Preparation of 2,6-naphthalic acid by liquid phase oxidation of 2,6-diisopropyl naphthalene

Article abstract of DOI:10.14233/ajchem.2013.12709

An orthogonal experiment was carried out and the effects of bromine, metal ions additives and organic nitrogen compounds on the yield of 2,6-naphthalic acid were discussed. The results shown that influence order for the yield of 2,6-naphthalic acid in the

Full text of DOI:10.14233/ajchem.2013.12709

Asian Journal of Chemistry; Vol. 25, No. 2 (2013), 735-738
http://dx.doi.org/10.14233/ajchem.2013.12709
Preparation of 2,6-Naphthalic Acid by Liquid Phase Oxidation of 2,6-Diisopropyl Naphthalene
GE PENG
Chemical Engineering College, Ningbo University of Technology, Ningbo, Zhejiang Province, 315016, P.R. China
Corresponding author: E-mail: gepengv@hotmail.com
(Received: 10 September 2011;
Accepted: 16 August 2012)
AJC-11966
An orthogonal experiment was carried out and the effects of bromine, metal ions additives and organic nitrogen compounds on the yield
of 2,6-naphthalic acid were discussed. The results shown that influence order for the yield of 2,6-naphthalic acid in the orthogonal
experiment were: reaction temperature > feed flux > catalyst dosage > Co/Mn. High temperature could cause the cleavage of naphthalene
ring and low temperature could not give rise to the full oxidation of intermediate products, then the suitable temperature could be 473 K.
It is good for producing 2,6-naphthalic acid to decrease the feed flux and increase the catalyst dosage. However, there was no significant
influence of Co/Mn for the yield of 2,6-naphthalic acid.
Key Words: Oxidation, 2,6-Naphthalic acid, 2,6-Diisopropyl naphthalene.
introducing nitrogen gas after tight sealing. Pressurization was
then carried out with nitrogen gas to a final pressure at ca. 1
MPa, the inlet valve and the outlet valve were shut off, the
power supply was switched on for heating, subsequently the
valve for cooling water in the end gas condensation tube at
the same time and the stirring was started. Nitrogen gas was
continuously introduced during the process for increasing
temperature in order to guarantee the synchronized increase
in pressure and temperature. After the temperature and the
pressure increased to the preset values, the heating power was
adjusted in order to maintain the temperature in the reactor
constant and the stirring speed was adjusted to 900-950 rpm.
Finally the nitrogen gas was switched to the air and the reaction
was triggered.
INTRODUCTION
2,6-Naphthalic acid (2,6-NDCA) and its derivates are im-
portant monomers for producing various kinds of polyester
and polyurethane materials as well as liquid crystal com-
pounds1. The major preparation methods included Henkel
method, isomerization method, carboxy group transfer reac-
tion, 2,6-dimethylnaphthalene oxidation, 2,6-diisopropyl naph-
thalene oxidation and other methods2-4. The raw materials for
2,6-diisopropyl naphthalene oxidation are easy to obtain and
the separation was relatively simple and the route for this syn-
thetic technology was the most economical. Thus this method
has become one of the hotspots for the investigations on the
preparation of 2,6-naphthalic acid5-7.
The purpose of the present study was to producing 2,6-
naphthalic acid by liquid phase oxidation of 2,6-diisopropyl
naphthalene. The effects of incipient concentration, catalyst
concentration, ratio between cobalt and manganese, reaction
temperature, different bromides, metallic compound additives
and nitrogenous compound additives on 2,6-diisopropyl naph-
thalene oxidation were examined. Finally the relatively
appropriate reaction conditions were screened out for the
reference for industrial design and implement process.
Analytical method: The sampling was carried out for
every a short period during the reaction and the ingredients in
the sample were determined by using a HP6890-MS5973 gas
chromatograph-mass spectrometer and quantitative analysis
was carried out by using a Shimadzu GC-9A gas chromato-
graph. The chromatographic column was a SE-54 capillary
column and the detector was a hydrogen flame ion detector
(FID).
RESULTS AND DISCUSSION
EXPERIMENTAL
Orthogonal experiment and the results: The factors of
experimental tests to be used in the orthogonal design included
reaction temperature, ratio between cobalt and manganese,
2,6-diisopropyl naphthalene feeding rate and molar ratio
between the catalyst and the raw materials and the parameters
The experiment was carried out in different batches, firstly
the reaction materials, solvents, catalysts were prepared into a
solution of 300 mL in proper proportions. The solution was
added into the reactor, the air in the reactor was exhausted by

