Cyclohexane Oxidation to Adipic Acid Under Green Conditions: A Scalable and Sustainable Process

Article abstract of DOI:10.1002/cctc.201800419

This work reports a 1 mol scale catalytic process to synthesize adipic acid directly from cyclohexane in solvent-free conditions using air as oxidant. Catalysts based on vanadium phosphorus oxides were prepared, characterized and tested. They showed good activity and remarkably high selectivity to adipic acid in comparison to other already known heterogeneous catalysts. The use of solvent-free conditions permits the easy separation of the product from the reactant mixture, which is very important from the industrial point of view. The process can be used at industrial scale for sustainable adipic acid synthesis.

Full text of DOI:10.1002/cctc.201800419

Accepted Article
Title: Cyclohexane oxidation to adipic acid under green conditions: a
scalable and sustainable process.
Authors: Robert Wojcieszak, Alberto Mazzi, Sebastien Paul, and
Fabrizio Cavani
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To be cited as: ChemCatChem 10.1002/cctc.201800419
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10.1002/cctc.201800419
ChemCatChem
COMMUNICATION
Cyclohexane oxidation to adipic acid under green conditions: a
scalable and sustainable process.
A. Mazzi [a],[b], S. Paul[a], F. Cavani[b] and R. Wojcieszak[a]*
Abstract:
global warming and ozone depletion [3]. Moreover, it is also
responsible for acid rains and smog. Every years more than
400,000 metric tons are emitted into the atmosphere, which
corresponds to about 5–8 % of the worldwide N2O production [3].
When developing a new process, which eventually could
be used at industrial scale, the choice of the starting reagent is
crucial. Such a process requires a starting material, which is
widely available, economically viable and suitable for green
reaction conditions (namely liquid phase oxidation, solvent free
conditions, use of air as oxidant and avoiding the use of any free
radicals initiators). Such a substrate would permit to obtain a
product with a competitive cost as compared with to the
traditional process. For these reasons cyclohexane (CH) was
chosen as the starting molecule as it is a largely available
chemical with a competitive cost.
This work reports a 1 mol scale catalytic process to synthesize
adipic acid directly from cyclohexane in solvent free conditions using
air as oxidant. Catalysts based on vanadium phosphorous oxides
were prepared, characterized and tested. They showed good activity
and remarkably high selectivity to adipic acid in comparison to other
already known heterogeneous catalysts. The use of solvent free
conditions permits the easy separation of the product from the
reactant mixture, which is very important from the industrial point of
view. The process can be used at industrial scale for sustainable
adipic acid synthesis.
Currently, one of the main challenges of the chemical industry is
the development of new sustainable and “green” processes
taking into account the numerous environmental issues that
affect our planet. Consequently, there is a renewed interest in
the production and use of chemicals obtained through
environmentally friendly technologies. Adipic acid (AA) is an
aliphatic dicarboxylic acid with high volume of production and it
is primarily used as starting reagent in the preparation of nylon-
6,6 polyamide, fibres, plasticizers, food additives and many
other applications [1]. Global production of adipic acid is around
2900 ktons/year and the demand continues to grow around
2%/year with non-nylon applications growing faster than the
nylon sector [2]. Industrially, adipic acid is synthesized by a two-
step process involving oxidation of cyclohexane to a mixture of
cyclohexanol and cyclohexanone (the so-called KA oil) further
oxidised by HNO3 to the final acid (Scheme 1).
Here, we reported on the synthesis of the adipic acid using
heterogeneous catalysts based on vanadium phosphorous
oxides. As prepared catalysts were tested in the oxidation of
cyclohexane, without any solvent and under mild conditions
(T<150°C, 13-25 bars of air).
As reported in the literature [4], the oxidation of
cyclohexane carried out in solvent-free conditions, using air as
oxidant and without the presence of free radicals initiators does
not permit to reach high level of conversion (Table 1), due to the
difficulty in the activation of the aliphatic ring of the alkane. In
particular, it is worth to note that the maximum cyclohexane
conversion (57%) was obtained after a long time of reaction (16
h) without the formation of significant amounts of adipic acid [5-
7] as could be seen from Table 1. 65% of selectivity to adipic
acid was reached in the case of Fe-AlPO-31 but only after 24
hours of reaction and at a very low CH conversion (7%).
