Synthesis of a thermostable polymer-supported strongly basic catalyst and its catalytic activity

Article abstract of DOI:10.1071/CH13027

A novel strongly basic polymer-supported catalyst with guanidine groups has been synthesized and its thermal stability has been investigated. To obtain a thermally stable, cross-linked structure, guanidine groups bound to a polystyrene matrix were allowed to react in a nucleophilic manner with p-xylylene dichloride. Compared with the conventional strongly basic anion-exchange resin 201, it possesses higher thermal stability when placed in deionized water at 95°C for 60h. This was confirmed by thermogravimetric analysis. The excellent thermal stability can be attributed to the unique structure of guanidine and the cross-linking connection mode between this base and the polymeric matrix. The obtained resin was found to efficiently catalyze Knoevenagel condensation reactions with remarkably high yields. Furthermore, the catalytic efficiency of the resin was found to remain unaffected for seven cycles, whereas that of 201 resin was reduced by 25% owing to degradation of the strongly basic groups.

Full text of DOI:10.1071/CH13027

CSIRO PUBLISHING
Aust. J. Chem. 2013, 66, 913–920
Full Paper
http://dx.doi.org/10.1071/CH13027
Synthesis of a Thermostable Polymer-supported Strongly
Basic Catalyst and its Catalytic Activity
A
A
A
,
A B
Huining Zan, Zhiai Hou, Rongfu Shi, and Chunhong Wang
A
Key Laboratory of Functional Polymer Materials of the Ministry of Education,
Institute of Polymer Chemistry, Nankai University, Tianjin 300071, China.
Corresponding author. Email: wch2004@nankai.edu.cn
B
A novel strongly basic polymer-supported catalyst with guanidine groups has been synthesized and its thermal stability has
been investigated. To obtain a thermally stable, cross-linked structure, guanidine groups bound to a polystyrene matrix
were allowed to react in a nucleophilic manner with p-xylylene dichloride. Compared with the conventional strongly basic
anion-exchange resin 201, it possesses higher thermal stability when placed in deionized water at 958C for 60 h. This was
confirmed by thermogravimetric analysis. The excellent thermal stability can be attributed to the unique structure of
guanidine and the cross-linking connection mode between this base and the polymeric matrix. The obtained resin was
found to efficiently catalyze Knoevenagel condensation reactions with remarkably high yields. Furthermore, the catalytic
efficiency of the resin was found to remain unaffected for seven cycles, whereas that of 201 resin was reduced by 25 %
owing to degradation of the strongly basic groups.
Manuscript received: 17 January 2013.
Manuscript accepted: 26 April 2013.
Published online: 17 June 2013.
Introduction
in which conventional strongly basic polystyrene resin can be
applied has an upper limit of 608C. Many efforts have been
devoted to improving the thermal stability of polymeric resins
by modificationof the linkagebetween the quaternary ammonium
Cross-linked, polymer-supported solid catalysts have recently
witnessed a resurgence of interest for applications in organic
synthesis, such as oxidations, reductions, alkylations, con-
[
1–6]
[10–12]
[13]
densations, and acetylations.
They have certain inherent
groups and the polystyrene chain.
For example, Tomoi
advantages over standard small molecular catalysts: (i) polymer-
supported catalysts can be easily separated from reaction mix-
tures by filtration or centrifugation, and (ii) the catalysts can, in
many cases, be reused many times without loss of efficiency.
Among the various polymer-supported solid catalysts, resins
with a polystyrene matrix and quaternary ammonium groups
have attracted unprecedented interest owing to their numerous
proposed that the incorporation of alkylene chains into a
polystyrene polymer would contribute to the improvement of
heat resistance. However, it was concluded that such modifica-
tions would not be economical for application in large-scale
preparations because of the complicated synthetic processes
required. Moreover, although these methods lead to slight
improvements, it is difficult to fundamentally enhance the heat
stability owing to the inherent nature of the quaternary ammo-
nium group.
From Scheme 1, it is apparent that two factors influence the
thermal stability of a strong-base catalyst: (i) the inherent
chemical stability of a strongly basic group bound to the
polystyrene matrix of a solid-supported catalyst, and (ii) the
binding mode between the polymeric chain and the strongly
basic group of the solid-supported catalyst. Therefore, two
approaches have been considered for fundamentally improving
[
7–9]
advantages.
