Successive hydrogenation and dechlorination systems using palladized ion exchange membranes

Article abstract of DOI:10.1149/1.1630807

Pd black was deposited on a cation or anion exchange membrane by electroless plating and the subsequent electroplating. Total amount of Pd black deposited on the ion exchange membranes was about one eighth of the mass of the corresponding Pd sheet used in the previous work. A hydrogenation system of styrene and a dechlorination system of 4-chlorotoluene were successfully constructed using a two-compartment cell separated by the resulting palladized ion exchange membranes. The sole hydrogenation product was ethylbenzene, and the dechlorination product was only toluene. In each case current efficiency for ethylbenzene and toluene production was markedly improved by using methanol with high polarity and was not less than that on a Pd sheet or palladized one as a reference. Considering these results, the plausible mechanism of the hydrogenation and dechlorination on the palladized ion exchange membranes was discussed.

Full text of DOI:10.1149/1.1630807

Journal of The Electrochemical Society, 151 ͑1͒ D1-D5 ͑2004͒
D1
0013-4651/2003/151͑1͒/D1/5/$7.00 © The Electrochemical Society, Inc.
Successive Hydrogenation and Dechlorination Systems Using
Palladized Ion Exchange Membranes
,z
*
Chiaki Iwakura, Yukiko Tsuchiyama, Koji Higashiyama, Eiji Higuchi,
and Hiroshi Inoue
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Sakai,
Osaka 599-8531, Japan
Pd black was deposited on a cation or anion exchange membrane by electroless plating and the subsequent electroplating. Total
amount of Pd black deposited on the ion exchange membranes was about one eighth of the mass of the corresponding Pd sheet
used in the previous work. A hydrogenation system of styrene and a dechlorination system of 4-chlorotoluene were successfully
constructed using a two-compartment cell separated by the resulting palladized ion exchange membranes. The sole hydrogenation
product was ethylbenzene, and the dechlorination product was only toluene. In each case current efficiency for ethylbenzene and
toluene production was markedly improved by using methanol with high polarity and was not less than that on a Pd sheet or
palladized one as a reference. Considering these results, the plausible mechanism of the hydrogenation and dechlorination on the
palladized ion exchange membranes was discussed.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1630807͔ All rights reserved.
Manuscript submitted December 16, 2002; revised manuscript received July 20, 2003. Available electronically December 9, 2003.
Heterogeneous hydrogenation with a Pd catalyst is one of the
important synthetic New
processes.1,2 hydrogenation,3-8
tilled water and then 0.92 M hypophosphorous acid (H3PO2) aque-
ous solution was poured into the cell for the electroless palladiza-
tion. After 24 h, the H3PO2 solution was removed from the system
and the palladized membrane with black color was washed thor-
oughly with distilled water again.
hydrogenolysis,9 and dehydrogenation10,11 systems using a two-
compartment cell separated by a Pd sheet or palladized Pd sheet
have been constructed by our group.3 This system was composed of
three processes which are electrochemical production of atomic hy-
drogen by water electrolysis on one side of the Pd sheet, permeation
of the electrogenerated atomic hydrogen through the Pd sheet, and
hydrogenation of an unsaturated organic compound on the other
side. The system was quite different from classical electrochemical
hydrogenations in the point that the first electrochemical process and
the last hydrogenation process proceed on different sides. Therefore,
after the reaction we did not need to separate a supporting electro-
lyte from products. The thinner Pd sheet can lead to lower cost of
material in addition to faster permeation of atomic hydrogen.
Ion exchange membranes such as Nafion are often used for a
matrix of active noble metal catalysts for electrochemical and
chemical hydrogenations, isomerization, and alcohol oxidations and
so on.12-15 If Pd or Pd black with hydrogen absorbability is loaded
on one side of the ion exchange membranes, we should be able to
apply it to the successive hydrogenation system in place of the Pd
sheet. In this work, single-sided palladized ion exchange membranes
were prepared by electroless plating and the subsequent electroplat-
ing, and the resulting palladized membranes were applied to the
hydrogenation system of styrene in place of a Pd sheet and the
dechlorination system of 4-chlorotoluene from the environmental
interest.
