Liquid-phase catalytic hydroxylation of phenol using metal crosslinked alginate catalysts with hydrogen peroxide as an oxidant

Article abstract of DOI:10.1016/j.molcata.2014.04.006

A series of divalent metal (Cu, Co, Ni, Fe, Mn, Zn and Pd) crosslinked alginate dry beads catalysts were prepared using metal chloride and sodium alginate. The resulting catalysts were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, energy-dispersive X-ray analyses and nitrogen physisorption measurements. The characterization results showed that a series of ion-crosslinked metal-alginate dry beads were synthesized through ion exchange, followed by the coordination of alginate and metal ions. These materials have been assessed as catalysts for liquid-phase hydroxylation of phenol to dihydroxybenzenes using H ₂O₂ as an oxidant. The results showed that Cu-based catalyst exhibited higher activity than other metal-based catalysts for phenol hydroxylation. The influence of key reaction parameters, including the reaction temperature, solvent, reaction time, initial pH, molar ratio of phenol/H ₂O₂ and the amount of catalyst on the reactivity and selectivity were also investigated. Finally, the reusability of Cu-based catalyst illustrated that it could be recovered and reused without notable loss of activity.

Full text of DOI:10.1016/j.molcata.2014.04.006

Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Liquid-phase catalytic hydroxylation of phenol using metal
crosslinked alginate catalysts with hydrogen peroxide as an oxidant
Fengwei Shia, Li Mub, Peng Yuc, Jianglei Hua, Long Zhanga,∗
a School of Chemical Engineering, Changchun University of Technology, Changchun 130012, PR China
b School of Bioscience and Technology, Changchun University, Changchun 130023, PR China
c Petrochina Jilin Petrochemical Company Acrglonitrile Factory, Jilin 132021, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of divalent metal (Cu, Co, Ni, Fe, Mn, Zn and Pd) crosslinked alginate dry beads catalysts were
prepared using metal chloride and sodium alginate. The resulting catalysts were characterized by Fourier
transform infrared spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma atomic
emission spectroscopy, scanning electron microscopy, energy-dispersive X-ray analyses and nitrogen
physisorption measurements. The characterization results showed that a series of ion-crosslinked metal-
alginate dry beads were synthesized through ion exchange, followed by the coordination of alginate and
metal ions. These materials have been assessed as catalysts for liquid-phase hydroxylation of phenol to
dihydroxybenzenes using H2O2 as an oxidant. The results showed that Cu-based catalyst exhibited higher
activity than other metal-based catalysts for phenol hydroxylation. The influence of key reaction param-
eters, including the reaction temperature, solvent, reaction time, initial pH, molar ratio of phenol/H2O2
and the amount of catalyst on the reactivity and selectivity were also investigated. Finally, the reusability
of Cu-based catalyst illustrated that it could be recovered and reused without notable loss of activity.
© 2014 Published by Elsevier B.V.
Received 12 October 2013
Received in revised form 5 April 2014
Accepted 5 April 2014
Available online 15 April 2014
Keywords:
Metal crosslinked alginate
Dry bead
Phenol hydroxylation
Heterogeneous catalysts
1. Introduction
nanoparticles [13] and mesoporous materials [14,15] have been
widely utilized in phenol hydroxylation reaction because of their
The products of phenol hydroxylation are hydroquinone (HQ)
and catechol (CAT), of which the scientific importance and commer-
cial value is demonstrated by their applications in pharmaceuticals,
drugs, polymerization inhibitors, photography and so on [1–4].
The most desirable method for producing dihydroxybenzenes is
direct hydroxylation of phenol with hydrogen peroxide. Hydrogen
peroxide which has high active oxygen content has been widely
explored as a green oxidant in oxidation reactions, because it is
readily available and the resulting products are water and oxygen,
which are environment-friendly [5]. For the hydroxylation reac-
tion, many homogeneous and heterogeneous catalysts have been
studied. While homogeneous catalysts such as mineral acids, metal
ions and metal complexes are difficult to be separated and recov-
ered from the reaction mixture, which prevent their applications
in practical utilization. Compared with homogeneous catalysts,
heterogeneous catalysts such as zeolites [6,7], heteropolyacids
[8], metal oxides [9], hydrotalcite-like compounds [10–12],
favorable activities, selectivities and easy separation [16–19].
