Every year, we produce more than 100 million tonnes of biowaste in the EU, and in the US it is double that. About 40 per cent of this biowaste ends up in landfills, where it is converted into methane, among other things. Landfills are thus the second largest man-made source of methane—a far more potent greenhouse gas than CO2, which we usually consider to be the primary culprit of climate change.
In Denmark, we burn our biowaste to produce heat and electricity. But this method also produces CO2, and then we are back at negatively impacting the climate. The same is true when converting straw and other biowaste into ethanol, which is added to petrol.
Associate Professor at DTU Martin Nielsen believed we should be able to do better, so he hired Sakhitha Koranchalil for a PhD project that investigates how to better utilize biowaste.
Spotted GVL
"I find it hard to understand why we should burn something that nature provides us for free—and which even contains a chemical complexity. Why not maintain that complexity?" asks Martin Nielsen.
A complex organic compound typically consists of carbon chains with more atoms than simple compounds. Such a compound can potentially be part of many more structures than a simple compound might, and thus there are more ways it can be used. And the substance that Martin Nielsen spotted during work with the project was gamma-valerolactone, commonly known as GVL.
GVL is a natural product that forms when biomass decomposes. It can be used for a wide range of purposes, for instance as a solvent or in the production of pharmaceuticals. It can also be used in plastic production and the manufacture of acrylic fibres, or it can also be added to fuels in the same way as ethanol. An internal market analysis conducted by DTU therefore also points to GVL as a valuable green substance of the future.
But even though GVL has been approved as a possible player in the green transition, there is currently no GVL industry as such. And there is a particular reason for this.
Indeed, the way in which the industry is currently able to produce GVL is rather inefficient. If you want to produce GVL from biomass such as wood, grass, or straw, it is only feasible to convert the cellulose content of the biomass. Cellulose, together with hemicellulose and lignin, is the main component of green plant material. In the same process, however, it is not viable to convert the content of hemicellulose, which makes up 15-35 per cent of the biomass. Furthermore, the process of converting cellulose stops when it reaches the compound called levulinic acid, which then must go through a new process before it is converted into GVL.
"It is extremely expensive, and much more expensive than what is needed to make it commercially viable," says Martin Nielsen.
Innovative method
The method used so far to produce levulinic acid and GVL uses a catalyst consisting of a solid substance mixed into the material from which levulinic acid and GVL is extracted. This leads to some limitations, and Martin Nielsen wants to work around them by using an innovative approach that DTU has chosen to patent.
"Our method differs in two ways in particular. One is that we use what is called a molecular catalyst. The second is that we use phosphoric acid," says Martin Nielsen.
A molecular catalyst is special because it is dissolved in the liquid that is to be converted. This makes it much easier to manage the process. The catalyst's job in this specific context is to ensure that hydrogen (H2) is added in the final steps of the process. The phosphoric acid, on the other hand, must ensure that protons (H+) is added in a wide range of steps during the process.
The catalyst chosen by the DTU researchers for the experiments goes by the not very colloquial trivial name Ru-MACHO-BH with the chemical formula C29H34BNOP2Ru.
"The fundamental question was whether our molecular catalyst could hydrogenate—i.e., deliver hydrogen to an organic molecule—under acidic conditions, because this has never been proven before," explains Martin Nielsen.
If possible, it would—theoretically—be possible to go all the way from biowaste to GVL in one single process.
A stew
The process that Sakhitha Koranchalil worked on in her PhD project is a so-called batch process. Martin Nielsen compares it to a stew, where everything is mixed together from the start and ends up as a finished dish without adding anything along the way. Here, the ingredients are biomass, phosphoric acid, hydrogen, water, and the catalyst. The process is then carried out under a hydrogen pressure of 30 bar and at a temperature of 140°C.
