Zeolitic Imidazolate Frameworks Crystals Research Articles

Vibrational dynamics dictate heat transport in metal-organic frameworks with gas adsorbates

Understanding the thermal conductivity (κ) of metal-organic frameworks (MOFs) with adsorbates is critical for tailoring their heat dissipation in practical applications. However, a detailed description of how adsorbates influence the heat transport of MOFs is still lacking. In this work, we employ molecular dynamics simulations combined with lattice dynamics calculations to investigate the effect of gas adsorbates on κ of representative crystalline and glassy zeolitic imidazolate frameworks (ZIFs). We identify that the diffusive adsorbate motions in the crystalline ZIFs with large pores can enhance lattice anharmonicity, serving as a source of phonon scattering and significantly suppressing κ. Conversely, the adsorbates with restricted dynamics in limited space (i.e., in the dense ZIF crystals or glasses) behave more like a heat transfer medium and facilitate heat transport. Besides pore size, gas dynamics are also proved to vary upon gas-lattice interaction strength, and dictate if gas adsorbates serve as a heat conduction suppressor or promoter. We anticipate that this governing role of gas dynamics is applicable to a diverse range of MOF-adsorbate systems. As such, our work offers guidance for the practical thermal management of MOF porous materials to ensure their desired storage or separation performance.

(Zn/Co)-ZIFs@TiO2 composite catalysts for oxidative desulfurization: Impacts of Zn2+/Co2+ on TiO2 interactions

Zeolitic imidazolate frameworks (ZIFs)-based catalysts with a high surface area and structural features have received great attention for potential use in complete eliminating refractory thiophenes from fuels. In this study, new composite frameworks of Zn/Co-based ZIFs@TiO2 were investigated for catalytic oxidative desulfurization (ODS) of dibenzothiophene (DBT). The effect of the type of metal centers (Zn2+ or Co2+) in the ZIF-8 and ZIF-67 frameworks on the coordination with TiO2 and eventually on the catalytic performance was targeted to be studied. The results verified that the intensity of effective N-Ti-O bond was influenced by the size of ZIF crystals and the type of metal ions. Over the ZIF-8-M with small size, the aggregation of TiO2 layer decreased the intensity of N-Ti-O bonds. Meanwhile, the lower energy of Zn-N bond and controlled particle size of ZIF-8-W were favorable for partially replacing Zn2+ with Ti4+ and forming extra N-Ti-O species which are supposed to enrich active Ti-peroxo intermediates for the ODS progress. In the case of ZIF-67, unsaturated Co-N bonds further improved DBT uptake. The maximum ODS efficiency of 98.6% was achieved using ZIF-8-W@TiO2 due to the high surface area (SBET=1548 m2/g), uniform TiO2 distribution, and further N-Ti-O bonds. A good recyclability was also verified for the proposed composite catalyst.

Tunable growth of a single high-density ZIF nanoshell on a gold nanoparticle isolated in an optical trap.

Here, we demonstrate an all-optical method using an optical tweezer to controllably grow high quality zeolitic imidazolate framework (ZIF) nanoshells on the surface of gold nanoparticles (AuNPs) and monitor the growth via darkfield spectroscopy. Our single particle approach allows us to localize an individual NP within a microscope slide chamber containing ZIF precursors at the focus of an optical microscope and initiate growth through localized heating without affecting the bulk system. Darkfield spectroscopy is used to characterize changes to the localized surface plasmon resonance (LSPR) of the AuNP resulting from refractive index changes as the ZIF crystal grows on the surface. We show that the procedure can be generalized to grow various types of ZIF crystals, such as ZIF-8, ZIF-11, and a previously undocumented ZIF variety. Utilizing both computational models and experimental methods, we identify the thickness of ZIF layers to be self-limiting to ∼50 nm or less, depending on the trapping laser power. Critically, the refractive index of the shells here was found to be above 1.6, indicating the formation of high-density crystals, previously accessible only through slow atomic layer deposition and not through a bulk heating process. The single particle method developed here opens the door for bottom-up controllable growth of custom nanostructures with tunable optical properties.

