Zinc/Gallium Modified ZSM-5 Catalyst For Methanol To Aromatics

Metal-modified ZSM-5 catalysts currently cannot combine high aromatic selectivity with long reaction life well in methanol to aromatics reaction(MTA).Loading metal with high dehydrogenation activity can improve the selectivity of aromatics but reduce the stability of reaction,which is not conducive to large-scale industrial production.Therefore,it is very important to adjust the acidity and pore structure of metal-modified catalysts to prolong the reaction life in MTA.In this paper,Zn、Ga modified ZSM-5 samples with high Si/Al ratio were prepared to investigate the effects of metal type,content,state and acidity on catalytic performance in MTA.Hierarchical and microporous ZSM-5 were loaded with Zn to study the effect of pore structure on catalytic performance of MTA.In-situ synthesis Zn and Ga modified ZSM-5 through ethylenediamine complexed metal ions have similar pore structure of ZSM-5,but great difference on acidic properties.With Zn content increased,initial aromatics selectivity of Zn-ZSM-5 decreased and relative content of Zn(OH)~+first decreased and then increased.The relative content of non-framework Ga in Ga-ZSM-5 is proportional to aromatics selectivity,which is more beneficial to improve MTA reactivity than other Ga species.Both Zn and Ga can significantly improve the aromatics selectivity of ZSM-5 at the same time samples have high stability.The reaction life of 2wt%Zn-ZSM-5 sample was 228 h,and the methanol conversion of 0.5wt%Ga-ZSM-5 could keep100%within 380 h.The aromatics selectivity of Zn/ZSM-5 prepared by impregnation is more than 3 times that of ZSM-5,the aromatics selectivity of Ga/ZSM-5 is 1.5～2 times that of the ZSM-5 sample,and1.0Zn-1.0Ga/ZSM-5 has the highest aromatics selectivity.It was found that Zn-Ga/ZSM-5 with a suitable metal amount had higher aromatics selectivity than Zn/ZSM-5 and Ga/ZSM-5.Moreover,the reaction stability of Zn-Ga/ZSM-5 was between Zn/ZSM-5 and Ga/ZSM-5.The reaction life of Zn-impregnated samples was significantly shorter than that of in-situ synthesis,but the aromatics selectivity was significantly increased;the aromatics selectivity of the Ga-impregnated samples was slightly higher than that of the in-situ synthesis.Ga modification is more conducive to the formation of light aromatics such as benzene and toluene,while Zn modification generates more heavy aromatics such as trimethylbenzene.There are more high-carbon olefins in the reaction product of Ga,while more low-carbon olefins produced by Zn.Therefore,Ga is more favorable for the stability of MTA reaction than Zn.Hierarchical Zn/ZSM-5 have longer reaction life than microporous Zn/ZSM-5,which are more favorable for the formation of light aromatics to improve the stability of the reaction.