Structure and hydrolysis of a complex alkoxide as a cordierite precursor

JOURNA L OF

Journal of Non-Crystalline Solids 139 (1992) 205-214 North-Holland

NON-CR~TALLINE SOLIDS

Structure and hydrolysis of a complex alkoxide as a cordierite precursor Toshimi Fukui 1, Chihiro Sakurai 2 and Masahiko O k u y a m a 3 Colloid Research Institute, 350-10gura, Yahata-higashi-ku, Kitakyushu 805, Japan Received 16 February 1991 Revised manuscript received 5 October 1991

A complex alkoxide as a cordierite precursor was synthesized by reaction of alkoxides. This precursor alkoxide structure was analyzed by IR, Raman, 27Al_ and 29Si-NMR spectroscopy. Hydrolysis and condensation of the complex alkoxide were investigated by 27A1- and 29Si-NMR spectroscopy. The proposed structure of the complex alkoxide consists of Mg-AI double alkoxide containing 4-coordinated AI atoms and silicates such as (EtO)3SiO- and -(EtO)zSiO-(EtO)2SiO- bonded to Al atoms. Although the complex alkoxide is partially dissociated by the addition of water, more than 60% of Al atoms maintain 4-coordination structures and the A l - O - S i bonds introduced into the complex alkoxide are maintained during gelation. Gelation results from formation of A1-O-AI linkages, but not from condensation at Si sites. The resultant gel was monolithic and transparent, and crystallized to ;x- and a-cordierite at 950 and 1050°C, respectively.

1. Introduction

The sol-gel method using metal alkoxides as starting materials is an attractive process for preparation of multicomponent glasses and ceramics at low temperature since chemical homogeneity at a molecular level can be easily achieved in solutions. It is difficult to prepare homogeneous multicomponent gels, especially in silicate systems, because of differences in hydrolysis rate between Si alkoxide and other alkoxides which cause partial precipitation of the latter [1]. To achieve homogeneity in silicate systems, Yoldas proposed synthesis of metallosiloxane derivatives by reactions of partially hydrolyzed Si alkoxides with other alkoxides [1-3]. For ternary systems containing silica, Gensse et al. reported powder

Present address: Technical Research Laboratory, Krosaki Co., Ltd., 1-1 Higashihama, Yawata-Nishi-ku, Kitakyushu 806, Japan. 2 Nippon Steel Co., 2-6-30temachi, Chiyoda-ku, Tokyo 10071, Japan. 3 NGK Spark Plug Co., Ltd., 2808 Iwasaki, Komaki 485, Japan.

preparation in the MgO-AlzO3-SiO 2 system by mixing partially hydrolyzed tetraethoxysilane (TEOS), aluminum tetra-sec-butoxide (A1 (OBuS)3) and magnesium di-sec-butoxide (Mg (OBuS)2 [4]. Suzuki et al. described the effects of various hydrolysis methods of alkoxides on the homogeneity of cordierite (2MgO-2A120 35SiO 2) precursors [5]. Bowen et al. reported the synthesis of a cordierite precursor by the reaction between aluminum magnesium octa-iso-propoxide (Mg[Al(OPr i)4]2) and triethoxysilanol from chlorotriethoxysilane ((EtO)3SiC1) [6,7]. The chemical structures of precursor gels may affect crystallization behavior and properties of the resultant ceramics. It is, therefore, important to analyze the chemical structures of precursor alkoxides and structural changes during hydrolysis and condensation. There are several studies on hydrolysis and condensation of Si alkoxides by infrared (IR) [8], Raman [9] and 298i nuclear magnetic resonance (29Si-NMR) [10-12] spectroscopy. Pouxviel et al. reported, for a binary system, the structure and hydrolysis of a Si-A1 complex alkoxide as determined by small-angle X-ray scattering (SAXS), 27Al-NMR and 29Si-

