VOC Oxidation Catalysts

The selectivity of the catalyst is of major importance in the case of chlorinated VOCs the oxidation products should not contain even more harmful compounds than the parent-molecule, for example, formation of dioxins should be avoided. In addition, the minimization of CI2 and maximization of HCl in a product gas should be achieved [61]. These are just a few examples of why researchers are continuing the search for VOC oxidation catalysts as well as new reactor concepts. The new possibilities include, for example, utilization of nanosized gold catalysts in the oxidation of sulfur-containing VOCs and microwave-assisted processes where combination of adsorption and oxidation is used in low-concentration VOC oxidation [62, 63]. 

The design employs filters for particulate removal, a single desiccant wheel coated with both low temperature VOCs oxidation catalyst based on nanostructured catalyst and regenerable VOCs adsorbent made from modified mesoporous silica. The distribution of the active elements along the wheel thickness was optimized and shown in Fig. 12.8-6. The adsorbents are coated along two-thirds of the wheel thickness and all catalysts are concentrated on one... 

The two catalysts reporetd here, both commercial VOC oxidation catalysts, are a cerium-promoted Hopcalite (Catalyst A), and a chromia/alumina (Catalyst B). catalyst B was designed primarialy for fluid-bed operation, but has been studied here in both fixed-and fluid-bed application. Catalyst A has been studied only in fixed-bed studies Further data on these materials, as well on the performance of a Pt/Ni/alumina catalyst, are given in [2]. 

Effects of Deposits on the Catalv.sts- In describing techniques for regenerating commercial VOC oxidation catalysts of noble metals on ceramic honeycombs, Heck et al. pointed out that generally below 450 C the presence (in the gas stream) of phosphorous and other metals, particularly as oxides, leads to catalyst deactivation by simple masking as opposed to poisoning by chemical interaction. These deposits can often be removed by chemical treatment with acid or alkaline solutions and sometimes by physically cleaning with a compressed air lance. When combustible... 

As with the prior discussion on VOC oxidation catalysts, deposits on the catalyst with commercial use are expected. These deposits are removable with a cleaning treatment by analogy to the VOC oxidation catalysts. In addition, thermal sintering behavior occurs in this service, by analogy to VOC oxidation behavior. However, with chlorinated hydrocarbons, it has been observed that exposure of the catalyst to halogenocarbons causes an accelerated rate of growth of metal particle size, and hence deactivation. The suppon on which the metal is dispersed also affects this particle size growth rate. 

This review focuses on deactivation of VOC oxidation catalysts by Si and P compounds. Deactivation by both types of compounds are used as examples for the application of mathematical models describing the deactivation process. 

Raciulete, M. and Afanasiev, P. (2009) Manganese-containing VOC oxidation catalysts prepared in molten salts. 

Blanco, J., Petre, A., Yates, M., etal, (2007). Tailor-made High Porosity VOC Oxidation Catalysts Prepared by a Single-step Procedure, Appl, Catal, B Environ., 73, pp. 128-134. 

The key elements in the design of catalytic oxidation systems are space velocity and temperature. Space velocity, as used in the design of VOC oxidation catalyst beds, is usually defined as standard cubic feet of gas per cubic foot of catalyst per hour. It has the units of reciprocal hours. Space velocity is not simply the reciprocal of residence time because (1) the gas volume is measured at standard not actual conditions, and (2) the catalyst volume is the total volume, including both solid catalyst and spaces for gas flow,... 

In a typical operation, 0.3-0.4% of mixed aromatic and oxygenated hydro-caibons are almost completely removed at a space velocity of 40,000 hrs and an inlet temperature of 280°C. The increase in temperatuie is about lOO C per 0.1% hydrocaibon. When the concentration of hydrocaibons in the feed gas is greater than about 0.2%, it may be necessary to carry out the oxidation in tube-cooled converters, to provide adequate control over the temperature of the reaction. Some examples of operation with VOC oxidation catalysts are given in Table 11.16. 

F. Wyrwalski, 2007, Additioiral effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts, Appl. Catal. B Environ., 70, 393-399... 

Ca.ta.lysts, A catalyst has been defined as a substance that increases the rate at which a chemical reaction approaches equiHbrium without becoming permanently involved in the reaction (16). Thus a catalyst accelerates the kinetics of the reaction by lowering the reaction s activation energy (5), ie, by introducing a less difficult path for the reactants to foUow. Eor VOC oxidation, a catalyst decreases the temperature, or time required for oxidation, and hence also decreases the capital, maintenance, and operating costs of the system (see Catalysis). 

