REACTIVE SILICA
321
Table 111: Comparison of Reaction Rates of Nucleophiles toward Methyl Iodide
Nucleophilic constant
Nucleophile
F-
-0.27 0 1.24 1.51 1.65 1.83 2.06 2.04 2.52
Ha0
c1-
BrOH SCN ICN SaOse-
a
NZH4
Bimolecular reaotion rate constant, 1. mol-' seo-1
7.08 X 1.4 x 3.30 x 4.16 X 6.36 X 3.58 x 4.71 x 5.76 x 2.84 X 1.1 x
lo-* 10-ob
10-4 10-4 10-4 loba 10-8
Activation energy, kcal mol-1
25.2 f 0 . 5 28.4' 21.97 f 0 . 3 4 19.31 f 0.50 22.22 Z!Z 0.23 19.95 Z!Z 0.40 17.58 f 0.20 20.47 f 0.12 18.88 20.2 * 0 . 4
Preexponential term, 1. mol-1 mc-1
2.08 X 1 x 4.16 X 6.00 X 1.24 X 1.49 X 0.93 X 6.01 X 2.01 x 1.7 X
10" 10'2 10'0 109 10l2 10" 1Olo 10" 10'2 10la
Ref
10 17 11 11 7 20 12 9 7 This work
'
Calculated froin unimolecular hydrolysis rate constant. ' Calculated between 30.03 and 38.10". The activaa Not available. tion energy goes through a minimum at approximately 60'. The term in column 5 was calculated at 25" assuming E ~ ( 2 5 ' )= 28.4 kcal/mol.
In summary, our understanding of the factors which influence the rate of reaction of this simple displacement reaction is unsatisfactory. Changes in the hydration of the r e a ~ t a n t s , formation ~~'~ of bonds in the reaction intermediate,'* nucleophilicity of the reactants,18 and influence of the solvent viscosity on the preexponential term20 have been considered in models which attempt to predict reaction rates and/or energies' None of these factors are by themselves sufficient to produce
Reactive Silica. I.
an adequate model. The relative contribution of each factor in a single model has not yet been accomplished.
Acknowledgments. The author wishes to thank l\frs, for her assistance in performing the experimen ts.
s. Sutter
(19) C. G. Swain and E. R. Thornton, J. Amer. Chem. Soc., 84, 817 (1962). (20) G, C. Lalor and E. A, Moelwn-Hughes, J. Chem, Sot., 2201
(1965).
The Formation of a Reactive Silica by the Thermal
Collapse of the Methoxy Groups of Methylated Aerosil by Claudio Morterra and M. J. D. Low1 Department of Chemistry, New Yorlc UniversBy, New York, New Y O T ~10463
(Received June 18, 1.968)
The degassing of methylated Aerosil silica was studied by infrared spectroscopic techniques. The surface methoxy groups were quite stable and were removed only very slowly at temperatures up t o 600'. At higher temperatures, however, the removal of methoxy groups became very rapid. The methoxy groups cracked, and the silica surface was drastically modified. Surface silanol and silane groups were formed. Also, the spectra indicate that hydrocarbonic surface species were formed which were acetylenic in nature and which showed extreme stability to degassing. Silane and other groups were only slowly removed a t 830'; their removal resulted in the formation of a silica surface exhibiting unusually high reactivity. A possible mechanism is discussed.
It is well known that silicas can react with methanol, and that the resulting methylated surfaces are unusually stable.2 The surface Si-O-CH3 groups are removed Only very s~owlyby pumping at temperatures to approximately 6000, Degassing at higher ternperatures, however, leads to rapid changes.a Essen-
tially, the thermally induced collapse of the methoxy groups results in a drastically altered surface. The (1) To whom inquiries should be directed. (2) (a) E. Borello, A. Zeochina, and C. Morterra, J. PhUS. Chem., 71, 2938 (1967),and references therein; (b) M. J. D. Low and Y. Harano, J . Res. Inst. Hokkaido Univ. (Anniversary Vol.), in press. (3) c. Morterra and M. J. D. LOW, Chem. Commun., 203 (1968). Volume 78,Number 9 Februaru 1969
322
CLAUDIOMORTERRA AND M. J. D. Low
new adsorbent produced in this manner, termed "reactive silica" for brevity, exhibits interesting properties which differentiate it from "normal" degassed silica, including an exceptionally high activity for chemisorption. The present paper deals with the surface species formed in the preparation of reactive silica, as studied by infrared spectroscopic techniques. The reactivity of the new surface will be described elsewhere.
