INTERACTION OF OXOMOLYBDENUM SPECIES WITH GAMMA-C-AL2O3 AND GAMMA-C-AL2O3 MODIFIED BY SILICON .1. THE SIO2 GAMMA-C-AL2O3 SYSTEM/

The combination of several characterization techniques, i.e., surface area and porosimetry measurements, X-ray microprobe, X-ray photoelectr on spectroscopy, Fourier transform infrared spectroscopy, temperature programmed desorption of ammonia, and basic site titration, allowed us to investigate a gamma(c)-Al2O3 support modified by grafting varying amounts of Si using tetraethoxysilane. Three Si concentration domains have been evidenced: (1) a first domain from 0 to 1 Si atom-nm-2 chara cterized by a strong interaction with the alumina support, (2) a secon d domain from 1 to approximately 5-6 Si atom.nm-2 corresponding to the formation of a species weakly interacting with alumina, and (3) for a Si loading higher than 6 Si atom.nm-2 a third domain where bulk SiO2 was detected. It appeared that only the basic hydroxyls of gamma(c)-Al 2O3 were reacting with the molecule used for depositing Si. Neither th e neutral hydroxyls nor the acidic sites were affected, except for som e enhancement in the strength of the acidic sites. Upon Si deposition, silanol groups also appeared.