The process of selective oxy-functionalization of hydrocarbons using peroxide, O
, H
O
, O
, and transition metals can be carried out by the reactive oxygen species such as hydroxyl/hydroperoxyl ...radical and/or metal oxygenated species generated in the catalytic reaction. Thus, a variety of mechanisms have been proposed for the selective catalytic oxidation of various hydrocarbons including light alkanes, olefins, and simple aromatics by the biological metalloproteins and their biomimetics either in their homogeneous or heterogeneous platforms. Most studies involving these metalloproteins are Fe or Cu monooxygenases. The pathways carried out by these metalloenzymes in the oxidation of C-H bonds invoke either radical reaction mechanisms including Fenton's chemistry and hydrogen atom transfer followed by radical rebound reaction mechanism or electrophilic oxygenation/O-atom transfer by metal-oxygen species. In this review, we discuss the metal oxide nano-catalysts obtained from metal salts/molecular precursors (M = Cu, Fe, and V) that can easily form
through the oxidation of substrates using H
O
in CH
CN, and be facilely separated from the reaction mixtures as well as recycled for several times with comparable catalytic efficiency for the highly selective conversion from hydrocarbons including aromatics to oxygenates. The mechanistic insights revealed from the oxy-functionalization of simple aromatics mediated by the novel biomimetic metal oxide materials can pave the way toward developing facile, cost-effective, and highly efficient nano-catalysts for the selective partial oxidation of simple aromatics.
We undertake silver cyanide (AgCN) powder for its catalytic epoxidation of cyclohexene or styrene in CH3CN with variable substrate‐to‐solvent volume ratios using H2O2(aq) at 60 °C. The reaction ...mixtures can facilely separate into organic and aqueous layers. Cyclohexene oxide can be produced in the organic layer with 100 % selectivity from the substrate cyclohexene, while styrene oxide was identified with 80 % selectivity against benzaldehyde in styrene oxidation. After reactions, we can recycle the AgCN particles with comparable bulk property clarified via XRD, XPS, XAS, FT‐IR, and 13C‐SS NMR spectroscopy. Using H218O2 as the oxidant, both epoxide products and acetamide are highly enriched with 18O‐atom, indicating that the π bond‐activation is essential for forming the cyclohexene/styrene oxides in the organic and acetamide in the aqueous layers. The oxidation of cyclohexene/styrene catalyzed by AgCN powder through surface activation by H2O2(aq) and assisted by the non‐innocent CH3CN‐originated acetamide can achieve highly selective π‐bond activation with high reactivity.
An AgCN nanocatalyst with high reusability is developed for epoxidation of cyclohexene and styrene using H2O2(aq) in CH3CN at 60 °C. 100 % cyclohexene oxide and 80 % styrene oxide selectivity in the organic layers can be achieved. Electrophilic adsorbed oxygen and hydroxide species are evidenced to be essential for the heterogeneous olefin epoxidation.
Abstract
We undertake silver cyanide (AgCN) powder for its catalytic epoxidation of cyclohexene or styrene in CH
3
CN with variable substrate‐to‐solvent volume ratios using H
2
O
2(aq)
at 60 °C. The ...reaction mixtures can facilely separate into organic and aqueous layers. Cyclohexene oxide can be produced in the organic layer with 100 % selectivity from the substrate cyclohexene, while styrene oxide was identified with 80 % selectivity against benzaldehyde in styrene oxidation. After reactions, we can recycle the AgCN particles with comparable bulk property clarified via XRD, XPS, XAS, FT‐IR, and
13
C‐SS NMR spectroscopy. Using H
2
18
O
2
as the oxidant, both epoxide products and acetamide are highly enriched with
18
O‐atom, indicating that the π bond‐activation is essential for forming the cyclohexene/styrene oxides in the organic and acetamide in the aqueous layers. The oxidation of cyclohexene/styrene catalyzed by AgCN powder through surface activation by H
2
O
2(aq)
and assisted by the non‐innocent CH
3
CN‐originated acetamide can achieve highly selective π‐bond activation with high reactivity.
•Fe-oxide CNC prepared from Fe(ClO4)2 using H2O2(aq) in C6H6-CH3CN.•Fe-oxide CNC composes of Fe2O3 nanoparticles and carbonaceous moiety.•Highly efficient Fe-oxide CNC for highly selective benzene ...oxidation to phenol.•EPR study shows DMPO-OOH adduct formation on heterogeneous Fe-oxide CNC surface.•Hydrophobic carbonaceous surface assists selective C6H6 oxidation to phenol.
Direct benzene hydroxylation under mild conditions using hydrogen peroxide can be essential in producing phenol. A new iron oxide-carbonaceous nanocatalyst (Fe-oxide CNC) is prepared in situ using a precursor of Fe(ClO4)2 and H2O2 in benzene and CH3CN. The obtained Fe-oxide CNC can efficiently catalyze benzene oxidation to phenol using H2O2(aq) in CH3CN. The highest phenol production was achieved with the turnover number (TON) of 122 in unit iron content and selectivity of 96% at 60 °C. The catalyst was characterized by SEM, TEM, XAS, XPS, Raman, XRD, BET, and TGA techniques.
Interestingly, materials identification and characterization of the Fe-oxide CNC provided evidence to support the presence of Fe2O3. In addition, the carbonaceous moieties mainly consisted of branched aliphatic hydrocarbons on the iron oxide surface, which enhance the catalytic performance for benzene oxidation to phenol. The Fe-oxide CNC can be recycled three times with comparable catalytic efficiency. Using H218O2 as an oxidant, the phenol product with highly enriched 18O-atom indicates that the reactive oxygen species (ROS) activated by H2O2(aq) is crucial for aromatic oxidation. Time-resolved spin trapping experiments displayed the formation of the intermediate adducts consisting of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and reactive oxygenated radicals. For 3‒24 h durations, higher steady-state concentrations of the oxygenated intermediates observed at 25 °C than 60 °C indicate that the catalytic oxidation of benzene mediated by Fe-oxide CNC performs much faster kinetics at 60 °C than the ambient condition.
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The major aim of our work is to explore various ways to achieve regioselective and stereoselective oxidations of hydrocarbons under ambient conditions with high efficiency and atom economy. When ...researchers seek to account for differences in the chemical selectivity of metallo‐enzymes or monooxygenases, they typically invoke differences in shape selection of substrate pockets as well as differences in specific nonbonding interactions, such as hydrogen bonding, electrostatic forces, and/or van der Waals interactions, in the interactions of the hydrocarbon substrates with the active sites of the metalloenzymes/monooxygenases. To explore these effects in depth, we have redesigned the substrate binding pockets of alkane hydroxylase (AlkB) and cytochrome P450 BM3 using a heterogeneous
Escherichia coli
recombinant system. These two redesigned proteins are deployed to achieve efficient oxidations of n‐butane at its C‐1 and C‐2 positions, respectively. These studies have enabled us to propose mechanisms used by AlkB and cytochrome P450 BM3 to selectively oxidize inert CH bonds and oxyfunctionalize a variety of hydrocarbons. The insights gained are expected to contribute to the design of biomimetics or artificial catalysts that selectively oxidize alkanes, including nanomaterial platforms of these biomimetics.
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