A polyhedral oligomeric silsesquioxane (POSS)-bridged oxo-molybdenum Schiff base complex with enhanced heterogeneous catalytic activity in epoxidation

Article abstract of DOI:10.1039/c3cy01014g

We have generated a new heterogeneous catalyst by bridging an oxo-molybdenum Schiff base on a polyhedral oligomeric silsesquioxane (POSS) via covalent attachment. The resulting POSS-bridged oxo-molybdenum Schiff base complex catalyst was fully characterized by ¹H NMR, XRD, FT-IR, SEM, TGA, and contact angle analysis, and its catalytic potential was studied for the epoxidation of alkenes using aqueous tert-butyl hydroperoxide (TBHP) as the oxidant. The catalyst was found to be highly efficient and showed higher catalytic reactivity than the corresponding homogeneous analogues with added benefits of facile recovery and recycling of the heterogeneous catalyst. The POSS-bridged oxo-molybdenum Schiff base complex was successfully reused for four runs without significant loss in activity. The unique three dimensional network catalyst structure and the hydrophobic properties of the POSS units in the catalyst are revealed to be responsible for the catalyst's excellent performance in epoxidation reactions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3cy01014g

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A polyhedral oligomeric silsesquioxane
POSS)-bridged oxo-molybdenum Schiff
(
Cite this: Catal. Sci. Technol., 2014,
base complex with enhanced heterogeneous
catalytic activity in epoxidation
4, 997
Yan Leng,* Jian Liu, Chenjun Zhang and Pingping Jiang
We have generated a new heterogeneous catalyst by bridging an oxo-molybdenum Schiff base on a
polyhedral oligomeric silsesquioxane (POSS) via covalent attachment. The resulting POSS-bridged oxo-
1
molybdenum Schiff base complex catalyst was fully characterized by H NMR, XRD, FT-IR, SEM, TGA, and
contact angle analysis, and its catalytic potential was studied for the epoxidation of alkenes using aqueous
tert-butyl hydroperoxide (TBHP) as the oxidant. The catalyst was found to be highly efficient and showed
higher catalytic reactivity than the corresponding homogeneous analogues with added benefits of facile
recovery and recycling of the heterogeneous catalyst. The POSS-bridged oxo-molybdenum Schiff base
complex was successfully reused for four runs without significant loss in activity. The unique three
dimensional network catalyst structure and the hydrophobic properties of the POSS units in the catalyst
are revealed to be responsible for the catalyst's excellent performance in epoxidation reactions.
Received 4th December 2013,
Accepted 4th January 2014
DOI: 10.1039/c3cy01014g
www.rsc.org/catalysis
1
3
cyclooctene. Grela reported a POSS-tagged Grubbs–Hoveyda-
type olefin catalyst, which can be recovered using nano-
filtration techniques and can be reused. These reports have
shown that POSS can function as carriers or activity promoters
for developing highly efficient heterogeneous catalytic systems.
The transformation of alkenes to corresponding epoxides
is both an important industrial technology and a useful
synthetic method for a wide range of products including
1
. Introduction
Polyhedral oligomeric silsesquioxanes (POSS), an intriguing
class of organic–inorganic hybrid materials, possess a cubic
structure represented by the formula R
central hydrophobic inorganic core (Si O ) is surrounded by
organic moieties at the eight vertices. Their unique star-
shaped nanostructures and chemical properties, such as
facile chemical modification, good pH tolerance, and high
temperature and oxidation resistance properties, make POSS
an ideal building block for constructing multi-functional
8 8
Si O12, in which the
8
12
,2
1
1
4–17
pharmaceuticals and agrochemicals.
Metal Schiff base
complexes are known to be good homogeneous epoxidation
1
8–20
catalysts due to their high activity and selectivity.
