Aqueous-phase catalytic hydroxylation of phenol with H2O2 by using a copper incorporated apatite nanocatalyst

Article abstract of DOI:10.1039/c9ra02021g

Copper incorporated apatite (Cu-apatite) nanomaterial was prepared by a co-precipitation method. The obtained material was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and Raman spectroscopy, scanning electron microscopy (SEM, STEM) and nitrogen adsorption-desorption. The as-prepared Cu-apatite was used to catalyze phenol hydroxylation with hydrogen peroxide as an oxidant. The influencing parameters including reaction time, temperature, H₂O₂/phenol ratio and catalyst mass have been investigated. Under the optimized conditions, the Cu-apatite catalyst gave a phenol conversion of 64% with 95% selectivity to dihydroxybenzenes. More importantly, the results of catalyst recycling indicated that the same catalytic performance has been obtained after four cycles with a slight loss of activity and selectivity.

Full text of DOI:10.1039/c9ra02021g

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PAPER
Aqueous-phase catalytic hydroxylation of phenol
with H O by using a copper incorporated apatite
2
2
Cite this: RSC Adv., 2019, 9, 14132
nanocatalyst
ab
a
a
a
Abdallah Amedlous, Othmane Amadine, Younes Essamlali, Karim Daanoun,
b
ab
Mina Aadil and Mohamed Zahouily
*
Copper incorporated apatite (Cu-apatite) nanomaterial was prepared by a co-precipitation method. The
obtained material was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray
photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and Raman spectroscopy, scanning
electron microscopy (SEM, STEM) and nitrogen adsorption–desorption. The as-prepared Cu-apatite was
used to catalyze phenol hydroxylation with hydrogen peroxide as an oxidant. The influencing parameters
Received 15th March 2019
Accepted 15th April 2019
2 2
including reaction time, temperature, H O /phenol ratio and catalyst mass have been investigated. Under
the optimized conditions, the Cu-apatite catalyst gave a phenol conversion of 64% with 95% selectivity
to dihydroxybenzenes. More importantly, the results of catalyst recycling indicated that the same
catalytic performance has been obtained after four cycles with a slight loss of activity and selectivity.
DOI: 10.1039/c9ra02021g
rsc.li/rsc-advances
phenol hydroxylation over heterogeneous catalysts has received an
increasing attention due to its easy separation from the products,
1
. Introduction
It is well known that phenol and its derivatives are considered as which makes its recycling and its reusability simple. Though various
hazardous pollutants due to their high toxicity even at low concen- types of heterogeneous catalysts have been studied for this reaction,
trations, which causes toxicity to human beings and animals. copper based catalyst have been fascinating due to its precise OHc
7
Indeed, phenol and phenolic compound removal from wastewater radical activating including mesoporous silica Cu/MCM-41, organic
8
represents an important area of research, especially with the new frameworks Cu-MOFs, alumino-phosphate molecular CuAPO-11
1
10
increasing stringent requirement of environmental laws. In recent (ref. 9) and mixed oxide CuO@CeO
2
.
years, phenol hydroxylation using hydrogen peroxide as an oxidant
Recently, apatite-based materials have attracted increasing
has received much attention due to its efficiency to convert phenol interest due to its interesting properties which are needed in
to high value phenolic compounds (catechol (CAT) and hydroqui- a wide range of potential application in pharmaceuticals,
11,12
none (HQ)), which are widely used as chemicals in the pharma- biomedical, and water treatment.
These materials showed
ceutical industry, polymerization inhibitors, photography various advantages such as, high adsorption capacity, acid–base
2,3
chemicals, antioxidants and cosmetic products. Hydroxyl radicals properties, high dispersion in aqueous phase, non-toxicity,
(
OHc) produced from hydrogen peroxide are considered as a strong chemical and thermal stability, and abundantly available in
13,14
oxidant with a high redox potential, and can react with many nature.
