Heterogeneous catalytic epoxidation of olefin over a hydrothermally synthesized 3D phosphate bridged copper(II) framework

Article abstract of DOI:10.1080/00958972.2013.806654

A 3D copper phosphate, [Cu₂(PO₄)(OH)] _n (1), has been synthesized hydrothermally and characterized by single-crystal X-ray diffraction analysis. In [Cu₂(PO₄)(OH)] _n, there are two types of copper centers having distorted trigonal bipyramidal geometry and distorted octahedral geometry that are connected by the μ₂-bridging of each phosphate oxygen ultimately forming a η⁸-PO₄ bridged 3D network. The compound exhibited excellent catalytic performance in olefin epoxidation. Epoxidation of styrene and substituted styrenes, as well as bulky olefins like cycloalkenes and long-chain alkenes, is efficiently catalyzed by [Cu₂(PO ₄)(OH)] _n using tert-butylhydroperoxide in acetonitrile. The results obtained in the heterogeneous catalytic reactions show that the olefins are converted to the respective epoxides in good yield with high selectivity. [Cu₂(PO₄)(OH)] _n was catalytically more active and selective in comparison to simple copper(II) phosphate salt in heterogeneous medium. The catalyst can be recycled and reused several times without significant loss of activity.

Full text of DOI:10.1080/00958972.2013.806654

This article was downloaded by: [University of New Mexico]
On: 14 October 2014, At: 15:54
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Coordination Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gcoo20
Heterogeneous catalytic epoxidation
of olefin over a hydrothermally
synthesized 3D phosphate bridged
copper(II) framework
Debraj Saha a , Tanmoy Maity a & Rupam Sen a b
a Department of Chemistry , Jadavpur University , Kolkata , India
b Department of Chemistry , CICECO, University of Aveiro ,
Aveiro , Portugal
Published online: 20 Jun 2013.
To cite this article: Debraj Saha , Tanmoy Maity & Rupam Sen (2013) Heterogeneous
catalytic epoxidation of olefin over a hydrothermally synthesized 3D phosphate bridged
copper(II) framework, Journal of Coordination Chemistry, 66:14, 2444-2454, DOI:
10.1080/00958972.2013.806654
To link to this article: http://dx.doi.org/10.1080/00958972.2013.806654
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Coordination Chemistry, 2013
Vol. 66, No. 14, 2444–2454, http://dx.doi.org/10.1080/00958972.2013.806654
Heterogeneous catalytic epoxidation of olefin over a
hydrothermally synthesized 3D phosphate bridged copper(II)
framework
DEBRAJ SAHA†, TANMOY MAITY† and RUPAM SEN*†‡
†Department of Chemistry, Jadavpur University, Kolkata, India
‡Department of Chemistry, CICECO, University of Aveiro, Aveiro, Portugal
(Received 30 July 2012; in final form 21 March 2013)
A
3D copper phosphate, [Cu2(PO4)(OH)]n (1), has been synthesized hydrothermally and
characterized by single-crystal X-ray diffraction analysis. In [Cu2(PO4)(OH)]n, there are two types of
copper centers having distorted trigonal bipyramidal geometry and distorted octahedral geometry that
are connected by the μ2-bridging of each phosphate oxygen ultimately forming a η8-PO4 bridged 3D
network. The compound exhibited excellent catalytic performance in olefin epoxidation. Epoxidation
of styrene and substituted styrenes, as well as bulky olefins like cycloalkenes and long-chain alkenes,
is efficiently catalyzed by [Cu2(PO4)(OH)]n using tert-butylhydroperoxide in acetonitrile. The results
obtained in the heterogeneous catalytic reactions show that the olefins are converted to the respective
epoxides in good yield with high selectivity. [Cu2(PO4)(OH)]n was catalytically more active and
selective in comparison to simple copper(II) phosphate salt in heterogeneous medium. The catalyst
can be recycled and reused several times without significant loss of activity.
