Hybrid Benzimidazole-Dichloroimidazole Zeolitic Imidazolate Frameworks Based on ZIF-7 and Their Application in Mixed Matrix Membranes for CO2/N2 Separation

Regulation of CO2 Pressure-Induced Flexibility of Zeolitic Imidazolate Framework-7 Using a Mixed Linker Strategy: An In Situ Positron Annihilation Spectroscopy Study

The external stimuli-induced framework flexibility of zeolitic imidazolate frameworks (ZIFs) is a fascinating physical phenomenon. Flexible ZIFs offer varying pore aperture under gas pressures leading to inferior separation selectivity because the flexibility-induced enlargement in the aperture size allows diffusion of larger size gas molecules. Desolvated ZIF-7 crystals show high flexibility and phase transformation from the narrow pore (np) to the open pore (op) phase under CO2 pressure. Regulation of the gas-framework-induced flexibility of ZIF-7 is of paramount importance for enhancing its selectivity for gas separation and storage. Herein, we have exchanged the benzimidazole linker of ZIF-7 with varying amount (up to 62%) of 4,5-dichloroimidazole (dcIm), maintaining the sodalite topology and intracrystalline porosity. As a result of loading of the halogenated linker, the op phase of ZIF-7 can be obtained at room temperature in a desolvated form, which is otherwise exhibited only by the solvated ZIF-7. Using positron annihilation lifetime spectroscopy (PALS), it has been established that the pore architecture is significantly varied depending on the extent of linker mixing. The framework flexibility of ZIF-7 is determined by indexing the vacant volume available at the pore sites under the increasing CO2 pressure using in situ PALS. The flexibility of the mixed linker framework indexed through vacant volume evolution at pore sites under CO2 pressure is observed to be drastically reduced as compared to that of ZIF-7 due to the presence of the halogenated linker in the framework. The present study confirms that the pore architecture and flexibility of ZIF-7 can be efficiently regulated by incorporating varying amounts of dcIm in ZIF-7 frameworks.

Thin Film Composite Mixed-Matrix Membranes Based on Matrimid and Zeolitic Imidazolate Frameworks for CO2/N2 Separation Performance

Membrane-based gas separation processes are a technology in continuous evolution. Various types of polymer membranes have been developed, many exhibiting high CO2 permeability and selective properties over competing gases such as N2 and CH4. In order to be competitive, membranes must be less-expensive, more stable, and more efficient, and their production must be scalable. One solution is to develop thin-film composites with mixed-matrix membranes (TFC_MMM) that have the potential to boost productivity while maintaining low costs. In this work, TFC_MMMs containing Matrimid mixed with 12 wt % ZIF-94 were prepared by kiss coating on a porous support. The SEM analysis showed that defect-free membranes with a 3 μm selective layer have been obtained. At 1 bar, the addition of ZIF resulted in improved the separation performance for the CO2/N2 pair, with CO2 permeance of 4 GPU and CO2/N2 selectivity of 40, surpassing neat TFC-Matrimid (CO2 permeance ≈ 3 GPU, CO2/N2 selectivity ≈ 29). The use of ZIF-94 also had a stabilizing effect on the membranes against CO2 plasticization at high pressure.

Zinc triazolate oxalate CALF-20 with platelet morphology and its PEBAX-based mixed matrix membranes for CO2/N2 separation

CALF-20, [Zn2(1,2,4-triazolate)2(oxalate)] shows remarkable performance in post-combustion carbon capture, even under humid conditions1 but its reported crystal morphology hinders its applicability in mixed matrix membranes (MMMs). Here, a route to its preparation as platelets a few tens of nm thick is reported. These were incorporated into a PEBAX MH1567 polymer matrix and the resultant MMMs display improvement in CO2 permeability and CO2/N2 selectivity.

Effect of Boron-Doping on Gate-Opening CO2 Adsorption in Zinc-Benzimidazolate Coordination Networks

Flexible metal-organic frameworks (MOFs) have attracted much attention as selective gas adsorption and storage. This report describes boron doping in zeolitic imidazolate framework-7 (B-ZIF-7), which exhibits reversible phase transition during CO2 adsorption/desorption. We have successfully prepared B-ZIF-7 coordination networks using boron-bridged benzimidazolate (B(bim)4-) as organic ligands. Powder X-ray diffraction (PXRD) measurements and infrared spectroscopy revealed that B-ZIF-7 has a crystal structure similar to that of ZIF-7 while containing boron bridging in the coordination network. Since B-ZIF-7 forms a cationic coordination network, the guest anions are encapsulated within the pore. CO2 adsorption/desorption measurements at 300 K showed that B-ZIF-7(NO3), which contains nitrate ions (NO3-) as guest anions in its pores, exhibits a S-shaped CO2 adsorption/desorption isotherm, which is characteristic of gate-opening type MOFs. Compared with ZIF-7, B-ZIF-7(NO3) has superior CO2 adsorption capacity in the low-pressure and superior CO2 storage capacity. The CO2 adsorption and desorption behavior of B-ZIF-7(NO3) was analyzed by in situ temperature-controlled PXRD measurements and thermogravimetric analysis under a CO2 atmosphere, and a reversible phase transition was observed. We have also successfully prepared B-ZIF-7(Cl) and B-ZIF-7(OTf) (OTf = CF3SO3-) with different guest anions. The CO2 adsorption/desorption behaviors of B-ZIF-7(Cl) and B-ZIF-7(OTf) were significantly different from those of B-ZIF-7(NO3) and ZIF-7. B-ZIF-7(Cl) showed gate opening at a higher pressure than ZIF-7, and B-ZIF-7(OTf) did not show S-shaped CO2 adsorption isotherm and showed adsorption behavior in micropores. These results indicate that the CO2 adsorption behavior of B-ZIF-7 depends on the interaction between the guest anions and CO2 molecules or the cationic framework and the bulkiness of the guest anions. Boron doping in a coordination network with boron-bridged imidazolate ligands is a promising strategy to increase the gas adsorption capability of porous materials.

