Organic salts and merrifield resin supported [PM12O40]3? (M = Mo or W) as catalysts for adipic acid synthesis

Article abstract of DOI:10.3390/molecules24040783

Adipic acid (AA) was obtained by catalyzed oxidation of cyclohexene, epoxycyclohexane, or cyclohexanediol under organic solvent-free conditions using aqueous hydrogen peroxide (30%) as an oxidizing agent and molybdenum- or tungsten-based Keggin polyoxometalates (POMs) surrounded by organic cations or ionically supported on functionalized Merrifield resins. Operating under these environmentally friendly, greener conditions and with low catalyst loading (0.025% for the molecular salts and 0.001–0.007% for the supported POMs), AA could be produced in interesting yields.

Full text of DOI:10.3390/molecules24040783

molecules
Article
Organic Salts and Merrifield Resin Supported
3
−
[
PM O ] (M = Mo or W) as Catalysts for
12 40
Adipic Acid Synthesis
Jana Pisk 1,2,* , Dominique Agustin 1,2,
*
and Rinaldo Poli 1,3
1
Centre National de la Recherche Scientifique (CNRS), Laboratoire de Chimie de Coordination (LCC),
Université de Toulouse, UPS, INPT, 205, route de Narbonne, 31077 Toulouse, France;
rinaldo.poli@lcc-toulouse.fr
2
Université de Toulouse, IUT P. Sabatier, Département de Chimie, Av. G. Pompidou, BP 20258,
8
1104 Castres CEDEX, France
3
Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France
Correspondence: jana.pisk@chem.pmf.hr (J.P.); dominique.agustin@iut-tlse3.fr (D.A.);
Tel.: +33-5-63-62-11-72 (D.A.)
*
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀ
ꢁꢂꢃꢄꢅꢆ
Received: 1 February 2019; Accepted: 15 February 2019; Published: 21 February 2019
Abstract: Adipic acid (AA) was obtained by catalyzed oxidation of cyclohexene, epoxycyclohexane,
or cyclohexanediol under organic solvent-free conditions using aqueous hydrogen peroxide (30%)
as an oxidizing agent and molybdenum- or tungsten-based Keggin polyoxometalates (POMs)
surrounded by organic cations or ionically supported on functionalized Merrifield resins. Operating
under these environmentally friendly, greener conditions and with low catalyst loading (0.025%
for the molecular salts and 0.001–0.007% for the supported POMs), AA could be produced in
interesting yields.
Keywords: polyoxometalates; Merrifield resin; cyclohexene; adipic acid; organic solvent-free process
1
. Introduction
Adipic acid (AA) is one of the reagents used for the production of nylon-6,6, a polymer that
exhibits high mechanical strength, rigidity, and good thermal and chemical stability [
processes for the AA production (2.3 Mtons/year) involve nitric acid oxidation of cyclohexanol (CHA)
and/or cyclohexanone (CHK) [ ]. This inexpensive acid, used in several industrial processes [ ],
however, leads to pollution, producing unwanted waste and various nitrogen oxides (NO ), including
ca. 400 ktons of the greenhouse gas N O per year [ ]. Although this emission can be significantly
], current academic and industrial process
11]. One possible alternative route for AA
production is the selective oxidation of cyclohexene (CH) with hydrogen peroxide (H O ) [12 13] or
molecular oxygen (O ) [14] as preferred oxidants (Scheme 1). Even though recent reports describe
1]. Most industrial
2,
3
4
x
5,6
2
reduced using a thermal decomposition process [
7,8
developments focus on cleaner technologies [2,3,9–
,
2
2
2
a very fast AA production under high pressure and high temperatures conditions [15
membrane reactors [20] or microwave conditions [21], our research goal is to apply user-friendly
and safe protocols (lower temperatures avoiding the H O decomposition, atmospheric pressure,
–19], using
2
2
one step reaction, recoverable catalyst), and to compare them with the classical protocols used in
industrial processes.
Molecules 2019, 24, 783; doi:10.3390/molecules24040783
www.mdpi.com/journal/molecules

