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Determination of hydroxyl groups and free water on silica gel in the near infrared region
Journal of Non-Crystalline Solids 116 (1990) 93-99 North-Holland
93
D E T E R M I N A T I O N OF HYDROXYL G R O U P S AND FREE WATER O N SILICA GEL IN T H E NEAR INFRARED R E G I O N Jan K R A T O C H V i L A , Zden~k SALAJKA, Antonin KAZDA, Zbyn6k KADLC, Jaroslav SOUCEK and Mihnea G H E O R G H I U Chemopetrol, Research Institute of Macromolecular Chemistry, 656 49 Brno, Czechoslovakia Received 10 April 1989 Revised manuscript received 20 September 1989
The paper deals with the interpretation of fundamental, combination, and first overtone vibration signals of OH groups and H 2° on silica gel. A signal at 4425 c m - 1 is assigned to vibrations of free O H groups, while a signal at 4505 c m - 1 is attributed to vibrations of the s u m of all O H groups, independently of their types. A molar absorption coefficient of H 2 0 signal at 5235 cm 1 was determined. Fundamental bands of H 2 0 and OH groups in the 3100-3800 c m - 1 range cannot be separated and interpreted quantitatively.
1. Introduction Fine-grained porous silica gel is widely used as a catalyst carrier for the gas phase olefin polymerization. Depending on the activating procedure and history of the silica gel sample, it contains variable amounts of OH groups and frequently considerable quantities of physically adsorbed H20. Hydroxyl groups and water molecules exhibit fundamental, first overtone, and some combination vibrations in the near infrared region (3000-10000 cm -1) [1-9]. The aim of this paper is a study of relations between vibration signals in the near infrared region and a surface structure of silica gel, considering the amounts of O H groups and physisorbed H20.
2. Experimental Davison silica gel, grade 952 (USA), was dehydrated in a quartz glass vessel in a nitrogen stream at 200-900 ° C for 6 h, then pumped for 30 min and cooled under nitrogen. Unheated samples of silica gel were prepared through evacuation in a quartz vessel at 2 0 ° C and 1 0 - 2 Pa for several 0022-3093/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
hours. In both cases a layer of silica gel was stirred mechanically or pneumatically so that the evolved water was efficiently removed. The specific surface area of the silica gel was measured using the BET method and the following values were obtained at various temperatures of heat treatment: 2 8 5 _ 2 m2/g (20-600°C), 278 + 2 m2/g (800°C), and 218 ___4 m2/g (900 o C). Pore volume values calculated using a nitrogen desorption isotherm were 1.8 + 0.1, 1.7 + 0.1, and 0.85 _+ 0.05 cm3/g, respectively; the range of pore diameter distribution was 6-30 nm with a mean of 18 nm. n-Heptane, analytical grade (Loba Feinchemie, Austria) was checked for the presence of aromatics by long-term shaking with concentrated sulphuric acid. When no discoloration appeared in the test, the n-heptane was distilled and stored. Immediately before use, n-heptane was freed from water and oxygen by distilling at least 15% of the initial volume by highly pure nitrogen at elevated temperature. Tetrahydrofurane, (THF) analytical grade (Loba Feinchemie, Austria) was predried by prolonged stirring with metallic sodium and then vacuum-distilled into a storage ampoule containing fresh sodium and benzophenone. Before each procedure, fresh T H F was distilled from the storage solution of radical anions into an evacuated
94
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
vessel. Preparation of radical-anion solutions was described earlier [10]. Phenanthrene and benzophenone (both Reachim, USSR) were recrystallized and volatiles were removed before use by evacuation (10 -2 Pa, 2 h). Pure sodium (Lachema, Czechoslovakia) was freed from impurities by a mechanical removal of the surface layer in an inert atmosphere and out to small pieces ( - 10 mg). Triethylaluminum and trimethylaluminum (both Texas Alkyls, USA), were distilled, diluted by heptane to - 1 mol 1-~ and stored in sealed ampoules under nitrogen. Carbon tetrachloride, analytical grade (Lachema, Czechoslovakia) was distilled and then freed from water and oxygen by stripping off 15% of its volume by pure nitrogen, Nujol, analytical grade (Slovakopharma, Czechoslovakia), was stripped off before use by highly pure nitrogen at 100 o C. Infrared spectra of silica gel slurries in nujol (thickness 0.2 mm) in the 3000-4000 cm-1 region were measured using a Perkin-Elmer 377 instrument. Spectra in the 4000-10000 cm 1 region were measured on 16 wt.% slurries in CC14 in a 1 cm quartz cell using a Perkin-Elmer 330 instrument. Peak intensities were evaluated planimetrically or from the second derivative value of the absorption curve, Methane or ethane evolution in the reaction of silica gel with trialkylaluminum was measured in the following way. The weighed glass ampoules with 0.3-0.5 g of silica gel were consecutively broken in a magnetically stirred heptane solution (15 cm 3) of trimethylaluminum or triethylaluminum (concentration about 0.3 mol 1-1) in a glass reaction vessel of predetermined volume. The temperature of the apparatus was set at 25 + 0.1 ° C. The amount of methane or ethane liberated was measured manometrically. The dependence of the increase of pressure on time was recorded and the amount of alkane was calculated after reaching equilibrium (1-3 h). The solubility of methane and ethane in heptane at 2 5 ° C and partial pressure of 101.3 kPa was found to be 0.029 + 0.001 and 0.221 ___0.004 mol 1-1, respectively. Corrections on volume changes and on nitrogen content in ampoules were made. The measurements of alkane amount evolved w e r e r e producible within 2-3%.
Distillations of H 2 0 with T H F were carried out in an all-glass Simax apparatus depicted in fig. 1. Glass ampoules with samples were located in the upper connecting tube T. About 100 mg of phenanthrene and 200 mg of sodium were placed into ampoule C. After evacuating the whole apparatus, pure T H F was distilled into ampoule C and a solution of radical anions was prepared in an excess of sodium so that all phenanthrene reacted. Concentration of radical anions was determined via an acidimetric titration of sample by 0.1 mol 1-1 HC1, using bromocresol green as an indicator. A part of the radical-anion solution ( - 15 cm 3) was poured from ampoule C to ampoule B by tilting the apparatus and electric conductivity was measured corresponding to the predetermined concentration. The connection between ampoules B and C was sealed off and thus the part of the apparatus with ampoules was separated from the vacuum line. The actual measurement consisted of multiple distillation of T H F from the radical anions contained in ampoule B into ampoule A and back, measuring simultaneously the conductivity and volume of the radical anions in ampoule B at 20 ° C after each distillation step. Thus, a diffusion rate of impurities through the greased ampoule breaker D could be determined. Then a vial with a silica gel sample was broken and the distillation of T H F between ampoules A and B was repeated several times, cooling down and warming the ampoules alternatively. As the T H F distillation caused a certain dusting of fine silica gel particles into ampoule B, a sintered glass S was placed into connecting tube T. The radical-anion concentration was - 0.05 mol 1-1. The amounts of decomposed radical anT===~D
T S
vocuum Nz,THF
===~====~ IU
=
Pt A
B
C
Fig. 1. Apparatus for distillation of H 2 0 with T H F into radical-anion solution. See text for definition of A - D , S, and
T.
