Synthesis of 3,3′-dimethyl-4,4′-diaminodiphenylmethane catalyzed by zeolites replacing mineral acids

Article abstract of DOI:10.1016/j.catcom.2013.11.001

Synthesis of 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDT) from o-tolylamine and formaldehyde over zeolites was investigated. Among the three tested zeolites, Hβ showed higher catalytic activity than HY and HZSM-5 for MDT synthesis. In the case of Hβ as a catalyst, the effects of feed composition, reaction time and temperature on the yield and selectivity of product MDT were further examined to optimize process conditions. In an o-tolylamine:formaldehyde = 8:1 molar ratio, the two-step reaction running at 413 K for 0.5 h and then 433 K for 0.5 h gave the MDT yield of 87.5%.

Full text of DOI:10.1016/j.catcom.2013.11.001

Catalysis Communications 45 (2014) 69–73
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of 3,3′-dimethyl-4,4′-diaminodiphenylmethane catalyzed by
zeolites replacing mineral acids
a
b
a
b
Xiao Lin a, Yingxian Zhao a, , Shengjian Zhang , Lingyun Wang , Yi Wang , Yanna Guo
⁎
a
Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, Zhejiang, China
b
Jiangsu Qingquan Chemical Co. Ltd., Yancheng 224555, Jiangsu, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 August 2013
Received in revised form 1 November 2013
Accepted 4 November 2013
Available online 11 November 2013
Synthesis of 3,3′-dimethyl-4,4′-diaminodiphenylmethane (MDT) from o-tolylamine and formaldehyde over
zeolites was investigated. Among the three tested zeolites, Hβ showed higher catalytic activity than HY and
HZSM-5 for MDT synthesis. In the case of Hβ as a catalyst, the effects of feed composition, reaction time and
temperature on the yield and selectivity of product MDT were further examined to optimize process conditions.
In an o-tolylamine:formaldehyde = 8:1 molar ratio, the two-step reaction running at 413 K for 0.5 h and then
433 K for 0.5 h gave the MDT yield of 87.5%.
Keywords:
MDT
© 2013 Elsevier B.V. All rights reserved.
Synthesis
o-Tolylamine
Formaldehyde
Zeolites
1. Introduction
diaminodiphenylmethane (MDA) by condensation of anilines and
formaldehyde, solid acids were suggested by several researchers to re-
3,3′-Dimethyl-4,4′-diaminodiphenylmethane (MDT) is an important
curing agent and polymer additive with a variety of industrial applications
[1–3]. For examples, MDT has been used in preparing electric cables, wires
and electromagnetic strings with good heat-resisting, chemical-resisting
and electric-insulating property [4]. Isocyanate prepared from MDT
can be used as room temperature curing agent with good adhesion
property [5].
MDT is traditionally synthesized by condensation of o-tolylamine
with formaldehyde catalyzed by mineral acids such as HCl [6]. The
synthetic process went through salinization, condensation, transposi-
tion rearrangement and neutralization. A large amount of waste acids
needed to be neutralized, and substantial aqueous salt waste could
cause environmental contamination seriously.
place mineral acids as catalysts. The selected solid acids included ion-
exchanged resins [9], metallic compounds [10–12], organosilanesulfonic
acid-functionalized Zr-TMS [13], clays [14,15], zeolites and modified ze-
olites [16–19].
In this work, synthesis of 3,3′-dimethyl-4,4′-diaminodiphenylmethane
by condensation of o-tolylamine and formaldehyde over zeolite cata-
lysts was conducted. The catalytic activity of Hβ, HY and HZSM-5 was
first scanned, and then, the catalytic performance of Hβ zeolite for this
synthetic reaction was further investigated. The out-coming results
would establish the foundation of a green and environment-friendly
technology for the synthesis of MDT and analog compounds.
As environmental awareness increasingly expanded, developing
new non-toxic and highly-effective catalysts is becoming a hot research
topic in the area of green catalysis [7,8]. While a green technology for
MDT synthesis waited to be developed, the replacement of homoge-
neous catalysts with heterogeneous catalysts might provide a solution
to the problems of waste pollution. Using a heterogeneous catalyst
allowed easy separation of catalyst from product without needing
neutralization, so that the aqueous waste stream could be eliminated,
and the catalyst could be recovered and reused. For synthesis of 4,4′-
2. Experimental
2.1. Materials
Chemicals o-tolylamine, formaldehyde (37 wt.% aqueous solution),
anhydrous ethanol, dimethyl-formamide (DMF) and potassium
dihydrogen phosphate were purchased from Sinopharm Chemical
Reagent Corporation Ltd., China. Methanol and methyl cyanide were
purchased from Tedia Corporation Ltd, United States. Zeolites Hβ was
supported by Tianjin NCIC catalyst Corporation Ltd., HY and HZSM-5
were supported by Shandong Qilu Huaxin Gaoke Corporation Ltd.,
China.
⁎
Corresponding author. Tel./fax: +86 574 88229075.
E-mail address: zyx@nit.zju.edu.cn (Y. Zhao).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.11.001