Keywords: Metal-inorganic framework; Copper(II) phosphate; Heterogeneous catalysis;
tert-Butylhydroperoxide; Olefin epoxidation
1. Introduction
Transition metal coordination polymers attract attention because of interesting molecular
topologies and applications in catalysis, magnetism, gas adsorption, etc. [1–3]. Hydrother-
mal process is an efficient method for synthesizing such materials by incorporating desired
metal ions with polydentate-bridging ligands. This method has been extensively used for
preparation of zeolites and mesoporous silica [4] and is now being used as an effective
technique for synthesizing coordination polymers. Copper phosphates and organophospho-
nates are a large class of compounds which have been studied in depth [5, 6]. The
literature shows only limited work on purely inorganic phosphates including some work
on phosphate ester hydrolysis [7], transition metal phosphonates [8], and amine-templated
metal-inorganic framework solids [9].
There have been extensive studies on selective catalytic oxidation reactions of alkenes,
aromatics, alcohols, etc. catalyzed by copper compounds in heterogeneous or homogeneous
*Corresponding author. Email: rupamsen@ua.pt
Ó 2013 Taylor & Francis

Phosphate-bridged copper(II)
2445
medium [10]. Transition metal complexes have been used as catalysts for epoxidation of
alkenes [11]. Amongst the oxidants used in catalytic reaction, alkyl-hydroperoxides are
receiving increased attention. tert-Butylhydroperoxide is used on a large scale in industrial
epoxidation, for example, in Halcon-Arco and Sumitomo processes [12]. Recycling of co-
products (e.g. tert-BuOH) has been realized in the Sumitomo process. The recycling of cat-
alysts is of great economic and environmental importance in chemical and pharmaceutical
industries, especially when expensive and/or toxic heavy metal complexes are employed.
Many attempts have been made, such as intercalating or encapsulating the metal complex
into layered compounds or within the cavities of a porous solid (e.g. zeolites) [13], binding
the metal complex into the polymeric matrix [14] and employing the steric hindrance [15]
to design recyclable catalysts.
Olefin epoxidation reactions by copper complexes under homogeneous condition are
well known [16]. Recently, application of copper(II) complexes in heterogeneous catalysis
has received attention [4, 17, 18]. Koner et al. explored the catalytic epoxidation of
alkenes over copper-Schiff-base complexes immobilized into MCM-41 matrix [18]. Never-
theless, copper-based metal-organic/inorganic framework compounds have scarcely been
used as catalyst in olefin epoxidation reactions [19].
Here, we report the catalytic efficiency of hydrothermally synthesized 3D framework,
[Cu2(PO4)(OH)]n (1), towards epoxidation of alkenes in heterogeneous medium.
2. Experimental
2.1. Materials and methods
Cu(NO3)2·3H2O, cyclooctene, cyclododecene, 1-hexene, 1-octene, styrene, 3-methyl
styrene, 4-methyl styrene, trans-stilbene, and tert-butylhydroperoxide (70 wt.% aq.) were
purchased from Aldrich and used as received. Ortho-phosphoric acid (ca. 85%,
density = 1.7), cesium hydroxide, and solvents were purchased from Merck (India).
Single-crystal X-ray diffraction (XRD) data were collected at 293(2) K on a Bruker
SMART APEX CCD X-ray diffractometer using graphite-monochromated MoKα radiation
(λ = 0.71073 Å). Fourier-transform infrared spectra of KBr pellets were measured on a
Perkin–Elmer RX I FT-IR spectrometer. The metal content of the sample was estimated on
a Varian Techtron AA-ABQ atomic absorption spectrometer. TG analysis was made using
a Perkin Elmer (SINGAPORE) Pyris Diamond TGA unit. The heating rate was
programmed at 5 °C minꢀ1 with a protecting stream of N2 flowing at a rate of
20 mL minꢀ1. Powder XRD patterns of the samples were recorded with a Scintag
XDS-2000 diffractometer using CuKα radiation. N2 sorption measurements were performed
on an Autosorb iQ (Quantachorme Inc., USA) gas sorption system at 77 K. The products
of the catalytic reactions were identified and quantified by a Varian CP-3800 Gas
Chromatograph using a CP-Sil 8 CB capillary column.