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
ions were corrected for diffusion rate of impurities in the blank experiment,
3. Results and discussion 3.1. Fundamental vibrations of H 2 0 and O H groups in silica gel
Figure 2 reveals fundamental vibrations of H 2 0 and O H groups in the 3000-4000 c m - 1 region measured in the nujol-suspended silica gel samples. The high-temperature activated silica gel samples exhibited a narrow absorption band 3660 cm -1 assigned to free O H groups [2-5,9]. The signal at wave number 3560 c m - 1 is probably due to the overtone vibration of nujol and contributes to the spectra of all samples. Absorption bands of O H groups (3400-3700 cm -1) are wide and cannot be resolved: they merge with the 3660 cm -1 band in the silica gel samples dehydrated at lower temperatures of dehydration. Free H 2 0 should
Wave Number [cm-I] 3600 4000
3200 I
I
I
g o <
kJ//
~
20"c
200"C 400"c ~ / 8 0 0 "600°C C 900°C ~ L.-_..-J
t t 2.8 2.6 2.4 WQvetength [# m] Fig. 2. Fundamental vibration spectra of H20 and OH groups measured for silica gel activated in the 20-900°C range; 15 wt.% slurry in nujol, 0.2 mm NaCI cell, Perkin-Elmer 377 instrument. The 20 °C silica gel sample was pumped for 18 h at 20°C and contained 0.06 mmol H20/g. Other samples were dehydrated for 6 h in a nitrogen s t r e a m , 32
30
95
exhibit a signal at 3300 c m - 1 , but the band is also wide. Infrared spectra of fundamental vibrations of the silica gel O H groups exhibit - apart from a large band width - additional disadvantages, such as unequal extinction coefficients of different vibrations and fluctuating background. The slope of the straight line connecting minima on both sides of the absorption bands decreases progressively from positive to negative values with decreasing dehydration temperature. The phenomenon is likely to be caused by the increasing content of free H 2 0 (a band with wave number around 3300 cm-1). These observations make dubious a comparison of spectra of various silica gel samples and a direct interpretation of a complicated set of the fundamental vibration absorption bands. The area of the fundamental vibration absorption bands (3000-4000 cm -1) decreases approximately linearly with increasing dehydration temperature of silica gel in the 200-900 ° C range. The pattern of this dependence corresponds to the temperature dependence of the gaseous decomposition products (methane, ethane) formed during the reaction of silica gel with trialkylaluminum. However, it should be noted that the area under the enveloping curve is a sum of incongruous terms, i.e. areas of overlapping bands with unequal extinction coefficients. Thus, they cannot be studied separately. 3.2. First overtone and combination vibrations of 1120 and O H groups
In order to identify the sources of signals in the near infrared region, samples of silica gel with various amounts of O H groups were prepared. Davison silica gel, grade 952, dehydrated at 20-900 ° C, or the same silica gel, where the O H group content was decreased through the reaction with an trialkylaluminum, were used for this purpose. The amounts of O H groups were determined using a manometric measurement of evolved methane under an excess of trimethylaluminum in heptane. It was found that part of the O H groups can be removed either by a chemical reaction with trialkylaluminum or by the temperature induced
96
J. Kratochvila et al. / Hydroxyl groups and free water on stltca gel
Wave Number [crn-~] 10000
6000
150
4000
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~6
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~, ..Q
~
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~
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~
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/
o
o
_ I
Lf// 04
O2 . . . . k . ~ 600" 900" I ~ ~ 1A
) ~ ~///
_
0 1.8 2.2 2.6 WaveLength [ ~ m ]
Fig. 3. Combination and first overtone vibration spectra of silica gel; 16 wt.% slurry in CC14, 1 cm quartz cell, PerkinElmer 330 instrument. The 20 o C silica gel sample was pumped for 2 h at 2 0 ° C and contained 0.16 mmol H 2 0 / g . Other samples were dehydrated for 6 h in a nitrogen stream,
dehydration. The same O H signals in infrared spectra are observable after either of the above treatments, suggesting that the same level of O H groups is obtainable using the above procedures, Figure 3 shows spectra of four samples of silica gel in carbon tetrachloride in a 1 cm cell in the 4000-10 000 c m - t region. Signals of 4425/4505, 5235 and 7140 cm -1 are easily distinguishable in the figure. The abscissa of fig. 2 points in the opposite direction to that in Fig. 3; it is caused by different thicknesses of cells (0.2 vs. 10 mm) and by different optical media used (nujol vs. carbon tetrachloride). Signals in the region of fundamental vibrations have many times higher extinction coefficients than overtone signals; therefore it is not possible to measure both spectra in the same conditions. Figure 4 presents the dependence of the signal intensities at 4425, 4505, and 7140 cm -a on the amount of O H groups. Twenty silica gel samples were dehydrated at temperatures 20-900 o C (see also fig. 5) and nine silica gel samples were dehydrated at 600 ° C; five of them reacted with 0.4-2.0
I
0.5
t
1.0
o/o
//i/.s
~ 5c
04
0
• •
I
1.0
1.5
2.0
OHgroupsamount[rnrnot.g -1]
Fig. 4. Dependences of 4425 (,a), 4505 (e), and 7140 ( o ) cm -1 signal intensities on the amount of silica gel O H groups. Conditions as in fig. 3.
mmol triethylaluminum per gram, four others r e acted with 0.4-2.5 m m o l trimethylaluminum p e r gram. The intensity of the strongest signal at 4505 c m - 1 is a linear function of the amount of O H group, independent of the silica gel dehydration temperature (i.e. 2 0 - 9 0 0 ° C ) or of the trialkylaluminum content. Based on the validity of the linear relation between the 4505 cm -1 signal intensity and the amount of O H groups, this signal can be assigned to vibrations of all O H groups, independently of their type (free O H and hydrogen bonded O H groups). Figure 5 demonstrates the dependence of the 4425, 4505, and 7140 c m - I
150 _~ ~
-.,.
oc
"
~'1 ~
~ 5c ~ ~ _~ ~
"
"~o..~i [
-. " " - . 8
~
~ 200
o"-.
i
i ~oo
x------...~...~ J 6oo 800
Dehyde ratoim tnperautre f
['C]
Fig. 5. Dependences of 4425 (zx), 4505 (O), and 7140 ( o ) cm-1 signal intensities on dehydration temperature of silica gel. Conditions as in fig. 3.