Molecules 2019, 24, 783
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Scheme 1. Proposed routes for the synthesis of adipic acid (AA).
z−
Heteropolyoxometalates—inorganic compounds with general chemical formula [X
n
M
p
O
q
]
,
M being commonly Mo or W, and X being usually a main group element (Si, P, B)—were described
as suitable for catalyst design at the atomic and molecular levels [22]. Among all structures, the one
n−
corresponding to the discrete assembly of 12 M around one X, with a general formula [XMo O ]
12
40
(called Keggin), is very stable. Different isomers of Keggin polyoxometalates (POMs) can exist,
according to the arrangement of the metal oxides around the heteroatom. Keggin-type POMs are
efficient catalysts for liquid phase oxidation [23], due to their stability towards oxygen donors and their
ability to incorporate different transition metals [24]. Immobilization of POMs through electrostatic
interactions in insoluble polymer resins can lead to easier separation and recovery of the molecular
catalysts [25,26]. Other interesting supports are SiO2 [27], mesoporous silica (SBA) [28,29] or certain
macroporous resins [30]. Among the resins, the low-cost and relatively robust Merrifield resins (MR),
originally developed for the solid-state synthesis of peptides [31], have the ability to undergo facile
functionalization [25]. Even though many transition metal catalysts have been supported on MR for
catalytic purposes [25,32,33], there is no report, to the best of our knowledge, on the application of
such ionically supported POM-based catalysts for AA production.
The AA production from a mixture of CHA and CHK (so-called KA “ketone-alcohol” oil [34]) has
previously been described using various homogeneous catalysts such as H WO [35] or Keggin-based
2
4
POMs, i.e., M
x
PMo O (M = Ni, Co) [36], H
Co
x
PMo O
[
37], or Y
x
12 40
Ni PMo O (Y = H
12
40
3−2x
12 40
3−2x
or NH , x = 0.25–1.5) [38]. With (C H N) [PMo O ], AA (18% yield) and CHK were produced
4
4
9
3
12 40
from CHA, while (C H N) [PW O ] produced CHK only [39]. The replacement of the substrates
4
9
3
12 40
(
1
CHA, CHK) used in industrial processes by other starting substrates (CH, cyclohexane oxide CHO,
,2-cyclohexanediol CHD), is guided by several economical parameters (substrate price, target yield,
availability, time cost analysis, etc.). An attractive starting material, CH, was successfully converted
to AA by aqueous H O using Na WO as a catalyst and the [CH (n-C H ) N]HSO phase transfer
2
2
2
4
3
8
17 3
4
agent [40]. The oxidation of trans-CHD is another suitable pathway for AA production [
providing the highest AA yields but CHD is the most expensive reagent among the listed substrates.
On the other hand, CH is largely available by benzene hydrogenation [43 45] or by cyclohexane
dehydrogenation [46 48] and, therefore, less expensive and more promising. Since CHD can be
3,10,41,42]
–
–
obtained by the oxidation of CH, a more direct and selective AA “one-pot” production from CH is of
great interest.
Different heterogeneous catalysts based on calcined POMs [49], mesoporous solids [50],
Ti-MMM-2 [51], Ce-SBA-15 [51], TAPO-5 [52], and Ti-AlSBA-15 [53
,54] were also tested for direct
CH oxidation, the AA yield being improved by the stepwise addition of H O to the reaction mixture.
2
2
The catalytic mechanism for the CH oxidation into AA through CHD was clarified with TAPO-5 as a
catalyst [52], showing that trans-CHD was formed from cyclohexene oxide (CHO) while the cis-CHD