2.2. Synthesis of [Cu2(PO4)(OH)]n (1)
The structure of [Cu2(PO4)(OH)]n was reported previously [20]. The phase-pure
compound was synthesized by hydrothermal treatment of copper nitrate (0.241 g, 1 mM)
with excess ortho-phosphoric acid (15 mM) in the presence of cesium hydroxide

2446
D. Saha et al.
(0.90 g, 6 mM) dissolved in 10 mL milliQ water in a 25 mL Teflon-lined autoclave for
three days at 170 °C, followed by cooling to room temperature. Green block-shaped
crystals (yield 50%) were separated out during hydrothermal treatment. The crystals were
filtered off by suction, washed with copious amounts of distilled water, and dried in air.
AAS analysis shows that [Cu2(PO4)(OH)]n contains 52.98% of copper (calcd 53.16%).
The FT-IR spectrum of [Cu2(PO4)(OH)]n shows two strong absorptions at 1151 and
961 cmꢀ1 assigned to ν(P–O) and an absorption at 3460 cmꢀ1 from ν(O–H).
2.3. Catalytic reactions
The catalytic reactions were carried out in a glass batch reactor according to the following
procedure. Substrate (1 g), solvent (8 mL), and catalysts (2 mg) were first mixed. The mixture
was then equilibrated to 65 °C in an oil bath. After addition of tert-butylhydroperoxide
(1.5 mL), the reaction mixture was stirred continuously. Reactions were performed in air. The
products of the epoxidation reactions were collected at different time intervals, and identified
and quantified by gas chromatography. Compound was dried under vacuum (100 °C, 12 h)
prior to reuse as catalyst.
3. Results and discussion
3.1. X-ray structure of [Cu2(PO4)(OH)]n (1)
Compound 1 possesses a densely packed 3D framework consisting of two metal centers,
Cu(1) and Cu(2), where Cu(1) centers are distorted trigonal bipyrimidal and Cu(2) centers
are distorted octahedral (figure S1). The phosphate anion assembles eight copper centers
by μ2-bridging of each phosphate oxygen, forming a η8-PO4 bridged network (figure S2).
The basal plane of the Cu(1) centers is formed by three oxygens of phosphates (Cu1–O2,
O2⁄, and O3; ⁄ = 1/2 + x, 1/2ꢀy, and ꢀ1/2 + z) and the two axial position are occupied by
two other phosphate oxygens (O3 and O1). Cu(2) is bound by six oxygens from phos-
phates. The basal plane is formed by O1, O1⁄, O2, and O1, and the two axial positions
are O1 and O1⁄. Cu(1) ions are bridged by μ2–O3 forming a consistent dimeric unit and
the Cu(2) centers are bridged by μ2–O1 forming another dimeric unit. The Cu(1) dimeric
units are perpendicular to each other in alternation within a layer, but the Cu(2) dimeric
units share the edge of the octahedron (figure S3). The Cu(2) ions are in the basal plane,
with a deviation of 2.44 Å. In the dimeric units with Cu(1) centers, the copper ions are
separated by 3.059 Å and for Cu(2) dimers, the centers are situated at 2.94 Å. Cu(1) and
Cu(2) centers are bridged by O2 and O1 oxygens. PO4 groups can be described as
distorted tetrahedra. The P–O distances, with a mean value of 1.54 Å, range from 1.524(2)
to 1.552(2) Å and the O–P–O angles are 107.82(9)°–111.44(9)° (mean value 109(2)°). The
PO4 groups are connected to the copper trigonal bipyramidal and octahedron by four
oxygen vertices, O(1), O(2), O(2)⁄, and O(3) (⁄ = x, y, z). All oxygens are bridged biden-
tate interacting with eight copper centers.
