Ag nanoparticles stabilized on cyclodextrin polymer decorated with multi-nitrogen atom containing polymer: An efficient catalyst for the synthesis of xanthenes

Article abstract of DOI:10.3390/molecules25020241

In attempt to broaden the use of cyclodextrin polymer for catalytic purposes, a novel covalent hybrid system was prepared through growth of multi-nitrogen atom containing polymer (PMelamine) derived from reaction of ethylenediamine and 2,4,6-trichloro-1,3

Full text of DOI:10.3390/molecules25020241

molecules
Article
Ag Nanoparticles Stabilized on Cyclodextrin Polymer
Decorated with Multi-Nitrogen Atom Containing
Polymer: An Efficient Catalyst for the Synthesis
of Xanthenes
Samahe Sadjadi *, Fatemeh Ghoreyshi Kahangi , Masoumeh Dorraj and Majid M. Heravi 3,*
1
,
2
1
1
Gas Conversion Department, Faculty of Petrochemicals, Iran Polymer and Petrochemicals Institute,
P.O. Box 14975112, Tehran 1497713115, Iran; masidor20@gmail.com
Department of Chemistry, University Campus 2, University of Guilan, Rasht 4199613776, Iran;
2
nf_ghoreyshi@yahoo.com
Department of Chemistry, School of Science, Alzahra University, P.O. Box 1993891176, Vanak,
3
Tehran 1993891176, Iran
*
Correspondence: s.sadjadi@ippi.ac.ir (S.S.); m.heravi@alzahra.ac.ir (M.M.H.); Tel.: +98-2148666 (S.S.);
98-2188044051 (M.M.H.); Fax: +98-2144787021-3 (S.S.); +98-2188041344 (M.M.H.)
+
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Paolo Lo Meo and Anna Trzeciak
Received: 15 November 2019; Accepted: 6 January 2020; Published: 7 January 2020
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: In attempt to broaden the use of cyclodextrin polymer for catalytic purposes, a novel
covalent hybrid system was prepared through growth of multi-nitrogen atom containing polymer
(PMelamine) derived from reaction of ethylenediamine and 2,4,6-trichloro-1,3,5-triazine on the
functionalized cyclodextrin polymer (CDNS). The resulting hybrid system was then utilized as a
catalyst support for the immobilization of silver nanoparticles through using Cuscuta epithymum extract
as a naturally-derived reducing agent. The catalytic activity of the catalyst, Ag@CDNS-N/PMelamine,
for the synthesis of xanthenes through reaction of aldehydes and dimedone in aqueous media was
examined. The results showed high catalytic activity and recyclability of the catalyst. It was believed
that cyclodextrin in the backbone of the catalyst could act both as a capping agent for Ag nanoparticles
and phase transfer agent to bring the hydrophobic substrates in the vicinity of the catalytic active
sites and accelerate the reaction rate. Multi-nitrogen atoms on the polymer, on the other hand, could
improve the Ag NPs anchoring and suppress their leaching.
Keywords: cyclodextrin polymer; catalyst; silver nanoparticles; xanthenes
Academic Editors: Paolo Lo Meo and Anna Trzeciak
1
. Introduction
Within the past decade, noble metal nanoparticles (NPs) with narrow particle size distributions
have received significant attention in various applications, such as catalysis. Metal NPs are known to
provide high surface area of catalytically active sites [ ]. However, the major problem for the noble
1–4
metal NPs is their high tendency to form aggregate, thereby leading to loss of their main characteristics.
A possible way to address this issue is to stabilize the NPs through immobilizing them on an
appropriate solid substrate such as inorganic oxides, carbon-based nanomaterials or insoluble polymers,
or using soluble capping agents such as surfactants, ligands, or polymers [5–9].
One of the most known oligosaccharides for the catalytic purposes is cyclodextrin (CD). CD is a
cyclic oligosaccharide with an exceptional cone-shape structure with hydrophobic exterior surface
and hydrophilic interior space [10
–12]. This feature allows CD to form an inclusion complex with
hydrophobic reagents. As the outer surface of CD is hydrophilic, the hosted guest can be easily
Molecules 2020, 25, 241; doi:10.3390/molecules25020241
www.mdpi.com/journal/molecules

