Ceria-supported copper nanoparticles: A highly efficient and recyclable catalyst for N-arylation of indole

Article abstract of DOI:10.1016/j.molcata.2014.08.009

CuCeO₂ catalytic nanomaterials have been synthesized by impregnation of ceria by copper acetate, followed by reduction and calcination under nitrogen. The components of these nanomaterials were characterized by means of XRD, XPS, N₂

Full text of DOI:10.1016/j.molcata.2014.08.009

Accepted Manuscript
Title: Ceria-supported copper nanoparticles: A highly efficient
and recyclable catalyst for N-arylation of indole
Author: Othmane Amadine Houda Maati Karima
Abdelouhadi Aziz Fihri Sa ¨ı d El Kazzouli Christophe Len
Abdeslam El Bouari Abderrahim Solhy
PII:
S1381-1169(14)00356-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.08.009
MOLCAA 9224
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
5-7-2014
3-8-2014
4-8-2014
Please cite this article as: O. Amadine, H. Maati, K. Abdelouhadi, A. Fihri, S.E.
Kazzouli, C. Len, A.E. Bouari, A. Solhy, Ceria-supported copper nanoparticles: A
highly efficient and recyclable catalyst for N-arylation of indole, Journal of Molecular
Catalysis A: Chemical (2014), http://dx.doi.org/10.1016/j.molcata.2014.08.009
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Ceria-supported copper nanoparticles: A highly efficient and
recyclable catalyst for N-arylation of indole
a,b
a,b
c
a*
a
Othmane Amadine, Houda Maati, Karima Abdelouhadi, Aziz Fihri, Saïd El Kazzouli,
d,e
f
a,g *
Christophe Len, Abdeslam El Bouari, Abderrahim Solhy
a
MASCIR Foundation, Rabat Design, Rue Mohamed El Jazouli, Madinat El Irfane 10100-Rabat, Morocco.
FST, Université Hassan II-Mohammedia B.P. 146, 20650, Morocco.
CNRST, Division UATRS, Angle Allal Fassi/FAR, B.P.8027, Hay Riad, 10000-Rabat, Morocco.
b
c
d
Transformations Intégrées de la Matière Renouvelable, UTC/ESCOM, Centre de Recherche Royallieu, B.P. 20529,
0205 Compiègne, France.
6
e
f
Department of chemistry, University of Hull, Hull, HU6 7RX, England.
Laboratoire de Physico-Chimie des Matériaux Appliqués. Faculté des Sciences Ben M'Sik, Université Hassan II-
Mohammedia, Casablanca, Morocco.
g
Mohammed VI Polytechnic University, Lot 660 - Hay Moulay Rachid. 43150 Ben Guerir, Morocco.
Highlights
►
N-arylation of indole with various aryl bromides was carried out in good to excellent yields
using a nanostructured catalysis: Cu@CeO2.
►
This nanostructured catalysis based on ceria-supported copper nanoparticles was synthetized
and caracterized by sevral advanced analysis techniques .
►
The initial trials were performed to optimize the reaction conditions by varying different
parameters using a model reaction.
►
Using the optimized conditions, the scope was extended of this catalyzed N-arylation of
indole to the use of various aryl bromides.
Abstract
*
Corresponding authors: Fax: +212 530279827 (A. Solhy);
E-mail addresses: a.solhy@mascir.com; Abderrahim.Solhy@um6p.ma (A. Solhy), a.fihri@mascir.com (A. Fihri).
1
Page 1 of 35

Cu@CeO
acetate, followed by reduction and calcination under nitrogen. The components of these
nanomaterials were characterized by means of XRD, XPS, N adsorption/desorption, TGA,
SEM, and TEM microscopy techniques. The XRD spectra of ceria and Cu@CeO _1 (3.4 wt%
2
catalytic nanomaterials have been synthesized by impregnation of ceria by copper
2
2
copper loading) revealed a slight shift of the position of the planes with a small increase of the
0
crystal lattice parameters, which can be explained by an eventual insertion of Cu in interstitial
4+
2+
2
sites and/or a partial substitution of Ce by Cu . Cu@CeO nanomaterials were found to be
efficient catalysts for the N-arylation of indole with various aryl bromides to produce good yields
of the desired products. The decrease in the catalytic activity of the catalyst during the reuse was
0
2+
probably due to oxidation of Cu to Cu and it poisoning of KBr that was formed in situ.
Keywords: Nanocatalysis; Ceria; Cu-catalyzed; N-arylation; Indole; Catalyst Reuse.
2
Page 2 of 35

