Synthesis of zeolite molecular sieves

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The widespread industrial use of zeolite molecular sieves has prompted people to adopt synthetic methods for their preparation. The most commonly used method at present is to synthesize molecular sieves by simulating the formation conditions of natural zeolite minerals, namely the hydrothermal synthesis method. (1) Hydrothermal synthesis: Hydrothermal synthesis must be carried out at an appropriate temperature. Depending on the temperature, it can be divided into low-temperature hydrothermal synthesis (25–150°C) and high-temperature hydrothermal synthesis (>150°C). When synthesized at higher temperatures, the silicoaluminate crystals obtained have a lower degree of hydration, whereas those synthesized at lower temperatures have a higher degree of hydration; in other words, more water is contained within the zeolite, resulting in zeolites with larger pore sizes. The zeolites obtained through low-temperature hydrothermal synthesis are mostly in a metastable, non-equilibrium state. Therefore, on the one hand, low-temperature hydrothermal synthesis allows for the creation of zeolite varieties that do not exist in nature; on the other hand, the relatively low reaction temperature (usually around 100°C) provides favorable conditions for the large-scale industrial production of zeolites. The raw materials used for synthesizing zeolites are mainly silicon-containing compounds, aluminum-containing compounds, alkalis, and water. Silicon-containing compounds can include silica gel, silica sol, sodium silicate (water glass), etc., among which water glass is the most commonly used. Aluminum-containing compounds can include various types of activated alumina, aluminum hydroxide, sodium aluminate, and aluminum sulfate, among which sodium aluminate is the most commonly used. The alkalis used in synthesis are usually sodium hydroxide, potassium hydroxide, etc.; to reduce costs and make use of natural resources, natural minerals such as clay and kaolin can also be used as raw materials. The raw materials are combined in a certain ratio to form a reaction mixture, whose composition is usually expressed in terms of the molar ratios of oxides, generally written in the form of xM2O•Al2O3•ySiO2•zH2O. Here, M represents alkali metals (mainly Na and K), while x, y, and z represent the molar amounts of each component respectively. It is sometimes also expressed in the form of three ratios: SiO2/Al2O3, M2O/SiO2, and H2O/M2O. The reaction mixture prepared in a certain ratio becomes a white, opaque gel after being mixed evenly (the gelation rate varies depending on the ratio and temperature). Place this reaction mixture in a reaction vessel and carry out the crystallization reaction at a certain temperature. It is worth noting that local overheating should be avoided during heating, as it will affect product quality. After crystallization is complete, the zeolite generally settles at the bottom of the reactor, while the upper part contains a clear solution (often referred to as the mother liquor). The precipitate is subjected to steps such as filtration, washing, and drying to yield white zeolite crystal powder. The mother liquor generally still contains sodium oxide and silica, equivalent to dilute water glass, and can be recycled. In industry, vacuum belt filters or plate and frame filter presses are commonly used for filtration and washing. Distilled water is preferred for washing, while deionized water is commonly used in industry. With the development of synthetic zeolite production, the synthesis techniques have also been continuously improved. In the early stages, improvements were mainly made in aspects such as the order of adding raw materials, the mixing method, as well as the speed of stirring and heating. Subsequently, it was proposed to add an aging step: before undergoing the hydrothermal reaction, the reaction mixture is allowed to stand at a certain temperature for a period of time, after which the temperature is raised to the crystallization temperature to carry out the crystallization reaction. This approach allows for a reduction in the crystallization time and an increase in the degree of crystallization. In the early synthesis of X-type and Y-type zeolites, aging procedures were employed. The use of seeding technology represents an important advancement in zeolite synthesis techniques; it involves adding the natural zeolite crystals to be synthesized into the reaction mixture used for synthesis, in order to promote their crystallization and growth. For example, when synthesizing X-type and Y-type zeolites, the corresponding molecular sieve crystal powders were added to the reaction mixture respectively, and it was found that the crystallization time was significantly reduced. It later evolved to use a partially crystallized reaction mixture as a seed crystal. For example, when synthesizing X-type zeolite, 10–15% of a partially crystallized reaction mixture (containing 75% X-type zeolite and 25% amorphous sodium aluminosilicate) is added. The crystallization time was reduced by nearly one-third, while the crystallization was more