IR spectroscopic study of silica—triethylamine interaction

Colloidsand Surfaces,63 (1992) 97-101

97

Elsevier Science Publishers B.V.,Amsterdam

IR spectroscopic study of silica-triethylamine interaction T . I . T i t o v a a n d L.S. K o s h e l e v a

Institute of Physical Chemistry of the USSR Academy of Sciences, Leninsky prospect 31, Moscow 117915, Russia (Received 8 August 1990; accepted 18 July 1991)

Abstract The molecular interactions occurring in the silica-triethylamine (TEA) system were studied by means of FT-IR spectroscopy on a hydrothermally treated superpure silica gel. A low-frequency shift of the yon band from 3750 to 3220-2770 cm- 1 as a result of slight, moderate and strong H-bonding of silanol groups to TEA molecules was found to correlate with a high-frequency shift from 2804 to 2812-2855 cm -1 of the vs vibration of C-H bonds attributable to .~Iq---CH2- fragments of TEA. Pre-adsorption of TEA onto silica gel followed by evacuation at temperatures not exceeding 473 K promoted the dehydroxylation process due to the interaction of TEA with free silanol groups, causing formation of Et 3HN +(OH)- species.

Keywords: FT-IR spectroscopy;hydrothermal treatment; silica gel; surface modification; triethylamine adsorption.

Introduction The strong organic base triethylamine, possessing a low value of ionization potential and exhibiting no self-association of its molecules [1], is widely used as a convenient probe for acidic sites on silica, which have become the subject of several adsorption-calorimetric and IR spectroscopic studies I-2-6]. As was shown in Ref. [7], adsorption of triethylamine (TEA) onto silica surfaces with different degrees of dehydroxylation, caused slight and moderate H-bonding of TEA molecules with silanol groups, together with the formation of T E A - S i O H associates with strong H-bonds, up to a complete proton transfer from SiOH to TEA, yielding the thermostable, salt-like (Et3HN) + ( S i - O ) - species. This explains why free

Correspondenceto: T.I. Titova, Institute of Physical Chemistry of the USSR Academy of Sciences, Leninsky prospect 31, Moscow 117915, Russia. 0166-6622/92/$05.00

silanol groups were shown I-7] to retain TEA most tenaciously. Thus, it was of interest to homogenize the silanol coverage, by elimination of associated silanol groups, in order to study the interaction between TEA and free SiOH groups in more detail. In the present work this aim was achieved by hydrothermal treatment of silica gel.

Experimental section Amorphous silicon dioxide with a specific surface area of 360 m 2 g - 1 and a level of inorganic contaminants of 5 - 1 0 - 4 % w t . produced by the hydrolysis of superpure tetraethoxysilane was used as the initial silica gel sample, and this was hydrothermally treated by steaming at 1073 K for 10 h under shallow-bed conditions. The specific surface area of the sample was measured by the nitrogen thermal desorption method. For IR spectroscopic analysis, self-supporting platelets of the samples

© 1992 - - Elsevier Science Publishers B.V. All rights reserved.

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T.I. Titova, L.S. Kosheleva/Colloids Surfaces 63 (1992) 97-101

containing 10-15mg of SiO2cm -2 were heatevacuated to a residual pressure of 10 -s Torr in a special quartz cell with CaF2 windows. The IR spectra were recorded on a Bruker IFS-115c Model FT-IR spectrometer over the range 12004000 cm-~ with a resolution of 4 cm-1. The flattened absorbance spectra were analyzed as obtained for specified quantities of the samples. Flattening of the absorbance spectra was employed to eliminate sloping of the base line caused by scattering of IR radiation by the sample.

