Zeolitic Imidazolate Framework ZIF-8 Research Articles

Nanoparticle-Directed Metal-Organic Framework/Porous Organic Polymer Monolithic Supports for Flow-Based Applications.

A two-step nanoparticle-directed route for the preparation of macroporous polymer monoliths for which the pore surface is covered with a metal-organic framework (MOF) coating has been developed to facilitate the use of MOFs in flow-based applications. The flow-through monolithic matrix was prepared in a column format from a polymerization mixture containing ZnO-nanoparticles. These nanoparticles embedded in the precursor monolith were converted to MOF coatings via the dissolution-precipitation equilibrium after filling the pores of the monolith with a solution of the organic linker. Pore surface coverage with the microporous zeolitic imidazolate framework ZIF-8 resulted in an increase in surface area from 72 to 273 m2 g-1. Monolithic polymer containing ZIF-8 coating was implemented as a microreactor catalyzing the Knoevenagel condensation reaction and also in extraction column format enabling the preconcentration of trace levels of toxic chlorophenols in environmental waters. Our approach can be readily adapted to other polymers and MOFs thus enabling development of systems for flow-based MOF applications.

Gate opening effect of zeolitic imidazolate framework ZIF-7 for adsorption of CH4 and CO2 from N2

Measurements of S-shaped adsorption isotherm as evidence of ZIF-7 gate opening in the presence of CH4 at 1245 kPa.

Gate Opening, Diffusion, and Adsorption of CO2 and N2 Mixtures in ZIF-8

Diffusion and adsorption of CO2/N2 mixtures in the zeolitic imidazolate framework ZIF-8 are investigated by molecular dynamics (MD) and Gibbs ensemble Monte Carlo (GEMC) simulations. Structural changes called “gate opening” could be found for the adsorbed single-component gases and for the mixture. The gate opening appears for the mixture at a total number of guest molecules per cavity between that for the pure CO2 and that for the pure N2 but closer to that of N2 which is lower. Due to the stronger dependence of CO2 adsorption upon the temperature in comparison with N2, the adsorption selectivity is predicted to be higher at lower temperatures, which is in accordance with experimental findings.

Effective Adsorption Separation of n-Hexane/2-Methylpentane in Facilely Synthesized Zeolitic Imidazolate Frameworks ZIF-8 and ZIF-69

A facile synthesis procedure was built to synthesize zeolitic imidazolate frameworks (ZIFs) ZIF-8 and ZIF-69 under ambient conditions with the prevention of organic solvents. ZIF-69 adsorbent was first introduced in the liquid-phase separation of hexane isomers. A sieving test was carried out using samples of n-hexane (nHEX) or 2-methylpentane (2MP) over two ZIFs and commercial 5A zeolite materials. According to batch adsorption performances and modeling, ZIF-8 exhibits the highest adsorption selectivity and capacity for nHEX, followed by ZIF-69 and 5A zeolite. The equilibrium data at 25 °C show that a significant amount (0.51 g/g) of nHEX and only 0.09 g/g 2MP can be adsorbed in ZIF-8. And the maximum adsorption capacity of nHEX and 2MP on ZIF-69 are 0.34 g/g and 0.10 g/g, respectively. Breakthrough experiments performed at 150 °C with a binary mixture of nHEX/2MP confirm the shape selectivity and adsorption capacities of ZIF-8 toward nHEX at various total pressures from 0.5 to 1.3 MPa and with different...

Toward an Understanding of the Microstructure and Interfacial Properties of PIMs/ZIF-8 Mixed Matrix Membranes.

A study integrating advanced experimental and modeling tools was undertaken to characterize the microstructural and interfacial properties of mixed matrix membranes (MMMs) composed of the zeolitic imidazolate framework ZIF-8 nanoparticles (NPs) and two polymers of intrinsic microporosity (PIM-1 and PIM-EA-TB). Analysis probed both the initial ZIF-8/PIM-1 colloidal suspensions and the final hybrid membranes. By combination of dynamic light scattering (DLS) and transmission electron microscopy (TEM) analytical and imaging techniques with small-angle X-ray scattering (SAXS), the colloidal suspensions were shown to consist mainly of two distinct kinds of particles, namely, polymer aggregates of about 200 nm in diameter and densely packed ZIF-8-NP aggregates of a few 100 nm in diameter with a 3 nm thick polymer top-layer. Such aggregates are likely to impart the granular texture of ZIF-8/PIMs MMMs as shown by SEM-XEDS analysis. At the molecular scale, modeling studies showed that the surface coverage of ZIF-8 NPs by both polymers appears not to be optimal with the presence of microvoids at the interfaces that indicates only a moderate compatibility between the polymer and ZIF-8. This study shows that the microstructure of MMMs results from a complex interplay between the ZIF-8/PIM compatibility, solvent, surface chemistry of the ZIF-8 NPs, and the physicochemical properties of the polymers such as molecular structure and rigidity.

