Toluene Decomposition in the Presence of Hydrogen on Tungsten

Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, South Carolina 29634-0909, and RTI International, Research Triangle ...
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Ind. Eng. Chem. Res. 2008, 47, 4077–4085

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Toluene Decomposition in the Presence of Hydrogen on Tungsten-Based Catalysts Sourabh S. Pansare,† James G. Goodwin, Jr.,*,† and Santosh Gangwal‡ Department of Chemical and Biomolecular Engineering, Clemson UniVersity, Clemson, South Carolina 29634-0909, and RTI International, Research Triangle Park, North Carolina 27709

Removal of NH3, tars, and H2S from biomass gasification gas represents a significant step in the commercial use of biomass gasification as a source of hydrogen, syngas, and electricity. This paper reports the results of an investigation into the use of W-based catalysts [tungsten carbide (WC), tungstated zirconia (WZ), and platinum supported on tungstated zirconia (PtWZ)] for catalytic tar removal. In this study, toluene was used as a model compound for tars. In the case of WZ, the effect of calcination temperature on the activity of toluene decomposition was also investigated. The toluene decomposition reaction was conducted at 1 atm, in the temperature range of 300-800 °C, and in the presence of 10% H2. CH4 and benzene were the only detectable products of toluene decomposition on all the catalysts. Incorporation of 5 wt % Pt with WZ was found to give the most effective catalyst considering initial rates of product formation. WC, WZ, and PtWZ each showed an initial partial deactivation for both CH4 and benzene formation due to coke deposition on active sites. Pt incorporation had a significant effect on the steady-state activity of WZ for toluene decomposition at temperatures below 600 °C; however, at temperatures above 700 °C, the effect was nullified. All W-based catalysts showed comparable performance to that of a commercial cracking catalyst (ultra-stable Y zeolite) above 700 °C. 1. Introduction Biomass, with 220 billion tons of annual worldwide production, represents a “green” and renewable source of hydrogen, synthesis gas, and electricity.1 When biomass is gasified in the presence of gasifying media (steam, oxygen, or air) in a gasifier, it yields a mixture of H2 and CO (syngas) along with CO2, CH4, and H2O. The gasification also produces impurities such as NH3 (1000-10000 ppm), tars (1000-10000 ppm), and H2S (20-100 ppm) that pose a significant problem in commercial application of biomass gasification. Among these impurities present, tars pose the greatest problem for various downstream operations. Tars can loosely be defined as condensable organic compounds. Although there is no universal definition for tars, an operational definition generally agreed upon is “organic contaminants with a molecular weight greater than that of benzene”.2 Tars act as a coke precursor for various downstream catalytic processes. They can also block downstream lines and foul engines and turbines when the gasification gas stream is used for power generation. There are two fundamental approaches that have been employed for minimization of tars.1 The first approach (primary treatment) aims to minimize the amount of tar formation in the gasifier itself by optimizing the properties of biomass feedstock and the gasifier operating conditions. The second approach (secondary treatment) consists of removal of tars in downstream operations based on physical methods (such as scrubbers) or catalytic strategies. Extensive work related to both approaches has led to the conclusion that necessary reduction in the quantity of tars cannot be achieved by means of only primary treatment, and secondary treatments are essential. The most economical way agreed upon for removing tars is to catalytically crack them to lower hydrocarbons, preferably to CH4. This catalytic tar removal is an important component in any catalytic hot gas cleanup. For tar decomposition, CH4 is not a desirable product * To whom all correspondence should be sent. E-mail: jgoodwi@ clemson.edu. Phone: (864) 656-6614. Fax: (864) 656-0784. † Clemson University. ‡ RTI International.

in itself. However, the quantities of products from tar decomposition are only on the order of 5000 ppm, and what is desired are products that will minimize downstream fouling and/or coking. The conventional catalysts that have been utilized for tar removal include acid catalysts (zeolites, silica-alumina), basic catalysts (CaO, MgO, dolomites), and supported metal catalysts (Fe and Ni-based).1 Although these catalysts are economical, they suffer from several disadvantages such as poor attrition resistance (for dolomites), sulfur poisoning (for Fe and Ni-based catalysts), and the need for severe reaction temperatures (>800 °C) for cleanup in the presence of gasification gas components. There have been ongoing efforts to develop better economical catalysts with improved sulfur tolerance and higher attrition resistance. Since the original work of Levy and Boudart,3 there has been much interest in tungsten carbide (WC) due to the exceptional properties that it offers. For WC, the carbon modifies the surface electronic properties of W due to transfer of electrons resulting in Pt-like behavior.3 Boudart and co-workers have shown that incorporation of O into WC in small amounts above 100 °C introduces WOx acid sites essential for cracking reactions.4–6 One of the most important properties offered by WC is its resistance to sulfur.7 WC also offers other excellent properties such as extreme hardness and thermal stability. Recently, we have shown that WC is highly active for NH3 decomposition.8 Thus, there is a possibility that WC could act as a multifunctional catalyst for the cleanup of biomass gasification gas. Another interesting W-based catalyst is tungstated zirconia (WZ).9–11 WZ is an acid catalyst with acid sites present in the form of WOx clusters on the surface that can catalyze cracking reactions.10 There is also a possibility of in situ formation of WC in the presence of H2 and CO, resulting in a bifunctional catalyst. WZ has optimal redox and acidic characteristics compared to WOx/Al2O3 and WOx/TiO2.12 Recent studies have shown that WZ is active for the decomposition of NH313 and, hence, it is also a strong candidate for simultaneous removal of NH3 and tars present in biomass gasification gas.

