Development of a force field for zeolitic imidazolate framework-8 with structural flexibility

A force field is developed for zeolitic imidazolate framework-8 (ZIF-8) with structural flexibility by combining quantum chemical calculations and classical Amber force field. The predicted crystalline properties of ZIF-8 (lattice constants, bond lengths, angles, dihedrals, and x-ray diffraction patterns) agree well with experimental results. A structural transition from crystalline to amorphous as found in experiment is observed. The mechanical properties of ZIF-8 are also described fairly well by the force field, particularly the Young's modulus predicted matches perfectly with measured value. Furthermore, the heat capacity of ZIF-8 as a typical thermophysical property is predicted and close to experimental data available for other metal-organic frameworks. It is revealed the structural flexibility of ZIF-8 exerts a significant effect on gas diffusion. In rigid ZIF-8, no diffusive behavior is observed for CH(4) within the simulation time scale of current study. With the structural flexibility, however, the predicted diffusivities of CH(4) and CO(2) are close to reported data in the literature. The density distributions and free energy profiles of CH(4) and CO(2) in the pore of ZIF-8 are estimated to analyze the mechanism of gas diffusion.     

Flexibility and swing effect on the adsorption of energy-related gases on ZIF-8: combined experimental and simulation study

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.     

低共熔溶剂中原位合成ZIF-8膜及其气体分离性能

在α-氧化铝载体上,采用原位合成法,在尿素/氯化胆碱低共熔溶剂中合成了ZIF-8膜。采用X射线衍射(XRD)和扫描电子显微镜(SEM)对合成的ZIF-8膜进行了表征。考察了反应溶液浓度、降温速率对ZIF-8膜合成的影响。通过优化合成条件合成了表面平整致密,厚度为8 μm的ZIF-8膜。采用Wicke-Kallenbach技术对优化条件下合成的ZIF-8膜进行了单组分气体渗透和双组分混合气体分离性能表征。在室温(293 K)下,ZIF-8膜对H2/CO2、H2/O2、H2/N2、H2/CH4双组分气体的分离系数分别为7.4、5.2、9.1、13.8,均大于相应的努森扩散分离系数,说明合成的ZIF-8膜具有分子筛分性能。     

A High Efficient Electrocatalytic Activity of Metal-organic Frameworks ZnO/Ag/ZIF-8 Nanocomposite for Electrochemical Detection of Toxic Heavy Metal Ions

A novel electrochemical sensor on ZIF-8 nanocomposites (Ag/ZnO/ZIF-8) was developed to analyze the mercury ions (Hg²⁺). The ZIF-8 materials are one of the 3-dimensional porous metal-organic frameworks with highly accessible pores and great surface area. The ZIF-8 nanocomposites were prepared through simple sol-gel methods and their physio-chemical properties were characterized via different analytical analyses. As a result of cyclic voltammetry, Ag/ZnO/ZIF-8 exhibited a better electrocatalytic behavior towards the detection of mercury ions (Hg²⁺). Furthermore, the composite modified electrode was then inspected as a sensor for DPV detection of mercury ions. The nanocomposite sensor performed a wide linear range from 0.5 μM to 140 μM with a low detection limit of 40 nM, and high sensitivity of 56.06 μA μM⁻¹ cm⁻². Moreover, the ZIF-8 composite sensor showed a higher selectivity toward the detection of mercury ions (Hg²⁺). The real-time applications of the ZIF-8 composites sensor were inspected in various samples with good sensitivity.     

Facile synthesis of size-tunable ZIF-8 nanocrystals using reverse micelles as nanoreactors

This paper describes a robust method for the synthesis of high-quality ZIF-8 nanocrystals using reverse micelles as discrete nanoscale reactors.The precise size control of ZIF-8 nanocrystals is conveniently achieved by tuning the concentration of precursors,reaction temperatures,the amount of water,and the structure of surfactants.The as-synthesized ZIF-8 nanocrystals are of narrow distribution and tunable size.A size-dependent catalytic activity for Knoevenagel condensation reaction is further demonstrated by using ZIF-8 nanocrystals with different sizes as the catalysts.This facile method opens up a new opportunity in the synthesis of various ZIFs nanocrystals.     