736 Peng
Asian J. Chem.
for examination were the yield of the product 2,6-naphthalic
acid and the yield of the side product trimellitic acid. The
levels and factors of orthogonal design was shown in Table-1,
the experimental conditions and the results were shown in
Table-2.
catalyst concentration > feeding rate > ratio between cobalt
and manganese; the optimal reaction conditions within the
examined range were as followed: the temperature was 170 ºC,
the catalyst concentration [Co + Mn]: diisopropyl naphthalene
(molar ratio) = 3:1, [Co]: [Mn] = 1:3, the feeding rate was 5
mL/min. Verification experiments were carried out at the
optimal conditions, the yield of 2,6-naphthalic acid was 74.9
% and the yield of trimellitic acid was 14.1 %, which was
higher than that at the conditions in the orthogonal experiment.
Effects of the promoter on the oxidation of 2,6-diisopropyl
naphthalene: Bromine is a kind of renewable source for chain-
initiating radicals as a kind of promoter and it can significantly
promote the oxidation reaction. The present study examined
the effects of different bromides on the liquid phase catalytic
oxidation of 2,6-diisopropyl naphthalene and related experi-
mental conditions and the results were shown in Table-3. Other
experimental conditions were as followed: the pressure P =
2 MPa, T = 170 ºC, acetic acid = 1000 mL, 2,6-diisopropyl
naphthalene = 10 g.
TABLE-1
UTILIZATION OF DIPN OXIDATION NDCA UTILIZATION
FACTOR LEVELS ORTHOGONAL DESIGN TO WATCH
Factor
Item
Co + Mn/
2,6-DIPN
Quantity of
flow (mL/min)
T(K)
Co/Mn
1
2
3
443
423
473
3:1
1:1
3:1
1:3
5
2
Factor
1.5:1
0.5:1
10
2,6-DIPN = 2,6-diisopropyl naphthalene
According to the experimental results in Table-2, we can
obtain the results as follows: 2,6-naphthalic acid was a target
product and its yield should be as high as possible, while
trimellitic acid was a major side product and it stood for the
probability for oxidative cleavage during 2,6-diisopropyl
naphthalene oxidation, which may seriously affect the yield
of the target product, thus its yield should be as low as possible.
It can be found from the results in Table-2 that among the four
factors for examination, the effects on the yield of the target
product 2,6-naphthalic acid were successively: reaction tem-
perature > feeding rate > catalyst concentration > ratio
between cobalt and manganese, while the effects on the yield
of trimellitic acid were successively: reaction temperature >
It can be found from Table-3 that the yields of 2,6-
naphthalic acid from 2,6-diisopropyl naphthalene oxidation
did not differ significantly when potassium bromide and
tetrabromoethane were used as the additives, but they were
significantly higher than the yields when hydrogen bromide
was used as the additive and the amount of trimellitic acid by
disrupting naphthalene ring also decreased when tetrabromo-
ethane was used as the catalytic additive, but the mechanism
for the promoter bromine on the disruption of naphthalene
ring is still unclear, further investigations are still required.
TABLE-2
UTILIZATION OF DIPN OXIDATION NDCA ORTHOGONAL DESIGN TABLE UTILIZATION
Experiment condition Experimental results
No
T (K)
443
Co+Mn/2, 6-DIPN
Co/Mn
1:3
Feeding flow rate (mL/min)
2,6-NDCA yield (%)
TMA yield (%)
1
2
3:1
3:1
2
5
72.8
48.8
47.9
70.2
32.1
48.9
42.1
33.7
48.7
–
13.5
7.40
27.4
17.4
13.8
27.7
20.7
15.0
28.9
–
423
1:1
3
473
3:1
3:1
10
4
443
1.5:1
1.5:1
1.5:1
0.5:1
0.5:1
0.5:1
1.695
0.4836
1.512
0.5894
1.245
0.6468
0.450
0.1632
3:1
5
5
423
1:3
10
6
473
1:1
2
7
443
1:1
10
8
423
3:1
2
9
473
1:3
5
K1N
K1T
K2N
K2T
K3N
K3T
RN
RT
1.851
0.5168
1.145
0.3628
1.455
0.8402
0.706
0.4774
1.536
0.5590
1.398
0.5983
1.518
0.5625
0.138
0.0393
1.677
0.5376
1.554
0.5622
1.221
0.6200
0.456
0.0824
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Remarks: in this table, K1N, K2N, K3N, RN were the sums and the ranges for the experimental results with the yield of 2,6-NDCA as the
examination parameters, while K1T, K2T, K3T, RT were the sums and the ranges for the experimental results with the yield of trimellitic acid as
the examination parameters; higher R value indicated that the effects of this factor on the experiment were much significant.
TABLE-3
DIFFERENT DIPN OXIDATION OF UTILIZATION ROHS THE INFLUENCE
Yield (%)
Co + Mn/2,6-DIPN
(mol/mol)
Feeding flow rate ×
Bromide
Co/Mn/Br
103/(mol/min)
0.472
2,6-NDCA
Trimellitic acid
Potassium bromide
Tetrabromo ethane
Hydrogen bromide
3
3
3
1:3:1
1:3:1
1:3:1
66.2
68.0
60.3
16.5
11.8
13.2
0.472
0.472

