Phosphoric acid modified SBA-15 as a highly efficient heterogeneous catalyst for the synthesis of 4,4′-diamino-3,3′-dibutyl-diphenyl methane

Article abstract of DOI:10.1016/j.catcom.2009.11.018

Phosphoric acid modified SBA-15 (P/SBA-15) was employed as catalyst in the synthesis of 4,4′-diamino-3,3′-dibutyl-diphenyl methane (MBTBA) via condensation of o-tert-butylaniline with paraformaldehyde. It performed more efficiently than HY, kaolinite, γ-Al₂O₃ in heterogeneous reaction and HCl in homogeneous catalysis did. The catalyst preparation and reaction conditions were investigated and the yield reached 50% under the optimized term. The BET and Py-FTIR results suggested that the excellent performance of P/SBA-15 in the synthesis of MBTBA should be attributed to the weak acidity of the material and its large pore size that facilitated the diffusion of substrate and product.

Full text of DOI:10.1016/j.catcom.2009.11.018

Catalysis Communications 11 (2010) 438–441
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Phosphoric acid modified SBA-15 as a highly efficient heterogeneous catalyst for
0
0
the synthesis of 4,4 -diamino-3,3 -dibutyl-diphenyl methane
*
Qiaoling Xu, Zhi Liu, Rui Ma, 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
0
Article history:
Phosphoric acid modified SBA-15 (P/SBA-15) was employed as catalyst in the synthesis of 4,4 -diamino-
3
Received 9 September 2009
Received in revised form 13 November 2009
Accepted 18 November 2009
Available online 26 November 2009
0
,3 -dibutyl-diphenyl methane (MBTBA) via condensation of o-tert-butylaniline with paraformaldehyde.
It performed more efficiently than HY, kaolinite, c-Al O in heterogeneous reaction and HCl in homoge-
2 3
neous catalysis did. The catalyst preparation and reaction conditions were investigated and the yield
reached 50% under the optimized term. The BET and Py-FTIR results suggested that the excellent perfor-
mance of P/SBA-15 in the synthesis of MBTBA should be attributed to the weak acidity of the material and
its large pore size that facilitated the diffusion of substrate and product.
Keywords:
Heterogeneous catalysis
P/SBA-15
Ó 2009 Elsevier B.V. All rights reserved.
0
0
4
,4 -Diamino-3,3 -dibutyl-diphenyl
methane
1
. Introduction
Aromatic polyimides [1] have received much attention for its
environment and increase the profit [12]. Recently, it is reported
that heterogeneous catalysis has been applied in similar reactions.
Corma et al. [13] employed delaminated zeolite instead of HCl in
0
excellent thermal stability, chemical resistance, outstanding
mechanical and electronic properties, and been employed broadly
in many high technique areas, such as electronics [2], membranes
the production of 4,4 -diaminodiphenylmethanes, which gave high
conversion and selectivity. Bahulayan et al. [14] developed an effi-
0
cient method of synthesis of 4,4 -diaminodiphenylmethanes cata-
[
3–5], coatings [6,7], and composite materials [8]. However, the
lyzed by an eco-friendly catalyst, kaolinitic clay. However, there
was no description of the synthesis of MBTBA under heterogeneous
catalysis, which might be the consequence of large molecule size of
the substrate and the product (ca. 1.1 nm and 1.5 nm, respectively,
measured by Gaussview 3.0).
commercial use of these materials is limited because of their poor
solubility and high softening or melting temperatures. To resolve
these problems, researchers focused on synthesis of soluble and
processable aromatic polyimides in a fully imidized form without
sacrificing their excellent properties. Introducing tert-butyl-
Mesoporous materials have received much attention because of
their unique properties with well-controlled pore structure, high
surface area, large and tunable pore size which facilitate the diffu-
sion of reactant and product inside the pore [15–19]. However,
purely mesoporous materials show limited application for catalysis
because of the lack of acidity and capacity for ion-exchange. Previ-
ous research in our group [20,21] and other groups [22,23] re-
vealed that phosphoric acid modified mesoporous materials
could exhibit Brønsted acidity and large pore size. And, Mauder
et al. [24] presented their detailed study on the hydrogen bond
interaction of pyridine with phosphonic acid moieties at the sur-
face of SBA-15 mesoporous silica by a combination of solid-state
NMR techniques. Moreover, Paul et al. [25] synthesized mesopor-
ous titanium–phosphorus mixed oxides that showed excellent cat-
alytic activity in acid catalyzed Friedel–Crafts benzylation of bulky
aromatics and partial oxidation of styrene to benzaldehyde with
0
substituted monomer, 4,4 -diamino-3,3’-dibutyl-diphenyl meth-
ane (MBTBA), into the backbone of aromatic polymers displayed
great improvement of the solubility and processability without
causing the unacceptable loss of thermal properties of the
polymer [9,10].
MBTBA can be used not only as monomer in the preparation of
advanced polyimide, but also as chain extender of polyurethane
and curing agent of epoxy resin [11]. Therefore, it is attractive to
explore the synthetic method of MBTBA. Traditionally, o-tert-
butylaniline is condensed with formaldehyde to give MBTBA in
the presence of HCl. This method suffers from the difficulty in sep-
aration and the environmental pollution. Heterogeneous catalysis
has received more and more attention, which gives an easier sep-
aration and offers the opportunity to reduce the impact on the
2 2
dilute H O oxidant. Panneerselvam et al. [26] reported that phos-
phoric acid modified b-zeolite was an efficient sorbent media for
*
Corresponding author. Tel./fax: +86 21 64253372.
E-mail address: lidao@ecust.edu.cn (D. Li).
the removal of copper ions from aqueous solution and wastewater.
1
566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2009.11.018

