Synthesis of new microporous layered organic-inorganic hybrid nanocomposites by alkoxysilylation of a crystalline layered silicate, ilerite

Article abstract of DOI:10.1039/b610088k

We have developed microporous organic-inorganic hybrid nanocomposites by alkoxysilylation of 4,4′-biphenyl-bridged alkoxysilane compounds, which contain triethoxysilyl, methyldiethoxysilyl, and dimethylethoxysilyl groups at each end of the 4,4′-biphenylene unit ((CH₃)_n(C ₂H₅O)_3-n-Si-C₁₂H₈-Si- (OC₂H₅)_3-n(CH₃)_n, n = 0, 1, or 2, abbreviated as BESB(0), BESB(2), or BESB(4), respectively, where the number in parentheses indicates the number of methyl groups in these molecules), in the interlayer of a crystalline layered silicate, ilerite. XRD, ²⁹Si solid-state NMR and fluorescence spectroscopy revealed the immobilization and bridging formation of the BESB molecules between the silicate layers by condensation, not only with H-ilerite, but also with the BESB molecules. The interlayer structures exhibited different molecular arrangements. BESB(0) and BESB(4) molecules are present as a monolayer arrangement in which BESB(0) molecules form the oligomeric species caused by close stacking like a dimer. BESB(2) molecules form mainly bilayer-like aggregates in the interlayer. The structural differences are caused by the different reactivities of the BESB molecules, which control their polymerization in the interlayer. The resultant BESB(0)- and BESB(2)-ilerite had high microporosity with BET surface areas (508 and 578 m² g^-1 for BESB(0)- and BESB(2)-ilerite, respectively). The micropores showed higher toluene adsorptivity than several other porous silica materials due to the successful surface modification. Consequently, this approach provides a new method for constructing novel microporous nanocomposites, the key to improved selectivity and activity in separation and catalytic applications. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610088k

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis of new microporous layered organic–inorganic hybrid
nanocomposites by alkoxysilylation of a crystalline layered silicate, ilerite
Ryo Ishii,* Takuji Ikeda, Tetsuji Itoh, Takeo Ebina, Toshirou Yokoyama, Takaaki Hanaoka and
Fujio Mizukami
Received 14th July 2006, Accepted 17th August 2006
First published as an Advance Article on the web 31st August 2006
DOI: 10.1039/b610088k
We have developed microporous organic–inorganic hybrid nanocomposites by alkoxysilylation of
4,49-biphenyl-bridged alkoxysilane compounds, which contain triethoxysilyl, methyldiethoxysilyl,
and dimethylethoxysilyl groups at each end of the 4,49-biphenylene unit ((CH3)n(C2H5O)32n-Si-
C12H8-Si-(OC2H5)32n(CH3)n, n = 0, 1, or 2, abbreviated as BESB(0), BESB(2), or BESB(4),
respectively, where the number in parentheses indicates the number of methyl groups in these
molecules), in the interlayer of a crystalline layered silicate, ilerite. XRD, 29Si solid-state NMR
and fluorescence spectroscopy revealed the immobilization and bridging formation of the BESB
molecules between the silicate layers by condensation, not only with H-ilerite, but also with the
BESB molecules. The interlayer structures exhibited different molecular arrangements. BESB(0)
and BESB(4) molecules are present as a monolayer arrangement in which BESB(0) molecules
form the oligomeric species caused by close stacking like a dimer. BESB(2) molecules form mainly
bilayer-like aggregates in the interlayer. The structural differences are caused by the different
reactivities of the BESB molecules, which control their polymerization in the interlayer. The
resultant BESB(0)- and BESB(2)-ilerite had high microporosity with BET surface areas (508 and
578 m2 g21 for BESB(0)- and BESB(2)-ilerite, respectively). The micropores showed higher
toluene adsorptivity than several other porous silica materials due to the successful surface
modification. Consequently, this approach provides a new method for constructing novel
microporous nanocomposites, the key to improved selectivity and activity in separation and
catalytic applications.
interlayer. Some research groups have successfully reported
1. Introduction
the formation of a three-dimensional open framework struc-
Construction of novel organic–inorganic hybrid nano-
sides of the interlayer.10–13 Alternatively, Kuroda’s group has
composites using crystalline layered silicates such as kanemite,
ture like zeolites by the condensation of silanols on both
ilerite, magadiite and other layered silicates has attracted much
alkoxychlorosilanes on the layered silicates.14,15 The periodic
attention in inorganic materials fields. Typical nanocomposites
demonstrated an indirect bridging formation by silylation with
are organically modified layered silicates that have been
silylation on the surface and the subsequent connection
synthesized by surface modification using silylation with
organochlorosilane compounds.1–9 The resultant nanocompo-
sites possess a hydrophobic liquid in the interlayer due to
intercalated alkyl groups, and have potential applications as
adsorbents or fillers in polymer composites. On the other
hand, the nanocomposites have no vacant spaces (micropores)
in the interlayer due to the bulky, flexible alkyl groups. This
reduces selectivity for the accommodation of specific guest
molecules in the interlayer, limiting their application as
adsorbents or functional materials.
between the layers also form an open framework in the
interlayer space. In addition, they have synthesized organically
modified kanemite by periodic silylation with monochloro-,
dichloro-, and alkyltrichlorosilanes.16 Some of the products
exhibited microporosity in their interlayer because the organic
moieties of the alkylchlorosilanes play the role of ‘pillars’
to preserve the interlayer space. Their work suggests that
alkylsilylation focused on the periodic silanols has the
potential to construct organic–inorganic nanocomposites with
an open framework structure.
