Preparation and characterization of H3?2(x+y)MnxCoyPMo12O40 heteropolysalts. Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide

Article abstract of DOI:10.1016/j.crci.2019.01.003

H_3?2(x+y)Mn_xCo_yPMo₁₂O₄₀ heteropolysalts (x + y ≤ 3/2 and x, y: 0–1.5) were prepared by a cationic exchange method based on barium sulfate precipitation. Structural and textural properties of salts were examined by several physicochemical techniques such as infrared, scanning electron microscopy-energy dispersive X-ray, and ³¹P nuclear magnetic resonance spectroscopies, X-ray diffraction diffraction, and thermogravimetric analysis, and their catalytic properties were evaluated in the cyclohexanone oxidation using hydrogen peroxide (30%). The reaction products, adipic, glutaric, succinic, hexanoic, 6-hydroxyhexanoic, 7,7-dimethoxy, and heptanoic acids and 1,1-dimethoxy octane were identified by gas chromatography–mass spectrometry analysis. Only adipic, glutaric, and succinic acids were quantified by chromatography (high-performance liquid chromatography), the other products were noted X. Adipic acid (AA) is the major product for all systems. The effects of molar ratios of catalyst/reactant and cyclohexanol/cyclohexanone, heteropolysalt composition, and reaction duration on AA yield were investigated. The stability of the catalytic system was also examined. H_3?2(x+y)Mn_xCo_yPMo₁₂O₄₀ catalysts were found to be efficient for the cyclohexanone oxidation with conversions >95%. Among them, H₁Mn_0.25Co_0.75 exhibits the highest AA yield (75%).

Full text of DOI:10.1016/j.crci.2019.01.003

C. R. Chimie xxx (xxxx) xxx
Contents lists available at ScienceDirect
Comptes Rendus Chimie
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ꢀ
Full paper/Memoire
Preparation and characterization of
H
3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts. Application to
adipic acid green synthesis from cyclohexanone oxidation
with hydrogen peroxide
ꢀ
ꢀ
ꢀ ꢀ
Preparation et caracterisation des heteropolysels
ꢁ
ꢁ
H
3ꢀ2(xþy)MnxCoyPMo12O40. Application a la synthese verte de l'acide
adipique par oxydation de la cyclohexanone avec du peroxyde
ꢁ
d'hydrogene
,
b
,
,
Sihem Mouanni a, Tassadit Mazari a *, Dahbia Amitouche a b, Sihem Benadji a,
Leila Dermeche a , Catherine Roch-Marchal , Cherifa Rabia
Laboratoire de chimie du gaz naturel, Faculte de chimie, USTHB, BP32, El-Alia, 16111 Bab-Ezzouar, Alger, Algeria
Laboratoire de chimie appliquee et de genie chimique, Universite Mouloud-Mammeri (UMMTO), Tizi-Ouzou, Algeria
ILVeUMR 8180 CNRS, Batiment Lavoisier, Universite de Versailles-Saint-Quentin-en-Yvelines, 45, avenue des Etats-Unis, 78035
,
b
c
a
ꢀ
a
ꢀ
b
c
ꢀ
ꢀ
ꢀ
ꢀ
^
ꢀ
Versailles cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts (x þ y ꢁ 3/2 and x, y: 0e1.5) were prepared by a
cationic exchange method based on barium sulfate precipitation. Structural and textural
properties of salts were examined by several physicochemical techniques such as infrared,
scanning electron microscopy-energy dispersive X-ray, and 31P nuclear magnetic reso-
nance spectroscopies, X-ray diffraction diffraction, and thermogravimetric analysis, and
their catalytic properties were evaluated in the cyclohexanone oxidation using hydrogen
peroxide (30%). The reaction products, adipic, glutaric, succinic, hexanoic, 6-
hydroxyhexanoic, 7,7-dimethoxy, and heptanoic acids and 1,1-dimethoxy octane were
identified by gas chromatographyemass spectrometry analysis. Only adipic, glutaric, and
succinic acids were quantified by chromatography (high-performance liquid chromatog-
raphy), the other products were noted X. Adipic acid (AA) is the major product for all
systems. The effects of molar ratios of catalyst/reactant and cyclohexanol/cyclohexanone,
heteropolysalt composition, and reaction duration on AA yield were investigated. The
stability of the catalytic system was also examined. H3ꢀ2(xþy)MnxCoyPMo12O40 catalysts
were found to be efficient for the cyclohexanone oxidation with conversions >95%. Among
them, H1Mn0.25Co0.75 exhibits the highest AA yield (75%).
Received 8 June 2018
Accepted 14 January 2019
Available online xxxx
Keywords:
Cobalt- and manganese-substituted Keggin
mixed salt
Cyclohexanone oxidation
Hydrogen peroxide
Adipic acid
© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
* Corresponding author.
