Mesoporous VMCM-41: highly efficient and remarkable catalyst for selective oxidation of cyclohexane to cyclohexanol

Abstract

Liquid phase oxidation of cyclohexane was carried out under milder reaction conditions over mesoporous VMCM-41 molecular sieve catalysts using aqueous hydrogen peroxide as oxidant, acetic acid as solvent, and methyl ethyl ketone as initiator. The catalysts showed high substrate conversion and excellent product (cyclohexanol) selectivity. Although the activity of the catalyst slightly decreased after first recycle, owing to leaching of small amount of non-framework vanadium ions, it, however, remained nearly same thereafter. This observation was further confirmed by washing experiments where the non-framework vanadium ions were removed upon ammonium acetate treatment. Further, the washed catalyst also showed an activity similar to that of the recycled catalyst. Thus, the recycled or washed VMCM-41 behaves truly as heterogeneous catalyst. This observation was complemented and confirmed by both filtrate and quenching studies. The effects of reaction time, temperature, Si/V molar ratio, and catalyst concentration on the catalyst performance were examined in order to optimize the conversion of cyclohexane and selectivity of cyclohexanol. However, the use of strong oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. In addition, the use of solvents like methanol, dioxan and acetone showed lower activity.