The chemical product used in the design project (chapter 12) is a household appliance designed to deliver clean air by removing and killing airborne microorganisms, and converting carbon monoxide and common VOCs found indoor into harmless carbon dioxide and water. It also dehumidifies indoor air and maintains a comfortable humidity level that suppresses fungal proliferation. The appliance is intended to maintain its performance without maintenance for at least two years and is expected to have a functional life of at least five years. The product contains an active formulation of (1) low temperature oxidation catalyst, (2) VOCs adsorbent and (c) desiccant. 

An efficient, low temperature oxidation catalyst was developed based on highly disperse metal catalyst on nanostructured Ti02 support. Addition of dopants inhibits metal sintering and prevents catalyst deactivation. The nanostructured catalyst was formulated to tolerate common poisons found in environments such as halogen- and sulfur-containing compounds. The nanocatalyst is capable of oxidizing carbon monoxide and common VOCs to carbon dioxide and water at near ambient temperatures (25-50 °C). 

The team agreed that the production of (1) low temperature oxidation catalyst, (2) VOCs adsorbent and (3) desiccant should be the core business of the new enterprise and the appliance manufacture should be conducted through strategic partnerships with Chinese OEMs. The enterprise will produce the catalyst, adsorbent and desiccant in suspension, paste and powder forms as needed for their incorporation in the appliance. Honeycomb filter made of the formulated powders will also be one of the main products. The enterprise will design the appliance and provide the blueprint along with components containing the active formulation to the OEM partners. The OEM manufacturers will be responsible for the manufacture of mechanical and electrical components and their assembly into an appliance. 

Both the MPT and DTS use CATOX units to destroy VOCs in the gaseous effluent streams. The CATOX units use a Pt/Pd oxidation catalyst. AEA uses a scrubber and filter upstream of the CATOX units to remove phosphorus, fluorine, and chlorine compounds that could poison the catalyst. 

The effect of water addihon on the complete oxidahon of benzene and propane VOCs by uranium oxide catalysts has been inveshgated [37]. Benzene oxidahon was studied using a silica-supported U3O8 catalyst Complete oxidahon was promoted by the addition of 2.6% water compared with the reachvity when no water was added to the reactant feed. Increasing the water concentrahon to 12.1% resulted in a suppression of oxidahon achvity. Inveshgahon of propane oxidahon using U3O8 showed a dramatic promohon of achvity. Propane conversion was ca 50% at 600 °C without added water, whilst it increased to 100% at 400 °C with the... 

One of the potentially wide-spread applications under development is catalytic filters for air pollution control. This combines separation and catalytic oxidation into one unit operation. One possibility is the oxidation of volatile organic carbon (VOC) by employing a porous honeycomb monolithic ceramic membrane filter. Inside the pores are deposited an oxidation catalyst such as precious metals. The resulting VOC removal efficiency can exceed 99% [Bishop et al., 1994]. 

Results for mixtures of TCE- (500 ppm) Heptane are shown in Figure 14 and for mixtures TCE-(300 ppm) Toluene in Figure 15. It is seen that i) Some (up to 1 vol%) steam in the flue gas enhances the catalytic activity (for Cl-VOC oxidation), ii) hqrtane enhances the catalytic activity while the aromatic hydrocarbon seems to decrease it. Of course, if these V-W-Ti catalysts be selected for a further research, a lot of other tests should be made with mixtures of VOCs and Cl-VOCs. 

Activity for Cl-VOC oxidation at lab scale of eleven catalysts in monolithic shape from eight different manufacturers has been here reported. When the ko (for Eapp=44 kJ/mol) values are compared with the similar ones for the Pt, Pd, and chromia based catalysts, indicated in Tables 4 and 5 of ref 7, it is concluded that ground (particles) V-W-Ti catalysts are, by average, more active than the competitive (used for the same application) Pt, Pd and Cr203 catalysts. Nevertheless, as monoliths they are not so active, and the activity of commercial V-W-Ti monoliths is similar to the commercial Pt or Pd spheres. Besides, these results about catalytic activity have to be used joint with the deactivation or life time results (for the same catalysts) shown in forthcoming part II of this paper. Their deactivation above 200-250° C for relative high contents of Cl-VOCs (besides their price) will be the key or limiting aspect in their use. 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

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