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How to Make Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are made by connecting metal centers with organic linkers through coordination bonds. The process of creating MOFs plays a crucial role in crystal structure formation. This determines their properties and how well they perform in various applications. With this in mind there are a variety of components to consider:

Reagents: Organic linker, metal center, modulator, solvent

Parameters: Concentration, ratio of reagents, temperature, time, pH

Synthetic methods:

Various other techniques like the diffusion method, spray-drying synthesis, ionothermal synthesis, and chemical vapor deposition have also been used in the creation of MOFs.

The diverse range of methods and reagents available has enabled the synthesis of 90,000+ different MOFs. Controlling the structure, size and functionality of MOFs has meant these materials are suitable for a huge range in applications of MOFs.

Once a synthetic method has been selected and MOF crystals are formed, the material is washed to remove unreacted reagents. This is done with the solvents used in the initial reaction as well as other more volatile solvents. Centrifugation is often used to collect MOF crystals and the washing supernatant is disposed of. Fresh solvent is used to resuspend MOF crystals and the process is repeated.

The MOF is then activated by removing the guest solvent from the MOF pores in order to access the full surface area of the MOF. Accessing the full porosity is crucial in ensuring full performance especially for storage and separation applications.

Synthesis of MOF
General synthesis of MOFs

Advantages and Disadvantages of MOF Synthetic Methods


Summary of the advantages and disadvantages of the different MOF synthetic methods:

Synthetic Method Advantages Disadvantages
Solvothermal/Hydrothermal
  • One-step synthesis
  • Can access single crystals
  • Moderate temperature
  • Long reaction time (hrs/days)
  • Requires more solvent
  • Easy to produce unwanted by-products
Microwave-assisted
  • Rapid (mins)
  • High purity
  • Uniform morphology
  • Eco-friendly
  • Difficult to get single crystals
  • Not yet scalable
Electrochemical
  • No need for metal salts
  • Mild reaction conditions
  • Quick (hrs)
  • Requires N2 atmosphere
  • Varied structure
  • Lower yield
Mechanochemical
  • Room temperature
  • Less hazardous by-products
  • Rapid (mins)
  • Eco-friendly
  • Decreased pore volume
  • Lower crystallinity
  • Lower yield
Sonochemical
  • Quick (tens of mins)
  • Eco-friendly
  • Room temperature
  • Homogeneous nucleation
  • Can vary crystallization time
  • Difficult to get single crystals

Solvothermal/Hydrothermal MOF Synthesis


Aprotic (can't hydrogen bond) Protic (can hydrogen bond)
  • DEF
  • DMF
  • NMP
  • DMSO
  • DMA
  • Acetonitrile
  • Methanol
  • Ethanol
  • Water
  • n-Propanol

Solvothermal synthesis is the most common way to make MOFs. Dissolve metal salts and organic linker compounds in protic or aprotic solvents, then heat to induce crystallisation. The components used dictates the solvent system selected. Sometimes a mixture of solvents is needed to ensure all components are dissolved in the reaction mixture.

The metal-linker solution is sealed in a glass reaction vial for low temperature reactions, or a Teflon-lined stainless-steel autoclave for high temperatures (400K). The pressure of the system increases as the temperature is increased to be above the solvent’s boiling point.

Hydrothermal synthesis: water is the solvent.

solvothermal/hydrothermal synthesis of MOF
Solvothermal/hydrothermal synthesis of MOF

Microwave-assisted MOF synthesis


Microwave-irradiation has aided the rapid synthesis of MOFs under hydrothermal conditions. Reaction vessels are sealed and then heated to temperatures beyond the boiling point of the solvent. Microwave irradiation provides efficient heating because of interactions between the electromagnetic waves and the mobile dipoles and/or ions that are present in the reaction mixture. The electromagnetic energy is converted to thermal energy through excitation and relaxation mechanisms.

microwave-assisted synthesis of MOF
Microwave-assisted synthesis of MOF

Dipole rotation: For polar solvent molecules with dipoles the applied electric field causes alignment. At the correct frequency molecules collide, increasing the temperature of the system.

