(257f) Gas Separation Performance Enhancement of Zeolitic Imidazolate Framework ZIF-8 Membranes Via Post Synthetic Ligand Exchange

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