1. Introduction
Catalysis is widespread in the manufacturing industry and over 60% of manufactured
products have undergone at least one catalytic cycle [1]. Although homogenous and
heterogenous catalysis are the two main methods of catalysis currently developed, nanocatalysis
is a third option that combines the advantages of both types of previously established catalysis
and is being explored by both the academic and manufacturing industries [2]. A large portion of
research regarding catalysis has often focused on the metal nanoparticles supported on
nanostructured materials, specifically mesoporous non-silicious oxides [3,4]. Indeed, ceria fits
into this category. It is one of the most important functional rare earth oxides [5,6], and has
attracted significant interest because of its wide applications in cutting-edge technology sectors
like automotive three-way catalysts, solid oxide fuel cells, UV absorption ability, gas sensors,
and antioxidant therapy [7-12]. All these uses are due to certain unique properties of ceria,
particularly its: inherent chemical component, high oxygen capacity, and high oxygen mobility
4+
3+
2
originating from storing/releasing O due to the Ce /Ce redox couple. However, this oxide has
been widely used as a catalyst or support-catalyst of several metal nanoparticles for various
chemical transformations [13-16]. Thus, Cargnello et al. demonstrated that ceria as a support has
a crucial role in the catalytic activity and the interaction between the metal nanoparticles (nickel,
palladium, and platinum) and this oxide (support) is decisive for the CO oxidation [17]. In
another work, Li et al. conducted a comparative study on several catalysts prepared by
2 2 2 3
supporting gold on different materials, namely: CeO , TiO , Al O , activated carbon, and
graphene [18]. This experimental and theoretical study demonstrated that the use of ceria as
support has a synergistic effect and has boosted the catalytic activity of gold. These observations
led Li et al. to conclude the presence of a strong interaction and an electronic exchange between
metal and ceria in comparison with the other oxides. The impregnation of ceria by copper
3
Page 3 of 35

nanoparticles or copper oxide, or vice versa (ceria supported on copper nanoparticles), is widely
studied for oxidation of CO [19,30], and organic volatile compounds [21,22], but especially for
Water-Gas-Shift reaction [23-25]. However, it should be noted that the majority of studies
revealed that there is a cooperative effect of ceria doesn’t just have an inert support role, but also
2
contributes to the catalytic activity of the system: metal@CeO . Given the central importance of
copper nanoparticles in boosting the catalytic activity of ceria, or reciprocally, this catalyst has
been studied by using theoretical tools to elucidate and corroborate experimental data [26-28].
Alternatively, N-aryl indoles have attracted a keen interest over the last few years due to
their diverse biological activities that have already been revealed and exploited [29,30].
Historically, the synthesis of these N-substituted heterocyclic compounds has been carried out
via the Ullmann coupling reactions. However, these reactions generally suffered from high
temperatures and the use of stoichiometric amounts of copper reagents, which make their further
applications limited [31,32]. To overcome these drawbacks, another class of catalysts was
developed by Buchwald & Hartwig, who separately reported that the N-arylation of nitrogen
containing heterocycles with aryl halides could be achieved in the presence of appropriate copper
compounds and ligands under mild conditions [33-38]. However, these homogeneous systems
pose significant issues related to product purification andtoxic waste produce after separation of
a copper catalyst. One of the most favorable ways to overcome these problems is the realization
of the N-arylation reaction via heterogeneous catalysts [39-44]. Most of these systems are
copper-based catalysts in various forms, including: i) copper nanoparticles alone or supported
1+
2+
[
45-47], ii) copper oxides [48-53], and iii) supported copper (Cu or Cu ) [54-59]. Relatedly,
Hyeon et al. prepared CuO coated copper nanoparticles by thermal decomposition, using copper
acetylacetonate. They then reported the use of this nanostructured material to catalyze the
Ullman amination coupling reaction of various aryl chlorides [60]. In another example,
Choudary et al. designed a catalyst for N-arylation of imidazoles via the cation exchange of
4
Page 4 of 35

fluorapatite and tert-butoxyapatite with copper [61]. In light of these facts, Choudary et al.
discovered this reaction by using arylboronic acids and obviously a copper-exchanged
fluorapatite-like catalyst [62]. Another kind of material to catalyze the N-arylation reaction has
been developed recently; it comes from a nanostructured mixed oxide, based on iron and copper
[63]. Zhang et al. synthesized and then tested a series of magnetic mixed oxides (CuFe
2 4
O ,
MgFe , NiFe , MnFe , CoFe ). These authors revealed that CuFe was the best of
2
O
4
2
O
4
2
O
4
2
O
4
2 4
O
all the prepared oxides, with an excellent yield obtained [64]. It is in light of these studies that
this work is situated. The objective of this study is the synthesis and characterization of ceria
doped with copper nanoparticles, and then its use as a catalyst in the reaction of N-arylation of
indole with bromo-aryl derivatives (Scheme 1).
Scheme 1.
2. Experimental
2.1.Materials and apparatus
All commercial reagents were purchased from Aldrich Chemical Company and were used
without further purification. Thermogravimetric analysis (TGA) was conducted under air or
nitrogen in a TA Instrument Q500 apparatus, with a 10 °C/min ramp between 25 and 1000 °C.
Fourier transform infrared (FT-IR) spectra of samples in KBr pellets was measured on a Bruker
Vector 22 spectrometer. X-ray diffraction patterns were obtained at room temperature on a
Bruker AXS D-8 diffractometer using Cu-K radiation in Bragg-Brentano geometry (-2).
TEM micrographs were obtained on a Tecnai G2 microscope at 120 kV. High-resolution
transmission electron microscopy analysis was carried out on a Jeol 2100F microscope, equipped
with a high-resolution pole piece, field emission gun and operating at 200 kV. The gas
adsorption data was collected using a Micromeritics 3Flex Surface characterization analyzer,
2 2
using N . Prior to N sorption, all samples were degassed at 150 °C overnight. The specific
5
Page 5 of 35

