Oxidative coupling of ortho-aminophenol over mesoporous silica containing copper(ii) diethylamino-substituted salen complex

Abstract

Copper(II) N,N’-bis[4-(N,N-diethylamino)salicylidene]ethylenediamine (CAS) complex, which has the metal-ligand coordination “CuN2O2” mimicking the active site of the enzyme galactose oxidase, has been synthesized by the reaction of copper(II) acetate monohydrate and the prepared ligand, N,N’-bis[4-(N,Ndiethylamino) salicylidene]ethylenediamine (A-salen). The structure of A-salen was confirmed by FTIR, 1H- and 13C-NMR spectra, while the CAS complex was characterized using FTIR and DR UV-Vis spectroscopy. The CAS complex was then immobilized on Si-MCM-48, NH2-MCM-48 and SO3H-MCM-48, respectively. After that, MCM-48 containing CAS complex was characterized by using FTIR, DR UV-Vis, powder XRD, N2 adsorption-desorption isotherm, AAS, TGA, FESEM and TEM. XRD patterns showed that the structure of mesoporous MCM-48 was preserved but the crystallinity of MCM-48 had decreased after modification and immobilization of CAS. Besides that, N2 physisorption measurement and TEM image showed that the pore channel of MCM-48 was well maintained in the meso range after modification and immobilization of CAS. The decrease in pore diameter of MCM-48 suggests that the modification had occurred in the pore channel. The catalytic activity of the supported CAS catalyst was tested in the heterogeneous oxidation of o-aminophenol (AP). The reaction was carried out in methanol containing H2O2 as oxidant at 70 oC for 24 hours. All the catalytic reactions were monitored using GC-FID. The supported CAS catalyst showed lower percentage selectivity and yield of 2-amino-3H-phenoxazin-3-one (APX), but gave higher TON than those obtained from homogeneous CAS. Oxidation of AP over supported CAS catalyst is considered as “green” process due to the recoverability of the catalyst. However, the leaching out of CAS catalyst from MCM-48 matrix has affected the catalytic performance of supported CAS catalyst. The supported CAS catalyst is proposed to have the catalytic activity mimicking the phenoxazinone enzyme, a copper-containing enzyme which catalyze the peptides-substituted o-aminophenol to phenoxazinone chromophore. This is because the supported CAS catalyst exhibits the catalytic behavior similar to phenoxazinone enzyme, which favors the oxidation of AP to APX via the formation of o-quinone imine (QI) intermediate. The effect of reaction time, temperature, molar ratio substrate to oxidant, different type of oxidant and different type of solvent to the oxidation of AP were also examined.