In addition, apatite can act as catalysts by them-
organic compounds. The activation of H O to produce OHc can be selves or as supports for other active phases due to their excel-
2
2
2+
2+
achieved by different ways, including the use of Fe and/or Co salt lent ion-exchange capacity where the divalent calcium ions are
used in the Brichima process), strong mineral acid (such as H
4
(
3
PO
4
substituted by other metal ions. Moreover, apatite has been
5
or HClO
reagent (used in the Hamilton process). However, most of them transformations namely, C–C homo-coupling reaction,
employed in the Rhone–Poulenc process), and a Fenton's widely used as catalyst or support catalyst for various chemical
4
15
6
16
have multiple disadvantages including the use of toxic solvents, high reduction of nitric oxide with ammonia, N-arylation of imid-
temperature reaction, and product contamination with metal ions, azoles, biodiesel production, synthesis of naphthopyran
which limit their industrial applications. Several attempts have been derivatives and cycloaddition reactions. To the best of our
devoted to overcome these limitations and to improve phenol knowledge, the use of copper incorporated apatite as a hetero-
hydroxylation process using heterogeneous solid catalysts. However, geneous catalyst for the phenol hydroxylation in water has not
been reported in the literature. Accordingly, in continuation of
17
18
19
20
our efforts to explore a green and performing nanocatalyst, we
were planning to prepare copper incorporated apatite and
investigate its catalytic performance on phenol hydroxylation
a
VARENA Center, MAScIR Foundation, Rabat Design, Rue Mohamed El Jazouli,
Madinat El Irfane, 10100-Rabat, Morocco. E-mail: m.zahouily@mascir.com
b
Laboratoire de Mat ´e riaux, Catalyse et Valorisation des Ressources Naturelles, URAC
2
´
4, FST, Universite Hassan II-Casablanca, Morocco
with H
2
O
2
as an oxidant under optimal conditions (Scheme 1).
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Furthermore, the structural, textural, surface and morpholog- in 100 mL of deionized water and stirred to form a clear solu-
ical properties of the prepared nanocatalyst, the inuence of tion. Subsequently, 100 ml of (NH ) HPO (0.3 M) was added
4
2
4
reaction temperature, phenol : H O molar ratio, solvent and into the prepared solution under continuous stirring ((Ca + Cu)/
2
2
catalyst mass were investigated. The reusability and leaching of P molar ratio was kept constant at 1.67), followed by the addi-
the catalyst were also evaluated.
tion of NH
4
OH to adjust until a pH of 10–11. The resulting
ꢀ
mixture was heated at 90 C for 12 h under continuous stirring.
The blue precipitate was collected by centrifugation, washed
several times with deionized water and ethanol, and then dried
overnight at 80 C for 12 h in hot vacuum desiccator. Finally, the
as-obtained product was calcined at 700 C for 3 h.
2. Experimental
ꢀ
2
.1 Materials and apparatus
The chemical reagents used in this study including calcium
nitrate Ca(NO $4H O, copper nitrate Cu(NO $3H O,
ammonium phosphate (NH HPO , ammonium hydroxide
NH OH 28–30%, phenol, and 30 wt% aqueous H were
ꢀ
3
)
2
2
3
)
2
2
4
)
2
4
2.3 Phenol hydroxylation over Cu-apatite catalyst
4
2 2
O
Phenol hydroxylation was carried out in a ask of 50 mL
equipped with magnetic stirrer and a reux condenser. In
a typical procedure, 0.47 g of phenol (5 mmol) and 0.05 g of Cu-
apatite catalyst were added into 10 ml of deionized water, fol-
lowed by added drop-wise 1 ml of H
mixture (The molar ratio of phenol : H
tion temperature was subsequently heated up to 60 C under
vigorous stirring for 2 hours. At the end of the reaction, the
catalyst suspensions were removed and the product was ana-
lysed and quantied using high-performance liquid chroma-
tography (HPLC Shimadzu Kyoto, Japan) equipped with
a reversed-phase C18 column (150 mm ꢂ 4.6 mm ꢂ 5 mm)
using the methanol/water ((40/60) volume ratio) as mobile
purchased from Aldrich Chemical Company. All chemicals were
analytical grade and were used directly without any further
purication. X-ray diffraction (XRD) patterns of the catalysts
were obtained at room temperature on a Bruker AXS D-8
diffractometer using Cu-Ka radiation in Bragg–Brentano
geometry (q–2q). SEM and STEM micrographs were obtained on
a Tecnai G2 microscope at 120 kV. The gas adsorption/
desorption data was collected using a Micromeritics 3Flex
Surface characterization analyzer, using nitrogen. Prior to
2 2
O (10 mmol) to the
2
O
2
was 1 : 2). The reac-
ꢀ
ꢀ
nitrogen sorption, all samples were degassed at 200 C over-
night. The specic surface areas were determined from the
ꢀ
nitrogen adsorption/desorption isotherms (at ꢁ196 C), using
the BET (Brunauer–Emmett–Teller) method. Pore size distri-
ꢁ1
phase at a ow rate of 0.5 mL min with UV detection at
90 nm. The degree of phenol conversion and the products
2
butions were calculated from the N adsorption isotherms with
1
the “classic theory model” of Barret, Joyney and Halenda (BJH).
