,3,5-Benzenetricarboxylate (BTC)
UiO-66	Universitetet i Oslo-66	Zr4+	1,4-Benzenedicarboxylate (BDC)
UiO-66-NH2	Amino-functionalized UiO-66	Zr4+	2-Amino-1,4-benzenedicarboxylate
UiO-66-Br2	Bromo-functionalized UiO-66	Zr4+	2,5-Dibromo-1,4-benzenedicarboxylate
MIL-101(Cr)	Matériaux de l’Institut Lavoisier-101	Cr3+	1,4-Benzenedicarboxylate (BDC)
MIL-53(Al)	Matériaux de l’Institut Lavoisier-53	Al3+	1,4-Benzenedicarboxylate (BDC)
MOF-5	Metal–Organic Framework-5	Zn2+	1,4-Benzenedicarboxylate (BDC)
MOF-74(Ni)	Metal–Organic Framework-74	Ni2+	2,5-Dihydroxy-1,4-benzenedicarboxylate
MOF-177	Metal–Organic Framework-177	Zn2+	1,3,5-Benzenetribenzoate (BTB)
Bio-MOF-14	Biological Metal–Organic Framework-14	Zn2+	9,10-Anthracenedicarboxylate (ADC)

lists the acronyms, full names, metal ions, and organic linkers of the MOFs discussed in this review.

2.1. MOF–Carbon Composites

Carbon-based composites are an important class of functional materials formed by combining MOFs with carbon materials such as graphene/graphene oxide (GO), carbon nanotubes (CNTs), porous carbon (PC), activated carbon (AC), and biochar. Carbon materials like graphene and CNTs possess high specific surface areas, providing additional adsorption sites for MOFs. Meanwhile, the oxygen-containing functional groups on their surfaces can act as nucleation sites, guiding the in situ growth of MOF crystals. Furthermore, the introduction of carbon materials can construct mesoporous or macroporous channels between MOF particles and at the MOF–carbon interfaces, forming a hierarchical pore system featuring synergistic microporous, mesoporous, and macroporous structures. This significantly alleviates the mass transfer limitations of pristine MOFs caused by their predominantly microporous nature [44]. In terms of stability, graphene and CNTs exhibit excellent mechanical properties and can serve as structural scaffolds, providing stable mechanical support for MOFs and effectively suppressing structural collapse and volume expansion during cycling [45,46]. Meanwhile, the hydrophobic surfaces of carbon materials can build protective barriers for MOFs, resisting water molecule attack and preventing MOF hydrolysis, thereby significantly enhancing the chemical stability of the composites under humid conditions [47,48].

2.1.1. Synthesis Strategies

MOF–carbon composites are typically prepared by in situ growth of MOF crystals on carbon substrates or by physical mixing of pre-synthesized MOFs with carbon materials. The oxygen-containing functional groups (carboxyl, hydroxyl, epoxy) on graphene oxide, carbon nanotubes, and activated carbon can coordinate with metal ions, acting as nucleation sites that guide the heterogeneous crystallization of MOFs. For example, HKUST-1 is grown on acid-treated multi-walled carbon nanotubes by dispersing the nanotubes in the MOF precursor solution under solvothermal conditions [49]. Graphene oxide sheets are mixed with MOF precursors, where zinc or copper ions bind to the oxygen groups, leading to a sandwich-like alternation of GO layers and MOF crystals [50]. Biochar-based composites are synthesized by oxygen-limited pyrolysis of agricultural wastes followed by in situ MOF growth, where the biochar surface provides abundant functional groups for anchoring MOF nanoparticles [51,52].

2.1.2. Graphene-Based Composites

Graphene is a two-dimensional carbon material possessing high electrical and thermal conductivity, an extremely high theoretical specific surface area, excellent mechanical strength and flexibility, good chemical stability, and unique optical properties [53,54]. Graphene oxide (GO) can be obtained by oxidizing graphite with strong oxidizing agents, and further reduction yields reduced graphene oxide (rGO) [55]. The sheet structures of GO and rGO are rich in reactive oxygen-containing functional groups such as hydroxyl (-OH), carboxyl (-COOH), and epoxy (C-OC) groups. These hydrophilic groups not only facilitate their dispersion in aqueous media but also provide an ideal platform for the in situ growth and stable anchoring of MOFs [56,57].

In the construction of MOF-based composite adsorbents, graphene and its derivatives play multiple roles. First, the oxygen-containing functional groups on the surfaces of GO and rGO can serve as additional adsorption sites, enhancing the affinity for polar greenhouse gases such as CO₂ through hydrogen bonding or electrostatic interactions, thereby creating synergistic effects with the pore adsorption of MOFs [58]. Second, graphene sheets can effectively prevent the agglomeration of MOF particles, while MOF particles can also prevent the restacking of graphene sheets [46]. This enables the construction of mesoporous channels between MOF particles and at the MOF–graphene interfaces, forming a hierarchical pore system featuring synergistic microporous and mesoporous structures, which alleviates the mass transfer limitations of pristine MOFs caused by their predominantly microporous nature [45]. Furthermore, the hydrophobic nature of graphene can build protective barriers for MOFs, suppressing the competitive adsorption of water molecules from flue gas. This is particularly relevant for practical applications such as CO₂ capture from wet flue gas in coal-fired power plants.

To date, the modification of various MOFs with graphene oxide has demonstrated excellent greenhouse gas adsorption performance. For example, MOF-5@GO exhibits excellent physical CO₂ adsorption performance at 25 °C and 1 bar. The SEM image shows that 3Li-MOF-5@GO retains a cubic morphology similar to that of MOF-5, with a reduced crystal size due to Li⁺ doping. After doping with 3% Li⁺, its specific surface area increases by 52.53%, and the CO₂ adsorption capacity reaches 98.32 cm³/g. This is attributed to the Li⁺ doping strengthening the Zn-O coordination bonds and amplifying the electrostatic interaction with the quadrupole moment of CO₂ [50]. In another study, Cirujano et al. [58] developed a Co-MOF-74@GO2 composite via a green aqueous room-temperature synthesis using non-graphitized carbon nanofiber-derived GO (GO2) (Figure 2a). Among the materials tested, Co-MOF-74@GO2 achieved the highest CO₂ uptake of approximately 116.5 cm³/g (5.2 mmol/g) at 0 °C and 1 bar, significantly outperforming the corresponding Zn- and Ni-based analogues. The superior performance is attributed to the smaller crystallite size of Co-MOF-74, which provides more surface-exposed active sites, and the higher density of oxygen-containing functional groups on GO2, which enhances interfacial interactions with the MOF and creates additional CO₂ binding sites (Figure 2b,c). Collectively, these studies indicate that graphene-based MOF composites hold great promise for greenhouse gas adsorption and separation applications, including CO₂ capture and CH₄ purification [59].

2.1.3. Carbon Nanotube-Based Composites

Carbon nanotubes (CNTs) possess a unique one-dimensional tubular structure, good mechanical strength, and excellent thermal stability. The nanoscale pore spaces within CNTs can generate significant confinement effects, which greatly influence gas adsorption behavior [60]. MOF-CNT composites exhibit significant synergistic effects by combining the high porosity and structural diversity of MOFs with the superior mechanical, electronic, and thermal properties of CNTs. The introduction of CNTs can enhance the structural stability of MOFs by dispersing electrostatic charges or providing internal confined spaces. The formation of ultramicropores at the MOF-CNT interfaces, together with the improved structural integrity of MOFs, contributes to enhanced storage capacities for gases such as hydrogen, carbon dioxide, and methane. Meanwhile, this synergy can optimize the adsorption selectivity of the composites toward specific gas mixtures (e.g., CO₂/N₂). Furthermore, the specific surface area and total pore volume of MOFs are often increased upon compositing with CNTs.

HKUST-1 (chemical formula [Cu₃(BTC)₂], BTC = 1,3,5-benzenetricarboxylate) is an important MOF material that has attracted considerable attention due to its open copper metal sites, high thermal stability, and excellent adsorption properties [61]. To further enhance its adsorption and separation performance, Xiang et al. [49,62] introduced carboxylated multi-walled carbon nanotubes into HKUST-1. For example, upon introducing CNTs into HKUST-1, the pore volume of the composite increased from 0.73 to 0.87 cm³/g. At 18 bar and 298 K, and the CO₂ and CH₄ uptake increased from 295 to 595 mg/g and from 72 to 120 mg/g, respectively [49]. In another study, using ideal adsorbed solution theory (IAST) based on the dual-site Langmuir–Freundlich equation, Xiang et al. [62] evaluated the CO₂/CH₄ separation potential of CNT@HKUST-1 and several representative MOFs, including MOF-177, UMCM-1, ZIF-8, MIL-53(Al), and HKUST-1 (Figure 3a). For an equimolar CO₂/CH₄ mixture at pressures ranging from 1 to 20 bar, CNT@HKUST-1 exhibited the highest selectivity among the tested materials, with values ranging from 5.5 to 7.0 (Figure 3b). Collectively, these studies demonstrate that CNT-based MOF composites hold great promise for greenhouse gas adsorption and separation applications, including CO₂ capture, CH₄ purification, and acid gas removal.

2.1.4. Porous Carbon-Based Composites

Porous carbon materials are considered one of the most promising candidates for gas-related applications owing to their low cost, high thermal and chemical stability, high adsorption capacity, and fast regeneration capability. Recent studies have demonstrated that incorporating microporous particles into the meso-/macroporous networks of carbon materials to create hierarchical porous systems can optimize structural features and enhance adsorption performance (Figure 4). For example, HKUST-1@ACM composites were prepared by in situ synthesis of HKUST-1 within a porous carbon monolith. The composite retained the monolithic shape and exhibited a unique hybrid structure combining features of both ACM and HKUST-1. Its volumetric CO₂ uptake reached approximately 24 cm³/g at 1 bar and 25 °C, which is comparable to that of pristine ACM. In terms of CO₂/CH₄ separation, the composite showed superior selectivity compared to pristine ACM. IAST calculations revealed that for an equimolar CO₂/CH₄ mixture, the selectivity factor of HKUST-1@ACM was approximately 5.5 at 1 bar, while that of pristine ACM was only 3.5 [63].

