Photocatalytic hydroxylation of phenol with Fe-Al-silicate photocatalyst: A clean and highly selective synthesis of dihydroxybenzenes

Article abstract of DOI:10.1016/j.catcom.2011.03.012

Photocatalytic hydroxylation of phenol to catechol and hydroquinone in liquid phase over a novel heterogeneous photocatalyst Fe-Al-silicate was investigated with the assistance of UV irradiation at 365 nm at room temperature. The catalyst was characterize

Full text of DOI:10.1016/j.catcom.2011.03.012

Catalysis Communications 12 (2011) 1022–1026
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Catalysis Communications
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Short Communication
Photocatalytic hydroxylation of phenol with Fe–Al-silicate photocatalyst: A clean and
highly selective synthesis of dihydroxybenzenes
a
a,
a
a
a
b
Shi Huixian , Zhang Tianyong ⁎, Li Bin , Wang Xiao , He Meng , Qiu Mingyan
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
Department of Chemical and Environment Engineering, Anyang Institute of Technology, Anyang 455000, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic hydroxylation of phenol to catechol and hydroquinone in liquid phase over a novel
Received 13 January 2011
Received in revised form 7 March 2011
Accepted 11 March 2011
Available online 17 March 2011
heterogeneous photocatalyst Fe–Al-silicate was investigated with the assistance of UV irradiation at
3
65 nm at room temperature. The catalyst was characterized by BET, BJH, FT-IR, UV–vis DRS and XRD. The
effects of various parameters (types and amount of co-solvent, amount of catalyst, reaction time and amount
of H ) on photocatalytic hydroxylation of phenol were studied to explore the better reaction conditions. In
this study, phenol conversion could reach high up to 64.9%, with a total selectivity of 95% and the yield of
9.3%, 22.3% for catechol and hydroquinone, respectively.
2 2
O
Keywords:
Photocatalyst
Hydroxylation
Silicate
3
© 2011 Elsevier B.V. All rights reserved.
Phenol
Catechol
Hydroquinone
1
. Introduction
over CoNiAl ternary hydrotalcites, and the maximum conversion of
phenol was 14.2%. K.M. Parida et al. [19] developed the Fe(III)
salicylamide immobilized on MCM-41 for hydroxylation of phenol,
and the highest conversion can reach 64.6% at 80 °C. However, all of
the above reactions are thermocatalysis which requires high energy,
and accompanies with the generation of undesirable by-products.
Samia Azabou et al. [20] studied the liquid-phase photooxidation of
tyrosol to hydroxytyrosol via a new Fe–Al-montmorillonite photo-
catalyst, and the yield of hydroxytyrosol can reach 64.36%. In this study,
a new type of photocatalyst based on silicate (Fe–Al-silicate) was
prepared and characterized with a goal to effective utilizing in
photocatalytic organic synthesis of dihydroxybenzenes from phenol.
The catalyst showed high activity and selectivity on photocatalytic
phenol hydroxylation, which was then pursued under various para-
meters in attempts to find the optimal reaction conditions, as well as to
establish related fundamental studies for a future clean industrial
application.
The application of photocatalytic reactions to organic synthesis has
attracted interest because of recent developments in environmentally
benign synthetic processes. Notably, TiO photocatalytic reactions for
organic synthesis have recently been the subject of many reports [1–5].
It is widely accepted that •OH radicals can be generated on illuminated
2
nanometer TiO
2
surfaces, which are considered to be efficient electro-
as
phillic substitution agents. Furthermore, the processes using H
2 2
O
oxidant are considered to be the better foreground technique, due to the
easy acquirement of their feed, mild reaction condition, and water as
main by-product without the environmental pollution.
2 2
The selective and one-step hydroxylation of phenol by H O is an
attractive and challenging subject from economical and environmental
point of view. The dihydroxybenzenes viz. catechol and hydroquinone
are two important products of phenol hydroxylation, widely used as
photographical chemicals, antioxidants, polymerization inhibitors,
pesticides, flavoring agent and medicine [6].
The choice and reaction conditions of catalysts are very important
in the phenol hydroxylation using H
2
O
2
as oxidant. Several attempts
2. Experimental
were made for this reaction using heterogeneous catalysts [7–16]. K.
