Developing Advanced Mg‐Based Solid‐State Materials for Gas Separation and Purification: A Review

Abstract

ABSTRACT

Magnesium (Mg) is globally abundant in resources, and Mg‐based compounds—such as magnesium based hydrides, hydroxides, oxides, and magnesium metal‐organic frameworks (Mg MOFs)—have shown significant application prospects in gas separation. This is largely due to the electronic characteristics of Mg or Mg²⁺ ions, which facilitate the capture of hydrogen (H₂) and acidic gases such as carbon dioxide (CO₂) and sulfur dioxide (SO₂) from other gases. Consequently, exploring the use of Mg‐based materials in gas separation and purification applications could not only advance the scientific understanding of solid‐gas interaction mechanisms but also provide cost‐effective solutions for gas separation technology at an industrial level. This review summarizes the recent practices and explorations of Mg‐based solid‐state materials in various gas separation and purification methods, including physical adsorption‐based separation, chemical absorption‐based separation, and membrane‐based separation. For each separation method, the relevant Mg‐based materials are discussed in detail, and key findings from existing research are presented and analyzed. Additionally, inspired by the straightforward design of air‐stable hydrogen storage materials, this review specifically addresses anti‐passivation strategies for Mg‐based hydrides, which are crucial for their applications in hydrogen gas separation and purification. Finally, this review highlights key issues and fields for future research and development in Mg‐based gas separation materials.

Abbreviation

• BTC
• benzene 1, 3, 5-tricarboxylate
• CA
• cellulose acetate
• CCD
• central composite design
• DFT
• density function theory
• DOBDC
• 2, 5-dihydroxyterephthalic acid
• ED
• ethylenediamine
• EGCS
• Exhaust Gas Cleaning System
• EVA
• ethylene vinyl acetate
• GCMC
• Grand Canonical Monte Carlo
• HRTEM
• high-resolution transmission electron microscopy
• LA-LB
• Lewis acid–base
• LDH
• layered double hydroxide
• MMMs
• mixed matrix membranes
• MOFs
• metal-organic frameworks
• M&SEGCS
• Mg-based seawater exhaust gas cleaning system
• PCT
• pressure-composition-temperature
• PEBAX
• poly(amide 12-b ethylene oxide)
• PEG
• polyethylene glycol
• PEGDME
• polyethylene glycol dimethyl ether
• PES
• poly-ether sulfone
• PMMA
• poly(methyl methacrylate)
• PSA
• pressure swing absorption
• PSF
• polysulfone
• PVAc
• polyvinyl acetate
• SEM
• scanning electron microscopy
• TPSA
• temperature-pressure swing absorption
• TSA
• temperature swing absorption
• VMHS
• vacancy-mediated hydrogen spillover
• VTSA
• vacuum-temperature swing absorption
• XRD
• X-ray diffraction

Introduction

Gas separation and purification are essential processes aimed at refining pure gases or removing undesirable impurities from gas mixtures, which play a critical role in modern society. To be specific, the production of various necessities, such as fuels, plastics, and polymers, lies on gas separation processes including hydrocarbon separation and carbon capture process for enough raw chemicals and decarbonization of industrial waste gases [1–3], while exhaust gases from marine navigation, containing significant amounts of sulfur dioxide (SO₂), are desperate for efficient desulfurization processes, which heavily relies on efficient gas separation methods [4]. Additionally, as advanced industries like the manufacturing of fuel cell and semiconductor are rapidly emerging, further purification of hydrogen gas (H₂) to ultrahigh purity and even electronic-grade is crucial, as impurities like nitrogen (N₂), carbon monoxide (CO), carbon dioxide (CO₂), and hydrocarbons can considerably degrade the performance of fuel cells and semiconductors even at trace concentrations [5]. Consequently, developing effective strategies for gas separation and purification is of paramount importance as society strives toward a more environmentally friendly future.

Gas separation and purification techniques are typically categorized into four main methods: low-temperature distillation, absorption, adsorption, and membrane separation [6, 7]. Low-temperature distillation involves the multi-stage gasification and liquefaction processes, exploiting the differences in the boiling points of various gases to achieve separation. While this method can yield gases of high purity, it is associated with high energy consumption, operation costs, and significant foot prints [6]. Consequently, there has been considerable interest in the other three separation techniques due to their relatively lower energy demands. Absorption separation, both liquid-gas and solid-gas, relies on the selective interaction between the gas molecules and sorbents. In liquid-gas absorption, a gas mixture is brought into contact with a liquid solvent in an absorption column (or scrubber), where a specific gas is dissolved, allowing for the release of a high-purity gas. In solid-gas adsorption, the gas mixture is introduced to an adsorption bed containing adsorbents that selectively bind gas molecules. Through cyclic adsorption processes and oscillation of conditions such as pressure, temperature, and electric fields, the gas mixture can be separated into pure gas products [8]. Membrane separation, on the other hand, leverages differences in gas molecule permeability. In a typical membrane separation process, the gas mixture is fed into one side of the membrane under an elevated pressure. The driving force of the partial pressure difference across the membrane causes gas molecules with higher permeation rates to pass through, accumulating on the permeate side, while the remaining gases concentrate on the retentate side [9].

Based on the gas separation and purification processes outlined above, it can be inferred that for physi/chemi- sorption processes, which are generally categorized as adsorption and absorption, the choice of appropriate sorbents is crucial for effective gas separation. In membrane separation, the development of mixed-matrix membranes (MMMs) incorporating selective additives and continuous polymeric membranes is promising, as it can enhance both permeability and selectivity. This approach has the potential to surpass the well-known permeability-selectivity trade-off, represented by the Robeson upper bound for pure polymeric membranes, while maintaining the ease of processing associated with polymeric materials [9, 10]. Numerous studies have demonstrated the efficiency of Mg-based compounds in various gas separation processes. For instance, Mg-based desulfurization of exhaust gases from marine diesel engines has proven to be more stable and effective, with easier handling of by-products and wastewater. Mg-based metal-organic frameworks (Mg-MOFs) have shown significantly higher CO₂/H₂ adsorption selectivity compared to other adsorbents. Mg-based hydrides are recognized as promising hydrogen storage materials with potential for effective hydrogen absorption. Additionally, MgO in MMMs can reduce polymer density and chain compression while exhibiting a high affinity for acidic CO₂ due to its polar nature [11–15], as shown in Figure 1. Given the abundance and effectiveness of Mg-based compounds in gas separation and purification applications, it can be concluded that these materials could play a vital role in the development of cost-effective gas separation and purification processes in the future. However, there is a noticeable lack of reviews focused on the utilization of Mg-based compounds in the gas separation and purification industry.

