Prussian blue analogues as heterogeneous catalysts for epoxidation of styrene

Article abstract of DOI:10.1039/c4ra16570e

The catalytic epoxidation of styrene was carried out using tert-butyl hydroperoxide (TBHP) in the presence of Prussian blue analogues (PBA) as catalysts and the reaction parameters of the epoxidation, such as temperature, solvent and reaction time, have been optimized. Optimum reaction conditions led to 96% conversion of styrene with selectivity in styrene epoxide equal to 64%. Furthermore, a kinetic investigation of the epoxidation of styrene with TBHP has been studied, a first order with respect to the concentrations of styrene, TBHP and catalyst were determined and an apparent activation energy value of 100.4 kJ mol^-1 was obtained. This journal is

Full text of DOI:10.1039/c4ra16570e

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Prussian blue analogues as heterogeneous catalysts
for epoxidation of styrene
Cite this: RSC Adv., 2015, 5, 17993
a
a
c
a
a
Yanling Liang, Chaoyong Yi, Simon Tricard, Jian Fang, Jihua Zhao*
b
and Weiguo Shen
The catalytic epoxidation of styrene was carried out using tert-butyl hydroperoxide (TBHP) in the presence
of Prussian blue analogues (PBA) as catalysts and the reaction parameters of the epoxidation, such as
temperature, solvent and reaction time, have been optimized. Optimum reaction conditions led to 96%
conversion of styrene with selectivity in styrene epoxide equal to 64%. Furthermore, a kinetic
investigation of the epoxidation of styrene with TBHP has been studied, a first order with respect to the
concentrations of styrene, TBHP and catalyst were determined and an apparent activation energy value
Received 17th December 2014
Accepted 5th February 2015
DOI: 10.1039/c4ra16570e
ꢀ1
www.rsc.org/advances
of 100.4 kJ mol was obtained.
20,21
epoxidation of styrene.
To the best of our knowledge, PBA
Introduction
catalysts have not been reported for the epoxidation of styrene.
In the present work, we explored the use of PBA catalysts for
Prussian blue analogues (PBA) possess zeolite-like cage struc-
tures and have gained considerable attention for their spec- the epoxidation of styrene with tert-butyl hydroperoxide (TBHP),
1
,2
3
4
tacular electrochemical, magnetic, photomagnetic and gas and studied the effects of temperature, solvent, and reaction
5,6
sorption properties. A general formula of PBA catalysts is time on the conversion and the selectivity of the reaction.
0
0
u n v 2
M [M (CN) ] $xH
O, where M and M are transition metals: they Secondly, we investigated the kinetic of the epoxidation of
can be the same kind of metal or different ones. PBA catalysts styrene with TBHP using the initial rate method. Our results
are part of the class of double metal cyanide (DMC) catalysts. conrmed that the PBA compounds as heterogeneous catalysts
Previous studies reported that DMC catalysts can be useful for have potential applications for low toxicity, lower dosage reac-
7
8
Prins condensation reactions, transesterication, copolymer- tions, simple preparation processes and cheap, thanks to their
9
,10
ization of epoxides and CO
ols,
2
,
synthesis of polyether-poly- high catalytic activity.
11,12
13
14
synthesis of b-amino alcohols and b-alkoxy alcohols,
production of biofuels and lubricants,
hyperbranched polymers.
Here, we use PBA as heterogeneous catalysts for the epoxi-
15,16
and preparation of
Experimental
Materials
17,18
dation of styrene as styrene oxide which is an important organic K Co(CN) ($90%) was procured from American Alfa Aesar, and
3
6
intermediate to synthesize ne chemicals and pharmaceuticals. was used without further purication. Styrene was distilled
Traditionally, styrene oxide is synthesized using two main under reduced pressure before use. Cu(NO ) $3H O ($99.5%),
3
2
2
methods, namely dehydrohalogenation of styrene halide tert-butyl hydroperoxide (TBHP, 70%), H O (30%), acetonitrile,
2
2
alcohol with sodium hydroxide and direct oxidation of styrene methanol, ethanol, hexane, cyclohexane, N,N-dimethylforma-
19
using organic peroxyacids. Both methods not only have low mide (DMF), ethyl acetate, toluene, chloroform, acetone,
selectivity for styrene oxide, but also involve corrosive and toxic cyclohexene, norbornene and bromobenzene were all analytical
chemicals giving rise to serious problems of equipment dete- grade reagents and were used without further purication.
