The difference between zeolite and molecular sieve
Molecular sieve is powder crystal with metallic luster, hardness of 3 ~ 5 and relative density of 2 ~ 2.8. Natural zeolite has color, synthetic zeolite is white and insoluble in water. Thermal stability and acid resistance increase with the increase of SiO2 / Al2O3 composition ratio. Molecular sieve has a large specific surface area, up to 300 ~ 1000m2 / g, and the inner crystal surface is highly polarized. It is not only a kind of efficient adsorbent, but also a kind of solid acid. The surface has high acid concentration and acid strength, which can cause positive carbon ion type catalytic reaction. When the metal ions in the composition are exchanged with other ions in the solution, the pore size can be adjusted to change its adsorption and catalytic properties, so as to prepare molecular sieve catalysts with different properties. Zeolite is the general name of zeolite group minerals. It is an aluminosilicate mineral containing aqueous alkali metal or alkaline earth metal. According to the characteristics of zeolite minerals, it can be divided into four types: frame, sheet, fibrous and unclassified. According to the characteristics of pore system, it can be divided into one-dimensional, two-dimensional and three-dimensional systems. Any zeolite is composed of silica tetrahedron and alumina tetrahedron. Tetrahedrons can only be connected by vertices, that is, they share one oxygen atom, not "edges" or "faces". The aluminum oxygen tetrahedron itself cannot be connected, and there is at least one silicon oxygen tetrahedron between them. The silicon oxygen tetrahedron can be directly connected. Silicon in silicon oxygen tetrahedron can be replaced by aluminum atoms to form aluminum oxygen tetrahedron. However, the aluminum atom is trivalent, so in the aluminum oxygen tetrahedron, the electricity price of one oxygen atom is not neutralized, resulting in charge imbalance, which makes the whole aluminum oxygen tetrahedron have negative points. In order to keep neutral, there must be positively charged ions to offset, which are generally compensated by alkali metal and alkaline earth metal ions, such as Na, CA, Sr, Ba, K, Mg and other metal ions. Because of its unique internal structure and crystalline chemical properties, zeolite has a variety of characteristics that can be used in industry and agriculture. The natural zeolite is light gray, and sometimes it has been found in the world. Holding it in your hand is obviously lighter than ordinary stones. This is because the zeolite is filled with subtle holes and channels, which is much more complex than the hive. If zeolite is compared to a hotel, there are 1 million "rooms" in this "Super Hotel" of 1 cubic micron! These rooms can automatically open or block the door according to the gender, height, weight and hobbies of "passengers" (molecules and ions), and will never let "fat" go to "thin" rooms, nor will tall people live in the same room with short people. According to this characteristic of zeolite, people use it to screen molecules and obtain good results. This is of great significance for the recovery of copper, lead, cadmium, nickel, molybdenum and other metal particles from industrial waste liquid. Zeolite has the properties of adsorption, ion exchange, catalysis, acid resistance and heat resistance, so it is widely used as adsorbent, ion exchanger and catalyst, as well as gas drying, purification and sewage treatment. Zeolite also has "nutritional" value. Adding 5% zeolite powder to the feed can accelerate the growth of livestock, make them strong, fresh meat and high egg laying rate. Due to the porous silicate nature of zeolite, there is a certain amount of air in the pores, which is often used to prevent explosion and boiling. During heating, the air in the small hole escapes, playing the role of gasification core, and small bubbles are easy to form on its corners. The main difference is that in their use, zeolites are generally natural with different pore sizes. As long as there are bubbles, they can prevent boiling. The function of molecular sieve is much higher, such as screening molecules, making catalysts, slow-release catalysts, etc. Therefore, it has certain requirements for pore size, which is often synthetic.
