A large-scale continuous-flow process for the production of adipic acid via catalytic oxidation of cyclohexene with H2O2

Article abstract of DOI:10.1039/c2gc35677e

The demand for a clean production process of adipic acid (AA) can be achieved by developing a synthetic route using H₂O₂ as the oxidant. In this paper, a green process with a recyclable catalyst system consisting of H₂WO₄, H₂SO₄ and H₃PO₄ was developed for the production of AA via catalytic oxidation of cyclohexene. A continuous-flow reactor was set up for the optimization of the reaction parameters and developing the industrial operation of this green process. The mixture of H₂SO₄ and H ₃PO₄ as acidic promoter displays a significant improvement in the activity of catalyst and the stability of H₂O₂. The catalyst could be recovered and reused 20 times, and no significant loss of catalytic performance can be observed. The effect of Fe³⁺ ion as a possible contaminant has no serious negative effect on this reaction, and the 316L stainless steel and glass-lined steel were selected as appropriate equipment material. Calorimetry and the scale-up in batch reactor demonstrate that the reaction could be operated safely on scale. The process was scaled up in a continuous-flow pilot plant, with excellent yield (94.7%) and purity (99.0%). Some advantages such as the solvent-, phase-transfer-catalyst-, and organic additive-free and low-cost light up the application of this process in the industrial production of AA.

Full text of DOI:10.1039/c2gc35677e

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PAPER
A large-scale continuous-flow process for the production of adipic acid via
catalytic oxidation of cyclohexene with H O †
2
2
Yiqiang Wen, Xiangyu Wang,* Huijuan Wei, Baojun Li,* Peng Jin and Limin Li
Received 3rd May 2012, Accepted 25th July 2012
DOI: 10.1039/c2gc35677e
The demand for a clean production process of adipic acid (AA) can be achieved by developing a synthetic
route using H O as the oxidant. In this paper, a green process with a recyclable catalyst system consisting
2
2
of H WO , H SO and H PO was developed for the production of AA via catalytic oxidation of
2
4
2
4
3
4
cyclohexene. A continuous-flow reactor was set up for the optimization of the reaction parameters and
developing the industrial operation of this green process. The mixture of H SO and H PO as acidic
2
4
3
4
promoter displays a significant improvement in the activity of catalyst and the stability of H O . The
2
2
catalyst could be recovered and reused 20 times, and no significant loss of catalytic performance can be
3
+
observed. The effect of Fe ion as a possible contaminant has no serious negative effect on this reaction,
and the 316L stainless steel and glass-lined steel were selected as appropriate equipment material.
Calorimetry and the scale-up in batch reactor demonstrate that the reaction could be operated safely on
scale. The process was scaled up in a continuous-flow pilot plant, with excellent yield (94.7%) and purity
(99.0%). Some advantages such as the solvent-, phase-transfer-catalyst-, and organic additive-free and
low-cost light up the application of this process in the industrial production of AA.
Introduction
N and O . Therefore, several environmentally benign methods
2 2
for the synthesis of AA have been designed and investigated,
9
Adipic acid (AA) is the main raw material for manufacturing
nylon 6,6 and many other products, with a global annual pro-
duction over 3.50 million tons. Currently, there are two commer-
such as the oxidative cleavage of 1,2-diol by HNO , the oxi-
3
1
,2
1
0–12
dation of 1,2-diol or cycloalkanones by hydrogen peroxide,
13,14
the synthesis from D-glucose or muconic acid,
the oxidation
and the bishydroformylation
of 1,3-butadiene. Nevertheless, novel environmentally benign
methods are still needed for the industrial production of AA.
One-step oxidative cleavage of cyclohexene to adipic acid
with H O has been considered to be one of the green synthetic
2
15–20
cial processes for the production of AA (Fig. 1). The traditional
of cyclohexane by dioxygen,
21
process developed by DuPont for the production of AA (Fig. 1a)
involves the hydrogenation of benzene to cyclohexane, the oxi-
dation of cyclohexane to KA oil (the mixture of cyclohexanol
and cyclohexanone) with air as oxidant, and the oxidation of KA
oil to AAwith HNO . The application of selective hydrogenation
of benzene to cyclohexene has provided a novel way to product
abundant and cheap cyclohexene, which was used for the indus-
2
2
1
3
processes with most potential up to date. H O is a cheap and
2 2
3–6
trial synthesis of AA.
