Confinement effect as a tool for selectivity orientation in heterogeneous synthesis of 4,4′-diamino-3,3′-dibutyl-diphenyl methane over montmorillonite catalysts

Article abstract of DOI:10.1016/j.molcata.2010.03.032

Montmorillonite was successfully employed as a heterogeneous catalyst in the condensation of o-tert-butylaniline with paraformaldehyde for its significant confinement effect on the synthesis of 4,4′-MBTBA.

Full text of DOI:10.1016/j.molcata.2010.03.032

Journal of Molecular Catalysis A: Chemical 325 (2010) 55–59
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Confinement effect as a tool for selectivity orientation in heterogeneous synthesis
of 4,4ꢀ-diamino-3,3ꢀ-dibutyl-diphenyl methane over montmorillonite catalysts
Zhi Liu, Qiaoling Xu, Xutao Peng, Dao Li∗, Xingyi Wang
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Montmorillonite was employed as a heterogeneous catalyst in the condensation of o-tert-butylaniline
with paraformaldehyde for its significant confinement effect on the selective synthesis of 4,4ꢀ-diamino-
3,3ꢀ-dibutyl-diphenyl methane (4,4ꢀ-MBTBA). Due to the appropriate d-spacing which is larger than
the molecular size of 4,4ꢀ-MBTBA but smaller than that of 4,4ꢀ-(4-amino-5-tert-butyl-1,3-phenylene)
bis(methylene)bis(2-tert-butylaniline) (PBMBA), montmorillonite favored the production of 4,4ꢀ-MBTBA,
comparing with HY, P/SBA-15 and PW/C catalysts. In addition, with the decrease of the d-spacing of mont-
morillonite, the formation of PBMBA was inhibited and the selectivity of 4,4ꢀ-MBTBA became more and
more remarkable, which verified the confinement effect as a tool for selectivity orientation on the het-
erogeneous synthesis of 4,4ꢀ-MBTBA over montmorillonite catalysts. When paraformaldehyde was used
as starting material and the ratio of o-tert-butylaniline to paraformaldehyde was as low as 4:1, the yield
of 4,4ꢀ-MBTBA reached 83.9% with butyl acetate as solvent, reaction temperature of 130 ◦C and catalyst
loading of 0.18 g.
Received 16 January 2010
Received in revised form 24 March 2010
Accepted 24 March 2010
Available online 2 April 2010
Keywords:
Confinement effect
Montmorillonite
Heterogeneous catalysis
Selectivity
4,4ꢀ-Diamino-3,3ꢀ-dibutyl-diphenyl
methane
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
thesis of 4,4ꢀ-MBTBA under heterogeneous catalysis and found
that the condensation of o-tert-butylaniline with paraformalde-
Aromatic polyimides, owing to the excellent properties of
thermal stability, chemical resistance, mechanical and electronic
properties, are widely applied in many high technology areas,
such as separation membranes [1–3], coatings [4–6], composite
materials [7], plastic film [8] and electronics [9,10]. However, the
commercial use of aromatic polyimide is limited because of their
poor solubility and high softening or melting temperatures. To
resolve these problems, researchers focused on synthesis of sol-
uble and processable aromatic polyimides in a fully imidized form
without sacrificing the other properties.
hyde catalyzed by P/SBA-15 could provide a considerable yield of
4,4ꢀ-MBTBA [14].
However, the drawback of the new route is its low selec-
tivity. With the continuous exploration of this reaction, we
find that the major by-products in this reaction are 2,4ꢀ-
diamino-3,3ꢀ-dibutyl-diphenyl methane
(2,4ꢀ-MBTBA) and
4,4ꢀ-(4-amino-5-tert-butyl-1,3-phenylene)bis(methylene)bis(2-
tert-butylaniline) (PBMBA) (see Scheme 1). The former is not
thermodynamic favorable and its formation could be controlled by
adjusting the catalyst properties and reaction conditions [15,16].
