Langmuir 1987, 3, 960-967
960
IR Study of Hydroxylated Silica? S. P. Zhdanov Institute of Silicate Chemistry of the USSR Academy of Sciences, Nab. Makarov, 2, Leningrad, USSR
L. S. Kosheleva and T. I. Titova* Institute of Physical Chemistry of the USSR Academy of Sciences, Leninsky prospect, 31, Moscow, USSR Received September 10,1986. I n Final Form: July 20, 1987 The contribution of IR Fourier spectroscopy to the investigation of the state of surface modes and molecular interactions occurring on adsorption of various compounds onto silica surfaces was demonstrated by the analyses of the FT-IR spectra of various amorphous and crystallinesilicas with specific surface area ranging from 1 to 1OOO m2 g-l, such as quartz fiber (QF), silica gel, aerosil, and silicalite. Thus, the Si-OH. .OH-Si silanol group pairs were found to represent those H-perturbed silanol groups of highly hydroxylated amorphous silica surfaces which initiate dehydration and/or dehydroxylation processes at room temperatures. Microporosity and surface permeability of "hydrophobous" QF with a specific surface area of 1-5 m2 g-l cause a tenacious retention by QF of molecular adsorbed water varied on chemical modifications of QF. Hydrophilic sites of silicalites, if any, are largely represented by residual alkali cations being doped on synthesis of silicalite. Adsorption of small amounts of triethylamine onto amorphous superpure silica surfaces causes acid-base type interactions of varying strength with Si-OH groups up to the formation of a surface chemical compound (Et,HN)+(Si-O)- stable under vacuum to over 773 K.
-
Introduction The surface of silica has been broadly considered and has become a subject of much research,l* which resulted in valuable information on the detailed characterization of ita properties. Nevertheless, the surface chemistry of silica in a highly hydroxylated state still presents many problems to be solved by IR spectroscopy that have not yet been investigated to a desirable extent. These problems are as follows: identification of the surface hydroxyl modes and investigation of their thermal stability; investigation of the mechanism of water adsorption and dehydroxylation, with emphasis on the peculiarities involved in these processes as presented by the microporous structure of amorphous and crystalline silicas. Great advances in the LR spectroscopicanalysis of highly surface-hydroxylated silicas may be gained by using spectrometers of great light-gathering power associated with high-speed digital computers. In the present work the analysis of the surface properties of some silica samples with specific surface area ranging from 1to lo00 m2g-l, exemplified by quartz fiber, aerosil, silica gel, and silicalite, has been performed by the FT-IR technique. Most attention has been paid to the investigation of the hydroxylated state of the above silica materials as the real state in numerous practical applications. Experimental Section The amorphous and crystallinesilica samples of various degrees of diaperaionand with a low level of inorganic contamination have been studied. The characteristicsof the investigated samples and the conditions of modification are collected in Table I. The IR spectroscopic analyses were run with the samples dependent on the silica type, as follows: as pellets pressed without the use of any immersion medium, containing on average 10-15 mg of silica/cm2,for fiiely dispersed amorphous silica;as 1% and 3% Kl3r pellets, for s i l i d i k , as compact wafera (prepared without any dispersion and/or compressionof the material) filling in the spectroscopic cell of calibrated volume, using the combined immersion and thermoevacuation method,?v8for quartz fiber. The Presented at the 'Kiselev Memorial Symposium", 60th Colloid and Surface Science Symposium, Atlanta, GA, June 15-18,1986; K. S. W. Sing and R. A. Pierotti, Chairmen.
evacuation of the pelletized samples was performed while they were heated from room temperature to 1273 K, at a residual pressure of 10" Torr in a special quartz cell with KBr or CaF, windows. Quartz fiber was investigated as packed into a sealed hydroxyl-freequartz cell. Measuring the spectra on a temperature scale from 73 to 473 K by using a special Zeiss Model cell provided an accuracy on the order of 0.5 K on thermostating the samples. We recorded the spectra on a Brucker Physic IFS-115c Model IR Fourier transform spectrometerover the range 400-4000 cm-' with a resolution of 1,2, and 4 cm-' using a Ge/KBr beam splitter (for measuring spectra in the 400-4000-cm-' region) and a Gel CaFzbeam splitter (for measuring spectra in the 1200-4000-cm-' region).
Results and Discussion Dehydration and Dehydroxylation of Amorphous Silica. Up to now the numerous IR spectroscopic investigations of the mechanism of dehydration and dehydroxylation of silicas have been reported1+,*12 as being mainly performed on low-hydroxylated silicas. As to silica surfaces in a highly hydroxylated state it presents the difficulties in the interpretation of the OH bands because of the overlapping of the absorbance bands attributed to the surface silanol groups perturbed by the multiple hy(1) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979; Chapter 6. (2) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface ComDounds: Wilev-Interscience: New York. 1975: Chaater 4-7. (3) &&i&er, H. In Hydrogen Bond, Recent Deuelopments in Theory
and Experiments; Schuster, P., Zundel, G., Sandorfy, C., Eds.;North Holland Amsterdam, 1976; Vol. 3, Chapter 27. (4) Kiselev, A. V. Discuss. Faraday Soc. 1971, 52, 14. (5) Klier, K.; Shen, J. H.; Zettlemoyer, A. C. J.Phys. Chem. 1973, 77, 1458.