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This mini-review examines the role of Mg-based compounds in gas separation and purification processes, focusing on materials, such as Mg(OH)₂, MgO, Mg-based hydrides, and alloys, as well as Mg-based nanoporous materials, such as Mg-MOFs, nanostructured MgO, and zeolites. These compounds are explored within the context of sorption, including absorption and adsorption, and membrane separation. Additionally, since numerous gas separation processes work in cycles, the reversibility of materials under sorption-desorption cycles is also crucial when applied in gas separation. Thus, this review also considers methods derived from the design of Mg-based hydrogen storage materials aimed at mitigating their passivation. These approaches may offer new insights for enhancing the cyclic performance of gas separation processes using Mg-based materials, especially for Mg-based hydrides that deteriorate more easily during separation process. Overall, this review aims to uncover new potential applications for Mg-based materials in gas industry while serving as a reference for the development of more cost-effective gas separation and purification technologies.

The Applications of Mg-Based Materials in Sorption Separation Process

The Principle of Separation Process Based on Sorption

According to the statement of sorbents, the sorption process can be categorized as liquid-gas sorption and solid-gas sorption processes. In typical liquid-gas sorption separation processes, which usually work as absorption, including once-through and regenerable processes, a general scheme is illustrated in Figure 2. The gas mixture containing the target gas (usually cooled before the separation process) is fed into a scrubber filled with a liquid solvent. The target gas components are absorbed or dissolved into the solvent, leading to their accumulation in the liquid phase [16]. Depending on the treatment applied to the solution with a high concentration of target gas components, the absorption separation process can be classified into once-through process and a regenerable process. In a once-through process, the absorbent with a high concentration of the target gas or its corresponding by-products is disposed of directly as waste or by-products, without regeneration for the next absorption cycle. Conversely, in a regenerable process, the gathered gas components are extracted as high-purity gases or other products through thermal or chemical treatment, allowing the absorbent to be reused until its capacity to absorb the target gas is fully diminished [17].

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For solid-gas sorption process, both adsorption and absorption can occur during the process. Generally, a solid-gas sorption separation process consists of two primary steps: sorption and regeneration. During the sorption process, the gas mixture is introduced into a sorbent bed, which is typically selective for a specific component. Unlike liquid-gas sorption, where the gas is homogeneously dissolved within a liquid solvent, solid-gas sorption involves gas-solid interaction. This process is driven by surface attachment and diffusion, meaning that most of the adsorption occurs at or near the surface of the sorbents [18].

Another key feature of solid-gas sorption is its regeneration process. Since gas separation primarily occurs on the surface of the adsorption bed, it is relatively straightforward to couple regeneration with sorption, creating a sorption-regeneration cycle rather than using once-through or establishing separate processes for regenerating sorbents. Typically, regeneration is achieved by altering pressure and temperature, as the sorption direction can be easily reversed by adjusting these parameters. Based on this principle, adsorption cycles can be categorized as follows and shown in Figure 3. It needs to be noted that these swing sorption processes, though being able to work on both absorption and adsorption, are usually called “adsorption” processes in gas separation and purification. Thus, the “adsorption” in the name of swing sorption processes does not mean that the principle is limited to physical adsorption.

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Temperature Swing Adsorption (TSA) Process

In a TSA process, the sorption-regeneration cycle is based on the alteration of temperature while the pressure is kept nearly constant. Traditionally, the temperature is altered through a heating/cooling medium, e.g. stream/water. The idea of using medium as heat exchangers is simple and reasonable, however, in practical cases, limitations like the hydration of sorbents and the low heat capacity of idle purging gas may occur [19, 20]. To address these issues, applying electric current through the sorbents can enable Joule heating, offering a potential solution by compensating for these limitations and allowing for independent control of the purge gas. This approach is known as electric swing adsorption (ESA).

Pressure Swing Adsorption (PSA) Process

A typical PSA process is based on the “Skarstrom cycle” named after Charles Skarstrom, who introduced the first application of PSA in 1960 [21]. In this process, a gas mixture is initially fed into a sorption column at a pressure higher than atmospheric pressure, enabling the selective sorption of target components until the sorbent reaches saturation. To regenerate the sorbent, the pressure within the system is then reduced, either by venting to the atmosphere (in the case of PSA) or by applying a vacuum (vacuum swing adsorption, VSA). The regeneration process is often supplemented with purging to ensure complete desorption of the sorbed gas species. After the sorbent is fully regenerated and the system is re-pressurized, the PSA cycle is complete [22]. Due to the absence of a heating process, PSA is advantageous in terms of lower energy consumption and extended sorbent life.

Besides, coupling of PSA and TSA is also attracting intense interest. By applying elevated temperature at depressurizing, energy consumption can be significantly reduced while the reversibility of sorbents can be improved [23]. Based on the PSA technology applied, the coupled swing sorption can be categorized as temperature-pressure swing adsorption (TPSA) and vacuum-temperature swing adsorption (VTSA), but with the same principle that elevates temperature while venting to air or evacuation. By applying TPSA or VTSA, neither a deep vacuum degree nor a high temperature is strongly required while the productivity and recovery of targeted gas can reach a high level [8, 24]. The feature of coupled swing-sorption process makes it possible for large-scale applications because of its low energy consumption and high efficiency for gas separation.

Mg-Based Materials for Absorption-Based Separation Process

Liquid-Gas Absorption: Desulfurization Using Mg(OH)₂/MgO-H₂O System

As previously mentioned, Mg and its compounds are widely utilized in the exhaust gas cleaning systems (EGCS) of marine diesel engines due to their low cost, stability, and high efficiency. The concept of using Mg-based compounds for desulfurization was first introduced in the 1970s and it demonstrated several benefits, including cost-effectiveness, the absence of solid waste, and ease of maintenance [25]. Given these advantages, Mg-based EGCS were first applied in the desulfurization of flue gas from power plants and paper mills, and introduced to desulfurization in maritime navigation as SO₂ exhausted by marine diesel engines contributes about 13% of annual global SO_x emissions. A typical technology of Mg-based desulfurization in maritime navigation is Mg-based seawater exhaust gas cleaning system (M&SEGCS), which was innovatively proposed by Tang et al. in 2012, combining Mg-based EGCS and seawater desulfurization [11]. A typical M&SEGCS operates based on the Mg(OH)₂-H₂O system, where the primary chemical reactions are described in Equations (1)–(3): 1${\text{SO}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\text{SO}}_3(\text{aq}),$ 2${\text{Mg}(\text{OH})}_2+{\text{SO}}_2\to {\text{MgSO}}_3+{\mathrm{H}}_2\mathrm{O},$ 3${\text{2MgSO}}_3+{\mathrm{O}}_2\to 2{\text{MgSO}}_4.$

