Copper-cobalt synergy in Cu1-xCoxFe2O4 spinel ferrite as a highly efficient and regioselective nanocatalyst for the synthesis of 2,4-dinitrotoluene

Article abstract of DOI:10.1039/c5ra11338e

Highly regioselective dinitration of toluene with nitric acid as nitrating agent in the presence of Cu_1-xCo_xFe₂O₄ (0 ≤ x ≤ 1) as nanocatalysts is described. The results of this protocol show that this nitration system can significantly improve the selectivity and the yield of 2,4-dinitrotoluene. The prepared spinel ferrites were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). Copper and cobalt synergy (50%:50%) in spinel ferrite also shows the best catalytic activity in the selectivity of dinitrotoluene under solvent-free conditions.

Full text of DOI:10.1039/c5ra11338e

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Copper–cobalt synergy in Cu1ꢀxCoxFe2O4 spinel
ferrite as a highly efficient and regioselective
nanocatalyst for the synthesis of 2,4-
dinitrotoluene†
Cite this: RSC Adv., 2015, 5, 71911
a
Reza Fareghi-Alamdari, Farzad Zandia and Mohammad Hossein Keshavarzb
*
Highly regioselective dinitration of toluene with nitric acid as nitrating agent in the presence of
Cu1ꢀxCoxFe2O4 (0 # x # 1) as nanocatalysts is described. The results of this protocol show that this
nitration system can significantly improve the selectivity and the yield of 2,4-dinitrotoluene. The
prepared spinel ferrites were characterized by scanning electron microscopy (SEM), energy dispersive
X-ray spectroscopy (EDX), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR).
Copper and cobalt synergy (50% : 50%) in spinel ferrite also shows the best catalytic activity in the
selectivity of dinitrotoluene under solvent-free conditions.
Received 14th June 2015
Accepted 18th August 2015
DOI: 10.1039/c5ra11338e
www.rsc.org/advances
produces 2,4-, and 2,6-dinitrotoluenes (2,6-DNT) in a ratio of
only four to one (4 : 1).11 However, separation of 2,4-DNT from
1. Introduction
2,6-DNT is a difficult and expensive work. In addition, the use of
sulfuric acid for generation of nitronium ions is a contributor to
environmentally harmful waste. In recent years, major
endeavors are therefore being made to develop environmentally,
friendly, safe, and clean benign processes, especially for the
regioselective para-substituted products in dinitrotoluene.10,12–15
On the other hand, it is well recognized that mesoporous
catalysts can display an important role through their ability to
act as reusable heterogeneous catalysts which can increase
product selectivities.16–18 Furthermore, they can be easily sepa-
rated from the reaction mixtures by simple ltration and reused
several times to give the same selectivity and yield. Most studies
in this eld have focused on the application of regioselective
heterogeneous catalysts between a precursor nitronium ion and
a benzene ring. There have been several attempts to improve the
selectivity of nitration of toluene using heterogeneous cata-
lysts.19–22 In the process of classical nitration based on nitric and
sulfuric acids or phosphoric mixed acids, the 2,4-DNT and 2,6-
DNT isomers were obtained in approximately a 4 : 1 ratio.23 2,4-
DNT is a main and basic chemical commodity of great market
demand. Aer the original work by Othmer and co-workers24 in
1944, further progress for nitration of aromatic compounds was
achieved by Adamiak and co-worker,25 Gao et al.,26 Liu et al.,27
and Green et al.28 in the recent years. For example, in 2011, Lu
and co-worker20 reported regioselective nitration of different
aromatic rings such as toluene, ethylbenzene and etc. using
N2O5 in the presence of ionic liquid. However, in this study, the
reaction carried out in CCl4 which is a hazardous solvent.
Moreover, conversion of reactants to products can give
moderate to good yields (61.3–89.9%). In another study in 2013,
Nitration of toluene is a very important process in the produc-
tion of different chemical commodities and is usually carried
out using nitric and sulfuric acids on a large volume scale.1,2 The
use of less hazardous chemicals, safer solvents and auxiliaries
are very important topics in the chemical industry with the aim
of introducing environmentally benign chemical processes.
