Highly active and green aminopropyl-immobilized phosphotungstic acid on mesocellular silica foam for the O-heterocyclization of cycloocta-1,5-diene with aqueous H2O2

Article abstract of DOI:10.1039/c0gc00691b

The heteropoly phosphotungstic acid, H₃PW₁₂O ₄₀ (HPW), was successfully immobilized on the surface of MCF, SBA-15 and MCM-41 by means of chemical bonding to aminosilane groups. The as-obtained materials were characterized by N₂ sorption, TEM, XRD, FT-IR, ¹³C-, ²⁹Si-, ³¹P-MAS NMR and XPS. Characterization results suggest that the surface area decreased after grafting amino groups to silica. The as-prepared HPW-NH₂-MCF is highly efficient in the O-heterocyclization of cycloocta-1,5-diene (COD) to 2,6-dihydroxy-9-oxabicyclo[3.3.1] nonane (1) and 2-hydroxy-9-oxabicyclo [3.3.1] nonane-6-one (2) with a COD conversion of 100% and (1 + 2) selectivity up to 98%. It is worth mentioning that this material could be reused six times without any significant loss of activity and selectivity. The good stability can be attributed to the strong interaction between the amino groups on the surface of MCF and HPW anions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00691b

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PAPER
Highly active and green aminopropyl-immobilized phosphotungstic acid on
mesocellular silica foam for the O-heterocyclization of cycloocta-1,5-diene
with aqueous H2O2†
Ruihua Gao,a,b Quanjing Zhu,a Wei-Lin Dai*a and Kangnian Fana
Received 15th October 2010, Accepted 6th December 2010
DOI: 10.1039/c0gc00691b
The heteropoly phosphotungstic acid, H3PW12O40 (HPW), was successfully immobilized on the
surface of MCF, SBA-15 and MCM-41 by means of chemical bonding to aminosilane groups. The
as-obtained materials were characterized by N2 sorption, TEM, XRD, FT-IR, 13C-, 29Si-,
31P-MAS NMR and XPS. Characterization results suggest that the surface area decreased after
grafting amino groups to silica. The as-prepared HPW-NH2-MCF is highly efficient in the
O-heterocyclization of cycloocta-1,5-diene (COD) to 2,6-dihydroxy-9-oxabicyclo[3.3.1] nonane (1)
and 2-hydroxy-9-oxabicyclo [3.3.1] nonane-6-one (2) with a COD conversion of 100% and (1 + 2)
selectivity up to 98%. It is worth mentioning that this material could be reused six times without
any significant loss of activity and selectivity. The good stability can be attributed to the strong
interaction between the amino groups on the surface of MCF and HPW anions.
of waste. Therefore, it is highly desirable to improve the
conventional process by safer and cleaner oxidizing agents,
and easily separable catalysts with more activity and selectivity.
One effective way to produce these two compounds (1 + 2) is
the selective oxidation of COD with environmentally benign
aqueous H2O2 as the oxidant. Since no noxious substances are
needed and no toxic waste would be generated in the reaction,
hence, it can be called as a green process.
1. Introduction
g-Butyrolactone structure is an important intermediate in
organic synthesis since lots of compounds containing this
functional group show interesting biological activity.1–3 For
instance, 5-hydroxy-g-decalactone is a potent cytotoxic agent
on different tumor cell lines.4
It is commonly accepted that 9-oxabicyclo[3.3.1]nonane-
2,6-dioles (1) and 2-hydroxy-9-oxabicyclo[3.3.1]nonane-6-one
In our previous work, tungstic acid was reported as an
(2) are the major starting materials for the synthesis of g-
butyrolactones. Therefore, it is of great significance to develop
efficient and accessible approaches to afford these products (1
and 2). Traditional routes for 1 and 2 employ peroxo-acid or
permanganate as oxidants. Behr et al. obtained 23.3% yield of
1 by using peracetic acid as the oxidant and cycloocta-1,5-diene
(COD) as the raw material.5 If peracetic acid was replaced by
permanganate, much lower conversion of COD was observed.
