The reaction pathways for thiophene can be of two types: Hydrogenolysis of the C-S bond before hydrogenation of the aromatic ring. Intramolecular HDS pathway. Hydrogen in H2S product originates from position β to the Sulphur atom of the thiophene.
(Thiophene ring splits due to attack by surface adsorbed hydrogen at the Sulphur atom. Sulphur is removed in the form of H2S, leaving butadiene as the other product. This pathway is known as hydrogenolysis pathway)
Both reaction occur at different catalytic sites.
Mechanistic possibilities:
Model 1:
According to this model Thiophene and hydrogen sulphide compete for the same site to get adsorbed and hydrogen goes for a different site. The surface reaction is the rate determining step and this model uses a CoO-MoO3/Al2O3 catalyst. The temperature range for this rate equation is 2500 to 3130 C. And the different partial pressure range for this model are:
0.0263 < pT < 0.21 atm, 0.724 < pH < pH2S < 0.138 atm.…show more content… Pre-adsorbed H2S selectively retarded butene hydrogenation in comparison to thiophene hydrogenolysis indicating a difference in sites for the two reactions. Primary products from thiophene were 1-butene, cis-trans 2- butene and H2S. Very small amounts of lighter hydrocarbons were formed at high temperature and low pressure of hydrogen. No butadiene was found under any condition. There was a significant inhibition of rate by H2S as well as inhibition by thiophene at higher