Kinetics and mechanism of thiophene hydrodesulfurization over carbon-supported transition metal sulfides

Results of a detailed kinetic study on the thiophene hydrodesulfurization reaction at atmospheric pressure over a set of carbon-supported transition metal sulfides, i.e., the sulfides of Co, Mo, Rh, and the mixed CoMo sulfide, are presented. It is found that (partially) hydrogenated thiophenes, i.e., 2,3-dihydrothiophene, 2,5-dihydrothiophene, and tetrahydrothiophene, are important intermediates in the reaction mechanism. The reaction orders of thiophene suggest that carbon-sulfur bond cleavage is rate limiting for most of the catalysts. The CoMo catalyst may have hydrogenative sulfur removal as the rate limiting step. This catalyst shows a strong decrease in apparent activation energy with temperature to be ascribed to a large change in steady state surface coverage by thiophene (or H2S) as a function of temperature. This is consistent with a strong interaction between catalyst and thiophene. The Rh catalyst most probably shows a phase transition leading to different kinetic parameters. A strong interaction between the metal sulfide and thiophene is important for a high HDS activity. (C) 1996 Academic Press, Inc.