The effect of encapsulated Zn-POM on the catalytic activity of MIL-101 in the oxidation of alkenes with hydrogen peroxide

Article abstract of DOI:10.1080/00958972.2011.648929

Zinc monosubstituted Keggin heteropolyanion [PZnMo₂W ₉O₃₉]^5- was electrostatically bound to nanocages of MIL-101 polymer matrix. The Zn-POM@MIL-101 catalyst was characterized by XRD, N₂ adsorption, atomic absorption (AAS), and FT-IR spectroscopic methods. The catalytic activity of the new composite material, Zn-POM@MIL-101, was assessed in the oxidation of alkenes using aqueous hydrogen peroxide as oxidant. Zn-POM@MIL-101/H₂O₂ catalytic system demonstrated good catalytic activity in the oxidation reactions. Zn-POM@MIL-101 was reusable for three catalytic cycles. While the MIL-101 matrix is an active catalyst in these oxidation reactions, the presence of Zn-POM significantly changed the selectivity and reaction times. Copyright

Full text of DOI:10.1080/00958972.2011.648929

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The effect of encapsulated Zn-POM on
the catalytic activity of MIL-101 in the
oxidation of alkenes with hydrogen
peroxide
Zahra Saedi a , Shaharm Tangestaninejad a , Majid Moghadam a
Valiollah Mirkhani a & Iraj Mohammadpoor-Baltork a
a Department of Chemistry, Catalysis Division, University of
Isfahan, 81746-73441 Isfahan, Iran
,
Published online: 30 Jan 2012.
To cite this article: Zahra Saedi , Shaharm Tangestaninejad , Majid Moghadam , Valiollah Mirkhani
& Iraj Mohammadpoor-Baltork (2012): The effect of encapsulated Zn-POM on the catalytic activity
of MIL-101 in the oxidation of alkenes with hydrogen peroxide, Journal of Coordination Chemistry,
65:3, 463-473
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Journal of Coordination Chemistry
Vol. 65, No. 3, 10 February 2012, 463–473
The effect of encapsulated Zn-POM on the catalytic activity of
MIL-101 in the oxidation of alkenes with hydrogen peroxide
ZAHRA SAEDI, SHAHARM TANGESTANINEJAD*, MAJID MOGHADAM*,
VALIOLLAH MIRKHANI and IRAJ MOHAMMADPOOR-BALTORK
Department of Chemistry, Catalysis Division, University of Isfahan,
81746-73441 Isfahan, Iran
(Received 6 October 2011; in final form 28 November 2011)
5ꢀ
Zinc monosubstituted Keggin heteropolyanion [PZnMo2W9O39
]
was electrostatically bound
to nanocages of MIL-101 polymer matrix. The Zn-POM@MIL-101 catalyst was characterized
by XRD, N2 adsorption, atomic absorption (AAS), and FT-IR spectroscopic methods. The
catalytic activity of the new composite material, Zn-POM@MIL-101, was assessed in the
oxidation of alkenes using aqueous hydrogen peroxide as oxidant. Zn-POM@MIL-101/H2O2
catalytic system demonstrated good catalytic activity in the oxidation reactions.
Zn-POM@MIL-101 was reusable for three catalytic cycles. While the MIL-101 matrix is an
active catalyst in these oxidation reactions, the presence of Zn-POM significantly changed the
selectivity and reaction times.
Keywords: Metal–organic framework; MIL-101; Supported polyoxometalate; Alkene
oxidation; Hydrogen peroxide; Nanoporous material
1. Introduction
Polyoxometalates (POMs), as a unique class of metal-oxide clusters [1–5], have many
properties that make them candidates for application in catalysis, medicine, and
materials science [6, 7]. Due to their advantageous properties such as inorganic nature,
thermodynamic stability toward oxidation, hydrostability, tunability of redox and acid
properties, etc. [8–10], the preparation of POM-based materials is still a vibrant field of
study [11]. For instance, polyoxometalates are applied as homogeneous catalysts in
oxidation reactions with hydrogen peroxide. Catalytic oxidations with hydrogen
peroxide have the following advantages: (i) water is the sole by-product, (ii) it has a high
content of active oxygen species, and (iii) it is much cheaper and safer to use than
organic peroxides or peracids [12].
Although many efficient oxidation systems with hydrogen peroxide in the presence of
POMs have been developed, most of them are homogeneous and have general
disadvantages such as difficult catalyst/product separation, poor catalyst reusability,
and recoverability [13–16]. Hence, practical application of POMs in the oxidation
*Corresponding authors. Email: stanges@sci.ui.ac.ir; moghadamm@sci.ui.ac.ir
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2011.648929

