Evidence for the presence of alternative mechanisms in the oxidation of cyclohexanone to adipic acid with oxygen, catalysed by Keggin polyoxometalates

Article abstract of DOI:10.1016/j.apcata.2010.04.032

This report deals with the results of a study on the oxidation of cyclohexanone to adipic acid with air, catalysed by Keggin-type polyoxometalates of composition H_3+xPMo_12-xV_xO₄₀ (x = 1 and 2), which was carried out in a semi-continuous stirred-tank reactor. It was found that when conducted in the presence of a water-only solvent, the reaction proceeds with a redox mechanism, in which the step of reoxidation of the reduced POM by oxygen is rate limiting. When, however, the reaction was carried out with an acetic acid co-solvent, a radical-chain autoxidation mechanism prevailed, especially when very low amounts of catalyst were used. Autoxidation overlapped with the redox mechanism when the catalyst-to- cyclohexanone ratio was increased. Moreover, the composition of the polyoxometalate, that is, the number of V atoms per Keggin unit, affected the relative importance of the two mechanisms. The selectivity to adipic acid achieved was a function of the reaction mechanism, but also was affected by cyclohexanone conversion, due to the presence of a complex reaction network.

Full text of DOI:10.1016/j.apcata.2010.04.032

Applied Catalysis A: General 391 (2011) 118–124
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Evidence for the presence of alternative mechanisms in the oxidation of
cyclohexanone to adipic acid with oxygen, catalysed by Keggin polyoxometalates
a,∗
a
a
a
a
Fabrizio Cavani , Laura Ferroni , Alessandra Frattini , Carlo Lucarelli , Andrea Mazzini ,
a
b
b
b
Katerina Raabova , Stefano Alini , Pasquale Accorinti , Pierpaolo Babini
Dipartimento di Chimica Industriale e dei Materiali, ALMA MATER STUDIORUM Università di Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
Radici Chimica SpA, Via G. Fauser 50, 20190 Novara, Italy
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
This report deals with the results of a study on the oxidation of cyclohexanone to adipic acid with air,
catalysed by Keggin-type polyoxometalates of composition H3+xPMo12−xVxO40 (x = 1 and 2), which was
carried out in a semi-continuous stirred-tank reactor. It was found that when conducted in the pres-
ence of a water-only solvent, the reaction proceeds with a redox mechanism, in which the step of
reoxidation of the reduced POM by oxygen is rate limiting. When, however, the reaction was carried
out with an acetic acid co-solvent, a radical-chain autoxidation mechanism prevailed, especially when
very low amounts of catalyst were used. Autoxidation overlapped with the redox mechanism when the
catalyst-to-cyclohexanone ratio was increased. Moreover, the composition of the polyoxometalate, that
is, the number of V atoms per Keggin unit, affected the relative importance of the two mechanisms. The
selectivity to adipic acid achieved was a function of the reaction mechanism, but also was affected by
cyclohexanone conversion, due to the presence of a complex reaction network.
Received 27 February 2010
Received in revised form 30 March 2010
Accepted 15 April 2010
Available online 22 April 2010
Keywords:
Adipic acid
Cyclohexanone oxidation
Keggin polyoxometalates
Reaction mechanism
Redox mechanism
Autoxidation
© 2010 Elsevier B.V. All rights reserved.
1
. Introduction
Adipic acid (AA) is one of the most used chemicals in the world.
final quality of the product, and the safety and environmental
impact of the process. In any case, the study of an alternative syn-
thetic pathway in which air is the oxidising agent is a challenge
for modern chemistry, with potential industrial applications due
to the savings resulting from the elimination of both the nitric acid
production and the recovery plant.
The global AA capacity is around 3 million metric tons per year;
currently, the growth of AA production is close to 3% per year;
nylon 6,6 accounts for the greater part of the total AA consumption
[
1–4].