2.1.5. Biochar-Based Composites

Biochar is a porous carbon material prepared from agricultural wastes (such as straw, nut shells, wood chips, and leaves) through oxygen-limited pyrolysis. It possesses three inherent advantages. First, its raw materials are widely available and low in cost, aligning with the principles of circular economy. Second, it features abundant surface functional groups (e.g., hydroxyl, carboxyl, and carbonyl groups), which facilitate its functional modification. Third, its porous structure can be tuned over a wide range of scales, forming hierarchical pore networks with coexisting micropores and mesopores [65,66].

Upon compositing biochar with metal–organic frameworks (MOFs), significant synergistic effects arise between the two components. Unlike synthetic carbon materials such as graphene and carbon nanotubes, biochar possesses unique structural and chemical advantages stemming from its natural biomass source and oxygen-limited pyrolysis preparation process. On one hand, the three-dimensional carbon skeleton of biochar and its naturally abundant oxygen-containing functional groups provide a low-cost, high-activity growth substrate for MOFs. These functional groups can coordinate with the metal nodes of MOFs, which not only inhibits the agglomeration of MOF nanoparticles but also significantly enhances the structural stability of the composite [51,52]. On the other hand, the inherent mesopores of biochar interconnect with the micropores of MOFs, forming a hierarchical pore network featuring coexisting micropores and mesopores. In this architecture, the mesoporous channels serve as fast diffusion pathways for gas molecules, while the micropores provide high-density adsorption sites. Together, they overcome the mass transfer bottleneck of pristine MOFs caused by their predominantly microporous nature, thereby substantially increasing the saturated adsorption capacity [67]. Most critically, active site synergy is achieved. The metal nodes (e.g., Zr-O clusters, Cu²⁺, Fe³⁺) and organic linkers (e.g., -COOH, -NH₂) provided by MOFs, together with the oxygen-containing functional groups (e.g., -OH, C=O) contributed by biochar, create a coexistence of multiple types of active sites within the composite [68]. This endows the material with multiple adsorption mechanisms for greenhouse gases. For instance, acidic CO₂ can be captured by the basic functional groups on the biochar surface through acid–base interactions, coordinated with the metal nodes of MOFs, and simultaneously form hydrogen-bonding networks with functional groups. This multi-mechanism synergy enables the composite to exhibit significantly superior adsorption performance in complex gas environments compared to single-component materials.

In terms of adsorption capacity, Wang et al. synthesized three BC/Mg-MOF-74 composites with different mass ratios (Figure 5a). Under a 15% CO₂ atmosphere at 100 °C, BC/Mg-MOF-74 (1:1) exhibited the optimal CO₂ adsorption performance, achieving an uptake of 6.86 mmol/g (Figure 5b) [69]. This excellent performance is mainly attributed to the synergistic effect between biochar and Mg-MOF-74: the two components are composited via an in situ growth method, forming multiple adsorption sites rich in oxygen vacancies, lattice oxygen, and chemisorbed oxygen. CO₂ forms ionic bonds with metal ions through electrostatic attraction, while simultaneously undergoing chemisorption with oxygen-containing functional groups, ultimately generating carbonate species (Figure 5a). In terms of stability, the Cu-BTC/biochar composite retained approximately 92% of its adsorption capacity after five cycles, demonstrating better regeneration performance compared to pristine Cu-BTC (Figure 5c) [70]. Compared with other carbon-based MOF composites such as those using carbon nanotubes or graphene, MOF–biochar composites significantly reduce raw material costs and preparation barriers while maintaining good adsorption performance, making them more amenable to large-scale production and industrial application. Therefore, these materials hold great promise for greenhouse gas adsorption and separation applications, including CO₂ capture from industrial flue gas, natural gas purification, biogas upgrading, and air purification in enclosed spaces [68].

2.2. MOF–Polymer Composites

2.2.1. Synthesis Strategies

Polymer composites are an important class of functional materials formed by combining MOFs with polymers such as cellulose, chitosan, and polyvinylidene fluoride (PVDF) through physical blending, in situ growth, or chemical crosslinking [71,72]. Among commonly used polymer matrices, cellulose and its derivatives (e.g., nanocellulose, carboxymethyl cellulose) have become promising MOF support substrates [73,74] due to their abundant availability, non-toxicity, renewability, excellent hydrophilicity, flexibility, high strength, and low cost [71,75]. Chitosan has attracted considerable attention for its abundant amino functional groups. Polyvinylidene fluoride (PVDF), polyimide, polyvinyl alcohol (PVA), and polyurethane (PU) are widely used in the construction of MOF composites owing to their good film-forming ability and mechanical properties [76].

The composite forms of MOFs and polymers are diverse, mainly including MOF gels (hydrogels and aerogels), MOF/polymer membranes (mixed matrix membranes, MMMs), and polymer@MOF core–shell structures. Two main synthetic routes are used to prepare these composites: (i) confined polymerization of monomers inside MOF channels, and (ii) coating of MOF crystals with a polymer shell. For confined polymerization, vinyl monomers (e.g., styrene, methyl methacrylate) are infiltrated into the MOF pores, followed by radical polymerization initiated by heat or UV light, yielding polymer chains with narrow molecular weight distribution and unusual conformations [77]. For core–shell structures, pre-synthesized MOF particles are dispersed in a monomer solution, and polymerization is triggered to form a protective polymer layer (e.g., PN@MOF-5 via in situ polymerization of polynaphthalene) [78]. Alternatively, Pickering emulsions stabilized by MOF nanoparticles can be used to fabricate MOF–polymer composite microcapsules, where the polymer membrane forms at the oil–water interface [79].

2.2.2. Structural Features and Synergistic Mechanisms

The design of such composites aims to address engineering issues associated with pristine MOF powders, including poor processability and insufficient mechanical strength, while also introducing additional adsorption sites through the functional groups of polymers to enhance greenhouse gas adsorption performance [72,80,81,82]. The synergistic effects between polymers and MOFs are primarily manifested in the following aspects. First, polymers can significantly improve the processability of MOFs, resolving engineering problems such as high bed pressure drops and dust pollution caused by pristine MOF powders in fixed-bed reactors. Studies have shown that incorporating 30 wt% UiO-66 nanoparticles into a polyurethane (PU) matrix yields composites with optimal mechanical properties, with both tensile strength and elastic modulus substantially enhanced [83]. The composite hydrogel formed by polyvinyl alcohol (PVA) and UiO-66 exhibits higher structural stability than the pure polymer [71]. Second, the functional groups on polymer surfaces can serve as additional adsorption sites, enhancing the affinity for CO₂ [84]. Cellulose surfaces are rich in hydroxyl (-OH) groups, and functionalization can further enhance specific CO₂ adsorption [67]. Chitosan contains amino (-NH₂) groups, which can form reversible chemisorption interactions with CO₂, significantly increasing CO₂ adsorption capacity and selectivity [85]. Third, the introduction of polymers is not a simple mixing process. Instead, it constructs a hierarchical pore system featuring synergistic microporous, mesoporous, and macroporous structures, substantially optimizing gas mass transfer efficiency [86]. For example, Zhu et al. [77] prepared a MOF/cellulose composite aerogel with a three-dimensional porous structure by mixing MOF crystals, surface-functionalized nanocellulose crystals (CNCs), and carboxymethyl cellulose (CMC) to form a suspension, followed by sol–gel reaction and freeze-drying. This material exhibits excellent mass transfer performance in gas adsorption and separation applications.

Finally, the hydrophobic nature of polymers can build protective barriers for MOFs, suppressing competitive adsorption of water molecules from flue gas. Many polymers, such as PVDF and polyimide, possess good hydrophobicity. When used as a shell or matrix to encapsulate MOFs, their hydrophobic surfaces can effectively block the contact between water molecules and the active sites of MOFs, preventing framework hydrolysis and collapse under humid conditions [87]. Studies have shown that encapsulating MOFs in hydrophobic polymer matrices significantly reduces the water uptake of the composites and greatly enhances their adsorption stability and recyclability in water-containing systems such as wet flue gas CO₂ capture. For example, the PN@MOF-5 composite, formed by in situ polymerization of polynaphthalene within the channels of MOF-5 (Figure 6a), retains over 90% of its dynamic CO₂ adsorption capacity under humid conditions (Figure 6b,c) [78]. This is mainly attributed to the hydrophobic nature of polynaphthalene (water contact angle increases from 0° to 135°) and its pore-partitioning effect, which effectively prevents water molecules from attacking the MOF and suppresses competitive adsorption. In another example, Choe et al. [79] coated diamine-grafted Mg₂(dobpdc) with a hydrophobic poly(divinylbenzene) (PDVB) layer to prepare een-MOF/PDVB-5 composites. After exposure to 90% relative humidity for 7 days, the CO₂ adsorption capacity of the composite remained almost unchanged (from 9.79 to 10.15 wt%), while that of the uncoated een-MOF drastically decreased from 18.9 to 7.1 wt%. After 90 humid CO₂ adsorption–desorption cycles, een-MOF/PDVB-5 retained approximately 96.5% of its initial capacity (decreasing from 9.79 to 9.45 wt%), whereas een-MOF retained only about 41%. These results demonstrate that the hydrophobic PDVB layer effectively protects the diamine-grafted MOF from water vapor attack, significantly enhancing its long-term stability and recyclability under high-humidity conditions. Collectively, these studies demonstrate that hydrophobic polymer coating is an effective strategy for enhancing the long-term stability of MOF materials under humid operating conditions.

