Carbon Dioxide Green Sorbents

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Carbon dioxide is a greenhouse gas. Copyright Image by Decoded Science, all rights reserved.

Carbon dioxide is a greenhouse gas. Copyright Image by Decoded Science, all rights reserved.

Researchers have developed materials that can absorb carbon dioxide (greenhouse gas) produced with a green synthesis.

The materials are oligo-urea-based (polymers of urea) and were prepared in a process that followed ten of the twelve principles of green chemistry.

Carbon Dioxide

Carbon dioxide (CO2) is a gas that is naturally present in the atmosphere. Since the industrial revolution, however, its concentration has increased remarkably, due to anthropogenic activities, such as fossil fuels combustion.

The higher carbon dioxide atmospheric content causes worries since CO2 is a greenhouse gas; according to some people this is one of the causes of global warming.

Further to this, higher CO2 concentration is also responsible for increased levels of acidity in the sea; this phenomenon is referred to as “ocean acidification.”

Limiting CO2 Emissions

To address these problems, several measurements have been, and are currently, implemented: the use of alternative fuels or fuels with lower emissions, for instance, is a solution that could contribute to reversing the current trend in increasing CO2 emissions. Another interesting approach is Carbon Capture and Storage (CCS); with this technique, CO2-containing gases (for instance, from a power plant) are passed through compounds which can capture CO2 before their emission into the atmosphere.

CO2 Sorbents

The most common way to capture CO2 is with the use of solid sorbents – materials used to absorb (soak up) or adsorb (retain on the surface) liquids or gases. For the process to be effective, however, the sorbent must have high efficiency, be stable in the conditions in which the sorption process is performed, and be selective towards CO2. Moreover, the materials also have to be made with a process that is cost-effective and sustainable for the environment.

Green Chemistry

Green Chemistry uses non-hazardous compounds. Photo by Nemo.

Development of Novel Green Sorbents

Researchers from the TU Munich (Technische Üniversität München, Germany) developed new CO2 sorbents based on oligo-urea materials. The researchers performed this study in cooperation with the University of Jordan, the German Jordanian University, and Qatar University.

They published their results in Chemistry and Sustainability (ChemSusChem) in April 2015.

Dr. Abdussalam K. Qaroush, one of the researchers involved in the project, explains to Decoded Science the work he and his coworkers did, and its importance.

“Although scientists developed several CO2 sorbents, their manufacture often requires the use of toxic chemicals and/or solvents; considering this, the overall impact that these materials have on the environment may not be as good as thought.

Because of this, we tried to have a greener approach, as we wanted to make a CO2 sorbent following as much as possible the 12 principles of the green chemistry.”

Oligourea-based Materials

Professor Bernhard Rieger, leading researcher of the study, and his coworkers decided to study oligourea-based materials among others. These are compounds in which more urea molecules are joined in a chain. Depending on the number of urea molecules, present, the characteristics of the materials may be different.

According to Dr. Qaroush:

“We chose these materials due to their structure; they are expected to capture CO2. What is important, however, is that we made the oligourea materials with a process much more green than conventional ones. In fact, we prepared the oligoureas without using any toxic compounds, such as phosgene or isocyanate, without any solvent and using a non-toxic molecule (polypropylene carbonate) as a carbonylating agent (the source of carbonyl group, needed for building up the material (building block A).

In this process, we followed ten out of the twelve green chemistry principles. To our knowledge, the first time that such “green” CO2 sorbents are made.”

 A (polypropylene carbonate) + B (diamine) ” AB ([n]-Oligourea)

Urea

The sorbents were made using urea. Image by Ben Mills

Testing Sorbents

The researchers prepared materials with carbon chains within the diamines (building block B) to be used, viz., 4, 6, 7, 10 and 12; then they tested the CO2 absorbing performance.

“The material which worked best was that with ([10]-Oligourea, [10]-OU: where the number in brackets stands for the carbon atoms within the building block). It absorbed about 24 mg of CO2 per gram of sorbent. We are aware that there are sorbents with better performance. We have to highlight, however, that the overall performance of the materials did not get worse after various cycles of CO2 sorbtion/desorbtion, this is an important issue, which in a way partially compensates for the lower CO2 absorption.”

Improving the System

As Dr. Qaroush says:

“This was the first attempt to produce green CO2 sorbents, and we got some interesting and promising results. We will continue to work on these materials, to improve them.

Our goal is to use renewables as starting materials; doing this, we will have eleven of the twelve green chemistry principles followed. We want also to increase the CO2 capture performance.”

Green Chemistry to Decrease Atmospheric CO2

The research described here shows how we can achieve a decrease in atmospheric CO2 while following the principles of green chemistry. Although the sorbents still have to be optimized, this research is a first step in this direction, to reduce CO2 emissions while minimizing the impact on the environment.

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