Synthesis and characterization of alkyl-imidazolium-based periodic mesoporous organosilicas: A versatile host for the immobilization of perruthenate (RuO4-) in the aerobic oxidation of alcohols

Article abstract of DOI:10.1002/chem.201200380

The preparation and characterization of a set of periodic mesoporous organosilicas (PMOs) that contain different fractions of 1,3-bis(3- trimethoxysilylpropyl)imidazolium chloride (BTMSPI) groups uniformly distributed in the silica mesoporous framework is described. The mesoporous structure of the materials was characterized by powder X-ray diffraction, transmission electron microscopy, and N₂ adsorption-desorption analysis. The presence of propyl imidazolium groups in the silica framework of the materials was also characterized by solid-state NMR spectroscopy and diffuse-reflectance Fourier-transform infrared spectroscopy. The effect of the BTMSPI concentration in the initial solutions on the structural properties (including morphology) of the final materials was also examined. The total organic content of the PMOs was measured by elemental analysis, whereas their thermal stability was determined by thermogravimetric analysis. Among the described materials, it was found that PMO with 10 % imidazolium content is an effective host for the immobilization of perruthenate through an ion-exchange protocol. The resulting Ru@PI-10 was then employed as a recyclable catalyst in the highly efficient aerobic oxidation of various types of alcohols.

Full text of DOI:10.1002/chem.201200380

FULL PAPER
DOI: 10.1002/chem.201200380
Synthesis and Characterization of Alkyl Imidazolium Based Periodic
Mesoporous Organosilicaꢀs: A Versatile Host for the Immobilization of
Perruthenate (RuO4 ) in the Aerobic Oxidation of Alcohols
Babak Karimi,*[a] Dawood Elhamifar,[a, b] Omolbanin Yari,[a] Mojtaba Khorasani,[a]
Hojatollah Vali,[c] James H. Clark,[d] and Andrew J. Hunt[d]
Abstract: The preparation and charac-
terization of a set of periodic mesopo-
rous organosilicas (PMOs) that contain
different fractions of 1,3-bis(3-trime-
thoxysilylpropyl)imidazolium chloride
(BTMSPI) groups uniformly distribut-
ed in the silica mesoporous framework
is described. The mesoporous structure
of the materials was characterized by
powder X-ray diffraction, transmission
electron microscopy, and N2 adsorp-
tion–desorption analysis. The presence
of propyl imidazolium groups in the
silica framework of the materials was
also characterized by solid-state NMR
spectroscopy and diffuse-reflectance
Fourier-transform infrared spectrosco-
py. The effect of the BTMSPI concen-
tration in the initial solutions on the
structural properties (including mor-
phology) of the final materials was also
examined. The total organic content of
the PMOs was measured by elemental
analysis, whereas their thermal stability
was determined by thermogravimetric
analysis. Among the described materi-
als, it was found that PMO with 10%
imidazolium content is an effective
host for the immobilization of per-
ruthenate through an ion-exchange
protocol. The resulting Ru@PI-10 was
then employed as a recyclable catalyst
in the highly efficient aerobic oxidation
of various types of alcohols.
Keywords: ionic liquids · mesopo-
rous materials · organosilicas · per-
ruthenate · supported catalysts
Introduction
rous materials can normally be achieved through co-conden-
ꢀ
sation of organotrialkoxysilanes [(R’O)3Si R] with tetraal-
Since the first discovery of surfactant-templated ordered
mesoporous silica by Beck et al.,[1,2] organically functional-
ized mesoporous silicas[3,4] have been extensively explored
for applications that range from gas sorption, chromatogra-
phy, and catalysis, to biological uses.[3–6] These materials ex-
hibit a large specific surface area and narrow pore-size dis-
tribution that ranges from 2 to 30 nm. For most applications,
the incorporation of organic groups inside ordered mesopo-
koxysilanes in the presence of a suitable structure directing
agent (SDA) to produce organofunctionalized ordered mes-
oporous materials.[3–6] In these types of materials, however,
due to a loss or a significant reduction in the degree of long-
range order, inhomogeneous distribution of organic groups
in the pores, and partial filling of pore spaces by pendant or-
ganic groups, thereby restricting substrate diffusion into the
channels, the organic content of these materials cannot
exceed approximately 25 molar percent of the total silicon
source.[7,8] To address these limitations, in 1999 a number of
groups including Inagaki,[9] Stein,[10] and Ozin[11] independ-
ently discovered a new class of hybrid organic–inorganic
materials called periodic mesoporous organosilicas (PMOs)
[a] Prof. Dr. B. Karimi, Dr. D. Elhamifar, O. Yari, Dr. M. Khorasani
Department of Chemistry
Institute for Advanced Studies in Basic Sciences (IASBS)
PO Box 45195-1159, Gava zang
ꢀ ꢀ
Zanjan, 45137-6731 (Iran)
Fax : (+98)241-4214949
E-mail: karimi@iasbs.ac.ir
built from bridge organosilica precursors, [(R’O)3Si R Si-
AHCTUNGRTEG(NNUN OR’)3], in which organic group R is an integral part of the
mesoporous walls. In these materials, the bridging organic
moieties in the framework of PMOs not only have improved
hydrothermal and mechanical stabilities, but also higher or-
ganic loading and greater avoidance of channel blockage
can be achieved relative to mesoporous materials functional-
ized with terminal organic groups.[7,8,12–14] These unique
properties allow PMOs to be excellent candidates for wide-
spread applications in material and chemical sciences.[7,8,12,13]
In particular, depending on the nature of the organic species
in the framework, the chemical and physical properties of
PMOs can be tuned towards specific applications. These in-
clude a plethora of bridged organic units that have been uti-
[b] Dr. D. Elhamifar
Department of Chemistry, Yasouj University
Yasouj, 75918-74831 (Iran)
[c] Prof. H. Vali
Anatomy and Cell Biology and Facility for
Electron Microscopy Research, McGill University
3450 University St, Montreal
Quebec, H3A 2A7 (Canada)
[d] Prof. J. H. Clark, Dr. A. J. Hunt
Green Chemistry Centre of Excellence
University of York, York, YO10 5DD (UK)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200380.
