Functionalization of graphene oxide sheets with magnetite nanoparticles for the adsorption of copper ions and investigation of its potential catalytic activity toward the homocoupling of alkynes under green conditions

Article abstract of DOI:10.1016/j.jcat.2020.04.019

This paper deals with the preparation of graphene oxide sheets (GO) modified with magnetite nanoparticles (Fe₃O₄NPs) for removing Cu(II) ions from aqueous solutions. Moreover, the recovered Cu(II)-based material was recycled as a cat

Full text of DOI:10.1016/j.jcat.2020.04.019

Journal Pre-proofs
Functionalization of graphene oxide sheets with magnetite nanoparticles for
the adsorption of copper ions and investigation of its potential catalytic activi-
ty toward the homocoupling of alkynes under green conditions
Laroussi Chaabane, Emmanuel Beyou, Dominique Luneau, Mohammed
Hassen V Baouab
PII:
S0021-9517(20)30154-8
https://doi.org/10.1016/j.jcat.2020.04.019
YJCAT 13721
DOI:
Reference:
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Journal of Catalysis
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Revised Date:
Accepted Date:
5 March 2020
20 April 2020
21 April 2020
Please cite this article as: L. Chaabane, E. Beyou, D. Luneau, M. Hassen V Baouab, Functionalization of
graphene oxide sheets with magnetite nanoparticles for the adsorption of copper ions and investigation of its
potential catalytic activity toward the homocoupling of alkynes under green conditions, Journal of Catalysis
(
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Functionalization of graphene oxide sheets with magnetite
nanoparticles for the adsorption of copper ions and
investigation of its potential catalytic activity toward the
homocoupling of alkynes under green conditions
a,b
a,b *
c
Laroussi Chaabane , Emmanuel Beyou , Dominique Luneau , Mohammed Hassen V
Baouabd
aIngénierie des Matériaux Polymères (IMP), Villeurbanne, F-69622, France.
bCNRS, UMR5223, Villeurbanne, F69621, Université de Lyon, F-69003, Lyon, France.
cLaboratoire des Multi matériaux et Interfaces, UMR CNRS 5615, Université de Lyon1,
Université Lyon1, Villeurbanne F-69622, France
dUnité de Recherche Matériaux et Synthèse Organique (UR17ES31) Institut Préparatoire aux
Etudes d’Ingénieurs de Monastir, Université de Monastir, Bd. de l’environnement, 5019
Monastir, Tunisie.
E-mail: beyou@univ-lyon1.fr
1

Abstract
This paper deals with the preparation of graphene oxide sheets (GO) modified with
magnetite nanoparticles (Fe O NPs) for removing Cu(II) ions from aqueous solutions.
3
4
Moroever, the recovered Cu(II)-based material was recycled as a catalyst for the homocoupling
of a series of alkynes. The adsorption behavior of magnetic GO-based materials towards Cu(II)
ions from aqueous solutions was studied as a function of the pH value and the contact time and
it was observed that the magnetic GO-based absorbent can be separated quickly from its
aqueous solution by using an external magnetic field. Its equilibrium adsorption capacity value
-1
for Cu(II) ions was as high as 3.808±0.125 mmol.g under neutral conditions. In addition, the
catalytic activity of the resulting [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) material toward
3
4
the homocoupling of phenylacetylene into 1,4-diphenylbuta-1,3-diyne was demonstrated with
a yield up to 99%. Importantly, the [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) supported
3
4
catalyst was consecutively reused for fifteen runs without a significant loss of its activity.
Keywords: Graphene oxide sheets; Removal of Cu(II) ions; Catalytic activity; Homo-
coupling of alkynes; Green Conditions.
1
.
Introduction
2

With the rapid growth of industrial activities, water contamination by heavy metals ions has
enhanced undesirable effects on both human health and environment considering their extreme
toxicity, persistence, non-biodegradability, and bioaccumulation in the food chain [1-3]. One
of the most diffuse toxic metal ions are copper ions (Cu (II)) coming from several industries
including battery manufacturing, fertilizer industry, mining, and electroplating [4-7].
Nowadays, various methods and processes such as flocculation, ultrafiltration, electro-dialysis,
reverse osmosis, and adsorption have been used to remove toxic metal ions from wastewater
and water [8-11]. However, most of these methods require high operating costs and produce
toxic sludge while the use of absorbents do not allow an easy recovering [12, 13]. Moreover,
traditional adsorbents such as active carbon, biomass materials, and polymer resins suffer from
poor adsorption sites and consequently display a low adsorption capacity [14-16]. Recently,
various novel adsorbents based on functionalized carbon nanotubes and graphene oxide (GO)
sheets have been developed [17].
In particular, GO displays a good adsorption capacity towards heavy metal ions due to the
presence of epoxy, carboxyl, and hydroxyl functional groups onto their surface [18-22] and its
functionalization can improve it [9, 23]. One of the attractive ways for modifying GO is the use
of multifunctional organic molecules containing strong chelating groups [24, 25]. For instance,
Laroussi et al., [25] modified GO with N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) for
removing Cu(II), Ni(II) and Co(II) ions from aqueous solutions. However, the the obtained
GO-based materials, having high dispersibility in aqueous solutions, did not permit to revover
it easily. In this context, a few authors [26-28] studied the modification of GO with magnetic
nanoparticles which allow its recovering by using an external magnetic field. Indeed, it is well
known that magnetite nanoparticles (Fe O NPs) can be combined with GO sheets by various
3
4
methods such as co-precipitation, solvothermal, and ionothermal [29]. The co-precipitation
method is the most used technique to synthesize Fe O NPs because of its eco-friendly
3
4
3

