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A comparison of nitrogen and mercury pore size distributions of silicas of varying pore volume
187 Powder Technology, 9 (1974) 187-190 0 Elsevier Sequoia S-A., Lausanne - Printed in The Netherlands
A Comparison of Nitrogen and Mercury Pore Size Distributions of Silicas of Varying Pore Volume
STANLEY
M. BROWN
and EDWIN
IV-R. Grace and Co., Davison (Received
June 18,1973;
W. LARD
Chemical
Division,
in revised form October
Columbia,
Md. 21044
(U.S.A.)
4, 1973) ---
Summary Although good correlation between the pore size distribution determined by nitrogen desorption isotherm and mercury penetration methods has been reported with low pore volume samples, large discrepancies in pore size distribution and in pore volume were found with high pore volume silicas and were shown to be a function of the pore volume of the silica. The mercury penetration method is belieuec-to compress these highly porous silicas and therefore reduces the pore volume and forms smaller pores.
EWERIMENTAL
AND
DISCUSSION
Good correlation between the pore size distribution determined by the nitrogen desorption and the mercury penetration methods has been reported in the literature [I- 3]_ Our experience with aluminas, silicas, silica aluminas, and various catalysts of less than
Fig. 1. N2 and Hg pore size distributions of sample 1; fresh (F) and after mercury penetration and removal (A).
_-_--
1.2 cm3/g pore volume has confirmed ihat results of both methods usually agree quite well, as shown in Figs. 1 - 3. In a study of high pore volume silicas, however, we found large discrepancies between nitrogen and mercury pore size distributions. The distribution of pore volume in 14 600 i-$ diameter pores was calculated from the nitrogen desorption isotherm data by the BJA method [4] using an Aminco Adsorptomat and a value of 16.2 A2 as the molecular coverage of nitrogen. The distribution in 35 10,000 A diameter pores was determined by the mercury penetration method with an Aminco 60,000 psi porosimeter using a contact angle of 140° in the calculations. Surface areas were determined by the BET method using nitrogen adsorption data. Pore volume and surface area data for an alumina (l), a silica - alumina containing 28% Al2 O3 (2), and nine silicas (3 - II) are summarized in Table 1. In order to qutirli;ify the discrepancies between nitrogen and mercury determinations, the differences in pore vol-
Fig. 2. Nz sample 2.
and
Hg pore
size
distributions
of
fresh
Fig. 3. N2 and Hg pore size distributions ple -I.
of fresh sam-
ume in pores under 600 A, the maximum to which the nitrogen method is generally used, are indicated in column four as a percentage of the nitrogen pore volume of the sample. Note that both the total mercury pore volume of pores from 35 to 10,000 A and the mercury pore volume of pores from 35 to 600 A are shown in column three_ The percentage differences in column four increase with pore volume. The differences in average pore size are also a function of the pore volume of silica. Figures 4 - 7 show the pore size distributions of these samples. Sample 3, Table 1, shows a much larger difference between the mercury and nitrogen values than sample 4, which is of larger nitroTABLE
of sample 3; and removal
gen pore volume. This apparent deviation from the otherwise observed relationship can be explained by the work of Ione et al. [ 53, who investigated the compressibility of tableted samples under the conditions of mercury penetration experiments. They noted that the compressibility of silica gel was greater than that of other materials having equal or larger pores such as an iron molybdenum catalyst and zeolites. In a comparison of silicas of similar pore volume but varying average pore diameter, they found that the silicas of largest pore diameter are most subject to tablet compression at a specific high pressure. However, these same large pore materials require less pressure for mercury to penetrate
1
Experimental Sample
Fig. 4. N2 and Hg pore size distributions fresh (F) and after mercury penetration (A).
