Synthesis Of ZSM-5/AlPO₄-5 Composite Zeolites In Ionic Liquids And Their Catalytic Performences In The MTO Reaction

Ionic liquids are often environmentally-friendly solvents with the advantages of having vapor-free pressure,being non-flammable,and having a wide liquid range,low volatility,and strong designability compared with conventional solvothermally synthesized molecular sieves.The reaction can be performed under normal pressure,avoiding the potential safety hazards of high pressure,and ionic liquids as a salt can be dissolved at room temperature,are soluble in water,and are green and pollution-free,avoiding environmental pollution caused by the volatilization of organic solvents.In view of the excellent physicochemical properties of ionic liquids,the ionothermal synthesis of molecular sieve materials have properties that traditional solvent synthesis molecular sieves do not possess.Therefore,ionothermal synthesis of ZSM-5/AlPO₄-5 is of great significance.Up to now,there has been no report on ionothermal synthesis of ZSM-5/AlPO₄-5 composite molecular sieves.For the first time,a pure ZSM-5/AlPO₄-5 composite molecular sieve was successfully synthesized in 1-butyl-2,3-dimethylimidazolium bromide?[BMMIm]Br?.The effects of crystallization temperature,crystallization time,ZSM-5 addition amount,IL content and TEA content on the structure of ZSM-5/AlPO₄-5 composite zeolite were investigated.The optimal conditions for the synthesis of pure ZSM-5/AlPO₄-5 core-shell composite zeolite with high crystallinity and uniform crystallinity in[BMMIm]Br were determined to be as follows:the addition amount of ZSM-5 was 1/8,the crystallization temperature was 150°C,the crystallization time was 24 h,and the molar ratio of the composite molecular sieve synthesis gel was:12IL:2.4P₂O₅:1Al₂O₃:1.5T-EA:0.1HF:2.5H₂O.The amount of TEA and IL added would affect the purity and crystallinity of the composite molecular sieve.If the amount of TEA and IL was too small,no pure composite molecular sieve could be synthesized.If the amounts were too large,the crystallinity would be affected.By changing the types of ionic liquids,pure ZSM-5/AlPO₄-5 composite molecular sieves were successfully synthesized in[EMMIm]Br and[HMMIm]Br,respectively.The effects of crystallization time,IL content and TEA content on the structure of ZSM-5/AlPO₄-5 composite zeolites were investigated,and the synthesis of ZSM-5/AlPO₄-5 composite molecular sieves in[EMMIm]Br was determined,and the molar ratio of the synthesis gel was 12IL:2.4P₂O₅:1Al₂O₃:1.5TEA:0.15HF,the crystallization temperature was 150°C and the crystallization time was 24 h.A spherical-like ZSM-5/AlPO₄-5 core-shell composite molecular sieve with high crystallinity,good degree of recombination,and uniform morphology was obtained,and the surface was loaded with a large amount of ZSM-5 particles.The amount of TEA had a great influence on the synthesis of the composite molecular sieve.Only when the amount of TEA was 1.5,could a pure composite molecular sieve be synthesized.In[HMMIm]Br,the molar ratio of the synthesized composite molecular sieve was12IL:2.4P₂O₅:1Al₂O₃:2TEA:0.1HF,the crystallization temperature was 150°C,and the crystallization time was 24h.A spherical-like ZSM-5/AlPO₄-5 core-shell composite molecular sieve with a large amount of ZSM-5 particles loaded on the surface was synthesized with high crystallinity,good recombination and uniform morphology.In order to compare with the ionothermal synthesis composite molecular sieves,ZSM-5/AlPO₄-5 samples were also synthesized in hydrothermal conditions.The morphologies of the ZSM-5/AlPO₄-5 composite molecular sieves synthesized by four different solvents were irregular large blocks with a large number of surface-loaded ZSM-5 particles,but the crystallite size,specific surface area,pore volume,and pore size were significantly different.Among them,the composite molecular sieves prepared ionothermally had smaller particle size,and their specific surface area decreased with the increase in the side chain of the alkyl chain,indicating that the ionic liquid with adjustable properties showed significant changes in the ionothermal synthesis of ZSM-5/AlPO₄-5 samples.On the other hand,the Z/P specific surface area was the largest,and the larger the specific surface area was,the more acid was distributed in the molecular sieve,which in turn was not conducive to the production of light olefins to some extent.Compared with Z/P,ionothermal composite molecular sieves with dual template agents had smaller pore volume and pore size,and their shape-selective effects were stronger.They were suitable for the production of small molecules like ethylene and propylene,while macromolecular alkanes and aromatic hydrocarbons were restricted.The MTO performance of the ZSM-5/AlPO₄-5 composites synthesized in four different systems was better than that of ZSM-5 molecular sieve and mechanically mixed molecular sieve.The reason was that the amount and intensity of acidity of ZSM-5/AlPO₄-5 samples were well modulated.Too weakly acidic or too acidic were not conducive to the formation of light olefins.The lower acid strength was not sufficient for methanol conversion.The higher acid strength would promote the circulation of alkanes.The influence of the crystallization time of the three ionic liquids on the performance of ZSM-5/AlPO₄-5 was that the crystallization time of 36h provided a higher selectivity to lower olefins,and the selectivity of light olefins increased as the amount of strong acid increased.This showed that within a certain range,a moderate increase in the acid strength could activate the alkane,thereby exacerbating the C-H bond cleavage and thus favouring the production of more target products.The yields of light olefins obtained from the synthesis of ZSM-5/AlPO₄-5samples in three ionic liquids were better than those from hydrothermal synthesis samples,which indicated that the ionothermal synthesis composite molecular sieves were truly modified compared with the hydrothermal.The lifetimes of ionothermal synthesis ZSM-5/AlPO₄-5 samples were all higher than the hydrothermal.The hydrothermal synthesis was only 6 h.The ionothermal synthesis increased with the increasing length of alkyl chain,and the lifetimes increased accordingly:they were 15h,37 h,and 72 h.