How-
3
–6
materials by introducing functional groups and elements.
ever, problems in catalyst separation and recyclability should
be addressed for these homogeneous catalysts. To this end,
several strategies involving immobilization of the active
catalytic moiety, viz. to zeolites, polymers, silica, layered double
hydroxides, and amine-functionalized hexagonal mesoporous
Despite the versatile applications of POSS in many materials,
such as dental restorative materials, drug delivery agents,
liquid crystal materials, and light-emitting materials among
7
–11
others,
POSS have not been extensively explored for cata-
2
1–26
lytic applications facilitating useful chemical transformations
even though this area has immense potential. Recently, only
a few studies have focused on investigation of POSS-based
heterogeneous catalysts. For example, Wada described the
preparation of silica-supported Ti oxide catalysts using
Ti-containing POSS as precursors, with the resultant product
showing high catalytic activity and stability in epoxidation of
hydroxides, have been reported.
Though good activity
was achieved in those cases, most of them still bear some
drawbacks like slow reaction speed, low catalyst loading,
leaching of active species, or limited accessibility to the reac-
tants. A new strategy for catalyst preparation to overcome
the above problems is therefore highly desirable.
In this paper, we utilize aminopropyl-POSS reagent as
the monomer for the preparation of a POSS-bridged oxo-
molybdenum Schiff base complex via covalent attachment
with an oxo-molybdenum Schiff base and then use it as the
catalyst for epoxidation reactions. It supplies a unique three
dimensional network structure and isolates each substituted
1
2
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
China. E-mail: lengyan1114@126.com; Fax: +86 510 85917763;
Tel: +86-510-85917090
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oxo-molybdenum Schiff base group on the vertices of POSS,
which would provide high stiffness and stability beneficial
for the homogeneous distribution of the active centers.
Moreover, hydrophobicity of the POSS core surrounded by
the oxo-molybdenum Schiff base would facilitate the access of
reactants to the reactive oxo-molybdenum centers. To the best
of our knowledge, there is no report on the use of POSS as
monomers for the immobilization of an organometallic
complex in order to facilitate its efficiency and recycling
during an organic transformation. Catalytic tests for the
epoxidation of various alkenes using aqueous tert-butyl
hydroperoxide (TBHP) as the oxidant, along with comparisons
to its homogeneous analogues, well demonstrate that this
newly designed solid complex is a highly active and selective,
reusable, and non-leaching catalyst for the epoxidation
reactions.
(1.19 g, 5 mmol), and the resulting mixture was refluxed for
12 h under a nitrogen atmosphere. The precipitated brown
solid oxo-molybdenum Schiff base SB-Mo was filtered off and
washed thoroughly by acetonitrile and diethyl ether, and
1
dried under vacuum. H NMR (300 Hz, CDCl ): δ (ppm) = 6.57
3
(m, 1H), 7.00 (m, 2H), 7.50 (d, 3H), 8.03 (d, 2H), 9.09 (s, 1H),
12.74 (s, 1H). Elemental analysis for SB-Mo (by the weight
percentage): C 55.11%; H 3.14%; N 4.58%.
Synthesis of octa(aminopropyl silsesquioxane) (POSS).
Octa(aminopropyl silsesquioxane) was prepared according to
7
,29
the previous literature.
Typically, deionized water (18 mL),
propyl alcohol (8 mL), acetonitrile (2 mL), and tetraethyl
ammonium hydroxide (0.4 mL) were added into a 500 mL
flask to obtain a heterogeneous solution. γ-Aminopropyl
triethoxysilane (44.0 g) was added into the solution, and the
obtained mixture was heated to 50 °C with vigorous stirring.
After reaction for 12 h, the formed white crystalline precipitate
product POSS was filtered and washed with cold methanol
three times, followed by drying in vacuum at 80 °C for 24 h.
Synthesis of the POSS-bridged oxo-molybdenum Schiff base
complex (POSS–SB-Mo). The carboxylic acid groups (–COOH)
2
. Experimental section
2
.1 Reagents and analyses
All chemicals were of analytical grade and used as received.
FT-IR spectra were recorded on a Nicolet 360 FT-IR instrument
in SB-Mo were converted to acid chloride (–COCl) using SOCl
2
−
1
1
(
KBr discs) in the 4000–400 cm region. H-NMR spectra were
measured on a Bruker DPX 300 spectrometer at ambient
temperature in D O or CDCl using TMS as the internal ref-
erence. TG analysis was carried out using a STA409 instru-
30,31
according to the previous reports.