464
Z. Saedi et al.
reactions requires the development of easily recoverable and recyclable catalysts. One
approach in this area is the heterogenization of catalytically active species [12].
Metal–organic frameworks (MOFs) have attracted attention because of their unique
combination of properties such as high surface area, crystalline open structures, tunable
pore size, and functionality [17–21]. These properties allow MOFs as prospective
catalytic materials and supports for immobilization of homogeneous catalysts [22–29].
Recently, new methods based on MOFs such as: (i) catalysis by homogeneous
catalysts incorporated as framework struts [30, 31], (ii) catalysis by MOF-encapsulated
molecular species [32], and (iii) catalysis by MOF-encapsulated clusters [10, 33–36] have
been reported for heterogenization of active homogeneous catalysts.
MIL-101 possesses a rigid zeotype crystal structure, extremely large surface area and
˚
quasi-spherical cages of two types (free internal diameters are close to 29 and 34 A,
˚
while the cages are accessible through microporous windows of about 12 and 16 A).
MIL-101 is resistant to air, water, common solvents, and thermal treatment (up to
320ꢁC) [18, 37]. Transition metal monosubstituted POMs (M-POMs) are valuable
single-site catalysts where active metal, M, is isolated and strongly bound to an
inorganic metal–oxide matrix and thus is prevented from oligomerization and
leaching [38, 39].
Therefore, MIL-101 supported polyoxometalates have attracted considerable atten-
tion [10, 33–36]. In this study, we report the preparation of a heterogeneous catalyst,
based on nanoporous MIL-101 metal organic framework, and catalytically active Zn-
monosubstituted Keggin type heteropolyanions, [(n-Bu)4N]5PZnMo2W9O39ꢂ3H2O,
which has been incorporated within the coordination polymer nanocages. The catalytic
performance of this catalyst was investigated in the oxidation of alkenes with hydrogen
peroxide.
2. Experimental
2.1. General
All reagents were obtained commercially and used without purification. Hydrogen
peroxide was used as 30% solution in water. The tetra-n-butyl ammonium salt of
monosubstituted POM [PZnMo2W9O39]5ꢀ was prepared from ꢀ-K7PMo2W9O39 by a
slight modification of the previously reported method [40].
FT-IR spectra were recorded using potassium bromide pellets from 400 to 4000 cmꢀ1
by a Jasco FT/IR-6300 instrument. GC analyses were performed using a Shimadzu
GC-16A gas chromatograph equipped with a flame ionization detector and a 2 m
column packed with silicon DC-200 or Carbowax 20 M. The products were identified
by comparing their retention times with known samples. Powder X-ray diffraction
˚
patterns were obtained on a D8 Avance Bruker using Cu Kꢀ (ꢁ ¼ 1.5406 A). Mass
1
analyses were performed using a Micromass platform 8379E spectrometer. H NMR
spectra were recorded by a Bruker 400 MHz instrument using CDCl3 as solvent.
Thermogravimetric analyses (TGA) were performed on a Mettler TA 4000 instrument
at a heating rate of 10 K ꢂ minꢀ1. The accelerated surface area and porosimetry (ASAP)
analyses were carried out at low temperature by adsorption of N2 gas with an ASAP
2000 micromeritics instrument.

