You can make diesel from natural gas. This process is called Gas-To-Liquids (GTL). The Fischer-Tropsch catalysts needed for this process lose part of their activity over time. Doctoral candidate Denzil Moodley (31, South Africa) of Eindhoven University of Technology (TU/e) investigated one of the culprits: carbon. His work sheds light on the mechanisms involved –knowledge that can be used to improve the catalysts. On 6 November, he defends his PhD thesis.

When oil prices are high, converting natural gas or coal to liquid fuel can become lucrative. Alternatively, for countries that have vast natural gas reserves at their disposal, such as Qatar in the Middle East, this is an attractive technology to monetize these resources.

Conversion occurs through the Fischer Tropsch process (FT), in which the natural gas is converted into a mixture of diesel and naphtha, a feedstock for many chemical compounds. Moodley’s employer, Sasol (a global petrochemical company originating in South Africa), used this process, in a joint venture with Qatar Petroleum, as a basis for a plant in Qatar for the production of both products (see photo, total of 34,000 barrels/day).

The plant first converts natural gas into a so-called syngas, a mixture of hydrogen and carbon monoxide, in large reactors, with catalysts as well as oxygen and steam. In the next step –where the actual FT process comes in– the syngas is converted into synthetic crude. This is then hydroconverted into synthetic fuels, predominantly diesel.

Better quality
The synthetic diesel fuel is of better quality than diesel derived from natural petroleum. Moreover, the synthetic fuel fires more easily when injected into an engine. And it is cleaner: synthetic diesel has fewer sulfur and nitrogen components, so its combustion produces fewer hazardous emissions.

As with all industrial catalysts, the cobalt catalysts that accelerate the FT reactions lose part of their efficiency over time. Pilot plant tests show that the activity of the catalysts falls after continued use and after two months of operation, they stabilize. As cobalt is an expensive metal, one wants to get the most out of it.

Carbon
Research conducted at TU/e has previously shown that this decrease in efficiency is not caused by oxidation, as frequently thought. This result was corroborated by Moodley and additionally, he showed that various forms of carbon slowly but surely cover the small cobalt particles and thus may be responsible for a part of the observed deactivation. He used several methods to determine this, including electron microscopy and low-energy ion scattering spectroscopy (LEIS), two TU/e specialties.

Moodley’s work helps to create a better understanding of deactivation, which is an important objective in the field. Sasol can use this knowledge to make its FT process more effective.

Partnership
The scientific research into catalysts being performed in the Netherlands is world class. That is the reason Sasol teamed up with TU/e starting in 2001. In fact, a few of the company’s employees currently work in the Physical Chemistry of Surfaces in Catalysis research group at TU/e, under Prof. Dr. Hans Niemantsverdriet.

Sasol benefits from not only the TU/e group’s wide-ranging knowledge of catalysis, but also its extensive network and ultra-modern characterization techniques. This working structure is interesting for the TU/e research group, too, because of the new knowledge acquired and the access to real-life problems and materials from industry.

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