Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and xylenes on Ca10(PO4)6(OH)2 – CaWO4

Article abstract of DOI:10.1016/j.crci.2015.10.013

A W-containing apatite (W/HAp) catalyst was prepared following a hydrothermal synthesis route and served as a model catalyst. Crystallographic analysis indicated that the resulting material contained hydroxyapatite, Ca_10?3xW_x(PO₄)₆(OH)₂, W-hydroxyapatite, calcium tungstate, CaWO₄, and tricalcium phosphate, Ca₃(PO₄)₂. The catalyst was investigated in liquid phase oxidation of benzyl alcohol and xylenes using hydrogen peroxide as an oxidant. For comparison, commercial calcium phosphate, hydroxyapatite and CaWO₄ were tested in the same reaction. Calcium phosphate and hydroxyapatite appeared as inactive and decomposed hydrogen peroxide non-selectively. A moderate activity but low hydrogen peroxide efficiency was observed for the CaWO₄ phase. In contrast, the W/HAp catalyst showed a reasonable activity and a better hydrogen peroxide efficiency in the oxidation of benzyl alcohol and xylenes. This new W/HAp catalyst showed, after six cycles, losses of the activity below 15% compared to the fresh catalyst with no effect on the selectivity. It is noteworthy that ICP-OES analyses showed no tungsten leaching that is the main advantage of this catalyst.

Full text of DOI:10.1016/j.crci.2015.10.013

C. R. Chimie xxx (2016) 1e10
Contents lists available at ScienceDirect
Comptes Rendus Chimie
www.sciencedirect.com
ꢀ
Full paper/Memoire
Liquid-phase oxidation with hydrogen peroxide of benzyl
*
alcohol and xylenes on Ca10(PO4)6(OH)2 e CaWO4
María Isabel Domínguez a, Bogdan Cojocaru b, Madalina Tudorache b,
a
a
,
, *
Jose Antonio Odriozola , Miguel Angel Centeno **, Vasile I. Parvulescu b
ꢀ
a
ꢀ
Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Avda Americo Vespucio 49, 41092 Sevilla, Spain
b Department of Organic Chemistry, Biochemistry and Catalysis, University of Bucharest, Bdul Regina Elisabeta 4-12, 030016 Bucharest,
Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
A W-containing apatite (W/HAp) catalyst was prepared following a hydrothermal syn-
thesis route and served as a model catalyst. Crystallographic analysis indicated that the
resulting material contained hydroxyapatite, Ca10ꢀ3xWx(PO4)6(OH)2, W-hydroxyapatite,
calcium tungstate, CaWO4, and tricalcium phosphate, Ca3(PO4)2. The catalyst was inves-
tigated in liquid phase oxidation of benzyl alcohol and xylenes using hydrogen peroxide as
an oxidant. For comparison, commercial calcium phosphate, hydroxyapatite and CaWO4
were tested in the same reaction. Calcium phosphate and hydroxyapatite appeared as
inactive and decomposed hydrogen peroxide non-selectively. A moderate activity but low
hydrogen peroxide efficiency was observed for the CaWO4 phase. In contrast, the W/HAp
catalyst showed a reasonable activity and a better hydrogen peroxide efficiency in the
oxidation of benzyl alcohol and xylenes. This new W/HAp catalyst showed, after six cycles,
losses of the activity below 15% compared to the fresh catalyst with no effect on the
selectivity. It is noteworthy that ICP-OES analyses showed no tungsten leaching that is the
main advantage of this catalyst.
Received 16 March 2015
Accepted 28 October 2015
Available online xxxx
Keywords:
W-containing apatite
Liquid phase oxidation of benzyl alcohol and
xylenes
Morphologic characterization of materials
ꢀ
© 2016 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
are among them. To overcome these drawbacks various
solutions are proposed in the recent literature. These
Chemistry and the chemical industry have been shaping
our lives since the beginning of the 20th century. Many
positive effects of chemistry and the chemical industry are
made explicit in most nowadays technologies that are pil-
lars of the development of modern societies. However,
many drawbacks still remain to achieve the expected effi-
ciency. Waste generation and pollution caused by the low
selectivity of reactions and the formation of by-products
include the use of alternative solvents, new catalysts and
alternative products with the same efficiency or even
greater, but with a reduced degree of toxicity [1].
Oxidation reactions fit in a category for which the
enhancement of the selectivity is still a target. Liquid phase
oxidation of organic substrates, improved by solid catalysts,
is considered a non-polluting and energetically favorable
oxidation process on using hydrogen peroxide as an
oxidizing agent. Hydrogen peroxide is one of the most
active sources of oxygen, and at the same time the most
acceptable oxidation agent from the point of view of
environmental protection [2,3].
*
This paper is a tribute to Prof. Edmon Payen on the occasion of his
65th birthday.
* Corresponding author.