rioration and environmental pollution. Thus current
approaches proposed the use of heterogeneous catalysts for
Catalyst preparation
The catalyst preparations involved slight modications from a
a
5
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu procedure from the literature. As an example, the Cu–Co PBA
ꢀ1
Province, College of Chemistry and Chemical Engineering, Lanzhou University,
catalyst was prepared as follows. 10 mmol L of K
3 6
Co(CN) was
Lanzhou 730000, P. R. China. E-mail: zhaojihua@lzu.edu.cn; Fax: +86 931 891
dissolved in 25 mL of double-distilled water to prepare solution
2
582; Tel: +86 931 891 2541
ꢀ
1
1
. 18 mmol L of Cu(NO ) was dissolved in 25 mL of double-
b
3 2
School of Chemistry and Chemical Engineering, East China University of Science and
Technology, Shanghai 200237, P. R. China
distilled water to prepare solution 2. Solution 1 was added
dropwise into solution 2 under magnetic stirring at room
temperature. The resulting precipitate was allowed to anneal in
c
Laboratoire de Physique et Chimie de Nano-Objets, Universit ´e de Toulouse, INSA,
UPS, CNRS, 135 avenue de Rangueil, 31077 Toulouse, France
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the mother liquor for 48 h, then ltered, and washed several
times with distilled water. The precipitate was then dried in air
to give Cu–Co PBA compound.
Characterization apparatus
X-ray energy dispersive spectroscopy (EDS) data were collected
using EDS Instrument on JSM-5600LV. Elemental analysis was
performed using a VarioEL cube (Elemetar analysensysteme
GmbH). Infrared spectra (IR) were recorded in a TENSOR27 FT-
ꢀ
1
IR spectrophotometer with a resolution of 4 cm and 32 scans
ꢀ1
in the region of 4000–400 cm using the KBr pressed disk
technique. X-ray powder diffraction (XRD) measurements were
performed on a Rigaku D/MAX-2400 X-ray diffractometer with
graphite-monochromatized Cu Ka radiation (l ¼ 0.15406 nm)
3 6
Fig. 1 Infrared spectra of (a) K Co(CN) , (b) the Cu–Co PBA catalyst.
ꢁ
ꢁ
ꢁ
ꢀ1
from 10 to 90 with a scanning speed of 10 min . Trans-
mission electron microscope (TEM) measurements were made
on a Hitachi H-600 transmission electron microscope (Hitachi,
Tokyo, Japan).
ꢀ1
ꢀ1
2
128 cm for K
3 6
Co(CN) to 2188 cm for the Cu–Co PBA
ꢀ
ꢀ1
catalyst, the n(CN) of free CN is equal to 2080 cm (aqueous
22
solution). The frequency of d(Co–CN) band increased from 416
Catalytic reactions
ꢀ1
ꢀ1
cm for K Co(CN) to 468 cm for Cu–Co PBA catalyst. Above
3
6
ꢀ
1
The PBA catalyst was used for the catalytic studies without 3500 cm , the n(OH) vibrations (asymmetric and symmetric) of
further activation. In a typical reaction, 3 mg of catalyst, 0.5 coordinated water were observed in the Cu–Co PBA catalyst,
mmol of styrene, 0.75 mmol of tert-butyl hydroperoxide were water molecules lled the interstitial cavities of Prussian blue.
added to a 10 mL round-bottomed ask in 2 mL of acetonitrile.
