New Process of 2-Nitrotoluene into 2-Methylaniline by Transfer Hydrogenation with Methanol over X Zeolite Catalyst

Article abstract of DOI:10.1007/s10562-015-1633-1

A new transfer hydrogenation of 2-nitrotoluene with methanol to 2-methylaniline was successfully implemented. The transfer hydrogenation of 2-nitrotoluene with methanol showed much faster rate than its hydrogenation with hydrogen over X zeolites. Based on the reaction results over different modified X zeolites and activated carbon, a radical mechanism was proposed. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-015-1633-1

Catal Lett (2016) 146:174–179
DOI 10.1007/s10562-015-1633-1
New Process of 2-Nitrotoluene into 2-Methylaniline by Transfer
Hydrogenation with Methanol over X Zeolite Catalyst
1
1
1
1
1
Guoqing Zhao
•
Huanhui Chen
•
Ying Yuan
•
Xiaoci Li
•
Zhirong Zhu
Received: 1 August 2015 / Accepted: 24 September 2015 / Published online: 19 October 2015
Ó Springer Science+Business Media New York 2015
Abstract A new transfer hydrogenation of 2-nitrotoluene
with methanol to 2-methylaniline was successfully imple-
mented. The transfer hydrogenation of 2-nitrotoluene with
methanol showed much faster rate than its hydrogenation
with hydrogen over X zeolites. Based on the reaction
results over different modified X zeolites and activated
carbon, a radical mechanism was proposed.
1 Introduction
As a very important organic intermediate, 2-methylaniline
is a widely used precursor to synthesize many useful
chemical materials, such as pesticides, medicines, and
dyestuffs, etc. Commonly, 2-methylaniline is produced by
the process of liquid-phase hydrogenation of 2-nitrotoluene
under high pressures in the tank reactor due to the rela-
tively higher boiling point of 2-nitrotoluene as shown
below.
Graphical Abstract
.
Keywords Transfer hydrogenation Á Methanol Á X
zeolite Á 2-Nitrotoluene Á 2-Methylaniline
However, these production processes always consume a
large amount of hydrogen and need the separation of sol-
vent, being lower production efficiency, expensive equip-
ment and higher energy consumption. More importantly,
the catalysts for liquid-phase hydrogenation, like Pt/Pd-
type catalysts, are very expensive and often could not be
able to recycle. Therefore, it is desirable to explore a high-
efficiency, more economic and environment-friendly
reaction route and inexpensive catalysts.
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1633-1) contains supplementary
material, which is available to authorized users.
Although catalytic hydrogenation is an environmental
friendly method for the reduction of aromatic nitro com-
pounds, it has some difficulties in safety. With the devel-
opment of green chemistry, many hydrogenation processes
can be realized by catalytic transfer hydrogenation (CTH)
&
Zhirong Zhu
zhuzhirong@tongji.edu.cn
1
Department of Chemistry, Tongji University,
Shanghai 200092, China
1
23

New Process of 2-Nitrotoluene into 2-Methylaniline by Transfer Hydrogenation with Methanol…
175
using molecules containing hydrogen atoms other than
molecular hydrogen as the source of hydrogen [1–4].
Compared with the traditional hydrogenation process, CTH
possesses the advantage of high safety, low equipment
requirement. At present, existing transfer hydrogenation
generally requires the participation of noble metals, such as
Ru, Pt, Pd, Ir etc. [5–10]. However, these metal catalysts or
metal-supported catalysts often accompany with the loss of
active components, which causing a short life and a high
catalyst cost. Furthermore, the reduction of nitroarenes to
aromatic amines needs to be provided a liquid environment
for the system and a longer reaction time. Therefore, the
transfer hydrogenation is always limited by the solvent to a
large extent. The kind of solvent, amount of solvent, and
contact time have significantly effect on the reaction
results. Meanwhile, the use of solvents likely to cause a
larger amount of wastes.
calcined at 793 K for 2 h with a rate of 3 K/min to obtain
H-type X, donated as HX.
