The use of acidic task-specific ionic liquids in the formation of high surface area mesoporous silica

Article abstract of DOI:10.1039/b907077j

The acidic ionic liquids 1-alkyl-3-methylimidazolium hydrogen sulfate [C_nMIM]HSO₄ (n = 8, 10, 12, 16) were synthesised using a one-pot method and then used as new acidic templates to generate high surface area ordered mesoporous sili

Full text of DOI:10.1039/b907077j

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LETTER
www.rsc.org/njc | New Journal of Chemistry
The use of acidic task-specific ionic liquids in the formation of high
surface area mesoporous silica
Ajit A. Pujari, Jessica J. Chadbourne, Antony J. Ward, Lorenzo Costanzo,
Anthony F. Masters and Thomas Maschmeyer*
Received (in Victoria, Australia) 7th April 2009, Accepted 16th July 2009
First published as an Advance Article on the web 2nd September 2009
DOI: 10.1039/b907077j
The acidic ionic liquids 1-alkyl-3-methylimidazolium hydrogen
sulfate [CnMIM]HSO4 (n = 8, 10, 12, 16) were synthesised
using a one-pot method and then used as new acidic templates
to generate high surface area ordered mesoporous silicas
(41000 m2 gÀ1) using a one-step synthetic route.
and as a catalyst for the reaction being performed. The
most obvious cases involve the use of acidic ionic liquids
(usually possessing the HSO4À anion) catalysing various reactions
such as the Biginelli reaction,14 Friedel–Crafts alkylation,15
hydrolytic reactions,16 and electrophilic substitution.17
Herein we describe the one-pot microwave synthesis of the
acidic ionic liquids 1-alkyl-3-methylimidazolium hydrogen
sulfate (where alkyl = octyl, decyl, dodecyl, hexadecyl) and
their subsequent application as both templates and acid source
for the synthesis of (short-range) ordered, high-surface-area
mesoporous silicas.
The discovery of three-dimensional ordered mesoporous
silica1 has generated wide interest in the synthesis of mesoporous
materials based on self-assembly of ordered, surfactant/inorganic
structures. Routes based on ionic, hydrogen and covalent
bonding between the surfactant and the inorganic precursors
have been developed for the synthesis of mesoporous silicas
and other metal oxides. Much effort has been focused on
developing ordered periodic mesoporous materials with
well-defined, controlled pore channels by sol–gel templating
using amphiphilic surfactants as templates.2,3
The acidic ILs [CnMIM]HSO4 (n = 8 (1), 10 (2), 12 (3), 16 (4))
were synthesised by a one-pot method using microwave
irradiation and obtained as a viscous orange oil in the case
of 1 and white waxy solids for 2–4. The method involved
microwave irradiation of equimolar quantities of 1-methyl-
imidazole, the appropriate alkyl halide and NaHSO4. The
counterion was added to the synthesis in three approximately
equal aliquots to allow for complete reaction to occur during
the irradiation process. With the counterion being present in
the reaction mixture, the high temperature of the reaction and
the solubilising power of the ILs allow for the anion exchange
to occur in situ, rather than post-synthetically. Post-synthetic
anion exchange can be difficult due to the low solubility of
salts such as NaHSO4 in organic solvents. Once the crude
reaction mixture had cooled, the product was extracted with
CH2Cl2 and filtered to remove unwanted salts (NaBr and
unreacted NaHSO4). Upon removal of the solvent in vacuo,
the product was washed with hexane and diethyl ether to
afford the products as viscous oils or white solids. The overall
yields of 1–4 were greater than 80% after purification.
Products 1–4 were characterised by 1H and 13C{1H} NMR.
Testing of the ILs for halides using silver nitrate revealed the
presence of trace amounts of halides in the final products.
This, however, does not affect their usefulness as templates or
as hydrolysis catalysts for the preparation of mesoporous
silicas.