The improved process developed by
Asahi Kasei (Fig. 1b) involves the selective hydrogenation of
benzene to cyclohexene, the hydration of cyclohexene to cyclo-
hexanol, and the oxidation of cyclohexanol to AA with HNO3.
Due to removing the procedure of cyclohexane oxidation with
air, the Asahi Kasei process improves the operational safety and
the utilization of carbon atom significantly. It is noteworthy that
both of the above two processes produce a large amount of N O,
2
7
,8
an environmentally harmful greenhouse gas. A supplementary
catalytic reactor must be employed for conversion of N O into
2
College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou 450001, China. E-mail: xiangyuwang@zzu.edu.cn,
lbjfcl@zzu.edu.cn
†
Electronic supplementary information (ESI) available: Effect of acidic
promoters and the pathway of oxidation of cyclohexene to adipic acid
with aqueous H . See DOI: 10.1039/c2gc35677e
Fig. 1 (a, b) Current industrial processes and (c) green processes for
production of AA.
2
O
2
2868 | Green Chem., 2012, 14, 2868–2875
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environmentally benign oxidant with water as the only by-
product, and thus it has played an important role in environmen-
AA with H O using peroxotungstate formed in situ as catalysts
in the absence of organic solvents and phase-transfer catalyst
2
2
2
2,23
tally benign methods in the chemical industry.
Currently,
were investigated and compared (Table S1, ESI†). With H SO4
2
H O is produced on an industrial scale by the anthraquinone
as acidic promoters, the highest yield of AA was obtained. The
yield of AA increased with the increase of acidity of acidic pro-
2
2
23
oxidation process. The cost of manufacturing H O has contin-
2
2
ued to decrease by the progress of process technology and the
increasing economies of scale. New promising methods for
H O production, such as synthesis from CO/O /H O mixtures,
moters except HCl and HNO , which was similar to the results
3
24,27,46
in previous reports.
Although HCl is very strong in
acidity, the yield of AA is low with HCl as acidic promoters.
2
2
2
2
direct synthesis from O /H , photocatalytic synthesis and bio-
The reason is that the volatility of the HCl (or HNO ), decrease
2
2
3
inspired production, will make this key reagent of green chem-
actual acid concentration in heated liquid phase that leads to the
decrease of catalyst activity. The stability of H O in the pres-
22,23
istry even cheaper.
In 1998, Noyori had obtained a historic
2
2
breakthrough in the green production of AA via one-step oxi-
ence of different acidic promoters was also investigated
1
dative cleavage of cyclohexene to AA with H O (Fig. 1c).
(Table S1, ESI†). The results demonstrate that H O is more
2
2
2 2
In Noyori’s system, the homogeneous tungstic catalysts
stable in the presence of H PO or 5-sulfosalicylic acid. The
3 4
showed excellent catalytic activity for oxidation of cyclohexene
reason is that H PO4 and 5-sulfosalicylic acid can sequester
3
1,24–33
to AA.
ammonium salt was employed as phase-transfer catalyst.
phase-transfer catalyst can be replaced by organic ligands, which
In order to accelerate the reaction, quaternary
many transition metals, reduce their catalytic activity for un-
productive decomposition of H O , and stop H O decompo-
1,24
The
2
2
2 2
47
sition as radical scavenger.
H SO was selected as the primary promoter for the oxidation
2
5–28
coordinate onto peroxotungstate.
Surfactant-type peroxo-
2
4
tungstate had also been synthesized and used in this organic
of cyclohexene to AA, and H PO was used as assistant promo-
ter to improve the stability of H O , because they are much more
2 2
3
4
2
9
solvent-free catalytic synthesis of AA. Na WO in acidic ionic
2
4
liquids and Dawson polyoxotungstate also were reported as
inexpensive and stable than 5-sulfosalicylic acid. Thereby a
system consisting of H WO , H SO and H PO was chosen as
30,31
effective catalysts.
The microwave reactor and PVDF flat
2
4
2
4
3
4
membranes as contactors for direct solvent-free biphasic oxi-
catalyst precursor. The influence of addition amount of H SO4
2
3
2,33
dation of cyclohexene to AA have been researched.