the OH stretching modes region reveals that after the removal of SiOH groups perturbed either by hydrogen (the band at 3473 cm-1) or by oxygen (the band at 3730 cm -1) [7], the silanol coverage of the sample investigated becomes much more homogeneous and is represented mainly by free silanol groups absorbing at 3750 cm -1 (curve 2). In line with the changes in the OH stretching modes region, a high-frequency (HF) shift and noticeable changes in intensity of the bands at 1985, 1875 and 1635 cm -1, assigned to the overtones and combination tones of silicon-oxygen fundamental vibrations [9], occur. The spectral changes of these bands appear to result from the change in the electron configuration of siloxane bridging oxygen on elimination of strained siloxane bonds through hydrothermal treatment. Elimination of strained siloxane bonds diminishes the negative charge of bridging oxygen and hence its proton accepting capability. It gives rise to hydrophobization of the surface, which is consistent with the above described change in absorbance at 3200-3700 cm- 1 Adsorption of gaseous TEA onto such a surface (see Fig. 2, curves 2 and 3) causes the 3750 cm-1 band of free SiOH groups to recede due to the perturbation by TEA molecules. In the corresponding difference spectrum (curve 4), the perturbation is revealed in the hatched continuum of the overlapping bands with maxima at ~ 3200, 2785, 2347 and 1736 c m - L This picture is very close to that observed by us on a silica gel with the same content of inorganic impurities but which had not been subjected to a hydrothermal treatment. It was interpreted in terms of the acid-base type interactions of TEA with SiOH groups of varying strength. Additionally, the difference spectrum, 4, indicates that the participation of SiOH groups in fine pores (the negative absorbance band at 3552 cm -~) in the interaction with TEA is, unlike free SiOH groups (the band at 3750cm-1), extremely limited. For this reason alone, the spectral behavior of adsorbed TEA is worthy of attention, for it is able in its turn to throw light upon the peculiarities involved in the interaction

Results and discussion The effect of hydrothermal treatment which caused the specific surface area of the silica gel investigated to contract from 360 to 160 m 2 g-~, is illustrated by the corresponding spectra of Fig. 1. Thus, a drastic decrease in absorbance occurs at 3200-3700 cm -1 (curve 1) involving that characteristic of the vibrations of molecular adsorbed water and associated silanol groups concentrated within the bulk and/or fine pores I-8], so that after the hydrothermal treatment (curve 2) those silanol groups are represented mainly by bulk SiOH groups exhibiting the band at 3665 cm-1. Analysis of the difference spectrum (curve 3) in

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Fig. 1. The absorbancespectra ofthe initial(curve 1)and hydrothermallytreated (curve2) silica gel after evacuationat 473 K for 6h, and the corresponding difference spectrum 2-1 (curve 3).

T.I. Titova, L.S. Kosheleoa/Colloids Surfaces 63 (1992) 97-101

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99 as a result of strengthening of the positive induction effect of its hydrogen a t o m on partial proton transfer from the S i - O H group to the N - a t o m of TEA. Additionally, magnification of the shape of this band revealed that it was split into two (2850 and 2830 c m - 1 ) which suggests (spectrum 4) the occurrence of differing degrees of H-bonding to TEA. To differentiate between those states, the adsorbed TEA was outgassed while varying the temperature and the duration of evacuation. Evacuation of the sample at r o o m temperature for 20 min and for 2 h (Fig. 3, curves 1 and 2) shifts the v,:,~c_m band to 2839 c m - 1 and to 2854 c m - 1 , respectively. Analysis of the corresponding difference spectrum 1-2 (curve 4) in the region of VCH modes of adsorbed TEA species outgassed at r o o m temperature for 1 h 4 0 m i n reveals that the

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Fig. 2. The absorbance spectra of gaseous TEA taken at P/P, .~ 0.4 (curve 1) and those of hydrothermally treated silica gel after evacuation at 473 K (curve 2), further exposure to TEA at room temperature for 20 min (curve 3), and the corresponding difference spectrum 3 - 2 (curve 4). of TEA with SiOH groups. Indeed, the spectrum of gaseous TEA in the C H stretching modes region (curve 1) is represented by the bands at 2976, 2947, 2885 and 2804 c m - 1,a with the last being assigned to the v~ vibration of C - H bonds in methylene groups neighboring the nitrogen a t o m possessing lone-pair electrons [10]. Hence, one can anticipate that the parameters of this band will alter depending on the state of these lone pair electrons, e.g. on proton transfer to the nitrogen atom of TEA. As a matter of fact, this is shown by the change in the position and the shape of this band (denoted below schematically as v~:,~c_m) in the spectrum of adsorbed TEA (curve 4) as compared with the same properties in the spectrum of gaseous TEA. Thus, centering of this band in the spectrum of adsorbed TEA at higher frequencies can reasonably be ascribed to a decrease in the C - H bond length aln our previous report I-7] a wrong value for the wavenumber of the vs(CH) band in the spectrum of TEA was given: 2904 cm- ~ instead of 2804 cm- 1.