procedure [30]. For instance, Liu et al., [31] prepared a magnetic-GO modified 1,2-
diamineocyclohexanetetraacetic acid composite (GO-Fe O NPs-DCTA) and they studied the
3
4
effect of multi-functional organic acid ligands on the adsorption of Cu(II) ions. The
incorporation of metal ions onto magnetic GO-based materials provided a new strategy for the
fabrication of atomic level-distributed metal catalysts.
In addition, the recyclability of GO-based absorbents containing metal ions is of great
interest in particular for the catalysis of some organic reactions such as the homocoupling of
alkynes. Indeed, homocoupling of alkynes is one of the conventional methods for the synthesis
of symmetric 1,3-diynes [32]. This reaction is usually carried out in presence of copper salts,
as catalyst [33]. For example, Wang et al., [34] studied the homocoupling of terminal alkynes
in the presence of CuCl and Et N as the base under solvent-free conditions. Moreover, Singh
2
3
et al., [35] reported the homocoupling of terminal alkynes to symmetric 1,3-diynes using CuI,
benzotraizole as the metal ligand and K CO as the base in dimethylformamide (DMF).
2
3
However, the heterogeneous catalysis is often preferred with regard to facile catalyst separation
and recycling [36]. Recently, several copper-based heterogeneous catalytic systems for the
oxidative homocoupling of terminal alkynes have been reported, such as CuI immobilized on
3
-(2-aminoethylamino)propyl-functionalized (MCM-41) [37] and copper(I)-modified zeolites
[38]. However, these catalytic systems require the use of high reaction temperatures and
stabilizing ligands while displaying a low selectivity [39].
In the present study, we have designed a novel multi-functional magnetic GO-based
adsorbent for high-efficiency removal of Cu(II) ions from aqueous solutions and having a
catalytic activity toward the homocoupling of terminal alkynes. First, the magnetite
nanoparticles (Fe O NPs) were successfully adsorbed onto GO sheets through a co-
3
4
precipitation method after the grafting of ethylenediamine (EDA) and chloroacetyl chloride
(CAC) onto the GO sheets through an experimental procedure described in a previous work
4

[25]. Then, the grafting of N,N-bis(2-pyridylmethyl)ethylenediamine (BPED) onto (GO-EDA-
CAC)@Fe O NPs was characterized by FTIR, XPS, TGA, SEM, EDX, XRD, and VSM and
3
4
the corresponding GO-based absorbent was used as an adsorbent to remove Cu(II) ions from
aqueous solutions. The influence of the pH and contact time onto the Cu(II) ions adsorption
capacities of the adsorbents were investigated, thoroughly. Finally, the resulting [GO-EDA-
CAC@Fe O NPs]-BPED-Cu(II) material was reused as a catalyst for the homocoupling of a
3
4
series of terminal alkynes under green experimental conditions.
. Experimental
.1. Reagents
Graphite powder was kindly provided by TIMCAL Graphite & Carbon, France (4 mm in
2
2
size). Graphite Oxide (GO) was prepared using a modified Hummers’ method by oxidation of
natural graphite powder (4 µm in size, 99.99% purity) [25]. Ethylenediamine (EDA),
chloroacetyl chloride, sodium carbonate (Na CO ), sodium hydroxide (NaOH), nitric acid
2
3
(
HNO ), n-butylamine, diisopropylamine, diethylamine, calcium carbonate (CaCO ),
3
3
tripotassium phosphate trihydrate (K PO .3H O), ferric chloride (FeCl .6H O) 98%, ferrous
3
4
2
3
2
chloride tetrahydrate (FeCl .4H O) 99%, copper (II) chloride dihydrate (CuCl .2H O) 99%,
2
2
2
2
sodium chloride, copper (I) bromide 99%, copper (I) chloride 99% and potassium hydrogen
phthalate were purchased from Aldrich. Solvents such as dimethylformamide (DMF), dimethyl
sulfoxide (DMSO), 1,4 dioxane, chloroform, tetrahydrofuran (THF), n-propanol, benzene,
glycerol, n-hexane, ethyl acetate (EtOAc), methanol, ethanol, diethyl ether, ammonium
hydroxide (NH OH) and all the substrates required for homocoupling were purchased from
4
Aldrich and used as received. All aqueous solutions were diluted with deionized water
-1
(
EC<0.05µs.m ; Millipore system).
2
.2. Instruments
5

Fourier transform-infrared spectroscopy (FTIR) were recorded using a Nicolet 460
spectrometer (Thermo Nicolet Corporation, USA) and samples in KBr pellet form.
Thermogravimetric analysis (TGA) was investigated by Perkin Elmer thermal analyzer
-1
instrument (Nicolet, TA SDT Q600, USA) with a heating rate of 5 °C.min and nitrogen gas
-1
flow at 50 mL min . Powder X-ray Diffraction (PXRD) measurements were collected on a
Siemens D500 diffractometer (Ni-filtered Cu, KR radiation, 1.5405 Ǻ, FEI, USA). X-ray
photoelectron spectroscopy (XPS) analysis were carried out on a Perkin Elmer PHI5300
physical spectrometer (SXM instrument, USA) with binding energy in the range -2 to 1400 eV.
Scanning electron microscope (SEM) images were taken by a commercial FEI Company,
Netherlands. Magnetic measurements were performed at 300 K on powdered samples packed
in polycarbonate capsules using a Quantum Design MPMS-XL SQUID magnetometer. NMR
1
spectra were recorded on a Brucker AC-600, USA spectrometer at 600 MHz ( H) and 150 MHz
(13
C). All chemical shifts were reported as δ values (ppm) relative to the residual non-deuterated
solvent. EI-MS was determined with an Agilent 5975N mass spectrometer. The concentration
of Cu(II) ions solutions were measured using an Uv-Visible Perkin Elmer, USA
spectrophotometer through the formation of Cu(II) hydrogen phthalate complex.
2
.3. Preparation of the GO-EDA-CAC)@Fe O NPs material
3 4
First, the GO-EDA-CAC substrate was prepared according to our previous work [25].
Then, 1.50 g of FeCl .6H O and 0.75 g of FeCl .4H O were added to a GO-EDA-CAC
3
2
2
2
dispersion in 10 mL of deionized water followed by a sonication at 60 °C for 1 h. Subsequently,
the pH of the corresponding aqueous suspension was adjusted to 10-11 by dropping 5.46 g of
NH OH [29]. At the end of the reaction, the (GO-EDA-CAC)@Fe O NPs material was
4
3
4
separated from the aqueous solution with the help of a magnet and washed repeatedly with
deionized water and ethanol and finally dried under vacuum at 80 °C overnight.
2
.4. Grafting of BPED onto the GO-EDA-CAC@Fe O NPs substrate
3 4
6

