Boosting one-step conversion of cyclohexane to adipic acid by NO2 and VPO composite catalysts

Article abstract of DOI:10.1039/c5cc09840h

We demonstrate VPO composites as efficient catalysts for highly selective oxidation of cyclohexane to adipic acid with NO₂. In particular, the Ni-Al-VPO composite catalyst exhibits the striking conversion of cyclohexane (60.6%) and exceptionally high selectivity towards adipic acid (85.0%). Moreover, N₂O is an environmentally harmful gas, and its yield in the present process is only 0.03 t/t adipic acid, which is far below that obtained using the industrial method (0.3 t/t adipic acid). This work provides a new strategy for the one-step synthesis of dicarboxylic acids from cycloalkanes.

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COMMUNICATION
Boosting one-step conversion of cyclohexane to adipic acid by NO2 and
VPO composite catalysts
Jian Jian a, Kuiyi You a,b,*, Xuezhi Duan c, Hongxu Gao a , Qing Luo a, Renjie Deng a , Pingle Liu a,b
Qiuhong Ai a,b and He’an Luo a,b,
,
*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
H2O2
H2
ROOC
ROOC
We demonstrate VPO composites as efficient catalysts for
COOR
highly selective oxidation of cyclohexane to adipic acid with
NO2. Especially, Ni-Al-VPO composite catalyst exhibits the
epoxidation
O
OH
OH
H2
O
H
r
o
O
OH
O
10 striking conversion of cyclohexane (60.6%) and exceptionally
high selectivity towards adipic acid (85.0%). Moreover, N2O
as an environmental harmful gas, its yield of N2O in the
present process is only 0.03 t/t adipic acid, which is far below
the industrial method (0.3 t/t adipic acid). This work provides
15 a new strategy for one-step synthesis of dicarboxylic acids
from cycloalkanes.
O
O3/UV
H2
HNO3
HO
C
O2
H2
C
O
OH
Adipic acid
OH
OH
H
,
F
e
OH
OH
r
O
C
m
e
OH
air
O
n
t
HO
H2
Figure 1 Currently developed routes for the synthesis of adipic acid
Adipic acid, the most widely used dicarboxylic acid, has long
been known as a versatile building block or intermediate for the
production of nylon 66, polyurethane, plasticizer and other minor
20 endꢀuse segments.1 Various approaches have been developed for
the synthesis of adipic acid, that is, oneꢀstep oxidation of
55 towards high selectivity of adipic acid, it is still a promising
route,because the usage of NO2 as oxidant replacing nitric acid
not only simplifies the technology process but also reduces the
environment pollution. Therefore, there is an exigent need to
develop an effective catalyst for highly selective oneꢀstep
60 conversion of cyclohexane to adipic acid.
Herein, we report vanadium phosphorus oxide (VPO)
composites catalysts as efficient catalyst for highly selective oneꢀ
step oxidation of cyclohexane to adipic acid with NO2, as is
shown in Scheme 1.27The present catalyst is facile separation,
65 recycling, and reuse. Moreover, the reduction product NO can be
effectively recovered for reuse by adding oxygen in this process.
This work will be much interesting from a fundamental as well as
from an applied point of view, to find a new, environmental
friendly catalytic process towards directly converting
70 cyclohexane into adipic acid. The detailed results obtained from
the oneꢀstep process will be reported in this paper.
cyclohexane2ꢀ11 or cyclohexene,12 twoꢀstep oxidation of
13ꢀ17
cyclohexane and other multiꢀstep process
(see Figure 1). In
terms of the cost of reactants (starting substrates and oxidants)
25 and the energy efficiency of process the twoꢀstep oxidation of
,
cyclohexane has been industrially applied in the production of
adipic acid: (i) the oxidation of cyclohexane by air to produce
cyclohexanone and cyclohexanol (KA oil) at around 125ꢀ165 °C
and 8ꢀ15 atm air pressure in the presence or absence of
30 homogeneous cobaltꢀbased catalyst; (ii) the further oxidation of
KA oil to form adipic acid with nitric acid.18ꢀ20 However, in order
to achieve a finally higher KA oil yield of 82ꢀ84%, the
cyclohexane conversion per pass in the step has to be kept only 3ꢀ
5%, thus causing a low efficiency. Meanwhile, the use of a large
35 amount of nitric acid inevitably leads to the formation of
undesirable nitrous oxide (N2O) (about 300 kg of N2O per tonne
of adipic acid), linking to global warming effect, ozone depletion,
acid rain and photochemical smog.21ꢀ23 Hence, it is highly
desirable to develop more efficient, economic and
40 environmentally benign process for the production of adipic acid.
