Pt catalysts supported on H-type zeolites for the catalytic combustion of chlorobenzene

Applied Catalysis B: Environmental 45 (2003) 117–125

Pt catalysts supported on H-type zeolites for the catalytic combustion of chlorobenzene Salvatore Scirè∗ , Simona Minicò, Carmelo Crisafulli Dipartimento di Scienze Chimiche, Università di Catania, Viale A. Doria 6, 95125 Catania, Italy Received 11 November 2002; received in revised form 2 March 2003; accepted 11 March 2003

Abstract The deep oxidation of chlorobenzene was investigated over Pt catalysts supported on H-type zeolites (H-ZSM5 and H-beta). Pt/zeolite catalysts showed a higher activity compared to Pt/␥-Al2 O3 samples which were tested for comparison. Within each class of zeolite, the activity of Pt/zeolite catalysts was found to be higher on the samples with lower SiO2 /Al2 O3 ratio. Amounts of polychlorinated benzenes (PhClx ) were produced in the order Pt/H-ZSM5 < Pt/H-beta < Pt/␥-Al2 O3 and were found to be roughly independent of the SiO2 /Al2 O3 ratio of the zeolitic support. The trend in the PhClx formation observed on Pt/zeolite samples, both in terms of total amount and relative distribution, was explained on the basis of a product shape selectivity effect induced by the zeolite, a lower size of zeolite channels hindering the further chlorination of PhCl to PhClx . © 2003 Elsevier Science B.V. All rights reserved. Keywords: Catalytic combustion; Volatile organic compound (VOC); Chlorobenzene; Platinum; Zeolite; H-ZSM5

1. Introduction Chlorinated volatile organic compounds (Cl-VOCs) are considered potent environmental pollutants due to their toxicity, high stability and widespread application in industry [1,2]. At present, the main industrial process for Cl-VOCs destruction involves thermal oxidation at extremely high temperatures (>1000 ◦ C). This is a rather expensive process, that can also lead to highly toxic byproducts, such as dioxins and dibenzofurans, formed by incomplete combustion [1,2]. Catalytic combustion is one of the most promising technology for the removal of volatile organic compounds from waste gases, due to its definitive character and safe use of energy [3,4]. The major ∗ Corresponding author. Tel.: +39-095-7385-034; fax: +39-095-580-138. E-mail address: [email protected] (S. Scir`e).

advantages of this approach are that the combustion can be carried out at lower temperatures (<500 ◦ C) and lower concentration of pollutants (<1%) than thermal oxidation. Hence, catalytic oxidation may be considered as a more appropriate method for end of pipe pollution control. Recently, catalytic combustion has been also applied to the destruction of Cl-VOCs [1,2,5–15]. However, the interaction of the catalyst with chlorine is the main problem met in the design of catalytic systems for the Cl-VOCs combustion. The optimal catalyst for this reaction should be active, stable and, above all, highly selective to CO2 , H2 O and HCl (this latter compound being easily eliminated from the stream by washing), limiting the formation of other environmentally hazardous organic compounds. Metal oxides or supported noble metals are the most studied catalysts for the combustion of Cl-VOCs [1,2,5–15]. Generally noble metals (Pt and Pd) exhibit a high activity for the

