Halogen type as a selectivity switch in catalysed alkane oxyhalogenation†
Catalysed alkane oxyhalogenation has recently been demonstrated as an attractive route for the selective functionalisation of natural gas into commodity chemicals such as alkyl halides or olefins. Herein, we assess the role of the hydrogen halide on the product distribution in ethane and propane oxyhalogenation over europium oxychloride and europium oxybromide catalysts. While olefins are the main reaction products in oxychlorination (selectivity ≥95%), the formation of alkyl bromides (selectivity ≤95%) is favoured in oxybromination, particularly at low conversion levels. At high conversion levels, olefins are also observed, although the selectivity is generally limited (≤60%) due to the formation of cracking and combustion products. A comprehensive kinetic analysis of the oxyhalogenation, halogenation, and oxidative dehydrogenation of the alkanes as well as of the dehydrohalogenation of the alkyl halides provide insights on the complex reaction network, which proceeds through alkyl halide formation followed by its conversion to olefins and/or halocarbons. The observed selectivity differences primarily originate from (i) the halide-dependent alkane activation path, which is surface-driven in oxychlorination, while it mostly occurs via gas-phase halogenation in oxybromination, along with (ii) the preferred tendency of alkyl chlorides towards catalytic dehydrochlorination pathways opposing alkyl bromides that undergo further bromination routes in the gas phase. Finally, detailed characterisation corroborated the above findings and revealed the higher structural stability of europium oxychloride compared to the analogous oxybromide phase under oxyhalogenation conditions. These results deepen the understanding of the alkane oxyhalogenation chemistry on catalyst surfaces for the manufacture of valuable commodities.