RESUMO
High-valent metal-oxo species are key reactive intermediates in many biological and biological oxidation reactions. Herein, allylic hydroxylation (C-H) versus epoxidation (CâC) reactions of propene with a model catalyst iron phthalocyanine (FePc) in the presence of hydrogen peroxide were investigated contrastively, aiming to probe the active intermediates, structure-activity relationship, and reaction pathways. Our results showed that H2O2 as an oxygen-donor reagent can be easily decomposed on FePc to produce key active intermediates OâFePc and OâFePcâO with the energy barriers of 19.57 and 23.89 kcal/mol, respectively. In the selective oxidation of propene, OâFePc has a small preference for CâC epoxidation over C-H hydroxylation while OâFePcâO has a small preference for C-H hydroxylation. Since the electron-withdrawing O axial ligand in OâFePcâO further increases the radical character (Fe-O·) and Fe-O bond length of the iron-oxo moiety, OâFePcâO has better catalytic performance in both CâC epoxidation and C-H hydroxylation than OâFePc. Furthermore, in the whole reaction processes, the dual-hydrogen bonds between the two terminal H atoms of the alkene and allylic groups of propene and oxygen atom of the iron-oxo moiety would lead to the reaction toward CâC epoxidation while the single-hydrogen bond between the terminal H atom of the allylic group and the oxygen atom of the iron-oxo moiety would lead to the reaction toward C-H hydroxylation, implying that the weakly interacting hydrogen bonds affecting oxidation pathways also play a very important role in the regioselectivity of CâC epoxidation and C-H hydroxylation.
RESUMO
Catalytic reduction of CO2 over Cu-based catalysts can produce various carbon-based products such as the critical intermediate CO, yet significant challenges remain in shedding light on the underlying mechanisms. Here, we develop a modified triple-stage quadrupole mass spectrometer to monitor the reduction of CO2 to CO in the gas phase online. Our experimental observations reveal that the coordinated H2O on Cu(I)-based catalysts promotes CO2 adsorption and reduction to CO, and the resulting efficiencies are two orders of magnitude higher than those without H2O. Isotope-labeling studies render compelling evidence that the O atom in produced CO originates from the coordinated H2O on catalysts, rather than CO2 itself. Combining experimental observations and computational calculations with density functional theory, we propose a detailed reaction mechanism of CO2 reduction to CO over Cu(I)-based catalysts with coordinated H2O. This study offers an effective method to reveal the vital roles of H2O in promoting metal catalysts to CO2 reduction.