M. Parida [17] studied the phenol hydroxylation over molybdovana-
dophosphoric acid modified zirconia, and giving 49% conversion and
2.1. Preparation of the catalyst (Fe–Al-silicate)
6
1% selectivity. V. Rives et al. [18] studied the phenol hydroxylation
The silicate inorganic gel (the product brand is SMF-LV) is
provided by Fenghong New Material Company (China). Its general
formula is (Mg, Na, Li, F)
55–57)%; MgO, (23.5–25.0)%; Na
and F, (5–5.8)%.
3
Si
4
O
10(OH)
2
·nH
2
O, which consists of SiO
2
,
(
2
O, (2.8–3.8)%; Li
2
O, (1.2–1.5)%;
⁎
Corresponding author. Tel./fax: +86 22 27406610.
E-mail address: tyzhang@tju.edu.cn (T.Y. Zhang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.03.012

H. Shi et al. / Catalysis Communications 12 (2011) 1022–1026
1023
Fe–Al-silicate was readily obtained by ion adsorption process
according to the following procedure. 1.22 g of FeCl ·6H O and 0.2 g
of AlCl (molFe3+/molAl3+ =3:1) were mixed into 100 mL of H O with
3. Results and discussion
3
2
3
2
3.1. Characterization of the catalyst
stirring until absolutely dissolved. 1.5 g of silicate was added to the
above solution. Then 0.2 mol/L of NaOH (50 mL) solution was slowly
added to the solution at 70 °C under stirring. After being aged for 24 h,
the catalyst was washed with distilled water until total elimination of
The XRD of Fe–Al-silicate, silicate and PDF 39-1425 are shown in
Fig. 1. The XRD patterns of the prepared photocatalyst give some
typical diffractions at 2θ=22°, 28.2°, 31.3°, and 36.1°, which are
identical to the typical pattern characteristic of cristobalite-like phase
(PDF 39-1425). In addition, the XRD patterns of the catalyst are
similar with the raw material silicate, which can infer that the main
structure of the catalyst is not changed.
−
Cl , then dried in the oven at 60 °C and finally calcined at 500 °C for
5
h.
The sample Fe–Al was prepared by the same process above, but no
silicate addition for the contrast test.
The catalyst is characterized using FT-IR with the goal to further
investigate Fe3 and Al ions incorporated into the silicate. The FT-IR
+
3+
2
.2. Characteristic of the catalyst
spectra of Fe–Al-silicate and silicate after calcination are shown in
−
1
To determine the crystal phase composition of the Fe–Al-silicate,
Fig. 2. The sharp and strength peak at 1060 cm
is the silicate
−
1
we carried out X-ray diffraction (XRD) measurements using a
PANAalytical X'pert Por X-ray diffractometer with Cu Kα radiation.
The diffraction spectrum was recorded in the 2θ range of 10–90° in
steps of 0.017°. FT-IR spectra were recorded on NICOLET 380
Thermo) using the KBr pellet technique. The specific surface area
BET method), and average pore diameter (BJH method) of the
catalysts were determined by nitrogen adsorption–desorption iso-
therms using Quantachrome NOVA-2000 sorption analyzer, and the
samples were analyzed at 77 K by nitrogen adsorption–desorption.
The UV–vis diffuse reflectance spectra (UV–vis DRS) were performed
with a Lambda 900 UV–vis spectrophotometer (Perkin-Elmer Co.).
characteristic peak. For Fe–Al-silicate, the peaks at 474 cm
and
are from the M–OH stretching and M–O–M′ bending
−
1
796 cm
modes, wherein M and M′ represent the Fe and Al elements. So we
preliminarily infer that the metal elements are incorporated into the
silicate. But the main structure of the catalyst was not changed
compared with the silicate, which is consistent with the XRD analysis
results.
(
(
The N
2
adsorption–desorption isotherm and the pore size
distribution curve measured from the adsorption branch of nitrogen
isotherm by BJH method (inset) of the catalyst are shown in Fig. 3. The
composite exhibits mesoporosity with an average pore size of
2
−1
approximately 9 nm and the BET specific surface area is 90 m g
2
.3. Photocatalytic hydroxylation of phenol and product analysis
when calcined for 5 h at 500 °C. The large specific surface area
confirms that the framework of the catalyst has high adsorption
ability.
