Synthesis Of ZSM-5Zeolites Without Organic Structure-directing Agent

Due to unique pore structure, good thermal stability and hydrothermal stability, ZSM-5zeolites were widely used in catalytic materials and adsorption materials. As a catalyst, the gas synthesized by ZSM-5zeolites with methanol as raw materials had caused a highly evaluation of the world scientific research. Moreover as a catalyst, the hydration of cyclohexene catalyzed by ZSM-5zeolites also showed good performance. Although the conversion was low, the selectivity of cyclohexanol was above99%, basically without by-products. However, for synthesis of ZSM-5zeolites, the traditional synthesis methods are to make use of the expensive structure directing agent (SDA). Despite the excellent template effect of TPA+cations, it causes many adverse problems such as high production cost, organic species contained in channel system due to incomplete decomposition and environment problems from thermal decomposition of organic species. Unfortunately, SDA removal to open the zeolite micropores, commonly done through high temperature calcination, has proved unsuitable for colloidal nanocrystals because it leads to significant irreversible aggregation. Since the synthesis of ZSM-5zeolites with the SDA was reported by Flanigen in1980s, the synthesis of ZSM-5zeolites in the absence of SDA for industrial application has been implemented and this method has been reported. From the point of industrial application and environment benign, the synthesis method of ZSM-5zeolite in the absence of SDA still has the important practical significance.In this paper, the synthesis of ZSM-5zeolites in the absence of SDA with different methods was reported. The factors such as crystallized temperature and time were studied with two-step method. The various factors with nucleation solution method were mainly reported. The crystal transformation between ZSM-5and MOR was studied also. The dispersive and sub-micro ZSM-5zeolites were captured by base source step-feeding method. The physical and chemical properties of ZSM-5zeolites were characterized by various methods. The acquired ZSM-5zeolites were used as catalysts for catalytic reation. This paper is focused on the following several aspects:(1) Zeolite ZSM-5small particle aggregates were synthesized with two-step method:The factors such as crystallized temperature and time with two-step method were discussed. Zeolite ZSM-5small particle aggregates were synthesized in the absence of template using sodium silicate as silica source by controlling nucleation time at high temperature with a simple two-step method by rotating autoclaves. The first stage at higher temperature is to accelerate the nucleation and the second stage at lower temperature is to obtain the small crystal size the absence of template. The resulting zeolites were characterized by X-Ray Powder Diffraction (XRD), scanning electron microscope (SEM), ammonia temperature-programmed desorption (NH3-TPD) and N2adsorption. Compared with micron-sized zeolite crystals synthesized by a one-step method, zeolite ZSM-5small particle aggregates prepared by the two-step method had higher specific surface area and equivalent acid amount. It is clear that the sample synthesized with the two-step method mainly had Br(?)nsted acid sites and less Lewis acid sites, indicating that the Al atom was incorporated into the framework of zeolite essentially.(2) ZSM-5zeolite synthesized with nucleation solution method:The effect of adding nucleation solution on the preparation of aggregate ZSM-5zeolite was studied by rotating autoclaves, completely without organic structure-directing agent. The influence on the crystallization rate and particle size was systematically investigated. Physicochemical characteristics derived from XRD, SEM, and N2adsorption. It is indicated that the crystallization rate of the ZSM-5zeolite was increased by the addition of nucleation solution. The crystallization time with adding33.3wt%nucleation solution is24h while without adding nucleation solution is48h. Adding33.3wt%nucleation solution, after crystallization at150℃for24h, the average size of aggregate was increased from5.5μm to26.3μm, the specific surface from292m2/g to394m2/g. The mechanism of the crystal growth of aggregate zeolite ZSM-5 without organic template by nucleation solution method was also discussed in this work.(3) The crystal transformation of zeolite ZSM-5and mordenite in the absence of organic structure-directing agent was studied in this paper. Zeolite crystals were synthesized at the lower temperature (150℃) by adding nucleation solution prepared at the higher temperature (190℃) into the mother solution. The effects of the ratio and the nucleation time of the nucleation solution, and the ratio of the mother solution on crystal transformation between ZSM-5and mordenite were investigated. By changing the overall SiO2:Al2O3ratio of the whole synthesis solution, the transformation between zeolite ZSM-5and mordenite could be controlled effectively. The Na2O:SiO2ratio of0.18was a boundary between the ZSM-5phase and mordenite phase when the composition of the whole synthesis solution was100SiO2:2.5Al2O3:4000H2O. Let Na2O:SiO2>0.18by altering the composition of the mother solution, the ZSM-5product formed at the higher temperature nucleation stage could be transformed into mordenite crystals at the lower temperature (150℃) crystallization stage; Let Na2O:SiO2<0.18, co-crystalline of ZSM-5and mordenite product obtained at the higher temperature nucleation stage could be transformed into ZSM-5crystals at the lower temperature crystallization stage. The precondition of crystal transformation between ZSM-5and mordenite was that the crystallinity of crystals formed at the higher temperature nucleation stage must be<30%. The crystal morphology of products changed with increasing the Na2O:SiO2ratio in the whole solution.(4) Dispersive and sub-micro ZSM-5zeolites were prepared with a new base source step-feeding method. It was reported a new base source step-feeding method for obtaining dispersive and sub-micro ZSM-5zeolites. Physicochemical characteristics derived from XRD, NMR, SEM/TEM, DLS and N2adsorption. The effects of addition of H2O2and NaOH during the synthesis process were studied. It indicated that highly dispersed ZSM-5zeolites with400-500run and high crystallinity were aqcuired with alkalinity control method. The specific surface is396m2/g. The Al atom was incorporated into the framework of zeolite essentially.(5) Hydration of cyclohexene catalyzed by ZSM-5zeolites:The effect of acid properties and acid amount on catalytic activity for hydration of cyclohexene were also investigated. It is found that hydration of cyclohexene was mainly depended on the Bronsted acid sites.