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

206

T. Fukui et a L / Complex alkoxide as a cordierite precursor

NMR spectroscopy [13,14]. Livage et al. reported studies of hydrolysis of TEOS and chemically modified Al(OBuS)3 with an aqueous solution of magnesium acetate (Mg(OAc) 2) employing 27A1and 29Si-NMR spdctroscopy [15] for ternary cordierites. Roy et al. noted differences in intermediate structures of cordierite gels, depending on preparation methods observed by 27A1and 298i magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy [16]. Prabakar et al. discussed the structure evolution of alkoxide-derived gels in SiO2-AlzO3-P20 5 system as determined by MAS-NMR spectroscopy [17]. We reported [18] three types of preparation methods of cordierite gels and noted the importance of introducing A1-O-Si structures into a precursor for low temperature crystallization of cordierite. The structure of the complex alkoxide as a cordierite precursor synthesized by reactions of Si, A1 and Mg alkoxides was examined by IR, Raman, 27A1- and 29Si-NMR spectroscopy is reported here. Structural changes of the complex alkoxide during hydrolysis were investigated by 27AI- and 29Si-NMR spectroscopy. Crystallization behavior of the resultant gel was examined.

2. Experimental

2.1. Synthesis and hydrolysis of complex alkoxides The complex alkoxide was synthesized from reagent grade TEOS, Al(OBuS)3 and Mg metal flakes. TEOS (10.42 g, 0.05 mol) in ethanol (25 ml) was partially hydrolyzed by equimolar water with hydrochloric acid (molar ratio of HCI/TEOS to 0.01) at ambient temperature for 30 rain. The solution of partially hydrolyzed TEOS was added to aluminum triethoxide (AI(OEt) 3) suspension in ethanol separately prepared by the alcoholysis of A l ( O B u S ) 3 (9.85 g, 0.04 mol) with ethanol (20 ml). A transparent solution of AI-Si complex alkoxide (AS) was obtained by refluxing the mixture for 30 min. That solution was refluxed with Mg (0.49 g, 0.02 tool) for approximately 15 h until the Mg metal was completely consumed, and a transparent solution of Mg-AI-Si complex alkoxide (MAS) was obtained. The MAS solution was hy-

Table 1 Measurement parameters of NMR spectra 29Si-NMR Observation frequency (MHz): 79.3 Reference: T M S a) Pulse width (~xs): 10 (45 ° pulse) Delay time (Us): 5 Number of scans: 1000-4000

27AI-NMR 104.0 Al(H20)63+ b) 35 (90° pulse) 0.5 256

TMS: tetramethylsilane. a) Internal reference. u) External reference.

drolyzed with five times molar distilled water to TEOS at ambient temperature. After about 80 h, the MAS solution gelled to form a monolithic, transparent gel.

2.2. Analytical technique The structures of AS and MAS were analyzed by IR, Raman, 27A1- and 29Si-NMR spectroscopy. The IR spectrum of AI(OEt) 3 and 27A1-NMR spectrum of aluminum magnesium octaethoxide (Mg[AI(OEt)4] 2 (AME) as a reference were also measured. Structural changes in MAS during hydrolysis and condensation were investigated by 27A1- and 29Si-NMR spectroscopy. NMR spectra were measured with a JNM GX-400 spectrometer (JEOL) using CDC13 for the internal lock. The measuring conditions of the NMR spectra are summarized in table 1. IR and Raman spectra of AS and MAS, both viscous liquids, were measured after removing the solvent under reduced pressure. The IR spectrum of AI(OEt) 3 was measured after being mixed with nujol. IR spectra were recorded with samples sandwiched between potassium bromide plates with a Fourier transform infrared spectrometer (FT/IR-5MP, Japan Spectroscopic Co., Ltd.). Raman spectra of the samples were recorded with an NR-1100 Laser Raman spectrophotometer (Japan Spectroscopic Co., Ltd.). Sample excitation was achieved using the 488 nm line of an NEC argon-ion laser. A T G - D T A profile of the gel dried at 100°C was measured to 1200°C at a heating rate of 10°C/min using TAS100 (Rigaku Co.). Crystalline phases of the gels fired at various temperatures for 1 h were identified by powder X-ray

207

7:. Fukui et a L / Complex alkoxide as a cordierite precursor

diffraction (XRD: RAD-C, Rigaku Co.) using Cu K s radiation with a graphite monochrometer.

