Hae-Kwon Jeong

Synthesis of Zeolitic Imidazolate Framework Films and Membranes with Controlled Microstructures

Zeolitic imidazolate frameworks (ZIFs) are hybrid organic-inorganic microporous materials that exhibit zeolite-like structures and can be synthesized with a wide range of pore sizes and chemical functionality. ZIFs as thin films and membranes are of interest for their applications in sensors and gas separation. Here, we report a method for ZIF film and membrane fabrication, based on support surface modification and in situ solvothermal growth, which has potential for general application to other ZIF membranes. Our simple surface modification method results in strong covalent bonds between α-Al(2)O(3) supports and imidazolate ligands, which promote the heterogeneous nucleation and growth of ZIF crystals. The microstructure of ZIF-8 films can be controlled by controlling the pH of the growth solution. ZIF-7 films were fabricated to demonstrate the potential for general applicability of our method. Finally, the separation performance of several ZIF-8 membranes was evaluated, revealing molecular sieving behavior with an ideal selectivity for H(2)/CH(4) of 13.

In Situ Synthesis of Thin Zeolitic–Imidazolate Framework ZIF-8 Membranes Exhibiting Exceptionally High Propylene/Propane Separation

Metal-organic frameworks (MOFs) are a class of hybrid porous crystalline materials comprising of metal centers coordinated to organic linkers. Owing to their well-defined pores and cavities in the scale of molecules combined with abundant surface chemistry, MOFs offer unprecedented opportunities for a wide range of applications including membrane-based gas separations. It is not straightforward (often requiring multiple steps) to prepare membranes of MOFs due to the fact that the heterogeneous nucleation and growth of MOF crystals on porous supports are not generally favored. Furthermore, the performance of polycrystalline MOF membranes strongly depends on the membrane microstructure, in particular, the grain boundary structure. Here we report a simple one step in situ method based on a counter-diffusion concept to prepare well-intergrown ZIF-8 membranes with significantly enhanced microstructure, resulting in exceptionally high separation performance toward propylene over propane.

Rapid fabrication of metal organic framework thin films using microwave-induced thermal deposition.

We have demonstrated a novel method to rapidly fabricate nanoporous MOF thin films and patterns on porous alumina substrates under microwave irradiation.

HKUST-1 membranes on porous supports using secondary growth

Here we report the synthesis of continuous HKUST-1 membranes on porous supports via the secondary (i.e., seeded) growth method. A new seeding technique (“thermal seeding”) was developed to strongly anchor HKUST-1 seed crystals on porous α-alumina supports. It was found critical to have both organic ligands and copper species in the seed suspension as well as to seed the crystals at elevated temperature. The HKUST-1 seed crystals on the supports were then hydrothermally grown into continuous HKUST-1 films. The formation of cracks and fractures in the films was prevented by controlling the cooling and the drying processes after crystallization. The permeation results of HKUST-1 membranes show moderate separation of hydrogen over other small gas molecules such as carbon dioxide, nitrogen, oxygen, and methane.

Building multiple adsorption sites in porous polymer networks for carbon capture applications

IAST calculations reveal that sulfonate ammonium salt grafting renders porous polymer networks (PPN-6-SO3NH4) with exceptionally high adsorption selectivity for CO2 over N2 and CO2 over CH4. Breakthrough experiments confirm the high CO2 adsorption capacity (1.7 mmol g−1, 75 mg g−1) by feeding 15% CO2 balanced with N2 at 295 K and 1 bar.

Isoreticular metal-organic frameworks and their membranes with enhanced crack resistance and moisture stability by surfactant-assisted drying.

