21114
J. Phys. Chem. C 2009, 113, 21114–21122
Immobilization of Phosphotungstic Acid (PTA) on Imidazole Functionalized Silica: Evidence
for the Nature of PTA Binding by Solid State NMR and Reaction Studies
L. T. Aany Sofia,† Asha Krishnan,† M. Sankar,‡,§ N. K. Kala Raj,*,‡ P. Manikandan,‡,⊥
P. R. Rajamohanan,† and T. G. Ajithkumar*,†
Central NMR Facility, and Catalysis and Inorganic Chemistry DiVision, National Chemical Laboratory,
Pune 411008, India
ReceiVed: June 30, 2009; ReVised Manuscript ReceiVed: October 20, 2009
Phosphotungstic acid (PTA) immobilized onto imidazole functionalized fumed silica and was used as an
efficient catalyst for epoxidation of a variety of olefins using aqueous H2O2 as an oxidant. Negligible leaching
of PTA under the reaction conditions employed indicates a strong interaction between PTA and imidazole.
The immobilized catalysts could be separated and reused after the catalytic cycle. Evidence for the
heterogenization of PTA on the imidazole functionalized fumed silica has been inferred from different
spectroscopic techniques like IR, UV-vis, and NMR. Importantly, the nature of binding of PTA on the
support has been studied in detail by solid state NMR spectroscopy using 15N labeled imidazole support. It
is clear from the NMR studies that the effective heterogenization of PTA is mainly due to imidazolium ion
formation on the support by the acidic protons of PTA and the resultant ion pair.
1. Introduction
Heteropolyacids (HPAs) are a large fundamental class of
inorganic compounds having both acidic and redox properties.
The first to be characterized and the best known among these
are the Keggin heteropolyanions, having the general formula
XM12O40x-8, where X is the central atom (Si4+, P5+, etc.) with
an oxidation state x, and M is the metal ion (Mo6+ or W6+).
The countercations present may be H+, H3O+, etc. HPAs exhibit
high proton mobility and high thermal stability and are highly
soluble in polar solvents. These properties give them the ability
to catalyze several acid, redox, and bifunctional reactions in
both homogeneous and heterogeneous systems.1-3
Although HPAs in their acidic form or transition metal ion
substituted forms are versatile compounds, their main disadvantages are high solubility in polar solvents and low surface
area (<10 m2/g).4 Therefore, in a homogeneous reaction the
isolation of the products and the reuse of the catalyst after
reaction become difficult. Heterogenization of homogeneous
catalysts has been an attractive strategy to overcome the
difficulties in the separation and reusability of homogeneous
catalysts. Hence, it is important to identify and implement clean
technologies such as the use of heterogenized catalysts that have
many advantages over their homogeneous counterparts like the
ease of separation, high surface area, and catalyst reusability.
Several studies were published on the immobilization of HPAs
on various supports and their use in catalysis as green
catalysts.5-7
Phosphotungstic acid (H3PW12O40, PTA) is the strongest HPA
among the Keggin series and is a well established catalyst for
* To whom correspondence should be addressed. E-mail: nk.kala@
ncl.res.in; Phone: +91 20 2590 2582; Fax: +91 20 2590 2633 (N.K.K.R.).
E-mail: [email protected]; Phone: +91 20 2590 2569; Fax: +91 20
2590 2615 (T.G.A.).
†
Central NMR Facility.
‡
Catalysis and Inorganic Chemistry Division.
§
Current address: Cardiff Catalysis Institute, School of Chemistry, Cardiff
University, Cardiff, CF10 3AT United Kingdom.
⊥
Current address: Dow Chemical International Pvt Ltd., Pune, India.
selective oxidation reactions and acid catalyzed reactions.8 They
are used in many applications such as the hydrolysis of propene
to 2-propanol9 as a homogeneous catalyst. Another important
application is the dehydration of 2-propanol to propene and
methanol to hydrocarbons where PTA is used as a heterogeneous
catalyst.10 For heterogenization of PTA, materials like silica,
alumina, zirconia, titania, and carbon11-15 are used as supports.
It is observed that the support plays an important role in
determining the nature of the heterogeneous catalyst.4,16 However, in most of the studies, silica was used as the support.
According to Vazquez et al.,16 the stability and acidity of HPA
supported on various supports decrease in the order silica >
titania > carbon > alumina.