According to the literature [25, 40], the preparation of N,N-bis(2-pyridylmethyl)-
ethylenediamine (BPED) can be achieved using the synthetic route presented in Scheme 1
(
Section 1, Supporting information †). So, 1 g of GO-EDA-CAC@Fe O NPs was dispersed
3 4
in 40 mL anhydrous DMF and sonicated for 30 min. Then, 3.63 g of BPED and 3.95g of Et N
3
were added in the suspension and stirred at 80 °C for 24 h. At the end of the reaction, the
suspension was centrifugated and washed with ethanol to remove the free BPED molecules.
The latter purification step was monitored by UV-Visible analysis. (Fig. S1†). Finally, the
powder was dried under vacuum at 80 °C overnight.
2
.5. Cu(II) adsorption experiments
The adsorption behavior of the two magnetic adsorbents (GO-EDA-CAC)@Fe O NPs and
3
4
[
(GO-EDA-CAC)@Fe O NPs]-BPED towards Cu(II) ions were studied in details. Thus, the
3 4
effect of some operating parameters such as the pH and the contact time on the Cu(II) adsorption
capcities were evaluated. First, the point of zero charge (pH ) defined as the point where the
pzc
density of the adsorbent electrical surface charge of adsorbent is zero (section 2, Supporting
information†). First, the effect of the pH on Cu(II) ions adsorption was studied in the range
3
-8 and adjusted with a Clark-Lub’s buffer solutions (0.1M HNO and 0.1M NaOH) [41].
3
Experiments were performed by mixing 100 mg of the adsorbent into 100 mL of the Cu(II) ions
-1
aqueous solutions with [C ]= 250 mg.L . Then, the suspensions were sealed and stirred for 24
0
h at room temperature. When adsorption equilibrium was reached, the mixture was centrifuged
at 5000 rpm for 10 min. In the case of the presence of a precipitate of Cu(OH) in the aqueous
2
solution at pH˃7, the suspension was filtered through a 0.45 µm membrane filter. The initial
and equilibrium concentrations of Cu(II) ions in the aqueous solution were determined by UV-
Vis spectrometry using calibration curves (Table S1† and Fig. S2†).
-1
The equilibrium adsorption capacity (Qe; mmol.g ) of the adsorbent was calculated
with the following equation (Eq. 1) [42, 43]:
7

V(C ― C )
0
e
Q =
e
(1)
m
-1
Where C and C are the initial and equilibrium concentrations (mmol.L ) of the Cu(II)
0
e
ions aqueous solutions, respectively. V is the volume of the Cu(II) ions solution (mL) and m
mg) is the mass of the adsorbent.
(
2
.6.
Homocoupling of alkynes in presence of the [GO-EDA-CAC@Fe3O4NPs]-
BPED-Cu(II) material
In a typical reaction, the terminal alkyne (1.00 mmol) and Na CO (1.20 mmol) were dissolved
2
3
in 2 mL of glycerol. Then, the supported copper catalyst [GO-EDA-CAC@Fe O NPs]-BPED-
3
4
Cu(II) catalyst (10 mg) was added in the mixture and stirred at 80 °C for 30 min to 320 min.
The reaction was monitored by TLC using EtOAc and n-hexane (1:4) as eluent and gas
chromatography. After completion of the reaction, the reaction mixture was cooled to room
temperature and diluted with water (5 mL), followed by the separation of the magnetic
supported copper catalyst from the reaction mixture by using an external magnet. Then, the
[
GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) catalyst was washed with methanol and dried
3 4
carefully before use in the next run. The remaining products in the supernatant mixture was
extracted with EtOAc (4×5 mL) and finally purified by column chromatography.
3
. Results and discussion
As described in our previous article [25], GO-EDA-CAC was easily synthesized in two
steps from GO sheets. However, the synthetic procedure involves a complementary step
based on the doping of GO-EDA-CAC with magnetite nanoparticles (Fe O NPs) through the
3
4
co-precipitation method (scheme 1). Indeed, it is expected that the presence of Fe O NPs onto
3
4
the GO-based surface will facilitate its recovering after the Cu(II) adsorption. Finally, the
grafting of N,N-bis(2-pyridylmethyl)ethylenediamine groups (BPED) onto GO-EDA-
CAC@Fe O NPs was realized in order to ensure higher Cu(II) ions adsorption (Scheme 1).
3
4
8

Scheme 1. Preparation of the GO-EDA-CAC@Fe O NPs-BPED absorbent followed by the
3
4
adsorption of Cu(II) from aqueous solutions.
3
.1.
Synthesis and characterization of the GO-EDA-CAC@Fe3O4NPs-BPED
absorbent
.1.1. Doping GO-based materials with magnetite nanoparticles
As discussed before, the synthesis and the characterization of GO-EDA-CAC are described
in our previous article [25]. Then, the magnetite nanoparticles (Fe O NPs) were in situ
3
3
4
produced onto the GO-EDA-CAC surface through the well-known co-precipitation method,
which requires the use of both ferric and ferrous ions in basic conditions [29]. First, the
formation of the Fe O NPs onto the GO-EDA-CAC surface was qualitatively evidenced by
3
4
FTIR analysis (Fig. S3a†). Indeed, the FTIR spectrum of the GO-EDA-CAC@Fe O NPs (Fig.
3
4
-1
S3a†) displays a peak at 635 cm which may be attributed to the stretching vibrations of the
9

-1
CH -Cl groups and another one located in the range 620-598 cm that is characteristic of
2
octahedral and tetrahedral stretching vibrations of the Fe-O bonds in Fe O NPs [5].
3
4
The immobilization of Fe O NPs onto the GO-based material surface was also
3
4
demonstrated by analyzing both the elemental composition of (GO-EDA-CAC)@Fe O NPs by
3
4
EDX (Fig. 1a) and its morphology by SEM (Fig. 1b).
Figure 1. (a) EDX spectrum and (b) SEM image of GO-EDA-CAC@Fe O NPs.
3
4
Fig. 1a shows the EDX spectrum of GO-EDA-CAC@Fe O NPs, which mostly displays
3
4
the presence of carbon, nitrogen, oxygen, chlorine, and iron, as expected. In addition, the SEM
image of GO-EDA-CAC@Fe O NPs (Fig.1b) shows the presence of sphere-like structures
3
4
with diameters ranging from 6 nm to 20 nm. A few larger aggregates with an average diameter
centered at 80 nm are also observed.
Then, the weight content of magnetite nanoparticles in the (GO-EDA-
CAC)@Fe O NPs was estimated by thermogravimetric analysis (TGA) (Fig. S4a†) and it was
3
4
calculated to be around 28wt % when compared to the curve of GO-EDA-CAC [25].
1
0

