Preparation of hierarchical SAPO-34 molecular sieve directed by defect crystal and its application in methanol to olefin
In order to prepare methanol to olefin(MTO)catalysts with higher performance and achieve efficient conversion of methanol to low-carbon olefins,crystal with defect sites was prepared by post-treatment of conventional SAPO-34 molecular sieve.And then hierarchical SAPO-34 molecular sieve was successfully prepared in the hydrothermal synthesis system.The crystal structures,morphologies,pores and acidities of the prepared SAPO-34 molecular sieve were characterized by X-ray diffraction(XRD),scanning electron microscope(SEM),N2 adsorption/desorption,mercury intrusion porosimetry and NH3 temperature programmed desorption(NH3-TPD).The catalytic activity of the prepared SAPO-34 molecular sieve for MTO reaction was carried out in a fixed bed reactor(reaction temperature of 460℃,atmospheric pressure,mass space velocity of methanol of 3.0 h-1 and raw material of pure methanol).The results indicate that the hierarchical SAPO-34 molecular sieve has a three-level pore structure of macropore,mesopore and micropore.The presence of macro/mesopores can,on the one hand,make the target products diffuse from the pore quickly,reducing the probability of secondary reactions,and on the other hand,make it easier for heavy hydrocarbon molecules to diffuse to the active sites for further reactions.Therefore,in the MTO reaction,the hierarchical SAPO-34 molecular sieve shows the highest selectivity of ethylene and propylene,which is 85.3%,and lower selectivity of C4 and C5+(hydrocarbons with carbon numbers of four and carbon numbers greater than or equal to five)and can maintain stable performance within 135 min.At the same time,after multiple reactions and regeneration,the catalyst can still maintain excellent catalytic performance.
defect crystalmolecular sievehierarchical SAPO-34methanol to olefin
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