The characterizations of the modified X zeolites are
shown in the supplemental file. The X-ray diffraction
patterns of catalysts were recorded on a BRUKER D8
ADVANCE diffractometer using Cu Ka radiation, which
was operated at 40 kV and 40 mA. A scanning rate of 5
8/min was used for Bragg’s angles 2h = 5–508. The
specific surface area of catalysts was determined by
nitrogen adsorption at 77 K on a Quantachrome Nova-
2200e sorptometer and calculated by the Brunauer–Em-
mett–Teller (BET) method. Samples were pre-treated at
573 K for 3 h in a vacuum prior to the measurement. The
elemental compositions of catalysts were determined by
inductively coupled plasma (ICP, OPTIMA2100DV, Per-
kin Elmer). The acidity of catalysts was determined by
pyridine-infrared spectroscopy (Py-IR) (Bruker IFS-88).
The basicity of catalysts was determined by test reaction
of the alkylation of toluene with methanol and charac-
terized by IR with phenol adsorption. The test reaction
conditions were as follows: reaction temperature 703 K,
In this work, we successfully complete the process of
the transfer hydrogenation of 2-nitrotoluene to 2-methy-
laniline with a new facile approach to use methanol as
hydrogen resource. This new process carried out in a gas–
solid phase reaction system at a fixed bed reactor over non-
noble metal X zeolite as a hydrogenation catalysts. Obvi-
ously, this continuous production process is no need to
separate the catalysts from the solvents, which improving
the production efficiency and simplifying the production
process.
-
1
Toluene/Methanol molar ratio of 5, WHSV of 1.0 h
,
with nitrogen as carrier gas at atmosphere pressure. TG
curves were measured with a NETZSCH STA449C for
the uncalcined samples, with a-Al O being used as a
2
3
reference sample. The sample was heated at a rate of
10 K/min in a static air.
2.2 Reaction Procedures
2
Experimental
.1 Catalyst Preparation
The Na-X (Tongxing zeolite industry, Shanghai, Si/
A fixed bed tubular reactor system was employed for car-
rying out three type reactions, i.e., 2-nitrotoluene react with
methanol, 2-nitrotoluene react with formaldehyde and
2
hydrogenation of 2-nitrotoluene with H . The specific
2
?
?
?
Al = 1.27) was ion-exchanged with NH4 , K , Cs to
reaction conditions as follows: reaction temperature of
-
718 K, weight hourly space velocity (WHSV) of 1.0 h ,
1
prepare NH -type X, KX, CsX, respectively. CsOH was
4
?
used as a source of Cs , NH Cl and KHCO were used as
atmospheric pressure. A catalyst sample (5 g) was packed
into a stainless steel with 1.5 cm inner diameter (50 cm
4
3
?
?
the sources of NH4 and K , respectively. An aqueous
solution of CsOH (400 mL, 0.75 M) and NaX (20 g) were
mixed, stirred for 5 min and kept for 4 h at 363 K. The
slurry was filtered with Buchner funnel. The filtered cake
was dried in an oven at 373 K for 12 h and calcined at
length) and pretreated in a flowing N at 718 K for 1 h
2
prior to the reaction. For the reaction of 2-nitrotoluene with
methanol, the mixture containing 1:1 molar ratio of 2-ni-
trotoluene to methanol was pumped into the reactor at the
rate of 0.11 mL/min by a metering pump. The raw material
was replaced with 2-nitrotoluene and formaldehyde in 1:1
molar ratio for the reaction of 2-nitrotoluene with
formaldehyde. These reactions were all conducted under
7
93 K for 4 h in a muffle furnace in air. The sample was
again immersed in an aqueous solution of CsOH (400 mL,
.75 M), stirred and kept for 4 h at 363 K. The above
0
procedures were repeated for two more times (total ion-
exchange for three times). The obtained catalyst was
pressed, crushed, and sorted to get parent CsX catalysts
the N flow of 40 mL/min with atmospheric pressure. For
2
hydrogenation of 2-nitrotoluene, 2-nitrotoluene was fed
(
particles of 12–20 mush, donated as CsX). KX and NH4-
into the reactor at the rate of 0.11 mL/min with a H flow
2
type X were prepared by following the same method,
replacing the precursor solution with 1.0 mol/L KHCO3
and 1.0 mol/L NH Cl, respectively. NH -type X was
of 40 mL/min. The reactor effluent was analyzed with
online gas chromatograph HP-5890 equipped with a FID
detector and a 50 m HP-FFAP capillary column.
