Investigation of “Rearrangement Step” in classical Beckmann rearrangement mechanism over solid acid by means of 18O isotopic labeling

The reaction mechanism of the Beckmann rearrangement over B2O3/γ-Al2O3 and TS-1 in the gas phase has been investigated by isotope labeling approach. The isotopic labeled products were measured by mass spectrometry method. By exchanging oxygen with H218O in the rearrangement step, it was found that the exchange reaction between cyclohexanone oxime and H218O over B2O3/γ-Al2O3 and TS-1 could only be carried out in some extent. It suggested that the dissociation of nitrilium over solid acids be not completely free as the classical mechanism. A concept of the dissociation degree (α) that is defined as the ratio of the dissociated intermediate nitrilium to the total intermediate nitrilium has been proposed. By fitting the experimental values with the calculation equation of isotopic labeled products, it is obtained that α values for B2O3/γ-Al2O3 and TS-1 are 0.199 and 0.806 at the reaction conditions, respectively.

Beckmann rearrangement mechanism in gas phase and catalyzed by B2O3/γ-Al2O3 and TS-1 have been investigated by isotope labeling approach. The formed water is only partially released or there is still some interaction between the formed H2O and the nitrilium cation. Dissociation degree (α) values for B2O3/γ-Al2O3 and TS-1 are 0.199 and 0.806 at reaction conditions, respectively. Figure optionsDownload as PowerPoint slide