Ind. Eng. Chem. Res. 1998, 37, 3343-3349
3343
KINETICS, CATALYSIS, AND REACTION ENGINEERING Alumina-Supported Noble Metal Catalysts for Destructive Oxidation of Organic Pollutants in Effluent from a Softwood Kraft Pulp Mill Qinglin Zhang and Karl T. Chuang* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada
The effectiveness of alumina-supported noble metal catalysts for the destructive oxidation of organic pollutants in effluent from a softwood kraft pulp mill was evaluated in a slurry reactor at 463 K and an oxygen pressure of 1.5 MPa. The effects of catalyst preparation procedures, such as metal loading, calcination, or reduction treatment on the catalytic activities, were also tested. Alumina-supported palladium catalysts were found to be more effective than supported manganese, iron, or platinum catalysts. The rate of oxidation over Pd/alumina catalyst was significantly higher than that of the uncatalyzed reaction. Adding Ce on the alumina support was found to promote the activity of alumina-supported Pt catalyst but inhibit the activity of alumina-supported Pd catalyst. The reaction mechanisms for the catalytic wet oxidation process and the roles of Ce on catalytic activity for destructive oxidation of organic pollutants in wastewater are discussed. Introduction Industrial effluent streams from pulp mills contain many organic materials. Wet air oxidation (WAO) is an accepted technology for the destruction of these organic compounds in the aqueous phase.1 Several investigators have studied noncatalytic WAO of the bleach plant effluents.2-5 Prasad and Joshi reported that pyrolysis (or thermal degradation) of the black liquor did not occur even at 543 K in the absence of a catalyst.6 The noncatalytic wet air oxidation of black liquor is slow and requires very severe temperature and pressure conditions, typically 473-573 K and a pressure over 10 MPa. A continuing need exists for the development of catalytic systems capable of degrading these organic compounds under relatively milder conditions of temperature and pressure and with reasonable retention time. Although homogeneous copper catalysts are effective for the wet oxidation of these effluents,7 heterogeneous catalysts are preferred to avoid the need for further treatment to remove the toxic catalyst from the water stream. Numerous metal oxides have been tested for the catalytic oxidation of individual organic compounds in water.8-17 Mn-Ce, Mn-Cu, and Cu-Zn oxide catalysts, supported copper oxide, and Co-Bi complex oxides are some of the promising catalysts. Cerium oxide is known to promote the redox properties of many catalysts for the oxidation of hydrocarbons, particularly the three-way catalyst for the oxidation of pollutants in automobile emission gases.18 Recently Leitenburg et al. have reported that the activities of mixed-metal oxides, such as ZrO2, MnOx, or CuO for acetic acid oxidation, can be enhanced by adding ceria as a promoter.19 * To whom correspondence should be addressed. Telephone: 4034924676.Fax: 4034922881.E-mail:
[email protected].
Imamura et al. also studied the catalytic activities of supported noble metal catalysts for wet oxidation of phenol and the other model pollutant compounds.13 Ruthenium, platinum, and rhodium supported on CeO2 were found to be more active than a homogeneous copper catalyst.13 Atwater et al. have shown that several classes of aqueous organic contaminants can be deeply oxidized using dissolved oxygen over supported noble metal catalysts (5% Ru-20% Pt/C) at temperatures 393-433 K and pressures between 2.3 and 6 atm.20 Mantzavinos et al. reported that palladium supported on alumina (0.5% Pd/alumina) is a very active catalyst for the oxidation of poly(ethylene glycol).21 This catalyst also has some activity for acetic acid oxidation. The organic pollutants in the waste streams from pulp mills include organic sulfur compounds, pulping chemicals, organic acids, chlorinated ligins, resin acids, phenolics, unsaturated fatty acids, terpenes, etc. Oxidation of such a multicomponent mixture is much more complicated than the oxidation of individual organic compounds. Although some metal oxides, such as CuO, MnO2, and ZnO, appear to be useful in oxidation of individual organic compounds, these metal oxides were only nominally effective as catalysts for the destructive oxidation of organic pollutants in the bleach plant effluents.1 Supported noble metal oxides are generally more active for the destructive oxidation of organic compounds in wastewater. However, the effectiveness of these supported noble metal oxides, particularly those containing ceria, was not known for the destructive oxidation of mixtures of organic compounds in industrial wastewater from pulp mills. To the best of our knowledge, no experimental data for the catalytic destructive oxidation of the industrial waste streams such as bleach plant effluent are known.
S0888-5885(98)00111-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/26/1998
3344 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
Figure 1. Schematic diagram of the experimental apparatus.
The key to the development of a catalytic oxidation process for the treatment of these industrial wastewater streams is catalyst design (or formulation). The objectives of this study are to evaluate (1) the effectiveness of alumina-supported noble metal catalysts for the treatment of the industrial wastewater from a softwood kraft pulp mill under milder reaction conditions; (2) the usefulness of cerium oxide as a promoter for the catalytic oxidation of the bleach plant effluent; (3) the influence of the initial valent state of metal, i.e., in reduced or oxidized form, and the metal loading of some promising catalysts; and (4) the mechanism of wet catalytic oxidation of organic compounds. Experimental Section Catalyst Preparation. Ultrafine γ-alumina (ALON, Cabot Corp.; BET surface area of 102 m2/g), was used as support. Tetraammine platinum(II) nitrate solution (Pt 5.5 wt %) and palladium chloride (Colonial Metals Inc.), cerium acetate (>99.9%, Rocky Mountain Research Inc.), manganese acetate (Fisher), and ferric nitrate (Baker) were used as received. Supported single-metal catalysts were prepared by incipient wetness impregnation. The mixture of the support and the metal-containing solution was dried for 12 h in an atmospheric rotary evaporator under infrared light. The dried mixture was then calcined in a tube reactor under an air flow at the desired temperature, typically 773 K, for 6 h. For comparison, some catalysts were reduced under hydrogen flow instead of air flow at 773 K for 6 h. The supported multicomponent catalysts were prepared by successive incipient wetness impregnation of the support with cerium and precious metals (by using either nitrates or chloride salts) followed by drying and calcination. Ce was added first on alumina support by the incipient wetness impregnation procedure. After drying and calcination, Ce/alumina was then impregnated with the active metals such as Pd or Pt. The final sample then underwent the same drying and calcination procedure. The metal loading of the catalyst was calculated from the weight of chemicals used for impregnation. For simplicity, each catalyst will be designated according to the active metal and its loading; for example, Pt1Pd1Ce4/Al2O3 denotes the alumina-supported catalyst containing 1 wt % Pt, 1 wt % Pd, and 4 wt % Ce. Experimental Procedure. The experimental setup is shown in Figure 1. The reaction vessel is a high-
pressure Parr reactor (model 4653, Parr Instrument Inc.). It is made of SS-316 stainless steel. A Pyrex glass liner is used. The reactor, equipped with a turbine-type glass impeller, has an effective volume of 300 mL. A thermal sensor and an external heating element are also provided in the reactor for temperature control to an accuracy of (1 K. The liquid sampling line consists of 1/ -in. o.d. Teflon tubing fitted with a filter and a 8 stainless steel sampling valve. The operating pressure of the oxidation reaction is controlled by a back-pressure controller in the exit line. In the experiments, typically 1 g of catalyst powder (
