Photocatalytic conversion of naphthalene to α-naphthol using nanometer-sized TiO2

Article abstract of DOI:10.1016/S1872-2067(10)60158-1

The effects of various parameters (co-solvents, electron acceptors, and surface modification) on the direct synthesis of α-naphthol from naphthalene using photocatalytic processes were investigated. The OH radicals generated on UV-illuminated TiO₂ photocatalysts led to the direct hydroxylation of naphthalene to α-naphthol. The addition of Fe ³⁺, Fe²⁺, Fe³⁺ + H₂O₂, and Fe²⁺ + H₂O₂ greatly increases the conversion of naphthalene and the yield of α-naphthol in TiO₂ suspensions. The addition of Fe³⁺ + H₂O₂ to a TiO₂ suspension increased the yield to 22.2. Surface modified-TiO₂ had a significant influence on the hydroxylation reaction. La-Eu/TiO₂, La-Y/TiO₂, H₃PW ₁₂O₄₀/TiO₂, H₃PMo₁₂O ₄₀/TiO₂, Fe/TiO₂, Ag/TiO₂, Cu/TiO₂, and N/TiO₂ enhanced the conversion and yield more than TiO₂. Fe/TiO₂ has the highest photocatalytic efficiency among these species.

Full text of DOI:10.1016/S1872-2067(10)60158-1

CHINESE JOURNAL OF CATALYSIS
Volume 32, Issue 1, 2011
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2011, 32: 46–50.
RESEARCH PAPER
Photocatalytic Conversion of Naphthalene to Ơ-Naphthol
Using Nanometer-Sized TiO2
1
1,2,
3
1
1
SHI Huixian , ZHANG Tianyong *, WANG Hongliang , WANG Xiao , HE Meng
1
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
2
State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University, Fuzhou 350002, Fujian, China
3
School of Pharmaceutical Science and Technology, Tianjin University, Tianjin 300072, China
Abstract: The effects of various parameters (co-solvents, electron acceptors, and surface modification) on the direct synthesis of Į-naphthol
from naphthalene using photocatalytic processes were investigated. The •OH radicals generated on UV-illuminated TiO photocatalysts led
greatly increases the
suspension increased the
, La-Y/TiO , H
2
enhanced the conversion and yield more than TiO . Fe/TiO has the highest
2
3
+
2+
3+
2+
to the direct hydroxylation of naphthalene to Į-naphthol. The addition of Fe , Fe , Fe + H
2
+
O
2
, and Fe + H
suspensions. The addition of Fe + H to a TiO
had a significant influence on the hydroxylation reaction. La-Eu/TiO
, Cu/TiO , and N/TiO
photocatalytic efficiency among these species.
2 2
O
3
conversion of naphthalene and the yield of Į-naphthol in TiO
yield to 22.2%. Surface modified-TiO
, Fe/TiO , Ag/TiO
2
2
O
2
2
2
2
2
3 2
PW12O40/TiO ,
H
3
PMo12
O
40/TiO
2
2
2
2
2
2
Key words: titanium dioxide; modification; photocatalysis; naphthalene; Į-naphthol
Į-Naphthol is considered to be an important organic
chemical and pharmaceutical intermediate because of its wide
use as a disinfectant, a rubber auxiliary, and a dye auxiliary.
The traditional production of Į-naphthol requires a multistage
process and a large amount of energy, and undesirable
by-products are generated. It is necessary to develop a new
synthetic process for Į-naphthol that is more efficient and
environmentally friendly. Thus, the direct hydroxylation of
naphthalene to Į-naphthol is a most challenging task.
the photocatalytic reactions of surface modified TiO are often
2
different from those of pure TiO [12–16].
2
Most studies focus on the hydroxylation of benzene and its
derivatives by photocatalytic reactions [1–3,6,8]. The reported
yield and selectivity for phenol production from benzene were
0.5% and 80% after TiO photocatalysis [17], respectively.
2
Tanarungsun et al. [8] studied the liquid phase hydroxylation of
benzene to phenol on Fe/TiO catalysts loaded with different
2
types of secondary metals and they found that doping with 1
Some methods are available for the direct hydroxylation of
aromatic organics such as oxidation with H O , the Fenton
wt% Pt or Pd on Fe/TiO improved the phenol yield noticeably.
2
Although their conversion efficiency was still low, the organic
synthesis process that they used was energy-efficient and en-
vironmentally benign. However, only a few reports exist about
the direct hydroxylation of naphthalene to Į-naphthol by sur-
2
2
process, and semiconductor photocatalysis [1–11]. Among
them, TiO photocatalytic reactions for organic synthesis have
2
recently been the subject of many reports [6–11]. It is widely
accepted that •OH radicals can be generated on illuminated
face modified TiO nanoparticles using photocatalysis. We thus
2
nanometer TiO surfaces. Hydroxyl radicals are considered to
investigated the hydroxylation reactions of naphthalene using
2
be efficient electrophilic substitution agents. Therefore, TiO2
photocatalysis has been used for the hydroxylation of aromatic
compounds. Surface modifications of TiO2 should change
reaction yields and selectivity. The kinetics and mechanisms of
several photocatalytic systems with TiO and surface-modified
2
TiO as heterogeneous photocatalysts. The effect of various
2
solvents (acetonitrile, acetone, acetic acid) and electron ac-
2
+
3+
ceptors such as Fe , Fe , H O , (NH ) S O , and KBrO was
2
2
4 2
2
8
3
Received 28 June 2010. Accepted 24 August 2010.
*
Corresponding author. Tel/Fax: +86-22-27406610; E-mail: tyzhang@tju.edu.cn
Foundation item: Supported by the State Key Laboratory Breeding Base of Photocatalysis, Fuzhou University (K-081027).
Copyright © 2011, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60158-1

