CD-MOF-2 is a new material recently developed to adsorb carbon dioxide. Adsorption is a process by which CO2, or carbon dioxide, gets fixed on the surface of the solid material. CD-MOF-2 is based on γ-cyclodextrin, a sugar which can be synthesized from renewable sources (starch). It shows high selectivity, adsorbing almost only the greenhouse gas CO2. Moreover, the adsorption is completely reversible, making the material reusable for successive cycles.
Carbon Dioxide (CO2) In the Atmosphere
Carbon dioxide content in the atmosphere has increased remarkably in recent years; this is due to human activities, especially the emissions into the atmosphere of the gases produced from the combustion of fossil fuels. This causes concern, as this gas is environmentally harmful.
In fact, it is known that CO2 is a greenhouse gas: this means that it can absorb some of the energy/heat emitted by the earth; consequently this energy gets trapped inside the atmosphere. It is thought that these increasing concentrations of this gas can be the cause of an increase in the Earth’s average temperature (Global Warming).
Furthermore, CO2 can react with water (H2O) to form carbonic acid (H2CO3). The presence of higher amounts of this acid in the atmosphere and/or in waters can affect the life of some species, and the equilibrium of the whole ecosystem.
The main solution to this problem is the reduction of the CO2 emissions; however, other short-term options, such as CO2 Capture and Storage (CCS), are also considered.
Capture and Storage: The Idea and the Problems
The idea behind CCS is to remove the CO2 from the gases before they are emitted in the atmosphere; then CO2 can be transported and stored in a place where it cannot cause any damage to the environment.
Several projects and studies were performed in this field. In the European Union, for instance, a detailed research strategy was developed; in the United States, the Environment Protection Agency has regular reports on this matter.
One method used for the capture is the adsorption of CO2 on a solid material, which acts as a filter. For this process to be effective and worthwhile, it is important that:
- The adsorption is selective towards CO2, i.e. no other compound (or very little) is adsorbed on the solid.
- The efficiency has to be high, i.e. a high amount of CO2 has to be removed.
- The adsorption has to be reversible. In this way the gas can then be desorbed and transported to the storage site.
A new material with all these characteristics was recently developed and tested by some researchers; the study was part of an international research project, involving universities from the US (Northwestern and California Universities), Saudi Arabia and South Korea.
The principle this material is based on is the Metal Organic Framework (MOF): this is a structure formed by a metal coordinated to a larger organic molecule, with a very specific geometry.
In this case, the MOF is formed by rubidium (Rb) coordinated to a sugar molecule, the γ-cyclodextrin (γ-CD). The material, indicated as CD-MOF-2, has an ordered crystalline structure, with the cyclodextrin molecules coordinated to each other through the rubidium in units of six; this structure is very important for the material to work as a CO2 adsorber.
CO2 adsorption and desorption
The principle for the adsorption of CO2 is a reaction between the CO2 itself and the hydroxyl (OH) groups present in CD-MOF-2, to form a carbonate ion (RCO3–).
This gives the material a very high selectivity towards CO2; other molecules, in fact, do not react (or react to much lesser extent) with the hydroxyl groups. When the same system was tested with methane (CH4), for instance, a volume of gas up to 3000 times smaller was adsorbed.
Furthermore, the desorption process took place very easily and quickly: the results showed that, as soon as CO2 was not in contact with the material, the reaction between the gas and the OH groups reversed. This can be seen clearly in the Figure on the side. In this way, CO2 can be released for storage, and the material can be used again.
In addition to all these good characteristics, another very important feature of CD-MOF-2 is that it can be produced from natural sources, without using petrochemical-derived products. Dr. Ross Forgan, one of the researchers involved in the project, explains: “γ-cyclodextrin is a sugar which can be obtained from starch with a simple reaction. Therefore, it can be obtained from a renewable source and it is quite cheap. No toxic solvents or other artificial compounds are necessary in this process. This makes CD-MOF-2 even more valuable – a selective and efficient material for CO2 adsorption which can be prepared with a green process.”
The paper of Dr. Forgan and his co-workers was published in the Journal of the American Chemical Society.
European Commission: “CO2 capture and storage projects.” Accessed September 2011.
US Environmental Protection Agency: “Carbon Sequestration in Agriculture and Forest.” Accessed September 2011.
J.J. Gassensmith, H. Furukawa, R.A. Smaldone, R.S. Forgan, Y.Y. Botros, O.M. Yaghi, J.F. Stoddart: “Strong and Reversible Binding of Carbon Dioxide in a Green Metal-Organic Framework.” Journal of American Chemical Society, dx.doi.org/10.1021/ja206525x.
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