EFFECT OF WO3 LOADING ON THE SURFACE-ACIDITY OF WO3/AL2O3 COMPOSITE OXIDES

High-resolution transmission electron microscopy, X-ray energy dispersive spectroscopy, colorimetric titration and inverse gas chromatography (IGC) were used to study the changes in acidity of a series of WO3/Al2O3 composite oxides with increasing loading of tungsten. The addition of WO3 increases the Lewis and Bronsted acidity of the series of composites. We find that, for the highly dispersed solid acid, WO3/Al2O3, the modified Benesi titration provides an estimate of the number of acid sites and that Lewis and Bronsted partitioning cannot be conclusively assessed. The inverse gas chromotography technique offers certain advantages in that it can be performed at elevated temperatures (close to reaction temperatures), in a gaseous environment, and on colored samples. The electron donating properties of the probes used in IGC have enabled us to speculate on the source of Bronsted acidity introduced into this system by the presence of the second-phase oxide. Charge delocalization into the support due to terminal oxygen atoms is proposed to be the source of this increased Bronsted acidity and is analogous to that occurring when halides are added to alumina.