Novel mild synthesis of high-added-value p -hydroxyphenyl acetic acid and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system

Article abstract of DOI:10.1016/j.crci.2015.11.011

Acid-activated clays KSF and KSF/0 were successfully used in the hydrogen peroxide conversion of phenyl acetic acid to high-added phenolic compounds: p-hydroxyphenyl acetic acid and 3,4-dihydroxyphenyl acetic acid, endowed with a powerful antioxidant capacity. The catalytic conversion enhancement could be correlated to the total surface acidity and the high iron content of the catalysts KSF/0 and KSF, respectively. The synthetic route described here was conducted under mild conditions with very low degree of mineralization and without significant Fe ion leaching observations. The synthesis reaction is operationally simple and could find application for industrial purposes.

Full text of DOI:10.1016/j.crci.2015.11.011

C. R. Chimie xxx (2016) 1e7
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Full paper/M eꢀ moire
Novel mild synthesis of high-added-value p-hydroxyphenyl
acetic acid and 3,4-dihydroxyphenyl acetic acid using the
acidic clay/hydrogen peroxide catalytic system
a
b
c
Soumaya Bouguerra Neji , Samia Azabou , Sandra Contreras ,
c
d, *
Francisco Medina , Mohamed Bouaziz
Laboratoire de Chimie appliqu ꢀe e H ꢀe t ꢀe rocycle Corps Gras Polym ꢁe res Facult ꢀe des Sciences de Sfax, 3038, Universit ꢀe de Sfax, Tunisia
Laboratoire Analyse Valorisation et S ꢀe curit ꢀe des Aliments, Ecole Nationale d'Ing ꢀe nieurs de Sfax, 3038 Sfax, Tunisia
Departament d’Enginyeria Química, Universitat Rovira i Virgili, Campus Sescelades, 43007 Tarragona, Spain
Laboratoire d'Electrochimie et Environnement, Ecole Nationale d'Ing ꢀe nieur de Sfax, BP 1173, 3038 Sfax, Universit ꢀe de Sfax, Tunisia
a
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Acid-activated clays KSF and KSF/0 were successfully used in the hydrogen peroxide
conversion of phenyl acetic acid to high-added phenolic compounds: p-hydroxyphenyl
acetic acid and 3,4-dihydroxyphenyl acetic acid, endowed with a powerful antioxidant
capacity. The catalytic conversion enhancement could be correlated to the total surface
acidity and the high iron content of the catalysts KSF/0 and KSF, respectively. The synthetic
route described here was conducted under mild conditions with very low degree of
mineralization and without significant Fe ion leaching observations. The synthesis reaction
is operationally simple and could find application for industrial purposes.
Received 15 September 2015
Accepted 17 November 2015
Available online xxxx
Keywords:
Acid-activated clays
Phenyl acetic acid
Oxidation
©
2015 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
3,4-Dihydroxyphenyl acetic acid
p-Hydroxyphenyl acetic acid
1. Introduction
[2,3]. From a chemical point of view, phenolic compounds
can be classified in different ways because they are
There is a growing interest in natural and unnatural
constituted by a large number of heterogeneous structures
that range from simple molecules to highly polymerized
compounds [4]. The antioxidant activity of phenolic com-
pounds depends largely on the chemical structure of these
substances [5] with flavonoids, tannins, chalcones and
coumarins as well as phenolic acids, being the most high-
lighted ones. For instance, antioxidant activity of phenolic
acids, especially hydroxycinnamic acids and hydrox-
ybenzoic acids, is usually determined by the number of
hydroxyl groups found in the molecule thereof [6].
Although a great amount of phenolic compounds can be
found in plants, it could be very interesting to easily syn-
thesize them with competitive prices, so that industrial
investigation of their biological properties can occur more
frequently. Accordingly, many different routes were pub-
lished for total synthesis of phenolic antioxidant com-
pounds. Among useful transformations, hydroxylation of
antioxidants as a protective strategy against various dis-
eases such as artherosclerosis, ischemia-reperfusion injury
and inflammatory injury by blockage or removal of oxida-
tive stresses [1]. Ascorbic acid, vitamin E and polyphenols
are the major antioxidants used as dietary supplements for
the prevention of such diseases. Phenolic compounds have
many industrial applications as natural additives with
antimicrobial and/or antioxidant properties that are
important in protecting the nutritive value and increasing
the shelf life of cosmetic and food products. In addition,
they are considered as one of the bioactive compound
groups which are responsible for beneficial health effects
*
Corresponding author.