Vol. 25, No. 2 (2013)
Preparation of 2,6-Naphthalic Acid by Liquid Phase Oxidation of 2,6-Diisopropyl Naphthalene 737
Effects of metals on the oxidation of 2,6-diisopropyl
naphthalene: The effects of metal ion additives in the
manganese-cobalt catalytic system were significant and they
can accelerate the main reaction. Therefore, the present study
carried out investigations on the effects of different metal ions
on the oxidation of 2,6-diisopropyl naphthalene, related
experimental results and the results were listed in Table-4 (other
experimental conditions were as followed: the pressure P =
2 MPa, T = 443 K, acetic acid = 1000 mL, 2,6-diisopropyl
naphthalene = 10 g).
acid both showed certain increased after small amount of
cerium acetate was added in the reaction system, thus it can
be considered that the inclusion of cerium acetate increase
2,6-diisopropyl naphthalene in general. Previous investigations
on the functions of cerium in the oxidation of aromatic hydro-
carbon showed that cerium acetate had two major functions
in the reaction system, one was that the rate for aromatic
hydrocarbon oxidation was improved in general by the syner-
gistic action between Co and Mn. On the other hand, it was
beneficial for inhibiting the precipitation of the catalyst Mn
and thus improving the water content in the reaction system.
Previous studies have shown that copper also showed relative
good synergistic action with cobalt and manganese.
Firstly, the effects of potassium ion on the oxidation of
2,6-diisopropyl naphthalene were examined. The yield of the
2,6-naphthalic acid continuously increased with the increase
in potassium ion and the effects of the increase in potassium
ion on the improvement in 2,6-naphthalic acid yield was very
significant when Co/Mn (molar ratio) was lower than 6, but
continuous increase in potassium ion can not further increase
2,6-naphthalic acid yield when it went beyond this range.
Moreover, the content of the side product trimellitic acid did
not change significantly with the increase in potassium ion,
indicating that the concentration of potassium ion did not
affect the disruption of naphthalene ring, but it may affect the
oxidation of isopropyl.
Conclusion
The optimal conditions for oxidation were determined by
examining the effects of different experimental conditions in
the orthogonal test: the reaction pressure was 2 MPa, the
temperature was 443 K, the catalyst concentration Co + Mn/
diisopropyl naphthalene (molar ratio) = 3:1, Co/Mn = 1:3, the
feeding rate was 5 mL/min furthermore, the effects on the
yield of the target product 2,6-naphthalic acid were succes-
sively: the reaction temperature > the feeding rate > the catalyst
concentration > the proportion of cobalt and manganese, while
the effects on the yield of trimellitic acid were successively:
the reaction temperature > the catalyst concentration > the
feeding rate > the proportion of cobalt and manganese.
Among the metal ion additives, base metal ions can
improve the yield of 2,6-naphthalic acid and the yield of 2,6-
naphthalic acid also gradually increased with the increase in
potassium ion concentration, however, continuous increase in
potassium ion concentration can not improve the yield of 2,6-
