Vapor-phase Beckmann rearrangement over silica monolayers prepared by chemical vapor deposition

Abstract

Silica monolayers prepared on γ-Al 2O 3, ZrO 2 and TiO 2 were used for the Beckmann rearrangement of cyclohexanon oxime. Formation of ε-caprolactam was enhanced by deposition of SiO 2, whereas side reactions and deactivation were weakened. On Al 2O 3, the yield of ε-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 ε-caprolactam on ZrO 2 and TiO 2 as well. Pure SiO 2 was inactive compared to the monolayers. This shows that the rearrangement requires acidity. The weak Brønsted 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 Brønsted acid sites.