Moreover, 1 yield can reach 70% if performic acid was used as
oxidant, as reported by Hegemann et al.6 However, the use of
performic acid, as well as peracetic acid and permanganate, is
unsafe, corrosive and costly, and will produce huge amounts
efficient homogeneous catalyst for the target reaction.7 However,
the difficulties of separating and recovering the catalysts from
the product mixture during the homogeneous process made
them impractical for a large-scale process. One of the most
promising approaches is to design a heterogeneous WO3-
containing heterogeneous catalyst. In our previous work, we
reported the synthesis of a novel tungsten trioxide containing
silica with ultra large mesopores and its catalytic application
in the O-heterocyclization of COD.8 Although WO3-containing
mesocellular silica foam (MCF) catalysts have been found to
show high activity in this reaction, there is detectable leaching
of tungsten species from 10% WO3-MCF according to ICP-
AES analysis. It is known that the leaching of tungsten species is
inevitable in most reactions with H2O2. Hence, there is a strong
driving force to find a more stable catalyst without any leaching
of tungsten species, so that no heavy metal containing waste
water is generated and the catalyst can be reused many times.
Heteropolyacids (HPAs) are early transition metal oxygen
anion clusters.9 Among the various HPA structural classes,
Keggin-type10 HPAs have been widely used as homogeneous
aShanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, Fudan University, Shanghai,
200433, P.R. China. E-mail: wldai@fudan.edu.cn;
Fax: +86-21-55665701
bResearch Center of Nano Science and Technology, Shanghai University,
Shanghai, 200234, P.R. China
† Electronic supplementary information (ESI) available: See DOI:
10.1039/c0gc00691b
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and heterogeneous catalysts for acid–base and oxidation
reactions.11–15 The Keggin-type HPA has a soccer ball-shape with
a molecular size of ca. 1 nm.16 One of the disadvantages of HPA
catalysts is that their surface area is very low (<10 m2 g-1).
To overcome this problem, HPA catalysts have been supported
on inorganic porous materials including mesoporous molecular
sieve (MCM-41,17 SBA-1518), carbon gel,19 and mesoporous
g-alumina20 by an impregnation method. However, there is
obviously detectable leaching of active species from these
catalysts. In recent years, another promising approach to obtain
HPA catalyst with high surface area is to take advantage of
the overall negative charge of the heteropolyanions. By using
this method, HPA was immobilized on the polymer materials
such as poly-4-vinylpyridine,21 polyaniline,22 and polystyrene23
to obtain molecularly dispersed HPA catalysts. However, such
procedure has been restricted when inorganic supporting ma-
terials are utilized due to the difficulty in forming a positive
charge on inorganic supporting materials. Recently, a successful
example for the immobilization of HPA catalyst on an inorganic
support has been found.24 It was reported that H3PMo12O40 was
chemically immobilized on a kind of porous carbon by forming
a positive charge on the support via surface modification.24
However, no attempt has been made to immobilize H3PW12O40
on the aminopropyl-functionalized MCFs.
Fig. 1 Schematic procedure for the surface modification of MCF silica
and the subsequent immobilization of HPW on the surface modified
MCF (NH2-MCF) silica.
constant stirring at room temperature. These solid products were
filtered and dried at 100 ◦C for 2 h to yield the NH2-MCF, NH2-
SBA-15 and NH2- MCM-41 silica, respectively.
The immobilization of HPW on the NH2-MCF silica support
was carried out as follows (shown in Fig. 1). 1 g of NH2-
MCF silica, NH2-SBA-15 or NH2-MCM-41 was added to
an aqueous solution containing HPW (0.36 g) with vigorous
◦
stirring at 60 C, and the resulting solution was maintained at
MCFs with well-defined ultra large mesopores and hydrother-
mally robust frameworks are firstly described through an oil-
in-water microemulsion method by Winkel et al.25 The MCF
materials gain many benefits from their facilitated synthesis
method, such as well defined pore and wall structure, thick
walls, and high hydrothermal stability, which give MCFs unique
advantages as catalyst supports. The HPW/MCF catalysts were
firstly synthesized by traditional impregnated method in the
present study. Although good performance of the HPW/MCF
catalyst was obtained in the title reaction, the obvious leaching
of the active tungsten species was also observed. In contrast,
the novel HPW-NH2-MCF catalyst shows better stability and
reusability.