Kinetic studies on the oxidation of KA oil with nitric acid
showed that the active species are not HNO3 but nitrous acid
(HNO2)24 or dissolved NO2 and N2O425. For the above industrial
process, the excessive nitric acid is needed to possibly provide
45 enough active species towards desirable yield of adipic acid, but
leads to form a large amount of undesired N2O, which can not be
reꢀconverted into HNO3. Interestingly, Hoot and coꢀworker
proposed the nonꢀcatalytic oxidation of cyclohexane to adipic
acid with NO2 26 in the reaction tube and the reduction product is
50 NO, which can be directly converted into NO2 for reuse.
Although the conversion of cyclohexane is only 2.5% at 43 h
The effects of reaction temperature, reaction time, molar
ratio of cyclohexane to NO2 on the conversion of
cyclohexane and selectivity of adipic acid were examined
75
(Figure S1 and S2, Table S1, ESI†). From the single factor
experiment results, the optimal reaction conditions were
obtained that the reaction time was 24 h, reaction
temperature was 80 °C and the molar ratio of cyclohexane to
NO2 was 0.2:1.
80
NO2, 80oC
COOH
+
NO
COOH
VPO composites
Scheme 1 Oneꢀstep synthesis of adipic acid from the selective oxidation
of cyclohexane with NO2
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Table 1 Effect of various VPO composite catalysts on the catalytic
VPO
VPO precursor
∆
O 1s
ꢀ
V 2p
3/2
a
oxidation reaction of cyclohexane with NO2
14.4 eV
∆
O 1s
ꢀ
V 2p
3/2
PꢀOH
PꢀOH
Selectivity (%) b
14.4 eV
12.9 eV
Conversion
Catalyst
V4+
V5+
(%)
Adipic acid
78.2
Nitrocyclohexane
V 2p
1/2
V 2p
1/2
V4+
None
40.4
50.2
56.1
57.6
60.6
14.8
13.0
11.3
12.3
10.7
VPO precursor
VPO
80.8
Al-VPO
Ni-Al-VPO
V 2p
3/2
82.3
∆
O 1s
ꢀ
V 2p
3/2
∆
O 1s
ꢀ
PꢀOH
AlꢀVPO
NiꢀAlꢀVPO
83.1
14.4 eV
12.9 eV
14.3 eV
PꢀOH
85.0
13.0 eV
V5+
V4+
aReaction conditions: the reaction time is 24 h; reaction temperature:80
oC; the molar ratio of cyclohexane to NO2 is 0.2:1; the mass of catalyst
is 0.1 g (the mass fraction (wt.%) is 2% based on the cyclohexane).
V4+
V5+
V 2p
1/2
V 2p
1/2
b
Other products include cyclohexanol, cycloexanone, succinic acid,
540 535 530 525 520 515 510 540 535 530 525 520 515 510
Binding energy(eV)
glutaric acid and cyclohexyl nitrate.
130
Figure 2 The XPS of typical VPO composite catalysts
85
Table 1 summarizes representative results for the catalytic
oxidation reaction of cyclohexane with NO2 over VPO composite
catalysts. In this catalytic system, the catalytic performances of
the present VPO composite catalysts were different. Although
adipic acid could be formed in the absence of any catalysts, the
selective oxidation of cyclohexane with NO2 can efficiently
enhance the conversion of cyclohexane and selectivity of adipic
acid.
