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

Journal of Porous Materials -     

Improving CH4 uptake and CH4/N2 separation in pillar-layered metal–organic frameworks using a regulating strategy of interlayer channels

A pore engineering strategy involving the regulation of pillars in a series of pillar-layered metal–organic frameworks was presented to promote pore size adjustment and pore environment optimization for efficient separation of CH₄ from coal-mine methane. Compared with the original Ni(BTC)(BPY) and Ni(BTC)(TED), the suitable pore size and rich supply of carboxylate oxygens in the newly constructed Ni(BTC)(PIZ) resulted in the strongest recognition of CH₄, leading to both high CH₄ uptake (1.62 mmol/g) and CH₄/N₂ (50/50) selectivity (7.32) at 298 K and 1 bar. Its optimal balance performance between uptake and selectivity was better than most reported adsorbents, which further led to the most efficient separation of CH₄ from CH₄/N₂ mixtures. The stability of the structure and performance was verified by repeated cycle tests. Overall, it is believed the structure constructed based on the regulation strategy of the interlayer channel would enable the application of promising metal–organic frameworks in industrial gas separation processes.     

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.     

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).     

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.     

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.     

Phenanthroline modulated self-assembly of nano/micro-scaled metal–organic frameworks

In stark contrast to the report that phenanthroline is an efficient chelate ligand for lanthanide-based metal–organic framework formation, here it is found that phenanthroline can act as a very useful regent to modulate self-assembly of nano/micro-sized yttrium benzenetricarboxylate (Y–BTC). 1D nanobelts of Y–BTC was firstly produced via a simple, mild one-step approach. Then the morphology was transformed to 3D urchin-like hierarchical architectures with the assistance of phenanthroline. Interestingly, the as-synthesized 3D urchin-like superstructures can be disassembled through simple ultrasonic treatment. Thus a large quantity of 1D nanorods was obtained. Additionally, brilliant-red light of the nanomaterials was realized by doping Eu^3 + ions.     

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.     

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.     

A computational analysis of a ZrO2–SiO2 scale for a ZrB2–ZrC–Zr ultrahigh temperature ceramic composite system

The success of a ceramic composite for ultrahigh temperatures (i.e., >1873 K) in an oxidizing atmosphere resides in the protective characteristics of a scale to limit oxygen ingress or to control the oxygen reaction into the substrate. With temperature changes from room temperature to ultrahigh temperatures, the mechanics of the scale and its reactivity becomes critical for ceramic composites to operate under extreme environments. A study was pursued to design computationally a SiO₂–ZrO₂ scale for a ZrB₂/ZrC/Zr–Si composite by using conventional finite element analysis, which was used as a baseline microstructure for the extended finite element method. The model of the Zr boride/carbide composite with a SiO₂/ZrO₂/ZrSi_x scale simulates the development of local strain energetics under a thermal load from 300 to 1700 K. The computational analysis determined that the size of the SiO₂ and ZrSi_x precipitates does not appreciably influence the durability of the microstructure. A simulated annealing optimization algorithm was also developed for an extended finite element program (called XMicro) with the purpose of optimizing the auto re-meshing of XMicro and thus minimizing its combinatorial selection of a composite's reinforcement architecture. After correcting for the overlapping of ZrO₂ precipitates within a matrix, XMicro determined that 1.96 μm as the optimal spacing of precipitates within a cluster and 20 μm between clusters within a silica matrix of the scale interphase. The strategic experimentation determined that porosity developed during oxidation should be incorporated into the simulation of a ceramic composite. To probe into the efficacy of the silica layer for the scale, oxidizing experiments were performed at 1973 K, as well as microstructural analysis of the scale interphase. The computational mechanics coupled with consideration of the thermodynamic stability of phases for the Zr–Si–O system to set the oxygen potentials between layers can design a scale interphase for an ultrahigh-temperature, ceramic composite system. The processing challenge would be to attain the optimal configuration of the microstructure, for example, silicide precipitates developed with the appropriate spacing along a scale/matrix interface or ZrO₂ clusters within a silicate phase.     

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     

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.     