On the other side, apatites and apatitic materials are
widely represented in nature [4]. The apatite family is
** Corresponding author.
E-mail addresses: centeno@icmse.csic.es (M.A. Centeno), vasile.
parvulescu@chimie.unibuc.ro (V.I. Parvulescu).
http://dx.doi.org/10.1016/j.crci.2015.10.013
ꢀ
1631-0748/© 2016 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

2
M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
formed by a large number of isomorphous compounds with
the general chemical formula Me10(XO4)6Y2, where Me is a
divalent cation, X, phosphorus and Y, a monovalent anion
[5]. Representative members of this group are calcium
phosphate fluorapatite, Ca10(PO4)6F2 and calcium hy-
droxyapatite (HAp) Ca10(PO4)6(OH)2. Apatites crystallize in
the hexagonal system (space group P63/m), with a struc-
ture described as a compact package of Me ions and
tetrahedral XO4 groups delimiting two types of uncon-
nected channels [6]. The first one with a diameter of 2.5 Å
surrounded by four Me is designated as MeI. The second
type of channel has six Me, is named MeII, placed in two
equilateral triangles at ¼ and ¾ and centered in a C6 axis.
The diameter of this latter type of channel is 3.5 Å and is
oriented along the c axis, hosting Y anions which balance
the positive charge of the matrix and have great mobility
[7].
with catalytic amounts of tungstate and phosphate ions
provides a synthetically useful procedure for the highly
selective oxidation or epoxidation of organic substrates.
However, tungstates suffer from a rather high leaching
under reaction conditions [26]. From a practical point of
view, this generates some inconveniences in purification of
the product and in the recycling of the catalyst. The
immobilization of peroxotungstic acid on silica-grafted
phosphoramides [27] or hydroxyapatite [28] led to a
certain improvement of both activity and stability but
leaching was not avoided. Well-defined complexes of
tungsten as polyoxometalate anions [29] succeeded in this
respect. However their use, for practical reasons, also re-
quires dispersion on a high surface area support [30]. It is
clear, however, from these studies the fact that association
of W and P is beneficial to the activity of tungsten in liquid
phase oxidation reactions.
One of the main characteristics of the apatite is the
flexibility of its structure; all the elements can be
exchanged if the charge balance is maintained. For
example, it is possible to prepare a complete series of ap-
Therefore, the aim of this work was to associate W and P
through the insertion of tungsten into an apatite structure
by means of a hydrothermal procedure. The scope of this
association was to suppress the leaching of tungsten
keeping the catalyst activity. To prove this concept the new
catalysts were investigated in the oxidation of aromatic
compounds (benzylic alcohol, o-xylene, m-xylene and p-
xylene) using H2O2 as the oxidizing agent.
4ꢀ
3ꢀ
atites by the substitution pair La3þ þ SiO4 ↔ Ca2þ þ PO4
[8]. Furthermore, these compounds present a high chemi-
cal and thermal stability and a weak and retrograde solu-
bility. These properties make apatites useful in applications
like biomaterials [9], chromatography [10], sensors [11],
optics, detoxification of water [12], radioactive and indus-
trial waste immobilization [13,14] or catalysis [15]. Doping
apatites with various cations enlarges the application field
of these materials into luminescence [16], fuel cells [17],
etc.
2. Materials and methods
2.1. Catalyst preparation
W-containing apatite (W/HAp) was prepared using
mono-calcium phosphate (MCPM, Panreac), ®-tricalcium
phosphate (®ꢀTCP obtained from the Apatite type TCP,
PROLABO Rectapur, after calcination for 3 h at 900 ꢁC),
tetracalcium phosphate (TTCP, obtained from an equimolar
mixtureꢁof MCMP and CaCO3 Panreac PA, calcined for 24 h
at 1350 C, in a N2 flow) and tungstic acid (H2WO4).
The hydrothermal synthesis followed the formulation of
Mejdoubi and Lacout [31]. This type of formulation using
three components (TTCP, ®-TCP and MCPM) allows varying
the constituent proportions, which affects the physical and
chemical properties, but one always obtains hydroxyapatite
according to reaction (1).
Hydrothermal synthesis of apatites is one of the
simplest and most economical methods of preparation.
Mixtures of calcium phosphates, in aqueous medium at
room temperature, set and harden resulting in apatitic
cements [18]. When using hydrothermal reactors, evolu-
tion from powdery to coherent materials occurs; this is due
to the combined action of both physical and chemical
phenomena: temperature, pressure and chemical reactions
between constituents. Usually, the initial powder is pressed
prior to its introduction into the autoclave, allowing the
particles to approach as well as the slow diffusion of water
through the pellet. In the hydrothermal process all re-
actions occur at temperatures above the boiling point of the
liquid [19].
Benzyl alcohol is the most commonly used model
compound in alcohol oxidation. Moreover, oxidation of
benzyl alcohol to benzaldehyde is an important organic
transformation since benzaldehyde is a very valuable
chemical with applications in perfumery, agro-chemical
industries, etc [20,21]. Conventional methods for per-
forming such transformations imply the use of toxic sol-
vents and, generally, involve the use of stoichiometric or
more than stoichiometric quantities of inorganic oxidants
[20]. On the other hand, oxidation of xylenes results in
useful products such as terephthalic acid, mostly used as a
precursor to the polyester PET.
xCaðH2PO4Þ2$H2O þ ð2 ꢀ 3xÞCa3ðPO4Þ2
H2
O
þ ð1 þ 2xÞCa4ðPO4Þ2O
Ca10ðPO4Þ6ðOHÞ2
(1)
H2WO4 was added to the three calcium phosphates in
adequate percentages to replace ~2% of the calcium atoms
in the apatite network (Table 1), and then the four com-
ponents were milled together in a mortar until producing
Table 1
Composition of the reactant mixture.
