Iron nanoparticles supported on chemically-derived graphene: Catalytic hydrogenation with magnetic catalyst separation

Article abstract of DOI:10.1002/adsc.201000877

Iron nanoparticles (Fe-NP) supported on chemically-derived graphene (CDG) were prepared and identified as an effective catalyst for the hydrogenation of alkenes and alkynes. The catalyst canh easily be separated by magnetic decantation.

Full text of DOI:10.1002/adsc.201000877

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DOI: 10.1002/adsc.201000877
Iron Nanoparticles Supported on Chemically-Derived Graphene:
Catalytic Hydrogenation with Magnetic Catalyst Separation
Mario Stein,a Jçrg Wieland,a Peter Steurer,b Folke Tçlle,b Rolf Mꢀlhaupt,b,c,*
and Bernhard Breita,c,*
a
Institut fꢀr Organische Chemie und Biochemie, Albert-Ludwigs-Universitꢁt Freiburg, Albertstrasse 21, 79104 Freiburg,
Germany
Fax : (+49)-761-203-8715; e-mail: bernhard.breit@chemie.uni-freiburg.de
Freiburger Materialforschungszentrum (FMF) and Institut fꢀr Makromolekulare Chemie, Albert-Ludwigs-Universitꢁt
b
Freiburg, Stefan-Meier-Strasse 21–31, 79104 Freiburg, Germany
Fax : (+49)-761-203-6319; e-mail: rolf.muelhaupt@makro.uni-freiburg.de
Freiburg Institute for Advanced Studies (FRIAS), Soft Matter Research, Albertstrasse 19, 79104 Freiburg, Germany
c
Received: November 22, 2010; Published online: March 4, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000877.
Abstract: Iron nanoparticles (Fe-NP) supported on
chemically-derived graphene (CDG) were prepared
and identified as an effective catalyst for the hydro-
genation of alkenes and alkynes. The catalyst can
easily be separated by magnetic decantation.
transfer hydrogenation of C=O bonds including
ligand-metal bifunctional catalysis.[15,16] Furthermore,
direct reductive aminations of carbonyl compounds
with iron were achieved employing an Fe-EDTA
complex.[17]
There are only few examples known for iron cata-
lysts that are active in the hydrogenation of C=C
bonds. Chirik and co-workers used an iron complex
with a tridentate N-based ligand for the hydrogena-
tion of various alkenes under mild conditions.[18]
Bhanage et al. could successfully apply the Fe-EDTA
complex to the selective hydrogenation of a,b-unsatu-
Keywords: graphene; hydrogenation; iron catalysis;
magnetic separation; nanoparticles
Hydrogenation reactions are considered one of the rated carbonyl compounds.[19] An interesting approach
most valuable synthetic transformations in organic towards catalytic hydrogenation with iron nanoparti-
chemistry. The hydrogenation of olefins and alkynes cles (NP) without the need of additional ligands was
is typically promoted by heterogeneous catalysts reported by de Vries et al.[20] In this case the nanopar-
based on precious metals such as palladium or plati- ticles were homogeneously dispersed in solution.
num.[1–5] In order to render catalytic hydrogenation However, in order to make the catalyst easily separa-
reactions more efficient, many efforts have been ble, it would be rather attractive to have solid-sup-
made to decrease the costs for these reactions either ported iron nanoparticles.
by obtaining highly active catalytic species or by sub-
stituting inexpensive metals such as iron for the com- of iron nanoparticles supported on chemically-derived
monly used noble metals Pd and Pt.[6–10]
graphene (CDG) as hydrogenation catalyst. This sup-
Herein, we describe the synthesis and application
The broad availability of iron makes it a desirable port features simple and inexpensive preparation as
metal for catalysis. Furthermore, iron compounds are well as very high specific surface areas. Graphene-
usually easy to handle and toxicologically inert. It is supported metal NPs were already successfully ap-
therefore not only economically but ecologically ex- plied in different catalytic reactions.[21] The synthesis
pedient to use catalysts based on iron.[11] Iron cata- of Fe-NP/CDG is described in Scheme 1. CDG was
lysts are of enormous industrial importance as they prepared in a two-step procedure starting from graph-
are an important part of the Haber–Bosch pro- ite. The first step is an oxidation, which was first de-
cess[12,13] as well as the Fischer–Tropsch process[14]
.
