Mixed-metal Metal-organic Frameworks Research Articles

Upgrading the Hydrogen Storage of MOF-5 by Post-Synthetic Exchange with Divalent Metal Ions

In metal-organic frameworks (MOFs), mixed-metal clusters have the opportunity to adsorb hydrogen molecules due to a greater charge density of the metal. Such interactions may subsequently enhance the gravimetric uptake of hydrogen. However, only a few papers have explored the ability of mixed-metal MOFs to increase hydrogen uptake. The present work reveals the preparation of mixed metal metal-organic frameworks M-MOF-5 (where M = Ni2+, Co2+, and Fe2+) (where MOF-5 designates MOFs such as Zn2+ and 1,4-benzenedicarboxylic acid ligand) using the post-synthetic exchange (PSE) technique. Powder X-ray diffraction patterns and scanning electron microscopy images indicate the presence of crystalline phases after metal exchange, and the inductively coupled plasma–mass spectroscopy analysis confirmed the exchange of metals by means of the PSE technique. The nitrogen adsorption isotherms established the production of microporous M-MOF-5. Although the additional metal ions decreased the surface area, the exchanged materials displayed unique features in the gravimetric uptake of hydrogen. The parent MOF-5 and the metal exchanged materials (Ni-MOF-5, Co-MOF-5, and Fe-MOF-5) demonstrated hydrogen capacities of 1.46, 1.53, 1.53, and 0.99 wt.%, respectively. The metal-exchanged Ni-MOF-5 and Co-MOF-5 revealed slightly higher H2 uptake in comparison with MOF-5; however, the Fe-MOF-5 showed a decrease in uptake due to partial discrete complex formation (discrete complexes with one or more metal ions) with less crystalline nature. The Sips model was found to be excellent in describing the H2 adsorption isotherms with a correlation coefficient ≅ 1. The unique hydrogen uptakes of Ni− and Co-MOF-5 shown in this study pave the way for further improvement in hydrogen uptake.

Enhancement of photocatalytic aerobic oxidation of benzyl alcohol with the incorporation of cobalt in Zn-based MOF via post-synthetic metal exchange

In this work to introduce Co(II) ions into TMU-22(Zn) metal–organic framework (MOF), we report a postsynthetic exchange (PSE) approach. Several analytical methods were employed to examine the exchange between Co2+ ions and Zn2+ ions in TMU-22(Zn) for the formation of the mixed-metal TMU-22(Zn/Co). It was found that the introduction of active Co2+ ions into the TMU-22(Zn) framework leads to the photocatalytic activity of TMU-22(Zn/Co) for benzyl alcohol oxidation. Since Co2+ ions were favorably substituted at positions more accessible to the substrate, the TMU-22(Zn/Co) showed superior catalytic performance. This study provides a facile method to develop mixed-metal MOFs for photocatalysis performance.

A direct mechanochemical conversion of Pt-doped metal-organic framework-74 from doped metal oxides for CO oxidation

Mixed-metal metal-organic frameworks (MM-MOFs) contain more than one type of metal node, which can improve or even introduce new features compared with the single metal frameworks. Traditionally, most of the MM-MOFs are synthesized by solution method which requires all the metal ions to form stable coordination with the ligand in a single framework and therefore limits the choice of the metal ions. In this work, Pt-doped Zn-MOF-74 (PtZn-MOF-74) has been prepared through a mechanochemical conversion approach from Pt-doped ZnO (Pt–ZnO) directly. The absence of a large quantity of solvent limits the solvation and diffusion of growth species during the mechanochemical conversion and therefore prevented the agglomeration of Pt dopants in PtZn-MOF-74. CO oxidation was used to evaluate the catalytic performance of the prepared MM-MOFs. Compared with inert Zn-MOF-74, PtZn-MOF-74 with unsaturated coordinated Pt at elevated temperature can act as the active sites to catalyze CO oxidation reaction. This study provides a novel design strategy for the synthesis of MM-MOFs with extended functionalities.