(6) Sermon, P. A. J. Chem. Soc., Faraday Trans. 1 1980, 76, 885. (7) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I.; Titova, T. I.; Shchepalin, K. L. Kolloidn. Zh.1976,38, 32. (8) Arutjunjan, B. S.; Kiselev, A. V.; Korolev, A. Ja.; Kosheleva, L. S.; Titova, T. I.; Mukhanova, E. E. Kolloidn. Zh. 1983,45, 195. (9) Benesi, H. A.; Jones, A. C. J.Phys. Chem. 1959, 63, 179. (10) Hambleton, F. H.; Hockey, J. A.; Taylor, A. J. C. Trans. Faraday Soc. 1966,62, 795. (11) Armistead, C.G.;Taylor, A. J. G.; Hambleton, F. H.; Mitchel, S. A,; Hockey, J. A. J.Phys. Chem. 1969, 73, 3947. (12) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I. Zh. Fiz. Khim. 1974,48,
1214.
0743-746318712403-0960$01.50/0 0 1987 American Chemical Society
Langmuir, Vol. 3, No. 6, 1987 961
IR Study of Hydroxylated Silica
sample
silica type
1
a er osi1 silica gelb quartz fiberc
2 3 4 5
Table I. Characteristics of the Investigated Silica Samples level of specific inorganic surface area contaminations, wt% m g conditions of modification
~p*:l
-3 x 10-2 -5 x 10-4 -1 x 10-2
0.9
6
7 8
205 340 5.6 1.1 1.2
silicalited
2.8 X 1.2 x 10-3
hydrothermal treatment with boiling water for 6 h a t 373 K treatment with aqueous 0.001 m NHIOH solution followed by calcination in air a t 393 K treatment with aqueous 0.1 m NHIF solution followed by washing with distilled water
1000 acid treatment with aqueous 0.03 m HC1 solution after calcination in air a t 873 K
a Specific surface area of the samples was measured by the nitrogen thermal desorption method using silica aerogel as a standard with specific surface area determined by the low-temperature cryptone adsorption method. Silica gel has been produced by the hydrolysis of superpure tetraethoxysilane with an aqueous ethanol ammonia solution to yield monosilicic acid sol that has undergone gelation through blowing off carbon dioxide and drying in air a t 573 K. Quartz fiber with an average diameter of 0.5-7 pm has been produced by drawing fused quartz through spinnerets and rapidly cooling the formed fibers down to ambient temperature. dSilicalite with an A1 content less than 0.001 w t % has been produced hydrothermally by autoclaving a t 423 K a reactive form of silica in the presence of tetrabutylammonium cations. The wt % of sodium for the silicalite samples investigated is given in column 3.
3.1
I -
u 1600
1800
2000 “1
Figure 2. HzO bending spectra of silica gel taken a t room temperature before (1)and after evacuation for 6 h a t 298 (2) and 473 K (3) and the corresponding difference spectra: 1- 2 (4) and 2 - 3 (5).
Figure 1. H 2 0 bending spectra of aerosil taken at room temperature before (1)and after evacuation for 6 h a t 298 (2) and 473 K (3) and the Corresponding difference spectra: 1- 2 (4) and 2 - 3 (5).
drogen bonding to adsorbed water molecules and/or to one another, on the one hand, and the absorbance bands attributed to the H-bond molecular associates, on the other hand. This is valid for both the fundamental and overtone regions of the spectra. To be able to unequivocally interpret the changes in the OH vibration region of such systems occurring on varying the conditions, e.g., heat treatment, it is necessary to find the temperature limits within which dehydration and/or dehydroxylation occurs. In the present work the temperature limits of dehydration occurring on heat evacuation of chemically pure aerosil and silica gel samples (see Table I) have been estimated by following the changes in the H20 6 band as the difference absorbance spectra band of initial and treated samples. Thus, the difference spectra obtained for aerosil and silica gel evacuated for 6 h at ambient temperature (see Figures 1 and 2, curve 4) reveal that such treatment removes molecular adsorbed water almost entirely. Evacuation at more elevated temperatures, Le., from 298 to 473
K, does not cause any changes in absorbance in this region, as is evidenced in the slight changes occurring merely with the background level of spectra (Figures 1and 2, curve 5). The residual water concentration assessed by the maximum absorbance of the 1632-cm-’ band (determined as the height of the band with respect to the base line tangent to the proper region of “flattened”38absorbance spectra) by using the H 2 0 6 absorbance coefficient reported in ref 10 was found to be 0.037 f 0.005 pmol m-2 for silica gel and 0.048 f 0.007 pmol m-2 for aerosil, thus not exceeding 0.007 of a complete monolayer. Figure 3 demonstrates the absorbance spectra of silica gel in the OH stretching region taken at ambient temperature before and after evacuation of the silica gel s m ple for 6 h at room temperature (curves 1and 2) and the corresponding difference spectrum (curve 3). In this difference spectrum the “positive absorbance” indicates that evacuation at 298 K results in removing molecular adsorbed water (the band at 3400 cm-’) and, as a consequence, in freeing the silanol groups perturbed by water molecules (the band at 3320 cm-’), while the “negative absorbance” manifested in the sharp bands at 3750,3737, and 3719 cm-’ reveals that just the Si-OH groups absorbing at these frequencies were perturbed by hydrogen bonding with adsorbing water molecules. The isolated
962 Langmuir, Vol. 3, No. 6, 1987
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I
10.2
3200
1
4000 om-’
3600
Figure 3. OH stretching spectra of silica gel taken at room temperature before (1) and after evacuation for 6 h at 298 K (2) and the corresponding difference spectrum 1 - 2 (3).
