Factors affecting the synthesis of zeolite molecular sieves

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The synthesis of zeolite is a rather complex process, which is affected by many factors. If these factors are not carefully controlled, unexpected results may be obtained, and even the synthesis may fail. This is briefly discussed below. 1. The composition of the reaction mixture. As can be seen from the above, the chemical compositions of various zeolites are different. In order to synthesize a certain type of molecular sieve, a reaction mixture with a certain ratio must be prepared. Different ratios of the reaction mixture will result in different types of zeolites. Therefore, the composition of the reaction mixture is the most important factor affecting the synthesis of zeolite, and the silicon-aluminum ratio and alkalinity are two important factors in the composition of the reaction mixture. Generally speaking, when other conditions remain unchanged, the higher the silicon-aluminum ratio of the reaction mixture, the slower the crystallization speed, and the higher the silicon-aluminum ratio of the product. ； On the contrary, if the silicon-to-aluminum ratio in the system is reduced, the crystallization speed will be accelerated, but the silicon-to-aluminum ratio in the product will also be reduced. The so-called alkalinity refers to the concentration of alkali in the system, which is determined by the water-sodium ratio in the system. When the amount of water in the system is fixed, the higher the excess alkali, the greater the alkalinity. Alkalinity has two main functions. One is to control the state of the silicate anion (especially its degree of polymerization), and the other is to control the position of the equilibrium state of each component in the system to ensure that the reaction proceeds in the direction of forming a certain zeolite under certain conditions. Different whiskites are produced at different alkalinities (and different silicon-to-aluminum ratios). For example, as the alkalinity goes from high to low (while the silica-aluminum ratio goes from low to high accordingly), basic sodalite, type A zeolite..., faujasite and mordenite are sequentially generated. This is because some of the structural units that make up these zeolites have different stabilities at different alkalinities. The structural units (β cage) of type A zeolite are connected to each other through four-membered rings with oxygen bridges, and they are stable in media with slightly lower alkalinity. ； The structural units of faujasite are connected by oxygen bridges through six-membered rings. They are stable in media with lower alkalinity, and the degree of stability decreases with the decrease in the number of aluminum-oxygen tetrahedrons in the ring. ； In the structural unit of mordenite, there is an average of one aluminum-oxygen tetrahedron for every five silicon-oxygen tetrahedrons. It can only be stable in a medium with very low alkalinity. Therefore, the synthesis of mordenite must be carried out in a medium with very low alkalinity. Within the range of the ratio to generate a certain zeolite, when other conditions are fixed, the greater the alkalinity, the faster the crystallization speed, and the lower the silicon-aluminum ratio of the product, the smaller the particle size. ； On the contrary, the smaller the alkalinity, the slower the crystallization speed, and the higher the silicon-to-aluminum ratio of the product, the larger the particle size. Since the composition of the reaction mixture has a great impact on the synthesis process and product quality, a uniform reaction mixture must be prepared, otherwise miscellaneous crystals may be produced or even failure may occur due to changes in the proportion of local materials. For this reason, strong stirring is necessary during the mixing process (especially when the silicon-aluminum ratio and water-sodium ratio are low), and an appropriate feeding sequence must be adopted to obtain a uniform reaction mixture.