Hydroxylation of phenol by hydrogen peroxide catalyzed by heteropoly compounds in presence of glycerol as green solvent

Article abstract of DOI:10.1016/j.cattod.2015.03.003

Cobalt, manganese, and iron in cationic positions of the secondary structure of Mo- or W-Keggin type heteropolyanions were investigated in the liquid-phase oxidation of phenol in glycerol. Iron tungstophosphate series is the most active in this reaction, yielding catechol and hydroquinone. On the other hand, manganese and cobalt salts of heteropolyacids demonstrate higher catalytic activity in the degradation of phenol to CO₂ in comparison with iron tungstophosphates. It is the first time that glycerol is used as alternative "green" solvent in phenol oxidation reaction.

Full text of DOI:10.1016/j.cattod.2015.03.003

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Hydroxylation of phenol by hydrogen peroxide catalyzed by
heteropoly compounds in presence of glycerol as green solvent
K. Pamina,∗, J. Połtowicza, M. Pron´ czukb, S. Basa˛gc, J. Maciejewskaa,
J. Krys´ciak-Czerwenkaa, R. Tokarz-Sobieraja
a Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland
b Faculty of Chemical Engineering and Technology, Cracow University of Technology, Warszawska 24, 31-155 Kraków, Poland
c Jagiellonian University, Department of Chemistry, Ingardena 3, 30-060 Kraków, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Cobalt, manganese, and iron in cationic positions of the secondary structure of Mo- or W-Keggin type
heteropolyanions were investigated in the liquid-phase oxidation of phenol in glycerol. Iron tungstophos-
phate series is the most active in this reaction, yielding catechol and hydroquinone. On the other hand,
manganese and cobalt salts of heteropolyacids demonstrate higher catalytic activity in the degradation
of phenol to CO2 in comparison with iron tungstophosphates. It is the first time that glycerol is used as
alternative “green” solvent in phenol oxidation reaction.
Received 2 November 2014
Received in revised form 27 February 2015
Accepted 2 March 2015
Available online xxx
Keywords:
© 2015 Elsevier B.V. All rights reserved.
Phenol oxidation
Heteropoly salts
Glycerol
Cyclic voltamerty
1. Introduction
hand, many natural products originated from the biomass utiliza-
tion such as soy methyl ester, lactate ester, or glycerol are available.
Catalytic hydroxylation of phenol leads to catechol and hydro-
quinone which are important intermediates for production of fine
chemicals. Total world production of phenol amounts to about 9 Mt
per year [1]. Phenol is also dangerous pollutant in the natural envi-
ronment because of its high toxicity and low biodegradability [2].
However, we are particularly interested in the oxidation of phenol
to catechol and hydroquinone. It is industrially important reac-
tion, because products have several important applications such as
production of medicines, pesticides, perfumes, photographic film
developer etc.
There are several technologies operated by chemical com-
panies producing hydroquinone and catechol in hydroxylation
of phenol. For example, Rhodia, Solvay owned company, uses
dichloromethane in acid catalyzed phenol hydroxylation while in
Enichem technology phenol is oxidized in the presence of TS-1 in
dioxane and tetrahydrofuran as solvents [3]. Most of the currently
running technologies use organic compounds as solvents in which
other substances can dissolve and deliver heat. Many organic sol-
vents are hazardous and toxic. They are volatile and may cause
environmental threats by polluting the atmosphere. On the other
Glycerol is trihydroxy alcohol, nontoxic, biodegradable, and recy-
clable medium manufactured from renewable sources and is an
attractive alternative to volatile organic solvents for catalytic reac-
tions. The exchange of volatile organic solvents used in chemical
processes for the environmentally friendly compound would have
a great impact on the reduction of the use of organic solvents on
industrial scale [4]. In the past glycerol was considered as expensive
solvent. Due to the rapid expansion of biodiesel industry the avail-
ability of glycerol has improved significantly and the price dropped
in 2010 to 0.50D for 1 kg of 99.9% pure glycerol, thus making it an
attractive “green” solvent.
The first example of the use of glycerol as a “green” solvent
was reported in 2006 by Wolfson group [5] in asymmetric hydro-
genation of prochiral ␤-keto esters and ketons. Both activity and
enantioselectivity were high and competitive with the reactions in
water. Another example of the use of glycerol as solvent is the sim-
ple reduction of benzaldehyde or ethyl acetoacetate with sodium
borohydride followed by easy separation of the products [6]. How-
ever, no attempts to apply glycerol as solvent in the oxidation of
phenol with hydrogen peroxide as oxidant in the presence of het-
eropoly compounds have been made so far.
