Enhancing the Chemo-Enzymatic One-Pot Oxidation of Cyclohexane via In Situ H<sub>2</sub>O<sub>2</sub> Production over Supported Pd-Based Catalysts
Joseph Brehm, Richard J. Lewis, Thomas Richards, Tian Qin, David Morgan, Thomas E. Davies, Liwei Chen, Xi Liu, Graham J. Hutchings
Abstract
The combination of an evolved unspecific peroxygenase (UPO), from Agrocybe aegerita (PaDa-I variant) and bimetallic Pd-based catalysts, is demonstrated to be highly effective for the one-pot oxidative valorization of cyclohexane to cyclohexanol and cyclohexanone (collectively KA oil), via the in situ formation of H2O2 from the elements. The alloying of Pd with Zn in particular is found to significantly enhance catalytic performance compared to bimetallic PdAu or monometallic Pd analogues. The improved activity of the PdZn/TiO2/PaDa-I system is attributed to the facile formation of PdZn alloys and the resulting electronic modification of Pd, which results in an inhibition of competitive chemo-catalyzed H2O2 degradation reactions and the total suppression of the overoxidation of cyclohexanol. By comparison, the large population of Pd-only clusters present in both the PdAu and monometallic Pd catalysts is considered to be responsible for the promotion of further oxidation products and the unselective conversion of H2O2 to H2O, which hampers overall process efficiency. Notably, given the susceptibility of the enzyme to deactivation at moderate H2O2 concentrations, the continual supply of low levels of the oxidant via in situ production represents a highly attractive alternative to the continuous addition of preformed H2O2 or co-enzyme-based systems.