Synthesis and characterization of copper (II)-poly(acrylic acid)/M-MCM-41 nanocomposite as a novel mesoporous solid acid catalyst for the one-pot synthesis of polyhydroquinoline derivatives

Article abstract of DOI:10.1016/j.poly.2019.114294

In this study, a novel mesoporous silica nanocomposite (MSNC) based on poly(acrylic acid) and amine-functionalized MCM-41 mesoporous silica as amplification agents was constructed via an ultrasonic-assisted method as a straightforward, fast, eco-friendly and it is denoted as M-MCM-41. Then Cu (II) was immobilized on PAA/M-MCM-41 surface and the novel heterogeneous reusable catalyst was fully characterized using appropriate techniques such as Fourier transform infrared (FT-IR), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), field emission scanning electron microscopy (FE-SEM), thermogravimetry analysis (TGA) and N₂ adsorption–desorption. The resulting catalyst was successfully used for the condensation of various aromatic aldehydes, dimedone, ?-ketoester and ammonium acetate under solvent-free and mild conditions to yield polyhydroquinoline derivatives in good to excellent yields. Also, the experimental evidence was demonstrated that Cu (II)-PAA/M-MCM-41 nanocomposite could act as a recoverable and reusable nanocatalyst without a meaningful drop in the yield and the reaction time at least four times.

Full text of DOI:10.1016/j.poly.2019.114294

Journal Pre-proofs
Synthesis and characterization of copper (II)-poly(acrylic acid)/M-MCM-41
nanocomposite as a novel mesoporous solid acid catalyst for the one-pot syn-
thesis of polyhydroquinoline derivatives
Shahla Vaysipour, Zahra Rafiee, Masoud Nasr-Esfahani
PII:
S0277-5387(19)30739-9
DOI:
https://doi.org/10.1016/j.poly.2019.114294
POLY 114294
Reference:
To appear in:
Polyhedron
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Revised Date:
Accepted Date:
27 August 2019
1 November 2019
27 November 2019
Please cite this article as: S. Vaysipour, Z. Rafiee, M. Nasr-Esfahani, Synthesis and characterization of copper (II)-
poly(acrylic acid)/M-MCM-41 nanocomposite as a novel mesoporous solid acid catalyst for the one-pot synthesis
of polyhydroquinoline derivatives, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.114294
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Revised
Synthesis and characterization of copper (II)-poly(acrylic acid)/M-MCM-41
nanocomposite as a novel mesoporous solid acid catalyst for the one-pot
synthesis of polyhydroquinoline derivatives

Shahla Vaysipour, Zahra Rafiee and Masoud Nasr-Esfahani
Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, 75918-74831, Iran

Corresponding author: E-mail address: z.rafiee@yu.ac.ir; Tel: +98-74-33223048
Synthesis and characterization of copper (II)-poly(acrylic acid)/M-MCM-41
nanocomposite as a novel mesoporous solid acid catalyst for one-pot synthesis of
polyhydroquinoline derivatives
Shahla Vaysipour, Zahra Rafiee* and Masoud Nasr-Esfahani
A novel, green and recoverable nanocomposite was designed, synthesized and fully
characterized. The resulting nanocomposite have successfully used as a catalyst for the
condensation of various aromatic aldehydes, dimedone, ß-ketoester and ammonium acetate
under mild and solvent free conditions to yield polyhydroquinoline derivatives in good to
excellent yields.

Abstract
In this study, a novel mesoporous silica nanocomposite (MSNC) based on poly(acrylic acid)
and amine-functionalized MCM-41 mesoporous silica as amplification agents was constructed
via an ultrasonic-assisted method as a straightforward, fast, eco-friendly and it is denoted as
M-MCM-41. Then Cu (ІІ) was immobilized on PAA/M-MCM-41 surface and the novel
heterogeneous reusable catalyst was fully characterized using appropriate techniques such as
Fourier transform infrared (FT-IR), X-ray diffraction (XRD), energy dispersive spectroscopy
(
EDS), field emission scanning electron microscopy (FE-SEM), thermogravimetry analysis
TGA) and N adsorption-desorption. The resulting catalyst was successfully used for the
(
2
condensation of various aromatic aldehydes, dimedone, ß-ketoester and ammonium acetate
under solvent-free and mild conditions to yield polyhydroquinoline derivatives in good to
excellent yields. Also, the experimental evidence was demonstrated that Cu (ІІ)-PAA/M-
MCM-41 nanocomposite could act as a recoverable and reusable nanocatalyst without a
meaningful drop in the yield and the reaction time at least four times.

