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A dual system comprising two catalytic reactors connected in series was developed for the direct conversion of ethene to vinyl chloride monomer (VCM). The first reactor uses ZrO2-supported ceria (CeO2/ZrO2) to perform ethene oxychlorination to 1,2-dichloroethene (EDC) that is dehydrochlorinated to VCM in the second reactor over calcium-promoted γ-Al2O3. The choice of carrier for ceria is of critical importance to maximize the EDC production by reducing combustion products. While MgO, SiO2, SiC, TiO2, ZSM-5, and γ-Al2O3 carriers induced higher overoxidation compared to bulk ceria, ZrO2 was the only carrier that suppressed COx formation. Moreover, the latter carrier led to the highest oxychlorination activity. The unique performance of the CeO2/ZrO2 catalyst was rationalized by its ability to promote chlorine evolution and to suppress the combustion of chlorinated products, as inferred from the activity evaluation in HCl and VCM oxidation, respectively. The outstanding redox properties, enabling operation at low temperature and thus high selectivity, are associated with the formation of defective CeO2 nanoparticles, contrasting the low activity over Ce–Zr mixed oxide. In order to subsequently form VCM, an efficient EDC dehydrochlorination catalyst was designed by moderating the acidity of γ-Al2O3via calcium doping and used in a reactor after CeO2/ZrO2. This dual catalyst system displayed 100% selectivity to VCM at 25% ethene conversion, surpassing the space time yield of the best ethene-to-VCM catalyst EuOCl by a factor of four, where the first step is operated at an elevated temperature of about 100–150 K with respect to cupric chloride benchmarks. In addition, the catalytic dehydrochlorination, operated at a lower temperature of 100 K than the current non-catalytic process, showed minimized coke formation. This developed system rendered stable after slight initial deactivation, offering promising potential to intensify VCM production.

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 (SCH3Br = 80–95% versus SCH3Cl = 52–90% at XCH4 = 5–18%). The formation of carbon oxides (SCOx = 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., Catalysis Science & Technology, 9 (17), ISSN:2044-4753, ISSN:2044-4761

Catalysed alkane oxyhalogenation has recently been demonstrated as an attractive route for the selective functionalisation of natural gas into commodity chemicals such as alkyl halides or olefins. Herein, we assess the role of the hydrogen halide on the product distribution in ethane and propane oxyhalogenation over europium oxychloride and europium oxybromide catalysts. While olefins are the main reaction products in oxychlorination (selectivity ≥95%), the formation of alkyl bromides (selectivity ≤95%) is favoured in oxybromination, particularly at low conversion levels. At high conversion levels, olefins are also observed, although the selectivity is generally limited (≤60%) due to the formation of cracking and combustion products. A comprehensive kinetic analysis of the oxyhalogenation, halogenation, and oxidative dehydrogenation of the alkanes as well as of the dehydrohalogenation of the alkyl halides provide insights on the complex reaction network, which proceeds through alkyl halide formation followed by its conversion to olefins and/or halocarbons. The observed selectivity differences primarily originate from (i) the halide-dependent alkane activation path, which is surface-driven in oxychlorination, while it mostly occurs via gas-phase halogenation in oxybromination, along with (ii) the preferred tendency of alkyl chlorides towards catalytic dehydrochlorination pathways opposing alkyl bromides that undergo further bromination routes in the gas phase. Finally, detailed characterisation corroborated the above findings and revealed the higher structural stability of europium oxychloride compared to the analogous oxybromide phase under oxyhalogenation conditions. These results deepen the understanding of the alkane oxyhalogenation chemistry on catalyst surfaces for the manufacture of valuable commodities.