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J. B. PERIAKD A. L. HENSLEY, JR.
The Surface Structure of Silica Gel
by J. B. Peri and A. L. Hensley, Jr. Research and Deoelopment Department, American Oil Company, Whiting, Indiana
46S9Q (Received March 18, 1968)
Information on the surface structure of silica gel was sought through the study of the reactions of AlC&and Sic14 with surface hydroxyl groups. The stoichiometry indicates that even on very dry surfaces residual hydroxyl groups are paired to a high degree. With a few exceptions, more than 95% were paired after drying at 400’, and often more than 85% were paired after drying at 600’. If hydroxyls exist as pairs, the surface cannot resemble a 111 face of cristobalite or a 0001 face of tridymite. When fully hydrated, the surface may resemble a 100 face of cristobalite on which each surface silicon atom holds two hydroxyls. Random partial dehydration of such a faceby a Monte Carlo method yielded a relatively stable surface holding 4.56 hydroxyls/ 100 all as either geminal or vicinal pairs. Such a model can explain the stoichiometry of reactions with reactive halides, infrared spectra of surface hydroxyls, and dehydration and subsequent chemisorption behavior of the surface in most cases.
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Introduction The surface properties of silica gels, being of major practical and theoretical importance, have been the subject of much research. Recent reviews1r2summarize present understanding in this area. Silica surfaces are usually covered with a layer of hydroxyl groups and adsorbed €I20 which can be largely removed by drying at high temperatures or through treatment with suitable reagents. This surface layer has been intensively studied by infrared spectroscopy, X-ray, and other techniques. 2-8 Early X-ray studies suggested that the structure of silica gel resembled that of cristobalite, but more recent studies have indicated a closer resemblance to silica glass, with a short-range order but with a random arrangement of more distant neighbors. However, some resemblance of the surface to a face (or faces) of crystalline forms of silica is usually postulated. Her9 concluded that a fully hydrated surface held about 8 hydroxyls/100 i2 and felt that a P-cristobalite 100 face more readily explained the data than a P-tridymite 0001 face. De Boer and VleeskenslO found, however, that silica gels approac) a constant surface coverage of 4.6 hydroxyls/100 A2 when fully rehydrated after previous annealing above roughly 450’. They noted that P-cristobalite normally exhibits octahedral faces similar to the 0001 face of, P-tridymite and holds 4.55-4.85 hydroxyl groups/100 A2 if each surface silicon atom holds one hydroxyl. They concluded, therefore, that the annealed and rehydrated surface of silica gel probably resembled such a face. Extra hydroxyls on virgin silica gels were attributed to surface irregularities and arrangements having more than one group per surface silicon atom and were removed by repeated drying and rehydration. Hockey1 has further suggested that this “annealing” process involves the removal of surface geminal hydroxyls through condensation with hydroxyls in the second layer. T h e Journal of Physical Chemistry
Davydov, et c ~ l . ,have ~ shown, however, that some silica hydrogels contain substantial amounts of bulk hydroxyl groups (ie., held internally rather than on the surface) and have concluded that the true surface concentration of hydroxyl! of a wide range of silica gels is fairly close to 4.8/100 A2, similar to the value found for annealed samples by De Boer and Vleeskens.lo Others4,*also found evidence for bulk hydroxyls in Aerosil silicas. Snyder and Ward3 concluded that the annealing described by De Boer serves to remove bulk hydroxyls but does not significantly alter the nature of the surface. Wide variations in surface properties exist among different silica gels.3*6,11t1zIf these differences are not caused by differences in the concentration of surface hydroxyl groups, they may reflect differing degrees of surface regularity or crystallinity. Lacking a better model, however, the silica surface has most often been pictured as described by De Boer and Vleeskens, holding widely spaced (5 8) hydroxyl groups.o Formation of siloxane links between silicon atoms 5 A apart should, (1) J. A. Hockey, Chem. I n d . (London), 57 (1965). (2) H. P. Boehm, Angew. Chem. Int. Ed., 5 , 533 (1966). (3) L. R. Snyder and J. W. Ward, J. Phys. Chem., 70, 3941 (1966). (4) F. H. Hambleton, J. A. Hockey, and J. A. G. Taylor, Trans. Faraday Soc., 62, 801 (1966).
