Photocatalytic reductive dechlorination of chlorobenzene in alkali-free aqueous alcoholic suspensions of palladium-loaded titanium(iv) oxide particles in the absence or presence of oxygen

Article abstract of DOI:10.1039/c3ra40411k

Photocatalytic reductive dechlorination of chlorobenzene in aqueous 2-propanol suspensions of palladium-loaded titanium(iv) oxide (Pd-TiO ₂) particles was examined in a wide range of aerated and deaerated solutions with different water and NaOH

Full text of DOI:10.1039/c3ra40411k

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Photocatalytic reductive dechlorination of
chlorobenzene in alkali-free aqueous alcoholic
suspensions of palladium-loaded titanium(IV) oxide
particles in the absence or presence of oxygen
Cite this: RSC Advances, 2013, 3,
6
058
Hiroshi Kominami,* Takuya Nishi, Kojirou Fuku and Keiji Hashimoto
Photocatalytic reductive dechlorination of chlorobenzene in aqueous 2-propanol suspensions of
palladium-loaded titanium(IV) oxide (Pd–TiO
deaerated solutions with different water and NaOH contents. In water-free 2-propanol, the presence of
dissolved NaOH and removal of oxygen (O ) were critical for the dechlorination of chlorobenzene at a
2
) particles was examined in a wide range of aerated and
2
sufficient reaction rate and a redox balance (stoichiometry with the oxidation of 2-propanol) close to
unity. In 50 vol% water–2-propanol solvent, the yield of benzene under aerated conditions was the same
as that under deaerated conditions and was very close to the yield of acetone under aerated conditions,
indicating that photogenerated electrons were selectively used for the reduction of chlorobenzene in the
2 2
water–2-propanol solvent even in the presence of O . The low solubility of O in the water–2-propanol
solvents was attributed to the high selectivity of photocatalytic dechlorination. In 50 vol% water–2-
propanol solvent, NaOH was unnecessary when the reaction was performed under deaerated conditions,
Received 24th January 2013,
Accepted 18th February 2013
2
suggesting that water played a role in removing chloride ions from the reaction site (surface of TiO ) and
in suppressing the consumption of chloride ions by another side-reaction even in the absence of NaOH.
Under ideal conditions, i.e., reaction in the absence of NaOH under aerated conditions, benzene was
obtained in a 71% yield and a high material balance of benzene and chlorobenzene was recovered.
DOI: 10.1039/c3ra40411k
www.rsc.org/advances
Based on the above considerations, we have examined the
photocatalytic reductive dechlorination of chlorobenzene in a
1
. Introduction
Chlorinated aromatic compounds are organic waste products
which are harmful to the human body, and are generally
treated by incineration or biodegradation. Since these meth-
ods aim to mineralize the chlorinated aromatic compounds, a
2-propanol suspension of metal-loaded titanium(IV) oxide
(
TiO ) nanocrystals in the presence of dissolved NaOH and
2
reported that benzene and chloride ions (as insoluble NaCl)
were recovered quantitatively when palladium (Pd)-loaded
large amount of carbon dioxide (CO
2
) is produced. In addition,
15,16
TiO2 nanocrystals were used as the photocatalyst.
reaction is described in eqn (1).
The
it is not possible to recover and re-use the aromatic rings of the
original chlorinated aromatic compounds by using these
methods. Therefore, the recovery of aromatics by dechlorina-
tion of chlorinated aromatic compounds is one of the most
promising methods for the treatment of chlorinated com-
pounds from the viewpoints of the environment and chemical
resources. Previous studies have shown that the effective
removal of chloride ions from the reaction system is one of the
key processes to enable the quantitative recovery of aromatics
PhCl + (CH ) CHOH + NaOH A
3
)
2
PhH + (CH
3
2
2
CO + NaCl Q + H O
(1)
In terms of its practical applications, photocatalytic
dechlorination of chlorobenzene would be more attractive if
the reaction could be applied to high concentration solutions.
