The Theoretical Study Of Effect Of Heteroatom On The Structure,Acidity Of Zeolites And MTO Conversion

Due to the unique pore systems and the the properties of adjustable solid acids of zeolite,it is widely used in various fields.Among them,methanol to olefins?MTO?is an important one carbon chemical reaction,and is also the most hopeful technology that substitute for the way of petroleum to olefins.The preparation and synthesis of catalysts with high activity and selectivity have become the focus of MTO research.Therefore,different contents of Zr doped zeolite,including sodalite?SOD?and CHA topology framework?Al PO-34 and H-SAPO-34?,are invesigated in this work.And the framework composition,the stability and acidity of zeolites were modified by doping Zr atoms.Then the influence of the different framework structural and acid strength of Zr modified zeolites on the activity and selectivity of MTO conversion were studied.The framework composition and acid properties of the catalyst with high activity and selectivity were confirmed.The results show that the incorporation of Zr into the zeolite framework lead to the stability enhance and the framework structure deformation due to the radius difference between the Si and Zr atom.The Br?nsted acidity of substituted zeolites strongly depends on the content of Zr.The Br?nsted acid strength of SOD zeolite with different Si/Al ratio decreases because of the incorporation of Zr,except for the low content of Zr substitution in SOD with lower Si/Al ratio.However,concerning the Zr-substituted forms of Al PO-34and H-SAPO-34 zeolites,the presence of Zr atoms substantially decreases at first and then increase the Br?nsted acidity of Zr-O?H?-Al acid site.The results in H-zeolite indicate that the more Zr atoms substantially increase the Br?nsted acidity of Si-O?H?-Al acid site.Compared with the MTO conversion on H-SAPO-34,the incorporation of Zr atom into Al PO-34 improves the overall activity of MTO conversion and the ethylene selectivity,but it has little effect on the selectivity to propylene.In addition,the catalytic activity of MTO conversion overand Alwith higher concentration of acid sites is higher.And the selectivity to ethylene is higher than that to propylene.In this paper,the structural stability and acidity of H-SAPO-34 modified by Fe,Co and Ni were also investigated,and the influence of different acid strength on the activity and selectivity of MTO conversion were also studied.The results show that the intrinsic acidity of H-SAPO-34 modified by Fe and Co increased,while that of H-SAPO-34 modified by Ni decreased in comparison with the acid strength of H-SAPO-34.And the apparent acidity of H-SAPO-34 modified by Fe increased,while that of H-SAPO-34 modified by Co and Ni decreased in comparison with the acid strength of H-SAPO-34.In this paper,the effect of apparent acid strength on catalytic activity and product selectivity of MTO conversion was analyzed and compared.The order for the activity of MTO conversion is Co APO-34>Fe APO-34>Ni APO-34.The selectivity of MTO conversion over Co APO-34 to propylene is better than that to ethylene,and the selectivity of MTO conversion over Fe APO-34 and Ni APO-34 to ethylene is better than that to propylene.In addition,the stability and acidity of H-ZSM-5modified by Zr were also investigated,and the influence of differenct location and acid strength of acid site on the activity and selectivity of MTO conversion were investigated.The results show that the incorporation of Zr into H-ZSM-5framework make the stability increased but acidity decreased,and the activity of MTO conversion over Zr modified H-ZSM-5 zeolites are higher than of H-ZSM-5-Al@T7/T9.The selectivity towards to ethylene are both improved by Zr incorpration into H-ZSM-5 framework.For the activity and selectivity of MTO converion,the diffusion of the related species is also an important factor determining the catalytic performance except for the thermodynamic and kinetic factors.Therefore,in this paper,the diffusion of CH₃OH,C₂H₄ and C₃H₆ in H-SAPO-34,Alandwith different acid strength under different loading and different temperatures were studied.The results of kinetic simulation show that the self-diffusion coefficients increase with temperature as the same species at the same adsorption capacity in the same zeolite.And ethylene molecules diffuse fastest in three zeolites with different acid strength.In addition,the diffustion activation barrier of CH₃OH is the minimum when the adsorption amount of CH₃OH is 64 over Alwith weaker acid strength.A small number of acid sites in zeolite is favorable for the diffusion of CH₃OH molecule,while many of acid sites is unfavorable for the diffusion of CH₃OH molecule.