The hydroxylation of phenol was investigated using cobalt(II) complexes of bidentate ligands including N,N′-ethylenebis(salicylideneamine)(salen), N,N′-propylenbis(salicylideneamine)(salpn) and N,N′-phenylenebis(salicylideneamine)(salophen) encapsulated in zeolite-Y. The catalysts were characterized by Fourier transform infrared and X-ray diffraction analyses to confirm the complex encapsulation. These complexes catalyze the liquid-phase hydroxylation of phenol, through H2O2, to catechol and hydroquinone as major and minor products, respectively. The activities of all prepared catalysts were tested for the oxidation of phenol and hydrogen peroxide. The overall reaction conditions were optimized to get maximum hydroxylation gain to consider the concentration of substrate and oxidant, amount of catalyst, and the reaction temperature. The zeolite encapsulated complexes of cobalt(II) were found to be catalytically active toward the hydroxilation of phenol. Under the optimized reaction conditions, [Co-(salen)]-Y showed the highest conversion, about 43 %, after 6 h which was followed by [Co-(salpn)]-Y with 39 % conversion and [Co-(salophen)]-Y showing the lowest efficiency with 35 % conversion. The hydroxylation of phenol without catalyst and with Co/zeolite-Y catalyst showed poor results.
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The authors are grateful to the financial support from the Islamic Azad University, South Tehran branch. This study has been done under the format of project research “Zeolite-Y encapsulated metal complexes of Cobalt(II) as catalyst for the hydroxylation of phenol”.
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