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transferred to aqueous media. In this way, CD can act as a molecular shuttle. On the other hand,
CD can serve as a stabilizing agent for nanoparticles and prevent their aggregation through efficient
capping [13,14].
It is worth to mention that the activity of heterogeneous catalysts strongly depends on the
loading amount of their immobilized homogenous moieties [15]. An innovative way for increasing the
loading amounts of NPs is immobilization of metal ions onto cross-linked polymeric networks [15,16].
These polymeric networks have high porosity, high surface area, and large amounts of attaching sites
to grab metal ions [17]. Polymeric networks have many coordination sites, which can adsorb the large
amounts of metal ions. Besides, they are more thermally and chemically stable than conventional
supports [8].
CD-based polymers (CDNSs) can be prepared through reaction of CDs with a cross-linking
agent. This class of compounds benefits from the features of CD and the polymeric network.
Nanocomposites of CDNs and metal nanoparticles are also very interesting materials. In these systems,
CDs can stabilize nanoparticles and improve catalytic properties [18].
Among the various noble metal NPs, silver nanoparticles (Ag-NPs) have received particular
attention in view of their unique applications and characteristics. In recent years, Ag-NPs witnessed
growing applications for catalysis, sensing, electronic, biological labeling [19], drug delivery [20], water
treatment [21], etc.. Mainly, the Ag-NP preparation method involves reduction of silver ions in the
solution or in gaseous environments [22]. Use of chemical reducing reagents is not environmentally
benign [22
,23]. Alternatively, naturally derived reducing agents can be used for the synthesis of
nanoparticles [16
–
19]. Use of this class of reagents benefits from some advantages such as low cost
and non-toxicity. Moreover, the reduction process mostly can be carried out in aqueous solution in
a single-step procedure. Nowadays, using plant extracts as reducing and stabilizing agents for the
synthesis of metallic nanoparticles is considered to be an eco-friendly and rapid strategy.
Xanthene derivatives are key biologically active chemicals with diverse important pharmacological
properties such as antibacterial, antiviral, and anti-inflammatory [24
,25]. These compounds are found
in the structure of drugs used in photodynamic therapy [26 28]. Moreover, xanthenes can be applied
–
for the development of pH-sensitive fluorescent materials and dyes. Considering high utility of
xanthenes, many researchers devoted their research to develop efficient procedures for the synthesis of
these chemicals [29
,30]. As examples, some catalysts such as ionic liquid [31], TiO -SO H [32], and
2 3
acid functionalized SiO [33–35] have been reported for xanthene synthesis.
2
In our following research on CD-based heterogeneous catalysts [36–40], herein we wish to report
a novel catalyst support based on growth of multi-nitrogen-containing polymer on the functionalized
CDNS. More precisely, amine functionalized CDNS was prepared and reacted with ethylenediamine
and 2,4,6-trichloro-1,3,5-triazine under basic condition to allow multi-nitrogen atom containing polymer
growth. Then, the hybrid system was applied for the immobilization of silver nanoparticles through
reduction with Cuscuta epithymum extract as a naturally-derived reducing agent. The reasons for use of
this extract was as follow: availability in large quantity in our local area, very low cost, non-toxicity,
and environmentally benign nature. The final hybrid system was then applied as a heterogeneous
catalyst for promoting the synthesis of xanthene derivatives from reaction of aldehydes and dimedone
in aqueous media. The generality of the developed protocol and the recyclability of the catalyst were
also studied. Furthermore, to elucidate the roles of multi-nitrogen containing polymer and CDNS in
the catalysis and disclose the merit of this catalyst, the catalytic activity of the catalyst was compared
with Ag@CDNS, Ag@multi-nitrogen atom-containing polymer and some previously reported catalysts.
2
. Result and Discussion
2
.1. Characterization of Ag@CDNS-N/PMelamine
The morphology of CDNS was studied by recording its FESEM images. The images showed that
bare CDNS showed plate-like morphology. Moreover, the EDS and elemental mapping analysis also

