Infrared studies of the reactions between silica and trimethylaluminum

REACTIONS BETWEEN SILICAAND TRIMETHYLALUMINUM

911

Infrared Studies of the Reactions between Silica and Trimethylaluminum by D. J. C. Yates, G. W. Dembinski, W. R. Kroll, and J. J. Elliott Corporate Research Laboratories, Esso Research and Engineering Company, Linden, New Jersey (Received September 9 , 1 9 6 8 )

We have studied spectroscopicallythe reaction between trimethylaluminumand silica. The reaction is shown to involve mostly the “free” hydroxyl groups of the silica rather than the “hydrogen bonded” hydroxyl groups. A rearrangement of the aluminum alkyl takes place, the final products being methyl groups bound to silicon atoms, and silicon-hydrogen groups. Confirmatory evidence for such a rearrangement was obtained by examining the spectra of the reaction products of silica with methanol and chlorinated methyl silanes. A possible mechanism for the reaction is suggested.

Introduction It is well known that aluminum alkyls are unstable in the presence of oxygen1or water12Jdecomposing very rapidly to hydrated alumina and hydrocarbons if excess water is used. Little is known of their reaction paths when limited amounts of water are p r e ~ e n t . ~Such circumstances can be found on the surface of finely divided stable oxides, which commonly contain OH groups even when all molecular water has been removed at high temperature. Surface hydroxyl groups can be reacted in various ways; they can be exchanged with deuterium,616 replaced with methoxy groups,’ or reacted with molecules such as silane^,^^^ boron trichloride,1° or diborane. l1 Silica hydroxyl groups have also been found to react with organometallic compounds such as alkyllithium and alkylmagnesium iodide,12 and also with aluminum alkyl~.’3-1~ I n many of these investigations, no spectroscopic experiments have been carried out, or when they have, only the OH region has been examined. I n the case of aluminum alkyls, no spectroscopic investigations have been reported; the only information known is the type and amount of hydrocarbon formed as a result of the reaction. We have determined, by infrared studies, that predominantly one type of hydroxyl group on silicas reacts with aluminum alkyls. I n addition, a very striking rearrangement of the alkyl molecule takes place, as chemisorbed silicon-methyl groups are formed on the surface, rather than aluminum-methyl groups which others have postulated,l* and might indeed have been expected to form. Experimental Section a. Materials. Three farms of silica were used: Cabosil, Davison silica gel, and porous glass. Most of the work was done with the Cabosil, Grade HS5, surface area about 300m2/g, obtained from the Cabot Go., Boston, Mass. The silica gel, Code No. 951, has a surface area of 600m2/g and was obtained from the W. R. Grace Go., Davison Chemical Division, Baltimore, Md. The porous glass used was in the form of a plate, 0.25 mm thick, similar to that used earlier.?

Trimethylaluminum was obtained from the Ethyl Corp., New York, N. Y., of purity 99.9%. Reagent grade methanol was used. Tetramethylsilane, of purity 99%, was obtained from N.M.R. Specialties, New Kensington, Pa. Hexamethyldisiloxane was obtained from K and K Laboratories, Plainview, N. Y. Trimethylchlorosilane and trichloromethylsilane were obtained from the Dow Chemical Corp., Midland, Mich. b. Apparatus. Because of the extreme chemical reactivity of trimethylaluminum, initial experiments were performed using a simple silica cell similar to that described elsewhere.? Later, it was found that the trimethylaluminum did not significantly attack either the vacuum grease, glyptal cement, or barium fluoride, and our usual rotatable dual cell@ were used. Details of the vacuum system have been given16 earlier. A Beckman IR7 grating spectrometer was used, with an external mirror system providing an accessible focus. Details of the resolution achieved are given in the legends of the figures. (1) K. Ziegler, F. Krupp, and K. Zosel, Lieb. A n n . Chem., 629, 241 (1960). (2) K. Ziegler in “Organoaluminum Compounds,” H. Zeiss, Ed., Reinhold Publishing Corp., New York, IS.Y.,1960,p 206. (3) G. E. Coates in “Organo-metallic Compounds,” Methuen and Co., Ltd., London, 1956, p 75. (4) CT. B. Sakharoskaya, N.N. Korneev, A. I?. Popov, E . I. Larikov. and A. F. Zhigach, Zh. Obshch. K h i m . , 34, 3435 (1964). (6) M. P.Boehm, Advan. Catal., 16, 179 (1966). (6) J. L. Carter, P. J. Lucchesi, P. Corneil, D. J. C. Yates, and J. H. Sinfelt, J. Phys. Chem., 69, 3070 (1965). (7) M. Folman and D. J. C. Yates, Proc. Roy. Sac., A246, 32 (1958). (8) T. E. White, Proceedings of the Society of Plastics Industry, 20th Annual Meeting, Chicago, Ill., 1966, Reinforced Plastics Division, Section 3B, p 1. (9) L. R. Snyder and J. W. Ward, J. Phys. Chem., 7 0 , 3941 (1966). (10) F. H. Hambleton and J. A. Hockey, Trans. Faraday Soc., 62, 1694 (1966). (11) H. G. Weiss. J. A. Knight, and I. Shapiro. J. Amer. Chem. SOC., 8 1 , 1823 (1959). (12) J. J. Fripiat and J. Uytterhoeven, J . Phys. Chem., 66, 800 (1962). (13) M. Lieflilnder and W. Stsber, Z . Naturforsch. 15B, 411 (1960). (14) M. Sato, T. Kanbayashi, N. Kobayashi, and Y . Shima, J . Calal., 7 , 362 (1967). (16) H. Jenkner, Z . Naturforsch., 14B, 133 (1959). (16) D. J. C. Yates and P. J. Lucchesi, J. Chem. Phys., 3J, 243 (1961). Volume 75,Number 4 April 1969

