Synthesis, characterization and optimization of heterogeneous catalytic cyclohexene oxidation by tungstophospho(aqua)ruthenate via the fractional factorial design methodology

Article abstract of DOI:10.1039/C8NJ02690D

Keggin-type tungstophospho(aqua)ruthenate (PRuW) was synthesized and supported on acid activated montmorillonite under mild conditions. The prepared catalysts were characterized using X-ray fluorescence (XRF) spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), surface area measurements and Fourier-transform infrared (FTIR) spectroscopy, and their catalytic activity in cyclohexene epoxidation was explored. X-ray diffraction indicated that PRuW was properly loaded on the acid activated montmorillonite support. Heterogenization of homogenous catalysts is really interesting as heterogeneous catalysts are recoverable. The optimum conditions were determined using fractional factorial design in which catalyst weight, catalyst loading, reaction temperature, time, reaction stirring and the molar ratio of cyclohexene/H₂O₂ were varied. A first order polynomial equation was developed to investigate the relationship between the responses and operational variables and the fitted model showed a good agreement between the predicted and actual responses. Furthermore, by employing regression analysis, an empirical model was developed to predict the recovery yield of the studied range of conditions.

Full text of DOI:10.1039/C8NJ02690D

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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DOI: 10.1039/C8NJ02690D
New Journal Of Chemistry
Synthesis,
characterization
and
optimization
of
heterogeneous catalytic cyclohexene oxidation by
tungstophos(aqua)ruthenate via the fractional factorial
design methodology
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
Mostapha Zerrouki , Souheyla Boudjema * , Abderrahim Choukchou-Braham and Ilhem Rekkab-
Hammoumraoui
DOI: 10.1039/x0xx00000x
a
www.rsc.org/
Keggin type tungstophospho(aqua)ruthenate (PRuW) was synthesized and supported on acid activated montmorillonite
under mild conditions. The catalysts were characterized by XRF, XRD, TGA, BET and FTIR, and the catalytic activity in
cyclohexene epoxidation. X-ray diffraction indicated that PRuW was properly loaded on acid activated montmorillonite as
a support. Heterogenization of homogenous catalysts is really interesting as heterogeneous catalysts are recoverable.
Optimum conditions were determined using fractional factorial design in which catalyst weight, catalyst loading, reaction
2 2
temperature, time, reaction stirring and the molar ratio Cyclohexene/H O were varied. A first order polynomial equation
is developed to relate the responses and operational variables. The fitted model shows a good agreement between
predicted and actual responses. Furthermore, by employing a regression analysis, an empirical model was developed to
predict
the
recovery
yield
in
the
studied
range
of
conditions.
n-
consisting of large polyanions, [XM O ] (X = P, Si. And M = W,
Introduction
12 40
Mo, V, etc…), counter-cations (protons, alkalines, transition metals,
Catalytic epoxidation of alkenes is both an important industrial
reaction and a useful synthetic method for the production of a wide
variety of fine chemicals because they are derived directly from
5-9
ammonium salts) and crystallization water . They exhibit well
elaborated substitution chemistry in which one or more of the
addenda atoms can be replaced with other transition metal ions,
1, 2
alkenes; a primary petrochemical source
. The oxidation of
10, 11
even by organometallic or organic groups
.
cyclohexene can give rise to a number of products (Scheme 1)
depending on the catalysts and reaction conditions. The 2-
cyclohexene-1-ol and 2-cyclohexene-1-one are used in the
OH
O
+
OH
Epoxide of cyclohexane
Oxidant
Cyclohexane-1,2-diol
OH
3
manufacture of high value pharmaceutical chemicals . Classically,
OH
O
O
Catalyst
epoxidation is carried out using organic peracids as epoxidizing
reagents, or by intramolecular etherification of intermediate
chlorohydrins formed by reaction of alkenes with hypochlorous
acid. Safety and environmental concerns have led to alternative
schemes most notably catalytic epoxidation with alkyl
hydroperoxides and hydrogen peroxide. On the other hand,
transition-metal-catalyzed oxidation of alkenes with molecular
oxygen leads primarily to allylic oxidation products via auto-
T., Slvt.
Cyclohexene
+
+
+
Cyclohex-2-enol Cyclohex-2-enone Cyclohexanol
Cyclohexanone
Scheme 1. Products resulting from the cyclohexene oxidation.
Ruthenium-substituted POMs are of particular interest in this
2
, 4
oxidation pathways rather than epoxides
.
12, 13
field because of unique redox properties of ruthenium
despite
Polyoxometalates (POMs) are early transition metal–oxygen
anion clusters that exhibit a variety of structures in chemical
composition and architecture. Among various POM structural
classes, the Keggin-type POMs occupy an important place in the
research domain. In the solid state, POMs are ionic crystals
they exhibit high reactivity and selectivity in the catalytic oxidation
10
of a variety of organic substrates by O
2
and H
2
O
2
. Nevertheless,
most of the reported studies which have been devoted to synthesis
and structural characterization of Ruthenium substituted silico and
7,
phosphotungstates have been carried out by a number of groups
1
4-18
.
