Synthesis of highly stable pure-silica thin-walled hexagonally ordered mesoporous material

Article abstract of DOI:10.1039/b901796h

Hexagonally ordered mesoporous silica with a very narrow mesopore size distribution and exceptionally high stability paired with unusually thin pore walls was prepared using piperidine and cetyltrimethylammonium bromide.

Full text of DOI:10.1039/b901796h

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Synthesis of highly stable pure-silica thin-walled hexagonally ordered
mesoporous materialw
a
a
b
b
Pieter Verlooy, Alexander Aerts, Oleg I. Lebedev, Gustaaf Van Tendeloo,
a
a
Christine Kirschhock and Johan A. Martens*
Received (in Cambridge, UK) 28th January 2009, Accepted 18th May 2009
First published as an Advance Article on the web 9th June 2009
DOI: 10.1039/b901796h
Hexagonally ordered mesoporous silica with a very narrow
mesopore size distribution and exceptionally high stability paired
with unusually thin pore walls was prepared using piperidine and
cetyltrimethylammonium bromide.
concepts, we planned to exploit the known stability of
the clathrasil family to obtain a highly stable mesoporous
material.
We developed a synthesis recipe for all-silica clathrasil with
3
2
MTN framework topology based on literature. A clear
solution was prepared from 3.34 mL piperidine (Acros,
99%) and 40.6 mL milliQ water to which 5 mL TEOS (Acros,
99%) were added under vigorous stirring. Dynamic light
scattering (DLS) revealed the formation of silica nanoparticles
measuring 1–2 nm. Heating the clear solution to 453 K
for 21 days resulted in the crystallisation of MTN type
zeolite. Calcination at 1273 K did not cause structural
damage, demonstrating the high thermal stability. Inspired
With the discovery of ordered mesoporous materials in the
early 1990s the pore size limitation imposed by zeolites has
1
been overcome. However, the lower (hydro)thermal stability
of ordered mesoporous materials compared to zeolites is a
major drawback for many applications. Amorphisation some-
times is even observed when calcined samples are stored under
ambient conditions, or when calcination is followed by
2
,3
4–31
wetting. Many strategies attempt to overcome this problem.
Adding mineral salts during synthesis can improve hydro-
1
7,18
by the synthesis of Zeolite-2, the clear solution was
added to an aqueous mixture of CTAB (Acros, 99%, 15 g in
4
,5
thermal stability of MCM-41.
pH during hydrothermal synthesis or using alternative silica
Maintaining a constant
4
,6
135 mL MilliQ water). Unlike the procedures reported in
8,9,16,19,26,30,31
3,7,16,21–29
sources such as amorphous silica,
1
or calcined MCM-41
have been found to increase pore wall thickness,
literature
the zeolite synthesis mixture was not
0–13
material
ordering, and stability. Pore wall thickness and stability are
also increased by reduction of the cetyltrimethylammonium
treated hydrothermally before adding CTAB. While under
these circumstances no extended zeolite like connectivity was
established the occurrence of silica nanoparticles indicated
specific interaction between piperidine and silica oligomers.
After CTAB addition the mixture was stirred for 1 h at room
temperature prior to hydrothermal treatment at 363 K for
7 days. The initial and final pH during synthesis were 11.5 and
11.6, respectively, revealing the pH buffering by piperidine. A
1
CTAB) to silica ratio, increasing hydrothermal synthesis
4
(
7
,11–14
7
or by a second hydrothermal treatment. The use of
time
triblock copolymers allows the synthesis of thick-walled,
more stable ordered mesoporous materials such as SBA-15.
Higher synthesis temperatures generally yield more hydro-
thermally stable SBA-15 materials with wider meso- and less
4
,6
constant pH is known to improve ordering and stability.
2
1–23
micropores.
mesopore walls, silylation, and Al grafting are other ways
Incorporation of hetero-elements into the
The suspension was filtered off, and the product, denoted
COK-11, was washed and dried at 333 K. For removing
the organic molecules, 1 g of as-synthesized COK-11 was
2
,3,7–9,15,16,19,21,24–26,30,31
to increase hydrothermal stability.
Despite the progress, a pure-silica thin-walled ordered meso-
porous material surviving demanding (hydro)thermal condi-
tions has not yet been reported.
¨
suspended in a mixture of 0.17 g acetic acid (Riedel de Haen,
99%) and 80 g ethanol (technical, 96%), refluxed for 1 h and
filtered. The leaching procedure was repeated once. The
leached material was calcined for 1 h at 573 or 898 K using
Zeolitisation of the mesopore wall to improve material
stability is a known concept but it generally results in
thicker walls. The incorporation of FAU and BEA* type
zeolite properties in ordered mesoporous silicates improved
ꢀ1
a heating rate of 0.5 K min
.
Mesoscale ordering was evaluated by XRD.z COK-11
shows seven low angle diffraction peaks in accord with a
hexagonal system (Fig. 1) and revealing the high quality of
ordering. Template leaching did not change the unit cell
constant. Calcinationz at 573 K caused contraction by
2–3%. After calcination at 898 K the contraction was 4–7%
8
,9,16,19,26,30
stability.
are obtained from MFI precursors.
Stable hierarchical materials, the zeolites,
1
7,18,31
Based on these
a
Centre for Surface Chemistry and Catalysis,
Catholic University of Leuven, 3001, Heverlee, Belgium.
