Combustion of Volatile Organic Compounds at Trace Concentration

Apr 8, 2010 - Department of Chemical Engineering and Nanoscience Institute of ... of the zeolite-coated microreactors were tested in the combustion of...
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Ind. Eng. Chem. Res. 2010, 49, 6941–6947

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Combustion of Volatile Organic Compounds at Trace Concentration Levels in Zeolite-Coated Microreactors Nuria Navascue´s, Miguel Escuin, Yolanda Rodas, Silvia Irusta, Reyes Mallada, and Jesu´s Santamarı´a* Department of Chemical Engineering and Nanoscience Institute of Aragon, UniVersity of Zaragoza, 50009 Zaragoza, Spain

A study is presented of the preparation of zeolite films (Pt/ZSM5 and Pt/Zeolite Y) on microreactor channels with a diameter of 500 µm. The catalysts were prepared and fully characterized as powders and then incorporated into the microreactors by a seeded hydrothermal synthesis procedure. The performance of the zeolite-coated microreactors were tested in the combustion of n-hexane, alone, and in mixtures with acetone at concentrations of 100 and 200 ppmV. The microreactors outperformed the fixed-bed reactor, giving lightoff temperatures in the 150-190 °C range. Introduction The combustion of pollutants at low concentrations in air presents a particularly challenging problem to reactor engineers. In this case, a technical problem (reacting a highly diluted stream with a few tens to a few hundred parts per million of organic matter) is compounded with an economic problem: the cost of heating a gas stream that consists largely of nonreacting components. Because of this, research efforts have been directed either toward improving the catalyst performance with the objective of attaining lower combustion temperatures or implementing a better heat management scheme in the reactor. An example of the latter approach is the use of reverse-flow reactors for the combustion of lean hydrocarbon mixtures with air.1 Regarding the improvement of the reactor performance, a central problem when dealing with highly diluted streams is to achieve an efficient contact between reactants and the catalyst. The excellent mass-transfer characteristics of microreactors2 make them ideal candidates for scenarios in which a highly efficient gas-solid contact is required. However, most volatile organic compound (VOC) combustion applications involve the processing of large gas volumes, which poses obvious problems for microreactors. An exception, however, would be the case of VOC combustion to remove pollutants from indoor air in domestic applications, where low feed rates (and therefore microreactors) can be contemplated. Indoor air quality problems stem from pollution sources inside buildings and are exacerbated by inadequate ventilation. This can lead to situations in which the concentration of certain pollutants inside buildings may be several times higher than the outdoor levels.3 The term “sick building syndrome” (SBS) is used to describe situations in which building occupants experience acute health and comfort effects that appear to be linked to time spent in a building but in which no specific illness or cause can be identified.4 A recent review5 presents VOC concentration measurements in homes, offices, and schools, with the total VOC concentrations being higher in the case of offices (up to 4600 µg/m3). Jia et al.6 reports the characterization of a wide range of VOCs across communities representing a gradient of the population density and industrial activity. Over a quarter of the residences had benzene, naphthalene, chloroform, and carbon tetrachloride at levels giving a chronic lifetime cancer risk of above 10-5 * To whom correspondence should be addressed. Phone: +34 986761153. E-mail: [email protected].

and sometimes much higher; maximum levels of total VOCs of 1200 µg/m3 could be reached. Indoor air pollution sources include adhesives, carpeting, upholstery, manufactured wood products, copy machines, pesticides, cleaning agents, and tobacco smoke. This investigation deals with the study of VOC abatement using microreactors coated with platinum/zeolite films. Noble metal catalysts such as platinum (Pt) and palladium (Pd) supported on zeolites have already demonstrated efficiency for the catalytic combustion of a variety of VOCs at relatively low temperatures in conventional reactors7-10 usually with moderate VOC concentrations in the feed (1000-3000 ppm). There are few examples of investigations dealing with lower VOC concentrations and even fewer where the problem has been addressed using microstructured catalysts or microreactors. Sanz et al.11 used structured reactors containing Pt-impregnated aluminum foam for the combustion of toluene at low concentrations (266 ppm). Carvalho et al.12 proposed an interesting threedimensional microchannel reactor to remove VOCs from air, although they only presented results in the batch mode, with extremely low loadings (in the microgram range). Other works13,14 have explored the use of microreactors in catalytic combustion for energy production applications, using relatively high hydrocarbon concentrations in the reactor feed (9% and higher). Previous works in our laboratory15 have also used microreactors to remove CO (at concentrations of around 1%) from reformer streams. In this work, we have carried out a study of Pt/ZSM5 and Pt/ZY catalysts in the combustion of traces of n-hexane (100-200 ppm; alone or in mixtures with acetone) as model compounds representative of VOCs. This range of concentrations is of interest for the compounds considered (for instance, the ACGIH Threshold Limit Values for hexane and acetone are respectively 50 and 500 ppmV). The catalysts were first compared in the powder form and then as catalytic films in microreactor channels. Special attention has been paid to the catalyst preparation and distribution of Pt in the best-performing catalyst (Pt/ZY). Experimental Section Catalyst Preparation. The zeolite supports selected for this study were ZSM5 (nominal Si/Al ) 100) and ZY (Faujasite), with a nominal Si/Al ratio of 4. The reactants for the hydro-