Therefore, the design of suitable heterogeneous catalysts for
hydroxylation reaction has been an important area of research.
However, there are also some problems with the preparation and
utilization of these catalysts, such as complexity, high cost and tox-
icity involved in the process. Consequently, the present research
focuses on exploring new catalysts which are cheap, widely abun-
dant and environment-friendly.
Alginate is both a biopolymer and a polyelectrolyte that is
considered to be biocompatible, non-toxic, non-immunogenic
and biodegradable [20]. It can be characterized as an anionic
copolymer comprised of (1 → 4)-linked ␤-d-mannuronic (M) and
␣-l-guluronic (G) residues [21–23]. In Scheme 1, the M and G blocks
of the alginate with their representative sequence are reported.
Because alginate has a number of free hydroxyl and carboxyl groups
distributed along the backbone, it is an ideal candidate for chemi-
cal functionalization, and has good adsorption capacity for metals
cations (e.g. Ca2+, Ba2+, Co2+, Cu2+, Ni2+ and Fe2+, except Mg2+
)
[24–26]. As well, alginate can be shaped as self-standing beads,
films or monoliths with good mechanical stability [21]. Thus, the
alginate became a desirable metal catalyst carrier and had been
utilized in some catalytic reactions [27–29]. However, few reports
∗
Corresponding author at: Yan’an Street No. 2055, Changchun, China.
Tel.: +86 431 85716785; fax: +86 431 85716785.
E-mail addresses: hujl863@163.com (J. Hu), zhanglongzhl@163.com (L. Zhang).
http://dx.doi.org/10.1016/j.molcata.2014.04.006
1381-1169/© 2014 Published by Elsevier B.V.

F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
67
NaOOC
OH
O
NaOOC
OH
O
OH
HO
O
O
O
O
O
HO
OH
OH
O
OH
NaOOC
OH
O
O
OH
NaOOC
NaOOC
Scheme 1. Molecular structure of sodium alginate.
focused on the synthesis and utilization of metal alginates as cata-
lysts for the hydroxylation of phenol.
at 40 ◦C for 3 days, the dry beads were obtained. The above catalysts
were denoted as M–ALG-3.
In the current study, a process was designed to prepare a
series of metals crosslinked alginate (M–ALG) dry beads catalysts
(Scheme 2). These coordination polymers were well-characterized
by inductively coupled plasma atomic emission spectroscopy (ICP-
AES), Fourier transform infrared spectroscopy (FT-IR), UV–vis
diffuse reflectance spectroscopy (UV–vis/DRS), scanning electron
microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photo-
electron spectroscopy (XPS) and nitrogen sorption analysis, to
prove the formation of the coordination binding between metal ion
and alginate, and to explore the morphology and surface textural
property of the prepared catalysts. The catalysts were subsequently
applied in phenol hydroxylation with H2O2 as an oxidant under
mild reaction conditions. The influence of key reaction parame-
ters for this reaction was investigated, and the reusability of the
catalysts was evaluated.
2.2.2. Synthesis of Cu–ALG
Four CuCl2 solutions with different concentrations (0.1, 0.2, 0.3
and 0.4 mol/L) were used to prepare the Cu–ALG by following the
above way. The prepared catalysts were denoted as Cu–ALG-x,
where x = 1, 2, 3 and 4 represent the molar concentration of CuCl2
being 0.1, 0.2, 0.3 and 0.4 mol/L, respectively.
2.3. Characterization of the sample
FT-IR spectra were recorded on a Nicolet IS10 IR spectropho-
tometer with n of scans to be 32, and the spectra were collected
within the range of 400–4000 cm−1. UV–vis/DRS were recorded
at ambient atmosphere in the wavelength range of 200–800 nm
using a Hitachi U-4100 spectrometer. The contents of metal ele-
ments in the alginates were determined by a Leeman Prodigy Spec
ICP-AES. All determinations were conducted at least in triplicate.