The
and H PO on the yield of AA and the stability of H O were
3
4
2 2
zeolite, mesoporous material or heteropolyacid supported on
metal oxide also showed somewhat activity in synthesis of
studied and the results are shown in Fig. 2. As shown in Fig. 2a,
34–39
AA with H O .
However, owing to the difficulty in recyc-
2
2
,34,35,38
1
ling catalyst,
these attractive methods for the synthesis of
AA via green oxidation of cyclohexene have not been industrial-
ized. Hence, this oxidation reaction requires further improve-
ments with particular focus on the recycle of catalyst, as well as
the effective utilization of H O by avoiding the unproductive
2
2
decomposition.
Compared to batch operation, the continuous-flow techniques
4
0
has significant processing advantages. A particularly attractive
feature of continuous-flow process is the ease with which reac-
tion conditions can be scaled up to achieve production scale. In
previous works, process development and theoretical studies of
the green oxidation of cyclohexene to AAwith catalyst system of
peroxotungstate have been carried out for several years in our
41–46
laboratory.
Herein, we report a large-scale continuous-flow process for
green production of AA via catalytic oxidation of cyclohexene
with H O as oxidant using H WO , H SO and H PO as cata-
2
2
2
4
2
4
3
4
lyst precursor. The present method possesses the following
advantages: (1) no N O produced; (2) organic-solvent- and
2
quaternary-ammonium-free system; (3) high yield and purity for
AA; (4) the catalyst can be reused and possesses excellent
stability; and (5) simple, inexpensive and safe.
Results and discussion
A recyclable catalyst system
Fig. 2 The effect of (a) H SO and (b) H PO on AA yield and H O
2
4
3
4
2
2
decomposition. Reaction conditions: (a) H
and H
given in the Experimental section). (b) H
and H O in a molar ratio of 1 : 1.04 : 50 : 220 (all other conditions were
2
WO
4 3 4
, H PO , cyclohexene
In order to solve the problem of recycling tungstate catalyst, our
group explored a recyclable catalyst system. As a starting point,
the effect of different acidic promoters on the yield of AA and
the stability of H O in the green oxidation of cyclohexene to
2
O
2
in a molar ratio of 1 : 0.5 : 50 : 220 (all other conditions were
WO , H SO , cyclohexene
2
4
2
4
2
2
unchanged).
2
2
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the ratio of H SO in catalyst system has clear effect on the cata-
and reused in the next run. This recycled catalyst (H WO ,
2
4
2
4
lytic performances. Based on the experimental data of pro-
duction yield, the simulated curve about the effect of sulfuric
acid on AA yield was given via cubic Spline function fitting.
The simulated curve showed that the suitable molar ratio of
H SO to H WO was 1.04, which was also validated by experi-
H SO4 and H PO4 as precursor) could be reused for twenty
2 3
cycles in the oxidation of cyclohexene to AA without significant
loss of catalytic activity (Fig. 3a). The average yield in twenty
cycles was over 93%, and only slight increase of unproductive
decomposition of H O was observed. At the end of the reaction,
2
4
2
4
2 2
ment. When the molar ratio of H SO to H WO was 2.2,
the byproducts (such as glutaric acid and succinic acid) in the
reaction solution were increased over 3%. It indicates that
the recovery efficiency of the catalyst was about 90 wt%. In con-
trast, the catalyst system with tungstatic acid and sulfuric acid as
catalyst precursor rapidly deactivated (Fig. 3b).
2
4
2
4
the excessive H SO4 caused oxidative degradation of inter-
The active structure of this catalyst system (with H WO ,
2
2
3−
4
4
6
mediate and reduced reaction selectivity. In addition, H SO4
H SO4 and H PO as precursor) is {PO [WO(O ) ] } syn-
2 3 4 4 2 2 4
2
4
8–53
caused the increase of H O decomposition, especially when
thesized in situ.
According to previous experiments and the
2
2
1,46
the molar ratio of H SO to H WO was above 1.2. It could be
reaction pathway proposed by Noyori,
this reaction should
2
4
2
4
seen in Fig. 2b that the suitable molar ratio of H PO to H WO
4
include the organic–water heterogeneous reaction stage (stage A
in Fig. S1, ESI†) and the homogeneous reaction stage (stage B
in Fig. S1, ESI†). The experimental results show that, with the
consumption of H O in reaction, the catalyst system is con-
3
4
2
was 0.56, and the highest AA yield up to 92.3% was obtained.