6
23
The ask was equipped with a water condenser and was
X-ray diffraction (XRD)
maintained in an oil bath at 345 ꢂ 2 K under continuous stir-
The crystal structure of the catalyst was conrmed by XRD
measurements. We observed that the catalyst had a very high
crystallinity (Fig. 2), and conrmed its cubic lattice structure –
typical of a Prussian blue structure – by comparison with XRD
analysis of the Cu [Co(CN) ] $11H O compound, already pub-
ring. Aer 6 h of reaction, the catalyst was separated by
centrifugation. The products are identied by analyzing the
reaction mixture with a gas chromatograph-mass spectrometer
(GC-MS, Shimadzu GC-MS-QP-2010SE) having a programmed
3
6 2
2
oven (temperature range 323–573 K). The conversion and
product selectivity were monitored by gas chromatographic
analysis with bromobenzene as internal standard. We used a
gas chromatograph (Varian CP-3380) equipped with a ame
ionisation detector and a KF1701 capillary column (15 m-long,
lished in XRD databases (JCPDS 51-1895).
Catalytic activity for styrene epoxidation with the Cu–Co PBA
catalyst
0
.2 mm-wide, and with a 0.5 mm-thick coated lm), a pro-
Catalytic performances of the Cu–Co PBA catalyst. First, the
effect of the reaction temperature on the conversion and
selectivity in the epoxidation products was investigated
2
grammed oven (temperature range 323–533 K), and N as the
carrier gas.
(
Fig. 3(a)). Increasing the reaction temperature sped up the
Results and discussion
reaction rate, but too high reaction temperature accelerated
Catalyst characterization
X-ray energy dispersive spectroscopy (EDS) and elemental
analysis. The EDS results indicated that the Cu : Co molar ratio
was close to 3 : 2. Combined with the data of elemental analysis,
the formula of Cu–Co PBA catalyst could be determined, namely
Cu [Co(CN) ] $11H O : Cu ¼ 61.86% (61.18%), Co ¼ 37.64%
3
6 2
2
(
(
38.34%), K ¼ 0.50% (0.48%), C ¼ 17.58% (17.65%), N ¼ 20.06%
20.04%), H ¼ 2.68% (2.65%) (where the contents of Cu and Co
were measured by EDS and the contents of C, N and H were
measured by elemental analysis – the values in brackets corre-
sponds to the percentage calculated from the formula).
Infrared spectroscopy (IR)
3 6
Infrared spectra of K Co(CN) and Cu–Co PBA catalyst (Fig. 1)
showed a shi of the cyanide band frequency n(CN) band from Fig. 2 Powder XRD pattern of the Cu–Co PBA catalyst.
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Table 1 Effect of different oxidants on conversion and selectivity
Selectivity (mol%)
Styrene conversion
(mol%)
Oxidants
So
Bza
Others
O
2
—
15
96
—
29
64
—
72
35
—
—
1
2 2
H O (30%)
TBHP (70%)
a
Reaction conditions: catalyst: Cu–Co PBA catalyst 3 mg, styrene: 0.5
mmol, solvent: acetonitrile (2 mL), reaction temperature: 345 K, reaction
time: 6 h, oxidants: H O (30%), 0.75 mmol; TBHP (70%) 0.75 mmol; O ,
2 2 2
bubbling; conversion and selectivity were determined by GC analysis. So:
styrene oxide, Bza: benzaldehyde, others: acetophenone.
a
Table 2 Effect of the solvents on conversion and selectivity
Selectivity (mol%)
Styrene conversion
Solvent
Polarity
(mol%)
So
Bza
Others
DMF
Acetonitrile
Acetone
Chloroform
Ethyl acetate
Toluene
Cyclohexane
Hexane
6.4
6.2
5.4
4.4
4.3
2.4
0.1
0.06
6.6
4.3
56
96
49
75
85
74
64
78
37
31
56
64
21
32
51
33
48
52
—
—
44
35
64
54
45
63
51
44
58
60
—
1
15
14
4
4
1
4
42
40
Methanol
Ethanol
a
Reaction conditions: catalyst: Cu–Co PBA catalyst 3 mg, styrene: 0.5
mmol, TBHP: 0.75 mmol, solvent: 2 mL, reaction temperature: 345 K,
reaction time: 6 h, conversion and selectivity were determined by GC
analysis. So: styrene oxide, Bza: benzaldehyde, others: acetophenone.
the decomposition of tert-butyl hydroperoxide as well as the
polymerization of styrene, and therefore was unfavorable for
the reaction. Then concerning the inuence of the reaction
time on the epoxidation reaction was observed (Fig. 3(b)).
When the reaction time exceeded 6 h, the conversion rate
stayed almost constant, while the selectivity began to decrease.