不同方法合成的沸石咪唑酯骨架结构材料(ZIF-8)的表征和催化性能

以二甲基咪唑为有机连接体和以Zn(OH)2或Zn(NO3)2·6H2O为Zn源,在甲醇与氨水的混合溶液、甲醇和DMF 3种不同的合成体系中合成了沸石咪唑酯骨架结构材料ZIF-8(分别记为ZIF-8(NH4OH)、ZIF-8(MeOH)和ZIF-8(DMF),并采用XRD、FTIR、N2吸附、SEM、TPD及Knoevenagel缩合反应等手段对所合成材料进行了表征。结果表明,采用这3种不同的合成方法均可成功制备出ZIF-8,所合成的ZIF-8的形貌基本一致,但其晶粒大小和酸碱性能有较大区别,同ZIF-8(NH4OH)和ZIF-8(DMF)相比,ZIF-8(MeOH)晶粒分布集中、平均粒径较小且具有较大的外比表面积和较多的酸碱位。不同方法合成的ZIF-8在苯甲醛和丙二腈的Knoevenagel缩合反应中的催化性能有很大差异,ZIF-8(MeOH)催化活性明显高于ZIF-8(DMF)和ZIF-8(NH4OH),其较高的催化活性,同其较大的外比表面积和酸碱性能密切相关。     

Experimental and molecular simulation studies of CO2 adsorption on zeolitic imidazolate frameworks: ZIF-8 and amine-modified ZIF-8

ZIF-8 has been rapidly developed as a potential candidate for CO₂ capture due to its low density, high surface area, and robust structure. Considering the electron-donating effect of amino functional groups, amino-modification is expected to be an efficient way to improve CO₂ adsorption of ZIF-8. In this work, grand canonical Monte Carlo (GCMC) simulation was performed to study the CO₂ adsorption isotherm based on ZIF-8, ZIF-8-NH₂, and ZIF-8-(NH₂)₂. ZIF-8 was synthesized and CO₂ adsorption isotherms based on ZIF-8 was measured. The experimental surface area, pore volume, and CO₂ adsorption isotherm were used to validate the force field. Adsorptive capacity of ZIF-8-NH₂, and ZIF-8-(NH₂)₂ were first estimated. The GCMC simulation results indicated that the order of increasing CO₂ capacity of the ZIF-8 in the lower pressure regime is: ZIF-8 < ZIF-8-NH₂ < ZIF-8-(NH₂)₂, and in the high pressure is: ZIF-8 < ZIF-8-(NH₂)₂ < ZIF-8-NH₂. New adsorption sites can be generated with the existence of-NH₂ groups. In addition, for non-modified and amino-modified ZIF-8, it was the first time to use density functional theory (DFT) calculations to investigate their CO₂ adsorption sites and CO₂ binding energies. The present work indicates that appropriate amine-functionalized can directly enhanced CO₂ capacity of ZIF-8.     

不同方法合成的沸石咪唑酯骨架结构材料(ZIF-8)的表征和催化性能

以二甲基咪唑为有机连接体和以Zn(OH)₂或Zn(NO₃)₂·6H₂O为Zn源,在甲醇与氨水的混合溶液、甲醇和DMF₃种不同的合成体系中合成了沸石咪唑酯骨架结构材料ZIF-8(分别记为ZIF-8(NH₄OH)、ZIF-8(MeOH)和ZIF-8(DMF),并采用XRD、FTIR、N₂吸附、SEM、TPD及Knoevenagel缩合反应等手段对所合成材料进行了表征。结果表明,采用这3种不同的合成方法均可成功制备出ZIF-8,所合成的ZIF-8的形貌基本一致,但其晶粒大小和酸碱性能有较大区别,同ZIF-8(NH₄OH)和ZIF-8(DMF)相比,ZIF-8(MeOH)晶粒分布集中、平均粒径较小且具有较大的外比表面积和较多的酸碱位。不同方法合成的ZIF-8在苯甲醛和丙二腈的Knoevenagel缩合反应中的催化性能有很大差异,ZIF-8(MeOH)催化活性明显高于ZIF-8(DMF)和ZIF-8(NH₄OH),其较高的催化活性,同其较大的外比表面积和酸碱性能密切相关。     

One-step fabrication of ZIF-8/polymer composite spheres by a phase inversion method for gas adsorption