10.1021/ie8002864 CCC: $40.75  2008 American Chemical Society Published on Web 05/22/2008

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Several model compounds have been used to represent tars in studies related to biomass gas cleanup. The most common compounds include heptane,14 benzene,15,16 and toluene.17 In the present study, toluene was selected as a model compound because of its similarity to tars and simplicity of use. Preliminary experiments conducted in the presence H2, CO, CO2, H2O, and NH3 (typical biomass gasification gas components) showed that both WC and WZ can catalyze toluene decomposition with considerable activity. Experiments conducted with only toluene and He present in the inlet stream showed negligible activity for toluene decomposition on both catalysts. H2 was found to be the key component in biomass gas in producing activity. Hence, in the present study, toluene decomposition in the presence of hydrogen was investigated in detail on WC and WZ at biomass gas cleanup conditions. In the case of WZ, effects of calcination temperature and Pt incorporation on the activity for toluene decomposition were also studied. 2. Experimental Section 2.1. Materials and Catalyst Preparation. WC was obtained from Alfa Aesar while WZ was donated by Magnesium Electron Inc. (MEI). H2 and He were UHP grade from National Specialty Gases. WC was characterized as received while it was pretreated before use in reaction studies in the presence of an 80/20 mixture of H2/CO at 650 °C for 1 h. Prior to characterization or use in reaction, WZ was calcined at various temperatures in the range 450-900 °C in static air (where WZ900 indicates WZ calcined at 900 °C). The temperature was ramped to the desired calcination temperature at 15 °C/min and held there for 3 h. Since the study of the effect of calcination temperature of WZ on toluene decomposition showed that WZ calcined at 900 °C gave the best steady-state activity, WZ900 was chosen as the support for Pt incorporation. Several catalysts with different Pt loadings were prepared by incipient wetness impregnation. Suitable amounts of H2PtCl6 · 6H2O (Alfa Aesar) were dissolved in distilled water to give desired Pt metal loadings of 3%, 5%, and 10 wt % in the catalysts. The solution was then added dropwise to the support until incipient wetness, and the resulting catalysts were oven-dried at 100 °C for 12 h followed by calcination in static air at 500 °C for 3 h. It has been reported in the literature that a calcination temperature for PtWZ above 500 °C could result in the migration of zirconia species over Pt, resulting in reduced H2-chemisorption.18 Hence, in the present study, 500 °C was chosen as the final calcination temperature for PtWZ. The catalysts with 3%, 5%, and 10 wt % Pt loadings are henceforth referred to as 3PtWZ, 5PtWZ, and 10PtWZ, respectively. 2.2. Catalyst Characterization. Brunauer-Emmett-Teller (BET) surface areas and pore-size distribution of the catalysts were determined by N2 adsorption at 77 K in a Micromeritics ASAP 2020 system. Before measurements, the catalysts were degassed (under vacuum at 10-3 mmHg) at 300 °C for 3 h. Static H2 chemisorption on PtWZ was carried out in a Micromeritics ASAP 2010 system at 35 °C. Before chemisorption, the catalyst was pretreated in the presence of H2 at 100 °C for 20 min and then ramped to 350 at 10 °C/min (in a flow of H2). The catalyst was then reduced for 2 h at this temperature in the presence of H2 followed by evacuation (at 10-6 mmHg) for 2 h. After pretreatment, the temperature was decreased from 350 to 35 °C, and then the chemisorption analysis was performed. BET, measuring only physisorption, required an evacuation at only 10-3 mm Hg.