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oxidation of many VOCs. However, on these metals chlorination of organic compounds besides their oxidation can also occur leading to considerable amounts of polychlorinated compounds [1,5,6], which are more toxic and re-calcitrant than the starting material. This is actually the major drawback for the application of these catalysts. Metal oxides are much less expensive than noble metals, but they are also inherently less active. Moreover, with metal oxides, the formation of volatile metaloxychlorides is a serious problem, leading to a relevant catalyst deactivation [5,6]. U3 O8 has been reported as the only exception, but an industrial application of this catalyst requires special procedures for its safe handling due to chemical toxicity considerations [1]. More recently zeolites has been reported as potential catalysts for the oxidation of halogenated hydrocarbons, because of their pore structures, acidic properties and thermal stability [11–15]. In this context, the aim of this work is to evaluate the performance of Pt catalysts supported on different H-type zeolites (H-ZSM5 and H-beta) towards the gas-phase catalytic oxidation of chlorobenzene (PhCl), focusing attention on the formation of byproducts, i.e. polychlorinated benzenes (PhClx ). In order to investigate the influence of the acidic properties of the support on the catalytic activity and products selectivity of supported Pt samples, different SiO2 /Al2 O3 ratios have been taken into consideration for each zeolite. It must be reminded that PhCl has been chosen as the reactant molecule considering that it is not only a pollutant itself, but also a particularly stable compound that is difficult to oxidize, owing to its stabilization by aromaticity. Finally, chlorobenzene is considered as a suitable probe for the destruction of polychlorinated aromatics [1,5–7,15].

2. Experimental Pt (0.5 wt.%) catalysts were prepared by incipient wetness impregnation of supports with appropriate amounts of aqueous solution of H2 PtCl6 (Alfa Aesar). One ␥-Al2 O3 (Harshaw with an average pore size of 110 Å), four H-ZSM5 (Zeolyst), with SiO2 /Al2 O3 ratios, respectively, of 30, 50, 150, 280, and two H-beta (Zeolyst), with SiO2 /Al2 O3 ratios of 75 and 300, were used as supports. Code of all samples, together with support used, SiO2 /Al2 O3 ratios of zeolites, H/Pt ratio

Table 1 Code and physico-chemical properties of supported Pt samples Code

Support

SiO2 /Al2 O ratio

H/Pt ratio

NH3 adsorbed (mmol g−1 )

PtAlHD PtAlLDa PtBEA75 PtBEA300 PtMFI30 PtMFI50 PtMFI150 PtMFI280

␥-Al2 O3 ␥-Al2 O3 H-beta H-beta H-ZSM5 H-ZSM5 H-ZSM5 H-ZSM5

– – 75 300 30 50 150 280

0.85 0.10 0.12 0.15 0.08 0.10 0.12 0.09

– – 0.85 0.30 1.16 1.05 0.34 0.13

a

Prepared by calcination at 700 ◦ C of the PtAlHD sample.

and total amount of adsorbed ammonia are reported in Table 1. It must be noted that the sample coded PtAlLD has been obtained by treating the PtAlHD sample in air at 700 ◦ C for 10 h. Catalytic activity tests were carried out in a fixed-bed reactor at atmospheric pressure in the 200–550 ◦ C range, using 0.1 g of catalyst. PhCl was fed to the reactor by a carrying gas of He flow through a saturator maintained at 2.8 ◦ C and then mixed with O2 and He before reaching the catalyst. The reactant mixture was 10% O2 and 2000 ppm PhCl diluted with He. The total gas flow was 44.3 cm3 min−1 with a GHSV of 18600 h−1 . The reaction products were analyzed using two on-line gas chromatographs, one equipped with FID detector and HP-INNOWax column for the analysis of PhCl and PhClx and the other with TCD detector and Octoil S at 3% on silica gel column for the CO/CO2 analysis. Tubing and injection valves were heated at 200 ◦ C to prevent condensation of products. For all experiments, CO2 was the main carbon-containing product, very small amounts of CO were found only at low conversions. The carbon balance was always higher than 97%. Before catalytic runs, all samples were reduced at 450 ◦ C in H2 for 1 h and then calcined in air at the same temperature for 3 h. Preliminary runs carried out at different flow-rates showed the absence of external diffusional limitations. The absence of internal diffusion limitations was verified by running experiments with crushed pellets at different grain size. H2 chemisorption was measured in a static system operating at room temperature. Before the measurements, the samples have been pre-treated as follows:

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reduction in H2 at 450 ◦ C for 1 h, outgassing at 450 ◦ C for 1 h and cooling at room temperature. TPD experiments of ammonia were carried out in a quartz U-shape reactor in a flow of He with a constant heating rate of 10 ◦ C min−1 . The desorbed products were detected by a quadrupole mass spectrometer (Sensorlab VG Quadrupoles). Before TPD all samples were reduced in flowing H2 for 1 h at 450 ◦ C, calcined in air at 450 ◦ C for 3 h, heated and maintained in flowing He for 30 min at 600 ◦ C and then cooled to 30 ◦ C in a flow of He. FT–IR spectra were recorded with a Perkin-Elmer System 2000 FT–IR spectrophotometer (resolution of 2 cm−1 ) using self-supporting pressed discs of the pure catalyst powders (25 mg cm−2 ). The disc, placed in an IR cell which allows thermal treatments in a controlled atmosphere, was reduced in pure H2 at 450 ◦ C for 30 min, evacuated a 450 ◦ C for 1 h, and finally cooled at room temperature. Commercial 1,4and 1,3-dichlorobenzenes (Fluka) were then vaporized in vacuum and adsorbed from the gas phase.

3. Results Fig. 1 reports the conversion of chlorobenzene (Fig. 1(A)) and the yield to PhClx (Fig. 1(B)) as a function of the reaction temperature over ␥-Al2 O3 supported Pt samples (PtAlHD and PtAlLD). It must be reminded that PtAlLD was obtained by calcining the PtAlHD sample at 700 ◦ C. This treatment resulted in a relevant change in the Pt dispersion, which decreased from 85% in the case of PtAlHD down to 10% for PtAlLD (see H/Pt ratio in Table 1). Fig. 1(A) shows that on both the Pt/␥-Al2 O3 samples, the PhCl conversion becomes perceptible above 250 ◦ C, achieves 50% at about 350 ◦ C and approaches completion at ca. 450 ◦ C. On both these samples significant amounts of polychlorinated benzenes are formed (Fig. 1(B)). The output of PhClx reaches the maximum (yields to PhClx of 2.3 and 1.7%, respectively, on PtAlHD and PtAlLD) at ca. 450 ◦ C (temperature at which PhCl conversion approaches 100%), then decreasing at higher temperatures. It must be underlined that the level of polychlorinated products remains still important up to the maximum temperature investigated (550 ◦ C). The above trend in the PhClx formation is in good accordance with data

Fig. 1. (A) PhCl conversions and (B) yields to PhClx as a function of reaction temperatures over ␥-Al2 O3 supported Pt samples: (䊉) PtAlHD; (䊐) PtAlLD.

reported in the literature over a 2 wt.% Pt/␥-Al2 O3 catalyst [5]. From Fig. 1(B) it can be also seen that on the sample with lower Pt dispersion (PtAlLD) PhClx production is a bit lower. This agrees with results of Van den Brink et al., who reported that larger Pt crystallites result in lower PhClx levels, attributing this to the fact that in large particles most of Pt, which is responsible for the PhClx formation, is not accessible to chlorobenzene [6]. However, we must note that in our case a big difference in the Pt dispersion (compare H/Pt ratios of PtAlHD and PtAlLD in Table 1) corresponds to a slight change in PhClx yields, thus pointing out that the PhClx formation is

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Fig. 2. PhClx distributions at different reaction temperatures for: (A) PtAlHD and (B) PtAlLD samples.

affected just to a small extent by the particle size of platinum. In Fig. 2, PhClx distributions at different reaction temperatures, respectively, for PtAlHD (Fig. 2(A)) and PtAlLD (Fig. 2(B)) are depicted. At reaction temperatures of 350–400 ◦ C, for both the Pt/␥-Al2 O3 samples, dichlorobenzene isomers were predominant over all PhClx , the amounts of PhCl2 following the order 1,3-PhCl2 > 1,4-PhCl2 > 1,2-PhCl2 . This PhCl2 distribution, with meta isomer as major dichlorobenzene, rules out the possibility that the chlorination of PhCl occurs via a classical aromatic electrophilic substitution mechanism, which should, on the contrary, favor ortho and para isomers. It seems, instead, more plausible that chlorination of PhCl occurs through a radical mechanism [5]. Besides, it has been reported in the literature that when chlorination of substituted aro-