However, the latter is hardly suppressed since it forms by the
condensation of 4,4ꢀ-MBTBA and formaldehyde at the similar
catalytic sites under the similar conditions to that of 4,4ꢀ-MBTBA.
Fortunately, there exists a significant difference in molecular size
between 4,4ꢀ-MBTBA and PBMBA (1.5 and 1.9 nm, respectively,
estimated by Gaussview 3.0). Therefore, confinement effect of
shape-selective catalysts is proposed to increase the selectivity of
4,4ꢀ-MBTBA.
Introducing tert-butyl-substituted monomer, 4,4ꢀ-diamino-
3,3ꢀ-dibutyl-diphenyl methane (4,4ꢀ-MBTBA), into the backbone
of aromatic polymers displays great improvement of the solu-
bility and processability without causing the unacceptable loss
of thermal properties of aromatic polyimide [11]. Traditionally,
o-tert-butylaniline is condensed with formaldehyde to give 4,4ꢀ-
MBTBA in the presence of HCl. This method suffers from the
difficulty in environment and separation problems. Compared with
homogeneous catalysis, heterogeneous catalysis has many advan-
tages, such as facile recovery of the solid catalyst from the reaction
mixture for recycling without tedious workup [12,13]. Therefore,
our research group pioneered new synthetic route for the syn-
Montmorillonite, with both Brønsted and Lewis acidic sites
available, is a kind of solid acid catalyst widely used in var-
ious alkylation reactions. And what is more, montmorillonite
possesses definite layered structure, which makes it an excel-
lent shape-selective catalyst. Suzuki and co-worker [17] found
metal-pillared montmorillonite could provide a significant shape-
selective effect for various reactions. Choudary and Valli [18]
employed vanadium-pillared montmorillonite for isomerization
∗
Corresponding author. Tel.: +86 21 64253372; fax: +86 21 64253372.
E-mail address: lidao@ecust.edu.cn (D. Li).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.03.032

56
Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 325 (2010) 55–59
Scheme 1. Condensation of o-tert-butylaniline with formaldehyde in presence of acidic catalysts.
of ␣-acetylenic alcohols, which could selectively generate ␣,␤-
ethylenic carbonyl compounds. Da and Enze [19] mentioned
that platinum-pillared montmorillonite exhibited a clear shape-
selective effect for isomerization of n-paraffins. Knifton et al.
[20,21] reported the selective synthesis of linear alkylbenzenes
by restrictions of montmorillonite d-spacing. Garade et al. [22,23]
research group had successively researched alkylation reaction
of phenol, p-cresol with formaldehyde, and found that mont-
and followed by filtration and wash with distilled water. Finally, the
sample was dried in air at 80 ◦C for 10 h and then at 120 ◦C for 4 h.
Phosphoric acid modified SBA-15 (P/SBA-15) was synthesized
according to [14]. PW/C and PW/H-Mont were prepared via wet
impregnation method: 0.3 g PW and 1 g active carbon or H-Mont-
200 was stirred with 4 ml absolute ethanol solution at room
temperature for 6 h. Then, the solvent was evaporated and the solid
was dried at 110 ◦C for 6 h and calcined at 285 ◦C for 3 h in air.
All the catalysts were activated at 120 ◦C before use [27].
morillonite gave
a higher conversion and selectivity owing
to its unique layered structure. Binitha and Sugunan [24,25]
reported alkylation reaction of benzene and toluene catalyzed
by an eco-friendly catalyst, titania-pillared montmorillonite and
chromia-pillared montmorillonites. The alkylation of benzene with
ethanol produced ethylbenzene as the only product by restrictions
of titania-pillared montmorillonite layered structure.
2.2. Characterization
X-ray diffraction (XRD) measurements were taken on
a
RINT2000 vertical goniometer using Cu K␣ radiation (40 kV,
200 mA, 1.2–10◦).
