Conversion of halon 1211 (CBrClF 2) over supported Pd catalysts

Abstract

Conversion of halon 1211 has been studied over γ-Al 2O 3 and supported 0.5% Pd catalysts (Pd/Al 2O 3, Pd/fluorinated Al 2O 3, Pd/AlF 3, Pd/Al 2O 3 pretreated with CH 4 and CHClF 2). The experiments were performed in the temperature range of 443–523 K, in a tubular alumina reactor. The temperature of the reactor was maintained uniformly by a three zone furnace. The reaction products were quantified with a micro gas chromatograph and identified with a gas chromatograph–mass spectrometer. The catalysts were characterised with XRD diffractometry and the content of halogen on the catalysts was determined with an ion chromatograph. The measurements were presented in terms of the conversion of halon 1211 and product selectivity, as functions of catalyst type, catalyst time on stream and composition of inlet gases. Transformation of Pd to Pd carbide is observed in the CH 4 treated Pd/Al 2O 3, but not in the CHClF 2 treated Pd/Al 2O 3 in which case Al 2O 3 was partially fluorinated. In the absence of hydrogen, the conversion of halon 1211 over Al 2O 3 and Pd/Al 2O 3 gives a similar product profile and the reactions follow a heterogeneous halogen exchange reaction pathway, which takes place on the positively charged aluminum ions. Introduction of hydrogen has no apparent effect on either halon 1211 conversion level or the product profile during the conversion of halon 1211 over Al 2O 3. Over supported Pd catalysts, major products are hydrogenated species which include CH 2F 2, CH 4, C 2H 6, C 3H 8, CH 3Br and CHF 3. The most striking feature of the hydrodehalogenation reactions is the increasing CH 2F 2 selectivity with time on stream, especially on Pd/Al 2O 3 and the CH 4 treated Pd/Al 2O 3. The changing selectivity during the catalytic hydrodehalogenation reactions is mainly ascribed to the interaction of support with Pd.