Relationships between Microstructure and Surface Acidity of Ti-Si Mixed Oxide Catalysts

The surface acidities and reactivities of titania, silica, and a series of Ti-Si mixed oxides have been investigated with a variety of surface-sensitive techniques, including temperature-programmed desorption/reaction of ammonia and 2-propanol, IR spectroscopy of adsorbed ammonia, and catalytic reaction of 1-butene. Results from TPD, TPR, and IR spectroscopy indicated that total acidity and relative acid strength decreased as silica was incorporated into titania. However, the areal rate of 1-butene isomerization was almost four times greater over a titania-rich mixed oxide than over pure titania. Infrared spectroscopy of adsorbed ammonia revealed that all of the acid sites on pure titania were of the Lewis type, whereas about 80% of the sites on the mixed oxides were of the Bronsted type. Therefore, the enhancement of butene isomerization activity over titania-rich mixed oxides was attributed to the Bronsted acid sites that were not present on pure titania. The appearance of Bronsted acidity in titania-rich mixed oxides was due to the local charge imbalance associated with tetrahedrally coordinated silica chemically mixing with the octahedral titania matrix. In silica-rich mixed oxides, Ti substituted isomorphously for Si in the tetrahedral silica matrix, thus eliminating the local charge imbalance that caused Bronsted acidity. The molar ratio of NH3(ads)/Ti for a silica-rich mixed oxide was approximately 0.3, which was nearly the same as the fraction of nontetrahedrally coordinated Ti in the sample.