2.3. MOF–Metal Oxide Composites

2.3.1. Synthesis Strategies

Metal oxide composites are an important class of functional materials formed by combining MOFs with metal oxides such as MgO, CaO, and ZnO through in situ growth, physical mixing, or chemical bonding [88,89,90]. Integrating metal oxides into MOF matrices has proven to be a particularly effective strategy. Metal oxides possess unique surface unsaturated sites and redox activity, while MOFs offer ultrahigh specific surface areas, ordered microporous structures, and abundant coordination sites. The synergistic integration of these two components endows metal oxide@MOF (MO@MOF) composites with enhanced stability, improved charge transfer characteristics, and greater accessibility of active sites.

Two common approaches are used. The first is the in situ generation of metal oxide nanoparticles inside MOF cavities. Volatile metal–organic precursors (e.g., ZnEt₂, Ti(O-iPr)₄) are introduced into MOF-5 or ZIF-8 via gas-phase infiltration (CVD), followed by oxidative annealing to form nanosized ZnO or TiO₂ particles [88]. Alternatively, metal nitrates (e.g., Co(NO₃)₂) confined in MOF pores are decomposed at low temperatures (≈200 °C) to produce highly dispersed Co₃O₄ NPs [91]. The second approach is the encapsulation of pre-synthesized metal oxide nanoparticles by growing MOF shells around them. For example, Fe₃O₄ nanoparticles modified with polyvinylpyrrolidone (PVP) are used as seeds for the growth of ZIF-8 or HKUST-1, yielding core–shell Fe₃O₄@MOF composites [92].

2.3.2. Structural Features and Synergistic Mechanisms

Commonly used metal oxides mainly include alkaline earth metal oxides (e.g., MgO, CaO), transition metal oxides (e.g., ZnO, Fe₃O₄), and composite metal oxides (e.g., NiCo₂O₄). Among these, MgO and CaO have attracted considerable attention due to their strong basic sites. They can form stable carbonates with CO₂, enabling efficient chemisorption. Studies have shown that compositing MgO with ZIF-8 significantly enhances the affinity of the composite for CO₂, maintaining high adsorption capacity even under low partial pressure conditions [93]. Second, metal oxides possess good mechanical strength and thermal stability, allowing them to serve as structural scaffolds that provide mechanical support for MOFs and effectively suppress structural collapse during processing and cycling. For example, an Mg-doped CaO adsorbent derived from a calcium-based metal–organic framework (Ca-MOF) precursor exhibited a capacity decay of only 0.7 mmol/g after 30 CO₂ adsorption–desorption cycles, demonstrating significantly enhanced cycling stability [91]. In addition to these conventional roles, metal oxides can also act as bifunctional components. For example, Wu et al. [94] developed a Ag NPs@Fe₂O₃@MOF composite via a modified coprecipitation method (Figure 7). The Ag nanoparticles provide preferential adsorption of C₂H₂ over CO₂ through π-complexation, while under visible light irradiation, they generate a plasmon-driven photothermal effect that leads to distinct release behaviors for the two gases. As a result, the C₂H₂/CO₂ selectivity of the composite under visible light reached 4.73 at 293 K and 100 kPa, approximately twice that of pristine MOF-74. Furthermore, magnetic oxides (e.g., Fe₃O₄) allow low-energy regeneration via magnetically induced swing adsorption (MISA), where alternating magnetic fields generate localized heat for rapid desorption [92]. This further expands the applicability of MOF–metal oxide composites in energy-efficient carbon capture processes.

Various methods are available for preparing MO@MOF composites, including hydrothermal/solvothermal methods, sonochemical methods, mechanochemical methods, and microwave-assisted methods. Among these, the in situ growth of MOFs on metal oxide surfaces under high-temperature and high-pressure conditions by introducing metal oxide precursors into the precursor solution is a common strategy for constructing core–shell structures. Kim et al. [95] employed a hydrothermal oxidation-assisted MOF construction strategy to successfully prepare an Al@MIL-53 core–shell microstructure on an Al substrate by controlling the metal surface oxidation–dissolution process and MOF crystallization kinetics, and verified the generalizability of this method on Cu and Zn substrates. Ben Neon et al. [96] further revealed the competitive mechanism between MOF growth and alumina crystal plane rearrangement during the oxide–MOF transformation process, identifying pH variation as a key factor influencing MOF crystal morphology and distribution. Romero-Guerrero et al. [97] achieved in situ growth of ZIF-8 crystals on TiO₂ micropillar films using a layer-by-layer immersion method. DFT calculations confirmed that the formation of covalent bonds between Zn²⁺ and the TiO₂ surface is a critical step for nucleation. Collectively, these studies provide important theoretical guidance and experimental foundations for the design and preparation of MOF–metal oxide composites.

2.4. MOF-on-MOF Composites

MOF materials possess high specific surface areas, tunable pore sizes, and functionalizable pore environments, making them ideal adsorbents for gas separation. Nevertheless, the structural and functional limitations of single MOFs have become increasingly evident with ongoing research. Compositing two or more MOFs with different functionalities yields composites with superior performance. MOF-on-MOF composites combine the structural advantages of different MOFs: abundant metal centers and ligand diversity endow them with high specific surface areas and hierarchical pore structures, which balance the high selectivity of micropores with the fast mass transfer of mesopores/macropores, thus overcoming the selectivity–capacity trade-off of single MOFs [98,99,100].

2.4.1. Synthesis Strategies

The design core of MOF-on-MOF composites lies in achieving controllable spatial assembly of different MOF units. Based on existing research, the synthesis strategies can be mainly summarized into the following categories (Figure 8a–e).

To achieve controllable spatial assembly of different MOF units, epitaxial growth (Figure 8a) is the most straightforward strategy. This strategy requires that the host MOF and guest MOF have similar lattice parameters on specific crystal planes, thereby inducing oriented nucleation and growth of the guest MOF on the host surface. Furukawa’s team first reported this strategy in 2009. They selected [Cu₂(ndc)₂(dabco)]_n and [Zn₂(ndc)₂(dabco)]_n, which share the same tetragonal topology and similar lattice parameters. By mixing a Zn-based precursor with pre-synthesized Cu-based MOF crystals, they successfully prepared core–shell single crystals. This work demonstrated for the first time that MOFs with different metal centers can be controllably composited via epitaxial growth [101].

To overcome the strict lattice matching requirement of conventional epitaxial growth, researchers have further developed the heteroepitaxial growth strategy (Figure 8b). This strategy takes advantage of the low elastic constant of MOF materials and the healing effect of special functional groups in ligands on interfacial vacancies, allowing oriented growth between host and guest MOFs even with a lattice mismatch as high as 26%. This greatly expands the range of combinable MOF types. For example, the Wöll team successfully achieved MOF-on-MOF heteroepitaxial growth with a lattice mismatch as high as 20%, and through theoretical calculations revealed that the mechanism lies in the low elastic constant of MOFs and the ligand healing of interfacial vacancies [102].

Different from epitaxial and heteroepitaxial growth, surfactant-assisted growth (Figure 8c) does not strictly require lattice matching, offering broader applicability. Surfactants such as PVP can enhance the interfacial affinity between the host and guest MOFs, mediating the adsorption of guest metal ions onto the host surface and inducing heterogeneous nucleation and growth. For example, Kitagawa’s team constructed an NH₂-UiO-66@NH₂-MIL-125 core–satellite structure using a PVP-assisted strategy. Control experiments showed that without PVP, the two MOFs nucleated independently and failed to form a composite structure, confirming the critical role of the surfactant in mediating heterogeneous nucleation [100].

In addition to the above two strategies, ligand/metal ion exchange (Figure 8d) provides an alternative approach by exploiting the diffusion and exchange capacity of ligands or metal ions within the MOF to construct heterostructures layer by layer from the outside in. By controlling kinetic parameters such as reaction time and concentration, the shell thickness and component distribution can be precisely tuned, enabling the preparation of complex structures such as core–shell and yolk–shell architectures. For instance, Li’s team used a ZIF-67@ZIF-8 core–shell structure as a precursor. Leveraging the lower stability of ZIF-67 compared to ZIF-8, they selectively converted the core in a Co²⁺/methanol solution, successfully preparing a yolk–shell structure [103]. Zhang’s team, through ion exchange between Fe-MIL-88B and Co²⁺, constructed a Fe-MIL-88B@Co-MIL-88B bimetallic core–shell structure. By controlling the exchange time, they achieved continuous tuning of the shell thickness [104].

Furthermore, nucleation kinetics-guided growth (Figure 8e) offers a distinct advantage by exploiting differences in nucleation rates among different MOFs to achieve controllable construction of core–shell structures in a one-pot reaction system. This strategy eliminates the need for pre-synthesizing the host MOF, simplifies the synthesis process, and is suitable for large-scale preparation. Zhou et al. reacted two ligands, TCPP and BPDC, with Zr⁴⁺. Owing to its stronger coordination ability and faster nucleation, TCPP preferentially formed a PCN-222 core. Subsequently, BPDC underwent heterogeneous growth on the core surface to form a Zr-BPDC shell, successfully yielding a PCN-222@Zr-BPDC core–shell structure. This composite combines the mesoporosity of the core with the microporosity of the shell, demonstrating excellent performance in size-selective catalysis. Based on the same strategy, the team also constructed a PCN-134@Zr-BTB system, validating the generalizability of this method [104].

