Comparative study on the hydrolysis kinetics of substituted ethoxysilanes by liquid-state 29Si NMR

Journal of Non-Crystalline Solids 343 (2004) 61–70 www.elsevier.com/locate/jnoncrysol

Comparative study on the hydrolysis kinetics of substituted ethoxysilanes by liquid-state 29Si NMR Ruili Liu

a,c

, Yao Xu a, Dong Wu a, Yuhan Sun a,*, Hongchang Gao Hanzhen Yuan b, Feng Deng b

b,c

,

a

b

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Tao Yuan North Road 27, Taiyuan 030001, PeopleÕs Republic of China State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, PeopleÕs Republic of China c Graduate School of the Chinese Academy of Sciences, Beijing 100039, PeopleÕs Republic of China Received 14 January 2004; received in revised form 20 June 2004

Abstract Liquid-state 29Si NMR was used to investigate the ammonia-catalyzed initial hydrolysis of different substituted ethoxysilanes dissolved in methanol. The following ethoxysilanes were used: tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DBS). Their reaction expresses were obtained by assuming first-order in the reactant ethoxysilanes and varying other reactant ammonia and water concentrations. Individual kinetic rate constants for the initial hydrolysis of each ethoxysilanes, the rate-limiting step for total reactions, were calculated, which were independent of any reactant concentration. The results indicated that the steric factors have more important effect on the reactive reactivity than inductive factors, so that the initial hydrolysis rate constants for the studied ethoxysilanes decreased in an unusual order DDS > TEOS > MTES. For these three ethoxysilanes, a common conclusion was drawn that the substitution of the hydroxyl group for the ethoxyl group resulted in a downfield shift. In the case of DDS, however, the trend was opposite to the previous studies in which the hydrolyzed resultants had upfield shift compared with DDS monomer.
2004 Elsevier B.V. All rights reserved.

1. Introduction Monodisperse organically modified silica nanoparticles have many applications in mechanical, chemical protection [1] and optic fields [2]. Sto¨ber synthesis [3] was the first method to produce this kind of uniform silica nanoparticles by ammonia-catalyzed hydrolysis and condensation of ethoxysilanes in alcohol solvents. Generally, the organically modified silica particles are prepared from two or more precursors, whose reactive *

Corresponding author. Tel.: +86 0351 4084072; fax: +86 0351 4041153. E-mail address: [email protected] (Y. Sun). 0022-3093/$ - see front matter
2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.07.032

compatibility is decisive of the homogeneity of product. It has proved that the final polymer properties are dependent on the early hydrolysis and condensation steps of ethoxysilanes [4], since they determine the subunits from which the further structures are made. To obtain a homogeneous distribution of all the organic groups throughout the product, it is important to know the relative reactivity of various precursors. In the last two decades, 29Si NMR was widely used to investigate the hydrolysis of non-substituted tetraethoxysilane (TEOS) [5–7], methyl-substituted ethoxysilanes such as methyltriethoxysilane (MTES) [5–9] and dimethyldiethoxysilane (DDS) [5,10]. However, these works mostly focused on the acidic conditions, based on a

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R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

common thought of fast hydrolysis at low pH. So far, few studies were about the hydrolysis and condensation of different substituted ethoxysilanes under basic-catalyzed conditions [11,12]. Furthermore, the kinetics of the individual reactions is still not well developed, and the key kinetic parameters have not been obtained, such as rate constant, rate order and activation energy. 29 Si NMR has proved a useful tool to follow the reactive process of the initial hydrolysis and condensation of silicon alkoxides [9,13]. It allows identification and quantification of each silicon species in solution, thus enabling reaction kinetic data to be extracted. In the present work, liquid-state 29Si NMR was employed to monitor the initial ammonia-catalyzed hydrolysis and condensation of substituted ethoxysilanes with different numbers of methyl, e.g., MTES and DDS, compared with non-substituted TEOS. On the basis of the detailed study, a conclusion different from the reference [11] was drawn that the initial hydrolysis rate constant decreased in an unusual order of DDS > TEOS > MTES.

