Synthesis of large-pore ordered mesoporous silicas containing aminopropyl groups

Article abstract of DOI:10.1039/b502848p

Ordered mesoporous silicas with large-pore diameters incorporating aminopropyl groups in variable quantity have been synthesized via the co-condensation of tetraethyl orthosilicate (TEOS) and 3-tert- butyloxycarbonylaminopropyltriethoxysilane templated wi

Full text of DOI:10.1039/b502848p

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P A P E R
Synthesis of large-pore ordered mesoporous silicas containing
aminopropyl groups
a
a
b
b
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Ahmad Mehdi, Catherine Reye, Stephane Brandes, Roger Guilard and
Robert J. P. Corriu*a
a
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Laboratoire de Chimie Moleculaire et Organisation du Solide, UMR 5637 CNRS, Universite
de Montpellier II, Sciences et Techniques du Languedoc, Place E. Bataillon,
F-34095 Montpellier Cedex 5, France. E-mail: reye@univ-montp2.fr
b
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Laboratoire d’Ingenierie Moleculaire pour la Separation et les Applications des Gaz,
LIMSAG, UMR 5633, Universite de Bourgogne, 6, Boulevard Gabriel, 21100 Dijon, France
´
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Received (in Montpellier, France) 24th February 2005, Accepted 13th May 2005
First published as an Advance Article on the web 9th June 2005
Ordered mesoporous silicas with large-pore diameters incorporating aminopropyl groups in variable
quantity have been synthesized via the co-condensation of tetraethyl orthosilicate (TEOS) and 3-tert-
butyloxycarbonylaminopropyltriethoxysilane templated with nonionic surfactant P123 under acidic
conditions. The deprotection of amino groups was then quantitatively achieved either by thermal treatment
or acid hydrolysis followed by Et3N treatment, both routes leading to exactly the same materials. We
showed that the free amino centers are fully accessible, by using the condensation of the amine
function with benzaldehyde.
Here we report the synthesis of large-pore aminopropyl-
Introduction
functionalized ordered mesoporous silica by using co-conden-
sation of TEOS and 3-tert-butyloxycarbonylaminopropyl-
triethoxysilane 1 in the presence of surfactant self-assemblies
under acidic conditions. We show that the quantitative carba-
mate deprotection renders the amino centers fully accessible,
which could allow the immobilization of bio-molecules.
Since the discovery of ordered mesoporous silica,1 many
research efforts have been made to modify interior pore
surfaces of mesoporous materials. Postsynthesis grafting of
an organotrialkoxysilane RSi(OR0)3 onto the pore surface was
often used.2 However, this method allows control of neither the
loading nor the distribution of the functional groups.3 More
recently, an alternative approach in one step, overcoming the
main restrictions of the postsynthesis method has been devel-
oped.4–8 It consists of the co-polymerization of tetraethyl
orthosilicate (TEOS) and an organotrialkoxysilane RSi(OR0)3
in the presence of a structure directing agent. However, this
method requires that the R group is sufficiently hydrophobic to
enter the core of the micelle and not too bulky to avoid its
perturbation.
Experimental
General procedures
Among the variety of functional groups that have been
incorporated into mesoporous materials by this route, the
amino group gave rise to numerous studies. Indeed, it is a
particularly useful functionality for many applications such as
entrapping enzymes, selective catalysis and adsorption. Ami-
nopropyl functionalized silicas were synthesized in the presence
of dodecylamine,5 cetyl-trimethylammonium bromide9 or
sodium dodecyl sulfate10 as surfactant by co-condensation of
TEOS and 3-aminopropyltriethoxysilane (APTES). However,
the pore sizes of all these materials are relatively small (less
than 3 nm), which is a drawback for many applications, in
particular for catalysis11 or immobilization of biomolecules. In
an attempt to overcome this drawback, Zhao et al.12 used the
triblock copolymer P123 as surfactant under acidic conditions.
