Dawson-type polyoxometalates as green catalysts for adipic acid synthesis

Article abstract of DOI:10.1016/j.molcata.2015.12.014

Dawson-type POMs series of formula α- and β-K₆P₂W₁₈O₆₂ isomers, α1- and α2-K₆P₂Mo₅W₁₃O₆₂ isomers, α-K₆P₂Mo₆W₁₂O₆₂ isomer and α1-K₇P₂Mo₅VW₁₂O₆₂ isomer, have been used as catalysts for the liquid phase oxidation of cyclohexanol, cyclohexanone and cyclohexanol/cyclohexanone mixture to form adipic acid in presence of hydrogen peroxide without solvent, without phase-transfer agents and without adding acid. ³¹P NMR spectroscopy of different POMs after cyclohexanol oxidation showed the formation of new active species that can be attributed to "peroxo-POMox" form. An adipic acid yield of 69% was obtained from the oxidation of a mixture of cyclohexanol (50%) and cyclohexanone (50%) with α-K₆P₂Mo₆W₁₂O₆₂ isomer.

Full text of DOI:10.1016/j.molcata.2015.12.014

Accepted Manuscript
Title: Dawson-type polyoxometalates as green catalysts for
adipic acid synthesis
Author: M. Moudjahed L. Dermeche S. Benadji T. Mazari C.
Rabia
PII:
S1381-1169(15)30177-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.12.014
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Journal of Molecular Catalysis A: Chemical
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Accepted date:
16-11-2015
14-12-2015
20-12-2015
Please cite this article as: M.Moudjahed, L.Dermeche, S.Benadji, T.Mazari, C.Rabia,
Dawson-type polyoxometalates as green catalysts for adipic acid synthesis, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.12.014
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Dawson-type polyoxometalates as green catalysts for adipic acid
synthesis
M. Moudjahed1, L. Dermeche1,2, S. Benadji1, T. Mazari1,2 and C. Rabia1,*
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,
Algérie
2 Département de Chimie, Faculté des Sciences, Université Mouloud Mammeri (UMMTO),
15000 Tizi-Ouzou, Algérie
(*) corresponding author: c_rabia@yahoo.fr/crabia@usthb.dz
Graphical abstract
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Highlights


Adipic acid synthesis over Dawson-type polyoxometalates with H2O2 in free solvent.
Formation of new ‘‘peroxo-POMox’’ active species evidenced by 31P NMR
spectroscopy.


α-K6P2Mo6W12O62 isomer is the most efficient in cyclohexanol and cyclohexanone
oxidation.
α-K6P2Mo6W12O62 gives 69% AA yield from oxidation of 50% cyclohexanol/50%
cyclohexanone mixture.
Abstract
Dawson-type POMs series of formula α- and β- K6P2W18O62 isomers, α1- and α2-
K6P2Mo5W13O62 isomers, α-K6P2Mo6W12O62 isomer and α1-K7P2Mo5VW12O62 isomer, have
been used as catalysts for the liquid phase oxidation of cyclohexanol, cyclohexanone and
cyclohexanol/cyclohexanone mixture to form adipic acid in presence of hydrogen peroxide
without solvent, without phase-transfer agents and without adding acid. 31P NMR
spectroscopy of different POMs after cyclohexanol oxidation showed the formation of new
active species that can be attributed to ‘‘peroxo-POMox’’ form. An adipic acid yield of 69%
was obtained from the oxidation of a mixture of cyclohexanol (50%) and cyclohexanone
(50%) with α-K6P2Mo6W12O62 isomer.
Keywords: Dawson-type polyoxometalates; Cyclohexanone; Cyclohexanol; Hydrogen
peroxide; Adipic acid.