results on fresh samples
N,PV (cm3/g)
Hg 35 -
PV (cm3/g) 600A
35 -
lo,0006
% Differences
Surface
Average
between N, and Hg FV’s
area (m2/g)
pore
1
0.64
0.71
1.02
253
100
2
0.85
0.84
0.91
1
401
84
3
0.99
0.82
0.93
17
566
70
4 5
1.09 1.72
1.14 1.45
1.22 1.89
16
305 279
144 247
6 7 8
1.76 2.02 2.08
1.51 1.49 1.49
2.23 2.64 2.88
14 26 28
276 448 284
254 180 293
9 10
2.34 2.66
1.57 1.71
2.53 2.84
33 36
316 446
296 239
11
2.85
1.72
3.63
40
473
241
4 x N, pore volume, cm3/g ’ Average pore diameter, a = ~ ---____N2 surface area, m*/g
-11
diameter (A )(l)
-5
x 104
189
Fig. 5. NZ and Hg pore size distributionsof sample 5; fresh (F) and after mercurypenetrationand removal
(Al.
their pores than samples having predominantly smaller pores. The small-pore silicas, therefore, undergo more compression during a mercury penetration experiment, and this causes a greater error in the determined pore size distribution. Sample 3, having much smaller pores than sample 4, requires a greater mercury breakthrough pressure. This then could be the cause of the greater nitrogen - mercury difference noted with that sample. In order to investigate this compressibility of certain silicas, the mercury was removed from samples after a mercury penetration experiment by heating at 540°C for four hours under 20 mm pressure. Analysis indicated that the samples having undergone this removal procedure contained less than 50 ppm mercury. This procedure was found to cause no thermal sintering of silica. Nitrogen and mercury pore size distributions were then determined on these samples, and the results are shown in Table 2 and Figs. 4 - 7.
Fig. 6. Nz and Hg pore size distributionsof sample 9; fresh (F) and after mercurypenetrationand removal
(Al.
Fig. 7. NZ and Hg pore size distributions of sample 11; fresh (F) and after mercury penetration and removal (A).
The pore size distribution determined by mercury penetration after mercury removal differs from the mercury pore size distribution of the fresh sample only in the loss of some macroporosity. The nitrogen pore size distribution after mercury removal, however, shows not only a loss of pore volume but a shift to smaller diameter pores. The nitrogen and mercury results after mercury removal, unlike those before, are similar in pore volume and pore size distribution_ Column four of Table 2 indicates that the differences between the nitrogen and mercury pore volumes after mercury removal do not correlate with pore volume_ It appears from these data that during a mercury penetration experiment, silica undergoes compression from the exterior of the particle and/or from within, as mercury entering large crevices and pores causes the collapse of pore walls, forming smaller pores. As mercury fills the voids resulting from the compression, an exaggerated macroporosity is recorded. One would expect that the compression of silica would not change the surface area of the sample, since only pore walls are compressed and the number and size of micelles are not altered. The data in Tables 1 and 2 show that the surface area does indeed remain unchanged, although the average pore diameter is reduced significantly. The compressibility of silicas was further demonstrated by exposing samples to 24,000 psi for 25 seconds in a hydraulic press. Nitrogen pore volume and surface area data for these samples are shown in Table 3. As in the
190 TABLE 2 Experimental Sample
results on samples having undergone
NZ PV (cm3 /g)
mercury
Hg TV (cm3/g) 35-6OOA
35 -
1
0.63
0.73
0.98
3
0.76
0.69
0.77
5
1.02
1.04
1.15
6 7 8
1.26 0.57 1.01
1.25 0.96
1.62 1.19
9 10 11
1.03 1.57 0.93
1.15 -
1.45 -
10,000
case of compression by mercury penetration experiments, the nitrogen pore volume of these samples was reduced but the surface area was not changed.