10.1021/ie901843t  2010 American Chemical Society Published on Web 04/08/2010

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Table 1. Summary of the Catalyst Preparation Conditions zeolite synthesis synthesis gel composition a

catalyst Pt/ZSM5 Pt/ZSM5 Pt/ZY0.1 Pt/ZY0.3

(P) (MR) (P or MR) (P or MR)

ion exchange

Na2O

SiO2

Al2O3

TPAOH

H2O

tmixing (h)

T (°C)

T (h)

[Pt] (mM)

T (°C)

T (h)

15 15 17 17

200 200 12.8 12.8

1 1 1 1

9.5 9.5

9400 18800 975 975

12 12 4 4

175 150 90 90

8 8 24b 24b

2.5 2.5 0.1 0.3

Troom Troom 60 60

24 24 24 24

a Note: P and MR indicate zeolite in the powder form or as a film grown on microreactor channels, respectively. b In this case, two synthesis cycles were performed.

thermal zeolite synthesis were colloidal silica (LUDOX AS40), a tetrapropylammonium hydroxide solution (TPAOH, 1.0 M), sodium hydroxide (Sigma-Aldrich), and sodium aluminate (Carlo Erba). The reactants were mixed under stirring during the time specified in Table 1 before hydrothermal synthesis, which took place in a Teflon-lined autoclave. Molar compositions of the gel and synthesis conditions are summarized in Table 1. After hydrothermal synthesis, ZSM5 materials were calcined at 480 °C with a heating rate of 0.5 °C/min for 8 h to remove the structure-directing agent (TPAOH) from the zeolite pores. The calcined samples were submitted to a Pt ion-exchange procedure, as previously described.16 The zeolite powder or the microreactors coated with zeolite films were immersed in an aqueous solution of [Pt(NH3)4](NO3)2, under the conditions presented in Table 1. Afterward, the zeolite-coated microreactor plates (or the zeolite crystals) were calcined at 350 °C for 3 h under a flux of 200 N · mL/min of air with a heating rate of 0.2 °C/min. For the case of a Pt/ZY catalyst, Pt was ion-exchanged with dissolution with a concentration of 0.1 or 0.3 mM of the Pt precursor. These samples are denoted as Pt/ZY0.1 and Pt/ZY0.3, respectively. Microreactors. The microreactors consisted of two plates with 50 mm length, 10 mm width, and a thickness of 2 mm manufactured at Institut fu¨r Mikrotechnik Mainz. In each plate, 14 microchannels (length ) 41 mm and diameter ) 500 µm) are connected to wider inlet and outlet sections. The preparation of the zeolite layer by seeded liquid-phase hydrothermal synthesis on the microchannels was previously optimized in our laboratory for different zeolites: ZSM517 and mordenite, zeolite Y, and ETS-10.18 Briefly, this method involved the preparation of a suspension containing colloidal zeolite crystals (20 g/L), which was then coated as seeds on the microchannels using a 1 µL syringe. To form the zeolite films, the seeded plates were subjected to hydrothermal treatment under the conditions reported in Table 1. For the case of ZSM5 films, silicalite-1 seeds (with the same MFI structure) were prepared according to the method previously described by Valtchev et al.19 The ZY seeds suspension was obtained by wet grinding of a zeolite Y powder in an agate mortar. The size of the seeds in both cases was measured by photon correlation spectroscopy (Brookhaven 90 Plus), and the average values were 200 ( 13 nm for silicalite-1 and 400 ( 20 nm for zeolite Y. To avoid zeolite synthesis outside the channels, the gels used to prepare zeolite films in microreactors were considerably more diluted than those used for zeolite powder,18 as can be seen in Table 1. In this way, the probability of nucleation (homogeneously in the bulk of the solution or heterogeneously on the microreactor surfaces) is reduced, and growth occurs mainly on the existing zeolite seeds. In the case of zeolite Y, two hydrothermal treatments were performed.18 Catalytic Activity Measurements. The zeolite powders (50 mg), diluted with ground quartz (100 mg), were tested in a

