Photocatalytic cyclohexane oxidation and epoxidation using hedgehog particles.

Inorganic particles are effective photocatalysts for the liquid-state production of organic precursors and monomers at ambient conditions. However, poor colloidal stability of inorganic micro- and nanoparticles in low-polarity solvents limits their utilization as heterogeneous catalysts and coating them with surfactants drastically reduces their catalytic activity. Here we show that effective photo-oxidation of liquid cyclohexane (CH) is possible using spiky particles from metal oxides with hierarchical structure combining micro- and nanoscale structural features engineered for enhanced dispersibility in CH. Nanoscale ZnO spikes are assembled radially on α-Fe2O3 microcube cores to produce complex 'hedgehog' particles (HPs). The 'halo' of stiff spikes reduces van der Waals attraction, preventing aggregation of the catalytic particles. Photocatalysis in Pickering emulsions formed by HPs with hydrogen peroxide provides a viable pathway to energy-efficient alkane oxidation in the liquid state. Additionally, HPs enable a direct chemical pathway from alkanes to epoxides at ambient conditions, specifically to cyclohexene oxide, indicating that the structure of HPs has a direct effect on the recombination of ion-radicals during the hydrocarbon oxidation. These findings demonstrate the potential of inorganic photocatalysts with complex architecture for 'green' catalysis.

Dispersion of Hydrophilic "Hedgehog" Microparticles in Liquid CO2 Mixtures.

Liquid and supercritical CO2 are nontoxic and nonflammable reaction media with pressure-variable physical properties. These states of CO2 also have high solubility limits for gas and liquid hydrocarbons, making them good candidates for "green" hydrophobic solvents in sustainable chemical technologies. However, the dispersion of hydrophilic colloidal nanoscale and microscale particles in CO2 is challenging due to the tendency of polar particles to aggregate in nonpolar media, limiting their available surface area and catalytic efficiencies. Here we show that native hydrophilic semiconductor particles can be effectively dispersed in a liquid CO2 mixture with acetonitrile (ACN) without additional chemical or mechanical dispersion techniques. Using surface corrugation as a method to prevent aggregation, we find that geometrically complex particles with a halo of stiff nanoscale spikes disperse and remain suspended longer in liquid CO2 than those without or with less prominent nanoscale corrugation. For the particles of this size and liquid CO2 mixtures, individual particle mass remains a prominent factor determining particle sedimentation rate even in the absence of aggregation. Particle dispersion and structural stability are confirmed using a combination of UV-vis spectroscopy, finite-difference time-domain modeling, and electron microscopy. The necessity of the cosolvent (ACN) indicates that particle behavior in liquid CO2 is vastly different than in traditional liquid-phase solvents and highlights the need for future studies to understand the wetting behavior of hydrophilic particles in high-pressure nonpolar environments.