Photocatalytic reductive dechlorination of chlorobenzene to benzene in 2-propanol suspension of metal-loaded titanium(iv) oxide nanocrystals in the presence of dissolved sodium hydroxide

Article abstract of DOI:10.1039/c0cc00589d

Photocatalytic reaction of chlorobenzene in 2-propanol suspension of palladium- and rhodium-loaded titanium(iv) oxide nanocrystals in the presence of dissolved sodium hydroxide under deaerated conditions, in which 2-propanol, metal loaded (palladium and rhodium) and sodium hydroxide effectively acted as hole scavenger, co-catalyst and chloride solidifier, respectively, quantitatively gave benzene and chloride ions with a high stoichiometry. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc00589d

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Photocatalytic reductive dechlorination of chlorobenzene to benzene in
2-propanol suspension of metal-loaded titanium(IV) oxide nanocrystals in
the presence of dissolved sodium hydroxidew
Kojirou Fuku, Keiji Hashimoto and Hiroshi Kominami*
Received 25th March 2010, Accepted 18th May 2010
First published as an Advance Article on the web 11th June 2010
DOI: 10.1039/c0cc00589d
Photocatalytic reaction of chlorobenzene in 2-propanol suspension
of palladium- and rhodium-loaded titanium(IV) oxide nano-
crystals in the presence of dissolved sodium hydroxide under
deaerated conditions, in which 2-propanol, metal loaded (palladium
and rhodium) and sodium hydroxide effectively acted as hole
scavenger, co-catalyst and chloride solidifier, respectively,
quantitatively gave benzene and chloride ions with a high
stoichiometry.
oxidation of chlorinated compounds has been studied widely.
However, selective recovery of dechlorinated product(s) was
difficult.6,7 Although reductive treatment of chlorinated aromatic
compounds by heterogeneous photocatalytic reaction was
reported, the recovery of dechlorinated aromatic compounds
was insufficient.8–11 Previous studies on photocatalytic reaction
have suggested that strong and selective electron injection to
chlorinated aromatic compounds and effective removal of
chloride ions from the reaction system are important for
quantitative recovery of aromatics from chlorinated aromatic
compounds. Based on the above considerations, we examined
photocatalytic reductive dechlorination of chlorobenzene in
2-propanol suspension of metal-loaded titanium(IV) oxide
(TiO2) nanocrystals in the presence of dissolved NaOH. Here
we briefly report that benzene and chloride ion were recovered
quantitatively when palladium (Pd)-loaded titanium(IV) oxide
(TiO2) nanocrystals were used as the photocatalyst.
The chemical industry has been contributing to the development
of human society and making our life comfortable. However,
some organic and inorganic wastes produced from the chemical
industry cause various problems. Chlorinated aromatic
compounds are one of the organic wastes and are harmful to
the human body. These chemical substances are stored in
nature as persistent organic pollutants. These chlorinated
aromatic compounds are generally treated by incineration or
biodegradation. Since these methods aim to mineralize the
chlorinated aromatic compounds, a large amount of carbon
dioxide (CO2) is produced. In addition, we cannot recover and
re-use aromatic rings of chlorinated aromatic compounds by
using these methods. Therefore, recovery of aromatics by
dechlorination of chlorinated aromatic compounds is one of
the promising methods for treatment of chlorinated com-
pounds from the viewpoints of the environment and chemical
resources. Although catalytic dechlorination of chlorinated
aromatic compounds has been examined under various
conditions,1–5 most of the methods used required specific
reaction conditions such as high temperatures, high pressure
and the presence of molecular hydrogen (H2). Recently, it has
been reported that aromatic compounds were recovered from
various chlorinated aromatic compounds under ordinary
pressure by a catalytic process in a 2-propanol–sodium hydroxide
(NaOH) system.3–5 However, elevated temperatures were
still required under these conditions. On the other hand,
photocatalytic dechlorination of chlorinated compounds has
attracted attention because photocatalytic reaction occurs
under mild conditions (at room temperature and under
atmospheric pressure) and can utilize solar energy. Since the
strong oxidation power of holes can be used, photocatalytic
Nanocrystalline TiO2 powder was prepared from
titanium(IV) tetra-n-butoxide at 573 K using the HyCOM
method12 and then calcined at 823 K. This HyCOM-TiO2
powder was suspended in 80 vol% aqueous 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 by a 400 W
high-pressure mercury arc, resulting in the formation of
Pd-loaded HyCOM-TiO2 (Pd–TiO2). Various metals other
than Pd were also deposited on HyCOM-TiO2 particles using
the photodeposition method. Photocatalytic dechlorination of
chlorobenzene was carried out in 2-propanol suspension of
metal-loaded TiO2 powder in the presence of dissolved NaOH.