Fourier transforms infrared (FT-IR) spectra of samples in KBr
pellets were measured on a Bruker Vector 22 spectrometer. The
copper content of Cu-apatite materials was determined by
a ame atomic absorption spectrometer A ame atomic
absorption spectrometer (AA-7000), equipped with an optical
double-beam photometric system is automatically set for ame
measurements, and the high-throughput, single-beam photo-
metric system is automatically set for furnace measurements.
EDS spectra from the single points and chosen microareas and
maps showing elemental composition and surface distribution
were obtained using the SEM-EDS method. NMR solid-state
experiments were recorded on a Bruker Avance 600 spectrom-
eter. The Raman spectra of the catalyst were recorded on
a DXR™ 2 Raman Microscope (ThermoFisher Scientic) with
a backscattering mode at room temperature in the range 400–
selectivity were calculated using the following equation:
Conversion of phenol:
0
phenol
n
ꢁ nphenol
phenol
X
Phenol
¼
ꢂ 100%
ꢂ 100%
ꢂ 100%
(1)
0
n
Selectivity of catechol:
n
CT
S
Catechol
¼
(2)
(3)
(4)
n
CT þ nHQ þ nBQ
Selectivity of hydroquinone:
n
HQ
SHydrquinone ¼ n
CT þ nHQ þ nBQ
ꢁ
1
1
200 cm . The samples were excited using 532 nm laser with
Selectivity of benzoquinone:
ꢁ1
a resolution of 4 cm . To determine the surface compositions
of catalyst synthesis, X-ray photoelectron spectroscopy (XPS)
analysis was performed with Shimadzu Co: AXIS ULTRA. The X-
ray source was Mg Ka operated at 15 kV, the anodic current was
n
BQ
SBenzoquinone ¼ n
ꢂ 100%
CT þ nHQ þ nBQ
0
where nphenol and nphenol are the initial and the nal amounts
moles) of phenol, respectively. Whereas nCT, nHQ and nBQ are
(
1
0 mA, and the operating pressure in the vacuum chamber was
ꢁ
7
the produced amounts (moles) of catechol, hydroquinone and
benzoquinone, respectively.
lower than 5 10 Pa.
2.2 Preparation of copper-incorporate apatite (Cu-apatite)
2
.4 Cu-apatite catalyst reusability
according to the method reported in the literature with few The reusability of Cu-apatite in the phenol hydroxylation was
adjustments. In a typical procedure, Ca(NO $4H O (9.446 g, investigated. Aer one catalytic test, the Cu-apatite was sepa-
.04 mol) and Cu(NO $3H O (2.416 g, 0.01 mol) were dissolved rated by centrifugation and washed with water and
Cu-apatite was synthesized using co-precipitation method
21
3
)
2
2
0
3
)
2
2
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ꢀ
dichloromethane several times, and calcined at 700 C for 3 h.
The regenerated Cu-apatite catalyst was reused for the next run.
3. Results and discussion
3.1 Catalysts characterization
Fig. 1 show thermogravimetric analysis curve of copper incor-
porated apatite obtained by co-precipitation method. According
to the TG curve of the Cu-apatite dried sample, two weight
losses steps were observed. The rst weight loss occurs from 100
ꢀ
to 150 C, and can be attributed to the loss of water molecules
2
2,23
adsorbed on the surface of Cu-apatite.