Polyoxometalates are known as active and selective catalysts in
different oxidation processes and they have received much atten-
tion in the field of catalytic oxidation [7]. They are metal–oxygen
∗
Corresponding author. Tel.: +48 126395153; fax: +48 124251923.
E-mail address: ncpamin@cyf-kr.edu.pl (K. Pamin).
http://dx.doi.org/10.1016/j.cattod.2015.03.003
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: K. Pamin, et al., Hydroxylation of phenol by hydrogen peroxide catalyzed by heteropoly compounds
in presence of glycerol as green solvent, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.003

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cluster compounds possessing both acidic and redox properties
which can be adjusted by a suitable choice of constituting elements.
Introduction of a cation with variable oxidation state makes it pos-
sible to obtain new charge transfer complexes. Polyoxometalates
are resistant to oxidative degradation and they continue to be active
in oxidation reactions for a long time. It is important to emphasize
that the complexes show multifunctionality in composition and
physico-chemical properties and, as a consequence, it is possible
to design the composition for a chosen reaction [8]. The nature of
the aforementioned compounds assures their proper performance
and high catalytic activity in homogeneous reactions.
The series of iron salts of dodecatungstophosphoric or
dodecamolybdophosphoric acids were synthesized by mix-
ing the stoichiometric amount of FeCl2 with an aqueous
solution of selected heteropolyacid. The resultant salts were
dried in the oven at 363 K. The following salts with varying
number of iron cation in the heteropoly structure were syn-
thesized: H2Fe0.5PW12O40, HFePW12O40, Fe3(PW12O40)2, H2Fe0.5
PMo12O40, HFePMo12O40, Fe3(PMo12O40)2, and for simplicity des-
ignated as H2Fe0.5PW, HFePW, Fe3PW, H2Fe0.5PMo, HFePMo,
Fe3PMo, respectively.
Therefore, we propose, for the first time, the use of glycerol as
an alternative “green” solvent for phenol oxidation reaction. The
purpose of the present study is to explain how the application of
glycerol as alternative solvent influences the catalytic activity of
cobalt, manganese, and iron in cationic positions of the secondary
structure of Mo- or W-Keggin type heteropolyanions in the liquid
phase oxidation of phenol. We have prepared a series of cobalt,
manganese, and iron salts of 12-tungstophosphoric (HPW) and 12-
2.2. Catalyst characterization
Fourier transform infrared (FT-IR) absorption spectra were
recorded using Nicolet 6700 spectrometer under atmospheric con-
ditions. Spectra of solid samples were recorded as KBr pellets in
range of 4000–400 cm−1 with resolution of 2 cm−1 and collecting
64 scans.
Cyclic voltammograms were recorded in a three-electrode cell
using 2.0 mm gold disc as the working electrode, platinum coil as
auxiliary electrode and Ag/AgCl as reference electrode. All com-
pounds were analyzed as 0.1 M water solutions in acetate buffer
of pH 5.0 combined with HCl addition (sufficient to keep Keggin
anion structure intact) as electrolyte. Prior to the measurements
the solution was deaerated with argon to keep air-free atmosphere
over the solution during the measurement.
molybdophosphoric (HPMo) acids: H2M0.5PX12O40, HMPX12O40
,
and M3(PX12O40)2, where M = Co, Mn, Fe, and X Mo or W, and
applied them as catalysts in the oxidation of phenol with hydro-
gen peroxide in glycerol. For comparison, the catalytic activities of
HPW and HPMo have also been studied.
2. Experimental
2.1. Catalysts preparation
The dehydration of ethanol was carried out in a conven-
tional flow type reactor under atmospheric pressure. Typically,
a 0.3 ml sample was placed in a quartz reactor. Ethyl alcohol
was introduced into the helium stream (5.7% mole in He) by an
evaporator–saturator set placed in a thermostat. The total flow rate
of alcohol in the stream of the feed gas was 1.8 L/h. Before the test
a catalyst was heated to 623 K at the rate 100 K/h and activated in a
helium flow for 2 h. Catalytic tests were run at 498 K. The products
were analyzed by means of the PerkinElmer 900 gas chromatograph
equipped with FID detector and Porapak S column.