The photocatalytic hydroxylation of phenol by H was carried
2 2
O
out in a standard photocatalytic reactor which consists of two parts: a
pyrex glass cylindrical reactor (160 mL) with an outer jacket and a
To investigate the optical absorption properties of the catalyst,
the UV–vis DRS of the catalyst is measured and the result is shown
in Fig. 4. The band gap of Fe–Al-silicate is 3.1 eV, so the absorption
edge wavelength (λ ) of the catalyst is 400 nm matched with the
g
lamp emission (365 nm). It is a benefit for the photocatalytic
1
25 W high pressure Hg lamp (main emission wavelength is 365 nm)
placed parallel to the reactor as a light resource. The following
solvents and reagents were used as purchased: phenol, H (30%),
2
O
2
distilled water, and co-solvents (acetonitrile, t-butyl alcohol, acetone,
acetic acid and methanol).
reaction.
All the experiments were conducted at ambient pressure and the
reaction temperature was kept at 25± 1 °C by the continuous
circulation of water through the jacket around the reactor. The typical
procedure follows. To a suspension of 0.1 g Fe–Al-silicate powder and
3.2. Preliminary experiments
Fe–Al-silicate can catalyze the phenol to catechol and hydroqui-
none in the presence of H O with the assistance of UV irradiation at
2 2
0
.5 g phenol in 15 mL distilled water, and then added 4 mL
the wavelength 365 nm. This oxidation relies on the generation of the
acetonitrile as co-solvent, the reaction system was stirred until well
powerful oxidant •OH radicals as H O reacts with a conduction band
electron (Eq. 1) and the redox process in presence of iron ions (Eqs. 2
2
2
mixed. Then 1 mL of H
reaction.
2
O
2
was added to the suspension to start the
The samples were collected at regular intervals during illumina-
tion. Phenol, catechol, and hydroquinone were analyzed with GC
(
Agilent 6890N) and GC–MS (gas chromatography–mass spectrom-
etry; Hiden HPR20-QIC).
The term of reaction performance was defined as follows:
mole of phenol reacted
initial mole of phenol
Conversion of phenol ¼
× 100%
mole of catechol produced
initial mole of phenol
Yield of catechol ¼
× 100%
mole of hydroquinone produced
initial mole of phenol
Yield of hydroquinone ¼
× 100%
yield of catechol þ yield of hydroquinone
Fig. 1. XRD patternsof Fe–Al-silicate, silicate calcinedat 500 °Cfor 5 h and the PDF 39-1425
(inset).
Selectivity ¼
× 100%:
conversion of phenol

1024
H. Shi et al. / Catalysis Communications 12 (2011) 1022–1026
1
0
0
0
0
.0
.8
.6
.4
.2
silicate
Fe-Al-silicate
2
00
300
400
500
600
700
800
4
000
3500
3000
2500
2000
1500
1000
500
Wavemumbers / cm-1
Wavelength (nm)
Fig. 4. UV–vis DRS of Fe–Al-silicate calcined at 500 °C for 5 h.
Fig. 2. FT-IR spectra of Fe–Al-silicate and silicate calcined at 500 °C for 5 h.
OH
OH
OH
OH
and 3), then phenol can be directly converted into catechol and
hydroquinone by reacting with •OH (Eqs. 4 and 5).
OH
OH
OH
H
OH
or
H
2
O
2
or
·OH
2
H O
−
−
:
H O þ e → OH þ OH
ð1Þ
ð2Þ
ð3Þ
2
2
cb
H
ð5Þ
−
þ Fe3þ→Fe2þ
Blank controls (without catalyst) and dark controls (without UV
irradiation) were run, and no significant catechol and hydroquinone
were observed (Table 1). It should be noted that, in the absence of UV
irradiation, phenol was not converted to catechol and hydroquinone
even with adding excessive H O .
e
cb
2
þ
3þ
−
:
Fe þ H O →Fe þ OH þ OH
2
2
2
2
We also studied the phtotocatalytic hydroxylation reaction of
phenol with the Fe–Al sample, but it has no photocatalytic activity
with very low conversion, no catechol and hydroquinone production.
So the silicate plays an important role for the Fe–Al-silicate in the
photocatalytic reaction.
