Development of polymeric palladium-nanoparticle membrane-installed microflow devices and their application in hydrodehalogenation

Article abstract of DOI:10.1002/cssc.201100418

We have developed a variety of polymeric palladium-nanoparticle membrane-installed microflow devices. Three types of polymers were convoluted with palladium salts under laminar flow conditions in a microflow reactor to form polymeric palladium membranes at the laminar flow interface. These membranes were reduced with aqueous sodium formate or heat to create microflow devices that contain polymeric palladium-nanoparticle membranes. These microflow devices achieved instantaneous hydrodehalogenation of aryl chlorides, bromides, iodides, and triflates by 10-1000 ppm within a residence time of 2-8 s at 50-90°C by using safe, nonexplosive, aqueous sodium formate to quantitatively afford the corresponding hydrodehalogenated products. Polychlorinated biphenyl (10-1000 ppm) and polybrominated biphenyl (1000 ppm) were completely decomposed under similar conditions, yielding biphenyl as a fungicidal compound. Streamlined nanoparticles: We developed a variety of polymeric Pd nanoparticle membrane-installed microflow devices. Instantaneous hydrodehalogenation of aryl halides can be achieved at short residence times and moderate temperatures using a safe, nonexplosive aqueous sodium formate to quantitatively afford the corresponding hydrodehalogenated products. PCB and PBB can also be fully decomposed under similar conditions to give a biphenyl. Copyright

Full text of DOI:10.1002/cssc.201100418

DOI: 10.1002/cssc.201100418
Development of Polymeric Palladium-Nanoparticle
Membrane-Installed Microflow Devices and their
Application in Hydrodehalogenation
Yoichi M. A. Yamada,[a] Toshihiro Watanabe,[a, b] Aya Ohno,[a] and Yasuhiro Uozumi*[a, b]
We have developed a variety of polymeric palladium-nanopar-
ticle membrane-installed microflow devices. Three types of
polymers were convoluted with palladium salts under laminar
flow conditions in a microflow reactor to form polymeric palla-
dium membranes at the laminar flow interface. These mem-
branes were reduced with aqueous sodium formate or heat to
create microflow devices that contain polymeric palladium-
nanoparticle membranes. These microflow devices achieved in-
stantaneous hydrodehalogenation of aryl chlorides, bromides,
iodides, and triflates by 10–1000 ppm within a residence time
of 2–8 s at 50–908C by using safe, nonexplosive, aqueous
sodium formate to quantitatively afford the corresponding hy-
drodehalogenated products. Polychlorinated biphenyl (10–
1000 ppm) and polybrominated biphenyl (1000 ppm) were
completely decomposed under similar conditions, yielding bi-
phenyl as a fungicidal compound.
Introduction
The further evolution of polymer-supported transition-metal
nanoparticle catalysts is an important objective for organic
synthetic chemistry and nanochemistry.[1,2] These catalysts are
expected to be highly active and more efficient than metal-
complex catalysts in facilitating reactions, including unantici-
pated reactions not possible with metal-complex catalysts. The
polymer support enables recovery and reuse of the metal
nanoparticles with no contamination of the products and sta-
bilizes the metal nanoparticles against aggregation and deacti-
vation. We previously reported the preparation of polystyrene–
polyethylene glycol resin (PS-PEG)-supported palladium and
platinum nanoparticles, which promoted the aerobic oxidation
of alcohols, hydrodechlorination of aryl chlorides, and hydroge-
nation of alkenes.[3] We also developed a polyviologen-convo-
luted palladium-nanoparticle catalyst for the a-alkylation of ke-
tones with alcohols and the ring-opening alkylation of dike-
tones with alcohols.[4] These reactions did not proceed effi-
ciently with our polymer-supported metal-complex catalysts.