X-ray diffraction analyses of WC, WZ, and PtWZ were performed using a Philips PW3050 X’Pert X-ray diffractometer. The diffractometer had monochromatized Cu KR radiation and a Ni filter. The acidities of WC, WZ, and PtWZ were determined using NH3-TPD as described elsewhere.19 Briefly, the catalyst was pretreated in the presence of He at 315 °C for 1 h followed by saturation with 10% NH3/He for 2 h at room temperature. The temperature was then raised to 60 °C, and the catalyst was flushed with He for 4 h to remove any physisorbed NH3. After this step, the temperature was raised to 600 at 10 °C/min and desorbed NH3 was measured using a thermal conductivity detector (TCD). WZ900, after reaction at several TOSs, was analyzed for coke deposition by Galbraith Laboratories (Knoxville, TN) using combustion/coulometric titration. 2.3. Isothermal Reaction Studies. The isothermal time-onstream (TOS) reaction studies of toluene decomposition were carried out in a plug-flow reactor at 1 atm and at different temperatures in the range of 300-800 °C. Approximately 50 mg of catalyst was placed at the center of the reactor sandwiched between quartz wool plugs. Before the reaction, WC was pretreated in the presence of an 80/20 mixture of H2/CO at 650 °C for 1 h. This specific pretreatment was chosen for WC in the present study because previous experiments in our laboratory have suggested that WC is most active following this particular pretreatment.8 The temperature was raised from room temperature (RT) to 650 at 5 °C/min. All other catalysts were pretreated in the presence of He by increasing the temperature from RT to the desired reaction temperature at a rate of 5 °C/min. The total flow rate was 100 sccm consisting of 10% H2 and 3000 ppm of toluene with the balance He. The desired concentration of toluene was obtained by flowing He through a saturator containing high-performance liquid chromatography (HPLC) grade toluene (Fisher Scientific) and heated at 45 °C. The concentration was monitored by a gas chromatograph. The effluent from the reactor was analyzed using a Varian 3800 GC equipped with three columns (Poraplot, CPSil5CB, and CPMolsieve 5A) and two detectors (one TCD and one flame ionization detector (FID)). All reaction measurements were free from external and internal mass transfer limitations as indicated by the Mears’ and Weisz-Prater criteria, respectively. 3. Results and Discussion 3.1. WC. 3.1.1. Characteristics. The physicochemical properties of WC are shown in Table 1. WC showed a very small surface area of 1.5 m2/g. X-ray diffraction (XRD) analysis revealed the hexagonal phase as the predominant phase, while any features corresponding to metallic W, WO3, or W2C were absent. The NH3-TPD (temperature-programmed desorption) experiments did not show any surface acidity for WC, suggesting that WOx sites responsible for such surface acidity were largely absent. 3.1.2. Reaction on WC. The TOS behavior of WC for toluene decomposition in the presence of 10% H2 at 700 and at 800 °C is shown in Figure 1. CH4 and benzene were the only products of the reaction detected by gas chromatography. The rates of CH4 formation are shown in Figure 1a, while the rates of benzene formation are shown in Figure 1b. From the figure, it is observed that the catalyst was clearly active in this temperature range. At 700 °C, the catalyst gave ca. 2% toluene conversion initially, while at 800 °C, the initial conversion was 24%. A small partial deactivation with TOS was observed at 800 °C for both CH4 and benzene formation. A carbon analysis of the catalyst with the TOS at 800 °C indicated that coke deposition may be the possible reason for the observed partial deactivation.

Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008 4079 Table 1. Physicochemical Properties of WC, WZ, and PtWZ Catalysts catalyst WC WZ WZ WZ WZ 5PtWZ a

calcination temperature (°C)

surface area (m2/g)a

W-surface density (W-atoms/nm2)

450 700 800 900 900/500

1.5 ( 0.3 173 86 68 57 31

2.3 5.2 6.6 7.7

XRD phases hexagonal WC t-ZrO2 t-ZrO2, WO3 t-ZrO2, m-ZrO2,WO3 Pt, t-ZrO2, m-ZrO2,WO3

surface acidity from NH3-TPD (µmol of NH3/g)b 0 100 54 44 18 45

Maximum error ) (4% unless otherwise indicated.b Maximum error ) (8%.

Figure 1. TOS behavior of WC for toluene decomposition in the presence of 10% H2 at 700 and 800 °C: (a) rate of CH4 formation and (b) rate of benzene formation (maximum error ) (5%).