E-mail address: tassa2002@gmail.com (T. Mazari).
https://doi.org/10.1016/j.crci.2019.01.003
1631-0748/© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003

2
S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
r é s u m é
ꢀ
ꢀ ꢀ
ꢀ ꢀ
ꢀ
ꢀ
Mots cles:
Les heteropolysels H3ꢀ2(xþy)MnxCoyPMo12O40 (x þ y ꢁ3/2 et x, y: 0e1,5) ont ete prepares par
ꢀ ꢀ ꢀ ꢀ
une methode d'echange cationique basee sur la precipitation du sulfate de baryum. Les
Cobalt
ꢁ
ꢀ ꢀ
ꢀ ꢀ
ꢀ
Phosphomolybdates de manganese
Oxydation de la cyclohexanone
proprietes structurales et texturales des sels ont ete examinees par plusieurs techniques
physico-chimiques telles que les spectroscopies IR, MEB/EDX et RMN 31P, la diffraction
ꢁ
ꢀ ꢀ
ꢀ ꢀ ꢀ
ꢀ
Peroxyde d'hydrogene
des rayons X et l'analyse TG. Leurs proprietes catalytiques ont ete evaluees dans l'oxydation de
ꢁ
ꢀ
Acide adipique
la cyclohexanone en utilisant le peroxyde d'hydrogene (30%). Les produits de la reaction,
ꢀ
acides adipique, glutarique, succinique, hexanoïque, 6-hydroxyhexanoïque, 7,7-dimethoxy,
ꢀ
ꢀ ꢀ
ꢀ
heptanoïque et 1,1-dimethoxy octane, ont ete identifies par analyse GC-MS. Seuls les acides
ꢀ ꢀ
ꢀ
adipique, glutarique et succinique ont ete quantifies par chromatographie (HPLC), les autres
ꢀ ꢀ
ꢀ
ꢀ
produits ont ete notes X. L'acide adipique (AA) est le principal produit de la reaction. Les
ꢀ
effets des rapports molaires catalyseur/reactif et cyclohexanol/cyclohexanone, de la compo-
ꢀ
ꢀ ꢀ ꢀ
ꢀ
ꢀ
sition du sel et du temps de reaction sur le rendement en AA ont ete etudies. La stabilite du
ꢁ
ꢀ
ꢀ ꢀ
ꢀ
systeme catalytique a egalement ete examinee. Les catalyseurs H3ꢀ2(xþy)MnxCoyPMo12O40 se
ꢀ ꢀ ꢀ
^
sont reveles etre efficaces pour l'oxydation de la cyclohexanone avec des conversions >95%.
ꢀ
ꢀ
Parmi eux, H1Mn0.25Co0.75 conduit au rendement le plus eleve en AA (75%).
© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
H3ꢀ2xNixPMo12O40 and (NH4)3ꢀ2xNixPMo12O40 [37],
(NH4)xAyPMo12O40 (Anþ
¼
Sb3þ
,
Bi3þ
,
or Sn2þ
)
[38],
2-K6P2Mo5W13O62
1-K7P2Mo5VW12O62 [40]. They
showed high activities and led to high AA yields. Seventy
One of the major drawbacks in the actual adipic acid
(AA) production process is the N2O emission [1e3]. It is one
of the ozone depleting agents with a strong greenhouse
effect and a very long residence time in the atmosphere,
thus leading to serious environmental problems [4,5].
AA is an important raw material for nylon-6,6 produc-
tion [6,7] and other compounds as synthetic fibers and fine
chemicals [1]. It is usually produced via the oxidation of a
mixture of cyclohexanone and cyclohexanol (KA oil) in the
presence of an excess of HNO3 (40e60%) using CuII/NH4VO3
as a catalyst [1,5]. The HNO3 reduction causes inevitably the
release of nitrous oxides (N2O, NO, and NO2), which are
harmful gases. Others powerful oxidants as Na2Cr2O7,
NaClO, MnO2, KMnO4 are cited in the literature [8e14], but
they are noxious too. Therefore, the development of a
process that would belong to the “green chemistry” domain
is more than necessary. So, several studies have focused on
the search for benign oxidants for environment as H2O2, O2,
and air. Among them, hydrogen peroxide is the most used,
easier to handle, and moreover, its reduction leads only to
water. It was used for KA oil production from cyclohexane
oxidation using transition metal based catalysts as Cu/
Cr2O3 [15], CoFe2O4 [16], Co3O4 [17], Ce1ꢀxMnxO2 [18], Cu/
Co/AC [19], WO3/V2O5 [20], HMS [21], MCM-48 [22], Cu/
Cr2O3, AlPO-5 modified with rare earth elements [23,24],
and A-HMS (A ¼ Ce, Ti, Co, Al, Cr, V, and Zr) [21] and
MnAlPO [25]. It was found that the most effective systems
for cyclohexane oxidation are those based on Mn and Co
elements [26,27].
