Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5-2.0 and their gels after heat treatment at 130°C. The presence of Si - O - Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2-3 ppm compared to corresponding signals of homo-condensed Si - O - Si bonds. The existence of Si - O - Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si - O - Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti - O - Ti and Si - O - Si bonds resulting in a decreased molecular homogeneity of the materials

Hoebbel, Dagobert

Nacken, Manfred

Schmidt, Helmut K.

Acronym

On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in theSystem: Glycidoxypropyltrimethoxysilane-Water-TitaniumtetraethoxideDAGOBERT HOEBBEL, MANFRED NACKEN AND HELMUT SCHMIDTInstitut fu¨r Neue Materialien, Im Stadtwald, 66123 Saarbru¨cken, GermanyAbstract. Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for opti-cal applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes,their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMRspectroscopy has been used to investigate sols with molar ratios Si : Ti D 1 and H2O : OR (H) D 0:5¡2:0 andtheir gels after heat treatment at 130–C. The presence of Si O Ti bonds in sols with a low water content (H< 0:2)and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of upto 2–3 ppm compared to corresponding signals of homo-condensed Si O Si bonds. The existence of Si O Tibonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavageof the Si O Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bondsand the building up of homo-condensed species leads to a separation into areas with predominantly Ti O Ti andSi O Si bonds resulting in a decreased molecular homogeneity of the materials.Keywords: heterometal bond, hydrolysis, condensation, 17O and 29Si NMR, titanosiloxane1. Introduction3-Glycidoxypropyltrimethoxysilane (GPTS) and tita-nium alkoxides are frequently employed for the prepa-ration of heterometal hybrid polymers which are usedfor example as hard coatings on organic polymers andcontact lens materials in the optical industry [1, 2].A homogeneous distribution of structural units at themolecular level is a prerequisite for materials for op-tical applications and thus phase separations or inho-mogeneities in composition should be kept at a levelas low as possible [3, 4]. In this respect heterometalSi O Ti bonds, their distribution and their hydrolyticstability during sol-gel processing, play an importantrole in the preparation of highly homogeneous mate-rials on a molecular scale. To date a detailed knowl-edge of the homo- and hetero-condensation reactionsof GPTS and Ti-alkoxides during sol-gel processing isnot available. The objective of this work is to attain abetter insight into the reactions and interactions of thecomponents of the system GPTS-H2O-Ti(OC2H5)4 insol and gel states, mainly by means of 29Si and 17ONMR spectroscopy.2. ExperimentalSolutions of the system GPTS-H2O-Ti(OC2H5)4 wereprepared at molar ratios Si : TiD 1 and H2O : alkoxidegroupD 0:5 to 2 (Table 1). 0.1 M HCl was used for allsteps of hydrolysis. The time interval used was 0.5 h.The water content of the sols was determined by Karl-Fischer-titration [5] 0.5 h after their preparation. Thegel samples were prepared from corresponding sols,after storage at room temperature for 24 h followed byheating to 130–C for 5 h. The solid products were thencrushed and heated again for 2 h at 130–C.The 29Si and 17O NMR spectra were obtained usinga Bruker AC 200 spectrometer (4.7 T). 29Si NMR: ex-ternal reference: tetramethylsilane, internal standard:phenyltrimethylsilane, repetition time (r.t.): 40 s, pulseangle (p.a.): 63–, number of scans (n.s.): 45–1500. 