Compound
Weight, %
TTCP
61.98
23.30
9.72
Tungsten was reported as an active oxidizing species in
number of oxidation reactions implying hydrogen
peroxide [22,23]. Venturello [24,25] reported that, under
acidic conditions, aqueous hydrogen peroxide together
®-TCP
MCPM
H2WO4
a
5.00
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
3
particle sizes smaller than 50
m
m. A 10 mm diameter
Iodometric titrations determine the transformation de-
gree of hydrogen peroxide. The selectivity was calculated as
the ratio between the moles of the product “i”, Ni, and the
total number of moles of the products, N:
infrared die was used to pelletize 1.25 g of the mixture at
5 Tm. The pellets were introduced into a hydrothermal
reactor with 100 mL of de-ionized water, and the reactor
was heated in a furnace for 48 h at 200 ꢁC (5 ꢁC/min). Once
the heating schedule was finished the reactor was allowed
to cool down in air to room temperature and the obtained
solid was taken away from the reactor and oven-dried at
100 ꢁC for 30 min.
Hydroxyapatite Ca10(PO4)6(OH)2 and Ca3(PO4)2 were
purchased from SigmaeAldrich. CaWO4 was prepared via
precipitation of Na2WO4 with CaCl2 and the precipitate was
further washed until it was free of chlorine. Then it was
dried under vacuum at 110 ꢁC.
Si ¼ Ni=SN
(2)
Dehydration of the H2O2 solution (30 wt %, Consors SRL,
Romania) was carried out in the presence of anhydrous
MgSO4, previously calcined at 300 ꢁC under vacuum for
24 h. Ten grams of anhydrous MgSO4 were added to a
mixture of 10 mL H2O2 and 50 mL dioxane. The mixture was
stirred for 2 h. After centrifugation and filtration, another
10 g of anhydrous MgSO4 were added and the solution was
stirred for 1 h more. A centrifuge allows separating the
MgSO4 from the solution that was finally filtered. To ho-
mogenize the substrate-hydrogen peroxide mixtures
dioxane was added [32]. It is a low toxicity versatile aprotic
solvent that exhibits weak interactions with the solid [33].
The H2O2 content before and after the reaction is
determined by iodometry. 0.5 mL of solution was poured in
an Erlenmeyer flask together with 10 mL HCl and 1 g KI. The
mixture was diluted with 100e125 mL water and kept in a
dark place for 15 min after that it was titrated in the
presence of 14 mL Na2S2O3 (0.1N) until it turned yellow. At
that point, 1 mL of starch solution was added and the so-
lution was further titrated until the color turned from dark
blue to colorless.
2.2. Catalyst characterization
Textural characteristics (surface area and pore diameter)
were determined from the adsorptionedesorption iso-
ꢁ
therms of nitrogen at ꢀ196 C using a Micromeritics ASAP
2010 Surface Area and ꢁPorosity Analyzer. Samples were
degassed for 2 h at 150 C in a vacuum.
Chemical compositions of the obtained catalysts were
measured by X-ray fluorescence (XRF), employing a PAN-
alytical AXIOS sequential spectrophotometer with
rhodium tube as a source of radiation. XRF measurements
were performed onto pressed pellets (sample including
6 wt.% of wax).
X-ray diffraction (XRD) patterns were obtained in an
X'Pert Pro PANalytical. Diffraction patterns were recorded
a
2KI þ 2HCl þ H2O2/I2 þ 2KCl þ 2H2O
(3)
using Cu K
a
radiation (
l
¼ 1.5404 Å) over a 5e90ꢁ 2
q range
I2 þ 2Na2S2O3/2NaI þ Na2S4O6
(4)
and a PIXcel solid state X-ray detector operating at 0.026ꢁ
step size and 147 s step time. The incident beam intensity
was optimized using a W/Si incident beam parabolic
mirror. The software X'Pert Highscore permits obtaining a
semiquantitative analysis of the phases present in the
sample.
The initial content of hydrogen peroxide determined by
iodometry was 38 wt % H2O2.
The reaction products were analyzed using an HPCL,
Agilent Technologies 1260 Infinity equipped with a DAD
detector, and a column Eurosphere C18 working with a flow
rate of 1 mL/min, AcN: H2O ¼ 40: 60,
l
¼ 274.5 nm, and
Scanning electron microscopy (SEM) observations were
carried out in a JEOL JSM-5400 instrument equipped with
an energy dispersive X-ray spectrometer (Oxford Link Tetra
1128-231).
Vinj ¼ 10
mL.
