criteria [5–7], one can obtain information about the called textural properties of the
solid, related with their surface and pore structure. Pores can be classified according
to their sizes as micropores (<2 nm; subdivided in ultramicropores (<0.7 nm) and
supermicropores (0.7–2 nm)), mesopores (2–50 nm), and macropores (>50 nm) [7].
The first zeolites were microporous solids, and due to their narrow pore size, they
presented diffusional problems for molecules and reagents with a size similar to
those of the micropores [8]. To broaden their applications, considerable efforts have
been made to produce materials with different porosity levels (micro-, meso-, and
macropores), sometimes interconnecting between them to obtain hierarchical porosity [9, 10].
The zeolite structures have regular channels and cavities [11], and its aperture
depends on both the number of connected tetrahedral units (membered rings, MR)
and the compensation cations [12], i.e., extra-large, large, medium, and small pores
are formed by >12-MR, 12-MR, 10-MR, and 8-MR, respectively. In order to
understand the various types of openings that are considered components that
form the pores in zeolites, the International Union for Pure and Applied Chemistry
(IUPAC) established some definitions [13]:
• Windows: rings formed of tetrahedral units that define the faces of polyhedral
pores.
• Cages: polyhedral pores with narrow windows that do not allow the passage of
molecules larger than water.
• Cavities: polyhedral pores with at least one face defined by a ring large enough to
allow the penetration of a guest species but is not infinitely extended.
• Channels: pores extended infinitely in one dimension and wide enough to allow
the diffusion of guest species along its length. The number of dimensions in
which the pore has infinite extension defined by the pore dimensionality.
The parameters of the pore system are defined by means of the pore descriptor,
which includes dimensionality, the shape of the pore, direction of the channel, and
the effective pore width [13, 14].
Another meaningful feature of zeolites, besides their narrow pore sizes, is the
presence of surface functional groups, displaying diffusional problems for molecules
to access inside them. This fact is a critical impediment to obtain an adequate textural
characterization of this class of materials by gas adsorption. To avoid this inconvenient, and to assess the presence of narrow micropores as well, the IUPAC recommends the use of different gases (e.g., N 2 , Ar, CO 2 , Kr, O 2 ) to guarantee an accurate
evaluation of materials porosity, regardless of the texture and surface chemistry of
the material. The characterization of the textural properties of the porous solids is
relevant to establish a further correlation between them and the performance in
specific applications.
In this chapter, we present an overview of the textural characterization by gas
adsorption applied to zeolites, pointing out the critical aspects of this procedure. We
will discuss the basic principles of gas adsorption, highlighting the main concepts of
this phenomenon, experimental requirements to carry out an accurate procedure and
data analysis.
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
33
solid, related with their surface and pore structure. Pores can be classified according
to their sizes as micropores (<2 nm; subdivided in ultramicropores (<0.7 nm) and
supermicropores (0.7–2 nm)), mesopores (2–50 nm), and macropores (>50 nm) [7].
The first zeolites were microporous solids, and due to their narrow pore size, they
presented diffusional problems for molecules and reagents with a size similar to
those of the micropores [8]. To broaden their applications, considerable efforts have
been made to produce materials with different porosity levels (micro-, meso-, and
macropores), sometimes interconnecting between them to obtain hierarchical porosity [9, 10].
The zeolite structures have regular channels and cavities [11], and its aperture
depends on both the number of connected tetrahedral units (membered rings, MR)
and the compensation cations [12], i.e., extra-large, large, medium, and small pores
are formed by >12-MR, 12-MR, 10-MR, and 8-MR, respectively. In order to
understand the various types of openings that are considered components that
form the pores in zeolites, the International Union for Pure and Applied Chemistry
(IUPAC) established some definitions [13]:
• Windows: rings formed of tetrahedral units that define the faces of polyhedral
pores.
• Cages: polyhedral pores with narrow windows that do not allow the passage of
molecules larger than water.
• Cavities: polyhedral pores with at least one face defined by a ring large enough to
allow the penetration of a guest species but is not infinitely extended.
• Channels: pores extended infinitely in one dimension and wide enough to allow
the diffusion of guest species along its length. The number of dimensions in
which the pore has infinite extension defined by the pore dimensionality.
The parameters of the pore system are defined by means of the pore descriptor,
which includes dimensionality, the shape of the pore, direction of the channel, and
the effective pore width [13, 14].
Another meaningful feature of zeolites, besides their narrow pore sizes, is the
presence of surface functional groups, displaying diffusional problems for molecules
to access inside them. This fact is a critical impediment to obtain an adequate textural
characterization of this class of materials by gas adsorption. To avoid this inconvenient, and to assess the presence of narrow micropores as well, the IUPAC recommends the use of different gases (e.g., N 2 , Ar, CO 2 , Kr, O 2 ) to guarantee an accurate
evaluation of materials porosity, regardless of the texture and surface chemistry of
the material. The characterization of the textural properties of the porous solids is
relevant to establish a further correlation between them and the performance in
specific applications.
In this chapter, we present an overview of the textural characterization by gas
adsorption applied to zeolites, pointing out the critical aspects of this procedure. We
will discuss the basic principles of gas adsorption, highlighting the main concepts of
this phenomenon, experimental requirements to carry out an accurate procedure and
data analysis.
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
33