Q. Xu et al. / Catalysis Communications 11 (2010) 438–441
439
Due to the need of acidic heterogeneous catalyst, phosphoric
Table 1
a
Screening catalysts for the synthesis of MBTBA.
acid modified materials highlight an opportunity in the develop-
ment of novel efficient catalysts for the synthesis of MBTBA. Here,
we present a novel synthetic method of MBTBA via condensation of
o-tert-butylaniline with paraformaldehyde, in which phosphoric
acid modified SBA-15 (P/SBA-15) is employed as catalyst.
NH2
Catalyst
+
(HCHO)n
Sovlent, 130 o
C
H N
NH2
2
2
. Experimental
Yield (%)b
Entry
Catalysts
HCl
Natural kaolinite
HY zeolite
1
2
3
4
5
44
1
13
2
2.1. Catalyst preparation
c
-Al
2
O3
Concentrated HCl, paraformaldehyde, natural kaolinite, HY and
-Al O catalysts were purchased from Sinopharm. o-Tert-butylan-
2 3
30P/SBA-15
50
c
a
iline (Aldrich) and butyl acetate (Qiangsheng Chemical) were used
as commercial products without further treatment.
Reaction conditions: o-tert-butylaniline (4 mmol), paraformaldehyde (1 mmol),
catalyst (0.18 g), in butyl acetate (3 ml) at 130 °C under N
2
for 4 h.
b
Isolated yields.
Mesoporous SBA-15 was synthesized via a surfactant-templat-
ing method in acid condition, according to [27]. The synthesized
product was then calcined at 550 °C in air for 4 h. P/SBA-15 was
prepared by incipient impregnation method. A certain amount of
phosphoric acid and 1 g SBA-15 were stirred with 4 ml absolute
ethanol solution at room temperature for 6 h. Then, solvent was
evaporated and the solid was dried at 110 °C for 6 h and calcined
at 400 °C for 4 h in air. The catalysts are denoted as xP/SBA-15,
where x represents phosphoric acid concentration.
distance of natural kaolinite and the pore diameter of HY are smal-
ler than 1 nm, the steric effect of t-butyl group on benzene ring
should be responsible for the inferior performance of kaolinite
and HY in the synthesis of MBTBA.
tested for the condensation of o-tert-butylaniline and paraformal-
dehyde and the results are presented in Table 1. -Al was gen-
2 3
c-Al O and P/SBA-15 were also
c
2 3
O
erally regarded as a heterogeneous Lewis acid catalyst. The yield of
the desired product was only 2%, which indicated that the reaction
was not catalyzed by Lewis acidic centers. Comparing with the
above catalysts, P/SBA-15 catalyst exhibited high activity in the
MBTBA synthesis. The yield was even higher than that of reaction
catalyzed by HCl.
The catalytic performance for P/SBA-15 catalysts under differ-
ent preparation conditions was tested and the results are listed
in Table 2. As can be seen, the yield of MBTBA increased moder-
ately with the increase of the concentration of phosphoric acid
on SBA-15 and the highest yield of 50% was achieved at 30 wt.%
phosphoric acid concentration. The increase of acid sites on the
catalysts should be the reason for the improvement of the catalytic
activity. The further increase of phosphoric acid loading to 40 wt.%
led to the distinct drop in MBTBA yield. This decrease might be due
to the blockage of pores and the collapse of structure of SBA-15
which was caused by the overweight of phosphoric acid. In addi-
tion, the calcination temperature of 30P/SBA-15 should be adjusted
to obtain the best catalytic performance. At the low temperature
2.2. Catalyst characterization
The X-ray diffraction (XRD) measurements were taken on a
RINT2000 vertical goniometer using Cu K
00 mA, 0.5°–8°).
adsorption–desorption isotherm was measured at 196 °C
a radiation (40 kV,
2
N
2
with Quantachrome NOVA instrument. The Brunauer–Emmett–
Teller (BET) method was utilized to calculate the specific surface
areas. By using the Barrett–Joyner–Halenda (BJH) model, the pore
volumes and pore size distributions were derived from the adsorp-
tion branches of isotherms.
FTIR spectra of adsorbed pyridine were collected on a Nicolet
–
1
Magna 550 Fourier transform infrared spectrometer at 4 cm
resolution. The sample was finely grounded and pressed into a
self-supporting wafer, and then placed into the measurement cell
with CaF windows. The evacuation at 300 °C for 2 h was carried
2
out prior to adsorption of pyridine at room temperature. IR spectra
were recorded after subsequent evacuation at increasing tempera-
tures from 50 to 300 °C.
(
300 °C), the drop of MBTBA yield (19%) might be caused by the
weak interaction between phosphoric acid and SBA-15 which re-
sulted in the leakage of surface acidity of the catalyst. At the high
temperature (500 °C), low yield was obtained (29%), which should
account for the decomposition of phosphoric acid and the follow-
ing decrease of surface acidity.
The effect of different reaction conditions for the condensation
reaction of o-tert-butylaniline and paraformaldehyde is given in
Table 3. Butyl acetate was the most effective solvent, whereas
2.3. Catalyst activity measurement
A mixture of o-tert-butylaniline (4 mmol), paraformaldehyde
(
1
1 mmol), catalyst (0.18 g) butyl acetate (3 ml) was stirred at
30 °C for 4 h under N atmosphere. After cooling to room temper-
2
ature, the catalyst was filtered off and the organic layer was
evaporated. The crude product was subjected to column chroma-
tography on silica gel with petroleum ether/EtOAc as eluent.
1
13
Isolated products were determined by H NMR and C NMR spec-
troscopes. Yield of MBTBA was calculated based on the input of
paraformaldehyde.
Table 2
a
Optimization of catalyst for synthesis of MBTBA.
Entry
P concentration (wt.%)
Calcination temperature (°C)
Yield (%)b
1
2
3
4
5
6
10
20
30
40
30
30
400
400
400
400
300
500
42
48
50
35
19
29
3
. Results and discussion
In literature, zeolite and kaolinitic clay [13,14] behaved as
0
efficient catalysts in the synthesis of 4,4 -diaminodiphenylme-
thanes. However, both of them showed very low activities in the
reaction of condensation of o-tert-butylaniline and paraformalde-
hyde, as shown in Table 1. Since the molecular sizes of o-butylan-
iline and MBTBA reach ca. 1.1 nm and 1.5 nm, while the layer
a
Reaction conditions: o-tert-butylaniline (4 mmol), paraformaldehyde (1 mmol),
catalyst (0.18 g), in butyl acetate (3 ml) at 130 °C under N2 for 4 h.
b
Isolated yields.