1. Introduction
The substrate liquid-phase oxidation in presence of the transition metal based catalysts
with a clean oxidant as hydrogen peroxide, can be an environmentally friendly alternative to
that using inorganic oxidants as HNO3 and HClO4. Contrarily, to the reduction of
conventional acids that lead to pollutant gas causing serious environmental problems, that of
hydrogen peroxide leads only to water. Moreover, in the oxidation mechanisms with
hydrogen peroxide, it was suggested that H2O2 can be involved in the peroxometal formation
(metal coming from the catalyst) that is expected to give stoechiometrically oxygen atoms to
the substrate [1] and on the other hand, it was observed that peroxometal active oxidant
species are favored when the metal of catalyst, in its highest oxidation state, is both a Lewis
acid and a weak oxidant, as transition metal ions with d0 or d1 electronic configurations
(Mo(VI), W(VI), V(V), Nb(V)) [2].
Keggin-type polyoxometalates (POMs) contain both redox centers and Lewis acid
sites (Mo(VI), W(VI) and V(V)) and were reported as efficient catalysts for liquid phase
oxidation [3]. The advantages of this type of compounds are their great stability towards
oxygen donors and the possibility of the incorporation of several transition metals [4].
Although, Keggin-type POMs (PM12 with M: Mo(VI), W(VI)) are the most studied, Dawson-
type POMs (P2M18), that have not received much attention as catalysts, may be interesting
candidates to catalyze oxidation reactions, because the number of elements with a high
oxidation state is greater than that corresponding to the Keggin anion (18 against 12). In this
context, this work was focalisated in the application of Dawson-type POMs as catalysts for
adipic acid synthesis from liquid-phase oxidation of cyclohexanol and/or cyclohexanone in
the presence of hydrogen peroxide.
Adipic acid (AA), main precursor for nylon 6,6, is considered as a valuable raw
material for industry. It is prepared, in industrial process, by oxidizing of cyclohexanol and
cyclohexanone, so-called ketone/alcohol oil (KA oil), coming from cyclohexane or benzene,
using nitric acid as oxidant, whose major disadvantage is the release of the nitrogen oxide
(N2O), source of greenhouse effect and ozone depletion [5,6].
The use of hydrogen peroxide as oxidant for the AA synthesis in the presence of
tungsten based catalysts was the subject of many works [7-12]. For increase oxidation
reaction rates, ionic liquid catalysts composed of tungstate based species and sulfonic,
sulphamic or p-toluene sulfonic acid groups [7-9], surfactant-type catalysts (STC) as phase-
transfer agents [10], and conventional acids (H2SO4, H3PO4) [11,12] were used. These
additives are harmful to the environment.

Several studies have examined the adipic acid synthesis from cyclohexanone
oxidation, in the presence of molecular oxygen or peroxide hydrogen. It was reported
that the cyclohexanone oxidation by molecular oxygen over H3+xPMo12-xVxO40 (x = 1 and
2) catalyst, in acetic acid medium, led to a strong increase of the conversion compared to
that obtained in the presence of water only [13]. The authors have suggested that the
addition of the acetic acid promotes a radical autoxidation mechanism while an aqueous
medium that of a redox mechanism. With Mn(OAc)2/Co(OAc)2 [14] and µ3-Oxo-
bridged Co/Mn cluster complexes systems [15], high catalytic activity and high AA
selectivity were also observed in acetic acid medium for cyclohexanone oxidation with
molecular oxygen. The positive effect, exerted by acetic acid to stabilize and prevent the
decomposition of hydrogen peroxide, was demonstrated.
With hydrogen peroxide as oxidant for cyclohexanone oxidation to adipic acid, it
was shown that tetrabutylammonium salts of both H3PW12O40 and H3PMo12O40, react
catalytically in the presence of H2O2; whereas, they react only stoichiometrically in its
absence [16]. Keggin-type transition metal-substituted phosphomolybdic as MxPMo12O40
(M: Ni, Co, Fe) [17], H3-2xCoxPMo12O40 [18], H3-2xNixPMo12O40 and (NH4)3-
2xNixPMo12O40 (x: 0.0–1.5) [19] are found to be efficient for adipic acid synthesis, from
liquid-phase oxidation of cyclohexanol and/or cyclohexanone in the presence of
hydrogen peroxide without solvent, without phase-transfer agents and without adding
acid because, the acidity necessary to the hydrogen peroxide reduction as well as the
oxidizing power needed for the oxidation reaction could come from the POM only. In
these works, it was shown that the molybdenum based POMS, are easily reducible to
give blue compounds without modifying their structures. This allows to follow the
reaction evolution. Cobalt based salts generally present a higher activity than nickel
based salts. Furthermore, it was observed that the cyclohexanol addition to
cyclohexanone caused a decrease of adipic acid yield [15,17-19].
In our previous works, we also showed that Dawson-type POMs, K6P2MoxW18-xO62
(x= 0, 5, 6) and α1- and α2- isomer compounds were found to be active in methanol oxidation
at 260 °C in the presence of molecular oxygen and the product distribution (formaldehyde,
methyl formate, dimethylether, dimethoxymethane) depends on the polyanion composition
and on the framework symmetry [20]. In this work, Dawson type POMs as α- and
β-K6P2W18O62 isomers, α1- and α2-K6P2Mo5W13O62 isomers, α-K6P2Mo6W12O62 isomer and
α1-K7P2Mo5VW12O62 isomer, have been used as catalysts for the liquid phase oxidation of
cyclohexanol, cyclohexanone and cyclohexanol/cyclohexanone mixture to form adipic acid in