In this work, MCF-type silica was prepared via oil-in-water
microemulsion method. The as-obtained MCF silica was then
modified by APTES to provide sites for the immobilization of
HPW. By taking advantage of the overall negative charge of
[PW12O40]3-, the HPW catalyst was chemically immobilized on
the surface of modified MCF (NH2-MCF) silica as a charge
matching component. The characteristics of HPW catalyst
immobilized on the NH2-MCF silica were extensively charac-
terized in terms of various physicochemical techniques.
60 ◦C for 24 h. These solid products were filtered, washed with
distilled water three times and then dried overnight at 80 ◦C to
yield the 16%HPW-NH2-MCF, HPW-NH2-SBA-15 and HPW-
NH2-MCM-41 silica samples. The 8%, 24% and 30% HPW-
NH2-MCF were synthesized using the same method, only with
different volumes of HPW aqueous solution.
The impregnation method derived HPW/MCF-1 catalyst was
synthesized as follows. 0.36 g of HPW was dissolved in deionized
water. Then 1 g of MCF was added into the stirred solution
at 60 ◦C. After being stirred for 8 h, the solid was filtered,
washed with distilled water, and dried at 80 ◦C (denoted as
16%HPW/MCF-1). Inductively coupled argon plasma (ICP)
analysis indicated that the tungsten weight content of the
16%HPW/MCF-1 was 0.04%, suggesting that more than 99%
loosely bonded HPW species were readily removed after washing
with distilled water.
The impregnation method derived HPW/MCF-2 catalyst
was synthesized as follows. 0.36 g of HPW was dissolved in
distilled water. Then 1 g of MCF was added into the stirred
solution at 60 ◦C. After being stirred for 8 h, the excessive
water was completely evaporated at the same temperature under
depressed pressure and the catalyst was finally obtained after
◦
the solid material was dried at 80 C in air for 3 h (denoted as
2. Experimental
16%HPW/MCF-2).
2.1 Catalyst preparation
2.2 Characterizations
The MCF materials were prepared following a procedure similar
to that reported by Winkel et al.25 Fig. 1 shows the schematic
procedures for the surface modification of MCF silica and the
subsequent immobilization of HPW on the surface of modified
MCF (NH2-MCF) silica. The surface modification of the MCF
silica was achieved by reacting the silanol group of the MCF
silica with APTES (Aldrich) under a nitrogen atmosphere.
2.3 mmol of APTES was slowly added to a dry toluene solution
containing 1 g of MCF silica, SBA-1526,27 and MCM-4128 with
The specific surface area, pore volumes and the average pore
size of the samples were measured and calculated according to
the BET method on Micromeritics TriStar 3000 equipment with
◦
liquid nitrogen at -196 C. Transmission electron micrographs
(TEM) were obtained on a JOEL JEM 2010 scan-transmission
electron microscope. The wide-angle X-ray powder diffraction
(WAXS) patterns were recorded on a Bruker D8 advance diffrac-
tometer with Cu-Ka radiation, operated at 40 mA and 40 kV.
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Table 1 Physico-chemical parameters of various samples
The FT-IR measurements were carried out with a Nicolet
Model 205 spectrometer, using KBr pellet technique. The 29Si,
13C and 31P CP-MAS NMR spectra of the solid catalysts were
collected in a Brucker DXR 400 spectrometer.
