Mixed-metal Metal-organic Frameworks Research Articles

Theoretical investigation of mixed-metal metal-organic frameworks as H2 adsorbents: insights from GCMC and DFT simulations

ABSTRACT Molecular hydrogen ( H 2 ) is a renewable energy carrier, however, its practical applications are limited due to the challenges of developing safe and efficient H 2 storage devices. Metal Organic Frameworks (MOFs) containing at least two different metal ions in their structures are called as mixed-metal MOFs (MM-MOFs) and they could adsorb H 2 in higher amounts compared to structures containing single metal nodes. We theoretically examined the H 2 storage capacities of 26 MM-MOFs having various physical and chemical properties applying Grand Canonical Monte Carlo (GCMC) and Density Functional Theory (DFT) simulations. H 2 adsorption isotherms were calculated using a five-site anisotropic H 2 model. QIXSOG, YOMVIG, OSOYUR, Cu-Mg-BTC, Fe-Mg-BTC, and Cr-Mg-BTC were selected as top-performing MM-MOFs maximising H 2 adsorption gravimetrically and volumetrically at near-ambient conditions (233 K and 100 bar), approaching the DOE targets. YOMVIG has the largest H 2 adsorption enthalpy, calculated as − 9.93 kJ/mol at 233 K and 100 bar. DFT simulations have been conducted to analyse preferable H 2 adsorption sites as well as identify guest-host interactions. Electron density difference analysis showed that adsorbed H 2 molecules in the OSOYUR, Cr-Mg-BTC, Cu-Mg-BTC, and Fe-Mg-BTC are polarised. Our study challenges existing literature by identifying promising MM-MOFs as potential next-generation hydrogen storage adsorbents at near-ambient conditions.

Enhanced Fenton catalytic degradation of methylene blue by the synergistic effect of Fe and Ce in chitosan-supported mixed-metal MOFs (Fe/Ce-BDC@CS)

Methylene blue (MB) is a refractory organic pollutant that poses a potential threat to the aquatic environment. Fenton reaction is considered a primrose strategy to treat MB. However, the traditional Fenton process is plagued by narrow pH application range, poor stability, and secondary pollution. To solve these problems, many Fenton-like catalysts including metal-organic frameworks (MOFs) have been prepared. Herein, a novel bimetallic MOF (Fe/Ce-BDC@CS) was prepared through simple adsorption for the effective removal of MB, where chitosan (CS) was used as the carrier. The degradation performance of Fe/Ce-BDC@CS (100 % within 20 min) was better than that of most reported monometallic MOFs. Moreover, Fe/Ce-BDC@CS exhibited good repeatability and its anti-interference performance of some inorganic ions was also remarkable. Column loading experiments showed that the removal efficiency of MB was still about 50 % over 155 h with a flowing speed of 0.30 L/h. Comparative analysis indicated that such excellent performances could be attributed to the synergistic effect between Fe and Ce. Furthermore, the results of quenching tests indicate that OH, O2−, and 1O2 contributed to MB degradation. In brief, Fe/Ce-BDC@CS has promising prospects in MB treatment, which can provide scientific references for the design and application of bimetallic MOFs.

Post‐Synthetic Modification of Mixed‐Metal UiO‐66 Framework for Rapid Neutralization of Chemical Warfare Agent Simulants

AbstractMetal‐organic frameworks (MOFs) are a promising class of materials for catalysis, offering unique opportunities for designing efficient and selective catalysts. In this study, we have focused on the synthesis and catalytic activity of a novel mixed metal MOF, Ce/Zr−UiO‐66@LiOtBu, designed for the rapid hydrolysis of chemical warfare agents simulants: dimethyl 4‐nitrophenyl phosphate (DMNP) and 2‐chloroethyl ethyl sulfide (CEES). This MOF was thoroughly characterized using powder X‐ray diffraction (PXRD), BET surface area, and inductively coupled plasma optical emission spectrometry (ICP‐OES). The catalytic performance of Ce/Zr−UiO‐66@LiOtBu was monitored in the hydrolysis of DMNP and CEES by using 31P NMR and GC‐MS respectively. Remarkably, the MOF demonstrated rapid and efficient catalytic activity, breaking the P−O bond in DMNP with 2 min and the hydrolysis of CEES was observed in less than 45 min The results suggest the potential application of Ce/Zr−UiO‐66@LiOtBu as a robust catalyst in the detoxification of chemical warfare agent simulants. Our findings highlight the novelty of Ce/Zr−UiO‐66@LiOtBu as a catalytically active MOF, showcasing the successful integration of cerium ions and lithium tert‐butoxide to enhance basicity.

