3 Textural Properties Analysis
Once the adsorption isotherm has been measured under the right conditions, the
results must be presented and analyzed using the proper units and methods/models,
respectively. It is important to point out that this technique can only analyze the
micro and mesopores of the sample.
The IUPAC in its technical report of 2015 [7] suggests the graphical representation of the adsorption isotherms in terms of the amount adsorbed in mol per
adsorbent gram (usually presented as mmol g
À1 ) versus the relative equilibrium
pressure ( p/p
o ); the latter may be changed to p when the analysis temperature is
above the critical temperature of the adsorptive, or it can be changed to gas fugacity
when the conditions deviate the gas behavior far from ideality (e.g., high-pressure
measurements). The resulting isotherm (with or without a hysteresis loop) provides
information regarding pore size and adsorption/desorption mechanisms that will be
essential to make a complete elucidation of the pore structure.
As above named, the textural properties obtained by the analysis data of the
isotherms are specific surface area, pore volumes, and pore size distribution. The
surface area assessment is based on the Brunauer-Emmett-Teller (BET) method [27],
which is the most used procedure for evaluating porous materials. By taking into
account specific criteria to ensure an objective analysis, even microporous materials
can be studied with this method [5, 7, 28]. The evaluation of the micropore volume
can be made with t-plot [29], α S -plot [30], and Dubinin-Radushkevich (DR) [31],
where some of them give us other properties as the internal and external surface area
of porous materials [5, 32]. The total pore volume (micro- and mesopores volumes)
can be calculated using the Gurvich rule [5]. Mesopore volume can be calculated by
subtracting the micropore volume to the total pore volume. Finally, the pore size
distribution (PSD) allows determining the volume corresponding for each pore size.
To obtain the PSD of a given porous material by gas adsorption, it is necessary to
know the filling/emptying mechanism of each pore size. For the micropore region,
this mechanism is not universally defined; nonetheless, there are available methods
to obtain a reliable micropore size distribution; some of them are based on the
Horváth-Kawazoe (HK) method [33]. However, for the mesoporous region, and
even more in the case of nitrogen at 77 K, the capillary condensation/evaporation
theory adequately describes its behavior. Based on this theory, the so-called macroscopic methods arise, which use the original and modified Kelvin equation [34];
these methods include Barrett-Joyner-Halenda (BJH) [35], Dollimore-Heal
(DH) [36], and Villarroel-Barrera-Sapag (VBS) [37]; and they can be applied to
obtain the mesoporous size distributions from the isotherms data. To obtain the PSD
in size range corresponding to micro- and mesopores, computational or also called
microscopic approaches were developed, where the most applied ones are those
based on Monte Carlo simulations (MC) [38] and the density functional theory
(DFT) [39]. It is noteworthy that despite the existence of several models, methods,
and equations to evaluate the PSD, it is necessary to consider the following aspects:
nature and pore geometry of the adsorbent, the adsorptive and its thermodynamic
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
39
Once the adsorption isotherm has been measured under the right conditions, the
results must be presented and analyzed using the proper units and methods/models,
respectively. It is important to point out that this technique can only analyze the
micro and mesopores of the sample.
The IUPAC in its technical report of 2015 [7] suggests the graphical representation of the adsorption isotherms in terms of the amount adsorbed in mol per
adsorbent gram (usually presented as mmol g
À1 ) versus the relative equilibrium
pressure ( p/p
o ); the latter may be changed to p when the analysis temperature is
above the critical temperature of the adsorptive, or it can be changed to gas fugacity
when the conditions deviate the gas behavior far from ideality (e.g., high-pressure
measurements). The resulting isotherm (with or without a hysteresis loop) provides
information regarding pore size and adsorption/desorption mechanisms that will be
essential to make a complete elucidation of the pore structure.
As above named, the textural properties obtained by the analysis data of the
isotherms are specific surface area, pore volumes, and pore size distribution. The
surface area assessment is based on the Brunauer-Emmett-Teller (BET) method [27],
which is the most used procedure for evaluating porous materials. By taking into
account specific criteria to ensure an objective analysis, even microporous materials
can be studied with this method [5, 7, 28]. The evaluation of the micropore volume
can be made with t-plot [29], α S -plot [30], and Dubinin-Radushkevich (DR) [31],
where some of them give us other properties as the internal and external surface area
of porous materials [5, 32]. The total pore volume (micro- and mesopores volumes)
can be calculated using the Gurvich rule [5]. Mesopore volume can be calculated by
subtracting the micropore volume to the total pore volume. Finally, the pore size
distribution (PSD) allows determining the volume corresponding for each pore size.
To obtain the PSD of a given porous material by gas adsorption, it is necessary to
know the filling/emptying mechanism of each pore size. For the micropore region,
this mechanism is not universally defined; nonetheless, there are available methods
to obtain a reliable micropore size distribution; some of them are based on the
Horváth-Kawazoe (HK) method [33]. However, for the mesoporous region, and
even more in the case of nitrogen at 77 K, the capillary condensation/evaporation
theory adequately describes its behavior. Based on this theory, the so-called macroscopic methods arise, which use the original and modified Kelvin equation [34];
these methods include Barrett-Joyner-Halenda (BJH) [35], Dollimore-Heal
(DH) [36], and Villarroel-Barrera-Sapag (VBS) [37]; and they can be applied to
obtain the mesoporous size distributions from the isotherms data. To obtain the PSD
in size range corresponding to micro- and mesopores, computational or also called
microscopic approaches were developed, where the most applied ones are those
based on Monte Carlo simulations (MC) [38] and the density functional theory
(DFT) [39]. It is noteworthy that despite the existence of several models, methods,
and equations to evaluate the PSD, it is necessary to consider the following aspects:
nature and pore geometry of the adsorbent, the adsorptive and its thermodynamic
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
39