J. Kratochvila et el. / Hydroxyl groups and free water on silica gel
97
signal intensities on the dehydration temperature for 20 silica gel samples. All three dependences exhibit linear sections in the range above 500600°C. The pattern of the dependences is similar to that observed for the dependence of the OH group content on the silica gel dehydration ternperature. The intensity of the 4425 and 7140 c m - I signals is a linear function of the amount of the OH group, up to approximately 1.0 mmol g - t (dehydration temperature of 500-600 ° C). At dehydration temperatures above 600 ° C, free OH groups prevail [2,9,11]. Thus, all three overtone and cambination signals in the spectra of samples treated in this temperature range are due to the free OH group vibrations. A decrease of the rate of the intensity increase of the 4425 and 7140 cm-1 signals observed when the OH group amount exceeds 1.0 mmol g-1 may be connected with the formation of hydrogen bonds with different vibration frequencies. The 4425 c m - ] signal intensity is virtually constant in the spectra of samples dehydrated in the 20-400 ° C range, while that of the samples dehydrated in the 6 0 0 - 9 0 0 ° C range is linearly proportional to the OH group content, Thus, the 4425 cm -] signal can be assigned to the free OH group vibrations, The intensity of the 7140 cm-~ signal increases linearly with the amount of O H groups up to about 1.0 mmol g-~; the rate of increase decreases at higher amounts. It can be assumed that the 7140 cm-~ signal corresponds to a certain combination of the vibrations of the free and hydrogen
3. 3. Calibration curve of 1-120 on silica gel
bonded OH group, e.g. it could be assigned to vibrations of all free OH groups and part of the hydrogen bonded O H groups. The dependence of the 7140 cm -] signal intensity on the amount of O H groups is intermediate between those of the 4425 and 4505 cm -] signals. It is likely that the 7140 c m - ' signal is a sum of the overtone vibrations of various O H groups; the fundamental vibrations of these groups are in the 3000-4000 cm-1 range, as suggested by Anderson and Wickersheim [6]. Different vibrations exhibit apparently unequal extinction coefficients thus leading to the observed non-linear dependence of the
~
signal intensity on the amount of OH groups,
In order to determine the extinction coefficient of H 2 0 on silica gel, infrared spectra of Davison silica gel, grade 952, samples dehydrated at 200, 600, and 800 ° C with added H 2 0 were measured. H 2 0 was added by means of a syringe to a weighed sample of silica gel, then the wetted sampie was stirred thoroughly for 2 h in a closed vessel. The samples were poured into quartz cells and carbon tetrachloride was added. Absorption spectra measured in different thicknesses of the silica gel layer were the same within the experimental error, thus demonstrating a homogeneous distribution of H 2 0 within the silica gel sample. Figure 6 presents the 4425, 4505, 5235, and 7140 c m - ] signal intensities of the absorption curve for silica gel dehydrated for 6 h at 600 ° C. Vibration modes appearing after H 2 0 addition (wave numbers 5100, 5235, 6800, 7090, 7190, and 8700 cm - ] ) show that a wide range of newly formed structures is present, the main component being hydrogen bonds. The 5235 c m - 1 signal exhibits the highest intensity and is measurable even at low contents of water. That is why it was used to measure the H 2 0 concentration on silica gel. The calibration curve of the 5235 cm -] signal is linear in the whole range of H 2 0 concentrations (0-1.1 mmol g - l ) , thus obeying the Lambert-Beer law. This signal occurs also in H 2 0 saturated C C I 4 , being
~40 1
2
0
_
~
~
.
~
o
~
~
.
~
~
~,I00 ~ 80 ~ 60 ~ 40! ~ 20 ~, 0
, 02
.
_
i 0.4
_
_
i 0.6
_
.
.
t 0.s
~
i to
Water content [rnmot.g -t1
Fig. 6. Calibration curves of H 2 0 on silica gel dehydrated at 6 0 0 ° C based on 4425 (r,), 4505 (O), 5235 ([]), and 7140 (©) cm -1 signal intensities; ( I ) H 2 0 saturated CCI 4. Conditions as in fig. 3.
J. Kratochoila et al. / Hydroxyl groups and free water on silica gel
98
an inactive solvent. The similarity of wetted CC14 and silica gel spectra, the sharpness of the spectra, and a linear dependence on H 2 0 concentration indicate that the signal can be assigned to a monomeric form of H20, i.e. to individually adsorbed water molecules, The increase of free H 2 0 content brings about a decrease of the 4425, 4505, and 7140 cm-1 O H group signal heights. The decrease is reversible as documented by removing H 2 0 at 20 ° C in vacuo, Absorption of H 2 0 in the vicinity of O H groups brings about a Van der Waals interaction of H 2 0 molecules with O H groups, thus changing the frequencies of the O H group vibrations. Newly appearing signals may be caused - apart from vibrations of H 2 0 and hydrogen bonds between H 2 0 and O H groups - by changed vibrations of O H groups,
3. 4. Dehydration of silica gel by pumping at 20 °C Figure 7 shows results of spectral measurement following the dehydration by pumping at laboratory temperature (20 o C). The abscissa features the time of evacuation by an oil diffusion p u m p after a preceding removal, taking - 1 h, of most of the H 2 0 (a turning-point is observed at 10 Pa pressure under conditions of intensively stirred 20
'~ ~2o "~
~
80 ~ 60
~0
E~° ~To_..r..__0~ 50
r~ a00
t ~s0
~ 20c
40 ~ "-= 0 ~
Oehydro~ion time [hn/ Fig. 7. Dependence of the amounts of gases (evolved during the silica gel-induced organometal decomposition), the amount of physically bonded H20 and signal intensities at (A) 4425 and (o) 4505 cm-1 (measured in the near-infrared region) on time of silica gel evacuation at 20 o C. (v) CH 4 from Me3AI; (V) C2H 6 from Et3AI; (D) H20 amount. Conditions as in fig. 3, details see text.
