relative pressure range to apply the BET method is not adequate to assess a reliable
S BET value.
Table 1 presents the S BET , the C value, and the linear regression coefficient (R
2 )
obtained with the p/p
o range proposed by the original BET method and by the
IUPAC suggestions. It must be highlighted that the S BET values obtained in each
range differ by 30%, where the value of 650 m
2 /g is adequate to represent the
apparent surface area of a Zeolite 13X by the BET method.
Figure 9 shows the nitrogen isotherms for two zeolites with different porosity,
also here, the relative pressure range obtained by IUPAC criteria is different from the
original proposed by the BET method. It is noteworthy that in new ranges, the region
where the knee is present is more representative.
Table 2 compiles the information obtained from these relative pressure ranges for
both zeolites. If the original BET range (0.05–0.35) is used, the C value is negative;
therefore, it is necessary to apply the IUPAC recommendations, and these results are
also shown in Table 2. It is worthy of highlighting that, for microporous materials,
this new relative pressure range is narrower and at lower relative pressures than the
original. Also, the new C values could be very high (typical of materials with narrow
microporosity), which leads to considering the specific surface area as “apparent.”
Despite this latter issue, by taking into consideration the IUPAC recommendations,
it is possible to obtain a reproducible S BET, which can be considered as a “fingerprint” of the sample.
Fig. 8 BET-plot with the
original relative pressure
range proposed by BET
method (blue squares), and
those based on the IUPAC
recommendations (empty
red circles)
Table 1 Values for Zeolite 13X obtained by BET method using the relative pressure ranges
proposed by the original method and the recommended by the IUPAC
p/p
o range
Zeolite 13X
S BET (m
2 g
À1
)
C
R
2
0.05–0.35
460
À35
0.990827
0.0027–0.020
650
8,739
0.999999
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
43
S BET value.
Table 1 presents the S BET , the C value, and the linear regression coefficient (R
2 )
obtained with the p/p
o range proposed by the original BET method and by the
IUPAC suggestions. It must be highlighted that the S BET values obtained in each
range differ by 30%, where the value of 650 m
2 /g is adequate to represent the
apparent surface area of a Zeolite 13X by the BET method.
Figure 9 shows the nitrogen isotherms for two zeolites with different porosity,
also here, the relative pressure range obtained by IUPAC criteria is different from the
original proposed by the BET method. It is noteworthy that in new ranges, the region
where the knee is present is more representative.
Table 2 compiles the information obtained from these relative pressure ranges for
both zeolites. If the original BET range (0.05–0.35) is used, the C value is negative;
therefore, it is necessary to apply the IUPAC recommendations, and these results are
also shown in Table 2. It is worthy of highlighting that, for microporous materials,
this new relative pressure range is narrower and at lower relative pressures than the
original. Also, the new C values could be very high (typical of materials with narrow
microporosity), which leads to considering the specific surface area as “apparent.”
Despite this latter issue, by taking into consideration the IUPAC recommendations,
it is possible to obtain a reproducible S BET, which can be considered as a “fingerprint” of the sample.
Fig. 8 BET-plot with the
original relative pressure
range proposed by BET
method (blue squares), and
those based on the IUPAC
recommendations (empty
red circles)
Table 1 Values for Zeolite 13X obtained by BET method using the relative pressure ranges
proposed by the original method and the recommended by the IUPAC
p/p
o range
Zeolite 13X
S BET (m
2 g
À1
)
C
R
2
0.05–0.35
460
À35
0.990827
0.0027–0.020
650
8,739
0.999999
Critical Overview of Textural Characterization of Zeolites by Gas Adsorption
43