Recently, many efforts have been devoted to the design and
construction of an open framework in the interlayer spaces of
crystalline layered silicates. The basic strategy is to create a
bridging formation between the silicate layers by utilizing
periodic and reactive silanol groups on both surfaces in the
We have also synthesized organic–inorganic nanocompo-
sites by creating a bridging formation between the silicate
layers of ilerite through alkoxysilylation with 4,49-bis(triethoxy-
silyl)biphenyl (BESB).17 BESB is one of the organic-bridging
alkoxysilane compounds ((R1O)3-Si-R2-Si-(OR1)3; R1 = CH3
or C2H5, R2 = -CH2-, -C2H2-, -C6H4-, -C12H8- , etc.) that
have been extensively studied in the development of novel
silica hybrid materials.18–22 The compound has reactive
ethoxysilyl groups at each end of the biphenylene unit,
leading to condensation with the silanol groups. The layered
Research Center for Compact Chemical Process, National Institute for
Advanced Industrial and Science Technology (AIST), Nigatake 4-2-1,
Miyagino, Sendai 983-8551, Japan. E-mail: ryo-ishii@aist.go.jp;
Fax: +81 22 237 7027.; Tel: +81 22 237 3092
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silicic acid of ilerite provides periodic and reactive silanol
groups on the interlayer surface, which function as host
compounds for the biphenylene-bridging alkoxysilane.
Intercalation in the layered silicic acid caused anisotropic
condensation with the silanols on both sides of the
interlayer, producing a bridging formation of biphenylene
moieties between the layers. The intercalation compound
showed a layered structure of silicic layers and biphenylene
units that play the role of pillars, retaining the interlayer
space due to its rigidity. As a result, this material can be
Synthesis of ilerite and the corresponding silicic acid
Ilerite was prepared from a suspension containing 2 g of silica
in 4 cm3 of 4 mol dm23 NaOH aqueous solution at 378 K for
216 h, as described by Kosuge and Tsunashima.23 Then, the
silicic acid of ilerite (H-ilerite) was obtained by adding 70 cm3
of 0.1 mol dm23 hydrochloric acid aqueous solution dropwise
at a rate of 0.7 6 1022 cm3 min21 to the synthesized ilerite.
The acid treatment was repeated twice. The white product was
washed with distilled water until Cl2 ions were no longer
detected in the filtrate. H-Ilerite was then dried at 323 K
overnight.
expected to be
a novel microporous organic–inorganic
nanocomposite that enables us to control the porosity and
surface properties utilizing alkoxysilane compounds with
various organic spacers.
Synthesis of BESB(0), BESB(2), and BESB(4)
BESB(0) was synthesized according to the method reported by
Shea et al.18 A solution of 4,49-dibromobiphenyl (10 g) in THF
(75 cm3) and TEOS (50 cm3) was added to magnesium turnings
(3 g), and the mixture was refluxed at 348 K for 5 days. The
mixture was then cooled to room temperature. After cooling,
THF was removed in vacuo from the mixture and n-hexane
(20 cm3) was newly added. The mixture was filtered under
argon to remove precipitates in the mixture, after which
n-hexane and the remaining TEOS were distilled off in vacuo.
The resultant brown oil was distilled under ca. 0.3 mmHg at
508–518 K to give a clear oil (2.5 g, 16%). BESB(2) and
BESB(4) were synthesized according to the same method
using methyltriethoxysilane and diethoxyldimethylsilane for
BESB(2) and BESB(4), respectively, as a silicon source.
Distillations were conducted under ca. 0.3 mmHg at 463–
473 K and at 453–463 K for BESB(2) and BESB(4), respec-
tively. They were obtained as colorless oils (BESB(2): 2.9 g,
22%; BESB(4): 3.1 g, 27%). NMR data of the compounds are
as follows: BESB(0): dH (500 MHz; CDCl3; Me4Si) 7.77–7.62
(8 H, dd, C6H4), 3.90 (12 H, q, OCH2CH3), 1.27 (18 H, t,
OCH2CH3); dC (126 MHz; CDCl3; Me4Si), 142.7 (C6H4, C),
135.3 (C6H4, CH), 129.9 (C6H4, C), 126.6 (C6H4, CH), 58.78
(OCH2CH3), 18.27 (OCH2CH3); BESB(2): dH (500 MHz;
CDCl3; Me4Si), 7.73–7.62 (dd, 8 H, C12H8), 3.85 (q, 8 H,
OCH2CH3), 1.26 (t, 12 H, OCH2CH3), 0.39 (s, 6 H, SiMe); dC
(126 MHz; CDCl3; Me4Si), 142.5 (C6H4, C), 134.6 (C6H4, CH),
133.7 (C6H4, C), 126.6 (C6H4, CH), 58.58 (OCH2CH3), 18.38
(OCH2CH3), 24.09 (SiMe); BESB(4): dH (500 MHz; CDCl3;
Me4Si), 7.67–7.63 (dd, 8 H, C12H8), 3.71 (q, 4 H, OCH2CH3),
1.21 (t, 6 H, OCH2CH3), 0.42 (s, 12 H, SiMe); dC (126 MHz;
CDCl3; Me4Si), 142.1 (C6H4, C), 136.9 (C6H4, C), 134.0
(C6H4, CH), 126.6 (C6H4, CH), 58.73 (OCH2CH3), 18.47
(OCH2CH3), 21.66 (SiMe2).