2
3
However, high concentration solutions, such as 0.2 mol dm
,
1
–5
cannot be used because it is difficult to increase the NaOH
from chlorinated aromatic compounds.
In photocatalytic
2
3
concentration to 0.2 mol dm due to the limited solubility of
NaOH in 2-propanol. One simple and plausible method to
increase the solubility of NaOH is to add water to 2-propanol.
In our previous paper, we reported that the addition of a small
amount of water (1–5 vol%) to 2-propanol caused a drastic
reactions, strong and selective electron injection into chlori-
nated aromatic compounds is also important to achieve
efficient dechlorination.
5
–14
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki
University, Kowakae, Higashiosaka, Osaka 577-8502, Japan.
1
6
decrease in the reaction rate and yield of benzene. It seems
that addition of water to 2-propanol has both positive and
E-mail: hiro@apch.kindai.ac.jp; Fax: +81-6-6727-2024; Tel: +81-6-6721-2332
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negative effects on the photocatalytic dechlorination of
chlorobenzene to benzene. In this study, we examined the
photocatalytic dechlorination of chlorobenzene in aqueous
capillary column (30 m 6 0.25 mm). The amount of acetone in
the liquid phase was determined with a Shimadzu GC-14A gas
chromatograph equipped with a fused silica capillary column
(HiCap-CBP20, 25 m 6 0.22 mm). Toluene was used as an
internal standard sample, and the amounts of benzene,
chlorobenzene and acetone were determined using the ratios
of the peak areas of benzene, chlorobenzene and acetone to
alcoholic suspensions of Pd–TiO particles with a wide range
2
of water contents and compared the results with those
obtained under water-free conditions. Here we report that
the reaction rate and benzene yield were improved in aqueous
alcoholic suspensions in the range 40–80 vol% water and that
the photocatalytic dechlorination of chlorobenzene to benzene
3
the peak area of toluene. The reaction solution (1 cm ) was
3
added to a diethyl ether–water mixture (2 : 1 v/v, 3 cm ). After
occurred even in the presence of oxygen (O
absence of NaOH in the 50 vol% water–2-propanol solvent.
2
) and in the
the mixture had been stirred for 10 min, the amounts of
benzene, chlorobenzene and acetone in the ether phase were
analyzed. The amount of chloride ions in the liquid phase was
determined with a Jasco PU-2800 plus ion chromatograph (IC)
equipped with an ICNI-424 column (Shodex, Japan).
2. Experimental
All of the reagents were commercial materials of reagent grade
and were used without further purification. Nanocrystalline
3
. Results and discussion
TiO
2
powder was prepared at 573 K using the HyCOM
1
7
method. Titanium(IV) butoxide was used as the starting
material and the product was calcined at 823 K. The HyCOM–
3.1 Effect of water content on the dechlorination of
chlorobenzene in the presence of NaOH under deaerated
conditions
TiO powder (148.5 mg) was suspended in an 80 vol% aqueous
2
solution of acetic acid containing palladium(II) acetate in a test
tube. The test tube was sealed with a rubber septum under Ar
and then photoirradiated at a wavelength, l . 300 nm using a
In our previous study, the effect of adding water in the
photocatalytic dechlorination of chlorobenzene to benzene
was investigated in the range up to 5.0 vol%. In this study,
16
400 W high-pressure mercury arc (Eiko-sha, Osaka, Japan) with
the water content was changed over a wide range (0–100 vol%)
and the effects on photocatalytic dechlorination (98 mmol) in
the presence of NaOH (200 mmol) under argon are shown in
Fig. 1. Chlorobenzene was almost completely dechlorinated to
benzene with a high stoichiometry in water-free 2-propanol
after photoirradiation for 15 min. The yield of benzene
drastically decreased in the presence of a small amount of
water and most of the chlorobenzene was recovered with 3.0
and 5.0 vol% of water. The negative effect of water on the
photocatalytic dechlorination of chlorobenzene in the range
up to 5.0 vol% water was reconfirmed in this study. We noted
that the conversion of chlorobenzene and the yield of benzene
increased with a high mass balance when the content of water
was further increased. Finally, the benzene yield reached 99%
at 40 vol% water content and most of the chlorobenzene was
dechlorinated to benzene with up to 90 vol% water content.
