Selective Catalytic Oxidation of Cyclohexene with Molecular Oxygen: Radical Versus Nonradical Pathways.

We study the allylic oxidation of cyclohexene with O2 under mild conditions in the presence of transition-metal catalysts. The catalysts comprise nanometric metal oxide particles supported on porous N-doped carbons (M/N:C, M=V, Cr, Fe, Co, Ni, Cu, Nb, Mo, W). Most of these metal oxides give only moderate conversions, and the majority of the products are over-oxidation products. Co/N:C and Cu/N:C, however, give 70-80 % conversion and 40-50 % selectivity to the ketone product, cyclohexene-2-one. Control experiments in which we used free-radical scavengers show that the oxidation follows the expected free-radical pathway in almost all cases. Surprisingly, the catalytic cycle in the presence of Cu/N:C does not involve free-radical species in solution. Optimisation of this catalyst gives >85 % conversion with >60 % selectivity to the allylic ketone at 70 °C and 10 bar O2. We used SEM, X-ray photoelectron spectroscopy and XRD to show that the active particles have a cupric oxide/cuprous oxide core-shell structure, giving a high turnover frequency of approximately 1500 h-1. We attribute the high performance of this Cu/N:C catalyst to a facile surface reaction between adsorbed cyclohexenyl hydroperoxide molecules and activated oxygen species.

Boosting the Supercapacitance of Nitrogen-Doped Carbon by Tuning Surface Functionalities.

The specific capacitance of a highly porous, nitrogen-doped carbon is nearly tripled by orthogonal optimization of the microstructure and surface chemistry. First, the carbons' hierarchical pore structure and specific surface area were tweaked by controlling the temperature and sequence of the thermal treatments. The best process (pyrolysis at 900 °C, washing, and subsequent annealing at 1000 °C) yielded a carbon with a specific capacitance of 117 F g-1 -nearly double that of a carbon made by a typical single-step synthesis at 700 °C. Following the structural optimization, the surface chemistry of the carbons was enriched by applying an oxidation routine based on a mixture of nitric and sulfuric acid in a 1:4 ratio at two different treatment temperatures (0 and 20 °C) and different treatment times. The optimal treatment times were 4 h at 0 °C and only 1 h at 20 °C. Overall, the specific capacitance nearly tripled relative to the original carbon, reaching 168 F g-1 . The inherent nitrogen doping of the carbon comes into interplay with the acid-induced surface functionalization, creating a mixture of oxygen- and nitrogen-oxygen functionalities. The evolution of the surface chemistry was carefully followed by X-ray photoelectron spectroscopy and by N2 sorption porosimetry, revealing stepwise surface functionalization and simultaneous carbon etching. Overall, these processes are responsible for the peak-shaped capacitance trends in the carbons.