Selective Methane Functionalization via Oxyhalogenation over Supported Noble Metal Nanoparticles

Abstract

This article investigates SiO2-supported Ru-, Pt-, Ir-, Rh-, and Pd-based catalysts (1 wt % metal loading) as new catalytic systems for the oxychlorination and oxybromination of methane, both pivotal steps in the halogen-mediated production of commodities from natural gas. Furthermore, this article provides insights into the structure–performance relationships and mechanism of these reactions as a function of the metal and the hydrogen halide. In-depth characterization of the equilibrated catalysts by X-ray diffraction, electron microscopy, Raman, and X-ray photoelectron spectroscopies demonstrate a fast restructuring of the starting oxide nanoparticles into metallic, metal silicide, or metal halide phases. The oxychlorination activity, which decreases in the order: Ru/SiO2 > Pt/SiO2 > Ir/SiO2 > Rh/SiO2 ≈ Pd/SiO2, is enhanced in the presence of metal oxides, while the activity order in oxybromination: Ru/SiO2 ≈ Ir/SiO2 ≈ Pd/SiO2 > Pt/SiO2 > Rh/SiO2 is less dependent on the phase composition. The highest selectivity to chloromethanes (78–83%) and bromomethanes (92–98.5%) at moderate methane conversion (20%), rivaling the performance of the best oxyhalogenation catalysts, is attained over Ir/SiO2, Rh/SiO2, and Pd/SiO2, and correlates with their ability to reduce and form metal halides. Catalyst propensity toward halogenation depends on the halide type, although it is less pronounced at higher loadings (up to 5 wt %), while supporting over other inert carriers has a marginal impact on the restructuring patterns. Finally, kinetic analysis coupled with detection of radical intermediates by operando photoelectron photoion coincidence spectroscopy indicates a significant role of gas-phase halogenation in methane activation via oxyhalogenation. In oxychlorination, the latter pathway has a similar contribution as surface activation for Pd/SiO2, Rh/SiO2, and Pt/SiO2, and major contribution for Ir/SiO2 and Ru/SiO2 catalysts, while in oxybromination, it dominates for all the systems, limiting the potential to enhance the selectivity to mono- over dihalomethanes.

Catalytic halogenation of methane: a dream reaction with practical scope?

“…[15][16][17][18][19] The implementation of this technology is contingent on the full recycling of the hydrogen halide (HX) formed in the halogenation and coupling steps and the ability to maximize the productivity of CH 3 X over polyhalomethanes and its coupling efficiency. 1,20 The recent decade demonstrated significant progress in HX recovery via cataloreactant systems, 21,22 electrochemical and thermochemical HX oxidation, [23][24][25][26] and methane oxyhalogenation, [26][27][28] as well as CH 3 X coupling, [16][17][18][19] which brought this technology closer than ever to practical implementation. In contrast, the selective monohalogenation was tackled in a much less depth.…”

A Review of Methane Activation Reactions by Halogenation: Catalysis, Mechanism, Kinetics, Modeling, and Reactors

“…Catalyst design for oxyhalogenation of methane and catalyst structure-performance relationships were also investigated by Paunović et al [49][50][51]. It was pointed out that the catalyst for oxybromination should be able to oxidize HBr on the surface, but at the same time it should not activate the C-H bond in methane and hydrocarbon products.…”

“…Their loss of activity was shown to be a consequence of LaOBr particle sticking in the former, and to the volatilization of free V2O5 in the latter catalyst [50]. Noble metals (Pd, Rh, Ir, Ru, Pt) supported on SiO2 also showed a good potential for methane oxybromination and oxychlorination [51]. The nominal metal loading of the prepared catalysts was 1 wt%.…”

“…When the Pd catalysts that were used for the oxybromination reaction were analyzed, the differences in XRD diffractograms of materials with a higher Pd loading were observed compared to the 1 wt% Pd/SiO2 catalyst. At higher Noble metals (Pd, Rh, Ir, Ru, Pt) supported on SiO 2 also showed a good potential for methane oxybromination and oxychlorination [51]. The nominal metal loading of the prepared catalysts was 1 wt%.…”

“…In the latter case it was estimated that at least half of the methane conversion rate can be attributed to the gas phase reactions. However, the formation of C-X (where X is Br or Cl) on the catalyst active sites was not undoubtedly excluded [51].…”