Hierarchical zeolites
- PDF / 477,291 Bytes
- 5 Pages / 585 x 783 pts Page_size
- 49 Downloads / 177 Views
Introduction Interest in porous materials, defined as solids comprising empty space in the form of channels or cavities, is related to a number of properties that are not available for bulk solids. For instance, high specific surface areas that can reach several thousands of square meters per gram are characteristic of porous solids. Another distinguishing property is the pore size, which can vary from a few Angstroms to hundreds of nanometers, and thus the pores can be used for selective separation and storage of different species. According to the classification of the International Union of Pure and Applied Chemistry (IUPAC), porous solids are divided into three categories: (1) microporous, with pores less than 2 nm in size; (2) mesoporous, with pores from 2 to 50 nm; and (3) macroporous, with pores between 50 and 1000 nm.1 Porous materials can be organic, inorganic, or hybrid with different structural organizations (crystalline or amorphous). All of these characteristics are important for the potential uses of a porous solid. Zeolites are crystalline microporous materials with a framework built of corner sharing SiO4– and AlO4– tetrahedral units that form channels and cages. There are naturally occurring and synthetic zeolites. At present, there are 231 zeolite framework types. The Structure Commission of the International Zeolite Association assigns a three-letter code to each distinct framework type according to the rules set up by an IUPAC Commission on Zeolite Nomenclature.2 The structure code is
generally obtained from the name of the zeolite, for instance, the synthetic zeolite structure MFI stands for “Mobil 5,” while FAU is an abbreviation of the name of the mineral faujasite. Depending on the framework composition, zeolites with a similar structure type can have different names. The MFI-type family includes an all-silica (Silicalite-1), an Al-containing (ZSM-5), and a Ti-containing (TS-1) member. The structure of a zeolite can also be built from two or more polymorphs. A typical example is zeolite beta, which always includes two polymorphs (BEA and BEB) that can be present in different proportions.2 Well-defined pore systems (Figure 1) and flexible chemical compositions are the origin of the unique characteristics of zeolites such as shape selectivity, high specific surface area, high thermal and chemical stability, and controllable hydrophilic/hydrophobic properties. These properties make zeolites indispensable materials for catalytic and separation processes.3 The small and regular pore dimensions of zeolites allow separation of molecules with size differences below 1 Å. Their shape selectivity, however, comes at the cost of diffusion limitations (due to the similar dimensions of the diffusing species and zeolite channels), which brings some undesirable effects. For instance, pore blocking and active site deactivation are the main drawbacks in the use of zeolites as catalysts. A decrease in zeolite crystal size is a possible solution to this problem owing to the shorter diffusion path length.4
Valentin V
Data Loading...
