Enhancing carbon dioxide capture under humid conditions by optimizing the pore surface structure

Abstract

Metal–organic frameworks (MOFs) exhibit significant potential for mitigating carbon emissions due to their high porosity and tunability. Despite numerous reports on CO₂ capture by MOF sorbents, a common challenge is their poor selectivity for CO₂ over water. Moreover, in-depth studies are much needed to elucidate the relationships among the pore surface structure, hydrophobicity, and CO₂ uptake capacity/selectivity. In this work, we investigate the factors influencing CO₂ adsorption capacity and selectivity under humidity in a series of isoreticular pillar-layer structures, Ni₂(L)₂(dabco) (L = bdc, ndc, adc). Our study shows that increasing ligand conjugation not only results in increased hydrophobicity, decreased pore size and BET surface area, but also leads to the change of primary binding sites of water molecules and higher binding energy of CO₂, all of which contribute to largely increased CO₂ uptake capacity under humid conditions. Additionally, increasing ligand conjugation and consequently hydrophobicity slow down and reduce competitive water adsorption drastically. Notably, the MOF made of ligand with the highest conjugation, Ni₂(adc)₂(dabco), exhibits significantly enhanced CO₂ adsorption in N₂/CO₂ binary mixtures under relatively high humidity (50% RH), with an increase of ∼31% and ∼36% for the composition of 15/85 and 50/50, respectively, compared to dry conditions. An experimental FTIR study and DFT theoretical calculations confirm that H₂O occupies different primary binding site in Ni₂(bdc)₂(dabco) and Ni₂(adc)₂(dabco), and under humid conditions a higher binding energy of CO₂ is achieved with preferential H₂O/CO₂ co-adsorption in Ni₂(adc)₂(dabco), potentially creating additional adsorption sites for CO₂.