OH
OH
OH
H
OH
ð4Þ
3.3. The effect of co-solvent
·OH
Selected solvents such acetonitrile, acetic acid, acetone, t-butyl
alcohol and methanol asthe co-solventwere studied respectively for the
photocatalytic hydroxylation of phenol, and the results were shown in
Table 2. When t-butyl alcohol and acetone were used as co-solvent, only
a trace amount of catechol and hydroquinone was detected. Only 8.3%
and 5% of phenol was converted, no significant catechol and hydro-
quinone was observed when acetic acid and methanol were the co-
solvent, indicating that acetic acid and methanol surrounding was
unfavorable for the reaction. The promotion effect of the co-solvent for
the hydroxylation of phenol can be rated according to the following:
acetonitrileNnoneNt-butyl alcoholNacetone.
H
OH
Acetonitrile was a typical solvent in the hydroxylation reaction by
thermocatalysis. Table 3 lists the terms of phenol hydroxylation
performance with the effects of acetonitrile as a co-solvent. The
0
10
20
30
40
50
60
70
80
90
Pore Diameter / nm
Table 1
a
Hydroxylation of phenol under different conditions.
Entry
Catalyst (g)
UV
Phenol conversion
%)
Yield (%)
Selectivity
(%)
(
CAT
HQ
0
.0
0.2
0.4
0.6
0.8
1.0
1
2
No
0.1
Yes
No
31.7
8.0
3.4
N.D
N.D
N.D
10.7
–
Relative Pressure (P/P0)
CAT: catechol; HQ: hydroquinone; N.D: no detected.
a
Fig. 3. N
2
adsorption–desorption isotherms and the pore size distribution curve
Phenol, 0.5 g; water, 15.0 mL; actetonitrile, 4.0 mL; H
4.0 h.
2 2
O , 1.0 mL; reaction time,
calculated from the adsorption branch of nitrogen isotherm by BJH method (inset).

H. Shi et al. / Catalysis Communications 12 (2011) 1022–1026
1025
Table 2
Table 4
a
a
Effect of co-solvents on hydroxylation of phenol.
Time dependence of phenol hydroxylation reaction.
Co-solvent
Phenol conversion
%)
Yield (%)
CAT
Selectivity
(%)
Reaction
time (h)
Phenol conversion
(%)
Yield (%)
CAT
Selectivity
(%)
(
HQ
HQ
CAT/HQ
None
72.0
64.9
67.4
57.9
8.3
24.5
39.3
3.6
1.3
N.D
N.D
13.6
22.3
1.9
0.8
N.D
N.D
52.9
95.0
8.2
3.6
–
1
2
3
4
5
6
33.0
46.0
66.5
64.9
66.9
72.3
3.5
7.8
26.4
39.3
33.4
11.5
2.1
1.67
1.63
1.60
1.76
1.69
1.69
17.0
27.4
64.5
95.0
79.5
25.3
Acetonitrile
t-butyl alcohol
Acetone
Acetic acid
Methanol
4.8
16.5
22.3
19.8
6.8
5.0
–
a
a
Phenol, 0.5 g; catalyst, 0.1 g; water, 15.0 mL; co-solvent, 4.0 mL; H
2 2
O , 1.0 mL; UV
2 2
Phenol, 0.5 g; catalyst, 0.1 g; water, 15.0 mL; acetonitrile, 4.0 mL; H O , 1.0 mL.
time, 4.0 h.
product yield and selectivity decreased gradually after 4 h due to the
increase of byproducts in the system, which was proposed to be caused
by oxidative degradation and mineralization [21,22].
It should be noted that the phenol conversion after 4 h of reaction
(64.9%), in Table 4, was found to be slightly lower than that after 3 h of
reaction (66.5%). It is additional evidence of reductive back reaction
which happened while HCHD radical meets a conduction band (CB)
electron and decreased conversion by regeneration of feed-in phenol
[23].
addition of acetonitrile up to 4 mL slightly increased the yield, the
conversion of phenol has no obvious changes, but any further addition
gradually reduced the conversion and yield. Therefore, 4 mL of solvent
was used throughout the remainder of this work. The solvent effect of
acetonitrile as co-solvent was in agreement with the results obtained
by Park et al. [1], they studied the hydroxylation of benzene with
2
modified TiO photocatalyst and achieved the best yield of phenol
with usage of 4 vol.% acetonitrile as co-solvent. Nevertheless,
additional introduction of acetonitrile also lead to gradual reduction
of yield, which is consistent with our findings in this study. The
photocatalytic hydroxylation of phenol by H
2
O
2
happened through a
2 2
3.5. Effect of the amount of H O
hydroxycyclohexadienly (HCHD) radical intermediate which formed
by attacks of phenol with •OH radical. Then hydrogen atom
Table 5 shows that the yield of catechol and hydroquinone can reach
up to 39.3% and 22.3%, respectively, as H concentration increases.
