{Mo72Cr30} nanocluster as a novel self-separating catalyst for hydrogen peroxide olefin epoxidation

Article abstract of DOI:10.1016/j.catcom.2017.03.006

Phase-transfer catalytic epoxidation activity of nanosized {Mo₇₂Cr₃₀} Keplerate polyoxometalate in ethanol in the presence of hydrogen peroxide is exploited. The nanocluster acts as a homogeneous catalyst upon treatment with hydrogen peroxide, while it can be easily filtered and reused as a heterogeneous catalyst. The high catalytic activity and durability of catalyst was demonstrated.

Full text of DOI:10.1016/j.catcom.2017.03.006

Accepted Manuscript
{Mo72Cr30} nanocluster as a novel self-separating catalyst for
hydrogen peroxide olefin epoxidation
Abdolreza Rezaeifard, Maasoumeh Jafarpour, Reza Haddad,
Fahimeh Feizpour
PII:
DOI:
Reference:
S1566-7367(17)30092-4
doi: 10.1016/j.catcom.2017.03.006
CATCOM 4963
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
22 November 2016
8 March 2017
10 March 2017
Please cite this article as: Abdolreza Rezaeifard, Maasoumeh Jafarpour, Reza Haddad,
Fahimeh Feizpour , {Mo72Cr30} nanocluster as a novel self-separating catalyst for
hydrogen peroxide olefin epoxidation. The address for the corresponding author was
captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi:
10.1016/j.catcom.2017.03.006
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ACCEPTED MANUSCRIPT
{Mo72Cr30} nanocluster as a novel self-separating catalyst for hydrogen
peroxide olefin epoxidation
Abdolreza Rezaeifard,* Maasoumeh Jafarpour,* Reza Haddad and Fahimeh Feizpour
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand,
Birjand, 97179-414 Iran.
*Corresponding author.
Abstract:
Phase-transfer catalytic epoxidation activity of nanosized {Mo72Cr30} Keplerate polyoxometalate in
ethanol in the presence of hydrogen peroxide is exploited. The nanocluster acts as a homogeneous
catalyst upon treatment with hydrogen peroxide, while it can be easily filtered and reused as a
heterogeneous catalyst. The high catalytic activity and durability of catalyst was demonstrated.
Keywords
Keplerate
Polyoxometalate;
Nanocluster; Phase-transfer catalyst; Epoxidation; {Mo72Cr30};
Heterogeneous catalyst
*Corrsponding author, Tel: +98 56 32202516; Fax: +98 56 32202515, E-mail address: rrezaeifard@birjand.ac.ir;
rrezaeifard@gmail.com; (A. Rezaeifard); mjafarpour@birjand.ac.ir (M. Jafarpour)

ACCEPTED MANUSCRIPT
The extensive applications of polyoxometalate (POM) molecular clusters in catalysis,
electrochemistry, biomedicine, and magnetism as sensors, surfaces and coatings has offered new
opportunities in many different fields.[1,2] The POMs with high solubility in polar solvents are
the most commonly used homogenous catalysts, owing to their good catalytic features such as
high activity and selectivity. Nevertheless, difficulty in catalyst recycling and contamination by
residual catalyst are a number of serious limitations for their industrial applications.[2] POM-
based heterogeneous catalysts prepared by various “immobilization” processes or “solidification”
with suitable cations, exhibit facile separation and reusability. Nonetheless, the use of
heterogeneous catalysts are limited by mass transfer, diffusion, and accessibility of the active
sites.[2,3]
The combination of the homogeneous catalysts advantages with heterogeneous ones generates a
“Phase-transfer catalyst” (PTC) which is the most adopted strategy to overcome the catalysis
limitations and
therefore have raised
a
kind
of promising catalyst accommodating
phase-transfer catalytic system,
environmentally-friendly sustainable development.[4,5] In
a
homogeneous and heterogeneous states can be switched, achieving homogeneous catalysis of
reaction and heterogeneous separation of the catalyst. POM-based PTCs, are mostly organic-
inorganic hybrid materials constructed by modification of the cations of POMs with organic
compounds such as ionic liquids, quaternary ammonium salts, oligomers and so on. [6,7]
Catalytic olefin oxidation mediated by transition metal complexes is the most elegant and
environmentally benign route for the production of epoxides as versatile intermediates in organic
synthesis and pharmaceutical products [1, 2]. In this regard, considerable efforts have focused on
removing or replacing toxic solvents, reagents, catalysts and additives with safe ones. Moreover,
substituting oxidants in particular, Oxone®, NaOCl, iodosylbenzenes, and m-CPBA, with more
atom economic oxidants such as O2 and especially H2O2 is a major challenge.[8,9] The
epoxidation of various olefins is one typical reaction most widely studied over POM-based
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PTCs.[5] Recently, various newly designed POM-based PTCs have been applied in other
reactions such as esterification, transesterification, hydroxylation of benzene and oxidation of
alcohols,[10,11] expanding the scope of POM-based PTCs.
