Tetrafluoroborate salts as site-selective promoters for sol-gel synthesis of mesoporous silica

Article abstract of DOI:10.1021/ja0478532

Tetrafluoroborate ion (BF₄^-) serves as a powerful and better-behaved promoter than fluoride ion (F^-) for hydrolytic condensation of alkoxysilanes, such as tetraethoxy orthosilicate, in aqueous media containing amphiphiles with onium ion headgroups as templates, affording thermally and hydrothermally stable mesoporous silica. According to ¹⁹F NMR spectral profiles, BF₄^- is localized on a positive-charged micellar surface, thereby allowing a site-selective growth of the silica framework. The resulting porous silica has an ordered hexagonal structure with a well-developed and thick silicate wall. Even without calcination, the condensation with BF₄^- as the promoter progresses to a large extent to furnish a [Si-(OSi-)₄]/([HOSi(OSi-) ₃] + [(HO)₂Si(OSi-)₂]) ratio of 6.2, which is greater than that of mesoporous silica formed without BF₄^- before (1.5) and even after calcination (3.5) to promote thermal condensation in the solid state.

Full text of DOI:10.1021/ja0478532

Published on Web 07/03/2004
Tetrafluoroborate Salts as Site-Selective Promoters for
Sol-Gel Synthesis of Mesoporous Silica
Akihiro Okabe, Takanori Fukushima, Katsuhiko Ariga, Makiko Niki, and
Takuzo Aida*
Contribution from the Aida Nanospace Project, Exploratory Research for AdVanced Technology
(ERATO), Japan Science and Technology Agency (JST), 2-41 Aomi, Koto-ku,
Tokyo 135-0064, Japan
Received April 14, 2004; E-mail: aida@macro.t.u-tokyo.ac.jp
Abstract: Tetrafluoroborate ion (BF
ion (F ) for hydrolytic condensation of alkoxysilanes, such as tetraethoxy orthosilicate, in aqueous media
containing amphiphiles with onium ion headgroups as templates, affording thermally and hydrothermally
4
-) serves as a powerful and better-behaved promoter than fluoride
-
19
-
stable mesoporous silica. According to F NMR spectral profiles, BF
4
is localized on a positive-charged
micellar surface, thereby allowing a site-selective growth of the silica framework. The resulting porous
silica has an ordered hexagonal structure with a well-developed and thick silicate wall. Even without
-
calcination, the condensation with BF
4
as the promoter progresses to a large extent to furnish a [Si-
]) ratio of 6.2, which is greater than that of mesoporous silica formed
(
OSi-)
4
]/([HOSi(OSi-)
3 2
] + [(HO) Si(OSi-)
2
-
without BF
state.
4
before (1.5) and even after calcination (3.5) to promote thermal condensation in the solid
to be considered between BF4- and F- is that the former is
Introduction
Fluoride ion is known to accelerate protonolysis of alkoxy-
much less hydrophilic and shows a greater ∆G for hydration
-
1
-1 5
1
-
(-45.4 kcal mol ) than the latter (-111.1 kcal mol ). Hence,
silanes, for which a hypervalent silicon species Si-F , though
-
proven only in limited cases,1a,2 has been considered responsible.
one may expect that BF4 may prefer to be localized on the
micellar surface. Here we report a detailed account on the effects
of NaBF4 in the sol-gel processing and thermal/hydrothermal
properties of resultant mesoporous silica.