However, they showed a strongly adverse effect of APTES on
the formation of mesostructure under these conditions due to
protonation of the amine functions. Recently, Katz et al.13
reported the synthesis of imprinted amines in bulk silica by
using the deprotection of carbamate groups, which are com-
monly used as protecting agents for amino groups in synthetic
chemistry. These observations prompted us to investigate the
co-condensation method on the protected amino group.
Triblock copolymer (EO20PO70EO20 with PEO ¼ poly(ethy-
lene oxide) and PPO ¼ poly(propylene oxide)) Pluronic P123,
3-aminopropyltrimethoxysilane,
tetraethyl
orthosilicate
(TEOS) and di-tert-butyl dicarbonate were purchased from
Aldrich and used as supplied. The CP MAS 29Si solid state
NMR spectra were recorded on a Bruker FTAM 300 as were
CP MAS 13C solid state NMR spectra, in the latter case by
using the TOSS technique. In both cases, the repetition time
was 5 and 10 s with contact times of 5 and 2 ms. Specific
surface areas were determined by the Brunauer–Emmett–Teller
(BET) method on a Micromeritics ASAP 2010 analyser and the
average pore diameters were calculated by the BJH method.
The X-ray measurements and data treatment were performed
at the ‘‘Groupe de Dynamique des Phases Condensees’’ in
´
Montpellier. The X-ray diffraction experiments were carried
out on solid powders in glass capillaries of 1 mm diameter in
transmission configuration. A copper rotating anode X-ray
source (4 kW) and 2D detector were used. The diffraction
curves were obtained giving diffracted intensity as a function of
the wave vector q. The diffracted intensity was corrected by
exposure time, transmission and intensity background coming
from diffusion by an empty capillary.
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 6 5 – 9 6 8
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Table
1
Textural and structural data for SBA-xNHBoc and
Preparations
SBA-xNH2
3-tert-Butyloxycarbonylaminopropyltriethoxysilane, 1. 3-tert-
Butyloxycarbonylaminopropyltriethoxysilane 1 was prepared
by mixing 3-aminopropyltriethoxysilane (11.30 g, 51.1 mmol)
and di-tert-butyl dicarbonate (12.50 g, 57.3 mmol) in 50 mL of
ethanol. The resulting mixture was stirred overnight at room
temperature. The solvent was removed under vacuum and the
residual liquid was distilled to afford 12.10 g of 1 (37.3 mmol,
Sample
SBET/m2 gꢀ1
Dpa/nm
Vp/cm3 gꢀ1
d100/nm
SBA-20NHBoc
SBA-10NHBoc
SBA-5NHBoc
SBA-20NH2
SBA-10NH2
SBA-5NH2
a
596
570
522
550
540
420
4.4
6.1
7.5
4.0
6.0
7.5
0.70
1.02
1.07
0.62
0.92
1.00
9.38
10.47
10.74
9.84
10.65
10.92
1
73%) as a colorless liquid (bp 95 1C at 0.05 Torr). H NMR
(dppm, 200 MHz, CDCl3): 0.60 (m, 2H, CH2Si), 1.20 (t, 9H,
3JHH ¼ 6.90 Hz, OCH2CH3), 1.41 (s, 9H, C(CH3)3), 1.54
(m, 2H, CH2CH2CH2), 3.09 (m, 2H, CH2N), 3.79 (q, 6H,
3JHH ¼ 7.00 Hz, OCH2), 4.76 (s, 1H, NH). 29Si NMR (dppm, 40
MHz, CDCl3):-45.40.
Calculated from desorption branch using the BJH method.
removed by washing to give the functionalized material de-
noted SBA-xNHBoc (SBA to recall the P123 surfactant used, x
to indicate the molar % of organic groups in the initial mixture
and NHBoc for functional groups) in high yield.
It is worth noting that no solid was obtained under the same
conditions by using APTES instead of 1. This result demon-
strates the importance of the approach with protected amino
groups. Some relevant textural and structural data for
SBA-xNHBoc are given in Table 1.