17O38 Hoebbel, Nacken and SchmidtTable 1. Flow chart of the preparation of solutions in the system GPTS-H2O-Ti(OC2H5)4at room temperature. Water is always added as 0.1 M HCl.GPTS4:52 mMSol CSi CHCl H2O/ORno. (mole/l) (mmole/l) molar ratio1 2.87 — — ¡ˆ Ethanol8:58 mM0.5 h2 2.66 7.2 0.5 ¡ˆ H2O6:78 mM0.5 h3 1.26 3.4 0.21 ¡ˆ Ti.OC2H5/4/Ethanol4:52 mM 16:12 mM0.5 h4 1.16 7.3 0.5 ¡ˆ H2O/Ethanol9:0 mM 2:78 mM0.5 h5 1.01 12.7 1.0 ¡ˆ H2O/Ethanol15:78 mM 4:88 mM0.5 h6 0.81 20.3 2.0 ¡ˆ H2O/Ethanol31:56 mM 9:74 mMNMR: single pulse, r.t.: 300 ms, p.a.: 90–, referenceH2O (1% 17O), n.s.: 8000–20,000. The 17O NMR mea-surements start from 17O labelled GPTS-hydrolysate(sol 2) using 0.1 M HCl derived from 17O (10%)enriched water. The subsequent hydrolysis steps (sol4–6) were carried out with non-labelled 0.1 M HCl.The NMR measurements of solutions 2–6 were started15 min after their preparation. The 29Si NMR spectrawere accumulated for 0.5 h (solution 2) or 15 h (solu-tions 3–6). The 29Sif1Hg inverse gated sequence wasused for solid state 29Si NMR spectra. External stan-dard: Q8M8, MAS: 3 kHz, p.a.: 63–, r.t.: 60 s, n.s.:200–1000.3. Results and DiscussionThe data in Table 2 show that the amount of waterin sol 2 decreases strongly from 0.5 to 0.13 H2O/ORwithin 0.5 h of starting hydrolysis. The remaining wa-ter in sol 2 is small enough to prevent rapid homo-condensation reactions of the Ti-ethoxide and precipi-tations [6]. Practically, the water in sol 2 is completelyconsumed by the added Ti-ethoxide for partial hydro-Table 2. Content of water in the standard sols 2–6 after0.5 h reaction time.Sol no. Remaining water(Table 1) Addition of water (0.5 h after addition)2 0.50 H2O/OR 0.13 H2O/OR3 (0.21 H2O/OR) 0.02 H2O/OR4 0.50 H2O/OR 0.12 H2O/OR5 1.0 H2O/OR 0.54 H2O/OR6 2.0 H2O/OR 1.53 H2O/ORlysis of its alkoxide groups (sol 3). The amount of freewater in the sols 4–6 is increased by the stepwise addi-tion of water. A nearly constant consumption of 0.46H2O/OR is obtained in the sols 5 and 6.29Si and 17O NMR Examinationon Heterometal BondsThe 29Si NMR spectrum of the GPTS-prehydrolysate(sol 2) shows a variety of signals in the region ofchemical shifts –D¡40 to ¡44 ppm. This region ischaracteristic for monomeric silanes whose assignmentTitanosiloxane Bonds in the System 39Figure 1. 29Si NMR spectrum of the GPTS-hydrolysate (0.5 H2O/OR, sol 2) and assignment of the signals.follows from Fig. 1. Practically, the intensity of theseven visible signals represents the total content ofSi-atoms so that condensed species can be neglected.From the spectrum it follows that about 50% of thealkoxy-groups are hydrolyzed. The dominant amountof monomeric silanols and the absence of condensedsiloxanes gives a good basis to examine the reaction ofthese Si-species with the Ti-tetraethoxide.The 29Si NMR spectrum of solution 3 after additionof Ti-ethoxide shows three sharp signals at –D¡42:44,¡43:58, and¡44:74 ppm which were identified as un-reacted GPTS (4% of the total signal intensity) and themonomeric silanes T0 (2 OCH3, 1 OC2H5) (18%) andT0 (1 OCH3, 2 OC2H5) (13%) (Fig. 2, I). Furthermore,two broad signals at –D¡53:2 (40%) and ¡61:1 ppm(25%) are visible in the region of T1 and T2 units im-plying that the predominant amount of Si in sol 3 ispresent as a condensed species. The stepwise additionof further water to sol 3 leads to the spectra of sols 4and 6 shown in Fig. 2, II and III; no further signalsof monomeric species are detected. The position ofthe three signals in the spectrum of sol 6 .