Selectivity control of benzene conversion to phenol using dissolved salts in a membrane contactor

Article abstract of DOI:10.1016/j.apcata.2010.12.018

Effect of salts, pH, type of acids and anion sulphate in the synthesis and separation of phenol through the hydroxylation of benzene by using a Fenton reaction in a biphasic membrane contactor has been investigated. The results indicated that sodium sulphate (1 M) increased phenol extraction in the organic phase (76.3%) but also increased reaction kinetics promoting over-oxidation products and a black solid (tar) formation. The acids delayed and in some tests avoided the tar appearance as precipitate but also gave a reduction of phenol selectivity. The sulphate absence, obtained by using iron(0), did not avoid the precipitate formation but only caused its decrease favouring a significant increase of the ratio productivity/amount of black solid from 4.6 to 62.4.

Full text of DOI:10.1016/j.apcata.2010.12.018

Applied Catalysis A: General 393 (2011) 340–347
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Selectivity control of benzene conversion to phenol using dissolved salts in a
membrane contactor
∗
Raffaele Molinari , Teresa Poerio
Department of Chemical Engineering and Materials, University of Calabria, Via P. Bucci, 44/A, I-87036 Rende (CS), Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2010
Received in revised form
Effect of salts, pH, type of acids and anion sulphate in the synthesis and separation of phenol through
the hydroxylation of benzene by using a Fenton reaction in a biphasic membrane contactor has been
investigated. The results indicated that sodium sulphate (1 M) increased phenol extraction in the organic
phase (76.3%) but also increased reaction kinetics promoting over-oxidation products and a black solid
2
4 November 2010
Accepted 9 December 2010
Available online 16 December 2010
(tar) formation. The acids delayed and in some tests avoided the tar appearance as precipitate but
also gave a reduction of phenol selectivity. The sulphate absence, obtained by using iron(0), did not
avoid the precipitate formation but only caused its decrease favouring a significant increase of the ratio
productivity/amount of black solid from 4.6 to 62.4.
Keywords:
Liquid phase benzene oxidation to phenol
Catalytic membrane contactor
Selective oxidation
©
2010 Elsevier B.V. All rights reserved.
Tar formation from benzene
Salts and phenol extraction (from aqueous
to organic)
1
. Introduction
mixture is one of the advantages in using a membrane in a reactor
and permits to obtain improvements in terms of yield and selec-
tivity in equilibrium-limited reactions and in consecutive catalytic
reactions.
A membrane contactor is a device that achieves gas/liquid or
liquid/liquid mass transfer without dispersion of a phase within
another [14].
Phenol is an important raw material for the synthesis of petro-
chemicals, agrochemicals, and plastics. The current worldwide
capacity for phenol production is nearly 7 million metric tonnes
per year [1]. Today, almost 95% of the worldwide phenol produc-
tion is based on a three-step process with the so-called “cumene
process”. Despite its great success, the cumene process has some
disadvantages: poor ecology, an explosive intermediate (cumene
hydroperoxide) and a multistep character, making difficult to
achieve high phenol yields with respect to benzene [2].
The search for new routes for phenol production based on the
one-step direct benzene oxidation became more intensive in the
last decade [3–10] but this reaction is a little selective because
phenol is more reactive than benzene and over-oxygenated by-
products occur.
To slow down consecutive catalytic reactions catalytic mem-
brane reactors (CMRs) can be employed. These systems have
attracted attention because of their advantages related to the syn-
ergy of the catalyst and membrane when implemented in the same
device [11–13]. Within this family, the membrane contactors also
permit to combine membrane separation and catalytic reaction in
one unit operation. The separation of a product from the reaction
As common feature of these processes the separation perfor-
mance is determined by the distribution coefficient of a component
in two phases. The membrane, which can be defined as a permse-
lective barrier between two homogenous phases, represents only
an interface. It accomplishes a particular separation transporting
a component more easily than another because of differences in
physical and/or chemical properties between the membrane itself
and the permeating components. Membrane contactors have a
number of important advantages compared to conventional dis-
persed phase contactors. Some of them are: no flooding at high
flow rates, no unloading at low flow rates, absence of emulsions,
no density difference between fluids required. They reduce the vol-
ume of equipment and offer more interfacial area in non-dispersive
contact across a membrane, leading to a decrease in the value of
a height transfer unit (HTU). The membrane should be accurately
chosen to enable as much as possible higher values of the mass
transfer coefficient. Additional advantages of this technique are a
high surface area per unit volume, when hollow fibre modules are
used, and the possibility of changing the hydrodynamics of both
phases independently. These contactors give any wanted shape of
∗ Corresponding author. Tel.: +39 0984 496699; fax: +39 0984 496655.