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YATES,DEMBINSKI, KROLL,4ND ELLIOTT

80

-

J

0

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3800

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3600

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I 1 I 3400 3200 Frequency in cm-'

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Figure 1. Interaction of trimethylaluminum with thin (about 15 mg/oma) silica (Cabosil): a, spectrum after evacuation at 500";b, after treatment at room temperature with Al(CH& vapor, and removal of excess vapor at room temperature. Resolution at 2800, 2900, and 3000 cm-I is 3.8, 3.0, and 2.3 ern-', respectively.

Frequency in an'' Figure 2. Interaction of trimethyl aluminum with thick (about 60 mg/cm2) Cabosil: a, after initial evacuation at 400"; b, after treatment with Al(CH& vapor at room temperature, and evacuation at room temperature for 10 min; c, after evacuation at 300" for 10 min. Resolution at 2200, 2800, 2900, and 3000 om-1 is 2.4, 4.5, 3.6, and 2.7 om-', respectively. The Journal of Physical Chern'istry

REACTIONS BETWEEN SILICAAND TRIMETHYLALUMINUM

Frequency in

913

cm"

Figure 3. Addition of water to methyl groups adsorbed on silica: a, sample as in Figure 20, but evacuated an additional 15 min a t 400"; b, spectrum run 40 min after adding 4 mm of water vapor; c, spectrum run after evacuating a t room temperature for 3 min, 100" for 15 min, and finally a t 200" for 20 min. Resolution identical with that used in Figure 2.

c. Procedure. After initial evacuation of the samples a t elevated temperature to remove adsorbed water, their spectra were recorded over the whole accessible region (about 1400 to 4000 cm-l). The trimethylaluminum was usually added as an excess of vapor at room temperature, but it was sometimes distilled over as a liquid by cooling the end of the cell containing the sample. After a few minutes, the unreacted compound, together with any methane formed, was removed by evacuation. If desired, a further dose was added using the same procedure. The trimethylaluminum was always purified immediately before use by freezing it at 77'K, evacuating until vacua in the region of Torr were obtained, and then warming to room temperature. A similar procedure was followed with the silanes and methanol, except that they were always added as vapor, in excess.

Results Data obtained with thin Cabosil samples are shown in Figure 1. It will be seen that a very extensive reaction has occurred, with a striking decrease in the intensity of the OH groups. Similar data were obtained with a thicker sample of silica (Figure 2), but the resulting CH stretching bands of the adsorbed methyl groups were much more intense. This sample was used to study the stability of these methyl groups

by evacuating at high temperatures (Figures 2 and 3 ) , by adding water (Figure 3) and by deuterium exchange (Figure 4). The experiments with Davison silica gel and porous glass were with relatively thick samples, and the spectra obtained were similar to those shown in Figure 2. However, no studies were made of the effects of water addition or deuterium exchange with these particular samples. To show that the adsorbed species formed by the reaction of trimethylaluminum were not methoxy groups, studies were made of the chemical reaction between methanol and the OH groups. Some of the data obtained are shown in Figure 5. Companion deuterium exchange studies were also made with the methoxylated silica (Figure 6) . To obtain data on known types of chemisorbed methyl groups, exchanges were performed between silica and tetramethylsilane (Figure 7) and trichloromethylsilane (TCMS) (Figure 8 ) . Although there are some data on adsorbed species obtained from TCMS,* no high-resolution spectra have been published and no high-resolution spectra have been given of adsorbed silicon methyl groups obtained from trimethylchlorosilane (TMCS) The frequencies of the various adsorbed species are given in Table I. There seem to be very few published data in the CH stretching region on the infrared spectra with high Volume 75, Number 6 April 1969

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YATES,DEMBINSKI, KROLL, AND ELLIOTT

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Position of -c 2985 cm-l Peoks on Spectrum b

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Figure 4. Deuterium exchange of silica with adsorbed methyl groups: a, sample as in Figure 30, but evacuated at 400" for 70 min; b, after heating in deuterium (12 om pressure) for 15 min a t 400'. Resolution identical with that used in Figure 2.

