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Winter 2018

The Importance of Being Defective

Defects as added value in well-designed porous materials for gas adsorption

Jenny G. Vitillo

(a) Design of MOFs as Geomag™ toys. (b) Defects introduced on purpose in a metal-organic framework material allowed the large adsorption of otherwise forbidden cyclohexane molecules. Image courtesy of Jenny Vitillo, ICDC

Researchers spend a lot of time trying to find the perfect recipe to have the perfect sample, but some others are simply looking for the perfect recipe for an imperfect sample.

Designing a defective material. Metal-organic frameworks (MOFs) are a new class of materials that have the same flexibility in design you can achieve with the Geomag™ toy. For those unfamiliar with the plaything, it is a toy construction system consisting primarily of metal spheres and short connecting sticks. Changing the way you connect the spheres with the sticks, you can obtain very different shapes.

In their work, the researchers at the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center, have used UiO-66, among the most famous MOFs. Among the different possible uses of MOFs, they would like to use them as gas “sponges.” Because gases need a container to be transported and used, they are often stored in cylinders at high pressure to have a large amount of gas stored per volume. High gas pressures are associated with an increase in safety issues related to their use, as well as the costs associated with the process. The use of materials as gas sponges allow scientists to significantly reduce the pressure of operation in the gas cylinders with an evident advantage in terms of cost, safety, and practicality.

Why defects can be useful. If you want to store gas molecules in a MOF, the molecule being captured should be smaller than the voids, or pores, in the structures. Moreover, if the pores are too small, a molecule will run slowly in the structure. Using a more technical expression, their diffusion is low. This is a problem, for example, if you would like to quickly charge your methane cylinder when it is empty! Let us imagine running marbles in a Geomag™ toy: The larger the pores, the faster the marbles will go from one side to the other. UiO-66 has pores that are too small to allow some gases to diffuse quickly. For example, it can host molecules having the dimensions of benzene but not those of cyclohexane, because cyclohexane is simply too large. This limits the number of gases that can be stored in the material.

You could think to make the sticks longer to have a UiO-66 with larger pores. However, this would require a significant amount of additional work to optimize a new good synthetic recipe. On the other hand, if you take out a stick here and there, you will get larger voids without the necessity to change the recipe.

How to introduce defects? If you would like to introduce defects in the Geomag™ toy, you can detach sticks from the spheres. In MOFs, scientists introduce defects by reacting the MOFs with water or by changing the construction materials. Researchers at UNCAGE-ME chose the latter method. However, its simplicity is deceptive. Some MOFs are so sensitive to water that the structure can completely collapse after a few minutes in air, just because of the humidity. Moreover, defective MOFs are generally very unstable in air. The necessity to avoid air during the MOF production, storage, and use is an important factor because it will cause an increase of the cost associated with the material. Zirconium-based MOFs have proven to be very resistant to water, especially UiO-66, making it a very interesting candidate for study. How to introduce defects in this MOF while maintaining the chemical stability of UiO-66?

Creating defects on purpose. The researchers at UNCAGE-ME added trifluoroacetic acid (TFAA) into the synthesis reaction. Essentially, TFAA is a stick that attaches to only one sphere per time. It is evident that this trick is enough to have defects in the MOF structure. Moreover, it is water fearing, so it would not increase the affinity of the MOF for destructive water. Because the pores of UiO-66 are too small for cyclohexane, the team used this molecule to verify that the pore dimension of the new material actually increased. The resulting defective structure showed larger cavities than pristine UiO-66, as verified by a significant cyclohexane intake by the “sponge,” whereas the “perfect” UiO-66 adsorbed very little.

Unlike similar results previously reported, the team’s MOF structure was also as chemically and thermally stable as the “perfect” UiO-66. This procedure can be, in principle, extended to other perfect MOFs because sometimes defects are not … a defect.

Acknowledgments

The work was sponsored by the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences. The Hazardous Gas Laboratory at Georgia Tech was used to conduct acid gas measurements. Solid-state nuclear magnetic resonance (NMR) spectroscopy measurements were performed at the Emory University Solid-State NMR Center.

More Information

Jiao Y, Y Liu, G Zhu, JT Hungerford, S Bhattacharyya, RP Lively, DS Sholl, and KS Walton. 2017. “Heat-Treatment of Defective UiO-66 from Modulated Synthesis: Adsorption and Stability Studies.” Journal of Physical Chemistry C 121(42):23471-23479. DOI: 10.1021/acs.jpcc.7b07772

About the author(s):

  • Jenny G. Vitillo is a postdoctoral researcher in physical chemistry at the Department of Chemistry of the University of Minnesota. She is a member of the Inorganometallic Catalyst Design Center (ICDC). Her research focuses on environmentally relevant problems by using experimental and computational approaches applied to systems based on metal-organic frameworks.

Flaws in the Architecture of Gas Sponges

Adding defects makes material adsorb 167 percent more of desired chemical

In many ways, MOFs resemble the Geomag™ construction toy. In the MOF UiO-66, introducing defects increases the material’s ability to adsorb gases of interest. Image courtesy of Nathan Johnson, PNNL

Storing chemicals efficiently can be a challenge. More efficient separation techniques require new materials. One option is a group of porous materials called metal-organic frameworks, or MOFs. The challenges are that the MOFs have small pores and are too delicate. Researchers tinkered with the architecture of the MOF UiO-66. They added defects by adding an acid and heat. The result? Larger pores, a stable structure, and a big increase in the adsorption of a molecule of interest, cyclohexane. In fact, the modified UiO-66 took up 167 percent more cyclohexane. This study helps scientists see how defects benefit MOFs and suggests ways to create defective but stable materials. The work was done at the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center led by Georgia Institute of Technology.

More Information

Jiao Y, Y Liu, G Zhu, JT Hungerford, S Bhattacharyya, RP Lively, DS Sholl, and KS Walton. 2017. “Heat-Treatment of Defective UiO-66 from Modulated Synthesis: Adsorption and Stability Studies.” Journal of Physical Chemistry C 121(42):23471-23479. DOI: 10.1021/acs.jpcc.7b07772

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.