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.
What is the difference between zeolite and molecular sieve?
Zeolite, molecular sieve, zeolite molecular sieve, these words are easy to confuse, today we will talk about the difference between them: Zeolite is only one type of molecular sieve. Because zeolite is the most representative among molecular sieves, the terms "zeolite" and "molecular sieve" are easily confused by beginners. Molecular sieves are crystalline silicates or aluminosilicates, composed of silicon-oxygen tetrahedrons or aluminum-oxygen tetrahedrons connected by oxygen bridges to form a molecular size (usually 0.3 nm to 2.0 nm) channel and cavity system , so as to have the characteristics of sieving molecules. Molecular sieve is powder crystal with metallic luster, hardness is 3-5, and relative density is 2-2.8. While natural zeolite has color, synthetic zeolite is white, insoluble in water, thermal stability and acid resistance increase with the increase of SiO2/Al2O3 composition ratio. The main difference between the two is in the use. Zeolite is generally natural, with different pore sizes. As long as there are cavities, it can prevent bumping; while the functions of molecular sieves are much more advanced, such as screening molecules, making catalysts, and slow-release catalysts. etc., so there are certain requirements for the aperture, and they are often artificially synthesized. I don't know if you have a deeper understanding of the relationship between zeolite and molecular sieves in today's explanation.
Method and characteristics of preparation of zeolite molecular sieve from natural silica-alumina clay
Zeolite molecular sieve is a kind of aluminosilicate crystal with regular pore structure, which is widely used in gas adsorption separation, industrial catalysis, heavy metal ion pollution control and other fields. The hydrothermal synthesis of traditional zeolite molecular sieves often uses chemical products containing silicon and aluminum and organic templates as raw materials, which is not only expensive, but also pollutes the environment. In recent years, with the popularization of the concept of "green chemical industry", natural silica-alumina clays such as kaolin, montmorillonite, rectorite, and illite have the advantages of abundant reserves and low price. It has shown great potential, and its synthesis methods 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 seed crystal, the seed crystal method can greatly shorten the synthesis induction period, inhibit the formation and regulation of heterocrystals. Excellent effects such as grain size, as well as the characteristics of green synthesis process, simple and convenient operation, no organic template agent for synthesis and greatly reducing production cost, have now become one of the representative routes for green synthesis of zeolite molecular sieves. The mechanism of synthesizing clay-based zeolite molecular sieves by seed crystals tends to the liquid phase synthesis mechanism, that is, zeolite seeds are partially dissolved in the early stage of crystallization to form small fragments with the primary unit structure of zeolite molecular sieves; at the same time, they are activated by natural silica-alumina clay The generated active silica-alumina species are dissolved-polycondensed to form aluminosilicate gel, which will gradually envelop the seed crystal fragments, and crystallize under the structural guidance of the seed crystal to form a shell structure with the seed crystal as the core. With the prolongation of crystallization time, the amorphous aluminate gel gradually generates primary molecular sieve structural units, which are deposited from the shell to the core through condensation-polymerization, and finally convert the active geomineral polymers formed by clay depolymerization. Become a zeolite molecular sieve. 2. Solid-phase synthesis method The feature of this technology is that the raw material for synthesizing zeolite molecular sieve is placed in the vapor phase of the reaction solvent and the structure-directing agent for crystallization synthesis by using the spacer. Compared with the traditional hydrothermal synthesis process, the solid-phase synthesis system has been widely used by researchers in recent years for ZSM-5, In the synthesis of zeolites such as SSZ-13 and SAPO-34. The crystallization process of natural silica-alumina clay-based zeolite molecular sieves prepared by solid-phase synthesis technology is more in line with the dual-phase crystallization mechanism between solid-phase and liquid-phase synthesis. That is, in the initial stage of crystallization of solid-phase synthetic zeolite molecular sieves, the natural silica-alumina clay is dissolved under the dual action of water vapor and strong alkaline hydroxide ions attached to the surface of the solid raw material, and active silicon and aluminum species are generated. , and took the lead in crystallization into zeolite molecular sieve crystallites. With the prolongation of crystallization time, zeolite crystallites absorb more active silicon and aluminum species from their surroundings, and grow gradually following the Oswald mechanism under the action of Na+ and structure directing agents. In the vapor environment, the mass transfer and heat transfer of the active silicon and aluminum species in the surrounding environment of the crystal nucleus are greatly increased, which not only reduces the activity of the surface of the geopolymer, but also makes the organic template easily attached to the surface of the solid raw material. It also promotes further depolymerization and rearrangement of geomineral polymers, thereby accelerating the growth rate of crystals. Although the preparation of clay-based zeolite molecular sieves by solid-phase synthesis technology overcomes the green synthesis characteristics of a large amount of synthetic solvents, the actual synthesis operation is too cumbersome, the pressure in the system is too large during crystallization, and the synthesis products are mixed. A series of practical problems are still unable to be applied industrially. 3. Solvent-free method In order to overcome the problems of large amount of alkaline solution discharge to pollute the environment, low yield per kettle and high pressure of synthesis system due to the use of solvent water in the traditional synthesis of zeolite molecular sieves, the technology of solvent-free synthesis of clay-based zeolite molecular sieves came into being. Since solvent-free synthesis of zeolite molecular sieve belongs to the interaction between solid and solid state, no solvent is added in the synthesis process, so the problem of solvent discharge and synthesis pressure caused by zeolite production is completely eliminated. At present, it is believed that the solvent-free synthesis of clay-based zeolite molecular sieves follows a solid phase transition mechanism. That is to say, in the process of zeolite crystallization, it goes through four stages of diffusion, reaction, nucleation and growth. The difference from hydrothermal seed crystal synthesis and steam-assisted solid-phase synthesis is that in the process of solvent-free synthesis of zeolite molecular sieves, there is neither the dissolution of solid-phase raw materials nor the direct involvement of liquid phase in molecular sieve nucleation and crystal growth. In the process of zeolite synthesis, prolonging the grinding time and increasing the grinding strength can not only increase the chance of intermolecular contact, which is conducive to the spontaneous diffusion of molecules, but also increase the surface free energy of the reaction components, thereby increasing the total free energy of zeolite synthesis. Purpose. During the crystallization process, depending on the abundant voids and concentration gradient differences between the phase interfaces, the active silicon and aluminum species generated by the activation and depolymerization of natural silico-alumina clays polymerize, gradually forming a primary "crystal nucleus", and then continuously Polycondensation, condensation form and finally combine into molecular sieve single crystals.