Highly efficient iron(0) nanoparticle-catalyzed hydrogenation in water in flow

Article abstract of DOI:10.1039/c3gc40789f

Highly efficient catalytic hydrogenations are achieved by using amphiphilic polymer-stabilized Fe(0) nanoparticle (Fe NP) catalysts in ethanol or water in a flow reactor. Alkenes, alkynes, aromatic imines and aldehydes were hydrogenated nearly quantitatively in most cases. Aliphatic amines and aldehydes, ketone, ester, arene, nitro, and aryl halide functionalities are not affected, which provides an interesting chemoselectivity. The Fe NPs used in this system are stabilized and protected by an amphiphilic polymer resin, providing a unique system that combines long-term stability and high activity. The NPs were characterized by TEM of microtomed resin, which established that iron remains in the zero-valent form despite exposure to water and oxygen. The amphiphilic resin-supported Fe(0) nanoparticles in water and in flow provide a novel, robust, cheap and environmentally benign catalyst system for chemoselective hydrogenations.

Full text of DOI:10.1039/c3gc40789f

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Highly efficient iron(0) nanoparticle-catalyzed
hydrogenation in water in flow†
Cite this: DOI: 10.1039/c3gc40789f
Reuben Hudson,a,b,c Go Hamasaka,b Takao Osako,b Yoichi M. A. Yamada,c
Chao-Jun Li,a Yasuhiro Uozumi*b,c and Audrey Moores*a
Highly efficient catalytic hydrogenations are achieved by using amphiphilic polymer-stabilized Fe(0)
nanoparticle (Fe NP) catalysts in ethanol or water in a flow reactor. Alkenes, alkynes, aromatic imines and
aldehydes were hydrogenated nearly quantitatively in most cases. Aliphatic amines and aldehydes,
ketone, ester, arene, nitro, and aryl halide functionalities are not affected, which provides an interesting
chemoselectivity. The Fe NPs used in this system are stabilized and protected by an amphiphilic polymer
resin, providing a unique system that combines long-term stability and high activity. The NPs were charac-
terized by TEM of microtomed resin, which established that iron remains in the zero-valent form despite
exposure to water and oxygen. The amphiphilic resin-supported Fe(0) nanoparticles in water and in
flow provide a novel, robust, cheap and environmentally benign catalyst system for chemoselective
hydrogenations.
Received 26th April 2013,
Accepted 13th June 2013
DOI: 10.1039/c3gc40789f
www.rsc.org/greenchem
iron-based nanoparticles (NPs).16–19 The de Vries group used
soluble Fe NPs for hydrogenation of alkenes and alkynes,7,20
Introduction
Hydrogenation, known to chemists for decades, remains one while the Breit group functionalized graphene sheets with Fe
of the most studied reactions. Its industrial applications span NPs21 to further aid recoverability and recycling. In these two
petrochemical conversion to pharmaceuticals synthesis; a systems, an accessible Fe(0) surface is responsible for the cata-
plethora of catalysts exists for this transformation. However, lytic activity.22 These two systems, however, exhibit great sensi-
hydrogenation reactions heavily rely on the chemistry of group tivity to traces of either oxygen or water, thus limiting use in
9 and 10 metals.1 These elements are very expensive and their practical applications.
price is highly volatile on the stock market.2 Regulatory organi-
Over the past two decades, the use of water has gained con-
zations, such as the FDA, limit residual levels in pharma- siderable momentum as a solvent for organic reactions.23 It
ceutical products to ppm or less levels because of their enables novel reactivity,24,25 speeds reactions by the hydro-
inherent toxicity. In response to these economic, regulatory phobic26 and ‘on water’ effects.27 Ohde et al.28 hydrogenated
and environmental concerns, efforts have been made to olefins with palladium nanoparticles in water-supercritical
improve recovery and limit leaching,3 or to seek metal-free CO2 microemulsions. More recently, Xiao et al. achieved asym-
solutions.4,5 Iron has also been at the centre of renewed inter- metric transfer of hydrogen ‘on water’.29,30 Amphiphilic poly-
est in both homogeneous and heterogeneous hydrogenation.6,7 mers have also been used as supports for metal complexes and
Iron complexes can catalyse the hydrogenation of alkenes,8,9 nanoparticles.31–33 They are able to extract organic substrates
carbonyls,6,10,11 imines,11 carbonates12 in addition to the selec- from the aqueous phase resulting in higher concentrations
tive hydrogenation of alkynes to alkenes,13,14 but such systems near the catalyst, speeding the reaction. These systems demon-
have limited recoverability. In contrast, heterogeneous catalysts strate efficiency in hydrogenation,34–36 oxidation,37,38 cross
are more amenable to recycling,15 and several groups turned to coupling39 and hydrodechlorination31,40,41 reactions in water.
