**2. Structure and chemical properties of zeolites**

As one of the most important materials used in catalysis, adsorption or ion exchange, zeolites have triggered the interest of scientists because of their structural elements, such as cavities, pores and channels as well as their catalytic properties [10, 17–22]. The molecular-sized open framework forming a periodic array of void spaces, enables confinement of guest particles or molecules and opened interesting research areas like metal-to-insulator transition, charge transfer, solvation and production of trapped electrons [12, 23]. Since their discovery in 1756 the molecular sieve properties are being increasingly used in industry in applications as chemical sensors, medical monitoring or air separation, to name a few [11].

The microporous crystalline aluminosilicate consist of 3-dimenisonal Si-O/Al-O bond tetrahedral framework of nearly spherical cages connected through subnanometric windows with alkali or alkaline earth metals as counterions (**Figure 1**) [24–27]. Basically, they consists of a structure of [SiO4] 4− and [AlO4] 5− tetrahedra connected via their oxygens. They have the general formula: (M<sup>+</sup> )x [(AlO2 − )x (SiO2)y] · *m*H2O where M+ indicates the alkali metal cation. Depending on how they are connected, the tetrahedra form a three-dimensional framework with pores, channels and cavities [11].

The framework is negatively charged in the zeolites containing aluminum and this is due to unbalanced charge of the [SiO4]4− and [AlO4] [5] primary building units (PBUs) which needs to be compensated by the cations. The cations and, eventually, water molecules are distributed inside the cavities which, when aligned, become channels. The presence of H2O molecules inside the cavities is the reason why zeolites can be hydrated/dehydrated by changing the sample temperature. The PBUs combine by sharing oxygens with adjacent tetrahedra to form a spatial

#### **Figure 1.**

*Framework structure of sodalite-based zeolites.*

arrangement of simple geometric forms named the secondary building units (SBU). The crystalline structure is a net product of the special arrangement of SBUs resulting in a large variation in type and morphology of different species of zeolites. The type of zeolites mostly employed in stabilization of metal clusters are A (LTA), FAU (faujasite), and ZSM-5. However, other zeolite morphologies exist while their composition is not limited to the aluminosilicate. For the purpose of this chapter we will limit the description of morphologies to the ones mentioned above.

The structure of zeolite A (LTA) is rather simple and results from connecting the same building unit (β-cage) through a pair of D4R rings (**Figure 1**). Such an arrangement generates a structure with three type of cavities: a D4R square cuboid, an α-cage and of course, the β-cage (**Figure 2**). The largest void present is the α-cage with a diameter of 11 Å and a window of 4.1 Å limiting the size of the potential guests.

Faujasite zeolites exists in two structures X and Y, both constituted from β-cages building blocks. The diameter of the β-cage is 6.6 Å while the window is 2.1 Å and connects to the frame via double D6R. This small window size prohibits the molecular oxygen entering the cage while this remains accessible to water molecules. The difference between X and Y frameworks consists simply in a different Si/Al ratio which is between 1.0 and 1.5 for zeolite X and 1.5 and 3 for zeolite Y. Interestingly, such assembly of β-cages gives rise to a quasi-spherical super cage (**Figure 1**) with a diameter of almost twice the β-cage 13 Å and a window size of 7.4 Å. The mobile counterions needed to compensate the framework charge are distributed inside the β-cage, on the hexagonal faces, at the interface of the supercage 4-ring and D6R unit.

In contrast to LTA and Faujasite topologies, ZSM-5 uses a 5 ring as building unit resulting in a zeolite framework with two type of channels having diameters of 5-6 Å and lengths up to 500 μm. L-type zeolite have a unidimensional channel system as in the mordenite, formed by 6- and 4-membered rings (**Figure 3**).

It is worth mentioning that the electronic structure of the material is in fact a superposition of that of the framework, co-cations, solvent molecules and, eventually, the guest molecules of clusters. The optical bandgap can be as high as 7 eV [2].

The concept of Zeolites as 'solvent' has been introduced by Hashimoto [11] to indicate that the pores and cavities can be used to "dissolve" guest entities. The combination of negatively charged framework and cations possessing a large degree

#### **Figure 2.**

*Schematic representation of (A) LTA unit cell displaying the sodalite (green dashed area) and super cage (gray dashed area); the eight ring (8R), six ring (6R), and double four rings (D4R) are highlighted in blue, yellow and green respectively, (B) isolated sodalite cage with D4R connectors.*

of freedom produce an electrostatic field in which the guest particles are dispersed. Locally, the electric field strength is believed to be extremely high as its intensity hinges on both cation size and Si/Al ratio [28]. In this light, a large number of

#### *Zeolites as Scaffolds for Metal Nanoclusters DOI: http://dx.doi.org/10.5772/intechopen.96876*

molecules have been encapsulated into the zeolite cavities to probe the local electrostatic field and the effects induced by the zeolite framework [29–31].

Exceptional photophysical and photochemical properties are observed when the zeolite crystal dimension is reduced from micrometer size to extremely small nanometer particles with a distribution centered at 10–15 nm [1]. While the micropore volume remains comparable to their corresponding micrometer-sized crystals, the resulted high external surface area, 180 m2 g−1, opens a number of opportunities for processes taking place specifically on this part of the zeolite. The presence of silanes changes the Si/Al ratio and the surface charge of the nanosized zeolites and also the crystal size [8].