Figure 5. Reaction between methanol and Cabosil (thickness about 40 mg/cm2): a, spectrum after evacuation a t 500' for 2 hr; b, methanol (13 cm pressure) added a t room temperature, then heated to 350" in 10 min, and methanol changed. Kept a t 360" for 45 min, then evacuated for 15 min a t 350", and cooled to room temperature, then the spectrum recorded. Resolution a t 1475, 2800, 2900, and 3000 cm-' is 3.6, 4.2, 3.2, and 2.6 cm-l, respectively. The Journal of Physical Chemistry

REACTIONS BETWEEN SILICAAND TRIMETHYLALUMINUM

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I I I 3000 2800 Freqwtcy in cm-'

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Figure 6. Deuterium exchange on silica with methoxy groups: a, sample as in Figure 5b, but after treatment with 10.0 om of Dt at 350" for 10 min; b, after treatment with 27.6 om of Dz at 500" for 15 min; c, after treatment with 63.7 cm of Dz at 550" for 25 min. Resolution identical with Figure 5.

resolution of compounds containing aluminum-methyl groups and silicon-methyl groups, so we have presented some data on these compounds in Figure 9 and Table 11.

Discussion Reaction between Trimethylaluminum and Silica. When trimethylaluminum (TMA) reacts with silica, it is known13J4that methane is formed. This reaction is presumed to go by the abstraction of a hydrogen atom (from an OH group) from the solid which combines with the methyl groups to form methane. This idea is shown to be correct, in a general sense, by the spectra shown in Figures 1 and 2. I n Figure 1, it will be seen that the hydroxyl band remaining in the silica is asymmetric. This is because the band is a composite of a sharp, narrow band a t 3750cm-' caused by the ~ - a~ ~ broader, weaker band isolated OH g r o ~ p s , ~and extending from about 375Ocm-I to 35OOcm-l, The latter band shows the presence of hydroxyl groups hydrogen-bonded to each other (see Figure 16 of ref 20). If sufficiently high temperatures are used during the ~

Table I: Frequencies of Adsorbed Methyl Groups on Silica Starting compd

CH stretching frequencies, cm-1

Diff erence in cm-1

Al(CHa)a Si(CHa)4 SiCl(CH& SiClaCHs CHIOH

2955,2902 2970,2915 2968,2910 2992,2925 3000,2962,2856

53 55 58 67 38,106

evacuation of the sample, the broad band can be removed on most forms of silica, leaving only the sharp band of the isolated hydroxyl groups.20 Figure 1 shows that a very striking difference in reactivity exists between these two types of hydroxyl groups. The 3750-cm-l band had a peak optical density of 1.45 before adding the TMA, but only about 0.05 afterward, showing that essentially all of these OH groups had been removed from the surface. There is no well-defined maximum in the broad band of the hydrogen-bonded hydroxyl groups, but the band a t 3700 cm-l was reduced to 4101, of its original intensity. The hydrogen-bonded hydroxyl groups seem much less reactive than the isolated ones. This is most surprising, as TMA is so very reactive with water-so much so, that the amount of methane produced from the reaction of oxides with trimethylaluminum has been proposed as a method for determining the total hydroxyl content of such solids.14 Clearly this is not correct, and one should not assume that because a given organometallic compound is very reactive with water it will necessarily provide an excellent way of measuring the total surface hydroxyl content of solids. No definite cause can yet be suggested for the nonremoval of the hydrogenbonded OH groups. It may partly be due to molecular size effects, as shown by the surface reactions with (17) V. Ya. Davydov, A. V. Kiselev, and L. T. Zhuralev, Trans. Faraday SOC..60, 2254 (1964). (18) R. S. McDonald, J . Amer. Chem. SOC.,79, 850 (1957). (19) R. S. McDonald, J. Phys. Chem., 62, 1168 (1958). ( 2 0 ) D. J. 0.Yates, Advan. Catal., 12, 265 (1960). Volume 73, Number 4 April 1969

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YATES,DEMBINSRI, KROLL,AND ELLIOTT

Frequency in cm'' Figure 7. Reaction between silicon tetramethyl and Cabosil (thickness about 50 mg/cma) : a, background spectrum after evacuation at 500" for 1 hr; b, after adding 19.6 om pressure of Si(CH3)a at room temperature, heating rapidly to 500" for 30 min, then cooling to room temperature and evacuating for 2 min. Resolution at 2800, 2900, and 3000 om-1 is 3.2, 2.5, and 2.0 om-', respectively.

