One-step hydroxylation of benzene to phenol over Schiff base complexes incorporated onto mesoporous organosilica in the presence of different axial ligands

Abstract

Liquid-phase hydroxylation of benzene to phenol using Schiff base complexes anchored on a mesoporous organosilica support was investigated in various solvents when molecular oxygen was utilized as a green oxidant. The mesoporous organosilica support was prepared through the condensation of tetraethyl orthosilicate (TEOS) and (3-aminopropyl)triethoxysilane (APTES) in different ratios, and then Schiff base complexes were anchored on it. Various aromatic heterocycles such as imidazole, 1-methyl imidazole, pyridine, and some pyridine derivatives were used as a proximal axial ligand, and their role in the reactivity of the catalyst was investigated experimentally and theoretically. Impressive promotion of the reactivity of the catalyst was the result of this coordination. Although all the synthesized catalysts showed catalytic reactivity, the vanadyl complex showed higher activity. The results show that SiO₂-Pro-Sal-VO was extremely stable concerning leaching of the vanadyl group and rendered excellent recyclability in the hydroxylation of benzene to phenol with a yield of 11.3% and selectivity of >95%. The activity of the other catalysts was as follows: SiO₂-Pro-Sal-Co > SiO₂-Pro-Sal-Cu > SiO₂-Pro-Sal-Fe > SiO₂-Pro-Sal-Zn > SiO₂-Pro-Sal-Mn.