Sulfur exchange via thiophene HDS

Exchange of radioactive sulfur and the hydrodesulfurisation mechanism has been studied intensively for the last two decades by the Moscow Zelinsky Institute Group. The first studies appeared in the 80s,P l and an increasing research activity has been apparent in the last few years.The method of their exchange studies is based on the preparation of S-labelled catalyst samples, followed by testing in the thiophene HDS reaction and 

Experimental data indicated that the isotope exchange process is described by first order equations, and in most cases, e.g., on supported C0M0/AI2O3 catalysts by a superposition of two curves representing two types of sulfur mobility, the more and the less mobile i.e., rapidly and slowly exchangeable sulfur. In general, the H2S molar radioactivity a (in percent of the initial molar radioactivity of catalyst sulfide sulfur), as a function of the produced H2S-[X(cm )], is given (Fig. 7) as a superposition of curves 

Ai is the proportion in thiophene, converted on the sites of the given type. 

Note that a near to similar ratio of rapidly and slowly exchangeable sulfur was determined when the catalyst was sulfided originally with thiophene instead of elemental irrespective of the 2.5 times lower S content of the sample, sulfided by thiophene. This indicates the similarity of active site distribution for catalysts of equal chemical composition. 

Experiments indicatedthat the molar radioactivity function of H2S production on M0S2 can be described by one equation (A = 100, i= 1) biexponential equations were required for promoted catalysts with different Aj and Aj. This indicates that the catalyst sulfur was not bonded homogenously, like that of the observations with H2 S or H2 S exchange.An important difference from those observations is that, in this case, we deal with H2 S, replaced by sulfur, formed in HDS, so that the H2S molar radioactivity represents sulfur formed on the (or via) cat-al Tically active sites. Consequently, the fact that function (7) is described by a biexponential equation with two A-s and A-s, indicates that there exist two types of catalytic sites, active in HDS on promoted Mo-based catalysts. 

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