4
4
123

1
76
G. Zhao et al.
3
Results and Discussion
2-nitrotoluene with methanol (the source of hydrogen).
Secondly, methanol interacted with basic zeolite and con-
verted into formaldehyde, then KX catalyzed the reaction
of formaldehyde with 2-nitrotoluene to 2-methylaniline.
Thirdly, 2-nitrotoluene reacted with hydrogen which was
originated in the decomposition of methanol.
3
.1 Reaction of 2-Nitrotoluene with Methanol
In the reaction of 2-nitrotoluene with methanol over KX at
a fixed bed reactor, the product of reduction of 2-nitro-
toluene, 2-methylaniline, was found as shown in Fig. 1.
Other by-products as being detected in the reduction of
Figure 2 shows the product distribution of the reactions
of 2-nitrotoluene with hydrogen, formaldehyde and
methanol over KX, respectively. In the same conditions,
the conversion of 2-nitrotoluene and the selectivity of
2
-nitrotoluene with hydrogen, such as toluene, aniline, also
were formed in the reaction of 2-nitrotoluene with metha-
nol. The highest conversion of 2-nitrotoluene and selec-
tivity of 2-methylaniline were 73.09 and 58.19 %,
respectively. However, nitrostyrene and nitroethylbenzene
which might be formed as the products of the side-chain
alkylation of 2-nitrotoluene with methanol as the alkylating
reagents were not produced. The conversion of 2-nitro-
toluene slighted decreased after 4 h. The catalytic perfor-
mance of KX decreased obviously after 8 h, and the
catalysts became deep black. From the characterization of
TG (as shown in Figure S4) and the experiments of the
transfer hydrogenation of 2-nitrotoulene with methanol
over regenerated X zeolite (as shown in Table S5), the
deactivation of catalysts was mainly due to coke
deposition.
2-methylaniline of hydrogenation with H (7.38, 27.64 %)
2
and reaction with formaldehyde (5.78, 43.78 %) were
much lower than the transfer hydrogenation (73.09,
58.19 %) over KX zeolite. Therefore, the transfer hydro-
genation of 2-nitrotoluene with methanol proceeded sig-
nificantly faster than the hydrogenation of 2-nitrotoluene
with hydrogen and the reaction with formaldehyde for KX
zeolite. The hydrogenation of 2-nitrotoluene and the
transfer hydrogenation with formaldehyde were so slow to
be excluded from the higher conversion of 2-nitrotoluene.
Comparatively, when 2-nitrotoluene reacted with methanol
under hydrogen atmosphere, the conversion of 2-nitro-
toluene (from 73.09 to 74.83 %) and the selectivity of
aniline (from 36.02 to 39.13 %) and 2-methylaniline (from
58.19 to 54.58 %) have no significant change over KX, as
3
.2 Reactions of 2-Nitrotoluene with Hydrogen,
Formaldehyde and Methanol
shown in Table 1.
From the above, it could concluded that 2-methylaniline
was mainly formed by the transfer hydrogenation of 2-ni-
trotoluene with methanol over KX zeolite.
Methanol reaction over alkali exchanged FAU(X) molecu-
lar sieves yielded surface formaldehyde, which was found
to decompose into CO and H [11, 12]. So the reaction
2
3.3 Transfer Hydrogenation over Different
Modified X Zeolites
process of 2-nitrotoluene with methanol over KX zeolite
might be performed by three pathways. Firstly, 2-methy-
laniline was formed by the transfer hydrogenation of
Different kinds of X zeolites modified by ion-exchange
were applied to the reaction of transfer hydrogenation, the
reaction results are shown in Table 2. For all catalysts, the
Fig. 1 Reaction of 2-nitrotoluene with methanol over KX zeolite.
