No abstract

Natural gas contains large volumes of light alkanes, and its abundant reserves make it an appealing feedstock for value-added chemicals and fuels. However, selectively activating the C-H bonds in these useful hydrocarbons is one of the greatest challenges in catalysis. Here we report an attractive oxybromination method for the one-step functionalization of methane under mild conditions that integrates gas-phase alkane bromination with heterogeneously catalysed HBr oxidation, a step that is usually executed separately. Catalyst-design strategies to provide optimal synergy between these two processes are discussed. Among many investigated material families, vanadium phosphate (VPO) is identified as the best oxybromination catalyst, as it provides selectivity for CH3Br up to 95% and stable operation for over 100 hours on stream. The outstanding performance of VPO is rationalized by its high activity in HBr oxidation and low propensity for methane and bromomethane oxidation. Data on the oxybromination of ethane and propane over VPO suggest that the reaction network for higher alkanes is more complex.

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) in a wide range of conditions over various materials having different oxidation properties to assess the impact of hydrogen halide (HX, 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 the 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. On contrary, CeO2 and Cu-based catalyst provide a high selectivity to CH3Cl, 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 compared to the metal-Br, 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 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 CeO2, by adjusting the feed HX content to curb the CO2 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 towards the design of an oxyhalogenation process.