3
.1.2. Grafting of BPED onto GO-EDA-CAC@Fe O NPs
3 4
The grafting of BPED groups onto the GO-EDA-CAC@Fe O NPs surface was
3
4
performed according to the experimental procedure described by Laroussi et al., [25] and it was
characterized by X-ray photoelectron spectroscopy (XPS). The XPS spectra of C 1s and N 1s
for (GO-EDA-CAC)@Fe O NPs and [(GO-EDA-CAC)@Fe O NPs]-BPED materials are
3
4
3
4
shown in Fig. S5† and Fig. S6†, respectively. Each individual peak of carbon and nitrogen
functional groups was fitted with a Gaussian function and their peak area ratios were calculated
(Table S2† and Table S3†). The C 1S spectrum of GO-EDA-CAC exhibits different peaks at
2
85.45 eV, 286.85 eV, 287.95 eV and 289.12 eV corresponding to C=C/C-C, C-N, O-C-O and
C=O bonds, respectively, (Fig. S5†) while the peak intensity of carbon atoms bonded to
nitrogen increases after BPED grafting, as expected (Fig. S6†). In addition, the C-N species
ratio content increases from 18.7% for GO-EDA-CAC@Fe O NPs to 32.5% for [(GO-EDA-
3
4
CAC)@Fe O NPs]-BPED (Table S2†). Furthermore, the high-resolution N 1s spectrum of
3
4
(
GO-EDA-CAC)@Fe O NPs (Fig. S6a†) displays three peaks centered at 400.11 eV (C=N),
3 4
4
00.18 eV (C-NR ) and 401.74 eV (N-C=O) and the amounts of C=N- (13.7%) and O=C-N
2
(
12.9%) (Table S3†) increase when compared to those of the (GO-EDA-CAC)@Fe O NPs
3 4
which attest for the successful grafting of BPED groups onto the (GO-EDA-CAC)@Fe O NPs.
3
4
Moreover, the presence of BPED groups onto the GO-EDA-CAC@Fe O NPs was
3
4
quantitatively studied by TGA and it was observed an increase of the weight loss of 10% in
comparison with the TGA curve of GO-EDA-CAC@Fe O NPs (Fig. S4a†). To obtain more
3
4
understanding of the effect of the grafting of BPED groups onto the morphology of the (GO-
EDA-CAC)@Fe O NPs surface, scanning electron microscopy (SEM) was also conducted.
3
4
However, no significant change was observed upon BPED grafting (Fig. S7†).
The XRD patterns of [GO-EDA-CAC)@Fe O NPs]-BPED is shown in Fig. 2.
3
4
1
1

Figure 2. XRD patterns of [(GO-EDA-CAC)@Fe O NPs]-BPED.
3
4
As it can be seen, the broad peak at 2ϴ= 25.12° may be attributed to the presence of
multi-functional groups such as amine, amide and pyridine moieties located onto the GO sheets
while the characteristic diffraction peaks of Fe O NPs are observed at 2ϴ= 31.19°, 35.61°,
3
4
4
3.25°, 54.47°, 57.41°, and 62.95°, corresponding to (220), (311), (400), (422), (511), and (440)
crystal plane, respectively [5].
Then, the size of Fe O NPs was estimated from the full-width at half maximum of the
3
4
most intense diffraction peak by Scherrer’s equation (Eq.2) [44]:
퐾 휆
훽 푐표푠ϴ
퐷 =
(2)
Where K is the grain shape factor (k= 0.89), λ is the wavelength of CuK radiation (λ =
α
0
.15406 nm), β is the Full-Width at half maximum of the most intense diffraction peak (311) in
the 2ϴ scale and ϴ is the Bragg’s angle. Using Eq.2, we obtained a diameter of 6.00 nm, which
is in the same size range that the one observed by the SEM image.
1
2

In addition, the magnetization proprieties of [GO-EDA-CAC@Fe O NPs]-BPED were studied
3
4
at 300 K and the plots of magnetization (M) versus magnetic field (H) are shown in Fig. 3a.
Figure 3. (a) Magnetization (M) versus magnetic field (H) recorded at 300K for (1)
Fe O NPs and (2) [GO-EDA-CAC)@Fe O NP ]-BPED. (b) Susceptibility of [GO-EDA-
3
4
3
4
S
CAC@Fe O NP ]-BPED (2), when magnetic field is near zero.
3
4
S
No coercivity or hysteresis can be observed in the magnetization curves, indicating that
the [GO-EDA-CAC@Fe O NPs]-BPED is superparamagnetic. In addition, the saturation
3
4
-1
magnetization (M ) of [GO-EDA-CAC@Fe O NPs]-BPED (22.45 emu.g ) is lower than that
s
3
4
-1
of the Fe O NPs (74.05 emu.g ) which may be due to the presence of non-magnetic GO sheets
3
4
and the smaller size of the supported Fe O NPs [45]. On the other hand, it is observed from the
3
4
inset in Fig. 3a that the [GO-EDA-CAC@Fe O NPs]-BPED material can be easily isolated by
3
4
applying an external magnetic field.
In case of almost zero coercivity (M ) and zero remanence (M ), the average size (D )
c
r
m
of the Fe O NPs can be calculated from the initial susceptibility (x ) using Eq.3 [46]:
3
4
i
1
8 × 퐾 × 푇 × 푥푖
퐷 =
푚
(3)
П × 휌 × (푀푠)²
1
3