The ratios of rates of CH4 and benzene formation calculated at 700 °C were ca. 1.0 ( 0.1 at all TOSs, indicating almost equimolar formation of CH4 and benzene. This suggests that, at 700 °C, the only possible reaction taking place, besides coke deposition, was toluene cracking (or toluene dealkylation), as shown in eq 1: C7H8 + H2 f C6H6 + CH4

(1)

This is not surprising since WC is known to catalyze the hydrogenolysis of 2-methylpentane and methylcyclopentane.20 On the other hand, at 800 °C, the ratios were ca. 1.5 ( 0.05 at all TOSs indicating larger amounts of CH4 formation compared to benzene. This suggests that reactions besides toluene dealkylation were contributing to CH4 formation. Given that no other products were detected, it is highly likely that the excess CH4 formed was via benzene cracking, as shown in eq 2: C6H6 + 9H2 f 6CH4

(2)

Thermodynamic calculations performed over a temperature range of 250-1000 °C indicated the feasibility of both reactions 1 and 2. According to Garin et al.,21 the reactant molecules undergo C-C bond breakage after adsorption on a WC surface. This is followed by either desorption of product molecules or further C-C bond cleavage. In the present study, the reaction data indicate that, at 700 °C, there was only first-stage C-C bond breaking (toluene dealkylation), leading to the formation of CH4 and benzene. Further C-C bond breaking would appear not to be taking place as the ratios of CH4 and benzene formation are ca. 1. However, at 800 °C, the data indicate second-stage C-C bond breaking, leading to an increase in CH4 formation. Only reaction 2 appears to take place since no other cracking products, besides CH4, were observedsperhaps due to the large excess (10%) of H2 relative to benzene (ppm). Thus, the results clearly indicate that WC has a potential to catalyze tar decomposition in biomass gas cleanup. A question can be raised about the possibility that some or all of the CH4, detected during the reaction, was formed by the reaction of H2 with WC. Thermodynamically, transformation of WC to metallic W, without any formation of W2C, is possible above 700 °C.22 Experiments performed indicated only a very small amount of CH4 ( 800 °C, formation of monoclinic ZrO2 was observed along with tetragonal ZrO2, as shown in Table 1. WO3 nanoparticles were also detected for calcination temperatures g 800 °C. The reason for this could be either that WO3 nanoparticles were completely absent below this calcination temperature or that their size was below the detectability limits of XRD. Barton et al.11 have reported that the detection of three-dimensional bulk WO3 species in WZ by XRD becomes possible only after exceeding

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Figure 2. Effect of calcination temperature of WZ on initial and steadystate rates of CH4 and benzene formation at 700 °C and 1 atm (10% H2 present for all reaction runs).

the WOx surface saturation coverage. This suggests that the surface saturation coverage of WOx species in the present study was reached around the calcination temperature of 800 °C. The evolution of the monoclinic ZrO2 phase with an increase in calcination temperature is well documented in the literature.11 Scheithauer et al.24 have suggested that WOx species on ZrO2 inhibit its transformation from tetragonal to monoclinic phase up to the surface saturation coverage, consistent with the results of the present study. The larger WOx clusters present above surface saturation coverage have been suggested to be possibly essential for creation of strong Bro¨nsted acid sites.24 All the above observations suggest that the surface saturation coverage of WOx species was reached at a calcination temperature of ca. 800 °C. The surface acidity concentration values obtained from NH3-TPD decreased monotonically with an increase in the calcination temperature. This was likely due, in part, to the decrease in BET surface area and, in part, to the formation of WO3 nanoparticles from WOx clusters. 3.2.2. Reaction on WZ. The calcination temperature of WZ has been reported to have an impact on rates of various reactions including NH3 decomposition13 and gas/liquid-phase esterification/transesterification.23 Hence, in the present study, the effect of calcination temperature of WZ on toluene decomposition was investigated. All the reaction runs were conducted in the presence of 10% H2 at 700 °C and at 1 atm. The initial and steady-state rates of formation of CH4 and benzene on WZ calcined at various temperatures are shown in Figure 2. From the figure, it is observed that the initial rates of CH4 and benzene formation decreased with an increase in the calcination temperature. On the other hand, the steady-state rates of CH4 and benzene formation were little affected by calcination temperature, although calcination at 900 °C did appear to produce a slightly better catalyst in terms of steady-state CH4 yield. The initial rates of CH4 and benzene formation followed the surface acidity profile obtained from NH3-TPD. As shown in Table 1, the surface acidity values decreased with an increase in the calcination temperature. The highest value of surface acidity was for WZ450, and that same catalyst showed the highest initial rates of CH4 and benzene formation. Because cracking reactions take place on acid sites (mainly Bro¨nsted sites), the higher the acidity, the more one would expect cracking to CH4 and benzene as observed in Figure 2. An initial partial deactivation was observed for all catalysts (Figure 2), and the highest deactivation was observed for WZ450. The least deactivation was observed for WZ900. The