The d-spacing of montmorillonite locates in the range of
1–2 nm, which is similar to the molecular sizes of 4,4ꢀ-MBTBA and
PBMBA. In this work, we employed montmorillonite as a hetero-
geneous catalyst for the condensation of o-tert-butylaniline with
paraformaldehyde. The confinement effect of montmorillonite on
the selectivity of 4,4ꢀ-MBTBA and PBMBA was investigated via the
subtle adjustment of the d-spacing. Besides, reaction conditions
were screened for the optimization of the yield of 4,4ꢀ-MBTBA.
Nitrogen adsorption/desorption: The nitrogen adsorption and
desorption isotherms were measured at −196 ◦C on an ASAP 2400
system in static measurement mode. The samples were outgassed
at 160 ◦C for 4 h before the measurement. The specific surface area
was calculated using the BET model.
2.3. Catalytic activity measurement
A mixture of o-tert-butylaniline (4 mmol), paraformaldehyde
(1 mmol), catalyst (0.18 g) and butyl acetate (3 ml) was stirred at
130 ◦C for 4 h under N2 atmosphere. After cooling to room tem-
perature, the catalyst was filtered off and the organic layer was
evaporated. The crude product was subjected to column chro-
matography on silica gel with petroleum ether/EtOAc (6:1) as
effluent. Isolated products were determined by 1H NMR and 13C
NMR. Yields of products were calculated based on the input of
paraformaldehyde.
2. Experimental
2.1. Catalyst preparation
Concentrated HCl, paraformaldehyde, phosphotungstic acid
(PW), HY zeolite and active carbon (C) were purchased from
Sinopharm. o-tert-Butylaniline (Aldrich) and butyl acetate (Qiang-
sheng Chemical) were used as commercial products without
further treatment. Montmorillonite was purchased from Zhejiang
Feng Hong Clay Chemical Co., Ltd. and designated as Na-Mont.
Acidified montmorillonite was prepared according to [26]:
montmorillonite (10.0 g) and 100 g of aqueous HNO3 (0.5 wt.%) was
stirred at 80 ◦C for 24 h. The slurry obtained was filtered, washed
with distilled water and calcined in air at a certain temperature
for 4 h. Depending on different calcination temperatures of 200,
300 and 400 ◦C, acidified montmorillonites were designated as H-
Mont-200, H-Mont-300 and H-Mont-400, respectively.
The preparation of PG-H-Mont and CTAB-H-Mont was as fol-
lows: H-Mont-200 (10.0 g) was mixed with polyethyleneglycol
(4.0 g) or cetyltrimethylammonium bromide (4.0 g) in 100 ml water
and then stirred at 80 ◦C for 24 h. The slurry obtained was filtered
and washed with distilled water, dried in air at 80 ◦C for 10 h and
then at 120 ◦C for 4 h.
3. Results and discussion
Traditionally, 4,4ꢀ-MBTBA was synthesized in homogeneous
condition in presence of HCl. However, the difficulty in separation
and the pollution made this route unfavorable for the environ-
ment. Our research group discovered a new synthetic route under
the heterogeneous catalysis by P/SBA-15 [14], with unsatisfactory
selectivity. To improve the selectivity of 4,4ꢀ-MBTBA, several cata-
lysts were tested on the condensation of o-tert-butylaniline with
paraformaldehyde and their catalytic performances are presented
in Fig. 1. Obviously, HY zeolite showed the worst catalytic perfor-
mance with only 12.9% for the yield of 4,4ꢀ-MBTBA. The result could
be explained by the narrow pore size of 0.75 nm of HY zeolite, which
led to the impossibility of the accessibility of o-tert-butylaniline
(ca. 1.1 nm) to the acidic sites in the pores. When P/SBA-15 and
PW/C were used as catalysts for the condensation, the products’
distribution was similar to that in homogeneous condition: about
50% yield of 4,4ꢀ-MBTBA, 20% of PBMBA and 8% of 2,4ꢀ-MBTBA.
