The Research On The Synthesis Of Hierarchical ZSM-5 Zeolites And Their Catalytic Performances In The Methanol To Propylene Reaction

The conversion of methanol to propylene?MTP?process,suitable for the resource situation of China,is regarded as a potential technology to prepare propylene.ZSM-5zeolite is commonly used in the MTP process.Because hierarchical ZSM-5 zeolite possesses secondary porosity,it can reduce diffusion limitation of reactants and products and enhance the performance of the catalyst.In this dissertation,a low cost synthetic method witnout mesopore template is used to obtain hierarchial ZSM-5zeolites.First,the effect of the composition of the gel for the synthesis of the hierarchical ZSM-5 was studied.The effect of the silica source,the aluminum source,the template content,the OH^-/H₂O molar ratio,the H₂O/SiO₂ molar ratio,the sodium content and the SiO₂/Al₂O₃ molar ratio were investigated.It is found that different silica sources and aluminum sources are able to synthesize hierarchial ZSM-5 zeolites.With the use of TPAOH as template,the boundaries between the zeolite crystals are fuzzy and the size of aggregates of crystals is relatively small,which is about 500 nm.When the SiO₂/Al₂O₃ molar ratio was selected in the range of 104⁴15,hierarchial ZSM-5zeolites can be only synthesized with the OH^-/H₂O molar ratio in the range of0.0096⁰.03.As the H₂O/SiO₂ molar ratio increase from 6.2 to 11.1,the crystal size increases from less than 100 nm to about 200 nm,and the aggregates size of crystals size increases from 500 nm to 2?m.With the increase of Na⁺/H₂O from 0.012 to 0.053,the zeolite crystal size increases from 40 nm to 1?m or more;the aggregate of crystals increases from 1?m to 10?m or more.With the decreases of the SiO₂/Al₂O₃ molar ratio,the synthesis of hierarchial ZSM-5 zeolites is difficult.When the OH^-/H₂O molar ratio is 0.024,it is not possible to synthesize a zeolite with a SiO₂/Al₂O₃ ratio at 34.Second,the effects of aging and crystallization conditions on the synthesis of ZSM-5 were investigated.Aging time does not affect the morphology of the catalyst.With the introduction of pre-crystallization technology,the crystal size of zeolite and the aggregate size of zeolite crystals decrease and the interface between the zeolite crystals is clear.For the crystallization time,the relative crystallinity of the synthesized zeolite is 98%and the crystallization has been basically finished in 12 h.Last,the catalytic performance of zeolite in the MTP reaction was investigated.In Comparison with the conventional ZSM-5,hierarchical ZSM-5 zeolite shows a higher propylene selectivity.With a low weight hourly space velocity?WHSV?of methanol at3 h^-1,hierarchical ZSM-5 zeolites with different crystal sizes show high activity.At a high WHSV at 6 h^-1,hierarchical ZSM-5 zeolite with a crystal size of 80 nm¹00 nm shows the longest lifetime.When the aggregate size of zeolite crystals decreases to 500nm¹?m,the lifetime increases to 115 h.