Enhanced stability and activity for solvent-free selective oxidation of cyclohexane over Cu2O/CuO fabricated by facile alkali etching method

Abstract

The easy deactivation of cuprous oxide (Cu2O) have significantly limited their practical applications in catalysis. Formation of composites with other semiconductors including CuO is an crucial strategy for stability improvement of Cu2O. However, this strategy developed so far has often focused on only Cu2O thin film systems. Herein, Cu2O/CuO composites were fabricated via a simple alkali etching method, for the first time, to stabilize the powdered Cu2O in liquid-phase selective oxidation of cyclohexane under solvent-free conditions. The etching method presented remarkable high dispersion of copper species and an appropriate Cu+/Cu2+ ratio on catalyst surface, which leads to superior catalytic activity and stability. Hot filtration experiment confirmed that Cu2O/CuO composite was a heterogeneous catalyst which could be reused at least five times without considerable loss of catalytic activity, whereas the pure Cu2O suffered from severe deactivation caused by oxidative decomposition of Cu2O and copper leaching. Moreover, the amount of Cu+ and Cu2+ sites on catalyst surface are dramatically affected by the amount of alkali during etching process and the catalytic performance can be further tuned by regulating the Cu+/Cu2+ ratio. The Cu2O/CuO with a molar ratio of NaOH/Cu2O = 1:1 exhibited the highest catalytic activity with a cyclohexane conversion of 84.3 % and cyclohexanone selectivity of 77.0 %. The present work provides a promising strategy to stabilize Cu2O catalysts in catalytic oxidation systems.