Research

Is there a smarter way to utilize biowaste?

Martin Nielsen asked himself that question three years ago. The answer is yes, but there is still a long way to go before his team’s method for a smarter use of biowaste is ready for industrial application.

Cross-section of a leaf under the microscope. Photo: Shutterstock — Cross section of a leaf seen through a microscope. The components that give plant material such as this its structure, can be used by DTU researchers to create a molecyle with many possible uses. Photo: Shutterstock

Wednesday 01 May 2024

Henrik Olsen

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.

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The stew was a success. All the types of biowaste studied could be converted into varying amounts of GVL. The experiment started with biowaste from nine different types of biowaste, including straw, grass, and beechwood sawdust. The GVL yield varied between 10 and 26 per cent (weight), which was a highly satisfactory result. The greatest yield was achieved with beech sawdust, where the 26 per cent corresponds to almost half of the cellulose and hemicellulose being converted. The lignin in the sawdust was not converted to GVL.

Similar positive results were then obtained with starchy biomass such as rice and potato flour (16-20 per cent yield), and finally, tests were carried out on chitinous biowaste in the form of shells from shrimp and other crustaceans. The chitin structure is similar to cellulose but harder to break down. In this process, GVL was also produced—although in slightly more limited quantities (13 per cent).

A big but

Now that the method has proven effective, you would think that the way was paved for scaling up and commercializing the process. If not for a big but, that is. Because when the stew is ready, GVL can indeed be isolated and used, but the catalyst is still dissolved in the remaining liquid and cannot be extracted and reused. And since the catalyst is extremely expensive, the price of producing GVL with the new method is 1,000 times the amount that would be commercially viable.

There is therefore a need of a solution that brings the costs down, and Martin Nielsen has a good idea of what that solution is. However, this is, so far, a well-guarded secret, and for patent reasons the associate professor cannot elaborate on what the next step should be.

However, Martin Nielsen can reveal that the chosen method has been successfully used for other chemical processes. You would think this to be an advantage in terms of moving forward with the project. But that is not the case—quite the opposite.

For this very reason, the next step is not basic research, and funding from foundations that traditionally support basic research will therefore not be relevant. The DTU researchers have also yet to demonstrate that the process they plan to industrialize actually works, which makes it difficult to obtain funding from foundations that support upscaling and commercialization of research methodologies. Finally, there is not currently an established market for GVL, meaning there is no guarantee the product resulting from the process can be sold. In other words, the researchers are currently stuck in a no-man’s land.

But even though it is a frustrating situation, Martin Nielsen remains optimistic. DTU’s Office for Research, Advice and Innovation, Law & Tech Transfer, and DTU Skylab are specialists in commercializing DTU inventions and assessing opportunities for funding. So, before embarking on any further development of the GVL process, he will join forces with colleagues from these areas.

Read more about the work to transform biomass to GVL in an article in EES Catalysis: Direct biomass valorisation to γ-valerolactone by Ru-PNP catalysed hydrogenation in acid.

Contact

Martin Nielsen Associate Professor Mobile: +45 24651045 marnie@kemi.dtu.dk