There have been some recent efforts in this regard, for example
by replacing the sulfuric acid with heterogeneous catalysts and
carrying out the reaction in a safer solvent. 2,4-Dinitrotoluene
(2,4-DNT) is a signicant product and a crucial intermediate for
the production of many industrially important materials
including explosives, and propellants,3 dyes,4 polyurethanes,5,6
pharmaceuticals,7 agrochemicals,8 and fragrances.9 As previ-
ously mentioned, 2,4-DNT is employed in double based propel-
lants as a plasticizer, and the existence of impurities in 2,4-DNT
can lead to mechanical and safety problems. Therefore, the high
purity of 2,4-DNT is an important requirement in the production
of this material. 2,4-DNT is produced annually on a scale of
many million metric tons worldwide and mostly converted to
tolylene diisocyanate, i.e. an important and necessary precursor
to polyurethane resins employed for rigid and exible foams,
coatings, elastomers, and bers.10 Unfortunately, the process of
dinitration of toluene with nitric and sulfuric acids (mixed acid)
aCollege of Chemistry and Chemical Engineering, Malek-Ashtar University of
Technology, Tehran, 16765-3454, I. R. Iran. E-mail: reza_fareghi@yahoo.com; Fax:
+98-2122970195; Tel: +98-2122970277
bDepartment of Chemistry, Malek-Ashtar University of Technology, Shahin-Shahr,
83145-115, I. R. of Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra11338e
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spinel ferrites Cu1ꢀxCoxFe2O4 are used for regioselective and
green synthesis of 2,4-DNT in this study.
2. Experimental
2.1. Materials and apparatus
Toluene and nitric acid were purchased from Merck chemical
company. All of the used solvents and FeCl3$6H2O, CoCl2$6H2O,
CuCl2$2H2O, and hexadecyltrimethylammonium bromide
(CTAB) were purchased from Merck, and Fluka chemical
companies. FT-IR spectra were obtained with KBr pellets in the
range of 400–4000 cmꢀ1 with FT-IR Shimadzu-8776 spectrom-
eter. X-ray diffraction (XRD) patterns were recorded by the X-ray
diffractometer (Philips PW-1800). The particle size and external
morphology of the ne calcined powders were characterized by
scanning electron microscopy (SEM) (Philips XL30). All samples
were coated with gold at 10 mA for 2 min prior to SEM analysis.
Energy dispersive X-ray spectroscopy (EDX) analysis was studied
by Philips XL30 microscope.
Fig. 1 Different products for nitration of toluene in the presence of
spinel ferrites.
the Smith's group29 reported the application of zeolite Hb in
dinitration of toluene. In this research, the mono substituted
toluene using propanoic anhydride was obtained along with
2,4-DNT and 2,6-DNT. Hence, so far there exists no general and
highly regioselective catalytic systems for production of 2,4-DNT
with good selectivity and yield.
Due to specic chemical and physical properties of spinel
ferrites, there have been several endeavors to investigate their
mechanisms.30,31 They are widely employed in gas sensor,32
electrode materials,33 adsorption,34 and catalysis.35,36 Ternary
2.2. Analysis and characterization of the products
Analysis of the products was carried out by gas chromatography
(GC) with these specications: (i) column: CP-SIL 8CB, L ¼ 25 m,
ꢁ
d ¼ 0.32 mm, lm ¼ 1.20 mm; (ii) inlet: injector (1079) 250 C;
(iii) split injection; (iv) injected volume: 0.1 mL; (v) carrier gas:
ꢁ
N2; (vi) pre-column pressure: 50 kPa; (vii) oven: 160 C (3 min)
with 20 ꢁC minꢀ1; (viii) isotherm 180 ꢁC (30 min); (ix) detector:
FID; (x) integration: Varian CP-3800; and (xi) internal standard:
para-nitroacetophenone. All of the expected nal products from
the nitration of toluene were purchased from Aldrich Chemical
Company in order to determine response factors and retention
times relative to ethyl acetate for each product.
and binary spinel ferrites such as Mg1ꢀxCoxFe2O4,37 Co0.2
-
Cu0.03Fe2.77O4,38 Cu1ꢀxCoxFe2O4,39–41 PrxCoFe2ꢀxO4,42 ZnFe2O4,43
48
NiFe2O4,44 MnFe2O4,45 CoFe2O4,46,47 and CuFe2O4 have been
reported to display better catalytic and electrochemical perfor-
mance than single component metal oxides due to their
achievable oxidation state and specic structure. The ternary
and binary spinel ferrites also have some application in organic
synthesis. Recently, Ghahremanzadeh and co-workers reported
the application of MnFe2O4 nanoparticles as a magnetic nano-
catalyst for the synthesis of spirooxindoles in water.49 In addi-
tion, the Cu1ꢀxCoxFe2O4 was employed in the synthesis of
different dibenzoxanthenes under solvent free conditions.35
As shown in Fig. 1, different products of the nitration of
toluene are described. Distribution of different products
depends on various parameters, e.g. the molar ratio of toluene
to nitric acid, temperature of the reaction, type of catalyst and
solvent. Due to the presence of desired physic–chemical prop-
erties of ternary spinel ferrites especially Cu1ꢀxCoxFe2O4, the
2.3. Preparation of Cu1ꢀxCoxFe2O4 with different molar ratio
of copper and cobalt
First, 1.0 g of CTAB was dissolved in 35 mL of deionized water.