70
X. Lin et al. / Catalysis Communications 45 (2014) 69–73
Table 1
Table 2
HCHO conversion and MDT selectivity in condensation of o-tolylamine and formaldehyde
Surface area, pore diameter, pore volume and acidity of Hβ, HY and HZSM-5 zeolites.
over various zeolites.
Zeolites
Surface area
(m2/g)
Pore diameter
(nm)
Pore volume
(cm3/g)
Acidity
(μmol/g)
Catalysts
HCHO conversion (%)
MDT
Yield (wt.%)
Selectivity (wt.%)
Hβ
HY
HZSM-5
461.3
684.0
352.7
0.66
0.74
0.56
0.28
0.36
0.19
517
381
643
Hβ
HY
HZSM-5
80.6
75.2
73.6
52.5
42.9
30.7
65.1
57.0
41.7
Reaction conditions: n(o-tolylamine):n(HCHO) = 3:1 (molar ratio), w(HCHO):
w(catalyst) = 1:1 (weight ratio), 1 h, 433 K.
30.7%, and MDT selectivity from 61.5 to 57.0 to 41.7% with shifting cat-
alyst from Hβ to HY to HZSM-5. By comparison, Hβ exhibited the best
performance, while HZSM-5 gave the poor performance to this reaction
system.
2.2. Procedures
Catalytic activity of a zeolite depended on its properties of acid,
surface and structure. As illustrated in Fig. 1, NH3 temperature-
programmed desorption (NH3-TPD) showed the difference of acid site
density and acid strength distribution among the three zeolites. One
NH3 desorption peak was detected in 450–500 K for each zeolite, and
second peak detected in 730–770 K for HZSM-5 and Hβ, indicating ex-
istence of stronger acid sites. Theoretically, the catalytic activity should
be in the order of HZSM-5 N Hβ N HY if the acid strengths would have
played a dominant role, and also the HZSM-5 should give the better
performance than Hβ if the acid amounts have played a critical role in
MDT synthesis. The data in Table 1, however, declared that Hβ gave
the best catalytic performance, indicating that the structure and surface
of zeolites might have determinant effects on this synthetic reaction.
Based on the data in Table 2, HZSM-5 with smaller pore opening
(0.56 nm) and volume (0.19 cm3/g) is poorly active because diffusion
of product MDT and rearrangement of reaction intermediate were
strongly hindered, while HY with pore diameter (0.74 nm) and volume
(0.36 cm3/g) was less active due to low acid density and weaker acid
strength. In addition, the effect of acid density (amount) on the activity
might be insignificant at a high ratio of catalyst to HCHO (1:1), and
therefore, the Hβ with moderate pore size (0.66 nm), pore volume
(0.28 cm3/g) and surface area (461.3 m2/g) exhibited the best overall
performance for catalytic synthesis of MDT.
In a typical run, zeolite (9.0 g), formaldehyde (0.3 mol), and
o-tolylamine (0.6–3 mol) were added into a 500 mL autoclave made
in Weihai Zhengwei Machinery Equipment Corporation Ltd, China.
The sealed reactor was purged with nitrogen and then heated up to
the designed temperature with stirring. After the reaction went through
the scheduled times, the product mixture was sampled for HPLC
analysis.
2.3. Analysis
Product samples were analyzed by EX1600SM High Pressure Liquid
Chromatography with conditions: detection wavelength of UV 254 nm,
C18 column, room temperature, methanol solvent, mobile phase of
acetonitrile/water (1:1 volume ratio) at flow rate of 1.00 mL min−1
.
The textural properties of zeolites were characterized by BET
method using a ASAP 2020 V3.01 H analyzer, and the acid properties
were measured by NH3 temperature programmed desorption (TPD)
analysis. The TPD experiments were carried out in a flow apparatus
with helium as carrier gas, and for evolved gas detection, a thermal
conductivity detector (TCD) was used. A small amount of the granulated
zeolite (100 mg) was heated at 823 K in a He flow (30 mL min−1) for
1 h, and then cooled to 373 K. The activated sample was saturated
with ammonia at 373 K for 1 h and, after being purged with pure He
for 1.5 h at the same temperature, was heated at 10 K min−1 to 973 K
under helium flow (20 mL min−1).
3.2. MDT synthesis over Hβ zeolite
3.2.1. Effect of reactant composition
3. Results and discussion
The effect of o-tolylamine/formaldehyde molar ratio on yield and
selectivity of MDT in the catalytic reaction over Hβ at 433 K for 1 h
was shown in Fig. 2. As the ratio increased from 2 to 10, MDT yield
increased from 39.8 to 74.6%, and selectivity increased from 58.3 to
88.7%, respectively. The significant effect of reactant composition on
MDT yield and selectivity indicated that the excess of o-tolylamine
favored the formation of product MDT. However, a high ratio of
3.1. Catalytic activity of zeolites
Catalytic activities of Hβ, HY and HZSM-5 zeolites for MDT synthesis
were tested at 413 K. As summarized in Table 1, HCHO conversion
changed from 80.6 to 75.2 to 73.6%, MDT yield from 52.5 to 42.9 to
100.0
90.0
80.0
70.0
60.0
HY
HZSM-5
H-Beta
MDT yield
50.0
MDT selectivity
40.0
30.0
1
3
5
7
9
11
300
400
500
600
700
800
900
1000
Molar ratio
Temperature / K
Fig. 2. Effect of o-tolylamine/formaldehyde ratio on yield and selectivity of MDT. Reaction
conditions: w(Hβ):w(HCHO) = 1:1; 433 K; 1 h.
Fig. 1. NH3-TPD curves of HY, HZSM-5 and Hβ zeolites.

