Effective solvent-free oxidation of cyclohexene to allylic products with oxygen by mesoporous etched halloysite nanotube supported Co2+

Abstract

One dimensional mesoporous etched halloysite nanotube supported Co²⁺ is achieved by selective etching of Al₂O₃ from halloysite nanotube (HA) and immersing the etched HA (eHA) into the Co(NO₃)₂·6H₂O solution consecutively. By facilely tuning the etching time and the weight ratio of Co(NO₃)₂·6H₂O to eHA, the morphology, specific surface area and the supported Co²⁺ content of the mesoporous material can be tuned. The method for mesoporous material is scaled up and can be extended to other clay minerals. The mesoporous eHA supported Co²⁺ is used as catalyst for the selective catalytic oxidation of cyclohexene in solvent-free reaction system with O₂ as oxidant. The results shows the catalytic activity is dependent on etching time, weight ratio of Co(NO₃)₂·6H₂O to eHA, calcination treatment and reaction time/temperature. Among them, mesoporous eHA supported Co²⁺ prepared with 18 h etching time and 2 : 1 Co(NO₃)₂·6H₂O/eHA weight ratio without calcination (HA/HCl-18 h/Co²⁺-2 : 1) demonstrates the highest catalytic activity under 75 °C reaction temperature and 18 h reaction time (58.30% conversion and 94.03% selectivity to allylic products). Furthermore, HA/HCl-18 h/Co²⁺-2 : 1 has exhibit superior cycling stability with 37.69% conversion and 92.73% selectivity to allylic products after three cycles.