Pore Engineering in Metal–Organic Frameworks for Enhanced Hydrocarbon Adsorption and Separation

Abstract

The separation and purification of hydrocarbons are crucially important processes in the petrochemical industry, as they are essential for producing high-quality chemicals and fuels. However, traditional thermal-driven separation practices, such as cryogenic distillation, are notoriously energy-intensive, accounting for a notable portion of the energy consumption in industrial operations. This has spurred the exploration and development of low-energy and sustainable alternative separation technologies, among which adsorption/desorption-based separation with porous materials has gained significant attention. Metal–organic frameworks (MOFs) are emerging as ideal porous materials for hydrocarbon separation due to their exceptional porosity and structural tunability. This Account delves into the latest advancements in microporous MOFs for hydrocarbon separation, categorizing them based on their pore structures: single array, tandem array, and orthogonal array. Single-array MOFs feature uniformly arranged channel-like pores along the axial direction, facilitating the incorporation of binding sites on the pore surfaces. One notable functional group used in these applications is open metal sites (OMSs), which can engage in strong metal-π interactions with unsaturated hydrocarbons such as acetylene. For example, JNU-1 demonstrates increased binding energy with the increasing pressure of acetylene due to the induce-fit effect, where framework contraction behavior is triggered by its OMSs. JNU-4 offers two binding sites per metal center for acetylene molecules, greatly improving the adsorption capacity. On the other hand, introducing low-polarity groups, as seen in JNU-6-CH₃, can effectively enhance the separation performance in favor of alkanes while maintaining structural integrity under humid conditions. Another methyl group-modified MOF, JNU-5-CH₃, exhibits an acetylene-triggered gate-opening effect due to the multiple supramolecular interactions with acetylene. Tandem-array MOFs provide enhanced selectivity and adsorption capacity through the interconnection of spacious cavities with narrow apertures. For instance, JNU-2 with pore-channel interconnected structure exhibits improved separation efficiency for C₂H₆/C₂H₄ and hexane isomers. The slim channels connecting the large cavities act as screening sites for matching-sized molecules to pass through, while the large cavities function as storage sites for large adsorption capacity. Orthogonal-array MOFs, like JNU-3a, feature one-dimensional (1D) channels that enable rapid diffusion, complemented by molecular pockets on both sides that facilitate selective recognition. The dynamic “gourd-shaped” opening of the pocket demonstrates notable adaptability when interacting with different hydrocarbons, allowing for sieving-like behavior in the separation of propylene/propane, as well as efficient separation of ethylene from its mixtures with alkynes of various sizes. Overall, the designability and tunability of MOF pore structures make them promising candidates for effectively discriminating targeted molecules from multicomponent mixtures, offering energy-efficient solutions for challenging industrial separations.