In society, CO₂ has obtained a negative reputation due to its contribution to global warming. However, carbon is one of the main building blocks in many industries. CADIAC aims to catalyze the conversion of CO2 to forms that could be directly used in production.

CO₂ is present around us in almost infinite quantities. If chemical technologies can be developed to exploit this molecule, CO₂ may become a valuable chemical resource instead of just being considered as a waste product. For many years, it has been common knowledge that the level of greenhouse gases has been increasing since the beginning of the 1960’s, which may be directly related to the global population growth and consequently the industrial expansion (1). Figure 1 shows the recordings of the atmospheric CO₂ concentration since 1960. However, the effect of the increase of greenhouse gas concentrations (such as CO₂) on the climate has not always been clearly connected. To observe and identify the connection between CO₂ and climatic changes, basic research has played an important role.

Figure 1: CO₂ concentration as a function of time. Data collected by NOAA-ESRL (1,5).

Climate scientists from all over the world have collected and compared data from scientific measurements of a variety of parameters over a timespan of many years. These measurements include parameters such as atmospheric composition (and thus CO₂ content), average temperatures, and ocean water levels. As is almost always the case with basic research, the time can be long before concrete results and trends are made available. Already back in 1975, Wallace Smith Broecker suggested a connection between the global temperatures and the CO₂ concentration in the atmosphere (2). Forty years later there is no longer any doubt about the correlation between greenhouse gas concentrations in the atmosphere and the observed climate changes.

Fossil fuels create climate change

The most abundant of these greenhouse gases is CO₂, which is also considered the major contributor to global warming. Thus, limitation of CO2 emission into the atmosphere is imperative. The burning of fossil fuels such as oil and gasoline represents one of the major sources of these CO2 emissions. Most of us rely on these fuels every day in one or another form of transportation.

Industry is another significant source of CO₂ emissions. Here, these emissions can arise from fossil fuels as an energy source, which drives the factories’ machineries, but there are also other sources of CO₂. For example, two types of contributions can be observed from the cement industry. One from the fossil fuel used for heating the cement kiln (furnace), and then an even larger contribution from the calcination of CaCO₃ (also called lime or limestone). This latter process involves the heating of CaCO₃ in the cement kiln to about 1450 ?C. In this process, CaO (which is the main ingredient in cement) and CO₂ are formed in such large quantities that cement production alone accounts for five percent of the annual CO₂ emissions on the global scene (3).

CO2: From waste to value

Preventing or limiting the CO₂ emission of today’s modern societies has proven to be an extremely difficult task. A new angle could therefore be to rethink our perception of CO₂ and instead consider this wasteful gas as a useful resource. Researchers from the Carbon Dioxide Activation Center (CADIAC) at Aarhus University funded by the Danish National Research Foundation are trying to answer this question through their research. The name of the Center (Carbon Dioxide Activation Center) describes in a simple way what is necessary for CO₂ to be considered as a chemical resource, namely that CO₂ must be activated. One of our projects involves the transformation of CO₂ to carbon monoxide through an electrochemical reduction process. As CO₂ is a thermodynamically and kinetically stable compound, catalysts are required to activate this molecule in order to carry out chemical transformations. Hence, much focus in the Center has been concentrated on the identification of inexpensive and effective catalysts for performing this reduction step.

Why carbon monoxide? Unlike its less reactive counterpart, this diatomic molecule is very reactive and has been used by the chemical industry for over 100 years in chemical synthesis for the preparation of hydrocarbons as synthetic fuels, and bulk oxygenated compounds, such as alcohols, aldehydes and carboxylic acid derivatives. It is currently formed from steam reforming and coal gasification, which both rely on fossil fuel sources. Furthermore, carbon monoxide is highly toxic, colorless and odorless, and hence its handling requires extreme caution. If methods could be found for the selective exploitation of this greenhouse gas for its transformation to carbon building blocks for the chemical industry, such as carbon monoxide, this could potentially have a significant and beneficial and thereby counteract global CO₂ emissions.

Other potential uses of CO₂ could come from the polymer industry which produces the plastics we use on a day-to-day basis. Today, almost everything is packed in plastic, so the need for technologies to produce sustainable packaging materials and bags, etc. is still increasing. Just like in the fuel industry, the starting material for plastic is crude oil. So, the plastic industry faces the same challenges with limitations in future fossil fuel resources. The transformation of CO₂ to the carbon building blocks of the polymer industry is therefore an area of high focus from many researchers including those from the Carbon Dioxide Activation Center.

Finally, the pharmaceutical and agrochemical industries are third examples of where carbon based building blocks are highly important. At the Carbon Dioxide Activation Center, we have demonstrated that CO₂ can be exploited as a building block to prepare different active pharmaceutical ingredients of commercial drugs (4). In particular, the Center has shown how the following active pharmaceutical ingredients can be produced with CO₂ as one of the starting materials:

• Moclobemide : treatment of depression and anxiety
• Butoxycaine : local anesthesia
• Tigan : nausea and vomiting
• Olaparib : ovarian cancer

In addition to this, CADIAC has also demonstrated how to capture the CO₂ from the exhaust gas of a car, and to exploit it as a chemical reagent for the synthesis of the antidepressant drug, moclobemide.

Basic research can transform CO₂ into a valuable resource

With the concentrations of CO₂ present in the atmosphere and considering how much of this gas is emitted every day, this triatomic molecule represents almost an infinite chemical resource. Nevertheless, basic research is essential in order to develop useful methods for the activation of CO₂ to fully exploit its potential as a chemical reagent. Such are the efforts of many researchers on a global basis including the Carbon Dioxide Activation Center. If new effective catalysts can be identified, undoubtedly such molecular machines will have a significant impact on the exploitation of CO₂ in the chemical industry.

References

1. https://www.co2.earth/daily-co2
2. Broecker WS. Climatic Change: Are We on the Brink of a Pronounced Global Warming? Science 1975;189:460-3.
3. Worrell E, Price L, Martin N, Hendriks C, and Meida LO. Carbon Dioxide Emission from the Global Cement Industry. Annual Review of Energy and the Environment 2001;26:303-29.
4. Pedersen BL. CO2 kan blive fremtidens ressource. Videnskab.dk d. 22. januar 2017. http://videnskab.dk/naturvidenskab/co2-kan-blive-fremtidens-ressource
5. By Scrippsnews – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=58530710