Keywords: Nanocomposite, Mesoporous silica nanoparticles, MCM-41, Poly(acrylic acid)
PAA), Copper (II), Polyhydroquinoline (PHQ)
(
1
. Introduction
Nanomaterials are especially of interest due to their distinct electronic, optical,
magnetic and catalytic properties [1,2]. The nanocomposites (NC)s with an organic
polymer matrix filled with organic or inorganic fillers are important members of NC
materials which offer a higher surface to volume ratio in collation to the pure polymer
[
3-5]. The polymeric NCs (PNC)s incorporate the favorable properties of inorganic and
organic ingredients, such as high thermal stability, mechanical, electrical, biocidal and
flexibility, to obtain new materials with considerably improved physical and/or
chemical properties [6-9]. Despite the diverse possibility of nanofiller candidates, an
important stage before use is the examination of their features such as composition,
morphologies, geometries, sizes, and porosities. Nanofiller components have been
categorized according to the pore diameter as micro-(2 < nm), meso-(2-50 nm), and
macroporous (> 50 nm) which play a basic role on the physicochemical properties of
NCs [10,11]. Among the wide range of materials, mesoporous silica nanoparticles
(
MSNs) with different pore sizes and structures including hexagonal MCM-41 [12-15],
cubic MCM-48 [16,17], hexagonal SBA-15 [18,19], wormhole framework MSU-J [20-
2], two-dimensional, hexagonally ordered MSU-H [23], and mesocellular silica foam
2
MSU-F silicas [24] are suitable candidates for the preparation of PNCs.
Among MSNs, MCM-41 as a hexagonal arrangement of unidimensional pore
channels has been attracted a great deal of attention over nearly two decades ago due to

its unique properties such as biocompatibility, abundant surface silanol groups (Si-
2
-1
OH), extremely large surface area (700-1500 m g ), tunable pore size (varying from
.5 to 10 nm), thermal and chemical stability for recyclability and relatively
1
hydrophobic nature, favourable surface for further functionalization [3,5,25-27]. Four
types of silanol groups exist on the surface of mesoporous MCM-41: 1) Terminal
silanol groups (isolated or free silanol); 2) Geminal silanol groups; 3) Ethereal silanol
groups (siloxane) and 4) Vicinal silanol groups (hydrogen-bonded) [28].
Notwithstanding these desirable properties, MCM-41 in pure silica form has low
inherent acidity and indicates weak hydrogen bonding sites which decreases its
performance in the fields of catalysis [29]. Also, surface modification of mesoporous
silica nanoparticles as nanofillers by an organic-inorganic compound as a connecting
agent is a general path to increase a strong interfacial interaction between the
nanofillers and polymer and to enhance the scattering of the nanofillers in a polymer
matrix [3,30,31].
Among different polymers that are being studied, poly(acrylic acid) (PAA) has
received considerable attention. PAA is a water-soluble polymer, non-toxic,
hydrophilic, biocompatible superabsorbent and is the simplest synthetic
poly(carboxylic acid). PAA is selected as a host polymer in this research due to its
charming properties such as excellent stability in acidic and basic, biodegradable,
permeability and high ability to attach with a variety of metal ions in solution [32-34].
Polyhydroquinoline (PHQ) derivatives are an important class of organic N-
heterocyclic compounds and have been attracted considerable attention due to their key
role in pharmacological and biological activities [35].

In view of the therapeutic importance of PHQ derivatives, organic chemists have
explored several procedures for the synthesis of these compounds. General method for
the synthesis of PHQ derivatives is multi-component reaction of aldehydes, β-
dicarbonyl compounds, active methylene compounds and ammonium acetate using
various catalysts such as ceric ammonium nitrate (CAN) [36], FSM-16-SO
pyridine-SO H]Cl [38], polymer [39], MCM-41 [40], Ni-nanoparticle (NP) [41], ionic
liquids [42], p-TSA [43], BINOL-phosphoric acid derivatives [44], montmorillonite K-
0 [45], TiO NPs [46], SnO NPs [47], HClO -SiO [48], Fe @B-MCM-41 [49],
MCM-41-OBF [50], silica-bonded imidazolium-sulfonic acid chloride [51], SBA-
5/SO H [52], IL-HSO @SBA-15 [53] and MCM-41@Serine@Cu(ІІ) [54].
However, there is a need to find new catalysts and green methods to minimize the
3
H [37],
[
3
1
2
2
4
2
3
O
4
2
1
3
4
drawbacks of existing methods such as use of toxic organic solvents, prolonged
reaction times, poor yields, difficulty in catalysts recovery, excess reagents and harsh
reaction conditions. In recent years with the development of green chemical
technology, the copper heterogeneous catalysts have been widely used for organic
synthesis under solvent-free conditions because of their chemical, economic, and
environmental aspects [54]. Solid catalysts due to lack of corrosiveness, safety, and
ability to reduce the amount of waste isolation are harmless to the environment.
Therefore many reactions in organic chemistry are performed in the presence of solid
catalysts.
In this paper, the preparation and characterization of Cu(PAA) complex grafted over
the modified surface MCM-41 using 3-aminopropyltriethyoxysilane (Cu(ІІ)-PAA/M-
MCM-41) as a novel acid nanocatalyst (Scheme 1) were reported and the applicability