I-
Experimental Section
o\"
Samples for spectroscopic study were prepared by compressing approximately 85 mg of Aerosil silica4 in a steel die at 35-40 kg/cm2. The resulting pellet (approximately 40 mg/cm2) was mounted in a cell similar t o that described by Peri and Hannan6 connected to a conventional vacuum system capable of 10-6 to 10-6 Torr. The sample was exposed t o 70 Torr of methanol a t 350". Nearly all of the surface hydroxyls of the silica were replaced by methoxy groups by this treatment after about 20 hr.2b The methylated sainple was the degassed as required. A Perlrin-Elmer Rlodel 621 spectrophotometer fitted with a Reedera thermocouple was used to record spectra. Scale expansions were frequently used t o observe small spectral changes and to study weak or sharp bands. In the various figures, OjoT is per cent transmittance and D is optical density.
I I
1-A
, 29bo
3100
Figure 1. Effect of outgassing at 750" on methylated aerosil: broken line, background. The sample was degassed at the following times in minutes: A, 0; B, 1; C, 2; D, 3; E, 5 ; F, 9; G, 16; H, 40.
600' -
Results and Discussion Demethoxylation. nlethylated samples were subjected to degassing under various conditions. As noted earlier,2 the spectra showed that the surface Si-O-CH3 groups were very stable to outgassing until quite high temperatures were reached. However, if the outgassing was carried out much above BOO", even relatively small increases in temperature markedly changed the decomposition rate of the methoxy groups. Such heating of methylated silica caused the desorption of water and methanol in the early stages of decomposition and the evolution of several hydrocarbons (mainly methane, ethylene, and acetylene), water, formaldehyde, CO, and H2,in the later stages.' The initial stages thus involved the elimination of water and methanol from surface structures2& such as I and I1
OH. *
I
* *
.OH
I
A\ 2; I
0-H
* * * *
I
A\
O-CI13
I
I1
A
followed by a cracking of the Si-O-CH3 groups. The changes in the infrared spectra occurring during the pyrolysis were complex, and are summarized as follows. When a methylated sample was degassed, the bands2 due to the C-H stretching vibrations of the methoxy groups declined in intensity. An example of the The Journal of Phywical Chemislvu
06''
3747 SI-OH A
Q Q
2300 Si-H A
50
100 I50 DEGASSING, hour8
200
Figure 2. Degassing at 600"
changes observed at 750" is given in Figure 1. The changes were similar a t other degassing temperatures and differed only in rate. The decline of the prominent 2859-cm-1 C-H band with degassing time for three temperatures is shown in Figures 2, 3A, and 4A. The time required to remove the methoxy groups completely at 750" varied from 30 min to about 2 hr depending on the degree of methylation. Demethylation was com(4) (6) (6) (7)
Aerosil2491/380, Degussa Inc,, Kearny, N. J. J. B. Peri"and R. B. Hannan, J. Phys. Chem., 64, 1626 (1960). C. M. Reeder Co., Detroit, Mich. Experiments of E. Borello, et al., at the University of Turin.
