Tuning catalytic activity between homogeneous and heterogeneous catalysis: Improved activity and selectivity of free nano-Fe2O3 in selective oxidations

Article abstract of DOI:10.1002/anie.200703418

(Chemical Equation Presented) A light sprinkling of rust: Nanoparticulate Fe₂O₃ is an active catalyst for the selective oxidation of alcohols and olefins. Nano-Fe₂O₃ can be easily isolated by using a magnetic stirrer bar and directly reused without any deactivation over at least five cycles (see picture).

Full text of DOI:10.1002/anie.200703418

Communications
DOI: 10.1002/anie.200703418
Nanoparticle Catalysts
Tuning Catalytic Activity between Homogeneous and Heterogeneous
Catalysis: Improved Activity and Selectivity of Free Nano-Fe O in
2
3
Selective Oxidations**
Feng Shi, Man Kin Tse, Marga-Matina Pohl, Angelika Brückner, Shengmao Zhang, and
Matthias Beller*
Nanocatalysis is considered to be a bridge between homoge-
Firstly, the oxidation of benzyl alcohol applying hydrogen
peroxide was investigated as a typical benchmark reaction for
[1]
neous and heterogeneous catalysis. Nanostructured com-
pounds may combine the advantages of both disciplines and
offer unique activity with high selectivity in catalysis.
Unfortunately, unsupported nanoparticles are usually unsta-
ble and the coagulation of the nanoparticles during reaction is
[
8]
liquid-phase oxidation (Table 1). As expected, bulk Fe O
2
3
[
a]
Table 1: Selective oxidation of benzyl alcohol to benzaldehyde.
[2]
frequently unavoidable. Thus, till now, the investigation of
free” nanoparticles as catalysts has been rare, although it is
an important tool to gain a fundamental understanding of
“
[
b]
[c]
Entry
Catalyst
H
2
O
2
[equiv]TON
Selectivity [%]
[3]
catalysis. Clearly, the development of “free” nanoparticles
with tunable catalytic activity is of great significance for both
academia and industry.
1
2
3
4
5
bulk a-Fe O
1
1
1
1
1
1
1
5
4
99
99
97
35
35
88
85
66
96
2
3
bulk g-Fe O
2
3
nano-g-Fe O 1
32
30
25
18
10
12
25
2
2
3
3
Iron compounds are abundantly available and play a
nano-g-Fe
O
2
[4]
Fe(NO ) ·9H O
3 3 2
nano-g-Fe O 1
2 3
crucial role in many biological systems. In comparison with
other transition metals extensively used, iron-based catalysts
are inexpensive, environmentally benign, and relatively non-
toxic. In the past decades, various iron salts have been applied
[
d]
e]
6
7
8
9
[
nano-g-Fe O 1
2
3
[f]
nano-g-Fe O 1
1.5
1
2
3
[g]
nano-g-Fe O 1
2
3
3
+
as Lewis acids (Fe ) in homogeneous catalysis and different
[
5]
[a]10 mmol benzyl alcohol (1.08 g), 1 equiv H
O (10 mmol, 1.0 mL,
2 2
catalytically active iron complexes were also reported. In
heterogeneous catalysis, iron oxides have been frequently
used as catalysts and supports in bulk industrial processes,
3
0 wt% in water), 1 mol% catalyst, 758C, 12 h. H O was added
2
2
continuously over 12 h. [b]Number of moles of aldehyde produced per
mole of catalyst. [c]Selectivity based on alcohol conversion. [d]2 mol%
catalyst. [e]4 mol% catalyst. [f]4 mol% catalyst, 1.5 equiv H 2O2.
[
6]
usually at high temperature (> 3008C) and pressure. As part
of our continuous efforts to develop iron-based catalytic
processes, herein, we report our use of “free” nano-iron
oxide as an oxidation catalyst under mild reaction conditions.
[g]200 mmol benzyl alcohol, 200 mmol H O (30 wt%), 1 mol%
2 2
catalyst, 758C, 12 h, repeated two times.
[7]
(
a or g, particle size ꢀ 100 nm; see Figure S1a,b in the
[
*]Dr. F. Shi, Dr. M. K. Tse, Prof. Dr. M. Beller
Leibniz-Institut für Katalyse e.V. an der Universität Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
and
Center for Life Science Automation (CELISCA)
University of Rostock
Supporting Information) displayed only low activity
(Table 1, entries 1 and 2). Next, we chose different particle
sizes of g-Fe O3 because of their ferrimagnetic property,
hence recovery and recycling of the catalyst can be easily
achieved (see below). In the presence of nano-g-Fe O 1, in
2
[4,9]
2
3
Friedrich-Barnewitz-Strasse 8
which the majority of the particles is 20–50 nm in size
(Figure 1a), the conversion reached 33% with an excellent
benzaldehyde selectivity of 97% (Table 1, entry 3). The
1
8119 Rostock-Warnemünde (Germany)
Fax: (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
Dr. M.-M. Pohl, Dr. A. Brückner
Leibniz-Institut für Katalyse e.V.
