Adsorptive Properties of Porous Silicas - Advances in Chemistry (ACS

25 Adsorptive Properties of Porous Silicas

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 2, 2016 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch025

M a r t y n B . Kenny and Kenneth S. W. Sing

1

Department of Chemistry, B r u n e l University, Uxbridge, Middlesex UB8 3PH, United Kingdom

Four main types of porous silica adsorbents have been identified: compacts of pyrogenic powders, precipitated silicas, silica gels, and zeolitic silicas. The importance of porosity relative to the adsorptive properties of each group is reviewed, with particular reference to the adsorption of nitrogen, argon, and water vapor. The differences in size and specificity of these adsorptive molecules may be exploited to explore the surface properties of each grade of silica. A notable feature of Silicalite I, which is the best known of the zeolitic silicas, is its remarkable hydrophobic character. Furthermore, the uniform tubular pore structure of this microporous silica is responsible for other highly distinctive properties.

/ A M O R P H O U S A N D C R Y S T A L L I N E F O R M S O F SILICA are now widely used as

industrial adsorbents and catalyst supports. The preparation of a highly active and inexpensive silica adsorbent is not difficult, but the fine tuning of the adsorbent activity is somewhat more demanding. Hence, over the past 40 years the upgrading of the adsorptive properties of silicas has presented a challenge to many academic and industrial research workers. The microstructure of various amorphous silicas was first discussed in a comprehensive manner by Her in his early book The Colloid Chemistry of Silica and Silicates (I). Her drew attention inter alia to the importance of the dense silica particle size and particle packing density in controlling the surface and colloidal properties of sols, gels, and precipitates. In particular, he showed how a change i n coordination number of these globular Current address: Department of Chemistry, University of Exeter, Devon EX4 4QD, United Kingdom. 1

0065-2393/94/0234-0505$08.00/0 © 1994 American Chemical Society

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


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

particles could affect the porosity and hence the adsorptive properties of the dried materials. Since 1955, many others have extended these principles of particle packing, and now secondary and tertiary assemblages can be identified within the microstructures of certain silica gels and precipitates (2, 3). Her had proposed (1) that the minimum size of the dense silica globule was about 1 nm. Later Barby (2), making use of transmission electron microscopy, came to the conclusion that in many amorphous silicas the primary particle size was indeed 1-1.5 nm. In their pioneering studies of silica sols, Alexander and Her (4) employed low-temperature nitrogen adsorption to determine the surface areas of the colloidal particles after removal of the aqueous medium. The Brunauer-Emmett-Teller (BET) areas were found to be only slightly larger than the values obtained from the particle size distributions as determined by light scattering and electron microscopy. These remarkable measurements indicated little change in the particle size or shape after the stabilized silica sols were carefully dried. Before the distinctive adsorptive properties of porous silica can be described, the different ranges of pore size that are of special importance to the mechanisms of physisorption must be identified. Micropores are the pores of the smallest width (d < 2 nm); mesopores are of intermediate size (d ~ 2-50 nm); macropores are the widest pores (d > 50 nm) (5). Amorphous silica gels tend to be mesoporous or microporous, whereas the crystalline zeolitic silicas possess intracrystalline microporosity. The pre cipitated silicas are macroporous and also, to a small extent, microporous. These and other aspects of the microstructures will be discussed in the following sections.

Compacts of Pyrogenic Powders From the standpoint of gas adsorption, the pyrogenic silicas can be regarded as essentially nonporous. Transmission electron microscopy has revealed that the high-temperature arc silicas, and to a lesser extent the flame-hydroryzed " f u m e " silicas (e.g., Degussa Aerosils), consist of dis crete spheroidal particles. According to Barby (2), these globules are i n fact composed of primary particles of about 1 nm. The coordination number is so high that there is virtually no microporous structure; i n their original, loosely packed state, these powders give physisorption isotherms (e.g., nitrogen at 77 K) of Type II in the Brunauer (6) and International Union of Pure and Applied Chemistry (IUPAC) (5) classification (Figure 1). This type of isotherm is associated with unrestricted monolayer-multilayer adsorption, the stage of monolayer completion being indicated by point Β in Figure 1 (7).



