Preparation, Characterization and Reactivity of Keggin Type Phosphomolybdates, H3-2x Ni x PMo12O40 and (NH4)3-2x Ni x PMo12O40, for Adipic Acid Synthesis

Article abstract of DOI:10.1007/s10562-014-1373-7

Two H_3-2x Ni _x PMo₁₂O₄₀ and (NH₄)_3-2x Ni _x PMo₁₂O₄₀ Keggin type heteroploysalts series (x: 0.25-1.5) were synthesized and characterized by IR spectroscopy, XR diffraction and thermal analysis (TG). Their catalytic properties were studied in the oxidation reaction of cylohexanone and cyclohexanone/cyclohexanol mixture to adipic acid in presence of hydrogen peroxide and in absence of solvent. The effects of catalyst/substrate molar ratios and heteropolysalt composition on AA yields were examined. TG analysis allowed to confirm the x value. Obtained catalytic results showed that both salts series are efficient for oxidation of cylohexanone and cyclohexanone/cyclohexanol mixture. Adipic acid yield achieved 45 % in the cyclohexanone oxidation in presence of (NH₄)_0.5Ni_1.25PMo₁₂O₄₀ with a catalyst/substrate molar ratio of 10^-3. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1373-7

Catal Lett
DOI 10.1007/s10562-014-1373-7
Preparation, Characterization and Reactivity of Keggin
Type Phosphomolybdates, H322xNixPMo12O40
and (NH4)322xNixPMo12O40, for Adipic Acid Synthesis
•
A. Tahar S. Benadji T. Mazari L. Dermeche
•
•
•
•
C. Marchal-Roch C. Rabia
Received: 31 May 2014 / Accepted: 29 September 2014
Ó Springer Science+Business Media New York 2014
Abstract Two H3-2xNixPMo12O40 and (NH4)3-2xNix
PMo12O40 Keggin type heteroploysalts series (x: 0.25–1.5)
were synthesized and characterized by IR spectroscopy,
XR diffraction and thermal analysis (TG). Their catalytic
properties were studied in the oxidation reaction of cylo-
hexanone and cyclohexanone/cyclohexanol mixture to
adipic acid in presence of hydrogen peroxide and in
absence of solvent. The effects of catalyst/substrate molar
ratios and heteropolysalt composition on AA yields were
examined. TG analysis allowed to confirm the x value.
Obtained catalytic results showed that both salts series are
efficient for oxidation of cylohexanone and cyclohexanone/
cyclohexanol mixture. Adipic acid yield achieved 45 % in
the cyclohexanone oxidation in presence of (NH4)0.5
Ni1.25PMo12O40 with a catalyst/substrate molar ratio of
1 Introduction
The Keggin type phosphomolybdates (PMo12) are known to
catalyze oxidation reactions [1–6]. The acid and oxidizing
properties of polyoxometalates (POMs) are very sensitive to
the composition and the nature of components and can be
modified according to the reaction needs. Several studies
have developed synthetic methods for introducing as coun-
ter-cations in the secondary structure of POM, a transition
element to increase its oxidizing power and ammonium or
cesium ions to obtain an insoluble and thermally stable salt.
Thus, the solubility of the polyoxometalate can be adjusted
as function of the counter-cation nature. Different synthesis
methods were proposed in the literature [1–8].
Mixed cesium salts as Cs2.5HxMyPMo11VO40 were pre-
pared by introducing a transition metal M in the aqueous
solution of 12-phosphomolybdic acid before precipitating of
the Cs2.5 salts [1, 2]. Mixed ammonium salts as (NH4)2.5
MzHyPMo12O40 (M = Ni, Co or Fe) [3–5] and (NH4)xXy
PMo12O40 (X = Sb, Bi or Sn) [6, 7] were prepared by
addition of aqueous solution of 12-phosphomolybdic acid to
aqueous solution containing the corresponding chloride salt
in excess (NH4Cl, MClx or XClx). In the mixed cesium or
ammonium salts, the transition metal atom number does not
exceed 0.08. Unlike cesium ions, in the case of ammonium
ions, it was shown that they adjust the oxidizing power and
acidity strength of POM [4]. K. Narasimha Rao et al. [8] have
developed a method that consists to prepare the ammonium
salt of 12-molybdophosphoric acid by solid-state interaction
of ammonium heptamolybdate and iron phosphate. The solid
mixture undergoes thermal pretreatment.
