Formation of a new crystalline silicate structure by grafting dialkoxysilyl groups on layered octosilicate

Article abstract of DOI:10.1021/ja027512t

Dialkoxydichlorosilanes ((RO)₂SiCl₂, R = alkyl) react almost completely with interlayer silanol groups in a layered silicate octosilicate to create a new crystalline silicate structure consisting of new five-membered rings arranged regularly on both sides of the silicate layers. The introduction of dialkoxysilyl groups to the interlamellar region of layered silicates with regular reaction sites provides a new methodology for the design and construction of novel crystalline silicate frameworks by a soft chemical route. Copyright

Full text of DOI:10.1021/ja027512t

Published on Web 09/19/2002
Formation of a New Crystalline Silicate Structure by Grafting Dialkoxysilyl
Groups on Layered Octosilicate
Dai Mochizuki,† Atsushi Shimojima,† and Kazuyuki Kuroda*,†,‡
Department of Applied Chemistry, Waseda UniVersity, Shinjuku-ku, Tokyo 169-8555, Japan, and Kagami Memorial
Laboratory for Materials Science and Technology, Waseda UniVersity, Shinjuku-ku, Tokyo 169-0051, Japan
Received June 29, 2002
Scheme 1. Grafting of Dialkoxysilyl Groups onto Octosilicate
Nanoarchitecture of ordered silicate frameworks is important
from the viewpoints of both basic chemistry and applications.
Various layered polysilicates and zeolites are basically prepared
hydrothermally.1 However, precise control of silicate frameworks
by hydrothermal synthesis is limited, and we have not yet
established a soft chemical methodology for the design of silicate
structures. Mesostructured silicas are synthesized by various
interactions between supramolecular assemblies and silicate spe-
cies.2 However, such methods do not afford an approach for
nanoarchitecture at a molecular level. Here we report the formation
of novel crystalline layered silicate as a novel approach by the
reaction of octosilicate with dialkoxydichlorosilanes.
Layered polysilicates are ideal as a substrate for this reaction
because of the ability of the two-dimensional silicate frameworks
to be modified variously.3 The most suitable silicate is layered
octosilicate (Na8[Si32O64(OH)8‚32H2O], Na-Oct).4 The layered
structure is composed of five-membered rings as the structural unit,5
as shown in Scheme 1a. The layer thickness is thicker than that of
a single layered polysilicate kanemite, resulting in the rigidity. The
distances among Si-OH and/or Si-O- sites arranged along one
axis are different from those along the other. Consequently, the
bonding direction on the interlamellar surfaces should be arranged
regularly when appropriate silylating reagents are grafted onto the
surfaces. We have recently reported the reaction of kanemite with
various alkylchlorosilanes, resulting in the formation of novel silica-
organic nanostructures with newly formed ring structures.6 How-
ever, because of both the flexibility of the single-layered structure
and the high silanol density, it is difficult to create a novel crystalline
silicate framework.
As the silylating reagent, we used dialkoxydichlorosilane
((RO)2SiCl2, R ) alkyl).8a Although both Si-OR and Si-Cl groups
are reactive, the reaction rate of Si-Cl groups is much higher than
that of Si-OR groups. By utilizing the difference, dialkoxysilyl
groups can be grafted onto octosilicate, as schematically shown in
Scheme 1. Up to now, there have been no reports on the design of
the framework by such a soft process. Also, the silylated products
can be regarded as a planar polymer of silicon alkoxide. Ordered
alkoxyl groups can be hydrolyzed, and the formed silanol groups
can condense to form novel silica-based materials.
The powder XRD patterns of (CnO)2Si-Oct exhibit different
profiles from those of Na-Oct and DTMA-Oct. The basal spacing
of DTMA-Oct (d ) 2.37 nm) changed to d ) 1.66 nm (n ) 4),
2.07 nm (n ) 6), 2.49 nm (n ) 8), 2.87 nm (n ) 10), and 3.20 nm
(n ) 12) after the reaction (Figure 1 and Supporting Information
Figure S1). The amounts of nitrogen in all of the silylated products
became zero, whereas that of DTMA-Oct was 2.4%, indicating
complete removal of DTMA ions. The basal spacing increased with
the increase in the carbon number (n) of alkoxyl groups. The relation
is linear, and ∆d/CH2 was 0.19 nm, being larger than that of a
monolayer (0.13 nm/CH2), suggesting a bilayer arrangement of the
organic groups. The interlayer alkoxyl groups (n ) 8-12) take
gauche-trans conformations, as proved by the chemical shift (δ
) 31 ppm) of the interior methylene chains in the 13C CP/MAS
NMR spectra of the products (Supporting Information Figure S2).10
The scanning electron microscopic (SEM) images of the silylated
products (Supporting Information Figure S3) showed the retention
of the particle size and morphology of starting Na-Oct.