Precise control over gas-transporting channels in zeolitic imidazolate framework glasses

Porous metal–organic frameworks have emerged to resolve important challenges of our modern society, such as CO2 sequestration. Zeolitic imidazolate frameworks (ZIFs) can undergo a glass transition to form ZIF glasses; they combine the liquid handling of classical glasses with the tremendous potential for gas separation applications of ZIFs. Using millimetre-sized ZIF-62 single crystals and centimetre-sized ZIF-62 glass, we demonstrate the scalability and processability of our materials. Further, following the evolution of gas penetration into ZIF crystals and ZIF glasses by infrared microimaging techniques, we determine the diffusion coefficients and changes to the pore architecture on the ångström scale. The evolution of the material on melting and processing is observed in situ on different length scales by using a microscope-coupled heating stage and analysed microstructurally by transmission electron microscopy. Pore collapse during glass processing is further tracked by changes in the volume and density of the glasses. Mass spectrometry was utilized to investigate the crystal-to-glass transition and thermal-processing ability. The controllable tuning of the pore diameter in ZIF glass may enable liquid-processable ZIF glass membranes for challenging gas separations.

Nanoengineered ZIF fillers for mixed matrix membranes with enhanced CO2/CH4 selectivity

Zeolitic-imidazolate frameworks (ZIFs) are considered as promising nanoporous solids for the development of membranes with exceptional gas separation properties. In this work, we show how molecular-scale modifications can lead to ZIF variants, which can greatly enhance the CO2/CH4 separation performance of mixed-matrix membranes (MMMs) when used as additives. First the permeabilities of CO2 and CH4 in ZIF-8 are estimated with the help of molecular simulations. Based on these results and a macroscopic model, the performance of MMMs with ZIF-8 as fillers is deduced. The validity of this approach is confirmed through experiments by developing flat-sheet ZIF-8 based nanocomposite membranes with two typical polymers (rubbery Pebax® MH1657 and glassy 6FDA-DAM) having different permeability properties. In a subsequent step, we design computationally new ZIF crystals, by changing the original metal, organic linker and functional group of ZIF-8, and the CO2/CH4 separation performance of MMMs with the new ZIF variants is predicted. Our results reveal that some of these MMMs exhibit an unprecedented CO2/CH4 separation performance, which surpasses any reported MMM and is directly attributed to the enhanced sieving capacity of the modified ZIF fillers. In conclusion, this work highlights the effectiveness of ZIF nanoengineering as a means to develop highly selective CO2 membranes.

Zeolitic imidazolate frameworks as effective crystalline supports for aspergillus-based laccase immobilization for the biocatalytic degradation of carbamazepine

In this study, zeolitic imidazolate frameworks (ZIF) were employed as effective porous supports for laccase enzyme attachment and further explored synergistic adsorption and biocatalytic degradation of carbamazepine (CBZ) in aqueous solutions. Characterization results from FTIR and NMR analysis confirmed successful incorporation of the laccase enzyme onto ZIF particles. Further analyses from SEM and TEM revealed rhombic dodecahedral morphologies of ZIF crystals with crusts of the enzyme observed on the particles' surface. The carbamazepine degradation results showed that immobilization of the laccase improved its stability and resistance at various pH's, in comparison to the free enzyme. The immobilized laccase also exhibited relatively higher activities across the studied temperature range compared to the free form. Kinetic studies revealed a negligible decline in velocity, Vmax after immobilization, evaluated to be 0.873 and 0.692 mg L−1 h−1 for the free and immobilized laccase, respectively. The immobilized laccase demonstrated improved stabilities towards organic solvents, which qualifies the composite's application in real wastewater samples. In which case, the laccase-ZIF composite proved effective in CBZ decontamination with an efficiency of ∼92%. Furthermore, the immobilized laccase exhibited appreciable storage stabilities (∼70% residual activity) for up to 15 days before any significant loss in activity.