E-mail address: r.molinari@unical.it (R. Molinari).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.12.018

R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
341
Fig. 1. Scheme of the reactor–membrane contactor.
96%, d = 1.84 g mL 1), nitric acid (HNO , purity 63%, d = 1.4 g mL
−
−1
)
fluid–fluid interface in contrast to conventional separation equip-
ments where the shape of the fluid–fluid contact is an accident of
nature. There are different membrane configurations like hollow
fibre, flat sheet, rotating annular and spiral wound that have appli-
cations in wastewater treatment, VOC removal from waste gas,
osmotic distillation, fermentation, pharmaceuticals, carbonation of
beverages, protein extraction [14]. The most important drawback,
however, is the slow mass transfer rate because of the additional
mass transfer resistance offered by the pores of the membrane.
In our research group a flat-sheet membrane contactor was used
for the one-step oxidation of benzene to phenol [15–18] reducing
by-products formation employing a liquid–liquid membrane con-
tactor. The membrane contactor system was formed by an aqueous
3
from Carlo Erba Reagenti and hydrochloric acid (HCl, 37%, w/w solu-
tion in water) by Riedel-De Haën, were the acids tested to achieve
acidic pH in the aqueous phase.
Sodium chloride and sulphate (NaCl purity 99.9%, Na SO purity
99.0%,) from Sigma–Aldrich were used in the aqueous phase to
modulate its ionic strength acting on phenol extraction.
2
4
Iron(II) sulphate (FeSO ·7H O, purity = 99%) and iron(0) in pow-
4
2
der form (Sigma–Aldrich) were employed as catalysts.
Hydrogen peroxide (H O , 30%, w/w solution in water) from
Sigma–Aldrich was used as the oxidant.
Hydrophobic polypropylene flat sheet porous membrane
(Accurel, manufactured by Membrana, thickness 142 ␮m; pore size
0.2 ␮m; porosity 70%) was used in the membrane contactor.
2
2
phase, containing dissolved FeSO as catalyst and hydrogen perox-
4
ide as oxidant, and by an organic phase containing benzene. The two
phases were separated by a hydrophobic membrane. In this sys-
tem the benzene permeates across the membrane from the organic
phase to aqueous membrane interface where the interfacial oxi-
dation reaction takes place. Then the formed phenol permeates
back across the membrane to the organic phase where it takes
shelter from over oxidation. The high value of phenol selectivity
2
.2. Methods and apparatus
Identification of the oxidation products was performed
analysing the solutions by gas chromatography mass spectrome-
tery (GC–MS QP2010S) from Shimadzu.
Concentrations of phenol and oxidation by-products in the
aqueous and organic phases were measured by high performance
liquid chromatography (HPLC, Agilent 1100 Series instrument)
using a GEMINI C18 (250 mm × 4.60 mm) column by UV readings
at 254 nm wavelength. The mobile phase consisted of an acetoni-
trile/water/acetic acid solution 50/49/1 (v/v/v) fed at a flow-rate of
1
volume was 20 ␮L.
Ultrapure water, used throughout the work, was obtained from
Milli-Q equipment by Millipore. A pH meter (WTW Inolab Terminal
Level 3), with a glass pH electrode SenTix 81 (WTW), was used for
pH measurements.
Experimental tests were conducted in a lab-made membrane
contactor built with a two compartment cell to separate the organic
phase and the aqueous reacting phase (Fig. 1).
(
98%) in the organic phase was obtained thanks to its extraction in
the organic phase so avoiding further contact between phenol and
the catalyst, which was soluble in the aqueous phase. The draw-
back of this system was the low rate of phenol extraction in the
organic phase. Indeed, phenol that does not cross rapidly the mem-
brane, reacts further to generate over-oxidation products such as
−1
.0 mL min . The column pressure was 110 bar and the injection
1
-4 benzoquinone, biphenyl as trace and tars (black solid).