2.4.2. Structural Features and Synergistic Mechanisms

MOF-on-MOF composites integrate two or more MOFs with different structures and functionalities at the atomic scale, forming novel heterostructures with unique structural features and synergistic effects. These composites exhibit significant advantages over single MOFs in terms of hierarchical pore synergy, complementary functional sites, and external protection effects. Li et al. [105]. successfully achieved the epitaxial growth of Bio-MOF-11/14@Bio-MOF-14 core–shell structures by modulating the lattice parameters of the core via mixed acetate/valerate ligands to match those of the shell (Figure 9a). In terms of adsorption capacity, compositing a highly porous core with a shell possessing specific gas affinity enables synergistic enhancement. As demonstrated in Figure 9b, the CO₂ uptake of this core–shell structure is approximately 30% higher than that of the pure shell material (bio-MOF-14).

For selective separation, the pore size of the shell MOF can precisely regulate the entry of gas molecules, enabling a molecular sieving effect. Klomklang et al. [106] synthesized ZIF-67@ZIF-8 core–shell structures using a seed-mediated growth strategy. The small-pore ZIF-8 shell selectively restricts N₂ entry while allowing higher CO₂ permeation, resulting in significantly higher CO₂ adsorption capacity of the core–shell structure compared to monometallic ZIF-8. Additionally, lattice mismatch at the core–shell interface can create unique nanoconfined spaces, providing extra molecular sieving channels and further optimizing separation performance.

Figure 9. (a) Synthetic scheme for the preparation of the Bio-MOF-11/14@Bio-MOF-14 core–shell crystal. (b) CO₂ adsorption isotherms at 273 K (core, navy; core–shell, dark red; bio-MOF-14, green). Filled and empty circles represent adsorption and desorption points, respectively. (c) Schematic illustration of the synthesis of HKUST-1@ZIF-8 core–shell structures, where the numbers in circles indicate the order of reagent addition. (d) Water vapor adsorption isotherms at 298 K for HKUST-1@ZIF-8 core–shell adsorbent, compared with those of pure HKUST-1 and pure ZIF-8 [105,107].

Figure 9. (a) Synthetic scheme for the preparation of the Bio-MOF-11/14@Bio-MOF-14 core–shell crystal. (b) CO₂ adsorption isotherms at 273 K (core, navy; core–shell, dark red; bio-MOF-14, green). Filled and empty circles represent adsorption and desorption points, respectively. (c) Schematic illustration of the synthesis of HKUST-1@ZIF-8 core–shell structures, where the numbers in circles indicate the order of reagent addition. (d) Water vapor adsorption isotherms at 298 K for HKUST-1@ZIF-8 core–shell adsorbent, compared with those of pure HKUST-1 and pure ZIF-8 [105,107].

Regarding stability, coating a hydrophobic or highly stable MOF shell (e.g., ZIF-8, Bio-MOF-14) can effectively protect the moisture-sensitive core from water vapor attack. Gebremariam et al. [107] successfully achieved uniform coating of hydrophobic ZIF-8 nanoparticles on the surface of hydrophilic HKUST-1 using a PVP-assisted in situ growth combined with a secondary growth strategy, forming the HKUST-1@ZIF-8 core–shell structure (Figure 9c). As shown in Figure 9d, the water vapor adsorption capacity of this core–shell structure is reduced by approximately 70% compared to pristine HKUST-1. Collectively, these synergistic mechanisms endow MOF-on-MOF composites with comprehensive advantages in adsorption capacity, selectivity, stability, and mass transfer efficiency, positioning them as ideal platforms for high-performance greenhouse gas adsorption and separation materials [108].

2.5. Other Types of MOF Composites

In addition to carbon-based, polymer, metal oxide, and MOF-on-MOF composites, researchers have also explored the compositing of MOFs with other functional materials such as porous SiO₂, ionic liquids, and aerogels. These materials each possess unique characteristics in terms of pore structure regulation, surface chemical modification, and macroscopic shaping, offering diverse solutions for greenhouse gas adsorption and separation.

2.5.1. MOFs/Porous SiO₂ Composites

Porous SiO₂ materials (such as mesoporous SBA-15 and MCM-41) feature well-defined pore structures, high specific surface areas, excellent hydrothermal stability, and easily functionalizable surfaces [109,110,111]. Compositing MOFs with porous SiO₂ can significantly enhance the hydrothermal stability and mechanical strength of the composites while retaining the high adsorption capacity of MOFs. Furthermore, the silanol groups on the SiO₂ surface can form covalent bonds with the metal nodes or organic linkers of MOFs, enhancing interfacial adhesion and inhibiting the agglomeration of MOF particles [112]. In the context of SBA-15-based composite systems, numerous studies have demonstrated that this support can effectively enhance the CO₂ separation performance of MOFs. The Cu-BTC@SBA-15 composite exhibits significantly enhanced selectivity in CO₂/CH₄ and CO₂/N₂ separation, with a CO₂/CH₄ selectivity 39% higher than that of pristine Cu-BTC. [113] Through amine functionalization, the MIL-101(Cr)/SBA-15/PEI-25 composite achieves a CO₂ adsorption capacity of 3.2 mmol/g, which is 146% higher than that of pristine MIL-101(Cr), and can be regenerated at a low temperature of 65 °C [114]. Furthermore, the Mg-MOF-74@SBA-15 composite, fabricated by confining Mg-MOF-74 nanocrystals within the mesopores of SBA-15 (Figure 10a), exhibits significant CO₂ adsorption at 293 K and 1 bar. The composite with 75 wt% MOF loading shows a CO₂ uptake of ~88 mL/g, which is only 45% lower than that of pristine Mg-MOF-74, while the 50 wt.% loading sample shows a 77% decrease (Figure 10b,c) [115].

In the context of MCM-41-based composite systems, the MCM-41/Cu(BDC) composite exhibits a 73% increase in specific surface area compared to pristine MCM-41, a 15% increase in CO₂/CH₄ selectivity at 4 bar, and only a 1% loss in adsorption capacity after three cycles [116]. Similarly, the MIL-101(Cr)@MCM-41 composite utilizes the confinement effect of MCM-41 to regulate the crystal growth of MOFs, resulting in reduced MOF particle size and increased specific surface area. At 298 K and 1 bar, its CO₂ uptake is 79% higher than that of pristine MIL-101(Cr) [117]. Collectively, these studies demonstrate that MOFs/porous SiO₂ composites hold great promise for CO₂ capture and gas separation membrane applications.

2.5.2. MOF/Ionic Liquid Composites

Ionic liquids (ILs) are low-temperature molten salts composed of organic cations and organic or inorganic anions, featuring low vapor pressure, high thermal stability, strong interionic interactions, and excellent CO₂ solubility [118]. Incorporating ILs into MOF pores combines the high specific surface area of MOFs with the specific affinity of ILs, enabling synergistic CO₂ adsorption. Functional groups such as amine groups and imidazolium cations in ILs can interact with CO₂ through acid–base interactions or form carbamates, significantly enhancing the CO₂ adsorption capacity and selectivity of the composites under low partial pressure conditions [119,120]. Studies have shown that loading the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) into ZIF-8 significantly improves the CO₂ adsorption performance of the composite. At 0.1 bar and an IL loading of 20 wt%, the CO₂ uptake increases by approximately 9% compared to pristine ZIF-8. Meanwhile, at a loading of 28 wt%, the CO₂/N₂ selectivity doubles compared to pristine ZIF-8 [121]. The presence of ILs can also modify the surface properties of MOFs, enhancing their affinity for polar molecules. Furthermore, IL-loaded MOF composites exhibit excellent performance in CO₂/CH₄ separation, which is attributed to the strong interactions between ILs and CO₂ as well as the size sieving effect of MOF pores. In recent years, the combination of functionalized ILs (e.g., amino-containing ILs) with MOFs has further enhanced chemisorption capacity, offering new routes for low-concentration CO₂ capture and industrial flue gas treatment.

3. MOF Composites for Different Gas Systems

3.1. CO₂ Capture

CO₂ is the primary greenhouse gas driving global climate change, accounting for more than 70% of total anthropogenic greenhouse gas emissions. CO₂ capture technologies can be classified into four categories based on their integration position in energy systems: post-combustion capture, pre-combustion capture, oxy-fuel combustion, and direct air capture (DAC) [122].

MOF/carbon composites have been widely applied in CO₂ capture. For pre-combustion capture, which operates under high pressure and requires good thermal stability, Liu et al. [123] found through computational simulations that the internal pores of carbon nanotubes exhibit higher adsorption energy for CO₂ than for N₂. Consequently, the MOF-177/carbon nanotube composite achieved an approximately 5-fold increase in CO₂/N₂ selectivity at a CO₂ partial pressure of 0.01 MPa. For oxy-fuel combustion, where flue gas CO₂ concentration exceeds 90% and high moisture resistance is required, Zeng et al. prepared a UiO-66-F4@rGO composite with a water contact angle of 169.3°, demonstrating excellent superhydrophobic properties [47].