In the reaction conditions as far optimized, we further tested
three kinds of oxidants for the epoxidation of styrene (Table 1).
When the reaction was carried out under bubbling O , no
2
epoxidation reaction occurred at all, this may be because there
was no a sacricial reductant in the reaction system which
could combine with O . Lastly, results in (Fig. 3(c)) reected the
2
Fig. 3 (a) Effect of temperature on conversion and selectivity with the inuence of TBHP/styrene molar ratio in the reaction mixture
Cu–Co PBA catalyst; reaction conditions: Cu–Co PBA catalyst: 3 mg, on the styrene epoxidation with the Cu–Co PBA catalyst. The
styrene: 0.5 mmol, TBHP: 0.75 mmol, acetonitrile: 2 mL, reaction time:
styrene oxide selectivity reached a maximum when the TBHP/
6
h, reaction temperature: 325 K, 335 K, 345 K, 355 K. (b) Effect of
styrene molar ratio was equal to 1.5. Therefore, the optimal
experimental conditions were 6 h, 345 K, TBHP/styrene ¼ 1.5.
The effect of various solvents on the epoxidation of styrene
reaction time on conversion and selectivity with the Cu–Co PBA
catalyst; reaction conditions: Cu–Co PBA catalyst: 3 mg, styrene: 0.5
mmol, TBHP: 0.75 mmol, acetonitrile: 2 mL, reaction temperature: 345
K, reaction time: 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h. (c) Effect of the TBHP/ was summarized in Table 2. We conrmed that aprotic solvents
styrene ratio on conversion and selectivity; reaction conditions: Cu–
Co PBA catalyst: 3 mg, styrene: 0.5 mmol, TBHP: 0.5 mmol, 0.75
facilitated styrene epoxidation, compared to protic solvents
(such as methanol, ethanol), but we did not observed that the
mmol, 1.0 mmol, 1.5 mmol, acetonitrile: 2 mL, reaction temperature:
45 K, reaction time: 6 h.
catalytic activity was directly related to the polarity of solvent,
3
with the Cu–Co PBA catalyst. The solvent effect on the
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Fig. 6 Reusability of the Cu–Co PBA catalyst.
Fig. 4 Infrared spectra of the Cu–Co PBA catalyst before (a) and after
(
b) catalysis.
before and aer catalysis. As shown in Fig. 4, the frequencies of
epoxidation reactions is very complex, as many parameters can the n(CN) band and the d(Co–CN) bands remained constant.
compete with each other: polarity, solubility of reactants and Meanwhile, there was no change in the morphology of the
products, diffusion effects, reactivity of the solvent with the catalyst (Fig. 5), the catalyst material had the mixed morphology
24,25
active center of the catalyst, etc.
In this study, acetonitrile of sphericity and cube.
was the best solvent for the epoxidation of styrene as it gave the
best conversion and epoxidation selectivity.
The catalyst was recovered from the reaction mixture by
ltration, washed with acetonitrile, dried at room temperature
and reused for the epoxidation reaction under the optimal
reaction conditions (acetonitrile, 6 h, 345 K, TBHP/styrene ¼
Stability of the catalyst
1
.5) without any further purication and regeneration process.
To study the stability of the structure of the Cu–Co PBA catalyst
aer the reaction, we compared its IR spectra and TEM images
The Cu–Co PBA catalyst could be reused with similar conversion
and selectivity as fresh catalyst at least four cycles, as shown in
Fig. 6.
Reaction conditions: catalyst: Cu–Co PBA catalyst 3 mg,
styrene: 0.5 mmol, TBHP: 0.75 mmol, solvent: acetonitrile (2
mL), reaction temperature: 345 K, reaction time: 6 h.
Epoxidation of other olens
A similar procedure was employed for the epoxidation of
cyclohexene and norbornene, the results were shown in Table 3.
The conversion of cyclohexene and norbornene were 93%
and 91%, respectively, and the product selectivity achieved 67%
and 99%, respectively. Therefore, it was noteworthy that the Cu–
Co PBA catalyst was not specic of styrene, but it was also a
highly efficient catalyst for epoxidation of other olens.
