Hydroxylation of phenol catalyzed by different forms of Cu-alginate with hydrogen peroxide as an oxidant

Article abstract of DOI:10.1016/j.catcom.2012.04.008

Copper alginate (dry beads and powder) catalysts were prepared using four concentrations of copper chloride dihydrate and sodium alginate. The resulting catalysts were characterized by FT-IR, ICP-AES, SEM, EDX and nitrogen physisorption measurements. The

Full text of DOI:10.1016/j.catcom.2012.04.008

Catalysis Communications 25 (2012) 102–105
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Hydroxylation of phenol catalyzed by different forms of Cu‐alginate with hydrogen
peroxide as an oxidant
Fengwei Shi , Yaguang Chen , Linping Sun , Long Zhang a,b,⁎, Jianglei Hu b,⁎
a
a
b
a
School of Chemistry, Northeast Normal University, Changchun 130024, PR China
School of Chemical Engineering, Changchun University of Technology, Changchun 130012, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper alginate (dry beads and powder) catalysts were prepared using four concentrations of copper chloride
dihydrate and sodium alginate. The resulting catalysts were characterized by FT-IR, ICP-AES, SEM, EDX
and nitrogen physisorption measurements. The results of phenol hydroxylation showed that the catalytic
activities of these catalysts are related to the Cu (II) content. The highest conversions of phenol were
Received 12 January 2012
Received in revised form 10 April 2012
Accepted 12 April 2012
Available online 21 April 2012
5
2.9% and 62.5% over dry beads and powder, respectively, with a catechol-to-hydroquinone ratio of 3:2.
2012 Elsevier B.V. All rights reserved.
©
Keywords:
Cu‐alginate
Powder
Dry bead
Catalyst
Phenol hydroxylation
1
. Introduction
The catalytic hydroxylation reaction has been a topic of concern due to
synthesis of biopolymer and metal‐cross-linked Cu‐alginate catalysts
for the phenol hydroxylation.
In the current study, a process was designed to prepare a series of
Cu‐alginate dry bead and powder catalysts. These catalysts were
characterized by inductively coupled plasma atomic emission spec-
troscopy (ICP-AES), Fourier transform infrared (FT-IR) spectroscopy,
scanning electron microscopy (SEM), energy dispersive X-ray (EDX)
and nitrogen sorption analysis. The catalysts were utilized in phenol
hydroxylation with hydrogen peroxide (30 wt.%) as an oxidant. The
influences of different forms of catalysts for this reaction were investi-
gated, and the results show that both the Cu‐alginate dry beads and
the powder have favorable catalytic activities for phenol hydroxylation.
its core role in a variety of chemical processes for producing bulk and fine
chemicals. Heterogeneous catalysts are particularly attractive ascribed to
their favorable activity, selectivity and easy separation. While researchers
have demonstrated that design of suitable heterogeneous catalysts
has been an important area in hydroxylation reaction. Recently, several
catalysts have been utilized in this reaction, such as zeolites [1–3],
heteropolyacids [4], metal oxides [5], hydrotalicite-like compounds
[6–8], nanoparticles [9] and mesoporous materials [10–12]. However,
complexity, high cost and toxicity in the preparation and utilization
process restrict on their wildly application. Consequently, the present
research focus is on exploring new catalysts that are cheap, widely
abundant and more environmentally friendly.
2. Experimental
Alginates are important, naturally occurring biopolymers and the
block copolymers of uronic acids, more specifically (1→4)‐linked β‐D‐
mannuronic (M) and α‐L‐guluronic (G) residues [13,14]. Sodium alginate
2.1. Catalyst preparation
2
Four CuCl solutions with different concentrations (0.1, 0.2, 0.3
and 0.4 mol/L) were prepared in 100 mL of distilled water. Sodium
alginate was dissolved in distilled water at a concentration of 4%
can be exchanged by metal cations (e.g. Ca2 , Ba and Fe ) and then
be coordinated with the –OH of the carboxylate groups to generate
the ion‐cross-linked alginates [15–17]. They have been used in several
applications including wound dressing and fiber materials [18,19].
Cu (II) alginate has been reported to have excellent activity for the
oxidation of organic substances, such as the cycloaddition of alkynes
and oxidative of naphthols [20]. However, no reports focused on the
+
2+
2+
2
(w/v) and then dropped into a CuCl solution through a syringe with
a 2 mm-diameter needle. The reaction proceeded at 50 °C for 6 h with
gentle (200 rpm) or intense (1200 rpm) stirring to form gel beads or
powder (Scheme 1), and then these catalysts were separated from the
solution and washed with distilled water. After dried at 40 °C for
3
days, the dry beads and powder were obtained. The above catalysts
were denoted as Cu‐ALG-d-x and Cu‐ALG-p-x, where ALG-d and ALG-p
represent alginate dry beads and powder, respectively, and x represents
the content of Cu (II) (wt.%) in the catalysts.
⁎
Corresponding authors at: School of Chemical Engineering, Changchun University
of Technology, Changchun 130012, PR China. Tel./fax: +86 431 85716785.
E-mail addresses: Zhanglongzhl@163.com (L. Zhang), Hujl863@163.com (J. Hu).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.04.008