H3ꢀ2xCoxPMo12O40 (x: 0e1.5) [39], and
-K6P2Mo6W12O62, and
a
,
a
a
percent of AA yield was attained with
from the mixture of ol/one oxidation.
a-K6P2Mo6W12O62
On the basis of the literature highlighting the efficiency
of Mn- and Co-based catalysts for the oxidation of cyclo-
hexane, in this study, we propose to introduce these two
elements as counterion of [PMo12O40
POMs of formula 3ꢀ2(xþy)MnxCoyPMo12O40, noted
]
3ꢀ. So, a series of
H
H3ꢀ2(xþy)MnxCoy (x þ y ꢁ 3/2 and x, y: 0e1.5) were pre-
pared, characterized by several physicochemical tech-
niques (infrared [IR], scanning electron microscopy-energy
dispersive X-ray [SEM/EDX], and 31P nuclear magnetic
resonance [NMR] spectroscopies, X-ray diffraction [XRD]
diffraction, and thermogravimetric analysis [TGA]) and
used as catalysts for the oxidation of cyclohexanone using
hydrogen peroxide (30%) as an oxidant. The reaction
products, AA, glutaric acid (GA), succinic acid (SA), hex-
anoic, 6-hydroxyhexanoic, 7,7-dimethoxy, and heptanoic
acids and 1,1-dimethoxy octane were identified by gas
chromatographyemass spectrometry (GCeMS) analysis.
Only AA, GA, and SA were quantified by high-performance
liquid chromatography (HPLC), the other products were
noted X. The effects of POMs' counterion nature, molar
ratios of catalyst/-one and cyclohexanone/cyclohexanol,
and reaction time on the reaction product distribution were
investigated. The used catalysts were analyzed by 31P NMR.
H1Mn0.25Co0.75PMo12O40 catalytic stability was examined
in five consecutive cycles.
Furthermore, polyoxometalates (POMs) have received
great attention in the catalysis field because of their
multifunctional properties. They were extensively studied
for various reactions as olefin epoxidation [28], organo-
silane oxidation [29], organic pollutant photo-oxidative
degradation [30], oxidation of alcohols [31e33], cyclo-
hexane [34,35], and cyclohexanol/cyclohexanone [36e40].
In this last case, the examined catalytic systems are
MxPMo12O40 (M ¼ Fe, Ni, Co and x ¼ 1 or 1.5) [36],
2. Experimental section
2.1. Material synthesis
H3PMo12O40 heteropolyacid (noted H3PMo12) was pre-
pared according to the classical method described by
Tsigdinos [41].
H3ꢀ2(xþy)MnxCoy mixed heteropolysalts
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003

S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
3
were synthesized by
a
two-step cationic exchange
hydrogen peroxide (30%) was added to restore the oxidant
state (VI) of Mo characterized by yellowcolor. This sequence
is repeated after each color change until there is no more
change of color. Depending on the nature of the catalytic
system, the added volume of H2O2 can vary between 5 and
12 ml. The reaction end, indicating that the reactant was
totally consumed, is observed, when the POM catalyst is no
longer reduced. It is noteworthy that up to 90 ꢃC, hydrogen
peroxide may be decomposed. The preliminary tests
showed that the AA formation did not take place when the
reaction mixture is constituted either of substrate and
catalyst only, or substrate, catalyst, and hydrogen peroxide
simultaneously or in the absence of catalyst.
[36,37,39] under the stochiometric conditions (x, y: 0e1.5).
The first step consists of an exchange of protons of H3PMo12
by Ba2þ ions according to Eq. (1). In the second step, Mn2þ
and Co2þ ions replace those of Ba2þ that precipitates as
BaSO4, according to Eq. (2). Ba(OH)2 was gradually dis-
solved in an aqueous solution of H3PMo12O40, 4.8 ꢂ 10ꢀ2 M,
to avoid a sharp increase in pH that will lead to the
decomposition of Keggin anion. The pH must be main-
tained at <2. Then, both Mn and Co sulfates, according to
the fixed stoichiometric coefficients, were added to the
previous solution. The Ba2þ ions precipitate as BaSO4. The
H
3ꢀ2(xþy)MnxCoyPMo12O40 soluble saltsꢃ were separated
from BaSO4 by filtration and dried at 50 C and the mixed
The homogeneous reaction mixture was analyzed by
HPLC and GCeMS. HPLC was connected to a quaternary
pump and a UVevis detector (210 nm). HPLC (YL9100
HPLC) analysis was carried out on a Tracer Excel 120 ODSB-
salts recovered as a powder.