–D¡49:7(14%), ¡58:0 (64%) and ¡65:6 ppm (22%)) is in ac-cordance with homo-condensed Si-sites in T1, T2 andT3 building groups of a long-term condensed GPTShydrolysate (Fig. 2, IV). Comparing spectrum I withIII a significant high-field shift of the T1 and T2 signalsof up to 2–3 ppm can be seen. This shift cannot becaused by the influence of the Ti-atom on the chemicalshift of the Si-atoms alone; this latter shift contributes0.3 ppm at the most. It is very likely that the two sig-nals in the spectrum of sol 3 reflect Si-atoms in hetero-metal Si O Ti bonds. Such a high field shift has beenfound for Si-atoms in Ti O Si O Ti bonds in pre-vious works on titanodiphenylsiloxanes [7]. From theresult it is concluded that after addition of water to sol3 a degradation of the titanosiloxane bonds occurs infavor of homo-condensed siloxane bonds.40 Hoebbel, Nacken and SchmidtFigure 2. 29Si NMR spectra of the sols 3, 4 and 6 (see Table 1) and of the GPTS-hydrolysate (2 H2O/OR) after 15d.The solid state 29Si NMR spectrum of the Ti-containing gel 3 (Fig. 3, II) shows three broad sig-nals with similar high-field shifts (¡53.3 (32%),¡60.3(47%),¡68.9 ppm (16%)) as those seen in the spectrumof the corresponding sol 3. This signal shift shows thatspecies with Si O Ti bonds remain after the sol-geltransformation. The spectrum of gel 6 has two over-lapped signals around –D¡58 (56%) and ¡66 ppm(44%) whose maxima are in good agreement with thoseof the signals for homo-condensed Si O Si bonds ofthe reference gel (Fig. 3, IV).17O NMR was used to prove the existence ofSi O Ti bonds. The 17O NMR spectrum of the 17Olabelled GPTS hydrolysate (sol 2) shows strong signalsat –D 26 and ¡8 ppm (Fig. 4, I) which are caused bythe Si 17OH groups of the prehydrolyzed GPTS andby 17O labelled water [8]. The signal at –D 575 ppmderives from deutero-acetone used as external lockand standard. A dramatic decrease in intensity of theSiOH and H2O signal appears after the addition of Ti-ethoxide to sol 2. Meanwhile a broad asymmetricalsignal centered around –D 340 ppm and signals at 540and around 750 ppm develop (Fig. 4, II). The resonanceat 340 ppm is attributed to O-atoms in Si O Ti bonds[7, 8], the signal at 540 ppm to „3- and those around750 ppm to „2-Ti O Ti bonds according to [9]. Itcan be concluded from the 17O NMR spectrum thatthe SiOH species in GPTS-hydrolysate are consumedTitanosiloxane Bonds in the System 41Figure 3. Solid state 29Si NMR spectra of the gels 2, 3, 6 and the reference gel derived from GPTS-hydrolysate (2 H2O/OR) by heat treatmentat 130–C.for hetero-condensation reaction with the Ti-ethoxideleading to Si O Ti bonds immediately after the Ti-ethoxide addition.After repeated addition of 0.1 M HCl the intensityof the signals at 340 and 540 ppm decreases and a sig-nal around 0 ppm appears (Fig. 4, III–V). This signalis mainly caused by 17O labelled water and possiblysome Si17OH groups but it can hardly be caused bycondensed Si 17O Si species which should appear inthe region at –D 50–80 ppm. The H217O signal mainlyderives from 17O-atoms in Si 17O Ti bonds. Thismeans that the Si 17O Ti bonds undergo a quick iso-tope exchange during their cleavage with non-labelledwater. The quick 17O isotope exchange leads even-tually to mostly non-labelled Si O Si bonds in thecondensation products which cannot be detected in the17O NMR but they are detectable in the correspond-ing 29Si NMR spectra. No significant signals can befound for homo-condensed Ti 17O Ti species. It isprobable that the signals are overlapped by the broadoscillation bands or that the 17O isotope is exchangedin favor of 17O labelled water.42 Hoebbel, Nacken and SchmidtFigure 4. 