Later, 1.0 g of FeCl3$6H2O was added to the beaker and it was
stirred for 10 min (S1). Then, the appropriate amounts of
CuCl2$2H2O and CoCl2$6H2O salts according to Table 1 were
dissolved in deionized water (S2). S2 was added by a burette into
S1 (S3). Aer that, an aqueous solution of NaOH (5 M) was
introduced drop-wise into the S3 solution (pH ¼ 10–11). The
mixture was also stirred for 30 min. The resultant materials
were transferred into an autoclave (L ¼ 7.3 cm, D ¼ 3.9 cm). The
autoclave was kept at 160 ꢁC for 72 h. Aer three days, the
Table 1 The amount of starting materials for the preparation of Cu1ꢀxCoxFe2O4 (0 # x # 1) spinel ferrite nanocatalysts
Entry
X
Cu1ꢀxCoxFe2O4
CoCl2$6H2O (g)
CuCl2$2H2O (g)
FeCl3$6H2O (g)
CTAB (g)
1
2
3
4
5
0
CuFe2O4
0
0.32
0.24
0.16
0.079
0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.25
0.5
0.75
1
Cu0.75Co0.25Fe2O4
Cu0.5Co0.5Fe2O4
Cu0.25Co0.75Fe2O4
CoFe2O4
0.11
0.22
0.33
0.44
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autoclave was allowed to cool down slowly to room temperature. results conrmed the presence of only one spinel phase without
The desired Cu1ꢀxCoxFe2O4 (0 # x # 1) was washed several any signicant impurities. CuFe2O4 spinel ferrite shows various
times with deionized water and dried at room temperature.
patterns due to cubic spinel phase and a considerable amount
of CuO and Fe2O3 (Fig. 3a). Substitution of copper with cobalt
increases the overall crystallinity of the spinel phase. XRD
pattern of CoFe2O4 (Fig. 3c) revealed the formation of single
phase cubic spinel structure with a sharp peak attributed to
(311) reection, which indicates that the crystallites are pref-
erentially oriented along the (311) plane. The breadth of the
Bragg peak is a combination of both instrument and sample
dependent effects. To decouple these contributions, it is
necessary to collect a various pattern from the line broadening
of a standard material such as silicon for determination of the
instrumental broadening. The corrected instrumentation
2.4. Typical procedure for dinitration of toluene using nitric
acid over Cu0.5Co0.5Fe2O4 under solvent-free conditions
First, in a 25 mL round bottom two-neck ask equipped with a
magnetic bar and condenser, 0.09 g (0.37 mmol) of Cu0.5Co0.5
-
Fe2O4 and 0.32 g (5 mmol) of 98% nitric acid were added and
ꢁ
heated at 90 C for 10 min. Then, 0.23 g (2.5 mmol) of toluene
ꢁ
was added to the reaction mixture and stirred at 90 C for 4 h.
The progress of the reaction was monitored by thin layer
chromatography (TLC) (n-hexane/EtOAc 9 : 1). Aer completion
of the reaction, the reaction mixture was slowly cooled to room
temperature as well as 25 mL of EtOAc was added and stirred.
Then, the reaction mixture was ltered over sinter glass (G-4)
under reduced pressure. Finally, for separation and isolation
of the pure product of 2,4-DNT from mononitrotoluene, the
reaction mixture was recrystallized in ethanol. The isolated
yield and melting point of 2,4-DNT were 88% and 69–71 ꢁC,
respectively. In addition, the pure product was identied by 1H
NMR, 13C NMR, and FT-IR spectroscopy.