This method has been applied to the sol-gel synthesis of
mesoporous silica, where the hydrolytic condensation of alkoxy-
silanes, in the presence of amphiphilic templates, is promoted
by fluoride salts, such as NaF and NH4F, to give thermally and
Results and Discussion
3
hydrothermally stable mesoporous silica. However, in our
experience, careful optimization of the molar ratio of fluoride
ion to surfactant is crucial to obtain ordered mesoporous
structures. In the present paper, we highlight that tetrafluorobo-
rate salts are better-behaved promoters than fluoride salts for
the sol-gel synthesis of ordered mesoporous silica. This is based
The alkoxy groups of alkoxysilanes are known to exchange
slowly with alcohols at room temperature. We found that the
addition of NaBF to a CD OD solution of tetramethyl ortho-
4
3
silicate (TMOS) results in a notable acceleration of the exchange
reaction. An example is shown by the reaction at an initial molar
ratio [NaBF ] /[TMOS] of 6:100 ([TMOS] ) 0.20 M) at 25
-
on our serendipitous finding that BF4 accelerates an alkoxy
4
0
0
0
1
group exchange between alkoxysilanes and alcohols under mild
conditions. Certain alkoxysilanes are known to be fluorinated
°C. H NMR spectroscopy of the reaction mixture showed the
appearance of a signal due to MeOD at 3.34 ppm, at the expense
of the signal due to MeO-Si at 3.55 ppm. The integral ratio of
these two signals indicated that the exchange reaction proceeds
to 80% in 10 h (Figure 1a), whereas only 6% conversion is
-
4
by BF4 under rather rigorous conditions. However, in general,
-
the fluorine groups in BF4 have been considered to possess a
much lower affinity than F toward silicon species due to the
-
high Lewis acidity of BF3. On the other hand, in view of the
attained in the absence of NaBF under conditions otherwise
4
sol-gel synthesis of mesoporous silica, an interesting contrast
identical to those mentioned above (Figure 1e). The observed
acceleration effect of NaBF4 appears to be almost comparable
to that of NaF (Figure 1b), while other salts, such as NaBr and
NaNO3, did not accelerate the exchange reaction (Figure 1c,d).
(
(
(
(
1) (a) Holmes, R. R. Chem. ReV. 1990, 90, 17-31. (b) Brinker, C. J.; Scherer,
G. W. Sol-gel science: the physics and chemistry of sol-gel processing;
Academic Press: London, 1990.
2) (a) Corriu, R. J.; Guerin, C.; Henner, B. J. L.; Man, W. C. Organometallics
1
988, 7, 237-238. (b) Holmes, R. R.; Deiters, J. A. J. Am. Chem. Soc.
Taking into consideration the above interesting observations,
we investigated hydrolytic condensation of tetraethyl orthosili-
cate (TEOS) in ethanolic hydrochloric acid in the absence and
1
990, 112, 7197-7202.
3) (a) Schmidt-Winkel, P.; Yang, P.; Margolese, D. I.; Chmelka, B. F.; Stucky,
G. D. AdV. Mater. 1999, 11, 303-307. (b) Kim, W. J.; Yoo, J. C.; Hayhurst,
D. T. Microporous Mesoporous Mater. 2002, 49, 125-137.
4) (a) Feher, F. J.; Phillips, S. H.; Ziller, J. W. J. Am. Chem. Soc. 1997, 119,
3
6
397-3398. (b) Farooq, O. J. Chem. Soc., Perkin Trans. 1 1998, 661-
(5) (a) Marcus, Y. J. Chem. Soc., Faraday Trans. 1991, 87, 2995-2999. (b)
Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81-91.
65.
10.1021/ja0478532 CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 9013-9016
9
9013

A R T I C L E S
Okabe et al.
Figure 3. XRD patterns of mesoporous silicates (A) 2[BF4] and (B) 2[F].
Precursor materials 1[BF4] and 1[F] were obtained, respectively, in the
presence of NaBF4 and NaF at [NaX]0/[CMICl]0 ) 0.1 (a), 0.5 (b), 1.0 (c),
and 2.0 (d). Inset in (A): a magnified XRD pattern in a range 2θ ) 3.5-
Figure 1. Time-courses of methoxy group exchanges between TMOS
(
initial concentration ) 0.2 M) and CD3OD in the absence (e) and presence
of NaX (X ) BF4 (a), F (b), Br (c), and NO3 (d)) at [NaX]0/[TMOS]0 )
:100 in CD3OD at 25 °C.
