Novel functional TiPO/SiO2 catalyst enriched surface hydroxyl groups for highly selective catalytic oxidation of cyclohexylamine to cyclohexanone oxime

Abstract

Developing a low-cost and environmentally benign alternative approach for the production of cyclohexanone oxime is strategic due to the increasing need for caprolactam, which is monomer for nylon-6. In this work, a novel and supported phosphoric acid modified TiO2 catalyst (TiPO/SiO2) had been successfully prepared for the catalytic oxidation of cyclohexylamine to cyclohexanone oxime. The results revealed that the supported TiPO/SiO2 catalyst exhibited promising catalytic activity and stability. Under the optimized reaction conditions, 49.4% of cyclohexylamine conversion with 87.8% of selectivity to cyclohexanone oxime was achieved over 20%TiPO/SiO2. The characterization results indicated that the TiPO/SiO2 possessed the abundant surface hydroxyls. XPS and DFT calculations verified that the surface titanium hydroxyls (Ti-OH) on TiPO/SiO2 were acted as the main catalytic active sites for activating cyclohexylamine. In situ FT-IR revealed the possible catalytic reaction pathway of cyclohexylamine with oxygen over TiPO/SiO2. The calculated energy profiles for the aerobic oxidation of cyclohexylamine showed that the energy barrier for NH bond cleavage was lower than that of α-CH bond cleavage, resulting in cyclohexanone oxime as main product. The present work afforded the new insights into the selective oxidation of cyclohexylamine to cyclohexanone oxime over functional TiPO/SiO2 catalyst, and has important industrial application prospects.