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lized for the preparation of different types of PMOs with
novel bulk properties, thus extending the concept of “pore
chemistry” into the “wall chemistry” inside the hybrid meso-
porous material world.[7,8,12,13,15–23]
These materials showed remarkable thermal and structur-
al properties over a relatively wide concentration range and
synthetic conditions. The possibility of using these materials
as an effective host for the immobilization of perruthenate
in the aerobic oxidation of various types of alcohols is also
investigated in some detail for the first time.
Ionic liquids (ILs) have received great attention in recent
years in a variety of areas that range from catalysis[24–27] and
separation science to material synthesis as well as environ-
mentally acceptable solvents for chemical reactions.[24–29]
However, despite promising results, their widespread appli-
cation in process chemistry is still hampered because many
of them are very expensive and are a cause of toxicological
concern.[30] Moreover, it is well known that the relatively
high viscosity of the ionic liquids in many of the liquid–
liquid biphasic reactions means that generally only a small
fraction of the so-called “diffusion layer” of ILs participates
in the overall reaction process.[31] Therefore, on account of
the slow mass transfer of highly viscose liquids, it is indeed
not necessary to use large amounts of ionic-liquid solvents.
To address these limitations, the concept of supported ionic-
liquid catalysis (SILC) has recently been introduced, which
combines the advantages of ionic liquids with those of heter-
ogeneous systems such as easy recyclability.[32–38] However, it
has been shown that in many cases the ionic-liquid layer,
which serves as the reaction phase, can be partially dissolved
in the organic phase, thus restricting catalyst recov-
ery.[32,33,39,40] Although drawbacks such as leaching can be
partly avoided by designing various types of chemically sup-
ported ionic liquids,[41,42] the low loading of immobilized
ionic liquid and their inhomogeneous distribution through
inorganic supports are still important issues that remain un-
resolved. Considering the unique characters of PMOs, we
recently prepared a novel PMO that contained 10% 1,3-
Results and Discussion
Nitrogen adsorption–desorption and pore-size distribution
isotherms of solvent-extracted PMO materials are shown in
Figure 1.
The isotherms indicate that the pore sizes of all samples
are in the mesoporous range. The organosilica materials
showed typically type IV N2-sorption isotherms with H1 or
bis(3-trimethoxysilylpropyl)imidazolium
chloride
(BTMSPCl) groups that were uniformly distributed in the
silica mesoporous framework.[43] In our follow-up study
herein, we wish to demonstrate the possibility of using a rel-
atively wide range of concentration of BTMSPCl ionic liq-
uids bound to the silica framework of the PMO materi-
als.[44,45] The preparation of ionic-liquid-based periodic mes-
oporous organosilicas (PMO-IL) was achieved by co-con-
densation of 1,3-bis(3-trimethoxysilylpropyl)imidazolium
chloride ionic liquid 1 and tetramethoxysilane (TMOS) in
the presence of Pluronic P123 under acidic conditions
(Scheme 1).
Figure 1. a) Nitrogen adsorption–desorption isotherms and b) pore-size
distributions of the PMO-IL material.
Scheme 1. Preparation of PMO-IL materials.
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H2 hysteresis loops according to the IUPAC classification,
which is characteristic of mesoporous materials. In particu-
lar, N2-adsorption isotherms for PI-25 and PI-35 showed a
broad hysteresis loop with a relatively sharp capillary con-
densation step at a relative pressure of approximately 0.70,
and a very sharp capillary evaporation step centered at ap-
proximately 0.45, which is very close to lower limit of ad-
sorption–desorption hysteresis. Since the course of the ad-
sorption and desorption branches are not exactly parallel,
“ink-bottle-like” pores should be considered to exist in both
samples. Similar shapes of adsorption–desorption isotherms
have been reported for FDU-1 and other PMOs or ordered
silicas with large cagelike mesopores.[17,46] On the other
hand, the shape of the resulting isotherms for PI-10 reveals
a typical sharp hysteresis loop similar to those of SBA-15
type materials, thereby confirming a two dimensional (2D)
hexagonal mesopore structure with a narrow pore-size dis-
tribution. For PI-10, PI-25, and PI-35, the Brunauer–
Emmett–Teller (BET) surface areas were 671, 483, and
372 m2 gꢀ1; pore diameters were 7.0, 5.4, and 4.2 nm; and
pore volumes 1.21, 0.82, and 0.63 cm3 gꢀ1, respectively
(Table 1).
flection and is usually observed for hexagonally ordered
mesoporous materials. The results of d100 reflections and ni-
trogen adsorption–desorption analyses of the as-prepared
and solvent-extracted PMOs indicate outstanding solvother-
mal stability, because none of the samples exhibited any ap-
preciable matrix contraction upon surfactant extraction. An-
other interesting observation was that by increasing the con-
centration of IL in the initial gel, the intensity of the diffrac-
tion peak gradually decreased, thereby confirming the loss
of structure ordering, as suggested from N2 adsorption–de-
sorption studies. This effect and also the absence of higher-
order reflections (d110 and d200 reflections) can be attributed
to either the large size of IL groups, which restrict their in-
corporation in the mesoporous walls, or the interference of
initial BTMSP ionic liquid at higher concentration in the
structure of the surfactant-template assembly. The appear-
ance of one broad peak in the small-angle regions of the dif-
fraction pattern of PI-50 is also supported by transmission
electron microscopy, in which a relatively amorphous meso-
pore structure was observed but with no significant meso-
scale ordering (see above).
Thermogravimetric analyses (TGA) of the PMO-IL sam-
ples were conducted in temperatures that ranged from 20 to
8008C under air flow (Figure S2 in the Supporting Informa-
tion). The first weight loss of about 4–8% appeared around
1008C, which corresponded to desorption of water and alco-
holic solvents that remained from the solvent-extraction
processes. The relatively large amount of water (ꢁ8%) in
samples PI-25, PI-35, and PI-50 might also suggest that alkyl
imidazolium groups are well enclosed within the walls by
Table 1. Structural parameters of all synthesized mesoporous materials
determined from nitrogen-sorption experiments.