H3PMo12O40 þðxþyÞBaðOHÞ2/H3ꢀ2ðxþyÞBaðxþyÞPMo12O40
þ2ðxþyÞH2O
C18 phase (25 cm ꢂ 4.6 mm, 5
mm particle size) using a 30%
(1)
(2)
methanol in 5 mM of ammonium acetate buffer (pH 3.3) at
30 ꢃC with a flow rate of 1 ml/min. GCeMS analysis was
carried out using a GCeMS (GC 6890 plus, MSD5973,
Hewlett Packard-5MS) with HP-INNOWAX column (30 m
ꢂ 0.25 mm). The mass analysis is of quadripole type
(150 ꢃC). Purified helium was used as the carrier gas with a
flow rate of 0.5 ml/min. The injector temperature and
H
3ꢀ2ðxþyÞBaðxþyÞPMo12O40 þ x MnSO4
þ y CoSO4/H3ꢀ2ðxþyÞMnxCoyPMo12O40 þ BaSO4Y
2.2. Characterization
injector volume were 250 ꢃC and 0.2
ml. The split ratio was
The composition and the weight percentages of P, Mo,
Co, and Mn in the samples were measured with EDX
microanalysis (SEM/EDX) using a HITACHI S3400 N tung-
sten filament microscope, equipped with an X-ray micro-
analysis system. Before starting the process of analysis, the
samples were covered with carbon to improve their con-
ductivity. Following this procedure the metal content could
be estimated within an experimental error of 0.5%. SEM
images were recorded using an ESEM xL30 microscope.
Thermal analysis was performed between room tem-
perature and 600 ꢃC, under airflow with a heating rate of
20:1. The ionizatiꢃon source (electronic impact) temperature
ꢃ
was kept at 230 C and that of the interface at 280 C.
Before catalytic test, hydrogen peroxide concentration
was verified by potassium permanganate titration.
3. Results and discussion
3.1. Characterization results
The heteropolysalt chemical composition determined
by SEM/EDX analysis is reported in Table 1. The stochio-
metric coefficients adjusted considering 12 Mo atoms per
Keggin unit were found in good agreement with desired
stoichiometries for the transition metal atom number (Mn
and/or Co). These results show a good correlation between
the experimental formula, deduced from SEM/EDX anal-
ysis, and theoretical one confirming thus the feasibility of
the cationic exchange method used in this study.
ꢃ
5 C/min using a TGA 2050 apparatus.
IR spectroscopy (1200e400 cmꢀ1) was performed using
a Nicolet 6700 Fourier transform infrared spectrometer
(CsI), including an attenuated total reflectance (ATR)
module with a diamond crystal.
XRD patterns were recorded at room temperature in the
2
q
range between 2ꢃ and 60ꢃ with a Siemens D5000
diffractometer using copper anticathode Cu
Ka
(
l
¼
1.5418 Å) at 40 kV and 40 mA. The indexing of the
diffraction lines and phase identification were performed
using the EVA software including a high score database.
Solid-phase 31P NMR spectroscopy was carried out at
room temperature using a BioSpin GmbH, Bruker spec-
trometer, equipped with a probe of 3.2 mm in diameter
(double mode). Eighty-five percent H3PO4 was used as an
external reference.
TGA curves of the samples (not represented here) show
different mass losses. The first loss of ca. 9e12% was
observed in the temperature range 50e180 ꢃC, corre-
sponding to the departure of 8e13 hydration water mole-
cules. The second one of ca. 1e3% observed in the range
180e380 ꢃC was attributed to the departure of constitution
water molecules that results from the combination of
protons with oxygen atoms of Keggin anion. Above 400 ꢃC,
the POM decomposition to P2O5, MoO3, MnO, and CoO
occurred. TGA results are reported in Table 2. A good cor-
relation between experimental and theoretical formulas
was observed, confirming also the reliability of the cationic
exchange synthesis method.
2.3. Catalytic reaction
The cyclohexanone oxidation was carried out under
reflux at 90 ꢃC according to the literature [42]. The reaction
principle is to oxidize the reactant by POM catalyst. A POM
color change from yellow to green or to blue-green (char-
acteristic color of MoV) was observed. Then, 0.5 ml of
The XRD patterns registered for H1Mn0.25Co0,75 and
H1Mn0.75Co0,25 with (x þ y) ¼ 1 (Fig. 1) are similar to that of
H3PMo12 acid that crystallizes in a triclinic system [43].
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003

4
S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
Table 1
Elemental analysis of H3ꢀ2(xþy)MnxCoy heteropolysalts obtained by SEM/EDX analysis.
H
3ꢀ2(xþy)MnxCoy Composition (Wt %)
Molar ratio
P
P
Mo
Mn
1.80
5.73
5.32
Co
Mn
Co
H1Mn0.25Co0.75
H1Mn0.75Co0.25
H0Mn0.75Co0.75
H0Mn1.50Co0.00
7.60
7.17
7.74
6.91
84.95
84.92
81.83
81.75
5.65
2.16
5.11
/
1.07 (1)
1.01 (1)
1.13 (1)
1.01 (1)
0.25 (0.25)
0.80 (0.75)
0.78 (0.75)
1.66 (1.50)
0.79 (0.75)
0.30 (0.25)
0.74 (0.75)
/
11.33
( ) Theoretical values; / absence.
Table 2
(H1Mn0.25Co0.75 and H1Mn0.75Co0.25) a small shoulder at
around 978 cmꢀ1 was observed on Mo]Od vibration bond.
This latter is very sensitive to the Keggin anion symmetry
because of its high charge density induced by the presence
of both cobalt and manganese ions.
Thermal analysis results of H3ꢀ2(xþy) MnxCoy solids.
