Interpretation of the Differences Between the Pore Size Distributions of

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17 Interpretation of the Differences Between the Pore Size Distributions of Silica Measured by Mercury Intrusion and Nitrogen Adsorption A. R. Minihan, D. R. Ward, and W. Whitby Unilever Research Port Sunlight, Quarry Road East, Bebington, Wirral, Merseyside, L63 3JW, United Kingdom

Measuring the nitrogen sorption isotherms of a number of silicas both before and after analysis by mercury intrusion demonstrates that mercury intrusion can lead to compression of silica structures and that this compression can account for differences in pore size distributions measured by the nitrogen sorption and mercury intrusion techniques. These techniques are widely employed in the structural characterization of porous solids, often independently, despite the fact that very often the pore size distributions obtained by the two techniques fail to agree. Compression effects must be recognized because use of incorrect information can lead to misconceptions regarding the structure of a material.

ERCURY INTRUSION AND NITROGEN SORPTION

are two common techniques used to analyze the structures of porous solids. However, they can give different pore size distributions or pore volumes for a given solid. Giles et al. (I) suggested that differences between pore size distributions as measured by mercury intrusion and by nitrogen sorption might be due to progressive rearrangement of the structure during mercury intrusion analysis followed by breakthrough into the voids between the globular particles when the particles reach a coordination number of 4. More recent work (2) on fume silicas and silica aerogels, which were examined by

0065-2393/94/0234-0341$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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T H E C O L L O I D CHEMISTRY OF SILICA

mercury intrusion after enclosure in an impermeable membrane, demonstrated that the bulk of the intrusion that takes place with such materials is associated entirely with compaction of the powder particles. The effect of mercury intrusion analysis on structure was examined for a series of

silica xerogels with different pore size distributions. This

analysis was achieved by applying nitrogen sorption analysis to the silicas both before and after mercury intrusion analysis. The study required the development of a method for the removal of mercury from a sample after the initial intrusion measurement that does not damage the structure. The


results show the potential for an elastic deformation of the structure during compression as well as irreversible compression during mercury intrusion.

Experimental

Details

Materials. The silica samples were selected on the basis of high purity, narrow pore size distributions, and availability with a large particle size, such that the inter- and intraparticle porosity regions are clearly distinguishable in the mercury intrusion curves. This feature allows the examination of the internal pore structure without the confusion of overlapping interparticle porosity. A n experimental sample of silica [similar to silica produced by Crosfield Chemicals, Warrington, United Kingdom, as a support for Phillips ethylene polymerization (EP) catalyst] and a series of silicas manufactured by Crosfield for chromatographic applications (Sorbsil C 6 0 , Sorbsil C 2 0 0 , and Sorbsil C500) were examined. The surface areas of these materials, determined from the nitrogen adsorption isotherms by the Brunauer-Emmett-Teller (BET) equation (3), are shown in Table I. The effect of mercury intrusion on a sample of silica spheres (S980 G1.7, manufactured by Shell) was also examined. To eliminate any errors due to moisture sorption by the silicas, all samples were predried at 120 °C for at least 2 h and stored in a desiccator until used.

Table I. Surface Areas of Silica Samples Sample E P silica Sorbsil C 6 0 Sorbsil C 2 0 0 Sorbsil C 5 0 0

BET

Surface Area 301 511 299 114

NOTE: Data are reported as square meters per gram.

Nitrogen Gas Adsorption Analysis. The apparatus used was a Micromeritics A S A P 2400, a fully automatic nitrogen gas sorption apparatus that can be programmed to measure gas adsorption and desorption isotherms and calculate surface areas and pore volumes by using a number of widely accepted procedures. A l l samples were outgassed initially at room temperature until a pressure of less than 100 mtorr (13 Pa) was achieved. Outgassing was completed by heating the samples to 120 °C and evacuating until a pressure of less than 5 mtorr (0.7 Pa) had been sustained for at least 2 h. The criteria for terminating the outgassing step was