The field of heterogeneous catalysis in water has been exten-
sively reviewed.42
aCentre in Green Chemistry and Catalysis, Department of Chemistry,
We recently demonstrated that core–shell iron/iron oxide
McGill University, 801 Sherbrooke St. West, Montreal, QC H3A 0B8, Canada.
nanoparticles are effective hydrogenation catalysts in protic
media.43 Exposure to oxygen and/or the presence of up to 1%
of water does not affect catalytic activity, thanks to the protec-
tive effect of the iron oxide shell. However, neither pure nor
water-rich mixtures could be used as a medium due to rapid
catalyst deactivation. Additionally, the presence of the oxide
E-mail: audrey.moores@mcgill.ca
bDivision of Complex Catalysis, Institute for Molecular Science, Okazaki, 444-8787,
Japan. E-mail: uo@ims.ac.jp
cRIKEN Center for Sustainable Resource Science, 2-1 Hirowasa, Wako, 351-0198,
Japan
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3gc40789f
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Scheme 1
Scheme 2
Fig. 1 Schematic of hydrogenation reactions undertaken with polymer sup-
ported iron nanoparticles, under flow conditions (PS = polystyrene).
LK-stabilized Fe NPs. These LKs are terminated with a func-
tional group (FG = NH2, COOH, Br) and may also contain a
polyethylene glycol (PEG) spacer. The Fe NPs were synthetized
in situ, in the presence of the polymer. We adapted two known
methods to produce our novels catalysts: (1) the thermal
decomposition50,51 of Fe(CO)5 (Scheme 1); (2) the reduction of
FeSO4 using black tea as a reducer52 (Scheme 2). The first
method was expected to afford salt free, smaller and more
reduced NPs than any other known method of Fe NPs pro-
duction. The second synthesis proceeds via reduction of iron
salts by tea polyphenols52 and was selected as a green alterna-
tive to the first synthesis. The catalysts reported herein are
notably different from already published methods in two
notable ways. First, rather than using oleyamine, or other
stabilizers, as a stabilizing agent, we use FG terminated PEGy-
lated spacers, which likely passivate the nanoparticle surface
(vide infra), dissuading formation of a surface oxide layer, as
generally observed.50,51 Second, because the polymer is
present during the time of nanoparticle seeding and growth, it
affords a robust resin that can be used in a flow system.
shell, although protective, limited access to the active surface
and forced the use of more drastic conditions and longer reac-
tion times.
Building upon those initial results, we investigated support-
ing catalytically active Fe NPs on an amphiphilic polymer resin
(Fig. 1). This combination provides the opportunity to use the
catalyst in a flow system.44 Flow systems are widely used to alle-
viate waste, work-up effort and scale-up problems.45,46 Hydro-
genation, in fact, has been one of the most researched
reactions in flow systems because they allow to greatly reduce
the volumes to pressurize, improving both gas consumption
and safety.47–49
We herein report catalytic and selective hydrogenation of
alkenes and alkynes, as well as aromatic imines and ketones,
involving three green chemistry themes—flow chemistry, water
as a benign solvent, and the use of cheap, non-toxic and bio-
logically-essential heterogeneous iron (Fig. 1). By adapting
known syntheses of reduced iron particles to the presence of a
stabilizing polymer, we report the discovery of novel polymer
supported iron nanoparticles that are uniquely robust toward
oxidation, and yet active for hydrogenation catalysis. Quite
remarkably, the polymer resin increased drastically the longevity
of the nanoparticles, resulting in catalytic activity in ethanol
and water–ethanol mixtures of up to 9 : 1. Interestingly, this
method is very selective and specifically preserves aryl halide
The resulting materials were characterized by transmission
electron microscopy (TEM) of microtomed slices of the
materials. This method allowed visualization of the Fe NPs
embedded in the linker covering the PS beads (Fig. 2).
functionalities,
a known limitation of palladium-based
systems. In a demonstration of durability, scale-up of the
system to 20 grams of substrate (styrene) can easily be achieved
by increasing reaction time. Because this system is robust to
water, iron can now be envisaged as a realistic competitor to
platinum series metals as a practical hydrogenation catalyst.