I
3000
3540 1
I
3400
‘ 3800 cm-I
Figure 5. OH stretching spectra of silica gel preevacuated from 298 to 473 K for 6 h as taken at 473 (l), 298 (21, 223 (3), 173 (4), and 123 K (5). Table 11. Frequency Maxima Positions (cm-’) of the OH and OD Y Bands of Freely Vibrating Silanol Groups of Silica Gel at Temperatures Ranging from 473 to 123 K
T,K YOH
YOD
-
I
473 3744 2760
423 3745 2760
373 3745 2761
323 3746 2762
298 3747 2762
223 3748 2763
173 3749 2764
123 3750 2765
either simultaneously with or after the removal of molecular adsorbed water. State of Surface Hydroxylic Modes of Some Amorphous and Crystalline Silicas. 1. Silica Gel. To find out the state of silanol groups covering the hydroxylated silica gel surface, the temperature changes in the state of free and associated silanol groups were followed by the spectra measured at temperatures ranging from 123 to 473 K by using a variable-temperature spectroscopic cell. Before spectra were measured the silica gel samples were preevacuated for 6 h from room temperature to 473 K. The OH stretching region of the obtained spectra is shown in Figure 5. Lowering the temperature results, as is readily seen from curves 1-5, in both a pronounced increase in the total intensity of the stretching band of hydrogen-bondedsilanol groups, with the shift of the band from 3640 to 3540 cm-l, and a decrease in the stretching band of freely vibrating silanol groups, with the shift from 3744 to 3750 cm-l. When the temperature was raised at the same rate within the same limits, the spectral changes were completely reversible. The changes in the band position of free silanol groups are evident from Table 11, wherein the frequency maxima of free Si-OH stretching bands are given versus temperatures ranging from 473 to 123 K at intervals of 50 K. In parallel to the high-frequency shifts of this band, the absorbance maximum of that band decreases by -0.2. It is noteworthy that absorbance of free Si-OH groups on highly dehydroxylated (at 1073 K) silica surfaced3 increases when the temperature is lowered. It appears that the extent of hydrogen bonding between the associated silanol groups increases as revealed in the above-mentioned shift from 3640 to 3540 cm-l and gives rise to such “behavior” of hydroxylated silica surfaces. To make the spectral changes in the Si-OH modes more distinct, the preevacuated silica gel samples underwent a deuteriation which apparently gains a higher resolution
1 2800
3200
3600
4ooo “I
Figure 4. OH stretching spectra of silica gel taken at room temperature after evacuation for 6 h at 298 (1) and 473 K (2) and the corresponding difference spectrum 1 - 2 (3). bands at 3737 and 3719 cm-’ may be assigned (in the absence of inorganic contaminants, which is the case with the silica gel investigated) to the stretching vibration of the oxygen-perturbed silanol groups represented schematically as OOH.O,H
surface species which were found to be generated on adsorption of water onto highly dehydroxylated silicas.12 After the entire removal of molecular adsorbed water from the silica surface, the observed changes in the OH vibration region at more elevated temperatures of evacuation are exceptionally due both to the reduction of the surface silanol groups concentration and to the changes in the state of these groups caused by dehydroxylation. Thus, after evacuation of silica gel sample from 298 to 473 K (Figure 4, curves 1 and 2) only “positive absorbance” enters the picture of the difference spectrum (curve 3), favoring the removal in this temperature interval not only of the associated silanol groups either H-perturbed (the band at 3390 cm-l) or 0-perturbed (the bands at 3720 and 3742 cm-l) but also of the isolated freely vibrating silanol groups (the band at 3750 cm-l) as well. From these data one can conclude that dehydroxylation of silica occurs
(13) Rayson, R. P.; Russell, B. G. J. Phys. Chem. 1975, 79, 1276.
IR Study of Hydroxylated Silica
I
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I
I
3200
Langmuir, Vol. 3, No. 6, 1987 963
I
I 3600
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"7
Figure 6. OH stretching spectra of silica gel preevacuated from 298 to 473 K for 6 h and thereafter exposed to saturated D20vapor for 1h and outgassed at room temperature as taken at 123 (l), 173 (2), 223 (3), 298 (4),323 (5),373 (61,423 (7), and 473 K (8).
with the OH bands, owing to the effect of narrowing these bands on substituting H with D. Due to the effect of isotopic substitution, the bands at 2760-2765,2727, and 2640-2584 cm-l in the spectra of deuteriated silica gel (see Figure 6) are observed as deuterioanalogues of the bands at 3744-3750, 3719, and 3640-3540 cm-l. The corresponding isotopic shift coefficients for these bands at 473 K are 1.357, 1.364, and 1.375, respectively. These values indicate reduction of the anharmonicity of surface silanol group stretching vibrations dependent on the strength of their perturbation by a hydrogen bond. When the temperature is lowered to 123 K, the coefficient of 1.375 of the 3640-cm-l band reduces to 1.370. Hence, the increase of the extent of H-perturbation, revealed, as said before, in the 3640-3540-cm-l shift, is not accompanied by a strengthening the H bond between the associated silanol groups. The changes in position of free silanol groups of the deuteriated silica gel sample correspond closely to those of initial silica gel samples (see Table 11); i.e., the highfrequency shifts of Si-OH and Si-OD v bands are 6 and 5 cm-l, respectively. The corresponding shifts for dehydroxylated (at 873 K) silica gel are reported14 to be equal to 18 and 12-13 cm-l. In other words, the anharmonicity of free Si-OH (Si-OD) stretching vibrations is less temperature dependent for a hydroxylated silica surface as compared to that of a dehydroxylated silica surface. This fact accounts for the interchangeability of free silanol groups of hydroxylated silica and those slightly perturbed by hydrogen, thus being in the dynamic equilibrium. When the sample temperature is varied, the reversible transformation of free and associated silanol groups proceeds via conventional association-dissociation type transformations. 2. Quartz Fiber. An analysis of the surface state and sorption properties of quartz fiber (QF), especially of water sorption, is of great interest, since the surface properties of this practically important material define to a great extent its adhesion, strength, and heat insulation characteristics. (14) Pery, J. B. J. Phys. Chen. 1966, 70, 2937.