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Fig. 3. The absorbance spectra of hydrothermally treated silica gel after evacuation at 473 K, exposure to TEA and evacuation at room temperature for 20 min (curve 1), 2 h (curve 2) and further evacuation at 473 K for 4 h (curve 3), and the corresponding difference spectra 1 - 2 (curve4) and 2 - 3 (curve 5). Above spectrum 3, the CH stretching region of this spectrum and that of the spectrum of the initial silica gel evacuated at 473 K after adsorption of TEA (the upper curve) are shown at a 3 x magnification.

100

T.I. Titova, L.S. Kosheleva/Colloids Surfaces 63 (1992) 97-101

vs:N~c_mband centers at 2812 cm-1, i.e. rather close to 2804 cm -1 characteristic of individual TEA molecules in the gaseous state. This suggests that in the TEA-SiOH surface species being evacuated at room temperature, TEA was associated with SiOH groups via only a weak H-bond. This conclusion is supported since the removal of this portion of TEA liberates SiOH groups absorbing at 3734cm -1, i.e. in the frequency interval characteristic of the absorbance of silanol groups slightly perturbed by hydrogen [7]. Liberation of those silanol groups is seen from the same spectrum (4) to be accompanied by the elimination of the corresponding H-perturbation as manifested in the broad hatched band with its maximum at ~3228 cm-1. Further evacuation of this sample at 473 K (curve 3) removes more tenaciously retained TEA-SiOH associates. Those associates exhibiting the vs:~c_m band at 2851 cm-1 (curve 5) were formed through H-bonding of TEA mainly with free silanol groups (3749 cm -~) and, to a minor extent, with silanol groups in residual fine pores (3552cm-~). So, the liberation of those groups on evacuation at 473 K parallels the elimination of a stronger perturbation of these silanol groups by hydrogen manifested in the continuum of overlapping hatched bands at ~3220, 2781, 2210 and 1727 cm -~. Analysis of the OH stretching mode region of the spectrum of the silica gel sample evacuated after adsorption of TEA at 473 K (curve 3) indicates that the 3746cm -1 band of free silanol groups has decreased in absorbance by ,~ 30% as compared with that in the spectrum of the initial hydrothermally treated silica gel evacuated at the same temperature (Fig. 2, curve 2). So, in the present case we are dealing with so-called chemically activated dehydroxylation:

As to the salt-like associates Et3HN+(SiO) -, formed mainly due to the interaction of TEA with free silanol groups [7] and which thus might give rise to the reduction of those groups, they are formed on the hydrothermally treated silica gel sample to an extent about 5-7 times less than on the untreated silica gel sample evacuated at 473 K. This is deduced by comparing the CH stretching region of the spectrum of the initial silica gel evacuated after adsorption of TEA at 473 K to the same region of the corresponding spectrum of the hydrothermally treated sample (curve 3), both of which are shown in Fig. 3 above the overall spectrum 3 at a 3 x magnification. The absence of a markedly pronounced absorption band in the region 2800-2850cm -1 in the CH stretching spectra of adsorbed TEA species remaining after evacuation at 473 K, implies the formation of saltlike Et3HN+(Si-O) - species, for TEA salts are known to exhibit a similar spectral pattern [12]. The essentially smaller extent of Et 3HN ÷ (Si-O)species formation on a silica gel surface after hydrothermal treatment seems very likely to be due to a redistribution and homogenization of acidic properties of silanol groups following such treatment, resulting in the decrease in that portion of Si-OH groups which are capable of the strongest acid-base type interactions with TEA. As was shown by us in Ref. [7], only 1-2% of free silanol groups of a hydrothermally untreated silica gel evacuated at 473 K participate in the formation of salt-like associates. Since the amount of such SiOH groups is noticeably less on a hydrothermally treated silica gel, it is obvious that in this case it has little effect on the observed reduction of absorbance of the silanol groups. Repeated adsorption of TEA under the same conditions onto the "non-thermally" dehydroxylated silica gel sample produces changes which are evidenced by the corresponding spectra of Fig. 4. Though the general picture of spectral changes is similar to that observed on the initial hydrothermally treated sample (see Fig. 2), there are some differences which become evident from corn-

Et3 N + 2~-SiOH ~ ~~--'sitO~si-~ + Et 3HN ÷ (OH)-

(1)

To achieve this degree of thermally activated dehydroxylation, one would need to raise the temperatures of evacuation to about 773 K [11].