The electroplating of Pd black was performed immediately after
the electroless plating. The single-sided palladized membrane was
set as shown in Fig. 1b. The 2.82 ϫ 10Ϫ2 M Pd͑NH3)42ϩ solution
was put in the cell and contacted with the palladized side of the
membrane ͑contact area: 0.28 cm2͒. The Pd black was galvanostati-
cally deposited at 10 mA cmϪ2 for given periods of time.
The single-sided palladization of the AEM was based on the
above, and a 1 M HCl aqueous solution containing 1.13 ϫ 10Ϫ2
M
PdCl2 , which is described as PdCl24Ϫ solution hereafter, was used
instead of the Pd͑NH3)24ϩ solution.
The amount of Pd͑NH3)24ϩ adsorbed on the CEM was evaluated
from the difference in the concentration of the Pd͑NH3)24ϩ solution
before and after contacting it with the CEM. The concentration was
determined by measuring absorbance at 296 nm in absorption spec-
trum of each solution and applying the absorbance into the Lambert-
Beer’s law. Then the experimental molar extinction coefficient at
296 nm was 1.95 ϫ 102 MϪ1 cmϪ1. In case of the PdCl24Ϫ adsorbed
on the AEM, absorbance at 473 nm was measured. The experimental
molar extinction coefficient at 473 nm was 1.61 ϫ 102 MϪ1 cmϪ1
.
The palladization on a Pd sheet ͑thickness: 50 ␮m͒, for compari-
son, was galvanostatically carried out at 10 mA cmϪ2 for given
periods.
Experimental
For the hydrogenation of styrene or dechlorination of
4-chlorotoluene, the palladized CEM or AEM was set in a cell illus-
trated in Fig. 1c. In the former reaction, substrate was pure ͓ϭ8.7
mol dmϪ3 ͑ϭM͔͒ styrene or a methanol solution containing 1 M
styrene, whereas in the latter reaction the substrate was a methanol
or cyclohexane solution containing 1 M 4-chlorotoluene or pure
͑ϭ8.5 M͒ one. Each substrate was put in the Pd black-side compart-
ment. 1 M H2SO4 solution for the CEM or 6 M KOH solution for
the AEM was put in the other compartment. The current density for
A cation exchange membrane ͑CEM, Selemion CMV, thickness:
100 ␮m͒ and an anion exchange membrane ͑AEM, Selemion AMV,
thickness: 100 ␮m͒ were provided from Asahi Glass Co. Ltd.
The palladization on a side of the CEM or AEM by electroless
plating and the subsequent electroplating was carried out using re-
action systems illustrated in Fig. 1a and b. In case of the CEM, a
1.13 ϫ 10Ϫ2 M Pd͑NH3)4Cl2 aqueous solution, which is described
as Pd͑NH3)24ϩ solution hereafter, was poured into the reaction sys-
tem shown in Fig. 1a. The apparent contact area of the membrane
with the Pd͑NH3)42ϩ solution was 0.64 cm2. The Pd͑NH3)24ϩ ions
were adsorbed on the membrane by ion exchange with original Naϩ
ions. After standing for given periods of time, the Pd͑NH3)24ϩ solu-
tion was moved from the system. The resulting
Pd͑NH3)24ϩ-adosorbed membrane was washed thoroughly with dis-
water electrolysis to produce atomic hydrogen was 36 mA cmϪ2
.
Products were qualitatively and quantitatively determined by gas
chromatography.