In the last two years, especially Neumann et al. have been
engaged in a large number of studies on the catalytic properties of
7
ruthenium containing polyanion salts . Rong and Pope reported the
III
4-
synthesis of [PW11
O
39R (H
2
O] by electrolytical reduction of a
O).5 H O and characterized in solution
by UV–Vis, P and W NMR, and cyclic voltammetry
solution of Cs [PW11
4
O
39Ru(H
2
2
3
1
183
19
.
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ARTICLE
Journal Name
The major disadvantages of heteropoly acids (HPAs), as Clay samples used in this investigation were obtained from the
catalysts, lie in their low thermal stability, low surface area (1–10 Hammam Boughrara, Tlemcen region (Roussel quarry), west of
2
20
m /g) and separation problem from reaction mixture
.
Algeria. The particles that were lesser than 2 µm fraction of the
Immobilizing soluble acids on an insoluble inorganic support is an sample were separated by sedimentation. The purified clay was
effective way to transform the high-performance of soluble acids activated with HCl in concentrations of 1 M by slowly adding the
2
1
into heterogeneous solid catalysts . Acid modified clay minerals solutions to dry materials at 80 °C, with continuous stirring for 4 h.
2
2
can be used as an efficient support because they exhibit higher The resultant obtained solution was cooled, filtrated and the
surface area, pore volume, pore diameter and higher surface resultant material was washed with deionized water and dried at 80
acidity. In particular, acid activated montmorillonite has been °C. The resulting material was called Hmont.
widely used as an inorganic support for the immobilization of
homogeneous catalysts to construct various heterogeneous Synthesis of catalyst (x % PRuW/Hmont)
23-26
catalysts for various chemical reactions
. Using clay catalysts,
one can conduct environmentally benign green chemistry both at Mono ruthenium substituted phosphotungstates/acid-activated
2
7
industrial level and on laboratory scale
.
montmorillonite was prepared according to the method described
3
4
in the literature . PRuW was dissolved in water, and the resulting
In this paper, we report the synthesis of mono ruthenium solution was added slowly to 10 or 20 wt.% of acid activated
3
-
40
substituted phosphotungstates (PRuW) from [PW12O ] (aq) and montmorillonite. The solvent was evaporated under vacuum and
RuCl (aq). The synthesized polyoxometalate was supported on acid wet samples were dried in an oven for 4 h at 100 °C. The catalyst
3
activated montmorillonite by impregnation method. The support was calcined at 250 °C for 4 h.
and the new catalysts have been characterized by FT-IR, XRD,
particle size distribution and surface area measurement (BET Characterization
method), and thermo gravimetric analyses (TGA). Their catalytic
properties were evaluated for cyclohexene epoxidation using The composition and weight percentages of P, Ru, and W in the
hydrogen peroxide as oxidant. This reaction is influenced by several samples were evaluated by the XRF technique using a Thermo
factors such as catalyst weight, catalyst loading (% of the active Electron ARL-ADVANT¨XP X-ray fluorescence spectrophotometer.
phase), reaction temperature (°C), reaction time (h), the molar ratio Quantization was carried out using Thermo Electron UNIQUANT
(
cyclohexene/oxidant H O rate stirring, etc... The experimental software. Powder X-ray diffraction (XRD) data were collected on
2 2),
design is a suitable tool to recognize significant variables that affect Rigaku D/max2500 diffractometer with Cu Kα radiation (λ =
the process and to determine optimal conditions in several 1.541874 Å) in the range 2θ = 2 ° - 70 ° with a step of 0.02 ° and an
2
8-30
processes with limited experimental runs
. The technique of acquisition time of 1 s. The thermogravimetry analysis (TGA) was
31-33
experimental design is largely used in chemical industry
. For conducted on TA instruments 2960 SDT V3 thermal analyzer with a
-1
this reason, the aim of our work is to optimize the conditions of the heating rate of 10 °C.min from room temperature to 800 °C. The
epoxidation reaction of cyclohexene using PRuW supported acid specific surface areas of the samples were determined by Brunauer-
activated montmorillonite as catalyst and
2 2
H O as oxidant Emmett-Teller (BET) method using N2 adsorption–desorption at 77
e
following a fractional factorial design looking to get a good K on Quantachrome Instruments (Nova 1000 ), and the porosity
conversion (Y
1
) with good selectivity (Y
2
) in cyclohexene oxide.
distribution was obtained from the desorption branch of the
isotherm using Barrett-Joyner-Halenda (BJH) analysis. The sample
was outgassed at 150 °C for 3 h. The FTIR spectra of the solid
samples were recorded using an Agilent Technology Cary 60 series
FTIR spectrometer, with ATR accessories, and a measuring range of
Experimental
Synthesis of mono ruthenium substituted phosphotungstates
PRuW)
-1
4
000–400 cm .