g silica gel sample in a 500 ml spherical glass vessel). A residual amount of H 2 0 (5235 cm -1 signal) decreases and free and hydrogen bonded O H groups (4425 and 4505 cm -1) increase with the increasing time of evacuation. After 100 h evacuation (pressure < 1 × 10 -2 Pa) at 2 0 ° C , silica gel contains only 0.01-0.02 mmol H 2 0 / g . On the other hand, the amount of O H groups does not change - after reaching a certain level - upon continuing evacuation. This observation proves (contrary to published considerations [12]) that virtually all physically bonded H 2 0 can be removed by pumping silica gel at room temperature within a reasonable time (100-200 h). In the evacuation experiments pore size distribution can play an important role. It can be expected that smaller pores bind water more firmly than larger ones and the times needed for water removal may be longer in the former case. In another experiment 10 g of heptane was added to 1 g of Davison silica gel, grade 952, and heptane was distilled to dryness. This procedure, conducted at 98.4 ° C followed by a short evacuation, allowed preparation within 1 h of a material containing less than 0.01 mmol H 2 0 / g and exhibiting the same infrared spectrum as that p u m p e d for 200 h at 20 ° C to 1 × 10 2 Pa. Both samples of silica gel also exhibit equal chemical properties (i.e. the reactivity with trialkylaluminums yielding alkanes) and have the same infrared spectra. Thus, both procedures can be considered as equivalent to obtain anhydrous silica gel with a maximum amount of
3.5. Distillation of anion solution
H20 with T H F into radical-
To check the reliability of the determination of free H 2 0 on the silica gel surface by the spectral method, the H 2 0 concentration was measured by distilling it from silica gel with T H F into a radical-anion solution. Radical anions react very fast and virtually quantitatively with compounds containing acidic hydrogen. In the case of H20, one proton neutralizes one anion and the remaining O H - group forms with a cation an ether-insoluble hydroxide [13]. This reaction can be used for the determination of trace amounts of H20.
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
If an inner surface of silica gel pores contains physically adsorbed H 2 0 , it can be transferred into the liquid phase by using a strong solvating agent and distilled together. Suitable solvating agents are ethers, such as T H F and diglymes, also used as solvating agents for radical anions. Considering a very fast release of H 2 0 from the silica gel surface during evacuation even at low temperature (20°C), an efficient removal of H 2 0 by a solvating ether (which in turn adsorbs on the surface) can be expected, Titration of H 2 0 distilled with T H F into a radical-anion solution led to a good agreement between the added amount of H 2 0 and the decomposed quantity of radical anions. It demonstrates the presence of free (adsorbed or cond e n s e d ) H 2 0 in the silica gel pores without reactions with siloxane bonds. Thus, H 2 0 adsorption on the silica gel surface is a reversible, physical process, By using the above method, 0.019 + 0.005 mmol g-~ of H 2 0 was found in silica gel dehydrated for 6 h in a nitrogen stream at 200 o C. The spectral method (5235 cm-~ signal) revealed 0.013 _+ 0.005 mmol g-~ in the same sample. No H 2 0 was found, using the above radical-anion method, in samples dehydrated for 6 h in a nitrogen stream at temperatures exceeding 400 o C. At the same time, the 5235 c m - l signal was not observed. The above findings indicate that both methods can be used for free H 2 0 determination.
4. Conclusions The results obtained demonstrate the suitability of the spectral determination of H 2 0 and O H groups on silica gel in a near infrared region [1,6,7]. Using a calibration curve, physically adsorbed H 2 0 (which may be present in limited amounts even after a long-lasting dehydration of silica gel, particulary at lower temperatures) can be determined quantitatively. A linear dependence of the 5235 cm -1 H 2 0 signal on the amount of H 2 0 (seen fig. 6) reveals the molar absorption coefficient value of 1.33 + 0.04 2 mol ~ cm 1. An accuracy of 0.01 mmol H 2 0 per gram of silica gel
99
was obtained, being sufficient for studies of the silica-based supported catalysts. The kinetics of silica gel dehydration during its evacuation at 20 ° C were followed by measuring the amount of free H20. Based on evaluation of the dependence of the 4425 and 4505 cm -1 combination signals of O H groups on the amount of O H groups (fig. 4) and on the silica gel dehydration temperature (fig. 5), it can be assumed that the 4505 cm -1 signal corresponds to the sum of O H groups (i.e. free and hydrogen bonded O H groups) while the 4425 cm-~ signal is related to the free O H groups only. The molar absorption coefficients are 0.89 + 0.03 and 0.35 +_ 0.02 mol-~ c m - ~ for signals 4505 and 4425 cm -~, respectively. The results obtained and the comparison with published data [1,6] reveal a necessity to verify the hypotheses using silica gel samples containing known amounts of different types of O H groups. The authors are indebted to Prof. M. KuEera for numerous helpful discussions and suggestions.
References [1] G. Wirzing, Naturwissenschaften 51 (1964) 211. [2] R.J. Peglar, F.H. Hambleton and J.A. Hockey, J. Catal. 20 (1971) 309.
[3] J. Murray, M.J. Sharp and J.A. Hockey, J. Catal. 18 (1970) 52. [4] A.V. Kiselev, V.A. Lokutsievskii and V.I. Lygin, Zh. Fiz. Khim. 49 (1975) 1796. [5] O.H. Ellestad and V. Blindheim, J. Molec. Catal. 33 (1985) 275. [6] J.H. Anderson and K.A. Wickersheim, in: Surface Science, ed. H.C. Gatos, Vol. 2 (North Holland, Amsterdam, 1964) p. 252. [7] D.L. Wood and E.M. Rabinovich, J. Non-Cryst. Solids 82
(1986) 171.
[8] Y. Yokomachi, R. Tohmon, K. Nagasawa and Y. Ohki, J. Non-Cryst. Solids 95&96 (1987) 663. [9] V.I. Lygin, Zh. Fiz. Khim. 58 (1989) 289. [10] Z. Salajka and M. KuEera, Collect. Czech, Chem. Corn-
mun. 48 (1983)3041.
[11] M.P. McDaniel, J. Phys. Chem. 85 (1981) 532. [12] H.P. Boehm, Angew. Chem. 78 (1966)617. [13] M. Szwarc, in: Carbanions, Living Polymers and Electron Transfer Processes (Interscience, New York, 1968).