Unravelling the structure of CO2 in silica adsorbents: an NMR and computational perspective

This comprehensive review describes recent advancements in the use of solid-state NMR-assisted methods and computational modeling strategies to unravel gas adsorption mechanisms and CO2 speciation in porous CO2-adsorbent silica materials at the atomic scale. This work provides new perspectives for the innovative modifications of these materials rendering them more amenable to the use of advanced NMR methods.

Degradation Kinetic Studies of BSA@ZIF-8 Nanoparticles with Various Zinc Precursors, Metal-to-Ligand Ratios, and pH Conditions

Nanosized zeolitic imidazolate framework particles (ZIF-8 nanoparticles [NPs]) have strong potential as effective carriers for both in vivo and in vitro protein drug delivery. Synthesis of ZIF-8 and stability of protein encapsulation within ZIF-8 are affected by several factors, notably the metal ion source and molar ratio. To systematically investigate these factors, we investigated such effects using BSA as a model test protein to synthesize BSA@ZIF-8 NPs at various metal-to-ligand (M:L) ratios. SEM, FTIR, XRD, and DLS were applied to characterize the morphology and structure of BSA@ZIF-8 NPs and their effects on protein loading capacity. Degradation kinetics and protein release behavior of BSA@ZIF-8 NPs were evaluated at pH 5.0 (to simulate the tumor environment) and pH 7.4 (to mimic the blood environment). Our objective was to define optimal combinations of the high protein loading rate and rapid release under varying pH conditions, and we found that (i) the yield of BSA@ZIF-8 NPs decreased as the M:L ratio increased, but the protein content increased. This highlights the need to strike a balance between cost-effectiveness and practicality when selecting ZIF-8 NPs with different molar ratios for protein-based drug formulation. (ii) BSA@ZIF-8 NPs exhibited cruciate flower-like shapes when synthesized using Zn(NO3)2 as the zinc precursor at M:L ratios of 1:16 or 1:20. In all other cases, the NPs displayed a regular rhombic dodecahedral structure. Notably, the release behavior of the NPs did not differ significantly between these morphologies. (iii) When Zn(OAc)2 was used as the zinc precursor, the synthesized ZIF-8 NPs exhibited a smaller size compared to the Zn(NO3)2-derived ZIF-8 NPs. (iv) The release rate and amount of BSA protein were higher at pH 5.0 compared to pH 7.4. (v) Among the different formulations, BSA@ZIF-8 with an M:L ratio of 1:16 at pH 5.0 was observed to have a shorter time to reach a plateau (0.5 h) and higher protein release, making it suitable for locally rapid administration in a tumor environment. BSA@ZIF-8 prepared from Zn(OAc)2 at an M:L ratio of 1:140 showed the slower release of BSA protein over a 24-h period, indicating its suitability for sustained release delivery. In conclusion, our findings provide a useful basis for the practical application of ZIF-8 NPs in protein-based drug delivery systems.

Hybrid Benzimidazole-Dichloroimidazole Zeolitic Imidazolate Frameworks Based on ZIF-7 and Their Application in Mixed Matrix Membranes for CO2/N2 Separation

Mixed-linker zeolitic imidazolate frameworks (ZIFs) with the sodalite (sod) topology type and based on ZIF-7 have been prepared by direct synthesis from the mixtures of benzimidazole (BzIm) and 4,5-dichloroimidazole (dcIm). Incorporation of dcIm into the ZIF-7 structure gives ZIF-7/COK-17 hybrids with rhombohedral symmetry that do not show the "open-to-closed form" structural transition upon solvent removal exhibited by ZIF-7. They show Type I isotherms for low molecular weight gases and high affinity for CO₂ even at low partial pressures. Synthesis under mild conditions gives ZIF nanoparticles (250-400 nm) suitable for incorporation into mixed matrix membranes (MMMs): these were prepared with both glassy (Matrimid) and rubbery (PEBAX 1657) polymers. Permeation tests at 298 K and 1.2 bar reveal that the incorporation of Zn(BzIm_0.55dcIm_0.45)₂ nanoparticles at up to ca. 12 wt % gives defect-free membranes with enhanced CO₂ permeability in both polymer matrices, with retention of selectivity (Matrimid) or with an enhancement in selectivity that is most pronounced for the smaller nanoparticles (PEBAX). The membrane with the best performance exhibits a selectivity of ca. 200 for CO₂/N₂ (a 4-fold increase compared to the pure polymer) and a CO₂ permeability of 64 Barrer. At the relatively low loadings investigated, the MMMs' performance obeys the Maxwell model, and the intrinsic property of diffusivity of the ZIFs can be extracted as a result.