MOF/polymer composites have shown great promise for post-combustion CO₂ capture, which targets flue gas with low CO₂ partial pressure (10–15%), high N₂ content, and water vapor. A recent breakthrough in the ionic hydrophobic gating (IHG) strategy offers a new approach for CO₂ capture from humid flue gas. Sun et al. [124] assembled a hydrophobic ionic liquid and fluorinated terephthalaldehyde on the MOF surface to construct a bifunctional shell. This shell simultaneously forms a H₂O permeation barrier and provides CO₂-selective transport channels (Figure 11c). Under saturated humidity (RH = 100%) flue gas, this material directly captured dry CO₂ with a purity of 99.999% (Figure 11a), achieving a CO₂/N₂ selectivity as high as 1780 at 298 K and 1 bar (Figure 11b). For direct air capture (DAC), which removes CO₂ from ambient air (~400 ppm), amine-functionalized polymer composites achieve efficient capture through chemisorption. Yan et al. [125] covalently grafted polyethylenepolyamine (PEPA) onto a defective chromium-based MOF (Figure 11d). The resulting PEPA-DM composite exhibited a CO₂ adsorption capacity of 1.26 mmol/g under ambient air conditions, with a regeneration energy consumption of only 75.1 kJ/mol (Figure 11e). Ionic liquid functionalization offers a new approach for low-energy regeneration. Zeeshan et al. [126] loaded functionalized ionic liquids into MOFs. The resulting IL/MOF composite achieved a CO₂ uptake of 0.5 mmol/g at 500 ppm CO₂ under varying relative humidity conditions (Figure 11f) and could be rapidly regenerated within 2–4 min using microwave dielectric heating, which delivers energy directly to the ionic liquid domains.

MOF/metal oxide composites offer advantages in high-temperature stability and cyclic performance. Zhao et al. [91] prepared an Mg-doped CaO adsorbent using a calcium-based MOF as a precursor. The optimal sample, Ca5Mg3, exhibited a capacity decay of only 0.7 mmol/g after 30 CO₂ adsorption–desorption cycles, with MgO serving as an inert high-temperature stabilizer that significantly enhanced the sintering resistance and cycling stability of the adsorbent. For oxy-fuel combustion, magnetic MOF composites such as Fe₃O₄@HKUST-1 can be aligned under an external magnetic field to optimize membrane performance. This material achieves a CO₂ uptake of 5 mmol/g at 273 K and 1 bar, with a CO₂/N₂ selectivity of 22.

MOF-on-MOF composites also contribute to CO₂ separation. Hou et al. [127] constructed a ZIF-8@NH₂-MIL-125 heterostructure featuring a “small pore–large pore–small pore” architecture, enabling a “select–diffuse–select” separation mechanism. When incorporated into a Pebax matrix to prepare mixed matrix membranes, the material achieved an ideal CO₂/N₂ selectivity of 103.2 at 308 K and 4 atm, representing a 102% increase compared to pristine Pebax membranes (51.0).

In summary, the integration of MOFs with various functional materials has emerged as a versatile strategy for CO₂ capture, offering unique advantages in enhancing adsorption capacity, selectivity, and regenerability. As summarized in

Table 3. Summary of MOF composites for CO₂ capture.

Composite	Type	Target Gas	Uptake	Selectivity	Test Conditions	Ref.
MOF-5@GO (3% Li doping)	MOF/carbon	CO2/N2	4.39 mmol/g	-	298 K, 1 bar	[50]
MOF-505@5GO	MOF/carbon	CO2/N2	3.94 mmol/g	37.2	298 K, 0.1 MPa	[128]
Mg/DODBC MOF@GO	MOF/carbon	CO2/N2	8.60 mmol/g	20.8	298 K, 1 bar	[129]
Ni/DODBC/GO	MOF/carbon	CO2/N2	10.50 mmol/g	20.8	298 K, 20 bar	[130]
MOF-74(Ni)@GrO	MOF/carbon	CO2/N2	5.76 mmol/g	44	298 K, 1 bar	[131]
CALF-20/GO-20	MOF/carbon	CO2/N2	3.65 mmol/g	277	298 K, 1 bar	[132]
UiO-66/GO-5	MOF/carbon	CO2/N2	3.37 mmol/g	277	298 K, 1 bar	[133]
UiO-66(OH)2/GO	MOF/carbon	CO2/N2	4.34 mmol/g	277	298 K, 1 bar	[134]
Cu3(BTC)2/GO-1	MOF/carbon	CO2/N2	3.13 mmol/g	20.8	298 K, 1 bar	[135]
CuBTC@1%GO	MOF/carbon	CO2/N2	8.90 mmol/g	186	273 K, 1 bar	[136]
PI-UiO/GO	MOF/carbon	CO2/N2	8.24 mmol/g	64.7	298 K, 30 bar	[137]
HKUST-1@GO	MOF/carbon	CO2/N2	6.00 mmol/g	44	273 K, 1 bar	[138]
Mg-MOF-74@GO	MOF/carbon	CO2/N2	5.64 mmol/g	185.64	298 K, 1 bar	[139]
Cu-BTC/MWCNTs	MOF/carbon	CO2/N2	1.63 mmol/g	181.63	297 K, 1 bar	[140]
CuBTC-BC-5 wt%	MOF/carbon	CO2/N2	3.70 mmol/g	67.46	1 bar	[71]
30%MIL-53(Al)/AC	MOF/carbon	CO2/N2	1.86 mmol/g	3.75	298 K, 1 bar	[141]
MOF-74(Ni)-Pd(AC)	MOF/carbon	CO2/N2	12.24 mmol/g	12.4	298 K, 32 bar	[142]
MOF-74(Co)-Pd(AC)	MOF/carbon	CO2/N2	11.42 mmol/g	12.4	298 K, 32 bar	[142]
HKUST-1@AC/2%	MOF/carbon	CO2/N2	5.50 mmol/g	51.6	298 K, 1 bar	[143]
HKUST-1@ACM	MOF/carbon	CO2/CH4	2.30 mmol/g	5.5	293 K, 1 bar	[63]
een–MOF/PDVB-5	MOF/carbon	CO2/N2	2.22 mmol/g	-	298 K, 150 mbar	[79]
MOF-CNFs	MOF/carbon	CO2/N2	2.76 mmol/g	66	298 K, 1 bar	[144]
Cu-MOF-GO	MOF/carbon	CO2/N2	1.79 mmol/g	-	15 bar	[145]
39 wt% GO/UTSA	Carbon/MOF	CO2/N2	3.26 mmol/g	183.26	298 K, 1 bar	[146]
2GrO@HKUST-1	Carbon/MOF	CO2/N2	9.02 mmol/g	186	273 K, 1 bar	[147]
2 wt% MWCNT/MIL-101(Cr)	Carbon/MOF	CO2/N2	5.88 mmol/g	185.88	298 K, 1 bar	[148]
2 wt% SWCNT@HKUST-1	Carbon/MOF	CO2/N2	8.23 mmol/g	188.23	196 K, 1 bar	[149]
5 wt% SWCNT@HKUST-1	Carbon/MOF	CO2/N2	3.75 mmol/g	183.75	298 K, 1 bar	[149]
CNT@MIL-100-En	Carbon/MOF	CO2/CH4	4.70 mmol/g	5.4	298 K, 1 bar	[150]
CNT@Cu3(BTC)2-En	Carbon/MOF	CO2/CH4	6.10 mmol/g	5.4	298 K, 1 bar	[150]
CNT@Ni-MOF-74/PZ	Carbon/MOF	CO2/CH4	4.30 mmol/g	6.2	298 K, 1 bar	[151]
CNT@MOF-199/30PZ	Carbon/MOF	CO2/N2	1.79 mmol/g	3.75	305 K, 1.09 bar	[152]
AC@ZIF-8	Carbon/MOF	CO2/N2	2.24 mmol/g	14.6	298 K, 32 bar	[153]
AX-Mg-MOF-74(15%)	Carbon/MOF	CO2/N2	35.50 mmol/g	51.6	300 K, 10 bar	[154]
AC/UTSA-16(Co)30%	Carbon/MOF	CO2/N2	3.35 mmol/g	67.46	298 K, 1 bar	[155]
6 vol% AC/HKUST-1	Carbon/MOF	CO2/N2	6.38 mmol/g	67.46	15% CO2, 85% N2	[156]
BC/Mg-MOF-74(1:1)	Carbon/MOF	CO2/N2	5.83 mmol/g	67.46	1 bar	[69]
RC-MIL-101(Cr)	Carbon/MOF	CO2/CH4	25.79 mmol/g	-	298 K, 35 bar	[117]
PN@MOF-5	Carbon/MOF	CO2/N2	3.53 mmol/g	212	273 K, 1 bar	[78]
Mg-MOF-74@PDCPD	MOF/Polymer	CO2	9.3 mmol/g	-	298 K, 1 bar	[157]
Co-MOF-74@PDCPD	MOF/Polymer	CO2	6.8 mmol/g	-	298 K, 1 bar	[157]
Zn-MOF-74@PDCPD	MOF/Polymer	CO2	5.8 mmol/g	-	298 K, 1 bar	[157]
20% TEPA@MOF-177	Polymer/MOF	CO2/N2	2.8 mmol/g	-	298 K, 1 bar	[158]
10% PEI@MOF-177	Polymer/MOF	CO2/N2	2.3 mmol/g	-	298 K, 1 bar	[158]
20% DETA@MOF-177	Polymer/MOF	CO2/N2	2.8 mmol/g	-	298 K, 1 bar	[158]
CS@MOF-74(Co)	Polymer/MOF	CO2/N2	3.1 mmol/g	-	298 K, 1 bar	[159]
COP@ZIF-8 (20%)	Polymer/MOF	CO2/N2	3.425 mmol/g	207.8	1 bar, 300.15 K	[160]
Ni-MOF-74@Al2O3	MOF/oxide	CO2	10.26 mmol/g	-	303.15 K, 5 bar	[161]
MOF-74(Zn)/Fe2O3/Ag	MOF/oxide	CO2	4.11 mmol/g	-	293 K, 1 bar	[94]
ZIF@ZnO95	MOF/oxide	CO2	0.34 mmol/g	-	298 K, 1 bar	[162]
CaO-M	Oxide/MOF	CO2	14.1 mmol/g	-	650 °C	[91]
CuO@UiO-67	Oxide/MOF	CO2/N2	2.76 mmol/g	54.3	298 K, 1 bar	[163]
Fe3O4/Mg-MOF-74	Oxide/MOF	CO2	10.00 mmol/g	-	273 K, 1 bar	[164]
MgFe2O4/UiO-66	Oxide/MOF	CO2	0.35 mmol/g	-	300 K, 0.15 bar	[165]
Fe3O4@HKUST-1	Oxide/MOF	CO2/N2	5 mmol/g	22	273 K, 1 bar	[166]
ZIF-11@ZIF-8	MOF/MOF	CO2/N2	8.21 mmol/g	40.23	298 K, 3.45 bar	[43]
ZIF-8@Zn-MOF-74	MOF/MOF	CO2/N2	3.27 mmol/g	11.5	308 K, 4 bar	[167]
MOF-S@MOF-C	MOF/MOF	CO2/N2	2.30 mmol/g	32.7	273 K, 1 bar	[168]
ZIF-8@NH2-MIL-125	MOF/MOF	CO2/N2	1.90 mmol/g	40	298 K, 1 bar	[127]
HKUST-1@ZIF-8	MOF/MOF	CO2/N2	2.9 mmol/g	41.4	298 K, 1 bar	[107]
ZIF-8/Mg-MOF-74 (1:1)	MOF/MOF	CO2	4.09 mmol/g	-	30 °C, 12% CO2	[169]

