Cu/Ni/Al layered double hydroxides as precursors of catalysts for the wet air oxidation of phenol aqueous solutions

Layered double hydroxides (LDHs) with a hydrotalcite-like structure containing different amounts of Cu2+/Ni2+/Al3+ cations with M2+/M3+ ratios between 0.5 and 3.0 were prepared. Well-dispersed mixed-oxide and spinel-like phases were obtained by calcining these hydrotalcite-type precursors at different temperatures between 373 and 1173 K. We performed thermogravimetric analysis, X-ray diffraction, BET areas and FT-IR spectroscopy to characterise the hydrotalcite-like compounds synthesised and the chemical changes which take place during their calcination process. We also tested their catalytic behaviour for the oxidation of phenol aqueous solutions using a trickle-bed reactor and a semi-batch reactor for comparison, and air with an oxygen partial pressure of 0.9 MPa at a reaction temperature of 413 K. The initial activities and the stability of the catalysts can be related to their chemical composition, structural phases and BET areas. Non calcined samples were practically inactive. At calcination temperatures of 623 and 673 K, copper catalysts showed higher initial activities than nickel ones (>90 and >70% of phenol conversion, respectively). However, the activity of them decreased continuously over time reaction, because of the elution of the active phase (CuO and/or NiO), and/or the formation of new phases due to the reaction conditions. Furthermore, nickel catalysts showed lower amounts of intermediate oxidation products such as quinones and carboxylic acids than copper ones. On the other hand, the spinel phases (obtained for the samples calcined at 1023 K) showed higher conversions (among 40–75%) and were stable at these reaction conditions. They did not show any loss in activity after a continuous working run of 15 days using a trickle-bed reactor. However, when the experiments were performed in an autoclave a loss of activity was observed for the copper spinel phase (probably due to polymer formation) but not for the Cu-Ni-spinel catalyst.