In situ growth of amino-functionalized ZIF-8 on bacterial cellulose foams for enhanced CO2 adsorption

Zeolitic imidazolate frameworks (ZIFs) hold great potential for carbon capture, while a major challenge for the practical application of ZIFs is the development of convenient three-dimensional bulk materials. Here, sustainable and biodegradable bacterial cellulose (BC) was used as the substrate for ZIF growth. Amino-functionalized ZIF-8 (ZIF-8-NH2) was prepared within BC substrate via an in situ growth approach. ZIF crystals were wrapped uniformly over cellulose fibers and the chelating effect between metal (zinc) ions and hydroxyl groups makes the composites have high interface affinity and compatibility. The resulting foams presented a high CO2 adsorption capacity of 1.63 mmol/g (25 °C, 1 bar). Moreover, ZIF-8-NH2@BC foams are facile to be regenerated by heating at 80 °C. This work provides a new avenue to construct ZIF/cellulose composites for gas treatment applications.

Design and Fabrication of the Evolved Zeolitic Imidazolate Framework-Modified Polylactic Acid Nonwoven Fabric for Efficient Oil/Water Separation

Design of durable and recyclable superhydrophobic materials for oil/water separation is a major concern in the field of wastewater treatment. Functionalization of a biodegradable matrix with controllable grown crystals brings out a new research perspective. In this study, multiscale zeolitic imidazolate frameworks (ZIFs) were grown and decorated on a polylactic acid (PLA) nonwoven fabric (NWF) to construct a superhydrophobic material by an in situ growth method and a spraying process. The stable superhydrophobic layer contains two kinds of ZIF crystals showing microscale flake-like structures and nanoscale particles. The morphology and surface energy of such a hierarchically structured ZIF-modified PLANWF is controllable by the adjustment of experimental parameters. The as-prepared PLA hybrid materials exhibit high separation efficiency and recyclability as for water-nitromethane and water-toluene mixtures. Based on the wetting envelopes of the ZIF-modified PLA material, its separation performance for various oil/water mixtures can be preliminarily assessed before the application.

Nanoarchitectured porous carbons derived from ZIFs toward highly sensitive and selective QCM sensor for hazardous aromatic vapors

Metal-organic frameworks (MOFs) are a versatile source of carbon nanoarchitectures in gas sensing applications (Torad et al., 2019). Herein, several types of nanoporous carbons (NPCs) have been prepared by in-situ carbothermal treatment of zeolitic imidazolate frameworks (ZIFs) under different inert atmospheres to achieve a highly sensitive discrimination of vaporized aromatic compounds. In this study, we demonstrate how different carbonization conditions under the flow of N2 or H2 gases affect the surface area and the degree of graphitization of the resulting NPCs polyhedrons, and their consequent effect on the sensing performance in terms of sensitivity and selectivity toward toxic volatile hydrocarbons. A growth of carbon nanotubes (CNTs) is observed on the surface of polyhedral NPCs after careful carbonization of ZIF crystals under H2 atmosphere. The fabricated quartz crystal microbalance (QCM) sensor with CNT-containing NPCs demonstrates increased sensitivity and selectivity towards toxic volatile aromatic hydrocarbons over the aliphatic analogues, suggesting the rich growth of hairy graphitic-like CNTs on the surface of carbon framework act as highly selective sensing antennae for vapor molecular discrimination of toxic aromatic hydrocarbons. Despite of increased selectivity towards volatile aromatic compounds, however, the surface area of CNT-rich NPCs derived from hybrid ZIFs and ZIF-67 is greatly sacrificed as compared to CNT-free NPCs from ZIF-8 polyhedron. In the case of Co-containing ZIF-67, the rich growth of hair-like CNTs, which is induced by the presence of Co, is observed during carbothermal reduction under a flow of H2 gas, thus allowing ultra-selective detection of aromatic hydrocarbons in the vapor phase, such as benzene (C6H6) and toluene (C6H5CH3) over their aliphatic analogue, c-hexane (c-C6H12) of same molecular mass, size and vapor pressure.