-1
Reaction conditions: 718 K, WHSV of 1.0 h , molar ratio of
-nitrotoluene/methanol of 1.0, N flow rates of 40 mL/min, atmo-
sphere pressure
2
2
Fig. 2 Reactions of 2-nitrotoluene with hydrogen, methanol and
formaldehyde over KX zeolite
1
23

New Process of 2-Nitrotoluene into 2-Methylaniline by Transfer Hydrogenation with Methanol…
Table 1 KX catalyze the
177
Atmosphere
Conversion (%)
Selectivity (%)
reaction of 2-nitrotoluene with
methanol under N and H
2
2
2-Nitrotoluene
Methanol
Toluene
Aniline
2-Methylaniline
N
2
2
73.09
74.83
49.71
60.45
5.79
6.21
36.02
39.13
58.19
54.58
H
The detailed calculation equation of the conversion and selectivity is shown in the supplementary material
with page 1, Mass balance obtained)
(
distribution of product did not change obviously, and dif-
ferent catalysts mainly affected the conversion of 2-nitro-
toluene and methanol. Transfer hydrogenation achieved the
highest conversion of 2-nitrotoluene (76.70 %) over NaX,
and the highest yield of main product 2-methylaniline
3.4 The Mechanism of Transfer Hydrogenation
of 2-Nitrotoluene with Methanol
The mechanism of side chain alkylation of toluene with
methanol to styrene, over X zeolites exchanged with alkali-
cation, postulated by Itoh et al. [13] was widely accepted,
as shown in Fig. 3. Generally, as a stronger electron-
withdrawing group, nitro could facilitate the polarization of
the methyl C-H on the aromatic ring to a negatively
charged methyl carbon to form the products of the side
(
42.53 %) over KX. As an acid zeolite, HX possessed a
distinctly different chemical properties according to the
classification of acid or basic zeolite. However, the reaction
of the transfer hydrogenation did not exhibited the similar
difference over HX. Although the conversion of 2-nitro-
toluene over HX was lower than other X zeolites, the
distribution of products was similar, the selectivity of
toluene, aniline, 2-methylaniline was 5.75, 42.37, 51.88 %,
respectively. From the above, X zeolites with various
acidity and alkalinity exhibited a similar tendency for the
transfer hydrogenation, high conversion of 2-nitrotoluene
and the selectivity of 2-methylaniline around 50 %. The
perceptible differences between four types of X zeolites
were the different conversion of 2-nitrotoluene and the
different utilization of methanol. These indicated that
acidity or alkalinity on X zeolite was not the main inter-
action factor for the transfer hydrogenation. When acti-
vated carbon was applied to catalyze the reaction of
chain alkylation. Moreover, the methyl hydrogen from
?
NO -Ph-CH (pKa = 20.4) is easier to dissociate into H
2 3
than that of toluene (pKa = 43), and p-nitro-benzyl anion
-
(NO
–Ph–CH
) is more stable than benzyl anion (Ph–
). If the transfer hydrogenation of 2-nitrotoluene with
2
2
-
2
CH
methanol follows the similar ion mechanism, the side-chain
alkylation products are easier to form than the reaction of
side-chain alkylation of toluene with methanol. However,
under the same reaction conditions, there were no nitros-
tyrene and nitroethylbenzene formation in the reaction of
2-nitrotoluene with methanol.
When the transfer hydrogenation reaction is interpreted
by other kinds of ion mechanism, the donor of hydrogen-
?
2
-nitrotoluene with methanol, the same products as transfer
H
is needed. It is easy to understand that methanol con-
?
–
hydrogenation over X zeolites were obtained, although the
catalytic activity and selectivity was much lower than X
zeolites (14.98, 29.29 %). However, when the reaction was
carried out in in the empty stainless steel reactor, toluene
and aniline were the main products because of the de-
methyl group and de-nitro group. The selectivity of
verts to H and CH OH on the base sites of basic zeolites
2
(such as KX and CsX), as shown in Fig. 4. The stronger
?
alkalinity, the easier formation of H . But it scarcely
occurs on the acid zeolite, HX and activated carbon. Nor-
mally, the alkalinity of CsX zeolite is stronger than KX and
?
NaX, it prefers to the formation of H . At the same reac-
2
-methylaniline was only 5.49 %.
