The HDS mechanism: Which “auxiliary” process takes place – sulfur isotopic exchange or replacement – and why is it important to know it?

In the light of general evaluation of the radioisotopic data obtained during the years of our research two possible ways of sulfur transfer from the catalyst into H2S in the course of thiophene HDS have been reconsidered. These are (i) sulfur exchange between H2S formed and the catalyst sulfur and (ii) extrusion of sulfide sulfur into the gas phase by the interaction of the catalyst SH group with H2 with the further formation of H2S. The data of our radioisotopic studies supports the second way, which proceeds within the “forcing out” mechanism. This mechanism assumes either of the two possible schemes of H2S formation—(a) H2 interaction with the catalyst bridge sulfur and (b) H2 interaction with the SH group. The radioisotopic studies support the second possibility.