Mixed-metal Metal-organic Frameworks Research Articles

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.

One-Pot Synthesis of Heterobimetallic Metal-Organic Frameworks (MOFs) for Multifunctional Catalysis.

A one-pot synthesis of bimetallic metal-organic frameworks (Co/Fe-MOFs) was achieved by treating stoichiometric amounts of Fe and Co salts with 2-aminoterephthalic acid (NH2 -BDC). Monometallic Fe (catalyst A) and Co (catalyst F) were also prepared along with mixed-metal Fe/Co catalysts (B-E) by changing the Fe/Co ratio. For mixed-metal catalysts (B-E) SEM energy-dispersive X-ray (EDX) analysis confirmed the incorporation of both Fe and Co in the catalysts. However, a spindle-shaped morphology, typically known for the Fe-MIL-88B structure and confirmed by PXRD analysis, was only observed for catalysts A-D. To test the catalytic potential of mixed-metal MOFs, reduction of nitroarenes was selected as a benchmark reaction. Incorporation of Co enhanced the activity of the catalysts compared with the parent NH2 -BDC-Fe catalyst. These MOFs were also tested as electrocatalysts for the oxygen evolution reaction (OER) and the best activity was exhibited by mixed-metal Fe/Co-MOF (Fe/Co batch ratio=1). The catalyst provided a current density of 10 mA cm-2 at 410 mV overpotential, which is comparable to the benchmark OER catalyst (i.e., RuO2 ). Moreover, it showed long-term stability in 1 m KOH. In a third catalytic test, dehydrogenation of sodium borohydride showed high activity (turnover frequency=87 min-1 ) and hydrogen generation rate (67 L min-1 g-1 catalyst). This is the first example of the synthesis of bimetallic MOFs as multifunctional catalysts particularly for catalytic reduction of nitroarenes and dehydrogenation reactions.

EPR characterization of vanadium dopant sites in DUT-5(Al)

The structural environment of the VIV ions doped in the metal-organic framework (MOF) DUT-5(Al) ((AlIIIOH)BPDC) was studied with electron paramagnetic resonance (EPR). At low V concentrations, two different vanadyl components are observed in the EPR spectrum: a broad-line component, best visible at ambient air conditions, and a narrow-line component, becoming more prominent at lower pressures. Recording the EPR spectra at both X- and Q-band, spin-Hamiltonian parameters were derived for both components. Similarities between the spectrum of the narrow-line component and the spectrum of VIV centers in the large pore state of a related MOF, MIL-53(Al) ((AlIIIOH)BDC), are discussed, including their sensitivity to oxygen. The broad-line component cannot conclusively be identified by comparison with VIV centers in MIL-53(Al). Increasing the concentration of VIV in the DUT-5(Al) framework, a third vanadyl component appears in the spectrum, together with a broad feature related to the spectrum of COMOC-2(V) ((VIV=O)BPDC). Surprisingly, this third component appears to be linked with a narrow pore state of the mixed-metal MOF.

Unraveling the Arrangement of Al and Fe within the Framework Explains the Magnetism of Mixed-Metal MIL-100(Al,Fe).

Properties of mixed-metal MOFs depend on the distribution of different metals within their frameworks. Determination of this distribution is often very challenging. Using an example of aluminum- and iron-containing MIL-100, we demonstrate that 27Al NMR spectroscopy, when combined with first-principles calculations and magnetic, X-band electron paramagnetic resonance, Fe K-edge extended X-ray absorption fine structure, and Mössbauer measurements, enables one to accurately determine the arrangement of Al and Fe within the metal trimers, which are the basic building units of MIL-100. In this particular material, the incorporation of Fe and Al on the framework metal sites is random. Crucial for deciphering the arrangement is detecting NMR signals, shifted because of the strong hyperfine interaction between the 27Al nuclei and the unpaired electronic spins of Fe3+ ions, assigning the shifted signals aided by first-principles calculations of hyperfine couplings, and quantitatively evaluating the NMR intensities and the measured effective magnetic moment.