T.I. Titova, L.S. Kosheleva/Colloids Surfaces 63 (1992) 97-101 ,L 29'28

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3746 cm -1 of about 25%. If one now compares the spectrum of the initial silica gel (curve 1) with that after a "double" chemical dehydroxylation (curve 4), it becomes clear, even without examination of difference spectra, that these spectra demonstrate distinct differences in the regions of Vsc. (2800 - 3 0 0 0 c m - 1 ) and VoH of free silanol groups (3750 cm- 1). Thus, the presence of CH stretching bands in spectrum 4 appears to be attributed to the absorbance of C - H bonds in the E t 3 H N + ( O - S i ~ ) surface species formed on interaction of TEA with isolated silanol groups:

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Fig. 4. The absorbance spectra of the hydrothermally treated silica gel before (curve 1) and after the primary exposure to TEA followed by evacuation at 473 K (curve 2) and repeated exposure to TEA at room temperature (curve 3) and evacuation at 473 K for 4 h (curve 4), and the corresponding difference spectrum 3 - 2 (curve 5).

paring the difference spectrum 5 of Fig. 4 with the corresponding spectrum 4 of Fig. 2. Thus, centering of the vs:N~c_m band 14cm -~ lower (at 2816cm-1), as well as propagation of the H-perturbation of silanol groups over a smaller frequency interval (down to 1747cm -1) in spectrum 5 of Fig. 4, as compared with spectrum 4 of Fig. 2, argues no doubt in favor of the fact that chemically activated dehydroxylation reduces that portion of free S i - O H groups which are capable of strongest acid-base interactions with adsorbed TEA molecules. Evacuation at 473 K of the sample after repeated adsorption of TEA is seen from Fig. 4 (curve 4) to result in the development of the chemical dehydroxylation process, yielding this time a decrease in absorbance at

(2)

Since no more than 1-2% of free silanol groups take part in this interaction, the drastic decrease in absorbance at 3750 cm -1 after evacuation of TEA at 4 7 3 K is mainly due to the chemical dehydroxylation by reaction (1). So, the results obtained reveal that both reactions (1) and (2) must be taken into consideration to be able to finely tune the surface chemistry properties of silica gel. References 1 A.J. Gordon and R.A. Ford, The Chemist's Companion, Wiley-Interscience, New York, 1972. 2 A.V. Kiselev, Discuss. Faraday Soc., 52 (1971) 14. 3 G. Curthoys, V.Ya. Davydov, A.V. Kiselev, S.A. Kiselev and B.V. Kuznetsov, J. Colloid Interface Sci., 48 (1974) 58. 4 E.A. Paukshtis and E.N. Yurchenko, Usp. Khim., 52(3) (1983) 426. 5 F.H. van Cauwelaert, F. Vermoortele and J.B. Uytterhoeven, Discuss. Faraday Soc., 52 (1971) 66. 6 M.L. Hair, J. Colloid Interface Sci., 59 (1977) 532. 7 S.P. Zhdanov, L.S. Kosheleva and T.I. Titova, Langmuir, 3(6) (1987) 960. 8 V.Ja. Davydov, A.V. Kiselev and L.T. Zhuravlev, Trans. Faraday Soc., 60 (1964) 2254. 9 H.A. Benesi and A.C. Jones, J. Phys. Chem., 63 (1959) 179. 10 K. Naskanisi, Infrakrasnye Spektry i Stroenie Organicheskikh Soedineniji, Mir, Moscow, 1965. 11 A.A. Agzamkhodzhaev, G.A. Galkin and L.T. Zhuravlev, in M.M. Dubinin and V.V. Serpinskiji (Eds), Osnovnye Problemy Teorii Fizicheskoji Adsorbtsii, Nauka, Moscow, 1970, p. 168. 12 V.P. Glasunov, A.A. Mashkovsky and S.E. Odinokov, J. Chem. Soc., Faraday Trans. 2, 4 (1979) 629.