The external morphology of the dry beads was examined using
a JEOL JSM-5600LV SEM with an EDX system (EDAX-Falcon). The
nitrogen adsorption/desorption measurement was performed on
a Micromeritics ASAP 2020 M surface area and porosity analyzer.
XPS was performed in Thermo ESCALAB 250 spectrometer.
2. Experimental
2.1. Materials
All chemicals used were of analytical grade, sodium alginate,
copper chloride dehydrate (CuCl2), cobalt chloride tetrahydrate,
manganese chloride tetrahydrate, ferrous chloride dehydrate,
nickel chloride hexahydrate, zinc chloride, palladium chloride,
hydrogen peroxide (wt. 30%) were obtained from Tianjin Guangfu
Fine Chemical Research Institute, Tianjin China. Phenol, catechol,
hydroquinone and benzoquinone were obtained from Aldrich. All
chemicals were used without further purification.
2.4. Activity test of the catalyst
Phenol hydroxylation with H2O2 was carried out in a three-
necked round bottom flask equipped with mechanical stirrer and
reflux condenser (Scheme 3). First, 1.0 g (10 mmol) of phenol was
dissolved in 30 mL of distilled water, and set amount of catalyst was
added. Then, H2O2 (30%) was added within 30 min. The amount
of H2O2 introduced was varied to obtain a desired phenol/H2O2
ratio. After a given reaction time, initial pH and temperature, the
resulting mixture was sampled. The products were analyzed by gas
chromatography (Agilent GC6890) through a HP-5 column (film
thickness, 0.25 ␮m; i.d., 0.25 mm; length, 30 m) using a flame ion-
ization detector (FID) and the concentration of residual H2O2 was
determined by the iodometric titration.
2.2. Catalyst preparation
2.2.1. Synthesis of M–ALG, M Co, Ni, Fe, Mn, Zn or Pd
The divalent metal (Co, Ni, Fe, Mn, Zn and Pd) chloride solutions
with molar concentration of 0.3 mol/L were prepared in 100 mL dis-
tilled water. Sodium alginate was dissolved in distilled water at a
concentration of 4% (w/v) and then dropped into the former metal
chloride solutions through a syringe with a 2 mm-diameter nee-
dle. The reaction proceeded at 50 ◦C for 6 h with a gentle stirring
(200 rpm) to form gel beads, and then these gel beads were sepa-
rated from the solution and washed with distilled water. After dried
The performance of the catalysts is evaluated quantitatively
by the conversions of phenol and H2O2, the selectivities to cate-
chol (CAT), hydroquinone (HQ) and para-benzoquinone (BQ), and
turnover frequency (TOF: moles of reacted substrate over moles
Scheme 2. Pathway of the synthesis of M–ALG dry beads catalysts.

68
F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
Fig. 1. FT-IR spectra of M–ALG dry beads.
Scheme 3. Hydroxylation of phenol to produce CAT and HQ with H2O2 catalyzed
by M–ALG.
Usually, the identification of the different oxidation states of the
transition metals in compounds can be easily carried out by XPS
because the different shapes or energy position of the photoelec-
trons. The XPS spectra of the M–ALG were shown in Fig. 2a–g. It
is known that the appearance of spin-orbit split of Cu (2p3/2 and
2p1/2) along with their shake up satellites is mainly a characteristic
of bivalent copper (Fig. 2a) [31]. The Cu (2p3/2 and 2p1/2) peaks were
at 935.2 and 955.2 eV, separated by 20.0 eV, indicating the Cu in the
catalyst was mainly Cu2+. Regarding Mn, Co, Ni, Zn and Fe–ALG, the
separated binding energy of their 2p3/2 and 2p1/2 peaks were 11.7,
15.5, 18.4, 23.1 and 13.6 eV, respectively (Fig. 2b–f) [32–34], indi-
cating the chemical valence of these metals were all bivalent. The
XPS analysis results of the Pd 3d were shown in Fig. 2g. It was char-
acterized by the two spin-orbit split of Pd 3d5/2 (343.3 eV) and Pd
3d3/2 (339.0 eV), separated by 5.3 eV, indicating that the Pd ions in
the catalyst were bivalent [35].
