Losing the Sulfur


	Bas Vogelaar

Dutch researchers have figured out why the activity of catalysts used to produce clean fuels gradually falls. Their findings show that loss of sulfur atoms from the catalyst itself is to blame and could lead to a way to remedy the situation.

Bas Vogelaar, and Jacob Moulijn at Delft University of Technology are investigating hydroprocessing catalysts, which remove sulfur compounds from petroleum and diesel fuels, and would otherwise form polluting sulfur oxides during the combustion process. The hydroprocessing catalysts used in oil refineries become irreparably poisoned after about two years and have to be replace. This is an intensive and costly operation.

Vogelaar has demonstrated and established the most important causes for the deactivation of hydroprocessing catalysts, in order to increase the lifetime of these.

Common hydroprocessing catalysts consist of an aluminum oxide carrier on to which the metals molybdenum and nickel or cobalt are added. During refining, these metals form a catalytically active layer with sulfur from the oil. Until now, chemists thought that "coking", the accumulation of soot particles on the catalyst's surface, was the cause of failing activity. But Vogelaar has discovered that the active phase has a "self-cleaning" effect to counteract coke precipitation. His research suggested that an alternative poisoning process must be occurring.

Vogelaar explains that catalysts convert sulfur compounds in two ways: the sulfur atom is either removed directly from the fuel contaminant or else hydrogenation takes place resulting in removal of the sulfur atom. Conventional catalytic theory has it that a sulfur atom must first be removed from the active phase to leave a vacant position to receive a sulfur atom from the contaminant. However, Vogelaar's experiments show that the second mechanism, hydrogenation, can take place on peripheral sulfur atoms rather than in the vacant positions. The researchers' model suggests that loss of sulfur atoms from the active phase plays an important role in the deactivation of the catalyst.


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In real industrial processes the feedstock is often a very complex mixture, which may all affect the catalyst performance in some way, so it is premature to extrapolate the research to working systems. However, Vogelaar believes his work has brought us a step closer to understanding how hydroprocessing catalysts work. "This knowledge may enable us to develop better and longer lasting catalysts in the future", he told Reactive Reports. "Hopefully, other researchers will follow up my work."