Theoretical
formula
TGA deduced
formula
Hydration water
molecules
H3.00Mn0.00Co0.00
H2.98Mn0.00Co0.00
H0.06(Mn Co)1.47
H0.96(Mn Co)1.02
H0.94(Mn Co)1.03
H0.02Mn1.49Co0.00
H0.06Mn0.00Co1.43
8
12
11
15
13
13
It is known that 31P NMR analysis is very sensitive to the
local chemical environment and surrounding symmetry of
phosphorus atom in the Keggin unit. Solid-state 31P NMR
H0.00(Mn Co)1.50
H1.00(Mn Co)1.00
H1.00(Mn Co)1.00
H0.00Mn1.50Co0.00
H0.00Mn0.00Co1.50
data (Table 4) of H1Mn0.25Co0.75, H1Mn0.75Co0.25PMo12
,
H0Mn0.75Co0.75, and H0Mn1.5Co0 salts show chemical shifts
at ꢀ4.33, ꢀ4.50, ꢀ4.30, and ꢀ4.16 ppm, respectively, values
close to that of H3PMo12 (ꢀ4.4 ppm), suggesting the same
phosphorus atom environment, evidencing thus the purity
of samples. The small difference in the chemical shift is
because of the proton substitution degree of H3PMo12 by
Mn and/or Co.
The SEM images show that the POM morphology is
sensitive to its chemical composition (Fig. 2). H3PMo12
shows
a
very porous texture whereas H0Mn0Co1.5
,
H1Mn0.25Co0.75
,
and H0Mn0.75Co0.75 show compact
a
texture constituted of aggregates of irregular particle sizes
and for H0Mn1.5Co0, an assembly of small groups of crys-
tals separated by voids leading to a nonhomogeneous
texture.
Fig. 1. XRD patterns of H3ꢀ2(x þ y)MnxCoy solids.
In the absence of proton (x þ y ¼ 1.5), H0Mn0Co1.5
,
3.2. Catalytic performances of H3ꢀ2(xþy)MnxCoy
H0Mn0.75Co0.75, and H0Mn1.5Co0 crystallize in a quadratic
system (high score database) [44].
The catalytic properties of Keggin-type mixed salts,
The Fourier transform infrared data of POMs, presented
in Table 3, show the characteristic vibration frequencies of
phosphoruseoxygen and metaleoxygen bonds of Keggin
anion in the spectral range 1100e600 cmꢀ1 [45]. Asym-
metric stretching phosphoruseoxygen bond, vas PeOa, was
observed at 1060 cmꢀ1 and those of metaleoxygen, vas
MoeOd, vas MoeObeMo, and vas MoeOceMo, at 962e955,
895e879, and 791e785 cmꢀ1 respectively. For x þ y ¼1.5,
the vibration frequency values of salts are close to those
obtained with H3PMo12 acid, whereas for x þ y ¼ 1
H
3ꢀ2(xþy)MnxCoy, were examined in the liquid-phase
oxidation of cyclohexanone (-one) and of mixture of
cyclohexanone and cyclohexanol (-ol/-one) in the presence
of hydrogen peroxide (30%) at 90 ꢃC.
The GCeMS analysis obtained with H1Mn0.25Co0.75 are
given as an example and results are presented in Table 5.
The results show, in addition to the formation of AA and
GA, that of following acids: hexanoic, 6-hydroxyhexanoic,
and 7,7-dimethoxy heptanoic acids and 1,1-dimethoxy
octane. The presence of both 6-hydroxyhexanoic and
hexanoic acids was already observed for the
BaeyereVilliger oxidation of cyclohexanone [46,47]. It is
Table 3
IR vibration wave numbers (cmꢀ1) of H3ꢀ2(xþy)MnxCoy.
H
3ꢀ2(xþy)MnxCoy v (PeOa) v (M]Ob) v (MeObeM) v (MeOceM)
Table 4
31P NMR data of H3ꢀ2(xþy)MnxCoy compounds.
X
Y
0.00
0.25
0.75
1.50
0.75
0.00
0.00
0.75
0.25
0.00
0.75
1.50
1058
1060
1060
1060
1060
1060
955
879
785
791
787
789
791
789
POM
d (ppm)
962 (980)a 879
958 (978)a 879
H3PMo12
ꢀ4.40
ꢀ4.30
ꢀ4.33
ꢀ4.50
ꢀ4.16
H0Mn0.75Co0.75
H1Mn0.25Co0.75
H1Mn0.75Co0.25
H1Mn1.50Co0
957
957
957
889
893
889
a
Shoulder.
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003

S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
5
Fig. 2. SEM images of H3ꢀ2(xþy)MnxCoy.
noted that SA was observed with other systems. In this
study, onlyAA, SA, and GA were quantified by HPLC, the
other products were noted X.