The liquid-phase oxidation of phenol was performed in a ther-
mostated glass reactor equipped with condenser and stirrer. In
a typical reaction, phenol (0.05 mole) and 30% aqueous H2O2
(0.05 mole) were dissolved in 2 ml of glycerol and the reaction
mixture was heated on to 353 K with continuous stirring. The reac-
tion started after the addition of 7.8 × 10−6 mole of catalyst. After
The series of cobalt and manganese dodecatungstophosphate
or dodecamolybdophosphate were obtained as precipitate by
mixing an aqueous solution of an appropriate amount of cobalt
(II) or manganese(II) carbonate (Aldrich) with an aqueous solution
of appropriate heteropolyacid. The solution was filtered and the
mother liquor evaporated to dryness in the oven at 363 K. The fol-
lowing salts with varying number of metal cations in the heteropoly
structure were synthesized: H2Co0.5PW12O40, HCoPW12O40, Co3
(PW12O40)2, H2Co0.5PMo12O40, HCoPMo12O40, Co3(PMo12O40)2,
H2Mn0.5PW12O40
,
HMnPW12O40
,
Mn3(PW12O40)2,
H2Mn0.5PMo12O40
,
HMnPMo12O40 Mn3(PMo12O40)2. These
,
samples are hereafter denoted by H2Co0.5PW, HCoPW, Co3PW,
H2Co0.5PMo, HCoPMo, Co3PMo, H2Mn0.5PW, HMnPW, Mn3PW,
H2Mn0.5PMo, HMnPMo, Mn3PMo, respectively.
Table 1
FTIR vibrations of iron, manganese, and cobalt dodecatungstophosphates or dodecamolybdophosphates.
Vibration mode
HPW
H2M0.5PW
HMPW
M3PW
HPMo
Fe
H2M0.5PMo
HMPMo
M3PMo
M
Fe
M
␯as (P Oa)
␯as (X Od)
1080
982
891
797
1080
982
889
800
1080
982
889
799
1080
981
892
804
1064
961
870
782
1064
961
869
782
1064
961
870
783
1064
961
871
784
␯
as(X Ob X)
␯as (X Oc X)
Vibration mode
HPW
H2M0.5PW
HMPW
M3PW
HPMo
H2M0.5PMo
HMPMo
M3PMo
M
Mn
M
Mn
␯as (P Oa)
␯as (X Od)
1080
982
891
797
1080
982
890
804
1080
981
893
807
1080
981
895
806
1064
961
870
782
1063
963
877
795
1062
965
880
799
1061
961
881
796
␯
as(X Ob X)
␯as (X Oc X)
Vibration mode
HPW
H2M0.5PW
HMPW
M3PW
HPMo
H2M0.5PMo
HMPMo
M3PMo
M
Co
M Co
␯as (P Oa)
␯as (X Od)
1080
982
891
797
1080
982
889
804
1079
980
894
807
1079
980
895
808
1064
961
870
782
1064
961
870
782
1064
961
870
782
1062
962
880
799
␯
as(X Ob X)
␯as (X Oc X)
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1 h of reaction time the oxidation was stopped and the products
were analyzed using PerkinElmer Clarus 500 gas chromatograph
equipped with FID detector and ZB 5MSi column. To analyze carbon
dioxide formed in the oxidation of phenol in glycerol the reaction
was performed in a stainless steel batch reactor of 50 ml volume in
the conditions described above. Gas products were analyzed chro-
matographically using PerkinElmer Clarus 500 gas chromatograph
equipped with FID detector connected with methanizer and ZB
5MSi column. Tar was analyzed by gravimetric method.