Although the catalytic abilities of polymeric
strong bases have been successfully demonstrated for many
reactions, broad applicability of such catalysts at high tempera-
tures is still lacking, especially in aqueous solution owing to the
desquamation or conversion of strongly basic groups. This
phenomenon arises as a result of Hofmann degradation of the
quaternary ammonium groups attached to the polymeric matrix
at high temperatures. The mechanism of Hofmann degradation
is presented in Scheme 1. Consequently, the temperature range
ꢀ
ꢁ
CH N(CH )3·OH
CH N(CH ) ꢀ CH OH
2 3 2
3
2
3
(1)
(2)
(
degradation to weak base)
ꢀ
ꢁ
CH N(CH )3·OH
CH OH ꢀ (CH ) N
2
3
2
3 3
(detachment of amino group)
Scheme 1. Hofmann degradation reaction.
Journal compilation Ó CSIRO 2013
www.publish.csiro.au/journals/ajc

9
14
H. Zan et al.
CH2
CH
CH
2
CH
CH2
CH
NH
C
NH
C
CH2 NH
A
NH2
CH
2
NH
NH CH
2
B
Fig. 1. Two connection forms of guanidine resin.
the thermal stability of strong-base catalysts. First, guanidine is
an organic strong-base reagent having three C–N bonds and,
with a pK of 13.5 for the unsubstituted base, its base strength is
reaction between benzyl chloride and guanidine, structure A
should be the main binding mode between guanidine and the
polymer skeleton. Thus, transforming structure A to structure
B without an excessive increase in complexity or cost of the
synthetic process was the challenge to be addressed in our
experiments. To obtain guanidine in a cross-linked and sym-
metrical structure, p-xylylene dichloride (XDC) was selected
as a post-synthesis cross-linking agent to react with guanidine
in structure A, and then the ideal symmetrical cross-linked
structure B could be easily obtained.
Finally, we report on the application of the solid-supported
guanidine catalyst in the Knoevenagel condensation reaction.
When the reactivity of the reactant is low, the condensation
reaction can be conducted at higher temperature to improve
efficiency, which imposes a strict requirement for thermal
stability in the strong-base catalyst used. The activity and
recycling properties of the catalyst are also reported herein.
a
comparable with that of sodium hydroxide with a pK ¼
a
[
14,15]
1
3.6.
According to X-ray structural data for guanidinium
salt crystals, the three C–N bonds are equal in length owing to a
conjugative effect, implying that the positive charge on the
guanidinium cation is delocalized over one carbon and three
nitrogen atoms. Solid-supported guanidine catalysts are suitable
for a wide range of base-directed reactions such as Michael
additions, Knoevenagel condensations, and transesterifica-
[
16–18]
[19]
Blanc et al.
tions.
reported on a series of catalysts
based on the attachment of 1,1,3,3-tetramethylguanidine to
different silicas, and their high catalytic activity in epoxidation
[20]
and condensation reactions. Schuchardt et al.
demonstrated
that alkylguanidines heterogenized on different substituted
polystyrenes are efficient catalysts for the transesterification
of soybean oil with methanol, but catalysts were found to
leach during the reaction. It should be pointed out that all the
work described above has focussed on the catalytic activity of
solid-supported guanidine and the influence of the guanidine
structure itself on the thermal stability of a solid catalyst. Thus,
the thermal stability of the strong-base guanidine catalyst is
expected to improve as a result of inhibiting the Hofmann
degradation reaction (1) illustrated in Scheme 1.
Results and Discussion
Characterization of the Prepared Solid-supported Strongly
Basic Catalyst
Scheme 2 illustrates the synthetic process for obtaining the
catalyst resins with guanidine functional groups (PG) and with
guanidine-XDC functional groups (PGX). The content of
strongly basic groups in resins PG and PGX is listed in Table 1.
The residual content of Cl inthe PG resin was close tozero, which
indicated that chemical modification of the chloromethylated
polystyrene resin PSCl had been successfully accomplished.
However, as shown in Table 1, the content of the strongly basic
Second, in the structure of a normal strongly basic solid-
supported catalyst, ammonium groups are attached to the
polystyrene matrix through benzyl C–N bonds. This is because
the benzyl carbocation has strong electrophilicity and readily
reacts with tertiary amines to form ammonium groups. This
reaction process is simple and inexpensive. From another
ꢀ
1
groups was lower than the theoretical value (4.69 mmol g ),
suggesting that not all of the benzyl chloride groups had reacted
with guanidine in an equimolar ratio and that to some extent, two
equivalents of benzyl chloride had reacted with one equivalent of
guanidine. Thiswould imply that structure B (illustratedin Fig. 1)
existed in the PG resin. Moreover, the base content per gram of
PGX resin was found to be slightly lower than that of PG resin by
acid–base titration owing to the weight gain of PGX resin after
reaction of PG with XDC, indicating successful formation of the
cross-linked structure between XDC and guanidine bound to the
polymeric chain in PG resin.
