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Modification of SiO2 Surfaces by Reaction with Trialkoxymethanes and Triphenoxymethane Bruno R. Guidotti, Walter R. Caseri,* and Ulrich W. Suter Eidgeno¨ ssische Technische Hochschule, Institut fu¨ r Polymere, CH-8092 Zu¨ rich, Switzerland Received December 5, 1995. In Final Form: June 19, 1996X SiO2 (Aerosil) reacts with trialkoxymethanes and triaryloxymethanes (orthoformates) of the formula HC(OR)3, R ) CH3, C2H5, n-C4H9, and C6H5. The modified surface was analyzed with IR and solid-state NMR spectroscopy and thermogravimetric analysis (TGA). The results show that only the alkyl (or aryl) residues are left on the surface (as part of alkoxy or aryloxy groups). Alkyl residues are more tightly bound to the surface than aryl residues. The reaction of SiO2 with trialkoxymethanes allows stable modifications with organic products in the range of monolayer coverage, with little effort.
Introduction SiO2 has been modified with a number of organic and inorganic substances for decades. Examples are alcohols1-3 and, most frequently used, reactive silanes such as chlorosilanes or alkoxysilanes.4-6 SiO2 modified with silanes has wide technical and scientific applications, e.g., as filler for polymers or as stationary phase in chromatography. Alcohols readily adsorb from the gas phase. However, they can also form “strong” bonds with SiO2 upon reaction from the gas phase or from solution at elevated temperature, e.g., using an autoclave. These “strong” bonds are commonly interpreted as SiOR bonds, although a coordination to hypercoordinated silicon atoms might also be considered.7,8 The coverage of the surface with “strongly” bound alcohols is often relatively low. Reactions of SiO2 with organic substances are usually performed with gaseous reagents at 200-300 °C or in solution at the boiling temperature of the solvent.9 Both types of reactions are easy to perform, provided the reagents foreseen for a gas phase reaction are volatile. Gas phase reactions are often studied in situ, i.e., without further purification of the reaction products. Here, the advantage is that air sensitive modifications can be investigated easily, provided the required equipment is available. A disadvantage of in situ studies might be the presence of adsorbed byproducts that may impede the interpretation of results. Here, the reaction of SiO2 with trialkoxymethanes and triphenoxymethanes is described. To our knowledge, this type of reaction has not been presented earlier in the literature. Experimental Section Chemicals. With the exception of triphenoxymethane, all the chemicals were purchased form Fluka, Buchs (Switzerland). Triphenyl orthoformate was prepared according to the literature10 X
Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) Bauer, G.; Sto¨ber, W. Kolloid Z. 1958, 169, 142. (2) Tuel, A.; Hommel, H.; Legrand, A. P.; Balard, H.; Sidqi, M.; Papirer, E. Colloids Surf. 1991, 58, 17. (3) Zaborski, M.; Vidal, A.; Papirer, E.; Morawski, J. C. Makromol. Chem., Macromol. Symp. 1989, 23, 307. (4) Plueddemann, E. P. In Silanes, Surfaces and Interfaces; Leyden, D. E., Ed.; Gordon and Breach: New York, 1986. (5) Hair, M. L. In Silanes, Surfaces and Interfaces; Leyden, D. E., Ed.; Gordon and Breach: New York, 1986. (6) Iler, R. K. The Chemistry of Silica; John Wiley & Sons: New York, 1979. (7) Low, M. J. D. J. Phys. Chem. 1981, 85, 3543. (8) Egorov, M. M.; Kvlividze, W. I.; Kisselev, V. F.; Krassil’nikow, K. G. Kolloid Z. Z. Polym. 1966, 212, 126. (9) Kiselev, A. V.; Lygin, V. I.; Shchepalin, K. L. Zh. Fiz. Khim. 1986, 60, 1701; Russ. J. Phys. Chem. 1986, 60, 1019.