Our previous report revealed that the bridging formation of
the BESB molecules is accompanied by polymerization with
each other, due to the excess number of ethoxy groups at each
end of the biphenylene unit.17 It suggests that the control of
the number of ethoxy groups provides an opportunity to
produce various bridging formations such as mono- or multi-
bridges, yielding various open framework structures. On the
other hand, the mechanical stability of the pillars would be
changed with the variations of the bridging formations,
inducing the deterioration of certain open framework struc-
tures. Thus, alkoxysilylation using not only trifunctional,
but also mono- and difunctional organic-bridging alkoxy-
silanes is a vital issue for the construction of this layered
nanocomposite. In this study, we have investigated the
bridging formation with three kinds of alkylalkoxysilane
compound containing triethoxysilyl, methyldiethoxysilyl, and
dimethylethoxysilyl groups at each end of the 4,49-biphenylene
unit ((CH3)n(C2H5O)32n-Si-C12H8-Si-(OC2H5)32n(CH3)n, n =
0, 1 or 2, abbreviated as BESB(0), BESB(2), or BESB(4),
respectively, where the number in parentheses indicates the
number of methyl groups in these molecules), for the first
time. In the first part of this study, structural analyses of
the resultant products were conducted in order to deduce the
molecular arrangements of the biphenylene units in the
interlayer. The porosity and adsorptivity of the products were
then evaluated in order to determine their surface properties.
Finally, we fabricated models of their open framework
structures in the interlayer.
2. Experimental
Materials
Silica (Wako gel Q-63, Wako Pure Chemical Industries,
Japan, reagent grade) was used for the synthesis of ilerite.
Magnesium, p-nitroaniline (pNA) and 4,49-dibromobiphenyl
(Wako Pure Chemical Industries, Japan, reagent grade) were
used as received. Tetraethylorthosilicate (TEOS, Wako Pure
Chemical Industries, reagent grade), methyltriethoxysilane
(Tokyo Kasei Kogyo Co., Ltd., Japan, reagent grade), and
dimethyldiethoxysilane (Shin-Etsu Chemical Co., Ltd., Japan,
reagent grade) were distilled from calcium hydride before
use. Tetrahydrofuran, ethanol and n-hexane (Wako Pure
Chemical Industries, dehydrated) were used without further
purification.
Condensation of BESB molecules in H-ilerite
H-Ilerite (0.2 g) was mixed with 1.2 cm3 of n-hexylamine and
placed in a sealed Teflon tube for 2 days at room temperature.
The H-ilerite and n-hexylamine mixture was stirred with
BESB(0) (0.5 cm3) and n-hexane (10 cm3) for 7 days at room
temperature. A white compound was separated from the
suspension by centrifugation. The compound was then air-
dried for 5 days at room temperature. After drying, the
compound was suspended in 20 cm3 of 1 mol dm23 HCl
ethanol solution for 5 days at room temperature for the elution
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of n-hexylamine. The final product (BESB(0)-ilerite) was
separated and air-dried at 323 K overnight. The same
procedure was conducted for the reaction of BESB(2) with
the n-hexane and H-ilerite mixture. The final product is
abbreviated as BESB(2)-ilerite.
Japan) at a heating rate of 10 K min21
.
29Si solid-state MAS-
NMR experiments were performed on a Bruker AVANCE
400 WB spectrometer operated at 79.495 MHz with a 7 kHz
spinning frequency using a 7 mm MAS probe. 29Si DDMAS
NMR spectra were obtained with 30u pulses of 1.6 ms and 30 s
cycle delay time. A total of 1024 scans were accumulated
for each sample. UV-vis diffuse reflectance spectra for the
The procedure with BESB(4) followed the same steps as
with the BESB(0)-ilerite until the air-drying of the mixture of
BESB(4), n-hexylamine, and H-ilerite for 5 days at room
temperature. The HCl ethanol treatment after the drying led to
the collapse of the interlayer space due to insufficient siloxane
formation between BESB(4) and H-ilerite. Alternatively, the
product was irradiated with microwaves for 30 min in an NE-
M310 microwave oven (Matsushita Electric Industrial Co.,
Ltd., Japan, 500 W). The compound was then suspended in
23 cm3 of 1 N HCl aqueous solution for 5 days. After
centrifugation, the product was stirred in ethanol at 343 K
for 5 h. These steps were effective for the retention of the
interlayer space. The final product (BESB(4)-ilerite) was air-
dried at 323 K overnight.
products were obtained on
a UV-3100PC spectrometer
(Shimadzu Co., Ltd. Japan) coupled with an integrating
sphere unit (MPC-3100, Shimadzu Co., Ltd. Japan) over the
range of 200 to 400 nm after the compounds were mixed
with barium sulfate powder and molded into pellet form.
Fluorescence emission spectra were collected by an F-4500
fluorescence spectrometer (Hitachi Co., Ltd, Japan) over the
range of 280 to 500 nm with excitation wavelength of 280 nm.
Nitrogen adsorption and desorption measurements at 77 K
were carried out on a Belsorp-28SA (Japan BEL Co. Ltd.,
Japan) for samples degassed at 423 K below 1023 mmHg for
4 h. Toluene adsorption isotherms at 298 K were collected
by a Belsorp-18 (Japan BEL Co. Ltd.) for samples degassed at
423 K below 1023 mmHg for 6 h.
Preparation of reference samples
As references, 4,49-biphenyl-bridged polysilsesquioxane (poly-
merized BESB(0)) and cetyltrimethylammonium (CTA)-inter-
calated ilerite were prepared. BESB(0) (2 cm3) and
n-hexylamine (2.3 cm3) were mixed and stirred for a day.
After stirring, distilled water (1 cm3) was added dropwise. The
solution immediately became a transparent gel. The gel was
washed with ethanol and dried at 423 K overnight. A trans-
parent solid was obtained (polymerized BESB(0), Brunauer–
Emmett–Teller (BET) surface area = 2 m2 g21). Ilerite (0.2 g)
was stirred in CTA chloride solution (0.3 mol dm23) at 423 K
overnight. The product was centrifuged and washed with
ethanol. A white product was obtained (CTA-ilerite).