Apart from the usual impregnation method for supporting
PTA on supports, other techniques like the sol-gel technique
with or without using ionic liquids as the template17 and
immobilization on chemically modified silica have been reported
in the literature. Since physically adsorbed PTA on the support
surface can leach out easily, immobilization by means of
chemical bonding via functionalized silica is the most effective
approach. Among the different silica supports that are used,
fumed silica has the advantages of low cost of production and
ease of availability. In addition, its physicochemical property,
which can be modified easily, becomes a crucial factor in
determining the property of the final grafted material. It has
already been shown that immobilization of heteropolyacids on
fumed silica is one of the effective approaches in designing
catalyst systems that give high selectivity to the desired product,
and the functionalized fumed silica has better affinity to anchor
HPAs including PTA over other silica supports like MCM-41.18,19
Immobilization of HPAs on a functionalized support containing nitrogen donors gives more stability and enhanced catalytic
activity.20 Kaleta et al.21 and Kala Raj et al.14 have reported the
immobilization of HPA inside the channels of Si-MCM-41 and
SBA-15, respectively. The HPA is bonded strongly to these
functionalized mesoporous supports by means of ionic interaction with the strong σ-donor amine groups, which prevents/
reduces the leaching of the catalyst in polar solvents. In all the
10.1021/jp906108e CCC: $40.75  2009 American Chemical Society
Published on Web 11/16/2009
Immobilization of PTA
above methods, the amine group was used as the anchoring
agent. Recently, Mirkhani et al. reported that silica modified
imidazole is a very good support for immobilization of Mn(III)
salophen and plays an important axial ligand role in the
oxidation reactions.22
Although there are many studies on the advantages of
heterogenizing a homogeneous catalyst, the exact nature of
bonding between the catalyst and the support is not well
understood. Elemental analysis gives information about the
average surface loading on the support materials. In order to
gain information on the functional group moiety and also to
understand the change in coordination environment of the
compound, spectroscopic techniques such as IR, UV-vis, etc.,
are used. Solid state NMR is a powerful technique which can
probe molecular level information, and this can be used to obtain
complete information of the modified surfaces. The 29Si NMR
studies of the surface and surface immobilized species were
reported by Fyfe et al.23 on silica gel and activated high surface
area glass beads. Caravajal et al.24 reported 29Si and 13C solid
state NMR experiments of (3-aminopropyl)triethoxysilane modified silica. In a recent study, functionalization of imidazole on
SBA-15 was confirmed by 13C solid state NMR.25 In another
work, Bordoloi et al. have used 31P NMR to get clear evidence
for the anchoring of heteropolyacids on the supports.26
The study reported in this paper was carried out with two
main objectives. The primary objective was the immobilization
of phosphotungstic acid (H3PW12O40, PTA) onto the surface of
3-(imidazolin-1-yl) propylsilane modified fumed silica to design
a catalyst that shows high selectivity in the epoxidation reaction.
Imidazolinyl based anchoring agent was chosen for many
reasons: (a) relative easiness to synthesize from chloropropylsilane functional unit, (b) imidazole nitrogen is a better
coordinating atom with its lone pair electron than the amine
based ligand, and (c) availability of 15N labeled imidazole.
Epoxidation of a variety of olefins using aqueous hydrogen
peroxide as an oxidant was carried out to demonstrate the
effective heterogenization of heteropolyacid in this catalyst. The
second objective was to carry out a complete physicochemical
characterization of this material using IR, UV, and an extensive
solid state NMR study with 13C, 15N, 29Si, and 31P to extract
detailed microscopic information on the catalyst system, especially to understand the nature of interaction between the
functionalized units. Since the natural abundance of 15N is much
less (≈0.3%), the 15N solid state measurements were carried
out on the Si-Imid-PTA system prepared using 98% 15N enriched
imidazole. As nitrogen atom is expected to play an important
role in the anchoring, 15N NMR should help us to identify the
changes in the bonding of C-NdC and H-N in the imidazole
group. This in turn is expected to give valuable information
about the structural features of the imidazole ring and its
interaction with PTA catalyst. There are only a few detailed
15
N solid state NMR studies on the nitrogen functionalized silica
surfaces.27-29
2. Experimental Methods
2.1. Chemicals. All the chemicals used were of analytical
reagent grade. Fumed silica, chloropropyltriethoxysilane, 15N
labeled imidazole, and imidazole were purchased from Aldrich
Co. and phosphotungstic acid (H3[PW12O40].xH2O) from Loba
Chemie and were used as received without further purification.
Diethyl ether (Merck India) and acetonitrile (S.D. Fine) were
of analytical grade and were used as received. The exact strength
of hydrogen peroxide (Loba Chemie) was determined by redox
titration with KMnO4 solution. The substrates, limonene, cis-
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21115
cyclooctene, 1-octene, norbornene, trans-2-octene, 1-methyl1-cyclohexene, etc., were procured from Aldrich and used as
received.
2.2. Characterization. 2.2.1. Elemental Analysis and IR
and UV Spectra. The elemental analysis of the samples was
done with an EA 1108 CHNS element analyzer. Tungsten
contents in the functionalized sample before and after catalytic
reactions were estimated by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES). FT-IR spectra of the powdered samples were recorded on a SHIMADZU FTIR-8300
spectrophotometer as KBr pellets. Diffuse reflectance UV-visible
measurements were recorded at room temperature with BaSO4
as a reference on a Perkin-Elmer Lambda 650 Spectrometer.
