Catalytic wet peroxide oxidation of phenol over iron or copper oxide-supported allophane clay materials: Influence of catalyst SiO ₂/Al ₂O ₃ ratio

Allophane clay materials with SiO ₂/Al ₂O ₃ ratios of 1.0 (AlSi1) and 2.2 (AlSi2) were synthesized by a co-precipitation route and further impregnated with iron or copper species. The structure of the parent AlSi1 sample is similar to that of a typical Al-rich soil allophane, while the parent AlSi2 material resembles the structure of a hydrous feldspathoid with a large interspherule surface, thereby exhibiting a high surface area. The ability of the various iron or copper-based allophane samples to behave as efficient and stable catalysts in phenol oxidation using H ₂O ₂ was investigated in ambient conditions for the first time. Their structural and textural properties were determined by X-ray diffractometry, N ₂ adsorption-desorption at 77 K, electrophoretic mobility measurements, infrared spectroscopy, transmission and scanning electron microscopy, as well as by thermo-gravimetric analysis. The catalytic activity of the iron or copper oxide-supported allophanes was markedly influenced by their SiO ₂/Al ₂O ₃ ratio and by their respective structure. The iron-based AlSi2 catalysts with tail-like structure and high surface area proved to be far more active than their corresponding AlSi1 counterparts. The highest catalytic efficiency in terms of total organic carbon abatement was obtained at 40 °C for the calcined iron oxide-supported AlSi2 allophane sample, for which very low leaching level of iron species was noticed (0.37 mg L ^-1). By contrast, large differences in terms of catalytic efficiency (conversion rates) and stability were observed for the copper-based counterparts, thereby indicating that the iron oxide-supported allophanes with a hydrous feldspathoid structure are highly active and stable in the catalytic wet peroxide oxidation of phenol.