Synthesis of a novel melamine-formaldehyde resin-supported ionic liquid with Bronsted acid sites and its catalytic activities

Article abstract of DOI:10.1007/s00706-013-1022-7

Bronsted acidic ionic liquid immobilized on a melamine-formaldehyde resin (AIL-MFR) was synthesized through the reaction of melamine-formaldehyde resin (MFR) with 1,4-butanesulfonate. Using PEG-2000 as the additive, the MFR can be prepared in regular microspheres with an average diameter of 3.97 μm and surface area of 9.09 m² g^-1. The AIL-MFR had high acidity of 2.93 mmol g^-1, mainly from the sulfonic groups. The catalysis results showed that the AIL-MFR had high activity and stability for acetalization with excellent conversions and yields for most substrates. Furthermore, immobilization of the acidic ionic liquid on the MFR made the recycling of the catalyst convenient.

Full text of DOI:10.1007/s00706-013-1022-7

Monatsh Chem (2013) 144:1369–1374
DOI 10.1007/s00706-013-1022-7
ORIGINAL PAPER
Synthesis of a novel melamine-formaldehyde resin-supported ionic
liquid with Brønsted acid sites and its catalytic activities
Guiying Xing
Received: 5 October 2012 / Accepted: 16 May 2013 / Published online: 12 June 2013
Ó Springer-Verlag Wien 2013
Abstract Brønsted acidic ionic liquid immobilized on a
melamine-formaldehyde resin (AIL-MFR) was synthesized
through the reaction of melamine-formaldehyde resin (MFR)
with 1,4-butanesulfonate. Using PEG-2000 as the additive,
the MFR can be prepared in regular microspheres with an
ILs in catalytic technologies for chemical industries [3, 4].
The alkane sulfonic acid group function is an efficient way to
synthesize acidic ILs and was first reported in 2002 by Cole
et al. [5]. So far, a number of sulfonic group functionalized
ionic liquids have been developed for various applications
[6–8]. Recently, Liang et al. [9, 10] reported that they had
synthesized two acidic ionic liquids, which were very effi-
cient for the acetalization and synthesis of biodiesel.
Recently, heterogenization of homogeneous catalysts
and reagents has been designed to increase the overall
productivity and cost effectiveness [11]. Moreover, the
heterogenization can also offer important advantages in
handling, separation, and recycling procedures. Therefore,
it is desirable to immobilize the ILs on solid supports to
minimize the amount of ILs utilized in a potential process.
Immobilized acidic ILs have the advantages of both
homogeneous ILs and heterogeneous catalysts, which
made the separation, recovery, and recycling of the catalyst
convenient. Some literature has reported the applications of
immobilized ILs in acetalization, esterification, and nitra-
tion [12–14].
2
-1
average diameterof3.97 lm and surface areaof9.09 m g
.
-1
The AIL-MFR had high acidity of 2.93 mmol g , mainly
from the sulfonic groups. The catalysis results showed that the
AIL-MFRhad highactivity and stability foracetalization with
excellent conversions and yields for most substrates. Fur-
thermore, immobilization of the acidic ionic liquid on the
MFR made the recycling of the catalyst convenient.
Keywords Acidity Á Heterogeneous catalysis Á
Aldehydes Á Green chemistry
Introduction
Ionic liquids (ILs) have received much attention as envi-
ronmentally benign media for catalytic processes or
chemical extraction [1, 2]. In contrast with conventional
organic and inorganic solvents, ILs have negligible vapor
pressure, excellent thermal stability, and special character-
istics. Endowed with a catalytic activity center, the
applications of ILs could be further expanded as both the
solvent and catalysts. One type of expanded IL is the acidic
IL. The acidic nature of these ILs has been explored for many
important organic reactions with excellent conversions and
selectivities, which demonstrated the great potential of these
In this article, melamine-formaldehyde resin (MFR) was
first used to immobilize Brønsted acidic ionic liquid
(Scheme 1). The catalytic performance of this heteroge-
neous catalyst was carefully investigated by acetalization.
The results showed that the immobilized acidic ionic liquid
catalyst was very efficient for the acetalization.
Results and discussion
Characterization of the AIL-MFR
G. Xing (&)
School of Medical Science, Shaoxing University, Shaoxing,
Zhejiang Province 312000, People’s Republic of China
e-mail: 312087578@qq.com
MFR is a type of crosslinked polymer with excellent
physical and chemical stability [15, 16]. It was found that
123