Nitrogen adsorption measurements were carried out on a
Quantachrome Autosorb-1 at 77 K. First, the samples were
activated at 423 K under vacuum, and then the adsorptiondesorption was conducted by passing N2 into the sample, which
was kept under liquid nitrogen. The specific surface areas of
the samples were calculated using the multiple-point BrunauerEmmett-Teller (BET) method in the relative pressure range
P/P0 ) 0.05-0.3.
2.2.2. Multinuclear Solid State NMR. Solid state 31P, 13C,
29
Si, and 15N MAS (magic-angle spinning) and CP-MAS (crosspolarization-magic angle spinning) experiments were performed
on a Bruker AV-300 NMR spectrometer equipped with a 7.05
T wide bore superconducting magnet, using a 4 mm BL MAS
probe resonating at 121, 75.4, 59.6, and 30.3 MHz, respectively
for 31P, 13C, 29Si, and 15N nuclei. The samples were packed in
a 4 mm zirconia rotor and were spun at 10 kHz for all the
experiments. A standard ramped-amplitude cross-polarization
(RAMP-CPMAS) pulse sequence was used with a CP contact
of 3, 4, and 4 ms for the 29Si, 13C, and 15N CPMAS, respectively.
For the 29Si CPMAS spectra of the functionalized samples,
8000-10 000 scans were recorded with a repetition time of 4 s.
For the 13C CPMAS spectrum of pure imidazole, 15 000 scans
were recorded with a repetition time of 3 s, and for the
functionalized samples, 10 000-15 000 scans were recorded
with a repetition time of 5 s. For the 15N CPMAS spectra of
15
N labeled imidazole, 2000 scans were recorded with a recycle
delay of 1 s, while for functionalized samples 4000-5000 scans
were recorded with a recycle delay of 1 s. For the 31P MAS
NMR spectra of dehydrated PTA 16 scans with a recycle delay
of 1000 s and for the functionalized PTA, 120 scans with a
recycle delay of 360 s were used. A single pulse excitation using
high values for the recycle delay (>5*T1) ensured a complete
relaxation of 31P nuclei. The 31P spin-lattice relaxation time
(T1) was measured using standard saturation recovery pulse
sequence and was 190 s for the dehydrated PTA and 70 s for
the functionalized samples. Chemical shifts were referenced to
the CH2 carbon of adamantane (38.48 ppm) for 13C, 2,2dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) for 29Si,
and 85% phosphoric acid for 31P, respectively. 15N chemical
shifts are given with respect to glycine at 34.5 ppm.
2.3. Catalyst Preparation. The PTA anchored on 3-(imidazolin-1-yl)propylsilane was prepared using a two-step synthesis method as described below.
2.3.1. Preparation of 3-(Imidazolin-1-yl)propylsilane (SiImid). A schematic representation of the synthesis of 3-(imidazolin-1-yl)propylsilane (Si-Imid) is shown in Scheme 1. A
mixture of fumed silica (1 g), 3-chlorotriethoxypropylsilane
0.241 g (1 mmol), and imidazole 0.0680 g (1 mmol) was
refluxed for 24 h in p-xylene (40 mL) under nitrogen atmosphere. After refluxing for about 24 h, the mixture was cooled
to room temperature, filtered, washed with xylene to remove
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J. Phys. Chem. C, Vol. 113, No. 50, 2009
Sofia et al.
SCHEME 1: Preparation of 3-(Imidazolin-1-yl)propylsilane (Si-Imid)
SCHEME 2: Anchoring of Phosphotungstic Acid onto
3-(Imidazolin-1-yl)propylsilane
Figure 1. FT-IR spectra of (a) fumed silica, (b) Si-Imid, (c) PTA,
and (d) Si-Imid-PTA.
any unreacted 3-chlorotriethoxypropylsilane, and dried. The
white product obtained is designated as Si-Imid.
2.3.2. Anchoring of PTA onto 3-(Imidazolin-1-yl)propylsilane. Scheme 2 represents the anchoring of PTA onto 3-(imidazolin-1-yl)propylsilane. Si-Imid (1 g) was added to an
acetonitrile solution of PTA; 2.88 g (1 mmol) in 40 mL was
taken in a round-bottom (RB) flask. The mixture was refluxed
for 24 h under nitrogen atmosphere. After 24 h, the mixture
was filtered, washed with acetonitrile and dichloromethane, and
dried at room temperature. The final product was a white powder
and is designated as Si-Imid-PTA. 15N labeled sample was
prepared in a similar manner using 15N labeled imidazole and
used for solid state NMR studies.
