Synthesized for the first time near the beginning of the 21st Century, metal-organic frameworks (MOFs) are still attracting a great deal of research investment as they become increasingly used in a commercial sense.
Of all known materials, crystalline macromolecules with controllable nano-scaled structure MOFs have the largest specific surface areas1. This is a particularly attractive property for applications which vary from drug delivery and catalysis to gas separation and storage.
Developing MOFs depends directly upon surface area and porosity data which is detailed and accurate. For researchers in this field, classic gas adsorption techniques are an incredibly useful tool.
This article provides an introduction to MOFs and how gas adsorption is used in their characterization. Unique insights regarding the potential of this exhilarating type of materials and the practicalities of their development are given by Dr Mircea Dinca, Associate Professor at MIT and a leading researcher in the area.
What are MOFs?
As is implied by their name, MOFs are hybrids of organic and inorganic materials assembled from metal ion clusters or single metal ions which work as nodes. These are connected through organic linkers or struts.
Under reaction conditions which are closely controlled, the periodic structures of MOFs self-assemble. This typically occurs in the presence of a solvent, however, processing routes which are solvent-free have also been commercialized1.
Learning how to develop structures which were stable and rigid was a crucial moment in MOFs’ evolution2, but since then, synthesis techniques have become firmly established. A wide variety of organic ligands and transition metals can now be confidently stitched together by chemists, in order to make MOFs which have properties that are relatively predictable.
Unprecedented flexibility and unique structural diversity are offered by MOFs, which allows them to produce the porosity profiles necessary in order to exert control at a molecular level. MOFS’ tenability reaches far beyond porosity and surface area. In order to enhance commercially useful properties, additional functionality such as hydrophilicity or hydrophobicity is readily incorporated.
Once MOFs could be made permanently porous people really began to recognize the potential not only to control porosity but to augment, for example, electrical, adsorptive or catalytic properties. This tunability of MOFs is an important commercial factor because it offers considerable scope for the development of intellectual property. Investment is fueled by the recognition that there are important societal challenges that MOFs are uniquely well-placed to help with, where we are reaching the limits of the performance with traditional materials
Dr Mircea Dinca, Associate Professor, MIT
The Potential of MOFs
In 2016, a fruit packaging system constituted the first commercial application of an MOF3. Working to slow the ripening process, the MOF releases 1-methylcyclopropene which binds with ethylene receptors in the fruit.
There is also now a commercially available MOF which is used to store toxic gases used as dopants in the semiconductor industry (such as boron trifluoride, phosphine, and arsine)4. This is able to reduce safety concerns as it facilitates storage under pressures which are significantly lower than those used conventionally. These applications are fairly niche and consequently the costs of MOFs, though still reducing, remain an issue for commodity applications.
Reducing cost is important for the commercialization of MOFs but shifting certain misconceptions about what MOFs are and what they are not is also critical. MOFs are not necessarily a replacement for zeolites, for example, rather they offer potential for applications where zeolites and other materials may not be suitable. And it’s vital to recognize that not all MOFs are made the same. MOFs are also not the answer for every application – no one class of materials is – but it is important that their value is not underestimated simply as a result of choosing a MOF with properties that are misaligned with the intended application.
Dr Mircea Dinca, Associate Professor, MIT
Since it is possible to uniquely tailor MOFs so that they retain specific molecules, they are useful for the capture, separation, or storage of liquids and gases. This means that there is the potential for storing enormous volumes of gas, or even for separating gases using minimal energy input. These possibilities offer a great deal of scope for reducing the consumption of energy which is associated with the processing of gas.
The use of MOFs in catalysis is already being explored and there are exciting applications sure to be developed in the near future, such as ‘water harvesting,’ where water is extracted from air using solar energy1.
Quantifying Surface Area and Porosity
The performance of an MOF is defined by its porosity and surface area. This means that measurement is crucial, and gas adsorption is considered the best technique for doing so.
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Figure 1. A simple schematic of a generic, volumetric gas adsorption analyzer showing, from left to right - degassing, charging of the manifold, and gas absorption onto the sample.
A generic volumetric gas adsorption apparatus is shown in Figure 1. Measurement starts with sample preparation – outgassing or degassing – which is usually undertaken at a slightly elevated or ambient temperature. Adsorptive gas is then used to charge the manifold of known volume to a specified pressure. The mass of gas or number of moles present can then be calculated using the gas law.
The adsorption of gas into the sample is enabled by opening the manifold to the sample tube. The amount of gas which is adsorbed is calculated once the pressure has reached equilibrium by difference using the gas law.
