Synthesis, characterization and catalytic hydroxylation of phenol over CuCoAl ternary hydrotalcites

Abstract

Copper–cobalt–aluminium hydrotalcites with (Cu + Co)/Al atomic ratio of 3.0 and Cu/Co atomic ratios 80:20, 75:25, 50:50, 33:67, 25:75 and 20:80 were synthesized by coprecipitation under low supersaturation and characterized by various physicochemical techniques. Powder X-ray diffraction (PXRD) showed a single phase with a resemblance to hydrotalcite, whose crystallinity increased with increasing copper concentration. A continuous blue shift of the ν_OH band with a concomitant red shift in the ν₂ vibration of the carbonate was observed with increasing cobalt content. Thermal analysis of these samples showed a significant variation depending on the nature of the atmosphere, more marked at higher temperatures, owing to the possibility of oxidation of cobalt. UV–vis spectroscopy showed absorption maxima close to 750 and 530 nm with the former becoming stronger and the latter weaker as the copper content is increased. TPR of fresh samples substantiated the lack of oxidation of cobalt, as evidenced by an M/H₂ (M = Cu + Co) ratio for these samples close to 1.0. N₂ adsorption measurements showed type II isotherms with a hysteresis loop closing at P/P₀ = 0.45. The total pore volume, the specific surface area and the area of the hysteresis loop increased with cobalt content. Thermal calcination of these samples at 500°C showed a diffuse pattern exhibiting both a tenorite and a spinel phase, whose peak intensities varied with Cu/Co atomic ratio. TPR for these samples showed higher M/H₂ values, suggesting partial oxidation of Co²⁺. Liquid phase hydroxylation of phenol was carried out over these catalysts using H₂O₂ as oxidant. Among the catalysts studied, that with a Cu:Co atomic ratio of 3:1 showed the maximum activity with a catechol: hydroquinone ratio close to 3.4. An increase in substrate:catalyst ratio enhanced the conversion of phenol. Oxidants other than H₂O₂ and solvents other than water did not show measurable conversion of phenol. Time-on-stream studies indicated that 80% conversion of phenol was achieved in 10 min. The normalized activity of these catalysts indicated the influence of cobalt, probably together with surface properties, on the intrinsic activity of copper in controlling the course of reaction. A reaction pathway involving hydroxyl radical is proposed.