quartz fixed-bed reactor (h ) 6 mm and d ) 6.8 mm). The total gas flow rate was 200 N · mL/min corresponding to a WHSV ) 240 000 mL/h · g, and the concentration of hexane was 200 ppm. The same value of WHSV was used for microreactors, taking as the zeolite weight the increase of the weight of the catalyst plate after synthesis and calcination (the zeolite weight is, in fact, likely to be less than the total weight increase because of partial oxidation of the supporting plate). Typically, the total weight increase was 25-30 mg for microreactors coated with Pt/ZY and around 100 mg in the case of Pt/ZSM5 coatings. Catalyst powders and microreactors were conditioned for 100 h at 300 °C under air flux, before catalytic activity tests. The catalysts were tested at predetermined temperatures, starting from 75 °C. Before measurement, the catalyst was stabilized at each temperature for 150 min to ensure stable conversion. The gases were analyzed by online gas chromatography (Agilent 3000 Micro GC). The detection limits for acetone and hexane were 4 and 3 ppm, respectively. Mass balance closures were (2%. Temperature profiles were measured with a movable thermocouple in the fixed-bed reactor. Maximum temperature deviations with respect to the set point were less than 5 °C and usually less than 2 °C. Characterization of the Catalysts. A battery of techniques were used to characterize the catalysts and microreactors. The amount of Pt incorporated onto the zeolite catalysts was determined by absorption emission spectroscopy (SpectraAA 110 Varian, with a Pt lamp working at λ ) 265.9 nm) after digestion with HF in a microwave oven (Milestone Ethos Plus). The surface composition of the catalysts was analyzed by X-ray photoelectron spectroscopy (XPS) with an Axis Ultra DLD (Kratos Analytical). In this case, the samples were mounted on a sample rod placed in the pretreatment chamber of the spectrometer and evacuated at room temperature. The spectra were excited by the monochromatized Al KR source (1486.6 eV) run at 15 kV and 10 mA. Analyses of the peaks were performed with the manufacturer software, using a weighted sum of Lorentzian and Gaussian component curves after Shirley background subtraction. The binding energies were referenced to the internal standard C 1s (284.9 eV). The specific surface area, Brunauer-Emmett-Teller (BET) method, and pore volume were measured by nitrogen adsorption at 77 K in a Micromeritics ASAP 2020; samples were outgassed at 26.7 Pa and 623 K for 6 h. The BET equation was limited to the pressure range where the term Q(P0 - P) continuously increases with P/P0.20 Temperature-programmed reduction (TPR) experiments were performed with a heating rate of 5 K/min under a 5:95 (v/v) H2/Ar stream. The nature, texture, and distribution of the zeolite films and chemical analysis were also characterized by field-emission scanning electron microscopy (SEM) using a FEI Inspect F instrument, equipped with an energy-dispersive X-ray detector. The particle size distribution of Pt in the zeolite Y catalyts was measured by transmission electron microscopy (TEM; FEI Tecnai, operating at 200 kV). The crystallinity,

Ind. Eng. Chem. Res., Vol. 49, No. 15, 2010 Table 2. Catalyst Characterization Results

catalyst Pt/ZSM5 (P) Pt/ZY0.1 (P)

N2 theoretical atomic adsorption (synthesis gel) absorption BET surface Al/Si ratio Pt (wt %) area (m2/g) 0.01 0.156