E-mail address: mohamed.bouaziz@fsg.rnu.tn (M. Bouaziz).
http://dx.doi.org/10.1016/j.crci.2015.11.011
631-0748/© 2015 Acad eꢀ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
http://dx.doi.org/10.1016/j.crci.2015.11.011

2
S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
aromatic compounds stands out as a fundamental reaction
due to its many uses in the manufacture of high-added-
value compounds, like orthodiphenol compounds such as
hydroxytyrosol (3,4-dihydroxyphenylethanol) and 3,4-
dihydroxyphenyl acetic acid [7].
catalyst was introduced into 50 mL of PAA aqueous solution
ꢁ1
(500 mg L ), under continuous stirring. The desired
quantity of H (0.02 M) was added indicating the start of
2 2
O
the reaction. During the experiments, aliquots were with-
drawn at regular intervals with the purpose of monitoring
the conversion of PAA to p-HPAA and then to 3,4-DHPAA
after being immediately filtered to completely remove
catalyst particles.
In the course of the development of an efficient syn-
thetic route of a potent antioxidant, we can describe herein
a new alternative method of a two-step-synthesis of 3,4-
dihydroxyphenyl acetic acid using the advanced oxidation
of phenyl acetic acid under mild conditions. Actually, this
synthetic route is based on literature analogies used for the
synthesis of hydroxytyrosol [8]. Though the difference with
the latter is the use of acid-activated clay instead of (AleFe)
pillared one without ultra-violet light. As a matter of fact,
acid-activated clays are one of the most widely studied
solid acid catalysts for many organic transformations due to
their environmental compatibility, low cost, high selec-
tivity and thermal stability [9].
2.4. HPLC analysis
Monomeric phenols (PAA, p-HPAA and 3,4-DHPAA)
were analysed on a high performance liquid chromato-
graph (Dionex HPLC) equipped with
a C18 column
(4.6 mm ꢂ 250 mm; Shimpack VPODS).
2.5. LCeMS/MS analysis
2
. Materials and methods
LC-UV-MS/MS analyses were performed as described by
2
.1. Reagents
Kite et al. [11] on a Thermo Scientific System consisting of
an Accela U-HPLC unit with a photodiode array detector
and an LTQ Orbitrap XL mass spectrometer fitted with an
electrospray source. Chromatography was performed with
Phenylacetic acid (PAA) was obtained from Sigma-
Aldrich (St. Louis, MO). para-hydroxyphenyl acetic (p-
HPAA) acid and 3,4-dihydroxyphenyl acetic (3,4-DHPAA)
used as standards for chromatographic calibration were
also purchased from Sigma-Aldrich. Hydrogen peroxide
5
m
l sample injections onto a 150 ꢂ 3 mm i.d., 3
mm, Luna
C18(2) column (Phenomenex) using a 400 l/min linear
m
mobile phase with a methanol/water/acetonitrile þ 1%
(
(
30% solution in water) was obtained from CePharma
Tunisia). Deionized water was used throughout the ex-
gradient. Formic acid changes from 0:90:10 to 90:0:10 over
2
0 min followed by an isocratic phase for 5 min afterwards,
periments and in the HPLC mobile phase. Pure HPLC sol-
vents were used in all cases.
a column washing phase for equilibrium per-pauses was
performed during 3 min before the next injection. The ESI
source of the mass spectrometer was operated in both
positive and negative modes under the recommended
manufacturers’ conditions for the mobile phase parame-
ters. An orbitrap mass analyser was set to scan in a range of
m/z 200e2000 at 30,000 resolutions in one polarity, while
the linear ion-trap analysis was performed. Analyses of MS
were due on the most abundant ions in both polarities
using an ion isolated window of þ2 m/z and relative colli-
sion energy of 35%. For accurate mass analyses of product
ions generated by MS2 in the ion trap, were scanned by the
orbitrap at 7000 resolutions. Negative mode ESI was used
since the phenolic compounds in question ionize better in
this mode, as described by other authors about these spe-
cies [12]. Acidification of the LC mobile phase allows the
best separation, since the hydroxyl groups in the com-
pounds are kept in their acidic form, thereby increasing
their retention on the column and decreasing peak
broadening.
2
.2. Catalysts
The commercial clays (Montmorillonite KSF/0 clay,
Montmorillonite KSF clay and Montmorillonite K10 clay)
used as catalysts in this study were purchased from Fluka
and used without further treatment. In fact, the relevant
physico-chemical properties of the catalysts used are given
in Table 1 [10]. The total acidity of the catalysts was
determined by NH
3
adsorption. Following a 2 h pretreat-
was adsorbed at ambient temperature.
ꢀ
ment at 300 C, NH
3
3
After evacuating, the amount of the chemisorbed NH was
ꢀ
ꢀ
measured applying 2 C/min heating rate till 500 C.