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the attainment of a stable pressure less than 5 mtorr and was not based on the outgassing time. Typical outgassing times were between 12 and 18 h, and the samples were held under vacuum until the start of the analysis. After outgassing, the samples were cooled under vacuum and the tubes were back-filled with helium before being transferred to the analysis ports. T h e surface areas in Table I were obtained from the adsorption isotherms by using the B E T method (3). T h e relative pressure range used for B E T analysis was selected to give the best linear correlation with the B E T function, and the surface areas were calculated by assuming a molecular cross section for the nitrogen molecule of 0.162 n m . T h e pore size distributions were calculated by using the desorption isotherm, following the method of Barrett, Joyner, and Halenda (BJH) (4). In this procedure the Kelvin equation is used to calculate the radius r of the capillaries, which are assumed to be cylindrical:


2

p

M

P )

=

Z^f

>o

cos

(D

0

r RT P

Here V is the condensed molar volume (34.68 cm /mol for nitrogen); 7 is the liquid-vapor surface tension (8.72 Χ Ι Ο - N/m for nitrogen); R is the gas constant; Τ is the temperature; ρ is the pressure of nitrogen above the sample; po is the saturation vapor pressure of nitrogen at temperature T; and η is a unitless factor. T h e contact angle Θ is assumed to be zero, and the value of η is set to 2 for the desorption branch of the isotherm. T h e pore radius is then calculated from r by adding the thickness of the adsorbed layer present before capillary condensation takes place. This thickness (t) is calculated by using the Halsey (5) equation: 3

m

3

p

A value of 0.354 nm is used for the average thickness σ of a single molecular layer of nitrogen. T h e algorithm used in the A S A P 2400 software is based on Faas's (6) implementation of the B J H method. M e r c u r y Intrusion Analysis. Mercury intrusion measurements were carried out with a porosimeter (Micromeritics 9220) capable of intruding mercury with intrusion pressures (p) up to 414 M P a (60,000 psi). Pore size distributions were calculated from the intrusion curve by using the Washburn (7) equation:

—2j cos Θ

^

A value of 140 °C was used for the contact angle of mercury on the solid (Θ), and the surface tension of mercury (7) was taken as 0.485 N/m. These values correspond to an effective working range for the instrument of 150 μπι to 1.7 nm in pore radius. T h e samples were outgassed at room temperature to a pressure of 50 mtorr (7 Pa) immediately prior to analysis to facilitate filling the penetrometers with mercury. A l l data were fully corrected for mercury compression with calibrated penetrometers.



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T H E C O L L O I D CHEMISTRY OF SILICA

Experimental Procedure. Each sample was first characterized by both mercury intrusion and nitrogen sorption. Mercury intrusion measurements were replicated at least four times, and the solid residues from each analysis were collected and combined after the bulk of the mercury was decanted. These samples were washed free of mercury by using 5 0 % nitric acid (25 m L per 0.5 g of solid) and then washed free of acid by filtering and reslurrying in demineralized water (six times with 50 m L per 0.5 g of solid). The washed samples were then rapidly cooled in liquid nitrogen and freeze-dried (Chemlab SB4). For comparison, samples of material that had not been analyzed with mercury intrusion were washed and dried in a similar manner to test for structural modification caused by the acid-washing technique. After being dried, the samples were reexamined by nitrogen sorption and mercury intrusion, and a portion of the material was analyzed to determine the residual mercury levels. This analysis was achieved by acid digestion (10 m L of 5 0 % aqua regia; sample sizes were approximately 0.2 g in all cases) in pressuresealed poly(tetrafluoroethylene) (PTFE) tubes heated to 140 °C for 10 min in a microwave oven (CEM). The solutions were analyzed after suitable dilution in distilled water with a graphite furnace atomic absorption spectrometer (Perkin Elmer 5100-PC). The detection limit for this method is estimated to be 6 ppm of mercury on the dry solid.

