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.
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.
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.
Separation and purification are very important in production and life. About 40-60% of the energy in the production process is used for separation and purification; the separation of substances with similar physical properties is also very difficult, such as the separation between isomers. Membrane-based separation methods, if the separation efficiency can be improved, can greatly reduce energy consumption. For example, organic solution nanofiltration membranes are used for the purification of high-value products, but cannot effectively separate molecules of similar molecular size due to insufficient molecular specificity. In order to obtain a better separation and purification method, effectively reduce the energy consumption, and improve the separation efficiency, researchers still need to continue research. Results introduction On August 19, Ryan P. Lively, School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, USA, reported an asymmetric carbon molecular sieve (CMS) hollow fiber membrane in Nature as a potential organic solvent reverse osmosis technology (OSRO). Material. The organic solvent reverse osmosis technology using carbon molecular sieve not only does not need to change the phase of the organic matter, reduces the energy loss in the separation process, but also effectively separates the organic matter with similar molecular sizes. The authors used the changes in the permeability of para-xylene and ortho-xylene in CMS films to reflect the permeation performance of CMS. Using carbon molecular sieve membrane, the reverse osmosis separation of organic liquid molecules can be achieved, and the separation can be efficiently completed without changing the phase morphology and reducing energy consumption. Outlook The use of the dialysis separation technology under the low temperature and high pressure of the separation membrane can greatly reduce the energy consumption, but the separation efficiency and separation selectivity are still great challenges, and the continuous efforts of the majority of researchers are still needed.
carbon molecular sieves Hydrogen (H2) production from natural gas is considered to be one of the most potential technologies for low-carbon energy in the future and reducing greenhouse gas emissions. Compared with conventional H purification technologies, membrane-based separation technologies have received extensive attention due to their higher energy efficiency and environmental friendliness. However, currently commonly used H2 and CO2 separation membranes usually suffer from low separation performance, high cost, and low stability under high temperature and high pressure. Therefore, it is still challenging to prepare commercially viable H2 purification membranes. Carbon molecular sieve (CMS) membranes are made by controlled carbonization of polymer precursors at high temperatures and have rigid pore structures. When the CMS membrane is made into a hollow fiber suitable for a membrane module, it is expected to have the properties of high temperature and high pressure resistance. Cellulose has strong interchain and intrachain hydrogen bonds, which makes it poorly soluble in most solvents, with only a few solvents such as N-methylmorpholine-N-oxide (NMMO), ionic liquids and Inorganic salts can effectively disrupt their hydrogen bond network. However, obtaining an accurate cellulose/solvent/non-solvent ternary phase diagram is still challenging due to the huge viscosity of this system. Based on this, Xuezhong He et al. from the Norwegian University of Technology prepared carbon hollow fiber membranes (CHFMs) by adjusting the solidification temperature and final carbonization temperature of the cellulose/ionic liquid/water system and used them for H2 separation. The researchers prepared asymmetric cellulose hollow fiber precursors through a dry and wet spinning process, and then exchanged with water to remove the original solvent EmimAc and DMSO, and finally obtained the corresponding microporous structure through high-temperature carbonization. From the SEM images, it can be found that the asymmetric structures of the outer selective layer and the porous inner support layer of about 3 μm are still maintained when different carbonization temperatures are used. CHFM-550 at the lowest carbonization temperature has the lowest hardness and Young's modulus. With the increase of carbonization temperature, the hardness and Young's modulus increase gradually. The increase in hardness and modulus can be attributed to the internal structural changes caused by the increase in carbonization temperature. At the same time, with the increase of carbonization temperature, the pore peaks >5 Å weakened, while the pore peaks <5 Å increased, which indicated that the average pore size of the carbonized CHFMs decreased at high temperature. Furthermore, both the surface area and pore volume of CHFM decreased with increasing carbonization temperature, suggesting that CMS films tended to have denser packing when carbonized at higher temperatures. The membranes prepared at higher carbonization temperatures have higher H2/CO2 selectivity, but lower H2 permeability, which indicates that the gas permeability is mainly determined by the motion diameter of gas molecules, i.e. molecular sieve transport transport mechanism. When the sp3/sp2 ratio decreased from 0.73 to 0.36, the H2 permeability decreased from 466.8 GPU to 148.2 GPU, while the H2/CO2 selectivity increased from 11.1 to 83.9, which also suggested that the gas separation performance can be tuned by adjusting the carbon structure. Due to the simultaneous presence of molecular sieve and surface diffusion transport of CO2 molecules, the apparent activation energy of CO2 is relatively lower compared to H2, which indicates that temperature has a greater effect on H2 permeability, so lower CO2 adsorption at higher temperatures This leads to an increase in H2/CO2 selectivity. When the membrane was exposed to the laboratory atmosphere for 50 days, its H2 permeability and H2/CO2 selectivity decreased by about 40% and 10%, respectively, and the gas permeability and gas permeability were effectively restored after heat treatment and helium purging. Optional. CHFM exhibits excellent H2/CO2 selectivity and high H2 permeability compared to other membranes, of which CHFM-850 shows the highest overall gas separation performance with an ideal H2/CO2 selectivity of 83.9 at 130 °C, exceeding non-polymer films. At the same time, the selectivity of CHFM-850 to H2/N2 is >800 and the selectivity of H2/CH4 is >5700, which provides the possibility for H2 purification in some processes. In summary, this work produced an asymmetric cellulose hollow fiber material by spinning microcrystalline cellulose and EmimAc. The obtained cellulose hollow fibers are carbonized at high temperature to obtain asymmetric hollow fiber membranes whose microporous structure helps them to separate H2 from other gases.