2. Experiment Solutions were prepared using the following ethoxysilanes as precursors: tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and dimethyldiethoxysilane (DDS). The molar ratio and reaction conditions are presented in Tables 1–3 for TEOS, MTES, and DDS, respectively. NMR sample was prepared by mixing two parts: solution A, ethoxysilanes was dissolved in half of the total methanol and stirred for 20 min; solution B, deionized water, the ammonia hydroxide and methanol was stirred for 5 min. The reaction was initiated by mixing solutions A and B and vigorously stirred for 5 min, and then the mixture was immediately transferred to an NMR sample tube (5 mm OD), and immediately analyzed. Chromium (III) acetyl acetonate Cr(acac)3 (1%) was added as the spin relaxation agent, and many

studies [12,14] have shown that Cr(acac)3 has no effect on the reactions. All 29Si NMR experiments were carried out in duplicate on a UNITY INOVA-500 Spectroscopy at a spectral frequency of 99.351 MHz. To achieve sufficient signal intensity, 168 scans were acquired for each spectrum with a 3 s pulse delay using a 90 pulse angle. During the experiments, gelatin did not occur and the transesterification was negligible [12].

3. Result 3.1.

29

Si NMR spectra and data analysis

In order to assign the 29Si NMR chemical shift for different silicon species, the traditional notation was adapted [15]: Q presents the tetrafunctional silicon in TEOS, T presents the trifunctional silicon in MTES and D presents the difunctional silicon in DDS. Then the symbols Qnm , Tnm , and Dnm denote the products of hydrolysis and condensation of TEOS, MTES, DDS, respectively; m is the number of siloxane bridges attached to Si atom; n is the number of silanol bonds on Si atom. Three examples of the time dependence of 29Si NMR spectra during the each precursor hydrolysis are shown in Figs. 1–3, and the values of the chemical shifts for the soluble Si-species formed are presented in Tables 4–6 (TEOS – Fig. 1 and Table 4, MTES – Fig. 2 and Table 5, DDS – Fig. 3 and Table 6). Fig. 1 and Table 4 show that the hydrolysis of TEOS resulted in the formation of Q30 and Q10 , peaks, and the chemical shifts of them are
78.3 and
80.4 ppm, respectively. While Q20 was not observed probably due to the high concentrations of NH3 and H2O under all studied reaction conditions [12]. Fig. 2 and Table 5 indicate that only two 29Si peaks were observed, a primary MTES or T00 peak and hydrolysis resultant T10 peak, whose chemical shifts appeared at

Table 1 Chemical composition, reaction conditions, the pseudo-first-order hydrolysis rate constant k 1h and the initial hydrolysis rate constant kh for TEOS studied by 29Si NMR Samples

TEOS/CH3OH/H2O/NH3

T (C)

k 1h  103 (min
1)

kh · 103 (mol
0.56 dm1.68 min
1)

(a) (b) (c) (d) (e) (f) (g) (h)

1/12.5/4.0/0.18 1/12.5/4.0/0.18 1/12.5/4.0/0.18 1/12.0/4.0/0.045 1/12.5/4.0/0.36 1/12.0/2.0/0.18 1/11.6/5.7/0.18 1/12.0/7.5/0.18

25 35 45 25 25 25 25 25

4.87 ± 0.02 6.70 ± 0.02 9.14 ± 0.02 2.61 ± 0.02 6.25 ± 0.02 4.17 ± 0.02 5.08 ± 0.02 5.13 ± 0.02

7.34 ± 0.4 10.68 ± 0.5 13.71 ± 0.7 7.35 ± 0.4 7.42 ± 0.4 7.34 ± 0.4 7.45 ± 0.4 7.59 ± 0.4

The error stated of k 1h is estimated from the fitting of the experimental data, and the error stated of kh comes from the error of k 1h and the 5% error of integration area.

