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Catalysis

Robust carbon-coupling catalysts sans precious metals

Advance may reduce cost of making synthetic fuels via Fischer-Tropsch chemistry

by Mitch Jacoby
June 6, 2019 | APPEARED IN VOLUME 97, ISSUE 23

 

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Credit: Trung Dung Tran/ Johnson Matthey
A new catalyst preparation yields fine cobalt particles on an alumina support (top left, blue) and distributes Co2+ species throughout the particles (top right, red). A standard method yields less-active, larger particles with a surface coating of Co2+(bottom).

A new, precious-metal-free catalyst could lower the cost of making synthetic fuels—and it even outperforms commercial catalysts when it comes to knitting together new carbon-carbon bonds using a common synthetic process.

Fischer-Tropsch synthesis is a century-old process for converting mixtures of CO and hydrogen, generally derived from natural gas and coal, to transportation fuels and other hydrocarbons. The process can be used to make fuels where crude oil is unavailable and can make fuels that are purer and higher performing. The F–T process, which generates tens of millions of liters of fuel per day, relies on oxide-supported cobalt, iron, and ruthenium catalysts. Cobalt is the most common choice, especially when the feedstock is natural gas.

Manufacturers often run the F–T process in slurry reactors, in which the solid catalyst and gas-phase reactants are stirred in a liquid. These reactors can provide energy and heat-management advantages relative to other types of reactors. But the mechanical agitation, coupled with heat and humidity, take a toll. The conditions can break down the catalyst’s support, typically a form of aluminum oxide called γ-alumina. This degradation ruins the catalyst’s performance.

Gamma alumina is not the material’s most stable phase. Other forms, such as α-alumina, can better tolerate the harsh reactor conditions, but it doesn’t usually work well as a support because the relatively non-porous substance does not take up much cobalt and does not disperse the particles finely. The result is an inactive catalyst. Precious metal additives, which are used in some commercial F–T catalysts, can improve performance, but they also add cost and complexity.

Now, Peter R. Ellis and colleagues at Johnson Matthey have come up with a way to capitalize on α-alumina’s stability to make precious-metal-free cobalt F–T catalysts that remain active in slurry-phase tests for more than 1000 h (Nat. Catal. 2019, DOI: 10.1038/s41929-019-0288-5).

To make the catalysts, the team treated cobalt metal with an aqueous solution of ammonium carbonate, ammonium hydroxide, and bubbling air, and then reacted the product with α-alumina. Microscopy studies show that the method coats the support with fine (~5-nm-dimater) cobalt oxide particles—a precursor to the catalytically active metallic phase. In contrast, a standard preparation method based on impregnation of α-alumina with cobalt nitrate generated much larger particles—up to 75 nm in diameter. Large particles lead to less surface area and lower activity. Other analyses show that the small and large particles differ in terms of the oxidation state of cobalt at their surfaces, which may also affect activity.

To assess catalytic performance, the researchers conducted various types of reactor tests. One test, which compared 2 α-alumina-supported catalysts, showed that the new catalyst is more than six times as active as the cobalt-nitrate-based material and generates a larger fraction of the desired C5 and longer hydrocarbons.

Another test showed that the activity of the new catalyst—made without precious metals—on α-alumina, is roughly equivalent to that of a reference commercial catalyst composed of ruthenium and cobalt on γ-alumina. In this test, too, the new catalyst exhibited higher selectivity for C5+ products.

Eric van Steen, a specialist in F–T chemistry at the University of Cape Town, notes that the researchers “show convincingly” that their catalysts exhibit high surface area on α-alumina and that the support is hydrothermally more stable than γ-alumina. Another F–T expert, Utrecht University’s Krijn P. de Jong, is impressed with the reported activity and product selectivity. However, both scientists note that additional studies are needed to further boost the catalyst’s stability.

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