5.0°.
6
Figure 4. TEM micrograph of calcined material 2[BF4]. Precursor material
[BF4] was prepared with CMICl as template in the presence of NaBF4 at
1
[
CMICl]0/[TEOS]0/[H2O]0/[HCl]0/[EtOH]0/[NaBF4]0 ) 1.0:3.7:530:32:20:
Figure 2. Hydrolytic condensation of TEOS at 25 °C in ethanolic aqueous
0.50.
HCl in the presence of NaBF4 (a), NaF (b), NaBr (c), and NaNO3 (d) at
[TEOS]0/[H2O]0/[HCl]0/[EtOH]0 ) 1.0:23:1.0:3.5. Times required to lose
Scheme 1
fluidity by gelation (tgel).
presence of the above four different sodium salts. Upon addition
of NaBF4 to an ethanolic hydrochloric acid solution of TEOS
at [NaBF4]0/[TEOS]0/[H2O]0/[HCl]0/[EtOH]0 ) 0.01:1.0:23:1.0:
3
.5, the reaction mixture underwent gelation and completely
lost its fluidity in 1.6 h ()tgel) at 25 °C as a result of the
hydrolytic condensation of TEOS (Figure 2a). Although the
addition of NaF to the system also resulted in gelation, the
hydrolytic condensation appears to be slower than with NaBF4,
where 4.5 h was necessary for complete gelation of the reaction
mixture (Figure 2b). Upon incrementally changing the molar
ratio, [NaBF4]0/[TEOS]0, from 0.01 to 0.025, 0.05, and then
chloride (CMICl) ([CMICl]0/[TEOS]0/[H2O]0/[HCl]0/[EtOH]0 )
6
1
.0:3.7:530:32:20). As shown in Figure 3A, the sol-gel
reactions in the presence of NaBF4 at [NaBF4]0/[CMICl]0 )
.1 (a), 0.5 (b), 1.0 (c), and 2.0 (d) proceeded to give insoluble
0
precipitates, all of which showed X-ray diffraction (XRD)
patterns with (100), (110), and (200) diffraction peaks, typical
of hexagonal structures (Figure 3A, only calcined materials 2[BF4]
are shown). Transmission electron microscopy (TEM) of, e.g.,
2[BF4] formed at [NaBF4]0/[CMICl]0 ) 0.5, showed a honey-
comb structure, with an interpore distance of 4.4 nm (Figure
4), which is in excellent agreement with that (4.2 nm) estimated
from the d-spacing of the (100) diffraction (d100 ) 3.7 nm) using
the relationship: interpore distance ) 2d100/x3. In contrast, the
sol-gel synthesis with NaF, in place of NaBF4, gave an ordered
mesoporous structure only at low [NaF]0/[CMICl]0 ratios, such
as 0.1 (a) and 0.5 (b), while higher [NaF]0/[CMICl]0 ratios
(g1.0, (c) and (d)) resulted in only broad XRD profiles (Figure
3B).
0
.1, tgel was significantly reduced to 30, 16, and then 5 min,
respectively. On the other hand, as expected from Figure 1, NaBr
and NaNO3 under conditions identical to those above did not
accelerate the gelation (Figure 2c,d). Thus, NaBF4 is a quite
efficient promoter for the hydrolytic condensation of alkoxysi-
lanes. Since the condensation reaction likely involves F3B-
F-Si(OEt)4 as the active species, we investigated, by means of
1
9
F NMR, a mixture of NaBF4 and TEOS in CD3OD, using
-
-
NaF as a reference. However, neither BF4 nor F gave any
unique NMR signals, even in the presence of a large excess of
TEOS, suggesting that such active hypervalent species are only
formed transiently to promote the reaction.1
a,2
We then investigated effects of NaBF4 on the sol-gel
synthesis of mesoporous silica under acidic conditions (Scheme
1
) including a surfactant such as 1-cetyl-3-methylimidazolium
(6) See Supporting Information.