The trials of M&SEGCS came out to be satisfying with efficient desulfurization, and appropriate working conditions were also discovered by experiments. The first trial of M&SEGCS applied on Binghe container ship confirmed an optimal condition for operation: 0.5 m/s for empty container gas velocity, 10 L/m³ for liquid to gas ratio and 7.5 for pH value and a maximum desulfurization efficiency of 95.5% was obtained [11]. Another shipboard trial of full-scale M&SEGCS performed on Ling Yun He container ship at 4 load points of main engine further unveiled the efficiency of Mg-based absorption process in desulfurization. Given the 1.75% (weight ratio) sulfur content of fuel, the sulfur content of outlet gas after M&SEGCS was proved to be lower than 4.3 ppm with additional fuel consumption and chemical cost both being only 1% of fuel cost [26]. Though the trials came out with satisfying results, further optimization is still needed for improving cost-efficiency. The high cost and low efficiency of the experiment hinder its further optimization design. Thus, a series of modeling and simulation studies were also carried out to navigate the design of M&SEGCS system, predicting more optimized conditions while trying to gain a deeper insight into the dynamics details in M&SEGCS. By using central composite design (CCD) coupled with response surface method (RSM), Tang et al. [27] analyzed the individual and combined effects of liquid-to-gas ratio, pH and empty container velocity in a MgO-seawater scrubber system, drawing the conclusion that liquid-to-gas ratio and pH played vital roles in desulfurization process while the effect of empty container velocity was limited. For the detailed absorption mechanism, Zhao et al [28]. established a detailed model concerning the motion of droplets, the absorption or desorption of SO₂ and CO₂, the mass transfer process in the liquid phase, the dissolution or crystallization of Mg-containing products and the equilibrium of the ion and charge, as illustrated in Figure 4. Based on this model, factors affecting SO₂ absorption rate were analyzed in detail and operation strategy of M&SEGCS for safety and efficiency was proposed according to different conditions. Flow-field based analysis was also performed based on multi-fluid model coupled with dispersive k–ε turbulence model and the simulation result of desulfurization efficiency matched well with the practical experiment, providing effective model for the prediction of desulfurization efficiency of M&SEGCS [29].

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It is important to note that the M&SEGCS mentioned above operates as a once-through process, meaning that the solvent containing a high concentration of MgSO₄ is directly discharged into the sea, which is not harmful due to the nature of Mg-based absorbents but collides with the idea of recycling. Thus, an alternative absorption-based gas separation system has been designed using MgO as the solvent for SO₂, which is regenerable. In this system, by-products of desulfurization, mainly consisting of MgSO₃, MgSO₄ and MgCO₃, are calcined in an industrial demonstration unit, producing MgO with high reactivity at temperatures between 900°C and 1000°C for next desulfurization cycle, achieving the regenerable Mg-based absorption separation process [30]. The challenge in achieving efficient regeneration of the solvent lies in the oxidation of MgSO₃, which can lead to increasing amount of MgSO₄ in by-products, consuming much more energy when calcinated to form MgO. The application of oxidation inhibitors, such as phenol, ethanol, and ascorbic acid, has been shown to be effective in preventing oxidation, with detailed kinetic analyses supporting their efficacy [31].

Solid-Gas Absorption: Separation and Purification of H₂ and CO₂ Using Mg─MgH₂ and MgO─MgCO₃ System

In the case of Mg-based solid-gas absorption process, magnesium itself is reported to have strong affinity toward hydrogen, making it possible to be utilized in hydrogen purification or removing hydrogen in gas mixture [32]. Meanwhile, Mg-based nanomaterials, especially nanoporous MgO, often possess a high specific surface area and high selectivity toward specific gas molecules due to exposed Mg²⁺, making them much more effective for absorption processes [33]. Furthermore, both magnesium hydrides and nanoporous MgO can be regenerated easily by heating and depressurization, which is suitable for swing technologies. Thus, Mg-based materials are well-suited for use in gas absorption cycles, which will be discussed in the following sections.

H₂ Separation and Purification: Utilization of Mg-Based Hydrides

A specific scenario for applying Mg-based hydrides in gas separation is the extraction of H₂ in mixed gas, including H₂ capture, purification and removal due to their capacity of selectively bond with hydrogen chemically, as described in Equation (4): 4$\mathrm{Mg}+{\mathrm{H}}_2\rightleftharpoons {\mathrm{MgH}}_2.$

The reaction process between Mg and H₂ is usually illustrated by a pressure-composition-temperature (PCT) curve. At the beginning of absorption process, hydrogen molecules are first disassociated and diffused into tetrahedron vacancies of Mg lattice, forming Mg─H solid solution or α-phase. As pressure rises, MgH₂, or β-phase, nucleates and grows inside the solid solution, which is presented by constant absorption pressure (plateau pressure) in PCT curve until complete transformation. The relationship between plateau pressure and temperature follows van't Hoff equation shown as Equation (5), where P₀ represents standard pressure, P_eq refers to plateau pressure, R is the ideal gas constant, T is the temperature, and ΔH and ΔS refer to the enthalpy and entropy change of absorption reaction, respectively. 5$\ln \left(\frac{P_{\text{eq}}}{P_0}\right)=\frac{\Delta H}{RT}-\frac{\Delta S}{R}.$

The absorption process, along with the PCT curve and van't Hoff diagram, is shown in Figure 5A,B. Through this chemical absorption process, 7.6 wt% or 110 g/L hydrogen can be captured and stored in Mg hydrides, making it competitive for H₂-related gas separation processes, including capture, purification and removal [34].

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Systematic investigations have been carried out to instruct the design of hydrogen separation set using Mg hydrides. For example, Guo et al [36]. proposed a flow-through metal hydride reaction model for hydrogen purification and a theoretical hydrogen absorption efficiency of 93.75% was achieved with LaNi₅. Further study gave a deeper insight into the behaviors of the impurity transport and separation process inside metal hydride operating under poisoning conditions. For Mg-based materials, though the absorption mechanism is similar to La-Ni, an elevated temperature is needed for both absorption and desorption of Mg─MgH₂ system. Figure 6 illustrates a conceptual TPSA design of Mg-based gas separation equipment based on the hydrogenation-dehydrogenation properties of Mg─MgH₂ system. In a separation cycle, first H₂ with impurities is fed at around 150°C–300°C with a pressure of around 30 bar for the absorption of H₂ while the exhaust gas mainly consists of impurities. When the concentration of H₂ in exhaust gas reaches a threshold, representing a complete phase transformation from Mg to MgH₂, the system is cooled down to 50°C–100°C while retaining high pressure, then inert gas is purged to remove the impurities inside the system. At last, the system is vented while heated to 300°C–350°C for MgH₂ dehydrogenation, releasing high-purity H₂. It is worth noting that Mg-based large-scale hydrogen storage system works on a similar principle, and the systematic controlling strategies have been extensively investigated, including material modification and heat management strategies, which provide the TPSA system with a stable engineering basis [37–39]. However, the TPSA system is not put into practice, as the operation of this TPSA process strongly depends on the toxic gas resistance of Mg─MgH₂ system, which will be discussed in detail in following part.