Kinetic measurements
In order to determine the kinetic parameters, the external or
internal diffusional effects were ignored in this work. As the
a
Table 3 Epoxidation of olefins catalyzed by Cu–Co PBA catalyst
Olen conversion
Selectivity of epoxide
(mol%)
Substrate
(mol%)
Styrene
Cyclohexene
Norbornene
96
93
91
64
67
99
a
Reaction conditions: catalyst: Cu–Co PBA catalyst 3 mg, substrate: 0.5
mmol, TBHP: 0.75 mmol, solvent: acetonitrile (2 mL), reaction
Fig. 5 TEM image of the Cu–Co PBA catalyst before (a) and after (b) temperature: 345 K, reaction time: 6 h, conversion and selectivity were
catalysis.
determined by GC analysis.
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(
1), the 1/(a ꢀ x) had a line relationship with time (t) as listed in
Fig. 7(a). That meant the reaction was second order in whole to
the concentration of TBHP and styrene. (a + b ¼ 2).
1
¼
k
obst þ C
(1)
a ꢀ x
g
a ¼ [styrene] ¼ [TBHP], C ¼ 1/a, kobs ¼ k[catalyst]
When the styrene and TBHP has different initial concentra-
tion, the styrene conversion (x) and the reaction time (t) can be
related applying the integral method through eqn (2).
1
bða ꢀ xÞ
aðb ꢀ xÞ
ln
¼ kobs
t
(2)
ða ꢀ bÞ
g
a ¼ [styrene], b ¼ [TBHP], kobs ¼ k[catalyst]
Fig. 7 (b) Displayed the graph obtained plotting expression (2)
versus reaction time (t). Therefore, the epoxidation of styrene was a

rst order respect to TBHP and styrene, respectively (a ¼ 1, b ¼ 1).
To determine g, the reaction was carried out at different
catalyst concentration and the other conditions was kept
Fig. 7 (a) Plot of 1/(a ꢀ x) vs. reaction time; (b) plot of [1/(a ꢀ b)]ln[b(a ꢀ
x)/a(b ꢀ x)] vs. reaction time; (c) plot of kobs vs. catalyst concentration.
Reaction conditions: acetonitrile (2 mL), reaction temperature: 345 K.
ꢀ
1
ꢀ1
(
0
a) Catalyst: 3 mg, styrene: 0.24 mol L , 0.35 mol L , TBHP:
ꢀ1
ꢀ1
ꢀ1
ꢀ1
.24 mol L , 0.35 mol L ; (b) styrene: 0.12 mol L , 0.24 mol L ,
ꢀ
1
ꢀ1
ꢀ1
TBHP: 0.35 mol L , 0.68 mol L ; (c) styrene: 0.24 mol L , TBHP:
ꢀ1
ꢀ1
ꢀ1
ꢀ1
0
.35 mol L , catalyst: 0.46 g L , 1.41 g L , 2.35 g L .
concentration of styrene, TBHP and catalyst might affect the
reaction rate. We supposed the kinetic equation was r ¼ k
a
b
g
[
styrene] [TBHP] [catalyst] when the reaction temperature was

xed (345 K). To determine a, b, the reaction was carried out
with the concentration of acetonitrile and catalyst xed. When
the styrene and TBHP had the same initial concentration (a),
the styrene conversion (x) and the reaction time (t) can be
Fig. 8 (a) Plot of [1/(a ꢀ b)]ln[b(a ꢀ x)/a(b ꢀ x)] vs. reaction time at
different temperatures; (b) Arrhenius plot of the specific rate kinetic
related applying the integral method through second order eqn constants of styrene oxidation over Cu–Co PBA.
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a
Table 4 Fitting parameters of the experimental points in Fig. 8(a)
Mechanism of styrene epoxidation with the Cu–Co PBA
catalyst
T (K)
Slope
Intercept
R-square
Based on the results of this study, a possible mechanism was
proposed (Fig. 9). The active metal of the PBA catalyst (species 1)
reacted with the TBHP to form a hydroperoxy compound
323
333
343
353
0.41
1.65
3.52
2.26
0.34
0.75
0.99
0.98
0.99
0.97
ꢀ1.02
(species 2) in a preequilibrium step. Species 2 lost a t-butanol
1.78
molecule to render the species 3, which interacted with styrene
to form a metalloepoxy intermediate (species 4), then species 4
decomposed to the product and regenerated to the PBA catalyst.
