Alumina Foam Coated with Nanostructured Chromia Aerogel: Efficient

R-Al2O3 ceramic foam was coated with chromia aerogel consisting of 1-2 nm CrOOH nanocrystals with surface area of 670 m2/g. The layer thickness change...
0 downloads 0 Views 570KB Size
Download PDF
7462

Ind. Eng. Chem. Res. 2006, 45, 7462-7469

Alumina Foam Coated with Nanostructured Chromia Aerogel: Efficient Catalytic Material for Complete Combustion of Chlorinated VOC M. V. Landau,*,† G. E. Shter,‡ L. Titelman,† V. Gelman,‡ H. Rotter,† G. S. Grader,‡ and M. Herskowitz† Blechner Center for Industrial Catalysis and Process DeVelopment, Department of Chemical Engineering, Ben-Gurion UniVersity of the NegeV, Beer-SheVa, 84105, Israel, and Chemical Engineering Department, TechnionsIsrael Institute of Technology, Haifa, 32000, Israel

Commercial volatile organic compound (VOC) combustion catalysts, including V-W-Ti-O, Mn-Al-O, and Mn-W-Al-O, that operate well below 600 K produce only dehydrochlorination of 2-chloropropane to HCl and propylene. Catalytic materials based on chromia aerogel, prepared in this study, yielded complete combustion with about 90% selectivity to CO2 at mild conditions. R-Al2O3 ceramic foam was coated with chromia aerogel consisting of 1-2 nm CrOOH nanocrystals with surface area of 670 m2/g. The layer thickness changed from 15 to 80 µm corresponding to CrOOH loading of 4.5-21.5 wt % with the total surface area of 40-105 m2/g. The chromia-loaded foams were packed in a tubular reactor and tested with a mixture of 1000 ppmv 2-chloropropane (2-CP) in air at 450-550 K and GHSV ) 60 000 h-1. Ceramic foam containing 7.7 wt % CrOOH yielded essentially complete conversion of 2-CP to mainly CO2/H2O/HCl at 550 K. Actually, those results were obtained despite apparent bypassing at high chromia loading indicated by the analysis of three catalysts tested in this study. Increasing chromia loading from 7.7 to 21.5 wt % little increased the apparent rate constant. This is further reflected by the relatively low apparent activation energy of 14 kcal/ mol calculated from the measured values. The measured pressure drop was low, as expected, in general agreement with the predictions of a literature correlation developed for various foams. The results of this study demonstrate the significant potential of ceramic foams. For high-loaded catalytic systems further work is needed to create the right configuration of the foam. Introduction Aerogels of metal oxides prepared via sol-gel processing followed by supercritical solvent removal display open structures, high porosities, and surface areas up to 800 m2‚g-1.1,2 These excellent texture characteristics could be combined with the high catalytic activity of transition metals as environmental catalysts in complete oxidation of hydrocarbons or SCR of NO with ammonia.3-5 The normal configuration of catalysts for those applications is a monolith tubular reactor. Another potential configuration is ceramic foam, which provides improved mass and heat transfer properties combined with a low pressure drop as well as negligible diffusion limitations.6,7 They are positive replicates of polymeric foams and exhibit high porosities (85-90%) with spherical-like cells connected to each other through openings or windows.8 Ceramic struts connecting the cells provide a surface for loading of catalytic components. High-surface-area washcoats added to the struts increase the BET surface area from 98%) to CO2, H2O, and HCl, with the latter being removed from the effluents by scrubbing.10 Bulk chromia aerogel prepared as mesoporous nanostructured oxide/ hydroxide CrOOH material with a surface area of >600 m2‚g-1 and packed as pressed pellets in a fixed bed displayed high activity, CO2 selectivity, and stability in the combustion of ethyl acetate, 1,2-dichloethane, and chlorobenzene.12,13 Application of such catalytic materials in commercial systems requires coating of monoliths or ceramic foams. Coating of nonporous or porous substrates (glass,14 silicon,15 carbon, alumina16 or borosilicate17 fibers, monolithic supports,18 or silica gel19) with a gel such as a metal oxide precursor followed by supercritical drying for production of aerogel films is a practical method. It improves the substrates’ hydrophilic properties and thermal insulation and increases their porosity for implementation as catalyst supports. In most published studies, substrates were coated with thin nanometer layers of pure silica or alumina aerogels having high integrity and adhesion characteristics. Such thin layers change little the surface area and porosity of substrates. Production of thicker micrometer layers of high porosity required mixing the preformed silica aerogel with a wet silica gel as a binder before substrate coating.19 No information was found on the coating of substrates with transition metal based catalytic aerogels that display relatively low integrities of primary particles and adhesion characteristics compared with silica or alumina. The scope of this study was to develop an improved method for coating R-alumina ceramic foam with a catalytic layer of chromia aerogel while controlling thickness at the micrometer scale. The rotational washcoating technique combined with instantaneous drying was elaborated in the present study. It demonstrated higher uniformity of the catalytic layer distribution and much better control of chromia loading compared with the