Where K is Boltzmann constant, T is the temperature (300 K), x is the susceptibility
i
-1
when the magnetic field is near zero (0.02611 emu.g .Oe) (Fig. 3b), 휌 is the density of
magnetite nanoparticles, M is the saturation magnetization. Assuming that the saturation
s
magnetization (M ) corresponds to that of the [GO-EDA-CAC@Fe O NPs]-BPED (22.45
s
3
4
-1
emu.g ), it was calculated a diameter of 6.01 nm which in good agreement with the results
obtained from calculated the Scherrer’s equation (XRD) (Eq.2) and nearly in the range of the
ones evaluated from the SEM image (Fig. S7†).
Moreover, Fig.4 shows the Zero field cooled (ZFC) and field cooled (FC) magnetization
of [GO-EDA-CAC@Fe O NPs-BPED] as a function of temperature. It can be observed that
3
4
both ZFC and FC curves of the [GO-EDA-CAC@Fe O NPs-BPED] absorbent have the same
3
4
magnetization behavior for temperature higher than 88K (Fig.4). The maximum of the ZFC
curve is called the blocking temperature (Tb) of Fe3O4NPs that can be also calculated by using
Eq.4 [47]:
Tb=(β×K×Vm)/(k_b×ln(τ /τ_0))
(4)
m
Where β is a constant depending on the crystal size distribution (β=9.2), K is the
-3
-26
3
anisotropy constant of the crystal (K= 5×104 J m ), Vm is the crystal volume (1.125×10 m ),
−
1
kb is the Boltzmann constant (kb = 1,380 649 × 10−23 J K ), τ is the measurement time and
m
τ_0 is a characteristic relaxation time of crystal of 10-11 s [47].
1
4

Figure 4. ZFC (Zero field cooled) and FC (Field cooled) temperature dependence of
magnetic susceptibility for [(GO-EDA-CAC)@Fe O NPS]-BPED.
3
4
A value of 87.67 K was calculated for Tb, which is close to the experimental one (88 K ;
Fig.4). The blocking temperature (T ) of the [(GO-EDA-CAC)@Fe O NPs]-BPED (88 K) is
b
3
4
lower than that of the Fe O NPs (226 K) [48] which may be explained by the increase of inter-
3
4
distance between Fe O NPs and consequently a decrease of dipole-dipole interactions.
3
4
3
.2. Effect of the pH and the contact time on Cu(II) ions adsorption by GO-EDA-
CAC@Fe O NPs and [GO-EDA-CAC@Fe O NPs]-BPED
3
4
3
4
3
.2.1. Effect of the pH onto Cu (II) adsorption capacities and zero-point charge
(pH zpc) studies
The pH of the metal ions containing aqueous solutions is a key parameter for the
obtained adsorption capacity, as reported by several authors in the literature [49]. Herein, we
investigated the pH range 4.0-8.0 and a blank experiment without the GO-based absorbent was
also performed in order to check the precipitation of Cu(II) ions, as illustrated in Fig. S8†. As
it can be observed, the Cu(II) ions adsorption capacities increase with increasing the pH from
4
to 8 whatever the GO-based absorbent and the adsorption capacities are higher for the GO-
1
5

EDA-CAC@Fe3O4NPs]-BPED. The low Cu(II) adsorption capacities obtained in acidic
experimental conditions may be explained by the positive surface charge of the absorbents at
low pH and the protonation of the BPED groups for the GO-EDA-CAC@Fe3O4NPs]-BPED
absorbent which partly prevent Cu(II) ions from adsorption. Indeed, the pH point zero charge
(
pH ) of the adsorbents were determined to evaluate their surface charges and the experimental
pzc
curves (plots of pH(Final) vs pH(Initial)) are shown in Fig. S9†. It is expected that the surface of
adsorbent becomes negatively charged when the pH of the aqueous solution is higher than pHpzc
leading to favorable electrostatic interactions with Cu(II) ions [49, 50] and we obtained a pHpzc
of 6.6 for the GO-EDA-CAC@Fe O NPs absorbent and a pH of 6.9 for the [GO-EDA-
3
4
pzc
CAC@Fe O NPs]-BPED one. Accordingly, the higher adsorption capcity (QCu(II)) is achieved
3
4
at pH=7 for the [(GO-EDA-CAC)@Fe O NPs]-BPED adsorbent with a value of 3.808±0.125
3
4
-1
mmol.g which is slightly higher than the one of GO-EDA-CAC-BPED (Q(Cu(II))= 3.464±0.100
-1
mmol.g ).
.2.2. Effect of the contact time on the Cu(II) ions adsorption capacities
The adsorption of Cu(II) ions at different contact times were performed with a Cu(II)
3
-1
ion concentration of 250 mg.L at pH=7 for GO-EDA-CAC)@Fe O NPs and [GO-EDA-
3
4
CAC)@Fe O NPs]-BPED absorbents (Fig. S10†). As it can been seen, the optimal contact time
3
4
depends on the nature of the adsorbent and the optimal contact time is 5h for (GO-EDA
CAC)@Fe O NPs and 3h for [(GO-EDA-CAC)@Fe O NPs]-BPED. Thus, the [(GO-EDA-
3
4
3
4
CAC)@Fe O NPs]-BPED absorbent achieves an equilibrium adsorption faster than the GO-
3
4
EDA CAC@Fe O NPs one and other GO-based adsorbents (GO, GO-EDA, GO-EDA-CAC,
3
4
and GO-EDA-CAC-BPED) that were studied in our previous work [25]. This could be ascribed
to a higher number of active sites (e.g. BPED groups) located onto the [(GO-EDA-
CAC)@Fe O NPs]-BPED sheets.
3
4
3
.3. Adsorption kinetics
1
6

Four kinetics models, including, the pseudo-first-order [51], the pseudo-second-order
52], the Roginsky-Zeldovich [53] and the intra-particle diffusion [54] ones were used to fit the
[
experimental data and to understand both the adsorption process and the diffusion of the Cu(II)
adsorbate through adsorbent pores. The linear forms of these models are described in supporting
information (Section 2). Linear fitting plots of pseudo-first-order and pseudo-second-order
models are shown in Fig. 5 and the usual calculated parameters from these models are
summarized in table 1.
Figure 5. (a) Pseudo-first-order and (b) Pseudo-second-order kinetics plots for adsorption of
Cu(II) ions onto (1) GO-EDA-CAC@Fe O NPs and (2) [GO-EDA-CAC@Fe O NPs]-BPED.
3
4
3
4
According to the correlation coefficientvalues, it is demonstrated that the adsorption data
fit better with the pseudo-second-order model than with the pseudo-first-order model while the
-1
calculated Q (Table 1) are close to the experimental ones (in the range 3.5-3.8 mmol.g ). Thus,
e
the adsorption of Cu(II) ions onto magnetic adsorbents is controlled by a chemical sorption
which includes exchange electrons between adsorbent and Cu(II) ions [55].
1
7