Polydopamine-reinforced magnetization of zeolitic imidazolate framework ZIF-7 for magnetic solid-phase extraction of polycyclic aromatic hydrocarbons from the air-water environment

Zeolitic imidazolate frameworks (ZIFs) consist of metal nodes connected to imidazolate linkers, having both the properties of metal-organic frameworks (MOFs) and inorganic zeolites, such as controllable pore sizes, high porosity and surface areas, as well as exceptional thermal and chemical stability, thereby making them a class of attractive materials for diverse analytical applications. In this study, we reported a facile magnetization process of ZIF-7 (zinc benzimidazolate) for simultaneous magnetic extraction of polycyclic aromatic hydrocarbons (PAHs) by simply mixing ZIF-7 and polydopamine (PDA)-coated Fe3O4 nanoparticles (PDA@Fe3O4) in solutions. Functional groups (−OH and −NH2), provided by PDA as a highly efficient molecular linker, could attract and anchor ZIF-7 through noncovalent adsorption and covalent cross-link interactions, thereby promoting the complete magnetization of ZIFs and enhancing their stability and reusability. The bridging ligand benzimidazolate, could be bonded with PAHs because of its high surface area, large pores, accessible coordinative unsaturated sites (π-complexation), and π-π stacking action. This ZIF-based magnetic solid-phase extraction (SPE), coupled with gas chromatography/tandem mass spectrometry (GC/MS), was further evaluated for analysis of PAHs from rainwater and air samples of particulate matter less than 2.5μm in diameter (PM2.5). The main effective parameters, including ionic strength, solution pH, extraction time, desorption solvent and desorption time, were investigated, respectively. Under optimized conditions, the developed method based on Fe3O4@PDA/ZIF-7 gave detection limits of 0.71–5.79ng/L, and quantification limits of 2.50–19.2ng/L for PAHs, respectively. The relative standard deviations for intra-day and inter-day analyses were in the range of 3.1–9.1% and 6.1–12.7%, respectively. The PAHs founded in PM2.5 were in the range of 0.40–6.79ng/m3. Good recoveries (>82%) with low relative standard deviations (≤9.2%) were achieved. This method was demonstrated to be an accurate, convenient and sensitive pretreatment procedure for trace analysis of environmental water and air samples.

Long‐Lived Room‐Temperature Phosphorescence of Coronene in Zeolitic Imidazolate Framework ZIF‐8

Long-lived phosphorescence is demonstrated with an emission lifetime of 22.4 s at room temperature from coronene-d 12 encapsulated in the zeolitic imidazolate framework ZIF-8. The guest emitter coronene-d 12 is well isolated in the rigid framework of ZIF-8, enabling long-lived triplet excitons even at higher than 300 K. It is also demonstrated that thermally activated delayed fluorescence can be achieved at temperatures >300 K.

Defect-induced ripening of zeolitic-imidazolate framework ZIF-8 and its implication to vapor-phase membrane synthesis

We report for the first time that ZIF-8 crystals undergo an Ostwald-ripening-like process without degradation in the presence of a ligand vapor. The ripening process is dependent on the defect density of the crystals: the more defective the more amenable to the ripening. The process was adapted to synthesize ultra-thin ZIF-8 membranes by vapor-phase secondary growth.

Controlling ZIF-67 crystals formation through various cobalt sources in aqueous solution

Zeolitic imidazolate frameworks ZIF-67 were prepared under hydrothermal (120°C) and non-hydrothermal (room temperature) from various cobalt sources and 2-methylimidazolate (Hmim) in aqueous solution within 30min. The particle size and morphology were found to be related to the reactivity of the cobalt salt, Hmim/Co2+ molar ratios and experimental condition. Using Co(NO3)2 as cobalt source, small-sized ZIF-67 crystals with agglomeration were formed. For CoCl2, small-sized rhombic dodecahedron were obtained. While large-sized crystals of rhombic dodecahedron structure were obtained from CoSO4 and Co(OAc)2. Under hydrothermal condition, the size of ZIF-67 crystals tended to be more uniform and the morphology were more regular comparing to non-hydrothermal condition. This study provides a simple way to control the size and morphology of ZIF-67 crystals prepared in aqueous solution.