CONCLUSION
The differences in pore volume and pore size distribution of silicas as determined by the nitrogen and mercury methods are believed to be caused by sample compression in the mercury penetration method. These discrepancies become pronounced in silicas of high pore volume. Whereas the compressibility of solids has previously been reported to be one of the possible soumes of error in the
TABLE 3 Comparison Sample
A
penetration
and removal
% Differences
Surface
Average
between N2 and Hg PV’s
area Cm2 /g)
pore
-16
236 9
-2 1 5 -12 -
diameter (IL) 106
572
53
254
161
259 391 279
194 58 145
285 421 468
145 150 79
porosimetry technique [S] , the above examples indicate that the effect is so severe that for certain materials this method cannot be accurately used. While limiting the applicability of the mercury penetration method of determining pore size distribution, the compressibility of materials has in fact been measured by this same technique [7] _
REFERENCES L.G. Joyner, E.P. Barrett and R. Skold, The determination of pore volume and area determinations in porous substances, J. Am. Chem. Sot., 73 (1961) 3155. N-M. Kamakin, The method of mercury under pressure and its application to the characterization of the porous structure of adsorbents, Akad. Nauk SSSR, Tr. Soveshch., 1951 (1953) 47.
S. Brunauer, New approaches to pore structure analysis. Chem. Eng. Progr. Symp. Ser., 65 (1969) of samples before and after compression Nr PV (cm3 19)
SA lm2 /a)
Fresh
Fresh
After compression
After cornpression(l)
3
0.99
0.77
666
593
5 9 11
l-72 2.34 2.85
0.81 0.92 1.07
279 316 473
263 285 474
’ 25 set at 24,000
psi.
1.
E.P. Barrett, L-G. Joyner and P.P. Halenda, The determination
of pore volume and area distribution Chem. Sot., 73
in porous substances, J. Am.
(1951) 373. KG. Ione, A.P. Karnaukhou and E.E. Kuon, Complex investigation of the porous structure of catalysts, Kinetka Kataliz, 12 (1971) 457. H.M. Rootare, A short review of mercury porosimetry as a method of measuring pore size distribution in porous materials, Aminco Lab. News, No. 4. P. Zwietering and D.W. van Krevelen, Chemical structure and properties of coal. IV. Pore structure, Fuel, 33 (1954) 331.
A Comparison of Nitrogen and Mercury Pore Size Distributions of Silicas of Varying Pore Volume
STANLEY
M. BROWN
and EDWIN
IV-R. Grace and Co., Davison (Received
June 18,1973;
W. LARD
Chemical
Division,
in revised form October
Columbia,
Md. 21044
(U.S.A.)
4, 1973) ---
Summary Although good correlation between the pore size distribution determined by nitrogen desorption isotherm and mercury penetration methods has been reported with low pore volume samples, large discrepancies in pore size distribution and in pore volume were found with high pore volume silicas and were shown to be a function of the pore volume of the silica. The mercury penetration method is belieuec-to compress these highly porous silicas and therefore reduces the pore volume and forms smaller pores.
EWERIMENTAL
AND
DISCUSSION
Good correlation between the pore size distribution determined by the nitrogen desorption and the mercury penetration methods has been reported in the literature [I- 3]_ Our experience with aluminas, silicas, silica aluminas, and various catalysts of less than
Fig. 1. N2 and Hg pore size distributions of sample 1; fresh (F) and after mercury penetration and removal (A).
_-_--
1.2 cm3/g pore volume has confirmed ihat results of both methods usually agree quite well, as shown in Figs. 1 - 3. In a study of high pore volume silicas, however, we found large discrepancies between nitrogen and mercury pore size distributions. The distribution of pore volume in 14 600 i-$ diameter pores was calculated from the nitrogen desorption isotherm data by the BJA method [4] using an Aminco Adsorptomat and a value of 16.2 A2 as the molecular coverage of nitrogen. The distribution in 35 10,000 A diameter pores was determined by the mercury penetration method with an Aminco 60,000 psi porosimeter using a contact angle of 140° in the calculations. Surface areas were determined by the BET method using nitrogen adsorption data. Pore volume and surface area data for an alumina (l), a silica - alumina containing 28% Al2 O3 (2), and nine silicas (3 - II) are summarized in Table 1. In order to qutirli;ify the discrepancies between nitrogen and mercury determinations, the differences in pore vol-
Fig. 2. Nz sample 2.