This could be due to the increase in hydroxyl radical concentration upon
photolysis of H (Eq. 1). A further increase of H concentration
higher than 1 mL decreases the catechol and hydroquinone production;
it was difficult to control the selective oxidation to produce catechol and
hydroquinone at a high oxidant concentration. The phenol conversion
was increased by a high concentration of oxidant but it decreased the
abstraction of HCHD radical by oxidants (H
2
O
2
) gives catechol and
2 2
O
hydroquinone subsequently (see Eqs. 4 and 5). The co-solvent of
acetonitrile shared the properties of electron acceptor as •OH radical
scavenger. Thus, both acetonitrile and phenol compete for •OH radical,
which induced the reduction of both conversion and yield by
decreasing the amount of •OH at the high concentration of
acetonitrile. Nevertheless, herein we found acetonitrile effectively
improve the reaction performance of photocatalytic hydroxylation of
phenol with proper dosage in the presence of acetonitrile as a co-
solvent because it can prevent the phenol in water from volatilization
during the reaction [1].
O
2 2
2 2
O
2 2
selectivity. H O is so reactive and non-selective that oxidative
degradation and mineralization are prevalent. Therefore, minimizing
unwanted pathways such as oxidative degradation is important to
achieve selective hydroxylation.
3
.4. Effect of reaction time
3.6. Effect of catalyst amount
Table 4 shows the influence of reaction time on the phenol
The influence of the catalyst loading on the conversion, yield and
selectivity is illustrated in Table 6. The conversion of phenol increased
when continuously increasing the amount of catalyst, but the yield
and selectivity decreased when the catalyst loading was increased to
0.15 g. The active catalytic sites may be considered to have respon-
sibility for these phenomena. The oxidation of the organic species
adsorbed on the catalyst active sites may be associated with the delays
for short reaction time, and the phenol was easy to be oxidized to
other products. Therefore, the yield did not increase as adding more
catalyst. 0.1 g of the catalyst was the optimum amount for
photocatalytic hydroxylation of phenol in this case.
hydroxylation reaction. The reaction occurs similar to Fenton
chemistry, through the participation of •OH in activating phenol
toward the formation of catechol and hydroquinone accumulation.
Longer reaction time directly increased the phenol conversion. During
the first 4 h of reaction, accumulation of catechol and hydroquinone
was concomitant with the removal of phenol. The yield of catechol and
hydroquinone can reach 39.3% and 22.3%, respectively, the highest
yields found in this study. Meanwhile, the selectivity can reach 95%.
However, the reaction performance data in Table 4 indicated that the
Table 3
a
Effect of the acetonitrile amount.
Table 5
Effect of H
amount.a
Phenol conversion
2
O
2
Acetonitrile
mL)
Phenol conversion
(%)
Yield (%)
CAT
Selectivity
(%)
(
H
2
O
2
Yield (%)
CAT
Selectivity
(%)
HQ
5.2
12.4
13.1
22.3
20.8
15.5
(
mL)
(%)
HQ
9.8
14.8
22.3
12.6
11.9
1
2
3
4
5
6
67.6
69.5
65.6
64.9
53.7
51.2
14.9
22.0
21.3
39.3
29.1
24.8
29.7
49.5
52.4
95.0
92.9
78.8
0.5
0.8
1.0
1.2
2.0
65.1
66.2
64.9
65.1
81.3
15.7
20.4
39.3
22.3
23.0
39.2
53.2
95.0
53.6
42.9
a
a
Phenol, 0.5 g; catalyst, 0.1 g; water, 15.0 mL; H
2
O
2
, 1.0 mL; UV time, 4.0 h.
Phenol, 0.5 g; catalyst, 0.1 g; water, 15.0 mL; acetonitrile, 4.0 mL; UV time, 4.0 h.

1026
H. Shi et al. / Catalysis Communications 12 (2011) 1022–1026
Table 6
Effect of catalyst loading.
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