Molecules 2019, 24, 783
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formation followed a radical mechanism. The CH oxidation to AA was also tested in CH CN using
3
TBHP (tert-butylhydroperoxide) in decane as oxidant and Ti-AlSBA-15 as a catalyst [54]. Some literature
data and reaction conditions with CH, CHO, CHD, CK, and CHA substrates are listed in Table 1.
In the present contribution, we report the preparation and characterization of new molecular
and resin-supported Mo- and W-based Keggin-type catalysts and their catalytic performance for the
production of AA starting from CH. The main objective of the presented research was to evaluate
a greener process, avoiding the use of organic solvent, using a new family of grafted catalysts and
minimizing the catalyst loading. Different starting substrates for AA production were tested, in order
to gain a better insight into the oxidation mechanism, especially by comparison between molecular
and grafted catalysts.
Table 1. Reaction conditions for the preparation of adipic acid from CH, CHO, CHD, CHK, CHA
(
see footer for abbreviations)—Some literature data with Mo, W-based polyoxometalates (POMs) and
mesoporous catalysts.
Cat/Sub
Loading
Ox. Agent
(eq)
Sub Conv
(%)
◦
Sub.
Cat.
T ( C)
Time (h)
AA Yield (%)
Ref.
Na2WO4·2H2O
CH
CH
1%
2.5%
1%
A
75–90
reflux
90
8
90
100
90
>99
89–97
>99
[40]
[55]
[35]
[
3 8 17 3 4
CH (n-C H ) N]HSO
Na2WO4 + ILs
4.4 A
10
20
CHK
CHA
H2WO4 + ILs
3.3 A 4.4 A
CH
CH
H2WO4 + IL
H4SiW12O40
2%
2%
4.4 A
5 A
73–87
US (25 kHz)
reflux
12
4
85–96
92
n.d.
[56]
[57]
[39]
CHA
(C4H9N)3[PMo12O40]
0.02%
3 A
17
18
>99
>99
H3PMo12O40
NiPMo12O40
CoPMo12O40
CHA
CHK
17–30
17–32
0.5%
1 A
90
14
[36]
H3−2xNixPMo12O40
CHK
CHA
(
NH
4
)
3−2xNi
x
PMo12
O
40
0.1–0.4%
1 A
1 A
90
90
20
20
34–45
30–50
>99
>99
[38]
[37]
(x = 0.0–1.5)
CHK
CHA
H3−2xCoxPMo12O40
0.15–0.66%
CHD
CHO
CHK
CHA
1
3–39
sub/cat mass
ratio = 9.7
24–60
59 36
Ti-Y zeolites
4 A
80
24
72
44 78
[42]
70
5
5
mmol CH
0 mg cat
CH
Ti-MMM-2 Ce-SBA-15
TAPO-5
3.6 A
3.5 A
80
80
80
10–30
>99
[51]
[52]
>
99
>99
72
CH CHO
CHD
62 mmol 0.5 g
cat
72 24
24
30 40
42
>
CHD
CHO
7 mmol sub
0.1 g
50–88
94
56–98
84
3
3
B
48
[53]
[58]
Ti-AlSBA-15 + 10 mL
CH3CN
A
70
80
24
2
80
10
7
CH
7.8%
1.5 B
15
CHO
CHD
CH
48
94
84
Ti(16)Al(DS)SBA + 10
mL CH3CN
9.7%
3 B
80
[54]
76–96
33–88
40–84
2
4–48
9.7–19.4%
4B
CH = cyclohexene, CHA = cyclohexanol, CHK = cyclohexanone, CHO = cyclohexene oxide, A = aqueous H
B = TBHP in decane AA: adipic acid, US = ultrasound, IL= ionic liquid.
2
O
2
,
2
2
2
. Results and Discussion
.1. Synthesis and Characterization of the Prepared Catalysts
.1.1. Synthesis
Four molecular catalysts, (Scheme 2, left), were straightforwardly prepared by the reaction of the
corresponding ionic liquid (1-hexyl-3-methylimidazolium bromide or 1-butyl-3-methylimidazolium

Molecules 2019, 24, 783
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bromide) and phosphomolybdic or phosphotungstic acid. The four isolated complexes are composed
3
−
by a [PM O ] (M = Mo or W) surrounded by three alkylmethylimidazolium cations.
12
40
Scheme 2. Catalysts used.
The synthesis of the polymer-supported catalysts (Scheme 2, right) was achieved as shown in
Scheme 3. Merrifield resins were used as polymeric supports. The resins were modified to obtain two
different types of imidazolium-functionalized supports (MR4 or MR12) in two steps (see Scheme 3).
The first step consisted of the addition of 2-methylimidazole in the presence of a base to the Merrifield
resin, leading to the MRIm
in the second step with the appropriate alkyl bromide to obtain the desired imidazolium moieties
MR4 and MR12). The different alkyl chain lengths were expected to increase the lipophilicity of
the catalytic object, enhancing the catalyst compatibility with the apolar substrate relatively to the
water present in the reaction media since carried by the oxidant (H O in water). In the last step,
[59]. The imidazole-functionalized intermediate MRIm was further treated
(
2
2
3
−
3−
the POMs ([PMo O ] or [PW O ] ) were then immobilized by simple ion exchange adding the
12
40
12 40
corresponding heteropolyacids (H [PMo O ] or H [PW O ]). All the synthesized catalysts were
3
12 40
3
12 40
characterized by FTIR and elemental analysis.
Scheme 3. Synthetic procedure for the preparation of the heterogeneous catalysts.
2
.1.2. Characterization
Infrared (IR) Spectroscopy
The IR spectra of the molecular catalysts Cn-M (n-4,6; M-Mo, W) were assigned according to
the literature data [60 63]. The main characteristic features of the Keggin structure were observed
•
–
−
1
−1
−1
−1
M), and 771–777 cm
at 1078 cm
(
νas
P
−
O
a
), 970 cm
c−M) for both M (W and Mo). For the catalysts grafted on Merrifield resins MRn-M (n = 4,
2; M-Mo, W), the IR analysis showed the characteristics bands of the 2-methylimidazolium cation,
(
νas
M
−
O ), 885 cm
d
(
νa
M
−
Od
−
(νas
M−O
1
1
7
−
1
1
420–1490 cm . The final POM-containing resins exhibit characteristic IR bands at 1060, 955, 880, and
−
−1
95–802 cm for MR -Mo and MR -Mo, and at 1080, 975, 890, and 795–810 cm for MR -W and
4
12
4
MR -W, corresponding to the Keggin anions (Figure 1 shows the specific area corresponding to the
12
POM backbone for C -Mo, MR -Mo and the corresponding starting H [PMo O ], proving that the
12
3 12 40
6
POM structure remains unchanged when grafted).