Synthesis, Characterization, and Catalytic Performance of a New Heterobimetallic Y/Tb Metal-Organic Framework with High Catalytic Activity.

A three-dimensional heterobimetallic porous structure with the formula {[Y3.5Tb1.5L6(OH)3(H2O)1.5 (DMF)1.5] n ·1.5H2O·DMF} n (L = 3-amino-4-hydroxybenzoate) (Y/Tb-MOF) has been synthesized and characterized by single crystal and powder X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma mass spectrometry (ICP-MS), electrophoretic mobility, and Fourier transform infrared (FTIR) spectroscopy. The structure presents two metal environments: a bioaugmented isosceles wedge (mm2) MO8 and a tricapped trigonal prism (-6m2) MN3O6. These configurations facilitate the creation of channels with a diameter of 10.7 Å, enabling its utilization as an active catalyst where the heterobimetallic nature of the assembly will be explored. This mixed-metal metal-organic framework has been tested in the cycloaddition of epoxides with carbon dioxide as well as in the cyanosilylation and hydroboration reactions of carbonylic substrates. Additionally, a monometallic Tb-MOF analogue has been synthesized for comparative evaluation of their catalytic performances. Both the mixed metal and monometallic variants exhibit outstanding activity in the cyanosilylation and hydroboration of carbonyls and in the synthesis of carbonates under CO2 pressure. However, only the latter exhibits high recyclability.

Synthesis, spectroscopic study and carbofuran adsorption of mixed metal (Co, Cu)@Ca-BTC frameworks aimed at wastewater cleaning

The extensive use of insecticides raises a great risk to humans because these compounds are human carcinogens and their agricultural application causes contamination of natural free water. One of the most applied insecticide is carbofuran, and the present study aims at removing it from water using novel mixed metal–metal-organic frameworks (M−MOFs) based on Ca-benzene-1,3,5-tricarboxylic acid (Ca-BTC). Different “guest” metal ions (M = Mn, Ni, Co, Cu; each at 20 %) and Ca (at 80 %) were mixed together with the organic linker BTC to produce Ca-M−BTC. Different geometrical shapes of Ca-M−BTC were observed depending on the type of the added “guest” metal ion. The EPR spectrum of both Ca-Cu-BTC and Ca-Co-BTC shows a doublet peak with line-broadening at g// = 2.34 and a broad peak at g ∼ 2.1, respectively, and the narrow peak provided information about the chelating atoms of the paramagnetic Cu2+ atom. Carbofuran adsorption follows the second order model and a Langmuir isotherm. The adsorption capacity of carbofuran reached 627.26 and 736.31 mg/g onto Ca-Co-BTC and Ca-Cu-BTC, respectively. Efficient carbofuran adsorption on these Ca-M−BTCs, hence removal from water, can be proceeded via both of physical deposition in pores and chemical interaction via H-bonding, coordination and π – π interaction.

Conversion of biomass-derived carbohydrates into 5-hydroxymethylfurfural over sulfonated zirconium/tin mixed-metal metal–organic frameworks in deep eutectic solvents-water based biphasic system

5-hydroxymethylfurfural (5-HMF) is a key platform chemical that can be applied to generate varied value-added products. Herein, the sulfonated zirconium/tin Mixed-Metal metal–organic frameworks (MOFs), Sn/UiO-66-SO3H catalysts were employed for catalytic conversion of biomass-derived carbohydrates (glucose, sucrose, maltose, starch, and cellulose) to 5-HMF in green deep eutectic solvents (DESs)-water based biphasic system. The Sn/UiO-66-SO3H catalysts were prepared by infiltrating Sn4+ into the UiO-66-SO3H via post-synthesis modification. The synthesized catalysts were characterized by PXRD, SEM, FTIR, N2 adsorption–desorption analysis, XPS, ICP-OES, NH3-TPD and Py-FTIR. The synergistic effect of Zr4+ and Sn4+ increased the total acidity and pore size of MOFs, thus promoting the catalytic performance of MOFs towards carbohydrates conversion to 5-HMF. The 24-Sn/UiO-66-SO3H with impregnation of Sn4+ for 24 h presented desired catalytic ability, which resulted in 81.5 % and 41.7 % 5-HMF yields from glucose and cellulose in DES betaine-malic acid–water/MIBK biphasic system, respectively. Moreover, the five-recycling test for 24-Sn/UiO-66-SO3H demonstrated excellent reusability of the prepared catalyst. This study offered a novel sulfonated Mixed-Metal strategy on MOFs in DES betaine-malic acid–water based biphasic system for the production of 5-HMF from biomass-derived carbohydrates.