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Setting the TPSA system aside, some practices focusing on the viability of Mg hydride-based gas separation are operated. Zhou et al. prepared VTiCr-catalyzed MgH₂ by high energy ball milling for capturing low-pressure hydrogen from a mixture of Ar/H₂ [40]. Under a H₂ partial pressure of 5%, the catalyzed MgH₂ showed a cyclable hydrogen capacity of around 1.3 wt% with a hydrogenation temperature of 160°C and a dehydrogenation temperature of 230°C, as shown in Figure 7A, indicating excellent capability of capturing low-pressure H₂ (0.04 bar). Another research focused on the application of fluorinated MgNi_0.1Fe_0.05Ti_0.05 alloy in hydrogen purification with a VPSA apparatus for further validation of hydrogen separation [41]. It is reported that flocculent precipitate MgF₂ inside the alloy, as shown by scanning electron microscopy (SEM) in Figure 7B, can significantly prevent the Mg alloy from being deteriorated by toxic gas like CO₂ and CO. Breakthrough experiments with feed gas of 80% H₂ + 20% CO₂ + 100 ppm CO performed under pristine and fluorinated Mg alloys were performed and the results, as shown in Figure 7C,D, demonstrated that fluorinated Mg alloys were resistant to CO and CO₂ poisoning and maintained gas separation performance after five cycles. In the one-bed VPSA test, H₂ was successfully separated with a recovery rate above 95%, showing the validation of fluorinated Mg─Ni─Fe─Ti alloy in hydrogen extraction.

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Furthermore, the research mentioned above also reveals the reason that the report of utilizing Mg hydrides in H₂-related gas separation is limited: sluggish kinetics and weak poisoning resistance of Mg-based metal hydrides. While an early literature reported the severe CO poisoning of Mg alloys at temperatures lower than 100°C and formation of CH₄ at higher temperatures when CO was fed [42], the sluggish kinetics of Mg-based hydrides have been extensively reported by researchers focusing on hydrogen storage materials [35]. Thus, further exploration of Mg-based hydrides in H₂-containing gas separation depends on the modification of both kinetics and anti-resistance performance.

Although research on the anti-passivation of Mg-based hydrides for gas separation is relatively limited, extensive studies have been conducted on developing air-stable Mg hydride-based hydrogen storage materials. These studies provide valuable references for designing Mg hydrides resistant to toxic gases, such as CO, H₂O, CH₄, and O₂. A common approach involves encapsulating MgH₂ particles within selective shells that allow the passage of H₂ while blocking larger, toxic gas molecules like O₂ and H₂O.

For example, as illustrated in Figure 8A, anti-passivation shells can be composed of polymer membranes such as poly(methyl methacrylate) (PMMA), 2D materials like graphene oxide, or activated carbon [43–45]. Jeon et al. prepared Mg nanocrystals/PMMA nanocomposites using a straightforward one-pot reduction reaction of an organometallic Mg²⁺ precursor. These nanocomposites could absorb up to 5.97 wt% of hydrogen in 80 min at 200°C under 35 bar H₂, and after 3 days of air exposure, their composition remained stable, as confirmed by X-ray diffraction (XRD) measurements [46]. Similarly, Chen et al. reported a facile plasma metal reaction method using CH₄ as the carbon source, which resulted in an ultrathin (~4 nm) carbon layer encapsulating Mg nanoparticles, as observed through high-resolution transmission electron microscopy (HRTEM) in Figure 8B [47]. This composite absorbed 4.8 wt% of hydrogen in 10 min at 573 K and desorbed 5.0 wt% in 20 min when heated to 350°C. Notably, after 3 months of air exposure, no formation of MgO was detected (Figure 8C), demonstrating the effectiveness of the activated carbon encapsulation in preventing passivation. More recently, Ali et al. synthesized nanosized MgH₂ encapsulated by nitrogen-doped graphene nanospheres, with Ni introduced as a catalyst, as shown in Figure 8D [48]. XRD analysis (Figure 8E) revealed that the structure and composition of the composite remained stable even after 3 months of air exposure. Furthermore, isothermal hydrogen absorption tests indicated that the composite retained approximately 85% of its maximum hydrogen absorption capacity (5.5 wt% out of 6.5 wt%), as shown in Figure 8F.

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Additionally, some studies have reported that an anti-passivation layer can be introduced in situ by using specific catalysts, such as transition metal hydrides. For instance, the air stability of the hydrogen storage material Mg₂NiH₄ was significantly improved by adding TiH₂ and graphite. After 30 days of air exposure, the material retained 76.9% of its desorption capacity and 75.6% of its absorption capacity, with an observed improvement in absorption kinetics [49]. A deeper investigation into the air stability mechanism of this composite revealed that during air exposure, TiH₂ underwent sacrificial oxidation, forming a passivation layer on the surface of nanosized Mg₂NiH₄. This layer had protected the hydrogen storage material from further deactivation, while the in situ formation of Ni/NiO/Ni(OH)₂ had enhanced hydrogen absorption, dissociation, and recombination [50]. Furthermore, Shi et al. proposed a vacancy-mediated hydrogen spillover (VMHS) mechanism to explain the improved hydrogen storage properties and air stability of water-activated Mg₂NiH₄ [51]. As illustrated in Figure 9A, a passivation layer can form after air exposure. Without water activation, H₂ must penetrate the dense MgO layer to be released, as shown in Figure 9B, which negatively impacts dehydrogenation kinetics. However, immersing Mg₂NiH₄ in water can in situ form Ni@Mg(OH)₂ nanosheets, which provide air-stable passivation layers that protect Mg₂NiH₄ from further degradation. Upon desorption, vacancies form due to the delocalization and transport of protons from the hydroxyl groups of Mg(OH)₂ toward Ni clusters. These vacancies diffuse to the Mg(OH)₂/Mg₂NiH₄ interface, driven by the concentration gradient, effectively “dragging” hydrogen atoms along. This process improves desorption performance, releasing 3.1 wt% of hydrogen within 300 s at 225°C, as illustrated in Figure 9C.

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It can be concluded that by introducing a protection layer either by coating or by in-situ forming, the poisoning caused by toxic gas can be minimized, leading to improvement of stability and reversibility of Mg-based hydrides. Though current research focusing on improving stability concentrates on air-stable Mg-based hydrogen storage materials, the principles are similar, and the same idea can be probably applied to developing high-reversibility Mg-based hydrides for H₂-related gas separation.