3. Results and discussion
The FT-IR spectra of iron, manganese, and cobalt dode-
catungstophosphates or dodecamolybdophosphates are shown in
Table 1. The samples were recorded as KBr pellets. Table 1 gath-
ers spectra of parent tungstophosphoric acid and its Fe, Mn, and Co
salts with 1, 2, or 3 protons substituted with the transition metal
cation. Both heteropolyacids show four characteristic bands in the
region 1100–700 cm−1, which can be described as stretching vibra-
tion of P Oa, X Od, X Ob X, and X Oc X [9]. The oxygen atoms
within Keggin anions can be identified as Oa—internal oxygen,
Od—terminal oxygen, Ob—corner sharing oxygen, and Oc—edge
sharing oxygen [10]. The spectroscopic measurements show that
the primary structure (Keggin) remains unaltered with the for-
mation of the salt. The positions of P Oa and W Od, or Mo Od
bands are quite similar for all samples within each series. The
bands associated with vibrations of bridging X
O X chains show
a slight shift toward higher frequencies upon stepwise introduc-
tion of the metal atom into the heteropoly structure. The changes
of positions of the observed bands may be interpreted as a result of
lowering the degree of hydroxylation of Keggin anions upon intro-
duction of metal cations in the position of compensating cations
[11]. Moreover, the shift toward higher wavenumber results from
the shortening of the bond length between substituted metal and
oxygen.
Figs. 1 and 2 present the results of voltammetric measurements
for the series of cobalt salts of Mo- and W-based heteropoly-
acids, respectively. The wave shape and peak separation indicate
reversibility. The upper figure presents voltammograms measured
for molybdophosphoric acid. Two one-electron signals in the upper,
oxidation wave are visible, and represent the 2-electron oxidation
of the Keggin anion. In the corresponding bottom reduction wave
two one-electron signals are also observed.
Fig. 1. Cyclic voltammograms of cobalt molybdophosphate series.
The substitution of protons for cobalt cations in the heteropoly
structure results in two different effects observed in cyclic voltam-
metric measurements. In the presence of cobalt atom an additional,
third signal appears in voltammograms (marked with arrows in
Figs. 1 and 2). As the number of Co atoms in the heteropoly struc-
ture grows, the intensity of that signal increases gradually (Figs.
S1, S2). Moreover, a shift to more negative potentials for CoxPW
and CoxPMo series in comparison to parent heteropolyacids was
also observed (Table 2). Similar observations can be made for man-
ganese and iron catalysts.
Table 2 presents data of the position of signal I for dode-
camolybdophosphate series and the position of signal II for
dodecatungstophosphate series in both oxidation and reduction
waves. HPMo is the reference sample for the molybdenum series
of heteropoly salts with the position of signal I at 140.2 mV in the
oxidation wave and 101.8 mV in the reduction wave (marked with
asterisk in Fig. 1). For HPW which is the reference sample for tung-
sten series of catalysts the positions of signal II reach 42.2 mV in
oxidation wave and −15.8 mV in reduction wave (marked with
asterisk in Fig. 2). The introduction of transition metals into molyb-
dophosphoric acid influences the position of signal I in cyclic
voltammetry which in consequence lowers in comparison with the
Table 2
Position of signal I for dodecamolybdophosphate series and the position of signal II
for dodecatungstophosphate series.
Dodecamolybdophosphate series (signal I)
Metal
Fe
Wave
HPMo
H2M0.5PMo
HMPMo
M3PMo
Oxidation
Reduction
Oxidation
Reduction
Oxidation
Reduction
140.2 mV
101.8 mV
140.2 mV
101.8 mV
140.2 mV
101.8 mV
132.6 mV
87.2 mV
98.2 mV
61.1 mV
123.2 mV
74.6 mV
116.6 mV
80.7 mV
124.1 mV
80.8 mV
129.4 mV
80.2 mV
119.8 mV
82.8 mV
133.1 mV
89.0 mV
128.4 mV
79.2 mV
Co
Mn
Dodecatungstophosphate series (signal II)
Metal
Fe
Wave
HPW
H2M0.5PW
HMPW
M3PW
Oxidation
Reduction
Oxidation
Reduction
Oxidation
Reduction
42.2 mV
−15.8 mV
42.2 mV
34.2 mV
−19.0 mV
31.2 mV
32.5 mV
−20.1 mV
35.0 mV
30.2 mV
−17.8 mV
39.0 mV
Co
−15.8 mV
−16.5 mV
−14.7 mV
−12.8 mV
Mn
42.2 mV
25.7 mV
26.9 mV
27.9 mV
−15.8 mV
−26.0 mV
−18.7 mV
−17.4 mV
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Scheme 1. Dehydration of ethanol.
100
80
60
40
20
0
H Fe PW
HFe PW
1
Fe PW
HPW
2
0.5
3
Fig. 3. Dehydration of ethanol for iron tungstophosphate series.
in heteropoly salt. The increase of reduction potential upon intro-
duction of the cation into the secondary structure of Keggin anion
results in easier oxidation and more difficult reduction with respect
to parent heteropolyacids.