Figure 7. OH stretching spectra of quartz fiber taken at room temperature before (1)and after evacuation at 298 (2) and 423 K (3) and the corresponding difference spectra: 1- 3 (4), 1 - 2 (51, 2 - 3 (6).
In the present work the IR spectroscopic analysis of the surface properties of fused QF (0.5-7 pm; refraction coefficient, 7293K= 1.458), initial and chemically modified (see Table I), has been carried out in the 2000-4000-~m-~ region involving the OH vibrational modes. Before measuring spectra (for the unified quantities of initial and modified samples) by the method described in the Experimental Section, we immersed the packed QF under a vacuum in carbon tetrachloride (refraction coefficient, q-293K = 1.4603). Figure 7 represents the absorbance spectra of initial QF before (curve 1) and after 6 h of evacuation at 298 (curve 2) and 423 K (curve 3). In the OH stretching region the spectra demonstrate similarity to those of fused monolithic quartz,'J5J6v2' crystalline quartz,15and microporous silica films,18as well as of silica sintered a t high temperature.' Due to the widely accepted interpretation of the bands of these systems, the broad diffused continuum at 3370-3450 cm-l is attributed to the vibration of molecular associates of adsorbed water. The band at 3640-3670 cm-l, which is difficult to replace by its deuterioanalogue, is therefore assigned to the vibration of the bulk silanol groups concentreated throughout the silica gel globuleslg or to the vibration of hydroxylic modes of hydrolyzed "patches" concentrated within the surface of quartz g1asses.l' Since the corresponding bands in the spectra of QF immersed in CCll do not exhibit any changes indicative of the perturbation by the CC14environment, with respect to those in spectra of monolithic quartz materials, they are to be attributed to the vibration of bulk OH species. However, evacuation of initial QF at 298 and 423 K, as seen from Figure 7, results in obvious changes in both the maximum positions and the intensity of the bands, thus providing evidence for the bands under discussion to be due to the vibration of the surface-permeablemodes. More distinctly, the above changes can be followed by the difference spectra shown in Figure 7 by curves 4 , 5 , and 6, which reveal, in (15) Wong, J.; Angell, C. A. In Glass Structure by Spectroscopy; Marcel Dekker: New York, 1976; Chapter 7. (16) Dunken, H.H.In Treatise on Materials Science and Technology; Doremus, R. H., Ed.; Academic: New York, 1982; Vol. 22, Chapter 1. (17) Bershtein, V. A.;Emeljanov, Ju. A.; Stepanov, V. A. Fiz. Khim. Stekla 1978,4, 549, 557. (18) Crigorovich, S.L.;Kiselev, A. V.; Lygin, V. I. Kolloidn. Zh. 1976, 38, 139. (19) Davydov, V. Ja.; Kiselev, A. V.; Kiselev, S.A. Kolloidn. Zh. 1979, 41, 227.
964 Langmuir, Vol. 3, No. 6, 1987
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Table 111. Silanol Groups and Adsorbed Water Relative Content of Initial and Modified Quartz Fiber silanol groups relative content per unit surface area adsorbed water silanol groups after evacuation specific surface relative content relative content before per unit mass per unit mass evacuation 298 K 423 K sample area CN2, m2 g-' initial 5.6 1 1 1 0.92 0.86 hydrothermally treated 1.1 0.27 0.84 3.1 2.35 1.03 NH40H treated 1.2 0.25 0.76 2.96 3.68 2.83 NH4F treated 0.9 0.22 0.74 3.38 2.63 2.58
A
,
l
l
3000
l
l
l
l
l
3500
,
i
I1
1 ~
o ,
,
cm-'
Figure 8. OH stretching room temperature spectra of initial quartz fiber (1) and that modified hydrothermally (2), with NH40H (3) and NH4F (4) taken under an ambient atmosphere.