ꢀ
perspective, however, ifastronglynucleophilicgroup, e.g. OH ,
is present around the ammonium groups, the benzyl C–N bond is
susceptible to nucleophilic attack, resulting in removal of the
whole tertiary amine, as illustrated in reaction (2) in Scheme 1. It
can thus be concluded that the binding mode of the strong-base
group is also a significant factor affecting the thermal stability of
a solid-supported strong-base catalyst. However, little attention
has been paid to the cross-linked connection mode between the
strong-base group and the polymer skeleton.
In order not to increase the complexity and synthetic cost of
the solid-supported catalyst, our preparation of the guanidine
catalyst is still based on electrophilic reaction between a
benzyl carbocation and guanidine. Fortunately, the special
structure of the guanidine group ensured that our experimental
design could be successfully accomplished. As the nitrogen
atoms in guanidine have the same reactivity, there are two
possible connection modes of the guanidine groups in the
solid-supported catalyst, as shown in Fig. 1, when benzyl
chloride groups on the polystyrene skeleton are reacted with
guanidine. According to the results of thermal stability experi-
ments, thermal stability is improved when the guanidine group
is in a symmetrical cross-linked arrangement, as illustrated in
Fig. 1 (structure B). Based on the steric hindrance of the
Fig. 2 shows the Fourier-transform (FT)IR spectrum of the
solid-supported strong-base catalyst PGX. Compared with the
infrared spectrum of the initial PSCl resin, the absorption bands
ꢀ
1
due to the C–Cl stretching vibration at 673 cm and the C–H
ꢀ
1
out-of-plane wagging vibration at 1276 cm are no longer seen.
These two absorption bands are characteristic of benzyl chloride
groups bound to PSCl. At the same time, two new absorption
ꢀ
1
bands at 1656 and 3376 cm , attributable to C¼N and N–H
stretching vibrations respectively in guanidine, were
[
21,22]
observed,
guanidine groups.
confirming the successful introduction of
Fig. 3 shows scanning electron microscopy (SEM) images
of the gel-type polymer-supported strongly basic catalyst with

Thermostable Polymer-supported Strongly Basic Catalyst
915
CH2
CH
NH
C
NaOH
DMF
ꢀ
H N
NH2
2
CH Cl
2
CH2
CH
CH2
CH2
CH
CH2
CH
CICH2
CH Cl
2
NH
C
NH
C
NaOH
CH2
HN
NH
CH2
HN
NH2
Resin PG
CH2
CH
CH2
CH
CH
NH
NH
C
CH2
HN
C
NH
CH2 HN
NH CH2
NH
C
HN
NH CH2
CH2
CH
CH2
CH
CH
NH
NH
C
NH
C
NH CH2
CH2 HN
NH
Resin PGX
Scheme 2. Preparation process for PGX resin.
Table 1. Parameters of solid-supported strong basic catalyst
(a)
Name
Chlorine
content
C, H, N analysis
[%]
Strong base content
ꢀ1
[mmol g
]
ꢀ
1
[
mmol g ]
C, H,
Acid–base
titration
N analysis
(b)
PSCl
PG
5.28
0.04
0.03
–
–
–
C 78.89, H 7.57, N 12.26
C 79.51, H 7.52, N 11.97
2.91
2.85
2.92
2.85
PGX
3
500
3000
2500
2000
1500
1000
500
Wavenumber [cmꢁ1]
guanidine groups. The particle shape was a regular spherical
structure on the exterior (Fig. 3a) and had no porosity in the
interior (Fig. 3b). An advantage of synthesized gel-type cata-
lyst over other commonly used solid basic catalysts is its
relatively high concentration of strong base sites at the surface,
and it is this factor that allows ready access of small molecules
to the polymer. In a gel-type strongly basic catalyst, the
reaction is not limited by internal diffusion, which affects
the reaction kinetics.
Fig. 2. Fourier-transform (FT)-IR spectrum of (a) PSCl and (b) PGX
resins.
Thermal Stability of the Prepared Strong-base
Catalysts PG and PGX
The content of strong-base groups in PG and PGX after different
thermal degradation times is listed in Table 2. The commercial

9
16
H. Zan et al.
(
a)
(b)
Mag
ꢂ 100
100 µm
Mag
ꢂ 15000
1 µm
Fig. 3. Scanning electron micrographs of PGX resin: (a) exterior; (b) interior.