of this catalyst for the synthesis of new PHQ derivatives at 80 °C under solvent-free
conditions was studied (Scheme 2).
2
. Experimental
2
.1. Methods and materials
All chemicals and reagents in this study were purchased from Merck and Aldrich
chemical companies. The reaction progress and the purity determination of the
compounds were monitored by thin-layer chromatography (TLC). Melting points were
determined using an Electrothermal KSB1N capillary apparatus and uncorrected.
Fourier transform infrared (FT-IR) spectra were recorded with a JASCO FT-IR/680
-
1
1
plus spectrometer in the scanning range of 4000-400 cm using KBr pellet. H NMR
and 13C NMR spectra were obtained with Bruker 400 and 100 MHz instrument
spectrometer on the solution in DMSO-d
6
or CDCl
3
. The X-ray diffraction (XRD) data
ractometer equipped with Cu K
were conducted on a Panalytical X' Pert Pro X di
ff
α
radiation (λ=0.1542 nm) at 40 kV and 40 mA. The elements in the sample were probed
by energy dispersive spectroscopy (EDS) using the TESCAN Vega model instrument.
The particle size and surface morphology of catalyst were examined by carrying out
field emission scanning electron microscopy (FE-SEM) using a SIGMA VP model
from ZEISS Company. Thermogravimetric analysis (TGA) of the samples was
performed on an STA 6000 instrument (Perkin Elmer, America) between 30 and 800
°
C at a heating rate of 20 °C/min under a nitrogen atmosphere. N adsorption-
2
desorption isotherms were obtained on a BELSORP-mini ІІ-REOTERM japan
instrument. The surface area was determined based on BET analyses. Pore size

distribution was derived from the desorption branches of the isotherms based on the
BJH model, and the total pore volume was calculated using BJH analyses.
2
.2. Preparation of MCM-41
MCM-41 mesoporous compound was synthesized according to the procedure described
in literature [55]. Succinctly, 2.4 g of cetyltrimethylammonium bromide (CTAB) as a
surfactant template was dissolved in 120 mL of deionized water and stirred until a
solution was formed. Then, 10 mL of ammonia was added dropwise with vigorous
stirring for 5 min. After that, 10 mL of tetraethylorthosilicate (TEOS) as a source of
silicon for the preparation of the mesoporous material was added into the solution to
form a gel composition (supersaturated solution). This solution was stirred for 72 h
under reflux conditions and N atmosphere, and then the mixture was allowed to cool
2
to room temperature. The solid product was separated by filtration, washed with
deionized water and ethanol. The obtained white solid was dried in an oven at 70°C for
12 h. The final step involved calcinating the sample at 550 °C for 12 h to remove the
organic template.
2
.3. Preparation of amino-functionalized MCM-41 (M-MCM-41)
MCM-41 was functionalized with 3-aminopropyltriethoxysilane (3-APTES) (a silane
coupling agent) as follows: MCM-41 (0.6 g) was dispersed in 30 mL of anhydrous
toluene by sonicating for 45 min at room temperature, and then 3-APTES (4.42 mmol)
was added to the mixture under reflux with stirring under an argon atmosphere for 24
h. The white solid MCM-41-(SiCH
2
CH
2
CH
2
NH
2
x
) was collected and washed three

times with ethanol and deionized water to remove unreacted 3-APTES, and dried in an
oven at 80°C for 24 h [31].
2
.4. Synthesis of polyacrylic acid (PAA)
A solution of 2 mmol of acrylic acid (AA) in dry CHCl
mmol of ethylene glycol dimethacrylate (EGDMA) and 0.05 g of azobisisobutyronitrile
AIBN) as an initiator. After complete addition, the reaction mixture was stirred under
reflux conditions at 60°C for 24 h under a N atmosphere. PAA was precipitated by
3
(40 mL) was added to 10
(
2
pouring into 20 mL of methanol. The obtained product was filtered, washed
successively with methanol and dried overnight at 40 °C [56].
2
.5. Preparation of PAA/M-MCM-41 NC
For the fabrication of PAA/M-MCM-41 NC using a green and rapid method, 3.0 g of
PAA was added to 30 mL of double distilled water at 87 °C and stirred for at least 30
min until a clear solution of PAA was obtained. Simultaneous, 0.06 g of M-MCM-41
was added to 10 mL of the same solvent and sonicated for 1 h. Then, M-MCM-41
solution was added to the polymer matrix solution to obtain a clear suspension, and the
suspension refluxed for 4 h at 80 °C followed by sonication for 40 min. In the final
step, the resulting clean homogeneous solution was poured onto a glass plate, and the
solvent was evaporated in an oven at 30 °C for 24 h.
2
.6. Preparation of Cu(ІІ)-PAA/M-MCM-41 NC
In the last step, for the synthesis of Cu(ІІ)-PAA/M-MCM-41 NC, 1 g of PAA/M-
MCM-41 and 0.604 g of Cu(NO .3H O were mixed in 30 mL of ethanol under reflux
3
)
2
2