When the molar ratio of H PO to H WO increases over
3
4
2
4
0
.56, the yield of AA decreases with the further increase of
2
2
H PO , although the decomposition of H O decreased slightly.
verted into soluble H PW O (Fig. S2a and S3a, ESI†), which
3
4
2
2
3
12 40
Thus a catalyst precursor system consisting of H WO , H SO
is a commonly used catalyst for oxidation using H O as the
2 2
2
4
2
4
3
−
and H PO in a molar ratio of 1.00 : 1.04 : 0.56 was employed,
oxygen donor and can rapidly form {PO [WO(O ) ] } in
4 2 2 4
3
4
11,50
and excellent results (92.3% product yield and 7.1% H O2
moderate concentration of H O solution.
Even in low con-
2
2 2
decomposition) were obtained under the optimized catalyst
precursor.
Fig. 3 shows the reuse results of the catalyst. This catalyst
could be recycled easily. After the AA crystal was separated
from the oxidation reaction mixture, the filtrate was condensed
centration of H O solution (5%), the catalyst system can retain
2 2
catalytic activity for the oxidation of 1,2-cyclohexanediol to AA
(Table S3, ESI†), which is the main reaction in the homogeneous
reaction stage. In the next run, when H O is added into the reac-
2
2
3
tion system, H PW O is reconverted into {PO [WO(O ) ] }
as active catalyst of oxidation reaction.
3
12 40
4
2 2 4
−
10,50
Therefore, the cata-
lyst can be recycled without significant loss in catalytic activity.
Comparatively, the colloidal tungstic acid is formed in the
absence of H PO with the concentration of H O decreasing
3
4
2 2
(Fig. S2b and S3b, ESI†), which leads the leaching of tungsten
and the deactivation of catalyst (Table S3, ESI†). Thus, the
H PO in catalyst system, not only restrains the decomposition
3
4
of H O , but also retains the active structure of catalyst in low
2
2
concentration of H O solution.
2
2
According to our experiments for verifying the promoting
action of H SO and H PO on this reaction (Table S2, ESI†), in
2
4
3
4
the organic–water heterogeneous reaction stage, the TOF of
cyclohexene and the yield of 1,2-cyclohexanediol decreased sig-
nificantly from 25.7 h− to 4.1 h , and 54.0% to 4.0%, respect-
1
−1
ively, while the yield of 1,2-epoxycyclohexane increased from
0
.0% to 6.1%, after adding Na CO into the reaction system to
2 3
adjust the pH value of aqueous phase of to 7. Finally, the yield
and purity of AA decreased significantly from 90.2% to 8.5%,
and 98.0% to 77.6%, respectively, concurrently with the increase
of the unproductive decomposition of H O to H O and O from
2
2
2
2
7
.2% to 35.8%. Even using 1,2-cyclohexanediol as substrate, the
yield and purity of AA were as low as 25.1% and 88.7% respect-
ively, concurrent with a highly unproductive decomposition of
H O (30.3%). H SO gives higher TOF of cyclohexene and
2
2
2
4
yield of AA than H PO , while H PO gives a lower unproduc-
3
4
3
4
tive decomposition of H O than H SO by sequestering transi-
2
2
2
4
tion metals and scavenging radical which cause decomposition
47
of H O . The results demonstrate that a highly acidic system is
2
2
Fig. 3 The reuse of catalyst (a) in the present of H
absence of H PO . Reaction conditions: (a) H WO
cyclohexene and H
other conditions were given in the Experimental section). (b) H
SO cyclohexene and H in a molar ratio of 1 : 1.04 : 50 : 220,
without H PO (all other conditions were unchanged).
3
PO
4
and (b) in the
efficient in the increase of the utilization rate of H O , and more
2
2
3
4
2
4
, H
2
SO , H PO
4
3
4
significantly in the acceleration of the acid-catalyzed hydrolysis
of 1,2-epoxycyclohexane to 1,2-cyclohexanediol and adipic
anhydride to AA. In combination with the stabilizing effect of
H PO4 on catalyst system and H O2 during the recycling
2
O
2
in a molar ratio of 1 : 1.04 : 0.56 : 50 : 220 (all
4
WO ,
2
H
2
4
2 2
O
3
4
3
2
2870 | Green Chem., 2012, 14, 2868–2875
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reaction, this formulation of catalyst system leads to the effective
oxidation of cyclohexene to AAwith H O .
