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.
Bibliographical noteFunding Information:
This work was supported by Academia Sinica, Taiwan (AS-iMATE-107-93), and grants from the “Taiwan's Deep Decarbonization Pathways toward a Sustainable Society” research program (AS-KPQ-106-DDPP) and the Ministry of Science and Technology, Taiwan ( MOST 108-2113-M-001-006 , 109-2113-M-001-003 and 110-2113-M-001-038 ). We are grateful to Drs. Jyh-Fu Lee, Yu-Chun Chuang, Chih-Wen Pao, Jeng-Lung Chen and staffs of the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan, for their kind assistance with the X-ray absorption spectroscopy and synchrotron radiation high-resolution powder X-ray diffraction studies.
- Carbonaceous materials
- Iron oxide
- Selective oxidation