Halogenated macroporous sulfonic resins as efficient catalysts for the Biginelli reaction

Article abstract of DOI:10.1016/j.catcom.2016.01.010

A series of halogenated macroporous sulfonic resins A-15-Cl, A-15-Br and A-15-I were synthesized from the precursor Amberlyst 15 by a typical halogenation reaction, and they were evaluated for the catalytic activities of the halogenated macroporous sulfon

Full text of DOI:10.1016/j.catcom.2016.01.010

Catalysis Communications 77 (2016) 18–21
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Halogenated macroporous sulfonic resins as efficient catalysts for the
Biginelli reaction
⁎
Pengfei Shen, Mancai Xu , Dulin Yin, Shaoan Xie, Chan Zhou, Fada Li
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of halogenated macroporous sulfonic resins A-15-Cl, A-15-Br and A-15-I were synthesized from the pre-
cursor Amberlyst 15 by a typical halogenation reaction, and they were evaluated for the catalytic activities of the
halogenated macroporous sulfonic resins via the Biginelli reaction in detail. These modified resins possessed a
larger contact angle of water droplet than the precursor and a significantly improved thermal stability, which
contributed to their higher catalytic activity. In particular, the resin A-15-Br containing 31.27% bromine exhibited
the best catalytic activity and excellent recyclability.
Received 27 October 2015
Received in revised form 7 January 2016
Accepted 8 January 2016
Available online 12 January 2016
Keywords:
© 2016 Elsevier B.V. All rights reserved.
Amberlyst 15
Halogenation
Biginelli reaction
1. Introduction
increasing the hydrophobicity of the sulfonic resins [13], and the
hydrophobic interaction of the gel cavities in the sulfonic resins may
Macroporous sulfonic resins are extensively used as catalysts in
various chemical reactions because of their high catalytic activity, low
toxicity, low cost and easy separation from the reaction system [1–4].
In general, they are sulfonated from the macroporous poly (styrene-
co-divinylbenzene) using concentrated sulfuric acid as the sulfonating
agent [5–8]. These resins can also be used as catalysts in aqueous or
nonaqueous medium, where the catalytic mechanisms differ consider-
ably [9,10]. In an aqueous medium, sulfonic groups of these resins are
completely dissociated as sulfonate anions and protons, where the latter
serve as active sites for the catalytic reaction. In a nonaqueous medium,
several clusters are fabricated by intermolecular hydrogen bonds
between the sulfonic groups on the polymer chains. Moreover, the clus-
ters have a higher catalytic activity and better catalytic stability than the
individual sulfonic groups because of the direct contact with the clusters
due to the adsorption effect [11]. Of course, water is formed as the by-
product for some special dehydration reactions such as condensation
reaction, which not only affects chemical equilibrium of the reaction,
but also causes damage to the sulfonic clusters in the polymer chains,
thereby decreasing the catalytic performance of the sulfonic resins.
Recently, Stoerzinger et al. [12] have reported that intrinsic hydro-
phobicity of the catalysts should be considered in the design of highly
active catalysts. They utilized the flexibility of the perovskite surface
chemistry to show that the tendency of the perovskite surface toward
hydroxylation was propitious to wetting and adverse to catalysis.
Halogens are proved to be the feasible hydrophobic groups for
be enhanced as some special halogenated groups are introduced in
the polymer chains. In particular, the sulfonic clusters are more stable
because of the inductive effect of halogenated groups, which enhances
the catalytic performance of the sulfonic resins.
In this study, the precursor Amberlyst 15 was halogenated by differ-
ent compounds (Cl2, Br2, and I2), and a series of halogenated
macroporous sulfonic resins A-15-X (X = Cl, Br, or I) was synthesized.
The Biginelli reaction was then selected as a model reaction to evaluate
the catalytic performance of the halogenated macroporous sulfonic
resins. The Biginelli reaction is well known for the synthesis of
dihydropyrimidinones (DHPMs) [14–16]. It generally involves one-pot
condensation of an aldehyde, β-diketonate, urea, or thiourea using
catalysts [17–22], and water is produced as the by-product in this
reaction. It is found that the Biginelli reaction catalyzed by sulfonic
resins produces a relatively low yield, and the catalyst exhibits poor
recyclability [23–25].
2. Experimental
2.1. Materials
Amberlyst 15 was supplied by Rohm and Haas Shanghai Chemical
Industry Co., Ltd. and all other reagents were purchased from Aladdin
Reagent Co., Ltd. (Shanghai), and used as received. Nitrogen adsorption
experiments were conducted using a TriStar 3000 surface area and
porosity analyzer. A TX 500 H spinning drop interfacial tensiometer
was used to measure the contact angle of water in air on the surface
of the sample.
⁎
Corresponding author at: College of Chemistry and Chemical Engineering, Hunan
Normal University, Changsha, Hunan 410081, PR China.
E-mail address: romann@hunnu.edu.cn (M. Xu).
http://dx.doi.org/10.1016/j.catcom.2016.01.010
1566-7367/© 2016 Elsevier B.V. All rights reserved.