.3. Phenyl acetic acid conversion
Phenylacetic acid oxidation was carried out in a 100 ml
2
Pyrex reactor equipped with a magnetic stirrer. The solid
Table 1
Some characteristic data of the investigated catalysts.
Catalyst
Textural characteristics
Analysis (%)
Al
Acidity
pH
2
Specific surface area (m /g)
Average pore diameter (Å)
Fe
IR spectra (AU)
Brønsted
Lewis
KSF/0
KSF
K10
117
30
249
74
50
56
6.16
8.62
7.71
1.37
3.17
1.99
1.3
1.5
4.5
1.03
0.59
0.33
0.20
0.15
0.29
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
http://dx.doi.org/10.1016/j.crci.2015.11.011

S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
3
2
.6. TOC removal and leaching-off of Fe ions
that the reaction rate increased (p < 0.05) when adding
acid-activated catalyst clays according to the following
order: K10 < KSF z KSF/0. The enhancement of PAA con-
version is due to the increase in hydroxyl radicals by the
combination of the oxidant hydrogen peroxide and the
redox process in the presence of iron ions.
The total organic carbon TOC of the solution (in mg of
carbon per litre of solution) was measured with a Shimadzu
Model 5050 TOC-analyzer. Iron leaching was evaluated by
measuring iron concentrations at the end of the conversion
reaction using an atomic absorption analyzer,
PerkinElmer 1101 B, with a hollow cathode lamp having
mA current.
a
Acid treatment in commercial-type montmorillonite
KSF/0, KSF and K10 is performed using H SO .This behav-
2 4
8
iour promoted an increase in the number of acid sites of
moderate strength compared to natural clay [15]. The acid
strength of the catalysts can be characterized by the pH
value of their 10% water suspension, while the number of
sites by the intensities of the Brønsted- and Lewis-
characteristic bands in the IR spectra after pyridine
adsorption as previously described (Table 1) [10]. The acid
strength increases from K10 to KSF and KSF/0. Based on the
data shown in Table 1, no correlation was found between
either the surface area or average pore diameter with the
catalytic activity. Indeed, K10, which recorded the highest
2
.7. Antioxidant activity
Evaluation of samples' antioxidant activity withdrawn
at regular intervals was assessed by the DPPH (1,1-
diphenyl-2-picryl hydrazyl) radical scavenging assay [13].
Aliquots (50 mL) were added to 5 mL of methanolic solution
ꢁ
6
ꢁ1
containing DPPH radicals (6 ꢂ 10 mol L ). After 30 min
of incubation period under room temperature and in the
dark, the absorbance was read at 520 nm using a Shimadzu
UV-160 A spectrophotometer. DPPH radical scavenging
activity was calculated as follows:
2
surface area (249 m /g), led to the slowest reaction rate.
The increase of the catalytic conversion could be correlated
to the total acidity surface of the catalyst. Besides, lower pH
clay values resulted in a significant increase (p < 0.05) of
the reaction rate. Both KSF and KSF/0 with initial pH of 1.5
and 1.3, respectively, led to total PAA conversion, where
only 30% PAA conversion yield was observed for the K10
catalyst at initial pH 4.5. KSF/0 and KSF are more active in
the conversion of PAA because of their stronger acidity. So
the catalytic activity of these K-catalysts is controlled by
their acidity.
A
control ꢁ Asample
DPPH radical
­
scavenging activity ¼
ꢂ 100
A
control
A lower absorbance of the reaction mixture indicates a
higher DPPH radical-scavenging activity. Butylated
hydroxyanisole (BHA) was used as a standard. The test was
carried out in triplicate.
2
.8. Statistical analysis
On the other hand, higher catalytic activity of KSF could
be attributed to its high iron content. As a matter of fact,
hydroxyl radicals can be easily produced through a redox
process known as the Fenton reaction in the presence of
iron ions [16]. They are generally schematized as follows:
Results were expressed as mean ± standard deviation
(
mean ± SD). The whole analysis was carried out with
GraphPad Prism 4.02 for Windows (GraphPad Software,
San Diego, CA). Significant differences for comparison of
the yield of PAA conversion were determined by one-way
ANOVA, followed by Tukey's post-hoc test for multiple
comparisons with a statistical significance of p ꢃ 0.05.