Figure 3. TOS behavior of WZ900 for toluene decomposition in the presence of 10% H2 at different temperatures: (a) rate of CH4 formation and (b) rate of benzene formation (maximum error ) (5%).

ratio of steady-state rates of CH4 and benzene formation was 1.5 for WZ450, while the ratio was 2.6 for WZ900. Thus, although higher calcination temperatures appeared to decrease the surface acidity concentration and resulted in lower initial activities, they produced more stable catalysts exhibiting less initial partial deactivation and higher CH4/benzene ratios. Since WZ900 showed the highest steady-state CH4 rate and the least deactivation for both CH4 and benzene formation, it was selected for further investigation. The TOS behavior of WZ900 at various temperatures and 1 atm in the presence of 10% H2 is shown in Figure 3. Similar to the case for WC, CH4 and benzene were the only products of reaction detected. The rates of CH4 formation are shown in Figure 3a, while the rates of benzene formation are shown in Figure 3b. The initial conversion of toluene at 700 °C was ca. 5 ( 1%, while it was 24 ( 3% at 800 °C. The ratios of steadystate rates of CH4 and benzene formation were ca. 1.6 ( 0.1 at all temperatures, indicating additional reactions, besides toluene cracking (eq 1), were taking place at or above 700 °C, most likely benzene cracking (eq 2). The ratio of 1.6 indicates that the overall rate of CH4 formation was greater than the rate of benzene formation, mostly due to reaction 2. From Figure 3, it is observed that both CH4 and benzene formation displayed an initial partial deactivation at all temperatures. The deactivation was smaller for CH4 formation compared to benzene formation, probably due to increased benzene cracking. The catalyst changed its color from pale yellow to black during reaction at all temperatures, indicating coke deposition. This coke deposition was likely the reason for

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Figure 4. Coke formation on WZ900 as a function of TOS at 700 °C and in the presence of 10% H2. Formation rate of benzene as a function of TOS is also shown for comparison.

the observed partial deactivation during the initial TOS. Besides toluene decomposition, benzene cracking, which was evident from the CH4 rate/benzene rate ratio of 1.6, could be another source of coke. According to Kuba et al.,25 the interconnecting polyoxotungstate clusters on the surface of WZ are responsible for the homolytic C-H bond cleavage. This results in reduction of W6+ centers to W5+ and formation of organic radicals that ultimately get converted to products. The reaction intermediates, adsorbed on the catalyst surface, can undergo polymerization and cracking, leading to coke deposition. In order to verify if coke deposition was the reason for the observed partial deactivation, various samples of WZ900 after reaction at different TOSs were sent to Galbraith Laboratories for carbon analysis. The carbon content of fresh WZ900 was also determined. Coke formation on WZ900 as a function of TOS of toluene decomposition at 700 °C is shown in Figure 4. For comparison, the TOS behavior of benzene formation is also shown in Figure 4. From the figure, it is observed that the fresh WZ900 had ca. 0.4 wt % carbon present. However, after the reaction was started, the amount of carbon deposited on the catalyst increased from 0.4 to 1.6 wt % during the first 5 min of TOS. This confirms that a large amount of carbon was deposited during the first few minutes of reaction, resulting in the initial partial deactivation. Little additional carbon appeared to be deposited after 100 min of TOS, and the carbon content remained constant at ca. 2.6 wt %, indicating why steady-state rates of CH4 and benzene formation were achieved. 3.2.3. Effect of Pt Incorporation. To study the effect of Pt incorporation with WZ900, a series of PtWZ catalysts with different Pt loadings were investigated. Reaction studies were conducted at 700 °C. Dispersing Pt on WZ900 resulted in an increase in the initial toluene decomposition activity at 700 °C. Such an increase in the activity of tungstated zirconia after incorporation of Pt has been widely reported in the literature for various reactions including n-pentane isomerization,26 nbutane isomerization,27 and cyclohexane dehydrogenation.27 According to Kuba et al.,25 incorporation of Pt significantly enhances desorption of reaction intermediates in the presence of H2, which, otherwise, would remain on the WZ surface as coke precursors, leading to partial deactivation. Identical to the results for WC and WZ900, CH4 and benzene were the only products observed at all reaction conditions studied. The initial and steady-state rates of CH4 and benzene formation as a function of Pt loading are shown in parts a and b of Figure 5, respectively. From the figure, it is observed that the initial rates of CH4 and benzene formation went through maxima at a Pt

Figure 5. Effect of Pt incorporation with WZ900 on the initial and steadystate rates of product formation at 700 °C: (a) rate of CH4 formation and (b) rate of benzene formation.

