Rapid and complete hydrodechlorination of 2,4-dichlorophenoxyacetic acid catalyzed by Pd/TiO2 with H2 in deionized water

Article abstract of DOI:10.1246/cl.2001.368

Pd/TiO₂ catalyzes rapid and complete hydrodechlorination of 2,4-dichlorophenoxyacetic acid dissolved in H₂O with H₂ to phenoxyacetic acid with a high turnover number of Pd at 303 K.

Full text of DOI:10.1246/cl.2001.368

3
68
Chemistry Letters 2001
Rapid and Complete Hydrodechlorination of 2,4-Dichlorophenoxyacetic Acid
Catalyzed by Pd/TiO with H in Deionized Water
2
2
Ichiro Yamanaka,* Koji Nishikawa, and Kiyoshi Otsuka
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8552
(Received, January 15, 2001; CL-010037)
Pd/TiO catalyzes rapid and complete hydrodechlorination
2
of 2,4-dichlorophenoxyacetic acid dissolved in H O with H to
2
2
phenoxyacetic acid with a high turnover number of Pd at 303 K.
Detoxification and disposal of halogenated aromatics is
one of the most serious problems in environmental chemistry.
Many methods, oxidative decomposition and reductive deharo-
genation, were proposed and examined. However, most of the
methods were operated under serious conditions, at higher tem-
perature, under higher pressure and using expensive oxidants
and reductants. These methods require much energy and cost
for the detoxification. Therefore, a new method to detoxify
1
–4
halogenated aromatics is desired under mild conditions.
A few catalysts (Pd/SiO , Pd/Al O , and Ru/carbon) were
2
2
3
reported for the hydrodechlorination of chloroaromatics with H
2
1–3
under mild conditions.
Active hydrogen species generated on
Pd or Ru was proposed to dechlorinate chloroaromatics. Thus, we
considered that a hydrodechlorination activity of hydrogen species
on the catalyst should be enhanced by the choice of a suitable sup-
This TiO was supplied from Catalysis Society of Japan as a
reference standard catalyst (JRC-TIO-4). The structure of TIO-4
2
5
port of Pd. We found Pd/TiO catalyst to enable to perform rapid
2
and complete hydrodechlorination of chloroaromatics.
consists of a mixture of anatase and rutile crystals about 7:3 con-
firmed by XRD. To get information for the synergism of Pd and
TiO , other three kinds of TiO (anatase and rutile) supplied from
Various Pd catalysts were prepared by a conventional
–2
impregnation method: i) impregnation of PdCl4 aq to supports,
ii) drying at 373K and calcination for 2h at 573 K, iii) reduction
2
2
Catalysis Society of Japan were examined as supports for the
with H for 2h at 573 K. The amount of Pd loading was 1 wt%.
hydrodechlorination of 2,4-D with H in Table 1. The reaction
2
2
2
,4-Dichlorophenoxyacetic acid (2,4-D) which is used as
time was reduced to 30 to 10 min to compare significant catalytic
herbicides, was chosen as a typical model of chloroaromatics.
Figure 1 shows catalytic activities for the hydrodechlorination of
activities. All kinds of TiO supported Pd were active for the
hydrodechlorination. Catalytic activities were Pd/TIO-4 >
Pd/TIO-2 > Pd/TIO-3 > Pd/TIO-5. This order was not correspon-
2
2
,4-D. The hydrodechlorination was started by bubbling H (1
2
–1
atm, 10 mL min ) into the aqueous solution of 2,4-D (2 mmol
ding to the order of each surface area of TiO . This result pro-
2
–1
–1
L , 40 mL) with the catalyst (0.125 mmol L ) and stirring with
a magnetic spin-bar. The reaction was continued for 30 min at
posed that the surface area of TiO does not directly affect the cat-
2
alytic activities.
303 K. Products, phenoxyacetic acid (PAA), 2-chlorophenoxy-
acetic acid (2-Cl), and 4-chlorophenoxyacetic acid (4-Cl), were
analyzed by HPLC. The Pd supported on carbon (carbon fiber,
active carbon, and graphite), SiO and Al O reported
2
2
3
previously1 catalyzed the hydrodechlorination of 2,4-D with H2
under this reaction condition. However, their yields of PAA,
complete dechlorinated product, were not enough. Therefore, we
have examined other oxides as the support of Pd. As shown in
–3
Figure 1, Pd supported on TiO performed the highest catalytic
2
activity for the hydrodechlorination. Of course, the TiO without
2
Pd was not active for the hydrodechlorination. On the other
hand, the catalytic activity of TiO supported Pt or Rh on which
2
H dissociative adsorbs was very low. A strong synergism of Pd
To get information for reaction scheme, kinetic experiments
of the hydrodechlorination were studied. Turnover number for
the hydrodechlorination (TON-Cl) was defined as the amount of
Cl dechlorinated (mol) per Pd (mol) to evaluate the catalytic
activity for the hydrodechlorination.
2
and TiO was observed for the hydrodechlorination. In addition,
2
the Pd/TiO catalyst showed the same catalytic activity for the
2
hydrodechlorination in the dark. Thus, TiO functioned as the
2
support of Pd but not photo-catalyst.
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
369
First, time course of the hydrodechlorination of 2,4-D cat-
atm) was studied, as shown in Figure 3. The conversion of 2,4-D