0.8 0.62

419 859

XPS atomic ratios Pt/Si

Al/Si Na/Si

0.0117 0.0027 0.013 0.22 0.19

purity, and structure of the catalytic films were studied by X-ray diffraction (Rygaku/Max System RU 300) before and after ion exchange. Results and Discussion Pt/ZSM5 and Pt/ZY Catalysts. Characterization. Table 1 summarizes the catalyst preparation conditions used for the zeolite preparation and subsequent ion exchange, and Table 2 gives the main characteristics of the catalyts prepared, including the Pt content, BET surface area, and M/Si atomic ratio determined by XPS (M ) Pt, Al, Na). It must be noted that the investigation of Pt-zeolite supported systems by means of XPS has some spectroscopic obstacles because the most intense platinum line, Pt 4f, usually used for XPS analysis of Pt lies in the same region of XPS spectra with a strong Al 2p line of alumina from zeolite. As a consequence, obtaining reliable information about Pt states in Pt-Al systems faces considerable difficulties.21 As an alternative, in the present work, Al 2s and Pt 4d were used to study the surface composition; however, this procedure presents the problem of overlap of Al 2s with the plasmon loss of Si 2p, which is especially important for zeolites with a high Si/Al ratio. Table 2 shows that the Pt contents (measured both as a weight percentage and as a Pt/Si ratio) were similar for the powdered Pt/ZSM5 and Pt/ZY0.1 catalysts, in spite of having been ionexchanged with very different concentrations of Pt (a concentration of about 25 times higher was used to impregnate the ZSM5 support). This reflects the much larger number of exchange sites in the zeolite Y support, as indicated by the Al/Si ratios determined by XPS (0.22 in the powdered Pt/ZY catalyst, which is somewhat higher than the theoretical value of around 0.16; in the case of the Pt/ZSM5 catalyst, it could not be determined because of the low Al content coupled with the already discussed difficulties of XPS analysis). In the case of Pt/ZY0.1 solid, the values of the Pt/Si and Na/Si ratios indicate a degree of exchange that is around 12%. In the case of Pt/ZSM5, the high Pt/Si ratio coupled to a Na/Si ratio of 1 order of magnitude lower indicates a high degree of ion exchange. Figure 1 shows the TPR diagrams obtained for both powdered catalysts Pt/ZY0.1 and Pt/ZSM5. On both zeolites, the first

Figure 1. TPR diagrams of Pt/ZY0.1 and Pt/ZSM5 powdered samples.

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reduction peak is observed around 140 °C, in agreement with our previous work.16 The Pt/ZY catalyst presents a second reduction peak at a temperature of around 205 °C and a third one at around 347 °C, while the corresponding peaks for the Pt/ZSM5 catalyst appear at higher temperatures (405 and 555 °C, respectively). As was already noted,16 the reduction temperatures on Pt/zeolite catalysts must be interpreted with caution because they also depend on a series of factors such as the preparation technique, calcination temperature, and type of support. In the case of Pt/ZSM5, the different ion-exchange positions (R, β, and δ) approximately correspond to those observed by other authors after ion exchange of Co2+22. Similarly, for Pt/ZY, the three peaks could be associated with sites with different Pt-support interaction and/or different accessibility.23 The dispersion of Pt obtained by H2 chemisorption gave values between 40 and 85% for the samples used in this work (assuming a 2:1 H/Pt stoichiometry). However, H2 chemisorption often gives unreliable results upon comparison of different supports, as shown by Ji et al.24 A better assessment of dispersion can be obtained by examination of the catalysts by TEM and measurement of their particle size distribution. The results are presented in Figure 2 for the three different powdered catalysts used in this work (Pt/ZSM5 and Pt/ZY, the latter with two different levels of Pt exchange, namely Pt/ZY0.1 and Pt/ZY0.3, respectively). The cluster size distribution is given as an inset in each figure. It can be seen that good dispersion was obtained in the three cases, with a relatively narrow poresize distribution. For Pt/ZY0.3 (the sample that gave the better results in VOC combustion, as will be shown below), the average size was 1.2 ( 0.04 nm. Catalytic Activity. The catalytic activities of Pt/ZSM5 and Pt/ZY0.1 catalysts for the combustion of n-hexane are compared in Figure 3. It can be seen that the light-off temperature is around 25 °C lower for the Pt/ZY0.1 catalyst, even though the total Pt load is somewhat higher for the Pt/ZSM5 catalyst. This can be explained by taking into account the higher specific surface of the Pt/ZY0.1 catalyst (859 vs 419 m2/g), its larger pore size (0.74 vs 0.55 nm), and probably also its better Pt dispersion as a consequence of the higher number of exchange sites (15 times higher Al/Si ratio). This would be in agreement with the TEM observations presented above, indicating that an excellent Pt dispersion is obtained even for the Pt/ZY0.3 catalyst, with a higher Pt load. Zeolite-Coated Microreactors. Characterization. The Pt/ ZSM5 and Pt/ZY coatings on the microreactor channels were prepared by seeded hydrothermal synthesis and then ionexchanged to incorporate Pt, under the conditions described in Table 1. The resulting coatings are shown as SEM pictures in Figure 4. It can be seen that a good coating was obtained, with complete coverage of the channel surface. A network of cracks running throughout the coating is evident for the case of Pt/ZY films (Figure 4b). Cross-sectional views (not shown) indicate that these cracks are several micrometers wide and run across most of the coating depth. They are thought to be formed as a result of thermomechanical stress during calcination. However, their existence seems to be beneficial in terms of reactor performance because they provide a rapid access route into the bulk of the zeolite film, which otherwise could present a significant diffusion resistance (except for the cracks, the appearance of the zeolite film is rather compact, with a thickness between 10 and 20 µm for the Pt/ZY coatings used in this work). This beneficial effect has been shown for the case of CO oxidation in simulated reformer streams.15