In a 100 ml round-
bottom flask equipped with a magnetic stirring bar, SOCl
2
2
3
treated SB-Mo (0.64 g) was dissolved in ethanol (30 mL), and
the solution was heated to 80 °C. Then, the aqueous solution
of POSS (0.22 g) was added dropwise to the above mixture. The
mixture was vigorously stirred at 80 °C for 12 h. Then, the green-
−
1
ment in dry air at a heating rate of 10 °C min . SEM was
performed on a HITACHI S-4800 field-emission scanning
electron microscope. XRD patterns were collected on a
Bruker D8 Advance powder diffractometer using a Ni-filtered
Cu/Kα radiation source at 40 kV and 20 mA, from 5 to 80°
with a scan rate of 4° min . The CHN elemental analysis
was performed on an elemental analyzer Vario EL cube. The
amount of leached molybdenum species in the filtrate was
measured using a Jarrell-Ash 1100 ICP-AES spectrometer.
3
colored precipitate was filtered and washed with CH CN three
times, and dried at 80 °C for 24 h to afford POSS–SB-Mo.
Elemental analysis for POSS–SB-Mo (by the weight percentage):
C 51.6%, N 7.11%, and H 4.09%, and the loading amount
of Mo in the catalyst POSS–SB-Mo is calculated to be
about 12.1%.
−
1
2
.3 Catalytic test
Cyclooctene (1 mmol), CHCl
2 mmol), and POSS–SB-Mo (0.048 mmol) were added to
2
.2 Preparation of catalysts
3
(5 mL), aqueous 65% TBHP
Synthesis of MoO (acac) . Molybdenyl acetylacetonate
2
2
(
2
7
MoO
2
(acac)
2
was prepared according to previous literature.
a 25 mL flask. The resulting mixture was heated at 70 °C
for 30 min. After completion of the reaction, the reaction mix-
ture was analyzed using a gas chromatograph (GC, SP-6890A)
equipped with a FID detector and a capillary column (SE-54
(NH ) MoO ·4H O (30.0 g) was dissolved in H O (100 mL)
4 6 24 2 2
and acetylacetone (40 mL) was added. The pH of the solution
was adjusted to 3.5 using 10% HNO with the appearance of
precipitated solids. Upon completion, the formed yellow
precipitate MoO (acac) (28.0 g, 51% yield) was filtered and
washed with H O, ethanol, and ether, respectively, followed
2
by drying in a vacuum.
Synthesis of the oxo-molybdenum Schiff base (SB). Oxo-
molybdenum Schiff base complex SB-Mo was prepared
according to previous literature.
3
3
0 m × 0.32 mm × 0.25 μm). The catalyst was recovered
2
2
by filtration and washed with ethanol, then dried in vacuum
to obtain the recovered catalyst, and finally reused in the
next run without addition of any fresh catalyst.
3
3
. Results and discussion
.1 Catalyst characterization
2
2,23,28
To a stirred solution of
4
-aminobenzoic acid (6.85 g, 0.05 mol) in ethanol (150 mL)
was added salicylaldehyde (6.10 g, 0.05 mol), and the resulting
mixture was stirred at room temperature for 30 min. The
precipitated yellow Schiff base was separated from the reaction
mixture by filtration and recrystallized with methanol to afford
SB. To a stirred solution of Schiff base SB (2.41 g, 0.01 mol) in
dry acetonitrile (20 mL) was added the prepared MoO (acac)
The synthetic functionalization route for POSS-bridged
oxo-molybdenum Schiff base complex POSS–SB-Mo is shown
in Fig. 1. Aminopropyl-POSS was synthesized according
to a procedure reported in the literature. The required
oxo-molybdenum Schiff base was readily prepared by the
two-step sequence involving condensation of salicylaldehyde
2
9
2
2
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Fig. 1 Synthetic pathway for the POSS-bridged oxo-molybdenum Schiff base complex.