Universität Rostock, Außenstelle Berlin
Richard-Willstätter-Strasse 3, 12489 Berlin (Germany)
Dr. S. Zhang
Special Functional Material Laboratory, Henan University
475001, Kaifeng (China)
[
**]This work was supported by the State of Mecklenburg-Western
Pommerania, the Deutsche Forschungsgemeinschaft (SPP 1118
and Leibniz prize), and the Deutsche FMER (BMBF). F.S. thanks the
Alexander-von-Humboldt-Stiftung for an AvH Fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Figure 1. TEM pictures of nano-g-Fe O 1 before use (a) and after
reuse five times (b).
2
3
8866
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8866 –8868
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Angewandte
Chemie
corresponding catalytic activity is higher by a factor of eight
relative to the corresponding bulk Fe O . By applying nano-
Variation of the amount of hydrogen peroxide (1.5 equiv) and
catalyst 1 (4 mol%) revealed an increased conversion of
benzyl alcohol and a product yield of 48% (Table 1, entry 8),
which makes the system already interesting for batch
applications. In this regard, it is interesting that the present
system can be easily scaled up. With 200 mmol (21.6 g) benzyl
alcohol as the starting material, a catalyst turnover number of
25 was obtained with 96% selectivity.
2
3
Fe O 1, the selectivity with respect to hydrogen peroxide
2
3
(
1 equiv) is approximately 30%. Interestingly, nano-g-Fe O3
2
2, which consists of rather uniform particles of markedly
smaller size (3–5 nm; see Figure S1d in the Supporting
Information), showed an even higher activity. Here, the
conversion was 85% but the selectivity was only 35%
(
Table 1, entry 4).
Next, the selective oxidation of various alcohols in the
presence of 1 mol% of “free” nano-g-Fe O was studied. In
2 3
[
12]
Benzoic acid, benzyl benzoate, and oligomers are
observed as side products by GC-MS analysis. Noteworthy,
general, the nanocatalyst exhibits good activity with excellent
selectivity (Table 2). Importantly, note that the ferrimagnetic
the activity and selectivity of nano-g-Fe O 2, which has a
2
3
particle size of 3–5 nm, is already close to that of a
homogeneous iron system in which Fe(NO ) served as a
3
3
[a]
Table 2: Selective oxidation of alcohols to aldehydes and ketones.
3
+
Fe source (Table 1, entry 5). The use of soluble FeCl gave
3
[
b]
[c]
Entry
1
Substrate
Product
TON
Select. [%]
similar results. To exclude the involvement of homogeneous
species in the oxidation with bulk and nano-Fe O , acetone
2
3
10
>99
was added after a catalytic run and the reaction mixture was
filtered. The filtrate was concentrated under vacuum, and the
remaining crude oil was analyzed by atomic absorption
spectroscopy (AAS). The measured iron content is below
the detection limit of our machine (Perkin Elmer Analyst
2
6
>99
3
4
6
>99
>99
3
00). These results clearly illustrate that by downsizing the
14
particle diameter in solid Fe O catalysts, it is possible to
2
3
[
d]
[e]
approach catalytic activity and selectivity values close to
5
70
>99
3
+
those obtained with homogeneous Fe single-site catalysts.
The catalytic activity increases while the selectivity decreases
with decreasing Fe O particle size. Hence, with a suitable
[
a]10 mmol benzyl alcohol (1.08 g), 1.5 equiv H O (10 mmol, 1.0 mL,
2 2
30 wt% in water), 1 mol% catalyst, 758C, 12 h. H O was added
2 2
2
3
continuously over 12 h. [b]Number of moles of aldehyde produced per
mole of catalyst. [c]Selectivity based on alcohol conversion. [d]The
catalyst was reused five times. [e]Total turnover number of five
experiments.
particle size, heterogeneous nano-Fe O particles can display
2
3
improved catalytic activity as well as excellent selectivity as
compared to bulk Fe O .