SYNTHESIS OF MIXED-METAL MIL(Ti-Fe) FROM VIETNAM ILMENITE ORE AND ITS APPLICATION FOR DEGRADATION OF DYE

In this work, the mixed-metal metal-organic frameworks MIL(Ti-Fe) were synthesized by the hydrothermal method. MIL(Ti-Fe) hybrid material was fabricated from ilmenite ore and 1,3,5-benzene tricarboxylic acid at a temperature of 130 oC for 24 hours. The prepared material was characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared spectroscopy (IR), and Brunauer-Emmett-Teller (BET) surface area. The obtained MIL(Ti-Fe) particles have a diameter of from 0.2-1.0 µm with a BET surface area of 85.482 m2 g-1. The influence of various vital parameters such as pH of the dye solution, initial dye concentration, adsorption time, and amount of the catalyst on the dye removal efficiency was investigated. The photocatalytic degradation rate of Rhodamine B was found to be 0.0074 min-1 at pH 7. The maximum removal of Rhodamine B was obtained at the catalyst dose of 1.0 g L-1. Under simulated sunlight irradiation, the resultant mixed-metals MOFs showed high photodegradation toward Rhodamine with degradation efficiency of approximately 99,97% after 6 hours. Furthermore, the resultant materials also showed remarkable absorption behavior toward Rhodamine B with the adsorption capacity of 70 mg g-1.

Mixed-metal MOFs as efficient catalysts for transfer hydrogenation of furfural, levulinic acid and other carbonyl compounds

Crystalline, porous Fe-Ni mixed-metal metal-organic frameworks (MOFs), namely MIL-88B(Fe2Ni) and NH2-MIL-88B(Fe2Ni), were synthesized and characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, atomic absorption spectrophotometry (AAS) and nitrogen physisorption measurements. The MOFs were then employed as catalysts for transfer hydrogenation (TH) of biomass derivatives such as furfural and levulinic acid, and various other carbonyl compounds using 2-propanol as a sacrificial hydrogen donor. These heterogeneous catalysts are deemed to be environmentally benign, efficient and recyclable. NH2-MIL-88B(Fe2Ni) showcased good catalytic activity with low catalyst loading in short reaction time. The catalysts were recovered and reused several times without significant degradation in catalytic activity. A mechanism for the TH reaction was also proposed. As far as we are aware, this is the first report on the synthesis of furfuryl alcohol and γ-valerolactone (GVL) via TH reaction in just 60 min using a MOF catalyst.

Hydrogen Evolving Activity of Dithiolene-Based Metal-Organic Frameworks with Mixed Cobalt and Iron Centers.

Electrocatalytic systems based on metal-organic frameworks (MOFs) have attracted great attention due to their potential application in commercially viable renewable energy-converting devices. We have recently shown that the cobalt 2,3,6,7,10,11-triphenylenehexathiolate (CoTHT) framework can catalyze the hydrogen evolution reaction (HER) in fully aqueous media with Tafel slopes as low as 71 mV/dec and near-unity Faradaic efficiency (FE). Taking advantage of the high synthetic tunability of MOFs, here, we synthesize a series of iron and mixed iron/cobalt THT-based MOFs. The incorporation of the iron and cobalt dithiolene moieties is verified by various spectroscopic techniques, and the integrity of the crystalline structure is maintained regardless of the stoichiometries of the two metals. The hydrogen evolving activity of the materials was explored in pH 1.3 aqueous electrolyte solutions. Unlike CoTHT, the FeTHT framework exhibits minimal activity due to a late catalytic onset [-0.440 V versus reversible hydrogen electrode (RHE)] and a large Tafel slope (210 mV/dec). The performance of the mixed-metal MOFs is adversely affected by the incorporation of Fe, where increasing Fe content results in MOFs with lower HER activity and diminished long-term stability and FE for H2 production. It is proposed that the FeTHT domains undergo alternative Faradaic processes under catalytic conditions, which alter its local structure and electrochemical behavior, eventually resulting in a material with diminished HER performance.

High energy density supercapacitor electrode materials based on mixed metal MOF and its derived C@bimetal hydroxide embedded onto porous support

