Synthesis and characterization of CuMgAl ternary hydrotalcites as catalysts for the hydroxylation of phenol

CuMgAl ternary hydrotalcites with a (Cu + Mg)/Al atomic ratio of 3.0 and a Cu/Mg atomic ratio of 5.0, 3.0, 1.0, 0.33, and 0.2 were synthesized by coprecipitation under low supersaturation. Powder X-ray diffraction (PXRD) of all of the samples showed the pattern characteristic of hydrotalcite without any detectable impurity phases, with the crystalline order, especially in the ab plane, improving with an increase in magnesium concentration. In situ PXRD studies revealed varying phase evolution processes depending on the concentration of magnesium; this observation was well complemented by in situ diffuse reflectance infrared Fourier transform (DRIFT) measurements. The thermal stability of these materials also improved with an increase in magnesium concentration, as indicated by TG-DTA-EGA measurements. However, a characteristic endotherm was noted at temperatures around 870 K accompanied by the evolution of CO2, the intensity of which increased with an increase in copper concentration. Catechol (CAT) and hydroquinone (HQ) were the main products noted in the hydroxylation of phenol over these materials, with H2O2 as oxidant and H2O as solvent. Variation of the substrate:catalyst mass ratio (10-500) showed that the activity passes through a maximum for a ratio of 100. The influence of reaction time indicated that the reaction was nearly complete in less than 60 min. Activity was enhanced by an increase in the concentration of copper, whereas the normalized activity (activity per unit copper concentration) showed an opposite trend. The structure-activity relationship of these samples, obtained through scanning electron microscopy, N2 adsorption measurements, X-ray photoelectron spectroscopy, and cyclic temperature-programmed reduction-oxidation measurements, corroborated such activity variations.