10-3
.
Keywords Heteropolysalts preparation ꢀ
Cyclohexanone ꢀ Cyclohexanol ꢀ Catalytic oxidation ꢀ
Hydrogen peroxide ꢀ Adipic acid
A. Tahar ꢀ S. Benadji ꢀ T. Mazari ꢀ L. Dermeche ꢀ
C. Rabia (&)
´
Laboratoire de Chimie du Gaz Naturel, Faculte de Chimie,
Universite des Sciences et de la Technologie Houari
´
`
Boumediene (USTHB), BP 32, El-Alia, Bab-Ezzouar,
16111 Alger, Algeria
e-mail: c_rabia@yahoo.fr; crabia@usthb.dz
T. Mazari ꢀ L. Dermeche
´
´
´
Departement de Chimie, Faculte des Sciences, Universite
Mouloud Mammeri (UMMTO), 15000 Tizi-Ouzou, Algeria
In the proposed aqueous methods, it is very difficult to
control the number of metal ions introduced into the mixed
salt, (NH4)xMyPMo12O40 or Cs2.5HxMyPMo11VO40 due to
the instantaneous precipitation of the salt. In this study, we
C. Marchal-Roch
ˆ
´
ILV-UMR 8180 CNRS, Batiment Lavoisier, Universite de
Versailles-St Quentin-en-Yvelines, 45 avenue des Etats-Unis,
78035 Versailles Cedex, France
123

A. Tahar et al.
chose the dry method to prepare (NH4)3-2xNixPMo12O40
with x ranging between 0.25 and 1.5. This technique allows
to fix the nickel atom number introduced in the secondary
structure of POM. In the second part of this study, we
examined the catalytic performance of this series of salts in
adipic acid synthesis (AA). This later was carried out from
homogeneous liquid phase oxidation, free solvent, of
cyclohexanone and cyclohexanone-cyclohexanol mixture
in the presence of H2O2 (30 %).
In a previous study conducted on a series of catalysts of
the formula H3-2xMxPMo12O40 (M: Fe, Co and Ni and x:
0.25–1.5), it was shown that cyclohexanone oxidation by
H2O2 in the absence of solvent, involves a redox mecha-
nism similar to that of Mars and Van Krevlen [17, 18].
In this work, the (NH4)3-2xNixPMo12O40 salts (with x:
0.25–1.5) were prepared using a dry method from a mix-
ture constituted of H3-2xNixPMo12O40 and urea. H3-2x
NixPMo12O40 salts were prepared by a cationic exchange
between H3-2xBaxPMo12O40 and NiSO4. (NH4)3-2xNix
PMo12O40 salts were characterized by IR spectroscopy, TG
and XRD analysis. The nickel atom number was correlated
with adipic acid yield. For a comparative study, the series
Adipic acid is one of the most widely used chemicals in
the world and it is among the ten products in terms of vol-
ume, used annually by chemical industry [9]. Nylon 6,6
production accounts for the greater part of the total AA
consumption [10]. The principal industrial process for AA
production is done in two steps. The first one is the cyclo-
hexane oxidation with air to form a mixture of cyclohexanol
and cyclohexanone (so-called KA oil). In a variant of this
first step, cyclohexanol is obtained by the hydrogenation of
phenol or the hydration of cyclohexene. The second step is
the oxidation of KA oil or cyclohexanol with an excess of
40–60 % HNO3 and a Cu/V catalyst. The molar yield
obtained at total KA oil conversion is as high as 95 %; the
by-products are glutaric (GA) and succinic (SA) acids. The
co-products of oxidation with nitric acid are nitrogen oxides;
NO and NO2 can be reoxidized and recycled, whereas N2O is
a pollutant gas that has to be abated before the emission of
tail-gases into the atmosphere [11–14]. Furthermore, in
current industrial processes, organic solvents are used as
phase-transfer agents to increase reaction rates [15].
H
3-2xNixPMo12O40 was tested in the same conditions.