10.1021/ie0606744 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/15/2006

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7463

dipping method used in ref 6 for coating the R-Al2O3 ceramic foam with pure alumina. The catalytic material was thoroughly characterized by physical methods (XRD, Raman scattering spectroscopy, EDX, SEM, TEM, N2 adsorption). The catalytic performance of this coated foam was tested in the combustion of 2-chloropropane as CVOC. It is probably the first published example of washcoating the structured cellular solid with a catalytic aerogel that displays excellent catalytic performance. Pressure drop measured over a range of air flow rates was compared with predicted values. Experimental Section Catalyst Preparation. The R-Al2O3 ceramic foam was prepared by the polymeric-sponge method.20 A polymeric sponge was infiltrated with a slurry of R-alumina powder (BA Chemicals, particle size 2-5 µm). Typically, an initial polymeric sponge (BULPREN S 28133, RECTICEL, density 30 kg/m3) was cut into cylindrical shapes, compressed to remove air, and immersed in the slurry; then it was allowed to expand. After the excess slurry was drained, the infiltrated foam was dried and slowly heated in air to 1093 K at 1 K/min. During this stage gaseous products are evolved in a considerable amount, and therefore, slow heating is critical to keep the ceramic structure intact. The next step is a densification of the ceramic matrix by sintering at 1923-1973 K for 9 h in air with bulk shrinkage of about 20%. Finally, the R-Al2O3 ceramic foam was shaped into strong cylindrical monoliths (D ) 15 mm; L ) 20 mm) with 50 PPI pore density, 0.5 mm pore diameter, 0.1 m2/g surface area, 3.89 g/cm3 solid density, and 0.2 g/cm3 bulk density. The chromia aerogel was prepared in a powder form by solgel processing combined with supercritical drying of obtained gel, as described in previous studies.12,13 It was synthesized by mixing a 0.1 M aqueous solution of urea (Aldrich Chemicals) with 0.038 M aqueous chromium nitrate nonahydrate Cr(NO3)3‚ 9H2O (Riedel de Haen) in a volume ratio of 1.5:1 followed by agitating the mixture at 368 K for 16 h and aging at room temperature for 16 h. The water in the wet gel was replaced with methanol (Frutarom), and the gel was dried at supercritical conditions (568 K, 135 bar, 3 h) followed by slow pressure release. The discharged aerogel was dehydrated in a vacuum at 85 mbar and 593 K for 16 h. It was a powder with particle diameters of < 20 µm determined by sieving. The alumina binder was prepared as a colloidal solution from aqueous (60 g of Al2O3/L) aluminum nitrate Al(NO3)3‚9H2O (Riedel de Haen) by oscillating basification-acidification21 in the pH range 3.5-7.5, adding sequentially aqueous aluminum nitrate (Riedel de Haen) and aqueous ammonia (15%, Frutarom) solutions to a stirring reactor at 343 K. Finally, the pH was increased up to 9.0 and the aluminum hydroxide was separated by filtration, washed, and redispersed in deionized water. The chromia aerogel was added to this colloidal solution and stirred constantly to keep the slurry homogeneous. The mixture of chromia aerogel and alumina binder was deposited at the surface of R-Al2O3 ceramic foam by rotational washcoating with this slurry in a heating camera. The electrical driver provided rotation at 40 rpm of a cylindrical foam monolith fixed between two stainless steel disks at 75 mm from the driver axis with the horizontally oriented axis of the monolith. To improve the adhesion characteristics of R-Al2O3 ceramic foam surface, it was covered first with a thin layer of γ-Al2O3 (∼5 wt %) by dip coating with an aqueous aluminum nitrate solution followed by drying at 393 K and calcination in air at 773 K for 2 h. An aqueous slurry of CrOOH powder in AlOOH sol

