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.