The current increase in gasoline costs and the many ways these costs are passed onto consumers remind us that much of the world’s economy still relies on the production of fuels that are refined from crude oil.
The refining process uses high temperatures and catalysts to perform a series of chemical reactions and physical separations that convert crude oil into the different kinds of fuels used to power automobiles, trucks and airplanes. These chemical reactions are performed at enormous scale in oil refineries around the country; almost 1 billion gallons of refined petroleum products are produced in the U.S. each year.
Isooctane is an important component in many hydrocarbon fuels and is made as part of the refining process using a method called alkylation. Alkylation forms chemical bonds between isobutane and isobutene as feedstock molecules to produce isooctane in a high-temperature process that uses concentrated sulfuric or hydrofluoric acids as the catalyst.
This process has a considerable environmental cost due to the production of large quantities of acid waste that are costly to remediate. A more recent method combines two isobutene molecules with addition of hydrogen using a platinum or palladium catalyst. But these metal catalysts are rare materials and consequently very expensive, and they have a host of environmental impacts associated with their extraction through mining. Current methods use dangerous acids and expensive catalysts and generate considerable toxic waste.
A team from the School of Molecular Sciences and the School of Earth and Space Exploration at Arizona State University has now developed a method for the production of isooctane from common feedstock molecules that uses just water, heat, and the inexpensive, Earth-abundant metals iron and nickel. The process is inspired by the chemical processes that occur naturally within the Earth’s crust. The process of learning from geology to develop new chemical processes is a strategy the ASU team calls “geomimicry,” the geologic equivalent of better-known biomimicry.
The chemistry is described by School of Earth and Space Exploration postdoctoral researcher Kirt Robinson: “The reactions take place in water, usually considered a poor solvent for organic chemistry reactions. But at high temperatures and pressures, water readily dissolves organic chemicals. It also automatically generates an acid catalyst as a result of increased autodissociation to form hydronium ions. In other words, the acid catalyst required for isooctane production is contained simply within the water solvent. Chemical reaction of isobutylene, tertbutanol or isobutanol under these conditions forms isooctane with virtually no unwanted side-products.”
At the end of the reaction the water is cooled, the acid catalysis is automatically removed by reassociation to form the water it came from, and the now-insoluble isooctane product floats to the top of the water for simple separation.
“The process eliminates the strong acid catalysts and allows the water to be reused for future reactions, in contrast to the disposal of carcinogenic solvents and acidic sludge waste in the conventional methods,” Robinson explained.
The chemistry emerged from the team’s work on the basic principles that control geochemical organic reactions.
“I was reading a review article on industrial uses of concentrated sulfuric acid and saw a comment about acid vapor plumes being emitted in isooctane manufacturing, and I thought, ‘That can’t be good. I wouldn’t want to live near that chemical plant,’ ” School of Molecular Sciences Professor Ian Gould said. “We discussed this in group meeting, and it occurred to us that we could perhaps do the same thing using the Earth chemistry we were studying at the time, and it worked!”
The method is described in a patent recently awarded to the ASU team (US 11,332,419, May 17, 2022). As described in the patent, the method can in principle also be used to form chemical bonds between other organic molecules in other industrial applications. The larger the scale of the chemical process, the more important it is to develop greener, cleaner and more efficient chemical methods for doing the process. Petroleum refining represents the largest-scale chemical process in the world, pointing to the importance of the ASU invention.
The team has been working for over 10 years on the hydrothermal organic reactions relevant to earth chemistry and to the search for new habitable worlds, and recently on new catalytic methods based on earth chemistry. The inventors who contributed to the patent are postdoctoral researcher Robinson, graduate student Christiana Bockisch (School of Molecular Sciences) and faculty leads Gould, Hilairy Hartnett (School of Molecular Sciences and School of Earth and Space Exploration), Everett Shock (School of Molecular Sciences and School of Earth and Space Exploration) and Lynda Williams (School of Earth and Space Exploration).
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