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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