Chemical modification of crystal-like mesoporous phenylene-silica with amino group

Article abstract of DOI:10.1039/b716141g

Amination of phenylene moieties in crystal-like mesoporous silica hybridized with phenylene is successfully achieved with close to 28% conversion of phenylene by a two-step chemical transformation process while preserving both the ordered mesostructure an

Full text of DOI:10.1039/b716141g

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Chemical modification of crystal-like mesoporous phenylene-silica with
amino groupw
a
Masataka Ohashi,ab Mahendra P. Kapoorz and Shinji Inagaki*ab
Received (in Cambridge, UK) 19th October 2007, Accepted 27th November 2007
First published as an Advance Article on the web 19th December 2007
DOI: 10.1039/b716141g
Amination of phenylene moieties in crystal-like mesoporous
silica hybridized with phenylene is successfully achieved with
close to 28% conversion of phenylene by a two-step chemical
transformation process while preserving both the ordered meso-
structure and crystal-like molecular scale periodicity of the
parent material.
Ph-HMM. Direct bromination,11 amination,12,13 hydroxyla-
tion,14 and Diels–Alder reaction15 of bridged organic moieties
have also been reported for the preparation of HMMs,
although such approaches have only been applied to mesopor-
ous ethenylene-silica with an amorphous pore-wall structure.
The incorporation of functional amino groups into the
framework organics of an HMM is expected to afford highly
useful materials, since the amino group can be transformed
into various functional groups via amide (–NCO–R), imide
(–NCH–R), and diazo (–NN–R) bond formation. Amino-
functionalized HMMs are also potentially applicable as solid
base catalysts. In the present report, the direct amination of
framework phenylene moieties of crystal-like Ph-HMM via a
two-step chemical transformation is demonstrated for the first
time (Scheme 1). This route achieves phenylene conversion of
close to 28% while preserving both the ordered mesoporous
structure and pore-wall crystallinity of the parent Ph-HMM.
The structure is maintained even upon treatment with very
strong acid solutions of HNO3–H2SO4 and SnCl2–HCl,
demonstrating the high chemical stability of Ph-HMM.
Mesoporous organosilica solids have attracted considerable
attention due to the broad range of potential applications of
such materials in catalysis, adsorption, sensors, and electrical
and optical devices.1–3 Hybrid mesoporous materials (HMMs)
having a periodic arrangement of organic and silica moieties
are expected to exhibit unique chemical and physical proper-
ties arising from the highly organized organic framework.2,3
Crystal-like pore-wall structures have been achieved for me-
soporous silica hybridized with phenylene (–C6H4–),4,5 biphe-
nylylene (–C6H4–C6H4–),6 vinylene-phenylene (–CHQ
CH–C6H4–CHQCH–),7,8 and ethenylene (–CHQCH–).9 In
these materials, the organic moieties are strictly assembled in a
circle around the pore channels. Fig. 1 shows a model of the
pore-wall structure of a crystal-like phenylene-bridged HMM
(Ph-HMM). Chemical modification of the framework organic
moieties in HMMs is an effective approach to the construction
of highly functionalized and controlled chemical environments
with uniform and stable mesopore spacing. Such environments
provide functionality similar to that of enzymes, with applica-
tion in selective catalysis and adsorption.
Crystal-like Ph-HMM was synthesized according to the
previously reported procedure.4 A typical modification in-
volves careful treatment of Ph-HMM (0.524 g) with a mixed
acid solution of 96% v/v sulfuric acid (15.2 g) and 69% v/v
nitric acid (3.47 g). After stirring the reaction mixture at 27 1C
for 3 days, the solution was poured into cold distilled water
(300 mL). The solid product was isolated by filtration and
repeatedly washed with a large amount of distilled water. The
nitrated Ph-HMM (NO2-Ph-HMM, 0.480 g) was obtained as
To date there have been only a few reports on the chemical
transformation of organic moieties in crystal-like HMMs. The
present authors have reported the direct sulfonation of frame-
work phenylene (–C6H4(SO3H)–) by treatment of Ph-HMM
with fuming sulfuric acid, although the conversion of pheny-
lene groups was less than 10%.4 Kamegawa et al. reported the
formation of arenetricarbonyl complexes (–C6H4M(CO)3–,
M = Cr, Mo)10 by treatment of Ph-HMM with Cr(CO)6 or
Mo(CO)6 vapor, achieving 15% conversion. The low rates of
conversion can be attributed to the low reactivity of covalently
bonded phenylene moieties in the siloxane network of
a Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan.