(
Catalytic performance tests
About 2.4 g of H
containing 0.6 mL acetic acid and the pH was adjusted to 5.2 using
2
sodium bicarbonate solution. The hot solution of 0.25 g RuCl .3H O
3 2
PW12O40.n H O was dissolved in 10 mL of water
The liquid-phase epoxidation of cyclohexene was carried out in a
3
round bottom flask, immersed in
a thermostated oil bath
dissolved in minimum amount of water was added to the previous
solution. The final pH of the solution was adjusted to 4.8 using
acetate buffer. The solution was heated at 80 °C by stirring for 30
min and filtered hot. The filtrate was refrigerated overnight. The
solution was then filtered and washed with acetone to obtain the
black brown product. The filtrate was used for the estimation of
maintained at required temperature. A typical reaction mixture
contains 30 mmol of cyclohexene; 30 mmol of 30 % H O , 10 mL of
2
2
acetonitrile, 25 mg of catalyst and 1 mL of 1,2 dimethoxyethane as
internal standard. The reaction mixture was stirred with a teflon-
coated magnetic bar. On completion of the reaction, the liquid
products were analyzed by GC. The conversion and selectivity were
calculated by means of the following expressions:
1
2
tungsten in order to see the loss during the synthesis
.
Synthesis of acid activated montmorillonite (Hmont)
2
| J. Name., 2012, 00, 1-3
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ARTICLE
ꢀ
ꢁꢂꢃꢄꢅꢆꢇꢁꢂꢈꢉ%ꢊ
Results and Discussion
ꢌꢁꢍꢄꢆꢈꢁꢎꢈꢇꢂꢇꢏꢇꢐꢍꢈꢆꢑꢒꢆꢏꢅꢐꢏꢄ ꢓ ꢈꢌꢁꢍꢄꢆꢈꢁꢎꢈꢅꢄꢆꢇꢔꢑꢐꢍꢈꢆꢑꢒꢆꢏꢅꢐꢏꢄ
ꢋꢈ
ꢈxꢈ100
ꢌꢁꢍꢄꢆꢈꢁꢎꢈꢇꢂꢇꢏꢇꢐꢍꢈꢆꢑꢒꢆꢏꢅꢐꢏꢄꢈ
Characterization of catalyst
ꢌꢁꢍꢄꢆꢈꢁꢎꢈꢇꢂꢔꢇꢃꢇꢔꢑꢐꢍꢈꢘꢅꢁꢔꢑꢖꢏ
ꢕꢄꢍꢄꢖꢏꢇꢃꢇꢏꢗꢈꢉ%ꢊ ꢋ
ꢈxꢈ100
ꢌꢁꢍꢄꢆꢈꢁꢎꢈꢏꢁꢏꢐꢍꢈꢘꢅꢁꢔꢑꢖꢏꢆ
The results of XRF analysis listed in Table 2 indicate that the actual
contents of tungstophospho (aqua)ruthenate in the samples are
similar to the contents (10 and 20 wt.%) in the synthesis mixtures.
Fractional factorial design and statistical analysis
2 2 3
The highest proportions of SiO and Al O agree with the nature of
the clay support. Moreover, for the impregnated samples, the initial
Si, P, W and Ru amounts in PRuW/Hmont were found again (within
1.5 wt.% experimental errors) at the end. In addition, the molar
ratio of W/P and Ru/P in the supported samples is close to 1:11,
suggesting that the lacunary Keggin structure unit is well preserved.
Table 1. Variables and experimental domains
Variables
Level (-1)
Level (+1)
A: catalyst weight (mg)
B :catalyst loading (% PRuW/Hmont)
C: temperature (°C)
D : reaction time (h)
E: stirring (Tr/min)
2
1
60
3
5
0
75
20
80
9
800
1/3
Figure 1 shows the X-ray powder diffraction patterns of acid
activated montmorillonite (Hmont), PRuW and PRuW/Hmont
catalysts with PRuW loading of 10 and 20 %. The XRD pattern of
PRuW shows its principal diffraction angles at 10.2 °, 14.5 °, 17.9 °,
F: molar ration cyclohexene/H
2
O
2
400
1
/1
Fractional factorial designs are designs of experiments consisting of
fraction runs of a full factorial design with less experimental runs to
3
6
2
0.7°, 29.5 °, 31.3 ° and 34.5 ° . The XRD patterns for Hmont
3
5
reveal one peak at 2 = 28.1 °, which is due to quartz, and there is no
effect on the 2 value of quartz even after supporting Hmont with
PRuW, either. Bulk PRuW is crystalline in nature, but it was
observed that while loading PRuW on Hmont, some of its
crystallinity was lost. After impregnation, the reflection of the
polyacid was not detected, indicating that PRuW was in amorphous
state and randomly distributed on the montmorillonite surface or
incorporated in the pore of the support.