Table 2. Results of the thermal stability experiments for 201, PG and PGX resins
Resin
Initial content of strong
Content of strong base groups after
ꢀ1
Loss of strong base
groups [%]
ꢀ
1
base groups [mmol g
]
different heating time [mmol g
]
2
4 h
36 h
48 h
60 h
PG
2.92
2.85
4.30
2.62
2.82
3.97
2.40
2.81
3.74
2.37
2.78
3.29
2.34
2.80
3.01
19.8
1.75
30.0
PGX
201
strong-base resin 201 with quaternary ammonium groups was
selected for comparison, and the results of its thermal degra-
dation are also listed in Table 2. It can be seen from Table 2 that
commercial resin 201 displayed inferior thermal stability and
the content of strong-base groups steadily decreased during
the course of heating. After treatment with hot water for 60 h,
there was an almost 30 % reduction in the number of strong-base
groups. However, the thermal stability of the PG resin with
bound guanidine groups was obviously greater compared with
that of resin 201. In contrast to quaternary ammonium groups,
guanidine groups have a conjugated structure, in which the three
bonds between C and N atoms are equivalent. This means that
the bond length in guanidine lies between those of C–N single
beginning of the heating experiment. However, according to the
synthesis process for PG illustrated in Scheme 2, the guanidine
groups can be connected in another mode, that is, in a cross-
linked structure involving two C–N bonds. These guanidine
groups were hypothesized to be much more thermally stable.
In order to validate this hypothesis, XDC was used to cross-link
the guanidine groups attached to the PG skeleton through a
single C–N bond. In this way, PGX was prepared, in which all of
the guanidine groups were in a cross-linked arrangement. The
results of a thermal stability experiment, shown in Table 2,
supported our hypothesis and only 1.75 % of the strong-base
groups were degraded after heating for 60 h. It was concluded
that both the structure and the connection mode of the guanidine
groups influenced the thermal stability of the solid-supported
strong-base catalyst.
[23]
and C¼N double bonds.
This stable structure of guanidine
may account for the stability of the PG resin when heated at high
temperatures. As shown in Table 2, the PG resin exhibited
interesting characteristics in thermal degradation experiments.
Although the thermal stability of PG was obviously improved
compared with that of resin 201, the content of bound strong-
base groups still decreased somewhat at the beginning of the
thermal degradation test. From the data in Table 2, it can be seen
that 17.8 % of the strong-base groups had been lost after heating
for 36 h. Nevertheless, the remaining strong-base groups were
not degraded on further heating and the strong-base catalyst
exhibited good thermal stability. Thus, although the PG resin
only bears one kind of strong-base functional group, this group
exhibited different thermal stabilities at the same heating tem-
perature. This was because of the two connection modes of
guanidine in PG shown in Fig. 1. If a guanidine group is bound to
the polystyrene matrix through just one C–N bond (structure A
in Fig. 1), it can be easily cleaved on nucleophilic attack by
In order to characterize the thermal stability of the PGX
resin, thermogravimetric (TG) analysis curves and differential
weight loss derivative thermogravimetry (DTG) curves for the
PGX and 201 resins were recorded and are presented in Fig. 4.
Comparing the TG and DTG curves of these resins, it can be
seen that they both showed two main discrete weight loss steps.
The first step (,12 % w/w), appearing at .1708C, corresponds
to the loss of guanidine or amino functional groups. The second
weight loss (at ,300–4008C), which amounted to ,60 %
(w/w), was related to decomposition of the polymer moieties.
For the PGX resin, the first peak appeared at ,2408C, and the
second thermal degradation process occurred at ,4208C,
whereas resin 201 showed a lower degradation temperature
of ,2278C for the first stage. All of the data revealed that the
PGX resin was more stable than resin 201, consistent with the
previously observed heat stability of the strong-base catalyst in
aqueous solution.
ꢀ
OH . Hence, the thermal stability of PG was poor at the

Thermostable Polymer-supported Strongly Basic Catalyst
917
(
a) 100
(b)
0
2
01 resin
PGX resin
ꢁ1
8
6
4
2
0
0
0
0
0
ꢁ
2
3
ꢁ
PGX resin
201 resin
ꢁ4
ꢁ
5
6
ꢁ
1
00
200
300
400
500
600
100
200
300
400
500
600
Temperature [ꢃC]
Temperature [ꢃC]
Fig. 4. (a) Thermogravimetric, and (b) derivative thermogravimetry curve of 201 and PGX resins.
R1
R1
R2
Table 4. Effect of the amount of catalyst on Knoevenagel condensation
reaction
Strong base
C
O
ꢀ
H C
C
2
C
(
aldehyde or ketone)
Reactants: cyclohexanone and ethyl cyanoacetate; molar ratio is 1 : 1, and
reaction temperature is 708C
R2
Scheme 3. Knoevenagel condensation.