Results and discussion
Synthesis and characterization of polymer resin stabilized
Fe(0) NPs
Amphiphilic polymer resins composed of polystyrene (PS)
beads (average size 90 micron), functionalized with a variety of
linkers (LK) were used to support Fe NPs (Fig. 1). The PS beads
Fig. 2 TEM images of images of (A) and (B) FeNP@PS-PEG-NH2 (thermal
decomposition); (C) FeNP@PS-PEG-NH2 (tea reduction); (D) FeNP@PS-PEG-Br
(thermal decomposition); (E) FeNP@PS-PEG-COOH (thermal decomposition);
serve as compact supports, whose surfaces are covered with (F) FeNP@PS-NH2 (thermal decomposition).
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Table 1 Ethylbenzene yield in batch and flow as a function of polymer-
immobilized Fe NP composition
Fe
Yield/
Yield/
Entry
Composition
loadinga
flowb (%)
batchc (%)
d
1
2
3
4
5
6
7
FeNP@PS-PEG-NH2e
FeNP@PS-PEG-NH2
FeNP@PS-PEG-COOHd
FeNP@PS-PEG-Brd
11.72
2.55
5.01
4.05
1.03
0
100
100
100
100
100
0
44
13
24
8
19
0
d
FeNP@PS-NH2
PS-PEG-NH2
FeNPf
All iron
N/Ag
26
a mg Fe g−1 polymer determined by ICP. b Reaction conditions: 100 °C,
40 bar, 1 mL min−1 through 300 mg polymer, 0.05 M styrene in EtOH
(residence time 53 seconds). c Reactions conditions: 100 °C, 40 bar,
0.05 M styrene in EtOH (17 mL), 6 hours. d Fe nanoparticles generated
by thermal decomposition of Fe(CO)5. e Fe nanoparticles generated by
black tea-extract reduction of FeSO4. f Reaction conditions: 100 °C,
40 bar, 0.05 M styrene in EtOH (17 mL), 5 mol% FeNP (generated by
thermal decomposition of Fe(CO)5 with oleyamine as a stabilizer),50
6 hours. g The catalyst could not be tested in flow without polymer
support.
Fig. 3 High resolution TEM image of a single Fe(0) NP exhibiting lattice fringes
in FeNP@PS-PEG-NH2.
PS-(PEG)-NH2 afforded the best results. Well dispersed and
monodisperse ∼5 nm Fe NPs were observed in the case of
thermal decomposition (Fig. 2A and B). At high resolution,
regular lines evince the crystal lattice of FeNP@PS-PEG-NH2
(Fig. 3). The lines are separated by 2.45 Å, which is in good
agreement with the interatomic spacing calculated to be
2.49 Å, from either bcc or fcc iron.53 These are very different
from spacing measured for iron oxides.54,55 They demonstrate
the Fe(0) nature of the nanoparticles. Fewer particles were
drastic effect on loading with a 10 fold drop in Fe content,
perhaps because the extra oxygen atoms help to coordinate
iron, helping seed the formation of nanoparticles.
Catalytic tests
visible when using the tea reduction method, most of them These polymer supported-Fe NPs were then assessed in both
having again a size of ∼5 nm (Fig. 2C). With both PS-(PEG)-Br flow and batch conditions for the hydrogenation of styrene in
and PS-(PEG)-COOH, the thermal decomposition afforded ethanol (Table 1). The flow system consists of a peristaltic
larger particles between 15 and 20 nm for Br and 5 and 20 nm pump that forces a solution of the solvent and substrate
for COOH. PS-NH2 afforded localized clusters of particles through a cartridge packed with the catalyst, heated and press-
between 5 and 10 nm. Large sections of the matrix did not urized with H2 gas.
contain Fe NPs. In no sample did we see any iron oxide layer at
All iron/polymer systems provided quantitative yields in the
the surface of the particles, as we had observed in previous flow conditions. However, only the FeNP@PS-PEG-NH2 gener-
work.43 This demonstrates the excellent stability of this system ated by thermal decomposition provided even a moderate yield
toward oxidation, presumably through a stabilizing effect of in batch conditions (Table 1, entry 1). Interestingly, the
the polymeric support.