P. Shen et al. / Catalysis Communications 77 (2016) 18–21
19
Scheme 1. Synthetic procedure for producing A-15-X.
2.2. Halogenation of Amberlyst 15
As shown in Scheme 1, the synthetic procedure for producing A-15-
X was performed according to the method described in Refs. [26,27].
Amberlyst 15 (50 g) and water (80 mL) were placed in an agitated reac-
tion vessel. Halogen (30 g) was mixed with the agitated suspension of
the resins and the mixture was heated to 50 °C shielded from light
and agitated at this temperature for different reaction times. The solid
product was filtered and washed by a large amount of distilled water.
Fig. 1. Water droplet contact angle on Amberlyst 15, A-(15), and A-15-X.
polymer chains. This might be attributed to the following two reasons:
(1) the mass increase of the modified resins after introduction of the
halogenated groups and (2) the hydrolysis of the sulfonic groups in
the acidic aqueous solution. In addition, it is observed that the BET
surface area, pore volume, and average pore size of the modified resins
are similar to those of the precursor, implying that halogenation reac-
tion has almost no effect on pore structure. Although the BET surface
area and pore volume decreased slightly, theoretically, they increased
in the absence of the mass of halogen, which may be attributed to the
hydrophilicity and hydrophobicity of the halogenated sulfonic resins.
After introduction of halogens on the surface of the sulfonic resins, a
small amount of sulfonic groups will be removed during the halogena-
tion reaction, and hence the hydrophilicity and hydrophobicity of the
modified sulfonic resins would be slightly changed. Therefore, a sulfonic
3. Results and discussion
3.1. Halogenation of Amberlyst 15
It is well known that halogenation of Amberlyst 15 follows the
typical electrophilic substitution mechanism, which was similar to the
halogenation of aromatic compounds such as benzene and toluene
[28,29]. Furthermore, the halogen content of the modified resins (A-
15-Cl and A-15-Br) was very low. It is interesting to observe that the
synthesis of A-15-Cl and A-15-Br was successful in aqueous medium,
and the electrophile X+ can be easily formed and has a small ion radius.
Furthermore, the sulfonic groups dissociate as sulfonate anions and
protons, which have a smaller hindrance for X+. However, synthesis
of A-15-I requires an acidic oxidizing reagent, such as nitric acid,
because of the chemical inertness of iodine.
resin with a smaller cation exchange capacity, A-(15) (Qv
=
1.02 mmol/mL), was prepared by the desulfonation of Amberlyst 15.
Moreover, the number of halogenated and sulfonic groups is shown to
be the primary deciding factor of the hydrophilicity and hydrophobicity
of sulfonic resins. Hence, halogenated resins having a similar molar ratio
(~1.6: 1) of halogenated groups to sulfonic groups were chosen as the
samples to measure the contact angle of water droplet. Fig. 1 shows
the results of Amberlyst 15, A-(15), A-15-Cl-2, A-15-Br-4, and A-15-I-
2 samples, respectively. It is evident from the figure that A-15-Cl-2, A-
15-Br-4, and A-15-I-2 have higher contact angles, indicating their
increased hydrophobic surface. As a result, hydrophobicity of the
macroporous sulfonic resins was enhanced by the introduction of
3.2. Performance analysis of the catalysts
The halogen content (WX), mass exchange capacity (Qm), volume
exchange capacity (Qv) of the resins were measured using procedures
reported in Refs. [30,31], whose results are summarized in Table 1. It
is found that the exchange capacity of the halogenated resins is
decreased after introduction of halogens in the benzene ring of the
Table 1
Characteristics of the sulfonic resins.
BET surface area (m2/g) Pore volume (cm3/g) Average pore size (nm)
a
Sample
Time (h) Cation exchange capacity
m (mmol/g) Qv (mmol/mL)
Halogen content (%) n(−X): n(−SO3H)
Q
A-15
A-15-Br-1
A-15-Br-2
A-15-Br-3 24
A-15-Br-4 36
A-15-Br-5 48
A-15-Br-6 72
A-15-Cl-1
A-15-Cl-2
0
2
8
4.30
3.58
3.13
2.72
2.45
2.32
1.97
3.41
3.10
2.98
2.35
1.91
1.73
1.50
1.39
1.35
1.12
1.08
0.98
0.64
1.30
0.95
0.88
1.20
0.94
0.72
0
–
38.5
–
–
–
34.5
–
–
–
37.7
–
–
0.246
–
–
–
0.208
–
–
–
0.218
–
–
0.183
–
25.7
–
–
–
24.1
–
–
–
23.2
–
–
10.03
16.20
26.79
31.27
34.94
38.98
11.35
17.90
21.50
30.58
39.69
42.36
0.35: 1
0.65: 1
1.23: 1
1.60: 1
1.88: 1
2.47: 1
0.93: 1
1.63: 1
2.03: 1
1.02: 1
1.63: 1
1.93: 1
2
6
A-15-Cl-3 12
A-15-I-1
A-15-I-2
A-15-I-3
48
72
96
29.4
–
24.8
–
a
Molar ratio of halogenated groups to sulfonic groups.