In the recent years, metal-organic frameworks (MOFs) and their derived mixed metal hydroxides/oxides structures have emerged as an engrossing category of functional materials with some unique properties such as high porosity and high specific surface area for energy storage applications. Here, a cathodic electrodeposition strategy was utilized to prepare a bimetallic NiCo metal-organic framework (NiCo-MOF) with flower-like morphology consisting of nanopetals onto Ni foam (NF) as free-standing electrode. Structural characterization revealed that Ni2+ and Co2+ metal ions are uniformly distributed on the deposited films on the nickel foam. Post-chemical treatment of NiCo-MOF/Ni foam electrode under basic condition (i.e. 4 M KOH) was concluded carbon coated hierarchical mixed hydroxide (i.e. C@Ni1-xCox(OH)2) structures onto Ni-foam with similar morphology as its pristine binary MOF. Both ready-to-use fabricated electrodes were characterized with various techniques of X-ray diffraction (XRD), Fourier-transform infrared (FT-IR), Field-emission scanning electron microscope (FE-SEM), Energy dispersive X-ray analysis (EDAX), Energy dispersive spectroscopy (EDS) mapping and Thermogravimetric-differential scanning calorimetry (TG-DSC). Supercapacitive behaviors of pristine NiCo-MOF/NF and its derived C@NiCo-hydroxide/NF electrodes were also measured using cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectroscopy (EIS) measurements in 2 M KOH as electrolyte. The C@NiCo-hydroxide/NF electrode exhibited better performance as compared to pristine sacrificial binary metallic MOF electrode, which was 1825 F g–1 at a discharge current density of 1 A g–1 which still preserved 66.5% of its initial capacitance even at a high-rate load of 15 A g–1. Additionally, the C@NiCo-hydroxide/NF showed 87.6% of its capacitance at the end of 8000th cycle.

Mixed Metal-Metal Organic Frameworks (MM-MOFs) and Their Use as Efficient Photocatalysts for Hydrogen Evolution from Water Splitting Reactions

Mixed Metal-Metal Organic Frameworks (MM-MOFs) and Their Use as Efficient Photocatalysts for Hydrogen Evolution from Water Splitting Reactions

A direct solvent-free conversion approach to prepare mixed-metal metal-organic frameworks from doped metal oxides.

We propose a novel strategy to introduce platinum into the metal nodes of ZIF-8 by preloading Pt as a dopant in ZnO (Pt-ZnO) and then convert it to Pt doped ZIF-8 (Pt-ZIF-8) through a chemical vapor deposition (CVD) approach. The solvent-free conversion of Pt-ZnO to Pt-ZIF-8 allows the Pt dopant in ZnO to coordinate with organic linkers directly without the formation of Pt nanoparticles, which is a general issue of many methods. This general synthesis strategy may facilitate the discovery of MMOFs that have not been reported previously.

Pre-programed hydroxy double salt templates for room-temperature controlled synthesis of mixed-metal zeolitic imidazolate frameworks

Bimetallic hydroxy double salts pre-programed through co-precipitation were found as excellent templates for mixed-metal MOFs.

Mixed-Metal Cerium/Zirconium MOFs with Improved Nerve Agent Detoxification Properties.

A series of Ce/Zr mixed-metal-organic frameworks with different topology/connectivity, namely, Ce/Zr-UiO-66 (U01, U02, and U03) (fcu (12-c)), Ce/Zr-DUT-67-PZDC (D01 and D02) (reo (8-c)), and Ce/Zr-MOF-808 (M01, M02, and M03) (spn (6-c)) were evaluated toward the detoxification of toxic nerve agent model diisopropylfluorophosphate (DIFP) at room temperature in unbuffered aqueous solution. Noteworthily, the catalytic rate for P-F bond cleavage increased with increasing Ce/Zr molar ratio. A further increase in catalytic activity can be achieved by Mg(OMe)2 doping of the mixed-metal MOFs as exemplified with M01@Mg(OMe)2 and M02@Mg(OMe)2 systems. The results show that Mg(OMe)2 incorporation into the mesoporous cavities of M01 and M02 give rise to P-F hydrolytic degradation half-lives of nearly 5 and 2 min with 100% degradation of DIFP after 55 and 65 min for M01@Mg(OMe)2 1:2 and M02@Mg(OMe)2 1:4, respectively.

Mixed-Metal Cu-BTC Metal-Organic Frameworks as a Strong Adsorbent for Molecular Hydrogen at Low Temperatures.

