沸石咪唑骨架膜的制备及其应用进展

近年来,将金属-有机骨架材料(MOFs)和膜基材料结合,制备新型MOFs分离膜成为膜领域研究的热点之一。由于MOFs具有类似分子筛结构和空间拓扑结构,在分离、催化等方面具有潜在的应用前景。沸石咪唑框架材料(ZIFs)作为MOFs中重要分支之一,因其具有优异的热稳定性和化学稳定性被应用于膜分离。本工作重点阐述了原位生长、界面反扩散、逐层组装、二次生长、气相沉积和微流体处理等方法制备ZIFs多晶膜和杂化膜,并系统介绍了ZIFs复合膜在染料与重金属离子去除、气体分离、天然气净化、生物医药和电化学传感中的应用。最后,总结了ZIFs复合膜制备过程中存在的问题和挑战,并对ZIFs复合膜未来研究的方向提出了展望。     

CO2/CH4 and CH4/N2 separation on isomeric metal organic frameworks☆

Two isomeric metal-organic frameworks(MOFs) with 2-dimensional(2D) and 3-dimensional(3D) topologies both comprised of Cu(Ⅱ) and OTf(OTf = trifluoromethanesulfonate) ions were synthesized and characterized.The CO_2,CH_4 and N_2 adsorption properties of the two isomeric MOFs were investigated from 263 K to 298 K at0.1 MPa.The results showed that the 2D MOF exhibited a higher selectivity for CO_2 from CO_2/CH_4 and CH_4from CH_4/N_2 compared to the 3D MOF,even though it possessed a lower surface area and pore volume.The higher adsorption heats of gases on the 2D MOF inferred the strong adsorption potential energy in the layered MOFs.Dynamic separation experiments using CO_2/CH_4 and CH_4/N_2 mixtures on the two MOFs proved that the2 D MOF had a longer elution time than the 3D MOF as well as better separation abilities.     

金属有机骨架材料储存CO_2的研究进展

随着温室效应日益严重,研发高效捕获CO_2的新型材料对于缓解环境压力具有重要意义。金属有机骨架材料具有高比表面积、结构多样性和孔道可调控性等优点,尤其在储存CO_2方面展现出惊人潜质。简要介绍金属有机骨架材料及其延伸化合物——沸石咪唑酯骨架材料和共价有机骨架材料作为新型储存CO_2材料,在结构、储存CO_2性能和机理等方面的研究进展,并阐述了该类材料在研究过程中存在的不足与发展前景。     

Gas adsorption in shaped zeolitic imidazolate framework-8☆

Zeolitic imidazolate framework-8 (ZIF-8) was prepared through a solve-thermal reaction method and then shaped using different additives. The influence of the shaping conditions on the microstructure of the shaped samples was characterized by the XRD, BET, and SEM techniques. The results demonstrate that the compressive strength of the various shaped tablets is greatly increased and capable of meeting the industrial requirements compared to the unshaped ZIF-8 and that the loss rate of specific surface areas was maintained at 10%after the addition of 10%(by mass) binder and 10%(by mass) solvent. The adsorption isotherms of CO2, CH4, C3H8, and C3H6 on powdery ZIF-8 and the shaped tablets (T-shaped ZIF-8, C-shaped ZIF-8, and N-shaped ZIF-8) were determined through volumetric measurements under different pressures and temperatures (298.2, 323.2, and 348.2 K). The adsorption capacities of the gases on both the ZIF-8 powder and the shaped tablets fol ow the order C3H6 N C3H8 N CO2 N CH4. Furthermore, the results show that the adsorption capacities of the gases on the shaped tablets are lower by approximately 10%–20%than those on the powdery ZIF-8. In fact, the adsorption equilibrium isotherms for CO2, CH4, C3H8, and C3H6 on both powdery and shaped ZIF-8 can be wel described by the Langmuir equation.     