Method and characteristics of preparing zeolite molecular sieve from natural silicoaluminescent clay
Zeolite molecular sieve is a kind of silicoaluminate crystal with regular pore structure. It is widely used in the fields of gas adsorption and separation, industrial catalysis, heavy metal ion pollution control and so on. The traditional hydrothermal synthesis of zeolite molecular sieve often takes chemical products containing silicon and aluminum and organic template as raw materials, which is not only expensive, but also pollutes the environment. In recent years, with the popularity of the concept of "green chemical industry", natural aluminosilicate clays such as kaolin, montmorillonite, rectorite and illite have shown great potential as raw materials for the synthesis of zeolite molecular sieves because of their rich reserves and low price. Their synthesis processes mainly include seed method, steam assisted solid-phase method and solvent-free method. 1. Seed method Since Holmes et al reported the production of high-purity ZSM-5 molecular sieve with natural kaolin as silicon source and commercial molecular sieve as crystal seed, the crystal seed method has greatly reduced the production cost because it can greatly shorten the synthesis induction period, inhibit the formation of hybrid crystals and regulate the grain size, as well as the characteristics of green synthesis process, simple and convenient operation and no organic template, It has become one of the representative routes of green synthetic zeolite molecular sieve. The mechanism of synthesizing clay based zeolite molecular sieve by seed method tends to liquid phase synthesis mechanism, that is, the zeolite seed is partially dissolved in the early stage of crystallization to form small fragments with the primary unit structure of zeolite molecular sieve; At the same time, the aluminosilicate gel formed by the dissolution polycondensation of the active aluminosilicate species produced by the activation of natural aluminosilicate clay will gradually wrap the seed fragments and crystallize under the structure guidance of the seed to form a shell structure with the seed as the core. With the extension of crystallization time, the amorphous aluminate gel gradually generates primary molecular sieve structural units, which deposit from shell to core through concentration polymerization, and finally convert the active geological and mineral polymer formed by clay depolymerization into zeolite molecular sieve. 2. Quasi solid phase combination method The technology is characterized in that the spacer is used to crystallize the raw material for synthesizing zeolite molecular sieve in the vapor phase of reaction solvent and structure directing agent. Compared with the traditional hydrothermal synthesis process, the quasi solid phase synthesis system has been widely used in the synthesis of ZSM-5, SSZ-13, SAPO-34 and other zeolites in recent years because of its advantages such as less amount of template, saving water and eliminating the separation steps between products and mother liquor. The crystallization process of natural silica alumina clay based zeolite prepared by quasi solid phase synthesis technology is more in line with the two-phase crystallization mechanism between solid-phase and liquid-phase synthesis. That is, in the early stage of crystallization of solid-phase synthetic zeolite molecular sieve, natural silicoaluminescent clay dissolves under the dual action of water vapor and strong alkaline hydroxide ions attached to the surface of solid raw materials, generates active silicon and aluminum species, and takes the lead in crystallization into zeolite molecular sieve microcrystals. With the extension of crystallization time, ZEOLITE CRYSTALLITES absorb more active silicon and aluminum species from their surroundings, and gradually grow according to Oswald mechanism under the action of Na + and structure directing agent. In the vapor environment, the mass transfer and heat transfer of active silicon and aluminum species in the environment around the crystal nucleus are greatly increased, which not only reduces the activity of the surface of geopolymer and makes the organic template easy to adhere to the surface of solid raw materials, but also promotes the further depolymerization and rearrangement of geopolymer, thus accelerating the growth rate of crystal. Although the preparation of clay based zeolite molecular sieve by solid-phase like synthesis technology overcomes the green synthesis characteristics of a large number of synthetic solvents, it is still unable to be industrialized due to a series of practical problems, such as cumbersome synthesis operation, excessive pressure in the system during crystallization and impurity of synthetic products. 3. Solvent free method In order to overcome the problems of large discharge of alkaline solution, environmental pollution, low yield of single kettle and high pressure of synthesis system caused by the use of solvent water in the traditional synthesis of zeolite molecular sieve, the technology of solvent-free synthesis of clay based zeolite molecular sieve came into being. Since the solvent-free synthesis of zeolite molecular sieve belongs to the interaction between solid and solid state, and there is no solvent addition in its synthesis process, the problems of solvent emission and synthesis pressure caused by zeolite production are completely eliminated. At present, it is considered that the solvent-free synthesis of clay based zeolite molecular sieve follows the solid-state transformation mechanism. That is, the formation of zeolite crystallization should go through four stages: diffusion, reaction, nucleation and growth. Different from hydrothermal seed synthesis and steam assisted solid-phase synthesis, there is neither the dissolution of solid-phase raw materials nor the direct involvement of liquid phase in the nucleation and crystal growth of zeolite in the process of solvent-free synthesis. In the process of zeolite synthesis, prolonging the grinding time and strengthening the grinding force can not only increase the opportunity of intermolecular contact and facilitate the spontaneous diffusion of molecules, but also increase the surface free energy of reaction components, so as to increase the total free energy of zeolite synthesis. In the crystallization process, depending on the rich voids and concentration gradient difference between the phase interfaces, the active silicon and aluminum species produced by the activation and depolymerization of natural silicoaluminescent clay polymerize and gradually form a primary "crystal core", and then they will continue to be polycondensated, condensed and finally connected into molecular sieve single crystals.