2 Experimental
2.1 Catalysts Preparation
H3PMo12O40 was prepared according to the classical
method and H3-2xNixPMo12O40 (x = 0.25–1.5) heteropo-
lysalts by a cationic exchange method in two steps [17, 18].
First, H3-2xBaxPMo12O40 salts were synthesized by addi-
tion of Ba(OH)2 to an aqueous solution of H3PMo12O40 in
stoichiometric ratios, followed by precipitating of BaSO4
using nickel sulphate. There is a cationic exchange
between H3-2xBaxPMo12O40 and NiSO4 according to
Eqs. (1) and (2) equations. After filtration, the resulting
heteropolysalt solution was evaporated to dryness at 50 °C.
POMs have demonstrated their efficacy in oxidation
reactions in both gas and liquid phases [1–6]. Oxidants
used are generally molecular oxygen, air or hydrogen
peroxide. Hydrogen peroxide is the most attractive oxidant
(after dioxygen) in liquid phase, because it is green, quite
cheap and easy to handle [16]. Its reduction requires an
acidic environment that can come from the POM. Thus, the
acidity as well as the oxidizing power needed for the oxi-
dation reaction could come from the POM only [17, 18].
In the case of the cyclohexanone oxidation to adipic
acid, over tetrabutylammonium salts of H3PW12O40 and
H3PMo12O40 catalysts in the absence of solvent, K. Nom-
iya et al. have shown that in the absence of H2O2, the
catalyst reacted with a stoichiometric and non-catalytic
manner and in its presence with a catalytic manner [19].
Whereas, F. Cavani et al. have shown that the use of acetic
acid, in the presence of H3?xPMo12-xVxO40 (x = 1 and 2)
heteropolyacids, led to a strong increase of the cyclohex-
anone conversion in the presence of molecular oxygen at
high pressure (4 bars) compared to the reaction carried out
in the presence of water only [20]. It was also reported in
this study, that the cyclohexanone oxidation involves a
redox mechanism in an aqueous medium and a radical
autoxidation mechanism when acetic acid is added.
H3PMo12O40 þ xBaðOHÞ2! H3ꢁ2xBaxPMo12O40
þ 2xH2O
ð1Þ
ð2Þ
H
3ꢁ2xBaxPMo12O40 þ xNiSO4
! H3ꢁ2xNixPMo12O40 þ xBaSO4 #
The method of (NH4)3-2xNixPMo12O40 preparation
consist in finely grind H3-2xNixPMo12O40 and urea in
stoichiometric ratios (Eq. 3). The solid mixture then
undergoes thermal treatments under molecular nitrogen
with a flow rate of 7 mL min-1, a first heating is at 150 °C
for 6 h, followed by another heating at 250 °C for 6 h
again. After cooling, the obtained salts are of yellow–green
color.
H3ꢁ2xðNiÞxPMo12O40 þ ðð3 ꢁ 2xÞ=2ÞCOðNH2Þ2
! ðNH4Þ3ꢁ2xNixPMo12O40 þ ðð3 ꢁ 2xÞ=2ÞCO2
2.2 Characterization
ð3Þ
Infrared spectroscopy was recorded on a Fourier Transform
IR Nicolet 550 apparatus, using KBr disks in the following
conditions: 2 mg of sample diluted in 200 mg of KBr.
X-ray diffraction patterns (XRD) were recorded at room
temperature with a Siemens D5000 diffractometer using
123

H3-2xNixPMo12O40 and (NH4)3-2xNixPMo12O40 Keggin Type
˚
Cu-Ka radiation (k = 1.5418 A). The indexing of the
reveal significant difference within 1,500–250 cm-1 range,
for x B 0.75 (Fig. 2). This result was expected, because the
Keggin unit, PMo12O403- has not been modified, since the
substitution is in the cationic position. However, for values of
x [ 0.75, an extension of the masMo–Ob–Mo vibration band
is observed with appearance of a shoulder at 904 cm-1. This
can suggest amodification of the Keggin anion with probably
the diffusion of nickel in the heteropolyanion.
diffraction lines and phase identification were performed
using the EVA software including a ICDD-JCPDS
database.
TG analysis was performed, under nitrogen, on a TGA
2050, 626 Polarecord Metrohm apparatus with a heating
rate of 5 °C min-1 from 25 to 600 °C (maximal tempera-
ture to avoid the MoO3 sublimation). The TG analysis was
performed immediately after the synthesis of the salt.