Synthesis,characterization, and tunable adsorption and diffusion properties of hybrid ZIF‐7‐90 frameworks

Hybrid zeolitic imidazolate frameworks (ZIFs), containing more than one type of imidazolate linker, can allow highly tunable molecular sieving and adsorption. Their crystallization becomes more challenging when the end‐member (single‐linker) ZIFs crystallize in different crystal systems. We demonstrate the controlled synthesis and detailed characterization of hybrid ZIF‐7‐90 frameworks containing linkers of ZIF‐7 (rhombohedral) and ZIF‐90 (cubic). ZIF‐7‐90 materials with SOD‐type topology are obtained in three crystalline phases depending on the linker composition and synthesis technique. The effect of synthesis conditions on the activation‐induced phase transition from rhombohedral to other topologies is studied. Nitrogen physisorption at 77 K and CO₂ physisorption at 273 K shows the tunability of the pore‐size distribution and the framework flexibility as a function of framework composition. Measurements of water adsorption and butane isomer diffusion illustrate the tunability of diffusivity over seven orders of magnitude and control of hydrophobic to hydrophilic adsorption behavior. © 2015 American Institute of Chemical Engineers AIChE J, 62: 525–537, 2016     

Synthesis of metal-organic frameworks: A mini review

Metal organic frameworks (MOFs) are porous crystalline materials of one-, two-, or three-dimensional networks constructed from metal ions/clusters and multidentate organic linkers via coordination bonding, which are emerging as an important group of materials for energy storage, CO₂ adsorption, alkane/alkene separation, and catalysis. To introduce newcomers in chemical engineering discipline to the rapidly expanding MOF research works, this review presents a brief introduction to the currently available MOFs synthesis methods. Starting from the conventional solvothermal/hydrothermal synthesis, microwave-assisted, sonochemical, electrochemical, mechanochemical, ionothermal, drygel conversion, and microfluidic synthesis methods will be presented. Examples will be limited to those representative MOF structures that can be synthesized using common organic ligands of 1,4-benzenedicarboxylic acid (and its functionalized forms) and 1,3,5-benzenetricarboxylic acid, in conjunction with metal nodes of Zn²⁺, Cu²⁺, Cr³⁺, Al³⁺, Fe³⁺ and Zr⁴⁺. Synthesis of widely-investigated zeolitic imidazolate framework (ZIF) structure, ZIF-8 is also included.     

Enhanced adsorption of phenol from aqueous solution by carbonized trace ZIF-8-decorated activated carbon pellets

Trace zeolitic imidazolate framework-8(ZIF-8)-decorated activated carbon(AC) pellets were synthesized by a facile wet impregnation technique. After pyrolysis of the above composite material, the obtained carbon had a large surface area and pore volume, with traces of Zn on its surface. Subsequently, the capacity of the ZIF8/AC samples to adsorb and remove phenol from aqueous media was evaluated in both batch and column experimental setups. The equilibrium adsorption capacity reached 155.24 mg·g~(-1), which was 2.3 times greater than that of the pure AC(46.24 mg·g~(-1)). In addition, adsorption kinetics were examined by pseudofirst and pseudosecond order models, and adsorption isotherms were fitted into Langmuir and Freundlich equations. The adsorbent could be easily filtered from the solution and washed with methanol and water, while maintaining an efficiency N 90% after 4 cycles. The above results make it a potentially reusable candidate for water purification.     

Stabilization of immiscible polymer blends using structure directing metal organic frameworks (MOFs)

We have developed a novel approach for compatibilizing immiscible polymer blends using metal organic frameworks (MOFs). For the first time we demonstrated that the droplet diameter of the dispersed phase in a 1:1 immiscible polymer blend composed of 6FDA-DAM:DABA [copolymer of 4,4-hexafluoroisopropylidene diphthalic anhydride (6FDA), 2,4,6-trimethyl-1,3-phenylenediamine and 3,5-diaminobenzoic acid (DABA)], and polybenzimidazole (PBI), is dramatically reduced obtaining a uniform microstructure with the incorporation of as low as 5% (w/w) of the zeolitic imidazolate framework-8 (ZIF-8). This indicates a large improvement in the compatibility of the immiscible polymers with the inclusion of ZIF-8. As the ZIF-8 loading was further increased to 10% (w/w), the droplet diameter further decreased resulting in even higher compatibility. The compatibilizing effect can be attributed to a reduction in the interfacial energy of the immiscible polymers due to the interfacial localization of ZIF-8. This MOF based compatibilization of immiscible polymer blends can open up opportunities for the combination of different properties of polymers in membrane-based separations and in more applications.     