FeNP@PS-PEG-NH2 produced using the tea extract method
In addition to TEM, the materials were characterized by ICP afforded a modest yield of 13% in batch conditions (entry 2),
(Table 1). The highest loading was obtained with FeNP@ which is superior to what could be expected from simply con-
PS-PEG-NH2 with 11.72 mg Fe g−1 (entry 1), confirming the sidering the 5-fold lower loading from the thermal decompo-
observation made by TEM. In terms of loading, thermal sition method. Replacement of the NH2 functionality by
decomposition was a more efficient method than tea reduction COOH (entry 3) or Br (entry 4), or the removal of the PEG
as the latter afforded material with about 5 times less iron spacer (entry 5), afforded lower yields than FeNP@
content (entry 2). Indeed, the reaction conditions of thermal PS-PEG-NH2, presumably because of the lower loading. The
decomposition, which occurs at 180 °C, were more favourable excellent performance of all these systems in flow compared to
to iron diffusion, than those of tea reduction (room tempera- batch conditions can be explained by the very high local cata-
ture). Additionally, the neutral nature of Fe(CO)5 is more lyst concentration within the flow cell. Fe NPs were critical to
adapted to the PEGylated environment surrounding the PS catalysis, PS-PEG-NH2 alone did not lead to any measurable
beads than Fe2+ salts. A change in the terminal group, from conversion in the conditions used (Table 1, entry 6). Nano-
NH2 to COOH (entry 3) or Br (entry 4) affected loading, particles generated without the polymer support (and instead
although all PEGylated systems could successfully immobilize with oleyamine as a stabilizer) could be tested in batch con-
Fe NPs. Amines are classical Fe NPs stabilizers, used notably ditions, but not in flow, because, with no support, they would
in the original synthesis of Fe NPs by thermal decomposition, not stay anchored in the flow system. Simple Fe NPs demon-
and are expected to be better ligands than either –Br or strated a reasonable activity in batch conditions, as expected
–COOH functionalized polymers.38,39 Removal of the PEG for small iron NPs (∼12 nm)50 protected from oxidation by
spacer while keeping the amine functionality (entry 5) had a air or water.7,43 Consistent with these results, this reaction is
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Table 2 Screening of hydrogenation conditionsa
Table 3 Functional group tolerance and selectivitya
Pressure
(bar)
Temp
(°C)
Flow
Yield
(%)
TOF
Yield–selectivity
(%–%)
Entry
(mL min−1
)
(h−1
)
Entry
1
Substrate
Product
1
2
3
4
5
6
9
10
11b
12c
13d
14e
40
60
20
40
40
10
40
40
40
40
40
40
100
100
100
100
100
100
80
2
2
2
1
0.5
1
1
1
1
92
95
85
100
94
54
95
94
100
95
88
0
106
109
97
57
26
30
54
53
57
54
50
0
100–100
2
3
73–91
67–100
4
5
79–87
60
98–100
100
100
100
100
1
1
1
6
100–100
a Reaction conditions: styrene (0.05 M) in EtOH was circulated through
250 mg of FeNP@PS-PEG-NH2 resin (generated by thermal
decomposition of Fe(CO)5). b EtOH–H2O = 1 : 99 v : v. c EtOH–H2O =
50 : 50 v : v. d EtOH–H2O = 10 : 90 v : v. e No catalyst present.
7
35–96
85–98
0–N/A
8b
9
expected to proceed through the classical heterogeneous
hydrogenation mechanism (see ESI†). Based on these prelimi-
nary results, we used FeNP@PS-PEG-NH2 generated by thermal
decomposition in the rest of the study.
10
11
12
0–N/A
Selective hydrogenation of styrene double bond served as a
model reaction for the optimization of reaction conditions
(Table 2). Conditions of 40 bar H2, 100 °C and a flow rate of
2 mL min−1 constituted the benchmark conditions, achieving
a 92% yield of ethyl benzene (Table 2, entry 1). Increasing the
pressure to 60 bar pushed the yield to 95% (Table 2, entry 2).