Molecules 2019, 24, 783
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Figure 1. Infrared (IR) spectra of the zone of POM vibrations for the commercial H PMo12O40 precursor
3
(grey), C6-Mo molecular catalyst (blue) and MR12-Mo grafted catalyst (orange).
•
31P Nuclear Magnetic Resonance (NMR) Spectroscopy
The 31P NMR spectra of C -Mo and C -Mo salts in the solution exhibit a characteristic peak at
4
6
−
4.15 ppm, while those of C -W and C -W salts show a peak at
−
15.5 ppm, in accordance with the
65]. In the solid-state P NMR spectrum of the Mo-grafted catalysts (Figure 2),
3.5 ppm in case of MR -Mo, while two resonances at 1.9 and 3.5 ppm
are observed for MR -Mo. Likewise, in the case of the W catalysts (Figure 3), one resonance is
4
6
31
literature data [64
,
the resonance is shifted to
−
−
−
4
12
observed for MR -W (at
−
15.1 ppm) and two for MR -W (at
−
12.4 and
−
15.2 ppm). The similar
4
12
behavior of both POMs seems to depend on the length of the imidazolium substituent and not on the
quality of the Keggin POM precursor (the 31P NMR in D O of the commercial Mo and W precursors
2
displayed only one signal at
−
3.9 and
−
15.3 ppm, respectively). The observed behavior indicates
the presence of different cation-dependent interactions at the resin surface, the two simultaneously
observed species possibly corresponding to species located in two different sites (i.e., closer to or
farther from the imidazolium moiety) [66,67].
(
a)
(b)
Figure 2. Solid-state 31P NMR of MR4-Mo (a) and MR12-Mo (b) supported catalysts.

Molecules 2019, 24, 783
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(
a)
(b)
Figure 3. Solid-state 31P NMR of MR4-W (a) and MR12-W (b) supported catalysts.
•
Elemental and Thermogravimetric Analyses
The EA data of the grafted resins show that 32% of Cl from the –CH Cl pending functions has
2
been replaced by the 2-methylimidazolium function.
The metal loadings were determined from TG analyses. The residue after thermal treatment was
identified as P O and MO (M = Mo or W), see Table 2. Figure 4 shows an illustrative example of the
2
5
3
TG curve for MR -W.
4
Table 2. CHN elemental analyses and metal loading.
M (Mo or W) Loading (µmol
EA Data
C (%)
H (%)
N (%)
Cl (%)
POM/g of Polymer) *
MR
MRim
MR4-Mo
MR4-W
MR12-Mo
MR12-W
79.1
74.6
59.5
55.9
40.9
30.0
6.4
8.4
6.8
6.0
4.1
3.0
-
14.5
10.1
n.d.
n.d.
n.d.
n.d.
-
-
6.9
5.0
4.5
4.3
3.5
66.7
12.3
55.6
18.9
n.d. = not determined, * based on the TG analyses.
Figure 4. Thermogravimetric curve of MR4-W.

