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.