The C/N molar ratio calculated from elemental analysis was
3.12, which is very close to the theoretically expected value 3,
indicating that the chloro groups from 3-chlorotriethoxypropylsilane are replaced by imidazole completely. The tungsten
content in the anchored PTA estimated by ICP-AES was 26.12
ppm.
2.4. Catalytic Activity. The expoxidations of alkenes (limonene, cis-cyclooctene, 1-octene, norbornene, trans-2-octene,
1-methyl-1-cyclohexene) were carried out in a two-necked RB
flask with anhydrous CaCl2 guard tube. All reactions were
carried out at 60 °C in acetonitrile solvent. Typically 5 mmol
of substrate and 5 mmol of hydrogen peroxide (30%) were taken
in 4.5 mL of solvent along with 0.05 g of Si-Imid-PTA as a
catalyst. The reaction mixture was stirred constantly at the
required temperature. Small aliquots of the samples were
withdrawn at regular intervals and subjected to GC analysis to
monitor the conversion of the substrate and selectivity to its
products using chlorobenzene as an internal standard. The
samples were analyzed using a HP-5890 gas chromatograph
fitted with a fused megabore column SE-52, HP-% (cross-linked
5% PhMe silicone), 30 m in length, 0.53 mm i.d., 0.3 m film
thickness, and a FID. The products were confirmed by GC-MS
using a QP-5000 SHIMADZU mass spectrometer.
2.5. Leaching and Recycle Tests. The possible leaching of
anchored PTA into the solution was studied by carrying out a
reaction with cyclooctene as a representative substrate under
the same reaction conditions. In a controlled reaction, the
catalyst was filtered out from the reaction mixture after 1 h of
reaction time, and the reaction was allowed to continue with
the remaining filtrate with fresh addition of aq. H2O2. Aliquots
of the samples were withdrawn at definite intervals and were
analyzed by GC, and the results were compared with a reaction
without removing the catalyst intermittently.
Recycling tests with the catalyst were also carried out with
the same substrate. A reaction was started using 5 mmol of
cyclooctene and 5 mmol of aq. H2O2 (30%) in 4.5 mL of
acetonitrile solvent and 0.05 g of Si-Imid-PTA catalyst. The
reaction was carried out at 60 °C for 4 h. At the end of the
reaction, the catalyst was filtered off, washed thoroughly with
acetonitrile, and dried. This dried catalyst was used again as
catalyst for a fresh reaction. It was reused for two more reaction
cycles.
3. Results and Discussion
3.1. Characterization of the Catalyst. 3.1.1. FT-IR. FTIR spectra of fumed silica, Si-Imid, PTA, and Si-Imid-PTA are
shown in Figure 1a, b, c, and d. Some of the important bands
which are observed in the spectra are given in Table 1. In fumed
silica, bands at 801 cm-1 (symmetric stretching frequency of
Si-O-Si), 965 cm-1 (stretching frequency of Si-O-H), and
1000-1200 cm-1 (antisymmetric stretching of Si-O-Si)30 are
observed which are also present in the spectra of Si-Imid and
Si-Imid-PTA. Peaks in the region of 3500 cm-1 are observed
which correspond to adsorbed water present in the system.31
Immobilization of PTA
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21117
TABLE 1: Infrared Spectroscopic Data for Fumed Silica,
Si-Imid, PTA, and Si-Imid-PTA before the Catalytic
Reaction
vibration mode
assignment
fumed
silica, cm-1
Si-Imid,
cm-1
PTA,
cm-1
Si-Imid-PTA,
cm-1
Si-O
Si-O-Si/W-O-W
W-O-W
Si-O-H
W-O
Si-O/P-O
CdN
467
801
965
1094
-
467
802
931
1094
1447
801
889
983
1081
-
467
814
897
955
981
1095
1456
The bands observed in the range of 2800-2950 cm-1 are due
to the propyl group present in Si-Imid and Si-Imid-PTA. The
band at 1447 cm-1 for Si-Imid is assigned to the CdNs of the
imidazole group, which is shifted toward 1456 cm-1 in Si-ImidPTA which could be due to the change in environment of the
double-bonded nitrogen in imidazole. A small band between
1400-1410 cm-1 in Si-Imid and Si-Imid-PTA is the characteristic band of the C-N bond which can be attributed to
anchoring of imidazole to the propyl group.25 PTA shows
characteristic IR bands at 1081 cm-1 (stretching frequency of
P-O in the central PO4 tetrahedron), 983 cm-1 (terminal bands
for WdO in the exterior WO6 octahedron), 889 cm-1, and 801
cm-1 (W-O-W bands). In Si-Imid-PTA, bands are observed
at 981 and 897 cm-1 which confirms the presence of PTA in
SI-Imid-PTA.22,25,32-36
3.1.2. UV-Visible. Diffuse reflectance UV-visible spectra
of fumed silica, imidazole, Si-Imid, PTA, and Si-Imid-PTA are
shown in Figure 2a, b, c, d, and e. Pure fumed silica shows no
absorption in the UV-vis region. Imidazole shows a UV
absorption peak at 210 nm.37 This peak is shifted to 226 nm in
Si-Imid due to the functionalization of imidazole to fumed silica.