Table 2 IR absorption bands of the complex alkoxides synthesized from individual alkoxides Assignment

TEOS

Al(OEt)3

AS

MAS

3. Results

CH 3 rocking

1173

3.1. I R s p e c t r a

C-Ostretching

1169 965 1105 1082

1169 961 1107 1083

1167 957 t110 1079

911

905

789

787

687

648

490

476

The IR spectra of TEOS, AI(OEt) 3, AS and MAS are shown in fig. 1. The I R absorption bands for AS and MAS were assigned as shown in table 2 based on the assignments for AI(OEt) 3 [19] and TEOS [20]. In the IR spectrum of AS, A I - O and C - O stretching bands in A I - O - C structures ( A I - O - C bands) were affected by reaction of AI(OEt) 3 and partially hydrolyzed TEOS, while S i - O and C - O stretching bands in S i - O - C structures ( S i - O - C bands) were not. Splitting of the A 1 - O - C bands due to terminal and bridging sites in AI(OEt) 3 [19] disappeared and three absorption bands of A I - O - C appeared

i

I

i

I

x

i

)

i

i

i

=

I

Si-O stretching A1-O stretching

793

OCC deformation

470

I

I

I

i

1500

I

I

(

i

i

1000

Wave

i

1

500

Number (cm -1)

Fig. 1. IR spectra of (1) TEOS, (2) AI(OEt)3, (3) AI-Si complex alkoxide and (4) Mg-AI-Si complexalkoxide.

704/T 640/B 515/T 461/B

Units in cm 1. B and T indicate the bands due to bridging and terminal structures in alkoxides, respectively. Assignments of TEOS and AI(OEt)3 were referred to (20) and (19), respectively.

at 911,678 and 490 cm -1. S i - O - C bands at 1107, 1083 and 789 c m - 1 were almost analogous to those for T E O S [20]. In the I R spectrum of MAS, two A 1 - O stretching bands shifted to frequencies 648 and 476 c m - 1, lower than those of AS. No significant shift was observed for S i - O - C bands or the C - O stretching band in MAS and AS. 3.2. R a m a n

i

1100/T 1076/T 1053/B 930/T 895/B

spectra

Figure 2 shows R a m a n spectra of TEOS, AS and MAS. There were major changes of the peak at 654 cm 1 due to SiO 4 breathing vibration of T E O S [20]. In the spectrum of AS, the peak intensity at 654 cm -1 decreased and two new peaks appeared at 610 and 585 cm -1. These peaks were observed frequencies about 10 c m - i higher than those of the dimer (Si20(OEt) 6) and trimer (Si302(OEt) s) of TEOS, as reported by Mulder and D a m e n [9]. Peak shifts of the SiO 4 structures to higher frequencies than the dimer and trimer can be explained by the lower reduced mass of an A 1 - O group than that of a S i - O group [21]. Thus, A I - O - S i and A I - O - S i - O - S i structures may possibly be formed after the reaction of AI(OEt) 3 and partially hydrolyzed TEOS.

208

T. Fukui et al. / Complex alkoxide as a cordierite precursor

o

.m ~o

~"

(3)

j

$2 i

(m 5

L~

I

i

I

[

I

1 O0

I

I

0

-20

Chemical shift (DDm) I

1500

I

1000 500 Raman Shift (cm "1)

00

Fig. 2. Raman spectra of (1) TEOS, (2) A I - S i complex alkoxide and (3) M g - A ] - S i complex alkoxide. ©, 2-butanol.

In the spectrum of M A S two peaks turned into one broad peak at 540 cm 1.

Fig. 4.27Al-NMR spectra of Mg-A1-Si complex alkoxide after hydrolysis: (1) immediately after, (2) t = 0.33tg and (3) t = 0.9tg, where tg indicates gelation time. Gelation time was about 80 h.