Probing Local Structural Changes at Cu2+ in a Flexible Mixed-Metal Metal-Organic Framework by in Situ Electron Paramagnetic Resonance during CO2 Ad- and Desorption

The flexible mixed-metal pillared-layered metal-organic framework (MOF) Zn1.9Cu0.1(BME-bdc)2(dabco) (BME-bdc2– = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = diazabicyclo[2.2.2]octane) with Cu2+ dopants as electron paramagnetic resonance (EPR) active probes was synthesized, and the guest-induced phase transition was studied by a newly developed in situ gas sorption EPR setup. Within the structure, the Cu2+ ions do not build any Cu2+–Cu2+ paddlewheels, and only mixed-metal Zn2+–Cu2+ paddlewheels were identified by EPR. Two distinct Zn2+–Cu2+ species (A and B) occur in the structure: Species A occurs in regions with low Cu2+ concentrations and has no adjacent mixed-metal paddlewheel units. Species B is present in domains with higher local copper concentrations, where adjacent mixed-metal paddlewheel units alter the EPR signal. The parent monometallic Zn MOF is known to undergo phase transitions from a narrow pore (np) to a large pore (lp) phase due to host-guest interactions with CO2. In situ ...

Mixed-metal metal-organic frameworks.

Mixed-metal MOFs are metal-organic frameworks that contain at least 2 different metal ions as nodes of their frameworks. They are prepared relatively easily by either a one-pot synthesis with a synthesis mixture containing the different metals, or by a post-synthetic ion-exchange method by soaking a monometallic MOF in a concentrated solution of a different (but compatible) metal-ion. More difficult is the accurate characterization of these materials. Is the formed product a mixture of monometallic MOFs or indeed a MOF with different metallic nodes? Are the metals randomly distributed or do they form domains? What is the oxidation state of the metals? How do the metals mutually influence each other, and impact the material's performance? Advanced characterization techniques are required e.g. X-ray absorption spectroscopy, magnetic resonance and electron microscopy. Computational tools at multiple scales are also often applied. In almost every case, a judicious choice of several techniques is required to unambiguously characterize the mixed-metal MOF. Although still in their infancy, several applications are emerging for mixed-metal MOFs, that improve on conventional monometallic MOFs. In the field of gas sorption and storage, especially the stability and affinity towards the target gases can be largely improved by introducing a second metal ion. In the case of flexible MOFs, the breathing behavior, and in particular the pressure at which the MOF opens, can be tailored. In heterogeneous catalysis, new cascade and tandem reactions become possible, with particular focus on reactions where the two metals in close proximity truly form a mixed-metal transition state. The bimetallic MOF should have a clear benefit over a mixture of the respective monometallic MOFs, and bimetallic enzymes can be a huge source of inspiration in this field. Another very promising application lies in the fields of luminescence and sensing. By tuning the lanthanide metals in mixed-metal lanthanide MOFs and by using the organic linkers as antennae, novel smart materials can be developed, acting as sensors and as thermochromic thermometers. Of course there are also still open challenges, as also mixed-metal MOFs do not escape the typical drawbacks of MOFs, such as low stability in moisture and possible metal leaching in liquids. The ease of synthesis of mixed-metal MOFs is a large bonus. In this critical review, we discuss in detail the synthesis, characterization, computational work and applications of mixed-metal MOFs.

Metal doping in cerium metal-organic frameworks for visible-response water splitting photocatalysts.

Cerium metal-organic frameworks (Ce-MOFs) show great promise for photocatalytic water splitting as they have low-lying unoccupied 4f orbitals with energies lower than the unoccupied linker orbitals to drive the ligand-to-metal charge transfer (LMCT) of the photo-generated electron at the linker to separate the photo-excited charges. Nevertheless, the large and negative LMCT energies of Ce-MOFs are often accompanied by high photon-absorption energies, which then limit the application of Ce-MOFs as visible-light-driven water splitting photocatalysts. In this work, we propose that metal (Zr or Ti) doping can raise the very negative LMCT energies of Ce-MOFs, consequently leading to the decrease of the absorption energy and promoting the response of Ce-MOFs to visible light. By functionalizing the linker of the mixed-metal MOFs, we found two possible visible-response photocatalysts for water splitting using a single photocatalyst.

Systematic Investigations of the Transition between Framework Topologies in Ce/Zr-MOFs.