broadening b corresponding to the diffraction peak of Cu1ꢀx
-
CoxFe2O4 (x ¼ 0.0, 0.5 and 1.0) was estimated by using the below
equation:
b ¼ b1 ꢀ b2
(1)
The crystallite sizes (Dc) of Cu1ꢀxCoxFe2O4 spinel ferrite were
calculated by using the Debey–Scherrer equation (Table 2):
2,4-Dinitrotoluene; IR (KBr): n (cmꢀ1) 3087, 3057, 2870, 1608,
1531, 1479, 1466, 1445, 1383, 1270, 1164, 1036, 843, 836, 791,
637, 1H NMR (400 MHz, CDCl3/TMS) d (ppm): 2.38 (3H, s, Me),
7.54 (1H, d, J ¼ 7.8 Hz, Ar), 8.08 (1H, d, J ¼ 7.8 Hz, Ar), 8.65 (1H,
s, Ar), 13C NMR (100 MHz, CDCl3/TMS) d (ppm): 20.48, 120.16,
127.05, 134.55, 141.01, 146.58, 149.19.
Kl
Dc ¼
(2)
b cos q
3. Results and discussion
3.1. Preparation of Cu1ꢀxCoxFe2O4
As shown in Fig. 2, Cu1ꢀxCoxFe2O4 spinel ferrites were prepared
in four steps, which give ve spinel ferrite nanocatalysts with
different molar ratios of copper and cobalt. Undoubtedly, the
synergy of copper and cobalt displays the best catalytic perfor-
mance, cubic shape, and overall yield of products.50,51
3.2. Characterization of Cu1ꢀxCoxFe2O4
3.2.1. XRD analysis. XRD patterns of the CuFe2O4, Cu0.5
-
Co0.5F2O4, and CoFe2O4 are displayed in Fig. 3. They revealed
the presence of (220), (311), (400), (422), (511) and (440) major
lattice plane for the cubic spinel phase with Fd3m space group.
Clear lattice plane of (111) and (422) is also present. These Fig. 3 XRD pattern of (a) CuFe2O4 (b) Cu0.5Co0.5Fe2O4 (c) CoFe2O4.
Fig. 2 Synthetic route for the preparation of different spinel ferrites.
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Table 2 Crystalline size of spinel ferrites Cu1ꢀxCoxFe2O4 according to
where b is the breadth of the observed diffraction peak at its
half-intensity maximum; K is the so-called shape factor, which
usually takes a value of about 0.9; and l is the wavelength of
X-ray source used in XRD instrument.
XRD patterns
Entry
x
Cu1ꢀxCoxFe2O4
2q (deg)
FWHM (deg)
D (nm)
The information of XRD patterns, such as FWHM, and XRD
diffraction data of Cu1ꢀxCoxFe2O4 for the most intense reec-
tion, are presented in Table 2. As shown from these data, the
crystallite sizes of CuFe2O4, Cu0.5Co0.5Fe2O4, and CoFe2O4 were
18, 22, and 20 nm, respectively.
1
2
3
0
0.5
1
CuFe2O4
Cu0.5Co0.5Fe2O4
CoFe2O4
35.4
35.4
35.2
0.24372
0.32946
0.32755
18
22
20
3.2.2. FT-IR study. Fig. 4 shows the FT-IR spectra of
CuFe2O4, Cu0.5Co0.5Fe2O4, Cu0.25Co0.75Fe2O4, and CoFe2O4
spinel ferrite. Two important broad bands of metal–oxygen are
observed in all of the spinel, especially spinel ferrites. The value
of n1, generally the highest one, was observed in the range of
550–600 cmꢀ1, which corresponds to the intrinsic stretching
vibrations of the metal at the tetrahedral site, [Mtetra 4 O].
Whereas the lowest band (n2) was observed in the range of 450–
385 cmꢀ1, which is related to octahedral metal stretching
[Mocta 4 O]. Since no clear peak attributed to octahedrally
coordinate metal ions has been observed, detection limit of our
FT-IR instrument is expected to be less than 400 cmꢀ1. The
bands within about 2845–2923 cmꢀ1 and 1060–1120 cmꢀ1 are
related to stretching modes of C–H and C–O groups, respec-
tively. The broad band centered about 3430 cmꢀ1 can be
assigned to O–H stretching mode for arising from the surface
hydroxyl groups on the spinel ferrite. The absorption band at
1630 cmꢀ1 on spectra refers to the vibration of remaining H2O
in the sample.