This paper reports a phase inversion method for the preparation of macroporous polysulfone (PS) composite spheres through a single orifice spinneret. Surfactant F127 was pre-added in the polymer solution as a surface pore-forming agent, and different amount of zeolitic imidazolate framework-8 (ZIF-8) particles were incorporated to form the ZIF-8/polysulfone (ZIF-8/PS)-composite spheres. ZIF-8 and polymer acted as the adsorbent and binder in the final composite spheres, respectively. The fabrication conditions, such as the types of the surfactant, the amount of the surfactant, and ZIF-8 added in the polymer solution, were investigated. Nitrogen and carbon dioxide sorption analysis indicated the ZIF-8/PS composite spheres had similar properties as the pure ZIF-8 particles, and the active sites of ZIF-8 in the polymer composites were well exposed. The composite spheres exhibited advantages of easy handling and recycling over ZIF-8 particles, and this phase inversion method can be extended to prepare other polymer composite spheres.     

Stability of ZIF-8 Nanoparticles in Most Common Cell Culture Media

Zeolite imidazolate framework-8 (ZIF-8) is a promising platform for drug delivery, and information regarding the stability of ZIF-8 nanoparticles in cell culture media is essential for proper interpretation of in vitro experimental results. In this work, we report a quantitative investigation of the ZIF-8 nanoparticle’s stability in most common cell culture media. To this purpose, ZIF-8 nanoparticles containing sterically shielded nitroxide probes with high resistance to reduction were synthesized and studied using electron paramagnetic resonance (EPR). The degradation of ZIF-8 in cell media was monitored by tracking the cargo leakage. It was shown that nanoparticles degrade at least partially in all studied media, although the degree of cargo leakage varies widely. We found a strong correlation between the amount of escaped cargo and total concentration of amino acids in the environment. We also established the role of individual amino acids in ZIF-8 degradation. Finally, 2-methylimidazole preliminary dissolved in cell culture media partially inhibits the degradation of ZIF-8 nanoparticles. The guidelines for choosing the proper cell culture medium for the in vitro study of ZIF-8 nanoparticles have been formulated.     

The calcined zeolitic imidazolate framework-8 (ZIF-8) under different conditions as electrode for supercapacitor applications

Zeolitic imidazolate framework-8 (ZIF-8) is synthesized by typical solvothermal method and subsequently calcined under air and nitrogen atmosphere, respectively. The carbon in the calcined ZIF-8 under nitrogen atmosphere was from the carbonization of the guest molecules, without adding any other carbon sources. The samples are characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and electrochemical analyzer system. When the ZIF-8 is used as electrode materials for a supercapacitor in 6 M KOH electrolyte, it displays a pseudocapacitive behavior. The untreated ZIF-8 and calcined ZIF-8 under air and nitrogen atmosphere electrodes exhibit a specific capacitance of 96, 156, and 185 F g^?1, respectively, at a scan rate of 5 mV s^?1 and good stability over 1,500 cycles. These results indicate that the ZIF-8 is a promising material for supercapacitors.     

Sandwich-like QDs/MOFs films for selective sensing and multicolor emitting

A novel type sandwich-like composite films composed of ZIF-8 and CdTe QDs were successfully constructed through facile layer-by-layer assembly strategy and their potential applications were also explored. Based on the limitation effects of the aperture of ZIF-8, CdTe QDs/ZIF-8 fluorescent composite films exhibit obvious selective optical response toward hydrogen peroxide and folic acid. Furthermore, tunable colorful light emission composite films in the red-green region are obtained through incorporating two different sized QDs and ZIF-8 films into one composite films.     

Zeolitic imidazolate framework-8 as a reverse osmosis membrane for water desalination: insight from molecular simulation

A molecular simulation study is reported for water desalination in zeolitic imidazolate framework-8 (ZIF-8) membrane. The simulation demonstrates that water desalination occurs under external pressure, and Na(+) and Cl(-) ions cannot transport across the membrane due to the sieving effect of small apertures in ZIF-8. The flux of water permeating the membrane scales linearly with the external pressure, and exhibits an Arrhenius-type relation with temperature (activation energy of 24.4 kJ∕mol). Compared with bulk phase, water molecules in ZIF-8 membrane are less hydrogen-bonded and the lifetime of hydrogen-bonding is considerably longer, as attributed to the surface interactions and geometrical confinement. This simulation study suggests that ZIF-8 might be potentially used as a reverse osmosis membrane for water purification.     