An adsorption isotherm is produced for the material by repeating this process at progressively increasing pressures. This is a unique fingerprint of the material’s textual characteristics. Displayed in Figure 2 is the process which occurs at the molecular level during the gas adsorption process.
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Figure 2. In a gas adsorption measurement, the pores of the sample progressively fill with the adsorptive gas as pressure is increased.
With the use of appropriate mathematical theories, porosity and surface area can be calculated using an adsorption isotherm. The most commonly used technique for the surface area is the Brunauer, Emmet, and Teller (BET) theory. In order to determine pore size distributions, the Barret, Joyner, and Halenda (BJH) method is used, using the Kelvin model of pore filling.
Pore volume distribution by pore size, total pore volume, and specific surface area (the surface area per unit of mass) are included within the parameters generated. The range of pore size which is covered extends from the ultra-microporous region up to the macropore – or 0.3 nm to 300 nm – which easily covers MOFs’ range of interest. More modern models for determining pore size distributions are based on statistical thermodynamics and account for the influence of confined space.
Using the Density Functional Theory to create models for pore size calculations has for instance become a well-established approach. Micromeritics have long pioneered the application of NLDFT models and ongoing and significant investment in NLDFT modeling is part of the company’s commitment to providing access to the very best porosity characterization technology. For example, by developing the first commercially available MOF model.
For MOFs surface area and porosity are defining characteristics so gas adsorption is essential both to characterize new materials and to check whether an existing material has been made reproducibly, essentially as a proxy for QC. I’ve been using gas adsorption systems from Micromeritics since graduate school and they deliver a level of dependability that is relatively rare in a research-grade tool. Our systems require minimal routine maintenance and incur very little in the way of downtime or repair costs.
Dr Mircea Dinca, Associate Professor, MIT
Optimizing Gas Adsorption for MOF Applications
While the porosity and surface areas of MOFs are characterized by physisorption, chemisorption can also be applied courtesy of advanced systems.
In chemisorption, an active adsorptive gas is used instead of an inert one in order to quantify the level of active sites on the surface of the sample. Being able to alternate between these two measurement modes enables more advanced MOF characterization to be performed. For instance, this allows the investigation of adsorption enthalpy as a function of ligand or linker structure.
More broadly speaking, a single analysis is able to simultaneously utilize more than one gas courtesy of cutting edge systems, in order to gain a more detailed understanding of mechanisms for adsorption site binging.
These systems provide the precise temperature control and gas management which is required for high resolution ultra-micopore and micropore measurements. They also enable the accurate collection of data at low pressures and the investigation of behavior at low loadings. These capabilities, when combined with software which is increasingly well-oriented to MOFs, significantly increase the contribution of gas adsorption to the optimization of MOFs.
An important strand of our research is MOFs that can be used in applications involving ammonia and water, including adsorbents for heat pumps and for gas masks, and the development of technology that can generate fresh water from air. Micromeritics has helped us to customize our gas adsorption systems so that we can use water and ammonia as the adsorptive gases to gain highly relevant information. The company’s responsiveness and knowledge are really valuable when it comes to issues such as this and help us to get the best out of the excellent hardware.
Dr Mircea Dinca, Associate Professor, MIT
Looking Ahead
There is an exciting prospect that MOFs could directly answer the questions posed by societal challenges relating to transport, pollution, and energy consumption, as well as the availability of water and healthcare.
The precise, accurate, and detailed characterization of the porosity and surface areas of these interesting materials is vital for progress. Analytical instrumentation which is able to deliver this information efficiently and reliably consequently plays a crucial part in establishing MOFs’ commercialization.
References and Further Reading
- Scott, A ‘Round two for MOF commercialization’ C&EN, Vol 95, Issue 24, Pages 18 – 19, Jun 2017
- Yaghi, O ‘The New Chemistry of Metal Organic Frameworks (MOFs) Video presentation available to view at: https://www.youtube.com/watch?v=bzDIH7olP0c
- ‘MOF Technologies announces world’s first commercial application of Metal Organic Framework technology by Decco Worldwide at MOF 2016’ News item available to view at: https://www.moftechnologies.com/
- ‘ION X. Engineering Solutions for Next-Generation Electronics’ whitepaper available for download at: https://www.numat-tech.com
This information has been sourced, reviewed and adapted from materials provided by Micromeritics Instrument Corporation.
For more information on this source, please visit Micromeritics Instrument Corporation.