with 4-aminobenzoic acid to give Schiff base SB followed by
its reaction with molybdenyl acetylacetonate MoO (acac) in
2
2
acetonitrile to yield oxo-molybdenum Schiff base SB-Mo in
nearly quantitative yield. POSS–SB-Mo was then prepared
as a green solid by reacting POSS with SOCl treated SB-Mo
2
in ethanol solvent at 80 °C for 12 h. Preparation of the
POSS–SB-Mo catalyst and the intermediate steps were exam-
1
ined and monitored by H NMR, XRD, FT-IR, SEM, and TGA.
Fig. 3 SEM images of (A) SB-Mo and (B) POSS–SB-Mo.
The resulting solid catalyst is insoluble in almost all the
commonly used solvents, like ethanol, acetonitrile, chloro-
form, THF, DMF, etc. Thus the catalyst synthesized herein is
favorable for heterogeneous catalysis.
The XRD patterns of both POSS and POSS–SB-Mo are
shown in Fig. 2. POSS displayed well resolved identical peaks
at 8° and 20° for the rhombohedral unit cell structure of the
POSS inorganic segment (Fig. 2, curve a). However, the XRD
pattern for POSS–SB-Mo indicates that the developed catalytic
material is almost amorphous in nature, which is revealed
by very weak and broad diffraction peaks as shown in
Fig. 2, curve b.
Fig. 3 shows the SEM images of SB-Mo and POSS–SB-Mo.
The oxo-molybdenum Schiff base is composed of irregular
spherical particles with a size in micrometers (diameter:
10–30 μm, Fig. 3A). However, the particle size drops signifi-
cantly with a diameter of about 50–200 nm upon using
aminopropyl-POSS reagent as the monomer to prepare
POSS–SB-Mo (Fig. 3B). The smaller particles of POSS–SB-Mo
should be attributed to the unique three dimensional
structure of the POSS monomer that isolates each substituted
oxo-molybdenum Schiff base group on the vertices of POSS,
which prevents the aggregation of SB-Mo into large particles.
The pore structure and the surface area for these catalysts
were characterized by nitrogen sorption experiments at 77 K.
The results show that the catalyst POSS–SB-Mo is almost
2
−1
nonporous with a BET surface area of 30 m g , while the
2
−1
BET surface areas of POSS and SB-Mo are lower than 5 m g .
The chemical nature of the POSS-bridged oxo-molybdenum
Schiff base catalyst and its intermediate products was revealed
by FTIR. As shown in Fig. 4, the IR spectra of SB-Mo (Fig. 4,
−1
curve a) showed bands at 1703 cm assigned to stretching
−1
vibrations of CO groups, 1606 cm due to CN of the
−1
Schiff base and the characteristic bands at 948 and 905 cm
for the symmetric and asymmetric stretching modes of the
cis-MoO fragment. An intense and broad peak which
2
−1
appeared at 2800–3500 cm in the spectrum of SB-Mo is
attributed to the stretching mode of the O–H band, revealing
Fig. 2 XRD patterns of (a) POSS and (b) POSS–SB-Mo.
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Fig. 6 TGA patterns of (a) POSS, (b) POSS–SB-Mo and (c) SB-Mo.
Fig. 4 FT-IR spectra of (a) SB-Mo and (b) POSS–SB-Mo.
early heating stage below 250 °C results from the release of
moisture and constitutional water. POSS shows about 45%
weight loss between 250–700 °C, evidently owing to thermal
decomposition of organic groups in POSS (Fig. 6, curve a),
and SB-Mo shows a drastic weight loss of about 70% in the
range 250–500 °C owing to the decomposition of the ligand
moiety SB (Fig. 6, curve c). The TGA pattern for POSS–SB-Mo
shows a weight loss of nearly 60% for the organic moiety
(Fig. 6, curve b). Moreover, the elemental analysis for POSS–SB-Mo
gave the data C: 51.6%, N: 7.11% and H: 4.09% (by weight
percentage). From the results of TGA and elemental analysis,
the molar ratio of POSS to SB-Mo is estimated to be 1 : 4
and the loading amount of Mo in POSS–SB-Mo is calculated
to be 12.1%, demonstrating a high SB-Mo content in the
catalyst.
