Abstract
AbstractMethane dehydroaromatization (MDA) over Mo/HZSM‐5 has been hypothesized in literature to proceed via a two‐step mechanism: methane is first converted to ethylene on the molybdenum (Mo) functionality and then ethylene is oligomerized, cyclized and dehydrogenated on the Brønsted acid sites (BAS) of the HZSM‐5 support. This hypothesis is tested by studying the conversion of ethylene at the same conditions as used for MDA, namely 700 °C, atmospheric pressure, and by co‐feeding experiments with H2 and CH4. Our results suggest that ethylene is not the main intermediate for MDA, because the aromatic selectivities obtained from methane conversion are higher than selectivities measured during ethylene conversion. Furthermore, carbonaceous deposits formed during MDA have a lower density, are more hydrogenated and more active than the ones formed during ethylene aromatization (EDA). Similarly as for MDA, an activation period in which Mo carburizes to its active phase and an induction period, in which aromatics formation rates increase to their maximum are observed for ethylene conversion. The induction period, which was explained by the buildup of a hydrocarbon pool (HCP) is much faster with methane than with ethylene. This period, is attributed to a slow buildup of hydrocarbons, strongly adsorbed on Mo sites, because it is only observed with catalysts containing Mo. Hydrogen co‐feeding with ethylene leads to the formation of more reactive coke species and a significantly prolonged lifetime of the catalyst, but not to a faster buildup of the HCP.
Highlights
Aromatics, such as benzene, toluene and xylene (BTX) are in demand, because they enhance the octane number of fuels and are used as feedstock for the fine-chemicals industry, commodity goods, plastics and medicine
Methane dehydroaromatization (MDA) over Mo/HZSM-5 has been hypothesized in literature to proceed via a two-step mechanism: methane is first converted to ethylene on the molybdenum (Mo) functionality and ethylene is oligomerized, cyclized and dehydrogenated on the Brønsted acid sites (BAS) of the HZSM-5 support
Since 100 mol% conversion of ethylene is achieved under the applied conditions it can be assumed that the concentration of ethylene throughout MDA at any given point is rather low, if ethylene were to be the main intermediate
Summary
Methane first converts to C2 hydrocarbons on the Mo site of the catalyst, which get further dehydrogenated and aromatized on the Brønsted acid sites (BASs) of the zeolite support Both ethylene and acetylene have been proposed as likely intermediates for this mechanism.[3] Mériaudeau et al investigated whether ethylene or acetylene is a more likely intermediate, comparing their reactivity at MDA conditions of 700 oC and atmospheric pressure with methane conversion.[4] They found that acetylene led to higher benzene formation rates, likely due to its higher reactivity. While methane cannot be activated directly on BASs, ethylene is activated on the BASs of the zeolite even at temperatures as low as 200 oC[5], but aromatics are formed at higher temperatures.[6] At these high temperatures, coking becomes significant leading to a fast drop-off in activity This can be mitigated to some extent by introducing a metal like Ga to the zeolite. The activation of Mo/HZSM-5 in ethylene, methane, in an ethylene/H2 mixture and in CO are compared to separately study the activation and induction period and to prevent coke formation during the activation period
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