Made up out of thin air

Innovations always emerge from something existing, something tried and tested. Approaching them in terms of their content requires a look at the fundamentals. This is also true for the question of how carbon dioxide (CO2) from exhaust gases can be used for the production of plastics.

The production of plastics requires chemical ingredients such as ethene, propene and ammonia as well as various other ingredients such as methanol, ethanol or acetic acid. For their extraction, elements are needed, including oxygen (O), nitrogen (N), sulphur (S), chlorine (Cl) or fluorine (F) and, last but not least, carbon (C) and hydrogen (H). Hydrocarbons (CmHn) form the chemical backbone of carbon-based polymers and thus of all common plastics.

Usually, hydrocarbons [CmHn] are obtained from fossil resources such as crude oil, natural gas or coal; a small part is produced from renewable raw materials. After appropriate pre-treatment and fractionation, smaller hydrocarbon units (monomers) are cross-linked by chemical reactions such as polymerisation, polycondensation or polyaddition to form larger units, the macromolecules (polymers). These become plastics. Which source the ingredients come from is irrelevant for the manufacturing process. What matters is the result.

In a different way, sustainability is emphasised. This does not only mean the use of renewable instead of fossil resources, a particularly economical use of energy, water and solvents, the use of green electricity generated in an ecologically valuable way. Sustainability also means: an ideally complete and zero-loss circular economy.

In the final analysis, this means that there is no waste, but only recyclable residues that arise in the course of or at the end of a process. Waste gases from oxidation or combustion processes must be included in this consideration. The focus here is on the carbon dioxide (CO2) contained in combustion gases, one of the relevant greenhouse gases along with methane (CH4) and water vapour (H2O). A look at the chemical formula CO2 reveals the reason.

Increasing global warming and its consequences have led to a rethink. The message: greenhouse gas emissions must be reduced in order to achieve the goal of the Paris Climate Agreement of limiting the increase in global warming to 1.5 degrees Celsius. But how?

By using "green" electricity, generated by wind or solar power plants, companies like Covestro [1] are trying to make their production completely climate-neutral by 2035. An important step. But what happens if emissions cannot be prevented? Pumping carbon dioxide from coal combustion into the earth and storing it in deep layers of rock?  Sounds interesting and works, and politically the way is clear [2]. Is this the solution to the problem?

Or is it more of a waste? After all, every molecule of CO2 contains a carbon atom that, simply thought, can be used? The English term for the recycling process that follows on from this is "Carbon Capture and Utilisation" (CCU) - the separation of carbon dioxide and its use as a valuable material in chemical processes.

What may seem simple in layman's terms is more complicated than expected. Carbon dioxide is chemically extremely stable. It takes a lot of energy to make the C in the CO2 molecule compliant. This also applies to hydrogen (H), which is also needed for the production of polymer materials. Energy is needed, and not in short supply. Where do we get it? From the conversion of coal, oil or natural gas into electricity? The wrong approach, because it is not sustainable. Nuclear energy? Nuclear power plants are considered low-emission, but are they environmentally friendly?

Sustainable "green" hydrogen can be produced, among other things, by electrolysis, i.e. the splitting of water (H2O) into oxygen (O) and two hydrogen atoms (2 H), using wind or solar energy generated from renewable sources [3]. Electrolysis with green electricity has the greatest potential from the point of view of climate protection. And what about carbon from gaseous carbon dioxide? Let's take a look at the practice of one of the world's leading companies in the chemical industry:

BASF is currently examining in several projects how carbon dioxide can best be used and introduced into production as a raw material in the long term. Would you like to see some examples?

• The way is home-grown: With the OASE® technology, for example, BASF offers a process with which CO2 can be isolated from waste gas streams and then further processed. [You can find out more about this at OASE® (basf.com)].
• Dry reforming: BASF's dry reforming technology is a catalytic solution with which CO2 can be used to produce synthesis gas. More can be found here: BASF Blue Map | BASF Catalysts and here: Dry Reforming Catalysts: SYNSPIRE™ | BASF Catalysts.
• Emission-free production of valuable, useful methanol - basic chemical for many products: During the production of methanol, waste gas streams consisting of methane, carbon monoxide, carbon dioxide and also hydrogen are produced. The exhaust gases are burnt with pure oxygen in the so-called oxyfuel process. This results in a minimum amount of flue gas with a maximum carbon dioxide content. The flue gas then passes through a gas scrubber using BASF's OASE® process to completely wash out the carbon dioxide it contains. The captured carbon dioxide is fed back into the process at the beginning. [Here's more info: New technologies (basf.com)] [4]

Other companies are also working on solving the CO2 problem through CCU. Evonik, for example, says it is a global leader in the field of speciality chemicals. However, the focus is different. When it comes to the use of gaseous, airborne carbon dioxide, Evonik is guided by the example of Mother Nature and photosynthesis, which is fundamental to life on earth: "Just as plants use solar energy to produce sugar, for example, from carbon dioxide (CO2) and water in several steps, artificial photosynthesis uses renewable energies to produce valuable chemicals from CO2 and water by means of electrolysis with the help of bacteria," it says on the Evonik homepage [5]. The fact that artificial photosynthesis works in the laboratory, it continues, was successfully demonstrated several years ago.

At the end of 2020, another milestone was reached on the way to industrial implementation: the experimental plant at Evonik's Marl site, funded by the German Federal Ministry of Education and Research (BMBF), went into operation. It uses electricity from renewable sources, CO2 and H2O to produce chemicals, demonstrating for the first time that artificial photosynthesis [6] also works on a larger scale.

The test plant in Marl consists of a CO electrolyser developed by Siemens Energy, a water electrolyser and the bioreactor with the know-how of Evonik. View of some relevant de-tails: In the electrolysers, carbon dioxide and water are converted into carbon monoxide (CO) and hydrogen (H2) with electricity in a first step. Special microorganisms use this synthesis gas to produce special chemicals, initially for research purposes. They are the starting materials for food supplements and special plastics, for example.

The attempt to find an efficient and sustainable solution for the recovery of carbon dioxide from the air requires the cooperation of many scientific disciplines. At this point, chemistry, mechanical engineering, process engineering, biotechnology and the energy industry interlock and interact. Concentrated know-how that is needed to preserve our basis of life.

[1] Press release, Covestro becomes climate neutral, COVESTRO, 1 March 2022

[2] Power giants can pump CO2 underground, Welt (08.07.2011)

[3] Flexible production: How is hydrogen produced?, BDEW Bundesverband der Energie- und Wasserwirtschaft e.V.

[4] Personal information BASF, Holger Kapp

[5] www.creavis.com/rheticus

[6] Haas et al, Technical photosynthesis involving CO2 electrolysis and fermentation, Nature Catalysis (2018)