Neat PTA shows absorption bands at 254 and 315 nm which
are attributed to oxygen-tungsten charge transfer absorption
bands for the Keggin anion.11,38,39 On anchoring with Si-Imid,
the 254 nm band is shifted to 266 nm and the 315 nm band
appears as a shoulder band reconforming the presence of PTA
in Si-Imid-PTA. Changes in intensity of these bands compared
to the neat PTA are due to the presence of another component
like imidazole.
3.1.3. Nitrogen Adsorption Studies. The nitrogen sorption
isotherms for fumed silica, Si-Imid, and Si-Imid-PTA were
carried out by the BET method at 77 K. The surface area of
fumed silica has reduced from 502 to 139 m2/g on imidazole
Figure 2. UV-visible spectra of (a) fumed silica, (b) imidazole, (c)
Si-Imid, (d) PTA, and (e) Si-Imid-PTA.
Figure 3.
29
Si CP-MAS spectra of (a) Si-Imid and (b) Si-Imid-PTA.
functionalization, and it was further reduced to 77 m2 /g upon
anchoring the PTA. This observation indirectly confirms the
anchoring of PTA onto Si-Imid. The reduction in surface area
may be due to the presence of the bulkier trisiloxypropyl
imidazole group and the heteropolyacid anion on the surface
of fumed silica.
3.1.4. Solid State NMR. Although the presence of PTA in
Si-Imid-PTA is confirmed using elemental analysis, IR and
UV-vis, and solid state NMR studies probed through 29Si, 13C,
and 15N NMR can bring out another dimension of the structural
aspects. The 29Si CP-MAS spectra of the Si-Imid and Si-ImidPTA samples are shown in Figure 3 in which prominent peaks
are observed at -110, -102, -60, and -68 ppm. The peaks at
δ ) -110 and -102 ppm can be assigned as the Q4 [Si(OSi)4]
and Q3 [Si(OSi)3OH] sites of fumed silica. The spectrum of
fumed silica has already been reported in the literature and is
known to have peaks at -109, -100, and -91 ppm corresponding to the Q4, Q3, and Q2 [Si-(OSi)2(OH)2]40,41 sites. The
Q4 structural units represent interconnected SiO4 tetrahedrons,
while the Q3 and Q2 structural units represent the silanol groups
associated with the surface of silica. The intensity of the Q2
site at -91 ppm is much less in the functionalized silica because
most of the silanol groups associated with Q2 sites of the fumed
silica are wiped out and get attached to 3-(imidazolin-1yl)propylsilane. During functionalization, new covalent Si-O-Si
linkages are formed which are designated as the T3 structural
unit [(-O-)3Si-CH2CH2CH2(C3H3N2)] and the T2 structural
unit [(-O-)2Si-CH2CH2CH2(C3H3N2)]. The T3 structural unit
which corresponds to the peak at δ ) -68 ppm indicates the
formation of Si-O-Si linkage of the imidazolin-1-yl-propyl
group on the surface of silicon atoms of fumed silica through
three siloxane bonds. The T2 structural unit gives a peak at
around -60 ppm, and the remaining OEt group of triethoxycholoropropylsilane that is not anchored onto the surface of silica
may undergo hydrolysis to form a Si-OH species on the
support. The relatively high intensity of the peak at -68 ppm
(T3 structural unit) confirms that the 3-(imidazolin-1-yl)propylsilane groups have attached to the surface of fumed silica.23,42
Since the PTA is expected to attach with the imidazole molecule,
we do not expect any change in the 29Si spectra of Si-ImidPTA from the Si-Imid, and that is seen in Figure 3b.
The 13C CP-MAS spectra of neat imidazole, Si-Imid, and SiImid-PTA are shown in Figure 4. The numbering sequence for
each element is given at the top of these figures. The spectrum
of imidazole was also recorded for comparison. In the 13C
spectrum of imidazole, peaks are observed in the aromatic region
at 135 ppm (C1), 127 ppm (C2), and 115 ppm (C3). For Si-Imid
(Figure 4b) and Si-Imid-PTA (Figure 4c), two sets of peaks
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J. Phys. Chem. C, Vol. 113, No. 50, 2009
Figure 4. 13C CPMAS spectra of (a) imidazole, (b) Si-Imid, and (c)
Si-Imid-PTA.
