2740
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
A. J. van Roosmalen and J. C. Mol
An Infrared Study of the Silica Gel Surface. 1. Dry Silica Gel A. J. van Roosmalen” and J. C. Mol University of Amsterdam, Institute of Chemical TechnologygAmsterdam, The Netherlands (Received May 15, 1978) Publication costs assisted by the University of Amsterdam
, Transparent plates of silica aerogel were studied by infrared spectroscopy. Experiments with D exchange, silanization, and heat treatment showed that dry silica gel, degassed at 875 K, has four absorption bands in the 0-H stretching region. These bands are assigned to four different types of surface hydroxyl groups, viz. isolated single and geminal silanols (absorbing at 3749 and 3742 cm-’, respectively), and asymmetric H-bridged vicinal silanol pairs (absorbing at about 3720 and 3500-3700 cm-I). Interfering water vapor proved not to cause the observed spectral features. It is concluded that silica powders (Cabosil, Aerosil), having only one absorption at 3748 cm-l after vacuum treatment at the same temperature, and silica gel do not have identical surfaces.
Introduction Silica gel is a highly porpus, amorphous silica. It is a polycondensation product of silicic acid, prepared by acid hydrolysis of sodium silicate or another suitable silicon compound, such as tetraethoxysilane. Because of its porosity, silica gel has a specific surface area of 100-1000 m2 g-1. Another silica with a large surface area is silica powder, or pyrogenic silica (Cabosil, Aerosil). This type of silica is nonporous; its large surface area is brought about by the low particle size (10-20 nm). It is usually prepared by flame hydrolysis of tetrachlorosilane. Hydroxyl groups are found on all silica surfaces. By heating the silica under vacuum these groups are removed, with the formation of water, in the following order: (1) physical adsorbed water; ( 2 ) “bound’ silanols, i.e., mutual hydrogen-bridged =Si-OH groups; (3) “free” silanols, i.e., =Si-OH groups not involved in hydrogen bonding. The main infrared absorbances of the hydroxyl groups lie in the 2.5-3.0-pm region. Silica samples suitable for infrared surface studies must fulfill two requirements: (1)they must be self-supporting plates having a weight less than 40 mg cm-2;(2) the particle size in the sample must be either far less, or much more than the wavelength of the applied radiati0n.l Commercial silica gel is difficult to mill and press, which means that reliable infrared measurements on this material are hardly possible. Spectroscopic studies on silica and silica-based systems are, therefore, usually done with silica powder, tacitly assuming that silica gel and silica powders have identical surface structures. It has been proved that on the silica powder, degassed a t about 700 K, the remaining hydroxyl groups are isolated, single surface silanols; no evidence was found for the existence of geminal silanediols, =Si(OH)2.2 This is surprising, since the latter groups should have a thermal stability comparable with that of single silanols; simple dehydration according to eq 1 is not likely a t moderate =Si0 + H20 (1) temperatures, as =Si0 is thought to be highly r e a ~ t i v e . ~ Although silica powder apparently does not hold geminal silanediols, it remains possible that on silica gel these groups do exist, e.g., because of the different methods of preparation. The indicated experimental problems with silica gel can be avoided by using transparent silica gel plates in one piece as described by Peria4 In this paper we present an infrared study on these preparations in order to find out
-
if dry silica gel holds groups, e.g., geminal silanediols, that are not present on the surface of silica powder. We used deuterium exchange, silanization, and heat treatment as surface-modification techniques.