Sample
BET surface area
Pore diameter
[nm]
Pore volume
[m2 gꢀ1
]
[cm3 gꢀ1
]
PI-10
PI-25
PI-35
PI-50
671
483
372
260
7.0
5.4
4.2
3.7
1.21
0.82
0.63
0.45
ꢀ
the surrounding hydrophilic Si OH moieties. It is well
In the case of PI-50, the isotherm indicates that the mate-
rial is still highly porous with a surface area of 260 m2 gꢀ1,
and a pore volume of 0.45 cm3 gꢀ1 (Table 1). The average
pore diameter of PI-50, calculated from the adsorption
branch of the isotherm using the Barrett–Joyner–Halenda
(BJH) method, showed a broad pore-size distribution with
pronounced maxima at 3.7 nm (Table 1). This indicates that
both pore volume and the long-range order of the materials
strongly decrease with increasing the concentration of bisily-
lated ionic liquid in the primary gel. A sharp rise in the iso-
therm for PI-25, PI-35, and PI-50 occurs at a relative pres-
sure higher than 0.8, and this increase intensifies with in-
creasing concentration of BTMSPI, which features in the
gradual development of textural mesoporosity.
Powder X-ray diffraction (PXRD) analyses were per-
formed on both as-prepared composites and the surfactant-
extracted PMOs. The PXRD pattern of the as-made prod-
ucts showed a weak diffraction peak with low-intensity at 2q
ꢁ1.1–1.4 (Figure S1 in the Supporting Information), thus re-
flecting a low electronic contrast between the inside of the
channels and walls. On the other hand, all surfactant-ex-
tracted samples exhibit a single prominent peak in the dif-
fraction pattern at approximately the same region as those
of the as-prepared materials, which is characteristic of mate-
rials with long-range periodicity (Figure S9 in the Support-
ing Information). This single peak is indexed as the d100 re-
known that weight loss at 200–2508C might generally be at-
tributable to the decomposition of the residual surfactant.
The absence of any significant weight loss in this range for
the solvent-extracted PMO samples indicated that the sur-
factant was successfully removed by means of solvent ex-
traction. The main weight loss (from 10.6% for PI-10 to
39.7% for PI-50), which was observed around 350 to 4508C,
with the same pattern, might be attributed to the thermal
dissociation of the alkyl imidazolium groups. Another inter-
esting observation is the broadening of the decomposition
range that starts at 4508C. This might be attributed to a par-
tial transformation of bridge ionic-liquid groups with two-
point attachment in the framework to ionic-liquid groups
with single-point attachment. The amount of the incorporat-
ed alkyl imidazolium moieties into the framework of the
material that was determined by elemental analysis showed
a good agreement with the amount of organic groups esti-
mated from the TGA data (Table 2). A comparison between
Table 2. Elemental analysis (EA) and TGA weight-loss data of the
PMO-IL materials with different loading of ionic liquid.
Sample
IL [mol%]
C [%]
N [%]
TGA weight loss [%]
PI-10
PI-25
PI-35
PI-50
10
25
35
50
14.0
19.4
22.7
24.6
3.4
5.0
6.1
6.7
10.6
24.5
29.2
39.7
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carbon atoms found and calculated values shows that about
90% of the ionic liquid in the original mixture is incorporat-
ed into the organosilica framework, most likely because of
the faster condensation rate of the tetramethoxysilane.[47]
The presence of alkyl-imidazolium moieties into the mes-
oporous network was identified by diffuse-reflectance infra-
red Fourier transform spectroscopy (DRIFTS) (Figure S10
in the Supporting Information). All synthesized samples dis-
play very similar DRIFT spectra with differences only in the
relative band intensities. The two sharp and broad peaks at
1090 and 925 cmꢀ1 are attributed to the asymmetric and
symmetric stretching vibration of the Si-O-Si in the frame-
work.[22] In addition, all PMO materials show absorption
peaks at 3125 cmꢀ1 (for unsaturated C H stretching); 3050,
ꢀ
2918 cmꢀ1 (aliphatic C H stretching); 1620 cmꢀ1 (C=N
ꢀ
stretching of imidazolium ring);[47] 1558 cmꢀ1 (C=C stretch-
ing of imidazolium ring); 1442 cmꢀ1 (C H deformation vi-
ꢀ
brations); 700–790 cmꢀ1 (for C Si stretching vibrations);
ꢀ
and also a broad peak around 3300 cmꢀ1 (stretching vibra-
[22,42,48,49]
ꢀ
tion of O H groups), respectively.
Furthermore, the
increase in intensity of absorption bands as the amount of
IL increases confirms the structure and stability of IL
groups during the processes of material synthesis and extrac-
tion.
The 29Si and 13C cross-polarization magic angle spinning
(CP/MAS) NMR spectra of all mesoporous organosilica
samples were also recorded to confirm the presence of IL in
the mesoporous framework (Figure 2 and Figure 3; also see
the Supporting Information for further images). The
29Si NMR spectra of the all PMO materials display three sig-
nals at d=ꢀ49.5, ꢀ58.5, ꢀ67.5 ppm, which correspond, re-
1
2
ꢀ ꢀ
ꢀ ꢀ
spectively, to T [C Si
U
ACHTUNGTRENNUNG
3
ꢀ ꢀ
and T [C Si
(OSi)3] sites for Si species that are covalently
bonded to carbon atoms. In addition, the 29Si NMR spectra
show three peaks at d=ꢀ111, ꢀ102, and ꢀ91.5 ppm, which
4
3
ꢀ
ꢀ
correspond to the Q [Si
(OSi)4], Q [Si
E
2
ꢀ
Q [Si
(OSi)2(OH)2] species of the silica framework, respec-
Figure 2. Solid-state 29Si MAS NMR of materials PI-10, PI-25, PI-35, and
PI-5.
tively. The degree of framework condensation was found to
be high, as observed from the presence of a higher percent-
age of fully cross-linked T3 and Q4 silicon sites. Comparison
of the solid-state 29Si NMR spectra (Figure 2 and Table 3)
shows that with increasing ionic liquid content in the initial
gel, the intensity of Tn bands with respect to Qn bands grad-
ually increased, which illustrates the successful condensation
and incorporation of IL moieties into the mesoporous walls.