3. Results and discussion
3-1. Intercalation and alkoxysilylation with BESB in the
interlayer of H-ilerite
XRD patterns of BESB(0)-, BESB(2)-, and BESB(4)-ilerite
are presented in Fig. 1. These basal spacings are observed at
2h = 4.56, 3.29, and 5.34u for BESB(0)-, BESB(2)-, and
BESB(4)-ilerite, respectively. The basal spacings shift to the
lower side of 2h than that of H-ilerite, indicating the
intercalation of the biphenylene units. The corresponding
d-values are 1.94, 2.68, and 1.65 nm for BESB(0)-, BESB(2)-,
and BESB(4)-ilerite, respectively. The gallery height, which is
Measurement of adsorption of pNA into BESB-ilerite’s
BESB(0)-, BESB(2)-, and BESB(4)-ilerite (0.05 g) were
degassed at 423 K below 1023 mmHg for 2 h. After drying,
they were stirred for 2 days in saturated pNA ethanol solution,
the concentration of which was ca. 0.4 mol dm23. The
pNA-adsorbed BESB(0)-, BESB(2)-, and BESB(4)-ilerite were
centrifuged from the solutions and then dried at 423 K
overnight.
Characterization
Silicon contents of the samples were determined using an
SPS7800 inductively coupled plasma (ICP) spectrometer
(SEIKO Co., Japan) after melting in a mixture of H3BO3
and Na2CO3 (sample : Na2CO3 : H3BO3 = 1 : 5 : 1) and
subsequently dissolved in a 5 mol dm23 HCl solution. Total
carbon (TC) and total nitrogen (TN) contents of the products
were measured by gas chromatography with a SUMIGRAPH
NCH-21 analyzer (Sumika Analysis Service Co., Ltd., Japan).
Powder X-ray diffraction (XRD) data were measured using
an M21X diffractometer (MAC Science Co., Ltd, Japan) with
curved graphite monochromator (Cu Ka radiation) operated
at 45 kV and 250 mA. DTA-TG curves were collected on
a Thermo plus TG8120 thermal analyzer (Rigaku Co., Ltd,
Fig. 1 XRD patterns of H-ilerite and biphenylene-bridged ilerites:
(A) H-ilerite, (B) BESB(0)-ilerite, (C) BESB(2)-ilerite, and (D)
BESB(4)-ilerite.
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derived by subtracting the layer thickness (0.74 nm) of
H-ilerite from the d-value, is calculated as 1.20 nm for
BESB(0)-ilerite. It corresponds to the molecular length of the
biphenylene unit (Si-C12H8-Si), indicating that the units are
intercalated perpendicular to the silicate layers. On the other
hand, the gallery heights are calculated as 1.94 and 0.91 nm for
BESB(2)-ilerite and BESB(4)-ilerite, respectively, showing
different intercalation from that of BESB(0)-ilerite. The
reduced gallery height of BESB(4)-ilerite indicates the inter-
calation of the biphenylene unit at an angle of less than 90u to
the surface. BESB(2)-ilerite exhibits a larger gallery height
than the molecular length of the biphenylene unit. It indicates
the intercalation of two biphenylene units in a direction
perpendicular to the silicate layers, or, in other words, a
bilayer arrangement. Moreover, BESB(2)-ilerite also exhibits a
small shoulder at 2h = 5.03u near the main peak. The d-value
(d = 1.76 nm) almost corresponds to that of BESB(4),
suggesting that BESB(2)-ilerite partially contains a monolayer
arrangement of the biphenylene unit similar to BESB(0)- and
BESB(4)-ilerite.
423 K due to desorption of water molecules. The curves also
show a small loss ranging from 423 to 600 K, which is caused
by the dehydration of the silanol groups. The loss is estimated
as 0.8, 3.6, and 3.2 wt% for BESB(0)-, BESB(2)-, and BESB(4)-
ilerite, respectively. This indicates that BESB(2)- and BESB(4)-
ilerite possess more residual silanol groups that are present in
H-ilerite and in hydrolyzed BESB molecules. In addition, a
large weight loss is observed in the TG curves ranging from
600 to 950 K. Because the losses correspond to TC contents in
the products, they are ascribed to the decomposition and full
oxidation of the biphenylene unit. The corresponding DTA
curves show an exothermic peak around 850 K (Fig. 2(b)). The
curves also show another exothermic peak at a temperature
lower than 850 K. The products exhibit different peak
behaviors: BESB(2)-ilerite (curve B) has two sharp peaks at
673 and 740 K, although BESB(4)-ilerite (curve C) has no peak
in the range of 673–740 K. The behaviors probably reflect the
structural differences between the products. XRD analyses
reveal that BESB(2)-ilerite has the largest d-value of 2.68 nm
among the products. The large expansion of the interlayer
space might expose the intercalated biphenylene units to the
external environment, leading to easy decomposition of the
biphenylene unit at a relatively low temperature.
29Si solid-state NMR spectra of H-ilerite and the products
are presented in Fig. 3. H-Ilerite shows two peaks at 299.7 and
2111.9 ppm (curve A). They represent Q3 ((SiO)3SiOH) and
Q4 ((SiO4)Si) structural units that constitute the framework of
the silicate sheet in H-ilerite. These peaks are still present in the
spectra of the products, although the intensities are changed.
The integral ratio (Q3/Q4) of H-ilerite is estimated as 1,
which is in agreement with the chemical formula reported by
Gies et al.24 The ratios of the products are summarized in
Table 1. They show lower values than that of H-ilerite,
indicating that the Q3 units change to Q4 units as a result of
TN contents were 2.98, 2.61 and 3.57 wt% for BESB(0)-,
BESB(2)- and BESB(4)-ilerite, respectively, before HCl–
ethanol or HCl aqueous solution treatment, respectively.