ꢀ
Fe2 þ H
þ
O
/ Fe þ OHꢁ
3þ
þ
OH
2
2
ꢀ
Fe2
þ
þ
OH / Fe þ OHꢁ
3þ
3
. Results and discussion
3
.1. Phenyl acetic acid conversion: preliminary experiments
ꢀ
ꢀ
RH þ OH / H
2
O þ
R
A variety of montmorillonite catalysts KSF, KSF/0 and
K10 were investigated in the PAA acid conversion. Mont-
morillonite is a member of the smectite clay family and has
a 2:1 structure i.e. an octahedral layer of Al is sandwiched
between two tetrahedral layers of silicon coordinated with
oxygen (TOT) exhibiting overall weak acidity. The crystal-
line sheets of negatively charged aluminosilicates are
balanced by hydrated cations (Na , K or Ca ) in the
interlayer spaces of montmorillonite. The most interesting
features of the smectite clay are their intercalation,
swelling and cation exchange capacity which improve the
catalytic properties of smectite clay [14]. Fig. 1 shows the
conversion profiles of the reaction times at the room tem-
perature over the studied catalysts in the presence of
hydrogen peroxide. Blank controls made without the
catalyst were also analysed, and no significant (p > 0.05)
PAA conversion was observed. However, it can be noted
ꢀ
R þ Fe3þ / R þ Fe2þ
þ
Even with its lower iron content compared to KSF's one,
the KSF/0 catalyst yielded high PAA conversion similar to
KSF. This result could be attributed to the high acidic pH
(2.97) of the aqueous phenolic solution mixed with KSF/0
particles (Table 2). Such acidic pH values (around 3) have
been reported to be optimum for homogenous Fenton
processes. The latter corresponds to the maximum con-
þ
þ
2þ
2
þ
centration of the active Fe species and the lowest rate of
parasitic decomposition. By increasing the pH, the
precipitation of the insoluble ferric hydroxides takes place
2 2
H O
and the decomposition of H
[17].
2 2
O becomes preponderant
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
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4
S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
] ¼ 2 ꢂ 10ꢁ M, and [catalyst] ¼ 0.5 g/L.
2
Fig. 1. PAA oxidation by: H
2
O
2 2
, H O
2
/KSF/0, H
2
O
2
/KSF, H
2
O
2
/K10. [PAA] ¼ 500 mg/L, T ¼ 298 K, [H
2
O
2
Table 2
spectra are then compared to those reported by the liter-
ature [18,19].
pH Values, TOC removal and leaching-off of Fe ions throughout the cata-
lytic reaction run.
Compound
1
was identified as being 3,4-
Catalyst
Time (h)
pH
TOC
Fe leaching (ppm)
dihydroxyphenyl acetic acid retained at 7.16 min (l ,
max
1
2
68) (Fig. 4). The MS spectrum (Fig. 4a) exhibited a mo-
KSF
0
2
4
6
8
3.5
359
342
230
323
325
326
359
341
335
325
330
329
0
e
ꢁ
2
lecular ion at m/z 167 [MꢁH] . The MS spectrum obtained
by fragmentation of the ion m/z 197 presented the
following m/z values: 167, 150 and 149. The pseudo mo-
2.97
2.95
2.94
2.91
2.58
2.97
2.94
2.92
2.91
2.87
2.67
e
e
e
ꢁ
lecular ion [MꢁH] fragmented at m/z 167 yielded a frag-
2
4
0
2
4
6
8
4
4.37
0
ment at m/z 149 by the loss of 18 mass units of
KSF/0
ꢁ
e
[MꢁH
2
OꢁH] . The structure of these compounds was
e
confirmed with those of an authentic standard [20].
Compound 2 (Rt, 9.31 min; max, 270 nm) (Fig. 4 and b)
e
l
e
was identified as being para-hydroxyphenyl acetic acid,
2
1.57
based on chromatography analysis with an authentic
ꢁ
standard and a mass spectrum with a [MꢁH] at 151 and
2
prominent MS fragments at m/z 151, 133. The ion m/z 133
obtained by the loss (ꢁ18) of a H
2
O molecule provided an
ꢁ
anion of [MꢁHꢁH
2
O] .
3
.2. Phenylacetic acid conversion: identification of reaction
Taking into account the LC-MS/MS finding, it is possible
to presume that the PAA conversion behaviour would be as
follows: PAA led to the p-HPAA which itself was converted
to the high-added value 3,4-DHPAA. Despite the complete
PAA conversion after the 4 h reaction, an accumulation of
products
A typical catalytic reaction behaviour is shown in Fig. 2a
and b. After a short induction period, PAA was rapidly
converted to new phenolic reaction intermediates until
complete disappearing. Throughout the reaction course, an
interesting observation was noted. The initial transparent
colour of the aromatic solution has been changed from
colourless to yellow to brown and the intensity of the latter
increased with time (Fig. 3).