Chemocatalysis of sugars to produce lactic acid derivatives on zeolitic imidazolate frameworks

Several research studies related to biorefining have focused on developing routes for biomass conversion into biomaterials or platform molecules. In this work, the zeolitic imidazolate frameworks (ZIFs) ZIF-8 and ZIF-67 have been tested as catalysts in the conversion of sugars (sucrose, glucose and fructose) into methyl lactate. ZIF-8 and ZIF-67 have the same sodalite type zeolite structure but behaved differently in the sugar conversion in methanol due to the respective presence of Zn and Co in their structures. ZIF-8 has been found to be the most active for the conversion of sugars into methyl lactate (yield 42%) and was reused in four catalytic cycles. The chemical and physical effects caused by these cycles on the catalysts have been studied by several techniques (X-ray diffraction, thermogravimetric analyses, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electronic microscopy and nitrogen adsorption).

Adsorptive Separation of Olefin/Paraffin Mixtures with ZIF-4.

The microporous zeolitic imidazolate framework ZIF-4 has been synthesized, and its ethylene/ethane and propylene/propane separation potentials have been evaluated by single-component adsorption isotherms and breakthrough experiments of the respective binary mixtures. In all experiments, a higher selectivity for the paraffin is observed that is manifested by a steeper equilibrium isotherm as well as a later breakthrough in the fixed-bed adsorber experiments. Microporous adsorbents with paraffin selectivity are rare but highly interesting for cyclic adsorption processes such as pressure-swing adsorption (PSA).

New High- and Low-Temperature Phase Changes of ZIF-7: Elucidation and Prediction of the Thermodynamics of Transitions.

We have found that the 3D zeolitic imidazolate framework ZIF-7 exhibits far more complex behavior in response to the adsorption of guest molecules and changes in temperature than previously thought. We believe that this arises from the existence of different polymorphs and different types of adsorption sites. We report that ZIF-7 undergoes a displacive, nondestructive phase change upon heating to above ∼700 °C in vacuum, or to ∼500 °C in CO2 or N2. This is the first example of a temperature-driven phase change in 3D ZIF frameworks. We predicted the occurrence of the high-temperature transition on the basis of thermodynamic arguments and analyses of the solid free-energy differences obtained from CO2 and n-butane adsorption isotherms. In addition, we found that ZIF-7 exhibits complex behavior in response to the adsorption of CO2 manifesting in double transitions on adsorption isotherms and a doubling of the adsorption capacity. We report adsorption microcalorimetry, molecular simulations, and detailed XRD investigations of the changes in the crystal structure of ZIF-7. Our results highlight mechanistic details of the phase transitions in ZIF-7 that are driven by adsorption of guest molecules at low temperature and by entropic effects at high temperature. We derived a phase diagram of CO2 in ZIF-7, which exhibits surprisingly complex re-entrant behavior and agrees with our CO2 adsorption measurements over a wide range of temperatures and pressures. We predicted phase diagrams of CH4, C3H6, and C4H10. Finally, we modeled the temperature-induced transition in ZIF-7 using molecular dynamics simulations in the isobaric-isothermal ensemble, confirming our thermodynamic arguments.

Encapsulation of red sulfur chromophores in a zeolitic imidazolate framework (ZIF-8) via solvent assisted linker exchange

Red color sulfur chromophores were entrapped inside the cages of zeolitic imidazolate framework ZIF-8 in order to prepare a ZIF analog of ultramarine red. ZIF-8 is made from Zn2+ and 2-methylimidazole linker and has sodalite topology. SIM-1, a ZIF isostructural to ZIF-8 that is made from Zn2+ and an imidazole linker containing a carboxaldehyde group was used as the starting material for the red pigment preparation. SIM-1 was converted to ZIF-8 via solvent assisted linker exchange (SALE) in DMF in the presence of sodium tetrasulfide as the sulfur source. The polysulfide was oxidized into the red sulfur chromophore S4 due to the presence of the carboxaldehyde group of the SIM-1 linker under the reaction conditions. During the transformation, the red sulfur species were successfully trapped inside the ZIF-8 cages generating a stable red solid pigment. To the best of our knowledge, this is the first example of low temperature ultramarine pigment preparation process. The red pigments were characterized by using PXRD, FTIR, UV–Vis, XPS and SEM.

Improved ZIF-8 membrane: Effect of activation procedure and determination of diffusivities of light hydrocarbons

Zeolitic imidazolate framework ZIF-8 has shown great potential for effective separation of hydrocarbon mixtures based on its intrinsic ultramicroporous feature. In order to explore the permeation and diffusion properties of hydrocarbons through ZIF-8 membrane, high-quality ZIF-8 membranes with a separation factor of ~90 for propylene/propane are successfully prepared via optimizing the activation processes. Single-component permeation data for hydrocarbons (C1–C4) through the improved ZIF-8 membrane are measured and analyzed by Maxwell–Stefan (MS) model to get the transport diffusivities of these hydrocarbons. The diffusivity values of hydrocarbon compare well with those obtained by other experimental techniques. Binary mixture permeation also can be well predicted through single-component adsorption parameters.

Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF-4.

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes prepared on coarse and macroporous tubes through repeated synthesis

Highly hydrogen-permselective zeolitic imidazolate framework ZIF-8 membranes are prepared on coarse and macroporous Al2O3 tubes through a repeated synthesis strategy for the separation of H2/CO2, H2/CH4, and H2/C3H8 mixtures. Since the amino-groups of 3-aminopropyltriethoxysilane (APTES) can coordinate to the free Zn2+ centers and thus bind the growing nano-crystals directly, the nucleation and growth of well intergrown ZIF-8 membranes could be promoted through a repeated synthesis of the ZIF-8 layer on APTES-modified macroporous Al2O3 tubes. The SEM and XRD characterizations indicate that a dense and phase-pure ZIF-8 membrane with a thickness of about 25.0μm can be formed on the alumina tubes by microwave synthesis at 105°C for 3h, and no cracks, pinholes or other defects were observed in the membrane layer. The ZIF-8 membranes were evaluated in single gas permeation and mixed gas separation. Attributed to the preferential adsorption affinity and capacity as well as a highly porous structure with nano-sized channels, the ZIF-8 membrane displays a high hydrogen permselectivity. For the separation of equimolar H2/CO2, H2/CH4, H2/C3H8 mixtures at 100°C and 2bar, the mixture separation factors of H2/CO2, H2/CH4, and H2/C3H8 are 5.6, 44.0, 328.6, with a H2 permeance of about 1.1×10−7molm−2s−1Pa−1, which is promising for potential applications in hydrogen purification and separation.

A combined theoretical and experimental analysis on transient breakthroughs of C2H6/C2H4 in fixed beds packed with ZIF-7

The selective separation of C2H6/C2H4 mixtures can be achieved by the zeolitic imidazolate framework ZIF-7, which is a well-known flexible microporous material due to its gate-opening effect in response to external stimuli such as pressure and temperature. Transient breakthrough experiments with C2H6/C2H4 mixtures were carried out at varying pressure and temperature conditions to confirm the potential application of ZIF-7 to selectively adsorb the saturated alkane and reject the unsaturated alkene in the gas phase during the adsorption cycle. Transient breakthrough simulations, including the influence of intra-crystalline diffusion, were compared with the experimental breakthroughs. The assumption of negligible diffusional limitations is able to capture the essential characteristics of the experimental breakthroughs. With the breakthroughs from both experiments and simulations, the adsorbed amounts of C2H6 and C2H4 in ZIF-7 were calculated to estimate the separation selectivity, which is in reasonable agreement with ideal adsorbed solution theory calculations. The derived isosteric heats of adsorption for both adsorbates are compared and used to explain the adsorption selectivity for C2H6 over C2H4 in ZIF-7. The good agreement between experiments and simulations verifies that the simulation methodology employed in the current study is a valuable and efficient tool for modeling the separation performance of C2H6/C2H4 mixtures in ZIF-7.

Zeolitic imidazolate metal organic framework ZIF-8 with ultra-high adsorption capacity bound tetracycline in aqueous solution

Synthetic process of ZIF-8 and a mechanism of tetracycline bound to ZIF-8 due to electrostatic attraction and π–π stacking interaction.

Efficient oxidation of ethylbenzene catalyzed by cobalt zeolitic imidazolate framework ZIF-67 and NHPI

Efficient catalytic oxidation of ethylbenzene to acetophenone was realized using the catalytic system of cobalt zeolitic imidazolate framework ZIF-67/N-hydroxyphthalimide (NHPI) under mild conditions. 95.2% conversion of ethylbenzene with 90.3% selectivity to acetophenone could be obtained at 373 K under 0.3 MPa O2 for 9 h. The results show that there exists synergetic effect between ZIF-67 and NHPI. 1-Phenylethyl hydroperoxide (PEHP) was generated via a radical process involving the hydrogen abstraction from ethylbenzene by phthalimide N-oxyl, and subsequently effectively decomposed to acetophenone by ZIF-67.

Bimetallic alloy nanocrystals encapsulated in ZIF-8 for synergistic catalysis of ethylene oxidative degradation.

Highly dispersed PtPd alloy nanocrystals (NCs) were firstly successfully encapsulated in microporous zeolitic imidazolate framework ZIF-8 (PtPd@ZIF-8) with tunable compositions by a "bottle around ship" approach. The PtPd@ZIF-8 catalyst showed excellent synergistic photocatalytic activity in the transformation of the adsorbed ethylene into CO2 and H2O at room temperature.