Therefore, the observed mass loss is linked only to the
departure of the ammonium ions as ammonia (NH3) and
water (1/2 H2O).
Figure 3 shows that the XRD patterns of the
H
3-2xNixPMo12O40 series are similar to that of the
parent acid, indicating that the salts crystallize in a
triclinic structure as in the case of the acid. The partial
or total substitution of the protons of H3PMo12 by Ni2?
ions does not affect the crystal system of the POM.
Figure 4 also shows that the XRD patterns of the
(NH4)3-2xNixPMo12O40 series for x B 0.75 are similar
to that of the parent salt, (NH4)3PMo12O40, indicating
that the salts crystallize in a cubic structure as the
unsubstituted salt. A crystallinity loss is observed for
the other salts at low ammonium ion content (x [ 0.75)
suggesting probably a partial migration of Ni2? ions
from the cationic position to anionic position (in the
primary structure of the polyanion) or amorphization of
salt for values of x [ 0.75, results consistent with those
obtained by FT–IR.
2.3 Catalytic Reaction
The synthesis method is based on that described in the
literature [19]. The liquid-phase oxidation of cyclohexa-
none (-one) or mixture of cyclohexanone (-one) and cy-
clohexanol (-ol) was carried out at 90 °C, using a 100 mL
round-bottomed flask equipped with a magnetic stirring bar
and a reflux condenser. The reaction mixture is constituted
by a calculated amount of catalyst and substrate; where into
one drop of hydrogen peroxide (30 %) was added when-
ever the salt showed a color change from yellow or light
green to blue. The reaction mixture was stirred at 800 rpm.
The end of the reaction was estimated when the catalyst no
longer reduced that corresponds to oxidation of substrate.
The reaction time was found to be of 20 h. It is noteworthy
that beyond 90 °C, hydrogen peroxide may decompose.
The resultant homogeneous mixture was cooled at 0 °C
overnight. Adipic acid, one of the oxidation products, was
isolated as crystals and identified by FT–IR spectroscopy
and melting point (151 °C). The AA yield (%) is AA mass
formed. 100/theoretical AA mass.
FT–IR and XRD results indicate that the ammonium–
nickel mixed salts can be synthesized by the proposed
method in this work for x B 0.75.
Thermal analysis (TGA) was used in our case to determine
the x value. The departure of water molecules of constitution
resulting from the combination of protons with oxygen atoms
of the solid is observed in the 180–250 °C range in the case of
H3-2xNixPMo12O40 series and the departure of ammonium
ions as ammonia is observed in the 350–450 °C range for
(NH4)3-2xNixPMo12O40 series (figures not shown). The stoi-
chiometric coefficients derived from TGA are close to those
calculated theoretically (Tables 1, 2). These results confirm
the reliability of the synthesis methods used in this work.
3 Results and Discussion
3.1 Catalysts Characterization
3.2 Catalytic Tests
Figure 1 shows the FT–IR spectra of the H3-2xNixPMo12O40
series. The characteristic bands of the Keggin structure,
Both H3-2xNixPMo12O40 (noted H3-2xNix) and (NH4)3-2x
NixPMo12O40 series (noted (NH4)3-2xNix) with x = 0.25–
1.5, together with H3PMo12O40 and (NH4)3PMo12O40 were
tested in the liquid-phase oxidation of (-one) and mixture
of (-one/-ol) to adipic acid (AA), in the presence of
hydrogen peroxide (30 %) under solvent-free at 90 °C. In
earlier work, it was shown that high AA yields are achieved
with a high stirring rate (800 rpm) [17, 18].
phosphorus–oxygen corresponding to masP–Oa (1,064 cm-1
)
and dP–O (594 cm-1) and metal–oxygen corresponding to
masMo–Od (962–960 cm-1), masMo–Ob–Mo (872–870 cm-1
)
and masMo–Oc–Mo (787–781 cm-1) were observed [21].