The second weight
ꢀ
loss from 180 to 650 C can be related to the precursor
decomposition and the elimination of structural water from Cu-
apatite. Similar results have been reported by Suresh Kumar
Fig. 2 XRD patterns of HAp and Cu-apatite samples.
24
ꢀ
et al. In addition, a weight loss at 800 C was observed, which
can be attributed to the decomposition of apatite structure to
2
5
form Ca
3
(PO
4
)
2
and CaO.
ꢁ1
Indeed, the absorption peaks detected at 3439 and 1636 cm
are corresponding to the bending and the stretching vibrations
Fig. 2 shows the X-ray diffraction patterns of the hydroxy-
apatite, and copper incorporated apatite prepared by co-
precipitation method. Accordingly, the XRD patterns of HAp
Fig. 2) conrmed the presence of peaks corresponding to
hexagonal structure Ca (PO ) (OH) (space group P6 /m (176);
27
mode of H
2
O molecules. Furthermore, the intensity of the
peaks located at 1420 cm is contributed to the presence of
species in Cu-apatite. It's worth nothing that all
absorption bands corresponding to the vibrations of adsorbed
ꢁ1
(
2ꢁ
28
CO3
1
0
4 6
2
3
2
+
JCPDS card no. 09-0432), where the Ca at site I are connected
water molecules and carbonate disappeared aer the
2
+
to 9 oxygen atoms, while Ca at site II are connected to only 5
oxygen atoms and 1 hydroxyl group (Scheme 2). Moreover, there
are some remaining Ca(I) and Ca(II) exposed on the surface of
the HAp, which can be easily substituted. On the other hand,
the diffraction patterns of Cu-apatite samples showed the
presence of hydroxyapatite and a secondary phase in which the
diffraction peaks correspond to Ca Cu (PO ) (JCPDS 46-
1
9
2
4 14
0
403).
ꢁ
1
Fig. 3a shows FTIR spectra in the range of 400–4000 cm of
ꢀ
the Cu-apatite before and aer heat treatment at 700 C. As it
can be seen, the bands observed at 570 and 601 cm are
attributed to antisymmetric deformation (n
intense absorption peak at 1042 cm is ascribed to the asym-
ꢁ
1
3
ꢁ
4
) of PO
4
and the
ꢁ
1
3
–26
metric stretching vibration (n ) of PO .
The peaks located at
3
4
ꢁ1
3572 and 633 cm are attributed to the stretching mode of
hydroxyl OH group located in the hydroxyapatite structure.
Fig. 1 TGA curve of dried Cu-apatite under air atmosphere.
Fig. 3 (a) FTIR and (b) Raman shift spectra of Cu-apatite.
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26
ꢁ1
group (P–O bond). The high intense peak at 954 cm corre-
sponds to the symmetric stretching mode (n ) of the tetrahedral
1
2
9
PO group (P–O bond). The modes (n ) located at ꢃ551, 565,
4
4
ꢁ
1
5
80, and (n ) located at ꢃ418, 437 cm can be addressed
2
4
fundamentally to the bending mode of the PO group (O–P–O
30
bond). This results was in agreement with the results obtained
from FTIR analysis.
To investigate the composition and surface chemical states
of Cu-apatite, XPS measurement was carried out. Fig. 4 shows
the XPS survey spectrum in which it is indicated the presence of
principal element namely, Ca 2p, P 2p, O 1s, Cu 2p and C 1s. The
2
ꢁ
presence of a small pic of carbon was due to (CO3 ) adsorbed
from air during synthesis.
The high-resolution XPS scan of Ca 2p as shows (Fig. 5 a)
Fig.
4
XPS total spectrum of the Cu-apatite catalyst at room exhibits two peaks Ca 2p1/2 at351.4 eV and Ca 2p3/2 at 347.51 eV,
31
temperature.
which are assigned to calcium apatite and Ca–O, respectively.
The binding energy peak at 133.58 eV that tted with the elec-
tron orbit of P 2p3/2 2p is ascribed to the P–O bonds of Hap
(
Fig. 5b). The XPS spectrum of oxygen O 1 s (Fig. 5c) can be
deconvoluted to three characteristic pics at 532.7, 531.5 and
30.6, which could be assigned to O–P–O, P–O and OH,
respectively. Indeed, as can be observed in Fig. 5d, the Cu 2p3/2,
ꢀ
calcination process at 700 C, which suggests that the organic
components have been removed from the Cu-apatite material.