The bond valence sum (S) for Cu(1) and Cu(2) centers in [Cu3(PO4)2]n (1) was
calculated according to equation (1) [21]:
S ¼ exp½ðr0 ꢀ rÞ=Bꢁ
ð1Þ

Phosphate-bridged copper(II)
2447
Figure 1. 3-D framework structure of 1.
where r is the observed bond distance, r0 and B are empirically determined parameters,
r0 (Cu–O) = 1.679(2) Å, and B is 0.37. The calculated values of S, 1.95 for Cu(1) and
2.12 for Cu(2), are in agreement with the expected value of 2 for Cu(II) [21]. The Cu
(1) TBP and Cu(2) octahedra are cross-linked by phosphate anions forming the 3D
network (figure 1).
3.2. Thermogravimetric analysis
To ascertain thermal stability of [Cu2(PO4)(OH)]n, thermogravimetric (TG) analysis was
undertaken. TG analysis reveals that the framework is thermally stable to 380 °C in air;
above this temperature, the framework starts to decompose, which completes at 990 °C
(see Supplementary material; figure S4). The complete decomposition of the compound
proceeds in one step, resulting in CuO, as shown by powder XRD measurements. The
calculated weight loss for the whole process (66.7%) is in agreement with the experimental
value of 66.4%.
3.3. Nitrogen sorption study
Nitrogen sorption measurements of [Cu2(PO4)(OH)]n were undertaken to verify the porous
nature of the compound as well as to calculate the surface area of the catalyst (see Supple-
mentary material; figure S5). Brunauer–Emmett–Teller (BET) [22] surface area of the com-
pound was calculated from N2 adsorption isotherms. The adsorption of N2 follows a type II
isotherm and the BET surface area is estimated as 24 m2/g, indicating that there are no
micropores or mesopores. The structure of [Cu2(PO4)(OH)]n is very densely packed. This
can be easily seen with a space-filling representation of the structure and it is supported by
PLATON [23] calculations. Careful examination of the crystal structure ensures that there
are no pores or cavities of any appreciable size whatsoever. Also, there are neither guest
nor coordinated solvent molecules, which could give a void after activation. Secondly, the
surface area is very low for truly microporous metal-inorganic frameworks. It can simply
indicate the external surface, rather than the surface of the microporous structure.
3.4. Catalytic epoxidation reactions
Olefin epoxidation catalyzed by various MOF/MIF-based heterogeneous catalysts is well
documented in the literature by using H2O2 [24] or TBHP [25]. Compound 1 catalyzes
alkenes from terminal linear to cyclic, styrene, and also stilbene. Results of the catalytic

2448
D. Saha et al.
epoxidation of different substrates are given in table 1. A graphical representation of rela-
tive efficacy of [Cu2(PO4)(OH)]n for epoxidation of various alkenes in acetonitrile with
time is given in figure 2. The compound is active towards epoxidation of styrene and its
derivatives, 3-Me styrene, and 4-Me styrene. Styrene and the substituted styrenes were
converted above 90% with the highest selectivity of 70%, along with the benzaldehyde/
substituted benzaldehyde as the usual side product. Epoxidation of styrene with
tert-BuOOH over a variety of catalysts under heterogeneous conditions has been studied
[18]. Xiao et al. reported styrene epoxidation with a similar [Cu2(PO4)(OH)] complex
[26], where selectivity reached 67% with the limited conversion of 44%. Seelan et al.
studied epoxidation of styrene over copper phthalocyanine immobilized NaY catalyst,
Table 1. Epoxidation of olefins catalyzed by [Cu2(PO4)(OH)]n and copper phosphate salt in acetonitrile at 65 °Ca.
