Amine-functionalized metal–organic frameworks/epoxy nanocomposites: Structure-properties relationships

Amine-functionalized MIL-101(Cr)-NH₂ metal–organic frameworks (MOF-N)/epoxy nanocomposites with Excellent cure label and high thermal stability were developed. Structure–property relationship was discussed by comparison of the cure state, thermal and viscoelastic behavior of epoxy nanocomposites containing pristine MOF or MOF-N applying differential scanning calorimetry (DSC), thermogravimetric analysis, and dynamic mechanical analysis. Epoxy containing 0.3 wt% MOF-N exhibited high glass transition temperature (T_g) of 96°C compared with 85°C observed for epoxy/MOF system. Thus, MOF-N played the role of catalyst in epoxy/amine curing reaction. Correspondingly, a lower activation energy was obtained based on cure kinetics modeling based on DSC measurements. Besides, incorporation of low amount (0.5 wt%) MOF-N induced an early-state resistance against decomposition, featured by 11°C rise in decomposition temperature at 5% weight loss. This was ascribed to the formation of porous metallic oxides during thermal decomposition of MOF-N in the epoxy system acting as a heat barrier, which increased the activation energy of decomposition. Amine-functionalization considerably prevented from further oxidation of the inner part of the matrix.     

Immobilization of lipase onto metal–organic frameworks for enantioselective hydrolysis and transesterification

Enzyme immobilization enhances the catalytic activity and stability of the enzyme, and also improves reusability. Metal–organic frameworks (MOFs), which possess diversified structures and porosity, have been used as excellent carriers for enzyme immobilization. Pseudomonas fluorescens lipase (PFL) has been successfully immobilized onto MOFs by covalent cross-linking to obtain a series of immobilized lipase (PFL@MOFs). PFL@MOFs are used for catalytic enantioselective hydrolysis of 2-(4-hydroxyphenyl) propionic acid ethyl ester enantiomers (2-HPPAEE) in aqueous medium and transesterification of 4-methoxymandelic acid enantiomers (4-MMA) in organic medium. The experimental results indicated that PFL@Uio-66(Zr) exhibits excellent enzymatic catalysis performances and high enantioselectives. In addition, to improve catalytic activity and reusability, PFL is modified by the polyethylene glycol (PEG) to prepare PEG-modified lipase (PFL-PEG), then PFL-PEG is immobilized onto Uio-66(Zr) to prepare PFL-PEG@Uio-66(Zr), demonstrating better reusability and catalytic activity compared with PFL@Uio-66(Zr).     

Encapsulation and controlled release of fragrances from functionalized porous metal–organic frameworks

In the fragrance and perfume industry, the encapsulation and controlled release of fragrance is important to appeal to consumers and promote the quality of products. Here, we demonstrate that porous metal–organic frameworks (MOFs) can effectively encapsulate and release fragrant molecules in a controlled manner. The incorporation of functional groups into MOFs can improve the adsorption and release behavior of fragrant molecules. We find that polar ester-type fragrances exhibit higher adsorption on polar hydroxyl-functionalized MOF [UiO-66-(OH)₂] than on nonpolar MOF (UiO-66), while nonpolar terpenoid-type fragrances show no adsorption difference between these two MOFs. The release profiles show that UiO-66-(OH)₂ can prolong the release of polar fragrances compared with nonpolar fragrances. Both the experimental results and computer molecular modeling demonstrate that the hydroxyl groups in UiO-66-(OH)₂ can form strong hydrogen binding with different ester fragrances. The releasing kinetics indicates that pore diffusion is the rate-limiting step of fragrance release from MOFs. © 2018 American Institute of Chemical Engineers AIChE J, 65: 491–499, 2019     

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.     

Selective adsorption of CO2/N2 promoted by polar ligand functional groups of metal–organic frameworks

Journal of Porous Materials - The structure modification of metal–organic frameworks (MOFs) is a promising technique to enhance its selective adsorption of carbon dioxide at room...     