Nickel-Iron-Modified 2D Metal-Organic Framework as a Tunable Precatalyst for Electrochemical Water Oxidation.

Mixed-metal metal-organic framework (MOF)-based water oxidation precatalysts have aroused a great deal of attention due to their remarkable catalytic performance. Yet, despite significant advancement in this field, there is still a need to design new MOF platforms that allow simple and systematic control over the final catalyst's metal composition. Here, we show that a Zr-BTB 2D-MOF could be used to construct a series of Ni-Fe-based oxide hydroxide water oxidation precatalysts with diverse Ni-Fe compositions. In situ Raman spectroscopy characterization revealed that the MOF precatalysts could be electrochemically converted to the active catalysts (NiFeOOH). In turn, it was found that the highest water oxidation activity was obtained with a catalyst containing a 47:53 Ni:Fe molar ratio. Additionally, the obtained catalyst is also active toward electrochemical methanol oxidation, exhibiting high selectivity toward the formation of formic acid. Hence, these results could pave the way for the development of efficient electrocatalytic materials for a variety of oxidative reactions.

Unraveling the molecular mechanism for enhanced gas adsorption in mixed-metal MOFs via solid-state NMR spectroscopy

The incorporation of multiple metal ions in metal-organic frameworks (MOFs) through one-pot synthesis can induce unique properties originating from specific atomic-scale spatial apportionment, but the extraction of this crucial information poses challenges. Herein, nondestructive solid-state NMR spectroscopy was used to discern the atomic-scale metal apportionment in a series of bulk Mg1-xCox-MOF-74 samples via identification and quantification of eight distinct arrangements of Mg/Co ions labeled with a 13C-carboxylate, relative to Co content. Due to the structural characteristics of metal-oxygen chains, the number of metal permutations is infinite for Mg1-xCox-MOF-74, making the resolution of atomic-scale metal apportionment particularly challenging. The results were then employed in density functional theory calculations to unravel the molecular mechanism underlying the macroscopic adsorption properties of several industrially significant gases. It is found that the incorporation of weak adsorption sites (Mg2+ for CO and Co2+ for CO2 adsorption) into the MOF structure counterintuitively boosts the gas adsorption energy on strong sites (Co2+ for CO and Mg2+ for CO2 adsorption). Such effect is significant even for Co2+ remote from Mg2+ in the metal-oxygen chain, resulting in a greater enhancement of CO adsorption across a broad composition range, while the enhancement of CO2 adsorption is restricted to Mg2+ with adjacent Co2+. Dynamic breakthrough measurements unambiguously verified the trend in gas adsorption as a function of metal composition. This research thus illuminates the interplay between atomic-scale structures and macroscopic gas adsorption properties in mixed-metal MOFs and derived materials, paving the way for developing superior functional materials.

Post-synthetic exchange in a zirconium metal–organic framework for efficient photoreduction of CO2 to formate

Post-synthetic modification of a water-stable Zr–NDI MOF to a Zr–Ti mixed-metal MOF results in an enhanced photocatalytic CO2 conversion to formate.

Direct Synthesis of Mixed-Metal Paddle-Wheel Metal-Organic Frameworks with Controlled Metal Ratios under Ambient Conditions.

Currently, synthesizing mixed-metal metal-organic frameworks (MM-MOFs) in a single step remains a challenge due to the varying reactivities of different metal cations. This often results in the formation of mixtures of monometallic MOFs or MM-MOFs with nonstoichiometric metal ratios. A promising approach to overcoming this issue is the controlled precursor method, which uses prebuilt polynuclear complexes with structures similar to the secondary building units (SBUs) of the desired MOFs. In this study, we report that metal acetates can serve as natural prebuilt SBUs, enabling the controlled synthesis of MBDs ([M2(BDC)2DABCO]n, M = metal, BDC = 1,4-benzenedicarboxylic acid, DABCO = 1,4-diazabicyclo[2,2,2]octane) under ambient conditions. By exploiting the fact that metal acetates readily form soluble paddle-wheel dimers similar to the SBUs of MBDs, we achieve the direct synthesis of mixed-metal MBDs at room temperature. The metal ratios (Zn, Co, and Ni) in the resulting MBDs are controllable, and the production yields exceed 90%. The use of metal acetates facilitates the fast and uniform nucleation of MBDs, regardless of the metal cations involved. This similarity in nucleation rates leads to the formation of bimetallic and trimetallic MBDs with predefined metal ratios and homogeneous metal distribution while maintaining the quality of the MOFs. Importantly, this strategy offers an efficient pathway for synthesizing mixed metal MBDs using stoichiometric amounts of metal salts without toxic additives, high energy consumption, and complex synthesis steps.