Fig. 3b shows the Raman spectrum of the synthesized Cu-
5
ꢁ
1
apatite. The peaks appearing at ꢃ1042, 1035 and 1067 cm
2
+
32
peak 1 at 933.07 eV is attributed to Cu /CuO, while the peak 2
are attributed to the asymmetric stretching mode (n ) of the PO
3
4
+
+
2
at 933.83 eV is assigned to Cu /Cu O. The formation of Cu can
Fig. 5 XPS at high-energy resolution on Cu-apatite at room temperature showing: (a) Ca 2p, (b) P 2p, (c) O 1s, and (d) Cu 2p peaks and respective
fitting peaks.
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36
transfer. Aer the incorporation of copper in apatite (Cu-
apatite), a new band has appeared at 260 nm attributed to low
2
ꢁ
2+
energy charge transfer O / Cu of copper ions in tetrahedral
37
coordination. In addition, the increase of intensity of base line
in the range of 550–800 identies the d–d transitions of Cu
2
+
tetrahedral coordination surrounding by oxygen in CuO.
Fig. 8a shows the isotherm of Cu-apatite sample. According
to the IUPAC classication, the isotherm of Cu-apatite is
assigned to type IV with a distinct hysteresis loop of type H3,
which is characteristic of mesoporous materials. These results
were further conrmed by the pore size distribution curve
determined by Barrett–Joyner–Halenda (BJH) method, which
˚
showed a centered pore size at 20.43 A (Fig. 8b). The BET surface
area, pore volume and pore diameter of the Cu-apatite material
are summarized in Table 1. The specic surface area and the
31
Fig. 6
P MAS-NMR spectra of HAp and Cu-apatite.
2
ꢁ1
pore volume of the Cu-apatite are found to be 17 m g and
3
ꢁ1
0
.00025 cm g , respectively (Table 1).
The morphology of Cu-apatite was investigated by SEM. As
be explained by the reduction of Cu during analysis under X- shown in Fig. 9, the Cu-apatite materials had a heterogeneous
ray irradiation in ultrahigh vacuum. The peak Cu 2p1/2 at microstructure that consisted of crystallites with heterogeneous
binding energy 953 eV (peak 3) corresponds to Cu . The peaks size and forms (Fig. 9a and b). Furthermore, the elemental
2
+
33
2
+
2
+
positions of the satellites indicate the presence of only Cu at analysis by energy dispersive spectroscopy (EDS) of Cu-apatite
the surface of catalyst.
was also performed in different selected zones of the Cu-
P solid state NMR was used to investigate the coordination apatite surface (Fig. 9c). From EDS analysis (Fig. 9d), it was
of phosphorus species (PO4 ) in the prepared hydroxyapatite conrmed that Cu-apatite was composed of Ca, Cu, P, O and C.
and Cu-apatite (Fig. 6). The P MAS NMR spectrum of the Indeed, the atomic distribution of these elements in the Cu-
hydroxyapatite shows one isotopic signal at 2.58 ppm suggest-
3
1
3
ꢁ
3
1
3
ꢁ
ing the presence of one phosphorus group (PO
4
) in the
34
prepared HAp. Moreover, when copper was incorporated to
3
ꢁ
the apatite, the signal of PO
4
was shied from 2.58 to
.89 ppm. Indeed, the phosphorus groups cannot be distin-
guished in the Ca (PO ) (OH) and Ca Cu (PO ) phases by
35
1
1
0
4 6
2
19
2
4 14
3
1
NMR P, which can be explain by the superposition of the two
peaks in the same environment.
The UV-visible diffuse reectance spectra (DRS) of HAp and
Cu-apatite were illustrated in Fig. 7. The pure HAp showed
2
ꢁ
2+
a characteristic peak at 205 nm assigned to O / Ca charge
Fig. 8 (a) Nitrogen adsorption–desorption isotherm and (b) BJH pore
size distribution of Cu-apatite.
Fig. 7 UV-visible profiles of the HAp and Cu-apatite.