Decoration of graphene network with metal–organic frameworks for enhanced electrochemical capacitive behavior

Well-intergrown nanocrystals of zeolitic imidazolate frameworks (ZIF-8) supported on three-dimensional (3D) graphene were prepared by a counter diffusion technique. The incorporation of ZIF-8 crystals greatly improves the surface areas of the graphene composites. The carbonized graphene–ZIF composites with hierarchical pore structures showed high electrochemical capacitance and good stability. This work provides an efficient method to synthesize porous carbon materials with high capacitance.     

Characterization of metal-organic frameworks by water adsorption

The water physisorption properties and the water stability of the metal-organic frameworks HKUST-1 (=(Cu₃(BTC)₂) (BTC = benzene-1,3,5-tricarboxylate)), ZIF-8, MIL-101, MIL-100(Fe) and DUT-4 (=Al(OH)(NDC)) (NDC = naphthalene-2,6-dicarboxylate) were studied. The water physisorption isotherms were compared to nitrogen physisorption isotherms and the chemical stability after water adsorption was investigated.Water adsorption does not only provide information about specific surface area, pore size and pore volume, but can also be used to estimate hydrophobicity and stability towards moisture. Both HKUST-1 and DUT-4 turned out to be unstable in direct contact with water, whereas the MIL-materials and ZIF-8 do show stability. The highest water adsorption affinity was observed for HKUST-1. Even though unstable in liquid water, for applications in trace water removal or molecular sensing HKUST-1 is a promising material. ZIF-8 is highly inert but hydrophobic. The MIL-materials are both likely candidates for water adsorption applications, but higher concentrations of the adsorbate are needed to attain an effective adsorption. DUT-4 shows insufficient adsorption capacity, but stability at ambient conditions is given.     

Water-stable ZIF-300/Ultrason(R) mixed-matrix membranes for selective CO2 capture from humid post combustion flue gas

Water stable mixed-matrix membranes (MMMs) were developed to help control the global warming by capturing and sequestrating carbon dioxide (CO2) from post-combustion flue gas originated from burning of fossil fuels.MMMs of different compositions were prepared by doping glassy polymer Ultrason(R) S 6010 (US) with nanocrystals of zeolitic imidazolate frameworks (ZIF-300) in varying degrees.Solution-casting technique was used to fabricate various MMMs to optimize their CO2 capturing performance from both dry and wet gases.The prepared composite membranes indicated enhanced filler-polymer interfacial adhesion,consistent distribution of nanofiller,and thermally established matrix configuration.CO2 permeability of the membranes was enhanced as demonstrated by gas sorption and permeation experiments performed under both dry and wet conditions.As compared to neat Ultrason(R) membrane,CO2 permeability of the composite membrane doped with 40 wt％ ZIF-300 nanocrystals was increased by four times without disturbing CO2/N2 ideal selectivity.In contrast to majority of previously reported membranes,key features of the fabricated MMMs include their structural stability under humid conditions coupled with better and unaffected gas separation performance.     

An improved theoretical procedure for the pore-size analysis of activated carbon by gas adsorption