Z-scheme heterojunction Bi2MoO6/NH2-UiO-66(Zr/Ce) for efficient photocatalytic degradation of oxytetracycline: Pathways and mechanism

In this study, Bi2MoO6/NH2-UiO-66(Zr/Ce) photocatalytic materials with heterojunction structure were synthesized by the hydrothermal method, and their photocatalytic degradation of oxytetracycline was researched. All of the composites exhibited better photocatalytic degradation properties than the pristine component. BMNU-2 showed the optimal photodegradation efficiency of 93.7 % after 150 min illumination, with the kinetic constant 7.22 and 3.37 times higher than that of NH2-UiO-66(Zr/Ce) and Bi2MoO6, respectively. The intermediates and degradation pathways of oxytetracycline were investigated by combining intermediate products testing and theoretical calculations. The ·O2–, h+ and ·OH are all the vital active species, and the degradation mechanism of oxytetracycline have been thoroughly investigated based on energy band theory. The toxicity test was conducted to investigate the environmental effect of OTC and the intermediates. The outstanding photocatalytic efficiency is mainly owing to three reasons: (1) The introduction of Ce in mixed-metal MOFs minished the band gap of the MOFs; (2) The Ce4+/Ce3+ cycle improved the charge separation efficiency; (3) The formation of heterojunction accelerated the charge transfer and expanded the extension of light absorption. This research offers neoteric thinking for the design of high-performance photocatalytic materials.

Encapsulation of Microperoxidase‐8 into MIL‐101(Cr/Fe) Nanoparticles: A New Biocatalyst for the Epoxidation of Styrene

AbstractA new biocatalyst MP8@MIL‐101(Cr/Fe) was prepared by immobilization of a heme octapeptide, Microperoxidase 8 (MP8) within a mixed metal MOF, MIL‐101(Cr/Fe). Both MIL‐101(Cr/Fe) and MP8@MIL‐101(Cr/Fe) were characterized by PXRD, FTIR spectroscopy and TGA. The catalytic activity of MP8@MIL‐101(Cr/Fe) for the oxidation of styrene by H2O2 and tBuOOH was then examined under various reaction conditions (nature of the co‐solvent and of the oxidant, concentration of the oxidant and of the substrate, time, pH) and compared to that of MP8 alone. Under the best conditions used, MP8@MIL‐101(Cr/Fe) was then shown to catalyze the oxidation of styrene about 3 times more efficiently than MP8 alone with approximately 50 % selectivity for styrene oxide.

A review on modified MOFs as CO2 adsorbents using mixed metals and functionalized linkers

The effects of greenhouse gases, like CO2, are becoming more and more visible, from drastic weather changes to global sea level rise. The resultant global warming has become an environmental issue of great concern in recent years. The potential for CO2 capture through metal-organic frameworks has been widely studied and is currently being explored as a way to reduce greenhouse gas emissions. The strong structural and electronic properties of these frameworks make them excellent candidates for capturing CO2 due to their high porosity, tunable composition and good chemical stability. Functionalized metal organic frameworks (MOFs) are important because it allows for the development of MOF materials with properties that are tunable for many different applications. Many of these functions are gas adsorption, catalysis, and separation. Depending on the composition of the linkers and nodes, various functional groups can be introduced into the network through the organic linkers and metal nodes, resulting in MOFs with different functions. Mixed-metal MOFs are composed of several different metals. The variety of metals in a mixed-metal MOF gives the MOF many options for tailoring its properties. The resulting bimetallic MOFs are not only more thermally and chemically stable, but also can absorb more gases. The other objective of this study was to observe the effect of organic functional groups on CO2 adsorption in MOFs. The study found that organic functional groups have a significant effect on CO2 adsorption in MOFs.

Investigating the effectiveness of bifacial mixed metal MOF electrodes for the photoelectro-catalytic treatment of municipal wastewater

The effectiveness of bifacial mixed metal-metal organic framework (MOF) anodes for municipal wastewater's photoelectro-catalytic (PEC) treatment has been rarely explored. Herein, pollutants removal efficiency from the municipal wastewater and the effect of operational parameters is verified under a visible range of light in a PEC system using TiO2–Bi(mixed metal)-MOF and TiO2–Sb(mixed metal)-MOF anodes, respectively. The comparable efficiency of pollutant removal from municipal wastewater in the PEC system with TiO2–Bi(mixed metal)-MOF anodes is high. At optimum operation parameters, TiO2–Bi(mixed metal)-MOF anodes in the PEC system play a positive role in the removal of micropollutants and the disinfection of municipal wastewater. This study reveals that PEC treatment of municipal wastewater contributes (35% of relative luminescence inhibition) to toxic biproducts.