E-mail: Johan.Martens@biw.kuleuven.be; Fax: +32 16 321998;
Tel: +32 16 321610
EMAT, Department of physics, University of Antwerp,
2020 Antwerpen, Belgium
w Electronic supplementary information (ESI) available: XRD
data of COK-11; pore wall thickness estimations and thermal and
hydrothermal stability of ordered mesoporous materials. See DOI:
(
ESIw).
Adsorption and desorptionz branches of N
2
physisorption
b
isotherms of leached and calcined COK-11 were found to
of ca. 0.35 is
physisorp-
coincide (Fig. 2). The adsorption step at P/P
0
ascribed to capillary condensation. According to N
2
tion COK-11 contains uniform mesopores (Fig. 2, inset). The
10.1039/b901796h
mesopore sizes were 3.0 ꢁ 0.1, 3.1 ꢁ 0.1 and 2.9 ꢁ 0.1 nm for
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Fig. 1 Powder XRD pattern of leached COK-11 obtained by
accumulating 8 times for 100 s (a) and by accumulating 55 times for
1000 s (b).
Fig. 2 Adsorption (J) and desorption (ꢃ) branches of N
2
-isotherms
of COK-11, leached, calcined 573 K and calcined 898 K. A horizontal
ꢀ1
ꢀ1
shift by 200 mL (STP) g (calcined 573 K) and by 400 mL (STP) g
calcined 898 K) was applied for clarity. Inset: BJH pore size distribu-
(
tion of COK-11 calcined at 573 K.
leached, calcined 573 K and calcined 898 K materials, respec-
tively. The narrow pore size distribution is unusual. MCM-41
materials of similar pore diameter present a standard deviation
2
,4,14,25–29
of ꢁ0.2–0.8 nm.
HRTEMz also revealed a high
ordering, uniform pores and thin pore walls of ca. 0.8 nm
3
Fig. 3). Using a geometrical method (r = 1.6 g cm ; Vp
3
ꢀ3
(
mesopore volume obtained from t-plot) the pore wall thick-
ness of COK-11 was determined to be 0.9 nm (ESIw). This wall
thickness is at the lower limit of MCM-41 materials which
5
,7,8,11–15,19,20,23,28,29
have walls of 0.9–1.7 nm.
pore wall is significantly thinner than in SBA-15, with walls
The COK-11
2
1–23
thicker than 4 nm.
After three years of storage at room temperature, a calcined
sample retained its crystallinity and unit cell dimension (ESIw).
Four diffraction peaks could still be observed (ESIw). The pore
size was 2.8 ꢁ 0.2 nm. The BET surface area of COK-11
Fig. 3 HRTEM images of as-synthesized (a), leached (b) and calcined
573 K (c) COK-11 material oriented along the [001] hexagonal zone
axis. Image (b) is imaged at a very slight underfocus, such that the
boundaries show up as dark and the holes as white. The width of the
boundary, averaged over several boundaries is 0.8 nm. Note that for
larger underfocus values (c) the contrast increases, but the width can
no longer be correctly measured.
2
ꢀ1
calcined at 573 K decreased from 1122 to 1027 m g after
storage. XRD revealed slight structural degradation after
calcination at 1273 K, although three diffraction peaks
remained (ESIw). This thermal stability is comparable to the
best values reported for thick-walled mesoporous materials
373 K for 24 h. Some decrease in the intensity of the higher
order X-ray diffraction peaks and a 5% contraction of the
hexagonal unit cell was observed (ESIw).
and is unique for
7
a
,11,13–15,24,27,28,29
thin-walled ordered mesoporous
A high stability of COK-11 towards
material
the electron beam was also found during HRTEM.
1
3
FTIR and C MAS NMR spectroscopy revealed that
as-synthesised COK-11 contained CTAB, overshadowing
Several methods for evaluating hydrothermal stability of
2
MCM-41 materials have been reported
,4,5,7,12–14,19,20,25–27,30,31
29
possibly present piperidine. According to Si MAS NMR
3
as-synthesised COK-11 contained an equal number of Q and
(
suspension of calcined material (2 g L ) in deionised water at
ESIw). The stability of COK-11 was investigated by heating a
ꢀ
1
4
Q
silicon sites. Similar silicate connectivities have been
4
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Notes and references
z X-Ray diffraction was performed on a STOE Stadi P using Cu-Ka
radiation and an image plate detector covering an angle range of 801.
physisorption isotherms were determined on a Micromeritics
Tristar instrument at 77 K. Leached samples were outgassed under
1
N
2
N
N
2
2
flow at 363 K during 12 h. Calcined samples were outgassed under
flow at 573 K during 12 h. The mesopore size distribution was
calculated from the desorption branch using the Barrett–Joyner–
Halenda (BJH) method. Scanning electron microscopy (SEM) was
performed on a Philips SEM XL30 FEG instrument. The samples
were gold-plated prior to imaging. HRTEM images were taken using a
CM20 electron microscope, operated at 200 kV.
Fig. 4 Representative SEM micrograph of leached COK-11.
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2
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73 K. In Si MAS NMR of COK-11 calcined at 898 K the
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3
and C MAS NMR, N physisorption and XRD and TEM
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1
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3
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P. V. acknowledges the Flemish IWT for a PhD grant; A. A.
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´ ´
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1
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29
performed SEM and K. Houthoofd C and Si MAS NMR.
This journal is ꢂc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4287–4289 | 4289