find out important variables using FFD . In this work, a two-level
6
-2
fractional factorial design 2IV , with six factors and two variation
levels, was used for the study of the cyclohexene oxidation reaction
being analyzed by the ANOVA test. The studied factors were:
catalyst weight (A), catalyst loading (B), temperature (C), reaction
time (D), stirring (E) and molar ration cyclohexene/H
2 2
O (F). The
variation levels in the codified form, based on literature data, are
6
-2
presented in Table 1. The 2IV
fractional factorial design is
4
obtained by writing down the complete 2 factorial as the basic
design (with the four variables A, B, C and D) and then equating
factors E and F to the ABD and BCD interactions, respectively. The
last conditions allow us to identify the independent generators (1)
and the corresponding complete defining relation (2) which is the
set of all columns that correspond to the identity column I:
E.E = I =ABDE & F.F = I = BCDF
I = ABDE = BCDF = ACEF
(1)
(2)
The experimental data on two responses (The conversion of
cyclohexene (Y ) and the combined selectivity of cyclohexene oxide
)) have been modeled by the first order regression equation as
follows:
1
(Y
2
ꢞ
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ꢚ
ꢛ ꢜ ꢒ
ꢝ
ꢡ
ꢝ
ꢛ
ꢜ
ꢝꢢ ꢝ ꢢ
ꢒ ꢡ ꢡ ꢛ ꢄꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢉ3ꢊ
ꢝꢟꢠ
ꢝꢟꢠꢈꢢꢟꢠꢣꢝ
Figure 1. XRD patterns of (a) HMont, (b) 10% PRuW/HMont, (c) 20%
Where Y is the predicted responses, X are the uncoded or coded
i
PRuW/HMont, (d) PRuW.
0
values of the factors, b is a constant, bi are the main effect
coefficients for each variable and bij are the interaction effect
coefficients. The significance of factors and their interactions have
been evaluated by the analysis of variance (ANOVA) using the
Design Expert®7 software.
.
Table 2. Chemical composition of catalysts from XRF.
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Journal Name
Catalyst
(wt.%)
[
a]
[a]
[a]
SiO
-
2
Al
2
O
3
Fe
-
2
O
3
CaO
-
MgO
-
Na
-
2
O
K
2
O
TiO
-
2
P
W
Ru
PRuW
Hmont
-
-
0.98
-
10.99
-
0.99
-
57.12
55.31
53.82
19.87
22.20
23.12
2.62
2.64
2.92
0.43
0.01
0.01
3.10
1.80
2.01
0.30
0.09
0.09
2.70
3.28
3.00
0.24
0.22
0.21
1
2
0%PRuW/Hmont
0%PRuW/Hmont
0.99
1.05
11.00
10.98
0.99
0.99
[a] These numbers correspond to the numbers of atoms deduced from the raw wt.% found from XRF, as the number of W atoms is fixed to 11.
The surface area and pore volume of Hmont and different wt% of
2
PRuW on Hmont are given in Table 3. The surface area (195 m
-1
g ) and total pore volume (0.27 cc/g) of Hmont decrease as the
amount of PRuW loading increases. For 20%PRuW loading the
2
-1
surface area becomes 42 m
g
while the total pore volume
becomes 0.06 cc/g only. The acid-leaching has fundamentally
transformed the layered Montmorillonite into an amorphous silica-
like structure. The high surface area of acid activated
montmorillonte is related to the removal of some of the aluminum
from octahedral sites in the clay. The reduction in surface area
and pore volume upon PRuW loading may be due to the blockage
3
7
of pores by PRuW molecules (~ 12 Å)
being smaller than the
average size of mesopore (~ 38 Å) which usually leads to a
decrease in pore diameter .
Table 3. Textural properties of various samples.
Catalyst
S
BET
2
Pore V
/cm g
Av.pore
radius/Å
d
001
-1
3
-1
/m g
/Å
Hmont
195
143
42
0.27
0.21
0.06
1.89
1.91
1.90
1.65
1.84
2.09
1
2
0%PRuW/Hmont
0%PRuW/Hmont
Similar observations have been reported by other authors
who used various montmorillonite (clay) as support
2
3-26, 34, 38-43
heteropolyacids,
decreased the surface area and
porosity (mesopores contribution) especially for Hmont. The 10%
deposit on PRuW Hmont has less significant impact on the specific
surface area and porosity in the case of 20% PRuW / Hmont. The
mesopores are partially filled with PRuW. The pore blocking effect
with PRuW is more noted for the catalyst 20% PRuW / Hmont.
Figure 2. N adsorption-desorption isotherms (A) and pore size
2
distribution curves (B) of: (a) 20%PRuW/Hmont, (b)
1
0%PRuW/Hmont and (c) Hmont.
Typical FT-IR spectra of the PRuW and supported catalysts are
shown in Figure 3. The spectra show a band around 3600 cm1 for all
the samples. This is due to asymmetric stretching of OH group. The
adsorption band at 1620–1640 cm1 is due to H–O–H bending
vibration in water. The broad band stretching from 1000 to 1200
The pore size distribution plot in Figure 2 (b) shows that the
average pore sizes of Hmont and different wt% of PRuW/Hmont
fall in the range of 25-30 Å suggesting that pore sizes of the
catalysts lie in the mesoporous region, i.e. > 20 Å. The adsorption–
desorption isotherms of Hmont and x% PRuW/Hmont in Figure 2(a)
show that they have the form of Type IV isotherms with the
hysteresis loop of type H3, which is a characteristic of a
mesoporous solid.
