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.
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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.
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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.