matic compounds is carried out at high reaction temperatures (300–400 ◦ C) ortho–para directing groups (such as chloro-) direct meta and vice versa, via a not yet well understood reaction mechanism [16]. Fig. 2 shows also that, on increasing reaction temperatures, percentage of PhCl2 sensibly decreases in favor of higher polychlorobenzenes, PhCl3 resulting the most abundant PhClx (1,2,4-trichlorobenzene is always the major PhCl3 ) at 450–500 ◦ C. This clearly indicates that at higher temperatures further chlorination of the organic compound becomes easier. From the comparison of Fig. 2(A) and (B) it can be seen that PhClx distribution patterns of PtAlHD and PtAlLD are quite similar, thus pointing out that platinum dispersion does not play an important role in determining the distribution of polychlorobenzenes on Pt/␥-Al2 O3 catalysts. It must be reminded that, under experimental conditions used in this work, PhClx higher than PhCl4 were present only in traces. Dibenzodioxins and dibenzofurans, whose formation has been reported may take place during the combustion of organo-chlorine compounds in the presence of metallic ions between 300 and 500 ◦ C [17], were never observed. Indeed, we wish to underline that our analytical system cannot detect trace quantities of these compounds which could be instead considered already significant from an environmental point of view. Nevertheless, it must be remarked that no dibenzodioxins and dibenzofurans were detected in the combustion of chlorobenzene over Pt/␥-Al2 O3 neither by Van den Brink et al., notwithstanding they employed more sensible analytical techniques [5]. The curves of PhCl conversion and PhClx yields as a function of the reaction temperature for H-beta supported Pt samples are shown, respectively, in Fig. 3(A) and (B). It is possible to note that on both PtBEA75 and PtBEA300 samples the light-off temperature is just above 200 ◦ C and PhCl conversion approaches 100% at 350–400 ◦ C (Fig. 3(A)). These data indicate that the Pt/H-beta system is considerably more active than Pt/␥-Al2 O3 , showing conversion curves shifted at 50 ◦ C ca. lower than those observed on alumina supported Pt samples. PhClx formation presents a maximum at 300 ◦ C, i.e. 150 ◦ C lower than in the case of Pt/␥-Al2 O3 samples. Maximum yields to PhClx are just slightly lower (1.4 and 1.6%) than that measured (1.7%) on the PtAlLD sample, which exhibits a Pt dispersion comparable to that of

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Fig. 4. PhClx distributions at different reaction temperatures for: (A) PtBEA75 and (B) PtBEA300 samples. Fig. 3. (A) PhCl conversions and (B) yields to PhClx as a function of reaction temperatures over H-beta supported Pt samples: (䊉) PtBEA75; (䊐) PtBEA300.

Pt/H-beta samples. It is interesting to highlight that on H-beta supported Pt samples the relative distribution of PhCl2 isomers (Fig. 4(A) and (B)), following the order 1,3-PhCl2 > 1,4-PhCl2 > 1,2-PhCl2 , resembles that of Pt/␥-Al2 O3 catalysts. On the other hand, the amount of PhCl3 formed on Pt/H-beta samples is relevantly lower compared to Pt/␥-Al2 O3 samples with no formation of PhCl4 or higher PhClx , thus pointing out that on Pt/H-beta samples further chlorination of PhCl2 is somewhat hindered. This also accounts for the fact that PhClx pattern does not change substantially with reaction temperature. It is also worthy noting that PhClx distribution does not

