MECHANISM OF GROWTH OF SILICA MONOLAYER AND GENERATION OF ACIDITY BY CHEMICAL-VAPOR-DEPOSITION OF TETRAMETHOXYSILANE ON ALUMINA

The mechanism of chemical vapor deposition of tetramethoxysilane on gamma-alumina was studied. A silica monolayer covered the suface almost completely upon the deposition at 593 K in a static system. Even at low temperatures (<473 K), tetramethoxysilane was deposited quickly, but the deposition was readily saturated at a low concentration of Si, and the silica layer thus covered only a part of the surface. The amount of methoxy groups on the surface was determined from gravimetry and product analysis by the pulse method; the ratio of CH3O-/Si was high at the low temperatures. The ratio decreased with increasing temperature, and, simultaneously, silanol appeared. On these findings, the following mechanism is proposed; gaseous tetramethoxysilane is deposited on alumina quickly until the silicon alkoxide and a fraction of methanol cover the surface; at high temperatures, contaminated water hydrolyzes the silicon alkoxide into silanol, which is reacted with another tetramethoxysilane; the successive deposition thus proceeds, and at last, the monolayer with a network of Si-O-Si covered the surface almost completely. The stoichiometry upon the proposed mechanism is consistent with the experiments. Catalytic activity for isomerization of butene was observed over the monolayer deposited at 493 and 593 K. The samples deposited at low temperatures (less than or equal to 423 K) showed almost no activity. It is suggested that Al-O-Si-OH in the network of Si-O-Si possesses the Bronsted acidity, whereas the dispersed silica species has no acidity. At 673 K, the random deposition formed a heterogeneous structure of silica including thick layers and/or particles, which showed low activity.