The leaching of tungsten was checked by analyzing
samples collected at the end of the reaction using an ICP-
OES-700/715 Agilent Technology equipment.
Raman spectra were collected with a Horiba Jobin Yvon
e Labram HR UVeVisibleeNIR (200e1600 nm) Raman
Microscope Spectrometer, using a laser with a wavelength
of 632 nm. The spectra were collected from 10 scans at a
3. Results and discussion
resolution of 2 cmꢀ1
.
3.1. Characterization
2.3. Catalytic tests
Table 2 presents the chemical analysis of the catalyst,
expressed as weight and molar percentages.
The catalytic tests were performed in a round bottom
stirred flask with a vertical refrigerant. A thermocouple
with a precision of 0.2 ꢁC controls the reactor temperature.
A heating bath using oil as thermostatic fluid ensures the
reactor isothermicity. Typical catalytic tests were carried
out in the presence of 50 mg catalyst with a molar
substrate-to-oxidant ratio of 1/4 (1 mmol of substrate) for
4 h at 55 ꢁC. A magnetic stirrer (4 mm) at 900 rpm ho-
mogenizes the reactant mixture. The benzyl alcohol (99.8%
purity) and xylenes (99% purity) were purchased from
SigmaeAldrich.
CaWO4 had a very low surface area (2.4 m2/g), while the
surface area of the W/HAp simple was much higher
(18.6 m2/g). Fig. 1 shows the diffractogram of the W/HAp
sample and that of a commercial pure calcium apatite. The
W/Hap pattern presents diffraction lines corresponding to
Ca10(PO4)6(OH)2 (01-073-0294), CaWO4 (01-077-2233) and
Ca3(PO4)2 (00-009-0169), the more intense reflections
corresponding to the apatitic phase, Ca10(PO4)6(OH)2. Cal-
cium tungstate, CaWO4, and tricalcium phosphate,
Ca3(PO4)2, are minor components of the W/Hap sample. A
semiquantitative analysis of the sample performed with
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

4
M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
Table 2
Elemental and structural composition of the synthesized W/hydroxyapatite.
Composition
Elemental compositiona
Structural compositionb
Ca10(PO4)6(OH)2
83
Ca
P
W
Ca3(PO4)2
11
CaWO4
6
Weight, %
Molar, %
63.20
60.70
30.70
38.03
6.13
1.27
a
Determined from XRF.
Determined from XRD.
b
the software X'Pert Highscore is presented in Table 2. An
estimate of the tungsten content of the W/HAp sample
from the amount of observed crystalline phases results in a
Ca:P:W atomic ratio of 62.4:37:5:0.1, which clearly evi-
dences that tungsten is partially replacing Ca in the apatitic
phase. This is also corroborated by the systematic shift of
Raman spectra of investigated W/HAp display lines that are
characteristic of apatites [34] (Fig. 8A). Several absorptions
are assigned to PeO bonds in the PO4 group. The absorp-
tions at 1079 and 1051 cmꢀ1 are assigned to the triply
degenerate asymmetric stretching mode (y3) of the PeO
bond, the strong line at 966 cmꢀ1 to totally symmetric
stretching mode (y1) of the PꢀO bond, the line at 594 cmꢀ1
to the triply degenerate bending mode (y4) of the OꢀPꢀO
bond and that at 436 cmꢀ1 assigned to the doubly degen-
erate bending mode (y2) of the OꢀPꢀO bond. These spectra
also indicate the presence of tungsten. In the iso-
polytungstate species, the W exhibits a distorted octahe-
dral geometry with generally more WO2(t) groups, where t
stands for a terminal oxygen atom. According to Payen and
co-workers the symmetric and antisymmetric stretching
modes of these groups are observed at 960e980 and
900e920 cmꢀ1, respectively [35,36]. However, the line
the diffraction lines of the W/Hap sample to smaller 2
q
angles with respect to the pure calcium hydroxyapatite
phase, for instance the diffraction line corresponding to the
[002] plane appears at 25.88ꢁ for Ca10(PO4)6(OH)2 and at
25,68ꢁ for the W/HAp sample that is in accordance with the
ionic radius of W6þ (0.6 Å) smaller than that of Ca2þ (1.0 Å).
Fig. 2 shows a representative SEM micrograph of the W/
HAp sample. Three types of crystals can be distinguished.
First, the needle-like crystals that are characteristic of the
apatite phase. The EDX analysis of these crystals shows the
presence of P and Ca, although the presence of small
amounts of W, Fig. 3, cannot be disregarded in accordance
with the FRX and XRD analysis.
The second type of crystal presents an octahedral
morphology (Fig. 4). In accordance with EDX (Fig. 5) and
XRD analysis they correspond to W-rich crystals, namely,
CaWO4.
The third type of crystal, without
a well-defined
morphology (Fig. 6), was attributed on the basis of EDX
(Fig. 7) and XRD analysis to un-reacted tricalcium phos-
phate Ca3(PO4)2.
Raman spectra provided additional structural informa-
tion and evidence on the formation of the different phases.
