Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performances

Abstract

Propylene/propane separation is one of the most challenging separations, currently achieved by energy-intensive cryogenic distillation. Despite the great potential for energy-efficient membrane-based separations, no commercial membranes are currently available due to the limitations of current polymeric materials. Zeolitic imidazolate framework, ZIF-8, with the effective aperture size of ∼4.0 Å, has been shown to be very promising for propylene/propane separation. Despite the extensive research on ZIF-8 membranes, only a few reported ZIF-8 membranes have displayed good propylene/propane separation performances presumably due to the challenges of controlling the microstructures of polycrystalline membranes. 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. The ZIF-67 membranes exhibited impressively high propylene/propane separation capabilities. 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.

Decoding Nucleation and Growth of Zeolitic Imidazolate Framework Thin Films with Atomic Force Microscopy and Vibrational Spectroscopy

“…In Figure 3a-c and Figure 6a,c haracteristic vibrations are highlighted as observedf or both Zn-ZIF-8a nd Co-ZIF-67 (thin films). [41,45,56,57,59] Ad etailed comparison between the IR spectra of bulk and thin film grown materials shows that more distinct IR bands are observable for bulk ZIFs, especially in the ring stretching and vibrational regions(% 830, 940,1 070, 1240 cm À1 ,e tc. ).This suggests that bulk ZIF framework is less defective/perturbed in nature relative to its thin film analogue, leading to less bond/ring disorder.N ote that the absence of broad NÀHs tretching bands in the region between 3200-3500 cm À1 in the DRIFT spectra of Zn-ZIF-8a nd Co-ZIF-67 demonstrate that the linker 2-methylimidazole is incorporated in the ZIF-8/-67 thin films in its deprotonated state.…”

“…[26,28,30,[41][42][43] By using Co 2 + instead of Zn 2 + cations, isostructural Co-ZIF-67 is obtained, with an identical structuret oZ n-ZIF-8 but with Co 2 + in the Zn 2 + lattice positions. [44][45][46][47] Several studies report on the crystallisationm echanismso f bulk ZIFs by using in situ static light scattering (SLS), [29] transmission electronm icroscopy (TEM), [32,48] scanning electron microscopy (SEM), [28,49] X-ray diffraction( XRD), [28,32] small angle and wide angle X-ray scattering (SAXS/WAXS), [31] electrospray ionisation mass spectrometry (ESI-MS), [50] and atomicf orce microscopy (AFM). [51,52] However,f or (reactive)m embrane and gas sensor applications,c onformal ZIF thin films with highly controllable morphologies are required insteado fb ulk powders.…”

Heteroepitaxial growth of vertically orientated zeolitic imidazolate framework‐L (Co/Zn‐ZIF‐L) molecular sieve membranes

“…However, as shown, it is challenging to prepare a high‐quality Co‐ZIF‐L seed layer because ZIF‐L crystals are disorderly arranged with large interfacial voids between two adjacent ZIF‐L sheets (Figure S6a,b). This is mainly ascribed to the faster crystallization of Co‐ZIF‐L in contrast to Zn‐ZIF‐L under the same experimental conditions 22 . The overly rapid nucleation and crystallization of Co‐ZIF‐L potentially accelerate homogenous nucleation in solution, which has a negative effect on the heterogeneous nucleation of Co‐ZIF‐L onto the PSS‐HNTs layer; this leads to a low‐quality Co‐ZIF‐L seed layer.…”

“…While the methodology of secondary growth notably facilitates the formation of continuous ZIF layers, poor interaction between the polymer and ZIF layers remains an issue to be addressed. On the other hand, it was reported that molecular sieving of continuous ZIF membranes is closely linked to their intrinsic material apertures (intracrystalline diffusion) and grain boundaries (intercrystalline diffusion) 22 . Due to the presence of nonselective intercrystalline diffusion, the optimization of grain boundaries via feasible processing techniques is of critical importance to increase the gas selectivity.…”

“…Analogous to the isostructural relation between ZIF‐8 and ZIF‐67, Zn‐ZIF‐L and Co‐ZIF‐L are expected to be isomorphic with different metal nodes (ZIF‐8, Zn; ZIF‐67, Co), and to have the same crystallographic results. In addition, several studies have reported the use of heteroepitaxial growth to design ZIF‐67/−8 Janus crystals, 31 ZIF‐8@ZIF‐67 core‐shell structures, 32 and ZIF‐67/−8 membranes 22 . These reports afforded sufficient evidence to design mixed‐component ZIF layers using heteroepitaxial growth; however, there has been no research on vertically oriented, intergrown, and mixed‐component ZIF‐L membranes for CO 2 /N 2 separation.…”