17O NMR spectra of the sols 2–6 (see Table 1).4. ConclusionsGPTS-prehydrolysates which contain monomeric sila-nols and a low content of free water (0.13 H2O/OR)react with titanium-tetraethoxide to form sols mainlyconsisting of titanosiloxane species. The heterometalbonds can be identified by the high-field shift of theT1 and T2 signals of up to 2–3 ppm in the liquid andsolid state 29Si NMR spectra of the sols and corres-ponding gels. The existence of titanosiloxane bondsis confirmed by the signal at –D 340 ppm in the 17ONMR spectra. Considerable hydrolytic cleavage of theheterometal bonds can be detected with increasingamounts of water. Mainly species with homo-conden-sed Si O Si bonds appear in the sols and correspond-ing gels after the hydrolysis at molar ratio H2O/ORD 2.The drastic cleavage of the heterometal bonds and thepreferential building up of homo-condensed Si O Sispecies and probably also the species with Ti O Tibonds lead to a separation in (RSiO1:5)x and TiO2 richareas which lower the homogeneous distribution of Siand Ti on a molecular scale. This result has to be con-sidered for more controlled syntheses of highly homo-geneous heterometal materials.Titanosiloxane Bonds in the System 43AcknowledgmentThe authors gratefully acknowledge Ms. S. Carstensenfor experimental work.References1. H. Schmidt, B. Seiferling, G. Philip, and K. Deichmann, in Ultra-structure Processing of Advanced Ceramics, edited by J.D.Mackenzie and D.R. Ulrich (J. Wiley, New York, 1988), pp. 651–660.2. JP 6126637 in C.A. 105, 80819 (1986).3. J.D. Basil and C.C. Lin, in Ultrastructure Processing of AdvancedCeramics, edited by J.D. Mackenzie and D.R. Ulrich (J. Wiley,New York, 1988), pp. 783–794.4. H. Schmidt, in Chemical Processing of Advanced Materials,edited by L.L. Hench and J.K. West (J. Wiley, New York, 1992),pp. 727–735.5. E. Scholz, Karl-Fischer-Titration (Springer-Verlag, Berlin,1984).6. H. Schmidt, in Structure and Bonding, edited by R. Reisfeld andC.K. Jorgensen (Springer-Verlag, Berlin, 1992), pp. 119–151.7. D. Hoebbel, M. Nacken, H. Schmidt, V. Huch, and M. Veith,submitted to J. Mater. Chem. (1997).8. F. Babonneau, Mat. Res. Soc. Symp. Proc. 346, 949–960(1994).9. V.W. Day, T.A. Ebersbacher, W.G. Klemperer, C.W. Park, andF.S. Rosenberg, J. Am. Chem. Soc. 113, 8190–8192 (1991).

On the existence and hydrolytic stability of titanosiloxane bonds in the system : glycidoxypropyltrimethoxysilane-water-titaniumtetraethoxide

Universaar

Scientific documents from the Saarland University,

On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in the
System: Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide
DAGOBERT HOEBBEL, MANFRED NACKEN AND HELMUT SCHMIDT
Institut fu¨r Neue Materialien, Im Stadtwald, 66123 Saarbru¨cken, Germany
Abstract. Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for opti-
cal applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes,
their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR
spectroscopy has been used to investigate sols with molar ratios Si : Ti D 1 and H2O : OR (H) D 0:5¡2:0 and
their gels after heat treatment at 130–C. The presence of Si O Ti bonds in sols with a low water content (H< 0:2)
and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up
to 2–3 ppm compared to corresponding signals of homo-condensed Si O Si bonds. The existence of Si O Ti
bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage
of the Si O Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds
and the building up of homo-condensed species leads to a separation into areas with predominantly Ti O Ti and
Si O Si bonds resulting in a decreased molecular homogeneity of the materials.
Keywords: heterometal bond, hydrolysis, condensation, 17O and 29Si NMR, titanosiloxane
1. Introduction
3-Glycidoxypropyltrimethoxysilane (GPTS) and tita-
nium alkoxides are frequently employed for the prepa-
ration of heterometal hybrid polymers which are used
for example as hard coatings on organic polymers and
contact lens materials in the optical industry [1, 2].
