Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol Using (NH4)xAyPMo12O40 (A: Sb, Sn, Bi) Mixed Heteropolysalts and Hydrogen Peroxide in Free Solvent

Article abstract of DOI:10.1007/s10562-017-2263-6

Abstract: Clean synthesis of adipic acid (AA) from oxidation of cyclohexanone, cyclohexanol or mixture cyclohexanol/cyclohexanone, was carried out at 90?°C, in the presence of hydrogen peroxide (30%) in free solvent, using Keggin-type polyoxometalates, (NH₄)_xA_yPMo₁₂O₄₀ (Aⁿ⁺=Sb³⁺, Bi³⁺ or Sn²⁺), as catalysts. HPLC analysis of reaction mixture showed the formation of adipic, succinic and glutaric acids and unidentified products. The salts were found to be effective for AA synthesis. Whatever the composition of the catalyst, the alcohol oxidation favors the formation of the unidentified products, unlike the ketone oxidation which favors that of the adipic acid. (NH₄)_0.5Sn_1.25PMo₁₂O₄₀ led to the highest AA yield (56%) from cyclohexanone oxidation, after 20?h of reaction. In addition, ³¹P NMR analysis showed that it has conserved the Keggin structure contrary to others catalysts and that it can be used at least 3 times with reaction times of 20?h, without regeneration. From different catalytic tests and ³¹P NMR data, reaction pathways have been proposed. The active species could be peroxo-polyoxometalates.

Full text of DOI:10.1007/s10562-017-2263-6

Catalysis Letters
https://doi.org/10.1007/s10562-017-2263-6
Clean Adipic Acid Synthesis from Liquid-Phase Oxidation
of Cyclohexanone and Cyclohexanol Using (NH4)xAyPMo12O40 (A: Sb,
Sn, Bi) Mixed Heteropolysalts and Hydrogen Peroxide in Free Solvent
Lynda Mouheb1,2 · Leila Dermeche1,2 · Tassadit Mazari1,2 · Siham Benadji1 · Nadine Essayem3 · Chérifa Rabia1
Received: 18 July 2017 / Accepted: 29 November 2017
© Springer Science+Business Media, LLC, part of Springer Nature 2017
Abstract
Clean synthesis of adipic acid (AA) from oxidation of cyclohexanone, cyclohexanol or mixture cyclohexanol/cyclohexanone,
was carried out at 90 °C, in the presence of hydrogen peroxide (30%) in free solvent, using Keggin-type polyoxometalates,
(NH4)xAyPMo12O40 (An+=Sb3+, Bi3+ or Sn2+), as catalysts. HPLC analysis of reaction mixture showed the formation of
adipic, succinic and glutaric acids and unidentified products. The salts were found to be effective for AA synthesis. Whatever
the composition of the catalyst, the alcohol oxidation favors the formation of the unidentified products, unlike the ketone
oxidation which favors that of the adipic acid. (NH4)0.5Sn1.25PMo12O40 led to the highest AA yield (56%) from cyclohexanone
oxidation, after 20 h of reaction. In addition, 31P NMR analysis showed that it has conserved the Keggin structure contrary to
others catalysts and that it can be used at least 3 times with reaction times of 20 h, without regeneration. From different cata-
lytic tests and 31P NMR data, reaction pathways have been proposed. The active species could be peroxo-polyoxometalates.
Graphical Abstract
Keywords Oxidation · Keggin-type polyoxometalates · Cyclohexanone · Cyclohexanol · Hydrogen peroxide · Adipic acid
* Chérifa Rabia
1 Introduction
c_rabia@yahoo.fr; crabia@usthb.dz
In the current chemical industry framework, adipic acid,
important precursor in the nylon production is obtained from
oxidation of a mixture of cyclohexanone and cyclohexanol
(KA oil) by nitric acid [1–4]. However, the reduction of this
latter leads to nitrogen oxides gases as by-products. Among
them, NO2 and NO are recycled and N2O5 and N2O are
vented to the outside, thus contributing to global warming
and ozone depletion.