Ionic conduction: Mobile charge carriers (electrons, ions, etc.) move back and forth through the material under the influence of the electromagnetic field. This creates an electric current. These induced currents cause collisions of charged species resulting in electrical resistance. This in turn causes heating in the sample.

Electrolyte solutions (water and salts) can undergo both dipole rotation and ionic conduction.

Dielectric polarization: Dielectric solid materials (eg. π-conjugated aromatic materials) with charged particles that are free to move (eg. π-electrons) can carry a current. The electromagnetic field induces the current but as it’s phase changes electrons cannot couple to it and energy dissipates as heat.

There is direct interaction between the radiation and system so heating is instant and energy efficient. Especially when compared to conventional heating methods. This means reaction times are significantly quicker. Microwave-assisted synthesis is an environmentally friendly approach where harmful solvents like DMF can be replaced with aqueous mixtures.

microwave-assisted synthesis of MOF
Comparison of conventional and microwave-assisted synthesis of MOFs

Electrochemical MOF Synthesis


Electrochemical synthesis is carried out in an electrochemical cell. A current or potential is applied through an electrolyte solution that contains organic linkers via electrodes. Direct electrosynthesis involves the direct formation of MOF on electrodes and indirect formation includes other steps as well.

Electrochemical synthesis of MOF
Anodic electrochemical synthesis of MOF

Most commonly, an applied voltage on the metal anode results in oxidation to ions which then react with the organic linker in solution. A MOF thin film is then formed on the electrode. Different electrode substrates have been used to grow MOFs including:

Anode: Zn plate/foil, Cu mesh/foam/sheet, Tb foil, Cu or Co/FTO, Zn/ITO, Al

Cathode: Aluminium oxide, Graphite, GCE, FTO

The substrate, potential, voltage, current density, distance between electrode, solvent and electrolyte concentration all play a role in MOF formation. By varying these components MOF structure, size and properties can be tuned. There are two key types of electrochemical synthesis:

Anodic dissolution/electrodeposition: As the metal anode is the metal ion source, there is no need for metal salts. Synthesis is not reliant on metal salt kinetics. Changing the electrochemical conditions means different oxidation states of the metals can be accessed from the same set up. This leads to more control over MOF properties.

Cathodic electrosynthesis/deposition: Electrodes act as an electron source rather than reagent source. Metal salts, organic ligand and pro-base (eg. (e.g. NO3, Et3NH+, H2O etc.) in electrolyte solution. When a potential is applied, MOF forms on the cathode. A pro-base isn't always used. As long as the pH by the cathode is high enough to cause the deprotonation of the organic ligand then MOF formation can occur.

Electrochemical synthesis of MOF can result in lower quality MOFs as reagents including the electrolyte salts can remain in the pores. MOFs grown on a substrate are harder to wash and activate.

Mechanochemical MOF Synthesis


Mechanochemical synthesis of MOF
Mechanochemical synthesis of MOF

Mechanochemical methods involve the grinding of solid reagents sometimes in a small amount of solvent. They typically rely on basic anions from metal salts deprotonating the organic linker. The mechanical breaking of intramolecular bonds leads to chemical transformation and MOF formation. Ball-mill or pestle and mortars are used to mill/grind the metal salts and organic linkers. Reactions are usually done at room temperature with heating required for the removal of solvents if used.

This synthetic approach is environmentally friendly as it uses little to no organic solvent. It can also be done:

  • On a large scale
  • Cheaply
  • Efficiently
  • With simple processing

and can overcome issues of low reactant solubility.

Sonochemical MOF Synthesis


Sonochemical MOF synthesis uses ultrasound. Ultrasonic frequencies (20 kHz to 10 MHz) are high energy and result in cavitation. This is where pressure changes causes bubbles to form, grow and collapse. The collapse of these cavitation bubbles generates local hot spots with very high temperatures and pressures. The collapse of bubbles by the solid metal salts and organic linkers causes activation. Energy is transferred from the collapsed bubble to the solid reagents. This splits large particles and causes rapid MOF formation.

