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Synthesis

Improved route from syngas to light olefins

Catalytic process has advantages over methanol- and Fischer Tropsch-based technologies

by Stu Borman
March 4, 2016 | APPEARED IN VOLUME 94, ISSUE 10

Credit: P. Huey/Adapted from Science
In the OX-ZEO technique, activation of syngas on a metal-oxide surface forms CH2 groups that help create ketene, which then diffuses to a zeolite with hydrogen active sites, giving rise to light olefins.

A catalytic technique has promise as a new way of industrially producing olefins such as ethylene and propylene.

Manufacturers use low-molecular-weight olefins to make plastics, solvents, paints, medicines, and other products. The largest-volume organic chemicals produced worldwide, these “light” olefins have traditionally been made, and are still primarily made, by catalytic cracking of crude oil.

Because of high oil prices and petroleum conservation efforts in past years, researchers developed two technologies as alternatives to catalytic cracking: the MTO (methanol to olefins) process and the FTO (Fischer-Tropsch to olefins) process. These methods use zeolite and metal catalysts, respectively, to convert syngas, a mixture of hydrogen and carbon monoxide, to olefins.

The OX-ZEO (oxide-zeolite) technique, developed by Xiulian Pan and Xinhe Bao of Dalian Institute of Chemical Physics and coworkers, provides a third alternative (Science 2016, DOI: 10.1126/science.aaf1835). The researchers optimized a catalyst system—the partially reduced metal-oxide surface catalyst ZnCrOx and a zeolite called MSAPO—to convert syngas to ketene (CH2CO) and then into light olefins.

OX-ZEO is highly selective for making light olefins over other products; it favors propene, which has been in short supply; its catalysts last a long time; its one-pot nature makes it streamlined; and it does not generate carbon deposits, which can degrade catalyst activity.

MTO’s selectivity for light olefins is similar to OX-ZEO’s. But its zeolite catalyst deactivates quickly, and its two separate reaction steps make it potentially less efficient. FTO has lower selectivity, producing higher proportions of methane and other low-molecular-weight alkanes in addition to light olefins, and it is sometimes plagued by carbon.

Eric van Steen of the University of Cape Town, a specialist in solid catalysts, comments that OX-ZEO “may find application in the direct conversion of syngas to predominantly propene, a fast-growing market.”

Krijn P. de Jong of Utrecht University, an expert on oil and syngas conversion and catalysis, estimates that worldwide production of light olefins is more than 200 million metric tons per year and that a few percent of that, perhaps 10 million metric tons, is currently made from syngas by MTO and FTO, with most of the rest made from crude oil.

A new technique competitive with MTO and FTO is thus not earthshaking in the short term. But de Jong notes that it could be important in the long term because the percentage of light olefins made by MTO and FTO has been growing significantly in the past few years as a hedge against high oil prices. He adds, however, that recent precipitous drops in the price of oil “spoil the party” and make future production trends much trickier to predict.

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