3.2.3. SEM and EDX studies. The morphologies of the
spinel ferrites were investigated by SEM micrograph that is
shown in Fig. 5. SEM images were employed to observe the grain
microstructure of the nanoparticles, which would provide a
better view of the grain development and grain sizes. These
gures indicate that the prepared spinel ferrites were obtained
by the hydrothermal method as being uniform in both
morphology and crystalline size. As shown in Fig. 5a, the cubic
Fig. 4 FT-IR spectra of (a) CuFe2O4 (b) Cu0.5Co0.5Fe2O4 (c) Cu0.25
Co0.75Fe2O4 (d) CoFe2O4.
-
Fig. 5 SEM micrograph of (a) Cu0.5Co0.5Fe2O4 (b) Cu0.25Co0.75Fe2O4 (c) Cu0.75Co0.25Fe2O4 (d) CoFe2O4 (e) CuFe2O4.
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Fig. 6 Distribution of particle sizes of prepared catalysts according to SEM images.
Table 3 The EDX analysis results of Cu0.5Co0.5Fe2O4, and CoFe2O4
consists of nearly octahedral crystals with an average size of
about 30 nm. The calculated grain sizes of Cu1ꢀxCoxFe2O4 (x ¼
0, 0.25, 0.50, 0.75 and 1) are 48, 34, 26, 24, and 29 nm,
respectively.
In addition, the distributions of grain size of spinel ferrites
are obtained in every prepared catalyst, and histograms of the
average size of spinel ferrites based on percentage of particles
are depicted in Fig. 6. The main percentage of distribution of
nanoparticles is nearly predicted by SEM, and XRD patterns. For
example in the spinel ferrite Cu0.5Co0.5Fe2O4, 69% of
Entry
Catalyst
Fe wt%
Co wt%
Cu wt%
1
2
CoFe2O4
Cu0.5Co0.5Fe2O4
65.70
63.49
34.30
18.26
0
18.25
morphology was observed for Cu0.5Co0.5Fe2O4. As can be seen
from SEM images, grains have different morphologies other
than spherical. A broad size distribution is observed, which
Fig. 7 EDX of (a) Cu0.5Co0.5Fe2O4 (b) CoFe2O4.
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nanoparticles are at approximately 26 nm. For other spinel of toluene in order to obtain the most appropriate conditions
ferrites such as Cu0.25Co0.75Fe2O4, Cu0.75Co0.25Fe2O4, CoFe2O4, such as type of the catalyst, medium and temperature of the
and CuFe2O4, the main particle sizes are also (74%, 24 nm), reaction, the amount of the catalyst, the molar ratio of the
(68%, 34 nm), (64%, 24 nm), and (55%, 48 nm), respectively.
reactant on the selectivity and overall yield of the reaction. Since
EDX was used for further clarify the existence of iron (Fe), toluene is moderately activated compound by the methyl group
copper (Cu), and cobalt (Co) ions and briey understand the in which the dinitration of toluene occur on active ring, the
information about the chemical composition of the resultant initial step of nitration of toluene is mostly complete and the
nanosized spinel ferrites. The resultants of numeric analysis are second nitration needs hard conditions.
given in Table 3. Moreover, EDX spectra of the Cu0.5Co0.5Fe2O4,
3.3.1. The effect of different spinel ferrites on nitration of
and CoFe2O4 are shown in Fig. 7. EDX spectra indicate the toluene. The reactions were carried out in the presence of
presence of 0.5 of Cu and 0.5 of Co for Cu1ꢀxCoxFe2O4 (Fig. 7a). different spinel ferrites and all of the results were depicted in
They also show the existence of the other elements such as Fe, Fig. 8. As shown in this gure, the binary spinel ferrite CuFe2O4
and O, which exist in the catalyst. As shown in Fig. 6b, although was used in this reaction. The yields of 2-nitrotoluene (2-MNT),
the presence of the element of Co is conrmed, there is no 4-nitrotoluene (4-MNT), and 2,4-DNT were 35.1%, 15.1%, and
evidence for the existence of Cu in the spectrum.
49.8%, respectively. In this experiment, the isomer 2,6-DNT was
obtained in trace amount so that the structure and shape of
prepared catalysts did not allow the formation of two nitro
groups in the vacant adjacent of methyl group of toluene.52
Meanwhile, the binary spinel ferrites CuFe2O4 preferred the
formation of 2-MNT as compared to 4-MNT (ratio: 2.3).