SHI Huixian et al. / Chinese Journal of Catalysis, 2011, 32: 46–50
investigated to determine the optimal conditions for Į-naphthol
Agilent 6890N).
synthesis. Surface modified TiO2 such as La-Eu/TiO2,
La-Y/TiO2, H PW O /TiO , H PMo O /TiO , Fe/TiO2,
The reaction performance was defined as follows: conver-
sion of naphthalene = moles of naphthalene reacted/initial
moles of naphthalene; yield of Į-naphthol = moles of
Į-naphthol produced/moles of naphthalene reacted.
3
12 40
2
3
12 40
2
Ag/TiO , Cu/TiO , and N/TiO were also used to investigate
2
2
2
how the surface properties affect the hydroxylation reaction.
This study mainly focused on the feasibility of the hydroxyla-
tion of naphthalene and should aid in the choice of a photo-
catalyst. The effect of catalyst characteristics will be studied in
further work.
2 Results and discussion
2.1 The effect of cosolvents
1
Experimental
Acetonitrile, acetic acid, and acetone were used as the
co-solvents for these reactions and the results are shown in
Table 1. When acetic acid and acetone were used as co-solvents
only a trace mount of Į-naphthol was detected. The reaction
media for the hydroxylation could be rated according to the
order acetonitrile > acetic acid > acetone.
1
.1 Preparation of the modified catalysts
Catalysts were prepared by the sol-gel process according to
4
the following procedure. Solution A, 34 ml Ti(OBu) (AR,
Kewei Chemical Reagents Company, Tianjin, China) was
dissolved in 44 ml absolute ethanol with stirring for several
minutes and then 3 ml acetic acid was added as a hydrolysis
suppressant. Solution B containing 44 ml absolute ethanol, 7.2
Figure 1 shows the effect of acetonitrile as a cosolvent on the
production of Į-naphthol. The addition of acetonitrile at up to
20 ml slightly increased the conversion and yield but any fur-
ther addition gradually reduced the conversion and yield.
Therefore, 20 ml of solvent per experiment was used
throughout the remainder of this work.
ml H O, and nitrates or polyoxometalate (AR, Kewei Chemical
2
Reagents Company, Tianjin, China) in the required
stoichiometry were slowly added to solution A. The mixture
was hydrolyzed at 25 ºC for 30 min under agitation and a
transparent sol was obtained. Gelation was complete after
aging the sol for 24 h at room temperature. The gel was dried at
Naphthalene can be directly converted into Į-naphthol by
reaction with •OH (Eq. (1)):
OH
H OH
[O]
·
OH
(1)
6
0 ºC, ground to a powder and calcined for 3 h at 500 ºC.
La-Eu/TiO , La-Y/TiO , H PW O /TiO , H PMo O /TiO ,
2
2
3
12 40
2
3
12 40
2
Fe-TiO , Cu-TiO , Ag-TiO , and N/TiO were prepared ac-
•OH attacks naphthalene to produce a hydroxycyclo-
hexadienly (HCHD) radical [6], which subsequently undergoes
H-atom abstraction by oxidants and gives Į-naphthol. Al-
2
2
2
2
cording to this process. SEM (scanning electron microscopy,
TESCAN VEGA TS-5130SB Czech) images of the catalysts
showed that the samples have a relative uniform particle di-
ameter distribution in a range of 80ꢀ90 nm.
2
Table 1 Hydroxylation of naphthalene using illuminated TiO and
various solvents
Solvent
Conversion (%)
Yield (%)
5.1
1
.2 Photocatalytic synthesis and product analysis
Acetic acid
Acetone
5.3
4.8
7.1
The photocatalytic conversion reactions of naphthalene were
Acetonitrile
19.5
22.2
carried out in an aqueous catalyst suspension and in a solvent.
2
TiO (P-25 from Degussa Co., BET surface area 50 m /g, mean
2
25
20
15
10
5
0
25
diameter 30 nm, 87% anatase and 13% rutile) or surface
Conversion
Yield
modified TiO powder (20 mg) was suspended in 30 ml of
2
20
15
10
5
distilled water and then 20 ml of cosolvent containing 0.5 g
naphthalene was added to the suspension and mixed well.
Depending on the experiment we then added electron donors,
acceptors, or other reagents to the suspension. The photocata-
lytic reactor consists of a 160 ml Pyrex glass bottle with an
outer jacket and a 125 W high pressure Hg lamp as a light
resource that was placed parallel to the reactor. In all the ex-
periments, the reaction temperature was kept at (25±1) ºC by
the continuous circulation of water through the jacket around
the reactor. The samples were collected at regular intervals
during illumination. Naphthalene, Į-naphthol, and other in-
termediates were analyzed with GC (gas chromatography,
0
0
5
10 15 20 25 30 35 40 45 50 55
Acetonitrile amount (ml)
Fig. 1. Effect of the acetonitrile co-solvent amount on the conversion of
naphthalene and the yield of Į-naphthol.