Fig. 1. Diffractograms of the commercial apatite (HAp) and the prepared
sample (W/HAp) a: Ca10(PO4)6(OH)2; w: CaWO4; t: Ca3(PO4)2.
Fig. 2. SEM image of the prepared sample.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
5
Fig. 3. EDX analysis of apatite crystals.
Fig. 8B presents the Raman spectrum of CaWO4 used as
the reference in this study and prepared by precipitation of
Na2WO4 with CaCl2. Six Raman phonon modes located 915,
839, 799, 398, 332, and 210 cmꢀ1, respectively, were
observed in this spectrum. The line at 915 cmꢀ1 has the
assignment discussed above. Those at 839 and 799 cmꢀ1
correspond to Bg and Eg modes that degenerated from
A1(y1) and F2(y3) vibration modes, while those at about 398
and 332 cmꢀ1 correspond to Bg and (Ag þ Bg) vibrations
which are attributed to the degeneration of E(y2) vibrations
of tungstate groups. The line at about 210 cmꢀ1 belongs to
the external mode of CaWO4 [37].
3.2. Catalytic tests
For the sake of comparison, blank experiments were
made using a commercial hydroxyapatite. No conversion
was observed using benzyl alcohol or xylenes. However,
apatite decomposed over 20% of hydrogen peroxide in a
non-selective way. Similar results were obtained for
Ca3(PO4)2.
Tests with the pure CaWO4 phase, also found in the new
W/HAp catalyst, indicated a catalytic activity that is in
agreement with the previous report of Noyori et al. on the
role of tungsten in liquid phase oxidation [22]. Thus for the
Fig. 4. SEM image of W-rich crystals.
assigned to symmetric stretching mode overlaps the signal
of the strong PeO bond and it is very difficult to be
decomposed. In addition to this line, the line at 337 cmꢀ1 is
typically assigned to the vibrations of W-O bonds in tung-
states [36] including CaWO4 [37]. These results are in good
agreement with the XRD results.
Fig. 5. EDX analysis of W-rich crystals.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

6
M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
conversion reached a plateau and the resulting benzalde-
hyde underwent further oxidation to benzoic acid. More-
over, the aromatic cycle starts to oxidize giving rise to three
more products which are clearly observed after four hours.
Thus, the selectivity decreased and up to 5 products were
identified, Scheme 1.
The advantage of using dioxane is its (http://en.
wikipedia.org/wiki/miscible) miscibility with water that
generates an azeotrope with a boiling point of around 90 ꢁC
[38]. Also, it has weak interactions with the catalyst
affording the access of the reactants to the active sites [33].
In this way its use in mixtures with hydrogen peroxide has
two advantages: (i) an advanced solubilization of the sub-
strate, and (ii) “solubilization” of the water produced in the
reaction, thus diminishing its competition with the sub-
strate for the catalyst active sites.
Xylenes are also oxidized on CaWO4 catalysts. 22.4%
conversion in the oxidation of p-xylene was achieved after
4 h reaction time. The evolution of the conversion and of
the product distribution with time is presented in Fig. 10.
The evolution of both conversion and reaction products
differ from that observed in the case of the oxidation of
benzyl alcohol. The p-xylene conversion increased contin-
uously with time, while the selectivity changed from
around 90% in p-tholylmethanol after 1 h to around 70% in
Fig. 6. SEM image of tricalcium phosphate crystals.
oxidation of benzyl alcohol 18.8% conversion was achieved
after 4 h reaction time. Time evolution of the conversion
and reaction products is presented in Fig. 9. In the first two
hours the oxidation occurred only with the selective for-
mation of benzyl aldehyde. After three hours the
Fig. 7. EDX analysis of tricalcium phosphate crystals.
Fig. 8. Raman spectra of the W/HAp sample in comparison with commercial HAp and CaWO4 (A); Raman spectra of the CaWO4 (B).
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
7
Fig. 9. Time evolution of the conversion and selectivity of the prepared CaWO4 in the oxidation of benzyl alcohol (50 mg catalyst, substrate-to-oxidant molar
ratio of 1/4 (1 mmol substrate), 4 h, 55 ꢁC and 900 rpm).
4-methylbenzaldehyde after 4 h reaction time. Scheme 2
depicts the evolution of the reaction products.
intermediate products to the total number of moles of
H2O2 transformed during the oxidation process. This ratio
expresses the percentage of the H2O2 molecules effec-
tively used for the oxidation of the substrate. Fig. 11a
shows the evolution with time of the number of moles of
H2O2 consumed per gram of substrate and reaction
products as a function of time during the oxidation of both
benzyl alcohol and p-xylene. A rather fast H2O2 decom-
position is observed in the first hour of the reaction while
the substrate conversion was only of around 4% for benzyl
alcohol and around 13% for p-xylene. This demonstrates a
non-selective decomposition of H2O2 to water and mo-
lecular oxygen. Although not identified by analytical
measurements it may also correspond to a certain leach-
ing of tungsten. The efficiency of the H2O2 decomposition
presented in Fig. 11b confirms the fact that on pure
CaWO4, H2O2 is decomposed non-selectively in a large
extent.