Dw
Material
SBET (m2 g-1) Vp (cm3 g-1) Dc (nm)a (nm)b
The XPS spectra were recorded on a Perkin-Elmer PHI 5000C
ESCA system equipped with a dual X-ray source, using the Mg-
Ka (1253.6 eV) anode and a hemispherical energy analyzer.
The background pressure during data acquisition was kept
below 10-6 Pa. Measurements were performed at pass energy
of 93.90 eV to ensure sufficient sensitivity for the acquisition
scan, and pass energy of 23.50 eV was used for the scanning
of the narrow spectra of Si 2p, W 4f, O 1s, N 1s and C 1s to
ensure sufficient resolution. All binding energies were calibrated
using contaminant carbon (C 1s = 284.6 eV) as a reference. The
tungsten content was determined by ICP method (IRIS Intrepid,
Thermo Elemental Company) after solubilization of the samples
in HF : HCl solutions.
MCF
NH2-MCF
8%HPW-NH2-MCF
16%HPW-NH2-MCF
24%HPW-NH2-MCF
30%HPW-NH2-MCF
SBA-15
NH2-SBA-15
16%HPW-NH2-SBA-15 284
MCM-41
NH2-MCM-41
592
303
263
170
122
122
760
407
1.37
0.88
0.83
0.55
0.35
0.33
1.37
0.80
0.56
0.84
0.54
0.21
1.0
18.9
15.8
12.8
12.7
12.1
11.7
7.3
6.9
6.5
3.2
3.0
5.1
4.5
4.0
4.9
4.8
4.5
—
—
—
926
759
16%HPW-NH2-MCM-41 434
16%HPW/MCF-1
16%HPW/MCF-2
3.0
16.8
10.5
—
4.0
3.6
545
439
0.8
a Cell diameter. b Window diameter, determined according to the BJH
method. The 16%HPW/MCF-1 and 16%HPW/MCF-2 was synthe-
sized as described in the experimental section.
2.3 Catalytic reaction: COD oxidation
The activity test was performed at 60 ◦C for 20 h with magnetic
stirring in a closed 25 mL regular glass reactor using 50%
aqueous H2O2 as oxygen-donor and t-BuOH as the solvent. In
a typical experiment, 1.06 mL of COD, 10 mL of t-BuOH and
0.2 g of the HPW-NH2-MCF material (16%) were introduced
into the regular glass reactor at 60 ◦C with vigorous stirring.
The reaction was started by adding 1.4 mL of 50 wt% aqueous
H2O2 into the mixture and was kept for additional 20 h. The
quantitative analysis of the reaction products were performed
by using GC method and the identification of different products
in the reaction mixture was determined by means of GC-MS on
HP 6890GC/5973 MS.
Table 2 ICP results of the different catalysts over different HPW
loadingsa
mmol am- mmol HPW/ mol HPW/
Samples
W (%) ine/g silica g silica
mol amine
8% HPW-NH2-MCF 6.2
16% HPW-NH2-MCF 15.6
24% HPW-NH2-MCF 23.0
30% HPW-NH2-MCF 24.2
2.2
2.2
2.2
2.2
0
0.028
0.071
0.104
0.109
0.0002
0.072
0.0127
0.0323
0.0473
0.0495
—
16% HPW/MCF-1
16% HPW/MCF-2
0.04
15.8
0
—
a Determined according to the ICP-AES method.
MCF silica. In this case, however, the HPW species were almost
completely dissolved out from 16%HPW/MCF-1 during the
washing step (see Table 2). In other words, HPW species could
not be successfully immobilized on the surface of unmodified
MCF silica, due to the absence of anchoring sites for the HPW
species. The above results imply that the surface modification
of MCF silica to provide anchoring sites for HPW species is
essential for the successful immobilization of HPW species on
MCF silica.