loading corresponding to 5 wt %. Although a considerable effect of Pt incorporation was observed on the initial rate of benzene formation, the steady-state rate did not show any variation with Pt incorporation or loading (Figure 5b). On the other hand, the steady-state rate of CH4 formation did increase somewhat with the addition of Pt (Figure 5a), but this effect was not affected by Pt loading for the amounts used. A considerable amount of coke was deposited on all the catalysts at steady-state, as was evident from the color change from dark yellow to black, and this may have been the possible reason why no really significant change in steady-state rates was observed with Pt incorporation. As no major dependence of product formation on Pt content was observed at steady-state and since 5 wt % Pt loading displayed maxima for initial rates of product formation, it was selected for further investigation of toluene decomposition. This catalyst is henceforth referred to as 5PtWZ. 3.3. 5PtWZ. 3.3.1. Characteristics. The BET surface area, XRD phases, and surface acidity of 5PtWZ obtained from NH3TPD are shown in Table 1. For 5PtWZ, the value of H2 uptake obtained from static H2 chemisorption was 53 µmol/(g of catalyst), which corresponds to a Pt dispersion of 23%. A 40% reduction in BET surface area of 5PtWZ was observed compared to WZ900. This could be possibly due to the recalcination step in the preparation of 5PtWZ. The XRD spectra of WZ900 and 5PtWZ showed peaks corresponding to tetragonal and monoclinic ZrO2 along with WO3 nanoparticles. Thus, there appeared to be no change in the bulk WZ structure of 5PtWZ during catalyst preparation. The surface acidity for 5PtWZ obtained from NH3-TPD was 45 (µmol of NH3)/(g of catalyst), while

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Figure 6. Effect of Pt incorporation on pore-size distribution of WZ900.

that for WZ900 was 18 (µmol of NH3)/(g of catalyst). Thus, the results indicate that incorporation of Pt appears to increase the surface acidity of WZ900 as measured by NH3-TPD. Although, the acidity value for 5PtWZ was higher than that of WZ900, both catalysts showed NH3 desorption peaks centered at 180 °C, indicating Pt incorporation may have increased the number of acid sites while the strength of acid sites was unaffected. The effect of Pt incorporation on the pore-size distribution of WZ900 is shown in Figure 6. The volume-weighted frequency for pore-size distribution (or change in pore volume/change in pore diameter) is plotted against pore diameter in Figure 6. From the figure, it is observed that WZ900 had an average pore size of 11 nm. However, as the amount of Pt incorporated increased from nil to 10 wt %, the average pore size decreased from 11 to 8.8 nm, indicating Pt particles were partially blocking the pores. The above results suggest that the incorporation of Pt has a small effect on pore structure, but the bulk structure remains essentially the same. The question can be asked whether the presence of residual Cl- from the Pt precursor had any poisoning effect on PtWZ. Several experiments were performed in order to elucidate the effect of the presence of Cl- on the activity of this catalyst. The calcined 5PtWZ catalyst was washed with hot deionized water to remove Cl- species. The Cl content of this catalyst, before and after washing, was determined by sending the samples to Galbraith Laboratories. The amount of Cl- before washing was ca. 360 ppm, while its value was ca. 230 ppm after washing. The TOS experiments performed at 700 °C and 1 atm and in the presence of 10% H2 indicated there was no change in the activity of PtWZ due to the change in Cl- content. This suggests that residual Cl- did not have any poisoning effect. 3.3.2. Reaction. The TOS behavior of 5PtWZ for toluene decomposition in the presence of 10% H2 at various temperatures and at 1 atm is shown in Figure 7. The rate of formation of CH4 is shown in Figure 7a, while the rate of formation of benzene is shown in Figure 7b. An initial partial deactivation, similar to that for WC and WZ900, was observed with TOS followed by steady-state activity. The steady-state rate of CH4 formation (corresponding to a toluene conversion of ca. 4 ( 1%) at 700 °C was ca. 1.8 µmol/(g of catalyst)/s, while that of benzene was ca. 1 µmol/(g of catalyst)/s. The most plausible reason for the partial deactivation, like for WZ900, is coke deposition on the active sites of 5PtWZ. The ratio of steadystate CH4 and benzene formation rates decreased from 2.3 at 700 °C to 1.6 at 800 °C. Figure 8 shows the pore-size distributions of 5PtWZ before and after toluene decomposition. The BET surface area of the

Figure 7. TOS behavior of 5PtWZ for toluene decomposition in the presence of 10% H2 at different temperatures: (a) rate of CH4 formation and (b) rate of benzene formation (maximum error ) (5%).