solution stabilized by stirring at a weight ratio CrOOH:AlOOH of 1:1 was used for washcoating. It was added by pipetting to the rotating cylindrical foam monolith, keeping a uniform slurry distribution along the cylinder axis. The total volume of aqueous slurry used for washcoating one piece of cylindrical foam was 10 mL, and the amount of (AlOOH + CrOOH) in this volume was adjusted according to the required loading of CrOOH in the final catalyst. The oven temperature was adjusted so that the temperature in the chamber with the rotating foam monolith was maintained at 423 K. After addition of the required amount of aqueous slurry, the foam monolith was kept under rotation at the same temperature for 1 h and then calcined in air in the oven at 593 K for 6 h. In a blowing gas stream the chromiacoated ceramic foam monolith displayed high stability without detectable weight loss. Characterization of Parent and Coated Ceramic Foam. The chemical compositions of the catalysts and their components (in weight percent, average of five measurements at different points of the solid) were obtained by energy-dispersive X-ray spectroscopy (EDX) analysis with a JEOL JEM 5600 scanning electron microscope (SEM). The SEM images of not grinded small fragments of parent and coated ceramic foams were obtained with the same instrument after coating with gold to avoid charging effects. Surface areas, pore volumes, and pore size distributions of component powders and grinded parent and coated ceramic foams were obtained from N2 adsorptiondesorption isotherms measured at 77 K in the pressure range used in conventional BET and BJH methods. The samples were outgassed under vacuum at 523 K. Isotherms were obtained at the temperature of liquid nitrogen with a NOVA-2000 (Quantachrome, version 7.11) instrument. The texture characteristics of whole coated foam monolith samples were derived from N2 adsorption-desorption isotherms measured in the relative pressure range P/Po ) 1 × 10-4-1 with the instrument ASAP 2010 (Micromeritics). The micropore size distribution was derived from the adsorption isotherms using the HorvathKawazoe method, while the mesopore size distribution was derived from the desorption isotherms by the BJH method. Samples for TEM analysis were prepared by depositing a drop of an ultrasonicated aqueous suspension on a carbon-coated Cu grid. The grid was dried at 373 K and mounted in the specimen holder. Micrographs were recorded with a JEM 2010 microscope operated at 200 kV. The wide-angle X-ray diffraction (XRD) patterns of powdered catalyst components and grinded parent and coated ceramic foams were recorded on a Guinier G 670 camera (Cu KR1 radiation) connected to a rotating anode X-ray source that was operated at 40 kV and 100 mA. The data were collected in range 2θ ) 3-70° with step of 0.005°. Exposition time was 30 min. The peak positions and the instrument peak broadening were determined by fitting each diffraction peak by means of APD computer software. The crystal domain size was determined from the Scherrer equation: l ) Kλ/(B2 - β2)0.5 cos(2θ/2), where K ) 1.000 and λ ) 0.154 nm. B is the peak broadening at 2θ ) 35.0° and 63.5° for chromia; at 2θ ) 20.4° for R-Al2O3; and at 2θ ) 14.1°, 28.9°, and 38.6° for AlOOH phases, respectively. The Raman spectra were excited using an Ar laser (λ ) 488 nm) and recorded with a Jobin-Yvon LabRaman HR 800 microRaman system, equipped with a liquid nitrogen cooled detector. The pressure drop in a tubular reactor packed with the parent and coated cylindrical R-Al2O3 ceramic foams was measured with a Matheson Instrument flowmeter (tube E753) and Dwyer Mark II, Model 25, micromanometer. The sample, wrapped with

