A Quantum Chemistry Study On The Adsorption Mechanism In Zeolitic Imidazolate Framework Membrane

Zeolitic imidazolate framework (ZIFs), a subclass of metal organic framework (MOFs), combines the properties of both zeolites and MOFs. Similar to zeolites, ZIFs has exceptional thermal stabilities. Similar to other MOFs, it could have diverse and tunable structures by changing metal ions and the organic linkers. Due to these advantages, ZIFs has been found to be a superior performance of separation membrane materials. It is of great importance to study the separation for designing and tuning ZIFs membrane. The separation mechanism of ZIFs is the combination of adsorption-based separation and the diffusion-base separation. And the adsorption of ZIFs directly relate to these two. So, it is necessary to study the adsorption mechanism for understanding the sepration mechanism. Owing to the limited of experiment conditions, there are some difficulties to study the adsorption mechanism. As a suitable computational chemical method to study a larger system, Density Functional Theory (DFT) provides an approach to study the mechanism of adsorption and separation of ZIFs.Firstly, a DFT theory and a large cage cluster which contains684atoms were applied to study the adsorption of H2, CH4, N2and CO2molecules in ZIF-8for the first time. Especially, N2and CO2molecular adsorption is the first time to reseatch. The result showed that there are four main adsorption sites in ZIF-8:The first two adsorption sites are located at both sides of C=C in mIm ligand, and the other two nearest the center of the six-membered channel and both sides of the six-membered ring plan. MIm site Ⅰ is the most stable one for the four small molecules.Secondly, for the most stable adsorption site, the MP2study on the local fragment cluster showed that:the binding energis of H2, CH4, N2and CO2in ZIF-8are-1.1Kcal/mol,-3.0Kcal/mol,-1.7Kcal/mol and-5.3Kcal/mol, respectively. The calculated values of H2and CH4have great agreement with the experimental binding energies. At mIm sites, there are not only interactions between the four small molecules and the nearest C=C double bond in mlm ligand, but also the interactions between the small molecules and the adjacent two mlm ligand at least. It is not enough to describe the adsorptions of ZIFs by only a mlm ligand.The study of two small molecules adsorbed at the neighbouringm mlm site Ⅰ and mⅠm site Ⅱ at the same time showed that mⅠm site Ⅰ have little impaction on mⅠm site Ⅱ. However, during the research of multi-molecules adsorption, the impaction between adsoprion sites must be considered. The study of small molecular adsorbed on a series of ZIFs ligands showed that the substituedt group can impact the binding energies. Meanwhile, the small molecular adsorption in ZIF-1and ZIF-3showed that the coordination structure can also impact the binding energies.