Figure 8. Pore-size distribution of 5PtWZ before and after toluene decomposition at 700 °C and in the presence of 10% H2.

catalyst did not change significantly after the reaction. On the other hand, the average pore size of 5PtWZ dropped from 8.8 to ca. 7.5 nm (Figure 8). This suggests that coke formed during the reaction partially blocked pores of the catalyst, although the overall surface area was less affected. An important question that can be asked is why 5PtWZ, especially since it contains metal surfaces, does not more selectively open the toluene/benzene ring, leading to even more CH4 and/or straight-chain alkanes? It has been reported in the literature that selective ring opening of aromatics requires solid acids with intermediate Bro¨nsted acidity.28 Strong Bro¨nsted acid sites with higher density lead to cracking and dealkylation of

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Figure 9. Comparison of steady-state rates of product formation on USY, WC, WZ900, and 5PtWZ from 300 to 800 °C: (a) rates of CH4 formation and (b) rates of benzene formation.

aromatics instead of selective ring opening,28,29 which is evident in the present study. Higher temperatures are responsible for more dealkylation than selective ring opening. In the present study, the temperatures of the reaction are relatively high (>700 °C). Hence, either of these reasons (too strong acid sites, too high reaction temperature) or both could be responsible for lower amounts of ring-opening activity of both 5PtWZ and WZ900 observed in the present study. 3.4. Comparison of Catalyst Activities: 300-800 °C. On the basis of the results reported to this point, one might be interested in how WC, WZ900, and 5PtWZ compare in activity to each other and to a commercial cracking catalyst such as ultra-stable Y (USY) zeolite. In addition, one might wonder if reactions carried out at temperatures lower than 700 °C would improve the secondary conversion of benzene. In order to answer these questions, several experiments were conducted at different temperatures in the range of 300-800 °C on WC, WZ900, 5PtWZ, and USY. A comparison of steady-state rates of CH4 and benzene formation on all catalysts is shown in parts a and b of Figure 9, respectively. The commercial proprietary USY zeolite used for comparison showed a BET surface area of 548 ( 2 m2/g. XRD confirmed the presence of only Y zeolite. USY, as one would expect, was highly acidic compared to other W-based catalysts, with the acidity value obtained from NH3-TPD experiments being 295 µmol/(g of catalyst). USY showed a small steadystate activity at 300 °C, and this activity increased monotonically with temperature until 575 °C. Surprisingly, after 575 °C, a drop in the steady-state activity was observed until 650 °C. From 650 °C, the activity again increased monotonically until 800 °C (the highest temperature studied). The precise reasons for this drop in the activity are presently unclear, although it is probably related to greater partial deactivation above 575 °C. The increase in the rate after 650 °C probably results from an effect of temperature compensation for deactivation. For WC, no activity was observed below 700 °C, while the activity increased with an increase in temperature up to 800

°C. WZ900 did not show any activity until 575 °C, where a small activity was observed. WZ900, like WC, showed a monotonic increase in activity after 575 °C. Both WC and WZ900 exhibited similar activities (on a “per-g-of-catalyst” basis) to USY for reaction at g700 °C. 5PtWZ, on the other hand, displayed a behavior and activity similar to USY for reaction in the range 575-800 °C, showing rate maxima at 575 and 800 °C. At 300 °C, 5PtWZ showed only a small activity for toluene decomposition. At 500 °C, 5PtWZ showed a steady rate of ca. 1.5 µmol/(g of catalyst)/s for CH4 formation and ca. 1.2 µmol/ (g of catalyst)/s for benzene formation. The activity at 575 °C was even higher and was 6 times that of WZ900 at similar conditions.It has been widely suggested that PtWZ may undergo a number of structural changes in the presence of a H2 atmosphere.30–32 The plausible explanation is that H-spillover, resulting from diffusion of H+ ions from Pt nanoparticles to the WOx sites, is occurring at these temperatures on 5PtWZ, leading to in situ formation of Bro¨nsted acid sites. This could result in increased surface acidity of PtWZ during reaction in H2 compared to that of WZ900 (where hydrogen activation and spillover is absent), leading to higher activity, as observed in parts a and b of Figure 9. In any case, the results of this study clearly indicate that Pt incorporation has a significant effect on the activity of WZ900 for toluene decomposition when reaction temperatures are e575 °C. As seen in parts a and b of Figure 9, an increase in the temperature from 575 to 650 °C saw a drop in the activity of 5PtWZ for both CH4 and benzene formation. Above 650 °C, however, 5PtWZ showed an increase in activity till 800 °C. Surprisingly, the activities of WZ900 measured in this temperature range were comparable to that of 5PtWZ, indicating that, at temperatures > 700 °C, 5PtWZ was behaving similarly to WZ900. The positive effect of Pt appears to have disappeared at these higher temperatures. A suppression in the hydrogenolytic ability of Pt in PtWZ has been reported for benzene hydrogenation in the temperature range of 200-300 °C, and the reason for this was attributed to strong metal-support interaction (SMSI) between Pt and WZ.32 Since, in the present study, the temperatures of the reaction were so high, the existence of SMSI between Pt and WZ900 would have been highly likely. The temperature range of 575-700 °C, thus, marked the apparent transition of catalytic behavior of 5PtWZ to that of WZ900. To understand more about the unusual behavior of 5PtWZ in this temperature range, the ratio of steady-state rates of CH4 and benzene formation was calculated and plotted as a function of the reaction temperature, as shown in Figure 10. The same ratio for WZ900 was also determined and is shown as a function of temperature in Figure 10. At lower temperatures (e500 °C), the ratios for 5PtWZ were ca. 1, indicating equimolar formation of products according to eq 1. Between 575 and 700 °C, the ratios for 5PtWZ and WZ900 showed contrasting, unusual (but reproducible) behaviors. The ratio for 5PtWZ increased and achieved the highest value of 3.4 ( 0.4 at 650 °C, while the ratio for WZ900 decreased after 575 °C and attained a minimum of 0.4 ( 0.2 at 650 °C. This may suggest that the presence of Pt may actually still be beneficial at 650 °C (affecting selectivity but not activity). After 650 °C, the ratio for 5PtWZ quickly decreased and the curves of 5PtWZ and WZ900 merged with each other, indicating both catalysts were apparently behaving similarly to each other above 700 °C. All of the W-based catalysts and USY exhibited somewhat comparable activities above 700 °C. However, only 5PtWZ and