Table 1. Usual calculated parameters calculated from the pseudo-first-order kinetic,
the pseudo-second-order kinetic, the Roginsky-Zeldovich model and the intra-particle
diffusion kinetic models for the adsorption of Cu (II) ions onto (1) (GO-EDA-
CAC)@Fe O NPs and (2) [(GO-EDA-CAC)@Fe O NPs]-BPED.
3
4
3
4
Models
Parameters
R2
Adsorbents
1
2
0.990
0.985
2
-1
First-order Kinetic model
K ×10 (min )
1.151±0.002
3.054±0.021
0.996
3.477±0.002
4.027±0.011
0.998
1
-1
Qe, cal (mmol.g )
R2
Second-order Kinetic model
Roginsky-Zeldovich model
2
-1
-1
0.697±0.005
3.125±0.007
0.939
0.211±0.004
3.817±0.005
0.923
K ×10 (g.mmol .min )
2
-1
Qe, cal (mmol.g )
R2
-1
-1
α (mmol.g .min )
0.258±0.197
1.712±0.041
0.998
0.717±0.272
1.557±0.057
0.997
-1
β (g.mmol )
R2
-1
-1/2
0.208±0.003
0.403±0.047
Ki (mmol.g .min )
1
-1
-1/2
-0.043±0.017 -0.070±0.202
L (mmol.g .min )
1
The intraparticle diffusion Kinetic
R2
0.992
0.991
-
1
-1/2
0.093±0.008
1.578±0.127
0.127±0.047
2.803±0.051
Ki (mmol.g .min )
2
-1
-1/2
L (mmol.g .min )
2
The experimental data were also fitted with the Roginsky-Zeldovich model by using
Eq. 3, (S.I†) and the corresponding calculated parameters are summarized in Table 1. In
particular, the low values of α suggest a fast equilibrium adsorption and a low activation energy
for the chemsorption [56].
1
8

In order to investigate the mechanism of the adsorption of Cu(II) ions onto the different
adsorbents, the kinetic results were also fitted with the intra-particle diffusion model by using
Eq. 4, (S.I†) and the corresponding calculated parameters are listed in Table 1. Considering
these latter results, Fig. 6 shows a multi-linear fit of the intraparticle diffusion model for the
adsorption of Cu(II) ions onto (GO-EDA-CAC)@Fe O NPs and [(GO-EDA-
3
4
CAC)@Fe O NPs]-BPED.
3
4
1
/2
Figure 6. Adsorption capacities versus t calculated from the intra-particle diffusion
kinetic model for the adsorption of Cu(II) ions by the adsorbents (a) (GO-EDA-
CAC)@Fe O NPs and (b) [(GO-EDA-CAC)@Fe O NPs]-BPED.
3
4
3
4
For both the adsorbents, the first linear stage corresponds to the diffusion of the Cu(II)
ions onto the surface of the magnetic adsorbent. The second linear stage describes the gradual
adsorption stage which may be attributed to the diffusion of Cu(II) ions inside the active sites
of the adsorbent and the beginning of saturation. Moreover, the intra-particle diffusion constants
show K (first stage) is higher than K (second stage) (Table 1), indicating that the second stage
i1
i2
corresponds to a slower transport of Cu(II) ions from the liquid phase up to the active sites of
the adsorbent during the adsorption process.
1
9

3
.4. Homo-coupling study of terminal alkynes in the presence of reused [(GO-EDA-
CAC)@Fe O NPs]-BPED-Cu(II), as supported catalyst
3
4
The recyclability of the [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) material is of great interest
3
4
in particular for the catalysis of organic reactions such as homocoupling of alkynes. Indeed, the
synthesis of 1,3-diynes through the homocoupling of alkynes requires the use of metal catalysts
such as palladium which is efficient at low loading but the price of this precious metal has led
to the use of a low-cost metal catalyst such as copper [57]. Therefore, we investigated the use
of [GO-EDA-CAC@Fe O NPs]-Cu(II) and [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) as
3
4
3
4
supported catalysts for the homocoulping of phenylacetylene.
3
.4.1. Effect of the catalyst nature, the solvent, the temperature and the base on
the homocoupling reaction of phenylacetylene
In this study, the homocoupling of phenylacetylene was selected as a model reaction to
evaluate the catalytic activity of the Cu(II) ions adsorbed onto the magnetic GO-based
adsorbents and in order to find the optimal experimental conditions. For comparison, the
homocoupling of phenylacetylene was first carried out in the presence of both usual
unsupported and supported copper catalysts with EtOH as solvent and Et N as a base in air at
3
8
0 °C. It should be noted that the weight content of copper ions located onto the [GO-EDA-
CAC@Fe3O4NPs]-BPED and [GO-EDA-CAC@Fe3O4NPs] are 0.24g/g and 0.22g/g,
-1
-1
respectively and corresponding to adsorption capacities of 3.8 mmol.g and 3.5 mmol.g ,
respectively. Arbitrarily, we have used all the recovered Cu(II)-GO-based substrates from the
adsorption experiment (about 10 mg per experiment) as supported catalysts for the
homocoupling of alkyne and it was compared to experiments containing a copper ions amount
of 4mg that is usually used for this reaction [35, 36]. The results are summarized in Fig. 7.
2
0