TEM images (Fig. 2) of 16%HPW-NH2-MCF materials
revealed a disordered array of silica struts composed of uniform-
sized spherical cells interconnected by windows with a narrow
size distribution, which is the characteristic structural feature
3. Results and discussion
3.1 Characterization of the HPW-NH2-MCF catalysts
N2 adsorption isotherms of different HPW-NH2-MCF samples
are recorded. The irreversible-type IV adsorption isotherms with
H1 hysteresis loop defined by IUPAC are observed, that is a
typical feature of mesoporous materials (not shown here). The
textural parameters of catalysts with different HPW loadings
were listed in Table 1. The surface area of the NH2-MCF
silica was decreased with incorporation of APTES. The HPW-
NH2-MCF silica showed a much lower surface area than the
corresponding NH2-MCF silica, due to the loading of the HPW
species. The surface area of the HPW-NH2-MCF silica dropped
along with the increasing amount of HPW. We can also find
the pore size order of these samples: 16% HPW-NH2-MCF >
16% HPW-NH2-SBA-15 > 16% HPW-NH2-MCM-41, which is
in the same trend with their parent materials.
ICP results of the catalysts with different HPW loadings
are shown in Table 2. The concentration of amino group was
2.2 mmol g-1 as determined by TG method. The tungsten
content of the HPW-NH2-MCF catalysts increases with the
increasing amount of HPW used. However, the tungsten content
of the 30%HPW-NH2-MCF is similar to that of the 24% one,
indicating that the modified silica has a maximal saturated
value for the percentage of HPW stably incorporated on the
surface. In addition, we also attempted to support HPW on pure
Fig. 2 TEM images of 16%HPW-NH2-MCF catalyst.
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of the MCF materials.25,29 This result clearly indicates that the
samples keep the pore structure of the support very well.
Fig. 3 shows the XRD patterns of pure HPW, NH2-MCF
and HPW-NH2-MCF samples. The pure HPW catalyst showed
the characteristic Keggin XRD pattern of heteropoly acids. On
the other hand, NH2-MCF silica and HPW-NH2-MCF samples
showed no characteristic XRD pattern due to the amorphous
nature of MCF silica. It is interesting that 8–24% HPW-NH2-
MCF all showed no characteristic Keggin structure in their
XRD patterns, indicating that the HPW species were not in a
crystalline state but in a highly dispersed amorphous-like state.
This finding demonstrates that the HPW species are finely and
molecularly dispersed on the surface of NH2-MCF via chemical
interaction. As attempted in this work, it is believed that
heteropolyanions were strongly immobilized on NH2-MCF as
charge compensating components. However, 16%HPW/MCF-
2 catalyst shows sharp XRD peaks of HPW (see Figure S1 in
ESI†), indicating that the keggin-type structure was kept well in
the impregnated-method derived catalyst.
Fig. 5 shows the solid-state 13C CP-MAS NMR spectra of
NH2-MCF silica and 16%HPW-NH2-MCF, which obviously
showed three chemical shifts at d = 43 (C1), 22 (C2), and 9(C3)
ppm, while MCF silica exhibited no resonance peak (not shown
here). The three resonance peaks observed in the NH2-MCF
were attributed to different carbon atoms (C1, C2, and C3) in
the molecular structure of APTES.32,33 This finding indicates that
aminopropyl functional groups were successfully grafted on the
MCF silica.
Fig. 5 The 13C MAS NMR of NH2-MCF and 16%HPW-NH2-MCF
catalyst.
The successful immobilization of the HPW catalyst on the
aminopropyl-functionalized MCF silica can also be confirmed
by FT-IR analyses as shown in Fig. 6. A band at 1630 cm-1
observed in all the samples can be assigned to the –OH vibration
of physisorbed H2O. In the case of NH2-MCF and HPW-
NH2-MCF, the Si–O–Si band originated from MCF silica were
observed at around 1000–1250, 805 and 475 cm-1. A broad
band at 2700–3400 cm-1 and a weak band at around 1520 cm-1
observed in NH2-MCF and HPW-NH2-MCF are attributed
+
to the –NH3 stretching vibration, indicating the presence of
aminopropyl functional group in NH2-MCF and HPW-NH2-
MCF.30,34 The primary structure of unsupported HPW can be
identified by the four characteristic IR bands appearing at 1076
cm-1 (P–O band), 982 cm-1 (W–O bands), 882 and 806 cm-1 (W–
O–W bands). The characteristic IR bands of [PW12O40]3- in the
HPW-NH2-MCF were different from those of the unsupported
one. The P–O band in the HPW-NH2-MCF sample was not
clearly identified due to the overlapping by the broad Si–O–Si
band. However, W–O and W–O–W bands of [PW12O40]3- in the
HPW-NH2-MCF sample appeared at slightly red-shift positions
(944 cm-1) compared to those of the unsupported one, indicating
Fig. 3 X-Ray diffraction patterns of HPW-NH2-MCF catalysts.