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

63

Table 2 Chemical composition, reaction conditions, the pseudo-first-order hydrolysis rate constant k 1h and the initial hydrolysis rate constant kh for MTES studied by 29Si NMR Samples

MTES/CH3OH/H2O/NH3

T (C)

k 1h  103 (min
1)

kh · 103 (mol
0.87 dm2.61 min
1)

(a) (b) (c) (d) (e) (f) (g)

1.0/13.5/3.0/0.21 1.0/13.5/3.0/0.21 1.0/13.5/3.0/0.21 1.0/13.3/3.0/0.11 1.0/13.4/3.0/0.46 1.0/14.1/1.5/0.21 1.0/12.4/6.0/0.18

25 35 45 25 25 25 25

2.64 ± 0.2 3.80 ± 0.2 5.27 ± 0.2 1.67 ± 0.2 4.71 ± 0.2 2.40 ± 0.02 2.94 ± 0.02

7.25 ± 0.4 10.44 ± 0.4 14.47 ± 0.7 7.25 ± 0.4 7.28 ± 0.4 7.37 ± 0.4 7.29 ± 0.4

The error stated of k 1h is estimated from the fitting of the experimental data, and the error stated of kh comes from the error of k 1h and the 5% error of integration area.

Table 3 Chemical composition, reaction conditions, the pseudo-first-order hydrolysis rate constant k 1h and the initial hydrolysis rate constant kh for DDS studied by 29Si NMR Samples

DDS/CH3OH/H2O/NH3

T (C)

k 1h  103 (min
1)

kh · 103 (mol
0.73 dm2.19 min
1)

(a) (b) (c) (d) (e) (f) (g)

1.0/14.5/2.0/0.21 1.0/14.5/2.0/0.21 1.0/14.5/2.0/0.21 1.0/14.5/2.0/0.11 1.0/14.5/2.0/0.42 1.0/15.0/1.0/0.21 1.0/13.8/4.0/0.21

25 35 45 25 25 25 25

10.30 ± 10.02 13.44 ± 0.02 18.35 ± 0.02 6.43 ± 10.02 15.801 ± 0.02 9.80 ± 0.02 10.87 ± 10.02

27.991 ± 1.4 36.52 ± 1.8 49.87 ± 2.5 27.31 ± 1.4 27.43 ± 1.4 28.14 ± 1.4 28.14 ± 1.4

The error stated of k 1h is estimated from the fitting of the experimental data, and the error stated of kh comes from the error of k 1h and the 5% error of integration area.

Fig. 1. 29Si NMR spectra of the solutions containing different molar ratio of TEOS:CH3OH:H2O:NH3 = 1:12.5:4:0.18 (25 C).

Fig. 3. 29Si NMR spectra of the solutions containing different molar ratio of DDS:CH3OH:H2O:NH3 = 1:12.5:4:0.18 (25 C).


41.8 and
40.5 ppm. Additionally, when the concentrations of NH3 and H2O were proper, T20 peak was observed and the chemical shift appeared at
40.5 ppm. In the case of DDS, Fig. 3 and Table 6 show a very complex situation emerged, that is, nine soluble intermediate species were detected during the hydrolysis and condensation process, and the details were discussed hereinafter. 3.2. Chemical reaction kinetics

29

Fig. 2. Si NMR spectra of the a solution containing the molar ratio of MTES:CH3OH:H2O:NH3 = 1:12.5:4:0.18 (25 C).

The relative concentration for all intermediate hydrolysis and condensation species were determined by integration over fixed individual frequency ranges

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R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

Table 4 29 Si NMR chemical shift d and silicate structures obtained with TEOS Q30 Q20 Q10 Q00

Structures

d (ppm)

Si*(OEt)(OH)3 Si*(OEt)2(OH)2 Si*(OEt)3OH Si*(OEt)4


78.3 –
80.4
81.3

Table 5 29 Si NMR chemical shift d and silicate structures obtained with MTES T20 T10 T00

Structure

d (ppm)

MeSi*(OEt)(OH)2 MeSi*(OEt)2OH MeSi*(OEt)3


39.2
40.5
41.8

D3 D00 D11 D01 D4 D02

Structure

d (ppm)

Me2Si*(OH)2 Me2Si*(OEt)(OH) (Me2SiO)3 Me2Si*(OEt)2 Unknown species „SiOSi*(OH)Me2 „SiOSi*(EtO)Me2 (Me2SiO)4 („SiO)2Si*Me2