Table 1. Literature results [5-7] obtained in solvent-free conditions using air
as oxidant and without free radicals initiators.
T (°C), P (bar), Reaction ratea / CH
Main product
selectivity (%)
Catalyst
time (h)
conversion (%)
Co-AlPO-5
Co-AlPO-36
Fe-AlPO-31
Fe-AlPO-5
Fe-AlPO-5
130, 15, 16
130, 15, 16
100, 15, 24
100, 15, 24
130, 15, 24
8 / 11
42 / 57
3 / 7
KA Oil 80
KA Oil 83
Scheme 1. Schematic representation of the current industrial method for
cyclohexane oxidation.
AA 65
2 / 5
KA Oil 77
4 / 7
KA Oil 51, AA 31
This process is seriously harmful for the environment and leads
to a huge mark on the global warming. Indeed, the nitrous oxide
(N2O) produced in this process is an unavoidable stoichiometric
waste that is commonly considered as a major factor of the
[a] [mmolCH*gcat-1*h-1]
In this work, heterogeneous catalysts consisting in supported
vanadium-phosphorus mixed oxides (VPO), which are known as
being excellent oxidation catalysts for n-butane conversion to
maleic acid, were developed. The first step was the synthesis of
the VOHPO4·0.5H2O precursor, obtained by refluxing a mixture
of vanadium pentoxide and phosphoric acid in isobutanol for 6
hours. In a second step, the precursor was supported on several
supports (CeO2, Fe3O4, H-mordenit and Y-goethite) then
calcined at 500 °C for 4 hours under static air. The precursor
used in the preparation of VPO catalysts was analysed by
[a]
[b]
*
Msc A. Mazzi, Prof. S. Paul, dr R. Wojcieszak, Univ. Lille, CNRS,
Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de
Catalyse et Chimie du Solide, F-59000 Lille, France.
Prof. F. Cavani, Dipartimento di Chimica Industriale “Toso
Montanari”, Università di Bologna, Viale Risorgimento 4, 40136,
Bologna, Italy.
Corresponding author: Robert.wojcieszak@univ-lille.fr
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10.1002/cctc.201800419
ChemCatChem
COMMUNICATION
means of XRD to verify its structure (data not shown here). The
results confirmed the presence of the characteristic diffraction
peaks of the VOHPO4 · 0.5H2O precursor validating in the same
time the synthesis method.
organic solvents and therefore impossible to analyse via GC or
HPLC.
The first catalytic screening was performed at 120°C using 5
wt% VPO supported on different supports (CeO2, Fe3O4, H-
mordenite, Y-and goethite (FeO(OH)). The catalytic results are
shown in Table 2. It could be seen that a remarkably high
reaction rate was achieved in the case of VPO and VPO/CeO2
catalyst, which was then chosen for further studies. Several
parameters such as temperature, pressure and time of the
reaction were studied. Blank tests performed with the support
and without catalyst clearly indicated the important role of VPO
as an active phase in the cyclohexane oxidation to adipic acid.
The effect of the pressure was studied in the 13-25 bar range,
but no significant variations in term of conversion and products
selectivity was observed. Tests at different temperatures were
performed at 100 °C, 120°C, 135 °C and 150 °C keeping
constant the others reaction parameters (time and pressure) in
order to improve the catalytic activity of the VPO-based catalysts.
Table 2. The reaction was carried out with 13 bar of air, air flow of 120 mL/min,
120 °C, 15 mg of catalyst, 20 mL of cyclohexane and 4 h of reaction time,
ammol of cyclohexane (CH) converted/(gram of catalyst)*(time)
Figure 1. Effect of the temperature on the catalytic performance of VPO/CeO2.
[mmolCH*gcat-1*h-1].
b
Selectivity
to:
KA
Oil
(mixture
Conversion of CH (X) and products selectivity (S) at 120 °C, 135 °C, 150 °C.
cyclohexanol/cyclohexanone), adipic acid (AA), c No catalyst was used, d
Cyclohexane conversion in %.