Figure 7. Effect of the different catalysts on the time-yield plot for the homocoupling of
phenylacetylene: (1) without any catalyst, (2) with (GO-EDA-CAC)@Fe O NPs (10 mg), (3)
3
4
with [(GO-EDA-CAC)@Fe O NPs]-BPED (10 mg), (4) with CuI (4 mg), (5) with CuBr (4
3
4
2
mg), (6) with CuCl (4 mg), (7) with (GO-EDA-CAC)@Fe O NPs-Cu(II) (10 mg ; 2.2 mg
2
3
4
Cu(II)) and (8) with [(GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II) (10 mg ; 2.4 mg Cu(II)).
3
4
Experimental conditions: phenylacetylene (1.0 mmol, 1.0 eq.), Et N (1.2 mmol, 1.2 eq.), and
3
EtOH (1.0 mL) at 80 °C under air, the error bars correspond to one standard deviation n=2.
As it can be seen, the homocoupling reaction of phenylacetylene is not efficient without
any catalyst though in the presence of GO-EDA-CAC@Fe O NPs and GO-EDA-
3
4
CAC)@Fe O NPs-BPED substrates, as expected. For the copper-based catalysts, we observe
3
4
the presence of a plateau corresponding to the higher reaction yield that can be obtained at
00% phenylacetylene conversion (determined by gas chromatography) due to the selectivity
1
of the homocoupling reaction (presence of secondary products). The CuX (X: Cl, Br)
2
unsupported catalysts are more efficient than the CuI one for reaction times higher than 150
minutes with a maximum reaction yield of 61% after 280 minutes suggesting that Cu(II) ions
play a key role in the homocoupling reaction. Moreover, despite their lower Cu(II) weight
content (about 2 fold lower), the supported copper catalysts such as (GO-EDA-
CAC)@Fe O NPs-Cu(II) and [(GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II) lead to higher
3
4
3
4
2
1

yields in a shorter time. Indeed, we obtained homocoupling yields in the range 80-90 % for
reaction times lower than 180 min (Fig. 7). The latter behavior may be due to the high Cu(II)-
GO-based suspension stability in glycerol and especially to the easier coordination of the
penylacetylene reactant onto the Cu(II) ions adsorbed onto the GO-based surface, as discussed
in 3.4.3. (Scheme 2). Moroever, it is observed that the time necessary to produce the 1,4-
diphenylbuta-1,3-diyne depends on the amount of Cu(II) ions that is adsorbed onto the GO-
based surface, so that it is 180 min for (GO-EDA CAC)@Fe O NPs-Cu(II) and 120 min for
3
4
[
(GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II). Then, the influence of the solvent onto the
3 4
homocoupling reaction yield was studied and the results are summerized in Table 2.
Table 2. Effect of the solvent on the time-yield plot for the homocoupling of
phenylacetylene in presence of [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II).
3
4
Solvents
THF
Yields (%)a
15±1
25±1
45±1
62±1
74±2
89±1
94±1
DMF
DMSO
n-propanol
Methanol
Ethanol
Glycerol
a
Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), Et N (1.2 mmol, 1.2 eq.), [GO-
3
EDA-CAC@Fe O NPs]-BPED-Cu(II) (10 mg), and solvent (1.0 mL) at 80 °C under air for 120
3
4
min; the error bars correspond to one standard deviation n=2.
It can be concluded that the use of polar aprotic solvents do not permit to obtain an
homocoupling reaction yield higher than 60% for reaction time higher than 100 min. In contrast,
the use of polar protic solvents, such as n-propanol and ethanol, leads to a homocoupling
reaction yield increasing from 62% to 89%, respectively, for a reaction time of 120 min (Table
2
). Moreover, to improve the greenness of the homocoupling reaction, it was also carried out in
2
2

a biocompatible and non-flammable solvent such as glycerol, and we obtained an excellent
yield of 94±0.62% for a reaction time of 120 min. Therefore, to investigate the effect of
temperature on the homocoupling reaction of phenylacetylene under the most greener
experimental conditions, glycerol was selected as the most suitable solvent and the reaction was
performed in a low temperature range (40-80°C ; Fig. 8). It should be also mentioned that the
homocoupling reaction of alkynes requires the use of low temperatures to avoid the presence
of side reactions that is why a temperature of 80°C is often used [35, 36, 58, 59].
Figure 8. Effect of temperature on the time-yield plot for the homocoupling reaction: (1) 40
°
C, (2) 60 °C, and (3) 80 °C. Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.),
Et N (1.2 mmol, 1.2 eq.), [(GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II) (10 mg), and glycerol
3
3
4
(1.0 mL); the error bars correspond to one standard deviation n=2.
Fig. 8 shows that the reaction yield increases by increasing both the reaction time and the
temperature and the highest yield is as high as 94% and was achieved at 80 °C for 120 min.
Thus, 80 °C was selected as the optimum temperature for studying the effect of the base nature
onto the reaction yield.
2
3

Table 3 summarized the results obtained with the use of some common organic and
inorganic bases under identical experimental conditions.
Table 3. Effect of the base nature on the time-yield plot for the homocoupling of
phenylacetylene.
Bases
Without base
n-butylamine
Diisopropylamine
Diethylamine
Triethylamine
CaCO3
Yields (%)b
0
35±1
45±1
69±1
94±1
90±1
92±1
99±1
K PO .3H O
3
4
2
Na CO
2
3
b
Experimental conditions: Phenylacetylene (1.0 mmol, 1.0 eq.), base (1.2 mmol, 1.2 eq.), [(GO-
EDA-CAC)@Fe O NPs]-BPED-Cu(II) (10 mg), and glycerol (1.0 mL) at 80 °C under air for
3
4
1
20 min; the error bars correspond to one standard deviation n=2.
As expected, the reaction could not be performed without any base and most of the
organic bases including tert-butylamine, diethylamine, and diisopropylamine gave poor yields
< 65 %) for a reaction time of 120 min. Triethylamine was the most efficient organic base with
(
a yield of 94±0.62% for 120min. In comparison, the inorganic bases including CaCO and
3
K PO .3H O lead to higher yields, especially Na CO with a yield to 99%. Thus, the optimal
3
4
2
2
3
greener experimental conditions for the homocoupling of phenylacetylene are based on the use
of 10 mg of [GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II) in glycerol at 80 °C, in presence
3
4
Na CO as a base. In this regard, it is interesting to measure the green performance of the
2
3
homocoupling reaction so that we have calculated some green metrics [60] such as :
the E-factor, defined by the ratio of the total mass of waste generated in the synthetic scheme
to the mass of product isolated (Eq. 5)
-
2
4

