Journal of Catalysis, Vol.151, No.1, 192-203, 1995
Silica-Alumina-Supported Mo Oxide Catalysts - Genesis and Demise of Bronsted-Lewis Acidity
Diffuse reflectance FTIR spectroscopy of chemisorbed pyridine was used to investigate the influence of catalyst composition and treatment on the content of Bronsted and Lewis acidity. This investigation included oxidic and reduced Mo oxide catalysts containing 2, 4, 8, and 12 wt% MoO3. The supports were seven silica-aluminas of composition between 0 and 100 wt% SiO2. The IR spectra were collected at 200 degrees C under helium flow, and the areas under the 1545 and 1450 cm(-1) absorbance bands were related to the concentration of Bronsted and Lewis acid sites, respectively. The results indicate that the ratio of Bronsted to Lewis acid concentration (B/L) increases with SiO2 content in the support and reaches a maximum for SiO2:Al2O3 = 75:25 wt%. For Al2O3 and alumina-rich supports B/L increases continuously with MoO3 loading because of the generation of new Bronsted acid sites and decrease of Lewis acid sites. For silica-rich supports, the B/L has a maximum at 2 wt% MoO3 and then decreases slightly as MoO3 loading is increased. The SiO2 support with MoO3 loading up to 12 wt% does not contain any Bronsted acidity in water-free environment. Upon reduction of the supported catalysts at 500 degrees C in hydrogen, the B/L ratio decreases irrespective of SiO2 content and MoO3 loading. A structural model that includes tetrahedral surface species of Mo oxide explains the results.