The 13C CP/MAS NMR spectra of all PMO materials
show the characteristic signals of the alkyl-imidazolium
bridging groups (Figures S3–S5 in the Supporting Informa-
tion). For example, Figure 3 shows the solid-state 13C NMR
spectrum of PI-35, which can be assigned to the C species as
follows: d=10.0 (SiCH2), 24.6 (CH2CH2CH2), 52.5 (CH2N),
123.2 (CHCH), and 136.0 ppm (NCHN). The absence of
13C NMR spectroscopic resonances that correspond to the
methoxy groups of the ionic-liquid organosilica precursor
further confirms a high degree of hydrolysis and efficient
cross-linking in the material. This is in good agreement with
the results obtained from elemental analysis. These solid-
state 13C NMR spectra show that the ionic-liquid groups
were indeed thoroughly incorporated into the mesoporous
channel walls. Moreover, the lack of any further carbon res-
ꢀ
onance clearly shows that almost all Si C bonds survived
intact during the acidic polycondensation and the hot-sol-
vent-extraction processes. Furthermore, the absence of sur-
factant peaks, which are located at d=30 and 70 ppm in the
13C NMR spectra of surfactant-extracted samples, indicates
complete removal of the surfactant under the extraction
conditions.
In a separate experiment and for the purposes of com-
parison, we also prepared a sample of pure self-assembled
ionic-liquid phase (SAILP; PI-100) through co-condensa-
tion of 1,3-bis(3-trimethoxysilylpropyl)imidazolium iodide
(BTMSPI) under the same acidic conditions but in the ab-
sence of pluronic P123. The SAILP solid was also character-
ized by simultaneous thermal analysis and solid-state NMR
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TEM images of the solvent-extracted materials also pro-
vided further evidence that the structural integrity of the
materials was retained after extraction of the template, a
result consistent with the PXRD results and N2-sorption
data. TEM images of PI-10 sample along different directions
shows 2D hexagonal symmetry throughout the sample with
superior uniformity in its framework (Figure 4a,b). TEM
images of PI-25 and PI-35 also show a uniform rod structure
with regularity in their channels (Figure 4c,d). However,
TEM images of PI-50 show a wormhole motif structure (Fig-
ure 4e) that is consistent with a low degree of structural or-
dering as also evidenced by PXRD and N2-sorption analy-
ses.
Figure 3. Solid-state 13C MAS NMR of PI-3.
Table 3. Solid-state 29Si NMR spectroscopic data of PMO-IL materials.
Sample
Q4 [%]
Q3 [%]
Q2 [%]
T3 [%]
T2 [%]
T1 [%]
PI-10
PI-25
PI-35
PI-50
41.2
29.9
23.5
17.3
29.3
24.1
21.9
12.7
5.0
3.7
4.3
1.9
44.9
27.4
32.2
43.0
11.0
13.1
16.2
21.6
1.7
1.9
1.9
3.4
spectroscopy. The uniform distribution of the alkyl imidazo-
lium group in the silica framework of the material was con-
firmed by 29Si and 13C CP/MAS NMR spectroscopy.
The 29Si spectrum of the SAILP displays three signals at
d=ꢀ49.1, ꢀ58.52, and ꢀ68.36 ppm, which correspond to T1
[C SiACHTUNGTRENNUNG ACHTUTGNRENNUGN
(OSi)(OH)2], T2 [C Si(OSi)2(OH)], and T3 [C Si
ACHTUNGTRENNUNG(OSi)3] sites, respectively, for Si species that are covalently
ꢀ
ꢀ
ꢀ ꢀ
attached to carbon atoms (Figure S8 in the Supporting Infor-
mation). In addition, the absence of any resonances at d=
ꢀ90 to ꢀ115 ppm that are attributed to Qn sites of the silica
ꢀ
framework clearly shows that no C Si bond cleavage of the
BTMSPI molecules occurred in the synthesis conditions of
SAILP (Figure S8 in the Supporting Information). The
solid-state 13C CP/MAS NMR spectrum of SAILP was also
measured to clarify the characteristic signals of the alkylimi-
dazolium bridging moieties in pure self-assembled ionic-
liquid phase (Figure S7 in the Supporting Information). This
spectrum illustrates carbon signals of the ionic liquid moiet-
ies as follows: d=10.5 (SiCH2), 24.7 (CH2CH2CH2), 52.8
(CH2N), 123.6 (CHCH), and 136.7 ppm (NCHN). This data
also suggests that the ionic-liquid groups were well incorpo-
rated into the material network. Moreover, the absence of
Figure 4. TEM images of materials a,b) PI-10, c) PI-25, d) PI-35, and
e) PI-5.
SEM images of surfactant-extracted materials display
well-defined external morphology (Figure 5). These images
show that with increasing IL in the synthesized materials, a
gradual change in the particle morphology could be ob-
served. This change is from ropelike for PI-10 to sharp-
edged particles for PI-50. PI-10 displays a ropelike structure
with superior regularity and a diameter of 2–4 mm.[19] PI-25
and PI-35 samples show a uniform spherical organosilica
ꢀ
any other carbon signals confirms that almost all of the Si
C bonds survived intact under the synthesis conditions.
Thermogravimetric analysis of the SAILP composite was
also conducted from room temperature to 9008C for purpos-
es of comparison (Figure S6 in the Supporting Information).