vary by changing the SiO2 /Al2 O3 ratio of the zeolite (compare Fig. 4(A) and (B)). Fig. 5 depicts the curves of PhCl conversion (Fig. 5(A)) and PhClx yields (Fig. 5(B)) as a function of the reaction temperature for Pt samples supported on H-ZSM5 zeolites with four different SiO2 /Al2 O3 ratios. For all the samples, the conversion of PhCl (Fig. 5(A)) becomes noticeable at ca. 250 ◦ C. It can be also seen that the activity towards the conversion of PhCl slightly decreases on increasing the SiO2 /Al2 O3 ratio of the zeolite, with 50% PhCl conversion ranging from 310 ◦ C on PtMFI30 to 350 ◦ C on PtMFI280. The yields to PhClx (Fig. 5(B)) are sensibly lower compared both to ␥-Al2 O3 and H-beta supported Pt samples and are roughly independent from the SiO2 / Al2 O3 ratio, reaching for all Pt/H-ZSM5 samples a

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Fig. 6. PhClx distributions at different reaction temperatures for: (A) PtMFI30 and (B) PtMFI280 samples.

Fig. 5. (A) PhCl conversions and (B) yields to PhClx as a function of reaction temperatures over H-ZSM5 supported Pt samples: (䊉) PtMFI30; (䊐) PtMFI50; (䉱) PtMFI150; (×) PtMFI280.

maximum value of around 0.6% at 350 ◦ C, then dropping down to <0.1% at T > 450 ◦ C. With regard to the PhClx distribution (Fig. 6), it is interesting to note that in the case of Pt/H-ZSM5 samples the formation of PhClx is almost limited to dichlorobenzenes, with very low amounts of PhCl3 isomers and no higher PhClx detected at any investigated temperature. Differently from that observed on alumina and H-beta supported Pt samples, on Pt/H-ZSM5 catalysts the para-isomer is at any temperature the most abundant PhCl2 it must be also mentioned that, analogously to that observed on the Pt/H-beta system, on Pt/H-ZSM5 catalysts PhClx distribution does not seem to be af-

fected by the SiO2 /Al2 O3 ratio of the zeolite (compare Fig. 6(A) and (B)). Finally, it must be reminded that in all experiments with Pt/zeolite catalysts dibenzodioxins and dibenzofurans were never detected. In terms of catalyst stability it was observed that, when catalysts were maintained under reaction conditions for 15 h in the 350–400 ◦ C range, PhCl conversion decreased 5% of ca. on Pt/␥-Al2 O3 and Pt-H-beta samples, whereas all Pt/H-ZSM5 catalysts resulted to be more stable, showing an activity lost lower than 1%.

4. Discussion Table 2 summarizes the main catalytic results (temperature at which 50% PhCl conversion was reached, reaction rates calculated at 300 ◦ C and maximum

S. Scir`e et al. / Applied Catalysis B: Environmental 45 (2003) 117–125 Table 2 Summary of catalytic activity data of supported Pt samples Code

T50% (◦ C)a

V ×104 (mol(gcat h)−1 )b

PhClx (%)c

PtAlHD PtAlLD PtBEA75 PtBEA300 PtMFI30 PtMFI50 PtMFI150 PtMFI280

340 350 300 320 310 320 340 340

1.8 1.0 13.3 5.0 5.2 2.9 2.3 1.8

2.3 1.7 1.6 1.4 0.6 0.6 0.6 0.6

(450 ◦ C) (450 ◦ C) (300 ◦ C) (300 ◦ C) (350 ◦ C) (350 ◦ C) (350 ◦ C) (350 ◦ C)

a

Temperature at which 50% PhCl conversion was reached. Reaction rate calculated at 300 ◦ C. c Maximum production of total polychlorinated benzenes expressed as percentage yield (in parentheses the temperature to match). b

