Fullerenes, particularly C60 and higher-order structures like C70 and C82, exhibit unique properties that make them promising candidates for gas storage and membrane-based separation applications. Their hollow, cage-like structures, tunable surface chemistry, and well-defined porosity enable interactions with small gas molecules through physisorption and chemisorption mechanisms. The potential of fullerenes in these applications stems from their ability to adsorb gases within their internal cavities or through interstitial sites in crystalline arrangements, as well as their capacity to selectively permeate gases when incorporated into membranes.

The adsorption of gases in fullerenes occurs primarily through van der Waals interactions, electrostatic forces, and, in some cases, covalent bonding. For hydrogen storage, the curvature of the fullerene cage creates a localized electron density that enhances the binding energy of H2 molecules. Studies have shown that pristine C60 can adsorb hydrogen at cryogenic temperatures, with uptake capacities ranging from 1 to 3 wt% at 77 K and moderate pressures. The adsorption is reversible, making fullerenes suitable for cyclic storage applications. However, the weak van der Waals interactions limit practical room-temperature storage. To address this, researchers have explored chemical modification of fullerenes, such as hydrogenation or doping with alkali metals, which can enhance the binding energy through spillover effects or Kubas-type interactions. For example, lithium-doped C60 has demonstrated improved hydrogen adsorption at near-ambient conditions due to the polarization of H2 molecules by the metal ions.

Methane storage in fullerenes follows similar physisorption mechanisms but benefits from the stronger polarizability of CH4 compared to H2. The methane adsorption capacity of fullerene-based materials depends on the packing arrangement of the cages. Face-centered cubic (FCC) or hexagonal close-packed (HCP) structures of C60 create interstitial pores with diameters of approximately 0.4 nm, which are optimal for accommodating methane molecules. Experimental measurements indicate that fullerite (crystalline C60) can adsorb up to 5 wt% methane at 298 K and 35 bar. The adsorption isotherm typically follows a Type I behavior, indicating monolayer coverage within the pores. Functionalization of fullerenes with alkyl groups can further enhance methane uptake by increasing the hydrophobicity and improving the compatibility with nonpolar gas molecules.

The pore structure of fullerene assemblies plays a critical role in gas adsorption and separation performance. Unlike amorphous porous materials, fullerenes form ordered crystalline lattices with uniform pore size distributions. The interstitial sites in these lattices act as molecular sieves, allowing for size-selective adsorption. For example, the pore aperture in FCC-packed C60 is large enough to admit hydrogen (kinetic diameter: 0.29 nm) and methane (0.38 nm) but excludes larger molecules like nitrogen (0.36 nm) or carbon dioxide (0.33 nm) under certain conditions. This selectivity is advantageous for gas separation applications, particularly in membrane systems where the goal is to discriminate between similarly sized molecules.

In membrane-based gas separation, fullerenes can be incorporated as fillers in mixed-matrix membranes (MMMs) or as standalone layers. The impermeable nature of the fullerene cage forces gas molecules to diffuse through the interstitial pathways, creating a selective barrier. The gas permeation properties depend on the packing density of the fullerenes and the presence of functional groups that modify the interaction with specific gases. For instance, hydroxylated fullerenes (fullerenols) exhibit higher affinity for polar gases like CO2 due to hydrogen bonding interactions, while alkylated derivatives favor nonpolar species such as CH4. The permeability and selectivity of fullerene-based membranes can be tuned by varying the degree of functionalization and the processing conditions used to fabricate the films.

Theoretical and computational studies have provided insights into the gas adsorption mechanisms in fullerenes. Molecular dynamics simulations reveal that gas molecules preferentially occupy the octahedral and tetrahedral interstitial sites in FCC-packed C60. The diffusion of gases through these sites follows an activated process, with energy barriers that depend on the size and shape of the molecule. Density functional theory (DFT) calculations have identified charge transfer interactions between fullerenes and adsorbed gases, particularly in systems where the fullerene is doped or chemically modified. These interactions can significantly alter the adsorption enthalpy and kinetics, leading to improved storage or separation performance.

Experimental challenges in utilizing fullerenes for gas storage and separation include achieving high packing densities without compromising accessibility to the internal pores. Solvent-assisted processing techniques, such as supercritical drying or thermal annealing, have been employed to optimize the crystalline order and porosity of fullerene assemblies. Another challenge is the cost and scalability of producing high-purity fullerenes, although advances in synthesis methods, such as plasma pyrolysis or flame synthesis, have reduced production costs in recent years.

Compared to other carbon-based materials like graphene or activated carbon, fullerenes offer distinct advantages in terms of structural uniformity and chemical tunability. The ability to precisely functionalize the outer surface of the cage enables the design of materials with tailored gas adsorption properties. Additionally, the spherical shape of fullerenes facilitates dense packing in membranes, minimizing non-selective voids that can degrade separation performance.

In summary, fullerenes present a versatile platform for gas storage and membrane-based separation applications. Their well-defined pore structure, coupled with the ability to chemically modify their surfaces, allows for precise control over gas adsorption and permeation properties. While practical implementation faces challenges related to material processing and scalability, ongoing research into fullerene derivatives and composite materials continues to expand their potential in this field. The unique combination of molecular-level uniformity and tunable chemistry positions fullerenes as a compelling alternative to conventional porous materials for selective gas interactions.