% Selectivity of
products
Substrate
Reaction time (h)
24
Conversion [wt.%]
Epoxide
Others
TONg
(a) 92
(b) 66
100
100
–
–
499
380
24
24
(a) 81
(b) 27
86
44
14b
56b
291
103
(a) 96
(b) 67
60
30
40c
70c
551
408
24
24
(a) 96
(b) 90
70
9
30d
91d
485
483
(a) 92
(b) 97
59
4
41e
96e
465
520
24
24
(a) 100
(b) 74
100
89
–
709
558
11
(a) 85
(b) 19
100
52
–
452
107
48
24
24
(a) 80
(b) 21
85
38
15f
62f
337
94
(a) 89
(b) 62
94
83
6
17
295
218
aReaction conditions: alkenes (1 g); catalyst (2 mg); tert-BuOOH (1.5 mL); and solvent (8 mL). The products of
the epoxidation reactions were collected at different time intervals, and were identified and quantified by Varian
CP-3800 gas chromatograph equipped with an FID detector and a CP-Sil 8 CB capillary column. (a) and (b) cor-
respond to the catalytic performance of [Cu2(PO4)(OH)]n and copper phosphate salt, respectively. bCyclododeca-
f
nol, cbenzaldehyde, d3-methyl benzaldehyde, e4-methyl benzaldehyde, 2-decanone, and gTON = moles converted/
moles of active site.

Phosphate-bridged copper(II)
2449
Figure 2. Reaction profiles for epoxidation of olefins with tert-BuOOH catalyzed by [Cu2(PO4)(OH)]n.
which shows over 95% conversion with epoxide selectivity of 24% [27]. In our earlier
study, we employed vanadium dihydrogen phosphate, where the conversion was 100% but
the oxide selectivity was limited to 44% [28]. Selectivity of the epoxide was improved
(53%) when copper-perchloro-phthalocyanine had been anchored onto MCM-41; however,
conversion remains at 47% [17]. In fact, in the epoxidation of styrene with tert-BuOOH
over copper/copper complex immobilized zeolite or molecular-sieves catalysts, epoxide
selectivity rarely goes above 40% [29]. A high selectivity of 78% was obtained by Maurya
and his group, where styrene was converted up to 60.2% with high turnover number
(TON) [30]. In a recent attempt, Koner et al. have improved the formation of styrene
oxide up to 86% using copper(II) Schiff-base anchored MCM-41 catalyst [18].
Cyclic olefins such as cyclooctene and cyclododecene were converted with good yield
(conversion 92 and 81%) to their corresponding oxides, with good product selectivity (100
and 86%, respectively). Product selectivity for cyclooctene rarely goes high in homoge-
neously catalyzed reaction because cyclooctane-1,2-diol is the commonly generated
Table 2. Epoxidation of alkenes with tert-BuOOH catalyzed by a variety of catalystsa.
Catalyst
Conversion (wt.%)
Epoxide (wt.%)
TON
Refs.
Styrene
b
[VO(H2PO4)2]n
100
60.2
40
48
67
44
78.0
8
5
41
60
1249
1464
76
155
404
551
[28]
[30]
This work
This work
This work
This work
[Cu(acpy-oap)Cl]-Yb
CuO
CuCl2
Copper phosphate salt
Compound 1
96
Cyclooctene
b
[VO(H2PO4)2]n
90
66.87
86.40
92
80
1064
669
864
499
[28]
[31]
[31]
This work
VOL5b
100
100
100
VOL5b
Compound 1
aReaction conditions were the same as given in footnote of table 1.bCompound detail and reaction condition are
in the references.

2450
D. Saha et al.
byproduct. In earlier study, we converted cyclooctene with 80% epoxide yield [28]. In a
recent study, Behzad et al. got an impressive selectivity of 100% [31]. Catalytic activities
and selectivity in styrene and cyclooctene epoxidation by tert-BuOOH over various cata-
lysts are presented in table 2.
Although terminal alkenes need higher activation energy, we got a fair conversion of
80–100%. Longer alkene, 1-decene, shows 80% conversion with 85% epoxide selectivity
while 1-octene and 1-hexene show epoxide conversion of 85 and 100%, respectively, with
100% oxide selectivity. Selectivity and conversion become less when the bulkiness or
chain length of the substrates is increased, which may be explained on the basis of steric
hindrance caused by the substrates.
