The conversion of the vegetable substance lignin into the fuel kerosene has become much more efficient. Dutch, Chinese and Swiss chemists have achieved this with a new catalyst, here a substance that facilitates chemical reactions. With their method, that they recently described in Nature Chemical Engineeringthe commercial conversion of lignin into sustainable fuels, especially kerosene for aviation, is coming a lot closer in the eyes of scientists.
To stop global warming, fossil fuels (coal, oil, gas) will have to be replaced by sustainable alternatives. The production of biofuels from plant material is such an alternative. A competitive alternative is electric transport, with batteries. “For cars and trucks, a lot is expected from electrification,” says Emiel Hensen, professor of inorganic materials and catalysis at TU Eindhoven, and coordinating researcher of the now published study. “But for heavier transport, with ships and aircraft, more attention is being paid to fuels based on biomass, among other things.”
Much biomass, such as wood and straw, consists mainly of lignocellulose, a mixture of cellulose, hemicellulose and lignin. They are the most common organic materials on Earth. Cellulose can be converted relatively easily into bioethanol, a fuel that is now mixed with gasoline.
More difficult to convert
But lignin is much more difficult to convert. “It is now usually incinerated, but that is a low-value application,” says Hensen. According to him, the conversion of biomass would become a lot more commercially interesting if lignin could also be converted into higher-quality products, such as kerosene.
The fact that lignin is so difficult to convert has to do with its structure, Hensen explains. Cellulose consists of repeating units of glucose, all linked together via the same bond. You only have to break that one bond to get individual units of glucose, which you can then convert into ethanol.
But lignin is a three-dimensional network made up of ring-shaped molecules that are linked via various bonds. “You want to keep the ring-shaped molecules, because they are suitable as fuels such as kerosene,” says Hensen. The bonds consist of carbon and oxygen atoms (COC), or carbon atoms (CC). The first is relatively easy to break, the second is much more persistent.
The catalyst that the chemists have now used focuses precisely on that CC bond and consists of a combination of platinum and a zeolite. A zeolite is a three-dimensional mineral with many cavities in which reactions can take place. The conversion with the catalyst is a two-step reaction. Platinum weakens the CC bond, and the zeolite then cracks it. The chemists discovered that the yield of ring-shaped molecules increases when platinum and zeolite are closer together. The reaction took place at a temperature of around 260°C. “Which is relatively mild,” says Hensen.
The chemists tried out their new approach on hardwood and softwood (birch). The yield of individual ring-shaped molecules was two to eleven times higher than in previously tested methods. The ring-shaped molecules consist of 6 to 12 carbon atoms, including some varying carbon side groups. Molecules with more than 9 carbon atoms are suitable as kerosene.
Less greenhouse gases
According to the chemists, their method is competitive with alternative techniques for making sustainable kerosene, such as the production of synthetic kerosene via the Fischer-Tropsch process. Kerosene is made up of a mixture of carbon monoxide and hydrogen. The chemists also calculated that their method releases half to three-quarters less greenhouse gases compared to those alternative techniques.
“The new catalyst not only works at low temperatures, but also produces very good yields, even with lignin streams that are difficult to convert,” says Bert Weckhuysen, professor of inorganic chemistry and catalysis at Utrecht University. He was not involved in the study.
According to him, the study is indeed an important step forward. The difficult-to-break chemical bonds in lignin are selectively tackled. This makes it possible to efficiently convert technical lignin flows, for example from the paper and pulp industry, into fuels. “If the catalyst also proves to be resistant to contaminants in technical lignin flows, such as sulphur, then it has great stability. Then the conversion has a lot of potential.”
According to Weckhuysen, future research could focus on improving the accessibility of the zeolite material, which further increases its reactivity.