The advancement of hydrogen and fuel cell technologies hinges on the development of hydrogen storage methods. Metal–organic frameworks (MOFs) are one of the most favorable materials for hydrogen storage. In this study, we synthesized a series of isostructural mixed-metal metal–organic frameworks (MM-MOFs) of 1,3,5-benzenetricarboxylate (BTC), M-Cu-BTC, where M = Zn2+, Ni2+, Co2+, and Fe2+ using the post-synthetic exchange (PSE) method with metal ions. The powder X-ray diffraction patterns of MM-MOFs were similar with those of single-metal Cu-BTC. Scanning electron microscopy indicates the absence of amorphous phases. Inductively coupled plasma mass spectroscopy of the MM-MOFs shows successful metal exchanges using the PSE method. The N2 adsorption measurements confirmed the successful synthesis of porous MM-MOFs. The metal exchanged materials Ni-Cu-BTC, Zn-Cu-BTC, Fe-Cu-BTC, and Co-Cu-BTC were studied for hydrogen storage and showed a gravimetric uptake of 1.6, 1.63, 1.63, and 1.12 wt %; respectively. The increase in hydrogen adsorption capacity for the three metal exchanged materials is about 60% relative to that of the parent MOF (Cu-BTC). The improvement of gravimetric uptake in M-Cu-BTC (where M = Ni2+, Zn2+, and Fe2+) is probably due to the increase in binding enthalpy of H2 with the unsaturated metal sites after the partial exchange from Cu2+ to other metal ions. The higher charge density of metal ions strongly polarizes hydrogen and provides the primary binding sites inside the pores of Cu-BTC and subsequently enhances the gravimetric uptake of hydrogen.

MOF-derived NiO/CeO2 heterojunction: a photocatalyst for degrading pollutants and hydrogen evolution

A NiO/CeO2 p–n heterojunction with the hierarchically porous microspheres was fabricated by annealing the precursor of the nickel/cerium mixed-metal metal–organic frameworks. The morphologies, microstructures, composition, specific surface area, photogenerated electron–hole pair separation/transfer, and photocatalytic activities were characterized for all acquired catalysts. The photodegradation rates of organic dyes including methyl orange and methylene blue were investigated under ultraviolet light. The NiO/CeO2 heterojunctions showed a much higher photocatalytic performance than that of pure NiO and CeO2. Among all photocatalysts NiO/CeO2-3 presented the best photocatalytic activity, i.e., the degradation of MO was 2.6 times higher than pure NiO under ultraviolet light, and the hydrogen generation with the rate of about 29.6 μmol h−1 g−1. The improved photocatalytic performance could be ascribed to the formation of the NiO/CeO2 p–n heterojunction, which is beneficial for effective separation and fast transfer of the photoexcited electron–hole pairs.

Tailoring the breathing behavior of functionalized MIL-53(Al,M)-NH2 materials by using the mixed-metal concept

Abstract Mixed-metal metal-organic frameworks with two or more metals distributed over crystallographically equivalent sites are gaining growing attention in various research areas. We have extended the range of available mixed-metal frameworks of the MIL-53-NH2 family by combining aluminum with scandium, vanadium, chromium or iron. In contrast to other already reported mixed-metal MIL-53 materials, the developed MIL-53(Al,M)-NH2 materials (M = Sc, V, Cr, Fe), which contain 2-aminoterephthalate as linker instead of terephthalate, will enable subsequent post-synthetic modifications. All developed materials have been thoroughly characterized with standard methods as well as X-ray absorption spectroscopy to prove the exclusive incorporation of the employed metals into the MIL-53-NH2 framework structure and to verify the absence of impurity phases. Nitrogen physisorption measurements revealed that the choice of the metal combinations and ratios had a strong influence on the breathing behavior. Furthermore, post-synthetic modification reactions of the amine groups with maleic anhydride showed that the metal combination had a dramatic impact on the resulting modification degrees. Thus, the flexibility of the pore systems can be tailored by a careful selection of suitable metal combinations for these materials.

Experimental Evidence for the Incorporation of Two Metals at Equivalent Lattice Positions in Mixed-Metal Metal-Organic Frameworks.

Metal–organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed‐metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed‐metal metal–organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed‐metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X‐ray diffraction, X‐ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed‐metal FeCuBTC material with HKUST‐1 structure, which contained bimetallic Fe−Cu paddlewheels as well as monometallic Cu−Cu and Fe−Fe units under optimized synthesis conditions. The in‐depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed‐metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed‐metal metal–organic framework materials and the thorough characterization thereof is particularly important to derive structure–property or structure–activity correlations.