The principal industrial process is done in two steps. The first
step is the oxidation of cyclohexane with air to form a mixture of
cyclohexanol and cyclohexanone—the so-called KA oil; the yield of
this step is between 85% and 90%. In a variant of this first step, cyclo-
hexanol is obtained by the hydrogenation of phenol. The second
Various catalysts were proposed in the literature for the aero-
bic oxidative cleavage of cyclic ketones, vicinal diols and ␣-ketols
under mild conditions [5–12], including Keggin-type polyoxomet-
alates [13], V oxo and dioxo salts and complexes [14], and
heterogenized VO catalysts [15]. Table 1 shows some literature
results for cyclohexanone oxidation to AA. Keggin-type polyox-
ometalates (POMs) offer a unique structural versatility that makes
it possible to tune the composition in function of the requirements
of the reaction [16,17]. For instance, POMs are amongst the very few
systems that catalyse the oxidation of organic substrates with air
at a moderate reaction temperature through a redox-type mecha-
nism [18–24]. To this regard, experimental evidence was reported
for O-transfer from the oxidized POM to anthracene (to produce
anthraquinone) and xanthene (to produce xanthone), whereby the
reduced lacunary POM is then reoxidized by O2 [21,22], in the same
step is the oxidation of KA oil with an excess of 50–60% HNO and a
3
Cu/V catalyst. The molar yield obtained at total KA oil conversion is
as high as 95%; the by-products are glutaric acid (GA) and succinic
(
SA) acid. The co-products of oxidation with nitric acid are nitro-
gen oxides; NO and NO2 can be reoxidized and recycled, whereas
N O is a pollutant gas that has to be abated before the emission of
2
tail-gases into the atmosphere.
This synthetic approach by two-step oxidation was developed
during the 1940s but several technological improvements have
been adopted with positive results on energy consumption, the
∗ Corresponding author. Tel.: +39 051 2093680; fax: +39 051 2093680.
E-mail addresses: cavani@fci.unibo.it, fabrizio.cavani@unibo.it (F. Cavani).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.04.032

F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
119
Table 1
A summary of catalysts used in the literature for the aerial oxidation of cyclohexanol (Ol) and cyclohexanone (One) to adipic acid (AA).
◦
Catalyst
Reactant
Solvent, T ( C)
Conversion (%)
Sel. AA (mol%)
Ref.
◦
◦
Pt/charcoal
Ol
Water, 150
Water, 140
C
C
100
100
100
99.8
97.5
99
50
21
77
75
93.4
51
55
[5]
[6]
[7]
[8]
Pt/Carbon/monolith
Mn(OAc)2/Co(OAc)2
Mn(OAc)2
Mn(NO3)2/Co(NO3)2
H7PMo8V4O40
One
Ol/One
One
One
One
One
One
◦
Acetic acid, 70
Acetic acid/CF3COOH, 65
Acetic acid + catalytic HNO3, 40
Acetic acid/water, 70
Acetonitrile/methanol, 60
Acetic acid/water (MEK), 100
C
◦
C
◦
C
[9]
◦
C
[10]
[11]
[12]
◦
H5PMo10V2O40
Co/Mn cluster
C
98
97.6
◦
C
86.6 (wt%)
way as it occurs in the Mars-van Krevelen mechanism with mixed
oxides catalysts in gas-phase oxidations.
balance, by comparing cyclohexanone conversion with the sum of
yields to all products, the latter obtained with the esterification
method described above. In general, atomic balances were good,
although not excellent; typically the ratio: (C atoms contained in all
products/C atoms in converted cyclohexanone) was between 0.90
and 0.95. However, this value, lower than 1.00, was mainly due to
the small but not negligible loss of cyclohexanone in the vapours
stream, as confirmed by a mass balance made by comparing the ini-
tial and final reactor weight loading (calculated also including the
amount of O incorporated in products and the C lost in the form of
In a previous paper, we reported on the catalytic behaviour of
Keggin-type P/Mo/V POMs in the aerobic oxidation of cyclohex-
anone to AA conducted in water solvent; it was found that reactivity
performance was greatly affected by the POM composition, e.g.
the number of V atoms incorporated in the Keggin anion and the
nature of the cation [20]. However, the cyclohexanone conversion
and AA yield achieved were relatively low. In this paper, we have
analysed the reactivity of the same catalysts under conditions that
are more favourable to achieving higher cyclohexanone conversion,
and investigated the reaction mechanism in the presence of acetic
acid solvent.
CO ).
2
The gas chromatography for the analysis of the reaction prod-
ucts used a capillary HP5 column and a FID (internal standard
◦
n-decane). The oven temperature was programmed at 50–280 C
(
◦
heating rate 10 /min). GC–MS was used for the identification of
2
. Experimental
products. Incondensable gases (carbon oxides) were analysed by
two methods: (i) by sampling, from time to time during the reac-
tion, 1 mL of gases in the effluent stream, and injecting it into
a GC equipped with a Carbosieve SII column (oven temperature
Catalytic experiments were carried out in a semi-continuous
stirred autoclave reactor (100 mL volume) made out of glass, at
an operating pressure of between 1 and 10 atm (Buchi Miniclave).