7464

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006

Results and Discussion

Figure 1. SEM micrographs of parent R-Al2O3 foam (a) and 7.7 wt % CrOOH-R-Al2O3 coated foam (b) obtained at magnifications of ×30, ×200, and ×2000.

a thin Teflon film to eliminate bypassing, was placed in a 20 mm inside diameter (i.d.) tube. Tests of Catalytic Performance. Two pieces of chromiacoated cylindrical R-Al2O3 ceramic monoliths were packed in series in a 16 mm i.d. reactor. The oxidation of 2 -chloropropane (2-CP) was studied at atmospheric pressure and 450-550 K. Air at 2950 mL/min and 2-CP at 3 mL/min, controlled by Brooks mass flowmeters, were mixed in a preheater kept at 400 K and fed to the reactor. It corresponded to a GHSV of 60 000 h-1 based on 4 cm3 ceramic foam. The effluent gas was analyzed by gas chromatography (GC). The main oxidation products were CO2, propylene, water, and HCl with traces of CO. HCl was absorbed in aqueous NaOH solution. The carbon balance was better than 98%. 2-CP conversion (X, %) and CO2 selectivity (SCO2, %) were calculated as X (%) ) (1 - CR/CR,0)(100) and SCO2 (%) ) [CCO2/(3(CR,0 - CR))](100), where CR is the concentration of 2-CP in effluent air, CR,0 is the concentration of 2-CP in the feed, and CCO2 is the CO2 concentration at the reactor outlet.

Catalyst Properties. Five samples containing 4.3, 7.7, 13.2, 16.5, and 21.5 wt % CrOOH were prepared by the rotating washcoating method. Figure 1 presents SEM micrographs of parent R-Al2O3 foam (a) and coated foam containing 7.7 wt % chromia (b) recorded at magnifications of 30, 200, and 2000. The uniform distribution of the active component (CrOOH) mixed with the binder over the internal surface of ceramic foam cavities clearly visible at low magnifications 30 and 200, with no plugging of macropores, indicates that the shape and curvature of the walls in the coated material were maintained. The higher magnification of 2000 shows the foam consisting of blocks of flat surface R-alumina. Coating with chromia substantially changes the surface morphology to a packed layer consisting of 0.5-5 µm particles bound to each other and to the monolith surface which represent agglomerates of the primary particles, crystals of CrOOH, and AlOOH. The SEM micrographs of partially cracked CrOOH-coated foam monolith containing 7.7 wt % CrOOH (Figure 2a, ×1000) and 21.5 wt % CrOOH (Figure 2b, ×250) show the thickness of the compact coating, being 20-30 and 50-70 µm, respectively. The average thickness of CrOOH coating increased from 15 to 70 µm as the loading increased from 4.3 to 21.5 wt % CrOOH. The characteristics of components of structured chromiacoated foam catalysts as pure materials are presented in Table 1. The TEM micrograph of the pure CrOOH used for coating of the ceramic foam is shown in Figure 3. It consists of 1-2 nm primary particles packed so as to create macropores as voids. A macropore of 10 nm is clearly observed on the micrograph. This is consistent with the crystal domain size of the R-CrOOH phase of 1.5-2 nm obtained from the XRD peak broadening. The XRD patterns of this phase shown in Figure 4 display three broad peaks reflecting d spacing of d1 ) 0.246; d2 ) 0.201, and d3 ) 0.148 nm. They correspond to the two-dimensional fragments (clusters) built on [Cr(OH)3O3] octahedra without bonding along the Z-axis in the R-CrOOH lattice that is characteristic of the structure of chromia aerogel.22,23 The wide bimodal pore size distribution in this phase centered at ∼3 and ∼15 nm (Figure 5) reflects the existence of large macropores in agreement with high-resolution transmission electron microscopy (HRTEM) data, while the high BET surface area of 670 m2/g (Table 1) indicates no blocking of the surface of primary nanocrystals of the chromia phase. Therefore, the