In this work, to avoid/reduce tar formation and enhance phenol
recovery in the organic phase, different aspects have been studied:
i) use of dissolved salts in aqueous phase; (ii) pH and anion type of
(
various acids; (iii) composition of aqueous phase before the black
precipitate appearance; (iv) role of sulphate anion present in the
reaction system by changing the FeSO4 catalyst with Fe(0).
2
. Experimental
The two compartment cells (each one with a volume of 130 mL)
were separated by a flat sheet polypropylene membrane with an
exposed membrane surface area of 28.3 cm . In the compartment
2
2.1. Materials
containing the aqueous phase the catalytic reaction takes place,
while in the second one, containing only benzene in the double
task of stripping solution and reagent, phenol is accumulated in
the time. In particular, use of benzene, as both substrate and extrac-
tant, avoids use of other organic solvents with benefit in product
recovery.
Benzene (C H , purity 99.8%) from Carlo Erba Reagenti was
6
6
used both as substrate and as organic extracting phase. Phenol
C H5OH, purity 99.99%), benzoquinone (C H O , purity 99.9%),
(
6
6
4
2
biphenyl (C12H10, purity 99.99%), hydroquinone (C H O , purity
6
6
2
9
9
9%), resorcinol (C H O , purity 98%) and catechol (C H O , purity
6 6 2 6 6 2
9.5%) from Sigma–Aldrich were used for analytical calibrations.
The phases were stirred by two motors and thermostated by a
◦
Acetic acid (CH COOH purity 99.9%), sulphuric acid (H SO , purity
water bath at 35 C. This value was chosen on the basis of the results
3
2
4

3
42
R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
obtained in our previous work [15]. Samples of organic phase were
periodically withdrawn and analyzed. The aqueous phases were
analyzed after quantitative extraction with diethyl ether at the end
of the catalytic tests.
the slope corresponding to the distribution coefficient of phenol
between the two phases.
Another important equilibrium parameter is the degree of
extraction E defined as:
In every test the aqueous phase contains 130 mL of ultrapure
water, 4 mmol of catalyst and 4 mmol of acetic acid. The acetic acid
was not used in the catalytic tests with different acids at constant
pH. The oxidant (18 mmol total of a run) was added step by step
KD
E% = (
× 100
(2)
KD + 1)
when the values of organic and aqueous phases are equal.
It must be noted that the system studied in this work did not
reach the equilibrium condition thus the parameters QE% and RD
previously defined have been used.
(
0.375 mmol every 5 min), last addition was done at 240 min and
all run lasted 270 min.
Black solid samples were recovered at the end of the catalytic
tests by filtration and weighed after being air dried. On the black
solid some studies were carried out as elemental analysis and
Fourier Transform Infrared Spectroscopy (FT-IR) analysis. Elemen-
tal analysis (C, H, N) was performed by the Microanalytical Service
of the University of Calabria and FT-IR analysis by FT-IR spectrom-
eter Spectrum One by Perkin Elmer.
2
.4. Flux
The transport of a component A through the membrane in a
membrane contactor, from the feed phase to the permeate phase,
occurs in three main steps: transport from the feed phase to the
membrane interface, diffusion through the membrane and transfer
from the other membrane interface to the permeate phase. Indeed,
in the studied system the aqueous and organic phases are well
stirred thus the liquid–liquid interfaces to both sides of the mem-
brane and concentration polarization effects, due to accumulation
of salt ions near the liquid–liquid interface, can be neglected.
Results of the experimental tests are reported using the follow-
ing parameters:
•
extraction quotient percentage Q % = (norg/ntot) × 100; where
E
norg is the mol number of phenol in the organic phase and ntot is
the sum of the mol number of phenol in the organic and aqueous
phases;
distribution ratio RD = [Phenol]org/[Phenol]aq, which is the distri-
bution of the compound between the two phases at the time
t;
The flux (J ) of component A, expressed in terms of overall mass
transfer resistance coefficient (kov,A), is as follows:
A
•
•
•
•
•
JA = kov,A ꢀcA
(3)
with
phenol selectivity (%) = Ph/(Bq + Ph + Biph) × 100 where Ph, Bq
and Biph are the mol numbers of phenol, benzoquinone and
biphenyl, respectively, detected in the organic phase;
1
kov,A
1
1
1
=
+
+
(4)
kA(feed)
kA(membrane)
kA(permeate)
phenol productivity P = (g Phorg + gPhaq) gcat−
1
−1
h , which is the
If the mass transfer resistance in the membrane phase is domi-
nant then Eq. (3) reduces to
total mass (g) of phenol produced per unit mass of the catalyst
per unit of time.