REACTIVE SILICA
323
.6
-
A
0
DEGASSING, mlnutrr
.4
n .2
DEGASSING, hbV8
Figure 3.
near 3747 cm-l, hereafter termed the Si-OH band, is generally attributed to "free" or isolated surface silanol groups. I n the present study, the intensity of the Si-OH band increased upon degassing, the increases closely following the decreases of the C-H bands. The changes occurring at three temperatures are shown in Figures 2, 3A, and 4A. The Si-OH band which was 10 em-l) and performed was quite sharp (Av1l2 fectly symmetrical, indicating that the newly formed silanols were completely free of both inter- and intramolecular interactions. Subsequent to the complete removal of the methoxy groups, the Si-OH band decreased slowly upon further degassing a t 750" or higher. It was not feasible to observe the decrease in the Si-OH band a t 600" because the removal of methoxy groups was incomplete even after 10 days. The formation of the surface silanols could arise through the rupture of the C-0 bonds of the Si-0-CHs groups, with subsequent abstraction of hydrogen from the gaseous pyrolysis products. The formation of large numbers of hydroxyls would recreate some structures of type I, which would then be immediately destroyed at high temperatures t80yield water molecules. The occurrence of such processes would agree with the observation of water in the gas phase during both early and late stages of pyrolysis. Surface Silane. The pyrolysis of the methoxy groups led to the formation of a strong and broad band near 2280 cm-l. The intensity of the latter increased rapidly until the number of methoxy groups was quite small (D 6 O.l), and declined very slowly thereafter. The band, hereafter termed the 2300-cm-1 band, shifted to 2300 cm-l after several hours of degassing. Changes of that band with continued degassing are shown in Figures 2-5. At temperatures around 600" the growth of the 2300-cm-' band was not complete even after 200 hr. At 830", however, the growth was very rapid and the decline of the band occurred even in the first few minutes of degassing. The 2 3 0 0 - ~ m -band ~ was clearly brought about by an Si-H stretching vibration, as suggested by the spectral position of the bandapeand demonstrated by the formation of an analogous band near 1650 cm-' when methanol-& was used in the surface rnethylati~n.~The simplest assignment of the 2300-cm-1 band would be to the Si-H stretching vibration of a grouping consisting of an hydrogen atom bonded to an Si atom of the SiOz surface or to an Si atom of a small aggregate or "island" of silicon. The latter hypothesis must be rejected because others have studied the adsorption of hydrogen on pure silicon and observed the Si-H band at quite different frequencies.'O Also, the position of the 2300-
Degassing a t 750".
n*p--&-= 2
2300 SI-H
i 830'
.4-'
n -2-
i .
B
,
SO 100 DEGASSING, houri
180
Figure 4. Degassing a t 830".
plete in a few minutes above 800" but required several days at 600". The decline of the C-H bands was accompanied by changes in other spectral regions. Hydyoxyl Formation. With spectra of silicas, a band
(8) E. A. V. Ebsworth, "Volatile Silicon Compounds," The Macmillan Co., New York, N. Y., 1963. (9) L, J. Bellamy, "The Infrared Spectra of Complex Molecules," John Wiley and Sons, 2nd ed, New York, N. Y., 1963. (10) G. E. Becker and G. W. Gobeli, J. Chem. Phus., 38, 2942 (1963). Volume 73, Number 8 Februaru 1989
CLAUDIO MORTERRA AND M. J. D. Low
324
t-
8
I-
8 F-
v B
2
2200
2400
2200 c"
Figure 5 . Changes of the silane band at 750": broken line, background. Part A, outgassing time in minutes: A, 1; B, 2 ; C, 3; D, 5 ; E, 16; part B, outgassing time in hours: F, 2 ; G, 18; H, 27; J, 42; K, 66; L, 130.