4
40
Q. Xu et al. / Catalysis Communications 11 (2010) 438–441
Table 3
Effect of reaction conditions on synthesis of MBTBA over 30P/SBA-15.
higher angles, were exhibited when the phosphoric acid loading
didn’t exceed 30 wt.%. It clearly indicated that the mesoporous
SBA-15 was stable and the highly ordered mesostructure was re-
tained after the appropriate impregnation of phosphoric acid.
However, 40 wt.% of phosphoric acid loading on the catalyst led
to the disappearance of all the diffraction peaks, which suggested
the damage of the mesostructure and comfirmed that a high
amount of phosphoric acid led to a decrease of activity due to
structure damage.
Entry Solvent
Mole ratio of o-
butylaniline to
Reaction
temperature loading
Catalyst Yield
a
(%)
paraformaldehyde
(°C)
(g)
1
1,4-
4:1
130
0.18
35
dioxane
2
3
Acetonitrile 4:1
130
130
0.18
0.18
45
40
Butyl
5:1
4:1
3:1
2:1
4:1
4:1
4:1
4:1
4:1
4:1
acetate
Butyl
4
5
6
7
8
9
1
1
1
130
130
130
90
0.18
0.18
0.18
0.18
0.18
0.18
0.09
0.12
0.24
50
47
35
26
32
45
35
45
19
Fig. 2 shows the result of N
2
adsorption–desorption isotherms
acetate
Butyl
and pore size distribution of 30P/SBA-15. The type IV isotherm
has the pore filling (by capillary condensation) restricted to a nar-
row range of the relative pressures of 0.5–0.9, which is a typical
acetate
Butyl
acetate
Butyl
2
feature of SBA-15. Both XRD and N adsorption results showed that
the uniform framework and pore structure of SBA-15 could still be
well maintained after the external introduction of phosphorus spe-
cies onto its pore surface. As shown in Fig. 2, the pore size distribu-
tion of SBA-15 is very sharp, indicative of uniform mesopore with
the size of 6.5 nm which is similar to that of SBA-15 before phos-
phoric acid modification. The pore diameter of 30P/SBA-15 is much
larger than the sizes of substrate and product, which allows the
adequate access and rapid diffusion of these molecules to the cat-
alytic centers. On the contrary, the layer distance of natural kaolin-
ite and the pore diameter of HY are smaller than 1 nm. Therefore,
the steric effect of t-butyl group on benzene ring should be respon-
sible for the inferior performance of kaolinite and HY in the synthe-
sis of MBTBA. And the ordered mesoporous structure of 30P/SBA-
15 should be one of the reasons that this material exhibits the
higher activity than HY and kaolinite.
acetate
Butyl
110
140
130
130
130
acetate
Butyl
acetate
Butyl
0
1
2
acetate
Butyl
acetate
Butyl
acetate
a
Isolated yields.
1
,4-dioxane and acetonitrile gave lower yield of MBTBA. When the
mole ratio of o-tert-butylaniline to paraformaldehyde increased
from 2:1 to 4:1, the yield of MBTBA increased from 35 to 50%,
which might be due to the reduction of the possibility of the reac-
tion of paraformaldehyde with product. Further increase of this ra-
tio decreased the product yield, which should be the result of small
amount of paraformaldehyde in reaction, similar to the results ob-
served by Ajaikumar [28]. With regard to reaction temperature, the