1
Laboratoire de Chimie du Gaz Naturel, Faculté de
Chimie, Université des Sciences et de la Technologie
Houari Boumediène (USTHB), BP 32, El-Alia,
16111 Bab-Ezzouar, Alger, Algeria
2
Département de Chimie, Faculté des Sciences, Université
Mouloud Mammeri (UMMTO), 15000 Tizi Ouzou, Algeria
3
Institut de Recherches sur la Catalyse et l’Environnement de
Lyon, Université Lyon 1 -CNRS- 2, Avenue Albert Einstein,
69626 Villeurbanne, France
Vol.:(0123456789)
1 3

L. Mouheb et al.
To avoid these drawbacks, many research groups are
trying to develop a process environmentally friendly, to
substitute nitric acid by oxidants such as air, molecular
oxygen [5–7] or hydrogen peroxide [8–13], so-called
“Green Chemistry”. The hydrogen peroxide is the oxidant
that has focused a considerable attention for the adipic
acid synthesis, because its reduction leads only to water
as by-product. The used catalysts are peroxotungstates,
peroxomolybdates, peroxotungstate-organic complexes
[8–11], silica-functionalized ammonium tungstate [12]
and alumina supported Fe2O3, mesoporous nanoparticles
[13].
not require the addition of reagents as solvent, co-catalysts,
phase transfer compounds, surfactants and/or acid.
As, it was shown that the presence of a less metallic ele-
ment than molybdenum in the POM, modified the redox
properties with a better distribution of both reduced Mo(V)
and oxidized Mo(VI) sites and in addition, facilitated elec-
tron transfer during redox process [33, 34], our choice was
made on element introduction as antimony, bismuth or tin.
Thus, in this work, the ammonium salts of 12-molybdophos-
phoric acid were partially substituted by these elements.
The POMs have as formula (NH4)2.988Bi0.004PMo12O40
(noted NH4BiPMo12O40), (NH4)0.75Sb0.75PMo12O40 (noted
NH4SbPMo12O40) and (NH4)0.5Sn1.25PMo12O40 (noted
NH4SnPMo12O40). This salt series was tested in the adipic
acid synthesis from the liquid phase oxidation of cyclohex-
anone, cyclohexanol or cyclohexanol/cyclohexanone mix-
ture at 90 °C, using hydrogen peroxide (30%). The operation
conditions to optimize the AA yield were determined. The
effects of chemical composition and reaction time on AA
yield were examined. Reaction products were analyzed by
HPLC and the POMs were characterized after catalytic test
by 31P NMR spectroscopy. The physico-chemical characteri-
zations of these materials were published previously [35].
Nevertheless, hydrogen peroxide, very sensitive to reac-
tion conditions, decomposes easily to water and oxygen
molecular, therefore its oxidizing power is found reduced.
To prevent its decomposition, several authors have suggested
the addition of co-catalysts, phase transfer compounds and
surfactants [8–11]. However, despite the obtained high AA
yields, the presence of these harmful reactants, that are rela-
tively expensive, makes the process of AA production, less
environmentally friendly.
Polyoxometalates (POMs), particularly those based on
molybdenum are known for their strong oxidizing power,
stability and high resistance to oxidizing conditions. In addi-
tion, to facilitate the oxygen transfer to the substrate, POMs
may undergo multi-electron redox process without any struc-
tural modifications [14–16]. Although, their catalytic per-
formance has been established, for oxidation of alkenes and
alcohols in presence of the hydrogen peroxide [17–23], their
degradation over time was also observed. It was showed by
31P NMR that the hydrogen peroxide led to the POM decom-
position into peroxo-species as {PO4[WO(O2)2]4}3− and
{PO4[WO(O2)2]2}2−, in the case of the H3PW12O40 use
[24–27].
2 Experimental
2.1 Synthesis
(NH4)3PMo12O40 (noted NH4PMo12O40) and the series of
mixed ammonium salts (NH4)xAyPMo12O40 with An+=Sb3+,
Bi3+ or Sn2+ (noted NH4APMo12O40) were precipitated at
pH 1 from ammonium heptamolybdate, phosphoric acid
and ACl3 (Sb or Bi) or ACl2 (Sn) with added hydrochloric
acid, according with the method described in literature [34,
35]. NH4PMo12O40 and NH4BiPMo12O40 are yellow and
NH4SbPMo12O40 and NH4SnPMo12O40 are light green.
So, to make more efficient the use of both POM and
hydrogen peroxide, Nomiya et al. [28], have adopted a
strategy that consists to perform the catalytic oxidation at
two-steps. In the first step, the reaction takes place between
the organic substrate and POM catalyst, leading to reaction
products and reduced form of the catalyst that is of blue
color, characteristic color of POM, in its reduced state. In the
second one, the reduced catalyst is oxidized by the hydrogen
peroxide that is manifested by a color change from blue to
yellow, characteristic color of POM, in its oxidized state.
