Catalytic hydrodehalogenation of halon 1211 (CBrClF2) over carbon-supported palladium catalysts

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Abstract

The hydrodehalogenation of halon 1211 over Pd/C catalysts was studied over a relatively narrow temperature range, from 453 to 573 K. The main products of the reaction are CH2F2 and CHBrF2 with CHClF2 formed as a minor product. Pd/C catalysts show high initial activities but undergo gradual deactivation before reaching a steady-state activity level. Experimental results suggest that CHBrF2 and CHClF2 are formed via the secondary reaction of CF2 with HBr and HCl.

Introduction

One of the major consequences of The Montreal Protocol on Substances that Deplete the Ozone Layer has been to intensify research efforts aimed at developing methods for disposal of stockpiled CFCs and halons [1], [2], [3], [4], [5]. Catalytic hydrodehalogenation is a promising non-destructive treatment process in which ozone-depleting bromine and chlorine atoms are removed from the parent halon or CFC. Fluorine atoms may or may not be retained in the final product, depending on reaction conditions. The resulting products are usually a mixture of perfluorocarbons (PFCs), hydrofluorocarbons (HFCs) and hydrocarbons. HFCs can be used as replacements for CFCs and halons or as precursors for the production of other chemicals.

Palladium has been widely used as a catalyst for the hydrodehalogenation of CFCs because it promotes the cleavage of carbon–halogen bonds and facilitates hydrogenation. Palladium is also resistant to catalyst poisoning by halide ions [6], [7], [8]. An example of a palladium-catalysed process is the conversion of CCl2F2 into CH2F2 (HFC 32) [9], [10], [11], [12], [13], [14], [15]:CCl2F2+2H2CH2F2+2HClA detailed assessment of this process for conversion of 10 000 t per year of CCl2F2 into CH2F2 suggests that, the process is both technically and economically feasible [15]. For other CFCs for which the above method is not suitable, an alternative catalytic process has been proposed which involves the complete dehalogenation of CFCs to hydrocarbons. These hydrocarbons can then be recycled or used as fuel for auxiliary heating. For example, the conversion of C1 CFCs into dechlorinated C2 products and mineral acids has been reported:2CCl2F2+6H2C2H4+4HF+4HClPalladium catalysts show high selectivities, up to 75%, for C2+C3 dehalogenated hydrocarbons [8]. This method is of particular interest for converting cleaning solvents, such as 1,2-dichloroethane [16], [17]:ClCH2CH2Cl+H2CH2CH2+2HClVirtually all research on the catalytic hydrodehalogenation has focused on the reactions of CFCs. Conversely, there appears to be very little work on the catalytic hydrodehalogenation of halons, especially halon 1211, which remains a significant contributor to depletion of the stratospheric ozone. While most gases regulated by the Montreal Protocol have shown a significant decline in atmospheric concentration, the level of halon 1211 in the stratosphere has remained relatively constant [18], [19]. One possible explanation for this observation is that production of halon 1211 is still allowed in developing countries and that halon 1211 remains in use in many industrialised countries for specific, critical applications. With its current emission rate, halon 1211 poses a more significant ozone destruction capability than any other halocarbon [20].

In the present study, we report catalytic reaction of halon 1211 with hydrogen over Pd/C catalysts under various reaction conditions. Palladium catalysts were used because they show high activity in carbon–halogen hydrogenolysis, have acceptable stability and have been widely used in the hydrodehalogenation of CFCs. Activated carbon was used as support, as it is relatively inert to HF, HCl and HBr released during reaction. The purpose of our study is to explore the applicability of palladium catalysts in halon 1211 hydrodehalogenation and to investigate the catalytic behaviour of palladium under various reaction conditions. The information gathered will be useful in finding a suitable catalytic system for treatment of halon 1211.

Section snippets

Preparation of Pd/C catalyst

Activated carbon (BG11-1, 10–30 mesh, 1050 m2 g−1 surface area) was used as catalyst support. It was impregnated with a solution of PdCl2 (Sigma) in 0.1 M HCl to obtain a nominal 3 wt.% palladium loading. The solvent (H2O) was then removed in a rotary evaporator at 45 °C, under vacuum. The material was placed in an air oven at 100 °C overnight and then charged into an alumina microreactor and calcined in flowing N2 (60 cm3 min−1) by being heated at a rate of 10 °C min−1 to 450 °C. This temperature was held

Results

The main products of hydrodehalogenation of halon 1211 over Pd/C are CH2F2 and CHBrF2. The secondary products include CHClF2, CH4, C2H4, C2H6, and C3H6. Other products detected in trace amounts include CH3F, CH3Cl, CH3Br, CH2ClF and C3H8.

Fig. 1 shows the conversion of halon 1211 and selectivity to major products as a function of time on stream at a temperature of 473 K and a GHSV of 1150 h−1 (the calculation is based on the flowrate of halon 1211 only). The selectivity towards carbon-containing

Discussion

The reaction mechanism involved in this intriguing chemistry has not yet been reported in the literature and is far from being fully elucidated. Intensive research effort has been made to understand the catalytic behaviour of palladium during the hydrogenolysis of CCl2F2 [11], [12], [14], [22], [23], [24], [25], [26], [27], [28]. Most researchers support a halogenation/dehalogenation mechanism, in which the palladium surface is halogenated by dissociative adsorption of CCl2F2 and the absorbed

Conclusions

Catalytic hydrodehalogenation of halon 1211 over Pd/C catalysts was conducted within the temperature window of between 453 and 573 K with an input volumetric ratio of N2:H2:CBrClF2 = 25:9:1, at various gas hourly space velocities. Pd/C catalysts show higher activities and the conversion of halon 1211 can reach up to 80% at 575 K. But catalysts suffer severe deactivation especially at higher temperatures. The main products are CH2F2 and CHBrF2. Secondary products include CHClF2, CH4, C2H4, C2H6

Acknowledgements

The authors wish to thank the Australian Research Council for financial support of this project.

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