The effect of the time of reaction (4 and 10 hours) was also
investigated in the range 100-135 °C. The results are given in
Table 3.
Reaction ratea
Selectivityb [%]
KA Oil
Catalyst
AA
Table 3. Effect of the time of reaction at 100 °C, 120 °C and 135 °C, using
VPO/CeO2. atemperature of the reaction (°C). btime of reaction (h).
cConversion of cyclohexane (CH). dSelectivity to KA oil (cyclohexanol (Chol) +
cyclohexanone (Chone)), adipic acid (AA) and other (glutaric acid (GA) and
succinic acid (SA)).
blank testc
VPO/CeO2
VPO/Fe3O4
<1d
24
4
99
93
98
98
99
95
0
6
0
0
0
2
Xc %
CH
Selectivityd %
AA
VPO/H-
mordenite
4
CeO2
VPO
2
Ta
tb
KA Oil
Other
12
4
10
4
0
1
0
0
0
0
0
100
The results obtained after 4 h of reaction are shown in
Figure 1. As expected the catalytic activity is strongly influenced
by the temperature of the reaction. No activity was observed at
100 °C. At 135 °C the rate of reaction reached 63 mmolCH*gcat-
1*h-1 while an increase of the temperature to 150 °C brings the
reaction rate to the maximum value of 162 mmolCH*gcat-1*h-1.
99
93
83
78
51
2
6
0
120
135
10
4
5
13
18
35
4
4
2
The highest selectivity to cyclohexanol, that is the primary
product of oxidation, was obtained at 120°C (80%). At 135 °C
and 150°C the cyclohexanol selectivity were respectively 69%
and 43%, which suggests that at higher temperatures the further
oxidation of cyclohexanol to secondary products prevails. Indeed
cyclohexanone selectivity increased from 11% to 15% at 135 °C,
but remained almost constant at 150 °C. In addition, the
selectivity to adipic acid reached the maximum also at higher
temperature. As shown in Figure 2 selectivity of 25% to AA was
obtained at 150 °C. It is worth to note that the tests at higher
temperature (170 °C), showed the presence of several
decomposition products, which were insoluble in both, water and
10
12
11
The tests confirmed that even after 10 h of reaction the
catalyst is inactive at 100 °C (not shown). Moreover at high
temperature (>150 °C) many by-products such as: formic acid
cyclohexyl ester, acetic acid cyclohexyl ester, pentanoic acid
cyclohexyl ester, butyrolactone, hexanoic acid, acetic acid, 2-
cyclohexen-1- one, formic acid, butanoic acid cyclohexyl ester
and hexanoic acid ethyl ester were identified.
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10.1002/cctc.201800419
ChemCatChem
COMMUNICATION
120 mL/min of oxygen was used in order to work in semi-batch
conditions. At the end of reaction 0.5 mL of the final liquid solution was
withdraw with a micropipette and kept for the GC analysis (Shimadzu
GC2010 Plus AF, FID detector, ZB-WAX plus column), and then a
certain amount of water (20-30 mL) was added in order to extract the
other products for HPLC analysis (Shimadzu UFHPLC, Rezex ROA
Organic acid column, trifluoroacetic acid as eluent with the total flow rate
of 0.5 mL/min). The catalyst was filtered and the two final solutions,
organic and aqueous solution separated.
It is clear that an increase in the time of the reaction
increases the cyclohexane conversion. Indeed, 12% conversion
was reached at 135 °C after 10 hours of reaction. In terms of AA
selectivity, the best performance was also obtained at 135 °C
(35%). This temperature seems to enhance the activity of the
catalyst without favouring the formation of by-products, as
observed at 150 °C. An increase in time of the reaction at
135 °C leads also to higher selectivity to secondary products
such as cyclohexanone and adipic acid. This trend suggests that
the oxidation of CH to CHOL is the rate-determining step and
this is proved also by the low cyclohexane conversion reached
after 10 hours. Once the concentration of the cyclohexanol starts
to increase it can be oxidised more easily to cyclohexanone and
adipic acid, leading to higher selectivity to these two products
Catalysts based on vanadium phosphorous oxides (VPO)
supported on ceria have been tested for direct oxidation of
cyclohexane to adipic acid under green conditions. This catalytic
system is able to activate the cyclohexane ring at relatively low
temperature, low pressure of air and without the use of any free
radical initiators. Moreover, it shows a good activity in terms of
cyclohexane conversion and adipic acid selectivity, especially in
comparison with the results reported in the literature for similar
conditions. In particular high reaction rates and good adipic acid
selectivity were obtained. Further studies are in progress in
order to improve the catalytic system based on vanadium
phosphorous oxides and also to better understand the reaction
pathway.