1.46
0.36
1.23
2.18
3.78
8.92
9.45
12
20.4

d½Si a b c ¼ k h ½Si ½NH3  ½H2 O ; dt

ð2Þ

where Si presents ethoxysilanes. The exponents a, b and c are the order of the reaction with respect to ethoxysilanes, NH3 and H2O, respectively. Since NH3 was used as the catalyst during the process of the hydrolysis and condensation, its concentration remained; the amount of water was in excess of the amount needed to completely hydrolyze monomer, and its largest change was less than 5% of the initial concentration [6], so it was also considered invariable. If we define k 1h ¼ b c k h ½NH3  ½H2 O , then the rate expression (2) can be simplified to r¼


Table 6 29 Si NMR chemical shift S and silicate structures obtained with DDS D20 D10

r¼


d½Si a ¼ k 1h ½Si : dt

ð3Þ

As can be seen from Figs. 4–6, the fitted curves for the concentration of each ethoxysilanes monomer are good approximation for the experimental data. In this case, it is assumed that the reaction is a first-order in [Si], i.e., a = 1, so the rate expression (3) becomes r¼


d½Si ¼ k 1h ½Si: dt

ð4Þ

Integration of Eq. (4) leads to Z t Z t d½Si ¼ k 1h
dt: ½Si 0 0

ð5Þ

This gives in the spectra. The integrated area for the initial Q00 peak of 1 M TEOS without catalyst or water was taken to be 100%. For each studied reaction mixture, the disappearance of monomers from their initial level of 1.0 M and the appearance of one or more intermediate species are presented in Figs. 4–6 (TEOS – Fig. 4, MTES – Fig. 5 and DDS – Fig. 6). The curves of the time dependent changes for each monomer concentration were obtain by doing an exponential decay of firstorder fitting, and the other curves for intermediate species were obtain by doing a Gaussian or multi-peaks fitting. The reaction of the initial hydrolysis of these three ethoxysilanes, is described as kh

ðCH3 Þð4
xÞ SiðORÞx þ H2 O ! ðCH3 Þð4
xÞ SiðOHÞðORÞx
1 þ ROH;

ð1Þ

where kh is the initial hydrolysis rate constant. On the hydrolysis and condensation of alkoxides, there are detailed rate models [16] and simpler models [17]. This type of simplified reaction scheme has been used to fit tractable rate models to the data shown in Figs. 4–6, where the rate is dependent upon the [NH3] and [H2O]. So the rate expression for the reaction (1) is

ln½Si ¼ ln½Si0
k 1h t:

ð6Þ

Thus a plot of ln[Si] versus time yielded a straight line with
k 1h as slope and ln[Si]0 as intercept (see Figs. 7– 9), and the values of the corresponding k 1h are listed in Tables 4–6. A plot of ln[TEOS] versus time is shown in Fig. 7, and a plot of ln[MTES] versus time is shown in Fig. 8, and a plot of ln[DDS] versus time is shown in Fig. 9, which show good linearity and proves a is equal to 1 indeed. A plot of ln k 1h versus ln[NH3] are shown in Fig. 10, indicating the plots are linear with a slope equal to b, i.e., b(TEOS) = 0.333, b(MTES) = 0.723 and b(DDS) = 0.652, respectively. Similarly, a plot of ln k 1h versus ln[H2O] are shown in Fig. 11, indicating the plots are linear with a slope equal to c, i.e., c(TEOS) = 0.227, c(MTES) = 0.144 and c(DDS) = 0.080, respectively. Those indicate that TEOS is more sensitive to water than MTES and DDS, which contradicts the tendency in acidic condition supported by Bruent [8]. Finally, the initial hydrolysis rate equations of TEOS, MTES and DDS are 0:333

r ¼ k h ½TEOS½NH3 

r ¼ k h ½MTES½NH3 

½H2 O

0:723

0:227

½H2 O

;

ð7Þ

0:144

ð8Þ

;

