**2.1. Zeolites**

Zeolites have been widely studied because of their H2 storage capacity. For equal surface areas, there is a smaller capacity of absorption in the case of zeolites in comparison with the ones which are bases on carbon structures.

Zeolites have structures mostly mesoporous, with volumes of relatively small microporous, which make them the least promising materials based on H2 storage, as referred to in [12]. Their abilities range from 2% to 2.5% based on the weight of the lattice. Zeolites are crystalline aluminosilicates with a structure consisting of a three-dimensional combination of TO4 tetrahedra (T =Si, Al) linked together via oxygen atoms. The structural formula of zeolites can be expressed as [13] *Mx/n [(AlO2)x (SiO) and]. wH2O*, where *M* is the exchangeable cation, *n* is the cation's valence, (*x*, and) are the total number of tetrahedra per unit cell, and *w* is the number of water molecules. The structure of zeolites presents channels and cavities of molecular dimensions, which are water molecules, adsorbates, and compensating cations' charge (negative charge created by the presence of AlO4 structure). These channels and cavities give zeolites a porous structure, which allows these materials to have a very large internal surface compared to the external.

Some zeolitic materials pass through an intermediary laminating these precursors during their training, evidenced by their X-ray diffractogram. Zeolitic laminars are attractive candidates for pillarization processes, which could result in very interesting features such as microporous sheets and activity type zeolite, together with their properties such as mesoporous adsorbents.

Leonowicz et al. [14] proposed two sets of independent pores for the so-called zeolite MCM-22 type. One of the systems of pores is defined by sinusoidal and bidirectional channels; the other consists of large supercavities with a 7.1-Å free inner diameter *A* and an internal height of 18.2 Å. Figure 1 schematically illustrates the structure of MCM-22, where you can see the two systems of pores.
