Surface properties of sulfided [formula omitted] catalysts

Abstract

Three types of measurement have been successively performed on a series of sulfided Mo Al 2 O 3 catalysts with initial contents ranging from 3 to 21 wt% MoO 3, namely (i) catalytic activity for thiophene hydrogenolysis and propene hydrogenation reactions (420 °C, 1 atm total pressure), (ii) dynamic oxygen chemisorption at 60 °C, and (iii) selective dissolution of molybdenum in a basic medium. Correlation between the three sets of data brings a new approach to the active surface species. They reveal that catalysts activated under thiophene bear different Mo species according to the loading. It is suggested that the molybdenum ions are grouped as polythiomolybdate clusters bound to the alumina support. The isolated polyions characterized in the less concentrated catalysts tend to polymerize further at higher Mo contents. Oxygen is a good probe molecule for titration of HDS active sites associated with the smaller Mo units. The stoichiometry of the polythiomolybdate entities cannot be precisely established, but it is clearly a function of the sulfiding conditions.