The dealumination of molecular sieves is one of the main topics in the secondary synthesis or modification of molecular sieves at present. Therefore, over the past few decades, scientists have been continuously studying the technical routes and methods and conditions used for aluminum removal based on the requirements of properties and functions. There are three routes summarized below.
(1) The thermal treatment and bydro thermal treatment routes of molecular sieves at high temperatures. Perform dealumination and metastability.
(2) Chemical dealumination route of molecular sieves. For decades, numerous scientists have used different methods and conditions to chemically remove aluminum from molecular sieves, mainly through the reactions of acids (including inorganic acids and certain organic acids), bases, and salts in solution with molecular sieves, as well as through the reactions of inorganic coordination ions such as F-1 with a large number of chelating agents such as EDTA and ACAC with aluminum, to carry out skeleton aluminum removal. There are also methods that rely on gas-solid reactions, such as using F2, COCl2, etc. at a certain temperature, The method of converting aluminum into a volatile substance and removing it from the skeleton. The SiCl4 method for aluminum removal and silicon supplementation is a commonly used method in this route.
(3) Optimization combination of high-temperature hydrothermal and chemical dealumination routes. Due to the dealumination of molecular sieves, not only does it increase the skeleton silicon aluminum ratio, but also different types of non skeleton aluminum (EFAL) chemical entities are generated during the dealumination process, which exist on the pores, cavities, or surface of molecular sieves. It is also possible that due to the special environment of dealumination and reaction, defects in the skeleton, collapse of a small amount of skeleton, and even blockage of certain pores may be caused. The final result is a change in the properties and functions of the molecular sieve, especially in the acidity, pore structure, thermal stability, and catalytic performance closely related to the structure. The generation of these results is closely related to the determination of the dealumination route, method, and conditions used. Overall, due to the advantages and disadvantages and unique characteristics of each of the three routes mentioned above, molecular sieve chemistry researchers have been continuously practicing, improving, and summarizing the laws for decades. They have also used modern research methods to study different dealumination routes and methods, and summarized the effects and laws caused by structure and performance. On this basis, they have continuously coordinated and optimized the combination of these two routes, In order to achieve better results.