The sonochemical method can replace classic solvothermal methods as it reduces reaction time whilst maintaining MOF crystallinity and size. This method is particularly useful for the synthesis of nanoscale MOFs and can lead to unique morphologies and properties suitable for various applications, including catalysis, gas storage, and separation.

MOF Modulators


Modulators help control MOF crystallization beyond what the synthesis method alone can achieve. A variety of modulators are available. Each are selected based on the desired influence on MOF formation. Particle size, phase purity and defects can be controlled by modulators. They can be categorized into two classes:

Coordinating Modulators: usually monotopic (one coordinating group) with similar binding chemistry as the linker. The coordinating group becomes deprotonated and competes to bind with the metal centers. With just one coordinating group they can only bind to one metal center. High temperature synthetic conditions increase the likelihood of coordinating groups dissociating from metal centers. Multitopic linkers can then substitute the monotopic modulators. Competition for metal binding, with increasing ratio of modulator as the synthesis progresses, increases the crystallinity of the MOF. This is due to a reduction in reaction kinetics.

Examples: Formic Acid (HCOOH), Acetic Acid (CH3COOH), Trifluoroacetic Acid (CF3COOH), Amino Acids, Pyridine (C5H5N), Water, Ammonia (NH3) etc.

Coordination modulator
Coordination Modulator

Brønsted acid modulators: slow down MOF self-assembly by protonating the linker’s coordinating group, preventing binding. Strong Brønsted acids are happy to lose their hydrogen ion (H+) and provide a higher concentration of H+ to protonate the linker. This slows the reaction and increases MOF crystallinity.

Examples: Hydrochloric Acid (HCl), Tetrafluoroboric Acid (HBF4), Sulfuric Acid (H2SO4), Phosphoric Acid (H₃PO₄) etc.

Some organic acids (formic acid, acetic acid) function as both coordinating and Brønsted acid modulators. In some cases, the modulator remains part of the MOF after crystallization in order to access specific particle shapes and sizes. The chemistry of the modulator is therefore incorporate much like with the metals and linkers.

MOF Characterization Methods


Many analytical techniques are used to characterize MOFs. They can be categorized by the characteristics that they determine. Here are some of the key MOF characterization methods:

Crystal Structure Stability and Composition Imaging and Particle Size Functional Groups Surface Area

X-ray Powder Diffraction (XRPD)

Single crystal X-ray (SCXR)

Small-angle X-ray scattering (SAXS)

Thermogravimetric Analysis (TGA)

Elemental Analysis (EA)

Atomic Force Microscopy (AFM)

Scanning Electron Microscopy (SEM)

Transmission Electron Microscopy (TEM)

Confocal Fluorescence Microscopy

Dynamic Light Scattering (DLS)

Infrared Spectroscopy (IR)

Nuclear Magnetic Resonance (NMR)

Brunauer-Emmett-Teller (BET) method

Barrett-Joyner-Halenda (BJH) method

MOF Ligands

MOF Ligand
  • Multitopic
  • Multifunctional
  • Modular

Available from £70

References


Yusuf, V. et al. (2022). Review on Metal−Organic Framework Classification, Synthetic Approaches, and Influencing Factors: Applications in Energy, Drug Delivery, and Wastewater Treatment., ACS Omega, 7.doi:10.1021/acsomega.2c05310

Kirlikovali, K. et al.(2023). Back to the Basics: Developing Advanced Metal–Organic Frameworks Using Fundamental Chemistry Concepts. ACS Nanosci. Au, 3, doi:10.1021/acsnanoscienceau.2c00046

Contributing Authors


Written by

Dr. Amelia Wood

Application Scientist

Diagrams by

Sam Force

Graphic Designer

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