-
1
cm is due to a Si-O stretching vibration, and the shoulder at 917
-
1
cm is indicative of -OH groups bonded to the Al atoms. The Keggin
structure of bulk PRuW was identified by five characteristic
-
1
absorption bands at 1056 cm (stretching frequency of P–O in the
-
1
central PO tetrahedron), 924 cm (terminal bands of exterior WO),
4
-
1
8
63 cm (stretching vibration of W–O
b
–W bridges between corner-
-
1
sharing octahedra) and 735 cm (stretching vibrations of W–O –W
c
4
4
-1
bridges between edge sharing octahedral) . The band to 631 cm
4
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is attributed to the grouping Ru-O. FT-IR spectra of 10%
The thermal analyses of selected catalysts have been carried
PRuW/Hmont and 20% PRuW/Hmont indicate that the primary out on PRuW, Hmont, 10% and 20% PRuW/Hmont catalysts to
Keggin structure is preserved in both cases on acid activated determine the thermal stability of the catalyst. The results of these
montmorillonite (Hmont). When the characteristic peaks of analyses are reported in Figure 4. Acid activated montmorillonites
unsupported PRuW and Hmont are superimposed, the peaks for the exhibit an endothermic signal below 100 °C which represents the
supported catalysts appear without any shift in the peak position. dehydration of the smectic structure. A steady loss of mass of about
-
1
+
Sharp peak at 1631 cm indicates the presence of H
3
O
ion 17 %, up to 500 °C, was attributed to the loss of physisorbed and
interlayer water and to the dehydroxylation caused by the breaking
4
5
(
bronsted acidity) .
4
6, 47
of structural OH-groups in the support
. The TGA of PRuW
shows a weight loss of about 10 % up to a temperature of 210 °C,
indicating loss of free and adsorbed water. The gradual weight loss
of about 3 % up to 500 °C corresponds to the mass loss due to the
reaction between acidic protons and structural oxygen from
tungstophos(aqua)ruthenate acid, releasing water, followed by
4
8
3 x
decomposition to WO and PO species .
Generally, TGA and DSC profiles for PRuW containing catalysts
are similar to those corresponding to appropriate montmorillonite
supports. The TGA of supported catalysts showed a mass loss of
about 18 % within the temperature range of 80 – 150 °C due to a
loss of adsorbed water molecules. A gradual mass loss of about 2 %,
up to 500 °C, was also observed, which indicates an increase in the
Figure 3. FTIR spectra of acid-activated montmorillonite and thermal stability of PRuW on Hmont support. This might be due to
PRuW/Hmont
catalysts
with
various
loadings
of the formation of intermolecular bondings between the support and
the heteropoly acid; it indicates the presence of a chemical
tungstophospho(aqua)ruthenate.
2
3, 49, 50
interaction between them
.
Fig 4. TGA and DTG curves of different materials
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A
[
a]
Table 4. Oxidation of cyclohexene using different catalysts
.
Selectivity
Catalyst
Conversion
Cyclohexene oxide
Cyclohexane-1,2 diol
Allylic products
TON*
TOF**
Hmont
PRuW
20
79
36
69
0
0
0
-
-
55
28
30
35
63
58
10
-
-
1
2
0%PRuW/Hmont
0%PRuW/Hmont
9
12
1078
1033
180
172
[a] Reaction conditions: 0.025 g of catalyst, 30 mmol of cyclohexene, 30 mmol oh H
2
O
2
30 %, 10 mL CH
3
CN, 70 °C, 6h
*Turnover number (TON) is given as moles of products/ mol of catalyst.
**Turnover frequency (TOF) is defined as: converted cyclohexene mole per hour divided by the Ru and W catalyst mole used.
decreases with the increase in the content of PRuW and of the TOF
-1
-1
of 180 h and 172 h were calculated for the contents 10 % and
Catalytic performance and screening
20%, respectively (Table 04).
Metal-catalyzed oxidations with hydrogen peroxide can be
The mains products of cyclohexene epoxidation are shown in conveniently divided into two categories, involving peroxometal
scheme 1. Oxidation of the double bond carbon-carbon of and oxometal species, respectively, as the active oxidant.
cyclohexene with peroxides yields the corresponding epoxide which Peroxometal pathways are typically observed with early transition
0
upon further reaction with water, produces diols (Scheme 1). metal ions with a d configuration, e.g. W(VI), that are relatively
Oxidation of the allylic C-H bond results in alcohol (-ol) which can weak oxidants. Oxometal pathways are characteristic of late
get further oxidized into a ketone product (-one).
transition elements and first row transition elements, e.g., Ru(VI)
and Ru(VIII), that are strong oxidants in their highest oxidation
states. It is also worth noting that an oxometal intermediate can be
formed via heterolysis of the O-O bond of an initially formed
peroxometal species. The metal ion increases the oxidizing power
For cyclohexene oxidation with H , solid acid catalysts
2 2
O
including 10% PRuW/Hmont, 20% PRuW/Hmont, PRuW and Hmont
were screened. The reaction without catalyst (blank) was also
studied by using identical reaction parameters: 0.025 g catalyst, 30
(
electrophilicity) of the peroxo moiety by withdrawing electrons, i.e.
mmol cyclohexene, 30 mmol H
2
O
2
, 10 mL CH
3
CN, 70 °C and 9 h
51
by acting as a Lewis acid
.
reaction time. Efficacy of these catalysts was found as follows:
PRuW > 20%PRuW/Hmont > 10% PRuW/Hmont > Hmont > blank
(
Table 4). Out of them, PRuW catalyst gave the highest cyclohexene
conversion of 79%, followed by 20% PRuW/Hmont, 10%
PRuW/Hmont and Hmont gives 69, 36, 20 %, respectively. The
catalytic systems of supported samples proved to be highly active
and selective towards diol. The reaction is directed towards
oxidation way and not towards allylic way. A blank reaction was
performed in the absence of catalyst and no product was detected.