∑
mass of wastes
퐸~factor =
(5)
mass of isolated product
-the Process Mass Intensity (PMI), defined as the ratio of the total mass of materials to the mass
of the isolated product (Eq. 6)
∑
mass of materials
PMI =
= 퐸 + 1
(6)
mass of isolated product
-the Atom Economy (AE), defined as the number of atoms from the starting materials that are
incorporated into the final product (Eq. 7)
Molecular weight of desired product
Atom economy (%) =
× 100
(7)
∑
Molecular weight of reactants
Moreover, an alternative mass-based metric such as the Reaction Mass Efficiency is usually
discussed and is defined as the percentage of actual mass of desire product to the mass of all
reactants used. It takes into account both atom economy and chemical yield (Eq. 8)
mass of desired product
mass of all reactants
Reaction mass efficiency (%) =
× 100
(8)
The results are summarized in Table 4.
Table 4. Evaluation of green chemistry metrics for the synthesis of a 1,3-diyne based on
phenylacetylene.
Reactant
Solvent
Base
Auxiliary
Product
(
Phenylacetylene) (Glycerol)
(Na CO )
2
3
Mass
1.02 g
12.6 g
----
1.27 g
----
----
----
1.01 g
Number of moles
Molecular mass
0.010 mol
0.012 mol
105.98 g.mol-1
Values
0.005 mol
102.05 g.mol-1
----
202.07 g.mol-1
E-factor
13.74 Kg waste/Kg product
14.74 kg waste/Kg product
97.1%
Process Mass Intensity
Atom economy
Reaction mass efficiency
99.0%
2
5

According to these results, we can conclude that our experimental conditions can enable the
synthesis of symmetrical conjugated 1,3-diynes at a large scale with an E-factor of 13.74 Kg/ 1
kg product, 97.13% of atom economy and a reaction mass efficiency of 99.0% (Table 4).
Therefore, these results are more interesting than the ones published elsewhere (E-factor of
5
3.51 Kg, PMI = 54.51 Kg/Kg, AE = 78.85%, and RME = 99.0% (Table S3†) [61].
3
.4.2. Scope and limitation of the Glaser homocoupling reaction
Considering the efficiency of the experimental conditions described above, the
homocoupling of other phenylacetylene-based molecules containing electron-donating groups
and electron-withdrawing groups was studied (Fig. 9).
a
Experimental conditions: Phenylacetylene (1.0mmol, 1.0 eq.), Na
2 3 3 4
CO (1.2 mmol, 1.2 eq.), [GO-EDA-CAC@Fe O NPs]-
BPED-Cu(II) (10mg), and glycerol (1.0 mL) at 80 °C under air.
Figure 9. Symmetric dialkynes synthesized in presence of [GO-EDA-CAC@Fe O NPs]-
3
4
BPED-Cu(II)a
2
6

The phenylacetylenes having chloro (1b) and fluoro (1c) atoms in para-positions and
trifluoromethyl groups (1h) in meta positions gave the higher yields, close to 98% (Fig.9) while
the presence of electron-donating substituents such as p-methyl (1d) and p-propyl (1e) onto
phenylacetylene did not affect the homocoupling yield. Finally, we have checked the efficiency
of the homocoupling reaction onto etheroxide-based alkynes and we also obtained high yields
(89% for (1k) and 84% for (1l), Fig.9).
All these results confirmed that the [GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II) can be
3
4
recycled as a supported catalyst for the homocoupling reaction of terminal alkynes.
3
.4.3. Mechanistic scheme for the homocoupling of terminal alkynes in the
presence of [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II)
3
4
According to the literature [62-65] and this work a proposal for the mechanism of the
preparation of 1,3-diyne derivatives in presence of [(GO-EDA-CAC)@Fe O NPs]-BPED-
3
4
Cu(II) as catalyst is shown in Scheme 2.
2
7

Scheme 2. A plausible mechanism for the [(GO-EDA-CAC)@Fe O NPs]-BPED-
3
4
Cu(II) catalyzed homocoupling of terminal alkynes.
In the initial step, the acetylene group is deprotonated in presence of the alkaline
conditions of Na CO . Then, it is expected that the high surface area of the GO-based adsorbent
2
3
and the high number of coordination sites for Cu(II) ions will facilitate the in situ formation of
complexes Cu(II)-acetylide (B). In the second step, the air oxidation of Cu(II) (B) leads to the
formation of the instable intermediate Cu(III) (C) whose decomposition allows the production
of the 1,3-diyne product (D) through a spontaneous reduction [62]. In the last step, the reduced
Cu(I) species are oxidized to Cu(II) and ensures the regeneration of the catalyst.
3
.4.4. Regeneration/reusability
2
8

Regeneration of the catalyst is a key parameter when assessing for commercial applications.
The reusability of the supported [GO-EDA-CAC@Fe O NPs]-BPED-Cu(II) catalyst for the
3
4
homocoupling reaction was investigated with phenylacetylene as a model molecule under our
optimal experimental conditions. After completion of the reaction, the catalyst was efficiently
separated from the reaction media by using an external magnetic field after each experiment.
Then, the recovered catalyst was washed with methanol several times and dried carefully before
reuse in the next experiment (Figure 10).
Figure. 10. Number of the catalytic runs for the homocouplig reaction of
phenylacetylene in presence of [(GO-EDA-CAC)@Fe O NPs]-BPED-Cu(II). Experimental
3
4
conditions: Phenylacetylene (1.0mmol, 1.0 eq.), Na CO (1.2 mmol, 1.2 eq.), and glycerol
2
3
(1.0 mL) at 80 °C under air for 120 min.
As shown in Fig. 10, the recovered catalyst was consecutively reused for fifteen runs without a
significant loss of its activity. It should be mentioned that the FTIR spectrum of the reused
[(GO-EDA-CAC)@Fe3O4NPs]-BPED-Cu(II) after fifteen runs (Fig.S11b†) still contains the
typical absorption band of the Cu-bond at 453 cm-1 while no significant differences are
observed between the positions of both the stretching vibrations and the absorption bands of
the samples before and after fifteen catalytic runs (Fig.S11†).
2
9