particular, that the 3221-3320-cm-l band of molecular adsorbed water is shifted to lower frequencies, with respect to that of liquidlike water associates, at 3420 cm-1,20,21 and is quite close to that of icelike structures.20 These results appear to indicate that the water molecules in the surface pores are drawn together so as to provide their peculiar state as intermediate to the liquid and solid one. The decrease in the 3648-cm-I band intensity on evacuation at ambient and/or elevated temperature indicating partial surface dehydroxylation is a further argument in favor of surface permeability of QF. The changes in the spectra of QF on chemical modification exhibited by Figure 8 suggest that all type modifications result in the detectable decrease in water and silanol group content. To characterize the alterations in water sorptive properties on modification, the content of silanol groups and molecular adsorbed water assessed by the maximum absorbance of corresponding bands was compared between initial and modified samples. The comparative values of molecular water and silanol group content for initial (denoted as l) and modified samples of QF are collected in Table 111. As is seen from column 3 of Table 111, all type modifications cause the molecular adsorbed water content to decrease about 4 times. Such reduction of water sorption capacity is accompanied by the analogous changes in specific surface areas measured by nitrogen. The content of silanol groups per unit mass of QF (column 4 of Table 111)decreases on modification by an average of 1525%. Evacuation of initial and modified samples at 298 K produces partial surface dehydroxylation of the samples, with the NH40H-treated one exhibiting, on the contrary, some increase in the Si-OH content per unit surface area (column 6 of Table 111). The latter appears to be related to (20) Karjakin, A. V.; Kriventaova, G. A. Sostojanie Vody u Organicheskikh i Neorganicheskikh Soedinenijakh; Nauka: Moskva, 1973; s 176. (21) Zolotarev, V. M.; Mikhailov, B. A.; Alperovitch, L. I.; Popova, S. I. Opt. Commun. 1970, 1 , 301.
the generation of hydroxyl groups in fine pores resulting from the NH, evolution.22 The extent of dehydroxylation on heat evacuation up to 423 K is seen from Table I11 (columns 5-7) to increase for the modified samples in the order of modifier type given by the succession H20 > NH4F > NH40H, which is certain to be consistent with the chemical treatments yielding fibers that differ in the structure porosity and, consequently, in the state of the strained siloxane bonds being rehydroxylated to a different extent on chemical treatments. Rehydroxylation of the strained siloxane bonds facilitates the dehydroxylation process, which is recognized to occur through the migration of protons via the mutually hydrogen-bonded silanol and/or the strained siloxane bonds forming on the Si-OH groups removal, preferentially in the surface fine pores." The increase in silanol group content per unit surface area of about 3 times (column 5 of Table 111)favors the generation of fine pores on the chemical modifications, with the NH40H and NHIF modifications appearing to yield most fine pores, as deduced from comparing the values of column 7 of Table 111. Hydrothermal treatment generates wider pores since dehydroxylation on evacuation from 298 to 473 K is much more markedly pronounced at this sample. The mechanism of the ultrapore formation seems to be unique for both hydrophilic and hydrophobic silicas. It consists of the hydrolytic cleavage by water molecules of the strained siloxane bonds, which extend from the surface to the intracrystalline volume. 3. Silicalite. Microporous crystalline silica polymorphs, silicalite-1 and silicalite-2, the structural analogues of ZSM-524and ZSM-1lZ5zeolites, respectively, represent a new type of molecular sieve adsorbent with an adsorption pore size near 0.6 nm, possessing a hydrophobic nonswelling structure of high thermostability.26 They have received the particular attention of researchers in different realms of science that has resulted in numerous studies on this topic. These were typified by the work of Flanigen,26wherein some emphasis has been placed on the fact that the first representation of new crystalline silica polymorphs was highly hydrophobic, yet exibited a slight hydrophilicity, revealing, e.g., in that its water sorption isotherm was somewhat convex. As to further experiments on silicalites, they can be deduced from extensively reviewed literature on this topic, dealing with silicalites which were not absolutely hydrophobic either. Thus, they may be said to be analogous, to a certain extent, to some lowhydroxylated amorphous silicas, despite the great difference in the specific surface area. In this respect, the investigation of the nature of hydrophilicity of silicalites can (22) Strelko, V. V.; Kanibolotaky, V. A. Kolloidn. Zh. 1971, 33, 750. (23) Arutjunjan, B. S.; Kiselev, A. V.; Titova, T. I. Dokl. Akad. Nauk SSSR 1980, 251, 1148. (24) Kokotailo, G. T.; Lawton, S. L.; Olson, D. M. Nature (London) 1978,272, 437. (25) Kokotailo, G. T.: Chu. P.: Lawton, S. L. Nature (London) 1978. 275, 119. (26) Flanigen, E. M.; Bennet, J. M.; Grose, R. M.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature (London) 1978, 271, 512.
IR Study of Hydroxylated Silica
Langmuir, Vol. 3, No. 6, 1987 965
3680
1635
E.l
r4 0
U
z
z
4 €4
4 H E.l
H
H
H
a
3
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z
z
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I
400
800
1200
1600
XI
I l l
800
I
1O b
,
I
I ,
1200
AL-1600
3200
3600 cm”
Figure 9. Absorbance spectra of silicalitetaken under an ambient atmosphere after calcination in air at 773 (l),873 (2), 1073 (3), 1173 (4), 1273 (5), and 1373 K (6).