Conversely, microflow reactor systems offer many fundamen-
tal as well as practical advantages and have been developed
as innovative devices for rapid organic transformations.[5] They
are not restricted to the microscale production of chemical
compounds, but sustainably provide closed chemical processes
for producing pharmaceutical and chemical compounds with
safety and continuation.[5g] Molecular transformation with a
catalyst-immobilized microflow reactor is a typical application
of these systems, in which the efficiency of various reactions
increases because of the vast interfacial area and the close dis-
tance of the molecular diffusion path in the narrow space of
the microflow reactor.[6] If a catalyst were installed as a mem-
branous composite at the center of the microchannel, two op-
positely introduced reactants could flow through the divided
channel, while remaining in contact with the vast interfacial
surface of the catalytic membrane from both the front and
back sides, thereby realizing an instantaneous chemical reac-
tion. We reported the formation of a variety of membranous
polymeric palladium complex catalysts inside a microchannel
reactor at the laminar flow interface of the channel by using
our molecular convolution methodology.[4a,7,8] The resulting mi-
croflow devices were applied to the Suzuki–Miyaura reaction
and allylic arylation under microflow conditions, for which the
instantaneous production of coupling compounds was quanti-
tatively achieved within a residence time of 1–5 s in the de-
fined channel region.
We envisaged the possibility of reducing the polymeric pal-
ladium complexes inside a microflow reactor and creating the
first polymeric palladium-nanoparticle membrane-installed mi-
croflow devices. Such devices should promote some organic
transformations that cannot proceed with metal-complex
counterparts, providing instantaneous completion of the reac-
tions. This sequential protocol is schematically depicted in
Figure 1.
Herein, we report the first development of a variety of poly-
meric palladium-nanoparticle membrane-installed microchan-
nel devices and their application in instantaneous, mild, safe,
and nonexplosive hydrodehalogenation. Six different new mi-
[a] Dr. Y. M. A. Yamada, Dr. T. Watanabe, A. Ohno, Prof. Dr. Y. Uozumi
RIKEN Advanced Science Institute
Wako, Saitama 351-0198 (Japan)
Fax: (+81)048-467-9599
E-mail: uo@riken.jp
[b] Dr. T. Watanabe, Prof. Dr. Y. Uozumi
Institute for Molecular Science
and the Graduate University for Advanced Studies
Okazaki, Aichi 444-8787 (Japan)
Fax: (+81)0564-59-5574
E-mail: uo@ims.ac.jp
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100418.
ChemSusChem 2012, 5, 293 – 299
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293

Y. Uozumi et al.
plex of a precatalytic membrane of poly(4-vinylpyridine) and
Na2[PdCl4] (composite A) was prepared inside a microchannel[11]
under similar conditions to those used in our previous studie-
s.[4a,7b] A saturated aqueous solution of HCO2Na was streamed
into the microchannel at 508C for 30 min at a flow rate of
0.5 mLminÀ1 to yield the black-colored polymeric palladium-
nanoparticle membrane-installed m-device 1 (Figure 2a). A pre-
catalytic membrane of poly[(N-isopropylacrylamide)5-co-(4-di-
phenylstyrylphosphine)] and (NH4)2[PdCl4] (composite B) was
reduced with aqueous HCO2Na under similar conditions to
form m-device 2 (Figure 2a). The reduction of composite C
inside a microflow device with aqueous HCO2Na gave the
black membrane-installed m-device
3 (Figure 2a). Figure 2
shows optical microscopy as well as TEM images of m-devices
1–3. Optical microscopy observations revealed that a sheet of
polymeric Pd membranes was continuously formed at the in-
terface of the laminar flow in all cases (Figure 2a). These obser-
vations also revealed that a black-colored membrane was pro-
duced in m-devices 1 and 3, whereas a yellow-colored mem-
brane still remained in m-device 2. TEM images indicated that
Pd nanoparticles had formed in a membrane of m-devices 1
and 3, with diameters of approximately 6 and 12 nm, respec-
tively (Figures 2b and d). Conversely, Pd nanoparticles were
barely observed in m-device 2 (Figure 2c). High-resolution (HR)
scanning transmission electron microscopy (STEM, Figure 3a)
and energy dispersive spectroscopy (EDS)/STEM (Figure 3e)
analysis of m-device 1 showed that Pd nanoparticles, with a di-
ameter of (6.0Æ1.4) nm, were dispersed in a polymer matrix.