Recent Advances on Multivariate MOFs for Photocatalytic CO2 Reduction and H2 Evolution (Adv. Sustainable Syst. 1/2023)

Solar Energy Conversion MTV-MOF-based photocatalyst is one of the significantly candidates in achieving solar-energy conversion. In article number 2200394, Falu Hu, Guowei Zhou and coworkers review the recent development of MTV-MOF-based photocatalysts, classed into mixed-metal MOF, mixed-ligand MOF, and mixed-metal and mixed-ligand MOF, which are used to achieve and regulate photocatalytic CO2 reduction or H2 evolution by introduction of multiple metals or different functional ligands.

Exploring cation distribution in ion-exchanged Al,Ga-containing metal-organic frameworks using 17O NMR spectroscopy.

A mixed-metal metal-organic framework, (Al,Ga)-MIL-53, synthesised by post-synthetic ion exchange has been investigated using solid-state nuclear magnetic resonance (NMR) spectroscopy, microscopy and energy dispersive X-ray (EDX) spectroscopy. 17O enrichment during the ion-exchange process enables site specific information on the metal distribution to be obtained. Within this work two ion-exchange processes have been explored. In the first approach (exchange of metals in the framework with dissolved metal salts), 17O NMR spectroscopy reveals the formation of crystallites with a core-shell structure, where the cation exchange takes place on the surface of these materials forming a shell with a roughly equal ratio of Al3+ and Ga3+. For the second approach (exchange of metals between two frameworks), no core-shell structure is observed, and instead crystallites containing a majority of Al3+ are obtained with lower levels of Ga3+. Noticeably, these particles show little variation in the metal cation distribution between crystallites, a result not previously observed for bulk (Al,Ga)-MIL-53 materials. In all cases where ion exchange has taken place NMR spectroscopy reveals a slight preference for clustering of like cations.

Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions.

Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.

Guest Editorial: Solar Energy Utilization

Guest Editorial: Solar Energy Utilization

Deconvolution of metal apportionment in bulk metal-organic frameworks.

We report a general route to decipher the apportionment of metal ions in bulk metal-organic frameworks (MOFs) by solid-state nuclear magnetic resonance spectroscopy. We demonstrate this route in Mg1-xNix-MOF-74, where we uncover all eight possible atomic-scale Mg/Ni arrangements through identification and quantification of the distinct chemical environments of 13C-labeled carboxylates as a function of the Ni content. Here, we use magnetic susceptibility, bond pathway, and density functional theory calculations to identify local metal bonding configurations. The results refute the notion of random apportionment from solution synthesis; rather, we reveal that only two of eight Mg/Ni arrangements are preferred in the Ni-incorporated MOFs. These preferred structural arrangements manifest themselves in macroscopic adsorption phenomena as illustrated by CO/CO2 breakthrough curves. We envision that this nondestructive methodology can be further applied to analyze bulk assembly of other mixed-metal MOFs, greatly extending the knowledge on structure-property relationships of MOFs and their derived materials.

Conversion of glucose into 5-hydroxymethylfurfural catalyzed by Cr-and Fe-containing mixed-metal metal–organic frameworks

Catalytic production of 5-hydroxymethylfurfural (5-HMF) from glucose is a promising routine for generation of varied value-added chemicals. Herein, Mixed-Metal metal–organic frameworks (MOFs)–Fe/MIL-101(Cr) and MIL-101(Cr, Fe) obtained from post-synthesis modification and one-pot synthesis approach respectively were applied for catalytic converting glucose to 5-HMF. The synthesized Mixed-Metal MOFs were characterized by PXRD, SEM, N2 adsorption–desorption analysis, TGA, XPS and NH3-TPD. Cr-and Fe-containing Mixed-Metal MOFs Fe/MIL-101(Cr) and MIL-101(Cr, Fe) presented synergistic effect with enhanced catalytic ability for glucose transformation to 5-HMF compared with typical single-metal MOFs and sulfonic acid-functionalized MOFs. Moreover, these two Mixed-Metal MOFs also presented better thermal stability than single-metal MOFs like MIL-101(Cr). Fe/MIL-101(Cr) was found to possess higher catalytic performance than MIL-101(Cr, Fe). 55.8 % 5-HMF yield was obtained with Fe/MIL-101(Cr) at 170 °C for 2 h in DMSO/water system. In addition, Fe/MIL-101(Cr) demonstrated desired recyclability for five times without incredible loss in its activity. This study provides a Mixed-Metal strategy on MOFs for improving catalytic efficiency of glucose conversion to 5-HMF.