To green chemistry with renewable raw materials

Prof. Dr. Hans-Willi Kling from the Chair of Management of Chemical Processes in Industry/Analytical Chemistry on the still untapped advantages of lignins

In the beginning there was the wood ...

When reading about the development of industrial chemistry, one realizes that from the middle of the 19th century many natural substances were modified by chemical processes in order to obtain cheaper and better materials. Above all, cellulose, as the main component of all plants, came more and more into focus. However, the introduction of far cheaper crude oil in the 20th century once again shifted the premise to the huge industrial plants that still exist today and cannot be converted at the moment.

At the University of Wuppertal, chemist Prof. Dr. Hans-Willi Kling has been researching natural raw materials and their future use in industry for years. "We simply have to go back a bit, to the time before 1950," he explains, "when the basis for the chemical industry was not crude oil at all, but rather, the basis for the chemical industry was coal, tar and pitch from coke production. And if you then go back a few more years, the basis for the chemical industry's raw materials was wood."

Lignins; renewable all-rounders

In translation, lignin simply means wood. Lignins are solid biopolymers that have a stabilizing and also protective function in the cell and are therefore stored by the plant. In combination with cellulose, they give the plant the necessary stability. Kling explains it using the example of the tree: "It builds a cell wall from three scaffolding substances, namely cellulose, hemicelluloses and lignin, and uses them to glue the fibers together. The outcome then is ultimately the wood. A tree needs lignin in order to stabilize the cellulose afterwards. In addition, lignin also prevents the penetration of rot. The wood becomes impervious and stable."

A waste product with potential

To Kling's chargin, lingin is only produced as a waste product during the extraction of cellulose. Cellulose is macerated into pulp, and is thus one of the most important raw materials in paper production, but is also used as a basis for other substances. "That is actually a bit sad," he says, "lingin is way to valuable to simply burn it as you do today to gain energy from it." In production wood is therefore firstly chopped into so-called chips for pulp production and then broken down in various processes. "What remains is the cellulose, the pure product. Unfortunately, that is only 40% from the tree. 60% basically goes into the lignins and hemicellulose. The lignins are then left as a 15% aqueous solution. Burning a 15% solution means that I have to put an insane amount of my energy, which I actually want to use, solely into evaporating the water. Right now it is worth it because it is heavily subsidized through renewable energies," but Kling knows what else lingin can be used for. At the moment, the environmentally friendly, globally available and inexpensive material is simply being used as fuel. "I believe lignin has a very, very great potential," the scientist asserts, suggesting that the degraded polymer could simply be rebuilt. "During the pulp extraction the polymer lingin is being broken down to its basic components or to shorter components. But, I could of course also rebuild. Then I would have sort of an artificial wood." The Academic Chair is already cooperating with some small companies that are using this approach.

Finding alternatives to petrochemicals

Parallel experiments take place at the university itself. In these experiments it is tried to convert these lingins into aromatics or substances similiar to aromatics and thus replace them as a natural substitute for the aromatics used in petrochemicals. "That is where I would start, with the petrochemical chain," says Kling, "except that I would not use petroleum in this process, since it does not regrow. But, I would use renewable raw materials. I would not burn them, but rather put them to material use."
There are researchers who see lignin as a possible alternative to petroleum in the plastics industry and speak of bioplastics. The chemist does not see it quite so simply, because the polyethylene and polypropylene used in vast quantities today can hardly be replaced with natural raw materials. However, polystyrene, known as styrofoam, could actually be used, since "I have an aromatic in there," he explains. "Ultimately, based on this aromatic chemistry, I could obtain very many substances that are currently obtained from petroleum on the basis of renewable raw materials, and thus, in that sense, build a green chemistry."
The road to this goal is long and rocky, because Kling also sees the problem that primarily occurs in the pulp industry. The possibilities of lignin are beyond discussion from an economical point of view. "In principle, we would have to match the demands of the pulp industry with the demands of the chemical industry, in order to always get the same qualities. Then, building corresponding large scale processes would be worth it."

The difficult path of resource conservation

Although the younger generation is becoming more and more interested in conserving resources with regard to in the climate debate, an implementation still seems to be difficult. The situation is complex, Kling states, "while producing clothes, I go into the cellulose fiber. Modified cellulose from wood can easily compete with cellulose fiber from cotton. That is not the problem," but lignin must first be chemically converted, he says. "You now have to convert the processes to large-scale chemical processes, because only then you can produce cheaply," he explains. But these plants do not yet exist. That is where a cycle begins. The producer does not yet have his raw material in the appropriate quantity, and thus stays with petrochemicals to be on the safe side. "That is where it is simply a matter of breaking through these knots! In pilot plant trials, we have already demonstrated that this lignin can be converted to aromatics and can be incorporated into the aromatics chemistry."
All of the processes have been known for years, Kling says: "At the beginning of petrochemicals oil was cheap. A barrel cost two to three U.S. dollars back then (in 2019, a barrel cost 62,4 USD ed.). Today, the chemical industry simply has these huge plants. They specialize in processing petroleum and cannot easily process other raw materials. In order to put chemistry back on a different footing, you would now have to look intensively at these old processes again, and combine them with the possibilities of today's technologies.

The new wood

The new wood with which Kling is experimenting, is a fiber-reinforced material that uses components of the natural material lignin. By purifying it and returning it to the polymer, the properties of the polymer can be controlled again. He has also already developed a deodorizing process for the momentary odor generated by the process.

"We have to get an insane amount of this raw material lingin. They are talking about 20 billion tons as nature's annual synthesis output. It is not just in wood, but it is found in grass, straw, many byproducts of agriculture. We have extract it and then make it compatible with existing processes and equipment. Then you have won."

Actually, everything is there, the processes are known, the material is available in abundance and can be used in a resource-saving way. Sooner or later, the chemical industry has to realize that it can make a decisive contribution to protecting our planet, because, Kling concludes, "chemistry does not care where the basic molecules come from, whether from petroleum or wood." And it all started with wood, after all.

Uwe Blass (Interview on November 27, 2020)

Prof. Dr. Hans-Willi Kling studied at the Ruhr University in Bochum and earned his doctorate at University of Wuppertal. He held various positions in the private sector. From 2003 on, he taught at the University of Wuppertal in parallel to his industrial activities and followed the appointment to the Chair "Management of Chemical Processes in Industry" in 2010. In 2012, the consolidation with the "Analytical Chemistry" followed under his leadership.