tion conditions, the conversion of methanol in the side-
Table 2 Transfer
Catalyst
Conversion (%)
Selectivity (%)
Toluene
hydrogenation of 2-nitrotoluene
with methanol over different
modified X zeolites
2-Nitrotoluene
Methanol
Aniline
2-Methylaniline
NaX
76.70
73.09
63.97
55.63
14.98
10.14
72.39
49.71
79.83
95.43
52.56
49.86
6.71
5.79
46.74
36.02
44.51
42.37
51.01
14.59
46.54
58.19
49.24
51.88
29.29
5.49
KX
CsX
6.25
HX
5.75
Activated carbon
None
19.70
79.92
The detailed calculation equation of the conversion and selectivity is shown in the supplementary material
with page 1, Mass balance obtained)
(
123

1
78
G. Zhao et al.
Fig. 3 The mechanism of side
chain alkylation of toluene with
methanol to styrene
chain alkylation of toluene with methanol over KX and
CsX zeolites was almost 100 %. However, the conversion
of methanol in the reactive system of transfer hydrogena-
tion over X zeolite was less than 80 %, as shown in
hard to prove that the transfer hydrogenation of 2-nitro-
toluene with methanol complies with free radical mecha-
nism because the radical trapper cannot be used at the
higher reaction temperature (718 K). Therefore, there was
no visualized scientific evidence to prove the existence of
radicals in the process of transfer hydrogenation of 2-ni-
trotoluene with methanol. Even so, some of the phe-
nomenon cannot be ignored. Nitro radicals from
2-nitrotoluene and hydrogen radicals from methanol are
relatively easy to form at the high reaction temperature.
And the nitroarenes are typical free radical scavenger
because nitro-group can react with active radicals to form
more stable radicals. From the above, a possible mecha-
nism with radical was predicted for the transfer hydro-
genation of 2-nitrotoluene with methanol, as shown in
Fig. 5.
?
Table 2. If the formed H participates in the transfer
hydrogenation process, it could not explain the lower cat-
alytic performance of CsX than KX and NaX in the transfer
hydrogenation of 2-nitrotoluene with methanol under the
premise of not completely decomposed in methanol.
Besides, the catalytic activity of HX and activated carbon
for the transfer hydrogenation in an ionization mechanism
was impracticable.
According to the report results in the preceding paper,
the side chain alkylation of toluene with methanol to
styrene might be conducted with a radical mechanism,
which has been validated by many experiments and char-
acteristic methods (Chinese Journal of Catalysis, in press).
According to the reactive performance over the activated
carbon and different modified X zeolites, it seems that the
transfer hydrogenation of 2-nitrotoluene with methanol
also can be interpreted by the radical mechanism reason-
ably. However, the common methods, such as the
spin trapping-electron spin resonance (ESR) technique, are
The formation of hydrogen radicals was the prerequisite
to the accomplishment of free radical process. Nitro-group
reacted with the active hydrogen radical to stabilize the
active free radicals and form the objective product
2-methylaniline eventually. Compared to the activated
carbon, X-type zeolites exhibit better catalytic perfor-
mance. It is probably due to the captodative effect among
Fig. 5 The radical mechanism of transfer hydrogenation of 2-nitro-
touene with methanol
Fig. 4 Methanol reaction over basic X zeolites
1
23

New Process of 2-Nitrotoluene into 2-Methylaniline by Transfer Hydrogenation with Methanol…
179
Like X molecular sieve, the activated carbon has a large
specific surface area and a certain channel environment,
which is beneficial to the adsorption of 2-nitrotoluene and
methanol and the reactive collision, thus more conductive
to the transfer hydrogenation reactions than that without
any catalyst.
4
Conclusions
The reaction of 2-nitrotoluene with methanol to 2-methy-
laniline was proved to the process of transfer hydrogena-
tion. As a new way to prepare 2-methylaniline, X zeolites
exhibited the higher catalytic performance for the transfer
hydrogenation of 2-nitrotoluene with methanol, especially
for KX. In comparison to the different catalytic perfor-
mance over activated carbon and different acid or basic X
zeolites, it is reasonable to interpret this phenomenon in the
radical mechanism.
Fig. 6 Adsorption isotherms of methanol on X zeolites
the X zeolites. When an electron-withdrawing (captor) and
an electron-releasing (donor) attach to the sides of a radical
center, it would greatly increase the stability of the free
radical. When the 2-nitrotoluene entered into X zeolites, it
was stronger adsorbed on the extra framework cations of
zeolites due to the interactions between the 2-nitrotoluene
p-electrons (electron pair donor) with the cation (electron
pair acceptor). The interactions between electron-with-
drawing of the aromatic ring and pushing electron of
methyl, the anion on the framework of zeolite donates the
electron to nitro group of 2-nitrotoluene. Therefore, the X
zeolite can stabilize CH –Ph–NO Á, which is beneficial to
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123