Layered zeolites structure modification and application

Zeolites are a class of crystalline aluminosilicates and silicalites in which silicon and aluminum atoms are tetrahedrally linked by oxygen bridges, constructing three-dimensional (3D) networks containing channels, pores, cages, and cavities. Zeolites then possess the properties of a high specific surface area and adsorption capacity. Having well-defined crystalline structures, zeolites contain quite uniform pores of molecular dimensions and the sizes of which are usually in the micropore region. This unique porosity endows zeolites with molecule-sieving abilities for discriminating guest molecules and partitioning of reactants/products. Only those molecules with the dimensions smaller than the entrance windows are allowed to diffuse into and out of the pores and channels, while the larger ones are excluded outside. The chemical compositions of the zeolite frameworks are changeable, not only in silica to alumina ratios but also in coordinated metal ions. Other than Si and Al, transition metals and many group elements, for example, P,³ B,⁴ Ga,⁵ Fe,⁶ Ti,⁷ Sn,⁸ Ge,⁹ Zr,¹⁰ V,¹¹ and so on, can also be incorporated into the framework occupying the tetrahedral sites. Among them, the zeotype microporous materials of titanosilicate, tinsilicate, gemenosilicate, and ALPO, SAPO, and MeAPO are also recognized as zeolites with the enlargement and definition of zeolites. By introducing metal ions into the zeolite framework, the catalytic active sites and ion-exchange sites are generated. Especially, the presence of trivalent metal ions enhances the negative charge of the framework, which is balanced by organic structure-directing agents (SDA) or metal cations. These cations are exchangeable and contribute to the application of zeolites as detergents. Introducing aluminum atoms into the silicalite framework develops the acid sites, which makes the zeolites applicable to chemical processes as solid acid catalysts. Meanwhile, strong electric fields are generated within zeolite pores. As heterogeneous catalysts, zeolites then may exhibit strong quantum effects in combination with the molecular confinement of micropores. The enhanced activity and product selectivity in zeolites are superior to nonporous or amorphous metal oxides.