yields to total polychlorinated benzenes) for all the investigated samples. It must be reminded that on pure supports (alumina and zeolites) the oxidation of PhCl has been found to occur at much higher temperatures (light-off >400 ◦ C) without substantial formation of PhClx . This clearly indicates that platinum strongly improves the oxidation activity of oxides [5,6], but it is also directly involved in the formation of polychlorinated benzenes, which has been reported to occur probably through chlorination of adsorbed chlorobenzenes species by platinum oxychlorides [5]. On the basis of the results summarized in Table 2 it appears clear that zeolite supported Pt samples exhibit better catalytic performance towards the combustion of chlorobenzene than alumina supported ones, both in terms of activity and byproducts formation. With regard to the activity, all Pt/zeolite samples show, in fact, higher reaction rates compared to Pt/␥-Al2 O3 samples. It is important to remark that on each class of zeolite the activity depends on the SiO2 /Al2 O3 ratio of the zeolite, being higher on samples with lower ratio. Bearing in mind that the number of acid sites of zeolites is known to decrease on increasing the SiO2 /Al2 O3 ratio [18], as also confirmed by the amount of NH3 adsorbed on our samples measured by TPD of ammonia (Table 1), it can be suggested that the acidity of the support somehow favors the PhCl oxidation over Pt/zeolite catalysts. This is reasonable considering that oxidation of halocarbons has been proposed to occur through the adsorption of the hydrocarbon on Brönsted acid sites and of the oxygen on active metal ion sites

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[14]. A relationship between activity and strong Brönsted acidity has been also described in the oxidation of aliphatic chlorinated hydrocarbons over H-zeolites [11]. On the other hand, it must be reminded that it has been recently reported that chloroaromatics adsorb on Lewis acid sites of oxides through the chlorine atom giving rise to chlorophenate species [19]. Following these considerations it seems plausible that both acidic properties and oxygen adsorption capacity of the catalyst affect the catalytic combustion of Cl-VOCs over supported metal systems. In terms of formation of polychlorinated aromatics Table 2 shows that employing Pt/zeolite catalysts results in lower amounts of PhClx than using Pt/␥-Al2 O3 samples. In particular, the yields to PhClx are in the order Pt/H-ZSM5 < Pt/H-beta < Pt/␥-Al2 O3 . It is also important to underline that on each zeolite the amount of PhClx is roughly independent of the SiO2 /Al2 O3 ratio of the zeolite used as support, thus pointing out that acidity is not responsible for the selectivity to PhClx . Reaction product distributions (Figs. 2, 4 and 6) appear also to depend mainly from the support and not from its acidity or Pt dispersion on the catalytic surface. The low PhClx output of Pt/H-ZSM5 samples makes this catalytic system quite attractive from a practical point of view, considering that the formation of polychlorinated compounds is the main drawback for the application of noble metal catalysts to the combustion of Cl-VOCs. Moreover, Pt/H-ZSM5 catalysts also exhibited the highest resistance to deactivation among all investigated samples. In order to explain the trend of PhClx formation let us consider the different structures of zeolites used as support of platinum. ZSM5 is a medium pore zeolite possessing two types of channels (straight and sinusoidal) both formed by rings of 10 oxygen atoms with openings, respectively, of 5.3 Å × 5.6 Å and 5.1 Å × 5.5 Å, whereas beta is a large pore zeolite with a 3D channel system (12-membered ring apertures), having two perpendicular straight channels each with a cross-section of 7.6 Å × 6.4 Å and a sinusoidal channel of 5.5 Å × 5.5 Å [20]. Catalytic data, reported in this paper, showed that Pt/H-ZSM5 samples produce a much lower amount of PhClx compared to Pt/H-beta catalysts. Therefore, it can be suggested that the zeolite pore dimension plays a key role in determining the selectivity to PhClx of the catalytic system. It is probable that the zeolitic structure induces a product shape