The latter continues to react with the remaining substrate
and so on until the reagent is exhausted or total reduction of
POM. This process was already used in our previous works
to examine the catalytic properties of Keggin type transition
metal substituted POM series of formula MxPMo12O40 (M:
H, Ni, Co, Fe), H3−2xCoxPMo12O40 and H3−2xNixPMo12O40
(x: 0.25–1.5) [29–31] and Dawson-type POM series of for-
mula K6P2MoxVyW18−xO62 (x:5,6 and y:0) and (x:5 and y:1)
[32] for adipic acid synthesis. It is a clean process that does
2.2 Catalytic Test
The adipic acid synthesis method is that described by
Nomiya et al. [28]. The liquid-phase oxidation of cyclohex-
anone (-one), cyclohexanol (-ol) or -one/-ol mixture 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 consists of calculated amounts of cata-
lyst and substrate. Whenever the catalyst exhibited a change
in color from yellow or light green to blue, the hydrogen
peroxide (30%) was added by fraction of 0.5 mL until the
color change from blue to the initial color of catalyst and
so on. The reaction mixture was stirred at 1000 rpm. The
catalytic cycle is finished when the POM catalyst is no
longer reduced after 20 h, indicating that the substrate was
1 3

Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol…
completely consumed. Hydrogen peroxide (30%) concentra-
tion was verified by potassium permanganate method prior
to use. It is noteworthy that beyond 90 °C, hydrogen perox-
ide may be decomposed.
3 Results and Discussion
The catalytic performances of the series of Keggin-type
salts, (NH4)xAyPMo12O40 (An+=Sb3+, Bi3+ or Sn2+) together
with H3PMo12O40 and (NH4)3PMo12O40, were examined in
the liquid-phase oxidation of -ol, -one and equimolar mix-
ture (-ol/-one) at 90 °C, in the presence of hydrogen peroxide
(30%) in free solvent.
To further verify the adipic acid yield, two separate
experiments were tested, with each catalyst. The reaction
mixture of the first one is analyzed by HPLC to determine
the substrate conversion and the selectivities of reaction
products. The reaction mixture of second experiment, after
completion of the reaction, is set to 4 °C overnight and the
adipic acid is recovered as crystals. These latter were iden-
Several preliminary tests were carried out with
NH4SnPMo12O40 catalyst. In the following different
mixtures: (i) POM + H2O2, (ii) -one + POM and (iii)
-one+POM+H2O2, a blue color characteristic to reduced
POM (Mo(VI) → Mo(V)) was observed. The (i) mixture
demonstrates that the POM has an oxidative power more
high than that of H2O2. This latter is oxidized to molecular
oxygen. The protons, required to H2O2 oxidation, come from
the ammonium ions of POM ((NH4)0.5Sn1.25PMo12O40). In
the case of (ii) mixture, the reaction take place, the sub-
strate was oxidized, but AA was not obtained suggesting that
probably intermediates species were formed. In (iii) mixture,
both substrate and H2O2 were oxidized by the POM but in
this case also, AA was not obtained. In conclusion, for that
the hydrogen peroxide to be able to act as an oxidant, it must
be added after reduction of the catalyst by the substrate.
These observations were already signaled in previous works
[20–23]. It should be noted that in the absence of catalyst,
AA was obtained as traces from -one oxidation by hydrogen
peroxide.
1
tified by melting point (152 °C) and FT-IR and H-NMR
spectroscopies.
The two experiments will allow to compare the AA yields
calculated as follows:
[
]
AA yield (%) = substrate conversion (%) × AA selectivity (%) ∕
100, for firstexperiment
AA yield (%) = AA recovered mass × 100∕
theoretical AA mass, for secondexperiment
The selectivities of products were calculated as follows:
Selectivity (%) ofidentified product = product mole number
× 100∕substratemole number
Selectivity (%) ofunidentified products
Moreover, it should be emphasized that the
(NH4)xAyPMo12O40 salts are not soluble in the presence of
substrate, but they become soluble in the reaction medium,
after addition of H2O2. The decomposition of this latter, very
sensitive to contact of the metal surface, could involve the
formation metal-peroxo species as observed in the presence
of metal ions with electronic configurations d0 [24, 25, 36,
37]. Therefore, the formation of metal-peroxo species can be
the cause of the solubility of ammonium salts.
= [substratemole number
ꢀ
−
identified products mole number]
× 100∕substratemole number
2.3 Analysis
31P MAS NMR spectra of used catalyst were recorded at
room temperature on a Bruker Avance 400 spectrometer.
85% H3PO4 was used as an external reference.
The reaction products were analyzed by HPLC (Dionex
Varian 380-LC) equipped of a 107H Corgel column and
a RID. Sulfuric acid diluted in degassed water is used as
mobile phase.