This phenomenon was assumed to be brought about by the reac-
The preparation of B-A-Mont was as follows: montmorillonite
(1.0 g) in 50 ml aqueous NaOH (0.1 mol/L) was stirred at 80 ◦C for
24 h. The slurry obtained was filtered, washed with distilled water
and dried in air at 80 ◦C for 10 h. Then, the obtained powder was
mixed with 50 ml aqueous HNO3 (0.2 mol/L), stirred at 80 ◦C for 24 h

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 325 (2010) 55–59
57
Table 1
Confinement effect on the synthesis of 4,4ꢀ-MBTBA over heterogeneous catalysts.
Entry
Catalyst
Yield (%)a
4,4ꢀ-MBTBA/ PBMBA
d-Spacingb
BET surface area (m2 g−1
)
2,4ꢀ-MBTBA
PBMBA
4,4ꢀ-MBTBA
1
2
3
4
5
6
7
8
B-A-Mont
Na-Mont
4.3
6.4
8.6
6.4
10.7
6.4
8.6
2.3
5.4
6.4
6.4
8.6
12.9
10.7
10.7
49.5
75.1
83.9
62.6
73.1
32.3
64.5
75.2
21.5
1.33
1.46
1.53
1.58
1.59
1.83
–
129.1
3.6
13.9
13.1
9.8
8.5
2.5
H-Mont-200
PG-H-Mont
H-Mont-300
CTAB-H-Mont
H-Mont-400
PW-H-Mont
89.8
57.2
75.1
45.4
–
6.0
7.0
8.6
–
55.6
The bold indicates the best catalyst and the highest yield in the reaction.
a
Isolated yield.
Calculated by XRD.
b
hyde, the d-spacing of montmorillonite was adjusted and the
catalytic performances were tested (see Table 1). Fig. 2 shows the
XRD patterns of montmorillonites with various modification meth-
ods and the corresponding values are listed in Table 1. As can be
seen, the d-spacing of CTAB-H-Mont reached 1.83 nm. And the ratio
of 4,4ꢀ-MBTBA/PBMBA produced on this catalyst was 2.5, which
was similar to the result obtained in the homogeneous condition.
As mentioned above, the molecular sizes of o-tert-butylaniline,
4,4ꢀ-MBTBA and PBMBA equal to 1.1, 1.5 and 1.9 nm, respectively.
Therefore, when the d-spacing of catalyst equals to or is larger than
the size of all products, the reactant could access to the acidic sites
inside the layers of CTAB-H-Mont and the condensation reaction
should occur. As a result, confinement effect could not put any influ-
ence on the selectivity of products and the products’ distribution
kept unchanged.
Fig. 1. Optimization of catalysts for synthesis of 4,4ꢀ-MBTBA.
However, when the d-spacing of catalyst became smaller than
the size of products, confinement effect was exhibited in the change
of the products’ distribution. H-Mont-300 showed a smaller d-
spacing of 1.59 nm and apparently, the formation of PBMBA was
limited by the shrink of layer distance. The selectivity of PBMBA
dropped and the ratio of 4,4ꢀ-MBTBA/PBMBA increased to 8.5. With
the further decrease of the d-spacing of montmorillonite, the con-
finement effect became more and more clear. For PG-H-Mont,
H-Mont-200 and Na-Mont, the d-spacing reduced from 1.58 to 1.46
and the 4,4ꢀ-MBTBA/PBMBA ratio increased from 9.8 to 13.9. When
B-A-Mont was used as catalyst, it presented the highest diffraction
angle of ca. 6.5◦, which corresponds to the d-spacing of 1.33 nm.
The yield of PBMBA on this catalyst dropped so much that the ratio
of 4,4ꢀ-MBTBA/PBMBA reached as high as 21.5, while the conver-
sion of o-tert-butylaniline decreases significantly, which may be
due to diffusional resistance in catalyst pores and interlayer [28].
