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Trimetallic Metal-Organic Frameworks: Advanced Synthesis, Structural Engineering, And Multifunctional Applications In Catalysis And Energy Conversion

MAR 27, 202656 MINS READ

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Trimetallic metal-organic frameworks (MOFs) represent a cutting-edge class of porous crystalline materials that integrate three distinct metal centers within a single coordination network, offering unprecedented opportunities for tuning catalytic activity, electronic properties, and structural stability. By incorporating multiple metal species—such as Co, Ni, and Bi or combinations involving Zr, Ce, and transition metals—these frameworks achieve synergistic effects that surpass the performance of monometallic and bimetallic analogues in applications ranging from photocatalytic hydrogen generation to selective gas adsorption and electrochemical reduction reactions123.
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Molecular Composition And Structural Characteristics Of Trimetallic Metal-Organic Frameworks

Trimetallic metal-organic frameworks distinguish themselves through the deliberate incorporation of three different metal ions into a unified coordination architecture, enabling precise control over electronic structure, redox behavior, and catalytic site distribution. The structural design typically involves pyrazole-based ligands, carboxylate linkers (such as 1,3,5-benzenetricarboxylic acid, BTC), and bipyridine derivatives (e.g., 4,4′-bipyridine) that bridge metal nodes to form three-dimensional porous networks123.

In the case of the trimetallic MOF reported with the formula BixCoyNi(1-x-y)(BTC)(4,4′-bipy), the optimal molar ratio of Bi:Co:Ni is approximately 0.3:0.3:0.3, which balances the electronic contributions of each metal center3. This stoichiometry is critical: bismuth provides heavy-atom effects and modulates charge transfer, cobalt contributes to redox-active sites with variable oxidation states (Co2+/Co3+), and nickel enhances structural stability and participates in electron-transfer processes during catalytic cycles3. The resulting framework exhibits a BET surface area typically in the range of 800–1200 m²/g (depending on activation conditions), with pore diameters of 6–12 Å that facilitate guest molecule diffusion while maintaining structural integrity under operational conditions3.

Another prominent example involves trimetallic nodes of the type M6O4(OH)412−, where M can be a combination of Zr4+, Ce4+, and Hf4+, coordinated by tricarboxylic aromatic ligands such as trimesic acid or 1,3,5-(4-carboxyphenyl)benzene1. These hexanuclear clusters provide exceptional thermal stability (up to 400–500 °C under inert atmosphere) and chemical robustness in aqueous media, with water contact angles ranging from 20° to 80° depending on ligand functionalization1. The presence of multiple metal species within a single node creates a gradient of Lewis acidity and redox potentials, which is instrumental in multi-step catalytic transformations such as tandem oxidation-reduction reactions1.

The coordination geometry around each metal center is predominantly octahedral, with metal-oxygen bond lengths of 2.0–2.2 Å for transition metals and 2.1–2.3 Å for lanthanides or actinides, as confirmed by single-crystal X-ray diffraction (SCXRD) and extended X-ray absorption fine structure (EXAFS) spectroscopy123. The organic linkers adopt planar or near-planar conformations to maximize π-π stacking interactions between aromatic rings, contributing to framework rigidity and preventing structural collapse during guest removal or solvent exchange23.

Precursors And Synthesis Routes For Trimetallic Metal-Organic Frameworks

The synthesis of trimetallic MOFs demands careful control of reaction stoichiometry, solvent composition, temperature, and reaction time to ensure homogeneous metal distribution and phase purity. A representative solvothermal protocol for preparing Bi0.3Co0.3Ni0.4(BTC)(4,4′-bipy) involves the following steps3:

  • Precursor preparation: Dissolve 1,3,5-benzenetricarboxylic acid (BTC, 0.5 mmol) and 4,4′-bipyridine (4,4′-bpy, 0.5 mmol) in a mixed solvent of N,N-dimethylformamide (DMF, 10 mL), ethanol (5 mL), and deionized water (2 mL). Sonicate the mixture for 15 minutes at room temperature to ensure complete dissolution and homogeneous ligand distribution.
  • Metal salt addition: Add CoCl2·6H2O (0.15 mmol), NiCl2·6H2O (0.15 mmol), and Bi(NO3)3·5H2O (0.15 mmol) to the ligand solution. Sonicate for an additional 20 minutes until all metal salts are fully dissolved, yielding a clear or slightly turbid solution with a pale green-brown color indicative of mixed-metal coordination.
  • Solvothermal crystallization: Transfer the reaction mixture to a Teflon-lined stainless-steel autoclave (23 mL capacity) and heat at 120–140 °C for 48–72 hours. The heating rate should be controlled at 2 °C/min to avoid rapid nucleation and ensure large, well-formed crystals. After the reaction, cool the autoclave to room temperature at a rate of 0.5 °C/min to minimize thermal stress on the crystals.
  • Product isolation and activation: Collect the precipitate by vacuum filtration, wash sequentially with DMF (3 × 10 mL), ethanol (3 × 10 mL), and acetone (2 × 10 mL) to remove unreacted precursors and occluded solvent molecules. Dry the product under vacuum at 80 °C for 12 hours, then activate by heating at 150 °C under dynamic vacuum (10−3 mbar) for 6 hours to generate open metal sites and maximize porosity3.

For trimetallic MOFs based on M6O4(OH)412− nodes (e.g., Zr/Ce/Hf-BTC), a modulated synthesis approach is often employed to control crystal size and morphology1:

  • Modulator selection: Add monocarboxylic acids such as acetic acid or benzoic acid (10–50 equivalents relative to metal) to the reaction mixture. These modulators compete with the multidentate ligands for coordination sites, slowing crystal growth and promoting the formation of defect-free frameworks with uniform particle sizes (50–200 nm)1.
  • Reaction conditions: Conduct the reaction in DMF or a DMF/water mixture (9:1 v/v) at 120 °C for 24 hours. The presence of water is critical for hydrolysis of metal precursors and formation of μ3-oxo and μ3-hydroxo bridges within the hexanuclear clusters1.
  • Post-synthetic metal exchange: To introduce a third metal species into a pre-formed bimetallic MOF, immerse the crystals in a solution of the desired metal salt (e.g., Ce(NO3)3 in methanol, 0.1 M) at 60 °C for 48 hours. Monitor the exchange process by inductively coupled plasma optical emission spectroscopy (ICP-OES) to confirm the target metal ratio1.

The choice of solvent is paramount: DMF provides high solubility for both metal salts and organic ligands, while ethanol and water modulate the dielectric constant and influence the coordination kinetics23. The use of mixed solvents also facilitates the formation of interpenetrated or non-interpenetrated frameworks depending on the ligand length and metal coordination preferences2.

Physicochemical Properties And Performance Metrics Of Trimetallic Metal-Organic Frameworks

Trimetallic MOFs exhibit a suite of physicochemical properties that are finely tuned by the identity and ratio of the constituent metals, as well as the nature of the organic linkers. Key performance metrics include:

  • Surface area and porosity: BET surface areas typically range from 800 to 1500 m²/g, with pore volumes of 0.4–0.8 cm³/g. For example, the Bi0.3Co0.3Ni0.4(BTC)(4,4′-bipy) framework exhibits a BET surface area of approximately 1050 m²/g and a pore volume of 0.52 cm³/g after activation at 150 °C under vacuum3. Pore size distributions, determined by non-local density functional theory (NLDFT) analysis of N2 adsorption isotherms at 77 K, reveal a bimodal distribution with micropores (6–8 Å) and mesopores (12–15 Å), facilitating both small-molecule adsorption and diffusion of larger substrates3.
  • Thermal stability: Thermogravimetric analysis (TGA) under N2 atmosphere shows that trimetallic MOFs remain stable up to 300–400 °C, with the first significant weight loss (5–10%) occurring between 100–150 °C due to desorption of physisorbed water and residual solvent molecules12. The framework decomposition temperature (Td), defined as the onset of ligand combustion, is typically 380–450 °C for carboxylate-based MOFs and 320–380 °C for nitrogen-rich linkers such as pyrazole or triazole derivatives12.
  • Chemical stability: Trimetallic MOFs demonstrate enhanced resistance to hydrolysis compared to monometallic analogues. For instance, Zr/Ce/Hf-BTC retains >90% of its crystallinity after immersion in water (pH 7) for 7 days at room temperature, as confirmed by powder X-ray diffraction (PXRD)1. In contrast, purely Zr-based MOFs may show partial degradation under the same conditions. The improved stability arises from the synergistic effect of multiple metal-oxygen bonds with varying bond strengths, which collectively resist hydrolytic cleavage1.
  • Electronic and optical properties: UV-Vis diffuse reflectance spectroscopy (DRS) reveals that trimetallic MOFs exhibit broad absorption bands in the UV-Visible region (300–600 nm), with band gaps (Eg) of 2.0–2.8 eV depending on the metal composition12. The presence of multiple metal centers creates a cascade of energy levels that facilitate charge separation and extend the photoresponse into the visible spectrum, which is advantageous for solar-driven photocatalysis12. Electron paramagnetic resonance (EPR) spectroscopy confirms the presence of mixed-valence states (e.g., Co2+/Co3+, Ni2+/Ni3+) that serve as electron reservoirs during redox reactions3.
  • Redox potentials: Cyclic voltammetry (CV) measurements in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as supporting electrolyte show that trimetallic MOFs exhibit multiple redox couples. For Bi0.3Co0.3Ni0.4(BTC)(4,4′-bipy), quasi-reversible peaks are observed at E1/2 = −0.45 V, −0.78 V, and −1.12 V (vs. Ag/AgCl), corresponding to the reduction of Bi3+, Co2+, and Ni2+, respectively3. These multiple redox events enable the framework to participate in multi-electron transfer processes, which are essential for catalytic applications such as CO2 reduction and water splitting3.

Photocatalytic Hydrogen Generation Using Trimetallic Metal-Organic Frameworks

One of the most compelling applications of trimetallic MOFs is in photocatalytic hydrogen production from water, a process that holds promise for sustainable energy conversion. The trimetallic MOF containing Zr, Ce, and Hf with pyrazole ligands has been demonstrated to generate H2 under simulated solar irradiation (AM 1.5G, 100 mW/cm²) with a production rate of 1200–1500 μmol g−1 h−1 in the presence of a sacrificial electron donor such as triethanolamine (TEOA, 10 vol% in water)12.

The photocatalytic mechanism involves the following steps12:

  • Photon absorption and exciton generation: Upon irradiation with photons of energy ≥ Eg, electrons are promoted from the valence band (primarily composed of O 2p orbitals from carboxylate ligands) to the conduction band (formed by metal d orbitals), generating electron-hole pairs (excitons).
  • Charge separation and migration: The presence of three different metal centers creates a built-in electric field that drives spatial separation of electrons and holes. Electrons migrate preferentially to the metal sites with the most negative reduction potential (e.g., Zr4+/Zr3+ at −1.0 V vs. NHE), while holes are trapped at the organic linkers or at metal sites with lower oxidation potentials (e.g., Ce4+/Ce3+ at +1.6 V vs. NHE)12.
  • Proton reduction: Electrons accumulated at the metal nodes reduce adsorbed H2O or H+ to H2 via a two-electron process: 2H+ + 2e → H2. The turnover frequency (TOF) for H2 evolution is approximately 0.8–1.2 s−1 per active site under optimal conditions12.
  • Hole scavenging: The sacrificial donor (TEOA) is oxidized by the photogenerated holes, preventing charge recombination and maintaining the catalytic cycle. The quantum efficiency (Φ) for H2 production at 420 nm is measured to be 3.5–5.0%, which is competitive with state-of-the-art semiconductor photocatalysts such as CdS or g-C3N412.

The stability of the photocatalyst is critical for practical applications. After five consecutive 4-hour irradiation cycles, the trimetallic MOF retains >85% of its initial H2 production rate, with PXRD and scanning electron microscopy (SEM) confirming minimal structural degradation12. The slight decrease in activity is attributed to the accumulation of oxidation products on the catalyst surface, which can be partially restored by washing with dilute HCl (0.1 M) followed by reactivation under vacuum12.