Furthermore, in accordance to SEM image of Fig. 5a, CuFe2O4
3.3. Catalytic evaluation of Cu1ꢀxCoxFe2O4 spinel ferrites in
the dinitration of toluene
Different experiments were used with toluene and nitric acid as
nitrating agent in the presence of Cu1ꢀxCoxFe2O4 for dinitration
Fig. 8 The influence of different prepared binary and ternary spinel ferrites on the nitration of toluene (reaction conditions: toluene : HNO3
98% : Cu1ꢀxCoxFe2O4 ¼ 1 : 2 : 0.15, solvent: n-hexane, T ¼ reflux, time ¼ 4 h).
Fig. 9 GC chromatogram (reaction conditions: toluene : HNO3 98% : Cu0.5Co0.5Fe2O4 ¼ 1 : 2 : 0.15, solvent: n-hexane, T ¼ reflux, time ¼ 4 h,
internal standard: 4-nitroacetophenone).
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Fig. 10 Effect of different solvent in the dinitration of toluene (reaction conditions: toluene : HNO3 98% : Cu0.5Co0.5Fe2O4 ¼ 1 : 2 : 0.15, time ¼ 4 h).
Table 4 Effect of temperature on dinitration of toluene with HNO3 in
the presence of Cu0.5Co0.5Fe2O4
CoFe2O4 show low activity for the formation of the main product
(2,4-DNT). Meanwhile, the y-y synergy between two metals
in the structure spinel ferrite can increase the yield of 2,4-DNT
to 78.9%.
Retention times of ethyl acetate (solvent), 2-MNT, 4-MNT, 4-
HNO3
(%)
T
Time
(h)
2-MNTa
(%)
4-MNTa
(%)
2,4-DNTa
(%)
Entry
(ꢁC)
1
2
3
4
5
6
7
8
9
65
65
65
65
98
98
98
98
98
r.t.
60
80
90
r.t.
60
80
90
100
6
6
4
4
5
4
4
1
1
56.0
52.3
39.8
34.8
52.3
39.2
28.0
9.7
35.0
19.2
25.4
22.7
37.2
24.8
21.5
0
8.8
28.5
34.8
42.5
10.3
36.0
50.5
90.2
44.0
nitroacetophenone (internal standard) and 2,4-DNT were 1,
3.3, 3.8, 7.0, and 9.2 min, respectively. Fig. 9 shows the GC
chromatogram of reaction mixture for previous experiment
(green legend) that has been used for determination of
different isomers of mono and dinitrotoluene. As shown in this
gure, the peak about 1 min is related to ethyl acetate as
solvent. In 3.2 min the peak of 2-MNT was revealed and at
about 3.8 min the peak of 4-MNT did not appear. Also, the
peaks at 7.0 and 9.18 min are related to internal standard and
2,4-DNT respectively.
37.9
18.0
a
GC yields.
3.3.2. The effect of solvent on the nitration of toluene. In
order to check the effect of solvent on the nitration of toluene,
the reactions were carried out in ve organic solvents, i.e. n-
hexane, CH2Cl2, CH3OH, EtOAc and CH3NO2, and also solvent-
free conditions. As shown in Fig. 10, the ve different solvents
of varied polarity employed in which n-hexane with a low
dielectric constant displayed the best results. For n-hexane with
has sphere-like shape. Since, the sphere shape in spinel ferrite
deals with the formation of 2-MNT and 4-MNT, little amount of
the desired product 2,4-DNT was formed by using cubic shape
of spinel ferrite. Moreover, the other binary spinel ferrite
CoFe2O4 prefers the formation of ortho-isomer relative to para-
isomer too (ratio: 3.35). Two binary spinel ferrites CuFe2O4, and
Fig. 11 Effect of catalyst amount in dinitration of toluene (reaction conditions: toluene : 98% HNO3 ¼ 1 : 2, T ¼ 90 ꢁC, solvent-free, time ¼ 4 h).
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Fig. 12 Optimizing of molar ratio of toluene and nitric acid (reaction conditions: T ¼ 90 ꢁC, solvent-free, catalyst: 15 mol%, time ¼ 3, 1, 1, 1, 0.5,
0.5 h for each molar ratio 1 : 1 to 1 : 5 respectively).
Fig. 13 Influence of the type of spinel ferrite on distribution of nitration system (reaction conditions: toluene : HNO3 98% : Cu1ꢀxCoxFe2O4
1 : 2 : 0.15, T ¼ 90 ꢁC, solvent-free, time ¼ 4 h).