Yield% = (Ph/18) × 100 where Ph is the mol numbers of phenol
detected in the organic phase and 18 is the mol numbers of fed
oxidant (limiting reagent).
dC
dt
D K
A
A
JA
=
=
ꢀcA
(5)
l
where ꢀcA is the bulk concentration difference between feed and
permeate phases, l is the membrane thickness, DA is the diffusion
coefficient of A in the membrane pores and KA is the distribution
coefficient of component A between the feed phase and the solu-
tion into the pores of the membrane phase. In most applications of
liquid/liquid extraction the lowest membrane mass transfer resis-
tance is obtained if the pores are filled with a fluid in which the
solute is highly soluble. This suggests the use of a hydrophilic or
hydrophobic membrane for a low or high KA solute, respectively.
That is, a low KA solute is more soluble in a polar solvent than
in a non-polar one, and the polar solvent will wet the pores of a
hydrophilic membrane, the situation is analogous for high KA solute
and a hydrophobic membrane [14].
flux (Jorg Ph) was calculated as variation of mmol of phenol
in the organic phase per unit area of membrane and time
−
2
−1
h ).
(mmol m
The separation performance of the membrane contactor was
determined by the distribution coefficient and the flux.
2.3. Distribution coefficient
The distribution coefficient (KD) of a component A between two
phases is generally an index that characterizes the goodness of the
separation. It is defined as the ratio of concentration of a component
in the organic phase to its concentration in the aqueous phase of a
two-phase organic/water system at equilibrium and at a specified
temperature:
3
. Results and discussion
3
.1. Distribution coefficient and extraction percentage
[
A]org
KD =
(1)
The effects of addition of salts in the aqueous phase on the distri-
[
A]aq
bution coefficient of phenol between water and benzene have been
observed as comparison with the system performance without salt
(water–benzene system only).
where [A]org is the concentration of the component A in the organic
phase and [A]aq is the concentration of the same component in the
aqueous phase at equilibrium.
The distribution coefficients of phenol between aqueous and
benzene phases of different compositions were determined using
different volume ratios of aqueous and organic phases (with a con-
stant initial concentration of phenol in the aqueous phase) [19,20].
The phenol concentration in the organic phase was plotted as
function of phenol concentration in the aqueous phase measured
at equilibrium after each single extraction. The obtained equilib-
The benzene–water distribution coefficient (KD) of phenol and
◦
its extraction percentage (E%) were determined at 25 C using the
method described above and the results are reported in Table 1.
It can be observed that KD increases significantly when Na SO4
2
is added; in particular, the addition of 2 M Na SO4 or 3 M NaCl
2
allows an increase of the distribution coefficient more than 3 times
compared with no salt addition (from 2.1 to 7.2).
The observed increase can be explained by a salt solvation phe-
nomena. Indeed, the studied salts can be considered as strong
◦
rium isotherm at 25 C was a straight line passing from zero with

R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
343
Table 1
◦
Distribution coefficient and extraction percentage at 25 C of phenol in the system
water:benzene without and with salts in the aqueous phase.
E (%)
KD
No salt
68.0
79.7
83.4
87.8
77.4
79.7
83.4
87.8
88.0
2.1
3.9
5.0
7.2
3.4
3.9
5.0
7.2
7.3
Na2SO4 1 M
Na2SO4 1.5 M
Na2SO4 2 M
NaCl 1 M
NaCl 1.5 M
NaCl 2 M
NaCl 3 M
NaCl 4 M
electrolytes completely dissociable permitting the formation of
more or less solvated ions. This solvation leads to a decrease in the
amount of water available for solvation of phenol which acquires
more affinity for organic phase thus decreasing its concentration in
the aqueous phase [21,22].
The obtained data indicate that using salts dissolved in the aque-
ous phase it is possible to transport phenol in the organic phase
at higher flux (according to Eq. (5)) thus avoiding its subsequent
oxidation.
3.2. Catalytic tests using dissolved salts in the aqueous phase
The catalytic tests, performed in the membrane contactor using
sodium sulphate (at concentrations of 1, 1.5 and 2 M) and sodium
chloride 4 M, were monitored for 240 min. The results (Table 2)
were compared using the parameter Q%, RD, selectivity%, phenol
productivity, yield%, phenol flux and amount of black solid.