(W~/;=

only one peak was observed at 69 p p m 2780 Hz). The chemical shift of A M E as a reference was 69 p p m = 1500) as in the case of MAS. Figure 4 shows changes in the eYAI-NMR spectra after hydrolysis of MAS. Immediately after hydrolysis, the peak at 69 p p m for MAS greatly decreased, and a sharp peak at 47.9 p p m and three broad peaks at 63, 58 and 5 p p m appeared. The intensity ratio of the peaks around 60 and 5 ppm, due to 4- and 6-coordinated AI atoms, was about 64 to 36. The peaks at 47.8 and 63 p p m progressively disappeared with time, whereas the broad peaks at 58 and 5 p p m increased and shifted to the higher magnetic fields. Immediately before gelation (t = 0.9tg), these peaks shifted to around 46 and 0 ppm, respectively, and their intensity ratio, almost unchanged from that observed immediately after hydrolysis, was about 60 to 40.

(W1/2

3.3. 27Al-NMRspectra Figure 3 shows 27A1-NMR spectra for AS and MAS. For AS, a sharp peak was observed at 5.8 p p m (half width: = 180 Hz) and a broad peak at 41 p p m = 4800 Hz). The intensity ratio of those peaks was 3 to 97. As for MAS,

W1/~ (W1/2

,9, I t

,,,I' I ~ (2)

3.4. 29Si-NMRspectra 2~o

i oo

;

- go

Chemical shift (DDm)

Fig. 3. 27AJ-NMR spectra of (]) A ] - S i complex alkoxide and (2) Mg-A1-Si complex alkoxide.

29Si-NMR spectra for AS and MAS are shown in fig. 5. For AS, three peaks, differing from those of T E O S or condensed T E O S species, were

209

T. Fukui et al. / Complex alkoxide as a cordierite precursor

0

970/

C

2o

DTA ~ / J 40

,

100

I

,

,

,

500

,

I

1000

Temperature (°C) -80

-90

- 100

Fig. 7. T G - D T A curves of the cordierite gel prepared from Mg-A1-Si complex alkoxide.

Chemical shift (ppm) Fig. 5. 29Si-NMR spectra of (1) AI-Si complex alkoxide and (2) Mg-AI-Si complex alkoxide.

observed at -87.1, - 8 9 . 0 and - 9 4 . 5 ppm. Peaks observed at - 8 1 . 7 and - 8 8 . 7 ppm are due to T E O S and its dimer, respectively [12]. For MAS,

,,,

ttttt

new peaks appeared at - 8 4 . 7 , - 8 5 . 2 , - 8 6 . 3 , - 9 1 . 8 and - 9 2 . 8 ppm. These peaks were observed at magnetic fields about 2 ppm lower than those for AS. Figure 6 shows changes in 29Si-NMR spectra after the addition of water to the MAS solution. Immediately after hydrolysis, the peaks around - 85 ppm for MAS became multiple at the center of - 8 4 ppm and broadened with time. Immediately before gelation, no peak due to highly condensed silicates, whose peaks appear at magnetic fields higher than - 1 0 0 ppm, was observed, although the peaks widened to low magnetic fields.

,

k3• L

_z (2)

-A____^

(1)

5 -80 -90 Chemical Shift (ppm)

-100

Fig. 6. 29Si-NMR spectra of Mg-A1-Si complex alkoxide (1) before and (2)-(4) after hydrolysis: (2) immediately after, (3) t = 0.3tg and (4) t = 0.9tg.

I

10

I

2(3

I

20

30

I

40

5(3

Fig. 8. X-ray diffraction patterns of the cordierite gel prepared from Mg-A1-Si complex alkoxide after firing at various temperatures; (1) 900, (2) 950, (3) 1050, (4) 1200, and (5) 1400°C.

210

T. Fukui et al. / Complex alkoxide as a cordierite precursor

3.5. Thermal and XRD analysis In the T G - D T A curves of the gel (fig. 7), there were three exothermic peaks. Peaks below 600°C accompanying weight loss were due to burning of residual organics and those at 970 and 1050°C are due to crystallization of the gel t o / x and a-cordierite, respectively, as confirmed by the X R D results (fig. 8). The gel was transformed to a-cordierite single phase at 1050°C for 1 h and no crystalline phase except cordierite could be detected at all heating temperatures.

