(696d) Heteroepitaxially-Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performance

Heteroepitaxially-grown
Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane
Separation Performance

Hyuk Taek Kwon¹,
Panagiotis Krokidas,³ Marcelo Castier,³ Ioannis G.
Economou,³ Salvador Moncho Escriva,⁴ Dusan Sredojevic,⁴
Ed Brothers⁴ and Hae-Kwon Jeong*^1,2

¹Artie
McFerrin Department of Chemical Engineering and ²Department of Materials
Science and Engineering, Texas A&M University, College Station, TX 77843-3122

³Chemical
Engineering Program and ⁴Science Program, Texas A&M University
at Qatar, PO Box 23874, Education City, Doha, Qatar

* Corresponding author: hjeong7@tamu.edu

Propylene/propane separation is one of the most challenging
separations, currently achieved by energy-intensive cryogenic distillation.
Despite the great potentials for energy-efficient membrane-based
propylene/propane separation processes, no commercial membranes are available
due to the limitations (i.e., low selectivity) of current polymeric materials.^1,2

Zeolitic imidazolate frameworks (ZIFs) are promising
membrane materials primarily due to their well-defined ultra-micropores with
controllable surface chemistry along with their relatively high
thermal/chemical stabilities.³ In
particular, ZIF-8 with the effective aperture size of ~ 4.0 A has been shown
very promising for propylene/propane separation.⁴ Despite
the extensive research on ZIF-8 membranes, only a few of ZIF-8 membranes have
displayed good propylene/propane separation performances^5-9
presumably due to the challenges of controlling the microstructures of
polycrystalline membranes. Since the membrane microstructures are greatly
influenced by processing techniques, it is critically important to develop new
techniques.

Here, we report the first well-intergrown membranes of
ZIF-67 (Co-substituted ZIF-8) by heteroepitaxially growing ZIF-67 on ZIF-8 seed
layers. ZIF-67 membranes were sub-micron thick and well-intergrown, exhibiting impressively
high propylene/propane separation capabilities. The presence of a methanol
co-solvent in the secondary growth solution was critically important to achieve
ZIF-67 membranes in a reproducible manner. Furthermore, when a tertiary growth
of ZIF-8 layers was applied to heteroepitaxially-grown ZIF-67 membranes, the
membranes exhibited unprecedentedly high propylene/propane separation factors
of ~ 200 possibly due to enhanced grain boundary structure.

The new material was studied also computationally using
molecular simulation. A new force field was generated for both ZIF-67 and ZIF-8
structures from ab initio calculations. Molecular Dynamics (MD) simulations of the
diffusion of the two C₃ components show a great enhancement on the
separation of propane/propylene in ZIF-67 compared with that in ZIF-8. A
detailed study in the structural behavior shows that the presence of Co in the
framework results in a shorter Me ? linkage distance and ?tighter? angles,
which renders the pores an aperture with better sieving function. Further
simulations using the Widom test particle insertion methodology shows that in
both structures, the propane and propylene isosteric heat of adsorption are
almost the same. Therefore, the separation of this mixture in ZIF-67 and ZIF-8
is primarily a diffusion-based process.

References and Notes

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3.         Park KS, Ni Z, Côté AP, et al. Exceptional
chemical and thermal stability of zeolitic imidazolate frameworks. Proceedings
of the National Academy of Sciences. July 5, 2006 2006;103(27):10186-10191.

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