Evolution And Catalytic Performance Of Active Species In Methane Dehydroaromatization

Methane is the main component of natural gas（≈95%）and one of the most abundant fossil resources on Earth.It is remains the cleanest among all fossil fuels.Natural gas will play an increasingly important role in the transition to a renewable chemical industry.In 1993,the Dalian Institute of Chemistry reported for the first time that methane could be dehydrogenated into aromatic hydrocarbons（MDA）under nonoxidative conditions,and many scientific research teams at home and abroad immediately carried out related research on this reaction.The products of the reaction are simple and easy to separate,and the C-H bond in CH₄ can be activated in a controlled manner to directionally produce aromatic mixtures.In this paper,a highly dispersed Fe-based ZSM-5 zeolite catalyst was prepared by metal ion exchange,and the placement of Fe phase on the zeolite and its evolution during the reaction were mainly studied.A series of Mo/ZSM-5 catalysts were prepared by impregnation method,and the effects of calcination temperature,loading and atmosphere on the distribution of Mo species and catalyst activity were investigated.And an effective way to slow down the catalyst deactivation was discussed by CO/H₂ and CH₄ co-feeding.（1）Fe/HZSM-5 catalysts prepared by impregnation method often have various Fe species phases,which brings trouble to correlate the catalytic activity and selectivity with the active species on the catalyst.The Fe/HZSM-5-IE catalyst prepared by the liquid-phase ion exchange method has a relatively single existence form of Fe species on the zeolite,which provides convenience for studying the metal active sites of the MDA reaction on the catalyst.The experimental results show that the Fe/HZSM-5-IE-S2 catalyst prepared by ion exchange exhibits excellent catalytic activity and high aromatics selectivity in the MDA reaction.At a relatively high CH₄ conversion of 16%,the maximum aromatics selectivity can reach about75～80%.Atomically dispersed Fe-oxo species anchored by framework aluminum on Fe/ZSM-5 catalysts prepared by Fe²⁺exchange were successfully identified as active sites for efficient conversion of methane to aromatic hydrocarbons using ⁵⁷Fe M?ssbauer.（2）Mo species acts as the active species for activating CH₄ in the MDA reaction,and the intermediate product is aromatized at the B acid position.The excellent spatial position between Mo species and B acid will have a certain impact on the activity of the catalyst.The Mo loading and calcination temperature of the catalyst were changed to control the distribution of Mo species on the zeolite catalyst,and the effect of Mo distribution on the catalytic activity was studied by means of an inert warming atmosphere/reducing warming gas.The results show that the loading is the key factor affecting the activity,and the calcination temperature will be affected by the atmosphere only at a certain loading,which will have a significant effect on the activity of the catalyst.A series of catalysts,1～5 wt%,showed significant differences in activity under different atmospheres.When the loading was increased to 7 wt%,the activity of the catalyst was not affected by the atmosphere.（3）Mo/ZSM-5 catalyst is the best catalyst for MDA reaction,but the severe carbon deposition during the reaction greatly reduces the service life of the catalyst（800℃,<3 h）.Strong oxidizing gases such as O₂ will cause certain damage to the structure of the catalyst while removing carbon deposits.However,CO/H₂,as a commonly used reducing gas,will remove part of the carbon deposits during the co-feed reaction with CH₄,which prolongs the life of the catalyst.The results of this study show that CO can stabilize the catalyst conversion rate,while H₂ has a removal effect on all carbon deposits.Through the study of Fe-based and Mo-based HZSM-5 zeolite catalysts in the MDA reaction,it is expected that the evolution of the metal active species of the catalysts in the reaction,the effect on the activation of C-H bonds of methane and the generation of aromatic products can be explored.which will provide a basis for the construction of high-performance catalytic system to improve methane conversion.