Scientists Develop New Catalyst to Reduce Cost of Recycling Organic Waste into Fuel

IN DEPTH: Zeolite Crystal Breakthrough to Boost Biofuel Production from Waste

Researchers at the Technical University of Munich (TUM) using zeolite crystals to significantly reduce the temperature and energy requirements of converting organic waste into fuel.

Members of Prof. Lercher’s team at the Catalysis Research Center: Dr. Yue Liu, Teresa Schachtl and Daniel Melzer (fltr; Image: Andreas Heddergott / TUM).

Image © Technical University of Munich

Researchers at the Technical University of Munich (TUM) using zeolite crystals to significantly reduce the temperature and energy requirements of converting organic waste into fuel.

According to the researchers, current methods of converting organic waste into fuel are not economically viable due to excessively high temperature energy requirement.

However, the scientists claimed to now be on the brink of solving this problem using a novel catalyst concept with the reaction takes place in very confined spaces inside zeolite crystals.

Prof. Johannes Lercher, who heads the Chair of Technical Chemistry II at TU Munich, explained that with ever more electricity is generated at decentralized wind, hydro and solar power plants, “It thus makes sense to decentralize chemical production, as well”.

However, to date, this was said to have not been possible because chemical processes require a great deal of energy – more than local renewable energy sources can provide.

“We thus aimed at finding new processes to lay the foundations for the distributed production of chemicals, which can be powered using renewable energy sources,” said the Lercher - who is also Director of the American Institute for Integrated Catalysis at Pacific Northwest National Laboratory.

His team has now fulfilled one prerequisite for a turnaround in chemical production: In the laboratory, the scientists demonstrated that the temperature required for splitting carbon-oxygen bonds in acidic aqueous solution can be drastically reduced using zeolite crystals. The process also ran much faster than without the zeolite catalysts.

Lessons for Nature
Nature provided the reference for the development of the new process. In biological systems, enzymes with small pockets in their surface accelerate chemical processes.

“We thought about how we could apply theses biological functions to organic chemistry,” explained Lercher. “While searching for suitable catalysts that accelerate the reaction, we stumbled upon zeolites – crystals with small cavities in which the reactions take place under cramped conditions comparable to those in enzyme pockets.”

But, do cramped quarters really increase the reactivity?

To answer this question, Lercher’s team compared the reactions of carbon compounds with acids in a beaker to the same reactions in zeolites. The result: In the crystal cavities, where the reacting molecules, for example alcohols, meet upon the hydronium ions of the acids, reactions run up to 100 times faster and at temperatures just over 100°C.

“Our experiments demonstrate that zeolites as catalysts are similarly effective as enzymes: Both significantly reduce the energy levels required by the reactions,” said Lercher. “The smaller the cavity, the larger the catalytic effect. We achieved the best results with diameters far below one nanometer.”

Tight Spaces
On the subject of why tight spaces foster the reactivity of molecules, Lercher explained: “The force that improves the reaction path is the same as the one that causes wax to stick to a tabletop and that allows geckos to walk on ceilings.”

“The more contact points there are between two surfaces, the larger the adhesion. In our experiments, the organic molecules, which are in an aqueous solution, are literally attracted to the pores in the zeolites,” continued the professor.

Thus, the hydronium ions within the cavities have a significantly greater likelihood of bumping into a reaction partner than those outside. The result is an acid catalyzed chemical reaction that takes place faster and with lower energy input.

From Waste to Fuel
When they come into contact with hydronium ions, organic molecules such as alcohols lose oxygen. This is said to make the process suitable to converting bio-oil obtained from organic waste into fuel.

However, according to Lercher it will take some time before the new process can be deployed in the field.

“We are still working on the fundamentals,” he said. “We hope to use these to create the conditions required for new, decentralized chemical production processes that no longer require large-scale facilities.”

The work was developed in a cooperation of the Chair for Technical Chemistry II and the Catalysis Research Institute at the Technical University of Munich (TUM) with the Pacific Northwest National Laboratory (PNNL). They were funded by the U.S. Department of Energy (DOE). Some of the NMR experiments were performed at the PNNL's Environmental Molecular Science Laboratory (EMSL). PNNL’s National Energy Research Scientific Computing Center (NERSC) provided computing time for simulations.

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