CO₂ Capturing: Utilization of Nanoporous MgO

Another series of intensively investigated Mg-based materials for gas separation is nanoporous MgO, which is believed advantageous in CO₂ sorption and has been recently reviewed by Zhu et al [52, 53]. As a basic oxide, MgO can act as a sorbent for acid gases both physically through the interaction between exposed Mg²⁺ sites and CO₂ and chemically through the following equation (Equation 6) [54, 55]: 6$\text{MgO}\left(\mathrm{s}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{MgCO}}_3\left(\mathrm{s}\right).$

In detail, the absorption of CO₂ in MgO can be divided into five successive steps including bulk diffusion, film diffusion, interparticle diffusion, intraparticle diffusion and surface absorption, as shown in Figure 10A [56]. Macroscopic analysis of absorption behavior shows that the absorption of CO₂ in porous MgO fits well with pseudo-second order absorption model, indicating the nature of chemisorption dominated absorption process.

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In a microscopic view, infrared spectra along with quantum calculations reveal that CO₂ absorbs as monodentate and bidentate carbonate on defective sites on MgO (terrace, edge and corner sites) with absorption energy varying from 115 to 154 kJ/mol related to the coordination number of surface O and Mg ions [58]. The absorption configurations of CO₂-MgO on different sites have been simulated by density function theory (DFT) method and shown in Figure 10B [57]. It can be observed that CO₂ can be chemisorbed on MgO surfaces and form carbonate species via electron donation from surface O sites to carbon atoms and secondary covalent binding of O atoms in CO₂ with surface Mg ions, except for Mg (100) surface where O atoms are five-coordinated, showing weak physisorption toward CO₂ molecules.

The interaction confirmed by both theoretical calculations and practical characterizations explains the strong selective absorption toward CO₂ in MgO adsorbent. Among other solid sorbents for CO₂ capture, MgO shows satisfying abundance, low cost and a theoretical CO₂ absorption of 24.8 mmol/g, higher than most sorbents, indicating its potential of large-scale application in CO₂ capture. Also, suitable binding energy between MgO and CO₂ molecules makes it capable of cyclic working under intermediate temperature range (200°C–400°C) compared with other alkali earth metal oxides like CaO [59]. Further, intrinsic anti-passivation effect is predicted in MgO for CO₂ capture by theoretical calculations, where the existence of water vapor can facilitate the CO₂ absorption [60]. A detailed investigation has revealed the mechanism of wet, CO-containing CO₂ sorption in MgO-based sorbents [61]. As illustrated in Figure 11A, the presence of H₂O can introduce surface OH groups to MgO-based sorbents, which are highly affinitive to acidic CO₂ gas molecules. As a result, the breakthrough time for CO₂ significantly increased with the increasing content of H₂O in the feed gas, as shown in Figure 11B. However, the presence of CO still hinders the absorption of CO₂ by interference and competitive sorption shown in Figure 11A,B. As a result, when fed with a gas mixture containing 5% CO, the breakthrough time of MgO-based sorbents showed an obvious drop, as shown in Figure 11C. All the above-mentioned properties indicate that MgO is a competitive adsorbent toward effective CO₂ capture process.

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As the potential of using MgO for effective CO₂ capture sufficiently indicated by full explanation via both experimental investigations and theoretical calculations, it's reasonable to optimize the absorption of CO₂ in MgO by various methods to fully discover the CO₂ capture potential. In practical cases, an obstacle faced by MgO for CO₂ absorption is the sluggish kinetics originating from the large lattice energy of MgO, thus the optimization centering improvement of both absorption and desorption kinetics of MgO─CO₂ system is the prerequisite for implanting MgO for CO₂ capture in large scale [62, 63].

General methods to improve CO₂ absorption and desorption kinetics include molten-salt promotion and structural engineering. The promotion of kinetics via molten salt can be achieved through facilitating the formation of carbonate groups and/or forming double carbonates, as depicted in Figure 12 [64]. To be specific, Gao et al. investigated the effect of molten NaNO₃ in the MgO─CO₂ system using DFT calculations and well-designed isotopic experiments [65]. They found that the molten NaNO₃ not only built a liquid channel for the fast reaction between CO₂ and O²⁻, but also acted as an important intermediate by generation of NO₂⁺ and O²⁻, altering the dissociation energy barrier while preventing the formation of CO₂-impermeable dense layer, as illustrated in Figure 12A. On the other hand, Kwak et al. have investigated the interfacial interactions in A₂CO₃-promoted MgO (A = Na, K, Rb, Cs), revealing that in carbonate-promoted MgO, CO₂ is first adsorbed on the basic sites at the defects of MgO, then forming double carbonate between A and Mg ions through diffusive processes, as illustrated in Figure 12B [66].

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Compared with promoting MgO─CO₂ kinetics via adding molten salt, the means of structural engineering seem to be more intrinsic and effective. Gao et al. prepared a series of MgO sorbents for CO₂ capture with controlled diverse basic sites by adjusting calcination temperature, resulting in controllable absorption sites and application under a wide range of temperature [67]. Another important method of structural engineering is to construct MgO particles at nanoscale. By calcinating an exquisitely designed MgO aerogel precursor, Ho et al. prepared mesoporous MgO with a stable CO₂ sorption capacity of around 6 wt% under a feeding of 15%CO₂-85%N₂ mixture [68]. Dispersing MgO onto porous supports can also be effective in preparing MgO at nanoscale. Han et al. prepared a series of MgO─Al₂O₃ aerogel by a single-step epoxide-driven sol-gel method followed by CO₂ supercritical drying [69]. By optimizing the Mg-Al molar ratio inside the aerogel, the highest sorption capacity of 2.59 wt% could be achieved when fed with 10%CO₂-90%N₂ mixture at 200°C. Examples using nano-MgO (both free-standing and dispersed on a certain support) for CO₂-related separation are summarized in Table 1, including the feeding gas composition, absorption/desorption conditions, and CO₂ sorption capacity.

Table 1 Summary of using nano-MgO for CO₂ related gas separation.

Feeding gas	Absorption conditions	Desorption conditions	Maximum CO2 capacity [mg CO2 per g sorbent]	Ref.
15%CO2-85%N2	30°C, 60 min	450°C, 30 min	61	[68]
15%CO2-5%H2O-80%N2	200°C, 30 min	450°C, 30 min	22	[67]
15%CO2-85%N2	50°C, 240 min	\	47.02	[70]
10%CO2-90%N2	200°C	\	110.88	[71]
10%CO2-90%N2	200°C	400°C, 30 min	73.48	[72]
10%CO2-90%N2	200°C	\	25.9	[69]
13%CO2-8%H2O-79%N2	60°C, 60 min	350°C, 40 min	57	[73]
10%CO2-10%H2O-80%N2	200°C, 60 min	600°C, 60 min	115	[74]
15.21%CO2-84.79%He	25°C	200°C	90	[75]
15%CO2-85%N2	27°C	500°C	77	[76]

In general, the unique binding characteristics of MgO─CO₂ absorption-desorption system provide it with a satisfying prospect in CO₂ capture at intermediate temperature range (200°C–400°C) due to its nature of being capable of absorbing and releasing CO₂ with high selectivity in this temperature range. Further optimization focusing on improving the kinetics of MgO─CO₂ absorption-desorption system is waiting to be made to expand its application in practical CO₂ capture applications.