Encapsulated Zn-POM
465
2.2. Preparation of catalyst
2.2.1. MIL-101 synthesis. The coordination polymer MIL-101 was prepared as
described by Fe
´
rey et al. [37]. In a typical procedure, a mixture of Cr(NO3)3ꢂ9H2O
(1.2 g, 3 mmol), terephthalic acid (H2bdc, 500 mg, 3 mmol), and HF (0.6 mL of
5 mol Lꢀ1, 3 mmol) in H2O (15 mL) was heated at 220ꢁC for 8 h in a Teflon-lined
stainless steel autoclave. The resultant green solid was passed through a coarse filter to
eliminate unreacted crystals of H2bdc and then filtered on a thick filter. Then the green
raw product was washed two times with hot DMF for 8 h (100ꢁC) and twice with hot
EtOH for 8 h (80ꢁC), filtered off, and dried overnight in an oven at 75ꢁC.
2.2.2. Immobilization of Zn-POM on MIL-101. Zn-POM@MIL-101 catalyst was
prepared as follows: [(n-Bu)4N]5PZnMo2W9O39ꢂ3H2O (22 mg) was dissolved in CH3CN
(2 mL) and MIL-101 (100 mg) was added to this solution and stirred for 6 h. The
resulting solid was stored overnight at room temperature and then filtered off. The
solids were washed with CH3CN and dried under air until the weight remained
constant. The complete disappearance of Zn-POM in the CH3CN solution was
established by UV-Vis spectroscopy. The Zn-POM loading was also evaluated by
atomic absorption spectroscopy (AAS). The catalyst was characterized by XRD,
TG/DTG, FT-IR spectroscopy, and adsorption–desorbtion of N2 gas.
2.3. General procedure for oxidation of alkenes
In a 25 ml round-bottom flask equipped with a magnetic stir bar and a condenser, the
catalytic experiments were carried out under reflux (70ꢁC). The H2O2 (5 mmol) was
added to a mixture of alkene (0.5 mmol) and catalyst (5 mg, 2.3 ꢃ 10ꢀ4 mmol Zn-POM)
in acetonitrile (3 mL). The reaction progress was monitored by GC. At the end of the
reaction, the catalyst was filtered and washed with acetonitrile (10 mL). The filtrates
were concentrated and purified on a short column of silica gel to obtain the pure
1
product. The corresponding carboxylic acids were characterized by FT-IR, H NMR
spectroscopic methods, and mass spectrometry (Supplementary material).
3. Results and discussion
3.1. Characterization of catalyst
MIL-101 was prepared by hydrothermal method as described by Ferey et al. [37]. The
´
matrix was characterized by X-ray diffraction, FT-IR spectroscopy, and N2
adsorption–desorption techniques. Figures 1(a) and 2(a) show the XRD pattern and
FT-IR spectrum of MIL-101 which is similar to the previously reported ones [10, 37].
Figure 3 shows the schematic representation of Zn-POM encapsulated in the MIL-101
matrix. This catalyst, Zn-POM@MIL-101, was prepared by encapsulation of Zn-POM
in acetonitrile solution at 25ꢁC. Similar methods were reported for encapsulation of
´
POM within the MIL-101 pores by Ferey and Maksimchuk [10, 37]. Comparison of
XRD pattern of Zn-POM@MIL-101 (figure 1b) with the original MIL-101 (figure 1a)

466
Z. Saedi et al.
Figure 1. XRD patterns of (a) MIL-101, (b) Zn-POM@MIL-101.
Figure 2. FT-IR spectra of (a) MIL-101 (A) and Zn-POM@MIL-101 (B), (b) Zn-POM (A) and Zn-
POM@MIL-101 after subtraction of the spectrum of MIL-101 (B).
shows that the MIL-101 matrix retains crystallinity upon encapsulation of Zn-POM.
This similarity indicates strong interaction of POM anion with MIL-101, therefore, the
positions of the X-ray reflections, compared to the initial MIL-101 sample, remained
unchanged. The interaction between POM anion and the positively charged interior

