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 include d 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 und er helium flow, and the areas under the 1545 and 1450 cm(-1) absorbanc e bands were related to the concentration of Bronsted and Lewis acid s ites, respectively. The results indicate that the ratio of Bronsted to Lewis acid concentration (B/L) increases with SiO2 content in the sup port and reaches a maximum for SiO2:Al2O3 = 75:25 wt%. For Al2O3 and a lumina-rich supports B/L increases continuously with MoO3 loading beca use 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 SiO 2 support with MoO3 loading up to 12 wt% does not contain any Bronsted acidity in water-free environment. Upon reduction of the supported ca talysts at 500 degrees C in hydrogen, the B/L ratio decreases irrespec tive of SiO2 content and MoO3 loading. A structural model that include s tetrahedral surface species of Mo oxide explains the results. (C) 19 95 Academic Press, Inc.