Catalytic Oxychlorination versus Oxybromination for Methane Functionalization

The catalytic oxyhalogenation is an attractive route for the functionalization of methane in a single step. This study investigates methane oxychlorination (MOC) and oxybromination (MOB) under a wide range of conditions over various materials having different oxidation properties to assess the effect of hydrogen halide (HX, X = Cl, Br) on the catalyst performance. The oxyhalogenation activity of the catalysts, ranked as RuO 2 > Cu–K–La–X > CeO 2 > VPO > TiO 2 > FePO 4, is correlated with their ability to oxidize the hydrogen halide and the gas-phase reactivity of the halogen with methane. The product distribution is found to be strongly dependent on the nature of the catalyst and the type of halogen. The least reducible FePO 4 exhibits a marked propensity to halomethanes (CH 3 X, CH 2 X 2 ), and the strongly oxidizing RuO 2 favors combustion in both reactions, while other systems reveal stark selectivity differences between MOC and MOB. VPO and TiO 2 lead to a selective CH 3 Br production in MOB and pronounced CO formation in MOC, whereby product distribution was only slightly affected by the variation of the HX concentration. In contrast, CeO 2 and Cu-based catalysts provide a high selectivity to CH 3 Cl but give rise to a marked CO 2 formation when HBr is used as a halogen source. The behavior of the latter systems is explained by the higher energy of the metal–Cl bond in comparison to the metal–Br bond, enabling more suppression of the unwanted CO and CO 2 formation when HCl is used, as also inferred from the more pronounced performance dependence on the HX content in the feed. Extrapolating this result, the highest reported yields of chloromethanes (28% at >82% selectivity) and bromomethanes (20% at >98% selectivity) are attained over CeO 2, by adjusting the feed HX content to curb the CO 2 generation. A vis-à-vis comparison of MOC and MOB presented for the first time in this study deepens the understanding of halogen-mediated methane functionalization as a key step toward the design of an oxyhalogenation process.