TUngsten catalysts sulfur effect

Dimethylsulfide was added to the n-heptane to give 1000 ppm S in the gas phase. Figure 22.3 shows the effect of sulfur on conversion. The platinum catalyst lost its activity in about 3 h, whereas the tungsten carbide catalyst was very slightly affected during the experiment. Sulfur tolerance is important in petroleum refineries, since it may allow the substitution of the costly noble metals with carbides in streams containing sulfur. 

The mechanism of the catalyst activation by sulfur is not understood. The amount of sulfur compounds necessary to maintain or increase the catalyst activity depends in some cases on the stability of the heavy-metal sulfide component of the catalyst. Thus molybdenum sulfide seems to require a higher hydrogen sulfide concentration than tungsten sulfide. However, some catalysts that do not contain elements that can form sulfides under reaction conditions also showed an increased activity when sulfur compounds were added to the feed. Hydrogen sulfide in many cases decreases the catalyst sensitivity to nitrogen compounds and thus causes an activity increase. Sufficient data for pure compounds are not available to permit segregation of these effects. 

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