The reactor was equipped with a vapours condenser (cooling fluid
◦
◦
programmed at 40–240 C, heating rate 10 /min) and a TCD; (ii)
by bubbling the effluent stream in a sodium hydroxide aqueous
solution and determining the final amount of excess NaOH by
titration—providing an integral amount of CO2 produced over the
entire experiment time.
◦
temperature: −10 C), to avoid major loss of cyclohexanone dur-
ing reaction. Oxygen was fed continuously to the reactor, with a
standard flow rate of 300 N mL/min. The liquid phase was loaded
batchwise; typical reaction conditions, unless otherwise specified,
were the following: 25 mL cyclohexanone (Sigma–Aldrich, purity
The catalysts, Keggin-type polyoxometalates with the compo-
sition H PMo11VO40 (POMV1) and H5PMo10V O (POMV2) were
4
2
40
>99.5%), and 25 mL of solvent (either water or water + acetic acid
prepared by following a procedure similar to that developed by
Matveev and co-workers [26]. An aqueous solution was prepared
mixtures; acetic acid Sigma–Aldrich, purity >99.7%) were loaded,
together with an amount of polyoxometalate (POM) ranging from
a minimum of 0.02 g to a maximum of 1.6 g. There was not a
single-phase liquid phase; because of the limited solubility of cyclo-
hexanone in water (approx 9 g/100 mL), an emulsion developed.
The POM completely dissolved in the solvent.
by dissolving MoO and H PO in an abundant amount of water, at
3
3
4
◦
a temperature close to 100 C (but avoiding boiling of the solution);
it took about 15–20 h for a complete dissolution. A second solution
was prepared by dissolving V O5 in water containing 5% H O , at
2
2
2
◦
4
C; in this case, complete dissolution was achieved in less than
The analysis of the reaction products was carried out by means
◦
1 h, even though it was then left at rest overnight. The former solu-
of the following procedure: the reaction mixture was heated (65 C)
◦
tion was heated again at 90 C, and the second solution was added
under vacuum to evaporate both the unconverted cyclohexanone
and the reaction solvent; the residual solid was mixed with an
◦
drop by drop; then the mixed solution was left at 90 C for 8 h, in
◦
order to let most of the solvent evaporate, and was finally left at
excess of methanol and with BF3 (dissolved in methanol), at 80 C
◦
8
0 C overnight in order to complete the evaporation of the sol-
for 1 h in a sealed vessel, in order to transform the diacids pro-
duced (AA, GA and SA) to the corresponding methyl diesters. The
esters were then extracted with n-hexane (this also lead to the
separation of the POM and of BF3 from the mixture); the extrac-
tion procedure was repeated several times. Finally, the solution
of diesters in n-hexane was analysed by GC (internal standard n-
decane). This method allowed the identification of acids in the
form of methyl esters, of some residual cyclohexanone (and, in
general, carbonyl groups in aldehydes or ketones) in the form of
the corresponding methyl acetals, and alcohols (and in general
hydroxy groups) as the corresponding methyl esters. Since the
caprolactone eventually formed would bring to the ring-opened
compound during methanolysis in acid conditions, the formation of
this product and of other by-products was checked by means of the
extraction of soluble compounds with n-hexane from the reaction
mixture (before esterification), and gas-chromatographic analysis.
The same procedure was also adopted to estimate the C atomic
◦
vent. The wet solid obtained was finally dried at 110 C for 4 h. The
dried solid was characterised by means of FT-IR and Raman spec-
troscopy in order to confirm the formation of the desired Keggin
compound.
3. Results and discussion
3.1. Experimental evidence of the presence of two different
mechanisms
The use of water as the solvent for cyclohexanone oxidation
has positive implications from the environmental point of view,
due to both lower corrosion problems and avoiding of toxic sol-
vents, although at the end of the process additional costs have
to be borne in order to degradate organic contaminants in water
before disposal. A solvent is necessary, however, because the reac-

1
20
F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
Fig. 1. Conversion of cyclohexanone and selectivity to products as functions of the
acetic acid-to-water volumetric ratio. T = 70 C, P = 1 atm, O2 feed 300 N mL/min,
reaction time 6 h. Liquid-phase composition: 25 mL cyclohexanone, 25 mL solvent
◦
Fig. 2. Conversion of cyclohexanone and selectivity to products as functions of
the POM-to-cyclohexanone molar ratio. T = 70 C, P = 4 atm, O2 feed 300 N mL/min,
reaction time 6 h. Liquid-phase composition: 25 mL cyclohexanone, 25 mL solvent
◦
(
acetic acid/water), catalyst POMV1, amount 1.6 g (POM-to-cyclohexanone molar
(
water); catalyst POMV1, in variable amounts. Symbols: conversion of cyclohex-
ratio 0.0037). Symbols: conversion of cyclohexanone (ꢀ), selectivity to AA (ꢀ), to
anone (ꢀ), selectivity to AA (ꢀ), to CO2 (ꢁ) and to GA + SA (ꢂ).