93
D E T E R M I N A T I O N OF HYDROXYL G R O U P S AND FREE WATER O N SILICA GEL IN T H E NEAR INFRARED R E G I O N Jan K R A T O C H V i L A , Zden~k SALAJKA, Antonin KAZDA, Zbyn6k KADLC, Jaroslav SOUCEK and Mihnea G H E O R G H I U Chemopetrol, Research Institute of Macromolecular Chemistry, 656 49 Brno, Czechoslovakia Received 10 April 1989 Revised manuscript received 20 September 1989
The paper deals with the interpretation of fundamental, combination, and first overtone vibration signals of OH groups and H 2° on silica gel. A signal at 4425 c m - 1 is assigned to vibrations of free O H groups, while a signal at 4505 c m - 1 is attributed to vibrations of the s u m of all O H groups, independently of their types. A molar absorption coefficient of H 2 0 signal at 5235 cm 1 was determined. Fundamental bands of H 2 0 and OH groups in the 3100-3800 c m - 1 range cannot be separated and interpreted quantitatively.
1. Introduction Fine-grained porous silica gel is widely used as a catalyst carrier for the gas phase olefin polymerization. Depending on the activating procedure and history of the silica gel sample, it contains variable amounts of OH groups and frequently considerable quantities of physically adsorbed H20. Hydroxyl groups and water molecules exhibit fundamental, first overtone, and some combination vibrations in the near infrared region (3000-10000 cm -1) [1-9]. The aim of this paper is a study of relations between vibration signals in the near infrared region and a surface structure of silica gel, considering the amounts of O H groups and physisorbed H20.
2. Experimental Davison silica gel, grade 952 (USA), was dehydrated in a quartz glass vessel in a nitrogen stream at 200-900 ° C for 6 h, then pumped for 30 min and cooled under nitrogen. Unheated samples of silica gel were prepared through evacuation in a quartz vessel at 2 0 ° C and 1 0 - 2 Pa for several 0022-3093/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
hours. In both cases a layer of silica gel was stirred mechanically or pneumatically so that the evolved water was efficiently removed. The specific surface area of the silica gel was measured using the BET method and the following values were obtained at various temperatures of heat treatment: 2 8 5 _ 2 m2/g (20-600°C), 278 + 2 m2/g (800°C), and 218 ___4 m2/g (900 o C). Pore volume values calculated using a nitrogen desorption isotherm were 1.8 + 0.1, 1.7 + 0.1, and 0.85 _+ 0.05 cm3/g, respectively; the range of pore diameter distribution was 6-30 nm with a mean of 18 nm. n-Heptane, analytical grade (Loba Feinchemie, Austria) was checked for the presence of aromatics by long-term shaking with concentrated sulphuric acid. When no discoloration appeared in the test, the n-heptane was distilled and stored. Immediately before use, n-heptane was freed from water and oxygen by distilling at least 15% of the initial volume by highly pure nitrogen at elevated temperature. Tetrahydrofurane, (THF) analytical grade (Loba Feinchemie, Austria) was predried by prolonged stirring with metallic sodium and then vacuum-distilled into a storage ampoule containing fresh sodium and benzophenone. Before each procedure, fresh T H F was distilled from the storage solution of radical anions into an evacuated
94
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
vessel. Preparation of radical-anion solutions was described earlier [10]. Phenanthrene and benzophenone (both Reachim, USSR) were recrystallized and volatiles were removed before use by evacuation (10 -2 Pa, 2 h). Pure sodium (Lachema, Czechoslovakia) was freed from impurities by a mechanical removal of the surface layer in an inert atmosphere and out to small pieces ( - 10 mg). Triethylaluminum and trimethylaluminum (both Texas Alkyls, USA), were distilled, diluted by heptane to - 1 mol 1-~ and stored in sealed ampoules under nitrogen. Carbon tetrachloride, analytical grade (Lachema, Czechoslovakia) was distilled and then freed from water and oxygen by stripping off 15% of its volume by pure nitrogen, Nujol, analytical grade (Slovakopharma, Czechoslovakia), was stripped off before use by highly pure nitrogen at 100 o C. Infrared spectra of silica gel slurries in nujol (thickness 0.2 mm) in the 3000-4000 cm-1 region were measured using a Perkin-Elmer 377 instrument. Spectra in the 4000-10000 cm 1 region were measured on 16 wt.% slurries in CC14 in a 1 cm quartz cell using a Perkin-Elmer 330 instrument. Peak intensities were evaluated planimetrically or from the second derivative value of the absorption curve, Methane or ethane evolution in the reaction of silica gel with trialkylaluminum was measured in the following way. The weighed glass ampoules with 0.3-0.5 g of silica gel were consecutively broken in a magnetically stirred heptane solution (15 cm 3) of trimethylaluminum or triethylaluminum (concentration about 0.3 mol 1-1) in a glass reaction vessel of predetermined volume. The temperature of the apparatus was set at 25 + 0.1 ° C. The amount of methane or ethane liberated was measured manometrically. The dependence of the increase of pressure on time was recorded and the amount of alkane was calculated after reaching equilibrium (1-3 h). The solubility of methane and ethane in heptane at 2 5 ° C and partial pressure of 101.3 kPa was found to be 0.029 + 0.001 and 0.221 ___0.004 mol 1-1, respectively. Corrections on volume changes and on nitrogen content in ampoules were made. The measurements of alkane amount evolved w e r e r e producible within 2-3%.
Distillations of H 2 0 with T H F were carried out in an all-glass Simax apparatus depicted in fig. 1. Glass ampoules with samples were located in the upper connecting tube T. About 100 mg of phenanthrene and 200 mg of sodium were placed into ampoule C. After evacuating the whole apparatus, pure T H F was distilled into ampoule C and a solution of radical anions was prepared in an excess of sodium so that all phenanthrene reacted. Concentration of radical anions was determined via an acidimetric titration of sample by 0.1 mol 1-1 HC1, using bromocresol green as an indicator. A part of the radical-anion solution ( - 15 cm 3) was poured from ampoule C to ampoule B by tilting the apparatus and electric conductivity was measured corresponding to the predetermined concentration. The connection between ampoules B and C was sealed off and thus the part of the apparatus with ampoules was separated from the vacuum line. The actual measurement consisted of multiple distillation of T H F from the radical anions contained in ampoule B into ampoule A and back, measuring simultaneously the conductivity and volume of the radical anions in ampoule B at 20 ° C after each distillation step. Thus, a diffusion rate of impurities through the greased ampoule breaker D could be determined. Then a vial with a silica gel sample was broken and the distillation of T H F between ampoules A and B was repeated several times, cooling down and warming the ampoules alternatively. As the T H F distillation caused a certain dusting of fine silica gel particles into ampoule B, a sintered glass S was placed into connecting tube T. The radical-anion concentration was - 0.05 mol 1-1. The amounts of decomposed radical anT===~D
T S
vocuum Nz,THF
===~====~ IU
=
Pt A
B
C
Fig. 1. Apparatus for distillation of H 2 0 with T H F into radical-anion solution. See text for definition of A - D , S, and
T.
