Improvement of Microporous Molecular Sieve Catalyst

Improvement of microporous molecular sieve catalyst and synthesis of micro-mesoporous composite molecular sieve

At present, a composite molecular sieve with better performance is obtained by improving the microporous molecular sieve or combining the mesoporous molecular sieve with the microporous molecular sieve. This material is expected to have broad application prospects in the adsorption and catalysis of macromolecules.

1. The first is to create mesopores in the micropores to achieve the purpose of changing the pore size. Depending on the method, it can be processed by dealumination, desilication and heat treatment .

1.1 dealumination method

In this method, the dealumination agent is used to complex with the Al of the molecular sieve framework to make it enter the solution. After the framework is dealuminated, a large number of hydroxyl holes are formed, and a hollow structure is formed after the high temperature dehydration. In addition, it is also possible to dealuminate by hydrothermal method, and the process is to exchange sodium in the molecular sieve with ammonium as the molecular sieve structure to stabilize the hydrothermal conditions. In general, the dealumination kit is used in combination with hydrothermal dealumination to form a stable and modified molecular sieve structure. For example, Cooper et al used the combination of soluble aluminum salt solution exchange and hydrothermal treatment to prepare Y zeolite whose unit cell constant was less than 2.414nm. The hydrothermal stability of the dealuminated Y molecular sieve has been significantly improved after the improvement, and it has been widely used in industry.

1.2 Desiliconization

Mainly, the molecular sieve is placed in an alkaline hydrothermal solution, and part of the skeleton silicon is removed to generate mesopores. This method has the advantages of simple operation, cheap raw materials, and is suitable for industrial production, but the mesoporous channels synthesized by this method are not uniform due to the limitation of Si/Al, which is the result of the original molecular sieve, such as: Groen et al. When Si/Al ≤ 15, it is difficult to remove framework silicon by alkali treatment, thus limiting the formation of mesopores; when Si/Al = 25-50, zeolite can be removed in alkaline solution Except for part of the framework silicon, mesopores of 5-20nm are formed; when Si/Al≥ 200, the zeolite framework silicon is dissolved in large quantities, forming zeolites with macroporous structure, and some even cause the collapse of the zeolite framework structure.

1.3 Heat treatment method 

This method generally heats the molecular sieve in high temperature (1000 ℃) air to expand the micropores. The key is to strictly control the heating conditions. This method is simple, low cost, easy to control and has good repeatability. However, the pore size distribution of the double-porous molecular sieve synthesized by this method is still not uniform.