Study Of In-Situ Synthesis Of Zeolite L On Kaolin Microspheres

Zeolite L is a synthetic zeolite which possesses one-dimensional pore structure.It's mole ratio of SiO₂/Al₂O₃ is higher than many other zeolites.After calcined at 700℃it's original structure still remains.Zeolite L is a promising catalyst not only because of it's particular adsorbability and catalytic performance,but also it's good thermal stability.Zeolite L can be used as a catalyst in the process of cracking,reforming,isomerization,aromatization and alkylation.Therefore,people pay more attention to the research and development of zeolite L.This work focused on the study of in-situ synthesis of zeolite L on kaolin microspheres.The zeolite L synthesized by in-situ technique grew on inner surface of the kaolin microspheres.The in-situ technique improved dissemination and utilizing efficiency of zeolite L.On the other hand,the crystal size of the synthesized zeolite is smaller than other zeolites synthesized by traditional method.This increases the active center of the external surface and reduces the diffusion resistance,which is favorable to the crack of the residue oil.The kaolin support is used as sites for nucleation and that a chemical bonding between crystals and outer support layers is formed,which results in a high thermal,chemical and mechanical stability of the zeolite/support and the outer support layers itself has a high heat capacity which can avoid the structural collapse of the catalyst at high temperature thus can extend the catalyst life.The kaolin was firstly calcined at different temperatures,meta-kaolin was obtained from kaolin which was calcined at relative lower temperature;pyrokaolin was obtained at relative higher temperature.The contents of the activated SiO₂ and Al₂O₃ were analyzed by chemical method.While using pyrokaolin as material there was no zeolite L in the product.This is because the mole ratio of SiO₂ and Al₂O₃ in the pyrokaolin is not in the range of the required ratio of SiO₂ and Al₂O₃ and the KAlO₂ solution was added to adjust the mole ratio of SiO₂ and Al₂O₃.The activity of the added aluminium source is higher than the aluminium in the kaolin.This results in the mole ratio of SiO₂ and Al₂O₃ in the gel precursor less than the required ratio of SiO₂ and Al₂O₃,so there is no zeolite L.The pyrokaolin and meta-kaolin was mixed according to a certain proportion and then hydrothermal crystallized in alkaline medium with appropriate amount of other constituents,such as sodium silicate,water,zeolite L initiator.Such preparative factors as crystallization temperature,crystallization time,aging time,the volume of the water and the mole ratio of n(SiO₂)/n(Al₂O₃) were systematically investigated to discuss their effects on the synthesis of zeolites on calcined kaolin supports.Crystallization temperature,time and amount of added water were the main parameters which affecting zeolite synthesis.The termperature is too low to synthesis zeolite L when reacted at 90℃.The higher temperature can accelerate crystal growth speed to shorten reaction time.But unwanted impurity phase may form at excess higher temperature.There is almost no zeolite in the product if the crystallization time is too short.When the crystallization time extended to 18 hours zeolite L with a higher relative crystallinity can be obtained.The functions of water can be described as follows.First,the water volume contributes to the alkalinity of the system and accordingly influences the dissolution of the SiO₂ and Al₂O₃ in the kaolin.Second the water volume influences the supersaturation of the potassium silicate gel which converted to zeolite L.When the water is not enough the reaction materials can't be mixed uniformly.But the system alkalinity and the supersaturation may decrease when the water is excessive.This indicates that too much or too little water may inhibit the formation of perfect zeolite L from kaolin.The optimum conditions to get zeolite L using kaolin microsphere by in situ hydrothermal crystallization was as follows:the mole ratio of n(SiO₂)/n(Al₂O₃) was 15 in the reaction mixture,aging the reactants at room temperature for 30h and crystallized at 130℃for 18h under agitation.The crystal phase of the prepared zeolite was characterized by XRD and SEM.On the basis of the bench scale experiment a better synthesis condition was selected and zeolite L was prepared in a 1L autoclave at this condition,the obtained zeolite had a high relative crystallinity.