analysis conditions because the adsorption-desorption isotherm will be the unique
information obtained from the experiment. Once the data are obtained, a careful
selection of methods and models to analyze them is mandatory to evaluate textural
properties of the samples in a reliable and reproducible manner. Particularly for
zeolites, due to their pore sizes and the presence of surface functional groups, the
application of this characterization technique is not straightforward, thus needing to
pay attention to the previous knowledge that exists about this type of materials, to
carry out the experiment as well as to choose the appropriate methodology for data
treatment. In this chapter, we introduce an overview of the experimental procedure
and data treatment to obtain the more reliable textural properties for zeolites.
Keywords Gas adsorption · Isotherms analysis · Porous materials · Textural
characterization of zeolites
1 Introduction
Zeolites are aluminosilicate compounds forming a framework consisting of tetrahedrons of silicon cations (Si
4+ ) and aluminum cations (Al
3+ ) that are surrounded by
oxygen anions (O
2À
). Oxygen form bonds between Si-O and Al-O tetrahedrons,
resulting in a three-dimensional framework of SiO 2 and AlO 2 building blocks, i.e.,
[SiO 4 ]
4À and [AlO 4 ]
5À
, denominated primary building units (PBUs) [1]. The different special arrangements that take place when PBUs share oxygen resulting in
simple geometric forms are called secondary building units (SBUs), and these SBUs
come in a variety of forms (i.e., rings and polyhedral) linked together to produce a
unique system of channels and cages. In addition to PBUs and SBUs, zeolites may
contain other components such as double rings, cages, and cavities; these are called
composite building units (CBUs). The different combinations of SBUs, CBUs, and
the presence of counterions (Na
+
, K
+
, Ca
2+ , etc.) result in a vast number of zeolite
structures (more information regarding zeolite structure and their denomination can
be found in specialized bibliography [1–3] and in the database of the International
Zeolite Association [4]). An interesting result of the different zeolite structures is the
presence of cavities and channels with different sizes (i.e., porosity), making the
zeolites a versatile material for numerous applications.
Porosity is defined as the ratio of the volume of pores (cavities or channels which
are deeper than wide) and voids to the volume occupied by the solid [5], and it is
crucial because its porous structure is relevant in several applications of these
materials. This property can be measured by several techniques (e.g., direct methods,
optical methods, computed tomography, water evaporation, among others). However, for porous materials, the most accepted technique to assess their porosity is gas
adsorption. After a careful analysis of the adsorption data, considering adsorption
mechanisms, pore shape and surface chemistry of the solid, probe gas, and other
32
J. Villarroel-Rocha et al.
information obtained from the experiment. Once the data are obtained, a careful
selection of methods and models to analyze them is mandatory to evaluate textural
properties of the samples in a reliable and reproducible manner. Particularly for
zeolites, due to their pore sizes and the presence of surface functional groups, the
application of this characterization technique is not straightforward, thus needing to
pay attention to the previous knowledge that exists about this type of materials, to
carry out the experiment as well as to choose the appropriate methodology for data
treatment. In this chapter, we introduce an overview of the experimental procedure
and data treatment to obtain the more reliable textural properties for zeolites.
Keywords Gas adsorption · Isotherms analysis · Porous materials · Textural
characterization of zeolites
1 Introduction
Zeolites are aluminosilicate compounds forming a framework consisting of tetrahedrons of silicon cations (Si
4+ ) and aluminum cations (Al
3+ ) that are surrounded by
oxygen anions (O
2À
). Oxygen form bonds between Si-O and Al-O tetrahedrons,
resulting in a three-dimensional framework of SiO 2 and AlO 2 building blocks, i.e.,
[SiO 4 ]
4À and [AlO 4 ]
5À
, denominated primary building units (PBUs) [1]. The different special arrangements that take place when PBUs share oxygen resulting in
simple geometric forms are called secondary building units (SBUs), and these SBUs
come in a variety of forms (i.e., rings and polyhedral) linked together to produce a
unique system of channels and cages. In addition to PBUs and SBUs, zeolites may
contain other components such as double rings, cages, and cavities; these are called
composite building units (CBUs). The different combinations of SBUs, CBUs, and
the presence of counterions (Na
+
, K
+
, Ca
2+ , etc.) result in a vast number of zeolite
structures (more information regarding zeolite structure and their denomination can
be found in specialized bibliography [1–3] and in the database of the International
Zeolite Association [4]). An interesting result of the different zeolite structures is the
presence of cavities and channels with different sizes (i.e., porosity), making the
zeolites a versatile material for numerous applications.
Porosity is defined as the ratio of the volume of pores (cavities or channels which
are deeper than wide) and voids to the volume occupied by the solid [5], and it is
crucial because its porous structure is relevant in several applications of these
materials. This property can be measured by several techniques (e.g., direct methods,
optical methods, computed tomography, water evaporation, among others). However, for porous materials, the most accepted technique to assess their porosity is gas
adsorption. After a careful analysis of the adsorption data, considering adsorption
mechanisms, pore shape and surface chemistry of the solid, probe gas, and other
32
J. Villarroel-Rocha et al.