CO2 (ꢁ) and to GA + SA (ꢂ).
tion is strongly exothermal, and a ballast component contributes to
the removal of the heat generated. However, the problem encoun-
tered with water as the only solvent is that of a limited conversion
achieved when homogeneous Keggin-type polyoxometalates are
used as catalysts for the reaction; usually, either long reaction
times, high pressure of oxygen or high catalyst-to-substrate ratio
are necessary to achieve high cyclohexanone conversion with
water solvent [10,20,23,24].
the protonation of cyclohexanone may eventually facilitate the dis-
solution of the substrate in the aqueous phase, where the POM is
dissolved.
Data in Fig. 1 show that in the solvent composition range consid-
ered, where a remarkable acceleration of the reaction rate occurred,
there was also an important effect on product distribution. We
expected a fall of selectivity to AA, because of the increased cyclo-
hexanone conversion experimentally observed after acetic acid
addition, which should favour the consecutive oxidative degrada-
The use of acetic acid as the co-solvent greatly enhanced the
reaction rate, as shown in Fig. 1 by plotting the conversion of
cyclohexanone in function of the acetic acid/water volume ratio
tions of AA to CO ; in contrast, the addition of acetic acid led to an
2
increased selectivity to AA and lighter acids, as well as to a pro-
(
whilst keeping constant the overall volume of solvent) with the
nounced decline of selectivity to CO . Then, a further increase of
2
POMV1 catalyst. The increase in the acetic acid concentration led
to a pronounced increase in cyclohexanone conversion, up to a
maximum value achieved for an equivolumetric ratio of the two
solvents. After that, higher concentrations of acetic acid did not
lead to a further increase in conversion. In regard to the distribu-
tion of products, Fig. 1 also reports the selectivity to AA, to GA + SA
acetic acid above the ratio of 1 caused a reduced selectivity to AA
and lighter acids, with a corresponding increase of selectivity to
CO , even though conversion was not affected. The highest cyclo-
2
hexanone conversion and selectivity to AA were reached at the
optimal volume ratio of 1. These effects suggest that acetic acid
did not simply accelerate the reaction rate, but favoured a change
in the reaction mechanism.
and to CO . No hydroperoxide was detected, which indicates that
2
these possible reaction intermediates (that indeed have never been
isolated in the literature on cyclohexanone aerial oxidation) are
quickly transformed into more stable diacids. Moreover, neither 6-
hydroxyhexanoic acid nor 6-oxohexanoic acid – possible reaction
intermediates in AA formation – were produced.
Fig. 2 shows the effect of the catalyst-to-cyclohexanone molar
ratio on catalytic behaviour for tests carried out with a water-only
solvent and POMV1 catalyst. There was an almost proportional
effect of the catalyst amount on cyclohexanone conversion,
whereas the selectivity to the products was not affected much by
the conversion. It is worth noting that in the absence of catalysts
there was no cyclohexanone conversion. Moreover, the addition of
very small amounts of a catalyst (with a POM-to-cyclohexanone
The presence of acids is known to catalyse autoxidation of cyclo-
hexane using Mn and Mn/Co catalysts [25–31]; in the Asahi process,
the one-step oxidation of cyclohexane to AA uses Co acetate cata-
lyst and acetic acid solvent (that instead is not used in the oxidation
of cyclohexane to KA Oil with air). In general, the stronger the acid,
the faster the oxidation rate. Rhodia recently reported on the use of
a lipophilic acid, 4-ter-butylbenzoic acid, instead of acetic acid, in
order to facilitate the separation and recycling, showing outstand-
ing effects in the acceleration of the reaction rate [32]. A similar
acceleration effect has also been reported in the aerial cyclohex-
anone oxidation to AA, catalysed by Co/Mn acetates, which was
developed by Asahi [7]. A possible explanation for this effect is
the acceleration of the tautomerism between cyclohexanone and
the corresponding enolic form, which is more easily activated by
radicalic H abstraction than cyclohexanone itself (Scheme 1). An
alternative explanation is the in situ formation of peracetic acid,
by acetic acid oxidation, that may eventually act as an oxidant on
cyclohexanone; however, separate tests of acetic acid oxidation
with oxygen and POMV1 catalyst led us exclude the formation of
any peracid, as inferred by iodometric titration. On the other hand,
−
5
molar ratio as low as 4 × 10 ) had practically no effect on con-
version, which remained below the detection limit; the same was
◦
observed also at a higher temperature, i.e. 90 C (conversion 0.50%,
yield to AA 0.35%). This observation is important, in view of the
behaviour shown by the same catalyst in the presence of acetic
acid (see below). These results indicate a direct participation of the
catalyst in the rate-determining step of the process, and suggest
that the POM has a catalytic effect on the process rate.
