The Catalytic Performance For Methane Dehydro-aromatization Over Mo/HZSM-5 Catalysts

The catalytic dehydroaromatization of methane to aromatics and hydrogen is ofgreat scientific importance and industrial interest in the direct conversion of naturalgas. Up to now, Mo/HZSM-5 has been regarded as the most promising catalyst sincenearly equilibrium conversion of CH₄ under the typical reaction conditions (973K)was achieved with high selectivity to aromatics. However, it was also observed thatmethane conversion decreased drastically with time-on-stream due to the heavyformation of carbonaceous deposits on the catalyst, which becomes a serious barrierfor the industrial application of the reaction. Therefore, the exploitation of theMo/HZSM-5 catalysts with high capacity of carbonaceous deposit and highcoke-resistance has been intensively drawn attention. In the present thesis, severalrestructuring methods have been introduced to modify the acidity and reconstruct thetexture properties of commercial HZSM-5 zeolites. The experimental results weresummarized as follows:1. It was found that the catalytic performance of Mo/HZSM-5 catalyst in methanedehydroaromatization reaction (MDA) has close relationship with the acidity of theHZSM-5 zeolite. After the N₂ thermal treatment, the framework compositions andacidity of the ZSM-5 zeolite would be significantly altered with the treatmenttemperatures. At 823 K, the N₂ thermal treatment induced a mild dealumination of theparent HZSM-5 and as a result, destroyed the origin strong Bronsted acid whileretained sufficient weak acidic sites. This treatment not only effectively restrains theformation of carbonaceous deposits on the catalyst but also satisfies the requirementof the aromatization of the intermediates during the MDA reaction, resulting in aremarkable improvement in the catalytic durability and the selectivity to aromatics.2. On parent HZSM-5 zeolite, the existence of some Bronsted acid sites with strongacidic strength promoted the formation of coke deposited on its corresponding Mo/HZSM-5 catalyst. At 823 K, the streaming treatment induced a milddealumination of the zeolite and as a result, destroyed the origin strong Bronstedacidic sites while kept a higher crystallinity. Accordingly, a remarkable improvementin catalytic durability and selectivity to aromatics was achieved on the moderateddealuminated Mo/HZSM-5 catalyst compared with the conventional Mo-basedcatalysts, mainly due to effective suppression of coke formation during thearomatization reaction.3. It was found that the catalytic performance of Mo/HZSM-5 catalyst in methanedehydroaromatization (MDA) has strongly dependent on the dispersion of Mo specieson the HZSM-5 zeolite. On the mechanically mixed Mo/HZSM-5 catalyst, the Mospecies mainly existed as MoO₃ crystallites on the external surface of the zeolite.Without protection by the shape-selective environment within zeolite channels, theMo crystallites promoted the coke formation during the MDA reaction. Aftersubjected to the streaming treatment, the dispersion of the loaded Mo speciespronouncedly increased, and some Mo species with small particle size migrated intothe zeolite channels and reacted with the Bronsted acid sites to form (Mo₂O₅)²⁺dimers. Thus, a remarkable improvement in catalytic durability and selectivity toaromatics was achieved on the streaming-treated Mo/HZSM-5 catalyst compared withthe mechanically mixed catalysts, mainly due to effective suppression of cokeformation during the MDA reaction.