29Si MAS NMR spectra (Fig. 4) of the amino group
functionalized MCF showed three up-field resonance peaks
corresponding to Q4 (-111 ppm), Q3 (-101 ppm) and Q2 (-92
ppm), and two down-field peaks, assigned to T3 (-67 ppm) and
T2 (-58 ppm), respectively, where Qn = Si(OSi)n(OH)4-n, n = 2–4
and Tm = RSi(OSi)m(OH)3-m, m = 1–3. The presence of T peaks
confirms the incorporation of the aminosilane moieties as a part
of the silica wall structure.30,31
Fig. 4 The 29Si MAS NMR of NH2-MCF material.
Fig. 6 FT-IR of different HPW-NH2-MCF catalysts.
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the presence of a strong interaction between [PW12O40]3- and
NH2-MCF silica.24
31P MAS-NMR is a useful tool in detecting the local envi-
ronments of HPW, since the chemical shift of the phosphorous
atom depends not only on its local environment within the metal
cluster but also on factors such as associated water molecules,
metal ions, solid supports, and thermal treatments.35,36 Fig. 7
shows the 31P MAS-NMR spectra of unsupported bulk HPW,
HPW-NH2-MCF, and HPW/MCF. Bulk HPW exhibited a
sharp, intense peak at -15.3 ppm, typical of Keggin structures.35
The sharp peak indicates the presence of P in a highly
uniform environment in a hydrated structure of POM. After
immobilization, the 31P NMR peaks were found to shift to up-
field. The up-field shift reflects the interaction of [PW12O40]3-
with the cationic mesoporous silica support. The shift in peak
position and decrease in the peak width can also be associated
with the loss of water molecules during POM loading.37 It is
interesting to note that the up-field shift was more exaggerated
for 8%HPW-NH2-MCF than that for 24%HPW-NH2-MCF.
This difference in behavior for the two samples can be attributed
to a higher HPW loading in the 24%HPW-NH2-MCF sample,
in which excessive HPW particles show some low condensed
polymeric HPW species character. Meanwhile, the HPW species
of 8%HPW-NH2-MCF are presented as isolated species. In
addition, the peak of 16%HPW/MCF-2 is similar to that of the
bulk HPW, indicating that the HPW species of 16%HPW/MCF-
2 shows some bulk character of HPW. Thus, the HPW species are
not well dispersed on the MCF support. This finding indicates
that there is weak interaction of HPW with the mesoporous
silica support for the traditional impregnation method.
Fig. 8 XPS spectra of the N 1s region for different HPW-NH2-MCF
catalysts.
Scheme 1 The products distribution of the oxidation of COD by H2O2.
variety of the byproducts, including 3, 4 and 5, are observed,
which are all the oxidation derivatives from the epoxide of one
C
C bond. It is a surprise to find that there are no cleavage
products from one or two C C bonds, suggesting that only
epoxidation of one C C bond and O-heterocyclization of two
C
C bonds occurred under the present conditions. Since the
two main products 1 and 2 can all be oxidized to the same
product – g-butyrolactones – the total yield of these two products
is used to determine the catalytic performance of different
catalysts.