Entry Molar ratio of catalyst to cyclohexanone Time [h] Yield [%]
1
2
3
4
5
1 : 20
1 : 10
1 : 5
10
8
68.3
81.3
86.4
86.6
86.7
Table 3. Effect of reaction temperature in Knoevenagel condensation
Reactants: cyclohexanone and ethyl cyanoacetate; molar ratio is 1 : 1, and
molar ratio of catalyst to cyclohexanone is 1 : 5. The number of moles of
catalyst refers to guanidine bound on the catalyst
4.5
1 : 3
4
2
1 : 2.5
Entry
Temperature [8C]
Yield [%]
The effect of the amount of PGX on this Knoevenagel
1
2
3
4
5
6
7
25
30
40
50
60
70
80
16.8
23.9
42.7
60.5
80.2
86.4
87.3
condensation of cyclohexanone and ethyl cyanoacetate (molar
ratio of reactants is 1 : 1) was investigated at 708C in ethanol. The
results are listed in Table 4. It is evident that the reaction
efficiency increased greatly as the amount of PGX catalyst was
increased, so that the reaction time required to reach the same
conversion was shortened markedly. When the molar ratio was
increased beyond 1 : 5 (as shown in Table 4), neither the reaction
time nor conversion showed any significant improvement. Thus,
amolarratioof1 : 5wasselectedastheoptimalamountofcatalyst
for further experiments. Fig. 5 illustrates the gas chromatogram
obtained for samples of the substrates and reaction product.
Further studies were performed to investigate the Knoeve-
nagel condensation of aromatic aldehydes and ketones with a
variety of active methylene compounds such as malononitrile,
ethyl cyanoacetate, and ethyl acetoacetate. The results are
summarized in Table 5. Compounds with more active methyl-
ene groups, such as malononitrile and ethyl cyanoacetate, gave
the corresponding alkenes in excellent yields in excess of 90 %
(entries 1–4) at room temperature. The reaction of ethyl aceto-
acetate with benzaldehyde required an elevated temperature
(708C) to afford a good yield (entry 5). Although ketones are less
reactive than aldehydes, a higher temperature (708C) resulted in
higher conversions (entries 6 and 7).
Catalytic Activity of PGX for Knoevenagel
Condensation Reactions
It is well known that strong-base catalysts can significantly
enhance many organic reactions. In our studies, the Knoeve-
nagel condensation was selected to investigate the catalytic
activity of the synthesized solid-supported guanidine catalyst.
The Knoevenagel condensation is an effective carbon–carbon
bond-forming reaction between a carbonyl compound and any
[
24,25]
compound having an active methylene group.
In order to
examine the catalytic activity of the strong-base catalyst at high
temperature, reactions of relatively inactive substrates have
been chosen. Scheme 3 illustrates the reaction scheme for a
Knoevenagel condensation.
The effect of temperature on the Knoevenagel condensation
reaction between cyclohexane and ethyl cyanoacetate in etha-
nol over 5 h catalyzed by the prepared PGX was investigated.
The results are listed in Table 3. It is evident that just raising
the temperature can ensure complete reaction for such an
inactive reactant. The yield of the reaction was unchanged
beyond 708C.
Stability of the Prepared Solid-supported
Catalyst PGX at High Temperatures
In order to assess the thermal stability and catalytic activity of
PGX at high reaction temperatures, experiments were

9
18
H. Zan et al.
(
a) 60
(b) 25
20
ethyl-2-cyano-2-cyclohexylideneacetate
cyclohexanone
ethyl cyanoacetate
5
4
3
2
1
0
0
0
0
0
0
1
5
0
5
0
1
0
1
2
3
4
5
6
7
8
9
10 11 12
0
5
10
15
20
25
30
Time [min]
Time [min]
Fig. 5. Gas chromatograms for samples of substrates and reaction product between cyclohexanone and ethyl cyanoacetate.
Table 5. Knoevenagel condensation with different substrates catalyzed by PGX
Molar ratio of reactants is 1 : 1, and molar ratio of catalyst to reactant is 1 : 5; rt, room temperature
Entry
C¼O (aldehyde or ketone)
R
1
CH
2
R
2
Temperature [8C]
Time [h]
Yield [%]
R
1
R
2
CHO
CHO
1
2
3
4
5
6
7
CN
CN
CN
rt
rt
1
94.4
95.4
91.2
93.5
83.1
89.8
86.4
CO
2
2
2
2
Et
4.5
5
HO
CHO
CHO
CN
CO
Et
Et
Et
rt
O N
2
CN
CO
CO
rt
0.5
4
CHO
COCH
CN
3
70
70
70
O
O
CN
1
CN
CO
2
Et
4.5
9
8
0
conducted in which the catalyst was reused seven times under
the optimized conditions. For comparison, the commercial
strong-base resin 201 was used for the same Knoevenagel
condensation reaction under the same conditions. As shown in
Fig. 6, resin 201 was significantly deactivated, and only a 60.4 %
yield was obtained after seven cycles, whereas the PGX resin
could be reused seven times without any apparent loss in activity
under the same reaction conditions.