Figure 5. 15N CPMAS NMR spectra of (a) imidazole, (b) Si-Imid,
and (c) Si-Imid-PTA.
are observed: one from the aromatic and the other from the
aliphatic region. Though a set of three peaks from the aromatic
region at 136 ppm (C1), 126 ppm (C2), and 120 ppm (C3) are
seen for Si-Imid, only two peaks at 134 and 121 ppm are
observed for Si-Imid-PTA in the same region. The second set
of peaks from propyl carbons (aliphatic region) are at 49.32
ppm (C4), 24.1 ppm (C5), and 9.7 ppm (C6) for Si-Imid and
51.38 ppm (C4), 23.38 ppm (C5), and 9.2 ppm (C6) for Si-ImidPTA.
The 15N CP-MAS spectra of neat imidazole, Si-Imid, and SiImid-PTA, are shown in Figure 5. 15N labeled imidazole and
functionalized materials were used for 15N NMR studies. In the
15
N spectrum of imidazole (Figure 5a), peaks at 172 and 244
ppm are observed. The peak at 172 ppm is assigned as the
protonated (pyrrole) type nitrogen atom43 labeled as N1, and
the peak at 244 ppm is assigned as the nonprotonated (pyridine)
type nitrogen atom labeled as N2 present in imidazole. In the
15
N spectrum of Si-Imid (Figure 5b), the N2 peak (244 ppm)
appears to be broad and reduced in intensity, and the chemical
shift value of the N1 peak has changed from 172 to 182 ppm
with a shoulder at 171 ppm. There is an overall broadening in
all the peaks which could be due to the chemical shift dispersion
or dynamics when attached to the silica surface. The 15N
spectrum of Si-Imid-PTA (Figure 5c) shows that the N2 peak
is absent, a prominent peak at 182 ppm, and a small peak at
171 ppm are observed. The peak at 171 ppm is more intense
than the similar one observed in Si-Imid.
The anchoring of imidazole to silica surface through the
propyl group of triethoxypropylsilane by means of chemical
bond will change the environment of the N1 of the imidazole
because the anchoring takes place at the N1 position leading to
a downfield shift of the N1 peak from 172 to 182 ppm in the
15
N spectrum. This also causes a downfield shift for C4 which
is attached to N1, resulting in a peak at 49.32 ppm in the 13C
Sofia et al.
Figure 6. 31P MAS NMR spectra of (a) dehydrated PTA, (b) Si-ImidPTA, and (c) Imid-PTA.
spectrum.44 The carbon atom attached to the electropositive Si
group (C6) is more shielded and therefore shows an upfield shift
resulting in the peak at 9.20 ppm. No protonation is expected
at the N2 position in the Si-Imid sample. However, a decreased
intensity of the N2 peak (244 ppm) in the 15N spectrum of SiImid indicates possible protonation at this nitrogen which may
be due to the H+ ions from HCl25 remaining in the system. It
may be noted here that HCl is a byproduct during the
immobilization of imidazole which is normally eliminated by
repeated washing but traces of which could remain in the system.
The peak at 171 ppm in the 15N spectrum is also due to the
protonated N2 nitrogen. The shift of about 1 ppm from the neat
imidazole N1 peak is likely due to the change in environment.
The evidence for protonation at N2 is also reflected in the 13C
spectrum of Si-Imid. The decrease in intensity and a downfield
shift of the C2 peak position compared to the imidazole peaks
could be due to the protonation of N2 in Si-imid which causes
the two carbon atoms (C2 and C3) to become nearly equivalent
due to the equal probability for the proton being on N1 and N2.
It is well-known that the acid form of imidazole, imidazolium
ion, exists as tautomers of two equally contributing forms in
which the proton is on either N1 or N2 which makes the carbon
atoms (C2 and C3) indistinguishable.27,45-48 The 15N spectrum
of Si-Imid-PTA (Figure 5c) shows that the N2 peak is absent.
However, the peaks at 182 and 171 ppm became prominent
and more intense in comparison to that in Si-Imid. This suggests
that there is a complete replacement of chloride by PTA resulting
in a complete protonation at N2, making the peak at 244 ppm
vanish and showing an increase in the intensity of the 171 ppm
peak. This is also supported by the 13C CP-MAS results of SiImid-PTA in which there are only two carbon peaks observed
at 134 and 121 ppm. The peak at 121 ppm is due to the merging
of C2 and C3 on complete protonation of the imidazole ring.
The protonation of imidazole due to PTA also results in the
downfield shift of C4 to 51.38 and an upfield shift of C6 to 9.2
ppm in the 13C spectrum. These observations from 13C and 15N
NMR measurements clearly demonstrate that PTA interacts/
binds with Si-Imid by forming an ion pair, i.e., [imidazolium]+[H2PW12O40]- (refer to Scheme 2).
