Oxy-functionalization of small hydrocarbons can be essential in producing crucial intermediate molecules for organic synthesis. We demonstrated catalytic activities towards either single or double oxygenation of benzene to form phenol/hydroquinone, selectively tuning sp3 vs sp2 C-H bond oxidations of toluene to form benzyl alcohol/aldehyde and/or o-/p-cresol, and selective hydroxylation of methane and ethane using oxidants H2O2, O2, by surface-functionalized metal oxide nanocatalysts. The surface modification strategy entails creating organic-inorganic hybrid structures of the nanocatalyst to alter the redox properties and the local coordination environment of the metal oxide and to increase the amount of highly dispersed active species and oxygen atom transfer capacity of the catalyst. In this manner, we have enhanced the catalytic performance and achieved the highest (84%) selective double oxidation of benzene to p-benzoquinone, switchable selective sp2 and sp3 C-H bond oxidation of toluene 75% and 95%, respectively, as well as the highest ~100% selective oxidation of methane and ethane to methyl/ethyl hydroperoxide at ambient temperatures. The nanocatalysts have been prepared from metal salt precursors (M = Cu, Fe, and V) that can form in situ through the oxidation of substrates and a solvothermal process using H2O2 in CH3CN, and are characterized by X-ray absorption spectroscopy, XPS, XRD, FT-IR, TGA-Mass, ICP-Mass, electron microscopy SEM/TEM-EDX, Electron paramagnetic resonance (EPR) spectroscopy techniques. The surface carbonaceous moieties of the nanocatalysts consist of branched aliphatic hydrocarbons with carboxyl, carbonyl, and hydroxyl functional groups. Mechanistic studies were conducted using isotope-labeled oxygen 18O2, H218O2, and probed by deuterated compounds or 13C substrates. The oxidative C–H bond activations were carried out by the reactive oxygen species such as hydroxyl/hydroperoxyl radical and/or metal oxygenated species generated in the catalytic reactions at ambient temperatures. Therefore, controlled production of the active radical species and increased reactivity of the oxygenated metal species are critical to modulating the selective oxidation of the small hydrocarbons.

Selective oxidation of small hydrocarbons to essential molecules using surface functionalized metal oxide nanocatalysts

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