S0743-7463(95)01513-7 CCC: $12.00
(Mp: 76 °C. Microanalysis (%): C, 78.06 (calc 78.12); H, 5.52 (calc 5.59); O 16.42 (calc 16.29). 1H-NMR (CDCl3, 300 MHz) (ppm): 6.61 (s, 1H), 7.06-7.15 (m, 9H), 7.27-7.36 (m, 6H). 13CNMR (CDCl3, 75 MHz) (ppm): 110.9, 118.1, 123.6, 129.7, 153.8. IR (KBr) (cm-1): 3070 (w), 3043 (w), 3016 (w), 2901 (w), 1599 (s), 1588 (s), 1489 (s), 1382 (m), 1366 (m), 1202 (s), 1171 (s), 1085 (s), 1068 (s), 893 (m), 756 (s), 691 (s)). Surface Modification. Aerosil 200 (Degussa; 200 mg) was placed in the reaction vessel and evacuated for 1 h at 120 °C and 0.005 mbar. The reaction vessel was filled with argon, followed by addition of 2 mL of the orthoformate in 10 mL of CCl4. The suspension was heated for 15 h under reflux. Then the powder was filtered under argon, washed with 10 mL of CCl4, and dried for 15 h at 0.005 mbar. IR Spectra. IR spectra were taken as suspensions of 20 mg of silica in 0.2 mL of poly(chlorotrifluoroethylene) oil (Merck) between KBr plates with a path length of 0.2 mm. The suspensions were prepared in a glovebox under a nitrogen atmosphere. The spectra were recorded on a Nicolet 5SXC FTIR spectrometer using as reference poly(chlorotrifluoroethylene) between KBr plates with a path length of 0.2 mm. NMR Spectra. Spectra of liquids were measured in CDCl3 at 300 MHz (1H) or 75 MHz (13C) on a Bruker AM 300 spectrometer. Solid-state spectra were measured with a Bruker AMX 400 WB using the CPMAS technique. The rotation frequencies were 8-12 kHz for the detection of 1H and 3-4 kHz for 13C nuclei. Occasionally the CRAMPS technique was applied for comparison, but no substantial changes in the spectra were found when compared to those recorded with CPMAS. TGA. Thermogravimetric analyses were performed using a Perkin Elmer Series 7 thermal analysis system. The measurements were carried out under nitrogen atmosphere with a heating rate of 20 °C/min. Specific Surface Area. The measurement of the specific surface area was performed according to the BET method using a Quantasorb gauge.
Results SiO2 (Aerosil 200, Degussa) was treated with solutions of trialkoxymethanes and triphenoxymethane of the formula HC(OR)3, R ) Me, Et, n-Bu, and Ph, in CCl4 at 75-78 °C (reflux) under argon. After reaction, the SiO2 was analyzed with IR and solid-state NMR spectroscopy and thermogravimetric analysis (TGA). The IR spectra were taken in poly(chlorotrifluoroethylene)11 at a concentration of 100 mg/mL and a cell path length of 0.2 mm, conditions that assure quantitative determination of surface species.12 The embedding liquid induces a shift (10) Baines, H.; Driver, J. J. Chem. Soc. 1924, 125, 907. (11) van de Venne, J. L. M.; Rindt, J. P. M.; Coenen, G. J. M. M. J. Colloid Interface Sci. 1980, 74, 287. (12) Guidotti, B. R.; Caseri, W. R.; Neuenschwander, P.; Suter, U. W. Compos. Interfaces 1993, 1, 429.
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Table 1. IR and NMR Data of Surface Species after Treatment with HC(OR)3 R ) Me
R ) Et
R ) Bu
IR (cm-1)
2956 2855
1H
NMR (ppm)
3.4
2984 2932 2902 1.1 3.8
NMR (ppm)
48.4
2964 2939 2878 0.8 1.3a 3.7 11.1 17.9 33.76 63.2
13C
a
15.4 58.9
R ) Ph 3067 3048 6.1-7.7 115.4 120.6 129.4 155
Probably two overlapping signals.
Figure 2. IR spectra (a) of “methylated” SiO2, (b) after reaction of “methylated” SiO2 with HC(O-n-Bu)3, and (c) of “butylated” SiO2.