Room temperature ionic liquids (RTILs) have attracted
much attention due to their unusual properties, especially their
low vapour pressure, high thermal and chemical stability,
and excellent solvating power for organic and inorganic
compounds.4–6 Since most long-chain RTILs possess both a
hydrophilic ionic head group and a hydrophobic organic
chain, they represent a category of surfactant and can form
liquid crystals in various solvents. Accordingly, they can be
used as templates to prepare microporous and mesoporous
materials.7,8 In addition, they have been widely utilised as
environmentally benign solvents for various processes, including
synthesis, separation and catalysis, frequently achieving higher
catalytic activity and/or selectivity than accessible with
traditional solvents.9,10 They are also a novel class of salts
that have already been found to perform well in the preparation
of nanostructured materials and nanoparticles for catalysis.11,12
Seddon et al.13 and other groups have been interested in
using ionic liquids as reaction media for synthesising advanced
materials. Herein, we report the dual use of a range of acidic
ionic liquids in which the cation acts as a template and the
anion as the acid source for the synthesis of mesostructured
hybrid silica materials.
The basic molecular structure of the new ionic liquids is
shown in Fig. 1. The head group of these new templates is the
alkylated imidazolium cation, which can interact not only with
negatively charged silica precursors but also potentially
with the hydrophobic organic bridges of silsesquioxanes.18
Cetyltetrabutylammonium bromide (CTAB), a frequently
used template for the preparation of mesostructured
materials,2,19 requires the addition of an acid or base to
catalyse the hydrolysis of the silica precursor and subsequent
As the name suggests, task-specific ionic liquids (TSILs)
perform a dual role in a process for which they are being
applied. Often, these dual roles are claimed to be as a solvent
Laboratory for Advanced Catalysis for Sustainability,
School of Chemistry F11, University of Sydney, Sydney, Australia.
E-mail: th.maschmeyer@chem.usyd.edu.au; Fax: +61 2 9351 3329;
Tel: +61 2 9351 2581
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Fig. 1 Dual characteristics of the acidic ionic liquids.
silica-forming condensation reactions. Thus, these new ILs,
when used in the preparation of mesoporous silicas, serve the
dual roles of template and acid source.
Fig. 2 X-Ray diffraction patterns of silicas I–IV.
The mesoporous silicas were prepared by a hydrothermal
synthesis procedure using tetraethyl orthosilicate (TEOS) as
the silica source and task specific ionic liquids (TSILs) as the
structure directing agent and acid source. The final product
was ground into powder for further characterisation. The
surface areas, pore sizes, and pore volume of mesostructured
materials obtained when the acidic ionic liquids are used as
templates are summarised in Table 1. The materials generated
show an increase in surface area with increase in the alkyl
chain length of the ionic liquid to a maximum of 1103 m2 gÀ1
when [C16MIM]HSO4 is used as the template. The pore sizes
show a significant decrease from 99 A to 41 A with an increase
in chain length from 8 to 16. The large difference in pore sizes
associated with the C-8 chain length versus the C-10
and higher is consistent with a different, i.e. non-micellar,
templating mechanism for C-8 (e.g. similar to that ascribed to
the TUD-1 structure family).20 The decrease in pore size going
from C-10 to C-11 can be attributed to the more efficient
packing of the longer alkyl chains.
The TEM images reveal that none of the siliceous materials
prepared by this method have any observable long-range
order. The images of I and IV (Fig. 3) show that the silicas
have a sponge-like topology. This observation is consistent
with the XRD data.
The nitrogen adsorption–desorption isotherms for each of
the silicas I, II, III and IV are classic Type IV curves which
indicates that the silica is mesoporous in nature (Fig. 4), as
such materials have pore sizes in the range of 2–50 nm and are
characterised by Type IV isotherms which have a hysteresis.
This type of isotherm indicates that capillary condensation is
occurring during the adsorption process. These results are
similar to Zhou’s work on ionic-liquid-casted supermicro-
porous lamellar phase silica,21 which shows that imidazolium-
based ionic liquids can template ordered mesoporous silica as
well as supermicroporous lamellar silica.
We have demonstrated
a self-assembly synthesis of
mesoporous materials using acidic ILs. This research clearly
opens an easy method of synthesis of mesoporous materials
using acidic ILs.