When the acid activated montmorillonite used as catalyst, only a
negligible catalytic activity was evidenced, and no desired product
was detected. The obtained results clearly indicate that the POM
species are responsible for the high oxidation activity of these
heterogeneous catalysts. Besides, the large pores of the Hmont
support permit the diffusion of a relatively large quantity of
cyclohexene and oxidation proceeds selectivity inside the pores of
the support. The catalytic performance of the heterogeneous
catalysts is also related to the fine dispersion of the active catalytic
species within the support as shown by different characterization
techniques. Moreover, the acid Brønsted sites of PRuW, acid Lewis
sites of support and redox properties of PRuW are shown to have a
crucial effect on the oxidation process.
Scheme 2. Peroxometal and oxometal pathways in alkene
5
1
epoxidation
.
The mechanistic pathway for the oxidation of alkenes using a
52
dioxygen species has been reported by Neumann and Dahan . It
has been proposed that oxidation of substrate in presence of
transition metal compound proceeds by a metal catalyzed auto
2
oxidation reaction by forming M–O intermediate. This type of auto
oxidation reaction; therefore, gives a possibility of achieving
epoxidation of alkenes by an addition reaction. As a result,
oxidation way is favored. As proposed by R. Neumann, the metal
centre, i.e. Ru (III) was first reduced to Ru (II) and this species then
attacks the reaction site and activation of catalyst takes place. In
the present catalytic system, ruthenium is already in Ru (II) state as
a result the catalyst directly interacts with the oxidant forming an
activated complex (i.e. metal-oxo intermediate). This intermediate
The TONs of the heterogeneous catalysts (x%PRuW/hmont)
are respectively 1078 and 1033. The increase of catalyst loading
induces a decrease in TON to 1033. The activity of catalysts
6
| J. Name., 2012, 00, 1-3
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ARTICLE
favors epoxidation of cyclohexene. As the reaction time increases, ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢛꢈ0.23ꢈꢈEꢈ ꢛ ꢈ2.98ꢈꢈFꢈꢈꢈ ꢓ 3.74ꢈꢈAB ꢛ ꢈ2.00ꢈDFꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢉ4ꢊ
more and more of the activated species interact with the
Yꢤ ꢋ 68,612 ꢛ 1,174ꢈA ꢛ 2,172ꢈB ꢓ 0,845ꢈC ꢛ 4,378ꢈD
cyclohexene giving rise to epoxide.
Generally, the reaction is believed to proceed through the
ꢓ0,469ꢈE ꢓ 3,650ꢈFꢈꢈ ꢓ 1,379ꢈABꢈ ꢛ 1,891ꢈACꢈ
mechanism postulated for Keggin type POMs. Monomeric, dimeric,
and tetrameric peroxo species are generated by the reaction of
ꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢓ1,462ꢈAEꢈ ꢓ 1,308ꢈDF ꢓ 1,021ꢈCEꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢈꢉ5ꢊꢈꢈꢈꢈ
polyoxometalates with hydrogen peroxide, and the peroxo species
2
are supposed to be the active species for epoxidation of olefins
The coefficient of determination (R ) values of these reduced
53-55
within a POM/H
2
O
2
system
. It has been postulated that the models (Equations (4) and (5)) was recalculated as 0.9692 and
epoxidation reaction rate and epoxide yield, for the same type of 0.9889, respectively. This indicated good correlation between the
5
6,
catalytic species, are determined by the catalyst’s acid strength
observed and the predicted values.
5
7
.
From the obtained results, estimates of aliases of main effects
and two variable interaction effects, which were computed using
Expert vcatalyst weight (A), catalyst loading (B), reaction time (D),
In conclusion, our reaction catalyzed by PRuW and
x%PRuW/Hmont can operate via both pathways, i.e. peroxometal
and oxometal pathways.
2 2
stirring (E) and the molar ratio cyclohexene / H O (F), represent
that these factors have positive effect on the cyclohexene
conversion while the temperature of reaction (C) has a negative
impact on response 1 by a negative coefficient. In the case of
Experimental design results
In the case of processes influenced by multiple variables, statistical response 2 (equation 5), the factors A, B, D have a positive
experimental design has been shown to be a powerful tool for coefficient and effect on the combined selectivity of cyclohexene
determining the effects of operational factors and their oxide. The temperatures of reaction and molar ratio cyclohexene /
interactions; this allows process optimization to be conducted H O have a negative effect.
