Nanospace Engineering of Metal–Organic Frameworks for Adsorptive Gas Separation

Abstract

Gas separation is a critical process in the industrial production of chemicals, polymers, plastics, and fuels, which traditionally rely on energy-intensive cryogenic distillation techniques. In contrast, adsorptive separation using porous materials has emerged as a promising alternative, presenting substantial potential for energy savings and improved operational efficiency. Among these materials, metal–organic frameworks (MOFs) have garnered considerable attention due to their unique structural and functional characteristics. MOFs are a class of crystalline porous materials constructed from inorganic metal ions or clusters connected by organic linkers through strong coordination bonds. Their precisely engineered architectures create well-defined nanoscale spaces capable of selectively trapping guest molecules. In contrast to traditional porous materials such as zeolites and activated carbons, emerging MOFs not only demonstrate exceptional capabilities for pore regulation and interior modification through nanospace engineering but also hold great promise as a superior platform for the development of high-performance functional materials. By virtue of the isoreticular principle and building unit assembly strategies in MOF chemistry, precise adjustments to pore structures─including pore size, shape, and surface chemistry─can be readily achieved, making them well-suited for addressing the separation of intractable industrial gas mixtures, particularly those with similar sizes and physicochemical properties.

This Account presents a comprehensive overview of our recent advancements in high-performance gas separation through nanospace engineering within porous MOFs. First, by strategically immobilizing open metal sites (e.g., Ag⁺) in the pore surface, the functionalized PAF-1-SO₃Ag demonstrates enhanced ethylene uptake capacity while maintaining exceptional structural stability under humid conditions. Furthermore, pore surface modification with low-polarity groups (e.g., −CH₃, −CF₃), as demonstrated in Ni(TMBDC)(DABCO)_0.5, leads to enhanced C₂H₆/C₂H₄ separation performance. To achieve strong guest molecule binding, we engineered novel ″nanotrap″ binding sites that synergistically integrate oppositely adjacent open metal sites and dense alkyl groups, as exemplified by the Cu-ATC framework. Remarkably, Cu-ATC achieves efficient separation of several challenging gas mixtures, including acetylene/carbon dioxide (C₂H₂/CO₂), xenon/krypton (Xe/Kr), and methane/nitrogen (CH₄/N₂). These innovations have resulted in the development of MOF materials with exceptional separation performance, tailored for specific industrial applications such as light hydrocarbon purification, rare gas separation, and coalbed methane enrichment. Our work not only advances the fundamental understanding of structure–property relationships in MOFs but also provides practical insights for the development of next-generation separation technologies. These advancements hold promise for drastically reducing energy consumption and operational costs in gas separation processes, contributing to more sustainable industrial practices. Future research on MOF materials is anticipated to play a pivotal role in addressing global energy challenges and advancing separation science.