20
P. Shen et al. / Catalysis Communications 77 (2016) 18–21
Scheme 2. Synthesis of DHPMs catalyzed by A-15-X.
3.3. Catalytic activity of the catalysts toward the Biginelli reaction
The Biginelli reaction was chosen as a model reaction to test the cat-
alytic activity of A-15-X. In order to evaluate the catalytic effect of vari-
ous sulfonic resins, we used A-15-X (0.5 mmol of acid sites) to catalyze
the model reaction of benzaldehyde (5.0 mmol), ethylacetoacetate
(5.0 mmol), and urea (5.0 mmol) in refluxing acetonitrile to afford
compound 4b (Scheme 2).
It can be observed from Table 2 that A-15-Br containing 31.27%
bromine shows an excellent catalytic performance, and as bromine
content increases further, the yield of compound 4b has no obvious
change. The catalytic activity of the modified resins is better than that
of the precursor and follows the order: A-15-Br N A-15-Cl ≈ A-15-
I N Amberlyst 15. With regard to the inductive effect of chlorinated
groups [32,33], A-15-Cl should display the best catalytic performance;
by contrast, it is not as good as that of A-15-Br, which might be attribut-
ed to the fact that poor hydrophobicity of chlorinated groups cannot ob-
viously improve catalytic activity of A-15-Cl. In addition, hydrophobicity
of iodinated groups is excellent, whereas electronegativity of iodine is
low.
We extended our study to different combinations of β-diketonates,
aldehydes, and ureas or thioureas, whose results are summarized in
Table 3. It is evident from the table that several β-diketonates reacted
very well and had no noticeable effect on the reaction, and a variety of
substituted aldehydes reacted in the presence of A-15-Br-4. Although
the reaction proceeded smoothly to produce the corresponding
DHPMs with moderate yields, there were some differences between
these reactions. Aromatic aldehydes carrying electron-donating substit-
uents, on the whole, reacted better than those carrying electron-
withdrawing substituents (Table 3, entries 6 and 9–17). Unfortunately,
steric hindrance was unfavorable for the process (Table 3, entries 7, 8,
11, and 15). A-15-Br-4 exhibited a general catalytic effect where the
reactions involved aliphatic aldehydes (Table 3, entries 18 and 19). In
addition, ureas reacted better than thioureas, particularly methyl
substituted urea (Table 3, entries 20 and 21).
Fig. 2. Ratios (χ) of the halogen content at different heating times.
halogen in the polymer chains, and A-15-Br-4 and A-15-I-2 showed
similar hydrophobicities.
The dehalogenating and desulfonating tendencies of A-15-Cl, A-15-
Br, and A-15-I were measured under a pressure of approximately
8 atm and temperature of 170 °C in water, and the effect of heating
time on the ratios (χ) of the halogen content after heating to the original
content was studied and the results are shown in Fig. 2. It is obvious that
the C–X bond has excellent stability for all halogenated resins. In
addition, it is clear that a lower bromine content of the modified resin
is more stabile, which may relate to the distribution of brominated
groups. With an increase of bromine content, the brominated groups
that are distributed close to the particle surface can be removed easily.
Fig. 3 presents the ratios (ψ) of the cation exchange capacity after
heating to the original content in the resins at different time points.
Notably, A-15-X possesses a higher thermal stability than the precursor
Amberlyst 15. Moreover, it is clear that the stability of the sulfonic
groups is significantly improved with the increase of bromine content,
and A-15-Br with a bromine content of 31.27% shows the best thermal
stability. However, its thermal stability shows a downward trend as
the resins continue to be brominated, because debromination of the
resins increases acidity of water, which promoted desulfonation of the
resin via an aromatic electrophilic substitution reaction.
Table 2
Catalytic activity of different resins for the synthesis of compound 4b.
Entry
Catalyst
Amount of catalyst a (g)
Time (h)
Yield b (%)
1
2
3
4
5
6
7
8
A-15-Br-1
A-15-Br-2
A-15-Br-3
A-15-Br-4
A-15-Br-5
A-15-Br-6
A-15
A-15-Cl-1
A-15-Cl-2
A-15-Cl-3
A-15-I-1
A-15-I-2
A-15-I-3
0.14
0.16
0.18
0.20
0.22
0.25
0.12
0.15
0.17
0.18
0.21
0.26
0.29
2
2
2
2
2
2
2
2
2
2
2
2
2
77
82
88
92
93
92
57
63
77
79
66
75
73
9
10
11
12
13
a
Concentration of acid sites in all resins was 0.5 mmol.
Isolated yields.
b
Fig. 3. Ratios (ψ) of the cation exchange capacities at different heating times.