(5) J. B. Peri, J . Phys. Chem., 70, 2937 (1966). (6) V. Ya. Davydov, A. V. Kiselev, and 1,. T. Zhuravlev, Trans. Faraday Soc., 60,2254 (1964). (7) M. L. Hair, “Infrared Spectroscopy in Surface Chemistry,” Marcel Dekker, Inc., New York, N. Y., 1967. (8) J. J. Fripiat and J. Uytterhoeven, J . Phys. Chem., 66, 800 (1962). (9) R. K. Iler, “The Colloid Chemistry of Silica and Silicates,” Cornell University Press, Ithaca, N. Y., 1955, pp 242-247. (10) J. H. De Boer and J. M.Vleeskens, Proc. Koninkl. Ned. A k a d . Wetenschap., B61, 2 (1958). (11) C. Naccache and E.Imelik, Bull. Soc. Chim. Fr., 553 (1961). (12) J. H. De Boer, M.E. A. Hermans, and J. M. Vleeskens, Proc. Koninkl. Ned. Akad. Wetenschap., B60, 44 (1957).
THESURFACE STRUCTURE OF SILICA GEL however, be extremely difficult, if not impossible. Barring highly unlikely distortions of the lattice or the equally unlikely creation of separated silicon and oxide ions on the surface, the concentration of surface hydroxyls could hardly be reduced much below 4.6/100 82. Actually, however, fewer than 2 hydroxyls/100 A2 remain on most silicas which have been dried under vacuum a t 800". If, as seems to be the case,5 the surface hydroxyls are not very mobile and are held a t fixed sites, difficulty also arises in explaining the stoichiometry of their reactions with molecules such as AlCla, BC13, and SiC14. On a surface resembling the tridymite 0001 face, only one hydroxyl would be expected to react with each halide molecule. However, even on dry silica holding few hydroxyls, two hydroxyls often react with each halide molecule.6 This suggests that the hydroxyls on the dry surface are located closer together than usually pictured. Attempts to detect "paired" hydroxyls, particularly geminal pairs, by infrared study of dry silica were unsuccessfu1,6 but the presence of such groups was not necessarily excluded. Further evidence on the extent to which paired hydroxyls occur on dry silica gel surfaces was therefore sought, primarily through the study of the stoichiometry of their reactions with AlC13 and Sicla.
Experimental Section Materials. The preparation and properties of pure silica aerogel plates (SG-1) have been described.6 Variation in surface area was found between different batches of aerogel plates, probably caused by minor differences in preparative procedure. The different batches, designated SG-1A, SG-lB, SG-lC, and SG-lD, had surface areas of 810, 550, 480, and 570 m2/g, respectively. The SG-1C had stood in water for 6 months before the final exchange with alcohol and autoclaving. Other pure silica gels were prepared from the reagents used in the preparation of SG-1 and had the following properties. SG-2. A 300-ml volume of concentrated HCI was added dropwise, with constant stirring, to 500 ml of a solution of ethyl orthosilicate (40 vol %) in methanol. The solution gelled in about 1 hr. After standing overnight, the gel was broken into small pieces and placed in 1 1. of 50 vol % methanol in distilled water. After 8 hr the solution was replaced with 3 1. of distilled water. The water was changed daily for 9 days. The gel was heated for 6 hr at 100" in water in an autoclave and then was cooled. It was washed again with water, filtered, and dried at room temperature under vacuum. When the silica appeared dry, it was ground to a fine powder and was stored in a screw-cap jar. After drying a t 300°, this material had a surface area of 807 m2/g.