of metal per hour) obtained at the selected conditions. Herein, the
conversion of phenol is expressed by Xphenol
:
Xphenol(%) = 100 × (Cb, phenol − Ca, phenol)/Cb, phenol
where Cb,phenol and Ca,phenol are the molar concentrations of phenol
before and after the reaction, respectively. The selectivity of the
product is shown by Sp:
Sp(%) = 100 × Cp/(Cb,phenol − Ca,phenol
)
where Cp is the molar concentration of the product and p = CAT, HQ
or BQ. The conversion of H2O2 is shown by XH
:
O
2
2
XH (%) = 100 × (Cb, H
− Ca, H )/Cb, H
O
O
2
O
O
2
2
2
2
2
2
2
SEM image in Fig. 3 were taken at 50× and 1000× magnifica-
tions to observe the external and internal morphology of the dry
beads, respectively. As shown in Fig. 3A, the Cu–ALG dry bead was in
spherical form and had a very rough surface, on which some macro-
pores or roughness could be found. Compared with the gel beads,
the dry beads were more compact, which enhanced the mechanical
stability. The stratified structures were found on the surface of the
blocks (Fig. 3B). As the gel beads dried, the interior shrinked to a
greater extent than the exterior regions, giving this kind of “crum-
pled” structure with decreased size without affecting surface area
significantly [36]. If the dry bead was cut (Fig. 3C), it could be found
that the centre of the bead was solid, which was consistent with
Fernandez’s report [37].
where Cb,H
and Ca,H
are the molar concentrations of H2O2
2
O
O
2
2
before and 2after the reaction, respectively.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. FT-IR spectral studies
The FT-IR spectra revealed the changes in the absorption bands
for the surface functional group of the sodium alginate before and
after reacting with the cationic solution (Fig. 1). The broad absorp-
tion band around 3400 cm−1 was due to the stretching vibrations of
the hydroxyl groups. In this study, the peak shifted from 3400 cm−1
to higher wavenumber after sodium alginate reacted with cations
to form the metal crosslinked alginates dry beads. The increase in
the adsorption peak should be attributed to the attachment of the
metal cations to the –OH group. The spectrum of Na–ALG showed
the absorption bands at 1639 and 1386 cm−1, which were assigned
to asymmetric and symmetric stretching peaks of the carboxyl-
Table 1
The content of metal in alginate catalysts before and after reaction.
Entry
Catalysts
Metal content
(before
Metal content
(after
reaction)/%
reaction)/%
ate salt groups. While after crosslinking, both the C O and C
O
1
2
3
4
5
6
7
8
9
10
Mn–ALG-3
Co–ALG-3
Fe–ALG-3
Ni–ALG-3
Zn–ALG-3
Pd–ALG-3
Cu–ALG-1
Cu–ALG-2
Cu–ALG-3
Cu–ALG-4
13.1
12.5
11.6
11.7
13.5
12.7
9.7
14.1
15.3
15.5
12.9
12.4
11.4
11.5
13.4
12.6
9.4
13.7
15.2
15.2
in the carboxyl group ( COO−) shifted slightly in frequency. The
red-shifting of the C O bond could be attributed to the C O coor-
dination with the metal cations, which led to the decrease in the
stretching force constant of the carboxyl group [30]. In addition,
the C O in the carboxyl group bond shifted to a higher frequency,
which was attributed to the coordination of metal cations to the
carboxylate ion ( COO−), as well the high electron density induced
by the cations sorption onto the adjacent hydroxyl group.

