Catalytic hydroxylation of phenol over ternary hydrotalcites containing Cu, Ni and Al

Abstract

Liquid phase catalytic hydroxylation of phenol was carried out over ternary hydrotalcites containing copper, nickel and aluminum using hydrogen peroxide as oxidant. The influence of various reaction parameters, namely, substrate:catalyst ratio, substrate:oxidant ratio, nature of oxidant, solvent, pH, time-on-stream, reaction temperature and calcination temperature on the activity and selectivity for the “sought for” reaction, were studied. The catalysts were synthesized by the coprecipitation technique using metal nitrates and a NaOH/Na2CO3 mixture. Hydroxylation of phenol over these catalysts resulted mainly in the formation of catechol and hydroquinone. Among the catalysts studied, CuNiAl3-5 ((Cu+Ni)/Al=3.0; Cu/Ni=5.0) and CuNiAl2-1 ((Cu+Ni)/Al=2.0; Cu/Ni=1.0) showed maximum activity with a catechol:hydroquinone ratio close to 1.6. An increase in the substrate:catalyst ratio enhanced the conversion of phenol over these catalysts. With respect to the influence of reaction temperature, the conversion increased up to 65 °C and decreased when the temperature was further increased. Oxidants other than H2O2 and solvents other than water have not showed measurable conversion of phenol. Time-on-stream studies indicated that around 90% of conversion of phenol was achieved in 10 min and longer reaction time did not significantly enhance the conversion. Among the calcined samples studied, that calcined at 800 °C showed a maximum activity for phenol hydroxylation; however, the activities of calcined catalysts were lower than those of fresh hydrotalcites. The observed variation in the activity may be attributed to the copper concentration, especially those present on the surface. A reaction pathway involving hydroxy radical is proposed for the formation of dihydroxybenzenes.