Multifunctional catalysts that transform CO2 into fuels and other useful petrochemicals have been developed at KAUST in Saudi Arabia.
The catalysts could help reverse the release of CO2 by preventing new emissions without requiring a radical overhaul of existing infrastructure, said Jorge Gascon, who led the research published in the Journal of the American Chemical Society.
CO2 can serve as a raw material for useful hydrocarbons but its high chemical stability presents challenges when transforming it into something more useful.
According to KAUST, several approaches are available to transform CO2 into various hydrocarbons using conventional heterogeneous catalysts. However, these catalysts are severely limited in their ability to tune the product distribution according to the target application, Ph.D. student Abhay Dokania said in a statement.
Gascon’s team devised an approach that exploits several catalysts acting together. The catalysts combine a metal-based catalyst with acidic zeolites to directly transform CO2 into multiple hydrocarbons, including light olefins, aromatics and paraffins.
According to the team, a mixture of a methanol-producing indium–cobalt catalyst with a zinc-modified zeolite that catalyses methanol-to-hydrocarbon reactions yielded gasoline-grade isoparaffins, such as isobutane and isooctane, with a selectivity of 85 per cent. These high-octane-number hydrocarbons are sought after for their anti-knock performance and fuel efficiency but had been previously ignored as target products, the team said. The high catalyst selectivity is consistent with the zeolite pore structure and tendency to produce branched hydrocarbons.
“We did not start this project from scratch,” said research engineer Adrian Ramirez Galilea. “Yet, we were very positively surprised to demonstrate such a high selectivity in the isoparaffin fraction. There is still work ahead but we believe that we are on the right track.”
“Through exhaustive spectroscopic detective work, the team unveiled unusual zinc clusters inside the zeolites, which can help determine the precise role of each catalyst component during the reaction and thus optimize the catalysts,” added Dokania.
KAUST added that propane is an essential commodity with a growing market share but its production from CO2 has been overlooked. Together with a team of European universities, the KAUST researchers synthesised propane using a palladium–zinc-based catalyst that forms methanol and a zeolite with high selectivity toward three-carbon compounds.
The catalytic system displayed a selectivity exceeding 50 per cent toward propane, with a CO2 conversion nearing 40 per cent and a CO selectivity of 25 per cent.
“We attribute these results to the intimate contact between catalyst components,” Ramirez said. This shifts the overall CO2 /methanol/CO equilibrium to maximise how much CO2 is converted while minimising how much CO is formed. The palladium component also boosted the paraffin selectivity to 99.9 per cent.
Multifunctional catalysts are expected to enhance control over the range of hydrocarbon products and generate petrochemicals that are usually inaccessible. However, further performance enhancements depend on being able to better understand the chemistry at play, especially the role of the zeolite in the overall reaction mechanism. The researchers combined an iron-based hydrogenation catalyst with eight different zeolites and identified the zeolite-trapped organic compounds to shed light on zeolite reactivity.
The team classified all the zeolites into four distinct groups in terms of selectivity: two groups that form light olefins and long olefinic hydrocarbons, and two groups that produce paraffins and aromatic compounds.
“Therefore, targeting a specific product from CO2 could be as easy as selecting the adequate zeolite in the multifunctional system,” Ramirez said.
The researchers are now optimising their multifunctional catalysts to get closer to a circular carbon economy, an initiative adopted at KAUST to support the reduction, reusability, recyclability and removal of carbon emissions.
“We have produced hydrocarbons that fall in the gasoline fuel range but require major additional processing before becoming usable. Thus, our next step is to apply what we have learned to directly produce drop-in fuels from CO2, which could be used without any additional processing,” Dokania said.