Catalyst design for natural-gas upgrading through oxybromination chemistry

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.

Selective Production of Carbon Monoxide via Methane Oxychlorination over Vanadyl Pyrophosphate

“…Recent reports demonstrate the high selectivity to halomethanes over LaOCl and CeO 2 in oxychlorination, and over CeO 2 and vanadyl pyrophosphate (VPO) in oxybromination . Moreover, the oxyhalogenation of methane over catalysts with mild oxidizing properties, such as LaOCl in oxychlorination and FePO 4 and VPO in oxybromination, led to CO as the dominant oxidation product, with only marginal CO 2 production . These findings hint at the extra potential of the route as an effective method to exploit the natural gas feedstock for on‐purpose production of CO, which is a key building block in the manufacture of numerous commodities .…”

“…The catalytic oxyhalogenation of methane, comprising its reaction with a hydrogen halide (HCl or HBr) and oxygen, is an attractive approach to satisfy these objectives as it enables direct methane functionalization under moderate operating conditions (ca. 1 bar, <853 K) . This route has traditionally targeted the production of methyl halides (CH 3 Cl and CH 3 Br), platform molecules equivalent to methanol that can be transformed into a wide spectrum of value‐added chemicals and fuels via a halogen elimination step (hydrogen halide is liberated).…”

“…This route has traditionally targeted the production of methyl halides (CH 3 Cl and CH 3 Br), platform molecules equivalent to methanol that can be transformed into a wide spectrum of value‐added chemicals and fuels via a halogen elimination step (hydrogen halide is liberated). Recent reports demonstrate the high selectivity to halomethanes over LaOCl and CeO 2 in oxychlorination, and over CeO 2 and vanadyl pyrophosphate (VPO) in oxybromination . Moreover, the oxyhalogenation of methane over catalysts with mild oxidizing properties, such as LaOCl in oxychlorination and FePO 4 and VPO in oxybromination, led to CO as the dominant oxidation product, with only marginal CO 2 production .…”

“…In this way, the highly endergonic steam reforming and coal gasification processes, commonly practiced today to obtain CO, could be substituted by an exergonic halogen‐mediated process. Nevertheless, this application of the oxyhalogenation reaction has not been considered to date; this is probably due to the fact that CO was never produced at a selectivity exceeding 50 %, which would necessitate a complex downstream separation train. Alternatively, a two‐step process can be sought, involving selective methane oxyhalogenation over one catalyst, followed by oxidation/hydrolysis into CO over a second catalyst .…”

Molten salt chemical looping for reactive separation of HBr in a halogen-based natural gas conversion process

“…A particular challenge is the separation of HBr from light alkanes and heat management of the exothermic HBr oxidation [12]. Modifications to the process employing oxybromination have faced similar challenges [13][14][15][16]. A continuous process approach to separation of HBr, and regeneration of bromine using a regenerable solid metal oxide in a switched-bed zone reactor, has been proposed [17].…”