3.2.1.1. Effect of POM counterion nature. As shown in Table 6,
the catalytic systems are very active with 96e100% of
cyclohexanone conversion. AA and X are the major reaction
products observed with selectivities varying between 41-
66% and 29e51%, respectively. GA was obtained with se-
lectivities inferior to 10%. Except for H1Mn0.75Co0.25 system,
the introduction of both Mn and Co favors the AA forma-
tion, evidencing the positive effect of the partial substitu-
tion of H3PMo12 protons by these elements. Thus, AA yields
are of 56e64% against 46% for H3PMo12. Among substituted
systems, H1Mn0.25Co0.75 is the best catalyst, with an AA
yield reaching 64%.
3.2.1. Cyclohexanone oxidation
To optimize the reaction conditions, H3PMo12 was used
as a catalyst for the oxidation of cyclohexanone. The ob-
tained results are the following: a catalyst/-one molar ratio
of 0.47 ꢂ 10ꢀ3 corresponding to a catalyst mass of 0.03 g
and substrate amount of 30 mmol, a reaction time of 20 h,
and an agitation rate of 1000 rpm.
In this study, the effects of POM counterion nature,
molar ratios of catalyst/-one and cyclohexanone/cyclo-
hexanol, and reaction duration on the reaction product
distribution were examined.
3.2.1.2. Effect of catalyst/-one molar ratio. The effect of
catalyst/-one molar ratio was performed with H1Mn0.25
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
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6
S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
Table 5
Results of GCeMS analysis obtained in the presence of H1Mn0.25Co0.75
.
Retention Product name
time (min)
Formula Molar mass
(g/mol)
06.78
07.96
08.43
Hexanoic acid
C6H12O2 116
C6H12O3 132
C6H10O4 146
6-Hydroxyhexanoic acid
AA (by removal of two
methyl groups)
08.94
09.18
7,7-Dimethoxyheptanoic acid
AA (by removal of a single
methyl group)
C9H18O4 190
C6H10O4 146
09.40
09.55
Thiohexanoic acid, s-butyl ester C10H20OS 188
1,1-Dimethoxyoctane 10H22O2 174
C
Fig. 3. Cyclohexanone conversion and reaction product selectivities as a
function of catalyst/-one molar ratio. Catalyst, H1Mn0.25Co0.75; Treact, 90 ꢃC;
agitation rate, 1000 rpm; and reaction time, 20 h.
Conditions: Treact, 90 ꢃC; mcat, 0.18 g; n-one, 30 mmol; agitation rate, 1000
rpm; H2O2, 0.5 ml; and t ¼ 20 h.
Co0.75, the most efficient catalyst. The cyclohexanone con-
version and reaction product selectivities are reported in
Fig. 3. Regardless the value of catalyst/-one molar ratio, the
cyclohexanone conversion is total, thus confirming the
high activity of catalyst. A catalyst/-one molar ratio of 2.84
ꢂ 10ꢀ3 leads to the highest AA selectivity (77%) and to the
lowest X selectivity (20%). With catalyst/-one molar ratios
superior to 2.84 ꢂ 10ꢀ3, AA selectivity gradually decreases
from 77% to 38% whereas that of X increases from 20% to
47%. The highest SA selectivity (20%) was obtained with a
catalyst/n-one molar ratio of 4.26 ꢂ 10ꢀ3 whereas GA was
practically not observed.
formation of some compounds X. The HPLC chromato-
gram (Fig. 5) shows the distribution of the reaction
products after 20 h of reaction. The formation of both GA
and SA varies little with the duration of the reaction. A
reaction time of 20 h, necessary to obtain the highest AA
yield, has already been reported in our previous work
[36e40] as well as by other authors [46,47].
3.2.2. Cyclohexanone/cyclohexanol mixture oxidation
In the case of cyclohexanone/cyclohexanol mixture
oxidation, only AA recovered by crystallization was
considered. The reaction was examined over H1Mn0.25
3.2.1.3. Effect of reaction time. Fig. 4 shows the reaction
time effect on the cyclohexanone conversion and reaction
product selectivities in the presence of H1Mn0.25Co0.75
catalyst. Each time reaction corresponds to a catalytic
test. With a time reaction of 10 h, the cyclohexanone
conversion attains more than 95%. The distribution of
reaction products is sensitive to the reaction duration.
Thus, in 10 h of reaction, X selectivity reached the
maximum (71%) and that of AA the minimum (27%).
Although within 20 h, the selectivity of AA attains the
maximum (77%) and that of X the minimum (22%).
Within 25 h of reaction, AA and X selectivities are of 52%
and 35%, respectively. From these results, it can be
concluded that in 20 h of reaction some X products could
act as intermediates for AA formation and within 25 h of
the reaction, AA could decompose and thus favor the
Co0.75 with a catalyst/(ol þ one) molar ratio of 2.84 ꢂ 10ꢀ3
.
The catalytic results (Fig. 6) show a gradual decrease in AA
yield from 75% to 0% when the percentage of added alcohol
increases from 0 to 100. Similar results are already reported
[48e53]. The oxidation of alcohol took place (a color
change in catalyst was observed) but the AA formation was
not observed. The negative effect of the alcohol addition
can be attributed to the formation of hydrogen bond be-
tween the C]O group of the ketone and the hydrogen of
CeOH group of the cyclohexanol, which makes difficult the
oxidation of cyclohexanone. Therefore, alcohol inhibits the
AA formation.