E-mail: inagaki@mosk.tytlabs.co.jp; Fax: +81-561-63-6507;
Tel: +81-561-71-7393
Fig. 1 Structural model of pore walls in crystal-like Ph-HMM.
b Core Research and Evolution Science and Technology (CREST),
Japan Science and Technology (JST), Japan
w Electronic supplementary information (ESI) available: Experimental
details, 13C and 29Si MAS NMR and N2 adsorption isotherms. See
DOI: 10.1039/b716141g
z Present address: Taiyo Kagaku Co. Ltd., 1–3 Takaramachi, Yok-
Scheme 1 Amination of phenylene-bridged moiety in crystal-like Ph-
HMM: (i) HNO3–H2SO4 and (ii) SnCl2–HCl.
kaichi, Mie 510-0844, Japan. E-mail: mkapoor@taiyokagaku.co.jp
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Fig. 3 XRD patterns of (A) Ph-HMM and (B) NH2-Ph-HMM.
very small difference in chemical shift of two carbon species in
the 1,4-substituted phenylene moieties. This peak split into
four resonance peaks at 130, 135 (initial), 138, and 150 ppm
upon nitration, corresponding to carbon in phenylene linked
to nitro groups (RSi–C6H4(NO2)–SiR) (Fig. S1w). The
subsequent reduction of nitro groups to amino groups resulted
in slight changes in the spectrum, with resonance peaks
assignable to carbon in phenylene linked to amino groups
(RSi–C6H4(NH2)–SiR).
Quantitative reduction of nitro to amino groups was con-
firmed by the complete disappearance of the N–O and C–NO
stretching bands in the FTIR spectrum, and by the resonance
of nitrated phenylene moieties in the 13C CP/MAS NMR
spectrum. Elemental analysis of NH2-Ph-HMM gives an
amino group density of close to 1.2 mmol gꢁ1, indicating that
nearly 28% of phenylene moieties were linked to amino groups
in the parent Ph-HMM. The 29Si MAS NMR spectrum also
indicates that no Si–C bond cleavage occurred during chemical
modification (Fig. S2w).
Powder X-ray diffraction (XRD) analysis of the parent Ph-
HMM and NH2-Ph-HMM (Fig. 3) reveals diffraction peaks at
low angles (2y o 5) attributable to an ordered arrangement of
meso-channels. Three sharp diffractions are apparent at inter-
mediate scattering angles (d = 7.6, 3.8, and 2.5 A), attribu-
table to the periodic arrangement of phenylene and silica
layers within the pore walls. These results confirm that both
the ordered mesoporous structure and the crystal-like mole-
cular-scale periodicity of the pore walls are preserved, even
upon treatment with very strong acid and a powerful reduc-
tion reagent. Nitrogen adsorption–desorption isotherms also
confirmed the complete retention of a uniform pore structure
(Fig. S3w). However, the XRD peak intensity of the NH2-Ph-
HMM material is slightly lower than that of the parent
material, indicated by the adsorption–desorption isotherms
to be due to a minor loss of surface area and pore volume, as
shown in Table 1.
Fig. 2 FTIR spectra of (A) Ph-HMM, (B) NO2-Ph-HMM, and (C)
NH2-Ph-HMM.
a pale yellow powder after drying to completeness. The NO2-
Ph-HMM powder (0.480 g) was suspended in 15 mL of 37%
v/v HCl solution containing SnCl2 (1.59 g) and the suspension
was stirred continuously at 27 1C for 3 days. After reaction the
suspension was poured into distilled water (300 mL) and
filtered to collect the products. The obtained material was
further washed with a large amount of distilled water, followed
by isopropylamine (10 mL) and ethanol (100 mL). Recovered
material was finally vacuum-dried to afford aminated
Ph-HMM (NH2-Ph-HMM, 0.445 g) as a pale pink powder.