Figure 10. Absorbance spectra of silicalite taken under an ambient atmosphere after acid treatment and calcination in air at
be approached in ways similar to those for the known low-hydroxylated silicas, e.g., quartz glass and quartz fiber. In Figure 9 the IR spectra of silicalite (see Table I) in the OH vibration and lattice vibration region are shown as taken before and after thermal treatment in air in a temperature range between 773 and 1373 K. In the OH stretching region the spectra demonstrate the bands at 3680 and 3475 cm-’. Of these the first is attributed to the stretching vibration of bulk silanol groups perturbed by the dielectric field of the oxygen environment,2J8while the second is attributed to the stretching vibration of molecular water hydroxyl groups and/or silanol group hydrogens perturbed by water molecules. In the H20 bending region the spectra are represented by the molecular adsorbed water band at 1635 cm-’. The position of these bands, closely corresponding to that of microporous silica film bands,18 indicates that the state of corresponding hydroxylic modes of silicalite is similar to the state of silanol groups and molecular adsorbed water of microporous silica. From that conclusion it may be inferred that the hydroxylic modes of silicalite are constitutents of the silica solid micropores. As can be seen from Figure 9, heat treatment of silicalite at temperatures above 1073 K results in the decrease of the intensity of the above bands to negligible values at 1273-1373 K, whereas the change in the intensity of the 1635- and 3475-cm-’ bands occurring in identical proportions permits us to identify these bands as attributing to the molecular adsorbed water associates. The described changes in the OH vibrational spectra of silicalite when it is heated above 1073 K correlate with the changes in the lattice vibrational spectra revealing reduction in the diffusion of the framework vibration bands along with their intensity, indicative of partial loss of crystallinity of the structure (Figure 9, curve 4) and a complete transformation of the overall spectrum into that characteristic of the a-crystobalite one, at temperatures above 1173 K (curves 5 and 6), indicative of the phase transition of silicalite into a-crystobalite. This fact of a comparativelylow temperature of a-crystobalite formation as compared to that of the transition of the chemically pure silica systems into ~u-crystobalite~~ was somewhat con-
vincingly explained in r e p by the presence of alkali cations being usually doped on hydrothermal synthesis of silicalites, which promoted the Si-0-Si bond destabilization preceding the following crystal structure transformation. Extraction of excessive Na+ cations, e.g., by acid treatment, provided the greater thermal stability of silicalite without any crystal rearrangements at elevated temperatures occurring (Figure 10). The analysis of the spectra of acid-treated silicalite in the OH vibration and lattice vibration region indicates that acid treatment does not give rise to the enrichment of spectra in the bands under discussion. However, the total band intensity of the OH vibration bands increases, with respect to those of the untreated sample (Figure 9, curve l ) , and remains up to 1373 K (Figure 10, curves 1-6). In attempting to clarify the nature of hydrophilic sites of silicalite, we found it reasonable to assume, on the basis of our data (and ref 28), that these sites are represented by sodium cations of sodium-silicate groupings concentrated at the sites of the siloxane network break and/or analogous silanol “defects”of the structure which may have been formed on the hydrolysis of sodium-silicate bonds as a result of acid-water treatments. The persistance of the OH vibrational modes in spectra of acid-treated silicalite, on heating in air to high temperatures, strongly suggests that the crystal structure of the silicalite becomes stable whenever alkali cations are substituted by OH groups capable of rehydroxylation after heat treatments in air. Surface Acidic Properties of Hydroxylated Silica. Investigation of the acidic properties of hydroxylated silica is of current interest, in connection with the practical methods in mind, e.g., selective molecular, ion molecular, and molecular sieve liquid chromatography utilizing silica gel as one of the basic adsorbents. The IR spectroscopy was so far applied with this purpose mainly to the highly dehydroxylated silica surfaces. An analysis has been approached for determining the energy of molecular interactions of some compounds specifically adsorbing onto such surfaces. From the spectral shift of the stretching band of freely vibrating surface silanol groups occurring
(27) Sosman,R. B. The Phases of Silica; Rutgers University Press: New Brunswick, NJ, 1965.
(28) Zhdanov, S. P.; Feoktistova, N. N.; Kozlova, N. I.; Pirjutko, M. M. Izu. Akad. Nauk SSSR, Ser. Khim. 1985, 2667.
773 (l),873 (2), 1073 (3), 1173 (4), 1273 (5), and 1373 K (6).
966 Langmuir, Vol. 3, No. 6, 1987 on adsorption of these compounds, the free silanol group ionization constant, pK,, has been deduced to be approximately equal to 7.29130J6It was of interest, by IR Fourier spectroscopy, to extend this analysis to highly hydroxylated silica surfaces, using some basic compounds with high ionization constants as acid site probes, such as ammonia: some derivatives of ~ y r i d i n e , and ~ ' aliphatic amine^,^'-^^ with triethylamine (TEA) being a strong organic base (with an ionization constant pKb = 11.5 and a spacing of 0.43 nm2)32exhibiting no self-association of its molecules. The difference between the values of the ionization constants for TEA and silica silanol groups, ApK,, = 4.5, is sufficient enough for a strong H-bond formation in this system to occur, which typically manifests in the perturbation of the OH v band, 3750 cm-', propagating over a large lower frequency interval. The propagation is revealed in the continuum of the overlapping bands of A, B, C, D, and E type (in close analogy to the same in condensed media%,%) shifted to within 2000-2500 cm-' with respect to the Si-OH stretching band. Most IR routine technique studies of such system were limited, as said before, to silicas in a highly dehydroxylated state, and the analysis of the perturbed OH stretching region was restricted to the most intensive A type band. Using a difference spectra manipulation procedure available from the FT-IR technique, an attempt was made to extend the analysis of this spectral region to lower frequencies. Figure 11 demonstrates the room