5
8
7
7
6
6
5
0
5
0
5
0
5
5
0
1st
2nd
3rd
4th
5th
6th
7th
PGX
01
86.4
85.4
87.1
84.7
86.3
84.9
84.8
83.5
83.3
80.2
84.0
74.8
82.5
60.4
Conclusion
2
In summary, a novel strongly basic polymer-supported catalyst,
PGX, was prepared based on the nucleophilic reaction of
Fig. 6. Reuse performances of PGX and 201 resins.

Thermostable Polymer-supported Strongly Basic Catalyst
919
guanidine with benzyl carbocations bound to the matrix of
chloromethyl polystyrene resin PSCl. From consideration of the
thermal degradation mechanism of the guanidine moieties, it was
clear that their cross-linkage would improve thermal stability. To
accomplish thiswithouta significantincreaseinthe complexity or
cost of the synthetic process, they were allowed to react in a
nucleophilic manner with p-xylylene dichloride to generate PGX.
The thermal stability of the resulting PGX was then assessed.
When immersed in water at 958C for 60 h, the degree of degra-
dation of the strongly basic groups was only 1.75%, which is far
less than that of commercial resin 201. Thermogravimetric
analysis further confirmed that the PGX resin possessed higher
thermal stability compared with commercial resin 201. It is
concluded that both the structure and the connection mode of the
guanidine groups influence the thermal stability of the solid-
supported strong-base catalyst. Lastly, the catalytic utility of PGX
for Knoevenagel condensation reactions was explored. The
results showed that all of the reactions examined could be carried
out in yields higher than 85 %, and even relatively inactive sub-
strates could be reacted at higher temperatures. When the PGX
catalyst was reused for seven cycles, its catalytic efficiency was
found to remain unaffected, whereas that of201 resin was reduced
by 25% owing to degradation of the strongly basic groups.
By virtue of its strong basicity and good stability, PGX can be
considered as a new member of the family of solid-supported
organic strong bases, and is an attractive candidate for many
other base-catalyzed reactions, such as Michael addition or the
transesterification of soybean oil, some of which are under
investigation.
Characterization of the Prepared
Solid-supported Catalyst
The content of strongly basic functional groups bound to the PG
and PGX resins was determined by acid–base titration, referring
[26]
to a literature method. Elemental analyses of the resins were
performed on an Elementar Vario EL analyser. The chlorine
content of PSCl and the PG and PGX resins was determined by
[27]
the Volhard method.
The infrared spectrum of the prepared
catalyst was measured on a Magna-560 FT-infrared spectro-
meter (Nicolet). Nujol mull was used as a background matrix.
The SEM images were recorded on a Shimadzu SS-550
instrument. The samples were loaded on stubs and sputter-
coated with a thin gold film to prevent surface charging and also
to protect them from thermal damage from the electron beam,
before scanning.
Evaluation of the Heat Stability
of the Prepared Solid-supported Catalyst
The heat stability of the prepared resin was assessed by means of
a water-bath experiment as described in ref. [28]. After the
water-bath experiment, the resin was collected by filtration and
regenerated with 1 M aqueous NaOH solution. The regenerated
resin beads were then washed with deionized water until the
washings were close to neutral. Finally, the content of the
strongly basic groups bound to the prepared catalyst was
determined, and C, H, and N elemental analysis was carried out
simultaneously. The thermogravimetric techniques of DTG and
TG were also applied in order to assess the heat stability of the
prepared catalyst. DTG and TG tests were carried out on a
DT-40 thermal analyser (Shimadzu) under a dynamic air (dried)
ꢀ1
atmosphere at a heating rate of 108C min from 25 to 6008C.
Experimental
Study of the Catalytic Activity of PGX Resin
for Knoevenagel Condensation
Materials
Gel-type cross-linked chloromethylated poly(styrene-co-
divinylbenzene) resin (PSCl; 5.28 mmol Cl g ) and resin 201
(4.30 mmol g strong-base content) were obtained from the
Chemical Plant of Nankai University (Tianjin city, China).
Guanidine hydrochloride (99 %), XDC (99 %), benzaldehyde,
cyclohexanone, malononitrile, ethyl cyanoacetate, and ethyl
acetoacetate were purchased from J&K Chemicals Ltd (Beijing)
or Tianjin Chemical Co. Hydrochloric acid (37 %), ethanol, and
N,N-dimethylformamide (DMF) were used without further
purification.