Results and

Discussion

The pore size distribution of the high-pore-volume silicas used for E P applications can be determined from the nitrogen adsorption isotherms by using the B J H method (3) described; a typical isotherm is shown in Figure 1. The isotherm has a "type A " hysteresis loop according to de Boer's classification (8), and this hysteresis indicates the pore structure has a uniform cylindrical form with no evidence of " i n k bottle" pores. These structures, however, have a mercury intrusion curve similar to that shown in Figure 2. Three distinct regions of intrusion are usually observed; the first intrusion step at around 30 μπι is associated with the voids between particles, and its exact position is dependent on the particle size distribu tion of the sample. This interparticle intrusion step is not of interest here, and the silicas selected for study were chosen because they have large particle sizes. Inter- and intraparticle pore size regions are thus easily resolved, and data interpretation in the remainder of this chapter concentrates solely on the internal porosity. The second and third intrusion steps are associated with the intraparticle porosity and should be directly comparable to the nitrogen desorption pore size distributions. Figure 3 shows such a comparison for the model EP-type silica. These curves show that the pore size distributions measured by the two techniques are different, although the total pore volumes are similar. This observation suggests that although the two techniques are measuring the same pore structure, either the models used in the interpretation of the data are inappropriate and do not adequately describe the pore structure or the structure is modified during analysis. Earlier workers ( J , 2)


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345


1200

1000h b)

ε 8003> "Ό Φ

JD

8
50 nm) pore sizes followed by a sharp increase in pore volume at pore diameters in the range 3 0 - 5 0 nm. Whereas this broad portion of the intrusion curve represents the majority of the pore volume for the E P catalyst support (Figure 3), it corresponds only to approximately 2 0 % of the total pore volume for the C 5 0 0 silica. This fact is consistent with the concept of a compression step followed by an intrusion step, because the crossover between compression and intrusion would be expected to occur at a lower pressure for the more highly aged, wider pore silica. Curve c i n Figure 6 represents the pore size distribution as measured by nitrogen desorption after mercury intrusion analysis and subsequent mercury removal. The mercury intrusion clearly results in a significant loss in pore volume, an observation consistent with an irreversible compression of the silica during the intrusion process. This loss i n pore volume as measured by nitrogen desorption is 0.32 cm /g, a value corresponding fairly closely to the 0.4 cm /g that represents the broad-diffuse portion of the intrusion curve. The reanalysis by nitrogen sorption thus provides strong evidence that the silica is irreversibly co American Chemical Society 3

3

Library 1155 16th St., N.W. Washington, DC 20036 In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

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T H E COLLOID CHEMISTRY OF SILICA 1


0.8h

Φ

o 0.4

0.2

0 10

1 100

* Curve (a) Curve (b)

1000

10000

Pore Diameter (Angstroms)

Figure 4. Effect of the mercury removal method on the pore structure of Sorbsil C60. Curve a is the material before treatment, and curve b is that after treatment. mercury intrusion experiment. True intrusion into the pores only appears to occur when the work required to cause further compression is less than that required to force mercury into the pores. The intrusion pressure (and hence the apparent pore size) at which this situation occurs will be a function of the original pore size distribution and the strength of the silica structure. Mercury Intrusion Experiments with Silica Spheres. These silica spheres (S980 G1.7 from Shell) were not examined i n the same detail as were the other silica samples, but the photographs are included because they illustrate the effect of mercury intrusion on the integrity of the solid. These particular spheres have a typical pore volume of 1 cm /g and a pore diameter of 60 nm. The particles are also much larger than the Sorbsil materials (1.7 mm in diameter, compared to 40 to 60 μπι for the Sorbsil materials). Despite the fact that the pores in this material are quite large, the deformation caused by the compression effect is clearly demonstrated in Figure 7. The most noticeable feature in these pictures is the cracked and 3


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349

2

1.5h

ε


ο, φ

• Curve (a)

1 1 ο

Curve (b)

> 2 ο ÛL 0.5

1000

10000


Figure 5. Effect of the mercury removal method on the pore structure of Sorbsil C200. Curve a is the material before treatment, and curve b is that after treatment. Table II. Mercury Levels After Intrusion and Washing Sample EP silica Sorbsil C60 Sorbsil C200 Sorbsil C500

Mercury Level Detected 320 90 1300 60

NOTE: Data are reported as parts per million.

broken nature of the particle surfaces and some slight indications of concave surfaces between the cracks. The integrity of the spheres is otherwise substantially maintained. These spheres have pores slightly larger than those of the Sorbsil C 5 0 0 , and a similar effect might be expected from the compression of the C 5 0 0 . Rather than create large cracks in the surface, however, the smaller particle size material probably simply fractures into even smaller pieces. If this breakdown occurs, internal porosity is lost i n favor of interparticle porosity.