Results and Discussion
Palladization on the CEM and AEM by electroless plating and
the subsequent electroplating.—Figure 2 shows time course of the
amount of Pd͑NH3)24ϩ and PdCl24Ϫ adsorbed on the CEM and AEM,
respectively, as a function of the contact time with the corresponding
solutions. In the case of the CEM, the amount of adsorbed
* Electrochemical Society Active Member.
z E-mail: iwakura@chem.osakafu-u.ac.jp
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Journal of The Electrochemical Society, 151 ͑1͒ D1-D5 ͑2004͒
Figure 1. Reaction systems for ͑a͒ electroless plating and ͑b͒ electroplating of Pd black on ion exchange membranes and ͑c͒ a hydrogenation and dechlorination
system.
Pd͑NH3)24ϩ greatly increased with the contact time because the
reduced to Pd black, and was around one fortieth of mass of the Pd
sheet.
Pd͑NH3)24ϩ ions adsorbed on the CEM by an ion-exchange reaction
Time course of the amount of the Pd black deposit in the elec-
and saturated at about 5 h, suggesting that the ion-exchange reaction
troplating is shown in Fig. 3. The amount of the Pd black deposit
came to equilibrium. When the Pd͑NH3)24ϩ-adsorbed membrane
increased linearly with electroplating time in both membranes, but
came into contact with the H3PO2 solution, the color of the mem-
over 3 h, a part of the Pd black deposit began to peel off. The total
amount of the Pd black deposited in the electroplating for 3 h was
5.1 mg for the CEM and 5.0 mg for the AEM, which was neverthe-
brane changed to black, suggesting that the Pd͑NH3)24ϩ is reduced to
Pd black. The amount of adsorbed Pd͑NH3)24ϩ after 24 h is evalu-
ated as 11.1 ␮mol from Fig. 2. Assuming that the adsorbed
Pd͑NH3)24ϩ was completely reduced to Pd, then the amount of the
Pd black deposit was 1.18 mg. Since the amount of a Pd sheet ͑50
␮m͒ corresponding to the apparent area ͑0.64 cm2͒ covered by Pd
black deposit is evaluated as 38.4 mg, the amount of Pd black de-
posited on the membrane by the electroless plating is less than one
thirtieth of the mass of a Pd sheet with a thickness of 50 ␮m.
In the case of the AEM, the time course of the amount of ad-
sorbed PdCl24Ϫ was similar to that for the CEM, as shown in Fig. 2.
less around one eighth of the mass of the Pd sheet.
Morphology of the Pd black deposited on the CEM and
AEM.—Figure 4 shows SEMs of the Pd black deposited on the
CEM or AEM by the electroless plating and the subsequent electro-
plating for 2 h. Final morphology of the Pd black deposits on the
CEM and AEM after the electroplating was similar to each other as
shown in Fig. 4. During the two-step palladization, the Pd black
particles grew up three-dimensionally on the membranes like on a
Pd aheet.4 In both cases, spherical deposits with various sizes cov-
ered the membrane surface. The difference in the morphology of the
Pd black deposits between the CEM and AEM seems to be ascribed
After 24 h, the adsorption of PdCl24Ϫ came to equilibrium. Then the
amount of adsorbed PdCl24Ϫ was 9.1 ␮mol, which corresponds to
0.97 mg, assuming that the adsorbed PdCl24Ϫ ions are completely
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Journal of The Electrochemical Society, 151 ͑1͒ D1-D5 ͑2004͒
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Figure 3. Time course of amount of Pd black deposited on the palladized
CEM and AEM in the electroplating. Current density: 10 mA cmϪ2
.
evaluated according to the previous method3 was 79% for the pal-
ladized CEM and 82% for the palladized AEM. The remainder was
ascribed to hydrogen evolution on the Pd black deposit and its hy-
drogen absorption. The conventional hydrogenation mechanism4 in
which electrogenerated atomic hydrogens permeate through a palla-
dized Pd sheet and add to styrene molecules adsorbed on the back
side must be accepted for the palladized CEM and AEM although
the mechanism for the production of the atomic hydrogens is differ-
ent in the following way
Hϩ ϩ eϪ → H for CEM
H2O ϩ eϪ → H ϩ OHϪ for AEM
͓1͔
͓2͔
The current efficiency for the palladized membranes, however,
was lower than that ͑ca. 93%͒ for the palladized Pd sheet. According
the previous paper,3 the current efficiency depended on the current
density for the production of atomic hydrogen because the current
density, that is the rate of the atomic hydrogen production, could
influence the diffusion rate of atomic hydrogen in the Pd and Pd
black. In the present case, the production of the Pd black deposits
Figure 2. Time course of amount of Pd͑NH3)2ϩ and PdCl42Ϫ adsorbed on
the CEM and AEM, respectively, as a function 4of the contact time with the
corresponding solutions.