exponentially increased with increasing the concentration of the
catalyst below 0.125 g L though the conversion of 2,4-D
–1
alyzed by Pd (1 wt%)/TIO-4 (0.125 g L ) was carried out. The
conversion of 2,4-D and the TON-Cl defined above smoothly
increased with reaction time and attained 90% and 300 at 30 min,
respectively. Major product was PAA (93.0% selectivity) and
minor ones were 2-Cl (6.5) and 4-Cl (0.5) at 30 min. On the other
hand, major product was 2-Cl at the early stage of the hydro-
dechlorination. The selectivity to PAA increased with reaction
time, in contrast, the selectivity to 2-Cl- and 4-Cl decreased. When
the selectivities to PAA, 2-Cl- and 4-Cl were extrapolated at 0 min,
these selectivities were respectively estimated to 0, 80, and 20%.
These results strongly suggested that the initial products were 2-Cl
–
1
increased. The complete hydrodechlorination of 2,4-D with H
was performed at the concentration of 0.625 g L within 10 min.
2
–1
(80% selectivity) and 4-Cl (20%), and PAA was secondary dechlo-
rinated product from the both primary ones. The initial product
selectivities suggested that the hydrodechlorination rate at p-posi-
tion of 2,4-D was faster than that at o-position. A steric hindrance
and electronic withdrawing effect of the [-O–CH CO H] group of
2
2
2,4-D should reduce the hydrodechlorination rate at o-position.
Second, effects of concentrations of 2,4-D, H , and the
2
Pd/TIO-4 catalyst on the hydrodechlorination were studied. Figure
shows the effects of the concentration of 2,4-D on the
2
hydrodechlorination. To reduce the conversion of 2,4-D, reaction
time and the concentration of the catalyst were reduced from 30 to
–
1
1
0 min and from 0.125 to 0.0125 g L . The TON-Cl increased
The TON-Cl increased with increasing the concentration of
–
1
with increasing the concentration of 2,4-D and attained the maxi-
mum at 1 mM. The conversion of 2,4-D smoothly decreased with
increasing the concentration. A higher concentration of 2,4-D (2
mM) is not favor for the effective hydrodechlorination.
catalyst and attained the maximum of 174 at 0.125 g L .
–1
Decrease in TON-Cl above 0.250 g L was due to the high con-
version of 2,4-D. The Pd/TIO-4 catalyst effectively works at the
higher concentration of the catalyst. When 5 wt% loading Pd/TIO-
4
(1 mg), in place of 1 wt% loading Pd/TIO-4 (5 mg), was used
for the hydrodechlorination, the TON-Cl was just 40. Though the
amounts of Pd in the both catalysts were the same, the TON-Cl of
1
5
wt% loading Pd/TIO-4 catalyst was larger than 4 times of that of
wt% loading one. This result suggests that an interface of Pd
and TiO is essential for the generation of active hydrogen
2
species. The data of the product selectivities versus the conver-
sions of 2,4-D in the time course experiment and Figures 2 and 3,
could be plotted on a smooth line. This fact proposed that the
product distribution determined by the conversion of 2,4-D.
We can propose the model of the hydrodechlorination on
Pd/TiO , as follows. Nature of the interface of Pd particle and
2
the surface of TiO controls the hydrodechlorination because the
2
strong synergism of Pd and TiO was observed. H dissociative-
2
2
ly adsorbs on the interface generating active hydrogen species
and 2,4-D competitively adsorbs. The active hydrogen species
successively dechlorinate 2,4-D to PAA through 2- or 4-Cl. The
active hydrogen species is not simple hydrogen atom (H·)
because hydrogenation of aromatic ring did not proceed.
In the case of the P(H ) dependence, the TON-Cl exponen-
2
tially increased with raising P(H ) from 0 to 1 atm. This result
As described so far, the Pd/TiO catalyst has very unique
2
2
proposed that the concentration of active hydrogen species on the
Pd/TIO-4 exponentially increased with P(H2).
catalytic ability for the hydrodechlorination with H . The
2
Pd/TiO catalyst is expected to show catalytic activity for the
2
These experimental facts mentioned above strongly suggest-
ed that competitive adsorption of 2,4-D and hydrogen species on
the catalysts should control the hydrodechlorination rate. At high-
er concentration of 2,4-D (>1 mM), adsorbed 2,4-D should
hydrodechlorination of other poisonous chloroorganic com-
pounds under mild conditions.
References
1
J. B. Hoke, G.A. Gamiccioni, and E.N. Kalko, Appl. Catal. B:
Envir., 1, 285 (1992).
depress the adsorption of H and the formation of active hydrogen
2
species on the catalyst.
2
3
C. Schüth and M. Reinhard, Appl. Catal. B: Envir., 18, 215 (1998).
V. Felis, C.D. Bellefon, P. Fouilloux, and D. Scweich, Appl. Catal.
B: Envir., 20, 91 (1999).
As mentioned above, the catalytic activity of the Pd/TIO-4
decreased at a higher concentration of 2,4-D. In order to perform
the complete dechlorination of 2,4-D, effect of the concentration
4
5
Y.Ukis, S. Kameoka, T. Miyadera, Appl. Catal. B: Envir., 27, 97
(
2000).
of the Pd/TIO-4 catalyst on the hydrodechlorination with H (1
2
wwwsoc.nacis.ac.jp/catsj