temperature spectra of gaseous TEA (curve 1) and silica gel preevacuated at 473 K (curve 2) and exposed after cooling to room temperature to TEA at p / ~=, 0.4 (corresponding to a complete monolayer formation; curve 3) and outgassed thereafter for 1h (curve 4). Adsorption of TEA onto silica gel is seen to cause the 3746-cm-l band of free silanol groups to reduce and the perturbed 3550-cm-' band of associated silanol groups to concurrently grow in intensity, thus generating the bands at 3352 and 3638 cm-'. As to TEA adsorbed onto silica, it is represented by the CH bands essentially the same as those in the spectrum of individual TEA. The difference spectrum obtained for curves 3 and 4 of Figure 11 (curve 5 ) indicates that a short evacuation at room temperature removes most slightly adsorbed TEA, thus liberating the 3580-cm-' band of that part of H-bonded silanol groups that interacted with it and removing the perturbation of these silanol groups (indicated as a cross-hatched continuum with a maximum at 3220 cm-' in Figure 11). The difference spectrum obtained for curves 2 and 4 of Figure 11 (curve 6) exhibits a complex set (cross-hatched) of the overlapping bands of A, B, C, D, and E type with maxima at 3200, 2780, 2350,1730, and 1420 cm-', attrib(29) Hair, M. L.; Hertl, W. J . Phys. Chem. 1970, 74, 91. (30) Tretjakov, N. E.; Filimonov, V. N. Kinet. Kutal. 1972, 13, 815. (31) Noller, N.; Mayerbock, B.; Zundel, G. Surf. Sci. 1972, 33, 82. (32) Curthoys, G.; Davydov, V. Ja.; Kiselev, A. V.; Kiselev, S. A.; Kuznetsov, B. V. J . Colloid Znterfuce Sci. 1974, 48, 58. (33) Van Cauwelaert, F. H.; Vermoorttelle, F.; Uytterhoeven, J. B. Discuss. Faradav SOC.1971.52. 66. (34) Van CauGelaert, F. H.; Jacobs, P. A,; Uytterhoeven, J. B. J. Phys. Chem. 1972. -, 76., 1434. - (35) Glazunov, V. P.; Mashkovsky, A. A.; Odinokov, S. E. Zh. Prikl. Spectrosk. 1975, 23, 469. (36) Zundel, G. In Hydrogen Bond, Recent Developments in Theory and Experiments; Schuster, P., Zundel, G., Sandorfy, C., Eds.; North Holland: Amsterdam, 1976; Vol. 2, Chapter 15. (37) Zhuravlev, L. T.; Kiselev, A. V.; Naydina, V. P.; Poljakov, A. L. Zh. Fir. Khim. 1963, 37, 2258. (38) Flattening of the absorbance spectra aimed to eliminate sloping of the base line due to scattering of IR radiation by the sample. (39) Modification of QF as described in Table I aimed at varying its hydrophilic properties, in connection with the practical applications in mind.
Zhdanou et al.
uted to the absorbance of the silanol groups perturbed by the multiple H bonds on interaction with TEA. Propagation of the H-perturbation over a large spectral interval is known to be a distinct spectral criterion for the occurrence of the acid-base type interactions of varying strength up to the strong ones and indicates that TEA interacts most strongly with isolated silanol groups (3746 cm-') and that part (3569 cm-') of associated silanol groups (3550 cm-l) that is of special steric fitting to adsorbing TEA molecules. A measure of the interaction energy in such systems is the spectral shift, Av, of the stretching Si-OH modes (at 3746 and 3580 cm-l, see curves 5 and 6 of Figure 11)with respect to those of the corresponding H-perturbed ones. We arrived at Av = 360 cm-l in the case of slight and moderate H bonding (3580 3220 cm-') and Av N 1120 cm-' in the case of multiple H bonding involving a strong H-bond component (3746 2625 cm-', with 2625 cm-' being an averaged maximum40 of the overlapping bands continuum). To remove the H-bonded species of adsorbed TEA, the temperature of evacuation was raised to 473 K (which is known to be enough to eliminate the H-bonds). This procedure resulted in a detectable decrease in the intensity of the CH stretching bands of TEA at 2981,2947, and 2908 cm-l, as seen from Figure 12, whereby on further elevation of temperature up to 773 K these bands did persist in spectra. This experimental fact appears to be related to the formation of a surface chemical compound of TEA with silanol groups of silica which is very likely to occur via conventional acid-base type interaction with a complete proton transfer from Si-OH to TEA yielding the (Et,HN)+(Si-O)- species. It appears that the observed high thermostability of such species4' is not surprising since it is generally characteristic of the (R,H4-,N)+A- type compounds, with A- being an inorganic anion. These are known to be stable in air over 500-600 K so that under vacuum one would expect them to be more stable. The "extraordinary character" of the (Si-0)- anion reveals that it is a constituent of the entire Si/O framework possessing a high oxygen negative charge stability, which will provide a peculiar thermostability of the above surface species. To verify the idea of the ionic binding of TEA to silanol groups yielding a surface compound of high thermostability, the samples of silica gel "modified" with TEA, i.e., exposed to it at p / p , = 0.4 for 6 h and outgassed thereafter at temperatures above 473 K, were subjected to a chemical wt% of nitrogen as a comanalysis. It yielded 4.7 X ponent of the surface chemical compound of TEA with Si-OH groups of the silica gel sample preevacuated at 473 K, exposed to TEA under the above conditions, and outgassed at 773 K. That is approximately six Si-OH groups per 100 nm2 that would be "out of play" on adsorption of TEA under these conditions onto such silica surfaces. From these data the acidic inhomogeneity of silica silanol groups is concluded as revealing that the Si-OH
-
(40) Determined as follows: If the A type band is considered as a triangle and if it is assumed that the position of its gravity center about the frequency axis coincides with that of ita absorbance maximum (which is valid in the case of low assymetry of the band), then replacing it by a rectangle with the same gravity center position and the same surface area and completing this thereafter with a rectangle of identical height and a value of surface area equaling that of the summarized areas of the B, C, D, and E bands would enable one to obtain the resulting rectangle with the gravity center position corresponding to the averaged frequency maximum position of the overlapping A, B, C, D, and E bands continuum. (41) Persistence of the corresponding CH bands was the case for the silica gel samples with different states of dehydroxylation prepared through heat evacuation of the samples from 473 to 1173 K so as to contain from 4.6 to 0.4 Si-OH groups per nm2(ref 37), whereby the higher the extent of dehydroxylation the less the intensity of the CH bands observed in the spectra.
