Characterization of acidic sites of silica-supported transition metal sulfides by pyridine and 2,6 dimethylpyridine adsorption:: Relation to activity in CH3SH condensation

The modification of the Bronsted and Lewis acidity induced by a reductive hydrogen treatment has been investigated on three silica supported sulfides, i.e. RuS2, MoS2, and a Co-promoted molybdenum sulfide. The catalytic properties of various reduced surfaces have been determined using a model acid demanding reaction such as the condensation of methylmercaptan into dimethylthioether, The acidic properties of the reduced solid were studied by FTIR spectroscopy using both pyridine and 2, 6 dimethylpyridine as probe molecules. Results show that these catalysts present distinct reactivities and stabilities towards a hydrogen atmosphere. For instance the reducibility of RuS2 is higher than that of MoS2. Similarly the number of Lewis acidic sites is higher on RuS2 than on MoS2 and no differences in Lewis acidic strength were detected on both systems. On RuS2 it is easier to create SH groups than on MoS2; however, on the latter the SH groups have a higher protonic character than over the Ru-based solid. Concerning the effect of Co, the promoter increases the reducibility of the molybdenum phase, inducing an augmentation of the number of both types of acidic centers. The variation of the activity in CH3SH condensation is very close to that reported for Lewis acidity. This confirms that methylmercaptan condensation is a suitable reaction to follow acidic properties of sulfided catalysts. This study provides evidence for the sensitivity of surface properties of sulfided catalysts to the reaction conditions. It points out that the reducibility of solids plays an important role since the lower the strength of the M-S bond, the easier the creation of numerous Lewis acidic sites and the higher the activity for a reaction requiring moderate acidity. (C) 1998 Academic Press.