Highly efficient CuCr-MMO catalyst for a base-free styrene epoxidation with H2O2 as the oxidant: synergistic effect between Cu and Cr

Abstract

The base-free styrene epoxidation with H₂O₂ as an oxidant was strongly in accordance with the requirements for a sustainable chemical industry. Aiming at the low efficiency of a non-noble metal catalyst system, we proposed utilizing the synergistic effect and manipulating the surface acid–basic property of materials to increase the catalytic performance in this reaction. Herein, to combine two promoting approaches, we synthesized a series of mixed metal oxide (MMO) materials to screen promising active components with a suitable acid–basic property for H₂O₂–styrene epoxidation. As a result, the highest styrene conversion (82.8%) and best styrene oxide selectivity (79.7%) were achieved over a CuCr-MMO catalyst. The catalytic performance was considerably enhanced compared with the CuO and CrOx references. Detailed characterizations were performed to investigate the dispersion, electronic structure, and acid–basic property to explain the structure–performance relationship. On one hand, the promoted dispersion of the Cu and Cr components contributed to more catalytic sites and the electron transfer between Cu and Cr improved the intrinsic activity, which were both responsible for the enhanced activity. On the other hand, the unique structure modulated the acid–basic properties to be suitable for this catalytic system. In particular, a suitable basicity was facilitated to selectively transform H₂O₂ into HOO˙ radicals, while a low acidity suppressed the epoxide ring opening to form phenylacetaldehyde. By combining the high accessibility, suitable basic–acid property, and fast electron transfer, CuCr-MMO exhibited a high styrene conversion and selectivity to styrene oxide.