Direct Synthesis And Characterization Of High-Silica Nanoaggregates Of Zeolite

Zeolite is a kind of aluminosilicate molecular sieve with a microporous structure throughout.Because of its large specific surface area,regular pore structure and adjustable silicon-aluminum ratio,as well as good adsorption and catalysis,it is widely used in modern petroleum.It has important application value in refining and petrochemical industry,and its synthesis is of great significance.Y zeolite has a FAU topology,and the internal framework can be viewed as SOD cages spliced by double six-membered rings,including ellipsoidal supercages（with a diameter of about 1.12 nm）and a three-dimensional twelve-membered ring pore structure（The pore size is 0.74 nm）,which has the larger pore size,larger specific surface area and higher acid density among zeolites.It is the catalyst used in important petroleum refining and petroleum processing processes such as fluidized catalytic cracking and hydrocracking.The important active ingredient is the molecular sieve with the largest industrial consumption.However,due to the inherent low-silicon framework of Y-type zeolite（Si/Al=1.5～3）and the resulting structural instability,it has always been the main bottleneck that plagued the direct application of this material.Many existing studies have shown that with the increase of the framework silicon to aluminum ratio（SAR）of Y zeolite,the structural stability,especially the hydrothermal stability,increases,and the strength of the acid center also increases,which is beneficial to the catalyst in the long cycle.Maintain performance while in use.The high-silicon Y zeolite（Si/Al>2.5）widely used in industry is mainly obtained by the subsequent treatment of conventional Y zeolite.This method is not only time-consuming and labor-intensive,but also has low yield,and its internal acid distribution is not uniform.On the other hand,the conventionally synthesized micron-scale Y zeolite,due to the long diffusion distance in the zeolite pores,the diffusion of reactants and synthesized products,especially heavy oil macromolecules,in the pores is limited,which reduces the probability of contact with the active center.,it is easy to cause the inactivation of zeolite carbon deposition,thereby reducing the catalytic efficiency and service life.Nano Y zeolite has higher reactivity and surface energy due to its small grain size,large external specific surface area,shortened pore channels,and increased exposed pores,which can effectively solve the diffusion problem.It can also effectively solve the problem that the nanocrystalline particles are difficult to handle in the process of synthesis and use.In this thesis,high silica Y zeolite molecular sieve nano-aggregates were successfully synthesized by organic amine-free hydrothermal system synthesis method,CTAB（hexadecyltrimethylammonium bromide）method and AESA（fatty alcohol polyoxyethylene ether ammonium sulfate）method.,and the crystallization conditions were investigated.The research work of this thesis mainly includes the following parts:（1）Synthesis method of organic amine-free hydrothermal system:under the ratio of 4.0Na₂O:Al₂O₃:12Si O₂:160H₂O:x（=0,6.86）C₂H₅OH,two-stage crystallization,the aging time was controlled for 24 h at 50°C and 95°C.The crystallization time at 50°C and 95°C was 24 h and96 h,respectively.By adjusting whether to add ethanol to the system,the influence of ethanol on the synthesis of samples was investigated.Agglomeration,increase the ratio of silicon to aluminum,reduce the grain size,and improve the crystallinity.Under the above ratio and process conditions,by changing the amount of ethanol,the effect of different ethanol contents on the synthesis of Y zeolite was studied.The results showed that the sodium to silicon ratio was 0.33,the ethanol amount was 6.86,and the aging time was 24 h,the crystallization time at50°C is 24 h,and the crystallization time at 95°C is 96 h.The samples synthesized under the condition of high silicon-aluminum ratio,high crystallinity,small grain size,relatively uniform morphology,and their skeleton silicon The aluminum ratio is 2.97,the BET surface area is 924m²g^-1,the external specific surface area is 27 m²g^-1,and the crystal size is about 300 nm.After that,the influence of different sodium-silicon ratios on the synthesis of Y zeolite was studied.The research showed that with the increase of sodium-silicon ratio,the silicon-aluminum ratio decreased,and when the sodium-silicon ratio was 0.33 and 0.38,the silicon-aluminum ratio was relatively high.were 2.74 and 2.88,respectively.Therefore,in the following experiments,all were carried out under the sodium to silicon ratio of 0.33 and 0.38.The synthesis process conditions under different sodium-silicon ratios were studied.Under the ratio of（4.0,4.5）Na₂O:Al₂O₃:12Si O₂:160H₂O:6.86C₂H₅OH,the aging time,50℃and 95℃crystallization The results show that when the sodium-to-silicon ratio is 0.33,the aging time is24 h,50℃（24 h）,95℃（96 h）时,the grain size of the synthesized Y zeolite sample is 400 nm,the framework silicon-alumina ratio is 2.97;The ratio of sodium to silicon is 0.38,the aging time is 24 h,and the Y zeolite synthesized at 50°C（24 h）and 95°C（60 h）has a framework silicon-alumina ratio of 2.63,The BET surface area is 981 m²g^-1,the external specific surface area is 31 m²g^-1,and the crystal size is about 300 nm.（2）Synthesis of high-silicon nano-Y zeolite by CTAB method:by adding cationic surfactant CTAB,under the ratio of x Na₂O:Al₂O₃:12Si O₂:160H₂O:y C₂H₅OH:z CT,the aging time was controlled for 24 h,The crystallization time at 50℃remained unchanged for 24 h.First,the effect of crystallization time at 95℃on theproperties of the synthesized samples was investigated,and the optimal crystallizationtime at 95℃was determined.The interaction between them was studied.Through characterization analysis,it can be concluded that when the sodium-silicon ratio is 0.33,the ratio of ethanol is 5.15,the aging time is 24 h,the crystallization time is50℃（24 h）,95℃（144 h）,the crystal size of the synthesized sample is determined.is 400 nm,the framework silicon-alumina ratio is 2.60,and the nitrogen adsorption data results show that the BET surface area is 937 m²g^-1,and the external specific surface area is 23 m²g^-1.Through XRD,SEM and nitrogen adsorption and desorption characterization analysis,it can be seen that CTAB can reduce the silicon-aluminum ratio of the product,and can also effectively reduce the grain size,and the accumulation between grains also becomes loose.After the addition of CTAB,the silicon-alumina ratio of the product first increased and then decreased with the prolongation of the crystallization time at 95°C.CTAB itself does not have the effect of agglomerating the grains,and ethanol is an important factor for agglomerating the grains.（3）Synthesis of high-silicon nano-Y zeolite by AESA method:By adding anionic surfactant AESA,the effect of crystallization time at 95°C on the performance of synthesized samples and the difference between AESA and ethanol were investigated under the sodium-silicon ratio of 0.33 and 0.38.Silica nano-Y zeolite interactions.In addition,the influence of the properties and morphologies of the samples synthesized under different AESA contents was also studied.And it was determined that when the ratio of sodium to silicon is 0.33,the ratio of die to silicon is 0.006,the crystallization time is 50°C for 24 h and 95°C for 96 h,the crystal size of the synthesized sample is 400 nm,the framework silicon to aluminum ratio is 3.02,and the nitrogen adsorption The data results show that the BET surface area is 965 m²g^-1 and the external specific surface area is 33 m²g^-1.