KAUST has developed a multifunctional catalyst for converting captured carbon dioxide (CO2) into fuels and other valuable petrochemical products, aiming to achieve a sustainable green economy independent of traditional fossil fuels. Jorge Gascon, who led the study, said the catalysts could help reverse the ever-increasing CO2 emissions by preventing new emissions without the need for a thorough overhaul of existing infrastructure.
CO2 is a key factor in global warming and it can also serve as a raw material for useful hydrocarbons. However, its high chemical stability makes it very challenging to convert it into something more useful.
There are several strategies for converting CO2 into various hydrocarbons using traditional heterogeneous catalysts. However, these catalysts are severely limited in their ability to adjust product distribution according to target applications, the PhD explained. Student Abhay Dokania. The team at
Gascon designed a method that uses multiple catalysts to work synergistically. The catalyst combines metal-based catalysts with acidic zeolites, an ordered microporous catalytic material, to convert CO2 directly into a variety of hydrocarbons such as light olefins, aromatics and paraffin.
A mixture of indium cobalt catalyst for methanol production and zinc-modified zeolites that catalyze the reaction of methanol to hydrocarbons yields gasoline grade isoalkanes such as isobutane and isooctane with a selectivity of 85%. These high-octane hydrocarbons are highly sought after for their explosive resistance and fuel efficiency, but have been previously ignored as target products. High catalyst selectivity is consistent with the zeolite pore structure and the tendency to produce branched hydrocarbons.
"We are not starting this project from scratch," said research engineer Adrian Ramirez Galilea. "However, we were very surprised that we showed such high selectivity in the isoalkane fraction. There is still work to be done, but we believe we are on the right track."
"With detailed spectral detection work, the team revealed unusual zinc clusters inside the zeolite, which helped determine the precise action of each catalyst component during the reaction, thus optimizing the catalyst," Docania said. Propane is an essential commodity with a growing market share, but its CO2 production is ignored. KAUST researchers, together with a team of leading European universities, synthesize propane using a palladium-zinc-based catalyst that forms methanol and zeolites with high selectivity to tricarbon compounds.
The catalytic system has a selectivity of more than 50% for propane, a CO2 conversion rate is close to 40%, and a CO selectivity is only 25%. “We attribute these results to close contact between the catalyst components,” Ramirez said. This changes the overall CO2/methanol/CO balance to maximize the conversion of CO2 while minimizing the amount of CO formed. The palladium component also increases paraffin selectivity to 99.9%.
multifunctional catalysts are expected to strengthen control over the range of hydrocarbon products and produce petrochemical products that are usually unavailable. However, further performance enhancement depends on the ability to better understand the chemical reactions therein, especially the role of zeolites in the entire reaction mechanism. The researchers combined an iron-based hydrogenation catalyst with eight different zeolites and identified organic compounds captured by the zeolite to elucidate the reactivity of the zeolite.
Despite the complex reaction mechanism, the team divided all zeolites into four different groups according to their selectivity: two groups form light olefins and long olefins, and two groups produce alkanes and aromatic compounds. “So, targeting a specific product from CO2 may be as easy as choosing the right zeolite in a multifunctional system,” Ramirez said.
researchers are now optimizing their versatile catalysts to get closer to the circular carbon economy, an initiative taken by KAUST to support reduction, reuse, recycling and elimination of carbon emissions. "We have produced hydrocarbons that fall within the gasoline fuel range, but there is a lot of additional processing required before use. So our next step is to apply what we have learned to produce plug-in fuel directly from CO2, and use without any additional processing," Dokania said.