Here we report a new strategy that can not only prevent the formation of cracks and fractures in the crystals and films of metal-organic frameworks (MOFs) but also substantially enhance their stability with respect to moisture. It involves the addition of surfactants during a drying process. Surfactants reduce interfacial tension, thereby repressing the formation of fractures and cracks during the final drying process. It was found that, once dried, surfactants adsorbed on the crystal surface render the surface hydrophobic, leading to the enhancement in the stability toward moisture. Using this new strategy, the first crack-free IRMOF-3 membrane was successfully prepared, and its gas permeation performance was tested. IRMOF-3 membranes are found to favor CO2 over C3H8 mainly due to the affinity of CO2 to the amine groups in the structure. In addition, crack-free IRMOF-3 membranes were postsynthetically modified with heptanoic anhydride, thereby changing the effective pore size and surface property of the MOF. Once modified with the anhydride, the membranes favor C3H8 over CO2 due to the increased solubility of C3H8 in the presence of the hydrocarbon moiety.

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

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.

An unconventional rapid synthesis of high performance metal-organic framework membranes.

Metal-organic frameworks (MOFs) are attractive for gas separation membrane applications due to their microporous channels with tunable pore shape, size, and functionality. Conventional MOF membrane fabrication techniques, namely in situ and secondary growth, pose challenges for their wider commercial applications. These challenges include reproducility, scalability, and high manufacturing cost. Recognizing that the coordination chemistry of MOFs is fundamentally different from the covalent chemistry of zeolites, we developed a radically different strategy for MOF membrane synthesis. Using this new technique, we were able to produce continuous well-intergrown membranes of prototypical MOFs, HKUST-1 and ZIF-8, in a relatively short period of time (tens of min). With a minimal consumption of precursors and a greatly simplified synthesis protocol, our new technique provides potential for a continuous, scalable, reproducible, and easily commercializable route for the rapid synthesis of MOF membranes. RTD-prepared MOF membranes show greatly improved gas separation performances as compared to those prepared by conventional solvothermal methods, indicating improved membrane microstructure.

Rapid microwave-assisted synthesis of hybrid zeolitic–imidazolate frameworks with mixed metals and mixed linkers

Herein we report a new microwave-assisted synthetic strategy to rapidly prepare hybrid zeolitic–imidazolate frameworks (ZIFs): ZIFs with mixed metal centers and/or mixed linkers. The microwave-based method significantly shortens synthesis time, produces a higher yield, substantially reduces the amounts of ligands, and eliminates the use of deprotonating agents. The X-ray diffraction pattern reveals that mixed metal CoZn–ZIF-8 (i.e., ZIF-8 with both Co and Zn centers) maintains the sodalite (SOD) zeolitic topology from the ZIF-8 parent. Elemental mapping using energy-dispersive X-ray spectroscopy (EDS) and electronic/geometric information obtained from X-ray absorption spectroscopy (XAS) confirm the uniform distribution of tetrahedral Co and Zn metal centers within the same framework of the mixed-metal ZIF. The metal to nitrogen (M–N) stretching frequencies of IR bands were observed to be systematically blue-shifted as the Co/Zn ratio in the mixed metal ZIF increases. Furthermore, for the first time, a hybrid ZIF with both mixed metal centers (Co and Zn) and mixed linkers (2-methylimidazolate and benzimidazolate) was prepared through one-step microwave synthesis. Finally, mixed metal CoZn–ZIF-8 with a Co/Zn ratio of ∼1 was grown as membranes on porous α-Al2O3 supports, showing a higher propylene/propane separation factor (∼120) when compared to pure Zn–ZIF-8 membranes (∼63) prepared by a similar method.

High-Flux Zeolitic Imidazolate Framework Membranes for Propylene/Propane Separation by Postsynthetic Linker Exchange

While zeolitic imidazolate framework, ZIF-8, membranes show impressive propylene/propane separation, their throughput needs to be greatly improved for practical applications. A method is described that drastically reduces the effective thickness of ZIF-8 membranes, thereby substantially improving their propylene permeance (that is, flux). The new strategy is based on a controlled single-crystal to single-crystal linker exchange of 2-methylimidazole in ZIF-8 membrane grains with 2-imidazolecarboxaldehyde (ZIF-90 linker), thereby enlarging the effective aperture size of ZIF-8. The linker-exchanged ZIF-8 membranes showed a drastic increase in propylene permeance by about four times, with a negligible loss in propylene/propane separation factor when compared to as-prepared membranes. The linker-exchange effect depends on the membrane synthesis method.