Encapsulated Zn-POM
467
Figure 3. Schematic representation of Zn-POM encapsulated within MIL-101 matrix.
Table 1. Physico-chemical properties of the catalyst studied.
Sample
Zn/Mo/W/Cra
A (m2 gꢀ1 b
)
Vp (cm3 gꢀ1 c
)
MIL-101
Zn-POM
Zn-POM@MIL-101
–
1.7/5/43/–
0.3/0.87/7.57/17.9
2125
1913
1.08
0.98
aEvaluated from the atomic absorption data.
bSpecific surface area.
cMesopore volume.
surface of the MIL-101 nanocages is an electrostatic interaction [36]:
n=0:2½Cr3OðbdcÞ3ðH2OÞ2F0:8ðNO3Þ0:2ꢄ þ POMnꢀ
ꢀ
! ½Cr3OðbdcÞ3ðH2OÞ2F0:8ꢄn=0:2POM þ nNO3
The FT-IR spectra of MIL-101 and Zn-POM@MIL-101 are shown in figure 2(a).
These spectra are similar and the high intensity peaks corresponding to MIL-101 solid
matrix. Strong bands at 1300–1800 cmꢀ1 belong to ꢂas(COO), ꢂs(COO), and ꢂ(C–C)
vibrations from the dicarboxylate linker in the MIL-101 framework. Narrow and weak
bands at 1017 and 749 cmꢀ1 can be attributed to ꢃ(C–H) and ꢄ(C–H) vibrations of
aromatic rings, respectively.
Figure 2(b) shows IR spectra of Zn-POM and Zn-POM@MIL-101 after subtraction
of the MIL-101 spectrum. The IR spectrum of Zn-POM@MIL-101 exhibits
the principal stretching modes of the Keggin POM unit: 1050 ꢂas(PO4) and 880 cmꢀ1
ꢂas(W–O–W), for Zn-POM. These show that the heteropolyanion structure was
sustained after immobilization.
Table 1 shows the physico-chemical characteristics for MIL-101 and Zn-
POM@MIL-101. The loading of the catalyst was determined by elemental analysis,
indicating that the amount of Zn-POM encapsulated in MIL-101 is 17.5%.

468
Table 2. Alkene oxidation with H2O2 in the presence of MIL-101 and Zn-POM@MIL-101.
MIL-101a Zn-POM@MIL-101b
Acid
Z. Saedi et al.
Conversion Time TOFc Conversion Time yield TOFc
Substrate
Acid
(%)
(h)
(hꢀ1
)
(%)
(h)
(%)
(hꢀ1
)
Cyclopentene Glutaric acid
75
90
98
100
90
80
85
8
8
4
3
8
8
5
7.10
8.52
18.56
25.25
8.52
100
95
100
94
85
84
72
3
4
4
3
4
4
4
98
70
23
62
15
12
7
723
516
542
680
462
456
391
Cyclohexene
Cyclooctene
Indene
Adipic acid
Suberic acid
Homophthalic acid
Heptanoic acid
Nonanoic acid
Undecanoic acid
1-Octene
1-Decene
1-Dodecene
7.57
12.88
a0.5 mmol alkene, 5 mmol H2O2, 5 mg MIL-101(66 ꢃ 10ꢀ4 mmol), 3 mL CH3CN, reflux (70ꢁC).
b0.5 mmol alkene, 5 mmol H2O2, 5 mg Zn-POM@MIL-101(2.3 ꢃ 10ꢀ4 mmol ([(n-Bu)4N]5PZnMo2W9O39ꢂ3H2O)), 3 mL
CH3CN, reflux (70ꢁC).
cTurn over frequency.
The specific surface areas and mesopore volumes of MIL-101 and Zn-POM@MIL-
101, obtained from the low temperature N2 adsorption measurements, are shown in
table 1 and figure S1 (Supplementary material). The pore volume and specific surface
area of MIL-101 decreased upon encapsulation of Zn-POM. Subsequently, in order to
investigate the influence of Zn-POM on the thermal stability of MIL-101 matrix, the
TG/DTG analysis was also performed. Thermograms are shown in figure S2.
Comparison of these thermograms indicated that Zn-POM increases the thermal
stability of MIL-101. Below 350ꢁC, the MIL-101 showed about 11% weight loss in
three steps, but Zn-POM@MIL-101 composite showed 6.2% weight loss in two steps.
At temperatures above 350ꢁC, both materials decomposed.
3.2. Catalytic oxidations
The catalytic performance of Zn-POM@MIL-101 was first assessed in cyclooctene
oxidation with aqueous H2O2 in acetonitrile. To achieve the highest catalytic
performance, parameters such as the type of solvent, amount of catalyst and oxidant
were optimized in the oxidation of cyclooctene. Acetonitrile, H2O2 (5 mmol), and Zn-
POM@MIL-101 (5 mg, 2.3 ꢃ 10ꢀ4 mmol Zn-POM) were chosen as optimal reaction
conditions. All reactions were carried out under these conditions and continued until no
further progress were observed. The results are shown in table 2. In the absence of
catalyst, the conversion was less than 30% and only 1,2-cyclooctanediol was produced.
In the case of cyclopentene (table 2, entry 1, figure 4) after 3 h, the conversion was
100% and the amount of glutaric acid was 98%. In the case of cyclohexene and indene
(table 2, entries 2 and 4), the conversions were 95% and 94% after 4 and 3 h,
respectively. In the oxidation of cyclohexene, adipic acid (70%) was produced as major
product and 1,2-cyclohexandiol and allylic oxidation products (enone and enol) were
also observed as minor products (figure 4). For indene, homophthalic acid was
produced in 62% and allylic oxidation products were also obtained (figure 4).
Cyclooctene (table 2, entry 3) was completely oxidized after 4 h. In this case, the main