Active sites modulation with Runge–Gross theorem: CO2 capture by porphyrinic metal–organic frameworks at excited states

Traditionally chemical modifications altering molecular skeletons (MSs) were the only solution to modulate material active sites at ground states. According to Runge–Gross theorem, the MS and the adjoint electron-configuration (MS-AEC) can be tuned at excited states (ESs), even without chemical modifications. A porphyrinic metal–organic framework PCN-222 and its metalloporphyrin homologs are used for adsorptive carbon capture both at ground states and with photoexcitation (350–780 nm). Instead of passive photothermal effects, the carbon capture performances of all the adsorbents get promotions. The dominant first ESs with long lifetimes meet the time-scale of molecular adsorption equilibrium, meanwhile tune the MS-AEC of the porphyrin ligands to generate new active sites with much more negative electrostatic-potentials, of which the distribution gradient is crucial for inducing CO₂ and can be further modulated by the central-coordinated metal cations at ground and excited states. This work demonstrates the availability of static ESs and possibility of nonchemical modifications.     

F3-functionalized nanoscale metal–organic frameworks for tumor-targeting combined chemotherapy and chemodynamic therapy

Nanoscale metal–organic frameworks (nMOFs) have attracted much attention as emerging porous materials as drug delivery carriers. Appropriate surface modification of them can greatly improve stability and introduce biocompatibility and cancer targeting functionality into drug delivery systems. Herein, we prepared nano-sized MIL-101(Fe)-N₃ and loaded anticancer drug doxorubicin (DOX) into it. The synthetic polymer layer Alkyne-PLA-PEG was then attached to the F3 peptide (labeled as Alkyne-PLA-PEG-F3), and the surface of DOX/MIL-101(Fe)-N₃ was covalently modified with it to obtain DOX/MIL-101-PLA-PEG-F3. Nano-sized MIL-101(Fe)-N₃ has high drug loading capacity and the modification of MIL-101(Fe)-N₃ by polymer Alkyne-PLA-PEG not only improved the dispersion, but also avoided the sudden release of the drugs and increased the biocompatibility of nanocarriers. The F3 peptide introduced into the nanocarriers also enabled it to specifically target tumor tissues and achieved active targeted drug delivery. As a nucleolin-mediated endocytosis drug delivery system, DOX/MIL-101-PLA-PEG-F3 can not only deliver anticancer drugs to tumors accurately, but also participate in Fenton-like reaction to generate hydroxyl radicals (•OH) for chemodynamic therapy (CDT), thus enabling combination therapy. It holds great promise as drug candidates to reduce systemic toxicity and improve the efficacy of cancer treatment.     

Functionalized metal–organic frameworks with strong acidity and hydrophobicity as an efficient catalyst for the production of 5-hydroxymethylfurfural

In the dehydration of fructose to 5-hydroxymethyl furfural(HMF), in situ produced water weakens the acid strength of the catalyst and causes the rehydration of HMF, causing unsatisfactory catalytic activity and selectivity. In this work, a class of benzenesulfonic acid-grafted metal–organic frameworks with strong acidity and hydrophobicity is obtained by the direct sulfonation method using 4-chlorobenzenesulfonic acid as sulfonating agent. The resultant MOFs have a specific surface area of greater than 250 m~2·g~(-1), acid density above 1.0 mmol·g~(-1), and water contact angle up to 129°. The hydrophobic MOF-Ph SO_3 H exhibits both higher catalytic activity and selectivity than MOF-SO_3 H in the HMF synthesis due to its better hydrophobicity and olephilicity. Moreover, the catalyst has a high recycled stability. At last, fructose is completely converted, and 98.0% yield of HMF is obtained under 120 °C in a DMSO solvent system. The successful preparation of the hydrophobic acidic MOF provides a novel hydrophobic catalyst for the synthesis of HMF.     

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.     

Synthesis and characterization of magnesium–carbon compounds for hydrogen storage

Magnesium (Mg) and carbon (C) compounds were synthesized by ball-milling a mixture of Mg and different graphites with different crystallinities. The materials were characterized by X-ray diffraction, X-ray absorption spectroscopy, and X-ray total scattering techniques. Hydrogen storage properties were also investigated. In the case of the material using low-crystalline graphite, a Mg and C compound was formed as main phase, and its chemical bonding state was similar to that of magnesium carbide (Mg₂C₃). The hydrogen absorption reaction of the Mg–C compound occurred at around 400 °C under 3 MPa of hydrogen pressure to form magnesium hydride (MgH₂) and the C–H bonds in the carbon material. The hydrogenated Mg–C material desorbed about 3.7 mass% of hydrogen below 420 °C with two processes, which were the decomposition of MgH₂ and the subsequent reaction of the generated Mg and the C–H bonds. From the results, it is concluded that the Mg–C compound absorb and desorb about 3.7 mass% of hydrogen below 420 °C.     

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.