Dominant anatase TiO2 (001) with rich surface hydroxyls as catalytic active sites for selective catalytic oxidation of cyclohexylamine to cyclohexanone oxime

Developing an environment-friendly and efficient approach for preparing cyclohexanone oxime is of great sig-nificance in nylon-6 production. In this work, a series of TiO2 nanocrystals with different facets ratios was prepared for the selective oxidation of cyclohexylamine to cyclohexanone oxime. The results indicated that the dominant anatase TiO2 (001) catalyst exhibited the promising oxidation results with 51.3% of cyclohexylamine conversion and 86.3% of selectivity to cyclohexanone oxime, and excellent stability in cyclic tests. The XPS and in situ FT-IR results indicated that the TiO2 (001) facet afforded the rich surface hydroxyl groups as catalytic active sites. The density functional theory calculation results demonstrated that the energy barriers of the rate determining steps for the oxidation of N-H bond in cyclohexylamine on TiO2 (001) facet were lower than those of (101) facet. Furthermore, the energy barriers for the cleavage of N-H were lower than those of C-H bond on TiO2 (001) facet in the cyclohexylamine oxidation. This work offered a new insight into the design of catalytic active sites for the selective oxidation of amines to corresponding oximes.

Automated summary

This study developed an environmentally friendly and efficient method for the selective oxidation of cyclohexylamine to cyclohexanone oxime by preparing TiO2 nanocrystals with different facets ratios. The dominant anatase TiO2 (001) catalyst showed promising results with 51.3% conversion of cyclohexylamine and 86.3% selectivity to cyclohexanone oxime, along with excellent stability. The TiO2 (001) facet provided abundant surface hydroxyl groups as catalytic active sites. DFT calculations revealed that the energy barriers for the rate determining steps in the oxidation of N-H bond on TiO2 (001) facet were lower than those on (101) facet, indicating better catalytic performance. This work offers new insights into the design of catalytic active sites for the selective oxidation of amines to oximes.