2
2
Process development and scale-up
In order to reasonably design the continuous-flow reactor, the
catalytic oxidation of cyclohexene with this catalyst system was
monitored (Fig. 4). The conversion of cyclohexene, the for-
mation of 1,2-cyclohexandiol and the formation of AA versus
time are shown in Fig. 4a. No 1,2-epoxycyclohexane was
detected in this reaction system, because of the rapid hydrolysis
of 1,2-epoxycyclohexane to 1,2-cyclohexandiol in the presence
of strong acid. Hence, the irreversible inactivation of catalytic
Fig. 5 The simplified process flow diagram. Abbreviations: CR, crys-
tallizer; P, pump; R, reactor; RC, reflux condenser; T, tank; TE, thermo
element, TS, thermo sensor; V, valve.
3
−
system ({PO [WO(O ) ] } ) at high concentrations of epoxide
4
2 2 4
50
is avoided. 1,2-Cyclohexandiol produced in stage A is rapidly
converted to other intermediates and AA in the first hour of
stage B. Interestingly, the conversion of cyclohexene is an accel-
erated process. Because cyclohexene is insoluble in water, the
turnover rate of cyclohexene is limited by organic–aqueous mass
transfer at first. With the increase of soluble organics such as
formation of AA increases rapidly in the first hour of stage B
with rising reaction temperature and then increases slowly
because of the decrease of H O and 1,2-cyclohexandiol. There-
fore, multiple continuous flow reactors in series should be
employed for high yield of AA in the stage B.
2
2
1
,2-cyclohexandiol, the solubility of cyclohexene in aqueous
To the best of our knowledge, a relatively small number of
reactors in series required in the reaction were usually chosen to
phase improves, so the turnover rate of cyclohexene increases.
Hence, the high conversion and turnover rate of cyclohexene can
be simultaneously achieved in one continuous-flow reactor. The
54,55
avoid excessive complication.
device consisting of four continuous stirred-tank reactors
CSTR) in series was designed and employed to examine the
A continuous-flow reaction
(
feasibility of the continuous-flow process and optimize process
condition, and the simplified process flow diagram of scale-up
reaction is shown in Fig. 5 (the reaction device of laboratory
scale is shown in Fig. S4, ESI†). The effects of residence time
(
τ) in continuous-flow reactor on the oxidation of cyclohexene to
AA with H O were investigated (Fig. 4b). The results indicate
2
2
that the optimal retention time in the continuous-flow reaction
device is 588 min (optimization via cubic Spline function
fitting), and the actual yield (94.1%) is slightly lower than the
yield of simulated result.
The effect of reaction temperature on the AA yield in the con-
tinuous-flow device was studied, and the results are listed in
Table 1. As shown in Table 1, with the increase of reaction temp-
erature in the first reactor of the continuous-flow reaction device,
the yield of AA increased remarkably. Due to the limitation of
the boiling point of water–cyclohexene mixture (70.9 °C), 73 °C
was selected as the reaction temperature in the first reactor. In the
subsequent reactors, the highest yield of AA was obtained at
9
0 °C. Because the oxidation of 1,2-cyclohexanediol and other
intermediates produce more byproducts at higher temperature,
and at lower temperature, incomplete oxidation of 1,2-cyclohexane-
46
diol results in low AA yield.
Hydrogen peroxide can decompose to oxygen and water with
3+
iron ion as catalyst. If some Fe leach from the material of
reactor and pipeline, this possible side reaction would decrease
3+
the utilization of H O . The influence of Fe ion on decompo-
2
2
sition of H O during the catalytic reaction was investigated. The
2
2
Fig. 4 The effect of (a) reaction time in batch reactor and (b) residence
time in continuous-flow reaction device. Reaction conditions: (a)
results are shown in Fig. 6a. The decomposition of H O distinc-
2
2
3+
tively increases with the increase of the Fe concentration, and
the decomposition ratio of H O is 14.2%, even if the concen-
H
2
WO SO
4
7.57 mmol, H
2
4 3 4
7.88 mmol, H PO 4.24 mmol, cyclohexene
2
2
3
80 mmol, and H O 1.67 mol (all other conditions were given in the
2 2
3+
tration of Fe is up to 10.0 ppm. The results demonstrate
that this reaction system can tolerate a certain concentration
Experimental section). (b) The reactants and catalyst were fed continu-
ously into the continuous-flow reactor (all other conditions were given in
the Experimental section).