Experimental Section Infrared spectra were recorded in the absorbance mode on a double-beam grating spectrometer (Grubb Parsons Spectromaster MK-111). Absorbances between 1.0 and 2.0 were measured by placing an optical attenuator in the reference beam. Slit widths and scanning rates were chosen so as to give minimal signal distortion. The spectral slit width was usually less than 4 cm-l, and the accuracy was better than 2 cm-l. Complete balance between sample beam and reference beam could not always be achieved. Therefore, in some spectra a weak, sample-absorption dependent, water vapor spectrum remained visible. We prepared the silica gel samples according to a method described by Peri.4 After autoclaving, a plate was cut to fit in a stainless steel sample holder. By means of a Ni-Cr thread and a winch, the holder could be moved between a 10-mm gas cell with NaCl windows and an oven, both being part of a conventional glass high-vacuum ~ y s t e m .A~ gas cell, identical with the measuring cell and filled with the same atmosphere, was placed in the reference beam. The samples were calcined during 2 h in 2 X lo4 N m-2 O2 a t 875 K, followed by 1-h evacuation a t the same temperature (the final pressure was always below N m-2). Sample heating by the infrared beam was reduced by filling the system with lo3N m-2 He if no other gases were present. Sample temperature in the beam was 335 f 5 K. The silica samples obtained were over 99.98% pure, as measured by atomic absorption spectroscopy (main contaminants Mg and Al). Their weight was about 20 mg cm-*, and the specific surface area after calcination was about 650 m2g-l (BET N2). Absorbances at 2200 and 8000 cm-l were less than 0.01. Hexamethyldisilazane (Aldrich-Europe, 98% ), D,O (Fluka, 97%), and doubly distilled water were degassed by repeated freezing and evacuation. Results Figure 1A shows the infrared spectrum of a freshly calcined silica gel sample (slit width 3 cm-l, scanning rate 6.25 cm-l min-l). Three successive scans were run over each other in order to reduce noise influences. As indicated in the Experimental Section, water vapor absorption could
0022-3654/78/2082-2748$0 1.0010 (C 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978 2749
Infrared Study of the Silica Gel Surface
I
I
,
1.6
1.4
-
12
-
m
e
2
Q
0.4
frequency crn-'
Figure 1. (A) Spectrum of a silica gel plate evacuated for 1 h at 875 K. (B) Water vapor spectrum at the same average absorbance. Scale refers to A; B is displaced 0.4 absorbance units upward.
frequency , cm-'
Flgure 3. Spectra of a silica gel sample before (A) and after (6) treatment with hexamethyldisilazane.
t
8 ' E ..i
1
0.4
2650
2750
3650
3750
frequency, cm"
Figure 2. Spectra of a silica gel plate during reaction with D20vapor: (A) after two exchange cycles; (B)after 20 cycles; (C) after treatment of B with vapor-phase H,O.
not be excluded in all experiments. Because there has been some discussion on this subject,@ the sample was removed, and an optical attenuator, set at an absorbance of 1.2, was placed in the sample beam. Under these conditions, three more spectra were run (Figure 1B). Combining Figures 1A and l B , it is evident that interfering water vapor cannot account for the observed maxima in the 3730-3750-cm-' region. In the following, we will refer to these three maxima as LF, MF, and HF, corresponding with bands a t low- (3734 cm-l), medium(3742 cm-l), and high-frequency (3749 cm-l), respectively. Deuterium Exchange. Silica gel plates were deuterated by exposing them to 2 X lo2N m-2 D20 at 700 K for 5 min, followed by evacuation at the same temperature. Figure 2A shows the spectrum after two such additions. Complete elimination of all bands in the 3730-3750-cm-l region could not be achieved after 20 exchange cycles (Figure 2B). Similar results were obtained when the sample was treated for 18 h with saturated D,O vapor a t room temperature. By treating the deuterated silica with H 2 0 vapor the original spectrum was restored, except for a small peak in the 0-D stretching region (Figure 2C). It can be seen that LF is the first band to be displaced on deuteration, followed by MF, while H F is left in place. The 0-D band has no clear fine structure, although there are distinct changes in band shape during deuterium exchange.
frequency
I
cm-'
Flgure 4. Spectra of a silica gel sample after heating at the indicated temperatures (see text). The dotted line shows the graphical difference between the two drawn curves.
Silanization. Because deuterium exchange has always been reported to occur rapidly on the silica surface,l0the assignment of HF to a surface hydroxyl vibration was doubted. Therefore, we treated a sample with hexamethyldisilazane, a highly effective compound for silanizing free si1anols:l' (CH3),Si-NH-Si(CHJ3
+ BSi-OH
-
2=Si-O-Si(CH3),
+ NH,
(2)
A sample was exposed for 1 h to 5 X lo2 N m-2 of hexamethyldisilazane a t 525 K, evacuated for 0.5 h a t the same temperature, and for 0.5 h a t 700 K. Spectra recorded before and after silanization are given in Figure 3. The residual bands in the 0-H stretching region can be ascribed to combinations of C-H (2909 and 2968 cm-l) and Si-C stretching vibrations (752 and 847 cm-1).12 No N-H compounds could be d e t e ~ t e d . ~ J ~The J * disappearance of H F clearly indicates that this band is caused by surface hydroxyl groups. Heat Treatment. During the experiments it was found that LF was much more sensitive to small variations in evacuation temperature than the other two bands. T o obtain more quantitative information on this phenomenon, a sample was subjected to a heat treatment.