F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
69
Mn 2p
3/2
Co 2p
3/2
a
Cu 2p
3/2
b
c
Cu 2p
1/2
Co 2p
1/2
Mn 2p
1/2
11.7
20.0
945
15.5
965
960
955
950
940
935
Bin6d5i0ng energy (eV)
Binding energy (eV)
660
655
645
640
635 810
800
790
780
770
Binding energy (eV)
Ni 2p
3/2
Zn 2p
f
e
3/2
Fe 2p
3/2
d
Fe 2p
1/2
Zn 2p
Ni 2p
1/2
1/2
13.6
18.4
23.1
890
880
870
860
850
840
1050
1040
1030
1020 740
730
720
710
700
Binding energy (eV)
Pd 3d
Binding energy (eV)
Binding energy (eV)
g
5/2
Pd 3d
3/2
5.30
346 344
340 338
B3in42ding energy (e3V36)
334
Fig. 2. XPS spectra of (a) Cu–ALG, (b) Mn–ALG, (c) Co–ALG, (d) Ni–ALG, (e) Zn–ALG, (f) Fe–ALG and (g) Pd–ALG.
The EDX spectrum of Cu–ALG dry beads analyzed by using ele-
mental microprobe method of SEM/EDX was illustrated in Fig. 4.
From Fig. 4a, it could be found that only O, C, Cu and Cl signals were
observed on the surface of catalyst, while Na signals could be found
only in the centre of the catalyst (Fig. 4b), which demonstrated
that Na ions on the surface of the catalyst had been exchanged
completely. EDX analysis therefore provided the direct evidence
for crosslinking of the Cu on the surface of alginates.
As SEM coupled EDX microprobe analysis is semi-quantitative
analysis, to precisely investigate the metals contents of the cata-
lysts, ICP-AES was applied (Table 1). Table 1 showed that different
metal elements had different crosslinked content with alginate,
among which the Cu content was the highest (reached to 15.3%),
and for the other dry beads, the crosslinked contents of metals were
between 11.6% and 13.5%. The results illustrated that the ability of
alginate crosslinking with metal was different.
In order to detect the relation of metal in catalyst and raw
material, a series of Cu–ALG dry beads were prepared using dif-
ferent concentration of CuCl2 (0.1–0.4 mol/L), the results (Table 1,
entries 7–10) showed that the Cu content of the Cu–ALG dry
beads increased with the increase of the CuCl2 concentration.
When the CuCl2 concentration increased from 0.1 to 0.3 mol/L
(Table 1, entries 7–9), the Cu content of the dry beads increased
significantly (from 9.7% to 15.3%). However, when the CuCl2 con-
centration exceeded 0.3 mol/L, the change of the Cu content in
dry beads was unapparent. This result should be attributed to
the saturation of the crosslinked products [29]. To test whether
metal ions were leaching out from the catalysts, ICP-AES was
used to detect the metals contents after once hydroxylation reac-
tion. The results showed that a trace amount of metals was
lost (lower than 3 wt.%) during the phenol hydroxylation reac-
tion.

70
F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
Two major products (CAT and HQ) and a small amount of byprod-
uct (BQ) were identified by GC. The catalytic performance of various
transition metals (Cu, Co, Ni, Fe, Mn, Zn and Pd) alginates catalysts
in phenol hydroxylation was listed in Table 2. The conversion of
phenol, conversion of H2O2 and turnover frequency (TOF: moles
of reacted substrate over moles of metal per hour) were used to
evaluate the catalytic activities of prepared catalysts. The result of
a blank experiment under the same reaction conditions showed
that in the absence of the catalyst, H2O2 alone was unable to oxi-
dize phenol to a significant extent (only 5.3%, entry 1) [39]. After
the prepared catalysts were added, the conversions of phenol were
increased in all experiments (entries 2–11). While concerning with
Mn–ALG, Zn–ALG and Pd–ALG (entries 2–4), the conversion of phe-
nol increased little, which indicated the low activities of them in the
hydroxylation reaction. For the Co, Fe, Ni and Cu alginates (entries
5–7 and entry 10), obvious increases in conversions were observed.
Prepared with the same concentration of chloride (0.3 mol/L), their
reactivities (conversion of phenol, conversion of H2O2, and TOF)
followed the order: Cu–ALG-3 > Fe–ALG-3 > Ni–ALG-3 > Co–ALG-3,
and the selectivities to dihydroxybenzenes were nearly the same
(above 97%). Transition metal complexes are known to have effect
on the decomposition of H2O2 either by a free radical mechanism
or through the formation of active peroxo species in the coordina-
tion sphere [40]. Because the decomposition behavior of H2O2 in
oxidation is resulted by the electronic and steric factors of the tran-
sition metal, the difference of the activities of these catalysts could
be due to the different transition metals chosen.