2
2
effectively. This technique has been widely used in the chemical
The best way to identify relation between the factors and
3
5
industry . A total of 16 experiments has been performed according responses is examination of surface plots as function of two factors
6
-2
58
to 2 FFD with sixteen duplicated experimental runs and four by holding the other factors at constant levels for each plot . The
central points. Table 5 shows the experimental design matrix and 3D surface plots between the responses and the factors are shown
the obtained results of responses. The cyclohexene conversion was in Figure S1 (See supporting information). These results illustrate
found between 34 % and 83 %, whereas the combined selectivity of the responses of different experimental factors and can be used to
cyclohexene oxide was ranging from 56 % to 86 %, at the studied identify the major interactions between the factors. The significant
conditions.
interactions were selected for the reason that they are the most
efficient for each response. It can be observed that cyclohexene
conversion increases with the increase of catalyst weight and
reaction time and the decrease of catalyst loading and the molar
The main results of ANOVA are represented in Table S1 (See
supporting information). P-value less than 0.05 indicated that the
model terms are significant. The model F-values were determined
ration of Cycohexene/ H
2 2
O (Figure S1). The response in Figure S2
1 2
as 10.85 and 38.02 for Y and Y , respectively, indicating that both
(
See supporting information) shows a curvature, which indicates a
models were significant. The correctness and goodness of the
models were checked by the coefficient of determination values.
good interaction effect between all factors on the selectivity of
cyclohexene oxide. The results showed also that the selectivity
generally increased with the increase in catalyst weight, catalyst
loading and the reaction time. The highest selectivity was obtained
when the catalyst weight, catalyst loading and reaction time were
at the maximum value and other factors (reaction stirring and molar
ratio) were at the minimum value within the range studied.
2
The R values of full first-order polynomial regression models were
1 2
found as 0.9197 (adj. 0.8869) and 0.9843 (adj. 0.9557) for Y and Y
respectively. For the conversion of cyclohexene (Y ), reaction time
1
was found to have the greatest effect with the highest F-value of
141.235 as shown in Table S1. However, the catalyst weight had
less effect than the reaction time as its F-value in Table S1 is 14.789,
while the stirring showed very little effect on response 1 with F-
value of 0.06. Furthermore, the variables A, B, C, D and F were the
most statistically significant model terms for the response 2 with p-
value < 0.0001.
The statistical analysis of the Design-Expert software gave a
numerical solution of optimum conditions with desirability value of
0
.995 for the epoxidation of cyclohexene based on maximum
conversion and selectivity. The optimum factors determined by
numerically using software with high desirability were as follows: 75
mg of catalyst, 10% of catalyst loading (10%PRuW/Hmont), reaction
temperature of 80 °C, 9 h of reaction, reaction stirring at 400 Tr/min
Table S2 (See supporting information) shows estimates of
these effects. The fitted model and the experimental data are
within 95 % confidence level. The fitted first order polynomial
models with significant coded factors are as follows:
2 2
and the molar ratio cyclohexene/H O of 1/1. Under these
conditions, the cyclohexene conversion was 80 % with 90 % of
selectivity of cyclohexene oxide.
Yꢠ ꢈꢋ 59.07ꢈ ꢛ ꢈ3.56ꢈꢈAꢈ ꢛ ꢈ1.62ꢈꢈBꢈ ꢓ ꢈ0.96ꢈCꢈ ꢛ ꢈ11.00ꢈDꢈ
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ARTICLE
Journal Name
A
6
- 2
Table 5. 2 IV
fractional factorial design experiment
a
b
Run
Catalyst
weight
A
Catalyst
loading
B
Temperature
Reaction
time
D
Stirring
Molar ratio
Conversion
(%)
Selectivity
2
Y (%)
TON
TOF
C
-1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
E=ABD
F= BCD
Y
1
1
2
3
4
5
6
7
8
9
-1
1
-1
-1
1
-1
-1
-1
-1
1
-1
1
-1
-1
1
33.91
49.50
46.14
45.23
37.28
53.16
45.82
41.89
72.20
83.16
71.14
69.58
51.50
75.14
76.09
75.67
66.25
68.11
71.86
66.10
56.41
62.06
66.51
76.28
72.72
71.05
81.37
83.12
77.61
85.69
71.23
69.96
1015
494
338
165
230
75
-1
1
1
691
1
-1
-1
1
1
225
-1
1
-1
-1
1
1
1116
530
372
177
229
70
1
1
-1
1
1
1
-1
-1
1
686
1
1
-1
1
209
-1
1
-1
-1
1
-1
-1
-1
-1
1
2161
830
240
92
1
1
1
1
1
1
0
1
2
3
4
5
6
-1
-1
1
1
-1
1
-1
-1
-1
-1
1
1065
347
118
39
1
-1
1
-1
-1
1
1
1541
750
171
83
1
-1
-1
1
-1
1
1
1139
377
126
42
1
1
1
1
a
Turnover number (TON) is given as moles of products/ mol of catalyst.
b
Turnover frequency (TOF) is defined as: converted cyclohexene mole per hour divided by the Ru and W catalyst mole used.
Table 6. A summary of catalysts used in the literature for the oxidation of cyclohexene with different oxidants.
Catalyst
Cs
Oxidant
Conversion
Selectivity
Ref.