For H-Mont-400 and PW-H-Mont, although the diffraction peaks
almost disappeared, the layered structures should not be destroyed
completely but became irregular. Thereafter, the catalytic perfor-
mances of these catalysts on selectivity were still higher than that
of CTAB-H-Mont because of the limitation of the layered structures
of montmorillonites.
tions occurred without spatial limitation, since the pore diameter
of P/SBA-15 of 6 nm is much larger than the molecular sizes of
all the products, while the reactions took place only on the out-
side surface of active carbon. However, when montmorillonite
was employed, a significant difference appeared. The yield of 4,4ꢀ-
MBTBA increased to 83.9%, while that of PBMBA decreased sharply
to 6.4%. As revealed by XRD (see Fig. 2 and Table 1), the d-spacing
of acidified montmorillonite is 1.53 nm, which could accommodate
4,4ꢀ-MBTBA molecule (1.5 nm), so that MBTBA’s formation pro-
ceeded successfully on it. But for PBMBA (1.9 nm), the d-spacing
is so small that the formation was confined and the yield dropped
accordingly.
For further clarification of the confinement effect on the selec-
tivity of the condensation of o-tert-butylaniline with paraformalde-
Anyway, with the decrease of the d-spacing of the montmoril-
lonites, the formation of PBMBA was inhibited and the selectivity
of 4,4ꢀ-MBTBA become more and more remarkable. Although the
other elements such as acid properties could not be ruled out for
the change of the activity and selectivity, the certainty is that, in
heterogeneous synthesis of 4,4ꢀ-MBTBA over montmorillonite cat-
alysts, confinement effect was employed successfully as a tool for
selectivity orientation.
The results of BET surface areas of various catalysts are pre-
sented in Table 1. Among them, Na-Mont showed the lowest surface
area of 3.6 m2 g−1. After acid treatment, the surface area of H-Mont-
200 reached 89.8 m2 g−1 and B-A-Mont presented the highest one
of 129.1 m2 g−1. However, the same amount of catalysts did not ren-
Fig. 2. XRD patterns of montmorillonites with different treatments.

58
Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 325 (2010) 55–59
Table 2
Effect of reaction conditions on the synthesis of 4,4ꢀ-MBTBA over H-Mont-200 catalyst.
Entry
Solvent
Reaction temperature (◦C)
Reaction time (h)
Catalyst loading (g)
Yield (%)a
1
2
3
4
5
6
7
8
Toluene
130
130
130
110
130
130
130
130
4
4
4
4
2
24
4
4
0.18
0.18
0.18
0.18
0.18
0.18
0.12
0.24
58.1
73.1
83.9
70.9
79.6
84.5
73.1
84.2
Acetonitrile
Butyl acetate
Butyl acetate
Butyl acetate
Butyl acetate
Butyl acetate
Butyl acetate
The bold indicates the best catalyst and the highest yield in the reaction.
a
Isolated yields.
Fig. 4 gives the influence of the ratio of o-tert-butylaniline to
paraformaldehyde on the reaction. When the ratio changed from
5:1 to 4:1, the products’ distribution did not change much, which
was similar to the result reported by Ajaikumar and Pandurangan
[16]. The further increase of the proportion of paraformaldehdye
led to the decrease of the selectivity of 4,4ꢀ-MBTBA and the opposite
trend of PBMBA, which was also caused by the increased concentra-
tion of the released formaldehyde species and the resulting reaction
between 4,4ꢀ-MBTBA and formaldehyde.
Table 2 presents the effect of the other reaction conditions on
the condensation reaction of o-tert-butylaniline and paraformalde-
hyde. It is indicated that butyl acetate was the most effective
solvent, whereas toluene and acetonitrile gave lower yield of 4,4ꢀ-
MBTBA of 58.1% and 73.1%, respectively, which was in accordance
with the report by Lipshutz et al. [29]. The poor performance in
toluene might be due to its weak polarity which led to the difficulty
of the solubility of substrates and products. On the other hand, ace-
tonitrile presented too strong polarity to bring about the problem
of the competitive adsorption between the solvent and substrates.