With regard to the Cu–ALG-x catalysts (entries 8–11) [39], the
conversion was increased as the Cu content increased. When the
concentration of CuCl2 increased from 0.1 to 0.3 mol/L (entries
8–10, the loaded Cu content increased from 9.7% to 15.3%, entries
7–11 in Table 1), the TOF value changed little, while the conver-
sions of phenol and H2O2 increased from 33.1% to 52.7% and 88.2%
to 96.7%, respectively, with a catechol-to-hydroquinone ratio of 3:2,
which is higher than the value reported in the literature [18,41]. It is
believed that a large number of Cu active sites increased the decom-
position rate of H2O2 [29]. The conversion changed little when the
concentration of CuCl2 reached to 0.4 mol/L (entry 11, the loaded
Cu content reached 15.5%), which was likely to be attributed to the
saturation of Cu–ALG. This result was consistent with the results of
ICP-AES.
Fig. 3. SEM images of Cu–ALG.
Besides, the elemental concentration distribution of the Cu–ALG
3.2.2. Catalytic behaviors under different reaction conditions
To investigate the influence of the reaction conditions of the
catalyst on the catalytic activity of phenol hydroxylation, the
Cu–ALG-3 catalyst was used to conduct the experiment. The effects
of solvent, temperature, reaction time, molar ratio of phenol/H2O2,
the amount of catalyst, initial pH, catalyst stability and reusability
on conversion profiles were examined in detail.
dry beads can be analyzed using the mapping analysis of SEM/EDX.
The Cu ions distribution mapping by EDX analysis of the catalyst
before and after regeneration (The spent catalyst was recovered by
filtration, washed thoroughly with ethanol and dried.) of hydrox-
ylation was illustrated in Fig. 4c and d. The bright white points
represented the signal of Cu element on the surface of the dry
beads. The result showed that after hydroxylation and regenera-
tion, the amount of Cu ions on the surface of the catalyst reduced
insignificantly, which was consistent with the ICP-AES.
To investigate the specific surface area and porosity of the pre-
pared alginate dry beads, a BET test was performed. The results
of nitrogen sorption (<1 cm3/g) and surface area (<1 m2/g), calcu-
lated by the BET method, indicated that the dry beads have no small
dimensions pores [38], which was consistent with the SEM result.
Thus, the sags on the surface were likely caused by the depressions
of the surface, which occurred during the drying phase.
3.2.2.1. The influence of solvents. It is well-known that the solvent
plays an important and sometimes decisive role in the catalytic
behavior of a catalyst [42,43]. The effect of solvents (ethanol, ace-
tonitrile, ethyl acetate and water) has been investigated in the
reaction. As listed in Table 3, the catalytic activities were strongly
dependent on the solvents. The conversion of phenol was 52.7%
with a catechol-to-hydroquinone ratio of 3:2 in water, which was
better than in other organic solvents. In ethanol, acetonitrile and
ethyl acetate, the conversion of phenol was very low that the
catalytic activities could be almost ignored. The polarity order of
the solvents is water > acetonitrile > ethanol > ethyl acetate, and the
hydroxyl radicals is more stable in polar solvents. Considering
the above results, it was found that polar solvent could promote
the oxidation of phenol, which suggested that the radical reaction
3.2. Catalytic phenol hydroxylation
3.2.1. Catalytic behaviors of various catalysts
Liquid-phase hydroxylation of phenol catalyzed by the metal
crosslinked alginate using H2O2 as an oxidant has been studied.

F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
71
Fig. 4. EDX analysis of (a) surface of the Cu–ALG; (b) centre of the Cu–ALG; mapping of Cu element in Cu–ALG catalyst before (c) and after regeneration of (d) hydroxylation
reaction.
mechanism was in operation on phenol hydroxylation over these
catalysts.
because the byproduct of BQ increased. Therefore, the suitable tem-
perature for this catalytic reaction was 60 ◦C.
