Electronic Effect of Substituents on Hydrodesulfurization of Thiophenes : Reactivity and Selectivity

Abstract

Despite the large scale-commercial application of heterogenous catalytic hydrodesulfurization (HDS) to the desulfurization of organosulfur compounds in petroleum, very little is known about the various mechanisms likely to be involved in this process. In this work, one possible mechanism of hydrodesulfurization of substituted thiophenes which is based on the electronic effects of the substituents is proposed. Organosulfur compounds studied are: alkylthiophenes, phenylthiophenes, pyridylthiophenes and thiophene. Under present conditions (T = 553 K; pH₂ = 50 bar), it is possible to formulate both global relative rate and selectivity hydrogenation/hydrogenolysis for substituted thiophenes. Electronic effects of substituents are responsible of the selectivity and lead to particular modes of adsorption on catalyst: more hydrogenation is observed with an electron-withdrawing substituent which would be adsorbed on an acceptor catalytic site. The hydrogenolysis is facilated by an acceptor substituent, adsorbed on a donor catalytic site.