Encapsulated Zn-POM
469
Conversion
Y (epoxide)
Y (-enol+ -enone)
Y (diol)
100%
100%
98%
95%
94%
100
80
60
40
20
0
Y (diacid)
70%
62%
41%
29%
25%
23%
20%
7%
7%
5%
2%
Cyclopentene
Cyclohexene
Cyclooctene
Indene
Figure 4. Oxidation of cyclic alkenes with Zn-POM@MIL-101.
Conversion
Y (epoxide)
Y (aldehyde)
Y (allyl alcohol)
Y (acid)
100
80
60
40
20
0
85%
84%
72%
38%
32%
29%
22%
14%
7%
21%
11%
20%
15%
12%
1-Octene
1-Decene
1-Dodecene
Figure 5. Oxidation of linear alkenes with Zn-POM@MIL-101.
product was 1,2-cyclooctandiol, and epoxycyclooctane and suberic acid were produced
in 29% and 23% yields, respectively (figure 4).
For more comprehensive study, oxidations of linear alkenes in the presence of this
new composite (Zn-POM@MIL-101) were also performed. The results are shown in
table 2 (entries 5–7) and figure 5. In the oxidation of 1-octene, the conversion was 85%
after 4 h. In this case, allylic alcohol and aldehyde were produced in 38% and 21%
yields (figure 5); other oxidation products like 1,2-epoxyoctane and heptanoic acid
were observed. For 1-decene and 1-dodecene, the acid production was decreased while
the yield of epoxides increased (figure 5).

470
Z. Saedi et al.
Table 3. Reusability of Zn-POM@MIL-101 in the oxidation of cyclooctene with H2O2.a
Epoxide
yield
(%)
Enoneþenol
yield
(%)
1,2-Cyclooctandiol
yield
(%)
Suberic
acid yield
(%)
Conversion
(%)
Run
1
2
3
100
100
94
29
34
56
7
7
14
41
44
24
23
15
0
aReaction condition: 0.5 mmol cyclooctene, 5 mmol H2O2, 5 mg Zn-POM@MIL-101 (2.3 ꢃ 10ꢀ4 mmol
([(n-Bu)4N]5PZnMo2W9O39 ꢂ 3H2O), 3 mL CH3CN, reflux (70ꢁC).
In an attempt to show the effect of incorporated Zn-POM on the activity of
MIL-101, the reaction of cyclooctene was carried out in the presence of homogeneous
Zn-POM (2.3 ꢃ 10ꢀ4 mmol). The results showed that only epoxycyclooctane was
produced in 85% yield after 8 h, and no by-product was detected in the reaction
mixture. When the reactions were carried out in the presence of pure MIL-101, the
major products were carboxylic acids. On the basis of these observations, in the
presence of Zn-POM@MIL-101 the reaction selectivity changed. Therefore, upon
incorporating Zn-POM into MIL-101, a new composite was synthesized that show
different catalytic activity than MIL-101 and Zn-POM.
3.3. Catalyst stability and recycling
Important properties of heterogeneous catalysts are the stability of the active site
toward leaching and other transformations and the possibility of catalyst reuse.
Therefore, the stability of Zn-POM@MIL-101 was studied in multiple oxidations of
cyclooctene with H2O2. The catalyst was separated from the reaction mixture after each
experiment by sedimentation, washed with the solvent (CH3CN), and dried carefully
before using it in the subsequent run. Indeed, Zn-POM@MIL-101 is recyclable under
the reaction conditions for 3 cycles (table 3, figure 6). The results showed that selectivity
of reaction considerably changed in the consequent runs. In the first run, 1,2-
cyclooctandiol was produced in 41% yield. While in the third run, epoxycyclooctane
was detected as a major product. This can be attributed to destruction of the catalyst
structure, in which the Zn-POM is leached to the reaction mixture and acts as a
homogeneous catalyst. This finding is in accord to results with pure Zn-POM. The FT-
IR-spectra (figure 7) showed the destruction of the MIL-101 structure after 3 catalytic
cycles, with the appearance of new bands corresponding to stretching C¼O vibrations
at 1721 and 1686 cmꢀ1 along with new stretching OH vibrations at 2940 cmꢀ1
,
presumably indicating that COO-groups were protonated. These bands can be assigned
to the production of terephthalic acid during the catalyst destruction.
The results on using different kinds of POMs in the oxidation of cyclooctene are
summarized in table 4. Using different POMs@MIL-101 with a 1/2 molar ratio of
cyclooctene/H2O2, the epoxide selectivity is 100%. When the molar ratio of
cyclooctene/H2O2 decreased to 1/10 (by increasing the amount of H2O2), the epoxide
selectivity decreased significantly. In the presence of homogeneous Zn-POM, both
cyclooctene/H2O2 ratios gave epoxide as sole product.