The amination of phenylene moieties in Ph-HMM was
confirmed by Fourier transform infrared (FTIR) (Fig. 2)
and 13C cross-polarization magic angle spinning (CP/MAS)
nuclear magnetic resonance (NMR) analyses (Fig. S1w). Me-
soporous powder was pressed into thin self-supporting wafers
and placed in a quartz cell for FTIR measurements. The FTIR
spectra were recorded in absorbance mode after evacuation at
ambient temperature for 30 min. The FTIR spectrum of the
parent Ph-HMM displayed absorption bands attributable to
the stretching modes of silanol (3200–3800 cmꢁ1), C–H species
(3010–3060 cmꢁ1), and the overtones of benzene ring vibra-
tions (1300–2000 cmꢁ1). These responses are similar to those
reported previously for dry Ph-HMM.16 Treatment with
H2SO4–HNO3 produces two strong bands at 1350 and 1550
cmꢁ1 assigned to the symmetrical and asymmetrical stretching
modes of N–O, respectively. The band at 1277 cmꢁ1 corre-
sponds to the stretching mode of C–N, indicating the covalent
linkage of nitro groups (–NO2) to phenylene moieties in the
parent Ph-HMM. Upon treatment with SnCl2–HCl, the N–O
stretching vibrations disappeared completely while new bands
emerged at 1628 and 3398–3487 cmꢁ1 attributable to the
bending and stretching modes of N–H.
The catalytic performance of NH2-Ph-HMM was studied by
The 13C CP/MAS NMR spectra further support the amina-
tion of phenylene moieties in Ph-HMM. The parent Ph-HMM
displayed a single resonance peak at 135 ppm attributable to a
conducting a typical base-catalyzed reaction, the Knoevenagel
¨
condensation reaction. This reaction is an important C–C
bond-forming reaction that is widely used in the synthesis of
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Table 1 Textural properties of Ph-HMM and NH2-Ph-HMM
The results of catalysis further support the successful amina-
tion of Ph-HMM and the usefulness of the resultant material
as a base catalyst.
Surface area/ Pore volume/
m2 gꢁ1
Pore diametera/
nm
cc gꢁ1
In summary, framework phenylene moieties in crystal-like
Ph-HMM were successfully modified with covalently linked
amino groups at high conversion rates (ca. 28%). The highly
ordered mesostructure and molecular-scale periodicity of the
pore walls of Ph-HMM were completely retained upon mod-
ification despite the use of strong acid and a powerful reduc-
tion reagent. The resultant NH2-Ph-HMM material
Ph-HMM
NH2-Ph-HMM
780
720
0.48
0.44
3.0
2.7
a
By the Barret–Joyner–Halenda method.
successfully catalyzed the Knoevenagel condensation reaction
¨
as a solid base catalyst, and is expected to be applicable as a
precursor material for the design of highly functionalized
mesoporous solids with strong shape-selectivity for catalytic
and adsorption processes.
Notes and references
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3 F. Hoffmann, M. Cornelius, J. Morell and M. Froba, Angew.
¨
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6 M. P. Kapoor, Q. Yang and S. Inagaki, J. Am. Chem. Soc., 2002,
124, 15176.
7 A. Sayari and W. Wang, J. Am. Chem. Soc., 2005, 127, 12194.
Fig. 4 Knoevenagel condensation reaction of malononitrile with
¨
benzaldehyde at 60 1C in toluene solution catalyzed by (&) Ph-
HMM, (n) NO2-Ph-HMM, (J) NH2-Ph-HMM, and (K) recycled
NH2-Ph-HMM.
8 M. Cornelius, F. Hoffmann and M. Froba, Chem. Mater., 2005,
17, 6674.
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¨
chemical intermediates and end products for pharmaceutical
products and perfumes.17 The reaction of benzaldehyde and
malononitrile over NH2-Ph-HMM proceeded to yield a quan-
titative amount of 1,1-dicyanophenylethylene (Fig. 4). Under
identical conditions, Ph-HMM and NO2-Ph-HMM exhibited
negligible activity for this reaction. Conversion of close to
80% was achieved after 2 h on stream, and none of the
Michael addition byproducts typically associated with the
reaction of malononitrile with the double bond of the reaction
product (1,1-dicyanophenylethylene) were detected. Further,
the reusability of the NH2-Ph-HMM was also assessed. After
reaction, the catalyst was filtered off and recycled by washing
and drying under vacuum for further use. No obvious loss of
its initial catalytic activity was monitored upon first recycle.
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