9014 J. AM. CHEM. SOC.
9
VOL. 126, NO. 29, 2004

Tetrafluoroborate Salts as Site-Selective Promoters
A R T I C L E S
Figure 5. Schematic illustrations for locations of (A) BF4 and (B) F- in
-
1
9
water containing surfactant micelles. (C) F NMR chemical shift changes
-
-
of the signals due to BF4 and F in D2O solutions of NaBF4 (a) and NaF
b) in the presence of micellar CTACl and those of NaBF4 (c) and NaF (d)
(
in the presence of nonmicellar TMACl at [NaX] ) 8.0 mM.
We assume that the above contrasting behaviors of NaBF4
and NaF in the mesostructure formation is due to the difference
in hydrophilic natures of these two salts. Namely, less hydro-
-
philic BF4 is possibly localized on a micellar surface in aqueous
media (Figure 5A) and can promote the silica formation site-
-
selectively around the template micelle. In contrast, F is
hydrated and may not be selectively localized in proximity to
the micellar surface (Figure 5B), so that it would promote the
nontemplated silica formation. To support this idea, F NMR
spectroscopy was conducted at 25 °C on D2O solutions of
NaBF4 and NaF in the presence of varying amounts of onium
salts, such as micelle-forming cetyltrimethylammonium chloride
Figure 6. XRD patterns of calcined materials (A) 2[BF4], (B) 2[F], and
(C) 2[nonsalt] before (solid black curves) and after being heated in air at
1
9
900 °C for 3 h (broken red curves) or in water at 100 °C for 32 h (broken
blue curves).
(
(
CTACl) and nonmicellar tetramethylammonium chloride
TMACl). Upon mixing of a D2O solution of CTACl with
40.0. On the other hand, when nonmicellar TMACl was used
-
-
in place of CTACl, neither BF4 (c) nor F (d) exhibited a
notable downfield shift (∆δ < 0.3 ppm at [TMACl]/[NaBF4]
) 40.0). These spectral features, along with the formation of
insoluble (hydrophobic) CTABF4, allowed us to conclude that
NaBF4, a white precipitate formed at a [CTACl]/[NaBF4] molar
ratio ranging from 0.01 to 3.5 ([NaBF4] ) 8.0 mM). The
precipitates at [CTACl]/[NaBF4] ) 1.0 and 2.0, isolated by
filtration, were both identified as chlorine-free CTABF4 by
elemental analysis (Calcd for C19H42BF4N: C, 61.45; H, 11.40;
B, 2.91; N, 3.77. Found: C, 61.7; H, 11.9; B, 3.0; Cl, <0.02;
N, 3.5 and C, 61.5; H, 11.5; B, 3.0; Cl, <0.02; N, 3.5,
respectively.) On the other hand, when the [CTACl]/[NaBF4]
ratio was further increased, the mixture became homogeneous,
without formation of precipitates. The 19F NMR spectrum of a
-
-
BF4 , in contrast with F , prefers to be localized in a less
hydrophilic environment around the CTACl micelle in aqueous
media.