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Table 1 Textural properties and copper content of Cu-apatite catalyst
a
b
2
ꢁ1
b
3
ꢁ1
b
˚
Catalyst
Cu (wt%)
S
BET (m g
)
Pore volume (cm g
)
Pore diameter (A)
Cu-apatite
1.39
17
0.00025
2.436
a
b
Determined by ame atomic absorption spectroscopy. From nitrogen sorption analysis.
Fig. 9 SEM images (a and b), EDS analysis (c) and EDS mapping image (d–f) of Cu-apatite.
apatite was Ca ¼ 31.31%, Cu ¼ 1.19%, P ¼ 18.13%, O ¼ 45% notice also that Cu-apatite dispersion is relatively heteroge-
and C ¼ 3.89%. The experimental Ca/P molar ratio is equal to neous, since the nanoparticles presented some agglomera-
1.73, which is higher than that of theoretical HAp composition tion, which can be explained by the Ostwald ripening
(
1.67), indicating an even incorporation of Cu in the apatite phenomena.
structure. Moreover, it can be seen, from SEM-EDS maps of Cu- To evaluate the catalytic performance of Cu-apatite material,
apatite (Fig. 9e–g), that Ca, P and Cu elemental were regularly the phenol hydroxylation with H O as oxidant was selected as
2
2
distributed over the surface of Cu-apatite.
catalytic reaction mode (Scheme 1). The preliminary experi-
STEM images of Cu-apatite are presented in Fig. 10. ments were started by screening the activities of hydroxyapatite
According to this gure, the Cu-apatite nanoparticles are well (HAp) and copper incorporated apatite (Cu-apatite) for catalyst
nanostrutured materials with uniform shape and size. We the phenol hydroxylation (Table 2). Firstly, it should be noted
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Fig. 10 STEM images of Cu-apatite (a and b).
Scheme 1 Phenol hydroxylation catalyzed by copper incorporated
apatite (Cu-apatite).
Fig. 11 The influence of the reaction time on the phenol hydroxylation
2 2
over Cu-apatite catalyst. Reaction conditions: phenol (5 mmol), H O
ꢀ
(
10 mmol), catalyst (50 mg); water (10 mL), and temperature (60 C).
The effect of the reaction parameters, namely: reaction time,
reaction temperature, H O /phenol molar ratio, type of solvent,
2 2
catalyst amount, in the phenol hydroxylation reaction was also
investigated. Firstly, the inuence of the reaction time on the
catalytic activity of phenol hydroxylation over Cu-apatite was
examined (Fig. 11). From this result, it was found that phenol
conversion increases with increasing reaction time. Aer 2 h,
the phenol conversion reached 64% with 95% of dihydrox-
ybenzenes selectivity. However, the increase of time reaction
Scheme 2 Structural representation of hydroxyapatite.
that the reaction didn't occur without adding the catalyst (Table
2, entry 1), also the hydroxyapatite was inactive for this reaction
(Table 2, entry 2). However, the copper incorporated apatite has
high catalytic activity for phenol hydroxylation under the same
conditions, indicating that the phenol hydroxylation is very
sensitive to the copper species.
a
Table 2 Evaluation and screening of catalysts
Sample
Catalyst
XPh (%)
YCat (%)
YHQ (%)
YBQ (%)
1
2
3
Free catalyst
HAP
Cu-apatite
n.r.
n.r.
64
—
—
68
—
—
27
—
—
5
a
Fig. 12 Effect of reaction temperature on phenol hydroxylation over
2 2
Reaction conditions: phenol (5 mmol); H O (10 mmol), catalyst (50
ꢀ
mg), water (10 mL), time (2 h) and temperature (60 C).
Cu-apatite catalyst. Reaction conditions: phenol (5 mmol), H
mmol), catalyst (50 mg), water (10 mL) and time (2 h).
2 2
O (10
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Fig. 13 The effect of phenol : H
O
2 2
molar ratio on phenol hydroxyl- Fig. 14 The effect of Cu-apatite catalyst amount on phenol hydrox-
ation over Cu-apatite. Reaction conditions: sample of phenol reacted ylation. Reaction conditions: phenol (5 mmol), H
2
O
2
(10 mmol), water
ꢀ
2
with H O
2
in different molar ratios over 50 mg of Cu-apatite in 10 mL (10 mL), time (2 h), temperature (60 C).