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Scheme 2. Preparation of Ru@PMO-IL 2.
employ PI-10 as the support, because of our previously suc-
cessful experience in preparing several catalytic systems
based upon this material.[43,53]
The selective oxidation of alcohols to carbonyl com-
pounds is a key process in organic synthesis and plays a cru-
cial role in the fine chemicals industries. There is a plethora
of stoichiometric oxidizing reagents to selectively effect this
transformation.[54] These oxidizing reagents are, however,
often very expensive and there are sometimes serious toxici-
ty concerns associated with them. Therefore, the oxidation
of alcohols by using molecular oxygen or air in the presence
of a recyclable heterogeneous catalyst system offers signifi-
cant advantages from both environmental and economic
points of view.[55] In this context, several supported catalysts
based upon Pd,[56] Au,[57] Pt,[58] and Ru[59] were found to
show interesting activity in the aerobic oxidation of alcohols.
Although a significant number of these catalyst systems
have given remarkable results in terms of selectivity and
substrate scope, in many cases, they are not suitable for non-
activated alcohols such as aliphatic and alicyclic alcohols.
Moreover, many of the catalysts can produce satisfactory
yields of products only in the presence of large excess
amounts of base additives.[56,57] Among the various heteroge-
neous catalysts exploited so far, ruthenium-based catalysts
show promising catalytic performances because they not
only catalyze the aerobic oxidation of alcohols in the ab-
sence of base additives,[59] but also offer excellent catalytic
activities in a variety of other important functional-group
transformations.[60]
The aerobic oxidation of benzyl alcohol to benzaldehyde
was used to probe the catalytic activity of Ru@PMO-IL.[61]
First, we examined the aerobic oxidation of benzyl alcohol
(1 mmol) by using molecular oxygen (1 atm) in the presence
of 5 mol% of 2, in toluene (2 mL) at room temperature.
Under these conditions, 53% of the corresponding aldehyde
was obtained after 72 h (Table 4, entry 1).
The same reaction at 508C smoothly went to completion
within 16 h to afford an excellent yield of 99% of benzalde-
hyde as the sole product (Table 4, entry 2). The reaction was
further optimized for the reaction temperature, and we
found that by increasing the reaction temperature to 708C,
the reaction proceeded more readily and the desired alde-
hyde was obtained in almost quantitative yields after 9 h
(Table 4, entries 1–4).
Figure 5. SEM images of materials PI-10, PI-25, PI-35, and PI-5.
with diameters that range between 0.5 and 2 mm. These
spherical mesostructures would be good candidates for chro-
matography, catalysis, and microelectronics applications due
to their high uniformity and density.[50,51] The PI-50 sample
consisted of isolated and also agglomerated particles with
different shapes and sizes.
Application in catalysis: As the initial objective of this re-
search was to develop PMOs with variable degrees of ionic-
liquid moieties for application in catalyst immobilizations,
we then turned our attention to the use of these materials as
supports to begin the development of a wide variety of
novel immobilized catalyst systems. In an earlier communi-
cation, we demonstrated that among different types of func-
tionalized organosilicas, self-assembled organic–inorganic
hybrid silica with an alkylimidazolium bridge (SAILP; PI-
100) was better suited as a support to immobilize a Yb-
ACHTUNGTRENNUNG(OTf)3–pybox (pybox=2,6-bis[(4R)-4-phenyl-2-oxazolinyl]-
pyridine) complex in catalyzing the enantioselective addi-
tion of trimethylsilyl cyanide (TMSCN) to aldimines.[52] We
have also investigated the ability of mesoporous organosili-
cas for actual use as a support for the immobilization and
stabilization of Pd nanoparticles in the Suzuki-coupling re-
action of aryl halides with arylboronic acids[43] and the aero-
bic oxidation of a diverse range of alcohols under mild reac-
tion conditions.[53] In these studies, we found that the PMO-
IL nanostructure could indeed act as a nanoscaffold to stabi-
lize Pd nanoparticles (PdNPs) in the mesochannels, thus
preventing undesired agglomeration of Pd nanoparticles. In
continuation of these studies, we therefore speculate that
this innovative system with tunable loading of imidazolium
moieties could be used as a support in a very similar way to
the preparation of a novel ruthenium-supported catalyst
ꢀ
through the anion exchange of RuO4 with chloride anions
inside the PMO-IL framework, which is denoted as
Ru@PMO-IL 2 (Scheme 2). In this regard, we chose to
Investigation of the impact of catalyst loading, reaction
solvents, and the temperature revealed that a catalyst load-
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Table 4. Effects of catalyst loading, solvent, and reaction temperature on the aerobic
were converted to their corresponding a,b-unsatu-
rated carbonyl compounds in moderate to excellent
yields (Table 5, entries 19–21). It is well known that
aliphatic alcohols are more reluctant to undergo se-
lective oxidation. Notably, Ru@PMO-IL was also
capable of catalyzing the oxidation of these alco-
hols, although it was found that secondary aliphatic
alcohols showed a higher activity than their primary
counterparts (Table 5, entries 22–24).
It should also be noted that the same transforma-
tions for open-chain secondary alcohols when using
our recently developed catalyst system (Au/
Cs2CO3) were very sluggish and produced their cor-
responding ketones in poor yields (Table 5, en-
tries 21 and 22).[57h] In the case of primary aliphatic
and allylic alcohols, by increasing either the reac-
tion time or reaction temperature so as to increase
the conversion, selectivity towards the aldehyde de-
creases significantly on account of the overoxida-
tion of the aldehyde to the corresponding carboxyl-
oxidation of benzyl alcohol catalyzed by Ru@PMO-IL 2.[a]
Entry
n
Solvent
T [8C]
t [h]
Yield [%][b]
TOF [hꢀ1
]
1
2
3
4
5
6
7
8
5
5
5
5
2.5
5
5
5
5
5
toluene
toluene
toluene
toluene
toluene
THF
H2O
H2O
TFT
TFT
RT
50
60
70
70
RT
RT
70
RT
60
72
16
13
9
14
72
72
72
72
6
53
>99
>99
>99
>99
20
14
17
70
>99
86
0.14
1.23
1.52
2.24
2.80
0.05
0.04
0.05
0.19
3.30
2.64
8.80
8.80
9
10
11
12
13
2.5
2.5
2.5
TFT
TFT
TFT
60
70
80
13
4.5
4.5
>99
>99
[a] Conditions: benzyl alcohol (1 mmol), O2 (1 atm). [b] GC yield.