After the treatment, the values decreased to less than 0.1 wt%
due to the elution of n-hexylamine. TC contents were 19.3,
26.4, and 14.4 wt% for BESB(0)-, BESB(2)- and BESB(4)-
ilerite, respectively, after the treatment. It originates from the
biphenylene units, indicating their successful immobilization in
H-ilerite. We evaluated the number of biphenylene units in a
unit cell of H-ilerite from the TC and Si contents. The numbers
were 4.2, 5.8 and 2.0 for BESB(0)-, BESB(2)-, and BESB(4)-
ilerite, respectively. The numbers increase in the order:
BESB(4)- , BESB(0)- , BESB(2)-ilerite. The order corre-
sponds to that of their basal spacings, supporting the notion
that the biphenylene units are loaded in the interlayer as a
result of the intercalation.
TG curves for BESB(0)-, BESB(2)-, and BESB(4)-ilerite are
presented in Fig. 2(a). The TG curves show a weight loss up to
Fig. 3 29Si solid-state NMR spectra of H-ilerite and biphenylene-
bridged ilerites: (A) H-ilerite, (B) BESB(0)-ilerite, (C) BESB(2)-ilerite,
and (D) BESB(4)-ilerite.
Fig. 2 (a) TG curves of (A) BESB(0)-ilerite, (B) BESB(2)-ilerite, and
(C) BESB(4)-ilerite. (b) The corresponding DTA curves of (A)
BESB(0)-ilerite, (B) BESB(2)-ilerite, and (C) BESB(4)-ilerite.
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Table 1 Relative Q3 and Q4 intensities of H-ilerite and the products, the number of siloxane bonds generated in the unit cell of H-ilerite and the
number of siloxane bonds generated between a biphenylene unit and H-ilerite
Generated siloxane bonds
between a biphenylene
unit and H-ileriteb
Generated siloxane
bonds (Ns)
Q3
Q4
Q3/Q4 a
BESB(0)-ilerite
BESB(2)-ilerite
BESB(4)-ilerite
a
0.66
0.61
0.82
1.34
1.39
1.18
0.49
0.44
0.69
5.4
6.2
2.9
1.3
1.1
1.5
b
Relative intensities were normalized by the total Q3 and Q4 area of H-ilerite. The total area is assumed to be 2.00. The number was
calculated as the division of Ns by the number of biphenylene units in the unit cell.
condensation with silanol groups. In addition, BESB(0)-ilerite
exhibits two new peaks appearing at 268.8 and 277.3 ppm
(curve B). They represent T2 and T3 structural units originat-
ing from the BESB(0) molecules. The appearance of the T
units results from the hydrolysis/condensation of ethoxy
groups combined with two silicon atoms on each end of the
biphenylene unit. The condensation indicates that the biphe-
nylene units are connected with the silanol groups on the
surface of H-ilerite, explaining the decrease of the Q3 silanol
sites. BESB(2)- and BESB(4)-ilerite show similar peaks at
228.5 and 233.2 ppm for BESB(2)-ilerite (curve C), and
around 8 ppm for BESB(4)-ilerite (curve D), respectively. They
represent D1 and D2 structural units for BESB(2)-ilerite, and
M1 for BESB(4)-ilerite, respectively. They also reveal con-
densation with H-ilerite.
molecule. In other words, these units reflect 2 or 3 siloxane
bonds on the Si atoms bonded to each end of the biphenylene
unit, whereas one BESB(0) molecule connects with only one
silanol group of H-ilerite. The large numbers of siloxane bonds
suggest the condensation with BESB(0)s using the free ethoxy
groups. The D1 and D2 structural units of BESB(2)-ilerite also
indicate condensation with BESB(2)s in the interlayer. As a
result, BESB molecules are condensed with H-ilerite, accom-
panied by considerable condensation with each other in
BESB(0) and BESB(2)-ilerite.
3-2. Structural behaviors and the interlayer structure of
BESB-ilerites
We investigated via the XRD patterns the time dependence of
the products in order to examine the stability of the interlayer
structures resulting from the condensation of the BESB
molecules. XRD patterns of BESB(0)-ilerite with a time-course
are presented in Fig. 4(a). The patterns are still retained after
the reaction for 499 days, indicating that the structure of
BESB(0)-ilerite does not collapse. On the other hand, the peak
at 2h = 4.56u (pattern A), corresponding to the basal spacing,
shifts to slightly higher 2h values with time. The time
We estimated the number of siloxane bonds generated
between the biphenylene unit and the layered framework of
H-ilerite from the chemical analyses and the Q3/Q4 ratios. The
number of siloxane bonds generated in the unit cell of H-ilerite
can be calculated as follows:
Ns = (1 2 Q3/Q4)/(1 + Q3/Q4) 6 Nsh
(1)
where Ns represents the number of siloxane bonds generated in
the unit cell, and Nsh represents the number of siloxane bonds
in H-ilerite. Based on the crystal structure of ilerite, Nsh is 16.