Electrochemical Reduction Catalysis With Trimetallic Metal-Organic Frameworks

Trimetallic MOFs also serve as efficient electrocatalysts for the reduction of small molecules such as CO2, nitrate (NO3), and

OrgApplication ScenariosProduct/ProjectTechnical Outcomes
CONSEJO SUPERIOR DE INVESTIGACIONES CIENTÍFICAS (CSIC), UNIVERSITAT POLITECNICA DE VALENCIA, PARIS SCIENCES ET LETTRES, ECOLE SUPERIEURE DE PHYSIQUE ET DE CHIMIE INDUSTRIELLES DE LA VILLE DE PARIS, CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE, ECOLE NORMALE SUPERIEURESolar-driven photocatalytic hydrogen generation from water splitting for sustainable energy conversion, renewable fuel production systems requiring robust and recyclable photocatalysts under aqueous conditions.Trimetallic Zr/Ce/Hf-Pyrazole MOF PhotocatalystAchieves H2 production rate of 1200-1500 μmol g⁻¹ h⁻¹ under simulated solar irradiation with quantum efficiency of 3.5-5.0% at 420 nm, retains >85% activity after five consecutive cycles, demonstrates exceptional water stability and thermal stability up to 400-500°C.
KING FAISAL UNIVERSITYElectrochemical reduction catalysis for CO2 reduction, nitrate reduction, and small molecule transformations in resource-limited environments requiring multi-step catalytic reactions with enhanced chemical stability.Bi₀.₃Co₀.₃Ni₀.₄(BTC)(4,4'-bipy) Trimetallic MOFExhibits BET surface area of approximately 1050 m²/g with bimodal pore distribution (6-8 Å micropores and 12-15 Å mesopores), demonstrates multiple redox couples at E₁/₂ = -0.45 V, -0.78 V, and -1.12 V enabling multi-electron transfer processes, maintains >90% crystallinity in aqueous media.
KOREA RESEARCH INSTITUTE OF CHEMICAL TECHNOLOGYSelective gas adsorption and separation applications requiring moisture-tolerant materials, industrial-scale CO2 capture systems, and humidity-responsive catalytic processes in chemical manufacturing.Al-based Mixed-Ligand MOF with Tunable HydrophilicityProvides adjustable surface hydrophilicity through controlled isophthalic acid to 3,5-pyridinedicarboxylic acid ratio, exhibits three-dimensional porous structure with high surface area (800-1200 m²/g), demonstrates superior gas adsorption capacity and chemical robustness in variable humidity conditions.
COLORADO STATE UNIVERSITY RESEARCH FOUNDATIONAntimicrobial surface coatings for medical textiles, bandages, and healthcare fabrics to prevent bacterial colonization and biofilm formation, water treatment applications requiring stable porous materials.Functionalized Cu₃(BTC)₂ Tricarboxylate MOFModified tricarboxylate ligands with aliphatic carbon chains enhance water stability and antimicrobial properties, prevents bacterial attachment and biofilm formation, maintains structural integrity under aqueous conditions with tunable hydrophobic-hydrophilic balance.
INDUSTRIAL TECHNOLOGY RESEARCH INSTITUTEGas storage and separation devices for industrial applications, adsorption-based purification systems requiring high thermal and chemical stability, heterogeneous catalysis in petrochemical processes.3,5-Pyridinedicarboxylic Acid MOF (Al/Cr/Zr-based)Coordinates 3,5-pyridinedicarboxylic acid with aluminum, chromium, or zirconium ions achieving excellent gas adsorbability and durability, thermal stability up to 380-450°C, enhanced Lewis acidity gradient for multi-step catalytic transformations.
Reference
  • Metal organic framework and use thereof for generating h2
    PatentPendingUS20240024861A1
    View detail
  • Metal organic framework and use thereof for generating h2
    PatentWO2022073979A1
    View detail
  • Tri-metallic organic framework (MOF) complex as a reduction catalyst
    PatentActiveUS12128390B1
    View detail
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