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

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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/γ-Al2O3 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/Al2O3 ratio. Amounts of polychlorinated benzenes (PhClx) were produced in the order Pt/H-ZSM5<Pt/H-beta<Pt/γ-Al2O3 and were found to be roughly independent of the SiO2/Al2O3 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.

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 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], [6], [7], [8], [9], [10], [11], [12], [13], [14], [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, H2O 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], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Generally noble metals (Pt and Pd) exhibit a high activity for the 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]. U3O8 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], [12], [13], [14], [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/Al2O3 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], [6], [7], [15].

Section snippets

Experimental

Pt (0.5 wt.%) catalysts were prepared by incipient wetness impregnation of supports with appropriate amounts of aqueous solution of H2PtCl6 (Alfa Aesar). One γ-Al2O3 (Harshaw with an average pore size of 110 Å), four H-ZSM5 (Zeolyst), with SiO2/Al2O3 ratios, respectively, of 30, 50, 150, 280, and two H-beta (Zeolyst), with SiO2/Al2O3 ratios of 75 and 300, were used as supports. Code of all samples, together with support used, SiO2/Al2O3 ratios of zeolites, H/Pt ratio and total amount of adsorbed

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 γ-Al2O3 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/γ-Al2O3 samples, the

Discussion

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

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

Acknowledgements

The financial support of MIUR (PRIN-2001) is acknowledged.

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