Q3/Q4 represents the integral ratio of the Q3 peak to the Q4
peaks for BESB(0)-, BESB(2)-, and BESB(4)-ilerite. The
resultant (1 2 Q3/Q4)/(1 + Q3/Q4) leads to the conversion
ratio from the silanol group to siloxane bonds after
condensation. The Ns values are shown in Table 1. Thus, the
number of siloxane bonds generated between a biphenylene
unit and H-ilerite is derived from the division of the Ns values
by the number of biphenylene units in the unit cell. The
numbers for the products are also shown in Table 1. The
number (= 1.5) for BESB(4)-ilerite indicates that two ethoxy
groups in the BESB(4) molecule are almost completely
condensed with H-ilerite. However, the numbers for
BESB(0)- and BESB(2)-ilerite are considerably smaller than
the numbers of ethoxy groups in these molecules, although
they possess 6 and 4 ethoxy groups for BESB(0)- and BESB(2)
molecules, respectively. It indicates that 5 or 3 ethoxy groups
in the molecules are not condensed with H-ilerite. The NMR
spectra of the products suggest that these ethoxy groups are
consumed to form condensation with BESB(0) or BESB(2)
molecules in the interlayer. As stated above, BESB(0)-ilerite
has T2 and T3 structural units on the spectra. The units reveal
the presence of at least 4 siloxane bonds in one BESB(0)
Fig. 4 (a) XRD patterns of BESB(0)-ilerite: (A) after the synthesis,
(B) 186 days after the synthesis, (C) 291 days, (D) 356 days, (E)
426 days, and (F) 499 days. The dotted line represents the position of
the basal spacing (2h = 4.56u) of BESB(0)-ilerite before the shift to the
higher side of 2h. (b) Time courses of basal spacings of biphenylene-
bridged ilerites: (A) BESB(0)-ilerite, (B) BESB(2)-ilerite, and (C)
BESB(4)-ilerite.
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dependence of the d-values is shown in Fig. 4(b). The basal
spacing of BESB(0)-ilerite decreases from 1.94 nm to 1.61 nm
up to 356 days (curve A). The decrease in the basal spacings
indicates that the interlayer space shrinks along the direction
of the stacking of the silicate layers. The shrinkage without
destruction suggests that the biphenylene units cause the
structural change in the interlayer, maybe changing their
sloping orientation relative to the surface. This may be induced
by insufficient mechanical stability of the biphenylene units
due to poor connection between the biphenylene units and
H-ilerite, as shown in the NMR results. BESB(2)- and
BESB(4)-ilerite also show similar behavior without change of
the XRD pattern. Their time dependences in Fig. 4(b) show a
decrease from 1.65 nm to 1.34 nm for BESB(4)-ilerite (curve C)
and a decrease from 2.68 nm to 2.39 nm for BESB(2)-ilerite
(curve B). After decreasing up to around 300 days, the basal
spacings become constant at 1.61, 2.39 and 1.34 nm for
BESB(0)-, BESB(2)- and BESB(4)-ilerite, respectively. As a
result, the structures of the products attain stability after the
shrinkage of the interlayer spaces.
BESB(4) , BESB(2) , BESB(0). Polymerized BESB(0) also
shows a peak at 390 nm that exists at a longer wavelength
than the others (curve A). Goldenberg et al. reported that
emission from the biphenyl monomer is observed at 319 nm,
the emission showing a red-shift to the higher wavelength
side, resulting from the formation of the excimer emission.25
The red-shift at 350–365 nm has also been observed in
biphenyl molecules on some clay minerals where the molecules
form aggregates on the surface.26 In addition, the conforma-
tional transition of bis(4-biphenylmethyl) ether revealed that
the excimer emission, originating from the biphenyl dimer,
brings about a red-shift to 380 nm.25 These reports indicate
that the red-shift from the monomeric emission is inversely
correlated with the distance between the neighboring biphe-
nylene units. In other words, the smaller the distance
between the biphenylene units, the longer the red-shift. This
emission behavior provides information on the molecular
arrangement of the biphenylene units in the interlayer. The
emission wavelength of BESB(4)-ilerite is close to the
monomeric emission. This indicates that the biphenylene units
are not interacting with each other but are isolated in the
interlayer. On the other hand, BESB(0)-ilerite exhibits
emission at a wavelength similar to that from the dimer,
demonstrating that the biphenylene units are close to each
other. Polymerized BESB(0), having an emission wavelength
near BESB(0)-ilerite, supports the notion of a close relation
between the neighboring biphenylene units in the interlayer,
because polymerized BESB(0) consists of closely packed
biphenylene units resulting from the direct connections
between the biphenylene units. The emission of BESB(2)-
ilerite shows at an intermediate wavelength between the
monomeric and dimer emissions, indicating that the biphenyl-
ene unit exists at an intermediate distance from its neighbors
in the interlayer.
Diffuse reflective UV-vis absorption spectra are presented
for BESB(0)-, BESB(2)-, and BESB(4)-ilerite in Fig. 5(a). The
samples were measured after the shrinkage of the interlayer
space. The spectrum of polymerized BESB(0) is also plotted
for comparison. Polymerized BESB(0) shows two peaks at
260 and 280 nm due to absorption from the biphenyl unit
(curve A). Similar peaks are observed in the spectra of
BESB(0)-, BESB(2)-, and BESB(4)-ilerite (curves B–D),
indicating the presence of the biphenyl unit in these products.
Fluorescence spectra are presented for BESB(0)-, BESB(2)-,
BESB(4)-ilerite, and polymerized BESB(0) in Fig. 5(b). The
peaks are observed at 372, 355, and 330 nm for BESB(0)-,
BESB(2)-, and BESB(4)-ilerite, respectively (curves B–D),
which are caused by fluorescence emission from the biphenyl-
ene unit. The peak shifts to longer wavelength in the order:
3-3. Porosity and surface property of BESB-ilerites
Nitrogen adsorption and desorption isotherms before the
shrinkage of the interlayer distance showed type-I isotherms of
BDDT classification for BESB(0)-, BESB(2)-, and BESB(4)-
ilerite, respectively.27 These BET surface areas are shown in
Table 2. The pore widths, which are derived from the division
of the N2 adsorption uptake around p/p0 = 0.9 by the BET
surface area, were estimated as 0.93, 0.91, and 1.00 nm for
BESB(0)-, BESB(2)-, and BESB(4)-ilerite, respectively. The
widths differ from the corresponding gallery heights (estimated
from the obtained d-values of the basal plane), being 1.20, 1.94
and 0.91 nm for BESB(0)-, BESB(2)-, and BESB(4)-ilerite,
respectively. This indicates that the widths reflect not the
interlayer distance but the interval between the biphenylene
units in the interlayer, assuming that the biphenylene units are
˚
present at several A intervals in the interlayer.