The 29Si MAS NMR spectra of the products (Figure 2) show
quite different profiles from those of DTMA-Oct. The signals due
to dialkoxysilyl ((RO)2Si(OSi)2, R ) alkyl) groups are observed at
-96 ppm, characteristic of dialkoxylated Q2 units.11 The Q2 signal
at -93 ppm is ascribable to ((RO)(HO)Si(OSi)2).11 The small broad
signals at around -104 ppm can be assigned to unsilylated Q3 units,
and the shift from -100 to -104 ppm is probably caused by the
distortion of neighboring silylated sites, being in agreement with
the small lattice expansion based on the powder XRD data. The
distortion also affects the profiles in the Q4 region, and the peaks
were split into several signals, -111, -113, and -114 ppm for
(C4O)2Si-Oct and -108, -110, -112, -114, and -115 ppm for
(CnO)2Si-Oct (n ) 8-12). The intensity of the Q3 signal of DTMA-
Oct (the ratio of the intensities due to Q3 (-100 ppm) and Q4 (-111
ppm) is 1:1) decreased substantially after silylation, and the degree
It is difficult to directly graft the interlayer surface of Na-Oct
with dialkoxysilyl groups. Dodecyltrimethylammonium-exchanged
octosilicate (DTMA-Oct)7 was used as an intermediate. Dialkoxy-
silylated derivatives of octosilicate ((CnO)2Si-Oct, n ) 4, 6, 8, 10,
and 12) were prepared by the reaction of DTMA-Oct with the
corresponding dialkoxydichlorosilanes.8b
* To whom correspondence should be addressed. E-mail: kuroda@waseda.jp.
† Department of Applied Chemistry.
‡ Kagami Memorial Laboratory for Materials Science and Technology.
9
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J. AM. CHEM. SOC. 2002, 124, 12082-12083
10.1021/ja027512t CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
and 1 for models A and B, respectively. The value for (C4O)2-Oct
is 1.6 (Table S1), suggesting the combination of both models. On
the other hand, the values for (C8O)2Si-Oct and (C12O)2Si-Oct
become 2.0, which strongly suggests the bonding shown by model
A. The difference with the carbon number is similar to that found
for the alkylsilylation of kanemite6 and is explained by the different
degree of silylation due to the steric hindrance of alkoxyl groups.
All of the results prove the formation of alkoxysilylated layered
polysilicates with a novel crystalline silicate framework. The
dialkoxysilyl groups are grafted in a controlled manner to form
new five-membered rings regularly on both sides of the silicate
layers (Scheme 1b). The products are a new type of layered silicates
with thicker layers where only Q4 and Q2 units are present, being
in clear contrast to all known layered silicate structures composed
of Q3 and Q4 units. This material is structurally unique and
potentially applicable as precursors for silicate-based materials by
hydrolysis and condensation. The interlayer alkoxysilyl groups can
act as functional groups and provide geminal silanol groups by
hydrolysis which could be utilized as a bridging part to form a
three-dimensional framework. Further silylation will also afford a
well-designed silicate framework, which means the viability of the
method for the design of silicate framework at a molecular level.
Figure 1. Powder XRD patterns of (a) Na-Oct, (b) DTMA-Oct, and (c)
(C8O)2Si-Oct. (Rigaku RINT-2500X diffractometer with graphite mono-
chromated Cu KR radiation.)
Acknowledgment. We thank Profs. Y. Sugahara and M. Ogawa
for discussion. The work was partially supported by a Grant-in-
Aid for COE research, MEXT, Japan. A.S. thanks a financial
support by Grant-in-Aid for JSPS Fellows from MEXT.
Supporting Information Available: XRD patterns of (CnO)2-Oct
(n ) 4, 6, 10, and 12), SEM images of Na-Oct and (C8O)2-Oct, 13C
CP/MAS NMR spectra of (CnO)2-Oct (n ) 8, 10, and 12), 29Si MAS
NMR data, and possible models for the bonding state of silyl groups
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 2. 29Si MAS NMR spectra of (a) DTMA-Oct, (b) (C4O)2Si-Oct,
(c) (C8O)2Si-Oct, (d) (C10O)2Si-Oct, and (e) (C12O)2Si-Oct. The spectra
were recorded on a JEOL JNM-CMX-400 spectrometer at a resonance
frequency of 79.42 MHz with a 45° pulse and a recycle delay of 200 s.
