Deuterium Tracer Experiments Prove the Thiophenic Hydrogen Involvement During the Initial Step of Thiophene Hydrodesulfurization

Abstract

We reassess literature data and demonstrate that the intermolecular hydrogen transfer occur likely during the primary catalytic interaction between thiophene molecules, rendering it the credible thiophene hydrodesulfurization (HDS) pathway. Deuterium tracer experiments prove that thiophenic hydrogen plays a part in direct C-S cleavage, in the initial step of thiophene HDS. Hypothetical surface intermediates for thiophene exchange do not precede, i.e. do not form prior to the surface intermediates for thiophene HDS. Therefore the established thiophene exchange/HDS scheme and the sequence of events are not the viable concepts. Since the deuterium data also indicate that C 4 H 5 S(a)-radicals are not formed this means there are no common surface or reaction intermediates for the thiophene exchange and HDS and both reactions proceed parallel.Keywords Thiophene hydrodesulfurization mechanism Á Deuterium tracer experiments Á Intermolecular hydrogen transfer Á HD exchangeThe mechanism of thiophene hydrodesulfurization (HDS) requires further discussion. The authors' interpretation of the previous works that used deuterium as a tracer is mostly inconsistent with their experimental results. At least six essential facts in these works must be discussed. The first work contains two crucial facts, which escaped the attention of the authors and also went unnoticed by the readers. These and other results invalidate the major proposal of their subsequent works, i.e. that dihydrothiophenes are possible reaction intermediates for thiophene HDS. In reality, exactly past works with the use of deuterium already contain several important deuterium tracer facts, which testified that the thiophenic hydrogen could play a part in direct C-S cleavage, in the initial step of thiophene hydrodesulfurization. Our aim is to illustrate that the intermolecular hydrogen transfer may likely occur during the primary catalytic interaction between thiophene molecules and that such mechanism of thiophene transformation can be considered as the credible thiophene HDS pathway.While thiophene hydrodesulfurization (HDS) has been extensively studied, the main contribution to understanding of potential HDS pathways was made by product distribution and kinetic studies as well as by deuterium tracer experiments. In spite of various reviews on this topic, only some [1-6] are cited here, the key details remain debatable; no agreement was reached even on the initial step of thiophene HDS. There are several proposals for the thiophene HDS pathway. Amberg [2] concluded after product distribution studies that thiophene undergoes hydrogenolysis without prior ring hydrogenation, and that it is followed by a sequence of olefin hydrogenations; i.e. the initial step of thiophene (Th) HDS is the C-S cleavage.Th þ H 2 ! H 2 S þ 1:3 butadiene; butenes which can further hydrogenateKraus and Zdrazil [7] observed the formation of tetrahydrothiophene (THT) during the thiophene HDS and interpreted this as evidence for a consecutive Electronic supplementary material The online ve...