X. Lin et al. / Catalysis Communications 45 (2014) 69–73
71
100.0
90.0
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
3.2.2. Effect of temperature
As shown in Fig. 3, HCHO conversion generally increased from 42.4
to 80.7% with increasing temperature from 393 to 433 K, and then
stabilized in 433–473 K. At the same time, both yield and selectivity of
MDT increased gently from 393 to 413 K, and then rapidly from 413
to 433 K. As temperature continually increased from 433 to 473 K,
MDT yield almost remained without significant variation, but MDT
selectivity slightly decreased, probably due to the generation of more
by-reactions in higher temperature. Overall, the optimal temperature
to this reaction system was about 433 K.
It was interesting to see the rapid increase of MDT yield and selectiv-
ity from 393 to 433 K (Fig. 3). HPLC chromatograms of the product sam-
ples from the synthetic reactions at 393, 413, 423, 433, 453 and 473 K
were presented in Fig. 4. One can see that a large peak of the intermedi-
ate aminal (N, N′-di-o-tolyl-methanediamine) appears at a retention
time of 9.2 min in the chromatograms of samples from the reaction at
393 and 413 K, becomes much smaller for the sample at 423 K, and
almost disappears for the sample at 433 K. At the same time, the
peak area of reactant o-tolylamine decreases and the peak area of
product MDT increases significantly with raising temperature from
413 to 423 to 433 K. Therefore, we deduce that synthesis of MDT from
o-tolylamine and formaldehyde over Hβ involves the formation of
N, N′-di-o-tolyl-methanediamine which is quite stable at temperature
below 413 K, and goes on rearrangement to form MDT over 413 K.
HCHO conversion
MDT yield
MDT selectivity
380
400
420
440
460
480
500
Temperature / K
Fig. 3. Effect of reaction temperature on yield and selectivity of MDT. Reaction conditions:
n(o-tolylamine):n(HCHO) = 3:1, w(Hβ):w(HCHO) = 1:1, 1 h.
o-tolylamine/formaldehyde implied that a large amount of un-
reacted o-tolylamine required to be recycled, and thus an optimal
ratio of o-tolylamine/formaldehyde should be selected for industrial
production of MDT.
Fig. 4. HPLC chromatograms of product samples from reaction at various temperatures. Reaction conditions: n(o-tolylamine):n(HCHO) = 3:1, w(Hβ):w(HCHO) = 1:1, 1 h.