Molecules 2019, 24, 783
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2
.2. Catalysis
The transformation of CH to AA is reported to involve olefin epoxidation, alcohol oxidations, and
Baeyer-Villiger oxidation (Scheme 4) [52].
Scheme 4. Steps for the oxidation of CH to AA in the presence of TAPO-5 catalyst. Adapted from [52].
For this reason, we studied the AA formation from different starting compounds (CH, CHO,
CHD), using molecular and grafted catalysts. CHO and CHD were used as substrates since they are
intermediates in the formation of AA from CH. The formation of the CHD intermediate has also been
monitored when starting from CH or CHO (see results in Table 3).
Table 3. Data of oxidation reactions for AA production starting from different substrates (CH, CHO,
CHD) and different catalysts.
Molecular Catalysts
C4 C6
Grafted Catalysts
MR4 MR12
Mo
Mo
W
Mo
W
Mo
W
W
%
Cat a
0.025
CH as substrate
0.004
0.007
0.003
0.001
CH conv. b
65
46
15
75
61
12
61
42
11
68
50
17
58
33
24
61
43
17
56
30
19
73
51
20
c
AA yield
CHD form d
CHO as substrate
>99
CHO conv. e
>99
c
AA yield
36
25
47
26
32
43
28
28
33
23
21
32
36
35
CHD form d
29
24
CHD as substrate
>99
CHD conv. f
AA yield
71
46
94
60
79
41
95
56
c
74
72
58
63
a
b
2 2
Molar ratio (in percentage) relative to the substrate. CH conversion (in %) after t = 3 days, T = 333 K, 4 eq of H O
were added in small portions. c isolated AA yield (in %). CHD formation (%) based on GC. CHO conversion (in
%
d
e
f
) after t = 1 day, T = 333 K, 2 eq of H
2
O
2
were added at the beginning of the reaction. CHD conversion (in %)
after t = 3 days, T = 333 K, 2 eq of H2O2 were added at the beginning of the reaction.
•
Swelling Tests
In order to verify whether all the MR-supported POMs are accessible to the substrates under
organic solvent-free conditions, swelling tests were performed on non-treated MR. Heating the MR in
the presence of CH or CHO led to a mass increase by ca. 2% in the case of CH and 8% in the case of
CHO, indicating substrate incorporation into the polymer pores and, therefore, catalyst accessibility
without organic solvent. This feature is of great importance since it justifies the use of MR as a support
in the catalytically solvent-free processes.

Molecules 2019, 24, 783
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•
Classical Conditions
Initial tests were performed with the molecular catalysts Cn-M (n = 4 or 6, M = Mo or W;
.025 mol% vs. substrate), i.e., a 40 times lower catalyst loading than that reported for Na WO [40].
0
2
4
All the catalysts were insoluble in the reaction media, but became partially soluble as the reactions
evolved, better solubilization was observed for the W-based ones.
•
CH as the Substrate
After 60 min, the CH conversion was between 7 and 10% (analyzed by GC) with a low selectivity
towards CHO (3%). After 6.5 h, all the molecular catalysts provided a 5–8% selectivity towards CHO
with a CH conversion between 12 and 18%. No CHD was observed in the organic layer. The slow
conversion is certainly due to the low catalyst loading and to the mild reaction conditions compared to
those described in the reference study [35]. After 24 h, however, reasonable conversions were achieved
and CHO was no longer detected by GC whereas CHD was present (see Table 1). AA was isolated as a
solid in relatively good yields. As stated in several mechanistic studies [52], the species unidentified by
GC might correspond to the product of Baeyer Villiger oxidation (see Scheme 4). The observation that
CHO is formed and then consumed indicates that this is involved in the AA formation mechanism,
via the conversion to CHD.
The homogeneous Cn-W catalysts were more active than the corresponding Cn-Mo, with AA
isolated yields ca. 10% higher on average (see Table 1). Control experiments without catalyst or in
the presence of the commercial MR yielded only a very low amount of cis and trans-CHD (<7%) after
2
4 h. The shorter carbon chain on the imidazolium cation afforded better results. The same trend was
previously observed for the same reaction with (Hgly) [PM O ] (M = Mo, W; gly = glycine) [68
]
3
12 40
3
−
or for the cyclooctene epoxidation with alkylpyridinium salts of [PMo O ]
[
61]. The stirred
12
40
biphasic reaction medium appeared as an emulsion, possibly stabilized by the solid POM salt
(Pickering-type) [69–72].
For the grafted version of the catalysts, the W-based were again more active, despite the lower W
content in the case of MR -W than for MR -Mo (see Table 1). In contrast to the molecular catalysts,
12
12
the longer chain on the imidazolium cation led to greater activity. This could be linked to a better
solubilization” of the substrate near the catalyst active site.
“
•
CHO as the Substrate
Complete substrate conversion was observed in this case, with the molecular or the grafted
catalysts, producing AA after one day in 21–47% isolated yields together with a relatively high amount
of trans-CHD (Table 1). This experiment shows that the limiting factor is not the formation of CHD,
as seen in other studies [52]. The molecular catalysts (especially those based on W) were more selective
towards AA than the grafted ones. It seems that the transformation of CHD to AA is quicker in the
presence of the molecular catalysts, since less CHD intermediate accumulates.
•
CHD as the Substrate
When trans-CHD was used as the starting substrate, the reaction produced AA in higher isolated
yields (59–74%) than when starting from CH within the same time period. 2-Hydroxycyclohexanone,
a known intermediate within the postulated oxidation pathway (Scheme 4), [52] was also observed
by GC but not quantified. The Cn-M molecular catalysts are highly active (>99% conversion), more
than the grafted ones for which the measured conversion is between 71 and 95%. In all the reactions,
the W-based catalysts show slightly better performances in terms of activity and AA yields.
•
Recycling
Catalyst recovery and recyclability were investigated for the most active grafted catalyst, MR -W
,
12
and CH as the starting substrate. The IR spectrum of the isolated MR -W after the first experiment
12
exhibited no change, except for the presence of additional vibrations attributed to CHD. After the 2nd