HPLC analysis of reaction mixture showed the forma-
tion of adipic (AA), succinic (SA) and glutaric (GA) acids,
in addition of unidentified products (UPs). An example of
chromatogram obtained from -one oxidation, in the presence
of NH4SbPMo12O40 catalyst, is given in Fig. 1. In the present
work, we were particularly interested to the AA formation.
Figure 2 shows the characteristic 1H-NMR and FT-IR spec-
tra of AA.
The 1H-NMR spectrum of adipic acid was recorded over
Bruker Ascend 400 spectrometer (1H-NMR 128 scans).
Dimethylsulfoxide-d6 (DMSO-d6) was used as solvent.
The FT-IR spectrum of adipic acid was recorded over
a Fourier Transformer Shimadzu FTIR-8400 on the
4000–400cm−1 range.
In order to fix the molar ratio, ncatalyst/n−one, leading to the
highest AA yield, the catalytic tests were carried out with
NH4SnPMo12O40. As shown in Table 1, there is a parallel
between the results, so the AA yield increases from 22 to
56% with the -one conversion from 77 to 100% and with
the ncatalyst/n−one molar ratio from 1.08×10−3 to 4.30×10−3.
From these results, all catalytic tests were carried out with
1 3

L. Mouheb et al.
Fig. 1 HPLC chromatogram of reaction products obtained from cyclohexanone oxidation, catalyst: NH4SbPMo12O40, ncatalyst/nsubstrat molar ratio:
4.30×10−3, reaction time: 20 h, reaction temperature: 90 °C
Fig. 2 1H-NMR and FT-IR
spectra of adipic acid
1 3

Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol…
a ncatalyst/n−one molar ratio of 4.30×10−3 corresponding to
125 mg catalyst and 15 mmol of substrate.
H3PMo12O40 and NH4PMo12O40 respectively. Glutaric acid
appears particularly in the presence of NH4BiPMo12O40,
NH4SbPMo12O40 and H3PMo12O40 (25, 24 and 7% of yield
respectively). Whatever the used POM, succinic acid is
obtained with a low yield (≤ 4%) while the unidentified
products are obtained with a high yield sum varying between
22 and 61%. In literature, only the formation of both glu-
taric and succinic acids as major by-products were reported
[8–11]. The results obtained indicate that the catalytic pro-
cess used in this work makes the action of both POM and
H2O2 more efficient with high cyclohexanone conversion,
but with also more unidentified products.
3.1 Cyclohexanone Oxidation
Figure 3 shows that the -one conversion is total after 5 h
of reaction for NH4SnPMo12O40, NH4SbPMo12O40 and
H3PMo12O40 evidencing the high activity of POMs. The
catalytic results summarized in Table 1, were recovered
after 20 h of reaction. The counter-cation has an influence
on reaction product distribution. Thus, the AA formation
is favored in the presence of substituted salts with yields
of 42, 47 and 56% for NH4BiPMo12O40, NH4SbPMo12O40
and NH4SnPMo12O40 respectively against 31 and 38% for
It should be emphasized that the obtained AA yields
after its cold crystallization are similar to those obtained
by HPLC analysis as shown in Table 2, with respec-
tively, 56 and 57% for NH4SnPMo12O40, 47 and 48% for
NH4SbPMo12O40, 42 and 42% for NH4BiPMo12O40, 38 and
38% for NH4PMo12O40 and 31 and 35% for H3PMo12O40.
From these results, it appears that the crystallization allows
practically to recover the totality of AA.
Table 1 AA yield and conversion as function of ncatalyst/n−one, cata-
lyst: NH4SnPMo12O40, reaction time: 20 h, reaction temperature:
90 °C
ncatalyst/n−one (103)
Conversion (%)
AA yield (%)
4.30
2.16
1.44
1.08
100
99
85
56
38
31
22
3.2 Cyclohexanol Oxidation
Figure 3b shows that -ol conversion in presence of
NH4SnPMo12O40 is practically total (97–100%) after 5 h
77
Fig. 3 Conversion of cyclohexanone (a) and cyclohexanol (b) as function of the reaction time. ncatalyst/nsubstrat molar ratio: 4.30×10−3, reaction
time: 20 h, reaction temperature: 90 °C
1 3