1- H-Pyrazole-3,5-dicarboxylic acid (H2PZDC), a small, strongly bent linker molecule with an angle of 147.4° between the carboxylate groups, was used in the synthesis of metal organic frameworks (MOFs) with fcu, bcu and reo topology. In systematic studies of the chemical system Ce4+/Zr4+/H2PZDC/HCOOH, their fields of formations were established. The decisive factors for the product formation and hence the transition between the framework topologies are the HCOOH/metal ratio and the molar ratio of Ce4+/Zr4+ employed in the synthesis. All title compounds crystallize with the well-known hexanuclear cluster {M6(μ3-O)4(μ3-OH)4(-CO2) n}, with n = 8 or 12 and M = Ce4+ and Zr4+, as the inorganic building unit (IBU). Connection through 12 or eight linker molecules leads to three framework topologies: fcu, bcu, and reo, respectively. The dominant phase observed in this system crystallizes with reo topology and is known as DUT-67. The pure Zr-MOF of composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Zr-DUT-67-PZDC) as well as the mixed-metal compounds Ce/Zr-DUT-67-PZDC are accessible and the molar ratio Ce4+/Zr4+ can be adjusted between 0 and 1. At low HCOOH/metal ratios, surprisingly, the UiO-66 type structure with fcu topology is formed despite the nonlinear geometry of the linker. Thus, using exclusively Zr4+ ions in the starting mixture the pure Zr-MOF with ideal composition [Zr6(μ3-O)4(μ3-OH)4(PZDC)6] (Zr-UiO-66-PZDC) was obtained. Variation of the Ce/Zr molar ratio leads to a continuous increase in linker defects with increasing Ce content in the MOF. At a Ce/Zr value of ∼ 1:1 a transition from the fcu to the reo framework topology takes place. Using high HCOOH/metal ratios, a transition from the reo to the bcu topology is observed when a molar ratio of Ce/Zr ≥ 1:5 is employed. Irrespective of the molar ratio used in the reaction mixture, the mixed-metal MOF of composition [CeZr5(μ3-O)4(μ3-OH)4(PZDC)4(OH)2(H2O)2] (Ce/Zr-CAU-38-PZDC) is always formed as confirmed by comprehensive EDX analyses. Rietveld refinement strongly indicates the presence of exclusively hexanuclear {CeZr5(μ3-O)4(μ3-OH)4} clusters and thus CAU-38 is the first Ce/Zr-MOF which solely occurs at a specific metal stoichiometry. In addition to the detailed synthetic study, the compounds were thoroughly characterized regarding their composition, lattice parameters, and porosity, as well as thermal and chemical stability.

CuPd Mixed-Metal HKUST-1 as a Catalyst for Aerobic Alcohol Oxidation

Metal–organic frameworks (MOFs) featuring isolated coordinatively unsaturated metal sites (CUS) have enormous potential as single-site catalysts. In particular, mixed-metal MOFs may exhibit unique catalytic properties compared to their monometallic counterparts. Herein, we report a thorough fundamental study on the mixed-metal CuPd-HKUST-1 ([Cu3–xPdx(BTC)2]n; BTC = 1,3,5-benzenetricarboxylate) including the two-step synthesis, characterization, and catalytic performance evaluation. The combined results from a multitechnique approach provide solid evidence that the chemical properties of HKUST-1 can be tuned via successful incorporation of Pd–CUS into the framework, leading to the formation of new Cu–Pd and/or Pd–Pd dimers. The introduction of Pd occurs exclusively at the metal nodes in a controlled manner while retaining the structural integrity. The incorporated Pd ions have an oxidation state of +2, whereas no PdO or metallic Pd nanoparticles embedded inside MOFs are detected. These mixed-metal CuPd-MOF...

Isolated Fe Sites in Metal Organic Frameworks Catalyze the Direct Conversion of Methane to Methanol

Hybrid materials bearing organic and inorganic motifs have been extensively discussed as playgrounds for the implementation of atomically resolved inorganic sites within a confined environment, with an exciting similarity to enzymes. Here, we present the successful design of a site-isolated mixed-metal metal organic framework (MOF) that mimics the reactivity of soluble methane monooxygenase enzyme and demonstrates the potential of this strategy to overcome current challenges in selective methane oxidation. We describe the synthesis and characterization of an Fe-containing MOF that comprises the desired antiferromagnetically coupled high-spin species in a coordination environment closely resembling that of the enzyme. An electrochemical synthesis method is used to build the microporous MOF matrix while integrating the atomically dispersed Fe active sites in the crystalline scaffold. The model mimics the catalytic C–H activation behavior of the enzyme to produce methanol and shows that the key to this react...

Rational route for targeted synthesis of microporous mixed-metal MOFs

Rational route for targeted synthesis of microporous mixed-metal MOFs

CO2 Dynamics in Pure and Mixed-Metal MOFs with Open Metal Sites

Metal–organic frameworks (MOFs), such as MOF-74, can have open metal sites to which adsorbates such as CO2 preferentially bind. 13C NMR of 13CO2 is highly informative about the binding sites present in Mg-MOF-74. We used this technique to investigate loadings between ∼0.88 and 1.15 molecules of CO2 per metal in Mg-MOF-74 at 295 K. 13C lineshapes recorded as a function of loading can be understood in terms of the dependence of the CO2 NMR frequency on the angle (θ) with respect to the CO2 axis and the channel of the MOF, reflected in the Legendre polynomial, P2. In the fast motion limit, the NMR spectra reveal the time-averaged value of P2, where θ is the angle between the instantaneous CO2 axis and the channel axis. DFT calculations were used to determine a weighted average of P2 in this regime and are in good agreement with experimental data. Static variable temperature 13C NMR from cryogenic temperatures to room temperature was used to investigate 13CO2 binding in Mg-MOF-74 loaded at two levels (∼0.88 a...