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Figure 2. TEM images of samples (a) Pt/ZSM5, (b) Pt/ZY0.1, and (c) Pt/ZY0.3 and statistical analysis of the particle size distribution (inset). A minimum of 100 particles were measured in each micrograph.

Microreactor Performance. In spite of the complexity of the preparation process (seeding the channel surface + hydrothermal synthesis + calcination + ion exchange), the performance of the coated microreactors is highly reproducible, as

shown in Figure 5 for the combustion of n-hexane in two different microreactors coated with Pt/ZY0.3 films. In both cases, the combustion curves are very similar, with light-off temperatures slightly over 160 °C. In addition, the microreactors

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Figure 3. Comparison of the performance of Pt/ZSM5 and Pt/ZY0.1 powdered catalysts in the combustion of n-hexane. Catalyst characteristics are given in Table 2. Conditions: 200 ppmV of n-hexane, atmospheric pressure, WHSV ) 240 000 mL/g · h.

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Figure 5. Reproducibility of the reactor performance. Combustion of n-hexane as a function of the temperature in two different microreactors coated with Pt/ZY0.3 films. Conditions: 200 ppmV of n-hexane, atmospheric pressure, WHSV ) 240 000 mL/g · h.

Figure 6. Combustion of mixtures of n-hexane and acetone in microreactors coated with Pt/ZY0.1 and Pt/ZY0.3 films. Conditions: 100 ppmV of n-hexane and acetone, atmospheric pressure, WHSV ) 240 000 mL/g · h. Table 3. Comparison of Light-Off (50%) and 90% Conversion Temperatures for the Combustion of n-Hexane Alone and in Mixtures with Acetone on Pt/ZY-Coated Microreactorsa reactor and catalyst

T50 (°C)

T90 (°C)

MR-2 Pt/ZY0.3 hexane (single gas) hexane (mixture) acetone (mixture)

162 171 152

173 185 166

MR Pt/ZY0.1 hexane (single gas) hexane acetone

182 191 164

212 215 194

a The concentrations used were 200 ppmV of n-hexane as a single compound and 100 ppmV each of acetone and n-hexane in mixtures.

Figure 4. SEM views of zeolite-coated microchannels. Top views of (a) Pt/ZSM5 and (b) Pt/ZY films.

showed a stable behavior under reaction conditions: an extended experimental run in which a Pt/ZY microreactor was used for the reaction of 200 ppm of hexane in air at 200 °C showed no apparent loss of activity after 280 h on stream.

The microreactors were also tested in the combustion of mixtures of n-hexane and acetone (100 ppmV). Figure 6 shows the results obtained for the Pt/ZY-coated microreactors with different Pt contents (to verify the sharpness of the light-off curve, the temperature step was 5 °C for MR ZY0.3), and the temperatures for 50 and 90% conversion can be seen in Table 3 and compared with the same temperatures when n-hexane was fed on its own at 200 ppmV. For both microreactors, it can be seen that the combustion of acetone in the mixtures takes preference, and, in fact, the combustion of n-hexane gathers momentum only after the conversion of acetone has reached relatively high values (ca. 70%). Table 3 shows that the lightoff (50% conversion) temperatures for n-hexane in the mixtures

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Figure 7. Comparison of the fixed-bed reactor and microreactor loaded with Pt/ZSM5. Conditions: 200 ppmV of n-hexane, atmospheric pressure, WHSV ) 240 000 mL/g · h.

