What's the difference between a MOF and a COF? And other questions on MOFs

To welcome the release of our CSD MOF collection, 10,636 structures available for free for academic research, we are answering some common questions on MOFs with the help of some of our team and collaborators.

What is a MOF?

A MOF, or metal organic framework, is a material formed of metal clusters or nodes, linked by organic molecules.

However, a more detailed description of the terminology (e.g. can a non-porous framework containing metal ions and organic linkers still be called a MOF?) is something that is still debated. The International Union of Pure and Applied Chemistry (IUPAC) published guidelines on MOF terminology in 2013 which discusses these challenges.

Certainly it can be said in many cases MOFs are porous, typically 3D, structures with a very high surface area. These properties make them of interest for catalysis, or for storage or adsorption of other molecules in the pores.

What is a MOF - a typical metal organic framework or MOF structure

CSD Refcode WUTXUH a typical MOF structure containing Molybdenum with organic linkers

 

What is the difference between a MOF and a COF?

A metal organic framework can be pictured as a combination of metal atoms (or clusters) as nodes, with organic linkers. In a COF (or Covalent Organic Framework) these metal 'nodes' are instead replaced by a non-metal such as boron or nitrogen.

These COF structures are much less common (with less than 600 structures reported so far) and even fewer with published crystal structures.

 

What are 2D MOFs?

The recommendations of the International Union of Pure and Applied Chemistry give terminology for MOFs in relation to a hierarchy of other definitions. The most broad of these is coordination polymer which covers any repeating polymeric structure in 1-, 2- or 3-dimensions. A subset of this group is a coordination network, which covers only 2- or 3-dimensional compounds.

Finally, the definition of a MOF by these guidelines states that a MOF is "...a coordination network with organic ligands containing potential voids." This definition therefore makes no distinction between a 2D or 3D MOF.

The CSD MOF collection, part of the CSD Software Portfolio, deliberately also takes a very broad definition of 'MOF-like' compounds to give the user the widest possible basis for their research.

 

How are porous MOFs defined?

Answer abridged from Dr David Fairen-Jimenez in the CCDC CrystEngComm joint webinar - watch the full webinar on-demand here.

Once free solvents have been removed from the original CIF structure, simulation methods are used to see if a sphere the size of a Nitrogen molecule could fit into the pores. If it can, the material is defined as porous. If this sphere cannot fit, it is defined as non-porous.

 

How will MOF research evolve in the next 5-10 years?

Answer abridged from Dr David Fairen-Jimenez in the CCDC CrystEngComm joint webinar - watch the full webinar on-demand here.

Machine learning and supercomputers will have an impact in the speed of analysis and predictions about MOFs. There has already been a move from standard Monte Carlo simulations into machine learning methods - this will accelerate the whole field. We went from years with experimental work, to weeks with Monte Carlo simulations, and now to seconds with machine learning. This ML then influences experimental work - we can optimise the synthesis process based on the predicted structure.

Robotics will also accelerate experimental work - leaving researchers free to answer other questions aside from synthesis.

 

How many MOFs are there?

At the time of writing there are 107,980 published MOF structures held in the Cambridge Structural Database (CSD).

Since the metal and the organic linkers can have so many variations it's predicted that over 500,000 structures are possible (ref).

Find the latest number in the CSD statistics updated weekly here.

 

Who are the biggest publishers of MOF structures?

In the last 20 years the number of MOF structures in the CSD has vastly increased; there are now more than 40,000 unique author names present in the MOF subset available in the CSD Software Portfolio.

Two of the most high-profile authors in the field of MOF chemistry are Omar M. Yaghi and Gérard Férey.

The author with the most MOF structures in the CSD is another well-known MOF chemist, Prof. Pascal D. C. Dietzel - his work using time-resolved synchrotron X-ray diffraction measurements leads to many individual data collections, currently over 1,900!

Market reports generally cite BASF as the biggest commercial company involved in MOF research.

 

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