Molecules 2020, 25, 241
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confirmed its formation (Supplementary Materials Figure S1). Then, to study the effect of incorporation
nitrogen functionality by treating with APTES on the morphology of CDNS, the FESEM image
of CDNS-N was recorded. Again, the EDS and mapping analysis confirmed the formation of this
compound (Figure S2). It was found that introduction of APTES can alter the morphology of the catalyst
and lead to the more compact morphology. Moreover, the elemental mapping analysis showed almost
uniform distribution of N atoms, confirming that functionalization was achieved throughout the CDNS
uniformly. Next, the morphology of CDNS-N/PMelamine was studied. The FESEM image of this sample
(Figure S3) was distinguished from CDNS and CDNS-N and showed aggregated-like morphology.
The FESEM images of Ag@CDNS-N/PMelamine catalyst is depicted in Figure 1A. As shown,
the catalyst showed aggregate-like morphology that was different from CDNS, CDNS-N, and
CDNS-N/PMelamine. The EDS analysis of the catalyst (Figure 1B) showed the presence of Si and O
atoms, which are mainly representative of APTES. Moreover, the presence of Ag atoms can confirm
the incorporation of Ag species in the hybrid catalyst. Also, the observation of C and N atoms can be
attributed to the presence of CDNS-N/PMelamine. Notably, the observation of Cl atoms showed that
in the course of condensation polymerization, some Cl atoms did not participate in the polymerization
process. This can be due to the steric hindrance. In Table 1, the quantitative results of EDS analysis
are summarized.
Figure 1. (A) FESEM image and (B) EDX analysis of the catalyst.
Table 1. The quantitative results of EDS analysis.
Element
Weight (%)
Atomic (%)
C
N
O
44.13
39.42
6.55
0.82
3.04
5.19
0.86
51.87
39.74
5.78
0.41
1.21
0.68
0.31
Si
Cl
Ag
K
In the following, the elemental distribution in the Ag@CDNS-N/PMelamine catalyst was examined
by elemental mapping analysis (Figure 2). As shown, Si atoms have been well distributed, confirming
that CDNS has been uniformly functionalized with APTES. On the other hand, Ag distribution is also
uniform. Similarly, this can be indicative of well dispersion of Ag on the support.

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Figure 2. Elemental mapping of catalyst.
Figure 3 shows the TEM image of Ag@CDNS-N/PMelamine. In the TEM image of the catalyst,
the clear sheet-like structure of PMelamine could be observed. Moreover, the spherical dark spots in
the photograph are representative of Ag NPs with average size of 16.5 nm
Ag NPs are dispersed on the support almost homogeneously and only on the edges of CDNS-N
PMelamine some aggregation is observed.
±
3.8. As shown in Figure 3,
/

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1
50nm
22
20
18
16
14
12
10
8
6
4
2
0
Mean:16.5
Std. Dev:3.8
8
10
12
14
16
18
20
22
24
26
Particle size (nm)
Figure 3. TEM image of catalyst.
The silver content of Ag@CDNS-N/PMelamine was evaluated by using ICP-AES analysis. To this
end, a known amount of Ag@CDNS-N/PMelamine was digested in a concentrated HCl and HNO3
solution. Subsequently, the resulting extract was analyzed applying ICP-AES. Using this approach, the
content of Ag nanoparticles was measured to be 0.4 wt%.
The FT-IR spectra of the CDNS, CDNS-N, and Ag@CDNS-N/PMelamine are exposed in Figure 4.
The CDNS spectrum is in good agreement with previous reports [40] and showed the characteristic
−1
−1
−1
bands at 3400 cm (-OH functionality), 2928 cm (–CH groups), and 1700 cm that can be assigned
2
to the ester –C=O functionality, indicating successful cross-linking between the CDs and diphenyl
carbonate. The FTIR spectrum of CDNS-N is very similar to that of CDNS. Noteworthy, the characteristic
bands of APTES overlapped with those of CDNS. In the FTIR spectrum of Ag@CDNS-N/PMelamine,
−1
a strong band observed at 1664 cm can be attributed to the vibration of the –C=N bonds that are
present in the melamine rings. Furthermore, the bands at 2874 is indicative of –CH groups, while the
2
−
1
bands at 3262 and 3400 cm are representative of –NH and –OH functionalities, respectively.

Molecules 2020, 25, 241
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Figure 4. FTIR spectra of pristine CDNS, CDNS-N, and catalyst.
In the following, the XRD pattern of Ag@CDNS-N/PMelamine was recorded. As shown in
◦
Figure 5, the XRD pattern of the catalyst showed a broad halo at 2
θ
= 19–30 that can be assigned to the
amorphous CDNS-N/PMelamine. This observation is in a good agreement with the previous reports,
in which CDNS prepared via melting method showed amorphous structure [41]. According to the
◦ ◦ ◦ ◦ ◦
θ = 38.06 , 44.1 , 64.5 , 77.6 , and 81.5 can be attributed to the Ag(0)
literature, the sharp bands at 2
species [42], confirming the successful reduction of silver salt to the Ag(0) via Cuscuta epithymum extract.
2
−1
.
Using BET, the specific surface area of Ag@CDNS-N/PMelamine was measured to be 19.4 m g
2
−1
This value is higher than the specific surface area of bare CDNS (4 m g ). This result confirmed
that conjugation of PMelamine could improve the specific surface area of CDNS that is instinctively
very low.
Figure 5. XRD pattern of the catalyst.