H2 S-Activable MOF Nanoparticle Photosensitizer for Effective Photodynamic Therapy against Cancer with Controllable Singlet-Oxygen Release.

Photodynamic therapy (PDT) has emerged as an important minimally invasive tumor treatment technology. The search for an effective photosensitizer to realize selective cancer treatment has become one of the major foci in recent developments of PDT technology. Controllable singlet-oxygen release based on specific cancer-associated events, as another major layer of selectivity mode, has attracted great attention in recent years. Here, for the first time, we demonstrated that a novel mixed-metal metal-organic framework nanoparticle (MOF NP) photosensitizer can be activated by a hydrogen sulfide (H2 S) signaling molecule in a specific tumor microenvironment for PDT against cancer with controllable singlet-oxygen release in living cells. The effective removal of tumors in vivo further confirmed the satisfactory treatment effect of the MOF NP photosensitizer.

From Mixed-Metal MOFs to Carbon-Coated Core-Shell Metal Alloy@Metal Oxide Solid Solutions: Transformation of Co/Ni-MOF-74 to CoxNi1-x@CoyNi1-yO@C for the Oxygen Evolution Reaction.

Calcination of the mixed-metal species Co/Ni-MOF-74 leads to the formation of carbon-coated CoxNi1-x@CoyNi1-yO with a metal core diameter of ∼3.2 nm and a metal oxide shell thickness of ∼2.4 nm embedded uniformly in the ligand-derived carbon matrix. The close proximity of Co and Ni in the mixed-metal Co/Ni-MOF-74 promotes the metal alloying and the formation of a solid solution of metal oxide during the calcination process. The presence of the tightly coated carbon shell prohibits particle agglomeration and stabilizes the CoxNi1-x@CoyNi1-yO nanoparticles in small size. The CoxNi1-x@CoyNi1-yO@C derived from Co/Ni-MOF-74 nanocomposites show superior performance for the oxygen evolution reaction (OER). The use of mixed-metal MOFs as precursors represents a powerful strategy for the fabrication of metal alloy@metal oxide solid solution nanoparticles in small size. This method also holds great promise in the development of multifunctional carbon-coated complex core-shell metal/metal oxides owing to the diversified MOF structures and their flexible chemistry.

Mixed-Metal Metal–Organic Frameworks as Catalysts for Liquid-Phase Oxidation of Toluene and Cycloalkanes

A series of isostructural mixed-metal metal–organic frameworks (MM-MOFs) of 1,3,5-benzenetricarboxylate (BTC), M–Zn–BTC, where M = Cu(II), Co(II) and Fe(II) have been synthesized using the post-synthetic exchange method. The catalytic activities of the M–Zn–BTC have been investigated for the oxidation of toluene and cycloalkanes in the presence of hydrogen peroxide as oxidant. The M–Zn–BTC catalysts show improved activity and selectivity for toluene oxidation. The introduction of the second metal to the node of the MOFs improved the toluene conversion from 22 to 70% and the selectivity of benzaldehyde improved from 22 to 57%. The Fe–Zn–BTC showed the best activity in terms of benzaldehyde selectivity for the oxidation of toluene. However, Cu–BTC gave the highest conversion for cyclohexane and methylcyclohexane oxidation.

A Solvent-Free Hot-Pressing Method for Preparing Metal-Organic-Framework Coatings.

Metal-organic frameworks (MOFs), with their well-defined pores and rich structural diversity and functionality, have drawn a great deal of attention from across the scientific community. However, industrial applications are hampered by their intrinsic fragility and poor processability. Stable and resilient MOF devices with tunable flexibility are highly desirable. Herein, we present a solvent- and binder-free approach for producing stable MOF coatings by a unique hot-pressing (HoP) method, in which temperature and pressure are applied simultaneously to facilitate the rapid growth of MOF nanocrystals onto desired substrates. This strategy was proven to be applicable to carboxylate-based, imidazolate-based, and mixed-metal MOFs. We further successfully obtained superhydrophobic and "Janus" MOF films through layer-by-layer pressing. This HoP method can be scaled up in the form of roll-to-roll production and may push MOFs into unexplored industrial applications.