conditions for 24 h under an argon atmosphere. The resulting catalyst was filtered,
washed with ethanol and dried in a vacuum to obtain the heterogenized complex
(Cu(ІІ)-PAA/M-MCM-41) as a novel nanocatalyst.
2
.7. General procedure for the synthesis of polyhydroquinoline derivatives via Hantzsch
reaction
To a test tube containing a mixture of aromatic aldehydes (1 mmol), dimedone (1
mmol, 0.14 g), ethyl (or methyl) acetoacetate (1 mmol) and ammonium acetate (1.5
mmol, 0.11 g) was added Cu(ІІ)-PAA/M-MCM-41 (0.02 g) as a catalyst. Then, the
reaction mixture was stirred at 80 °C under solvent‐free conditions. The progress of the
reactions was monitored by TLC (ethyl acetate/n-hexane, 1:4). After the completion of
the reaction, the resulting mixture was cooled to room temperature and then cold water
was added. The obtained solid was collected by filtration and purified by
recrystallization from hot ethanol to afford the pure product. In this study, the catalyst
was recycled and reused five times without significant loss of its catalytic activity.
Ethyl-4-(3-ethoxy-4-hydroxyphenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-
hexahydroquinoline-3-carboxylate (5i)
-1
M.P: 198-200°C; IR (KBr) υ (cm ) = 3448 and 3283 (OH and NH stretching vibration),
2
3
3
077 (=C-H stretching vibration, sp ), 2964 (C-H stretching vibration, sp ), 1689 (C=O ester
stretching vibration), 1613 (C=O ketone stretching vibration), 1489 (C=C, Ar, stretching
1
vibration), 1269 (C-O, ether, stretching vibration), 1214 (C-O, ester, stretching vibration); H
NMR (400 MHz, DMSO-d ) δ (ppm): 0.89 (s, 3H), 1.02 (s, 3H), 1.17 (t, J=12 Hz, 3H), 1.30
6

(
t, J=7.2 Hz, 3H), 2.01 (d, J=8 Hz, 1H), 2.18 (d, J=8.2 Hz, 1H), 2.27 (s, 3H), 2.29 (d, J=5.6
Hz, 1H), 2.42 (d, J=8.6 Hz, 1H), 3.86-3.95 (q, 2H), 3.96-4.07 (q, 2H), 4.75 (s. 1H), 6.50 (dd,
=1.0 Hz, J =0.8 Hz, 1H), 6.59 (d, J=4 Hz, 1H), 6.70 (d, J=1.0 Hz,1H), 8.56 (s, NH), 9.03 (s,
OH); C NMR (100 MHz, DMSO-d ) δ (ppm): 14.7, 15.3, 18.7, 26.8, 29.7, 32.6, 35.4, 50.8,
9.4, 64.2, 104.5, 110.7, 113.9, 115.5, 120.1, 139.4, 144.9, 145.3, 146.2, 149.7, 167.5, 194.8
J
1
2
1
3
6
5
3
. Results and discussion
3
.1. Preparation of Cu(ІІ)-PAA/M-MCM-41 NC
The schematic pathway for the fabrication of Cu(ІІ)-PAA/M-MCM-41 NC is depicted
in Scheme 1. Due to mesoporous silica MCM-41 physicochemical properties with
hexagonal order arrays, this compound was selected as a nanofiller. Recently, it has
aroused great research interest because of its significant advantages including the
hydrophilicity of its surface, good thermal stability, degradability, high surface area,
large pore volume, easy handling and low cost of manufacture as well as good
chemical and mechanical stability. The most prominent feature of mesoporous MCM-
41 is its ability to grafting with other active sites of molecules due to the abundance of
active hydroxyl groups on the internal and external surfaces of its 3D framework. First,
MCM-41 NPs were prepared through a sol-gel method in a basic solution by CTAB as
a template. Then, MCM-41 surface was modified by 3-APTES under ultrasonic
irradiation to improve the dispersibility of M-MCM-41 in water as a solvent. The
alkoxy groups of the silane coupling agent react readily with the abundant hydroxyl
groups on the surface of MCM-41. Hence, it can be used as a good filler for the
preparation of the NC based on PAA. The possible interaction between carbonyl and