complete, and it contained no B-type zeolite adulterants. Further development in synthesis technology involves using amorphous materials as seeds. For example, a colloidal solution is first prepared according to the composition ratio required for the formation of octahedral zeolites; after aging for a certain period of time, it is used as a seed crystal for synthesizing X-type or Y-type molecular sieves. This substance is actually a transition state that already contains crystal cells; it are colloidal particles ranging in size from 0.01 to 0.1 micrometers. In essence, it can be regarded as a substance containing crystal nuclei, and it plays a role in facilitating \"directed\" crystallization during the crystallization process of molecular sieves. Such amorphous substances are generally referred to as \"crystallization directors\" or \"crystallization guides\". In the process of synthesizing high-silica Y-type molecular sieves using water glass as a raw material, the use of crystallization promoters enables **shorter crystallization times**, while also ensuring stable product quality and high purity, thus providing a very convenient approach for the synthesis of high-silica Y-type molecular sieves. (II) Factors affecting the synthesis of zeolite molecular sieves The synthesis of zeolites is a rather complex process that is influenced by many factors. If these factors are not carefully controlled, unexpected results may occur, or even the synthesis may fail. It will be briefly discussed below. 1. Composition of the reaction mixture As mentioned earlier, various zeolites have different chemical compositions. To synthesize a certain type of molecular sieve, it is necessary to prepare a reaction mixture with specific proportions. Different proportions of the reaction mixture result in different types of zeolites; therefore, the composition of the reaction mixture is the most important factor affecting zeolite synthesis. The silicon-aluminum ratio and alkalinity are two key factors within the composition of the reaction mixture. Generally speaking, under constant other conditions, the higher the silica-to-alumina ratio of the reaction mixture, the slower the crystallization rate, and the higher the silica-to-alumina ratio of the product ； Conversely, if the silica-alumina ratio in the system is reduced, the crystallization rate increases, but the silica-alumina ratio in the product also decreases. The so-called alkalinity refers to the concentration of bases in the system, which is determined by the water-sodium ratio in the system. When the water content in the system remains constant, the higher the excess alkali, the greater the alkalinity. Alkalinity has two main functions: one is to control the state of silicate anions (particularly their degree of polymerization), and the other is to determine the equilibrium state of various components in the system, thereby ensuring that the reaction proceeds in the direction of forming a certain type of zeolite under specific conditions. Different pumice forms under different alkalinity levels (and different silica-aluminum ratios). For example, as the alkalinity decreases from high to low (with the silica-alumina ratio increasing correspondingly from low to high), basic natrolite, type A zeolite... octahedral zeolite, and mordenite are formed successively. This is because certain structural units that make up these zeolites have different stabilities at various levels of basicity. The structural units of Type A zeolites (β cages) are connected to each other by oxygen bridges formed by four-membered rings, and they remain stable in media with slightly lower basicity ； The structural units of octahedral zeolites are all linked by oxygen bridges through six-membered rings; they are stable in media with lower basicity, and their stability decreases as the number of aluminooxide tetrahedra in the rings decreases ； In the structural unit of mordenite, there is on average one aluminoxy tetrahedron for every five silicoxy tetrahedra; it can remain stable only in media with very low alkalinity. Therefore, the synthesis of mordenite must take place in a medium with very low alkalinity. Within the range of ratios for synthesizing a certain type of zeolite, with other conditions held constant, the higher the alkalinity, the faster the crystallization rate; simultaneously, the silica-alumina ratio of the product is lower and its particle size is smaller ； Conversely, the lower the alkalinity, the slower the crystallization rate; as a result, the silicon-aluminum ratio of the product increases and its particle size becomes larger. Since the composition of the reaction mixture has a significant impact on the synthesis process and product quality, it is essential to prepare a homogeneous reaction mixture; otherwise, changes in the local ratio of components can lead to the formation of impure crystals or even failure of the process. To this end, strong stirring is necessary during the mixing process (especially when the silicon-to-aluminum ratio and the water-to-sodium ratio are low), and an appropriate order of adding materials must be followed to obtain a homogeneous reaction mixture.