Since water that has dissolved in 2-propanol would have
various effects on the solubility and adsorption behavior of the
continuous magnetic stirring in a water bath kept at 298 K.
The Pd source was reduced by photogenerated electrons and
Pd metal was deposited on the HyCOM–TiO2 particles,
resulting in the formation of Pd-loaded HyCOM–TiO
2
(Pd–
TiO ). The resultant powder was washed with acetone and
2
distilled water and then dried at 310 K overnight under air.
The content of Pd was fixed at 1.0 wt% in this study.
Photocatalytic dechlorination of chlorobenzene in water-
free 2-propanol under deaerated conditions was carried out
1
5,16
according to the method previously reported:
Pd-loaded
3
TiO2 powder (50 mg) was suspended in 2-propanol (5 cm )
containing NaOH in a test tube. After the mixture had been
bubbled with argon and sealed with a rubber septum,
chlorobenzene (98 mmol) was injected into the mixture. The
test tube was photoirradiated in the same way as for
photodeposition. During photoirradiation, the test tube was
placed in a water bath kept at 298 K to avoid thermal reactions.
Water–2-propanol solvents having various water contents
(shown in vol%) were used to examine the effect of the water
content, and dechlorination of a high concentration of
2
3
3
chlorobenzene (0.2 mol dm , 980 mmol in 5 cm ) was
performed using 50 vol% water–2-propanol solvent.
2
Photocatalytic reactions in air and O were also examined to
determine the effect of O on the reaction. The effect of the
2
amount of NaOH on the photocatalytic dechlorination was
also examined by reducing the amount of NaOH from 200
mmol to 0 mmol. The amount of hydrogen (H ) in the gas phase
2
was measured using a Shimadzu GC-8A gas chromatograph
equipped with an MS-5A column. The amounts of benzene and
chlorobenzene in the liquid phase were determined with a
Shimadzu GC-14B gas chromatograph equipped with a DB-1
Fig. 1 Effect of water content on the photocatalytic dechlorination of
chlorobenzene (98 mmol) in water–2-propanol suspensions of Pd–TiO
presence of NaOH (200 mmol) under argon for 15 min.
2
in the
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starting compounds and products, there are various possible
reasons for the unusual water content dependency of the
photocatalytic dechlorination of chlorobenzene. The addition
of a small amount of water to 2-propanol might disturb the
solidification (removal) of chloride ions as NaCl by sodium
ions; however, the diffusion of chloride ions in the solvent (2-
propanol with a small amount of water) might be inhibited
due to the low solubility of NaCl in the solvent, resulting in
condensation of chloride ions on the hydrophilic surface of
2
TiO particles and a drastic decrease in reactivity. The
solubility of NaCl sufficiently increased with the increase in
water content, enabling Cl ions to diffuse in the solvents and
minimize the negative effects of chloride ions on the
dechlorination of chlorobenzene. Reaction in water again
resulted in a low yield of benzene due to the poor solubility of
chlorobenzene in water.