Enhanced Activity of Enzyme Immobilized on Hydrophobic ZIF-8 Modified by Ni2+ Ions

The development of efficient enzyme immobilization to promote their recyclability and activity is highly desirable. Zeolitic imidazolate framework-8 (ZIF-8) has been proved to be an effective platform for enzyme immobilization due to its easy preparation and biocompatibility. However, the intrinsic hydrophobic characteristic hinders its further development in this filed. Herein, a facile synthesis approach was developed to immobilize pepsin (PEP) on the ZIF-8 carrier by using Ni²⁺ ions as anchor (ZIF-8@PEP-Ni). By contrast, the direct coating of PEP on the surface of ZIF-8 (ZIF-8@PEP) generated significant conformational changes. Electrochemical oxygen evolution reaction (OER) was employed to study the catalytic activity of immobilized PEP. The ZIF-8@PEP-Ni composite attains remarkable OER performance with an ultralow overpotential of only 127 mV at 10 mA cm⁻², which is much lower than the 690 and 919 mV overpotential values of ZIF-8@PEP and PEP, respectively.     

Fischer–Tropsch synthesis using zeolitic imidazolate framework (ZIF-7 and ZIF-8)-supported cobalt catalysts

The Fischer–Tropsch synthesis using cobalt catalysts supported on zeolitic imidazolate frameworks (ZIFs), ZIF-7 and ZIF-8, has been investigated. ZIF-7, ZIF-8 and corresponding cobalt-containing catalysts, after preparation, were characterized using various techniques. Thermogravimetric analysis results show that ZIF-7 and ZIF-8 supports have good thermal stability for the Fischer–Tropsch synthesis reaction, and weaker interaction between cobalt and zinc in the ZIF-7 and ZIF-8 supports results in more cobalt reduction. The catalytic performance was evaluated in Fischer–Tropsch synthesis and compared with that of a cobalt catalyst supported on SBA-15 promoted with zinc. The pore structure of the ZIF supports plays an essential role in product selectivity for the prepared catalysts. The carbon number in hydrocarbon products and olefin selectivity depend on cobalt dispersion and support structure owing to the impacts of site density and carrier skeleton on the speed of diffusion-enhanced olefin re-adsorption reactions.     

Self-Assembled pH-Responsive Metal-Organic Frameworks for Enhancing the Encapsulation and Anti-Oxidation and Melanogenesis Inhibition Activities of Glabridin

Metal organic frameworks (MOFs) are formed by self-assembly of metal ions and organic ligands. A special type of MOF called ZIF-8, which is formed by self-assembly of zinc ions and 2-methylimidazole, shows excellent stability in aqueous solutions and disintegrates under acidic conditions. These properties make ZIF-8 a suitable carrier material for pH-stimulated drug delivery systems. Glabridin is an isoflavane compound that is widely present in the roots of licorice. Because of its outstanding skin whitening properties, glabridin is widely used as a whitener in the cosmetics industry. In this study, ZIF-8 was employed to encapsulate glabridin. Glabridin-loaded ZIF-8 was successfully prepared with a drug encapsulation efficiency of 98.67%. The prepared sample showed a fusiform or cruciate flower-like structure, and its size was about 3 μm. ZIF-8 enabled pH-controlled release of glabridin. Moreover, ZIF-8 encapsulation significantly enhanced the intracellular anti-oxidant activity and melanogenesis inhibitory activity of glabridin. This study provides a new approach that shows great potential to improve the biological application of glabridin.     

ZIF-8 degrades in cell media,serum, and some—but not all—common laboratory buffers

ABSTRACT

Drug delivery using metal-organic frameworks (MOF) has elicited interest in their biocompatibility; however, few studies have been conducted on their stability in common buffers, cell media, and blood proteins. In particular, the use of ZIF-8, a MOF interconnected by Zn and methylimidazole, has been frequently employed. In this study, we tested single crystals of ZIF-8 with common laboratory buffers, cell media, and serum, and noted several issues. Buffers containing phosphate and bicarbonate alter the appearance and composition of ZIF-8; however, these buffers do not appear to cause cargo to leak out even when the ZIF-8 itself is displaced by phosphates. On the other hand, serum dissolves ZIF-8, causing premature cargo release. Our results show that ZIF-8 undergoes surface chemistry changes that may affect the interpretation of cellular uptake and cargo release data. On the other hand, it provides a rational explanation as to how ZIF-8 neatly dissolves in vivo.     