The thicknesses of the membrane of m-device 1 under dry and
wet (iPrOH/H2O) conditions were 14.3 and 17.1 mm, respective-
ly (Table 1). The results indicated that the swelling ratio of the
membrane in m-device 1 was 1.2 in iPrOH/H2O (Table 1). An
HRTEM image of one batch of the membrane of m-device 1
shows that the lattice width is 0.38 nm (Figure 3b). SEM
images of the membrane revealed that the composite was a
homogeneous and mesoporous material (f=tens of nanome-
ters, Figures 3c and d).
Figure 1. Preparation of polymeric palladium-nanoparticle membrane-instal-
led microflow devices for hydrodehalogenation of aryl halides.
croflow devices were prepared from three types of linear poly-
mers with palladium salts through molecular convolution and
reduction. The microflow hydrodehalogenation of aryl chlor-
ides, bromides, iodides, and triflates (OTfs) was performed
within a residence time of 2–8 s at 50–908C by using aqueous
sodium formate (HCO2Na) as a reducing agent without the use
of explosive hydrogen gas or toxic organic solvents. Hydrode-
halogenation proceeded instantaneously to quantitatively
afford the corresponding dehalogenated products. Surprisingly,
although the ease of reductive hydrodehalogenation of organ-
ic halides follows the general order of I>BrꢀOTf>Cl,[9] we
found that the order became Cl>BrꢀOTf>I when using our
system. Moreover, the microflow system quantitatively decom-
posed 10–1000 ppm concentrations of a polychlorinated bi-
phenyl (PCB) and a polybrominated biphenyl (PBB) under simi-
lar conditions.
Thermal decomposition of composites A–C in a microflow
device was also examined (Figure 4). Composites A–C were
heated in an electric furnace at 250–3508C for 30 min to form
m-devices 4–6, during which time the color of the membranes
changed from yellow to black (Figure 4a). TEM observations in-
dicated that Pd nanoparticles were present in all cases in m-de-
vices 4–6 (Figure 4b–d).
Results and Discussion
Preparation of polymeric palladium-nanoparticle mem-
brane-installed microchannel devices
Hydrodehalogenation of aryl chlorides, bromides, iodides,
and triflates with the polymeric Pd-nanoparticle membrane-
installed microflow devices
Polymer deposition at the laminar interface was originally re-
ported by Whitesides et al. for the reaction of a polymeric sul-
fonate salt and a polymeric ammonium salt.[10] However, the in-
terfacial deposition of transition-metal complexes with a view
to using them as catalytic membranes has not been reported
by other researchers.
To explore the utility of the microchannel devices in organic
transformations, the catalytic ability of the six types of micro-
channel devices prepared above (m-devices 1–6) was examined
for the hydrodehalogenation of aryl halides. The decomposi-
tion of low concentrations of haloaryl compounds, such as en-
docrine-disrupting chemicals, polychlorinated biphenyls (PCBs),
and polybrominated biphenyls (PBBs), in water and organic
solvents to harmless substances is a serious global issue.[9]
New polymeric palladium-nanoparticle membrane-installed
microchannel devices were prepared from linear polymer li-
gands and palladium species through molecular convolution[7]
followed by reduction. Thus, a yellow-colored palladium com-
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Polymeric Nanoparticle Membrane-Installed Microflow Devices
iPrOH and an aqueous solution
of HCO2Na were streamed into
m-device 1 at 508C from two
separated inlets, with both flow
rates equal to 0.5 mLminÀ1 and
a residence time of 8 s, we
found that hydrodechlorination
proceeded instantaneously to
afford a quantitative yield of
1,3-dimethoxybenzene (8a).[15]
In contrast, the reaction of 7a
with m-devices 2 and 3 under
similar conditions gave 8a in 10
and 35% yield, respectively. The
thermal decomposition of a cat-
alytic membrane in a palladium-
nanoparticle membrane was
also effective. Thus, the hydro-
dechlorination of 7a with m-
device
4 was performed at
508C with a residence time of
8 s under similar conditions to
give 8a with 99% yield, al-
though that with m-devices 5
and 6 afforded 8a with 0 and
65% yield, respectively.
