Modifying ZIF-8 framework through metal substitution towards separation improvement of gas mixtures

Nano-scale porous solids are alternate candidates in the development of materials for gas separation with low energy demands. ZIF-8 is one of the most investigated Metal Organic Frameworks (MOFs) for separation of gas... [ view full abstract ]

Nano-scale porous solids are alternate candidates in the development of materials for gas separation with low energy demands. ZIF-8 is one of the most investigated Metal Organic Frameworks (MOFs) for separation of gas mixtures^[1] In this work, substitution of the Zn metal atom is studied in terms of separation efficiency of the framework. Four different modifications are investigated with the help of molecular simulations based on new-developed force fields for all the cases:[2]^,[3] a newly proposed structure with Be metal which we name BeIF-1, the recent synthesized framework with Cd (CdIF-1),[4] the Co substituted framework (ZIF-67)[5] and the original ZIF-8 (Zn). The results show that the metal substitution controls the size and flexibility of the aperture connecting the ZIF cages, which governs the diffusion selectivity of species (Figure 1).

The diffusion of molecules varying in size is simulated in all the metal-substituted modifications. The results are presented in Figure 2 as a function of a newly proposed scale, called Expansion Ratio,[6] which is the penetrant size against the expanded aperture size at the moment the molecule passes through it.

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[[2]] Krokidas, P., Castier, M., Moncho, S., Brothers, E. and Economou, I. G., Molecular simulation studies of the diffusion of methane, ethane, propane, and propylene in ZIF-8, J. Phys. Chem. C, 119, 27028-27037 (2015).

[[3]] Krokidas, P. Castier, M. Moncho, S., Sredojevic, S. D., Brothers, E. N., Kwon, H. T., Jeong, H.-K., Lee., J. S., and Economou, I. G., ZIF-67 framework: A promising new candidate for propylene / propane separations – experimental data and molecular simulations, J. Phys. Chem. C, 120, 8116–8124 (2016).

[[4]] Tian, Y-Q, Yao, S-Y, Gu, D., Cui, K-H, Guo, D-W, Zhang, G, Chen, Z-X and Zhao, D-Y, Cadmium Imidazolate frameworks with polymorphism, high thermal stability, and a large surface area, Chem. Eur. J., 16, 1137-1141 (2010)

[[5]] Banerjee, R., Phan, A., Wang, B., Knobler, C., Furukawa, H., O’Keefee, M. and Yaghi, O. M., High-Throughput synthesis of zeolitic imidazolate frameworks and application to CO₂ capture, Nature, 319, 939-943 (2008).

[[6]] Krokidas, P., Castier, M. and Economou, I. G., J. Matter. Chem. A, 2017, submitted.