ꢀ1
A carbonyl compound (1 mmol), an active methylene reagent
(1 mmol), and ethanol (10 mL) were agitated in a 50-mL conical
flask and heated to a specific temperature. PGX resin was then
added to the reaction mixture. The mixture was agitated for a
specified period of time. The progress of the reaction was
monitored by TLC, with light petroleum/ethyl acetate as an
eluent. After completion of the reaction, the resin was separated
from the reaction mixture. The excess ethanol was removed by
evaporation and the crude product was purified by column
chromatography to obtain the corresponding product (EtOAc/
light petroleum 1 : 5–1 : 10 v/v as eluent). All products were
characterized by comparison of their physical data (mp), FTIR,
ꢀ1
Synthesis of Solid-supported Catalyst bearing
Guanidine Groups
1
H NMR, mass spectrum, and elementary analysis with those
[
29,30]
reported in the literature.
In order to calculate the yield
PSCl resin (15 g) was swollen overnight in DMF (150 mL) in a
5
quantitatively, we use pure product as a standard in GC analysis
to calculate the yield on the basis of the peak area of the product.
The yield was calculated from the following equation:
00-mL three-necked flask fitted with a reflux condenser and a
thermometer. Guanidine hydrochloride (11.35 g, 118.8 mmol)
was added, which raised the pH of the reaction solution to
1
resultant resin was filtered off and washed with distilled water
0–11. The reaction mixture was kept at 708C for ,10 h. The
Wp
p ¼ Wi ꢁ 100
ð1Þ
until the pH of the eluate was 6.8–7.0. The resin was dried at
23 K under vacuum to furnish the solid-supported strongly
3
where p is the reaction yield (wt-%); W is the actual output,
p
basic catalyst with guanidine functional groups, designated PG
resin. PG resin (15 g) was swollen in DMF (150 mL) in a
which can be calculated on the basis of peak area of the pure
product; and W is the theoretical output. Theoretical yield can
be calculated according to the amount of reactant.
i
2
50-mL three-necked flask fitted with a reflux condenser and a
thermometer. XDC was then added and the mixture was heated
at 708C for 12 h. The beads were then collected by filtration and
successively washed with ethanol and water until the washings
were neutral. The beads were dried at 323 K under vacuum and
are designated PGX resin.
CN
O
COOC H5
2
1 mmol
1
mmol (theoretical yield)

9
20
H. Zan et al.
GC analysis was performed on a Shimadzu GC-9A gas chro-
[7] G. Gelbard, F. Vielfaure-Joly, React. Funct. Polym. 2001, 48, 65.
matograph equipped with a hydrogen flame ionization detector
using an SE-54 column (30 m ꢁ 0.530 mm ꢁ 1.50 mm). The tem-
perature program started with an isothermal hold at 373 K for
doi:10.1016/S1381-5148(01)00039-6
8] M. Benaglia, A. Puglisi, F. Cozzi, Chem. Res. 2003, 103, 3401.
doi:10.1021/CR010440O
[
[
9] Y. Chang, X. L. Shi, D. M. Zhu, Y. Liu, Polym. Adv. Technol. 2008,
2
min, followed by an increase to 493 K at a gradient of 278 K
ꢀ
1
19, 877. doi:10.1002/PAT.1053
min , at which temperature the column was maintained for
0 min; the detector temperature was 523 K, and the carrier gas
was nitrogen. Melting points were determined on a Yanaco
[
[
[
[
10] A. T. Dzhalilov, M. A. Askarov, SU 675056 Sovient Union, 1976.
11] M. Henmi, M. Noyoro, T. Yoshioka, JP 0175041 Japan, 1989.
12] T. Onozuka, M. Shindo, T. Ito, JP 0724334 Japan, 1995.
13] M. Tomoi, K. Yamaguchi, R. Ando, Y. Kantake, Y. Aosaki,
H. Kubota, J. Appl. Polym. Sci. 1997, 64, 1161. doi:10.1002/(SICI)
1097-4628(19970509)64:6,1161::AID-APP16.3.0.CO;2-Z
[14] D. P. Fairlie, W. G. Jackson, Inorg. Chem. 1990, 29, 140. doi:10.1021/
IC00326A030
15] S. E. Schneider, P. A. Bishop, M. A. Salazar, O. A. Bishop,
2
1
MP-500 apparatus and are uncorrected. H NMR spectra were
recorded on a Bruker Avance 400 NMR spectrometer. Electron
impact MS analyses were performed using a VG ZAB-HS
alliance spectrometer. FTIR measurements were done on a
Magna-56 spectrometer as KBr pellets.