Figure 8. Comparison of the fixed-bed reactor and microreactor loaded with Pt/ZY0.1 and Pt/ZY0.3. Conditions: 200 ppmV of n-hexane, atmospheric pressure, WHSV ) 240 000 mL/g · h.

are around 18 (Pt/ZY0.3) and 27 °C (Pt/ZY0.1) higher than those for acetone, respectively, and in both cases, the light-off temperatures are around 9 °C higher than those for n-hexane as a single VOC. This behavior can be explained as a result of the preferential adsorption of acetone in the combustion sites of the ZY support, which would be in agreement with its higher polarity compared to that of n-hexane. Furthermore, O’Malley and Hodnett25 studied the reactivity of a range of VOCs over Pt supported on β-zeolite and established the order alcohols > aromatics > ketones > carboxylic acids > alkanes, with the C-H bond cleavage being the slow step in the catalytic oxidation. Microreactors and Fixed-Bed Reactors. The pressure drop in fixed-bed reactors increases rapidly as the particle size decreases. Because of this, comparisons between the performances of fixed-bed reactors and microreactors are often done under conditions that are intrinsically favorable to the microreactor: a catalytic film of 10-20 µm in the microreactor channels is compared with a fixed-bed reactor loaded with particles whose sizes are measured in tens or even hundreds of micrometers. This is a valid practical comparison because, in most cases, operating considerations (loading, unloading, and pressure drop) would make it unrealistic to reduce the particle to a size comparable to the thickness of the catalytic film. In this case, however, we have attempted to carry out a comparison of the fixed-bed reactor and microreactor under comparable sizes of the particle diameters and the catalytic film thicknesses. To this end, in the fixed-bed reactor, we have used zeolite particles of both Pt/ZSM5 and Pt/ZY catalysts with a size of around 10 µm. Comparisons of microreactors and fixed-bed reactors loaded with Pt/ZSM5 and Pt/ZY catalysts under the same conditions (temperature and WHSV) are shown in Figures 7 and 8, and the values of the temperatures required for 50% and 90% conversion can be seen in Table 4. It can be observed that the temperatures in the microreactors for any of the catalyts (Pt/ZSM5, Pt/ZY0.1, and Pt/ ZY0.3) are consistently below their equivalent counterparts in the fixed-bed reactor. This was an unexpected result because the traditional advantages of the microreactor (enhanced mass transfer and shorter diffusion paths) cannot be easily invoked in this case. One possible explanation is that, because of their small size, the particles in the fixed-bed reactor would behave cohesively (like a type C Geldart solid), giving rise to particle agglomeration and channeling between the agglomerates. The uneven flow distribution and partial bypass would then decrease the performance of the fixed-bed reactor. Another possible explanation would be related to the different Pt distributions in the catalyst. The catalyst particles

Table 4. Comparison of Light-Off (50%) and 90% Conversion Temperatures for the Combustion of n-Hexane (200 ppmV) on the Different Catalyts and Reactors Used in This Worka

a

reactor and catalyst

T50 (°C)

T90 (°C)

FB Pt/ZSM5 MR Pt/ZSM5 FB Pt/ZY0.1 MR Pt/ZY0.1 FB Pt/ZY0.3 MR-1 Pt/ZY0.3 MR-2 Pt/ZY0.3

219 191 187 182 189 162 162

278 221 225 212 223 175 173

MR and FB denote microreactor and fixed-bed reactor, respectively.

Table 5. XPS Atomic Ratios Determined by XPS Analysis of Zeolite Y Particles and Zeolite Films on Microreactor Channels

sample MR-1 Pt/ZY0.3 MR-2 Pt/ZY0.3 Pt/ZY0.3 (P) MR Pt/ZY0.1 MR Pt/ZY0.1, etcheda Pt/ZY0.1 (P)

concn of Pt precursor used in ion exchange (mM) 0.3

0.1

Pt/Si

Al/Si

Na/Si

0.22 0.21 0.038 0.076 0.063 0.013

0.37 0.31 0.21 0.34 0.38 0.22

0.22 0.23 0.17 0.25 0.25 0.19

a Etching was performed after light surface cleaning by sputtering the surface with an Ar+ ion source (4 keV energy).