4. Discussion

4.1. Structures of complex alkoxides Cordierite has a (Si, A1)O 4 tetrahedral framework with Mg atoms incorporated into spaces of the octahedral sites [22]. To obtain a precursor with a cordierite-analogue structure, (Si, A1)O4 tetrahedral framework, the synthesis of the M g A1-Si complex alkoxide containing a A 1 - O - S i structure was conducted. The following schematic reactions were assumed in the synthesis of the complex alkoxide: Si(OEt)4 + H 2 0 ---, (EtO)3SiOH + E t O H ,

(1)

(EtO)3SiOH + AI(OEt)3 (EtO)3SiOAI(OEt)2 + E t O H , 2 AI(OR)3 + Mg(OEt)2 ~ Mg[AI(OR)4]2,

(2) (3)

where Et is C z H 5 and R is Et or (EtO)3SiO-. Introduction of the A 1 - O - S i structure into the complex alkoxide was first attempted by reaction between AI(OEt) 3 and partially hydrolyzed TEOS (eqs. (1) and (2)), as proposed by Yoldas [1]. Complexing of the products in eq. (2) and Mg(OEt) 2 was attempted via the double alkoxide formation (eq. (3)).

4.1.1. Al-Si complex alkoxide The IR spectrum for AS (fig. 1) suggests decrease in association degree of AS compared with that of AI(OEt) 3 because of disappearance of the terminal-bridge separation of the A 1 - O - C bands.

27A1-NMR spectra and Al-associated structures were investigated by Kriz et al. [23] who observed: (1) chemical shifts of 4-, 5- and 6-coordinated A1 atoms in the range of 35-67 ppm, 29-44 ppm and 3-7.5 ppm, respectively; (2) the alkoxide consisting of only 4-coordinated A1 atoms should have a dimeric structure, (3) trimer of A1 alkoxide with two peaks due to 4- and 5-coordinated A1 atoms and these peaks clearly separated, and (4) cyclic tetramer has only one peak due to 4-coordinated A1 atoms accompanied by a sharp peak due to 6-coordinated A1 atoms. In a previous N M R study on aluminosiloxane [24], Pouxviel and Boilot discussed the structure of a commercial aluminosilicate ester (ASE). ASE with ethoxy and butoxy groups has been proposed as dimer a n d / o r cyclic tetramer because of the presence of peaks in ZVA1-NMR spectrum at 50 ppm (14/2/1 = 5000 Hz) and 7 ppm (W1/2 = 270 Hz) due to 4- and 6-coordinated A1 atoms, respectively. AS with only ethoxy groups, which have smaller steric hindrance than butoxy groups, is expected to be highly associated compared with ASE. If AS has association degree below four (dimer, trimer or cyclic tetramer), the peak separation of the 4and 5-coordinated A1 atoms [13,23] a n d / o r high intensity peak of 6-coordinated A1 atoms [23,24] should have been observed, but they were not in the spectrum for AS. Therefore, polymeric structures with association degree exceeding five should be present in AS. Because 4-coordinated A1 atoms form only dimer structure, the presence of 5-coordinated A1 atoms is essential to form polymeric structures with high association degree. The peak at 41 ppm is thus assigned to the 5-coordinated A1 atoms for the formation of highly associated polymeric structures. These results are not in conflict with (1)-(4) described above. The peak at 5.8 ppm for AS (fig. 3), due to the 6-coordinated A1 atoms, shows the presence of a tricyclic tetramer, representing only 12% of the A1 atoms from the intensity ratio. In the Raman spectrum (fig. 2), the peak shifts of SiO 4 bands of AS to higher frequencies compared with dimer and trimer of TEOS, which is due to the reduced mass of an A1-O group lower than that of a S i - O group [21], suggest the formation of A 1 - O - S i and A 1 - O - S i - O - S i structures