3.2 Photocatalytic dechlorination of chlorobenzene in 50 vol%
water–2-propanol solvent under deaerated conditions
Since the photocatalytic dechlorination of chlorobenzene
sufficiently occurred in the 50 vol% water–2-propanol solvent
in the presence of NaOH, reactions in this solvent under
various conditions were investigated. Table 1 shows the effects
of UV light and the Pd/TiO photocatalyst on the dechlorina-
2
tion of chlorobenzene in the solvent under argon. Benzene was
quantitatively obtained after 15 min irradiation when the
Fig. 2 Time course diagrams for the photocatalytic dechlorination of chlor-
obenzene ((a) 98 mmol and (b) 930 mmol) in 50 vol% water–2-propanol solvent
in the presence of Pd–TiO
2
and dissolved NaOH (200 mmol and 930 mmol,
respectively) under argon.
reaction was carried out in the presence of a Pd/TiO
2
photocatalyst under UV irradiation (condition 1) as shown in
section 3.1. No reaction occurred when the reaction system
chlorobenzene consumes an equivalent amount of NaOH, as
shown in eqn (1). This means that photocatalytic dechlorina-
tion cannot be applied to high concentration chlorobenzene
solutions. The increase of water content in 2-propanol
improves the solubility of NaOH, and the amount of
chlorobenzene can be increased in the water–2-propanol
solvent system. Therefore, the photocatalytic dechlorination
of larger amounts of chlorobenzene in 50 vol% water–2-
propanol solvent was examined. Fig. 2 shows time course
diagrams for the photocatalytic dechlorination of 98 and 930
mmol chlorobenzene in 50 vol% water–2-propanol solvent in
was irradiated in the absence of a Pd/TiO
2
photocatalyst
(condition 2), indicating that the photochemical dechlorina-
tion of chlorobenzene did not occur in the solvent. Benzene
was not formed in an aqueous alcoholic solution of chlor-
obenzene and NaOH in the dark at room temperature
(
condition 3), indicating that thermochemical dechlorination
of chlorobenzene by NaOH did not occur. In the fourth
condition in a suspension of Pd/TiO in the dark at room
2
temperature, no benzene was detected, as in the cases of
conditions 2 and 3. This result indicates that no thermo-
catalytic dechlorination of chlorobenzene occurred over Pd/
TiO
2
at room temperature. These results obtained in the 50
2
the presence of Pd/TiO and NaOH under UV irradiation. In
vol% water–2-propanol solvent under the four conditions show
that Pd/TiO and UV light are essential for the dechlorination
contrast to the results for the water-free system, chloride ions
were observed in the liquid phase in the 50 vol% water–2-
propanol solvent. When 98 mmol of chlorobenzene was used,
the chlorobenzene was almost completely consumed and
benzene was obtained in a 97% yield within a short time (15
2
of chlorobenzene, i.e., dechlorination of chlorobenzene occurs
photocatalytically.
Since the amount of NaOH which can be dissolved in water-
free 2-propanol is limited, the amount of chlorobenzene to be
dechlorinated is also limited because dechlorination of
min) as shown in Fig. 2(a). After consumption of chloroben-
+
zene, H
2
was formed as a product of proton reduction (2H +
2
2e
A H
2
). This reaction can be expressed as the dehydro-
genation of 2-propanol (eqn (2)).
Table 1 Results of photocatalytic dechlorination of chlorobenzene (98 mmol)
and blank reactions in 50 vol% water–2-propanol solvent in the presence of
NaOH (200 mmol) under argon for 15 min
(CH
3
)
2
CHOH A (CH
3
)
2
CO + H
2
(2)
2
-Propanol was oxidized and acetone was formed in both
reactions. As shown in Fig. 2(a), switching between the
reduction processes (chlorobenzene dechlorination and H
formation) was very quick. To understand the reaction in
detail, the redox balance (ROB), i.e., the ratio of the amount of
2 6 5 6 6
Condition UV light 1.0 wt% Pd–TiO /mg C H Cl/mmol C H /mmol
2
1
2
3
4
on
on
50
0
,1
97
96
95
98
0
0
off (dark) 0
off (dark) 50
0
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consumed photogenerated electrons to that of consumed
holes, was defined as shown in eqn (3):
decreased to 30% in contrast to the reaction under deaerated
conditions (.99%), indicating that the reductions of chlor-
obenzene and O
solvent as predicted. A large amount of acetone was formed
368 mmol), resulting in a very small value of ROB (0.08). This
means that the unwanted partial oxidation of 2-propanol to
acetone was the predominant reaction in water-free 2-propanol
under air as shown in eqn (4).