Molecular fingerprint and machine learning to accelerate design of high-performance homochiral metal–organic frameworks

Computational screening was employed to calculate the enantioseparation capabilities of 45 functionalized homochiral metal–organic frameworks (FHMOFs), and machine learning (ML) and molecular fingerprint (MF) techniques were used to find new FHMOFs with high performance. With increasing temperature, the enantioselectivities for (R,S)-1,3-dimethyl-1,2-propadiene are improved. The “glove effect” in the chiral pockets was proposed to explain the correlations between the steric effect of functional groups and performance of FHMOFs. Moreover, the neighborhood component analysis and RDKit/MACCS MFs show the highest predictive effect on enantioselectivities among the four ML classification algorithms with nine MFs that were tested. Based on the importance of MF, 85 new FHMOFs were designed, and a newly designed FHMOF, NO₂-NHOH-FHMOF, with high similarity to the optimal MFs achieved improved chiral separation performance, with enantioselectivities of 85%. The design principles and new chiral pockets obtained by ML and MFs could facilitate the development of new materials for chiral separation.     

Heterogeneous heck coupling in multivariate metal–organic frameworks for enhanced selectivity

Highly selective heterogeneous Heck coupling has been demonstrated inside a series of metal–organic frameworks (MOFs). These MOFs, Zn₄O(BDC-NH₂)_n(BDC)_(3 − n) (n = 3, 2.4, 1.8, 1.2, 0.9, 0.75, 0.6, 0.3, and 0.15), have been synthesized with different amounts of 1,4-benzenedicarboxylate (BDC) and 2-amino-1,4-benzenedicarboxylate (BDC-NH₂) incorporated into their structure. The BDC-NH₂ is functionalized by covalent postmodification with salicylic aldehyde for binding catalytically active Pd(II) ions. The catalytic activity of the embedded Pd(II) ions was tested via heterogeneous Heck coupling to produce resveratrol trimethyl ether, a pharmaceutically relevant precursor. It is also found during catalytic testing that a trade-off exists between amount of metalation and pore blocking.     

Two new metal–organic frameworks based on cyclic dodecanuclear copper units

Two new metal–organic frameworks based on cyclic-type dodecanuclear copper units, (Me₄N)₆[Cu₁₂(OMe)₆(pz)₆(BTC)₆]·18H₂O 1 and (Me₄N)₆[Cu₁₂(OH)₆(pz)₆(BTC)₆]·21H₂O 2 (pz = pyrazolate, BTC = 1,3,5-benzenetricarboxylate), have been prepared by the solvothermal reactions of copper salts, Hpz and H₃BTC ligands. The cyclic-type Cu₁₂ unit in 1 and 2 is constructed by twelve Cu^II ions linked together by μ₂-OH or μ₂-OMe and unidentate carboxylate groups at the inner- and μ-pz and bidentate carboxylate groups at the outer surface of the toroid. In 1 and 2, each Cu₁₂ unit is connected to 12 other units by BTC linkers, leading to 12-connected three-dimensional porous frameworks. The photocatalytic investigations indicate that compounds 1 and 2 exhibit photocatalytic activity for the degradation of RhB.     

Enhancement of permittivity in P(VDF-CTFE)/metal–organic frameworks mixed matrix membranes

A novel family of mixed matrix membranes (MMMs) with the combination of ferroelectric fluropolymers matrix (P(VDF-CTFE)) and ferroelectric MOFs fillers ([NH₃(CH₂)₄NH₃][M(HCOO)₃]₂(MCo^II, Mg^II, and Mn^II), have been synthesized and characterized, including the morphology, structures, and dielectric properties. Dielectric measurements revealed that the permittivity of the composites improved notablely with the introduction of ferroelectric MOFs fillers, while the dielectric loss was comparable to that of the polymer matrix. And also, interestingly, the enhancement of permittivity evidently dependent on the quantity and dielectric behavior of MOFs fillers, which may be valuable to the dielectric regulation of the polymer/MOFs MMMs. In addition, two different preparation methods were adopted respectively, including blending and electrospinning-hot pressing. The comparison of the two preparation methods revealed that the blending MMMs exhibit higher permittivity and higher dielectric loss than that of the electrospinning-hot pressing MMMs. To our knowledge, this is a novel research of high-ε_r dielectric MMMs fabricated by ferroelectric MOFs and ferroelectric fluropolymers, and this work may provide a new perspective on the further research of high-κ dielectric MMMs and the industrial application of ferroelectric MOFs.     