Table 11: Frequencies of Methyl Groups in Pure Compounds Containing Aluminum-Methyl or Silicon-Methyl Groups Compound

Al(CH& Si(CH3)4 Sic1(CH& SiClrCHs O[Si(CH&12 Silicone grease

State, thickness in cm Pure, 0.0024 Solution (0.1%) in cc14, 1.0 Pure, 0.0023 Solution (0.1%) in cc14, 1.0 Solution (0.01%) in CCL, 1.0 Smear on NaCl Plate

methanol. I n this contrasting reaction, the smaller methanol molecule mainly reacts with the hydrogenbonded OH groups, leaving the free hydroxyl groups essentially unaffected (Figure 5 ) . Even though the TMA is much larger than methanol, this cannot, of itself, be the only reason for lack of removal of some of OH groups. For example, that fraction of the hydrogen-bonded OH groups that are on what may be considered to be t'flat'r portions of the surface should react with TMA. However, those OH The Journal of Physical Chemistry

OH stretching frequencies, cm-1

Difference in cm-1

CH deformation frequencies. cm-1

2940,2898 2960,2895 2968,2908 2985,2918 2965,2910 2968,2910

42 65

1250,1205

60

1255

67 55 58

1262

...

...

...

groups which are in narrow crevices will be beyond the radius of action of the TMA. It is known that Cabosil consists essentially of nonporous spheres and it is probable that the only crevices and pores that exist which are inaccessible to TMA are those formed as a result of pressing the powdered material into selfsupporting disks. The absorption in the 2800-3000-~m-~region (Figure 1) shows that the species formed during the displacement reaction of the hydroxyl group contains a hydro-

917

REACTIONS BETWEEN SILICAAND TRIMETHYLALUMINUM

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3200 Frequency in cm-I

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Figure 8. Reaction between trichloromethylsilane and Cabosil (thickness about 35 mg/cm2): a, background spectrum after evacuation at 500" for 2 hr; b, after treatment with TCMS vapor (about 10 om pressure) at 400" for 15 min, and evacuation at 400" for 5 min; c, as for b, but treatment at 500". Resolution at 2800, 2900, and 3OOO cm-1 is 4.0, 3.1, and 2.4 crn-l, respectively.

1

O'

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Frequency in em"

Figure 9. Spectra in the CH stretching region of compounds containing aluminum-methyl groups and silicon-methyl groups. Details of the state of the samples, and thicknesses, are identical with those given in Table 11. Pure Al(CH&, a; solution of Si(CH&, b; and silicone grease, C. Resolution at 2800, 2900, and 3000 cm-l: a, 1,6, 1.2, and 1.0 om-'; b, 2.8, 2.2, and 1.7 om-'; and 3.2, 2.5, and 2.0 om-'. Volume 75,Number 4 April 10@7

918 carbon. It will be seen that there are predominantly two bands, a t 2955 and 2902cm-’, although weaker bands can be seen a t 2965 and 2835 cm-’ (see Table I also). Similar bands are seen with the thicker sample (Figure 2) but now the band near 2960 cm-l is too intense to be precisely located. This hydrocarbon was not thought to be a methoxy group, because of the simplicity of the spectrum. Methoxy groups (Figure 5 ) show three strong bands in this region of the spectrum (see next section) in contrast to the species made from TMA, which has only two. With compounds containing a methoxy group, the methyl deformation is found close to 1450 cm-l, both for methyl alcohol and dimethyl ether, as well as with surface methoxy groups. The presence of a band a t 1450 cm-I permits21J2interaction by Fermi resonance between the first vibrational overtone and a fundamental carbon-hydrogen stretching vibration, with a resulting increase in the number of observed bands in the spectrum in the 2800-3000-~m-~ region. When methyl groups are attached to a silicon atom, as with silicone oils and greases, or in halogenated methylsilanes, the methyl deformation band is found close to 1260 cm-l. Consequently, the frequency of the first overtone would be in a region where interaction with the carbon-hydrogen stretching vibration cannot occur. A similar argument holds for a methyl group attached to an aluminum atom, as in dimethyl aluminum chloride, where the deformation vibration is found2a near 1200 cm-I. As shown in Figure 5, we found two overlapping bands near 1470 cm-l when methanol was reacted with silica, whereas no bands were found in that region when trimethylaluminum or chloromethyl silanes were reacted with silica. For both samples, additional bands were seen in the 2100-2400 cm-1 region after reaction with TMA, although only shown for the thicker sample. The most intense bands (and the only ones seen with the thin sample) are at 2160 and 2195 cm-l. A shoulder a t 2255 cm-1 is seen in Figure 2. It seems very unlikely that these bands are those of adsorbed hydrocarbons, as the only vibrations in such molecules which occur anywhere in this region are those of the triple bonds in acetylenic molecules.24 It is, of course, most improbable that such entities would be formed under our reaction conditions. These bands are probably those of SiH groups, as they usually absorb24in the 2100 to 2200 cm-I region. Such groups have been found26on adding atomic hydrogen to silicon, at 2100 cm-l. It is unlikely they are due to AI-H vibrations, as these usually occur26between 1700 and 1800 cm-l. It was suggested by Lieflander and Stober’* that aluminum-methyl groups

