Zeolite molecular sieve materials play a huge role in petroleum fine chemicals and environmental governance. Usually, most zeolite molecular sieves need to be synthesized with the participation of organic templates. However, most of the template agents used are toxic, which has a strong impact on the actual production and application of zeolites. Green synthesis routes refer to using greener raw materials to synthesize target products, and reducing or even eliminating negative impacts on the environment, reducing waste emissions and improving efficiency during the synthesis process.
First, after the raw materials required for zeolite molecular sieve are mixed, the main species silicate and aluminate are polymerized to form an initial aluminosilicate gel. This aluminosilicate gel is formed rapidly under high concentration conditions and therefore has a high degree of disorder. However, this aluminosilicate gel may contain certain primary structural units, such as: four-membered rings, Six-membered rings and so on. At the same time, a dissolution equilibrium is established between this gel and liquid phase. In addition, the solubility product of aluminosilicate ions is closely related to the structure and temperature of the gel. As the crystallization temperature changes, a new equilibrium between the gel and the liquid phase is established between the gel and the solution. Secondly, the increase in the concentration of polysilicate and aluminate in the liquid phase leads to the formation of crystal nuclei and then the growth of zeolite molecular sieve crystals. During the nucleation and crystal growth process of zeolite molecular sieves, the polysilicate and aluminate ions in the liquid phase are consumed, causing the continued dissolution of the silica-alumina gel. Since the solubility of zeolite crystals is less than that of amorphous gel, the final result is complete dissolution of the gel and complete growth of zeolite molecular sieve crystals.
For the synthesis of zeolite molecular sieves, temperature is a very important factor. Temperature changes will affect the changes in water pressure in the reactor, the polymerization state and polymerization reaction of aluminosilicates, the formation, dissolution and transformation of gels, the nucleation and growth of molecular sieves, and the crystallization between metastable phases. The same system may obtain completely different phases at different temperatures. The higher the temperature, the smaller the size and pore volume of the zeolite obtained, and the density of the crystal skeleton increases accordingly. Generally speaking, below 150C, the primary structure is often a four-membered ring or a six-membered ring, and when the temperature is higher than 150C, it is often the primary structural unit of a five-membered ring. It can be seen that under high-temperature hydrothermal conditions, there is a close relationship between the pore-forming rules of inorganic substances (mainly aluminosilicate species) and the crystallization temperature and water vapor pressure.
In order to overcome problems such as the discharge of alkali-containing wastewater due to the introduction of solvent water during the conventional hydrothermal synthesis of zeolite molecular sieves, the pressure of the synthesis system is too high, and the yield of a single reactor is too low, people have developed a new method. Synthetic route of green zeolite molecular sieve by solvent method. Through the characterization of the crystallized products during the crystallization process, it was found that the synthesis of zeolite molecular sieves by the solvent-free method goes through the following process: in the early stage of crystallization, the solid phase raw materials gradually diffuse in amorphous silica, accompanied by the polymerization of silicon species; As the crystallization time increases, amorphous silica gradually transforms into crystal. In general, the solid-phase synthesis reaction process has gone through the mixing and diffusion of initial raw materials, the continuous condensation of silanol groups, etc., and finally the reaction raw materials are converted into silicalite-1 zeolite molecular sieve in the solid-phase state.