Construction of Dual Active-Site NH2-MIL-125(Ti) for Efficient Selective Oxidation of Cyclohexylamine to Cyclohexanone Oxime

In this work, dual active-site Ti-incorporated metal-organic frameworks (MIL-125 and NH2-MIL-125) were synthesized by a simple solvothermal process and applied to prepare cyclohexanone oxime from cyclohexylamine oxidation. A low-temperature thermal calcination strategy was used for the modulation of surface properties while maintaining the crystal structure and morphology. The results demonstrated that novel bifunctional NH2-MIL-125@250 °C obtained from thermal calcination possessed a large surface area with both oxygen vacancies and surface hydroxyl-active sites, promoting the adsorption and activation of cyclohexylamine and oxygen molecules, respectively. Under the optimum conditions, the cyclohexylamine conversion was 44.3%, and the selectivity to cyclohexanone oxime was 83.0%. By comparison, the stability of MIL-125 and NH2-MIL-125 was investigated separately in cyclic tests, and the crystal structure and catalytic properties of NH2-MIL-125 have been shown to be more stable than those of MIL-125. Combined with density functional theory, it was further shown that NH2-MIL-125 displayed a higher adsorption and activation ability toward cyclohexylamine and oxygen than MIL-125 and had a more stable metal-organic ligand structure. Finally, a plausible reaction pathway for selective cyclohexylamine oxidation to cyclohexanone oxime was proposed. This work can give new insights into designing novel dual active-site catalysts for the efficient catalytic transformation of organic primary amines to corresponding oximes.