The results of the selective oxidation of COD to 1 and
2 over the HPW-NH2-MCF catalysts with different HPW
loadings and pure HPW are listed in Table 3. The homogeneous
HPW show very high catalytic performance for the reaction,
however, it is difficult to separate the HPW from the product
mixture. For the purpose of comparison, the catalysts used
in these experiments contain the same amounts of HPW. It
can be seen that the catalytic performance of 16%, 24% and
Fig. 7 The 31P MAS-NMR of various samples.
To understand the interaction between the silane moieties
and the HPW species, an XPS measurement was performed to
compare the N 1s binding energy before and after the adsorp-
tion. When compared with the one before HPW adsorption
(Fig. 8), the N 1s spectrum showed a high-energy shift from
399.4 to 401.9 eV (~2.5 eV) after the adsorption. This chemical
shift has been assigned to an acid–base interaction between the
NH2 groups in the silane moieties and HPW molecules,38 thus
confirming the strong chemical bonding interaction between
HPW and the amino groups on the surface of MCF.
Table 3 Catalytic performance of the selective oxidation of COD over
various catalystsa
Catalyst
CCOD (%)
S1 (%)
S2 (%)
S1+2 (%)
HPW-NH2-MCF (8%)
HPW-NH2-MCF (16%)
HPW-NH2-MCF (24%)
HPW-NH2-MCF (30%)
HPW/MCF (16%)-1
HPW/MCF (16%)-2
HPWb
91.0
100
100
100
0.02
100
100
31.0
68.8
62.0
45.7
2.9
36.4
62.1
6.9
37.9
98.0
99.0
98.4
6.6
97.0
98.9
29.2
37.0
52.7
3.7
60.6
36.8
3.2 Activity test
a Reaction conditions: reaction temperature 60 ◦C, H2O2 22.5 mmol, cat.
According to the GC-MS analysis of the product mixture, as
shown in Scheme 1, besides the two main products 1 and 2, a
0.2 g, reaction time 12 h, t-BuOH 10 mL. b HPW = 0.202 g.
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Table 4 Reusability of 16%HPW-NH2-MCF and 16%HPW/MCF-2a
Catalyst Entry CCOD (%) CH (%) S1 (%) S2 (%) S1+2 (%)
species or obvious loss of tungsten in the 16%HPW-NH2-MCF
catalyst could be observed, however, the leaching of tungsten
species from 16%HPW/MCF-2 is 0.12% in one reaction cycle.
Therefore, it can be concluded that the interaction between the
active HPW species and the silica-based matrix of MCF by
immobilized method is much stronger than that of HPW/MCF-
2 prepared by traditional impregnated method. To the best of
our knowledge, the as-prepared 16%HPW-NH2-MCF material
in the O-heterocyclization of COD is the first example for HPW-
based catalysts without any leaching of tungsten species and loss
of catalytic activity with H2O2 as the oxidant.
The results of COD oxidation over HPW-containing MCF,
SBA-15 and MCM-41 are shown in Fig. 9. Comparison of
16%HPW-NH2-MCF, 16%HPW-NH2-SBA-15 and 16% HPW-
NH2-MCM-41 reveals that 16%HPW-NH2-MCF is the most
effective one. The results of 16%HPW-NH2-SBA-15 is similar
to that of 16%HPW-NH2-MCF. The 16%HPW-NH2-MCM-
41 shows the worst catalytic performance. This phenomenon
uncovers that the ultra large pores and the unique three
dimensional cell-window of MCF are more favorable for the title
reaction than the MCM-41 counterpart owning only the small
pores and two dimensional pore structures. The good catalytic
performance of 16%HPW-NH2-SBA-15 can also be attributed
to its large pores. These results indicate that the pore size may
be an important factor on the catalytic performance.