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Relative Pressure

Figure I. Types of physisorption isotherms given by porous and nonporous solids. Well-defined pore structures are developed as a result of compaction of the fume silicas (8, 9). If the compaction pressure is not too high [—10 tons i n . (245 giganewtons m )] the nitrogen isotherm becomes similar to Type IV in Figure 1. The initial part of the isotherm is scarcely changed: The area of particle-particle contact is low, so there is little overall loss of surface area. However, at higher relative pressures capillary condensation occurs in the newly created mesopores and causes the isotherm to swing upwards away from the original (Type H) path until the pores are all filled and the isotherm reaches a plateau. Very small particles of fume silica were compacted i n the work of Avery and Ramsay (8), who found that high compaction pressures resulted in the conversion of the isotherm type from IV to I. A drastic loss of B E T area accompanied this change (from 630 to 219 m /g), and Avery and Ramsay concluded that this change was associated with a marked increase in particle packing density. The shape and reversibility of this Type I isotherm was a clear indication that the effective pore width had been reduced to below 2 nm, that is, that the compact had become microporous. Clearly, highly active mesoporous or microporous silicas cannot be produced by the compaction of nonporous powders, but Avery and Ramsay's (8) and other compaction studies (10-12) confirmed the importance of particle coordination in determining porosity and hence, adsorptive properties. - 2

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Precipitated Silicas The surface properties of precipitated silicas have not been studied in as much detail as those of the fume silicas or gels. O n the other hand, the



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extensive patent literature is an identification of the industrial importance of these materials (2, 3). The gas adsorption measurements by Zettlemoyer and co-workers (13, 14) appeared to indicate that some precipitated silicas (e.g., H i S i l 233 from Pittsburgh Plate Glass Company) behaved as nonporous adsorbents. Thus, reversible Type II isotherms of nitrogen and argon were obtained by Bassett et al. (14), who concluded that unrestricted monolayer-multilayer adsorption had occurred. More recent work (15) showed that this interpretation is probably an oversimplification of the physisorption process. In contrast to the behavior of the pyrogenic silicas, the level of nitrogen physisorption by several precipitated silicas was especially sensitive to change of outgassing temperature. Analysis of the adsorption data by the conventional B E T method indicated an apparent increase in the surface area of —26% over the range of outgassing temperature of 2 5 - 2 0 0 °C (see Figure 2). A more rigorous interpretation of the nitrogen isotherms, by application of the: a -method (16), revealed that this change was misleading and that the increase in B E T area was associated with the development of microporosity (15). A is the external area obtained by affiliation of the a -method (16), and Vmic is the derived micropore volume. s

s

s

Outgassing Temperature (*C)

Figure 2. The effect of outgassing temperature on the BET area, a external area, and micropore volume (Vmic) for precipitated silica VN3. s

The behavior of the precipitated silicas with respect to the adsorption of water vapor was even more anomalous (17, 18). Kiselev (11) and others (19) had demonstrated that in the fully hydroxylated form, a wide range of nonporous pyrogenic silicas gave rise to a common reduced water isotherm (i.e., adsorption per unit area versus relative pressure). However, in the



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precipitated silicas, the level of water adsorption was much higher than expected for monolayer adsorption. There seemed little doubt that water molecules were able to penetrate into very narrow pores that could not accommodate nitrogen or other molecules (17). The abnormal behavior of the precipitated silicas appears to be due to the presence of trapped hydroxyl groups within the secondary particles. Thus, although the primary globules are densely packed within the secondary agglomerate, they apparently remain partially hydroxylated. The internal hydroxyls undergo hydrogen-bonding with water molecules, which are able to move in and out of the secondary particles. It is evident that the removal of these hydrogen-bonded water molecules also leads to the development of the small micropore volume.