the presence of COOH groups. Additionally, the bands around
−1
1
206 and 1060 cm are assigned to stretching vibrations of
−1
the aromatic ring, and the peak at 776 cm represents the
ortho-substituted aromatic rings of the Schiff base. For the
FTIR spectrum of POSS–SB-Mo (Fig. 4, curve b), these peaks
appeared distinctively. Meanwhile, POSS–SB-Mo shows many
new absorption peaks, in particular, peaks at 3200 and
−
1
7
00 cm reveal the presence of –NH groups of amide. The
−
1
peak at 1124 cm illustrates the presence of Si–O–Si bonds.
−
1
Furthermore, the CN band at 1667 cm is broadened and
the intensity of the bands corresponding to CO and OH of
the carboxylic group decreased. These vibrational signatures
confirm the successful attachment of SB-Mo with POSS by
covalent interactions.
The UV-vis profiles of POSS–SB-Mo and SB-Mo are com-
pared in Fig. 5. POSS–SB-Mo showed an absorption band in
the range of 600–800 nm that is assignable to the hydrogen
bonds and intramolecular charge transfer from the amino
3
.2 Catalytic activity in the epoxidation reaction
The catalytic efficiency of the prepared POSS-bridged oxo-
molybdenum Schiff base complex POSS–SB-Mo and comparison
with its homogenous analogue were studied for epoxidation
of cyclooctene using TBHP as the oxidant. The results of
conversion and selectivity at different reaction times are
given in Table 1. In the absence of the catalyst, the reaction
did not proceed at all even after prolonged reaction times
(Table 1, entry 1). The MoO (acac) complex dissolved in the
3
2–34
groups –NH in POSS to MoO species;
was undetectable for SB-Mo, suggesting the reduction of Mo
however, the band
2
2
6
+
in POSS–SB-Mo by amino groups.
Fig. 6 illustrates TGA patterns for POSS–SB-Mo, POSS, and
SB-Mo. The catalyst was found to be stable thermally up to
ca. 250 °C, which is above the reaction conditions required
for the liquid phase catalysis. The slight weight loss at the
2
2
reaction medium to give a homogeneous system showed a
high conversion of 95% with 98% selectivity at 0.5 h reaction
time (Table 1, entry 2). A longer reaction time can increase
the conversion of cyclooctene; however, this leads to a very
low selectivity of 81%. SB-Mo exhibited higher catalytic
activity than MoO (acac) . Good conversion of 99% with
2
2
1
00% selectivity could be obtained over a reaction time
of 1 h (Table 1, entry 3), and the selectivity almost remained
in a constant value with the increasing of the reaction
time. However, SB-Mo was partly dissolved in the reaction
medium during the epoxidation reaction, which leads to a
low catalyst recovery ratio.
Interestingly, the POSS–SB-Mo catalyst showed the highest
catalytic activity, where the catalytic epoxidation of cyclo-
octene was completed in about 30 min at a molar ratio of
TBHP to cyclooctene of 2.0, giving a conversion of 99% and
Fig. 5 UV-vis spectra of (a) SB-Mo and (b) POSS–SB-Mo.
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Table 1 Catalytic results of epoxidation of cyclooctene using TBHP by various catalystsa
b
Conversion/selectivity at different reaction times (%)
Entry
Catalyst
Solubility
0.5 h
1 h
1.5 h
c
1
—
—
n.d.
n.d.
n.d.