The overall catalytic efficiency of [Cu2(PO4)(OH)]n in epoxidation reactions was
significant, reflected in the high TONs (table 1). The copper(II) binds the peroxo-group on
treatment with peroxides [32] to form the pre-catalyst containing LxCu–OOH (where
L= ligand), which are capable of transferring the oxo-functionality to the organic substrates
to produce the oxidized products [33]. We assume that in our case, a similar kind of mech-
anism is operative. X-ray crystal structure analysis shows the presence of five-coordinated
Cu(II) in [Cu2(PO4)(OH)]n. So, the coordination environment around copper(II) is easily
accessible for an external ligand, and as a result, tert-butylhydroperoxide gets enough
space to bind copper in the intermediate stages of the catalytic cycle.
To optimize the catalytic reaction, we studied some control experiments by varying the
oxidants, solvents, and the ratio of the oxidant. Different oxidants like tert-BuOOH, H2O2,
and NaOCl have been used in cyclooctene and 1-hexene epoxidation. The results of this
study are given in table 3. tert-BuOOH is most efficient amongst them for higher conver-
sion and selectivity. Solvent polarity also plays an important role for epoxidation. The best
performance of the catalysts was observed in acetonitrile (see Supplementary material;
table S1). The optimum polarity of acetonitrile to dissolve both olefin and tert-BuOOH
might be facilitating the epoxidation. A graphical representation of the relative efficacy of
the [Cu2(PO4)(OH)]n catalyst for the epoxidation of various alkenes in different solvents
has been given in figure 3. The efficiency of catalyst, in general, followed the order:
acetonitrile > acetone > chloroform ꢂ THF > dichloromethane. To determine the optimum
reaction temperature, we studied the reactions by varying the temperature from room
temperature to 75 °C for different olefins in acetonitrile (figure 4 and see Supplementary
material; table S2). The reaction profile clearly indicates that a desired temperature of
65–70 °C is needed to activate the catalyst.
It is occasionally assumed that TBHP acts as an initiator but not a true oxidant,
therefore the reaction was also performed in inert atmosphere by varying the amount of
a
Table 3. Epoxidation of cyclooctene and 1-hexene with various oxidants over [Cu2(PO4)(OH)]n
.
Substrate
Oxidant
Conversion (wt.%)
% Yield of epoxide
None
Not traceable
–
H2O2
tert-BuOOH
NaOCl
68
92
28
62
92
10
None
Not traceable
–
H2O2
tert-BuOOH
NaOCl
65
100
22
65
100
12
aReaction condition was the same as given in footnote of table 1.

Phosphate-bridged copper(II)
2451
Figure 3. Comparison of catalytic efficacy of 1 in different solvents.
Figure 4. Comparison of catalytic efficacy of [Cu2(PO4)(OH)]n at different temperatures.
Figure 5. Plot showing performance of the catalysts in epoxidation of 1-hexene and cyclooctene with varied
amount of tert-BuOOH.

2452
D. Saha et al.
Table 4. Catalytic efficacy of the recovered compound ([Cu2(PO4)(OH)]n) in successive runs for 1-hexene and
cycloctene epoxidationa.
Compounds
1-hexene
Cycles
Conversion (wt.%)
Selectivity of product (wt.%)
TON
1
2
3
4
5
1
2
3
4
5
100
100
100
98
98
91
90
90
88
90
100
100
100
100
100
100
100
100
100
100
709
709
709
695
695
493
488
488
477
488
cyclooctene
aReaction condition was the same as given in footnote of table 1.
the TBHP and the results prove the vital role of TBHP. When the amount of TBHP
was increased, the conversion gradually goes to completion (figure 5). The optimum
amount of tert-BuOOH needed for completion of catalytic reaction was 1 : 1.5–2 M
equivalent (substrate: tert-BuOOH). In epoxidation, oxidant requirement is generally of
this order [34, 35]. No induction period was observed in any of the reactions.
Simple copper phosphate salt was compared with 1 to see the structural effect (table 1).
Although, in some cases, conversion (e.g. 3-Me styrene and 4-Me styrene) was impressive
(90 and 97%, respectively) catalyzed by copper phosphate salt, they showed only poor
selectivity. For linear chain olefins, e.g. 1-octene, 1-decene, and bulkier cyclododecene,
conversion was very poor for copper phosphate salt in comparison with [Cu2(PO4)(OH)]n.