Mg-Based Materials for Adsorption-Based Separation Process: Utilization of Mg-Based MOFs

Origin of Adsorption Preference in Mg-Based MOFs (Mg-MOF-74)

Among Mg-based nanoporous materials, metal-organic frameworks (MOFs) containing Mg²⁺ sites are of particular interest for gas separation. Being able to fit all adsorption swings, these materials have demonstrated strong affinity for various gases including CO₂, PH₃, NH₃ and NO_x [62, 77–79], being applicable to be utilized in separation of numerous gas mixtures. Thus, the significant research has been undertaken centering Mg-based MOFs, especially Mg-MOF-74 (also referred to as Mg/2, 5-dihydroxyterephthalic acid), structure shown in Figure 13A for its unique honeycomb-like pore structure with a pore diameter of around 12 Å and a high open metal site density of 0.0045 sites per ų, which are known as preferable features for effective gas separation [80, 81].

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Systematic investigations have been performed to reveal the interaction between different gas molecules and Mg-MOF-74 in detail. Hou et al. have applied DFT and grand-canonical Monte Carlo (GCMC) simulations to the adsorption behavior of C₂H₂, CO₂ and CH₄, showing that open metal sites exposed in the Mg-MOF-74 structure predominate the adsorption process [82]. In the structure of Mg-MOF-74, the Mg²⁺ ion is surrounded by five oxygen atoms, constructing a MgO₅ pyramid structure with an average distance of 2.006 Å and a natural charge of +1.73. Figure 13B–E show the adsorption configuration of CO₂, C₂H₂ and CH₄ on Mg-MOF-74 surface. For CO₂, the Lewis acid–base (LA-LB) interaction between either ${\mathrm{O}}_{{\text{CO}}_2}$ and metal ions or ${\mathrm{C}}_{{\text{CO}}_2}$ and oxygen atoms on the square plane of MgO₅ pyramid contributes mainly to the adsorption, accompanied by the bending of O─C─O to 173.5° coming from the deviations from sp-hybridization of carbon atoms and a back-donation of σ*_C─O ← Mg²⁺CO, resulting in preferred CO₂ adsorption on Mg-MOF-74. For C₂H₂, at the most stable configuration, a cation-π interaction is formed between the metal ion and the entire π-cloud of C≡C bond while for CH₄, only a weak hydrogen bond contributes the adsorption, leading to high selectivity performance of Mg-MOF-74 in case of CO₂/CH₄ separation.

Since LA─LB interaction plays an important role in the adsorption of CO₂, one can deduce that Mg-MOF-74 also exhibits adsorption preference for gas molecules with features of strong Lewis acid or base, such as H₂S, SO_x, NH₃, PH₃ and NO_x, which are also toxic gases desperate for effective separation. First-principles study on the adsorption of a series of gas molecules including CO, H₂S, SO₂, NH₃, NO and NO₂ shows that gas molecules with strong Lewis acidity or alkalinity (SO₂, NH₃) proved by charge analysis while for NO_x, the unpaired electrons can strongly interact with Mg ions, resulting in high binding energy [79].

In general, the strong adsorptive sites originating from exposed Mg²⁺ ions lead to selective adsorption behavior of Mg-MOF-74 for gas molecules with high local electron density or unpaired electrons. Thus, it's quite reasonable to apply Mg-MOF-74 for adsorption-based gas separation processes including decarbonization and depletion of toxic gases from a gas mixture.

Mg-MOF-74 for CO₂ Capture

The high surface area and intrinsic electronic structure of Mg-MOF-74 have rendered its appealing performance for gas adsorption and purification, including specific adsorption of various gas molecules with high reversibility and fast kinetics.

An extensive utilization of Mg-MOF-74 in gas separation and purification processes is CO₂ capture due to its extreme adsorption preference toward CO₂. Generally, CO₂ capture methods can be categorized as absorption, cryogenic distillation, adsorption, and membrane separation, and for Mg-MOF-74, utilization in solid-gas adsorption is extensive [7]. As previously mentioned, Mg-MOF-74 has shown extremely high CO₂ adsorption at room temperature (296 K), with around 170 mL CO₂/g adsorption (35.2 wt%) at 1 atm and around 120 mL CO₂/g adsorption (23.6 wt%) at 0.1 atm, showing overwhelming performance among MOFs even with the same structure but different metal ions [77]. Thus, it's reasonable to use Mg-MOF-74 for CO₂ capture and further gas separation processes. An early study synthesized Mg-MOF-74 via a simple solvothermal method and employed breakthrough measurements to evaluate its CO₂ separation capacity. The study reported a reversible CO₂ adsorption capacity of 7.8 wt%, with good resistance to water vapor when purged with a 20% CO₂/CH₄ mixture, significantly outperforming Zn-MOF-74, which had a CO₂ capacity of 0.35 wt% [83]. Choi et al. further enhanced the cyclability of Mg-MOF-74 by functionalizing it with ethylenediamine (ED), achieving a stable CO₂ adsorption capacity of approximately 1.50 mmol/g over four cycles under simulated ambient air (400 ppm CO₂) [84]. Additionally, compositing Mg-MOF-74 with a microporous spatial skeleton structure, as illustrated in Figure 14A, has been shown to significantly improve the performance in fast dynamics gas separation. The resulting composite achieved a CO₂ adsorption capacity of 5.07 mmol/g and demonstrated a high dynamic selectivity under a mixed gas purge of 20% CO₂ and 80% CH₄ at a flow rate of 50 N mL/min. Moreover, it retained over 90% of its original capacity after four cycles, as shown in Figure 14B [85]. For practical application, Mansour et al. performed a TSA practice using Mg-MOF-74 along with numerical simulations, showing great potential for Mg-MOF-74 in practical gas separation applications [86]. Under feeding of CO₂/N₂ mixed gas with 15 vol% of CO₂, the optimized TSA system (200 s of heating time at 393 K, 400 s of cooling time at 300 K) could separate CO₂ from gas mixture efficiently with CO₂ purity of 96.22% and a rate of 0.279 kg CO₂ per hour per kg of Mg-MOF-74, consuming 663.8 kWh per ton of CO₂ separated. The outlet molar flowrate of CO₂/N₂ of 11 TSA cycles shown in Figure 14C shows that the flow rate of CO₂ and N₂ dominates at different stages, indicating satisfying separation performance. It is worth noting that the appealing performance of Mg-MOF-74 in CO₂ capture also indicates the probability of using Mg-MOF-74 in a variety of scenarios apart from post-combustion CO₂ collection and purification which is commonly realized when mentioning CO₂ capture. For example, Mg-MOF-74 can be utilized in H₂ purification as CO₂ is considered a main impurity gas, especially for gray H₂ which is produced by conventional reforming technologies [87]. Thus, Mg-MOF-74 can be applied to pre-combustion H₂ purification to ensure the purity of H₂ for fuel cell applications.