Molecules 2019, 24, 783
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run, the CH conversion was 54% and the AA yield was 29%. The lower activity after reuse could result
from the grafted catalyst inhibition by the presence of CHD within the resin.
•
CHK/CHA as the Substrate
In order to mimic a greener industrial process (replacing the traditionally used and
environmentally harmful oxidants) the most active molecular catalyst C -W (0.025% loading) was also
4
tested for the oxidation of CHK and CHA by H O (Scheme 5), leading after 24 h to isolated AA yields
2
2
of 41% and 35% respectively. These results show that C -W is less active than H WO and certain
4
2
4
Dawson POMs (58 and 59% with 0.5–2% cat loading with CHA as the substrate) [73], quite similar to
n+
3+
3+
2+
(
x y
12 40
NH4) A PMo O (A = Sb , Bi , Sn ) species with CHK as the substrate and better with CHA
with a lower POM/substrate ratio [74] but more active than other described POMs (see Table 1), and,
therefore, also seems promising for the solvent-free oxidation of CHK and CHA [35].
O
O
O
OH
O
O
2
H O
OH
OH
[O]
[O]
[O]
OH
O
O
CHA
CHK
[O]
HO
OH
O
AA
Scheme 5. Oxidation steps of CHA and CHK to AA in the presence of H WO catalyst. Adapted
2
4
from [2].
3
. Materials and Methods
3
.1. Materials
1
-Hexyl-3-methylimidazolium bromide (Aldrich, Saint Louis, MO, USA), H PMo O
·
26H O
3
12 40 2
(
(
(
99.9% Merck, Kennallsburg, NJ, USA), H PW O
Aldrich), bromobutane (Aldrich), bromododecane (Aldrich), Merrifield resin (Aldrich), EtOH
Aldrich), DMF (Aldrich), dichloromethane (Aldrich), Acetone (Aldrich), Cyclohexene (Aldrich),
·
15H O (purum, Aldrich), 1-methylimidazole
3
12 40 2
3
5% H O (Acros, Geel, Belgium) were used as received
2 2
3
.2. Physical Methods
Elemental analyses (EA) were provided by the Analytical Services Laboratory of LCC, Toulouse.
The thermogravimetric analyses (TGA) were performed on a SETARAM TGA 92-16.18 thermal analyzer.
−
1
The sample was placed into an alumina crucible and heated at 0.83 K
·
s
in a reconstituted air flow
◦
◦
from 25 C to 600 C. An empty crucible was used as a reference. The POM percentage in the grafted
species was calculated from the percentage of residue measured in the TGA traces divided by the
molar mass of PM O
38.5.
12
Infrared spectra were recorded using the ATR technique with a Perkin Elmer FTIR/FIR
00 spectrometer.
4
A Bruker Avance DPX-300 spectrometer was used to record 31P NMR spectra in the solution at
1
21.5 MHz. All solid-state 31P NMR experiments were recorded on a Bruker Avance 400 spectrometer
equipped with a 3.2 mm probe. Samples were spun at 16 kHz at the magic angle using ZrO rotors.
2
31
◦
For P MAS single pulse experiments, small flip angle (~30 ) were used with recycling delays of 20 s.
P-CP/MAS spectra were recorded with a recycle delay of 3 s and contact times of 2 ms. 31P chemical
31
shifts were referenced to an external 85% H PO sample.
3
4
All the catalytic reactions were followed by gas chromatography on an Agilent 6890 A
chromatograph equipped with FID detector, a HP5-MS capillary column (30 m 0.25 mm 0.25 m)
×
×
µ
and automatic sampling, or on a Fisons GC 8000 chromatograph equipped with FID detector and with
a SPB-5 capillary column (30 m × 0.32 mm × 0.25 µm).