A significant leaching of tungsten was detected by ICP-
OES for both the oxidation of benzyl alcohol and xylenes
on CaWO4 catalysts. Tungsten concentration was in the
range of 600e900 ppm for the oxidation of benzyl alcohol,
while for the oxidation of xylenes it was in the
500e850 ppm range.
The efficiency of the H2O2 decomposition is a very
important parameter in liquid phase hydrogen peroxide
oxidation since H2O2 is still expensive enough. Obviously,
the efficiency of the hydrogen peroxide decomposition is
referred to as the ratio of the number of moles of H2O2
consumed in the oxidation of the reactants and
Fig. 12 presents the conversion of benzyl alcohol and
xylenes on the W/HAp catalyst. On this catalyst 27.6%
conversion in the oxidation of benzyl alcohol was achieved
after 4 h reaction time with a selectivity of 100% for benzyl
aldehyde. It is noteworthy that the content in tungsten in
W/Hap was 6.1 wt.% compared to 63.8 wt.% in CaWO4.
Based on these it may be estimated that the W/HAp catalyst
was 15.6 times more active than the CaWO4 one.
Oxidation of xylenes on these catalysts depends on the
nature of the isomer. Thus, p- and o-xylenes led to higher
conversions than m-xylene (p-xylene (32%) > o-xylene
(30.8%) > m-xylene (25%)). The selectivity in this reaction
also depends on the substrate. After 4 h, the selectivity for
3-methylbenzaldehyde was 93.8% for m-xylene, with m-
tolylmethanol and 2,6-dimethylphenol as by-products. The
oxidation of p-xylene oxidation resulted in 45.5% p-tho-
lylmethanol, 40.1% p-tolualdehyde and 14.4% p-toluic acid.
The oxidation of the second methyl group (4-
carboxybenzaldehyde or therephthalic acid) was not
observed in this reaction. It is very probable that the
remaining methyl groups were deactivated by the electron-
withdrawing effect of the carboxyl group. Oxidation of o-
xylene led to 100% o-tolualdehyde. The pathways of xylene
oxidation are presented in Scheme 2.
Scheme 1. Evolution of the reaction products in the oxidation of benzyl
alcohol on the W-apatite catalyst.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

8
M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
Fig. 10. Time evolution of the conversion and selectivity of the prepared CaWO4 in the oxidation of p-xylene (50 mg catalyst, substrate-to-oxidant molar ratio of
1/4 (1 mmol substrate), 4 h, 55 ꢁC and 900 rpm).
sample. In all cases, the hydrogen peroxide efficiency for
the W/HAp catalyst was 22e24% and followed the con-
version order (Fig. 13). These values are higher than those
for pure CaWO4 but still below 25%. The observed efficiency
is rather high for a tungsten based catalyst although
smaller than those previously reported for titania [3,27,28].
According to the model we proposed in Scheme 3, the
increased values of hydrogen peroxide efficiency and also
of the conversions could be associated to the existence of
tungsten cations in lattice sites of the hydroxyapatite
structure
and/or
to
the
interaction
between
Ca10(PO4)6(OH)2 and CaWO4. Contrarily, on Ca3(PO4)2, as
mentioned above, hydrogen peroxide nonselectively
decomposed to water and oxygen.
Scheme 2. Evolution of the reaction products in the oxidation of xylenes on
the W-apatite catalyst.
Previous reports state the role of phosphorus in accel-
erating the epoxidation of olefins [22]. It was considered
that a phosphonate or a phosphate anion coordinating the
W center could increase the electrophilic nature of the
peroxo moiety of the tungstate anion. The model presented
in Scheme 3 is in line with these findings. It can also explain
the increase of the activity of the W/hydroxyapatite
compared with that of the pure CaWO4. Besides this, a
secondary effect of the texture cannot be neglected. CaWO4
exhibits a very low surface area (2.4 m2/g) while for W/HAp
the surface area was much higher (18.6 m2/g). This makes
Leaching measurements confirmed the protection effect
of the phosphate groups. Irrespective of the fact that the
substrate has been oxidized or the reaction conditions the
leaching was smaller than 12 ppm, accounting for the
reduced fraction of tungsten present in the form of CaWO4
since metal leaching is extremely reduced in apatitic
structures [13].
Hydrogen peroxide efficiency was also substantially
much higher for the W/HAp than that of the CaWO4
Fig. 11. a) Time evolution of H2O2 decomposition and b) efficiency using the prepared CaWO4.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
9
Fig. 12. Conversion of benzyl alcohol and xylene substrates on the W/HAp
catalyst (50 mg catalyst, substrate-to-oxidant molar ratio of 1/4 (1 mmol
substrate), 4 h, 55 ꢁC and 900 rpm).
the 6.1 wt.% W more accessible in W/HAp than the
63.8 wt.% W in CaWO4.