T. Fukui et aL / Complex alkoxide as a cordieriteprecursor

by the reaction of Al(OEt) 3 and partially hydrolyzed TEOS (eq. (2)). No change in the IR band at 965 cm-~ due to CH 3 rocking of the ethoxy groups in TEOS (table 2) indicates the presence of S i - O - C H 2 C H 3 structures in AS. Both results suggest the presence of (EtO)3SiO-Al= and -(EtO)2 SiO-(EtO)2 SiO-Al= structures. 29Si-NMR peaks of TEOS, Si*(OEt)4, as a starting material and its condensed species, (EtO)3Si*-OSi- and -SiO-Si* (OEt)2-OSi-, are reported to be observed at - 82, - 89, and - 95.2 (cyclic) and - 9 6 . 4 ppm (linear), respectively [12]. The three new peaks at -87.1, - 8 9 . 0 and -94.5 ppm in the 29Si-NMR spectrum for AS (fig. 5) were different from those of TEOS and its condensed species, and shifted to lower magnetic fields. Bonding of Al atoms with decreasing electron-acceptor property of oxygen atoms ( S i * - O Al structure) lowers electron density surrounding 29Si nucleus from that of Si atoms ( S i * - O - S i structure). This decreasing electron density implies deshielding of the 298i nucleus [25,26]; thus, resonance peaks of 298i nucleus in A1-O-Si structures would shift to magnetic fields lower than those in S i - O - S i structures. The new peaks are probably a consequence of formation of A1O-Si structures. Pouxviel et al. reported [14] that after hydrolysis of a solution containing TEOS and Al alkoxide, the peaks observed at -87.5 and - 9 4 .5 ppm in 29Si-NMR spectrum are assigned to (EtO)3SiO-A1- and -Si(OEt)zOSi(OEt)zO-Al-, respectively. Thus, we consider that the peaks at -87.1 and -94.5 ppm for AS may be best assigned to (EtO)3Si*O-Al= and bridging Si atoms in -Si(OEt)20-Si * (OEt)z-AI-, respectively. The peak at - 89. 0 ppm, which is not observed in the spectra of Pouxviel et al. [14], showed the largest shift to the low magnetic field among the peaks for AS (fig. 5) by the reaction of AS with Mg(OEt) 2. This result indicates that the 29Si nucleus line at - 8 9 . 0 ppm is strongly affected by the coordination of Mg(OEt) 2 to Al atoms. The chemical shift of the peak is probably due to (EtO)zSi-(OAI=)2, where two aluminates bond to Si atoms. Although residual TEOS and the dimer were detected, no condensed species, larger than dimers, were observed. These findings support the presence of (EtO)3SiO-AI= and

211

-(EtO)2SiO-(EtO)2SiO-AI= structures noted by the IR and Raman spectra. 4.1.2. M g - A l - S i complex alkoxide

The reaction of AS and Mg(OEt) 2 (eq. (3)) greatly affected the structures of A1 segments. The IR spectra (fig. 1) indicate that environment of the A l - O bonds were significantly changed compared with that of the Si-O bonds, suggesting that Mg(OEt) 2 coordinates to the A1-O bond, but not to the Si-O bond in MAS. The 27Al-NMR spectrum of MAS (fig. 4) shows only 4-coordinated Al atoms at 69 ppm ( W 1 / 2 = 2800 Hz). These changes of the IR and 27A1-NMR spectra suggest the formation of double alkoxide with the coordination of Mg(OEt) 2 to A1 atoms. 27A1-NMR spectra of double alkoxides containing A1 atoms have been reported [27,28]. The peaks for KAI(OPri)4 [28], Mg[Al(OPri)4]2 [27] and Nd[Al(OPri)4]3 [28] are observed at 68.5 ppm with a sharp line, 61 ppm with a half width of 900 Hz and 27.4 ppm with a broad line, respectively. The chemical shift for MAS shows that the environment of the tetrahedral Al (AlO 4) in MAS is most analogous to that in KAI(OPri)4 among the above three double alkoxides. Difference in the degree of ionic dissociation of the atoms coordinating to the A1-O bonds is thought to produce differences of A1-O binding strength, resulting in the appearance of different chemical shifts of the 27A1 nucleus. The K-A1 double alkoxide, which dissociates, may be bipolar (K+[AI(OPri)4] -) [29]; thus, MAS may also be an ionic bipolar, but not covalent structure. Further, MAS had a wider half width than AME. The 27A1 nucleus, which has quadrupolar moment, interacts with surrounding electric field gradients, and the linewidth in the 27Al-NMR spectrum reflects the degree of symmetry and ligand arrangement around the 27Al nucleus [30]. Thus, peak broadening for MAS is thought to be due to anisotropic Al atoms possessing higher electric field gradients at the A1 center than that in AME, Mg[AI(OEt)4]2, suggesting bonding of the siloxy groups to A1 atoms in the double alkoxide a n d / o r the presence of slightly condensed Al atoms. The structures of Si segments are determined from the IR and 29Si-NMR spectra. In the IR