Production of biodiesel through transesterification of soybean oil using ZIF-8@GO doped with sodium and potassium catalyst

One pot encapsulating (hydrothermal) method was used to synthesize ZIF-8@GO hybrid nanocomposites. Na/ZIF-8@GO doped with potassium were synthesized by the hydrothermal treatment of ZIF-8@GO precursor with a 10 M alkali solution containing both NaOH and KOH. The final product (KNa/ZIF-8@GO) was characterized by FTIR, XRD, BET, TGA, and SEM. The structure of graphene oxide remains intact following various modifications, as shown by absorption–desorption analysis, while spectral techniques indicate successful immobilization of the neat ZIF-8 between the GO sheets. Furthermore, soybean oil has been used as the feedstock in the preparation of biodiesel by KNa/ZIF-8@GO-catalyzed transesterification process. The effects of various reaction parameters, including methanol/oil molar ratio, reaction time, catalyst mass, and reaction temperature were investigated. A remarkable conversion of 98% was obtained using a 8% (wt/wt oil) of KNa/ZIF-8@GO catalyst, methanol/oil molar ratio of 18 : 1, and reflux temperature of methanol over a period of 8 h. The solid catalyst can be reused over at least three cycles under mild reaction conditions.

     

CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation using mixed matrix membrane-based polyurethane incorporated with ZIF-8 nanoparticles

This study presents using zeolitic imidazolate framework-8 (ZIF-8) as porous filler dispersed phase and polyurethane (PU) as continuous phase to synthesis mixed matrix membranes (MMMs). ZIF-8 nanoparticles were synthesized using centrifugal method. The synthesized nanoparticles were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermal gravimetric analysis (TGA). In order to investigate the effect of ZIF-8 loading on the membrane performance in CO₂/CH₄ separation, different membranes were prepared with various amounts of ZIF-8 (0–50 wt%). Membranes properties were characterized by SEM, XRD, TGA, differential scanning calorimetry (DSC), and tensile analysis. SEM images exhibit that the ZIF-8 is dispersed uniformly in cross section of membrane. Thermal stability of membranes increases with addition of the ZIF-8 nanoparticles into the polymer matrix. Both tensile strength and strain at break in the MMMs increase with the ZIF-8 loading. To study the effect of feed pressure on CO₂ and CH₄ transport properties of the membranes, single gas experiments were conducted at 4, 8, and 12 bar feed pressures. Incorporation of ZIF-8 crystals into continuous PU matrix resulted in high-performance gas separation membranes. Increasing feed pressure, significantly, increased separation performances in all the membranes.     

不同尺寸ZIF-8对U(VI)的吸附性能

以经典的金属有机骨架(MOFs)材料ZIF-8为吸附剂,研究尺寸效应对铀吸附性能的影响。 通过3种方法合成不同粒径的ZIF-8,利用扫描电子显微镜(SEM)、X射线粉末衍射仪(XRD)、表面积与孔隙度分析仪等对其进行了表征,测试了相同条件下不同尺寸的ZIF-8对硝酸铀酰溶液中U(VI)吸附,分别对其吸附过程的动力学和吸附等温线进行了考察,并测试了材料的可重复利用性。 结果表明,成功制备了高结晶性、高纯度的ZIF-8,产物形貌呈菱形十二面体,颗粒均匀,粒径分别为约50 nm、150 nm及2 μm;3种ZIF-8具有单一均匀的微孔结构和与粒径高度相关的比表面积;不同尺寸的ZIF-8均能快速吸附溶液中的U(VI),在室温pH=3下,在70 min左右时即可吸附初始质量浓度为200 mg/L的U(VI)溶液中90%以上U(VI);其中较小尺寸(约50 nm)的ZIF-8吸附性能最好,单位质量ZIF-8吸附U(VI)的饱和吸附量达到520.26 mg/g;ZIF-8对U的吸附动力学上符合二级动力学方程,吸附等温线符合Langmuir模型,说明ZIF-8对U(VI)的捕获属于化学单层吸附;经过4个吸附-解吸循环后,3种尺寸的ZIF-8均依然保持了70%以上的去除率。