to that of the kind of Pd ions, that is Pd͑NH3)24ϩ for the CEM and
PdCl24Ϫ for the AEM. Such three-dimensional growth of the Pd
black deposits should cause the increase in the surface area irrespec-
tive of the morphology, leading to the improved reactivity.4,5 The
same effect is also expected in the present case.
Hydrogenation system of styrene on the palladized CEM or
AEM.—The hydrogenation of styrene was carried out using the two-
compartment cell separated by the palladized CEM or AEM. The
membranes were palladized by an electroless plating after contacted
with the Pd͑NH3)24ϩ or PdCl42Ϫ solution for 24 h and the subsequent
electroplating at 10 mA cmϪ2 for 3 h. The current density for water
electrolysis to produce atomic hydrogen in the hydrogenation was
36 mA cmϪ2. The results are shown in Fig. 5. In the previous paper
which was reported on the hydrogenation of pure styrene on a Pd
sheet,3 a sole hydrogenation product of styrene was ethylbenzene
and its production rate increased linearly with time. Then current
efficiency for the ethylbenzene production was about 93%. In the
present case, the hydrogenation product was only ethylbenzene and
other products such as oligomer or polymer of styrene were not
detected. As can be seen from Fig. 5, the amount of produced eth-
ylbenzene linearly increased with time irrespective of the kind of
membrane. This indicates that the catalytic activity of the Pd black
deposited on both membranes was kept during the present reaction
period. The current efficiency for the ethylbenzene production
Figure 4. Scanning electron micrographs of Pd black deposited on a CEM
and AEM after the electroless plating and the subsequent electroplating for
2 h.
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Journal of The Electrochemical Society, 151 ͑1͒ D1-D5 ͑2004͒
Figure 6. Schematic illustration for hydrogenation mechanisms of styrene
dissolved in methanol for a palladized CEM.
largement of active reaction area for the hydrogenation. The styrene
molecules penetrated in the pores can directly react with atomic
hydrogen produced on the Pd black surface, as shown in Fig. 6. This
reaction is likely to contribute to the improvement of the current
efficiency for the hydrogenation.
Dechlorination system of 4-chlorotoluene on the palladized
CEM or AEM.—The dechlorination of 4-chlorotoluene was carried
out using the same cell as the hydrogenation. The CEM and AEM
membranes were palladized by an electroless plating after contacted
with the Pd͑NH3)24ϩ and PdCl42Ϫ solution for 24 h, respectively, and
then the resulting membranes were still palladized at 10 mA cmϪ2
for 2 h. The current density for water electrolysis to produce atomic
hydrogen in the dechlorination was 36 mA cmϪ2. The results are
shown in Fig. 7. Regardless of the kind of membrane, the dechlori-
nation system successfully operated and toluene was obtained as a
dechlorination product together with hydrogen as a byproduct. The
other products like biphenyl were not obtained. These suggest that a
Cl group of 4-chlorotoluene was substituted with atomic hydrogen
produced on the Pd black catalyst as follows.
Figure 5. Time course of amount of ethylbenzene produced in the hydroge-
nation of 1 M styrene dissolved in methanol and pure styrene on a palladized
CEM and AEM. The condition for palladization and hydrogenation is shown
in the text. The current efficiency for toluene production is shown in paren-
thesis.
led to the extension of the surface area for the electrochemical pro-
duction of atomic hydrogen as well as the hydrogenation. This
means that the decrease in the real current density and seems to be a
part of the reason for the lower current efficiency.