scribed by Hummers and Offeman, resulting in graph-
Consequently, the iron-catalyzed hydrogenation of ite oxide (GO).[22] It consists of non-planar carbon
coal, carbon monoxide and carbon dioxide is heavily sheets due to introduced functionalities like epoxy,
investigated. Progress has also been made in the hydroxy, carbonyl and carboxyl groups. In the second
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Scheme 1. Synthesis of Fe-NP/CDG from graphite. )))=ultrasound.
step, GO was reduced to CDG by flash heating, ac- The catalyst prepared as described above was tested
companied by exfoliation due to the release of CO, in olefin hydrogenation reactions.
CO2 and H2O.[23] The deposition of iron NPs on the
In initial attempts, 1-hexene was chosen as a test
functionalized graphene layers was achieved by sono- substrate for the catalytic hydrogenation with Fe-NP/
chemical treatment of Fe(CO)5 in a suspension of CDG (Scheme 2). At 1008C, a hydrogen pressure of
CDG in diphenylmethane at room temperature. TEM 20 bar, and a comparatively low catalyst loading of
images showed NPs with an average size of 4.37Æ 0.9 mol%, a conversion of 43% towards n-hexane was
1.62 nm deposited on the CDG sheets (Figure 1). Dif- observed. Complete conversion was achieved by addi-
fraction patterns and ESI images indicated the pres- tion of a small amount of Grignard reagent (EtMgCl)
ence of Fe(0). The loading of Fe-NPs was 3.66 wt%. under otherwise identical conditions. Presumably, the
Grignard reagent acts as a surface activator and re-
duces remaining oxide shells of the Fe-NPs which
may form via remaining oxidic functions on CDG. In
order to have the most active catalyst in hand, in all
Figure 1. TEM image and histogram showing the particle
size distribution.
Scheme 2. Catalytic hydrogenation of 1-hexene.[a] Addition
of 5 mol% EtMgCl.
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Iron Nanoparticles Supported on Chemically-Derived Graphene: Catalytic Hydrogenation
further experiments the catalyst was preactivated in version again (Table 1, entry 6). Furthermore, Fe-NP/
this manner.
CDG gave good results in the hydrogenation of cyclic
To demonstrate the involvement of Fe-NP/CDG as olefins (Table 1, entries 7 to 10). Unfortunately, this
catalytically active species in the hydrogenation, sev- catalyst revealed to be inapplicable in the hydrogena-
eral control experiments were performed. Thus, at- tion of internal double bonds of acyclic systems
tempted hydrogenation on CDG in the absence of (Table 1, entries 11 to 14). Little to no product was
Fe-NPs and in the presence of EtMgCl or MgCl2 did identified with tri- and tetrasubstituted olefins
not furnish even traces of hydrogenated products.
It was recently published that metal contaminants
(Table 1, entries 12 and 13).
The catalyst was also active in the hydrogenation of
of the iron source such as copper may act as the real alkynes (Table 1, entries 15 to 18). Surprisingly, disub-
catalyst.[24] Consequently, we checked the presence of stituted alkynes gave better results than the terminal
all relevant metals for hydrogenation catalysis in Fe- 1-octyne. Full conversion in the hydrogenation of 1-
NP/CDG as well as in the magnesium source by phenyl-1-propyne to n-propylbenzene (Table 1,
AAS. In all cases, all concentrations were below the entry 17) raised the speculation that electron-with-
detection limits. Hence, it can be assumed that Fe- drawing groups favor the reaction. However, tolane
NP/CDG acts as the real catalyst and other metals was hydrogenated with rather moderate yield
have no influence on the catalytic activity.