Heterometallic Titanium-Organic Frameworks as Dual Metal Catalysts for Synergistic Non-Buffered Hydrolysis of Nerve Agent Simulants

Heterometallic or mixed-metal Metal-Organic Frameworks (MOFs), incorporating two or more metal ions to the inorganic node of the frameworks, are increasingly gaining importance as a route to produce materials with increasing chemical and functional complexity. Heterometallic MOFs can offer important advantages over their homometallic counterparts to enable targeted modification of the adsorption properties, structural response, electronic structure or chemical reactivity of the framework. This field is still in its infancy likely due to the difficulties of controlling the formation of heterometallic nodes by direct synthesis. This restriction is even more acute in the case of titanium frameworks for which their challenging chemistry renders post-synthetic doping of preformed materials as the only route available. However, this often results in partial or non-homogeneous metal substitution in detriment of the potential benefits of controlling metal distribution at an atomic level toward performance improvement. We report the first family of heterometallic titanium frameworks that can be prepared by direct synthesis from metal precursors and trimesic acid. MUV-101 frameworks [TiM2(µ3-O)(O2CR)6X3] (M = Mg, Fe, Co, Ni; X = H2O, OH-, O2-) combine mesoporosity with good chemical stability. We use these materials to exemplify the advantages of controlling metal distribution across the framework in heterogeneous catalysis by exploring their activity toward the degradation of a nerve agent simulant of Sarin gas. MUV-101(Fe) is the only pristine MOF capable of catalytic degradation of (diisopropyl-fluorophosphate) DIFP in non-buffered aqueous media without the presence of a basic/nucleophilic co-catalyst. Compared to MUV-101(Fe), other titanium heterometallic and homometallic MOFs as MUV-101(Mg, Co and Ni), MUV-10(Mn), MIL-100(Ti and Fe) or UiO-66(Zr), all display a poorer performance or are poisoned by the degradation products. The catalytic activity of MUV-101(Fe) cannot be explained only by the association of Ti(IV) and Fe(III) but to their synergistic cooperation. Our simulations suggest that the combination of Ti(IV) Lewis acid and Fe(III)-OH Bronsted base sites in this dual metal catalyst leads to a much lower energy barrier for more efficient degradation of DIFP in absence of a base. Overall, this mechanism resembles the activity of the metalloenzyme purple acid phosphatase that displays also bimetallic active sites.

Recent highlights and future prospects on mixed-metal MOFs as emerging supercapacitor candidates.

Mixed-metal metal-organic frameworks (M-MOFs) consist of at least two different metal ions as nodes in the same framework. The incorporation of a second or more metal ions provides structural/compositional diversity, multi-functionality and stability to the framework. Moreover, the periodical array of different metal ions in the framework may alter the physical/chemical properties of M-MOFs and result in fascinating applications. M-MOFs with exciting structural features offer superior supercapacitor performances compared to single metal MOFs due to the synergic effect of different metal ions. In this review, we summarize several synthetic methods to construct M-MOFs by employing various organic ligands or metalloligands. Further, we discuss the electrochemical performance of several M-MOFs and their derived composite materials for supercapacitor applications.

Solvent-free mechanochemical route for the construction of ionic liquid and mixed-metal MOF composites for synergistic CO2 fixation

IL and metal ions were introduced into a MOF by a grinding approach and the resulting IL@ZIF-8(Zn/Co) exhibits synergistic effects for catalysis.

Metal–Organic-Framework-Derived Co/Cu–Carbon Nanoparticle Catalysts for Furfural Hydrogenation

The transformation of Co, Cu, and mixed Co/Cu MOF-74 crystals into bimetallic, carbon-supported Co–Cu catalysts is investigated via high-temperature pyrolysis. Mixed-metal MOFs prepared via a one-s...

Mixed-Metal MOFs: Unique Opportunities in Metal-Organic Framework (MOF) Functionality and Design.

Mixed-metal metal-organic frameworks (MM-MOFs) can be considered to be those MOFs having two different metals anywhere in the structure. Herein we summarize the various strategies for the preparation of MM-MOFs and some of their applications in adsorption, gas separation, and catalysis. It is shown that compared to homometallic MOFs, MM-MOFs bring about the opportunity to take advantage of the complexity and the synergism derived from the presence of different metal ions in the structure of MOFs. This is reflected in a superior performance and even stability of MM-MOFs respect to related single-metal MOFs. Emphasis is made on the use of MM-MOFs as catalysts for tandem reactions.