Table 2 Comparison of BET surface areas of the products before and
after the decrease in the basal spacing
Fig. 5 (a) UV-vis diffuse reflectance spectra of biphenylene-bridged
ilerites and polymerized BESB(0): (A) polymerized BESB(0), (B)
BESB(0)-ilerite, (C) BESB(4)-ilerite, and (D) BESB(2)-ilerite. (b)
Fluorescence emission spectra of biphenylene-bridged ilerites with
polymerized BESB(0): (A) polymerized BESB(0), (B) BESB(0)-ilerite,
(C) BESB(2)-ilerite, and (D) BESB(4)-ilerite.
BESB(0)-ilerite
BESB(2)-ilerite
BESB(4)-ilerite
Before
After
616 m2 g21
508 m2 g21
669 m2 g21
578 m2 g21
241 m2 g21
35 m2 g21
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Fig. 6 N2 adsorption–desorption isotherms of H-ilerite and biphenyl-
ene-bridged ilerites after decrease in their basal spacings: (A) BESB(0)-
ilerite (#: adsorption and $: desorption), (B) BESB(2)-ilerite (e:
adsorption and r: desorption) and (C) BESB(4)-ilerite (n: adsorption
and m: desorption).
Fig. 7 (a) Toluene adsorption isotherms of H-ilerite and biphenylene-
bridged ilerites after decrease in their basal spacings: (A) H-ilerite, (B)
BESB(0)-ilerite, (C) BESB(2)-ilerite and (D) BESB(4)-ilerite. (b) A
comparison of the toluene adsorption isotherm with those of other
silica materials: (A) BESB(0)-ilerite, (B) silicalite, and (C) FSM-22.
The isotherms after shrinkage are presented for BESB(0)-,
BESB(2)-, and BESB(4)-ilerite in Fig. 6. BESB(0)- and
BESB(2)-ilerite retain the type-I isotherms that represent
microporous materials (curves A and B), although BESB(4)-
ilerite has a type-IV isotherm (curve C). This means that only
BESB(4)-ilerite changed its isotherm form from type-I to type-
IV as a result of shrinkage. Their BET surface areas decreased
after shrinkage, as shown in Table 2. The surface area of
BESB(4)-ilerite corresponds to that of H-ilerite (BET surface
area = 26 m2 g21), indicating that it is limited to the external
surface area. This is caused by the shrinkage of the interlayer
space, resulting in the disappearance of vacant spaces. The
pore widths are estimated as 0.94 and 0.92 nm for BESB(0)-
and BESB(2)-ilerite, respectively. The differences in pore
widths before and after shrinkage are less than 0.03 nm,
showing that the widths are almost unaffected by shrinkage. In
other words, the interval between the biphenylene units is
almost constant in spite of the decrease in the interlayer
distance. This suggests that the arrangement of the biphenyl-
ene units in the interlayer is not destroyed by the shrinkage.
Toluene adsorption isotherms are given for BESB(0)-,
BESB(2)-, BESB(4)-ilerite, and H-ilerite in Fig. 7(a).
BESB(0)- and BESB(2)-ilerite (curves B and C) exhibit
extensively higher toluene uptakes than does H-ilerite (curve
A), indicating enhanced adsorptivity for toluene. This is due to
the formation of hydrophobic micropores whose framework
consists of biphenylene units. Interestingly, BESB(2)-ilerite
shows a lower uptake than BESB(0)-ilerite below P/P0 , 0.5.
This fact indicates that the surface modification of BESB(2)-
ilerite is inferior to that of BESB(0)-ilerite. The lesser
modification is probably due to the incomplete condensation
of BESB(2) molecules in the interlayer. This results in
residual hydrophilic silanols at the ends of biphenylene units
disturbing the hydrophobic surface. The toluene isotherm of
BESB(0)-ilerite was subsequently compared with those of
hydrophobic silicalite (curve B, BET surface area
374 m2 g21) and mesoporous silica (curve C, FSM-22, BET
surface area = 958 m2 g21 28
in Fig. 7(b). BESB(0)-ilerite
=
)
shows superior adsorptivity below P/P0 , 0.5 to other
silica materials. The high adsorptivity will lead to applications
as adsorbents, separation devices, and sensor devices for
aromatic compounds.