ing of 2.5 mol% in TFT as solvent at 708C are the best con-
ditions for this system (Table 4, entry 12). Whereas the oxi-
dation reaction proceeded well at room tempera-
ture in TFT (Table 4, entry 9), increasing the tem-
ic acid (Table 5, entries 19 and 22). Remarkably, sterically
encumbered alcohols such as borneol were equally oxidized
Table 5. Aerobic oxidation of alcohols catalyzed by Ru@PMO-IL 2.[a]
perature to 708C resulted in significantly higher ef-
ficiency at lower catalyst loading, which led to the
quantitative formation of benzaldehyde (Table 4,
entries 9–12). It was also noted that the use of a
higher reaction temperature is not beneficial for im-
proving the reaction efficiency (Table 4, entry 13).
With optimized reaction conditions, we then in-
vestigated the applicability of our method with re-
spect to other alcohol structures (Table 5). As
shown in Table 5, a wide range of either primary or
secondary benzylic alcohols were converted to their
corresponding aldehydes or ketones in good to ex-
cellent yields (Table 5, entries 1–24). Notably, this
method is highly selective for the oxidation of pri-
mary alcohols to their corresponding aldehydes
without the formation of appreciable amounts of
the corresponding carboxylic acids even after pro-
longed reaction times (Table 5, entries 1–11). Al-
though substituted benzyl alcohols that contain
electron-donating groups (such as CH3, OCH3, or
iPr; Table 5, entries 2–4) are more easily oxidized
than those that bear electron-withdrawing groups
(Table 5, entries 6–8), the substituents did not affect
the selectivity of the process or the resulting deacti-
vation of the catalyst. It is also worth noting that
Ru@PMO-IL displays a high activity for the oxida-
tion of secondary benzylic alcohols either cyclic or
acyclic and gives their corresponding ketones in
good to excellent yields with high selectivity
(Table 5, entries 13–18). The catalyst Ru@PMO-IL
was also found to be highly selective for the oxida-
tion of organic substrates that contain C=C double
Entry
R1
R2
t [h]
Yield
[%][b]
TOF [hꢀ1
]
1
2
3
4
5
6
7
8
C6H5
H
H
H
H
H
H
H
H
4.5
6.5
4.0
8.5
6.0
17
28
36
36
22
>99
>99
>99
95
95
99
8.80
6.15
10
ꢀ
4-Me C6H4
ꢀ
4-MeO C6H4
ꢀ
4-iPr C6H4
4.47
6.33
2.33
0.99
0.96
1.04
1.18
1.13
1.07
6.98
5.90
ꢀ
4-Cl C6H4
ꢀ
3-Cl C6H4
ꢀ
2,6-Cl2 C6H3
70
87
94
ꢀ
2-NO2 C6H4
ꢀ
9
2,4-Cl C6H3
H
H
10
11
12
13
14
2-furyl
cyclopropyl
3-pyridyl
C6H5
91[c]
95[c]
78[d]
96
cyclopropyl
24
24
5.5
6.5
H
Me
Et
C6H5
91
15
24
65
1.08
16
17
18
19
20
C6H5
C6H5
C6H5CH2
C6H5
33
24
15
24
12
91[d]
>99[d]
92[d]
0.55
0.82
1.22
1.16
1.50
ꢀ
ꢀ
4-MeO C6H4
4-MeO C6H4
ꢀ
Ph CH=CH
H
70
90[d]
ꢀ
Ph CH=CH
CH3
21
18
91
2.02
53[d]
75[d]
0.757
0.45
ꢀ
22
23
Ph CH2CH2CH2
H
Me
14
33
CH
3A
24
36
73[d]
0.40
[a] Conditions: alcohol (1 mmol), 2 (2.5 mol%), O2 (1 atm), TFT (2 mL), at 708C
unless otherwise stated. [b] GC yields using internal standard method. [c] Conditions:
bonds. Thus, primary and secondary allylic alcohols 2 (3.5 mol%), at 708C. [d] Conditions: 2 (5 mol%), at 858C.
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to the corresponding ketone, although in these cases larger
molar percentages of Ru were necessary (see footnotes to
Table 5) to achieve high yields. Another important issue that
can be understood from Table 5 is the remarkable activity of
Ru@PMO-IL in promoting the aerobic oxidation of alcohols
that contain heteroatoms such as oxygen, and nitrogen het-
erocycles, because the strong coordination of these alcohols
to a metal center deactivate the catalyst. However, these al-
cohols could also be easily oxidized to their corresponding
aldehydes in moderate to high yields under the optimized
reaction conditions (Table 5, entries 10 and 12). Product
analysis showed that selectivity decrease is a result of the
overoxidation of aldehydes to carboxylic acids and also to
the formation of an array of byproducts that are derived
from the oxidative decomposition of the heterocycles. Even
dicyclopropyl carbinol, which is remarkably sensitive, could
also be oxidized to dicyclopropyl ketone in excellent yields
in 24 h (Table 5, entry 11). The recyclability of 2 was also ex-
amined by separating it from the reaction mixture of the
aerobic oxidation of benzyl alcohol through successive cen-
trifugation, washing with toluene, and drying. In each run,
the leaching of the Ru species into solution was negligible,
as determined by inductively coupled plasma-atomic emis-
sion spectroscopy (ICP-AES) (lower than the detection
limit of 1 ppm). Furthermore, no significant catalytic activity
was observed from the residue of toluene extraction of 2
even after prolonged reaction times. The recycled catalyst
was then used in five reaction cycles under the same reac-
tion conditions (99, 99, 91, 89, and 75%). The catalytic re-
sults indicate that a slight decrease in catalytic activity of
the recovered catalyst for the aerobic oxidation was ob-
served between the first and fourth runs. To clarify this
issue, we also measured the Ru content of the recovered
catalysts by means of ICP-AES. The average result of three
individual experiments showed a Ru content of 0.30 and
0.29 mmolgꢀ1 for the fresh and the recovered catalyst, re-
spectively. As can be seen, the amount of leached Ru in the
recovered samples is negligible. This highlights the notion
that the drop in catalytic activity might be mainly due to the
loss of catalyst mass during the recycling and washing
stages.
