Direct Synthesis Of Hierarchical ZnZSM-5 Zeolite And Its Catalytic Properties In Methanol To Aromatics Reaction

Aromatics,especially light aromatics?benzene,toluene,and xylene?,are important raw materials in the organic chemicals and the polymer industry.In recent years,with the depletion of fossil fuel and the sharp increase of aromatic demand,traditional route such as catalytic cracking and catalytic reforming cannot meet the growing needs of aromatics.The methanol-to-aromatics?MTA?process is considered to be a promising method and has attracted both industrial and academic interests,as methanol can be abundantly available via syngas from various sources,such as biomass,natural gas and coal,and the aromatics produced from this method do not include phosphorus,sulfur and other harmful ingredients.ZSM-5 zeolite is the major catalyst for the MTA reaction.However,traditional ZSM-5 zeolite suffers low activity,low selectivity and short lifetime.Thus,developing suitable ZSM-5 catalysts with high activity,high aromatics selectivity and long lifetime has become a key of MTA technology development.In this work,firstly,ZnZSM-5 zeolite was directly synthesized by using silicalite-1?S-1?as seeds and with the addition of zinc nitrate in the synthesis.Its catalytic performance for MTA reaction was studied.The results showed that Zn can be successfully incorporated into the zeolite framework by the direct synthesis method.The addition of Zn species can reduce the crystal size and acidity of the zeolite.Moreover,the catalytic performance of the Zn-containing zeolite in the MTA reaction was apparently better than that of the conventional non-loaded ZSM-5 zeolite synthesized by the same method.Then,CTAB was used as the mesoporous structure-directing agent to synthesize hierarchical ZnZSM-5 zeolite.The effects of the CTAB amonts on the textural properties and catalytic performance were studied.The results indicated that the addition of CTAB can induce the generation of mesopores in the zeolite,reduce the primary crystal size of the zeolite and increase the specific surface area and mesopore volume.Compared with the microporous ZSM-5 zeolite prepared without addition of CTAB,the hierarchical ZnZSM-5 zeolite showed much longer lifetime.Finally,under the optimal CTAB/SiO₂ molar ratio?0.03?,the effects of Zn content on the textural properties of ZnZSM-5 and the catalytic performance were investigated.The results showed that the relative crystallinity,the primary crystal size and acidity of the zeolites gradually decreased with increasing the amounts of Zn,while the BET surface area,external surface area,total pore volume and mesopore volume first increased and then decreased.Among them,ZnZSM-5 zeolite with the ZnO/SiO₂ molar ratio of 0.015 had the largest specific surface area and mesoporous volume,and exhibited the best catalytic performance in the MTA reaction.