Cl-C6H4-CH3 ϩ 2H → C6H5-CH3 ϩ HCl
͓3͔
The current efficiency for the ethylbenzene production was im-
proved to 88% for the palladized CEM and 93% for the palladized
AEM, respectively, with the use of methanol as a solvent in spite of
the low concentration of styrene as shown in Fig. 5. This suggests
that the other plausible hydrogenation mechanism can be also ac-
cepted. The CEM and AEM used in the present study were com-
posed of segregated domains of hydrophobic polymer bones and
hydrophilic pores of hydrated ion-exchange sites like Nafion. Ions,
cations in the CEM, or anions in the AEM, incorporated in the
membranes permeated through a network of hydrophilic pores. Pd
black was likely to be deposited not only on the membrane surface
but also in the membranes by the electroless plating, as shown in
Fig. 6, because Pd͑NH3)42ϩ and PdCl24Ϫ ions can easily penetrate in
the hydrophilic pores of the CEM and AEM, respectively. A part of
Pd black deposits in the hydrophilic pores seem to make an electric
network with surface Pd black deposits in electrochemical palladiza-
tion after the electroless plating. Because methanol with high polar-
ity is miscible with water, the miscibility of methanol with water in
the hydrophilic pores seems to allow the penetration of styrene into
the membranes more or less in the present case, leading to the en-
The production of HCl was qualitatively confirmed with a silver
nitrate solution in all cases.
The amount of toluene production linearly increased with time in
all cases, suggesting that the activity of the Pd black catalyst is
maintained for at least 5 h. Current efficiency for toluene production
was evaluated based on the method reported previously3 and the
results are also shown in Fig. 7. The current efficiency for pure
4-chlorotoluene was about ten times higher than that in 1 M
4-chlorotoluene dissolved in cyclohexane probably due to the in-
crease in substrate concentration. Moreover, the current efficiency in
methanol, a protic solvent with higher polarity, was about sixty
times higher than that in cyclohexane, an a protic solvent with lower
polarity. This result is consistent with general tendency that polar
solvents such as alcohol facilitate dehalogenation.1
The dechlorination of 4-chlorotoluene in methanol was also car-
ried out by using a palladized Pd sheet instead of the palladized ion
exchange membranes. For obtaining the maximal current efficiency
for the toluene production, the palladization was performed galvano-
statically at 36 mA cmϪ2 for 1.5 h and the production of the atomic
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Journal of The Electrochemical Society, 151 ͑1͒ D1-D5 ͑2004͒
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The maximal current efficiency was achieved at the concentration of
2 M and it was 90% for the palladized CEM and 93% for the
palladized AEM. Over 2 M the current efficiency gradually de-
creased probably due to the lowering of the miscibility of methanol
with water in the hydrophilic pores of the membranes.
Conclusion
We have succeeded in reducing the amount of Pd used in the
successive hydrogenation system by using ion exchange membranes
palladized by electroless plating and the subsequent electroplating in
place of a Pd sheet. The hydrogenation and dechlorination system
using the palladized ion exchange membranes was successfully con-
structed, and ethylbenzene and toluene were exclusively produced
from styrene and 4-chlorotoluene, respectively. The present system,
however, showed low current efficiency in comparison with the pre-
vious system using a Pd sheet or palladized Pd sheet. This was
markedly improved by dissolving substrates into methanol with high
polarity. Considering these results, the hydrogenation and dechlori-
nation was suggested to proceed based on not the conventional
mechanism in which electrogenerated atomic hydrogens permeating
through the palladized Pd sheet and add to substrate molecules but
the other mechanism in which electrogenerated atomic hydrogens
directly added to the substrate molecules.
Acknowledgment
The authors wish to express their thanks to Asahi Glass Co., Ltd.
for providing them the Selemion CMV membrane.
Osaka Prefecture University assisted in meeting the publication costs of
this article.
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