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
ions were corrected for diffusion rate of impurities in the blank experiment,
3. Results and discussion 3.1. Fundamental vibrations of H 2 0 and O H groups in silica gel
Figure 2 reveals fundamental vibrations of H 2 0 and O H groups in the 3000-4000 c m - 1 region measured in the nujol-suspended silica gel samples. The high-temperature activated silica gel samples exhibited a narrow absorption band 3660 cm -1 assigned to free O H groups [2-5,9]. The signal at wave number 3560 c m - 1 is probably due to the overtone vibration of nujol and contributes to the spectra of all samples. Absorption bands of O H groups (3400-3700 cm -1) are wide and cannot be resolved: they merge with the 3660 cm -1 band in the silica gel samples dehydrated at lower temperatures of dehydration. Free H 2 0 should
Wave Number [cm-I] 3600 4000
3200 I
I
I
g o <
kJ//
~
20"c
200"C 400"c ~ / 8 0 0 "600°C C 900°C ~ L.-_..-J
t t 2.8 2.6 2.4 WQvetength [# m] Fig. 2. Fundamental vibration spectra of H20 and OH groups measured for silica gel activated in the 20-900°C range; 15 wt.% slurry in nujol, 0.2 mm NaCI cell, Perkin-Elmer 377 instrument. The 20 °C silica gel sample was pumped for 18 h at 20°C and contained 0.06 mmol H20/g. Other samples were dehydrated for 6 h in a nitrogen s t r e a m , 32
30
95
exhibit a signal at 3300 c m - 1 , but the band is also wide. Infrared spectra of fundamental vibrations of the silica gel O H groups exhibit - apart from a large band width - additional disadvantages, such as unequal extinction coefficients of different vibrations and fluctuating background. The slope of the straight line connecting minima on both sides of the absorption bands decreases progressively from positive to negative values with decreasing dehydration temperature. The phenomenon is likely to be caused by the increasing content of free H 2 0 (a band with wave number around 3300 cm-1). These observations make dubious a comparison of spectra of various silica gel samples and a direct interpretation of a complicated set of the fundamental vibration absorption bands. The area of the fundamental vibration absorption bands (3000-4000 cm -1) decreases approximately linearly with increasing dehydration temperature of silica gel in the 200-900 ° C range. The pattern of this dependence corresponds to the temperature dependence of the gaseous decomposition products (methane, ethane) formed during the reaction of silica gel with trialkylaluminum. However, it should be noted that the area under the enveloping curve is a sum of incongruous terms, i.e. areas of overlapping bands with unequal extinction coefficients. Thus, they cannot be studied separately. 3.2. First overtone and combination vibrations of 1120 and O H groups
In order to identify the sources of signals in the near infrared region, samples of silica gel with various amounts of O H groups were prepared. Davison silica gel, grade 952, dehydrated at 20-900 ° C, or the same silica gel, where the O H group content was decreased through the reaction with an trialkylaluminum, were used for this purpose. The amounts of O H groups were determined using a manometric measurement of evolved methane under an excess of trimethylaluminum in heptane. It was found that part of the O H groups can be removed either by a chemical reaction with trialkylaluminum or by the temperature induced
96
J. Kratochvila et al. / Hydroxyl groups and free water on stltca gel
Wave Number [crn-~] 10000
6000
150
4000
o.8
~6
..(3
o~
~, ..Q
~
<
0.6
~
•
>.I00
L ~-
~
%"
20"
~
I
-
~
y~
/
/
o
o
_ I
Lf// 04
O2 . . . . k . ~ 600" 900" I ~ ~ 1A
) ~ ~///
_
0 1.8 2.2 2.6 WaveLength [ ~ m ]
Fig. 3. Combination and first overtone vibration spectra of silica gel; 16 wt.% slurry in CC14, 1 cm quartz cell, PerkinElmer 330 instrument. The 20 o C silica gel sample was pumped for 2 h at 2 0 ° C and contained 0.16 mmol H 2 0 / g . Other samples were dehydrated for 6 h in a nitrogen stream,
dehydration. The same O H signals in infrared spectra are observable after either of the above treatments, suggesting that the same level of O H groups is obtainable using the above procedures, Figure 3 shows spectra of four samples of silica gel in carbon tetrachloride in a 1 cm cell in the 4000-10 000 c m - t region. Signals of 4425/4505, 5235 and 7140 cm -1 are easily distinguishable in the figure. The abscissa of fig. 2 points in the opposite direction to that in Fig. 3; it is caused by different thicknesses of cells (0.2 vs. 10 mm) and by different optical media used (nujol vs. carbon tetrachloride). Signals in the region of fundamental vibrations have many times higher extinction coefficients than overtone signals; therefore it is not possible to measure both spectra in the same conditions. Figure 4 presents the dependence of the signal intensities at 4425, 4505, and 7140 cm -a on the amount of O H groups. Twenty silica gel samples were dehydrated at temperatures 20-900 o C (see also fig. 5) and nine silica gel samples were dehydrated at 600 ° C; five of them reacted with 0.4-2.0
I
0.5
t
1.0
o/o
//i/.s
~ 5c
04
0
• •
I
1.0
1.5
2.0
OHgroupsamount[rnrnot.g -1]
Fig. 4. Dependences of 4425 (,a), 4505 (e), and 7140 ( o ) cm -1 signal intensities on the amount of silica gel O H groups. Conditions as in fig. 3.
mmol triethylaluminum per gram, four others r e acted with 0.4-2.5 m m o l trimethylaluminum p e r gram. The intensity of the strongest signal at 4505 c m - 1 is a linear function of the amount of O H group, independent of the silica gel dehydration temperature (i.e. 2 0 - 9 0 0 ° C ) or of the trialkylaluminum content. Based on the validity of the linear relation between the 4505 cm -1 signal intensity and the amount of O H groups, this signal can be assigned to vibrations of all O H groups, independently of their type (free O H and hydrogen bonded O H groups). Figure 5 demonstrates the dependence of the 4425, 4505, and 7140 c m - I
150 _~ ~
-.,.
oc
"
~'1 ~
~ 5c ~ ~ _~ ~
"
"~o..~i [
-. " " - . 8
~
~ 200
o"-.
i
i ~oo
x------...~...~ J 6oo 800
Dehyde ratoim tnperautre f
['C]
Fig. 5. Dependences of 4425 (zx), 4505 (O), and 7140 ( o ) cm-1 signal intensities on dehydration temperature of silica gel. Conditions as in fig. 3.