cm-l band is rather high in comparison to those of Si-H stretching bands of many silanes." Absorptions a t such high wave numbers have only been observed when several electronegative atoms were bonded to the Si-H structure, or when the Si atom was part of a polysiloxnnic structure.l' The present surface compound is analogous to the latter, if it is supposed that the hydrogenated Si atom is still a part of the silica surface. The assignment of the 2300-cm-' band to a surface silane, with the Si atom being a part of the silica surface, would therefore appear to be a reasonable one. However, this is an oversimplification, on the basis of the observed reactions of the reactive Aerosil. These, and the nature of the species responsible for the 2300cm-l absorption, will be discussed in the paper following. The shape and changes of the 2300-cm-l band a t 750" are shown in Figure 5. The continuous shift of the band position, both during the formation and the decline of the band, toward higher wave numbers as well as the narrowing of the band during the decline, suggest that the Si-H bond varied over a wide range of energies, possibly as a consequence of surface surroundings of quite different polarity. The grouping of lesser stability, namely those least hydridic in nature, would be formed first and cause absorption a t the lower frequencies. Such species would also be the first removed, and their removal would lead to a shift in the band position and a narrowing of the band. The existence of interactions between the groups which give rise to the low wave number absorption would seem to be excluded, because it seems quite unlikely that, interacting groups would be the first ones to form, weakly interacting or entirely free groups being formed later. The Journal of Physical Chemistry
Figure 6. Formation and decrease of the 2983-cm-' band at 750": broken line, methylated sample. The outgassing times were: A, 5 min; B, 15 min; C, 3 hr; D, 13 hr.
The formation of the surface silane structures during the high-temperature outgassing appears to be connected with the breaking of the Si-0 bond of the Si0-CHa groups, followed by reaction with the hydrogen of the products of the pyrolysis. The nature of the latter is uncertain. Also, the slow elimination of the silane grouping, as shown in Figures 3, 4, and 5B, does not lead to the formation of infrared-active species on the surface. However, the final degassing leads to a silica exhibiting unusually and unexpectedly high activity. Aerosil which has been subjected to a methylation-demethylation cycle as described is termed "reactive silica." Its reactive properties will be described in a subsequent paper. Surface Hydrocarbon. A small band was formed at 2983 cm-l during the last stages of the pyrolysis, e.g., Figure 6. This band was always very weak and was hardly observable in the 600" experiment. The 2983cm-1 band was not found when deuteriomethanol was used, but an extremely weak band was then observed at 2237 cm-l. This indicates that a hydrogen-containing species was involved and, in view of the spectral position of the band, the 2983-cm-I band is attributed to the C-H stretching of an otherwise unidentified surface hydrocarbon structure. The latter was quite stable to outgassing at 750" (Figure 6), but declined and completely disappeared at 800-850" in several hours, This unusually high stability and the time at which the band was formed suggest that the species causing it were formed at surface sites at which other highly stable species were formed which gave rise to absorptions at 3311 and 2070 crn-l. Xurface Acetylide. Two bands were formed which were quite sharp although weak, and were stabIe in position at 3311 and 2070 cm-1. The two bands appeared and disappeared together, and are consequently as(11) A. L. Smith and N. C. Angelotti, Spectrochim. Acta, 15, 412 (1959).
REACTIVE SILICA
325
cribed to the same species. Also, they seem to be connected with the elimination of silane groups rather than with the disappearance of methoxy groups. The bands, and their behavior on degassing, are shown in Figures 7 and 8. The absorptions are ascribed to the C-H stretching and the C=C stretching vibrations of a surface CzC-H structure, respectively. The observed frequencies of the bands are in good agreement with those expected for an asymmetric acetylene which has a mass much greater than a hydrogen a t one end of the molecule.12 Also, using deuteriomethanol, the band attributed to C r C was shifted to 1936 cm-I, as would be expected for a deuterioacetylene;12 using tenfold ordinate scale expansion, a weak band was observed a t 2588 cm-l which can be assigned to the C-D stretch of the deuterated acetylenic structure.12 It is difficult to explain the formation of such an unusual surface species and also its great stability to outgassing at high temperature. The acetylenic molecule could be bonded to the surface either through an oxygen atom (structure 111)or directly to a silicon atom (structure IV). However, the rapid decomposition of methoxy groups, involving the scission of Si-0 and 0-C bonds, suggests that a structure such as I11 would not 0-CEC-H
I
I
A
3350
3290
CEC-H
I
Si
Si
111
IV
exhibit the extraordinarily high stability which was observed, Rather, structure IV is indicated. The observed higher stability of the Si-C bonds in comparison to that of the 0-C bond13 would explain the stability of IV, and thus lends some support for the assignment. Comparison of Figures 7l3, 3A, and 4A shows that IV was mainly formed when methoxyl groups were no longer present on the surface and when the gaseous products of the pyrolysis had been pumped away. The initial, increasing parts of the curves of Figure 7B seem to be connected with the initial, steep decline of the Si-H curves of Figures 3B and 4B, thus pointing to a relation between the elimination of surface silane and the formation of surface acetylene. The following mechanism is suggested. It is postulated that reactivc sites of the type formed by the elimination of the silane are created in the initial stages of the decomposition of the methoxy groups. Some of these sites can then chemisorb hydrocarbon from the gas phase to produce a structure responsible for the weak but stable band observed a t 2983 em-l. Some of the sites could also be suitably spaced to hold an acetylene (or ethylene) molecule to yield, after a dehydrogenation step, a structure such as V.