surface areas were determined from the nitrogen adsorption/desorption isotherms (at -196 °C),
using the BET (Brunauer–Emmett–Teller) method. Pore size distributions were calculated from
the N
BJH) [65]. XPS studies were carried out in a Kratos Axis Ultra DLD spectrometer equipped
with a monochromatic Al Kα X-ray source (h = 1486.6 eV) operating at 150 W, a multi-
2
adsorption isotherms with the ‘‘classic theory model’’ of Barret, Joyney and Halenda
(
-9
channel plate and delay line detector under 1.0x10 Torr vacuum. The survey and high-
resolution spectra were collected at fixed analyzer pass energies of 160 and 20 eV, respectively.
The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy of
83.95 eV. The spectrometer dispersion was adjusted to give a binding energy of 932.63 eV for
metallic Cu 2p3/2. Samples were mounted in floating mode in order to avoid differential
charging; charge neutralization was required for all samples. The electronic binding energy of C
1
s (284.80 eV) was used as the internal standard. The data was analyzed with commercially
available software, CasaXPS. The individual peaks were fitted by a Gaussian (70%)-Lorentzian
30%) (GL30) function after Shirley type background subtraction.
(
2
2
.2. Preparation of the catalyst
.2.1. Preparation of ceria
For preparation of ceria powder by the co-precipitation method, high purity (99.5%) ammonium
cerium (IV) nitrate (NH Ce(NO and ammonia (NH OH) were used. In a typical procedure,
4
)
2
)
3 6
4
ammonia (25%) was added by the drop to an aqueous solution of ammonium cerium (IV) nitrate,
which was prepared by solubilizing 1 g of cerium precursor in 100 mL of water. The ammonia
was added until a basic pH (~12) was reached. Thus, the reaction took place under vigorous
stirring during 4 hours, at 70 °C. The obtained light yellow precipitate was separated by
centrifugation and washed with water several times. The obtained solid was dried overnight at
80 °C under vacuum and calcined for 3 hours at 550 °C to lead to cerium oxide.
6
Page 6 of 35

2.2.2. Preparation of Cu@CeO
2
The preparation of Cu@CeO
2
was performed by impregnating the ceria with copper acetate with
2+
a loading of 3.4 and 7 wt%, followed by the reduction of the Cu to copper nanoparticles. The
first step was to dissolve copper acetate (~0.54mmol) in 10 mL of diethylene glycol, where
another aqueous solution of sodium dodecyl sulphate (0.1M), was subsequently added. In the
second step, 1 g of previously prepared ceria was added to the first mixture and stirred at 70 °C
4
for 24 hours. The pH of this mixture was adjusted to pH>10 by adding NH OH. Then, hydrazine
2+
hydrate (80%, 1 mL) was added to reduce Cu to copper nanoparticles. In the final step, the
resulting powder was filtered and sequentially washed with ethanol and acetone, dried at 80 °C
under vacuum, and finally calcined at 550 °C under nitrogen for 3 hours. According to the
measurement of copper content, the aforementioned procedure led to two catalysts: Cu@CeO
3.4 wt%) and Cu@CeO _2 (7 wt%).
2
_1
(
2
2.2.3. General procedure for the N-arylation of N-H heterocycles with aryl bromides
5
mol% of catalyst was added to a stirred mixture of indole (0.5 mmol), 4-bromotoluene (1
mmol), KOH (1 mmol), and 1,10-phenanthroline (0.256 mmol). The reaction mixture was
refluxed in 2 mL of dimethylsulfoxyde for 24 hours. The reaction was monitored by thin layer
chromatography. After completion of the reaction, the catalyst was removed by filtration, and the
mixture was diluted with water and extracted with dichloromethane (3x10 mL). The organic
phase was dried over anhydrous sodium sulfate, concentrated and then subjected to flash column
chromatography on silica gel, which led to the isolation of the desired product. The structures of
1
the prepared products were confirmed by H NMR and were assigned on the basis of their
spectral data in comparison to those reported in other literature. The recovered catalyst was
7
Page 7 of 35

washed twice with dichloromethane and water, and dried at 100 °C before being reused in
subsequent reactions to demonstrate its prolonged activity.
3. Results and Discussion
3.1. Catalyst characterization
Once the materials were prepared, a systematic analysis was needed in order to correlate their
structural, textural and morphological properties to their catalytic activity. Therefore, TGA was
used to determine the calcination temperature and the thermal stability of the products. It should
be noted that all samples that were prepared as part of this study have been previously analyzed
2
by this technique (See Supporting Information). Thus, the thermal stability of dried Cu@CeO _1
was examined by TGA analysis under nitrogen (Fig. 1), and under air (Fig. S3). It is evident that
the analysis under nitrogen clearly exhibited three steps of weight loss, and while under air it
decomposed in two major steps. In the nitrogen analysis, the first weight loss of 70 to 110 °C
relates to the loss of water molecules. The second weight loss was 150 to 280 °C, which was due
to the degradation of organic molecules. The third loss above 500 °C relates to maturation and
crystallization of copper nanoparticles. TGA under air shows similar characteristics, but weight
loss occurred toward higher temperatures (700-860 °C). This observation corresponds to the
formation of CuO@CeO
2
_1, which was subsequently confirmed by XRD (Fig. S4).
Figure 1
Fig. 2 represents the powder X-ray diffraction spectra of the ceria obtained by co-precipitation,
and ceria-supported copper at two different copper loadings (Cu@CeO _1, Cu@CeO _2).
Firstly, the XRD pattern (Fig. 1a) confirmed that the as-synthesized ceria exhibits a face-
centered cubic fluorite structure (Fm3m; JCPDS card no. 34-0394). In the Cu@CeO _1 sample,
all the peaks are attributable to the ceria. By cons, the diffractogram of the Cu@CeO _2 sample
2
2
2
2
showed the presence of a secondary phase in which the diffraction peaks correspond to metallic
8
Page 8 of 35