Molecules 2020, 25, 241
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+
0
The reduction of aqueous Ag ions to Ag by the Cuscuta leaf extract was confirmed by UV-visible
spectroscopy. According to the literature [43 44], the characteristic band of Ag nanoparticles in the
UV-visible spectrum occurs near λmax = 430 nm. As depicted in Figure 6, in the UV spectrum of the
mixture of the extract and AgNO no band at λmax = 430 nm was observed, while, after reduction, the
,
3,
Ag(0) band appeared in the UV-visible spectrum, confirming the successful reduction of Ag(I) to Ag(0).
Figure 6. UV-visible spectrum of Ag nanoparticles, and the mixture of AgNO and plant extract.
3
2
.2. Study of the Catalytic Performance
Confirming the formation of Ag@CDNS-N/PMelamine, its catalytic performance was scrutinized.
Initially, two-component reaction of dimedone and benzaldehyde for the synthesis of xanthene was
targeted as a model reaction. The reason behind this selection was the importance of xanthene
derivatives as biologically active chemicals and their wide use for the synthesis of more complex
chemicals. First, the reaction condition for the model reaction was optimized (Table 2). In this line,
the model reaction was first performed in water as a solvent at ambient temperature in the presence
of 0.02 mg Ag@CDNS-N/PMelamine. The result confirmed that under this condition, high yield of
product (76%) was achieved. To increase the yield of the reaction, it was performed in various solvents
with different polarities. The reason for use of water as a solvent was its environmentally benign
nature. EtOH was also selected as a potential solvent. The reason for this choice was availability
and non-hazardous nature of EtOH. Moreover, it was assumed that due to the different polarity, the
solubility of the reagents can be improved in EtOH compared to pure water. The mixture of H O:EtOH
2
was also examined to provide a more ecofriendly solvent with improved potential for dissolving the
reagents. THF and CH CN were selected as non-polar solvents. The reasons for their selection were
3
their low boiling points and their capability to dissolve the reagents. As tabulated, the mixture of
H O:EtOH with ratio of 2:1 resulted in the best result. Hence, it was selected as the reaction solvent.
2
Subsequently, the effect of the reaction temperature was studied by elevating the reaction temperature
◦
to 50 C. The results confirmed that increase of the reaction temperature led to the improvement of the
reaction yield. However, there was not a linear relationship between the reaction temperature and
◦
the reaction yield and by increasing the reaction temperature from 50 to 70 C, no improvement was
observed in the reaction yield.
Then, it was investigated whether the increase of the amount of the catalyst from 0.02 to 0.03 g
could increase the reaction yield. Gratifyingly, use of higher amount of the catalyst led to the higher