The Journal of Physical Chemistry

YATES,DEMBINSKI, KROLL, AND ELLIOTT would be left on the surface of silica after treatment with TMA. If this is so, in view of the violent reaction between TMA and water or oxygen, we might expect that such methyl groups would readily react with other adsorbed molecules, such as water or oxygen. Figure 3 shows that these reactions did not occur. When water was added, large amounts were adsorbed, giving the typical very broad band characteristic of hydrogenbonded systems.27 This broad band made it difficult to determine the intensity of the CH bands, although it was obvious that they were only but little affected by the adsorbed water. The water was removed by a series of evacuations at increasing temperatures, and the final treatment, after evacuation at 200°, gave spectrum c (Figure 3). The CH band near 2960 cm-I was not noticeably changed in intensity, while the band near 2900 cm-l is a little weaker. The very stable configuration of these methyl groups is shown also by other experiments. On several occasions a t the end of the experiment, dried air was added to the cell and the spectrum was rerun. No effect was ever detected on the adsorbed species. Finally, these methyl groups show an entirely remarkable and unexpected resistance to deuterium exchange. From Figure 4, it will be seen that after adding water and evacuating a t 400°, the sharp 3750-cm-I band is again present, together with a broad band of hydrogenbonded hydroxyl groups. Some of the methyl groups had been removed by the addition of water and subsequent high-temperature evacuation (compare Figure 2b and 4a). The first deuteration lasted 15 min a t 400°, and it will be seen that both types of OH groups are converted into OD groups but that no decrease in the CH bands took place, and no CD groups could be detected. A further deuteration (not shown in the figure) was done (with a fresh dose of Dz) a t 400’ for 2.25 hr, but although further OH-OD exchange occurred, no CH-CD exchange took place. The sample was then evacuated at 500’ for 0.5 hr, and the spectrum was rerun. This spectrum is not shown, but the C R bands became slightly weaker after this treatment. A third deuteration was then done at 500’ for 3 hr. After this, nearly all of the OH groups were converted into OD groups but there was still no effect on the CH groups, and no CD groups could be detected. This is very surprising indeed, as it is quite probable that the mechanism of exchange between deuterium and surface (21) G. Hersberg, “Infrared and Raman Spectra of Polyatomic Molecules,” D. Van Nostrand, Inc., Princeton, N. J., 1945. (22) M. Falk and E. Whalley, J. Chem. Phys., 34, 1554 (1961). (23) E. Hoffmann, Z. Elektrochem., 64, 616 (1960). (24) L. J. Bellamy, ”The Infrared Spectra of Complex Molecules,” 2nd ed, John Wiley and Sons, Inc., New York, hT.Y., 1968. (25) G. E. Becker and G. W. Gobeli, J. Chem. P h y s . , 38, 2942 (1963). (26) E. G . Hoffman and G . Schomburg, “Advances in Molecular Spectroscopy,” Pergamon Press, London, 1962. (27) G . C. Pimentel and A. L. McClellan, “The Hydrogen Bond.” W. H. Freeman and Do., San Francisco, Calif., 1960.