F. Shi et al. / Catalysis Communications 25 (2012) 102–105
103
Scheme 1. Schematic presentation of the preparation of Cu‐alginate.
2
.2. Catalyst characterization
(Fig. S1). After drying, the gel beads shrunk to 1–2 mm and became
more compact, and their surfaces became tough. The powder was
brown and ground to 80 mesh for phenol hydroxylation.
FT-IR spectra were recorded on a Nicolet IS10 IR spectrophotometer.
The samples were finely ground with KBr and pressed into a self-
supporting wafer. The content of Cu (II) in the alginates was determined
by a Leeman Prodigy Spec ICP-AES. All determinations were conducted at
least in triplicate. The external morphology of the dry beads and powder,
and the elemental concentration distribution were 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.
The FT-IR spectra reveal the changes in the absorption bands for
the surface functional group of the sodium alginate before and after
reacting with CuCl
form a broad band in the region of 3100–3700 cm . In this study, the
2
(Fig. 1). The stretching vibrations of –OH usually
−
1
−
1
peak shifted from 3400 to 3426 cm
with CuCl to form the Cu‐alginate. The increase in the adsorption
peak should be attributed to the attachment of Cu
group. In addition, both the C_O and C\O in the carboxyl group
−COOH) shifted slightly in frequency after cross-linking. The red-
after sodium alginate reacted
2
2+
on the –OH
(
−
1
2
.3. Catalyst test
shifting (from 1639 to 1618 cm ) of the C_O band could be attributed
to the –OH coordination with Cu2 , which led to the decrease in the
stretching force constant of the carboxyl group [21,22]. In addition, the
C\O in the carboxyl group shifted to a higher frequency (from 1386 to
+
2 2
Phenol hydroxylation with H O was carried out in a three-necked
round bottom flask equipped with a reflux condenser. First, 1.0 g
10 mmol) of phenol was dissolved in 30 mL of distilled water, and
.05 g of catalyst was added.Then, 2.0 mL (20 mmol) of H (30%)
was added within 30 min. The solution was stirred for 2 h at 70 °C.
The concentration of residual H was determined by the iodometric
−
1
(
1411 cm ), which was attributed to the high electron density induced
by the copper sorption onto the adjacent hydroxyl group. The stretching
vibrations of C\O for both sodium alginate and copper alginate located
0
2 2
O
−
1
O
2 2
at 1028 cm
in the coordination. Compared with Fig. 1A, B and C both have absorption
indicated that the C\OH on the ring did not participate
titration. The products were analyzed by gas chromatography (Agilent
GC6890) through an HP-5 column using a flame ionization detector
−
1
bands at 673 cm , which are assigned to the stretching vibration of the
(
FID).
The characterization results of the concentrations of phenol, catechol
CAT), hydroquinone (HQ) and p-benzoquinone (BQ) were evaluated
Cu\O bond.
(
using the equations given below. The conversion of phenol was
expressed by Xphenol
:
Xphenolð%Þ ¼ 100 ꢀ ðCb;phenol–Ca;phenolÞ=Cb;phenol
where Cb, phenol and Ca, phenol are the molar concentrations of phenol be-
fore and after the reaction, respectively. The selectivity of the product is
shown by S :
p
S ð%Þ ¼ 100 ꢀ C =ðCb;phenol–Ca;phenol
Þ
p
p
where C
BQ. The conversion of H
p
was the molar concentration of the product and p=CAT, HQ or
is shown by XH2O2
O
2 2
:
XH2O2ð%Þ ¼ 100 ꢀ ðCb;H2O2–Ca;H2O2Þ=Cb;H2O2
where Cb, H2O2 and Ca, H2O2 are the molar concentrations of H
2 2
O before
and after the reaction, respectively.
3
. Results and discussion
The gel beads had regular sizes, and their diameters were approx-
imately 3–5 mm, with a certain mechanical strength and elasticity
Fig. 1. FT-IR spectra of sodium alginate (A), Cu‐alginate gel beads (B) and Cu‐alginate
powder (C).