presence of hydrogen peroxide and free solvent. The physico-chemical characterizations
(Fourier Transform Infrared spectroscopy (FT-IR), 31P NMR spectroscopy, UV-Visible
spectroscopy and thermal analysis (TGA and TDA) of these POMs were published previously
[20].
2. Experimental
2.1 POMs preparation
α- and β- isomers of K6P2W18O62 were prepared according to the literature data [21].
Mixed W/Mo/V polyoxometalates α-K6P2Mo6W12O62 isomer, α1- and α2-K6P2Mo5W13O62
and α1-K7P2Mo5VW12O62 isomer, were obtained from the hexavacant anion [H2P2W12O56]12-
according to the method described by Contant et al. including the successive condensation
and hydrolysis [22]. The different POMs were noted: α-P2W18, β-P2W18, α1-P2Mo5W13, α2-
P2Mo5W13, α-P2Mo6W12 and α1-P2Mo5VW12. Scheme 1 (Fig.1) summarizes the different
steps of the preparation of substituted POMs from α-P2W18.
2.2 Catalytic test
The synthesis method is based on that described in the literature [16]. The liquid-phase
oxidation of cyclohexanol (-ol) or mixture of cyclohexanol (-ol) and cyclohexanone (-one)
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
whenever the catalyst showed a color change from yellow or light green to blue, characteristic
color of reduced molybdenum based POMs. The reaction mixture was stirred at 800 rpm. The
catalytic cycle is finished when the POM catalyst is no longer reduced, indicating that the
substrate was completely consumed. The reaction time was found to be of 20 h. Hydrogen
peroxide (30 % solution in water) concentration was verified by potassium permanganate
method prior to use. It is noteworthy that beyond 90 ˚C, hydrogen peroxide may be
decomposed. The resultant homogeneous mixture was cooled at 4 ˚C overnight. Adipic acid,
one of the oxidation products, was isolated as crystals and identified by FT-IR spectroscopy
and melting point (152 ˚C). The AA yield (%) is: AA mass formed ×100/theoretical AA mass.
13
2.3 P NMR characterization
31P MAS NMR spectra were measured at room temperature on a Bruker Avance 400
spectrometer. 85% H3PO4 was used as an external reference.

3. Results and discussion
The catalytic properties of α-P2W18, β-P2W18, α1-P2Mo5W13, α2-P2Mo5W13,
α-P2Mo6W12 and α1-P2Mo5VW12 were examined in the liquid-phase oxidation, of
cyclohexanol (-ol) and mixture of the two reagents (-ol/-one) at different percentages (10-
90%) in presence of hydrogen peroxide (30%) and free solvent. It was shown that the reaction
did not take place when the initial mixture is composed simultaneously of catalyst, H2O2 and
substrate (-ol or -one). The importance of gradual addition of H2O2 after each reduction of the
catalyst by the substrate is confirmed by preliminary tests. The oxidation reaction leads to the
formation of adipic, glutaric and succinic acids. In the present work, only adipic acid
formation was taken into account, it is isolated from the other products by cold crystallization.
The adipic acid purity was verified by the melting point measurement (152 °C) and the record
of its IR spectrum.
Figure 2 shows the polyhedral representation of the Dawson polyanion structure,
[P2M18O62]6- and numbering of the metallic atoms according to IUPAC recommendations. In
Dawson heteropolyanions, two different types of clusters are present: two terminal trimetallic
groups M3O13 (numbers 1 to 3 and 16 to 18) and six dimetallic groups M2O10 (numbers 4 to
15) arranged in a double crown. The overall framework having a D3h symmetry corresponds
to α isomer. M3O13 and M2O10 clusters are connected to each other only by single µ-oxo-
bridges and the local symmetry of all W(VI), Mo(VI) and V(V) is octahedral. Based on the
results of tungsten, vanadium and phosphorus NMR spectroscopy, Contant et al [22]
assigned and marked the positions of molybdenum and vanadium atoms in the mixed
W/Mo/V Dawson heteropolyanions as follows: 1,4,9,10,15,16-P2Mo6W12 (α-P2Mo6W12),
1,9,10,15,16-P2Mo5W13 (α1-P2Mo5W13), 4,9,10,15,16-P2Mo5W13; (α2-P2Mo5W13), and
1,9,10,15,16-4-P2Mo5VW12 (α1-P2Mo5VW12).
-
Cyclohexanol oxidation
Different preliminary tests, carried out with P2W12Mo6 showed that the AA formation
requires the existence of both -ol substrate and catalyst. Hydrogen peroxide must be added
progressively by fraction after every catalyst reduction by the substrate. AA is not observed in
the absence of catalyst or hydrogen peroxide. The catalyst reduction is manifested by the
appearance of the blue color characterizing Mo(V) in POM. Thereafter, the H2O2 injection
leads to POM oxidation that comes back to its yellow initial color characterizing Mo(VI) in

