Preparation method of zeolite molecular sieve

Hydrothermal and solvent-thermal synthesis method

The hydrothermal (solvent) synthesis method involves the pre-dispersion of the precursor of the synthetic zeolite in aqueous (solvent) solution, followed by nucleation, growth, and crystallization at a certain temperature and autogenous pressure to form the zeolite. the first low-silica zeolites were synthesized by Barrer using the low-temperature hydrothermal method in the 1940s, and other types of molecular sieves have been successfully synthesized since then. 

The advantages of this synthesis method are the strong solubility of water for most substances, especially ionic and polar compounds (water is a polar molecule and similarly soluble principle), and the mild reaction conditions, low pollution and low cost of this synthesis method; the disadvantages are the long synthesis period, impure zeolite molecular sieve formed, prone to heterogeneous phase, and limited variety of zeolites synthesized. Later, people tried to use organic solvents instead of water, Bibby and Dale first used ethylene glycol and propanol as solvents to synthesize zeolites with a wide range of adjustable silica-alumina ratio and pure silica zeolites with SOD structure; later, Xu Ruren and others used a variety of organic solvents to synthesize ZSM-39 and ZSM-48 type molecular sieves. 

The advantages of this method: solvents can dissolve reactants that are insoluble in water or unstable when dissolved in water, and organic solvents have diverse physical and chemical properties, providing a broader scope for the synthesis of zeolites; at the same time, it helps to generate single crystals with fewer defects under the solvent heat system; the disadvantage is that the synthesis process uses a large amount of organic solvents, which increases the danger of operation and is not suitable for actual industrial production.

Vapor-phase synthesis method

The gas phase synthesis method is to prepare the precursor of molecular sieve without template agent into dry gel, and then transform it into molecular sieve by a certain temperature under the atmosphere of small amount of water and organic amine as liquid phase components. 1990 Xu et al. firstly proposed this method and successfully synthesized MFI type molecular sieve; after that, ZSM-5 and ZSM-35 molecular sieve membranes have been synthesized by this method. The advantages and disadvantages of this method are very obvious. 

The advantages are that no waste water is generated in the synthesis process, the mixed solvent can be recycled, the amount of template agent is reduced, and the cost of synthesizing molecular sieves is lower; the disadvantages are that the crystallization time is long, the synthesis cycle is long, and the products are prone to heterogeneous phases. For synthetic molecular sieve film, because of slow heating and uneven heating, zeolite will not nucleate on the surface of the support body at the same time, which in turn affects the thickness of the molecular sieve film.

Dry gelation method

The dry gel method is to mix a certain amount of zeolite synthesis raw materials, template agent and deionized water, then filter and wash them to obtain an amorphous gel, dry them into dry powder after the gel is formed, and finally finish the synthesis under water vapor atmosphere. 

This method was proposed by Wenyang Xu, a Chinese scholar, in 1990, and successfully synthesized ZSM-5 molecular sieve. Later, MFI-type zeolite nanocrystals with medium pore size were synthesized by this method. The advantages of this method are: less amount of template agent, less waste emission and lower synthesis cost, and higher yield to some extent; the disadvantage is that the dry gel used in the synthesis process should be prepared first and then the hydrated gel should be evaporated to dryness, which makes the preparation process relatively complicated and limits the industrial application.

Ion-thermal synthesis method

In 2004, Morris group reported the ionothermal synthesis of molecular sieves, and in 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences used ionothermal synthesis to synthesize superporous (20-membered ring) aluminum phosphate molecular sieves. 

The advantages of this method are: the organic cations in the ionic liquid play the role of solvent and structure guide; there are strong interactions between ions in the ionic liquid, it is difficult for individual molecules to detach from the whole system, and the heating process generally does not generate vapor pressure, which reduces the danger caused by high pressure and is more suitable for industrial production; the disadvantages are long synthesis period, high energy consumption and low efficiency.