Amorphous carbon materials play a vital role in adsorbed natural gas (ANG) storage.One of the key issues in themore prevalent use of ANG is the limited adsorption capacity,which is primarily determined by the porosity and surface characteristics of porous materials.To identify suitable adsorbents,we need a reliable computational tool for pore characterization and,subsequently,quantitative prediction of the adsorption behavior.Within the framework of adsorption integral equation (AIE),the pore-size distribution (PSD) is sensitive to the adopted theoretical models and numerical algorithms through isotherm fitting.In recent years,the classical density functional theory (DFT) has emerged as a common choice to describe adsorption isotherms for AIE kernel construction.However,rarely considered is the accuracy of the mean-field approximation (MFA) commonly used in commercial software.In this work,we calibrate four versions of DFT methods with grand canonical Monte Carlo (GCMC) molecular simulation for the adsorption of CH4 and CO2 gas in slit pores at 298 K with the pore width varying from 0.65 to 5.00 nm and pressure from 0.2 to 2.0 MPa.It is found that a weighted-density approximation proposed by Yu (WDA-Yu) is more accurate than MFA and other non-local DFT methods.In combination with the trapezoid discretization of AIE,the WDA-Yu method provides a faithful representation of experimental data,with the accuracy and stability improved by 90.0％ and 91.2％,respectively,in comparison with the corresponding results from MFA for fitting CO2 isotherms.In particular,those distributions in the feature pore width range (FPWR)are proved more representative for the pore-size analysis.The new theoretical procedure for pore characterization has also been tested with the methane adsorption capacity in seven activated carbon samples.     

Partial pore blockage and polymer chain rigidification phenomena in PEO/ZIF-8 mixed matrix membranes synthesized by in situ polymerization

Nanostructured zeolitic imidazolate frameworks(ZIF-8) was incorporated into the mixture of poly(ethylene glycol) methyl ether acrylate(PEGMEA) and pentaerythritol triacrylate(PETA) to synthesize mixed matrix membranes(MMMs) by in situ polymerization for CO_2/CH_4 separation. The solvent-free polymerization between PEGMEA and PETA was induced by UV light with 1-hydroxylcyclohexyl phenyl ketone as initiator. The chemical structural characterization was performed by Fourier transform infrared spectroscopy. The morphology was characterized by scanning electron microscope. The average chain-to-chain distance of the polymer chains in MMMs was investigated by X-ray diffraction. The thermal property was evaluated by differential scanning calorimetry. The CH_4 and CO_2 gas transport properties of MMMs are reported. The relationship between gas permeation–separation performances or physical properties and ZIF-8 loading is also discussed. However, the permeation–separation performance was not improved in Robeson upper bound plot compared with original polymer membrane as predicted. The significant partial pore blockage and polymer rigidification effect around the ZIFs confirmed by the increase in glass temperature and the decrease in the d-spacing, were mainly responsible for the failure in performance improvement, which offset the high diffusion induced by porous ZIF-8.     

The comparison of experimental and calculated pore size distributions of activated carbons

Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO₂ isotherms have been measured at 273 K on seven well-characterized microporous carbons with average micropore widths between 0.65 and 1.5 nm and analysed with model isotherms obtained with standard Monte Carlo simulations. The resulting PSD are in good agreement with those obtained from a modified Dubinin equation, from liquid probes of molecular dimensions between 0.4 and 1.5 nm, from STM and from modelling based on CH₄ adsorption at 308 K. The present study validates the determination of micropore distributions in active carbons based on CO₂ isotherms, provided that no gate effects are present.     

沸石咪唑骨架N_2吸附的分子模拟与BET分析

采用分子模拟的方法模拟77 K下N2在沸石咪唑骨架材料(ZIFs)上的吸附等温线,并与实验结果比较,确定合适的力场参数,提供用BET方法分析ZIFs材料比表面积的吸附等温线。通过BET吸附方程中的参数c与单层吸附压力(p/p0)m之间的关系为(p/p0)m=(槡c+1)-1,确定应用BET方程分析时的压力区间。采用这样的方法计算5种ZIF材料的比表面积并与根据一致性原则(Rouquerol,2007)的结果进行比较,结果表明方法是可靠的,这说明采用合适的压力区间时,BET方法同样适用于分析微孔ZIF材料的比表面积。计算得到的5种ZIF材料比表面积均在1 000 m2/g以上,说明ZIF材料是一种具有高比表面积、有应用前景的吸附材料。     