2
3
4
MoO
SB-Mo
POSS–SB-Mo
2
(acac)
2
Soluble
Partly soluble
Insoluble
95/98
82/100
99/100
99/93
99/100
100/100
100/81
100/100
100/100
a
b
3
Reaction conditions: catalyst (0.048 mmol), cyclooctene (1 mmol), CHCl (5 mL), TBHP (2 mmol), 70 °C. Selectivity for the epoxide product.
c
Byproducts for entry 2 are 2-cycloocten-1-ol and 2-cycloocten-1-one. The reactions are performed in the absence of the catalyst, and the conversions
are too low to be determined (abbreviated as “n.d.”).
selectivity of 100% (Table 1, entry 4). The conversion of 27%
with 100% selectivity was obtained with an equimolar
amount of TBHP to cyclooctene. It is worthwhile to notice
that the suitable reaction time, 30 min, is much shorter
than those for the previously reported catalysts, such
as molybdenum dioxo complexes, oniumsilica-immobilized
polyoxometalate, supported tungsten oxide, and metal–
organic framework immobilized manganese(II) acetylacetonate
on it. It is thus proposed that the hydrophobic three dimen-
sional network structure of the catalyst POSS–SB-Mo should
have supplied a unique microenvironment for the catalyst to
involve the substrates alkene and TBHP during the epoxidation
reaction, which allows Mo species in the bulk of the catalyst
fully act as active centers.
3.3 Solvent effects in the epoxidation reaction
14,35–37
complexes,
verifying the high efficiency of the present
The catalyst POSS–SB-Mo exhibits different performances in
POSS-bridged oxo-molybdenum Schiff base catalyst POSS–SB-Mo.
Moreover, facile recovery and recycling ability of POSS–SB-Mo
make it a more superior catalyst. The results of the above control
experiments imply that the featured three dimensional network
framework of POSS–SB-Mo endows the catalyst with an insoluble
nature in the reaction media, and that the POSS units in the
complex POSS–SB-Mo play an essential role in improving the
overall activity.
the epoxidation of cyclooctene with TBHP using various
solvents. As shown in Fig. 8, in the solvents such as CHCl
and CCl with no coordination ability to the metal center and
low dielectric constant, high epoxidation conversions were
obtained. In particular, for the CHCl solvent, excellent
3
4
3
conversion of 99% with 100% selectivity was observed in a
short reaction time of only 0.5 h. The reactions in solvents
such as CH Cl and CH COOC H were found to be slow,
2 2 3 2 5
Heterogeneous catalysts generally exhibit much lower
activities than homogeneous catalysts because the degree of
exposure of the catalytically active sites to reactants is much
presumably due to the higher dielectric constants than that
of CHCl . On the other hand, in the coordinated solvents,
3
3
such as CH CN with high coordinating ability to the metal
center and high dielectric constant, very low epoxidation
conversions were observed at every hour of the reaction time.
3
8,39
lower over homogeneous catalysts.
geneous catalyst POSS–SB-Mo exhibited a much higher activity
than its homogenous analogues SB-Mo and MoO (acac)
However, the hetero-
2
2
with the same number of active sites. It is noteworthy that
the addition of POSS can improve the hydrophobic and
3
.4 Epoxidation reactions with various substrates over
4
0,41
POSS–SB-Mo and SB-Mo catalysts
oleophilic properties of the functional materials.
In the
present study, the enhanced catalytic activity for this hetero-
geneous catalyst POSS–SB-Mo may be attributed to the
hydrophobic property of the POSS monomer. To understand
the role of POSS in catalysis, we carried out the contact angle
We assessed the catalytic activity of POSS–SB-Mo and SB-Mo
under the described reaction conditions, using a series of
various types of aromatic and aliphatic alkenes. The results
(CA) tests. As shown in Fig. 7, when a droplet of cyclooctene or
TBHP was brought in contact with the surface of POSS–SB-Mo,
the CA was measured to be less than 5°. However, for a water
droplet on the surface of POSS–SB-Mo, the CA was measured
to be 105°. These phenomena indicate that POSS–SB-Mo is
relatively hydrophobic and the reactants have good wettability
Fig. 8 Catalytic performance of POSS–SB-Mo in epoxidation of cyclo-
octene with TBHP in different solvents. Reaction conditions: solvent
(5 mL), cyclooctene (1 mmol), catalyst POSS–SB-Mo (0.048 mmol),
TBHP (2 mmol), 70 °C.
