Hydrophobic zeolitic imidazolate framework shell on a copper-based metal-organic framework for efficient and stable capture of carbon dioxide under humid conditions

Efficient CO₂ capture from humid industrial flue gas faces significant challenges due to competitive water adsorption and the moisture sensitivity of many high-capacity porous adsorbents. This paper employs a metal-organic framework-on-metal-organic framework (MOF-on-MOF) strategy to construct a Cu(BTC)@ZIF-67 core-shell structure, in which a relatively hydrophobic zeolitic imidazolate framework-67 (ZIF-67) outer layer is introduced onto Cu(BTC) to enhance adsorption capacity under humid conditions. Structural and elemental characterization, including Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDS) line scan analysis, supported the formation of a ZIF-67-enriched outer protective region on the Cu(BTC) surface. The composite retained a high CO₂ adsorption capacity of 149.79 cm³/g, compared with 171.85 cm³/g for Cu(BTC), while achieving a substantially higher ideal adsorbed solution theory (IAST) CO₂/N₂ selectivity of 201.15. More importantly, at 298 K and 50% relative humidity (RH), the CO₂ breakthrough time of Cu(BTC)@ZIF-67 remained at 3124.6 s/g, and the dynamic CO₂ adsorption capacity was 51.43 cm³/g, approximately 94% of its dry-state performance. H₂O breakthrough tests further demonstrated that the composite material's water uptake was substantially lower than that of Cu(BTC), maintaining excellent CO₂ separation performance even after full humidification. The composite material also exhibited stable wet-state breakthrough performance throughout five adsorption-regeneration cycles and retained most of its porous structure and CO₂ adsorption capacity after direct contact with water. Density functional theory (DFT) calculations, as well as molecular electrostatic potential (MEP), density of states (DOS) and projected density of states (PDOS) and differential charge density analyses, indicated that the composite material's interaction with CO₂ was superior to that with N₂. These results suggest that adjusting the balance between pore accessibility and hydrophobic protection through MOF-on-MOF interface engineering is an effective strategy for developing moisture-resistant CO₂ adsorbents.