L. Mouheb et al.
Table 2 Catalytic performances
of POMs for cyclohexanone
oxidation, ncatalyst/n−one molar
ratio: 4.30×10−3, reaction time:
20 h, reaction temperature:
90 °C
Catalysts
Cold crystalliza- HPLC analysis
tion of AA
AA Yield (%)
Conversion
Selectivities (yields) (%)
(%mol)
AA
GA
SA
UPs
H3PMo12O40
35
38
41
48
57
100
100
96
97
100
31(31)
38(38)
44(42)
48(47)
56(56)
7 (7)
1 (1)
26 (25)
25 (24)
<1 (<1)
1 (1)
<1 (<1)
3 (3)
4 (4)
<1 (<1)
61 (61)
NH4PMo12O40
NH4BiPMo12O40
NH4SbPMo12O40
NH4SnPMo12O40
61 (61)
27 (26)
23 (22)
43 (43)
achieves ca.60% in the second one. The observations made
from the Figs. 3 and 4, are confirmed by the results of
Tables 2 and 3, that show high yields toward unidenti-
fied products (UPs) from -ol oxidation (80%) compared to
those obtained from -one oxidation (43%).
The AA yields obtained after its cold crystallization are
similar to those obtained by HPLC analysis as shown in
the Table 2, H3PMo12O40 (5–6%), NH4PMo12O40 (6–7%),
NH4BiPMo12O40 (2–2%), NH4SbPMo12O40 (10–10%) and
NH4SnPMo12O40 (17–17%). These results are consistent
with those observed with the ketone oxidation which con-
firms that the AA loss after crystallization is negligible.
Whatever the composition of the catalyst, the obtained
AA yields from -ol oxidation are much lower than those
obtained from -one oxidation (2–17 against 31–56%),
whereas, the unidentified products represent 80–96 against
22–61% of yields. These results, showing that the forma-
tion of UPs favored from -ol oxidation, can be explained
by the presence of the more reactive hydroxyl group than
the ketone group, so the oxidation of the alcohol will lead
to more products.
Fig. 4 Adipic acid yield as function of reaction time, catalyst:
NH4SnPMo12O40; Substrate: -one and -ol, ncatalyst/nsubstrat molar ratio:
4.30×10−3, reaction time: 20 h, reaction temperature: 90 °C
of reaction as in the case of the -one conversion. It is noted
that, after 4 h of catalytic test, the -ol conversion is more
important than that of -one (ca.90 against ca.65%). These
results indicate that the alcohol oxidizes more readily.
However, the curves of the Fig. 4 show a slow progres-
sion of AA production with the time from -ol oxidation,
while it is brutal beyond 10 h from -one oxidation. The
AA yield does not exceed 17% in the first case, while it
3.3 Oxidation of Cyclohexanol and Cyclohexanone
Mixture
In the industrial process, adipic acid is obtained from oxida-
tion of KA oil (mixture of cyclohexanol(50%) and cyclohex-
anone(50%)) by nitric acid [2–4]. In this context, the POMs
were used for KA oil oxidation in the same operation
Table 3 Catalytic performances
Catalysts
Cold crystalliza- HPLC analysis
of POMs for cyclohexanol
oxidation, ncatalyst/n−one molar
ratio: 4.30×10−3, reaction time:
20 h, reaction temperature:
90 °C
tion of AA
AA yield (%)
Conversion
(%mol)
Selectivities (yields) (%)
AA
GA
SA
UPs
H3PMo12O40
6
7
2
97
97
99
6 (5)
7 (6)
2 (2)
10 (10)
17 (17)
1 (1)
1 (1)
<1 (<1)
1 (1)
3 (3)
<1 (<1)
<1 (<1)
<1 (<1)
<1 (<1)
<1 (<1)
93 (90)
92 (90)
97 (96)
89 (87)
80 (80)
NH4PMo12O40
NH4BiPMo12O40
NH4SbPMo12O40
NH4SnPMo12O40
10
17
98
100
1 3

Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol…
conditions (ncatalyst/nsubstrat molar ratio: 4.30 × 10−3, reac-
tion time: 20 h, reaction temperature: 90 °C) and AA was
recovered by cold crystallization. The obtained AA yields,
from oxidation of -one, -ol and -ol/-one, are displayed in
Fig. 5. The results show that the AA formation is very
sensitive to the substrate nature, thus, for all catalysts, the
highest AA yields were obtained from -one oxidation with
35–57% against 17–39 and 2–17% from -ol/-one oxidation
and -ol oxidation, respectively. Regardless of the substrate
nature, the tin based catalyst led to best catalytic perfor-
mances with AA yields of ca.57, 39 and 17% against ca. 48,
34 and 10% for NH4SbPMo12O40 and ca. 41, 25 and 2% for
NH4BiPMo12O40, obtained from oxidation of -one, -ol/-one
and -ol respectively. The observed low AA yields from oxi-
dation of -ol/-one mixture compared to those obtained from
-one oxidation can be attributed to the formation of hydro-
gen bonds between the C=O group of -one and the hydrogen
of C–OH group of -ol, that makes difficult the oxidation of
both substrates. Therefore, the alcohol has inhibited the -one
oxidation. Results are in agreement with those observed in
literature [29–32, 38].
atoms per Keggin unit for Sb and Bi respectively) and its
high reducibility, leading thus to a better distribution of ions
with different states of oxidation.