SBA-10NHBoc. 4.0 g of P123 (EO20PO70EO20) were dis-
solved in an aqueous HCl solution (160 mL, pH E 1.5). This
solution was poured on to a mixture of TEOS (8.41 g, 40.4
mmol) and 1 (1.44 g, 4.4 mmol) at ambient temperature. The
molar composition of the reaction mixture was: 0.04 Fꢀ : 1
TEOS : 0.11 1 : 0.02 P123 : 0.12 HCl : 220 H2O. The mixture
was stirred for 2 h giving rise to a microemulsion. After heating
this perfectly transparent solution at 60 1C, a small amount of
NaF (75.4 mg) was added under stirring to induce the poly-
condensation. The mixture was left at 60 1C under stirring for
48 h. The resulting solid was filtered and washed with ethanol
and ether. The surfactant was removed by hot ethanol extrac-
tion in a Soxhlet apparatus during 24 h. After filtration and
drying at 60 1C under vacuum, 3.10 g (4.2 mmol, 95%) of
SBA-10NHBoc were obtained as a white solid.
The powder small-angle X-ray pattern of SBA-10NHBoc
(Fig. 1a) exhibits an intense diffraction peak corresponding
to d100 spacing (10.47 nm) and a second weak and broad peak
centered at 5.96 nm. Further evidence for an ordered hexago-
nal structure was provided by transmission electron micro-
scopy (TEM) image (see the inset in Fig. 2). The nitrogen
adsorption–desorption isotherm for SBA-10NHBoc is dis-
played in Fig. 2. The sample showed type IV isotherm with
clear H1-type hysteresis loop at relative high pressure, char-
acteristic of mesoporous materials with large pores and narrow
pores size distribution. The BET surface area increases, but
pore volume and pore size decrease with increasing the
concentration of 1 (Table 1).
The incorporation of carbamate groups in the mesoporous
materials and the removal of surfactant were confirmed by
solid-state NMR spectroscopy. The 13C CP/MAS NMR spec-
trum of SBA-10NHBoc (Fig. 3) demonstrates that the carba-
mate group remains intact as shown by the signals at 27.31
ppm (CH3 resonances of tert-butyl), 158.00 ppm (carbonyl
resonances) and three additional signals (43.00, 22.27 and 9.85
ppm) attributed to the propyl spacer. The 29Si MAS NMR
spectrum displays signals at ꢀ101.89 ppm and ꢀ111.60 ppm
attributed to the substructures Q3 and Q4, respectively, denot-
ing high cross-linking of the siloxane species. An additional
signal at ꢀ66.75 ppm assigned to the substructure T3 showed
the fully cross-linked organosilsesquioxane species.
SBA-10NH2 by thermal treatment. 2.00 (2.7 mmol) of solid
SBA-10NHBoc were introduced into a one-necked round bot-
tomed flask. The flask was heated at 160 1C under vacuum
during 12 h. The resulting solid was washed with ethanol and
ether. After filtration and drying at 60 1C under vacuum, 1.65 g
(2.6 mmol, 95%) of SBA-10NH2 were obtained as a white solid.
SBA-10NH2 by acidic hydrolysis. To a 6 M aqueous solution
of HCl (40 mL) placed in a one-round bottomed flask, 2.05 (2.7
mmol) of solid SBA-10NHBoc were introduced. The resulting
mixture was heated at 100 1C during 12 h. The obtained solid
was filtered and washed with water (3 ꢁ 100 mL) and then
added to a solution (30 mL) of triethylamine–dichloromethane
(1 : 5 v/v) to deprotonate the amino groups. After 1 h the solid
was recovered by filtration and washed with ethanol and ether.
After filtration and drying at 60 1C under vacuum, 1.45 g (2.3
mmol, 85%) of SBA-10NH2 were obtained as a white solid.