Molecules 2020, 25, 241
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yield of the desired product. However, further increase of the catalyst content had no positive effect
on the yield of the product. Considering all of the results, the optimum reaction condition was using
◦
0
.03 g of Ag@CDNS-N/PMelamine at 50 C in the mixture of H O:EtOH.
2
Table 2. Optimization of reaction condition for the synthesis of model xanthene.
◦
Entry
Solvent
Temp. ( C)
Catalyst Amount (g)
Yield (%)
1
2
3
4
5
6
7
8
9
H O
EtOH
H O:EtOH (1:2)
25
25
25
25
25
50
70
50
50
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.03
0.04
76
79
80
70
72
85
85
92
92
2
2
THF
CH CN
3
H O:EtOH (1:2)
H O:EtOH (1:2)
H O:EtOH (1:2)
H O:EtOH (1:2)
2
2
2
2
In the following, the role of PMelamine in the catalysis was elucidated (Table 3). To this purpose,
Ag@CDNS was synthesized. First, the optimum reaction condition for this catalyst was obtained (use
◦
of 0.04 g of Ag@CDNS at 60 C in the mixture of H O:EtOH). Under the optimum reaction condition,
2
the catalytic activity of Ag@CDNS was 78% which was lower than that of Ag@CDNS-N/PMelamine.
Moreover, examining of the catalytic activity of Ag@CDNS for promoting the model reaction under
optimum reaction condition found for the catalyst confirmed that under that optimum reaction
condition, Ag@CDNS led to lower yield of the corresponding product (70%). Notably, examining
the catalytic activity of Ag@CDNS-N confirmed that the catalytic activity of this control sample was
similar to that of Ag@CDNS and much inferior compared to that of the main catalyst.
This observation indicated the contribution of PMelamine to the catalysis. It was postulated that
the abundant nitrogen containing functionalities on PMelamine could enhance the anchoring of Ag
NPs. To verify this assumption, the Ag contents for Ag@CDNS and Ag@CDNS-N/PMelamine were
measured via ICP. The results showed that the Ag loading in Ag@CDNS-N/PMelamine (0.4 wt%) was
higher than Ag@CDNS (0.27 wt%). This observation confirmed that PMelamine effectively contributed
to the Ag stabilization.
Next, the contribution of CDNS in the catalysis was studied. Similarly, the optimum reaction
◦
condition for this catalyst was first found as use of 0.035 g of Ag@PMelamine at 50 C in the
mixture of H O:EtOH. The catalytic activity of this control catalyst was 80%. The comparison of the
2
catalytic activity of Ag@PMelamine with that of Ag@CDNS-N/PMelamine under the optimum reaction
condition found for the catalyst also showed the inferior activity of Ag@PMelamine (72%) compared to
Ag@CDNS-N/PMelamine. According to the literature [10,11], the role of CDNS in the catalysis can be
assigned to the capability of CDs to encapsulate the hydrophobic substrates and formation of inclusion
complex. This feature allows CDs to act as phase transfer agents. On the other hand, CD can act as a
capping agent for Ag NPs [45,46].
Table 3. Comparison of the catalytic activity of Ag@CDNS-N/PMelamine with some control catalysts.
Yield at Optimum
Yield at Optimum Reaction
Condition of Each Control
Sample (%)
Entry
Catalyst
Reaction Condition of
a
the Catalyst (%)
78 b
1
2
3
4
Ag@CDNS
Ag@CDNS-N
Ag@PMelamine
Ag@CDNS-N/PMelamine
70
70
72
92
b
78
80
c
a
92
a
◦
O:EtOH. b use of 0.04 g of Ag@CDNS at 60 C in the mixture of
◦
0
.03 g of catalyst at 50 C in the mixture of H
2
c
◦
H2O:EtOH. 0.035 g of Ag@PMelamine at 50 C in the mixture of H2O:EtOH.

Molecules 2020, 25, 241
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Next, it was elucidated whether this protocol could be generalized to other aldehydes. In this regard,
various aldehydes with different functional groups and electron densities were examined. The results
(Table 4) confirmed that Ag@CDNS-N/PMelamine could promote the reaction of various aldehyde
derivatives with electron donating or electron withdrawing groups to furnish the corresponding
products in high yields. Studying the furfural as substrate, it was found that this protocol can be
generalized to heterocyclic substrates.
Table 4. Synthesis of various xanthenes under Ag@CDNS-N/PMelamine catalysis.
Entry
Substrate
Yield (%) a
1
2
3
4
5
6
7
8
Benzaldehyde
92
95
93
98
95
90
95
90
4-NO -benzaldehyde
2
2-NO -benzaldehyde
2
4-Me-benzaldehyde
4-MeO-benzaldehyde
2-MeO-benzaldehyde
4-Cl-benzaldehyde
Furfural b
a
Isolated Yield. b 91% in 4 h (65% in 3 h).
In the following, to investigate the efficiency of Ag@CDNS-N/PMelamine, the reaction condition
and the efficiency of the catalyst were compared with those of some of the previously reported catalysts.
Notably, as the reaction condition for each reported catalyst is different, the comparison cannot be
accurate and only provides an insight into the activity of the catalyst. The results, tabulated in
Table 5, revealed that various metallic catalysts have been reported for this organic transformation.
As shown, this model reaction has been performed both in solvent and under solvent-free condition.
From the data in Table 5, it can be concluded that some of the metallic catalysts such as Nano-NiO and
Nano-ZnO were not effective for this reaction. Comparing the reaction temperatures of the tabulated
catalysts, it can be seen that Ag@CDNS-N/PMelamine could promote the reaction in lower reaction
temperature to furnish the product in comparative yield. Regarding the reaction time, it can be seen
that this reaction was reported both in very short and very long reaction time and the reaction time
of that Ag@CDNS-N/PMelamine can be considered as a relatively short one. On the other hand,
Ag@CDNS-N/PMelamine could catalyze the reaction in aqueous media that is environmentally benign
solvent. Considering all of these results, it can be concluded that Ag@CDNS-N/PMelamine can be
classified as an efficient catalyst.