Figure 2. SEM micrographs of cracked CrOOH-coated R-Al2O3 foam monoliths containing 7.7 (a) and 21.5 wt % (b) CrOOH.

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7465 Table 1. Characteristics of Components of Structured CrOOH/Ceramic Foam Catalysts catalyst component

surface area, m2/g

pore volume, cm3/g

average pore diameter, nm

phase composition

average crystal size, nm

ceramic foam CrOOH AlOOH binder

0.1 670 254

3.4 0.56

20 5.6

R-Al2O3, corundum R-CrOOH, quasi-2D boehmite

>50 1.5-2 3-4

Figure 3. HRTEM micrograph of pure chromia aerogel after calcination at 593 K.

Figure 4. XRD patterns of pure chromia (1), alumina binder (2), ceramic foam (3), and structured catalyst (4) containing 21.5 wt % CrOOH coated on ceramic alumina foam.

surface of chromia nanocrystals is accessible to gases through small meso- and micropores of 5 nm disappeared after mixing chromia phase with alumina binder in a coated layer at the surface of the ceramic foam (Figures 5 and 7). The pore size distributions calculated separately for micro- and mesopores based on the N2 adsorption-desorption isotherms recorded with

7466

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006

Figure 5. Pore size distributions in pure ceramic foam, chromia, alumina binder, and structured catalyst containing 21.5 wt % CrOOH coated on ceramic alumina foam (measured as a powder after grinding).

Figure 6. Raman spectra of pure chromia aerogel (1) and structured catalyst (2) containing 21.5 wt % CrOOH coated on ceramic alumina foam.

the intact coated ceramic foam containing 16.5 wt % CrOOH are shown in Figure 7. It actually represents the porosity of the coated layer and contains only ∼3 nm mesopores and 0.5-1 nm micropores. Other catalysts with different loadings displayed, as expected, a similar pore size distribution. This is a result of coating the foam cavity walls with the same AlOOH-CrOOH composite being a source of micro- and mesoporosity. The mesopore size distribution in the coating layer resembles the pure chromia aerogel after blocking the large 4-80 nm pores. Blocking could be a result of intimate mixing of CrOOH and AlOOH nanoparticles in the aqueous slurry used for coating, so that small 3.4 nm particles of the alumina binder penetrate the large pores in the texture of the chromia aerogel. This decreased the total pore volume of the layer by a factor of 6 compared to that expected from the weighted contributions of the pure components. Increasing the chromia content in the range 4.3-21.5 wt % increased the total pore volume of coated ceramic monoliths from 0.025 to 0.13 cm3/g. Accessible surface area in the coated chromia-alumina layer measured for the intact ceramic foam after each preparation step starting from the parent foam, after coating with a thin γ-Al2O3 layer, and after coating with a layer of CrOOH-AlOOH at different chromia loadings are presented in Table 2. As expected, the surface areas of the binder and chromia are significantly higher than those of the other components. A comparison of

measured surface areas with the sum of surface areas of catalyst components (Table 2) indicates that the accessible surface area is about 60%. This means that a fraction of the surface was blocked as a result of aggregation of primary particles belonging to the active chromia phase and the binder and their packing in a layer at the foam surface. However, increasing the loading of the active chromia phase does not have an effect on the surface blocking. Therefore, the value of accessible surface area increased proportionally with increasing thickness of the coated catalytic layer. Pressure Drop. One of the important advantages of using ceramic foams is the low pressure drop generated at the high superficial velocities normally applied in processes that require a low residence time. Richardson et al.6 developed correlations for parameters a0 and a1 in the Forscheimer equation, based on a series of ceramic foams, without and with washcoats:

∆P/L ) a0V + a1V2

(I)

where ∆P/L is the pressure drop per unit length and V is the superficial velocity. The two parameters were expressed as a function of the geometric surface area per unit volume of solid, the porosity of the foam, and the density and viscosity of the fluid. Each parameter includes a constant calculated from the average pore diameter and the porosity. The geometric surface area per unit volume of solid was also correlated as a function of the porosity and average pore diameter. The correlations and parameters used in calculation of the pressure drop are listed in the Appendix. Experimental pressure drop measured with the parent and two coated ceramic foams is compared with the values calculated from eq I. The discrepancy between measured and calculated pressure drops, illustrated in Figure 8, shows that at relatively low chromia loading of 7.7 wt % the model predicts the pressure drop with high accuracy similar to that obtained for the parent foam monolith without any coating. At high chromia loading of 21.5 wt % corresponding to the total content of (chromia + alumina binder) of 43 wt %, the measured pressure drop significantly deviates from the model predictions, especially at high superficial gas velocities (Figure 8). A major problem could be the bypassing that may occur in such complex configurations coupled with the small pressure drop. At high loadings of the

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7467

Figure 8. Pressure drop data in ceramic foams and predictions based on correlations.6

Figure 9. SEM micrograph of cracked CrOOH-coated R-Al2O3 foam monolith with 21.5 wt % CrOOH content demonstrating a cavity filled with mesoporous CrOOH-AlOOH material.

along the ceramic struts, as was demonstrated in ref 6 for ceramic foam coated with pure alumina. Catalyst Performance. The conversion of 2-CP and selectivity to carbon dioxide as a function of the chromia loading and temperature are shown in Table 3. The data were measured at a constant space time of 0.69 g‚s/cm3. Assuming that the reaction is first order with respect to 2-CP, the reaction constants were calculated for each loading. The values of the apparent rate constants of 0.47, 0.59, and 0.65 cm3/(g‚s) were calculated at 450 K for 4.3, 7.7, and 21.5 wt % loading, respectively. A similar value was calculated for the combustion of dichloroethane12 on a bulk chromia aerogel catalyst at 593 K. This indicates that dichloroethane is a more refractory compound. The Thiele modulus calculated at 550 K is 50 h, displaying no measurable changes in activity and selectivity. This is a result of high resistance of chromia phase to deactivation with chlorine compounds in contrast to Mn-based catalysts30 that lose their combustion activity with time.

Conclusions The 670 m2/g chromia aerogel powder was rotary washcoated at the surface of R-Al2O3 ceramic foam cavities with a boehmite binder, yielding uniform distribution of the active component at chromia loading below 10 wt %. This approach can be used for preparation of structured solids from a wide range of catalytic nanomaterials that are generally prepared in the form of fine powders. Washcoatingdrying followed by calcination at 593 K did not affect the structure of chromia aerogel strongly bonded to the foam as a layer including agglomerated packed nanoparticles of the active component and the alumina binder. The surface area of the material accessible for gas adsorption increased proportionally to the thickness of the washcoated layer. The measured pressure drop was low, in general agreement with the correlation in the literature developed for various foams. The activity of the ceramic foam catalyst in the combustion of 2-chloropropane in air was high. At 550 K and GHSV ) 60 000 h-1 the catalyst yielded full combustion of 2-chloropropane to mainly CO2 and water. The apparent rate constant did not increase proportionally with loading of chromia aerogel. The catalyst utilization decreased at high (>10 wt %) chromia loading due to bypassing flow caused by nonuniform distribution of active material, thus requiring further improvement of the preparation.