Here 2-propanol acted as both reaction medium and hole
scavenger. One wt% metal-loaded TiO2 powder (50 mg) was
suspended in 2-propanol (5 cm3) containing NaOH (220 mmol)
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 photodeposition. During photoirradiation,
the test tube was set in a water bath kept at 298 K to avoid
thermal reaction. The amounts of H2 and CO2 in the gas phase
and benzene, chlorobenzene and acetone in the liquid phase
were measured using gas chromatographs. The amount of
chloride ion was determined with a JASCO co-2060 Plus ion
chromatograph. Details of preparation, photocatalytic reaction
and product analyses are provided in Supplementary Information
(ESIw).
Department of Applied Chemistry, Faculty of Science and
Engineering, Kinki University, Kowakae, Higashiosaka,
Osaka 577-8502, Japan. E-mail: hiro@apch.kindai.ac.jp;
Fax: +81-6-6727-2024; Tel: +81-6-6721-2332
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c0cc00589d
Photocatalytic dechlorination of chlorobenzene was examined
in 2-propanol suspensions of various 1.0 wt% metal-loaded
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HyCOM-TiO2 samples containing NaOH under irradiation of
UV light for 30 min. Since chloride ions were not observed in
the liquid phase (in 2-propanol solution) after photoirradiation,
water (2 cm3) was added to the reaction mixture and the
resultant liquid phase was analyzed using an ion chromatograph.
Fig. 1 shows the effect of loaded metal (1.0 wt%) on the yield
of chloride ion eliminated from chlorobenzene. For comparison,
results for a bare TiO2 sample are also shown in Fig. 1. When a
bare TiO2 sample was used, no chloride ions and no other
products were observed, indicating that neither hole scavenging
by 2-propanol nor electron scavenging by chlorobenzene
occurred under the present conditions. When Rh-, Ru-, Cu-,
Pt-, Pd- and Ag-loaded TiO2 samples were used under the
reaction conditions, chloride ions were observed in the liquid
phase after addition of water. Since NaOH was soluble in
2-propanol and sodium chloride (NaCl) was not, these results
indicate that chloride ions eliminated from chlorobenzene
were solidified with NaOH as NaCl, i.e., chloride ions were
removed from the present photocatalytic reaction system.
These results also show that these metals loaded on TiO2
particles worked as co-catalysts (reduction site) in photo-
catalytic dechlorination of chlorobenzene. The extent of
dechlorination increased in the order of loaded metals:
Ag E Cu o Pt o Ru { Rh E Pd. It should be noted that
Rh- and Pd-loaded TiO2 samples showed extraordinarily high
levels of activity in photocatalytic dechlorination of chloro-
benzene. Hydrogen over-voltage (HOV) of Pd and Rh electrodes
is slightly higher than that of Pt and much lower than those of
Ag and Cu, suggesting that a slightly higher HOV, moderately
suppressing hydrogen formation,13 is important for selective
reduction of chlorobenzene. However, Au did not show any
effects on photocatalytic dechlorination of chlorobenzene.
From these results, it can be concluded that Rh and
Pd were efficient co-catalysts for reductive dechlorination of
chlorobenzene.
with a high stoichiometry (ca. 100%), indicating that almost
complete recovery of benzene from chlorobenzene was
achieved by the present conditions. It is known that 2-propanol
acts as an efficient hole scavenger yielding acetone as shown in
eqn (1) and promotes reduction of various species by photo-
generated electrons in photocatalytic reaction.
(CH3)2CHOH + 2h+ - (CH3)2CO + 2H+
(1)
Therefore, in the present system, chlorobenzene was effectively
reduced by photogenerated electrons as shown in eqn (2).
PhCl + H+ + 2eÀ - PhH + ClÀ
(2)
Chloride ions were not observed in the liquid phase (in
2-propanol solution) also in this case. To confirm the formation
of chloride ions, water (2 cm3) was added to the reaction
mixture and the resultant liquid phase was analyzed using an
ion chromatograph. The amount of chloride ions (98 mmol)
was in good agreement with that of benzene determined by
GC, indicating that chlorobenzene was completely dechlorinated,
forming benzene and chloride ions. The photocatalytic reaction
shown in Fig. 2 is expressed as eqn (3).
PhCl + (CH3)2CHOH + NaOH - PhH + (CH3)2CO
+ NaClk + H2O
(3)
Chloride ions eliminated from chlorobenzene were immediately
solidified as NaCl by NaOH dissolved in 2-propanol and
separated from other products such as benzene and acetone
dissolved in 2-propanol. Isolation of chloride from the reaction
system suppressed consumption of chloride by other side reactions
that caused insufficient stoichiometry in dechlorination.