Keplerate-type polyoxometalates as a significant class of inorganic nanocapsules have attracted
more and more attention of scientists in the areas of chemistry, physics, biology, and materials.
Because they have overwhelming variety of fascinating and unusual properties, which make the
supramolecular clusters become good candidates to construct functional materials in the fields of
magnetics, optics, electrochemistry and materials science.[12,13] However, remarkably little work
has been done with respect to their catalytic performance[14-24] and to the best of our knowledge
a partial result on photo-controlled phase-transfer catalysis of azo derivative of {Mo132-SO4} is
the only report on PTC activity of Keplerates.[25]
Following our discovery on catalytic efficiency of {Mo132-OAc} nanoball in aerobic olefin
epoxidation [16] as well as oxidation of sulfides and olefins with H2O2 [17] a few works on
oxidation activity of other Keplerates has been done.[18-21] Catalyst degradation under oxidation
conditions is a major concern for using reduced POM such as {Mo132} nanocluster. Further
investigations, demonstrated that the {Mo132} acts as a pre-catalyst.[20] and our finding in Fig. S1
in SI)] It will change to several smaller nuclearity species by treatment with H2O2 which could
become active species and responsible for the oxidation of substrates.[20]. The residual catalyst
(Fig. S1) could be reused efficiently in the oxidation system.[17,20] In addition, solubility of
POM catalysts in water suffering its separation from the reaction solution and recycling, impede
its use in industrial processes. These results, induce us to search a robust cluster with desired
activity under heterogeneous conditions. Mixed metal oxide buckyball {Mo72Cr30} Keplerate
nanocluster
(2.5nm) “[{Na(H2O)12}∁{MoVI72CrIII30O252(CH3COO)19(H2O)94}]Ca.120H2O”[26] is
formed either as a water-soluble tiny crystals with a “pale” green colour in very low yield (<1%)
at earliest time and also as a “dark” green water-insoluble crystals in desired yield after 5 days.
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[26] In this work, for the first time the water-insoluble “dark” green {Mo72Cr30} (see Fig. S2-S7 in
ESI for full characterization of nanocluster) is introduced as a novel phase transfer catalyst for
efficient olefin epoxidation using 30% aqueous hydrogen peroxide as an oxidant in ethanol as a
safe solvent.
D
A
B
C
2.5 nm
{Mo72Cr30
}
O
H2O2/Ethanol
Fig. 1. Epoxidation of olefins using H2O2 in ethanol catalyzed by insoluble {Mo72Cr30} nanocapsule. (A)
Photograph of the catalyst in ethanol at 70 °C before addition of H2O2, (B) after addition of H2O2, (C)
after completion of the reaction, (D) after cooling to room temperature.
The solid catalyst is dissolved easily upon treatment with hydrogen peroxide in ethanol which
produces a deep “blue” solution exhibiting high activity and excellent selectivity in the oxidation
of different olefins. Nonetheless, it is filtrated easily at the end of reaction as a “dark” green solid.
(Fig. 1). The nanocluster maintains its icosahedral structure as well as its activity after recycling.
The catalytic activity of {Mo72Cr30} was examined in the oxidation of cyclooctene as a model
olefin under different conditions knowing that H2O2 has not ability to trigger the reaction under
catalyst-free condition (Table S1 in ESI). The results for screening of different solvents (entries 1-
9), using different amounts of oxidant (entries 5, 10-12) or catalyst (Table S1, entries 5, 13-15) at
various temperatures (Table S1, entries 5, 17-20) are given in Table S1. The “dark” green
{Mo72Cr30} which is insoluble in all solvents examined at 70 °C, dissolved rapidly in ethanol at
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70 °C after treatment with hydrogen peroxide (and to a lesser extent in other solvents). A
homogeneous “blue” solution (Fig. 1B) with characteristic absorption band at 590 nm assigned to
peroxide-Cr charge-transfer [27,28] was appeared (Fig. S8). The Raman spectra of “blue”
solution (Fig. S9) showed new vibrations at 530 and 1195 cm-1 rationalized to Cr-OO and O-O
stretching of a superoxo species [29-31] or a Cr-peroxo species [28] attached to numerous strong
electron deficient Mo(VI) centers. More evidences should be gathered to elucidate the actual
active species involved in the reaction mechanism.