For a heterogeneousꢀcatalyzed liquid phase reaction, the
135 recycling and reusability of catalyst have received considerable
attention, especially for the practical applications. In present
recycling test, NiꢀAlꢀVPO composite as a solid heterogeneous
catalyst can be easily separated and recovered from the reaction
mixture by filtering. The catalytic performance of the recovered
140 catalyst after five recycling are depicted in Figure 3. It can be
seen that, compared to the fresh catalyst, no evident decrease in
conversion and selectivity was observed in the five runs, and the
leaching of NiꢀAlꢀVPO catalyst was only ca 1.4% in each
recycling process (Table S3, ESI†). The results indicated that
145 the NiꢀAlꢀVPO composite showed excellent reusability and
stability in this catalytic oxidation process.
In additions, in the present reaction products, the qualitative
analysis results indicated that the liquid phase products were
mainly nitrocyclohexane with small amount of cyclohexyl nitrate,
150 the solid phase products were mainly adipic acid with small
amount of glutaric acid and succinic acid. The components of
tailꢀgases were mainly NO2, NO and N2 with very small amount
of N2O and CO2, as shown in Table 2. Especially, the yield of
N2O in the present process is ca 0.03 t/t adipic acid, which is far
155 below the industrial method (0.3 t/t adipic acid) (Entry 4). It was
noted that the yield of N2O was obviously reduced under the
relative lower temperature and shorter reaction time (Entry 2 and
3). CO2 was originated from the formation of a small amount of
glutaric acid and succinic acid in this oxidation process.
90 selectivity to adipic acid was only 78.2% with 40.4% of
cyclohexane conversion. The conversion was obviously increased
from 40.4% to 50.2% while the reduced VPO composite was
introduced to reaction system. It is interesting that the
cyclohexane conversion and selectivity to adipic acid were
95 obviously enhanced when the VPO and AlꢀVPO composites were
introduced to reaction system, respectively. Especially, the Alꢀ
VPO catalysts combined transition metals could further improve
the conversion and selectivity to adipic acid. Among these
composite catalysts, the NiꢀAlꢀVPO composite gave the best
100 results with 60.6% of conversion and 85.0% of selectivity to
adipic acid under the present conditions. The catalytic effects
may be connected with the generation of V5+ sites close to the
abundant V4+ center of the pyrophosphate structure. It can not be
discarded, however, that the presence of a certain amount of
105 either element at the surface level may contribute to the C–H
bond breaking of the starting paraffin. In order to obtain the
information of the vanadium oxidation state in VPO composites,
the XPS spectra of several typical VPO composites samples are
shown in Figure 2. The results indicate the V5+ (βꢀVOPO4) and
110 V4+ ((VO)2P2O7) species are present in the typical VPO
composite catalyst samples. In addition, the presences of βꢀ
VOPO4 and VO)2P2O7 species were also confirmed by XRD
patterns (Figure S3 ESI†), FTꢀIR spectra (Figure S4, ESI†),
UVꢀvis spectra (Figure S5, ESI†) and H2ꢀTPR (Figure S6,
115 ESI†). The morphology and microꢀstructure of VPO composite
catalysts were characterized by TEM (Figure S7, ESI†), and the
composition of the elements in VPO composite catalysts were
determined by ICP (Table S2, ESI†). Among these VPO
composites, NiꢀAlꢀVPO catalyst shows more V4+ state relative to
Table 2 The tailꢀgases analysis for the selectivity oxidation of
cyclohexane with NO2 under the different reaction conditions
Selectivity (%)
Adipic acid
78.2
Tail-gases (t/t adipic acid)
Conversion
Entry
N2
N2O
NO
CO2
(%)
1 a
2 b
3 c
4 d
5 e
40.4
23.2
24.3
60.6
3.5
0.27
0.23
0.17
0.22
0.24
0.03
0.01
0.01
0.03
0.30
0.1
0.12
0.02
0.06
0.03
0.05
120
V
5+ state, and the percentage of V5+ and V4+ is approximately 39
86.7
0.07
0.10
0.08
trace
76.5
% and 61 % respectively by the XPS analysis. The results reflect
that doped Ni component can effectively stabilize V4+ oxidation
state and limits the overoxidation of (VO)2P2O7 to βꢀVOPO4.
Furthermore, it has also been proved that a suitable V4+/V5+
85.0
81.0
aReaction conditions: the reaction time is 24 h; reaction temperature:80
oC; the molar ratio of cyclohexane to NO2 is 0.2:1.
b the reaction temperature: 60 °C.