Use of molecular simulation for evaluating adsorption equilibrium of inhalation anaesthetic agents on metal–organic frameworks

Metal–organic frameworks (MOFs) were studied as alternatives to zeolites and activated carbon for adsorptive removal of wasted inhalation anaesthetic agents (IAA). Monte Carlo simulation was used to predict equilibrium adsorption isotherms of IAA on selected MOFs. Rather than generic forcefields (FFs), the all-atom FF parameters published by Arcario were used for IAA modelling. Continuous fractional component Monte Carlo (CFCMC) proved crucial for speedy simulation of large molecules. We found that allocating 70% probability to the CFlambdaSwap move gave optimum fits between simulation and experiment. The simulations provided us with an insight into the adsorption mechanisms of IAA in these structures. Heats of adsorption, Brauner-Emmet-Teller (BET) surface area, and total pore volume were deduced to be the crucial parameters for low, medium, and high range of relative pressures in the isotherm. Therefore, the chromium atoms in MIL-101-Cr are better adsorbers of IAA than MIL-100-Al at lower pressures despite the similarities in terms of the type of linkers and topology. Our simulation results corroborated the earlier published studies on the self-association behaviour of sevoflurane molecules based on the experimental isotherms reported for MOF-177-Zn. Finally, the high polarity of IAA is thought to explain good low-pressure simulation/experiment data agreement for the MOFs possessing coordinatively unsaturated sites (CUS) despite using generic DREIDING FF for the framework atoms. Our in-house parsing code helped realize that the grand-canonical Monte-Carlo simulation speed is not the same for all pressure points but decreases for higher pressure points. This can be explained by increased density of the adsorbates making successful trial moves less probable.     

Double-layer structure,sorption and magnetism properties of metal–organic frameworks with trigonal planar ligand

A new double-layer metal–organic framework [Co₃(tcpt)₂(H₂O)₂] (1) has been synthesized using trigonal planar ligand 2,4,6-tris(4-carboxyphenoxy)-1,3,5-triazine (H₃tcpt) as a bridging ligand and characterized by single-crystal X-ray diffraction, elemental analyses, IR, PXRD and TGA. Structure analysis reveals that compound 1 has a double-layer structure. Gas sorption measurements indicate that compound 1 exhibits selective adsorption capabilities for CO₂ over CH₄ and N₂. Furthermore, the magnetic studies of compound 1 show antiferromagnetic interactions between Co(II) ions.     

Synthesis and characterization of homo- and heterometallic metal–organic frameworks based on 1,2,4,5-benzenetetracarboxylate ligand

Two new three-dimensional metal–organic frameworks, [H₂N(CH₃)₂]₂[Zn(btec)]·DMF (1, H₄btec = 1,2,4,5-benzenetetracarboxylate acid) and [H₂N(CH₃)₂][ZnLi(btec)]·DMF (2), have been solvothermally synthesized and structurally characterized by single crystal X-ray diffraction. Compound 1 based on μ₄-btec features an anionic homometallic framework with 4-connected pts topology. Compound 2 is a heterometallic organic framework with rare (4, 4, 8)-connected network topology, which can be considered as constitute of a Li-btec (pts) net and a Zn-btec net. Moreover, the luminescent properties of two compounds are investigated in the solid state at room temperature.     

Constructing two new crystalline metal–organic frameworks based on a mixed-donor ligand

The self-assembly of a prominent ligand, 5-(4-(1H-tetrazolyl)phen)isophthalic acid (H₃TZPI), with Zn^2 +/Cu^2 + centres generates two new MOFs, [Zn₂(TZPI)(μ₃-OH)(H₂O)₂]·2H₂O (JUC-161) and [Cu(HTZPI)(μ₂-H₂O)_0.5(H₂O)_1.5]·5(H₂O) (JUC-162). JUC-161 exhibits fascinating three-dimensional structures containing rectangular channels with dimensions of 6.5 × 3.0 Å², and JUC-162 displays 3D supramolecular structures with 12.5 × 14.6 Å² hexagonal channels. The observed structural diversity in these frameworks is due to the two distinct coordinating moieties of the H₃TZPI ligand. The photoluminescence of JUC-161 and the magnetic properties of JUC-162 are also measured at room temperature. Photoluminescence investigations reveal that JUC-161 displays a strong main emission spectrum peak at 388 nm. JUC-162 is found to exhibit an antiferromagnetic interaction between Cu^2 + ions.     