ꢀ
of water at 60 C for 2 h.
phenol/H O molar ratio of 1 : 2 was chosen as the optimum for
2
2
phenol hydroxylation.
beyond 3 h led to a decrease in the selectivity of hydroquinone,
which can be explained by their transformation to the benzo-
quinone and subsequently to the tar formation. This indicates
that the extension of the reaction time is not desirable for
phenol hydroxylation.
The inuence of solvent in the phenol hydroxylation over Cu-
apatite catalyst was investigated with various solvent. The
results of this study are presented in Table 3. The phenol
hydroxylation showed higher conversion and selectivity for DHB
when water was used as solvent. However, using methanol,
ethanol and acetone as solvent for phenol hydroxylation gives
a lower conversion. These results can be explain by the high
38
The reaction temperature is an important factor affecting the
phenol hydroxylation, so we investigated the effect of temper-
ꢀ
ature reaction from 25 to 100 C. The results of this study are
solubility of phenol and H
stability of free radicals OHc in water.
2 2
O in water and also to the higher
shown in Fig. 12. When the reaction temperature increases
39,40
ꢀ
from 25 to 90 C, the phenol conversion increased from 23 to
The inuence of the amount of Cu-apatite catalyst ranging
from 20 to 60 mg on phenol hydroxylation was also investi-
gated (Fig. 14). As can be seen, the phenol conversion was
slightly increasing from 57 to 58% with the increase of the
amount of Cu-apatite catalyst from 20 and 40 mg and reached
a maximum value when the catalyst amount was increased up
to 50 mg. Further increase in Cu-apatite catalyst does not
affect signicantly the conversion of phenol. Therefore, 50 mg
was chosen as the optimal catalyst amount for the phenol
hydroxylation.
7
2%. This result can be explained by the formation of more OHc
radicals at higher temperatures. However, the presence of
a high concentration of free radical led to the oxidation of
hydroquinone to benzoquinone. Moreover, when the reaction
temperature reached 100 C (decomposition temperature of
H O ), the phenol conversion decreased due to the decompo-
ꢀ
2
2
sition of H O to O and H O.
2
2
2
2
The phenol conversion and DHB selectivity over Cu-apatite
catalyst was studied by varying the amount of H at a xed
2 2
amount of phenol. Fig. 13 shows the inuence of phenol/H O
molar ratio from 0.25 to 3. From this study, we noticed that the
phenol conversion increased from 24 to reach 76% by varying
the phenol/H O molar ratio from 3 : 1 to 1 : 4. This result may
2 2
be explained by the large generation of OHc radicals using an
excess amount of H O . However, the excess amount of H O is
undesirable as it decreases the selectivity of DHB. Finally,
2 2
O
Several mechanisms have been proposed in the literature to
explain the oxidation of aromatic compounds using transition
41–43
metals.
The plausible reaction mechanism of phenol
hydroxylation in the presence of Cu-apatite catalyst is proposed
in Scheme 3. Firstly, the catalytic reaction is stated by the
2
2
2 2
2
+
2 2 2
reduction of H O by Cu -apatite to produce HO c and OHc
radicals via a redox mechanism (step 1). Then, the hydroxyl
radicals formed oxidize phenol to catechol, hydroquinone and
benzoquinone can be obtained by consecutive oxidation of
hydroquinone (step 2).
At industrial scale, the reusability of the catalyst is one of the
most important parameters of its application as heterogeneous
Table 3 The effect of solvent on the phenol hydroxylation over Cu-
apatite catalyst
a
Solvent
X
Ph (%)
Y
Cat (%)
Y
HQ (%)
Y
BQ (%) catalyst. Therefore, the reusability and the stability of Cu-apatite
in the phenol hydroxylation were investigated (Fig. 15). The
regenerated Cu-apatite catalyst was reused for the next run. As
Water
64
5
6
68
27
5
Methanol
Ethanol
Acetone
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
shown in Fig. 15, the Cu-apatite demonstrated its ability to be
reused for four consecutive cycles with a slight decrease in the
activity, which can be due to the gradual poisoning of the
3
a
Reaction conditions: phenol, 5 mmol; H
2
O
2
, 10 mmol; catalyst (50
ꢀ
mg), solvent (10 mL), time (2 h) and 60 C.