SHI Huixian et al. / Chinese Journal of Catalysis, 2011, 32: 46–50
H OH
OH
3
though the presence of CH CN as a co-solvent can prevent the
naphthalene in water from volatilizing during the reaction it is
also a •OH scavenger. Therefore, the high concentration of
Fe3+
Fe2+ H+ (4)
CH CN can decrease the amount of •OH and thus reduce the
3
conversion and yield [6]. In addition, water is an essential
precursor of •OH because its production cannot be sustained in
a non-aqueous environment. Therefore, the conversion and
yield in high concentrations of acetonitrile were low.
2
.3 Effect of TiO
2
surface modification
2
Table 3 summarizes the effects of TiO surface modification
on the hydroxylation of naphthalene. Simultaneously doping
TiO with two kinds of rare earth ions can enhance the photo-
2
2
.2 Effect of electron acceptors
catalytic reactivity significantly compared with undoped TiO2.
The data listed in Table 3 shows that the conversion of naph-
thalene increased rapidly although the yield of Į-naphthol was
The type of electron acceptor available in the TiO suspen-
2
sion is important in determining yield because a reductive
electron transfer and reactions of •OH radicals are involved in
the overall reaction mechanism. Conduction band (CB) elec-
tron transfer to electron acceptors should improve the reaction
process because of a retarded charge pair recombination.
Therefore, the presence of suitable electron acceptors is es-
3
+
3+
not very high. The ionic radii of La (10.61 nm), Y (9.0 nm),
3
+
4+
and Eu (9.47 nm) are larger than that of Ti (6.05 nm).
3+
3+
3+
Therefore, it is difficult for La , Y , and Eu to enter the TiO
2
3
+
3+
lattice. This might be due to the substitution of La , Y , and
3
+
4+
Eu atoms in the lattices of La O , Y O , and Eu O by Ti .
2
3
2
3
2
3
4
+
Ti-O-La, Ti-O-Y, and Ti-O-Eu bands can be formed at the Ti
sential for photocatalytic oxidative reactions. In the TiO sus-
2
4+
3+
3+
3+
interface. If Ti replaces La , Y , and Eu a charge imbal-
ance would result. Therefore, the formation of Ti-O-La and
Ti-O-Eu bonds and the charge balance might affect the
photocatalytic activity of La-Eu/TiO and La-Y/TiO catalysts.
pension dissolved O2 serves as an electron acceptor. The
presence of additional electron acceptors may enhance the
conversion of naphthalene. Results from the hydroxylation of
2
2
naphthalene in a TiO suspension in the presence of additional
2
ꢀ
With a charge imbalance more OH would adsorb onto the
electron acceptors are shown in Table 2.
ꢀ
surface to balance the charge. These surface OH ions can
The addition of (NH ) S O and KBrO can increase the
4
2
2
8
3
accept the holes generated by light irradiation to form hydroxyl
radicals, which leads to the oxidation of adsorbed substrates
naphthalene conversion slightly but Į-naphthol was not de-
tected in the reaction products. However, the addition of H O
2
2
[