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XG-5. This gel was prepared and washed as described for SG-2. Instead of being autoclaved for 6 hr at loo", it simply was heated for 4 hr a t 90-95" in open beakers. The water was then replaced by methanol, which was changed frequently over the next 10 days. The resulting alcogel was dried in an autoclave by bleeding off the alcohol slowly at 260-320". The surface area was found to be 648 m2/g. Analysis showed 0.064% of impurities (including Fe, 0.046% ; Mg, 0.005%;;; and Al, 0.0068% and traces of Cu, Ni, and Mn). Davison 62 silica, which was used as supplied, was found to have a surface area of 297 m2/g after drying at 300". Silicon tetrachloride (Matheson, 99.8% minimum) and aluminum chloride (Mallinckrodt AR or Fisher Certified) were purified by vacuum distillation or sublimation before use. All other reagents were of high purity. Equipment and Procedures. Conventional glass and silica vacuum equipment was used in most of the work. Procedures were similar to those previously described,5-I3 except that larger samples were usually studied and removal of hydroxyl groups was not monitored by infrared spectroscopy. Conditions ensuring complete reaction of hydroxyls, as established by previous infrared study, were used. Glass break-seals were used in all vacuum-system work with AIClr. A flow system was used in some experiments involving AlCl,. In these the silica gel (40 g) was supported on a sintered Vycor plate in a vertical Vycor tube (35-mm i.d.) and heated by an external furnace. The silica was normally heated for 2 hr in flowing 02 a t 600", then cooled to 100" and saturated with water by passing wet nitrogen through the bed. It was held at 100" overnight, then heated gradually in dry flowing nitrogen over a period of several hours to the predetermined drying temperature. After the silica had been calcined, rehydrated, and dried at 220, 400, or 600" overnight, it was exposed to A1C13 vapor, which was carried through the bed in the stream of nitrogen. After the excess AlCla was removed (by an ice-cooled trap packed with glass wool), the exit gas was bubbled through two traps holding 1 N NaOH solution, The flow was reversed through the bed several times to ensure complete reaction, the AlCl, being revolatilized and condensed in traps at the ends of the tube. The bed temperature was finally raised to 600" and dry nitrogen was swept through for 8 hr. The NaOH solution in the traps was titrated to determine the amount of I-ICl produced. The bed was then cooled to 220' and met nitrogen was swept through for several days. During this procedure, the bed temperature was cycled between 100 and 400'. The HC1 liberated by hydrolysis was again trapped in NaOH and was determined by titration. In two instances the final samples were analyzed (13) J .
B.Peri, J . Phga. Chem., 70, 3168 (1966). Volume 72, Number 8 August 1988
J. B. PERIAND A. L. HENSLEY,JR.
2928 for aluminum and chloride. Small corrections for chloride remaining on the silica were added to the final HC1 titers. Aluminum analyses in these and in two other instances previously reported13 were all within 20% and mostly were within 10% of the calculated values. Calculation of Paired OH Groups. An A1Cb molecule may react with n surface hydroxyls, where n is from 1to 3, leaving 3 - n chlorine atoms attached to the aluminum atom. Such reaction would liberate n molecules of HC1, and 3 - n additional molecules of HC1 would be evolved by subsequent complete hydrolysis of the surface. Let
R =
HC1 on hydrolysis HCl on initial reaction
Then
n=- 3 R + 1 I n the case where Sic14 reacts with surface hydroxyls,
1000/T 'K
Figure 1. The change in the concentration of hydroxyl groups on the surface of several silica gels with predrying temperature: A, Naccache and Imelik,ll gel A; A, Naccache and Imelik,11 gel P; 0, DeBoer, et al.,'z gel BF-105; 0, present study. The diagonal line represents the Lowen and Broge14 relation.