3+
of Fe , because phosphoric acid, 1,2-cyclohexandiol,
This journal is © The Royal Society of Chemistry 2012
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a
Table 1 The AA yield at various reaction temperatures in CSTRs
Temperature (°C)
R1
R2
R3
R4
Yield of AA (%)
7
6
6
7
7
3
8
3
3
3
90
90
90
95
85
90
90
90
95
85
90
90
90
95
85
94.1
85.4
78.5
91.4
80.9
Fig. 7 The calorimetric profiles in (a) CSTRs and (b) batch reactor.
a
Reaction conditions: H
2
WO
4
, H
2
SO
4
, H
3
PO
4
, cyclohexene and H
2
O
2
Reaction conditions: (a) H
2 4 2 4
WO 15.14 mmol, H SO 15.76 mmol,
in a molar ratio of 1 : 1.04 : 0.56 : 50 : 220, the total retention time in
CSTRs was 580–590 min (all other conditions were given in the
Experimental section).
H PO 8.48 mmol, cyclohexene 760 mmol, and H O 3.34 mol (all
3
4
2 2
other conditions were given in the Experimental section). (b) Reaction in
× 1 L CSTRs (all other conditions were given in the Experimental
4
section).
Table 2 The continuous-flow reaction and batch reaction on a large
a
scale
Volume of reactor
(L)
Yield of AA
(%)
Purity of crude product
(%)
Reactor
Batch
Batch
Batch
Batch
CSTRs
CSTRs
CSTRs
5
100
2000
5000
4 × 0.5
4 × 100
4 × 5000
93.0
89.5
96.3
95.8
94.1
92.0
94.7
98.0
99.1
98.9
99.2
98.8
98.7
99.0
3
+
Fig. 6 (a) The influence of Fe and (b) various materials on AA yield
and H
2
O
2
decomposition. Reaction conditions: (a) H
2 4 2 4
WO , H SO ,
H
1
3
PO
4
,
cyclohexene and in molar ratio of
H
2
O
2
a
a
: 1.04 : 0.56 : 50 : 220, the total retention time in CSTRs was
2 4 2 4 3 4 2 2
Reaction conditions: H WO , H SO , H PO , cyclohexene, and H O
in a molar ratio of 1 : 1.04 : 0.56 : 50: 220, at 73 °C (all other conditions
were given in the Experimental section).
5
80–590 min (all other conditions were given in Experimental section).
2
(b) The surface area of material for test is 0.01 m per kg reaction
mixture and the time over test is 100 h (all other conditions were
unchanged).
essential in scale-up reactor. The results demonstrate that no
uncontrollable heat accumulation occurred in the continuous-
flow reactor and the reaction could be operated safely on scale.
Fig. 7b shows the representative heat output with reaction time
in this oxidation. Oxidation of cyclohexene was marked by a
mild exothermal event devoid of significant thermal activity until
the system was converted to homogeneous phase. A significant
exothermic event took place upon the second hour since the
beginning of reaction, with a maximum heat release rate of
220.2 W kg− observed at about 123 min, indicating the for-
mation of AA, and then the heat release rate decreased and a
subsiding of heat release was observed, which indicated that the
reaction rate decreased because of the decrease of H O and 1,2-
cyclohexandiol. In comparison with the reaction in batch reactor,
excellent thermal management was obtained within continuous-
flow reactor. At first scale-up was carried out in batch reactors,
and the results shown in Table 2 demonstrate that the reaction
heat of large-scale reaction is controllable and the reaction
system can be operated safely on large scale. The results of an
experiment to validate reproducibility of scale-up are shown in
Table S4 in the ESI,† indicating that the reactions of scale-up
were reproducible.
3
+
2
-hydroxycyclohexanone and AA could sequester Fe and
quench the chain mechanism of H O decomposition as radical
2
2
47
scavengers. Furthermore, the test of gas explosion indicates
that the gas mixture in the reactor headspace is outside the
explosion limits and thus the oxidation processes can be handled
safely.