References
(1) (a) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press:
London, 1982. (b) Lagaly, G. AdV. Colloid Interface Sci. 1979, 11, 105.
(2) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999,
38, 56.
(3) (a) Ruiz-Hitzky, E.; Rojo, J. M. Nature 1980, 287, 28. (b) Yanagisawa,
T.; Kuroda, K.; Kato, C. React. Solids 1988, 5, 167. (c) Ogawa, M.;
Okutomo, S.; Kuroda, K. J. Am. Chem. Soc. 1998, 120, 7361.
(4) Iler, R. K. J. Colloid Sci. 1964, 19, 648.
of the decrease (Supporting Information Table S1) clearly indicates
that about 90% of the interlayer surface reactive sites are silylated.
The amounts of the introduced alkoxyl groups, based on the 29Si
MAS NMR data, were 0.8-1.1 per Si-OH, which are larger than
those reported for esterified layered silicates12 and silicas.13
The XRD patterns of the products exhibited many peaks at higher
angles (2θ ) 10-60°). The peaks are due to the ordering of the
framework, which has not been observed for all other silylated
derivatives of layered polysilicic acids.3,6 All of the diffraction peaks
in the pattern of (C8O)2Si-Oct, for example, are easily assigned to
a tetragonal cell (space group I41/amd), the same group as that of
Na-Oct. The same structure is retained because the reactive sites
are specifically arranged on each side of the silicate layers. The
lattice constants of the a-axis for all of the products are a ) 0.743
nm, and the value is slightly larger than that of Na-Oct (a ) 0.733
nm).5 This slight difference is ascribable to the distortion in the
silicate framework by forming new ring structures.
The bonding state of the silyl groups can take two types
(Supporting Information Chart S1). One silylating reagent reacts
with two confronting silanol groups on the surface to form a cyclic
siloxane ring (model A). The other type is the formation of siloxane
bonds between adjacent Si-Cl groups of silylating reagents in
which one group reacts with the surface of octosilicate (model B).
The signal intensity ratios of Q4 and Q2 ((Q4 - 1)/Q2) should be 2
(5) Vortmann, S.; Rius, J.; Siegmann, S.; Gies, H. J. Phys. Chem. B 1997,
101, 1292.
(6) Shimojima, A.; Mochizuki, D.; Kuroda, K. Chem. Mater. 2001, 13, 3603.
(7) Endo, K.; Sugahara, Y.; Kuroda, K. Bull. Chem. Soc. Jpn. 1994, 67, 3352.
(8) (a) SiCl4 was dissolved in hexane, and n-alcohol (CnOH, n ) 4, 6, 8, 10,
or 12) was added dropwise (SiCl4/CnOH ) 1:2) under N2 flow. The
mixture was allowed to react at room temperature for 1 h. The products
were the mixtures of (CnO)mSiCl4-m (m ) 0-4), and the dialkoxy-
dichlorosilanes were purified by distillation (0.1 Torr, bp; 340 K (n ) 4),
360 K (n ) 6), 380 K (n ) 8), 400 K (n ) 10), and 420 K (n ) 12)).
Each 29Si NMR spectrum of the products showed a signal at -55.8 ppm
for (CnO)2SiCl2. Each 13C NMR spectrum showed a signal due to SiOCH2
at 65.4 ppm.9 (b) DTMA-Oct (1.5 g) dispersed in dehydrated toluene (30
mL) containing dehydrated pyridine (15 mL) was mixed with an excess
amount (32 mmol) of dialkoxydichlorosilane and stirred at room temper-
ature for 2 d. The products were centrifuged and washed with toluene to
remove unreacted silylating reagents, followed by washing with dichlo-
romethane to remove pyridine hydrochloride and deintercalated DTMACl.
The resulting products were dried in vacuo to yield (CnO)2Si-Oct.
(9) Liepins, E.; Xicmane, I.; Lukevics, E. J. Organomet. Chem. 1986, 306,
167.
(10) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir
1997, 13, 115.
(11) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: San
Diego, CA, 1990.
(12) Mitamura, Y.; Komori, Y.; Hayashi, S.; Sugahara, Y.; Kuroda, K. Chem.
Mater. 2001, 13, 3747.
(13) Ossenkamp, G. C.; Kemmitt, T.; Johnston, J. H. Chem. Mater. 2001, 13,
3975.
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