Turning sunlight into plastics

As many good researchers biocatalysis expert Dr. Frank Hollmann (Faculty of Applied Sciences) has had his fair share of grant proposals being rejected. This winter, however, he hit the jackpot. The European Research Council awarded him with a two million euros ERC consolidator grant. And no more than two weeks after having received this joyful news, message came that NWO also awarded him a Vici grant worth 1.5 million euros.

Hollmann specialises in redox reactions, reactions that involve the transfer of electrons from one atom or molecule to the other. This type of chemical conversions are found everywhere. Many important biological processes involve redox reactions, such as cellular respiration and photosynthesis. But they also occur in industry, for instance, in  galvanizing steel or to produce chemicals for the pharmaceutical industry.

Prior to coming to Delft in 2008, Hollmann, who is originally from Germany, acquired experience in redox reactions while completing his PhD at the Swiss Federal Institute of Technology (ETH Zurich) and as post doc at the Max-Planck-Institute for Coal Research (Mulheim/Ruhr, Germany).

In Delft the chemist is focussing on new biotechnological routes to produce compounds that find applications in various sectors, including plastic precursors. One critical step in such a process is the hydroxylation reaction that turns cyclohexane into cyclohexanol, a precursor molecule for nylon. In this reaction, an oxygen atom and a hydrogen atom are added to cyclohexane. More specifically, cyclohexane has to be burned in a highly controlled fashion. For this, chemical catalysts are used.

Yet the catalysts do a poor job. “The reaction is very difficult to control, and lots of waste products are formed”, said Hollmann. “If we use enzymes instead of chemical catalysts, we will end with cleaner products and the whole process will be less detrimental to the environment. What we thus want to do is use biological catalysts to produce chemical compounds. We work at the interface of bioscience and chemistry.”

Hollmann chooses to work with enzymes called monooxygenases, which are found in all living organisms. In humans, they play a vital role in breaking down toxic substances in the liver.

Monooxygenases are not easy to get to work though. To activate them, they need to obtain electrons and nature has devised complicated molecular architectures involving many different types of collaborating enzymes in order for this to happen. “For nature this makes sense”, said the researcher. “It allows living cells to balance individual reactions. But for industrial chemistry it is a nuisance since the goal there is to run reactions non-stop at full speed.”

The solution is simple. Water can also serve as an electron donor for the enzymes if kick-started with the right stimulant. Sunlight can do the trick. In combination with the catalyst titanium dioxide, a compound found in white wall paint, sunlight will split water and produce electrons for the monooxygenases, just like it does in naturally occurring photosynthesis.

“The water oxidation technology with titanium dioxide is developed by the photovoltaics industry. We can just copy paste it. I’m astounded that nobody did this before.”

In his lab, Hollmann and his team, are performing the first experiments. Under a hood, a bright yellow light is shining. Around it ten small reaction vessels are turning at high speed. They contain water, the enzyme, titanium dioxide and the cyclohexane that must be turned into the precursor molecule for plastic. “The lightbulb costs only five euro’s”, says Hollmann. “I want everybody to be able to repeat the experiment without needing to buy fancy equipment.”

So far, the technique seems promising. In the Netherlands, artificial light is required in order for the chemical reactions to occur. The Delft researcher is however collaborating with chemists of the University of Crete and they have their setups on the roof of the building using natural light.

“If we are successful we will provide the chemists in academia and industry with a simple, robust and versatile tool to perform difficult chemical reactions. We hope thereby to make the chemical industry a little bit greener.”