3.2.2.2. The influence of temperature. The effect of the reaction
temperature on the conversion of phenol, the selectivity of dihy-
droxybenzene in the range from 20 to 80 ◦C was showed in Fig. 5.
It could be found that the reaction temperature was an important
parameter. When the temperature increased from 20 to 80 ◦C, the
conversion of phenol increased from 22.1% to 55.2%. In addition, the
selectivity of dihydroxybenzenes increased with the temperature
increasing from 20 to 60 ◦C. While going on increasing the tem-
perature to 80 ◦C, the selectivity of dihydroxybenzenes dropped,
3.2.2.3. The influence of reaction time. In the catalytic system, the
duration of the hydroxylation reaction was studied within 5 h
keeping other parameters constant to optimize reaction time.
Generally, the conversion of phenol increased with increasing
of reaction time. From the result displayed in Fig. 5, the con-
version of phenol increased rapidly within the first 2 h, while
when the reaction time increased beyond 2 h, the conversion
of phenol increased in a small range. (The conversions of phe-
nol were 52.7%, 53.9% and 54.9%, at 3, 4 and 5 h, respectively.)
Table 2
Catalytic activities and selectivity in phenol hydroxylation.
Entry
Catalyst
Xphenol (%)a
XH
(%)
2
Selectivity (%)b
CAT
TOF (h−1
)
O
2
HQ
BQ
1
2
3
4
5
6
7
8
Blank
5.3
7.2
7.8
61.4
70.3
64.0
62.7
84.4
91.2
93.9
88.2
94.0
96.7
96.8
59.1
52.4
55.0
57.2
60.9
53.5
50.1
58.9
57.1
59.5
57.4
38.8
46.7
43.8
42.4
36.8
44.1
47.9
38.2
40.0
38.4
40.6
2.1
0.9
1.2
0.4
2.3
2.4
2.0
2.9
2.9
2.1
2.0
–
Mn–ALG-3
Zn–ALG-3
Pd–ALG-3
Co–ALG-3
Ni–ALG-3
Fe–ALG-3
Cu–ALG-1
Cu–ALG-2
Cu–ALG-3
Cu–ALG-4
0.29
0.37
0.52
1.82
2.02
2.09
2.10
2.13
2.14
2.13
6.3
39.5
41.3
44.2
33.1
48.6
52.7
52.9
9
10
11
a
Phenol, 1.0 g; catalyst, 50 mg; phenol/H2O2 molar ratio, 1:2; water, 30 mL; 60 ◦C; 2 h.
Product distribution given on a tar-free basis.
b

72
F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
Fig. 5. Temperature and time studied over Cu–ALG-3.
Table 3
3.2.2.5. The influence of the amount of catalyst. The influence of the
catalyst amount on the hydroxylation of phenol was also studied.
Three amounts of catalyst viz., 10, 50 and 100 mg were used. These
results showed that the phenol conversion increased from 19.6 to
62.4% when the catalyst amount increased continuously from 10 to
100 mg, but the selectivity to dihydroxybenzene decreased when
the catalyst increased to 100 mg. The increased active catalytic sites
may be considered to be responsible for these phenomena, which
led to the fast decomposition of H2O2 and oxidize phenol to other
products. Therefore, 50 mg of the catalyst was the optimum amount
for hydroxylation of phenol in this case.
Effect of solvents in the phenol hydroxylation catalyzed by Cu–ALG-3.
Solvent
Xphenol (%)a
Selectivity (%)b
CAT
HQ
BQ
Water
Acetonitrile
Ethanol
52.7
11.5
7.2
–
59.5
27.1
62.3
–
38.4
72.7
37.5
–
2.1
0.2
0.2
–
Ethyl acetate
a
Phenol, 1.0 g; catalyst, 50 mg; phenol/H2O2 molar ratio, 1:2; solvent, 30 mL;
60 ◦C; 2 h.
b
Product distribution given on a tar-free basis.