¼
ꢁ
boiling point 68 C, the progress of the reaction was slow. In temperature of the reaction (r.t. to 90 ꢁC). The products of 2-
accordance to the plausible mechanism, the rst stage of this MNT, 4-MNT, and 2,4-DNT were produced in 56.0%, 35.0%, and
reaction is the formation of nitronium ion. Therefore, for 8.8% respectively for 6 h under solvent-free conditions by using
providing the activation energy, high temperature of the reac- 65% nitric acid at room temperature (Table 4, entry 1) for which,
tion medium accelerate the formation of nitronium ion that can the yield of 2,4-DNT was very low (8.8%). Two important reasons
inuence on the selectivity, distribution and yield of the prod- exist for this situation: (1) low concentration of nitric acid (65%)
ucts. The solvent-free conditions exhibited the best yield for 2,4- can cause the protonation of other molecules of nitric acid; and
DNT (90.9%). The isomer 4-MNT did not form under this (2) the low temperature of the reaction was not sufficient for the
condition. However, high temperature improves the selectivity second nitration on para-position of toluene. However, when
and the rate of formation of nitronium ion, which are important the temperature of the reaction was increased to 90 ꢁC, the
for the formation of isomer 2,4-DNT. The formation of 2,4-DNT reaction gave 34.8% of 2-MNT, 22.7% of 4-MNT, and 42.5% of
was decreased by increasing in dielectric constant of the used
solvents. According to the plausible mechanism, the solvated
nitronium ions by polar solvents can affect the direction and
approach of the substrates. However, the use of polar solvents
can decrease the yield of 2,4-DNT.10 Therefore, the addition of
organic solvent to the nitration system has no benet on either
selectivity or yield of the reaction.
3.3.3. The effect of temperature and concentration of nitric
acid on the nitration of toluene. The inuence of temperature
on dinitration of toluene, and concentration of nitric acid were
tabulated in Table 4. As shown in Table 4, two different
concentrations of nitric acid (65% or 98%) were used at various
Fig. 14 Nitration of toluene under optimum conditions.
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Fig. 15 The proposed mechanism for the reaction in the presence of cubic spinel ferrite.
2,4-DNT for 4 h in the presence of Cu0.5Co0.5Fe2O4 under catalytic activity for the yield and selectivity was observed by
solvent-free conditions (Table 4, entry 4). increasing the ratio of cobalt element up to 0.5. Homogeneity of
Concentration of nitric acid was improved to 98% in order to the structure of catalyst was destroyed and the yield of 2,4-DNT
achieve high yields of 2,4-DNT, which correspond to entries 5–9 was decreased to 19.6% with using of high ratio of 0.5, e.g.
of Table 4. For entry 5 of Table 4, the yield of 2,4-DNT was Cu0.25Co0.75Fe2O4. Nitration of toluene was also checked in the
10.3%, that gives only 1.5% higher yield relative to the same absence of spinel ferrites so that the selectivity of 2,4-DNT was
conditions for 65% nitric acid. Therefore, the effect of temper- far from the best conditions. Hence, the inuence of spinel
ature is important, which was revealed with two tests (Table 4, ferrites on the distribution of products nitration of toluene was
entries 1 and 5). The reactionsꢁ were checked at different approved with this experiment.
temperatures 60, 80, 90, and 100 C by using 98% nitric acid.
In order to show the best conditions of this study for dini-
The best result was obtained at T ¼ 90 ꢁC in which the yields of tration of toluene, Fig. 14 was depicted. As shown in this gure,
2-MNT, 4-MNT, and 2,4-DNT were 9.7, trace, and 90.2%, the spinel ferrite Cu0.5Co0.5Fe2O4, solvent-free, 90 ꢁC, and nitric
respectively.
acid (98%) were the optimum conditions for this study. Addi-
3.3.4. The effect of amount of the catalyst on the nitration tionally, the amount of catalyst and molar ratio of (tolue-
of toluene. Mole percent of catalyst is dened as (mmol catalyst/ ne : nitric acid-98%) were 15 mol% and 1 : 2, respectively.
