**2.2. Structures of carbon-based materials**

Materials with carbon-based structures offer potential to adsorb H2, values of relatively low density, appropriate chemical stability, and large pore structure, and they can be found in a wide variety of structural forms. These forms are closely related to the conditions of synthesis, carbonization, and activation, employed during its preparation [15]. Among the materials based on carbon are the so-called activated carbons (AC). In this type of material, the existence of a porous structure is determined by the spatial arrangement of the grapheme, which can be

Synthesis of Nanostructured Materials for Storing Hydrogen as an Alternative Source to Fossil Fuel Derivatives http://dx.doi.org/10.5772/61076 47

**Figure 1.** Diagram of the zeolite MCM-22 structure [1].

and the separation of gases and solvents, for example, as "molecular sieves" to the dehydration

In the case of activated carbons, they have large specific surface area as well as high porosity; however, they have a disordered structure. These materials are widely used in the processes of separation, catalytic converters, capacitors, storage of gas, and biomedical engineering

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

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

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

Materials with carbon-based structures offer potential to adsorb H2, values of relatively low density, appropriate chemical stability, and large pore structure, and they can be found in a wide variety of structural forms. These forms are closely related to the conditions of synthesis, carbonization, and activation, employed during its preparation [15]. Among the materials based on carbon are the so-called activated carbons (AC). In this type of material, the existence of a porous structure is determined by the spatial arrangement of the grapheme, which can be

of organic solvents.

46 Advances in Petrochemicals

applications [11].

which are bases on carbon structures.

compared to the external.

systems of pores.

**2.2. Structures of carbon-based materials**

**2.1. Zeolites**

stacked to give place to the development of a porous structure relatively little polar. The adsorption/desorption processes of H2 in such materials are characterized by a relatively rapid kinetics, and corresponding isotherms are hysteresis, in which results are attractive in systems that require a high-speed H2 loading and unloading. However, the use of these materials is limited by highly dependent temperature and pressure adsorption capabilities [16, 17].

In materials based on structures of carbon, whose surface is not chemically modified, the physical adsorption of H2 is only due to the existence of weak van der Waals adsorbent– adsorbate interactions. At environmental temperature, the energy corresponding to these interactions is similar to the energy of the thermal motion of the molecules of the gas, so by only decreasing the temperature, the adsorbent–adsorbate interaction energy becomes greater than the thermal movement, which decreases proportionally with the temperature [18, 19].

It has been reported that at low pressures, the amount of adsorbed H2 increases with the increasing density in carbon nanostructures because the pores favor narrowing H2– surface interactions. At high pressures, the specific surface area available for the adsorp‐ tion of H2 is the determining factor, and the amount adsorbed increases by decreasing the material density [20].
