Two-dimensional (2D) material membranes have emerged as promising candidates for gas separation applications due to their atomic thickness, high surface area, and tunable physicochemical properties. Among these materials, porous graphene and molybdenum disulfide (MoS₂) stand out for their potential in catalytic gas separation, particularly for mixtures such as H₂/CO₂. These membranes combine selective permeation with catalytic conversion, enabling simultaneous separation and reaction processes. The performance of such membranes depends on precise pore size engineering, surface functionalization, and maintaining mechanical integrity under operational conditions.

Pore size tuning is a critical factor in achieving selective gas separation. Theoretically, single-layer graphene and MoS₂ can exhibit near-perfect selectivity when pores are tailored to the kinetic diameters of target molecules. For H₂/CO₂ separation, the kinetic diameters of H₂ (2.89 Å) and CO₂ (3.30 Å) necessitate pores in the range of 3-4 Å to achieve molecular sieving. Experimental approaches to pore creation include plasma etching, chemical oxidation, and electron beam irradiation. For example, oxygen plasma treatment can introduce sub-nanometer pores in graphene with controlled density and size distribution. In MoS₂, sulfur vacancy engineering through annealing or chemical treatment creates active sites that can serve as selective pores. Advanced techniques such as block copolymer templating or atomic layer lithography further enhance precision in pore formation. The challenge lies in achieving uniform pore distribution without compromising membrane integrity.

Surface functionalization plays a dual role in enhancing selectivity and introducing catalytic activity. Functional groups such as hydroxyl, carboxyl, or amine moieties can be grafted onto graphene or MoS₂ to modify interaction strengths with gas molecules. For instance, amine-functionalized graphene exhibits higher affinity for CO₂ due to chemisorption, while H₂ permeates via size exclusion. In MoS₂, edge-site sulfur vacancies act as catalytic centers for reactions like CO₂ hydrogenation. Transition metal dopants (e.g., Ni, Co) or nanoparticle decoration (e.g., Pt, Pd) further enhance catalytic performance. The synergy between pore geometry and surface chemistry enables simultaneous separation and conversion, as seen in membranes where CO₂ is selectively adsorbed and converted to methane or methanol while H₂ permeates unaffected.

Simultaneous catalytic conversion during permeation is a unique advantage of 2D material membranes. Traditional gas separation processes require separate units for purification and reaction, increasing energy and capital costs. In contrast, catalytic membranes integrate these steps. For example, a porous graphene membrane decorated with palladium nanoparticles can separate H₂ from CO₂ while catalyzing CO₂ reduction to formic acid. The atomic thickness of 2D materials minimizes mass transfer resistance, allowing high reaction rates. However, the trade-off between permeance and conversion efficiency must be carefully balanced. High catalytic activity often requires dense functional sites, which may reduce gas flux. Optimizing the density and distribution of active sites is essential for maximizing overall performance.

Scalability remains a significant challenge for 2D material membranes. While lab-scale demonstrations show promising results, manufacturing large-area, defect-free membranes is non-trivial. Chemical vapor deposition (CVD) can produce meter-scale graphene films, but transferring these films onto porous supports without cracks or wrinkles is difficult. MoS₂ growth faces similar issues, with additional complications arising from stoichiometric control during synthesis. Roll-to-roll processing and interfacial polymerization are being explored to address these limitations. Another concern is the trade-off between selectivity and permeance at scale. As membrane area increases, maintaining uniform pore distribution and catalytic activity becomes harder, often leading to performance degradation.

Mechanical integrity is another critical consideration. 2D materials are inherently strong but prone to tearing or delamination under pressure gradients. Graphene’s high tensile strength (130 GPa) makes it resilient, but its flexibility can lead to deformation under transmembrane pressures. MoS₂, with its layered structure, may suffer from interlayer slippage. Composite approaches, such as embedding 2D materials in polymer matrices or using hexagonal boron nitride (hBN) as a reinforcing interlayer, improve mechanical stability. Additionally, porous supports must be carefully selected to minimize stress concentrations. Brittle supports can cause membrane failure, while overly flexible ones may not provide sufficient anchoring.

Environmental and operational stability further influence practical deployment. Exposure to high temperatures, corrosive gases, or humidity can degrade 2D membranes over time. Graphene is susceptible to oxidation at elevated temperatures, while MoS₂ may undergo phase transitions or sulfur loss. Protective coatings or passivation layers can mitigate these effects, but they must not compromise permeability or catalytic activity. Long-term testing under realistic conditions is necessary to validate durability.

In summary, 2D material membranes with catalytic functionality represent a transformative approach to gas separation and conversion. Pore size tuning and surface functionalization enable selective and reactive permeation, while scalability and mechanical challenges must be addressed for industrial adoption. Advances in synthesis, transfer techniques, and composite design will be pivotal in realizing the full potential of these membranes. Future research should focus on optimizing the interplay between separation efficiency, catalytic activity, and durability to enable next-generation gas processing technologies.