ZSM-5 molecular sieve pore structure modification

In recent years, in order to improve the shape-selective properties and diffusion properties of catalysts, the modification of the pore structure of ZSM-5 molecular sieve catalysts has attracted more and more attention. Porous zeolite molecular sieves provide the possibility for the entry and exit of reactants and product molecules on their active sites, and greatly improve the catalytic ability of molecular sieves, which is also an effective way to shorten the micropore diffusion path through the synthesized secondary pore structure.

ZSM-5 molecular sieve pore structure modification: desilication (alkali treatment), dealumination (acid treatment), additive pore expansion modification

Desiliconization--------Use organic alkali (TPAOH, TBAOH) to treat ZSM-5 molecular sieve with a silicon-to-aluminum ratio of 42. Compared with the traditional molecular sieve treated with inorganic alkali NaOH, NH4NO3 is used after NaOH treatment is omitted. The ammonium exchange step: dissolves silicon much faster than NaOH and also makes the desilication process controllable. The principle is that part of the silicon on the framework of the zeolite is removed during the alkali treatment process, causing part of the framework to collapse, resulting in a mesoporous structure. Many researchers have explored the optimal treatment conditions for this method, and the results do not affect the acidity of molecular sieves. And it can effectively increase the specific surface area of molecular sieves, but the selectivity of desiliconization by organic alkali treatment is poor, and a large amount of aluminum will be dissolved in the organic alkali solution.

Dealumination————The treatment of zeolite molecular sieve in acid solution can cause partial framework dealumination reaction, resulting in partial framework collapse, accompanied by the dissolution of impurities between molecular sieve crystals, resulting in mesopores. The use of HCl and hydrothermal ZSM-5 molecular sieve was calcined at high temperature. The results showed that a large amount of extra-framework Si-Al phase was generated by hydrothermal treatment, but the dealumination effect was not good when treated with relatively strong HCl, while the treatment with AHFS proved that this was a good solution. Suitable means for moderately dealuminated zsm-5 molecular sieve samples.

Desiliconization and dealumination processes are simple and effective methods for synthesizing micro-mesoporous porous materials, but these two methods not only affect the pore structure of molecular sieves, but also change the distribution of B and L acids on the molecular sieves. Although many literatures point out that acid treatment can generate mesopores, acid treatment seriously affects the acidity of molecular sieves, resulting in a significant reduction in acid content. Therefore, compared with alkali treatment, acid treatment needs further research and exploration.

Additive pore expansion modification---------The method of adding other additives to form mesopores during the synthesis of molecular sieves has become a research hotspot in recent years. This method, also known as the "second template method", is to add another template agent with a structure-directed effect to the initial solution for the synthesis of molecular sieves to generate porous ZSM-5 molecular sieves. Using excess gel, the molecular sieve is grown based on the porosity of carbon black particles, and the carbon black framework is removed by calcination. The pore size distribution of ZSM-5 molecular sieve produced by this method is between 10-100 nm, and most of the pores are larger than 50 nm. .

3.2 Particle size modification of ZSM-5 molecular sieve: reducing crystallization temperature, confined space method, adding alkali metal salt, adding seed crystal, adding amphoteric silicone surfactant

The grain size of nano ZSM-5 molecular sieve is between 1-100nm, and the decrease of the size of ZSM-5 molecular sieve will reduce the length of the diffusion path, thereby improving the diffusion performance of molecules. In most catalytic reactions, molecular sieves The reduction in grain size favors the intergranular mesoporous diffusion step.

3.2.1 Lower the crystallization temperature

Scientists synthesized ZSM-5 molecular sieve at a low temperature of 70-90 °C, and its grain size decreased significantly, but also reduced the molecular sieve yield. This lower temperature condition can avoid the polymerization of the primary groups of nano-sized molecular sieves. The synthesized molecular sieve The size is about 200nm, and it has a high specific surface area, and its molecular sieve framework structure has high and low hydrothermal stability.

3.2.2 Confined space method

Nano ZSM-5 molecular sieve was synthesized by confined space method. This method is that in the pores of the activated carbon inert medium, the particle size distribution of the product obtained by crystallization of the zeolite is related to the pores of the inert medium, and its particle size is smaller than the pore size inside the activated carbon, and has nothing to do with the composition of the reactant gel. method, the nano ZSM-5 molecular sieve with controllable particle size can be prepared, and its particle size is between 20-40nm.

3.2.3 Adding metal salts

The added alkali metal salts include NaCl, KCl, Na2SO3 and NaH2PO3, etc. The grain size of the added NaH2PO3 is the largest, about 300nm, and the grain size of the added NaCl and KCl is about 40-60nm, while the alkali metal particles such as Na+ and K+, in During the synthesis, the crystallization rate of molecular sieves was accelerated.

3.2.4 Adding seeds

During the synthesis of ZSM-5 molecular sieve, nano-sized all-silicon particles were added as seed crystals, and the grain size distribution of the product obtained by the reaction was determined by the concentration of all-silicon particles. In the reaction, 10(wt)%, 23(wt)%, 33 (wt)% of all-silicon seed crystal particles, the single crystal formed by the reaction has a smooth surface and edge, and the grain diameter is about 80-100 nm, and the Si/Al ratio of the reaction product is effectively controlled.