Decreasing the flow rate to 1 mL min−1 afforded a quantitative
yield, by improving residence time on the catalyst (Table 2,
entry 4). The reaction still proceeded to 95% yield in 50 : 50
ethanol–water (Table 2, entry 12), and an 88% yield in 10 : 90
ethanol–water (Table 2, entry 13).56 This constitutes a great
improvement compared to our previously reported iron/iron
oxide core–shell system, where a 50 : 50 ethanol–water mixture
significantly affected hydrogenation catalysis.43 The increased
stability of the Fe NPs in a 90% water medium arises from the
embedding of the particles in lipophilic pockets of the
polymers, preventing water oxidation of their surface. Both
Fe(0) NP syntheses tested – namely the thermal decomposition
of Fe(CO)5 and the greener black tea extract reduction of FeSO4 –
afforded quantitative yields under benchmark conditions
(40 bar, T = 100 °C, 2 mL min−1, with PS-PEG-NH2 resin)
(Table 1). FeNP@PS-PEG-COOH, FeNP@PS-PEG-Br, FeNP@
PS-NH2 were also equally efficient under the same conditions
(Table 1).
100–100
100–100
13
0–N/A
14
15
0–N/A
99–100
16
84–100
a Reaction conditions: 0.05 M substrate in EtOH, 100 °C, 40 bar H2,
1 mL min−1, 300 mg FeNP@PS-PEG-NH2 (residence time 53 seconds).
b Reaction conditions: 0.05 M substrate in EtOH, 100 °C, 60 bar H2,
1 mL min−1 300 mg FeNP@PS-PEG-NH2 (residence time 53 seconds).
With optimized conditions in hand, functional group toler-
We performed ICP analysis of the digested catalysts and ance and selectivity was explored (Table 3). The catalyst system
could not detect any other metal, not even nickel, a common is highly active for aromatic alkene hydrogenation (entries 1, 5,
contaminant of iron known to be active for hydrogenation. 15 and 16). The catalytic conditions are moderately efficient
This result is consistent with the fact that Fe(CO)5 purity is toward aliphatic alkenes (entry 3) and alkynes, both internal
claimed to be 99.999%. This demonstrates that the cata- (entry 4) and terminal (entry 2). The system demonstrates
lytic activity measured originates solely from iron. Addition- selectivity for C–C double and triple bonds over ketones
ally, ICP analysis of the product solution indicated only (Table 3, entries 6 and 11), esters (entry 5), nitriles (entry 14),
0.007 ppm soluble iron strongly suggesting a heterogeneous arenes (entries 1, 5, 6, 15, and 16). The system also selects
mechanism.
against aliphatic aldehydes (entry 9) and imines (entry 13).
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Table 4 Catalytic selectivity of the polymer-immobilized Fe NPs
Table 6 Activity of styrene derivativesa
Substrate
Catalytic conversion Entry Substrate
Product
Yield (%) Selectivity (%)
Alkenes and alkynes
Aldehydes and imines
Yes
1
45
52
48
>99
>99
>99
Yes for aromatic,
no for aliphatic
No
2
3
Ketone, ester, nitro, arene, benzyl carbamate,
reductive elimination of aryl halides
Table 5 Reactivity of various types of alkenesa
4
5
42
18
>99
>99
Entry
Substrate
Yield (%)
1
2
90
87
6
7
35
58
>99
>99
3
4
83
14
5
6
6
8
54
50
39
>99
>99
>99
Trace
9
a Reaction conditions: 40 bar H2, 100 °C, 0.5 mL min−1, 0.05 M
substrate in EtOH (residence time 116 seconds).
10
a Reaction conditions: 0.05 M substrate in EtOH, 80 °C, 20 bar H2,
3 mL min−1, 300 mg FeNP@PS-PEG-NH2 (residence time 18 seconds).
When activated by an aromatic ring, however, aldehydes
(entries 7 and 8) and imines (entry 12) react. The greater
activity demonstrated by aromatic activated substrates relative
to their aliphatic analogues could be attributed to the lower
LUMO of the former relative to the latter. The reductive elimin- achieve greater separation of chemical yields (Table 6). This
ation of aryl halides (entry 15) or reduction of nitro groups comparison suggests that sterics affect reactivity more than
(entry 16) does not occur under these mild conditions— electronics. For example, the difference in yield between ortho
opening the doors for selective synthesis. Given the sensitivity (entry 6, 35%) and meta (entry 7, 58%) chloro substituted
of aryl halides and aryl nitro groups to reducing conditions styrene overshadows the difference in yield between electron
with platinum series catalysts,57 these two examples of chemo- donating (NH2, entry 9, 50%) and electron withdrawing (NO2,
selectivity open a land of opportunities in synthesis. A entry 10, 39%) para substituted styrene. The trend for methyl-
summary of observed chemoselectivity is provided in Table 4.