REACTIONS BETWEEN SILICAAND TRIMETHYLALUMINUM OH groups is by the deuteriuminitially being dissociated; such deuterium atoms would then readily exchange with any hydrogen present. This may occur on pure silica (such as Cabosil) as shown by our data with a surface covered with methoxy groups (Figures 5 and 6, and next section). Both the CH and OH groups are then rapidly exchanged with D2 a t 500' and the simplest mechanism of exchange would seem to be that which involves atomic deuterium as an intermediate. The lack of reactivity would seem to make it unlikely that the groups left after treating silica with TMA contain A1-C linkages, but that they might be Si-C groups. The presence of bands in the Si-H stretching region also leads to the same conclusion. Unless some oxygen were removed from the surface by a strong reducing agent (such as TMA) it is very difficult to account for the presence of these Si-H bands. This whole question will be discussed in more detail later. Reaction between Methanol and Silica. We wished to study surface methoxy groups a t comparable resolutions to that used in our TMA work, in order to determine the differences in the spectra. Methanol was reacted with Cabosil a t 350' for 1 hr, and the results are shown in Figure 5. It will be seen that three sharp CH bands are present: a t 3000,2962, and 2856 cm-l. I n addition, shoulders are present a t 3040 and 2935 cm-l. It is of interest to note that the 2856-cm-1 band is very narrow, with half-width 11 cm-I. Our spectrum is quite similar to that given by McDonald,1gbut his samples of Cabosil were much thinner, and the bands consequently weaker. I n addition, we have found two overlapping CH deformation bands near 1470 cm-1 (Figure 5 ) , which is further evidence that we have formed surface methoxy groups. No results in this region were given in the earlier work. It is well e ~ t a b l i s h e d ~that ~ the deformation frequencies of -0CHa groups occur close to 1460cm-l. We note that C-CH3 groups have a deformation band24 a t 1450 f 20 cm-l, but it is difficult to see how such groups could have been formed from methanol on silica. I n contrast to the reaction with TMA, it will be seen that the isolated OH groups a t 3750 cm-l are but little affected by this reaction, as their peak intensity after the methanol treatment is 80% of their intensity after' the initial evacuation of the sample. Those OH groups mainly affected seem to be hydrogen bonded OH groups of frequency close to 3720 cm-l. At this frequency, the OH band after methanoltreatment is only 43% of its initial value. However, it should be noted that the methanol treatment, in addition to removing OH groups, forms new OH groups, or possibly rearranges the existing OH groups. This is shown by the fact that curves a and b of Figure 5 cross a t 3600 cm-l, and a t frequencies below this the OH band on spectrum b is more intense than that of a. The processes taking place during high-temperature reaction of methyl

919

alcohol with silica are clearly most complex, and will need considerable further study before they are understood in any degree of detail. Deuteration of the methoxy groups has also been studied (Figure 6). After the initial deuteration a t 350', only a very small band a t 2768 cm-', of OD groups, and no CD groups, could be detected. However, after deuteration at 500', both OD and CD groups could be seen. Further exchange a t 550' removed most of the CH-containing species, while leaving quite a strong OH band a t 3750cm-1. Thus, in very marked contrast to the results with the surface methyl groups derived from trimethylaluminum, the methoxy groups exchange very readily with D2. I n fact, the data of Figure 6 indicate that all of the CH groups will be converted into CD groups before all of the OH groups are exchanged. After the deuteration experiments shown in Figure 6, the sample was remethylated with CH80H. This was done initially at 350' for 15 min and the CH bands were restored to their original intensity, as seen in Figure 6a. The OD band was nearly all removed, but the CD bands remained quite strong. On further methylation, a t 600' for 20 min, all of the OD and CD groups were removed, but surface OH groups remained. It was noticed that after this methylation a t 600°, the sample was still as white as it was initially. This observation should be contrasted with earlier work on porous glass,' where 360' was used for methylation. At that time, in an attempt to remove all of the OH groups, higher temperatures than 360' were used, but the samples went brown and black, as the methyl alcohol was decomposed by the surface. This relative reactivity of porous glass may be due to the sodium impurity it contains, as shown by work on the effect of y radiatio@ on this material. Formation of Silicon-Methyl Groups on Silica. I n another approach to show that the methyl groups formed from TMA and silica are silicon-methyl groups, we have studied the properties of surface silicon-methyl groups, made from a series of silicon compounds and silica. The first compound used was tetramethylsilane, which is very stable and easy to handle. Figure 7 shows that, as might be expected, the reaction does not take place readily, as temperatures of 500' had to be used. While there is some reduction in the intensity of the 3750-cm-l band, the main effect is on the hydrogen-bonded band between 3750 and 3500cm-'. The adsorbed species, most probably in the trimethyl form, -Si(CH3)8, have CH stretching bands a t 2970 and 2915 cm-l. We have also studied the reaction between trimethylchlorosilane (TMCS) and Cabosil, but have not given the spectra as they are similar to those published ( 2 8 ) D. J. 0.Yates and P. 4258 (1964).