After completion of the reaction, the catalyst self-precipitated from the reaction medium (Fig. 1C,
D) making it amenable to recyclability by filtration. For other solvents little amounts of “green”
solids were precipitated at room temperature, requiring a high speed centrifuge for catalyst
separation. Thus, ethanol is the best offer for the reaction media, based on the high catalytic
efficiency and self-separation of catalyst. Next, the temperature effect on the catalytic efficiency
was screened. The best catalytic performance was observed at 70 °C leading to full conversion of
cyclooctene within 45 min and 96% cyclooctene oxide was achieved, while, no diol by-product
was detected in the reaction. (Table S1, entry 5, Table 1, entry 1). At this temperature the
decomposition of hydrogen peroxide after formation of “blue” solution in the presence of
{Mo72Cr30} in ethanol proceeded slowly (<10% per hour, Fig. S10). However, based on the
results presented in Table S1 (entry 12) a molar equiv. amount of H2O2 was not enough and the
use of at least 1.5 equiv. of oxidant is necessary to the complete conversion of cyclooctene (Table
S1, entry 5, 11). The screening of catalyst amount (Table S1, entries 5, 13-15) demonstrated that
the reaction required 0.4 mol% of {Mo72Cr30} to proceed with high performance after 45 min (27
and 65% cyclooctene oxide yields at 15 and 30 min, respectively). It is worth mentioning that,
performing the reaction using lesser amount of catalyst (0.1 mol%) gave 70% cyclooctene
conversion after 1 h illustrating a turnover frequency of 700 for title catalyst. Moreover, the
durability of {Mo72Cr30} nanocluster was established by a TON of 15750 resulted from 63%
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conversion during 24 h under controlled conditions (0.004 mol% catalyst, 60 °C) (see infra).
When the catalyst was replaced by CrCl3.6H2O (12 mol%) and NaMoO4.2H2O (29 mol%) with
the same metal concentration as is in {Mo72Cr30}, no oxidation products were observed under the
same conditions during 12 h. Nevertheless, the use of (NH4)6Mo7O24·4H2O (4 mol%) as catalyst
gave 28, 50 and 69% cyclooctene oxide after, 1, 4 and 12 h respectively. Further, the addition of
CrCl3.6H2O to the above mentioned Mo salts did not promote the catalytic activity.
Next, we test the catalyst activity and stability after self-precipitation. Oxidation reaction was
carried out with the regenerated catalyst under the optimized conditions. Our founding showed no
change in selectivity, and only a 4 percentage decrease in conversion was observed after 5 cycles,
probably resulting from minor loss of catalyst during filtration and washing (Fig. S11). The
leaching of Mo and Cr from catalyst was ruled out by AAS analysis of filtrates. The analysis of
the product mixtures after centrifuge of hot solution (Fig. 1C) or filtration at room temperature
(Fig. 1D) showed no Mo or Cr within the limit of detection (<1 ppm). Moreover, the solid-free
filtrate was then subjected to fresh oxidant and stirred under standard reaction conditions. The
catalyst-free reaction produced <5% cyclooctene oxide after 12 h which is consistent with
preliminary control experiments (Table S1, entry 16).
Fig. 2 shows the FT-IR spectra of fresh catalyst (Fig. 2a and Fig. S2), after treatment with H2O2
(Fig. 2b and Fig. S12), and the recycled catalyst (Fig. 2c and Fig. S13). No vibrational changes
were observed after recycling, indicating good stability of the catalyst (see also Fig. S14 for FT-
IR spectra of catalyst after second recycling). There were also no noticeable changes in the UV-
Vis (Fig. S15), XRD pattern (Fig. S4, S16) as well as magnetic hysteresis curves (Fig. S7 and
S17) of {Mo72Cr30}, which further reveals that the reaction had no effect on the structure and
properties of catalyst. These results suggest that contrary to {Mo132}, [17,20 and Fig. S1]
{Mo72Cr30} rather than the products of its destruction acts as an active catalyst in the oxidation
process.