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selectivity effect, with lower size of channels hindering the chlorination of PhCl to PhClx . A further evidence of this hypothesis derives from the analysis of PhClx distribution patterns. In fact, in the case of Pt/H-ZSM5 (Fig. 6) the further chlorination of PhCl was limited, at any reaction temperature, to the formation of dichlorobenzenes, whereas on H-beta supported Pt samples some PhCl3 isomers were also detected, even though their amount was substantially lower than that observed on Pt/␥-Al2 O3 samples. The hypothesis of a hindrance effect of the support also matches well with the relative distribution of the PhCl2 isomers observed. In fact on H-ZSM5 supported catalysts the formation of 1,4-PhCl2 , which is the PhCl2 isomer with the lowest steric hindrance (molecular diameter of 6.2 Å [21]) resulted to be favored compared to that of the more bulky molecules 1,2- and 1,3-PhCl2 (diameter of 6.8 Å [22]). This shape selectivity effect is not observed when H-beta is used as support in so that channel dimensions of this zeolite are quite big to permit a relatively easy access to all dichlorobenzenes. In order to verify experimentally that during PhCl combustion the PhClx formation is controlled by a molecular sieving effect, an FT–IR study on the accessibility of PhCl2 isomers into the ZSM5 cavities was carried out. FT–IR spectra of the PtMFI30 sample, recorded in the 3800–3000 cm−1 region, before and after admission, respectively, of 1,4- and 1,3-PhCl2 are reported in Figs. 7 and 8. The spectrum of the catalyst before admission of the organic molecule (spectrum a) presents bands at 3746, 3670 and 3610 cm−1 . According to the literature [23], the band at 3746 cm−1 is due to terminal silanol groups on the external surface of the ZSM5, the weak band at 3670 cm−1 is related to OHs of extra-framework alumina and the 3610 cm−1 band is associated with the internal bridged OH groups of zeolite. This latter band is, therefore, highly informative on the accessibility into zeolite pores of molecules able to interact with hydroxy groups. The contact of the catalyst with 1,4-PhCl2 vapors causes, already at room temperature (Fig. 7, spectrum b), the complete disappearance of the 3610 cm−1 band with the corresponding increase of a broad band at lower frequency, centered at 3250 cm−1 , due to perturbed OH groups [23]. No substantial changes in the spectrum are observed by the treatment at higher temperatures (Fig. 7, spectra c and d). The behavior above observed clearly indi-

Fig. 7. FT–IR spectra of: (a) the PtMFI30 sample and (b) put in contact with 1,4-PhCl2 vapor at RT, (c) after 15 min at 150 ◦ C and (d) after 15 min at 300 ◦ C.

cates that 1,4-PhCl2 enter easily the ZSM5 channels interacting with all internal OH groups of the zeolite. Fig. 8 shows, instead, that the band of bridged OHs is still evident after contact with 1,3-PhCl2

Fig. 8. FT–IR spectra of: (a) the PtMFI30 sample and (b) put in contact with 1,3-PhCl2 vapor at RT, (c) after 15 min at 150 ◦ C and (d) after 15 min at 300 ◦ C.

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at room temperature (spectrum b), decreasing significantly in intensity only when contact is carried out at higher temperatures (spectra c–d). This indicates that the access of 1,3-PhCl2 and, therefore, its diffusion into zeolite pores, is more hindered with respect to that of 1,4-PhCl2 . A similar results has been reported concerning xylenes–ZSM5 interaction [23,24]. Therefore, FT–IR results here reported point out that the molecular sieving effect of the zeolite has a key role in determining the level and the distribution of polychlorinated compounds during the chlorobenzene combustion over Pt/zeolite catalysts. 5. Conclusions On the basis of the results reported in this paper it can be concluded that Pt/H-ZSM5 catalysts appear to be promising for application in the catalytic combustion of chlorobenzene, mainly due to the fact that the amount of PhClx produced on this system resulted to be very low. It has been suggested that zeolite structure induces a product shape selectivity effect, which hinders the chlorination of PhCl to PhClx and affects the PhClx distribution, favoring the formation of isomers with lower steric hindrance. Acknowledgements The financial support of MIUR (PRIN-2001) is acknowledged. References [1] G.H. Hutchings, C.S. Heneghan, I.D. Hudson, S.H. Taylor, Nature 384 (1996) 341.

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