styrene further demonstrates this effect. para Methylstyrene
Variously substituted, non-functionalized alkenes were (entry 2, 52%) and meta-methylstyrene (entry 3, 48%) both
assessed (Table 5). The flow speed was divided by 2 because reacted faster than unsubstituted styrene (entry 1, 45%).
such alkenes, when not activated by an aromatic ring, are less However, ortho-methylstyrene reacts slower (entry 2, 42%).
reactive (see Table 3, entries 1 and 3). Hydrogenation of mono- Once in the ortho position, the negative steric effect of the
substituted alkenes (Table 5, entry 1) proceeds in good yields, methyl group outweighs the positive electron-donating effect.
cis alkenes (Table 5, entry 2) reacted slightly faster than trans It is therefore not surprising that the example with the largest
alkenes (Table 5, entry 3). Geminal substitution is more pro- ortho substituent (Br, entry 5) exhibits the lowest overall yield
blematic (Table 5, entry 4). Considering this, it is not sur- (18%). For each entry in Table 6, no side products were
prising that tri-substituted alkenes (Table 5, entry 5) reacted observed.
exceptionally slowly and tetra-substituted alkenes exhibit
Ease of reaction scale-up represents one of the most attrac-
negligible reactivity (Table 5, entry 6). Although the greater tive benefits of flow chemistry. We therefore performed a
degree of substitution would electronically favour hydrogen- scale-up test and hydrogenated five grams of cinnamyl acetate
ation in these substrates over the less substituted analogue, (Fig. 4) in the course of 5.7 hours. This experiment demon-
the reverse reactivity can be attributed with the difficulty of strated the robust nature of the catalyst in prolonged reactions.
coordinating more sterically hindered substrates to a hetero- The hourly snapshots indicate that the yield incrementally
geneous surface.
decreased from 97% to 94%; this very modest yield decrease
For the sake of comparing the reactivity of various styrene may be caused by several factors, including a slight oxidation
derivatives, we used milder reaction conditions in order to of the Fe NPs or an excessive packing of the catalyst beads over
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S COOH, Tentagel S Br and Tentagel S NH2 were purchased
from RAPP Polymere (Germany). Aminomethylated polystyrene
was purchased from Nova Biochem (Germany). High purity
water was obtained by the use of a Milli-Q-Millipore with
0.22 μm filter, Q-guard1 and an ultrapure organex cartridge.
Instruments
An Agilent Technologies 6850 series II Network GC System,
fitted with a flame ionization detector (GC FID). The GC MS
was an Agilent 5973 Network Mass Selective Detector. High
pressure hydrogenation flow reactor catalytic tests were per-
formed on an H-Cube (Thales Nanotechnologies). The trans-
mission electron microscope (TEM) used for imaging was a
JEM-2010F (HR7). Microtome slices were prepared using a
Leica EMFCS. The inductively coupled plasma (ICP) measure-
ments were recorded on a Leeman labs, inc. Profile Plus high
dispersion ICP.
Fig. 4 Scale up and long-term catalyst performance for the hydrogenation of
5 grams of cinnamyl acetate over 5.7 hours (TON = 434).
Synthesis of FeNP@PS-LK by thermal decomposition
time. This equates to a turn over number (TON) of 434. In
another scale-up experiment, we fed the system with 20.7 gram
of styrene over 29 hours, and obtained a total TON of 1685.
In conclusion, we describe a novel synthesis of polymer
supported Fe NPs, which display excellent reactivity for the
selective hydrogenation of alkenes, alkynes, aromatic imines
and aldehydes in a flow system. The catalyst is robust in the
presence of water, surpassing hydrogenation reaction yields
for aqueous mixtures of all other iron nanoparticle systems
reported to date.7,43 Very interesting selectivity was achieved
and complete protection of the sensitive aryl halides during
hydrogenation is an important progress provided by the novel
catalysts presented herein. This catalytic flow system functions
well at the multi gram scale. This work reports for the first
time the convergence of three green chemistry conditions: flow
hydrogenation with H2, use of water and ethanol as benign sol-
vents and the use of heterogeneous iron as a catalyst. More
importantly, it opens the possibility of using iron as a replace-
ment to platinum series metals for hydrogenation reaction
under realistic, industrial conditions. Current efforts in our
labs are focused on achieving a better understanding of the
stability of this system toward oxidation and of the mechanism
of the reaction.