for the fixed-bed reactor are impregnated in solution, and their small size guarantees an even distribution of Pt. In the microreactor, however, the impregnation occurs from one side only, with the zeolite film already deposited on the microreactor channels. This might concentrate the Pt near the surface of the film, giving better access from the gas phase. To confirm this, we have used XPS analysis to measure the Pt atomic concentration at the catalyst surface of zeolite films and particles after ion exchange with dissolution with the same concentration (0.1 or 0.3 mM) of the Pt precursor. The results show (Table 5) that the surface Pt concentration is significantly higher for the catalytic films in the microreactor than for the catalyst particles in the fixed-bed reactor. Measurements carried out on the MR Pt/ZY0.1 sample after a light etching process already reveal a significant decrease of the Pt content, suggesting that enrichment of the zeolite films takes place mainly on the outer regions of the film. Conclusions Zeolite-coated microreactors are highly efficient contactors for the combustion of VOCs (n-hexane and acetone), giving

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light-off temperatures in the150-190 °C region, depending on the catalyst support used and on the Pt loading. The best results in this work were obtained with a Pt/zeolite Y catalyst film, where acetone light-off temperatures were as low as 152 °C, at a WHSV value of 240 000 mL/g · h. The Pt dispersion on the Pt/ZY0.3 catalyst was excellent, with cluster sizes slightly above 1 nm, and the preparation procedure could be readily reproduced, with different microreactors giving virtually the same performance. The microreactors clearly outperformed the fixed-bed reactors for all of the catalysts and Pt loadings tested. The reasons for the better performance of the microreactors with respect to the fixed-bed reactors when the film thicknesses and particle diameters are of a similar size are still under investigation in our laboratory. XPS analysis data suggest that surface enrichment of Pt in the microreactor catalytic films may play a role in the observed improvement. Acknowledgment Financial support from MICINN, Spain, is gratefully acknowledged. The authors also acknowledge Dr. Alejandro Go´mez-Roca for his assistance in the study of the particle size distribution using TEM. Literature Cited (1) Marin, P.; Ordon˜ez, S.; Diez, F. V. Procedures for heat recovery in the catalytic combustion of lean methane-air mixtures in a reverse flow reactor. Chem. Eng. J. 2009, 147, 356. (2) Kolb, G.; Hessel, V. Microstructured reactors for gas phase reactions. Chem. Eng. J. 2004, 98, 1. (3) http://europa.eu/rapid/pressReleasesAction.do?reference)IP/03/ 1278. (4) http://www.epa.gov/iaq/pubs/sbs.html. (5) Barroa, R.; Regueirob, J.; Llompartb, M.; Garcia-Jaresb, C. Analysis of industrial contaminants in indoor air: Part 1. Volatile organic compounds, carbonyl compounds, polycyclic aromatic hydrocarbons and polychlorinated biphenyls. J. Chromatogr., A. 2009, 1216, 540. (6) Jia, C.; Batterman, S.; Godwin, C. VOCs in industrial, urban and suburban neighborhoods, Part 1. Indoor and outdoor concentrations, variation, and risk drivers. Atmos. EnViron. 2008, 42, 2083. (7) Tsou, J.; Pinard, L.; Magnoux, P.; Figueiredo, J. L.; Guisnet, M. Catalytic oxidation of volatile organic compounds (VOCs) oxidation of o-xylene over Pt/HBEA cataysts. Appl. Catal., B 2003, 46, 371. ´ rfao, J. J. M.; Figueiredo, J. L. (8) Tsou, J.; Magnoux, P.; Guisnet, M.; O Catalytic oxidation of volatile organic compounds (VOCs) oxidation of methyl-isobutyl-ketone over Pt/zeolite cataysts. Appl. Catal., B 2005, 57, 117. (9) Beauchet, R.; Magnoux, P.; Mijoin, J. Catalytic oxidation of volatile organic compounds (VOCs) mixture (isopropanol/o-xylene) on zeolite catalysts. Catal. Today 2007, 124, 118. (10) Lopez-Fonseca, R.; Gutie´rrez-Ortiz, J. I.; Gutie´rrez-Ortiz, M. A.; Gonza´lez-Velasco, J. R. Catalytic oxidation of aliphatic chlorinated volatile

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ReceiVed for reView November 20, 2009 ReVised manuscript receiVed March 19, 2010 Accepted March 22, 2010 IE901843T