Molecules 2019, 24, 783
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The GC parameters were quantified with authentic samples of the reactants and products. The CH
conversion, the CHD formation, and the CHO conversion/formation were calculated from calibration
2
curves (r = 0.999) relative to an internal standard (acetophenone). Acetophenone and CHD are present
both in the organic and in the water phase at the end of the catalytic experiments. Therefore, extraction
with AcOEt was carried out before the GC analysis. The AA yield was calculated from the isolated
mass recovered from the filtered reaction mixture at the end of the reaction.
3
.3. Preparative Part
Molecular Catalysts
-Hexyl-3-methylimidazolium bromide (1.65 mmol, 0.41 g) was mixed with aqueous H PM O
•
1
3
12 40
(
(
M = Mo or W) (0.54 mmol, 1.0 g H PMo O or 1.55 g H PW O ) and stirred for 2 h. Yellow
3 12 40 3 12 40
C -Mo) or white (C -W) solids precipitated immediately. The solid was filtered, washed with acetone
6
6
and ether and dried in air.
The preparation of the C -M (M = Mo or W) catalysts involved the synthesis of the corresponding
4
ionic liquid [75]. 1-methylimidazole (0.05 mol, 4.1 g) and bromobutane (0.05 mol, 6.85 g) were stirred at
◦
7
0 C for 24 h in EtOH (20 mL). After the reaction, EtOH was removed by evaporation under reduced
pressure. The formation of the corresponding ionic liquid BMImBr was confirmed by IR [76]. BMImBr
-
3
(
4.4 g, 0.02 mol) was directly mixed with aqueous H PM O (M = Mo or W) (6.67
×
10 mol) and
3
12 40
stirred for 2 h. A yellow (C -Mo) or white solid (C -W) precipitated immediately. The solid was
4
4
filtered, washed with acetone and ether and dried in air.
C -Mo: Yield: 10.3 g (69%). EA: calculated for C H Mo N O P (%), C: 12.9, H: 2.0, N: 3.6,
4
24 45
12
6
40
−
1
31
found (%), C: 12.9, H: 1.9, N: 3.7. IR-ATR (cm ): 1060, 945, 867, 763, 732. P NMR (ppm): −4.15.
C -W: Yield: 18.6 g (85%). EA: calculated for C H W N O P (%), C: 8.7, H: 1.4, N: 2.6, found
4
24 45 12
6
40
31
−
1
(
%), C: 8.9, H: 1.5, N: 2.7. IR-ATR (cm ): 1053, 945, 870, 775, 722. P NMR (ppm): −15.5.
C -Mo: Yield: 11.4 g (74%). EA: calculated for C H Mo N O P (%), C: 15.5, H: 2.4, N: 3.6,
6
30 57
12
6
40
−
1
31
found (%), C: 16.0, H: 2.3, N: 3.8. IR-ATR (cm ): 1078, 970, 885, 771, 732. P NMR (ppm): −4.15.
C -W: Yield: 17.6 g (78%). EA: calculated for C H Mo N O P (%), C: 10.7, H: 1.7, N: 2.5,
6
30 57
12
6
40
−
1
31
found (%), C: 10.8, H: 1.6, N: 2.5. IR-ATR (cm ): 1072, 969, 885, 777, 726. P NMR (ppm): −15.5.
•
Grafted Catalysts
The Merrifield resin (1.4–1.6 mmol Cl/g, 5.6–6.4 mmol, 4 g), 2-methylimidazole (0.05 mol, 4 g), and
◦
Na CO (0.01 mol, 1.3 g) were stirred in DMF (20 mL) at 80 C during 24 h [77]. After washing the resin
2
3
with dichloromethane (to remove the organic compounds) and then in water until neutral pH, the resin
was dried in air. This resin was then placed with 5 mL bromobutane (MR4-) or bromododecane
◦
(
MR -) and stirred for 72 h at 80 C. The resin was then washed with dichloromethane, ethanol, and
12
−
5
acetone. Finally, it was treated with 5 mL of aqueous solution of H PMo O (5
×
10 mol, 0.09 g)
10 mol, 0.15 g). After filtration and washing with distilled water and acetone,
around 4 g of light yellow resins (MR -Mo, MR -W, MR -Mo, MR -W) were obtained.
3
12 40
−
5
or H PW O (5
×
3
12 40
4
4
12
12
MR -Mo: EA: found (%), C: 59.5, H: 6.8, N: 5.0. TGA (residue) 11.9%. Mo loading (
µ
mol POM/g
4
−
1
31
of polymer): 66.7. IR-ATR (cm ): 1060, 953, 878, and 802. P NMR (ppm): −3.5.
MR -W EA: found (%), C: 55.9, H: 6.0, N: 4.5. TGA (residue) 3.5%. W loading (
µmol POM/g of
4
−
1
31
polymer): 12.3. IR-ATR (cm ): 1079, 975, 894, and 811. P NMR (ppm): −15.1.
MR -Mo EA: found (%), C: 40.9, H: 4.1, N: 4.3. TGA (residue) 9.99%. Mo loading (
µ
mol POM/g
1
2
−
1
31
of polymer): 55.6. IR-ATR (cm ): 1060, 952, 876, and 794. P NMR (ppm): −1.9, −3.5.
MR -W EA: found (%), C: 30.0, H: 3.0, N: 3.5. TGA (residue) 5.3%. W loading (µmol POM/g of
12
polymer): 18.9. IR-ATR (cm ): 1078, 974, 893, and 797. 31P NMR (ppm): −12.4, −15.2.
−
1

