Greener Chemistry Microwaves Enable 4.5× More Efficient Chemical Reactions

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The University of Tokyo

KNF Neuberger GmbH

A new approach using microwave excitation of specific catalyst sites could make energy-intensive chemical reactions, such as methane conversion or CO₂ recycling, far more efficient.

Some industrial processes used to create useful chemicals require heat, but heating methods are often inefficient, partly because they heat a greater volume of space than they really need to. Researchers including those from the University of Tokyo devised a way to limit heating to the specific areas required in such situations. Their technique uses microwaves, not unlike those used in home microwave ovens, to excite specific elements dispersed in the materials to be heated. Their system proved to be around 4.5 times more efficient than current methods.

While there’s more to climate change than power generation and carbon dioxide (CO₂), reducing the need for the former and the output of the latter are critical matters that science and engineering strive to tackle. Under the broad banner of green transformation, Lecturer Fuminao Kishimoto from the Department of Chemical System Engineering at the University of Tokyo and his team explore ways to improve things like industrial processes. Their latest development could impact on some industries involved in chemical synthesis and may have some other positive offshoots. And their underlying idea is relatively straightforward.

“In most cases, chemical reactions occur only at very small, localized regions involving just a few atoms or molecules. This means that even within a large chemical reactor, only limited parts truly require energy input for the reaction,” said Kishimoto. “However, conventional heating methods, such as combustion or hot fluids, disperse thermal energy throughout the entire reactor. We started this research with the idea that microwaves could concentrate energy on a single atomic active site, a little like how a microwave oven heats food.”

As Kishimoto mentions, the process is similar in concept to how a microwave oven works, only in this case, rather than having microwaves tuned to heat polar water molecules at around 2.45 gigahertz (which is also a common Wi-Fi frequency in case you’ve ever noticed that your internet connection becomes unstable when you’re heating leftovers), their microwaves are tuned to much lower frequencies around 900 megahertz. This is because those are ideal to excite the material they wished to heat up, zeolite.

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“The most challenging aspect was proving that only a single atomic active site was being heated by the microwaves. To achieve this, we spent four years developing a specialized experimental environment at Japan’s world-class large synchrotron radiation facility, SPring-8,” said Kishimoto. “This involved using spongelike zeolite, which is ideal because we can control the sizes of the sponge cavities, allowing us to balance different factors of the reactions. Inside the sponge cavities, indium ions act like antennas. These are excited by the microwaves which creates heat, which can then be transferred to reaction materials passing through the sponge.”

By selectively delivering heat to specific materials, lower overall temperatures can be used to achieve reactions which are otherwise very demanding, such as water decomposition or methane conversion, both useful to create fuel products. They can further improve selectivity by varying the pore size of the zeolite sponge, with smaller pores yielding greater efficiency and larger pores enabling greater control over reactions.

And one key advantage is that this technique can even be used in carbon capture, recycling CO₂ as part of the methane conversion, and even recycle plastics more easily.

The challenge now will be in how to scale this up to encourage industrial adoption — things that work in the lab don’t directly translate into large industrial settings easily. And there are some limitations to the research that would also need to be addressed first. The material requirements are quite complex and aren’t simple or cheap to produce; it’s hard to precisely measure temperatures at the atomic scale, so current data rely on indirect evidence and more direct means would be preferred. And, despite the improvements in efficiency, there is still room for improvement here too as there are heat and electrical losses along the way.

“We aim to expand this concept to other important chemical reactions beyond CO₂ conversion and to further optimize catalyst design to improve durability and scalability. The technology is still at the laboratory stage. Scaling up will require further development of catalysts, reactor design and integration with renewable power sources,” said Kishimoto. “While it is difficult to give an exact timeline, we expect pilot-scale demonstrations within the next decade, with broader industrial adoption depending on progress in both technology and energy infrastructure. To achieve this, we are seeking corporate partners to engage in joint development.”

Journal: Ryo Ishibashi, Fuminao Kishimoto, Tatsushi Yoshioka, Hiroki Yamada, Koki Muraoka, Toshiaki Ina, Hiroki Taniguchi, Akira Nakayama, Toru Wakihara, Kazuhiro Takanabe, “Focused Thermal Energy at Atomic Microwave Antenna Sites for Eco-catalysis”, Science Advances, DOI: 10.1126/sciadv.ady4043, https://doi.org/10.1126/sciadv.ady4043

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