hydroxyl groups of the polymer moiety and -OH and -NH
functionalized mesoporous silica may efficiently interact by hydrogen bonding [57].
Finally, the reaction of PAA/M-MCM-41 with Cu(NO .3H O in ethanol under reflux
2
groups on the surface of the
3
)
2
2
conditions for 24 h lead to the production of new catalyst, Cu(ІІ)-PAA/M-MCM-41
NC using immobilization of copper (ІІ) on the surface of MCM-41 mesoporous and
PAA by coordination bonding through hydroxyl and carbonyl groups interaction.
3
.2. Characterization of catalyst
Cu(ІІ)-PAA/M-MCM-41 NC was fully characterized by a variety of techniques such as
FT-IR, XRD, EDX, FE-SEM, TGA, and BET.
Fig. 1 shows the FT-IR spectra of pristine MCM-41 (a), M-MCM-41 (b), neat PAA (c),
PAA/M-MCM-41 NC (d) and Cu(ІІ)-PAA/M-MCM-41 NC (e). The absorption band around
-1
3
500 cm observed in all spectra is related to both the O-H stretching of the surface silanol
groups and the hydrogen-bonded adsorbed water molecules. The absorption bands observed at
-1
1
083 and 807 cm are characteristics of the Si-O-Si asymmetric and symmetric stretching
-1
vibrations and the band at 458 cm can be assigned to the Si-O-Si bending mode (Fig.1a). For
M-MCM-41, new bands were observed that is attributed to the organic moiety of the attached
-1
silane coupling agent (Fig. 1b). The broad peak at about 3400 cm region is attributed to both
the hydroxyl and amine groups present on the surface of MCM-41 as well as surface silanols.
-1
A new band around 2938 cm is assigned to the C-H stretching vibration of methylene of the
-1
propyl group and a weak peak at 689 cm is attributed to the Si-C stretching vibrations which
is absent in the spectrum of pristine MCM-41. Fig. 1c and 1d show the FT-IR spectra of neat
PAA and PAA/M-MCM-41 NC, respectively. Neat PAA shows a broad absorption band

-1
around 3600-3300 cm due to the presence of -OH groups of acid. The absorption peaks
-1
3
around 2986 and 2953 cm corresponded to the sp C-H stretching vibrations of alkyl groups
-1
in PAA. The absorption band around 1700 cm is attributed to the stretching vibration of the
carbonyl group of carboxylic acid, and also, the occurrence of the peak at 1158 cm-1 is
-1
attributed to C-O stretching vibration of PAA. The peak at 1454 cm is assigned to CH
2
bending. The FT-IR spectrum of NC (Fig. 1d) is also exhibited the presence of both the
polymer and modified filler. The linkage of M-MCM-41 with PAA caused scarce changes in
the intensities of absorption bands of neat PAA as well as the addition of new absorption
-1
bands in the range of 800-400 cm owing to the Si-C stretching and Si-O-Si bending as
mentioned above (Fig. 1c). The intensity of the broad band around 3600-3000 cm-1 is
decreased after adsorption of Cu(II) (Fig. 1 e). It may be due to the interaction of copper with
hydroxyl, amines and carbonyl groups. A significant decrease is observed in the relative
-1
intensity of the characteristic band for C-O located at 1073 cm , denoting the formation of the
O-Cu band. It may be as an outcome of the formation of a complex between Cu(II) bonded to
PAA/M-MCM-41 NC surface and weakening of Cu-N and Cu-O bonds.
XRD patterns of the MCM-41 (a), M-MCM-41 (b) and Cu(II)-PAA/M-MCM-41 (c) in the
range of 0 < 2θ < 10 are presented in Figure 2. The XRD pattern of MCM-41 shows three
peaks assigned to 100, 110, and 200 reflections which are attributed to the ordered 2D-
hexagonal structure (Fig. 2a). XRD pattern of M-MCM-41 shows that the intensities of these
peaks decreased. Pore filling of MCM-41 by 3-APTES can be the probable reason for the
reduction of peaks intensity (Fig. 2b). In the XRD pattern of Cu(II)-PAA/M-MCM-41 NC
(Fig. 2c), the 100 diffraction peak of the M-MCM-41 can still be observed in the polymer
matrix NC but with less intensity, as well as the disappearance of the signals at higher angles.