Silica Gels A large number of gas adsorption studies (2, 3, 7, 11, 20) have been reported on high-area silica gels, but unfortunately much of this work was carried out on ill-defined samples of unknown origin. Silica hydrogels are usually prepared by reacting silicate and acid in an aqueous medium. The properties of the final product (normally a xerogel) are controlled by the conditions under which the condensation-polymerization reaction occurs and by the after-treatment (washing and removal of the liquid phase). Syneresis takes place when the wet hydrogel is allowed to stand, and considerable further shrinkage of the solid accompanies the hydrogel-xerogel conversion. The dependence of the xerogel porosity on the conditions of gelation has been investigated (22). In the early work of Sing and Madeley (21, 22) a microporous product was obtained from the hydrogel prepared from sodium silicate and sulfuric acid at p H 3.5. Mesoporous structures developed when the reaction was carried out at higher p H (—6). O n the other hand, changes in the silicic acid concentration over a wide range had very little effect, provided that the gelation was conducted in a buffered aqueous medium (21). A c i d washing appears to result in partial depolymerization of the hydrogel (23, 24), which in turn leads to some enhancement in the adsorption activity. The effect is illustrated by the results in Figure 3. In this case, the original hydrogel was prepared at p H 5.4 and portions were then subjected to different forms of after-treatment (25). Soaking in H C l (at p H 2.0 for 24 h) resulted in significant upward movement of the nitrogen isotherm, that is, increase in both the B E T area (from 284 to 380 m /g) and the pore volume (from 0.44 to 0.55 cm (liquid)/g). However, the shape of the hysteresis loop remained almost unchanged, a result suggesting that the mesopore size distribution was not altered to any significant extent. 2

3


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The most striking result illustrated in Figure 3 was obtained when the hydrogel was washed with ethyl alcohol. The vacuum-dried alcogel so obtained gave a very much larger uptake of nitrogen over the complete range of relative pressure. A twofold increase was evident in both the B E T area and pore volume. Thus, by replacing water as the continuous liquid phase, it was possible to reduce the capillary forces that normally bring about a drastic shrinkage of the open hydrogël during normal drying.

The alcogel featured in Figure 3 had a B E T area of 641 m /g and a pore volume of 0.93 cm (liquid)/g. A n even larger pore volume can be obtained if the fluid phase is removed under supercritical conditions to give an aerogel, that is, a product having a very low particle coordination number. Such materials are macroporous and have a high surface area (Table I), but they are usually mechanically weak and unstable when exposed to water vapor. The upper limiting surface area of a silica composed of discrete primary particles would be —2000 m /g, but so far it has not been possible to obtain areas approaching this magnitude. Conventional silica gels [termed S-type by Barby (2)] are produced by the roasting (or oven-drying) of low-density hydrogels, which undergo drastic shrinkage with considerable loss of pore volume and surface area. A different type of adsorption is produced from the same initial hydrogel if it is subjected to hydrothermal aging prior to the final drying. In this case packing and fusion of the primary particles takes place (26), so that after drying the pore space is largely confined to the interstitial space between 2

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the secondary particles. The resulting G-xerogel has a somewhat lower surface area and larger and more uniform mesopore volume (Table I and Figure 4).


Table I. Typical Surface Areas and Pore Volumes of the Silica Gels

Type Aerogel G-xerogel S-xerogel S-xerogel

Porosity macro meso meso micro

BET-area (m*/g) 800 350 500 700

Pore Volume (cm /g) 3

2.0 1.2 0.6 0.4

Low-temperature nitrogen adsorption is normally used for the determination of surface area and pore size distribution of porous materials. However, specific field-gradient quadrupole interactions play a significant role i n the adsorption of nitrogen on hydroxylated silicas or other polar surfaces (7). Accordingly, some authors (14, 27) have proposed that a nonpolar adsorptive such as argon should be used instead of nitrogen for the determination of surface area. The difference i n shape of the nitrogen and argon isotherms, both determined at 77 K, on a mesoporous silica is illustrated in Figure 4a. In the middle range of relative pressure, the isotherms follow almost identical



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paths. The divergence at relative pressure p/p° > 0.7 is associated with the onset of capillary condensation of nitrogen i n the mesopores and confirms that argon cannot be employed at 77 Κ for the assessment of the mesopore size distribution (7). O f particular interest is the difference in shape of the isotherms at p/p° < 0.2, that is, in the monolayer region. The use of highresolution adsorption (HRADS) (28) allowed the initial part of the isotherm to be explored in considerable detail (see Figure 4b): It reveals that the nitrogen isotherm is extremely steep at fractional coverages

Adsorptive Properties of Porous Silicas - Advances in Chemistry (ACS

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