(Table 1, entry 18). This might be caused by the steric
Different olefins were hydrogenated with Fe-NP/ hinderance of the triple bond and the stability of the
CDG in order to define the scope of substrates fully conjugated substrate.
(Table 1). Complete or almost complete conversion
Subsequent to the hydrogenation reactions the cat-
was obtained with terminal olefins (Table 1, entries 1 alyst was recovered by centrifugation. Fe-NP/CDG
to 4 and 6). 1-Hexene and 1-octene were also tested was washed twice with degassed H2O and CH2Cl2,
under milder reaction conditions. 1-Hexene was quan- dried under vacuum and reapplied in a second cycle
titatively hydrogenated at 258C (Table 1, entry 3) of hydrogenation of 1-hexene. Again, complete con-
while for 1-octene, the yield dropped to 11% version to n-hexane was observed.
(Table 1, entry 5). However, it was shown that a fur-
A rather simple and attractive way of catalyst sepa-
ther increase of the hydrogen pressure led to full con- ration was the magnetic decantation (Figure 2).[25]
Therefore, a constant magnetic field of a Nd-Fe-B
based magnet was applied to the reaction mixture
after hydrogenation on homogeneous catalyst slurries.
Induced by magnetic separation, the catalyst re-
Table 1. Hydrogenation of different olefins and alkynes with
H2 using Fe-NP/CDG.[a]
Entry
Substrate
Yield [%][b]
mained at the wall of the glass tube and the clear so-
lution was ready for decantation. The recovered cata-
lyst was again successfully recycled without any loss
of activity and without renewed activation with ethyl
magnesium chloride reagent.
In principal, the catalysis with Fe-NP/CDG could
proceed in either a heterogeneous or homogeneous
manner, which should be clarified by a three-phase
1
styrene
99
2
1-hexene
1-hexene
1-octene
1-octene
100
100
100
11
100
100
73
79
99
0
0
3[c]
4
5[c]
6[c,d]
7
8
9
10
11
12
13
14
15
16
17
18
1-octene
norbornene
cyclopentene
cyclohexene
cyclooctene
trans-2-hexene
2-methyl-1-butene
2,3-dimethyl-2-butene
trans-stilbene
1-octyne
0
7
0
4-octyne
1-phenyl-1-propyne
tolane
7[e]
100[e]
24[f]
[a]
Olefin (1 mmol) in THF (1 mL), Fe-NP/CDG
(0.9 mol%), EtMgCl (2M/THF, 5 mol%), H2 (20 bar),
1008C, 24 h.
Yields determined by GC and NMR analysis.
Reaction temperature was 258C.
H2 pressure was 30 bar.
Only the corresponding alkane was identified as product.
21% stilbene, 3% dibenzyl.
[b]
[c]
[d]
[e]
[f]
Figure 2. Fe-NP/CDG after hydrogenation reaction dis-
persed (left); magnetic separation of the catalyst at ambient
temperature with a Nd-Fe-B based magnet (right).
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drogenation products were known and identified by GC,
GC/MS and NMR. Conversion was determined by GC.
General Procedure for Hydrogenation Reactions at
High Pressure
Scheme 3. Three-phase hydrogenation control experiment
with an alkenyl-functionalized Wang resin.
The reaction mixture was prepared as described above, and
was transferred via syringe into a stainless steel autoclave
containing a glass inlet. The autoclave was purged three
experiment (Scheme 3). For this reason, 1-hexenoic times with H2. The reaction mixture was stirred for 24 h at
the temperature and H2 pressure as specified in Table 1. The
pressure was relieved and the catalyst was separated from
the reaction mixture as described above. The hydrogenation
products were known and identified by GC, GC/MS and
NMR.
acid attached via esterification to a Wang resin was
subjected to our hydrogenation conditions. The spatial
separation of the substrate from the catalyst will pro-
hibit the reaction if the catalyst acts heterogeneously.