n is 1-4, with 4 - n chlorine atoms left on the silicon atom. Defining R as before
gels were similarly prepared and were very similar in other respects; they should hardly differ in this respect. 4 n=:Davison 62 silica may have held bulk hydroxyl groups, R + 1 but it gave results comparable with those from the other gels. However, a reaction with more than two hydroxyls Paired Hydroxyl Groups. Table I summarizes preseems highly unlikely. Assuming that n always falls treatment conditions and results for the various samples between 1 and 2 (i,e., that neither AlCla nor Sic14 ever and gives the calculated percentage of paired hydroxyls reacts with three hydroxyls a t once), the percentage of on the surface. All surface hydroxyls are believed to the original hydroxyls which are "paired" can be calhave been reacted with the reagent. This conclusion culated from n was verified for most of the SG-1 samples by direct 200(n - 1) percentage paired = infrared study. That removal was essentially complete n on other gels seems to be supported by the comparison of the results obtained with CH3MgI (SG-2C) with All reacting hydroxyls were assumed to be surface those obtained with A1c13 and Sic&(SG-2A and SG-2B) groups. on samples dried at 100". Results and Discussion The apparent incidence of paired hydroxyls was Retention of Hydroxyl Groups After Drying. The quite high, usually exceeding 95% after drying at 400" surface concentration of hydroxyl groups depended and often over 85% after drying at GOO". Even after mainly on the drying t e m p e r a t ~ r e . ~Water was drying at 800", over GO% of the hydroxyl groups were evolved only very slowly after 1 hr. As illustrated in paired in two cases. However, some exceptions were Figure 1, surface concentrations of hydroxyls left after found. The six samples which showed less than 50% drying a t 400" or above were comparable with those reof the hyodroxyl groups as paired all held less than 1 . G ported by othersll812 and usually somewhat lower than OH/100 A2. However, three other samples which held those predicted by the Lowen and Broge re1ati0n.l~ 1.4-1.5 OH/100 A2 showed G3-71% of the hydroxyls Similar data have also been shown by Davydov, et ~ 1 . to ~ be paired. No correlation was evident between the Two low-area gels studied by Naccache and Imelikl' extent of hydroxyl pairing and surface areas of the showed major deviations from the plot, however, holdsamples. Those samples which held few paired hying four to six hydroxyl groups after drying in the droxyls had either been exposed to the atmosphere 500-800" range. (These results are not plotted.) for over 2 years before calcination in 0 2 to remove surThe nature and surface areas of most of the silica gels face methoxy groups5 or had first been heated under of the present study were such that no appreciable vacuum a t 800" before calcination. content of bulk hydroxyl groups would be expected. The significance of the calculated incidence of paired The SG-1 gels have been shown to hold only surface hydroxyl groups after heating a t GOOO.6 The other SG (14) W. K. Lowen and E. C . Broge, J. P h y s . Chem., 65, 16 (1961). The Journal of Physical Chemistry
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THESURFACE STRUCTURE OF SILICA GEL Table I: Concentration and Extent of Pairing of Surface Hydroxyl Groups on Silica Gels Predrying temp,
Gel
O C
a
Pretreatment
Sample
% paired
97 81 19 (triple)
None None
A1Cla Sic14
5.5 6.8
C
None Steamed a t 600', rehydrated at 100' Heated at 600' in 02, rehydrated at 100' None Heated at 600' in 0 2 , rehydrated at 100' None None Heated a t 600" in 0 2 , rehydrated at 100' None Heated at 600" in 0 2 Heated at 600' in 0 2 Stood 2 years, heated in O2 at 600" Stood 3 years, heated in O2 at 600" Heated in O2 at 600' Heated in 02 at 600' Heated in 02 at 600°, rehydrated at 100' Heated in 02 a t 600' SG-3A rehydrated a t 100' Heated in 0 2 at 600' Heated in 0 2 at 600' Heated under vacuum at SOO', heated in 0 2 at 600' Heated under vacuum at SOO', heated in 0 2 at 600' Stood 2 years, heated under vacuum to SOO', heated in 02 at 600' Stood 2 yearsJ heated in 02 at 600"
CHaMgI NCla AlCls Alcls AlCla Sic14 AlCla AICls Sic14 AlCla SiC14 Sic14 Sic14 Sic& Sic14 AlCls SiCla Sic14 AlCla Sic14 Sic14
5.0 4.9 3.9 3.9 3.7 4.2 4.0 4.0 2.9 2.8 3.1 1.6 1.5 1.8 2.3 1.5 2.2 2.6 1.5 1.4 1.1
Sic14
1.4
22
Sic14
0.7
