Characterization and activity of copper and nickel catalysts for the oxidation of phenol aqueous solutions

Catalysts based on CuO/gamma-alumina, CuAl2O4/gamma-alumina, NiO/gamma-alumina, NiAl2O4/gamma-alumina and bulk CuAl2O4 have been structurally characterized by BET, porosimetry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Their catalytic behaviors have also been tested 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 of 413 K. The copper and nickel catalysts supported on gamma-alumina have surface areas of the same order as the support gamma-alumina of ca. 190 m(2)/g and high active phase dispersions which were also confirmed by SEM, whereas the bulk copper aluminate spinel has a surface area of ca. 30 m(2)/g. XRD detects the phases present and shows a continuous loss of CuO by elution and the formation of a copper oxalate phase on the surface of the copper catalysts which also elutes with time. The NiO was also eluted but less than the copper catalysts. Only the copper and nickel spinel catalysts were stable throughout the reaction. Phenol conversion vs. time shows a continuous overall decrease in activity for the CuO/gamma-alumina and NiO/gamma-alumina catalysts. In turn, the copper and nickel spinel catalysts reach steady activity plateaus of 40 and 10%, respectively, of phenol conversion. The bulk copper aluminate spinel shows an activity plateau of 20% of the conversion which is lower than that from the copper aluminate/gamma alumina catalyst due to its lower surface area. Nickel catalysts always have lower activities than the copper catalysts for the phenol oxidation Fraction. The copper catalysts drive a mechanism of partial phenol oxidation to carboxylic acids and quinone-related products with very high specific rates, and the nickel catalysts mainly drive a mechanism of COP formation with lower conversion but with a potential higher catalyst life. The triphasic tubular reactor using trickle-bed regime largely avoids the mechanism of polymer formation as a catalyst deactivation process. (C) 1998 Elsevier Science B.V. All rights reserved.