A homogeneous distribution of structural units at the
molecular level is a prerequisite for materials for op-
tical applications and thus phase separations or inho-
mogeneities in composition should be kept at a level
as low as possible [3, 4]. In this respect heterometal
Si O Ti bonds, their distribution and their hydrolytic
stability during sol-gel processing, play an important
role in the preparation of highly homogeneous mate-
rials on a molecular scale. To date a detailed knowl-
edge of the homo- and hetero-condensation reactions
of GPTS and Ti-alkoxides during sol-gel processing is
not available. The objective of this work is to attain a
better insight into the reactions and interactions of the
components of the system GPTS-H2O-Ti(OC2H5)4 in
sol and gel states, mainly by means of 29Si and 17O
NMR spectroscopy.
2. Experimental
Solutions of the system GPTS-H2O-Ti(OC2H5)4 were
prepared at molar ratios Si : TiD 1 and H2O : alkoxide
groupD 0:5 to 2 (Table 1). 0.1 M HCl was used for all
steps of hydrolysis. The time interval used was 0.5 h.
The water content of the sols was determined by Karl-
Fischer-titration [5] 0.5 h after their preparation. The
gel samples were prepared from corresponding sols,
after storage at room temperature for 24 h followed by
heating to 130–C for 5 h. The solid products were then
crushed and heated again for 2 h at 130–C.
The 29Si and 17O NMR spectra were obtained using
a Bruker AC 200 spectrometer (4.7 T). 29Si NMR: ex-
ternal reference: tetramethylsilane, internal standard:
phenyltrimethylsilane, repetition time (r.t.): 40 s, pulse
angle (p.a.): 63–, number of scans (n.s.): 45–1500. 17O
38 Hoebbel, Nacken and Schmidt
Table 1. Flow chart of the preparation of solutions in the system GPTS-H2O-Ti(OC2H5)4
at room temperature. Water is always added as 0.1 M HCl.
GPTS
4:52 mM
Sol CSi CHCl H2O/OR
no. (mole/l) (mmole/l) molar ratio
1 2.87 — — ¡ˆ Ethanol8:58 mM
0.5 h
2 2.66 7.2 0.5 ¡ˆ H2O6:78 mM
0.5 h
3 1.26 3.4 0.21 ¡ˆ Ti.OC2H5/4/Ethanol4:52 mM 16:12 mM
0.5 h
4 1.16 7.3 0.5 ¡ˆ H2O/Ethanol9:0 mM 2:78 mM
0.5 h
5 1.01 12.7 1.0 ¡ˆ H2O/Ethanol15:78 mM 4:88 mM
0.5 h
6 0.81 20.3 2.0 ¡ˆ H2O/Ethanol31:56 mM 9:74 mM
NMR: single pulse, r.t.: 300 ms, p.a.: 90–, reference
H2O (1% 17O), n.s.: 8000–20,000. The 17O NMR mea-
surements start from 17O labelled GPTS-hydrolysate
(sol 2) using 0.1 M HCl derived from 17O (10%)
enriched water. The subsequent hydrolysis steps (sol
4–6) were carried out with non-labelled 0.1 M HCl.
The NMR measurements of solutions 2–6 were started
15 min after their preparation. The 29Si NMR spectra
were accumulated for 0.5 h (solution 2) or 15 h (solu-
tions 3–6). The 29Sif1Hg inverse gated sequence was
used for solid state 29Si NMR spectra. External stan-
dard: Q8M8, MAS: 3 kHz, p.a.: 63–, r.t.: 60 s, n.s.:
200–1000.
3. Results and Discussion
The data in Table 2 show that the amount of water
in sol 2 decreases strongly from 0.5 to 0.13 H2O/OR
within 0.5 h of starting hydrolysis. The remaining wa-
ter in sol 2 is small enough to prevent rapid homo-
condensation reactions of the Ti-ethoxide and precipi-
tations [6]. Practically, the water in sol 2 is completely
consumed by the added Ti-ethoxide for partial hydro-
Table 2. Content of water in the standard sols 2–6 after
0.5 h reaction time.