The effect observed when an equivolumetric acetic acid/water
mixture (Fig. 3) was used was quite different from that observed
without acetic acid. Firstly, even with no catalyst at all, there was
a non-negligible cyclohexanone conversion (1.6% in 6 h reaction
time) that provides additional evidence for the role of acetic acid
in promoting a radicalic-type, thermal oxidation of cyclohexanone.
Moreover, even a truly negligible amount of catalyst (correspond-
−
5
ing to a molar ratio POM-to-cyclohexanone as low as 4 × 10
)
caused a rapid increase of conversion. This clearly indicates that the

F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
121
Scheme 1. The activation of cyclohexanone via protonation and tautomerisation.
Fig. 3. Conversion of cyclohexanone and selectivity to products as functions of
the POM-to-cyclohexanone molar ratio. T = 90 C, P = 4 atm, O2 feed 300 N mL/min,
reaction time 6 h. Liquid-phase composition: 25 mL cyclohexanone, 25 mL solvent
Fig. 4. Conversion of cyclohexanone and selectivity to products as functions
of the reaction time. T = 70 C, P = 4 atm, O2 feed 300 N mL/min. Liquid-phase
composition: 25 mL cyclohexanone, 25 mL solvent (water), catalyst POMV1 (POM-
to-cyclohexanone molar ratio 0.002) Symbols: conversion of cyclohexanone (ꢀ),
selectivity to AA (ꢀ), to CO2 (ꢁ) and to GA + SA (ꢂ).
◦
◦
(
acetic acid/water 1/1), catalyst POMV1, in variable amounts. Symbols: conversion
of cyclohexanone (ꢀ), selectivity to AA (ꢀ), to CO2 (ꢁ) and to GA + SA (ꢂ).
catalyst indeed acts as an initiator of the reaction, and is strongly
in favour of a radicalic-type autoxidation process.
After the initial rapid increase, a further addition of catalyst
led to a less pronounced effect on conversion; this saturation
effect might be due to the well known chain-termination effect
that may occur in the presence of relatively large amounts
of catalyst-initiator [31]. Moreover, it cannot be excluded that
under conditions of high POM-to-cyclohexanone molar ratio, a
redox-type mechanism may either prevail over autoxidation, or
nevertheless give an important contribution to cyclohexanone
conversion. The distribution of products showed only marginal
variations in function of the amount of catalyst.
The data obtained suggest the presence of two different reaction
mechanisms for cyclohexanone conversion—a radicalic autoxi-
dation mechanism and a redox-type mechanism. The former is
preferred in the presence of acetic acid, whereas the latter may
occur in the presence of a water-only solvent. The two mechanisms,
however, may overlap in the presence of acetic acid co-solvent
when relatively high POM-to-cyclohexanone ratios are used. As
shown in Fig. 1, the two mechanisms also show different selectivity
with regard to AA; in fact, when the reaction is carried out in the
presence of acetic acid, the selectivity to AA achieved is higher than
that achieved without acetic acid, despite the higher cyclohexanone
conversion.
Other evidences supporting the overlapping of two different
reaction mechanisms, where the prevailing one is a function of
reaction conditions used, were obtained by examining the effect of
both reaction time and of partial oxygen pressure on the catalytic
behaviour.
Figs. 4 and 5 compare the effect of reaction time on both the
cyclohexanone conversion and the distribution of products, for
tests carried out with either a water-only solvent (Fig. 4) or the
acetic acid/water mix (Fig. 5). In view of the different reaction per-
formances in the two cases (see Fig. 1) we carried out experiments
in different conditions, in order to obtain similar values of cyclo-
hexanone conversion in the two sets of experiments. For example,
Fig. 5. Conversion of cyclohexanone and selectivity to products as functions of the
reaction time. T = 90 C, P = 4 atm, O2 feed 300 N mL/min. Liquid-phase compositions:
25 mL cyclohexanone, 25 mL solvent (acetic acid/water 1/1), catalyst POMV1 (POM-
to-cyclohexanone molar ratio 0.00046). Symbols: conversion of cyclohexanone (ꢀ),
selectivity to AA (ꢀ), to CO2 (ꢁ) and to GA + SA (ꢂ).