The influence of various materials (such as glass-lined steel,
graphite, 316L and 304 stainless steels) for manufacturing
reactor and pipeline on the reaction was investigated (Fig. 6b).
The results demonstrate that glass-lined steel and 316L stainless
steel are suitable material of the equipment for this green oxi-
dation. The glass and 316L stainless steel can resist the corrosion
of this reaction system, and the increase of the concentration of
metal ions (such as Fe, Cr, Mn, Co, Ni, and Mo) due to leaching
from the reactor material in mother liquor is under the detection
limit. Therefore, the glass-lined steel was selected as the chemi-
cal tanks material and the 316L stainless steel was used as the
material of pipelines and mechanical parts of reactor.
With the optimized reaction conditions, we paid attention to
the scale-up. Safety assessment on the oxidation reaction was
conducted by calorimetry in continuous-flow reactor (Fig. 7a)
and batch reactor (Fig. 7b). Fig. 7a shows the heat release rate of
the reaction in each reactor of CSTRs on steady-state conditions.
1
2
2
Scale-up and pilot-scale continuous-flow reactions were per-
formed in a reaction device consisting of four 100 L continuous
stirred-tank reactors in series (4 × 100 L CSTRs) and reaction
device consisting of four 5000 L continuous stirred-tank reactors
in series (4 × 5000 L CSTRs), representing a 200-fold scale-up
−
1
The maximum heat release rate of 93.9 W kg observed at the
second reactor, indicating the rapid oxidation of 1,2-cyclohexan-
diol to AA. Thus, the transfer of heat via cooling system is
2872 | Green Chem., 2012, 14, 2868–2875
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and 10 000-fold scale-up from the laboratory-scale reaction. The
product was sampled and analyzed every 10 h by HPLC to
monitor constancy of the reaction. The results were shown in
Fig. S5 in ESI,† indicating that the large-scale continuous-flow
reactions were constant. In the pilot plant, the reaction was suc-
cessfully run for 350 h, and both AA yield (94.7%) and product
purity (99.0%, crystalline solid without recrystallization) were
excellent. The results indicate that the optimized laboratory
process was successfully scaled up 10 000-fold in the pilot plant,
giving good agreement between both product yield (94.7% in
pilot plant and 94.1% in the laboratory) and product purity
0.5 L continuous stirred-tank reactors in series (4 × 0.5 L
CSTRs). The scale-up experiments of batch operation were
performed using glass reactor (5 L) and glass-lined reactors
(100–5000 L) of geometric similarity equipped with heating
jacket, agitator and reflux condenser. Calorimetric experiments
for safety assessment were performed using Mettler-Toledo RC1
reaction calorimeter equipped with a 1 L jacketed glass vessel, a
teflon straight blade agitator and a reflux condenser. The scale-
up and pilot-scale of continuous process were performed using
four 100 L continuous stirred-tank reactors in series (4 × 100 L
CSTRs, glass-lined) and four 5000 L continuous stirred-tank
reactors in series (4 × 5000 L CSTRs, glass-lined).
(99.0% in pilot plant and 98.8% in the laboratory).
Compared with the current industrial processes, this green
process is environmentally benign and more simple. From cyclo-
hexene (or cyclohexane) to crude product, the current industrial
processes require five units (Fig. 1a) for reaction, separation and
waste gas treatment, the DuPont process consists of five units,
such as cyclohexane oxidation, distillation separation of KA oil,
oxidation of KA oil, crystallization separation and waste gas
treatment, and the Asahi Kasei process consists of five units
General analytical methods
For the determination of catalysts composition, inorganic acids
were analyzed by ion chromatography (IC) using a Dionex
ICS-2100 equipped with an electrical conductivity detector.
Reaction process and soluble organics were evaluated by HPLC,
using a Hewlett-Packard Series 1100 HPLC equipped with a
UV detector and a differential refraction detector. The cyclo-
hexene conversion was monitored by internal standard method
using a Techcomp 7890 GC equipped with OV-1701 capillary
column and a FID detector. The gas from the reaction system
was monitored by an Albright gas analyzer. The test of gas
explosion was carried out in a detonation reactor.