Acknowledgements
The REALCAT platform is benefiting from a state subsidy
administrated by the French National Research Agency (ANR)
within the frame of the ‘Future Investments’ program (PIA), with
the contractual reference ‘ANR-11-EQPX-0037’. The European
Union, through the ERDF funding administered by the Hauts-de-
France Region, has co-financed the platform. Centrale Lille, the
CNRS, and Lille 1 University as well as the Centrale Initiatives
Foundation, are thanked for their financial contributions to the
acquisition and implementation of the equipment of the
REALCAT platform.
Keywords: Adipic acid • VOPO4 • heterogeneous catalysis •
green oxidant • solvent free
[1]
[2]
C. Jan, J. Bart, S. Cavallaro, Ind. Eng. Chem. Res., 2015, 54, 1–46
http://www.pcinylon.com/index.php/markets-covered/adipic-acid,
18/05/2016
Experimental Section
[3]
[4]
A. Rahman, M. Mupa, C. Mahamadi, Catal. Lett., 2016, 146, 788-799
F. Cavani, G. Centi, S. Perathoner, F. Trifirò, Sus. Ind. Chem., 2007, 7,
367 – 414
Synthesis of the VOHPO4 · 0.5H2O precursor:
The appropriate amounts of vanadium oxide (≈6g) and phosphoric acid
(≈8.3g) (P/V ratio 1.1) were added into a three-neck round-bottom flask
with 125 mL of isobutanol. The solution was then heated under reflux at
100°C for 16h. The final blue solid was filtered and washed with acetone
(3x100mL) and dried in the oven overnight (90°C).
[5]
M. Dugal, G. Sankar, R. Raja, J.M. Thomas, Angew. Chem. Int. Ed.,
2000, 39, 2310
[6]
[7]
S. Sankar, R. Raja, J.M. Thomson, Catal. Lett. 1998, 55, 15
J.M. Thomas, R. Raja, Catal. Today, 2006, 117, 22
Preparation of the supported VPO/CeO2 (5% wt):
The appropriate amount of vanadium precursor was dispersed in few
millilitres of ethanol and the solution was dropped on cerium oxide until
the mud point was reached. The wet solid was then dried in the oven
(100°C) and the operation repeated till all the solution containing the
VPO species was adsorbed on ceria. Subsequently the catalyst was
calcined under static air atmosphere at 500 °C for 4 hours with a
temperature rate of 20 °C/min.
X-ray diffraction:
The X-ray analysis was carried out by using Bruker D8 Advance
diffractometer operated at an accelerating voltage of 40 kV and an
emission current of 40 mA with Cu radiation.
Catalytic tests:
20 mL of cyclohexane and 15 mg of catalyst were introduced into a
Teflon vessel installed inside of the reactor system. At the beginning the
temperature of the reflux condenser started to cool down at the minimum
temperature of -10 °C, once the temperature was reached the pressure
was increased with an airflow to the set point value. Only when the
pressure was constant the stirring system started to work and the vessel
temperature started to increase until the desired value, this step is
important to avoid the evaporation of the volatile cyclohexane. Airflow of
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10.1002/cctc.201800419
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COMMUNICATION
Getting greener with a new one-pot
sustainable and scalable synthesis
of adipic acid via free solvent air
oxidation of cyclohexane.
A. Mazzi [a],[b], S. Paul[a], F. Cavani[b]
and R. Wojcieszak[a]*
Page 1 – Page 3
Title:
Cyclohexane oxidation to
adipic acid under green conditions:
a scalable and sustainable process
This article is protected by copyright. All rights reserved.