J. Luccesi, J. Amer. Chem. SOC.,8 6 ,

Volume YS,Number 4 April 1969

9 20 earlier.17 The effects in the OH region are quite pronounced, and we found them to be similar to those reported elsewhere,17in that mainly the free OH groups react with TMCS (presumably with the formation of HCI) . It has also been shown8 with dimethyldichlorosilane that mostly the free OH groups are removed a t 400’. It should, however, be noted that when a wide range of silicas is used with varying pore sizes and surface areas, the situation becomes very c ~ m p l e x . ~ In addition, as mentioned by Snyder and WardjQwe agree that there is really no clear-cut differentiation of hydroxyl groups into “free” and “hydrogen-bonded” types. The fact that the band caused by the hydrogenbonded species extends continuously from 3750 cm-1 to lower frequencies demonstrates this. The spectra of the methyl groups which remain on the surface after TMCS treatment have been given by only one group of workers,17 and no CH frequencies were given. For our samples, the bands were found a t 2968 and 2910 cm-l (see Table I ) . These bands are a t almost the same frequency as those surface groups put on silica from silicon tetramethyl, and this agreement substantiates the idea that both treatments give adsorbed silicon trimethyl groups. I n the case of these groups formed from TMCS, we have performed deuterium exchange experiments, The initial deuteration was a t 400’ for 15 min, while the next was a t 500’ for the same time. In agreement with the idea that silicon-methyl groups are formed in the trimethylaluminum experiments (Figure 4) none of the methyl groups from TMCS exchanged into CD groups, while quite extensive OH-OD exchange took place. As no information seems available on adsorbed silicon-methyl species containing one methyl group, we have briefly studied the reaction of trichloromethylsilane (TCMS) with silica. Figure 8 shows that extensive removal of the surface OH groups took place, with predominantly the free OH groups being removed, as with TMCS. However, some support for a n intermediate frequency OH group being most reactiveQis given by spectrum b. At this intermediate stage, it will be seen that the OH band a t 3740 cm-1 is weaker than the band at 3750 cm-1 (the “free” hydroxyl band) while initially (spectrum a) they were of very similar intensity. Finally after the 500’ treatment (c) , both these bands have gone, leaving a weak, broad band due to hydrogen-bonded hydroxyl groups with a peak a t 3680 cm-1. Clearly the reactions between these chlorinated silanes and silica are very complex. The CH stretching frequencies of these monomethyl silane groups are a t 2992 and 2925 cm-’, which is significantly higher in frequency than the corresponding trimethylsilane species, Finally, it should be noted that the difference in the two CH stretching frequencies of the silicon-methyl groups put on the surface from tri- and tetramethylsilanes are essentially the same as are found in the The JOuTnal of Physical Chemistry

YATES,DEMBINSKI, KROLL,AND ELLIOTT methyl groups put on the surface by reaction with TMA (see Table I). This reinforces our view that siliconmethyl groups are found on silica after reaction with TMA under our conditions.

Conclusions The stability to water, air, and deuterium of the surface species obtained when trimethylaluminum reacts with silica, together with the close similarity of the spectra of the product with that of silica known to contain silicon-methyl groups, strongly suggests that the surface species formed from TMA and silica contains silicon-methyl groups rather than aluminummethyl groups. Unexpected though this is, it is not inconsistent with the energies required to make and break the various bonds involved in such a rearrangement. The heat of formation of fused silica,29AH(SiOz) is - 198 kcal/mol, giving -49.5 kcal/mol of Si-0 bonds. The heat of formation of alumina,29AH(A1209) is -390 kcal/mol, giving a value of -65 kcal/mol per A1-0 bond. This indicates that the AI-0 bond is more stable than the Si-0 bond by some 15 kcal/mol. Cottrell gives30 the bond energy of the C-AI bond in A1(CH3)3as 61 kcal/ mol and the energy of the C-Si bond in Si(CH3)4as 72 kcal/mol, so the latter is 11 kcal/mol more stable than the former. It follows that a combination of A1-0 and Si-C bonds in the surface structure would be more stable than a combination of AI-C and Si-0 bonds, providing a thermodynamic driving force for the proposed rearrangement. A mechanism for the reaction can be suggested. From the observed loss of free (nonhydrogen bonded) hydroxyl groups of the silica, it is reasonable to suppose that the initial interaction between the trimethylaluminum and the surface takes place via these particular hydroxyl groups, liberating methane. For these isolated hydroxyl groups, the lone pairs of electrons on their oxygen atoms are readily available to interact with molecules having electron acceptor properties which come close to the surface. The oxygen atoms in the hydrogen-bonded hydroxyl groups, in contrast, have their lone-pair electrons participating in the hydrogen bond. Such oxygen atoms will necessarily be less reactive toward electron-acceptor molecules, which can explain the lack of reaction between trimethylaluminum and hydrogen-bonded hydroxyls. The above reaction, if the only one occurring, might well leave a dimethylaluminum species on the surface, as suggested by Lieflander and Stober.la On the contrary, we find silicon-methyl groups (and some siliconhydrogen groups) t o be the final product of the reaction. (29) “Handbook of Chemistry and Physics,” 44th ed, The Chemical Rubber Go.. Cleveland. Ohio, 1963. (30) T. L. Cottrell, “The Strength of Chemical Bonds,” 2nd ed, Butterworth and 00.Ltd., London, 1958, p 275.

