Direct Methane Conversion to Olefins and Aromatics Under Non-Oxidative Condition

With the discovery of new large methane reserves (e.g. shale gas and methane hydrate), the direct conversion of methane into high value-added fuels and chemicals such as light olefins and aromatics, has ignited interest worldwide. ¹ Significant progress has been made in the development of several catalytic processes, such as oxidative coupling of methane (OCM),^2,3 methane dehydroaromatization (MDA)^5-10. We previously reported that iron single sites embedded within the lattice of silica are active in catalyzing methane conversion to olefins, aromatics and hydrogen (MTOAH) under non-oxidative conditions.¹¹ Characterization showed that the reaction involves catalytic activation of methane generating methyl radicals, which subsequently go through gas phase C-C coupling forming ethylene, aromatics and hydrogen as the products.¹¹ DFT calculations suggested that the H· free radicals may also play an important role in the selective reaction pathway of the MTOAH reaction.¹¹ Therefore, in this study, we further investigate the role of methyl radicals and hydrogen radicals. Furthermore, the strong endothermal reaction causes a steep temperature gradient in the catalyst bed along the radial direction. In addition, the gas phase reaction is also affected by the space of the reactor due to the lifetime of radicals. Thus, we explore the type of reactors containing catalysts directly on their walls, by coating a layer of catalysts onto the inner wall of the reactors including quartz and SiC-based reactors.

A catalytically active layer of around 15-20 μm thick SiO₂ containing the iron single sites with a loading in a range of 0.16-0.34 wt.% was coated on the inner wall of a quartz reactor with an inner diameter (id) of 14 mm, outer diameter (od) of 16 mm, and length of 100 mm. It gives a methane conversion of 7% at 1273 K and the conversion increases to 17.5% at 1323 K and a flow rate of 60 ml/min. The conversion and product selectivities can be tuned by changing the temperatures and flow rate. Interestingly, by providing hydrogen free radicals, generated by thermal decomposition of tetrahydronaphthalene (THN) or benzene, the MTOAH reaction is enhanced and methane conversion increases to 25.5% without changing much the products’ distribution. The involvement of hydrogen in the activation of methane is verified by the isotope experiments.

The same strategy can be scaled up to a catalytic reactor with id = 15.8 mm, od = 20 mm, and isothermal length as long as 700 mm. At a flow rate of 3800 ml/min of 50%H₂/45%CH₄/5%N₂ and 1463 K, methane conversion is 23.6%, which can be increased beyond 30% with a C₂ selectivity of 66.8% and and BTX 22.2% without coke formation in the presence of hydrogen radicals.

In order to meet the requirement of applications, we also develop a new type of reactor based on SiC containing iron sites in the reactor wall with a loading of 0.37wt%. Methane conversion reaches about 30% at 1423 K with the feeding of 50%H₂/45%CH₄/5%N₂ at a flow rate of 1700 ml/min. A 700 h test showed a rather good stability.

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