Heteropolyacids and their salts are active catalysts in alcohol
dehydration [12–14]. The dehydration of alcohol is a model reac-
tion to assess acid–base and redox properties of catalysts. During
dehydration of alcohol and in the presence of acid centers, alcohol is
transformed into ethers and olefins. In the presence of redox cen-
ters alcohol is converted via oxidative dehydrogenation pathway
into aldehyde or ketone.
Catalytic tests were performed in a flow reaction system, in
quartz reactor, under the atmospheric pressure at 498 K. All cat-
alysts were activated in helium stream, at 623 K, for 2 h. Catalytic
properties of all the heteropoly salts were studied in ethanol dehy-
dration (Scheme 1).
Figs. 3 and 4 show the results of catalytic tests performed in
the dehydration of ethanol for FexPW and FexPMo series, respec-
tively. Fig. 3 demonstrates the catalytic results for FexPW series,
where tungstophosphoric acid was compared with iron dode-
catungstophosphates. The only reaction product for this catalytic
series is ethylene. A gradual exchange of protons for iron atoms
results in the decrease of acid strength. Therefore the neutral salt,
Fe3PW, shows the lowest activity in this reaction.
The substitution of tungsten for molybdenum has a major influ-
ence on the acidic properties of the studied catalysts. The catalysts
of iron dodecamolybdophosphate series (Fig. 4) are substantially
less active in dehydration of ethanol (small yield to ethylene)
mainly due to their weaker acidic properties. Oxidative dehydro-
genation, which is carried out on the redox centers, leads to the
formation of acetaldehyde.
Fig. 2. Cyclic voltammograms of cobalt tungstophosphate series.
potential of HPMo. For tungstophosphoric acid-based catalysts a
similar tendency as for the former catalytic series was observed.
Moreover, 1st one-electron reduction potentials for all the
synthesized heteropoly salts were calculated and the results are
gathered in Table 3. As can be seen, the values of reduction poten-
tials gradually grow with the increase of number of metal atoms
Table 3
1st one-electron reduction potentials of synthesized heteropoly salts.
Dodecamolybdophosphate series (signal I)
Metal
H2M0.5PMo
HMPMo
M3PMo
Fe
Co
Mn
95.9 mV
79.7 mV
98.9 mV
98.7 mV
102.5 mV
104.8 mV
101.3 mV
111.1 mV
103.8 mV
Cobalt tungstophosphates (Fig. S3) and manganese
tungstophosphates (Fig. S4) as well as cobalt molybdophos-
phates (Fig. S5) and manganese molybdophosphates (Fig. S6)
demonstrate a similar tendency in ethanol dehydration.
Synthesized catalysts were applied in the phenol oxidation
(Scheme 2) in glycerol which is inert solvent in these reaction
Dodecatungstophosphate series (signal II)
Metal
H2M0.5PW
HMPW
M3PW
Fe
Co
Mn
7.6 mV
7.4 mV
0.4 mV
11.0 mV
11.1 mV
4.1 mV
12.5 mV
13.5 mV
5.3 mV
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100
80
60
40
20
8
hydroquinone
catechol
E - ethylene
AO - acetaldehyde
7
6
5
4
3
2
1
0
0
Fe3PMo
HPMo
H2Fe0.5PMo
HFe1PMo
Fe PW
H Fe PW
HFe PW
1
HPW
3
2
0.5
Fig. 4. Dehydration of ethanol for iron molybdophosphate series.
Fig. 5. Oxidation of phenol with iron tungstophosphate catalysts in glycerol.
8
hydroquinone
catechol
7
6
5
4
3
2
1
0
Scheme 2. Hydroxylation of phenol with hydrogen peroxide.