This phenomenon may be due to decreased pore size as well as the increase of wall thickness.
Nevertheless, the XRD results showed that the mesoporous hexagonal channels were still
present after modification.
Surface morphology of Cu(II)-PAA/M-MCM-41 NC was investigated via FE-SEM
analysis. As can be seen, the NC is formed in small particle size and approximately uniform
(Fig.3).
The EDX analysis of the NC is shown in Figure 4. The EDX spectrum of the NC has all
bands related to the elements that exist in the NC structure (Si, O, N, C, and Cu). The strong
band related to the C atom at the spectrum confirms that polymer grafted successfully on the
mesoporous silica (M-MCM-41) surface.
Thermogravimetric analysis (TGA) of MCM-41, M-MCM-41, and Cu(II)-PAA/M-MCM-
4
1 NC is presented in Figure 5. TGA of MCM-41 indicated two steps of weight loss. The
weight loss is 1.99% in the first step from 25°C to 200 °C that is related to loss of moisture
inside the MCM-41 channels and in the second step in the range of 450-900 °C, the weight
loss (3.19%) is mainly due to water loss formed by the density of silanol groups (Fig. 5a). The
thermogram of functionalized MCM-41 (MCM-41-n Pr-NH
at temperatures ranging from 25 to 900 °C (Fig. 5b). The first region at temperatures between
5 °C and 200 °C corresponds to the removal of physisorption water held in the pores. The
2
) showed a three-step weight loss
2
weight losses in the second and third steps are chiefly due to the decomposition of the organic
propyl amino fragments on the surface of the synthesized mesoporous silica MCM-41 NPs.
The TGA thermogram of the Cu(II)-PAA/M-MCM-41 NC was provided in Fig. 5c. As can be
seen from Fig. 5c, the thermal analysis identified three separate weight loss regions on the TG
curve. The first decomposition stage at 25-150 °C probably corresponded to the removal of

water or silane coupling agent that had been physically absorbed on the surface of the M-
MCM-41. The TGA results showed that the most weight loss was occurred at about 200-350
°
C, that can be associated to the decomposition of functional groups of PAA and the final
weight loss step begins around 400 °C and ends about 800 °C is due to the
decomposition/combustion of organic ligand present in the NC. Overall, these results show
that by introducing M-MCM-41 as a nanofiller into PAA, the thermal stability of NC has been
dramatically improved; this improvement could be due to good interaction between the
polymer matrix and mesoporous materials as well as the inherently good thermal stability of
the mesoporous MCM-41. Noteworthy, due to the catalyst stability of up to 200 °C, this new
NC can be used in organic reactions.
The N
2
adsorption-desorption isotherm of Cu (ІІ)-PAA/M-MCM-41 NC is shown in Figure
6
. N sorption measurements have been applied to determine the physical properties including
2
surface area, pore size distribution, and pore volume. Type IV isotherm is recognizable for
MCM-41 with H1 hysteresis loop. On modifying MCM-41 with APTES, the surface area of
2
MCM-41 reduced from 986.16 to 694.98 m /g and the pore size decreased from 3.56 to 3.3
nm [58]. On Cu (ІІ)-PAA/M-MCM-41, a further reduction in the surface area from 694.98 to
2
1
01.5 m /g and pore size from 3.3 to 1.22 nm was observed. Moreover, the results presented
3
in Table 1 show that the pore volume Cu (ІІ)-PAA/M-MCM-41 is 0.114 cm /g. The reduction
of BET surface area, volume and pore size of Cu(ІІ)-PAA/M-MCM-41 NC clearly is related
to pore entrance blocking.
3
.3. Study of Cu(ІІ)-PAA/M-MCM-41 NC catalytic activity through the synthesis of
polyhydroquinoline derivatives

After approving of the presented catalyst using the aforementioned technical skills, the
catalytic application of Cu(ІІ)-PAA/M-MCM-41 NC was investigated in the synthesis of
several polyhydroquinoline derivatives. Initially, the reaction conditions were optimized with
the model reaction of benzaldehyde (1), dimedone (2), ethyl acetoacetate (3) and ammonium
acetate (4). The effect of various solvents, temperature and amount of catalyst was studied to
obtain optimal conditions and the results were summarized in Table 2.
To optimize the required catalyst amount, different amounts of the nanocatalyst were
applied and according to the obtained results (Table 2, entries 1-6), 0.02 g of the catalyst was
chosen as the best amount with the highest yield in the model reaction. Moreover, this
reaction was performed in the absence of nanocatalyst. The product was obtained only in trace
amounts even after an extended time. Therefore, the presence of the catalyst was essential for
the completion of the reaction. Increasing the number of acidic active sites by increasing the
amount of catalyst resulted in decrease of concentration of reactants at the active sites and
finally a decline in yield (Table 2, entries 5 and 6) [59].
The effect of solvent and temperature was investigated by performing the model reaction in
the presence of 0.02 g of catalyst in various solvents and temperatures (Table 2, entries 7-16).
The reaction was carried out in the presence of various solvents like ethanol, methanol,
chloroform, water and acetonitrile and also under solvent-free conditions. The polar solvents
such as EtOH and MeOH were found to be better solvents than the non-polar solvents. The
low efficiencies in non-polar solvents may be due to the poor solubility of the reactants in
these solvents and low catalyst dispersion. The yields were better under solvent-free
conditions than solvents (Entries 1-11). It is also important to note that increasing the