◦
tests with acetic acid co-solvents were carried out with a minimal
catalyst amount, that is, under conditions in which the redox mech-
anism involving the direct oxidation of the substrate by the catalyst
should be minimal; on the other hand, tests with the water-only
solvent were carried out with a higher POM-to-substrate ratio, in
order to favour the redox mechanism.
It is shown that the two cases showed quite different behaviour,
especially in regard to both the selectivity to AA and the nature of
the reactions yielding GA and SA by-products. Firstly, the selectiv-
ity to AA was greater in the presence of acetic acid (in average,
about 50% without acetic acid, above 50% with acetic acid co-
solvent, in both cases for a cyclohexanone conversion lower than
15%). Secondly, in the former case, AA was stable and did not
undergo significant consecutive reactions of oxidative degrada-

1
22
F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
◦
Fig. 6. Conversion of cyclohexanone in function of the oxygen pressure. T = 70 C,
O2 feed 300 N mL/min, reaction time 6 h. Liquid-phase composition: 25 mL cyclo-
hexanone, 25 mL solvent (water ꢁ,ꢁ; acetic acid/water 1/1 vol ratio ꢂ, ꢃ),
other conditions as in Fig. 1. Catalysts: POMV1 (ꢁ,ꢂ); POMV2 (ꢁ,ꢃ) (POM-to-
cyclohexanone molar ratio 0.0014).
Fig. 7. Conversion of cyclohexanone in function of the POM-to-cyclohexanone
◦
molar ratio. Catalysts: POMV1 (᭹) and POMV2 (ꢀ). T = 90 C, P = 4 atm, O2 feed
3
2
00 N mL/min, reaction time 6 h. Liquid-phase composition: 25 mL cyclohexanone,
5 mL solvent (acetic acid/water 1/1).
This hypothesis was confirmed by the experiments shown in
tion; the low selectivity to AA was due to the presence of parallel
Fig. 7, when comparing the conversion of the two POMs as a func-
tion of the POM-to-cyclohexanone ratio. The two catalysts showed
quite similar behaviour for low POM-to-cyclohexanone ratios, that
is, under conditions that favoured the autoxidation mechanism, but
their behaviour diverged when higher ratios were used. That con-
firms that it is possible to accelerate the reaction rate by using a
catalyst with enhanced redox properties, even under conditions in
which the POMV1 did not provide any further improvement of the
cyclohexanone conversion. With POMV2, the higher redox poten-
tial leads to a more pronounced effect of the catalyst amount on
substrate conversion.
reactions for the formation of GA, SA and CO . In the presence of
2
acetic acid co-solvent, instead, the initial selectivity to AA was very
high (apparently almost total if extrapolated to nil conversion), but
AA readily underwent consecutive oxidative degradations to form
lighter diacids and CO . Remarkably, the two lighter acids (and also
2
possibly CO ) did not form by direct, parallel reaction, but exclu-
2
sively via secondary reactions. We found the same behaviour – with
a substantial absence of parallel oxidative degradation reactions –
when experiments were carried out with acetic acid co-solvent, but
◦
at milder reaction conditions (T = 70 C, P = 1 atm). This difference
once again confirms the existence of different reaction mecha-
nisms, which implies different reaction networks occurring with
either water-only or the acetic acid/water mix.
3.2. Reactivity tests carried out in the presence of a radical
scavenger
Another important difference concerned the effect of oxygen
pressure. This is shown in Fig. 6, when comparing the effect of oxy-
gen pressure on cyclohexanone conversion, with and without acetic
acid co-solvent, but with other reaction conditions remaining sim-
ilar in the two cases. In the case of tests carried out without acetic
acid, the cyclohexanone conversion appeared to be proportional to
the oxygen partial pressure; this means that oxygen is involved in
the rate-determining step of the reaction. This agrees with find-
ings from Matveev and co-workers [33]. The authors showed that
in the aerobic oxidation of methylnaphthol to menadione – catal-
ysed by Keggin POMs, in which the mechanism is redox-type with
electron transfer from the substrate to the POM, excess electrons
being then released to molecular oxygen – the rate limiting step
In order to find out whether radicalic intermediates play a
role in the reaction under the different reaction conditions used,
we added increasing amounts of 2,6-di-ter-butyl-1-hydroxy-4-
methylbenzene (BHT), which is known to be amongst the most
efficient radical scavengers. Experiments were carried out using
acetic acid co-solvent, at both low and high POM-to-cyclohexanone
feed ratios, because the results obtained suggested that the two
different mechanisms – i.e. a redox mechanism with direct involve-
ment of the POM and an autoxidation mechanism – gave a
different contribution in function of the POM-to-cyclohexanone
ratio used.