copper. However, the Rietveld refinement allowed the estimation of the lattice parameters of the
2 2
three samples: ceria, Cu@CeO _1 and Cu@CeO _2 (Table 1, Fig. S6). The criteria (Rp, Rwp
2
and χ ) indicate the quality of this refinement. The value obtained for the undoped sample (ceria)
is 0.54112 nm, which is close to the standard value previously reported (a = 0.541134 nm,
JCPDS 34-0394). An increase in the lattice parameter is observed in the doped samples (Table
1), which is consistent with the shift of the diffraction peaks (Fig. S5). The increase in the lattice
parameter is linear up to a doping level of 7%, loading corresponding to the appearance of the
0
secondary phase (Cu NPs). The conservation of the crystallographic structure of the ceria and
the increase of the lattice parameter suggest that the samples are solid solutions of substitution in
which the cerium is replaced by copper. The solubility limit of copper in the ceria lattice seems
to be between 3.4% and 7% loading into copper or doping percentage. The evolution of the
lattice parameter of ceria dependent upon the loading of copper is shown in Fig. 3. The lattice
parameter of ceria increases with the increase of the percentage of doping of copper. This is in
accordance with the observed shift of the diffraction peaks. It should be noted that the ionic
2+
4+
4+
radius of the cupric ions (rCu = 0.081 nm) is lower than that of Ce ions (rCe = 0.097 nm).
Figure 2
Figure 3
Table 1
Both scenarios may be presented in the case of the insertion of a small doping element within the
unit cell of ceria. In the first scenario, the doping element can occupy the atomic positions of
4+
Ce ions. In this case, the lattice parameter varies with the size of the doping element. With
copper, the positive charge deficiency leads to the creation of oxygen vacancies. The formula for
the substituting solid solution is then Ce1-xCu
x
O2-x, where x is the doping percentage into copper.
In the second scenario, the doping element is able to occupy interstitial sites, apart from the
4+
atomic positions of Ce ions. In this situation, the lattice parameter may increase even if the
9
Page 9 of 35

4+
dopant is smaller than Ce cation. The formula of the interstitial solid solution is
2+
x 2
Ce1-(1/2)xCu O in the case of Cu . This insertion mechanism does not cause deficiency in
positive charges, and thus does not lead to the formation of oxygen vacancies. In our case, the
increase of the lattice parameter suggests that copper ions are not substituted by cerium ions but
occupy interstitial sites in the structure of ceria, which causes an expansion of the lattice and an
increase in its volume. The increase of the lattice parameter seems to reach a threshold with the
appearance of the secondary phase when the ceria is doped by 7% of copper. The metallic copper
0
appearance of the secondary phase (Cu of cubic structure, JCPDS no. 85-1326) indeed indicates
that copper saturates the unit cell. The cupric ions can no longer get into the unit cell and no
longer affect its volume. The projection of crystal unit cell of ceria and Cu@CeO
shown in Fig. 4. This projection viewed along the b-axis shows the spatial distribution of ceria
and the unit cell of Cu@CeO (3.4 wt%), illustrating the sequence of units and the location of
copper ions within the crystal lattice.
2
(3.4 wt%) is
2
Figure 4
The FTIR spectra of CeO
2
, Cu@CeO
2
2
_1 and Cu@CeO _2 are presented in Fig. 5. The three
-1
spectras show the bands at 3300 and 1640 cm correspondingto the -OH stretching vibration and
to the H O bending vibration, respectively [66]. The fingerprint of ceria is assigned in FTIR
2
-1
spectroscopy to the Ce–O–Ce stretching vibration, which is detected toward 400 cm [67]. It is
noteworthy that the three spectras are similar in the existence of the characteristic band of ceria
in the three cases, which proves that the base framework of the structure does not change when
ceria is loaded with copper.
Figure 5
1
0
Page 10 of 35