o
temperature improves the yields of the reaction (Entries 12-16). The best result was at 80 C
under solvent-free conditions (Entry 4).
After establishing the optimal conditions, in order to verification of applicability and
efficiency of this catalyst, we were studied the synthesis of a variety of substituted
polyhydroquinoline (5a-5p) in the presence of Cu(ІІ)-PAA/M-MCM-41 NC as a
heterogeneous and recoverable catalyst using different aromatic aldehydes bearing both
electron-donating and electron-withdrawing substituents under the determined conditions. The
afforded results showed that in all instances the corresponding polyhydroquinoline derivatives
were obtained in good to excellent yields in suitable time (Table 3, entries 1-16). The reaction
of aromatic aldehydes having electron-withdrawing groups was rather faster than electron-
donating groups. To show the advantage of the present study, we collated the catalytic activity
of Cu(ІІ)-PAA/M-MCM-41 in the synthesis of polyhydroquinoline derivatives with other
catalysts. As shown in Table 4, Cu(ІІ)-PAA/M-MCM-41 NC is better than and/or comparable
with most of the previous studies in terms of applied solvent, reaction time, temperature and
catalyst amount due to good surface area, high reactivity and excellent stability. These
findings considerably confirm the high efficiency of the present catalyst in the synthesis of
polyhydroquinoline derivatives.
A proposed mechanism pathway for the synthesis of polyhydroquinoline derivatives
is portrayed in Scheme 3. Initially, dimedone is converted to its active enol form using
Cu (ІІ)-PAA/M-MCM-41 and reacted to the activated aldehyde by the catalyst for the
formation of the corresponding Knoevenagel intermediate I. On the other hand, the in-
situ generated NH
3
from ammonium acetate reacts with activated β-ketoester to yield
the corresponding intermediate ІІ. Then, the reaction between these two intermediates

generates intermediate III. Tautomerization of intermediate III leads to the formation of
intermediate IV. Afterward, the intramolecular nucleophilic attack of the amino group
to the activated carbonyl group leads to intermediate V. At the end of the removal of
the water molecule of this intermediate leads to the production of polyhydroquinoline
derivatives 5a-5p [54,60].
According to the green chemistry viewpoint, recoverability and recycling of the
catalyst is an important issue in modern catalysis researches. The reusability of Cu(ІІ)-
PAA/M-MCM-41 was investigated after the completion of the reaction for the one-pot,
four-component reaction of benzaldehyde, dimedone, ethyl acetoacetate and
ammonium acetate. Cu(ІІ)-PAA/M-MCM-41 NC was successfully recycled by
filtration and washed with MeOH, and dried in 100 °C for the next run without a
considerable decrease in its catalytic activity. It was found that the catalyst can be
recovered and reused in five reaction cycles with only a slight decrease in the reaction
yield (Fig. 7).
4
. Conclusions
In summary, in this research for the first time the preparation of a novel and efficient
heterogeneous catalyst was reported. Applicability of the catalyst Cu(ІІ)-PAA/M-
MCM-41 was tested for the synthesis of polyhydroquinoline derivatives by
condensation of dimedone, β-ketoester, ammonium acetate as a source of nitrogen and
a range of aromatic aldehydes in the absence of solvent. The encouraging points of this
study are eco-friendly and mild reaction conditions, easy separation and reusability of
the catalyst, short reaction time with high product yields, and easy work-up.

Acknowledgment
The authors gratefully acknowledge partial support of this work by Yasouj University,
Yasouj, Iran.
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Caption for Schemes
Scheme 1. Schematic illustration for the fabrication of Cu (ІІ)-PAA/M-MCM-41 NC.
Scheme 2. Synthesis of polyhydroquinoline derivatives using a one-pot, four-component
reaction in the presence of Cu(ІІ)-PAA/M-MCM-41 NC.
Scheme 3. Suggested mechanism for the synthesis of polyhydroquinoline derivatives
using Cu(ІІ)-PAA/M-MCM-41 NC.
Legend for the Figures
Figure 1. The FT-IR spectra of pristine MCM-41 (a), M-MCM-41 (b), neat PAA (c),
PAA/M-MCM-41 NC (d) and Cu(ІІ)-PAA/M-MCM-41 NC (e).
Figure 2. XRD patterns of MCM-41 (a), M-MCM-41 (b) and Cu(II)-PAA/M-MCM-41(c).
Figure 3. The FE-SEM image of Cu(II)-PAA/M-MCM-41 NC.
Figure 4. Energy-dispersive X-ray spectroscopy (EDS) spectrum of Cu(II)-PAA/M-MCM-41
NC.
Figure 5. TGA thermograms of MCM-41 (a), M-MCM-41 (b) and Cu(II)-PAA/M-MCM-41
(c).
Figure 6. Nitrogen adsorption-desorption isotherms of Cu(ІІ)-PAA/M-MCM-41 NC.
Figure 7. Reusability of the NC catalyst in the reaction of benzaldehyde, dimedone, ethyl
acetoacetate and ammonium acetate.