POM, as in free catalyst. The substrate oxidation lasted 20 h after starting the addition of
H2O2.
In order to optimize the reaction conditions, the effect of catalyst mass (0.032-0.250 g)
on the adipic acid yield, was examined with P2W12Mo6 catalyst with a rate agitation of 400
rmp. The results are reported in Table 1. An increase of the catalyst mass from 0.032 to 0.250
g led to an increase of the AA yield from 16 to 59%. It should be noted that when the catalyst
mass increases from 0.125 to 0.250 g, the difference in the AA yields is of 3% only, while
when a catalyst mass increase from 0.063 to 0.125 g, AA yield increases by a factor greater 2
(from 26 to 56%). These results may suggest that 15 mmol of substrate are not enough to
reduce 0.250 g of POM catalyst. As 0.125 and 0.250 g lead to similar AA yields (56-59%),
the catalyst mass was fixed at 0.125 g corresponding to 7.4 10-5 mol. In earlier work, it was
shown that the maximum AA yield is achieved with a high stirring rate (800 rpm) [17-19] and
many researches also recommend the use of vigorous stirring rate of the reaction mixture
[15,23,24]. With a catalyst mass of 0.125 g, it was observed that an increase of stirring rate
from 400 to 800 rpm led to an AA yield increase from 56 to 59%, value close to that obtained
with a catalyst mass of 0.250 g. This result confirms the reported observations in the
literature.
Table 2 shows the catalytic performances of different POMs in the cyclohexanol
oxidation with H2O2. α- and β-P2W18 isomers have similar catalytic behavior (5 and 6% of
AA yield, respectively) that seems to be independent of the Dawson anion symmetry. The
substitution of W atoms by Mo atoms led to a strong increase of the AA yield with 35, 45 and
59% in the presence of α1-P2W13Mo5, α2-P2W13Mo5 and α-P2W12Mo6, respectively. The
introduction of molybdenum in Dawson unit indicates a strong increase of the oxidative
power of the catalyst and this latter increases with Mo atom number. The strong oxidative
character of P2W12Mo6 compared to that of P2W13Mo5 was already observed for the methanol
oxidation [20].
Contrary to P2W18, in the case of P2W13Mo5, the AA production is sensitive to position
of Mo metal in the framework. It is favored when the Mo atoms are in the double crown (4, 9,
10 and 15 position) and in the terminal trimetallic group (1or16 position), with 45% against
35% of AA yield in the presence of α2-P2W13Mo5 and α1-P2W13Mo5, respectively.
In P/W/Mo/V system (α1-P2Mo5VW12), the substitution of one Mo atom situated in
position “4” by a V atom in α-P2W12Mo6 led to an AA yield decrease from 59 to 11%. These
results seem to indicate that a strong oxidative character of the catalyst is not favored to AA
formation but probably it’s in favor to that of the other carboxylic acids. Another hypothesis