2
by photogenerated electrons competed in the
ROB = [n(benzene) + n(H )]/n(acetone)
(3)
2
(
where n(benzene), n(H
benzene, H and acetone, respectively. The value of ROB at 15
min was determined to be 1.13. After 60 min reaction time, the
value of ROB decreased to 1.0, although a large amount of H
2
) and n(acetone) are the yields of
2
2
was formed. The value of ROB close to unity indicates that
(CH
3
)
2
CHOH + 1/2O
2
A (CH
3
)
2
CO + H
2
O
(4)
both of the photocatalytic redox reactions, i.e., dechlorination
of chlorobenzene and formation of H
stoichiometry without side reactions and formation of
byproducts.
2
, occurred in a high
We noted that the yield of benzene increased with
increasing water content in the water–2-propanol solvents
even under air and reached .99% in the 50 vol% water–2-
propanol solvent after photoirradiation for 15 min. The yield
of benzene was the same as the yield obtained in the 50 vol%
water–2-propanol solvent under deaerated conditions as
shown in Fig. 1. Surprisingly, the yield of acetone was 106
Fig. 2(b) shows that the photocatalytic dechlorination of
chlorobenzene also occurred when the amount of chloroben-
zene was increased to 930 mmol. The amount of chlorobenzene
linearly decreased, while the yields of benzene, acetone and
chloride ions linearly increased with photoirradiation.
Overlapping of the three yields within the experimental errors
indicates that stoichiometric dechlorination of chlorobenzene
occurred even in the ca. ten-times concentrated conditions in
the 50 vol% water–2-propanol solvent.
mmol, very close to that of benzene, and a small amount of H
2
(8 mmol) also formed in the solvent under air. The value of
ROB was determined to be 1.01. These results indicate that the
photogenerated electrons were selectively used for the reduc-
+
tion of chlorobenzene (and only partly for H reduction) in the
5
0 vol% water–2-propanol solvent even in the presence of O2.
3
.3 Effect of water content on the dechlorination of
The selective and quantitative dechlorination of chloroben-
zene to benzene under air continued until 70 vol% water.
To confirm the high selectivity for dechlorination of
chlorobenzene in the 50 vol% water–2-propanol solvent even
under air, reactions in water-free 2-propanol and in the 50
vol% water–2-propanol solvent under various partial pressures
of O2 were examined, and the results are summarized in
Table 2. The yield of benzene drastically decreased under air
chlorobenzene in the presence of NaOH in air
When a photocatalytic reaction is applied for the reduction of
a substrate, O is generally removed from the reaction system
2
because O always acts as a strong acceptor of photogenerated
electrons and decreases the efficiency of photocatalytic
reduction of the target. To counteract the negative effect of
2
O , photocatalytic reduction has been performed under
2
vacuum or under inert gases such as nitrogen and argon.