naphthalic acid when the concentration of potassium ioin was
increased to a certain level, but it can decrease the yield on the
contrary, moreover, base metals had no significant effects on
the disruption of naphthalene ring. The addition of magnesium
ion and cerium ion can increase the yields of 2,6-naphthalic
acid to some extent, but the addition of copper ion can inhibit
the oxidation of 2,6-diisopropyl naphthalene.
According to the data in Table-5, it was found that sodium
acetate also showed the same effects and the effects of sodium
acetate and potassium acetate at the same concentration on
2,6-diisopropyl naphthalene oxidation were the same, indi-
cating that base metals had significant effects during the
oxidation of 2,6-diisopropyl naphthalene.
Effects of other metal ions were also examined, magne-
sium acetate tetrahydrate was added in the reaction system, it
was found the addition of magnesium acetate tetrahydrate de-
creased the probability of naphthalene ring disruption and the
yield of trimellitic acid in comparison to the effects of potassium
acetate at the same concentration, but it also decreased the
yield of 2,6-naphthalic acid at the same time.
Small amount of rare earth metal compound cerium
acetate in the reaction system, it can be found from the results
in Table-4 that the yields of 2,6-naphthalic acid and trimellitic
TABLE-4
DIFFERENT METAL EFFECT ON DIPN OXIDATION
Yield (%)
Trimellitic acid
Co+Mn/2,6-DIPN
(mol/mol)
Co/Mn/Br/metal
ion
Feeding flow rate ×
103 (mol/min)
Base metal
Potassium acetate
2,6-NDCA
60.3, 66.8
71.9
3, 3
3
1:3:1:0, 1:3:1:1
1:3:1:2
0.472, 0.472
0.472
13.2, 15.9
14.2
Potassium acetate
Potassium acetate
Potassium acetate
Sodium acetate
3
1:3:1:6
0.472
75.7
15.5
3
1:3:1:9
0.472
74.9
14.1
3
1:3:1:1
0.472
67.1
14.1
Magnesium acetate tetrahydrate
Cerium acetate
3
1:3:1:2
0.472
62.2
11.8
3
1:3:1:0.2
1:3:1:1
0.472
63.6
20.5
Cupric acetate
3
0.472
40.7
2.5
TABLE-5
NITROGEN COMPOUNDS EFFECT ON DIPN OXIDATION
Yield (%)
Co+Mn/2,6-DIPN
(mol/mol)
Feeding flow rate ×
103 (mol/min)
Base metal
Co/Mn/Br
2,6-NDCA
49.8
Trimellitic acid
Guanidine carbonate
Tricyanic acid
3
3
3
1:3:1
1:3:1
1:3:1
0.472
0.472
0.472
7.7
9.2
8.4
64.1
Tricyanic acid
67.0

738 Peng
Asian J. Chem.
Nitrogenous compound additives can inhibit the disrup-
ACKNOWLEDGEMENTS
tion of naphthalene ring to some extent, particularly that
cyanurotriamide and tricyanic acid were very beneficial for
decreasing trimellitic acid production and it did not affect the
oxidation of 2,6-diisopropyl naphthalene and the subsequent
transformation into 2,6-naphthalic acid. The addition of guani-
dine carbonate can also inhibit the disruption of naphthalene
ring, but guanidine carbonate may be involved in the oxidation
of isopropyl and then other intermediate products may be
produced, thus the yield of 2,6-naphthalic acid was reduced,
therefore, it can be considered to carry out investigations on
how to improve the rate for 2,6-diisopropyl naphthalene
oxidation at the same time of adding nitrogenous compounds.
This research was supported by The Natural Science
Foundation of Ningbo, China (No. 2011A610143).
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