Figure 1. TGA diagram of SiO2 after treatment with HC(OEt)3.
of the surface hydroxyl groups to 3715 cm-1 from ca. 3750 cm-1 in self-supporting disks12 but is otherwise inert. The NMR and IR spectra exhibit the typical signals of the respective R groups (Table 1). No evidence is found for the presence of CH moieties stemming from the central carbon atom in the trialkoxymethanes or triphenoxymethane. The IR spectra in the region of C-H stretching vibrations agree with the spectra of ROH but not with those of HC(OR)3 (see also refs 13-15). Finally, the IR signal at 3715 cm-1, due to initially present surface hydroxyl groups, disappears completely after treatment with the trialkoxymethanes and triphenoxymethane. A new, relatively weak band appears at 3650-3660 cm-1 after reaction, indicating the presence of O-H stretching vibrations, either due to surface hydroxyl groups or adsorbed water or alcohol (O-H stretching vibrations of alcohols not involved in H bridges appear typically16 at 3590-3650 cm-1). TGA of orthoformate-treated SiO2 indicates weight losses of 1.5-3% at ca. 450-500 °C for the trialkoxymethanes (an example is displayed in Figure 1) and at 190 °C for triphenoxymethane. We interpret these weight losses as desorption or decomposition of the components of the organic surface species. The number of R groups per unit area, estimated for an SiO2 of specific surface area of 170 m2/g (measured with the BET method), is 2-3 alkyl groups/nm2. This value is in the range of surface hydroxyl groups initially present on SiO26 and hence, in agreement with the assumption that the surface hydroxyl (13) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1974. (14) Alpert, N. L.; Keiser, W. E.; Szymanski, H. A. Theory and Practice of Infrared Spectroscopy; Plenum Press: New York, 1970. (15) Sadtler Standard Spectrum 33826K. Sadtler Standard Grating Spectra; Sadtler Research Laboratories: Spring Garden, PA, 1974. (16) Hesse, M.; Meier, H.; Zeeh, V. Organische Chemie; Georg Thieme: Stuttgart, 1984.
Figure 3. 13C NMR spectra (a) of “methylated” SiO2, (b) after reaction of “methylated” SiO2 with HC(O-n-Bu)3, and (c) of “butylated” SiO2.
groups have reacted to a high extent with the trialkoxymethanes and triphenoxymethane, is further indicated by the complete disappearance of the IR band of the surface hydroxyl groups at 3715 cm-1. If SiO2 with surface methyl groups is treated with HC(O-n-Bu)3, a fraction of the methyl groups is replaced by butyl groups, as is evident from NMR and IR spectra (Figures 2 and 3). We have also treated SiO2 with corresponding alcohols, ROH, for comparison, under the same reaction conditions as used for the trialkoxymethanes and triphenoxymethane. Alcohols also adsorb on the SiO2 surfaces in these cases, but the reaction with the trialkoxymethanes and triphe-
Reactions of Orthoformates on SiO2 Surfaces
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Figure 4. Possible reaction product of surface silanol groups with trialkoxymethanes.