And styrene oxide attacked the species 3 to form the species 6,
which would generate the PBA catalyst and benzaldehyde, it
explained the formation of the benzaldehyde was more favored
a
Reaction conditions: catalyst: Cu–Co PBA catalyst 3 mg, styrene: 0.5
mmol, TBHP: 0.75 mmol, solvent: acetonitrile (2 mL), conversion and
selectivity were determined by GC analysis.
(
acetonitrile 2 mL, 6 h, 345 K, TBHP/styrene ¼ 1.5), we found
20,28
when the reaction time exceeded 6 h.
that the rate constant (kobs) increased linearly with increasing
catalyst concentration as revealed in Fig. 7(c) suggesting a rst
order dependence to catalyst concentration.
Conclusions
Therefore, the above results suggested a rst order respect to
TBHP, styrene and catalyst, respectively. And the kinetic
expression was r ¼ k[styrene][TBHP][catalyst].
An efficient catalytic system has been studied for liquid-phase
epoxidation of styrene in acetonitrile with tert-butyl hydroper-
oxide. Reaction conditions such as temperature, time of reac-
tion, nature of the solvent were optimized. The catalysts could
be reused four times aer centrifuging and washing with
acetonitrile without signicantly losing their initial epoxidation
activity. In addition, the Cu–Co PBA catalyst was also a highly
efficient catalyst for epoxidation of other olens such as cyclo-
hexene and norbornene. A rst reaction order was found respect
to styrene, TBHP and catalyst concentration. The apparent
To obtain the apparent activation energy of the epoxidation
of styrene, a series of experiments were carried out at 325 K,
3
35 K, 345 K and 355 K, respectively, as shown in Fig. 8(a).
Table 4 listed the tting parameters and the correlation coef-
cients, and the k values were the slope of these ttings. It can

be seen that the plot of ln k versus 1/T was linear between 325 K
and 345 K as shown in Fig. 8(b). An apparent activation energy
ꢀ1
of 100.4 kJ mol was obtained for styrene epoxidation in the
25–345 K, using the Arrheniu ´s equation. However, the line-
ꢀ1
3
reaction activation energy obtained was 100.4 kJ mol . Our
results conrmed that the PBA compounds as heterogeneous
catalysts have potential applications for lower dosage reactions,
and simple preparation processes, thanks to their high catalytic
activity. Further work is under investigation to describe the
mechanism of the epoxidation reaction in detail.
arity was broken at 355 K, this was because the epoxidation of
styrene was a multi-step reaction, and the effect of tempera-
ture was complex, meanwhile, the adsorption quantity of every
step decreased when the reaction temperature exceeded
2
6,27
3
45 K.
Fig. 9 Mechanism proposed for styrene epoxidation with Cu–Co PBA catalysts.
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3 L. Saikia, J. K. Satyarthi, R. Gonnade, D. Srinivas and
P. Ratnasamy, Catal. Lett., 2008, 123, 24–31.
Acknowledgements
This work was supported by the National Natural Science 14 S. H. Lee, I. K. Lee, J. Y. Ha, J. K. Jo, I. Park, C. S. Ha, H. Suh
Foundation of China (Projects 20603014, 20673059, 20973061
and I. L. Kim, Ind. Eng. Chem. Res., 2010, 49, 4107–4116.
and 20903051), the Chinese Ministry of Education (Key project 15 F. Yan, Z. H. Yuan, P. M. Lu, W. Luo, L. M. Yang and L. Deng,
05074), the Committee of Science and Technology of Shanghai
Renew. Energ., 2011, 36, 2026–2031.
Projects nos 0652nm010 and 08JC1408100), and the Funda- 16 J. K. Satyarthi, D. Srinivas and P. Ratnasamy, Appl. Catal., A,
1
(
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17 J. Sebastian and D. Srinivas, Chem. Commun., 2011, 47,
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8 J. Sebastian and D. Srinivas, Appl. Catal., A, 2013, 464–465,
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