Highly Efficient CuCr-MMO Catalysts for Base-free Styrene Epoxidation with H2O2 as Oxidant: Synergistic Eﬀect between Cu and Cr
The base-free styrene epoxidation with H2O2 as oxidant is strongly in accordance with the requirements of sustainable chemical industry. Aiming at the low efficiency of non-noble metal catalyst system, we proposed utilizing the synergistic effect and manipulating surface acid-basic property of materials to increase catalytic performance in this reaction. Herein, to combine two promoting approaches, we synthesized a series of mixed metal oxides (MMO) materials to screen promising active components with suitable acid-basic property for H2O2-styrene epoxidation. As a result, highest styrene conversion (82.8%) and best styrene oxide selectivity (79.7%) were achieved over CuCr-MMO catalyst, the catalytic performance of which was much enhanced compared with 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 the one hand, promoted dispersion of Cu and Cr components contributed to more catalytic sites, and the electron transfer between Cu and Cr improved 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. Specifically, suitable basicity facilitated to selectively transform H2O2 into HOO• radicals, while low acidity suppressed the epoxide ring opening to form phenylacetaldehyde. By combination of the high accessibility, suitable basic-acid property and fast electron transfer, the CuCr-MMO exhibited high styrene conversion and selectivity to styrene oxide.