Scheme 3. Oxidation of xylene and benzyl alcohol on Ca10(PO4)6(OH)2
CaWO4 catalysts.
e
After the reaction the catalysts were separated by
centrifugation and recycled in a new catalytic run for six
successive cycles. For the W/HAp catalyst, after six cycles
the loss of activity was less than 15% compared to the fresh
catalyst with no effect on the selectivity. Combining the
weight percentages of the different elements and the phase
percentages determined by XRD, Table 2, the apatite phase
may be formulated as Ca9,58W0,14(PO4)6(OH)2 accounting
this phase for around 66% of the total amount of tungsten
present in the W/HAp catalyst. Thus, the observed leaching
should be ascribed to the dissolution of the CaWO4 phase
that, as demonstrated above, shows low activity and a poor
selectivity. This demonstrates the robustness of the new
catalyst. Further work is in progress to increase the H2O2
selectivity.
the advantages of the W/HAp catalyst described in this
study. Comparable conversions but a more severe leaching
were observed for 25 wt.% 12-tungstophosphoric acid
deposited on different mesoporous molecular sieves
(19.1 wt.% W) [39]. Moreover, homogeneous catalysts based
on tungsten also required higher tungsten loadings and
have the typical disadvantage of catalyst recovery [40]. It is
also worth noting that this catalyst has a behavior com-
parable to HAp containing ~5 wt.% Ru and Ru co-doped
with Co, Ni, Pb or Fe using molecular oxygen as the
oxidant [41]. The oxidation of benzyl alcohol at 60 ꢁC on
these catalysts presented a TOF of about 11 hꢀ1 for the
sample containing only Ru, and 14, 23 and 38 hꢀ1 for the
sample co-doped with Fe, Ni, and Ni and Pb, respectively.
These values are comparable with the one obtained in the
present study, i.e. 23 hꢀ1. Other apatite reported systems
required tens of hours to reach significant conversion in the
oxidation of alcohols [42].
A comparison with literature reports on liquid phase
oxidation on apatites or tungsten related catalysts reveals
4. Conclusions
In conclusion, the new W/HAp catalyst prepared using
the described hydrothermal approach is composed from a
mixture of W-hydroxyapatite, as a major component, cal-
cium phosphate and CaWO4, as minor components. The
tungsten environment generates a co-factor effect. This
favors both an activity increase and a stabilization of the
catalyst against leaching. The catalyst was investigated in
the liquid phase oxidation of benzyl alcohol and xylenes
using hydrogen peroxide as the oxidant. For comparison,
calcium phosphate, hydroxyapatite and CaWO4 were tested
in the same reaction. Calcium phosphate and hydroxyapa-
tite appeared to be inactive and decomposed non-
selectively hydrogen peroxide. A moderate activity but
low hydrogen peroxide efficiency was observed for the
Fig. 13. Hydrogen peroxide efficiency on the W-apatite catalyst.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013

10
M.I. Domínguez et al. / C. R. Chimie xxx (2016) 1e10
[15] M.I. Domínguez, F. Romero-Sarria, M.A. Centeno, J.A. Odriozola,
Appl. Catal. B: Environ. 87 (2009) 245e251.
[16] M. Que, Z. Ci, Y. Wang, G. Zhu, S. Xin, Y. Shi, Q. Wang, Cryst. Eng.
Comm. 15 (2013) 6389e6394.
CaWO4 phase. In contrast to these solids the W/HAp cata-
lyst shows a reasonable activity and a better hydrogen
peroxide efficiency in the oxidation of benzyl alcohol and
xylenes. This new W/HAp catalyst showed, after six cycles,
losses of the activity below 15% compared to the fresh
catalyst with no effect on the selectivity.
[17] S.-F. Wang, Y.-F. Hsu, W.-J. Lin, K. Kobayashi, Solid State Ionics
247e248 (2013) 48e55.
ꢁ
[18] J. Carpena, B. Donazzon, J.-L. Lacout, M. Freche, J. Lacout, Process for
production of apatite ceramics, especially for biological use, French
Patent, FR2772746, WO9933766, (1999).
[19] B. Donazzon, G. Dechambre, J.-L. Lacout, Ann. Chim. Sci. Mat. 23
(1998) 53.
Acknowledgments
[20] M. Hudlicky, Oxidations in Organic Chemistry, ACS, Washington, DC,
1990.
[21] T. Mallat, A. Baiker, Chem. Rev. 104 (2004) 3037.
[22] R. Noyori, M. Aoki, K. Sato, Chem. Commun. (2003) 1977e1986.
[23] Y. Su, Y.M. Liu, L.C. Wang, M. Chen, Y. Cao, W.L. Dai, H.Y. He, K.N. Fan,
Appl. Catal. A: Gen. 315 (2006) 91e100.
Financial support for this work has been obtained from
the Spanish Ministry of Economy and Competitiveness
(ENE2012-37431-C03) and the Andalucía Government
(Junta de Andalucía TEP8196), both co-financed by FEDER
funds from the European Union. Also, Romanian team
acknowledges UEFISCDI for the project Idei/275/2011.
[24] C. Venturello, E. Alneri, M. Ricci, J. Org. Chem. 48 (1983) 3831.
[25] C. Venturello, M. Ricci, J. Org. Chem. 51 (1986) 1599e1602.