212

T. Fukui et al. / Complex alkoxide as a cordierite precursor

spectrum for MAS (fig. 1), no change in the S i - O - C bands indicates that (EtO)3SiO- and -(EtO)2SiO-(EtO)2SiO- structures are maintained after the reaction of AS and Mg(OEt) 2. In the 29Si-NMR spectrum for MAS (fig. 5), the peaks were observed at -84.7, -85.2, -86.3, - 9 1 .8 and - 9 2 . 8 ppm of lower magnetic fields of about 2 ppm than those of the A1-O-Si structures in AS. The coordination of Mg to the tetrahedral Al (AlO4) increases the electron-acceptor property of oxygen atoms bonding to A1 atoms, causing deshielding of 298i nucleus decreasing the electron density surrounding those [25]. Low magnetic field shifts of the resonance peaks of 29Si nucleus for MAS are predicted by these assumption. Although detailed assignments of individual peaks could not be made, the peaks from - 8 4 to - 8 7 ppm and from - 9 1 to - 9 3 ppm may be due to Si bonded to the A1 atoms in (EtO)3SiO-AI= and - ( E t O ) z S i O - ( E t O ) z S i * O -

nation, and AS has the polymeric structures containing the A I - O - S i bondings. A small amount of other structures, such as tetramer, coexists. After reaction of AS and Mg(OEt) 2, Mg-A1 double alkoxides with 4-coordinated A1 atoms and (EtO)3SiO- a n d / o r -(EtO)2SiO-(EtO)2SiObonded to the A1 atoms are formed.

4.2. Hydrolysis of Mg-AI-Si complex alkoxide Changes in the 27Al-NMR spectra (fig. 4) during hydrolysis of MAS solution suggest that MAS was rapidly hydrolyzed and that four intermediate species formed in the initial stage of hydrolysis. Tetrahedral Al atoms at 63 and 58 ppm are probably due to hydrolyzed and slightly condensed species from the remaining complex structures. The sharp peak at 47.8 ppm, representing less than 10% of the A1 atoms, is probably due to symmetric tetrahedral AI atoms, resulting from partial dissociation of the complex alkoxide. The structures of the octahedral A1 atoms at 5 ppm are not clear. The intensity ratio of the peaks around 60 and 5 ppm, due to 4- and 6-coordinated Al atoms, was about 64 to 36; thus, about 64% of Al atoms remain at tetrahedral

AI=.

According to the above results, structural changes i n alkoxides are proposed as shown in fig. 9. After the reaction of Al(OEt) 3 as a highly associated solid with the partially hydrolyzed TEOS, most A1 atoms in AS have 5-fold coordi-

8i(OR)3

Et

/-

RO ~ a l ~ O / 8 i (OR)a

"'/'~

RO./

(Ro)~si AI

|AI

[M

/ \o( EtL (1)

(EtO) a SiOH

\o t Et-]m m ~ ~

\

/ ~ O~7.al"~7-

> (RO,aSi/O//

'o

\

] I

R

!_.dR

', R (2)

' n - ~"~''RO/ ;Si(OR) a n >

Ro

M

2

(3) n - 1 (4)

OR

n -- 0

(5) Mg (OEt) z

R R R0.~ ."0~" / - 0 ~ /.OR RO A I ~ O Mg~o AI'~OR R R (6) Fig. 9. Proposed structures of synthesized complex alkoxides. R is Et, (EtO)3SiO- or -(EtO)zSiO-(EtO)2SiO-.