Results and discussion
First, we prepared SBA-15 mesoporous silica containing vari-
able amounts of tert-butyloxycarbonylamino (NHBoc) groups
called carbamate groups (Scheme 1). The synthesis of these
materials was achieved by co-polymerization of 1 and tetra-
ethyl orthosilicate (TEOS) in the presence of P123 as structure
directing agent (see Experimental section). The surfactant was
Scheme 1 Preparation of SBA-xNH2 material by (i) thermal treat-
ment or (ii) acid hydrolysis of SBA-xNHBoc solid followed by Et3N
treatment.
Fig. 1 SAXS patterns for (a) SBA-10NHBoc and (b) SBA-10NH2.
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N e w J . C h e m . , 2 0 0 5 , 2 9 , 9 6 5 – 9 6 8

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Scheme 2 Imine formation by coupling between the amino groups of
SBA-xNH2 materials and the benzaldehyde.
very similar (Fig. 2) to that of the starting material and
revealed that the mesoporosity was maintained as well as a
narrow pore size distribution.
The accessibility of the amino centers in SBA-xNH2 material
was investigated by using the condensation reaction of the
amine function with benzaldehyde to give imine (Scheme 2).
The solid SBA-xNH2 were treated with an excess of benzalde-
hyde (2 equiv. of benzaldehyde per NH2 moiety) in anhydrous
toluene under reflux for 12 h. The resulting solids, called
SBA-xImine, were copiously washed with toluene to eliminate
the unreacted benzaldehyde. The filtrate including the excess
benzaldehyde was analyzed by gas chromatography.
It was found that in all cases only one equivalent of
benzaldehyde remained in solution. In addition, the 13C
NMR spectra of SBA-xImine revealed the presence of aromatic
groups. These results indicate that all the amino groups were
accessible. It is worth noting that the same reaction does not
take place when SBA-xNHBoc solids were used instead of
SBA-xNH2. We observed that the lower the % (x ¼ 5), the
easier the transformation of amino groups in the imine,
probably due to steric reasons. Thus, when x ¼ 5 the whole
transformation takes 4 h and when x ¼ 20, it takes 12 h.
Fig. 2 N2 adsorption–desorption isotherms of SBA-10NHBoc and
SBA-10NH2. The inset shows the TEM image of SBA-10NHBoc.
Two independent routes for carbamate deprotection were
used leading to exactly the same materials (Scheme 1). Thermal
treatment of SBA-xNHBoc at 160 1C under vacuum gave rise
quantitatively to mesoporous silica containing aminopropyl
groups (SBA-xNH2). The amino groups can also be recovered
by treating SBA-xNHBoc with a 6 M aqueous solution of HCl
for 12 h under reflux. The resulting solids were then subse-
quently treated with CH2Cl2 triethylamine solution to depro-
tonate the amino groups giving rise to SBA-xNH2 in 95%
yield. Some relevant textural and structural data for
SBA-xNH2 are given in Table 1.
In both routes, the transformation of carbamate groups to
the corresponding primary amine was quantitative. This is
clearly reflected in the 13C CP/MAS NMR spectrum (Fig. 3)
by the disappearance of resonances associated with the carba-
mate protecting group (27.31 and 158.00 ppm) while the signals
(42.72, 21.54 and 9.34 ppm) of the propyl spacer were retained.
Interestingly, no significant change was observed on the
SAXS pattern for the SBA-10NH2 material (Fig. 1b) indicating
a very stable structure even after chemical transformation. The
nitrogen adsorption–desorption isotherm for SBA-10NH2 was
Conclusion
We have shown that it is possible to obtain large-pore ordered
mesoporous silica functionalised with aminopropyl groups
within the channel pores via tert-butyl carbamate as protecting
agent. This protecting group allowed us not only to obtain
material, whereas no material was obtained starting from the
free amino group (under the same conditions), but to obtain an
ordered material in spite of its bulk. Hydrolysis or thermal
treatment of tert-butyl carbamate generates quantitatively the
amine functions, which are fully accessible and could allow the
immobilization of bio-molecules.
Acknowledgements
The authors thank Dr Philippe Dieudonne
Montpellier, France) for SAXS measurements, the CNRS
and the Universite Montpellier II for financial support.
´
(GDPC-UMII,
´
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