2
The surface compositions and chemical states of Cu@CeO _1 were studied by using X-ray
photoelectron spectroscopy. Fig.6, spectra a, shows the Ce 3d core-level spectra of the
4+
2
Cu@CeO _1. Indeed, we detected the existence of two oxidation states of cerium (Ce and
3+
Ce ), which can be explained by the complex structure of the core-level Ce 3d spectra [68]. The
highest binding energy peaks located at approximately 916.9 and 898.3 eV, respectively, are the
9
0
4+
result of a Ce 3d 4f O 2p6. These satellite peaks are characteristic of the presence of Ce ions.
3+
However, the other peaks are characteristic of Ce . In addition, the O 1s core-level peak from
the lattice oxygen in Cu@CeO was detected at about 529.44 and 531.48 eV, respectively
spectra c). The presence of these two peaks is due to the presence of Ce-O-Cu bonds and Ce
2
3
+
(
0
states [69]. The Cu 2p3/2 peak at 932.68 eV was assigned to Cu (spectra b). The Cu 2p3/2 peak
2+
at 934.28 eV suggested the presence of a Cu states. This can be alternatively interpreted with a
possible electronic exchange between copper and ceria, which has been partially reduced to
CeO
x 2
. The XRD corroborate these observations, showing only the characteristic peaks of CeO
that are incorporated into its structure of copper.
Figure 6
The UV-visible spectroscopy analysis of all prepared materials also confirms the XPS
observations. Thus, the UV-Vis spectra of CeO , Cu@CeO _1 and Cu@CeO _2, show three
characteristic peaks at 222, 307 and 370 nm, respectively (Fig.7). These peaks are assigned to f-d
2
2
2
3+
4+
transitions of Ce species, low coordination Ce -O charge transfer and inter-band transitions in
CeO , respectively. The shoulder at 400-560 nm can be attributed to d-d transition of Cu in
Cu@CeO [70] or absorption from the conduction band of metallic copper [71]. The UV-visible
spectra analysis of the Cu@CeO samples is more complex due to overlapping of the above
mentioned bands, related to the presence of Ce and Ce cations and bands assigned to the
2
2
2
3+
4+
0
2+
various forms of copper: Cu and Cu .
1
1
Page 11 of 35

Figure 7
2
SEM was used to study the morphology of the surface of ceria and Cu@CeO _1 (Fig. 8). These
micrographs show that both materials had a heterogeneous microstructure that consisted of
crystallites of various sizes and forms, with an irregular surface roughness. It should be noted
that the morphology of ceria does not undergo a major change after loading with 3.4% of copper.
Figure 8
Table 2
Figure 9
TEM analysis of ceria and ceria-supported copper revealed that both samples are well
nanostructured with a quasi-spherical morphology and a particle size range from 9 to 15 nm
(
Table 2). It should be noted that the TEM images of the Cu @ CeO2_2, presented in the
Supporting Information (Fig. S7), are typical of a nanostructured material with an average size of
0 nm. In addition, the estimated crystallite size by Scherrer's equation was also summarized in
1
Table 2. Since the nanoparticles are agglomerated, we note that the dispersion of both samples is
relatively heterogeneous, especially after the doping of ceria by copper. This observation can be
explained by the Ostwald ripening phenomena. The HR-TEM technique was also used in order
to analyze mostly the inter-planar distances of the local structure of the catalyst. The images (c),
2
(d) and (e) in Figure 8 show the representative HR-TEM images of Cu@CeO _1. These images
show the arrangement of nanoparticles in this structure, which is characterized by its special egg
or ellipsoid shape (e). The measured interplanard-spacing on the image (d) is near 3.14 Å, which
corresponds to the atomic plane (111) of the fluorite structure of ceria. This slight increase in the
distance confirms the swelling of the crystal lattice of the cerium oxide after its doping by
1
2
Page 12 of 35

copper. This observation is in perfect harmonization with the results of XRD. Furthermore, we
note that these observations are corroborated by the SEAD (Fig.9f).
Figure 10
The calculated texture parameters of ceria, Cu@CeO
2
2
_1 and Cu@CeO _2 are summarized in
2
Table 2. It can be seen that the BET surface area of ceria is 31 m /g and its pore volume is
3
2
0
.0521 cm /g, while the surface area of the Cu@CeO
2
_1 is 45 m /g and its pore volume 0.1655
3
cm /g. The increase of the copper content has a significant effect on the surface area and pore
2
volume. Thus, for the sample prepared with 7 wt% copper, the surface area is 12 m /g and the
3
pore volume is 0.0566 cm /g. The nitrogen sorption isotherm of Cu@CeO
2
_1 is of type IV,
displaying hysteresis loops in the relative pressure range of 0.6-0.9 and plateaus in the adsorption
branches, indicating its mesoporous nature (Fig. 10a). The corresponding pore size distribution
curve (Fig. 10b) indicates that Cu@CeO
areas towards 4 and 17 nm, corresponding to mesoporous materials. The shape of the isotherm of
ceria (Fig. S8) is not exactly identical to that of Cu@CeO _1. It was initially characterized as
2
_1 has a centralized pore size distribution within two
2
either type IV or V, since the hysteresis loop manifests the existence of open mesopores, which
are characteristic of type IV or V isotherms. The hysteresis loop is of type H3 according to the
IUPAC description system. As shown in Figure S9, the nitrogen sorption isotherms of the ceria
2
and Cu@CeO _2 exhibits a hysteresis loop in the relative pressure range above 0.90, and a
plateau does not appear in the adsorption branch. These observations indicate the existence of
open mesopores.
3.2. N-arylation of indolesover nanostructured catalysts
1
3
Page 13 of 35

