| ZSM-5 zeolite,with excellent hydrothermal stability and rich solid acidity,has been widely applied in catalytic reactions such as aromatization and alkylation.However,as for traditional ZSM-5,the microporous structure pore size limits the contact between the macromolecule and active sites,inducing the deactivation of the catalyst's carbon deposits.Currently the research on hierarchical ZSM-5 zeolites containing mesopores or macropores has become a hot spot due to the optimization of the above issue to some extent.However,most of above starategies are synthesized in the presence of organic template agents and mesoporous directing agents,with several problems to be solved urgently.Hence,this paper takes the green synthetic hierarchical ZSM-5 zeolite without any mesoporous directing agent as the ultimate goal,utilizing the seed-directed method and one-step synthesis method to synthesize ZSM-5 aggregates and hollow ZSM-5 single crystals,respectively.The details are as follows:?1?Firstly,80 nm silicalite-1 seed crystals were prepared by using silica sol as the silicon source and tetrapropylammonium hydroxide?TPAOH?as the template.After that we got the ZSM-5 aggregates with“hedgehog”morphology in hydrothermal system based on silica sol and sodium metaaluminate.Moreover,we systematically investigated the effects of the amount of seeds and the synthesis conditions that determined the crystal morphology and structure.The results suggest that:The single aggregate assembled from 5090 nm nanoparticles was 5001200 nm,which the amount of seeds was only 0.5%2%of the mass of the liquor SiO2.In addition,we found that increasing the amount of seeds added and reducing the amount of water can decrease the particle size of aggregate.Interestingly,the ratio of SiO2/Al2O3 and alkali metal salt could play an important role in modifying morphology of product:the"coffin"zeolites with smooth surface were obtained by multipling the ratio of SiO2/Al2O3;while the auxiliary substance KH2PO4 and NaH2PO4 changed the concentration of liquor ions,resulting in the formation of zeolite with"nanorod"shape.Finally,deriving from the synthesis of the best conditions,we successfully conducted a small-scale enlargement of the 3 L synthesis kettle.?2?The"bowl-like"hollow ZSM-5 crystals were directly synthesized by one-pot method in amine hydrothermal condition with ethyl orthosilicate?TEOS?,TPAOH and sodium metaaluminate.Then we thoroughly optimized the conditions and formation mechanisms that affect the product morphology and hollow proportion.The results show that:when the ratio of OH-/SiO2 was only 0.4,the particle size was adjustable from 300 nm to 700nm and the hollow proportion was as high as 88.4%.Accidentally,the optimal value TPA+/Na+=10:2 was the key to the direct synthesis of products.What's more,the strategies of lowering the hydrolysis temperature,raising the crystallization temperature,adding the ethanol and accelerating the TEOS dropping velocity could all increase the particle dimension and aggravate porosity,hollowfication degree?void size?.Moreover,the ratio of SiO2/Al2O3exerted great effects on the distribution of silicon and aluminum elements in the zeolite framework.When SiO2/Al2O3=60120,hollow ZSM-5 single crystals with aluminum-rich exterior suface and silicon-rich interior core were prepared.Nevertheless,the values for SiO2/Al2O3 which were extremely high or low were not conducive to hollow structure.Finally,the formation of the hollow crystals seemed to mainly involve three processes:the generation of mesoporous defects in the hydrolysis stage,the shaping of interface pore in the early period of crystallization,the connection between internal mesoporous and interface pore gradually. |
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