T H E COLLOID CHEMISTRY OF SILICA

350


2,

10

100

1000

10000


Figure 6. Effect of mercury intrusion on Sorbsil C500. The pore size distribution of the original material measured by nitrogen desorption is shown as curve a and that measured by mercury intrusion is shown as curve b. Curve c shows the nitrogen desorption pore size distribution after mercury intrusion and the removal of mercury. Pore Structure of Sorbsil C 2 0 0 . The results obtained for the pore size distributions of Sorbsil C 2 0 0 material are shown i n Figure 8. Nitrogen adsorption (curve a) and mercury intrusion (curve b) again give different pore size distributions, the differences in the distributions being similar to those observed for the E P support (Figure 3). The internal pore volume as measured by mercury porosimetry is about 0.1 cm /g less than that obtained by nitrogen sorption. This difference may be due to the somewhat arbitrary choice of 1000 nm as the cutoff point between interand intraparticle porosity; some intrusion-compression may occur at larger pore sizes-lower pressures. As was observed for the C 5 0 0 silica, reanalysis of the pore structure by nitrogen sorption (Figure 8, curve c) following intrusion and removal of mercury indicates that the porosimetry experiment results in a permanent loss in pore volume and a shift to smaller pore sizes. In C 2 0 0 , however, this loss in pore volume no longer approximates that associated with the broad-diffuse area of the intrusion trace. Instead, the loss represents only 3


MINIHAN ET AL.


351


17.

Figure 7. Effect of mercury intrusion on silica spheres (Shell). Top, starting material; and bottom, material after mercury intrusion (no attempt was made to remove the mercury). about 4 0 % of this region. Furthermore, a second intrusion experiment yields a trace (curve d) that confirms the loss in pore volume, but shows the same two regions, with an apparent compression phase followed by an intrusion step at virtually the same apparent pore size as i n the initial intrusion experiment (curve b). From this data the structure can be concluded to be compressed during the first mercury intrusion, but the compression region (i.e., the


352

T H E COLLOID CHEMISTRY OF SILICA


2

100

1000

10000


Figure 8. Effect of mercury intrusion on Sorbsil C200. The pore size distribution of the original material measured by nitrogen desorption is shown as curve a, and that measured by mercury intrusion is shown as curve b. The pore size distribution after removal of the mercury is shown as curve c (nitrogen desorption) and curve d (mercury intrusion), initial gradual slope) must contain two contributions, one associated with an irreversible collapse and a second associated with elastic compression. In Sorbsil C 5 0 0 , the irreversible collapse appears to account for virtually all of this region, whereas the two phenomena are of approximately equal magnitude in C 2 0 0 silica. Pore Structure of Sorbsil C60. Examination of a silica with a relatively small pore size, such as Sorbsil C 6 0 , produces a different picture. The results (Figure 9) indicate differences in both pore size distributions and pore volumes as measured by nitrogen desorption (curve a) and mercury intrusion (curve b). Pore volumes measured by mercury intrusion are now significantly lower than those measured by nitrogen sorption, and the intrusion trace does not show a sharp step as the intrusion pressure increases, but rather only the initial gradual slope, which extends throughout the measured range of the intrusion plot (40-1000 Â pore diameter). This observation suggests that the structure is still being


17.

