Catalytic oxidation of aromatic oxygenates by the heterogeneous catalyst Co-ZIF-9

Abstract

The catalytic activity of Co-ZIF-9, a zeolitic imidazolate framework of sodalite topology, in the oxidation of small aromatic molecules including phthalan, vanillyl alcohol, guaiacol, syringol, veratryl alcohol, and cinnamyl alcohol by molecular oxygen has been investigated. Co-ZIF-9 selectively oxidized the substrates and thus represents a promising heterogeneous catalyst for the oxidation of small aromatic substrates. Phthalan was converted to three different products, i.e. phthalide, phthalaldehyde, and small amounts of phthalic acid. In addition to phthalan, the alcohol functionality of vanillyl alcohol, veratryl alcohol, and cinnamyl alcohol was also similarly oxidized to yield the corresponding aldehydes in high yields and excellent selectivity. The effect of solvent, temperature and NaOH addition on the Co-ZIF-9 catalytic system has been studied. The presence of NaOH in the solution greatly enhanced the oxidation activity. The structure and stability of the heterogeneous catalyst was probed by UV–vis diffuse reflection spectroscopy, confirming the coordination geometry of cobalt in the catalyst framework, thermal gravimetric analysis, establishing the thermal stability of the structure, and XRD to monitor crystallinity of the catalyst. The heterogeneous nature of the catalyst was established by a hot filtration experiment, ICP analysis of the liquid phase, and by UV–vis absorption spectroscopy on the hot filtered reaction mixture.