Microporous ZIF-8 and ZIF-67 membranes grown on mesoporous alumina substrate for selective propylene transport

Fabrication of Polycrystalline Zeolitic Imidazolate Framework Membranes by a Vapor-Phase Seeding Method

The reliable fabrication of polycrystalline zeolitic imidazolate framework (ZIF) membranes continues to pose challenges for their industrial applications. Here, we present a vapor-phase seeding approach that integrates atomic layer deposition (ALD) with ligand vapor treatment to synthesize ZIF membranes with high propylene/propane separation performance. This method began with depositing a ZnO coating onto the support surface via ALD. The support underwent treatment with 2-methylimidazole vapor to transform ZnO to ZIF-8, forming the seed layer. Subsequent secondary growth was employed at near-room temperature, allowing the seeds to grow into a continuous membrane. ZIF-8 membranes made on macroporous ceramic support by this method consistently demonstrated propylene permeances above 1 × 10-8 mol Pa-1 m-2 s-1 and a propylene/propane separation factor exceeding 50. Moreover, we demonstrated the effectiveness of the vapor-phase seeding method in producing the ZIF-67 membrane.

Fine-Tuning of the Pore Aperture and Framework Flexibility of Mixed-Metal (Zn/Co) Zeolitic Imidazolate Framework-8: An In Situ Positron Annihilation Lifetime Spectroscopy Study under CO2 Gas Pressure

The mixed-metal (Zn/Co) strategy has been used to enhance the gas separation selectivity of zeolitic imidazolate framework-8 (ZIF-8)-based membranes. The enhancement in selectivity has been attributed to possible modifications in the grain boundary structure, pore architecture, and flexibility of the frameworks. In the present study, we used in situ positron annihilation lifetime spectroscopy (PALS) under varying CO2 pressure to investigate the tuning of the pore architecture and framework flexibility of mixed-metal (Zn/Co) ZIF-8 frameworks with varying Co contents. The random distribution of Zn and Co metal nodes within the highly crystalline frameworks having an SOD topology was established using electron microscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy. The inherent aperture as well as cavity size of the frameworks, and the pore interconnectivity to the outer surface, were observed to vary with the Co content in ZIF-8 due to the random distribution of Zn and Co metal nodes in the frameworks. The aperture size is reduced with the incorporation of an additional metal (Zn or Co) in ZIF-67 or ZIF-8, respectively. The aperture size remains the smallest for a lower Co content (∼0.20) in ZIF-8. The framework flexibility determined by in situ PALS measurements under CO2 pressure continuously reduces with increasing Co content in ZIF-8. A smaller aperture size as well as low flexibility of ZIF-8 with a low Co content is seen to be directly correlated to a higher separation selectivity of membranes prepared with this mixed-metal composition.

Advances in metal-organic framework-based membranes

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

The Reversible Removal of SO2 by Amino Functionalized ZIF8 with 5-Aminotetrazole via Post-Synthesis Modification

The post-synthesis modification is a highly efficient method for the modification of Metal-organic framework (MOF) materials, which has been used to synthesize MOF materials purposefully that cannot be prepared by direct synthesis and impregnation method. In this work, amino modified ZIF8 with 5-aminotetrazole was prepared by the post synthesis modification method and was employed to reversibly remove SO2 from flue gas. Based on the characterization and analysis of X-ray diffraction (XRD), Scanning Electron Microscope (SEM), and Brunner Emmet Teller (BET), it was found that the functionalized ZIF8 (Zn(5-ATZ)1.5) was a microporous material with a two-dimensional nano-layered structure. According to the SO2 adsorption experiments, the adsorption capacity of SO2 at the concentration of 1.6% vol can reach to 122 mg/g under the optimal conditions (25 °C, 2865 h−1). Five successive adsorption-desorption experiments exhibited that Zn(5-ATZ)1.5 had excellent regeneration performance. The characterization results of Raman Spectra (Raman) and X-ray photoelectron spectroscopy (XPS) as well as the DFT simulation calculations revealed that SO2 mainly interacted with Zn(5-ATZ)1.5 by hydrogen bonds between O of SO2 and amino H in the Zn(5-ATZ)1.5, and the interaction of SO2 with amino N and 5-aminotetrazole N by forming a non-covalent charge transfer complex. This work suggested that Zn(5-ATZ)1.5 is an excellent potential sorbent for SO2 removal.