Acknowledgment The authors gratefully acknowledge Dr. V. Ezersky for conducting the HRTEM experiments and Dr. A. Erenburg for XRD characterizations of catalysts.

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7469

Appendix The correlations and parameters used in the calculation of the pressure drop are expressed in the following equations:

)1-

SV )

Fb Fs

12.979(1 - 0.971(1 - )0.5) dp(1 - )0.5

(1)

(2)

R ) (9.73 × 102)dp0.743(1 - )-0.0982

(3)

β ) (3.68 × 10-4)dp-0.7523‚(1 - )0.07158

(4)

a0 )

RSV2µ(1 - )2

a1 )

3 βSVF(1 - ) 3

∆P ) (a0V + a1V2)L

(5)

(6) (7)

Nomenclature a0 ) first constant of Forscheimer equation, kg‚m-3‚s-1 a1 ) second constant of Forscheimer equation, kg‚m-4 dp ) mean pore diameter, m L ) bed length, m ∆P ) pressure drop, Pa SV ) surface area per unit volume, m-1 V ) superficial velocity, m/s Greek Symbols R ) parameter of Forscheimer equation β ) parameter of Forscheimer equation  ) solid porosity µ ) air viscosity, kg‚m-1‚s-1 F ) air density, kg‚m-3 Fb ) bulk density of the foam, kg‚m-3 Fs ) solid (He) density of the foam, kg‚m-3 Literature Cited (1) Fricke, J.; Tillotson, T. Aerogels: production, characterization and applications. Thin Solid Films 1997, 297, 212. (2) Hu¨sing, N.; Schubert, U. AerogelssAiry Materials: Chemistry, Structure and Properties. Angew. Chem., Int. Ed. 1998, 37, 22. (3) Pajong, G. M. Aerogels in Catalysis. Appl. Catal., A 1991, 72 (2), 217. (4) Pajong, G. M. Some catalytic applications of aerogels for environmental purposes. Catal. Today 1999, 52, 3. (5) Orlovic, A. M.; Janackovich, D. T.; Skala, D. U. Aerogels in Catalysis. In New deVelopments in Catalysis research; Bevy, L. P. Ed.; Nova Science Publishers: Hauppauge, NY, 2005; p 39. (6) Richardson, J. T.; Peng, Y.; Remue, D.; Properties of ceramic foam catalyst supports: pressure drop. Appl. Catal., A 2000, 204, 19.