1.0

1.0

0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

0.6 0.4 0.2 0.0 0

0.4 0.2

50 100 150 200 250 300 350 400 Time(min)

0

Time(min)

1.0

1.0

0.8

0.8

0.6 0.4 0.2

0.6 0.4 0.2

0.0

0.0 0

50

100

150

200

250

300

Time(min)

(c)

0

50 100 150 200 250 300 350 400

(d)

Time(min)

1.0

1.0

0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

50 100 150 200 250 300 350 400

(b)

[Soluble silica](M)

[Soluble silica](M)

0.6

0.0

(a)

0.6 0.4 0.2 0.0

0.6 0.4 0.2 0.0

0

0

50 100 150 200 250 300 350 400 (f)

Time(min)

(e)

1.0

0.8

0.8

0.6 0.4 0.2

50

100 150 200 250 300 350 400 Time(min)

1.0 [Soluble silica](M)

[Soluble silica](M)

65

0.6 0.4 0.2 0.0

0.0 0

50 100 150 200 250

(g)

0

300 350 400 (h)

Time(min)

50 100 150 200 250 300 350 400 Time(min)

Fig. 4. The time dependences of all the soluble silica concentrations at the molar ratios of TEOS:CH3OH:H2O:NH3 are (a) 1/12.5/4/0.18 (25 C), (b) 1/12.5/4/0.18 (35 C), (c) 1/12.5/4/0.18 (45 C), (d) 1/12.0/4.0/0.045 (25 C), (e) 1/12.5/4.0/0.36 (25 C), (f) 1/12.0/2.0/0.18 (25 C), (g) 1/11.6/5.7/0.18 (25 C), (h) 1/12.0/7.5/0.18 (25 C); j – Q00 , d – Q10 , m – Q30 (lines are given as guides for the eye).

0:652

r ¼ k h ½DDS½NH3 

½H2 O

0:080

:

ð9Þ

The temperature dependence of rate constants can be presented by an Arrhenius equation ln k h ¼ ln A
Ea =RT :

ð10Þ

In the case of each ethoxysilane, the temperature effect on the rate constant was conducted at 25 C (a), 35 C (b) and 45 C (c), respectively (Fig. 12). The standard method for obtaining Ea is to graph experimental rate constant data on an Arrhenius plot, i.e., lnkh versus

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

1.0

1.0

0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

66

0.6 0.4 0.2 0.0

0.6 0.4 0.2 0.0

0

50

0

100 150 200 250 300 350 400 Time(min)

(a)

50

100 150 200 250 300 350 400

(b)

Time(min) 1.0 0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

1.0

0.6 0.4 0.2

0.4 0.2 0.0

0.0 0

50 100 150 200 250 300 350 400

(c)

Time(min)

0

1.0

0.8

0.8

0.4 0.2

100 150 200 250 300 350 400 Time(min)

1.0

0.6

50

(d)

[Soluble silica](M)

[Soluble silica](M)

0.6

0.0

0.6 0.4 0.2 0.0

0

50 100 150 200 250 300 350 400

(e)

Time(min)

0

50

100 150 200 250 300 350 400

(f)

Time(min)

[Soluble silica](M)

1.0 0.8 0.6 0.4 0.2 0.0 0 (g)

50

100 150 200 250 300 350 400 Time(min)

Fig. 5. The time dependences of all the soluble silica concentrations at the molar ratios of MTES:CH3OH:H2O:NH3 are (a) 1.0/13.5/3.0/0.21 (25 C), (b) 1.0/13.5/3.0/0.21 (35 C), (c) 1.0/13.5/3.0/0.21 (45 C), (d) 1.0/13.3/3.0/0.11 (25 C), (e) 1.0/13.4/3.0/0.46 (25 C), (f) 1.0/14.1/1.5/0.21 (25 C), (g) 1.0/ 12.4/6.0/0.18 (25 C); j – T00 , d – T10 , m – T30 (lines are given as guides for the eye).

1/T. The slope gives
Ea/R, where R = 8.3145 J mol
1 K
1. Thus Ea were obtained, i.e., Ea(TEOS) = 24.85 kJ mol
1, Ea(MTES) = 27.24 kJ mol
1 and Ea(DDS) = 22.72 kJ mol
1.