High Purity/Recovery Separation of Propylene from Propyne Using Anion Pillared Metal-Organic Framework: Application of Vacuum Swing Adsorption (VSA)

Propylene is one of the world’s most important basic olefin raw material used in the production of a vast array of polymers and other chemicals. The need for high purity grade of propylene is essential and traditionally achieved by the very energy-intensive cryogenic separation. In this study, a pillared inorganic anion SIF62− was used as a highly selective C3H4 due to the square grid pyrazine-based structure. Single gas adsorption revealed a very high C3H4 uptake value (3.32, 3.12, 2.97 and 2.43 mmol·g−1 at 300, 320, 340 and 360 K, respectively). The values for propylene for the same temperatures were 2.73, 2.64, 2.31 and 1.84 mmol·g−1, respectively. Experimental results were obtained for the two gases fitted using Langmuir and Toth models. The former had a varied degree of representation of the system with a better presentation of the adsorption of the propylene compared to the propyne system. The Toth model regression offered a better fit of the experimental data over the entire range of pressures. The representation and fitting of the models are important to estimate the energy in the form of the isosteric heats of adsorption (Qst), which were found to be 45 and 30 kJ·Kmol−1 for propyne and propylene, respectively. A Higher Qst value reveals strong interactions between the solid and the gas. The dynamic breakthrough for binary mixtures of C3H4/C3H6 (30:70 v/v)) were established. Heavier propylene molecules were eluted first from the column compared to the lighter propyne. Vacuum swing adsorption was best suited for the application of strongly bound materials in adsorbents. A six-step cycle was used for the recovery of high purity C3H4 and C3H6. The VSA system was tested with respect to changing blowdown time and purge time as well as energy requirements. It was found that the increase in purge time had an appositive effect on C3H6 recovery but reduced productivity and recovery. Accordingly, under the experimental conditions used in this study for VSA, the purge time of 600 s was considered a suitable trade-off time for purging. Recovery up to 99%, purity of 98.5% were achieved at a purge time of 600 s. Maximum achieved purity and recovery were 97.4% and 98.5% at 100 s blowdown time. Energy and power consumption varied between 63–70 kWh/ton at the range of purge and blowdown time used. The VSA offers a trade-off and cost-effective technology for the recovery and separation of olefins and paraffin at low pressure and high purity.

High performance polyvinylidene fluoride (PVDF) mixed matrix membrane (MMM) doped by various zeolite imidazolate frameworks

Zeolitic imidazolate framework (ZIF-8) in three particle sizes (40, 70 and 100 nm) was prepared through both solvothermal and hydrothermal methods and employed to decorate polyvinylidene fluoride (PVDF). The finger-like macro-voids, sponge-like poly-porous morphology and surface roughness of prepared membranes were characterized by SEM and AFM microscopy. The FTIR spectrum and XPS analysis bear out the chemical component. ZIF-8 has the characteristics of higher porosity and appropriate pore size, which is a condition for improving the permeability and pollution resistance of the modified membrane. Results indicated that different ZIF-8s have different enhancement effects on PVDF MMM. 100 nm ZIF-8 membrane possessed pure water flux (PWF) of 350 L m−2h−1, which was 10 times more than the bare membrane (30 L m−2h−1), and OVA flux recovery ration (FRR%) is 98%. 40 nm ZIF-8 membrane owned BSA FRR% of 98.4%. The 70 nm ZIF-8 showed the best mechanical properties. The dynamic contact angles of UP-Z70 ranged from 104.5° to 62.5° within 180 s. Furthermore, pore size distribution, molecular weight cut-off (MWCO) and porosity were also researched to evaluate the MMM. The dislodge of Reactive Black KN-B, Reactive Red 3BS and Reactive Brilliant Blue KN-R dyes by MMM were studied under different dye concentrations and transmembrane pressures. The membrane can provide selective separation methods for dyes and Reactive Brilliant Blue KN-R up to 99%. Overall, the permeability, hydrophilicy, anti-fouling performance and wastewater treatment of modified membranes were regulated by the ZIF-8 in a steerable blending reaction modification process.

Balancing the Grain Boundary Structure and the Framework Flexibility through Bimetallic Metal-Organic Framework (MOF) Membranes for Gas Separation

Separation is one of the most energy-intensive processes in the chemical industry, and membrane-based separation technology helps to reduce the energy consumption dramatically. Supported metal-organic framework (MOF) layers hold great promise as a molecular sieve membrane, yet only a few MOF membranes showed the expected separation performance. The main reasons include e.g. nonselective grain boundary transport or the flexible MOF framework, especially the inevitable linker rotation. Here, we propose a crystal engineering strategy that balances the grain boundary structure and framework flexibility in Co-Zn bimetallic zeolitic imidazolate framework (ZIF) membranes and exploit their contributions to the improvement of membrane quality and separation performance. It reveals that a good balance between the two trade-off factors enabled a "sweet spot" that offers the best C₃H₆/C₃H₈ separation factor up to 200.