[
E. V. Anslyn, Tetrahedron 1998, 54, 15063. doi:10.1016/S0040-
Stability of the Prepared Solid-supported
Catalyst PGX Resin
4
020(98)00900-4
[
[
16] D. Brunel, Microporous Mesoporous Mater. 1999, 27, 329.
doi:10.1016/S1387-1811(98)00266-2
17] D. Meloni, R. Monaci, Z. Zedde, M. G. Cutrufello, S. Fiorilli, I. Ferino,
After isolation of the products from the reaction mixtures by
filtration, the PGX resin was washed with ethanol and reused
under identical reaction conditions to those in the first run. For
comparison purposes, reuse of the commercial strong-base resin
Appl. Catal. B 2011, 102, 505. doi:10.1016/J.APCATB.2010.12.032
[18] R. Sercheli, R. M. Vargas, R. A. Sheldon, J. Mol. Catal. Chem. 1999,
148, 173. doi:10.1016/S1381-1169(99)00054-0
2
01 was also tested.
[
[
[
19] A. C. Blanc, D. J. Macquarrie, S. Valle, G. Renard, C. R. Quinn,
Supplementary Material
D. Brunel, Green Chem. 2000, 2, 283. doi:10.1039/B005929N
20] U. Schuchardt, R. M. Vargas, G. Gelbard, J. Mol. Catal. Chem. 1996,
The Supplementary Material contains the determination of the
chlorine content in the solid-supported strong basic catalyst, and
characterization data of the reaction product. The Supplemen-
tary Material is available on the Journal’s website.
1
09, 37. doi:10.1016/1381-1169(96)00014-3
21] B. N. Kolarz, A. W. Trochimczuk, D. Jermakowicz-Bartkowiak,
J. Jezierska, W. Apostoluk, React. Funct. Polym. 2002, 52, 53.
doi:10.1016/S1381-5148(02)00076-7
[
22] Y. Zhang, J. Jiang, Y. Chen, Polymer 1999, 40, 6189. doi:10.1016/
S0032-3861(98)00828-3
Acknowledgements
This project was supported by the National Natural Science Foundation of
China (grant no. 21074060) and PCSIRT (IRT1257).
[23] P. J. Bailey, S. Pace, Coord. Chem. Rev. 2001, 214, 91. doi:10.1016/
S0010-8545(00)00389-1
[
24] M. L. Kantam, A. Ravindra, C. V. Reddy, B. Sreedhar,
B. M. Choudary, Adv. Synth. Catal. 2006, 348, 569. doi:10.1002/
ADSC.200505266
References
[
[
[
[
1] R. W. Hein, M. J. Astle, J. R. Shelton, J. Org. Chem. 1961, 26, 4874.
[
[
[
[
[
[
25] I. Rodriguez, S. Iborra, A. Corma, F. Rey, J. L. Jorda, Chem. Commun.
doi:10.1021/JO01070A022
1
999, 593. doi:10.1039/A900384C
26] B. L. He, W. Q. Huang, Ion Exchanger and Adsorptive Resins 1995
Shanghai Scientific & Technological Press: Shanghai).
27] A. Vogel, Handbook of Quantitative Inorganic Analysis 1978
Longman: London).
28] G. L. Xiao, Y. G. Fan, P. L. Helan, Z. Q. Shi, Ion Exch. Adsorp. 2005,
1, 514.
2] A. Akelah, D. C. Sherrington, Chem. Rev. 1981, 81, 557. doi:10.1021/
CR00046A003
3] A. Bouillaud, M. Dargelos, M. E. Borredon, Aust. J. Chem. 1994,
(
47, 2123. doi:10.1071/CH9942123
(
4] D. Simoni, R. Rondanin, M. Morini, R. Baruchello, F. P. Invidiata,
Tetrahedron Lett. 2000, 41, 1607. doi:10.1016/S0040-4039(99)
0
2
2340-0
5] K. S. Kim, J. H. Song, J. H. Kim, G. Seo, Stud. Surf. Sci. Catal. 2003,
46, 505. doi:10.1016/S0167-2991(03)80432-9
29] J. A. Cabello, J. M. Campelo, A. Garcia, D. Luna, J. M. Marina,
J. Org. Chem. 1984, 49, 5195. doi:10.1021/JO00200A036
30] M. Waibel, J. Hasserodt, J. Org. Chem. 2008, 73, 6119. doi:10.1021/
JO800719J
[
[
1
6] G. Gelbard, Ind. Eng. Chem. Res. 2005, 44, 8468. doi:10.1021/
IE0580405