A R T I C L E S
Mochizuki et al.
factants.9 However, the silicate structure of the product does
not completely reflect the original structure. Recently the 3-D
zeolite structures (CDS-1, ERS-12, MCM-65, MCM-69, Nu-6,
and RUB-41) were synthesized through the topotactic conver-
sion of 2-D precursors by careful calcination.10 Also, a new
type of silica zeolite (RUB-24) containing 8MR was synthesized
from intercalated octosilicate by the condensation of the silicate
layers during calcination.11 This method can serve as a promising
route to produce a 3-D structure that reflects the crystallinity
of the original layered structures, but basically imposes a
limitation on the use of other layered silicate because of
difficulty in the connection of Si-OH groups between adjacent
layers.
A new approach proposed here is to manipulate silica
frameworks based on a stepwise reaction of monomeric
Si species within the interlayer spaces of layered silicates.12
Silylation of the surface silanol groups of layered silicates with
organochlorosilanes has been utilized to prepare a variety of
layered silica-organic nanomaterials.13 We reported the forma-
tion of new ring structures on the silicate layers by silylation
of kanemite with alkylchlorosilanes.13e However, the single
silicate sheets of kanemite deteriorated upon silylation. Our
interest has therefore been focused on layered octosilicate14 (also
known as ilerite14a or RUB-1814c), which has a more rigid
structure consisting of four-, five-, and six-membered rings
(abbreviated as 4MR, 5MR, and 6MR, respectively) and
Si-OH/Si-O- groups on its surface. By using dialkoxydichlo-
rosilanes [(RO)2SiCl2, R ) alkyl] as silylating agents, we
obtained a unique crystalline 2-D silica structure where di-
alkoxysilyl groups are regularly grafted on the silicate layers
to form new 5MR.15 The products are potentially applicable as
precursors for new silica-based nanomaterials either by ex-
changing alkoxy groups or by hydrolysis and condensation.16
However, the dialkoxy groups on the interlayer surfaces
remained intact even when the products were dispersed in an
aqueous or alcoholic solution under acidic conditions. It appears
that highly hydrophobic interlayer spaces consisting of bilayers
of alkoxy groups blocked the intercalation of water molecules
to hydrolyze Si-OR groups.
In this paper, we report the formation of novel silica structures
by silylation of octosilicate with alkoxytrichlorosilanes
[ROSiCl3]. By controlling the reaction conditions, two Si-Cl
groups of an alkoxytrichlorosilane molecule react with two
silanol groups on a silicate layer to form new 5MR. Importantly,
the silylated product is bifunctional having both Si-Cl and
Si-OR groups. Although Si-Cl group is useful for various
reactions,17 we focus on its role as “trigger” for hydrolysis of
the alkoxy groups. The high reactivity of Si-Cl group with
water leads to the formation of Si-OH and HCl; the former
creates less hydrophobic interfaces for further intercalation of
water molecules, and the latter catalyzes the hydrolysis of alkoxy
groups.16 As a result, we succeeded in the fabrication of novel
crystalline 2D silica with geminal silanol groups. Moreover, a
3-D silica structure with 12MR was formed by condensation
between the silanol groups on adjacent layers. Such a route
opens a new possibility to create unique 2-D and 3-D silica
structures that are not accessible by conventional methods.
Experimental Section
Synthesis of Na-Octosilicate and C16TMA-Octosilicate.
Na-octosilicate (Na-Oct; Na8[Si32O64(OH)8‚32H2O]) was synthesized
by the method reported previously.18 SiO2 (special grade, Wako
Chemicals), NaOH, and distilled water were mixed at a ratio of SiO2:
Na2O:H2O ) 4:1:25.8. The mixture was treated at 100 °C for 2 weeks
in a sealed Teflon vessel. Hexadecyltrimethylammonium-octosilicate
(C16TMA-Oct) was prepared by the ion exchange reaction of
Na-Oct with hexadecyltrimethylammonium chloride [C16H33N(CH3)3Cl,
C16TMACl] (Tokyo Kasei Co.).18 Na-Oct (12 g) was dispersed in an
aqueous solution of C16TMACl (0.1 mol/L, 400 mL). The mixture was
stirred at room temperature for 24 h and then centrifuged to remove
the supernatant. This procedure was repeated three times. The resulting
slurry was washed with water and air-dried at room temperature.
Synthesis of Alkoxytrichlorosilanes. Alkoxyltrichlorosilanes
[(CnH2n+1O)SiCl3; 1(Cn) (Scheme 1), n ) 6, 8, 10, and 12] were
synthesized by a similar method for dialkoxydichlorosilane.15 In a
typical procedure, n-alcohol (CnOH, n ) 6, 8, 10, or 12, Tokyo Kasei
Co.) was added dropwise to a vigorously stirred mixture of SiCl4 (Tokyo
Kasei Co.) and hexane (SiCl4/CnOH ) 1) under an N2 flow. The
mixture was allowed to react at room temperature for 1 h, yielding the
mixture of (CnO)mSiCl4-m (m ) 0-4). Alkoxytrichlorosilanes (m )
1) were separated by distillation (0.1 Torr, bp; ∼50 °C (n ) 6), ∼70
°C (n ) 8), ∼90 °C (n ) 10), and ∼110 °C (n ) 12)). The 29Si NMR
spectrum of 1(C12), for example, exhibited a single signal at -38.5
ppm assignable to (CnO)SiCl3 (Figure S1A).19 In addition, the 13C NMR
spectrum showed the signals assigned to the alkoxy groups (Figure
S1B), where the signal of R carbon (SiOC) appeared at 66.6 ppm, being
shifted from that of dodecyl alcohol (62.5 ppm). The 13C and 29Si NMR
spectra for other 1(Cn) (n ) 6, 8, and 10) also confirmed the formation
of these reagents (Figure S1).
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