Fig. 7 Contact angle for a droplet of (A) cyclooctene, (B) TBHP and
(
C) water on the surface of POSS–SB-Mo.
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Table 2 Epoxidation of various substrates with TBHP over catalysts POSS–SB-Mo and SB-Moa
Con (%)
Con (%)
Entry
1
Substrate
Product
t (h)
POSS–SB-Mo
Sel (%)
100
SB-Mo
82
Sel (%)
100
0.5
99
2
3
2
6
99
67
100
75
93
92
100
61
4
2
97
88
45
80
5
6
6
6
85
35
100
100
86
34
100
84
7
6
8
30
18
83
55
11
17
48
53
8
a
3
Reaction conditions: solvent (CHCl , 5 mL), alkene (1 mmol), catalyst (0.048 mmol), TBHP (2 mmol), 70 °C.
are summarized in Table 2. It can be seen that the heterogeneous
catalyst POSS–SB-Mo gave good to excellent conversions for the
epoxidation of a range of alkenes and, encouragingly, in the
majority of cases outperformed homogeneous SB-Mo. These
results further suggest a catalytically promotional role of POSS
for the epoxidation reaction. Moreover, as well known, higher
electron donation ability of the olefin double bond is expected
to show greater epoxidation reactivity. Therefore, cyclooctene,
cyclohexene, and limonene with inner double bonds exhibit
more activities in comparison with 1-hexene and 1-octene
which contain terminal double bonds. Styrene and indene
afforded very low conversions and selectivities mostly due to
the presence of more electron withdrawing groups connected
to the double bond.
Fig. 9 Catalyst recycling in epoxidation of cyclooctene with TBHP
over POSS–SB-Mo.
3
.5 Catalyst reusability
Further experiments were conducted to examine the recycla-
bility and reusability of the POSS–SB-Mo in the epoxidation
of alkenes using cyclooctene as a model substrate. At the end
of the reaction, the catalyst recovered by simple filtration
was washed with ethanol, dried and reused for the next run
without adding any fresh catalyst. Fig. 9 shows the catalytic
recycling properties for POSS–SB-Mo, suggesting quite a steady
reusability for this complex catalyst without observing signifi-
cant loss of conversion. In each run, only the corresponding
epoxide was detected and no other ring-opening byproducts
were found. In addition, the profiles of IR spectra for
the recovered catalyst (Fig. 10) were in good agreement with
that of the fresh one, demonstrating a very durable catalyst
structure.
In order to test the possible catalysis of the slightly
leached metal species in the reaction solution from the
catalyst, an additional experiment was carried out. Initially,
3
the mixture of the catalyst and TBHP in CHCl was stirred
at 70 °C for 20 min. The catalyst was then removed and the
reaction proceeded for another 40 min with the homogeneous
filtrate. It can be seen in Fig. 11 that no oxidation occurred.
Furthermore, the result of ICP-AES elemental analysis for the
reacted filtrate shows that less than 0.5 wt.% Mo in the catalyst
leached into the reaction media. The above results strongly
indicate the heterogeneous nature of the present catalyst
POSS–SB-Mo for the epoxidation reaction. The slight decrease
of the activity in the fourth run is mostly due to the weight loss
in the operation for recovering the catalyst.
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. Conclusions
In summary, we have demonstrated the successful application
of aminopropyl-POSS as a building block for immobilizing
the oxo-molybdenum Schiff base via covalent attachment. The
prepared POSS-bridged oxo-molybdenum Schiff base complex
POSS–SB-Mo was found to be a highly efficient catalyst for
epoxidation of alkenes using aqueous TBHP as the oxidant.
The catalyst was recovered easily and reused for four runs
without observing significant loss in catalytic activity. The over-
all superior catalytic activity, high selectivity and excellent
recyclability of POSS–SB-Mo as a heterogeneous catalyst
could be attributed to its excellent structural stability and
unique hydrophobic properties, which allow for a rapid and
easy diffusion of the reactants into the reactive Mo centers.
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