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.