Potentials and challenges of high-field PFG NMR diffusion studies with sorbates in nanoporous media

High magnetic fields (up to 17.6 T) in combination with large magnetic field gradients (up to 25 T/m) were successfully utilized in pulsed field gradient (PFG) NMR studies of gas and liquid diffusion in nanoporous materials. In this mini-review, we present selected examples of such studies demonstrating the ability of high field PFG NMR to gain unique insights and differentiate between various types of diffusion. These examples include identifying and explaining an anomalous relationship between molecular size and self-diffusivity of gases in a zeolitic imidazolate framework (ZIF), as well as revealing and explaining an influence of mixing different linkers in a ZIF on gas self-diffusion. Different types of normal and restricted self-diffusion were quantified in hybrid membranes formed by dispersing ZIF crystals in polymers. High field PFG NMR studies of such membranes allowed observing and explaining an influence of the ZIF crystal confinement in a polymer on intra-ZIF self-diffusion of gases. This technique also allowed measuring and understanding anomalous single-file diffusion (SFD) of mixed sorbates. Furthermore, the presented examples demonstrate a high potential of combining high field PFG NMR with single-crystal infrared microscopy (IRM) for obtaining greater physical insights into the studied diffusion processes.

Towards large-size bulk ZIF-62 glasses via optimizing the melting conditions

Production of large-size, bulk, and bubble-free melt-quenched Zeolitic Imidazolate Framework (ZIF) glasses is a challenge due to the difficulty in up-scaling of the synthesis of the initial ZIF crystals, to the extremely high viscosity at melting point and to their relatively high brittleness. In this work, we optimize the melt-quenching process to increase the size of ZIF-62 glass products and to increase their homogeneity and transparency. By finding the optimum melting time (i.e., <5 min), we succeeded in producing >10 mm large glass samples with a certain degree of transparency and low bubble content, which can be used for mechanical characterizations. We have found the condition (e.g., melting time >30 min) for obtaining ZIF-62 glass foams, which result from partial decomposition during melting. This work provides some information for up-scaling the ZIF glass production.

Quantitative Predictions of Molecular Diffusion in Binary Mixed-Linker Zeolitic Imidazolate Frameworks Using Molecular Simulations

Experimental studies have shown that adsorbate diffusion in zeolitic imidazolate frameworks (ZIFs) can be tuned by incorporating two different imidazolate linkers in the ZIF crystals. We demonstrat...

Rapid room-temperature synthesis of hierarchical porous zeolitic imidazolate frameworks with high space-time yield

Hierarchical porous zeolitic imidazolate frameworks (ZIFs) have potential for adsorption, catalysis and chemical sensing applications. Ultrafast synthesis of ZIFs at room temperature and pressure is particularly desirable for large-scale industrial production. Here, we developed a green and versatile method using organic amines as supramolecular templates (organic amine-template) to rapidly synthesize hierarchical porous ZIFs (ZIF-8, ZIF-61 and ZIF-90) at room temperature and pressure. The synthesis time was reduced dramatically to within 1 min, and the resulting ZIFs had multimodal hierarchical porous structures with mesopores/macropores interconnected with micropores. Notably, the space–time yield (STY) of hierarchical porous ZIF-8 was up to 1.29× 104 kg m–3 d–1, which is more than three times higher than that reported using other methods. Furthermore, the morphologies and porosities of the produced ZIFs could be readily tuned by controlling the synthesis time or type of organic amine. The organic amine played two roles in the synthesis: (1) a protonation agent to deprotonate organic ligands, facilitating the formation of ZIF crystals, and (2) an structure directing agent to direct mesopore/macropore formation. The resulting hierarchical porous ZIF-8 exhibited enhanced uptake capacities and diffusion rates for guest molecules relative to its microporous counterpart. This work provides a new direction for the green and efficient synthesis of various hierarchical porous ZIFs with high STYs for a wide range of applications.