We found that mesoporous silica 2[BF4], prepared by this
-
accelerated sol-gel synthesis with BF4 , is thermally and
hydrothermally stable, even more so than that synthesized with
-
F (2[F]). For example, upon heating at 900 °C for 3 h, reference
1
9
11 -
D2O solution of NaBF4 alone showed a signal due to F4 B
mesoporous silica 2[nonsalt], formed without any salt addition,
lost the majority of its hexagonal structure, as observed by XRD
(Figure 6C; from solid black to broken red curves), where the
%-survival value, as evaluated from a decrease in intensity of
the (100) diffraction peak, was only 4% (Figure 7A). In sharp
contrast, 2[BF4] preserved its hexagonal structure to a greater
extent with a %-survival value of 65% (Figure 7A). This value
is also definitely higher than that of 2[F] (49%) (Figures 6B
and 7A). On the other hand, 2[Br] and 2[NO3], obtained with
NaBr and NaNO3, respectively, lost their hexagonal structures
considerably, where the %-survival values were only as low as
19 and 9% (Figure 7A). Upon being heated in water at 100 °C
at -151.10 ppm (Figure 5C, (a)), along with a minor isotope
signal due to 19F4 B at -151.04 ppm. Upon mixing the
solution with CTACl, these signals shifted to, e.g., -149.61
and -149.56 ppm, respectively, at [CTACl]/[NaBF4] ) 4.0.
However, even upon further incremental changing of [CTACl]/
10
-
6
[NaBF4] from 4.0 up to 40.0, these signals hardly showed further
downfield shifts, suggesting that the spectral change is nearly
saturated, even at [CTACl]/[NaBF4] < 4.0. In contrast, although
fluoride ion, upon mixing with CTACl, exhibited a similar
downfield shift of the signal (b), the extent was much smaller
-
than that in the case of BF4 (a), even at [CTACl]/[NaF] )
J. AM. CHEM. SOC.
9
VOL. 126, NO. 29, 2004 9015

A R T I C L E S
Okabe et al.
the other hand, 29Si MAS NMR spectroscopy of uncalcined
1
[BF4], obtained directly from the sol-gel reaction system,
4
showed a major signal at -110 ppm due to Si(OSi-)4 (Q ),
whereas signals at -100 and -90 ppm due to HOSi(OSi-)3 (Q )
and (HO)2Si(OSi-)2 (Q ), respectively, were negligibly small.
The integral ratio of the signals Q /(Q + Q ) was calculated
to be 6.2, which is much greater than those of 1[Br] (1.5) and
3
2
6
4
3
2
1
[nonsalt] (1.5) and even higher than that of 1(F) (4.8). It is
4
known that the content of Q is increased on calcination as the
result of thermal-induced condensation. In fact, the Q /(Q +
Q ) ratio of 1[nonsalt], after the initial programmed heating at
3.5 °C min , followed by constant heating for 3 h at 550 °C,
was increased to 3.5, which is, however, still lower than that of
uncalcined 1[BF4] and much lower than that after the calcination
4
3
Figure 7. Stabilities of calcined materials 2[X] on (A) thermal (900 °C, 3
h) and (B) hydrothermal (100 °C in water, 32 h) treatments. Precursor
materials, 1[X], were obtained with CMICl as the template in the absence
2
-
1
(
none) and presence of NaX (X ) BF4, F, Br, and NO3) at [NaX]0/[CMICl]0
)
0.5. %-Survival values were given by I/I0 × 100, where I0 and I are
intensities of (100) diffraction before and after the treatments, respectively.
(
7.6). Thus, as-synthesized 1[BF4] already possesses a well-
Table 1. Structural Properties of Mesoporous Silicates
developed silicate framework. Such a high degree of condensa-
tion and the thick silicate framework (Table 1) both are likely
responsible for the high thermal and hydrothermal stabilities
of the mesoporous silica. That the site-selective hydrolytic
condensation of TEOS promoted by BF4 enables the formation
of highly condensed silica without spoiling the mesostructural
integrity of the material takes great advantage of BF4 over
F .
pore
diameter
(nm)
wall
thickness
(nm)b
surface
area
(m g-1)
2
4
3
2
Q /(Q + Q )
d
(
100
nm)a
uncalcinedc
calcined
7
2
2
2
2
[BF4]
[F]
[Br]
3.77
3.71
3.29
2.1
2.1
2.1
2.1
2.3
2.2
1.7
1.7
810
860
1480
1260
6.2
4.8
1.5
1.5
7.6
5.6
3.7
3.5
-
-
[nonsalt] 3.29
-
a
b
d-spacings of (100) diffraction in XRD. Wall thickness ) [interpore
c
29
Conclusions
distance] - [pore diameter]. Evaluated by Si MAS NMR spectroscopy
of uncalcined 1[X].