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Table 5. The comparison of the catalytic activity of the catalyst with some other reports for the synthesis
of 3,4,6,7-Tetrahydro-3,3,6,6-tetramethyl-9-phenyl-2H-xanthene1,8-(5H,9H)-dione. a
Time
h:min
Temp.
( C)
Yield
(%)
Entry
Catalyst
Solvent
Quantity
Ref.
◦
1
2
3
4
5
6
7
8
Ag@CDNS-N/PMelamine
Fe3O4@SiO2–SO3H
Silica-bonded S-sulfonic acid (SBSSA)
Nano-ZnO
H
2
O:EtOH
-
EtOH
03:00
00:4
50
110
Reflux
100
Reflux
90
100
0.03 g
0.05 g
0.03 g
10 mol%
15 mol%
0.013 g
10 mol%
10 mol%
92
97
98
Trace
95
91
-
[33]
[34]
[32]
[47]
[32]
[32]
[48]
10:00
02:00
03:00
01:10
02:00
01:30
-
Barium Perchlorate
Nano titania-supported sulfonic acid (n-TSA)
Nano-NiO
EtOH
-
-
-
Trace
86
Fe2(SO4)3.7H2O
120
a
The catalyst quantity was measured for 1 mmol benzaldehyde.
2
.3. Reaction Mechanism
Synthesis of xanthenes using silver nanoparticles has been previously reported [49]. According to
the literature, the catalyst can activate aldehyde. On the other hand, CDNS in the structure of the
catalyst can effectively act as a phase transfer agent and bring the hydrophobic substrates in the
vicinity of the catalytic active sites. In the next step, the enole formed from dimedone reacted with
the activated aldehyde to furnish an intermediate that then tolerates dehydration and reaction with
second dimedone molecules. Finally, dehydration and cyclization leads to the formation of the desired
product (Figure 7).
Figure 7. Plausible reaction mechanism for the synthesis of xanthenes.
2
.4. Catalyst Recyclability
The final feature of Ag@CDNS-N/PMelamine which was studied was its recyclability. This is an
important characteristic of heterogeneous catalysts that renders them suitable for large scale use and
industrialization. To this purpose, the recovered Ag@CDNS-N/PMelamine from model reaction was
washed and dried and then applied as catalyst for the next run of the same reaction under similar
reaction condition. This cycle was repeated for five reaction runs. In Figure 8, the yields of the desired
product in the presence of fresh and recycled catalysts are summarized and compared. As shown,
Ag@CDNS-N/PMelamine showed high recyclability and could be successfully recycled with slight loss
of the catalytic activity.

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Figure 8. Recyclability of Ag@CDNS-N/PMelamine for the synthesis of model reaction.
To further study the effect of recyclability on the catalyst, the recycled Ag@CDNS-N/PMelamine
after five consecutive reaction runs was analyzed with ICP. Gratifyingly, the ICP results confirmed
that recycling did not cause significant leach of Ag NPs and only slight loss of Ag NPs (3 wt% initial
loading) was detected. On the other hand, the recycled catalyst (after five reaction runs) was also
characterized via FTIR spectroscopy to elucidate whether recycling could destruct the structure of the
catalyst. The FTIR spectrum of the recycled Ag@CDNS-N/PMelamine (Figure 9) was very similar to
that of the fresh one, indicating that Ag@CDNS-N/PMelamine was stable under recycling.
Figure 9. Comparison of FTIR spectrum of the recycled Ag@CDNS-N/PMelamine after five reaction
runs with that of the fresh catalyst.
The morphology of the recycled catalyst after five reaction runs was also studied by recording
its TEM image. As shown in Figure 10, the morphology of the recycled Ag@CDNS-N /PMelamine is
similar to that of the fresh catalyst. The measurement of the Ag average particle size (16.8 nm
also confirmed that recycling did not induce significant aggregation.
±
4.5)