2 O2
HPW-NH2-MCF 1
100
100
100
100
100
99.0
100
99.0
45.0
98.0
97.5
94.5
97.0
96.7
95.8
96.0
91.5
21.0
68.8 29.2
67.6 28.8
66.5 31.3
64.4 32.9
67.2 29.2
67.5 28.9
36.4 60.6
60.2 32.1
16.4 11.6
98.0
96.4
97.8
97.3
96.4
96.4
97.0
92.3
28.0
2
3
4
5
6
HPW/MCF-2
1
2
3
a Reaction conditions: reaction temperature 60 ◦C, H2O2 22.5 mmol, cat.
0.2 g, reaction time 12 h, t-BuOH 10 mL.
30%HPW-NH2-MCF are better than that of 8%HPW-NH2-
MCF for this reaction, in terms of the conversion of COD and
the selectivity of (1 + 2). When tungsten content is higher than
16%, much higher conversion (100%) and excellent selectivity
at (~98%) were obtained. The different catalytic performance
of these catalysts can be attributed to the different content of
active tungsten species. Because the amount of the catalyst is
0.2 g, the total tungsten is different among various catalysts.
The tungsten content of the HPW-NH2-MCF catalysts increases
with the increasing amount of HPW used. However, the tungsten
content of the 30%HPW-NH2-MCF is similar to that of the 24%
one. The active species of 8%HPW-NH2-MCF are not enough
for the title reaction, and that of the 16% and 24%HPW-NH2-
MCF are enough for the reaction. It should be pointed out that
pure MCF is inert to this reaction as confirmed in our previous
work. It is also found that the catalytic performance of the fresh
16%HPW/MCF-2 is similar to that of 16%HPW-NH2-MCF,
not only in terms of the conversion of COD, but also in terms
of the selectivity to the target products. However, the catalytic
performance of 16%HPW/MCF-1 is very poor, similar to that of
the blank MCF support. Therefore, it can be speculated that the
low tungsten content may be the key factor for the extremely low
catalytic activity. From the above characterisations, the HPW
species in all the catalysts with different loadings are highly
dispersed on the surface of NH2-MCF. This result suggests that
the HPW species incorporated into the uniform framework of
mesoporous MCF materials act as the precursor of active centers
for the selective O-heterocyclization of COD.
The reusability and regeneration of 16%HPW-NH2-MCF
and 16%HPW/MCF-2 are also listed in Table 4. The COD
conversion and total selectivity of (1 + 2) over the immobilized
method derived 16%HPW-NH2-MCF catalyst decrease slowly
and still keep above 99% and 96% after the 6th cycle, while these
two values over 16%HPW/MCF-2 catalyst are only 45 and 28%
after 3rd cycles of the reaction. This finding clearly indicates that
the immobilized method derived 16%HPW-NH2-MCF catalyst
shows much better stability and can be reused for at least 6
times (selectivity of (1 + 2) > 96%), while the impregnation-
method derived one can only be used once. That is why the
HPW materials are very difficult to be applied in commercial
plants. To investigate the stability of the active tungsten species in
the 16%HPW-NH2-MCF catalyst, the reaction mixture and the
tungsten remaining in the catalyst were also determined by ICP
analysis after five reaction cycles. No detectable leaching of W-
Fig. 9 The conversion of COD (a) and the selectivity to (1 + 2) (b) with
time over the different catalysts.
In addition, another experiment has been carried out to test
whether this novel 16%HPW-NH2-MCF catalyst is actually a
heterogeneous one (Fig. 10). When the reaction over 16%HPW-
NH2-MCF catalyst has been carried out for 1 h, the catalyst
is removed through simple filtration and the reaction solution
is stirred for another 4 h. No detectable increase of COD
conversion in the next 4 h of reaction is observed, indicating that
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Notes and references
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bonding to aminosilane groups. Characterization results from
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We thank the Major State Basic Resource Development Pro-
gram (Grant No. 2003CB 615807), NNSFC (Project 20973042),
the Research Fund for the Doctoral Program of Higher
Education (20090071110011) and the Science & Technology
Commission of Shanghai Municipality (08DZ2270500) for
financial support.
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708 | Green Chem., 2011, 13, 702–708
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The Royal Society of Chemistry 2011
©