However, epoxidation of cyclooctene with copper phosphate salt showed 66% conversion
with 100% selectivity. So, the overall performance of copper phosphate salt in catalytic
reactions is poorer than 1.
3.5. Separation, catalyst reuse, and heterogeneity test
The major advantages of the use of heterogeneous catalysts are recovery of the catalyst
from the reaction mixtures and its possible reuse. To test if metal is leached from the solid
catalyst during reaction, the liquid phase of the reaction mixture was collected by filtration
at the reaction temperature after 30% of the epoxidation reaction was completed for 1-hex-
ene, and the residual activity of the supernatant solution after separation of the catalysts
was studied. Atomic absorption spectrometric analysis (sensitivity up to 0.001 ppm) of the
supernatant solution of the reaction mixture thus collected by filtration confirms the
absence of copper ions in the liquid phase. The filtrate mixture did not show any catalytic
activity towards epoxidation (figure S6). These experiments clearly demonstrate that metal
is not leaching from the solid catalyst during epoxidation. In order to check the stability of
the catalysts, we characterized the solids after completion of reactions. After the catalytic
reactions were over, solid catalyst was recovered by centrifugation, washed with fresh ace-
tonitrile several times, and dried in air oven. The recovered catalyst was then subjected for
X-ray powder diffraction analysis and IR spectral analysis. Comparison of IR spectra and
XRD patterns of the pristine complexes and recovered catalysts convincingly demonstrate
that the structural integrity of [Cu2(PO4)(OH)]n is unaltered after epoxidation (see Supple-
mentary figures S7 and S8). Notably, the recovered catalyst can be reused in epoxidation

Phosphate-bridged copper(II)
2453
reactions for several times with no significant loss of activity (table 4). The kinetic profiles
of the epoxidation reaction of 1-hexene and cyclooctene using virgin and recovered cata-
lysts are virtually the same (figure S9).
4. Conclusion
[Cu2(PO4)(OH)]n demonstrates good catalytic activities in epoxidation towards various
industrially important olefinic substrates. Low surface area obtained in the nitrogen sorp-
tion measurement indicates that [Cu2(PO4)(OH)]n is not porous to accept guest molecules
in the bulk of the solid. Therefore, epoxidation reactions take place on the surface of the
catalysts. The use of different solvent and temperature influences the catalytic reactions.
Maximum conversion is observed in acetonitrile at 65–70 °C. No decomposition or
leaching of the metal centers during the catalytic reactions proved the heterogeneous nature
of the catalyst. The catalyst is recyclable up to five cycles without significant loss in their
catalytic activity.
Supplementary material
Tables S1 and S2 and figures S1–S9 are available free of charge via the Internet.
Supplementary data associated with this article can be found in the online version.
Acknowledgment
We acknowledge DST for funding the Department of Chemistry, Jadavpur University to
procure single crystal XRD machine under DST-FIST programme.
References
[1] J. Zhang, A.V. Biradar, S. Pramanik, T.J. Emge, T. Asefa, J. Li. Chem. Commun., 48, 6541 (2012).
[2] Y.-F. Zeng, X. Hu, F.-C. Liu, X.-H. Bu. Chem. Soc. Rev., 38, 469 (2009).
[3] M.B. Duriska, S.M. Neville, J. Lu, S.S. Iremonger, J.F. Boas, C.J. Kepert, S.R. Batten. Angew. Chem. Int.
Ed., 48, 8919 (2009).
[4] R.M. Barrer. Hydrothermal Chemistry of Zeolites, Academic Press, London (1982).
[5] X.H. Chen, G.B. Yang, L.H. Kong, D. Dong, L.G. Yu, J.M. Chen, P.Y. Zhang. Cryst. Growth Des., 9, 2656
(2009).
[6] V. Chandrasekhar, T. Senapati, A. Dey, S. Hossain. Dalton Trans., 40, 5394 (2011) and references therein.
[7] F.H. Fry, A.J. Fischmann, M.J. Belousoff, L. Spiccia, J. Brügger. Inorg. Chem., 44, 941 (2005).