4. Discussion Firstly, the comparison was discussed about the substituent effect on the chemical shift of monomer and

1.0

1.0

0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

0.6 0.4 0.2

0.6 0.4 0.2 0.0

0.0 0

0

50 100 150 200 250 300 350 400 Time(min)

(b)

1.0

1.0

0.8

0.8

[Soluble silica](M)

[Soluble silica](M)

(a)

0.6 0.4 0.2 0.0

50 100 150 200 250 300 350 400 Time(min)

0.6 0.4 0.2 0.0

0

(c)

50 100 150 200 250 300 350 400 Time(min)

0

(d)

50 100 150 200 250 300 350 400 Time(min)

1.0

1.0 0.8

[Soluble silica](M)

[Soluble silica](M)

67

0.6 0.4 0.2

0.8 0.6 0.4 0.2 0.0

0.0 0

(e)

50 100 150 200 250 300 350 400 Time(min)

0

(f)

50 100 150 200 250 300 350 400 Time(min)

[Soluble silica](M)

1.0 0.8 0.6 0.4 0.2 0.0 0

(g)

50 100 150 200 250 300 350 400 Time(min)

Fig. 6. The time dependences of all the soluble silica concentrations at the molar ratios of DDS:CH3OH:H2O:NH3 are (a) 1.0/14.5/2.0/0.21 (25 C), (b) 1.0/14.5/2.0/0.21 (35 C), (c) 1.0/14.5/2.0/0.21 (45 C), (d) 1.0/14.5/2.0/0.11 (25 C), (e) 1.0/14.5/2.0/0.42 (25 C), (f) 1.0/15.0/1.0/0.21 (25 C), (g) 1.0/ 13.8/4.0/0.21 (25 C); n – D20 , . – D10 , m – D3, j – D00 , q – unknown species, h – D1, d – D4, s – D02 (lines are given as guides for the eye).

hydrolyzed reactants. From Figs. 1–3 and Tables 4–6, on the one hand, it can be observed that the values of 29 Si NMR chemical shift (d) for each pure ethoxysilanes are very different, which was due to the inductive effect of the substituent groups [5,18]. Generally, the increase of the net positive charge on the silicon (Si+) leads to

an upfield shift (more negative d). The methyl group is a weak electron donor, while the ethoxyl is a moderate electron withdrawer [15]. Hence, sequential replacement of ethoxyl groups by methyl groups would cause a decrease in positive charge on the silica, which leads to a downfield shift (more positive d than non-substituted

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R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

0.0

0

-0.4

-1

-0.8

-2 [DDS](M)

ln[TEOS]

-1.2 -1.6 -2.0 -2.4

(b) (d) (g)

(a) (c) (f)

-2.8 -3.2 -3.6 0

50

100

-3 -4 (a) (c) (e)

-5 (e) (h)

150

200

-6 250

300

350

400

-7

0

50

(b) (d) (f) 100

Time(min) Fig. 7. The time dependences of TEOS monomer concentration at the molar ratios of TEOS:CH3OH:H2O:NH3 are (a) 1/12.5/4/0.18 (25 C), (b) 1/12.5/4/0.18 (35 C), (c) 1/12.5/4/0.18 (45 C), (d) 1/12.0/4.0/0.045 (25 C), (e) 1/12.5/4.0/0.36 (25 C), (f) 1/12.0/2.0/0.18 (25 C), (g) 1/ 11.6/5.7/0.18 (25 C), (h) 1/12.0/7.5/0.18 (25 C). The slope of the straight line is the pseudo-first-order rate constant (k 1h ) for the hydrolysis reaction.