In order to further substantiate the above points, additional
experiments with 31P MAS NMR were carried out on neat PTA
and Si-Imid-PTA. In addition, a comparison has been made with
a sample that was prepared by interacting PTA with neat
imidazole. The 31P MAS spectra of PTA (dehydrated), PTA
anchored on Si-Imid-PTA, and PTA reacted with neat imidazole
named as Imid-PTA are shown in Figure 6. In the neat
dehydrated PTA, a sharp peak at -15.6 ppm (line width ≈ 30
Hz) is observed. It can be recalled here that PTA has a cubic
structure of symmetry with a central P atom. For Si-Imid-PTA,
Immobilization of PTA
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21119
TABLE 2: Oxidation of a Few Representative Alkenes with Aqueous Hydrogen Peroxide Using Si-Imid-PTAa
a
Experimental conditions: catalyst ) 0.05 g; substrate ) 5 mmol; aqueous H2O2 (30%) ) 5 mmol, temperature ) 60° C, time ) 4 h,
solvent ) acetonitrile, a ) cyclooctanol and cyclooctanone, b and c ) please see the text, d ) thrice recycled catalyst.
peaks (Figure 6b) at -15.3 ppm (line width ≈ 90 Hz) and -13.6
ppm (line width ≈ 120 Hz) and for Imid-PTA two peaks (Figure
6c) at -15.6 and -13.7 ppm are observed. These results can
be rationalized as follows. PTA on reacting with imidazole
makes a new environment which may be due to the distortion
of the cubic structure of PTA upon interaction with imidazole
during functionalization. The peak at -13.6 ppm for Si-ImidPTA and for Imid-PTA at -13.70 ppm can be assigned to the
phosphorus atom in the new environment. Even after functionalization, the peak due to the symmetric central P atom of PTA
is present with significant intensity. This is due to the fact that
there are eight molecules of phosphotungstic acids present in a
unit cube, the centers of the PTA anions being arranged in
positions corresponding to the diamond structure.49 These
molecules may remain as clusters which are strongly attached
to the functionalized silica. The broad peaks at -15.3 ppm for
Si-Imid-PTA and at -15.6 ppm for Imid-PTA may be due to
this type of phosphorus present in the system.50-52 Thus, 31P
NMR data clearly support the above model of PTA anchoring
on imidazole of Si-Imid by forming an ion pair complex.
3.2. Catalytic Activity. The catalytic activity of Si-ImidPTA was studied for the oxidation of different alkenes such as
limonene, cis-cyclooctene, 1-octene, norbornene, trans-2-octene,
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J. Phys. Chem. C, Vol. 113, No. 50, 2009
Sofia et al.
TABLE 3: Oxidation of cis-Cyclooctene with Aqueous Hydrogen Peroxide Using Si-NH2-PTA and PTA Impregnated on Fumed
Silicaa
a
Experimental conditions same as Table 1 and the products formed given as Others* could not be identified.
1-methyl-1-cyclohexene, etc., using aqueous hydrogen peroxide
as oxidant at 60 °C in acetonitrile solvent. These experiments
were carried out mainly to demonstrate the efficiency and
heterogeneous nature of the present catalyst. Additionally,
controlled experiments with neat phosphotungstic acid and SiImid were also carried out under identical reaction conditions.
The resulting data of substrate conversion and selectivity to the
main products at the end of 4 h of reaction time are summarized
in Table 2. As seen in the table, this catalyst system is active
for epoxidation reaction and also selectively produced the
desired epoxide compounds in most cases. Oxidation of ciscyclooctene, 1-octene, and norbornene gave corresponding
epoxide with more than 99% selectivity with substrate conversion of 90, 34, and 85 mol %, respectively, and no efforts were
made to optimize the conversion. With limonene, limonene
diepoxide (46 mol %) and limonene epoxide (29 mol %) were
formed with moderate conversion of 76 mol %. A certain
amount of carvone (17 mol %) along with products like carveol,
glycols, perillyl alcohol, etc. (included in Table 2 as Othersb
and amounting to a total of 8 mol %) were also formed during
the epoxidation of limonene. The oxidation of trans-2-octene
and 1-methyl-1-cyclohexene also gave the respective epoxides
as the major product with 91 mol % conversion. trans-2-Octene
gave mixtures of 2, 3, and 4-octanones (23 mol %, marked as
Othersc in Table 2), and 1-methyl-1-cyclohexene gave ketocarboxylic acid (29 mol %) and L-methyl-trans-1,2-cyclohexanediol
(11 mol %). Formation of dihydroxy compounds is formed
mainly due to the acidity arising from aq. H2O2/protons of
PTA.53
As noticed, the selectivity to the epoxide product was always
higher with the heterogeneous Si-Imid-PTA catalyst. It was
observed that the conversion of the substrate as well as
selectivity to the epoxide had reduced when neat PTA was used
as catalyst (Table 2). This may be due to the fast decomposition
of aq. H2O2 upon addition of neat PTA. When PTA impregnated
on fumed silica was used as the catalyst, cis-cyclooctene, a
conversion of 92% could be observed with a selectivity of 75%
to the epoxide (Table 3). A comparative study using Si-NH2PTA as catalyst for the oxidation of cis-cyclooctene was also
carried out, and the results are incorporated in Table 3. With
the controlled experiments using Si-Imid as the catalyst under
similar experimental conditions, no olefin conversion could be
observed. The above experiment indicates that the epoxidation
reaction is catalyzed purely by PTA of Si-Imid-PTA and no
contribution from Si-Imid, as expected, for the epoxidation of
these alkenes under the reaction conditions. Further, the catalytic
activity shown by the immobilized catalyst proves the effective
anchoring of phosphotungstic acid successfully onto Si-Imid.