Finally, we investigated the presence of the bridging
formation between the silicate layers resulting from the
connection between H-ilerite and the biphenylene unit. The
adsorption experiment is an appropriate test for evaluating
the bridging formation. pNA was used as an adsorbate because
pNA is a nonvolatile aromatic molecule that has high affinity
for the micropores of BESB-ilerites. We also used BESB(0)-,
BESB(2)-, and BESB(4)-ilerite after synthesizing for 499, 261
and 455 days, respectively, together with CTA-ilerite as an
adsorption reference. XRD patterns before and after the
adsorption of pNA are presented in Fig. 8. CTA-ilerite shows a
basal spacing of 2.71 nm (Fig. 8(a), dotted curve). The
adsorption of pNA enhances the basal spacing to ca. 4.0 nm
(Fig. 8(a), solid curve), which is estimated as further expansion
by 1.3 nm of the interlayer space. The large expansion is
caused by the swelling of pNA in the interlayer because CTA-
ilerite possesses a flexible interlayer space free of volumetric
constraint between the layers. The same pNA treatments
caused a slight increase of the basal spacing from 1.61 to
1.68 nm for BESB(0)-ilerite (Fig. 8(b)), and from 1.34 to
1.37 nm for BESB(4)-ilerite (Fig. 8(d)), respectively. The
expansion is estimated as 0.07 and 0.03 nm for BESB(0)- and
BESB(4)-ilerite, respectively, and smaller than that of CTA-
ilerite. The limited expansion indicates that the biphenylene
unit is connected with both surfaces of the silicate layers of
H-ilerite, preventing the swelling of pNA. Thus, it indicates
that the biphenylene units in BESB(0)- and BESB(4)-ilerite
make a bridging formation between the layers. On the other
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Fig. 8 XRD patterns of biphenylene-bridged ilerites and CTA-ilerite
before and after pNA treatment: (a) CTA-ilerite (dotted line: before
and solid line: after), (b) BESB(0)-ilerite (dotted: before and solid:
after), (c) BESB(2)-ilerite (dotted: before and solid: after) and (d)
BESB(4)-ilerite (dotted: before and solid : after). The data were
smoothed by the Savitzky–Golay method.
hand, BESB(2)-ilerite shows different behavior (Fig. 8(c)). The
XRD pattern after adsorption shows a peak at 2h = 3.18u
(solid curve). The peak results from a shift lower than 2h =
3.70u (dotted curve) of the basal spacing caused by the bilayer
arrangement of the biphenylene unit in the interlayer space.
The corresponding basal spacing shifts from 2.39 nm to
2.78 nm after the adsorption, calculated as an expansion of
0.39 nm. This expansion is larger than those of BESB(0)- and
BESB(4)-ilerite, indicating that the biphenylene units do not
completely connect with the consecutive layers. In addition, a
new peak is observed at 2h = 5.03u after the adsorption. This
peak corresponds to the shoulder on the main peak before
the pNA adsorption measurement. It is caused by the
monolayer arrangement of the biphenylene unit that is present
as a minor phase, indicating connections between the layers
similar to those in BESB(0)- and BESB(4)-ilerite. As a result,
BESB(2)-ilerite exhibits insufficient bridging formation
between the layers.
Fig. 9 Model structures of (a) BESB(0)-, (b) BESB(2)-, and (c)
BESB(4)-ilerite.
structural change of the oligomeric species in the interlayers.
The resultant structure exhibits an open framework containing
the oligomeric species in a sloping position at an angle of ca.
46u to the silicate surface. BESB(4)-ilerite (model c) also forms
an intercalation compound of biphenylene units that are
connected with H-ilerite at each end of the biphenylene unit.
They are orientated to the silicate layers at an angle of ca. 30u,
induced by the shrinkage of the interlayer space. The final
structure reveals that the biphenylene units are isolated from
each other, resulting in the observation of the monomeric
emission in the fluorescence spectrum. On the other hand,
BESB(2)-ilerite (model b) shows a different structure in the
interlayer. XRD results suggest that the biphenylene units
mainly form a bilayer arrangement in the interlayer space.
The pNA adsorption reveals that the biphenylene units do
not completely connect with each side of the bilayer, resulting
in insufficient bridging formation between the layers. The
bilayer arrangement, however, possesses micropores in the
interlayer because of the presence of vacant spaces between
the biphenylene units. The resultant interlayer structure of
BESB(2)-ilerite exhibits an open framework with a flexible
interlayer space.
3-4. Model structures of BESB-ilerites
Structural models of BESB(0)-, BESB(2)-, and BESB(4)-ilerite
are given in Fig. 9, based on the discussion above. BESB(0)-
ilerite (model (a)) forms an intercalation compound of
biphenylene units that bridge the interlayer spaces between
the consecutive layers. The bridging formation is achieved by
connections between the silanols of H-ilerite and the oligo-
meric species of the biphenylene units. The oligomeric species
consist of condensed biphenylene units such as the dimer in
which the phenylene rings are closely parallel to each other.
The oligomeric species form vacant spaces between themselves,
leading to the creation of slit-shaped micropores that are
expected to have high adsorptivity for toluene molecules. The
micropores are retained even after the shrinkage caused by
The differences in their interlayer structure are caused by the
different reactivity in the three BESB molecules. BESB(0) has
three reactive ethoxy groups at each end of the biphenylene
unit. The ethoxy groups can not only react with silanols of the
H-ilerite layers but also promote the polymerization with each
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other. The polymerization induces the formation of the
oligomeric species in which the biphenylene units are close to
each other because BESB(0) molecules are orientated perpen-
dicular to the silicate surface by the assistance with co-
intercalated n-hexylamines. BESB(2) and BESB(4) molecule,
however, do not cause the polymerization similar to that of
BESB(0) molecules in the interlayer. It would be due to the
presence of methyl groups which control the condensation
between these BESB molecules. The BESB(2) molecule has two
methyl groups that might have a role to suppress the ‘face-to-
face’ condensation of the biphenylene units, leading to the
bilayer-like aggregates in the interlayer. The higher numbers of
the methyl group in BESB(4) molecule has a possibility to
inhibit the condensation with the neighboring ethoxy group,
forming the isolated bridging formation of BESB(4) molecules
in the interlayer. Thus, the control of the reactivity suggests the
formation of the various microporous interlayer structures,
giving a high possibility to construct novel microporous
nanocomposites.29
will lead to the construction of novel microporous materials,
the key to improved selectivity and activity in separation and
catalytic applications.
Acknowledgements
R. I. is grateful to Dr Y. Shinohara (Tsukuba University,
Japan) for many helpful discussions.
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