4084 Ind. Eng. Chem. Res., Vol. 47, No. 12, 2008

Dr. Dora Lo´pez and Kaewta Suwannakarn for BET, XRD, and surface acidity data of the WZ catalysts. We also thank Dr. David Bruce, Dr. Walter Torres (Department of Chemical and Biomolecular Engineering, Clemson University), and Dr. S.-J. Hwu (Department of Chemistry, Clemson University) for thoughtful discussions. Literature Cited

Figure 10. Ratios of steady-state rates of CH4 and benzene formation on 5PtWZ and WZ900 in the presence of 10% H2 at various temperatures.

USY showed appreciable activities of product formation below 575 °C. This might seem surprising since USY did not contain Pt. Perhaps the comparable activity of USY with that of PtWZ was due to its high acidity. The incorporation of Pt had a significant effect on the activity of tungstated zirconia below 600 °C, while at temperatures above 700 °C, there was no significant advantage of Pt incorporation. 4. Conclusions WC and WZ were found to be effective for the decomposition of toluene in the presence of 10% H2, especially at temperatures g 700 °C. The calcination temperature of WZ had a significant effect on the initial rates of product formation, while a negligible effect was observed on the steady-state rates. WZ calcined at 900 °C showed the least deactivation and the highest steadystate activity for CH4 formation. Incorporation of 5 wt % Pt with WZ900 resulted in the highest initial rates of product formation. Pt incorporation had a small effect on pore structure, but the bulk structure of WZ was essentially the same. The effect of Pt incorporation with WZ900 on toluene decomposition was significant below 600 °C, while the effect was nonexistent above 700 °C. CH4 and benzene were the only products of reaction observed on any of these catalysts. All catalysts showed an initial partial deactivation due to coke deposition on active sites. Neither WZ900 nor 5PtWZ showed good ring-opening activity above 700 °C, the reasons for which could be either the higher reaction temperatures and/or the presence of strong Bro¨nsted acid sites. The same was true in the case of WC and USY. All of the W-based catalysts and USY exhibited comparable activities above 700 °C. WC, however, because of its sulfur tolerance, extreme hardness, and thermal stability, was a more superior catalyst than WZ, PtWZ, and USY for toluene decomposition from a hot gas cleanup viewpoint. Only 5PtWZ and USY showed appreciable activities for toluene decomposition at or below 575 °C. Decomposition of tars to lower hydrocarbons represents an important step in biomass gas cleanup. Developing catalysts that can effectively catalyze tar decomposition is the key to the commercialization of biomass gasification. Acknowledgment The authors acknowledge financial support from RTI International and the Department of Energy. We thank Magnesium Electron Inc. for providing the tungstated zirconia. We thank

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ReceiVed for reView February 18, 2008 ReVised manuscript receiVed March 24, 2008 Accepted March 29, 2008 IE8002864