Sol no. Remaining water
(Table 1) Addition of water (0.5 h after addition)
2 0.50 H2O/OR 0.13 H2O/OR
3 (0.21 H2O/OR) 0.02 H2O/OR
4 0.50 H2O/OR 0.12 H2O/OR
5 1.0 H2O/OR 0.54 H2O/OR
6 2.0 H2O/OR 1.53 H2O/OR
lysis of its alkoxide groups (sol 3). The amount of free
water in the sols 4–6 is increased by the stepwise addi-
tion of water. A nearly constant consumption of 0.46
H2O/OR is obtained in the sols 5 and 6.
29Si and 17O NMR Examination
on Heterometal Bonds
The 29Si NMR spectrum of the GPTS-prehydrolysate
(sol 2) shows a variety of signals in the region of
chemical shifts –D¡40 to ¡44 ppm. This region is
characteristic for monomeric silanes whose assignment
Titanosiloxane Bonds in the System 39
Figure 1. 29Si NMR spectrum of the GPTS-hydrolysate (0.5 H2O/OR, sol 2) and assignment of the signals.
follows from Fig. 1. Practically, the intensity of the
seven visible signals represents the total content of
Si-atoms so that condensed species can be neglected.
From the spectrum it follows that about 50% of the
alkoxy-groups are hydrolyzed. The dominant amount
of monomeric silanols and the absence of condensed
siloxanes gives a good basis to examine the reaction of
these Si-species with the Ti-tetraethoxide.
The 29Si NMR spectrum of solution 3 after addition
of Ti-ethoxide shows three sharp signals at –D¡42:44,
¡43:58, and¡44:74 ppm which were identified as un-
reacted GPTS (4% of the total signal intensity) and the
monomeric silanes T0 (2 OCH3, 1 OC2H5) (18%) and
T0 (1 OCH3, 2 OC2H5) (13%) (Fig. 2, I). Furthermore,
two broad signals at –D¡53:2 (40%) and ¡61:1 ppm
(25%) are visible in the region of T1 and T2 units im-
plying that the predominant amount of Si in sol 3 is
present as a condensed species. The stepwise addition
of further water to sol 3 leads to the spectra of sols 4
and 6 shown in Fig. 2, II and III; no further signals
of monomeric species are detected. The position of
the three signals in the spectrum of sol 6 .–D¡49:7
(14%), ¡58:0 (64%) and ¡65:6 ppm (22%)) is in ac-
cordance with homo-condensed Si-sites in T1, T2 and
T3 building groups of a long-term condensed GPTS
hydrolysate (Fig. 2, IV). Comparing spectrum I with
III a significant high-field shift of the T1 and T2 signals
of up to 2–3 ppm can be seen. This shift cannot be
caused by the influence of the Ti-atom on the chemical
shift of the Si-atoms alone; this latter shift contributes
0.3 ppm at the most. It is very likely that the two sig-
nals in the spectrum of sol 3 reflect Si-atoms in hetero-
metal Si O Ti bonds. Such a high field shift has been
found for Si-atoms in Ti O Si O Ti bonds in pre-
vious works on titanodiphenylsiloxanes [7]. From the
result it is concluded that after addition of water to sol
3 a degradation of the titanosiloxane bonds occurs in
favor of homo-condensed siloxane bonds.
40 Hoebbel, Nacken and Schmidt
Figure 2. 29Si NMR spectra of the sols 3, 4 and 6 (see Table 1) and of the GPTS-hydrolysate (2 H2O/OR) after 15d.
The solid state 29Si NMR spectrum of the Ti-
containing gel 3 (Fig. 3, II) shows three broad sig-
nals with similar high-field shifts (¡53.3 (32%),¡60.3
(47%),¡68.9 ppm (16%)) as those seen in the spectrum
of the corresponding sol 3. This signal shift shows that
species with Si O Ti bonds remain after the sol-gel
transformation. The spectrum of gel 6 has two over-
lapped signals around –D¡58 (56%) and ¡66 ppm
(44%) whose maxima are in good agreement with those
of the signals for homo-condensed Si O Si bonds of
the reference gel (Fig. 3, IV).