FT–IR spectra of (NH4)3-2x Nix PMo12O40 salts show the
presence of vibration bands characteristic of the Keggin
anion for all salts, in addition to the band observed at
1,410 cm-1 associated to the ammonium ions, its intensity
increases with the content of NH4?. The comparison of the
FT–IR vibration bands of the H3-2xNixPMo12O40 series,
with those of the (NH4)3-2xNixPMo12O40 series did not
In this work, only AA formation was taken into account.
The other reaction products observed are the succinic and
glutaric acids. It is worth noting that the -one oxidation did
not proceed when the reaction mixture is constituted of
123

A. Tahar et al.
g
f
e
d
c
b
a
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
Wavenumber (cm-1)
Fig. 1 IR spectra of H3-2xNixPMo12O40 salts x = 0 (a); x = 0.25 (b); x = 0.5 (c); x = 0.75 (d); x = 1 (e); x = 1.25 (f) and x = 1.5 (g)
Fig. 2 IR spectra of
(NH4)3-2xNixPMo12O40 salts
f
x = 0 (a); x = 0.25 (b);
x = 0.5 (c); x = 0.75 (d);
x = 1 (e) and x = 1.25 (f)
e
d
c
b
a
1600
1500
1400
1300
1200
1100
1000
900
800
700
600
500
400
300
Wavenumber (cm-1)
substrate, catalyst and hydrogen peroxide simultaneously
or when no catalyst was used. On the other hand, the
oxidizing power of POM is generated only after addition of
hydrogen peroxide (transition from blue to yellow). The pH
values of the solution after reaction, tested by pH meter are
around 2 for all catalytic tests, acidity necessary to the
H2O2 reduction, indicating also that the POM has oxidized
the -one to carboxylic acids. The ammonium salts are not
soluble in the cyclohexanone or the mixture of
cyclohexanone and cyclohexanol reactant, but become
soluble in the reaction medium, after addition of H2O2. The
decomposition of H2O2, very sensitive to temperature and
in contact with the metal surface, could involve the for-
mation of metal-peroxo species as observed in the presence
of metal ions with d0 electronic configuration as Ti(IV),
V(V) and W(VI) [22–24]. Therefore, the formation of these
metal-peroxo species can be the cause of the solubility of
ammonium salts.
123

H3-2xNixPMo12O40 and (NH4)3-2xNixPMo12O40 Keggin Type
Table 1 Thermal analysis results of H3-2xNixPMo12O40 salts
Theoretical formula
TG deduced formula
H3.00Ni0.00PMo12O40
H2.50Ni0.25PMo12O40
H2.00Ni0.50PMo12O40
H1.50Ni0,75PMo12O40
H1.00Ni1.00PMo12O40
H0.75Ni1.25PMo12O40
H0.00Ni1.50PMo12O40
H3.00Ni0.00PMo12O40
H2.53Ni0.23PMo12O40
H1.96Ni0.52PMo12O40
H1,57Ni0.71PMo12O40
H0.94Ni1.03PMo12O40
H0.60Ni1.20PMo12O40
H0.00Ni1.50PMo12O40
Fig. 3 XRD patterns of H3-2xNixPMo12O40 salts x = 0 (a); x =
0.25 (b); x = 0.5 (c); x = 0.75 (d); x = 1 (e); x = 1.25 (f) and x =
1.5 (g)
The effects of catalyst/substrate molar ratios and POM
composition on AA yields were examined. The substrate
oxidation reaction lasted 20 h.
Table 2 Thermal analysis results of (NH4)3-2xNixPMo12O40 salts
Theoretical formula
Number of
(NH4)/HPA
Table 3 shows the obtained AA yields with catalyst/
substrate (cat/-one) molar ratios of 4 9 10-3 (catalyst:
0.125 g and cyclohexanone: 15 mmol) and 10-3 (catalyst:
0.0625 g and cyclohexanone: 30 mmol).