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2 2
Scheme 3 Proposed reaction mechanism of hydroxylation of phenol with H O catalyzed by Cu-apatite.
ꢀ
stirred for additional 1 h at 60 C, the portion containing the
Cu-apatite catalyst showed 52% of phenol conversion, while the
no increment of phenol conversion was obtained in the catalyst-
free portion, which put in evidence the heterogeneity of the Cu-
apatite catalyst. Moreover, copper leaching was studied by
atomic absorption spectrometer of the Cu-apatite catalyst aer
the phenol hydroxylation reaction. The amount of copper
incorporated in apatite-based catalyst was found to be 1.24%
aer one catalytic cycle which conrms no signicant copper
leaching. The phenol conversion over Cu-apatite was compared
with other catalysts reported in the literature as tabulated in
Fig. 15 Reuse performance of Cu-apatite catalyst in phenol hydrox- Table 4. From this, we can show clearly that our Cu-apatite
ylation. Reaction conditions: phenol (5 mmol), H
catalyst (50 mg), water (10 mL), time (2 h), temperature (60 C).
2
O
2
(10 mmol),
catalyst exhibited a best catalytic activity of phenol conversion
comparing with other catalytic systems. In the other hand, in
order to study the stability of Cu-apatite aer the phenol
hydroxylation reaction, SEM, STEM and FTIR analysis were
investigated. The STEM images showed that the size and the
form of Cu-apatite nanoparticles remained the same with
a heterogeneous dispersion (Fig. 16a). Indeed, the SEM analysis
of Cu-apatite aer one cycle didn't show any changes in its
morphology (Fig. 16b). Also, all absorption bands in the IR
spectrum were corresponding to vibrations of original Cu-
apatite catalyst (Fig. 16c).
ꢀ
surface and pores of Cu-apatite catalyst by tar formation in the
phenol hydroxylation.
To investigate the heterogeneity of the Cu-apatite catalyst
and the copper leaching, the hydroxylation reaction of phenol
was maintained for 1 h in the presence of Cu-apatite catalyst.
Aer that, the catalyst was ltered and the ltrate obtained was
Table 4 Comparison of the phenol conversion catalyzed by different catalytic systems
ꢀ
Catalyst
W% Fe/Cu
Phenol : H
2
O
2
Solvent
Temperature ( C)
Time (min)
Phenol conversion (%)
Ref.
Fe/WAC
Cu/MCM-S
Cu-alginate
6.94
1.50
17.7
37.13 (Fe)
1 : 1
1 : 1
1 : 2
1 : 1
1 : 2
1 : 1
Water
Water
Water
Water
40
60
70
55
40
360
20
30
56
51
37
61
35
44
7
45
46
CuFe
2
O
4
21.18 (Cu)
Cu-apatite
1.39
Water
60
120
51
64
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: 2
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Fig. 16 STEM image (a), SEM image (b), FTIR (c) and EDS (d) of Cu-apatite after phenol hydroxylation reaction.
2
S. Briganti, E. Camera and M. Picardo, Pigm. Cell Res., 2003,
6, 101–110.
4
. Conclusion
1
Copper incorporated apatite is an active and promising nano-
catalyst for high conversion of phenol to high value phenolic
compounds. Physicochemical properties of Cu-apatite were
evaluated by XRD, FTIR, SEM, STEM, and adsorption–desorp-
tion of nitrogen. Moreover, the optimal reaction condition was
investigated, and water was proved to be the optimum solvent of
the phenol reaction over Cu-apatite. In addition, the Cu-apatite
separable catalyst was showed a good catalytic activity with high
conversion and selectivity on phenol even aer four cycles. The
lixiviation study was suggesting negligible copper leaching from
Cu-apatite catalyst, which conrms the heterogeneity of our
catalyst. Also, the analysis of Cu-apatite recovered catalyst by
SEM, STEM and FTIR corroborated the stability of Cu-apatite
catalyst.
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Conflicts of interest
The authors declare that there is no conict of interests
regarding the publication of this paper.
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this research is hereby acknowledged.
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