104
F. Shi et al. / Catalysis Communications 25 (2012) 102–105
The Cu (II) content of the products before and after phenol hydrox-
ylation was determined by ICP-AES (Table S1). The results showed that
the Cu (II) content of the Cu‐alginate dry beads and powder increased
2 2
with the increase in the CuCl concentration. When the CuCl concen-
tration increased from 0.1 to 0.3 mol/L, the Cu (II) content of the dry
beads and powder increased significantly (from 9.7% to 15.3% and
1
2
1.4% to 17.7%, respectively). However, when the CuCl concentration
exceeded 0.3 mol/L, no changes in the Cu (II) content in the Cu‐alginate
were apparent. This result should be attributed to the saturation of the
cross-linked products. To test if metal ions were leaching out from the
catalysts, which were analyzed using ICP-AES to detect the Cu (II)
content after once hydroxylation reaction. The results showed that a
trace amount of Cu was lost (The leaching amount of Cu ions was
lower than 3 wt.%.), which accounts for the recoverability and reusability
of the Cu‐alginate catalysts.
The external morphology of the powder and dry beads was examined
by SEM (Fig. 2). As shown in Fig. 2A and B, the powder particles were
present in a bulky form, and their sizes were at the micron-scale level.
Some floccules and stratified structures were found on the surface of
the blocks. Fig. 2C and D showed that the dry beads had a rough surface,
on which some sagging was noted. Compared with the gel beads, the dry
beads were more compact, which enhanced the mechanical stability of
the dry beads. The SEM images of the Cu‐alginate dry beads after reacting
three times were shown in Fig. 2E and F. From these figures, it could be
determined that after washing with ethanol, the surface of the dry
beads was covered by tarry by-products of the hydroxylation reaction,
Fig. 3. Mapping EDAX analysis of Cu element for Cu‐ALG dry beads before (A) and after
B) phenol hydroxylation reaction.
(
that after hydroxylation and without any washing, the amount of Cu
ions on the surface of the catalyst reduced a lot. Combined with the
results of the ICP-AES, the reduction of the Cu ions should be attributed
to the coverage of the catalysts' surface.
The spectrum of dry beads by using elemental microprobe analysis
of EDX is illustrated in Fig. S2. From these figures, it could be found
that the wt.% of Cu ions on the dry beads surface increased from
27.37% to 35.69% with the concentration of CuCl
0.1 to 0.4 mol/L, which indicated that the active sites increased with
the CuCl concentration increasing [24].
2
increasing from
2
To investigate the specific surface area and porosity of the Cu‐alginate
dry beads and powder, a BET test was performed. The results of nitrogen
3
2
sorption (b1 cm /g) and surface area (b1 m /g), calculated by the BET
method, indicated that the dry beads and powder have no pores with
those small dimensions [25]. Thus, the sags on the surface were likely
caused by the depressions of the surface, which occurred during the
drying phase.
which reduced the accessibility of the active sites for H
23]. This should be one of the reasons that caused the activities' reducing
of the catalyst.
2 2
O and phenol
[
Besides, the elemental concentration distribution and the quanti-
tative analyses of the dry beads can be analyzed using the analysis
of SEM/EDX. The Cu ions distribution mapping of EDX analysis for
the catalyst before and after hydroxylation is illustrated in Fig. 3A