(7) Richardson, J. T.; Garrait, M.; Hung, J.-K. Properties of ceramic foam catalyst supports: mass and heat transfer. Appl. Catal., A 2003, 255, 69. (8) Gibson, L.G.; Ashby, M. F. Cellular Solids. Structures and Properties; Pergamon Press: Oxford, 1988. (9) Twigg, M. V.; Richardson, J. T. Theory and applications of ceramic foam catalysts. Chem. Eng. Res. Des. 2002, 80 (A2), 183. (10) Spivey, J. J. Complete catalytic oxidation of volatile organics. Ind. Eng. Chem. Res. 1987, 26, 2165. (11) Everaert, K.; J.Baeyens, J. Catalytic combustion of volatile organic compounds. J. Hazard. Mater. 2004, B109, 113. (12) Rotter, H.; Landau, M. V.; Carrera, M.; Goldfarb D.; Herskowitz, M. High surface area chromia aerogel efficient catalyst and catalyst support for ethylacetate combustion. Appl. Catal., B 2004, 47, 111 (13) Rotter, H.; Landau, M. V.; Herskowitz, M. Combustion of Chlorinated VOC on Nanostructured Chromia Aerogel as Catalyst and Catalyst Support. EnViron. Sci. Technol. 2005, 39, 6845. (14) Yamamoto, H.; Suzuki, K. Water-repellent materials having aerogel intermediate layers on the surface and manufacture of them. Japan Patent 10045429, 1998. (15) Brinker, C. J.; Prakash, S. S. Method for preparing reliquefied sols for aerogel film formation at ambient pressure. U.S. Patent 5,948,283, 1999. (16) Ratke, L. Ultralight composites. German Offen. 10300979, 2004. (17) Pakowski, Z.; Maciszewska, K. Evaluation of the suitability of methods for hydrophobization of nonwoven filters coated with silica aerogel. Przem. Chem. 2003, 82, 1243. (18) Smith, D. M.; Johnston, G. P.; Ackerman, W. C.; Jeng, S. Rapid aging technique for aerogel thin films. U.S. Patent 5,753,305, 1998. (19) Liu, Y.; Zhang, L; Yao, X.; Xu, C.; Development of porous silica thick films by new base-catalyzed sol-gel route. Mater. Lett. 2001, 49, 102. (20) Schwartzwalder K.; Somers, A. Method of making porous ceramic articles. U.S. Patent 3,090,094, 1963. Brockmeyer, J. W. Process for preparing a ceramic foam. U.S. Patent 4,610,832, 1986. (21) Landau, M. V.; Herskowitz, M.; Givoni, D.; Laichter. S. Mediumseverity hydrotreating and hydrocracking of Israeli shale oil 1. Novel catalyst systems. Fuel 1996, 75, 858. (22) Abecassis-Wolfovich, M.; Rotter, H.; Landau, M.V.; Korin, E.; Erenburg, A. I.; Mogylyansky, D.; Gartsein, E. Texture and nanostructure of chromia aerogels prepared by urea-assisted homogeneous precipitation and low-temperature supercritical drying. J. Noncryst. Solids 2003, 318, 95. (23) Erenburg, A.; Gartstein, E.; Landau, M. Structural characterization of nanocrystalline CrOOH‚2H2O aerogel by X-ray diffraction. J. Phys. Chem. Solids 2005, 66, 81. (24) Moller, H. Automatic profile investigation by the Rietveld method for standardless quantitative phase analysis. ZKG Int. 1998, 51, 40. (25) Zuo, J.; Xu, C.; Wang, C.; Xie, Y.; Qian, Y. Raman Spectra of Nanophase Cr2O3. J. Raman Spectrosc. 1996, 97, 921. (26) Wells, A. F. In Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984; p 642. (27) Maslar, J. E.; Hurst, W. S.; Bowers, W. J.; Hendricks, J. H.; Aquino, M. I.; Levin, I. In situ Raman spectroscopic investigation of chromium surfaces under hydrothermal conditions. Appl. Surf. Sci. 2001, 180, 102. (28) Finocchio, E.; Baldi, M.; Busca, G.; Pistarino, C.; Romezzano, G.; Bergani, F.; Toledo, G. A study of the abatement of VOC over V2O5-WO3TiO2 and alternative SCR catalysts. Catal. Today 2000, 59, 261. (29) Pistarino, C.; Finocchio, E.; Romezzano, G.; Brichese, F.; Felice, R. D.; Busca G. A study of the Catalytic Dehydrochlorination of 2-Chloropropane in Oxidizing Conditions. Ind. Eng. Chem. Res. 2000, 39, 2752. (30) Busca, G.; Baldi, M.; Pistarino, C.; Amores, J. M. G.; Escribano, V. S.; Fincchio, E.; Romezzano, G.; Bregani, F.; Toledo, G. P. Evaluation of V2O5-WO3-TiO2 and alternative SCR catalysts in the abatement of VOCs. Catal. Today 1999, 53, 525. (31) Larrubia, M. A.; Busca, G. An FT-IR study of the conversion of 2-chloropropane, o-dichlorobenzene and dibenzofuran on V2O5-MoO3-TiO2 SCR-DeNOx catalysts. Appl. Catal., B 2002, 39, 343.

ReceiVed for reView May 27, 2006 ReVised manuscript receiVed July 19, 2006 Accepted August 8, 2006 IE0606744