MOF@MOF core-shell hybrid adsorbents with controlled water vapor affinity towards enhanced and steady CO₂ capture in moist conditions

Metal-organic frameworks (MOFs) are promising adsorbents for CO₂ capture due to their highly tuneable chemical and structural properties. However, most MOFs exhibit a strong affinity for moisture, an ubiquitous component of CO₂-containing mixtures such as flue gas and air, which can lead to a decline in CO₂ capture performance due to competitive adsorption between water vapor and CO₂. This can also increase the energy required for adsorbent regeneration and result in MOF framework decomposition due to the hydrolysis of weak metal-ligand bonds. Therefore, MOFs must possess low water vapor affinity and high CO₂ affinity to be effective in practical CO₂ capture applications. Hybridizing MOFs with other MOFs combines the distinct features of the individual MOF materials and results in unique properties that cannot be achieved by individual components. This study presents a versatile strategy for fabricating novel MOF@MOF core-shell structures with reduced water vapor affinity by in-situ growth of hydrophobic ZIF-8 shells on the surface of hydrophilic HKUST-1 crystals. The resulting core-shell hybrid adsorbent exhibited low moisture affinity, achieving up to a 70% reduction in water vapor adsorption capacity compared to pure HKUST-1. It also demonstrated an IAST CO₂/N₂ selectivity of 41.4 for a binary gas mixture containing 15 vol.% CO₂ and 85 vol.% N₂ at 1 bar and 298 K, which is 73% higher than that of HKUST-1 and 211% higher than that of ZIF-8, due to the presence of the ZIF-8 shell with low N₂ adsorption capacity. The reduced water vapor affinity and excellent CO₂ capture performance, with CO₂ uptake of 2.9 mmol g^-1 at 1 bar and 298 K, of the developed core-shell adsorbent, combined with its cyclability in vacuum swing adsorption (VSA)-based experiments without requiring thermal regeneration, make it promising for practical CO₂ capture applications.