(NH4)3.0Ni0.00 PMo12O40
(NH4)2.5 Ni0.25 PMo12O40
(NH4)2.0Ni0.50PMo12O40
(NH4)1.5Ni0.75PMo12O40
(NH4)1.0Ni1.00PMo12O40
(NH4)0.5Ni1.25PMo12O40
3.10
2.53
1.99
1.56
0.96
0.60
With a cat/-one ratio of 4 9 10-3, (NH4)3PMo12 salt
exhibited higher efficiency than the parent acid in the -one
oxidation, with a higher AA yield of about two times (31
against 16 %), indicating the beneficial effect of the protons
substitution of the acid by the ammonium ions and sug-
gesting that the reaction of -one oxidation may require only
weak acid sites that come from the hydrolysis of ammonium
ions and on the other hand, the oxidizing power of catalyst
became higher. However, it has currently no parallel
between the AA yields and the partial substitution of protons
or ammonium ions by the Ni (II) ions in H3-2xNix or
(NH4)3-2xNix salts respectively. In the H3-2xNix series, the
variation of Ni atom number from 0.25 to 1.5 increases the
AA yield from 19 to 25 against 16 % for unsubstituted acid,
excepted for H0.5Ni1.25PMo12 (15 %). In the (NH4)3-2xNix
series, for x values varying between 0.25 and 0.75, the AA
yields are similar (22–24 %), lower than that of unsubsti-
tuted salt (31 %). For a value of x = 1, the AA yield (31 %)
is equal to that of (NH4)3PMo12 (31 %). The highest AA
yield is obtained when x reaches 1.25 (35 %), superior to that
of Ni1.5PMo12 (25 %).
A decrease by a factor of 4 of cat/-one ratio (from
4 9 10-3 to 10-3) leads to an increase of the AA yield for
the two series of catalysts, from 22–35 to 33–45 % for
(NH4)3-2xNix and from 15–25 to 15–31 % for H3-2xNix. It
is noteworthy that practically all yields increased by 10 %
in the case of the (NH4)3-2xNix series and from 3 to 9 % in
the case of the H3-2xNix series with the ratio decrease.
Comparing the yields of the two series, it can be con-
cluded that the catalytic activity of (NH4)3-2xNix salts is
superior to that of H3-2xNix salts. Thus, it is apparent that
the introduction of a transition metal ions and ammonium
ions, as counter-cations is more favorable for the AA for-
mation. This may be related to both the weak acidity,
associated to ammonium ion hydrolysis, and to the oxi-
dizing power modification resulting also of the ammonium
ion presence. The high AA yield obtained with the lowest
Fig. 4 XRD patterns of
(NH4)3-2xNixPMo12O40 salts
x = 0 (a); x = 0.25 (b);
x = 0.5 (c); x = 0.75 (d);
x = 1 (e); x = 1.25 (f) and
x = 1.5 (g)
(g)
(f)
(e)
(d)
(c)
(b)
(a)
5
10
20
30
40
50
60
2-Theta - Scale
123

A. Tahar et al.
Cat/-one 10-3
Table 3 Adipic acid yields (%) as a function of catalyst/-one molar ratios and POM composition
POMs
Cat/-one 4 10-3
Cat/-one 10-3
POMs
Cat/-one 4 10-3
H3.0Ni0.00PMo12
16
21
24
19
20
15
25
15
26
27
22
29
16
31
(NH4)3.0Ni0.00PMo12
(NH4)2.5Ni0.25PMo12
(NH4)2.0Ni0.50PMo12
(NH4)1.5Ni0.75PMo12
(NH4)1.0Ni1.00PMo12
(NH4)0.5Ni1.25PMo12
31
22
24
24
31
35
39
34
33
33
41
45
H2.5Ni0.25PMo12
H2.0Ni0.50PMo12
H1.5Ni0.75PMo12
H1.0Ni1.00PMo12
H0.5Ni1.25PMo12
H0.0Ni1.50PMo12
Reaction conditions Treact: 90 °C, reaction time = 20 h, Cat/-one ratio = 4 9 10-3 (mcat = 0.125 g, n-one = 15 mmol), Cat/-one
ratio = 10-3 (mcat = 0.0625 g, n-one = 30 mmol)
50
45
40
35
30
25
20
15
10
5
Fig. 5 Adipic acid yields as a
function of the composition
of -one/-ol mixture over
(NH4)0.5Ni1.25PMo12O40
(Reaction conditions Treact
:
90 °C, Cat/substrate
ratio = 10-3, agitation rate:
800 rpm, reaction time: 20 h)
0
0
10
20
30
40
50
60
70
80
90
100
% of cyclohexanol
amount of catalyst (cat/-one = 10-3) may indicate that AA
formation rate is higher.
reaction was conducted with the most active catalyst,
(NH4)0.5Ni1.25PMo12O40 (cat/-one ratio: 10-3). It was
found that addition of (-ol) decreases the AA yield (Fig. 5).
