Abstract: This paper presents a novel modification of silicalite-1 with hydrofluoric acid treatment. Silicalite-1 treated with 0.04%HF solution exhibited remarkable improvements in stability and selectivity for Beckmann rearrangement of cyclohexanone oxime(CHO). Particularly, silicalite-1 catalyst with ammonium nitrate pre-treatment before HF modification showed the best catalytic performance: the conversion of CHO remained nearly 96% after 53 h reaction, and the selectivity for caprolactam during reaction was as high as 96.1%. XRD results showed that all samples hold the MFI structure after HF treatments, but the structural symmetry was changed from orthorhombic to monoclinic. FT-IR spectra revealed that the intensity of silanols, including isolated silanols and associated silanols on silicalite-1 catalysts, decreased severely after HF modification, while the ratio of associated silanols to isolated silanols became much higher. These results indicate that a high ratio of associated silanols to isolated silanols is favorable to the Beckmann rearrangement reaction. However, a 0.45%HF solution used in the modification led to a decrease in activity and selectivity because the arrangement of silicon atoms was greatly changed, which was proved by 29Si MAS NMR. It is inferred that the arrangement of silicon atoms on the surface of silicalite-1 is of significant importance to the catalytic performance of Beckmann rearrangement reaction.

Key words: silicalite-1, hydrofluoric acid, cyclohexanone oxime, vapor-phase Beckmann rearrangement, caprolactam