Fig. 2. Concentrations of detected products at the end of a run in aqueous and
organic phases using H2SO4. (Operative condition: T = 35 C; H2O2 = 18 mmol; cata-
lyst = 0.4 mmol FeSO4; pH = 2.8; run time = 270 min).
◦
From Table 2 it is seen that using a greater amount of Na SO ,
2
4
dissolved in the aqueous phase, a greater amount of phenol is
recovered in the organic phase (Q% = 83.7%), according to the previ-
ous results of liquid–liquid extraction. Despite this advantage the
other effects of salt addition on the catalytic reaction were neg-
ative concerning selectivity, productivity, yield% and flux. Indeed
of Q% (95.5%) reported in Table 2 when using NaCl is misleading:
it was caused by a disappearance of phenol in the aqueous phase
to form benzoquinone and not by an increase of extraction in the
organic phase.
−
1
−1
the phenol productivity decreased from 0.64 to 0.30 gph gcat
h
using sodium sulphate from 1 M to 2 M, respectively. This behaviour
could be explained by the work of Le Troung et al. [23] where the
authors studied the effects of sulphate ion on the oxidation rate of
3.3. Catalytic tests at different pHs and different acids
Some catalytic tests at different pHs were carried out by using
sulphuric acid to change the pH of the aqueous phase from the
value of 2.8, obtained with 4 mmol of acetic acid alone, to 1.5, 1.1
and 0.8, respectively. The acetic acid has been used in many works
which study the benzene oxidation to phenol as it is considered a
co-catalyst in the Fenton reaction [5,9]. Indeed, also in our previ-
ous work [15] the high phenol production (1560 mg) was obtained
when acetic acid was used at pH 2.8. This co-catalyst did not avoid
tar formation thus, in the present work, a study on the combined
effect of the co-catalyst and an inorganic acid at different pHs has
been performed.
ferrous ion by H O2 showing that in presence of sulphate the oxi-
2
dation rate of Fe(II) was faster and also depended on the sulphate
concentration Furthermore, the sulphate ion may have an effect
on the efficiency of Fe(II)/H O2 and Fe(III)/H O2 systems owing to
2
2
sulphate complexes formation with Fe(II) and Fe(III) [24].
This means that reduction of phenol productivity was probably
caused by an increase of phenol degradation to other by-products.
As a result, also tar formation increased by increasing the sodium
sulphate concentration while phenol concentration, yield% and
selectivity decreased.
In the catalytic test with NaCl 4 M a further reduction of selec-
tivity, productivity and flux was observed. The chloride ion also
increased the oxidation rate of Fe(II) [23] but, in the present study,
biphenyl was not produced and an almost equal amount of phenol
Other oxidation tests were carried out to investigate the influ-
ence of anion type of the acid (HCl, HNO3 and H SO ) on tar
formation.
From Fig. 2 it can be observed that too acidic condition (pH = 0.8)
slow down the reaction kinetics reducing both phenol and by-
2
4
(
selectivity = 53%) and benzoquinone, was obtained. The high value
Table 2
Results of catalytic tests with salts at different concentrations.
−
1
−2
m )
Salts
Q (%)
RD
Selectivity (%)
Productivity
Yield%
Flux (mmol h
Amount of black
solid (g)
−
1
−1
h )
(
gph gcat
Na2SO4 1 M
Na2SO4 1.5 M
Na2SO4 2 M
NaCl 4 M
76.3
76.3
83.7
95.5
3.2
3.2
5.1
–
91
90
88
56
0.64
0.40
0.30
0.07
13.2
8.1
6.8
188.5
116.2
97.3
0.27
0.47
0.70
0.17
1.8
25.8
◦
Operative condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol FeSO4; pH = 2.8 with 4 mmol of acetic acid; run time = 270 min.

3
44
R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
Table 3
Results of catalytic tests at different pHs.
−
1
−1
−1
−2
m )
pH
Q (%)
82.3
55.6
46.4
–
Selectivity (%)
Productivity (gph gcat
h
)
Yield%
Flux (mmol h
Amount of black solid (g)
0.8
1.1
1.5
2.8
89
88
95
98
0.21
0.38
0.91
3.59
4.8
5.8
11.4
38.5
68.6
82.3
162.9
594.5
0.094
0.154
0.197
0.150
◦
Operative condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol FeSO4; acid = 4 mmol acetic acid and H2SO4 1 M to reach the reported pH; run time = 270 min.