An increase of the percentage of (-ol) from 25 to 75 % in
the reaction mixture caused a decrease of AA yield from 45
to 25 %. Only, 17 % of AA yield were obtained from
(-ol) oxidation. It is noteworthy that the composition of the
reaction mixture leading to the higher yield (34 %) is that
which corresponds to industrial process (KA oil). For a
(-one)/(-ol) mixture of 75/25 or 25/75, AA yields are
similar with 25–28 %. In the first case, the addition of
25 % of alcohol can lead to the formation of hydrogen
bonds between the C = O group of the ketone and the
hydrogen of C–OH group of the cyclohexanol, this makes
difficult the oxidation of substrate mixture. In the second
case, the low AA yield (25 %) can be explain by the
amount of alcohol that is three times higher than that of the
The highest AA yields, 45 and 35 %, were obtained, in
the presence of (NH4)0.5Ni1.25PMo12 catalytic system with
the two cat/-one ratios 10-3 and 4 9 10-3 respectively.
This result suggests that with a composition of the counter-
ion of 0.5 ammonium ion and 1.25 nickel atom per Keggin
unit, the catalyst presents acid and oxidative characteristics
favoring AA formation. Thus, the acidity is necessary for
hydrogen peroxide reduction and carbon–carbon bond
cleavage of the substrate, and the oxidizing power is nec-
essary for oxidation of terminal carbons.
The nitric acid based industrial process for AA pro-
duction proceeds by oxidation of a mixture of cyclohexanol
(50 %) and cyclohexanone (50 %) [12–14]. It seemed
interesting to examine the influence of the (-ol) addition
(25–75 %) to the (-one) on AA yield. In this study, the
123

H3-2xNixPMo12O40 and (NH4)3-2xNixPMo12O40 Keggin Type
ketone (75/25) and by the rate of (-ol) oxidation that is
slower compared with that of the (-one) oxidation. After
20 h of reaction, the amount of AA obtained from the
(-one) oxidation is therefore greater than that obtained from
the alcohol.
oxidation with hydrogen peroxide and among them,
(NH4)0.5Ni1.25PMo12O40, is the catalyst that leads to the
highest AA yield (45 %).
The uses of bifunctionnal catalyst (acid and oxidative)
and a green oxidant (H2O2) whose water is the only
reduction product and the no use of solvent are positive
conditions for an environment more friendly in the process
of adipic acid production.
In this study, (NH4)0.5Ni1.25PMo12O40 is the catalyst that
leads to the highest AA yield. This chemical composition
presents appropriate acidic and oxidizing properties to the
AA formation. The homogenous catalytic oxidation pro-
cess of cyclohexanone may be summarized as follows:
(i) the first step corresponds to direct oxidation of substrate,
by the POM which acts as oxidant, step evidenced by the
color change from yellow–green to blue, characteristic
color of reduced POM, (ii) in the second step, hydrogen
peroxide acts to regenerate the catalyst by oxidation as
evidenced by the color change from blue to the initial color
of catalyst with probably formation of peroxo-POM spe-
cies, activated species that can function a oxygen donor
with withering-opening and insertion of oxygen atoms. The
catalytic cycle is finished when the POM catalyst is no
longer reduced, indicating a direct participation of the
catalyst in the rate-determining step of the process, and
suggesting that the POM has a catalytic effect on the
process rate. It should be pointed that if the reaction is
carried out with the simultaneous presence of substrate,
catalyst and hydrogen peroxide, the substrate is not
oxidized.
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4 Conclusion
The preparation method that was used in this work, allowed
to obtain two soluble H3-2xNixPMo12O40 and insoluble
(NH4)3-2xNixPMo12O40 POMs series with a well-defined
chemical composition (x: 0.25–1.5).
Their catalytic properties examined in the cyclohexa-
none oxidation to adipic acid showed that the AA yield is
sensitive to the chemical composition and the x value.
POMs are found to be efficient as catalysts for liquid phase
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123