In an effort to findthe best catalytic system, variouscatalysts such as ceria, Cu@CeO
2
_1,
Cu@CeO _2, CuO@CeO , Cu nanoparticles, and Cu(OAc) were screened for the N-arylation of
2
2
2
indole with 4-bromotoluene (Scheme 2). The results are summarized in Table 3.
Scheme 2.
First, it should be noted that a reaction without a catalyst did not occur (Table 3, entry 1), and the
ceria alone as a catalyst was also inactive in this reaction (Table 3, entry 2). However, the doping
of ceria by copper had a significant effect on catalytic activity (Table 3, entries 3 and 4). The
yields were, respectively, 86 and 80% for Cu@CeO
Cu@CeO _1 gave a yield slightly higher than Cu@CeO
explained by the difference in crystallographic and electronic structures between the two
2
_1 and Cu@CeO
2
_2. We note that
2
2
_2. This slight difference may be
4+
3+
2+
0
samples. Thus, the existence of two couples Ce /Ce and Cu /Cu , which were revealed by
2-
2
XPS, with the high concentration of O in the support (ceria), makes the Cu@CeO _1 catalytic
system more effective, since it is highly probable that there is an electronic exchange between
these couples red/ox existing in the same crystal lattice. This increases in the concentration of
0
copper metal is a key element in this chemical process because the limiting step requires Cu . To
argue this interpretation, we conducted this reaction in the absence of any ligand and the yield
obtained was 51% (Table 3, entry 5). This demonstrated the existence of some percentage of
0
Cu , which is perfectly correlated with the XPS data. The use of mixed oxide CuO@CeO
2
prepared by co-precipitation with a loading of 3.4% copper gave a very low yield(Table 3, entry
6), corroborating with the above interpretation, and revealing that the electronic structure of the
0
copper is crucial to catalyze this reaction. By cons, catalysis by copper nanoparticles (Cu )
formed in-situ, translated to a yield of 79%, which is somewhat similar to the yield obtained by
2
Cu@CeO2_1 (Table 3, entry 7).The last test was performed using homogeneous Cu(OAc) as
1
4
Page 14 of 35

the catalyst and led to very low yield. These results clearly show the importance of our catalytic
system developed in this work in comparison with the homogeneous catalysis.
Table 3
The effect of other parameters, namely: type of solvent, nature of the base, reaction temperature,
and catalyst loading was also investigated. The results obtained are summarized in Table 4. We
initially noted that the DMSO and DMF, which have the highest dipole moments and boiling
temperatures, gave the best yields (Table 4, entries 4 and 8). Acetonitrile and dioxane are both
highly polar solvents in the same range as DMSO and DMF and have boiling temperatures less
than the other two solvents (DMSO and DMF). We obtained yields unless in both other cases
(Table 4, entries 5 and 7). With the use of toluene (Table 4, entry 6), we obtained 66% yield,
which justified the influence of solvent polarity. Interestingly, when DMF was used as the
solvent, we observed that this reaction was sensitive to temperature (Table 4, entries 1-
4).According to these studies (nature of solvents and reaction temperature), we can conclude that
temperature is a key parameter in this type of reaction. The study of the influence of the nature of
the base shows that the best yields were obtained by using strong bases such as: NaOH and KOH
2 3
(Table 4, entries 8 and 9). By cons, when we used K CO (pKa = 10.2) and pyridine
(pKa = 5.229), the obtained yields were 60 and 57%, respectively. The use of cerium oxide,
which inherently has a basic character [72], only gave 25% in yield. However, the absence of the
base completely inhibits the N-arylation of indole (Table 4, entry 13) (knowingly, the base has a
key role in the catalytic cycle by snatching the acid proton of the indole). The comparison
between 2,2’-bipyridine and 1,10-phenanthroline shows that these ligands give quantitative
yields (Table 4, entries 8, 14), but with a higher performance like 1,10-phenanthroline. This
dissimilarity can be explained by the variation in the spatial distribution of each ligand. Thus,
0
1
,10-phenanthroline, which has a rigid and stable structure can stabilize more Cu . By cons, the
1
5
Page 15 of 35

electron cloud is so delocalized and the rotation of the C-C bond is so strong in 2,2’-bipyridine
0
that the stabilization of Cu is a bit difficult. On the other hand, the decrease of the catalyst
loading by half gave a yield of 72% (Table 4, entry 14), which demonstrated the influence of the
mass of catalyst in this type of reaction.
Table 4
Once the optimal conditions for this reaction model were established, we looked at examining
the utility of this methodology. In order to generalize the reaction of N-arylation, we used
2
Cu@CeO _1 as the catalyst, KOH as the base, 1,10-phenanthroline as the ligand and DMSO as
the solvent under refluxing condition. Thus, the scope of the N-arylation of indole was tested
with respect to other aryl bromides; the results of the test are shown in Table 5.Initially, the
substitution effects of aryl bromides were investigated. Thus, the substituted aryl bromides gave
quantitative yields independent of the kind of substitutions used (electron withdrawing, electron
donating) (Table 5, entries 1-4) and their positions (Table 5, entries 1 and 3). Under the above
optimal conditions, 1-bromonaphthalene underwent the C-N coupling reaction with excellent
yield, in spite of the effect of steric hindrance (Table 5, entry 5). Thereafter, 2-bromopyridine
was effectively coupled with an indole to afford the desired coupling product in a good yield,
under the same experimental conditions (Table 5, entry 6).
Table 5
Besides activity and selectivity, one expected advantage of supported catalytic systems lies in the
easy separation of catalytic materials and possible recycling of the system (depending on its
2
robustness). After a catalytic test, the Cu@CeO _1 powder was filtered, rinsed with a small
portion of dichloromethane, and dried under vacuum for 4 hours. A second catalytic cycle was
successfully achieved for the N-arylation of indole with 4-bromotoluene under the same
1
6
Page 16 of 35