T. Fukui et al. / Complex alkoxide as a cordierite precursor

sites immediately after hydrolysis. On the other hand, the 29Si-NMR spectrum showed the multiple peaks around - 8 5 ppm for MAS (fig. 6). This indicates diversification of the environment surrounding A1 atoms bonded to SiO4, due to the hydrolysis and condensation of A1 segments. In the intermediate stage, tetrahedral species at 63 ppm in the 27Al-NMR spectra disappeared and peaks for other species shifted to higher magnetic fields, indicating condensation of the hydrolyzed and slightly condensed species. Decrease in the symmetric tetrahedral species shows that they condense with other species. The 29SiNMR spectra are broadened, but no peak due to highly condensed silicates or dissociation of the AI-O-Si structures could be found. Immediately before gelation, the ratio of 4- to 6-coordinated Al atoms (about 3/2) rarely changed from that in the initial stage. Thus, the condensation of A1 segments in MAS proceed, the tetrahedral or octahedral structures were preserved, as formed immediately after hydrolysis. More than 60% of A1 atoms would maintain the 4-coordinated structures after gelation. The peaks in the 29Si-NMR spectra widened to low magnetic fields, but no new peak appeared at higher magnetic field during gelation. These indicate partial hydrolysis of Si-OEt, but not condensation. According to reports by Pouxviel and Boilot [14,24], after the hydrolysis of aluminosilicate ester (ASE) the 27A1-NMR spectra show three peaks at 49.4 ppm with a sharp line, 56 and 7 ppm, and then the growth of the peak at 56 ppm due to tetrahedral AI atoms. During the gelation, no 29Si-NMR spectra changes owing to the condensation of Si segments were observed. Based on their results, they suggested that gelation resulted from formation of AI-O-A1 linkages. In the present study, changes in the 27A1-NMR spectra of MAS are analogous to those described in their reports and no condensation of Si segments was observed. Accordingly, we conclude that gelation of MAS is also due to formation of AI-O-AI linkages. Based on the changes in the NMR spectra, the hydrolysis and condensation of the complex alkoxide are considered as follows. By the addi-

213

tion of water, A1-OEt in the complex alkoxide is predominantly hydrolyzed (eq. (4)) and then condensed (eqs. (5) and (6)), but the Si-OEt groups are maintained. No condensation at Si sites for the formation of the Si-O-Si and Si-O-AI structures proceeds, although the Si-OEt groups are partially hydrolyzed with time (eq. (7)): =AI-OEt + H 2 0

fast, =AI-OH + EtOH,

(4)

--AI-OH + EtO-AI ~ =AI-O-AI= + EtOH,

(5)

=AI-OH + HO-AI= ~ =AI-O-AI= + H20,

(6)

-Si-OEt + H 2 0

(7)

slow - S i - O H + EtOH.

Gelation proceeds by the formation of AI-O-A1 linkages, but not by the formation of Si-O-Si linkages, and in more than 60% of A1 atoms 4-coordination is maintained until gelation. The AI-O-Si structures introduced into the complex alkoxide are also maintained during hydrolysis. The crystallization behavior (fig. 8) of the gel obtained was analogous to that of cordierite component glasses [22], suggesting homogeneity of the precursor gel prepared from the Mg-AI-Si complex alkoxide.

5. Conclusions

The complex alkoxide as a cordierite precursor was synthesized by reactions of Si, A1 and Mg alkoxides. The AI-Si complex alkoxide as an intermediate product has A1-O-Si bonds and polymeric structures with 5-coordinated A1 atoms. The Mg-AI-Si complex alkoxide has a Mg-A1 double alkoxide structure with 4-coordinated A1 atoms and silicates such as (EtO)3SiO- and -(EtO)aSiO-(EtO)2SiO- bonded to A1 atoms. Hydrolysis and condensation of the Mg-AI-Si complex alkoxide proceed while more than 60% of A1 atoms are maintained in the 4-coordination structure, and gelation takes place via formation of AI-O-AI linkages by condensation at A1 sites. Although the complex structure is partially dissociated, the A l - O - S i structures introduced into the complex alkoxide are maintained during gelation. The resultant gel is monolithic and transparent, and crystallized to/x- and a-cordierite at 950

214

T. F u k u i et al. / Complex alkoxide as a cordierite precursor

a n d 1050°C, r e s p e c t i v e l y , w i t h o u t a n y o t h e r crystalline phases.

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