Linker-terminated polystyrene/(polyethylene glycol) beads
(Tentagel from RAPP Polymere or aminomethylated poly-
styrene from Nova, 1 gram) and 1-octadecene (60 mL, 90%
Aldrich) were combined with a magnetic stir bar in a 200 mL
round bottom Schlenk flask. The mixture was purged with N2
at 120 °C for 30 minutes. The temperature was then raised to
180 °C, at which point Fe(CO)5 (2.1 mL, 99.99%, Aldrich) was
quickly injected. The reaction was stirred for 30 minutes at
180 °C under a blanket of N2, then allowed to cool to room
temperature. The resulting polymer-supported iron nanoparti-
cles (FeNP@PS-(PEG)-FG) were washed 3 times with hexanes
(50 mL, 99% Aldrich) and dried under vacuum.
Synthesis of FeNP@PS-(PEG)-NH2 by black tea reduction
Red Label black tea (20 g) was brewed with boiling water (1 L)
and cooled to room temperature. The brewed tea was then
added to a solution of amine-terminated polystyrene/polyethy-
lene glycol beads (1 gram), FeSO4 (3.767 g) and water (2 L) with
a magnetic stir bar in a 4 L glass jug. After stirring for
24 hours, the polymer was filtered and collected.
Characterization of PS-LK supported Fe nanoparticles
To visualize the FeNP@PS-(PEG)-FG catalysts, the polymer was
sliced with a Leica EMFCS microtome. The resulting slices
were loaded onto carbon/Formvar grids and subjected to TEM
analysis on a JEM-2010F (HR7) operated at 120 kV. The inter-
atomic spacing was measured on Fig. 3 using the measuring
tool of the GIMP software over 14 lines. The lattice parameter
is 2.87 Å for bcc iron and 3.515 Å for fcc,53 and thus an inter-
atomic spacing of 2.49 Å.
Experimental section
Chemicals
Styrene (99.0% w/∼0.003% p-t-butylcatechol stabilizer) and
p-methoxystyrene (99% w/200 ppm p-t-butylcatechol stabilizer)
were purchased from Wako Chemicals. Cinnamyl alcohol
(98.0%), trans-2-heptene (99%), cis-2-heptene (97%), Fe(CO)5
(99.999%) and 1-octadecene (90%) were purchased from
PS-LK supported Fe nanoparticles for flow hydrogenation
Aldrich. 2-Phenylpropionaldehyde (98%), cinnamyl acetate A cartridge packed with 300 mg of PS-LK supported Fe nano-
(99%), 4-methylstyrene (99%), benzylideneaniline (98%), and particles was connected to an H-Cube flow hydrogenation
benzylcarbamate (97%) were purchased from TCI. Ethanol system. Each substrate (0.05 M) in ethanol, water, or a mixture
(99.5%) was purchased from Kanto Chemical Co. Tentagel of the two was forced through the system at different rates,
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temperatures and hydrogen pressures. The resulting solution 13 S. Enthaler, M. Haberberger and E. Irran, Chem.–Asian J.,
was characterized by GC-MS and quantified by GC-FID.
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Acknowledgements
We thank the Natural Science and Engineering Council of
Canada (NSERC), the Canada Foundation for Innovation (CFI),
the Canada Research Chairs (CRC), the Fonds de Recherche
du Québec – Nature et Technologies (FRQNT), the Centre in
Green Chemistry and Catalysis (CCGC), the Green Chemistry –
NSERC Collaborative Research and Training Experience
(CREATE) Program, the Riken-McGill fund, McGill University,
JSPS (Grant-in-Aid for Scientific Research #20655035 and
#24550126; Grant-in-Aid for Scientific Research on Innovative
Areas #2105), the CREST program “Elemental Strategy” and JST
ACT-C for their financial support. Lynn Leger (Green Centre
Canada), Shatha Qaqish (Green Centre Canada), Bruce Lennox
and Tomislav Friscic are thanked for their useful comments.
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