Multiscale Computational Study on the Adsorption and Separation of CO₂ /CH₄ and CO₂ /H₂ on Li⁺ -Doped Mixed-Ligand...

The quantum mechanics (QM) method and grand canonical Monte Carlo (GCMC) simulations are used to study the effect of lithium cation doping on the adsorption and separation of CO₂ , CH₄ , and H₂ on a twofold interwoven metal-organic framework (MOF), Zn₂ (NDC)₂ (diPyNI) (NDC=2,6-naphthalenedicarboxylate; diPyNI=N,N'-di-(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide). Second-order Moller-Plesset (MP2) calculations on the (Li⁺ -diPyNI) cluster model show that the energetically most favorable lithium binding site is above the pyridine ring side at a distance of 1.817 Å from the oxygen atom. The results reveal that the adsorption capacity of Zn₂ (NDC)₂ (diPyNI) for carbon dioxide is higher than those of hydrogen and methane at room temperature. Furthermore, GCMC simulations on the structures obtained from QM calculations predict that the Li⁺ -doped MOF has higher adsorption capacities than the nondoped MOF, especially at low pressures. In addition, the probability density distribution plots reveal that CO₂ , CH₄ , and H₂ molecules accumulate close to the Li cation site. The selectivity results indicate that CO₂ /H₂ selectivity values in Zn₂ (NDC)₂ (diPyNI) are higher than those of CO₂ /CH₄ . The selectivity of CO₂ over CH₄ on Li⁺ -doped Zn₂ (NDC)₂ (diPyNI) is improved relative to the nondoped MOF.