Table 1
Surface properties of Cu(ІІ)-PAA/M-MCM-41 NC.
Pore diameter by
BJH method (nm)
3
S
m /g)
BET
2
Pore volume (cm /g)
Sample
(
Cu(ІІ)-PAA/M-
1
01.5
1.22
0.114
MCM-41

Table 2
Optimization of the amount of catalyst, solvent and the reaction temperature for the
synthesis of polyhydroquinolines.
a
Yieldb
(%)
Entry
Catalyst
Solvent
Temperature
(°C)
Time
(min)
(g)
1
2
3
4
5
6
7
8
9
-
-
-
-
-
-
-
80
80
80
80
80
80
60
80
65
80
100
r.t
190
35
30
25
25
25
120
90
55
45
70
95
40
30
22
27
35
75
87
94
92
86
50
63
90
90
78
70
82
89
90
92
0.01
0.015
0.02
0.025
0.03
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
CHCl
3
CH CN
3
MeOH
EtOH
1
1
1
1
1
1
1
0
1
2
3
4
5
6
H
2
-
-
-
-
-
O
60
70
90
100
a
Solvent volume = 5 mL
Isolated yield.
b

Table 3
Preparation of polyhydroquinolines 5a-p in the presence of Cu(ІІ)-PAA/M-MCM-41
NC.a
Time
Yieldb
(%)
Mpc
(°C)
Entry
1
Ar
Product
5a
R
(min)
C
6
H
5
OEt
25
94
96
85
87
92
90
89
92
88
86
93
91
95
93
90
89
203-205
242-244
141-144
255-257
242-244
252-254
260-263
243-245
198-200
206-209
209-212
220-222
210-212
219-222
252-255
270-274
2
3
4
5
6
7
8
9
4-Cl C
4-CN C
4-OMe C
2,4-Cl C
4-Br C
4-Me C
4-NO
3-OEt-4-OH C
3-OMe-4-OH
6
H
4
5b
5c
5d
5e
5f
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OMe
OMe
OMe
OMe
20
28
34
24
28
30
40
30
31
28
33
30
27
26
36
6
H
4
6
H
4
6
H
3
6
H
4
6
H
4
5g
5h
5i
2
C
6
H
4
6
H
3
1
1
1
1
1
1
1
0
1
2
3
4
5
6
5j
C
6
H
3
2-Cl C
6
H
4
5k
5l
3-OH C
6
H
4
C
6
H
5
5m
5n
5o
5p
4-Cl C
2,4-Cl C
4-Me C
6
H
4
6
H
3
6
H
4
aReaction conditions: aldehyde = 1 mmol; dimedone =1 mmol; ethyl (or methyl)
acetoacetate = 1 mmol; ammonium acetate = 1.5 mmol; Cu(ІІ)-PAA/M-MCM-41 = 0.02 g.
b
Isolated yield.

cMelting points are uncorrected.
Table 4
The comparative study of the activity of Cu(ІІ)-PAA/M-MCM-41 NC with other
catalysts for the synthesis of compound 5a.
Time Yield
Entry
1
Catalyst
TiO NPs
Conditions
Ref.
[46]
(min)
(%)
2
EtOH, 80 °C, 10
mol% of catalyst
EtOH, reflux, 30
mg of catalyst
EtOH/reflux, 40
mg of catalyst
EtOH/reflux, 50
mg of catalyst
EtOH, reflux
EtOH, ultrasonic,
70 mg of catalyst
EtOH/80 °C, 50
mg of catalyst
Solvent-free, 90
°C, 1 mol% of
catalyst
105
92
2
3
4
5
6
7
8
9
Co
FSM-16-SO
Fe @B-MCM-41
MCM-41-OBF
SBA-15/SO
3
O
4
-CNTs
40
30
40
50
25
210
15
8
95
93
90
91
92
91
90
95
94
[60]
[37]
3
H
3
O
4
[49]
2
[50]
3
H
[52]
MCM-41@Serine@Cu
ІІ)
MCM-41
[54]
(
[40]
HClO
4
-SiO
2
Solvent-free, 90
[48]
°
C, 50 mg of
catalyst
1
0
Cu (ІІ)-PAA/M-MCM-
25
This Work
4
1
Solvent-free, 80
°
C, 20 mg of
catalyst

Figures
Click here to access/download;Figures;fig 1.tif

Figures
Click here to access/download;Figures;fig 2.tif

Figures
Click here to access/download;Figures;Fig 3.tif