Indeed, after cleavage from the solid support only
starting material was detected and no hydrogenation
product was observed. Contrary to former similar ex-
periments with Pd/CDG catalysts,[20a] iron nanoparti-
cles supported on CDG appear to be truly heteroge-
neous catalysts without noteworthy leaching of Fe-
NP.[26]
In conclusion, we have established a simple, cheap
and rapid synthesis for Fe nanoparticles supported on
chemically derived graphene by decomposition of
Fe(CO)5 with ultrasound. The obtained material (Fe-
NP/CDG) was highly active in the hydrogenation of
terminal and cyclic olefins. Furthermore, the catalyst
was separated in an elegant manner by simple mag-
netic decantation and recycled without loss of activity.
Work is in progress to enhance catalytic activities and
to expand the application to other substrates.
Acknowledgements
This work was supported by the Fonds der Chemischen In-
dustrie, the DFG “Catalysts and Catalytic Reactions for Or-
ganic Synthesis” (GRK 1038), and the Krupp Foundation
(Alfried Krupp Award for young university teachers to B.B.).
We thank BASF SE, Umicore, Chemtura, and Wacker for
generous gifts of chemicals and Dr. R Thomann, Dr. Y. Tho-
mann, Dr. M. Keller, and C. Warth for analytical help.
References
[1] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. 2005,
117, 8062–8083; Angew. Chem. Int. Ed. 2005, 44, 7852–
7872.
[2] ꢄ. Molnꢅr, A. Sꢅrkꢅny, M. Varga, J. Mol. Catal. A
2001, 173, 185–221.
Experimental Section
Preparation of Fe-NP/CDG
[3] M. Studer, H.-U. Blaser, C. Exner, Adv. Synth. Catal.
2003, 345, 45–65.
[4] T. Burgi, A. Baiker, Acc. Chem. Res. 2004, 37, 909–
917.
[5] M. Irfan, M. Fuchs, T. N. Glasnov, C. O. Kappe, Chem.
Eur. J. 2009, 15, 11608–11618.
[6] B. R. James, in: Homogeneous Hydrogenation, Wiley-
Interscience, New York, 1973, pp 64–72.
[7] S. Gaillard, J.-L. Renaud, ChemSusChem 2008, 1, 505–
509.
[8] C. Bianchini, A. Meli, M. Peruzzini, P. Frediani, C. Bo-
hanna, M. A. Esteruelas, L. A. Oro, Organometallics
1992, 11, 138–145.
[9] S. Enthaler, B. Hagemann, G. Erre, K. Junge, M.
Beller, Chem. Asian J. 2006, 1, 598–604.
[10] V. V. K. M. Kandepi, J. M. S. Cardoso, E. Peris, B.
Royo, Organometallics 2010, 29, 2777–2782.
[11] S. Enthaler, K. Junge, M. Beller, Angew. Chem. 2008,
120, 3363–3367; Angew. Chem. Int. Ed. 2008, 47, 3317–
3321.
CDG (920 mg) was suspended in a solution of Fe(CO)5
(447 mg, 2.28 mmol) in diphenylmethane (200 mL). The re-
action mixture was ultrasonicated three times for 10 min at
room temperature with an ultrasonic lancet (Bandelin, So-
noplus HD 2200, lancet KE76).
General Procedure for Hydrogenation Reactions at
Atmospheric Pressure
In a dry and argon-flushed Schlenk tube, Fe-NP/CDG
(14 mg, 9.1 mmol, 0.009 equiv.) was suspended in THF
(1 mL). EtMgCl (25 mL, 2M in THF, 0.05 mmol, 0.05 equiv.)
and olefin (1 mmol, 1.0 equiv.) were added. The Schlenk
tube was purged 5 times with H2 from an H2 gas filled bal-
loon. The reaction mixture was stirred for 24 h at a pressure
of 1 bar H2 and a temperature as specified in the text. The
catalyst was removed by one of the following procedures:
(a) Fe-NP/CDG was separated by condensation of all other
compounds with a flask cooled to À1968C in high vacuum
(10À2 mbar); (b) A constant magnetic field was applied to
the reaction vessel. The liquid was removed by syringe while
Fe-NP/CDG remained on the inner wall of the reaction
tube. The residue was washed with THF (2ꢃ1 mL). All hy-
[12] G. Ertl, J. Vac. Sci. Technol. A 1983, 1, 1247–1253.