ULTRASONIC ABSORPTION IN HYDRATED MELTS It seems very likely that, after the initial loss of one methyl group from the trimethylalumnium, the aluminum atom reacts further with nearby oxygen atoms of the silica, through a concerted 4-center reaction involving the unfilled fourth-coordinate position of the aluminum atom. This results in the breaking of two silicon-oxygen linkages and the formation of two silicon-methyl bonds, giving the observed adsorbed species. This assumption is also supported by the

921 observed16 reactions in the liquid state between trialkylaluminums and octamethylcyclotetrasiloxane in which the Si-0-Si bond can be cleaved. I n these reactions, the trialkylaluminum acts as an alkylation agent by transfer of one alkyl group to the silicon moiety. The alkylation action of trialkylaluminums has been well established.3' (31) H. Jenkner, Chem. Z., 8 6 , 527 (1962).

Ultrasonic Absorption in Hydrated Melts. I. Systems of Ca(N03)2.4Hz0and Its Mixtures with Water by G. S. Darbari and 5. Petrucci Polytechnic Institute of Brooklyn, Brooklyn, New York

11.201

(Received September 16, 1968)

Results of ultrasonic absorption measurements of Ca(N03)2.4Hz0melts and of 1:4.3, 1z4.7, and 1:5.0 Ca(NOa)2-HzO mixtures are presented. The temperature range 25-75' and the frequency range of 3-195 MHa have been investigated. A relaxation process at 20 A 5 MHa for Ca(NO3)2.4H2O,shifting to higher frequencies by addition of water, is reported. This relaxation tends to disappear by increasing the temperature. The volume viscosities are calculated in the frequency region below and above the observed relaxation region. The conclusion is drawn that two overlapping sources of the volume viscosities exist. A t frequencies below the relaxation region, structural and nonstructural molecular rearrangements contribute to the excess sound absorption over the classical Stokes value. The energy barrier of this over-all process is larger than the one for shear viscous flow as indicated by the decrease of the ratio qv/v. with temperature. A t frequencies larger than the observed relaxation region only the structural contributions to the sound absorption remain. The barrier of energy for the compression volume viscosity qp' is the same as the one for the shear viscosity q.. This indicates that all of the possible translation molecular rearrangements have been frozen out after the relaxation except the ones with a barrier of translational energy equal to the viscous flow.

Introduction Theories of equilibrium and mass transport properties of diluted electrolytic solutions are based on the classical Debye-Huckel model. Transport theories start a t infinite dilution and dissect a transport property like electrical conductance into the ionic components and calculate the ionic interactions a t nonzero concentration. The Fuoss-Onsager' and other similar theories, with all of the complexity of the mathematical derivation and sophistication in predicting the various interionic effects, are rarely valid a t C 2 10-2-10-1 M for a 1:l electrolyte in water a t room temperature. For concentrated solution the situation for transport theories has begun to improve in the past few years. As predicted a long time ago by FUOSS,~ the most promising development seems to be derived from the fused-salt end of the concentration scale. Angel13 has applied the free-volume theory of Cohen and Turnbul14with remarkable success to transport theories

of fused salts and glass-forming mixtures like the system Ca ( NO3)2-KN03,3to other fused salts,6 to highly concentrated glass-forming electrolyte solutions like Ca(N03)z.4Hz0 and Mg(NO3)z.nHzO ( n variable from 3 to 6 ) , and to mixtures of Ca(NO3)z.4H2OKNOa.637 According to the Cohen and Turnbull theory as well as to similar ones,* liquids that show a glass transition have an apparent energy of activation for shear flow that changes rapidly with temperature and (1) R . M. Fuoss and F. Accascina, "Electrolytic Conductance," Interscience Publishers, Inc., New York, N . Y., 1959: R. M . Fuoss and K.-L. Hsia, Proc. Natl. Acad. Sci. U.S., 57, 1550 (1967). ( 2 ) R . M . Fuoss. Chem. Rev., 17, 27 (1935). (3) C. A. Angell, J. Phys. Chem., 6 8 , 218 (1964); 6 8 , 1917 (1964). (4) M . H . Cohen and D. Turnbull, J. Chem. Phys.. 31, 1164 (1959). (5) C. A. Angell. J. Phys. Chem., 6 9 , 399 (1965). (6) C. A. Angell, ibid., 6 9 , 2137 (1965). (7) C. A. Angell, J. Electrochem. Soc., 112, 1224 (1965). (8) A. Doolittle, J. A p p l . Phys., 22, 1471 (1951); 23, 238 (1951); M . Williams, R . Landel, and J. Ferry, J. Am. Chem. Soc., 77, 3701 (1955) ; J. Beuche, J . Chem. Phys., 3 0 , 748 (1959). Volume 73, Number 4 April 1969