Robust TiO2/Hβ catalyst for selective synthesis of cyclohexanone oxime from the oxidation of N-cyclohexylcyclohexanimine with O2 under mild conditions

Abstract

N-cyclohexylcyclohexanimine (N-CCA), a by-product of cyclohexylamine (CHA) oxidation to cyclohexanone oxime (CHO), was used as the starting reactant for the efficient conversion to CHO under an oxygen atmosphere, utilizing the constructed surface hydroxyl-rich Ti-based catalysts. Of these, the 20% TiO₂/Hβ catalyst exhibited unique catalytic activity and stability, achieving 45.9% N-CCA conversion and 82.1% CHO selectivity, with the content of high-boiling by-products not exceeding 1.0%. The superior catalytic performance, as evidenced by characterization results, stems primarily from: (i) the combination of TiO₂ and Hβ, which promotes the formation of a high specific surface area with fine TiO₂ nanoparticles and high-density Ti^δ+ active sites, which facilitate the adsorption and activation of N-CCA; (ii) the construction of rich hydroxyl groups, which enhances the Brønsted acid sites, thereby improving the generation of active oxygen species; (iii) the intercrystalline embedding of TiO₂ and Hβ, which forms Ti–O–Si bonds, effectively inhibiting Ti leaching and ensuring excellent catalyst stability. The conversion of N-CCA, the main by-product of CHA oxidation, to the desired CHO using an efficient and robust 20% TiO₂/Hβ catalyst represents an attractive pathway.