[26] J.F. Deng, X.H. Xu, H.Y. Chen, A.R. Jiang, Tetrahedron 48 (1992)
3503e3514.
^
[27] G. Gelbard, T. Gauducheau, E. Vidal, V.I. Parvulescu, A. Crosman,
References
V. Pop, J. Mol. Catal. 182e183 (2002) 251e266.
[28] A. Crosman, G. Gelbard, G. Poncelet, V.I. Parvulescu, Appl. Catal. A:
Gen. 264 (2004) 23e32.
[29] H. Furukawa, T. Nakamura, H. Inagaki, E. Nishikawa, C. Imai,
M. Misono, Chem. Lett. (1988) 877e880.
[1] P.T. Anastas, J.C. Warner, Green Chemistry: Theory and Practice,
Oxford University Press, New York, 1998.
[2] C.W. Jones, Applications of Hydrogen Peroxide and Derivatives,
Royal Society of Chemistry, Cambridge, 1999.
[3] G. Strukul, Catalytic Oxidations with Hydrogen Peroxide as Oxidant,
Kluwer Academic, Dordrecht, The Netherlands, 1992.
[4] H. Aoki, Science and Medical Applications of Hydroxyapatite, Japa-
nese Association of Apatite Science, Tokyo, Japan, 1991.
[5] T.S.B. Narasaraju, D.E. Phebe, J. Mat. Sci. 31 (1996) 1e21.
[6] J.C. Elliot, Structure and Chemistry of the Apatites and Other Cal-
cium Orthophosphates, Elsevier Science, 1994.
[7] Z. Boukha, M. Kacimi, M.F.R. Pereira, J.L. Faria, J.L. Figueiredo,
M. Ziyad, Appl. Catal. A 317 (2007) 299e309.
[8] J. Carpena, L. Boyer, M. Fialin, J.R. Kienast, J.-L. Lacout, C. R. Acad. Sci.
Paris, Ser. IIa 333 (2001) 373.
[9] N.O. Engin, A.C. Tas, J. Eur. Ceram. Soc. 19 (1999) 2569e2572.
[10] G.W. Boers, J.J. Kettenes-van den Bosch, Anal. Chim. Acta 279 (1993)
89e94.
[30] J. Kasai, Y. Nakagawa, S. Uchida, K. Yamaguchi, N. Mizuno, Chem.
Eur. J. 12 (2006) 4176e4184.
[31] E. Mejdoubi, J.-L. Lacout, M. Hamad, Bioceramics 8 (1995) 457e460.
[32] G. Gelbard, T. Gauducheau, E. Vidal, V.I. Parvulescu, A. Crosman,
V.M. Pop, J. Mol. Catal. A: Chem. 182-183 (2002) 257e266.
[33] Y.-M. Chung, H.-K. Rhee, J. Mol. Catal. A: Chem. 175 (2001)
249e257.
[34] S. Koutsopoulos, J. Biomed. Mater. Res 62 (2002) 600e612.
[35] K.B. Tayeb, C. Lamonier, C. Lancelot, M. Fournier, E. Payen,
A. Bonduelle, F. Bertoncini, Catal. Today 150 (2010) 207e212.
[36] A.-S. Mamede, E. Payen, P. Grange, G. Poncelet, A. Ion, M. Alifanti,
^
V.I. Parvulescu, J. Catal. 223 (2004) 1e12.
[37] Y. Su, G. Li, Y. Xue, L. Li, J. Phys. Chem. C 111 (2007) 6687.
[38] C.H. Schneider, C.C. Lynch, J. Am. Chem. Soc. 65 (1943) 1063e1066.
[39] Y. Chen, Y. Cao, G.-P. Zheng, B.-B. Dong, X.-C. Zheng, Adv. Powder
Technol. 25 (2014) 1351e1356.
[40] M.P. Chaudhari, S.B. Sawant, Chem. Eng. J. 106 (2005) 111e118.
[41] Z. Opre, J.-D. Grunwaldt, M. Maciejewski, D. Ferri, T. Mallat,
A. Baiker, J. Catal. 230 (2005) 406e419.
[11] N. Takeda, Y. Itagaki, H. Aono, Y. Sadaoka, Sens. Actuators, B 115
(2006) 455e459.
ꢀ
[12] M. Vila, S. Sanchez-Salcedo, M. Vallet-Regí, Inorg. Chim. Acta 393
(2012) 24e35.
[13] M.I. Domínguez, J. Carpena, D. Borschnek, M.A. Centeno,
J.A. Odriozola, J. Rose, J. Hazard. Mat. 150 (2008) 99e108.
[14] T.A. Ionannidis, A.I. Zouboulis, J. Hazard. Mater. B97 (2003)
173e191.
[42] Y. Maeda, Y. Washitake, T. Nishimura, K. Iwai, T. Yamauchi,
S. Uemura, Tetrahedron 60 (2004) 9031e9036.
Please cite this article in press as: M.I. Domínguez, et al., Liquid-phase oxidation with hydrogen peroxide of benzyl alcohol and
xylenes on Ca10(PO4)6(OH)2 e CaWO4, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.10.013