P. Shen et al. / Catalysis Communications 77 (2016) 18–21
21
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Table 3
A-15-Br-4 resin-catalyzed synthesis of compound 4b.
Entry R1
R2
R3
X
Product Time (h) Yield a (%)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
OMe
OEt
O-i-Pr
O-t-Bu
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
S
4a
4b
4c
4d
4e
4f
2
2
90
92
90
89
85
88
85
80
90
90
83
88
82
82
83
95
90
78
80
83
98
2.5
2.5
2
2.5
3
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Ph
4-OMe-Ph
OEt
4-OH-3-OMe-Ph OEt
4g
4h
4i
3,4,5-triOMe-Ph
4-Me-Ph
4-Cl-Ph
2-Cl-Ph
4-F-Ph
3-NO2-Ph
4-NO2-Ph
2-OH-Ph
3-OH-Ph
4-OH-Ph
Me
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
5
2
9
10
11
12
13
14
15
16
17
18
19
20
21
4j
2
4k
4l
2
2
4m
4n
4o
4p
4q
4r
2
2
3
2.5
2.5
4
Et
Ph
Ph
4s
4t
4u
4
3.5
1
O
a
Isolated yields.
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Table 4
Recycling of the catalyst used for the synthesis of compound 4b.
Cycles Time Yield a Catalyst
Cation exchange
Bromine
(h)
(%)
recovered (%) capacities Qm (mmol/g) content (%)
Native
2
2
2
2
2
92
92
90
89
88
99
98
97
95
92
2.45
–
–
–
2.32
31.27
–
–
–
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31.12
a
Isolated yields.
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.01.010.
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