In situ growth of microporous ZIF-8 nanocrystals on a macroporous phyllosilicate mineral

Microporous zeolitic imidazolate frameworks (ZIFs) have shown great potential in many application fields. However, the small dimensions of ZIF nanoparticles make them difficult to employ in practical systems. To overcome this problem, a novel and efficient strategy has been developed to immobilize ZIF crystals on a macroporous phyllosilicate mineral. As an example, water-stable ZIF-8 nanocrystals were synthesized and assembled on the dopamine-modified surface of expanded vermiculite using the sequential addition of methanol solutions of Zn(NO3)2·6H2O and 2-methylimidazole. The ZIF-8 nanoparticle-decorated mineral exhibited a dense growth of microporous ZIF-8 nanocrystals of 80±10nm, resulting in 9.8wt% of ZIF-8 loading and a BET surface area of 120m2/g.

Lattice-Gas Modeling of Adsorbate Diffusion in Mixed-Linker Zeolitic Imidazolate Frameworks: Effect of Local Imidazolate Ordering.

The rates of adsorbate diffusion in zeolitic imidazolate frameworks (ZIFs) can be varied by several orders of magnitude by incorporating two different imidazolate linkers in the ZIF crystals. Although some prior measurements of short-range order in these mixed-linker materials have been reported, it is unclear how this short-range order impacts the net diffusion of adsorbates. We introduce a lattice diffusion model that treats diffusion in ZIF-8x-90100-x crystals as a series of activated hops between cages, allowing us to assess the effects of short-range imidazolate order on molecular diffusion.

Synthesis of ZIF-8 and ZIF-67 nanocrystals with well-controllable size distribution through reverse microemulsions

With the particles size down to nanoscale, porous materials with well-controllable dimension will have more promising applications in biologically relevant fields and even in the areas of traditional selective separation and catalysis. By using reverse microemulsions the nanoscale zeolitic imidazolate frameworks (ZIFs) such as ZIF-8 and ZIF-67 were synthesized at room temperature with uniform grain size distribution and extremely small crystal dimension (mean particle size less than 5nm). The ZIF-8 and ZIF-67 synthesized by reverse microemulsion have much larger surface areas and micropores volume than samples obtained from water, but are comparative to those obtained from methanol. The thermal stability of the synthesized ZIF-8 and ZIF-67 by microemulsion is also excellent. The DLS characterization revealed that it is the size of micelles in microemulsion that controls the dimension of ZIF crystals. This is different comparing to the controlling mechanisms of other common synthesis methods of MOFs. We believe that the microemulsion method developed in this work can enrich the synthesis strategies of other nanoscale ZIF or MOF particles.

Dual-ligand zeolitic imidazolate framework crystals and oriented films derived from metastable mono-ligand ZIF-108

Post-synthetic ligand exchange of metal-organic frameworks (MOFs) is a feasible target-oriented method of preparing dual-ligand MOF crystals with fine-tuneable compositions and functions. Herein, a series of dual-ligand zeolitic imidazolate framework (ZIF) crystals with SOD, GME and GIS topologies were constructed by ligand exchange of the mono-ligand parent materials ZIF-108, Zn(2-nitroimidazolate)2. In-depth investigations on the formation processes of the daughter ZIFs indicated that ligand exchange is a heterogeneous nucleation process using ZIF-108 as seeds. The metastability of ZIF-108 makes the ligand substitution of ZIF-108 require lower activation energy than homogeneous nucleation. This ligand exchange process paves the way for facile synthesis of dual-ligand and dual-metal-dual-ligand ZIF crystals with diverse porous networks. Based on these results, c-out-of-plane oriented dual-ligand ZIF-78 films were synthesized through the evolutionary selection in a van der Drift's type growth originated from randomly oriented ZIF-108 seed layer.

Synthesis of Zeolitic Imidazolate Framework Core-Shell Nanosheets Using Zinc-Imidazole Pseudopolymorphs.