17O NMR was used to prove the existence of
Si O Ti bonds. The 17O NMR spectrum of the 17O
labelled GPTS hydrolysate (sol 2) shows strong signals
at –D 26 and ¡8 ppm (Fig. 4, I) which are caused by
the Si 17OH groups of the prehydrolyzed GPTS and
by 17O labelled water [8]. The signal at –D 575 ppm
derives from deutero-acetone used as external lock
and standard. A dramatic decrease in intensity of the
SiOH and H2O signal appears after the addition of Ti-
ethoxide to sol 2. Meanwhile a broad asymmetrical
signal centered around –D 340 ppm and signals at 540
and around 750 ppm develop (Fig. 4, II). The resonance
at 340 ppm is attributed to O-atoms in Si O Ti bonds
[7, 8], the signal at 540 ppm to „3- and those around
750 ppm to „2-Ti O Ti bonds according to [9]. It
can be concluded from the 17O NMR spectrum that
the SiOH species in GPTS-hydrolysate are consumed
Titanosiloxane Bonds in the System 41
Figure 3. Solid state 29Si NMR spectra of the gels 2, 3, 6 and the reference gel derived from GPTS-hydrolysate (2 H2O/OR) by heat treatment
at 130–C.
for hetero-condensation reaction with the Ti-ethoxide
leading to Si O Ti bonds immediately after the Ti-
ethoxide addition.
After repeated addition of 0.1 M HCl the intensity
of the signals at 340 and 540 ppm decreases and a sig-
nal around 0 ppm appears (Fig. 4, III–V). This signal
is mainly caused by 17O labelled water and possibly
some Si17OH groups but it can hardly be caused by
condensed Si 17O Si species which should appear in
the region at –D 50–80 ppm. The H217O signal mainly
derives from 17O-atoms in Si 17O Ti bonds. This
means that the Si 17O Ti bonds undergo a quick iso-
tope exchange during their cleavage with non-labelled
water. The quick 17O isotope exchange leads even-
tually to mostly non-labelled Si O Si bonds in the
condensation products which cannot be detected in the
17O NMR but they are detectable in the correspond-
ing 29Si NMR spectra. No significant signals can be
found for homo-condensed Ti 17O Ti species. It is
probable that the signals are overlapped by the broad
oscillation bands or that the 17O isotope is exchanged
in favor of 17O labelled water.
42 Hoebbel, Nacken and Schmidt
Figure 4. 17O NMR spectra of the sols 2–6 (see Table 1).
4. Conclusions
GPTS-prehydrolysates which contain monomeric sila-
nols and a low content of free water (0.13 H2O/OR)
react with titanium-tetraethoxide to form sols mainly
consisting of titanosiloxane species. The heterometal
bonds can be identified by the high-field shift of the
T1 and T2 signals of up to 2–3 ppm in the liquid and
solid state 29Si NMR spectra of the sols and corres-
ponding gels. The existence of titanosiloxane bonds
is confirmed by the signal at –D 340 ppm in the 17O
NMR spectra. Considerable hydrolytic cleavage of the
heterometal bonds can be detected with increasing
amounts of water. Mainly species with homo-conden-
sed Si O Si bonds appear in the sols and correspond-
ing gels after the hydrolysis at molar ratio H2O/ORD 2.
The drastic cleavage of the heterometal bonds and the
preferential building up of homo-condensed Si O Si
species and probably also the species with Ti O Ti
bonds lead to a separation in (RSiO1:5)x and TiO2 rich
areas which lower the homogeneous distribution of Si
and Ti on a molecular scale. This result has to be con-
sidered for more controlled syntheses of highly homo-
geneous heterometal materials.
Titanosiloxane Bonds in the System 43
Acknowledgment
The authors gratefully acknowledge Ms. S. Carstensen
for experimental work.
References
1. H. Schmidt, B. Seiferling, G. Philip, and K. Deichmann, in Ultra-
structure Processing of Advanced Ceramics, edited by J.D.
Mackenzie and D.R. Ulrich (J. Wiley, New York, 1988), pp. 651–
660.
2. JP 6126637 in C.A. 105, 80819 (1986).
3. J.D. Basil and C.C. Lin, in Ultrastructure Processing of Advanced
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On the existence and hydrolytic stability of titanosiloxane bonds in the system : glycidoxypropyltrimethoxysilane-water-titaniumtetraethoxide

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