Published: July 22, 2010 r2010 American Chemical Society 2495 dx.doi.org/10.1021/ie100694g | Ind. Eng. Chem. Res. 2011, 50, 2495–2500 ARTICLE Catalytic Wet-Air Oxidation of p-Cresol on Ag/Al 2 O 3 -ZrO 2 Catalysts Francisco N u~ nez, † Gloria Del Angel,* ,† Francisco Tzompantzi, † and Juan Navarrete ‡ † Departamento de Química, Universidad Aut onoma Metropolitana-Iztapalapa, Avenida San Rafael Atlixco No. 186, C. P. 09340, M exico D.F. M exico ‡ Instituto Mexicano del Petr oleo, Eje Central L azaro C ardenas 152, 07730 M exico, D.F. M exico ABSTRACT: Catalytic wet-air oxidation of p-cresol was carried out on Ag/Al 2 O 3 -ZrO 2 catalysts at 1, 5, 10, and 20 wt % of ZrO 2 in a reactor-type batch and 160 °C and 15 bar of oxygen. Al 2 O 3 -ZrO 2 supports were prepared by the simultaneous hydrolysis of the boehmite and zirconium alkoxide. Ag/Al 2 O 3 -ZrO 2 catalysts prepared by deposition-precipitation showed higher surface area (202-216 m 2 /g) and lower Lewis acids sites than the Ag/Al 2 O 3 (161 m 2 /g) and Ag/ZrO 2 (25 m 2 /g) reference catalysts. FTIR-CO adsorption showed a shift to lower frequencies on Ag/Al 2 O 3 -ZrO 2 catalyst indicating electron-rich metallic particles. The p-cresol conversion, TOC abatement, and CO 2 selectivity were improved when Ag was supported on Al 2 O 3 -ZrO 2 mixed oxides. A modification of the surface properties of Ag on the Ag/Al 2 O 3 -ZrO 2 catalysts due to a metal-support interaction leading to a faster oxidation of the adsorbed reactant species is proposed. ’ INTRODUCTION Phenols are environmental pollutants present in discharge wastewaters from fossil fuel refining processes, phenol manufac- turing plants, and a variety of industries. These phenolic com- pounds are toxic and nonbiodegradable by conventional methods, and the U.S. Resource Conservation and Recovery Act (RCRA), classifies the effluents containing phenols as highly dangerous. 1,2 Processes such as the catalytic wet advanced oxida- tion route (CWAO) are favorable for converting the organic pollutants matter into carbon dioxide and water. 3-6 In this way, metallic oxides like CuO have been successfully used as catalyst for the degradation of phenols (phenol, cresols, chlorophenols, nitrophenols, etc.) via CWAO. 7,8 A combination of various metallic oxides as CuO-ZnO supported on alumina, 9 CeO 2 , or CeO 2 promoted with CuO and MnO, and noble metals as Rh and Pt have also been reported as good alternative catalysts for phenols degradation. 10-14 Catalytic wet-air oxidation on carbon support 15 as well as transition metals oxides as FeO, Co 2 O 3 , NiO, and CuO provide efficient methods 16 for phenol degradation. Support effects on noble metals were observed for CWAO of phenol; thus the support stabilizes the metallic surface area of Ru, Rh, Pt, Pd, Ag, and Au noble metals when they are supported on Al 2 O 3 , SiO 2 , or TiO 2 . The support provides high dispersion, enhanced activity, and stability 4,6,16-21 to the active metal. Zirconia, as a support, has also been used for the CWAO pro- cess in which Kraft-bleaching wastewater is successfully treated using Ru-supported on ZrO 2 . However, zirconia has the dis- advantage of having a very poor area. 22 This takes into account that zirconia can be a good support for the CWAO process. In the present work the preparation of alternative supports for noble metals involving zirconium oxide was studied in the CWAO of p- cresol. Specifically, the catalytic wet-air oxidation of p-cresol, using silver supported on Al 2 O 3 -ZrO 2 mixed oxides (1, 5, 10, and 20 wt % ZrO 2 ) is reported. The characterization of the mixed oxides was made by means of nitrogen adsorption, XRD, H 2 - TPR, FTIR-pyridine adsorption, and FTIR-CO chemisorption. ’ EXPERIMENTAL SECTION The Al 2 O 3 -ZrO 2 mixed oxides were prepared at different contents of ZrO 2 (0, 1, 5, 10, 20 wt %) by means of a method that was named “boehmite peptization”. The method consists of the addition of a metallic aloxide to the boehmite contained in an acid solution. The peptization of the boehmite and the alkoxide hydrolysis occurs simultaneously. With this method a strong interaction of the alkoxide with the Al-OH groups of the peptized boehmite is obtained, causing a high dispersion of ZrO 2 on the surface of alumina. Supports and Catalysts Preparation. The γ-Al 2 O 3 support was obtained by calcination of boehmite (CONDEA, high purity 99.999%, 74% AlOOH, 26% H 2 O) under air flow (3.6 L/min) at 650 °C for 12 h. Al 2 O 3 -ZrO 2 supports were prepared by boehmite peptization as follows: Catapal boehmita was put in contact with a solution containing 100 mL of butyl alcohol and the water stoichiometrically required to hydrolyze the zirconium alkoxide (Stream Chemicals 98%). For boehmite peptization the pH of the suspension was adjusted with nitric acid to a value of 3; afterward the temperature was increased to 80 °C under agitation for 12 h. Then the suspension was cooled down to room temperature, and the solution containing the Zr(OC 4 H 9 ) 4 was added drop by drop in adequate amounts to obtain 0, 1, 5, 10, and 20 wt % of ZrO 2 in the Al 2 O 3 -ZrO 2 mixed oxides. The formation of the mixed oxide was then obtained by increasing the temperature to 100 °C under vigorous agitation and reflux, during 12 h. Afterward, the solids were dried in vacuum until dryness. Finally the solids were maintained in an oven at 120 °C for 12 h and then calcined under air flow at 650 °C for 6 h (60 mL min -1 ). Special Issue: IMCCRE 2010 Received: March 20, 2010 Accepted: July 12, 2010 Revised: July 5, 2010