Capturing CO₂ under Dry and Humid Conditions: When Does the Parent MOF Outperform the MTV MOF?

A key challenge in capturing CO₂ from postcombustion gases is humidity due to competitive adsorption between CO₂ and H₂O. Multivariate (MTV) metal-organic frameworks (MOFs) have been considered a promising option to address this problem, e.g., combining CO₂-affinitive and hydrophobic groups. Here, we synthesized a series of amine and methyl cofunctionalized MTV MIL-53(Al)-xNH₂(1 - x)CH₃ and their parent materials. All the mixed linker MIL-53(Al)-xNH₂(1 - x)CH₃ showed amino linker enrichment compared to the synthesis ratio, yet the linkers were distributed relatively homogeneously from the bulk to the surface. Material hydrophobicity or hydrophilicity varied with methyl or amino group content, respectively. The single-component adsorption indicated that certain mixed linker MIL-53(Al)-xNH₂(1 - x)CH₃ might outcompete the parent materials. In CO₂-H₂O competitive adsorption, however, the hydrophobic parental MIL-53(Al)-CH₃ outperformed the mixed linker MOFs. CO₂ adsorption capacities of 5.4, 4.9, and 3.6 wt % were found for 0.3 bar of CO₂ under 0, 5, and 10% RH, respectively. The results highlight that materials with enhanced hydrophobicity and tight-fitting pores can outperform groups with high CO₂ affinity in the CO₂ capture under humid conditions.