mmol toluene) ꢂ 100 to check an improvement of the yield and
The plausible mechanism was presented in Fig. 15. As
selectivity even further, which can change from 10 to 40 mol%. shown in Fig. 15, the metal ions Fe, Cu, and Co have positive
All of the results were depicted in Fig. 11. As shown in this charge and the oxygen ions have negative charge in the cubic
gure, 15 mol% of Cu0.5Co0.5Fe2O4 gave the best result with the spinel ferrite Cu0.5Co0.5Fe2O4. Therefore, the catalyst acceler-
+
production of 9.7% 2-MNT, 90.2% 2,4-2DNT, and trace amount ates the formation of electrophile NO2 in the corner of the
of 4-MNT. The main product of reaction was produced lower cubic spinel ferrite Cu0.5Co0.5Fe2O4, which attacks the electron
than 90% by increasing in catalytic amount of Cu0.5Co0.5Fe2O4. rich ortho and para positions of toluene to produce of 2,4-DNT
High amount of catalyst caused that the effective collisions product. Since the special shape and structure of the Cu0.5
between substrates were decreased and the best catalytic Co0.5Fe2O4 is cubic, the formation of 2,4-DNT is more prefer-
activity did not exhibit. ence relative to 2,6-DNT. The orientation of catalyst and
-
3.3.5. The effect of different molar ratio of toluene : nitric toluene was also better in solvent free conditions when polar
acid. Reactions were also conducted in different molar ratio of solvent was used.
toluene : 98% nitric acid so that the results were shown in
Fig. 12. As can be seen from Fig. 12, with the molar ratio of 1 : 1
of substrates, the product 2,4-DNT was only produced in 7.1%.
4. Conclusion
Meanwhile, the best result was obtained in 90.4% of 2,4-DNT
and 9.5% of 2-MNT with the molar ratio of 1 : 2. In molar ratio
1 : 1, the ratio of ortho/para was 4.3. This ratio in molar ratio of
1 : 2 was the best and successful result because 4-MNT did not
produce. With increasing in molar ratio of toluene : nitric acid,
the yield of 2,4-DNT was decreased from 90.4% to 24.1% in
molar ratio of 1 : 5. It is not clear whether decreasing in 2,4-DNT
was real as a result of production of 2,4,6-trinitrotoluene, which
would not have been detected by the used GC system, or
whether the results reect experimental error in the
measurements.
It was established that the copper and cobalt synergy in Cu0.5
Co0.5Fe2O4 is an efficient and heterogeneous catalyst for the
high regioselective and green synthesis of 2,4-dinitrotoluene.
-
The use of 1 : 2 : 0.15 ratio of toluene : nitric acid : Cu0.5Co0.5
-
Fe2O4 gives 2,4-DNT in 90.2% yield. 98% nitric acid is more
attractive than 65% nitric acid in terms of double nitration of
toluene, and time of the reaction. The best temperature for the
nitration of toluene in this protocol was 90 ꢁC when the reaction
was performed in any organic solvent. To the best of our
knowledge up to date, there is no report on the use of spinel
ferrites in the nitration of toluene. The current work obtained
the best regioselectivity for the production of 2,4-DNT without
the formation of 2,6-DNT. 2,4-DNT compound was also sepa-
rated from 4-MNT via recrystallization in ethanol, while the
As shown in Fig. 13, the binary spinel ferrite such as
CuFe2O4, and CoFe2O4 could not be effective catalysts for
dinitration of toluene under solvent-free conditions. The best
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separation of 2,4-DNT and 2,6-DNT is a hard and intolerable 21 R. P. Claridge, N. Llewellyn Lancaster, R. W. Millar,
work. Therefore, this protocol represents an efficient method
for the production of 2,4-DNT in the polyurethane industry and
R. B. Moodie and J. P. B. Sandall, J. Chem. Soc., Perkin
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double based propellants as precursor and plasticizer, 22 D. Vassena, A. Kogelbauer and R. Prins, Catal. Today, 2000,
respectively.
60, 275–287.
23 Method for the determination of 2,6-dinitrotoluene [Air
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24 D. F. Othmer and H. L. Kleinhans, Ind. Eng. Chem., 1944, 36,
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25 J. Adamiak and M. Chmielarek, J. Ind. Eng. Chem., 2015, 27,
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26 M. Kong, K. Wang, R. Dong and H. Gao, Enzyme Microb.
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Acknowledgements
We gratefully acknowledge the nancial support by Malek-
Ashtar University of Technology (MUT). Also, the authors
acknowledge Dr Farhad Panahi, Dr Mohsen Golestanzadeh, and
Dr Mojtaba Khorasani for helpful discussions and providing GC
chromatograms.
27 W. Zhang, J. Zhang, S. Ren and Y. Liu, J. Org. Chem., 2014, 79,
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28 J. Green, J. Cao, U. K. Bandarage, H. Gao, J. Court,
C. Marhea, M. Jacobs, P. Taslimi, D. Newsome,
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