Although chlorobenzene was almost consumed at 15 min,
the yield of acetone still increased and a small amount of H2
formed after 15 min (Fig. 2). These results indicate that hole
scavenging by 2-propanol (eqn (1)) still continued and that
photogenerated electrons were used to reduce H+ as shown in
eqn (4).
Fig. 2 shows the time courses of photocatalytic dechlorination
of chlorobenzene in 2-propanol suspension of 1.0 wt%
Pd–TiO2 containing NaOH under irradiation of UV light.
Chlorobenzene was completely consumed within a very short
time (20 min). It should be noted that benzene was formed
2H+ + 2eÀ - H2
(4)
Fig. 1 Dechlorination in photocatalytic reaction of chlorobenzene in
2-propanol suspension of 1.0 wt% metal-loaded HyCOM-TiO2 in the
presence of dissolved NaOH under UV light irradiation (>300 nm)
for 30 min.
Fig. 2 Time courses of the amounts of chlorobenzene (closed
squares), benzene (closed triangles), acetone (open squares) and H2
(open circles) in 2-propanol suspension of 1.0 wt% Pd–TiO2 in the
presence of dissolved NaOH under UV light irradiation (>300 nm).
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Fig. 3 shows the effect of the amount of NaOH on the yield
of benzene in photocatalytic dechlorination of chlorobenzene
in 2-propanol suspension of 1.0 wt% Pd–TiO2 under irradiation
of UV light for 30 min. Although dechlorination of chloro-
benzene occurred in the absence of NaOH, the reaction rate
was low, resulting in low yield of benzene. The reaction rate
and benzene yield were improved with maintenance of a high
material balance by increasing the amount of NaOH up to ca.
100 mmol. The benzene yield was constant at larger contents of
NaOH because almost 100% conversion of chlorobenzene had
been achieved at ca. 100 mmol. These results indicate that
reductive dechlorination of chlorobenzene in 2-propanol was
promoted by addition of a small amount of NaOH and that
chlorobenzene can be almost completely dechlorinated only in
the presence of NaOH satisfying the stoichiometry shown in
eqn (3). Therefore, it should be noted that removal of chloride
ions from the reaction system, i.e., solidification of chloride,
avoiding consumption of chloride by other reactions, is very
important for effective recovery of benzene from chloro-
benzene in the photocatalytic process.
Fig. 3 Effect of the amount of dissolved NaOH on the yield of
benzene in photocatalytic dechlorination of chlorobenzene (98 mmol)
in 2-propanol suspension of 1.0 wt% Pd–TiO2 under irradiation of UV
light for 30 min.
Formation of H2 is totally expressed as eqn (5):
In conclusion, photocatalytic dechlorination of chloro-
benzene in 2-propanol suspension of Pd– and Rh–TiO2 in
the presence of dissolved NaOH under irradiation of UV light
was completed in a very short time and quantitatively yielded
benzene and chloride.
(CH3)2CHOH - (CH3)2CO + H2
(5)
From these results, it can be concluded that photogenerated
electrons were almost selectively used for dechlorination of
chlorobenzene just before its consumption in the present
system. Acetone can be further oxidized by holes, resulting
in formation of CO2; however, CO2 was not observed in any
reaction time. Since a large excess of 2-propanol was present in
this reaction system, the rate of acetone oxidation was probably
much lower than that of 2-propanol oxidation.
This work was partly supported by a Grant-in-Aid for
Scientific Research (No. 16560679) from the Ministry of
Education, Culture, Science, and Technology (MEXT) of
Japan. One of the authors (H. K.) is also grateful for financial
support from the Faculty of Science and Engineering, Kinki
University.
Since an almost stoichiometric material balance among the
amounts of consumed chlorobenzene, formed benzene and
chloride ions was confirmed, redox balance (ROB), i.e., ratio
of the amount of consumed photogenerated electrons to that
of consumed holes, in this reaction system was defined as
eqn (6):
Notes and references
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ROB = [n(benzene) + n(H2)]/n(acetone)
(6)
where n(benzene), n(H2) and n(acetone) are yields of benzene,
H2 and acetone, respectively. The value of ROB at 10 min, at
which time only dechlorination of chlorobenzene occurred (no
H2 formation), was determined to be 1.04, indicating that
photocatalytic reduction (dechlorination of chlorobenzene to
benzene) and photocatalytic oxidation of 2-propanol to acetone
took place with a ROB close to unity. The values of ROB at 15
and 20 min were determined to be 0.98 and 1.00, respectively.
The values of ROB close to unity indicate that both of the
photocatalytic redox reactions shown in eqn (3) and (5), i.e.,
dechlorination of chlorobenzene and formation of H2,
occurred with a high stoichiometry without side reaction(s)
and formation of byproduct(s).
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