30
60 SO MQASSNO. h w i
120
I
Figure 7. Effect of outgassing on surface acetylene: parts A, B: degassing at 830'; broken line, background. The degassing times in minutes were: A, 2; B, 5 ; C,20; D, 90; E, 240; part C: optical density of the 3311-cm-l band as a function of degassing time at 750 and 830".
The C z C stretching of V would not be observed hecause of the symmetry of the structure. During the slow decomposition of the 2300-~m-~ silane some of thc hydrogen atoms formed could react with the bridges of structure V, thus leading to the formation of structure IV and the 3311- and 2070-cm-' bands (see the increasing portions of the curves of Figure 7B). The acetylenic structure IV would then be slowly eliminntjed (decreasing parts of the curves of Figure 7B), probably through the reaction of IV with additional hydrogen with subsequent desorption of acetylene. The reactive (12) D. J. C. Yates and P. J. Lucchesi, J. Chem. P ~ M s35, . , 243 (1961). (13) Reference 8, p 82 ff.
Volunae 73,Number 8 Februnru
1889
CLAUDIOMORTERRA AND M. J. D. Low
326
Table I : Summary of Bands Cm-1
3747 3032 2996 2958 2929 2857
I-
o\"
D
28bo
Figure 8. Effect of degassing at 830" on deuterioacetylene. The outgassing times in hours were: A, 1; B, 16; C, 40.
sites, unsaturated silicon atoms discussed in a subsequent paper, would be regenerated by this desorption process. The above mechanism receives some support from the observation that the acetylenic structure IV can be eliminated by reaction with hydrogen. The process is quite slow at 450" but is faster a t higher temperatures. Hydroxyls were not formed during this reaction, thus again excluding structure 111,and surface hydrocarbons were not formed. At these temperatures (450-550') the sites could also react with hydrogen and contribute to the formation of a band at 2227 em-l. This will be discussed elsewhere. The surface silicon acetylide was also quite stable toward oxygen and water vapor. The reaction with oxygen started a t 350-400" and proceeded very slowly, more than 10hr being required for the complete oxidation a t such temperature. The oxidation rate increases with increasing temperatures. Water vapor had no effect on
The Jouvnal of Physical Chemislry
3311 2070 2588 1936
Isolated Si-OH Surface Si-O-CH8
H-CSCC--n
Surface acetylide Structure IV
2983 2237
C-H C-D
Unidentified surface species
2300 1650
Si--H Si-D
(Si&; see part 11)
the 3311- and 2070-cm-' bands at room temperature or a t temperatures up to 400". This is further evidence in favor of structure IV and against structure 111, because one would expect the latter to be more reactive to water in analogy to the reaction of methoxy groups with water to yield surface hydroxyl^.^^ Note also that the species responsible for the small 2983-em-' band showed an even greater stability to oxygen, being oxidized only at temperatures as high as 500-600". For a summary of the bands, see Table I.
Acknowledgment. Support from the National Center for Air Pollution Control is gratefully acknowledged. (14) L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, New York, N. Y.,1967, p 244 ff.