The higher conversion and selectivity of the selected alkenes
to the desired products using Si-Imid-PTA catalyst is attributed
to the formation of tungsten-peroxo species for the formation
of the selected products.54
Turnover frequency (TOF, h-1), defined as moles of product
formed per mole of PTA per hour, were calculated for Si-ImidPTA and neat PTA (Table 2), and the results indicate that SiImid-PTA is a better catalyst than neat PTA for oxidation
reactions when aq. H2O2 is used as the oxidant.
3.3. Catalyst Reuse and Leaching Studies. The stability
of Si-Imid-PTA was studied in repeated epoxidation reactions
under same reaction conditions using cyclooctene as a model
substrate. After 4 h of cyclooctene epoxidation reaction, the
Si-Imid-PTA catalyst was filtered from the reaction mixture
and washed with acetonitrile and is referred here as recovered
catalyst. The recovered catalyst was dried and used as a
catalyst for the fresh reaction as above under identical
experimental conditions, and the cycle was repeated thrice.
It was found that the recycled catalyst was stable and active
with selectivity of >99 mol % to epoxide although the
substrate conversion was slightly low (see Table 2, entry 7).
Even after three reaction cycles using the same catalyst, not
much loss in activity could be observed. Both conversion as
well as selectivity to the desired product remained almost
similar to that of the fresh catalyst.
Leaching experiments were performed with cyclooctene to
confirm that the catalyst is heterogeneous in nature. The tungsten
content present in the catalyst after final recycling studies was
16.853 ppm. The solid catalyst was separated from the reaction
mixture at the end of 2 h, and the reaction was continued further
only with the filtrate. Only traces of products were observed,
showing that there is negligible leaching of the active species
into the solution. There was no marked difference in the diffuse
reflectance UV-visible spectra and 13C CPMAS (Figure 7 and
Immobilization of PTA
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21121
interaction between PTA and imidazole. While IR, UV-vis,
and other routine NMR studies provide evidence for the
heterogenization of PTA on the imidazole functionalized fumed
silica, the nature of binding of PTA on the support has been
obtained from solid state NMR studies using 15N labeled
imidazole support. Effective heterogenization of PTA is mainly
due to imidazolium ion formation on the support due to protons
of PTA, and they form an ion pair compound.
Figure 7. UV-visible spectra of (a) Si-Imid-PTA before catalytic
reaction and (b) Si-Imid-PTA after catalytic reaction.
Acknowledgment. N.K.K.R. thanks the Department of Science & Technology (DST, New Delhi) for financial support.
M.S. thanks the Council of Scientific and Industrial Research
(CSIR, New Delhi) for a Senior Research Fellowship. The
authors thank Dr. R. Nandini Devi, Dr. Maya Devi, Dr. Sujatha
Mandal, Dr. R. H. Ingle, and Mr. Renny Mathew for useful
discussions.
References and Notes
Figure 8. 13C CPMAS NMR spectra of (a) Si-Imid-PTA before
catalytic reaction and (b) Si-Imid-PTA after catalytic reaction.
Figure 9. 31P MAS NMR spectra of (a) Si-Imid-PTA before catalytic
reaction and (b) Si-Imid-PTA after catalytic reaction.
8) of both used and fresh catalyst, whereas in the case of 31P
MAS spectra (Figure 9), it is observed that an additional peak
appears at around 8 ppm and there are broad peaks from 8 ppm
continuing until about -16 ppm for the used catalyst. These
peaks can be attributed to the presence of phosphorus of the
distorted Keggin structure of PTA which is formed after the
reaction.55,56 The distorted PW11O397- species could have been
formed by the interaction of leached PTA with the support
material. UV-visible spectra and 13C CPMAS spectra indicate
that the catalyst is intact even after catalytic activity.
4. Conclusions
The PTA anchored silica-imidazole system was used for
epoxidation of a variety of symmetric and asymmetric olefins.
The high olefin conversion and high selectivity to epoxide
demonstrate that the present system is an efficient catalyst for
such type of reactions. A negligible amount of leaching of PTA
under the present reaction condition indicates the strong
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