(293f) Control over the Gas Separation Range of Zeolitic Imidazolate Framework-8 Based Membranes: Metal Replacement and Linkage Exchange

Separation of hydrocarbon mixtures is one of the most important processes in the petrochemical industry. The high energy demands of the most common methods such as distillation and absorption create a need for cheaper and more environmentally friendly separation techniques. Membranes of nanoporous media (pore size on the order of 10^-9 m) are a new and promising approach to separation. The aim of this project is the design of novel nanoporous materials for challenging industrial gas separations. The materials under study are Zeolitic-Imidazolate Frameworks (ZIFs), a subfamily of Metal-Organic Frameworks that consist promising materials for gas separation, due to their unique separation properties and their adaptability to structural modifications which lead to changes in separation efficacy. These modifications are carried mainly through the so-called linker-exchange and metal replacement.^1,2,3,4

ZIF-8 is the main focus of this work and consists of 2-methylimidazolate anions tetrahedrally bridged with a zinc (Zn) cation. This sequence forms cages connected through apertures, which constitute the main energy barrier for the diffusion of penetrants between the cages. Three metals to replace the Zn ion were investigated along with the original ZIF-8 framework: the recently synthesized framework with cadmium (Cd) (CdIF-1),⁵ the cobalt (Co) substituted framework (ZIF-67)⁶ and a newly proposed structure by the authors using beryllium (Be), which we name BeIF-1. The second approach is to replace the one out of the three mim linkages of ZIF-8 aperture, with bim, which results in a hybrid topology, called ZIF-7-8.

We developed force fields using Density Functional Theory (DFT) calculations for the description of the framework and implemented them in molecular simulations. This constitutes the first attempt to simulate variations of this material. Our results revealed that the four frameworks exhibit a sequence of descending aperture sizes, following the increasing metal-nitrogen stiffness. The diffusivity of gas species was calculated with the help of molecular dynamics simulation and transition state theory (for slow-diffusing species) and the ratio of diffusivities of species provided the ideal kinetic separation for the desired mixtures. According to our calculations, ZIF-67 is one of the most promising candidates for propylene/propane separation,^7,8 and also appealing for ethylene/ethane separation.⁹ Our results¹⁰ show that Be-IF1 can consist a highly competitive candidate for the separation of small gases, like CO₂/CH₄ and CO₂/N₂, which are regarded as very difficult for sieving separations because of their small size differences. CdIF-1, which demonstrates the largest aperture, is suitable for separation of mixtures of large hydrocarbons, such as n/iso-butane. The ZIF-7-8 is a new system^11,2 for which neither structural (aperture size) measurements nor gas diffusion/separation values have been reported so far. Here, we will report such values for the first time. This system proves to be ideal for small gas separations. It exhibits high CO₂/CH₄ and CO₂/N₂ diffusion separations.

References

(1) Jayachandrababu, K. C.; Sholl, D. S.; Nair, S. J. Am. Chem. Soc. 2017, 139, 5906–5915.

(2) Hillman, F.; Zimmerman, J. M.; Paek, S.-M.; Hamid, M. R. A.; Lim, W. T.; Jeong, H.-K. J. Mater. Chem. A 2017, 5 (13), 6090–6099.

(3) Jayachandrababu, K. C.; Verploegh, R. J.; Leisen, J.; Nieuwendaal, R. C.; Sholl, D. S.; Nair, S. J. Am. Chem. Soc. 2016, 138 (23), 7325–7336.

(4) Zheng, B.; Wang, L. L.; Du, L.; Huang, K. W.; Du, H. Chem. Phys. Lett. 2016, 658, 270–275.

(5) Tian, Y.-Q.; Yao, S.-Y.; Gu, D.; Cui, K.-H.; Guo, D.-W.; Zhang, G.; Chen, Z.-X.; Zhao, D.-Y. Chem. - A Eur. J. 2010, 16 (4), 1137–1141.

(6) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science. 2008, 319 (5865), 939–943.

(7) Krokidas, P.; Castier, M.; Moncho, S.; Sredojevic, D. N.; Brothers, E. N.; Kwon, H. T.; Jeong, H. K.; Lee, J. S.; Economou, I. G. J. Phys. Chem. C 2016, 120 (15), 8116–8124.

(8) An, H.; Park, S.; Kwon, H. T.; Jeong, H.-K.; Lee, J. S. J. Memb. Sci. 2017, 526, 367–376.

(9) Krokidas, P.; Castier, M.; Economou, I. G. J. Phys. Chem. C 2017, 121 (33), 17999–18011.

(10) Krokidas, P.; Moncho, S.; Castier, M.; Brothers, E. N.; Economou, I. G. Phys. Chem. Chem. Phys. 2018, 20 (7), 4879–4892.

(11) Thompson, J. A.; Blad, C. R.; Brunelli, N. A.; Lively, R. P.; Lydon, M. E.; Jones, W.; Nair, S.; Lively, R. P.; Jones, C. W.; Nair, S. Chem. Mater. 2012, 24 (10), 1930–1936.