noxymethane is far more effective. This finding agrees with literature reports according to which SiO2, treated with methanol or ethanol at 30-50 °C in the gas phase, adsorbs only a small amount of “strongly” bound alkoxy groups,17,18 and reactions with alcohols to obtain “strongly” bound alkoxy groups are usually performed above 130 °C.19 TGA results indicate that the “strongly” bound methoxy and ethoxy groups obtained by direct exchange are stable in vacuum up to ca. 400 °C,17,20 corresponding to the stability of the corresponding surface species obtained after trialkoxymethane treatment. Substitution of the surface hydroxyl groups with methanol, ethanol, or butanol renders the SiO2 hydrophobic.21,22 We have observed the same behavior after treatment with the trialkoxymethanes (the samples float on water, while untreated SiO2 immediately sinks). It has also been reported that days are required for liquid water to substitute to a substantial degree the surface groups obtained after treatment with these alcohols,22 and we have found the same results after trialkoxymethane treatment. The trialkoxymethanes yield surface species with good resistance to organic solvents, while for triphenoxymethane the surface layer is removed by washing with toluene. TGA measurements also suggest that the species obtained with trialkoxymethanes are bound more strongly to the surface than those from triphenoxymethane. Discussion The surface species obtained after reaction with HC(OR)3 could be the same as would be obtained by treatment with ROH, but the reaction is much faster with the trialkoxymethanes and triphenoxymethane. Under our reaction conditions, i.e. chemical reactions in solvents, the presence of traces of water cannot be avoided. If the trialkoxymethanes and triphenoxymethane were hydrolyzed by traces of water in the solvent prior to adsorption, leaving alcohols that adsorb, the reaction with the trialkoxymethanes and triphenoxymethane could not proceed to a higher extent than that with pure alcohols. This suggests that a reaction of the trialkoxymethanes or triphenoxymethane and surface hydroxyl groups is indeed involved in the formation of the surface species. Naively, one would expect that reaction of surface silanol groups with orthoformates resulted in a transesterified product (Figure 4). This product is not present on the (17) Acosta Saracual, A. R.; Pulton, S. K.; Rochester, C. H. J. Chem., Faraday Trans. 1 1982, 78, 2285. (18) Jeziorokowski, H.; Kno¨zinger, H.; Meyer, W.; Mu¨ller, H. D. J. Chem. Soc., Faraday Trans. 1 1973, 69, 1744. (19) Eckert-Lill, C.; Lill, N. A.; Rupprecht, H. Colloid Polym. Sci. 1987, 265, 1001. (20) Mertens, G.; Fripiat, J. J. Colloid Interface Sci. 1973, 42, 169. (21) Ballard, C. C.; Broge, E. C.; Iler, R. K.; St. John, D. S.; McWhorter, V. J. Phys. Chem. 1961, 65, 20. (22) Sto¨ber, W.; Bauer, G.; Thomas, K. Ann. Chem. 1957, 604, 104.
Figure 5. IR spectra of untreated silica in poly(chlorotrifluoroethylene) oil: (top) before and (bottom) after evacuation at 120 °C and 0.005 mbar.
Figure 6. Possible reaction product of vicinal surface silanol groups with HC(OEt)3.
modified surfaces, as evident from the IR and NMR spectra. One could, however, argue that a further reaction of this transesterified product with an alcohol molecule, formed in the initial reaction, could produce an Si-OR surface group (Figure 4). IR spectra of silica evacuated at 120 °C show, before treatment with trialkoxymethanes, absorptions that could stem from vicinal hydroxyl groups (Figure 5).6,11 Reactions with vicinal surface hydroxyl groups must, therefore, also be considered. On the one hand, these groups could react with trialkoxymethanes analogously to the reaction shown in Figure 4. On the other hand, the reaction of a pair of vicinal surface hydroxyl groups with one trialkoxymethane molecule could result in a bridged species, as shown in Figure 6. Since we never observed the central carbon atom of such a structure by NMR spectroscopy, we surmise that the related species are probably not significantly present at the surface. The bridged species might, however, further react with two alcohol molecules, analogously to the reaction in Figure 4, leaving two Si-OR groups. On methanol-treated vanadia the C-O stretching mode of adsorbed methanol is visible at 1380 cm-1.23 The ν(C-O) region is not available for silica due to the strong Si-O stretching modes of the SiO2 lattice vibrations.23 In the ν(C-H) region signals were found at 2958 and 2855 cm-1 on vanadia-treated silica after exposure to methanol.23 These frequencies agree with those found after treatment of silica with HC(OMe)2 (2956 and 2855 cm-1, see Table 1). There appeared again and again unsettling reports that well-known aspects of SiO2 surfaces could not consistently be explained with common models.8,12,24-31 It has been (23) Busca, G. J. Mol. Catal. 1989, 50, 241. (24) Miller, J. D.; Ishida, H. In Fundamentals of Adhesion; Lee, L. H., Ed.; Plenum Press: New York, 1991. (25) Urban, M. W.; Koenig, J. L. Appl. Spectrosc. 1985, 6, 1051. (26) Chuiko, A. A.; Sobolev, V. A.; Tertykh, V. A. Ukr. Khim. Zh. 1972, 38, 774; Sov. Prog. Chem. 1972, 38 (8), 32. (27) Gavrilyuk, K. V.; Gorlov, Yu. I.; Konoplya, M. M.; Furman, V. I.; Chuiko, A. A. Teor. Eksp. Khim. 1979, 15, 212; Theor. Exp. Chem. 1979, 15, 167.