(in the presence of 20% O ) in water-free 2-propanol and no
2
However, removal of O
2
by vacuuming or replacement with
dechlorination of chlorobenzene occurred under 100% O . On
2
these gases requires great care and consumes large amounts of
energy. Reactions under air, i.e., reactions free from any
additional operations, are most favorable from the viewpoint
of practical applications. In this study, the photocatalytic
dechlorination of chlorobenzene in water–2-propanol solvents
with various water contents under air was examined and the
results after 15-min photoirradiation are shown in Fig. 3. The
yield of benzene in water-free 2-propanol (at 0 vol%) drastically
the other hand, the negative effect of the presence of 20% O
2
on the dechlorination of chlorobenzene to benzene was
negligible in the 50 vol% water–2-propanol solvent. Only a
slight decrease in chlorobenzene conversion and benzene yield
were observed under 100% O . The results of these compar-
2
2
able experiments indicate that the negative effect of O on the
photocatalytic dechlorination of chlorobenzene to benzene
was greatly suppressed in the 50 vol% water–2-propanol
solvent and that this reaction can be carried out even under
air. The excellent non-sensitivity of the photocatalytic dechlor-
ination towards O
2
in the 50 vol% water–2-propanol solvent is
related to the solubility of O in the solvent. The solubility of a
2
Table 2 Effect of O
mmol) in suspensions of 1.0 wt% Pd–TiO
200 mmol) under UV irradiation for 15 min
2
on the photocatalytic dechlorination of chlorobenzene (98
2
in the presence of dissolved NaOH
(
Solvent
O
2
/%
C
6
H
6
/mmol
6 5
C H Cl/mmol
2
2
2
-propanol
-propanol
-propanol
0
98
30
,1
96
95
87
,1
69
97
,1
,1
6
20
100
0
20
100
Fig. 3 Effect of water content on the photocatalytic dechlorination of
50 vol% water–2-propanol
5
5
0 vol% water–2-propanol
0 vol% water–2-propanol
chlorobenzene (98 mmol) in aqueous 2-propanol suspensions of Pd–TiO
presence of NaOH (200 mmol) under air for 15 min.
2
in the
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gas is expressed in terms of the Ostwald coefficient. The
Ostwald coefficient (L) is defined as ‘‘the ratio of the volume of
gas absorbed to the volume of the absorbing liquid’’. The L
values of O
2
in water and 2-propanol were reported to be
0
.0347 and 0.246, respectively, at 293 K under standard
1
8
1
pressure. The mole fraction of 2-propanol (x ) of the 50
vol% water–2-propanol solvent was 0.277. Since the L value of
O2 in the water–2-propanol solvent with x = 0.2985 was
1
1
8
reported to be 0.0917, the L value of O in the 50 vol% water–
2
2-propanol solvent should be smaller than 0.0917. Therefore,
the low solubility of O in water is attributable to the high
2
selectivity of photocatalytic dechlorination in water–2-propa-
nol solvents. Solubility of a gas in an aqueous electrolyte
solution generally decreases due to the salting-out effect and it
has been reported that the L value of O
2
in an aqueous NaOH
solution decreased with an increase in the concentration of
1
9
2
NaOH, suggesting that the L value of O in the 50 vol%
water–2-propanol solvent in the presence of dissolved NaOH
would be smaller than the L value in the absence of NaOH.
3.4 Effect of the amount of NaOH on the dechlorination of
chlorobenzene under deaerated conditions
The effects of the amount of NaOH on the benzene yield in
1
5,16
water-free 2-propanol were previously reported.
The results
obtained in the reaction under argon for 30 min are shown in
Fig. 4(a) and the features are briefly described for comparison
with those obtained in the 50 vol% water–2-propanol solvent.
Although dechlorination of chlorobenzene occurred in water-
free 2-propanol in the absence of NaOH, the yield of benzene
was low. The benzene yield was improved by the maintenance
of a high material balance by increasing the amount of NaOH
up to ca. 100 mmol. The benzene yield was constant at higher
concentrations of NaOH because almost 100% conversion of
chlorobenzene had been achieved at ca. 100 mmol. From these
results and the results of other additional experiments, it was
concluded that the removal of chloride ions from the reaction
system, i.e., solidification of chloride ions, avoiding consump-
tion of chloride by other reactions, is important for the
effective recovery of benzene from chlorobenzene in water-free
Fig. 4 Effect of the amount of dissolved NaOH on the photocatalytic
dechlorination of chlorobenzene (98 mmol) in (a) a 2-propanol suspension under
argon for 30 min and (b) a 50 vol% water–2-propanol solvent under argon for
1
2
5 min. Photocatalyst: Pd–TiO .