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Figure 10. TEM image of the recycled catalyst after 5 reaction runs.
3
. Experimental
3
.1. Materials and Instrumentation
All chemicals and reagents utilized for the synthesis of the catalyst and investigation of its
catalytic activity, including 2,4,6-trichloro-1,3,5-triazine (TCT), ethylenediamine (EDA), (3-amino
propyl) triethoxysilane (APTES), AgNO , K CO , diphenyl carbonate, -cyclodextrin, aldehydes,
β
3
2
3
dimedone were purchased from Sigma-Aldrich and used as received without any further purification.
To reduce Ag(I) to Ag(0), the extract of leaves of Cuscuta epithymum that were collected from Banaruiyeh
District, in Larestan, Iran was used.
The synthesized hybrid catalyst was characterized using energy dispersive X-ray
spectroscopy(EDS), FESEM, ICP, X-ray diffraction (XRD), Fourier transform infrared (FT-IR)
spectroscopy, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and
Brunauer-Emmett-Teller measurements (BET). FESEM and EDS analyses were done using a Bruker
◦
XFlash 6/100. XRD pattern of the as-synthesized sample was recorded from 2
θ
8 to 90 on a (Siemens,
model D5000, Karlsruhe, Germany), using Cu K
α
radiation. TG analysis was performed with
◦
−1
a (Mettler-Toledo, model Leicester, Leicester, UK) at a scanning rate of 10 C min from room
◦
temperature up to 800 C under nitrogen flow. FT-IR spectra were undertaken with a PerkinElmer
Spectrum 65 instrument. TEM analysis was performed using a Philips CM30 electron microscope
operating at 300 kV. To perform this analysis, the samples were prepared by evaporating much diluted
suspensions on carbon-coated cupper TEM grids. A BELSORP Mini II apparatus (BEL Japan, Inc.,
Osaka, Japan) was utilized to study the textural properties of the catalyst. To perform ICP analysis,
ICP-AES Varian, Vista-pro (Salt Lake City, Australia) was used.
3
.2. Synthesis of the Catalyst
3
.2.1. Synthesis of CDNS
To prepare CDNS, the melting method was used. Briefly, β-CD (1 mmol) as monomer was added
to the melted cross-linking agent, diphenyl carbonate (8 mmol). The polymerization reaction proceeded
◦
under stirring at 120 C under atmospheric condition for 10 h. At the end of the reaction, the obtained
white solid was cooled to room temperature and crushed to fine powder. Next, CDNS purification was
achieved by addition of an aqueous NaOH solution (1 M) to an aqueous suspension of CDNS in order
to let the phenol by-product become soluble as sodium phenoxide. The CDNS was then filtered off,