(Fig. 1b), such as cyclohexene hydration, distillation separation
of cyclohexanol, oxidation of cyclohexanol, crystallization separ-
ation and waste gas treatment. As an improvement, this green
continuous-flow process requires only two units consisting of
cyclohexene oxidation with H O and crystallization separation.
2
2
The simplification of process increases the production efficiency
of AA and reduces the cost of chemical plant.
Catalytic oxidation
Conclusions
Typical procedure of catalytic oxidation in batch reactor. A
2
In conclusion, the oxidation of cyclohexene to AA with H O2
batch reactor was charged with H WO , H SO , H PO , cyclo-
2
4
2
4
3
4
has been studied using a catalyst system synthesized in situ by
H WO , H SO and H PO in H O , under the solvent-, phase-
hexene and aqueous H O2 in
a
molar ratio of
2
2
4
2
4
3
4
2
2
1 : 1.04 : 0.56 : 50 : 220. For lab-scale reaction, the oil–water
mixture was heated to 73 °C and refluxed for 2 h with violent
stirring. After the organic phase was consumed the homo-
transfer-catalyst- and organic additive-free conditions. A continu-
ous-flow reactor was set up for the industrial operation of this
green process and the optimization of the reaction parameter.
The activity and stability of the catalyst depended strongly on the
acidic promoters. Under the optimal experimental conditions, the
catalyst system exhibits an excellent catalytic performance and
can be reused for twenty cycles with an average yield over 93%
in batch reactor. A green and efficient continuous-flow process
for AA production has been developed using this recyclable
catalyst system. The laboratory-scale reaction was successfully
scaled up 10 000-fold in the pilot plant. Both the product yield
geneous reaction mixture was heated to 90 °C with the rate of
1
1
°C min− . For large-scale reaction, the oil–water mixture was
heated to 73 °C by steam-heating unit, and then heat exchange
system was switched to water-cooling unit to remove the reaction
heat. After the organic phase was consumed the water-cooling
unit was turned up with the temperature rising of reaction system
to avoid temperature runaway. The homogeneous reaction con-
tinued at 90 °C for 6 h. Meanwhile, the gas from the reaction
system was monitored by an Albright gas analyzer to evaluate
the decomposition of H O and performed by the test of gas
(
94.7% in pilot plant and 94.1% in laboratory) and the purity of
2
2
crude product (99.0% in pilot plant and 98.8% in laboratory) are
excellent. The results of calorimetry, scale-up and pilot-scale
continuous-flow reaction indicate that the industrialization of this
green synthesis is safe and feasible. It will be an alternative for
the current industrial process.
explosion. After completion of the reaction, the reaction solution
was kept at 0 °C for 12 h, and the resulting white crystals was
separated by filtration and dried in vacuum at 60 °C. The reac-
tion solution and product were determined by HPLC. After the
AA crystal was filtered, the filtrate was condensed and reused for
the next run (for large-scale reaction, the filtrate was treated by
acid resin-bed before concentration).
Experimental
Typical procedure for lab-scale catalytic oxidation in continu-
ous-flow device. The continuous-flow reaction device is shown
in Fig. S4, ESI.† This device was made up of CSTRs, equipped
with Teflon agitator and reflux condenser, peristaltic pump,
reagent container, temperature controller and thermo element.
The reagent containers, CSTRs and pumps were connected with
Materials and apparatus
All materials were used as received except 30% H O2 was
diluted to 27.5%. Optimization of catalyst and process develop-
ment experiments in lab were performed using glass flasks
2
(0.5 L), and a continuous-flow reaction device consisting of four
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pipe of viton. The system was filled with mother liquor of batch
reaction at first. After temperature stabilization (the reaction
temperature in R1 reactor of CSTRs is 73 °C, and in other
reactor is 90 °C), the reactants and catalyst consisting of cyclo-
hexene, aqueous H O , H WO , H SO and H PO in a molar
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Acknowledgements
Financial supports from the Innovation Fund for Elitists of
Henan Province, China (Grants No. 0221001200), the China
Postdoctoral Science Foundation (No. 2012M511121) and the
Joint Project of Zhengzhou University and Shanxi Yangmei
Fengxi Fertilizer Industry (Group) Co., Ltd for the clean
production of adipic acid are acknowledged. The authors are
highly indebted to large teams of collaborators both from
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