As discussed in section 3.1 (reactions in the presence of
NaOH), the addition of water increased the solubility of the
byproduct (NaCl), enabling Cl ions to diffuse in the solvents
and minimize the negative effects of chloride ions on the
dechlorination of chlorobenzene. The results obtained under
the NaOH-free conditions support this explanation and also
show that water played a role in removing chloride ions from
1
5,16
the reaction site (surface of TiO ) even in the absence of NaOH
2-propanol.
The effect of the amount of NaOH on the
2
and in suppressing the consumption of chloride ions by
another side-reaction. Increasing the amount of NaOH added
to the 50 vol% water–2-propanol solvent to 50 mmol, which was
smaller than that required for the stoichiometric removal of
chloride (98 mmol), resulted in an improved benzene yield (95
mmol), and almost quantitative conversion of chlorobenzene
was achieved in the presence of 100 mmol of NaOH as well as
in the reaction in water-free 2-propanol. As clearly shown by
comparison of Fig. 4(a) and (b), a higher benzene yield was
obtained in the 50 vol% water–2-propanol solvent. The use of
the 50 vol% water–2-propanol solvent strongly contributes to
‘‘simple chemistry’’, i.e., reduction of the amounts of
2-propanol and NaOH. If the oxidized product (acetone) is
regarded as a useful product, the atom efficiency of the
reaction (eqn (5)) was calculated to be 79%. The value was
larger than that of the reaction using NaOH (64%, eqn (1)).
benzene yield in the 50 vol% water–2-propanol solvent was
examined in this study, and the results obtained in the
reactions under argon for 15 min are shown in Fig. 4(b). We
found that dechlorination of chlorobenzene to benzene
occurred in the 50 vol% water–2-propanol solvent only for 15
min even in the absence of NaOH, i.e., the reaction yielded 92
mmol of benzene, which corresponds to a high conversion of
chlorobenzene (94%) and almost complete selectivity (.99%).
Since a small amount of H2 (7.2 mmol) was formed,
dehydrogenation of 2-propanol (eqn (2)) partly occurred. The
value of ROB was determined to be 1.0 as shown in Fig. 4(b),
indicating that only dehydrogenation of 2-propanol (eqn (2))
and dechlorination of chlorobenzene along with oxidation of
2-propanol to acetone occurred under these conditions.
Therefore, most of the 2-propanol was consumed for the
dechlorination of chlorobenzene and the reaction in the
absence of NaOH is expressed as eqn (5).
PhCl + (CH
3
)
2
CHOH A PhH + (CH
3
)
2
CO + HCl
(5)
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NaOH and that the rate of dechlorination of chlorobenzene
increased, in other words, the rate of O
2
reduction (electron
scavenging by O ) was suppressed in the presence of a small
2
amount of NaOH. As discussed in section 3.3, the L value of O2
in an aqueous NaOH solution decreased with increasing
1
9
concentration of NaOH. Reduced solubility of O in the 50
2
vol% water–2-propanol solvent in the presence of dissolved
NaOH is attributed to the decrease in the rate of O reduction,
2
i.e., an increase in the value of ROB. When NaOH (100 mmol)
was added to the solvent, only dechlorination of chloroben-
zene occurred as stated in section 3.3, indicating that the
2
reduction of O was almost completely suppressed.
Fig. 5 Effect of the amount of dissolved NaOH on the photocatalytic
dechlorination of chlorobenzene (98 mmol) in 50 vol% water–2-propanol
2
solvent under air for 15 min. Photocatalyst: Pd–TiO .