Because m-device 1 was readi-
ly prepared to provide the high-
est conversion of 7a, we exam-
ined the hydrodehalogenation
of diverse aryl chlorides, bro-
mides, iodides, and triflates by
using m-device 1 with a resi-
dence time of 2–8 s (Table 2).
Thus, the reaction of 7a afford-
ed 8a with 99% yield
(Scheme 1 and Table 2, entry 1).
The reaction of electron-rich
aryl halides 4-methoxyphenyl
Figure 2. a) Optical microscopy images of catalytic membranes in m-devices 1–3. TEM images of m-devices b) 1,
c) 2, and d) 3 (scale bars=20 nm).
Treatment for higher concentrations of such compounds
through catalytic hydrodehalogenation reactions with explo-
sive hydrogen gas has been reported, but the development of
a catalytic reaction system for treating low concentrations (in
the order of ppm) with safe, nontoxic, nonexplosive HCO2Na
remains a major challenge.[3b,d,12,13] As the reaction rates for
chemical transformations generally depend on substrate con-
centrations,[14] it is difficult to promote an efficient reaction of
the substrates present in concentrations in the order of ppm.
To explore the utility of the m-device containing polymeric
Pd-nanoparticle membranes in the hydrodehalogenation of
aryl halides, first we studied the reaction of 5 mm (1000 ppm)
of an electron-rich aryl chloride [1-chloro-3,5-dimethoxyben-
zene (7a)] in a harmless alcohol (iPrOH) with a saturated aque-
ous solution of HCO2Na. We decided to use safe, inexpensive
HCO2Na in water as the reducing agent instead of dangerous,
explosive hydrogen gas in toxic organic solvents. Representa-
tive results are shown in Scheme 1. When a solution of 7a in
chloride (7b), bromide (9a), and triflate (10a) also proceeded
under similar conditions to give 8b in quantitative yield
(Table 2, entries 2–4). Surprisingly, the more reactive 4-methox-
yphenyl iodide (11 a) did not react at 508C (8b: 9% yield;
Table 2, entry 5), although the general ease of the reductive
hydrodehalogenation of organic halides followed the order of
Scheme 1. Hydrodehalogenation of 1-chloro-3,5-dimethoxybenzene (7a)
with aqueous HCO2Na by using polymeric palladium-nanoparticle mem-
brane-installed m-devices 1–6.
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295

Y. Uozumi et al.
Table 1. Thickness of the dry and wet m-device 1 and its swelling rate (in
iPrOH/H2O).
Thickness [mm]
Swelling ratio
wet/dry
dry
14.3
wet
17.1
1.2
Scheme 2. Plausible catalytic cycle.
H with the co-formation of NaX and CO2. Elimination allows
the formation of ArÀH and regeneration of Pd0. The rate-deter-
mining step should be hydrogenation, so the order of the hy-
drogenation reaction rate is X=Cl>BrꢀOTf>I (Table 2, en-
tries 6–9). However, the ease of oxidative addition follows the
order of X=I>BrꢀOTf>Cl. Therefore, the reaction of a mix-
ture of 7b and 11 b at 908C under similar conditions resulted
in the complete recovery of 7b and the quantitative conver-
sion of 11 b (Scheme 3). The reaction of a mixture of 9b and
11 b also gave similar results.
