Synthesis Of SAPO-34 Molecular Sieve And Its Catalytic Properties In Methanol To Olefins Reaction

Light olefins,especially ethylene and propylene,play an important role in the modern chemical industry.At present,ethylene and propylene are obtained primarily from the cracking of naphtha and consume large amount of crude oil.Owing to the decrease of oil reserves,methanol to olefins(MTO)reaction has received increasing attention,which is an alternative way to produce light olefins from non-petroleum based routes.SAPO-34 molecular sieve becomes the best catalyst for MTO reaction due to its excellent light olefin selectivity.However,the narrow microporous structure of SAPO-34 molecular sieve would result in coke formation,thus lead to the rapid deactivation of SAPO-34 molecular sieve.Hierarchical or nano-sized SAPO-34 molecular sieve could decrease the diffusion paths of the products and supress the formation of coke species,which could increase the lifetime of the catalyst.As a result,the preparetion of hierarchical or nano-sized SAPO-34 molecular sieve become a hot spot in research.In order to improve the catalytic performance of SAPO-34 molecular sieve in MTO reaction,hierarchical SAPO-34 molecular sieve,hierarchical SAPO-34 molecular sieve and nano-sized SAPO-34 molecular sieve were prepared in this work.The results obtained in this dissertation are as follows.Firstly,hierarchical SAPO-34 molecular sieve were hydrothermally prepared with the assistance of N-[3-(Trimethoxysilyl)propyl]ethylenediamine(TPED)as the mesoporous template,during which morpholine was used as the microporous template.The influence of TPED on the morphologies,textural properties and acidic properties of the resultant hierarchical SAPO-34 molecular sieve were investigeted.Different from the conventional SAPO-34 molecular sieve with cubic-like morphology,the hierarchical SAPO-34 molecular sieve is the aggregation of nanocrystals with the crystal size ranging from 200–500 nm.The hierarchical SAPO-34 molecular sieve possess higher external surface areas and lower concentration and weaker strength in strong acid sites than the conventional SAPO-34 molecular sieve.In addition,the formation process of the hierarchical SAPO-34 molecular sieve was investigeted,indicating that the addition of TPED is crucial for the formation of Hierarchical SAPO-34 molecular sieve.Hierarchical SAPO-34 molecular sieve exhibit a significant improvement in MTO reactions due to the improvement of the diffusion property and mild acidic properties of the catalyst.Secondly,hierarchical plate-like SAPO-18/34 intergrowth and SAPO-34 molecular sieve were synthesized by adding SAPO-34 seed crystals to a SAPO-18 precursor in an aminothermal system,in which triethylamine was used as both the solvent and the template.The influence of the seeds on the phase,morphologies,textural properties and acidic properties of the resultant products were investigeted.The result indicate that as the amount of SAPO-34 seed crystals increased,the phase of the products changed from SAPO-18 molecular sieve to SAPO-18/34 intergrowth and then to pure-phase SAPO-34 molecular sieve.This is owing to the structure-directing activity of SAPO-34 seeds,which could produce more SAPO-34.In addition,the formation process of the hierarchical plate-like samples was investigeted.The result reveal that the stacking of nanoplate-like particles result in the formation of hierarchical plate-like samples.The hierarchical plate-like SAPO-18/34 intergrowth and SAPO-34 molecular sieve had higher external surface areas and higher concentration and stronger strength in strong acid sites than the single-phase SAPO-18 molecular sieve.The SAPO-18/34 intergrowth and SAPO-34 molecular sieve with the hierarchical plate-like structure exhibted superior catalytic properties in MTO reaction due to enhanced diffusion efficiency.Thirdly,uniformly nano-sized SAPO-34 molecular sieve were synthesized using morpholine treated micrometer-sized SAPO-34 seeds.The influence of the amount of morpholine treated micrometer-sized SAPO-34 seeds added and treatment time on the crystal size,textural properties and acidic properties of the resultant hierarchical SAPO-34 molecular sieve were both investigeted.Using 4% morpholine-treated micrometersized SAPO-34 seeds decreased the crystal size of the SAPO-34 molecular sieve to200–500 nm.Meanwhile,when the micrometer-sized seeds were treated in morpholine solution for 30 h,the micrometer-sized seeds were broken into nanoparticles.In addition,the samples prepared with morpholine treated micrometer-sized SAPO-34 seeds had larger external surface areas and lower concentration and weaker strength of the strong acid sites than conventional SAPO-34 molecular sieve prepared without morpholine treated micrometer-sized SAPO-34 seeds.The nano-sized SAPO-34 molecular sieve exhibited excellent catalytic performance in MTO reaction owing to the shortened transportation path and appropriate acidic properties of the catalyst.