The XRD patterns of the mesostructured materials are
shown in Fig. 2. These patterns correspond closely to the
structured mesoporous materials reported by Kresge et al.,19
Zhou et al.21 and Stucky et al.3 Miskolczy et al.22 reported that
as the alkyl chain length of the template increased, the materials
showed improved X-ray diffraction patterns. The same group
also reported that ionic liquids should have at least nine carbon
atoms in their hydrophobic tail to form micelles in aqueous
solution which suggests that in the case of the template with the
C8 alkyl chain (1) it would be very difficult to form a structured
mesoporous material. This hypothesis matches with the data
presented in Fig. 2. In the case of I, the XRD pattern does not
have the (100) reflection present in silicas II, III and IV
indicating that this silica does not possess any regular repeating
structure. The presence of the peak at B31 2y indicates that the
pore sizes in the silicas II, III and IV have a narrow pore size
distribution which gives rise to regions of short-range order.
The lack of long-range order is indicated by the absence of
additional reflections in the XRD patterns.
Experimental
Microwave irradiations were performed using a Milestone
BatchSYNTH in a 2-necked round-bottom flask fitted with
a reflux condenser. 1H NMR (300.13 MHz) spectra and
13C{1H} NMR (75.48 MHz) spectra were recorded using a
Bruker DPX300 NMR spectrometer and were referenced
internally to residual proteo-solvent impurities (CD3CN,
d 1.94 (1H NMR) and d 1.32 (13C NMR)). Mass spectra (ESI)
were recorded on a Finnigan LCQ MS detector. X-ray
diffraction patterns were recorded using a Siemens D5000
diffractometer using CuKa radiation at 40 kV. Diffraction
patterns were recorded in the 1–601 2y range in steps of
0.02 s with 15 s per step. The nitrogen adsorption and
desorption isotherms at 77 K were measured using a
Micromeritics ASAP2020 surface area and porosity analyser
Table 1 The surface area, pore sizes and pore volumes of the mesostructured silicas I–IV
Silica
TSIL
Surface area/m2 gÀ1
Pore volume/cm3 gÀ1
Pore size/A
I
II
III
IV
[C8MIM]HSO4
[C10MIM]HSO4
[C12MIM]HSO4
[C16MIM]HSO4
737
991
1017
1103
1.820
1.475
1.149
1.020
98.85
59.54
45.19
41.22
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addition, the reaction mixture was irradiated for 12 min at
300 W at 110 1C. The product was extracted with CH2Cl2
(100 mL) and filtered to remove the salts. The solvent was
removed in vacuo. The resultant oil was washed with diethyl
ether (2 Â 100 mL). The product was then dried in vacuo at
80 1C to afford the product.
1-Methyl-3-octylimidazolium hydrogen sulfate 1. 1H NMR
(CD3CN): d 9.97 (s, ImC2-H), 7.60 (m, 1H, ImH), 7.44 (m, 1H,
ImH), 4.24 (t, 2H, 3JHH = 7 Hz, N-CH2), 4.04 (s, 3H, N-CH3),
3
1.84 (m, 2H, NCH2CH2), 1.21 (m, 10H), 0.76 (t, 3H, JHH
=
7 Hz, CH3) ppm. 13C {1H}NMR (CD3CN): d 137.17, 123.78,
122.03, 67.73, 50.02, 36.60, 31.57, 30.21, 28.92, 28.85, 26.15,
22.46, 13.95 ppm. m/z (ESI+): 195 (M+, 100%).
Fig. 3 TEM images of silica I (a) and IV (b).
1-Methyl-3-decylimidazolium hydrogen sulfate 2. 1H NMR
(CD3CN): d 9.22 (s, ImC2-H), 7.51 (m, 1H, ImH), 7.48
(m, 1H, ImH), 4.19 (t, 2H, JHH = 7 Hz, N-CH2), 3.89
3
(s, 3H, N-CH3), 1.82 (m, 2H, NCH2CH2), 1.24 (m, 16H),
3
0.85 (t, 3H, JHH = 7 Hz, CH3) ppm. 13C {1H}NMR
(CD3CN): d 137.60, 124.47, 123.92, 50.32, 32.55, 30.66,
30.27, 30.18, 30.05, 29.93, 29.60, 26.65, 23.30, 14.35 ppm.
m/z (ESI+): 223 (M+, 30%).