Figure 3. a) STEM, b) HRTEM, c,d) SEM images, and e) EDS/STEM of a catalyt-
ic membrane in m-device 1.
Finally, the decomposition of PCB and PBB was investigated.
The hydrodehalogenation of 1000 ppm of 7j (one of the PCBs)
and 9c (one of the PBBs) occurred at 508C with a residence
time of 8 s to quantitatively give a fungicide, 8 f (Scheme 4).
I>BrꢀOTf>Cl.[9] The issue was overcome by heating at 908C
to afford 8b in quantitative yield within a residence time of 8 s
(Table 2, entry 5). The electron-deficient 4-acetyl-
phenyl chloride (7c), bromide (9b), triflate (10b),
and iodide (11 b) were quantitatively converted into
acetophenone (8c) at 508C (11 b: 908C) in 2–8 s
(Table 2, entries 6–9). Not only p- (7c), but also m-
(7d) and o-chloroacetophenone (7e) were readily
dechlorinated to give 8c in quantitative yield
(Table 2, entries 10 and 11). Dechlorination of acetyl-
(7c), nitrile- (7 f), and formyl-substituted chloroben-
zenes (7g) gave 8c, benzonitrile (8d), and benzalde-
hyde (8e) in quantitative yield, and these substitu-
ents were not damaged during the reaction (Table 2,
entries 6, 12, and 13). Di- (7h) and trichloroaceto-
phenone (7i), model compounds for PCB treatment,
were smoothly decomposed to give 8c with 96 and
92% yield, respectively, under similar conditions
(Table 2, entries 14 and 15).
Table 2. Hydrodehalogenation of aryl halides and triflates performed by using m-
device 1.[a]
Entry Aryl halide
Residence time Product
[s]
Yield
[%]
1
2
3
4
5
6
7
8
9[c]
10
11
12
13
14
15
7a
8
8
8
8
8
2
4
4
8
3
4
8
8
2
4
C6H4-1,3-(OCH3)2 (8a) 99
C6H5OCH3 (8b)
8b
4-(OCH3)C6H4Cl (7b)
4-(OCH3)C6H4Br (9a)
4-(OCH3)C6H4OTf (10a)
4-(OCH3)C6H4I (11 a)
4-(COCH3)C6H4Cl (7c)
4-(COCH3)C6H4Br (9b)
4-(COCH3)C6H4OTf (10b)
4-(COCH3)C6H4I (11 b)
3-(COCH3)C6H4Cl (7d)
2-(COCH3)C6H4Cl (7e)
4-(CN)C6H4Cl (7 f)
99 (99)[b]
99
8b
8b
99
9 (99)[b]
98
C6H5COCH3 (8c)
8c
98
8c
8c
8c
99
16 (99)[b]
97
8c
97
C6H5CN (8d)
99
4-(CHO)C6H4Cl (7g)
4-(COCH3)C6H3-1,2-Cl2 (7h)
4-(COCH3)C6H2-1,2,3-Cl3 (7i)
C6H5CHO (8e)
99
8c
8c
96
92
Although the catalytic pathway for hydrodehalo-
genation has not yet been proven, a plausible work-
ing model is shown in Scheme 2.[12a,b] The oxidative
addition of ArX to Pd0 gives ArÀPdÀX, which under-
goes hydrogenation with HCO2Na to afford ArÀPdÀ
[a] Conditions: Aryl halides 7, 9–11 (5.0mM in iPrOH; flow rate: 0.5 mLminÀ1), saturat-
ed aqueous solution of HCO2Na (flow rate: 0.5 mLminÀ1), m-device 1 (catalytic mem-
brane=8 mg), 508C. [b] Performed at 908C. [c] 5.0mm substrates in iPrOH/EtOAc (5:2).