IR Study of Hydroxylated Silica
Langmuir, Vol. 3, No. 6, 1987 967
w U
Z 4 F9 0: 0
cn m 4
cm-1
Figure 11. Room temperature spectra of gaseous triethylamine (1)and silica gel after evacuation at 473 K (2), further exposure to TEA at p / p , = 0.4 for 6 h (3), and outgassing for 1h (4) and the corresponding difference spectra: 3 - 4 (5) and 4 - 2 (6).
2800
2900
3000
3100
om
Figure 12. CH stretching room temperature spectra of triethylamine, adsorbed onto silica gel (preevacuatedat 473 K) and outgassed for 6 h at 298 (l),373 (2), 473 (3), 573 (4), 673 (5), 723 (6), and 773 K (7).
groups with pK, > 7 are capable of slight and/or moderate H bonding with TEA, whereby the Si-OH groups with pK, I7 are capable of strong H bonding with TEA, up to the ionic binding producing the surface compounds of high thermostability. The first type of silanol groups is very likely to be represented by those silanol groups giving rise to the band at 3550 cm-l, which sterically fit to adsorbing TEA molecules and which appear to predominate in fine pores. The second type is largely represented by free silanol groups (3746 cm-’) and especially that part of these groups that gives rise to the low-frequency assymetric side of the 3746-cm-l band. Conclusion Application of the FT-IR technique to hydroxylated silicas provided the key to refining the state of the Hperturbed silanol groups at the hydroxylated amorphous silica surface in that they were found to be present as Si-OH- -0H-Si silanol group pairs associated by intra6
molecular hydrogen bonding and in the thermal association-dissociation type equilibrium with freely vibrating silanol groups. These silanol groups initiate the dehydroxylation process which develops simultaneously with or after the evolution of molecular adsorbed water. The latter, when held to the surface of chemically pure and ultrapores-free silicas through H bonding to the Si-OH pairs and/or isolated silanol groups, may be removed by evacuation at ambient temperature. Microporosity of the structure and availability of “hydrophilic” sites on the surface, and/or within the surface of the so-called “hydrophobous” amorphous and crystalline silicas, e.g., quartz fiber and silicalite, are the major factors in more tenacious retention by these of molecular adsorbed water. It appears that the hydrophilic sites of silicalites, if any, are largely represented by residual alkali cations (being purposely doped on the synthesis of these) and/or hydroxyl groups forming on the hydrolysis of the Si-O-Na+ bonds in the micropores. Adsorption of small amounts of triethylamine (within a complete monolayer) as a nitrogen base with rather strong acidity (pKb = 11.5) onto amorphous superpure silica surface causes the acid-base type interactions of varying strength with silanol groups. These interactions may be specified in terms of hydrogen bonds of various relative extent: slight and moderate H bonds of TEA (AvOH = 300-500 cm-l) with silanol groups exhibiting values of pKa > 7; strong H bonds (AvoH = 1000-1200 cm-l) of TEA with Si-OH groups exhibiting values of pKa I 7; up to the ionic binding of TEA with silanol groups producing the surface (Et,HN)+(Si-O)species as a result of a complete proton transfer from the Si-OH group to the TEA molecule. The IR spectroscopic analysis of the above terms enables one to distinguish between silanol groups with different states and hence gives a key to the designed regulation of the acidic properties of the silica systems. Acknowledgment. We have been honored by working for years in close collaboration with Professor A. V. Kiselev. The fruitful idea of the silica-active surface to be covered with silanol groups was given the first experimental support by A. V. Kiselev as early as 1936. It has definitely affected the direction of future numerous silica surface chemistry studies as one of the focal activities of the researchers of the Adsorption and Chromatography Laboratory of the Chemistry Department of Moscow State University and the Surface Chemistry Laboratory of the Institute of Physical Chemistry of the USSR Academy of Sciences, which were headed for years by Professor A. V. Kiselev. Having pioneered much research on this topic, he paid his great attention to the application of a complex set of instrumental methods of physicochemical analysis to the problems of silica. Among them it was vibrational spectroscopy that he set his great hopes on and that became a valuable aid in providing information on surface chemistry of amorphous silica. It was summarized in 1972 by A. V. Kiselev and V. I. Lygin in their well-known and widely cited monograph entitled “Infrared Spectra of Surface Compounds”. Professor Kiselev’s pioneering efforts in advanced studies of silica by means of FT-IR technique have resulted in the initiation of the present work, which was started under his direct guidance at the Institute of Physical Chemistry and was further developed in close collaboration with the Institute of Silicate Chemistry of the USSR Academy of Sciences, in keeping up their joint research traditions of many years standing. We gratefully dedicate this paper to the memory of A. V. Kiselev. Registry No. SiOz,7631-86-9;HzO, 7732-18-5; EGN, 121-44-8.
