Mesoporous (Cr)MCM-41: A mild and efficient heterogeneous catalyst for selective oxidation of cyclohexane

Abstract

Liquid-phase oxidation of cyclohexane was carried out at moderate reaction conditions over mesoporous (Cr)MCM-41 molecular sieves using acetic acid as solvent, hydrogen peroxide as oxidant, and methyl ethyl ketone as initiator. Under these conditions, the catalysts showed high substrate conversion and excellent product selectivity. Although the activity of the catalyst decreased after the first recycle, owing to leaching of nonframework chromium ions, it, however, remained nearly the same thereafter. This observation was further confirmed by washing experiments where the nonframework chromium ions were removed upon ammonium acetate treatment or under reaction conditions. Further, the washed catalyst also showed a activity similar to that of the recycled catalyst. The recycled or washed (Cr)MCM-41, therefore, behaves truly as a heterogeneous catalyst, and thus it forms the first example of a nonleaching chromium-based catalyst reported so far. The use of a stronger oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. On the other hand, the use of solvents like methanol, dioxan, and acetone showed lower conversion. Likewise, the use of initiators such as acetone, cyclohexanone, and acetaldehyde also resulted in lower activity.