Activated Carbon and Tungsten Oxide Supported on Activated Carbon Catalysts for Toluene Catalytic Combustion M. A. ALVAREZ-MERINO, † M. F. RIBEIRO, ‡ J. M. SILVA, § F. CARRASCO-MARI Ä N, | AND F. J. MALDONADO-HO Ä DAR* , | Departamento Quı ´m ica Inorga ´nica y Orga ´nica, Facultad de Ciencias Experim entales, Universidad de Jae ´n, 23071, Jae ´n, Spain, Departamento Engenharia Quı ´m ica, Instituto Superior Te ´cnico, Avenue Rovisco Pais, 1049-001, Lisboa, Portugal, Departamento Engenharia Quı ´m ica, Instituto Superior Engenharia de Lisboa, Rua Cons, Em ı ´dio Navarro, 1949-014 Lisboa, Portugal, and Departamento Quı ´m ica Inorga ´nica, Facultad de Ciencias, Universidad de Granada, 18071, Granada, Spain We have used activated carbon (AC) prepared from almond shells as a support for tungsten oxide to develop a series of WO x /AC catalysts for the catalytic combustion of toluene. We conducted the reaction between 300 and 350 °C, using a flow of 500 ppm of toluene in air and space velocity (GHSV) in the range 4000-7000 h -1 . Results show that AC used as a support is an appropriate material for removing toluene from dilute streams. By decreasing the GHSV and increasing the reaction temperature AC becomes a specific catalyst for the total toluene oxidation (S CO2 ) 100%), but in less favorable conditions CO appears as reaction product and toluene-derivative compounds are retained inside the pores. WO x /AC catalysts are more selective to CO 2 than AC due to the strong acidity of this oxide; this behavior improves with increased metal loading and reaction temperature and contact time. The catalytic performance depends on the non- stoichiometric tungsten oxide obtained during the pretreatment. In comparison with other supports the WO x / AC catalysts present, at low reaction temperatures, higher activity and selectivity than WO x supported on SiO 2 , TiO 2 , Al 2 O 3 , or Y zeolite. This is due to the hydrophobic character of the AC surface which prevents the adsorption of water produced from toluene combustion thus avoiding the deactivation of the active centers. However, the use of WO x /AC system is always restricted by its gasification temperature (around 400 °C), which limits the ability to increase the conversion values by increasing reaction temperatures. 1. Introduction The catalytic combustion of volatile organic compounds (VOCs) to control gaseous industrial emissions is one of the most promising environmental technologies. VOCs are typicallyoxidized usinga catalyst at temperatures below 600 °C,while thermalincineration processes need temperatures higher than 1000 °C. The catalytic process permits energy saving and minimizes other negative effects such as NOx formation,although the formation ofintermediate oxidation products, such as dioxins, should be controlled. Catalysts based on noble metals (Pt, Pd, Rh) supported on different materials present high activityand selectivityto the complete pollutant oxidation (1-4). However they are expensive, susceptible to poisoning (2, 5-8) and show, in some cases, a poor thermal stability. Therefore, there is a clear need to develop cheaper active and selective catalysts by improving either the support or the active phase. It has been pointed out previously that the complete oxidation of VOCs is achieved at lower temperatures using hydrophobic supports (activated carbon, polymers) (9, 10). The water vapor generated from VOCs oxidation, mainly at low temperatures,can deactivate the active sites ofinorganic supports, which show a hydrophilic behavior. Moreover, organic compounds are more readily adsorbed on hydro- phobic surfaces (3). Acidity is also expected to be a favorable factor for combustion catalysts since it is known that the acid character of supports or catalysts favors the oxidation reactions of organic compounds (1, 11, 12). Transition metaloxideshave been used forVOCsoxidation (13-16). The catalytic performance shown by these oxides is in general lower than that for noble metals, although they could tolerate higher levels of poisons. Many catalytic oxidation behaviors can be represented by a simple oxida- tion -reduction mechanism: Thus, the ability of oxide catalysts to develop reduction/ oxidation cycles is an important step in the combustion process and controls catalytic behavior (17, 18). Tungsten oxide is a solid with an acidic character which has many applications in heterogeneous catalysis and has an ability to form a large variety of stoichiometric and nonstoichiometricoxidesdependingon theatmosphereand thermal treatment used (19-24). Taking these factors into account, we expect the WO x/AC system to be a good combustion catalyst. In previous papers, we have described the preparation, characterization, and catalytic behavior in the skeletal isomerization of 1-butene (22), decomposition reactions ofalcohols (23), or ethylene hydrogenation (24)of different series of WOx/AC catalysts. The aim ofthis workis to investigate the catalytic behavior of tungsten oxide deposited on activated carbon for deep oxidation of toluene. The influence of tungsten loading and pretreatment conditions and the role of the support were analyzed. The role of the support was also evaluated by comparison of the WOx/AC system with catalysts based on tungsten impregnated in different materials,namelyalumina, silica, anatase, and Yzeolite. 2. Experimental Section A sample of activated carbon was prepared from almond shells in two steps: carbonization at 850 °C under N2 flow and activation on steam for 13 h at 750 °C, using in both cases a gas flow of 18 L h -1 . The activation degree was 40 wt %. Commercial Al2O3, TiO2, and SiO2 were used, as well as *Correspondingauthorphone: 34958240444;fax: 34958248526; e-mail: fjmaldon@ugr.es. † Universidad de Jae ´n. ‡ Instituto Superior Te ´cnico. § Instituto Superior Engenharia de Lisboa. | Universidad de Granada. VOC + oxidized catalyst f reduced catalyst + oxidized product (1) reduced catalyst + O 2 f oxidized catalyst (2) Environ. Sci. Technol. 2004, 38, 4664-4670