The synthesis of Co-doped SAPO-5 molecular sieve and its performance in the oxidation of cyclohexane with molecular oxygen

Abstract

Silicoaluminophosphate (SAPO) molecular sieves doped with cobalt (Co-SAPO-5) were synthesized hydrothermally with different concentrations of Co. Each sample was characterized by X-ray diffraction, N2 adsorption-desorption, scanning electron microscopy, ultraviolet-visible spectroscopy, temperature-programmed desorption of NH3 (NH3-TPD), and infrared spectrascopy of adsorbed pyridine (Py-IR). The results showed that Co was highly dispersed in the Co-SAPO-5 samples. In addition, a part of the Co content had been incorporated into the SAPO-5 framework, while the remainder existed on the surface as extra-framework Co. The surface areas of the Co-SAOP-5 samples were similar to the SAPO-5 sample. However, the pore volumes of the Co-SAOP-5 samples were lower than that of the SAOP-5 sample. As the concentration of Co increased, the pore volume gradually decreased because extra-framework cobalt oxide was present on the catalyst surface. NH3-TPD and Py-IR results revealed that the amount of Brønsted acid and the total amount of acid for the Co-SAPO-5 samples were higher than that for the SAPO-5 sample. These values were also higher for samples with higher Co content. The catalytic activity of the Co-SAPO-5 samples was evaluated for the oxidation of cyclohexane with molecular oxygen. When Co was added to the SAPO-5 catalyst, the catalytic activity of the Co-SAPO-5 catalysts improved. In addition, the conversion of cyclohexane increased as the Co content in the Co-SAPO-5 catalysts increased. However, with a high conversion of cyclohexane (>6.30%), the total selectivity of cyclohexanone (K) and cyclohexanol (A) decreased sharply. The K/A ratio ranged from 1.15 to 2.47. The effects of reaction conditions (i.e., reaction temperature, reaction time, initial oxygen pressure, and the catalyst amount) on the performance of the Co-SAPO-5 catalysts have also been measured. Furthermore, the stability of the Co-SAPO-5 catalyst was explored and found to be good for the selective oxidation of cyclohexane by molecular oxygen.