Zeolitic imidazolate frameworks (ZIFs) are an emerging class of microporous materials that possess an organic flexible scaffold and zeolite-like topology. The catalytic and molecular-separation capabilities of these materials have attracted considerable attention; however, crystal-shape engineering in ZIF materials remains in its infancy. This is the first study to report an effective method for tailoring the near-spherical crystal morphology of ZIF-8 using its leaf-like pseudopolymorph, ZIF-L. A thin, uniform layer of ZIF-8 is formed on ZIF-L through heterogeneous surface growth to produce a ZIF-L@ZIF-8 core-shell nanocomposite. This results in ZIF-8 with a crystal morphology comprising two-dimensional nanoflakes. We characterized the resulting core-shell crystals using a number of solid-state techniques, including powder X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and nitrogen physisorption. Approximately 16 mass% of ZIF-8 in the core-shell composites heterogeneous surfacely grown on ZIF-L core crystals. We also investigated the effects of zinc salts, which were used as a source of zinc in the formation of the ZIF-L@ZIF-8 core-shell nanocomposites. Finally, we assessed the CO2 adsorption properties of ZIF-8, ZIF-L, and ZIF-L@ZIF-8 core-shell crystals, the results of which were used to deduce the dynamic and equilibrium adsorption characteristics of various microporous ZIF crystals. The core-shell materials present hybridized CO2 uptake and diffusivity of the parent crystals. The proposed method for the synthesis of core-shell nanocomposites using pseudopolymorphic crystals is applicable to other ZIF systems.

Highly tunable molecular sieving and adsorption properties of mixed-linker zeolitic imidazolate frameworks.

Nanoporous zeolitic imidazolate frameworks (ZIFs) form structural topologies equivalent to zeolites. ZIFs containing only one type of imidazole linker show separation capability for limited molecular pairs. We show that the effective pore size, hydrophilicity, and organophilicity of ZIFs can be continuously and drastically tuned using mixed-linker ZIFs containing two types of linkers, allowing their use as a more general molecular separation platform. We illustrate this remarkable behavior by adsorption and diffusion measurements of hydrocarbons, alcohols, and water in mixed-linker ZIF-8(x)-90(100-x) materials with a large range of crystal sizes (338 nm to 120 μm), using volumetric, gravimetric, and PFG-NMR methods. NMR, powder FT-Raman, and micro-Raman spectroscopy unambiguously confirm the mixed-linker nature of individual ZIF crystals. Variation of the mixed-linker composition parameter (x) allows continuous control of n-butane, i-butane, butanol, and isobutanol diffusivities over 2-3 orders of magnitude and control of water and alcohol adsorption especially at low activities.

Stability of ZIF-8 membranes and crystalline powders in water at room temperature

The perceived robust stability of zeolitic imidazolate frameworks (ZIFs) was regarded as an important feature of ZIFs that have intrigued intensive research efforts. However, hydrolysis of ZIF crystals was recently identified under hydrothermal conditions. In this work, the dynamic stability of α-alumina supported ZIF-8 membranes and static stability of both ZIF-8 membranes and powder in water were studied via pervaporation and batch durability tests at room temperature. The sharp increase in water pervaporation flux from 0.04kgm−2h−1 to 1.39kgm−2h−1 and the disappearance of ZIF-8 structure after pervaporation are consistent with morphology and phase structure data showing that ZIF-8 layer on the α-alumina support underwent degradation during water pervaporation. The gradual degradation of ZIF-8 layer was also experimentally observed in batch durability tests in pure water. To better understand the mechanism of stability of ZIF-8 structure, control experiments on hydrolysis of ZIF-8 powder in aqueous conditions were designed and performed. The continuous release of Zn2+ from ZIF-8 structure to the aqueous solution was detected along with the change in pH value of the aqueous system. The presence of 2-methylimidazolate in aqueous solution can stabilize the ZIF-8 structure of both membrane and crystals. These evidences confirm that ZIF-8 crystals can degrade in water due to the dissolution of ZIF-8. The rate and extent of the dissolution depends on the ZIF-8 crystal to water mass ratio.