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Mg-MOF-74 for Toxic Gas Removal

Mg-MOF-74 can also be implanted in gas purification processes involving the removal of toxic gases. Theoretical analysis has shown that toxic gases, such as PH₃, NH₃, NO_x and SO_x, can be adsorbed by Mg-based MOFs effectively, as mentioned in the previous section. An early report from 2015 revealed that Mg-MOF-74 is also capable of adsorbing toxic PH₃ [88]. Among various M-MOF-74 (M = Co, Mn, Mg, Cu), activated carbon, and MOF-5 materials tested, Mg-MOF-74 exhibited a PH₃ uptake of 7.80 mmol/g, ranking third among the porous materials studied. This result suggests that Mg-based MOFs are promising candidates for phosphorus hydride separation. It was also reported that the preferential adsorption of SO_x is significant in Mg-MOF-74, making it capable to capture low concentrations of SO₂ (62–96 ppm) in water vapor [89]. Structural characterization after the saturated absorption of SO_x showed that Mg-MOF-74 was able to maintain full structural integrity and crystallinity while the adsorbed SO_x showed partial stability which indicated that the adsorbent could be regenerated under mild regeneration conditions. Furthermore, due to the specific structure of Mg-MOF-74, it was also reported to be capable for selective adsorption toward SF₆ and its decomposed gas including H₂S, SO₂, SO₂F₂ and SOF₂ [90, 91]. Adsorption isotherms at different temperatures showed that Mg-MOF-74 demonstrated the highest SF₆ uptake among different MOFs, showing its preferential adsorption toward SF₆ while Bader charge analysis under DFT calculations revealed that significant charge transfer happened between molecules, indicating strong interactions between Mg-MOF-74 and gas molecules from the decomposition of SF₆. However, the practical application of Mg-MOF-74 in toxic gas separation remains limited and further exploration is still to be made to release the potential of selective adsorption of toxic gases in Mg-MOF-74.

In general, Mg-based MOFs are obsessed with high specific surface area, strong affinity toward specific gas molecules and high reversibility with their adsorption nature of strong physical adsorption. The delicate porous structure with highly exposed Mg adsorption sites provides Mg-MOF-74 with significant adsorption preference toward different gas molecules and in most cases, the nature of physisorption indicates that Mg-MOF-74 can be regenerated under mild conditions, which further implies its application in practical gas separation processes as the regeneration process will not be energy consuming. To expand the practice of Mg-based MOFs in gas separation, efforts in improving specific surface area and reducing the complexity of production are probably needed and practices of using Mg-based MOFs for separating different gas mixtures also expect further exploration.

The Applications of Mg-Based Materials in Membrane Separation

Different from liquid-gas absorption-based or solid-gas absorption-based gas separation which separates gases by selectively capturing specific gas molecules, in a typical membrane gas separation process operates on the principle of selective permeation of gas molecules without bonding to gas molecules themselves. As illustrated in Figure 15, the inlet stream is fed at one side of the membrane at an elevated pressure. As different gas molecules have different permeability through the membrane, driven by the pressure gradient, more permeable gas molecules will pass through the membrane and accumulate on the so-called permeate side, while other less permeable gas molecules accumulate on the other side or retentate side [9].

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The need to use mixed-matrix membranes (MMMs) for gas separation emerges with the obstacles faced by traditional polymer membranes and inorganic membranes. It has been widely acknowledged that polymer membranes have advantages including high energy efficiency, low cost and ease of processing. However, polymer membranes are reported to be limited by the permeability-selectivity trade-off generated from the solution-diffusion gas permeation process which can be described by the Robeson upper bound P_i = kα_ijⁿ, where P_i refers to the permeability of the more permeable gas and α_ij indicates the gas selectivity [92]. Problems like plasticization, physical aging and swelling are also constraining the performance of polymer membranes. On the other hand, inorganic membranes made of metals or oxides like palladium alloys, amorphous SiO₂ and so on are proved to be stable enough for gas separation while breaking through the Robeson upper bound [93].

Despite the advantages, inorganic membranes are also facing obstacles such as brittle texture, low processibility and high cost. Thus, it's natural to seek ways to cooperate with the advantages of polymer membranes and inorganics, filling inorganic fillers into polymers and mixed matrix membranes or MMMs are fabricated. It has been revealed that by incorporating nanoparticle fillers into polymer matrix, the permeability of different gas molecules can be altered by introducing molecular sieving effect, structure disruption and physical blocking [94]. Since its beginning in 1970s, enormous research has been performed to find an optimal MMM system for certain gas separation and a wide range of inorganic fillers have been investigated. Like what has been discussed in the former section, in membrane gas separation, the utilization of Mg-based materials also concentrates on Mg-based porous materials like Mg-MOF-74, MgO in nanoscale and so on due to their intrinsic affinity of acidic gases like CO₂ [15, 95].

A deeper investigation can be conducted from the perspective of multi-dimensional construction of nanoporous Mg-based materials as fillers with different dimensions can modify the performance of MMMs in different aspects. For one-dimensional fillers like nanotubes or nanorods, structural stability of polymeric matrices can be improved due to high aspect ratio, which means large specific area, giving rise to a reinforced interfacial interactions between polymer and the nanofillers [96]. Enhancing interfacial interactions are reported to be beneficial for the gas separation performance of MMMs like preventing nonselective voids from forming. Rajpure et al. have fabricated cellulose acetate-based MMMs filled with MgO nanorods and the scanning electron microscopy in Figure 16A–D has shown that MgO is loaded successfully at different loading ratio. Permeability and selectivity of the prepared MMM for different gas molecules at different MgO loading shown in Figure 16E–H indicates the effect of cooperating MgO with MMMs, where the permeability of CO₂ shows a general trend of descending while the permeability of H₂ and CH₄ both show a rising trend [97]. For two dimensional fillers, the layered structure can provide MMMs with a longer path, resulting in a much harder movement of larger molecules thus the permeability decreases. Layered double hydroxide, or LDH for short, comes for Mg-based 2D fillers in MMMs. A delaminated LDH has been facilely designed and prepared by Wu et al using Mg²⁺ and Al³⁺ as metal ions [98]. By effective delamination, the LDH nanosheets have established numerous transport channels adjacent to the polymer chain while maintaining its selectivity by introducing tortuosity, thus achieving both highest permeability and highest selectivity at 4 wt% loading of LDH in a long term, as shown in Figure 16I–K. In case of three dimensional fillers, MOFs are probably the most fascinating choice for their tunable porosity, ultrahigh surface area (> 1000 m²/g), superior physicochemical properties and structural flexibility [99]. Apart from common Mg-based MOFs like Mg-MOF-74, Novita et al. cooperated [Mg₃(BTC)₂] with poly-ether sulfone (PES) polymer matrix for improved CO₂ gas separation and it turns out that under a [Mg₃(BTC)₂] loading of 20 wt%, the permeability of CO₂ had increased five times while the selectivity of CO₂/N₂ and CO₂/O₂ separation reached 200% and 150%, respectively [100]. Other cases utilizing Mg-based materials in MMMs for gas separation are summarized in Table 2.