Figure 6. TEM image of the recovered catalyst after five reaction runs
(scale bar: 20 nm).
droxyl apatite (HAP), MCM-41, and other types organic–in-
organic hybrid Ru-based catalysts under nearly the same re-
action conditions (Table 6, entries 1, 2, 5, 6, 9, and 10),
whereas they are inferior to those obtained from Ru-sup-
ported catalysts on the inorganic oxides such as Al2O3,
Fe3O4, and SiO2 (Table 6, entries 3, 4, 7, 8, and 13 versus 15–
18). It is very important to point out that a higher TOF of
23.2 hꢀ1 can be also obtained for our catalyst system based
on the calculations that the Hutchings group[57b] used in
0.5 h reaction time for the same process. Given the excellent
yields and selectivities and acceptably high TOFs obtained
at lower reaction temperature using our catalyst system in
the aerobic oxidation of a relatively wide range of alcohols,
it is reasonable to consider this system a high-performance
alternative for this purpose.
It is interesting to note that both N2-sorption analysis and
transmission electron microscopy (TEM) showed that the
ordered mesoporous structure of the PI-10 host matrix re-
mained mostly unchanged after five runs of catalysis
(Figure 6 and Figure S12 in the Supporting Information).
Furthermore, TEM images of the recovered catalyst reveal
that the Ru nanocluster with average diameters of 1.5–
3.0 nm were well dispersed over the PMO-IL matrix (see
the Supporting Information for high-resolution (HR) TEM
images).
As the final part of this study, we were also very interest-
ed to compare the catalytic performance of our system to
the ones already reported in the literature in terms of the
observed turnover frequencies (TOFs; Table 6).
As can be clearly seen, the TOFs in our protocol are su-
perior to those of the Ru-supported catalysts on carbon, hy-
Conclusion
In conclusion, a number of new alkyl imidazolium ionic-
liquid-based periodic mesoporous organosilicas were synthe-
sized by co-condensation of 1,3-bis[3-trimethoxysilylpropy-
l]imidazolium chloride and tetramethoxysilane (TMOS) by
using a triblock copolymer templating approach in acidic
media. PXRD, solid-state NMR spectroscopy, DRIFTS, ni-
trogen-sorption data, TEM images, and TGA analysis of the
synthesized materials showed that ionic-liquid blocks are
thoroughly incorporated into the wall mesostructures, and
the PMO materials that contain 10–35 mol% IL have high
surface area and narrow pore-size distributions with 2D hex-
agonal symmetry. Our studies showed that the structural
properties and morphology of these materials strongly
depend on the initial concentration of the precursor ionic
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Table 6. Oxidation of some alcohols using different ruthenium catalysts.
Entry
Alcohol
Catalyst
Mol%
T [8C]
t [h]
Yield [%]
Ref.
TOF[a] [hꢀ1
]
1
2
3
4
5
6
7
8
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
4-Cl-benzyl alcohol
1-phenyl ethanol
benzyl alcohol
HB Ru[b]
HB Ru
Ru/Al2O3
5
5
110
110
83
83
80
9
6
1
1
2
3
2
1
5
98
93
99
99
98
99
99
99
99
82
100
quant
98
96
96
95
99
[55b]
[55b]
[59b]
[59b]
[59a]
[59a]
[59e]
[59e]
[59h]
[59h]
[59k]
[59k]
[59l]
2.17
3.10
39.60
39.60
2.88
1.94
13.02
26.05
3.96
2.73
3.20
-
61.25
3.26
6.9
6.3
8.8
2.5
2.5
17
17
3.8
3.8
5
Ru/Al2O3
Ru/HAP c
Ru/HAP
80
Ru(OH)x/Fe3O4
Ru(OH)x/Fe3O4
Ru/C
105
105
70
50
80
80
75
75
70
9
10
11
12
13
14
15
16
17
18
Ru/C
5
6
24
1
0.16
3
5.5
6
4.5
0.5
RuO4ꢀ@MCM-41
RuO4ꢀ@MCM-41
RuO4ꢀ/SG[d]
RuO4ꢀ/SG
1.3
1.5
10
10
2.5
2.5
2.5
2.5
[59l]
[e]
1-phenyl ethanol
4-Cl-benzyl alcohol
benzyl alcohol
Ru@PMO-IL
Ru@PMO-IL
Ru@PMO-IL
Ru@PMO-IL
–
–
–
–
[e]
70
70
70
[e]
[e,f]
benzyl alcohol
29
23.2
1
yield½%ꢂ
[a] All TOFs are calculated by this equation: cat:½mol%ꢂ ꢃ
. [b] Organic–inorganic hybrid ruthenium (HB Ru). [c] HAP=hydroxyapatite. [d] Sol–gel-
time½hꢂ
encapsulated perruthenate. [e] The present protocol. [f] TOF was calculated after 0.5 h.
ution was removed with a pipette. The resultant oil was first washed with
liquid (BTMSPI) in the synthetic solution. However, both
PXRD and TEM studies revealed that the hexagonal order-
ing of the materials was retained over a relatively wide
range of BTMSPI (10–35 molar percent of total silicon
source). Diffraction patterns and nitrogen adsorption–de-
sorption analyses of the as-prepared and solvent-extracted
PMO-ILs also indicate outstanding solvothermal stability,
because none of the samples exhibited any appreciable
matrix contraction upon surfactant extraction. Given the
unique character of the ordered mesoporous materials and
ionic liquids, the described PMOs promise high-loading,
stable, and tunable heterogeneous versions of ionic liquids
for applications in different areas of nanotechnology, materi-
als chemistry, catalysis, and chromatography. Further work
on the practical applications of these materials as heteroge-
neous nanoreaction media and also as innovative supports
for the immobilization of various types of transition-metal
catalysts through anion-exchange capabilities of the materi-
als is underway in our laboratories.
