CO2 capture in rht metal-organic frameworks: multiscale modeling from molecular simulation to breakthrough prediction

A multiscale modeling study is reported for CO2 capture in a series of metal-organic frameworks (Cu-TDPAT, PCN-61, -66, -68, NOTT-112, NU-111 and NU-110). These MOFs share the same rht topology, though with different ligands. The predicted adsorption isotherms of pure gases (CO2, N-2, H-2 and CH4) agree well with experimental data. The structure-property relationships between adsorption capacity and ligand size are established. With increasing ligand size, adsorption capacity drops at a low pressure, but increases near saturation conditions. Due to the presence of small ligands, unsaturated metals and amine groups, Cu-TDPAT exhibits the highest adsorption capacity and separation performance among the seven rht-MOFs. On this basis, a new structure, Cu-TDPAT-N, is designed by substituting the phenyl rings in Cu-TDPAT by pyridine rings. Compared to Cu-TDPAT, the N-rich Cu-TDPAT-N possesses higher capacity, isosteric heat and selectivity. From simulated results, the breakthrough profiles for CO2-containing mixtures (CO2/N-2, CO2/H-2 and CO2/CH4) are predicted, and the breakthrough time for CO2 in Cu-TDPAT-N is found to be extended by two-fold. This study provides microscopic insights into gas adsorption and separation in rht-MOFs, establishes quantitative relationships, and suggests that N-substitution is effective to enhance the performance for CO2 capture.