, rational composite design including MOF/carbon hybrids, MOF/polymer composites, MOF/metal oxide systems, and MOF-on-MOF architectures significantly improves CO₂ capture performance through synergistic effects at the interface between components. MOF/carbon composites achieve enhanced CO₂ uptake by creating additional micropores at the MOF–carbon interface, while carbon/MOF hybrids exhibit substantially improved CO₂/N₂ selectivity through surface functionality modulation. MOF/polymer composites, particularly those functionalized with amine-rich polymers such as TEPA and PEI, achieve high CO₂ uptake under ambient conditions by combining the high surface area of MOFs with the strong chemical affinity of amines for CO₂. MOF/metal oxide composites leverage the strong chemical reactivity of metal oxides (e.g., CaO, MgO, Fe₃O₄) for enhanced CO₂ chemisorption, while also enabling magnetic separation and low-energy regeneration. Furthermore, MOF-on-MOF core–shell architectures achieve high CO₂/N₂ selectivity through precise pore engineering and tailored interface design (e.g., ZIF-8@NH₂-MIL-125). These composite design strategies collectively provide a versatile platform for developing high-performance adsorbents tailored to specific CO₂ capture scenarios, including post-combustion flue gas, pre-combustion syngas, and direct air capture.

However, several challenges remain. The trade-off between CO₂ uptake and selectivity persists, as materials with high surface area often exhibit reduced selectivity due to non-specific adsorption. Interfacial compatibility between MOFs and functional components requires further optimization to avoid pore blockage and ensure uniform dispersion. The long-term stability of these composites under realistic operating conditions, including humidity, trace impurities (e.g., SO_x, NO_x), and cyclic adsorption–desorption, demands systematic evaluation. Moreover, the energy consumption for regeneration remains a key concern, particularly for amine-functionalized composites where strong chemisorption necessitates high-temperature desorption. Building on the composite design strategies discussed above, future research should focus on developing novel MOF-based composites that combine high CO₂ uptake with excellent selectivity, low regeneration energy, and long-term cycling stability, while advancing scalable processing and real-world validation under practical conditions.

3.2. CH₄ Capture

Methane (CH₄) is not only an important clean energy gas but also the second largest greenhouse gas after CO₂, with a global warming potential (GWP) approximately 25 times that of CO₂. The main sources of CH₄ include natural gas, coalbed methane, and biogas. Among these, natural gas purification primarily involves CH₄/CO₂ separation, while coalbed methane/biogas upgrading mainly involves CH₄/N₂ separation. Both separation systems face challenges arising from similar molecular sizes and physical properties. For example, the kinetic diameters of CO₂ and CH₄ are 3.30 Å and 3.80 Å, respectively, while those of N₂ and CH₄ are 3.64 Å and 3.80 Å, respectively. Furthermore, CH₄ is a non-polar molecule, making it difficult to capture selectively through polar interactions. Therefore, developing MOF composites with both high selectivity and high adsorption capacity has become a research priority in this field.

MOF/carbon composites have been widely explored for CH₄ capture due to their enhanced porosity and improved gas uptake. Zheng et al. doped activated carbon into MIL-101 via an in situ hydrothermal method, tuning the pore structure to develop more micropores. The resulting DFAC-MIL-101 composite exhibited a CH₄ uptake of 11.5 mmol/g at 6 MPa and 30 °C, with a CH₄/N₂ selectivity exceeding 4.0 at 1.5 MPa [170]. In terms of high-pressure CH₄ storage, HKUST-1@rGO achieved a CH₄ deliverable capacity of 193 cm³/cm³ at 298 K and 5.8–65 bar, representing a 30% improvement over pristine HKUST-1, attributed to the formation of additional micropores at the MOF–carbon interface (Figure 12a,c) [171]. Similarly, Cui et al. developed a bromine-functionalized UiO-66-Br₂@AC composite (Figure 12d), also a MOF–carbon material, achieving a CH₄/N₂ IAST selectivity of 8.94 at 298 K and a CH₄ uptake of 1.16 mmol·g⁻¹ (Figure 12e,f), with excellent breakthrough performance under dilute CH₄ conditions [172]. Additionally, Li et al., combining GCMC simulations with experiments, identified Ni-MOF-74 as a promising candidate, and the resulting 5 wt% GO/Ni-MOF-74 composite achieved an IAST selectivity of 25.94, far exceeding that of pristine Ni-MOF-74 (15.23) [173].

MOF/polymer composites offer advantages in shaping and practical handling for CH₄ storage applications. Dou et al. developed a phase conversion method to fabricate PC-HKUST-1 nanofibers by decorating HKUST-1 on polyacrylonitrile (PAN) nanofibers. The resulting MOF/polymer nanofiber composite achieved a high MOF loading of 52 wt% and a CH₄ uptake of 86 cm³/g (3.84 mmol/g) at 298 K and 35 bar, demonstrating that polymer nanofiber matrices can effectively structure MOFs into mechanically robust and flexible adsorbent forms without sacrificing gas storage performance [174].

Figure 12. (a) Schematic illustration of the synthesis of MOF-GO nanocomposites. (b) N₂ physisorption isotherms of pristine HKUST-1 and MOF-GO nanocomposites at 77 K. (c) Volumetric CH₄ adsorption isotherms of pristine HKUST-1 and MOF-GO nanocomposites at 25 °C. (d) Schematic of the synthesis of UiO-66-Br₂@AC. (The numbers “1” and “2” indicate the sequential order of addition during the synthesis process.) (e) The CH₄ adsorption isotherms at (a) 298 K, (c) 273 K. (f) IAST selectivity of CH₄/N₂ (5: 5, v/v) [171,175].

Figure 12. (a) Schematic illustration of the synthesis of MOF-GO nanocomposites. (b) N₂ physisorption isotherms of pristine HKUST-1 and MOF-GO nanocomposites at 77 K. (c) Volumetric CH₄ adsorption isotherms of pristine HKUST-1 and MOF-GO nanocomposites at 25 °C. (d) Schematic of the synthesis of UiO-66-Br₂@AC. (The numbers “1” and “2” indicate the sequential order of addition during the synthesis process.) (e) The CH₄ adsorption isotherms at (a) 298 K, (c) 273 K. (f) IAST selectivity of CH₄/N₂ (5: 5, v/v) [171,175].

In summary, the integration of MOFs with carbon-based materials, polymers, zeolites, and silica has emerged as a powerful strategy for CH₄ capture and separation. As summarized in

Table 4. Summary of MOF composites for CH₄ capture.

Composite	Type	Target Gas	Uptake	Selectivity	Test Conditions	Ref.
MOF-505@5GO	MOF/Carbon	CH4	0.85 mmol/g	-	298 K, 1 bar	[128]
HKUST-1@ACM	MOF/carbon	CH4/CO2	2.30 mmol/g	5.5	293 K, 1 bar	[63]
HKUST-1@rGO (10 wt%)	MOF/Carbon	CH4	270 cm3(STP)/cm3	-	298 K, 65 bar	[171]
HKUST-1@GO (10 wt%)	MOF/Carbon	CH4	247 cm3(STP)/cm3	-	298 K, 65 bar	[171]
HKUST-1@fGO (10 wt%)	MOF/Carbon	CH4	220 cm3(STP)/cm3	-	298 K, 65 bar	[171]
10GrO@MIL-53(Cr)	Carbon/MOF	CH4/CO2	0.80 mmol/g	48	298 K, 5 bar	[176]
CNF/UiO-66-NH2	Carbon/MOF	CH4/CO2	-	3.81	298 K, 4 bar	[177]
Maxsorb-III/MIL-101	Carbon/MOF	CH4	3.87 mmol/g	-	300 K, 10 bar	[178]
DFAC-MIL-101	Carbon/MOF	CH4/N2	10.79 mmol/g	2.5–4.5	30 °C, 6 MPa	[170]
NMAC-MIL-101	Carbon/MOF	CH4/N2	8.87 mmol/g	2–4	30 °C, 6 MPa	[170]
CNT@Ni-MOF-74/PZ	Carbon/MOF	CH4/CO2	4.30 mmol/g	6.2	298 K, 1 bar	[151]
RC-MIL-101(Cr)	Carbon/MOF	CH4/CO2	25.79 mmol/g	-	298 K, 35 bar	[117]
5GO/Ni-MOF-74	Carbon/MOF	CH4/CO2	-	25.94	0.1 MPa, 298 K	[173]
Zeo-A@MOF-74-1	Zeolite/MOF	CH4	7.7 mmol/g	-	298 K, 20 bar	[179]
ZnY-pIM	Zeolite/MOF	CH4/N2	-	7.56	298 K, 1 bar	[175]
ZnZSM-5-pIM	Zeolite/MOF	CH4/N2	-	8.44	298 K, 1 bar	[175]
Ni-MOF/SBA-15	MOF/SiO2	CH4/N2	-	11.1	298 K, 1 bar	[180]
UiO-66-Br2/SBA-15	MOF/SiO2	CH4/N2	Enhanced CH4	20.06	-	[181]
PC-HKUST-1 NFs	MOF/Polymer	CH4	3.84 mmol/g	-	298 K, 35 bar	[174]
ZnO@ZIF-8	Oxide/MOF	CH4/N2	0.95 mmol/g	3.63	298 K, 1 bar	[182]

