Conversion of halon 1211 (CBrClF2) over supported Pd catalysts

Conversion of halon 1211 has been studied over γ-Al2O3 and supported 0.5% Pd catalysts (Pd/Al2O3, Pd/fluorinated Al2O3, Pd/AlF3, Pd/Al2O3 pretreated with CH4 and CHClF2). 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 CH4 treated Pd/Al2O3, but not in the CHClF2 treated Pd/Al2O3 in which case Al2O3 was partially fluorinated. In the absence of hydrogen, the conversion of halon 1211 over Al2O3 and Pd/Al2O3 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 Al2O3. Over supported Pd catalysts, major products are hydrogenated species which include CH2F2, CH4, C2H6, C3H8, CH3Br and CHF3. The most striking feature of the hydrodehalogenation reactions is the increasing CH2F2 selectivity with time on stream, especially on Pd/Al2O3 and the CH4 treated Pd/Al2O3. The changing selectivity during the catalytic hydrodehalogenation reactions is mainly ascribed to the interaction of support with Pd.