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 (FIX, X = Cl, Br) on the catalyst performance. The oxyhalogenation activity of the catalysts, ranked as RuO2 > Cu-K-La-X > CeO2 > VPO > TiO2 > FePO4, 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 FePO4 exhibits a marked propensity to halomethanes (CH3X, CH2X2), and the strongly oxidizing RuO2 favors combustion in both reactions, while other systems reveal stark selectivity differences between MOC and MOB. VPO and TiO2 lead to a selective CH3Br 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, CeO2 and Cu-based catalysts provide a high selectivity to CH3CI but give rise to a marked CO2 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 CO2 formation when HCl is used, as also inferred from the more pronounced performance dependence on the FIX 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 CeO2, by adjusting the feed FIX content to curb the CO2 generation. A vis-a-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.