Research On The Direct Hydroxylation Of Aromatic Compounds Catalyzed By The Iron-containing Catalysts

Direct hydroxylation of aromatic compounds to the corresponding phenols is of great interest as green approaches for the high atom economy. However, the inert aromatic C-H bond causes the low efficiency of the selective hydroxylation of the aromatics and the hydroxylated products usually undergo further oxidation which results in the low selectivity. In this paper, the iron-containing catalysts are used in the selective hydroxylation of aromatic compounds under mild conditions in the presence of H2O2.A mono iron(II) complex, coordinated by three 1-nitroso-2-naphtholate ligands was initiated to catalyze the direct hydroxylation of aromatic compounds to phenols in the presence of H2O2 under mild conditions. Various reaction parameters such as the temperature, reaction time, mole ratio of H2O2 to benzene which could affect the hydroxylation activity of the catalyst were investigated systematically for benzene hydroxylation to get ideal benzene conversion and high phenol selectivity. Under the optimum conditions, the phenol selectivity was 100%. The hydroxyl radical formed due to the reaction of the catalyst and H2O2 was determined to be the crucial active intermediate in the hydroxylation. A rational pathway for the formation of the hydroxyl radical was proposed and justified by the density functional theory calculations. Polynuclear complex was synthesized based on the special structure of the 1-nitroso-2-naphtholate ligand as an efficient building unit and applied as the heterogeneous catalyst for the direct hydroxylation of benzene to phenol which could be recovered and recycled easily.Three iron complexes were synthesized using 2,3-naphthalenediol, 2-hydroxy-3-naphthoic acid and 2-hydroxy-3-naphthanilide as the raw materials. The three iron complexes were proved to be highly selective and active for hydroxylation of aromatics in which the iron complex synthesized from the 2,3-naphthalenediol was more active with the phenol selectivity higher than 90%. The hydroxyl radical was the active intermediate. By comparing the hydroxylation activities of the three complexes we draw a conclusion that the complexes coordinated by the electron-donating ligands are more active.Three [FeFe]-hydrogenase model complexes were synthesized and used as efficient catalyst for hydroxylation of the aromatics to phenols. Under the optimum conditions, the phenol selectivity was higher than 90%. The diiron model complex with electron-donating ligand was proved to be more active towards hydroxylation. Based on the fact that the dithiolato-sulfur site and the Fe-Fe bond were the possible active oxidation site in the model complexes, density functional theory calculations were used to investigate the oxygenated products i.e. the S-oxygenated products or the Fe-based oxygenated products of the model complex which may be involved in the catalytic cycle. The experimental and computational results indicated that the thermodynamically favored Fe-based oxygenated μ-O intermediate may be the active intermediate in the hydroxylation.The supported bimetallic Cu-Co-MCM-41 and the silicate supported iron Fe-SMF were used as heterogeneous catalysts for the direct hydroxylation of phenol and benzene respectively. The selectivity of the dihydroxybenzenes was 89.1% and the phenol conversion was 12.8% in the Cu-Co-MCM-41 catalyzed hydroxylation of phenol. The environmentally friendly water was used as the solvent in the hydroxylation of benzene catalyzed by the Fe-SMF and the phenol yield was 13.4%.