Converting biomass, like used cooking oil, into valuable chemicals through catalysis can pave the way for a more eco-friendly chemical industry. Traditional methods, however, consume a lot of energy and produce harmful byproducts, which also shorten the lifespan of catalysts. Recently, scientists have introduced a zeolite catalyst that can be effectively heated using microwaves.
A team from Kyushu University has demonstrated that a zeolite known as Na-ZSM-5 is effective in the microwave-assisted transformation of biomass into olefins—key chemicals used to manufacture a range of products from plastics to pharmaceuticals. Their findings, published in Chemical Engineering Journal, suggest that using microwave heating with Na-ZSM-5 could make the chemical industry more energy-efficient and sustainable.
When creating complex organic substances, such as plastics, medicines, or food additives, it’s essential to begin with simpler chemical precursors. Thus, enhancing the efficient and sustainable production of these precursor chemicals is a significant area of research.
A commonly employed technique for producing these vital chemicals is the reforming of naphtha. Unfortunately, this method requires substantial energy and emits carbon dioxide. As alternatives, waste cooking oils and microalgal oils have emerged as low-cost sources for synthesizing simpler chemicals.
These oils can be transformed through a process called ‘catalytic cracking’ using zeolite as a catalyst. Zeolite is a naturally porous material often utilized as a catalyst or adsorbent. In catalytic cracking, the materials need to be heated to extremely high temperatures—about 500-600°C. This method is not only energy exhaustive but can also lead to undesirable deposit formations, known as coking, which shortens the catalyst’s life.
In this study, Associate Professor Shuntaro Tsubaki and his team at the Faculty of Agriculture, Kyushu University, explored microwave technology for heating zeolite catalysts to the necessary temperatures while avoiding issues like coking.
“Microwaves interact directly with materials, allowing precise energy delivery and thus substantial energy savings compared to traditional convection heating methods,” says Tsubaki. “Specifically, microwaves enhance gas-solid catalytic processes by passing through the gaseous phase and selectively heating the solid catalyst, creating hot spots in the catalyst bed.”
During their research, the team examined various zeolite catalysts to identify those that could be efficiently heated by microwaves and showed excellent catalytic performance. They ultimately focused on Na-ZSM-5, which is a sodium ion-substituted zeolite.
To demonstrate the benefits of microwave heating compared to traditional heating, the researchers conducted a catalytic conversion of methyl oleate. The results revealed that Na-ZSM-5, when heated using microwaves, significantly surpassed other catalysts, showing a high conversion efficiency of fatty acid esters to olefins with remarkable selectivity. Furthermore, carbon dioxide emissions were limited to just 1.3% of the total reaction output, and no carbon monoxide was produced.
Crucially, microwave heating allowed Na-ZSM-5 to produce four times more olefins at 500°C than conventional heating methods. This enhancement was largely due to Na-ZSM-5’s greater selectivity for olefins over other compounds. Notably, coking did not occur during microwave heating, even at the elevated temperature of 600°C.
To understand why microwave heating improves the catalytic process, the researchers studied the zeolite’s local structural changes under microwave exposure. Astonishingly, they found that microwave absorption led to localized temperatures exceeding 1000°C within the zeolite’s crystal lattice, while the overall temperature remained at 500°C. These elevated local temperatures likely facilitated the selective production of olefins.
The application of microwave heating to catalysts could significantly enhance the conversion of biomass into valuable chemicals, furthering the sustainability goals of today’s chemical industry.
“Our results aim to contribute to the increased electrification of the chemical sector. Since microwaves can be generated from renewable sources such as solar and wind energy, we can reduce the environmental footprint associated with the production of these essential chemicals,” states Tsubaki.
The research team is planning to refine microwave-driven catalytic processes further, aiming to boost yield and energy efficiency while expanding their scalability. They envision that their work could herald a new phase in sustainable chemical manufacturing.
