CO₂, N₂ and H₂ adsorption in Zn²⁺-containing zeolites and metal-organic frameworks

We employ a chemically accurate (±4 kJ mol^-1) embedding approach to calculate enthalpies for CO₂, N₂, and H₂ adsorption in the Zn²⁺-containing zeolites FAU and CHA, and for CO₂ and H₂ in the metal-organic frameworks (MOFs) Zn-MOF-74 and CALF-20. Using MP2 as the high-level method and PBE+D4 as the periodic low-level method (MP2:PBE+D4), we obtain CO₂ adsorption enthalpies of -46, -34, and -36 kJ mol^-1 for Zn-CHA, Zn-MOF-74, and CALF-20, respectively, all within chemical accuracy limits of the experimental values of -42, -30, and -39 kJ mol^-1. For CO₂ in Zn-FAU, where no experimental data exist, we provide an MP2:PBE+D4 prediction of -49 kJ mol^-1. For N₂, we predict MP2:PBE+D4 adsorption enthalpies of -41 and -39 kJ mol^-1 in Zn-CHA and Zn-FAU, respectively. CO₂ adsorption is stronger in the zeolites than in the studied MOFs. Across all systems, CO₂ adsorption tends to be stronger than N₂ adsorption and both are largely favoured over H₂. We observe inconsistent accuracy of PBE+D4 for the two MOFs, despite their similar characteristics, underscoring the need for high-level approaches in predictive screening. Additionally, we conclude that Zn²⁺ cations with open coordination sites act as the primary binding sites and that these cations preferentially occupy 6-membered rings in zeolites.