2.1 Analysis Conditions
A relevant factor for obtaining reliable adsorption data is the establishment of the
appropriate operating conditions. Once the gas adsorptive is selected, it is important
to know its thermodynamic characteristics and to choose an adequate adsorption
temperature, which is usually below the critical point. Defining the temperature of
the analysis for the chosen gas, it is necessary to select the relative pressure ( p/p
o )
range, where p is the measured equilibrium pressure, and p
o is the saturation vapor
pressure of the adsorptive at the adsorption temperature, which is advisable to
measure during the experiment. To obtain a high-resolution isotherm, the p/p
o
range must be between 10
À7 and 1, being advisable to use pressure transducers of
100 Pa, 1 kPa, and 0.1 MPa; to achieve this, low pressure is necessary the use of a
turbomolecular pump. In manometric equipments, once the analysis conditions were
established, the determination of the dead space (void volume) under the operational
conditions is usually carried out with helium [17]. However, it has been confirmed
that materials with narrow micropores, as in zeolites, may adsorb helium at 77 K,
changing the adsorbed amount of the adsorptive at ultralow relative pressures
[18]. Thus, for materials with ultramicropores, this determination should be carried
out once the adsorption isotherm has been obtained. Finally, if the analysis starts
from that low relative pressure, the equilibrium time should be long enough to
guarantee suitable isotherm data.
2.2 Sample Pretreatment
In order to remove all physisorbed molecules from the adsorbent surface, a
pre-treatment of outgassing is necessary to ensure a clean initial surface of the
adsorbents before the measurement. This procedure must be carried out by vacuum
pumping below 0.5 Pa and at an advisable temperature, depending on the nature and
synthesis conditions of the sample. The selected temperature should not cause
irreversible changes on the adsorbent surface during the outgassing process; this
temperature can be determined through thermogravimetric analysis. Thommes [19]
recommends, for zeolites, temperatures below 200
C to remove physisorbed water
from the adsorbent surfaces and high temperatures up to 400
C for water adsorbed in
pores smaller than 0.7 nm and not less than 8 h [20].
An example of the importance of the outgassing treatment is performed for a
Zeolite 5A, which has a window size of 0.49 nm is shown in the following figure.
The thermogravimetric analysis revealed that the advisable temperature to use must
be above 320
C (curve not shown). In Fig. 2, the CO 2 adsorption isotherms at 273 K
for Zeolite 5A are displayed with an outgassing process at different temperatures for
12 h. As it can be seen for outgassing temperature equal to or higher than 350
C,
isotherms are coincident.
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
35
A relevant factor for obtaining reliable adsorption data is the establishment of the
appropriate operating conditions. Once the gas adsorptive is selected, it is important
to know its thermodynamic characteristics and to choose an adequate adsorption
temperature, which is usually below the critical point. Defining the temperature of
the analysis for the chosen gas, it is necessary to select the relative pressure ( p/p
o )
range, where p is the measured equilibrium pressure, and p
o is the saturation vapor
pressure of the adsorptive at the adsorption temperature, which is advisable to
measure during the experiment. To obtain a high-resolution isotherm, the p/p
o
range must be between 10
À7 and 1, being advisable to use pressure transducers of
100 Pa, 1 kPa, and 0.1 MPa; to achieve this, low pressure is necessary the use of a
turbomolecular pump. In manometric equipments, once the analysis conditions were
established, the determination of the dead space (void volume) under the operational
conditions is usually carried out with helium [17]. However, it has been confirmed
that materials with narrow micropores, as in zeolites, may adsorb helium at 77 K,
changing the adsorbed amount of the adsorptive at ultralow relative pressures
[18]. Thus, for materials with ultramicropores, this determination should be carried
out once the adsorption isotherm has been obtained. Finally, if the analysis starts
from that low relative pressure, the equilibrium time should be long enough to
guarantee suitable isotherm data.
2.2 Sample Pretreatment
In order to remove all physisorbed molecules from the adsorbent surface, a
pre-treatment of outgassing is necessary to ensure a clean initial surface of the
adsorbents before the measurement. This procedure must be carried out by vacuum
pumping below 0.5 Pa and at an advisable temperature, depending on the nature and
synthesis conditions of the sample. The selected temperature should not cause
irreversible changes on the adsorbent surface during the outgassing process; this
temperature can be determined through thermogravimetric analysis. Thommes [19]
recommends, for zeolites, temperatures below 200
C to remove physisorbed water
from the adsorbent surfaces and high temperatures up to 400
C for water adsorbed in
pores smaller than 0.7 nm and not less than 8 h [20].
An example of the importance of the outgassing treatment is performed for a
Zeolite 5A, which has a window size of 0.49 nm is shown in the following figure.
The thermogravimetric analysis revealed that the advisable temperature to use must
be above 320
C (curve not shown). In Fig. 2, the CO 2 adsorption isotherms at 273 K
for Zeolite 5A are displayed with an outgassing process at different temperatures for
12 h. As it can be seen for outgassing temperature equal to or higher than 350
C,
isotherms are coincident.
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
35