of the process is the reoxidation of the reduced POM by O . More-
The results of these experiments are summarised in Fig. 8, which
shows the conversion of cyclohexanone as a function of both the
BHT-to-cyclohexanone ratio (top figure) and the BHT-to-POM ratio
(bottom figure), for two different POMV1 catalyst amounts, that
implied different POM-to-cyclohexanone feed ratios. It is shown
that the effect of the added BHT was quite different in function of
the POM-to-cyclohexanone feed ratio used. In fact, for tests car-
ried out in conditions of low feed ratio, the BHT caused a rapid
fall of cyclohexanone conversion in correspondence of very low
BHT-to-cyclohexanone ratio; on the contrary, for tests carried out
with high POM-to-cyclohexanone feed ratio, there was almost no
effect on cyclohexanone conversion up to a BHT-to-cyclohexanone
ratio of ca 0.1. It is worth noting that cyclohexanone conversion
was almost nil in the case of tests carried out at low POM-to-
cyclohexanone ratio, when a BHT-to-cyclohexanone ratio of 0.1
was used. It is evident that the added BHT – even a small amount
– has a remarkable effect on the reaction rate, under conditions in
2
over, again in agreement with Matveev’s findings, the reactivity
of the POMV2 was higher that that of POMV1. Conversely, when
the same experiments were performed with acetic acid co-solvent,
the cyclohexanone conversion was not greatly affected by oxy-
gen partial pressure—a clear indication that the steps involving
molecular oxygen are not rate limiting. It might be objected that
in such conditions, mass transfer of oxygen may become limit-
ing; however, when we used POMV2 (a compound showing a
good propensity for electron-transfer interactions with hydrocar-
bons and functionalised hydrocarbons [18]) as the catalyst, the
increase in O pressure (from 4 to 6 atm) had an important effect on
2
cyclohexanone conversion. This last experiment provides further
information: the type of prevailing mechanism may be a function
of the POM composition, and it may be expected that in the co-
presence of both redox and autoxidation mechanisms, the former
may become more important when POMV2 is used.

F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
123
3.3. Some considerations on the reaction mechanism
The mechanism of the aerobic oxidative cleavage of cyclic
ketones, vicinal diols, and ␣-ketols, catalysed by V(V) complexes,
precursors of pervanadyl (VO2)+ species, and by POMs has been
investigated by Brégeault and co-workers [14,23,24]. The inter-
action between the ketone (or the hydroxyketone) and the V(V)
complex is driven by the Lewis-type acid properties of the latter. As
•
a key step in the process, the authors suggested that the ROO per-
oxyl radical – formed after the addition of O to the radicalic species
2
•
formed by H abstraction from the substrate – might oxidize V(IV)
to V(V) forming a V(V) peroxo derivative. The simultaneous C–C
and O–O cleavage in this complex, then leads to ring opening and
oxoacid formation, still coordinated to the V(V) atom, which is the
precursor for the formation of AA and, eventually, also of partially
oxidized by-products. The authors, however did not exclude the
2+
possibility that the reoxidation of the reduced [V(IV) O] complex
might occur via the 4-electron redox process and O2 reduction. In
an alternative mechanism, the interaction of the radical interme-
diate with O2 may produce an endoperoxide species that, by the
simultaneous fission of C–C and O–O bonds, finally yields AA.
In our case, under conditions leading to the redox mechanism
as the prevailing one, and after the proposal of Brégeault of coordi-
3+
nation of the substrate to the monovanadyl species (V O) in the
POM and formation of a ring-opened intermediate, the develop-
•
ment of a –C(O)O group at the terminal C atom might lead either
to the production of AA or to the release of CO and the consequent
2
production of lighter diacids. Therefore, a common intermediate
for the diacids would explain the parallel formation of these com-
pounds (Fig. 4). On the other hand, it is likely that the reversible
complex between AA and the POM may help in saving AA from
consecutive oxidative degradation (to either light diacids or CO2),
which occurs in the liquid phase via the autoxidation process.
The mechanism of thermal and catalytic cyclohexane autoxida-
tion has also been studied in depth [1,3 and refs therein]; however,
Fig. 8. Cyclohexanone conversion as a function of the BHT-to-cyclohexanone molar
ratio (top) and of the BHT-to-POM molar ratio (bottom). Catalyst POMV1. POM-to-
−
5
−3
◦
cyclohexanone molar ratio 4.5 × 10 (ꢀ), 1.4 × 10 (᭹). T = 90 C, reaction time 6 h.