conditions. The catalyst was recyclable for up to three cycles, although a progressive decrease in
activity was observed (Table 6).
Table 6
2
In order to clarify the behavior of Cu@CeO _1 during recycling experiments, post-catalysis
characterizations were conducted. The oxidation state of the catalyst after one catalytic cycle was
examined by XPS. The obtained results revealed that the degree of oxidation state of cerium and
copper were not changed (Fig. 10) except that their concentrations were reduced in comparison
+
with XPS of fresh catalyst. By cons, we note the presence of the K with values compatibles to
+
K of the KBr in the recovered catalyst, which may explain the linear decrease of yield after each
re-use (as has already been reported) [73].To investigate the heterogeneity of the catalyst, the
solid catalyst was suspended in DMSO for 24 hours under reflux. After that, the catalyst was
filtered and the reaction mixture of indole, 4-bromotoluene, base and 1,10-phenanthroline were
added to the filtrate. After 24 hours, there was not any product was detected. These results
suggest that the solid catalyst caused the N-arylation reaction. Also noteworthy is the reuse of
2
Cu@CeO _2 that gave 74% yield in comparison with the fresh catalyst. The XRD analysis
revealed more significant and informative differences. In the recovered catalyst, we observed the
appearance of a third crystal phase that corresponded to copper oxide. It is clear that copper
nanoparticles in the used catalyst (Cu@CeO
Fig. S10).
2
_2) are oxidized to copper (II)
(
Figure 10
Conclusions
In summary, the work undertaken was aimed at exploring new ways to synthesize nanostructured
catalysts based on ceria-supported copper. The impregnation of the ceria by copper acetate
followed by reduction and calcination under nitrogen, leads to the formation of two
1
7
Page 17 of 35

nanostructured catalysts. Physicochemical properties of these materials were evaluated by
FT-IR, XRD, SEM, TEM, TGA, BET, XPS as well as UV. A Rietveld refinement was
2
performed to provide a clear idea about the structure of the Cu@CeO , especially on the site
where copper exists in the crystal lattice of ceria. First, a study of the evaluation and screening of
catalysts was achieved to find the best catalyst for N-arylation reaction of indole with
4-bromotoluene. This study revealed that the Cu@CeO2_1 gave the best yield compared to the
other catalytic systems tested. This can be explained by an eventual interaction of copper with
the support, which stabilized the metallic states of copper. Thereafter, we studied the effect of
several parameters to optimize the catalytic process. The said optimum conditions were adopted
to generalize this process to different aryl bromides. The yields obtained were generally
quantitative. The possibility to reuse the catalyst was also realized with the use of XPS, which is
2
an important analytical tool for this type of study. From characterization of the Cu@CeO _1, we
0
2+
showed that Cu was present in the Cu and Cu states before and after reaction. This was
assigned to a reversible exchange of electrons during the reaction between the support (ceria)
2
+
0
and the couple Cu /Cu . The decrease in the catalytic activity of Cu@CeO
by a catalyst poisoning of K and Br ions, which are formed in situ during the reaction and get
stuck on the catalyst surface even after washing. By cons, the study of the reuse of Cu@CeO _2
7 wt%) shows that the yield of the model reaction was decreased, and the analysis of the
2
_1 can be explained
2
(
recovered catalyst by XRD revealed the existence of a third phase of crystallographic that
corresponded to the copper oxide (CuO). This indicated that the isolated copper nanoparticles
were oxidized. The characterization and the application of this material in the catalyzation of the
N-arylation reaction of indole were major assets, illustrating the originality of this research work.
The transfer of this kind of catalysts in other catalytic processes is presently being examined in
our laboratory, and further research will focus on the development of new nanostructured
catalysts.
1
8
Page 18 of 35

Acknowledgments
The financial assistance of the Office Chérifien des Phosphates in the Moroccan Kingdom
towards this research is hereby acknowledged. We equally thank the support of the OCP
Foundation.
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2
1
Page 21 of 35

Table 1. The results emerging from the Rietveld refinement of CeO , Cu@CeO _1 and Cu@CeO _2.
2
2
2
Copper
loading
Lattice
parameter
a (Å)
5.41122
5.41494
5.41755
Cell
2
Sample
CeO2
a (Å)
volume
Rp
Rwp

3
(wt%)
(Å )
0
3.4
7
0.00059
0.00118
0.00094
158.44756
158.77457
159.00427
9.3
8.2
12.4
13.7
12.2
16.3
1.61
2.28
1.29
Cu@CeO
Cu@CeO
2
_1
_2
2
Table 2. Lattice parameter, crystallite size, nanoparticle diameter, BET surface area and pore
volume for all as-synthesized samples catalysts
c
c
c
Dmeso
/
a
b
S
m g
BET
/
V /
meso
Sample
Crystallite size /nm
Particle size /nm
2
-1
-1
mL g
nm
CeO2
Cu@CeO _1
Cu@CeO _2
2
12.9
13.4
12.2
9
15
10
31
45
12
0.0521
0.1655
0.0566
7
14
17
2
a
b
c
Calculated by Debye-Scherer equation. Evaluated by TEM. From nitrogen sorption analysis.
a
Table 3. Evaluation and screening of catalysts
b
Entry
Catalyst
Yield (%)
1
2
3
4
5
6
7
8
---
CeO
Cu@CeO
Cu@CeO
Cu@CeO
n.r.
n.r.
86
2
2
2
2
_1
_2
80
c
_1
51
d
CuO@CeO
2
35
79
41
e
Cu NPs
Cu(OAc)
2
a
Reaction conditions: catalyst (5 mol%), 4-bromotoluene (1 mmol), indole (0.5 mmol), KOH (1 mmol), 1,10-phen
(
0.256 mmol), DMSO (2 mL), reflux for 24 h.
b
c
d
e
Isolated yield.
Reaction in the absence of ligand.
CuO@CeO
2
was prepared via co-precipitation method.
Cu NPs was obtained in situ by reduction of Cu(OAc)
2
2 2
with NH NH .
2
2
Page 22 of 35