Therefore, the medium polarity of solvent, like butyl acetate, should
be advantageous in this reaction.
With regard to reaction temperature, the highest yield was
achieved at 130 ◦C. Lowering the temperature caused the decrease
of the yield of 4,4ꢀ-MBTBA (see Table 2, entry 4). Two reasons
should be responsible: (1) paraformaldehyde cannot decompose
completely at lower temperature [30]; (2) 4,4ꢀ-MBTBA is ther-
modynamic stable and higher temperature favors its formation.
As to the reaction time, the yield reached 79.6% at 2 h and
did not show significant increase after 4 h (see Table 2, entry
6), which was similar to the result reported by Siddhartha
[27]. The optimized catalyst loading was 0.18 g. The less cat-
alysts loading of 0.12 g gave lower yield of 73.1%. But higher
loading could not improve that (see Table 2, entry 8), which
might be due to the diffusional resistance in catalyst interlayers
[31,32].
Fig. 3. Effect of formaldehyde species on synthesis of 4,4ꢀ-MBTBA over H-Mont-200
catalyst.
der the significant discrepancy in their reactivity. Therefore, the BET
area should not be a decisive factor in terms of activity of catalysts.
Since H-Mont-200 gave the best yield of 4,4ꢀ-MBTBA and sat-
isfactory ratio of 4,4ꢀ-MBTBA/PBMBA, the catalytic performance
on H-Mont-200 under different reaction conditions was evacuated
for condensation reaction of o-tert-butylaniline and paraformalde-
hyde. Fig. 3 exhibits the effect of formaldehyde species on the
reaction. As can be seen, the use of formaldehyde and trioxymethy-
lene gave lower yield of 4,4ꢀ-MBTBA than paraformaldehyde did.
When paraformaldehyde was employed, considering its slow
decomposition, the released formaldehyde consumed quickly to
form 4,4ꢀ-MBTBA, which led to the very low concentration of
formaldehyde species in the reaction system. As a result, the for-
mation of PBMBA was inhibited and the selectivity of 4,4ꢀ-MBTBA
increased.
4. Conclusions
Since the d-spacing of montmorillonite is wide enough to
accommodate 4,4ꢀ-MBTBA molecule, but too narrow to hold
PBMBA, montmorillonite showed excellent catalytic activity in the
synthesis of 4,4ꢀ-MBTBA, comparing with HY zeolite, P/SBA-15 and
PW/C catalysts. With the decrease of the d-spacing of montmoril-
lonite, the formation of PBMBA was inhibited and the selectivity
of 4,4ꢀ-MBTBA became more and more remarkable. Therefore, the
confinement effect on the selectivity of the condensation of o-tert-
butylaniline with paraformaldehyde was verified to be successful
as a tool for selectivity orientation in heterogeneous synthesis of
4,4ꢀ-MBTBA over montmorillonite catalysts.
Different reaction conditions were evacuated for condensation
reaction of o-tert-butylaniline and paraformaldehyde. Due to the
low concentration of formaldehyde species released in the reaction
Fig. 4. Effect of mole ratio of o-tert-butylaniline to paraformaldehyde on synthesis
of 4,4ꢀ-MBTBA over H-Mont-200 catalyst.

Z. Liu et al. / Journal of Molecular Catalysis A: Chemical 325 (2010) 55–59
59
system, the formation of PBMBA was inhibited and the selectivity of
4,4ꢀ-MBTBA increased when paraformaldehyde was employed and
the ratio of o-tert-butylaniline to paraformaldehyde was lower than
4:1. Butyl acetate was the most effective solvent, due to its medium
polarity. The optimized reaction temperature and catalyst loading
were 130 ◦C and 0.18 g.
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Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2010.03.032.
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