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 contain ing 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 phas es were obtained by calcining these hydrotalcite-type precursors at differe nt temperatures between 373 and 1173 K. We performed thermogravimetric anal ysis, X-ray diffraction, BET areas and FT-IR spectroscopy to characterise t he hydrotalcite-like compounds synthesised and the chemical changes which t ake 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 pa rtial pressure of 0.9 MPa at a reaction temperature of 413 K. The initial a ctivities and the stability of the catalysts can be related to their chemic al 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% o f phenol conversion, respectively). However, the activity of them decreased continuously over time reaction, because of the elution of the active phas e (CuO and/or NiO), and/or the formation of new phases due to the reaction conditions. Furthermore, nickel catalysts showed lower amounts of intermedi ate oxidation products such as quinones and carboxylic acids than copper on es. 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 the se reaction conditions. They did not show any loss in activity after a cont inuous working run of 15 days using a trickle-bed reactor. However, when th e experiments were performed in an autoclave a loss of activity was observe d for the copper spinel phase (probably due to polymer formation) but not f or the Cu-Ni-spinel catalyst. (C) 2001 Elsevier Science B.V. All rights res erved.