Table 4
Results of catalytic tests using different inorganic acids.
−
1
−2
m )
Type of acid
Q (%)
Selectivity (%)
Productivity
Yield%
Flux (mmol h
Amount of black
solid (g)
−
1
−1
h )
(
gph gcat
HCl
HNO3
H2SO4
71.5
75.1
79.4
71
80
75
0.15
0.05
0.08
2.9
1.1
1.7
41.6
15.0
23.9
0.195
0.234
0.598
◦
Operative condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol FeSO4; pH = 2.8 adding 1 M acid solution; run time = 270 min.
◦
Fig. 3. Detected products and selectivity% in the organic phase versus the time in the catalytic tests using nitric acid. (Operative condition: T = 35 C; H2O2 = 18 mmol;
catalyst = 0.4 mmol FeSO4; pH = 2.8; run time = 270 min).
Table 5
Results of catalytic tests stopped when a cloudy solution appeared.
−
1
−1
−1
−2
m )
Catalytic medium
Q (%)
Selectivity (%)
Productivity (gph gcat
h
)
Yield%
Flux (mmol h
Stopped time (min)
FeSO4 4 mmol + Na2SO4 2 M
FeSO4 4 mmol
FeSO4 1 mmol
59.9
44.1
75.6
66
95
96
0.88
1.98
1.90
1.8
8.8
10.7
25.4
126.2
152.6
30
80
240
pH = 2.8 using 4 mmol acetic acid.
expense of phenol productivity (0.08 gph gcat 1
(75%).
−
−1
h
) and selectivity
products concentration as well as black solid amount (Table 3). At
higher pH (1.1 and 1.5) process performance improves compared to
pH 0.8, but the amount of black solid increases giving values higher
than that obtained at pH 2.8 with acetic acid alone. Thus this last
acid gives best results on reduced tar formation and improves selec-
tivity, productivity, yield and flowrate compared to the inorganic
acid.
In Table 4 the influence of the anion type of the mineral acid
on tar formation, operating at pH 2.8, is reported. It should be
noted that acetic acid was not added and that effect of the inor-
ganic anions is a general decrease of the selectivity promoting over
oxidation products (tar formation) compared to the presence of
acetic acid alone (see Table 3). The sulphate anion gave the worst
performance promoting the formation of black solid (0.598 g) at the
The hydrochloric acid in the acidic aqueous phase generates
chloride that leads to the formation of the FeCl+ complex and
various chlorinated inorganic radicals. Among these ones, the
dichloride radical anion Cl2 , which is predominant in the pres-
ence of millimolar concentrations of chloride, is a strong electron
acceptor [23]. Thus a greater loss of selectivity and phenol produc-
tivity is observed although tar formation did not increase.
Similar behaviour was obtained when nitric acid was used. The
concentration trend of phenol (Fig. 3) showed a very slow initial
increase and then reached a plateau. This behaviour can be caused
by the nitrate anion which is a good electron acceptor and compete
with the catalyst regeneration as can be seen by the higher standard
•
-

R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
345
Fig. 4. Detected products in the aqueous phase (a) and in the organic phase (b) in the stopped catalytic tests using different concentrations of catalyst and presence of sodium
◦
sulphate. Numbers on the bars indicate concentrations of products in mg/L. (Operative condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol FeSO4; pH = 2.8 using 4 mmol
of acetic acid).
Fig. 5. Concentration of phenol, benzoquinone and biphenyl and phenol selectivity in the time in the catalytic tests using suspended powder iron(0) at pH 2.8. (Operative
◦
condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol Fe(0); pH = 2.8 with 4 mmol of acetic acid; run time = 270 min).

3
46
R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
Table 6
Results of catalytic tests using suspended powder of Iron(0).
−
1
−2
m )
Catalyst
Q (%)
Selectivity (%)
Productivity
Yield%
12.7
Flux (mmol h
181.0
Amount of black
solid (g)
−
1
−1
h )
(
gph gcat
Iron(0)
66.1
94.3
8.11
0.13
◦
Operative condition: T = 35 C; H2O2 = 18 mmol; catalyst = 0.4 mmol Fe(0); pH = 2.8 using 4 mmol acetic acid; run time = 270 min.
Fig. 6. Further oxidation reactions of phenol.