1-Methyl-3-dodecylimidazolium hydrogen sulfate 3. 1H NMR
(CD3CN): d 9.18 (s, ImC2-H), 7.49 (m, 1H, ImH), 7.46 (m, 1H,
ImH), 4.18 (t, 2H, 3JHH = 7 Hz, N-CH2), 3.88 (s, 3H, N-CH3),
3
1.82 (m, 2H, NCH2CH2), 1.25 (m, 18H), 0.86 (t, 3H, JHH
=
7 Hz, CH3) ppm. 13C {1H}NMR (CD3CN): d 137.03, 123.96,
122.62, 66.90, 50.95, 36.30, 32.06, 30.15, 29.78 (2 carbons),
29.69, 29.56, 29.50, 29.10, 26.15, 22.81 ppm. m/z (ESI+): 251
(M+, 100%).
1-Methyl-3-hexadecylimidazolium hydrogen sulfate 4. 1H
NMR (CD3CN): d 9.12 (s, ImC2-H), 7.47 (m, 1H, ImH),
7.44 (m, 1H, ImH), 4.18 (t, 2H, JHH = 7 Hz, N-CH2), 3.88
Fig. 4 Nitrogen adsorption–desorption isotherms recorded at 77 K
for silicas I (A), II (B), III (C), and IV (D).
3
(s, 3H, N-CH3), 1.82 (m, 2H, NCH2CH2), 1.26 (m, 26H), 0.87
3
(t, 3H, JHH = 7 Hz, CH3) ppm. 13C{1H} NMR (CD3CN):
using B100 mg of the siliceous material. The data were
obtained by liquid nitrogen adsorption and desorption at
various nitrogen partial pressures and were analysed by the
BJH (Barrett–Joyner–Halenda) and the BET (Brunauer–
Emmett–Teller) methods. The pore size distribution curve
was derived from the analysis of the adsorption branch of
the isotherm. Transmission electron micrographs were
recorded digitally with a Gtan slow-scan charge-coupled
device (CCD) using a Phillips CM120 Biotwin electron
microscope operating at 120 kV. The samples were prepared
by dispersing the powder products as a slurry in ethanol,
which was then deposited and dried on a holey carbon film on
a Cu grid.
d 137.46, 124.28, 123.14, 68.69, 50.36, 36.81, 32.56, 30.63,
30.27 (4 carbons), 30.18, 30.05, 29.99, 29.58, 26.64, 23.31,
14.33 ppm. m/z (ESI+): 307 (M+, 100%).
Synthesis of mesoporous silica templated with acidic ionic
liquids
In a typical synthesis [CnMIM]HSO4 (1.0 g, 3.42 mmol) and
tetraethoxysilane (TEOS, 0.71 g, 3.42 mmol) were dissolved in
ethanol under mild magnetic stirring. After homogenisation of
the mixture, water (0.68 g, 37.6 mmol) was added dropwise
and the mixture stirred until a homogenous gel was obtained.
The molar compositions of the starting mixtures were
1 TEOS : 1 [CnMIM]HSO4 : 11 H2O. The resulting mixtures
were then transferred to a PTFE-lined steel autoclave and
heated at 160 1C for 118 h. After hydrothermal treatment, the
mixtures were dried at 190 1C for 124 h and then calcined at
550 1C for 110 h with a temperature ramp rate of 165 1C minÀ1
under a stream of air to remove the template.
Synthesis of acidic ionic liquids
A typical synthesis involved mixing a 1.05 molar excess of
1-alkyl bromide (60.9 mmol) to 1-methylimidazole (4.76 g,
58.0 mmol). To this mixture was added NaHSO4 (B2.0 g).
The mixture was then irradiated in a microwave for 5 min at
300 W to a maximum temperature of 110 1C. After this time
additional NaHSO4 (B2.0 g) was added and irradiated again.
This procedure was repeated until a molar equivalent of
NaHSO4 (7.32 g, 60.9 mmol) had been added. After the final
Acknowledgements
The authors would like to thank the Australian Research
Council for funding.
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11 A. Riisager, P. Wasserscheid, R. V. Hal and R. Fehrmann,
J. Catal., 2003, 219, 452.
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2000 | New J. Chem., 2009, 33, 1997–2000