could be the decomposition of H2O2 to O2 and H2O by the vanadium, similar to Fenton
mechanism [25].
The oxidative power introduced by Mo and V atoms has already been observed in the
case of both Keggin and Dawson type heteropolyanions [26,27]. The redox sites were
attributed to the metal-terminal oxygen bond (M=Od).
From these results, P2W12Mo6 displays as the most efficient catalyst for cyclohexanol
oxidation with 59% of AA yield. Its efficacy has already been observed for methanol
oxidation [20].
-
Cyclohexanone-Cyclohexanol mixture oxidation
The nitric acid based industrial process for AA production proceeds by oxidation of a
mixture of cyclohexanol (50 %) and cyclohexanone (50 %) [28-30]. It seemed interesting to
examine the influence of the (-one) addition (10–90 %) to the (-ol) on AA yield. In this study,
the reaction was conducted with the most active catalyst, P2W12Mo6 (cat/-one molar ratio:
4.93 10-3). The catalytic results (Fig.3) show that the obtained AA yields from -ol oxidation
and from -one oxidation are very similar (59 and 58%, respectively). Similar AA yields equal
to 46-51%, 48-51%, 52-51%, 42-41% were also obtained for the composition of reaction
mixture corresponding to -ol/-one ratio of 60/40 or 40/60 , of 70/30 or 30/70, of 80/20 or
20/80 and of 90/10 or 10/90, respectively. It should be emphasized that 50% of -ol and 50%
of -one composition, corresponding to industrial process (KA oil) exhibits the best
performance with 69% of AA yield.
This similarity for AA yields might suggest that alcohol oxidation directly give the
AA acid without going through the ketone oxidation as would be expected. Oxidation of the
two substrates would be on different catalytic sites. The simultaneous presence of alcohol (10-
90%) and ketone (90-10%) mixtures inhibit the oxidation of each of the reagents except for
the -ol (50%)/-one (50%) mixture. These results differ from those obtained with Keggin-type
POMs, where, it was observed that the yields obtained from the alcohol are much lower than
those derived from the ketone oxidation indicating that the following steps take place:
-ol→ -one→ AA [17-19].
31
Table 3 and the figure 4 present the results of P NMR spectroscopy of different
POMs before and after cyclohexanol oxidation. In the case P/W systems, chemical shifts after
reaction showed two characteristic peaks of both isomers P2W18 (α- and β-) indicating that
POM structure was conserved after reaction. For the P/W/Mo and P/W/Mo/V systems, the 31P

NMR results show the appearance of a new chemical species presenting only one signal at -
0.22 and -0.49 ppm, respectively.
31
Based on P NMR results, the appearance of a single peak seems to indicate that the
environment of the two phosphorus atoms of P2W12Mo6 and those of P2W12Mo5V Dawson
POMs are identical respectively, thereby suggesting the formation of a new species that
would be the active species favorable for the carboxylic acid formation. It was reported that
the presence of H2O2, in addition to its function to oxidize the POM, lead to the formation of
‘‘peroxo-POMox’’ [18,19]. On the other hand, it was also suggested that H2O2 can be
involved in the peroxometal formation that is expected to give stoechiometrically oxygen
atoms to the substrate [1]. In the case of P/W/Mo/V system, vanadium with its very high
oxidation power would behave like a free ion and would decompose hydrogen peroxide and
the active species « peroxo-POMox » would be in a reduced number contrary to the P/W/Mo
system. This might explain its poor performance in adipic acid formation.
It was reported, in the synthesis of adipic acid via nitric acid oxidation of cyclohexanol
(or cyclohexanone) in a two step continuous process in the presence of V/Cu based catalyst,
that in the first process step, there is formation of soluble oxidized intermediates as
cyclohexanediol (or cyclohexanedione) that are then converted into adipic acid in the second
step [31]. In the first process step, nitric acid reacts as oxidant and the VV species of catalyst,
are involved only to a small extent and in the second step, the catalyst becomes more effective
and the intermediates products are easily oxidized to AA.
In this work, the different preliminary tests, carried out with P2W12Mo6 showed that
the AA formation requires the existence of both substrate and catalyst. Hydrogen peroxide
must be added progressively by fraction after every catalyst reduction by the substrate. AA is
31
not observed in the absence of catalyst or hydrogen peroxide. On the other hand, P NMR
results have suggested the formation of ‘‘peroxo-POMox’’ active species. Based on these
results and according to the hypothesis of the adipic acid synthesis via nitric acid oxidation in
two step continuous process, the following steps can take place in the cycle:
Substrat + POM → oxidized intermediate products + POMred
POMred + H2O2 → ‘‘peroxo-POMox’’
Oxidized intermediate products + ‘‘peroxo-POMox’’ → AA + POMox