3.2.2.6. The influence of the initial pH. The effect of the initial pH has
also been investigated in the reaction. From the result displayed in
Fig. 6, both acidic and alkaline solutions have negative effect on the
conversion of phenol. The conversion increased from 12.1% to 37.7%
as the initial pH increased from 2 to 7, while went on increasing the
pH to 12, the conversion decreased to 8.3%. An interesting discovery
is that acidic solution could enhance the further oxidation of HQ to
BQ, which lead to the selectivity of HQ decreasing, while alkaline
solutions could increase the selectivity of HQ. The specific influence
of initial pH on the hydroxylation reaction is expected to be further
study.
The selectivity to hydroquinone decreased with prolonging the
reaction time. It is proposed that at the later reaction stage, an
oxidation of hydroquinone in the reaction medium with H2O2
might cause the formation of p-benzoquinone in relatively large
amount.
3.2.2.4. The influence of molar ratio of phenol/H2O2. The effect of
phenol/H2O2 molar ratios (2:1, 1:1, 1:2 and 1:3) on the conversion
of phenol, the selectivities to CAT, HQ and BQ for the phenol hydrox-
ylation catalyzed by Cu–ALG-3 was studied. As shown in Table 4,
under the given experiment conditions, the conversion of phe-
nol increased from 16.1% to 57.3% as the phenol/H2O2 molar ratio
was changed from 2:1 to 1:3, which could be due to the increase
in hydroxyl radical concentration upon decomposition of H2O2.
However, when the phenol/H2O2 molar ratio was lower than 1:2,
the selectivity to dihydroxybenzene decreased apparently, and the
lowest selectivity to dihydroxybenzene (94.1%) was obtained at the
phenol/H2O2 molar ratio being 1:3. The above result suggested that
a large amount of oxidant was not an essential condition in improv-
ing the catalytic performance of the catalyst. In order to obtain
a higher selectivity to dihydroxybenzene under the given cat-
alytic system, the molar ratio of phenol/H2O2 should be controlled
at 1:2.
3.2.2.7. Catalyst stability and reusability. The Cu–ALG-3, which
has favorable catalytic activity, was used three times for phenol
hydroxylation to investigate its reusability (Fig. 6). The hydrox-
ylation reaction was repeated three times under the condition:
the amount ratio of phenol to catalyst was 20:1 and the molar
ratio of phenol to H2O2 was 1:2. After each run, the spent catalyst
was recovered by filtration, washed thoroughly with ethanol and
dried. The weighed masses of the Cu–ALG-3 catalyst were 50, 44
and 37 mg, corresponded to the conversions of phenol were 52.7%,
47.7% and 44.4%, for the 1st, 2nd and 3rd runs, respectively. The
selectivities of CAT to HQ were all 3:2. Combining the results of the
Table 4
Effect of molar ratio of phenol/H2O2 in the phenol hydroxylation.
Entry
Molar ratio of phenol/H2O2
Xphenol (%)a
Selectivity (%)b
CAT
HQ
BQ
1
2
3
4
2:1
1:1
1:2
1:3
16.1
35.2
52.7
57.3
55.1
55.9
59.5
54.9
44.4
43.2
38.4
39.2
0.5
0.9
2.1
5.9
a
Phenol, 1.0 g; catalyst, 50 mg; water, 30 mL; 60 ◦C; 2 h.
Product distribution given on a tar-free basis.
b

F. Shi et al. / Journal of Molecular Catalysis A: Chemical 391 (2014) 66–73
73
Fig. 6. Effect of pH on the phenol hydroxylation (left) and the catalytic performance of Cu–ALG-3 in the phenol hydroxylation reaction for three runs (right).
ICP-AES (Table 1) and mapping of Cu element of the catalyst (Fig. 4),
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reaction. Therefore, the reduction of the activity of the Cu–ALG cat-
alyst should be attributed to both the loss of the catalyst during the
recycling processes and the surface’s coverage by tarry byproducts.
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Acknowledgments
This work was supported by the Natural Science Fund Council of
China (21203013). The authors gratefully acknowledge the Centre
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