Immobilization of urease in metal–organic frameworks via biomimetic mineralization and its application in urea degradation

Enzyme immobilization has been accepted as an efficient technique for improving the stability and recyclability of enzymes. Herein, biomimetic mineralization strategy was employed to achieve the immobilization of urease in a type of metal–organic frameworks(zeolite imidazolate framework-8, ZIF-8), and the immobilized enzyme urease@ZIF-8 was systematically evaluated for its structure, activity, stability and recyclability, using the hydrolysis of urea as a model. The entrapment of urease was found to be realized in a synchronous manner with the formation of ZIF-8 crystal. The loading of urease in ZIF-8 was measured to be ca. 10.6% through the bicinchoninic acid(BCA) protein assay. The encapsulated urease could efficiently maintain its native conformation, which endowed the immobilized urease with excellent activity and stability, even in harsh conditions(e.g., in the presence of trypsin, acidic or alkali conditions, or at high temperature). Further, urease@ZIF-8 exhibited good recyclability during the degradation of urea, in which it could keep 58.86% of initial activity after being used for 5 cycles. Thus, biomimetic mineralization could be potentially utilized as a promising method to prepare immobilized ureases with superior activity, stability and recyclability, thereby facilitating the construction of efficient catalysts for industrial biocatalysis and biosensing.     

Designed metal–organic framework based on metal–organic polyhedron: Drug delivery

A new metal–organic framework having formula {[NH₂(CH₃)][Cu₆(L)₃(OAc)(H₂O)₄]·xsolvent} (H4L = 3,5-bis(isophthalic acid)-1H-1,2,4-triazole) (1) was synthesized, in which two types of polyhedral cages and one-dimensional channels coexist. The incorporation of the drug 5-fluorouracil (5-FU) into the desolvated 1 was around 24.9 wt% per gram of dehydrated 1. 5-FU is released in a controlled and progressive fashion with 98% of the drug released after 120 h at PBS. The result from this work provides a new avenue for MOF to be used as potential drug delivery.     

Correction to: Recent advances in the design of metal–organic frameworks for methane storage and delivery

Journal of Porous Materials -     

Solvent-induced generation of two cadmium-based metal–organic frameworks from 1, 3, 5-benzenetricarboxylic acid ligand

Two cadmium-based metal–organic frameworks (MOFs), namely [Cd₅(BTC)₄(DMAc)₂(H₂O)₂DMF]·2[H₂N(CH₃)₂]·4DMAc (JUC-133) and [Cd₃(BTC)₂(DMSO)₄] (JUC-134) (JUC = Jilin University, China), based on 1, 3, 5-benzenetricarboxylic acid ligand (H₃BTC) were synthesized by solvent-induced strategy under solvothermal conditions and characterized by single crystal X-ray diffraction, elemental analysis, FT-IR spectroscopy, luminescent spectroscopy, TGA analysis, and powder X-ray diffraction. The ligand is connected to cadmium metals to form different cores with distinctive coordination modes to generate different structures. JUC-133 displays a new three-dimensional (3D) topology with the point symbol (4·6²)₄(4·8²)₄(4²·6⁵·8³)₄(8⁶) based on Cd₂(COO)₅ and Cd₂(COO)₄ as secondary building units (SBUs), which is unknown in previously reported MOFs. JUC-134 can be symbolized as a rutile (rtl) topology with the point symbol (4·6²)₂(4²·6¹⁰·8³) based on Cd₃(COO)₆ as SBUs. Furthermore, the luminescent of the ligand H₃BTC and compounds were measured at room temperature. It is worth noting that the solvent could play an important role in the structural assembly process.