Several properties of zeolite molecular sieves

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Above, we discussed the crystal structures of several major zeolite molecular sieves. The structure of a substance determines its properties; therefore, the unique structure of molecular sieves gives rise to their unique properties. These properties mainly include adsorption capacity, ion exchange capacity, and catalytic activity. They are discussed separately below. (1) Adsorption properties: Compared with commonly used solid adsorbents such as silica gel, activated carbon, and activated alumina, zeolite molecular sieves exhibit two notable characteristics in terms of adsorption properties: selective adsorption and high-efficiency adsorption. Let’s discuss them separately below. 1. Selective adsorption is manifested in the following aspects: (1) Selective adsorption occurs based on the size of molecules. Due to their open skeletal structure, molecular sieves contain many neatly arranged and regular pores (cages), and the diameter of these pores is quite uniform, being on a similar order of magnitude as that of ordinary molecules. They can only allow molecules with a diameter smaller than that of the pores to pass through, while molecules with a larger diameter are rejected. Therefore, zeolite molecular sieves have a screening effect on molecules during adsorption; in other words, they exhibit selectivity based on the shape and size of molecules. Taking advantage of this property, molecular sieves can separate substances based on their shapes during adsorption. For example, the critical cross-sectional diameter of n-alkane molecules is 4.9 angstroms, while that of other hydrocarbons is greater than 5 angstroms. When a 5A molecular sieve with a pore size of 5 angstroms is used as an adsorbent, only n-alkanes can enter the pores and be adsorbed, whereas other hydrocarbons are rejected. This allows n-alkanes to be separated from other hydrocarbons; this process is known in industry as molecular sieve dewaxing. In the field of catalysis, the sieving properties of molecular sieves can also be utilized for so-called shape-selective catalysis. (2) Selective adsorption based on molecular polarity: Zeolite molecular sieves have a very large internal surface area, typically ranging from 600 to 1000 m3 per gram. An electrostatic field exists on this internal surface, which gives them polarity. It can adsorb polar substances containing polar groups (such as -OH, -NH2, -SH, =CO) or substances that are easily polarized (such as unsaturated hydrocarbons). Moreover, the greater the polarity of a substance or the easier it is to polarize, the more readily it will be adsorbed. Therefore, zeolite molecular sieves exhibit selectivity based on the polarity of molecules, allowing them to be separated according to their different polarities. For example, the molecular sizes of carbon monoxide and argon are very similar, but since carbon monoxide is a polar molecule while argon is a non-polar molecule, the amount of carbon monoxide adsorbed by 5A molecular sieve is much greater than that of argon. At -75°C and a pressure of 100 millimeters of mercury, 5A molecular sieve adsorbs 1.7% (by weight) of argon, compared to 11% (by weight) of carbon monoxide. For example, at 25°C and a pressure of 1 mmHg, the 4A molecular sieve adsorbs 0.3% ethane, 1.4% ethylene, and 3.8% acetylene. 2. High-efficiency adsorption: Water is a highly polar substance that can be easily adsorbed by zeolites; therefore, zeolites are often used as desiccants. Moreover, compared to other desiccants, they have notable advantages. For common adsorbents such as silica gel and alumina, their water absorption capacity is very poor when the partial pressure or concentration of water vapor is low, or when the adsorption temperature is high and the gas flow velocity is high. However, zeolite molecular sieves maintain excellent water absorption efficiency even under conditions of low partial pressure, low concentration, high temperature, and high flow rates. (II) Ion exchange capacity: Zeolite molecular sieves contain a large number of cations, which are all sodium ions in the case of synthetic zeolites. These sodium ions can undergo reversible exchange with other cations; such exchange alters the electrostatic field within the crystal, thereby changing its adsorption properties. The extent of this change varies depending on the properties of the cations and the degree of exchange. Thus, different cations can be used for varying degrees of exchange in order to adjust the adsorption properties of molecular sieves, as well as to modify their catalytic properties. Therefore, ion exchange is an extremely important property of zeolite molecular sieves. The reason why zeolite molecular sieves are used so extensively is closely related to their ion exchange properties. The properties of cations mentioned above include the magnitude of their charge, their electron configuration, and various characteristics related to their radius. For different cations, these properties vary; they can cause changes in the adsorption rate, adsorption selectivity, and adsorption