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Table 2 Summary of using Mg-based nanomaterial in MMMs for gas separation.

Nanomaterial filler	Polymer	Maximum permeability and gas/Barrer	Maximum selectivity gas pair	Maximum selectivity	Ref.
MgO	Matrimid	CO2, 10.5	CO2/CH4	26.40	[101]
MgO	PSF	H2, 15.65	H2/N2	42.30	[102]
MgO	PEBAX	CO2, 83.3	CO2/CH4	25.30	[103]
MgO	PEBAX/PEG	CO2, 210.1	CO2/N2	60.9	[104]
MgO nanorods	CA	H2, 77.8	CO2/CH4	24.29	[97]
Mg-Al LDH	PEGDME	CO2, 613	CO2/N2	188.7	[98]
Mg-Al LDH	EVA	CO2, 119.21	CO2/N2	20	[105]
Mg-MOF-74	Matrimid	CO2, 41.1	CO2/N2	36.9	[106]
Mg-MOF-74	PVAc	CO2, 2.42	CO2/CH4	21.2	[107]
Mg3(BTC)2	PES	CO2, 227.131	CO2/N2	7.99	[100]

Summary and Outlooks

Magnesium is of great abundance around the world and making full use of it in a wide range meets the demand of further development for the magnesium industry. The nature of Mg and its compounds makes it possible to play an important role in gas separation involving H₂ and other gases like CO₂ and SO₂, shedding new light on the way to improve more effective and less costly gas separation processes. This review comprehensively explores the advancements in Mg-based materials for gas separation and purification, particularly focusing on Mg-based materials in gas separation processes of different principles: Mg(OH)₂ and MgO in absorption separation, magnesium hydrides, Mg-based MOFs and nanoporous MgO in absorption separation, and multi-dimensional Mg-based compounds as fillers in MMM separation. In all cases of gas separation, the Mg-based materials have demonstrated significant potential due to their unique properties and capabilities in selective gas capturing, effectively improving the gas separation process.

• In liquid-gas absorption-based gas separation, Mg(OH)₂ has been effectively used for the desulfurization of waste gases from diesel engines, showcasing its ability to remove sulfur compounds from marine emissions. It is expected that employing Mg-based ECGS can effectively reduce the cost of gas cleaning system onboard while maintaining satisfying desulfurization performance. Additional utilization of MgO can also make the absorption process regenerable instead of cleaning marine flue gas in a once-through way.

• In solid-gas absorption/adsorption-based gas separation, various Mg-based compounds can be employed for capturing specific gas molecules like H₂ and CO₂, resulting in fast and effective gas separation, including Mg-based hydrides, Mg-based MOFs and nanoporous MgO. Besides, methods to improve air stability of MgH₂ are highlighted with inspirations from designing air-stable Mg-based hydrogen storage materials, including the encapsulation of MgH₂ nanoparticles and coupling with sacrificial catalysts to enhance kinetics and air stability, which may provide new insights into designing Mg-based gas separation materials with high reversibility.

• In membrane gas separation, the use of MMMs has been concentrated on and the incorporation of Mg-based compounds in nanoscales or Mg-MOFs as fillers in MMMs has led to significant improvements in gas separation performance. These MMMs benefit from the synergistic effects of the polymer matrix and the high selectivity of the Mg-based fillers in different dimensionalities including nanorods, nanosheets and 3D porous structures, resulting in enhanced permeability and selectivity for target gases.

The future of Mg-based materials in gas separation and purification seems promising. However, from a general perspective, the investigations into utilizing Mg-based materials are still limited despite the potential of effectively capturing certain gases like H₂ and CO₂. Further explorations with the following avenues may boost the development of applying Mg-based materials in gas separation and purification:

• 1.

  Further modification of Mg-based gas separation materials. Continued efforts to improve the performance of Mg-based materials are crucial, including selectivity, stability and reversibility. Special attention is expected to be paid to improving the stability of Mg-based gas materials under the flow of toxic gas as cases of separating toxic gas from gas mixtures are common, especially for Mg-based hydrides which are too sensitive to toxic gases like CO, O₂, and H₂O. Advanced coating techniques and the development of robust composite materials could protect Mg-based adsorbents from oxidation and degradation, thereby extending their operational lifespan. Also, modification in synthesis methods for Mg-MOFs, MgH₂, and nanoporous MgO expects further development as promoted synthesis can lead to more efficient and cost-effective production processes. This includes exploring greener synthesis routes and scaling up laboratory methods to industrial levels.

• 2.

  Improvement in Advanced Simulation processes. Advanced simulation processes based on first-principle calculation, density function theory and other calculation methods can offer deeper insight into the details of reaction between Mg-based materials and different gas molecules, giving instructions for further design of Mg-based gas separation materials and optimal working conditions including temperature, pressure and other parameters. Also, appropriate simulation can reflect the mechanism of the deterioration of Mg-based materials under gas flow containing toxic impurities, inspiring the design of Mg-based gas separation materials with promoted reversibility.

• 3.

  Development and application of key equipment. To further integrate Mg-based materials into industrial gas separation and purification systems, development and application of key equipment are needed to fit the practice of using Mg-based materials for gas separation, including membrane modules, absorption columns, absorption towers and systems for regeneration and compression. Developed equipment for gas separation which fits the working process of Mg-based materials can probably promote the gas separation efficiency of the whole system while reducing the cost.

Overall, the continuous development and optimization of Mg-based materials holds significant potential for advancing gas separation and purification technologies, contributing to more sustainable and efficient industrial processes.

Acknowledgments

This work was supported by National Key Research & Development Program of China (2023YFB3809103 and 2023YFB3809100) and National Natural Science Foundation of China (52201266, 52171186), Startup Fund for Young Faculty at SJTU (SFYF at SJTU), Young Elite Scientists Sponsorship Program by CAST (2023QNRC001).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The authors have nothing to report.

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