Chemists make hydrogen from breadcrumbs in groundbreaking reaction that could replace some fossil fuels

Breadcrumbs from food waste could replace fossil fuels as a source of hydrogen in one of the most common chemical reactions used in chemical manufacturing, new research suggests.

The new process, reported Feb. 23 in the journal Nature Chemistry, combines natural fermentation processes in bacteria with metal catalysis to generate an array of valuable chemical products from simple food waste. Calculations showed that this hybrid procedure was carbon negative overall, and the authors think it could be the first step in reimagining chemical manufacturing as a more sustainable industry.

Hydrogenation is a chemical process that inserts a molecule of hydrogen across a double bond and is a staple reaction in food production, plastics manufacturing and the synthesis of drug compounds.

However, the bulk of the hydrogen gas used in this reaction is derived from fossil fuels through a dirty and energy-intensive process called steam reforming, which produces 15 to 20 kilograms of carbon dioxide for every kilogram of hydrogen generated . Consequently, hydrogenation is a huge sustainability challenge for the chemical industry, and scientists are urgently searching for greener alternatives.

Turning to nature, Stephen Wallace, a professor of chemical biotechnology at the University of Edinburgh, decided to investigate whether it was possible to harness the power of biology to tackle this chemistry problem. Many bacteria naturally produce hydrogen when they are forced to respire anaerobically (without oxygen), and they release a constant stream of this gas into their surroundings. If this could be linked to a compatible chemical system, it would be theoretically possible to use bio-hydrogen in a hydrogenation reaction, thereby eliminating the need for fossil fuels in this process, Wallace reasoned.

"The main challenge was finding a catalyst that can operate in a living system ‪—‬ in water, at mild temperatures, and without harming the cells," he told Live Science in an email. "We had to balance both sides: a catalyst that stays active in a complex biological environment, and microbes that continue functioning in the presence of the catalyst."

Culture shift

The team cultured E. coli bacteria in a glucose-containing medium, adding a commercial palladium catalyst and a test substrate before sparging the mixture to remove oxygen. The oxygen-free reaction was incubated at 98.6 degrees Fahrenheit (37 degrees Celsius) for a day, and subsequent analysis revealed that the top-performing strain had produced the expected hydrogenation product in 94% yield.

"The metal catalyst comes in and is essentially bound to the cell membrane," Simone Morra, a biotechnologist at the University of Nottingham who wasn't involved in the work, told Live Science. "The cell itself will produce the hydrogen, and then as soon as the hydrogen starts to diffuse out of the cell, it will hit this metal catalyst, which will do the second part of the reaction and produce a hydrogenation product."

With a biocompatible system established, Wallace next sought to replace the expensive glucose feedstock with a cheaper and more sustainable alternative. Focusing on bread waste, the team used microbial enzymes to break the complex carbohydrate molecules within breadcrumbs into simple glucose units. This waste-derived fuel was then fed directly to the E. coli cultures, effectively converting breadcrumbs into hydrogen.

But the researchers had one final trick up their sleeves: Instead of feeding a precursor molecule to the bacterial culture, they genetically engineered certain strains to produce the required substrates within the cells themselves. "It’s brilliant and very inspiring," Morra said. "They show that they can capitalize on the synthetic abilities of E. coli. Essentially they can make use of the carbon pathways of the cell to make any substrate they want."

The use of bio-generated hydrogen resulted in a three-fold decrease in greenhouse gas emissions compared to using fossil fuels. The breadcrumb-powered hydrogenation process, in particular, reduced the global warming potential by more than 135%, corresponding to a carbon-negative footprint.

The team is now working to increase the number of possible substrates and developing the process to accept more types of biowaste. Ultimately, they hope the method could be incorporated into industrial chemical synthesis.

"Right now, the system works best with simpler alkenes," or molecules containing a carbon-carbon double bond, Wallace said. "It's not yet as efficient as industrial processes, but it demonstrates a fundamentally new way of doing hydrogenation. To make it viable, we need to improve efficiency, scale the biology, and develop catalysts that remain stable and cost-effective at industrial scale."