2750
The Journal of Physical Chemistry, Vol. 82, No. 25, 1978
TABLE I: Position and Half-Width of v ( 0 H ) and v ( O D ) ~
4OD)
v(OH) wave halfnumber, width, curve cm-' cm-'
C A B
7 17 23
2762 2760 2760
wave number, cm-
halfwidth, cm- '
3734-3742-3749 3742-3749 3749
40 25 9
a For A, B, and C see Figure 2. Accuracy in the indicated numbers * 2 cm-'.
TABLE 11: Literature Values for v ( 0 H ) in Some Silicas
ref
silica type
degas temp, K
wave number, cm-I
halfwidth, cm-'
4 15 15 16 2
gel gel powder powder powder
875 775 775 950 1075
3740 3740 3748 3748 3748
40 50 11 8 5
I
I
SI
-
ESI-O-SI~
+
H20
'OH
(3)
Ill
3600 cm-'
vb
2
vf
= 3 7 2 0 cm-1
hydration (Figure 2C and Table I) is narrow, and shifted somewhat to higher frequencies, so that it is believed to be the deuterated HF. With respect to the nature of MF, the following observations are noteworthy: (a) neither the intensity ratios of H F and MF, nor their absolute heights change appreciable on degassing a t temperatures up to 1075 K; (b) MF exchanges easier with D,O than HF, although not as fast as LF; (c) MF is markedly broader than HF. These observations are in agreement with an assignment of MF to a very weakly coupled ~ ~ pair in ~ geminal ~ silanediol:
i; = 3 7 4 2 cm-I
Discussion We have shown that the silica gel plates show three absorption bands in the 3730-3750-cm-' region: LF, MF, and HF. These bands do not behave similarly under the applied surface modifications. From this, and from the fact that all three bands disappear on silanization, we conclude that they originate from at least three different types of surface silanols. A plausible explanation for LF is the stretching vibration of weakly hydrogen-bonded silanol pairs that condense upon heating under the evolution of water: SI Ill
van Roosmalen and J. C. Mol
= sI / O H
A silica gel sample was calcined a t 1075 K for 1 h in a pot oven. Next, it was quickly transferred to the vacuum line, where it was evacuated for 0.5 h at 875 K. Figure 4 shows spectra before and after this treatment. By substracting these two curves from each other, an indication for the nature of the removed groups is obtained (dotted line in Figure 4). It is obvious that LF originates from a very asymmetrical band having its maximum at about 3720 cm-l. The heat treatment hardly affected shape and intensity of MF and HF.