capacity of zeolites for adsorbate molecules. In addition to these properties, the influence of factors such as the number, size, and position of cations on the performance of zeolites must sometimes also be considered. This effect is more pronounced in zeolites with smaller pore sizes. For example, the pore size of NaA molecular sieve is 4 angstroms, which is why it is also called 4A molecular sieve. When one-third of the sodium ions in NaA molecular sieve are replaced by calcium ions, since one calcium ion replaces two sodium ions, the number of cations decreases, creating space that allows the pore size of the molecular sieve to increase to 5 angstroms; hence, CaA is also known as 5A molecular sieve ； When ~25% of the sodium ions are replaced by potassium ions, the pore size of the molecular sieve decreases significantly because potassium ions are larger than sodium ions, becoming 3 Å; hence, KA is also known as 3A molecular sieve. A change in pore size inevitably leads to changes in screening performance. For example, NaX is generally referred to as 13X; if the sodium ions in it are replaced by calcium ions, its effective pore size becomes around 10 angstroms, resulting in an lOX-type molecular sieve. Ion exchange also affects the thermal stability of zeolites; after NaY is exchanged with rare earth metal ions, both its thermal and hydrothermal stability increase significantly, as indicated by an increase in the temperature at which the crystal structure is destroyed. In NaY with a SiO2/Al2O3 ratio of 4.8, when the degree of Na substitution reaches about 80%, the temperature at which the crystal structure begins to degrade rises from 700°C to 840°C. However, some cations such as Ba2+, Cu2+, Ni2+, Fe4+ 등 can reduce the stability of zeolites, especially in zeolites with a low silica-alumina ratio. For example, in NaA zeolite with a Si/Al ratio of 2, after being exchanged with Ba2+, the temperature at which its structure begins to degrade drops from 600°C to 90°C. The ion exchange of molecular sieves is generally carried out in an aqueous solution of metal salts. The metal salts used include chlorides, nitrates, sulfates, etc. During the exchange, the metal cations in the solution enter the zeolite, while the cations in the zeolite are exchanged out and enter the solution. It can be expressed by the following formula: A+ Z– + B+ ≒ B+ Z– + A+ (6). In this formula, Z– represents the anionic framework of the zeolite, A+ denotes the cations present in the zeolite before exchange (usually sodium ions), and B+ refers to the metal cations in the aqueous solution. Under certain conditions, ion exchange can reach equilibrium. Cation exchange of zeolites can also be carried out in non-aqueous solutions. When the metal to be exchanged is in an anion ； Or, although metal ions are cations, their salts are insoluble in water ； Or, although salts such as AlCl3 and FeCl3 dissolve in water, the resulting solution is highly acidic, which can easily damage the zeolite structure ； In such cases, organic solvents such as dimethyl sulfoxide and acetonitrile can be used for anhydrous solution ion exchange. During ion exchange, some cations can be easily exchanged onto the zeolite, while others find it more difficult to do so. Some cations, once exchanged onto the zeolite, can be easily displaced by other cations; in other words, zeolite exhibits selectivity toward ions during ion exchange. This selectivity arises from the structure of the zeolite, as well as from the properties of the cations (charge number, ionic radius, degree of hydration, etc.) and the exchange conditions. During ion exchange, the outcome of the exchange reaction is usually expressed in terms of exchange degree or exchange capacity. The exchange degree refers to the percentage of sodium ions that have been exchanged on the zeolite, relative to the original amount of sodium ions in the zeolite. Exchange capacity, on the other hand, represents the number of milliequivalents of cations exchanged per 100 grams of zeolite ； The utilization efficiency of cations in the solution is also expressed using an exchange rate; this rate represents the percentage of cations that are exchanged onto the zeolite from those originally present in the solution. Therefore, exchange degree and exchange capacity pertain to zeolites, whereas exchange rate pertains to the exchange solution. The degree and rate of exchange are influenced by factors such as the concentration and amount of the exchange solution, as well as its pH; the temperature and number of exchange cycles; and whether intermediate calcination is performed. These aspects will be discussed in detail when examining the rare earth exchange in Y-type molecular sieves, so they will not be covered in detail here. (III) Catalytic performance: Zeolite molecular sieves possess excellent catalytic properties. So, how exactly does the catalytic performance of molecular sieves arise? When discussing the structure of zeolite molecular sieves, we already know that they are composed of a silico-aluminosilicate framework (with negative charges) and metal ions (with positive charges), which depend on each other