Regulation On The Crystal Growth And Crystallization Mechanism For ZSM-5 Molecular Sieves By Steric Hindrance And The Enhancement Of Mass Transfer

Due to its high specific surface area,hydrothermal stability,shape-selective catalysis and good adsorption properties,ZSM-5 molecular sieves are widely used in petrochemical,adsorption separation and pollutant control.The catalytic and adsorption properties of ZSM-5 molecular sieves are closely related to their grain size and pore volume.Therefore,it is important to adjust the particle size and pore structure of ZSM-5 molecular sieve in the nucleation and grain growth process to improve the performance of ZSM-5 molecular sieve.Firstly,the hydrothermal synthesis method was used to investigate the synthesis parameters of ZSM-5 molecular sieves in different template,silicon and aluminum source synthesis systems under static conditions.The crystal phase,morphology,functional group and pore structure were analyzed by XRD,SEM,FT-IR and BET.Secondly,the crystallization kinetics of ZSM-5 molecular sieves was studied by steric hindrance.The regulation of grain size of ZSM-5 zeolite during grain growth was obtained,and its regulation mechanism was proposed.Then,the mechanism of the regulation of grain growth by enhanced mass transfer during dynamic crystallization is explored,and the phase distribution and state of change of elements such as silicon and aluminum in the reaction system by XRD,SEM,PSD,ICP,FT-IR characterization is analyzed.Finally,the adsorption performance of ZSM-5 zeolite by Zn²⁺ion adsorption experiment was evaluated,and the recycling of elements in the mother liquor after the reaction was explored to provide a theoretical basis for its clean production.The specific research results are as follows:?1?Under static crystallization conditions,ZSM-5 molecular sieves were optimized in different systems:TPAOH as template,TEOS as silicon source,Al?OH?₃as aluminum source,n?SiO₂?:n?Al?OH?₃?:n?TPAOH?:n?H₂O?=60:1:12:2400,crystallized at 140°C for 24 h,the prepared ZSM-5 molecular sieve particle size is380-410 nm,specific surface area is 397 m²/g and total pore volume of 0.39 cm³/g.?2?The addition of a steric hindrance to the synthesis system has a regulatory effect on the ZSM-5 molecular sieve.After the addition of polyvinylpyrrolidone?PVP?,the nucleation phase of the crystal nucleus is shortened,but the grain growth period is prolonged.When w?PVP?/w?SiO₂?is 0.25,the synthesized molecular sieve has a particle size of 180-210 nm,and its particle size is reduced by about 48%compared with static crystallization conditions.When w?PVP?/w?SiO₂?is 0.05,the prepared molecular sieve has the highest specific surface area and pore volume,up to 491 m²/g and 0.55 cm³/g,respectively.?3?Under dynamic crystallization conditions,the mass transfer between the liquid phase and the solid phase is enhanced by mechanical stirring,which is beneficial to the regulation of grain size during the grain growth of ZSM-5 zeolite.At a rotational speed of 100 rpm and a temperature of 140°C,the grain size is 310-330nm,which is reduced by about 20%compared to static crystallization.?4?In a dynamic synthesis system,the crystallization process can be considered to be consistent with the crystallization mechanism dominated by solid phase transformation.During the induction period,5%silicon and 22%aluminum are converted to micron-sized amorphous aluminosilicates.During the nucleation period,the amorphous aluminosilicate is converted into a nucleus with a particle size of170-600 nm.During the growth spurt period,the grain grows continuously,and its particle size gradually decreases from 310-650 nm to 310-330 nm.In the later stage of crystallization,the distribution of silicon-aluminum elements in the liquid-solid phase can be considered to be in dynamic equilibrium.?5?The experimental results of ZSM-5 molecular sieve adsorption on Zn²⁺are as follows:ZSM-5 molecular sieve with large specific surface area and high total pore volume can be considered to have stronger adsorption capacity for Zn²⁺.Among them,the ZSM-5 molecular sieve synthesized at w?PVP?/w?SiO₂?=0.05 reached the adsorption equilibrium at 30°C for 30 min,and the Zn²⁺removal rate reached 98%,compared with the ZSM-5 molecular sieve without PVP.The adsorption equilibrium time was shortened by 10 min and the Zn²⁺removal rate was increased by 18%.?6?The recycling of the elements in the mother liquor after the reaction was initially explored.When the amount of mother liquor is less than 40%,ZSM-5molecular sieve with good crystal phase and no crystal can be synthesized,which provides a theoretical basis for its clean production.