VAPOR-PHASE BECKMANN REARRANGEMENT OVER SILICA MONOLAYERS PREPARED BY CHEMICAL-VAPOR-DEPOSITION

Silica monolayers prepared on gamma-Al2O3, ZrO2 and TiO2 were used for the Beckmann rearrangement of cyclohexanon oxime. Formation of epsilon-caprolactam was enhanced by deposition of SiO2, whereas side reactions and deactivation were weakened. On Al2O3, the yield of epsilon-caprolactam showed a maximum at ca. 15 Si nm(-2), where the monolayer covered the surface almost completely. The active sites were shown to be formed on the monolayer. The deposition enhanced the formation of epsilon-caprolactam on ZrO2 and TiO2 as well. Pure SiO2 was inactive compared to the monolayers. This shows that the rearrangement requires acidity. The weak Bronsted acid sites on the monolayers are assumed to be active, A significant difference of the acidic properties of M-O-SiOH, where M = Al, Ti and Zr, is proposed based on activity for isomerization of butene. However, the difference between the activities for the Beckmann rearrangement over the monolayers was small. Therefore, the rearrangement is considered to be independent of the strength of the Bronsted acid sites.