and B. The bright white points in Fig. 3A and B represented the signal
of copper element on the surface of the dry beads. The result showed
Phenol hydroxylation catalyzed by copper has long been a subject
of study. However, no reports have been published with Cu‐alginate
as the catalyst for this reaction. The catalytic performance of various
Cu‐alginate catalysts in phenol hydroxylation was listed in Table 1.
The result of a blank experiment under the same reaction conditions
2 2
showed that in the absence of the catalyst, H O alone was unable to
oxidize phenol to a significant extent (only 5.3%, entry 1). After the
catalyst was added, an obvious increase in conversion was observed.
For both the dry beads and powder, the conversion increased as the Cu
(II) content increases (entries 2–9). Additionally, the turnover frequency
(TOF: moles of reacted substrate over moles of Cu per hour) was also
employed to evaluate the reaction rate. With regard to the dry beads,
when the Cu (II) content increased from 9.7% to 15.3% (entries 2–4),
the TOF value changed little, while the phenol conversion and H O
2 2
conversion increased from 33.1% to 52.7% and 88.2% to 96.7%, respective-
ly, which is higher than the value reported in the literature [26,27]. It
is believed that a large number of copper active sites increased the
Table 1
Catalytic performance of Cu‐alginate for phenol hydroxylation.
Entry
Catalyst
X
(
phenol
X
Selectivity/%c
TOF
%)a
(h−1)d
H2O2
%)b
CAT
HQ
BQ
(
1
2
3
4
5
6
7
8
9
Blank
5.3
61.4
88.2
94.0
96.7
96.8
90.2
95.3
99.4
59.1
58.9
57.1
59.5
57.4
57.1
57.3
56.5
57.4
38.8
38.2
40.0
38.4
40.6
40.2
40.3
41.4
40.4
2.1
2.9
2.9
2.1
2.0
2.7
2.4
2.1
2.2
–
Cu‐ALG-d-9.7
Cu‐ALG-d-14.1
Cu‐ALG-d-15.3
Cu‐ALG-d-15.5
Cu‐ALG-p-11.4
Cu‐ALG-p-14.9
Cu‐ALG-p-17.7
Cu‐ALG-p-18.1
33.1
48.6
52.7
52.9
39.2
51.4
61.2
62.5
2.10
2.13
2.14
2.13
2.12
2.14
2.15
2.15
100.0
a
All reactions were performed under the following conditions: phenol 1.0 g, catalyst
molar ratio 1:2, solvent (water) 30 mL, temperature 70 °C, time 2 h.
H2O2 (%)=100×(Cb, H2O2−Ca, H2O2)/Cb, H2O2
Product distribution given on a tar-free basis.
5
0 mg, phenol/H
O
2 2
b
c
X
.
Fig. 2. SEM images of Cu‐alginate powder (A and B), Cu‐alginate dry beads before (C and
D) and after (E and F) phenol hydroxylation.
d
TOF: moles of substrate converted per mole of metal (in the catalyst) per hour.

F. Shi et al. / Catalysis Communications 25 (2012) 102–105
105
reusability of Cu‐ALG-d-15.3 indicate that after simple treatment, the
catalyst retained the majority of its initial activity.
Acknowledgments
The authors gratefully acknowledge the Centre of Analysis and
Testing of Northeast Normal University for providing the ICP-AES
analysis and Changchun University of Technology for all of the support
that was provided.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1
016/j.catcom.2012.04.008.
Fig. 4. Catalytic performance of Cu‐ALG-d-15.3 in the phenol hydroxylation reaction.
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A series of ion‐cross-linked Cu‐alginate dry beads and powder
were synthesized through ion exchange, followed by the coordina-
tion of alginate and Cu (II) ions. The catalysts exhibited outstanding
catalytic performance for phenol hydroxylation with H O as an oxidant
2 2
in water. The catalytic activities were associated with the immobilized
content of the Cu (II) ions. The results of the experiment tested the
[
[