, rational composite design including MOF/carbon hybrids, MOF/polymer composites, zeolite/MOF composites, and MOF/silica systems significantly improves CH₄ uptake and selectivity through synergistic effects. MOF/carbon composites achieve enhanced deliverable capacity by creating additional micropores at the MOF–carbon interface. MOF/polymer nanofibers enable high MOF loadings while maintaining excellent CH₄ uptake. Zeolite/MOF and MOF/silica composites have also demonstrated improved selectivity through subunit incorporation and pore engineering, respectively. These strategies collectively provide a versatile platform for developing high-performance adsorbents for CH₄/N₂, CH₄/CO₂, and high-pressure CH₄ storage.

However, several challenges remain. The trade-off between gravimetric and volumetric CH₄ uptake persists, as high-surface-area materials often suffer from low packing density. Interfacial compatibility between MOFs and carbon/zeolite matrices requires further optimization to avoid pore blockage and ensure uniform dispersion. The long-term stability of these composites under realistic operating conditions, including humidity, trace impurities, and cyclic adsorption–desorption, demands systematic evaluation. Moreover, the scalable and cost-effective production of high-performance MOF composites remains a significant hurdle for industrial deployment. Building on the composite design strategies discussed above, future research should focus on developing novel MOF-based composites that combine high CH₄ uptake with excellent selectivity and mechanical stability, while advancing shaping technologies and real-world validation under practical conditions.

3.3. Separation of Fluorinated Greenhouse Gases

In recent years, metal–organic frameworks (MOFs) and MOF-based composites have shown broad application prospects in gas adsorption and separation. Benefiting from their tailorable pore structures and tunable surface chemistry, these materials have also attracted growing attention for the capture of fluorinated greenhouse gases. However, compared with mature systems for CO₂, CH₄, and light hydrocarbons, studies on CF₄, NF₃, and SF₆ separation remain relatively limited. In this field, the research on MOF composites has mainly focused on MOF/polymer composites, while other composite types such as MOF/carbon and MOF/metal oxide have been less explored.

For CF₄/N₂ separation, various porous materials have been explored, but each suffers from certain limitations. Pure carbon materials (e.g., PNN) show moderate CF₄ uptake (4.89 mmol/g) but lack selectivity. Pure MOFs (e.g., SIFSIX-2-Cu) offer better selectivity (~5) but face challenges in shaping and practical handling [183]. To address these issues, Miao et al. [184] constructed a hierarchical porous UiO-66-Br₂@PS/DVB packed column via a high internal phase emulsion strategy (Figure 13a). Compared to the pristine PS/DVB polymer, the composite showed enhanced CF₄ uptake (1.54 vs. 0.94 cm³/g) and improved CF₄/N₂ selectivity (2.67 vs. 1.51) at 298 K and 1 bar (Figure 13b,c). Further improving the selectivity, the same group later designed a core–shell PSD@HKUST-1-AcOH composite, which achieved a CF₄ uptake of 5.28 cm³/g and a CF₄/N₂ selectivity of approximately 27 at 298 K and 1 bar. These examples demonstrate that compositing MOFs with polymer matrices can simultaneously improve separation performance and processability, providing a promising direction for fluorinated gas capture.

For SF₆/N₂ separation, pure MOF powders often suffer from high mass transfer resistance and poor processability, limiting their practical application. To address these issues, Li et al. [185] composited Al(fum) with hydroxypropyl cellulose (HPC) and kaolin, preparing Al(fum)@2%HPC and Al(fum)@5%Kaolin shaped pellets via wet granulation (Figure 13d). These composites not only retained the excellent SF₆/N₂ selectivity (>2 × 10⁴) and dynamic adsorption capacity (~2.7 mmol/g) of pristine Al(fum) powder but also significantly reduced bed pressure drops (Figure 13e). After 1000 adsorption–desorption cycles, the composites maintained stable performance (Figure 13f). Combined with a two-stage temperature-vacuum swing adsorption (VTSA) process, the SF₆ recovery rate reached 99.91%, with an energy consumption of only 18.7% of that of cryogenic distillation [185]. Further improving the selectivity–kinetics trade-off, Miao et al. [186] who prepared a core–shell PSD@HKUST-1-AcOH composite by growing HKUST-1 on mesoporous poly(styrene-divinylbenzene) spheres. By introducing acetic acid to create additional defects, the optimized material achieved an SF₆/N₂ IAST selectivity of 924 at 298 K and 1 bar, markedly higher than that of pristine HKUST-1, while also reducing material cost by approximately 25%. The hierarchical architecture, consisting of a microporous HKUST-1 shell and a mesoporous PSD core, was proposed to facilitate gas diffusion, reduce mass-transfer resistance, and mitigate the trade-off between selectivity and adsorption kinetics that is often encountered in purely microporous adsorbents. These examples demonstrate that shaping strategies and hierarchical architecture design can effectively address the practical challenges of MOF-based adsorbents for SF₆ capture.

In summary, the integration of MOFs with polymer matrices has emerged as a promising strategy for fluorinated gas separation, offering unique advantages in balancing selectivity, processability, and engineering applicability. As summarized in

Table 5. Summary of MOF composites for fluorinated gas capture.

Composite	Type	Target Gas	Uptake	Selectivity	Test Conditions	Ref.
UiO-66-Br2@PS/DVB	MOF/Polymer	CF4/N2	1.54 cm3/g	2.67	298 K, 1 bar	[184]
Al(fum)@2%HPC	MOF/Polymer	SF6/N2	~3.7 mmol/g	>2 × 104	298 K, 1 bar	[185]
Al(fum)@5%Kaolin	MOF/Polymer	SF6/N2	~3.7 mmol/g	>2 × 104	298 K, 1 bar	[185]
BUT-321/HEC pellets	MOF/Polymer	SF6/N2	2.72 mmol/g	133.4	298 K, 1 bar	[187]
PSD@HKUST-1-AcOH	Polymer/MOF	CF4/N2	5.28 cm3/g	27	298 K, 1 bar	[186]
PSD@HKUST-1-20	Polymer/MOF	SF6/N2	11.98 cm3/g	191	298 K, 1 bar	[186]
PSD@HKUST-1-AcOH	Polymer/MOF	SF6/N2	29.39 cm3/g	924	298 K, 1 bar	[186]

, rational composite design including hierarchical porous structures, core–shell architectures, and shaped pellets significantly enhances separation performance through improved pore engineering, interfacial interactions, and defect modulation. Hierarchical porous structures facilitate fast gas diffusion while maintaining high selectivity. Core–shell architectures effectively mitigate the trade-off between selectivity and adsorption kinetics. Shaped pellets address critical engineering issues such as high bed pressure drops and dust pollution, while preserving excellent gas uptake and selectivity over extended cycling. Defect engineering creates additional unsaturated metal sites that strengthen host–guest interactions with fluorinated gases, substantially boosting selectivity. These strategies collectively provide a versatile framework for designing high-performance adsorbents for fluorinated gas separation.

However, several challenges remain. High-selectivity, high-capacity adsorbents for CF₄ and NF₃ are still scarce, and the efficient separation of more challenging gas pairs (e.g., C₂F₆/CF₄, C₃F₆/C₃F₈) remains a major hurdle. Interfacial compatibility and structural homogeneity between MOFs and polymer matrices require further optimization to avoid non-selective defects. Moreover, the trade-off between selectivity and adsorption kinetics persists, necessitating continued refinement of shell thickness and pore architecture. The long-term stability of polymer matrices under realistic conditions (e.g., humidity, trace impurities) also demands systematic evaluation, and the complex composition of multi-component composites poses challenges for cost-effective large-scale production. Building on the composite design strategies discussed above, future research should therefore focus on developing novel MOF-based composites that combine strong affinity with precise molecular sieving capabilities, while advancing scalable processing and real-world validation. Maintaining the synergistic stability of MOF/polymer composites over extended cycling and under industrial conditions remains a significant challenge that warrants further investigation.

3.4. Structure–Property Relationships of MOF Composites for Gas Adsorption

The adsorption and separation performance of MOF composites is governed by the interplay of several structural parameters, including functional groups, organic linkers, central metal ions, and the molecular properties of the target gases. A deeper comparison of these factors reveals clear structure–property trends.

Polar functional groups (e.g., -NH₂, -Br, -OH) can significantly enhance the affinity of MOF composites for polar or polarizable gas molecules. For example, amino-functionalized UiO-66-NH₂@GO composites exhibit improved CO₂ capture via acid–base interactions between -NH₂ and CO₂, leading to reversible chemisorption and higher selectivity over N₂ [78,188]. Moreover, bromine substitution in UiO-66-Br₂@AC increases surface polarity, which strengthens CH₄ adsorption while sterically hindering N₂ diffusion, resulting in a CH₄/N₂ selectivity of 8.94 [172]. In contrast, hydrophobic polymer coatings (e.g., polynaphthalene on MOF-5) do not directly participate in gas binding but create a water barrier, preserving CO₂ uptake under humid conditions.

