Simulations of model metal-organic frameworks for the separation of carbon dioxide

Multiscale Computational Study on the Adsorption and Separation of CO2 /CH4 and CO2 /H2 on Li+ -Doped Mixed-Ligand Metal-Organic Framework Zn2 (NDC)2 (diPyNI)

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.