Nanosized CuO-CeO 2 catalysts with high surface area (>90 m 2 g -1 ) were prepared by a modified citrate sol-gel method with incorporation of N 2 thermal treatment. CO temperature-programmed reduction results indicated that there are three CuO species in the catalyst, namely, the finely dispersed CuO, the bulk CuO, and the Cu 2+ in the CeO 2 lattice. Using CO oxidation as a model reaction, catalytic activity of each species was evaluated. It was found that the finely dispersed CuO species had the highest activity (183.3 mmol CO g Cu -1 h -1 ) and the bulk CuO had medium activity (100.4 mmol CO g Cu -1 h -1 ), while the Cu 2+ in the CeO 2 lattice had the lowest activity (21.3 mmol CO g Cu -1 h -1 ). Furthermore, the Cu 2+ species in the CeO 2 lattice could migrate to the catalyst surface to form finely dispersed CuO under high-temperature calcination or under reaction conditions, which could consequently enhance the catalytic activity.

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The demand for a clean production process of adipic acid (AA) can be achieved by developing a synthetic route using H 2 O 2 as the oxidant. In this paper, a green process with a recyclable catalyst system consisting of H 2 WO 4 , H 2 SO 4 and H 3 PO 4 was developed for the production of AA via catalytic oxidation of cyclohexene. A continuous-flow reactor was set up for the optimization of the reaction parameters and developing the industrial operation of this green process. The mixture of H 2 SO 4 and H 3 PO 4 as acidic promoter displays a significant improvement in the activity of catalyst and the stability of H 2 O 2 . The catalyst could be recovered and reused 20 times, and no significant loss of catalytic performance can be observed. The effect of Fe 3+ ion as a possible contaminant has no serious negative effect on this reaction, and the 316L stainless steel and glass-lined steel were selected as appropriate equipment material. Calorimetry and the scale-up in batch reactor demonstrate that the reaction could be operated safely on scale. The process was scaled up in a continuous-flow pilot plant, with excellent yield (94.7%) and purity (99.0%). Some advantages such as the solvent-, phase-transfer-catalyst-, and organic additive-free and low-cost light up the application of this process in the industrial production of AA. † Electronic supplementary information (ESI) available: Effect of acidic promoters and the pathway of oxidation of cyclohexene to adipic acid with aqueous H 2 O 2 . See