Stability of Highly Propylene-Selective Zeolitic-Imidazolate Framework, ZIF-8, Membranes

Currently propylene/propane separation is performed with a highly energy-intensive cryodistillation process mainly due to their close boiling points. Although a variety of membranes with different materials have been studied for membrane-based propylene/propane separation, there are currently no membranes and membrane processes that are commercially available. This is mainly due to the fact that the majority of new membrane materials proposed failed to meet the separation performance requirements with respect to the propylene permeability and propylene/propane separation factor.^1,2,3

Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic framework with zeolite topology, are composed of divalent metal nodes (typically Zn or Co) interconnected with imidazolate-based ligands, forming three-dimensional crystalline structures with pores and cavities in the scale of molecules.⁴ ZIFs are considered very promising gas separation membrane materials because of their chemical and thermal stability in combination with their ultra-micropores (less than 0.5 nm). For example, membranes of ZIF-8, made of Zn and 2-methyl-imidazolate with a SOD structure, were reported having excellent propylene/propane separation due to the fact that its effective aperture size lies between propylene and propane.⁵ Highly propylene-selective ZIF-8 membranes were synthesized using several methods including in situ counter diffusion and microwave-assisted secondary growth methods reported by Kwon et al.^6,7

For industrial applications of ZIF-8 membranes, their long term stability is critically important. There are, however, only a few such studies have been reported.⁸ In our long-term stability studies, we observed that time-dependent propylene/propane permeation and separation behaviors depend on synthesis methods.

Here, we would like to discuss the relationship between the membrane stability and inherent surface defects resulting from different synthesis conditions and to show a post-synthetic treatment as an effective means to stabilize membranes.

References and Notes

1 Koros, William J. and Fleming, G.K. (1993), “Membrane-based gas separation”, Journal of Membrane Science, 83, pp l-39

2 Baker RW., (2002) “Future Directions of Membrane Gas Separation Technology” Industrial & Engineering Chemistry Research., 41 (6), pp 1393-1411

3 Tanaka, K., Taguchi, A., Hao, JQ, Kita, H., and Okamoto, K., (1996), “Permeation and separation properties of polyimide membranes to olefins and paraffins”, Journal of Membrane Science, 121, pp 197-207

4 Park KS, Ni Z, Côté AP, et al., (2006), “Exceptional chemical and thermal stability of zeolitic imidazolate frameworks”, Proceedings of the National Academy of Sciences, 103 (27), pp 10186-10191

5 Zhang C, Lively RP, Zhang K, Johnson JR, Karvan O, Koros WJ., (2012), “Unexpected Molecular Sieving Properties of Zeolitic Imidazolate Framework-8”, The Journal of Physical Chemistry Letters., 3 (16), pp 2130-2134.

6 Kwon HT, Jeong H-K., (2013), “In Situ Synthesis of Thin Zeolitic–Imidazolate Framework ZIF-8 Membranes Exhibiting Exceptionally High Propylene/Propane Separation”, Journal of the American Chemical Society, 135 (29), pp 10763-10768.

7 Kwon HT, Jeong H-K., (2013), “Highly propylene-selective supported zeolite-imidazolate framework (ZIF-8) membranes synthesized by rapid microwave-assisted seeding and secondary growth”, Chemical Communications, 49, pp 3854-3856

8 Liu, D., Ma, X., Xi, H., Lin, YS., (2014),  “Gas transport properties and propylene/propane separation characteristics of ZIF-8 membranes”, Journal of Membrane Science, 451, pp 85–93