0.0

-4.4

-0.4

-4.8 lnk1h

ln[MTES]

150 200 250 Time(min)

300

350

400

Fig. 9. DDS monomer concentration at the molar ratios of DDS:CH3OH:H2O:NH3 are (a) 1.0/14.5/2.0/0.21 (25 C), (b) 1.0/ 14.5/2.0/0.21 (35 C), (c) 1.0/14.5/2.0/0.21 (45 C), (d) 1.0/14.5/2.0/0.11 (25 C), (e) 1.0/14.5/2.0/0.42 (25 C), (f) 1.0/15.0/1.0/0.21 (25 C), (g) 1.0/13.8/4.0/0.21 (25 C). The slope of the straight line is the pseudofirst-order rate constant (k 1h ) for the hydrolysis reaction.

-4.0

-0.8

(g)

TEOS MTES DDS

-5.2 -5.6

-1.2 (b) (d) (f)

(a) (c) (e)

-1.6

-6.0 -6.4 (g)

-6.8

-2.0 0

50

100

150 200 250 Time(min)

300

350

-3.2

-2.8

400

Fig. 8. The time dependences of MTES monomer concentration at the molar ratios of MTESD:CH3OH:H2O:NH3 are (a) 1.0/13.5/3.0/0.21 (25 C), (b) 1.0/13.5/3.0/0.21 (35 C), (c) 1.0/13.5/3.0/0.21 (45 C), (d) 1.0/13.3/3.0/0.11 (25 C), (e) 1.0/13.4/3.0/0.46 (25 C), (f) 1.0/14.1/1.5/ 0.21 (25 C), (g) 1.0/12.4/6.0/0.18 (25 C). The slope of the straight line is the pseudo-first-order rate constant (k 1h ) for the hydrolysis reaction.

-2.4

-2.0 -1.6 ln[NH3]

-0.8

-0.4

Fig. 10. The pseudo-first-order rate constant k 1h versus ammonia concentration for TEOS, MTES and DDS system.

-3.4 -3.6 -3.8

TEOS MTES

-4.0

lnk1h

TEOS). At the same time, similar to the viewpoint provided by Lippert et al. [19], the more ethoxyls are substituted for methyls, the more positive d is observed. On the other hand, all of the hydrolysis reactants had downfield shifts compared with that of each corresponding monomer. The fact could be explained that, when ethoxyl group is substituted for hydroxyl group, the positive charge on silica decreases owing to the higher ionic character of the hydroxyl group than the ethoxyl group [7]. A common conclusion is drawn that the chemical shift has an approximately +2 ppm shift after each replacement of the ethoxyl group by the hydroxyl group, whatever the ethoxysilanes is TEOS, MTES or DDS.

-1.2

DDS

-4.2 -4.4 -4.6 -4.8 -5.0

0.0

0.4

0.8 1.2 ln[H2O]

1.6

2.0

Fig. 11. The pseudo-first-order rate constant k 1h versus water concentration for TEOS, MTES and DDS system.

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

-3.0

lnk1h

-3.5

TEOS

-4.0

MTES DDS -4.5

-5.0

3.15

3.20

3.25

3.30

3.35

1/T*103 Fig. 12. The pseudo-first-order rate constant k 1h versus temperature for TEOS, MTES and DDS system at 25, 35 and 45 C.

Comparing to TEOS and MTES, the hydrolysis and condensation resultants of DDS were very complex.

69


8.92 ppm and D11 at
9.45 ppm) of Sugahara et al. [9] and Civaller et al. [18]. It is worthy to notice that cyclic trimer and tetramer, as well as linear species, formed by the hydrolysis and polycondensation reactions of DDS. According to the previous report of acid-catalyzed DDS [9], the cyclic trimer D3 appeared at
1.23 ppm and the cyclic tetramer D4 appeared at
12 ppm. These cyclic polymers have large downfield shift from the linear D02 region, which may be attributed to reduction of the SiAOASi angle in more strained cyclics structures [21]. Secondly, a comparison was discussed about the substituent effect on the relative reactivity of ethoxysilanes, and then the hydrolysis mechanism must be understood well. Under basic conditions, it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid step, and then the hydroxyl anion then attacks the silicon atom. Iler [22] and Keefer [23] proposed an SN2-Si mechanism in which OH
displaces OR
with inversion of the silicon tetrahedron.