conditions. Figs. 5 and 6 and S7–S10 present the results of cat-
alytic tests in phenol oxidation with hydrogen peroxide and in
glycerol solvent for iron, manganese, and cobalt tungstophosphates
or molybdophosphates with respect to appropriate parent het-
eropolyacids. The catalytic activity of iron tungstophosphates is
shown in Fig. 5. For the H2Fe0.5PW catalyst, having the lowest con-
tent of iron in the catalytic series, the introduction of iron results
in a significant increase of catalytic activity, which is accompa-
nied by a small decrease of its acidic properties. For HFePW with
one iron cation in the heteropoly structure, the increased number
of iron cations causes a further increase of catalytic activity fol-
lowed up by a progressive decrease of acid strength. Finally, for
neutral salt Fe3PW the yield of desired products decreases in com-
parison with the FeHPW catalyst, which means that not only the
metal centers but also the acidic properties are necessary to assure
higher catalytic activity in this reaction. According to Ref. [15] phe-
nol undergoes oxidation with H2O2 in the presence of Brønsted or
Lewis acids catalysts. This is why iron tungstophosphates are more
active in this reaction than iron molybdophosphates which have
Fe PMo
H Fe PMo
HPMo
HFe PMo
3
2
0.5
1
Fig. 6. Oxidation of phenol with iron molybdophosphate catalysts in glycerol.
much weaker acidic properties (Fig. 6). The lowest activity for this
catalytic series is observed for HPW.
Cobalt tungstophosphates (Fig. S7) and cobalt molybdophos-
phates (Fig. S8) show low catalytic activity in the oxidation of
phenol. Similarly, manganese salts of tungstophosphoric acid
(Fig. S9) and molybdophosphoric acid (Fig. S10) demonstrate low
activity in the studied reaction. The catalysts of the MnxPW series
are the least active in the liquid-phase oxidation of phenol. It is not
Table 4
Catalytic activity of iron, cobalt, and manganese heteropoly salts in degradation of phenol to CO2.
Catalyst
Conversion of phenol [%]
Yield of [%]
CO2
Catalyst
Conversion of phenol [%]
Yield of [%]
CO2
tara
tara
HPW
H2Fe0.5PW
HFePW
37.8
46.5
44.6
47.2
43.1
52.1
46.3
43.9
43.9
43.3
34.9
36.8
31.5
36.6
39.3
48.1
43.3
41.1
41.4
41.0
1.7
1.9
1.8
1.9
1.6
2.5
1.7
1.6
1.6
1.5
HPMo
H2Fe0.5PMo
HFePMo
39.3
39.0
27.4
22.9
40.6
43.8
45.0
42.9
40.7
41.4
35.0
33.8
24.1
21.0
37.0
40.3
41.0
40.4
36.9
37.9
1.5
1.6
0.9
0.9
1.7
1.9
1.9
2.1
1.7
1.7
Fe3PW
Fe3PMo
H2Co0.5PW
HCoPW
Co3PW
H2Mn0.5PW
HMnPW
Mn3PW
H2Co0.5PMo
HCoPMo
Co3PMo
H2Mn0.5PMo
HMnPMo
Mn3PMo
a
Tar recalculated to CO2.
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a surprise that iron catalysts are the most active in this reaction
since in phenol oxidation by Fenton reagent (H2O2 + Fe2+) iron
plays a key role [16,17]. Preliminary DFT calculations for H2O2
activation on cation saturated by solvent ligands, which show that
energetically favorable activation process occurs on Fe complex
[18], additionally justify high activity of iron tungstophosphate
salts in oxidation of phenol. This is why the majority of synthe-
sized catalysts, except iron tungstophosphate series, show small
catalytic activity in the oxidation of phenol.
Poulenc, Brichima, Enichem) [15]. Mn and Co salts of heteropoly-
acids are active catalysts of phenol degradation to carbon dioxide.
Acknowledgement
The authors acknowledge financial support of the Polish
National Science Center—research project 2011/03/B/ST5/01576
Appendix A. Supplementary data
Phenol is not only an important substrate to produce catechol
and hydroquinone but it is also a dangerous pollutant. Many
efforts have been taken to convert phenol into more biodegradable
molecules [19,20]. It is important to stress that all the manganese-
and cobalt-modified heteropoly compounds demonstrate higher
catalytic activity in the degradation of phenol to CO2 in comparison
to iron catalysts. All catalysts show relatively high conversion of
phenol in its degradation to CO2 (Table 4). In fact, cobalt and
manganese catalytic series are the most active in this reaction.
Also, some polymeric species in the form of tar are present in the
reaction mixture.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.cattod.2015.03.003.
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Please cite this article in press as: K. Pamin, et al., Hydroxylation of phenol by hydrogen peroxide catalyzed by heteropoly compounds
in presence of glycerol as green solvent, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.03.003