O/Hti'.(fHf
A. J.
This would explain the fast deuterium exchange of LF, as hydrogen bonding enhances the dipole strength and, therefore, the efficiency in adsorbing polar species, e.g., (heavy) water. A further discussion on these groups and other mutual hydrogen-bonded groups on silica gel will be presented in a following paper. Band half-widths and peak positions from Figure 2 are summarized in Table I. Table I1 shows some values, reported by others, for the 0-H stretching vibration in dry silicas. A comparison of Table I and Table I1 strongly suggests that H F and the 3748-cm-l band on silica powder are identical, which means that H F is caused by an isolated, single surface silanol: =Si-OH, u = 3749 cm-l. The difficulties in exchanging the proton in this group with vapor-phase D,O are probably caused by low physisorption of water. The band left in the 0-D region after re-
This assignment is not in contradiction with the absence of fine structure in the 0-D band, if we &tribute the frequency difference between H F and M F to a slight hydrogen bonding between the two hydroxyl groups in a geminal gr0up.l' The hydrogen bonding in deuterated species is weaker than in the original compound,18so that the spacing between HF and MF after deuterium exchange can be less than the band half-width. This is supported by the fact that the difference between u(OD) in Figure 2B and 2C is only 2 cm-l (Table I). Recently, Morrow and Cody published an extensive study on very dry silica p o ~ d e r . ' ~ JThey ~ J ~ observed ~~~ that, after degassing Cabosil above 1350 K, readsorption of water led to the formation of new bands a t 3741, 3620, and 3520 cm-l. They assigned the 3741-cm-' absorption to a hydrated electron-deficient siloxane bridge, and the bands a t 3720 and 3520 cm-l to asymmetric hydrogenbonded silanol pairs. Similar spectra were obtained by Volkov et aL21 The close similarity between the bands observed by these authors, and LF and MF in the present study suggests that they correspond to the same species, although the difference in intensity indicates that the relative abundance on silica powder is less than on silica gel. If we adopt the above hypothesis, the explanation given by Morrow and Cody for their 3741-cm-l band becomes less likely, since it is improbable that coordinatively unsaturated species are formed a t the moderate temperatures under which the silica gel samples were prepared. It is possible that the extreme conditions in the experiments of Morrow and Cody induced a rearrangement of the pressed silica powder to a more gellike structure. During this rearrangement =Si0 groups could be formed, having the possibility to give geminal silanediols on hydration. These =Si0 groups could account for the observed reactivity of very dry silica powder, e.g., the formation of amines after exposure to "3:"
The Si-0 stretch and the Fermi resonance intensified first overtone of the Si-0 bending vibration, then, could give rise to the infrared bands a t 908 and 888 cm-l observed by the above authors.16 In accordance with this assignment, pyridine adsorption has been reported to have a greater effect on the 888-cm-l band (Si-0 bend) than on the band at 908 cm-l (Si-0 stretch).16 Measuring the stoichiometry of the reaction of hydrogen sequestering agents, e.g., (CH3)2SiC12, with silica has been proposed as a method for the detection of geminal sil a n e d i o l ~ . ~In~ ,our ~ ~ opinion, this is not realistic: the
-
u
The Journal of Physical Chemistry, Vol. 82,
Communications to the Editor
assumed 1:2 reaction (eq 5) would produce a highly (CH~)~SIC f I =~S I ( O H ) ~
2(CH3)2S1C12t =SI(OH);!
-
h
+
= S I \ /'u'S~(CH3)2
O S-,
I
2HCI
(5)
t 2HCl ( 6 ) '0-S
2751
Acknowledgment. We thank J. W. Elgersma from the Laboratory for Analytical Chemistry, University of Amsterdam, for analyzing our silica gel samples. References and Notes
(CH3)zCI
=SI
No. 25, 1978
I (CH 3) 2 C I
strained four-membered cyclic siloxane, a compound reported to be highly unstable.24 It seems, therefore, reasonable to assume that with geminal silanediols a 1:l reaction (eq 6) will occur, as with single hydroxyls. The question remains whether the surfaces of commercial silica gel, and the silica gel plates used in this study, are identical. From Tables I and 11, it can be seen that band shape and band position are the same for both types of silica. The reason why earlier workers with silica gel did not find a fine structure in the 0-H stretching band might be insufficient resolution, the use of too large silica gel particles, or recording in the transmission instead of absorbance mode. Surface-hydroxyl concentrations, measured by following the weight change of silica gel samples on silanization with hexamethyldisilazane, gave essentially the same results for a silica gel plate and two commercial silica gels.25 We conclude that the silica aerogel samples, used in this study, and commercial silica gel are closely similar in surface properties. Silica gel and silica powder are obviously not similar, neither in bulk structure, nor in surface properties. Therefore, results obtained from studies on silica powder should not be extended directly to practical systems, e.g., catalysts and drying agents, that are based on silica gel.
