(217by) Highly Propylene-Selective Supported Zeolitic-Imidasolate Framework (ZIF-8) Membranes By Rapid Microwave-Assisted Seeding and Subsequent Secondary Growth

Highly Propylene-Selective Supported Zeolitic-Imidasolate Framework (ZIF-8) Membranes by Rapid Microwave-assisted Seeding and Subsequent Secondary Growth

Hyuk Taek Kwon¹, and Hae-Kwon Jeong*^1,2

¹Artie McFerrin Department of Chemical Engineering and ²Materials Science and Engineering Program

Texas A&M University, College Station, TX 77843-3122

* Corresponding author: hjeong7@mail.che.tamu.edu

Zeolitic-imidazolate frameworks (ZIFs) are a sub-set of metal organic frameworks (MOFs) with zeolite topologies, comprising of divalent metal cations (Co²⁺, Zn²⁺) bridged by imidazole-based ligand.¹ Their exceptional thermal and chemical stabilities coupled with ultra-microporosity make them desirable candidates especially for gas separation membranes.²

So far, diverse synthesis routes for ZIF films and membranes have been reported and can be broadly classified into two categories: in situ growth and secondary (seeded) growth.² Given the importance of microstructure (particularly grain boundary structure) of polycrystalline membranes, though more complicated as compared to in situ method, secondary growth is preferred in which preformed seed crystals are anchored on porous supports followed by the growth of the seed crystals. Indeed, ZIF-8 membranes synthesized by Pan et al.³ using secondary growth method showed unprecedented propylene/propane gas separation performance. In secondary growth method, strong attachment of seed crystals on porous supports is the key step for the successful preparation of well-intergrown polycrystalline membranes. However, conventional seeding methods such as dip-coating, slip-coating, and manual rubbing often lead to poor reproducibility mainly due to the weak attachment of seed crystals and the inconsistency in obtaining uniform distribution of seed crystals.

In this context, we report a new strategy to rapidly prepare supports strongly attached with seed crystals under microwave irradiation. Using this new seeding technique, we were able to prepare seeded supports with a high packing density in a couple of minutes, which subsequently grown into continuous well-intergrown ZIF-8 membranes. The ZIF-8 membranes are highly propylene-selective with an average propylene/propane selectivity of ~40. Microwave-assisted seeding was also used to prepare well-intergrown films of other ZIFs including ZIF-7 and SIM-1, suggesting the possibility of its general applicability.

References

1.         K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeffe and O. M. Yaghi, Proc. Nat. Acad. Sci. U.S.A. 103 (27), 10186-10191 (2006).

2.         M. Shah, M. C. McCarthy, S. Sachdeva, A. K. Lee and H.-K. Jeong, Ind. Eng. Chem. Res. 51 (5), 2179-2199 (2011).

3.         Y. Pan, T. Li, G. Lestari and Z. Lai, J. Membr. Sci. 390–391 (0), 93-98 (2012).