Encapsulated Zn-POM
471
100%
100%
94%
100
80
60
40
20
0
Conversion
Y (epoxide)
Y (enol+enone)
Y (diol)
Y (diacid)
56%
44%
34%
41%
29%
24%
14%
23%
15%
7%
7%
1
2
3
Catalytic cycles
Figure 6. Reusability of Zn-POM@MIL-101 in the oxidation of cyclooctene.
Figure 7. FT-IR spectra of ZnPOM@MIL-101: (a) initial, (b) after first cycle and (c) after three cycles of
cyclooctene oxidation with H2O2.
4. Conclusions
The findings in this study demonstrated that Zn-monosubstituted Keggin POM can be
incorporated electrostatically to nanoporous MIL-101 coordination polymer. The
composite Zn-POM@MIL-101 showed catalytic activity in alkene oxidation with H2O2
with changed selectivity and reaction times.

472
Z. Saedi et al.
Table 4. Results obtained in the oxidation of cyclooctene with H2O2 in the presence of different
POMs@MIL-101.
Epoxide
selectivity
(%)
T
(ꢁC)
Time
(h)
Conversion
(%)
Catalyst
Substrate/H2O2
Ref.
PW12@MIL-101a
Zn-POM@MIL-101b
Zn-POM@MIL-101c
V-POM@MIL-101d
V-POM@MIL-101e
Zn-POMf
1/2
1/10
1/2
1/10
1/2
50
70
70
70
70
70
70
3
4
8
4
3
8
8
76
100
86
84
32
99
29
100
25
100
100
100
[33]
–
–
–
–
1/10
1/2
85
81
–
–
Zn-POMg
a28 mg catalyst (5.8 ꢃ 10ꢀ3 mmol W, H3PW12O40).
b5 mg catalyst (2.3 ꢃ 10ꢀ4 mmol Zn, [(n-Bu)4N]5PZnMo2W9O39ꢂ3H2O).
c7 mg catalyst (3.2 ꢃ 10ꢀ4 mmol Zn).
d7 mg catalyst (9.3 ꢃ 10ꢀ4 mmol V, Na5[PMo10V2O40]ꢂ14H2O).
e3 mg catalyst (4 ꢃ 10ꢀ4 mmol V).
f0.88 mg homogeneous catalyst (2.3 ꢃ 10ꢀ4 mmol Zn).
g1.2 mg homogeneous catalyst (3.2 ꢃ 10ꢀ4 mmol Zn).
Acknowledgments
The authors would like to acknowledge the Center of Excellence of Chemistry of
University of Isfahan (CECUI) in supporting this study.
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