The series H3ꢀ2(xþy)MnxCoyPMo12O40 showed its effi-
ciency in the oxidation of cyclohexanone, exhibiting high
catalytic performances that can be explained by their
dissolution in the reaction medium, thus making all of the
catalytic sites more accessible to the substrate. The effi-
ciency of cobalt- and manganese-based catalysts was
already reported by other authors for the oxidation of
cyclohexane by molecular oxygen. This efficiency was
attributed to the existence of redox couples, MnIV/MnIII and
CoIII/CoII [54]. Molybdenum-based POMs are also known for
their high oxidative power associated with the high
oxidation state of the molybdenum (MoVI) [55e58]. In the
presence of elements as CoII, SnII, and SbIII, the polyanion,
Table 6
Effect of H3ꢀ2(xþy)MnxCoy composition on cyclohexanone oxidation con-
version and reaction product selectivities (yields).
POM
Conversion (%) Selectivities (yields) (%)
X
AA
GA
SA
H3PMo12
100
96
97
46 (46)
58 (56)
66 (64)
41 (40)
58 (58)
3 (3)
9 (9)
5 (5)
8 (8)
7 (7)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
51 (51)
33 (31)
29 (28)
51 (50)
35 (35)
H0Mn0.75Co0.75
H1Mn0.25Co0.75
H1Mn0.75Co0.25
98
[PMo12O40
reduced POM with coexistence of several elements with
different oxidation states as in our case
3ꢀ2(MnIV/
MnII)x(CoIII/CoII)yP(MoVI/MoV)12O40
This was obtained
]
3ꢀ, can easily be reduced, thus leading to partial
H2Mn0.25Co0.25 100
Reaction conditions: Treact, 90 ꢃC; catalyst/-one molar ratio, 0.47 ꢂ 10ꢀ3
;
H
agitation rate, 1000 rpm; mode addition of the oxidant, 0.5 ml; and re-
.
action time, 20 h.
X, Unidentified products.
AA, adipic acid.
AG, glutaric acide.
As, succenic.
during its preparation confirmed by its blue color. Partial
reduced POMs are known as “heteropoly-blue”. This aspect
can facilitate the electron transfer during the oxidation of
cyclohexanone.
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7
Fig. 4. Cyclohexanone conversion and reaction product selectivities as a function of reaction time. Catalyst, H1Mn0.25Co0.75; Treact, 90 ꢃC; agitation rate, 1000 rpm;
and catalyst/-one molar ratio, 2.83 ꢂ 10ꢀ3
.
Fig. 5. HPLC chromatogram of obtained mixture in the presence of H1Mn0,25Co0,75. Catalyst, H1Mn0.25Co0.75; Treact, 90 ꢃC; catalyst/-one molar ratio of 2.84 ꢂ 10ꢀ3
;
agitation rate, 1000 rpm, reaction time, 20 h.
Among the tested systems, the best catalytic perfor-
mance was obtained by the formulation, H1Mn0.25Co0.75
catalyst preparation would lead to several different peroxo
,
species, after the addition of hydrogen peroxide. These
peroxo species, in turn, could oxidize and provide oxygen
atoms to the reaction intermediates, leading to the final
products., among which AA.
that exhibited an AA yield of 75%, which was higher than
those obtained with Keggin and Dawson series [36e40]
and lower than those obtained from the oxidation of
cyclohexane and cyclohexene (80e90%) [59e62]. Its
effectiveness may be related to the presence of the proton
that can accelerate the cyclohexanone molecule activation
3.2.3. Catalyst stability
To examine the H1Mn0.25Co0.75 catalyst stability, five
cycles of oxidation reaction were performed. After the first
cycle that lasted 20 h, the AA was recovered after cold
crystallization and then 30 mmol of cyclohexanone was
added to the filtrate and the oxidation reaction was
renewed under the same experimental conditions. Fig. 7
through
a ketoniceenolic tautomer equilibrium, thus
leading to the 2-hydroxy cyclohexanone formation in a first
step [63]. In a second step, the system consisting of several
redox couples, MnIV/MnII, CoIII/CoII, and MoVI in MoV,
formed from a partial reduction of MoVI to MoV during the
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S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
Fig. 6. AA yield as a function of the composition of mixture -one/-ol. Catalyst, H1Mn0.25Co0.75; Treact, 90 ꢃC; agitation rate, 1000 rpm; catalyst/-one molar ratio,
2.83 ꢂ 10ꢀ3
.
shows that the AA yield gradually decreases from 61% to 0%.
It can be suggested that this yield decrease is due to
following parameters: (1) the salt acidity decreases after
each reduction of H2O2, oxidant needing the presence of
the protons, and (2) the reaction medium comes more
diluted resulting from the H2O2 reduction to water.