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353

1


0.8h


Figure 9. Effect of mercury intrusion on Sorbsil C60. The pore size distribution of the original material measured by nitrogen desorption is shown as curve a, and that measured by mercury intrusion is shown as curve b. The pore size distribution after removal of the mercury is shown as curve c (nitrogen desorption) and curve d (mercury intrusion). compressed and that no intrusion has taken place when the upper pressure limit of the porosimeter has been reached. Curves c and d in Figure 9 show nitrogen desorption and mercury intrusion traces, respectively, for pore size distributions after the initial intrusion experiment and subsequent mercury removal. In a surprising result, no permanent modification to the structure was caused by the initial intrusion experiment. In view of the results obtained with the silicas of larger pore size (Sorbsil C 2 0 0 and C500), the results can be rationalized only i n terms of a completely elastic compression of the structure during the intrusion experiment.

Conclusions These results confirm the suggestions of earlier workers (I, 2) that the mercury intrusion method can lead to structural deformation of solids during analysis. For silicas, there appears to be both an elastic deformation and an irreversible compression effect that contribute to the differences in



354

T H E C O L L O I D CHEMISTRY O F SILICA

pore size distributions measured by nitrogen sorption and mercury intrusion. The irreversible compression appears to dominate in wide-pored silicas, which are presumably highly aged and therefore strong but brittle, whereas high-surface-area, low-pore-diameter silicas such as C 6 0 undergo an almost completely elastic deformation. W i t h large-pore-size silicas (e.g., Sorbsil C500) the compressive effects of mercury intrusion are minimized because intrusion can take place into the pores at lower pressures. Pore volumes of such materials measured by mercury intrusion can be greater than those determined from nitrogen sorption because a significant fraction of pores lies outside the nitrogen sorption measurement range. Hence the pore size distributions measured by mercury intrusion can be more useful than the nitrogen sorption results. Smaller pore size materials are subjected to greater compressive forces than wide-pore materials during mercury intrusion, because intrusion occurs at higher pressures and more marked structural changes occur before intrusion into the pores takes place. Some of this compression is an elastic deformation rather than an irreversible compaction. The pore sizes as measured by mercury intrusion for such materials are complex and depend not only on actual pore size distribution but on particle strength and deformability. As such, mercury intrusion is inappropriate for determining pore size distributions of these materials, although the technique can be used to determine total pore volumes as long as a sharp intrusion step is observed in the intrusion trace. Nitrogen sorption methods should therefore be used for the most realistic assessments of pore sizes for this type of material. W h e n the pore size of silicas is very small (as i n C60), the compression of the structure caused by the mercury intrusion decreases the pore size to such an extent that it is outside the range for mercury intrusion analysis. The pore volumes measured represent only the compression region of the curve, and as a consequence the total pore volumes measured are lower than those measured for nitrogen. Nitrogen sorption methods are the only appropriate technique for this type of material. The data presented show clearly the importance of using a combination of both mercury and nitrogen sorption methods if a complete understanding of porous solid structures is to be achieved. Even when mercury pore size distributions are capable of replication, they may not represent the true pore structure of the solid being examined.

Acknowledgments W e thank N . Whitehead for assistance in the development of the methodology for and determination of the residual mercury levels in the silica samples.


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References 1. Giles, C . H . ; Havard, D . C.; M c M i l l a n , W.; Smith, T; Wilson, R. In Characterisation of Porous Solids; Gregg, S. J.; Sing, K. S. W.; Stoeckli, H . F., Eds.; Society of Chemical Industry: London, 1979; pp 267-284. 2. Smith, D . M . ; Johnston, G . P.; H u r d , A . J. J. Colloid Interface Sci. 1990, 135, 227-237. 3. Brunauer, S.; Emmett, P. H . ; Teller, E . J. Am. Chem. Soc. 1938, 60, 309. 4. Barrett, E . P.; Joyner, L . G . ; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373-380. 5. Halsey, G . D . J. Chem. Phys. 1948, 16, 9 3 1 . 6. Faas, G . S., Masters Thesis, Georgia Institute of Technology, Atlanta, G A , 1981. 7. Washburn, E . W . Phys. Rev. 1921, 17, 273. 8. de Boer, J. H. The Structure and Properties of Porous Materials; Butterworth: London, 1958; p 68. RECEIVED 1992.

for review December 18, 1990. ACCEPTED revised manuscript March 20,


Interpretation of the Differences Between the Pore Size Distributions of

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