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Polymeric Nanoparticle Membrane-Installed Microflow Devices
Conclusions
Diverse palladium-nanoparticle
membranes
were
installed
inside a microchannel reactor
through the “ship-in-a-bottle”
protocol of our molecular con-
volution method, creating cata-
lytic microflow devices. More-
over, we developed a new cata-
lytic microflow system for the
hydrodehalogenation of aryl
chlorides, bromides, iodides,
and triflates. The microflow
device system facilitated instan-
taneous quantitative conversion
to dehalogenated products at
50–908C within
a residence
time of 2–8 s. Chlorobiphenyl
(7j) at concentrations of 10–
1000 ppm was readily decom-
posed to give a less harmful
fungicide (8 f) in quantitative
yield.
Experimental Section
Preparation of PdCl2/polyvinyl-
pyridine membrane inside a
microchannel (composite A)
A solution of poly(4-vinylpyridine)
in ethyl acetate/MeOH (3:1;
5.0 mm; solution A) and an aque-
ous solution of PdCl2 (1.7 mm)
and NaCl (17 mm; solution B)
were introduced into the micro-
channel (100 mm width, 40 mm
Figure 4. a) Optical microscopy images of catalytic membranes in m-devices 4–6. TEM images of m-devices b) 4,
c) 5, and d) 6 (scale bars=20 nm).
depth, 40 mm length) from two separated inlets at 258C for 1 min
at a flow rate of 20 mLminÀ1. A two-phase parallel laminar flow was
formed under the flowing conditions, and a yellowish polymer
membrane was precipitated out at the interface between the two
parallel flows (membrane thickness=10 mm). The device was then
Scheme 3. Hydrodehalogenation of a mixture of 7b and 11 b, and 9a and
11 b with aqueous HCO2Na by using m-device 1.
Furthermore, 10 and 100 ppm of 7j were perfectly decom-
posed to provide 8 f in quantitative yield; no 7j was detected
by using GC/flame ionization detector (FID) analysis.
Scheme 4. Decomposition of 10–1000 ppm of PCB and PBB.
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Y. Uozumi et al.
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Preparation of polymeric Pd-nanoparticle membrane-instal-
led m-device 1 and general procedure for the hydrodechlori-
nation promoted by m-device 1 with HCO2Na
A solution of 7 (5.0 mm) in iPrOH and a saturated aqueous solution
of HCO2Na were introduced into the abovementioned device from
two separated inlets at 508C at a flow rate of 0.5 mLminÀ1 for
30 min to give the polymeric palladium-nanoparticle membrane-in-
stalled m-device 1. Into the membrane-divided channel of m-device
1, a solution of 7 (5.0 mm) in iPrOH and a saturated aqueous solu-
tion of HCO2Na were introduced from two separated inlets at 508C
at a flow rate of 0.5 mLminÀ1, and two parallel laminar layers
flowed through the channel with a residence time of 8 s. The re-
sulting organic/aqueous microstream was collected from the outlet
of the channel to afford a quantitative yield of the hydrodechlori-
nated 8. The chemical yield and structure of the product were de-
termined by performing GC analysis (based on an authentic
sample).
Acknowledgements
Partial financial support was provided by JSPS (Grant-in-Aid for
Scientific Research #20655035; Grant-in-Aid for Scientific Re-
search on Innovative Area #2105) and the RIKEN Advanced Sci-
ence Institute (Fund for Seeds of Collaborative Research). We
thank Dr. G. Hamasaka at IMS and Y. Sakamaki at the RIKEN Ad-
vanced Technology Support Division for TEM and EDS/STEM anal-
yses.
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Keywords: heterogeneous catalysis · membranes · microflow
reactors · nanomaterials · palladium
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Received: August 1, 2011
Revised: December 9, 2011
Published online on January 13, 2012
[15] The reaction did not proceed without the catalytic membrane under
similar conditions. The resulting aqueous alcohol microstream was col-
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www.chemsuschem.org
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