Table 4. N-arylation of indole with 4-bromotoluene in various solvents and bases in the presence
a
of Cu@CeO
2
_1 as catalyst
b
Entry
Solvent
DMF
DMF
DMF
DMF
Dioxane
Toluene
Acetonitrile
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
Base
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOH
Temp (°C)
r.t.
Yield (%)
n.r.
n.r.
n.r.
83
1
2
3
4
5
6
7
8
9
80
110
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
reflux
73
66
57
86
80
60
57
25
1
1
1
1
1
1
0
1
2
3
4
5
K
2
CO
3
Pyridine
CeO
---
2
n.r.
c
KOH
KOH
72
72
d
a
Reaction conditions: Cu@CeO2_1 (5 mol%), 4-bromotoluene (1 mmol), indole (0.5 mmol), KOH (1 mmol), 1,10-
phen (0.256 mmol), DMSO (2 mL), reflux for 24 h.
Isolated yield.
Reaction using 2,2’-Bipyridine as ligand.
Reaction using 2.5 mol% of catalyst.
b
c
d
2
3
Page 23 of 35

2
Table 5. N-arylation of indole with various substituted aryl bromides using Cu@CeO _1 as
catalyst
b
Entry
1
Aryl-Br
Product
Yield(%)
88
N
Br
Br
2
3
86
89
N
N
Br
Br
4
5
6
85
87
82
Cl
N
Cl
Br
N
N
Br
N
N
a
2
Reaction conditions: Cu@CeO _1 (5 mol%), aryl bromide (1 mmol), indole (0.5 mmol), KOH (1 mmol), 1,10-
phen (0.256 mmol), DMSO (2 mL), reflux for 24 h.
Isolated yield.
b
2
Table 6. Recycling study of Cu@CeO _1 in the N-arylation of indole with 4-bromotoluene.
Conditions are the same as those given in Table 5.
Cycle
Product yield (%)
Fresh
86
79
70
67
1
2
3
2
4
Page 24 of 35

Ceria-supported copper nanoparticles: A highly efficient and
recyclable catalyst for N-arylation of indole
a,b
a,b
c
a*
a
Othmane Amadine, Houda Maati, Karima Abdelouhadi, Aziz Fihri, Saïd El Kazzouli,
d,e
f
a,g*
Christophe Len, Abdeslam El Bouari, Abderrahim Solhy
a
MASCIR Foundation, Rabat Design, Rue Mohamed El Jazouli, Madinat El Irfane 10100-Rabat, Morocco.
FST, Université Hassan II-Mohammedia B.P. 146, 20650, Morocco.
CNRST, Division UATRS, Angle Allal Fassi/FAR, B.P.8027, Hay Riad, 10000-Rabat, Morocco.
b
c
d
Transformations Intégrées de la Matière Renouvelable, UTC/ESCOM, Centre de Recherche Royallieu, B.P. 20529,
0205 Compiègne, France.
6
e
f
Department of chemistry, University of Hull, Hull, HU6 7RX, England.
Laboratoire de Physico-Chimie des Matériaux Appliqués. Faculté des Sciences Ben M'Sik, Université Hassan II-
Mohammedia, Casablanca, Morocco.
g
Mohammed VI Polytechnic University, Lot 660 - Hay Moulay Rachid. 43150 Ben Guerir, Morocco.
2
5
Page 25 of 35

Cu@CeO (5 mol%)
2
Aryl Br
DMSO, KOH, 1,10-phen
N
N
H
Aryl
Scheme 1.
1
00
0
0
0
0
0
0
,10
,08
,06
,04
,02
,00
9
9
8
8
5
0
5
0
-0,02
0
100 200 300 400 500 600 700 800 900 1000
Temperature (°C)
2
Figure 1. TGA spectra of dried Cu@CeO _1 under nitrogen atmosphere.
2
6
Page 26 of 35


*
CeO2
Cu








*






(
c)
*
*


(b)








(a)
0
10 20 30 40 50 60 70 80
(Degree)
2
Figure 2. X-Ray diffraction patterns of some representative samples: ceria (a), Cu@CeO
and Cu@CeO _2 (c).
2
_1 (b),
2
2
7
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5,418
5,417
5,416
5,415
5,414
5,413
5,412
5,411
0
1
2
3
4
5
6
7
8
Copper loading (wt%)
Figure 3. Lattice parameter in function of loading into copper
2
Figure 4. Projection view of ceria (a) and Cu@CeO _1 (b).
2
8
Page 28 of 35

(c)
1
640
3
300
(b)
~500
1
640
3
3
300
300
(a)
~500
1
640
~500
4000 3500 3000 2500 2000 1500 1000 500
-
1
Wave number (cm )
2 2
Figure 5. FT-IR spectra of CeO (a), and Cu@CeO _1 (b).
2
9
Page 29 of 35