and
Hg pore
size
distributions
of
fresh
Fig. 3. N2 and Hg pore size distributions ple -I.
of fresh sam-
ume in pores under 600 A, the maximum to which the nitrogen method is generally used, are indicated in column four as a percentage of the nitrogen pore volume of the sample. Note that both the total mercury pore volume of pores from 35 to 10,000 A and the mercury pore volume of pores from 35 to 600 A are shown in column three_ The percentage differences in column four increase with pore volume. The differences in average pore size are also a function of the pore volume of silica. Figures 4 - 7 show the pore size distributions of these samples. Sample 3, Table 1, shows a much larger difference between the mercury and nitrogen values than sample 4, which is of larger nitroTABLE
of sample 3; and removal
gen pore volume. This apparent deviation from the otherwise observed relationship can be explained by the work of Ione et al. [ 53, who investigated the compressibility of tableted samples under the conditions of mercury penetration experiments. They noted that the compressibility of silica gel was greater than that of other materials having equal or larger pores such as an iron molybdenum catalyst and zeolites. In a comparison of silicas of similar pore volume but varying average pore diameter, they found that the silicas of largest pore diameter are most subject to tablet compression at a specific high pressure. However, these same large pore materials require less pressure for mercury to penetrate
1
Experimental Sample
Fig. 4. N2 and Hg pore size distributions fresh (F) and after mercury penetration (A).
results on fresh samples
N,PV (cm3/g)
Hg 35 -
PV (cm3/g) 600A
35 -
lo,0006
% Differences
Surface
Average
between N, and Hg FV’s
area (m2/g)
pore
1
0.64
0.71
1.02
253
100
2
0.85
0.84
0.91
1
401
84
3
0.99
0.82
0.93
17
566
70
4 5
1.09 1.72
1.14 1.45
1.22 1.89
16
305 279
144 247
6 7 8
1.76 2.02 2.08
1.51 1.49 1.49
2.23 2.64 2.88
14 26 28
276 448 284
254 180 293
9 10
2.34 2.66
1.57 1.71
2.53 2.84
33 36
316 446
296 239
11
2.85
1.72
3.63
40
473
241
4 x N, pore volume, cm3/g ’ Average pore diameter, a = ~ ---____N2 surface area, m*/g
-11
diameter (A )(l)
-5
x 104
189
Fig. 5. NZ and Hg pore size distributionsof sample 5; fresh (F) and after mercurypenetrationand removal
(Al.
their pores than samples having predominantly smaller pores. The small-pore silicas, therefore, undergo more compression during a mercury penetration experiment, and this causes a greater error in the determined pore size distribution. Sample 3, having much smaller pores than sample 4, requires a greater mercury breakthrough pressure. This then could be the cause of the greater nitrogen - mercury difference noted with that sample. In order to investigate this compressibility of certain silicas, the mercury was removed from samples after a mercury penetration experiment by heating at 540°C for four hours under 20 mm pressure. Analysis indicated that the samples having undergone this removal procedure contained less than 50 ppm mercury. This procedure was found to cause no thermal sintering of silica. Nitrogen and mercury pore size distributions were then determined on these samples, and the results are shown in Table 2 and Figs. 4 - 7.
Fig. 6. Nz and Hg pore size distributionsof sample 9; fresh (F) and after mercurypenetrationand removal
(Al.
Fig. 7. NZ and Hg pore size distributions of sample 11; fresh (F) and after mercury penetration and removal (A).
