Computational insights into efficient CO2 and H2S capture through zirconium MOFs

Separation of CO2 involving mixtures is relevant to the various industrial settings and preserving environment for which different classes of materials including metal-organic frameworks (MOFs) have been researched. Herein, CO2/CO, CO2/H-2, CO2/N-2, and H2S/CO2 separation properties of the zirconium MOFs are computationally investigated mimicking vacuum swing adsorption (VSA) process. Structure-performance relationships are established and the best performing adsorbent materials are determined considering three performance metrics: adsorption selectivity, working capacity, and regenerability. For CO2/CO separation in dry conditions, PCN-59, BUT-10, and PCN-58 are identified to be the top three materials with CO2/CO selectivities of 219.8, 47.2, and 28.6, CO2 working capacities of 6.9, 5.3, and 4.0 mol/kg, CO2 regenerabilities of 63.3, 82.1, and 87.2 %, successively. In humid conditions, UiO-66-OH and MOF-805 appear promising for CO2/CO separation. Regarding CO2/H-2 separation in dry conditions, PCN-59, BUT-10, and LIFM-94 are ranked as the top three MOFs exhibiting CO2/H-2 selectivities of 1445.6, 378.1, and 411.3, CO2 working capacities of 3.6, 2.4, and 2.2 mol/kg, and CO2 regenerabilities of 56.6, 84.9, and 83.9 %, successively. These three materials are also found to be the top three materials for CO2/N-2 separation in dry conditions with CO2/N-2 selectivities of 346.0, 53.3, and 54.9, CO2 working capacities of 3.6, 2.3, and 2.2 mol/kg, and CO2 regenerabilities of 56.3, 84.1, and 83.9 %, successively. For CO2/H-2 and CO2/N-2 separation in humid conditions, UiO-66-NH2 is potentially useful. Considering H2S/CO2 separation in dry conditions, NU-1101, PCN-58, and LMOF-1 are identified to be the best three MOFs attaining H2S/CO2 selectivities of 109.7, 30.9, and 90.7, H2S working capacities of 1.6, 2.3, and 1.2 mol/kg, and H2S regenerabilities of 43.0, 56.4, and 43.7 %, respectively. All top materials for H2S/CO2 separation show relatively large water affinities (PCN-57 having the smallest affinity) which might render them inefficient for H2S/CO2 separation in humid conditions. Adsorbate density profiles are generated for the top 3 materials to elucidate the adsorption mechanisms for each gas separation. A comparison of predictions based on PACMOF and EQeq charges demonstrates drastic differences in material rankings, and separation performance metrics.