Microwave, as an energy source, is entering chemical reactions at a very rapid pace and its applications are becoming increasingly widespread. The synthesis and modification of molecular sieves under microwave irradiation is an important aspect. In principle, microwave dielectric heating effect, microwave ion conduction loss, and local overheating effect are the main factors that accelerate chemical reactions. Microwave, an in situ energy conversion heating mode, has many unique features. The coupling ability between microwave and molecules depends on the properties of molecules, which may control the properties of materials and produce selectivity of reactions, that is, a reactant or a transition state complex or intermediate that has reached a decisive reaction rate can selectively absorb microwave energy, thus causing a large rate increase.
In addition to the heating effect, microwave may also change the spatial structure of some molecules, break some chemical bond or activate molecules, thus promoting various types of chemical reactions. At present, the theoretical and experimental explanations for the non thermal effects of microwaves are not yet complete, and there are conflicting views, such as those related to the theory of dissipative structures. Further in-depth research is needed on the theoretical issues of microwave induced chemical reactions and molecular sieve crystallization.
The microwave radiation crystallization method of microporous compounds was a new synthesis route developed in the 1970s. This method has the characteristics of mild conditions, low energy consumption, fast reaction rate, and small particle size. For example, Na-A type molecular sieves can synthesize crystals with high crystallinity in a short period of time, even 1 minute, under the action of atmospheric pressure microwave. Therefore, this new synthesis route is expected to achieve the goals of rapid, energy-saving, and continuous production of molecular sieves. Other successful examples include the microwave synthesis of heteroatom microporous compounds such as FeAPO4-5, CoAPO-5, CoAPO-44, and the microwave synthesis of microporous compounds such as NaY-type molecular sieves, ZSM-5, TS-1, VPI-5, etc., which are used to prepare molecular sieve membranes, disperse salts or oxides into molecular sieve pores, and modify and modify pore components and structures.
In summary, it is feasible to conduct zeolite ion exchange under microwave assistance. It is convenient, fast, highly exchangeable, and can exchange ions that are not easily accessible by conventional methods. It is particularly suitable for preparing small batches of ion exchange zeolite molecular sieve samples in the laboratory. Based on the current results, the microwave method has shown certain success and characteristics in the synthesis, modification, and secondary synthesis of microporous crystals, as well as the preparation of microporous materials such as ultrafine particles, nanostructures, and membranes. Currently, it has attracted widespread research interest in the field of molecular sieve chemistry.