Conclusions
We examined the photocatalytic dechlorination of chloroben-
3.5 Effect of the amount of NaOH on the dechlorination of
zene in aqueous 2-propanol suspensions of Pd–TiO particles
2
chlorobenzene in 50 vol% water–2-propanol solvent under air
over a wide range of water and NaOH contents under
deaerated and aerated conditions and compared the results
with results previously obtained in water-free 2-propanol with
dissolved NaOH under deaerated conditions. The representa-
tive results are summarized in Table 3. The addition of a small
amount of water drastically decreased the yield of benzene in
the presence of dissolved NaOH. However, the yield of benzene
increased with high mass balance when the content of water
was further increased. Finally, the benzene yield reached 95%
at 40 vol% water content and most of the chlorobenzene was
dechlorinated to benzene until the water content reached 90
vol%. The yield of benzene under aerated conditions was the
same as the yield obtained in the 50 vol% water–2-propanol
solvent under deaerated conditions, and the yield of acetone
was very close to that of benzene. Dechlorination of
chlorobenzene to benzene occurred in the 50 vol% water–2-
propanol solvent only for 15 min even in the absence of NaOH
It was found that dechlorination of chlorobenzene occurred 1)
even under air in the 50 vol% water–2-propanol solvent in the
presence of NaOH (section 3.3) and 2) even in the absence of
NaOH under deaerated conditions (section 3.4). In this study,
the effect of the amount of NaOH on the benzene yield in the
50 vol% water–2-propanol solvent under air was examined and
the results obtained in 15 min reactions are shown in Fig. 5.
Under ideal conditions, i.e., in the absence of NaOH, 70 mmol
of benzene was formed, while 26 mmol of chlorobenzene was
recovered. These results indicate that the material balance of
benzene and chlorobenzene was preserved even in the absence
of NaOH under air. The yield of acetone (106 mmol) was larger
than that of benzene and the value of ROB decreased to 0.67,
2
indicating that O reduction resulting in the unwanted partial
oxidation of 2-propanol to acetone (eqn (4)) partly occurred
under the conditions. The ratio of eqn (1) and (4) under these
conditions was calculated to be 2 : 1 from the values of ROB,
indicating that dechlorination of chlorobenzene was predo-
minant even under air in the absence of NaOH. In the
presence of 50 mmol of NaOH, which was smaller than the
amount (98 mmol) required for stoichiometric dechlorination
shown in eqn (1), the yield of benzene increased to 90 mmol,
while the yield of acetone was almost unchanged, resulting in
an increase in the value of ROB (0.84). This result indicates
that the rate of 2-propanol oxidation (hole trapping by
under deaerated conditions and
a high conversion of
chlorobenzene (94%) and almost complete selectivity (.99%)
were obtained. Under ideal conditions, i.e., reaction in the
absence of NaOH under aerated conditions, benzene was
obtained in a 71% yield, with the material balance of benzene
obtained and chlorobenzene recovered being preserved.
The present study revealed that the use of the water–2-
propanol solvent has various advantages for the photocatalytic
dechlorination of chlorobenzene to benzene. 1) Since the
solubility of NaOH increased in the water–2-propanol solvent,
2-propanol) was almost independent of the presence of
Table 3 Results of the photocatalytic dechlorination of chlorobenzene (98 mmol) in suspensions of 1.0 wt% Pd–TiO
2
under various conditions under UV irradiation
a
Solvent
NaOH/mmol
Atmosphere
Time/min
C
6
H
5
Cl/mmol
6
C H
6
/mmol
ROB
2
2
2
5
5
5
5
-propanol
-propanol
-propanol
0 vol% water-2-propanol
0 vol% water-2-propanol
0 vol% water-2-propanol
0 vol% water-2-propanol
200
200
0
100
100
0
Ar
air
Ar
Ar
air
Ar
15
15
30
15
15
15
15
,1
69
67
,1
,1
5
98
30
28
98
95
92
70
1.1
0b.082
ND
1.0
0.96
1.0
0
air
26
0.67
a
b
Redox balance determined by eqn (3). Not determined.
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be increased. 2) Dechlorination of chlorobenzene occurred
while maintaining a high material balance and excellent ROB
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Acknowledgements
One of the authors (H. K.) is grateful for financial support from
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064 | RSC Adv., 2013, 3, 6058–6064
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