1
370
G. Xing
Scheme 1
N
N
N
N
N
N
C
C
O
O
H
H
2
2
N
N
N
N
N
N
3
HCHO
S
O
HSO4
1:
OH
+
S
CH2
N
O
CH2
N
O
H N
2
N
NH2
2: H SO4
2
HSO4
OH
N
N
N
N
H2
C
N
N
S
NH2
O
N
CH2
O
N
Fig. 1 SEM images of MF
resin (a), without addition of
PEG-2000 and AIL-MFR (b)
-
1
thermal treatment at 200 °C could further increase the
chemical stability of MFR. Moreover, there are plenty of
binding sites (N atom) in MFR that could react with 1,4-
butanesulfonate to introduce the sulfonic group. Deionized
water was first employed as the reaction media in the
synthesis of MFR. Unfortunately, the synthesized resin was
too hard to be fabricated into particles. It was found that the
addition of PEG-2000 could facilitate the preparation of
MFR particles. As shown in Fig. 1, compared with the
sample without the addition of PEG-2000, addition of
PEG-2000 could promote the formation of regular micro-
spheres of MFR. Besides the ease of handling, the surface
The acidity of the AIL-MFR was 2.93 mmol g , which
was determined through the neutralization titration [17].
The novel AIL-MFR had much higher acidity than the
common heterogeneous acid catalysts, such as Nafion and
-
1
Amberlyst-15 (0.8 mmol g ) [18]. The elemental analysis
gave the results: N: 41.22 %; C: 24.19 %; H: 4.44 %; S:
8.33 %. The S content demonstrated that the acidity of
-
1
AIL-MFR was 2.60 mmol g , indicating that the acidity
of AIL-MFR was mainly from the sulfonic groups. The
result of the thermal gravimetric analysis (TGA) of AIL-
MFR is shown in Fig. 3. The results indicate that AIL-
MFR was stable at temperatures below 300 °C, but
decomposed rapidly at temperatures between 300 and
650 °C. Thus, the AIL-MFR was stable at temperatures
below 300 °C.
2
-1
area of MFR was also increased to 9.09 m g from the
2
-1
original 1.72 m g after the addition of PEG-2000. As
we know, the larger surface area can increase the contact
area between the catalytic centers and substrates, which
would in turn increase the catalytic activity of the catalyst.
The IR spectra of the MFR and AIL-MFR are shown in
Fig. 2. The appearance of absorbance at 1,090 and
The catalytic activities of AIL-MFR
for the acetalization
-
1
1
,057 cm (due to S=O) confirmed the existence of the
Various carbonyl compounds and diols have been used as
substrates for the acetalization over the AIL-MFR catalyst.
The results in Table 1 clearly demonstrate that the AIL-
MFR was very efficient for the acetalization reaction.
Aliphatic aldehydes were transformed to the corresponding
acetals smoothly with excellent yields (Table 1, entries 1,
sulfonic groups. Also, it was found that the intensity of the
-
adsorption peak at 1,512 cm , attributed to –CH –, was
1
2
increased. All these spectroscopic results demonstrated that
the ionic liquid had been successfully introduced into the
MFR.
1
23

Synthesis of a novel melamine-formaldehyde
1371
The recycling of the catalyst
MFR
Recycling experiments were carried out to investigate the
stability of the AIL-MFR catalyst. In each cycle, the cat-
alyst was recovered by filtration and then reused directly in
the next run without any more treatment. The recovered
activity of the AIL-MFR catalyst was investigated through
the acetalization of cyclohexanone and 1,2-ethanediol. The
conversions and yields remained unchanged even after the
catalyst had been recycled seven times, which confirmed
the excellent stability of the catalyst (Fig. 4). The acidity of
AIL-MFR
-
1
the recovered catalyst was 2.06 mmol g after recycling
seven times, which clearly demonstrated that the loss of the
acid site was not significant in the reaction.
-
CH2-
S=O S=O
2000
1800
1600
1400
1200
1000
800
-1
Wavelength/cm
Fig. 2 FT-IR spectra of MFR and AIL-MFR
The comparison of catalytic activities
Compared with the homogenous catalyst, the reduction of
catalytic activity of the heterogeneous catalyst is associated
with the disfavored kinetics of the biphasic catalytic system
100
[
12–14, 19, 20]. A comparative study of the catalytic
activity of the AIL-MFR with traditional homogeneous
catalysts was carried out using the acetalization of cyclo-
hexanone and 1,2-ethanediol. As shown in Table 2, the
conversions and yields showed that the catalytic activity of
AIL-MFR was only a little lower than that of the homo-
geneous catalysts. However, the values of TOFs showed
that AIL-MFR was very efficient for the acetalization.
Moreover, it can be recovered and reused conveniently,
which could increase the overall productivity and cost
effectiveness.
8
0
0
0
0
6
4
2
0
0
100
200
300
400
o
500
600
700
Temperature/ C
Conclusion
Fig. 3 TGA curve of the AIL-MFR catalyst
2
, 10, 11). As the seven-membered ring was not as stable as
A novel melamine-formaldehyde resin-supported ionic
liquid with Brønsted acid sites has been synthesized, and its
catalytic activity toward acetalization was carefully
investigated. The results showed that the catalyst was
efficient for the acetalization reaction. The supported cat-
alyst can be easily recovered and reused without any
considerable loss of their initial activity. The novel
advantages of excellent catalytic performance and easy
recycling gave the catalyst great potential for applications
in the chemical industry.
the five- or six-membered ring, a prolonging reaction time
was required to obtain satisfactory yields for the acetal-
ization of 1,4-butanediol (Table 1, entries 3, 12). Because
the aromatic structure reduced the reactivities of the
attached carbonyl groups, benzaldehyde was acetalized to
corresponding acetals with moderate yields (Table 1,
entries 10–12). Acetalization of ketone was mainly influ-
enced by the steric hindrance. As the linear chain ketone
had relatively high steric hindrance and low reactivity for
the acetalization, acetalization of butanone needed 4 h to
complete the reaction (Table 1, entries 4–6). However,
cyclohexanone worked very well with high conversions
and yields for the low steric hindrance of carbonyl group
Experimental
(
Table 1, entries 7–9). As the five- and six-membered ring
All organic reagents were commercial products of the
highest purity available and were used for the reactions
without further purification. Melamine, formaldehyde,
1,4-butanesulfonate, cyclohexanone, and butanone were
was more stable than the seven-membered ring, the yields
generally decreased as follows: 2,2-dimethyl-1,3-propane-
diol [ 1,2-ethanediol [ 1,4-butanediol.
123