3.2.5. Oxidation reaction pathways
For the cyclohexanone oxidation process in the pres-
ence of H3ꢀ2(xþy)MnxCoy catalysts, several steps were
evidenced: (1) oxidation of the substrate confirmed by a
color change of catalyst from yellow, characteristic color of
MoVI species (POMox) to blue, characteristic color of MoV
species (POMred), involving a Mars and Van Krevelen type
mechanism in which the oxygen atoms of the crystalline
lattice participate in the oxidation process (Eq. 3); (2)
oxidation of POMred to POMox by H2O2 (a color change
from blue to yellow) with simultaneously the peroxo-POM
formation as revealed by 31P NMR analysis (Eq. 4), and (3)
the peroxo-POM species can react with some products
from Eq. 3 that could be the intermediates to form the
carboxylic acids whose AA (Eq. 5). By analogy with the
explanation of Eq. 3 (Mars and Van Krevelen mechanism),
the peroxo-ions (O22ꢀ) of the peroxo-POM would intervene
in the oxidation. It was also reported that the peroxo-POM
was considered as an electrophilic agent, which would
produce an electrophilic attack on the oxygen atom of the
intermediate specie, generating then the corresponding
carboxylic acid as observed in the case of sulfide oxidation
by H2O2 on POMs [69].
3.2.4. Characterization of the used catalyst
The used catalysts were analyzed by 31P NMR after 20 h
of reaction. Table 7 shows that they exhibit several peaks
evidencing phosphorus-based species formation. The
chemical shifts were observed at ꢀ3.14 and ꢀ5.5 ppm for
H3PMo12, ꢀ3.75 and ꢀ6.89 ppm for H0Mn0.75Co0.75, and
ꢀ3.76, ꢀ6.19, ꢀ6.84, and ꢀ12.94 ppm for H1Mn0.25Co0.75
.
These values are different from that of the oxidized specie,
[PMo12O40
]
3ꢀ (ꢀ4.30 to ꢀ4.40 ppm). It was reported that in
the presence of hydrogen peroxide, the POM was broken
down into several species identified by 31P NMR as peroxo
species. In the case of the H3PW12O40, the species are
{PO4[WO(O2)2]4}3ꢀ and {PO4[WO(O2)2]2}2ꢀ [64e68]. From
these observations, it can be assumed that the peroxo-POM
species could be the active species for the formation of
carboxylic acids.
Cyclohexanone þ POMox/POMred þ products
þ intermediates
(3)
(4)
H2O2 þ POMred/peroxo ꢀ POMox
Table 7
Results of 31P NMR analysis of different POMs before and after cyclohex-
anone oxidation.
POM
d
(ppm) before reaction
d(ppm) after reaction
H3PMo12
ꢀ4.40
ꢀ3.14/ꢀ5.52
Fig. 7. AA yield variation as
a function of cycle number. Catalyst,
H0Mn0.75Co0.75 ꢀ4.30
ꢀ3.75/ꢀ6.89
H1Mn0.25Co0.75; Treact, 90 ꢃC; catalyst/-one molar ratio, 2.84 ꢂ 10ꢀ3; agitation
H1Mn0.25Co0.75 ꢀ4.33
ꢀ3.76/ꢀ6.19/ꢀ6.84/ꢀ12.94
rate, 1000 rpm; reaction time, 20 h.
Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
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S. Mouanni et al. / C. R. Chimie xxx (xxxx) xxx
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4. Conclusions
In this study, a series of Keggin-type POMs of formula
H3ꢀ2(xþy)MnxCoyPMo12O40 was prepared, characterized,
and tested in homogeneous oxidation reactions of cyclo-
hexanone and cyclohexanol/cyclohexanone mixture using
H2O2 as an oxidant.
Physicochemical analysis has confirmed the formulation
of H3ꢀ2(xþy) MnxCoy with x þ y ¼ 3/2 and x, y: 0e1.5, thus
demonstrating the feasibility of the used cationic exchange
method.
The catalytic results showed that all studied systems are
very active in the cyclohexanone oxidation (conversions
>95%) that leads to several products as AA, GA, SA, hex-
anoic, 6-hydroxyhexanoic, and 7,7-dimethoxy, and hepta-
noic acids and 1,1-dimethoxy octane, identified by GCeMS.
Among reaction products (AA, GA, and SA) quantified by
HPLC, AA is the major product for following reaction con-
ditions: a catalyst/substrate molar ratio of 2.84 ꢂ 10ꢀ3 and
20 h of reaction. With a composition of H1Mn0.25Co0.75, the
AA yield attains the maximum (75%). The peroxo-POM
species could be the active species for the formation of
carboxylic acids from products coming from the first step,
substrate oxidation by POM.
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Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003

10
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Please cite this article as: S. Mouanni et al., Preparation and characterization of H3ꢀ2(xþy)MnxCoyPMo12O40 heteropolysalts.
Application to adipic acid green synthesis from cyclohexanone oxidation with hydrogen peroxide, Comptes Rendus Chimie,
https://doi.org/10.1016/j.crci.2019.01.003