Study Of In-Situ Synthesis Of ZSM-5 On Kaolin Microspheres

Zeolite ZSM-5 is widely used in petrochemical processing for improving gasoline quality and producing important chemical materials, such as ethylene and propylene. The zeolite material, which was synthesized by in-situ technique, grew on inner surface of kaolin microspheres. The in-situ technique improved dispersedness and utilizing efficiency of zeolite. The matrix provides active acid sites for feedstock pre-cracking, and it provides high porosity so that the zeolite can be used to its maximum capacity and regeneration can be made easy. At the same, it provides the bulk properties that are important for heat transfer during regeneration and cracking and heat storage in large-scale catalytic cracking. Compared with semi-synthetic catalyst, the catalyst prepared by in-situ technique exhibited improved properties, such as activity and activity stability, anti-heavy metals ability, resid cracking and yield distribution, etc.This work focused on study of in-situ synthesis of ZSM-5 on kaolin microspheres with kaolin as material. Properties of kaolin microspheres during calcinations and acid or caustic-modification were investigated, so as to realize matrix property of catalyst prepared by in-situ crystallization technique. The zeolite ZSM-5 synthesis on kaolin microspheres in different systems and conditions was also investigated in detail. In this study, material containing 27% ZSM-5 was successfully synthesized on kaolin microspheres under hydrothermal conditions with or without organic amine. In the synthesis system without organic amine template, there is no environmental pollutionproblem brought by using of organic amine, and the materials are cheap and obtained easily, which reduced the preparing cost of zeolite. Also, small crystal size zeolite ZSM-5, which size was 0.2 urn ~ 1 urn, was synthesized on kaolin microspheres under hydrothermal crystallization condition using H-butylamine as template. The zeolite products were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), IR and adsorption and desorption isotherm plot, etc. Some new findings and interesting results have been obtained via detailed investigation of the in-situ synthesized catalyst material performance, synthesis condition and mechanism of ZSM-5 material, and the relationship between the physical and chemical property and catalytic reaction performance of catalysts.(1) Meta-kaolin was obtained from kaolin which was calcined at relative lower temperature, out of which active alumina can be leached. Pyrokaolin was obtained at relative higher temperature, out of which active silica can be leached, while mullite was formed for vanadium passivation. Calcination and alkali or acid leaching modified the mesoporous structure of kaolin microspheres. The products had Lewis acid activity but no obvious Bronsted acidity. Additives made from the modified kaolin microspheresexhibited improved coke selectivity and yield distribution.(2) hi the system of crystal seeds and additional silica and alumina source, zeoliteZSM-5 was synthesized with alkali leaching modified pyrokaolin under hydrothermal conditions without organic amine. Temperature and system alkalinity were the main factor which affecting zeolite synthesis. The higher temperature can accelerate crystal growth speed to shorten reaction time. But unwanted impurity phase may form at excess higher temperature. The reaction speed can also accelerated with temperature increased, but excess higher temperature led to form impurity phases instead. There was much powder in the products. XRD results showed that there was little zeolite ZSM-5 in the microspheres, which indicated that the synthesized ZSM-5 was exist alone but not or almost not loaded on the microspheres.(3) In the alkaline system, zeolite ZSM-5 was synthesized with kaolin microsphereswhich was calcined at 1000 °C under hydrothermal conditions without organic amine. ZSM-5 was used as crystal seed and water glass as additional silica source. The results showed that the purity of the synthesized ZSM-5 was affected by ratio of the water glass, water and reaction time. So it is important to control reaction conditions. The material containing ZSM-5 synthesized in the system of kaolin microspheres-additional silica-water had high surface area and pore volume, especially an improved micropore surface area and micropore volume. This synthesis process easily led to form polycrystals. The zeolite on the surface of the microspheres existed as slice shape, which diameter was about 5 fim. But the zeolite powder formed via non-in-situ crystallization was about 32% in the products.(4) Silica gel as additional silica, ZSM-5 crystal seed and kaolin were blended to form an aqueous slurry, then sprayed dried to obtain kaolin-containing microspheres. In the alkaline system, material containing 27% zeolite ZSM-5 was synthesized with calcined kaolin-containing microspheres under hydrothermal conditions using organic amine as template. There was no obvious zeolite powder formed via non-in-situ crystallization in the products. XRD, SEM and chemical analysis have been used to examine the mechanism of synthesis of zeolite ZSM-5 on kaolin microspheres. When treated with NaOH solution, active silica in kaolin microspheres slowly solved into liquid phase to form sodium silicate gel, which converted into zeolite ZSM-5 under both effect of organic template and crystal seed. The process of the gelation of solved active silica and the crystallization of zeolite ZSM-5 from such gel were simultaneous till the end of the crystallization. Liquid phase took an active part in the gelation of solved active silica and the growth of zeolite ZSM-5. The crystallization of zeolite ZSM-5 on kaolin microspheres easily led to form large numbers of polycrystals. The crystallization was affected by the following factors, such as ratio of OH'/SiC^, Na2O/SiO2, content of template and water glass in the reaction system, etc. The higher ratio of OH7SiO2, NaaO/SiCh and higher content of water glass in the reaction system can accelerate crystal growth speed, butunwanted impurity phase may form synchronously. The above factors cooperated with each other. In this study, there was no obvious influence of SiCVAbOa ratio of ZSM-5 on its purity.(5 ) Small crystal size zeolite ZSM-5, which size was 0.2 fim~ 1 \im, was synthesized on kaolin microspheres under hydrothermal crystallization condition using ?-butylamine as template in stirring autoclave. The synthesis process easily led to form polycrystals and the zeolite on the surface of the microspheres existed as slice shape. Compared with calcined kaolin microspheres, the crystallization products had higher surface area, especially an improved micropore surface area and micropore volume. The additive obtained from the crystallization products had both Lewis and Bronsted acid sites, which Lewis acid was mainly weak acid and Bronsted acid was stronger. The active alumina in the calcined kaolin microspheres took part in the crystallization reaction process. The ZSM-5 SiO2/Al2O3 ratio of the synthesized ZSM-5 additive was about 56 to 214. The additive provided excellent thermal stability, which zeolite framework collapsed at about 1130 "C. When added as additive, reaction results in small-scale fluid-bed reactor showed that LPG and low carbon olefin yield were improved with good cracking selectivity, and the gasoline quality improved significantly.