J. Kratochvila et el. / Hydroxyl groups and free water on silica gel
97
signal intensities on the dehydration temperature for 20 silica gel samples. All three dependences exhibit linear sections in the range above 500600°C. The pattern of the dependences is similar to that observed for the dependence of the OH group content on the silica gel dehydration ternperature. The intensity of the 4425 and 7140 c m - I signals is a linear function of the amount of the OH group, up to approximately 1.0 mmol g - t (dehydration temperature of 500-600 ° C). At dehydration temperatures above 600 ° C, free OH groups prevail [2,9,11]. Thus, all three overtone and cambination signals in the spectra of samples treated in this temperature range are due to the free OH group vibrations. A decrease of the rate of the intensity increase of the 4425 and 7140 cm-1 signals observed when the OH group amount exceeds 1.0 mmol g-1 may be connected with the formation of hydrogen bonds with different vibration frequencies. The 4425 c m - ] signal intensity is virtually constant in the spectra of samples dehydrated in the 20-400 ° C range, while that of the samples dehydrated in the 6 0 0 - 9 0 0 ° C range is linearly proportional to the OH group content, Thus, the 4425 cm -] signal can be assigned to the free OH group vibrations, The intensity of the 7140 cm-~ signal increases linearly with the amount of O H groups up to about 1.0 mmol g-~; the rate of increase decreases at higher amounts. It can be assumed that the 7140 cm-~ signal corresponds to a certain combination of the vibrations of the free and hydrogen
3. 3. Calibration curve of 1-120 on silica gel
bonded OH group, e.g. it could be assigned to vibrations of all free OH groups and part of the hydrogen bonded O H groups. The dependence of the 7140 cm -] signal intensity on the amount of O H groups is intermediate between those of the 4425 and 4505 cm -] signals. It is likely that the 7140 c m - ' signal is a sum of the overtone vibrations of various O H groups; the fundamental vibrations of these groups are in the 3000-4000 cm-1 range, as suggested by Anderson and Wickersheim [6]. Different vibrations exhibit apparently unequal extinction coefficients thus leading to the observed non-linear dependence of the
~
signal intensity on the amount of OH groups,
In order to determine the extinction coefficient of H 2 0 on silica gel, infrared spectra of Davison silica gel, grade 952, samples dehydrated at 200, 600, and 800 ° C with added H 2 0 were measured. H 2 0 was added by means of a syringe to a weighed sample of silica gel, then the wetted sampie was stirred thoroughly for 2 h in a closed vessel. The samples were poured into quartz cells and carbon tetrachloride was added. Absorption spectra measured in different thicknesses of the silica gel layer were the same within the experimental error, thus demonstrating a homogeneous distribution of H 2 0 within the silica gel sample. Figure 6 presents the 4425, 4505, 5235, and 7140 c m - ] signal intensities of the absorption curve for silica gel dehydrated for 6 h at 600 ° C. Vibration modes appearing after H 2 0 addition (wave numbers 5100, 5235, 6800, 7090, 7190, and 8700 cm - ] ) show that a wide range of newly formed structures is present, the main component being hydrogen bonds. The 5235 c m - 1 signal exhibits the highest intensity and is measurable even at low contents of water. That is why it was used to measure the H 2 0 concentration on silica gel. The calibration curve of the 5235 cm -] signal is linear in the whole range of H 2 0 concentrations (0-1.1 mmol g - l ) , thus obeying the Lambert-Beer law. This signal occurs also in H 2 0 saturated C C I 4 , being
~40 1
2
0
_
~
~
.
~
o
~
~
.
~
~
~,I00 ~ 80 ~ 60 ~ 40! ~ 20 ~, 0
, 02
.
_
i 0.4
_
_
i 0.6
_
.
.
t 0.s
~
i to
Water content [rnmot.g -t1
Fig. 6. Calibration curves of H 2 0 on silica gel dehydrated at 6 0 0 ° C based on 4425 (r,), 4505 (O), 5235 ([]), and 7140 (©) cm -1 signal intensities; ( I ) H 2 0 saturated CCI 4. Conditions as in fig. 3.
J. Kratochoila et al. / Hydroxyl groups and free water on silica gel
98
an inactive solvent. The similarity of wetted CC14 and silica gel spectra, the sharpness of the spectra, and a linear dependence on H 2 0 concentration indicate that the signal can be assigned to a monomeric form of H20, i.e. to individually adsorbed water molecules, The increase of free H 2 0 content brings about a decrease of the 4425, 4505, and 7140 cm-1 O H group signal heights. The decrease is reversible as documented by removing H 2 0 at 20 ° C in vacuo, Absorption of H 2 0 in the vicinity of O H groups brings about a Van der Waals interaction of H 2 0 molecules with O H groups, thus changing the frequencies of the O H group vibrations. Newly appearing signals may be caused - apart from vibrations of H 2 0 and hydrogen bonds between H 2 0 and O H groups - by changed vibrations of O H groups,
3. 4. Dehydration of silica gel by pumping at 20 °C Figure 7 shows results of spectral measurement following the dehydration by pumping at laboratory temperature (20 o C). The abscissa features the time of evacuation by an oil diffusion p u m p after a preceding removal, taking - 1 h, of most of the H 2 0 (a turning-point is observed at 10 Pa pressure under conditions of intensively stirred 20
'~ ~2o "~
~
80 ~ 60
~0
E~° ~To_..r..__0~ 50
r~ a00
t ~s0
~ 20c
40 ~ "-= 0 ~
Oehydro~ion time [hn/ Fig. 7. Dependence of the amounts of gases (evolved during the silica gel-induced organometal decomposition), the amount of physically bonded H20 and signal intensities at (A) 4425 and (o) 4505 cm-1 (measured in the near-infrared region) on time of silica gel evacuation at 20 o C. (v) CH 4 from Me3AI; (V) C2H 6 from Et3AI; (D) H20 amount. Conditions as in fig. 3, details see text.
