The Synthesis, Characterisation and Uses of MOFs Flashcards
Like discrete coordination complexes, MOFs are also synthesised by a…
Self-assembly process
(i.e. reversible bond formation)
- When constructing a MOF, we are aiming to make crystalline solid material with the largest possible crystals, as this makes the resultant MOF much easier to characterise
- However, as such crustals would have orders of magnitude more pieces than even the largest discrete coordination complexes, there is more room for errors to occur during the synthesis, leading to imperfections in the structure
- How is this avoided?
As a result, MOF synthesis often takes place under harsher conditions than for discrete coordination complexes to allow more error checking
The most common MOF synthesis technique is referred to as solvothermal crystalliation synthesis
What are steps 1 and 2
In a typical solvothermal synthesis, the metal salt and organic ligand are mixed and heated in a solvent for a few days
The most common solvent is DMF, which is used because…
- Its polar (so can dissolve charged species)
- It can occupy vacant coordination site on metal ions during synthesis
- Has a high boiling, allowing the mixture to be heated to high temperatures (for error checking)
Why is MOF synthesis commonly perfomed in pressuried vessels?
- DMF boils at 153°C, but even that is not enough for error checking
- Hence MOF synthesis is commonly performed in pressuried vessels that allow DMF to be heated above its boiling point at atmospheric pressure
Other synthesis routes can also be attempted
What is hydrothermal synthesis?
If the solvent synthesis is conducted in water as a solvent
What is the issue with hydrothermal synthesis however?
Sometimes the coordinated solvent molecules from a solvothermal or hydrothermal synthesis prove too strongly coordinated to remove under vacuum
If we want to access structures without this coordinated solvent, then we need to perform a solvent free or solid phase synthesis
Without solvent, vigorous mechanical activation and mixing is required to encourage the bond formation and error correction processes required in the self-assembly
This is achieved in a grinding machine called
a ball mill
Doing this allows the reaction rate to be greatly increased
Another method is diffusion synthesis
What is the process and the benefits?
- In this method, two immiscible solvents are layered on top of each other and one of the starting materials in dissolved in each layer
- There is then slow diffusion of the starting material from one layer into the other
- The benefit of this method is that crystals which form slowly are often much larger
What is a drawback of diffusion synthesis?
Is that this process can take months to complete
Why is NMR spectroscopy not a routine characterisation technique for MOFs
- Although running an NMR spectrum of solid material is possible, it is much harder than running an NMR spectrum of material dissolved in solution
- As a result NMR spectrscopy is not a routine characterisation technique for MOFs
Why is Mass spectrometry not useful for analysis?
Mass spec relies on well defined, identifiable discrete ions
As MOFs are polymeric, only fragments of MOFs can be observed by mass spec, and this is often not useful for analysis
What is the best method for characterising MOF structure?
- Single crystal X-ray crystallography
- This give information about the 3D structure that no other method can
Whilst it is often possible to grow single crystals, foor some MOFs too many structural defects are present to obtain crystals of sufficient quality suitable for single crystal X-ray data collection
How can this be overcome?
In this case, powder X-ray diffraction must be used
This also gives useful structural information, particularly about the lattice type present in the MOF, but gives less information than single crystal analysis
Vibrational spectroscopy is made up of two main techniques: IR and Raman
These are used to investigate bond vibrations
How are they useful for charactering MOFs?
- They give information on the symmetry in materials, and the nature and strength of bonds
- They are useful for the characterisation of organic functional groups such as hydroxyl groups
- They can be used to determine the phase purity of MOFs and studying the interaction of MOFs with guest molecules
Infrared spectroscopy detects vibrations where there is a …
change in dipole moment
Raman spectroscopy detects vibrations where there is a…
… change in polarisability
What would combustion analysis be useful for determining in MOFs?
Combustion of the material can determine the carbon, nitrogen, oxgyen, and sulfur content; hence it is most useful is MOFs for elucidating information about the organic linker
It gives less information about the metal ions as these do not form volatile oxides
How are inductively coupled plasma atomic emission spectroscopy (ICP-AES) ised to analyse MOFs
- Determining the identity of metal ions can be done with ICP-AES
- This uses plama to excite electrons to higher orbitals
- When these electrons fall back to the ground state, they emit radiation of a particular wavelength which is a characteristic of that element allowing its identity to be determined
How can Thermogravimetric Analysis be used to analyse MOFs
Thermogravimetric analysis is a very routine technique for MOF analysis, where the sample is heated up whilst the cahnge in mass is measured
This allows the amount of absorbed solvent to be determined as well as the thermal stability of the material to be analysed
Describe the changes in this graph produced by Thermogravimentric analysis
- At point 1, the MOF has lots of solvent remaining in the structure from solvothermal synthesis
- Heating to modest temperatures in a vacuum leads to this solvent being driven out of the structure and the first loss in mass
- There is then typically quite a wide temperature range (2) where the material is stable and there is no major change in mass
- At very high temperatures the MOF decomposes to leave just the metal oxides behind with the organic compoents being vapourised (3)
How can Sorption Experiments be used to analyse MOFs
MOFs are of interest due to their permanent porosity
They have very high internal surface areas for gas molecules to adsorb
The amount of gas that can be taken up is measured in sorption experiments
How do Sorption experiments work?
- The data is typically collected in both directions
- As the gas pressure is increased, gas is taken into the material (adsorption or ads)
- When the pressure is decreased, the gas comes back out of the material (desorption or des)
- If this process is reversible, then these two curves should overlay each other
- (Useful MOFs show selectivity for absorbing certain gases over others)
The amount of gas that can be adsorbed in Sorption Experiment is influenced by….
- The internal surface area
- The volume of the pores
- And the chemical functional groups present in the MOF
MOFs are highly porous material
They can have very high percentage of free space in the structure and very high internal surface areas
What other classes of porous materials are known?
- Zeolites
- Porous Carbons
- Porous Organic Molecular Crystals
What are Zeolites?
Are microporous aluminosilicate materials
They have no organic components, and many exist within nature
What are Porous Carbons
Are made by the high temperature combustion of carbon-containing materials
What are Organic Molecular Crystals?
Are gaint covalent organic solids that are held together by reversible bond formation
This uses dynamic covalent chemistry, such as the formation of imine function groups from amines and aldehydes
As MOFs can be robust, solid state materials, they have more potential real-world uses than many of the discrete coordination structures
What can they be used for?
They have been investigated for use in chemical catalysis, for chemical seperation, and for gas capture and storage
NOTT-122 is a MOF which contains voids of three different sizes
With solvent and water removed, the bare metal site in the copper paddlewheels are an excellent site to bind
hydrogen gas, as hydrogen has a high affinity for copper
(there are van der Waals interactions with the benzene rings in this material which aid the uptake of hydrogen)
A MOF has aluminium in an octahedral coordination with four oxygens from the carboxylate and two from bridging hydroxyl groups
How does this affect its porosity?
- Gas sorption studies reveal that the material has a high selectivity for the uptake of both CO₂ and SO₂ with a very low uptake of other gases
- This is due to the hydroxyl groups which can form hydrogen bond interactions with CO₂ and SO₂, but not with other gases
- (can be beneficial for removing these gases from flue gas)
