Turning emissions into raw materials

In order to synthesize higher alcohols from a molecule such as CO, a catalyst must enable a series of very different reaction steps. To this end, the scientists need various active centers on the catalyst’s surface. A homogeneous material consisting of a single substance would not be adequate, so the team is using a combination of cobalt and copper.

There are good reasons for this choice. The experts are relying on know-how that was gained previously from similar processes. Cobalt catalysts have proved their worth in the conversion of syngas into longer-chain hydrocarbons by means of the Fischer-Tropsch process.

Copper, in turn, is a tried and tested component of the production process for the simplest alcohol, methanol. “In the synthesis of higher alcohols we are trying to combine these two well-established processes in such a way that they initially form hydrocarbon chains that still contain the alcohol group OH at the end of the process,” Reinsdorf says.

The idea of using a combination of cobalt and copper was only the beginning of working out the details. These included questions regarding the optimal proportions of the two elements and the optimal chemical structures they should have for the process—for example, as metals or in an oxidized form, separated into individual particles or in a mixed phase. The researchers also knew that a trace amount of manganese would help to kick-start the catalyst, but they still had to find out whether it would be preferable to position it on top of, next to, or between the copper and cobalt particles.

They had already carried out numerous tests to determine the optimal proportion of manganese to use. This testing was supported by the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT) in Oberhausen. That’s where the catalyst samples from Hanau are being tested in a parallel reactor system made of stainless steel. Under precisely defined conditions of temperature and pressure, the experts are guiding syngas through the reactors and subsequently analyzing the results to find out what substances have been produced in what quantities. Only the catalysts that have successfully passed the testing process at Fraunhofer UMSICHT will now be subjected to the next stage of testing in the project’s second phase at thyssenkrupp in Duisburg. The technical center that was built there specifically for the Carbon2Chem project has a test setup that is similar to the first one but has three important differences: the reactors are bigger, the syngas supplied to them is extracted directly from the smelter gas of the neighboring steelworks, and the test series will run much longer so that the lifespan of the catalysts can be examined.

The results of the initial test series with more than 40 catalyst variants from Hanau were clear: It is definitely possible to use syngas to produce higher alcohols. This is a major success, but so far the process has generated too many unwanted byproducts—in particular, methane and short-chain alkanes. The newest generation of catalysts results in a more than 50 percent proportion of higher alcohols. “This means we’ve reached the target of the first project phase,” says Wolf. But she’s still not completely satisfied. “We need higher selectivity and even better yields,” she adds.

That’s why the experts are continuing to work on optimizing the catalyst. The first test series showed them that a ratio of four parts of cobalt to one part of copper works well. Now they are specifically investigating process conditions such as the pressure, temperature, and retention time in the reactor. Other significant parameters could include the size of the metal crystallites and thus the incidence of crystal surfaces, corners, and edges in the powder. In order to gain a better understanding of these parameters with the help of experiments, the Evonik researchers are working together with experts from Ruhr-University Bochum and RWTH Aachen.

In parallel to this research, Dorit Wolf and Arne Reins­dorf are addressing another topic: also using carbon dioxide for the reaction. There was no carbon dioxide in the syngas that was used in the tests so far, but carbon dioxide is a major component of smelter gas. “We are now trying to design our catalyst in such a way that CO₂ can be integrated into the synthesis,” says Wolf, adding that “this work is absolutely not trivial.” If this effort succeeds, in the future it could also be possible to use other gas mixtures containing CO₂  as raw materials for the process.

But at the moment the researchers are focusing on increasing the catalyst’s selectivity. They want to not only increase the yield of higher alcohols but also to more effectively control the length of the molecular chains that are produced. So far, the distribution of the chain lengths in the product mixes ranging from ethanol (with two C atoms) to hexanol (with six C atoms) has been sufficient. However, for later users of this technology it would be very useful to be able to precisely control the length of the chains. That’s because, depending on the length of their molecular chains, various alcohols are of interest to very different market participants. Whereas the longer alcohols are especially in demand among producers of specialty chemicals, shorter alcohols such as ethanol and propanol could be used to produce synthetic fuels, and monomers could be used to make monomers for polyolefin production. Bernd Jaeger believes that there is great potential for a catalyst platform in this area. “Ideally, in the future we will offer a whole range of customized versions from which every user can choose the catalyst that matches his requirements,” he says.

Jaeger is also thinking one more step ahead. “Of course the ultimate goal must be to close material cycles,” he says. And the process developed in the higher alcohols project could one day help to achieve exactly this goal. “We’re developing a technology for the recovery of syngas, and this gas can also come from other sources,” Jaeger adds. As soon as the process is established, biogas plants could also be considered as a source of raw material.

In the future, waste plastics could also supply the desired gas mixture if the plastics were decomposed at a high temperature and high pressure in the presence of small amounts of oxygen. Some companies are currently working to scale up this gasification process to the industrial level. “That would be interesting for processing waste flows that cannot be mechanically recycled, for example,” says Jaeger, referring in particular to dirty or mixed waste that contains the plastics polyethylene or polypropylene. “These two polymers in particular are produced in large amounts, but to date they have seldom been chemically recycled,” he says.

Both of these polymer types can in principle be produced from two higher alcohols: ethanol or propanol. In this process, the alcohols are initially converted into ethylene and propylene—substances that previously were extracted from petroleum. The individual building blocks ethylene and propylene could then be used to make new polymers—and thus new plastic products. A material cycle would be closed.

Photos: ThyssenKrupp Steel Europe AG, Robert Eikelpoth (4), Stefan Wildhirt / Evonik
Illustration: Oriana Fenwick / Photo template from Private
Infographic: Maximilian Nertinger