Newly appearing phenolic compounds were identified
by LC-MS/MS, equipped with an electrospray mass spec-
trometer in the negative mode, was carried out by
comparing retention times and mass spectra of the un-
known peaks with the standards'. The structure assign-
ment of phenolic compounds was based on UV absorbance
spectra with a systematic search for molecular ions, using
extracted ion mass chromatograms. The recorded MS/MS
3
,4-DHPAA was continuously observed.
3.3. pH, TOC and Fe leaching-off evolution
Table 2 presents the pH value, TOC removal and Fe ions'
leaching-off throughout the catalytic reaction conducted
by the most efficient catalysts (KSF and KSF/0). It was
observed that the pH decrease was more significant
(p < 0.05) for KSF (from 3.5 to 2.5) than KSF/0 catalysts
(from 2.97 to 2.67). The latter were associated with less Fe
leaching-off varying between 4.37 ppm and 1.57 ppm for
KSF and KSF/0, respectively. This finding is of a great
importance since the leaching content of iron species was
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
http://dx.doi.org/10.1016/j.crci.2015.11.011

S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
5
] ¼ 2 ꢂ 10ꢁ M, and [catalyst] ¼ 0.5 g/L. b. PAA oxidation by H
2
O
2
/KSF/0. [PAA] ¼ 500 mg/
Fig. 2. a. PAA oxidation by H
2
O
2
/KSF. [PAA] ¼ 500 mg/L, T ¼ 298 K, [H
2
O
2
2
ꢁ
2
L, T ¼ 298 K, [H
2
O
2
] ¼ 2 ꢂ 10 M, and [catalyst] ¼ 0.5 g/L.
very small in spite of the initial acidic pH of the used clays,
as it was reported in the literature [21,22]. Thus, PAA con-
version under our catalytic system could be regarded as an
ecofriendly process for the synthesis of new compounds
with high biologically added values.
Unexpectedly, TOC values remained quite constant
during the catalytic PAA conversion using either the KSF or
KSF/0 catalyst, which indicates a low mineralization phe-
nomenon (<10%).
3.4. Antioxidant activity evaluation of the newly synthesized
p-HPAA and 3,4-DHPAA mixture
Fig. 3. Colour aqueous solution before and after the PAA conversion with
KSF and KSF/0 clay.
In order to evaluate the antioxidant properties of the
newly synthesized phenolic compounds, we first examined
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
http://dx.doi.org/10.1016/j.crci.2015.11.011

6
S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
a
b
Fig. 4. Detection by LC of para-hydroxyphenyl acetic acid and 3,4-dihydroxyphenyl acetic acid. a. LC-MS/MS spectrum (MS
acetic acid. b. LC-MS/MS spectrum (MS and MS ) of para-hydroxyphenyl acetic acid.
1 2
and MS ) of 3,4-dihydroxyphenyl
1
2
Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
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S. Bouguerra Neji et al. / C. R. Chimie xxx (2016) 1e7
7
chemical approach since it does not use toxic reagents
without significant iron catalyst leaching. At the end,
catalyst particles can be removed by simple filtration and
the target compound could be purified to obtain pure
antioxidant in huge amounts. Finally, this process may
prove its usefulness not only for laboratory applications but
also for potential industrial ones, such as cosmetic and food
industries.
Acknowledgement
The authors would like to thank the Ministry of Higher
Education and Scientific Research of Tunisia (Contract
programme LR14ES08), for the support of this research and
for giving us the opportunity to work on it. We would like
to appreciate Dr Mourad Jridi's help on the antioxidant
activity.
Fig. 5. Free radical scavenging activity of the ortho-diphenolic compound
during PAA oxidation.
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the radical scavenging efficiency of the mixture reaction,
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DHPAA. This newly synthesized ortho-diphenolic com-
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. Conclusion
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As a conclusion, the method described here for the
(2013) 46.
chemical synthesis of p-hydroxyphenyl acetic acid and 3,4-
dihydroxyphenyl acetic acid exhibited a promising alter-
native to other previously published methods dealing with
unnatural antioxidant invention. To our knowledge, this is
the first study showing the ability to synthesize 3,4-
dihydroxyphenyl acetic acid, a high-added-value phenolic
compound, by hydrogen peroxide catalytic oxidation of
phenylacetic acid using acid-activated clays. This is a very
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Please cite this article in press as: S. Bouguerra Neji, et al., Novel mild synthesis of high-added-value p-hydroxyphenyl acetic acid
and 3,4-dihydroxyphenyl acetic acid using the acidic clay/hydrogen peroxide catalytic system, Comptes Rendus Chimie (2016),
http://dx.doi.org/10.1016/j.crci.2015.11.011