| Methanol to olefin(MTO)is the most promising non petrochemical technology to produce olefins by coal or natural gas instead of oil.SAPO-34 shows excellent catalytic performance in MTO,at present,SAPO-34 zeolite is one of the typical MTO catalysts.However,SAPO-34 has the problems of fast deactivation and short life.In order to prolong the life of the catalyst and improve the selectivity of ethylene and propylene,it is necessary to modify the catalyst.The results show that the acidity(distribution,strength,etc.)of zeolite can affect MTO activity,product selectivity and catalyst life.Therefore,adjusting the acidity of SAPO-34 zeolite is an effective modification method to improve the MTO reaction performance.The addition of heteroatoms to the zeolite framework will change the acid strength of the zeolite,and then affect the catalytic performance.The results show that the activity of MTO can be improved and the life of catalyst can be prolonged by substituting Zn into SAPO-34 zeolite framework.However,the experiment can not give a clear explanation for this phenomenon.In this paper,the density functional theory method including van der Waals interaction correction is used to calculate the change of Bronsted acid strength in Zn doped SAPO-34 zeolite.The mechanism of aromatics cycle and olefins cycle in Zn-SAPO-34 zeolite is studied systematically,and the effect of Zn-SAPO-34 zeolite on the selectivity of ethylene and propylene is discussed.The main results are as follows:For aromatics cycle,Firstly,the structure of the most stable polymethyl benzene was determined to be 1,2,4,5-tetratoluene.Taking 1,2,4,5-tetramethylbenzene as the initial hydrocarbon pool species,the aromatics circulation route in Zn-SAPO-34 was obtained,including the elementary reaction steps of methylation,deprotonation,methyl transfer and alkyl side chain cracking.The analysis of energy span model shows that the total Gibbs free energy barrier for the reaction of ethylene and propylene is 167 k J/mol.compared with the parallel reaction energy barrier for the formation of ethylene and propylene,it is found that ethylene(121 k J/mol)has better product selectivity than propylene(166 k J/mol).Taking 2,3-dimethyl-2-butene as the initial hydrocarbon pool species,the olefin cycle mechanism in Zn-SAPO-34 was studied.The results show that in the olefin cycle,the cracking of alkyl side chain is the key step to produce ethylene,Moreover,the methylation side chain growth in the second step is a key step in the production of propylene.The corresponding total Gibbs free energy barriers are 146 k J/mol and 89 k J/mol,which means that in the Zn-SAPO-34 zeolite the olefin circulation route is more conducive to the production of propylene.The comparison of aromatics and olefins in Zn-SAPO-34 zeolite shows that the latter has better reactivity than the former,which means that propylene has higher product selectivity than ethylene in Zn-SAPO-34 zeolite.However,the intermediates are complex and diverse in MTO.Aromatics cycle and olefin cycle do not exist independently,but are interrelated.Therefore,we have further studied the aromatization of the precursor(2,3,3,4-tetramethylpent-2-carbocation,M7)and the chain growth of propylene in the olefin cycle.M7 can generate aromatics through the reaction route of Deprotonation ? Hydrogen transfer ? Oligomerization ? Cyclization,and realize the conversion from olefin cycle to aromatics cycle.At the same time,propylene can further chain grow to produce high-carbon olefins,which to a certain extent consumes propylene and promotes the generation of ethylene.In addition,the amount of Bronsted acid in Zn-SAPO-34 zeolite is more than that in SAPO-34 zeolite,so that some macromolecular intermediates can be easily adsorbed on the weak acid site(Zn-O2(H)-Si)in the zeolite cage,thus hindering the diffusion of propylene.Our theoretical calculation can well clarify the role and correlation of the two circulation routes in the SAPO-34 zeolite modified by Zn,and provide theoretical basis for the experimental study. |
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