5
9
3
PW12O
40
H
2
O
2
/
Cyclohexane 1,2-diol = 47 %
Adipic Acid= 24%
Allylic products = 90.4 %
2
PW11Co
ZrO -PW12O40
H
H
2
O
O
2
2
27.9 %
28 %
6
6
6
6
0
2
2
cyclohexenone =34 %
cyclohexenenol = 27 %
Cyclohexene oxide = 0 %
Cyclohexane 1,2-diol = 17 %
cyclohexenone = 34 %
cyclohexenenol = 30 %
Cyclohexene oxide = 0
Cyclohexane 1,2-diol = 17 %
cyclohexenone = 17 %
cyclohexenenol = 9%
Cyclohexene oxide = 9
Cyclohexane 1,2-diol = 51 %
cyclohexenone = 49 %
cyclohexenenol = 8%
Cyclohexene oxide = 25 %
Cyclohexane 1,2-diol = 14 %
Cyclohexene oxide 90%
0
0
0
ZrO
2
-PW11TiO40
H
H
H
H
2
O
2
O
2
O
2
O
2
2
2
2
43 %
57 %
33 %
H
3
PW12O
40
3 40
TBA [PW12O ]
1
0% PRuW/ Hmont
80%
Present work
6
1
1
% Au/GaW11-APTES@SiO
2
O
2
69.22 %
Cyclohexene oxide = 4.6 %
cyclohexanol= 56..87 %
cyclohexanone = 28.38 %
Cyclohexene oxide = 95.9 %
6
6
6
2
2
3
PW12/Mg-Al LDHRe
PW12/Mg-Al LDHRe
O
O
2
2
100 %
100 %
22 %
Cyclohexene oxide = 95.9 %
5
% Ru/H-Mont
TBHP
cyclohexenenol = 33 %
cyclohexenenone = 67 %
6
3
5
% Ru/Ti-PILC
TBHP
59 %
cyclohexenenol = 13 %
cyclohexenenone = 87 %
8
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8NJ02690D
New Journal Of Chemistry
For the factorial design, we note a reduction in turnover frequency 5. M. T. Pope, Y. Jeannin and M. Fournier, Heteropoly and
TOF) with the increase in the catalyst weight, the temperature and isopoly oxometalates, Springer-Verlag Berlin, 1983.
(
catalyst loading (% PRuW/Hmont) beside an increase in TOF with
the increase in stirring time and molar ratio (table 5).
6
. M. Pope and A. Müller, Polyoxometalate chemistry from
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. L. Bi, F. Hussain, U. Kortz, M. Sadakane and M. H. Dickman,
Chem. Commun., 2004, 1420-1421.
. C. Hu, X. Zhang, L. Xu, B. Mu, W. Zu and E. Wang, Appl. Clay
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ChemCatChem, 2017, n/a-n/a.
7
Table 6 shows some literature results for cyclohexene oxidation.
However, our results were better to the values reported in many
8
works
.
9
Conclusions
1
2
1
1
3
1
0. L.-H. Bi, M. H. Dickman, U. Kortz and I. Dix, Chem. Commun.,
Keggin type tungstophospho(aqua)ruthenate (PRuW) was
synthesized by one pot and easy method from the parent 12-
tungstophosphoric .The synthesized polyoxometalate was
successfully supported on acid activated montmorillonite by
impregnation process. Characterization of these catalysts showed
that PRuW is well dispersed on the surface of montmorillonite. IR
bands show the Keggin structure of HPA and XRF analysis showed
that the procedure used gives the expected materials. The prepared
heterogeneous catalysts showed high catalytic activity for
cyclohexene epoxidation. They are not expensive and can be used
005, 3962-3964.
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,
796-3800.
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1
1
1
1
993, 32, 1068-1071.
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6
-2
in heterogeneous media. In the present work, a 2 fractional
6. R. Neumann and C. Abu-Gnim, J. Chem. Soc. Chem.
factorial design of experiments has been employed to study and Commun., 1989, 1324-1325.
optimize the impact of catalyst weight, catalyst loading, 17. M. Sadakane, D. Tsukuma, M. H. Dickman, B. Bassil, U. Kortz,
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the conversion and selectivity. The optimal process conditions 19. C. Rong and M. T. Pope, J. Amer. Chem. Soc., 1992, 114
were: 75 mg of catalyst, 10
of catalyst loading 2932-2938.
0. D. P. Sawant, A. Vinu, J. Justus, P. Srinivasu and S. B.
2 2
O
on the epoxidation of cyclohexene to maximize 18. M. Higashijima, Chem. Letter., 1999, 28, 1093-1094.
,
%
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(
10 %PRuW/Hmont), reaction temperature of 80 °C, 9 h of reaction,
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reaction stirring at 400 Tr/min, and the molar ratio
cyclohexene/H of 1/1. Under these conditions, the cyclohexene
2
6
2
2
3
2
2
1. A. Liu, B. Liu, Y. Wang, R. Ren and Z. Zhang, Fuel, 2014, 117
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2
O
2
8-73.
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Acknowledgements
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The authors would like to thank the General Directorate for
Scientific Research and Technological Development (DGRST) for the
financial support.
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