MECHANISMS OF THIOPHENE HYDRODESULFURIZATION ON MODEL MOLYBDENUM CATALYSTS

Hydrodesulfurization (HDS) activities and selectivities were measured for thiophene, tetrahydrothiophene (THT), and 1-butanethiol on silica- supported molybdenum catalysts at a pressure of 1 atm and temperatures ranging from 530 to 795 K. The model catalysts, which mere previously characterized, feature isolated molybdenum atoms in the +2, +4, and 6 oxidation states and molybdenum dimers with each molybdenum atom in the +4 oxidation state. Silica-supported MoS2 was used for reference. Activities for thiophene and THT HHDS correlate with oxidation state. Mo(II) is most active among dispersed catalysts. 1-Butanethiol activit ies were much larger than thiophene or THT activities and were roughly equal on ail dispersed catalysts. Apparent activation energies of 43. 4 and 48.5 kJ/mol were determined for thiophene HDS on Mo(II) and MoS2 /SiO2, respectively. Apparent activation energies of 132 and 174 kJ/mo l were determined for THT HDS on Mo(II) and MoS2/SiO2, respectively. D ihydrothiophene, THT and 1-butanethiol were formed in thiophene HDS ov er Mo(II) and MoS2/SiO2. The major products of thiophene and THT HDS w ere 1-butene, 2-butene, and n-butene. Butadiene, i-butane, i-butene, m ethane, ethane, ethene, propane, and propene were formed in small amou nts. Butadiene is thought to be the initial product of thiophene and T HT desulfurization and undergoes subsequent hydrogenation and isomeriz ation to yield the observed products. A common mechanism for HDS of th iophene and THT with 2,5-DHT as an intermediate is discussed. (C) 1998 Academic Press.