The Structure and Properties of Molecular Sieves
Molecular Sieves (1) Control of grain size and shape The pore size of most zeolite molecular sieves is less than 1 nm. When small molecular organics react in the zeolite pores, the diffusion will be restricted to a certain extent, which will affect the pore utilization and catalytic performance. Reducing the grain size and changing the shape of the grain is the means to improve the molecular diffusion performance and the utilization rate of the pore channels. The diffusion path of the small grain or nano molecular sieve is shorter than that of the large grain molecular sieve, the utilization rate of the pore channel will be greatly improved, and the catalytic activity will also be reduced. There is improvement. (2) Multi-level pore compound Most of the mesoporous materials reported so far have shortcomings such as poor thermal stability, lack of surface acid centers with a certain strength, and easy loss of acid centers. The main reason is that although the above materials have ordered mesoporous channels, their The skeleton is an amorphous structure. Although zeolite molecular sieves have good structural stability and strong acid centers, there are limitations in molecular diffusion, which affect their catalytic activity and selectivity. The microporous and mesoporous or macroporous hierarchical porous composites are expected to combine the advantages of both and exert their advantages in practical applications. Hierarchical pore zeolite molecular sieves are expected to be used in some larger molecular catalytic reactions and liquid-phase catalytic reactions. (3) Co-crystal molecular sieve The catalytic nature of co-crystalline molecular sieves is actually the fine adjustment of pores and acidity, which is a means to improve the performance of catalysts. The catalytic performance of crystalline molecular sieves has been greatly improved. For example, when ZSM-5/ZSM-11 (MFI/MEL) co-crystalline molecular sieves are used in MTG reaction, gasoline components can be adjusted in a wide range. (4) Surface modification of molecular sieve and improvement of its hydrothermal stability Thermal stability and hydrothermal stability are one of the important properties of molecular sieve catalysts to be investigated. Many industrial catalytic reactions require high thermal stability of catalysts, especially hydrothermal stability. They often determine the life of catalysts and the selection of reaction processes. key. Taking the catalytic cracking reaction of CTE as an example, because the reaction is carried out under the condition of steam, improving the hydrothermal stability of the catalyst is the key to the development of CTE catalysts. The results show that the stability of the active center of the catalytic material under water vapor can be improved by assembling and modifying the catalytic active center of the porous material with phosphorus oxide compounds and introducing framework heteroatoms.
Catalytic Properties of Molecular Sieves
(1) Activity requirements for catalytic reaction: Large specific surface area, uniform pore distribution, adjustable pore size, good shape selection for reactants and products; stable structure, high mechanical strength, high temperature resistance (400 ~ 600 ° C), good thermal stability, after activation and regeneration Reusable; non-corrosive to equipment and easy to separate from reaction products, basically no "three wastes" are generated in the production process, and the waste catalyst is easy to handle and does not pollute the environment. For example, the research system of shape-selective catalysis includes almost all the conversion and synthesis of hydrocarbons, as well as the catalytic conversion of alcohols and other nitrogen, oxygen, sulfur-containing organic compounds and biomass, which are fundamental research, applied research and industrial. Development has opened up a vast field. Some transition metal-containing zeolite molecular sieves are not only used in traditional acid-base catalysis systems, but also in oxidation-reduction catalysis processes. (2) Efficient catalysis of zeolite molecular sieves For zeolite molecular sieves used in industrial catalysis, high performance is the basic requirement and goal. The type and number of active centers of catalytic materials and the diffusion performance of micropores are the intrinsic factors that affect their catalytic activity. Catalytic selectivity is closely related to the shape selectivity of micropore channels, the occurrence of side reactions, and the diffusion speed of each reaction molecule. Lifetime has always been an important indicator to measure the performance of catalytic materials. The eternal topic of the process. On the premise that the catalyst activity meets the requirements, if the deactivated catalyst is easy to regenerate and the structure can be recovered, that is, it can be regenerated repeatedly, and then with a suitable reaction process, the purpose of prolonging the life of the catalyst can be achieved. Therefore, high performance not only puts forward higher requirements for zeolite molecular sieve materials, but also requires multi-scale combination and coordination of catalytic materials, reaction processes and reaction engineering systems, and finally enables catalysts to achieve high performance in industrial applications.
Molecular Sieve Catalyst | Classification of molecular sieve catalysts
Molecular Sieve Catalyst Molecular sieves are divided according to the size of the pores, and there are molecular sieves smaller than 2 nm, 2-50 nm and larger than 50 nm, which are called microporous, mesoporous and macroporous molecular sieves respectively. Molecular sieves can be divided into three categories according to the pore size: microporous, mesoporous and macroporous molecular sieves. Microporous molecular sieves have the advantages of strong acidity, high hydrothermal stability, and special "shape-selective catalysis" performance, but they also have disadvantages such as narrow pore size and large diffusion resistance, which greatly limit their application in macromolecular catalytic reactions. Mesoporous molecular sieves have the characteristics of high specific surface area, large adsorption capacity, and large pore size, which can solve the problem of mass transfer and diffusion to a certain extent. However, their weak acidity and poor hydrothermal stability limit their industrial applications. In order to solve the above problems, researchers have developed hierarchical porous molecular sieves, which combine the advantages of mesoporous and microporous molecular sieves and have immeasurable application prospects in the petrochemical field.
what molecular sieve？
Molecular sieve, often called zeolites or zeolite molecular sieves, are classically defined as "aluminosilicates with a pore (channel) framework structure that can be occupied by many large ions and water". According to the traditional definition, molecular sieves are solid adsorbents or catalysts with a uniform structure that can separate or selectively react molecules of different sizes. In a narrow sense, molecular sieves are crystalline silicates or aluminosilicates, which are connected by silicon-oxygen tetrahedra or aluminum-oxygen tetrahedra through oxygen bridges to form a system of channels and voids, thus having the characteristics of sieving molecules. Basically, it can be divided into several types of A, X, Y, M and ZSM, and researchers often attribute it to the solid acid category.