3.3.1 Characterization of the Used Catalyst
31P NMR results of H3PMo12O40, NH4BiPMo12O40 and
NH4SbPMo12O40, used catalysts (Table 4) exhibit, after
20 h of reaction, chemical shifts at − 0.48, − 0.46 and at
−1.68 ppm, respectively, values, different to those of fresh
POMs that were observed at −4.4 ppm, characteristic value
of the oxidized species, [PMo12O40]3−, [35, 39]. These
results evidenced the presence of new phosphorous based
species. It was reported that in the presence of hydrogen per-
oxide, POMs usually decomposes into peroxo-based meta-
late complexes. In the case of H3PW12O40, the identified spe-
cies by 31P NMR were principally {PO4[WO(O2)2]4}3− and
{PO4[WO(O2)2]2}2− [24–27, 40]. Whereas in our case,
this is the reduced POM by the substrate that reacts with
H2O2, confirmed by blue-to-yellow color change. This color
change can suggested that H2O2 could simultaneously oxi-
dize the reduced POM and form peroxo-oxidized POM spe-
cies (noted peroxo-POMox). These latter could react with
intermediates species yielding oxygenated products whose
adipic acid. It was emphasized that peroxo-based POM spe-
cies can play a key role in oxidation reactions of organic
substrates [26, 27]. In the case of NH4SnPMo12O40, one peak
located at −4.1 ppm was observed, demonstrating that the
Keggin structure was conserved with probably a substitution
of some oxygen atoms bridge, (O2−), by oxygen “peroxo”
(O22−) in a symmetrical manner which would explain the
chemical shift value of −4.1 close to 4.4 ppm. These obser-
vations seem indicate that NH4SnPMo12O40 can undergo
stepwise multielectron redox process without any structural
modification contrarily to H3PMo12O40, NH4BiPMo12O40
and NH4SbPMo12O40.
All these results showed that the introduction of Sn, Sb
or Bi as counter ion into the polyoxometalate is favorable
to the AA formation. Indeed, the high catalytic activity of
POM, partially substituted by an element less metallic than
transition metal, Mo, can be attributed to the multifunctional
character of active sites (Lewis acid and oxidative) associ-
ated to the presence of redox couples, Mo(VI)/Mo(V) and
Sn(IV)/Sn(II), Sb(V)/Sb(III) or Bi(V)/Bi(III) as reported in
previous work [35] and the presence of a Brönsted acidity
coming from ammonium ions. These different sites intervene
simultaneous in the activation of substrate, oxidant (H2O2)
and oxygen-transfer and multielectron transfer. The better
catalytic performance exhibited by NH4SnPMo12O40 can be
lied to the high Sn atom number (1.25 against 0.75 and 0.004
3.4 Catalyst Stability
NH4SnPMo12O40 catalyst was reused in order to test its
catalytic stability. After each catalytic cycle, the adipic
acid is recovered after cold crystallization, then 15 mmol
of cyclohexanone were added to reaction mixture and the
Table 4 31P NMR results of POMs before and after ketone oxidation
POMs
δ (ppm) of fresh
catalyst
δ (ppm) after
catalytic test
H3PMo12O40
−4.4
−4.4
−4.4
−4.4
−0.22
−0.46
−1.68
−4.0
NH4BiPMo12O40
NH4SbPMo12O40
NH4SnPMo12O40
Fig. 5 Adipic acid yield as function of the POM composition, sub-
strate: -one, -ol and -one/-ol, ncatalyst/nsubstrat molar ratio: 4.30×10−3
reaction time: 20 h, reaction temperature: 90 °C
,
1 3

L. Mouheb et al.
oxidation reaction was carried out with the soluble used
catalyst, under the optimized conditions. Each cycle lasts
20 h. The catalytic activity results are represented in Fig. 6.