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.
IV. Conclusion
The catalytic properties of α-P2W18, β-P2W18, α1-P2Mo5W13, α2-P2Mo5W13, α-
P2Mo6W12 and α1-P2Mo5VW12 Dawson-type POMs were examined in the liquid-phase
oxidation, of cyclohexanol and/or cyclohexanone in the presence of hydrogen peroxide.
Among the catalysts tested, α-K6P2Mo6W12O62 isomer exhibits the best performance with
59% of AA yield from both oxidation of cyclohexanol and cyclohexanone and 69% of AA
yield from oxidation of cyclohexanol (50%) and cyclohexanone (50%) mixture, composition
31
corresponding to industrial process (KA oil). P NMR analysis showed that active species in
the adipic acid formation can be ‘‘peroxo-POMox’’ form. This method presents an
environmentally friendly for adipic acid synthesis, in absence of the solvent, phase-transfer
agents and adding acid.

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[31] A. Castellan, J.C.J. Bart, S. Cavallaro, Catal. Today 9 (1991) 301-322.

α1-K7P2W12Mo5VO62
H+
V
Mo , H+
W , H+
K2CO3
α-K6P2W18O62
α-K12H2P2W12O48
α1-LiK9P2W12Mo5O61
α1-K7P2W13Mo5O62
Mo H+
W , H+
KHCO3
α2-LiK9P2W12Mo5O61
α2-K6P2W13Mo5O62
α-K6P2W12Mo6O62
Figure 1: Successive steps of Dawson POM synthesis [22].

Figure 2: Polyhedral representation of the Dawson structure of [P2M18O62]6- polyanion.

Cyclohexanone (%)
0
20
40
60
80
100
10
30
50
70
90
70
68
66
64
62
60
58
56
54
52
50
48
46
44
42
40
100
90
80
70
60
50
40
30
20
10
0
Cyclohexanol (%)
Figure 3: Adipic acid yields obtained from the oxidation of the cyclohexanol/cyclohexanone
mixture at different percentages over P2W12Mo6.

a
c
- 13.12
-13,12
- 0.22
0
80
- 80
- 60
- 240
0
48
24
- 24
- 48
δ (ppm)
δ (ppm)
b
d
-0,49
- 0.49
-11.80 -12.61
-11,80 -12,61
0
38
19
- 19
- 38
0
10
-10
-21
-31
δ (ppm)
δ (ppm)
Figure 4: 31P NMR Spectrum of α-K6P2W18O62 (a), β-K6P2W18O62 (b), α-K6P2Mo6W12O62 (c)
and α1-K6P2Mo5VW12O62(d)

Table 1: Adipic acid yield as a function of the catalyst weight
m (g)
n (mol)
n(catalyst) / n(-ol) Reaction
(rmp)
rate AA Yield (%)
0.032
0.063
0.125
0.250
0.125
1.85 10-5
3.7 10-5
7.4 10-5
14.8 10-5
7.4 10-5
1.23 10-3
2.46 10-3
4.93 10-3
9.86 10-3
4.93 10-3
400
400
400
400
800
16
26
56
59
59
Reaction conditions: catalyst: K6P2W12Mo6O62, ncyclohexanol: 15mmol, H2O2 (30%): 0.5 mL/h,
T: 90°C, reaction time: 20 h.

Table 2: Adipic acid yield as a function for the POM composition
POM
Notation
AA Yield (%)
α-K6P2W18O62
α-P2W18
5
β-K6P2W18O62
β-P2W18
6
1,9,10,15,16-K6P2Mo5W13O62
4,9,10,15,16- K6P2Mo5W13O62
1,4,9,10,15,16- K6P2Mo6W12O62
α1-P2W13Mo5
α2-P2W13Mo5
α-P2W12Mo6
35
45
59
1,9,10,15,16-4- K7P2Mo5VW12O62 α1-P2W12Mo5V 11
Reaction conditions: Catalyst weight: 125mg, ncyclohexanol: 15mmol, H2O2 (30%): 0.5 mL/h, T:
90°C, agitation rate: 800 rmp and reaction time: 20 h.

Table 3: Results of 31P NMR analysis of different POMs before and after cyclohexanol
oxidation
(ppm) of the POM before (ppm) of the POM after
POM
reaction
-12.51
reaction*
-13.12
α-P2W18
-12.51
-9.51
-13.12
α-P2W12Mo6
-9.51
-0.22
-0.49
α1-P2W12Mo5V -9.73
-10.01
*chemical shifts measured on a different apparatus.