125 balance of surface VPO composite catalysts played an important
role in the selective oxidation of light alkanes with oxygen.28,
29These results fully illustrate that the synergisticꢀcatalytic action
of V5+ and V4+ state in VPO composites for the
c the reaction time is 12 h.
d 0.1 g NiꢀAlꢀVPO as catalyst was added in the reaction system.
e
the conversion and selectivity were based on cyclohexane from the twoꢀstep
oxidation.
2
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DOI: 10.1039/C5CC09840H
from the oxidation of cyclohexane with NO2 over VPO
composites in the present work. It has been demonstrated that
VPO composites were efficient catalysts for the direct conversion
of cyclohexane to adipic acid with NO2. The results indicate that
195 NiꢀAlꢀVPO composite catalyst gives the best results with 60.6%
of cyclohexane conversion and 85.0% of selectivity to adipic
acid. Furthermore, the yield of N2O in the present reaction system
is only 0.03 t/t adipic acid, which is far below the industrial
method (0.3 t/t adipic acid). The present catalyst can be facilely
200 separated, recycled, and reused. Such oneꢀstep method makes this
oxidation process safe, highly efficient and environmentally
benign by using NO2 as oxidizer compared to traditional
industrial twoꢀstep oxidation process. Further studies for the
selective catalytic oxidation are now in progress. The method
205 provides a novel strategy for highly selective oneꢀstep synthesis
of other dicarboxylic acids from cycloalkanes.
This work was financially supported by the National Natural
Science Foundation of China (21276216 and 21176204),
Specialized Research Fund for the Doctoral Program of Higher
210 Education (20124301130001), Project of Technological
Innovation & Entrepreneurship Platform for Hunan Youth (2014)
and Hunan Provincial Innovation Foundation for Postgraduate
(CX2014B268).
Conversion
Selectivity to adipic acid
Selectivity to nitrocyclohexane
90
75
60
45
30
15
0
1
2
3
4
5
Runs
160
Figure 3 The reuse of NiꢀAlꢀVPO composite catalyst in the catalytic
oxidation reaction of cyclohexane with NO2
On the basis of these results in this work, and our previous
work,30 the possible reaction pathway for the catalytic oxidation
165 of cyclohexane with NO2 over VPO composites catalyst is
proposed in Scheme 2. Firstly, the V5+ species in the VPO
composites abstracts the Hꢀtom of cyclohexane and were reduced
to V4+ species, then the resulting V4+ species contained hydrogen
were oxidized to V5+ species by NO2, and the nitrous acid
170 (HNO2) was formed. In this process, V5+ species maybe play an
important role in activating CꢀH bond from alkanes 31, 32 in our
present oxidation system. That is to say, it appears that dispersed
V5+ (βꢀVOPO4) species in combination with (VO)2P2O7 (V4+) are
important for the oxidation of cyclohexane whereas the V5+
175 species act as a dynamic oxidizing center.33 Meanwhile, the
activated cyclohexane reacted with the generated nitrous acid
(HNO2) to form cyclohexanol, which was further converted to
adipic acid by multiꢀstep oxidation process, and the activated
cyclohexane was also nitrated to nitrocyclohexane by NO2. In
180 addition, V5+ can accelerate the oxidation of intermediate
cyclohexaneꢀ1,2ꢀdione to adipic acid.34 Obviously, the nitration
and oxidation reaction of cyclohexane with NO2 happened
simultaneously in this reaction system. Further studies for the
reaction mechanism of cyclohexane with NO2 are now in
185 progress.
Notes and references
215 a School of Chemical Engineering, Xiangtan University, Xiangtan 411105,
P.R., China.
b National & Local United Engineering Research Center for Chemical
Process Simulation and Intensification, Xiangtan University, Xiangtan
411105, P. R., China
220 c State Key Laboratory of Chemical Engineering, East China University of
Science and Technology, Shanghai 200237, P. R. China
*Eꢀmail: youkuiyi@126.com (K. You); hluo@xtu.edu.cn (H. Luo)
†Electronic Supplementary Information (ESI) available: Experimental
details, Figures and Tables captions, XRD, FTꢀIR, UVꢀvis, H2ꢀTPR, TEM
225 and ICP etc.
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