A progressive decrease of adipic acid yield from 56 to 0% is
observed after five reaction cycles. The significant decrease
in AA yield after each cycle would probably arise from the
action of several factors such as the decrease in acidity from
ammonium ions, which results in reduction of the oxidizing
power of H2O2 (the protons being necessary for its reduc-
tion), and subsequently decrease of peroxo species, active
sites, necessary for the formation of oxygenated products.
These results showed that NH4SnPMo12O40 catalyst can be
used at least 3 times with reaction times of 20 h, without
regeneration.
POMred + H2O2 → peroxo − POMox
(2)
Peroxo−POMox + intermediate species
→ reaction products + POMox
(3)
The first stage (Eq. 1) occurs between the organic sub-
strate (-one) and POM, leading to reaction product interme-
diates and to the reduced form of the catalyst (blue color).
In the second stage (Eq. 2), the reaction takes place between
reduced POM (noted POMred) and hydrogen peroxide lead-
ing to simultaneously oxidation of reduced POM (yellow
color) and formation of peroxo-oxidized POM (noted per-
oxo-POMox), active species. This second step can be con-
sidered as the rate-determining step. The color change from
blue to yellow can last 10 min. After a time of 9–10 h, no
color change was observed suggesting that in the reaction
mixture, only peroxo-POMox species are present. These lat-
ter react with intermediate species to give oxygenated prod-
ucts and oxidized POM (POMox) (Eq. 3). The total reaction
time must be of 20 h, required time to obtain the maximum
AA yield. Whatever the stage, the predominant mechanism
is of the redox type, with direct involvement of the POM in
the reaction in oxidized, reduced and peroxo species forms.
In conclusion, these observations would indicate that the
formation of the oxygenated compounds would be initiated
only after addition of hydrogen peroxide and the POM alone
is not able to oxidize the organic substrate, to oxygenated
compounds.
3.5 Reaction Pathways
It is known that clusters POMs are good electron acceptors
exhibiting fast reversible multielectron redox transforma-
tions under mild conditions. In the case of molybdenum
based POMs, the color is sensitive to the oxidation state of
Mo. Thus, the POM changes from yellow, characteristic of
Mo (VI), to blue, characteristic of Mo (V), in the presence
of reducing compound without any structural modification.
It should be pointed out that only two Mo(VI) per Keggin
anion can undergo a reduction and therefore, result POM
with mixed valence. The change of color would allow us to
propose possible reaction pathways.
From the method of Nomiya et al. [28] that allows to
visualize the different catalytic oxidation steps and 31P NMR
results that show the formation of a new specie attributed
to peroxo-POMox, the following pathways could can be
proposed:
This proposition of reaction pathways is according with
those reported by others authors which have suggested that
the peroxo species could be the key active catalytic interme-
diate, in the one-step oxidative cleavage of cyclohexene to
adipic acid [8, 10, 11, 41] and in the liquid phase oxidation
reactions of the alkenes and alcohols [24, 25].
Organic substrate + POMox → POMred + intermediatespecies
(1)
4 Conclusion
In this work, the heteropolysalts, (NH4)xAyPMo12O40
(An+=Sb3+, Bi3+ or Sn2+), in the presence of hydrogen per-
oxide (30%), were found to be very active for both cyclohex-
anone and cyclohexanol oxidation with 96–100% of conver-
sion. For all catalysts, the formation of adipic acid is favored
from cyclohexanone oxidation while that of unidentified
products from cyclohexanol oxidation. Among, the tested
catalysts, (NH4)0.5Sn1.25PMo12O40 is the most effective
towards adipic acid with 56% of yield. The high catalytic
activity of (NH4)xAyPMo12O40 salts can be due to the mul-
tifunctional character of active sites and to the presence of
peroxo-polyoxometalate species.
Fig. 6 Adipic acid yields after several reaction cycles, catalyst:
NH4SnPMo12O40, ncatalyst/n−one molar ratio: 4.30×10−3, reaction time:
20 h, reaction temperature: 90 °C
This green process of AA production can be an alterna-
tive to that of current industrial using nitric acid as oxidant.
