For when the oil runs out

Producing plastic and nylon from plant waste

Bart Braun

Two recent Leiden papers reveal how to make useful materials from plant waste, such as nylon from wood instead of oil. But can we build a bio-based economy on that?

“The chemical industry will have to stop using petroleum sooner or later”, explains Marc Koper, an electrochemist. “Not only because of the limited amount of oil in the world, but also, primarily even, because a more sustainable economy is on the political agenda. However, it would be good to have the necessary technology by the time the politicians are ready and that will require a lot of new chemistry.”
Over the past year, two Leiden papers in ChemSusChem, the journal for sustainable chemistry, discussed this new kind of chemistry. This month, Koper, one of his former PhD students, Youngkook Kwon and two employees of the Amsterdam-based company Avantium published their work. Koper says: “They’re looking for a smarter way to produce fructose to build bioplastics for coke bottles that will decompose organically, and so on.”
The authors describe the conversion of sorbitol – relatively easily produced from glucose, a form of sugar. In turn, glucose is easy to make from cellulose, the stuff surrounding plant cells. Using a platinum electrode, Kwon and his colleagues converted glucose into fructose and sorbose, two other kinds of sugar.
“Although platinum is a precious metal, it reacts with more substances than you might think”, explains Koper. “It’s a good catalyst for many chemical conversions, but not a very selective one: it will convert sorbitol back into glucose again.” Chemists solve that problem by adding a promoter to change or enhance the effect of the catalyst.
“In this case, the promoters are things like bismuth and antimony, which ensure that the reaction we want is one-way to fructose instead of glucose. As a chemist, I find controlling the reaction very exciting, of course, but we don’t quite understand how it works.” He hopes that he and some scientists from Brazil, a country investing heavily in the use of biomass, can do more research on that subject next year. “To see whether we can find a better means of guiding the reactions or steering it in another direction.”
Earlier in the year, Professor Lies Bouwman, who works on the same floor of the Gorlaeus Building as Koper, wrote an article on a new process for manufacturing nylon. “Modern factories make caprolactam, the building block for nylon, from petroleum but that process produces four to five kilograms of ammonium sulphate and harmful nitrogen compounds for every kilogram of fabric”, says the professor of Inorganic Chemistry, “While our process only produces a tiny bit of water.”
It’s not just a difference in chemical method, their starting points differ too. Instead of using petroleum products, Bouwman and her colleagues used gamma-Valerolactone (GVL), which relatively easily produced from plant material such as wood waste.
The conversion from GVL to caprolactam passes through a number of different stages, the last of which are carried out in one vessel. A crucial ingredient is the catalyst, a compound of the metal rhodium in this case. For the fans: its full name is rhodium-4,5-bis(2,8-dimethyl-10-phenoxaphosphino)-9,9,-dimethylxanthene.
“We can influence the reactivity of the metal part by binding a group onto it which allows us to steer the reaction in a certain direction. We have tried a good many catalysts and this was the best so far” – “so far” because the squeaky clean reaction that produces nothing but water is only 87 per cent of what happens during the reaction. Bouwman continues: “Thirteen per cent by-product is a huge amount: we need a 98-per cent efficiency before it can be used by the industry. We hope we can avoid undesirable by-products by changing the reaction conditions and looking for a more suitable catalyst.”
In addition, the current catalyst wears out quickly. On average, after a hundred reactions, the group surrounding the rhodium starts to oxidise or disintegrate. “The catalyst will need to be usable for about a million times for commercial purpose. We’re working on that too, but I can’t say much at the moment because we might want to apply for a patent.”
Then there is the eternal problem of university research: things that work in the lab are not necessarily are cost-effective or safe enough to be produced in a factory, a problem Bouwman knows all too well. Nevertheless, she has succeeded before. “They use chlorine to produce epoxy resin but thanks to our research, less chlorine is needed in one of the manufacturing stages. A former post-doctoral researcher from our group is currently working with a plant in the United States that uses our improved method.”
Will the same thing happen with nylon? Bouwman explains that doesn’t just depend on her catalysts: “Nylon manufacturers already have factories that produce nylon and a new factory would cost billions while the profit margins for nylon are low. A company like BASF won’t build a factory for plant-based nylon if rival companies in China are using the dirty method.”
The problem is that the “dirty method” won’t last forever: the amount of oil the earth has is limited and the available oil is more and more difficult to acquire. Besides, it is theoretically possible that humanity will actually do something about climate change and if we do, drilling for oil for fuel will not be allowed. Koper’s and Bouwman’s research were both funded by Catchbio, a project for university researchers and the industry to find catalyst-chemical alternatives for our oil-based economy. The source of the products is to be biomass, and Catchbio’s website reports excitedly on the “transition to a bio-based economy”.
However, the big question is where all that biomass is to come from. If we grow plants specifically for that purpose on agricultural land, we will have less food and nobody is in favour of clearing areas of forest.
“If we’re talking about processing waste, it means, by definition, that the scale of production will be small,” says Ester van der Voet from the Institute for Environmental Sciences in Leiden. “And where do we find that waste? The remains of plants can be given to cattle to eat too. If you don’t have any waste to give them, you’ll have feed them something else. Every case will have to be assessed separately to see which option is the best for the environment.”
Forestry produces plenty of biomass: if you take a really suitable tree, you can use about forty per cent of it for timber. The rest of the tree can be used too: for paper, compressed fire bricks and that curiously soft material IKEA uses to build furniture. Other options need to be found for those materials, the environmental effects of which are not yet clear.
Van der Voet adds: “It’s a pity that Dutch programmes don’t consider these side-effects in economic terms – it’s always included in European programmes. If you don’t, they’ll send it back and you’ll have to go back and study the impact. You have to look further in Europe.”
Nonetheless, there are more options for forestry than agriculture, according to the environmental scientist. “We can’t have a bio-based economy in which we acquire our fuel from plants: we’d need six earths to grow all the plants. That’s less important for materials: the amount of the oil used for nylon or plastics is much smaller.” Bouwman adds: “Maybe it would be better to make implements from oil and reduce the oil consumption in the transport industry. Anyway, we’re learning from it. Even if it turns out we can’t apply it here, we can apply it somewhere else. Fundamental knowledge is always good for something.”

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