(
Elaborated from Bremner, 2006 [26]).
potential of nitrate:
Cl2 + e− → 2Cl− (E = 2.09 V)
•
-
◦
Fe3 + e− → Fe2+ (E = 0.77 V)
+
◦
•
-
Also the dichloride radical anion Cl2 , with an high standard
potential, can compete with the catalyst regeneration. Probably it
reacts also with organic substances in the solution [23] and then
−
◦
2
NO3 + 12H+ + 10e− → N + 6H O (E = 1.25 V)
2
2
Fig. 7. FT-IR analysis in ATR (Attenuated Total Reflectance) of black solid.

R. Molinari, T. Poerio / Applied Catalysis A: General 393 (2011) 340–347
347
the catalyst deactivation was not evidenced as in the case of nitric
acid. Taking into account the above considerations the combined
effect of nitrate–acetic acid and chloride–acetic acid was not tested
experimentally.
generated by the presence of intermolecular hydrogen bonds and
the C C bonding, which is indicative of the presence of aromatic
−
1
ring, generated by the signal at around 690 cm . Other func-
tional groups are present as the group C O which absorbs at
−
1
1
770 cm
The elemental analysis of tar on an instrument that was able to
.
3.4. Catalytic tests stopped before tar appearance
analyze only C, N and H gave: C = 63.92% and H = 3.36%. The differ-
ence could be mainly attributed to oxygen. These values are only
indicative because the tar is a complex mixture of various com-
pounds [26] and an “ad hoc” study should be carried out to find the
exact composition.
Some oxidation tests were carried out to stop the reaction just
before the black precipitate appearance in order to analyze the
composition of the aqueous phase when it started to become turbid.
The results (Table 5) evidenced that precipitate formation was not
caused by saturation of an oxidation product (in Fig. 4 no particular
increase of benzoquinone and biphenyl is observed) and that black
solid appears at very different reaction times depending on the
composition of the catalytic medium. Indeed, in the tests carried out
4. Conclusions
The effects of salts, pH, type of acids and anion sulphate in
the synthesis of phenol through the hydroxylation of benzene
using a membrane contactor and a catalytic Fenton system have
been investigated. The best results were obtained using powder
of iron(0) as catalyst. Indeed a phenol concentration of 1650 mg/L
in the organic phase and of 845 mg/L in the aqueous phase was
obtained recovering 66.3% of phenol with a selectivity of 94.3% and
using 4 mmol FeSO4 and 2 M Na SO4 the time of black appearance
2
was 30 min while using 4 mmol FeSO4 and 1 mmol FeSO , with-
4
out the salt, the appearance time was 80 and 240 min, respectively.
Thus, a higher initial amount of sulphate anion in the aqueous phase
causes solution clouding in lower time confirming that this anion
is involved in the black solid formation.
−
1
−2
a transmembrane flux of 181.05 mmol h
m .
3
.5. Catalytic tests using iron(0)
The sulphate absence, obtained by using iron(0), did not avoid
tar formation but it was the only cause of its decrease giving a signif-
icant increase of the ratio productivity/amount of black solid (from
4.6 to 62.4).
Further studies will be devoted to test other catalysts
that will avoid or reduce tar compounds and enhance phenol
formation and recovery using also different membrane configura-
tions.
The last aspect studied in this work was the use of suspended
powder Fe(0) as catalyst to investigate the difference of the cat-
alytic performance compared to the sulphate anion present in the
reaction system when FeSO4 was used. The classical Fenton oxida-
tion process utilizes the reaction of iron(II) with hydrogen peroxide
to generate hydroxyl radicals, which then give chemical oxidation
or degrade organic pollutants. Recently, an advanced Fenton pro-
cess (AFP) has been described, which uses metal iron surfaces in
acidic conditions in conjunction with hydrogen peroxide to gen-
erate hydroxyl radicals [25,26]. The corrosion of the metal iron
generates in situ ferrous iron giving rise to a potent Fenton-type
reaction. Particular advantages of this process are the cost-savings
owing to the use of metal iron rather than iron salts.
The results of the tests carried out using Fe(0) as catalyst in terms
of products detected in the organic phase are reported in Fig. 5. The
increasing phenol concentration in the time and an almost constant
and very low concentration of benzoquinone and biphenyl can be
observed.
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6
4
2
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6
6
4
−
1
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−
1
the presence of OH groups such as the OH stretching at 3373 cm