1 3

Clean Adipic Acid Synthesis from Liquid-Phase Oxidation of Cyclohexanone and Cyclohexanol…
23. Mizuno N, Kamata K, Yamaguchi K (2011) Top Organomet Chem
References
37:127–160
24. Salles L, Aubry C, Thouvenot R, Robert F, Doremieux-Morin C,
Chottard G, Ledon H, Jeanin Y, Bregault P (1994) Inorg Chem
33:871–878
1. Lindsay AF (1954) Spec Suppl Chem Eng Sci 8:78
2. Castellan A, Bart JCJ, Cavallaro S (1991) Catal Today 9:255
3. Castellan A, Bart JCJ, Cavallaro S (1991) Catal Today 9:285–299
4. Castellan A, Bart JCJ, Cavallaro S (1991) Catal Today 9:301
5. Chavan SA, Srinivas D, Ratnasamy P (2002) J Catal 212:39
6. Cavani F, Ferronia L, Frattini A, Lucarelli C, Mazzinia A,
Raabova K, Alini S, Accorinti P, Babini P (2011) Appl Catal A
391:118
25. Duucan DC, Chambers C, Hecht E, Hill CL (1995) J Am Chem
Soc 117:681–691
26. Kuznetsova LI, Kuznetsova NI, Maksimovskaya RI, Aleshina GI,
Koscheeva OS, Utkin VA (2011) Catal Lett 141(10):1442
27. Mengs LY, Zhai SR, Sun ZC, Zhang F, Xiao ZY, An QD (2015)
Microporous Mesoporous Mater 204:123
7. Piyali B, Kajari G, Sk. Safikul I, Astam KP, Sk Manirul I, Asim
B (2016) ACS Sustain 4:7147–7157
28. Nomiya K, Miwa M, Sugaya Y (1984) Polyhedron 3:607–610
29. Mazari T, Benadji S, Tahar A, Dermeche L, Rabia C (2013) J
Mater Sci 3:146–151
8. Blach P, Böstrom Z, Franceschi-Messant S, Lattes A, Perez E, I.
Rico-Lattes (2010) Tetrahedron 66:7124–7128
9. Penate IQ, Lesage G, Cognet P, Poux M (2012) Chem Eng J
200–202:357–364
30. Benadji S, Mazari T, Dermeche L, Salhi N, Cadot E, Rabia C
(2013) Catal Lett 143:749–755
31. Tahar A, Benadji S, Mazari T, Dermeche L, Roch-Marchal C,
Rabia C (2015) Catal Lett 145:569–575
10. Alcaniz-Monge J, Trautwein G, Garcia-Garcia A (2014) J Mol
Catal A 394:211–216
32. Moudjahed M, Dermeche L, Benadji S, Mazari T, Rabia C (2016)
J Mol Catal A 414:72–77
11. Shang M, Noël T, Wang Q,SuY, Miyabayashi K, Hessel V, Hasebe
S (2015) Chem Eng J 260:454–462
33. Cavani F, Mezzogori R, Pigamo A, Trifiro F (2001) Chem Eng J
82:33
12. Vafaeezadeh M, Hashemi MM (2014) Catal Commun 43:169–172
13. Astam K. Patra AK, Dutta A, Bhaumik A (2013) Chem Eur J
19:12388–12395
34. Cavani F, Mezzogori R, Pigamo A, Trifiro F (2000) Surf Chem
Catal 3:523
14. Misono M (1987) Catal Rev Sci Eng 29:269–321
15. Hill CL, Prosser-McCartha CM (1995) Coord Chem Rev
143:407–455
35. Dermeche L, Thouvenot R, Hocine S, Rabia C (2009) Inorg Chim-
ica Acta 362:3896–3900
36. Clerici MG (1990) Heterogeneous Catal Fine Chem III, E. P.
0.412596 A1. ENI
16. Kozhevnikov IV (1998) Chem Rev 98:171–198
17. Vazylyev M, Dorit S-R, Haimov A, Maayan G, Neumann R (2005)
Top Catal 34(1–4):93–99
37. Ingle RH, Kola NK, Manikandan P (2007) J Mol Catal A
262:52–58
18. Neumann R, Gara M (1994) J Am Chem Soc 116:5509–5510
19. Zhang X, Chen Q, Duncan DC, Campana CF, Hill CL (1997)
Inorg Chem 36:4208–4215
38. Chavan SA, Srinivas D, Ratnasamy P (2002) J Catal 212:39–45
39. Chami F, Dermeche L, Saadi A, Rabia C (2013) Appl Petrochem
Res 3:35–45
20. Bösing M, Nöh A, Loose I, Krebs B (1998) J Am Chem Soc
120:7252–7259
40. Maksimovskaya RI (2013) Polyhedron 65:54
41. Quesada Penate I, Lesage G, Cognet P, Poux M (2012) Chem Eng
J 200–202:357–364
21. Mizuno N, Seki Y, Nishiyama Y, Kiyoto I, Misono M (1999) J
Catal 184:550–552
22. Ben-Daniel R, Khenkin AM, Neumaun R (2000) Chem Eur J
6:3722–3728
1 3