highest yield was achieved at 130 °C, since paraformaldehyde can-
not decompose completely at lower temperature [29]. The opti-
mized catalyst loading was 0.18 g, because the catalytic centers
were not enough and the reaction proceeded slowly at lower cata-
lyst loading, while a large quantity of by-products formed with
excessive P/SBA-15.
Py-FTIR analysis was used to discriminate acid type and
strength on the surface of 30P/SBA-15. Fig. 3 shows the evolution
of the IR spectra of 30P/SBA-15 with temperature during the evac-
uation after adsorption of pyridine of the catalyst. As revealed, the
ꢀ1
bands at 1450, 1545 and 1490 cm
(representing Lewis acid,
Brønsted acid and both, respectively [30]) were observed for the
catalyst at desorption temperature of pyridine of 50 °C, which sug-
gested that both Brønsted and Lewis acid sites existed on the phos-
phoric acid modified SBA-15 catalyst. However, all these bands
decreased sharply when the evacuation temperature rose to
100 °C and almost disappeared after the evacuation at 200 °C,
which indicated that the strength of the acid sites were weak on
the surface of 30P/SBA-15. As a consequence, the acid sites with
weak strength were supposed to be the catalytic centers for the
Fig. 1 shows the X-ray diffraction patterns of P/SBA-15 catalysts.
Representative small-angle XRD patterns of the two-dimensional
(
2-D) hexagonal mesostructure, namely, one strong (1 0 0) diffrac-
condensation
reaction
of
o-tert-butylaniline
and
tion at 2h = 0.5–1.5° and two weak peaks ((1 1 0) and (2 0 0)) at
paraformaldehyde.
7
6
5
4
3
2
1
00
00
00
00
00
00
00
1
00
4000 6.5
500
000
500
000
3
3
2
2
1500
1000
1
10
2
10
SBA-15
500
0
0
10
20
30
40
50
60
pore diameter (nm)
1
0%P/SBA-15
2
0%P/SBA-15
3
0%P/SBA-15
4
0%P/SBA-15
3
0.2
0.4
0.6
0.8
1.0
1
2
4
Relative pressure (P/P0)
2
theta (degree)
Fig. 2. Nitrogen adsorption–desorption isotherms and pore size distribution of 30
Fig. 1. XRD patterns of xP/SBA-15.
P/SBA-15.

Q. Xu et al. / Catalysis Communications 11 (2010) 438–441
441
1
5. Therefore, the excellent performance of P/SBA-15 in the syn-
1450
1490
thesis of MBTBA should be attributed to the weak acidity of the
material and its large pore size.
5
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In summary, P/SBA-15 was found to be a highly active heteroge-
neous catalyst for o-tert-butylaniline condensation with parafor-
maldehyde. The optimized conditions for the catalyst preparation
were phosphoric acid concentration of 30 wt.% and calcination
temperature of 400 °C. Butyl acetate was found to be the best sol-
vent and the mole ratio of o-tert-butylaniline to paraformaldehyde
should be 4:1. Under the reaction temperature of 130 °C and the
catalyst weight of 0.18 g, the yield of MBTBA reached 50%, which
was even higher than that in homogeneous reaction in presence
of HCl, and the further improvement of the efficiency of this meth-
od is under the way. The sorption isotherm of P/SBA-15 indicated
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[
[
[