ð12Þ

Not only hydrolyzed monomers but also dimers, trimers, and tetramers peaks were observed in Fig. 3. Firstly, the chemical shifts for the hydrolyzed resultants of DDS (D10 and D20 ) have downfield shifts compared with that of DDS monomer (D00 ) under basic conditions. The trend is a good agreement with TEOS and MTES in this paper, but is contrary to the results of Hook [5] and Sugahara et al. [9]. In their studies, under acid conditions, the chemical shifts for the hydrolyzed resultants of DDS appeared at the more negative d relative to the DDS peak. Under acid-catalyzed conditions, the hydrolyzed species of DDS were likely to be protonated [20], which was illuminated by the reaction (11):

ð11Þ

The protonation of the silanol increases the positive charge on silica, which leads to an upfield shift (more negative d). However, under basic conditions, it is difficult for the protonation of the silanol. This explanation could be applied to reassignments of D11 and D01 , that is, a reverse occurred between them (D11 at
8.92 ppm and D01 at
9.45 ppm), opposite to the observations (D01 at

With monomer hydrolysis proceeding, AOR is gradually substituted for AOH. The electron withdrawing capabilities of AOH is stronger than that of AOR, which benefits the nucleophilic attack of OH
to Si+, so subsequent hydrolysis occurs more quickly. At the same time, it is a common that the condensation rate of ethoxysilanes is faster than hydrolysis rate under basic conditions. From the standpoints above, a conclusion can be drawn that the initial hydrolysis of each monomer is the rate-limiting step of the total reactions [14,24]. That is to say, as long as the rate constant of the rate-limiting step for different monomer could be obtained, the characters of the total reaction would be known. From Tables 1–3, the averages of the hydrolysis rate constant kh at 25 C for each ethoxysilane were obtained: kh(TEOS) = 7.41 · 10
3 mol
0.56 dm1.68 min
1, kh(MTES) = 7.27 · 10
3 mol
0.6 dm1.8 min
1 and kh(DDS) = 2.78 · 10
2 mol
0.73 dm2.2 min
1. Thus, it has quantificationally proved that DDS presents the highest reactivity and MTES has the lowest reactivity among the three studied ethoxysilanes. This unexpected order of the initial hydrolysis rate constant disagrees with the result of Schmidt et al. [11], who found that the reactivity order of hydrolysis was DDS < MTES < TEOS. Generally, the methyl group is classified to a weak electron donor, while the ethoxyl is a moderate electron

70

R. Liu et al. / Journal of Non-Crystalline Solids 343 (2004) 61–70

withdrawer. For SN2-Si mechanism, the silicon acquires little negative charge in the transition state, so the replacement electron-providing methyl groups with electron-withdrawing ethoxyl groups resulted in a reduce of the reactive rates. On the contrary, since the hydroxyl anion attacks the silicon atom from the rear, the substituents that reduce crowding in the transition state will enhance the hydrolysis rate. On the whole, the steric effects predominate over inductive effect and become a dominant influence on the reactivity [25]. In details, when one methyl group was substituted for one ethoxyl group, the hydrolysis rate of MTES is slower than that of TEOS. When two methyl groups were substituted for two ethoxyl groups however, the steric hindrance was deduced largely so that the attacking ability of OH
to silicon is increased. Therefore, the initial hydrolysis rate of DDS is the fastest among them, supported by Pouxviel [26], who reported both the hydrolysis- and condensation-rate coefficients of TMOS were significantly greater than TEOS. 5. Conclusions The initial basic-catalyzed hydrolysis and condensation of different substituted ethoxysilanes were studies by liquid-state 29Si NMR. The hydrolysis reactants of DDS had downfield shifts compared with that DDS monomer, which was consistent with the trend of TEOS and MTES, while the result was opposite to the previous studies. The kinetic constants of the initial hydrolysis of ethoxysilanes were obtained, which provided the quantitative evidence that the relative reactivities increased in the order of MTES < TEOS < DDS. This result disagreed with the trend of DDS < MTES < TEOS presented by Schmidt [11] and implied that the second substituent gives a mutational enhancement on the hydrolysis rate. Acknowledgment This work was supported by the National Key Native Science Foundation (20133040).

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