Molecules 2019, 24, 783
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3
.4. Swelling Test for MR
The MR (0.5 g) was placed in a glass test tube with stopper and CH or CHO was added.
◦
The reaction mixture was heated at 60 C for 6 h. The mixture was filtered and the recovered resin was
dried in air and weighted.
3
.5. General Procedure for Catalytic Oxidation to AA
3
.5.1. CH Oxidation (Procedure A)
A round-bottom flask equipped with a magnetic stirring bar and a reflux condenser was charged
with CH (80 mmol, 8 mL), acetophenone (3.5 mmol, 0.4 mL), and catalyst (0.02 mmol of molecular
◦
catalyst or 0.06 g of grafted catalyst). After the stirred reaction mixture reached 60 C, H O (320 mmol,
2
2
31.1 g, 35%) was added and stirring was continued for 72 h. The final mixture was filtered and the
filtrate was left to stand in the refrigerator for one day. The resulting white precipitate was filtered and
identified as AA by IR, NMR, and DSC.
3
3
3
.5.2. CHO Oxidation (Procedure B)
Similar to procedure A, with a reaction time of 24 h and 2 eq of H O (160 mmol, 35%).
2
2
.5.3. CHD Oxidation (Procedure C)
Similar to procedure A, with a reaction time of 72 h and 2 eq of H O (160 mmol, 35%).
2
2
.5.4. Recycling of the Resins
The resins were isolated at the end of the catalytic reaction, washed with diethylether at room
temperature, and dried under ambient air.
3
.5.5. CHK or CHA Oxidation (Procedure D)
A round-bottom flask equipped with a magnetic stirring bar and a reflux condenser was
charged with cyclohexanone (80 mmol, 7.85 g) or cyclohexanol (80 mmol, 8.0 g) and 10 mL of water.
Acetophenone (3.5 mmol, 0.4 mL) was added as an internal standard and the catalyst (0.02 mmol) was
◦
added. After the stirred reaction mixture reached 80 C, H O (320 mmol, 31.1 g, 35%) was added and
2
2
◦
stirring was continued for 24 h. The homogeneous solution was allowed to stand at 0 C for 12 h, and
the resulting precipitate was separated by filtration and washed with 20 mL of cold water. The product
was dried in a vacuum to give AA as a solid.
4
. Conclusions
Immobilization of the Keggin phosphomolybdate and phosphotungstate on functionalized
Merrifield resins has led to the development of new heterogeneous catalysts for AA production
in a one-step procedure from CH. Different intermediates were detected and the key steps could
be determined by running comparative oxidation tests form each one of the possible intermediates.
The substrate itself is able to swell the MR allowing access to all catalytic sites without the use
of the classical swelling organic solvents. This atom economical methodology obeys a few of the
green chemistry principles (greener oxidant, no phase transfer catalysts, no organic solvents added (safer
reagents and atom economy), minimal catalyst loading and recyclability). The developed protocol
is straightforward, safe, and environmentally benign and can be extended to a variety of other
POM-based catalyzed oxidations.
Author Contributions: Conceptualization, J.P. and D.A.; methodology, J.P. and D.A.; validation, J.P., D.A. and
R.P.; formal analysis and investigation, J.P., D.A.; writing—Original draft preparation, J.P., D.A.; writing—Review
and editing, J.P., D.A. and R.P.

Molecules 2019, 24, 783
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Funding: The research leading to these results has received funding from the European Union Seventh Framework
Programme (FP7 2007–2013) as the project “Diligent search for chemical bio-sources: Solvent-free homogeneous
◦
and heterogeneous oxidation processes catalyzed by polyoxometalates”, under grant agreement n 291823 Marie
Curie FP7-PEOPLE-2011-COFUND (The new International Fellowship Mobility Programme for Experienced
Researchers in Croatia-NEWFELPRO).
Acknowledgments: The authors acknowledge LCC-CNRS for elemental analyses and especially. Y. Coppel for
the NMR and the Department of Chemistry of IUT at Castres for the facilities in synthesis, catalysis, and TGA.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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