The organic linker—its length, rigidity, and functionalization—determines pore size and shape, which affect molecular sieving. For instance, in ZIF-8-based composites such as ZIF-8@ZIF-67/GO and 15C5@ZIF-7/PI, the short 2-methylimidazole linker creates a narrow aperture (~0.34 nm) that effectively excludes N₂ (kinetic diameter 3.64 Å) while allowing CO₂ (3.30 Å) to pass, contributing to high CO₂/N₂ selectivity [43,189,190]. In addition, in MOF-74-based composites such as BC/Mg-MOF-74 and CNT@Ni-MOF-74/PZ, the DOBDC linker with two -OH groups provides open metal sites after activation, enabling strong coordination with CO₂ [69,151]. Furthermore, extending the linker length in isoreticular MOFs generally increases pore volume but may reduce selectivity due to less confined diffusion pathways.

The molecular structure of the target gas—kinetic diameter, polarizability, and quadrupole moment—directly influences which composites perform best. For instance, CO₂ (3.30 Å, high quadrupole moment) is effectively captured by composites with open metal sites (BC/Mg-MOF-74) or amine groups (UiO-66-NH₂@GO) [78,188]. Likewise, CH₄ (3.80 Å, non-polar) relies more on van der Waals interactions and surface polarity; thus, brominated composites such as UiO-66-Br₂@AC enhance its uptake [172]. In contrast, N₂ (3.64 Å, low polarizability) is often excluded by molecular sieving in ZIF-8-based composites (ZIF-8@ZIF-67/GO, 15C5@ZIF-7/PI) or steric hindrance in brominated composites [43,190]. For larger fluorinated gases like SF₆ (5.50 Å) and CF₄ (4.70 Å), composites with hierarchical pores (PSD@HKUST-1) or defect sites (PSD@HKUST-1-AcOH) provide sufficient space and strong interactions (F···H hydrogen bonds), achieving ultrahigh selectivities [186].

In summary, the best performance for a given gas is achieved by matching the composite’s functional groups, metal ions, and pore architecture to the gas’s molecular properties. For CO₂ capture, open metal sites or basic functional groups on a high-surface-area support are most effective. For CH₄/N₂ separation, increasing surface polarity while maintaining microporosity enhances CH₄ affinity over N₂. For fluorinated gases, a combination of large enough pores to accommodate the bulky molecules and specific interaction sites (e.g., -OH defects, crown ether cavities) is key. This analysis underscores that rational design of MOF composites requires simultaneous optimization of multiple structural parameters.

4. Challenges and Perspectives

Despite the significant progress and superior performance of MOF composites for greenhouse gas adsorption and separation, translating fundamental research into industrial applications still faces numerous challenges. This section summarizes the key bottlenecks and outlines future perspectives.

4.1. Summary

Metal–organic frameworks (MOFs) are promising for greenhouse gas capture due to their high surface area and tunable pores, but pristine MOFs suffer from poor water stability, high bed pressure drops, limited mass transfer, and low mechanical strength. To overcome these bottlenecks, integrating MOFs with functional materials such as carbon, polymers, metal oxides, or other MOFs has proven effective. This review systematically summarizes four representative composite types, emphasizing design strategies and synergistic mechanisms. The synergistic effects operate across multiple scales: MOF–carbon composites introduce hydrophobic barriers and hierarchical porosity; MOF–polymer composites enable shaping and provide protective coatings; MOF–metal oxide composites combine physical adsorption with chemisorption and offer low-energy regeneration pathways; MOF-on-MOF heterostructures achieve atomic-level interfacial integration, balancing selectivity and permeability. These multiscale synergies position MOF composites as versatile platforms for greenhouse gas adsorption and separation, with tunability enabling applications from CO₂ capture and CH₄ purification to fluorinated gas recovery.

4.2. Challenges

Despite notable progress, several critical challenges impede the industrial deployment of MOF composites for greenhouse gas separation. These challenges can be grouped into five closely related aspects, namely mechanism understanding, performance evaluation, material preparation, functional innovation, and application expansion, which also define the priorities discussed in the following Perspectives section. Mechanistic understanding remains limited. The interfacial interactions between MOFs and functional components such as carbon materials, polymers, and metal oxides are central to the properties of the resulting composites, yet the microscopic origin of the observed synergistic effects is still poorly resolved. Fundamental questions persist regarding whether chemical bonds are formed at the interface, how the local electronic structure is reorganized, and how pore connectivity and mass-transfer pathways are modified. These issues still lack systematic experimental evidence and robust theoretical interpretation.

Performance evaluation is often disconnected from practical operating conditions. Most studies are still performed using pure gases or binary mixtures under idealized laboratory conditions, whereas real flue gas and industrial exhaust streams typically contain multiple impurities, including water vapor, SO₂, NO_x, and O₂. Water can compete with target molecules for adsorption sites and even induce framework hydrolysis, while acidic species may accelerate structural degradation. Under low-concentration conditions, such as direct air capture of CO₂ or trace recovery of SF₆/CF₄, equilibrium uptake alone is insufficient to predict process performance. Long-term working capacity, cyclic stability, and tolerance toward impurities therefore remain inadequately assessed.

Material preparation continues to be constrained by cost and scalability. Many synthetic routes depend on energy-intensive solvothermal processes, costly organic ligands, and high-purity metal salts, while the incorporation of additives such as graphene or carbon nanotubes further increases the economic burden. Scaling production from gram-scale synthesis to kilogram-scale manufacture remains challenging in terms of reproducibility, yield, and structural integrity. In addition, converting powdered composites into pellets, monoliths, or membranes often requires binders or mechanical processing, both of which can compromise porosity and separation performance.

Functional innovation has not yet advanced far beyond conventional adsorption enhancement. Much of the current work remains focused on maximizing adsorption capacity or equilibrium selectivity, whereas the concurrent optimization of selectivity, diffusion kinetics, moisture resistance, mechanical robustness, and regenerability has received far less attention. Low-energy regeneration strategies, including magnetic swing adsorption, microwave-assisted regeneration, and photothermal swing adsorption, have shown promise at the laboratory scale, but they have not yet matured into broadly applicable composite design concepts.

Application expansion is still at an early stage. Reported studies are largely limited to batch experiments or small fixed-bed demonstrations, with comparatively little attention paid to continuous operation, process integration, pilot-scale validation, or techno-economic assessment. More demanding scenarios, such as the separation of complex fluorinated gas mixtures or the treatment of streams containing corrosive impurities, also remain underexplored. Consequently, a substantial gap persists between laboratory demonstrations and practical deployment.

4.3. Perspectives

To address the above challenges, future research should be advanced in the same five aspects identified above: mechanism understanding, performance evaluation, material preparation, functional innovation, and application expansion. These five priorities are derived directly from the main bottlenecks currently limiting the practical deployment of MOF composites, and organizing the discussion in this way helps clarify the pathway from material design to real application. A clearer mechanistic picture will depend on the integration of multiscale characterization with theory and simulation. In situ and operando techniques, including synchrotron diffraction, infrared spectroscopy, XPS, and electron microscopy, when combined with DFT calculations, GCMC simulations, and molecular dynamics, can provide direct insight into interfacial bonding, charge redistribution, adsorption-site evolution, transport pathways, and degradation processes. Such knowledge is essential for moving MOF composite design beyond empirical optimization toward predictive, mechanism-guided engineering.

Equally important is the establishment of standardized, application-relevant evaluation protocols. Benchmark conditions covering temperature, pressure, gas composition, humidity, impurity level, and flow rate should be defined so that different materials can be compared on a meaningful basis. Beyond equilibrium uptake and ideal selectivity, future studies should routinely report breakthrough behavior, working capacity, regeneration energy, cyclic stability, mechanical strength, and cost-related indicators. Particular emphasis should be placed on multicomponent, humid, and low-concentration gas streams that more faithfully represent real operating environments.

On the synthesis side, greener and more scalable manufacturing strategies are urgently needed. Water-based, room-temperature, solvent-minimized, and continuous-flow routes offer promising pathways to lower energy consumption while improving reproducibility. Economic viability may be further enhanced through the use of earth-abundant metal nodes and biomass-derived carbons in place of more expensive components. At the same time, shaping strategies for pellets, monoliths, and membranes must be refined to preserve pore accessibility and separation performance while delivering the mechanical strength required for industrial operation.

Future functional design should emphasize multifunctional integration rather than the improvement of a single metric. In addition to high adsorption capacity, next-generation MOF composites should combine strong and selective host–guest interactions with rapid mass transfer, moisture tolerance, structural stability, and low-energy regenerability. Regeneration strategies driven by magnetic fields, microwaves, or photothermal effects merit deeper investigation because of their potential to reduce the energy penalty associated with conventional temperature-swing processes. Dual-functional composites that couple greenhouse gas capture with catalytic conversion may also open new avenues for increasing overall process value.

Finally, broader application development will depend on tighter coupling between materials and process engineering. More systematic work is needed on continuous adsorption systems, hybrid separation processes, long-term pilot demonstrations, and techno-economic analysis. Beyond conventional CO₂ capture, future efforts should also address more challenging targets, including trace SF₆/CF₄ recovery, mixed fluorinated gas separations, and the treatment of impurity-laden streams. Defining the operating windows and performance boundaries of MOF composites in these scenarios will be critical for identifying where they can deliver genuine practical advantages. Overall, the path forward for MOF composites lies not in isolated improvements to individual properties, but in the coordinated advancement of interfacial science, realistic performance assessment, scalable manufacturing, multifunctional design, and process-level deployment. Only by aligning materials innovation with engineering implementation can these systems be translated from promising laboratory materials into viable technologies for greenhouse gas separation.