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proposed that water and alcohol molecules might be strongly bound to SiO2 via coordination to pentacoordinated silicon atoms.8,26-29 As a consequence, we cannot exclude that pentacoordinated silicon atoms might be involved in the reaction of orthoformates with SiO2. The reaction with orthoformates could then be explained as a hydrolysis with adsorbed water to yield an alcohol as product, and this alcohol would occupy the vacant surface site previously held by adsorbed water. It is not obvious if we can exclude that the OR residues are bound to SiO2 as an alcohol. In the IR spectra, O-H vibrations were always present, but they might belong to species other than alcohols (e.g., water). In 1H solid-state NMR spectra, the chemical shifts of surface hydroxyl groups, adsorbed water, and aliphatic alcohols are reported to be 3.0-3.5 ppm,32-34 so that the signals of these OH groups are expected to overlap with the CH peaks of the aliphatic alcohols and might not be resolved because of the broad lines in the 1H solid-state NMR spectra. For R ) Ph, however, the aromatic hydrogen atoms appear at 6.1-7.7 ppm (in the same range that the signal of the alcohol group in phenol is expected16), so that signals at
0-5 ppm should be evident if the hydroxyl bands in the IR spectra belonged to water or surface hydroxyl groups; however, no resonance was detected between 0 and 5 ppm; i.e., for R ) Ph, the OH signals in the IR spectra might be attributed to alcohol groups. Also, the low desorption temperature of the surface phenyl groups (190 °C) and the low solvent stability of the adsorbed phenyl species might indicate that surface phenyl groups are present as an alcohol. The reaction of butyl orthoformate with SiO2 pretreated with methyl orthoformate could be explained in terms of a transesterification reaction if the methoxy groups would be present as methanol. The small nucleophilicity of oxygen in an Si-OR group35-37 does not make the model with Si-OR groups seem a likely reactant for the transformation of “methylated” SiO2 with HC(O-n-Bu)3. In this context it is interesting that even the detailed structure of silane-modified SiO2 is not completely evident.24,30 Whatever the detailed structure of the adsorbed species is, the reaction of SiO2 with trialkoxymethanes leads to a surface that is durably modified with organic groups.
(28) Gorlov, Yu. I.; Golovatyi, V. G.; Konoplya, M. M.; Chuiko, A. A. Teor. Eksp. Khim. 1980, 16, 202; Theor. Exp. Chem. 1980, 16, 164. (29) Konoplya, M. M.; Gorlov, Yu. I. Teor. Eksp. Khim. 1980, 16, 166; Theor. Exp. Chem. 1980, 16, 135. (30) Herzberg, W. J.; Erwin, W. R. J. Colloid Interface Sci. 1970, 33, 172. (31) Michel, D. Z. Naturforsch. 1967, 22, 1751. (32) Bronnimann, C. E.; Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1988, 110, 2023. (33) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I.; Khlebnikov, V. B. Zh. Fiz. Khim. 1972, 46, 502.; Sov. Prog. Chem. 1972, 46, 290. (34) Geschke, D. Z. Phys. Chem. Leipzig 1969, 242, 74.
Acknowledgment. We would like to thank Felix Bangerter for taking the NMR spectra, Dr. W. Saur of Gurit-Essex, Freienbach (Switzerland) for stimulating discussions, and Gurit-Essex for financial support. LA9515130 (35) Kiselev, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; John Wiley & Sons: New York, 1975. (36) Schneider, M.; Boehm, H. P. Kolloid Z. Z. Polym. 1963, 187, 128. (37) Boehm, H. P. Angew. Chem. 1966, 78, 617.