[13] G. Ertl, Nachr. Chem. Tech. Lab. 1983, 31, 178–182.
[14] E. de Smit, B. M. Weckhuysen, Chem. Soc. Rev. 2008,
27, 2758–2781.
[15] R. H. Morris, Chem. Soc. Rev. 2009, 38, 2282–2291.
526
asc.wiley-vch.de
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 523 – 527

Iron Nanoparticles Supported on Chemically-Derived Graphene: Catalytic Hydrogenation
[16] C. P. Casey, H. Guan, J. Am. Chem. Soc. 2007, 129,
5816–5817.
W. J. Stark, O. Reiser, Angew. Chem. 2010, 122, 1911–
1914; Angew. Chem. Int. Ed. 2010, 49, 1867–1870.
[22] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc.
1958, 80, 1339.
[17] M. D. Bohr, M. J. Banushali, N. S. Nandurkar, B. M.
Bhanage, Tetrahedron Lett. 2008, 49, 965–969.
[18] a) R. J. Trovitch, E. Lobkovsky, E. Bill, P. J. Chirik, Or-
ganometallics 2008, 27, 1470–1478; b) A. M. Archer,
M. W. Bouwkamp, M.-P. Cortez, E. Lobkovsky, P. J.
Chirik, Organometallics 2006, 25, 4269–4278; c) S. C.
Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc.
2004, 126, 13794–13807.
[23] a) M. J. McAllister, J. L. Li, D. H. Adamson, H. C.
Schniepp, A. A. Abdala, J. Liu, M. Herrera-Alonso,
D. L. Milius, R. Car, R. K. Prud’homme, I. A. Aksay,
Chem. Mater. 2007, 19, 4396–4404; b) H. C. Schniepp,
J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso,
D. H. Adamson, R. K. Prud’homme, R. Car, D. A. Sa-
ville, I. A. Aksay, J. Phys. Chem. B 2006, 110, 8535–
8539.
[24] S. L. Buchwald, C. Bolm, Angew. Chem. 2009, 121,
5694–5695; Angew. Chem. Int. Ed. 2009, 48, 5586–
5587.
[25] For a review about magnetically separable nanocata-
lysts, see: S. Shylesh, V. Schꢀnemann, W. R. Thiel,
Angew. Chem. 2010, 122, 3504–3537; Angew. Chem.
Int. Ed. 2010, 49, 3428–3459.
[19] M. D. Bhor, A. G. Panda, S. R. Jagtap, B. M. Bhanage,
Catal. Lett. 2008, 124, 157–164.
[20] a) P.-H. Phua, L. Lefort, J. A. F. Boogers, M. Tristany,
J. G. de Vries, Chem. Commun. 2009, 3747–3749; b) C.
Rangheard, C. de Juliꢅn Fernꢅndez, P.-H. Phua, J.
Hoorn, L. Lefort, J. G. de Vries, Dalton Trans. 2010, 39,
8464–8471.
[21] a) G. M. Scheuermann, L. Rumi, P. Steurer, W. Bann-
warth, R. Mꢀlhaupt, J. Am. Chem. Soc. 2009, 131,
8262–8270; b) S. Wittmann, A. Schꢁtz, R. N. Grass,
[26] An Fe leaching of 1% was obtained by AAS from the
reaction mixture after separation of the catalyst.
Adv. Synth. Catal. 2011, 353, 523 – 527
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