20]. Therefore, the surface state modification of the catalyst
significantly increased the conversion and yield compared with
TiO because H O generates additional •OH radicals as it
might result in an effective separation of electron-hole pairs.
As a homogenous photocatalyst, polyoxometalate has been
widely investigated for its high oxidizing power, which is
strong enough to oxidize a variety of organic compounds.
2
2
2
3
+
reacts with a conduction band electron (Eq. (2)) [18]. Fe has
also been reported to be a conduction band electron acceptor
(
[
Eq. (3)) and a good oxidant of HCHD radicals (Eq. (4))
Polyoxometalate is similar to the heterogeneous TiO photo-
3
+
2
19,20]. The addition of both Fe and H O to a TiO suspen-
2
2
2
catalyst in terms of light absorption and electrochemical band
sion increased the conversion and yield even more compared
edge positions [7,19,21,22]. As shown in Table 3, TiO modi-
3
+
2
with systems where only Fe was added or only H O was
2
2
fied with polyoxometalate greatly enhances the reaction proc-
ess and the conversion of naphthalene increased about 20 times
3+
added. The addition of Fe + H O to a TiO suspension in-
2
2
2
creased the yield to 22.2%, which was the highest yield ob-
tained in this study mainly because of the photo-Fenton reac-
tion.
compared to that for pure TiO . Additionally, the yield of
2
Į-naphthol increased and this can be attributed to the dual roles
of polyoxometalate as both a photocatalyst and an electron
shuttle [6].
–
–
H O + e ĺ •OH + OH
(2)
(3)
2
2
cb
–
3+
2+
e
cb + Fe ĺ Fe
2
Table 3 Hydroxylation of naphthalene in a surface-modified TiO sus-
Table 2 Hydroxylation of naphthalene in illuminated TiO
2
with various
pension
electron acceptors
Catalyst
Conversion (%)
Yield (%)
System
Conversion (%)
Yield (%)
not detected
10.2
TiO2
2.3
41
not detected
TiO
TiO
TiO
TiO
TiO
TiO
TiO
TiO
2
2
2
2
2
2
2
2
2.3
20.5
43.1
89.5
7.3
La-Y/TiO2
La-Eu/TiO2
H PW O /TiO
2
2.9
4.1
2.3
2.8
5.6
5.5
6.6
4.9
+ H
2
O
2
46
2
+
+ Fe + H
2
O
2
2
8
5.3
35
3
12 40
3
+
+ Fe + H
+ (NH
+ KBrO
2
O
22.2
H PMo O /TiO
3 2
47
12 40
4
)
2
S
2
O
not detected
not detected
7.6
Fe/TiO2
Ag/TiO2
Cu/TiO2
N/TiO2
50
3
5.6
6.5
11.4
4.9
3
+
+ Fe
+ Fe
17.0
25.3
2
+
9.1

SHI Huixian et al. / Chinese Journal of Catalysis, 2011, 32: 46–50
Metal ions that deposit onto the TiO
2
surface often enhance
catalytic synthesis was carried out at ambient temperature and
at atmospheric pressure and no complex equipment was used,
which make this process energy-efficient and environmentally
benign.
photocatalytic activity because they trap conduction band
electrons by reducing the charge pair recombination and pro-
moting interfacial electron transfer [13,16]. Fe/TiO is the most
2
effective among those photocatalysts for this conversion and it
can reach 50% but the yield was only 5.6% mainly because the
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