1
372
G. Xing
Table 1 The acetalization of various carbonyl compounds and diols
Entry
1
Substrate
Product
Reaction time/h
2
Conversion/%
92
Yield/%
90
O
Cl
CHO
CHO
Cl
O
2
3
2
99
99
O
Cl
Cl
Cl
Cl
O
2
4
77
99
69
99
O
CHO
O
4
5
4
4
95
98
95
96
O
O
O
O
O
O
O
6
4
70
69
O
O
7
8
2
2
99
99
98
98
O
O
O
O
O
O
9
2
94
92
O
O
O
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Synthesis of a novel melamine-formaldehyde
1373
Table 1 continued
Entry
Substrate
Product
Reaction time/h
2
Conversion/%
85
Yield/%
85
1
0
1
O
O
CHO
CHO
1
2
4
89
92
85
91
O
O
1
2
O
CHO
O
purchased from Shanghai Chemical Co. GC measurements
were carried out on a Shimadzu (GC-14B) gas chromato-
graph. The IR was analyzed on the NEXUS 670 FT-IR
from Nicolet Corp., USA. The elemental analysis was
made on the EuroEA 3000 from Leeman, USA. The sur-
face area was measured by TriStar II 3020 from
Micromeritics, USA. The thermal stability of AIL-MFR
was analyzed with a TGA thermogravimetric analyzer
(
Beijing Scientific Instrument Factory, China) from 20 to
-
1
6
50 °C at a heating rate of 10 °C min under nitrogen
atmosphere.
Conversion
Yield
1
00
Synthesis of the melamine-formaldehyde resin (MFR)
8
0
0
0
0
3
In a 250-cm round-bottom flask containing 19.8 g PEG-
2
000 aqueous solution (PEG-2000 content 30 wt%), 60 g
of 36 % aqueous solution of formaldehyde (0.74 mol),
50 mg hexamethylenetetramine, and 32.0 g melamine
0.25 mol) were added and stirred at 90 °C for 5 h. Then,
6
4
2
1
(
the reaction temperature was reduced to 50 °C, and the
pH value of the reaction media was reduced to 3–4 by
addition of aqueous HCl solution to stop the reaction. On
completion, the solid was filtered and then washed with
deionized water repeatedly at 80 °C to remove the PEG-
0
1
2
3
4
5
6
7
Recycle times
2
000 residue. The synthesized MFR was dried at 200 °C
Fig. 4 The reuse of the AIL-MFR catalyst
for 3 h.
Table 2 Comparison of different catalysts for the acetalization of cyclohexanone and 1,2-ethanediol
-
1
Entry
Catalyst
SO
Conversion/%
Yield/%
TOF/s
1
2
3
H
2
4
99
99
88
87
91
85
8.09
p-Toluenesulfonic acid
28.38
50.06
AIL-MFR
Catalytic conditions: 10 mg catalyst amount, 20 mmol cyclohexanone, 24 mmol 1,2-ethanediol, reflux, 20 min; with Dean-Stark apparatus
123

1
374
G. Xing
Synthesis of MFR-supported Brønsted acidic ionic
liquid (AIL-MFR)
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flask equipped with a magnetic stirrer and a thermometer,
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2
1
(
1
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48:609
Acknowledgments The authors acknowledge the financial support
from the Shaoxing University.
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