absolute toluene (5ꢁ30 mL), and then CH2Cl2 (50 mL) was added for the
precipitation of NaCl from the reaction mixture. The solution in dichloro-
methane was transferred into another well-dried flask. Pale yellow IL 1
was obtained after removal of solvent and dried under vacuum at room
temperature. 1H NMR (250 MHz, CDCl3, 258C, TMS): d=10.00 (s, 1H;
NCHN), 7.46 (d, 3J=1.7 Hz, 2H; CHCH), 4.32 (t, 3J
NCH2), 3.60 (s, 18H; OCH3), 2.00 (m, 4H; CCH2C), 0.62 ppm (t, 3J-
(H,H)=8.1 Hz, 4H; SiCH2); 13C NMR (63 MHz, CDCl3, 258C, TMS):
ACHTUNGTREN(NGNU H,H)=7.1 Hz, 4H;
AHCTUNGTRENNUNG
d=136.1 (NCHN), 122.2 (CHCH), 51.8 (NCH2), 50.8 (OMe), 24.1
(CH2CH2CH2), 5.8 ppm (SiCH2).
Synthesis of ionic-liquid-based periodic mesoporous organosilica (PMO-
IL, PI-10) materials in the presence of triblock copolymer P123: The syn-
thetic procedure was mainly based on a previous report on the synthesis
of the 1,4-diethylenebenzene-bridged PMOs.[43] Typically, P123 (1.58g)
and KCl (8.3g) were dissolved in deionized water (9.9g) and HCl solu-
tion (43.6g, 2.0m) with stirring at 408C for 3h. A pre-prepared mixture
(18.9mmol) of tetramethoxysilane (TMOS) and IL in absolute methanol
was added to this homogeneous solution and then stirred for 24h at the
same temperature. The resulting mixture was then transferred into a
Teflon-lined autoclave and heated at 1008C for 72h under static condi-
tions. The solid products were obtained by filtration, washed thoroughly
with deionized water, and air-dried at room temperature. The surfactant
was removed four times by Soxhlet extraction with ethanol (100mL) and
concentrated HCl (3mL each time) for each sample (1g) for 12h. The re-
sulting PMO materials were denoted as PI-n, for which n (n=10, 25, 35,
50) is the mol% of IL in the initial mixture.
Experimental Section
Preparation of Ru@PMO-IL (2) catalyst: Ru@PMO-IL was prepared on
the basis of a simple ion-exchange technique according to a previous
report.[62] For a typical method, PMO-IL (0.5 g, 1.0 mmol IL gꢀ1) was
added to deionized water (10 mL) and sonicated for at least 10 min. A
solution of KRuO4 (0.034 g, 0.016 mmol) in deionized water (3 mL) was
gradually added to said suspension and stirred at room temperature for
5 h. The resulting system was filtered and washed with deionized water
(3ꢁ10 mL) and acetone (2ꢁ10 mL), respectively. The resulting solid was
dried at room temperature in vacuo. The loading of Ru was calculated to
be 0.3 mmolgꢀ1 by means of ICP.
Preparation of 1,3-bis(3-trimethoxysilylpropyl)imidazolium chloride (1):
IL 1 (see the Supporting Information) was synthesized by means of a
modification of a literature procedure.[44] All reactions were carried out
under an atmosphere of dry argon and all solvents were dried in the pres-
ence of suitable reagents and freshly distilled prior to use. Sodium imida-
zolide (90% purity, Fluka) and 3-chloropropyltrimethoxysilane (98%,
Merck) were used as received. In a well-dried, two-necked 200 mL
Schlenk flask, (3-chloropropyl)trimethoxysilane (8.11 g, 40.0 mmol) was
added to a solution of sodium imidazolide (4.00 g, 40.0 mmol) in absolute
THF (120 mL). The system was heated to reflux in the absence of light
for 24 h and then allowed to cool to room temperature. After removal of
solvent under vacuum, the resulting mixture was added to a solution of
3-chloropropyltrimethoxysilane (8.11 g, 40.0 mmol) in absolute toluene
(120 mL). The reaction mixture was heated to reflux with exclusion of
light for 48 h, and after cooling to room temperature, the supernatant sol-
General procedure for alcohol oxidation with Ru@PMO-IL (2): Catalyst
(80–160 mg, 2.5–5 mol% (with regard to substrate)) was added to a solu-
tion of alcohol (1 mmol) in TFT (2 mL), and then the suspension was
heated at the 708C for the requisite time under an O2 atmosphere
(1 atm). The progress of the reaction was monitored by GC, and after the
end of the reaction, the resulting mixture was cooled to room tempera-
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B. Karimi et al.
ture. Finally, the catalyst was isolated with centrifugation, washed with
toluene (2ꢁ10 mL), and dried under vacuum. The recovered catalyst was
used in the recycling procedure in the same manner as reported in the
first run.
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Received: February 4, 2012
Revised: June 25, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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ÞÞ
These are not the final page numbers!

B. Karimi et al.
Go green: The synthesis and character-
ization of a range of periodic mesopo-
rous organosilica materials with differ-
ent compositions of alkyl imidazolium
ionic-liquid moieties (PMO-IL) is
described (see scheme). As part of this
study, the materials were also used for
immobilization of perruthenate
Mesoporous Materials
B. Karimi,* D. Elhamifar, O. Yari,
M. Khorasani, H. Vali, J. H. Clark,
A. J. Hunt ..................... &&&& — &&&&
Synthesis and Characterization of
Alkyl Imidazolium Based Periodic
Mesoporous Organosilicas: AVersatile
Host for the Immobilization of
through an ion-exchange protocol. The
resulting Ru@PI-10 was then
employed as a recyclable catalyst.
ꢀ
Perruthenate (RuO4 ) in the Aerobic
Oxidation of Alcohols
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!