g silica gel sample in a 500 ml spherical glass vessel). A residual amount of H 2 0 (5235 cm -1 signal) decreases and free and hydrogen bonded O H groups (4425 and 4505 cm -1) increase with the increasing time of evacuation. After 100 h evacuation (pressure < 1 × 10 -2 Pa) at 2 0 ° C , silica gel contains only 0.01-0.02 mmol H 2 0 / g . On the other hand, the amount of O H groups does not change - after reaching a certain level - upon continuing evacuation. This observation proves (contrary to published considerations [12]) that virtually all physically bonded H 2 0 can be removed by pumping silica gel at room temperature within a reasonable time (100-200 h). In the evacuation experiments pore size distribution can play an important role. It can be expected that smaller pores bind water more firmly than larger ones and the times needed for water removal may be longer in the former case. In another experiment 10 g of heptane was added to 1 g of Davison silica gel, grade 952, and heptane was distilled to dryness. This procedure, conducted at 98.4 ° C followed by a short evacuation, allowed preparation within 1 h of a material containing less than 0.01 mmol H 2 0 / g and exhibiting the same infrared spectrum as that p u m p e d for 200 h at 20 ° C to 1 × 10 2 Pa. Both samples of silica gel also exhibit equal chemical properties (i.e. the reactivity with trialkylaluminums yielding alkanes) and have the same infrared spectra. Thus, both procedures can be considered as equivalent to obtain anhydrous silica gel with a maximum amount of
3.5. Distillation of anion solution
H20 with T H F into radical-
To check the reliability of the determination of free H 2 0 on the silica gel surface by the spectral method, the H 2 0 concentration was measured by distilling it from silica gel with T H F into a radical-anion solution. Radical anions react very fast and virtually quantitatively with compounds containing acidic hydrogen. In the case of H20, one proton neutralizes one anion and the remaining O H - group forms with a cation an ether-insoluble hydroxide [13]. This reaction can be used for the determination of trace amounts of H20.
J. Kratochvila et al. / Hydroxyl groups and free water on silica gel
If an inner surface of silica gel pores contains physically adsorbed H 2 0 , it can be transferred into the liquid phase by using a strong solvating agent and distilled together. Suitable solvating agents are ethers, such as T H F and diglymes, also used as solvating agents for radical anions. Considering a very fast release of H 2 0 from the silica gel surface during evacuation even at low temperature (20°C), an efficient removal of H 2 0 by a solvating ether (which in turn adsorbs on the surface) can be expected, Titration of H 2 0 distilled with T H F into a radical-anion solution led to a good agreement between the added amount of H 2 0 and the decomposed quantity of radical anions. It demonstrates the presence of free (adsorbed or cond e n s e d ) H 2 0 in the silica gel pores without reactions with siloxane bonds. Thus, H 2 0 adsorption on the silica gel surface is a reversible, physical process, By using the above method, 0.019 + 0.005 mmol g-~ of H 2 0 was found in silica gel dehydrated for 6 h in a nitrogen stream at 200 o C. The spectral method (5235 cm-~ signal) revealed 0.013 _+ 0.005 mmol g-~ in the same sample. No H 2 0 was found, using the above radical-anion method, in samples dehydrated for 6 h in a nitrogen stream at temperatures exceeding 400 o C. At the same time, the 5235 c m - l signal was not observed. The above findings indicate that both methods can be used for free H 2 0 determination.
4. Conclusions The results obtained demonstrate the suitability of the spectral determination of H 2 0 and O H groups on silica gel in a near infrared region [1,6,7]. Using a calibration curve, physically adsorbed H 2 0 (which may be present in limited amounts even after a long-lasting dehydration of silica gel, particulary at lower temperatures) can be determined quantitatively. A linear dependence of the 5235 cm -1 H 2 0 signal on the amount of H 2 0 (seen fig. 6) reveals the molar absorption coefficient value of 1.33 + 0.04 2 mol ~ cm 1. An accuracy of 0.01 mmol H 2 0 per gram of silica gel
99
was obtained, being sufficient for studies of the silica-based supported catalysts. The kinetics of silica gel dehydration during its evacuation at 20 ° C were followed by measuring the amount of free H20. Based on evaluation of the dependence of the 4425 and 4505 cm -1 combination signals of O H groups on the amount of O H groups (fig. 4) and on the silica gel dehydration temperature (fig. 5), it can be assumed that the 4505 cm -1 signal corresponds to the sum of O H groups (i.e. free and hydrogen bonded O H groups) while the 4425 cm-~ signal is related to the free O H groups only. The molar absorption coefficients are 0.89 + 0.03 and 0.35 +_ 0.02 mol-~ c m - ~ for signals 4505 and 4425 cm -~, respectively. The results obtained and the comparison with published data [1,6] reveal a necessity to verify the hypotheses using silica gel samples containing known amounts of different types of O H groups. The authors are indebted to Prof. M. KuEera for numerous helpful discussions and suggestions.
References [1] G. Wirzing, Naturwissenschaften 51 (1964) 211. [2] R.J. Peglar, F.H. Hambleton and J.A. Hockey, J. Catal. 20 (1971) 309.
[3] J. Murray, M.J. Sharp and J.A. Hockey, J. Catal. 18 (1970) 52. [4] A.V. Kiselev, V.A. Lokutsievskii and V.I. Lygin, Zh. Fiz. Khim. 49 (1975) 1796. [5] O.H. Ellestad and V. Blindheim, J. Molec. Catal. 33 (1985) 275. [6] J.H. Anderson and K.A. Wickersheim, in: Surface Science, ed. H.C. Gatos, Vol. 2 (North Holland, Amsterdam, 1964) p. 252. [7] D.L. Wood and E.M. Rabinovich, J. Non-Cryst. Solids 82
(1986) 171.
[8] Y. Yokomachi, R. Tohmon, K. Nagasawa and Y. Ohki, J. Non-Cryst. Solids 95&96 (1987) 663. [9] V.I. Lygin, Zh. Fiz. Khim. 58 (1989) 289. [10] Z. Salajka and M. KuEera, Collect. Czech, Chem. Corn-
mun. 48 (1983)3041.
[11] M.P. McDaniel, J. Phys. Chem. 85 (1981) 532. [12] H.P. Boehm, Angew. Chem. 78 (1966)617. [13] M. Szwarc, in: Carbanions, Living Polymers and Electron Transfer Processes (Interscience, New York, 1968).