Molecules 2020, 25, 241
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washed with acetone, and distilled water. Further purification was carried out by Soxhlet extraction
◦
with EtOH for 4 h. The resulting CDNS was then dried in the oven at 90 C for 10 h.
3
.2.2. Synthesis of Amine-Functionalized CDNS (CDNS-N)
APTES solution (4 mL in 20 mL of dry toluene) was added in a dropwise manner to a stirring
suspension of CDNS (1.2 g) in dry toluene (40 mL). The obtained mixture was then irradiated with
ultrasonic irradiation at a power of 100 W for 30 min. The resulting suspension was subsequently
◦
refluxed at 110 C under nitrogen atmosphere for 24 h. After completion of reaction, the white solid
◦
was filtered off and washed with dry toluene repeatedly and then dried at 80 C overnight to afford
1
.3 g CDNS-N.
3
.2.3. Growing Polymer on CDNS (CDNS-N/PMelamine)
CDNS-N (1.0 g) was dispersed in 20 mL dry THF in a clean round-bottom flask and then sonicated
for 20 min. Subsequently, TCT (2.0 g, 10 mmol) was added to the suspension and afterwards, 2.0 g
(
32 mmol) of EDA was slowly dropped into the stirring mixture. The resulting mixture was then
◦
kept at 0 C. After the addition of 2.0 g (14 mmol) of K CO , the resulting mixture was stirred under
atmospheric condition for 4 h at room temperature and then refluxed at 50 C for 24 h. Upon completion
2
3
◦
of the polymerization reaction, the obtained solid support (denoted as CDNS-N/PMelamine) was
◦
filtered and washed three times with methanol and then dried at 60 C for 12 h to afford 1.28 g
CDNS-N/PMelamine.
3
.2.4. Preparations of Cuscuta Epithymum Extract
Fresh leaves of Cuscuta epithymum were collected from Banaruiyeh District, in Larestan, Iran. First,
the collected Cuscuta epithymum (2 g) were crushed in porcelain mortar. Then, the resulting powder
◦
was mixed thoroughly with 100 mL deionized water (DW) and boiled for 60 min at 80 C. The extract
was then obtained by cooling the mixture and simple filtration.
3
.2.5. Synthesis of Ag NPs and Their Embedding into CDNS-N/PMelamine: Synthesis of
Ag@CDNS-N/PMelamine
CDNS-N/PMelamine (1 g) was dispersed into a solution containing 0.1 g of AgNO in 20 mL DW
3
and kept under stirring at room temperature under atmospheric condition for 30 min. The adsorption
+
of Ag ions on the surfaces of CDNS-N/PMelamine was conducted using electrostatic attraction. Then,
the fresh extract (2 mL in 20 mL DW) as a reducing agent was added immediately to the suspension.
Upon addition of the bio-based reducing agent, the solution turned black, confirming the reduction of
+
Ag to metallic silver (Ag). The mixture was then stirred continuously under atmospheric condition
for 12 h to immobilize Ag NPs into the CDNS-N/PMelamine and forming Ag@CDNS-N/PMelamine.
Finally, the resulting product was separated and repeatedly washed using EtOH/DW and then
◦
dried in electronic oven at 60 C for 12 h to obtain 0.9 g catalyst. Figure 11 presents a schematic
illustration of synthesis of the proposed structures. Notably, to prepare the control catalysts, Ag@CDNS
and Ag@PMelamine, the same protocol was applied, except CDNS and PMelamine were used as
supports respectively.
3
.3. General Procedure for the Synthesis Xanthenes Derivative
In a typical procedure, to a mixture of aldehyde (1 mmol) and dimedone (2 mmol) in 1:2 H O:EtOH
2
(
3 mL), Ag@CDNS-N/PMelamine (0.03 g) was added and the mixture was stirred under atmospheric
◦
condition at 50 C for 3 h. The reaction progress was monitored by TLC and at the end of the reaction,
EtOH (20 mL) was added to the reaction mixture and the catalyst was separated via simple filtration.
To purify the organic product, it was recrystallized from EtOH.

Molecules 2020, 25, 241
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Figure 11. The schematic procedure of the synthesis of the proposed structures.
4
. Conclusions
A novel covalent hybrid system composed of CDNS and multi-nitrogen atom-containing polymer
were prepared through simple procedure, including functionalization of CDNS followed by its reaction
with ethylenediamine and 2,4,6-trichloro-1,3,5-triazine. The resulting compound was then applied
as a catalyst support for the immobilization of Ag(0) nanoparticles, reduced by Cuscuta epithymum
extract as a naturally derived reducing agent. The catalyst, Ag@CDNS-N/PMelamine, was successfully
applied for promoting the two component reaction of aldehydes and dimedone for the formation
of xanthenes in aqueous media under mild reaction condition. The catalytic activity of the catalyst
was superior to Ag@CDNS and Ag@PMelamine and showed high recyclability with low Ag leaching.
The high catalytic performance of the catalyst was attributed to the capability of CD for the formation
of inclusion complex with hydrophobic substrates and its role as a phase transferring agent in bringing

Molecules 2020, 25, 241
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the substrates in close contact with the catalytic sites. Moreover, CD could act as capping agent for Ag
NPs. On the other hand, the multi-nitrogen functionalities on the polymer backbone could improve
anchoring of Ag NPs and suppress their leaching.
Supplementary Materials: The following are available online, Figure S1. SEM image, EDS analysis and Elemental
mapping analysis of CDNS, Figure S2. SEM image, EDS analysis and Elemental mapping analysis of CDNS-N,
Figure S3. SEM image, EDS analysis and Elemental mapping analysis of CDNS-N/PMelamine.
Author Contributions: The author contributions is as follow: Conceptualization, S.S.; methodology, F.G.K. and
M.D.; validation, S.S., F.G.K. and M.D., formal analysis, F.G.K. and M.D.; investigation, S.S., F.G.K. and M.D.;
writing—original draft preparation, S.S., M.D.; writing—review and editing, S.S.; visualization, F.G.K. and M.D.;
supervision, S.S.; project administration, S.S.; funding acquisition, S.S., M.M.H. All authors have read and agreed
to the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: The authors gratefully acknowledge the partial support of Iran Polymer and Petrochemical
Institute, Alzahra University and INSF.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of CDNS is available from the authors.
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).