72
X. Lin et al. / Catalysis Communications 45 (2014) 69–73
Table 3
synthesis of MDT from o-tolylamine and formaldehyde was suggested
in Fig. 5.
Effect of two-step heating and one-step heating on Hβ catalytic performance.
The suggested sequence involves the formation and reactions of four
intermediates:
Two-step reaction
413 K for 0.5 h
21.3
One-step reaction
433 K for 1 h
433 K for 0.5 h
87.5
MDT (%)
71.6
(1) direct reaction of an o-tolylamine with a formaldehyde to form
an o-tolylamino-methanol;
Reaction conditions: n(o-tolylamine):n(HCHO) = 8:1, w(Hβ):w(HCHO) = 1:1
(2) conversion of the o-tolylamino-methanol into a methylene-o-
tolyl-amine by loss of a water;
3.2.3. Combinatorial effect of temperature and time
(3) reaction of methylene-o-tolyl-amine with o-tolylamine to form
N, N′-di-o-tolyl-methanediamine;
(4) rearrangement of N, N′-di-o-tolyl-methanediamine to 2-methyl-
4-(o-tolylamino-methyl)-phenylamine;
(5) rearrangement of 2-methyl-4-(o-tolylamino-methyl)-phenylamine
to final product MDT.
To optimize the synthetic conditions, a two-step heating method
was tested. The reaction was first carried out at 413 K for 0.5 h, and
then went through another 0.5 h at 433 K. As presented in Table 3,
a high MDT yield of 87.5% was obtained from the two-step heating
reaction, in comparison with the MDT yield of 71.6% obtained from
the one-step reaction at 433 K for 1 h. The significant difference was
an inevitable result from the reaction mechanism which was still
waiting to be verified. Further improvement of process performance
could be expected by combinatorial optimization of reaction time,
temperature and other conditions.
The above description on the reaction sequence of MDT synthesis
from o-tolylamine and formaldehyde was preliminary rather than pro-
found. We will present a thorough discussion on the details of reaction
mechanism elsewhere in the future.
3.3. Overview of reaction mechanism
4. Conclusions
The mechanism of MDT synthesis from o-tolylamine and formalde-
hyde over solid acids has, to the best of our knowledge, not been
interpreted so far. As well known, there were both Bronsted acid and
Lewis acid sites on Hβ zeolite. In general, the synthesis of MDT might
be catalyzed by Bronsted acid or Lewis acid, or both simultaneously.
There were no enough evidences to confirm the role of different acids
at moment. Nevertheless, an overview of reaction sequence for the
Zeolites Hβ, HY and HZSM-5 are found to be active for synthesis of
MDT by condensation of o-tolylamine and formaldehyde, and the cata-
lytic activity increases in the order of Hβ N HY N HZSM-5. Microporous
Hβ is capable to replace mineral acids for the acid-catalyzed synthesis of
MDT. The synthetic methodology is novel in its simple operation proce-
dure, shorter reaction time, easy catalyst recovery, and eco-friendly
nature.
H
CH
CH
3
3
CH
C
O
3
H
N
- H O
H
2
CH
N
CH
NH
2
2
2
HO
Methylene-o-tolyl-amine
o-Tolylamine
o-Tolylamino-methanol
H C
3
NH
H
C
2
2
H C
3
N
H
CH3
NH2
2-Methyl-6-(o-tolylamino-methyl)-phenylamine
CH3
H3C
H2
C
H
H
N
N
H C
3
Hβ
Hβ
H
C
2
N,N'-Di-o-tolyl-methanediamine
N
H
H N
2
CH
3
2-Methyl-4-(o-tolylamino-methyl)-phenylamine
Hβ
CH3
NH2
H2
C
H3C
H2N
MDT
Fig. 5. Overview of reaction sequence for MDT synthesis over Hβ zeolite.

X. Lin et al. / Catalysis Communications 45 (2014) 69–73
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