Preparation and activity of Cu-Al mixed oxides via hydrotalcite-like precursors for the oxidation of phenol aqueous solutions

We performed thermogravimetric analysis (TGA), X-ray diffraction (XRD), BET areas, and FT-IR spectroscopy to characterize copper-aluminium mixed-oxide samples with Cu/Al ratios between 0.5 and 3.0. The thermal stability, crystallinity, and purity of the materials depended on the Cu/Al atomic ratio. The FT-IR and TG detected carbonate (mainly) and nitrate as counteranions which interact in the interlayer region. We found loosely bound carbonate and nitrate anions and one strongly bound type of carbonate. We used dynamic XRD experiments to study the evolution of phases during calcination. All the samples after calcination showed well-dispersed CuO and/or CuAl2O4 phases. We also tested their catalytic behaviour for the oxidation of 5 g/l phenol aqueous solutions using a triphasic tubular reactor working in a trickle-bed regime and air with an oxygen partial pressure of 0.9 MPa at a temperature reaction of 413 K. Phenol conversion decreased continuously over time for the samples calcined at lower temperatures (673 K). This is because of continuous loss of the CuO phase by elution and the formation of a new phase like copper oxalate on the surface of the copper catalysts which also elutes with time XRD shows that samples calcined at higher temperatures (1073 K) and after HCl treatment (0.1 M) to avoid the CuO phase, have a pure copper aluminate phase. This CuAl2O4 phase reaches steady activity plateaus in the 55-65% range of phenol conversion. The triphasic tubular reactor using trickle-bed regime largely avoids polymer formation as a catalyst-deactivation process. (C) 1999 Academic Press.