G. Duykaerts, Analyst, 84, 201 (1959). P. R. Ryason and 8. G. Russell, J. Phys. Chem., 79, 1276 (1975). M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", Marcel Dekker, New York, 1967, p 87. J. B. Peri, J . Phys. Chem., 70, 2937 (1966). A. A. Olsthoorn, Ph.D. Thesis, University of Amsterdam, The Netherlands, 1974. M. L. Hair and W. Hertl, J. Phys. Chem., 73, 2372 (1969). J. A. Hockey, J. Phys. Chem., 74, 2570 (1970). F. H. van Cauwelaert, P. A. Jakobs, and J. B. Uytterhoeven, J. Phys. Chem., 76, 1434 (1972). B. A. Morrow and I. A. Cody, J. Phys. Chem., 77, 1465 (1973). V. Ya. Davydov, A. V. Kiselev, and L. T. Zhuravlev, Trans. Faraday Soc., 60, 2254 (1964). W. Hertl and M. L. Hair, J. Phys. Chem., 75, 2181 (1971). J. J. Fripiat, J. Uytterhoeven, U. Schobinger, and H. Deuel, Heb. Chim. Acta, 43, 176 (1960). C. G. Cannon, Spectrochirn. Acta, 10, 425 (1958). B. A. Morrow, I. A. Cody, and L. S.M. Lee, J. Phys. Chem., 79, 2405 (1975). R. S. McDonald, J. Phys. Chem., 62, 1168 (1958). B. A. Morrow and I. A. Cody, J. Phys. Chem., 80, 1995 (1976). E. R. Lippincott and R. Schroeder, J. Chem. Phys., 23, 1099 (1955). S.Pinchas and I. Laulicht, "Infrared Spectra of Labelled Compounds", Academic Press, London, 1971. B. A. Morrow and I. A. Cody, J. Phys. Chem., 80, 1998 (1976). B. A. Morrow, I. A. Cody, and L. S.M. Lee, J. Phys. Chem., 80, 2761 (1976). A. V. Volkov, A. V. Kiselev, and V. I. Lygin, Russ. J. Phys. Chem., 48, 703 (1974). C.G. Armistead, A. J. Tyler, F. H. Hambieton, S.A. Mitchell, and J. A. Hockey, J. Phys. Chem., 73, 3947 (1969). J. B. Peri and A. L. Hensley, J . Phys. Chem., 72, 2926 (1968). J. Greene and M. D. Curtis, J. Am. Chem. Soc., 99, 5176 (1977). A. J. van Roosmalen, to be published
COMMUNICATIONS TO THE EDITOR Three-Electron Oxidation. 14. Carbon-13 Isotope Effect in the Three-Electron Cooxidation of Isopropyl Alcohol and Oxalic Acid'
Sir: The proposed mechanism for the cooxidation of oxalic acid and isopropyl alcohol by chromium(V1) envisages a three-electron transfer with the simultaneous breaking of C-H and C-C bonds in the rate limiting step2 (Scheme I). The observed deuterium isotope effect in the cooxidation kH/kD = 5.9, constitutes proof of 2-deuteri0-2-propano1,~ for C-H bond cleavage in the rate-determining step. The assumption that the C-C bond is broken simultaneously was based on the simple stoichiometric ratio of oxidation products, particularly in the presence of free-radical scavengers ((CH,),CO:COZ = 1:l)and further on the high rate of the reaction which was interpreted as resulting from the system's ability to avoid the formation of an unstable high energy chromium(1V) intermediate. The force of the second argument was, however, weakened by subsequent studies which revealed that rate accelerations of comparable magnitude can also be observed in reactions where oxalic acid acts as mere catal y ~ t . ~In, ~the light of these findings it is necessary to consider an alternate mechanism in which the three0022-3654/78/2082-2751$01 .OO/O
Scheme I
Cr(V1) + CO,' Cr(V) t (CH,),CHOH Cr(V)
+ (CO,H),
--f
-f
-f
Cr(V) + CO,
+ (CH,),CO Cr(II1) + 2C0, Cr(II1)
(3) (4) (5)
electron oxidative decomposition of the intermediate complex is replaced by a two-step sequence (Scheme 11). In this modified mechanism the termolecular complex undergoes a two-electron oxidation decomposition (reaction 2a) to yield an unstable chromium(1V)-oxalic acid complex which subsequently decomposes in a one-electron reaction (reaction 2b). Such a mechanism would be consistent with the observed results provided that the chromium(1V) intermediate was too short lived to permit any ligand exchange with the solvent or other substrates.
0 1978 American
Chemical Society