-
We have demonstrated that BF4 is a powerful and better-
-
for 32 h, 2[BF4] also showed a high hydrothermal stability,
without any loss of the hexagonal structure (Figures 6A and
behaved promoter than F for hydrolytic condensation of
alkoxysilanes, such as TEOS, in aqueous media. BF4 is less
hydrophilic than F and localized more selectively on a micellar
surface so that it can site-selectively accelerate the hydrolytic
condensation of TEOS, affording ordered mesoporous silica with
a highly condensed and thick silicate wall. This phenomenon
is reminiscent of biological mineralization, which is triggered
by the adsorption of minerals on organic surfaces. Considering
an increasing interest in organic/inorganic hybrids, the site-
selective growth of silicate framework with BF4 , reported
herein, is considered quite important for the fabrication of
nanocomposite materials with functional organic domains in
ordered inorganic frameworks.
-
-
7
B: from solid black to broken blue curves), whereas 2[F]
obviously lost the structural regularity (%-survival value ) 74%)
Figures 6B and 7B). As expected, 2[Br] and 2[NO3], as well
(
as 2[nonsalt] (%-survival value ) 42%), underwent a serious
structural disordering under the above hydrothermal conditions,
where the %-survival values were 35 and 53%, respectively
8
(
Figure 7B). In relation to these observations, when an onium
-
-
ion surfactant bearing BF4 as counteranion was used, reinforced
silica was obtained without external addition of NaBF4. For
example, in water at 100 °C for 32 h, the mesoporous silica
templated by 1-cetyl-3-methylimidazolium tetrafluoroborate
(
)
CMIBF4) hardly lost the hexagonal structure (%-survival value
96%).
Some structural properties of mesoporous silicates are sum-
Supporting Information Available: Experimental details; 19
F
6
NMR spectra of D2O solutions of NaBF4 and NaF in the
29
presence of onium salts; Si MAS NMR spectra of uncalcined
marized in Table 1. Elemental analysis showed that calcined
material 2[BF4] is boron- and fluorine-free (found: B, <0.05;
F, <0.002; Si, 42). N2 adsorption/desorption isotherms indicated
that the pore diameters of 2[BF4], 2[F], 2[Br], and 2[nonsalt]
are all 2.1 nm. On the other hand, the interpore distances, as
estimated by the XRD analysis, suggested that the silica walls
of 2[BF4] and 2[F] were 2.2 and 2.1 nm thick, which were much
thicker than those of 2[Br] and 2[nonsalt] (1.7 nm). In
conformity with this observation, the surface areas of 2[BF4]
1
[BF4], 1[F], and 1[nonsalt]; XRD patterns of calcined meso-
porous silica, obtained with CMIBF4 as template, before and
after being heated in water at 100 °C for 32 h. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0478532
(7) (a) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. (b) Zhao,
D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.;
Stucky, G. D. Science 1998, 279, 548-552. (c) Mokaya, R. J. Phys. Chem.
B 1999, 103, 10204-10208.
(8) (a) Mann, S.; Burkett, S. L.; Davis, S. A.; Fowler, C. E.; Mendelson, N.
H.; Sims, S. D.; Walsh, D.; Whilton, N. T. Chem. Mater. 1997, 9, 2300-
2310. (b) Stein, A. AdV. Mater. 2003, 15, 763-775.
2
-1
and 2[F] in the N2 adsorption were 810 and 860 m g , which
are smaller by 30-55% than those of 2[Br] and 2[nonsalt]. On
9016 J. AM. CHEM. SOC.
9
VOL. 126, NO. 29, 2004