[8] S. Konar, A. Clearfield. Inorg. Chem., 47, 5573 (2008).
[9] H. Yang, S. Guo, J. Tao, J. Lin, R. Cao. Cryst. Growth Des., 9, 4735 (2009).
[10] M. Bahidsky, M. Hronec. Catal. Today, 91, 13 (2004).
[11] K.C. Gupta, A.K. Sutar. Coord. Chem. Rev., 252, 1420 (2008).
[12] J. Halcon Kollar. US Pat, 3 350, 422 (1967).
[13] R. Ferreira, M. Silva, C. Freire, B. de Castro, J.L. Figueiredo. Microporous Mesoporous Mater., 38, 391
(2000).
[14] H.U. Blaser, B. Pugin, F. Spinder. J. Mol. Catal. A: Chem., 231, 1 (2005).
[15] M.R. Maurya, S.J.J. Titinchi, S. Chand. J. Mol. Catal. A: Chem., 201, 119 (2003).
[16] M.M. Díaz-Requejo, T.R. Belderraín, P.J. Pérez. Chem. Commun., 1853 (2000).
[17] P. Jana, P. Roy, M. Nandi, M. Manassero, M. Riccό, M. Mazzani, A. Bhaumik, P. Banerjee. Dalton Trans.,
9543 (2009).
[18] S. Jana, B. Dutta, R. Bera, S. Koner. Langmuir, 23, 2492 (2007).

2454
D. Saha et al.
[19] K. Brown, S. Zolezzi, P. Aguirre, D. Venegas-Yazigi, V. Paredes-García, R. Baggio, M.A. Novak, E. Spodine.
Dalton Trans., 1422 (2009).
[20] O.V. Yakubovich, O.K. Melnikov. Kristallografiya, 38, 63 (1993).
[21] I.D. Brown, D. Altermatt. Acta. Cryst, B41, 244 (1985).
[22] S. Brunauer, P.H. Emmett, E. Teller. J. Am. Chem. Soc., 60, 309 (1938).
[23] PLATON: A.L. Spek. Acta. Cryst., D65, 148 (2011).
[24] C. Adhikary, R. Bera, B. Dutta, S. Jana, G. Bocelli, A. Cantoni, S. Chaudhuri, S. Koner. Polyhedron, 27,
1556 (2008) and references therein.
[25] W. Adam, C.R. Saha-Möller, O.J. Weichold. J. Org. Chem., 65, 5001 (2000).
[26] X. Meng, K. Lin, J. Sun, M. Yang, D. Jiang, F.-S. Xiao. Catal. Lett., 71, 241 (2001).
[27] S. Seelan, A.K. Sinha, D. Srinivas, S. Sivasanker. J. Mol. Catal. A: Chem., 157, 163 (2000).
[28] R. Sen, R. Bera, A. Bhattacharjee, P. Gütlich, S. Ghosh, A.K. Mukherjee, S. Koner. Langmuir, 24, 5970
(2008).
[29] D.E. De Vos, M. Dams, B.F. Sels, P.A. Jacobs. Chem. Rev., 102, 3615 (2002).
[30] M.R. Maurya, P. Saini, C. Haldar, A.K. Chandrakar, S. Chand. J. Coord. Chem., 65, 2903 (2012).
[31] A. Ghaffari, M. Behzad, G. Dutkiewicz, M. Kubicki, M. Salehi. J. Coord. Chem., 65, 840 (2012).
[32] T. Osako, S. Nagatomo, Y. Tachi, T. Kitagawa, S. Itoh. Angew. Chem. Int. Ed. Engl., 41, 4325 (2002).
[33] S.T. Prigge, B.A. Eipper, R.E. Mains, L.M. Amzel. Science, 304, 864 (2004).
[34] V.R. Choudhary, R. Jha, P. Jana. Catal. Commun., 10, 205 (2008).
[35] P.P. Knops-Gerrits, I.F.J. Vankelecom, E. Béatse, P.A. Jacobs. Catal. Today, 32, 63 (1996).