铪金属-有机骨架材料的孔尺寸调控及其吸附性能

利用三种不同长度的有机配体--反丁烯二酸（H₂FUM）、对苯二甲酸（H₂BDC）和联苯二甲酸（H₂BPDC）,合成了一系列具有不同孔尺寸的新型铪（Hf）金属-有机骨架（MOF）材料（Hf-FUM、Hf-BDC和Hf-BPDC）,并考察了CO₂、N₂和CH₄三种气体在这些材料中吸附分离行为。研究结果表明,这三种材料具有和UiO-66（Zr）相同的拓扑结构,且具有很好的热稳定性。Hf-FUM和Hf-BDC的结构在水中能够保持稳定,而Hf-BPDC在水中会发生降解。同时,具有最小孔尺寸的Hf-FUM材料对CO₂/N₂以及CO₂/CH₄体系具有最好的分离性能。这为以后设计用于CO₂分离的新型纳微结构材料提供了参考依据。     

Rapid construction of hierarchically porous metal–organic frameworks by a spray-drying strategy for enhanced tannic acid adsorption

Metal–organic frameworks (MOFs) have attracted much attention owing to their tailored pore environment and surface functionality. However, most of the currently reported MOFs are confined to small pore sizes (<5 nm), limiting their practical applications. Here, a facile and versatile strategy for rapid construction of hierarchically porous MOFs (HP-MOFs) by spray-drying is reported, in which presynthesized nanosized MOFs (N-MOFs) can be assembled to form MOF microspheres with meso/macropores resulting from the interspace among closely arranged N-MOFs. This strategy enables the construction of multicomponent HP-MOFs with various functions. As a proof of concept, we show that the adsorption for tannic acid (TA) can be significantly enhanced using HP-MIL-101(Cr). Particularly, HP-MIL-101-30 (30 represents the primary nanoparticle size) exhibits a record-high adsorption capacity (1175 mg g⁻¹), and the adsorption rate of HP-MIL-101-30 is increased by 50% compared to that of N-MIL-101-30. These findings have important implications for the rapid construction of HP-MOFs, which is beneficial for the adsorption of large molecules.     

Crystal formation and size control of zeolitic imidazolate frameworks with mixed imidazolate linkers

The effect of synthesis parameters, such as batch composition and synthesis temperature, on the formation and crystal size of zeolitic imidazolate frameworks (ZIFs) using mixed linkers, imidazole (Im) and benzimidazole (bIm), were studied by X-ray powder diffraction (XRPD) and scanning electron microscopy (SEM). ZIF-7 and ZIF-62 were identified as the main phases and a phase diagram relating the batch composition and the final product was constructed based on XRPD and SEM. The range of batch composition giving rise to ZIF-7 and ZIF-62 was identified from the phase diagram. Changes of crystal size and morphology of ZIF-7 and ZIF-62 in terms of batch composition and synthesis temperature were studied by SEM and different trends were observed for the two ZIFs. Thermogravimetric analysis (TGA) and in situ XRPD showed that ZIF-62 possesses a similar high thermal stability as ZIF-7.     

Determination of pore size distribution and adsorption of methane and CCl4 on activated carbon by molecular simulation

A combined method of grand canonical Monte Carlo (GCMC) simulation and statistics integral equation (SIE) for the determination of pore size distribution (PSD) is developed based on the experimental adsorption data of methane on activated carbon at ambient temperature, T=299 K. In the GCMC simulation, methane is modeled as a Lennord-Jones spherical molecule, and the activated carbon pore is described as slit-shaped with the PSD. The well-known Steele’s 10-4-3 potential is used to represent the interaction between the fluid molecule and the solid wall. Covering the range of pore sizes of the activated carbon, a series of adsorption isotherms of methane in several uniform pores were obtained from GCMC. In order to improve the agreement between the experimental data and simulation results, the PSD is calculated by means of an adaptable procedure of deconvolution of the SIE method. Based on the simulated results, we use the activated carbon with the PSD as the prototype of adsorbent to investigate adsorption. The adsorption isotherms of methane and CCl₄ at 299 K in the activated carbon with the PSD are obtained. The adsorption amount of CCl₄ reaches 20 mmol/g at ambient temperature and pressure. The results indicate that the combined method of GCMC and SIE proposed here is a powerful technique for calculating the PSD of activated carbons and predicting adsorption on activated carbons.