Solvent 25 mL (acetic acid/water 1/1).
it has been revisited recently by Hermans and co-workers [34–37].
These authors reported that a OH-radical breaking away from
cyclohexylhydroperoxide – the latter formed by reaction between
•
which reactivity tests suggested the presence of a radicalic-chain
autoxidation mechanism.
On the other hand, BHT caused a decline of cyclohexanone
conversion, although only for high BHT-to-cyclohexanone ratios,
even under conditions in which the redox mechanism contributed
to cyclohexanone conversion (i.e., at high POM-to-cyclohexanone
feed ratio). This suggests that in the case of the redox mechanism,
the activation and oxidation of cyclohexanone also occurs with
the involvement of radicalic intermediates. In this case, however,
the mechanism does not occur via a chain-reaction with free rad-
cyclohexyl radical and O to generate the peroxyl radical – abstracts
2
a weakly bonded ␣H-atom from cyclohexanone (one reaction prod-
uct), forming water and the ketonyl radical, stabilized by resonance.
The ketonyl radical then reacts to finally yield AA. According to
these authors, however, in the absence of acetic acid the main route
to AA is not that involving cyclohexanone, but rather the transfor-
mation of the intermediate cyclohexylhydroperoxide radical (a key
•
component in the reaction network) into the CyO radical, that con-
icals reacting with O ; rather, it would likely involve the electron
2
verts into 6-hydroxyhexanoic acid, the latter being finally oxidized
to AA. The hydroxyacid is also the precursor for the formation of
lighter diacids: that would imply a kinetic parallel scheme if the
intermediate hydroxyacid is readily converted into the final com-
pounds. In the mechanism proposed by Shimizu et al. [7] for the
Co/Mn acetate catalysed autoxidation of cyclohexanone, carried
out in acetic acid solvent, AA, GA and SA also form by parallel routes.
In our case, we observed that the formation of lighter diacids
mainly occur via consecutive reaction upon AA, with negligible
contribution of parallel reactions. The instability of AA under con-
ditions that favour autoxidation indicates that this compound may
still undergo activation by radicalic species and decarboxylation.
Moreover, we did not detect the formation of any partially oxi-
dized compound, e.g. either 6-hydroxy- or 6-oxohexanoic acid.
The mechanism proposed in Scheme 2 might explain the absence
of these compounds during cyclohexanone oxidation, because
the ring opening via C–C scission occurs before the forma-
transfer from the substrate to the POM and the development of
a complex by coordinating the substrate and intermediates [14].
This hypothesis is also confirmed by the plot in Fig. 8 (bottom).
It is shown that when the BHT-to-POM ratio was taken into con-
sideration, the effect of BHT on cyclohexanone conversion was
more pronounced for experiments carried out using high POM-to-
cyclohexanone feed ratio. In fact, with low feed ratio (i.e. with a
low POM amount in the reaction medium), the BHT added had a
non-negligible effect on the reaction rate only for extremely high
BHT-to-POM ratio.
With a water-only solvent, an experiment was carried out
at both high POM-to-cyclohexanone (0.012) and high BHT-to-
◦
cyclohexanone (0.25) molar ratio, at 90 C and a pressure of 4 atm.
Again in this case, cyclohexanone conversion dropped from 7%
(
measured in the absence of BHT) to 2% (in the presence of BHT);
moreover, in the latter case the selectivity to AA was almost nil,
with CO2 being the only product formed. This confirms the role of
radicalic intermediate species even under conditions at which the
redox mechanism prevails.
tion of the hydroxylactone [7] and leads to the radical species
•
OHC–(CH ) –C( O) ; successive steps bring about the production
2
4
of AA without forming any partially oxidized compounds.

1
24
F. Cavani et al. / Applied Catalysis A: General 391 (2011) 118–124
Scheme 2. A possible mechanism for the formation of adipic acid from activated cyclohexanone without intermediate formation of partially oxidized products.
4
. Conclusions
The reactivity of homogeneous Keggin-type P/Mo/V poly-
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cyclohexanone with oxygen was investigated under various con-
ditions. The use of acetic acid as a co-solvent led to a strongly
enhanced cyclohexanone conversion with respect to the reaction
carried out with water-only solvent. The conditions used for cat-
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of the POM in the reaction. Conversely, when the catalyst-to-
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acid co-solvent, the prevailing mechanism was a radicalic chain-
reaction autoxidation. The latter was more selective to AA than the
redox mechanism, due to the lower contribution of parallel reac-
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2
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