The pore size distribution determined by mercury penetration after mercury removal differs from the mercury pore size distribution of the fresh sample only in the loss of some macroporosity. The nitrogen pore size distribution after mercury removal, however, shows not only a loss of pore volume but a shift to smaller diameter pores. The nitrogen and mercury results after mercury removal, unlike those before, are similar in pore volume and pore size distribution_ Column four of Table 2 indicates that the differences between the nitrogen and mercury pore volumes after mercury removal do not correlate with pore volume_ It appears from these data that during a mercury penetration experiment, silica undergoes compression from the exterior of the particle and/or from within, as mercury entering large crevices and pores causes the collapse of pore walls, forming smaller pores. As mercury fills the voids resulting from the compression, an exaggerated macroporosity is recorded. One would expect that the compression of silica would not change the surface area of the sample, since only pore walls are compressed and the number and size of micelles are not altered. The data in Tables 1 and 2 show that the surface area does indeed remain unchanged, although the average pore diameter is reduced significantly. The compressibility of silicas was further demonstrated by exposing samples to 24,000 psi for 25 seconds in a hydraulic press. Nitrogen pore volume and surface area data for these samples are shown in Table 3. As in the
190 TABLE 2 Experimental Sample
results on samples having undergone
NZ PV (cm3 /g)
mercury
Hg TV (cm3/g) 35-6OOA
35 -
1
0.63
0.73
0.98
3
0.76
0.69
0.77
5
1.02
1.04
1.15
6 7 8
1.26 0.57 1.01
1.25 0.96
1.62 1.19
9 10 11
1.03 1.57 0.93
1.15 -
1.45 -
10,000
case of compression by mercury penetration experiments, the nitrogen pore volume of these samples was reduced but the surface area was not changed.
CONCLUSION
The differences in pore volume and pore size distribution of silicas as determined by the nitrogen and mercury methods are believed to be caused by sample compression in the mercury penetration method. These discrepancies become pronounced in silicas of high pore volume. Whereas the compressibility of solids has previously been reported to be one of the possible soumes of error in the
TABLE 3 Comparison Sample
A
penetration
and removal
% Differences
Surface
Average
between N2 and Hg PV’s
area Cm2 /g)
pore
-16
236 9
-2 1 5 -12 -
diameter (IL) 106
572
53
254
161
259 391 279
194 58 145
285 421 468
145 150 79
porosimetry technique [S] , the above examples indicate that the effect is so severe that for certain materials this method cannot be accurately used. While limiting the applicability of the mercury penetration method of determining pore size distribution, the compressibility of materials has in fact been measured by this same technique [7] _
REFERENCES L.G. Joyner, E.P. Barrett and R. Skold, The determination of pore volume and area determinations in porous substances, J. Am. Chem. Sot., 73 (1961) 3155. N-M. Kamakin, The method of mercury under pressure and its application to the characterization of the porous structure of adsorbents, Akad. Nauk SSSR, Tr. Soveshch., 1951 (1953) 47.
S. Brunauer, New approaches to pore structure analysis. Chem. Eng. Progr. Symp. Ser., 65 (1969) of samples before and after compression Nr PV (cm3 19)
SA lm2 /a)
Fresh
Fresh
After compression
After cornpression(l)
3
0.99
0.77
666
593
5 9 11
l-72 2.34 2.85
0.81 0.92 1.07
279 316 473
263 285 474
’ 25 set at 24,000
psi.
1.
E.P. Barrett, L-G. Joyner and P.P. Halenda, The determination
of pore volume and area distribution Chem. Sot., 73
in porous substances, J. Am.
(1951) 373. KG. Ione, A.P. Karnaukhou and E.E. Kuon, Complex investigation of the porous structure of catalysts, Kinetka Kataliz, 12 (1971) 457. H.M. Rootare, A short review of mercury porosimetry as a method of measuring pore size distribution in porous materials, Aminco Lab. News, No. 4. P. Zwietering and D.W. van Krevelen, Chemical structure and properties of coal. IV. Pore structure, Fuel, 33 (1954) 331.