2
3
X-ray diffraction (XRD) powder patterns confirm that
bulk and nano-Fe O have the same crystal structure. From
2
3
the binding energies derived from X-ray photoelectron
spectroscopy (XPS), it is clear that the surface iron ions are
trivalent in both samples, although minor reduction to Fe
property of g-Fe O made the isolation and reuse of this
catalyst very easy. In the presence of a magnetic stirrer bar,
2
3
2
+
nano-g-Fe O 1 moved onto the stirrer bar steadily and the
2
3
cannot be completely excluded for nano-g-Fe O3 1 as
suggested by the weak shoulder of the main Fe2p3/2 peak at
reaction mixture turned clear within 10 s. The catalyst can be
isolated by simple decantation. After washing with acetone
and drying in air, the nano-Fe O can be directly reused
2
lower binding energies. For nano-g-Fe O 2, the characteristic
2
3
2
3
XRD reflections are not observed. This might be due to the
very low particle size of 3–5 nm which is at the detection limit
of XRD. However, measurements of ferromagnetic reso-
nance indicate that this sample consists of superparamagnetic
particles. This suggests that despite the much smaller particle
size, the molecular structures of nano-g-Fe O 1 and 2 should
without any deactivation even after five rounds of selective
oxidation of cyclooctanol to cyclooctanone. The character-
ization of the nano-g-Fe O before and after reuse five times
2
3
showed the same particle size by transmission electron
microscopy (TEM; Figure 1b) and the same crystal structure
by XRD. The only difference is visible from XPS, which
showed lower peak intensity after the fifth use. This is due to
the increased carbon content of the surface. The C/Fe ratio
rose from 0.64 atom% in fresh nano-g-Fe O 1 to 2.82 atom%
2
3
be similar.
The reason for the improved activity of nano-iron oxide
most probably originates from the nanometer size of the bulk
iron oxide. In general, nanoscale heterogeneous catalysts
should offer higher surface areas, low-coordinated sites, and
surface vacancies, which are responsible for the higher
2
3
after the fifth use in cyclooctanol oxidation. We believe that
this is also the possible reason for the high stability of the
nano-Fe O3 presented herein. A thin layer of carbon is
2
[
10]
catalytic activity.
Theoretically, it can be assumed that
formed during the reaction which prevents significant coag-
ulation of the nano-Fe O . Obviously, carbon-containing
with a decrease of the particle size down to a “molecular”
level, the nanocatalyst behaves as a homogeneous system in
which the catalytic activity is not controlled by the surface
2
3
deposits cover the iron oxide particles partly during reaction
which, however, seems not to be detrimental to the activity.
Notably, the nano-Fe O is not only active for selective
[11]
area of the catalyst but governed by the concentration. The
most important significance of these results is that “free”
nano-Fe O , but not immobilized nano-Fe O , is highly active,
2
3
alcohol oxidations. Aromatic olefins can also be oxidized to
the corresponding aldehydes smoothly in the presence of
nano-g-Fe O 1 (Table 3). For 2-chlorostyrene, the catalyst
2
3
2
3
selective, and stable by merely controlling the particle size.
2
3
Angew. Chem. Int. Ed. 2007, 46, 8866 –8868
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8867
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Communications
turnover number reached a value of up to 31 with greater than
9% chemoselectivity (Table 3, entry 3), while the highest
catalyst activity was exhibited with 4-methylstyrene as
reactant (48% conversion, 75% selectivity).
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Abdelsayed, E. Alsharaeh, M. S. El-Shall, J. Phys. Chem. B 2006,
[
a]
Table 3: Nano-Fe O -catalyzed reaction of olefins to aldehydes.
2
3
110, 19100 – 19103.
[
b]
[c]
Entry
Substrate
Product
TON
Select. [%]
[
4] R. M. Comell, U. Schwertmann, The Iron Oxide: Structures,
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2 3
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yield are based on GC-FID analysis with an external standard of dioxane.
2
2
[
[
b]Number of moles of aldehyde produced per mole of catalyst.
c]Selectivity based on the olefin conversion.
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aldehydes: All reactions were carried out with a multireactor
(
Carousel 12 station, RADLEYS). Benzyl alcohol (1081 mg,
In conclusion, unsupported “free” nano-g-Fe O has been
2
3
10.0 mmol) and nano-Fe O 1 (16.0 mg, 1 mol%) were added to
2
3
shown to be an active, stable, and highly selective catalyst for
various oxidations applying hydrogen peroxide. Clearly, for
general applications in organic synthesis, the activity of the
catalyst system should be further improved. Nevertheless,
because of its simple recyclability, the catalyst is well-suited
for continuous processes. From a scientific point, our results
expand the application of “free” nanoparticles. They should
be helpful to understand the relationship between homoge-
neous and heterogeneous catalysis and for the development
of new catalytic systems.
a glass reactor (ca. 50 mL). The reaction mixture was vigorously
stirred (500–750 rpm) at 758C. H (30 wt% in water, 1.0 mL,
0.0 mmol; VWR) was added continuously over 12 h. The
mixture was then cooled to room temperature, and dioxane
1760 mg, 20 mmol) was added for qualitative analysis by GC-
O
2
2
1
(
FID.
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Received: July 30, 2007
Revised: August 28, 2007
Published online: October 10, 2007
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[
[
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temperature 758C). The olefin (10.0 mmol), H O (30 wt% in
Keywords: alcohols · aldehydes · iron · nanoparticles · oxidation
.
2
2
[
1] a) G. A. Somorjai, A. M. Contreras, M. Montano, R. M. Rioux,
water, 2.0 mL, 20.0 mmol; VWR), and nano-Fe O 1 (16.0 mg, 1
2 3
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8868 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8866 –8868
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