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Fig. 2. FT-IR spectra of fresh catalyst (a), after the reaction with H2O2 (b), and the recycled
catalyst (c).
Under optimized conditions, different olefins subjected to the oxidation procedure under catalytic
influence of {Mo72Cr30} (Table 1). Cyclohexene and norbornene were also oxidized completely
within 45 min and pertinent epoxides were isolated in high yields (Table 1, entries 2,3) and no
allylic oxidation by-product was observed. Hindered olefins such as 1-substituted cyclohexenes
(Table 1, entries 4 and 5) displayed less reactivity than that of non-substituted molecule.
Moreover, indene (Table 1, entry 6) required longer reaction time for its oxidation. An interesting
aspect of title methodology that needs to be highlighted is absolute stereospecificity in the
oxidation of cis- and trans-stilbenes as well as norbornene.
Despite the desired epoxide selectivity obtained in the oxidation of the above mentioned olefins,
terminal C=C bond conjugated with phenyl rings gave mainly the related carbonyl compounds
(Table 1, entries 10-12). Ring-opening reactions occurred at high temperature which supply high
energy to break the C=C bond.[32]
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Table 1. Hydrogen peroxide olefin oxidation catalyzed by {Mo72Cr30} nanocluster in ethanola
Entry
Olefin
Product
Yield%b
Time/min
1
2
3
4
5
6
96
91
97
75
36
80
45
45
45
60
120
90
8
9
94
69
180
180
O
10
11
88
85
45
45
12
72
45
aThe reactions were run under air at 70 °C using 1.5 equiv. H2O2 in the presence of {Mo72Cr30}(0.4 mol%).b The
products were identified by comparison with authentic samples. The selectivity of products were >99% except
for entry 10 and 11 which were 93% and 95% (5-7% styrene oxides). Yields and selectivity were determined by
1
GC-FID except for entries 2, 8 and 9 which did by H NMR analysis.
In conclusion, {Mo72Cr30} nanocluster was used successfully as a phase-transfer catalyst for
olefin epoxidation with hydrogen peroxide in ethanol. The polyoxometalate acts as
a
homogeneous catalyst while it can be easily filtered and reused like a heterogeneous catalyst. The
reactions proceeded with high selectivity for production of epoxides or carbonyl compounds. The
use of ethanol as a safe solvent, H2O2 as an eco-friendly oxidant in combination with {Mo72Cr30}
as an easily recyclable catalyst offer a novel sustainable oxidation protocol. The spectral data
revealed that the catalyst preserved its integrity make it appropriate for industrial application.
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Acknowledgement
Support for this work by Research Council of University of Birjand and “Iran Science Elites Federation”
is highly appreciated. We also thank Prof. A. Muller and S. Garai from Bielefeld University and T. Liu
and F. Haso from the University of Akron for their valuable comments on Keplerate properties.
Supporting Information
The procedure for the synthesis of catalyst and epoxidation of olefins, characterization of as-prepared and
reused catalyst including FT-IR, Raman, UV-Vis, XRD, TGA, EDS, magnetic hysteresis curves and
recycling results “This material is available free of charge via the Internet at http://....”
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produced under controlled conditions. After mixing CrCl3.6H2O with sodium molybdate in
35% water/acetic acid mixture, the solution is acidified using HCl. Then the solution needs to
be heated under reflux and left to cool at room temperature. After that we need to keep
watching the solution if there areany small “pale” green crystals grow! (having a similar IR
spectrum to the final product). The amount is very tiny (<1%) and need to FILTER THEM
RIGHT AWAY. The “pale” green crystals are soluble in water. Nevertheless, if the solutions
kept for a longer time (5 or more days), those “pale” green crystals grow gradually and the
resulting “dark” green crystals (in desired yield) will not be soluble in water, ethanol, ethyl
acetate, dichloroethane, chloroform and dichloromethane. It is only solved slightly in
methanol under sonication at higher temperature (>40 oC) (see supporting information for
complete characterization of as-prepared “dark” green {Mo72Cr30}).
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Research Highlight
 Phase-transfer catalytic activity of {Mo72Cr30} Keplerate polyoxometalate
 The high catalytic activity and selectivity as well as durability of catalyst
 Acting as a homogeneous catalyst and reusing like a heterogeneous one
11