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

Abstract

Development of catalysts that could surpass the activity and selectivity constraints of the non-catalytic radical-mediated halogenation of methane constitutes a long-standing challenge, which exhibits great potential to valorise this readily available resource for the production of commodities. This study presents comprehensive performance maps of a large library of materials, comprising carriers (quartz, SiO₂, SiC, α-Al₂O₃, γ-Al₂O₃ and carbon), noble metals (Pt, Pd and Ru), metal oxides (Fe₂O₃ and CeO₂), chlorides (PdCl₂ and CuCl₂) and oxyfluorides (TaOF₃) supported on SiO₂, γ-Al₂O₃, carbon or H-ZSM-5 carriers, sulfated systems (S-ZrO₂, S-ZrO₂-SBA-15, S-TiO₂, S-Nb₂O₅, S-Ta₂O₅ and Nafion) and zeolites (3A, H-USY, H-MOR, H-SAPO-34, H-BETA and H-ZSM-5), in the chlorination and bromination of methane under practically relevant conditions and gains insights into the nature of the catalytic effects as a function of the catalyst and halogen of choice. The chlorination activity of different catalyst beds at low temperatures (473–523 K) was 2–5.5 times higher compared to that of the empty reactor of identical volume, while the bromination rate was almost unaffected by the solids in the whole temperature range (643–723 K). Except for zeolites and Pt/carbon, which promoted polyhalogenation, selectivities to halomethanes over most of the catalysts were similar to those in the non-catalytic reactions and were higher in bromination (S_CH3Br = 80–95% versus S_CH3Cl = 52–90% at X_CH4 = 5–18%). The formation of carbon oxides (S_COx = 2–28%) over several materials in chlorination and virtually all systems in bromination implied the decomposition of halomethanes, which at higher temperatures led to coking, particularly in the latter reaction. The kinetic fingerprints along with the marginal impact of the Si : Al ratio, counter ions and extraframework aluminium species on the performance of the most active H-ZSM-5 catalyst indicated that methane chlorination over various materials is governed by the radical-chain mechanism, which limits the scope for breaking the selectivity–conversion relationships by tailoring the catalyst acidity. Nonetheless, the enhancement of chlorination activity over zeolites that followed a volcano-like dependence on their micropore size coupled with a more significant impact of the intracrystalline mesoporosity and crystallite size on the product distribution revealed the important role of confinement effects in this reaction, which may pave the way for advancements in the production of chloromethanes.